What's the difference between Holding Point and Hold Short? I heard some ATCos requesting pilots to report, on holding point, and also hold short. Is there any difference on them?

The U.S. and holding point
It depends on the locale and context. For taxiing in the US holding point is not used as the FAA ATC manual refers to it as an ICAO term for a visual (or otherwise) holding point in the sky.

HOLDING POINT [ICAO]− A specified location,
identified by visual or other means, in the vicinity of
which the position of an aircraft in flight is
maintained in accordance with air traffic control
clearances.

Elsewhere and hold short
Elsewhere for taxiing, holding point is where the intended departure will take place from a runway. Check eurocontrol.int for examples (direct linking not available), but here's an example they give:

TAXI TO HOLDING POINT [number] [RUNWAY (number)] VIA (specific route to be followed), [HOLD SHORT OF RUNWAY (number)] or [CROSS RUNWAY (number)]. [REPORT VACATED]

Taxi to holding point A, runway 36, via A.

That means take A all the way to the end where it meets runway 36 and stop at the hold short line or behind whoever is ahead.

Taxi to holding point C, runway 36, via A and C.

Here the runway was shortened (an intersection departure) and the pilot will hold at C and then line up from C when cleared.
Report holding point means report when you get there.

Tower, Stack 1 is at holding point A, fully ready.

Hold short is the same everywhere and applies to any intersection or runway on the way to the runway/gate.

Taxi to holding point A, runway 36, via A, hold short C.

Here the pilot will hold where C meets A and awaits clearance to continue.

Hold your position is an instruction to stop right now.
Some airports employ LAHSO (Land and Hold Short Operations), where a hold short line exists on the runway you are landing on.

Example: Cleared to land runway 27, hold short runway 22L.

Example in Boston Logan (blue circle):


What do aerodynamic flow cones measure? This picture of an Airbus A350 test aircraft shows multiple instruments on the flap fairings, wing fairing and on the empennage:

Source: A380_TLS_A350 AIB199GG (high-res version)

They are being described as 'aerodynamic flow cones'.

What are these -- what do they measure, and how?

To expand a bit on the other explanations here, "flow cones" are one of the many methods used for flow visualization.

When something is being tested in a wind tunnel or in flight, you usually can't see the air flow around the body. To be able to visualize the flow, something is added to the air, like smoke, or to the body, like flow cones or tufts. Tufts are basically just short lengths of string taped to the body at one end. Flow cones are small cones attached at their apex with string.

These items on the body have two main functions. Primarily, the flow cones or tufts will align them selves with the flow, allowing observers to see the direction of the air flow in that area.

They can also show flow separation. When the flow is attached to the body, it flows fairly smoothly along the surface. When the flow separates from the surface, it becomes much more turbulent and chaotic. The tufts or cones will show this as they are moved by the air.

Flow cones are larger than tufts, so they are much easier to see, which is important in cases like the picture you posted, where they will probably be observed from a chase plane at some distance.

For more on flow separation, see this question, or this video of tufts on a stalling wing. For a comparison of tufts and cones, zoom in on this photo of the X-29A. The darker, larger objects behind the cockpit are cones, while the reddish, smaller ones further aft are tufts. You can also see smoke at the nose being used to visualize the flow.


Do any ejector seat designs include controllable thrusters? Are there any ejector seat designs with built-in thrusters, or boosters, which the pilot can deploy to maneuver himself out of, or away from enemy territory?

A use case might be that after the parachute is deployed, the ejected pilot uses the thrusters to change direction or propel himself in a different path.

Yes, modern ejection seats have rocket motors, but they are only for propelling the seat away from the aircraft and upwards, high enough for safe parachute deployment. An escape from enemy territory is not possible with them.

They are controlled by a guiding system inside the seat, but not by the pilot. Ejection by itself is stressful enough, controlling the seat would be too much for any pilot.

Nevertheless, this is impressive technology. Even when ejected pointing downward at low altitude, the seat will change course and lift the pilot up.

The seat featured in the linked video is the Russian Zvezda K-36, the only ejection seat which meets or exceeds all US Air Force performance demands.

To get downed pilots out from enemy territory the armed forces use helicopters. To build such a capacity into the seat would make it impossibly heavy and dangerous - would you like to sit on hundreds of kilos of jet or rocket fuel while being shot at? Just for the chance of flying home after ejection? And how deep are you allowed to fly into enemy territory for the system to work? No, this idea is a dead end.

The most promising concept today is to separate the pilot from the airplane, so he sits thousands of miles away while his craft flies deep into enemy territory.


Manufacturers are narrowing seats. Any possibilities for price cut? To squeeze more profits from each plane, the airline industry in the
  last few years has begun to install extra rows of seats,  leading to a
  crunch in legroom that has sparked tiffs and feuds among frustrated
  passengers. (Source)

.

  Airbus has suggested 3-5-3 configuration for A380, making airlines
  have many extra seats.Source

A similar plan is there for the B777. Is it likely that airlines will pass on some of greater earnings to their passengers, in return of the lack of comfort?

You will likely not see a price cut, but you also might not see a price increase.

Seat pricing (along with just about any other service in a competitive market) is a complex function of actual cost to the company + desired profit + market factor. The last one is the difference between what they want to get and what the market is willing to pay. It is sometimes negative.


What are the different steps to certify a new aircraft in Europe? I know that to be allowed to fly an aircraft has to be certified by an agency and that this one is not the same for European or American (for instance) aircraft. What are the different steps that an aircraft designed to fly in Europe has to go through in order to be certified? Proof on the paper of some features? Ground tests (which one)? Flight tests (which one)?

The EASA certification requirements are all publicly available online. Actually proving that an aircraft meets those criteria will certainly include all the steps you mentioned although exactly what happens probably depends a lot on the specific aircraft and manufacturer.


How does the ski shape of an aircraft carrier deck help for take off? Why is a ski-shaped deck used on an aircraft carrier for take off?

The aircraft needs a particular lift for a particular weight, how does ski assist in take off?

The Ski jump was originally used by the UK Royal Navy to assist Harriers in getting airborne.

The Harrier cannot take-off vertically when fully loaded.  In all models, the thrust is lower than the maximum weight.  Therefore, they need to do a "short take-off" where the thrust nozzles are angled rearwards to provide thrust, then partially angled downwards as speed is gained to produce enough lift from the wings, and thrust angled downwards, to climb away.  Watch the nozzles closely at about 17 and 36 seconds in this video.  Try not to watch the poor chaps forehead too closely at 20 seconds.  Also watch this video.  At about 16 seconds, you can see the pilots left hand pull back on the nozzle angle lever as he passes the end of the deck.

Now imagine a flat deck on an aircraft carrier.  Operations must be able to carry on in rough seas.  You accelerate down the deck and fly off the end, just as the deck dips downwards as the carrier rides a large wave.  You are pointed straight at the sea.  In order to avoid flying into the sea, you need to climb, which means pulling G and a lot of power at low speed which the Harrier just didn't have.

Now consider the ski jump.  There are two effects.

The jump is angled so that even in the worst seas in which operations will continue, the bow pitching down will not point an aircaft leaving the end of the jump towards the sea.  The aircraft will always be pitched upwards.
Imagine driving a fast car up the jump and off the end.  It will be thrown up and forward in a parabola.  Even a car, which cannot fly, will be pointing up for a few seconds, then level for few seconds before falling into the sea.  But instead of a car, you are driving a jet which can fly and has thrust.

The aircraft will angle the nozzles rearwards for maximum acceleration until it leaves the jump. The wings are now generating significant lift, let's say 50% of that required to climb away.  As you leave the end of the jump, you angle the nozzles downwards so that you have the momentum of the aircraft and some rearward thrust maintaining lift on the wings whilst adding lift from the thrust vector having a downward component.

Those seconds of pointing up, then level, enable the aircraft to continue to accelerate until all of the lift required can be gained from the wing when the nozzles can now be angled rearward again to continue the acceleration. Take-off speed is therefore reduced and take-off distance is about halved meaning smaller, cheaper carriers.

That's how Harriers do it.  I imagine it's the same story for any other VSTOL aircraft.


Single-engine plane with long range flyable in Europe Not 100% sure if choice of airplane is on-topic here, please let me know if not.
Just got my PPL, and looking for a choice of single-engine meeting a few conditions:

Long-range, definitely >800nm, ideally >1200nm
Can be flown IFR with only IR (no special type/class ratings) under both FAA and EASA
Available used for under $100K, ideally less

Are there planes out there that meet these conditions?

There are definitely aircraft which meet some of your criteria, namely range above 800nm and can be flown IFR. I'm not going to list them all, notable examples would be the Mooney M20 series, the Cirrus SR22, Cessna 182 and Cessna 210. No doubt there are more. The issue will be cost, you can find older M20s and Cessnas for your budget, the cheaper they are the shorter time the engines have until they need a costly overhaul and the more work they'll need.


The F-16s that were scrambled to intercept United 93 did not have time to arm the jets, so the pilots were going to “ram it” how does that work? The Wikipedia page says

Two F-16 fighter pilots from the 121st Fighter Squadron of the D.C. Air National Guard, Marc Sasseville and Heather "Lucky" Penney, were scrambled and ordered to intercept Flight 93. The pilots intended to ram it since they did not have time to arm the jets; this was in the days before armed jets stood ready to take off at a moment's notice to protect the capital's airspace.[140] They never reached Flight 93 and did not learn of its crash until hours afterwards.[141]

Is this exactly what I think it means? Crashing into another aircraft? What I’m picturing is the fighter pilots coming in above and behind, and using the nose of their aircraft in either a downward or upward motion to eradicate the stabilizer and elevator completely.
Any info is greatly appreciated!

(disclaimer - Former USAF armament specialist)
The intention was not necessarily to "ram", but rather get eyeballs on the situation and take whatever action was needed. That may have, eventually, been a "ramming".
Obviously, that was not their first choice of action.
And ramming a 757 was not a thing that was practiced or planned for.
As far as " this was in the days before armed jets stood ready to take off at a moment's notice", that is false. There were 7 bases around the country, with fighter jets on alert. On that specific day, 2x F-15 from Otis AFB, MA and 2x F-16 from Langley AFB, VA had already launched to intercept the other airliners that were in play. Those 4 were fully armed.
Andrews AFB MD was not one of those few alert bases.
The pilots from the DC Guard knew that the WTC had been deliberately targeted, and would take whatever action that turned out to be needed.
For them being 'unarmed', that is the usual case for 99% of fighter jets on US soil. They don't all sit around with missiles and hot guns.
And to put missiles on them takes time. Time they didn't have. It would be a minimum of 30 minutes to get the missiles out of the storage area, tow them to the flightline, and load them on the jets.
Munitions are NOT stored right next to the jets.
While the DC Guard jets were being launched, other jets were being prepped with missiles. Turned out not to be needed.
The question often asked is how the USAF was caught with their pants down.
Well, the alert jets around the country were mainly for threats coming in from the coasts or over Canada. Russian Bears, for instance. Perimeter defense.
Not a known friendly civilian airliner, suddenly turning hostile once it is already inside the perimeter.
And up until the mid-90's, there were many more jets on alert around the country. East coast, probably every couple hundred miles. Loring ME, Griffiss NY, Langley VA, Shaw, SC, on and on all around the coast and borders.
The drawdown of the mid 90's did away with most of that tasking.


What are the US definition and restrictions on aerobatic flight? In the US, what is the legal definition of aerobatic flight? What are the restrictions (both for the aircraft and its location/maneuvers) applied to aerobatic flight? I remember reading something about a restriction on degrees of pitch or bank, but I was looking around for it and can't seem to find it again.

Aerobatic flight is "an intentional maneuver involving an abrupt change in an aircraft's attitude, an abnormal attitude, or abnormal acceleration, not necessary for normal flight."

According to the FAA (91.303), no person may operate an aircraft in aerobatic flight:

Over any congested area of a city, town, or settlement;
Over an open air assembly of persons;
Within the lateral boundaries of the surface areas of Class B, Class C, Class D, or Class E airspace designated for an airport;
Within 4 nautical miles of the center line of any Federal airway;
Below an altitude of 1,500 feet above the surface; or
When flight visibility is less than 3 statute miles.

Aerobatic flight is sometimes considered to be flight with more than 30 degrees pitch or more than 60 degrees bank. This is incorrect. It applies to wearing a parachute. (FAR 91.307)

A normal category aircraft is certified for the following "non-acrobatic" operation: Stalls, lazy eights, chandelles, and steep turns, in which the angle of bank is not more than 60°.

A utility category aircraft is certified for spins (if approved for plane), lazy eights, chandelles, and steep turns, or similar manoeuvres, in which the angle of bank is more than 60 degrees but not more than 90 degrees.

An acrobatic category aircraft is required for other aerobatics.


How do pilots manage the speed during the approach? Once the initial approach fix is reached, what are the different "speed stages" to the runway?

I often read about landing speed and approach speed. For example on this chart, Boeing indicates various approach speeds.

I don't understand from what point an aircraft is supposed to be at the approach speed. Is there a specific distance threshold that you cannot cross above the approach speed? What about for the landing speed?
Are there other key speeds than those two ones between the IAF and the runway?

The Boeing chart you reference appears to use for the approach speed column the Vref speed of the aircraft at its max landing weight in the landing configuration. This is the speed you want to be at or close to when you start the landing flare (the two 747 carriers I flew for used Vref+5).

Absent charted speed limits, controller instructions, or company policy, there is no specific distance threshold along the approach path that you cannot cross above that speed. The requirement is to be at the proper speed when you enter the landing flare.

However, if you were, say, flying the classic stabilized approach, you would be in the landing configuration at this speed from the FAF (final approach fix) to the landing flare. The FAF is typically around 5 miles from the runway.

Now, insofar as speed stages, the requirement is not one of specific distance points along your approach path (again absent charted limits, etc.), but rather what you have to do speed wise while changing from a clean configuration to the landing configuration. You're approaching the airport with the gear and flaps up. The critical element speed wise on the approach is the wing configuration.

Given the 747-200 speed of 150 kts in that chart, if you slow to that speed with the flaps up, you will stall. The minimum safe speed would be around 80 kts greater than that. Go to flaps 1 (which really doesn't put the trailing edge flaps at 1 degree but rather puts out the leading edge flaps or slats as some call them). Once they're out you can safely slow to Vref+60. At flaps 5 you can slow to Vref+40. At flaps 25 you'll be able to safely slow to the Vref speed in normal operating conditions.

Other aircraft have different flap settings and different speeds. The important thing to remember is that you're changing the shape of the wing to be able to fly more slowly. Where you will be along your approach path when you do this is typically not determined by distance per se, but by time. Do it too soon and you burn more fuel. Do it too late, and the problem becomes one of not being at Vref when you need to be.


If Space Shuttle flies "like a brick", why does it need the wings? I have seen the Shuttle named a "flying brick" multiple times.  However a brick, as understood in the building industry, is a simple rectangular shape without wings and without any adaptation of the form. If the Shuttle was only capable of that much, why could not it be a lifting body like  Martin Aircraft X-24?

The small wings make it fly like a brick. Without the wings it would fly like a stone.

Seriously, you are taking the expression too literally. The Space Shuttle is landing like a glider plane with a (not so good) glide ratio of about 4.5:1 (see What was the Space Shuttle's glide ratio?). No brick would be able to achieve that.

Designing the Space Shuttle as a lifting body ( ≠ brick) was actually considered. It appears that a lifting body design was not able to comply with the flight envelope requirements.


A320/321 FMA indications with Both Flight Director Channel Failed I hope that someone can answer my question. My friend had “FD 1+2 Failure” in simulator during the low visibility approach. Flight Directors on PFD disappeared, but no FMA change and no approach capability downgrade. When a go-around was executed, lateral and vertical guidances of FMA went blank (SPD/ / ).
On the next approach, he captured G/S and LOC with both FDs failed, but no ROLLOUT indication after touch down.
Anybody can explain the logic behind this?
Thank you.

On the A320, the FAC takes commands from the FMGC to compute the FD bars. The FD bars and  certain FMA annunciations and speeds are provided to the DMC for display on the PFD.

Your FD loss scenario is a partial FAC failure which is why AP continued to work, since the FG function of the FMGC was operational, but the FMA annunciations are faulty. Yaw control is also a function of the FAC which affects ROLLOUT.

See Airbus Safety First Magazine 23 for a similar scenario: the aircraft was operating with an ADR channel disabled under MEL, encountered windshear causing discrepant AOA measurements, the FACs only had two measurements so they tripped off causing the crew to lose FD, characteristic speeds, and FMA.


What is the difference between single and dual annular combustor? CFM56 uses single annular and GEnX uses dual annular combustion chamber. somebody please explain with pics.

Here's an engine with old combustor cans:

Here's a single annular combustor:

And in cross section:

And now here's a dual annular combustor (DAC)  (pic from here) in cross section:

See how it's like one ring (or donut)  inside another ring.

Wikipedia states "like an annular combustor, the DAC is a continuous ring without separate combustion zones around the radius. The difference is that the combustor has two combustion zones around the ring; a pilot zone and a main zone. The pilot zone acts like that of a single annular combustor, and is the only zone operating at low power levels. At high power levels, the main zone is used as well, increasing air and mass flow through the combustor."


Does airspeed impact fuel flow? Let's say an F-104 Starfighter is flying at sea level at 600 KIAS with throttle at 100% RPM.
Then, let's say the F-104 is still flying at sea level, but the pilot reduces throttle and airspeed falls to 250 KIAS. The pilot then pushes the throttle to 100% RPM and starts accelerating.
My question is, at 100% RPM, will fuel consumption be greater at 250 KIAS and less at 600 KIAS?
Does airspeed impact fuel consumption? The higher the airspeed, the less fuel consumption, and the lower the airspeed the greater the fuel consumption?
note: I added this chart after I made this post:

I don’t believe that airspeed directly affects fuel flow, but it may impact other variables that can have an influence.
I don’t have a flight manual for an F-104, but for the EA-6B the manual says the following about the fuel control unit on the J52P-408A engine: (straight turbo jet, non-after burning, 1970s technology)

“The engine fuel control is divided into a computing and metering
section.  The computing section monitors throttle position, burner
pressure, engine speed, and compressor inlet temperature.  These data
are translated into a fuel demand that is relayed to the metering
section, which measure the correct amount of fuel from the engine fuel
pump the fuel pressurizing and dump valve.”

Of those four inputs the primary would be throttle position, with metering occurring to dampen out pilot inputs and ensure smooth operation.  The other three may be influenced by environmental conditions such as airspeed, altitude, etc. but airspeed itself is not an input.
Modern FADEC, (Full Authority Digital Engine Control) engines might be different.


Does an aircraft carrier have a VOR onboard? I wouldn't think so due to the impracticality of VOR on a moving platform, but aircraft still need to navigate to the aircraft carrier. Would the radial shift around as the boat rocked around? The enemy could use the VOR to literally home in on the carrier as well.

Aircraft carriers use a TACAN, which stands for Tactical Aid to Navigation.  (The Navy is really good at making acronyms that don't follow normal conventions for abbreviation)

Functionally the TACAN is like a VOR with DME, providing bearing to and from the ship, as well as range.  The system is not susceptible to rocking motion, and least not perceptibly.  (and if you are already familiar with VORs, a VORTAC is a landbased co-located VOR and TACAN station.)

And true, an enemy could pick up the signal.  For this reason carrier airwings need to prove themselves proficient in EMCON (Emission Controlled) launch and recovery operations before being certified ready for deployment.  During an EMCON recovery crews are told where to expect the ship, what the expected recovery course will be, and  will be given a marshalling lat/long.

At the appointed time, and with no radio communication, formation flights will peel off the bottom of the holding stack, drop to break altitude,  head towards where the ship should be, and sequence themselves in with traffic in the pattern.  It is a pretty amazing thing when it works.


How would an airplane land if the on-board radio breaks down? Is there a special protocol for a situation where the on-board radio communication equipment suddenly breaks down? Would the pilot be able to land on a busy towered airfield or would they be forced to look for a small landing strip in the middle of nowhere?

It is important to remember that it is often difficult to diagnose radio malfunction during flight. Therefore it is hard to know whether you have a malfunctioning transmitter, a malfunctioning receiver, or perhaps both. You also have the point-of-failure that is the headset you are using. For this reason it is quite usual to carry a spare headset and the first piece of diagnostics is to swap headsets. Many General aviation aircraft also have a handheld microphone and speakers integrated into the radio stack.
Once you've ascertained that its the actual transmitter and/or receiver that has failed it is common to set the transponder to 7600. For landing, the relevant detail for the procedure to follow is contained within Chapter 4, Section 2-13 of the AIM:
Receiver Inoperative

Remain outside class D airspace until the direction of air traffic can be ascertained
Notify the tower your type, position, altitude, intention to land and request you are controlled with light signals
At 3 -5 miles, advise the tower of your position and join the traffic pattern
Watch the tower for light signals
Continue to advise of position (downwind, turning base, etc)

Transmitter inoperative

Remain outside class D airspace until the direction of air traffic can be ascertained
Monitor the primary local control frequency as depicted on sectional charts for landing or traffic info.
Look out for light signals that may be directed to your aircraft
During hours of daylight, acknowledge tower transmissions or light signals by rocking your wings
At night, acknowledge by blinking landing of navigation lights

Transmitter & Receiver inoperative

Remain outside class D airspace until the direction of air traffic can be ascertained
Maintain visual contact with the tower and acknowledge light signals as above.

The light signals used for an aircraft in flight are as followssource:
Steady green - Cleared to land
Flashing green - Cleared to approach airport, or return to land
Steady red - Continue circling, give way to other aircraft
Flashing red - Airport unsafe, do not land
Alternating red and green - Exercise extreme caution
They can be seen in this video.


Are pressurized passenger aircraft usually completely airtight? Various sources on the internet indicate that the passenger cabins on pressurised airplanes are not 100% airtight. I'm not talking about ventilation/pressure valves/cabin pressurisation system, but about unintentional leakages due to various imperfections in the construction.

Is that true? If yes, what would be the "usual" value for some commonly used airplane in easy to understand units?

I'm trying to imagine if the (possible) unintended leakage might cause any trouble in flight, if the pressurization system failed.

No, the are not even nearly airtight, and leak air quickly on their own.
Air pressure is maintained with bleed air or a powered pump. FAA has a thorough, well-illustrated guide to the systems involved.

As for why they're not built completely airtight, aircraft are mostly riveted and glued, not welded, so their structure is inherently not solid. You'd need to use sealants throughout the structure, install and maintain seals on every opening, and compartmentalize a few systems that need outside air by design.

That's extra weight and maintenance, which doesn't have to be there, as cargo doesn't need pressure and humans need fresh airflow. The minimum rate is ~1.6 L/s/pax for survival and ~8 L/s/pax for comfort. More flow means more O2, giving more consciousness time if decompression happens (breathing low ppO2 greatly accelerates the onset of hypoxia), less CO2 (affecting cognitive ability and comfort), and less humidity, so that less odor is perceived and less water accumulates in the insulation.

With outflow valves closed, airliners normally lose about 400 feet (~5%) of their pressure difference per minute when new, this varies per airframe. The permitted limit is typically 750 FPM. High-flying aircraft like A380, A350, 787 are built with considerably lower leak rates, a safety margin that allows them to get certified above FL400 by providing more time to get to 10k ft in emergency.

As aircraft age, their leak rate can double and more, usually being close to the limit for an average airliner. A common offender is window frames rattling themselves a bit of clearance. When tested and found to exceed the limits, maintenance is required to bring the leak rate down. So even the current level of airtightness takes effort to maintain.


Why is a 737 Original speed-restricted below 10 kft with inoperative windscreen heating? According to this page on the 737's operational limitations, the Original 737s (737-100/-200) are limited to 250 KIAS below 10 kilofeet if the windscreen-heating system is inoperative:

Anti-Ice & Rain
[...]
737-1/200:
[...]
Window heat inop: max speed 250kts below 10,000ft.  [My emphasis.]

Why is this?

To reduce damage in case of a bird strike.
The restriction is not only for the 737-100 and -200 models, the 737 NG QRH says:

WINDOW HEAT OFF
In flight:
WINDOWS HEAT switch (affected window) ..... OFF
Limit airspeed to 250 knots maximum below 10,000 feet.
Pull both WINDSHIELD AIR controls. This vents conditioned air to the inside of the windshield for defogging.

The reason for restricting the speed is that the windows are less elastic at lower temperatures and can therefore not absorb a bird strike as well as at nominal temperature. Since most bird strikes occur below 10,000 feet, the restriction only applies there.
I could not find a good source for this, but it is e.g. discussed on pprune.org:

Window heat also ensures the screen vinyl layer is kept elastic and resilient and better to absorb damage in case of a bird strike - not so if the screen is cold and hard.

and airliners.net:

cold windows become very brittle whereas warm windows have a degree of elasticity in them that makes them withstand birdstrikes better.

A new window was developed for the 737 and is installed on all models built since mid-September 2010. The technical data by PPG Aerospace list the following among the improvements:

Elasticity at low temperatures reduces potential for delamination and cold chipping

So low temperature elasticity seems to be improved. I do not know, if they removed the restriction from the QRH for the newer models.
Airbus also does not seem to have such a restriction. From pprune.org:

When I asked Airbus exactly this question their reply was that the windscreens had been tested at 350+ knots and cold.....therefore no problem!


Is separation provided in class F airspace? According to the Wikipedia page on different airspace classes, separation is provided for IFR/SVFR to other IFR/SVFR in class F airspace. In ICAO Annex 11 from 2018 (p.36 in the pdf file), I can't find anything about separation being mentioned.

  Class F. IFR and VFR flights are permitted, all participating IFR flights receive an air traffic advisory service and all flights receive flight information service if requested.

I'd like to know if I have missed something and the Wikipedia page is right nevertheless?

RE I can't find anything about separation being mentioned:

Check appendix 4 on PDF page 103 (APP 4-1):

The third column is for the separation provided, where for Class F it is:

  IFR from IFR as far as practical

And click here for the full table (red highlight mine).


Why do delta-wings perform so well on paper airplanes? Based on this question, it seems that delta-shaped wings do not perform well at low airspeeds. But why do paper airplanes, particularly the most well known one, fly well even though the airspeed they fly in is at most 15 knots on average?

Your interpretation of the answer to that question is a bit wrong.

It would be an error to claim that delta wings don't perform well at low speed. Delta wings perform exceptionally well at low speeds.

The correct interpretation is delta wings are not fuel efficient. And this is true at low and high speeds.

There are advantages to delta wings at both high and low speeds. The following list are some of the known advantages:

Improved handling at high Mach numbers due to swept leading edge (note: this is the same advantage as swept wings but with the down side of being draggy when performing manoeuvres - swept wing planes retain much more kinetic energy after a turn).
Intrinsically strong structurally (it's a triangle!) thus can be built much lighter.
Retains quite a lot of rudder effectiveness at low speeds and high angles of attack (this is unintuitive because you'd think the wing would block airflow to the rudder but the strong vortex shedding on the leading edge maintains flow around the rudder).
Have very wide CG range compared to conventional aircraft. This is especially useful in bombers.
Have very benign and gentle stall behaviour. This is especially useful for paper airplanes since it can prolong flight at low speeds (paper airplanes simply dip their noses and regain speed instead of simply diving and crashing).

The major disadvantage is that it is draggy. Thus not fuel efficient. Thus in a business world where profit and cost matters delta wings are not attractive.

For supersonic flights the drag is apparently roughly on par with swept wings. Except, as mentioned above, due to the geometry of the wing it can be built lighter thus save fuel - so it sort of cancels out.

For toy planes, where efficiency is not of great concern (fun and "cool" is more important) delta wings are OK.

Still, if you use the same sheet of paper and built a long, narrow, cambered wing like this:

.. you'll find that the long narrow winged plane flies much further than the delta wing paper airplane given the same launch speed.


What is (are) the accidental cause(s) most feared by the flight crew? As the question says, which is the accidental cause most feared (in general) by the flight crews and what's the reason of it?

Apart from hijacking, pilot incapacity or that kind of issues, I am referring to causes like aircraft systems failure, fire on cabin, etc.

Observation: I'm not asking this question to be an opinion question, since I want to know in general, what is this cause and reason(s) to be the most feared accidental cause.

What crews should be most concerned with is indeed crew error.  In the 1930's and 1940's, more accidents were caused by mechanical failures -- things failed that made the airplane unrecoverable.  Several decades ago, crew errors became the larger source of accidents, where a flyable aircraft (albeit with perhaps some issue, but flyable nevertheless) crashed because the crew did something that made the situation worse.  The hardware these days is pretty darned reliable, and if "you" do what you're supposed to do, chances are really excellent that you can get it on the ground safely.

That said, pilot psychology plays a major role in answering the question, since pilots strongly prefer to be "in control."  Or, less formally, we all tend to be control freaks.  (Not necessarily in every area of our lives, but certainly in the cockpit, we tend to have "take charge" personalities.  If something isn't right, we want to make it right, and a laissez faire "eh, whatever" attitude doesn't tend to be found among pilots at work.)

So the things that most concern us at a more visceral level, I'll suggest, tend to be the things that we can NOT control.  If an engine catches fire, I'm confident I can put it out with the procedures and tools that I have, and I can get the aircraft back on the ground because I'm trained to do that even with one less engine than usual.  But if there is a raging fire someplace that I can't control (electrical wires -- SwissAir Flight 111 or cargo hold -- ValueJet Flight 592), then that bothers me.  Similarly, Terry's scenario of cargo loading done wrong -- outside my control, and the pilots have to trust that somebody else did their job correctly.  And by the nature of that being entirely outside my control, that tends to be more of a troubling scenario than any of the typical list of things that I can control and have been trained for: engine fire, depressurization, electrical issues, etc.


Why, and how, are barometric altimeters accurate enough for approach and landing? The barometric altimeter is used as the primary altitude indicator during approach and landing right up until the aircraft crosses the threshold (after which the radar altimeter takes over and makes the “thirty...twenty...ten” calls), as it - unlike the radar altimeter - can’t be fooled by mountains or canyons lying across the descent path.

Yet barometric altimeters have problems of their own:

They require an accurate knowledge of the local surface pressure at all times... and atmospheric pressure can change very rapidly, both from time to time and from place to place.
They require that the aircraft be at a specific, constant angle of attack in order to give accurate readings; if the aircraft is flying at a higher angle than that for which the pitot-static system is calibrated, ram air will be forced into the static ports, resulting in a falsely low altimeter reading.
They rely on the aircraft’s static ports, which can be blocked by things such as rain, icing, or duct tape (fortunately, this is uncommon nowadays, as most static ports are heated, and most maintenance techs quadruple-check that they haven’t left anything covering the ports).

Given these vagaries of barometric altimeters, why don’t we see lots of crashes from situations like these:

You break out of the clouds at what your altimeter says is 3000 feet; unfortunately for you, it turns out the pressure’s dropped since the airport’s altimeter setting was last broadcast, and you’re about to hit the ground (either this or the following could, in the wrong circumstances, be exacerbated by torrential rain, which commonly occurs in conjunction with low-pressure systems, and can potentially be ingested by insufficiently-heated static ports, resulting in an even-more-falsely-high altimeter reading).
While on approach, you enter a localised low-pressure cell that the airport’s AWOS, being at the airport rather than back along the approach path, doesn’t know about.  Seeing your altitude apparently ballooning up, you idle the throttles and push the nose over to correct the altitude excursion; unfortunately, since you were never actually too high in the first place, this action instead sends you below the glidepath, and straight into the ground.
During a go-around, you set TOGA power on the throttles and pull back on the yoke; as the elevators are quicker to respond than the engines, the aircraft pitches up before it can accelerate much, resulting in the aircraft’s angle of attack temporarily increasing.  Ram air enters the static ports, the altimeter says you’re lower than you actually are, and, in an attempt to avoid hitting the ground, you pull the yoke as far back as it’ll go, stalling the aircraft.

A difference of 1 hPa results in an error of 27ft in the altimeter. To have the altimeter showing 3000ft when the aircraft is actually at sea level, the altimeter must be off by 111hPa. Standard air-pressure at sea level is 1013 hPa (that's converted to 29.92 inHg for you Americans).

For comparison, the pressure in Hurricane Katrina reached as low as 902 hPa. So if you were flying into a hurricane but left the altimeter setting at 1013 (which is used as the standard setting at cruising altitude), you could theoretically pop out of clouds at what you thought was 3000ft but instead hit the ground.

Ignoring the fact that nobody tries to land in a hurricane, this mistake is prevented by the airfield pressure being reported by ATC, in the ATIS, and even an alert from the aircraft if it is advanced enough.

  While on approach, you enter a localised low-pressure cell that the airport’s AWOS, being at the airport rather than back along the approach path, doesn’t know about. Seeing your altitude apparently ballooning up, you idle the throttles and push the nose over to correct the altitude excursion; unfortunately, since you were never actually too high in the first place, this action instead sends you below the glidepath, and straight into the ground.

Pockets of low pressure as you describe just don't exist to that degree. Hurricanes take hundreds of kilometres to reach their low point. And anyway, this is why airliners have a requirement for the approach to be stabilized at a specific point -  typically 1000ft above the aerodrome level. Any significant deviation from the glideslope below this point mandates a go-around.

  During a go-around, you set TOGA power on the throttles and pull back on the yoke; as the elevators are quicker to respond than the engines, the aircraft pitches up before it can accelerate much, resulting in the aircraft’s angle of attack temporarily increasing. Ram air enters the static ports, the altimeter says you’re lower than you actually are, and, in an attempt to avoid hitting the ground, you pull the yoke as far back as it’ll go, stalling the aircraft.

Altimeters are calibrated with multiple static ports and designed so AoA changes do not noticeably affect the altimeter. For the rest of this example, I mean I suppose a pilot could just forget the basics of flying but it's unlikely. Pilots know how to select the correct pitch and power to guarantee maximum climbing performance.

You didn't mention that altimeters are also affected by temperature, and in cold air they will read higher than reality.  There is no correction for this, and so some approaches will have limitations in very cold weather. EDIT: See John K's comment below.


What is the history behind CAR 105.01(2)? Canadian Aviation Regulation 105.01(2) states:

"No person shall conduct an aerial sightseeing flight, or any portion of an aerial sightseeing flight, in the control zone of the Québec/Jean Lesage International Airport unless the flight commences at that airport."

I find it odd that this is a law, and not just a NOTAM.

Does anyone know why this was added to the CARs in 1998?

In short, it's because of complaints about noise. From the Canada Gazette, Vol. 132, No. 2, pp.98-103:

  The regulation reflects the Minister’s desire to have increased
  oversight of sightseeing flights within the Québec/Jean-Lesage
  International Airport control zone.

  [...]

  The regulations are
  intended to prevent frequent low level flights for aerial sightseeing
  purposes over the built-up area within the Québec/Jean-Lesage
  International Airport control zone and to stop commercial
  passenger-carrying aerial sightseeing flights from using the Lac
  St-Augustin, Quebec aerodrome for either take-offs or landings. The
  prohibition of low level commercial aerial sightseeing flights within
  the entire Québec/JeanLesage International Airport control zone is
  intended to ensure that such operations do not relocate to another
  nearby lake and recommence sightseeing operations from that location.

  [...]

  Since the late sixties and, increasingly, in recent years,
  Transport Canada has received numerous complaints from citizens about
  the noise of floatplanes flying over several towns and cities in the
  Quebec City area.

The Gazette goes into a lot more detail about the background, impact, consultation process etc. 25 local mayors signed a letter to Transport Canada asking for action, so I guess it came down to political pressure in the end.


Materials for temporary dent repair? On a recent commercial flight (A320neo), I noticed some materials attached to the leading edge of a slat (no photo available). The attached material was not smooth, it rather looked like a small ball of steel wool. Making pilots aware of it, I was told this was a temporary repair for a dent.

I was satisfied for the moment, but rethinking the matter later, I wondered if dents can be temporarily fixed by spoiling the airflow, I would rather have expected a smooth foil or plate to restore the nominal flow. The aircraft continued its next legs without noticable delay.

How are dents in wings temporarily fixed?

Can pilots judge, if a damaged fix is within allowable limits?

What usually happens is the crew doing the walkaround spots something that looks like damage, calls airline maintenance, and techs come look at it.  For a ding, they will consult the Structural Repair Manual guidance on allowable limits on dents.  Based on the SRM it may be ok to leave as is, or require repair, or require further evaluation.

If the SRM doesn't cover it, or not all the eventualities, they will call the OEM's product support organization for an assessment, repair scheme, and a temporary "fly-by" (a fix deadline) if feasible.  A really big airline may have its own internal structural engineering team with its own design approval authority up to some level of severity, in which case it's all done in-house.  Smaller airlines will call the OEM and pay the OEM for its engineering services.

In any case if it's outside the SRM limits, somebody somewhere with authority has to review the dent and decide what to do.

We don't really know what was done in the case you saw, so I'll speculate that the "ball of steel wool" you saw was actually the dent but looking like a bump due to lighting, and the area was probably roughened with Scotchbrite or something similar during the inspection, creating a steel wool look.  There could have been a Non-destructive Test done (usually dye penetrant) to look for cracks in the surface where the metal was bent, which would make the surface look different from the surrounding area with the treatment process.  Also if there were sharp defects like a gouge, there was probably some "blending" of the local material to minimize the stress riser effect of the gouge and the blending with emery cloth (sandpaper) would certainly leave a steel wool look to the area.


Fans of tubofan jet engines Does the thrust of the fan in a turbofan jet engine decrease proportionally with decrease in the diameter of the fan?

As the fan in a jet engine is responsible for over 70% of the total thrust can a spark ignition engine driven fan of 1 meter give an equal thrust of a 1 meter diameter turbofan jet engine with the jet exhaust thrust removed form the sample? All inlet duct and shroud would be identical, also the fans.

You have asked two questions here. The first one:

  Does the thrust of the fan in a turbofan jet engine decrease proportionally with decrease in the diameter of the fan?

We need to clarify what you mean by "proportionally". If you mean if you halve the fan diameter, the thrust goes down by half, the answer is no. If you mean if you reduce the fan diameter, the thrust goes down, the answer is yes.

For a subsonic engine, net thrust is given by mass flow (kg/s), and the change in velocity of the mass flow between the inlet and exhaust exit. Exactly how the mass flow and exhaust velocity will change as the fan diameter changes is complex and will vary with each engine, so a general statement cannot be made.

Question two:

  As the fan in a jet engine is responsible for over 70% of the total thrust can a spark ignition engine driven fan of 1 meter give an equal thrust of a 1 meter diameter turbofan jet engine with the jet exhaust thrust removed form the sample? All inlet duct and shroud would be identical, also the fans.

Firstly, you have said "the fan in a jet engine is responsible for over 70% of the total thrust" as if this is a fact. It's not - what you can say is something like "in a high bypass ratio engine, the fan can be responsible for over 70% of the total thrust".

So, now to the question part:

  Can a spark ignition engine driven fan of 1 meter give an equal thrust of a 1 meter diameter turbofan jet engine with the jet exhaust thrust removed form the sample? All inlet duct and shroud would be identical, also the fans.

If, as you state, the inlet, fan, and exhuast duct are the same, and we ignore the thrust from the core (turbine exhaust flow), then if the fan driven by the spark ignition engine and a fan driven by a turbofan engine are at the same RPM, then, yes, the thrust produced by both fans is the same. The fan doesn't care where the torque required to turn it comes from, so long as the torque is adequate to achieve the same RPM.


What are the ways to perform air traffic control on long distance flights? I have a few questions on air traffic control.

From my understanding, there are three major ways of sending control messages to an aircraft:

1) Direct radio link from control as long as the aircraft is in the line of sight.

2) Satellite

3) Ground stations, e.g., TRACON.

My questions are:

a) For an intercontinental flight, how does the control know which ground station to use (since it wouldn't make sense to connect all ground stations to each airport) for control messages?

b) Are the control links (e.g., control-to-ground-station-to-aircraft) to communicate with an aircraft predetermined before take-off?

Direct link vs satellite

  There are three major ways of sending control messages to an aircraft:
  Direct radio link [when] in line of sight, satellite, ground stations,
  e.g., tracon.

The way the question starts brings confusion so the many comments. There are two categories of radio links involved in voice/data communication: Between a ground station and the aircraft and via a relay, e.g. a satellite.

Satellites are merely used today for air traffic control, it's a thing of the future (2 to 3 years for tracking the aircraft using ADS-B). However satellites are sometimes used for aircraft communication with the airline technical staff and/or the manufacturers. However the information exchanged here is not ATC related at all: It can be technical, e.g. live ACARS sent to airline technical staff or to the engine manufacturer, to prepare maintenance at the destination airport. It can also be internal exchange with the airline operations. Satellite communication is also used for passenger services like telephone and Internet access aboard the aircraft.
So ATC uses the other kind of links, the direct ones, which means a transmitter on the ground. However these "ground stations" can serve different purposes because ATC is organized so that ATC facilities are specialized.

Simple ATC organization

There are ATCOs on the site who may add details to this answer, but I'll draw the mainlines. In the US, ATC operational facilities are divided into 4 categories:

ATC Tower. Controllers are located in the airfield tower and they control aircraft on the ground (e.g. at their gates, or on the taxiways), aircraft on the runways (landing or taking off), aircraft approaching the airfield to land, aircraft leaving the airfield and climbing to their cruise altitude. The tower controllers have a good sight on all those aircraft.
Tracon (terminal radar approach control). Controllers maybe located in the tower or at other locations. They are in charge of taking over the aircraft that just took off and are now departing. They continue the control that was started by their colleagues of the Tower. They also take care of the descending aircraft before they are handed over to their colleagues of the Tower for landing. As you see, tracon actually takes aircraft from cruise altitude to near the landing runway, and from near the takeoff runway to cruise altitude.
Next, there is the enroute ATC center for controlling aircraft in cruise, between the departing tracon and the arrival tracon. This facility is also known as air route traffic control center (ARTCC). The enroute controllers can be located anywhere, they don't need to see the sky, and usually they don't see it at all, they work in a dark room using only displays and microphones. There are a small number of centers in the US to control the whole US airspace.

This split is shown on this figure:

that is extracted from this question.

The last type of ATC facility is the oceanic ATC which controls an ocean area, because the procedures and tools used are a bit particular. Oceans are not belonging to a particular country, however all countries have agreed to delegate the control to a particular country for a particular area. In the US, for the East coast, New-York oceanic center will control an area, Canada will control another, UK and France will manage the other side of the pond. These centers control areas named OCA, for oceanic control area. See this map for North Atlantic. When the aircraft progress along its route, it is handed over to the next area controller.

This was the first part: We have clarified who are the controllers and ATC facilities, where they are and what phase of flight they control. Now let's see the radio equipment they use.

Radio links

First let's clarify between radio and radar.

Radar is used to determine the position of the aircraft. This can be done everywhere, except over oceans. The reason is radar has a limited range, and there are no radar stations in the ocean (at least not for civil aviation).
Radar stations are located at selected locations, on hills and other high places, to increase their range. some radar are located on airfields too, but that's not a requirement. Actually, all information collected by all radar stations are gathered and distributed to all control centers as a synthetic picture.
Radio is used to exchange voice messages and data. As I wrote, all radio links have a ground station at the center. This ground station is usually at the same location than the controllers. So some ATC radio equipment is at the airfield, some at the enroute centers, etc. For OCA, the equipment can be remote from the controllers, but this is just a technical detail.

Aviation radio for ATC messages can use two types of frequencies:

VHF (very high frequency) which works mostly in line of sight, so the range is something around 50 km to 200 km depending on propagation conditions and location. VHF is used when the aircraft is flying over lands. Controllers at the towers and tracons use VHF, as they deal with aircraft that are either under they eyes (tower) or at a moderate distance from the airfield (tracon). Enroute use them too as they control the part of the sky around the transmitter.
HF (high frequency) works on long distance, like between continents. HF is used by OCA controllers, enroute ATC can also use them. The particularity of HF links is they work somehow erratically, the links can be broken at any time when an aircraft flies over an ocean. The reason is HF waves are reflected by the high ionized atmosphere and the ground, but the degree of ionization varies with time. OCA facilities have procedures to deal with that.
A special word on ATC communication in OCA: Satellites are not used by ATC today. In OCA, ATC communicates with aircraft using HF transmitters located on the ground. When the distance or propagation conditions make the HF communication impossible, ATC (or the aircraft crew) will ask another aircraft to relay the message. Aircraft equipped with satellite phone can also try to contact the OCA center using telephone. If communication is still impossible, then the crew will continue as already cleared by ATC, and after they will continue as indicated in the flight plan which was filed with ATC before the flight. ATC will clear enough airspace around the assumed position of the aircraft to prevent collisions. This is not very different than when there is a loss of communication over land. See section Communications Failure After Entering NAT Region at the end of this document (NAT is the standard North Atlantic route system often used to cross the Atlantic Ocean).

  For an intercontinental flight, how does the control know which ground
  station to use (since it wouldn't make sense to connect all ground
  stations to each airport) for control messages?

I hope you have got a better understanding, and know now this is not the case. Each ATC facility (tower, tracon, enroute center) uses different radio stations and a particular tower controls only the aircraft at this airport, using the local transmitter. The key is that ATC works with pre-agreed procedures to hand off control to the next facility when the aircraft leaves a given control area. An ATC unit controls aircraft in its area, and uses a ground radio station nearby the aircraft. OCA ATC indeed works with aircraft that are remote, and sometimes very far.

  Are the control links (e.g., control-to-ground-station-to-aircraft) to
  communicate with an aircraft predetermined before take-off?

Yes they are, not on a paper, but by the procedures. When the aircraft starts its engines, the crew is in contact with the "ground controller" who is a controller in the tower, specialized in controlling aircraft which aren't on the runways. When the aircraft is ready for takeoff, the ground controller hands over to someone else in the tower who manages the takeoffs (this is "the tower controller"), then after take off the tower hands over to the nearby tracon, then to enroute (there will be several successive enroute centers), etc.

The current controller hands over to the next controller on the aircraft route, when the aircraft is leaving their area of responsibility, this is part of the ATC organization itself.

"Plates" used by crew indicate how to reach a given control facility. For example, on this chart, the frequency of the Denver ARTCC is mentioned explicitly.

Source

This is in case you are lost because this information is normally given by the previous controller prior to handing you over to Denver enroute center:

And here is how to contact the different ATC units at Paris - Charles De Gaulle:

Source

More: ATC Facilities


Is every emergency landing of a commercial flight documented? A friend told me she was on a Royal Jordanian commercial flight in 2006 that had an emergency and was forced to land in a field in Greece.

She said the final destination was supposed to be Italy, with no scheduled stop in Greece. Nobody was hurt, and passengers were transported by bus to the Athens airport, which was not far. Perhaps the pilots had been attempting to reach Athens for an emergency landing.

Neither her nor I know much about aviation, but she described the plane as a small commercial jet holding 50-70 or so passengers.

I would like to know whether this kind of incident would create some kind of publicly-accessible record that I could look up. A commercial flight landing in a field seems significant enough to be recorded. But maybe as long as there were no injuries it isn't?

Generally there should be some kind of record of this incident but even more broadly speaking not all aviation accidents are necessarily documented or reported on a global scale.

It depends on what country you are in and who the reporting agency is. Here in the USA the NTSB is responsible for investigations of aircraft incidents. Investigations and reports are generally the responsibility of the country in which they happen but not all countries have investigation agencies nor the means to carry out complex investigations. Historically the manufacturers as well as other nations have offered assistance when need be as the outcome is as important to them as it is to the investigating agencies.

As for Greece their reporting agency can be found here and the list of 2006 reports here although they seem to publish the list by year that the report came out (not the year of the accident) so the 2007 list may also be of interest. Most of the reports are published in greek which I cant read and there does not seem to be one relating to Royal Jordanian.

Not that it is a complete list by any means but wiki does not list any similar Greek aviation incidents on their compiled list.

For the sake of completeness, what appears to be a grass field to some may actually be a grass runway (more common than you would think) and its possible what your friend described was a landing at a grass runway (field, or strip) done for a precautionary reason. A diversion like this may not cause an investigation to ensue.


How to deal with the noisy cockpit? From the pilot perspective I want to know how to deal with the noise in the cockpit on the turboprop aircraft while long-run flights.
The reason is old type aircraft with bad headphones which doesn't protect from noise than they should be.

The most common way to deal with noise in the cockpit is to get yourself a noise canceling headset. There are a bunch of models out there but the most common seem to be the Bose and the Lightspeed. I personally have the lightspeed and will attest to at least a 30%-50% perceived noise reduction over my old passive headset.


Why is cabin pressure increased above ambient pressure on the ground? As far as I know at least some (e.g. some Bombardier Dash-8 series) airplanes during takeoff may increase cabin pressure above air pressure on the ground, before starting to decrease it. In one case pressure was increased to around 760 mm Hg (1013 hPa) when pressure on ground was around 730 mm Hg (973 hPa).

Is this increase of cabin pressure desired for some reason or is it just a side effect of something? And why does it happen?

Short answer

The purpose of pre-pressurization is to avoid discomfort for passengers and crews during takeoff and landing.

By storing air in the cabin before the aircraft rotation or landing, the pressurization controller is able to smooth out:

The variations of bleed air supply when the engines deliver the highest thrust and need all available air.
The variations in both air supply and air release, due to the aircraft leaving or entering the ground effect. In particular this ground effect would force air into the cabin through the forward outflow valve when the angle of attack is increased for takeoff rotation or landing flare out.

However pre-pressurization involves relatively limited pressure differentials.

236 ft for a B737-NG (9 hPa).
140 ft for a Dash8-200-300 (5 hPa).

The case you mention seems ways higher with 40 hPa.

Pressurization system

The source of cabin air for the aircraft is commonly bleed air from the engine compressor stages. Air is sent to packs for conditioning and enters the cabin (see a more complete description in this answer):

(B737-800, source)

Bleed air pumped into the cabin then escapes it by one or two configurable exits, the outflow valves:

Outflow valve (and near to it the safety valve) on a Boeing 737-NG. Source

Cabin pressure is obtained by controlling the rates the air enters and leaves the cabin. This is indeed the job of the pressurization controller. This job is more difficult at takeoff and landing:

Air taken from the compressor is not used for the production of thrust, which can be a problem when thrust is critically required, e.g. for takeoff roll and rotation, or go-around. For these sequences, bleed air may be not available, or may be delivered at reduced or unstable rates. Normal bleed air delivery capability is restored after takeoff when the engines need less air.
The ambient pressure around the aircraft is subject to variations when the aircraft rotates and leaves the benefit of the ground effect, and also when it enters this effect before the flare. So the differential pressure at the outflow valve varies as well as the bleed air supply, and both need to be compensated too.

Pre-pressurization

Unmanaged variation of bleed air supply or cabin air outflow create air bumps in the cabin. To avoid discomfort, on some aircraft the cabin is pre-pressurized at a pressure slightly higher than the ground ambient pressure.

The pre-pressurization occurs during the takeoff roll and shortly before landing. It creates a stock of air the pressurization controller can use to smooth out bleed air bumps.

Source: B737-800 FCOM:

  The air/ground safety sensor signals whether the airplane is on the
  ground or in  the air. On the ground and at lower power settings, the
  cabin is depressurized by  driving the outflow valve to the full open
  position.

  The cabin begins to pressurize on the ground at higher power settings.
  The controller modulates the outflow valve toward close, slightly
  pressurizing the cabin. This ground pressurization of the cabin makes
  the transition to pressurized flight more gradual for the passengers
  and crew, and also gives the system better response to ground effect
  pressure changes during takeoff.

This additional amount of pressure mustn't compromise the ability to quickly open the doors in case of emergency, so pressure differential is limited. On a B737-NG, the differential is 0.125 PSI (9 hPa). This is equivalent to an altitude 236 ft below the runway. The system has a fail-safe mode which automatically opens the outflow valves quickly after the aircraft has landed (unless the Ditching switch has been pressed).

After the aircraft is airborne, or has landed, the pressurization system releases air previously stocked, to recreate a smooth altitude increase.

For the sake of completeness: The cabin will not "climb" or "descend" as fast as the aircraft, and will have its ceiling at 8,000 ft to maintain oxygen pressure at a safe level to breath without a mask.

Cabin pressure vs. atmosphere pressure

Here is a representation of the pressure differential during a flight (with the takeoff/landing highlighted. Note the cabin (dashed line) "is lower" than the aircraft.

(Source)


Are jet engines designed in a way to make an explosion result in the least damage possible? I know most engines are super reliable today, but do manufacturers do anything to at least make an explosion result in the least damage possible?

The most (kinetic) energy is in the fan and turbine blades and disc. The engine is enclosed in a containment chamber whose purpose it is to protect the rest of the airplane from shrapnel in case of a fan disc or fan blade failure. Here's a video of a test where a fan blade failure is simulated:

             As you can see, the fan blade more or less completely destroys the engine, but no debris or shrapnel leaves the containment chamber.

Unfortunately, the turbine disc and blades have too much energy to be realistically containable. (In other words: the required shielding would be too thick and heavy.)

Engine failures where debris or shrapnel is able to leave the containment chamber is called an uncontained engine failure, and they are very rare, but they still do happen.

Since it is impossible to contain a turbine blade or disc failure, risks are mitigated in different ways, e.g. by requiring regular, very detailed inspections and extremely tight tolerances.


Should landing light be off during line up and wait position? What is the proper position for the landing lights when on the runway in a line up and wait position?

Counter-intuitively, the FAA answer (see chart on page 1-9 and text on page 1-10) is actually, they should be off during the line-up-and-wait phase.

When cleared to ...

  “Line up and wait”—when entering the departure
  runway without takeoff clearance, turn on all exterior
  lights (except landing lights) to make your aircraft
  more conspicuous.

Then when takeoff clearance is received, the expectation is that landing lights will be turned on.

The best rationale I've seen for this is so that Tower controllers (and other aircraft) have an immediate visual cue that an aircraft "on the numbers" understands that it has been cleared for takeoff, or not.

Another FAA document, SAFO 11004 (these are "best practice" recommendations, not requirements):

  Exterior Lighting:

  o Taxi with taxi light on when moving, off when stopped.

  o Turn on all exterior lights when crossing any runway.

  o If cleared to “Line Up and Wait”, turn on all exterior lights except landing lights.

  o When “Cleared for take-off”, turn on all exterior lights, including landing lights

  o If you see an aircraft in take-off position on a runway with landing lights ON, that aircraft has
  most likely received its take-off clearance and will be departing immediately.


Is Assumed Temperature used in conjunction with a Derate in the B737 NG? When using an Assumed Temperature for Takeoff, is it common operational practice to also use a Derate?

Yes, that is allowed.
You can combine selection of a fixed derate and an assumed temperature for the takeoff thrust on the FMC N1 Limit page, but only up to a reduction of 25%.

Takeoff Derate
Fixed derates can be selected on the N1 LIMIT page. [...]
Derated takeoff rating can be further reduced by assumed temperature. [...]
Assumed Temperature Thrust Reduction Takeoff
A takeoff thrust less than the full rated thrust may be achieved by using an
assumed temperature that is higher than the actual temperature. The desired thrust
level is obtained through entry of a SEL TEMP value on the N1 LIMIT page or
TAKEOFF REF page 2. [...]
The maximum thrust reduction authorized is 25 percent below any certified rating. [...]
If the assumed temperature method is applied to a fixed derate, application of
additional power should not exceed the fixed derate N1 limit as loss of directional
control could occur while on the ground.
When the assumed temperature method is used with full rate, the reduced thrust
setting is not considered a limitation. If conditions are encountered where
additional thrust is desired, the crew can manually apply full thrust.

(Boeing 737NG FCOMv2 11.32.3 - Flight Management, Navigation - Flight Management Computer)
You can see the current selection on the Upper Display Unit:

2 Thrust Mode Display
[...] Thrust mode display annunciations are:

TO – takeoff
TO 1 – derated takeoff one
TO 2 – derated takeoff two
D-TO – assumed temperature reduced thrust takeoff
D-TO 1 – derate one and assumed temperature reduced thrust takeoff
D-TO 2 – derate two and assumed temperature reduced thrust takeoff

(Boeing 737NG FCOMv2 7.10.2 - Engines, APU - Side by Side – Displays)
What would actually be used by an airline depends on the exact situation. The derate and the thrust reduction via assumed temperature are not the same thing because the derate affects Vmcg, but the assumed temperature does not (see also What is the difference between Assumed Temperature and Derate takeoff thrust?). Today, most airlines will use an EFB (or a separate tablet computer) to perform the TO calculations and the computer will optimize the thrust reduction while taking all legal requirements for V1 (which may be affected by Vmcg, which is a lower limit for V1), second/fourth segment climb gradient, obstacle clearance and possibly noise reduction into account.


What is the lift force per unit span? I got Aircraft Design: A conceptual approach for Christmas, and I'm having a hard time with lift coefficients because I honestly have no idea what "lift force per unit span" means, so can someone please explain this to me?

The concept of lift force per unit span comes from potential flow theory. It will need some background information to explain what it means, so bear with me.

In the early years of flight, electricity was new and exciting, and it just happened that the equations which could calculate the strength of an electromagnetic field worked equally well when calculating the local flow change effected by a wing. What is the electrical current in a wire became the vorticity in a vortex, and the strength and orientation of the induced magnetic field were equivalent to the induced flow changes. So the vocabulary of electricity was copied over to aerodynamics, just like brain research used vocabulary from computer science when that was a hot topic.

Now we are left with abstract concepts like induced drag or lift per unit span. It would be so much more descriptive to use proper names, but the authors of technical books learned it that way and are much too lazy to explain aerodynamics any better.

In potential flow theory, you have sources, sinks and vortices. Sources and sinks are used to generate the displacement effect of a physical body moving through air, and vortices are used to explain why wings bend the flow and create lift. In order to calculate the lift force $L$ of a single vortex in two-dimensional flow, the circulation strength $\Gamma$ of the vortex is multiplied by the airspeed $u_{\infty}$ and air density $\rho$. You will find an equation like $L = -\Gamma\cdot u_{\infty}\cdot\rho$ in many treatises about numerical aerodynamics.

To expand that into the third dimension (and, consequently, into reality), you need to add something measured in spanwise direction - but you have already lift, and adding the third dimension would give a moment (lift times distance) where only lift would make sense. Therefore, this two-dimensional lift is now called "lift per unit of span" so there is still space for a third dimension where two-dimensional flow did already produce lift (counter to any sound intuition).

And no, this is never constant over span. In all cases the vorticity is gradually reduced towards the tips, or explained in a better way, the suction force acting on the wing is gradually reduced when you approach the tips because when the wing ends, nothing can prevent the air from flowing from the high-pressure region below to the low-pressure region on the upper surface of the wing.

While the potential flow mentioned above is the mathematical way of looking at aircraft, lift coefficients are the engineer's way of expressing things. From tests it was soon clear that the lift force of a wing scales with the dynamic pressure $q$ of the flow, that is the product of air density and the square of airspeed: $q = \frac{\rho}{2}\cdot v^2$.

The next observation of engineers was that lift also scales with wing area $S$. To make the lifting force independent of wing size and dynamic pressure, they stripped both from the lift (physical unit of Kilopond, Newton or pound-force) so they arrived at a dimensionless figure which they called lift coefficient $c_L$. Doing so made it much easier to compare measurements or scale up known designs for the next, better design. The lift equation now becomes $L = c_L\cdot S\cdot\frac{\rho}{2}\cdot v^2$


Why do aircraft models end their life as freighters? There seems to be a tendency that aircraft models are more long-lived for cargo than for passenger service.

For example, both the A300 and MD-11 are all but extinct in scheduled passenger service, but FedEx and UPS still fly scores of them.

My understanding is that the reason an aircraft model goes out of service is not that the planes are too old to possibly continue flying, but that it becomes economically favorable to replace them with newer models with better fuel or maintenance economy etc. Shouldn't those considerations apply with equal strength to cargo planes?

Which factors contribute to passenger aircraft obsolescence that do not apply equally well to freight?

Passenger operations are overwhelmingly scheduled whereas cargo is on demand and thus less easy to plan. The upside: A cargo plane crew will fly quite diverse routes. The downside: Capacity utilisation is worse. This shifts the balance of cost such that fixed cost must be lower than what is possible with scheduled services. As a consequence, older aircraft with lower depreciation cost are preferred. With them it is less costly if they don't move for some hours or even days.

Caveat: Integrators (freight companies which ship door-to-door like FedEx or UPS) run their freighter fleet like a scheduled service, but still their utilisation is lower. Please see @TomMcW's answer for a detailed analysis.

Operating cost are more important when the aircraft flies as much as technically possible. Especially low cost airlines need to operate hardware with the lowest operating cost possible so they survive the next price war. Premium airlines also have a need for the newest planes: This makes them look premium and more safe. Cargo operators have no such incentive to prefer the newest airplanes; they profit from the need of passenger airlines to sell their older hardware off.

Next caveat: Older airplanes are at least as reliable as new ones when maintained well. The need of premium airlines for newer planes is only to impress their ignorant customers, not to increase real reliability numbers.

The Economist wrote in April 2013 about Norwegian Air Shuttle:

  As fuel prices soared in the mid-2000s, and competition from the likes
  of Ryanair and easyJet intensified, Mr Kjos realised that the only
  route to survival was to buy lots of new, fuel-efficient planes to
  achieve economies of scale, and to base some of them in cheaper
  countries. Boeing and Airbus are straining to keep up with booming
  demand for new aircraft, so by reserving a large chunk of their
  delivery slots NAS is denying potential rivals the chance to refresh
  their fleets with better planes.

If an airline is large enough, ordering new aircraft has the added benefit of saturating supply and denying the same aircraft to its competitors. If oil prices are high, this airline will have won the price war before it started in earnest.


Would flying below FL285 be feasible for aircraft that don't comply with EASA CPDLC? I am not a pilot, my understanding about flight is limited.
EASA CPDLC will require airplanes to be capable of PM-CPDLC data communication to fly above FL285.

For an airline with multiple 767-300 and 757-200 airplanes, would it be feasible to consider the option of flying below FL285 in European airspace, instead of ceasing operation in the region or investing millions of dollars for avionics upgrades?

Unlikely. The extra fuel burned would cost a lot more than the required Avionics upgrade.

Both the 75/767 would be well below optimum cruise level.


Can quadcopters save energy by flying in formation? If two or more quadcopters are aligned, would the effect of drag due to the lead quadcopter's slipstream lead to measurable energy savings? And if so, how would one determine the optimal distance between the quadcopters?

My thought are that some energy savings are possible, but that there must be some distance between the quadcopters to avoid the high pressure air behind the lead quadcopter interfering with the quadcopter that's behind. How great this distance is depends on the tilt of the lead quadcopter. Since the drag force is proportional with the square of the velocity this means that energy savings should be larger at high velocity, but unfortunately the tilt of the quadcopters will also be higher causing the minimal distance needed between the quadcopters to increase.

Yes, drag can be reduced by formation flying.

Geese using a low drag "V" formation. The authors of a 2001 Nature article stated that pelicans that fly alone beat their wings more frequently and have higher heart rates than those that fly in formation. It follows that birds that fly in formation glide more often and reduce energy expenditure (Weimerskirch, 2001).

Any flying object (plane, bird, helicopter, etc) that creates a down-wash to create lift must also produce an opposite and equal up-wash. A trailing aircraft positioned properly can ride this "wave".

Research aircraft flying a test point (low drag) for the Autonomous Formation Flight project over California's Mojave Desert. From the NASA Dryden Flight Research Center Web site.

The goal of the Autonomous Formation Flight project was to demonstrate 10 percent fuel savings of the trailing aircraft. The project extended the symbiotic relationship of migrating formations of birds. In December 2001, an F/A-18 flying in the wingtip vortex behind another F/A-18 exhibited a 14-percent fuel savings. The project ended in late 2001


How are flaps sized in order to give necessary delta CLmax? Given an airfoil and wing planform, what is the process to find the flap size and deflection necessary to give a certain change in CLmax? Flaps are usually sized as a percentage of the chord, with 0.2c to 0.3c being the normal range for most aircraft. But how do you find the length of the flaps spanwise?

This is to answer your follow-on question in the comment;

  That answer did not cover the analytical method of determining CL change. I'm unsure what this iterative method is. Is the only method using CFD or wind tunnel testing? How were they sized before these technologies were available?

I feel the main question has been answered already here.

Flaps, especially the ones with gaps between the wing and the flap, depend a lot on how the boundary layer forms and when separation occurs. Therefore, CFD is still struggling to get everything right and wind tunnel testing is still needed at least to verify the computational simulation.

Before CFD, the average aircraft engineer would experience a number of designs in his (rarely her) lifetime which gave him a feeling what works and what not. Still, surprises were not uncommon and late fixes were needed to arrive at the desired performance.

If you desire to use a simple formula which precisely predicts the maximum lift coefficient, including size, speed, temperature, airfoil shape and manufacturing imprecisions: It does not exist.


How does the NATO code name system work, are they just random names, or is there some sort of rule(s)? When an aircraft is designated a NATO code name, are there a certain rule(s) that contribute to the naming of an aircraft.

From Wiki

  They provide unambiguous and easily understood English words in a
  uniform manner in place of the original designations, which either may
  have been unknown to the Western world at the time or easily confused
  codes.

Much the like the NATO alphabet rules the names but be easy to understand (possibly under multiple speaking accents) and must be non ambiguous.

The following rules are also applied

  Aircraft

  The first letter indicates the type of aircraft, like
  Bear for a bomber aircraft, or Fulcrum for a fighter aircraft.

  F —
  fighter aircraft, also later ground attack aircraft: List of NATO
  reporting names for fighter aircraft

  B — bomber aircraft: List of NATO
  reporting names for bomber aircraft

  C — commercial aircraft and
  airliners, and cargo aircraft: List of NATO reporting names for
  transport aircraft

  H — helicopters: List of NATO reporting names for
  helicopters

  M — Miscellaneous; trainers, reconnaissance, seaplanes,
  tankers, AEW etc.: List of NATO reporting names for miscellaneous
  aircraft

  For fixed-wing aircraft, one syllable names were used for
  propeller-powered craft (turboprops included), while two-syllable
  names indicated jet engines.


What is the definition of mountainous area? MEA, MORA, grid MORA, and MOCA for a non-mountainous area gives 1,000 ft clearance from the highest obstacle, and in mountainous area gives 2,000 ft clearance from the highest obstacle.

But what is the exact definition of a mountainous area?

I have also heard that MOCAs have a different clearance calculation for the mountainous area depending on the terrain/obstacle elevation, (3000 ft - 5000 ft gets 1500 ft clearance) and (above 5000 ft gets 2000 ft clearance) can someone clarify this.

(I'm assuming you're asking about the USA.)

14 CFR 91.177(a)(2)(i) says (emphasis mine):

  In the case of operations over an area designated as a mountainous
  area in part 95 of this chapter, an altitude of 2,000 feet above the
  highest obstacle within a horizontal distance of 4 nautical miles from
  the course to be flown; or [...]

If you then go to 14 CFR 95, you'll see that it defines mountainous areas by latitude and longitude. It's entirely useless for practical purposes, but at least it's precise :-)

As for the MOCA, I can't find anything to support the different definitions that you mentioned. 95.1(e) simply says:

  (e) The MOCA assures obstruction clearance on an ATS route, ATS route
  segment, or other direct route, and adequate reception of VOR
  navigation signals within 22 nautical miles of a VOR station used to
  define the route.

The AIM and PC/G support that; there's no other definition that I could find.


Does every state in the US require aircraft and pilot registration? I'm aware of both Washington and Oregon requiring, by law, registration of aircraft and pilots who are based in the state.

Is this something every state does? Do states enforce it?

No.

It varies from state to state. New York, for example, requires tax payment but no aircraft or pilot registration. New Jersey doesn't require any registration and didn't mention any taxes. In Delaware, registration is required but taxes can be waived if you register your aircraft under your corporate, instead of your own name. Iowa requires registration and so does Ohio.

I could not find a single resource which lists all states and their aircraft and pilot registration requirements. FAA does not have this information, probably because FAA does not care about state regulations.

We can create a Wiki on this topic here.


Could liquid alkali metal fuels be used for ramjet/scramjet propulsion? In particular, I'm thinking about lithium, but sodium could be considered as well.

Lithium has low molecular weight, very high propellant density, and low melting point. At sufficiently high temperatures (above 500 degrees Fahrenheit) it ignites spontaneously in air with explosive force.

A lithium scramjet/ramjet would require an onboard power supply to heat the fuel to liquid and then pump it into the combustion chamber. The idea is that the combustion chamber would need to be hot enough so that the lithium droplets would quickly evaporate and ignite making the mixture hypergolic.

I'm surprised that I have found no current R&D into alkali metal fuels for rocket propulsion(let alone hypersonic propulsion). It seems to me that to make longer range hypersonic flight possible, exotic fuels will be absolutely necessary.

How long does the lithium take to evaporate and then to react with oxygen? Air in a scramjet is moving extremely fast (supersonic by definition!), so there is very little time left for mixing and combustion if you desire to create thrust from the the combustion heat energy.

Your lithium engine will most likely trail a bright contrail of burning lithium but develop rather little thrust. The high flame speed of a mixture of hydrogen and oxygen makes hydrogen the preferred scramjet fuel.


Are the window wipers on jet aircraft ever deployed in flight? Many commercial airliners have car-like window-wipers. I would presume this is for taxiing only, as the wind would surely keep the windows free from water when airborne,  or do they turn them on in flight?

Also, don't these wipers cause a lot of drag, and wouldn't it be more economical to have some kind of air-jet blowing from nozzles to keep the windshield clear?

In the Falcon 50 and 900, we have windshield wipers that we are allowed to use in flight (up to 205 KIAS), but I have never used or seen them used in the eight years that I have been flying them because they simply aren't needed.  They are usually stowed out of the wind stream so don't really cause much drag.

The Learjet's on the other hand DO have hot air that blows on the windshield and we sometimes used it to blow water droplets off of the windshield when on the ground.  However, the primary purpose of the hot air was to heat and defog the windshield.

On a larger airplane, the additional piping, valves, etc. required to get the hot air all of the way up to the windshield actually weighs and costs more than just putting the windshield wipers up there along with electrically heated windshields to keep them from fogging up.


When calculating lift, using the the NASA lift equation, can any value be increased, ie. velocity or surface area, for increased lift? I am trying to understand the breakdown of the NASA lift formula, which is written as such

$L = C_L \frac{\rho v^2}{2}A$

If I take for example CL=0.87, v=20m/s, ρ=1.22kg/m3, and A=9.02m2, my equation looks like this so far: Lift = (1/2) * 0.87 * 1.22 * 20^2 * 9.02. Can any one of those values be increased for greater lift? If I doubled my velocity would I double the lift that the velocity provides? But if I added 25% to the surface area of the wing, my drag also increases. How does that factor into lift? By decreasing velocity?

Does this equation scale in that way? I apologize if it doesn't make sense, I am not sure how to word the question well.

Can any one of those values be increased for greater lift?

Certainly.  But increased lift will not be the only effect.

  If I doubled my velocity would I double the lift that the velocity provides? But if I added 25% to the surface area of the wing, my drag also increases. How does that factor into lift? By decreasing velocity?

It doesn't directly factor.  The formula gives you the approximate lift given certain conditions.  It doesn't tell you if those conditions are reasonable or achievable.

You're correct that increasing the wing area or speed will increase drag.  But this equation doesn't tell you how much.  You'd need to look elsewhere to find out the specifics.  The drag increase will not be linear with either one.


Does the validation of an ICAO licence allow you to fly internationally? Hypothetical situation:
A pilot with an ICAO-compliant PPL licence from Australia goes to France and France issues a licence validation in accordance with the Chicago Convention, thus allowing him to fly aircraft registered in France.
If this pilot wants to take a French-registered aircraft to another country is this possible? From Article 33 of the chicago convention it seems like this should be allowed, but is it actually in practice?

Article 33
Recognition of certificates and licenses
Certificates of airworthiness and certificates of competency
and licenses issued or rendered valid by the contracting State
in which the aircraft is registered, shall be recognized as valid
by the other contracting States, provided that the requirements
under which such certificates or licences were issued or
rendered valid are equal to or above the minimum standards
which may be established from time to time pursuant to this
Convention.

When doing the validation e.g. in France you will get a validation from an EASA member which enables you to fly an EASA registered plane world-wide.

EASA includes registered planes from

Austria
Belgium
Bulgaria
Croatia
Cyprus
Czech Republic
Denmark
Estonia
Finland
France
Germany
Greece
Hungary
Iceland
Ireland
Italy
Latvia
Liechtenstein
Lithuania
Luxembourg
Malta
Netherlands
Norway
Poland
Portugal
Romania
Slovakia
Slovenia
Spain
Sweden
Switzerland
United Kingdom


If a plane had a laser/infrared ruler its four extremities pointing downward, could that work as an altimeter device? Laser noob here. I don't know what the maximum range on a high powered laser is, nor a low-powered laser for that matter. Just wondering if this is at all possible. I mention four extremities because it would a combination of the four different laser readings of height. I guess it could be most useful over water, but things could get complicated if flying over rough regions with mountains and valleys, but could be helpful if use in conjunction with a map.

Yes, you could create an altimeter using a laser.  And they exist; See Laser altimetry for aircraft and UAVs by Renishaw as one example.

Laser altimeters, however, have a significant disadvantage over radar altimeters: They can be obscured by weather, like clouds and rain.


Why can't model aircraft be used to discover spin and stall behavior of manned aircraft designs? I was reading some literature on stalls and spins (which are almost always preceded by a stall) of aircraft when a thought hit me. Why can't a model aircraft (1/4 scale of manned aircraft) be used to inexpensively explore the stall and spin behavior of manned aircraft?

(One disclaimer for this, I know this can't be done at ALL speed ranges or all scales. Airliners are too big to make inexpensive 1/4 scale R/C aircraft, and aircraft that cruise at transsonic speeds will not benefit from this since the effects seen there only happen at that particular reynolds and mach number combination)

However, I was thinking more for aircraft that spend a lot of time at fairly low speed (below Mach 0.3) such as ultralight or light sport aircraft. Here the air is effectively incompressible, meaning flow physics wise, the reynolds number is dominant in determining flow separation. And because of that, all a 1/4 scale model has to do is fly four times faster to get the same flow separation characteristics as the full scale aircraft.

So for an ultralight aircraft, which is speed capped at 30 m/s, a 1/4 scale model would have to fly at 120 m/s to see similar stall behavior, which is at the bottom end of where the flow starts to become compressible, meaning most of the flow structures seen at full scale will be seen at the model scale. Accounting for the lower moment of inertia of the model, the spin behavior of the model should predictably scale to reveal the behavior of the full scale aircraft.

My question is, what am I getting wrong in my reasoning? Why can't this work?

Who says scale models cannot be used for spin modeling?

The key information is that the Reynolds number does not affect the behavior of separated airflow as much, so you do not need to increase speed to compensate for the smaller dimensions. Only when the stall behavior is studied would the Reynolds number matter, but here also meaningful results are possible at lower speeds.

Models are routinely used in spin tunnels. Here both the speed and the dimensions are smaller, but the results can be transferred to the original aircraft. There are two sorts of spin tunnels:

Free tunnels with an open, divergent section in which the model has to be thrown by a skilled operator so it will settle in a spin within the upwards flowing air stream. The section is divergent, so the speed changes with height and allows the model to find a matching speed.

Picture from spin tests on a Grumman E-2 model in the NACA 20 foot spin tunnel (picture source)

Closed tunnels in which the model is mounted on a sting. The sting is connected to a rotating balance, which can be set at different roll and yaw rates. The resulting matrix of coefficients over angle of attack, angle of sideslip and over the three rotation speeds is fed into a computer model which computes the equilibrium points.

The models used in free tunnels must be scaled both geometrically and inertially, so their mass distribution matches that of the original. If the spin tunnel uses a rotating balance, not even the dynamic scaling of masses and inertias is needed, and a regular tunnel model can be used. However, if the resulting spin is of an oscillating nature, the free spin tunnel is at an advantage, because this becomes readily apparent in the test. In a closed tunnel you just get two equilibrium points and must make the connection yourself.

A third way are free-flight tests of models, but they are much more expensive and allow less observations than wind tunnels.

The stall characteristic is a little harder to predict using models, but again you can draw conclusions from what can be seen in the tunnel. To arrive at the maximum lift coefficient starting from the measured value at Re = 1,000,000, you may use this scaling: $$\Delta c_{L_{max}} = \frac{log_{10}\left(Re^{\frac{1}{6}}\right)}{3.5}$$

The plot below shows the difference in lift coefficient between a small model and a full-scale airplane. The biggest differences are around stall, and at the high angles of attack seen in spins both show fairly similar behavior.

Lift coefficient over angle of attack for model and full-scale aircraft, taken from Joseph Chambers' monograph on testing with models (Modeling flight : the role of dynamically scaled free-flight models in support of
NASA’s aerospace programs).

For small aircraft, the relative expense of a tunnel test is normally inadequate. Instead, the real thing is used. Since the speeds involved are small, spin tests are carried out with a spin chute or a releasable mass at the tail of the aircraft. On modern designs, even a full-aircraft ballistic parachute system can be employed to prevent a crash if the spin test ends in an unrecoverable situation.

Spin chute installation on a Columbia 400 (picture source)


Can the MEL be equal to MMEL? I am performing a review of aircraft maintenance. i intend to reconcile the MMEL for airbus 320 along with the operators MEL. Based on my information, the MEL is supposed to be more restrictive than the MMEL as it considers operational hours, operating conditions and many such factors. In this case, is it OK it the MMEL and the MEL have the same item list? Shouldn't the MEL have some more items?

I am no authority on the topic, but I'll at least try to summarise the bits from comments:

MEL can contain additional items not on the MMEL only if:

They concern after-market modifications or additional equipment and the corresponding STC permits it. Typically this includes things like IFE, wifi-routers and similar items for passenger comfort, not necessary for safely conducting the flight.
Sometimes it might be possible to add a normally required item if additional equipment is installed that can work as a backup, subject to overseeing authority approval.

MEL can omit items from the MMEL if:

They concern optional equipment that is not installed on the particular aircraft.
The operator chooses to omit them. This can be done because the associated restrictions would not allow the plane to complete the typical mission profile with that operator anyway or if the operator considers the restrictions to be too much trouble for what they are worth (e.g. would require otherwise unused procedures that would have to be trained).

If all those categories happen to be empty, the MEL will be equal to MMEL.