What is the difference between a 'live' NLG axle and a 'fixed' NLG axle? What are the advantages / disadvantages of a live and fixed nose landing gear axle and how do they work? Maybe certain aircraft need a particular type fitted?
I can't see the need of a moving axle (assuming that's what 'live' means) unless it is somehow powered by a motor or something similar for push back..

Or..
Maybe on an aircraft a live axle is one where it rotates with the wheel and a fixed one is where the wheel rotates around the axle?

The reason I ask is that I saw in a textbook that nose wheels are either on a fixed or live axle and nose wheel shimmy is prevented by using a twin nose wheel set up attached to a live axle.

I'm now not sure if the terms live and fixed axle are in relation to the vertical component or horizontal component of the nose gear.

A live axle is when the two wheels rotate together. Its function is to reduce shimmying.

The Lockheed JetStar and the Space Shuttle Orbiter have live NLG axles.

References:

Source: Aircraft Landing Gear Design: Principles and Practices

  The nose wheel is a split, forged aluminum alloy design but its bearings are located on a rotating or "live" axle rather than in the hub such as in the main wheel design. Both nose wheels are splined to the "live" nose axle providing a corotating feature which improves stability or reduces the tendency to shimmy.

Source: Orbiter wheel and tire certification, NASA

  Tire friction can also be used to increase the damping in the castor mode. If the gear has two wheels, connecting them by a live axle can have this effect. In the case of a single wheel, it is possible to obtain ‘anti-shimmy’ tires incorporating two contact areas.

Source: Landing Gear (Aircraft), sciencedirect.com


Why is the A330/A340's angle-of-attack protection disabled in alternate law, even if the AoA vanes are operating normally? Reading about Air France Flight 447, to quote Wikipedia (emphasis added):

  ... A second consequence of the reconfiguration into alternate law was that stall protection no longer operated. Whereas in normal law, the aircraft's flight management computers would have acted to prevent such a high angle of attack, in alternate law this did not happen. (Indeed, the switch into alternate law occurred precisely because the computers, denied reliable speed data, were no longer able to provide such protection – nor many of the other functions expected of normal law).[55] The wings lost lift and the aircraft stalled.

But whether an aircraft stalls or not has nothing to do with its airspeed - the only things that matter are the wing's angle of attack and configuration (e.g., are the slats extended? is the leading edge damaged? have we got ice on the wings?).  The aircraft's configuration was completely normal, and the AoA vanes were unaffected by the blocked pitot tubes (as evidenced by the fact that the aircraft's stall warning correctly sounded due to its excessive AoA during the period of time that the pitot tubes were blocked and valid airspeed data was unavailable).  Why does the A330/A340 disable high-AoA protection in alternate law if airspeed data is lost - even if AoA data remains perfectly valid?

1. AoA vanes are speed sensitive

An AoA vane is in essence a weather vane, but due to various aerodynamic interactions it's not perfect. From flight testing they would have worked out how speed affects the position, and based on that the sensed position is corrected by speed input.

From the A330/A340 FCTM (Flight Crew Training Manual) section 8.110.4:

  The ADRs provide a number of outputs to many systems and a blockage of the pitot and/or static systems may also lead to the following:

  (...)
  Alpha floor activation (because AOA outputs from the sensors are corrected by speed inputs)
  (...)

2. Alpha floor inhibition

The above suggests alpha floor could have been activated due to the blockage, but unlike the A320, alpha floor is inhibited on the A340 above Mach 0.53 (no reason is given). (Source: A340 FCOM 1.22.30)

3. Crickets + stall

That stall warning in alternate law is based on a computed VSW (V Stall Wanring), and not the AoA. (A340 FCOM 1.27.30)

The A340 FCOM and FCTM are obscure as to what goes into the VSW calculation, but one of the inputs for that speed for the A320 is the calculated gross weight.


Is the Auxiliary Power Unit used after landing when ground power is available? Assuming the airport provides ground power at the gate, will the Auxiliary Power Unit (APU) be started and used after landing? If not, I guess the engines will only be shut down when the Ground Power Unit (GPU) is connected. In this case, how do pilots know that the ground power is connected, so that they can shut down the engines?

How do pilots know that the ground power is connected?
The ground crew hooks a headset to the plane to communicate with the flight deck.
Once the ground power is connected, they confirm it, also a light comes on in the flight deck confirming external power is now available.
(Source) Ground power switch and light on a Boeing 737.
In a twin-jet airliner, they usually (subject to company) taxi-in with one engine, the other being shut down after it had time to cool down after the reversers are stowed.
They usually shut down the number 2 engine, the starboard side, where the GPU and other ground crew members approach the plane as soon as it is parked.
Many airports have restrictions with regard to keeping the APU running at the gate, due to emissions and noise, unless the plane is pushing back.
Related: Do airliners have horns for use on the ground?


Can I work on aircraft without an A&P license? Can I work at a place where I pull parts, drop engines, hang engines which get sold back into the aircraft market? Am I allowed to do so without a A&P License? I've been told that it is legal if you're supervised. If so what is considered supervision? I live in California with no aircraft schooling at all if that matters.

Anyone can work on a plane.  Only an A&P can certify the work and return the airplane to service, with a few exceptions.  A plane cannot (legally) fly unless the work has been signed off by an A&P, with a few exceptions found in pt43 Appendix A Para C (preventive maintenance).  Even if the work is allowed by pt43 App A Para C, the person performing the maintenance must be at least a private pilot, the airplane must belong to (or be operated by) that pilot, and the airplane cannot be in commercial service.

"Supervision" is loosely defined and is up to the A&P to decide in the field.  The idea is that the supervising mechanic should be observing while you work but the language allows the mechanic to determine exactly what must be observed while work is being performed.  Some will want to be over your shoulder the whole time, some will trust your word, but most are somewhere in the middle (trust but verify).

You may work in a shop as an apprentice and do all of the work you describe but you will be subject to the directions of your supervising mechanic.  Your time as an apprentice counts toward A&P certification, too.  They will likely start you at very low pay and assign menial tasks until you prove yourself to be reliable.  Sweep the floors with a smile and don't be offended that they don't throw cylinder replacements at you on day one.  You need to build up trust before shops will let you do any fun work on customers' airplanes that can easily exceed the price of most middle class homes.


FL(Flight Level) vs Altitude in my radio book i came across this communications.

Control: "Pacific 800 report passing FL 150"

Aircraft: "Pacific 800 Wilco"...
           "Pacific 800 passing FL150".

next.

Control: "Fiji 58 maintain 2500 feet"
Aircraft: "Maintaining 2500 feet Fiji 58"

What is the formal explanation for FL and altitude, cause i am confused both are the same thing.

This depends on where you are in the world but under the FAA here in the US the FAA defines it as

  Flight level means a level of constant atmospheric pressure related to
  a reference datum of 29.92 inches of mercury. Each is stated in three
  digits that represent hundreds of feet. For example, flight level 250
  represents a barometric altimeter indication of 25,000 feet; flight
  level 255, an indication of 25,500 feet.

In other words its the altitude if you set your altimeter to 29.92. In the US all aircraft in class A airspace (18,000 and up generally) set their altimeter to 29.92 thus flight levels are used at that altitude. When an aircraft is below Fl 180 the local altimeter setting is used.


How are "Local Flights" defined in European regulations? Looking at some Greek aerodrome information charts, I discovered that there are airports with the designation "Local flights not permitted". I speculate this is because the areas are near sensitive/military installations.
My question is: How is a "local flight" defined?

Is it a flight where the departure as well as arrival point is the airport in question?
Is it a request for touch & go?
Is it a flight around the airport area, beyond what is required for departure/landing (though I wonder how easy is that to define, particularly in case of e.g. meteorological conditions, such as local clouds)?
All of the above? Something else?

FWIW, I noticed this looking at older charts (suitable only for simulation purposes), so I don't know if this reflects up-to-date information.

In the UK at least a local flight is one that begins and ends at the same aerodrome. This flight could be remaining in the circuit (pattern) or flying a 3 hour navigation exercise a long distance but as long as you come back to the same place without landing away it's a local flight. It is the same in all the other European countries I've flown to, I haven't been to Greece though so there's a chance their definition may be different.


Do aircraft that operate at lower altitudes tend to have more cycles? I've heard that aircraft cycles depend on the number of missions, regardless of the distance, because the pressurisation of the aircraft at altitude is what causes cumulative damage to the fuselage. If that's the case then does it tend to be true that aircraft that fly lower have more cycles? I'm particularly wondering about the difference between turboprop (FL240) and jet aircraft (FL410). Do turboprop fuselages tend to have more cycles?

It's a lot more than just pressurization cycles (all sorts of things are being flexed), but yes. Turboprops are somewhere around 1 hour per cycle and regional jets about 1.2 or 1.3 hours per cycle (a typical regional jet will do around 3000 flying hours and about 2400 cycles in a year).  Mainline jets are closer to 2+ hours per cycle, with long haul jets around 3-4 hours per cycle.
So a turboprop with 50000 hours will tend to have 50000 cycles (averaged across the fleet), the RJ with 50000 hours will have around 40000 cycles, while a 777 with 50000 hours might only have 20000 cycles (keeping in mind wide variations from aircraft to aircraft).
So yes the short haul commuter and regional equipment does get flogged like mules compared to their luxuriating mainline big brothers.  Whether this becomes a "premature" problem depends on how robust the original design was.  The beefier the metal with a given load profile, the more times you can flex it before it starts to crack.  But when you make it beefier, you are adding "ballast" from the perspective of the short term mission. So it's a trade off between performance now, and viability when the airframe gets old (the extra beef reduces payload upfront, but adds greatly to the residual value of the airframe 20 years later - some operators will prefer one approach, some the other).
When the first high-time DeHavilland Dash 8 got to 80000 cycles, the original structural life expectancy, they only found a couple of minor cracks. This eventually led to an extension of the structural life limit to 120,000 cycles, giving a huge boost to the market value of old Dash 8s.  Airlines that own their Dash 8 cared greatly about this.  Airlines that leased their airplanes knowing they'll turn them in at a certain point long before, not so much.
CRJs on the other hand, are bit less robust and a CRJ200 needs quite a bit of patching of the structure after 40000 cycles to keep it viable to its 80000 cycle airframe life (a consequence of not doing a full fatigue test program at the very beginning - they only tested a fuselage barrel, with the full program being done about 8 years later). You can keep flying it past 40000 cycles without the reinforcements, but the frequency of the repetitive inspection requirements imposed make that uneconomical.
On the other hand, the cost of all the patches was brutal, and a lot of CRJ200s were simply retired at 40k, and their airframes stripped for spares, because the operators didn't want to spend the money to incorporate all the structural Service Bulletins (Embraers like the 145 are even lighter in their construction and considered even more "disposable" - a 145 once had its back broken, became an instant tail dragger, on a hard-ish landing in Brazil, a landing that wasn't hard enough to blow tires).
None of this really impacts safety if the maintenance program was developed properly, because inspection intervals and procedures prevent structural issues from progressing to a dangerous level, plus damage-tolerant structural design and very large (3:1) scatter factors applied to fatigue test results (if you bend a component repetitively in a fatigue rig, and it starts to crack at 60000 cycles, you have to impose an inspection or repair or life limit of some kind at 20000 cycles).
Notwithstanding "outlier" events like Aloha Airlines. Things always get through the cracks.


Do wheels continue spinning when gear retracted? Do landing gear wheels continue spinning inside the landing gear compartment once the gear has been retracted or is there a mechanism that will stop them?

They continue spinning but there is a simple friction mechanism that immediately stops them. You can see how it works in this video at 10:50.


Can the landing gear be pulled up while the plane is on the ground? If a pilot makes a mistake and inadvertently touches the gear up lever, will it actually activate while on the ground and make the plane drop onto its belly?

I suppose that there would be some sensor to prevent this, but I would like clarification on that.

There is a weight sensor which senses if the plane is on the ground. This sensor prevents gear retraction while the plane is still on the ground. Failure of this sensor would prevent gear retraction after takeoff.

If you note closely, the landing gears (even the non-retractable ones) are not connected using a simple metal pole; rather, there is an oleo strut which is compressed by the weight of the aircraft. Besides airborne / ground detection, the struts absorb the vertical energy during touchdown.

I recall some decades ago engineers experimented obtaining the plane's gross weight by installing weight scales to each landing gear (as opposed to just an air/ground detection). The readings were found to be inaccurate and they soon abandoned the idea.

Landing gear stories cannot be complete without mentioning this incident in 1990:

A training captain of a Saab 340 was betting with his students that the weight-on-gear mechanism would prevent gear retraction while on the ground. On the accident airplane type, the mechanism would lock the gear handle, but the lock can be overridden if the pilot manually pull out and move the handle. The instructor confidently pulled out the handle and to his surprise, the hydraulics started to move and the gears were retracted while the plane was still on the ground.

(image source)

The aircraft was written off. This incompetent instructor pilot was killed 11 years later in another accident.


Is there any airline company owner who also works as a pilot? Do you know any airline company owner who holds ATPL licence and flies one of the company's aircraft occasionally in normal operations just because (s)he likes to do it?

Yes, Niki Lauda, former Formula 1 pilot and ATPL holder, founded Lauda Air, for which he also worked as PIC.

  Lauda returned to running his airline, Lauda Air, on his second Formula One retirement in 1985. During his time as airline manager, he was appointed consultant at Ferrari as part of an effort by Montezemolo to rejuvenate the team. After selling his Lauda Air shares to majority partner Austrian Airlines in 1999, he managed the Jaguar Formula One racing team from 2001 to 2002. In late 2003, he started a new airline, Niki. Lauda holds a commercial pilot's licence and from time to time acted as a captain on the flights of his airline. Lauda Air ceased operations in July 2013.


What does "design-induced pilot error" means? In the Slashdot article Is the Tesla Model S Pedal Placement A Safety Hazard? it says:

  what aviation analysts call a 'design-induced pilot error'

I tried to search for design-induced pilot error on Google, but found things too complex to understand! What does this mean?

The phrase is basically saying that bad design causes people to make mistakes.

Design-induced - the interaction design or user experience is problematic.

Pilot error - the pilot made a mistake.

An example of this would be the placement of a flap handle next to a gear lever, making it possible to raise the gear rather than the flaps after landing.

Engine controls in modern designs now feature different shapes for each control knob for similar reasons. There have been many accidents where pilots have grabbed a mixture knob — and shut an engine off completely — when they meant to grab the throttle and reduce power. Different textures and shapes make that harder to do:


B-52s that are not in service demolished? Have all the B-52s that are no longer in service already been "guillotined," i.e. chopped in two to make them no longer flyable?

No, some B-52H's in long term storage at the Aerospace Maintenance and Regeneration Group are in Type 1000 storage, which means,

  Aircraft stored in near-flyaway condition. Can be stored without re-preservation for a period of 4 years. Aircraft stored under this category may be downgraded to Type 2000.

Note: This screengrab is from Google Maps and I'm not sure when it was taken.

The USAF is under the process of reclaiming a B-52H from the Boneyard as a replacement.

Source: foxtrotalpha.jalopnik.com

However, all the B-52G models have been destroyed in compliance with the New START treaty.


Why is Hydrazine used to power the F-16's EPU? Why does the military use hydrazine to power the F-16's Emergency Power Unit (EPU)? What is so unique about Hydrazine fuel that it's  needed to power the EPU? Why not use a less volatile and less hazardous fuel?

To fill the voids in aeroalias' answer:

Hydrazine is a monopropellant, something which does not need to be mixed and burned to free up the energy contained in its chemical bonds. This energy is freed by letting the hydrazine stream over a catalyst, in case of the F-16 EPU that is iridium. This breaks the chemical bond, producing ammonia, nitrogen gas, hydrogen gas and heat which converts the water into steam. Heating increases the volume of the gasses and the steam so they pick up speed, just like the gasses in the combustion chamber of a jet engine. In the catalyst chamber of the F-16's EPU the temperature reaches more than 800°C in just a couple of milliseconds, and no combustion is involved! This high-pressure, high-speed gas flows through a small turbine which in turn drives a generator and supplies hydraulic pressure.

Compression is not needed because pressure is supplied from the bottle containing the hydrazine-water mixture. Using a monopropellant also avoids any ignition problems and makes the setup very simple. Since hydrazine is quite stable at high pressure and ambient temperature, the shelf life of a hydrazine engine is high, and the liquid state of the hydrazine-water mixture makes its storage very compact. Just what you need in a fighter aircraft.


Is there an abnormal condition that could have caused this fatal accident involving a stall? In 2009, a Grob 103 glider stalled and crashed at Richmond Field, Michigan, killing the passenger but not the pilot. (NTSB accident report: CEN09LA353)

The accident occurred after a failed winch launch; during the winch launch, the tow cable broke while the glider was about 400 feet AGL. The pilot correctly decided to turn around and return to the airport.

Here's what happened next, according to a statement the pilot later gave:

  I could feel in the controls that something was not right and the glider was not responding in the manner that I am accustomed. There were none of the signs of a stall and the glider did not behave like it has any time that I have practiced stalls. Before I could determine the cause or take any action, the nose abruptly dropped and we dove toward the intended runway.

The NTSB report about the accident states that the probable cause of the accident was simply the pilot's failure to maintain adequate airspeed, resulting in a stall. Judging by this, the NTSB didn't seem to put much stock in the pilot's report that "something was not right".

Suppose that we take the pilot at their word, however. Is there any type of abnormal condition that might have caused a "feeling in the controls that something is not right", and which could cause the nose to drop with "none of the signs of a stall" occurring beforehand?

Your question:

  Suppose that we take the pilot at their word, however. Is there any type of abnormal condition that might have caused a "feeling in the controls that something is not right", and which could cause the nose to drop with "none of the signs of a stall" occurring beforehand?

A couple of thoughts:

In a 60 degree level bank turn, stall speed increases by about 40%.
See below for information regarding indications of an approaching stall in the G103. (from the G 103 C Twin III Acro manual)

The flight manual, section 4, indicates that with aft cg there is a stall warning 2-3 kts before reaching stall speed - (read) "There is a tail vibration which increases with continuing pull on the control stick." I think the note above (highlighted in yellow) means that with forward CoG, at max weight, there is no stall warning at all, i.e., the glider just stalls (nose drops without warning). I could be wrong, but this seems to me to be what the note means.

Here is a copy of the manual for the Grob G103 Twin III Acro: Grob G103 flight Manual

Here is a copy of the NTSB report I believe covers this accident: Grob 103 Accident - NTSB Identification: CEN09LA353


Could a quadcopter fly if its propellers were encased in cylindrical tubes? Picture a standard quadcopter, with regular old propeller guards, which are generally just circles around the propellers. Now, take the propeller guards and stretch them so that they're cylinders surrounding the immediate cross-section of propeller blades, as well as some distance above/beneath them.

Now I am fairly certain that it would be able to go up/down. But would it be able to turn? I know it is a relatively vague question, but after thinking about it for some time, I really have no idea. I might just modify the propeller guards on my cheap little drone to see if this works.

Yes. It would be able to turn. Of your 4 propellers, 2 spin clockwise and 2 spin counterclockwise.  The torques cancel out.  To make it spin, you run the 2 clockwise propellers a little faster and the other 2 a little slower.  Now the torques do not cancel out and the quadcopter spins.  This does not change if you have a ducted fan versus a propeller


Phoenix Area Helicopter ATC: What is "Hee-lah" (Gila?) One, and what is the 101 transition? I've been listening to the Phoenix Tower LiveATC feed, and I hear references to "Hee-lah" (Gila?) One and the 101 transition with helicopters (see audio files below):

Angel-Five

TV-3

TV-15

Does anyone know what these are? References would be appreciated if possible. Thanks!

The Gila is a river that runs through Arizona, so that may be a visual reporting point for incoming aircraft.


What is the width of a high altitude airway? Victor airways (airways that are below 18,000 ft) has a width of 8 NM. How about those airways that operates higher than 18,000 ft, do they have specific width you must be in?

Or is there no width because you would be using GPS? (ones with an RNP of 2 NM)

At least in the US airspace definition, High Altitude Airways (Jet Routes) have no defined width regardless of the airway being navaid-based or RNAV/RNP

This is briefly stated in the FAA training manuals:
http://tfmlearning.faa.gov/Publications/atpubs/AIR/air2004.html


What can a new student pilot do on their own to prepare for class and instruction? What is the best way to start for beginners learning to fly? I have a class set up and a instructor but not for another week or two. I just want to know if there is anything extra that I can start now, so it will become easier. I have started reading about the basics to get my mind going.

Usually the best way is to call your local flight school and ask to take a discovery flight.  It’s a 30 minute or so flight in a Cessna 172 or similar aircraft and you’ll get a good introduction to flying, even some stick time in the airplane.

If you’re looking for material to read on the subject of learning to fly, I suggest getting copies of the pilots handbook of aeronautical knowledge, and the airplane flying handbook. Both are available for free as PDFs on the FAA’s website.  They will be the fundamental textbooks for your private pilot training and most of the knowledge and practical information on flying that the FAA will test you on are going to come from these documents.


How do I estimate piston aircraft fuel efficiency? Is there any way to figure out or calculate fuel consumption of piston engines for aircraft? I'm looking at different engines in a book, and while jet engines have useful values like thrust, bypass and pressure ratio, piston engines only have power output and weight.
Propeller efficiency plays a role here, I guess, but it's not at hand so I assume all engines use the same propeller in this case.
Would the easiest way to calculate such a number be to find the maximum fuel capacity and range and from there find out how much fuel each engine uses per hour?

I've asked a similar question before about turbojets and turbofans and received good answers - sadly, piston engines are completely different...

EDIT: To clarify, I am looking at airliners powered by piston engines and wish to compare them.

Yes there is a simple rule of thumb for what is known as Specific Fuel Consumption that you can apply to estimate the fuel burn of most piston engines:

An air cooled carbureted engine, leaned, will burn roughly 0.44 to 0.45 lb/hp/hr.
With Fuel Injection, a bit less, maybe 0.41 or 0.42-ish.

Cars with electronic sequential fuel injection are in the high 0.3s.  Diesels are in the low 0.3s

2 strokes are up in the mid 0.5s.

Turbo props are very thirsty and are in the .6 range.

Take a Cessna 150.

75% cruise is 75 hp.
X 0.45 = 33-34 lb/hr.
= 5.6 usg/hr.

Which is about what you will find in the POH, or close enough anyway.

Want to know the fuel burn of an R-985 powered airplane?

450 X .75 = 337 hp
@ .44 lb/hp/hr = 148 lb/hr
= 26 usg/hr, which sounds about right for something like a DeHavilland Beaver.


Why are passenger and cargo airplane noses manufactured separately from fuselages and then put together? I was wondering why the fuselage of a passenger airplane is not built along with the nose? It might be difficult to build fuselage and wings together as one unit, but nose is a small part. Then why is it made separately?

A radome, or nose cone, is essentially a structural cover serving a different purpose than the fuselage. These are made separate because they a special material invisible to the radar mounted beneath them. They are different shapes or sizes depending on the customer specific component they cover (radar antennas) and can be easily removed or replaced.

The radome serves two essential purposes. 1) It covers electronic equipment mounted to the forward bulkhead of the fuselage to protect it from weather, or from view. 2) It acts as an aerodynamic cover for the leading edge of the aircraft.

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


What are the differences between the proportional flow control FCU and pressure control FCU? what are the basic differences between the proportional flow control FCU and pressure control FCU? ( FCU : fuel control unit )

The pressure control system is a typical system as fitted to a turbo-propeller engine where the rate of engine acceleration is restricted by a propeller speed controller

The proportional flow control is a typical system where H.P. compressor shaft r.p.m. is governed by a hydro-mechanical governor which uses hydraulic pressure proportional to engine speed as its controlling parameter

Therefore the pressure control system is one step ahead as it looks at the tendance of speed, of course the pressure control system alone is not sufficient because even if the rate of speed increase is not excessive, the final speed could become excessive, that’s why also a speed limiter is included in the system

Today’s FADECs directly protect the engines for excessive speed and for excessive speed rate

Please refer to to the following Website

http://aeromodelbasic.blogspot.com/2012/01/fuel-control-system.html

Also please refer to the following website

http://aeromodelbasic.blogspot.com/2012/02/proportional-flow-control-system.html


Why is laser illumation of a cockpit an emergency? Another answer pointed out that laser illumination of a cockpit may be considered an emergency, and that pilots may take evasive action when under laser illumination.

Why is laser illumination considered an emergency?

Have any documented instances of laser illumination resulted in injury to or loss of crew, passengers, or plane?

Let me posit a hypothetical game ("Don't Try This at... well Anywhere").
It's simple - there are just 3 rules:

  Give your friend a nice laser pointer and have them stand at the end of a road
  A half-mile or so should be plenty.
  Get in your car at the other end of the road and drive toward your friend.
  Your friend's goal is to shine the laser in your eyes. Your goal is simple: try not to crash.

Clearly this is not a very safe game.
It should be clear that playing the same game with aircraft is substantially less safe. Someone attempting to "play" without your consent certainly rises to the definition of "emergency" in my book.

If you actually play this game you'll note that the laser isn't a neat pinpoint when it's shining at you.
Between scratches in the aircraft's window and the way your eye perceives light the laser pointer can completely blind the victim - check out the examples in the graphic below.

[

To date I don't believe any accidents have been attributed to laser illumination, but it is well within the realm of possibility, and the upward trend in laser illuminations of aircraft means the likelihood of such an accident is also increasing.

For more information check out the FAA's Laser Safety Initiative -- and particularly the video here.


Can you be a commercial pilot if you have a criminal record? Is it possible to become a pilot in the UK with a criminal record?

British Airways has this to say:

  If you have been cautioned, warned or convicted of a criminal offence (other than parking offences) we suggest you check whether you will be able to obtain an airside pass by visiting www.dft.gov.uk.

  In order to be employed as a pilot with British Airways you will be required to undergo a Criminal Record Check for every country in which you have been a resident, or worked in, for 6 months or more in the past 3 years.

Admittedly, the DFT's website is a maze of information, and it seems very difficult to find any information; I'd suggest giving them a call, to find out for your particular convictions.

Airside Manager has some more detailed information on airside passes, in general.


Are fixed wing aircraft with gimbal thrust feasible? Elon Musk has suggested that he'd look at implementing VTOL electric aircraft with a "gimballed thrust" system similar to the one used in rockets.

  Aircraft have all these unnecessary things like tails and rudders and elevators - not needed. Just gimbal.. anyway.. gimbal the electric fan. For some weird reason gimbaling motors is normal in rockets and not in aircraft. Why not? [Do you have specific plans?] I've been sort of toying with the design for an electric supersonic vertical take-off and landing electric aircraft for a while, I'd love to do it, but I think my mind would explode

What are the challenges to implementing this? Is it technically feasible?

Could you create an aircraft using an jet engine/propeller/ducted fan on a gimbal?  Sure you 'could'; it is feasible by the laws of physics.  But just because you can does not mean that you should.

Setting up multiple points of lift on a manned aircraft seems like a good idea with a lot of advantages but is a Pandora's Box of problems, the not the least of which is the grave risks of single point failure.  Lift and control is totally reliant on balanced lift between multiple engines.  Lose one engine and the craft departs from controlled flight and crashes.  You better hope you have an ejection seat and/or parachute on board.

Some solutions to these problems come in the form tiltrotor designs like the Agusta Westland 609 and the V-22.  But these aircraft have multiple redundancies in their design including a common transmission system between the rotors; in case of a single engine failure, the good engine has enough power to drive both rotors.  Tiltrotors can also glide like regular airplanes and utilize control surfaces during this and cruise flight.

As a final note:  Be careful in putting too much faith in Musk and other Silicon Valley whiz kids speculating about other design endeavors.  The skills required to run a successful software company are often very different from that required to develop other artifacts like aircraft and automobiles, etc.  Musk was a genius with PayPal but nearly went bankrupt starting an electric car company and his development work with SpaceX has been punctuated with a series of spectacular failures.  He also quietly withdrew from his proposed 'Hyperloop' project, I suspect, because he really didn't understand what he was getting into there.


Why was NWS shifted from green hydraulic to yellow hydraulic system in the A320? In older Airbus A320s, the NWS (nose wheel steering) was on the green hydraulic system and now it's on yellow. What's the reason for the change?

This enhanced package (green to yellow change) since serial number 1939[1] had the following objectives in mind:[2]

Objectives:

Avoid simultaneous loss of Normal braking and nose wheel steering functions on a single hydraulic failure. [normal braking is green]
Avoid loss of nose wheel steering function due to ADIRU or LGCIU failure. [ADR 1+3 failure results in NWS failure; ADIRU 1 and 3 control the Green shut off valve once the L/G is retracted and speed is above ~260 knots]
Make the nose wheel steering function available after a landing gear free fall extension. [L/G extension is green]

(My explanation is in square brackets.)
As a bonus, it would have permitted the eTaxi option (announced in 2013; scrapped in 2019). This way, the APU would have supplied power to the powered-wheels, as well as hydraulics for steering and braking (alternate), all without a tug, and without having to power the Green system via the PTU.[1]

1: Airbus FAST magazine #51
2: A320-family flight deck and systems briefing for pilots


What is the pressure in a civil aircraft fuselage at flight ceiling? What is the pressure (in bar) in the fuselage at flight ceiling of large passenger airliners?
Is this pressure the reason for the difficulty in having large windows? If so, how can the window size be increased?

The pressure inside the cabin is usually more than the outside pressure at the cruise altitude. The pressure altitude (ISA altitude with the same pressure) inside the cabin of civil aircraft is called the equivalent effective cabin altitude or normally cabin altitude.

As the aircraft climbs, the cabin altitude is gradually raised so that it reaches around 8000 ft for the aircraft cruising altitude of ~40000 ft. This level is maintained at flight ceiling too.

Source: www.flyertalk.com

The pressure is around 0.75 bar inside the cabin. The outside pressure at this point is around 0.2 bar, resulting in a pressure differential of 0.56 bar. This pressure differential causes stresses on the aircraft body.

Source:ecnmag.com

In addition to the stresses, pressurization - de-pressurizatoin cycle can cause fatigue in the windows.

As reducing the cabin altitude will improve the passenger comfort, newer aircraft tend to have lower cabin altitudes. For example, the Boeing 787 has a pressure altitude of around 6000 ft and the Bombardier Global Express jet has a cabin altitude of around 4500 ft.

This is achieved by the use of advanced composite materials, which allow a higher pressure differential have excellent fatigue properties compared to the metals. As a result, the windows of newer aircrafts are considerably larger than their predecessors.

Image: Boeing


Why and how would I land straight-in from a circle-to-land approach? I'm currently studying for an IR (US). One of the exam practice questions concerns an IAP having only circling minimums, and references 5-4-20(c) of the AIM, which reads:

Straight-in Minimums are shown on the IAP when the final approach course is within 30 degrees of the runway alignment (15 degrees for GPS IAPs) and a normal descent can be made from the IFR altitude shown on the IAP to the runway surface. When either the normal rate of descent or the runway alignment factor of 30 degrees (15 degrees for GPS IAPs) is exceeded, a straight-in minimum is not published and a circling minimum applies. The fact that a straight-in minimum is not published does not preclude pilots from landing straight-in if they have the active runway in sight and have sufficient time to make a normal approach for landing. Under such conditions and when ATC has cleared them for landing on that runway, pilots are not expected to circle even though only circling minimums are published. If they desire to circle, they should advise ATC.

I'm having trouble understanding the bolded sentence. What does it mean to land straight-in from an approach that is more than 30 degrees from runway alignment?
I've been understanding a circling approach as something like a pattern landing where you start the maneuver once you have passed the FAF and have - and can keep - the runway environment in sight. But this seems to imply something different. What am I missing? What does "a normal approach to landing" mean here?

The sentence before the one you bolded, with my bolding applied, says:

  When either the normal rate of descent or the runway alignment factor of 30 degrees (15 degrees for GPS IAPs) is exceeded, a straight-in minimum is not published and a circling minimum applies.

So you can have a "circling approach" that is lined up straight in to a runway, but with only circling mins, not straight-in minimums, published.  This could be driven by the need for a high FAF altitude, high enough that you can't get from that FAF at that altitude down to the runway, with a descent rate that's within the TERPS criteria.  So they build an approach that assumes you maneuver to lose the excess altitude, and give you circling mins so that you can.

However -- and this is what the exception that you bolded in the question is about -- if you're in, let's say a STOL airplane, you may very well be able to lose the excess altitude and be established on a normal visual glidepath to the runway with ample time to make a "normal approach and landing" without any S-turns or other maneuvering required.  And in that case you're absolutely allowed to fly the approach straight in.  The guy behind you in a Lear Jet may not be able to do the same thing that you did, and the TERPS descent profiles criteria are written to guarantee that if he has straight-in mins, he CAN get from the FAF at its crossing altitude to the runway without having to use what would be, for him, a dangerously high descent rate (angle).  But in the STOL aircraft, a steep descent is perfectly normal, so you can go ahead.  The Lear shouldn't try to mimic the Maule's descent profile, but that doesn't mean that the Maule has to mimic the Lear's descent profile.


How attitude, speed, and altitude vary when increasing pitch? I have been struggling with the following question for a while, would greatly appreciate some help and explanation why.
Scenario:

I'm cruising in my Cessna- cruise attitude 2300 RPM trimmed for 95.

I pitch up, say 1 inch of back pressure, and hold it.

Initially the plane starts climbing and slows down, but what happens next?
How will the plane look in say 5 minutes with me still holding that 1 inch pitch up, what attitude, speed, and altitude will it have?

The general answer goes something like this:
Level flight, at it's most basic involves a balance of 3 factors:

How fast are you going, relative to the wind?
What is the angle of attack (AOA) of your wings, relative to the wind?
How dense is the air that you are flying through, based on altitude, temperature, humidity, etc.?

Speed and AOA are variables that you indirectly control, via throttle (by increasing thrust) and yoke (by using elevator to alter pitch) respectively. The thing is, changing one variable (speed, AOA, or density) requires that one or both of the others change to maintain equilibrium. Want to stay in level cruise at a given density altitude after lowering throttle? AOA must increase. If the AOA stays the same, the aircraft will begin to sink because the current density cannot provide enough lift at that combination of AOA and speed.
That explains some of the "Why". Now let's consider your scenario. You have taken throttle out of the equation. Thrust is fixed and cannot be changed. This means that for any change to AOA, density must provide the equilibrium. Unless you are already at your stall speed, a slight increase in AOA without lowering throttle will begin a climb at slower forward speed because this new combination of AOA and speed are producing excess lift at the current air density. You have traded forward speed for extra lifting force. Because the throttle and new pitch are now fixed, the airplane will simply climb at it's new speed and AOA until altitude and temperature lower the outside air density to the new equilibrium point. At this point, the aircraft will once again be configured for level flight at the new density altitude. Also, because the density will change gradually, so too will the rate of climb decrease gradually. It will probably take a good while to fully level out.
ADDENDUM:
The "lift" topic always sparks arguments, because technically, lift can be generated in any direction, depending on the definition of lift you are considering. An aircraft moving through the air on its side, may still create some small amount of force laterally, which opposes gravity, even though the wings may be producing no force at all. A helicopter in hover is utilizing all of its accelerated airflow for to counteract gravity, but a change in rotor disc tilt divides that total force between lift (opposing gravity) and thrust (opposing drag). An aerobatic airplane hanging on its prop is utilizing accelerated airflow in much the same way. An aircraft flying upside down is creating lift that is simultaneously against the pull of gravity AND toward the bottom of the aircraft.
Additionally, Bernoulli's principles work equally well in any orientation. An asymmetrically cambered airfoil mounted vertically will still create an area of lower pressure on one side than on the other, and is still capable of altering angle of attack to adjust deflection. Is this lift? The simple physics definition would say "No" because it is not opposing gravity. An aerodynamic analysis of the airfoil would say "Yes", the airfoil is creating lift according to its orientation, which in this case, affects only yaw.
It would be nice if the entire world could agree that Lift is the sum of all forces that act in opposition to gravity, and Thrust is the sum of all forces that counteract drag - regardless of whether these forces are provided by airfoil or prop (which is still an array of airfoils!). Unfortunately, for now, clarification is often required when discussing this problem in polite company.
In closing, I would like you to consider this: in your scenario, the reason that your airplane slows down is because, much like the helicopter, your prop disc is being tilted, dividing its power between lifting and thrusting. The drag on the airplane remains, so the added burden of lift is what leeches away the speed. The wings, on the other hand, no longer bear the full burden of opposing gravity, so the lift that they are producing DOES become less, but only because they are now sharing that load with the prop.


Why were the fronts of the Messerschmitt Me-109 painted yellow? After seeing the movie Dunkirk I learnt that the Luftwaffe didn't actually use the yellow coloring on the Me-109 until a few months after the battle of Dunkirk. Was this perhaps done to avoid friendly fire? I'm eager to hear other possible explanations.

Thanks!

You're right that the yellow tactical markings were used in order to reduce the friendly fire incidents (similar to the Allied D-Day stripes). As noted by Alan W. Hall in Messerschmidt Bf 109 warpaint:

  Quick identification has always been a necessity in aerial conflict ... Channel front fighters of the Luftwaffe started carrying yellow cowl and rudder markings. Some painted the entire cowling forward of the windscreen yellow, while others just painted the undersides of the cowling.

The image below shows some of the various tactical markings used at the time.

Bf 109 tactical markings; image from asisbiz.com

While the yellow paint seems to have been used before, it saw widespread use from August 1940 during Battle of Britain, and was used in some cases in other theaters too. Also, other aircraft like FW 190 also used some variations, like the yellow painted rudder.


Why does a turn and slip indicator matter? I'm studying the principles of flight, and I'm having some unanswered questions in my mind regarding the turn and slip indicator.

I'm reading that a turn and slip indicator displays the slip or skid of the turn. It probably has something to do with directional stability.

But the questions I'm having are:

Why does it matter if the airplane is in a skidding or slipping turn?
What are the consequences of skidding or slipping turn?
Is it dangerous to be in a skid or slip?
Why would a pilot would like and/or not like to be in a skid or slip?

The turn and slip instruments are combined for convenience and by convention more than anything else, slip is important whether you are in a turn or not. When you slip it means one wing is more directly pointed into the airflow, which increases lift for that wing in comparison to the other. At a high angle of attack, for instance a climb and close to stall speed, a slip can put one wing into a stall, causing the non-stalled wing to flip the airplane. When done deliberately it's an aerobatic maneuver called a snap roll, when it's not deliberate it's a major cause of stall-spin accidents on take-off and approach. So slip is dangerous in some situations.

In level flight slip creates drag, so it's worth keeping the ball in the middle when safety is not a concern as well.

As for deliberate slip as I said before slip can be induced for aerobatic maneuvers like snap rolls and spins, slip can also be used for non-aerobatic maneuvers like side-slips, where you put the airplane out of balance to increase drag, allowing more rapid descents without increasing airspeed.


When ATC uses distances, are they using nautical miles or statute miles? "Skyhawk 872ND, airport at your 12 o'clock in 15 miles."

"Baron 981K, traffic, 2 o'clock 5 miles, opposite direction, a Piper southbound, 1000 feet below you."

Does ATC use nautical miles or statute miles?

ATC uses nautical miles, this is because it's the ICAO standard documented in Annex 5 - Units of Measurement to be used in Air and Ground Operations, which states that Nautical Miles are to be used for distance and Knots for speed as an alternative non-SI measurement. Kilometers are actually the primary distance measurement because they are the SI standard, however nautical miles are so entrenched they've kept them and there's no termination date.

The ICAO annex is hard to find because everyone wants you to buy it, however the standards have been copied into this CAAS document word for word if you want some light bedtime reading.


How much drag do the gear doors create during retraction? It's been said that if you retract the gear at a critical moment that performance will actually be worse than if you left the gear down. However, is this really true, and just how long is the performance worse for? Obviously in the long run it'd be better to have the gear up, so how much drag is there actually?

Of course gear doors create drag when open.  The question is "how much?"

As you suspect, door drag is not the critical factor here.  As usual the answer to your question is aircraft-specific, and your question is pointing to a generalized "old wive's tale" which is only true sometimes.

OK, what is the story behind the story?  Look at this Youtube video.  Watch the main gear as it enters/exits the storage wells in the fuselage.  The axles of the wheels are close to parallel to the direction of flight, which means the wheels are being dragged broadside in the slipstream.  A very high drag scenario which will affect aircraft performance.  The POH recommendation is (from memory, now) to climb over critical obstacles on takeoff with the gear down, and raise the gear only when the obstacle is cleared.  The attached video shows why.

This advice does not pertain to all planes.  Know your plane, read the POH.  Oh... and ask questions here!


What does it mean when the controller uses the word "request"? I recently had the opportunity to fly a PAR approach into Büchel Airbase in Germany. It was a ton of fun and I'll definitely try it again when I get the chance. However, as we were getting set up for the approach I received the following call

  (callsign) request heading

It caught me off guard, and it took a while but I eventually interpreted it as "say heading" and gave him my current heading. He didn't complain, but I'm still not sure if that's what he wanted.

A bit later I got a similar call

  (callsign) request QNE

However, I was unfamiliar with that Q-code1 and only later found out it means "pressure altitude". Q-codes suck. Anyway I said "Say again" and he came back and asked how many were on board (which I incorrectly assumed at that point was what QNE meant), again, using "request".

Anyway, I've never heard a controller say "request" before, is it just army version of "say"? I'm pretty sure the guy was German, so it might be a German military thing, or a NATO thing, or just a simple mistake on his side?

1: As a private pilot in Europe, you pretty much need to know QNH, QFE, QDM and QTE

In civil aviation the ICAO standard phraseology should be used. This phraseology is described in ICAO document 9432.
The standard ICAO phraseology is to use the word report in the first case.
According to the definitions it means: "Pass me the following information ..."

  [callsign] report heading

The second phrase is even stranger. QNE is the standard pressure altimetry setting of 1013.25 Hectopascal (29.92 inches of mercury in the US). When an altimeter is set to standard pressure, the normal way of reporting the altitude is in Flight Levels. So in this case the standard phraseology should have been

  [callsign] report Flight Level.


How realistic is the plane scene in Captain America: The First Avenger? In Captain America: The First Avenger, there's a scene in which Captain America gets into the villain's plane by holding onto the landing gear as it's taking off and waiting for it to be retracted. How realistic is this scene, when applied to modern airplanes?

Could you theoretically gain access to a large, modern airplane by just holding onto the landing gear - assuming, of course, you managed to hold on? Do the landing gears of large modern planes retract into interiors that have access to the rest of the plane, or do those interiors tend to be sealed off? And for that matter, how hard would it be to hold on to a plane's landing gear as it's taking off?

Though 'getting on/off aircraft through landing gear' is a favorite trope in movies, it is practically impossible in most of the modern airliners. Airliners are pressurized, and as a result, the cabin (and cockpit) area is in reality one big cylinder, cut off from everything else. So, no accessibility from the landing gear area.

For example, the following image shows the landing gear bay (the area into which the landing gear retracts into) of a Boeing 787.

Image from dailymail.co.uk

As a result, though the list of stowaways in landing gear area is long, to the best of my knowledge, no one has entered the aircraft through it.


Lift Pressure Distribution If the pressure goes always from high to low, we should have a compression, due the lower pressure of the air stream over the upper surface, and an expansion in the airstream in the lower surface.

So What causes the "suction" on the upper surface of an airfoil and the "pushing" in the lower one?

I undestand why there is a region of high pressure and a low pressure around the airfoil. But I am a little bit confused about the effects of the external pressure (Free stream) on the airfoil stream.

The key to the lift generation is the curvature of the streamlines. Forget the popular, but unfortunately wrong explanations based on the non-equidistant particle paths.
The simplest example will be if you place a plate with an angle of attack in the subsonic flow: the streamlines will get curved on the both plate sides as the flow turns.
Therefore, even a flat plate will generate a lift once its angle of attack is different from zero. As the streamlines are turning on the upper surface, the pressure will get reduced from the free-stream to the airfoil surface.
In other words, moving away from the airfoil, the pressure will increase - in the direction of the radius of curvature of the streamlines.
Important to note is, that this pressure drop is normal to the streamlines. This change in pressure is simply an effect of turning the flow, i.e. changing its direction.
Mathematically, the pressure gradient normal to the streamline, dp/dn, can be expressed as
$$ \frac{dp}{dn} = \rho \times \frac{V^2}{R} , $$
where R is the radius of curvature the streamline. Derivation of this equation can be obtained if writing the momentum equations in the natural coordinates.


Are pilots allowed to let passengers fly the plane? Is a private pilot license holder in the USA who is giving a scenic tour with friends allowed to let them fly the plane? Are there any restrictions on when this can be done?

In my mind, this could be done safely while at altitude and the pilot could let them do a few shallow turns.  They wouldn't want them doing anything complex and would want them to instantly let go if asked.

For a non-commercial flight under Part 91 rules there are no issues I'm aware of with letting a non-rated passenger handle the controls - and it's done quite frequently (I can't think of any pilots I know offhand who haven't had someone else take the controls for one reason or another).

The rated pilot is still pilot-in-command of the flight and ultimately responsible for everything that happens, so they need to be relatively certain their friend will let go of the controls if told to do so, and they need to maintain situational awareness (traffic scan, airspace, etc. -- if the friend blunders into Class B airspace it's going to land in the pilot's lap) and be ready to take over if the friend starts doing something that could jeopardize the safety of the flight.

There are some common-sense limits here: something insane like have two non-rated friends at the controls the rated pilot being PIC from the back seat giving them instructions over the intercom would be a no-go.
Similarly while this sort of thing is a total non-event in an aircraft with typical dual controls you would probably want to have a higher level of trust in your friend if you're flying something with a throw-over yoke.


Is there an upper age limit for pilots freight only? When I asked my original question below, I should have been more specific. I will rephrase my question, however, I do wish to extend my thanks & appreciation to those who answered my question as previously written!

PLEASE ADVISE WHETHER THERE IS AN UPPER AGE LIMIT FOR PILOTS FLYING STRICTLY AIR FREIGHT AND/OR FOR COMPANIES LIKE fEDEX, UPS, DHL, ETC. THAT FLY A MIX OF AIR FREIGHT, DOCUMENTATION, ETC. FLYING ON AMERICAN (USA) FLAG CARRIERS WITHIN THE USA? AND, WHETHER IT MAKES A DIFFERENCE IF THE SAME USA CARRIER FLIES OUT OF THE USA TO ANOTHER COUNTRY SUCH AS CANADA, MEXICO, CENTRAL AMERICA, SOUTH AMERICA, EUROPE, ASIA, AFRICA, ETC.?  DOES THE US POSTAL SERVICE ITS OWN FLEET OF AIRCRAFT, AND, IF SO, DOES IT HAVE AN UPPER AGE LIMIT FOR FLYING US MAIL AND/OR PARCELS?  THANK YOU!
PRIOR QUESTION LEFT HERE FOR REFERENCE ONLY:
(Please advise if there is an upper age limit for pilots flying aircraft carrying only freight (not passengers) into and out of the USA?)

The only situation in which a US-licensed pilot is subject to an age limit is part 121 scheduled airline service. Other than that there are no age limits, you just have to be able to pass your medical exam.

However, you state that in your scenario the pilot will have to be able to fly cargo in and out of the US. That implies the pilot may have a foreign license, in which case you must refer to that country's regulations.


Are tripropellant jet engines a viable way to increase fuel efficiency for large commercial airplanes? I was just reading about a tripropellent rocket engine that apparently passed all tests and was ready for flight before the program was cancelled in the early 90's. The engine was the RD-701 for the MAKS spaceplane program. It used kerosene and liquid oxygen before switching to liquid hydrogen.
So I'm now thinking about tripropellant jet engines. If we had a small tank of liquid hydrogen to either mix with the regular jet fuel or replace it for a short while, would this viably improve fuel economy for large commercial airplanes?
(I've heard that jet engines can easily run on almost anything that burns, so presumably it would be easy to get a jet running on hydrogen. Correct me if I'm wrong.)
There are many types of efficiencies, so let me be clear about which one I'm interested in: Fuel economy, as in miles per passenger-gallon.
Let's ignore any logistics on the ground about obtaining and storing the hydrogen. However, storage aboard the airplane itself should certainly be considered and might be a very detrimental factor, as might flammability. I'm pretty sure we would at least need double-walled, vacuum insulated tanks to let us store that hydrogen throughout the flight. They might be twice as heavy as normal tanks.
If you know anything about hydrogen, its chemical energy (per mass) is greater than gasoline....by a factor of 3 IIRC. The Lower Heating Values are around 120 versus 40. So we might all ask why not just use liquid hydrogen in everything and what is taking so long? Well apart from cryogenic considerations, it's not very dense at all. We would need a huge tank maybe even bigger than the airplane itself to store the necessary hydrogen, even tho that huge volume of hydrogen would still weigh less than the necessary jet fuel.
I think the idea of tripropellant engines is to allow some hydrogen with a not-so-overwhelming tank. The tank would only be as big as we can reasonably handle. Therefore we use some hydrogen to get a bonus in fuel economy.
P.S. I'm not really asking this from a "green" perspective. Hydrogen and oxygen burn to form pure water exhaust, which sounds so harmless, but water vapor is actually a very potent greenhouse gas, much more potent than CO2 molecule-for-molecule---especially when released at high altitudes. I'm asking this purely from a fuel-economy standpoint.

First of all: jets are not "n-propellant". Rockets are bipropellant. That's because rocket needs to carry fuel and oxidizer, while air-breathing jet gets the oxidizer (plus huge amounts of reaction mass) for free from the air. So jets carry only one propellant (which doesn't make them monopropellant either). The tripropellant rockets were imagined to overcome problems with specific impulse. The most complicated and impractical rocket engine could go as high as 542s - while average jet operates around 3000s easily. So specific impulse is the problem jets never had in the first place.
The biggest advantage of RD-701 was to use the kerosene-hydrogen mix on low altitude (where kerosene is more efficient) and then switch to pure hydrogen in space (where hydrogen is more efficient). They started with a hydrogen engine (RD-0120) and then added kerosene to increase efficiency by reducing the amount of hydrogen needed - so the exactly opposite reasoning. The main problem with your idea here is that jets never leave the kerosene zone.
The main problem of liquid hydrogen is that the fuel is lighter, but the tanks are so heavy it completely obliterates any savings. Rockets here get it easy - rocket working lifetime is measured in minutes (Space Shuttle burn: 8m30s Soyuz burn: bit over 10m50s). A plane on the other hand would need to store the hydrogen for a dozen of hours = even more weight. There is no weight-efficient way of keeping the cryogenic fluid on the go. All you can do is to let some of it "boil-off". Twice as heavy as kerosene tank is a huge understatement IMHO, as in modern jet liners the tanks - "wet wing" - are almost free (in terms of mass). Can't do that with hydrogen - you'll need to sacrifice cargo space for dedicated LH2 tank.
Another issue is that hydrogen burns in higher temperature. We're already having problems with making the turbine survive kerosene temperatures, adding hydrogen would make the engine design even more difficult. From the green perspective it's not all roses either, because the higher combustion temperature, the more NOx created.
I have to agree with others here - hydrogen planes won't happen unless we'll run out of easier options. Having 2 kinds of fuel is the most difficult of them all.


Can a US pilot fly a foreign registered aircraft in the US? Is it legal for a pilot with an FAA license to fly a foreign registered aircraft within the United States?  Is it covered by FAA regulations, or the country of aircraft registration?

Yes. 14 CFR 61.3(b) covers this (my emphasis):

  (b) Required pilot certificate for operating a foreign-registered
  aircraft within the United States. No person may serve as a required
  pilot flight crewmember of a civil aircraft of foreign registry within
  the United States, unless—

  (1) That person's pilot certificate or document issued under §61.29(e)
  is in that person's physical possession or readily accessible in the
  aircraft when exercising the privileges of that pilot certificate; and

  (2) Has been issued in accordance with this part, or has been issued
  or validated by the country in which the aircraft is registered.

So a US license (i.e. one issued under part 61) is allowed as an alternative to a foreign license for operating the foreign aircraft. The other country could have rules on this too, but I have no idea if they would be enforceable (obviously, one country's laws are usually not valid in another one).


Stick in center console, how to easily write down clearances and other similar information? I write with my right hand. I can make sort of intelligible scrawls with my left hand, but that is much slower for me.

The airplane I'm training in is a single-engine prop plane with the control stick (not yoke) in the center console, between the seats, rather than directly in front of the pilot. (For those curious, it's an Ikarus C42B. Here and here are some photos of what the interior looks like; your favorite search engine will show you many more, including all kinds of variations to the panel layout.)

Sitting in the left-hand seat, this means that my right hand is operating the control stick, which also has the PTT and trim controls plus the main brake handle. (I typically keep my left hand on the throttle lever, which in this aircraft is between the legs of each pilot. Yes, this means there are two throttle controls and one control stick.)

This combination presents an obvious obstacle when trying to write down things like clearances, as I'll have to either:

use my left hand to write, resulting in difficult-to-read scribbles (certainly not ideal), or
use my right hand to write, requiring reaching over with my left hand to the control stick (awkward), or
use my right hand to write, letting go of the control stick and trusting that the aircraft will fly reasonably straight and level (requires much trust in the aircraft)

Any practical tips on how to handle a situation like this? (Surely I can't be the only person in this situation.)

The aircraft is equipped only for VFR; no autopilot of any kind, no IFR instruments, etc. So I'm only interested in VFR during VMC, but with the possibility of some turbulence and possibly in controlled airspace. (If I somehow manage to end up in IMC, ATC will just have to put up with me until I'm back in VMC...)

Since there seems to be some confusion, here are some examples of the kind of information I'd like to be able to write down:

taxi instructions
landing instructions, including e.g. runway in use
altimeter settings (e.g. QNH value, transition level or transition altitude)
frequencies
reporting positions and altitudes ("report 3,000 feet climbing", "report BAZ")
instructions for where to enter or exit controlled airspace, or where to transition from one controlled airspace to another ("cleared entry via FOO, 2,500 feet or below")
transponder codes, especially when assigned a code different from the local default VFR code (1200 in the US, 7000 in Europe, ...)
cleared altitude or flight level

There's probably much more that I'm just not thinking of right now. While some of this is in a relatively relaxed environment on the ground, some of it needs to be handled in the air.

One of these days I plan on trying to simply let go of the stick for a few moments (after trimming for level flight) to see how well the plane actually maintains straight and level flight on its own with at most rudder inputs, but other than that, I'm having trouble coming up with ways of actually handling this situation. How do other pilots do it?

One technique many airline pilots use is to enter letters and numbers on the FMC keypad to use as a reminder for complicated taxi instructions.

“taxi via Hotel, cross 15L, right turn Bravo, hold short Alpha One” could be entered on the keypad as: HX15LB/A1 or possibly H X15L B/A1

There really is no requirement to write down this type of instruction, or any VFR clearance. It is just a reminder and makes for good airmanship.

Many FOs in the right seat seem to use their left hand when entering data in the FMC as it can feel awkward to reach over and punch buttons with your right hand.

In your case you might be able to use an iPad or some other type of electronic device on a knee pad to punch in some “shorthand” with your left hand to help you remember what ATC says.


Why is the flight director at 9 degrees nose high when hitting TOGA? Why is the flight director set to 9 degrees pitch up, when pressing the takeoff / go-around (TOGA) button? In all the jets I fly, this is the case and I would be curious to know if other jets set their pitch attitude to a different value.

curious to know if other jets set their pitch attitude to a different value.

747-100/200 aircraft didn't have a TOGA button to press, but at the two 747 carriers I flew for in the 1990s, the flight engineer filled out a data card for takeoff and later for landing that listed the pitch attitude target for takeoff and go-arounds, both for 3-engines and 4-engines. He/she looked up the values in tables in the QRH (Quick Reference Handbook), a copy of which each cockpit crewmember had. The card was placed against the instrument panel such that all three cockpit crew members could see it.

The target attitude depended not only upon the number of operating engines, but also engine type, aircraft weight, flap setting, and OAT. I just looked in my old QRH, and the lowest target I found was 9°, the highest was 19°. As I remember, typical actual operational values ran from 10° to 14°. Values higher than 14° were for very light weights and thus would be rarely used.

The 4-engine target pitch was generally 2° higher than the 3-engine pitch except that at very low weights, the difference was 3°.

Adherence to the target attitude was up to the flying pilot as he/she would be hand flying the airplane without flight director guidance.


What is the format of data stored by a Flight Data Recorder? Is the raw data fetched from the FDR available only in binary format and if so what are the software tools that are used to convert it to readable format?

To simplify explanations:

FDR: flight data recorder (flight parameters),
DFDR: digital FDR,
CVR cockpit voice recorder (audio recording).

In commercial aircraft, aside the mandatory FDR/CVR used by investigation agencies (the scope of the question), there are recorders used for maintenance and operational purposes by the operator. The progress made around these additional recorders influences the regulation around the mandatory recorders.

Short answer to the questions:

Sensor data are collected by a central acquisition unit which, for commercial aircraft, is compliant to standards, e.g. a digital acquisition unit will be compliant to ARINC 717 and use the frame format from the standard.
Data are then distributed to other units under this format, this directly influences the format used by recorders. Today most FDR store data as bits in a digital memory or a on long term media (e.g. optical disc). The FDR required by regulation uses a crash-resistant digital memory.
Since a few years, recorded data format can be described using ARINC 647A specifications in an XML file, known as FRED file. This file simplifies FDR data exchange and interoperability, a current requirement for aircraft parameters monitoring and analysis using products from different manufacturers/editors.
A FRED file can also be stored along with data, and as a matter of fact, mandatory FDR regulation now requires the FRED file to be embedded in FDR.
Readout hardware/software is provided by the recorder manufacturer.

As mandatory and non-mandatory recorders from a given manufacturer are susceptible of using a common format, readout devices are usually combined with monitoring and data analysis software.

Details: Regulation

Quite surprisingly, there is no mandatory format for storing flight data. International regulations don't require or recommend a specific scheme. They only require data to be provided to the aeronautical authorities in a timely manner if an investigation has to be carried out after an incident/accident.

ICAO Chicago Convention, annex 6 states:

  6.3.5.2 In approving the means to make flight recorder data available in a timely manner,
  the State of the Operator shall take into account the following:

  a) the capabilities of the operator;
  b) overall capability of the aeroplane and its systems as certified by the
  State of Design;
  c) the reliability of the means to recover the appropriate CVR channels and
  appropriate FDR data;
  d) specific mitigation measures.

ICAO member States implement this international requirement, by requiring a documentation describing how data were calibrated and converted for recording and how to read them back. The related documents are known as Data Frame Layout and Conversion Data. E.g. for the US this is found in FAR 125.226:

  Documentation sufficient to convert recorded data into the engineering
  units and discrete values specified in the applicable appendix must be
  maintained by the certificate holder.

and more details for DFDR in AC 20-141:

  The data stream must be correlated to EU or to discrete states for an
  accident investigator to use the data.  This appendix provides the
  specific information that operators must provide to the NTSB
  investigators decoding and converting the raw data stream recorded by
  the DFDR.  This data format information can be contained within an
  electronic file or in hardcopy document format.

The description of the data frame layout and conversion method encompasses:

The origin of the recorded parameter, and any computation done
The method used for parameter calibration
The method used to encode the value of the parameter
The unit used.
The method to be used to convert recorded values into readable values.

In addition there are requirements about the certification of the mandatory recorders themselves, for most countries, the acceptable means of compliance directly or indirectly (e.g. via TSO-124 wrapper) refer to EurOCAE Document 112A (ED-112A) Minimum Operational Performance Specification for Crash Protected Airborne Recorder Systems.

Since 2016, ED-112A forbids the use of analog recorders, including magnetic tapes, for mandatory recorders. Non volatile solid state memory (EEPROM, typically Flash memory) is used instead.

More: What data file format is used by FDRs?

Recording format is influenced by the FDAU

Overall, data are recorded in any efficient format, not a user readable format. Data are compacted as much as possible to save memory and volume requirements of the crash survivable memory unit (CSMU), because of the cost of making this unit highly reliable and protected from the effects of an accident.

On large aircraft, parameters are collected by a flight data acquisition unit: Analog FDAU are compliant to ARINC 573 and digital DFDAU to ARINC 717). Digitalized data can be distributed to other avionics units using a local area network, e.g. the common ARINC 429 LAN. Units receiving data include cockpit displays, ACARS, and mandatory and non-mandatory recorders.

  Is the raw data fetched from the FDR available only in binary format?

If we refer to large commercial aircraft, the answer is yes. Using ARINC 429 LAN already means data have been digitalized and streamed over the bus according to a well documented format. The FDAU choice greatly influences in practical how data will be recorded. It is easier to design a DFDR aligned on DFDAU ARINC 717 stream. ARINC 747 is an alternative with data compression for solid state memory.

ARINC 717 DFDR

The logical choice of the DFDR manufacturer is to use ARINC 717 both for a mandatory FDR and the various maintenance recorders. In this standard, the recorded data stream consists of a frame with four subframes.

Source

Each subframes is recorded at the rate of one per second, and contains a certain number of bits grouped into 12-bit words. The actual number of words depends on the need, hence on the aircraft. The cycle repeats after 4 seconds.

DFDR data example: Magnetic heading

Delays are to be taken into consideration, sensor output from the IRU takes some time to get to the recorder, this must be described by the manufacturer as part of the information needed for the recorded parameter readout:

Source CAP 731

Data transits on ARINC 429 bus identified by label 320, a BNR data:

Ibid

While no recording format is mandatory, the data sampling rate is critical: Transient values may be invisible if sampling is too slow. Therefore regulation mandates sampling rates.

A given parameter may appear only in some subframes, due to its sampling rate, and specific frames (superframes), recorded once per 64 seconds, are used for parameters which vary very slowly (e.g. gross mass). Examples of sampling rates:

Source

Data is recorded at some location in the overall framing of the FDR, this location is a choice of the DFDR manufacturer. For non-mandatory recorders (e.g. Quick Access Recorder, QAR) recorded parameters are chosen by the operator, as well as their location in the frames. For our example:

Ibid

Last but not least, the documentation describes how the recorded value must be processed to get the original parameter value and unit:

Heading Degree = 0.1758 x FDR Count (11 bit) - 180

Compliance to regulation

In spite of regulation, there are cases where data layout is unknown, e.g. BEA, the French investigation agency noted for a case:

  The operator was not able to provide the BEA with any data
  frame layout documents. A list of recorded parameters and
  their allocation in the data frame was obtained, but the
  conversion equations and the calibration reports were missing.
  The FDR data could not be analyzed.

(Flight Data Recorder Read-Out, §2.3.2.1)

In the same document you may read other cases where the FDR data readout has been difficult or impossible.

The mandatory data frame layout and conversion method documents can be embedded into the FDR, to facilitate the readout and produce a file which can be understood by any compliant software. There is a standard for the documentation: The Flight Recorder Electronic Documentation (FRED) which is described in ARINC 647A.

There have been cases where the missing documentation prevented data interpretation (see below), but since 2016 and ED-112A, the FRED file must be carried in the recorder itself with the data, to ensure the data decoding.

FDR readout

  What are the software tools that are used to convert it to readable format?

FDR manufacturers provide the tools required to read out data, based on their own recording techniques. Readout is done using simple hardware/software.

Readout is facilitated by reusing the embedded FRED file or using an external file (a FRED file is an XML file describing in detail the way the recording was done, and the format used):

Source: ARINC 647A

It should be noted manufacturers usually also sell non mandatory recorders (QAR and the many variants of QAR) and they provide not only readout tools, but also analysis/monitoring tools. Example:

ROSE analysis unit, source

Other readout solutions:

KGB Aviation
ATM

You can see the FDR itself is used for the readout. In case the FDR unit is broken (which is frequent after accidents), the protected memory unit is extracted and read with another working FDR.

Extraction of the memory module protected in temperature resistant foam:

(Source: ATSB accident report)

Readout facilities and readout certification

FDR/CVR readout cannot be trusted unless it's done with appropriate knowledge, skills and documentation. This is the reason readouts used by official agencies, which can be used at courts to try determining responsibilities after an accident, are required to be done by certified personnel in approved facilities.


What are typical resins used in carbon fiber for aircraft? Are any resins impossible to recycle? What types of resins are typically used in elements of aircraft, such as structural beams or sheets?

Are some of these commonly used resins more easy to recycle than others? I've that Pyrolysis and Solvolysis are used to recycle the carbon fibers, but it is unclear to me which resins make the carbon fiber composites able to be recycled.

This will give you some detail on thermoset resins, which solidify through chemical reaction. In summary:

Epoxy is the primary resin used in aerospace, at least for structures, as it is strong and wear resistant.
Phenolic is the primary non structural resin used in aerospace, as it is fire resistant. This is important in aircraft interiors.
Polyester is cheaper than epoxy but weaker, so it shows up in less structural or weight critical applications. Boats might use this.
Vinyl ester is somewhere in between epoxy and polyester, so not as good as either. I have never used this.
Bismaleimide (BMI) is used for high heat applications where maintaining strength is important. This is a slightly less expensive member of the Polyimide family, used for similar purposes.
Cynate ester apparently has military applications, with which I am not familiar.
PEEK as suggested in comment, is the only thermoplastic in this list, which is formed by melting and solidification as by injection molding. It can be recovered for reuse with reduced mechanical properties.

Thermoset resins are generally not recyclable. When composites are intended for recycling the fibers are the object, not the resin. The resin has to be burnt or dissolved away, creating an emissions problem that needs to be dealt with.


Have attitude indicators ever had triple failure in a commercial aircraft? I'm wondering about couple of well known crashes (mainly AF447 and AirAsia 8501) and thinking: is there any case where triple redundant attitude sensor failure has ever occurred? In those accidents it seems that the pilots assumed that they cannot trust the indicators and forced the airplane to stall to ground.
I understand that air speed sensors can have triple failure (e.g. AF447 had such a situation for a minute) because those depend on measuring external environment. However, the attitude indicators are fully closed system which doesn't require any external inputs. As a result, having failure of three fully independent attitude sensors at the same time looks impossible to me.
Why do commercial pilots keep ignoring the attitude indicators in case other instruments are assumed to have faults?

Yes! More or less, it depends on your definition of failure (see later).
As you said, the probability that three Inertial Reference Units (IRUs) fail is extremely rare. This is because IRUs are self-contained, internal to the aircraft, and independents. Instead, air data probes are subject to ice and external factors (for instance, bird strikes), thus more subject to faults. The IRS failure probability is so low, that in the non-neo A320/A330 the ECAM message "TRIPLE IRS FAILED" doesn't even exist (it shows all the three messages: IR1+IR2 FAULT, IR2+IR3 FAULT, IR1+IR3 FAULT).
However, yes, it happened: This is a real picture of an A330-220 (F-GRSQ) flying at FL240 with all IRUs failed in 2014:

According to the BEA report of the F-GRSQ incident (published in 2018), only 2 triple IRU failures incidents in Airbus aircraft happened since 1999: this one and another one in 2002.
In both cases the reasons of the triple failure are unclear. However, the BEA report suggests that the triple failure was triggered by erroneous pilots actions responding to a single IRU failure. In the F-GRSQ the possible explanation is that the pilots identified IR2 drifting too much and decided to switch it to ATT mode (degraded mode, providing only attitude), but they erroneously switched IR3 to ATT. Hours later, when IR2 drifted too much, Autopilot and FBW computer detected the data inconsistency and started to consider IR2 as failed, while IR3 was in ATT mode (and then not used by AP/FBW). Probably, at this point, by following a non-standard procedure, the pilots tried to restart all the IRs to regain the position on Navigation Display, including the  working IR1, which then lost alignment and caused the triple failure condition. Unfortunately, the lack of CVR recordings didn't allow the verification of this theory, but no malfunction was found in IR1 or IR3.

Why do commercial pilots keep ignoring the attitude indicators in case other instruments are assumed to have faults?

I don't think that pilots thought it was an IR problem in the sort of incidents you mentioned.


Can ATC handle a flight as emergency although the pilots have not declared an emergency? The preliminary report for the LNI610 crash shows how much distractive conversation there was between ATC and LIN610 at a time when the pilots clearly didn't have the aircraft under control – as evidenced by the FDR data – and should have talked to each other to diagnose the problem, rather than to ATC. This violates the principle "aviate - navigate - communicate".

While it clearly falls within the responsibility of the crew to declare an emergency, my question is posed from the point of view of air traffic control:

(from the preliminary report, p.1+2)

  At 23:21:53 UTC, the LNI610 SIC requested approval to the TE controller “to some holding point”. The TE controller asked the LNI610 the problem of the aircraft and the pilot responded “flight control problem”.

  At 23:24:51 UTC, the TE controller added “FLIGHT CONT TROB” text for LNI610
  target label on the controller radar system as reminder that the flight was
  experiencing flight control problem.

The preliminary report reveals how much dispensable talk there was between ATC and LNI610 in this crucial moments even after the flight was marked as experiencing flight control problems.

My question is:

Seeing that the flight was marked by ATC as "FLIGHT CONT TROB", could ATC have handled the situation differently, e.g. bothering them less or even handling the situation outright as PAN PAN (don't restrict navigation, reroute other traffic, avoid communication as much as possible) even though it was not declared as such by the crew ?

Or did they essentially have their hands tied, obliged to treat the flight as any other as long as no emergency is officially declared via either MAYDAY or PAN PAN ?

I very much welcome any answers, including from pilots or ATC controllers who are unfamiliar with this particular accident, but can share their work experience (how this is handled in practice) AND/OR point to specific rules (how this should be handled in theory).

FDR data (timestamps + pink vertical lines added by myself at 23:21:53 and 23:24:51 UTC corresponding to the ATC excerpts quoted above):

In the US, a controller has the authority to treat any situation as an emergency, and they do.  I have had controllers "declare" an emergency multiple times, even when I thought I did not have an emergency.
There are some advantages to the controller.  His work gets shed to others, and his supervisor is at his side to assist.
At one point, after ice, delays and trying to get into a DC airport close to the Christmas holiday, I declared "minimum fuel" which I didn't consider an emergency.  I just wanted to let the controller know that if they vectored me towards West Virginia again, I would be landing close to or at reserves.  The weather at the destination was far better than the ice I have been flying through at 13,000, but I wanted to avoid plunking down at Martinsburg, away from my destination to buy another load of fuel.
Immediately I got handed off to a Dulles controller, whose radar had better coverage than Baltimore Approach who handled approaches normally for my destination.  I got kid glove treatment, and did not get further delays, and landed at my destination without any incident.  After the paperwork, I dipped the tanks, and figured I still had about 27 minutes more fuel than my minimums for IFR night flight with a diversion to better weather.
Similarly, having gear problems almost always gets an "emergency" status.
I will say one thing, when I was dealing with a cockpit fire, and the controller kept asking again and again the same questions it was distracting, but they are trying to get their job done the best as they can, just as the pilot is.  In my case, I just stopped answering lower priority questions.  In the case of the cockpit fire, I killed the master switch (at night), and another pilot in the back brought ATC up on his handheld radio.  Up front we dealt with the fire, and delegated the communications to a guy in back.
So in application to the LNI610 and other situations, it makes sense that the pilots exercise their ability to tell a controller to "standby."  It works some of the time.
I am not as familiar with the US Controllers Handbook as I have been in the past, but I am confident that the controllers have authority to declare an emergency.  But keep in mind that they are products of their environment, just as pilots are, and they have training, objectives, experiences and organizational expectations which change and vary.


Where did the SOE agents sit in the 161 squadron Short Stirlings? I asked this question on history.se, where it was recommended I repost here.

During World War II Special Operation Executive (SOE) agents were dropped into occupied Europe by No. 161 squadron of the RAF. This squadron used the Short Stirling bomber up until March 1945.

In the Short Stirling, the pilots were separated from the flight engineer and wireless operator. There is also this picture on Wikipedia, which appears to show the interior of a modified Short Stirling that could carry passengers?

My question would be: In a typical 161 squadron mission, where in the Short Stirling would the agent(s) be located?

IIRC, the agents dropped (with parachute) through a hole in the belly of the aircraft, just aft of the bomb-bays. So they would sit in that general area, so that they wouldn't have to move far with the 'chute attached. The position of the pilots is immaterial to this.


What terminologies are used when ATC instructs a pilot to stop or proceed with his transmission? If some pilot forgets to release his mic button, distracting everyone on the frequency, what terminology can be used by an ATC to stop it?

And if another pilot stop transmitting his message until the former pilot release his mic button, what terminology can be used by an ATC to make him resume and finish his transmission?

If a pilot forgets to release his mic button, you can't talk to him. VHF radio is simplex, so only one station can transmit at the time. If more than one station transmits at the same time, the signal will sound garbled. As a result, there is no standard phraseology to use in such a situation. All you can do is wait for the pilot to discover the issue. See this question for additional info.

CPDLC (text based communication between ATC and the cockpit) does have a "CHECK STUCK MIC" message that can be sent to aircraft on a frequency with a stuck mic. But CPDLC is not globally available, and only used in the en-route environment.

When someone is waiting to transmit, ATC will use the phrase "Go ahead" to indicate that they can start their call. This will not be very useful in the case of a stuck mic, for the reason described above, but the "Go ahead" phrase is often used in other cases where ATC has told a pilot to wait ("Standby"), typically because ATC is busy on the phone or another, more important radio call has to be made first.


What are these hexagonal things under the Concorde's neck? The Concorde had a pair of dark-coloured hexagons under each of its forward boarding doors, as seen in this picture (showing the hexagons on the left side of the Concorde's neck):

British Airways Concorde, Pictured by LondonAirTravel at the BA Engineering Base, London Heathrow (link)

What are they, and what purpose do they serve?  They obviously aren't passenger or crew windows, and they're far too big to be static ports yet far too small to be cargo doors - some sort of maintenance access panel, maybe?

(source)

  This is the side loading door. Under this door are the static pressure
  panels that provide data to the Air Data Computers. They have been
  painted over now.

Here is a nice close up image

(source)


What website(s) can I use to plan an IFR route along airways? WORLDWIDE I'm looking for a good website where I can create a IFR route including the airways. I tried route finder but it doesn't give me the airways.

For the United States, Skyvector is easy and free.
Use the buttons in the top-right corner to switch to "World Lo", that will give you instrument charts.


What is the 3 click sound when the autopilot is disconnected on Airbus aircraft? On Airbus aircraft what is the 3 click tone that sounds once autopilot is disconnected? I have tried researching on the internet but could not find anything

There are two separate aural warnings that you might be referring to: one is indeed called 'Triple Click' and consists of three brief clicks, but is emitted when the landing capability becomes degraded or the autopilot otherwise changes mode.

The warning generated upon autopilot disconnection, instead, is the so-called 'Cavalry Charge' and consists of three longer beeping sounds; it's either played once or repeated continuously (cases 1 and 2 below).

From a table in the Airbus A320 FCOM, DSC-31-10 P 4/6:

  TRIPLE CLICK

  Meaning: Landing capability downgrade or some cases of mode reversion.

  Duration: 0.5 s (3 pulses)

  CAVALRY CHARGE

  Meaning:

  A/P disconnection by takeover push-button.
  A/P disconnection due to failure.

  Duration:

  1.5 s
  Permanent

  Cancellation:

  Second push on takeover push-button.
  Press Master Warning light or takeover push-button.

See this YouTube video (at 2:52) for a cavalry charge immediately followed by a triple click (thank you Jack).