Do air-driven gyroscopic instruments suffer from icing or any blockage of filter? Do gyro instruments suffer from icing in an air-driven gyro system? Do they have any particular way to protect them against blockages of filter or any type of other blockage? (Alternate source?)

I am more concerned on light aircraft C152, C172, PA28, etc.

From all that I have seen on that type of aircraft, there is merely a filter.  If one was operating in a dusty environment (eg the bush) then I would add those to a 100 hour check.  But honestly, I have never seen one clogged to the point it would not work.

They are not suspectable to icing as there is a low pressure drop across them, and they get their air from inside the cabin, where there is not a cold mist. It's usually rather dry and at low temperatures, it is probably heated.

The alternate air is for the static port, which is exterior, and will ice up, although it is a rather infrequent occurrence. The altimeter, airspeed and VSI are not gyro instruments, and are impacted by the static port. Keep in mind that if your altimeter is not reading correctly because of static port ice, then most likely your encoding altimeter is similarly off, and your mode C (or other) readout at ATC will be off. In my experience, the Cessna models you listed tend to ice less than the PA28 models, but again, it is rare, even for weather flyers. And if it is happening to you, you need to reexamine your weather choices.

While we are talking blockages, the pitot tube is the most frequent, and there have been times that I have had to remove a bug from the tube on the Cessnas. Let a mechanic do it the first time, so that you get some of the tricks. I usually use a straightened paper clip or a piece of safety wire, with a sharp hook, about 0.3mm bent in it. That will help you scoop out the bug.  Again, this doesn't happen often, as usually we have covers on pitot tubes, but I have had it happen in remote areas (like Central American hilltop unimproved airports) so if you are flying to those places you need to make friends with your mechanic so that you can manage the lots of different things that come up.

In summary, the vacuum gyros have a filter, and they don't tend to clog often. Some of them are rather coarse, but modern ones have 10 micron filters or better. If you are in an extremely dusty area, they should be periodically inspected. For most people, they will never have a problem with them.  (If you have that much dust in the cockpit, then you will have other problems, including avionics switches, dirt creeping into displays, etc.)


Does the airflow from a propeller over a wing contribute to lift? How I understand it, the perfect propeller would move air rearward at the exact same velocity as the free stream, i.e. the propeller air will merge with the free stream and have no real influence on the flow-field over the wing.

However a non-ideal propeller (thus all real propellers) accelerates the air to a higher speed than the free-stream and thus alters the flow-field over the wing.

My question is: Does this airflow from the propeller increase the lift generated by the wing?

My question obviously assumes that the propeller and wing is in the same plane and that the air from the propeller thus hits the wing.

A propeller not affecting the velocity of the free stream air would not generate any thrust (or drag), as the propeller works by the principle of creating thrust by accelerating a large mass of air, $F = m * a$.

The accelerated airflow from the propeller does generally increase the lift generated by the wing. The amount of lift increase depends on the configuration (how exposed the wing is to the slipstream of the propeller). This tends to be particularily noticeable on light twins:

(Image attribution: Wikimedia Commons)

There's also another effect on lift, coming from the corkscrew-shaped vortex of the propeller slipstream, perhaps more noticeable on single engine aircraft:

(Image attribution: FAA Pilots Handbook of Aeronautical Knowledge)

The image is a little bit unfortunate, as it demonstrates another effect as well (yaw from the slipstream), but you can also see how propeller slipstream will affect the flow pattern around the wing roots: on one side, it will tend to increase the effective angle of attack, leading to higher lift on that side, and on the other side it will decrease the effective angle of attack, decreasing lift on that side, leading to a roll-moment. This effect of course exists in twin engine aircraft too, but may be less noticeable, as the effect is more symmetrical, in the case of co-rotating propellers, or even symmetrical (in the case of counter-rotating propellers).


Is there any way I can check the status of my medical? I was deferred my third class medical because of a certain condition and the doctor said I’ll hear from the FAA in about 3-4 weeks with either my certificate  or they’ll ask for more information.  It’s been 3 weeks going on 4th week, and I’ve heard of them taking 2 months or longer so I was just wondering is there any way I could check the status of my medical like it can tell me it’s been sent back or is currently being reviewed or etc?

The standard way to check the status of your medical application is to call the Aerospace Medical Certification Division at (405) 954-4821.

When you call, it's very likely that you will hear a recorded message saying that due to high call volumes, they are unable to assist you. If you do, call again later. When I was checking on the status of my own application, I had to call quite a few times before my call was finally answered.


How much physical strength is required to control a Cessna 172? I have heard a number of folks tell me that one needs a certain amount of physical strength to be able to control the Cessna 172. Is that true? I'm flying sims now, just wanted to know.

I will take this from a more general perspective.

A Cessna 172 is part of the CS-23 category (in EASA land, see the equivalent Part 23 for FAA land).

In the relevant document, point CS 23.143, we find the maximum forces a pilot might be required to exert on the control columns so that the aircraft can be certified.

As the Cessna 172 is certified, we then know that it respects these requirements, so the maximum forces required to pilot a Cessna 172 (and any other CS-23 aircraft) are:

\begin{array}{l|c|c|c|}
\textrm{Values in Newton applied} & & & \\
\textrm{to the relevant control} & \textrm{Pitch} & \textrm{Roll} & \textrm{Yaw}\\
\hline
\ \ \ \ \textrm{For temporary application –} & & & \\
\textrm{Stick} & 267 \textrm{ N} & 133 \textrm{ N} & \\
\textrm{Wheel (two hands on rim)} & 334 \textrm{ N} & 222 \textrm{ N} & \\
\textrm{Wheel (one hand on rim)} & 222 \textrm{ N} & 111 \textrm{ N} & \\
\textrm{Rudder pedal} & & & 667 \textrm{ N} \\
\hline
\ \ \ \ \textrm{For prolonged application –} & 44.5\textrm{ N} & 22\textrm{ N} & 89\textrm{ N} \\
\end{array}


What is the current status of the deployment of RNP 1 and RNP 2 airspaces and approaches in Europe? Is there an overview of the deployment of RNP 1 and RNP 2 airspaces and approaches in Europe?

I'm looking for a map, that shows this information. Maybe even 'special' solutions that are not quite RNP.

A few years ago, each country in Europe had its own authority and plan for air traffic management. But this little continent is today the most busy in regard to air traffic, and without action, growth is nearly impossible. Thus countries have agreed to build a "single sky" system, the initiative is known as SESAR (here is the master plan). Those actions are part of the "Pilot Common Project" (PCP), sub-projects include "ATM functionalities 1" (AF 1). From the master plan:

  Enhanced terminal airspace using RNP-based operations consists of the
  implementation of environmentally friendly procedures for
  arrival/departure and approach using PBN in high-density TMAs, e.g.
  RNP 1 standard instrument departures and arrivals (RNP 1 SIDs and
  STARs).

and:

  Enhanced terminal airspace using RNP-based operations with RNP 1 SIDs,
  STARs and transitions (using radius to a fix-RF) and RNP APCH (lateral
  navigation/vertical navigation (LNAV/VNAV)) is planned for
  synchronised deployment at the 24 busiest airports in Europe by 1
  January 2024

The schedule:

The current implementation status for the 24 airports selected (from SESAR monitoring view):

While initially planned for 2018, RNP operational use is now planned for January 2024. Out of the 24 high density TMA (equivalent of FAA TCA), two are already ready (and RNP 1 is already operational for helicopter low routes in Oslo -- this early use is likely for environmental reasons).

I made this question a community question, it is open to improvement by anyone.


Do autopilots have an 'activate' switch/button, in addition to 'on' and 'off'? Say cruising at FL310 and on a 270 heading with autopilot on. If pilot decides to descend to say, FL180 on a 180 heading, is simply changing the altitude and heading numbers on the autopilot control panel enough for the autopilot to get going with the new commands, or after changing the altitude and heading request, there is some sort of button that commands the autopilot to execute the new commands? And if so, where is that switch (or button) and what is it called?

It is complicated (-;

First thing first, autopilot systems come with a variety of forms and complexity. The autopilot on a Cessna 172 or Piper Cherokee is going to be very different than the one on a Airbus or Boeing. Different manufacturer will also design their autopilot interface differently; while there are certainly similarities, there is no set standard in the aviation industry for autopilot interface and behavior.

  (...) is simply changing the altitude and heading numbers, or (...)
  there is some sort of button that commands the autopilot to execute
  the new commands?

Both.

Here I will discuss using the Boeing 777 autopilot, which I am familiar with. On the autopilot panel, there is a heading window, which displays a 3-digit number. Directly below the window is a knob, turning the knob counterclockwise will decrease the heading, while turning clockwise will increase it. The knob can also be pushed.

The heading section of the Model Control Panel on a Boeing 777. The knob is a 5-in-1 control: the outer ring can be rotated, the knob can be rotated, and the center of the knob is a push button.

If the autopilot is in "HDL SEL" mode, it will fly the heading specified in the window. In this mode, if the pilot rotates the knob to adjust the heading, the new heading will take effect immediately. However, if it is in "HDL HOLD" mode, turning the knob will adjust the number, but it will have no effect - the plane will hold the current heading. Pushing the knob will change the mode from "HDL HOLD" to "HDL SEL", and the plane will then turn towards the selected heading.

A similar logic exists when climbing using the "VNAV" mode. Let's say the plane is initially cleared to 5000 feet after takeoff. The pilot set the altitude window to 5000 prior to the takeoff roll. Once autopilot is engaged, it will climb to 5000 feet, after which it will hold that altitude. At any point, the altitude knob can be rotated to set a new altitude, say 7000 feet, however it will not take effect - the plane will still climb to 5000 feet, then stop the climb there. To climb to a higher altitude, the pilots have to push the knob, after which the plane will initiate a new climb from 5000 feet to 7000 feet. This is because ATC often clear planes to "step climb", i.e. climbing then holding at certain altitudes until reaching the cruise altitude. The autopilot function is designed around that.

  where is that switch (or button)?

It depends on the model. Other manufacturers call them differently, but the logic is similar. Also, advanced autopilots have a lot of functions; the ones I discussed are just selected examples about heading and altitude selections. Pilots can also "arm" an autopilot mode, i.e. mark it so that it will become engaged when certain conditions meet. The specifics of that are too long to describe in this post.

Like I said, it gets complicated.


What is the procedure if both pilots are incapacitated? Let's say both pilots passed out , is there anyone else from cabin crew that is trained enough to land a plane?

To put it short: no.

Unfortunately it might even take a while for anyone to notice both pilots are out. Most likely the first would be an air traffic controller since he/she couldn't reach the pilots on radio.
If the cabin attendants find both pilots incapacitated, they would most likely

try to find a doctor or nurse to help recover a pilot
try to find if any of the passengers was a pilot


Is there a disadvantage to the "fan in fin" design seen for the tailrotor on many Eurocopter helicopters? A quick google search will show that a lot of Eurocopter models have their tail rotors contained within their tail booms, while a look at Bell or Sikorsky models (and I'm sure others: I'm using Bell and Sikorsky because they're major non-European manufacturers) shows out-of-boom tail rotors. I can see a couple of advantages to the Eurocopter design:

Less danger: the fan blades can be protected from harm and be further removed from people.
Less noise: I don't have empirical evidence for this, but drawing on knowledge of other ducted systems, this design probably produces less noise, at least in the directions perpendicular to airflow.
Pure symmetry: maybe I just like this from an aesthetic perspective, but unlike the unbalanced external tail rotor design, the in-boom design is perfectly balanced.

Is this design less efficient? Is it the space constraints placed on the rotor by the boom?

The fenestron is a lot heavier than a free tail rotor. This is probably the main disadvantage; helicopters are more sensitive to extra mass than aircraft.

At the time Eurocopter was produced by joining Aerospatiale's and MBB's helicopter divisions in 1992, a follow-on to the successful Bo-105 was under development. That was the Bo-108. Immediately, development was stopped to implement a fenestron into the finished design in order to give all Eurocopter products a common, distinctive feature. This resulted in major structural changes and a heavier helicopter. The result is now known as the EC135.

From this Aerospaceweb article:

  The only significant drawback to ducted fans like the Fenestron is
  that the shroud adds weight that offsets at least some of the
  improvements in performance.

As @Koyovis' answer points out, the low position of the lateral thrust vector is also a clear disadvantage. See here for more.


Why isn't the U-2 used for commercial purposes? There are some companies that use decommissioned military aircraft for commercial high-altitude flights (for, say, tourists). Examples would be the MiG-31 and the MiG-29:

Why isn't the Lockheed U-2 used for such flights? Here is a two-seat version of the U-2:

( Image source )

Is the U-2 still secret? Or is it expensive to build?

It is expensive to operate, and there is no economic value in what it does.

Apart from reconnaissance and espionage, the U-2 is used for high-altitude atmospheric research, but since it is operated by NASA and man-carrying, it is subject to an immense bureaucracy. When you want to fly your own scientific instrument on it, you need to go through years of certification procedures. The instruments which are rated to fly on the U-2 are ancient and much less precise than what can be built today. Therefore, most high altitude research is done by balloons, where the bureaucracy is much less suffocating.


Does propeller size affect total drag force of plane? Integration of pressure distribution of  each blade  get net force(thrust) which is allways pointing forward.So there is no drag force at  any propeller ,that tells that size of propeller doesnt change total drag force.
So plane with propeller of 1m diameter will has same total drag force as plane with propeller of 2.5m diameter?
(if both planes are same and both propellers have same thrust force,and both propellers are fixed at tail so wash not blow at plane body,causing aditional skin friction and pressure drag)
boat analogy:
Why all speeds boats use small propellers,instead big ones?I allways think reason was big propellers increase boat overall hydrodynamic drag,but when I better think propeller cant cause drag when producing thrust!

the drag force that a propeller generates while under power is expressed as a torque applied to the engine's crankshaft and arises because of skin friction on the propeller blade surfaces. as long as the prop is generating net thrust it is not generating drag in a direction opposite to its direction of travel.
Prudent design of the prop blades guarantees that no portion of the blade is ever generating backwards thrust for all possible combinations of airspeed, RPM, and blade pitch.
A propeller that is not generating thrust is generating drag which does act against the airplane's direction of travel.


How to design a propeller for UAV? I need some advice on designing a propeller for a UAV and choosing the right motor for it. I am doing a project which requires the fix-wing UAV to travel at cruise speed of 20 m/s and weighs a total of 12kg, the propeller will have to be a pusher and the aircraft has a L/D of 35 at cruise. I am told by the lecturer to ignore take off velocities as it takes off on a seperate VTOL system before the propeller is activated at 300m for horizontal flight.

As a mechanical engineering student I don't have much experience in aerodynamics. So far I've looked into blade element theory and airfoil designs but there are many different variables to consider such as airfoil profile, pitch, angle of attack, diameter and I'm at a bit of a lost on how to determine these values to match the UAV requirements. So I would like to ask what is the best way I can go about doing this? The ultimate goal is to maximise flight time by designing a propeller and reducing power requirements.

It's been a long time since I did this but here's then general idea.

Work out your requirements.
Thrust, speed (usually cruise speed) and diameter (usually determined by the available space) being the key ones.
Use a piece of software such as this one to optimise other design parameters for your requirements.
You may also consider off-design (non-cruise) performance.

The link above also contains more details about the design process and considerations.


What is the effect of opening airplane bay doors on the radar cross section of the airplane? How does opening airplane bay doors (especially in the case of bombers), increase the radar cross section of the airplane and make it more visible to the radar?

  These are usually combined with active measures such as carefully planning all mission maneuvers in order to minimize the aircraft's radar cross section, since common actions such as hard turns or opening bomb bay doors can more than double an otherwise stealthy aircraft's radar return

Source

Radar waves are sent back to the radar by two mechanisms, reflection and diffraction.  Opening a bay door may present a surface that is angled such that waves will be reflected to the radar.  As for diffraction, low observable (stealth) aircraft are designed so that a radar wave encountering the aircraft's skin will travel along the skin and be gradually absorbed.  Opening the bay doors creates a break in the skin, causing waves on the skin to diffract.  Some of the radar energy is returned to the sender.

For an amateur looking at "stealth" diffraction is often not accounted for.


Where can I find data on the number of single propeller aircraft currently in operation in the EU and US? Are there publicly available data for aircraft registered in operation?

Specifically I am looking for number of single propeller aircraft in US/EU, ideally with aircraft types as well. However, any useful data of this sort would be appreciated.

You can download the entire FAA aircraft registration database and analyze it however you want.

I have no idea about EASA, but because aircraft registration is delegated to member states you'll probably have to look at each one individually (e.g. the UK). I don't know if all EASA countries consider aircraft registration information to be public or not.

You could also look for a commercial data provider that sells the data you need, of course.


What is a "cross check"? Shortly before take-off, and again before landing, the cockpit (no way to tell whether it was the Captain or First Officer) came on over the speakers, stating:

  Flight Attendant's, prepare for take-off/landing and cross-check

What is a cross-check?

This website has the definition:

  Crosscheck is a generic term used by pilots and flight attendants
  meaning that one person has verified the task of another. In the
  cabin, flight attendants crosscheck one another’s stations to make
  sure the doors are armed or disarmed as necessary.


What are cowl flaps? What are cowl flaps and what are they used for? It seems I have only heard of them in connection with old planes, are they peculiar to radial engines?

They are flaps or doors on the engine cowling that open up in order to provide increased cooling airflow for the engine. They look like this when open. They are generally controlled by a handle in the cockpit (some aircraft have electric cowl flaps with an open/close switch).

The disadvantage is that the open flaps also increase drag. So on airplanes equipped with cowl flaps, they are generally opened for low-speed, high-power situations such as takeoff. This is the harshest operational regime for the engine since it is at its highest power and relatively low cooling airflow due to low airspeed. They are then closed either reaching cruise or reaching a point where the airplane can transition to a lower-power, higher-speed climb profile.

They're not specific to radial engines, but do tend to be more common in older aircraft (there are exceptions though). Newer airplane and engine designs are more likely to have sufficient inherent cooling airflow to not require cowl flaps.

The photo above comes from AVweb, which also has a nice article explaining more about the function and usage of cowl flaps.


Are compressor maps for multistage compressors drawn for one blade row or the whole compressor? Apologies if this is a stupid question.
I am a little confused about compressor maps.
Are compressor maps such as this one, drawn for one blade row only or the whole compressor?

(source: Wikimedia)

If it is for one blade row, can you predict the behavior of the next blade row from the compressor map of the previous row? Like, I am guessing that the blade rows of say the HP compressor have the same speed lines because they are on the same spool, so would they move up or down the speed line?

If those maps are drawn for the whole compressor, then does that mean that all blade rows have the same operating conditions (in my opinion, they should have different operating conditions)?

Any insight would be really appreciated! Thank you!

The image you are referencing is definitely an entire compressor.  You can tell because the pressure ratio axis goes up to 17:1.  You won't get 17:1 out of a single blade row, ever. One-tenth of that would be more likely for a single stage.  i.e. this could be a 10 stage compressor with each stage contributing a 1.3 - 1.4:1 pressure ratio.

I've only ever seen compressor maps like this drawn for entire compressors.  Not to say you couldn't do a single stage, but I've not seen it done.


Are there any guidelines on airframe age and metal fatigue in light aircraft? Are there any guidelines or rules of thumb about when airframes become affected by metal fatigue?

More specifically, is something like a 1950s vintage Cessna still safe, or should I be worried about metal fatigue? I'm thinking of buying and completely overhauling a C172, but how can I determine if I should be concerned about metal fatigue or not? The planes are so old, and it doesn't seem possible to me that their structure has not been compromised because of their age.

The short answer is no, age or airframe hours alone are not something to be concerned about on these light Cessnas.

A 1975 C172M I flew for work had about 20,000 hours, or more, on the airframe (can't remember exactly), largely in low level flight at or below 500 ft AGL, often in high winds, moderate turbulence, low level wind shear, etc., and flight including plenty of relatively high G maneuvering, and lots of crosswind landings far exceeding the demonstrated crosswind component. Years ago, before my employer bought it, someone managed to flip it over on it's back while taxiing. The thing has also had several bird strikes.

As an A&P mechanic, I have thoroughly inspected this airframe, to the level of an annual/100 hr inspection. Keeping in mind the airframe's age and history, my inspections included paying special attention to items of structural interest such as spar and strut attach points, control surface attach points, firewall, spars, etc. I have no concerns about the structure of this particular airframe, and I've trusted my life to it over 840 flight hours.

I have quite a bit of experience maintaining a number of single engine Cessna models, including quite a number of older, well worn individuals. I cannot recall encountering any structural issues that were due to metal fatigue alone. Most of the structural instances I can think of would be due to one time events such as a hard landing causing a firewall to wrinkle or crack. Some, such as cracked gear strut castings—in a U206G for example—are relatively common on such airframes that are used for rough, backcountry flying. I suppose that could be chalked up to a type of metal fatigue, but I don't think it is what you are concerned about with this question. Such items are a known issue and won't cause you to fall out of the sky.

The real age-related concerns regarding structural issues will be corrosion. Obviously, corrosion can occur over a short time frame, but give an airframe an extended stay in a harsh, corrosion conducive environment and it may fare more poorly than in a "good" environment. That being said, there are plenty of great airframes from the '50-'70s that have been hangered or sitting in barns in humid climates that are in excellent shape. The same will probably not be true of the engines if they sat unused, but the airframes may be in great shape.

Every airframe is different, and a thorough pre-buy inspection will be necessary to ensure that yours does not suffer from some major structural issue. But age and hours alone should not be of real concern.

I would not hesitate to buy a high-time, vintage Cessna airframe merely on account of those factors.


Have the results of the USAF's $AVE program research been implemented? According to this Aviation Week article from October 2012, the US Air Force Research Laboratory ran a program called Surfing Aircraft Vortices for Energy ($AVE) to test/prove that autopilot settings could be adjusted to allow C-17s to fly in formation using the wingtip vortices of the lead plane to reduce fuel consumption of the following planes. They seem to have found upwards of a 10% reduction in fuel for the following planes.

The article indicated that previous experiments involved hand-flying of the aircraft in formation which required the equivalent concentration, focus and exhaustion levels of aerial refueling and was unfeasible for long duration flights, therefore the tests proposed to incorporate the formation flying into the autopilot

Does anyone know if these auto-pilot configurations have been put into use in line C-17s or other USAF transport aircraft?

The C-17 has a special autopilot system including a component called the MILCAS-FR formation flying system by Honeywell and here is a link a sales brochure.

My military flight time was not in a C-17 but in my opinion once the precise formation locations were determined that achieve the fuel savings in the article it would be a simple decision to use them on any given formation flight based on the tactical situation.  The requirements of each mission would drive that decision.


Why do pilots' seat covers almost uniformly use sheepskin? While browsing this answer I noticed the sheepskin cover and recalled that I couldn't remember a cockpit that didn't have them. It's not entirely surprising that they're used, but I am surprised that they seem to be always used. Are they indeed ubiquitous? Are there any competing alternatives? Do modern airliners use them as much as GA/military/sport aircraft?

They aren't "almost uniformly sheepskin", but they are absolutely uniformly flame-resistant (or self-extinguishing) as required for aircraft certification.

Sheepskin (and other natural wool fabrics) are both flame resistant and self-extinguishing (which is why industrial fire blankets are often made of wool), and sheepskin tends to wear well, so it's a good choice for the two seats that will absolutely always be occupied when an aircraft is being operated.
In terms of comfort, sheepskin covers also breathe well (so the pilots aren't sweating profusely while operating the aircraft - one of the reasons they're popular accessories for cars).

Other popular fabric choices are natural leather, flame-resistant synthetic leather (vinyl), and various other textiles which pass the FAA "burn test" requirements (test procedures are described in Part 23 Appendix F for light aircraft, Part 25 appendix F for transport category aircraft).


Where to find a waypoints/fixed points, navaids and airports database? I wonder if it is possible to download a database with waypoints, navaids, airports and airways between them. I've been working with AIP documents but it's difficult to get raw information from them and they only have information about waypoints.

I happened to find skyvector.com and I thought there would be an easier way to obtain that amount of information.

Nice question. There are several ways how to get these information.

http://www.fallingrain.com/: free to use, no registration needed. World database of the airports and waypoints. From these information you could easily make DB for waypoints.
EUROCONTROL EAD (https://www.ead.eurocontrol.int/eadcms/eadsite/index.php.html): free to use, registration needed. Big advantage - real and up to date information for all ECTL countries. But a bit difficult navigation.
JeppView (http://ww1.jeppesen.com/): very expensive SW, but the best in this time. You will find there all information which pilot needs. But it is not real database.
Garmin for pilots (http://www.garmin.com/en-US/explore/intheair/): another not free SW, very similair to JeppView but lightweighted.
NAVIGRAPH (http://www.navigraph.com): Great DB of all things which you need in FMC. But I guess that they have it just for flight simulator.
OpenFlights (http://openflights.org/data.html): free DB of airports, airlines and routes. You could downlod all data in one file :-)


Why does cabin air have to be dehumidified? When air is piped in from outside and into a pressurized cabin, it is also dehumidified in the process.  All airlines provide beverage service on even short-haul flights to combat the dryness of the air.  Why is it necessary to dehumidify cabin air?

A lot of consideration is placed on the airframe (which has to hold for 20+ years) rather than the passengers.

One of the primary reasons is to slow corrosion of the  aluminium airframe. This is also why the Boeing 787 is a lot more humid - corrosion is no longer an issue in a composite fuselage (or a lot less at least).

Boeing and the Economist sum it up nicely I think:

  the greatest issue lies with the cold fuselage skin interacting with
  moist air, which will condensate and speed up the rate of corrosion.

  But condensation in the gap between cabin and hull can be lethal.
  This gap contains much of an aircraft's wiring, and water can damage
  that wiring's insulation. Such a problem is thought to have
  contributed to the loss of a Swissair DC-11 off the Atlantic coast of
  America a few years ago. Too much condensation causes other
  difficulties, as well. There have been cases when ice has built up
  inside the tailplanes of aircraft, causing their rudders to freeze and
  thus preventing their pilots from steering them.

  An aircraft such as a Boeing 747-400 can accumulate as much as 700kg of condensation before it reaches equilibrium. Even though
  that is only about 0.17% of the 400 tonnes such an aircraft weighs
  fully laden, it is enough to cause problems for the pilot when he
  tries to trim the balance of the aircraft. And 700kg is about the
  weight of nine men. So the aircraft is carrying the equivalent of at
  least nine non-fare-paying passengers, in an industry where every
  extra kilogram affects fuel consumption and profit margins.


What is the low resistance design path in an aircraft to withstand a lightning strike? I know in a building we have lightning rods that conduct a lightning strike into the earth, but for an aircarft there is no ground to 'ground' so I assume the lightning enters and passes through the aircraft. Therefore, since some areas must have high resistance paths of protection, for example a path through the antennas or main electronics bay, they must be an alternate low resistance path capable of conducting a very high power flow with minimum damage and burning. What is this design path?

The conductive exterior body

Almost all aircraft have metal bodies, the electric current flows through the exterior metal.

Aircraft with composite hulls can have conducting material added to the composites to provide a path for lightning. This is typically a metal mesh.

Some smaller composite aircraft, lacking this sort of provision, simply cannot fly near clouds or anywhere where there is a risk of lightning.

photo by Boeing - see references

The outer conducting surface acts as a Faraday cage preventing the high voltages and currents from affecting the interior.

 a Faraday cage photo by Antoine Taveneaux

  It is estimated that on average, each airplane in the U.S. commercial
  fleet is struck lightly by lightning more than once each year.

  Initially, the lightning will attach to an extremity such as the nose
  or wing tip. The airplane then flies through the lightning flash,
  which reattaches itself to the fuselage at other locations while the
  airplane is in the electric "circuit" between the cloud regions of
  opposite polarity. The current will travel through the conductive
  exterior skin and structures of the aircraft and exit off some other
  extremity, such as the tail. Pilots occasionally report temporary
  flickering of lights or short-lived interference with instruments.

  Every circuit and piece of equipment that is critical or essential to
  the safe flight and landing of an aircraft must be verified by the
  manufacturers to be protected against lightning in accordance with
  regulations set by the Federal Aviation Administration (FAA) or a
  similar authority in the country of the aircraft's origin.

From Scientific American - What happens when lightning strikes an airplane?

  Planes are now built to absorb 250,000 amps, whereas the average
  strike generates 32,000 amps.

  "It only gets really serious when the radome [nose cone] is struck,
  the only part of the plane's shell not made of metal, as this is where
  the radar is located. But nose cones have special lightning conductors
  for just this reason."

From The Guardian - Should you be worried about lightning striking your plane?

  The 787 flight test team gathered the unexpected data last month after
  one of the Dreamliner test aircaft was struck by lightning. Unlike
  traditional aluminum aircraft where the entire aircraft is conductive,
  on a composite airplane the charge from a lightning strike would find
  its way to the conductive parts such as wiring or hinges. In order to
  avoid the risk of the charge damaging these kinds of parts, Boeing
  had to add conductive material to the composites in order to provide a
  pathway for lightning strikes.

From Boeing 787 Withstands Lightning Strike

Other references

Boeing - Lightning Strikes, Protection, Inspection and Repair
Protecting the 787
Lightning strike protection strategies for composite aircraft


How do planes respond aerodynamically when bringing them into inverted flight by rolling? For the sake of the question the plane is fully capable of inverted
flight, no problems with engines etc.

Background of the question is the observation that the airflow around
the wing must also invert the direction. If I look from the end of the right wing in direction of the fuselage, I see the air moving around the wing counterclockwise, while inverted it must spin clockwise. So at one point the relative air flow must cease completely on average, there is no difference anymore between top and bottom of the wing.

While the airflow is not turbulent, this should feel for the pilot like stalling because there is no lift, also the wingtip vortices should stop. The "no lift" position should be somewhere near the point when the plane has its wings vertical.

So once a pilot brings a plane into a roll, what exactly happens? Is it a smooth transition from full lift to no lift to full lift back or is there a sudden loss of lift which must be overcome by spin inertia? I have also the suspicion that trying a roll with a too high angle of attack risks a real stall because the restart of the circulation around the wing is hampered.

I fly such a plane.  My Laser has full inverted systems (oil, gas) and symmetrical airfoils.

You're right that when in upright flight the wings are producing lift in the direction away from the wheels and when in inverted flight the lift vector points in the opposite direction.

You're also correct that the wings are producing zero lift at 90 degrees of bank.

As for what happens during the roll, there's a smooth transition.  Assuming you're maintaining a constant heading, the lift provided by the wings will decrease to zero by the 90 degree of bank point, then resume in full at 180 degrees of bank in the opposite direction relative to the wing.  As lift decreases, the nose of the aircraft will typically decrease precipitously.  It's not an aerodynamic stall and there's no sudden onset.  It's just like rolling into a diving turn with the turn.

This is why trying to roll an airplane with no previous aerobatic training can be fatal.  That "simple roll" you tried can easily result in a >Vne dive.

There are 3 ways to not lose altitude during the roll:

As you suggested, roll fast enough.  My plane will roll a full 360 degrees in a bit over 1 second.  There's not much time to lose altitude :-)
Use "top rudder" (the foot closest to the sky) to lift the nose up.  At the 90 degree point you're using the side of the fuselage as a crude wing.  It works in the right airplane.  It requires an unusually large rudder and a lot of practice.
Anticipate the loss by pitching up a LOT before rolling.  You'll get more of a barrel roll, but it's still a roll.

Please don't try this without competent training.  Stupid people die this way.


How will a damaged logbook affect pursuing a career as a professional pilot? I recently damaged my log book by putting it in the washer machine. I have picked through all the pieces and saved what little legible logs are left. I have requested a copy of my 8710 from the FAA. With sudden funding I have decided to pursue a career as a professional pilot. I have contacted the owner of the previous aircraft flown and requested copies of the aircraft logs. With all the information, I plan on reconstructing my log book. My question is, with my 8710 proving that I did receive my license, I also had an additional 40-50 hours of flight time that I would like to put in my log book. Should I expect any future hiccups with additional ratings and employment as a pilot? What else could I do to make this process smoother?

Well, you've already covered the "What to do if you've lost your logbook" bit.  The remaining bit of your question is a little trickier to answer, but not as bad as you might think.

The good part about renting is that as you've noted you can get the rental records & that gives you a good start on at least the hours portion of reconstructing your logs. The Day/Night, Instrument/Simulated Instrument, and destinations (Cross-Country) times may be a bit harder to reconstruct, but with a little luck you'll be able to piece that together too.

Will you have trouble with additional ratings? No.
This is the tricky bit though: You will need logs that prove that you meet the aeronautical experience requirements for any new rating you're pursuing, so if you're working through your instrument/commercial/multi and you just washed the records of that training time, cross-country time, etc. you're going to need to either reconstruct that (from rental records & landing fee receipts), fly the hours over again, or some combination of the two.
The same goes for any endorsements you have (tailwheel, high-performance, etc.) - you'll need to get re-endorsed to have a record of those (and if you show up to your next practical test in an aircraft that requires an endorsement the examiner may ask to see that endorsement).
Will you have trouble with employment? I doubt it.
Your certificates are your major qualification, and you'll have those (subject to reconstructing the requisite aeronautical experience entries like I mentioned above).
You'll want to reconstruct as much of your logs as possible because all that experience should count when you go sit down with an employer, but if you've "only" washed ~50 hours you'll make up most or all of that experience in your future training.

A few related questions you might want to look at:

What is the best way to keep logbook entries backed up?
(For my part: I scan each page once I've filled it.)
Are there any best practices for transitioning from paper logbooks to electronic logbooks?
(if you decide to go that route)


Can someone identify this scrapped high-wing plane? I consider myself to be a decent aircraft identifier, and when I saw this plane when exploring an aircraft graveyard, I was not able to identify it. I took a pic and showed it to several aviation Facebook groups, and nobody was able to identify it. Maybe we’re just being dumb and it’s something simple, but can somebody help? Thanks!

It's an Aermacchi AL-60-B1 (original design by Al Mooney, Lockheed L-402 Bushmaster aka Lockheed 60)

(source)

(source)


Why do people say aviation regulations are written in blood? I have heard it said that all aviation regulations are "written in blood". I think this means that every rule in the book exists because someone died - and there was a lesson learned. Is this accurate? Is there anything else to the story of this phrase?

No, that's all it means. Aviation regulations tend to be put in place in response to accident reports, where a plane crashed or almost crashed for lack of the regulations. So, if you're confused why a stupid-seeming regulation exists, there's a good chance that people died because of the thing the regulation changed.

Obviously, not all regulations are like that; for instance, noise limits are based on quality of life for those near airports, not crashes. However, a good number of aviation rules came into effect as recommendations following a crash report.


What is the impact of one trailing edge rotary actuator failing? I know aircraft commonly have rotary actuators to extend and retract the flaps.  I am not sure how many but I think I read two per flap on a 747. My question is what is the result if one actuator fails? I don't know if more then one needs to fail in order for a flap not to extend or retract.  I am mostly wondering if could cause an aircraft turn-back because somebody told me it could. However that does not seem right to me.  I thought the flaps are extended before take-off so that worst case scenario prior to flight is a minor flight delay to replace it.

I suppose if it failed just prior to landing that would be the other scenario and am not sure what the worst impact on a pilots ability to land would be (assuming one failed unit would prevent a flap extension?).

Any experienced pilot probably knows more then I do and any insights are welcome.

The 747 has two motors per flap group.  The primary means of moving them is through a hydraulic motor, while the second motor is a backup electric motor which is only used if the primary doesn't work correctly.

There are also two groups of flaps on each wing, and they are mechanically linked to the same flap group on the other wing so that they always extend at the same time.

You are correct about a failure on the ground.  The pilots would take the airplane back for maintenance before they departed.

In the unlikely event that both the primary and secondary motors failed, or if the flaps were to somehow become jammed and not extend once airborne, the flaps would not be able to do their job.  Since they wouldn't make the wing bigger or change it's shape to create more lift, the aircraft would be forced to land at a higher airspeed, which would cause it to use more runway.  This isn't a big deal if there is sufficient runway at their destination, but if not they would have to divert to a suitable airport.


How does a pilot tell passengers to prepare for severe turbulence without causing panic? At the start of a flight, the pilot of an aircraft often talks about the expected weather on the flight path, and tells passengers if there is to be some turbulence during the flight.

How does a pilot explain that there will be severe turbulence to their passengers without inciting panic?

Turbulence is reported on a scale of light, moderate, severe, and extreme.  The full definitions are available in the FAA Aeronautical Information Manual. As experienced in the cabin,

For Light Turbulence, "Occupants may feel a slight strain against belts or shoulder straps. Unsecured objects may be displaced slightly. Food service may be conducted and little or no difficulty is encountered in walking."
For Moderate Turbulence, "Occupants feel definite strains against seat belts or shoulder straps. Unsecured objects are dislodged. Food service and walking are difficult."
For Severe Turbulence, "Occupants are forced violently against seat belts or shoulder straps. Unsecured objects are tossed about. Food service and walking are impossible."
Extreme Turbulence is "turbulence in which the aircraft is violently tossed about and is practically impossible to control. It may causes structural damage."

As a pilot, once the ride gets to the point of light turbulence (and not just light chop), I'll probably ask the flight attendants to go ahead & sit down.  Even though they might be able to serve, I don't want them out in the middle of the aircraft if things get worse & then they'd have nowhere to sit.  That may not be a PA announcement; I may just call them on the interphone & say something to the effect of, "ATC says this is probably going to last for a while, so why don't you guys go ahead & take your seats for a bit."  If we find out that the ride is about to get bad soon & suddenly, that's when I'll typically use the PA and be more directive: "Flight attendants, take your seats; flight attendants please take your seats."

In moderate turbulence, there's no question but that everybody needs to be sitting down.  And I'll be looking for better options (typically, changing altitude) in order to get a better ride.  Sometimes a better altitude is available, and some times you just have to ride it out.

I won't intentionally fly into severe turbulence.  If I know before takeoff that the only way to complete the flight is to transit an area of severe turbulence, I won't take off.  If it's possible to work out a different routing and/or altitude(s) that avoid all the areas of known & forecast severe turbulence, then we can plan on that, but you won't find me ever explaining to passengers before the flight that we'll be flying through severe turbulence today.  Not unless we're the last flight out of Saigon before it gets overrun by the murderous hordes, but I don't really foresee that scenario.

If you do fly into severe turbulence despite the best precautions, you've (hopefully!) already got everybody seated since you were probably expecting moderate turbulence anyway.  After landing, the aircraft will need to be inspected, because any encounter with turbulence classified as severe requires an inspection before the aircraft can fly again.  Typically, no problems are found (airliners are built pretty sturdy these days), but a mechanic will check all the things that might be over-stressed or damaged in that sort of an encounter in order to ensure that everything is still good to go fly again.

As far as preflight PA's when we're expecting light or moderate turbulence, that depends a little bit on the situation.  If it's a short flight with enough time in some sort of turbulence, I'll coordinate with the flight attendants ahead of time to not bother trying to serve, and then on the "welcome aboard" PA I'll explain that situation to the passengers.  The ride is forecast to be pretty bumpy, and so to keep our flight attendants safe, and since I'm sure nobody wants hot coffee spilled in their laps, I've asked our flight attendants to remain seated for the flight and not attempt any beverage service. So sorry, it's all about safety, thank you for your understanding, blah blah blah, and since they'll be staying seated, we do ask that you also observe the Fasten Seatbelt sign and remain seated for the short duration of the flight today as well."

If only a small part of the flight is expected to be turbulent, I probably won't say much about it at the gate (the Flight Attendants cover "stay seated while the seatbelt sign in on" at length), and then maybe as we're getting close, I'll mention something just to reinforce what they say on the PA.  But if there's nothing special to communicate, I don't see great value in creating apprehension before the flight about an interval of turbulence that we may end up avoiding altogether.

Some pilots to make it part of their preflight PA's to re-emphasize the Fasten Seatbelt sign and stress again that we want you seated when it's on.  That's an individual pilot's preference (at least at my company).  Our guidelines recommend against using the word "turbulence" because it does cause apprehension, and instead talk about a choppy ride or rough air or bumps in the road or whatever.  Conveys the same meaning to the passengers, while hopefully scaring the nervous fliers a little less.


Can I take my Canadian PPL with flight hours logged in the US? I was wondering if I can combine my flight training hours in the US with my Canadian ones if I want to take my PPL in Canada?

This is a little excerpt from a Canadian document, as it relates to the conversion of licences back and forth between the two countries.

  The Agreement provides, in pertinent part, that the FAA and TCCA will pursue mutual cooperation in the evaluation and acceptance of each other’s systems and standards in the issuance of certificates and licences.

Keep in mind, this text is for conversion after you already have a full license, but I believe each country recognizes the hours in a log book from the other. Best bet would be to call or email the flight schools in the area you plan on going, and get the official word on it.

Edit: I should have put this here in the first place, but here is the link to the Conversion agreement between U.S. and Canada. You will find the snippet that I pasted in the 'Purpose and Scope' section.


Has the design-flaw with the Boeing 737's cabin-pressure switch been fixed yet? At 42.21 minutes of this video about the crash of Helios' Flight 522 (a Boeing 737) caused by cabin depressurization, it was established that a major cause of the crash was a design flaw -- the main wire and the backup of a wire running from the cockpit to the depressurization valve runs through the same channel. So, if one wire burns up and fails, they both do. This is evidently contrary to safe design practices. Also, it seems that the cockpit instruments in the cockpit do not give sufficient indication during the preflight and takeoff checks that this problem exists.

From this video, it appears that this problem has not been addressed by Boeing. Is this still the position on current date?

The Wikipedia article on Helios Airways Flight 522 also mentions this wiring issue, and cites a separate report regarding a May 2003 incident in which a 737 had issues with pressurization. During the investigation it was discovered that wiring had shorted and burned in a particular bundle of wires, called a loom, which was important for control of pressurization. The AAIB issued a safety recommendation regarding this issue:

  Safety Recommendation 2004-33

  It is recommended that in order to prevent failure of the cabin pressure control system in the event of damage to wiring loom W298, the Boeing Commercial Airplanes should consider, on the Boeing 737-436 and similarly configured models, separating or protecting the wiring associated with the different modes of operation of this system, which connects the cabin pressure controller to the rear outflow valve, such that any single point failure of the loom would not result in effective failure of the pressurisation control system.

In their 2006 progress report, the AAIB followed up on this recommendation:

  After a review of the issue and the service history of the pressurization system for the 737-100 thru - 500 model airplane, Boeing felt that modifications of the wiring with the intent of separating the outflow valve functions consistent with the subject safety recommendation would not significantly contribute towards a reduction in the overall rate of depressurization events. Their rationale was based on the basic design philosophy and concept of the original system which includes:

  1) A single, mechanical outflow valve which is controlled by a single multi-mode analog pressure controller and driven by independent AC or DC powered electrical motors. Each motor can also be manually commanded open and closed via separate switch on the pressure control panel.

  2) In the event of a complete power/control loss (e.g. - electrical interruption or damaged wiring) either motor will remain in last commanded position. The airplane is equipped with an independent aural warning system (cabin above 10,000 feet) and visual cues (cabin above 14,000 feet) should this condition lead to a cabin pressure loss.

  3) The airplane is equipped with both fixed and portable oxygen supply systems for passengers and
  crew.

  4) If pressure cannot be reliably controlled/restored within a short period of time, the flight crew will utilize procedures (such as rapid reduction in altitude, declaration of emergency and landing at nearest suitable airport) which mitigate the exposure time to low pressure.

  Status - Rejected - closed

So the rationale is that if the system fails, the crew is alerted and may take appropriate action. Note the following regarding an AD published in 2011 with a deadline of March 2014:

  The FAA released Airworthiness Directive AD-2011-03-14 requiring all Boeing 737-100, -200, -300, -400 and -500 types to install two additional warning lights, one for takeoff configuration and one for cabin altitude warning, and introduce updated documentation within 36 months following March 14th 2011.


Have landing gear collapsed at the gate? Reading an answer to this question piqued my curiosity. Has there ever been an instance of an airliner's landing gear collapsing while loading/unloading passengers?

I do not include after emergency landings, but just while "sitting at the gate."

A good landing gear design locks it into the extended position under the aircraft's weight. This makes it physically impossible to retract it at the gate, but when the switch is in the wrong position the gear starts to retract during liftoff, when the wings take over the duty of carrying the weight.

This doesn't mean that careless operation or maintenance procedures can't make it retract, however. When the aircraft is poorly balanced, say during loading and unloading, the weight on the nose gear might not suffice to lock it, so a wrong switch position can cause an unintended retraction.


What is this aircraft with a crew member in an exposed nose compartment? Possibly from around 1935, possibly involving one of the two airplane-supplying-powers in the Spanish Civil war.

(Seen in a documentary about the 20th century inter-war era...it's only onscreen for a few frames.)

(It looks like one of the fantastical, almost-plausible air vehicles seen in Ghibli films!)

I believe this is a Dornier Do 18 flying boat / patrol bomber. The cockpit with its central piece of glass is quite distinctive. It entered service in 1938, and doesn't appear to have been involved in the Spanish Civil War.

There is a short film on YouTube which has the same sequence as the documentary you saw. Looks like it was part of a movie which featured the Do.18; the cockpit fills with smoke and the "bloke in the nose" (the gunner) starts to look for somewhere to land is what I think is going on.

The Dornier 18 has the distinction of being the first German aircraft to fall to British gunfire in WW2: on September 26th 1939 a Do.18 was felled by a Blackburn Skua.

Since there is some astonishment at the position of the forward position, it's worth noting that it was extremely common for inter-war military flying boats to allow someone to be stationed up front in this manner, for both reconnaissance and gunnery, and for ropes and other maritime uses. For instance most of the designs for medium and large seaplanes from Short Brothers and Supermarine have such a front station in this period.

There are too many to list, but since you mention Studio Ghibli you may want to reference Italian seaplane designs by CANT (e.g. Model 25, Z.501), Macchi (e.g. M.18, M.24) and Savoia-Marchetti (e.g. SM.62, SM.78) from where a lot of inspiration was derived.

I took a look at the documentary you saw, the aircraft registration is 6L+E05. So this was a Do 18D-1 which entered service in June 1938 in Travemünde and ended its days two years later in Putzig (Puck, Poland) after an engine fire on takeoff.

source


If your plane is out-of-control, why does military training instruct releasing the joystick to neutralize controls? I quote from Sunday Night's June 3 2019 episode with Capt Kevin Sullivan. I ask about Capt Sullivan's remark with quotation marks below.

Releasing the joystick is undoubtedly counter-intuitive to me, as I haven't been trained in military aviation! Can someone please expound? Please make this counter-intuitive neutralisation of controls intuitive?

  12:20
  Kevin Sullivan's next move is
  12:23
  completely counter-intuitive. Instead of
  12:26
  pulling back on the control stick, he
  12:29
  lets it go. "I have a choice to make. Do I
  12:33
  hold on to it or do I release it? And my
  12:37
  military training for out-of-control is
  12:40
  to release. Neutralize controls." It works.
  12:45
  The plane is no longer falling from the
  12:48
  sky, but now passengers
  12:51
  are being smashed back down to the floor.

Many abnormal situations can be at least partly pilot-induced.

Most civil airplanes (modern military ones are another topic - but computers add to that... ) are aerodynamically stable. Releasing the controls gets rid of probable faulty input and gives the plane a chance to stabilize. This might work (did in this case), but if there is a situation where the normal stability is no longer present - it might increase the problem.


What are the common mnemonic devices used to remember equipment required by 14 CFR 91.205? I'd like for this question's answers to be a references of the mnemonic devices pilots can use to assist with memorization of the equipment required by the FAA for flight in powered civil aircraft. What are the common mnemonic devices used to remember equipment required by 14 CFR 91.205?

For VFR Day Flights, you can use "A TOMATO FLAMES":

A - Altimeter

T - Tachometer for each engine.

O - Oil temp indicator for each engine

M - Manifold pressure gauge for each altitude engine

A - Airspeed indicator

T - Temperature gauge for each liquid cooled engine

O - Oil Pressure gauge for each engine

F - Fuel quantity gauge for each tank

L - Landing gear position lights (if retractable gear)

A - Anti-collision lights (aircraft certificated after March 11, 1996)

M - Magnetic direction indicator

E - ELT, if required by §91.207

S - Safety belts / Shoulder harnesses

For VFR Night Flights, you can use "FLAPS". This will be in addition to all the equipment required for a VFR Flight in Daylight.

F - Fuses (spare set)

L - Landing light (if for hire)

A - Anti-collision light

P - Position lights (navigation lights)

S - Source of power (such as battery)

Additionally, as @wbeard52 pointed out below, you can use "GRABCARD" as a mnemonic device to memorise the equipment required for IFR Flight. Again, these would be in addition to all the equipment required for VFR Day and Night Flight.

G - Generator/alternator

R - Radios (two way and navigational equipment
appropriate for the ground facilities to be used)

A - Altimeter (sensitive) adjustable for barometric pressure

B - Ball (slip-skid indicator, which is part of your turn coordinator)

C - Clock (shows hours minutes and seconds and installed as part of aircraft equipment)

A - Attitude indicator

R - Rate of turn indicator

D - Directional gyro (Heading indicator)


What does the snowflake with temperature symbol on an approach plate mean? I've only seen the snowflake symbol with temperature listed on this one approach plate for KASE airport in Aspen, CO. I looked for it in the legend but didn't find it. Does anyone know what the snowflake symbol with temperature means?

From FAA INFO 15002:

  A [snowflake] -XX°C icon will be incrementally added to airport approach plates, beginning Mar 5, 2015. The
  icon indicates a cold temperature altitude correction will be required on an approach when the reported
  temperature is, “at or below” the temperature specified for that airport.

This looks to be a work-in-progress, as they started adding the icons in March, and compliance with applying the cold temperature altitude corrections becomes mandatory in September, 2015.

More info from the NBAA:

  Background: In response to aviation industry concerns over cold weather altimetry errors, the FAA
  conducted a risk analysis to determine if current 14 CFR Part 97 instrument approach procedures, in the
  United States National Airspace System, place aircraft at risk during cold temperature operations. This study
  applied the coldest recorded temperature at the given airports in the last five years and specifically determined
  if there was a probability that during these non-standard day operations, anticipated altitude errors in a
  barometric altimetry system could exceed the Required Obstacle Clearance (ROC) used on procedure
  segment altitudes. If a probability, of the ROC being exceeded, went above one percent on a segment of the
  approach, a temperature restriction was applied to that segment. In addition to the low probability that these
  procedures will be required, the probability of the ROC being exceeded precisely at an obstacle position is
  extremely low, providing an even greater safety margin.

  Action:
  Pilots must make an altitude correction to the published, “at”, “at or above” and “at or below” altitudes on
  designated segment(s) (see list below), on all published procedures and runways, when the reported airport
  temperature is at or below the published airport cold temperature restriction.

  Pilots without temperature compensating aircraft are responsible to calculate and make a manual cold
  temperature altitude correction to the designated segment(s) of the approach using the AIM 7-2-3, ICAO Cold
  Temperature Error Table.

  Pilots with temperature compensating aircraft must ensure the system is on and operating for each segment
  requiring an altitude correction. Pilots must ensure they are flying at corrected altitude. If the system is not
  operating, the pilot is responsible to calculate and apply a manual cold weather altitude correction using the
  AIM 7-2-3 ICAO Cold Temperature Error Table. PILOTS SHOULD NOT MAKE AN ALTIMETER
  CHANGE to accomplish an altitude correction.

  Pilots must use the corrected MDA or DA/DH as the minimum for an approach. Pilots must meet the
  requirements in 14 CFR Part 91.175 in order to operate below the corrected MDA or DA/DH. Pilots must
  see and avoid obstacles when descending below the MDA.

  The temperature restriction at a “Cold Temperature Restricted Airport” is mutually exclusive from the charted
  temperature restriction published for “uncompensated baro-VNAV systems” on 14 CFR Part 97 RNAV
  (GPS) and RNAV (RNP) approach plates. The charted temperature restriction for uncompensated
  baro-VNAV systems is applicable to the final segment LNAV/VNAV minima. The charted temperature
  restriction must be followed regardless of the cold temperature restricted airport temperature.

There's more info in the NBAA document (I only included the exciting parts), including a list of affected airports, listed by state.  Lots in Alaska, none in Florida, unsurprisingly.  Applying this in flight looks like an absolute can of worms, as far as I can tell from the examples presented.

We'll probably see more come out about this as cold weather approaches, so stay tuned!


Why do anti-ice systems on airplanes with automatic ice detectors still need to be manually selected on? As even minuscule amounts of ice accumulation on aircraft can cause tremendous loss of lift, increases in drag, decreases in engine thrust, and/or flight-control impairment, some method of detecting ice is essential for safe flight, even for aircraft not certified for flight into icing conditions.  Various methods exist for doing this, ranging from examination with the Mk. I eyeball (for many light general-aviation craft, and even some larger planes) to sophisticated electromechanical systems utilising vibrating probes which change frequency when made more massive by ice accumulation and set off an alarm in the cockpit.
For aircraft not certified or equipped for flight in icing conditions, the detection of ice lets the flightcrew know that they need to get out of the ice ASAP.  For aircraft which can fly in ice, it signals the need to activate the aircraft’s built-in deice/anti-ice systems; these come in three main varieties (melting the ice using hot engine bleed air, melting it with resistance heating from electrical heating elements, and mechanically breaking it off with pneumatically-operated deicing boots), but all of these still need to be manually turned on by the pilots, even for aircraft that can automatically detect icing.
Given the potentially-catastrophic consequences of allowing ice to accumulate unchecked on an airframe, and the ease with which a flightcrew (especially if fatigued or already task-saturated) could overlook or forget about an ice warning, why don’t aircraft with automatic ice detectors and built-in anti-icing link the two together, and have the ice detector automatically turn on the anti-ice system when it detects icing?

Because most of the time the system has to be on even when there isn't detectable ice accumulation.  Because of what I like call "technical icing conditions" but which are just called "icing conditions":
ICING CONDITIONS:

TAT (total air temperature - accounts for temperature rise from compresibilty) less than 10C

Visible moisture (cloud, haze, anything that is water)

SAT above -40C

You also have to have WAI on for takeoffs when OAT is less than +5C with visible moisture (including visibility under 1 mile) regardless of whether there is actual ice formation (water streaming back on the wing on takeoff can freeze upon rotation due to the temperature drop from the low pressure over the wing).
When you get the wings deiced, you still have to have the WAI on at the start of the takeoff roll if departing in technical (or actual) icing.  You don't want to cook the thick type IV holdover fluid, so you leave WAI on selection the very last thing as you pull out to line up for the TO roll (the CRJ200 has an evaporative WAI system which meant the leading edges get VERY hot, about 105C, which could cook the fluid and make it crusty).
When flying, the only time you can leave the WAI off when in technical icing conditions is
above some speed (on the CRJs it was 230kt), unless you get an ICE caution from the ice detector.
The result is a system that automatically turns the WAI on when ice is detected would only cover about, oh, maybe 10 - 20% of the time you are legally required to have WAI on.
Cowls are similar, but have to be on even more often because there isn't the 230kt threshold for actual accumulations only. They have to be on all the time you are on the ground less than +10C with visible moisture (freezing around cowl inlets from velocity temperature drop).
Without the 230kt exclusion the wings have, you have to have the cowls on pretty much all the time you are in cloud, actual icing or not, so you are turning them on and off quite a lot if climbing/descending through layers (some pilots may just leave them on), whereas outside arrivals and departures the wings are only on when you get actual ice.
So you could automate the system like that, if you could figure out how to make the system reliably detect "visible moisture" outside somehow (probably possible, but very expensive).   Otherwise, the decision to turn it on is a crew decision based on the view outside, probably 80% of the time.


How to ballpark IAS vs TAS? Is there some reasonably accurate ballparking formula to allow for quick conversions between IAS and TAS as a function of pressure altitude and difference of ground temperature relative to standard atmosphere? That is,

E.g. "for each 1,000ft subtract X%, for each degree F/C subtract Y%" so that if I'm at 8,000ft and ground temperature is 10 C, I simply subtract 8X% and add 5Y% to get a good approximation.

My idea is to have a quick ballpark to start working with, in case there is no time to start fiddling with the flight computer, or in cases where the error would be insignificant for practical purposes.

Indicated vs. True is a hard one, as every aircraft has an instrument error and a position error. Once calibrated into Calibrated Air Speed however, this document gives CAS as a function of TAS:

All of them more or less straight lines, gradients a function of the altitude.

For temperature: the rule of thumb of density altitude can be used. From this website:

  Keep in mind the standard temperature is 15 degrees C but only at sea level. It decreases about 2 degrees C (or 3.5 degrees F) per 1,000 feet of altitude above sea level. The standard temperature at 7,000 feet msl, therefore, is only 1 degree C (or 34 degrees F).

So the procedure is:

Convert pressure altitude with temperature deviation. Ground temperature = 10 °C so density altitude = (10 - 15) * (1000/2) = -2,500 ft
Find TAS at 30,000 ft by subtracting 2,500 ft, then applying the TAS thumb rule of 2% / 1,000 ft: TAS @ 27,500 ft = 27,500 * (1.02/1,000) = 1.55 CAS.

At 30,000 ft and ground temperature of 10°C , TAS = 1.55 * CAS


What is the fastest recorded backward flying plane? It is a well known fact that some planes like the Antonov An-2 can fly backwards since their stall speed is lower than the speed of the winds they can stably fly into. My question is what is the fastest recorded backward flying plane and is there video of this phenomenon?

In response to the comments by @Simon et al the term backwards was vague - backwards relative to the ground would have been more accurate.

EDIT: This first paragraph previously only referenced the Harrier, but I've since found evidence of the Osprey doing the same, and the F-35B has since been reported to perform the feat.

The only true answers I'm aware of are the Hawker Harrier (or Harrier derivatives thereof), the Osprey, and the F-35B. These are all VTOL aircraft which can fly backwards, and can also fly very fast forward... thus they are the "fastest" aircraft which can also fly backwards.

Previously this crown belonged to the Harrier, but since the F-35B is faster it now takes the accolade of being the "fastest recorded backward flying plane": it can fly backwards, but also fly at mach 1.6.

As to your original question, the answer is that any aircraft is capable of this feat of "moving in reverse relative to the ground", if the wind speed is higher than that aircraft's stall speed.

I'm not quite sure what your question is, as you ask what is the fastest plane which can do this: Do you mean "What is the maximum speed of any aircraft which can achieve this?" or did you mean "How high can this relative speed be"

For the former: With the maximum wind speed recorded on Earth being over 230mph, any aircraft with a stall speed of 230mph or lower (ie, almost any plane), can do it. Therefore the fastest aircraft which can achieve this is the fastest aircraft on the planet. I'm not 100% sure on the fastest aircraft, but the SR-71, for example, can fly at well over Mach 3, but is also capable of flying "backwards" if the wind speed is above around 180 knots.

In theory the SR-71 could therefore fly at Mach 3 "forwards" or around 50mph "backwards" (in the context of this question) by travelling at 180 knots into a 230mph headwind. I wouldn't recommend attempting this, however....

Or to answer the real question I think you're asking which is "How fast can an aircraft performing this feat travel relative to the ground", the answer would be "the aircraft with the slowest possible stall speed" and the relative speed would be approximately 200mph.

An aircraft which can fly at under 30 knots, flying into that 230mph headwind, would be travelling "backwards" at 200mph relative to the ground, and at 30 knots relative to the wind.

So actually with it's very low stall speed the An-2 is probably a contender, although I believe there are aircraft with lower stall speeds.

(Apologies for mixing Knots and MPH almost at random in this reply: all numbers are approximate, however, so it probably doesn't matter too much. For the purposes of this answer, I'm treating 1 kt and 1 mph as approximately equal)


Was "the right stuff" ever really the right stuff? In the history of the culture of aviation, to what extent has the popular image of "the right stuff" required of a pilot ever matched the characteristics actually sought in aviation?

In this popular image, the pilot with "the right stuff" is imbued with, amongst other characteristics:

fearlessness
adventure
masculinity
a strong streak of individualism
stubborn single-mindedness
quick-thinking
sexiness
cool
self-sufficiency
a disregard for limits that bind lesser individuals
a distaste for rules
a stronger belief in himself than in authority
a strong intuitive sense of how to deal with problems in the air
... and others corresponding to the picture of the pilot who gets to walk around with a swagger because he deserves it, as vividly described in Tom Wolfe's The Right Stuff, but also represented that way through countless other descriptions and depictions in popular culture.

Well, kind of,

The technical portrayal of test pilots in that film received negative reviews with real test pilots.  I know Chuck Yeager did not like his portrayal by Sam Shepherd and director Philip Kaufman, particularly with the NF-104 accident at the end of the film.  While the scene is based on a real accident which Yeager did have, it was part of a scheduled test flight with established test points and not Yeager throwing on a pressure suit and jumping into the cockpit for a joyride with a YOLO type attitude.

Flight test and test pilot work is rigorously planned out, methodically didacted and cogitated, with multiple contingency plans and contingency plans for those if they don’t work.  Long gone are the days where you strapped your little pink fanny into a crate, pushed it off a cliff, and hoped for the best on the way down.  Test pilots and flight test engineers are very bright and methodical people who are intimately familiar with the systems of the aircraft they are flight testing and have spent a great deal of time thinking about all the ways something can go wrong and what they intend to do if they encounter this problem.  Hundreds, if not thousands of hours have been spent in systems and and flight simulation scenarios, not only evaluating these hypotheses, but making detailed plans for their test points they want to evaluate during flight.  Finally the flight testing itself is a slow and methodical affair, slowly and carefully increasing the performance envelope with each flight and thoroughly evaluating and addressing anomalies once encountered before continuing.  As one test pilot once put it, “We try to make flight testing as boring as possible.”

As to being cool, calm, unflappable under pressure, quick thinking and courageous being the defining qualities of 'the right stuff' in a pilot.  I would argue that they ARE useful to a pilot anywhere.  Remember US Airways Flight 1549's Capt Chesley 'Sully' Sullenberger?  That guy was faced with one of the most challenging scenarios out there and was a cool as a refrigerated cucumber throughout.  You may not be exploring and mapping out the absolute edges of an F-35's flight envelope, but those skills can come in handy with any kind of emergency situation which you might face.

Remember there is a difference between these qualities and having hazardous attitudes e.g. anti-authority, machismo, etc.  As Yeager once put it "If you want to grow old as a pilot, you gotta know when to push it and when to back off."  In addition flight test is a considerably different animal than it was back in Yeager's day, thanks in part to Yeager and a few unfortunates who now have a street at Edwards named after them.

Test flying isn't about being taking big risks; its a gradual and methodical process of exploring the flight characteristics of an aircraft and having multiple contingency plans to make it as safe as possible.  In short, good Aeronautical Decision Making really IS 'The Right Stuff'.  Risk takers, the macho guys, or the just plain stupid get weeded out, either by their superiors or by Natural Selection.


Is the destination of a commercial flight important for the pilot? As has been recently advertised (in here, here, here also), a recent British Airways flight from London City Airport destined for Düsseldorf mistakenly flew its passengers to Edinburgh instead.

I'm a layman in terms of aviation, but I (and I'm sure many more do) do see aviation as a strictly regulated area full of double-checks, double instruments (even 2 pilots!) to make sure an airplane is doing exactly what it's supposed to be doing.

Perhaps mine is a rather car-centric view, but I can't imagine starting the engine of the car without knowing clearly where I'm heading, which landmarks, roads (and so on) I'm supposed to pass by during the route.

TL;DR:

Must (or should) both pilots know where he airplane they are flying is going to? Or do they simply follow an approved flight path?

The pilots in this case did know where they were going: Edinburgh.

BA said a paperwork error was to blame, with the pilot following orders from Germany, where WDL’s head office had filed the incorrect flight plan.  The pilots flew according to the flight plan they were given, and ATC happily routed their plane along that flight plan as well.  The pilots had no reason to suspect the flight plan was incorrect, and safety was never compromised.

The dispatchers involved could use improvement, though.


Does two-way communication failure include one-way communication failure? Code 7600 means lost communication, more specifically two-way communication failure. I wonder whether two-way communication failure means losing the capability of maintaining two-way communication or  not? If the pilot can only hear but not transmit, is it also included in two-way communication failure?

7600 means a loss of two way communications, it's a way of letting ATC know you have a problem sending or receiving, or both.


Do pilots using an electronic flight bag need to carry paper charts? If a pilot uses an electronic flight bag (say, an iPad with ForeFlight or WingX), are they required to carry paper charts as a backup?

Do different rules apply to operations under Parts 91, 121, and 135?

A EFB is a legal replacement for paper charts and you are not required to carry backup if you are operating as part 91 single engine piston. Advisory Circular 120-76B is in reference to part 91F (Large and Turbine-Powered Multiengine Airplanes).

AC 91-78 is aimed at Part 91 operators, VFR or IFR and states that EFBs can be used in all phases of flight in lieu of paper.

A backup data source is suggested, but is not required. Note that this backup can be another electronic device.

See iPad Legal Briefing for Pilots.


How do I fix a slipping throttle friction lock in my C182? The throttle friction lock in my C182 works poorly in cruise, it slowly eases out to a lower power setting. What can I do to make it better? I've attempted to turn the lock nut harder but without result.

I suspect there is only one sensible answer to this, and that is to have a qualified mechanic check/replace as necessary.
To continue flying in the meanwhile I would add a check for throttle friction to my FREDA check which you probably already do every 6-10 minutes during cruise.


Which American twin tilt-rotor aircraft is this a scale-model of? What aircraft is this model?

(photos: own work)

Looks like the Bell XV-3, "an American tiltrotor aircraft developed by Bell Helicopter for a joint research program between the United States Air Force and the United States Army in order to explore convertiplane technologies."


What could cause uneven fuel levels in a Cessna 172? I went for a 1.5 hour flight in a 1977 Cessna 172N, with several takeoffs and landings. I was alone. When I left, I had 18 gallons in each tank. When I arrived back, I had 15 gallons in the left tank, and 10 in the right tank.

Why was there such a disparity? Is this at all dangerous? Why did the right tank have less?

Consulting your handy POH (or one for a 172N that I Googled up) you'll find the fuel system diagram looks like the one below (click to enlarge - most high-wing gravity-fed fuel systems are similar).

So there are a number of ways you could wind up with uneven fuel burn with this plumbing.
The most common ones are:

(Uncoordinated) Turns
If you're flying lots of left-hand traffic patterns (or you're making lots of left turns for some other reason) that means your left wing is frequently lower, and gravity favors pulling fuel from the right tank. (Note that it doesn't have to be a turn - prolonged slips can also cause uneven fuel flow.)
If I had to take a guess, I'd say this is probably your culprit. Were you making a lot of left turns?
Pressure
The fuel vent tube on the left tank faces into the wind - this pressurizes the left tank first (and the right tank through the breather tube connecting the two) - it's a little extra encouragement for fuel to leave the tanks and flow into the engine. In some aircraft this leads to the left tank draining faster (because it's at a slightly higher air pressure than the right tank) - this can become particularly pronounced if the breather tube between the two tanks is blocked.

Various blockages
A partial blockage in either of the tank fuel lines, the fuel selector valve, the vent, or the breather line between the two tanks could theoretically cause uneven fuel burn (the vent/breather blockages being a special case of "pressure" from (2) above).
Pilot Error
It's possible to mis-position the fuel selector in such a way that you're getting adequate fuel flow to run the engine, but one tank is more restricted than the other.
This is unlikely if the aircraft is properly maintained (the fuel selector should have nice, identifiable detents that it clicks into, letting you know it's in position).

For more detail on causes for uneven fuel burn (and tips for troubleshooting it with your mechanic) check out Cessna Pilot's Association Technote 003.

Is uneven fuel burn dangerous? Maybe.
It's certainly something you should be aware of (part of why you want your fuel gauges to be at least somewhat accurate is so you can tell if your fuel burn is uneven in flight and correct it by switching to the fuller tank briefly).

Uneven fuel burn can be an annoyance (a 5 gallon difference is about 30lbs - potentially enough to notice on the controls), but it can also be a safety hazard: If allowed to go uncorrected you could theoretically get yourself into a situation where the right fuel tank is empty, and in a steep enough left turn you may not have enough pressure from gravity to get fuel from the left tank into the engine.

In an extreme case your right tank may be empty and your left tank may be so low as to unport the fuel pickup tube in a turn, potentially leading to attitude-induced fuel starvation ("The noisy part of the plane gets quiet until you level the wings").


What is the purpose of the vertical splitter and the NACA duct at the MQ-9 Reaper engine scoop? The engine scoop/intake on the MQ-9 features a vertical splitter at the inlet face, as well as another smaller NACA duct on the top cowl surface just aft of the inlet lip.

I am guessing the former functions as a vane or diverter to channel airflow to different parts of the engine presumably for cooling, but the additional NACA duct at that location seems unusual (another inlet for an inlet?).

Is that NACA duct meant to modify the airflow within the engine scoop? Or is it specifically routed to another part of the engine altogether?

The NACA duct is probably routed to a gearbox oil cooler. The turboprop engine has its air inlet at the prop end of the engine, which means a splitter is needed to divide the incoming air flow into two ducts, one on each side of the engine, in order to conduct the air back to the inlet.


What type of airspeed are wind speeds given in? When you get wind speed from an ASOS/AWOS or from ATC, are those speeds in true airspeed? Are the instruments weighted for elevation?

Say you're on approach to 05R at MEX and you're given wind speed of 25 kt at 140° and the OAT is 35°C. Do you need to take density altitude into account to determine if it's safe to land? Or is there not enough of a difference to matter?

Wind is measured in true airspeed. That's why whether it's surface wind or a jet stream, subtracting the head/tail wind component from the airplane's TAS yields the ground speed (tailwind being a negative value). And a 25 knots crosswind—the example you give—will create the same crab angle for the same TAS being flown. But since pilots fly using IAS, the crab angle will be smaller the higher the airport.
Cup
Think of a cup anemometer, if the air is lighter (high altitude, high temp), there will be lower resistance and the same wind speed will result in the same rotation:

The remarkable thing is that the calibration does not depend on the air density. Who would have guessed? The cup anemometer responds to wind forces, and those wind forces have a magnitude directly proportional to density. Yet because in a steady state those forces are balanced, density vanishes as a factor.

The same thing applies to the propeller-vane anemometer.
Tube
In tube anemometers (pitot-static on a stick), corrections are applied:

[Due] to differing temperature, elevation or barometric pressure, a correction is required to obtain the actual wind speed. Approximately 1.5% (1.6% above 6,000 feet) should be added to the velocity recorded by a tube anemometer for each 1000 ft (5% for each kilometer) above sea-level.

Ultrasonic
Ultrasonic anemometers are like cup anemometers, density is cancelled out since measurements are taken in multiple directions:

[Scalar] values (temperature, humidity, density) are eliminated and the result comprises wind speed.


What are the unique sounds emitted by Embraer E190-E2 on final? I live close to an airport where these aircraft frequently land. When the E2 is between 2-3 thousand feet, something which I can only describe as whale moans, is heard.

I say they are unique because I can’t hear them from other aircraft. However, this airport seldom get heavy traffic, so maybe this sound is common among heavier aircraft—I wouldn’t know.

This was answered by an pilot acquaintance of mine:
The most recent generation of engines on the E2 comes with a geared turbofan. In the final stages of descent, the engines rotate at speeds which causes resonance in the combustion chamber. The distinct sound is that vibration.
This sound can also be heard when departing the gate, when the engine experiences similar rotational speeds.
Mitigation of this requires a redesign of the combustion chamber.


Why did TWA800 not stall? I believe the NTSB account, but based on this graph, http://twa800.com/lahr/zoom-climb.htm, why did the flight not stall after the nose separated?

In a nutshell, the aircraft had enough energy to climb without exceeding its stall AOA.  The expert stated the plane should have "immediately stalled", but the incredible amounts of mass and velocity associated with a plane that large demand a closer analysis to the physics.
When the nose breaks free, lift and weight are out of balance. When an aerobatic plane applies full up elevator, lift and weight are out of balance.
There are two "bookends" to what happens next.  The upper is G limits are exceeded and the wings tear off.  The lower is stall AOA is exceeded.  In between is an attempted loop.
Apparently, the rugged 747 had enough velocity to climb, sans nose, for a while.  Really a testament to its design.
Even after stalling, its momentum would have carried still higher, so it is difficult to determine when exactly it stalled.
Factors which determine "stallability" include wing loading, wing configuration, and tail volume. A very high wing loading with a very thin wing (as seen on many supersonic fighter aircraft) will stall much faster.  Deploying slats and partial flaps greatly helps a plane tolerate higher angles of attack.  Large tail volume also increasingly inhibits higher and higher AOA.
Essentially, it is a race between change in flight path and change in AOA within the G limits of the aircraft.


What is the difference between a high bypass turbofan and a low bypass turbofan? I see some questions on the site that talk about the relative positives and negatives of a high bypass turbofan (HB) vs a low bypass turbofan (LB).  But I don't see anything explaining what actually makes a turbofan engine HB or LB?  I assume it has to do with the ratio of air that is bypassing the compressor stages and just going through the fans.  But what ratio makes an engine HB v. LB?  And why was that ratio chosen?

Please note: I am not looking for usage cases, I am looking for differences in design specifications.

The first turbofan, the RR Conway, had just a bypass ratio of 0.25. It was designed for installation into the wing root of airplanes like the HP Victor bomber or the Vickers V-1000 transport project, so RR strived to keep the diameter small. Of course, back then nobody distinguished low- and high-bypass engines. The P&W JT-3D, which powered most of the second generation of jet airliners (Boeing 707 and DC-8), had a bypass ratio of less than 1.5.

But when the new generation of engines (GE CF-6, JT-9D) with bypass ratios of around 5 were developed for the first generation of wide body jets, marketing needed a term to make clear that this was a new generation of engines, and the high-bypass ratio engine category was born. Generally, the threshold is around 5, but note that some CF-6 versions have bypass ratios as low as 4.24 and still fall into the high bypass ratio category. Therefore, Wikipedia sets the limit at a bypass ratio of 4.

There is no technical difference between a low- and a high-bypass engine, and the limit is arbitrary. The jump in bypass ratio from 1.5 to 5 brought significant improvements in specific fuel consumption and noise reduction, but part of the technical progress was also from better materials and aerodynamics, which allowed higher compression ratios and turbine entry temperatures.

Now, almost 50 years later, we see the first engines with bypass ratios above 10 entering service. Designs are being readied with bypass ratios of 12, so marketing will need a new word to describe those creations. With "ultra high bypass" already taken by the unducted fan crowd, the search is on.


Which airfoil to use for high subsonic flight? I am desigining an aircraft, which shall fly up to Mach=0.9 in cruise flight in an altitude of 10 km.
So when I think of which airfoil to choose, I have to consider aspects like

max. lift coefficient required
max. necessary/allowed angle of attack
...

But what comes first in my mind is, that I have to prevent shock waves forming on my airfoil, because drag will dramatically increase. So actually, this is my first demand for the airfoil.
NACA 64006 is a supercritical airfoil. In order to prevent Mach=1 on the airfoil, I would need a swept wing with an angle of more than 50° to reach Ma_crit=0.88 .
I am wondering now, if this is normal that I need such a high sweep angle? Or did I choose the wrong airfoil. So normal commerical aircraft have angles between 30 to max. 40° ?
So the only way to have less sweep angle: find another airfoil (I think 64006 is already one of the best free available airfoils)  or I accept higher values of drag coefficient ?
How do airliners tackle the problem? I think I read that they allow mach numbers a little bit higher until the drag divergence velocity?

Yes, you chose the wrong airfoil.
While the NACA 6-digit series was among the first set of airfoils computed from a design pressure distribution, they will suffer from shocks when operated above their critical Mach number just as any other airfoil.

Comparison of drag rise over Mach for 6-series and early supercritical airfoils from NASA Technical Paper 2969. It should be obvious which one is to prefer.
When transsonic research started, inverted airfoils paradoxically turned out to perform better at moderate lift coefficients and high Mach numbers than regular airfoils. Key is the low curvature on the suction side which makes a shock-free pressure rise possible. Practical designs aim for a weak shock over a range of lift coefficients. In oder to produce the most lift at a give Mach number, the pressure difference between upper and lower side can be maximized where thickness is low, i.e. in the rear area of the airfoil. This is called "rear loading". Supersonic flow on the upper side allows to create more lift over the full chord. On the most recent designs another small lift contribution results from a slower pressure drop on the forward edge of the lower side pressure distribution (the forward lower-surface undercutting of Phase 3 airfoils).

Effect of forward lower surface undercutting, from NASA Technical Paper 2969. The added lift at the forward part also lowers the pitching moment of supercritical airfoils.
Supercritical airfoils have negative camber over most of their forward part and strong positive camber only in the rearmost 20 - 30% of their chord. This NASA report (PDF!) lists a range of supercritical airfoils with different thicknesses and design lift coefficients. But it can only serve as a starting point since wing sweep will change the pressure distribution locally and demands a shift in the point of maximum thickness (forward at the root, backward at the tip) and more negative camber at the root in combination with a higher local incidence.
Mach 0.9 is only used by business jets where owners care more about bragging rights than fuel consumption. Airliners normally fly at Mach numbers ranging between 0.78 (short range) and 0.84 (long range) and for best efficiency should stay below Mach 0.82. They fly a bit above their drag divergence Mach until the gains in speed are balanced by the losses in drag. Of course, they can be operated at higher speeds but do so only to make up for a delay.


In constant speed propeller, if i increase RPM lever from 2300 to 2500, will the Blade AOA decrease? Assume I am flying straight and level with power setting 21 inches of MAP and 2300 RPM with TAS=100 kts, now I change my Prop lever only (MAP = no change & TAS = no change) from 2300 to 2500, since now with same power output from engine and demand for propeller to spin faster, the governor will fine-tune the pitch, now keeping the TAS same and propeller RPM increase thereby leading to fining the pitch, the blade AOA will reduce, or doesn't it?
The image given is for constant pitch propeller, is there a similar diagram for constant speed propeller?

When you adjust a constant speed propeller to a higher rpm, this will be done by making the propeller blade AoA smaller.
Contrary to your question though, manifold pressure and power output will not remain constant unless you adjust the throttle. Neither will your airspeed, not surprisingly.
Exact effect will depend on the engine, but basically power is torque x rpm, so for a constant throttle position, increase in rpm will increase power output unless we are near max rpm.
See What is the relation between manifold pressure, RPM, and power, in a constant speed prop? for reference.


What type of engine is mounted on the Cessna Citation Hemisphere? What type of engine is mounted on the Cessna Citation Hemisphere?

Image Source

That does not look like a high bypass turbo jet engine.  If its not a high by pass turbo jet engine, and given a high bypass turbo jet engine is the most efficient engine, why was it not used for this particular aircraft and why was the particular engine chosen for this aircraft given its cruise speed is similar to that of aircraft that use the high bypass tubo jet engine?

The Cessna website mentions the engine type used (emphasis mine):

  The Citation Hemisphere will be powered by two next-generation Safran™ Silvercrest™ engines delivering over 12,000 pounds of thrust. The Silvercrest engines incorporate the latest advanced, field-proven technologies to offer unrivaled performance in its class in terms of direct operating cost, propulsion efficiency, reliability and environmental friendliness. These engines lower fuel consumption by up to 15 percent, reduce NOx emissions by up to 40 percent under CAEP/6 standards and half the noise footprint compared to other engines in its class.

According to Wikipedia, this engine will have a bypass ratio of

$$5.9 \, : \, 1 $$

which would certainly count as a high-bypass turbofan engine. Also see: What is the difference between a high bypass turbofan and a low bypass turbofan?