Is it permitted to deviate from yellow lines, during taxi? During taxi procedures, the pilot should follow the yellow lines, to avoid a collision with any obstacle or even with another aircraft. Is the any situation, which the pilot can deviate from the yellow line at their own discretion?

There's no rule that you have to follow the lines, they're there purely as a guide, a reference for pilots to know where the center of the taxi lane is. Following it is at a pilot's discretion, to be deviated from at any point. Generally you follow the line because it's easy and it should keep you out of trouble, common reasons for deviating are:

Obstructions: vehicles, people, debris
Clearance: airplanes with large wingspans may need to hug one side of a taxiway to get around parked airplanes or buildings. In a small airfield you'll often have to get around a parked airplane or two


Why can a plane land in zero visibility conditions but not take off in zero visibility? After waiting hours for a flight departure delayed because of heavy fog, I wonder why planes can perform an instrument landing with no visibility, but cannot take off without a minimum visibility.

Using a HUD, an airliner can take off with visibility as low as 300’, and land with visibility as low as 600’. Without a HUD, you need 500’ (and appropriate runway lights/markings, plus regulatory approval) for takeoff, and whatever your autopilot Cat III autoland system (along with the landing runway) is approved to for landing. (Or 1800’ for cases of no Cat II / Cat III capability.)

As noted, a pure “zero visibility” landing (a Cat IIIc) doesn’t yet exist, because while the autopilot can bring the aircraft to a stop on the runway, some visibility (300’) is still required to taxi clear and get to the parking spot.

Thankfully, visibilities below 500-600’ are pretty rare in most places. Not all airports have the taxiway lighting required for low-visibility (typically, below 1200’ lateral visibility) taxi operations. That is an airport limitation, not an aircraft limit.

Talking about so many feet of lateral visibility, the actual nomenclature is Runway Visual Range, or RVR. That is reported in feet in the U.S., and often in meters elsewhere.


What procedures will cockpit crew follow during a sudden depressurization? When pilots face a sudden depressurization, they will follow some procedures.
What are their sequences and if there is any addressed to the cabin crew?

In the EMB-145 I flew, the procedure for rapid depressurization was:

                   RAPID CABIN DEPRESSUIZATION

         Aural Warning                    Condition
         ━━━━━━━━━━━━━               ━━━━━━━━━━━━━━━━━━━━
         Voice Message            CABIN ALTITUDE INDICATION
            "CABIN"               HAS EXCEEDED 10,000 FT AND
                                        BECOMES RED
┏━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┓
┃ Crew Oxygen Masks ........................................ DON ┃
┃ Crew Communication ................................. ESTABLISH ┃
┗━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┛

  Passenger Oxygen ................................. AS REQUIRED
  Maximum Altitude .......................... MEA OR 10,000 FT.,
                                             WHICHEVER IS HIGHER
 ┏━━┓
 ┃IF┃  Emergency Descent Required:
 ┗━━┛  ATC .............................................. NOTIFY
       Emergency Descent Procedure ..................... PERFORM

                              * * *

The priority is getting your O2 mask on and then make sure the other pilot was able to get his on as well and help if needed.  The passenger O2 will automatically deploy if the cabin altitude gets high enough but you can force deployment from the cockpit.  If the depressurization is rapid and/or sudden an emergency descent will be performed to get down into breathable atmosphere ASAP (regulatory requirement time limit).  In the case of emergency descent our procedure did have a PA to make sure everyone was seated and seat belted for the maneuver.

The cabin crew will have their own procedures and while I'm not familiar with them they are equipped with portable oxygen cylinders and have the ability to move around the cabin and assist passengers with masks.


What is the section of the fuselage below the wings called? Is there a specific name for the protruding section of fuselage beneath the wings? Im talking about the area where the landing gear bay often is. In the photo it's where the red Emirates logo is painted.

source

Some older airliners don't have them, such as the A300

source

The outer covers at-least, are called wing to body fairings in this Airbus document- 'Airbus Future Composite Wing'.

Wing to body fairings; image from Airbus Future Composite Wing

Elsewhere, the covers are called belly fairings by the manufacturers themselves, and also in the US patent US 20060065784 A1- Aircraft provided with a belly fairing, and corresponding belly fairing.

Anyway, the fairings enclose the center wing box and the main landing gear wheel well.


What is the best altitude to penetrate a thunderstorm? 135.293(a)(7)(iii) requires that pilots be tested on:

  (iii) Operating in or near thunderstorms (including best penetrating
  altitudes), turbulent air (including clear air turbulence), icing,
  hail, and other potentially hazardous meteorological conditions;

So what are the best penetrating altitudes when forced to operate in a thunderstorm?

The most obvious answer is to stay out of it in the first place, but the reg requires us to come up with an answer....

The obvious answer you mentioned should be reiterated: Avoid thunderstorms whenever possible! Just because there are "best penetrating altitudes" doesn't change the fact that thunderstorms are extremely hazardous to aircraft and should be avoided in the first place.

With that said, the FAA's Advisory Circular 00-24C mentions that, if unable to avoid penetrating a thunderstorm:

  To avoid the most critical icing, establish a penetration altitude below the freezing level or above the level of -15ºC.

This is to help minimize rapid accumulation of clear ice:

  Supercooled water freezes on impact with an aircraft. Clear icing can occur at any
  altitude above the freezing level but at high levels, icing from smaller droplets may be rime or
  mixed rime and clear. The abundance of large, supercooled water droplets makes clear icing very
  rapid between 0ºC and -15ºC, and encounters can be frequent in a cluster of cells. Thunderstorm
  icing can be extremely hazardous.

Another point to mention is that, regardless of there being a recommended "penetration altitude," it is important to maintain a constant attitude, not altitude:

  It is almost impossible to hold a constant altitude in a thunderstorm, and maneuvering
  in an attempt to do so greatly increases stress on the aircraft. Stresses are least if the aircraft is
  held in a constant attitude.

But again, emphasizing the dangers of thunderstorms, the circular states:

  Never regard any thunderstorm lightly, even when radar
  observers report the echoes are of light intensity. Avoiding thunderstorms is the best policy.

and

  Weather recognizable as a thunderstorm should be considered hazardous,
  as penetration of any thunderstorm can lead to an aircraft accident and fatalities to those on
  board.


What is "airside inspection"? Today, I flew on a commercial airline European flight. Upon arrival, as I was walking from the airplane to the arrivals gate (across tarmac, no jetways), a yellow car labelled "Airside inspection" sped up to the plane with blue lights and sirens wailing.

An officer ran out and blocked people from exiting the plane (some had already gotten out and were allowed to leave). Then, other two people in orange jackets followed him into the plane.

Google searches for airside inspection don't produce a lot of material, but from what I could understand these people are charged with checking that the runways etc. conform to safety standards.

What reason would they have to approach a plane, sirens on, and block the passengers?

As the comments have explained, "airside inspection" is tasked with addressing issues "airside," which means pretty much everything inside the airport perimeter, and outside of the terminals.

Just about anything airside will affect the safety of aircraft operating at the airport. For this reason, there are generally many regulations and rules that apply to operations here. Airside inspection involves making sure all of these rules and regulations are followed. This can be anything from addressing security issues to removing FOD (foreign object debris) and ensuring markings and signage are in working order.

Since they are responsible for managing activity on the airport apron, they absolutely have the authority to stop anyone that is in the wrong place at the wrong time. Although there may not have been any apparent issue, there could be regulations or dangers that not everyone is aware of. If the flight crew sends passengers out onto the apron when they shouldn't, this is an issue that the airside employees would want to address.

There are various descriptions available for positions that involved airside inspection, including some of the duties involved.

  Maintains safety and security on airport premises; ensures TSR 1542 compliance; patrols,inspects, searches and escorts as necessary; conducts identification checks and permits access only to authorized individuals; provides security training as necessary.
  Patrols and inspects airport operational areas for potential hazards; ensures compliance with FAR 139 and other federal and local rules; monitors and controls vehicle traffic on taxiways and runways; patrols all airport facilities, airside and landside.
  Monitors and controls vehicular traffic on taxiways and runways and public roadways and parking lots; checks for proper ID on vehicles and personnel entering the Airport Air Operations Areas (AOA).
  Ensuring compliance with all safety/security regulations and standards within our activities and ground handling operations.
  The management of a safe and efficient airside operation by carrying out Airside Inspection Audits and Flight Turnaround Audits.


What can be logged as Total Time? The FAA regulates logging time in FAR 61.51. There are plenty of questions about logging PIC time, as well as SIC time. Another type remains: what can be logged as total time? It's only mentioned once:

  Total flight time or lesson time.

It is clear that any time logged as dual, PIC, or SIC can be logged as total time. Is total time merely the sum of those, minus any overlap, or is there a situation where total time can be logged but none of the others?

In particular, two situations arise:

I'm at the controls of an aircraft I'm not rated for (i.e. an ASEL pilot in the right seat of a multi-engine aircraft, where the other pilot is appropriately rated and not a CFI). I can't log PIC, SIC, or dual - but can I log total time?
In a plane I'm rated for, I pass the controls to a non-certificated passenger. I'm not a CFI. I cannot log PIC, SIC, or dual while my passenger is the sole manipulator of the controls, but I am acting as the PIC for this portion of the flight. Can I log this as total time?

"Total time" isn't an officially defined term as far as I know. Most people seem to use it to mean "total flight time", i.e. the total of their PIC, SIC and flight training time (excluding simulator training).

In general, you can log and count whatever you want in your logbook provided that you log at least the items required in 61.51(a):

  (a) Training time and aeronautical experience. Each person must
  document and record the following time in a manner acceptable to the
  Administrator:

  (1) Training and aeronautical experience used to meet the requirements
  for a certificate, rating, or flight review of this part.

  (2) The aeronautical experience required for meeting the recent flight
  experience requirements of this part.

So, the bottom line is that "total time" is whatever you want it to be, and if you want to log 'fun time' in aircraft that you aren't qualified to fly then go ahead; the time just won't count for anything as far as the FAA is concerned.

As for your two scenarios:

You can't log any 'FAA time' because you're not certified to fly the
aircraft as PIC or SIC, and you aren't receiving training. You might note it in your logbook for your own reasons but you can't count it towards anything for the FAA's purposes.
You can't log 'FAA time' here either, for the reasons you mentioned. You're acting PIC for the entire flight, of course, but acting as PIC doesn't automatically let you log PIC unless other conditions are met. 61.51(e) lists all the ways you can log PIC time, e.g. sole manipulator, acting as PIC when more than one crewmember is required.


Does ETOPS apply to Part 125 operations? Does ETOPS apply to aircraft operated under Part 125? I see ETOPS mentioned in Part 121 Appendix P and Part 25 Appendix K, but it would appear that ETOPS wouldn't apply to Part 125 operated aircraft?

Reviewing FAR Part 125, I concur with the last sentence in the question "It would appear that ETOPS wouldn't apply to Part 125 operated aircraft."

Since there are no references associated with ETOPs contained in the regulation, ETOP's criteria, limitations, operating specifications, etc. do not apply when conducting "operations" in aircraft under Part 125.


Why not more Max 8 accidents? Why did "only" two Max 8 accidents occur? Before the grounding, there must have been at least dozens but surely 100s if not 1000s of flights with this model. How come the vast majority of those flights didn't experience any problems?

With a well functioning angle of attack sensor the 737 MAX was safe. Only a malfunction which let the sensor report a higher angle than real would result in the repeated attempt of the MCAS to trim the airplane nose down.
Such a malfunction is a rare event (10 per year across all types flying in the US). This should explain the many uneventful 737 flights. However, to rely on only one sensor was criminally negligent. A safe design would have redundancy built in. Ironically, the 737 MAX has such a second sensor (just opposite of the one which fed the MCAS) but the software did not crosscheck the readings.

Additional remarks
Now there have been so many comments, and they all have been caused by my rather cursory description of the problem, that I better expand this answer so people will not feel a need to add more comments.
MCAS was already introduced with the Boeing 707 and the KC-46 (tanker version of the 767), but would only add nose-down trim once if a prescribed angle of attack is exceeded.
With the more powerful LEAP engines of the 737 MAX, pitch stability at high angle of attack and high thrust would become negative, something that would make any airplane immediately impossible to certify under current regulations for civil aircraft. The more forward position of the intakes and the high suction on the intake at high angle of attack would had required a larger horizontal tail if proper design rules would had been applied.
But Boeing is run by ex-McDonnell-Douglas management where cost-cutting is more important than applying proper design rules. A larger horizontal tail would result in structural changes throughout the rear part of the airplane, require new jigs and tests and a much more complex certification. Simpler solutions did not work out, either: Several aerodynamic solutions were introduced such as revising the leading edge stall strip and modifying the leading edge vortilons but they were insufficient to pass regulation. So the instability had to be fixed in software.
The result was a system that read inputs from a single sensor, and under the right conditions (flaps up and autopilot disengaged) changed pitch nose-down by 2.5 degrees stabilizer rotation when a specified angle of attack was exceeded and repeated this mindlessly after five seconds if the angle of attack was still too high. No provisions had been made for a defective sensor. Concerns by engineers and test pilots about the hazards resulting from a sensor failure were ultimately dismissed. Eventually, Boeing officially assumed that pilots would respond to an unexpected MCAS activation as if it were a runaway stabilizer trim event, and do so within four seconds.
All 737s have two AoA sensors, but the MAX MCAS relied on only one sensor at a time, switching between the left and right one each flight. By the way, the KC-46 MCAS, although less powerful, uses the inputs of two sensors and trims the stabilizer by just 0.6°, and only once per activation.
Next, in order to limit the amount of pilot training for the new type to Level B (no simulator), the MAX should feel to the pilot like a next generation 737. This made it possible to fly the MAX with the type rating for a regular 737, a big selling argument to airlines which already operated the 737. MCAS helped to achieve this, and information about its details was not part of mandatory training (which consisted of a two hour iPad tutorial). By repeating the pitch trim change, the MCAS had become safety critical where earlier versions were not. Nevertheless, information about it was hidden and most pilots did not even know of its existence after their transition to the MAX.
Another factor was the insufficient funding and regulatory capture of the FAA in the last decades which had less personnel to check new designs, so it gladly handed much of the testing for certification to the manufacturers: Boeing Authorized Representatives are Boeing employees who are granted special permission to represent the interests of the FAA and to act on the FAA’s behalf in validating aircraft systems and designs’ compliance with FAA requirements. The core function of the certification process, to let a second pair of trained eyes look over the new stuff, was outsourced to the same people who had authorized those designs. Also, the capacity of MCAS was misrepresented to the FAA as an addition to speed trim in order to avoid more scrutiny and testing. Later investigations found a disturbing trend of FAA management treating operators favorably and overruling the work of technical experts.
As usual, several opportunities to avoid what happened were missed and several causes conspired to make MCAS the trap it became.


What happens without trimming a statically stable aircraft? if I have a statically stable aircraft, it will always return to its neutral position. What happens now if I do not trim my aircraft? Will it move/osicallate around its neutral position and be statically stable but dynamically neutral?

Trimming or not trimming your aircraft should not change its inherent stability. It will, however, change the airspeed, vertical speed, and attitude of the aircraft based on power settings. Control input would have to be applied in order to maintain these parameters. The more out of trim the aircraft is, the more force/pressure will need to be applied to the controls.


What are these openings around the Comet's engine intakes? What are all these openings surrounding the Comet 1's engine intakes...

(Original image by Tony Hisgett at flickr, via Wikimedia Commons; modified by me)

and what is this opening between the Comet 4's intakes?

(Original image by Ian Dunster at Wikimedia Commons; modified by me)

Short answer

Top (Comet 1)

1: Air conditioning -- 2/3: Engine cooling -- 4: Likely fuel venting -- 5/6: Engine cooling

Bottom (Comet 4)

1: Engines cooling

Details

The De Havilland 106 (DH 106) "Comet" was a UK airliner, the first commercial one with jet engines, introduced in 1952 under the colors of BOAC. The last aircraft was retired in 1997. The Comet 1 is known as a great engineering achievement, but also for its initial series of catastrophic failures. These accidents led to the understanding of metal fatigue caused by decompression cycles (after a famous structural test) and the generalized use of openings with rounded corners by all airliners. Beside technical achievements, the aircraft and other of the same era are also remembered for their beautiful shapes.

Cutaway, showing how engines are buried in the wings:

Source

The Comet 1 was an interim version until the more powerful Rolls-Royce Avron engine development was complete. The Comet 1 engines were Halford H.2 Ghost aka De Havilland Ghost 50 Mk1 or simply Ghost.

De Havilland Comet 1XB - From photo by Ruairidh MacVeigh, Flickr

From left to right:

I cannot definitely identify the smaller hole near the fuselage, it might be the ram air intake used for fuel venting. A system which was used quite frequently for emergency landing until the engine bearing overheat problem had been fixed. In some description this hole is completely on the right of the outboard engine.
Ram air intake below inboard engine intake is for cabin air conditioning. Ram air goes to the heat exchanger which cools cabin air circulated in the heat exchanger. The Comet was one of the first aircraft to use bleed air from the compressor and the turbine to provide comfortable cabin pressure at high altitude.
Ram air intakes on each side of the outer engine are used for engine cooling.

On the top of the wing, there are scoops, more visible in the Comet 4 picture below.

De Havilland Comet 4 -- From photo by edk7, Flickr

The Comet 4 engines were Rolls-Royce Avon AJ.65 with twice the thrust of Halford H.2 Ghost.

The engine cooling intake between the two engines has been enlarged and the second one (outboard) removed.
The four air scoops over the wings are additional intake for engine (e.g. to ventilate fuel vapors) and accessory cooling (e.g. for alternator).

Additional information:

DH 106 Comet 1XB at Canada Aviation and Space Museum
Airliner Tech Series 07 - De Havilland Comet
De Havilland 106 Comet presentation


What is this old US Army Air Corps plane? This photo was taken by my Grandfather in 1918 at Rich Field in Wako, TX.  I don't think that it is a Taube because of the front structure, engine type, 4 wheels, fuselage shape, etc.

Well gosh, this is a rare bird! The 'DEHAN' monoplane:

From this forum:

  More about Dr.Ing. Jacob Emil Noeggerath (Nöggerath ?),from the first
  Lange's book :

  He had in 19011/12 the HUNGARIAN " DEHAN ":) monoplane designed ,wich
  was flown at München-Oberwiesenfeld.

  There is another short passage about it in the Etrich book by Hanuš
  Salz.

  Dr. med. Theodor Dieterle let built this monoplane in the Otto works
  in Munich in 1913. He was supported by J.E. Noeggerath (not Nöggerath)
  who corresponds with Etrich since 1911 and was probably involved in
  the design too.

  It had an unusual longitudinal stabilisation system with a sort of
  tank? at the front that later was removed.

  Hanuš Salz named it "Dieterle Schwalbe" or "Taube" only. I prefer the
  term "Schwalbe" as the wings are peaked and the tail even it is not
  splitted but inward curved and not Taube-like.

  After this machine was tested with the swiss army, they contacted
  Etrich as well. Only an order for his machines was not realised.

So, it would appear that the plane was a one-off, commissioned by two medical doctors in Zurich, designed by Etrich (hence the similarity of the wings to those of the Taube), built in Munich and flown in Switzerland.

Which does, of course, leave another question: how did it turn-up in Texas in 1918? (Or did it?)

Edit:
There is quite a lot more info on this page under the name 'Dieterle Schwalbe'. In particular:

  Als Besonderheit verfügte die ‚Dieterle-Schwalbe‘ vor der Rumpfnase über eine mit Bleischrot gefüllte, jederzeit verstellbare Metallkapsel von ca. 10 kg. Mit diesem Vorgewicht konnte der Aeroplan während des Fluges ausbalanciert werden. Mit der Lenkbarkeit und dem Sturzflugverhalten von Flugapparaten hat Dieterle sich intensiv beschäftigt und seine Erkenntnisse in mehreren Bulletins des Aero Clubs 1914/15 veröffentlicht. Ob das Resultat seiner Forschungsarbeit in die Entwicklung späterer Sturzkampfbomber wie der Ju 87 eingeflossen ist, kann vermutet werden.

Google translation:

  As a special ordered that, Dieterle Swallow 'in front of the fuselage nose over a column filled with lead shot, at any time adjustable metal capsule of about 10 kg. With this pre-Wicht of Aeroplan could be balanced in flight. With the steerability and the nosedive of Aerial equipment Dieterle has intensively and published his findings in several bulletins of the Aeroclub 1914/15. Whether the result of his research has gone into the development of later dive bombers like the Ju 87, can be suspected.

In other words, Dieterle specially ordered that there be 'pre-weight' canister of about 10kg lead shot attached forward of the fuselage and which could be adjusted in flight to balance the aircraft.

Caption reads: "The 'Dieterle Swallow' with the characteristic Pre-weight; Oberwiesenfeld January 1913"


Which external instruments are visible on the A350's nose section? Most pictures of airliner's nose sections show small dark tube- or fin-shaped instruments sticking out of the fuselage below the cockpit windows.

For example this Airbus A350 shows them well:

Source: Airliners.net

Other airliners (737, A380, 787) carry similar instruments at the front.

I'm sure Google would be able to tell me if only I knew how to correctly phrase my question, but...

These instruments sticking out of the fuselage, what purpose do they serve?

Here's a better picture showing all probes, and labeled:

Link to original, high-resolution, unnumbered picture

They are:

Ice detector
Multi-function probe 1
Static port
TAT probe
Side-slip vane
Pitot probe
Angle-of-attack vane

Airbus.com has a PDF (pdf page 125) about the various probes.

1
As the name suggests, the multi-function probes record different types of data. Here are more details:

  The piloting of any aircraft involves knowing the relative speed of
  the aircraft with respect to the airflow surrounding it, that is to
  say to the relative wind. This speed is determined with the aid of
  sensors of the static pressure Ps, of the total pressure Pt and of the
  angle of incidence “alpha”. “Alpha” provides the direction of the
  speed vector in a reference system tied to the aircraft and Pt-Ps
  provides the modulus of this vector. The three aerodynamic parameters
  therefore make it is possible to determine the speed vector of an
  airplane.

These are covered by a US Patent.

Someone asked a similar question here.


What type of microcontrollers are used in large scale UAVs? What type of microcontrollers (in terms of processing power, clock speed) are used in large scale UAVs such as the Northrop Grumman RQ-4 Global Hawk, the General Atomics MQ-1 Predator etc. ?

Also, are smaller, low powered microcontrollers used for controlling subsystems such as the INS and control surface movements. If so, then in what range does their processing power and clock frequency lie?

Airbus used standard Intel 80186 and Motorola 68010 CPUs with 2.5MiB of memory in the FBW system for A320. Exactly the same chips that were used for computers at the time (around 1985) so they were not expensive and were already tested. Over time they upgraded to newer version of those CPUs. Each computer is composed from two boards with different CPUs, one produces the control output and the other verifies it and shuts the unit down if they disagree. The boards run different standard embedded operating systems and the software was written by two independent teams to minimize risk of the same failure mode in both boards.

Each board only does a little bit of work and the important units have backups. There are 4 units processing the sidestick input, 3 controlling elevator and ailerons, 3 controlling spoilers and elevator trim, IIRC 2 for yaw damper (FAC), 2 for each engine digital control, 1 for each display etc.

The requirements didn't really increase much since then. It is still desired that the tasks are split across many independent boards to minimize effects of one of them failing. So a CPU with few MHz clock rate and few MB of memory would still be able to do the job. But such slow chips are not manufactured anymore, so they are probably using whatever was most common chips few years ago.


Is it possible to accurately measure airspeed without pitot tube? This is a follow-up to my previous question:

How does this IMU work and how to convert its output into meaningful information?

for which many people asserted that I need a pitot tube and a static port in order to calculate airspeed and pressure altitude respectively, and errors will build up if I just integrate the accelerometer values from the IMU.

I'd like to know whether it is possible to measure airspeed without a pitot tube, accurately? Is there any electronic sensor that can be used for this purpose?

Also are there any airplanes around the world, Airliners, GA, Military, or even Experimental, that operate without any pitot tubes?

Technically speaking, inertial measurements are not sufficient to derive airspeed. You need pressure measurement (or airspeed measurement, see LIDAR).  Pitot-static probes are the most usual and conventional devices to measure pressure.

However, several researches have been conducted, to replace pitot-static tubes with different technologies. These technologies are mainly based on measuring surface pressure on a part of the fuselage. Of course the measurements (not a single value but several values from various locations) have to be calibrated via flight tests.

With the current popularity of machine learning I think the calibration algorithms will not be the main problem.

Some other technology being investigated

there's also ongoing research and trials about using LIDAR to
directly measure airspeed. Which won't need pressure measurement.
ultrasonic devices are also able to measure airspeed directly,
however their real flight usage is unknown (to the author).


Aged aircraft type ratings Looking around on the internet you can still find a lot of aged aircraft such as the 727,737-200,a300/310, DC9/MD8X and DC-10 that are still being used as freighters or in "poorer" airlines.

I suppose a lot of pilots that were certified on these aircraft are getting older and retiring. Would it make sense for a younger guy to get certified for these airplanes rather than running for a newer aircraft certifications where the competition would be harsher in order to pursue a career?

I suppose that these aircraft will still have service time left as converted freighters thus extending the retirement dates further on in time.

It doesn't really make sense to "go and get" a type rating for any large aircraft. A type rating on a 727 would be very expensive for most people, when the airline that needs pilots will likely train you when you're hired. If the bulk of people that have the rating are gone, and the airline needs pilots, it will either have to be very patient when searching for pilots, or it will train new pilots.


Who decides what measurement units are used for altitude, and why is there no standard? I know that some countries (Russia and China for instance) use meters to measure altitude while the rest of the world uses feet. Why isn't this standardized around the world?

I guess all modern cockpit instruments nowdays offer both unit system measurements, but this must be creating some confusion for pilots.

Who decides which measurement units should be used for aircraft altitude? The airlines? ATC? Each country?

Why are both units of measure used?  Are there any pros or cons with using meters?

Unfortunately, countries tend to want to decide themselves how stuff is supposed to work once you're within their borders. That's why we have different rules and laws, and when you go somewhere else in the world, you might need to know some of the differences.

In England they drive on the wrong side of the road, imagine that! :)

In order to have things run smoothly in something so utterly international as aviation, the organization called ICAO (International Civil Aviation Organization) where most countries of the world are members, dictates the rules, (I believe they are) called the ICAO drafts recommendations which the member states should incorporate into their respective laws and regulations. If they choose not to, they need to inform the ICAO of such deviation, don't know exactly what happens then, but I believe the idea is to at least let everybody know there's a discrepancy.

The ICAO Annex 5 specifies units to use, including units for altitude and actually specifying meters as the primary unit, with feet being accepted as non-SI alternative. Why on earth you'd want to go against the majority on this I don't know, perhaps due to the fact that it's an SI unit and they want to be as much SI as they can. So it's basically up to each country how they want to play it. Most countries stuck with feet.


Do jackscrews suffer from blowdown? With hydraulic control surfaces, at high airspeeds, there is a point where aerodynamic loads exceed the capability of the actuators. This limits control authority and can result in the control surface not being in the position commanded by the pilot.

Jackscrews are commonly used for horizontal stabilizer control. I know they can jam and be difficult to move under high loads, but can they be blown back from their set position or do they stay put?

Acme screw type screw jacks with the square threads, as used in stab trim systems (as opposed to a recirculating ball screw), are usually inherently irreversible because of the higher friction of the direct sliding contact of the square sided threads vs a rolling ball interface (as a sliding interface, it's totally dependent on the grease to keep friction in check).

This is why the trim screw jack of the 737 becomes extremely difficult to move manually when the jack is heavily loaded, even though the movement input is coming from input side that is supposed to be the low effort side, requiring a technique of unloading it with elevator inputs to manually trim when the airplane's actual speed and trim speed are far apart.

Ballscrew type jacks however, typically used for flap actuators, have very low internal friction and very high efficiency thanks to the ball interface between the "nut" and screw (rolling, not sliding).  As a result they can be back-driven more easily; how easily depends on the gear reduction within the worm drive gearbox.

Ballscrew operated flap systems typically require friction brakes in the drive line, or anti-backdrive devices incorporated internally in each actuator (basically a clutch device that is disengaged only when the torque is coming from the input side), to prevent a flap surface that is disconnected from its drive motor from creeping up from air loads.

Acme screw actuators also have internal brakes but these mainly function to lock an inoperative motor when there are dual motors, so that the live motor can't backdrive the dead one instead of the downstream gear train.  They also serve to lock the acme screw when both motors are off, but this isn't as critical with an acme screw.


What is the maximum number of infants allowed on a flight? Is there any authority regulation which limits the maximum number of infants allowed on a flight?

Some airlines set a limit at 10% of total seats, some airlines have a different limit. Normally, the number of extra oxygen masks is much higher than 10%.

Since the question was only referring to infants, my answer treats them as a subgroup of SCPs (Special Categories of Passengers). SCPs are Persons with Reduced Mobility (PRMs), infants and unaccompanied children, deportees, inadmissible passengers, or prisoners in custody.
EASA's CRD containing the comments received to NPA 2014-01 has the following statement regarding SCP in chapter 2.5:

The Agency’s NPA did not establish a maximum number of SCPs on board an aircraft. The NPA proposed to clarify the existing AMC (AMC1 CAT.OP.MPA.155(b)) relating to the operator’s procedures that the number and subcategories of SCPs should not exceed the number of passengers capable of assisting them in case of an emergency.
In addition, the NPA proposed a new GM (GM4CAT.OP.MPA.155(b)) that clarifies  that  those passengers, who could assist the SCPs in case of an emergency, should not have another responsibility on board.

It's the closest I could find on this topic. Read the whole NPA, as it is a generally interesting read on SCP considerations (for example if using the word passengers or persons is more adequate to refer to the individuals on a plane).
Essentially, the CRD goes on stating that:

The  AMC  referring to operators’ procedures to establish the maximum number of SCPs has already been adopted by the Agency in 2012 and is based on the JAA Guidance Material.
Following the recommendations from the TÜV Rheinland study, the Agency does not propose any additional limits.
This  CRD  proposes a new  GM  (GM4  CAT.OP.MPA.155(b))stating that  a passenger capable  of assisting in case of an emergency means a passenger who is not an SCP and has no other role or private  responsibility  that  would  prevent  him/her  from  assisting  the SCP.  For  example,  an  adult travelling alone has no other role or private responsibility, unlike a family travelling with children.

In the US, I would assume that due to legal constraints originating on discriminatory aspects, there is also no hard limit as to how many infants are allowed on a flight, and the decision is left with the airline association and the NAA, with existing physical and safety constraints.


Does a constant speed prop deliver max speed at max manifold pressure and max RPM? I was reading a discussion in flight simulator forum and I found this statement:

Ignoring the issue of engine overheating, max MAP and max RPM is the
best thing for max speed

Do you agree with that?
Assuming a Constant Speed Propeller.

For maximum speed, you need maximum power from your engine. This means running it at its maximum rated RPM with the throttle wide open, and maximum manifold boost pressure for a supercharged engine. The prop governor will then adjust the prop pitch to put the biggest possible load on the engine at that RPM.


Can I fly a STAR (standard terminal arrival route) if I can't maintain the minimum speed for it? I fly a single-engine airplane not capable of more than 120 KIAS. I'd love to fly a STAR (standard terminal arrival route) into my local airport (US), but want to make sure I fully understand the STAR procedure.
Most of these STARS I'm looking at flying are RNAV stars with "expect" altitudes, which I understand fully, but have published speed segments of around 250 to 210kts. It's my understanding that if I'm cleared for the arrival "cleared the Tops 5" I'm to follow published speeds and lateral guidance and the altitudes I'm given.
Is there ever a case I could fly a STAR and not have to follow published speeds or is it implied when flying the STAR? If my understanding is correct, I wouldn't be able to fly a STAR with published speeds if I can't meet them?
Could I file for a STAR in my flight plan and not get whacked with a pilot deviation if I'm cleared for the arrival and can't meet the speeds. I would think I couldn't fly them if I can't meet the speeds, but want to make sure I'm not missing something.

According to the FAA's Instrument Procedures Handbook, chapter 3 (p. 3-16):

Pilots may have noticed that minimum crossing altitudes and airspeed
restrictions appear on some STARs. These expected altitudes and
airspeeds are not part of the clearance until ATC includes them
verbally. A STAR is simply a published routing; it does not have the
force of a clearance until issued specifically by ATC. For example, minimum en route
altitude (MEAs) printed on STARs are not valid unless stated
within an ATC clearance or in cases of lost communication.

So unless ATC explicitly, verbally assigns you a minimum airspeed, you don't have to maintain it. If they do assign one and you can't maintain it, you can simply say "unable" and request to fly it at a lower airspeed. If ATC can't allow that for some reason (like spacing) then they'll likely vector you instead.
That means that if you are simply cleared for the STAR with no specific airspeed it's not a pilot deviation to fly it more slowly than the airspeed on the procedure.
I have no idea how likely or not it is that ATC would allow you to fly the STAR you have in mind. That would depend heavily on traffic, and things could change at any time.


Power changes and trim forces. related to position of Center of Mass (CoG), or Center of Pressure? Theoretically (and Actually) do power increases cause a pitching tendency (and a need to adjust pitch trim), because the thrust line is not aligned with the Center of pressure, or, as I understand it, because the thrust line is not aligned with the center of mass (CG)?
I was recently told that the Cessna 182 is particularly sensitive to abrupt power changes in the landing phase (at slow airspeed) because it is a high wing aircraft, and the thrust line is below the Center of Pressure. i.e., Increases in Power generate a pitch up moment, requiring down trim to compensate, and reductions in power generate a pitch down moment, requiring nose up trim to compensate.
This strikes me as wrong, I believe it the alignment of the thrust vector with the center of mass of the aircraft, (the Center of Gravity (CG) as it is called), that determines the moment generated by thrust vector.

Your intuition is correct.  Each of the three principle axes of the aircraft originate at the center of gravity.  Therefore, all rotation in either pitch, roll, or yaw occur around the CG.
The center of pressure is a place along the chord where the lifting force acts on the wing.  It is not a pivot point like the CG is.  In fact, an increase or decrease of lift will actually produce a pitching moment around the CG if they are not co-located along the longitudinal axis.  This contributes to the relative stability (or instability) of an aircraft.


What are the drawbacks for manufacturing a large airliner capable of water landing? Emergencies are rare but still occur. Apart from weight and extra fuel costs, What are the drawbacks for manufacturing a large airliner capable of water landing in case of an emergency?

Assuming the extra weight problem could be mitigated through design and cost cutting techniques and advances in material science would this be feasible for airliners?

Maintenance. Ditching survivability calls for overwing engines, as underwing ones act as scoops and torque the aircraft. But underwing engines are quieter for the pax and much easier to access. Plus, you'll need to make most of the openings airtight, and regularly inspect and replace the seals.

But of course weight, drag and extra fuel are a big deal - with better materials you'll still be picking between better reducing fuel usage or improving ditching survivability. The latter is also only helpful in controlled ditchings, which are mostly being prevented through engine reliability.

That said, airliner design has shifted from "don't crash" to considering crashworthiness at every stage. So it's possible that ditch-worthiness might become a larger factor in design. But it's going to take a couple high-profile high-fatality controlled ditchings for the authorities to make the call to sacrifice fuel efficiency for it, which is currently a big deal due to rising oil costs and AGW concerns.


What's the pattern of airflow over the airfoil at high angle of attack? When at high angle of attack the boundary layer tends to:
A. stagnate and stop
B. loop around the upper camber line
C. decrease air pressure along the chord

The answer is A, but as I see it, B seems like a better answer. What's your opinions?
For your information, the material explains that B is the wrong answer because "As the boundary layer becomes stagnate and stops, the airflow will separate and loop around the upper surface and a stall occurs," which I don't find satisfying.

The question is about the boundary layer, which indeed stagnates and stops at high AoA, as shown for the laminar boundary layer in the picture. Then the airflow will detach from the upper surface, after the point where the boundary layer has stopped.

B. makes no sense to me, I'm only aware of one camber line.


Why aren't ceramics used in turbines? I have seen that ceramics take a lot of heat to melt (3240º F). So, I'm wondering, why aren't ceramics used as the compressor levels in turbines? Is there something to do with the strain? or is ceramic too heavy? Is it something to do with the way the crystals align?

Ceramic materials are starting to be used in jet engines.  E.g. the LEAP-1A is using a few ceramic matrix composite in the high pressure turbine.   (http://aviationweek.com/mro/ceramic-composites-enter-service). Future designs will likely expand the use of CMCs (e.g. http://www.geaviation.com/press/ge90/ge90_20150303.html)


How long does it take Boeing to develop an aircraft? How long does it typically take Boeing to make a new plane model (e.g the 787), from the moment they decide to make a plane (the moment they decide to create a new 7X7) to the moment the first plane (in this case the first 787) flies commercially?

For the general question, the answer will be "it depends on the aircraft." Specifically for the 787, though, the '7E7' program was officially announced on January 29, 2003. The program was later renamed to 787. It received its type certificate on August 26, 2011 and flew its first passenger revenue flight on October 26, 2011.

So, from the time the program began to the time of the first revenue flight was about 8 years and 9 months.

Of course, this is not including the R&D time invested on different projects whose R&D ended up being used either partially or completely in the 787 design before the 787 program itself began.

It should also be noted that not every airframe they start developing ever actually flies. Notably, Boeing worked on designs including larger 747 variants and the Sonic Cruiser (which would have cruised at Mach 0.98) before beginning the 7E7 program.

In addition to completely new airframes, aircraft manufacturers also commonly make derivative airframes based on existing product families. Current examples of derivatives in development include the 737MAX, A320neo, 777X, and A330neo ("neo" stands for "new engine option.") However, development time on derivatives can vary widely depending on how much modification is being made to the existing airframes and the level of resources being devoted to the project (which depends on market pressure.)

For a past example, the 2nd-generation Boeing 777 aircraft were announced on February 29, 2000 with the first aircraft, the 777-300ER, being certificated on March 16, 2004, and first service entry on May 10, 2004. This gives a time of about 4 years and 2 months from announcement to first service entry. This was likely slowed by the sharp downturn in aircraft orders following the 1999-2000 ".com bust" and, especially, the September 11, 2001 terrorist attacks in the U.S.

Also important is the FAA rule for applying for a type certificate. This will apply for new programs like the 787, but not for derivative programs like the 777X, which amend the original type certificate and do not need a new one. In 14 CFR §21.17:

  An application for type certification of a transport category aircraft is effective for 5 years and an application for any other type certificate is effective for 3 years...

So once Boeing applies for a new type certificate, they have 5 years to get the type cetificate approved, unless they convince the FAA upfront that more time is needed. If the time runs out they must reapply, and the new type certificate would require the aircraft to comply with all new regulations passed since their original application.


If the number of flights in the world exceed 2^24 how will ICAO allocate the address to the flights? What is the approximate number of commercial flights in the world as of now?
If the number of flights do exceed will they use the concept of frequency reuse in cell phones(dynamic allocation of addresses)?

Addresses are assigned to suitable equipped aircraft, not to flights. The total of 16 777 214 available adresses is distributed in blocks that are assigned to individual states. Block size depends on the size of the states, e.g. the US and Russian Federation have block with 1048576 addresses, while e.g. Grenada or Mauritius only have blocks of 1024 addresses.

The allocation of addresses is decribed in ICAO Annex 10, Vol. III, Chapter 9.

An extract of the relevant part can be found here.

As long as the addresses are just assigned to conventional Mode-S transponder equipped aircraft there should be sufficient free addresses. This could change if all airspace users would be assigned a unique address, e.g all drones participating in future UTM (urban or unmanned traffic management).


Question about Eurofighter structure I was researching for the Eurofighter and I came up with a picture of its structure. I have three questions about its parts. I can't understand the role and the usage of them. Can anyone help me?

These 3 parts are the following:

Radar mounting bulkhead
Strobe light/anti-collision beacon
Engine bleed-air primary heat exchanger

*All of them are circled with a different color from me so to be focused and easy to find them. Also, their definition exists in the picture but I would like to also know, as I mentioned above, their role and their usage.

PICTURE

Thank you for your time in advance!

Radar mounting bulkhead: this is the vertical plate to which the radar antenna is attached. This is just a structure.
Strobe light/anti-collision beacon: this is a bright light that flashes to make the aircraft more visible and prevent collisions.
Engine bleed-air primary heat exchanger: this is a heat exchanger for bleed air from the engine. Bleed air from the engine is hot and needs to be cooled down for various applications, this cooling down is done in the heat exchanger.

It's quite easy these days to find out what an unknown term means. Just go to Wikipedia and enter the unknown term into the search box.


61.113 - is paying your pro rata share limited to the time you are in the plane? Is the pro rata share that a passenger pays limited to the time he spends in the plane, or can it apply to the whole “operation”?
Let’s say my friend, who lives 100 miles away, asks me to fly him on a tour of his hometown. I take off from my home airport, fly 2 hours to his local airport, and land to pick him up. After flying him on a 1 hour tour, I drop him off and return, solo, to my home airport. Upon landing, 2 hours later, I refuel and, finding that I spent 100 dollars on gas for the entire day, prompt him to pay me 50 bucks. Given I made all legs of the flight for the sole purpose of entertaining his desire. Is my request for 50 dollars legal? Or, given that he was only in the plane for 1 hour out of 5, am I only entitled to 20?

The situation you're describing is a commercial operation. Without a commercial certificate (and a part 135 operation) you may not receive any compensation at all.

The FAA has consistently held that the 61.113's non-compensation exemption in 61.113(c) requires common purpose for the pilot and passengers (Bobertz - 2009, p.2):

  Absent a bona fide common purpose for their travel, reimbursement for the pro rata share of operating expenses constitutes compensation and the flights would be considered a commercial operation for which a part 119 certificate is required.

There is unfortunately not a strict definition of common purpose. However, the basic ground rule is that to fall under 61.113(c), the pilot must have their own reason for making the flight, independent of the passenger's.

The Bobertz interpretation presents an illustrative example: if a pilot wanted to go to a race, they could bring contestants to the race as passengers, and receive compensation. Because the pilot was flying there for the purpose of attending the race, common purpose is established, and pro-rata cost sharing is permitted.

However, if not all of the members of the race team could fit in the plane, the pilot could not share the cost to go back to the origin to make a second trip: there's no independent reason to do so. This crosses the line into a commercial operation, so no compensation is allowed.

Applying these guidelines to your question, the key phrase here is:

  I made all legs of the flight for the sole purpose of entertaining his desire.

There is clearly no common purpose. Therefore, accepting any compensation for the flight, even a pro-rata cost, is compensation in violation of 61.113.


How many flight training hours can a student realistically do in a week? How many flight training hours can a student realistically do in a week?

I have 11 PIC flight hours (and solo'd at 7hrs) but I havent flown (PIC time) since 2008. I want to finally get my license but am unsure if I can realistically do it before I leave the country in one month.

In one week, I can actually devote my entire day to flight training so I'm wondering how many PIC flight hours a student can realistically do in one day and thus a week. Is 2-3hrs flight time (2-3 actual sorties) for 7 days realistic?

Also, I know this is highly dependent on my CFI but let's assume I can find one to match my speed...

There are a few external factors

Your CFI:
If we are talking about dual time (or supervised solos) here your CFI will need to be free.
The plane:
Keep in mind that school may have other students who may also book the plane, or it may be out for its 100 hour inspection or it is broken etc.
The weather:
Depending on where you are located, you may be limited by the weather grounding you for VFR flight.
The Wind:
A lot of flight schools (at least the one I go to) impose student maximum limits when it comes to crosswinds. At single runway airports this could cause an issue. You can mitigate this by training at a place with two runways.

Now if we look past all those and assume you have a working plane, a CFI, and clear weather, then I would think (assuming the school is open) you could log 3 hours in the morning and 3 in the afternoon if you really pushed it. If you started your day with 1 hour of ground from 8-9 then flew from 9-noon. Lunch. Then another hour of ground from 1-2 and flying from 2-5 that is putting you at close to 6 hours a day. Throw a few night flights in there, some more ground lessons at night and you may be able to squeeze the time in. Keep in mind you will also need to study for the written exam.

Note: The above projection is strictly based on everything working all the time which in aviation is often not the case. Your results may vary.


Can a helicopter fly VFR at night in India? As per the title, can a helicopter fly VFR at night in India?.
If yes, can the pilot ask for special VFR?

In short, yes, a helicopter can fly at night, in the US.  Special VFR requires that the aircraft and pilot be instrument capable, but that requirement is not applicable to helicopters.  VFR weather minima are different for helicopters than airplanes.


Are there surveillance cameras in aircraft cargo bays? I've heard that there are cameras on the fuselage and wings of some passenger aircraft. Is there a similar surveillance system in-built at the cargo bay/compartment of a passenger aircraft?

During the baggage loading / unloading of passenger aircraft, many burglary events can be experienced. I think that if there was a camera surveillance system, theft could be reduced significantly. At the same time, when the fire warning was received from the compartment, the pilot could confirm by monitoring the cargo compartment whether the warning was correct and avoid unnecessarily diverting. In addition, the perpetrators of compartment damage occurring in cargo loading / offloading could also be easily identified.

Why did the aircraft manufacturers not consider installing in-built cameras in the cargo compartment?

Any burglary of cargo would happen outside the aircraft, after unloading the cargo. External cameras on the ramp are much more effective for this purpose. I don't know of any aircraft that has internal cameras just for the cargo area.


What can airline pilots do if they are no longer in the position to fly, career-wise? I was having a discussion with a friend about the incident in the past days (4U 9525) and more specifically the what could be described as 'Taboo' of Psychological problems among pilots. We came across a few points:

They will only reluctantly admit that something is wrong, since risk losing their Medical (hence being 'grounded'). It will only be easier to loose and harder to get back I suspect after this event.
At least for the younger bunch who might have difficulties they will have substantial debt from training that is difficult to pay off. Furthermore, they will have worked hard to get where they are.
Any airline that does suspect something will most probably not allow them near the cockpit again. Should anything happen the airline will be held to blame, since they did allow him to fly despite knowing of the problem.

This brings up the question: What can a pilot do, career-wise, if they feel they are unable to exercise the profession? It seems pretty cornered to me. Perhaps work in dispatch/ network control was the only one I figured their knowledge could come to use. Switching to the simulator I suspect might also be possible, but not for the younger individuals.

When I worked in the design department of an aircraft company, I had two types of coworkers: Those with and those without a pilot's license. You could easily spot the difference. In aircraft-related engineering decisions it was soon obvious. I mightily preferred to work with other pilots - they would have a much better grasp of "what looks about right" and not pursue hare-brained, outlandish ideas.

See it this way: All managers in car companies have a driver's license, so they have at least some contact with what their companies make. All higher managers where I worked had no pilot's license, and it made work outright awful. They were trained as beancounters, and that restricted their viewpoint. They were simply incapable of telling good engineering from bad.

If you have managed to get a CPL, adding an engineering degree on top is not hard. That is my advice to grounded pilots: Go to engineering school and enrich the workforce with your experience.


By what angle does the rotor disc change with max. cyclic input? If maximum longitudinal/lateral cyclic blade pitch input is given, by how many degrees does the thrust vector (or rotor disc) change? Is the magnitude in the order of $\pm5^\circ$ or more in the order of $\pm15^\circ$?

Can this be assumed to be the same as the maximum blade flapping angle?

Assuming you mean, in-flight, it depends on the main rotor design. A fully articulated head can allow for angles great enough for an H53 to chop-off it's own refueling boom (See Here: https://www.bing.com/videos/search?q=h53+chop+fueling+boom&qpvt=h53+chop+fueling+boom&view=detail&mid=1AD065C255D15A52813F1AD065C255D15A52813F&&FORM=VRDGAR ).

Helicopter designs employing non-articulated rotor systems often have a cockpit indicator for "Mast Moment" in order to alert the pilot that excessive cyclic input is being commanded, risking Rotor/Head/Mast failure. These types require the blades themselves to absorb most or all of the rotor's dynamic articulation forces, hence much less angular deflection of the disc.

Additionally, the duration of the deflection needs to be factored-in. During maintenance test flights of the old UH-1 Huey, a "Pylon Rock Check" is performed. From an IGE hover an instantaneous full deflection and return to neutral (less than 1 sec for the full maneuver) is conducted either left or right. Somewhat violent and not something you would want to do every day for the sake of the transmission mounts. Sorry I can't provide exact disc angles though.


Is it allowed to let the engine of an aircraft idle without a pilot in the plane. (For both helicopters and aeroplanes) Is it allowed to let the engine of an aircraft idle without a pilot in the plane. (For both helicopters and aircr).

Would there be a difference between a C172 and a 737?

You won't see it done in the fixed wing world unless the aircraft is tied down or otherwise securely restrained (like when you tie off the tail to something when hand starting your no-starter taildragger; some pilots just use chocks or parking brakes to hand bomb their airplane, but it's a terrible idea).

However, it's common in the helicopter world especially in bush operations.  With the collective and cyclic friction locks tightened down and the engine in ground idle, it can't really go anywhere and a pilot who lands in a remote area without assistance to hook up a sling load or lug something on board will often do it without shutting down.


How does stall risk in paramotors differ from that of conventional fixed-wing aircraft? I happened to be walking past a small outdoor fair today, and saw someone flying a paramotor. The craft took off and started a steep ascending turn losing speed rapidly, seemed almost to come to a stop (relative the ground, but the flags nearby didn't show much wind, either) before the pilot brought it back to more level flight and got some way again.

I'm not a pilot, but I'm roughly aware of the mechanism of stalls and the consequent danger of high angle of attack, low speed maneuvers and I was briefly worried that there was going to be a problem.

In hindsight, the craft clearly has a very low stall speed (not much more than walking speed, I guess) but it did leave me wondering.

Are these craft are more or less at risk of stalling than conventional fixed wing aircraft?

I suppose with paramotor you mean a powered paraglider, as pictured below.

Paramotor (picture source)

Since they obey the same laws of physics, they also stall, just like a fixed-wing aircraft does. There are, however, some peculiarities which should be mentioned:

The shape of the wing is produced by ram air filling the parafoil, so speed will help to keep the wing inflated and in shape. At low speed and in gusty winds the risk of the parafoil collapsing (severe deflation) is higher. This was the reason for several fatal accidents in the early times of paragliders, when the aspect ratio of high-performance parafoils was increased beyond safe limits. Today's designs will normally quickly recover from a deflation event, but not without a substantial loss of altitude.
The pressure forces on the parafoil will change its shape depending on its angle of attack. At low speed the angle of attack is higher, resulting in a suction peak close to the leading edge which in turn will increase the local camber of the foil. This leads to severe lift loss in a stall, and recovery needs some altitude. A fixed-wing aircraft, on the other hand, can employ an airfoil with only little lift loss in a stall, so recovery needs less altitude. This is by no means universal - there are also fixed-wing aircraft with less benign stall characteristics.
If the stall on a fixed wing starts near the wing tip, the aircraft might enter an uncontrollable roll. This can happen in steep turns and with badly designed aircraft. This is not possible in a parafoil; it might instead slide sideways with a partially collapsed parafoil, but stay upright due to its low center of gravity. If the pilot panics and pulls his brake lines on one side during a stall, the parafoil can enter a spin. Similarly, if the pilot of a fixed-wing aircraft insists on applying rudder when stalled, the aircraft can also enter a spin.

Generally, due to their low wing loading and the drag of the raiser lines, paramotors fly at much lower speeds than aircraft, so accidents involve less energy, but the pilot has no protection around her/him, so fatal accidents do occur more regularly than with fixed-wing aircraft. For paragliders, Wikipedia gives a number of 2 fatalities per year for 10.000 pilots.


Does NASA use T-38's to train all crew members of a space flight as pilots? Did all NASA flight crew recruits learn to fly when preparing for Space Shuttle missions?

Obviously, the pilot — and I am guessing the commander, too — would be USAF / USN / USMC pilots, but what about the specialists, engineers, etc? Do these have to learn to fly a jet?

Was this flight training used to simply train all crew members on the effects of traveling in a rocket into space? Or did (do?) all crew members have to learn how to fly a Northrop T-38?

These questions are based on documentaries I have watched where NASA uses the T-38 for flight training. I have wondered about this for a long time and could never get a satisfactory answer.

I have no idea if NASA still uses the T-38. Again, I am basing that on what I have seen and read.

Non-pilot astronauts are not required to become pilots. They fly in the backseat of T-38's partially for transportation purposes, and partially for flight regime training. They are called "mission specialists" and since their job does not require piloting skills, there is no reason to train them as such.

NASA literature says "The T38 is used for
flight readiness training to help the astronauts become adjusted to
the flight environment, including the g-forces experienced on launch" and that non-pilots fly at least four hours per month in a T-38. Pilot astronauts fly at least 15 hours per month to maintain proficiency.

You can look up whether someone has a US Pilot's license using the FAA's Airmen Inquiry Page. A quick search turns up several astronauts who have never been licensed. For example, the mission specialists on the final shuttle mission, Sandra Magnus and Rex Walheim, are not pilots. That being said, I wouldn't be surprised if most, if not all, of the non-pilot astronauts got some stick time in a T-38 since it is a trainer with dual controls.

  Obviously the pilot and I'm guessing the commander would be airforce pilots

In the NASA vocabulary, commander is equivalent to what you would think of as pilot or captain, whereas pilot is equivalent to what you would think of as co-pilot. In the Apollo program, the commander landed the lunar module on the moon, not the "Lunar Module Pilot," and in the shuttle program, the commander landed the shuttle, not the pilot.

So, you are correct that both the commander and pilot must be pilots, and some of them did come from the Air Force, although US Navy was more common, and a few came from the Marine Corps. Almost all of them were test pilots.

A few astronauts joined NASA with the qualifications to be pilot-astronauts, but only flew as mission specialists (for example, James Voss and Carl J. Meade). I am honestly not sure whether or not NASA allowed them to fly the T-38's; however, the only photo evidence I can find shows them in the rear seat, which leads me to believe they were not allowed to fly solo.

  I have no idea if NASA still uses T-38's

NASA does still use T-38's because NASA still sees a need for pilot astronauts in the future, and the pilots need a way to maintain flying proficiency. In the 2013 Astronaut Candidate Class, four out of the eight were test pilots of some sort, although it's unclear how many will be considered pilot-astronauts.

There is anecdotal evidence that the number of T-38's which NASA operates does appear to be shrinking, which is likely a reflection of a reduced astronaut corps size, budget constraints, and the fact that the aircraft fleet is about 50 years old. According to unconfirmed sources (wikipedia), the number of NASA T-38's is expected to be reduced from 32 to 16 by 2015.

Other Space Shuttle-specific aircraft have been retired entirely, such as the Gulfstream II Shuttle Training Aircraft and the Shuttle Carrier Aircraft (the latter wasn't flown by astronauts).


Why we need 1 hour for cooling after 3 consecutive attempts for starting the APU? As it's said here(APS-3200 training manual (pg 268)):

we need 1 hour for cooling after 3 consecutive attempts for starting the APU. But why?
why not after each attempt? and why 1 hour?
is it valid for all APUs?
What about the jet engines of aircrafts? is there something like that for them?

That is typical of any electrically started component.  During qualification testing of the APU, tests will be done to measure the heat build-up in the starter motor to determine how many start attempts it takes to get the motor's temperature to a critical level.  Other tests determine how long it takes to cool down enough to operate again, after it gets to that temperature.
In a nutshell, the limits are to protect the APU's starter/generator from heat damage if multiple start attempts are needed. Turboprop engines with battery powered starter/generators will have similar limitations.
Jet engines typically use bleed air driven turbine drive starters to start, and may have similar, if somewhat looser, limitations compared to an electrically started engine (to control the starter's gearbox oil temperature).
For example, on the CF34, you're allowed to crank for 90 seconds, and then there's a 10 second cool down after the 1st two attempts, and a 5 minute cool down after each subsequent attempt, up to 5 tries (after that, it's expected that you've given up and called for maintenance).  Other than that, no limitations.


What is a listening squawk? A NATS video recommends the use of a listening squawk as one of the means to avoid infringing airspaces.

What is it? And what is the equivalent of it outside of the UK? Say in the US.

A listening squawk is a transponder code you set to indicate to an ATC controller that you are listening out on a particular frequency.

Unlike squawk codes assigned by the controller, the listening squawk is not unique to your aircraft but is a set (and published) code for the particular area. In the UK it is commonly used in the uncontrolled airspace around a major airport's terminal control area.

Examples in the UK include the airspace surrounding Luton, Stansted, Leeds, and one dedicated to the narrow "low level corridor" allowing VFR flight between the class D airspaces of Manchester and Liverpool.

The procedure is: after signing off from whatever facility you were previously in contact with, set your com radio to the assigned listening frequency and set your transponder to the published listening squawk. Do not contact the controller, instead listen out for any broadcast which may be intended for you (eg the controller may ask the aircraft in a particular location to respond). They will know you are on frequency because of the code you are squawking.

The controller will generally only attempt to contact you if you are about to violate controlled airspace or have a traffic conflict. Note that this should be treated as a backup only, the responsibilities for navigation and see-and-avoid remain as ever with the pilot.

Once out of the area, or when you need to change frequency to your destination or next enroute contact, turn the transponder back to the VFR conspicuity code 7000.

If I recall correctly my instructor told me that this is a UK/CAA-specific initiative and is not found in other countries.


Is on-board equipment for in-flight de-icing also used on the ground? Aircraft operating in icy conditions are equipped with de-icing equipment to remove ice from specific parts of the aircraft (wing, sensors,...) in flight.

When there is snow or icy conditions while the aircraft is still on ground, it is de-iced with ground equipment (de-ice fluid, towel,...).

Is the aircraft's own de-icing equipment also used?

Is this equipment enough to de-ice an aircraft before take-off (lets say if this is a sunny day and the temperature is just below 0°C)?

Usually, no.

There are two main methods used to deice an airplane in the air. One is letting warm air that is bleed from the jet engine to heat the wing leading edge. The other is to have have rubber air bladders (called deice boots) on the leading edge of the wing and tail surfaces that are pumped up and the ice falls off. Both of those methods are only useful to deice the leading edge of the wing and tail surfaces which is pretty much the only dangerous place where an airplane can gather ice while it is flying.

Airplanes on the ground are a different story. Ice can be built up pretty much anywhere as there is no airflow to prevent it. Excessive ice on control surfaces can make them behave "unevenly" or in extreme cases even unresponsive. Bigger chunks of ice breaking off the fuselage can get ingested by the engine and cause problems. It is therefore important that the airplane takes off ice-free. In fact, as little as frost on the surface of the wings can cause a significant loss of lift and can even cause a plane to crash.


Would a coaxial propeller with the second prop slower and larger improve speed? Looking at various propeller designs, I started wondering about these limitations:

Large propellers at high speeds have tips going supersonic
Scimitar props attempt to keep prop tip from going supersonic
Counter-rotating props absorb more power at smaller sizes

This made me wonder if a coaxial prop had a larger prop spun at slower speeds and a smaller prop spun at high speeds would increase thrust better, or do other designs already deal with that problem?

I think what you'd ideally want to do is to have a fast-rotating "inner" propeller and a slower "outer" one, or actually a propeller whose RPM gradually decrease from the inside to the outside. Although that would look cool, it doesn't work with non-fluid materials for the rotor :o) So, the next best thing is to have two behind each other, where the downstream one spins a bit slower.

How I understand your proposition

Counter-rotating props do have the advantage that the second one can "straighten" out the flow from the first, effectively working similar to (but not entirely like) a rotor/stator couple in a turbojet. This means they can transfer more power per rotor area than single rotors. By putting the slower-spinning one behind, this couple will leave less swirl in the flow than the other way round, which sounds like a good idea to me.

Difficulties with regular counter-rotating rotors

In a counter-rotating rotor couple, the second rotor is exposed not just to the mean swirl coming from its upstream partner (for which it is designed, and which can increase its effectiveness), but also the pressure signatures and wakes coming off each blade. Every time a blade passes through one of these, the stagnation pressure at its leading edge goes down very sharply and the increases again. That makes a lot of noise, and also causes vibration in the blade, which then has to be made strong enough to deal with this. This is also the reason why most such rotors have swept blades like these:

This means that at no moment will the entire blade of the downstream rotor be in the wake of an upstream blade, but it will gradually pass through it. Still the blades have to be quite sturdy, which also prevents them from being very long. That, in turn, limits the ability to increase efficiency by making long, slender blades which produce less thrust per rotor area but make up for it in radius (like wind turbines, for example). That's one reason why most such configurations these days are seen in military airplanes where large elegant propellers would reduce maneuverability but lots of thrust is needed, pronto. Also, these machines are built very robustly anyway, so the added vibrations can be dealt with. In a passenger aircraft, the noise alone would be hard to sell to airlines, but developing the mechanical components would also not be easy. At the moment it seems that only Antonov has experience with large configurations of this type.

That said: These concepts keep cropping up, and it seems as if it should be possible and efficient to use counter-rotating rotors with strongly swept blades to replace turbofan engines at speeds that would seem a bit high for regular propellers (but maybe a bit low for turbofans).
Here are two sources I just found which give you some idea of what the flow looks like, and what types of aerodynamic and acoustic problems the designers have to content with:

A 2014 paper summarizing design and research activities
Slides from a 2010 presentation on simulating the flow, and on efficiency and acoustics -- very nice pictures, and a good way to get an idea of what propeller flow looks like.

The problem with having a larger downstream prop

You can also see in the references above that usually the downstream blades are actually shorter than the upstream blades. The reason is that otherwise the tip vortices from the upstream propeller would hit the downstream one, and that's acoustically, aerodynamically and structurally not nice. Even without those vortices, it would make sense because the downstream prop gets more vibrations either way, so it has to be sturdier (shorter, thicker blades will help!).
I think some of the existing configurations (the AN-70 above for example) do have equal blade lengths, but they are very much known for being loud, and any new civil application cannot have that.

This, by the way, is also why the configurations under discussions are rear-fuselage mounted rearward-facing rotors: That puts the source of the noise a good way downstream of the passenger cabin, instead of right next to their windows (which is loud enough with regular props already, as any passenger in a Saab 2000 can confirm). It also has the benefit of preventing accidents where a blade breaks off and hits the passenger cabin ...


What are the legal effects of an inoperative anti-collision light? In an airplane which is only equipped with a beacon for anticollision lights (1967 Piper Cherokee, for this example), what is the effect of an inoperative beacon?  Obviously, per part 91.205, anticollision lights are required equipment for VFR Night.  But, since the aircraft was certificated before 1996, it is not required under 91.205 for VFR Day.

Now, the interesting part is, under 91.209, since the aircraft was equipped with anticollision lights, it sounds like the aircraft would not be airworthy.

The guidance I've heard on this seems to be conflicting.  One argument was since the equipment was disabled and placarded, it doesn't qualify.  Alternatively, in 91.213(d)(2)(iii) (also referenced by AC 97-61 figure 2), it states that if the equipment is required by part 91 FARs, (91.209 seems to apply here), it is unairworthy (and deactivating/placarding is not an option).

A final point of confusion, specific to this in example: the beacon was an optional feature of the 1967 Cherokee according to the manual... which brings me here: any other opinions, short of a more formal clarification? I'm inclined to err on the side of caution, especially since the definition of "equipped" isn't getting any clearer (other than seeing that little red light on there and the "inop" marking on the panel).

The answer to your question is in FAR 91.205:

91.205(b) - Visual-flight rules (day):

  (11) For small civil airplanes certificated after March 11, 1996, in accordance with part 23 of this chapter, an approved aviation red or aviation white anticollision light system. In the event of failure of any light of the anticollision light system, operation of the aircraft may continue to a location where repairs or replacement can be made.

and 91.205(c) - Visual-flight rules (night):

  (3) An approved aviation red or aviation white anticollision light system on all U.S.-registered civil aircraft.  (it goes on to talk about color, which I've cut out) In the event of failure of any light of the anticollision light system, operations with the aircraft may be continued to a stop where repairs or replacement can be made.

So as you surmised in your question, a 1967 Cherokee is required to have an operating anti-collision light for Night-VFR operations (because the night anti-collision requirement is retroactive and doesn't care about certification date), but not for day Day-VFR operations (because it was certificated prior to 1996).

If you HAVE an anti-collision light it is required to be operating subject to FAR 91.213. The relevant part of that regulation says that you can take off with inoperative equipment provided that equipment is not:

  (i) Part of the VFR-day type certification instruments and equipment prescribed in the applicable airworthiness regulations under which the aircraft was type certificated

You're OK for Day VFR because CAR3 (and FAR 23 prior to 1996) didn't require an anti-collision light for daytime operations.

  (ii) Indicated as required on the aircraft's equipment list, or on the Kinds of Operations Equipment List for the kind of flight operation being conducted;

Your Cherokee predates the concept of a Kinds of Operations Equipment List in small aircraft by several decades, so your reference is 91.205.

  (iii) Required by §91.205 or any other rule of this part for the specific kind of flight operation being conducted;

If the kind of flight is "Day VFR" the anti-collision light is not required by 91.205, but if you want to fly at night it is required (as we determined previously from 91.205).

  (iv) Required to be operational by an airworthiness directive

No such Airworthiness Directive exists for anti-collision lights on the PA-28 family.

…so per 91.213 you can placard the beacon switch INOP and "deactivate" the anti-collision light (simply putting a bit of masking tape on the switch holding it in the off position & writing INOP on that is generally considered adequate) and fly in Day-VFR conditions (ideally to an airport where you can get a replacement lamp for your beacon).

Note that the fact that your beacon was optional equipment doesn't enter into the decision tree for 91.213: Any installed equipment gets evaluated according to this regulation. As an example my (1965) Cherokee has both a tail beacon (originally installed by Piper) and a belly strobe (added by someone over the years): if either is inoperative it needs to be "deactivated and placarded" appropriately.


What is this extension/device on top of the wing of this ultralight? This a shot of the Aero Synergie Papango ultralight, the succesor of the Aero Synergie J300 Joker.

The wings are more elaborate compared to its predecessor's and include on their top two additional extensions/devices.

What are they? Any aerodynamic role or just for aesthetic sake?

Those are wing fences and they prevent airflow perpendicular to the direction of travel. This isolates the portions of the wing and allows the inner wing to stall independently of the outer wing.

Since the wing uses flaps, the inner portion will create more lift and stall (separation of airflow on the upper wing leading to loss of lift) at a lower angle of attack. Wing fences prevent this stall from extending over the outer part with the ailerons, so the airplane will stay controllable in a stall and will not suddenly drop one wing.


Why are the leading edges of wings not always made as 'sharp' as possible? Why is an airplane's wing (the leading edge) not made as sharp as possible to break the air, in the same way that a ship's hull is made sharp to easily break the water?

This is unlike Concorde's nose and most jet fighters' noses, which are made very sharp.

A blunter (less sharp) leading edge allows the wing to operate effectively for a wider range of angles of attack (AoA). The angle of attack is the angle between the approaching airflow and the chord line (as illustrated below)

As you can see, the air has to curve around the leading edge on the upper side of the airfoil. This curvature accelerates the flow and thus creates an area of reduced pressure and thus contributes to the lift of the airfoil.

A sharper leading edge will cause a more intense curvature of the flow and therefore reduce the local pressure even further.
A higher angle of attack will also cause a more intense curvature of the flow with the same effects.

Here's the problem: In order to reach pressure equality between the upper and lower side at the trailing edge of the wing, the pressure will have to increase again as you move further towards the trailing edge.
The boundary layer of the flow doesn't like this at all. Strong pressure gradients will cause the boundary layer detach (generally, the boundary layer is already turbulent for commercial aircraft). A detached boundary layer (stall) causes a drastic increase in drag and decrease in lift and is therefore highly undesirable.

To increase the range of angles of attack at wich the boundary layer stays attached and to allow for a more gradual transition between normal flow and stall, the designers favour a quite blunt leading edge for most commercial and general aviation aircraft. This will increase the drag at zero AoA, but won't force the air to follow a high curvature at positive AoAs.

Supersonic aircraft such as the Concorde and fighter aircraft have to deal with a new type of drag called wave drag. Generally wave drag can be reduced by having sharp leading edges.
Most boats also have to fight against some kind of wave drag (although the fluid dynamics are very different) and therefore also have sharp hulls.

This is a broad generalisation and there are a lot more things to consider in reality. If you want I can go into more detail.


When do I execute a missed approach on a LOC approach when I can't identify the MAP? According to AIM 5.4.21(b):

  When an early missed approach is executed [...] fly the IAP as
  specified on the approach plate to the MAP at or above the MDA or DH
  before executing a turning maneuver.

Hence if a pilot decides on a missed approach, he needs to wait until over the MAP to execute any turns on that procedure. One can only assume that is meant to prevent a climbing turn into a nearby protected airspace.

The following scenario puzzles me.

A pilot executes a LOC approach with a missed approach consisting of "Climb to 1000' then left climbing turn to 5000' direct to..." Sometime after the FAF the cockpit timer breaks. The MAP can no longer be identified by time. The pilot immediately starts climbing to 1000'. But when to start the left climbing turn to 5000'?

The aircraft does not have other equipment that would positively identify the MAP, when is that pilot required to begin that turn?
What if the aircraft has a DME? Is the pilot allowed to identify the MAP by distance rather than by time?

Well, I’m not an FAA guy so I can’t find the relevant FAR quote, but going more general:

ICAO Doc 8161 Vol 1 - Aircraft Operations I-4-6-1

6.1.4 Note 2

  In the case of a missed approach with a turn at an altitude/height, when an operational need exists, an additional protection is provided for the safeguarding of early turns. When it is not possible, a note is published on the profile view of the approach chart to specify that turns must not commence before the MAPt (or before an equivalent point in the case of a precision approach).

There is a nuance difference between AIM’s “you are not supposed to turn” and the ICAO’s  “if the turn area is not protected there must be a warning label”. The first is just procedural, the second ensures you will not hit a mountain.

If the missed approach only specifies ‘Climb 1000’ then climbing left turn to..’ then the airspace is protected even for early turns (between IAF and MAPt) and you can start turning as soon as reaching 1000’. It might get you in trouble with ATC (ie. they were not expecting you to do it), but it will not get you into a mountain side. If you can call ATC and explain that you are turning early because of a navigation/timing fault, you will be allright.
If there is also a ‘do not turn before [...]’ then you shouldn’t turn before, regardless if that fix is the MAPt or not. How to determine that fix or MAPt then becomes a navigation issue:can you use alternative methods? Gps? Dme? Vor radials? are you under radar contact? How about using the VSI climb rate to estimate 1 minute (climbing 700’ at 700 fpm). If neither, then you might be in an emergency situation.


What is this USAF transport aircraft from possibly the 50-60s? I came across the following image (scan of a photograph) in an archive of photographs related to the 1962 Sino-Indian War.

Which particular aircraft could this be?
Update: This answer identifies it as belonging to 322d Airlift Division.

It is indeed a C-130A. The large tail numbers indicate it's from the late 50's or early 60's. In the late 60's, the USAF went with smaller numbers forward of the passenger door. Here's a similar a/c image from C-130.net

Further searching of the C-130 Aircraft Database shows the only "014" to be be s/n 55-0014. The history shows it was assigned to the 40th TAS which was part of the 322nd AD.

In 1968 it was converted to an AC-130A gunship. It is currently on display at the Museum of Aviation at Robins AFB.


What are the visual differences between Trent 892 and 900 engines? Are there any visual differences between a Trent 892 and a Trent 900 engine?
Particularly referring to the ones mounted on the 777-200.

The main difference between the two installed engines would be the fan blades. The 892 uses hollow wide chord Ti blades, like shown below.

By Cory W. Watts from Madison, Wisconsin, United States of America - Rolls Royce Trent 892 [777-200ER], CC BY-SA 2.0, https://commons.wikimedia.org/w/index.php?curid=43524027

The Trent 900, on the other hand, uses a swept design to reduces the effect of shock waves. From RR site:

  The engine’s 24 fan blades are to a new swept design that reduces the effect of shock waves, as the tip of the fan rotates supersonically, making it lighter, quieter and more efficient

Image from imagesourcemedia.com; appears to be taken from RR site.

Also, the Trent 900 is exclusively for A380.


How to safely wash a Nomex flight suit? Flight suits made of Nomex are flame retardant, which is important for flying some kinds of aircraft. However, they're not oil, dirt, or sweat retardant, so sooner or later they need a wash - a wash strong enough to remove all that muck.

Is there anything special you need to do to get them clean without losing their flame retardant quality?

Firstly to clarify:

What are the differences between “inherent” and “treated” flame resistant fabrics?
Inherently flame resistant fibers are materials that have flame resistance built into their chemical structures. Aramid fibers from companies like DuPont create inherently flame resistant fibers, with brands such as DuPont™ Kevlar® and DuPont™ Nomex®.....

With that information:

Laundering’s impact on flame resistance
An essential thing to keep in mind when assessing flame-resistant technologies is that inherent flame resistant properties cannot be washed out or damaged through exposure to chemicals in the workplace or laundering practices, whether at-home or commercial.
FR-treated garments, however, may be damaged by chlorine bleach, the combination of hydrogen peroxide (“oxygen bleach”) with hard water, or exposure to oxidizing chemicals in the workplace. Source

An extensive DuPoint Nomex Laundering Guide
For the 'treated' fabrics, reading this DuPont document might be interesting.


What is the function of the hole at the rear of the airplane? What is the function of the hole at the rear of a plane?
I searched online but didn't get a satisfying answer.

This one? It's the APU(auxiliary power unit)'s exhaust.

An auxiliary power unit (APU) is a device on a vehicle that provides energy for functions other than propulsion.

  The primary purpose of an aircraft APU is to provide power to start
  the main engines. Turbine engines must be accelerated to a high
  rotational speed to provide sufficient air compression for
  self-sustaining operation. Smaller jet engines are usually started by
  an electric motor, while larger engines are usually started by an air
  turbine motor. Before the engines are to be turned, the APU is
  started, generally by a battery or hydraulic accumulator. Once the APU
  is running, it provides power (electric, pneumatic, or hydraulic,
  depending on the design) to start the aircraft's main engines.

  https://en.wikipedia.org/wiki/Auxiliary_power_unit


Is "laze" (lava + haze) as hazardous as volcanic ash to turbo engines? Reading an article about Pahoa, I read that erupting Hawaiian volcanoes produce a combination of lava and haze - resulting in clouds that contain hydrochloric acid.
 Are these as dangerous as volcanic ash clouds?  What are the possible consequences for nearby flights?

Laze is produced by lava running into the ocean, which limits both volume and dispersion of harmful products. Volcanic ash can be blown high into the air in astonishing amounts by explosions at a volcano summit, making it possible for wind to spread the particles over a wide area.

The 2010 eruptions in Iceland blew ash several kilometers into the atmosphere and across much of Northern Europe, shutting down air traffic for days. Both types of emissions have abrasive particulates and corrosive chemicals, so the biggest determinants of harm are dispersion and volume. Ash is by far the more dangerous.

see Kileua sends ash 30,000 feet high and this isn't even considered a very explosive volcano.


During spin recovery, can you just release the yoke/stick to neutralize the ailerons? I have been watching spin recovery videos on YouTube for the last few days. Scary stuff and I haven't even started flying yet :o haha

Anyway, the recovery seems to be commonly described as PARE:

Power idle
Ailerons neutral
Rudder opposite of spin
Elevator forward

Do you physically neutralize the ailerons or does releasing the yoke/stick do that? Is that specific to certain airplanes?

You do not want to put aileron inputs during a stalled condition, as this can induce and/or further aggravate a spin.  So a neutral control position should be assumed with the stick prior neutralizing the spin with opposite rudder input.

Spin recovery techniques vary a little between manufacturers and specific aircraft but the PARE method (Beggs/Muller technique) is one of the most common recovery methods out there.  Alsways use the approved spin recovery technique listed in your airplane’s AFM.


Which is more efficient: a jet engine or a rocket-powered engine? Knowing that rocket powered engines have higher speeds than jet engines, it raises the question: are jet engines more efficient than rocket powered engines?

This is of course in terms of fuel consumption per distance traveled, as I am curious whether a rocket engine allows an airplane to travel a distance more quickly compared to a jet engine when consuming the same amount of fuel.

Jet engines are far more efficient. The metric to compare them is 'Thrust-specific fuel consumption', i.e. the amount of fuel used to produce one unit of thrust. Fuel consumption per distance traveled is proportional to this.

A few figures from that page:

Rocket engine: Space Shuttle main engine, 225 g/kN.s

Jet engine in afterburner: EJ-200: 48 g/kN.s

Jet engine, dry: CF-6: 17 g/kN.s

So a rocket uses ~10 times as much fuel to produce the same thrust. Even if you add afterburning, a jet is more efficient.

During and after World War 2, there was some interest in rocket-engined aircraft. The Me-163 saw service in the Luftwaffe.

The interest continued for a decade or so, until jet engines with afterburners were developed. One exponent of that interest was the Saunders-Roe SR-53, a short-range interceptor with both a jet engine and a rocket.

It was developed in response to an RAF requirement for an aircraft that could take off and get to high altitudes very quickly; more quickly than could be attained with the jet engines of the day.

Jet: Armstrong Siddeley Viper, 1640 lb thrust.
Rocket: De Havilland Spectre, 8000 lb thrust.

Fuel carried:
jet: 500 lbs
rocket: 10,500 lbs

Endurance:
jet: 45 minutes
rocket: up to 7 minutes

speed:
jet: 180 kts
rocket: accelerate to Mach 2 once, then the fuel ran out

(source: mostly 'British experimental jet aircraft', Barrie Hygate.)

It was superseded by the EE. Lightning, which had 2 afterburning jets. It had 4 times the range despite carrying less fuel and more payload.


Why does the angle of descent increase in a constant headwind while decreases in a sudden encounter with a headwind, in a descend? I'm currently studying aerodynamics.
I have realized that a sudden encounter with a headwind during descent
will result in the aircraft generating more lift, thus angle of descent decreases.
I haven't studied the descent in a constant headwind, but through research I've come across that a constant headwind during descent will increase the angle of descent.
This does not make sense. Shouldn't the angle of descent decrease in both situations? Will appreciate it if someone could explain this.

Consider descending in zero wind, trimmed to an airspeed of 80m/s (288km/h) and a sink rate of 2m/s. You won't touch stick, trim tabs or throttles during the descent, but you will encounter a headwind at a later time. For now, ground speed is equal to airspeed and the angle of descent is 1:40.

You suddenly encounter a headwind of 20m/s. Ground speed remains at 80m/s due to inertia; Airspeed rises to 100m/s. The rise in airspeed increases lift and reduces sink rate to 1m/s. The angle of descent is now much shallower, at 1:80.

After a couple of seconds, airspeed bleeds off and the aircraft resumes its trimmed state at an airspeed of 80m/s and a sink rate of 2m/s. This time, the ground speed is only 60m/s due to the headwind and the angle of descent is now 1:30.