Is it a mistake or standard procedure to lit the landing lights only on very short final? Yesterday I was spotting at Barcelona International (LEBL). I noticed that at least two airliners were approaching with their landing lights OFF, just to light them on very short final (about 0.4NM from runway threshold).

I thought landing lights were required to be ON at all times below 10000 feet. Am I wrong? Is there any situation when landing lights are not required below 10000 feet?

Visibility was excellent, I'm going to see if I can find the actual METARs but I would say CAVOK conditions. see the METARs:

LEBL 161630Z 17007KT 140V210 CAVOK 19/11 Q1019 NOSIG
LEBL 161700Z 20007KT 150V230 CAVOK 19/11 Q1019 NOSIG
LEBL 161730Z 22007KT 170V240 CAVOK 19/12 Q1019 NOSIG
LEBL 161800Z 21007KT CAVOK 18/12 Q1019 NOSIG
LEBL 161830Z 20005KT 170V230 CAVOK 18/12 Q1020 NOSIG
LEBL 161900Z 21005KT CAVOK 17/12 Q1020 NOSIG

It is not an ICAO requirement to operate landing lights whenever you are below 10,000 FT. The mandatory aircraft lights you will see in regulations is strobe lights, nav lights and anti collision beacon lights.

It is standard operating procedure at several airlines to only turn on the landing lights when a landing clearance is received by air traffic control. This is most likely what you have observed. The landing clearance is usually given at very short final, and so crews will turn on the landing lights on very short final (unless you are in FAA territory, where multiple aircraft can be cleared to land on the same runway at the same time ...)


What switch was Skiles talking about shortly before the Hudson landing? I'm always amazed by the "miracle of the Hudson River landing" and just now stumbled across this video by the NTSB.

At 3:52, Jeffrey Skiles appears to say "* * switch" shortly before they ditch in the Hudson. I am assuming it was unreadable on the recording what he said before "switch". Does anyone know? Any ideas what and why he said that?

Side question: On the video it is not identified who is speaking but rather what channel it was spoken on. HOT-1, I assume, is the captain, HOT-2 the first officer. PA and stuff is obvious, but what is CAM, why did it stop recording on impact and why does the first officer sometimes speak on CAM, sometimes on HOT-2?

It's explained in the final report. Most definitely they asked him what he meant, as the transcript itself is:

  * * switch?

"*" denoting unintelligible words by the transcribers.

  The first officer was referring to the cabin emergency notification switch, which provides a signal to the cabin crew members indicating that an emergency has occurred.

To which Sully replied "yes", then the GPWS announced 30 or 50 (transcribers not sure), then the RETARD callout (for the thrust levers) was announced (computers eh). The recording ended about 5 seconds after asking about the switch.

(wikimedia.org)

Putting the switch to ON does three things:

lights the overhead emergency lights (more here on this cabin indicating system)
lights the exit signs
lights the exit markings on the cabin floor (proximity marking system).

Regarding the HOT and CAM, see: What is the meaning of HOT-1, HOT-2, CAM, etc., in cockpit voice recorder transcripts?

  CAM Cockpit area microphone voice or sound source
  HOT Hot microphone voice or sound source
  -2 Voice identified as the First Officer

The audio source indicated is usually the clearest one.


What would the altimeter indication be through a cold front? Cold air is heavier than hot air because of its greater density, so the QNH of an airport inside a cold front would be higher. Flying towards the cold front would be, "low to high, hello sky".

But I also know that for altimetry, cold air generates lower pressure inside the pitot-static system than warm air, and it would increase the altimeter reading, so "hot to cold, look out below".

I'm confused because of the 2 contradictory factors. If we fly towards a cold front without adjusting the altimeter setting, would there be an increase or a decrease in the indicated altitude?

There's an ATP question:

  Event after an aircraft passes through a front into the colder air.

Answer:

  Atmospheric pressure increases.

Well, yes, that's why QNH would be higher, but our altimeter would sense the temperature and "feel" a decrease in atmospheric pressure.

So, what would be the real indication of the altimeter in this situation?

There are two altimeter reading questions here, what happens when:

flying towards the cold front (third paragraph)
flying into the cold front (headline)

According to this University of Illinois web article following things happen:

Flying towards a cold front (or in the article the front is approaching you, but it makes no difference):

pressure drops steadily
temperature is steady

Passing through the front:

pressure reaches minimum, then rises sharply
temperature drops suddenly.

So, to answer both questions, assuming you are flying at relatively low altitude and maintaining it in reference to your altimeter and you do not adjust QNH:

Approaching the cold front you would steadily drift downwards: from high to low, look out below!
Passing through the front you would need to climb to maintain steady altimeter reading: low to high, hello sky!

You see, the question is about what happens to altimeter reading, but since the vertical position of the aircraft is maintained in reference to the altimeter reading, whether it's you or the autopilot flying, the altimeter reading would not change. The vertical position of the aircraft would be instinctively or algorithmically adjusted to keep the reading at a selected value.

What happens at altitude, is (for now) somewhat beyond me (pressure gradients and stuff), so  I dare not answer that scenario here yet.

P.S. a private pilot should not fly through a cold front: it's not a good place for small planes to be in...


How is fuel stored in a tanker aircraft? How is fuel stored in a tanker aircraft? What arrangements are made to make the aircraft liquid tight?

You might be surprised to learn that most refueling tankers hold a relatively small percentage of their total fuselage volume in fuel. The reason why is simple; a plane stuffed to the gills with fuel would be so grossly overloaded it wouldn't make it off the runway. Tankers, therefore, can double as medium transports with little modification, as they have a sizable upper deck which is more or less empty space when serving as a tanker, but which can ferry troops, light vehicles or other materiel from Stateside bases to various theaters (a dedicated heavy transport like the C-5 or C-17 would be used for transporting large units or for specialized troop transport like airbornes).

Here's the layout of the KC-135 Stratotanker:

To my knowledge, all of this fuel is available both to power the plane and to refuel other aircraft, however the plane would likely draw primarily from the wing tanks for flight, as this is the "standard" configuration for the 707 the KC-135 is built on. Transport of the fuel from these tanks to the refueling systems would be accomplished with an integrated pump and pipe system that could move fuel from anywhere to anywhere.

The bulk of the fuel load, as you can see, is contained in four "body tanks" totaling over 144,000 pounds of fuel. For comparison, the F-15E strike fighter can carry a maximum, with CFTs and 3 droptanks, of just 35,550 pounds.

These tanks are lined with plastic bladders, which contain the fuel (so the entire fuselage doesn't have to be leak-proof which is much more difficult and expensive to ensure) and also allows the bladder to collapse as the fuel inside it is depleted, reducing the volume of air in direct contact with the fuel, thus reducing the vapor hazard. AFAIK the wing tanks do not have these bladders, but they're much smaller spaces overall and more easily accessed.

Separating the fuel into multiple tanks has several advantages:

The tanker is still serviceable with one or more of the tanks unable to carry fuel (leak in the bladder, pump failure, etc).
Sloshing of the fuel in the tanks has less effect on the weight distribution of the aircraft. Imagine trying to take off with a full Olympic swimming pool in your cargo hold; any pitch up or down will cause several tons of weight to move "downslope", unbalancing the aircraft (probably fatally).
Fuel can be pumped between these tanks to fine-tune or "trim" the aircraft's weight distribution and handling characteristics, a process known as "tank trimming" which is more aerodynamically efficient than elevator trimming.


What effect does a direct crosswind have on takeoff performance? Is the take-off distance increased by a direct crosswind?

The effects of a constant crosswind up to the demonstrated value are insignificant (at least for airliners), as ymb1's answer already says. I checked some Boeing FCOMs and crosswind is never mentioned for takeoff performance calculation. It can therefore not have a significant impact. Boeing recommends this technique:

Any deviation from the centerline
during thrust application should be countered with immediate smooth and positive
control inputs. Smooth rudder control inputs combined with small control wheel
inputs result in a normal takeoff with no overcontrolling. [...]
Note: Excessive control wheel displacement during rotation and liftoff increases
spoiler deployment. As spoiler deployment increases, drag increases and
lift is reduced which results in reduced tail clearance, a longer takeoff roll,
and slower airplane acceleration.
[emphasis mine]

A gusty crosswind may however require larger control wheel inputs, which result in spoiler deployment. This will impact takeoff performance. Boeing therefore recommends to use more than the minimum required thrust in these situations:

For takeoff in gusty or strong crosswind conditions, use of a higher thrust setting
than the minimum required is recommended. When the prevailing wind is at or
near 90° to the runway, the possibility of wind shifts resulting in gusty tailwind
components during rotation or liftoff increases. During this condition, consider the
use of thrust settings close to or at maximum takeoff thrust.

Source: Boeing 737 NG FCTM 3.15 - Takeoff and Initial Climb - Crosswind Takeoff


What should I look for when choosing a kneeboard? There's a lot of variety in kneeboards. Some pilots fly with an improvised one: just a small clipboard with some scrap paper on, with a strap glued to the back. At the other extreme, fancy ones come with pen loops, pockets, and maybe even a fold-out area big enough to rest a map.

I'm not looking for a specific recommendation: I'm looking for how to choose. If I decide to make something homebrew, what should I bear in mind? If I buy a commercial product, which features are actually useful and which are "box features"?

You don't want something so big that it interferes with fully moving your control yoke/stick.  I have one that is about 1/2 sheet of paper in size (or about the size of an approach chart), that works good for me. Decent width of velcro strap to go around your leg.
Sportys.com has Aluminim VFR and IFR kneeboards, with good info printed right on it.  If you roll your own, they also have the same info available as a placard you could stick on.  The kneeboard and a few pieces of paper are all I've used.
I fly my own plane, so I can leave pens/pencils in places that are convenient for me.  If you are switching between planes, then maybe a kneeboard with holders for a pen or two would be handy also.
I also have a yoke mount clip that holds an approach chart. I found that trying to read a chart on my leg under the yoke was too difficult.  I may change to an ipad holder on the yoke as that seems to be the trend now.


Do military jets' radar jammers block their own radar? And, if not, why not? Will a noise jammer block the fighter's own radar? If I have understood it, a noise jammer, e.g., a barrage jammer, will emit a noise signal across a wide frequency band. Won't this make it hard to interpret its own radar "hits"?

I and a buddy are trying to make a more advanced Mech game where we want to model the ECM war a bit more. So we are trying to learn :).

If you are emitting RF, you can be seen.  There are two ways to emit RF: 1. Transmit (or leak) a RF signal, and 2. Reflect a RF signal.

A jamming device emits (usually) high amounts of RF, with the general idea that the receiver of someone's radar unit will be desensed or otherwise overwhelmed by the signal.  Like putting a multicolored sun into a small dark room.

So with multiple receivers (as in multi-static radar) a jamming aircraft can normally be rather precisely located (assuming omnidirectional uniformity of the jamming signal).

When noise jamming, your aircraft may not broadcast on, near or on harmonics of a given frequency within a temporal frame of when you are emitting on that frequency, so your receiver has the opportunity to detect returns. More importantly, your noise emitter may drop power and your radar receiver may have cans (well, cavities) tuned to protect the sensitivity of your radar receiver when expecting a return signal.

There are many books on radar technology, but I believe EW102 covers radar, deceptions, noise jammers and other techniques in a manner that most mildly technical people can read and understand.  There are other books, but I use that series in an Electronic Warfare course I teach, and they are reasonably current. The open literature is rich with advances in jamming and other stealth methodologies that have advanced tremendously in the last 50 years.


How does the PTT button on the radio work? I've always seen a button either built into the yoke or attached to the yoke with velcro on older aircraft. However, the other day my instructor used an older-looking headset that had a push to talk button built into the side of the headphones. He had no cable plugged in except the mic/headphone jacks and never touched the yoke's PTT button to transmit. My question is: how is the PTT function activated? Is this done with a load resistor or something that loads down the mic line, triggering radio transmission? I've done some research and looked at various manuals of audio panels and navcoms and haven't found an answer. The audio panel in use in this case is the Garmin GMA-430 audio panel, pictured here:

The standard microphone plug on GA planes is the PJ-068 plug, which has three electrical contacts (tip, ring, and sleeve). The tip of this connector is a push-to-talk signal (the ring carries the actual audio signal, the sleeve is ground).


Why would an F16 battery make a buzzing sound? According to this YouTube Video I heard the mechanism that moved the canopy, and then I heard a small buzzing sound. The video description says that it is spooling something, but I don't know what exactly.

Also, I am completely blind, so I have nothing to look at and only rely on sounds to know what is happening.

The relevant part from the video description that you referred to:

  You'll here a buzz, thats the battery, then you'll here it spooling thru N (nancy) stages.

The word "spooling" does not refer to the battery, it refers to the engine. The battery is static and does not spool.

There are several measurements or indications used to convey a turbine engine's state. One of them is N2 (others include N1, Engine Pressure Ration, Exhaust Gas Temperature, Interstage Turbine Temperature and several others).

Back to the video description. In this case I believe the author was referring to the N2 measurement that raises ("spools") and needs to be monitored during engine start.

N1 is the low-pressure compressor speed, and N2 is the engine's high-pressure compressor speed (both usually expressed in percentages rather than actual RPM).

More information about N1 and N2 can be found here: What are N1 and N2?


Is this a reasonable way of thinking about an auto-rotation? Related to this question.

My instructor described an auto-rotation as:

Entry.  Lever down, stick back, pedal in.
Fly normally, with a little tweaking of RRPM to keep it in the green, until about 50 feet above the ground.
Perform a quick stop.
Perform an engine failure in the hover.

I can see the logic in this but, all of the autos that I have done have been far too busy to think about it like this and all the ones I've done to the ground have been run-ons so step 4 was not an option.  Without significant extra training and experience I would not even contemplate a zero-zero landing so in practice, I will probably never get to test the theory.

Is this a valid way of thinking about the manoeuvre?  I'm not suggesting that anyone does anything other than what they were taught, just thinking about how these four manoeuvres roll into one.

First off, auto-rotation is a general term for all helicopters and the process varies between different air-crafts and the cause of the auto-rotation. For example, in auto-rotation due to loss of tail rotor thrust you can (and should) descend with some collective power and reduce descent rate. Also, in such auto-rotation there is also a mandatory step 2.5 - cut the power. Otherwise, the craft will rotate out of control in the quick stop.

Also various helicopter types behave differently while auto-rotating such as bigger helicopters start flaring at least 125 feet above ground.

As for the steps described - I'll agree that they break down nicely the process into other familiar processes, even though obviously not entirely accurate. I think the main difference is that flaring from altitude in a 1500-3000 ft/second descend and approaching a quick stop usually with speed forward rather than downward - require significantly different flight technique and practice.


Do you ever get to practice ejecting out of a plane as a fighter pilot? The military throws away a lot of equipment simply because it doesn't hold up to current standards. So I was wondering the following.

As a fighter pilot, would you ever get a practice run ejecting out of a plane, or is it just kind of one of those things you read about and hope you never have to do it? What is it like ejecting out of a plane at 500 knots? 1000?

Air forces use special training rigs rather than real ejection seats. Example video

Plenty of pilots have survived ejection (Wikipedia says "As of 20 June 2011 ... the number of lives saved by Martin-Baker products was 7,402 from 93 air forces")
The types of injury are illustrated by this example

The ejection seat has been responsible for saving the lives of thousands of pilots around the world since its introduction in the late 1940s. Typical survival rates quoted in the literature vary from 80–97%.
It is generally accepted that radiographic evidence of vertebral fracture can be found in 30%–70% of aircrew after ejection

The reason real ejection seats are not used is almost certainly the high risk of injury. Ejecting clear from a high speed jet requires a lot of acceleration.


Does thrust output in jet airplane remains constant when altitude increases? I’m a noob so sorry for my ignorance. The question is: do we need (in jet airplane) to increase the throttle to maintain constant thrust output if we want to fly higher (not considering climb phase), I mean whether on 2000 ft and 20000 ft altitude throttle is in the same position or we need to push it forward if we fly at higher altitudes to maintain constant power (as in piston airplanes).

It depends, as so often.
With the thrust available at low altitude, the indicated flight speed will be much higher than at high altitude. There, the reduced density and thrust will only allow flight close to maximum thrust and over a small speed range around the minimum drag polar point. In practical terms, at low altitude the airliner will not be able to fly level at the maximum thrust setting because it will exceed its Mach limit. But since the speed of sound is higher in warmer air, it will still fly significantly faster than at high altitude when it flies at the same Mach number at both altitudes.
If, on the other hand, you compare states at the same indicated air speed, the required thrust at low altitude will be a bit smaller due to the higher Reynolds number in warmer air. On the other hand, fuel flow will be a bit higher since the engine is not running at its optimum design point and the higher ambient temperature reduces its thermal efficiency.
While thrust will go down with air density (or dynamic pressure, for the same Mach number at low and high altitude) in the first case, in the second case it will increase slightly.


What is the main source of yaw stability? I am creating a flight simulator and I need to implement lateral and longitudinal stability.
Roll stability is now working because I am computing the lift of each wing independently.
But yaw stability is confusing to me.

What is the main source of yaw stability : is it the weathervaning effect of the tail or is it the sweep angle of the wing ? Which one is the more responsible of stability ?

When I implement weathervaning effect by applying the wind at a certain point of the tail, the problem is that the oscillation will continue and the plane will always overshoot as he is not losing any of its angular velocity. So it will never align into the wind but will keep overshooting from negative to positive angle.

So my other question is : in real life, what does prevent the plane to overshoot wind direction?
When I just apply some scaled-angular drag value to the plane, it helps to dampen the oscillations, but the plane is wayyy to slow to align into the wind and thats just aweful to controll and not realistic at all. So I guess angular drag is not the answer why oscillations stops in real life. There must be another force depending on wind speed or I don't know...

And longitudinal stability is also confusing to me.
I know that it depends on the position of CG, CL and the downforce generated by the tail horizontal stabilizer.
But what about the weathervaning effect ?? Isn't it the main force that explains why the plane align into the wind on its pitch axis ?

As you can see I am a bit confused. I already looked many websites and I mainly understood how stability and flight dynamics work, but as I am working on a simulation, I need some equations to implement, and some concrete values to understand which forces are bigger than others.

Thank you !

Wing sweep does affect yaw stability, but all planes with a vertical tail will "weathervane" into the wind.  It seems you are on the right track considering angular drag as a mechanism of stability, angular lift works even better.  A vertical stabilizer can act as a vertical wing.  Symmetrically, it can generate lift against either deflection.

Secondly, the "overshoot" is a real concern in design, but will be a function of weight as well as "angular drag". Much harder to stop a piano on ice compared to a light cardboard box.  A tail will typically be light, pivoting around the center of gravity.  Ample tail area will make it easier to control.

Finally, as far as "damping" is concerned, one must realize the "viscosity" of air will resist motion as well.  Once a force is removed, drag will help it slow to a stop, so the opposite "correcting force" need not be as large.


How long does it take for an airline to establish a new international destination/route? This post discusses the process of opening a new route. I wonder how long does it take for an airline to get permission to fly a new route. I am interested on this because, say an airline goes bust during the current covid-19 crisis. If another airline wants to fly a route that was previously flown by the busted company, it will take some time for that to happen. Naturally, conditions might be particularly different in the current situation but it would be interesting to know the length of the process in normal times.

This article says it might take "years to develop". Anyone with a more specific estimate?

In Australia:

Our major airlines quality for what is called an "area AOC". That means they already have permission to operate ad hoc services to any port in Australia and in fact most other countries. All they need to do is an internal risk assessment specific to the new port and address any findings from that (e.g simulator sessions, maybe an on-site inspection), and share it with the regulator. This process can take as little as a few days if it's a simple well-established airport.

If they want to start new regular or RPT services, it's a little more complex in that they need to have a line maintenance facility at the new port, or contract another company to do it for them. This facility is inspected and approved by the regulator which can take a few months to arrange.

You also need government approvals to operate the route, both for the arrival country and overflight arrangements (though typically those are already in place from existing routes). Some countries can be a beaurocratic nightmare and take months to give this, others much quicker. Airports are similar, negotiating parking fees and reserving slots if required.

So to summarize, approval for a one off charter to a familiar port can usually be arranged within a week or two, a regular service needs a couple of months. Of course it can vary wildly depending in the scope of the operation, and I'm ignoring the sometimes years of economic research and monitoring that goes into deciding whether a route is even financially viable.


What is the result of increasing number of propeller blades on the same engine For example Rotax 912. We can meet this engine with 2 and 3 propellers mounted. Or compare airplanes equipped with engines with pretty the same power output i.e. C152 with Lycoming O-235 - 110 HP and 2-blade propeller versus AT3 with Rotax 912 - 100 HP and 3-blade propeller (also both engines have similar max propeller RPM (Rotax has reduction gearbox)). I know that increasing number of propeller blades alllow better power absorption, but what does it change if the engine has pretty low max power?

What it means in practical terms is if you replaced a 2 blade Warp Drive prop on a 100 HP engine with a 4 blade prop with double the total blade area, and if the blades of the 4 blade were set to the same pitch as the 2 blade, the torque requirement of the 4 blade would be significantly higher and you'd find the engine can't make the same static RPM it was making with the 2 blade at wide open throttle.
You would have to re-pitch the blades of the 4 blade prop to a lesser angle and retry until you regained the original static RPM.  You're basically offloading the individual blades until the total torque demand matches the original prop.  You might find you've lost a lot of efficiency from the blades now operating at less than their optimum blade angle of attack.
If the 4 blade prop had much smaller individual blades, so the total blade area was the same as the 2 blade, the difference in pitch would be minimal, although some adjustments in pitch would still need to be made to match static RPMs, and the static thrust you are getting at the target RPM would be reduced somewhat from the 2 blade because of the lower efficiency of the 4 blade.
If it was a constant speed propeller, the propeller governor would be doing all that for you.
If you are after the maximum thrust efficiency for a given horsepower, 2 blades are always better if the power can be absorbed and the target RPM can be achieved with 2 blades.  What makes multiple blades attractive, as a conversion to an existing engine, is the desire to limit noise, or limitations on diameter due to tip speed limits or ground clearance requirements. For example, 3 blade propellers are popular on DeHavilland Beaver seaplanes because the noise levels are lower because the tips speeds are lower, and the shorter blades get a little bit less water spray erosion damage, benefits sufficient to live with a small reduction in prop efficiency compared to the 2 blade paddle prop it originally had.


What were the procedures to follow in US Airways Flight 1549 during descent? In the documentaries, and in the texts about the US Airways Flight 1549, procedures and checklists are mentioned and shown as a good example.

What procedures were they following in that case, and what could have gone wrong if they were not followed?

17 seconds after bird impact, they used the "QRH Engine Dual Failure checklist" as per the NTSB report at:

https://www.ntsb.gov/investigations/AccidentReports/Reports/AAR1003.pdf

  Even though the engines did not experience a total loss of thrust, the
  Engine Dual Failure checklist was the most applicable checklist contained in the US Airways QRH, which was developed in accordance with the Airbus QRH, to address the accident event because it was the only checklist that contained guidance to follow if an engine restart was not possible and if a forced landing or ditching was anticipated (starting from 3,000 feet). However, according to postaccident interviews and CVR data, the flight crew did not complete the Engine Dual Failure checklist, which had 3 parts and was 3 pages long. Although the flight crewmembers were able to complete most of part 1 of the checklist, they were not able to start parts 2 and 3 of the checklist because of the airplane’s low altitude and the limited time available.

The "procedure" that the captain followed was the mantra taught to student pilots very early in their training: aviate, navigate, communicate. The ultimate priority in all flight circumstances is to first fly the aircraft. By realizing that 1) continued flight was impossible, 2) attempting to restart the engines was futile, and 3) deviating to the softest and least congested improvised landing site was necessary, in fact in these circumstances the best possible option, the captain saved his own life and the lives of his crew and passengers.

The captain was almost a 20,000 hour pilot at the time of that accident and the first officer had accumulated over 15,000 hours of pilot time. This cannot be understated. This was an extremely experienced flight crew.

In many years of dealing with imminently life-or-death circumstances, I have seen two categories of people: those who act and those who freeze. Obviously, the crew in this situation was composed of people in the former category. That is the category in which I place myself, but thus far almost all of the life-or-death circumstances I have faced have been ones I have trained heavily to mitigate. My actions have been almost reflexive based upon that training, but in my craft I am also well over the 10,000 hour mark.

The ultimate answer to your underlying question of how did this flight end successfully is the right kind of experience of the flight crew, and this cannot be summarized in a checklist or procedure. They knew how to fly. They intimately knew the systems of their aircraft. Most importantly, they worked together as a team to accomplish the mission even though that mission had changed significantly since the time when they were cleared for takeoff.


What are the effects of separation at hypersonic rarefied flows? Shock wave causes an adverse pressure gradient which causes an airfoil to stall. Also because gas density is much lower boundary layer becomes laminar and unable to prevent separation.

Correct me if information above is incorrect. Also are there any sources of stall? Like how much AoA affect separation? Can an airfoil stall (just because of hypersonic rarefied conditions) at 0° AoA?

Hypersonic rarified conditions means you're almost in space.
You are asking can you stall? Maybe the first question should be can you fly?
Correct me if I'm wrong but it seems that you are trying to apply a real life exemple to a totally different area of flight. The stall you are referring to happens in transonic flight when the shock wave appearing on the extrados forces the flow to detach.
In subsonic/transonic flight lift is generated through an acceleration of the flow around the extrados resulting in a pressure loss on the succion side of your wing. If the flow detach you're losing this acceleration effect of the flow following the surface of the wing and thus you lose lift.
In supersonic flight the shock waves starts attached to the leading edge of the wing. The shock deviates instantly the flow and the more the flow needs to be deviated the stronger the shock and the more pressure increases. Lift is generated through the difference of deviation in the flow betew intrados and extrados resulting in pressure difference on both side. The boundary layers here doesn't play a huge role and I've never heard of supersonic flight stall.
In hypersonic rarefied condition the shock detach from the body and you have very high temps, and complexe chemistry hapenning inside the layer between the shock and your surface thus making the flow hard to simulate. First of all you don't really use wings in this mixture of high energetic particles so speaking of stall doesn't make sens. Then lift is mostly done the same way as in supersonic, through the differential increase of pressure being the shock but the chemistry between the few molecules of air can create weird stuff resulting for exemple in asymmetrical lift on a surface for very complexe not always well known reason.


When does fuel indicator indicate red index? C172 Full fuel is 56 gal and usable fuel is 53 for c172r. Other than when the fuel indicator is broken, when the fuel indicator indicate red index? Is there any fuel quantity setting to indicate red?

As always, the answer to this question is in the POH. You should always check the specific POH for the aircraft in question but you can usually find one for common aircraft (such as a 172) online. One such example is here.

Under the section on instruments, and specifically "fuel systems"

  Fuel quantity is measured by two float-type fuel quantity transmitters
  (one in each tank) and indicated by two electrically-operated fuel quantity
  indicators on the left side of the instrument panel. An empty tank is
  indicated by a red line and the letter E. When an indicator shows an empty
  tank, approximately 1.5 gallons remain in a standard tank, and 2 gallons
  remain in a long range tank as unusuable fuel. The indicators cannot be
  relied upon for accurate readings during skids, slips, or unusual attitudes.

I'm pretty sure the "r" variant in your question is the 172 fitted with long range tanks.


Why are there no large turboprop passenger airliners? If turboprop engines are more efficient than turbofan/jet engines at lower speeds, and have a lower specific fuel consumption, why are there no large passenger airliners that utilize the turboprop engine?

I expect this would increase flight times, but say for example an airline was to only operate short-medium haul flights within Europe, wouldn't it be in the interest of minimum fuel burn to use the more efficient engine, the turboprop?

Most modern airliners seem to use two turbofan engines. I am curious as to why the largest turboprop aircraft have a passenger capacity of below 100 (ATR 72 and DHC Dash 8) and why there are no 150+ pax turboprop airliners.

With minimum fuel burn as the priority, ignoring costs and flight times, would this be a desirable and achievable design option?

Large airliners don't fly at slower speeds.
It's simple economics.

If you have to connect two destinations 10,000 miles apart, you'll do maybe 2 round trips a week. Your passengers have no other choice, except a flight that takes even longer. You also have the luxury of flying between two very large cities.

If you have to connect two destinations 100 miles apart, you can do 3+ round trips a day. Your passengers choose and pick among a dozen other carriers that do the same, and many will just go by car or bus. And at this distance, at least one of the cities being services is small.

As a result, a single 50-passenger turboprop running a local route can transport as many passengers as a 500-passenger ultra-long-range turbofan. At the same time, the local route will have far fewer passengers per day.

Fast long-range turboprops are possible (see the Tu-95 and -114), and only a little slower than turbofans. But they're still louder. All in all, for large planes on long trips, both the flying public and people on the ground choose quieter faster planes over somewhat more fuel-efficient ones.

Extra noise for an hour is less of a pain, speed matters much less, and the smaller turboprops working these routes are less of a pain on the ground too. The fuel economy gap isn't all that large.


Why is mixture lean recommended on the ground? Mixture lean is typically recommended during the ground operations at sea-level pressure in the POHs of most small aircraft equipped with a carburetor system. Why is it?

I heard some people saying it's because the engine requires less power for the ground operations, but that doesn't persuade me. If it's the amount of power output at stake, we just need to pull back the throttle and reduce power, which we exactly do for taxiing and holding short of the runway. Mixture is about controlling the ratio of fuel and air entering the cylinders, and we change it according to the variations in air density, not the power output required. If that's the case, then why do the POHs recommend a mixture lean for the ground operations at sea level?

Edit) I guess my question is put too poorly for some people to understand easily. Let me try again. Plug foulings happen when mixture is set too rich for the given density altitude. For this reason, many of the GA aircraft's POHs recommend pilots to start leaning mixture from 3000 MSL up. That means as long as the aircraft stay below 3000 MSL, the pilots have no reason to lean mixture, at least for the purpose of avoiding plug foulings. Yet, the same POHs recommend pilots to lean mixture for ground operations even when the elevation where ground operations are performed is below 3000 MSL. So my question is: "Why do the POHs recommend pilots to lean mixture for ground operations even when the density altitude of the ground does not require it?".

The new insurer for the Cessna 210 I fly is only charging \$300 to add me to the policy—instead of $1,300 the old one wanted—so I’ve been reading the 210 manual for the first time in several years. I noticed that the Owner’s Manual doesn’t mention anything about leaning on the ground and gives advice about leaning in the air that has been superseded by newer best practices. I looked in the manuals for the Cherokee and Cessna 182 and they don’t mention anything about leaning on the ground either.

One of the first things I learned when flying the Cessna 182 is to lean aggressively on the ground. There were several flights when I first started flying it that were not able to be started because, on runup, we couldn’t get the mags to clear. After a couple of expensive trips to the A&P, I started leaning aggressively on the ground. At the time, I was too busy with the firehose of information to even think about why I needed to lean, just that it saved a lot of money if I did. This post is an explanation of why you need to lean on the ground—especially older small-bore engines.

Fuel is different from the 1960s and 1970s

The POH for the Cherokee and 182 specify 80/87 fuel. The 210 specifies 100/130 and that 100/130 low lead is also approved. So what do these numbers mean?

Shell explains it really well, (bolding is mine)…

  Avgas grades are defined primarily by their octane rating. Two ratings are applied to aviation gasolines (the lean mixture rating and the rich mixture rating) which results in a multiple numbering system e.g. Avgas 100/130 (in this case the lean mixture performance rating is 100 and the rich mixture rating is 130).

  In the past, there were many different grades of aviation gasoline in general use e.g. 80/87, 91/96, 100/130,108/135 and 115/145. However, with decreasing demand these have been rationalised down to one principle grade, Avgas 100/130. (To avoid confusion and to minimise errors in handling aviation gasoline, it is common practice to designate the grade by just the lean mixture performance, i.e. Avgas 100/130 becomes Avgas 100).

  Some years ago, an additional grade was introduced to allow one fuel to be used in engines originally designed for grades with lower lead contents: this grade is called Avgas 100LL, the LL standing for ‘low lead’.

  All equipment and facilities handling avgas are colour coded and display prominently the API markings denoting the actual grade carried. Currently the two major grades in use internationally are Avgas 100LL and Avgas 100. To ease identification the fuels are dyed i.e. Avgas 100LL is coloured blue, while Avgas 100 is coloured green.

EAA explains the change as well…

  Avgas 80/87 is used in low compression ratio aircraft engines, and contains little or no lead (up to .5 grams of lead per gallon is allowed, but none is required). It is red in color, and should not be used in any automotive engine due to a low motor octane number of about 80.

  Avgas 100/130 can be used in some automotive engines. It has both research and motor octane numbers slightly over 100. Avgas 100/130 is green in color, contains four grams of lead per gallon, and is becoming hard to find.

  Avgas 100 LL (the LL stands for “low-lead”) contains two grams per gallon, half the lead contained in the avgas 100/130 it was designed to replace. It has research and motor octane numbers very similar to 100/130 avgas. The color is blue. This product sometimes has a high level of aromatics.

  Avgas 115/145 was developed for high performance piston aircraft engines used in World War II and the Korean conflict. It is very hard to find today due to lack of demand, and is usually only produced on special order. The color is purple.

Purvis Brothers has similar information in metric units:

  Tetraethyl lead content:

    80/87 Max of .14g/l
    100/130 has 1.12 g/l
    100LL has .56 g/l

So why do plugs foul?

The engines on the Cherokee and the 182 were designed for fuel that has little or no lead. 100LL has at least four times as much lead as 80/87. At annual there is a substantial amount of lead that needs to be removed from them. On the other hand, the 210 was designed for fuel that had twice as much lead. I don’t recall ever fouling the plugs and they don’t usually have much in the way of lead deposits at annual. I spoke to two other 210 drivers and they don’t recall fouling the plugs either—though they do lean aggressively on the ground.

As Steve Ells explains “The additive in Avgas that helps scavenge the lead in the fuel doesn’t do much until the combustion temperatures get up to around 1,400 degrees F.”

At idle, your engine isn’t generating much heat. If you remember from studying for your PP test, fuel is a coolant. So the more fuel you give an idling engine the cooler it runs. Leaning the engine makes it run hotter—which is what you want on the ground.

Leaning

None of the owner’s manuals I’ve looked at mention leaning on the ground. Probably because, when the planes were manufactured, fouling plugs wasn’t an issue. Fuel prices were low, so saving a half-gallon or so wasn’t that big of a deal. The truth is that a full-rich mixture combined with the low power setting is the perfect scenario for lead fouling your spark plugs and valve stems.

So why don’t instructors teach new students to lean aggressively? Maybe so low time pilots won’t forget to go full-rich before take-off. But if you lean appropriately, this will never be a problem. Lean the plane at taxi RPM (usually 1000 RPM) so that it starts to stumble. Then enrich just a bit. You’ll be able to taxi and do your runup at this mixture setting without worrying about taking off with less than full mixture. As soon as you advance the throttle a couple hundred RPM the engine will stumble. If you tried to advance to takeoff power, the engine would die from lack of fuel, before you even started your take-off run. To avoid the embarrassment of stalling when I leave the runup area, I put my hand on the mixture knob when I make my call to the tower or CTAF. And I don’t take it off until I’ve pushed the mixture to full rich for takeoff. (Or less that that for high-density altitude.)

John D. Collins explains his procedure for high density altitude:

  At sea level up to a density altitude of approximately 3000 feet, I would push the mixture full rich for takeoff. At higher density altitudes, I would do a full power run up and set the mixture to achieve the highest RPM as this will give you the most power available. As you climb above 3000 MSL, I would lean the engine for the highest RPM.

How octane numbers are calculated.

From Shell:

  Engine knock, which describes explosive detonation of the fuel/air mixture or preignition, can cause severe engine damage and subsequent failure in a short period of time. Anti-knock ratings are expressed as Octane Numbers for those of 100 or less and as Performance Numbers for those ratings above 100. These numbers relate the fuels performance compared to a reference fuel of pure isooctane. Because the anti-knock characteristics are influenced by the air/fuel mixture ratio, ratings are developed for both rich mixture performance and lean mixture performance. Rich mixture settings yield higher octane or performance numbers since the added fuel acts as an internal coolant and suppresses knock. Prior to 1975, both numbers were reported as the grade designation but current specification utilize only the lean mixture rating. Currently, ASTM (American Society for Testing and Materials) specifies five grades – 80, 82UL, 91, 100, and 100LL (low lead). In practice only 100LL is widely available. Production of Grade 80 has generally ceased due to small demand.

I don’t know when 80/87 started to become less available, but Service Letter L185B from Lycoming dated January 19, 1988 states that “Whenever 80/87 is not available you should use the lowest lead 100 grade fuel available.” It goes on to say that, “higher lead fuels can result in increased engine deposits both in the combustion chamber and in the engine oil. It may require increased spark pull maintenance and more frequent oil changes.…Operation at full rich mixture requires more frequent maintenance periods; therefore it is important to use proper approved mixture leaning procedures.”

The Service Letter doesn’t mention leaning on the ground, but it does mention that:

  At engine speeds of 1000-1200 RPM on the ground, the spark plug temperatures are hot enough to activate the lead scavenging agents in the fuel which retards the formation of salt deposits on the spark plugs and exhaust valve stems.

Since the leaner the mixture, the hotter the cylinder temperatures, leaning aggressively would help with reducing lead deposits.

The letter provides a method for leaning in the air to reduce deposits.


Twist optimization for a low speed aircraft I'm currently designing an aircraft with the following specifications-

s=1.8m^2, AR=7-8, trapezoidal planform with a taper ratio of about 0.45, a load of 25KGs and a cruising speed of 13-15m/s.

I'm barely getting any improvement in drag reduction or tipstall delay after testing twists upto -4deg.
A twist of -3 is delaying tipstall by about 0.7deg while reducing Cl by 0.08 in xflr.

Is a twist even necessary for my model?

Also I've read that a twist optimizes drag reduction and lift distribution for a single aoa in non-elliptical planforms.
Should this angle be the aoa for the Cl I get for my cruise speed?

Also how would my planes angle of incidence depend on the twist that I select?

Is a twist even necessary for my model?

Yes, with that amount of taper I would recommend twist. Those 4° look rather low on first sight. The tapered wing will produce induced loads on the outer wing due to the amount of suction of the deep inner wing.  Below is a plot of where flow separation starts for different sweep angles and aspect ratios (source):

I would classify your wing as a high taper wing (second from top, right column). Without twist it will start to stall from the tip. My recommendation is to use 0.7 taper even if that means higher root bending moments. With 0.7 taper and the same stall angle of the inner and outer wing airfoil, 2° of twist are usually sufficient.

  Twist optimizes drag reduction and lift distribution for a single aoa in non-elliptical planforms. Should this angle be the aoa for the Cl I get for my cruise speed?

Yes.

  Also how would my planes angle of incidence depend on the twist that I select?

Proportionally to the lift loss on the outer wing with increasing twist, the whole wing needs more angle of attack for the same lift. You will, however, need many degrees of twist before the wing's incidence will increase by more than one degree.


Airline flight scheduling method differences What are the differences between American major mainline passenger airline scheduling policies?

Presumably, each airline runs a model or optimization problem for determining

where (markets/airports),
when (timing of the flights throughout the day), and
how many seats (plane capacity).

Are there any specific examples of different scheduling policies, methods, or algorithms among airlines - within the context of each airline doing the best it can to match its schedule to market demand, according to its business plan and strategy?

There are many academic papers available for general airline scheduling topics. Airline flight schedule planning is a huge topic, existing within the more general field of Operations Research. The models and algorithms used are not simple by any means, and thus there could exist many kinds of differences between the methods used by each of the different airlines.

Considering such models are part of how airlines gain a competitive advantage, specific airline methods, at least for now, are proprietary information.  Perhaps this will not always be the case, but it would take a change in the  way the airline industry operates for this type of information to be publicly available.

That being said, the data about the output of those models are freely available, collected by the U.S. Government (see the Bureau of Transportation Statistics).

Theoretically, one could mine the available data to reverse-engineer some differences, but these will be theories at best.


Why would a C150's tachometer fluctuate? Why does a Cessna 150 aircraft tachometer fluctuate between a specific rpm between 800 and 1100 rpm? The idle rpm of the aircraft is 800 rpm.

It has a little internal generator that creates a voltage to drive the needle.  The generator, and a gearbox that runs the hour meter, is spun by a flex shaft from the engine.  You can have problems stemming from the flexshaft binding and winding up/releasing, but there is probably an electrical issue with the internal generator or the circuit that drives the pointer.  A mechanic might swap out the flex shaft and see if it still does it, and if yes, swap out the tach.  Or vise versa (If I had an other tach handy, I'd disconnect the flexshaft at the tach and work it out from under the panel and connect it to my spare tach and try it out).

In any case, if it's a Cessna 150, and the tach's hour meter shows the airplane's original hours, no doubt it's 43 or more years old and is toast.


Why do I missing 1090 Rebroadcast Traffic (ADS-R) Targets from Ground Station as part of 978 Uplink? For context I have an ADS-B 1090 OUT and 978 IN. I also have a Stratus portable Dual channel receiver in my plane, so I have the option of comparing the traffic between systems.

What I’ve been noticing is that lots of 1090 targets that I can see on Stratus connected iPad, I can never ever see it on my display connected to 978 IN avionics. My understanding is that my setup should:

A) Receive direct reports from aircraft equipped with UAT/978 OUT hardware. (Air-to-Air) (Working Good)

B) Receive ADS-R rebroadcast traffic from ground stations, for 1090 targets. (Not receiving)

C) Receive TIS-B traffic from ground stations, for Mode-C targets that aren’t participating in ADS-B network. (Good)

So my question is, why do scenario B seem not to be working for me?

I pulled ADS-B Performance Report for my flights and I see:

OUT capability is correctly listed as 1090 and IN capability is listed as UAT. This should indicate the Ground Stations that I need 1090 targets as part of rebroadcast (ADS-R) to me. I contacted FAA on it and they agree too

Any thoughts? is it something I am missing in the ADS-B OUT configurations or could it be Ground Station issue?

Ground stations only send ADS-R/TIS-B messages for targets within a certain distance horizontally and vertically of ADS-B targets with the CDTI (Cockpit Display of Traffic Information, aka "ADS-B In") flag set. Assuming your CDTI flag is set correctly, the targets you're missing probably aren't close enough to qualify.


What are the specs for runways at international airports? 1) What are the materials that international airport runways are made of? If runways consists of several layers, what are they made of, and what is their depth and other specifications?

2) What are the surface specifications/properties required of such runways?

3) What kind of routine maintenance drills do such runways require?

Runways for international airports are usually man made, which means that they are made up of asphalt or concrete and sometimes are a mixture of both. The depth for the runways are 29cm to 33cm.
Runways are usually around 150ft wide. For an international airport will have a runway around 6,000ft long, but depending on the size of the aircraft that land there a runway will need to be 8,000ft or more.
I found this link which should give you information on runway maintenance.


What do I look for when getting a PPL at a college or local school? I am interested in getting a private pilots license and I know I can go to a near by airfield to get one. However I would like to know if anyone has received one through college and if it would be best just to wait until then? The cost at the airfield near me is around $12,000 but i have not found a cost for getting one in college.

There are 2 major drawbacks to getting a PPL through a college program.

1) Admissions

In the comments you mentioned Purdue University. An excellent school, but they have very high academic standards (my son was interested in going there - didn't get accepted). There may be some variation for admission to the flight school program vs other degree tracks, but you'll still have to get admitted to the university of choice.

2) Cost

When getting your PPL in conjunction with a college degree, you're going to be taking a whole lot of general education courses that don't apply to flying. To go back to your Purdue example, they will not want you parading yourself as a proud Purdue alumn, yet not have the basic education levels that people would expect from a Purdue grad (IU v Purdue sparing aside). That means you're going to be taking writing and basic math and humanities classes, none of which have anything to do with getting your PPL. Each one of those credit hours will cost you. As will your flight hours. A school like Purdue is a prestigious, Big 10 school and tuition there ain't cheap. Purdue (again, mentioned in the comments and I have some personal experience) will run somewhere in the neighborhood of $30,000 per year for in-state tuition. I don't know the length of their flight school program, but you could do 2.5 trips through the flight school you mentioned for that kind of money.

If you go the straight up flight school route, you're going to be paying for flying lessons and flight time and that's it. Not having gone to flight school, I don't know what the admissions criteria are, but I'd venture it's not much more than having a medical certificate and the cash to start paying. If you run out of cash, you can put your training on hold for a while.

Of course, if you're going to school to get a degree and you want to make getting your PPL part of the process, not the sole reason for being there, that's a different story as far as classes go, but you're still paying a lot for the privilege of taking flight lessons through a university.


Is it possible to use a split rudder as an air brake? This question (which is the first result for a Google search for "Split rudder aircraft", btw), talks about some of the advantages of the split rudder, including the fact that the lower rudder is smaller for finer adjustments while flying.

If the rudders were equal size, or if there were three pieces, two of them being equal to the third, could they be used as an air brake?

Would there be advantages to doing so vs building in a dedicated air brake?

It is possible but there are side effects.

If both rudders would be deflected to opposite sides to cancel out each others yaw moments, their drag will act as an air brake. But since the top rudder is further from the center of gravity than the bottom rudder, the resulting roll moments of the rudders don't cancel each other out. As a result the aircraft will start to roll. This can then be counteracted with the ailerons.

Since the rudders don't deflect as far as air brakes would, they would be less effective. If they would be designed to deflect more, the increased forces upon them and on the control system would require a heavier construction and thus add weight to the aircraft.


What do the Airway Letters and the following numbers mean? For example, from RCTP - ZGGG the route is AJENT MKG A1 ANPOG R200 GLN W22 IDUMA via the AJENT SID to the IDUMA STAR. What does "A1 or R200 or W22" mean? Are they just randomly allocated letters with random numbers for routes?

RCTP - Taipei, Taiwan Airport
AJENT SID - Standard Instrument Departure leading to the Navigation fix named AJENT
MKG - fly to the MAGONG VOR Navigation beacon
A1 - follow airway A1
ANPOG - until the fix named ANPOG
R200 - follow airway R200
GLN - until the GUANLAN VOR Naviation beacon
W22 - follow airway W22
IDUMA - until the fix named IDUMA
IDUMA STAR - follow the IDUMA Standard Arrival Route
ZGGG - land at Guangzhou Baiyun International Airport

A1, R200 and W22 are airways. They are defined by lists of waypoints. The
waypoints between the first and last used on the airway are omitted and replaced by the airway name, otherwise the flightplan would be extremely long.

As an analogy one could describe a route on the road. For example Frankfurt - Amsterdam could be described as:

FRA - 44 - FRA KRZ - A3 - BORDER - A12 - KNP OUDENRIJN - A2 - KNP AMSTEL- S110 - AMS

Without mentioning all the highway intersections and exits, the route is described in sufficient detail to drive it. The numbers of the highways appear to be somewhat random, but I am sure somebody has thought about how to best assign the numbers.


Is it useful for twin propeller airplanes to have twin vertical stabilizers, in line with the propellers? In some twin propeller airplanes, there are two vertical stabilizers which are on the same line as the propellers.

Is this shape helpful in terms of efficiency?

Like this one (twin boom tail):

The vertical tailplane is dimensioned as a tail volume: total vertical area times the moment arm to the CoG, to tackle the following issues:

Control after engine failure.
Lateral Stability.
Crosswind landings.
Spin recovery.

The vertical tail is subject to side forces, and the aircraft structure must be able to withstand the resulting reaction forces. The fuselage is very suitable for absorbing these by applying counter-torque, and is the go-to place for mounting the fin.

A large, long fin has a relatively long moment arm, and it can be beneficial to divide the vertical tail into multiple surfaces, provided there are suitable mounting spots. And behold, the P-38 had two tail booms and a horizontal connection bar, and the natural mounting method is two fins each at the end of a boom.

The fins are indeed in the prop wash and this does help their efficiency - in straight flight. At a sideslip, large angle of attack or in a spin, the prop wash will be directed away from the vertical tails, and these will be dimensioned as if there was no prop wash at all.


What is the drag (or $C_D$ vs $C_L$) of the Airbus A320 in cruise flight? Is there any website or data-sheet that I can find drag of airplane for cruise conditions. $C_D$ vs $C_L$ graph is also acceptable.

To be more specific I am searching drag values of A320 in cruise flight.

I need to make a few assumptions to close the gaps in the data you provide.

The aircraft flies close to its optimum L/D - which is 18 for the A320-200 in cruise. So the $c_D$/$c_L$ is 0.055556.
The aircraft is close to its maximum take-off mass, minus fuel for climb. Let's pick 76 tons.

Now the drag force is 1/18 of the weight force. Weight is mass times gravitational acceleration: 745,300 N. So drag would be 41,406 N at this point.
At the end of the trip the aircraft has burnt through 15 tons of fuel; now the drag is a mere 34,300 N. Note also that it now is flying quite a bit higher than initially in the flight, so both friction and induced drag have become smaller.


When a hold is distance based, do you still used a timed entry? Let's say you have to do a parallel or tear-drop entry to enter this distance-based holding pattern.  Do you still fly outbound from the fix for one minute and then make your turn inbound to re-intercept the fix or are you taking it to 4 NM?

I haven't found any specific guidance for hand flying entries to RNAV (RNP) holding patterns. But part of the concept of RNAV is that the navigation system can provide the complete desired path including hold entries.

Referring to RTCA DO-236C RNP MASPS and the requirements for holding operations, I find the following: The dimensions of the hold pattern are defined based on altitude, airspeed, bank angle, and an allowance for wind. The length of the inbound/outbound leg is defined by the database or entry. In your case this is 4 NM, and is defined as d1. The width of the pattern (d2) is computed based on the above variables.

Without the details, we can still make an approximation. If you're flying at 120 kts and performing a 2 min turn (1 min for 180 deg), your pattern width (d2) would be about 1.3 NM.

The rules include the following statement:

  When performing an entry into the holding pattern, the navigation
  system is permitted to use an inbound leg length (d1) that is equal to
  the holding pattern width (d2) instead of the length defined in the
  database or from pilot entry.

So the end result is that your entry cannot exceed the 4 NM leg length, and could have an inbound leg length as short as 1.3 NM.


What is the inner dashed line on this helicopter chart? Inside the delta for KSGR is another blue dashed circle. What is this?

First of all, sorry for the confusing original image. It comes from foreflight and has three charts superimposed. 1) the sectional, 2) the houston helicopter route chart (white background) and 3) the foreflight aeronautical chart showing the airpace again. I'm attaching a simpler one which shows just the sectional and the helicopter route chart.

It turns out that the helicopter route chart is simply out of date (!?). I called the Sugarland tower and they confirmed that 2 years ago the Delta expanded from a 4.2 mile radius to a 5 mile radius. The Helicopter route chart still depicts this as 4.2 (https://aeronav.faa.gov/content/aeronav/heli_files/PDFs/Houston_Heli_9_P.pdf) Hopefully the next edition will correct this.


FAA Commercial Pilot Knowledge Test: Sign Question This image appears in the FAA Test Supplement Commercial Test FAA-CT-8080-1E but I am unable to find it in the AIM or the Pilot’s Handbook of Aeronautical Knowledge. I looks like a don’t taxi here pavement marking, but I’d like confirmation of my guess.

The closest I can get to an official explanation is in the AIM 2-3-2 b

  Markings for taxiways, areas not intended for use by aircraft (closed
  and hazardous areas), and holding positions (even if they are on a
  runway) are yellow.

But the examples don’t match this graphic.

This sign is a taxiway ending marker.  I retrieved this graphic from the CFI Notebook Website.


Is it enough for crossing/ascending/descending IFR-flights to adhere to the separation minima? The separation minima in Europe are in general 5NM lateral or 1000ft vertical. If the safety distance is too small, there is a conflict (loss of separation). When both aircraft are on the same track, the adherance to the minimal separation is relatively easy. But if two trajectories cross the same point horizontally, for example over a beacon, the adherance to the separation minima seems not that easy anymore. The same is true for ascending or descending flights which burst through different flight levels.

Is the adherence to the separation minima for IFR flights enough to avoid airborne conflicts for crossing/ascending/descending flights, too?

If not, what is the minimum distance between two flights in space and time, that precludes all the different possible conflict types [page5]? For example there are local conflicts, which occur, when two aircrafts cross a given point from different directions in under 4 minutes.

This questions refers to the european airspace at best. See the linked question How much is the minimum safe distance between two planes in-flight?

This might not be the answer to your question, but maybe it will help you understand separation.

Your question specifically asks for how separation is ensured between climbing, descending and crossing traffic over navigational fixes (NAVAIDs).

Flight routes are planned for traffic to use certain flight levels en-route, which follow the semicircular/hemispherical rule in effect in that country or are bound to the airway being used. In my example picture below I will use the German semicircular rule of eastbound flights using odd flightlevels.

(Image Source: Own Work - Program used: Aircraft Situation Editor (ASE))

Both aircraft will cross the HMM VOR roughly at the same time, based on the ground speed displayed. The westbound aircraft (DLH123) is at FL200 (even FL), the eastbound aircraft (BER456) is at FL210 (odd FL). The separation between both aircraft overhead HMM VOR will reduce to 0nm lateral separation, but 1.000ft of vertical separation, and thus, the separation criteria are met.

(Image Source: Own Work - Program used: Aircraft Situation Editor (ASE))

In this example, I need BER456 to descend to FL100, but I have crossing traffic from the right which would cause a cleared conflict, if I issue a descent clearance directly to FL100 without any conditional instructions. I can use a bit of mathematics here to ensure separation however and issue a clearance nevertheless.

Both aircraft are travelling at 420kt GS. An aircraft will also descend 1.000ft in 3nm, rule of thumb from every flight instruction book available. To ensure 5nm of lateral separation, I would need a buffer of 2.000ft during descent to the crossing traffic, not factoring in that the other traffic will be on a divergent track anyway. Let's just measure the distance to the HMM for simplicity.

My instruction to BER456 would be:

  R: BER456, when ready descend FL100, cross HMM VOR at FL230 or above (rate of descend 2.000ft/min or less)

With this instruction, I can issue a clearance to BER456 that will allow the aircraft a continuous descent to FL100, while ensuring minimum separation is kept to the other traffic. Overhead HMM VOR, both aircraft will have 3.000ft of separation. After BER456 has descended a further 2.000ft, it will have 6nm and 1.000ft of separation, making it well clear of a conflict, especially since the traffic is divergent.

The dirty solution would have been to clear BER456 to FL210 and after 5nm issue another clearance to FL100, but this doubles the frequency time and also interrupts the descent for BER456.


Is there a minimal approach speed for airliners during rush hour? For busy airport, sequencing can be complexe. If the ATC put all aircraft in front of each other with the correct separation (which can be really small for busy airports) in final, if the airliners' approach speed vary significantly, one plane could catch up the preceding one. Can the ATC ask for a pilot to increase its approach speed up to a minimum speed?

ATC can not only ask for a minimum speed, but rather instruct each aircraft to (more or less) exactly fly at a particular speed. This is essential for maintaining separation. As J. Hougaard pointed out in the comments, the speed on the last 4 NM is always up to the pilot to slow down to final approach speed.

It is the duty of the pilot to evaluate if it is safe to comply with this restriction. In case the aircraft cannot fly as slow as asked for, the pilot can simply reply UNABLE and ATC will either try to make it work or instruct the aircraft to go around.

Some airports also publish speed restrictions on the approach charts. This is an example of a minimum speed from the ILS approach into 01L/R at Stockholm-Arlanda (ESSA):

More precise speed restrictions are for example given on the STAR charts of the (now closed) Istanbul-Atatürk airport (LTBA):


Does an amended shortcut clearance change the clearance limit? A typical IFR cleared route is like,

KABC VOR1 V12 VOR2 V23 VOR3 V34 VOR4 KXYZ

Suppose the verbal ATC clearance is "N7890, you are cleared to KXYZ airport, KABC.ZYBUD departure, radar vector VOR1, V12 VOR2 V23 VOR3 V34 VOR4 then direct, climb and maintain 3000, expected 5000 5 min after departure, departure frequency is 123.45, squawk 4321"

While you are en-route on V12, ATC says "cleared direct to VOR3". Does this new clearance change the clearance limit from KXYZ to VOR3? If no further clearance is issued while approaching VOR3, should we hold at VOR3 or continued with the cleared route firstly obtained?

POST EDIT:
The context is within US.

I assume that this question is about the US, based on your examples.

No, it doesn't change the clearance limit. For some reason I could only find the documentation in the ATC orders, not the AIM. Section 4-2-5 of the orders says:

  Issue a clearance “direct” to a point on the previously issued route.

  PHRASEOLOGY−
  CLEARED DIRECT (fix,waypoint).
  Or
  CLEARED DIRECT (destination) AIRPORT.

  NOTE− Clearances authorizing “direct” to a
  point on a previously issued route do not require the phrase “rest of
  route unchanged.” However, it must be understood where the previously
  cleared route is resumed. When necessary, “rest of route unchanged”
  may be used to clarify routing.

If the clearance limit changes, the controller will usually give you a completely new clearance to copy.


How can the lift induced drag for a wing be calculated? I have found a lot of information to calculate drag coefficients and I know that total drag can be calculated by
$$ D = \frac{1}{2} \rho u^2 S C_d $$
However I was wondering, if I substitute the zero lift drag coefficient into this equation is it correct to assume the value produced for the drag force would be the parasitic drag whilst the remainder of the total drag is made up of lift induced drag?

Induced drag is the portion of the lift force that opposes motion.  Induced drag exists even when the drag coefficient CD in your equation is zero.

Lift is perpendicular to the air flow relative to the wing.  Imagine a wing moving right to left across your screen.  The air is moving left to right relative to the wing.  If there's no vertical induced airflow then the lift is vertical and there's no induced drag.  However, lift deflects the air down, so typically the air relative to this wing is not moving exactly left to right, but is tilted down slightly.  Hence, the lift, which is perpendicular to that airflow, is actually tilted back (right) slightly.  That means a (typically small) portion of the lift is "pushing this wing back," opposing its right-to-left motion.
If you know trigonometry, induced drag is lift times the sine of the inflow angle, $ L\sin \phi $.  The inflow angle is the angle of the airflow relative to the horizontal in this picture.
There's a picture of exactly this at the bottom of this page in the induced power section.  (Induced power is the power required to overcome induced drag.)


When flying VFR without GPS, how do pilots know if they are inside controlled airspace or not? How do VFR pilots know whether or not they are inside controlled airspace if they don't have GPS onboard?

VFR aviation maps called "sectionals" (and now GPS map displays) depict the types of airspace through borders with different colors and dashed lines. You can buy or download the maps for free from this FAA site.

It is always the responsibility of a pilot to know where they are and follow all applicable laws. In the US, a pilot that breaks a rule because they didn't check NOTAM's can expect certificate action. NOTAM's will inform the pilot of TFR's, "hot" military zones, and other important regional and local (i.e. airport) regulatory information.

I got my PPL in 1975 and still carry maps with me. I plot an "X" along my route every 10min of flight time (20-30mi). As I reach each checkpoint, I follow a very old axiom, "Never.. Ever.. EVER!.. proceed to the next checkpoint until you identify where you are and what corrections are needed for the next checkpoint". In this manner, you can never be further than 1 or 2min and 1-2 mi from where you should be.

Even when sightseeing with no particular route, I routinely mark a map about every 10min with my location. In my 2500hrs of flying, the most I have ever been off  in my navigation was 10mi (even when flying 1500mi x-country). So, knowing where you are is not difficult if you practice the skill.

The front of the map has a legend to remind pilots what the coloring stands for.

Sectional map (US Gov public domain)

Front map legend (US Gov public domain)


How are the system development and integration procedure separated for Integrated Modular Avionics? I'm reading this paper about IMA (Integrated Modular Avionics) and I don't get the splitting of the development and integration procedure. In page 4 it says:

  Airbus has to modify the conventional development processes and responsibilities. The IMA system development and integration procedure is split between

  the Avionic Module Supplier
  the Aircraft System Manufacturer
  Airbus

Who is the Aircraft System Manufacturer? Which is its function?

My doubt is principally about this paragraph that is few lines later:

  The open IMA concept puts the assembler into the position of the IMA system integrator. The
  development responsibility for the single systems and their functions remains at the system
  manufacturers. The avionic module supplier provides “empty” IMA modules to both, the
  system manufacturers and to assembler. The system manufacturer applies the CPIOM for the
  local function implementation and qualification. The assembler, in turn integrates the own
  empty CPIOM with the ADCN on the aircraft in order to establish the basic computing resource
  for different system functions.

The Aircraft System Manufacturer is the company developing the system that is being implemented on IMA hardware (supplied by the Avionic Module Supplier). They will likely develop software to load onto the IMA hardware to perform certain functions.

For example:
Airbus would like to implement a Thrust Reverser Control System for their new airplane. The system would take in some parameters gathered from various sensors within the aircraft, perform the necessary logic and/or calculations, and then provide the necessary control commands to the thrust reversers.

Now Airbus doesn't want to develop this specific system, so they contract it out to another company ACME Avionics. They provide ACME with the System Requirements Data, which describes how the system must operate, what inputs it will use, the Design Assurance Level of the system, and the IMA modules the system must use to implement the requirements.


Why don't airplane piston engines have mufflers? It is understandable jet engines couldn't have mufflers, but even long before the jet age, airplane piston engines never had mufflers the way cars do.
Is there any specific reason?

Today, most GA aircraft in Europe do have mufflers so they can pass the ever more stringent noise requirements. Here is one mounted to a Cessna 172 (picture source):

Historically, mufflers were considered as a disadvantage because they reduce engine performance by increasing back pressure. By partially blocking the exhaust flow, the muffler increases cylinder pressure on the last upstroke of the cycle (the exhaust stroke) so it faces more resistance against moving up. Also, more of the burnt gasses remain in the cylinder so less fresh fuel-air mixture can be ingested in the next cycle. At most, in aircraft with an open cockpit a tube was installed to duct the exhaust gasses away from the pilot. Below is a picture of a Mercedes D.IIIa in a Fokker D VII (source):

However, night flying would make aircraft easily detectable by their exhaust flames, so in WW II tubes were added to the exhaust stack in order to avoid optical detection. Below is a picture of an Avro Lancaster with Bristol Hercules engines (source):

Our much better understanding of piston engines now allows to design mufflers which do not cause much power loss and in case of a tuned resonance muffler you even get more power at the design speed because a reflected pressure wave will arrive from the muffler at the exhaust port just when the valves close and will increase cylinder pressure. However, given the low RPM speeds of our old-fashioned aviation piston engines, those mufflers need to be large and the equally old airframe designs have no space for them. So in aviation the mufflers are still causing a power loss.


What are the reliability statistics for modern jet engines? What is the actual numerical reliability statistics of the modern, large-aircraft, commercial turbofan engines available today?

e.g. How many running hours can we expect a failure after? Are there empirical or projected MTBF statistics?

How is credible MTBF established for a new engine model?

Is there a significant difference in the failure rate between engine models or manufacturers?

I was motivated to ask by this snippet on another answer on SE Aviation:

  In the days of the 747 regulations required at least 3 engines if you
  ventured more than 60 mniutes of flying time away from a suitable
  diversion airport (in case the second one also failed). However as
  reliability of engines increased this got extended with ETOPS
  certificates.

How rapidly has engine reliability improved? Do we have a time trend of reliability numbers?

Why do modern airliners have only two engines?

What is the actual numerical reliability statistics of the modern, large-aircraft, commercial turbofan engines available today?
e.g. How many running hours can we expect a failure after? Are there empirical or projected MTBF statistics?

Buried in this GE press release is the comment that the GE90 has 'only one engine IFSD  per one million engine flight-hours'.
This ICAO presentation has a lot more info, including these threshholds for ETOPS IFSD rates:

Propulsion system should achieve an in-flight shutdown (IFSD) rate of:

0.05 / 1000 engine hours for EDTO 120 min
0.02 / 1000 engine hours for EDTO 180 min
0.01 / 1000 engine hours for EDTO beyond 180 min
Rate computed based on world fleet data, on a 12-month rolling average

This strongly implies that the reliability targets are empirical, not based on theoretical numbers.

Is there a significant difference in the failure rate between engine models or manufacturers?

Page 56 of the presentation above has some data, but IMHO not enough to draw any conclusions.

How rapidly has engine reliability improved? Do we have a time trend of reliability numbers?

All I can say is it's definitely improved massively, but I'm not sure where to find the info.


What is meant by a "pressure system" in FAR 91.205? FAR 91.205 states:

  § 91.205 Powered civil aircraft with standard category U.S. airworthiness certificates: Instrument and equipment requirements.

  (a) General. Except as provided in paragraphs (c)(3) and (e) of this section, no person may operate a powered civil aircraft with a standard category U.S. airworthiness certificate in any operation described in paragraphs (b) through (f) of this section unless that aircraft contains the instruments and equipment specified in those paragraphs (or FAA-approved equivalents) for that type of operation, and those instruments and items of equipment are in operable condition.

  (b) Visual-flight rules (day). For VFR flight during the day, the following instruments and equipment are required:
  ...

  (5) Oil pressure gauge for each engine using pressure system.

What is a "pressure system" in this regard?

Background: Our aircraft has an oil pressure gauge that erroneously reads high pressure. The A&P checks this every year and notes that the oil pressure is within the normal range.

EDIT: "I guess I should be clear about what "erroneously high" means, the gauge pegs the needle so it isn't just a little high". – Ron Beyer

The sender has been replaced and the only thing left is the gauge. We've been flying like this for years. Recently one of our CFI's has determined that we are in violation of 91.205 and has told us to ground the aircraft until it can be repaired.

Have we been flying against FAR's? Is a "pressure system" meant to mean a pressurized oil system, or does it mean an aircraft with an altitude engine? Like I said, our A&P (two different ones in fact) have signed off on this since the engine was replaced 10-ish years ago.

EDIT:    With the additional information that the oil pressure gauge is "pegged", the CFI is correct in calling it un-serviceable.

It sounds like the A&P has checked the engine oil pressure with another means and finds that the actual engine oil pressure reading is within limits. The A&P then signs it off thinking that a correct gauge reading is not required. The A&P is incorrect as a serviceable gauge is required as per FAR 91.205

Oil pressure gauges are required for engines with pressurized lubrication systems,  but not for engines with "splash" oiling.

Older engine designs, and may of today's smaller engines (like lawn mowers), use a "splash" oiling system that does not supply oil under pressure to the bearing journals, or to other friction points in the engine.

In a "splash" system, oil is applied to the cylinders and pistons by rotating dippers on the connecting-rod bearing caps. As they they rotate, the dippers pass through the oil pan and slash oil into the areas that require lubrication.

If the oil pressure gauge in your "pressure system" is reading high, outside acceptable limits, it should be replaced. It is easy to confirm using another gauge. High oil pressure can also be caused using the wrong viscosity oil, incorrect pressure relief valve on the oil pump, or bearing clearances that are to tight.

High (or low) oil pressure could cause an engine to fail. If the gauge was out of calibration, it would be impossible to tell what the actual pressure was. If you are accustomed to seeing high oil pressure, how could you possibly identify what was too high, or what was too low?

I would not want to fly any aircraft with an inaccurate oil pressure gauge because it would rob you of valuable time to deal with an engine that might fail at any time.


Can I use a personally owned aircraft in the EU for PPL flight training? Can I use my own C152 to gain my PPL (EASA) with a qualified instructor?
Is this a considered a commercial flight?
Can you define a 'training flight'?
Is hour building a commercial flight?
If I use my private 152 how about insurance?

Can I use my own C152 to gain my PPL (EASA) with a qualified instructor?

It doesn't matter who the owner is, so yes. As long as you pay to keep it in an airworthy condition and fuel her up, she's all yours.

  Is this a considered a commercial flight?

The fees the instructor is getting is for teaching you to fly, so no. Also you'll be the one who's paying.

  Can you define a 'training flight'?

Definition (my own, Google not helping): Any flight with a CFI on board, or approved by your CFI, with the goal of getting a higher certification.

Which in the broadest sense, every single flight is a training flight. Don't let the over confident pilots fool you, every flight is indeed a lesson.

  Is hour building a commercial flight?

Any flight builds hours. The answer is no if you're training, in your own plane or otherwise.

  If I use my private 152 how about insurance?

The EU is big. Check with insurance companies. But most likely it's no different than normal insurance. What if a flight school owner (who is not a pilot) decided to learn in one of his planes and the instructor is one of his employees, would that change the insurance terms? Nope.

Last paragraph actually answers all questions.

Fly safe!


What does the airport fuel market look like? The single biggest cost for many airlines is fuel. Given this, I'm sure they work hard to reduce the cost they pay per gallon of fuel as much as possible.

How does that translate to the way fuel is bought at major airports? I know there's a fuel truck that comes and fills up the planes. Does each airport partner with one company to provide fuel? Are the multiple providers on each airport? Does the airport itself sell fuel? Is fuel lumped into the landing fee?

Large airlines have standing, negotiated contracts with major airports to buy fuel (and almost always also other services like cleaning, catering, etc).  Some times these are fixed-price contracts, sometimes they float in relationship to the spot price of jet fuel, sometimes they are a hybrid.  You can think of these as any other complex financial relationships: almost any way you can imagine a contract to be drawn up, someone has likely tried it.

Airlines' operations desks also constantly monitor the price of fuel at different airports and different times - and in some cases airplanes carry enough fuel to make a return trip without refueling because it ends up being cheaper to carry the fuel rather than purchase it at the next stop.

For general aviation, the airport FBO (fixed base operation) generally has one and sometimes multiple fuel vendors on-site, that generally sell at a fixed price -- very much like a gas station for cars.  Obviously, airports that have more than one fuel vendor generally have more competitive prices, but even for single-vendor operations they can't make the fuel costs TOO high, or pilots will purchase just enough fuel to take off and go somewhere that gas is cheaper.


What is "airspace underlying a Class B airspace area designated for an airport"? For the purposes of airspeed restrictions within airspace, the FAA regulations list the following area description (emphasis mine):

14 CFR 91.117 (C)

  No person may operate an aircraft in the airspace underlying a Class B airspace area designated for an airport [...] at an indicated airspeed of more than 200 knots (230 mph).

Should we understand the described airspace to be any and all airspace underlying a Class B where that specific Class B is designated for an airport? Or, should we understand the described airspace to be any and all airspace designated for an airport (Class C, D, or E) which is underlying a Class B?

Are there letters of interpretation or other regulations which clarify the grammatical ambiguity of this regulation?

Yes, there is a legal interpretation on this:

  [...] under  the  express  language  of  the  regulation,  aircraft
  operating  in  the  airspace  underlying  Class  B  airspace -
  irrespective  of  whether  the  underlying  airspace  is  controlled
  or  uncontrolled - may  not  exceed  the  200-knot  speed  limitation

So it is indeed any and all airspace underlying class B.

And note the exception to the rule:

  We  note  §
  91.117  (d)  does  provide  that,  if  the  minimum  safe  airspeed  for  any  particular  operation  is  greater  than  the  maximum
  speed  prescribed  in  §
  91.117,  then  an  aircraft  may  be  operated  at  that  minimum  speed.


What is the maximum altitude allowed to fly a model plane/copter in Germany? What is the maximum altitude I'm allowed to fly a model plane or 'copter in Germany?

The maximum altitude you can operate your RC plane or copter vertically without any clearance from ATC or another government body is the upper limit of class G airspace in Germany, which is the first available airspace from GND to the lower vertical limit of controlled airspace C/D/E, which can be 2.500ft AGL1, 1.700ft AGL or 1.000ft AGL, depending on surrounding airspace and airports in the vicinity. You can register at the website of the DFS AIS to obtain the ICAO charts for your area and take a look at the airspace structure near you.

The actual ceiling of airspace G can be significantly lower in the vicinity of larger airports, where the controlled airspace C/D/E has been lowered. The below picture shows how the dark green section around Frankfurt Airport has an airspace C with the vertical limits 1.500ft (MSL)/FL100. The airspace E in that area (not visible in this picture) has a vertical limit of 1.000ft AGL/C. Some areas within that section already have an elevation of 700-1100ft MSL, which leaves less than 100-400ft of airspace G before the overlying airspace C begins.

If you want to get your RC plane or copter into class C/D/E airspace, which is controlled airspace, you will need to obtain a clearance through the ATC unit in control of the sector or the FIS sector responsible, as postulated in LuftVO §16a

  § 16a LuftVO
  Besondere Benutzung des kontrollierten Luftraums
  (1) Bei Inanspruchnahme des kontrollierten Luftraums ist von der zuständigen Flugverkehrskontrollstelle eine Flugverkehrskontrollfreigabe einzuholen für
  1. Fallschirmsprünge und den Abwurf von Gegenständen an Fallschirmen;
  2. Aufstiege von Flugmodellen und anderen fern- oder ungesteuerten Flugkörpern mit Eigenantrieb;
  3. Aufstiege von unbemannten Freiballonen mit einer Gesamtmasse von Ballonhülle und Ballast von mehr als 0,5 kg sowie Aufstiege von gebündelten unbemannten Freiballonen und Massenaufstiege von unbemannten Freiballonen.

Please also mind LuftVO §15a, which dictactes that any RC plane or UAV needs to remain within the visbility range of the controller, so the maximum altitude can vary depending on the size of your RC plane or copter, the weather minima at your location and your own eyesight:

  (3) Der Betrieb von unbemannten Luftfahrtsystemen ist verboten, wenn

  1. er außerhalb der Sichtweite des Steuerers erfolgt oder
  2. die Gesamtmasse des Geräts mehr als 25 Kilogramm beträgt.

1: AGL = Above Ground Level
2: MSL = Mean Sea Level


Which FAA regulations require passengers to comply with flight crew instructions? Recent unfortunate news headlines prompted a quick search for the almost universally un-cited regulations governing passenger compliance with flight crew, and all I could find were regulations that seem to be quite specific in application (14 CFR 125 - interference, etc.).

While it makes sense that passengers are expected to comply with reasonable requests, one would not expect the law to require compliance with unreasonable requests unrelated to safety, or requests that actually endanger passengers.

So, for example, if a crew member requests that a seated and non-disruptive passenger leave a plane for purely commercial reasons unrelated to anyone's safety, is there a law or regulation that supports this for US carriers?

Again, I expect the answer is "yes," but am interested in the legal basis for it, and do not think the self-serving fine print on an airline ticket qualifies.

On the day itself, the captain decides who gets to be on the aircraft when it departs. End of story, full stop.

Not abiding with this falls under the interference provisions you already mention -- or, more elementarily, it is trespass. Local law enforcement will help enforce the decisions if necessary.

Afterwards, if someone who didn't get to fly contends they had a valid and applicable contract from the airline that entitled them would fly, they get to duke it out in court for monetary damages.  How that turns out would depend on a lot of factors that are better discussed on Travel.SE or Law.SE.

The airline may lose in court, and if the captain's decisionmaking was sufficiently egregious (say, "there'll be no [racial slur]s aboard the plane I'm flying"), he may soon find himself out of job.  But the police who enforced the captain's decision on the day were still in the right to do so.  It's not their job to attempt to unravel contract disputes on the spot.


What operational aspects of Alphabet's Wing drone delivery service will be the same as that of an airline? The NPR News article and podcast FAA Certifies Google's Wing Drone Delivery Company To Operate As An Airline begins with:

The Federal Aviation Administration has certified Alphabet's Wing Aviation to operate as an airline, in a first for U.S. drone delivery companies. Wing, which began as a Google X project, has been testing its autonomous drones in southwest Virginia and elsewhere.
"Air Carrier Certification means that we can begin a commercial service delivering goods from local businesses to homes in the United States," Wing said in a statement posted to the Medium website.

The article, plus the BBC News's Google Wing drones approved for US home deliveries indicate that Wing (owned by Alphabet, which also owns Google) will be he first US drone delivery company to be run as an airline.:
See also Medium.com's Wing becomes first certified Air Carrier for drones in the US
Question: What operational aspects of Wing will be the same as that of other airlines? They won't sell seats of course, but will the file flight plans for example? Will they have licensed human pilots to oversee the drone's flight?

The Wing company, a Google spinoff, has won federal approval to operate its drone delivery system as an airline in the U.S.  Credit: Wing, Source

Customers who took part in Wing's drone delivery test program in Virginia approach their package after it was dropped on their lawn. Credit: Wing Source

Under FAR Part 117, only certified "air carriers" are allowed to sell "aviation services" to the general public. Certification means that the FAA has determined the proposed operations, whatever they may be, will be reasonably safe.

FAR Part 121 covers most scheduled air carriers, which are commonly called "airlines". These are the only air carriers that the media knows about, so it's understandable that they'd conflate the two.

FAR Part 135 covers unscheduled and very small scheduled air carriers. I suspect this is where Google Wing would fall, but I haven't seen anything definitive so far.


How does the LIP package which is standard on the A320NEO enable to approach at a slower speed? Could someone kindly explain as to how the lift improvement package on the A320NEO enable it to approach at a slower speed compared to the IAE CEO. I have the list of the modifications which is done but it doesn’t explain how it helps.

They are:

Slats (de icing slay/track shutter modifications and stiffened slat/track seals)
Slat 1 enlarged horn
Adding an outboard nacelle strake
Fuselage wing fairing root fillet aerodynamic improvement
Wind shroud close to flap 1 and 2 junction
Introduce extended trailing edges on spoilers 1 and 2 and on fixed wing box trailing edge between spoilers 1 and 2.

These modifications mainly allow the aircraft to approach up to 5 knots slower than our CEOs. But how exactly do the modifications work in reducing the approach speed?

On first impressions, all of those (except the nacelle strake and the fillet) seem to modify the high lift system (slats/flaps), presumably to optimise air flow in the gaps between slat and main wing or main wing and flaps. This is probably done to increase the maximum attainable angle of attack or lift coefficient, which will reduce stall speed and thus allow a lower approach speed.

The nacelle strakes normally are used to optimise flow around the engine nacelle at high angles of attack, either to prevent premature wing stall around the nacelle or to at least improve aircraft handling qualities by preventing localised flow separation at the nacelle. Adding another strake on the other side will, I imagine, again allow a slightly higher angle of attack to be sustained and thus will lower the permitted approach speed.

The wing body fairing modification most probably, too, will allow a more optimal local air flow.

All of this is my speculation based on the list of changes in the question; I don’t have additional knowledge on this particular aircraft performance package.


Are there fees for international flights crossing through another country's airspace? When an aircraft flies over another country are there fees associated with such flight? I am thinking particularly of international airline flights, though this might also apply to flights with the same country of origin and destination that transit another country's airspace.

Are there fees for flying through another country's airspace in general?
Are there fees for flying through the airspace associated with an airport in another country?

Most countries have implemented route charges billed to aircrafts overflying their territory and by doing so using their control infrastructure. Those fees are usually computed according the distance flown and aircraft weight, small aircrafts (eg. less than 2,000 kg in Europe) may be exempt. They are collected for every flight over the territory, both domestic and international.

In the same way, some countries charge terminal fees when the airport control infrastructure is used.

Those fees are most usually levied for flight performed according IFR (Instrument Flight Rules), not VFR (Visual Flight Rules).

In Europe, the EUROCONTROL organization is empowered by States to collect route charges and terminal fees.