How close to a Restricted Area can a pilot be cleared to fly? In my local area there is an instrument approach that skirts the edge of a restricted area by a mile or two. The TRACON for this approach is not the controlling agency for the restricted area. When the restricted area is active, the TRACON will not give clearance for the instrument approach unless the weather is VMC. Is there a requirement that ATC cannot have pilots flying too close to Prohibited/Restricted areas in IMC? Even if they're flying a published procedure?
Is this different for VFR flights?

ATC will separate IFR traffic from active special-use airspace according to the separation minima prescribed in the 7110.65 9–3–2:

Vertical separation: 500 feet above or below a prohibited/restricted/warning/MOA/ATCAA/ALTRV area (1000 feet above or below, if above FL290).
Radar separation: 3 miles from the periphery of special-use airspace; or for specific areas that have been designated by management as not needing 3 miles, simply "clear of" the periphery.
Non-radar separation: On airways whose protected airspace does not overlap the special-use airspace. (Remember that Federal airways extend four or more miles from each side of the centerline.)

Note that VFR traffic are explicitly mentioned as not prohibited from flying through a transitional hazard area (a term I think is used in conjunction with spacecraft launch and reentry). It is also "common knowledge" that VFR aircraft may fly through an active MOA but not active R- or P-airspace; however I can't seem to find anything that says this explicitly. R- and P- airspace is discussed at 14 CFR 91.133, which does not mention MOAs.

Without knowing the specific situation you're talking about, my guess is that when the restricted area is hot the TRACON is withholding an approach clearance using that approach for all IFR aircraft whether IMC or VMC, and only permitting VFR practice approaches to use it.


Are aircraft still guided by ground marshals? Are ground marshalling operations still conducted by personnel on the ground? Are there any airports that still do this procedure?

From Wikipedia.

Typically, marshalling is used to guide aircraft to their parking stand.

There are alternatives available (Visual Guidance Docking Systems) that allow pilots to park their aircraft at the right spot without the aid of a marshaller.

In its simplest form, the parking spot has a big identifier to allow the pilot to select the right stand, a centreline for the nose wheel to follow, a marking where the nosewheel should stop and a mirror so the pilot can see where the nose wheel is relative to the marking.

In its more advanced form, there are pressure plates in the stand that determine the position of the aircraft, a laser scanning system that determines the exact position of the aircraft and whether the stand is obstacle free, and a digital display guiding the pilot during parking.

In case the VDGS is not available or cannot handle the specific aircraft type, a marshaller will have to step in.


Is there simpler way than scraping flightradar to determine Number of Flights between Two Countries? Is there a simple way to procedurally determine how many flights are flown between two countries on a given day?  Maybe even get an inbound/outbound break down?

I consulted the FAA data site (https://www.faa.gov/data_research/aviation_data_statistics/) but I didn't find anything promising.  I was also considering scraping flight radar (https://www.flightradar24.com/) but is a somewhat complicated endeavor.  Is there a simpler solution?

ADS-B Exchange could be an alternative to FlightRadar24. They have (free if you host a feeder station) APIs so you wouldn't need to go through the headache of scraping. The data should be comparable and with ADS-B becoming mandatory in more and more regions (US since the beginning of the year and an EU mandate covering pretty much all business flights coming in June) you should be able to track anything but small VFR only aircraft.


Do pilots ever increase the cabin altitude intentionally? Is it true that some pilots increase the cabin altitude slightly to calm down potentially disruptive passengers for safety reasons? For example, footballers who have just won a championship.

I've never heard of this "trick" being used or recommended, although the limits of what I've heard are pretty much limited to US commercial aviation.

If you climb the cabin from the usual 8000' to 10,000', the incremental gain in "sleepy" factor is a little bit but not all that much.  If you climb it above 10,000', then you might "encourage" a few of the more tired/drunk/elderly folks to go to sleep, but now you're into the realm of operating outside of normal practices, and you need to start considering things like, the pilots both need to be on oxygen, the flight attendants may get "tired" quicker than the passengers do, anybody who is elderly or in poor health may start suffering consequences, and then there are company policies and national regulations to consider.

If you raise the cabin above about 14,000 - 15,000' depending on your specific aircraft, then you'll end up activating the automatic passenger oxygen masks, and now you've got some serious 'splainin to do!  (#1, it's a bunch of work for maintenance to repack them, and the aircraft is grounded until they do, and #2 you open yourself up to all sorts of issues if you still have a long way to fly at high altitude to get to your destination!)

I'm going to guess that in the history of aviation, this sort of trick probably has been used at some point, but the opportunity for the uncomfortable questions along the lines of, "okay, tell me again, you did what exactly???" to which the best available answer is along the lines of, "um, well, it, ah, seemed like a good idea at the time, sir..." is awfully high.

I certainly wouldn't recommend it.


Is there a V speed for the point an aircraft starts an ascent at 0 angle of attack? Some aircraft with an angle of incidence or cambered airfoils are capable of climb without any angle of attack, Is there a V speed for the speed at which an aircraft starts a positive climb?

No, there is no V-Speed associated with zero angle of attack, nor is climb associated with angle of attack. Since zero angle of attack does not directly translate to minimum drag for a production aircraft in operation, it is not a meaningful performance gate, which is what V-Speeds are designed to represent.

However, this is not to say that having a zero flow incidence against the fuselage is not desirable:

If the body-axis is aligned with the fuselage longitudinal axis, it would be desirable from a passenger comfort standpoint to fly at zero AOA during level cruise. We don't want to have an inclined martini glass for the whole duration of the flight.
Non-zero flow incidence against the fuselage does create drag. It would be desirable from a performance perspective to decrease any form of drag during cruise or climb.

This is achieved during aircraft design. The wing setting angle against the fuselage is tuned such that the average operational flow incidence angle against the fuselage is minimized.


How often is communication lost with aircraft? I just read the NTSB press release of Northwest Airlines Flight 188 that flew more than one hour without making any radio contact. I imagine there may be many causes for losing radio contact with the ground (distraction, electrical failure,...) and I found a video (in French) saying that one way to solve the situation is to send a fighter to have a closer view of the situation and possibly assist the pilots.

I wonder if the situation of losing contact with an airplane is frequent? I imagine it happens between one and ten times per month over a region as big as the USA, but that is just a guess.

According to this FAA paper, Table 3, ASRS reports were analyzed for 16 months between 1978 and 1979. This included 553 incidents where the recipient was not monitoring, which seems to be the closest category to loss of communications. This equates to a little more than 1 incident a day. Note that this only includes incidents that were reported, and air traffic has increased since then.

This report contains info from ICAO in in Europe. The focus is on the security issues from unresponsive aircraft. In just the northern region, there were 230 flights over just 6 months in the beginning of 2004. This number inclues airlines, business jets, and military transports. This frequency is a little higher than the previous report, but is more recent and probably includes less flights.

Other reports like this one don't include a time frame for the reports analyzed, but look at other factors involved. For example, pilots with fewer hours and GA pilots are more likely to be involved in such an incident. The average duration of these incidents was 7.6 minutes.

This page also has some good information about this type of incident, including a list of some of the many possible reasons for lost communications and excerpts from some of the reports.


How to apply the TO/GA time and EGT limits? If there is a published 1) EGT limit and a 2) TO/GA time limit, which one should be applied when the EGT is well below the EGT limit and possibly even below the 'max continuous' EGT. Particularly with consideration of terrain avoidance procedures that require climbing to a high altitude (10,000 ft) for terrain avoidance.

The TO/GA time limit and the EGT limit are hard limits mandated by the approved flight manual. They must be adhered to independently and cannot be traded off between the two.

Takeoff performance is calculated so that if a safe altitude above terrain and obstacles cannot be reached within the time limit, aircraft mass must be reduced rather than the TO/GA time increased.

Having said that, if things don’t go as planned during takeoff e.g. for reasons not known beforehand, I‘d recommend to bust the time limit rather than fly into the side of a mountain, if the choice is only between the two...


Are customs stops required for private flights within the Schengen Area? I would like to make a VFR flight from Switzerland to Germany and Germany to France and then France to Switzerland.

For the first step: Switzerland to Germany I know that I have to fill a custom form AND a flight plan while taking off and landing on an airport with custom services. The same for France to Germany.

HOWEVER from Germany to France is this rule applicable (note that the plane is registered in Switzerland)? I cannot find an official website?

The European Union has also something called EEA - European Economic Area. Within the EEA it is not required to fill out anything except a flight plan between Germany and France.
Switzerland, as stated, is not part of the European Economic Area and therefor customs are required.

Just in addition, Schengen is for passport free travel and not for taxes.


What is the operating temperature of a turbo bypass engine, and how is it kept at that temperature? What is the operating temperature of a turbo bypass engine? If advancements were made to materials through material science for the internal components would increasing the operating temperature result in better fuel efficiencies, better thrust and optimal engine performance?
How is the jet engine cooled to operate at the desired temperature?

Yes, if you could increase the combustion temperature in the hot section of a gas turbine, you could dramatically increase the efficiency of the engine.  The problem of course is finding materials which can sustain structural integrity in these regimes for long service lives and still contain the hot gas; we are currently at the thermal limits of existing materials.

The key problems here are:

Temperatures - gas turbine hot sections operate at 1600°F all the way up to 2600° F for high performance military engines in order to yield high thrust or high useful work for a unit of fuel burned.  No known metals can survive these upper limits and still maintain high yield strengths.  It also causes great thermal expansion problems between non homogenous materials and coatings as well as oxidation and galvanic corrosion.

Stresses - the spools of a gas turbine can spin as fast as 40,000 rpm in the cases of high pressure compressors and high pressure turbines.  This creates tremendous centripetal loads on the blades and other components which they must be able to endure and not exceed the yield strength of the material.

Extended operation between servicing and reliability - the engine must survive these loads for several hundred cycles over a period of several thousand hours between major overhauls.  Redundancy as well as resistance to damage must be built into the design as well.  The consequences are grave; the trustworthiness and reliability of two gas turbines engines are all that separates a successful Seattle to Moscow polar flight in a 777 and 320 people freezing to death in the Arctic Ocean.

Historically gas turbine hot sections have been constructed from alloys of metals with high melting points and good yield strengths like nickel and cobalt of a very high purity.  Titanium and Molybdenum are also beginning to be used in this as well.  Additionally forming techniques such as single crystal casting are used to remove crystal grains from the material to improve its strength.  The blades ('buckets') of a high pressure turbine are also cast with channels in them and have fine holes laser drilled in their surface in order to take compressor bleed air and 'bathe' the boundary layer of the bucket with cooler air, effectively wrapping it in a cool gas blanket to protect it from the high temperature exhaust.

More exotic materials and techniques are being considered for this.  GEAE and P&W both are exploring the use of composite and ceramic blades in hot sections.  These offer high yield strengths at high temperatures but again pose challenges in terms of oxidation and thermal expansion problems.


Why can't runway 18 at Frankfurt Airport accept takeoffs or landings in the opposite direction? Runway 18 at Frankfurt Airport is one example of a runway which is numbered on only one side. Aircraft can only takeoff in the 18 direction, and cannot takeoff in the 36 direction, nor land in any direction except in an emergency situation.

What is the reason behind it?

Thomas Römer/OpenStreetMap data [CC BY-SA 2.0], via Wikimedia Commons

Let us consider the operations other than takeoff on runway 18.

Takeoff Runway 36

There are buildings not far off the departure end and directly after the end of the runway is a movement area and ramp.  To safely takeoff they would have to clear that area and probably replace all the concrete with crushable concrete (more on this later).  The buildings probably post obstacle clearance issues for engine out climb performance.  The east-west runway north of 36 would have conflicting traffic and would have to be carefully sequenced.  Lastly, that purple area to the north of the runway is probably a noise sensitive area you aren't allowed to fly over at low altitude.
Landing Runway 18

You approach over a noise sensitive area, cross over operations on the northern east-west runway, overfly a building at extremely low levels (wheels on the roof) and have to make sure there are not airplanes on the ramp between the building and runway.  Landing short would be nothing short of disastrous and every landing would be a risky operation with the close proximity of airplanes and buildings.  Glideslope services would probably not be available due to ground obstacles.
Landing Runway 36

This is the least problematic approach.  The problem is in missed approaches and long landings.   There is no safety area at the end of 36 and a runway excursion past the end of the runway would be catastrophic.  To mitigate this they could re-pour the ramp concrete with crushable safety concrete, but that would probably make using ramp and building unsuitable.  If traffic goes around they conflict with aircraft on the northern east-west runway and have a noise sensitive area to deal with.

In comparison to the above, taking off to the south only requires a blast fence just north of the runway to protect the ramp and building from jet blast.  There are no logistic issues in aircraft movement on the ramp and the building is not in danger of being hit.


What does an "X flagged" VOR mean on an en-route IFR chart? Source: skyvector.com

Maybe I'm looking at the wrong sections / using wrong keywords in the chart user's guide.

The X in a flag for a intersection, waypoint or fix regardless of if it is a NavAid means there is a Minimum Crossing Altitude. An MCA present means you must be at or above a certain altitude when passing over the fix in the direction specified. This would provide obstacle clearance for that particular Victor airway. The MCA usually indicates a transition from one leg or section of the airway to another with a higher Minimum En Route Altitude.
In the case of the LLC VORTAC noted above, that altitude is 8500 feet MSL when traveling Northeast on the V6 Victor Airway. However, it does not necessarily apply to the other airways. In this case, it only applies to aircraft continuing on V6 or joining V6 going Northeast. The MEA for the upcoming airway section is 12000 feet MSL. Where as some joining airways such as V32 have an MEA as low as 8000.
This differs from the Minimum Reception Altitude. The MRA is used to ensure proper signal reception of the NavAid used to identify a fix. The fix will be on an airway. But the cross referencing NavAid used to identify the fix does not have to have an airway associated with it.


What is the most carbon-efficient way to take off in a passenger plane? Consider two possible flight paths for a jet passenger plane; the first is a slow climb, held low over 20 or so miles to avoid other flight paths, the second a more rapid climb over 10 miles to the capacity of the plane rather than restricted.

It has been said to me that the more rapid climb will reduce CO2.
But logic to me is that the rapid height gain will burn more fuel and so increase CO2 production (or perhaps are the 2 not linked I wonder).
I would appreciate being put out of my mystery on this?

You are right, climbing adds to the fuel consumed, and increases the energy which is added to the plane, but this energy is conserved and can be reclaimed later, on descent. There are two kinds of energy to consider, potential and kinetic energy. Potential energy is weight times height, and kinetic energy is mass times speed squared, and both can be interchanged.
$$E = \frac{m}{2}\cdot v^2 + m \cdot g \cdot h$$
Nomenclature:
    $E \:\:\:$ energy
    $m \:\:\:$ mass
    $v \:\:\:\:$ speed
    $g \:\:\:\:$ gravitational constant
    $h \:\:\:\:$ height

The thrust of the engines over what is needed to overcome drag, accumulated over time, builds up energy. Lift and Drag, in turn, are proportional to airspeed squared times density of air, so if the airplane climbs, it needs to fly faster to compensate for the thinner air, and drag will stay roughly constant. Thrust is also proportional to the density of the air around the aircraft, and the same goes for fuel consumption. This means that the aircraft will approximately consume the same amount of fuel over time when it flies low and slow, and when it flies high and fast. Since the task is to transport people or goods to a specific destination, flying high and fast will result in less running time at the same fuel flow. This is the reason why flying high results in less CO2 produced!

When the aircraft comes close to its destination, it can now put the potential energy which has been stored as altitude to good use: In a descent, the thrust generation can be reduced by the amount of energy which is now reclaimed, just as you can ease off the gas pedal in your car when coasting downhill. So for the last half hour of the flight, the aircraft will now consume less fuel than before, and by the time it has reached its destination, the difference is just as big as the excess in consumption which was needed during climb.

For the nitpickers: The aircraft will weigh less at the end of the trip (fuel has been consumed), so the descent will not completely compensate for the climb. On the other hand, the lower temperature at altitude will reduce fuel consumption by letting the engines run more efficiently, so in the big picture a definite gain remains.


Which (if any) is the "most major" airport / field / strip on Antarctica? Which airport (/field, /strip, /skiway) is the "biggest", "most supported" on Antarctica?

So,

has the most permanent staff (do any?)
has the most runway facilities such as say runway lights, markings, length, aircraft weight possible, surfaced (are any?), etc?
has the most or any fueling facilities?
the most or any ground support facilities? repair facilities?
is opened all or at least the most months of the year?

What's the state of the "best" airport (or analogue) in Antarctica?

You can find a full list of airports on the continent here, the majority are ice/snow runways with two gravel runways at Rothera Air Facility and TNM. Supported is a loose term down there, however Williams Field which services McMurdo Station is groomed and has fuel available. Phoenix Airfield which also serves McMurdo can accept wheeled landings and seems to be marked in some capacity. Marble Point (a helicopter landing site) is also operated by the US, has fuel available and is operable 24 hours a day in the summer and has weather reporting.

The full operations of the (US) bases can be found in this document which states

  Williams Field is serviced by MLS, TACAN, and RNAV (GPS) approaches.

  ...

  Phoenix Airfield is serviced by TACAN, MLS and RNAV approaches.

which is a solid set of approaches all things considered.

At least two fields have runway lighting

  Airfield lighting systems are established at Phoenix and Williams
  Fields. The NSF Prime Contractor maintains airfield markings and
  lighting systems in accordance with AFMAN 32-1076

McMurdo even offers ATC!

All that considered Williams/Phoenix are your best bets but if you are trying to fly there on your own keep in mind:

  All navigational and approach aids are intended strictly for use by
  USAP (United States Antarctic Program) approved aircraft only. The use of NAVAIDs by non-USAP aircraft
  are strictly at the pilot’s own risk.

All of the fields are "weather permitting" as are the flights so "open" all year is relative to weather conditions and other considerations.


Are there diversion points for southern Pacific great circle route flights? At first I wasn't even sure if any airlines flew southern Pacific great circle route flights, but it looks like Qantas (QFA27) operates a flight between Sydney and Santiago, Chile.

For northern Pacific greate circle routes, you are flying near Canada/Alaska and then Russia for most of the flight, so you have some airports you could potentially divert to if an emergency arose. I've search online for more details about how this works for these flight, but haven't had much luck finding information.

Are there any point on the southern Pacific great circle route for flights to divert to? Or is there not really any backup plan if something goes wrong? Are there specific requirements that must be met for an aircraft to fly this route?

This particular flight was a Boeing 747 flying from YSSY (Sidney) to SCEL (Santiago).  Because a 747 has more than two engines, they don't have specific time limits on how far from land that they can fly like two engine airplanes do.

That being said, they do have to carry enough fuel to make it to a suitable airport in the event that the airplane depressurizes or it has an engine failure at any point along their route.1.  This doesn't mean that they have to make it all of the way to their destination in the event of a problem though.  They use en-route diversion airports and have predefined points on their flight plan which tell them where to divert to at any given point during the flight.

You will notice on this flight (see the images below) that their planned route is considerably further North than the shorter great circle route.  This is probably so that they can use NTAA (Tahiti) as a diversion airport if they have a problem.  The route is also optimized for the current winds at the cruising altitude, so it may be more fuel efficient to go out of the way in order to have more favorable winds.

In this case, without seeing their actual dispatch paperwork we can't know for sure whether or not they could have even flown the great circle route with the amount of fuel that the airplane can hold, but we'll just assume that they are using NTAA as an ETOPS airport for the purposes of this discussion.  That being said, we will use the following airports as our ETOPS diversion airports:  NZCH, NTAA, and SCEL.  In this case, their diversion plan in the event of an emergency would probably be something like this:

From takeoff (YSSY) until a point a little less than half way to NZCH (New Zealand)2 they would divert back to their point of departure2
From a little less than half way to NZCH until a point around 46W52 they would divert to NZCH.
Between the points that would be around 46W52 and 47W25 they would divert to NTAA.
After 47W25 they would continue on to their destination (SCEL) because it would be faster than turning around and going back to any of the other airports.

Each of the points takes wind into consideration, and as they pass each one they will always know which airport that they can reach in the least amount of time.  The most important thing is that they always have enough fuel to make it somewhere safe in the event of an emergency.

Filed Route

Great Circle Route

1 They also have to carry enough fuel to fly to their alternate airport and additional contingency fuel.  One thing to keep in mind is that a jet burns considerably more fuel at low altitudes than they do at their optimum altitude, so they have to carry a lot of extra fuel to meet this (and other) requirement.

2 If they turned around at the exact halfway point, it would take them longer to return than to continue because of the East winds.

Regulations

For those of you interested in the actual fuel requirements for a flight like this, the US regulations are listed in 14 CFR 121.  I'll use these as an example (other countries will be very similar, but I know these.)

You will notice that almost every line of the regulations listed here will require extra fuel to be on board the airplane. (Look for the ✈ symbol next to each one that I have added.)  Fortunately, it isn't quite as bad as it looks though, because some of the fuel that is pumped into the airplane can be used for more than one purpose.  For instance, a portion of the fuel may be used to divert to an ETOPS airport and used to fly to your destination, because they will only be doing one or the other.  The pertinent regulations are summarized here:

  §121.645 - Fuel supply: Turbine-engine powered airplanes...

  ...

  (b) For any certificate holder conducting flag or supplemental
  operations outside the 48 contiguous United States and the District of
  Columbia, unless authorized by the Administrator in the operations
  specifications, no person may release for flight or takeoff a
  turbine-engine powered airplane (other than a turbo-propeller powered
  airplane) unless, considering wind and other weather conditions
  expected, it has enough fuel—

  ✈ (1) To fly to and land at the airport to which it is released;

  ✈ (2) After that, to fly for a period of 10 percent of the total time
  required to fly from the airport of departure to, and land at, the
  airport to which it was released;

  ✈ (3) After that, to fly to and land at the most distant alternate
  airport specified in the flight release, if an alternate is required;
  and

  ✈ (4) After that, to fly for 30 minutes at holding speed at 1,500 feet
  above the alternate airport (or the destination airport if no
  alternate is required) under standard temperature conditions.

  ...

  §121.646 - En-route fuel supply: flag and supplemental operations.

  (a) No person may dispatch or release for flight a turbine-engine
  powered airplane with more than two engines for a flight more than 90
  minutes (with all engines operating at cruise power) from an Adequate
  Airport unless the following fuel supply requirements are met:

  (1) The airplane has enough fuel to meet the requirements of
  §121.645(b);

  ✈ (2) The airplane has enough fuel to fly to the Adequate Airport—

  ✈ (i) Assuming a rapid decompression at the most critical point;

  ✈ (ii) Assuming a descent to a safe altitude in compliance with the
  oxygen supply requirements of §121.333; and

  ✈ (iii) Considering expected wind and other weather conditions.

  ✈ (3) The airplane has enough fuel to hold for 15 minutes at 1500 feet
  above field elevation and conduct a normal approach and landing.

  (b) No person may dispatch or release for flight an ETOPS flight
  unless, considering wind and other weather conditions expected, it has
  the fuel otherwise required by this part and enough fuel to satisfy
  each of the following requirements:

  ✈ (1) Fuel to fly to an ETOPS Alternate Airport.

  ✈ (i) Fuel to account for rapid decompression and engine failure. The
  airplane must carry the greater of the following amounts of fuel:

  ✈ (A) Fuel sufficient to fly to an ETOPS Alternate Airport assuming a
  rapid decompression at the most critical point followed by descent to
  a safe altitude in compliance with the oxygen supply requirements of
  §121.333 of this chapter;

  ✈ (B) Fuel sufficient to fly to an ETOPS Alternate Airport (at the
  one-engine-inoperative cruise speed) assuming a rapid decompression
  and a simultaneous engine failure at the most critical point followed
  by descent to a safe altitude in compliance with the oxygen
  requirements of §121.333 of this chapter; or

  ✈ (C) Fuel sufficient to fly to an ETOPS Alternate Airport (at the one
  engine inoperative cruise speed) assuming an engine failure at the
  most critical point followed by descent to the one engine inoperative
  cruise altitude.

  ✈ (ii) Fuel to account for errors in wind forecasting. In calculating
  the amount of fuel required by paragraph (b)(1)(i) of this section,
  the certificate holder must increase the actual forecast wind speed by
  5% (resulting in an increase in headwind or a decrease in tailwind) to
  account for any potential errors in wind forecasting. If a certificate
  holder is not using the actual forecast wind based on a wind model
  accepted by the FAA, the airplane must carry additional fuel equal to
  5% of the fuel required for paragraph (b)(1)(i) of this section, as
  reserve fuel to allow for errors in wind data.

  ✈ (iii) Fuel to account for icing. In calculating the amount of fuel
  required by paragraph (b)(1)(i) of this section (after completing the
  wind calculation in paragraph (b)(1)(ii) of this section), the
  certificate holder must ensure that the airplane carries the greater
  of the following amounts of fuel in anticipation of possible icing
  during the diversion:

  ✈ (A) Fuel that would be burned as a result of airframe icing during 10
  percent of the time icing is forecast (including the fuel used by
  engine and wing anti-ice during this period).

  ✈ (B) Fuel that would be used for engine anti-ice, and if appropriate
  wing anti-ice, for the entire time during which icing is forecast.

  ✈ (iv) Fuel to account for engine deterioration. In calculating the
  amount of fuel required by paragraph (b)(1)(i) of this section (after
  completing the wind calculation in paragraph (b)(1)(ii) of this
  section), the airplane also carries fuel equal to 5% of the fuel
  specified above, to account for deterioration in cruise fuel burn
  performance unless the certificate holder has a program to monitor
  airplane in-service deterioration to cruise fuel burn performance.

  ✈ (2) Fuel to account for holding, approach, and landing. In addition to
  the fuel required by paragraph (b)(1) of this section, the airplane
  must carry fuel sufficient to hold at 1500 feet above field elevation
  for 15 minutes upon reaching an ETOPS Alternate Airport and then
  conduct an instrument approach and land.

  ✈ (3) Fuel to account for APU use. If an APU is a required power source,
  the certificate holder must account for its fuel consumption during
  the appropriate phases of flight.

  §121.647 - Factors for computing fuel required.

  Each person computing fuel required for the purposes of this subpart
  shall consider the following:

  ✈ (a) Wind and other weather conditions forecast.

  ✈ (b) Anticipated traffic delays.

  ✈ (c) One instrument approach and possible missed approach at
  destination.

  ✈ (d) Any other conditions that may delay landing of the aircraft.

  For the purposes of this section, required fuel is in addition to
  unusable fuel.


Is Flight Idle a constant? Is "Flight Idle" for a jet engine a constant N1% value or does it vary with Altitude, and OAT?

This will vary from engine to engine and with modern engines controlled by computers for max fuel efficiency, the computer will constantly adjust the idle during the descent unless you manually select otherwise. However a minimum idle speed is necessary to prevent flame-out, and is as high as 70%** during severe icing conditions although 55% is the minimum. There are also different idle regimes, flight idle, approach idle and ground idle, controlled automatically.
The last aircraft that I flew, the B727, did not have any fancy FADEC and we manually closed the throttles at top of descent. The fuel control unit would adjust the fuel flow throughout the descent to keep a minimum 55% N1. This was evident from observing the fuel flow increase throughout the descent even though the throttles were at idle. If we needed more N1 due to icing, we would have to advance the throttles manually. Some planes had autothrottle but I don't think it compensated for icing.
Approach idle is also controlled automatically and is somewhat higher to allow a quick spool-up in the event of a go-around. It is deactivated after touchdown.
The 737Ng has similar regimes controlled by the EEC (Electronic Engine Control).
** Thrust requirements for for engine and wing anti-ice:

If Icing conditions exist with TAT -7° or above: 55% N1
If Icing conditions exist with TAT below -7° or moderate to severe icing observed at any temperature: 70% N1
Only one outboard engine supplying bleed air for wing 75% N1 on that engine.

To answer your question below, a jet engine works much the same as a piston engine wrt air/fuel ratio and the compression ratio is determined by its construction (see NASA). In a jet engine however you don't have a butterfly valve to vary air input, you can only vary fuel input-(there are exceptions with bleed-off valves, but let's keep it simple). The speed at which the engine turns determines air input and this speed is regulated by the amount of fuel injected into the combustion chamber. The jet engine will automatically settle at an equilibrium speed when the best air/fuel ratio is achieved.
So assuming an engine with only a variable fuel input (no automatic compensation for any variable parameters such as altitude)--- if for instance the altitude at start of descent is higher, less air is going to be taken in by the compressor, the fuel/air ratio will change resulting in a different engine speed than if the start of descent altitude was lower.
Ram air effect (Indicated airspeed) will also change the fuel/air ratio once again causing a different equilibrium engine speed.
Temperature will also affect the density of the air and cause a different rotation speed.
This is clearly unacceptable therefore some kind of governor must be provided to maintain stable parameters.
You can use the NASA engine simulator here to play around and test various scenarios.


Do military bases still employ wheel watchers? When I was in the air wing in the '70s, I had the unpleasant opportunity of pulling wheel watch. The unfortunate individual sat in a small shack (maybe 3 x 6 ft.) with a pair of high-powered binoculars ensuring that each and every aircraft on final had their gears down. If not, there were two flare guns pointed at opposite right angles to the runway that had to be fired to warn the pilot off. I know I had to use them on more than one occasion. I think it was a thirty-day hitch. It seems like such a primitive way to ensure the gears were down, and I wonder if it's still being used today?

No

They do not continue this practice anymore. This is likely because landing gear are more reliable now than they were in the 70's. There is also the practice of Air Traffic Control prompting military pilots to check their gear is down, by verbally adding "Check Gear Down" to their landing clearance.

Source: Checked in with some of the Air Force / Air National Guard pilots in the office. They're not aware of any such practice and decided it must have been before their time. Oldest pilot started in the 90's, so that leaves you with a 20 year gap where they abolished the practice.


How do acrobatic pilots avoid pulling too many G’s and passing out? I just watched a video of a acrobatic pilot doing stunts, it involved a type of stall, not sure which one though. Long story short he overcompensated(?) which caused him to pass out, and then he crashed in the lake. Honestly I’m surprised this doesn’t happen more often. What kind of safety measures do they take when pulling off maneuvers that come close to the ground and would require a fast change in angle of attack?

There are certain things you can do to increase your G tolerance without a G suit, mostly static tension exercises like squeezing your diaphragm, lower torso and thigh muscles, isometric exercise style, to slightly slow down the blood flow to your legs during the pull.  Plus just really good physical fitness so your body can handle weighing 4 or 5 times normal for short periods (neck muscles are critical for this, more so if you're wearing a helmet).

Beyond that, and without a G suit, it's just awareness of how hard you can pull and knowing where to ease off.  You have a G meter in the plane to indicate where you are, but if experienced you should be able to tell within reason by the sensation.

You start to lose your vision (grey out) before you pass out completely, so that is a key warning sign that you'd better back off right now.  However someone who pulls too hard too fast may pass through the grey out period to pass out phase too quickly and it's lights out for a bit.   In the end, the pilot just misjudged and screwed up, probably by getting himself into a situation where the choice was to pull too hard or hit the ground.

Great link provided by a commenter for more info.


What is the differences of ADS-B and geometric height? Why does ADS-B and geometric height (calculated height) differ from each other?

If the geometric heights are calculated at night where the temperature is lower, then the result should be different during the day, right?

Is ADS-B with night or day calculated height smaller?

What is the more accurate result to be referenced?

ADS-B, and the older Transponder Mode C, report pressure altitude. That is static pressure where the plane is expressed as altitude at which that pressure occurs in international standard atmosphere.

In low pressure area, the same pressure density corresponds occurs at lower geometric altitude with similar difference at all altitudes. In higher temperature, the air is denser and the isobars closer together, so the same pressure altitude occurs at lower geometric altitude and the difference increases with altitude.

However for purpose of traffic separation, pressure altitude is always used. Due to physical properties of air, the pressure always decreases with altitude and at the same altitude changes only slowly with position, so we can always rely on close aircraft observing sufficiently different pressure altitude being at sufficiently different geometrical altitude.

The reason is that while barometric altimeter is simple, accurate to less than 100 ft and every aircraft has it since the early days of aviation, there was no way to measure geometric altitude before selective availability was turned off in GPS in 2000.

Geometrical altitude is altitude above the reference geoid (usually WGS84), a.k.a the sea level. It is only used internally in EGPWS where it is provided by GPS.

In contrast height is in aviation always used to mean height above terrain. It may be measured directly using radar altimeter or it can be estimated from pressure altitude by correcting for sea-level pressure at the nearest airfield.

Radar altitude is used in GPWS as a backup datum and in older such systems  from before GPS was available (those systems could warn if you were too low, but not if you were approaching high ground, because the radar altimeter only measures down while the new GPS-based systems have a terrain map and compare the GPS position with it) and is shown to pilot for reference, but it is not broadcast anywhere.

Approaches are normally flown in reference to the barometric altitude that is corrected for pressure at airport (the altimeter has setting for reference sea-level pressure, QNH), but not temperature. So the barometric altimeter shows true altitude at the airport, but when it's cold, it shows more than true altitude above. That's why some approaches have minimum temperature.

However, both Mode C and ADS-B broadcast pressure altitude, i.e. as calculated for standard pressure of 29.92 inHg/1013 hPa.


How would you use the mental clock code to solve this dead reckoning problem? Can someone explain the question below in details?

You plan to fly a cross country flight at 2 000 feet at a true airspeed of 120 knots. Your track is 270° (True) and the wind is from 225° (True) at 24 knots. Using the Mental Dead Reckoning Clock Code, what true heading would you steer, and what would be your magnetic heading if the local magnetic variation is 4° West?

Step 1: Calculate maximum drift.
Max drift $ = \frac{wind speed}{TAS (NM/min)} = \frac{24}{120/60} = \frac{24}{2} = 12$
That means if you have a direct crosswind, you'll need to correct 12$^{\circ}$.
Step 2: Determine how much of the wind is crosswind
The mental clock code is a way of estimating crosswind component from total wind. There are 60 minutes on a clock face.  If the wind direction is 60$^{\circ}$ or more, it's considered all crosswind, just as 60 minutes is considered the whole clock face.
By extension, a wind angle of 30$^{\circ}$ means 30 minutes, which is half a clock, so half the wind would be considered crosswind.  15 degrees would be a quarter of a clock, therefore a quarter of the wind, and so on.
Your maximum drift is 12 degrees with a full crosswind, but you don't have a full crosswind. Instead you have a 45$^{\circ}$ angle, so 45 minutes of a clock, which is $\frac{3}{4}$.
$ 12 * \frac{3}{4} = 9$ so you'll need to apply 9 degrees of crosswind correction toward the wind.  $270 - 9 = 261^{\circ}$
Step 3: Correct for variation
"East is least, West is best".  If variation is west, add to true to get magnetic.
$261 + 4 = 265^{\circ}$


Why would an airplane tip backwards during unloading? I flew on JetBlue Flight 1011 BOS-ORD today, 2017-01-14. At the arrival gate, after 2/3 of the passengers had deplaned, a flight attendant asked us to stop and wait. He told us that the pilots were concerned that the plane was "tipping" to aft. I overheard the pilot say that the front landing gear was fully extended. The pilot also mentioned that they had "never seen anything  like this," that there was no cargo on board, and that the baggage handlers had 30-40 bags remaining at the rear of the plane. After about ten minutes, we were all allowed off.

Why would something like this happen?

A picture is worth a thousand words... Passenger airplanes do not fasten baggage in the belly down, occasionally there can be a concern from the baggage pilling up at the tail (i.e. turbulence). New baggage employees may also have offloaded the aircraft wrong - unloading the front first.

I have been on airplanes where the flight attendants moved passengers from the rear of the airplane to first class because the aircraft was too tail heavy.

Most newer jets have pressure sensors on the gear. This allows the flight computer to show the crew how the aircraft is loaded in real time. The pilot may have received a warning the nose gear was too light or he may have notice the aircraft tipping up.


How to lean with constant speed prop on multi-engine aircraft without EGT This question is more about the indication of engine roughness. The procedure for leaning without an EGT is clear in the POH and Lycoming documentation which basically states as follows:

Best power:

lean until "engine roughness is noted."
Move mixture forward approx. 1/2 inch toward full rich.

Best Economy (75% power or less):

lean until "engine roughness is noted."
Enrich mixture until engine runs smoothly.

My question is what is engine roughness as defined by Lycoming in this situation?

I like to lean it until engine is clearly sputtering (no question its running rough), before I enrich, but most CFIs I've flown with lately seem to "detect" roughness with some magical senses, I think they are just going with their gut as opposed to empirical indications such as drop in MP or objectively detectable drop in engine performance.

My understanding of "engine roughness noted" would be a clear audible roughness of the engine being leaned, along with a slight, yet detectable drop in manifold pressure of said engine.

Engine Roughness is not a defined term in the POH or Lycoming documents I've read. So in my opinion it should be referring to a clearly objective condition that any person can easily identify.

I'm hoping someone can confirm or provide some objective guidance on how to identify "engine roughness" when leaning an engine on a twin with constant speed props.

There is a big difference between "by feel" leaning of a carburated vs fuel injected engine.  Carburated engines have less even fuel distribution and when leaning, one cylinder, the already leanest one, will start to misfire quite a bit before the others.  So with a carb it's easy to detect one cylinder starting to misfire (the cylinder usually doesn't totally quit right away - you will pick up random misses in the vibration rhythm).

With a fuel injected engine the mixture distribution is more even and misfire of one cylinder will be accompanied by misfire of its sisters almost immediately.  So on an injected engine you will lean and suddenly the engine will really start to stumble, whereas with a carburated engine there will just be these random dropouts in the engine's "beat", getting worse as you continue leaning.

So if you are leaning on an aircraft with fuel injection it is really fairly straightforward.  The engine will go from smooth to pretty rough right away, whereas with a carb it takes careful attention to detect the early onset of misfire. You are at a disadvantage to some degree because the airplane is less familiar than to the instructors.  It's a skill you will pick up over time as you develop at ear for it.

Don't worry too much about it. Err on the side of smoothness, knowing that the difference in fuel consumption between perfect and good enough is negligible (and if you're paying hourly on a hobbs meter, there is no benefit to you anyway), and it's wise to err on the side that is beneficial to the engine (too rich).


How does an A330 detect stall without airspeed? On Air France 447 the crew had experienced complete failure of the pitot static system, which meant they lost their readings on their airspeed indicators, but according to the Mayday / Air Crash Investigations episode the aircraft had detected that the plane was about to enter an aerodynamic stall and the stall warning systems were activated just before the crash.

How did the A330 in question detect that the plane was on the verge of stalling without the airspeed indicators working?

Stalls occur based on a wing's angle of attack rather than the aircraft's airspeed.  (In fact, one of the basic facts that all pilots learn in their initial training is that an airplane can stall at any airspeed).  The A330 measures angle of attack using vanes mounted on the fuselage:

However, below 60 knots, these vanes become ineffective.  During the AF447 accident, the stall warning only engaged intermittently because the computers detected that the airspeed readings from the blocked pitot tubes were invalid, and therefore silenced the stall warning system.

Furthermore, because of the lack of valid airspeed data, AF447 was no longer in normal law (and therefore had no automated angle-of-attack protection), a fact which the pilots may not have been aware of.  By the time the deicing equipment had cleared the pitot tubes, the aircraft was at such an extreme pitch that airspeed indications were still considered invalid, and the stall warning system did not activate.  The computers only activated the stall warning system when the crew pitched down, which brought valid airspeed data back into the cockpit.

An Airbus press release suggests that this may have contributed to the accident:

  We also note that the aircraft, or more specifically the design of the stalling warning system, misled the pilots: each time they reacted appropriately, the alarm triggered inside the cockpit, as though they were reacting wrongly. Conversely, each time the pilots pitched up the plane, the alarm shut off, preventing a proper diagnosis of the situation.

(Emphasis mine)


Why doesn't the pressure inside an airliner push the doors open? When flying at cruise altitude, the pressure inside the aircraft is higher than outside and that means a lot of air is pushing the door outwards. So how does the door stay in place and not open?

I'm not asking if a passenger could open a door, because that's almost impossible. I'm asking how does the door not open if there is a lot of air pushing it outwards?

Most of the airline doors are plug type- i.e. they are wedge shaped, so that the pressure differential pushes it against the fuselage and prevents opening. For example, the following image shows the Boeing 737 door.

Image from airteamimages.com

Note that the door is bigger on the inside, so that when it is locked in position, the pressure will push it against the fusealge, preventing it from opening. For opening the door, it has to be swung inwards before moving out.


Why are almost all STOL aircraft high-wing? Isn't ground effect a big factor in STOL operations?  Don't low wings produce greater ground effect than high wings?

Or is it a practical concern, that most real-world STOL operations are in unimproved or slightly-improved locations, and high wings are more likely to clear "runway" obstacles?

You're correct that the low wings produce more ground effect compared to their high winged counterparts. However, during takeoff and landing, it can result in undesirable handling qualities- the aircraft can 'float' the runway; on contrary, the high wing design is more stable. There are other reasons for STOL aircrafts to have high wing designs, most of which has to do with the location of their operation, as you noted:

Most of them are operated from austere airfields. The high wing location gives some protection from the debris (and obstacles).
For the same reason, engines mounted on the high wings have better protection compared to the low wing mounted ones from FOD ingestion.
Most STOL aircraft use large TE flaps. In case a low wing is adopted (and also if propeller is used, like Twin Otter, for example), the required ground clearance will require long and consequently heavy landing gear. This can be avoided in case of high wing design.
The visibility is better compared to low and mid wings.

Historically, STOL aircraft have been high wing. Also, some of the STOL aircraft are (military) cargo aircraft- the high wing means there is no wing spar across the cargo compartment; plus, the low fuselage means easier loading/unloading of cargo.


How does distributed propulsion work in a pusher configuration? How does distributed propulsion work in a pusher configuration?

In a tractor configuration, the wing benefits from a portion of the prop wash flowing over the wing and producing extra lift ( I stand to be corrected).

In a pusher configuration, there is no prop wash flowing over the wing.

So how does distributed propulsion created extra lift and/or thrust in a pusher configuration?

The wings don't benefit all that much.  Look at a small airplane from head on - how much wing is directly behind the prop?

My airplane for example, the prop is 86" diameter. The fuselage is 46" wide - so 20" of wing on either side is behind the prop.
The wing itself is 35'6", or 426", so less than 1/10" is in direct prop airflow.

The prop pulls the plane forward, airflow over/under the wing then creates the lift.


Is it practical to use wires to stiffen rotor arms? (Related to my last question on ducting.)

Wires have been used in the past to stabilize wings with a very low weight cost. Could something similar be use to stabilize the arms for a UAV's rotors?

Assuming I don't have enough money to build a custom 8 arm (based on previous advice) carbon fiber frame, could the wiring approach be applied to UAVs? I ask because for a V1 prototype PVC is cheap and relatively strong, but I could enhance it with a bit of aluminum plate in the center and small aircraft cable in tension.

I see little reason why small gauge wire couldn't be used to strengthen rotor arms on a multirotor. The only major issue I could see developing is that the wire on the top side of the arm might accidentally intersect the prop circle.

MS Paint sketch of a possible design idea:


When is a VOR shown on maps? I do not understand when the Compass around a VOR is and is not present on maps.

For example, around Kansas City:

VOR OJC has been deactivated for quite some time now.  But its compass is still present on the latest map revision. (the frequency has hash-marks through it, indicating the deactivated status.

Why hasn't the map been updated to remove the compass?

VOR RIS is still active, but does not have a compass around it.

Why isn't the compass on this active VOR?

Are there any general guidelines for when a compass is present on the map?

See the Interagency Air Cartographic Committee (IACC) Specifications:

  [IACC 2] 3.9.3.8 Compass Roses

  Compass roses, as illustrated in Appendix 6, shall be shown centered on all VHF NAVAIDs, when space allows.

  Preference shall be given to those serving enroute functions or located in areas not served by other VHF NAVAIDs.

  [IACC 5] 3.7.4.5 Compasses Roses

  Compass roses shall be charted centered on those NAVAIDs which have routes predicated on them. Compass roses may be shown around other NAVAIDs where placement of the compass rose does not create additional unnecessary clutter or congestion. If compass roses overlap and congestion hinders readability, the compass rose of the NAVAID with the most congestion may be deleted. Compass roses shall be shown oriented to slave magnetic North. Cardinal direction values shall not be shown. Compass roses shall be .75" in diameter and in very congested areas may be reduced in size to .5" in diameter.

In groundschool I was taught that this is actually based on VOR service volumes (which otherwise aren't explicitly marked at all on VFR sectional charts): the compass rose is not shown around VORs with Terminal service volumes. This might be a general rule, but it is not entirely true however. You provide a perfect example: OJC and RIS are both Terminal VORs.

The key differences: 1) OJC has the V10-12 airway, and 2) map is more congested around RIS.


Can you become a commercial airline pilot in Europe while being monocular I’m not sure if I could get a EASA medical class 1 while being monocular. But I think that you can get a soda in the us and I want to know, if I could take a FAA medical class 1 and then fly in Europe with a FAA. Or even better if I could get a soda in Europe and get a EASA medical class 1, and become a commercial airline pilot in Europe while being monocular.

I know this has been asked before but that only covers about getting the license, and I want to know if I could get a soda.

Pilot certificates don't give you permission to fly in a location, they give you permission to fly an airplane registered by that authority anywhere in the world. In other words, if you have an FAA pilot certificate you can fly a US registered airplane no matter where it is. If you have an FAA ATPL and appropriate class of medical you can work for any airline that flies US registered airplanes. The same is true for any licensing authority.

The reverse is not true, you could not get a FAA certificate and fly a UK registered airplane, for example, even if it was in the US. There are some cases where you can get a local certificate on the basis of another equivalent certificate, but I believe you always need to be able to pass the local medical.


Is a Cessna 172S still airworthy if factory-installed instrumentation not required for day VFR flight is inoperative? A C172S I occasionally rent had the EGT gauge placarded INOP when it had a JPI EDM-700 engine monitor installed. The placard sits on the gauge so you can no longer see the needle. Probably the single point EGT probe has been removed so the needle wouldn't work anymore. Of course that is fine - the EDM-700 is TSO'd.
Now recently the EDM itself is placarded INOP and its circuit breaker collared so it cannot be reset.

So now this 172S has two INOP EGT gauges, and I'm down to leaning by ear. I know the EGT gauge is not required daytime VFR equipment. However, since the airplane was certificated having a factory EGT gauge installed, I'm wondering whether it's airworthy like this. The Plane is N-registered and I'm asking within FAA jurisdiction.

Part 91 does not require an EGT probe for any operations so the next thing to look at is your AFM.

Your C172S has an AFM that includes a Comprehensive Equipment List. It is probably similar to this one that indicates that the EGT probe is standard equipment—not required for certification, this airplane can fly with it placarded.


How to do Weight & Balance for Cessna 172? What is the best method to do Weight & Balance for a Cessna 172? (If it's possible with a small tutorial!)

Thanks

The Calculation section of “Center of gravity of an aircraft” on Wikipedia gives an overview of the steps.

  Determine the weights and arms of all mass within the aircraft.
  Multiply weights by arms for all mass to calculate moments.
  Add the moments of all mass together.
  Divide the total moment by the total weight of the aircraft to give an overall arm.

The Basic Empty Weight (BEW) of the aircraft will come from the Weight & Balance information that is unique to each aircraft. The computed arm for the BEW may have some rounding error, so treat it as informational. Use the precise figures for Weight and Moment from your aircraft’s W&B.

The arms, distances from some reference datum, will also come from your aircraft’s W&B info: fuel, front seats, rear seats, and baggage areas on a Cessna 172. Copy those into your table as well.

Avgas (100LL) weighs 6 pounds per gallon, so the weight is 6 times the number of gallons.

I like to work top-to-bottom and left to right. For the grand finale, divide the sum of the Moment column by the sum of the Weight column to get the overall center of gravity.

The example below is from my PP-ASEL checkride taken in a Cessna 172. The items in boldface are computed values. The digits after the decimal are there for typographical purposes and do not necessarily indicate significant figures.

$$\begin{array}{c|r|r|r|}
                     & \textit{Weight} & \textit{Arm} & \textit{Moment} \\ \hline
\text{Airplane}      & 1{,}665.6     & 39.2       & 65{,}928.93      \\
                     &               &            &                  \\
\text{Fuel (50 gal)} & \bf{300.0}    & 48.0       & \bf{14{,}400.00} \\
                     &               &            &                  \\
\text{Bacon}         & 180.0         & 37.0       & \bf{6{,}660.00}  \\
\text{Shelton}       & 172.0         & 37.0       & \bf{6{,}364.00}  \\
                     &               &            &                  \\ \text{Duffel}        & 10.0          & 73.0       & \bf{730.00}      \\
\text{Shelton bag}   & 10.0          & 73.0       & \bf{730.00}      \\
                     &               &            &                  \\
\text{Backpack}      & 15.0          & 93.3       & \bf{1{,}399.50}  \\ \hline
                     & \bf{2{,}352.6}& \bf{40.9}  & \bf{96{,}212.43}
\end{array}$$

In this particular case—given for information only—the center of gravity (40.9 ≈ 96,212.43 / 2,352.6) at the total weight is within limits for the Normal category as given in the aircraft’s Pilot Operating Handbook (POH). Your case may vary. The Pilot-in-Command is responsible for computing Weight & Balance for each flight.

As a point of interest, my DPE was Clyde Shelton who recently flew his ten thousandth checkride.


Why is the fuel:air ratio of a turbojet engine not around ~1:15 (the stoichiometric ratio)? Also, what is the maximum fuel:air ratio possible in a combustion chamber of a jet engine?

Edit: to honor my original answer, I shall keep it as it is. The question was edited so much I think it would have constituted a whole new question. In it's original form, the Q presented jet engine f/a ratio as 1/4 - 1/3 and compared it to the stoich ratio of 1/15.
Your numbers seem way off, except for the stoichiometric fuel/air ratio (for kerosene).
Please see the following NASA article: Beginner's Guide to Propulsion and EngineSim
Fuel and Air Relationships
Activity
A quote from there:

For jet engine combustion, the important parameter is the fuel/air ratio (f/a) which is the ratio of the mass of fuel to the mass of air being burned in the engine.

The stoichiometric ratio of 1/15 can also be written as approximately 0.067
The article further elaborates on the subject:

For example, at sea level an airliner taking off at 375 mph with the throttle at 100% has an f/a of .017

This 0.017 translates into about 1/59
No matter how you look at the ratio, mass or volume, in no case will a jet engine run at f/a of 1/4~1/3. The mixture will simply be too rich to ignite and burn.
Here is a link to a Wikipedia graph about the range of typical jet engine air:fuel ratios:
Combustion stability limits of aircraft gas turbine.svg
Please note the graph displays the ratio in reverse order from what was used before in this answer, ranging from ~25/1 to ~180/1.


What does it mean to maintain a positive g loading? When we hear about a "g", if I am not mistaken, one "g" is an equivalent force to that which is experienced when we are standing at sea level.  So what does it mean if a plane maintains a positive g loading and vice versa when a plane maintains a negative g loading along with how it feels to someone inside.

Short Answer

In straight and level flight, it means don't push over too fast.  Doing so gets you "light in the seat" and things may fly about the cockpit/cabin.  In some aircraft, negative "g" can make the oil "flow up" away from the sump and begin to starve your engine of much needed lubrication. (Example: T-34C, trainer, limited to 10 seconds inverted flight.  Straight and Level inverted flight is -1 "g.")

More Answer

You want to avoid flying with negative "g" in a variety of circumstances,  depending upon how the aircraft is designed.

Example: In a helicopter, negative "g" can have dire consequences.
On the teetering, semi rigid rotor heads in the original Huey, for
example, unloading the head can lead to mast bumping, which can
lead to catastrophic failure of the mast, and then some death.
Counter Example: Flying a Pitts Special, or an F-18, the aircraft is
built to withstand some negative "g" due to their needing to fly
particular maneuvers.

"G" is a natural part of flying, but "how much" varies with design.

Whenever you turn the aircraft, you will add a little bit of "g" load since "g" is a measure of acceleration.  Vector sums based on angle of bank will tell you how much "g" you can expect in a level turn at constant speed.

Example: a 60 degree angle of bank adds an additional "g" compared to
level flight, and you feel a bit of a push into your seat.


Why Western fighters' afterburner glow is reddish orange while Eastern Bloc fighters' is blue? Recalling what little physics I know, this could be explained by two factors:

The blue color is given by the characteristic EM radiation of C-H bond breaking, C- and H- radicals forming and recombining with O- radicals to form CO2 and H2O. This is independent of temperature and is the reason why a new gas oven's flames are blue.
The orange color is definitely the characteristic blackbody radiation wavelength of about 1500K, ~1200 degree Celsius. Now the problem is what the source of this blackbody radiation is. The way I see it it's possibly from three sources:

The hot afterburner inner wall,
The soot produced by partial combustion, or
The incandescent exhaust itself.

If the above recap is correct, this means either

the Western afterburners either run a lot dirtier than Eastern afterburners, which is unlikely because F35 and F22 have orange afterburner flames and they must be running squeaky clean to not have their afterburner inner walls coated with soot (amorphous carbon) thus having a huge IR signature because of the high emissivity of carbon, and we do not see black smoke coming from any jet since F4,

or

Western afterburners are a lot hotter than Eastern afterburners, to the point that the blackbody radiation color, which is dependent on temperature, dominates the radical color, which is independent of temperature. This seems to be the logical conjecture, given that the Russians have always been pretty poor on metallurgy.

Does anyone have authoritative knowledge on this subject? This rumor seems to be circulating on the internet forever.

EDIT: OK, the exhaust may seem a little purplish, that is caused by the emissive characteristic of H2O.

EDIT: Or maybe the Su27 and Mig29 just had a cold start and the inner walls of their afterburners still cold?

EDIT: On closer inspection, the orange color seems to be solely coming from the glowing afterburner inner wall, as opposed to the exhaust, although the exhaust is glowing also. This seems to confirm my guess that Western afterburners run hotter.

EDIT: OK, I've spoken to some guys from one of the ex-soviet design bureaus. One of them said the blue color is the result of cold afterburner and Sukhoi afterburner design, the fuel being dumped at acute angles into the center of the cylinder using a few big injectors.

It all depends. I don’t think there’s a distinctive color between the two.   I would submit that the photos of the Flanker may be in a early stage of the afterburners lighting versus the photos of the Raptor or the F-35, which appear to be in full afterburner.   The color of the exhaust gas is a function of the stoichiometric mixture of fuel and air ratio. It full burner the mixture is quite rich, resulting in a orange colored flame.   Lower stages of afterburner are probably  much closer to peak stoichiometric mixture.  Incidentally these blue colored flames are indicative of a much hotter exhaust gas temperature than an orange colored exhaust.   I’m not sure of the reason why one is selected over the other.   I can submit that there are plenty of photos of Russian aircraft in afterburner with orange colored exhaust and other pics of western military airplane with blue colored exhaust as seen below.


How do I find the three factors of the hinge moment coefficient? How do I find the three factors $c_{h0}$, $c_{h\alpha}$ and $c_{h\eta}$?

First download and install xfoil from this site
http://web.mit.edu/drela/Public/web/xfoil/
if you use ubuntu - sudo apt-get install xfoil

user manual and typical session inputs are also there. please refer to them as you haven't used it before.

And there are many video tutorials in youtube.
[Link removed I cant put more than 2 links here- Just search xfoil tutorial in youtube]

For this specific case;

open your airfoil in xfoil
xfoil name.dat
Set the flap as you want
enter GDES and FLAP and follow the prompts.
Type these commands in sequence
exec
[enter]
pane
Enter OPER menu and set the RE number
oper
visc 1e6 (1 million Re for this case)
type  these commands in sequence
hinc     (hinge moment togle enabled)
    pacc     (accumilate polar - give a file name)
aseq 0 10 0.5 ( sequence of alfa to run - here from 0 to 10 in 0.5 increase)
Open the resulting file. which has two coloums along with other parameters to show alfa and hinge moment. (first and sixth)

plot it or linearize it or whatever you like to do to find the above parameters.

PS: Another suggestion is XFLR5. It is same as XFOIL but with GUI. you may find it easier.


What is the US FAA concerned about when it comes to psychiatric medications? I don't understand the FAA's policy for psychiatric medications (specifically anti-depressants). Obviously I'm glad there is oversight into who can obtain a pilot's license for the safety of the pilot, crew, passengers, and the people on the ground. And obviously people need to be in a stable condition before they eligible to fly. But some of these rules sound ridiculous to me.

First, they won't approve you to fly while taking a psychiatric medication, but they may approve you to fly if you decide to go off of it for 60 days.

What?

So basically if you have a chronic mental health condition, and you are taking a medication for it, and it's working well without side effects, it's better to go off the medication before you try flying? Wouldn't the disease be more apt to return if you were off the medication, than on it? That sounds like a very stupid risk for the FAA to take.

Are they worried about side-effects? Most of the side effects for modern SSRIs are very limited and/or transient and/or don't apply to flying (like constipation), so I know they can't be worried about those.

Are they just concerned about withdrawals if you miss a dose? That seems pretty unlikely to happen, and probably less likely to cause trouble than to go off meds entirely.

Basically, I don't get it. It seems that if you are fully-treated for a condition without side effects that you should be eligible to fly, regardless of whatever drugs are put in your body. Honestly, I'd be more concerned if people were taking less medications for passing a test, than what they were on before.

The FAA is concerned that individuals taking anti-depressant medication will not have the mental focus required when juggling multiple tasks in real time in the cockpit.

anti-depressant medications are psychotropic -- They are specifically intended to alter the takers' mental state.  The assumption is that any artificial modification of mental state will negatively impact aeronautical decision making.

The SSRI special issuance protocol requires, among other things, the applicant to demonstrate the ability to quickly and accurately perform multiple tasks simultaneously.  The idea is to make sure that you aren't being slowed down by the medications to the point where you can't make the right decisions rapidly enough to ensure safety of flight.


Can private pilots drop skydivers if the pilot pays a pro-rata share? This question was inspired by this other question. If a private pilot was to pay the pro-rata share of a flight for some skydiving friends, could they take those friends up and let them jump out?
If so, what counts as "pro rata share"? Is it expenses for the whole flight from takeoff to landing, or does the pilot assume 100% of the cost after the point where the skydivers leave the plane?

This is not so difficult. Many people like to refer to the regs without actually looking at the regs. This is all taken from §61.113.
Pro rata share is not the same as getting compensated.

(c) A private pilot may not pay less than the pro rata share of the operating expenses of a flight with passengers, provided the expenses involve only fuel, oil, airport expenditures, or rental fees.

In this scenario, the pilot isn’t holding out as a commercial operator; these are his friends.
According to any online legal dictionary, the pro rata share is defined as the total cost divided by the number of shares. In other words, the total cost of fuel, oil, airport costs, and rental fees split evenly between everyone in the plane, including the pilot. As long as it is split evenly, it’s not compensation or hire, it’s cost-sharing.
All that said, NEVER do anything because some guy on stack exchange told you that it was ok. Check the regs for yourself and contact your local FSDO for clarification.


How is DME station ident coded in ARINC 429? I understand that labels 242,244, 246, 247 are set for DME Station Id or DME Station Ident. they are marked as discrete. How should I convert the station code from alphabet to these 'discretes'? The station codes are something like AGG.

Labels 246 and 247 describe up to 4 7-bit ASCII characters.

1th char: Bits 11-17 of 246
2th char: Bits 19-25 of 246
3th char: Bits 11-17 of 247
4th char: Bits 19-25 of 247

For AGG, in ASCII we get 0x41, 0x47, 0x47, then:

Label 246 = 0x11D04A6
Label 247 = 0x11CA7

SSM encoding is the same as regular discrete labels.
I am not familiar with labels 242/244.


How can shifting to TCP/IP make current aviation systems more delay-tolerant? I've recently been reading a number of articles on the current aviation systems. Many of them say that the current aviation infrastructure is 'inefficient' since it is based on OSI model and, so, Eurocontrol/FAA are trying to shift to a TCP/IP model (without further elaboration as to why it is inefficient.)

As far as I know OSI and TCP/IP are both protocol stacks and pretty much follow the same ideas.

I'm not quite sure why they say that the current OSI model is inefficient? In what sense is it inefficient?

Another point, in the articles I read, which I thought was a bit vague, was that it said that the TCP/IP model is more 'modular' and can improve 'flight delays'.

Again, weren't both OSI and TCP/IP models created to make systems more modular? How can shifting to TCP/IP make current aviation systems more delay-tolerant given that the main cause of delay is air traffic and weather?

Articles I've read.

This one is the main article, published by Eurocontrol/FAA, that I read. It suggests a transition from a voice-based to data-based services network.
This one is a PhD thesis. See Section 3.1 for the main references. They refer to the TCP/IP ATN as ATN/IPS. Search for the word 'ATN/IPS'.
This one gives a detailed description of ATN/IPS.
The following claims that a TCP/IP model is more modular than the current OSI based model.

The current air/ground systems (based on OSI) are aviation unique. And they are limited by design to "messages no larger than 3.5 kilobytes". This limits how much information can be sent to/from an airplane, and is a major roadblock towards time based navigation and the broader data-hungry Global Air Navigation Plan (GANP), which includes SWIM for example.

SWIM: an advanced technology program designed to facilitate greater sharing of Air Traffic Management (ATM) system information, such as airport operational status, weather information, flight data, status of special use airspace, and National Airspace System (NAS) restrictions.

Option one, develop better aviation-unique systems (OSI) that can handle more data. Very expensive to certify, very hard to get everyone to agree on the system. Option two, use the IPS (Internet Protocol Suite) that has proven itself. And thus commercial off-the-shelf equipment can be used.

The reasoning at the time was that the OSI protocols were more formally specified and included such things as detailed protocol implementation conformance matrices and combinations of functions at each protocol level called profiles, and thus were more suitable for avionics certification.
(...)
However, as is common knowledge, the TCP/IP protocols have long since been proven in the World Wide Web and therefore are able to offer potential economic benefit and provide for the rapid introduction of new services in the global aviation environment.

OSI means OSI-based (such as ARINC 429), and is unique to aviation. Example: A router on a Boeing 777 is not sold on the non-aviation market. What they want to do is use off-the-shelf solutions to be used on airplanes. Using ready-made solutions is more economical than creating solutions for a specific industry. The current solutions can't handle the data requirements of the future.

IPS will also allow easier integration with the already IPS based ground/ground communication between airlines and air traffic management. The current plan is to have a seamless transition from OSI to OSI+IPS to IPS in the next 15 years or so.
Why is IPS more delay-tolerant?
Example: When every single user (pilot, airline, ATC, airport, etc.) has access to the same real-time weather data (e.g. via SWIM), decisions can be made more correctly, in a timely manner, and consistently across the users. Just like what FANS-1 did for the Pacific operations in the 90's, but way bigger (history below).
This is the gist of it and explains why the shift. For technical standards there are plenty of ICAO documents on the web: ICAO Doc 9705 (OSI) and ICAO Doc 9896 (IPS).
Also related: Why Boeing and Airbus are Pushing for IPS (aviationtoday.com)

A brief history of aviation data comms:

ACARS started as a punch clock for the flight crews by auto reporting the different flight phases (aka OOOI)
Then Europe expanded on what the US has done with it and used it for operational control, e.g., by keeping track of where the fleets are, etc. (ops driven)
Then British Airways took it a step further for airplane health monitoring (engineering driven)
Then the Pacific region wanted to benefit from it, and so came Boeing's FANS-1, enabling flights that took off with +11 hours old weather forecasts to readjust their routes mid-flight. The flights can now receive from the airlines the weather updates, new flex-routes, etc., and communicate it to the ATC. Shaving off 15+ minutes on long transpacific flights, and enabling more payload (2+ tonnes per flight)
Now FANS serves ATC (CPDLC and ADS-C) in oceanic airspace, and operators (AOC)
Airbus also made its own FANS-A, now they are known as FANS-1/A
Then they became FANS-1/A+ by adding a latency timer for the ATC messages (CPDLC), so a plane would not receive a delayed message that was meant for a previous flight this plane flew
Then PM-CPDLC (aka LINK2000+ and ATN-B1) came along in Europe, which requires yet again different equipment.

For ATC, FANS-1/A+ is mainly used in oceanic airspace and in delivery clearances in the US, and CPDLC (ATN-B1) in parts of the European airspace, and the hardware for both is not compatible (if used together) on older planes.
FANS-1/A+ combined with ATN-B1 creates FANS-2/B as one package (like on the Boeing 787). The plan is to make all systems use a standard network with higher bandwidth, which is the current goal of FANS-3/C (FANS-1/A+ and ATN-B2) using an IPS infrastructure while supporting the legacy planes. Check the progression of the 3 stars I added below.
(ICAO)

Note: the acronyms above may seem to overlap, e.g., for ACARS and CPDLC, but as the uses expanded and new systems were added, the meaning now depends on the context. For example the early FANS-1 CPDLC is still ACARS at its core, and also CPDLC can refer to ATN-B1 in Europe, or DCL-CPDLC (DCL = Delivery Clearance) in the US for example. It's an alphabet soup.

Further reading:

All You Need to Know About CPDLC ATN-B1
What you should know about FANS-1/A+ ATN-B2


Could a fighter aircraft be operated indefinitely? Is it possible for an air force to operate a fighter aircraft indefinitely?

Suppose a country has some Mig-21s or Mirage-IIIs. Is it possible for them to operate those aircraft indefinitely by continuously overhauling and upgrading them?

Yes and no. In Cambridgeshire we still have some Spitfires and other Second World War aircraft which are still operated more than 60 years after being built. The Shuttleworth Collection even flies some First World War and pre-war aircraft. It's demonstrably possible to keep operating a military aircraft for at least a century after it's built.

But that's not useful if the aircraft has no military value. None of the examples I've mentioned are still operated as military aircraft: they're in civilian hands, performing for displays, mostly for their historic value (though many ex-military aircraft in civil use are performing other functions, such as air transport or crop spraying).

Aircraft get retired for two reasons. The most obvious is that they're superseded by newer types, newer designs with better technologies and materials. The first air force that tried to field one of these Spitfires in combat against modern jet fighters would quickly realise it had no military value whatsoever.

The second reason is that they get increasingly expensive to maintain. Military aircraft especially can be very fussy about fuels and parts. Maintenance might require special fabrication of small numbers of parts or tools, which gets more expensive when the rest of the industry has moved on. (For example, the Tiger Moths I fly have one particular part that's impossible to obtain, because the alloy it's made of isn't produced any more.)

For this reason, although there are still surviving, operating aircraft from the earliest days of military aviation, it wouldn't be feasible or useful to continue operating them as combat aircraft.


How can I acquire an N number that's showing as deregistered? I'd like to use an N number that is listed as available. But when I request it, it shows as being deregistered. How can I get this number for my aircraft?

If the number is deregistered, you cannot re-register it unless you have the previous registered owner of that number submit a request to relinquish the number to you.

So you must get in touch with the old owner of that N number, and if he is willing to write a letter relinquishing it, all you should have to do is send his letter in with your request and a check.

If you have any other doubt, this page has information about your issue:
About Aircraft Registration Renewal

If you need any other information, do not hesitate to comment and I will try to give you a more detailed answer for your problem


What is a "Descend Via" clearance? When flying to Teterboro, NJ (KTEB) and cleared via the JAIKE THREE ARRIVAL from Flat Rock (FAK):

What does ATC expect when they give a clearance like:

  N1234, Descend VIA the JAIKE THREE ARRIVAL

Then, what if they give the next clearance when over JAIKE:

  N1234, Descend and maintain 7000

I believe that there is a difference in ATC procedures over whether or not the crossing restrictions still need to be made between US and Canadian rules, what should be done in both cases?  (And out of curiosity, does ICAO have anything to say about this situation?)

The "DESCEND VIA" clearance is described in FAA order 7110.65U (pdf) Section 4-5-7 paragraph h, which defines:

  h. Instructions to vertically navigate on a STAR/RNAV STAR/FMSP with published >restrictions.

  PHRASEOLOGY-
  DESCEND VIA (STAR/RNAV STAR/FMSP name and number)
  TERMINAL: DESCEND VIA (STAR/RNAV STAR/FMSP name and number and runway number).

  EXAMPLE-
  “Descend via the Mudde One Arrival.”
  “Cross JCT at flight level two four zero, then descend via the Coast Two >Arrival.”
  TERMINAL: “Descend via the Lendy One Arrival, Runway 22 left.”

This instruction authorizes the pilot to navigate the STAR laterally as published and to navigate vertically to meet all published altitude restrictions.

  Clearance to “descend via” authorizes pilots:

  To vertically and laterally navigate on a STAR/RNAV STAR/FMSP.
  When cleared to a waypoint depicted on a STAR/RNAV STAR/FMSP, to descend from a previously assigned altitude at pilot's discretion to the altitude depicted for that waypoint, and once established on the depicted arrival, to navigate laterally and vertically to meet all published restrictions. ATC is responsible for obstacle clearance when issuing a “descend via” clearance from a previously assigned altitude.

Though speed restrictions are not specifically addressed in the above paragraph, it does note that you will meet "all published restrictions" to include speed restrictions.  This is supported by a later note:

  EXAMPLE-
  “United 454 descend via the Haris One Arrival, except after Bruno, maintain one zero thousand.”

  NOTE-
  The aircraft should track laterally and vertically on the Haris One Arrival and should descend so as to comply with all speed and altitude restrictions until reaching Bruno and then maintain 10,000. Upon reaching 10,000, aircraft should maintain 10,000 until cleared by ATC to continue to descend.

This example and note pertain to modifications to a published altitude, but the note establishes that you comply with all published altitudes and speeds when flying the procedure as published.

Furthermore, InFO 12014 explicitly states:

  Published speed restrictions are always mandatory. Published speed must be complied with regardless of the use of “Climb/Descend Via” clearance.

For the example of the JAIKE 3 Arrival, the clearance

  N1234, Descend VIA the JAIKE THREE ARRIVAL

will be issued if you are already established on the arrival laterally and authorizes you do follow the published route and altitudes.  Assuming the FAK transition is your cleared route when you receive the descend via clearance, you would fly the lateral route

  FAK QBEES VAALI PALEO PEEDS FUBRR BUZIE SPNCR JAIKE ILENE WACKI REGLE SBJ

and you would descend at your discretion to be at 13,000 ft at JAIKE, at 13,000 ft at ILENE, at 11,000 ft at WACKI (and at 250 kts) and 7000 ft at REGLE.  You descend to and cross the fixes at these altitudes because you have been authorized to vertically navigate the STAR via the "DESCEND VIA" clearance.

NOTE: This arrival contains an "EXPECT" altitude and the controller should not issue a DESCEND VIA clearance until you are past this fix unless you gives you specific instructions as in:

  N1234 cross PEEDS at FL240 then descend via the JAIKE 3 arrival

If the controller at any time issues an instruction e.g.

  N1234, Descend and maintain 7000

then:

In the USA

then you would continue to navigate laterally on the STAR but would descend to 7000 ft, disregarding any published altitudes on the STAR.  You are no longer authorized to navigate vertically on the STAR.  If the controller requires you to still cross certain fixes at certain altitudes, they will specify those crossing restrictions in the descent clerance, e.g.

  N1234 Cross WACKI at 11000 250 kts then descend and maintain 7000 ft.

In Cadanda / ICAO rules

you shall continue to meet all published crossing altitudes on your descent to your cleared altitude.  For reference on this difference, see FAA InFO 12003


What is the function of interplane strut of a Biplane? What kind of load does the interplane strut(s) carry,is it compression, tension or shear?
In order words,what's their function and how do you calculate the force that the interplane strut carries?

The wing strut prevents the lower wing from bending upwards due to the lift forces.

The biplane is a truss structure: rectangular shapes are "crossed out" by diagonal structural members, in order to create triangular shapes with only tension and compression in their structural members, no bending. If the diagonal is a rod, it can transfer both compression and tension forces and only one diagonal is required. If it is a cable, it can only transfer tension forces, and the other diagonal needs to be crossed out by a cable as well.

In flight, lift wants to bend the wings upward and lengthen the diagonal, which becomes tension loaded. The upper wing is then prevented from bending upwards and becomes compression loaded.

The interplane strut connects the lower wing with the upper wing, and prevents the lower wing from bending upwards. It is compression loaded: the diagonal exerts a downward force, lower wing lift exerts an upward force.

The method of how to compute the force is in this answer. In a truss structure the intersection points are modelled as hinges, and the structure members are dimensioned to carry the resulting tension/compression loads.


How does the FAA decide if a PPL applicant with ADHD can fly? For the past few years, I've been struggling with fidgeting, difficulty spending long periods of time on creative tasks, difficulty finishing long and complicated tasks, and a few other symptoms. These are difficult in my professional life as I'm a journalist, so long periods of time doing research and writing is a necessary part of the process. My physician mentioned ADD/ADHD as something to look into, with potential benefit if treated with medication.
The trouble is, I'm also looking into getting my PPL next year, and I noticed that the FAA outright bans a number of ADHD medications, and seems to have a whole extra evaluation process you have to go through if you get diagnosed with ADHD. Nowhere could I find a good explanation of what this means if you're considering getting ADHD treatment but you also love, love, love to fly.
What is the FAA's policy, specifically covering PPL medicals for those being treated for ADHD?

This article from AOPA outlines the issues with ADHD pretty well.
This article is from Canada but the medical policies are quite similar and you might find the basic information useful.
Bottom line seems to be that once you are formally diagnosed with the condition, that becomes a problem.  You can't be on ADHD medications at all (within the last 6 months) and if you are diagnosed (officially) and are not on medications, you have to go through a testing protocol to satisfy the FAA that are fit to fly without medication.
There seems to be a lot of misdiagnosis of ADHD and the symptoms you describe may not be severe enough to be considered a real case (you may be getting too hard on yourself; I get sidetracked on tasks by things like shiny bottle caps on the floor, get bored easily, and if you give me a Rubik's Cube, I'll tire of it and put it down after about 2 minutes, and I've been flying since the mid 70s).
When I read about the symptoms of hard core ADHD (especially over-diagnosis of kids - it's criminal in my mind) it sounds like you would be barely able to function in the world, and your description doesn't sound like it rises to that level.  I'd be wary of proceeding along the path to an official diagnosis unless you've decided it's so bad you can't function without medication, and at that point flying is out.
If you do nothing and carry on, and you have a significant case, it'll become obvious during training and you'll probably never get to the solo stage.  FAA protocols are based on a diagnosis, which should be by a psychiatrist, so I wouldn't bring it up to an AME until you get to that point.  I would explore other non-psychiatric therapies or techniques that could help with focus and discipline first, and would avoid the medical route unless it was a last resort.  Your doctor may not have been doing you any favors by suggesting you might have a serious psychiatric condition based on some comments.


What is the structural significance of 'dimples' on the fuselage of a Boeing 747? I recently watched the following video:

In the video there is a mention that there are 'dimples' visible on the fuselage of a Boeing 747, when it is on the ground. It then goes on to explain that once the fuselage is pressurised - the expansion helps to "smooth" out the dimples and it will be flat.

I am curious as to what the structural significance is for 'dimples' on the aircraft?

That's just oil canning of the unsupported areas of skin between stringers and formers, and when pressurized the skin stretches enough to remove them (18:00 in the vid).  The fuselage diameter will swell a little bit, maybe 1/8 or 1/4 inch, when pressurized.


What are typical airspeeds and lift coefficients for UAVs at different flight stages? I've been trying (unsuccessfully) to figure out the relationship between airspeed of a fixed wing UAV at different flight stages. What I mean is, I would like to know the order of airspeeds from largest to smallest that would be expected for an aircraft (in particular an UAV with say 5m wingspan) to have during climb, cruise and descent. For example, the answer I'm looking for is something like: from largest to smallest, airspeed is expected to be ordered like this -> cruise>climb>descent (this would be my guess by the way).

Also I know that lift coefficient varies with angle of attack, airspeed and air density, but is it possible to make such a qualitative analysis to find an expected ordering like this for different flight stages? Looking only at the relationship between lift coefficient and airspeed, according to my previous guess, I would expect the lift coefficient to be smaller during cruise and larger during climb and descent. Is this correct?

Observation UAVs are flown in a narrow range of lift coefficients. This can be seen from their wings: Like glider wings, they have a very high aspect ratio and a highly cambered airfoil. If you look at the optimum speeds for propeller and turbofan-powered aircraft, a high aspect ratio drives the optimum lift coefficient to high values which translates into a low calibrated air speed. Note that flying high does not necessarily mean that they operate at a low true air speed, however. Details depend on the altitude in which they are operated.

Let's look at the MQ-1 Predator for an example. Its aspect ratio is 19, and with all the protrusions I would guess the zero-lift drag coefficient to be no less than 0.025. This means the optimum lift coefficient for loiter is close to 2, which is probably above the maximum lift coefficient of the wing airfoil. For piston powered aircraft the best climb speed is identical to the optimum loiter speed, so the Predator should be flown close to its stall speed of 54 knots over the whole mission in order to get the most endurance out of this airframe. Note that the maximum g force the airframe can withstand is only 2, which means that top speed needs to be limited to 76.4 knots in order to avoid structural damage from flying into gusts.

Another giveaway is the low ground clearance of the tail surfaces: The predator lands at its regular flight attitude; there is no need to slow the aircraft down when it comes in for landing.

Short answer: The lift coefficient in all flight stages is high (0.8 to 1.4) while the true airspeed depends on the altitude. Turbofans tend to fly faster than piston-powered aircraft, but the difference is marginal.


How can jet engines be more efficient than propellers? If jet engines are smaller in diameter than propellers, is it fair to say that propellers accelerate a lot of air a little while a jet engine accelerates a little air a lot? Basically, a jet engine accelerates the air flowing through it by more than a propeller engine of the same thrust, right?

If so, propellers should be way more efficient because, by accelerating a larger mass of air, they're able to generate more force (thrust) while performing less work (change in kinetic energy of the accelerated air) due to the fact that force is proportional to $\Delta V$ but energy is proportional to $\Delta V^2$, but both are proportional to mass.

So, if this is true, shouldn't propulsion systems try to maximize the mass of air that it accelerates in order to minimize the $\Delta V$ required? This is, after all, the whole concept behind using large wings to stay aloft (they can grab onto a large mass of air).

So how can jet engines be more efficient than propeller engines? What other factors contribute to the efficiency of the propulsion system besides just the mass and velocity of accelerated air? And similarly, why aren't propellers way larger than they typically are? Because by this logic airplanes should have propellers the size of helicopter rotors.

Why aren't propellers way larger than they typically are?

There are some "propellers" these days with a 400 foot disk diameter, known as windmills.
But, they are doing their best to extract energy from the airstream and here lies the issue.

"Relative wind" is conveniently used to combine the net direction and force of two aerodynamic effects, but let's look at an old question of:  can a sail boat ever sail faster than than wind?  This is very easy to explain by separating the "wind" components, rather than combining them.  Ice boats can go several times faster than the wind by using wind across the beam as "thrust", and using "wind" from the motion of the boat to create lift.

This is exactly how powered aircraft fly, the ice boat is simply doing it on its side (hence the tiller is the "elevator").

Which brings us to propellers, which can be more accurately described as generating lift than "moving air".  But, as the graph shows, net thrust of props decreases as speed increases because disk drag increases with speed.

Turbojets become more efficient at higher speeds than props because they have much less disk drag subtracting from their thrust.  Even though they are less efficient at producing thrust, their net thrust out put is much more linear over speed as "ram effect" (increasing amount of air available to produce more thrust) and drag cancel each other out, while the propeller falls victim to its own disk drag.