Why could increases in cockpit voice and image recording potentially raise privacy concerns? A major concern someone or other brings up against practically every suggestion to broaden the uses of cockpit voice recordings or add cockpit image/video recorders to the mix (both of these, especially the latter, being actions that accident investigators have long urged regulatory agencies and aircraft operators to implement in the name of air safety, and which the regulators and the operators have, for just as long, worked to weasel out of), and one which dates right back to when CVRs were first introduced, is that doing so would supposedly be a violation of the pilots' privacy.1

This concern seems somewhat misplaced:

For pilots operating commercially (including those operating the vast majority of the flights that would fall under the aforementioned proposed recording-expansion mandates), the cockpit is their workplace and the aircraft an item of company equipment, and there has generally never been any expectation of privacy for workers in the workplace while on duty, especially not as regards the use of company-owned equipment.3
For general-aviation pilots, the only time when they would be required to turn the recorded data over to anyone would be following an accident or incident, when the pilot(s) would have little reason (and, depending on the nature of the occurrence, may have been definitively rendered unable) to object to handing the data over.

Where, then, is the justification for these alleged concerns?

1: Other excuses offered for not installing cockpit image recorders are the large amounts of oh-so-expensive storage required to record and store cockpit video (which most likely stopped being a valid concern in the early 1970s, with the advent of miniaturised videotape systems with recording lengths of up to an hour - twice the length of contemporary CVRs! - and is a frankly ridiculous argument to make nowadays, when a tebibyte of solid-state flash memory, enough for dozens to hundreds of hours of high-res cockpit video from multiple vantage points, can be had for less than a hundred bucks in a package the size of a candy bar2), and the supposed uselessness of recorded image data for accident-investigation purposes (a contention easily refuted by a cursory glance at the many accidents and incidents that have prompted accident investigators to repeatedly and consistently urge for the installation of CIRs, as well as by five minutes’ or so thought about all the things that could be useful to an accident investigation and which would show up on a video recording, but not - or, at least, not identifiably - on an audio recording).

2: In fact, the storage for a CIR would probably be less expensive than that in a consumer-grade SSD, as flight recorders, which record data in strict sequential order and where data recoverability is far more critical than data access control, would not use the complex encryption and wear-levelling circuitry found in most consumer flash devices.

3: This is why (for example) it is both legal and ubiquitous for companies to install software on their work computers to remotely monitor what their employees are doing on said computers.

I will answer this question with the caveat that it is asking for an opinion, and therefore your experience may vary due to different regulatory authorities. I answer this question from an Australian CASA context but note that while there are differences between CASA, EASA and FAA, they operate so similar that in many cases the courseware I teach to students to become an engineer under those systems is almost identical. Therefore, I feel like I am able to make an informed opinion on the matter even though the courseware I teach is primarily designed for a CASA context. Having said that, If your aviation regulatory authority is not based under those I have just mentioned, I have no idea. Go consult their regulations if that is the case.

I will now paraphrase from an avionics textbook on module 13 (avionics instruments other - Cockpit Voice Recorder system) under the CASA part 66 licence outcomes:

-It is an offence under the Air Navigation Act 1920 to disclose any information on a cockpit voice recorder for the purposes other than that listed in part 2A of this act. This means that unless it is directly related to the investigation of an accident it will not be made public.

-CVR recordings may not be used as evidence in any criminal or civil proceedings within Australia

-CVR recordings cannot be used as grounds for any disciplinary action by employers

-It is an offence to make public any CVR recording. Large fines apply

-Only the investigative authority in the course of investigation or court order is to disclose the recording for the express purpose that it is relevant to understanding the cause of an accident

My text goes on to mention things like if the recording is distressing then a transcript should be made and the original recording not played if it does not add anything to the investigation.

My textbook does not specifically mention video recording systems but I would say they fall under the definition in the Air Navigation Act 1920 and therefore treated the same. But this is something lawyers can fight over.

To some up then, due to the points raised above, I feel like there are no grounds to feel concerned about your privacy if there were increases in recording. This is due to the protection given for your personal data under the act. Each nation should have a similar act that protects people in a similar way. If it doesn't, I would hope your workers union could lobby for change. There are no downsides I can see to increasing recording time and increasing the use of video in the name of safety, especially where a rigid protocol of non-disclosure exists in the form of appropriate legislation. Privacy concerns can only exist in a world where you are not confident in the system providing the protections.

I will finish by adding another opinion on the subject. The only time I wouldn't recommend increasing recording is on very light airframes where space and weight is an issue. There is an ongoing trend in the last 40 years or more and more flight parameters being recorded under the regs, so it makes sense to also find new and innovative ways to record video and voice in the cockpit as well.


How to determine the number of compression stages given a compression ratio? I have a personal project to design a gas turbine engine, the main problem that puzzles me is how do I determine the pressure rise in each stage of an axial compressor?
Say, I'm planning on a 38:1 engine so how do I determine how many stages I might need and how much will be the pressure rise?

You'll have to look up what the technology level can bring you these days. Usually, cycle design is s joint effort of what is needed and what can be done. Engine manufacturers have an aerodynamic department that is in close contact with the engine cylce designers and marketing.
An OPR (overall pressure ratio) of a design is based on the maximum turbine inlet temperature and depending on the cooling technology. Once selected, the aerodynamacists will tell you how many stages you're going to need. Since you're not a manufacturer, you'll need to choose a value, 1.2 : 1 is a value you can use. Note that a turbofan has a higher stage pressure ratio.


What does Lockheed have to do to mark the end of the F-35's SDD? What does Lockheed Martin have to do to mark the end, or close out, the System Development and Demonstration phase of the F-35?

The clearest explanation I have found, comes from the Congressional Research Service (www.crs.gov), from RL30563.pdf:

  The F-35 is currently in low-rate initial production, with 280
  aircraft delivered as of April 2018. At least 250 of those were in
  U.S. service. Four to five aircraft are currently delivered each
  month, with the production rate scheduled to increase to 120 per year
  by 2019. In keeping with the acquisition plan that overlapped
  development and production (known as “concurrency”), the F-35 was also
  in system development and demonstration (SDD), with testing and
  software development ongoing, from October 2001 until April 11, 2018.
  The SDD phase will formally continue until the end of Initial
  Operational Test and Evaluation, when a “Milestone C” full-rate
  production decision will be made.

A copy of the full PDF is available here:

https://fas.org/sgp/crs/weapons/RL30563.pdf

We know from other sources that Initial Operational Test and Evaluation (IOT&E), will formally begin later this year, 2018.


How can best glide speed be lower than best rate of climb speed? I was looking up V-speeds for C172. I don't have easy access to an official copy of the POH, but online I found:

$V_y$, best rate of climb speed (at sea level): 76 KIAS (p22)
$V_{glide}$, maximum glide speed: 65 kts (p15).

But I always thought that $V_y$ is the speed for highest L/D, which should give both highest rate of climb and lowest rate of descent, and that longest glide is achieved by flying slightly faster than that, because drag grows with square of speed, and therefore it initially grows slower than speed above the minimum drag point.

So why is the $V_{glide}$ quoted lower than $V_y$?

The best glide speed is the speed where tangent line from the origin (zero horizontal and vertical speed) touches power curve (for unpowered airplane).

Best rate of the climb is the highest point of the power curve.

When we start with unpowered airplane, the direct equivalent of best rate of climb is lowest rate of sink. The speed for lowest sink will be clearly lower than the best glide speed.

If we would assume that aircraft engine is a magical device, which simply adds some fixed amount of mechanical energy to the airplane regardless of what, then the "powered" power curve would be simply the "unpowered" one shifted up in the graph. Under such assumptions the speed for best climb would be the same as the speed for minimum sink and in both cases lower than best glide speed.

Nevertheless mentioned Cessna converts fuel to mechanical energy through not-so-magical fixed pitch propeller which pushes some amount of air around. Such propeller would be typically optimized for highest efficiency at the cruise speed, so with decreasing speed the amount of available mechanical energy dereases. Which results in changed shape of power curve and maximum point moves towards higher speeds compared to the "unpowered" power curve.

This shift can be high enough that Vy ends up higher than Vg.

The Vy is always higher than the speed for the best climb angle, because these two speeds are found on the same (max power) power curve. For the same reason best glide speed will be always higher than speed for minimum sink. But speeds for best rate of climb and for the least sink does not need to be the same because of effects of real propeller.

For related images and more information, have a look at great online text See How It Flies.


Is the compressibility of air the reason high aspect ratio wings are more effecient As stated many times here 1, 2, 3 wings generate lift by moving air downwards. This downward air creates an equal and opposite force pushing the aircraft up.

High aspect ratio wings have a greater lift/drag ratio because they are moving large amounts of air a little bit, vs lower aspect ratio wings which move a little bit of air a lot.

Now my question is why does this matter?

When using "solid" objects to generate "lift" this doesn't matter as long as the solid object is strong enough not to break. For example the image below shows 2 different sized poles (different aspect ratios) resisting the same amount of force.

Air is not a solid, and so when high pressure is exerted against it, air will compress absorbing some of that pressure and converting it into heat and potential energy.

There is also pockets of low pressure that will form which can try to "suck" the aircaft back into it creating drag. In some cases these low pressure pockets cause the water vapor in the air to even evaporate:

Is this the reason it's more effective to move a lot of air slowly, vs a little bit of air a lot? To prevent the energy from being lost to heat conversion/ storing potential energy that your aircraft will not beneficent from as you already flew out of the area of the release?

If that is true would the same apply to water which is a non compressible liquid? Is this why we don't often see boats with "high aspect ratio" displacements of water?

No. In fact, compressibility does not really come into play until you come into ground effect or high Mach numbers.

Let's consider placing an object (why don't you try your (empty) coffee cup?) in mid-air. The moment you release it, it will rapidly accelerate downwards and eventually disassemble spontaneously. This is because air can move out of the way of the coffee cup. The only reason the cup experiences some resistance is due to the viscosity in air (very minor effect) and the inertia of the air that's in the way.

Lift generation relies almost solely on the inertia of the air. You accelerate air downwards, and due to the inertia of the air mass, you feel a force opposite of this, that is, upwards.

The reason high AR wings are more efficient at this process has to do with reducing induced drag. Specifically, it is more efficient to accelerate a large mass of air a little bit, than a little bit of air a lot. Copying from an earlier answer of mine,

  The force to keep the object aloft is $$F=m_{object}g$$ The force generated by downwards momentum transfer is $$F=\dot{m}v$$ with $\dot{m}$ indicating mass flow (kilogram per second) of the air (not the mass of the object). The energy flow (power) required to  impart this momentum on the airflow is $$ P=\frac{1}{2}\dot{m}v^2$$ Here we can draw an important conclusion. The power requirement is arbitrarily small, by increasing the mass flow and decreasing the downwards velocity.

This is the main reason for high AR wings: by increasing the wingspan (so, for equivalent surface, we increase the AR), we increase the amount of mass flow $\dot{m}$ affected by the wing.


Which ATC instructions should be repeated back by the pilot? AIM 4-4-7 (b) says:

  Pilots of airborne aircraft should read back those parts of ATC
  clearances and instructions containing altitude assignments, vectors,
  or runway assignments as a means of mutual verification.

As a student pilot soloing into class C airspace, on one flight ATC kept repeating everything back until I spoke it back to him. As I approached the airport he informed me of the bearing (12 o'clock) and distance to the airport until I acknowledged him (comms were pretty busy so I didn't initially repeat back his information).

Since then I've noticed that if I don't repeat back wake turbulence warnings verbatim they will repeat the warning until I do. With a traffic alert I repeat back immediately 'looking for traffic' because if I look first they'll keep repeating the warning.

Aside from AIM 4-4-7 is there any more specific guidance?

ATC is, broadly speaking, required to ensure that pilots hear, understand, and acknowledge information that is delivered.  The way they do that is via read-backs, and although the AIM mentions a few of the things that are required it's a good idea to read back anything that's important (If you're not sure, err on the side of "It's important"), with your aircraft callsign (No callsign? Doesn't count!).

What are some important things controllers usually expect you to read back?

Clearances (headings, vectors, altitudes, transponder codes, and radio frequencies).
Like the AIM says, mutual verification.
Runway assignments, hold short instructions, line-up-and-wait, and takeoff clearances.
Taxi routes Instructions, ESPECIALLY runway crossings & hold-short instructions.
Wake Turbulence advisories.
Traffic point-outs.
The magic words here, by the by, are "Negative Contact" and "Traffic In Sight".
I think if you say "Tally Ho!" you're required to buy everyone on the frequency a bottle of 30-year-old scotch.
Instructions to "Maintain visual separation with" some traffic
ATC is handing off collision avoidance responsibility to you as the pilot.
They need to hear "Will maintain visual separation with" the traffic in question.
They also need you to tell them if you lose visual contact.
Airport Point-Outs (Especially if there are several airports close to each other)
You can also pre-empt this by calling the controller: "Cessna 12345 has Podunk Municipal in sight", possibly with a bearing and approximate distance.
Landing Clearances
If you're in a busy pattern and they give you a sequence ("Number 3, cleared to land") they want to hear that part too.
That altimeter setting you get after a handoff.
(I blame Don Brown for getting me into this habit, but it IS a good one unless the frequency is crazy)

This list is by no means exhaustive, but it's most of the big stuff.


Why do I have to turn off my (non-cellular) iPad before takeoff? Every time I travel on an airplane, the stewardess forces me to shut down my iPad.
I just lock it, and start using it again 5 minutes later.

I wonder why this is? Is it because I have to have my hands free if the airplane crashes? Or is there any other logical explanation?

Actually, they don't!  Well, not always anyway.

The FAA recently issued a press release titled FAA to Allow Airlines to Expand Use of Personal Electronics.  This press release allows for the use of some electronics, including iPads and similar devices, during all phases of flight,.  However, it is up to each airline to change their procedures and to determine which devices can be safely used.

This is a small excerpt from the press release:

  Passengers will eventually be able to read e-books, play games, and
  watch videos on their devices during all phases of flight, with very
  limited exceptions. Electronic items, books and magazines, must be
  held or put in the seat back pocket during the actual takeoff and
  landing roll. Cell phones should be in airplane mode or with cellular
  service disabled – i.e., no signal bars displayed—and cannot be used
  for voice communications based on FCC regulations that prohibit any
  airborne calls using cell phones.    If your air carrier provides
  Wi-Fi service during flight, you may use those services.  You can also
  continue to use short-range Bluetooth accessories, like wireless
  keyboards.

I flew on Jet Blue the day after that press release and they were already allowing iPads and similar devices as long as they were in airplane mode.  Other airlines will follow soon I'm sure, but their manuals and procedures must be updated and approved by the FAA first.


What's the difference between CPM and LDM? I know CPM is cargo pallet message and LDM is load distribution message, but what's the difference between them ?

Purpose of the LDM is for the load-planner on the next leg of a multi-sector flight knows the figures for the next leg.

Example.. if the flight is routed SIN-DXB-LHR, the LDM from SIN will be broken up into 2 parts, one for SIN-DXB and and one for SIN-LHR. The loadmaster in DXB will use the data in the SIN-LHR part of the LDM telex for his load-sheet and then add in the load joining from DXB.

The CPM is used for aircraft where ULDs are loaded. This will give the loaders info as to which containers are for which destination and also which ULDs contain bags and which contain cargo/mails etc.

A CPM is mandatory for all sectors of ULD equipped aircrafts. An LDM is required for all sectors on all aircraft types except the last sector for an aircraft with ULDs as the CPM is sufficient. As aircraft with only bulk does not have any CPM, the LDM, while not required by most airlines, is still a way to inform the next station of what to expect on the incoming flight.


When buying airliners what is the difference between "orders", "options" and "purchase rights"? I've often seen announcements by aircraft manufacturers or operators along these lines "Qantas announced today that they will buy 10 A380's and have taken options on another 10".

What does it mean to "have an option"? I mean what level of contractual obligation exists between the manufacturer and the operator? Has a non-refundable deposit been paid - if so what sort of proportion of the eventual cost?

Does "having an option" mean much more than the manufacturer hopes you'll order those planes and has pencilled you in on the manufacturing schedule?

I also came across this on Virgin Atlantic's website:

  Virgin Atlantic announced in March last year that it was ordering 15
  of the 787-9 Dreamliners - with options on ordering another eight
  787-9s and purchase rights on a further 20 aircraft.

I've never heard of "purchase rights" so I'd be interested to know about the significance of "purchase rights" as well.

Manufacturers typically plan their aircraft production well into the future, which includes having a buyer lined up for each aircraft, so they will know their position in the production line and have an anticipated date for delivery.

An order means that the buyer has agreed to purchase a certain aircraft in the production line, at a certain price. The buyer may cancel, but there will probably be a substantial penalty for doing so.

An option means that the manufacturer has reserved a certain aircraft in the production line for the buyer, usually at a certain price. However, the buyer will make the decision on whether to convert this to an "order" at a later date. There is less commitment than with an "order".

A purchase option is even less of a commitment than an "option." This just means that the buyer has agreed to purchase aircraft at a certain price, but it's not tied to a specific aircraft at that time.

It is becoming more common for buyers to mix these three types of orders, which allows them to have a balance of locking in an aircraft and price with orders, and less risk and commitment with options and purchase options.

These can all be canceled, though of course there will be more penalties for breaking a more firm commitment. Buyers may also change their order, either for their own or the manufacturer's business reasons.

The detailed finances of how much is actually paid for aircraft, and how much it costs to make or break any of these contracts, is generally not disclosed. Negotiations can be very specific to each order. For a rough estimate, this book considers that for 6 aircraft worth 45 million per unit, options would be worth 2.3 million each to the airline, or about 5%. This book estimates the down payment on a firm order at 10 to 15 percent, but notes that at least in the case of business and commuter aircraft, depending on market conditions, this may not always be required.


Why do we say "climb" and not "ascend"? The term for moving to a lower altitude is "descending." Why, then, is the term for moving to a higher altitude "climbing" and not "ascending"? The latter goes better with the term "descending."

Does it have anything to do with "ascend" and "descend" sounding similar on the radio?

One of the most important goals of standard phraseology in radio communications is to ensure that words are distinct enough that a momentary garble doesn't ruin understanding of the transmission. (This is also why a phonetic alphabet is used -- Alpha, Sierra, Foxtrot, etc., rather than just saying the letters A, S, F, ... Imagine if ATC told you on voice something like "taxi to runway 7 via S, hold short F4" and now instead make it "taxi to runway 7 via Sierra, hold short Foxtrot four". Which is less likely to be misinterpreted?)

Another goal, of course, is to have a limited vocabulary of distinct terms that can be used to state things between people who may not be all that familiar with the language.

As you correctly note, "ascend" and "descend" sound very similar. If you for whatever reason miss out on the first syllable, they're outright identical! I'd call it a Bad Thing (tm) to have a single syllable potentially making the difference between just another day at the office, and a mid-air collision.

So when faced with the choice of whether to say "climb" or "ascend", in light of the fact that we're already using the term "descend", it makes sense to use a clearly distinct term for the opposite case to clearly differentiate the two. Hence "climb". The same argument can be applied to the case of both terms being selected together, without either having been selected before the other.

As an aside, "maintain" is also clearly distinct from both, further reducing the risk of confusion. If your altimeter reads 5,000 ft and are asked to "climb to 4,000 feet" or "maintain 6,500 feet" (as opposed to climb and maintain), then you know that someone needs a reality check, and can ask for clarification, even if that request for clarification is only along the lines of "say again assigned altitude and QNH".


Would there be a purpose for a set of compressors in an electric turbo fan? This is actually a two part question. Would there be a purpose to a compressor in an electric turbofan where there is no need for a combustion chamber, and if not, is there then a purpose for a bypass as well?

Sorry if I'm unclear, as i am not an industry professional perhaps my wording may not be clear. What I mean to say is that in a regular turbofan engine there are compressors that push air into a combustion chamber with higher density (presumably to get more potential energy per volume of air) where fuel is introduced and ignited to create higher pressure/kinetic energy to run the turbines,  which in turn move the front compressors and the fan. However, in an electrically driven fan, since there is no combustion happening, and the fan is driven by an electric motor, does the engine then need the compressors? Do they perform any benefit in a system that does not use combustion to create pressure?

Then if not, is there any benefit to the bypass system in an electrically driven fan engine? Or does it make more sense to simply switch to a ducted fan system?

No.  The only purpose of the compressor is to provide high pressure air for the combustion chamber and turbine.  If there is no combustion chamber and no turbine, then you do not need a compressor.  Just drive a big ducted fan (or propellor, your choice) with your electric motor.


How do the flight decks work in unison on the Stratolaunch? This article implies that the Stratolaunch has two flight decks. How are the dual flight decks arranged on the Stratolaunch aircraft?

I'm curious if things are fly by wire or if there are separate controls?
Are the flight decks both fully operational and mirror images of each other?
If there are duplicate switches and controls, how do the counterparts reflect changes initiated from the cross side?

On Stratolaunch Systems, there will only be one cockpit as mentioned here:

  The pilot, co-pilot and flight engineer will be in the right fuselage
  cockpit, while the left fuselage one is empty and unpressurized.

News can be misleading. This article's title says that each fuselage has its own cockpit but later, it clarifies:

  The left-hand fuselage has what looks like a cockpit with windows to
  onlookers, but the section is in fact empty and unpressurised.

It does look like there both fuselages have cockpits.

Image Source


What body creates legally binding aviation regulations in Europe? Currently trying to get general picture of aviation organisations in Europe. At what stage do regulations get the status of a law?

ICAO issues SARPs, they are certainly not a law. Then, on the basis of SARPs, EASA issues legislation. Still, EASA is not a regulatory body.

Consider PART FCL, for instance. Does it have the force of law, or is it just a guidance for the local CAA to issue a regulation, which will be considered as law only for that specific country?

EASA regulations,  as emited by the European Council, automatically become law in all EU member states (it is part of EU adheration treaty).

PART-FCL, for instance begins with this text:

  COMMISSION REGULATION (EU) No 1178/2011

  of 3 November 2011

  laying down technical requirements and administrative procedures
  related to civil aviation aircrew pursuant to Regulation (EC) No
  216/2008 of the European Parliament and of the Council

Because of the way these regulations are framed, local regulations (emited as law by local parliaments) will take precedence over regulations emited by the European Parliament, so some local authorities (I’m looking at you UK) will amend/change/adapt these regulations as they see fit.

Make no mistake, EASA regulations are law within the European Union’s member states.


How often is a tire replaced on a 777? Given that it is long haul aircraft, and is most widely used by airlines in that capacity:

How often do they replace the whole tire?
How much does 1 tire cost the airline?

Aircraft tires are replaced on condition, but maingear tires usually last about 250 cycles. A 777 in longhaul usage might only get two cycles a day, so a set of tires would last about four months.

The 777 uses a 50x20.0r22 with 32ply rating. I couldn't find that exact tire but a similar one costs $4700 new. It is common practice to retread or remold worn tires after inspecting the carcass to ensure its serviceability, and these cost about half of new.

Interestingly, this study by Embry-Riddle suggests that 99% of tire wear could be eliminated by prespinning the tires prior to touchdown, eliminating the puff of smoke you see as a part of the tire burns away as it matches up to relative runway speed. If correct, a tire would effectively last forever, until it was damaged as by debris or a hard landing.


Why does maneuvering speed vary with weight? Aircraft certified by the FAA under 14 CFR Part 23 have many V speeds, one of them which manufacturers are required to determine and publish in the POH is maneuvering speed, or Va.

  In aviation, the maneuvering speed (Va) of an aircraft is an airspeed limitation selected by the designer of the aircraft. At speeds close to, and faster than, the maneuvering speed, full deflection of any flight control surface should not be attempted because of the risk of damage to the aircraft structure.

This speed varies with the weight of the aircraft. Why does this speed vary with weight? Additionally since manufacturers often only provide that value at max gross weight, how does one figure out what that Va airspeed is when not at max gross weight?

At the normal stall speeds given by $V_s$, the load on the aircraft is 1g, and the lift is equal to weight. i.e $L = W$.

In case of maneuvering, the load factor is greater than one, and we have $L = nW$, with $n$ being the load factor.

We have,  $L = nW = \frac{1}{2}  C_L  \rho  V^2 S$.

This gives maneuvering speed , $V_a$ = $V_s \sqrt{n}$

It can also be written as $V_s$ = $\sqrt{\frac{2 n W}{\rho C_{Lmax} S}}$

At maximum weight, this gives,
$V_A$ = $\sqrt{\frac{2 n W_{max}}{\rho C_Lmax S}}$

For other weights, we have,
$V_a$ = $\sqrt{\frac{2 n W_{a}}{\rho C_Lmax S}}$

For the same load factor, we then have,
$V_{a} = V_{A} \sqrt{\frac{W_{a}}{W_{max}}}$,
where
$V_{a}$ is the maneuvering speed at weight $W_{a}$ and $V_{A}$ is the maneuvering speed at maximum weight $W_{max}$

As the maneuvering speed depends on the load factor and the stall speed, it depends on the weight of the aircraft (which decides the stall speed). It is basically a structural limit.

Another thing to note is that the aircraft can sustain structural failure even below maneuvering speed when multiple large control inputs are given.


Is there a source of information on the Aerodynamics of Non Rotating Round Disc Wings? Is there any published information on the aerodynamics of Fixed round disc wings like that used on the Nemeth Parasol?

https://disciplesofflight.com/nemeth-parasol-strange-aircraft/

All I've found so far is info on rotating discs like the Frisbee or Olympic discus.

Yes.

The first published results of measurements made by O. Flachsbart of the AVA Göttingen on a flat disk can be found in the collection Ergebnisse IV (PDF!). He reported on a hysteresis in the angle of attack range between 39° and 42°, where the wing is either stalled or not, depending from which side this angle of attack range is approached.

Lift and pitching moment coefficients of a flat disk, page 97 of Ergebnisse IV

When C. H. Zimmerman tested disk-shaped wings with an Clark Y airfoil section in the early 1930s in the NACA 7 x 10 ft wind tunnel, he called them circular airfoils. The results of this testing can be found in NACA Technical Note 539 (PDF!) from August 1935.

Figures 10 and 11 from the last page of NACA TN 539

Sighard Hörner collected the little data available on circular wings in his book Fluid Dynamic Drag (page 17-8) and concluded:

  Disregarding any dreams of "flying saucers", the athletic implement
  called discus seems to be the only practical application of a circular
  planform.

However, Zimmerman went on to inspire the Vought V-173, the "flying pancake" of 1942, from which the Vought XF5U was developed.

Vought V-173 in flight (picture source)


What are the reasons for the particular range of VHF and UHF frequencies? Generally, VHF frequency range is 30~300MHz and UHF is 300~3,000MHz. (according to Wikipedia). However, in ATC, VHF frequency range is 117.975-137MHz and UHF is 225MHz-400MHz.

Why is there a difference? And what are the reasons for these differences?

Short answer
The reason for a difference in ranges is historical. The figures from Wikipedia are the recommended ones (by ITU, the reference in radio spectrum matters), however the abbreviations VHF/UHF are also used in related contexts, often in an attempt to discriminate similar uses when being short of other means. In the current case, FAA likely wanted to give different names to different ranges:
118-137.0 MHz: VHF Air/Ground Communications
225-328.6 MHz: UHF Air/Ground Communications

This kind of approximation has actually no practical consequences, except adding confusion to a so much misunderstood field.
Classification of waves according to their frequency is just to determine ranges where waves behave similarly, e.g HF are known to be well reflected by ionosphere, while VHF are less sensitive to such reflection and propagate more along direct light of sight. However this is a progressive and slow change, crossing the border between HF and VHF is like crossing the equator: It has no particular effect (though for the latter, there are lots of good 'proofs' I'm wrong)
There are many examples of radio concepts used approximately, e.g. when we're talking about the 'FM band' to refer to the frequency range used to broadcast using 'wideband FM' (WBFM) modulation required to transmit the large bandwidth of a high quality stereo signal; this range has no particular properties and is not a 'band' and is indeed not technically more appropriate for using WBFM, except regulation forbid other uses.

Close frequencies behaves identically
While there are significant changes between 50 and 600MHz, there is none between 100MHz and 130, 220MHz and 300, or 320MHz and 340.
The only exceptions are not relevant to aviation. Sudden changes in behavior of waves with close frequencies are possible due to absorption of narrow isolated ranges by atmospheric components, e.g. water molecules are small antennas absorbing signals at their resonance frequencies, but resonance occurs at 22 GHz.
Significant differences in radio waves relate to frequencies which are remote
The most significant change in propagation is how much waves can be reflected by the atmosphere and the ground. When such reflections are possible, the communication range is spectacularly increased, as waves can be guided like in an optical fiber towards the receiver area. But again there is no big difference between frequencies belonging to adjacent classes, but near their common border. Broad modes of propagation:

Below 3MHz (a wavelength of 100m), transmission takes advantage of the ground wave (ground currents) to travel on long distances.

Between 3Mhz (100m) and 100MHz (3m), long distance links use the sky wave (reflection on ionosphere, during specific periods of time and according to solar activity, for more on that, see What is the night effect?). Current propagation predictions with close frequencies tending to propagate identically (thanks and credits: Paul L. Herrman et al $\tiny 73$):

Above 100MHz (3m), transmission strictly requires a line of sight and no obstacle in the 1st Fresnel ellipsoid which volume is dependent on the distance between stations, else significant losses occur.

These three first classes are perhaps the most fundamental, then additional divisions have been created above 100MHz (which at the time was already in the "very" high frequency class), because materials can behave significantly differently, e.g. at 150MHz, the only acceptable type of transmission line is coaxial, non-coaxial lines have too much losses, at 15GHz metallic lines are not usable anymore, due to skin effect, dielectric behavior, thermal noise, to name only the first reasons, hollow waveguides are used.
Why are there different classifications for radio waves?
Different segmentations of the whole electromagnetic spectrum have been proposed by different organizations, to focus on the differences that are important for them. These classifications have not to be particularly strict for the reasons explained above, and the limits in publications have varied.
The one Wikipedia is referring to uses segment frequency limits in "3" (3, 30, 300), these limits when expressed in wave lengths become 10m (30MHz), 1m (300MHz), 10 cm (3GHz), etc ($\small \lambda = c / f$).

Source
These limits are the ones recommended by the ITU in ITU-R  V.431, and they are the most commonly used. Not that there are bands without names (but all bands have a specific number).
Sometimes it is useful, or necessary to subdivide the spectrum into smaller chunks. For example TV broadcasting has operated different ranges of frequency spanning HF/VHF/UHF, transmitting at high power. However in the beginning it was not possible to build reliable powerful amplifiers at economical price, except at lower frequencies, so TV started in HF/VHF. As technology improved and higher frequency ranges became workable for amplifiers, new names were used to refer to frequency and associated electronics, exactly like cellular infrastructures and phones are known as 3G, 4G, 5G. We have all seen those names, when tuning the channels: Band I (broadly the "FM broadcast" band), II, III (174-230MHz), IV, V (582-960MHz).
Space communication engineers have their own classification of the SHF and upper bands as letters (L, S, C, Ku, Ka...). This split is well suited to describe the communication capabilities.

For example the US Orbiter communicated with a built-in S band antenna (2GHz), but could deploy a Ku band antenna (16GHz) from the payload bay when in orbit, to increase the transmission rate, e.g. for video streaming.

Radar people have also invented their own bands, for their specific use of the radio spectrum:

Source
And for many persons, waves are also known as short waves and long waves, categories which have been seen during years on radio receivers:

FM/SW/MW/LW on a legacy Grundig Satellit receiver
Likewise many engineers also use the term 'decametric' ('deca') to refer to MF/HF, or 'centimetric' and 'millimetric' for waves in the hyperfrequency domain. Those names have also been recognized by the ITU, though the mapping is somehow approximate.
Indeed aviation has had a need to create frequency segments.
VHF/UHF in aviation
"Aviation bands" include:
 - 108 - 112  MHz NAVAID (VOR, ILS Localizer)
 - 112 - 118  MHz NAVAID (VOR, LASS and SCAT-I))
 - 118 - 137   MHz VHF Air/Ground Communications
 - 62 -  174   MHz Fixed, mobile Communications

 - 225.0 - 328.6 MHz UHF Air/Ground Communications
 - 328.6 - 335.4 MHz NAVAID (ILS Glide Slope)
 - 335.4 - 399.9 MHz UHF Air/Ground Communications
 - 406.0 - 406.1 MHz Satellite Emergency Position Indicating Radio Bcn

Source: FAA order 6050.32 (page 29).
FAA has chosen to see the 225.0 - 328.6 MHz segment as being UHF, even if it starts before 300 MHz. This has no technical impact or implication, wave at 225MHz behave like these at 300, the actual difference from a technology standpoint is more between 150 and 350MHz.
Frequency allocation
In the US, the radio spectrum is allocated to users and final use according to this map (year 2016):

Source
In Europe, spectrum allocation is under ECC responsibility. Current status can be found in "ERC Report 25", aka the European Common Allocation (ECA) Table.


How many flight hours do the first retiring A380s have? I read news here that an A380 previously owned by Singapore Airlines has been decommissioned and scrapped. Another A380 from Air France was also retired. Others are nearing retirement as well. But it is not clear for me whether the retirement is due to the aircraft reaching the end of their design life or due to some other reason. If it is due to the design life, why are they still able to sell the scrapped parts (as done by the TARMAC)? Most comments I read are negative, which is sad.

Then my question, how many flight hours do these aircraft have? Is the retirement purely due to its design life being used up? Or for other reasons?

The A380 that was being scrapped was the 3rd aircraft on the production line. After being used for a short time in the test fleet of Airbus, it became the first line number that was delivered to a customer. (Line No. 1 and 2 are still owned by Airbus).

The first few aircraft leaving a new production line are usually heavier and have different wiring than later builds, because the production process has not yet been optimised. For example, the wiring in early A380 fuselage sections was too short, so that after assembling the fuselage they couldn't connect the electronic cables. This led to rework that added weight and complexity.

Therefore, the first few aircraft from the production line have higher fuel burn (due to extra weight) and higher maintenance cost (due to the one-off quirks that are not found on later aircraft). This makes such aircraft uneconomic to operate and difficult to sell.
If it is hard to make money with such an aircraft anyway (the long-range market clearly prefers twin engine aircraft with approx 300-350 passengers), scrapping it will bring the most value.

The aircraft was not at the end of its technical design life. The design life of long range aircraft is in the order of 30-40 thousand cycles (flights). With two flights per day it would take approx 45 years to reach that limit. See also  What is the lifespan of commercial airframes (in general)?

I wouldn't be surprised if a few other A380s with low production line numbers will be scrapped in the next few years. However, in general I expect that the A380 will be operated for another 20+ years,  especially on high volume routes to airports with limited capacity.


What is the average number of flight hours that US private pilots require before passing their checkride? It's easy to find comments and claims online (including on this site) about the average number of hours that's required for a new US private pilot to pass their checkride. Some are very precise - 67 hours seems popular - but no one appears to have any actual data to support their statements.

This comment is typical:

  We aren’t sure that 67 hours is the actual average. You can find as
  low as 55 and as high as 85 as the national average. A quick Google
  search found a dozen different answers to the average flight hours.
  None of the sources seemed authoritative. The FAA doesn’t seem to have
  a published number.

It's quite surprising (to me) that the average number of hours is so mysterious. Checkride applicants have to report their hours in the IACRA application process anyway, so presumably the FAA has the data. And it would be a very useful piece of information for schools and pilots to have for planning/budgeting purposes. In fact, I would expect part 141 schools in particular to have very detailed records on their students' hours, and perhaps even use them as a marketing tool ("get your private certificate in 10hrs less than the national average!").

But despite all that, there's no official, reliable number that I can find. So, what is the average number of flight hours from zero to passing the checkride?

Details:

Number of flight hours required to successfully pass the initial private ASEL checkride
Data from reliable sources only, i.e. FAA or a credible third-party study that identifies its sources and methodology
FAA private ASEL ab initio candidates only (no add-on ratings)
Part 61 and/or 141

This document from and published by the FAA does include a range although they do not cite their source or where they pulled the data from. It is as official a number as I can find.

  One difference between a part 141 school and a part 61 school is that
  fewer flight hours are required to qualify for a pilot certificate in
  a part 141 certificated school. The requirement for a private pilot
  certificate is 40 hours in a part 61 school and 35 hours in a part 141
  certificated school. This difference may be insignificant for a
  private pilot certificate because the national average indicates most
  pilots require 60 to 75 hours of flight training.

The range most likely varies heavily due to the way people train. I know people who have gotten it in <50 and people who are up around 90 and still have not flown the check ride. It varies largely with how much time you have to train as well as local weather. I was flying once a week (occasionally twice) during my training) and I was somewhere around 63 when I got my ticket.


What is the purpose of blades on a compressor? What is the point of having blades on a compressor. I heard on NASA that it alters the flow but how would this help. As the flow of a fluid increases, the pressure decreases, and vice versa. The rotors increase the speed of the air and descreases the pressure and the stators decrease the speed of the air and increase the pressure. How would this help create higher pressure. Why not just have the rotors? I know that the ducts this whole thing goes in gets smaller as it goes on, does this play a role? Don't get all complicated here on me, I'm still learning about airplanes and flying. Try to be as simple as possible, thank you.

The statement about the rotor blades ("the rotors increase the speed of the air and descrease the pressure") is not correct, because Bernoulli's Equation does not apply in this situation. Hence, the statement "as the flow of a fluid increases, the pressure decreases, and vice versa" isn't appropriate for the rotating compresser blades.

Bernouilli's equation (below) assumes no energy is being added or removed from the fluid. This is true when the fluid is flowing through the stationary stator vanes, but not the rotating blades.

The rotating blades increase the velocity, but do not decrease the static pressure, becuase they are adding mechanical work (energy) to the fluid. This energy (or power) comes from the turbine.

When no energy is being added to the fluid:

$$
total\space pressure = static\space pressure + 1/2 \rho v^2
$$

where
$\rho$ is the density and $v$ is the velocity.

This is the mathematical equivalent of your statement "as the flow of the fluid increase, the static pressure decreases".

But, the rotating blades add total energy. So this equation does not apply, so as the velocity increases, static pressure does not go down. But, total energy has increased, by the mechanical work done by the blades. So the value on the left of the equation goes up as the fluid flows through the rotating blades. Next, the fluid flows through the stationary stator vanes. Because they are static, they add no energy, so Bernoulli's equation now does apply. Hence, as the velocity is decreased, the static pressure goes up (i.e. there is no change in total pressure).

If you just have the rotor blades, and no stator vanes, the compressor would just make the fluid move (i.e an increase in the total pressure) but with no increase in static pressure. The stator vanes slow the fluid down, and convert the velocity into an increase in static pressure.

You need to understand the difference between total and static pressures.


Nose gear failure in single prop aircraft: belly landing or nose landing? Let's say we are flying a single prop airplane with tricycle format retractable gear, such as a Mooney Bravo or a Piper Comache. The nose gear fails mid-flight and is unable to be lowered. Is it safer to attempt a belly landing, or to actually try and make use of the functional main landing gear? What is the standard procedure?

Edits and updates:

Here is the video of a very smooth looking nose landing with a RCMP Pilatus PC-12 I could find
Here is the video of a smooth looking belly landing find I could find

My major concern with the nose landing would be the increased risk of the nose digging in, causing a flip and/or the fuselage to break in half. As pointed out by @acpilot, the probability of this increases according to runway type.

The standard procedure is whatever is in the POH for your aircraft. This is from a C182RG POH:

  LANDING WITH A DEFECTIVE NOSE GEAR (Or Flat Nose Tire)

  Movable Load -- TRANSFER to baggage area.
  Passenger -- MOVE to rear seat.
  Before Landing Checklist -- COMPLETE.
  Runway -- HARD SURFACE or SMOOTH SOD
  Wing Flaps -- 40°
  Cabin Doors -- UNLATCH PRIOR TO TOUCHDOWN.
  Avionics Power and Master Switches -- OFF when landing is assured.
  Land -- SLIGHTLY TAIL LOW.
  Mixture -- IDLE CUT-OFF.
  Ignition Switch -- OFF.
  Fuel Selector Valve -- OFF.
  Elevator Control -- HOLD NOSE OFF GROUND as long as possible.
  Airplane -- EVACUATE as soon as it stops.

That assumes that you've already run the gear extension failure checklist without success, of course.


Where to find general A320 E&E Compartment Information? I'm student in electrical engineering and write my thesis on some topic of the e&e compartment. The main focus is on the computers of the A320 flightcraft, actually called E&E Compartment. Sadly because of closed source of this avionics part there is no general information of what it is. This is also clear why, most infos can only accessed by technician because of several security reasons. But maybe there is still some general descriptions and pictures that shows for example the location of e&e and text like: e&e has distributed system architecture where each computer has specific role, they are all connected by arinc... blabla.. and similiar and so on :)

It can be all, books, patents, googling keywords.. It would all help!

Thank you!

The term E&E is used only in Boeings aircraft. For Airbus try to Google "avionics compartment" as this is the correct term.

This specific data you Will find in wiring diagram manual and schematic diagram manual, sadly due the complexity of the systems you won't find only one file for all systems.


How and how often are the oxygen masks in the cabin checked? Looking at this old answer I can see that oxygen masks check is mandatory for the flight deck. What about passenger masks?

Is this check done daily on airliners?
How is this check conducted and what is actually being checked?

There is a maintenance schedule to test the mechanism that causes the passenger masks to deploy(drop down). It is usually performed each 12 months. During this test, the masks themselves are inspected, any discrepancies found dealt with, and re-stowed in the little overhead box until the next scheduled inspection.


Does an IFR flight at night in VMC count as “night VFR conditions” (FAR 61.129)? FAR 61.129 several times uses the phrase “night VFR conditions.” Does this mean strictly flights conducted under VFR at night, or does it include night flights conducted under IFR but in VMC?

"Night VFR conditions" describes the flight conditions, not the flight rules. The PCG says:

  VFR CONDITIONS− Weather conditions equal to or better than the minimum for flight under visual flight rules.

If the requirement was to conduct the flight under VFR, the regulation would say "under VFR". For example, 91.185(b) shows that the FAA sometimes makes that distinction (emphasis mine):

  If the failure occurs in VFR conditions, or if VFR conditions are
  encountered after the failure, each pilot shall continue the flight
  under VFR and land as soon as practicable.

So you can do the flights under IFR in VMC if you like.


Need more volume with my aviation headset (David Clark OneX). I wear hearing aids Does anyone know of a way to get more volume out of my David ClarK ANR aviation headset.  I wear hearing aids and often fly in older airplanes with older radios that are scratch/weak/etc.  I select full volume but still have trouble hearing the communications.  Is there any inline booster or something like that?  Thanks....larry

This is a decent article to take a look at but you may want to consider the clarity aloft headsets which use hearing aid speakers and seem to be able to serve as a nice drop in replacement.

  I have a hearing loss. Can I use this headset with my hearing aid?

  Because our headset uses high quality hearing aid speakers, the pilots
  who have hearing impairment just remove their existing hearing aids
  before putting on our headset. A balance control can be adjusted to
  favor one ear with louder sound. We have many pilots who have a range
  of hearing problems from mild to moderate.


What are U-2 loiter and stall speeds at low altitude? I've read that a U-2's takeoff speed is around 115 mph (or kts?). Given those glider wings, how slow could one fly, lightly loaded, at low altitude? Say, less than 10,000 feet?

A copy of the flight manual was released under a FOIA request and can be found here. It actually does not say a whole lot about stalls but it does reference some minimum speeds in the emergency engine out procedure section under "COMPLETE POWER FAILURE IMMEDIATELY AFTER TAKEOFF"

Maintain a safe flying speed and control until contact is made

NOTE: The following minimum safe speeds (10kts above stall) are recommended for the accompanying take off fuel loads
...
Fuel  |      Min Speed
...
685 | 90

So at ~runway altitude lightly loaded the plane could fly as low as 90Kts which is 10 Kts above the stall speed which is in line with other answers here. (Operations Manual Page: 3-3). At full fuel the stall speed rose to ~105Kts.
The manual devotes a whopping 2 paragraphs to stalls and lists no performance figures for them in section 6.
Strangely (or perhaps not so strangely) the the document lists an "Appendix I" which should contain the Operating Data (performance figures) but is absent from the document and regrettably where you would find the true answer.
Keep in mind this is the Handbook from March 1st 1959 and there have been several variants since so the numbers may vary.
Note the takeoff speed for a lightly loaded U2 was indeed 95-100Kts (Operations Manual Page: 2-12)


Clarification on flight hours as compensation I've seen the phrase "flight hours as compensation" before, and would like some clarification on it. What I've read seems to generally concern operating an aircraft on a PPL for a commercial purpose, basically meaning if you fly an aircraft and don't cover "your share" of the aircraft expenses, then recording your flight time may be considered compensation by the FAA. What I'm wondering is, does it stop being considered compensation after you get a commercial license? If I have a CPL and fly for hire, paying no expenses of the flight, are my flight hours considered compensation in addition to my wage? Do I have a legal right to those hours? Is a company allowed to, as a condition of employment, require that I don't log hours while flying their aircraft? If a company suddenly and without reason sticks me behind a desk instead of having me piloting an aircraft,  would the loss of flight hours be considered a pay cut?

There are a few different questions in here, some are FAA regulated and some are a matter of the terms of your employment

  If I have a CPL and fly for hire, paying no expenses of the flight,
  are my flight hours considered compensation in addition to my wage?

Maybe, its somewhat up to your employer, but more importantly its not really what the FAA cares about. You have a CPL and are presumably flying for a legal carrier/operation. In the eyes of the FAA they don't much care how you are paid beyond that. Practically the legislation is more to ensure that people who are getting paid are properly licensed and working for a proper operator.

  Do I have a legal right to those hours?

As Juan notes if you were PIC (or SIC) properly certified in the aircraft then the time is yours.

  Is a company allowed to, as a condition of employment, require that I
  don't log hours while flying their aircraft?

That seems like it would be encouraging you to not follow a FAR which is not a great idea.

  If a company suddenly and without reason sticks me behind a desk
  instead of having me piloting an aircraft, would the loss of flight
  hours be considered a pay cut?

This depends two things, your existing contract with your employer, and if that contract changes when your job changes. Often times with a role change your contract, or the terms of the original undersigned contract can be changed. If you were a pilot with a Minimum Monthly Guarantee in your contract you may be entitled to keeping your pay-rate. However if the new job comes with a contract change or shifts where you are under your current contract you may not be entitled to anything.


Does a multi-engine rating propagate down from commercial to private? I have a private pilot license with airplane single engine land and instrument ratings. I'm about to get the commercial with a airplane multi engine land class rating.

Let's say I'm not currently flying for hire and my second class medical is more than a year old, so I cannot exercise my commercial privileges. Can I then legally fly a multi-engine plane exercising my private privileges, carrying, say, a friend or other non-paying passengers in a part-91 operation?

In other words, does the class-rating propagate down to my private?

Note Part 61.31(c):

  (c) Aircraft category, class, and type ratings: Limitations on the
  carriage of persons, or operating for compensation or hire. Unless a
  person holds a category, class, and type rating (if a class and type
  rating is required) that applies to the aircraft, that person may not
  act as pilot in command of an aircraft that is carrying another
  person, or is operated for compensation or hire. That person also may
  not act as pilot in command of that aircraft for compensation or hire.

Since you will have the Category and Class rating, then you may carry another person.

Your old license says:

Private Pilot
Airplane Single-engine Land
Instrument Airplane

Your new license will say:

Commercial Pilot
Airplane Multi-engine Land
Instrument Airplane

Private Privileges
Airplane Single-engine Land

You may fly under part 91 ASEL, AMEL, and IFR

You may fly part 119 or 135 (or other commercial operations) AMEL and IFR1. You may NOT fly part 119 or 135 or any commercial operations ASEL. If you want to fly ASEL in a commercial operation, you will need to upgrade your ASEL by passing another commercial checkride in an ASEL.

This is the back of my certificate. I took the ASES checkride when I had a Private certificate.

1. Assuming you have the correct medical and any other checkouts you may need


What does it mean when ATC says climb maintain 7000, block 8000? I understand the climb and maintain, but what does block mean?

It means you've been allocated a block altitude - you can fly whatever altitude you want between 7000 ft and 8000 ft.

Typically this would be something you'd request, e.g. to practice unusual attitude recovery or to avoid having to hold a hard altitude in mountain wave.  It's unusual for ATC to volunteer a block altitude without the pilot asking for it, but maybe if previous aircraft along the same route reported difficulty maintaining altitude then ATC might offer it to you unprompted.


What is the difference between turbojet and turbofan engines? What is the difference between a turbojet engine and a turbofan engine?

I know that both of them have a fan and have turbines but how to distinguish them?

Turbojets and turbofans are very similar indeed:

both are turbine engines;
both create thrust from jet exhaust;
and both have a rotating implement in front that can be called a fan. Although in the case of the turbojet, it isn't called a fan but the compressor first stage.

$\ $ Junkers Jumo 109-004

So what is the difference? There are five types of turbine engines, named after their main means of generating thrust:

Turbojets. The very first type of jet engine as used in the Messerschmitt 262: a compressor, combustion chamber, and a turbine. The turbine extracts enough power from the combustion chases to drive the compressor. All air flows through the engine itself, and thrust is generated by the exhaust flow only. Bypass ratio zero.
Low Bypass Turbine Engines. A bit of a lengthy, non-catchy description. Part of the airflow bypasses the main engine, and is compressed by the first stage of the compressor which has a larger diameter than the main engine tube. The turbine drives the compressor and the (low bypass) fan, exhaust flow from the turbine still generates a sizeable proportion of the thrust - inversely proportional to the bypass ratio, which is 0 < BPR < about 2.
Turbofans. Same as above, named after the large visible fan-like first stage. These engines are high bypass turbine, 5 < BPR < 15 and ever increasing.
Turboprops. Main source of thrust is a propeller, which has longer, fewer blades than a fan to limit drag penalties. The turbine drives the compressor and the propeller, some exhaust flow is still used for thrust generation - according to Torenbeek section 4.3.5, about 5 - 10% of the thrust is generated by the . BPR up to about 50.
Turboshaft. Main thrust source is a rotor or other torque requiring device, like an electrical generator in an APU. All net turbine power is applied to the drive shaft, no practical thrust from the exhaust flow.

The five types incrementally use bypass airflow to generate thrust. Increasing bypass air allows for accelerating a larger mass of air at a lower speed, generating the required thrust at a higher efficiency. But bypass air volume is inversely proportional to airspeed: the faster the aircraft, the lower the amount of bypass air that can be utilised. At supersonic speeds, bypass air is very problematic for thrust generation.

The picture below shows the Olympus engine used for propelling Concorde, a pure turbojet with 2 axes and 7 compressor stages per axis.

The GE CF6 with BPR of about 5 depicted below was used for B747, A300 and DC10, and is a turbofan: the first stage of the compressor sticks out over the remaining stages, and bypasses most of the air inflow outside of the main turbojet engine. This bypass air is the main distinguishing feature between turbojet and turbofan. Visually, the large fan is very distinguishable of course.


What are these accelerating flare countermeasures called? There seems to be a type of flare countermeasure, that not only gets ejected from the flare dispenser, but also gets accelerated in the direction of the plane. So fare I´ve only noticed it on super and legacy Hornets.
My question is, whether anybody knows the actual name for those kinds of flares, and if there is any kind of information on how they work.

Video of legacy Hornet dispensing a normal and an accelerating flare

On the left Hornet you can see one of the two flares accelerating forward in a spiral after it is out of the dispenser.

Video of super Hornet dispensing a normal and an accelerating flare

These are generally called "propelled flares", or sometimes "aerodynamic propelled flares". Some antiaircraft systems are able to distinguish between a normal flare and the aircraft, as the flare is not travelling along the expected path of the aircraft. Aerodynamic flares were first developed to allow the flare to continue along the path, but as antiaircraft systems developed further, they added a method of propulsion (which uses the same fuel source as the flare itself), to ensure that the flare continued alongside the aircraft, slightly angled away, at a similar speed. This does a better job of confusing these aircraft systems, but it's a fairly "niche" type of flare that is meant to combat conical scanning.

I believe the ARM-031, MJU-47/B, and MJU-71/B are all forms of propelled flares.


"Total time away from gate", "Longest time away from gate" - what does this mean? In the US, airlines or airports report two figures regarding canceled flights:

  the total time away from the gate and the single longest time away from the gate.

and regarding diverted flights:

  the total time on the tarmac at the diversion airport and the longest single time on the tarmac.

I don't understand this distinction, nor the individuall phrases.

What is a "single time"? A single time period out of multiple ones?
Why would a plane be away from the gate multiple times? i.e. leave the gate, return to the/a gate, leave again and return again?
Why would a diverted plane be on the tarmac multiple times at the same airport?

The US regulations have the tarmac delay rules, which apply to extended delays where passengers are left sitting on the tarmac. This followed a variety of incidents where passengers were kept on board non-moving aircraft for long periods of time, sometimes without access to food and water. The rules are a bit complicated with various exceptions, but broadly:

  How long can an airline keep me on a DEPARTING flight before the
  airline is required to start moving the airplane to a location where
  passengers can safely get off?

  For flights departing from a U.S. airport, airlines are required to begin to move the airplane to a location where passengers can
  safely get off before 3 hours for domestic flights and 4 hours for
  international flights.

So yes, a flight can potentially have multiple tarmac delays. For example, a flight could potentially leave the gate on time, sit on the tarmac for 2.5 hours, return to the gate (to allow passengers a chance to get off, as is required), leave the gate, wait another hour, be cancelled, and return to the gate. The airline would report both all the time the plane was away from the gate (2.5 + 1 = 3.5 hours) and the longest period the plane was away from the gate (2.5 hours).

  What is a "single time"? A single time period out of multiple ones?

Yes. If the flight sits waiting on the tarmac for more than one period, the "single time" is whichever period was the longest.

  Why would a plane be away from the gate multiple times? i.e. leave the
  gate, return to the/a gate, leave again and return again?

It could happen for a number of reasons. One would be that the plane returned to the gate to comply with the rule and allow passengers an opportunity to get off after 3-4 hours delay. The plane might return to the gate for a maintenance issue. They could be waiting out bad weather or a situation where an airport is not accepting flights. Most airlines seek to avoid this, but it can happen, and the reporting requirement is intended to capture data about how long it takes when it does.

  Why would a diverted plane be on the tarmac multiple times at the same airport?

The same reasons. I had this happen once due to a tornado. The flight diverted, we went to a gate, we went back to the tarmac to wait for better weather, the flight was cancelled, so we went back to a gate again. It's an unusual situation, but it can happen.


Does the captain always control thrust on takeoff? I just read an article about a day in the life of a pilot and it states "The Captain always controls the thrust in the unlikely event that the take-off needs to be aborted for safety reasons." Is this correct?

It was always my understanding that the roles of PF and PNF can be alternated between the captain and first officer, and controlling the thrust would fall into one role or the other. I also recall seeing videos of the flight engineer pushing the throttles.

If the take-off needs to be aborted, can't the captain just command the FO to "abort" or take control by saying "I have control" or similar?

Finally, will pulling back the throttles be enough to abort takeoff safely without braking? Surely it would just cause more problems?

I believe your referenced material is mixing up "Captain" with "Pilot Flying". The person in control of the aircraft should have their hands on the throttle, Captain or not.

  If the take-off needs to be aborted, can't the captain just command the FO to "abort" or take control by saying "I have control" or similar?

Having your hands on the throttles has a number of advantages:

You don't have to waste seconds (which at 200mph means about 300 feet per second)
You don't have any confusion (i.e. "What did you say?")
You don't need to discuss it with the other person (you say abort, FO says OK)

A lot of companies require both the PF and PNF's hands to be on the throttle at the same time, which has the advantages of 1) assuring that the throttle is set to its maximum power, and 2) either one can abort without having to command the other to do it. Either pilot (or the FE if they exist) have the ability to call an abort before V1, and the entire cockpit crew should follow that course without discussion until clear.

  I also recall seeing videos of the flight engineer pushing the throttles.

There are also cases where if a FE is required (which usually is not the case anymore), that the FE also has their hands on the throttles along with the pilot/copilot.

  Finally, will pulling back the throttles be enough to abort takeoff safely without braking? Surely it would just cause more problems?

Depending on how fast you are going, it is almost always the case that you will have to brake as well. Airplanes are pretty slick at rolling down the runway and relying on drag alone to stop you will probably result in an overrun. Usually the PF will be the one on the brakes, but the PNF may also be applying pressure just in case.


What is the difference between component service and inspection? What is the difference between component inspection and component service? And is overhaul / restoration a service or an inspection?

Servicing is lubrication or replenishment as required for a component to continue in service - greasing, adding oil etc.  Inspection is verification that condition of a component is acceptable, either visually, or using an enhancement method like Xray, ultrasonic, or various non-destructive test methods.

Overhaul/Restoration is disassembly, inspection and replacement of internal parts to bring the unit to some minimum dimensional/performance standard.

The term Restoration is generally associated with MSG-3 based maintenance programs and is something like overhaul but often is restricted to specific components identified in endurance testing, or even just something like a grease charge that requires disassembly to remove/replace, again based on endurance testing.

A restoration doesn't "zero time" a unit; it's more of a mid-life refresh.  MSG-3 based parts can be run forever theoretically, because the whole idea is you run the component until periodic functional testing in the field detects a performance degradation, but parts usually also have a structural life limit imposed on a root component like a housing, based on endurance/fatigue testing.  Once the root component reaches its life limit, the part is scrap.  That clock does not get reset by rebuilding the unit.


Do modern Terrain awareness and warning system (TAWS) have blind spots? Do Terrain awareness and warning system (TAWS) have blind spots when the helicopter is in unknown territory with unpredicted terrain or the TAWS systems coming nowadays have each and every terrain on earth?

This answer is only applicable to civil airline type operations. Military systems do have significantly different requirements which I cannot address.

Quick Answer:

No, they don't have "blind spots" in the sense of areas with no data available.

A TAWS system works differently:

In the airline I've worked for, I was "exposed" to Honeywell Mark V EGPWS systems, but I assume that other systems are similar. These systems have a world-wide terrain database. That database and the algorithms take a normal terrain clearance into account as well as the performance of the aircraft.

"Holes" or "Corridors" are "cut" into this "terrain clearance carpet" to avoid nuisance warnings when an aircraft needs to descent below the normal margins. I.e. during landing and takeoff.

So, these aircraft can safely operate world-wide unless there are new mountains growing (which actually happens with volcanos) or large man-made obstacles are built.

As with all databases on board of an aircraft, they are updated on a regular basis.

Except for this link, I was not able to find an awful lot of information from manufacturers.


Calculate resultant force from pressure distribution? Can you explain with example how can I calculate magnitude and direction of resultant force from pressure distribution over airfoil?

I don't know exactly you maths background, so i'll start using it for the sake of expressing it generally, and in the second part i'll try to simplify things.
Making use of the definition of pressure p as force F per Area A.
Mathematically, the shortest answer would be to integrate the (local) pressure over the surface, including the normal vector:
$\vec F = \int_{A} p* \vec n dA$
some notes on the symbols:
pressure p is not directional, it is defined as pressing perpendicularly on the surface. Using $\vec n dA$ as "directionalized" surface area element, the normal vector $\vec n$ perpendicular to the surface would take care of the direction of in which the pressure is exercised. Note that the normal vector is "normalized" to the length of 1, so to preserve the pressure.
F as resulting force has a direction, making it a vector.
In real life (as in your diagrams) the distributions usually aren't known analytically, but just as a table of values. So you would discretize the above surface integral into segments of finite area, and then calculate the forces for each element, and at the end sum everyting up.
So I tried running a basic example: airfoil discretized in 5 segments (i through v), suction side pressure = 1bar (abs), pressure side 2 bar(abs),
ran the calculation "p * n * deltaA " and the summation in the Table below.
The force units are "kPond" = 10 N, as result of choosing pressures in bar and surface area in cm^2.

Interpretation of the Result: We get a aero force of
10N in positive x direction: 10N of Drag, and
50N in positive y direction: 50N of Lift.
Total aero Force is 50.99N in the direction of the vector [1,5]
Additional notes:
If you want to get more precise:
the finer the discretization, the more precise your result will be, or you can increase the order of your numerical integration..
https://en.wikipedia.org/wiki/Numerical_integration
hope that helps! have fun integrating..


How can an F-22 Raptor reach supersonic speeds without having supersonic inlets? The F-22 Raptor and some other jets do not have inlet spikes, yet they are able to achieve supersonic speeds. How is that possible?

The jet uses a fixed, rhombohedral shaped jet intake combined with a serpentine intake duct.  It allows for speeds just north of Mach 2 without the need for a variable geometry inlet to prevent shockwave ingestion.


What about hours flown solo without a student pilot certificate? My instructor has ruined my training. 2.5 months ago when I started private pilot training (FAR61) I had zero knowledge about aviation.  After 1.5 months he signed me off for solo and I finished all the requirements and we sat together to register through IACRA for my checkride, and guess what? He had forgotten to tell me I needed a student pilot certificate before I go soloing.

Besides the risk he made on my life ( imagine I crashed during solo and insurance definitely would deny any claims since I was not legally allowed to fly! ) and that I will be grounded for weeks until I get student pilot certificate, I don’t know what will happen to those solo hours ?

Do I have to re-do the whole solo requirements or they will still count towards my hours?
Do I need to file a complaint through AOPA or FAA or contact my AOPA representative?
What are the rules and who is responsible for that?

Do I have to re-do the whole solo requirements or they will still count towards my hours?

You will probably not have to re-do the hours. An FAA examiner will look at your log, test you and decide whether to give you a license. It is unlikely that they would notice that your solo time was done before the student license was issued and then disallow the hours. If they did notice and disallowed the hours, you could appeal that decision.

As long as a CFI certified you for solo flight, the hours you did should count.

Do I need to file a complaint through AOPA or FAA or contact my AOPA representative?

No.

What are the rules and who is responsible for that?

It's your job as a pilot to know the regulations and follow them, including requirements such as obtaining student licenses and medicals. The CFI is not your nanny. It is YOUR responsibility to know the rules and follow them.


Do aircraft obtain better milage by flying high or by using "ground effect?" It has been said the most fuel efficient way to fly is by flying in thinner air. On the other hand the largest plane in the world was the Russian "Caspian Sea Monster" Ekranoplan, which is said to be the most efficient plane for its weight.

Which method of flying is the most efficient and by how much when it come to time and fuel?

There really isn’t any strict single rule on whether an aircraft should fly high or use ground effect to increase endurance. It all depends on the design requirements and the purpose of the aircraft.
For cargo type aircrafts, it would make sense to use ground effect and fly over the sea, since speed is not really a major concern.
For reconnaissance aircraft, using ground effect is obviously impossible. For that they have to fly high and the optimum altitude would depend on aircraft parameters, and be a strong function of weight and possible induced drag.


What are the mechanical deflection angles for Airbus side-stick controllers? Does anyone know the angular deflection limits of the longitudinal (pitch) and lateral (roll) axis of Airbus's side-stick?

From this site:

So max deflection in pitch is +/- 16 deg, in roll it is +/- 20 deg


Can I get a FAA medical certificate and apply in an Indian airline? I am an Indian, and was wondering if I could get an FAA medical certificate and apply in an Indian based airline.

Is there a problem if I do that? Should I get the DGCA medical certificate itself?

Well Air India is hiring qualified pilots here. The advertisement says you need a medical certificate issued by the DGCA. There is no mention of accepting an FAA certificate.

I'm not completely sure how it works in India but usually you need to get a medical certificate from the country in which the airline is based (or to be specific, the country in which the aircraft is registered). I'm not aware of any exceptions to this rule except for temporary operations, but I stand to be corrected.


How do tyre pressure sensors in aircraft work? I am specifically interested in the sensors used on the B777 and B787.

On a Boeing 777, a tire pressure sensor/fill valve is installed on each of the 12 main gear wheels and two nose gear wheels. There is an in-axle transformer in each of the axles.

The tire pressure sensor/fill valve is made of an air fill valve and three independent pressure sensing channels which provide accurate tire pressure.

The tire pressure sensor/fill valve is installed in the wheel.

A relay transformer hubcap assembly connects to the tire pressure sensor/fill valve. The left 28V DC MAIN bus supplies the electrical power to operate the tire pressure indication system.

(Image source: 777 AMM)

You can see the sensor in these images:

(Source: airliners.net)

(Source)


Does the direction of rotation of the propeller affect the direction of the thrust generated? Does changing the direction of rotation of the propeller from clockwise to anti-clockwise change the direction of thrust it generates?

It depends.

For that simple kind of a fan with flat blades, yes. For anything that was ever installed on a full scale airplane, it will generate no or very, very little thrust, more likely backward.

The blades of a propeller are airfoils—like wings—mounted with suitable angle of incidence and usually slightly cambered. Any efficient airfoil is necessarily designed for one direction. If you move them backward—by turning the propeller the other way than it's designed for—they will generate very little lift. And the remaining bit may be cancelled by the camber, because the incidence will be for generating thrust backward, but the camber will still favor forward thrust.
For much more details, see
Is there an aerofoil that gives reasonably good lift for both air flow directions: forward and backward?

Only flat plates do generate lift both ways, so a simple fan with flat blades would indeed generate backward thrust. But it's not efficient in either direction in the first place, so it may be appropriate as a ceiling fan, but definitely not as aircraft propeller.


Why aren't large, low-speed propellers widely used? I am trying to calculate different propellers for the simple case like helicopter moving slow up like 1 m/s or staying/hanging in one place.

From generic physics point of view it is more efficient to push back more mass with low speed, then to push back less mass with higher speed.

Efficiency (Kg/Watt) I mean how many thrust in kg I will get from 1 Watt of power.

I did calculation using Prop selector software as shown in screenshot below big propeller 4 meters with only 32 RPM procude 1kg of thrust and power is just 13 Watts. Speed of the end of propeller will be 6.7 m/s.

Small propeller 27 cm with 7000RPM with the same 1 kg thrust needs 176 Watts of power. Speed of the end of propeller will be 99 m/s. 13 times more power!

Red thrust in the screenshot means "propeller is stalled", but maybe this "stalled" does not make sence for static case, when propeller is not moving.

My question is Why this type of propeller (big diameter, but low speed) is not in use? Is it just practical consideration like big field will be needed if I make very big propeller, or my calculation is wrong or something else?

You are not wrong, it is more efficient to accelerate a large mass by a little than a small mass by a lot.

This is due to momentum being linear with speed and mass, while energy is linear with mass but quadratic with speed, so the same momentum can be obtained more efficiently by slowly pushing a large amount of air, e.g. with a large propeller.

The reasons against this are as you imagined, like clearance from the ground and other parked craft. Also, extremely long propeller blades will suffer from high inertia and bending moments, without the benefit of centrifugal stiffness. And then there is the issue of keeping the propeller tips subsonic, to avoid wasting a lot of energy in the form of sound.


Does every control surface need to have a counterweight? Is it not a good idea to balance all control surfaces, no matter what they weigh?

I am building a small plane, with small control surfaces. There are many of the same type of plane flying. Most have no type of counterweight "balance" on the elevator, or rudder. I am told that is because the surfaces are small and light.

Flutter is an energy storage and release phenomenon.  Surface moves, applies a load to the fixed surface it's attached to, fixed surface bends under the load, stores some of the energy like a spring, then releases it in moving back to its original position, and some of this released energy is transferred to the moving surface attached to it, which is also stored and released back into the fixed surface in a way that adds more total energy than the first time, magnifying the original movement, repeat, and away you go.

For the this to happen the moving surface has to be unbalanced with a fair amount of mass away from the hinge line, the fixed surface has to be flexible (spring like), the whole system has to have a natural frequency that is close to the frequency of the natural oscillation frequency of the moving surface, and there has to be enough energy present in the first place to get the whole thing going.

Generally airplanes that go less than say 80-100 mph don't have the energy to induce flutter if there is reasonable stiffness in fixed surface like the horizontal stab.  Light airplanes with fabric covered wire braced tails have stabilizers that are quite stiff due to the wire bracing, and the combination of high natural frequency resulting from the rigid system and the low input energy at the speeds they fly at means they don't really need a balanced surface (look at a Champ's unbalanced elevator; the C of G of the surface is probably at 30% of chord - no big deal because it simply can't excite sympathetic movement in the stabilizer it's attached to at the energy level present at its operating speed range).

If the fixed surface is not so stiff, the moving surface is really heavy, or you want go faster, then you start having to consider mass balancing.  To really know for sure what the threshold is, you'd have to have someone that knows what they are doing do the analysis. But generally low speed light airplanes have the inherent rigidity to be immune from flutter in their speed range without balancing control surfaces.

You still have to confirm it in the real world though.  When you test fly your creation, you do some rudimentary flutter testing by diving to a speed a bit above your Vne (actually creeping up to it in a series of steps), pull up so you are decelerating, and as you pass the speed of your test point you slap the control stick sharply, trying to induce an oscillation.  By doing it while decelerating, the risks of an oscillation starting and building into full fledged flutter are low. But, if you see an oscillation that starts but then goes away, well you've "tickled the tiger's tail" and you'll need to balance the surface or reduce the airplane's speed range.  You're wearing a parachute while doing this of course (or whomever you're able to talk into doing your test flying).


Why are yaw rate and pitch rate obtained by these equations? I am reading "Introduction to Aircraft Aeroelasticity and Loads" by Jan R. Wright and Jonathan E. Cooper.

The book said: In steady bank turn (as shown in picture below), The rate of turn is given by:
$\omega_{turn} = V/r $  (I understand this)

And so the aircraft experiences steady yaw and pitch rates given
by':

$n_{yaw}= \omega_{turn}cos \phi$

$q = \omega_{turn}sin \phi$

I don't understand 2 equations above. I think the yaw rate would be equal to the turn rate and the pitch rate is zero.

Turn rate is referenced to earth axes, yaw rate to aircraft axes, hence the $cos\Phi$:

If you turn at $\Phi$ = 0, your rate of turn is the yaw rate and the pitch rate is zero.
If you turn at $\Phi$ = 90°, your yaw rate is zero and your pitch rate = rate of turn.

In order to maintain a steady turn and not lose altitude the pilot does need to pull on the stick when turning..


Is an FAA Part 121/135 checkride conducted under Part 91 or Part 121/135? Is a checkride which is required in order to qualify a pilot for 121 or 135 operations itself conducted under Part 91 or Part 121/135 rules?

For instance, is a first class medical required in order to be PIC for the line check required by 135.299 or is only a second class medical required?  Is a fully qualified 121/135 SIC required in order to do the flight (in non-revenue service), or could it be done with an SIC qualified only for Part 91 flights?

I understand that in order to act as PIC of a Part 121/135 flight you need those, but what about for the actual checkride?

Any flights operated by a certificate holder must be operated in accordance with the authorization of their certificate. Authorization will vary.

An Air Carrier may conduct training1 flights under §91 if authorized to do so. If not authorized, any flights operated by that Air Carrier—including training flights—must be conducted under the part of 14 CFR (§121, §135, etc) specified by their Air Operating Certificate (AOC).

An Air Carrier may be authorized to conduct training flights under 14 CFR 91 in accordance with their Operation Specifications authorized by the FAA.

The exception to this would be if that Air Carrier's Operation Specifications do not authorize the Air Carrier to conduct flights under §91. I know of no reason for such exception, nor if any such exception has been withheld from any actual Air Carrier.

The authorization is found within the Operation Specifications of the company in question, not necessarily in 14 CFR, though it can be inferred there. See Operation Specification A001 for authorization to conduct training flights under 14 CFR 91, for example:

  d. The certificate holder is authorized to conduct flights under 14 CFR Part 91 for crewmember training, maintenance tests, ferrying, re-positioning, and the carriage of company officials using the applicable authorizations in these operations specifications, without obtaining a Letter of Authorization, provided the flights are not conducted for compensation or hire and no charge of any kind is made for the conduct of the flights.

(The above is an example only; actual authorization may vary.)

An Air Carrier may also be authorized by A031 of their Ops Spec to make arrangements with an outside party for Contract Training, including instruction and evaluation. An authorized outside party under this specification may include a §142 Training Center and/or a different §119 Certificate holder authorized to conduct training and/or checking. Any training flights operated by either such certificate holder must likewise be authorized by the terms of their certificate.

1Note: Regarding the question of what is included under the definition of the term training—particularly whether checkrides are included in that definition—no explicit guidance is offered. However, a definition might well be inferred from—among other things—the definitions for the following two terms as found in Ops Spec A002, Definitions and Abbreviations:

  Major Contract Training
  Any flight training, flight testing, or flight checking leading to and maintaining certification and qualification of air carrier flight crew members in accordance with the requirements (maneuvers and procedures) explicitly stated in 14 CFR Parts 61, 121, or 135; or in SFAR 58 Advanced Qualification Program (AQP), as applicable.

  Outsourced Training
  Any training, testing, or checking activity which an air carrier certificate holder provides by way of a contract arrangement with another party.


How much is the fuel consumption of a Tecnam P2002? So I'm gonna have the training for PPL and I wanted to know the fuel consumption of a Tecnam P2002 aircraft. But I couldn't find any aircraft information for this particular plane.

Make your search based on the engine model, Rotax 912S2, not the aircraft.

However you can work it out easily for any aircraft and engine if you remember that carbureted gasoline engines burn about .4 to .45 lbs/hp/hr. The rotax is pretty efficient so you can use .4 (old air cooled direct drive carbureted engines like Lycomings are closer to .45).  So at 75hp in cruise, that's 30 lb/hr at cruise or 4.4 US gal/hr or about 17l/hr.


Why don't we use elevator trim as the primary flight control? One thing you're repeatedly told as a student pilot is to fly with the stick (or yoke), not the trimmer. To select a new pitch attitude, fly to it with the stick, wait for the aircraft to settle, and then trim away constant forces.

But the position of the elevator trimmer is roughly the same for the same cruise configuration. It would be a lot easier to mark the trimmer with "level flight at 80", "gliding descent", and "best climb" positions, and just move it to those positions (before or after setting the throttle appropriately) directly. The trimmer would be the primary means of setting attitude, with the front-back axis of the stick or yoke for temporary adjustments (e.g to bring the nose up a little in a turn).

So why aren't controls designed to be used that way, and why do we fly with the stick?

Proper trim control technique involves exerting control pressure to attain the desired attitude then using the trim to alleviate the requirement of holding that pressure. There can be a temptation for pilots to make small adjustments using trim alone. Resist the temptation and use the proper procedure.

We use the proper technique because it's the most precise way to use trim. If adjustments are made with the trim directly we'll end up over-controlling and chasing the correct trim position.

Trim isn't just a convenience. Especially for pilots flying in instrument conditions, alleviating the load of holding control pressure and ensuring that if the pilot lets go of the controls the airplane maintains its attitude are a matter of safety.

The Airplane Flying Handbook explains Trim Control starting on 3-6. It lists the proper technique in greater detail and provides a complete rationale.

There's also the matter of doing things the same way every time. Once we've established the right procedure, we should always use it to enforce the habit. This also benefits the pilot when things get busy. Proficient instrument pilots are experts in using trim effectively.

Predefined trim positions are impractical because they vary based on weight, CG, airspeed and other factors. These factors change the amount of downforce required on the elevator and therefore the trim position. Situations like "level at 8000" or "gliding descent" won't always result in the same trim position. For other control surfaces there are similar factors. We do have predefined trim positions for takeoff, but loading often makes those undesirable and trimming is required immediately.

During flight training, good trimming technique becomes especially important during instrument training. There's a lot of good information about trim and its importance in the Instrument Flying Handbook.


Are Air Traffic Flow Management Algorithms mostly based on linear programming techniques? I'm really interested in knowing the methods used by the Air-Traffic-Flow-Management (ATFM) providers to increase the throughput of the airspace by balancing safety, efficiency and cost. I've been reading different research papers and most of them conclude on using algorithms based on linear programming to manage the arrival and departure slots.

For instance, there are many articles explaining a possible resolution of the Ground Holding Problem, that allocates departure slots so that no sector is violated, by using linear programming and mathematical optimization techniques. Airport capacity as well can be improved by the use of linear programming.

I know that linear programming is based on minimizing/maximizing a certain objective function, subject to a set of constraints.

I see that these techniques can be a good idea to have a big picture of an optimal configuration of slots, for instance. But are these linear programming algorithms actually used?

Is linear programming so much effective to carry out air traffic management according to this method? What other methods are powerful and used nowadays to manage air traffic?

Linear programming (LP) is an optimization method that finds the highest (or lowest) possible value of a set of linear equations given a number of linear constraints.

There are a number of objectives in Air Traffic Management that may be described by linear equations. Runway throughput for example: twice as many aircraft can be seen as "twice as good". Or delay: one minute of delay of a passenger aircraft with 400 passengers is as bad as 2 minutes of delay for a passenger aircraft with 200 passengers (both have 400 passenger-minutes-of-delay). Mathematical models based on these linear relationship can help to make decisions for air traffic controllers/planners.

There are obvious limitations to linear programming. Not everything can be modeled as a linear relationship. Often the relations are more complex and linear models will be a poor fit because the relationship is not linear at all. In ATFM modelling, usually binary integer variables are used that can only take values of zero or one, thus a Binary Integer Linear Program is solved (BIP). And is 40 minutes of delay of a 10 passenger aircraft as bad as 1 minute of delay of a 400 passenger aircraft? These kind of relationships tend to prevent the small aircraft from ever taking off: it always pays off to let a bigger aircraft jump the queue. Perhaps a quadratic component in the  'cost-function' would bring a solution here.

Therefore more advanced models are needed. Methods like Integer Linear Programming (ILP), Mixed Integer Linear Programming, Mixed Integer Quadratic Programming and more advance optimization algorithms are used as well.

Another problem is that while these optimization methods may be able to find a best possible strategy, they are not necessarily easy to understand. Air traffic controllers don't like to be just told what to do, they need to understand why a strategy is proposed so they can think ahead and create what-if scenarios. It is impossible to program every what-if scenario in the optimization model, so at times the controller will need to deviate from the proposed solution. Without understanding what the algorithm had planned, it is difficult for a controller to predict the knock-on effects of his deviation from the proposed plan.

Early arrival management software worked reasonably well when the weather was nicely playing along, but gave hopelessly wrong suggestions when a couple of thunderstorms would render the normal approach routes unusable. Flexibility and two way communication between the system and the controller is essential to make a good product, there is much more to it than choosing the right optimization algorithm.