Why do turbofans have different nozzles? The CFM 56-7BE (on the 737) has a long and narrow nozzle, the RR Trent 900 (on the A380) has a sharper nozzle, and the CF-6 (on the 767) has a wider opening at the nozzle. Why do the nozzle sizes vary among turbofans?

The nozzle, apart from its main function, that is accelerating the air in order to propel the vehicle, it determines what goes into the front of the engine, it is a downstream restrictor along with the turbine nozzle.

The areas of both the propelling nozzle and turbine nozzle set the mass flow through the engine and maximum pressure. For different engine operational purposes, it is used one shape or another. These engines have different shapes due to optimization reasons; the engine, is designed in such a way that the aircraft features will have the best balance in terms of fuel consumption, thrust needed, and many other parameters.

Each manufacturer design their engines in the most convenient way and then, it is determined which engine is better for a given aircraft.

Different kind of geometries, (e.g. convergent-divergent) also affect on the engine operation regime (supersonic, subsonic, transonic), but this does not affect here since all the engines are turbofans and thus, subsonic.


Does a safety wire prevent a fuel filter to detach during flight? I know that fuel filters on aircraft engines have some features that avoid the bowl from detachment during flight, and that they can be secured using safety wire as shown below.

I've also heard that installing safety wire is very time consuming. Is that true?
What is your experience with installation & safetying of fuel filter bowls?

Safety wire is almost never "efficient", but efficiency is not the goal: Safety is.
You want to be certain that once the fuel strainer has been secured it's not going to fall off in flight (which will result in the engine being starved of fuel).

On many light aircraft that's accomplished by wrapping safety wire on the fuel strainer to ensure it can't come loose in flight:

A skilled mechanic who works with safety wire every day can safety a fuel strainer in about a minute - probably even faster. It's pretty impressive to watch.

A bungling incompetent like me who does it maybe once a year takes about 5 minutes, often punctuated with "colorful metaphors" and comments about the questionable parentage of the engineer responsible for the fuel strainer.

Either way, in the grand scheme of things that's not a lot of time spent on securing a flight-critical component.

Because there are far more bungling incompetents than skilled mechanics (and because even good mechanics are frequently not fond of safety wire) there are other fuel strainer designs which don't use safety wire.
The one I linked to uses a bayonet lock for the bowl, and a cotter pin to keep it from coming loose in flight. Anyone can bend a cotter pin in a couple of seconds, and as effective as the safety wire technique, if not moreso.


Is there a 3rd-party API for aviation weather forecast, which includes additional cloud information? I posted this question on StackOverflow here and terriblememory suggested I post it here.  I didn't even know there was an aviation stack exchange.  Cool!

  I have been scowering the internet all day for a third-party weather
  API that reports more than just the cloud coverage for cloud
  information.  I need to know the height of the cloud.

  I need something like an aviation weather API that reports either the
  cloud height, the cloud max and min vertical position, or the cloud type (Cirrus, Stratus, Cumulus, etc.) from which we could then derive the relative height.

  We want the API to be free at least during development.

I found some relative information in this stack exchange and one answer here that tells how to find the cloud tops but I'm posting this question anyway in case there is an API yet to be suggested.

Meanwhile, I'll be taking a look at the other information I found in this stack exchange.  Thank you in advance for suggestions.

Short answer:

I wish this information were available, but what you are looking for is simply not available.

Longer answer:

  There has to be something out there else how would pilots know to
  climb to a destination altitude to get above the clouds?

You are looking for data that you rightly think should exist. I would that it did, but it doesn't. Thats where the art of airmanship comes in. The answer to your question I quoted above is a complex mix of years of experience reading the weather charts, eyeballing the view out the window, and networking through the radio.

Real world weather is too hard to predict, and is constantly in flux. The only way to know the actual cloud conditions is to get out there and see it in the moment. The ground based weather observation stations like AWOS and ASOS do a decent job, but they are very limited and often wildly inaccurate. They only provide an assessment what is visible straight up.

PIREPS are the only partial answer you will get, and these are sparse and dated. A PIREP is only a snapshot of what a pilot observed at one place at one time.

Area forecasts will sometimes include a cloud top forecast (look for "TOP FL200" or similiar). However, a forecast—especially an area forecast which often covers half a state—is vastly different than an observation.

You've already mentioned Skew-T charts, and they do have useful data. Depending on your application, this might end up being your best bet. If you want something that doesn't have a real world aviation application (e.g. for the SIM community), you could derive your sought information from this.


Does an increase in air temperature actually improve aircraft performance? Mainly because of what pilots learn about density altitude and aircraft performance, any idea that an increase in temperature could improve performance seems counterintuitive. And indeed it is. Here's what got me thinking about it.

In studying for a written test I ran across a question phrased something like this:

  An aircraft flying at a constant power setting flies from a colder temperature to a warmer temperature. What happens to true airspeed and true altitude?

The correct answer turned out to be that true airspeed and true altitude both increased. I knew immediately on the altitude because I remembered reading how very cold temperatures can cause dangerous errors in altimeters, but the airspeed didn't make sense.

I still don't understand why the increased temperature lead to an increase in true airspeed. Obviously a reciprocating engine doesn't perform better in warmer air. We don't install "interwarmers." Does it really mean that warmer air can actually improve performance? Or does this need a deeper explanation?

Drag (and lift) increases with density. Density decreased and so did drag. So at the same power, you can fly faster.

Now I don't know whether reduction of power of a normally aspirated spark-ignition reciprocating engine at constant throttle setting would be higher or lower than the reduction of drag. But the question says power setting.


Helicopters: why hasn't NOTAR been more popular? NOTAR

No tail rotor: use of a ducted fan within the helicopter's body, and venting the air through the side of the tail boom to get the desired anti-torque.

One big advantage is simplicity and ruggedness: no tail rotor to get damaged, no tail rotor gearbox or driveshaft to wear and/or malfunction.

Another is noise: NOTAR is quieter.

And maybe lower cost: the simpler design and fewer components moving under high torque should be less expensive to build, definitely less expensive to maintain.

One possible disadvantage might be fuel consumption - it is quite possible that the ducted fan and the air turning a corner isn't as efficient as a propeller, so fuel consumption might be higher, but not that much higher. I haven't seen any figures on that, one way or another.

As of today, only Boeing (via purchase of M-D) uses the ducted fan in three MD helicopters, and even they haven't sold that many helicopters with this design. In all my aircraft watching, I have seen only one MD 520N in flight.

I'm just curious - other than maybe patent royalties, is there a mechanical reason this design hasn't been more popular? On the safety and maintenance aspects alone, it seems like a winner. And for military helicopters, the lower vulnerability and lower radar return would seem like big advantages.

Though NOTAR looks simple from outside, its actually a bit complicated. Let me use the following figure to elaborate on this point.

By Voytek S - Own workThis vector image was created with Inkscape., CC BY-SA 3.0, Link

Details from the figure: 1. Air intake
    2. Variable pitch fan
    3. Tail boom with Coanda Slots
    4. Vertical stabilizers
    5. Direct jet thruster
    6. Downwash
    7. Circulation control tailboom cross-section
    8. Anti-torque lift

You have the ducted fan (2), which is driven using power from the gearbox, which is used to produce the anti-torque effect through Coandă effect. Note that this fan has a variable pitch, which is adjusted similar to the tail rotor.

The Coandă effect actually provides only about two thirds of the required anti torque. The other one third is provided by the directed jet thruster (5) which rotates about the tail boom axis (that is why it is projecting outside) like a rotating can.

The problem is that in cruise, the effect is markedly reduced by the airflow over the tail boom, which is overcome by making the vertical stabilizers (4) adjustable. So basically, you have three systems for adjusting the anti-torque, compared to one in the conventional tail rotor. More systems translates into more parts, more complexity, more chances of failure and more maintenance, not to mention that this makes the helicopter tail heavy. For example, when using the same engine, the NOTAR MD 520N is heavier than the conventional MD 500E (from which it was developed) (Source:1, 2).

As you've noted, this system is not as fuel efficient and sacrifices speed and range for lesser noise and improved safety. Its a tradeoff and most operator would take the better performance over others.

In case of military helicopters, the main problem for helicopters, the infra-red detection, is improved in case of NOTAR only if the engine exhaust (or part of it) is routed through the fan, as the South Africans did with the Aérospatiale Alouette III III in their Cirstel (Combined Infra Red Suppression and Tail rotor Elimination) project.

Denel Cristel; image from stingrayslistofrotorcraft

As the site explains,

  The Cirstel principle splits the high pressure air from the engine for use in the Coanda slots, while the low pressure air is bled-off for the tail thruster and to mix and cool the engine exhaust. The thruster nozzle is a Denel-patented clamshell design, unlike McDonnell Douglas' `rotating can' concept.

As you can see, the system is further complicated. In addition, routing the hot air through the tail boom brings its own headaches and is better avoided. So, even in that sense, the advantage is not striking.


What is the one-engine-inoperative maximum range flight configuration? I was talking with a friend about what things would happen if you experienced engine problems in cruise. We were wondering the following:

What things would need to be changed to arrive in the optimal configuration for range?

Here I found that the speed is reduced to approximately 80% of the normal cruise value, but I can image that more things change, perhaps the altitude?

The optimization problem is unchanged from the all engines operative (AEO) cruise optimization. You should still:

Climb as high as practical (colder, thinner air is better for range)
Set the lift coefficient such that drag is minimized for propeller aircraft ($c_{Di} = c_{D0}$). For turbojets, minimize energy per flight time ($c_{Di} = \frac{1}{3}\cdot c_{D0}$). For turbofans, pick a value between those two extremes.

For the computation of the theoretical optimum polar point please see this answer for a complete derivation including wind, or this answer for a shorter explanation.

Your flight altitude will certainly be lower, because you need to generate more thrust on the remaining engine (requiring higher density), which in turn will result in a lower TAS (true air speed). The CAS (calibrated airspeed) should stay about the same, however. The configuration (flaps, gear) should stay unchanged, only rudder deflection will be added for asymmetric thrust compensation. The resulting sideslip will increase the zero-lift drag of the airplane a little, so a slightly lower speed than in the AEO case would result.

In the unlikely case that the required density will result in an altitude below ground level, you will need to slow down and need to set flaps to stay airborne, and now the new optimum with flaps will result in a markedly higher lift coefficient because deploying flaps will increase the zero-lift drag quite a bit. If you have the flaps-down polar, the optimum is easy to find.


Would the benign spiral maneuver be one alternative to dealing with being "stuck above" or inside a cloud? Recently I provided an alternate answer to a question involving "hands off" recovery techniques.  In this I briefly illustrated the benign spiral maneuver.  For completeness I will reiterate what I said in that answer:

This (benign spiral) maneuver is considered to be the safest way of recovering in a glider when one has found themselves above a hole-free cloud layer or inside a cloud.  My club teaches this maneuver as part of initial glider pilot training, though it is not (yet) part of the US FAA Glider Practical Test Standards.

The steps my club recommends are:

Full spoilers
Trim slightly aft (Other references suggest 1.5 times the stall speed)
Hands off stick
Feet off rudders

The effect of this maneuver is the glider will settle into a gentle spiral with a roughly constant (and not load-exceeding) airspeed and altitude loss.  The maneuver is also taking advantage of the aerodynamic stability mentioned in other answers which is inherent in most (all?) modern gliders.

Here are some additional sources that discuss this maneuver:

Scottish Gliding Centre's Airfield Procedures, Page 17 - 18
This post, which quotes "Transition to Gliders, A Flight Training Handbook for Power Pilots" by Thomas L. Knauff.

That got me to thinking about whether this technique would be applicable to recovery from the same situation in a powered airplane.  Being "stuck above" or even inside a cloud has certainly been the cause of accidents.  One harrowing story that really hit home for me is told in this video from the Air safety institute.

So, my question: Would a benign spiral be one alternative to dealing with being "stuck above" or inside a cloud?  Lets assume for the sake of argument that the airplane involved is a typical single-engine, forward prop GA aircraft (e.g. Cessna 172, Piper Cherokee, etc.)

It works in most gliders because they tend to be neutrally stable in roll and - the big one - have speed limiting spoilers/dive brakes.  I'm not sure it's safe to do that in an older glider without speed limiting spoilers.

In any case, most power planes, once disturbed beyond the ability of dihedral effect to restore level flight, will naturally descend in an ever steepening spiral without pilot input.  There aren't any ways to limit the speed on a GA power plane (gear and flaps won't cut it) the way spoilers/dive brakes on a glider can.  It would need a speed brake that can hold the plane to below Vne in a vertical dive.


Is there a detection method available to detect a plane without using RADAR? If we don't have RADAR, is there any other method to detect flying objects?

All flying objects disturb the air around them. Is there any method by which we can detect that this air turbulence is from an artificial flying object or a bird etc.?

Radar is, by far the most widely known and used of the active detection methods.

Passive methods include the use of visual and infrared search and track systems such as the F-14's Television Camera System or the Eurofighter Typhoon's PIRATE infra red search and track systems.


What parts of Austrian airspace are not covered by primary radars? I'm writing a story about the pilot of the Austrian Air Force, who uses his Eurofighter Typhoon aircraft for unintended purposes. In scope of another question we figured out that by turning off the transponder the pilot could make the aircraft invisible for the cooperative surveillance.

However, the primary radars detect a flying object regardless of whether it responds or not. This means that if a pilot of a Eurofighter turns off the transponder, civilian ATCs wouldn't see his plane, but he would remain visible for the military ones.

I assume that there are "blind spots", i. e. areas where primary radars can't detect aircraft. I believe this to be true because if primary radars covered 100 % of the airspace, the location of the Malaysia Airlines Flight 370 would be known.

Where exactly are these blind spots (if any) in Austria (areas in which primary radars of the Austrian Air Force would not detect flying objects) ?

I'm looking for a map like this for Austria:

Source of the image

The radar system in Austria is know by the name Goldhaube (Golden Hat). It is based on 3 fixed sites and several mobile stations. The coverage is this one:

Source

Another view:

Source

The three primary surveillance radars (PSR) are located at:

Kolomannsberg (Salzburg),
Großer Speikkogel (Koralpe)
Buschberg.

and are (or were) Selenia RAT-31 with a range of about 300 NM.

Source

Side notes

The difference between Austria and some incompletely covered areas is that Austria is not a large territory like China nor a water area like the Indian Ocean. It is relatively easy to get a full coverage.

In addition Austria is mountainous (famous ski resorts), so it's quite handy to use peaks for radar stations with increased range.

However, radio waves are mostly traveling in straight line. At low altitude in the valleys an aircraft is invisible. Military use this tactic under the name "ground hugging" or "nap-of-the-earth" to escape unfriendly eyes. Austrian army use mobile radars to make this possibility more complex:

Source

And to fuel imagination, there is this story of aircraft vanishing over Austria. Possibly a Bermuda triangle at the core of Europe.


What is the influence of drag on an airliner's block fuel consumption? I am not sure how widely the software is used, but I am using Piano-X to investigate the influence of total aircraft drag on block fuel consumption and noticed something odd. It is my hope that someone familiar with the software can assist me.

If I decrease the total aircraft drag both the block time and fuel consumed increases, which is the opposite of what I would expect. As expected however, the block fuel/min does decrease.

Shown below is the results from the investigation for the drag systematically decreased :

I expect that the block fuel/time would decrease but don't understand how the total time and fuel will increase for decreased drag.

Is there a rational explanation for this?

The software documentation only states the following concerning drag calculation:

"Aerodynamic drag is calculated in detail (as a function of lift coefficient, Mach number, and Reynolds number) and tuned with actual lift/drag data (‘polars’) whenever these are known. You can either factor the drag uniformly using one overall value or you can adjust certain items individually."

My approach was to vary total drag, i.e. induced and parasite drag simultaneously.

I will add that the software calculated total block fuel including estimates for both taxi's and the climb-out etc. Now in taxi drag doesn't have an influence, but I doubt that this will make such a large contribution.

I would suspect that reducing drag in this case would be result of reducing airspeed. If your drag coefficient 1 is for a certain speed, slowing down would cause smaller fuel flow but increase flight time and hence increase total consumption.


Can planes flap their wings like birds? Birds use their strong breast muscles to flap their wings and give them the thrust to move throught the air and fly. In a way, birds use a swimming motion to get the lift needed to fly. Plane wings have a similar shape as bird wings, but instead of flapping their wings, we use engines to thrust them into the air and create the lift needed to fly. (Source)

Can we design an aircraft which could also "flap" its wings to fly?
Yes, we would need fuel to anyways do that.  But, from a design point of view, is it possible to do that?

From a biological point of view, flapping wings is a viable means of flight. However, this presents some problems when scaled up for human flight.

One issue is the square-cube law: as the wings are scaled up, the area scales as a square (relating to lift), but the volume scales as a cube (relating to weight). This means the wings increase in weight faster than they increase in lift, resulting in less effective wings. The higher weight presents issues in making them flap.

ratchet freak pointed out that we do have such machines, and they are called ornithopters. Although there is some potential at smaller sizes (for small UAV's), the weight and force issue prevents them from being very useful at larger scales. Ornithopters were some of the first unsuccessful designs for heavier than air flight.

The best solution we have found is propellers. This doesn't work as well as biological propulsion, but it's much easier to spin a prop than flap the whole wing, and works much better for the larger scales we need to move humans. Flapping is also not effective for reaching higher speeds.

You can also see from the biological side that there is a size limitation. The largest birds are much smaller than our airplanes.


What is the youngest age allowed to fly any sort of plane in the USA? I have wondered this for quite some time, but what is the youngest age to fly any sort of plane in the USA? Does it also vary on what type of plane you're flying? (Commercial, Private, Training.)

The easiest way to figure this out for the US is to look at 14 CFR Part 61

  §61.83   Eligibility requirements for student pilots. To be eligible
  for a student pilot certificate, an applicant must:

  (a) Be at least 16 years of age for other than the operation of a
  glider or balloon.

  (b) Be at least 14 years of age for the operation of a glider or
  balloon.

That is the age at which you can solo. Lots of airplane pilots take lessons and solo on their 16th birthday. I don’t follow the gilder world and balloon world, but I assume lots of kids solo on their 14th birthday.

To get a license you must be a bit older:

  61.103   Eligibility requirements: General. To be eligible for a private pilot certificate, a person must:

  (a) Be at least 17 years of age for a rating in other than a glider or
  balloon.

  (b) Be at least 16 years of age for a rating in a glider or balloon.


How to start an approach in IMC loss of communication under IFR? Suppose I lost my radio communication while en-route, I am following the cleared route to the last fix. I am flying the higher of my last assigned altitude and MEA. Then, I am holding at my last en-route fix and the filed expected arrival time is coming. In this case, how may I start to descent and commerce an approach? regarding which IAP to use and which IAF to head for, provided that the last en-route fix is none of IAFs in any IAPs and my last altitude is much higher than the minimums prescribed in the IAPs.

POST EDIT:

Transponder is on 7600
No VMC within the range of fuel
Destination airport is tower-controlled

The FAA says (per 14 CFR 91.185):

  If the clearance limit is not a fix from which an approach begins,
  leave the clearance limit at the expect-further-clearance time if one
  has been received, or if none has been received, upon arrival over the
  clearance limit, and proceed to a fix from which an approach begins
  and commence descent or descent and approach as close as possible to
  the estimated time of arrival as calculated from the filed or amended
  (with ATC) estimated time en route.

This seems to answer your question unless you have a more specific one. You can descend to an IAF as you see fit and commence any approach. ATC will clear the way.


What visibility and cloud clearances are required for airplanes conducting visual approaches? What visibility and cloud clearances are required for airplanes conducting visual approaches during IFR while flying under FAA regulations? Is the visual approach a purely VFR maneuver requiring the usual VFR weather minimums for the airspace being flown? Or does the visual approach maneuver flown under IFR supersede normal VFR weather minimums?

This depends:

Under part 91, you just have to remain clear of clouds and have a minimum of a 1,000 ft. ceiling and 3 SM visibility.

Commercial operations (121 and 135) are further restricted by their Operations Specifications and have to maintain VFR weather minimums appropriate for the airspace that they are in (among other things, see below).

Here is an excerpt from the AIM (there is more details there for different operations, with the differences between controlled and uncontrolled fields, etc.)

  5−4−23(a). Visual Approach

  a. A visual approach is conducted on an IFR flight plan and authorizes a pilot to proceed visually and clear of clouds to the
  airport. The pilot must have either the airport or the preceding
  identified aircraft in sight. This approach must be authorized and
  controlled by the appropriate air traffic control facility. Reported
  weather at the airport must have a ceiling at or above 1,000 feet and
  visibility 3 miles or greater. ATC may authorize this type approach
  when it will be operationally beneficial. Visual approaches are an IFR
  procedure conducted under IFR in visual meteorological conditions.
  Cloud clearance requirements of 14 CFR Section 91.155 are not
  applicable, unless required by operation specification.

The Part 135 Ops Spec for this contains the following requirements for a visual approach:

  b. Terminal arrival IFR - Visual approach or a Charted Visual Flight
  Procedure (CVFP).
  The flightcrew may accept a visual approach or a
  CVFP provided all the following conditions exist. The flightcrew may
  not accept a visual approach or a CVFP unless the limitations and
  provisions of subparagraph f. of this operations specification are
  met.

  (1) The flight is operated and remains in Class B, C, or D
  airspace, within 35 miles of the destination airport in Class E
  airspace, or the airspace beneath the designated transition area.

  (2)
  The flight is under the control of an Air Traffic Control (ATC)
  facility.

  (3) The flightcrew must maintain the basic cloud clearance
  as specified in Section 91.155.

  (4) For a visual approach without a
  CVFP - The flightcrew must be able to establish and maintain visual
  contact with the airport or maintain visual contact with the traffic
  to be followed as directed by ATC. In addition, the following
  provisions and weather conditions at the airport during the approach
  must be met:

  (a) Reported visibility must be as specified in Section
  91.155, but not lower than a visibility of three miles and reported ceiling must be 1,000 feet or greater, or

  (b) When in the terminal
  area with the reported visibility not lower than three miles and
  ceiling not reported, the flightcrew may continue to a landing if the
  runway of intended landing is in sight and the flightcrew can maintain
  visual contact with the runway throughout the approach and landing,
  and

  (c) Ceiling and cloud clearance must be as such to allow the
  flightcrew to maintain the minimum altitudes prescribed in Section
  91.129, 91.130, or 91.131, as applicable for the airspace class in which the flight is operated.

  (5) For a CVFP - The flightcrew must be
  able to establish and maintain visual contact with the airport or the
  charted visual landmark(s) for the CVFP throughout the approach and
  landing. In addition, the weather conditions at the airport at the
  time of the approach must be reported to be at or above the weather
  minima established for the CVFP, but never lower than the VFR landing
  weather minima stated in Section 135.205 in uncontrolled airspace.

  ...

  f. Special Limitations and Provisions for Visual Flight Rules. All VFR operations authorized by this operations specification shall be
  conducted in accordance with the following limitations and provisions.

  (1) The certificate holder must identify obstacles and use airport
  obstacle data which ensures that the performance requirements of Part
  135 are met.

  (2) The weather conditions must allow the flightcrew sufficient
  visibility to identify and avoid obstacles and safely maneuver using
  external visual references and to maintain minimum altitudes.


Is there a height limit to national airspace? Given the following facts:

satellites can operate at an altitude of 200km without triggering frontier violation incident
aircraft can trigger diplomatic protestations if they cross a frontier at an altitude of few kilometers
there is a gap between high altitude and space

There must be a altitude or height limit between national and international airspace. If so, what is this limit?

As it stands, that 'border' between FL600 and above is undefined, and unregulated.

  There is no international agreement on the vertical limit of
  state sovereignty. Today, the need to settle this gap in the law is
  growing in importance.

  More and more states are developing their own domestic
  space launch capability. Few of these new space powers will be
  able to freely access space, or utilize the most efficient launch
  azimuths, if neighboring states can claim sovereignty up to even
  62 miles (100 kilometers). They will have even more difficulty
  returning objects to Earth if the boundary is set at that altitude.
  Even the United States and Russia are facing limitations on their
  ability to freely access space. Setting a low vertical limit on state
  sovereignty will ensure all states have equal access to space.

  Dean N. Reinhardt, The Vertical Limit of State Sovereignty, (2007)

and

  Vertical Sovereign Territory.

  Interestingly, there is no international law (or agreement) defining the vertical boundary of a nation’s sovereign territory/airspace (the boundary between outer space which is not subject to national jurisdiction and national airspace). Obviously, there is a “white” unregulated area that should be defined, not at least from national security concerns. Presently the airspace between FL 600 and up to the border between the earth’s atmosphere and outer space (generally an altitude of approximately 100 km (62 mi) (the Kármán line)) is not regulated.

  H. Bertil Nordin, U-2 and the Vertical Boundary of Sovereign Territory, (2017)


What do I need to get my pilot license? I am thinking of getting my pilot license, I would love to fly a helicopter or a small plane, but I don't know where to start. What do I need to get a pilot's license?

Things like:

How much does it cost
Get a cheap plane or chopper
Where to take lessons
Theory and practical
Is there a difference which plane or helicopter I may fly, especially in size

Any info is welcome.

Some answers:

  1) How much does it cost.

Cost will vary based on numerous factors, such as your age; being older seems to take more hours than younger - different reflexes, different reactions compared to experience from other motor vehicles. At the same time, being older one can probably be able to fly more regularly, which really helps. Or perhaps not if being older one is stuck with house and car payments and saving for college, etc.
For a full-up private license, a minimum of 20 hours of dual and 20 hours of solo time is needed.  I started at 33, could afford twice a week, missing a lesson seemed like two steps back at times. Ended up with about 70 hours total by the checkride, and my wife ended up with similar when she got her license a couple of years later.

  2) Get a cheap plane or chopper

I believe Plane will be less cost - helicopter lessons are more. Must be a maintenance thing - they do seem to just want to vibrate themselves apart! The times I have looked into it, helicopters seemed to be twice as much.

  3) Where to take lessons

Someone else already answered - find an airport near you and ask around.  If it becomes too impractical for you to get there on a regular basis, you may not be likely to keep doing it.

  4)Theory and practical

Depending on the school, you may or may not be required to take "ground school" with them, where a lot of basics are covered, and prep for the knowledge exam and the checkride is done.  Being a college grad, I took the self study path instead, and anything I couldn't figure out, or didn't understand, I brought up during flying lessons.  Jeppesen.com has good study materials, as well as courses you can find at Sportys, King Schools, etc. Once you apply for the student license/medical, you will get a lot of stuff in the mail.  Do that early to make sure you can pass the medical (it's very basic).  Ask at the airport for recommendations on nearby Aviation Medical Examiners (AME).

  5) Is there a difference which plane or helicopter I may fly, especially in size

Airplanes, you may start in a small 2-seater (I started in a Cessna 150) or a somewhat larger 4-seater (Cessna 172 is very popular), or a similar low wing plane from Piper (like a Cherokee).  Helicopter, the Robinson R-22 is a popular 2-seater for starting. R-44 for 4-seats.

Which license you go for depends on your "mission".  Want to take some friends up and go sightseeing without spending a fortune? Plane I suspect is more affordable.  You can also look at "trade a plane" magazine to see prices on what's for sale in the used market. There are also plenty of kit planes and helicopters and gyrocopters you can build if so inclined.

There are also plenty of very light sport aircraft (LSA) planes, basic 1 and 2 seaters with limited speed and fuel capacity, not intended for more than just local flying, but longer trips would be possible with some tenacity.


What factors cause a turbine aircraft to have decreasing fuel burn over time? Consider a fictional aircraft with a fuel schedule similar to the following:
1st hour - 10,000 (lb or kg, doesn't matter)
2nd hour  - 8,000
3rd hour  - 6,000
4th* hour - 5,000
 * and subsequent

As we can see the fourth hour of flight burns only half the fuel of the first hour, so fuel consumption is a function of time.  Is the decreasing weight of the aircraft as fuel is burned the only reason for this increase in fuel efficiency, or are there other influences?  For example if a jet launches with half tanks versus full tanks, would you use the same 1st hour fuel schedule?
I may be way off base here, so if there are fundamental errors in my question, please correct me.

Per this paper:

Fuel consumption increases (approximately) linearly with aircraft weight, because weight corresponds to required thrust and thrust corresponds to fuel consumption.

The paper lists other fuel consumption factors as well.  But, as you suspected, aircraft weight is a dominant factor.


Can a person with unilateral anosmia in the right nostril become a commercial pilot with a class I medical? I had a minor road traffic accident in November 2014, since then I cannot smell through my right nostril, where my left nostril is working normal. I am looking to know if you know any pilot you have met with who has no sense of smell or lost it due to any event like accident in my case and is still flying.
I am interested in FAA and DGCA India regulations.

We aren't medical professionals here, and no Aviation Medical Examiner (AME) posts on this board as far as we know.
The best answer is to schedule an appointment with an AME and have him review your case. If the answer is "yes" you can get the medical, this is a cost you'll be paying anyway (and I'd suspect that is the answer you're likely to get). If the answer is "no," better to get it from an authority, rather than an internet message board.
AOPA can help you locate an AME who specializes in "interesting" cases, some of whom may well be able to review everything over a phone call or video chat, at least to the point of recommending your best course of action.
Go with an AME's answer, rather than ours. Best of luck to you!


What is the technical term for ARINC 429 data injected into a heartbeat stream going to an LRU? I am going to describe a type of communication protocol that I've seen used with ARINC 429 and would like to know if there is a proper name and classification for this type of communication... Anyway, I'll try to describe as accurately as possible.

So there is an outgoing stream of communication (like a heart-beat) that an LRU expects to receive. The LRU also expects to receive certain data on that same heart-beat bus... So, you'd have to sort of inject the ARINC word data into the heart-beat stream temporarily to send it to the LRU.

Is there a name for that sort of communication?

I think I've also seen similar protocols with serial communications... Where a data stream (heart-beat) is always being sent, but the pay-load changes when you need to transmit/receive meaningful data.

I am looking for the technical terms to describe this communication process.

ARINC 429 is both an electrical bus standard and a data standard.

The electrical standard defines a unidirectional, one to many (Tx to Rx) serial data bus that is transmitted using bipolar Return to Zero (RtZ). This makes the signal self clocking. If no data is being transmitted, the line stays at 0 VDC (Null) which consequently removes the clock as well.

Since there is only one transmitter on a line, there isn't any "injection" of data onto the bus by another source.

The ARINC 429 Data standard uses the serial bus to transmit packets of information, called labels. These labels are 4 bytes (32 bits) long.

An ARINC 429 receiver is designed to expect a constant stream of labels to update the receiver's internal data within a certain time limit. If a label isn't received within the specified period it is considered stale and is left to the receiving LRU to decide how to handle stale data.


In aircraft construction, what is the difference between a firewall and a bulkhead? I'd like to know the difference between a firewall and a bulkhead.  They both separate segments of an aircraft, so how are they different as it pertains to their design and purpose in aircraft construction?

A bulkhead is any kind of structural panel separating two spaces in a fuselage.  Like a solid panel in the tail cone that has only small holes for cables to pass, or the panel that forms the end of the baggage area.  Pretty much any solid lateral partition with a structural use. On a large aircraft, any structural partitions separating parts of the cabin, like the flight deck bulkhead, or terminating the pressure hull at each end, the "pressure bulkheads".

A firewall is a firebreak.  It's a bulkhead that is fireproof to separate a "hot" area like an engine compartment from adjacent structure, so that if you have an engine fire, you'll have more time to try to land, or if at high altitude, say your prayers.  To be a firewall, the material of the bulkhead has to be flame resistant, with a much higher melting point than aluminum.

On GA airplanes, the firewall is usually stainless steel, but can be galvanized steel (antiques and homebuilts), or even special flame proof coatings on a normal bulkhead (some composite homebuilts).  On transport airplanes, a firewall will be stainless or titanium.

So in a nutshell, a bulkhead is a structural lateral panel in a fuselage.  A firewall is a "fireproof" bulkhead normally separating the engine compartment (or other flame source) from the rest of the plane.


What really causes a low-pressure gradient over the top of an aerofoil? As air flows over the top of an aerofoil the pressure drops. Compared to the high / normal pressures on the underside, a pressure gradient is formed with higher pressure further above the wing. You get:

less pressure = less particles = less density

This sketch summarizes what I know so far:

My questions are:

Does lower air density above the wing mean the air weighs less compared to the weight of the air underneath the wing? If the difference is negligible then it's OK, we can forget it.
What forces act on the air particles causing them to speed up?

This is my understanding: A combination of the shape, angle of attack and movement of the wing creates high pressure on the front side of the wing and low pressure on the back side. By front and back I mean the part that is facing the oncoming air and the part that cannot "see" the oncoming air.

The low pressure is there because of this. This is the pressure difference which causes lift. Some lift also comes from the downdraft caused by air moving downwards at the trailing edge of the wing due to the angle of attack.

This article has a video that shows airflow over a wing.
http://www.cam.ac.uk/research/news/how-wings-really-work


Why does the use of reverse thrust on the DC-8 generate a pitch-up moment rather than a pitch-down moment? Unlike most aircraft, the DC-8 is certified to use reverse thrust in flight (albeit only on its inboard engines). This serves as a very effective airbrake, although it also produces considerable buffeting (due to the large areas of separated and/or reversed airflow it produces over the wing’s surface).

It also, more interestingly, produces a pitch-up moment (at least according to the CAB AAR on the crash of EA304):

  [...]  The pilots in this case, if the drive system failed during a PIO rather than earlier, had no way of knowing the real reason for its failure to operate in the ANU direction.  In the split seconds available to them for analysis they could easily have concluded that the failure was due to heavy stick forces.  Reverse thrust, in addition to drag, produces a nose up pitching moment, a fact known to the first officer if not to the captain, and as indicated previously, they did employ this aid.  It is also true that during the time, no matter how short, required to go from forward thrust to reverse, the noseup pitching moment of forward thrust has been removed and therefore contributes to the severity of the dive.  Small as it may be, this factor becomes more significant at very low initiating altitudes.  [Page 25 of the aforementioned AAR; my emphasis.]

For an aircraft with engines mounted below the aircraft’s center of mass, like the DC-8, only the use of forward thrust would be expected to produce a pitch-up moment (due to the torque exerted by the offset net thrustline), and reverse thrust should produce a pitch-down moment (for the same reason):

Yet, for the DC-8, the use of reverse thrust not only creates a nose-up pitching moment, but, apparently, creates one that is stronger than that produced by forward thrust!

Why?

What you're missing from your diagram is @JohnK's drawing (which I'll borrow) from his answer to your related question, Why does the use of reverse thrust in flight on the DC-8 cause areas of separated/reversed airflow over the wing?

The use of reverse thrust increases the wing's angle of attack (AOA) more than it does for the tailplane (as opposed to if the whole plane had pitched up). And not only that, the inboard thicker part of the wing now starts with turbulent air, this reduces the amount of lift at the neutral point (aka aerodynamic center), unlike what happens in a normal AOA increase.

Since the AOA of the tailplane hasn't changed much, it can't contribute to the stability, and with the big reduction in lift, the tailplane wins and the plane pitches up.

Think of it as deploying spoilers, but spoilers that affect the wing from before the leading edge:

  When extended, wing-mounted speed brakes usually produce a nose-up pitching moment (Boeing).


What kind of motion will an airplane enter into when disturbed? When an airplane is disturbed from a longitudinal equilibrium position, what sort of motion will the aircraft experience? Will it be a phugoid or a short period motion? Or will it be both?

The separation into short-period and phugoid motion is somewhat artificial. There is just motion. It just happens that when you fly a 'real' airplane (or solve its equations of motions), you'll have quite different timescales for angular motion (isolated pitch; low seconds) and linear motion (changes of altitude and speed; tens of seconds and even minutes).

Broadly speaking, heavier and faster airplanes with swept wings demonstrate more of this separation. At the other end of the spectrum, smaller airplane models have similar timescales for both the short- and long-period motion, and for them the separation is not meaningful (which makes it more difficult for analysis!)

So, to answer the question, both will usually occur. In some cases, depending on the airplane and your point of interest, you can ignore one or the other. On airplanes where separation is significant and motion is stable and well damped, it may also be possible to produce a disturbance that results in (practically) one kind of motion: either short-period (e.g. a small elevator impulse) or long-period (e.g. a brief application of pitch-neutral air brakes).


Can a FAA "Letter of Authorization" be used when there are no pilots that are current on a type rating? A pilot has multiple type ratings and wants to train for another aircraft (e.g. DC-4) which is on his Part 125 certificate. Unfortunately, there are no available pilots with currency on this aircraft type, but some do have the rating.

Can the FAA authorize a "Letter of Authorization" for the pilot to train based on his other type ratings? Or, could the FAA issue a letter of authorization to a pilot with the appropriate type rating to be used as a training instructor?

As of today the FAA has a few examiners who can issue the type rating for this aircraft, but have lapsed the currency requirement. Therefore, the question is one of training and how to obtain it; not the actual addition of the rating which they can provide.

If I understand the question correctly;

Maybe

The FAA outlines a generic type rating curriculum in this AC which is worth going through as it outlines what training would potentially be required for the type cert,

FAA FAR 61.31 does allow for a waiver

  § 61.31 Type rating requirements, additional training, and authorization requirements.

  (b)Authorization in lieu of a type rating. A person may be authorized
  to operate without a type rating for up to 60 days an aircraft
  requiring a type rating, provided -

(1) The Administrator has authorized the flight or series of flights;

(2) The Administrator has determined that an equivalent level of
 safety can be achieved through the operating limitations on the
 authorization;

(3) The person shows that compliance with paragraph (a) of this
 section is impracticable for the flight or series of flights; and

(4) The flight -

      (i) Involves only a ferry flight, training flight, test flight, or
      practical test for a pilot certificate or rating;

      (ii) Is within the United States;

      (iii) Does not involve operations for compensation or hire unless the
       compensation or hire involves payment for the use of the aircraft for
       training or taking a practical test; and

      (iv) Involves only the carriage of flight crewmembers considered
         essential for the flight.

(5) If the flight or series of flights cannot be accomplished within
   the time limit of the authorization, the Administrator may authorize
   an additional period of up to 60 days to accomplish the flight or
   series of flights.

Section 3 is the important one. You would have to show that it is "impractical" for you to obtain the type rating with out the waiver to operate the aircraft and fly the required training time.

You may also have a compounded issue of then needing a second crew member who is typed in the aircraft and can provide training/co-pilot duties. Further more you cant conduct any commercial operations while doing this so you need an empty DC-4 and a lot of time (read money) to fly it. Keep in mind you may only have 60 days to accomplish all the training as well or you will need to get an extension on the waiver.

Just a general note. These kinds of situations are often best suited for an aviation lawyer who has experience dealing with the FSDO. It would be wise in this case to seek the aid of someone like that. You also may want to look for remote training if there is another operational DC-4 flying elsewhere and an arrangement could be made for you to go train with them for some time that situation may, in the long run be simpler.


What would happen if the wing of a supersonic plane goes into the shock wave from the nose? What happens if the wing of a supersonic plane goes into the shock wave from the nose?

A plane have this shock wave at the nose going back at an angle, but it also have shock waves going back (in the same angle) from the intake or the joint between the wing and the body, shocks that do go over the wing.

Since the wing can live with that, why is it then a problem if they go into the shock wave from the nose (which I have always learned)?

In case anyone want to know, then the question arose when an friend and me discussed chock-cone angles, and the the X-29 seem to have an angle between nose and wing-tip that relates to the angle a schock will have at M1.8, the plane´s top speed.

-And the same seems to be the case with many other fast planes.

-By the way, I chose thát image as the angle of the schock is higher than the angle of the leading edge of an F-18.

I haven't been able to find a better image than this, I believe it is an F-18 with pretty straight wings:

Not much.

The outer wing would fly in undisturbed flow and an inboard wedge would fly inside of the Mach cone of the nose. This is similar to a wing with a supersonic leading edge; something that has been quite common with Mach 2+ designs before swing-wing aircraft. The nose shock of a pointed nose is a weak, oblique shock which does not alter flow parameters dramatically,


Do jets use tinted windows? For cars, I've heard of LLumar used as window tint.
Do jet aircraft use window tint? If so, what type of window tint do they use?

Yes, airplanes (all types including jets) can use window tints.

Ultraviolet "A"
Tints for airplanes are with enhanced protection from UVA radiation. UVA exposure can increase the risk of skin cancer and will damage most organic materials. Its intensity increases about 5% with every 1000 feet of altitude. Unlike car tints, airplane tints need to block the additional exposure to UVA.

Solar Energy Control
Tints also block near-infrared radiation which results in lesser interior heat.

LP Aero Plastics (product listing) and PolyOne are the two manufactures I found for aircraft tints.


Are warnings generally "source-focused" or "effect-focused"? I have trouble wording this question, please bear with me.
Sullenberger recently stated that the pilots of the fatal MAX flights where likely under sensory overload caused by "multiple, compounded alerts". Which makes me wonder: As a general rule of thumb in modern commercial aviation, do we try to alert pilots of the cause of an issue (this or that part is out of order, e.g. we lost a hydraulic pump), or of the effect (we cannot move the control surface anymore)?

Only alerting of the cause might make them overlook effects.
Only alerting of effects might take away a chance to solve/remedy the issue.
Alerting of both might be the overload mentioned before.

Or is there another system in place, like filtering or at least ordering warnings by severity? I am a software architect by trade, and we always go with "display the error in the earliest possible step, and ignore the rest." The rational is: Do not distract me with the facts that my paint job is getting blisters and my tires are flat, if we already determined that the car is on fire.

They are source focused. They display precisely the condition that exists, not the follow-on effects.
WARNING indications that could be dangerous if not acted on right away are Red.  Indications that must be taken into account with some action but are not an immediate danger are called CAUTIONS, and are Yellow.  Indications that just indicate a status condition, like a loss of redundancy which does not degrade function but needs to be addressed on the ground, are Status messages, and are White.
Most indications are just exceedances of some parameter above or below a limit, like an idiot light on your car.
Examples:
"GEAR BAY OVERHEAT" would be a red WARNING that tells you that the temperature in the landing gear bay exceeded a certain temperature.  Not necessarily that there is a fire.
"L ENG LOW OIL PRESS" would be a red WARNING that tells you the oil pressure dropped below x psi.  Not that the engine is about to blow up from lack of lubrication.
"R SCAVENGE EJECTOR" would be a yellow CAUTION indicating that the output pressure of the scavenge pump in the right fuel tank is below X psi. As a Caution, it needs to be acted on in some way, but it's not a danger right now.
The protocol is, a message tells the pilot exactly what condition was exceeded or what failed.  What to do about it is found in the Quick Reference Handbook that is usually stowed at the base of the center console where both pilots can reach it.  It's a binder with a tall narrow page format, with all of the  Caution and Warning messages cataloged by system, and a logic flow chart presentation for each with a procedure for each CAUTION and WARNING that is to be followed, that is meant to be easy to follow in a dark cockpit while under stress.
Only a small number of WARNING conditions expect the pilot to deal with the condition from memory.  These are called "Memory Items" or "Immediate Action Items", and you're expected to be proficient and doing steps A B C D immediately from memory.  The QRH is consulted later to perform an expanded procedure when safe to do so. There might be half a dozen or a dozen WARNINGs that have memory items that need to be performed as soon as the warning appears, and you have to have nailed down in your mind so your hands just fly around automatically when they happen.
For example, an engine fire on takeoff has a series of Memory Items, done as a standardized drill: thrust lever to idle (with a protocol where each crew cross checks and confirms the actions), thrust lever to fuel shutoff, Fuel and Hydraulic Shutoff button push, Fuel boost pump off, Fire Extinguisher Push if the fire indication hasn't gone out, and so on.  For  that sort of thing, you drill those sequences until you're blue in the face and can perform the steps without having to think about them, while under intense pressure and distraction.
Another one would be a depressurization event.  After the crew gets their masks on and establishes communication with each other through the mask mics, you declare "emergency descent", slam the thrust levers to idle, pop flight spoilers fully out, dial the autopilot altitude select to 10000 ft, engage autopilot speed mode, causing the plane to pitch over to maintain the speed with the thrust removed, and dial the speed setting to maximum operating speed (Vmo).  Those are the memory items. Once settled down in the descent with the autopilot holding the airplane in a shallow dive at redline speed (if you think something structurally is compromised after a "big bang", company procedures may advise staying at current airspeed in the dive instead of accelerating to Vmo), the non-flying pilot gets out the QRH and you go through the expanded depressurization procedure together.
Messages will generally be prioritized when multiple messages appear, and lesser importance ones filtered out when a lot of conditions are set. And a lot of messages are inhibited during the takeoff process because they can wait.
WARNING messages will often be accompanied by CAUTIONS, but multiple WARNINGS are unusual.  Getting multiple WARNINGS in a situation where you are already surprised by seemingly nonsensical activity (like trim wheels spinning madly for no apparent reason) can push you toward brain lock unless you've trained yourself, or have the right instincts. Boeing thought that the trim wheel spinning madly for no reason could be dealt with using the Stab Trim Runaway procedure, but stab trim runaways don't generally include the trim running wildly, then stopping, then starting again, then stopping again.
At some point you have to just ignore all the data and just fly the plane.  But that's pretty tough when the plane is trying to dive itself into the ground seemly possessed by a demon, in addition to all the garbage being presented as messages.  It's not hard to imagine anybody going into brain lock with that kind of data explosion combined with bizarre behaviour.


Can you be cleared to cross a runway *behind* an aircraft on takeoff? Taking KEWR as an example, let us say that aircraft are taking off from 4L at N and landing on 4R.  Can I be cleared to cross 4L at say V, behind the aircraft taking off on 4L, without interrupting the departure flow? Or will the runway need to be clear full-length to let me cross it?

You can cross behind the aircraft on departure roll. This type of operation is done heavily at ATL, on the middle complex during an east flow. Aircraft will depart  runway 9L at M2, and arrivals from either of the south runway will cross behind them the whole time at M1, to get up to the gates.


What is the decision- and workflow during a strategical rerouting? Consider a commercial longhaul flight receives a warning about severe weather on its intendend route, a few hours of flight time out. What would be a typical work- and decisionflow between pilots, dispatchers and controllers for the rerouting of this flight?

To have a more specific scenario, lets assume the flight is bound to the US westcoast from Europe and the pilots receive the weather forecast when already in Canadian airspace (not oceanic).

Good question! One major problem of long-haul flights is the age of the wx forecasts in your flight preparation papers vs. the quick and unexpected development of especially convective adverse weather.

If the wx is of such magnitude that passing through it is not possible, ATC will offer reroutings to pilots. But in most occasions, the decision rests on the aircrew, as they have to consider other operational aspects as time in their tanks, navigation equipment etc.
If the adverse weather consists of ISOL CB which can easily be circumvented, ATC will most likely not issue big reroutings and let the pilots do the wx avoidance on a tactical / ad hoc level.

If the aircrew is lucky to work at an operator with savvy dispatchers, they will pro-actively send an ACARS message to the crew, so they can prepare for the wx development ahead.

So most of the time, the flight crew has to come up with an idea. And their means of situational awareness are limited. As of now, the only means to really see what is ahead is the wx radar, usually having a range of 120-200NM. So it is common practice to ask aircraft ahead on track about the wx situation.

After gathering all the facts, the classic decision workflow starts. At my company, we use F-O-R-D-E-C, which stands for

Facts
Options
Risks
Decision
Execution
Control

...where "D" is the point at which you make your decision. Depending on the time available and the task to be solved, FORDEC may take 10 seconds (fire on board) up to half an hour or more (I guess https://en.wikipedia.org/wiki/Qantas_Flight_32 must have been such a case).

The "C" part (for "control" or "check") is a reminder to revisit your decision at a later point in time and check if it is still valid.

In the future, we will see more and more "connected" flight decks, where pilots can receive long-range wx information as live radar pictures directly into their electronic flight bag. This could have saved the lives of the people onboard AF447:

The yellow circle represents the range of the wx radar (what the pilots of AF447 could see) and the red polygons represent the known wx situation from a satellite picture.


Why don't we have helicopter airlines? Just a curious thought. Why isn't there any helicopter airline? Airlines that operate large helicopters that can transport 10 or 20 people a short distance?

They do in fact exist, though they often use smaller helicopters. A Google search on the term "heli taxi" yields thousands of results, and while a lot are probably irrelevant, the first few pages give hundreds of operators around the world, ranging from companies ferrying passengers between airports and major cities to companies servicing oil platforms at sea, islands too small to have airfields, etc.


How fast do the F-35 weapons bay doors open/close during combat ops? (YouTube)

I came across the following Norwegian F-35 video launching a live missile from its right bay door. Of course, opening the door and launching the missile drastically increases RCS, but it seemed to operate quite slowly, about 3-4 seconds for the whole operation.

My questions are:

Is combat-operation of the bay door really so slow or does it speed up for a lesser chance of detection by today's powerful A2A radars?
Is 3-4 seconds of an exposed RCS increasing surface tactically significant? Obviously, if it wasn't there would be no need to overly stress the components if unnecessary.

3-4 seconds may lead to a detection, but then again 1-2 seconds could too with a sophisticated radar system. The important thing is that 3-4 seconds is short enough that a detection would be extremely hard to exploit; by the time a missile could be launched the contact would have disappeared.

If by some chance a station managed to get a missile off in that short time-frame it would probably miss. There are 2 types of radar guided missiles:

Semi-Active radar homing: semi-active missiles only have a radar receiver, they home in on a target using radar painted by a ground station or airborne radar. If the targeting radar station loses lock then the missile will go wide
Active radar homing: active missiles have their own radar emitters which get switched on when they get close to their target, so they are no longer relying on the ground station which is much farther away. They still rely on the ground station to get close enough to use it though, so while active missiles have a better chance it's still very unlikely

So shaving a second off that time would give very little benefit. Having a door open longer would be bad though. In the book "Skunk Works" by Ben Rich and Leo Janos (former director of the Lockheed Skunk Works) Major Miles Pound wrote about his experiences in the first Iraq war flying an F117 when a bomb bay door got stuck open after dropping:

  We came in at three in the morning using only eight airplanes and
  needing only two tankers to get us there and back, and took out three
  of the four nuclear reactors and heavily damaged the fourth. Once that
  first bomb hit all hell broke loose. I dropped my bombs, but I
  couldn’t get my bomb-bay door closed. That was as bad as it could get
  because a right angle is like a spotlight to ground radar and
  a bomb-bay door is a perfect right angle. And out of the corner of my
  eye I saw a missile firing up at me. I had one hand on the eject lever
  and the other trying to manually close that stalled bomb bay. As the
  missile closed on me, the door finally did, too, and I watched that
  missile curve harmlessly by me as it lost me in its homing. About an
  hour later I began breathing again.


How able were pilots of three-person cockpits at running the normal FE work? This question is about whether a FE could fly solo.

I'd like to know to what degree the pilots could operate the FE console, e.g. if the FE was incapacitated. How much was covered as part of the type rating, and how much did they just pick up while flying?

First, these answers insofar as the licenses and airline operations apply to the U.S. system since that is all I am familiar with.

You must have a Flight Engineer license to operate as an FE. Just as a pilot's license has ratings on it, so does an FE license, and you have to have a rating that matches what you're flying. My FE license has a single rating, TURBOJET POWERED, so I could not sit as an FE in other than a jet aircraft.

In the days when 3-man cockpits were the norm, the usual career course at large carriers was to start out as an FE (second officer), progress to being an FO (first officer), and then a captain. In that system, if the FE was incapacitated, it was likely that one if not both of the pilots had actually been an FE.

The U.S military continued with FEs well after the 3rd seat came out of civilian aircraft (I think they still do, but am not sure). Military FEs were typically never intended to become pilots. When all new civilian aircraft being manufactured were 2-man cockpits, but lesser airlines were still flying the old 3-man cockpits (think freight carriers in particular), former military FEs became the source for their FEs and were considered professional FEs, in other words they would not be progressing to a pilot seat. A few did, but they were definitely the exception.

The two 747 carriers I worked for hired pilots into the FO seat. The FEs they hired had to have A&P licenses as well as an FE license, the reason being that the company wanted them to be able to perform and sign off minor maintenance problems that occurred in remote places (think Africa especially) where maintenance wasn't locally available. With only a couple of exceptions, every FE I ever flew with was ex-military with an A&P, and they knew the airplane mechanically better than I.

In the airline ground schools, especially the initial groundschool, the FE panel and its operation were thoroughly covered in the material presented and in the testing. The oral exam for the aircraft type rating thoroughly covered that FE panel. As I remember my 747 type rating oral was 3 to 4 hours long. Fortunately I got through that and the sim ride on the first try. Not all do.

So, in the case of an incapacitated FE in the 10 years I was on the 747, it was likely that neither pilot had actually operated as an FE. However, he would have studied it in ground school and had demonstrated an understanding in an oral exam. There is, though, nothing like experience and currency.

Every once in awhile I would get out to a cold airplane before the FE had arrived and would want to start the APU. This was usually at a remote station without local maintenance. There was, of course, a procedure for doing this, and I had a copy. But even with that check list and doing everything very carefully, I always breathed a sigh of relief when the APU started and there were no problems. If the APU failed to start on the first try, I usually waited for the FE to arrive. I might try a second time if I knew he was going to be awhile, but never a third.

The old 747-100/200 had a lot of switches, and interesting things could happen if the APU was started and the switches were not properly set. I always did an outside check to make sure all gear doors were closed and that nobody was around the bottom of the airplane. If a gear door was down, and switches were not set right, when the APU started, the doors would immediately close when the hydraulics were pressurized. Anyone in the way would certain get hurt.

Individual pilots on the line varied greatly in their knowledge of the FE panel. FOs were at a disadvantage insofar as watching it be operated. They can't see it well. Captains can see it. However, some FEs resented captains looking at it too much, feeling that was their turf. What I often did during cruise, as much to pass the time as learn, was to let the FE be my tutor.


Why does the use of reverse thrust in flight on the DC-8 cause areas of separated/reversed airflow over the wing? The DC-8 is one of only a few jetliners that can safely use reverse thrust in flight; even so, reversing the inboard engines (the outboard engines can only be reversed on the ground) while flying still produces extensive airflow disruption over the wing’s upper surface in the vicinity of the reversed engine(s), as detailed in this excellent PDF documenting a series of flight tests carried out by NASA on their DC-8-70.  A significant area of separated flow extends from leading to trailing edge (wider at the trailing edge) whenever the inboard engines are reversed below approximately mach 0.88, and becomes very large (covering over a third of the upper wing surface) below approximately mach 0.6; additionally, at mach numbers of 0.6 and below, an area of full-on reverse airflow appears over the trailing edge, in line with the inboard engine.

This would make perfect sense if the engines extended aft of the trailing edge, in which case the reversers would blow air forward and over the wing, but that isn’t the case for the DC-8; instead, the DC-8’s tailpipes (even on the DC-8-50 and -60, with their long, cigar-shaped JT3Ds) are placed under, or just behind, the wing’s leading edge.  If anything, one would expect the use of reverse thrust on a DC-8 to strengthen the airflow over the wing, instead of disrupting it, due to all that forward-blowing air piling up around the engine and under the leading edge and helping to partially block air from passing under the wing, forcing it to go over the top instead.  So why does in-flight thrust reversal on a DC-8 instead produce large areas of airflow separation (and, at lower speeds, even airflow reversal)?

That's not surprising at all.  What the pictures of the tufts show, and exactly what I would expect to see, is a massive injection of turbulent reverser air into the upwash just ahead of the LE, disrupting everything behind it, and which would include a significant component of flow over the top of the wing.  The tufts show some straight and some pointing forward, which suggests a layer of very turbulent air with random eddies.  Like this:


How is a standard holding pattern flown? Unless otherwise specified, in what direction and what duration of leg should be flown if assigned a hold?

Holding procedures are defined by each country for their own airspace. With that said, member countries of the International Civil Avation Organization (ICAO) generally follow the guidance of Part I, Section 6, Chapter 1 in ICAO Doc 8168, Aircraft Operations, Volume I, Flight Procedures:

  Section 1.1.3 - The procedures described in this chapter are related
  to right turn holding patterns. For left turn holding patterns, the
  corresponding entry and holding procedures are symmetrical with
  respect to the inbound holding track.

While the document does not explicitly state that right-hand holding patterns are the default, it is implied.

  Section 1.4.9 - The still air time for flying the outbound entry
  heading should not exceed: a) one minute if at or below 4 250 m (14
  000 ft); or b) one and one-half minutes if above 4 250 m (14 000 ft).
  Where DME is available, the length of the outbound leg may be
  specified in terms of distance instead of time.

Member countries can deviate from these standards, but if they do, they are supposed to notify ICAO and publish these differences in their respective Aeronautical Information Publication.


Is it legal to use a home-made Airband radio receiver in the United States? I work at a local small airport (very small, if we get more than about 10 or 15 airplanes taking off/landing it's a "busy day") and I mow the runways (we have multiple grass ones).

Because of the visual obstructions of the tractor's cabin that I use I have to be even more careful than normal to check for in-coming planes and so I started looking into getting an air band radio for the tractor. However, they can be fairly expensive - not something I'd invest in unless I absolutely had to - and so I started looking into making my own.

I'd figured this was possible because I'd occasionally picked up communications on a cheap radio I had around my house (I live quite close to the airport, about a 5 minute bike ride). So, I went searching, and sure enough - this is a thing that is possible and has been done.

I found this post on Electrical Engineering.SE, Modify an AM/FM radio to an airband receiver, describing basically what could be done; and then this video done by MAKE, Weekend Project: Aircraft Band Receiver, showing it actually being done to cheap transistor radio.

Now, what I'm wondering is, is this a legal thing to do - or does the FAA have regulations on this sort of thing?

FYI, the runway is supposed to be closed XXX with NOTAM when it is mowed.

There is no FAA or FCC limit on constructing homemade airband receivers in the United States.

You can also construct transceivers. As long as you do not sell a transmitter you are allowed to make and operate up to 5 of them.

The exemptions covering low power transmitters are covered in Part 15 of Title 47 in FCC regulations.

The maximum unlicensed radiated output power for airband is 500 μV/m @ 3 m. Typical commercial airband handheld transmitters go up to about 1 Watt and base stations up to 50 Watts. For your purposes a 1 Watt transceiver would probably be reasonable.

Note that typical receiver/scanner designs have an oscillator in their mixer that can generate a small amount of RF noise. Normally the signal is so weak it is completely undetectable. Nevertheless, there are specialized receivers that have no oscillator that some people use to listen to airband with zero chance of interference.


Why is there a white line on Russian fighter jets' instrument panel? In several Russian fighter jets (e.g., MiG-29, Su-27) there is a white line on the stick that matches with another one on the instrument panel. Is there any reason for this?

Cockpit of a mig29

Cockpit of a Su27

A former flight instructor told me that it was to help pilots recover from a spin: when the two lines match, the stick is in the proper position to stop the spin. Is it true or is there any other reason?

I believe it is exactly what your instructor has said - it helps when regaining control of the aircraft during a spin.

The correct procedure for recovering from such a maneuver is to apply rudder in the opposite direction. Then, keeping the control column centered, move it forward (for an upright spin) or backward (for an inverted spin).

The problem is, when you're in a spinning aircraft everything outside is a blur and the G-forces involved can throw off the pilot's perception - this is of course especially true of inexperienced pilots. The white line painted on the control column in your picture was one method for helping the pilot to keep the stick centered. Having something simple to concentrate on helps immensely, and is more intuitive than looking at the yaw indicator.

Source: Flying at the Edge: 20 Years of Front-Line and Display Flying in the Cold War Era,
by Tony Doyle.

Edit: This book is an autobiography of a Cold War era RAF pilot, and while it does directly mention the white line painted in the cockpit, it does so in reference to Vampire and Meteor aircraft. I cannot find any reference specifically to Russian aircraft anywhere in the book, and neither does there appear to be any picture evidence of the stripe in the cockpit of any British aircraft. Perhaps someone else can source more evidence.


Why does each fan blade have a different mass and frequency? From a documentary (at 16:39) on building a Rolls-Royce jet engine:

  ... no two finished blades are exactly alike, and with twenty in each fan, it will only spin smoothly if the blades are perfectly balanced, so every one is precisely measured and weighed then rung like a bell. Each blade has a different mass and different frequency ...

Is it hard to develop blades of identical mass, or are there other reasons for the different masses and frequencies?

Because manufacturing processes are not perfect, and the minute differences between parts as-designed and as-produced are amplified by the high rotational speeds and large diameters of modern jet engines.

Take a following back-of-the-envelope calculation: disregard incoming airflow velocity and assume a hypothetical fan with a 1 meter diameter rotates at such rpm that the blade tip has a tangential velocity of 80% of the speed of sound at 35kft in ISA. The equivalent rpm would be 2817.
$$
a_c=\frac{v^2}{r}=\frac{(295\cdot0.8)^2}{1}=55.7\cdot10^3 \; \mathrm{[m/s^2]}
$$

That's the acceleration on the blade tip. If this blade were to be just 1 gram overweight, the force exerted on it would be:
$$
F_c=m\cdot a_c=0.001 \cdot 55.7\cdot10^3=55.7 \; \mathrm{[N]}
$$

that force is more than the weight of a 5kg dumbbell and it alternates its direction almost 3000 times per minute.

Its not that fan blades are hard to manufacture precisely, but that the application in which they are used is unusually demanding and any small imperfection in the blade will make itself known very loudly.

As for why the frequency is different, it is because the vibration modes of any system depend on the spatial mass distribution, like a pendulum which changes its natural frequency if you shorten the distance between the mass and the fulcrum. So the two issues you mention are directly related: a few milligrams of metal missing at the tip will shift the center of mass, and with it the natural frequencies of the blade.

In reality the whole vibration issue is a bit more complicated for continuous bodies, which effectively have infinite natural frequencies, because unlike ideal pendulums, their mass is distributed over infinite points in space, but that is a bit beyond the current point.

The important part is not to have any natural frequency coincide with the rotational frequency of the fan, because that would quickly lead to rapid unplanned disassembly: the vibration amplitude would be amplified with each revolution until the blade failed.


Can a passenger request to visit the cockpit during a flight? As a kid, I remember going to the cockpit of a plane with my mom (I believe the airline was AOM French Airlines).

Is this still possible now?

I understand the answer might differ with different airlines/countries, I'm mostly interested in European and Asian airlines/countries, specifically Japanese.

Is there some specific regulations for international flights, or does it depends on departure/arrival locations?

This stopped after the World Trade Center attacks.

You might be able to get a kid in briefly before the plane takes off.


What dictates the A320's 'emergency ram air inlet' operation? Why does the emergency ram air inlet flap open when delta p is greater than or equal to 1 psi on the A320?

Below is the extract I was reading from the FCOM. Why has the figure of 1 psi been set for opening or closing the flap?

  The emergency ram air inlet opens
  ‐ If Δp ≥ 1 PSI: The outflow valve control remains normal. No emergency ram air flows in
  ‐ If Δp < 1 PSI: The outflow valve opens to about 50 % when under automatic control. It does not automatically open when it is under manual control. Emergency ram airflow is directly supplied to the mixer unit.

(wikimedia.org)

You have misinterpreted the text. Given that the DITCHING pb is not pushed, the RAM AIR pb opens the emergency ram inlet.

The differential pressure (DP) being above or below 1 psi is for the outflow valve (not the inlet). If the DP < 1 psi, the outflow valve opens—this is the opposite to the former title (now the first line) of the question.

This serves two purposes:

Clear smoke from the airplane
Bring in fresh air if the packs fail.

When the system operates in this mode, there is no fine tuning for the outflow valve. By testing, they would have figured out with the fixed ~50% open position, any DP above 1 psi results in rapid decompression.

Note: the protection is via a check valve, so no system logic is involved. It's mechanical.


What accidents have had crew accustomization to warnings among their causes? A non-answer on this question claimed that:

  the idea that maneuvering with the stall warning activated will somehow damage us, causing us to ignore its warning when the chips are down and AOA is up, is just silly

which sounds wrong to me, as I vaguely remember reading about crews becoming accustomed to frequent warnings, spurious or real, and disregarding them when they became critical.

Which, if any, aviation accidents or incidents have had among their direct or contributing causes a crew that ignored a warning due to being used to it being spurious or unimportant?

I am looking for substantiated answers only, remember that this site is not a venue to speculate on aviation accidents.

A notable accident was the Helios Airways Flight 522 crash, where the pressurization system was turned off and the pilots ignored the warning horn because they thought it was the takeoff config warning. From the Wikipedia page:

  As the aircraft climbed, the pressure inside the cabin gradually decreased. As it passed through an altitude of 12,040 feet (3,670 m), the cabin altitude warning horn sounded. The warning should have prompted the crew to stop climbing, but it was misidentified by the crew as a take-off configuration warning, which signals that the aircraft is not ready for take-off, and can only sound on the ground.

The official accident report explains the source of the confusion in more detail (emphasis mine):

  The Board examined the flight crew’s actions to disengage the autopilot and auto-throttle,
  and to retard the throttles upon onset of the warning horn. Given that the expected
  reaction to a cabin altitude warning horn would have been to stop the climb (there was no
  evidence to this effect), the Board considered such actions to signify that the flight crew
  reacted to the warning horn as if it had been a Takeoff Configuration Warning (the two
  failures use the same warning horn sound). [...]

  The warning horn was designed to signal two distinctly different situations. The Board
  considered the role of experience in interpreting and reacting to the warning horn. In the
  course of his career, a pilot is generally likely to only hear the warning horn when it is
  associated with a takeoff and a takeoff configuration problem. [...]

  Most pilots are not very likely to experience a cabin pressurization problem and the
  associated warning horn at any time during their line flying.

Conclusion: Never use the same warning for multiple situations, if the pilots will only get used to one of them, otherwiese they will ignore the warning for the other situtations!


How long does it take for a regular size airliner jet engine to be warm? How long does it take from starting of turbine or turbofan or turboprop, either by using an APU or other methods, until the engine is warm enough for safely extracting full power for takeoff?

A320 with IAE engines has one of the longest shafts in business, and thus is most sensitive to temperature variations/deformations.

Don’t know what the producer requires, but operator procedure is 5 minutes from start to take-off power if the engine was shut down for more than 2 hours before.

Interestlingly enough, this is on top of other temperature limitations (eg min oil temp) and is the same time, regardless of outside temperature ( eg oil on cold day may take longer to warm up)


Modern airliners are designed to be able to withstand the worst (vertical) turbulence. But what about the rarer lateral (horizontal) turbulence? 'Extreme' turbulence is a derived vertical gust velocity of >3,000 fpm (50 fps) that causes a vertical G-load of >3. The limit turbulence intensity for a modern day airliner is 90 fps. (there is likely a 50% safety factor, so the plane is capable of withstanding more).
Lateral turbulence is the type that puts most stress on things like engine pylons and the vertical tail, rather than the wings. An accident involving lateral turbulence is BOAC Flight 911 and the B-52 crashes (Savage Mountain and Elephant Mountain).

Yes they are.  FAR 25.427 has the requirements for lateral gust loads.
I don't know if there was FDR information for the BOAC flight, but it may very well have been a similar event to AA Flight 587, the one that lost its vertical fin over NYC when the pilot got on the pedals in reaction to a wake turbulence encounter that induced a large yaw, and made rudder input loads that the fin is not designed to handle.
In that case the fin broke off not because of the gust, but because of the control response.  You will likely be out of phase in the attempt, and your inputs become "doublets"; amplifying inputs that make the yawing worse.  You don't normally touch the rudder pedals on an airliner between departure and landing unless an engine quits; you just let the yaw damper take care of yaw excursions like that and it'll have no problem staying in phase to dampen the motion.  I'll bet that the BOAC incident was that sort of case, where a gust reaction was followed by an input, at high speed.


What is a running rabbit? My father used to work as an air traffic controller, and often I heard mention of a 'running rabbit'.

Obviously, vermin on the runways of airports can be a bad thing, but I don't think he was literally talking about animals.

Background
The Sequenced Flashing Lights are part of some approach lighting systems and are a row of strobe lights that flash in sequence to direct the pilots eyes to the runway.  They are useful in conditions of bad weather, as they quickly catch the eye and help the pilot locate the runway threshold which might otherwise be hidden; however, during normal operations they can actually be distracting.  These lights appear to run towards the runway and are often called chasers, the rabbit, or the running rabbit.
Kill the Wabbit
Because these lights are very bright, and often distracting during normal operations in clear weather, it is not uncommon to hear pilots ask the controllers to kill the w/rabbit on their approach.  I personally find them overkill, and have heard many other pilots asked them to be turned off as well.

Photo credit ATG Airports


how does recovering a plane from stall lead to exceeding the critical mach speed? Recovering from stall itself states that the aircraft is below stall speed. So how does recovering from that stall lead to exceeding critical mach.

There are two types of stall: One is when speed drops below minimum speed, the lift curve slope flattens or inverses, flow separates and lift drops below weight. This is explained here.

The second type is a high speed stall and is caused by shock-induced flow separation when the aircraft approaches Mach 1. In this case, the aircraft will accelerate to a higher flight Mach number where its maximum lift coefficient is smaller and causes lift to drop below weight. Here stall is caused by a speed increase, hence the name.

Lift coefficient variation over Mach number (picture source)

In the coffin corner, the highest point in the envelope of subsonic high performance aircraft, both speeds coincide. Now the aircraft can fly only at one speed, both a speed decrease and a speed increase will cause stall. When the plane recovers from a low speed stall while flying near the coffin corner, it needs to speed up and consequently runs the risk of accelerating beyond the critical Mach speed at which the lift coefficient drops and causes a high speed stall. Recovery is only possible by sinking into higher density and warmer air where the difference between both stall speeds widens and more lift can be created.

Generally, this happens to jets with straight or moderately swept wings which are powerful enough to fly high and fast but are not designed to exceed Mach 1. The Lockheed U-2 would be a prime example.


Why does the European MALE RPAS UAV use twin engines? I don't know why the European MALE RPS UAV designers chose to use twin engines?

I know two engines are expensive but add safety...

Does choice is to obtain mass flow @ Medium altitude(low air density)?
especially with relatively low speed(maybe 0.4~0.6 Mach)...

I guess that as density drops off at higher altitudes, the machine ingests less air mass, resulting in reduced power and reduced thrust...

To need larger flow areas (larger diameter)?

Unlike RQ UAV series...

According to the Airbus press release:

  The MALE RPAS is the first unmanned aerial system designed for flight
  in non-segregated airspace and will be capable of performing a range
  of missions in intelligence, surveillance and reconnaissance, both
  wide area and in theatre. A twin-turboprop propulsion system will
  deliver energy for the mission system and provide proper redundancy to
  limit restrictions when operating over European densely populated
  ground and unrestricted airspaces.

Essentially, the aircraft is intended to be operated in the same airspace as existing air traffic, over large urban areas. The twin engine configuration is to provide redundancy and thus stop a robot plane from crashing on people.


Have any commercial airline passengers ever been injured from hypoxia due to a depressurization event? This question is inspired by the question "What can I do to save my life if oxygen masks fail to drop down? Is it possible?" One of my first thoughts upon seeing that question was: do you need to do anything, or will you survive regardless of what you do?

Note that I'm not asking about situations like Helios Airways Flight 522, where the crew became incapacitated due to hypoxia, resulting in a crash which killed all on board. I'm wondering if a passenger has ever gotten hypoxia and suffered from health problems (or death) as a result.

The passengers on Helios 522 were incapacitated or died due to hypoxia. Those masks in the aircraft only have enough oxygen to sustain the passengers while the plane descends to a safer altitude, or about 12 minutes. Helios 522 was on autopilot at 34,000 feet for 2 and a half hours. The autopsy showed that the passengers were alive at the time of the crash, but it could not determine if they were conscious. The aircraft was unpressurised at an altitude as high as the peak of Everest, the crew were incapacitated, so it follows that the passengers would have been as well, despite the fact that the autopsy showed that they were still alive at the time of the crash.

Now, let's suppose that they had survived the crash. They were hypoxic for 2 hours, which means they would have suffered brain damage and would be suffering with the long term effects of that (Insomnia, amnesia, etc) as well as injuries sustained from the crash itself.


Is it possible for an airliner to safely fly with doors open? From another question asks about the possibility of dropping bombs from converted airliners. My question: is it possible to safely fly aircraft with a door open for the whole flight envelope? Would the aerodynamics be affected if we did not close the door?

I am not asking about the need of oxygen for the crew, just about the aerodynamics involved. I know the plane would not be pressurized.

The Stratospheric Observatory for Infrared Astronomy (SOFIA) is a modified 747 that has an 5.5 m x 4.1 m door that is opened during flight for the installed in infrared telescope. But the door is usually closed during takeoff and landing.

But according to this story there are emergency procedures to land with an open door and it had to do so once, when it became stuck half open.

Images courtesy Wikipedia


Structural rating of aircraft to withstand storm induced structural stresses I was watching an accident report where an aircraft crashes when  a storm destroys a wing spar.

I'm curious is this only a danger for small aircraft or are there situations when a storm has caused  structural damage to large commercial aircraft?

Even among smaller aircraft are different models built to withstand different limits of such storm induced structural stresses & what would be the metric to compare them by?

All aircraft can in theory be overstressed by either a vertical gust (flying into an updraft) that is severe enough or a maneuvering input (pulling on the stick) that is hard enough.  Both increase angle of attack suddenly increasing the lift force, and bending, on the wings.  Increase the lift force enough to exceed the bending strength of the wing's spar, and that's all she wrote.

The protection from this comes from the fact that if you are below a certain speed, you will exceed the stalling AOA before the bending load exceeds the limit, and the stall will partially unload the wing before it gets over stressed.

So there are two speeds you have to observe; Maneuvering Speed and (on larger airplanes) Turbulent Air Penetration Speed (also called Rough Air Speed).   Maneuvering Speed is the speed below which you'll stall before over stressing the wings if you just yank the control column back as hard as you can to full travel.  Instead of pulling the wings off you'll just snap roll (which is in fact, with rudder added, how you do a snap roll).

Turbulent Air Penetration Speed is the speed below which an extreme vertical gust will take the wing past its stall AOA before the gust can exceed the wing's bending strength.

On transport airplanes, if you know you will, or expect there is a significant risk that you will, enter turbulence, you slow the airplane to Turbulent Air Penetration Speed as protection against over stress.  You would do this if there are thunderstorms in the area (you still give storm cells a wide berth in any case), or there are reports of Clear Air Turbulence, or you are crossing the boundary of a jet stream at altitude, where CAT is typically found and you are being cautious.

On the CRJ700, the Turbulent Air Penetration Speed is actually in the lower end of the normal cruise range, 280kt/.75 Mach, so it's not a major inconvenience.  Maneuvering speed is well below that.

Light aircraft like the Piper Saratoga in the video don't usually publish a Turbulent Air Penetration Speed, just a Maneuvering Speed (134kt for the Saratoga), and the pilot should have slowed to that to avoid being over stressed from turbulence.

Problem is, light airplanes bumbling into thunderstorms usually end up in spiral dives where limiting the speed becomes impossible and they typically come out the bottom of the storm in pieces from over speed and/or over stress, and it comes down to staying out of them at all costs.


How do Air Traffic Controllers determine which planes take off in a certain order? Does it depend on the weight of an aircraft? If two planes requested take off permission and taxied out to the runway at the same time what would determine which plane takes off first (is it to do with weight or flight path?)

If you have an A320 and an A380, which would take off first?

The busier airports around the world are slot controlled, meaning that each flight is assigned a designated departure or arrival time that they must be ready for. In this case it’s pretty straightforward who goes first - whoever has the first slot.

Non-slot controlled airports are usually on a first come first served basis, with some exceptions. Private flights have to wait for scheduled airline flights, and airliners wait for air ambulances. If two aircraft are departing on the same route, the faster one will go first if they have a significant speed difference. And the heaviest aircraft need greater separation for wake turbulence, so an A320 might get preference over an A380.

That doesn’t mean your A380 will have to wait for 15 A320s though. In practice these are only guidelines and the decision as to who goes first is based on the needs of the traffic system on the whole considering all of the factors at the time.


What is the purpose of several mandatory stops (RWY AHEAD) at busy airports? I learned that red signs mean: full stop is mandatory.

Do aircraft have to stop before CAT III 08L sign AND before CATI 26R-08L sign?
What is the purpose of making two stops?
Is the area between these two mandatory stops special?

A full stop at a red sign is only mandatory if ATC has not given you explicit clearance to proceed further. Aircraft will be instructed to "hold short" of the runway, and these signs indicate where the aircraft should stop.

The CAT II/III signs are only for holding when CAT II/III operations are active on that runway. This refers to categories of the Instrument Landing System (ILS) that are currently in use. An aircraft only needs to stop at the sign applicable for the current conditions, as instructed by ATC. Generally if an aircraft must stop at the CAT III holding point, they will later be cleared either to take off or line up and wait on the runway. That clearance means they do not need to also stop at the CAT I point.

On page 47 of this ICAO Aerodrome Standards manual there is a table showing the required distances from runway center line for holding positions. For example, a long runway (code 4) would require 75 m normal distance, and 90 m for CAT I/II/III approaches. These numbers increase with runway altitudes over 700 m (2,300 ft) elevation. There are also many other factors in the airport configuration that might require a greater distance.

The different holding points are to ensure that in lower visibility conditions, there is more room for added safety. It also helps to ensure that holding aircraft do not interfere with the precision approach signals. The CAT III holding point is further from the runway to ensure that the guidance equipment will provide CAT III accuracy and avoid interference from aircraft and vehicles. If there is not a clear procedure in effect to keep these areas clear, interference can happen, and the faulty signals can result in the airplane following an incorrect path.