What are the barriers to feasibility of stratospheric cruising for commercial air travel? (i.e. Concorde?) In 2003, British Airways retired the Concorde for a few reasons.

Rising maintenance costs
Lack of demand due to safety concerns
Further decrease in demand due to 9/11 attacks

additionally, due to lack of competition, the Concorde was never properly maintained. Because no other airline had such a plane, British Airways kept the analog cockpit from 1970 up until the planes retirement.

Well, we live in a world where safety in aviation is continuing to increase, and the technology we have to utilize the world around us constantly gets better.

What engineering breakthroughs will allow commercial aviation to use the upper atmosphere and supersonic cruising speeds?

--EDIT--

I don't mind if you downvote my question, but it certainly annoys me if you downvote and you don't leave a comment telling my why. I've tried to make this question as objective as possible, if you're going to take the time to downvote, take the time to explain yourself, and I would be happy to try and mitigate your concerns.

I would argue that the Concorde was from the beginning economically unfeasible and instead served as a prestige technology for the British and French governments. Even the name Concorde, indicates that the two governments working together to produce the aircraft seemed the most important aspect of it.

Both British and France at the time had completely nationalized airlines and almost all their major aviation companies as well. Americans were going to the moon so Britain and France were looking around for a prestige technology of their own. Bigger and faster airplanes had been the rage for twenty years so the Concorde seemed like a cool thing to do. The Soviets came to the same conclusion.

In the US, civilian airline development was still wholly private so when when Boeing , Douglas et crunched the numbers they saw little value in supersonic transport with  it's non-linear cost. US supersonic transport never made it off the ground, but in Europe, the politically driven Concorde did because the people making the decisions weren't paying the bills.

As noted by others, the Concorde was essentially a military aircraft, produced by political compromise with the primary goal of expressing the technical powers of the governments involved. It was more an art project like building the Pyramids than it was a piece of mobile transportation infrastructure.

Edit: (May 14, 2014)

Some sources on the economics of the Concorde:

The Atlantic August 1977:

  Already Britain, and France spent £1.46 billion \$2.3 billion to reach this  (and a London political economist has recently argued that
  the true cost is roughly  three times this amount)...

  ...In May 1976, Professor David Henderson, newly appointed professor of political  economy at University College, London, argued
  that the government's figure of  £1.46 billion shared between Britain
  and France was a drastic underestimate.  It had been reached by adding
  the yearly expenditure on the project at the current prices.  If these
  were adjusted to 1975 prices, and interest charges of 10 percent
  added, then  the cost of Concorde was not £1.46 bilion but £4.26
  billion (\$6.82 billion at the  present exchange rate of \$1.60).

That would be $22 billion US (2014 dollars.) Just to get the thing into the air.

They built 26 of the aircraft, 20 of which that actually flew paid flights. So, around 1.1 billion per plane, roughly the cost of a B2 stealth bomber. I can't find any hard numbers on operating cost but with 1.1 billion per plane sunk capitial cost, it's clear it would never operate at a total profit.

I believe the plane only flew for 20+ years because the British government essentially gave it away, eating the billions of in development and deployment cost and leaving the plane to just have to cover operating cost. I'm not clear that it even really did that.

Considering how much money was thrown at the problem, not particularly elegant or interesting.

Edit 02:

In rereading the parent, I realized I didn't make my answer explicit to the original question. He wanted to know what it would take to bring a Concorde like plane back.

My answer was really just saying, "the Concorde itself had no economic foundation so firstly, you'd have to find an economic niche for SST."
There was never an economic demand for the Concorde's speed of travel and there doesn't seem to be one now, either.
Technologically, you'd probably need something hyper radical like a nuclear powered air spike so the plane wouldn't burn tons of fuel and could fly over land masses without sonic booms or destroying smaller aircraft. (Seriously, it would have to be that advanced.)
In our current luddite era, it would be saner to think about inventing anti-gravity drives than think that the political regulators would let that type of radical technology fly, even if it existed.


What is wrong with a high-speed landing for an ultralight or GA aircraft? Is it okay to land an ultralight or a general aviation plane at a speed higher than the recommended landing speed for such plane? Will there be any damage if the descent rate is very low?

Keeping too much speed in an approach in any airplane can be risky depending on how much runway you have as it all has to bleed off before you can stop. It's a great way to end up in a hedge. Extra airspeed means you'll either float in ground effect for a very long time, or if you somehow manage to get the wheels down you won't get much braking action as the wheels won't have much weight on them on a tricycle configuration, in a tailwheel configuration you will have braking action but you'll have to be gentle or you'll nose in. In fact, trying to hold down an airplane that is far over rotation speed is tricky, and it can all go wrong. There's no good reason you'd want to try this unless you had some sort of condition in the airplane that necessitated it.

Landing fast isn't good for your tires, the extra ground speed will cause more wear on initial contact as the wheel has to spin up that much faster. Your brakes could overheat or at least would wear out faster as well. Other than that I can't think of any particular damage or wear.


why are regional cruising altitudes higher than those of international much longer flights? How come, for example, the average cruising altitude of let's say the Westjet Boeing 737-600 from CYWG to CYVR flies at an altitude of 38,000 feet-40,000 feet, yet the Air Transat Airbus 330-200 flying from CYVR to Man EGCC flies between 33,000 feet and 35,000 feet?
Why is the air transat one flies lower when you'd think that with the Vancouver to Manchester route being more of a long-range flight than the Winnipeg to Vancouver one, it'd want to fly above the regional/domestic flights.

Why is this so?
to illustrate, here are flightaware records for two recent flights:
Westjet: https://flightaware.com/live/flight/WJA307/history/20150903/1210Z/CYWG/CYVR
Air Transat: http://flightaware.com/live/flight/TSC284

Flight altitudes have relatively little to do with whether a flight is regional or international per se. More important factors include the winds aloft and the individual aircraft capability.

In the case of the Winnipeg to Manchester flight, they were probably using the North Atlantic Track System (NATS). The winds aloft over the North Atlantic are generally out of the west. The routes of the tracks are arranged each day to take advantage of the those winds. The track system puts the eastbound aircraft on tracks and at altitudes that would maximize the tailwinds, and westbound aircraft on tracks and at altitudes that minimize the headwind.

Since the winds aloft in the northern hemisphere are more often than not westerly, the Winnipeg to Vancouver flight might well have been fighting a headwind, but the strongest winds aloft tend to occur in the mid 30s, which can mean that you can lessen the headwind by climbing higher.

Aircraft capability also plays a major role, and aircraft vary widely in what they can do altitude wise. For example, a 747-200 up against its max takeoff weight of 800,000+ lbs can initially climb to only about 31,000 feet (FL310), but towards the end of the flight can reach FL390 and above.


How may one notify the FAA of the apparent flaw in the airspace design at KEGE? Several questions and answers have addressed the design of the airspace around KEGE.  The situation is perhaps best summarized in this answer -- Can the Class-E-to-surface airspace near Eagle County Regional airport KEGE be ignored when the tower is open? .  How may one notify the FAA of this apparent flaw in the design of the airspace?  Note that this is not a charting error, but rather an apparent error or omission in the way the airspace has been designated, as described in the "Airspace Designations and Reporting Points" document JO 7400.11C.

From the JO 7400.11C, page (ii)

  Request for Information.  For further information, contact the Airspace Policy Group, Federal Aviation Administration, 800
  Independence Avenue, SW, Washington, DC  20591; telephone: (202)
  267-8783.

They will likely connect you with the specific airspace coordinator for the region of interest.


How does the fan in the jet engine suck in air while the engine is stationary? This question is related to the fans and compressors in jet engines.

(Please Note: I know how a propeller driven airplane works. The propeller has the shape of the airfoil and when this propeller moves through air, a pressure difference is created between the front and the back of the propeller. This causes the airplane to move forward)

Most of the fans that I have encountered till now merely recirculate the air in an enclosed area (for example: the ceiling fan recirculates the air already present in the room)

I am comfortable with the idea that fans recirculate air when they rotate. But my intuition is failing me when I think of fans (exhaust fans) that suck air out of an enclosed area.

The exhaust fan sucks out air from an enclosed area (ex: room). The fan in a jet engine sucks in air from outside into a enclosed area.

I would appreciate it if someone could explain how these fans manage to suck-in/suck-out air. Does it have anything to do with the shape/orientation of the blades of the fan?

The answer in all cases is the fan accelerates air, creating a low-pressure area, and more air moves in to fill the low pressure.

Consider a ceiling fan.  As it runs, the air in the disk of the fan is accelerated downwards.  This means that the space where that air used to be is now at lower pressure, since some of the air has been removed from it. Air below the fan is moving downwards, away from the fan, so it's difficult for it to fill the low-pressure zone. So the air that moves in will come from the top and the sides.

If you put the fan inside a tube (a duct), the air can't come in from the sides, so it must come from above.  You now have something conceptually similar to a vertically mounted jet engine, except that it's much simpler (actual jet engines have multiple fans) and it's powered by a little electric motor, rather than by injecting and burning fuel.

If you want a pump, the fan doesn't care if you put in a false ceiling that divides the space above the fan from the space below it, with the duct and the fan as the only connection between the two.  Since no air can get into the area above the fan, the fan will pump the air out of that zone.  Eventually, the pressure in above the fan will get so low that air the fan is accelerating downwards will start to flow back up into the evacuated zone.  This is why any particular fan will only lower the pressure by a certain amount.


Why so much ground time at Frankfurt Airport? Disclaimer: Total layman here.

I frequently fly into and out of Frankfurt Airport (FRA) with regional jets (flight time advertised as 45 minutes) and one thing that everyone is always annoyed about is that you spend approximately the same amount of time in transit on the ground in Frankfurt as you spend in the air.

I understand FRA is a huge airport, but what does not make any sense to me is that you...

...have to walk 10 min to the gate
...must spend 10 min on a bus to get you to the aircraft
...then spend at least 10-15 min on the aircraft as it taxis to its start position on the runway

Given that these regional connection flights are always on a tight schedule, I do not quite get why they do it this way.

Specifically, can anyone explain why — in FRA — these small jets are parked so far away from their landing/starting positions when you have to get there by bus anyway?

Why bus
The typical reason you use a bus to your low-cost carrier (LCC) or regional jet is the cheaper 'remote stands'. But this doesn't mean the stand is necessarily closer to the runway where the planes take off and land.
See: Who decides whether an airline docks at a jetbridge or parks at a remote stand?
Can't be too close
A big area around the runway needs to be clear of tall tail fins, light poles, equipment, etc., as it may affect the navigation equipment near the runway or pose a hazard to landing/departing aircraft.
Example: Why do some airports park the aircraft backwards at the gate?
Queue
An LCC plane won't get ahead in line just because it's parked closer to the runway. If a plane at the terminal pushed back and started its engines before the one at a remote stand, it will typically be ahead by the time the LCC one is ready to go.
Shorter flights also typically use a more congested airspace (they don't fly too high, the flight is short, and it's busy near airports), so there might be a delay in releasing that flight.
Remote stand analogy
I'm not sure which remote stand you typically use, but also in airport design it's fairly common to have the remote stand share the same taxiway (the road planes use to get to the runway) as the terminal.

(Google Earth - FRA)
Think of it like this, there's a road with a mall and its fancy parking on one side (1), and the cheap parking across the road (2). Regardless of where you park, when it's time to reach the highway, cars from the two parking lots will eventually merge (3).

That being said, 15 minutes to taxi to the takeoff position on the runway (not including the time to start the engines and to get the taxi clearance) is actually really good for a major airport such as FRA. Jetliners typically take 90-degree turns at slower than 15 km/h, and 40 km/h is common for straight lines. With a taxi distance of 2.25 km (example shown above), we are looking at a delay of ~12 minutes. FRA handles ~74 movements per hour (takeoffs and landings), assuming they use 3 runways, it's a takeoff every 2.4 minutes on your runway, or like waiting behind 5 other planes.


Can a person legally mount a fake gun on an amateur-built aircraft? Is there an FAA rule that states that no amateur-built aircraft are allowed to have a weapon mounted to it? (If so, I'd certainly like to see this rule and its conditions) Say I want to build an airplane that a friend and I made blueprints for a while ago, and it was about 60% fantasy. But the plane doesn't look the same without the guns. The guns really add a different feel to the plane, and to have no guns makes it feel as though it's missing something. But clearly I'd never use it. I just wanted to know if it is even legal to put a gun on an amateur-built aircraft,  which I'd assume it isn't. So to avoid any trouble, I think it'd be best not to put any functioning guns on my plane. So, this begs the question, Is it legal to put a fake gun on an airplane, or to put a functioning ball turret that has fake guns?

Empirically, yes it's legal. I've seen several reproduction WW1 aircraft myself at airshows with fake guns, missiles or bombs attached. And I was once at an EAA chapter meeting where a member demonstrated his fake gun 'firing' using compressed air. There are also factory-built aircraft with fake weapons added later, by the way.
As to regulations on this, I've never seen one. Typically US laws and regulations state what you can't do, not what you can do. For example, there is a law that prohibits attaching dangerous weapons to drones.


Why are the controls of transport category aircraft much heavier than light aircraft? Anyone who has transitioned from light GA aircraft to large turbofan airliners will notice the difference: the control column on a large aircraft is much heavier. While still within limits of a healthy adult, it takes a considerably larger force to move the controls.

Why is this? Is this intentional, or just a by-product of the early days mechanical linkages? Airbus has shown that even on airliners, the controls can be made as light as simple joystick. So why can't the controls of a Boeing 747 feel the same as a Cessna 172?

The control forces are there by design.  All airliners have artificial force feel from springs, dampers etc. A B747 flies through the skies a lot faster than a Cessna 172 and bad things could happen if it were easy to deflect the flying controls at M 0.85.

The B737 elevator is a good case in point. It has q-feel (artificial feel) provided by a spring device, and the spring gradients stiffen up as a function of airspeed.  At higher speeds it is quite hard to deflect the column by pushing/pulling and much easier to use the trim button to control the pitch. For Level D simulator control forces we measured the forces necessary to deflect the column fully when trimmed in mid position, which turned out to be a serious body-building exercise:

About 200 N at zero airspeed
About 500 N at 400 knots 20,000 ft. (Measurement taken on ground with a pressure hose attached to the pitot tube, and aircraft on jacks with the landing gear retracted).

By contrast, helicopter flying controls are light because helicopters are inherently unstable and a lot of stick action is necessary, particularly in the hover. In most helicopters the feel forces can be switched off completely with the trim button, resulting in a very light stick, perfectly OK with the relatively docile response of the helicopter.

But a 747 is a stable airframe. A fast aircraft with large control deflections results in high accelerations, which can result in Pilot Induced Oscillations or structural damage to the airframe.

The two measurements of B737 control column, digitised from the original plots and on the same scale: column position on the x-axis, force on the y-axis. By design, force required to deflect the column fully from about mid position is about 40 lbf at zero knots, 100 lbf at 400 knots. This is from an artificial q-feel system and the feel is implemented by design. The vertical distance between the sides of the curve is caused by friction.

At 0 knots:

Notice that around mid the spring gradient is very steep: the breakout, to indicate neutral position.
Then between 0 and 5 inch a spring gradient of 4 lbf/inch.
Beyond 5 inch a relatively flat gradient, to make it not too hard to deflect the column fully.

At 400 knots: it is made hard to fully deflect the column. 100 lbf pull from a seated position is nothing to sneeze at.


Are there pilot weight limits for the Rutan Vari Eze? In this question on the Rutan Vari Eze it is said one of the reasons it parks nose down (gear up) is to bring the Cg forward for stability (so it doesn't tip over) when there is no pilot in the aircraft.

If the pilots weight is that important to move the Cg to a safe position, is there a minimum/maximum weight the pilot must be to bring the Cg to be within acceptable limits?

According to the Owner's manual,

  The air plane can structurally accommodate pilots or passengers weighing up to 250 lbs.

Of course, this is subject to the maximum allowable takeoff weight.

The manual also gives the forward and aft cg limits (in page 30). In case of very light pilots, the aft cg limit may be exceeded and it may be required to carry ballast for bringing the cg within limits.


Would fitting a T-tail on the 737 Max make MCAS unnecessary? This is pretty far out, so I just pose it as a question for notion or ideas, rather than conclusive answers.
Considering the differences in flight characteristics between the Boeing 737 Max-8 and older models demanding the creation of MCAS and subsequent problems and crashes;

would it be possible to bring the characteristics back into the existing 737 envelope by lifting the horizontal stabilizers to the top of the tail, rendering MCAS obsolete?

Would certification heritage from the 727 allow for such a change without demand for recertification?

If so, to what extent?

No. A t-tail would worsen the characteristics of the airplane. While not necessarily a horrible idea in its own right, the already-nasty slow-flight/high alpha/stall characteristics of the aircraft that necessitated MCAS in the first place would make this idea dangerous. The pitch-up tendency could lead to a stall, which in a T-tail can lead to a deep stall. A deep stall is a condition in which the wing blanks the tail surfaces. Since the tail is no longer effective, recovery is EXTREMELY difficult, if not impossible. So really, a T-tail offers no advantages and may actually be MORE dangerous. Regarding your second question, no. This proposal is such a drastic departure from both the 737 and 727 that recertification would be unavoidable.


What is the link OR difference between change in momentum & pressure difference? I ask in regards to the the thrust formula, which includes change in momentum, as well as difference in pressure between inlet & outlet of the engine to calculate generated thrust; this implies that momentum change & pressure difference are clearly two (2) different phenomenons.

But in reaction turbine blade cascades, we can observe a pressure distribution similar to that of an aircraft wing, producing lift which causes the turbine to spin, & a load component acting on the turbine bearings. Also, the inlet & outlet momentum change is seen to cause the turbine to spin. I would expect that a turbine's rotation can be explained by either of the explanations but not both together, since the pressure distribution over the blade profile is a consequence of the deflection of air by the blade, which also causes the change in momentum of the flow exiting the turbine. So the force acting upon a turbine blade can be calculated either by calculating momentum change, or by calculating pressure difference, but not as a sum of both.

Now, returning to the case of the engine, of course, there isn't a pressure difference vertically to the axis of an engine, so no force in this direction. But the pressure difference in line of the axis is a direct consequence of the acceleration of the flow (momentum change) through the engine:

So why is pressure difference & momentum change summed together to get thrust?

It's mr Bernouilli's signature discovery: total pressure is dynamic pressure (momentum change) plus static pressure. Both static and dynamic pressure are the result of the impact of air molecules upon the wing or turbine blade, which acts as the lifting surface. The main distinction between the two is that static pressure is the result of random velocity vectors averaging zero, while dynamic pressure has an average molecule velocity unequal to zero.

Bernouilli's law can also be written as

$$p_t = p_s + \dot{m} \cdot V$$

With $\dot{m}$ the mass of air that impacts the lifting surface per second, and $V$ its velocity. Since that is not a convenient way to make computations regarding aircraft structures, we usually see it in a format that uses air density and wing area.

There is an analogy with potential and kinetic energy. Both are forms of energy, and one form can be freely exchanged for another one. A falling object transforms $m\cdot \Delta h$ into $\frac {1}{2} \cdot \rho \cdot \Delta V^2$, but the total energy remains constant at any point in time.

The same goes for pressure, which is actually a measure of the energy of air molecules. A jet engine adds energy to the air molecules going through it. A turbine blade can be driven by either the dynamic pressure, or by the difference in static pressure before and behind the blade - there are different design blades according to the proportion of static & impulse. The exhaust shape of the engine can leave the outflowing air with a higher static pressure than surrounding air, or expand the higher static pressure completely into dynamic pressure.

Static pressure and impulse change are summed together because they are two forms of exactly the same entity: the energy added to the air stream.


What is the maximum taxi speed and who defines it? Which is the maximum speed that an airliner is limited to while taxiing?
I know that during 90 degree turns it is 10 knots, but what about long, straight taxiways? 20 knots? 25? 30?

More broadly, where I can find this information?
Would it be in company SOPs? Manufacturer's FCOM?

And here, a Reference from Boeing's 737 FCTM (pdf). Ch2 for ground operations.


Can the 747 be re-engined as a twin? Since more and more airlines are dropping the 747 for the 777 or equivalent due to fuel efficiency, a theoretical question came to mind.

What do you think is preventing Boeing from creating a 747 twinjet (two engines instead of 4).

I know there are probably design/engineering issues associated with such a conversion, but I reckon that if it was done it may increase sales to go up against the A380? Not sure the math as well, but perhaps the engine from the 777 could be used or configured for higher thrust?

Currently, the 777 has engines that have a max thrust of 115,000 lbf, for a total of 230,000 lbf of thrust. The 747-8 has engines with a max thrust of 66,500 lbf, each, for a total of 266000. And just to throw in for comparison, the A380, currently has engines each producing 72,000 lbf of thrust, and a total of 288,000 lbf.

Right now the 777 has some of the highest thrust producing engines in commercial service. This does not take into account the extra thrust that is required for engine out issues. With a 2 engine design you have to have enough extra thrust for loss of engine during take-off at max take off weight. Whereas with a 4 engine design, you only have to have enough thrust to account for loss of 1 engine. So for a 747 twin engine aircraft, as a rough estimate you'd need to have each engine produce at least 150,000 lbf of thrust if not more.


Can a "crash landing" be used for FAR 61.57 currency? FAR 61.57 (a) requires that one must make 3 takeoffs and landings within the preceding 90 days in order to carry passengers.

Does a "crash landing" count for the 61.57 currency? By "crash landing" I mean a landing on which an incident/accident occurs under the NTSB definitions.

For example, if I'm in the circuit to gain 61.57 currency, and on my 3rd landing attempt, I crash the aircraft into a tree well short of the runway. Would I then be allowed to go rent another aircraft and carry a passenger?

As far as I can find, the FAA doesn't define the term landing so it could be argued either way.  The "typical" definition of landing is:

  1: an act or process of one that lands; especially :  a going or bringing
  to a surface (as land or shore) after a voyage or flight

That definition doesn't say that it has to be "controlled" or that the airplane has to be usable again!  Even if you decide that it wasn't a "successful landing", you could always get your third landing later if you are still allowed to fly, and then carry the passengers.

So this brings us to the second part of your question:  Are you allowed to fly after an accident?

Since you had an accident (as defined by 49 CFR 830) you must immediately notify the NTSB of the accident.  Once you do that, you have 10 days to file the required report.

At this point, you still have a valid pilot license and can legally fly another airplane.

Chances are however, that you will be receiving a certified letter from the local FSDO requiring a reexamination in order to keep your license because your competence has been called into question.  If you don't call them to make an appointment, they will start the process to suspend your certificate.  During this time, you can also continue to fly because you still have a valid license.

The FAA can also use their emergency authority to immediately suspend your license if they feel that it is warranted.  This can be appealed to the NTSB board if you feel that it isn't justified, and they have up to five days to respond.  At this point you are not allowed to fly pending the response from the board.

This, and a lot more information can be found on AOPA's FAA Enforcement Webpage.


What are some of the longest flights without modern navigation systems? Context: As a private pilot, I enjoy preparing my flights with a mix of paper charts and iPad apps. In flight, my navigation is now often very assisted by the G1000 or iPad, relegating the paper maps to backup. But I remember that, when I was only using paper maps during training, the workload could quickly rise and that I was sometimes exhausted after a short navigation because of that.

What are the longest (either by time or distance) known flights ever flown without any "modern" navigation system ?

I'm interested by flights that comply to the following criteria:

Did not use any GPS or inertial navigation system (maps and stars are ok)
Radio navigation is ok (but bonus point if the aircraft is not equipped for that)
Have only one leg
Take off and landing fields can be the same if the flight was long enough to require actual navigation (recon missions, circumnavigation...)
Can be historical stories (war heroes, military experiment, pioneers...) as well as modern ones (record attempts...)
May have a flight crew as large as Pilot + Co-Pilot + Navigator + Engineer, but...
...don't have any rest period for the crew (no backup crew on board)

This list of flight endurance record on Wikipedia lists as number 3 an endurance record of 84 hours 32 minutes between May 25th and 28th 1931. It seems reasonable to assume that no modern navigation means were used in 1931.
However, the flight was mostly local, so navigation was not really required.

The all time record is from Dick Rutan and Jeanna Yeager in 1986. The Rutan Voyager had state-of-the-art navigation on board, including an Omega Navigation System and a GPS receiver. The latter was only functioning for approximately 4 hours per day, because the GPS constellation was not yet complete and many satellites were missing.

The Double Sunrise flights seem to qualify, with times in the air between 27 and 33 hours.


Why was a complete test flight of Hughes Hercules H-4 (Spruce Goose) never undertaken? I get that ,the max altitude it was flown to was 70 ft. after its 3rd test taxi.I also am aware that even this was done to prove a point. But,why was there no attempt later on to complete a test flight and to check if it was Air worthy? Was no one curious at all ?

By the time the government had pulled the plug on the program. Even that hop got Hughes into trouble because they had been expressly forbidden from taking off (he got off on a technicality, claiming it'd been unintentional, unexpected behaviour from an experimental machine, etc.).

If it had been anyone except Hughes I'm pretty sure the H4 would have been destroyed rather than preserved as a museum piece, the US government was that intent on killing the project there and then. They had a lot of air bases with long runways to find a purpose for, and long range air travel was an excellent purpose. They also wanted to find customers for the thousands of surplus transport aircraft (and bombers to be converted to that role). So getting rid of the traditional way to fly intercontinental, the flying boat, was important.

Were people curious? No doubt. But they also were budget conscious, the H4 had no more military application so there was no money coming out of the Pentagon. It had no civilian application, as for large payload ships are cheaper and for small, fast payloads it was too large. So even if there had not been active hostility towards the flying boat concept at the time from people with the power to release or deny budgets, there would have been little point in implementing an expensive flight test program (which usually requires multiple vehicles...) for an aircraft for which it was highly unlikely there would ever be any peacetime orders.

Of course 50 years later the idea was kinda revived with a short period of interest in Wing In Ground effect vehicles to fly troops and supplies over the feared Soviet submarine fleet in the Atlantic. But that didn't start up until near the demise of that threat in the 1990s and nothing ever made it past concept drawings (except paradoxically a few Soviet craft in the Caspian and Black Sea regions, the Ekranoplans).

I've little doubt the H4 would have worked as advertised, seeing it it looks sound in design, basically a scaled up B314. But there was just no market for it. It was an example of too big, too late.


Has anyone considered filling aircraft voids with helium to reduce the risk of fire? Watching air crash investigations, and aware of problems with batteries, the issue of fire and sparking is recurrent. As a noble gas, pure helium would not burn, and if I understand correctly would reduce though not necessarily eliminate sparks.
While any additional lift would be marginal, it would likely be sufficient to compensate for the materials necessary to ensure a sufficiently sealed envelope.

So... Has anyone considered filling aircraft voids with helium?

Yes, it has been considered, but with some differences.

First, the inerting gas used is not helium - it's nitrogen. Nitrogen also doesn't support combustion, and is for practical purposes inert. It also has major advantages over helium in this role. For one, nitrogen is very close to air in density, and doesn't leak as easily as helium, so it's easier to retain.

Nitrogen is also much easier to obtain by separating air. It can be done on the ground with nitrogen carried in cylinders, or generated onboard with OBIGGS. If you were to use a more exotic gas, it would be halon, which also breaks the chemical reaction sustaining most fires.

In practice, inerting is only applied to fuel tanks, not to avionics compartments (which house the batteries). One reason is that inerting won't prevent lithium fires: the batteries already contain all the reagents they need.

Nitrogen would prevent a secondary fire, but another reason comes in play: avionics compartments aren't sealed. Fires in these bays, like in the cabin, can be addressed by the crew. This requires insulation for the lithium cells (as has been installed) and fire detection in the compartment to allow response before the secondary fire is out of control.

But, as the first linked paper states, it's certainly possible to improve on that with quick automated fire suppression. Aircraft safety design is a set of tradeoffs and it's an evolving field. It's possible that in some years, as the amount and importance of avionics inevitably increase, more weight will be spent on their fire protection.


Are return springs on a carburetor throttle mandatory or not? In many GA aeroplanes throttle control is connected mechanically to the throttle of carburetor via a cable. I read in some AFMs that in case of a cable failure, return springs will open the throttle so an engine will be at full power.
Is it mandatory to have those springs installed (a certification requirement or something) or it is optional? I'm asking about EASA regulations, but a reference to FAA regulations will also be helpful.

The FAA doesn't require it, but some carburettor manufacturers may have them. You can get return springs for Rotax engines, but only for non-certified aircraft and then again only to prevent throttle creep


Does the Ground Proximity Warning System (GPWS) see through clouds? Or can it only detect terrain on a clear day or night? I’m mostly asking because of PIA268
I know GPWS is much more reliable in todays  time, and I’m unsure if the accident flight even had a GPWS system, but is it safe to say the chances of that scenario repeating, are minimal? Assuming the system is functioning properly and or the crew reacts properly to the warning.

Modern airliners typically use a Terrain Awareness and Warning System (TAWS) with two sources of information for triggering warnings: the radio altimeter and a global terrain database. See e.g. the Boeing 737 NG, which uses EGPWS (Enhanced Ground Proximity Warning System):

  Ground Proximity Alerts

  The GPWS provides alerts for potentially hazardous flight conditions involving
  imminent impact with the ground.

  The GPWS monitors terrain proximity using an internal world wide terrain data
  base. Proximate terrain data shows on the navigation display. If there is a potential
  terrain conflict, alerts are provided based on estimated time to impact. These alerts
  are “look-ahead terrain alerts.”

  The GPWS provides alerts based on radio altitude and combinations of barometric
  altitude, airspeed, glide slope deviation, and airplane configuration. The alerts are
  for:

  excessive descent rate
  excessive terrain closure rate
  altitude loss after takeoff or go-around
  unsafe terrain clearance when not in the landing configuration
  excessive deviation below an ILS glide slope
  excessive deviation below glidepath

  These alerts are “radio altitude based alerts.”

(Boeing 737 NG FCOM 15.20.11 Warning Systems - System Description, emphasis mine)

For the terrain database to work, the aircraft needs to know its position from a GNSS source (like GPS), but can also use another source (like inertial position) as backup:

  5.3 Primary Horizontal Position Sources. Horizontal position for TAWS must come from a GNSS source
  meeting ETSO-C129a or any revision of ETSO-C145, ETSO-C146, or ETSO-C196 (or subsequent). As an exception, TAWS equipment limited to installation in aircraft where the EU Regulation on Air Operations
  does not require such equipment may be configurable to operate solely on a non-GNSS position source.

  5.4 Alternate Horizontal Position Sources. Retaining TAWS functionality during GNSS outage or
  unavailability provides a safety benefit. It is acceptable and recommended to incorporate a secondary, non
  GNSS position source, to provide horizontal position when the GNSS is not available or reliable.

(ETSO-C151c, thanks to DeltaLima for pointing this out)

So there are two signals that need to work in clouds for full TAWS capabilities.

Radio Altimeter:

For the radio altimeter to work in a cloud, the radar signal needs to be able to penetrate the cloud and reflect off the ground. Only the part of the cloud below the aircraft is relevant here. Airbus published a nice overview of factors that could result in erroneous RA values:

  The external causes may be linked
  to aircraft flying over:

  Other aircraft, hail clouds or
  bright spots, i.e. terrain presenting
  reflectivity variations.
  Runways contaminated with water or snow.

  In these cases, the RA condition
  may not be detected by the systems,
  which continue to use the erroneous
  RA values.

(Airbus Safety First Magazine - Radio Altimeter erroneous values, emphasis mine)

So normal clouds should not affect the radio altimeter, but clouds with dense precipitation (like hail) can have such an affect. Note however, that if the radio signal reflects off the cloud itself, it could result in a false positive alert, but not a false negative.
GPS signal:

Here, the clouds above the aircraft are relevant, since they could block the signal to the satellite. The GPS signal is however largely unaffected by weather:

  Does RAIN or SNOW or CLOUDS affect the reception of my GPS receiver?

  Answer:  No.  Not so as the user can tell without instrumentation.

  [...]

  The GPS signal frequency of about 1575mhz was chosen expressly because it is a "window" in the weather as far as signal propagation is concerned.

(gpsinformation.net)

Even if the GPS signal is temporarily lost, the Alternate Horizontal Position Source can take over. The inertial position accuracy from the ADIRU would slowly degrade over time, but provide a good position estimate just after the GPS signal is lost.


Why do stealth aircraft generally lose in maneuverability? Whilst stealthy built aircraft have (or claim to have) many anti-detection systems, they seem to have to compromise on less maneuverability features, and even have to keep their missiles inside, not on the outside.

Why is it so these days?

Based on the above what could be the tendency for the future military aircraft (the jet fighters in particular)?

Radar stealth is achieved by carefully designing the aircraft to reflect radar waves in specific directions. It helps to keep the aircraft shape as simple as possible.

External weapons make this more complex: each missile adds a large number of surfaces that can reflect radar. The missile can also interact with the airplane fuselage to reflect radar waves several times. And once the missile has been launched it leaves an empty pylon behind with yet more surfaces capable of reflection.
It's much easier to maintain stealth if the missiles are stored inside the aircraft.

Some aircraft (e.g. the F-117) also try to achieve IR stealth by hiding the exhaust, mixing the exhaust gases with air etc. This is difficult to combine with vectored thrust.

Still, an aircraft like the F-22 has pretty good maneuverability, thanks to good aerodynamics and huge amounts of engine power.

The Su-37 uses vectored thrust and canards (again, extra reflection surfaces) to achieve high maneuverability.

So, there's a tradeoff going on. Do you want to blow your enemy out of the sky before he's seen you (which requires stealth)? Or do you focus on close combat (where maneuverability is most important)?


How can you find the best altitude for range or speed? How can you select best cruise altitude (altitude for best range velocity) and maximum speed altitude?

The specific case is a turboprop aircraft of gross weight 8000 lb.

Generally, climbing as high as you can would be close to the optimum strategy. In the end, the altitude is given by the optimum lift coefficient in cruise, which can be approximated as $$c_L = \sqrt{\frac{1-n_v}{n_v+3}\cdot \pi \cdot AR \cdot \epsilon \cdot c_{D0}}$$
Now it will be important to get the thrust over speed dependency right, which is expressed with $n_v$. A turboprop should have $n_v \approx -0.7$. The rest is determined by the aerodynamics of your plane.

Nomenclature:
    $c_L \:\:\:$ lift coefficient
    $n_v \:\:\:$  thrust exponent, as in $T \sim v^{n_v} $
    $\pi \:\:\:\:\:$  3.14159$\dots$
    $AR \:\:$  aspect ratio of the wing
    $\epsilon \:\:\:\:\:$  the wing's Oswald factor
    $c_{D0} \:$  zero-lift drag coefficient

If you have the optimum $c_L$, you need to find the altitude where level flight at cruise power will require this lift coefficient. Depending on the range of your small aircraft, you might spend most of the flight in climb and descent.

Maximum speed altitude: Draw the envelope at max power, and look for the rightmost point which can be trimmed. The equation above uses the quadratic polar approximation which works well at high lift coefficients, but for maximum speed the details of friction and parasitic drag are important, and they cannot be captured with a simple formula.


Are passengers really not allowed to change seats before take-off due to weight and balance? Just boarded an A320 with many open seats (Spirit), and the flight attendant said we may not change seats until at altitude due to weight and balance.  Is he just trying to prevent pax from moving around while the crew is busy moving around the cabin, or is this a legitimate issue?  I feel it must be the former, in which case I feel they're being a bit deceptive.

The crew gave you the correct reason, but it would had taken a co-ordinated effort from the majority of the passengers to upset the aircraft's balance.

But in smaller aircraft even a single person moving around can shift the center of gravity outside its safe limits. The de Havilland company once produced such an aircraft, the DH86 Express. Wikipedia reports:

  On 2 October 1935 Holyman's VH-URT Loina was also lost in Bass Strait,
  again with no survivors. This time a significant amount of wreckage
  was recovered from the sea and from beaches on Flinders Island.
  Investigation of the wreckage revealed a section of charred carpet on
  a piece of cabin flooring from just ahead of the lavatory door. It was
  thought possible that a small fire from a dropped cigarette had led to
  someone running aft suddenly to stamp it out – the sort of sudden
  change in weight distribution that could set up a fatal loss of
  directional control while the aircraft was on a low-speed landing
  approach.

A dramatic example what a shifting load can do to the controllability of an aircraft was National Airlines Flight 102, a Boeing 747-400 freighter conversion which took off from Bagram on April 29, 2013. The most likely cause was insufficiently secured payload which shifted back when the aircraft pitched up during take-off. A single passenger will not be as heavy as a MRAP, but several of them moving over a significant part of the cabin length in combination with a position of the center of gravity close to the limit could well upset the flight.

CG at the forward limit, passengers moving to the front: The aircraft will need higher speed and much more elevator deflection for lift-off. Especially in combination with an engine failure late in the take-off run, a crash is a distinct possibility.
CG at the rear limit, passengers moving to the back: The static stability and, in manually controlled aircraft, the stick forces are reduced. Flying the aircraft needs more attention, and in extreme cases the aircraft will become longitudinally unstable.

In any case, the pilots need to adjust trim.


Why does the Grumman Bat feature inverted winglets? I've been fiddling around with trying to make a blended wing body aircraft. While modelling, I heard from someone that the design reminded them of the Swift Killer Bee (now Grumman Bat). One distinct feature that I noticed from the design of the Bat is their use of an inverted winglet. Does anyone know why the design features an inverted winglet rather than a conventional one? The more recent designs of the UAV also features a conventional winglet in addition to the inverted one. Are there any benefits for this design?

I would also like to know if anyone has an idea on how the UAV handles landing? Is the inverted winglet capable of supporting the stresses experienced during landing? I can only find that the Bat comes with an optional landing gear while normally it is supposed to be caught by a net.

Without knowing what was going on in the minds of the designers it's difficult to tell exactly, however there are a few reasons as to why the vertical stabilizers are featured both above and below the wing.

In the sUAS market there are a huge amount of products which offer very similar capabilities. It is quite possible that the stabilizer design of above and below rather than in a single direction is to differentiate the product in a crowded market. This is similar to business jet design wingtip design - while it is known drag may be reduced the devices offer aesthetic appeal and different designs stand out to buyers.

The design also allows for the same surface area with a smaller packing footprint. Due to the bulbous shape of the fuselage the potential package size of the bat is smaller with the stabilizers as designed rather than simply above or below.

The stabilizer design is symmetric around the horizontal plane, meaning that in a side gust condition the UAS will feature less tendencety to roll and will instead simply be 'shifted'.

Again in a side gust condition the balancing of forces above and below the attachment point cancels out the moments and therefore may allow for a less strong joint (weight savings).


Can I land at an uncontrolled airport under class C without talking to ATC? In this situation, I would be flying from Richmond Executive-Chesterfield (FCI) to New Kent Co (W96).

Can I land at an uncontrolled airport under class C without talking to ATC?

Picture source: SkyVector

It looks to me like New Kent is entirely below the shelf of the class C space.   You don't need to go through the Class C Core at all, so I don't see that you need to speak to them.  The detour to avoid the core looks minimal

You can see the shelf height shown as 42/14, meaning that the class C space starts at 1400 feet, and goes up to 4200 feet.   Just stay below the shelf, at about 1100 feet. At that altitude, pay extra attention to any obstacles like radio antennas!  I do see several obstacles going up to 500 / 600 feet!

If you want to let them know of your intentions, I'd just say:

"Potomac Approach: N12345, Departing Richmond Exec, to New Kent, VFR below Class C.  On Frequency."

They will likely respond with:

"N12345: Proceed as requested. Maintain VFR"

Link to Flight Diagram


What is this seaplane shown in this photo? I searched up into a seaplane page and found a particular seaplane photo that I have never seen before. While the other images showing other seaplanes do have the name, the description of this seaplane image, however, does not show its name. I also couldn't even see the registration on its hull.
So, is this seaplane possibly manufactured by Cessna? If not, then does anybody know what the name is?

That's a DHC-2 Beaver on floats. The dead giveaway is the unique trapezoid window.


Do Instrument Landing Systems restrict building heights and can these be overcome? A recent territorial spat between the countries Malaysia and Singapore over an ILS implementation made me curious. I'm interested in finding out about ILS and how an implementation of one would affect the surrounding urban restrictions.

In that specific example, one country is saying that the ILS implementation would stifle its economic growth by restricting building height if the flight path went according to the yellow path showed on this picture

The question is, can the ILS still work if the flight path is altered to, say, an angled landing going through the water (along the thin white line) from North to South? Is it possible for a plane to turn and land into the airport (and bypass the land, hence removing building height limits) while still benefiting from the ILS in this case?

Note: The airport is 2km away from the other side of the shore (which is a separate country)

This is a fairly interesting case as it crosses international borders. Technically, as a sovereign nation they are free to build and approve construction as they see fit. Depending on where you are in the world this is handled in all kinds of different ways. Blocking the runway of another nations airport may not be great for international relations but there is nothing stopping them from doing it. There have been some fairly infamous real estate holdouts that test the limits of what you describe.

As for the latter part of your question, there have been instrument approaches that don't necessarily point you right at the runway initially. as @mfarver notes in the comments the classic example of this is the old approach into Kai Tak Airport which basically pointed you to the side of a mountain. The mountain face was painted in a checked pattern, when it came into view a right turn was executed and you were lined up with the runway. The approach is no longer in use but has been pretty notorious, you can see it in action here. With the prevalence of GPS approaches coming into use all sorts of things can be done that cant be done with a traditional straight line radio signal ILS. The issue with your described approach is that the airplane lands on the runway in the reverse direction which may not always be possible with the current winds.

The other option if things do get in the way is to have a really steep, slam dunk style final approach as they do in St. Barts. This, however can lead to other issues.


On a ducted propeller, how does duct length affect thrust? So if we are looking at an aircraft with a ducted propeller (or a high bypass turbofan engine), does it matter how long the duct is? As an extreme example, would having a duct that extends 100 feet behind the prop give any added thrust (let's ignore drag/weight effects of the 100 foot duct)? Because from my understanding, thrust is applied directly at the propeller, so it shouldn't matter how long the duct is just as long as it prevents tip vortex effects.

So as a follow up, why do most high bypass turbofan engines have their bypass duct extend back to where the engine core ends?

Generally, there are losses due to friction drag in any tube carrying a fluid; therefore, the longer the duct, the more energy is lost from the air stream (this loss appears as back pressure against the fan, so the same power is absorbed at the reduced flow rate).

The higher the flow velocity, the greater these losses, but in a high bypass turbofan, there are other considerations.  Extending the bypass duct past the engine core allows the cool, relatively slow bypass air to mix with the hot, high velocity turbine exhaust, with the main effect of reducing exhaust noise.  The loss with a duct 2-3 meters long rather than half a meter is small; the gain in noise reduction can be large, and mixing the bypass exhaust with the turbine exhaust inside the duct may also give enough augmentation effect to offset the duct losses.


Does too big an angle of decalage make the horizontal stabilizer to stall? This styrofoam airplane has two positions for stabilizer. When put in the upper one, the plane flies in a circle.
Is the stabilizer now stalled because the decalage angle is very big?What happens with the airflow over wing and stabilizer in this configuration?

Declage controls the AOA to which the static stability forces will be focused with no control input; in other words, the hand's free AOA/speed.  A higher declage angle is the same as increasing elevator input and holding it there.
It changes the trim force balance (nose down pitching moment from the main wing, C of G relative to wing center of pressure, and other appendages that generate pitching moments, opposed by the nose up pitching moment from the tail) to occur at a higher AOA.  Airplanes with moveable horizontal stabilizers, like most jets and a lot of light planes like the Cessna 180/185 family, have what is in effect, adjustable declage as their method of changing trim.  What you are doing by sticking the tail surface in the different slot, they achieve by cranking a screw jack to move the front of the same surface up and down for the same result.
So the tail won't stall at the moderate angle variation there, it'll just drive the main wing to a higher AOA to achieve trim equilibrium, and your glider will trim to a slower speed. If designed properly, if you keep increasing the declage, the main wing will be driven to its stalling AOA, and the aft Center of Pressure shift that occurs at stall will cause the nose down pitching moment to overpower the tail's downward lifting force and the glider pitches over before the tail's airfoil itself stalls.
The result is, when you throw it with the tail stuck in the upper slot, it will pitch up harder for the same throwing velocity as it seeks its higher trim AOA/lower flying speed (if you throw it at exactly its trim speed, it's already in trim when you release it and will just glide straight without pitching).
So bottom line; the lower slot is a "fast slot" and the upper slot is a "slow slot".  The glider will trim to a lower AOA and a higher speed on the lower slot, and vice versa on the slow slot.  That's the practical result of the two options.


What is the origin of the practice of numbering runways by magnetic heading? Today, runways are always named according to the magnetic heading. How did this practice start? When and where was the practice invented? Whose idea was it, and how did it catch on?

A pprune.org thread mentions that the UK borrowed the numbering system from the US.

  I believe the magnetic headings were adopted by the RAF in the UK early in 1944 and were inspired by American practice. Prior to that, RAF runways were numbered clockwise from north.

From the Department of Commerce Appropriation Bill for 1943 on page 515 from the snippet view (unfortunately can't find a fully scanned version) there's mention of a plan based on recommendations to number the runways.

This is confirmed by Flying Magazine Jul 1943:

So the when is circa 1943, the who is US Department of Commerce. It caught on by regulation based on recommendations. Naming runways north-south or 1, 2, 3, 4 were probably very confusing as the number of users grew.


What is the actual exhaust velocity of the exhaust from the J58 engines at cruise (Mach 3.2)? The SR-71 ran on two J58 turbojet engines and cruised at Mach 3.2.

What I want to know is, what is the actual exhaust velocity (relative to the airplane itself) of the jet exhaust at those conditions?

It's not a cut and dried single figure but  this
SR-71 PROPULSION SYSTEM - P&W J58 ENGINE (JT11D-20) seems to have enough information to allow you to arrive at a reasonable figure with 'some work'. If I had to choose one information source to get a feel for the answer this would be on the short short list (so far :-) ). Unlike most other sources, the result will be at least somewhat meaningful, if not overly accurate.

It also seems to make it 'clear enough' that if someone gives you a simple unqualified numerical answer it will be meaningless, or wrong, or both :-). It's a 44 page PDF slideshow - but has enough detail to show how different the J58 is to what you may expect.

He also supplies these references which will almost certainly be interesting, and possibly also useful.

Connors, Jack, The Engines of Pratt & Whitney: A Technical History, AIAA (American Institute of Aeronautics and Astronautics, Inc.), Reston, VA, 2010, pp. 321-332.
St. Peter, J., The History of Aircraft Gas Turbine Engine Development in the United States: A Tradition of Excellence, ASME, New York, 1999, pp.264-275.
Campbell, David H., F-12 Series Aircraft Propulsion System Performance and Development, AIAA No. 73821, 1973. Also found in AIAA Journal of Aircraft, Vol. 11, No. 11, November
1974, pp. 670-676. Note: This paper presents information only up to Mach 3.0 due to security restrictions at that time.
California Institute of Technology, Course Ae107, Case Studies in Engineering: The Lockheed SR-71 Blackbird, Spring 1991. Published by Cal Tech for course students and presenters only.
Anderson, J. Thomas, unpublished training course syllabus, How Supersonic Inlets Work: Details of The Geometry And Operation Of The SR-71 Mixed Compression Inlet, 2013.

If you do want a single misleading and probably wrong figure :-) then this book say 18,587 m/s at Mach 3.2 (table 1 page 175)

More M&PW answers:

Page 39-40

and here

Somewhere in here probably :-).

Related;

Comment in passing bottom of page 1

and

Here


Can a slow small plane, flying at a few meters above the ground, drop to the ground if the headwind abruptly stops? Why do I ask this question? Because Wilbur Wright sent a letter to Georges Spratt (an aviation enthusiast he personally knew) saying that his plane, Flyer II, once in flight, accelerated easily to 45 mph, by itself, after reaching 30 mph. (The plane had no throttle.)
I know that once in flight, the airspeed of a plane is independent of the headwind, so it simply does not matter how fast the wind blows from in front of the apparatus. Those lulls in the wind Wilbur feared about would not have had an impact on the course of the aeroplane unless the wind suddenly dropped in intensity (this is more a theoretical case). Only then for a short time the airspeed of the plane decreases to its former airspeed minus the drop in the headwind speed, but the plane regains quickly its former airspeed.
The question is, can a low flying plane really touch the ground just because the intensity of the headwind diminishes?

We have made forty five starts with our 1904 Flyer. Unless the relative speed at starting is 27 miles in a calm and two or three miles more than that in a wind, the machine will gradually slow down till unable to fly. After the relative speed passes thirty miles the velocity accelerates till a relative speed of 45 to fifty miles is reached. We found it difficult in practice to get a speed down the track greater than 20 miles an hour, so that unless we had a wind of about 10 miles we were not sure of being able to fly for a lull in the wind would let us drop below the real flying limit. … We have now finished a starting apparatus which gives a speed at start of 27 miles an hour in a dead calm, and expect shortly to begin circling. (Wilbur Wright, “Letter to G. A. Spratt”, Dayton, Sep. 10, 1904)

Source Small plane flying at low altitude.
Update:
The topic Does a sudden drop of the velocity of the headwind affect the airspeed by lowering it? and its explanations do not answer my question. I was already aware that a sudden drop of the headwind intensity reduces for a short time the airspeed of a plane and I have already written this thing to make it clear I knew about this phenomenon.
The plane of Wilbur flew a few meters off the ground.
So, assuming Flyer II traveled at an airspeed of 30 mph against a headwind of 10 mph and suddenly the machine entered a region of perfect calm, its airspeed would have reduced to 20 mph instantly, but did the plane need to drop more than a few meters to regain its former airspeed of 30 mph?

Yes.  The theoretical concept of "moving with the airmass" essentially says that for any given constant wind or no wind, airspeed is dependent on thrust and drag and reaches a steady state.
However, in nature, especially close to the ground, obstacles such as buildings, trees and hills can make airflow anything but constant.
This is why the modern aviator adds a little airspeed when landing on windy days.  A gust or a lull can momentarily change your airspeed faster than the plane can restore its thrust/drag balance.
Wind shears are an extreme example of this.  Were it not for the wind, flying that Cessna would not be more difficult than driving a car.


Should I use rudder to recover from power-on stalls? I was executing a power-on stall, and upon stalling the nose dropped and the plane rolled hard right.  I promptly applied left rudder and released yoke backpressure to recover, and the plane recovered normally.  However, my instructor told me that I should not use rudder when recovering from a power-on stall, even when the plane rolls one way or the the other.  This confused me, because a DPE had told me not to use ailerons when recovering from a power-on stall if the plane rolls one way or other other, so I'm not sure how I'm supposed to level the plane if I can't use either rudder or aileron.  Also, I had thought the roll was the beginning of a spin, but my instructor told me that it was not and that we were nowhere near a spin.  Can someone clarify the use of rudder (or aileron) on recovery from a power-on stall?  And if I'm in a situation where my plane is stalled and I'm rolling hard one direction, if that's not a spin, can someone please clarify what a spin is?

Your instructor is correct.
I've never seen a C172 where the POH actually discusses stall recovery besides saying it's 'standard'. I was taught to keep the airplane in balance using rudder in the approach to the stall, and manage any wing drops with rudder in a fully developed power off stall (I'm in the UK, not sure how the US is different). However, that isn't the procedure for a power on stall. The FAA Airplane Handling Handbook section 4-8 on power on stalls states:

The pilot must promptly recognize when the stall has occurred and take
action to prevent a prolonged stalled condition. The pilot should
recover from the stall by immediately reducing the AOA and applying as
much nose-down control input as required to eliminate the stall
warning, level the wings with ailerons, coordinate with rudder, and
smoothly advance the power as needed. Since the throttle is already at
the climb power  setting,  this  step  may  simply  mean  confirming
the  proper power setting.

In a power on stall if you get a big wing drop the rudder is not going to lift the wing, and coarse rudder movements could stress the airplane, see the AA 587 crash. So balance with rudder, but most importantly reduce your angle of attack with as much forward stick as you need.


How can business jets use slot coordinated airports? Many capacity-limited airports in Europe are so-called coordinated airports, meaning an airline must have a slot allocated to land at this airport. In Germany for example, these are FRA, DUS, HAM, MUC, STR, TXL and SXF. Often times, these airports are also used by general aviation aircraft, i.e. business jets, ambulance flights, government flights, etc.

Do airports "reserve" a given number of slots for such flights? Or are those flights exempted from the slot system?

I'm mostly interested in European regulations.

Depends on the particular airport, but I will take The Netherlands (Which has three slot-coördinated airports, including EHAM, EHRD and EHEH) as an example here.

According to the ACNL all flights to and from a coordinated airport can only be executed when an air carrier or any other aircraft operator has been allocated an airport slot. According to Article 2, paragraph g, of the Regulation as state flights, emergency landings and humanitarian flights are exempted. General aviation is therefore subject to slot coordination (at EHAM and EHRD). However, according to the LOC GA AMS, helicopter and GA propeller-driven aircraft under 5700kg MTOW are exempted from the slot system.

In conclusion: Regulations depends on country and airport. You should carefully read them before starting your flight to and/or from a coordinated airport.

A detailed PDF on how to request a slot can be found here.

GA slots for Europe can be requested through e-Airportslots


Reason why heavy aircraft are allowed to land but not to start I stumbled upon this article (German)
It says that a few of Lufthansas 747 are grounded at Twente airport. They were parked there because of COVID-19 at an aircraft recycling company.
Now the airport does have the license for such heavy aircraft to land, but not to start, which normally isn't a problem as they land there to be recycled.
Unfortunately the article doesn't state the exact requirements the airport doesn't fulfill. It just says something about safety issues. What could it be that allows the aircraft to land safely but not to start?

What could it be that allows the aircraft to land safely but not to start?

In the USA at least, to allow takeoff, a runway must be long enough to let an airplane reach takeoff speed, decide to abort, and reach a full stop.  That's longer than just landing.
The more ancient answer is: takeoffs are optional, landings are mandatory.  Refusing permission to land would damage Twente's reputation.
Even more ancient, before aviation, a ship in distress would not be forbidden to dock and ordered to head back into the storm.  (Only because COVID is mentioned, ships suspected of carrying plague are a different matter.  See the etymology of "quarantine.")

Greg's answer uncovers the Dutch report, which quotes Meiltje de Groot hoping that the 747's could be scrapped ("ontmanteld") in situ because of lack of demand, because of COVID's reduction of passengers, at Twente because they could land there with no intention of departing.


Swing-up canopy not shut in high-speed flight, what would happen? What would happen to a canopy like that on the Eurofighter Typhoon that swings up if it wasn't shut properly and the plane was going at high speeds, would the canopy blow back open?

(Source) T-38 Talon

Yes, it's possible. If it didn't separate, then it needs to be checked after the scary and cold flight is completed.

There will be warnings if the canopy is not closed shut.

(T-38 Talon Pilot's Flight Operating Instructions)


How is structural damage to a composite fuselage detected? When a service vehicle collides with the aircraft, the plastic nature could hide any subsurface fractures which could propagate as the aircraft goes through cycles.

How is potential damage diagnosed on the A350 or 787?

I can't personally speak for the Airbus, or Boeing methodology regarding damage diagnosis, however speaking as an A&P IA FCC, experienced in the Business, and General Aviation Industry, the diagnosis will employ, one or a combination of all of the following inspection methods:

Visual
Coin-Tap
X-Ray
Ultrasonic


How do I use the FAA's WINGS program as a flight review? I know pilots can substitute WINGS phases for the Flight Review otherwise required every two years. How do I do this? Do I have to attend a class to meet all the equivalency requirements?

The FAA Safety Team's WINGS program allows pilots to bypass the traditional FAR 61.56 flight review by completing a "phase" of the WINGS program.  The program and requirements are detailed in FAA Advisory Circular 61-91 (currently 61-91J).

The short version is that a "phase" of WINGS consists of three credits of "flight activities" and three credits of "knowledge activities", which must be earned with a 12 calendar month period.

Flight credits are earned by flying with an instructor and demonstrating safe & proficient flying skills in specific areas of operation.
For example, one "Basic WINGS" flight activity (worth 1 credit) is demonstrating basic air work - forward slips, steep turns, rectangular course, S-turns, and turns around a point - to Private Pilot practical test standards.
When you have successfully completed a flight activity your instructor can log in to the FAA Safety / WINGS site and give you credit for completing the activity.

Knowledge credits are earned by attending seminars and classes - these can be in-person seminars set up by the FAA Safety Team, or on-line webinars.  Many of the AOPA Air Safety Institute online courses qualify for WINGS credit.

When you complete a phase of wings your flight review clock resets, and your flight review becomes due 24 calendar months from the date you completed that phase.

Ultimately participating in the WINGS program probably means you will be spending more than the "bare minimum" 1 hour of ground time and 1 hour of flight time that FAR 61.56 requires. While you can often complete flight activities in an hour (or much less if you're proficient) the knowledge topics often take longer: the webinars are typically 30-40 minutes each, and in-person seminars can easily last an hour or more.

The major benefit to the WINGS program is increased safety through building your knowledge and proficiency - the seminars are actually informative (and the in-person ones can even be fun, depending on who is running them), and the flight activities are pretty easy (though there are certainly challenging ones available if you want to push yourself and really sharpen your skills). Resetting the flight review is an enticement to encourage all of us to participate.


How are this helicopter's blades perfectly timed with the camera shutter? This is a video of the Mi-24 helicopter. The rotor blades appear not to be moving.
Did the pilot and photographer sync the camera and helicopter to make this video and how? I do not know the true source but the video is remarkable.

If the rotor is spinning at 360 RPM, typical of helicopters, that's 6 revolutions per second, and 5 blades go by each revolution.  6 x 5 is 30, so you have 30 blades passing point X in any 1 second period of time.  Unlike film at 25fps, video is shot at 30 fps, in perfect sync with the blade passage, so the blades look stopped.  If the rotor was going a little faster or slower, the blades would start to move.

The video camera is in effect working like a stop action strobe camera.


Do propeller planes give contrails like jets do? Last weekend, while watching the second world war film "Fury" with a friend, there was a scene near the end with a large set of bombers flying overhead, leaving large contrails. That seemed wrong to me, as I'd thought that contrails were a jet thing, but the friend I was watching it with thought it was right. (Was a lovely shot though, accurate or not!)

That has me wondering - do propeller planes generate contrails like jets do? And if so, is it only turborops that would, or is it something all propeller planes can create?

Yes they can. That image from wikipedia showing B-17s is more descriptive

Contrails originate from condensation of vapor coming from internal combustion engines, regardless of being turboprop, turbojet/turbofan or cylinder engine. Vapor is a byproduct of the hydrocarbon (fuel) being combusted with the atmospheric oxygen.


What happens if an engine fails between V1 and V2? What happens if an engine fails between V1 and V2 on a modern twin airliner?

As I understand it, at V1 you have enough runway remaining to safely stop and abort the takeoff. V2 is the lowest single engine flying speed.

If an engine is going to fail, surely it will be at the worst possible time.

If an engine failure occurs above V1 the aircraft can accelerate to the Vr and take-off, retract gear and accelerate to V2 and pass the end of the runway at a height of at least 35 feet. From there, flying at V2 or above the aircraft can climb at angle that keeps it clear of nearby obstacles.

Part of the aircraft's certification is to demonstrate that this is possible.
This question is related to the exact requirements.

To determine the minimum runway length required, two scenarios are calculated. The first scenario has the aircraft accelerating to V1, suffering an engine failure at V1 and then aborting the take-off. This gives the Accelerate - Stop Distance Required (ASDR). The higher V1, the higher the ASDR.

The second scenario has the aircraft accelerating to V1, suffering an engine failure at V1 and then continuing the take-off. This take-off involves accelerating from V1 to Vr, lifting off, retracting gear while accelerating and climbing, reaching V2 and 35 feet at the end of the runway. The runway length required for this scenario is the Take-Off Distance Required (TODR). The higher V1 the lower the TODR. This because when V1 is higher the part of the scenario that involves accelerating on a single engine becomes smaller.

These calculations take into account aircraft weight, engine thrust, aircraft configuration and runway condition.

For the V1 that gives the same ASDR as TODR, the required runway length is shortest. This is called the balanced field length.

This video is an example of an engine failure between V1 and V2. While the aircraft is rotating the engine ingests a heron resulting in big balls of fire. The aircraft lifts of, accelerates and climbs away at angle steep enough to avoid obstacles.

Rejecting a take-off above V1 will likely result in a runway excursion.


How do pilots travel from remote private airports after landing? What if you fly your own personal plane to a small private airport in the countryside that has no public transportation? I imagine that most pilots don't have a car parked at each private airport they intend on flying to. That is, of course, unless your car is small enough to fit inside your plane. If you want to go somewhere twenty miles from the airport, for example, do you just call a taxi or ask a friend in that town to pick you up in their car?

You do whatever you can.

Sometimes you call a taxi/Uber, sometimes you walk, sometimes you pack a small folding bicycle into your plane, and sometimes you borrow the airport car.

Many small airports in the US have a car (often a retired police car) available for itinerant pilots to borrow.  It's usually a scary rusted out pile of junk, but it works.  If you borrow it you're expected to return it promptly & with a full tank of gas.

It's wise to make arrangements for ground transportation before you arrive - there's no guarantee that any particular option (except walking) is going to be convenient and available.


How can I calculate stall speed using foilsim? I need to find the stall speed of an airfoil using foilsim not the formula (https://www.grc.nasa.gov/WWW/K-12/airplane/foil3.html)

All you can do with any airfoil analysis software is to calculate the stall angle of attack and lift coefficient. To get to the stall speed, you need to have knowledge about the wing loading and wing shape of the particular aircraft you need to calculate the stall speed of. For a single airfoil no stall speed can be defined - this is only possible for a specific airfoil on a specific airplane.

Foilsim allows you to add this information via the "size" button. Next, click on the "shape" button and increase the angle to the maximum where the flow on the upper side does not yet separate from the airfoil contour. If you now change to the "Flight" button, you can enter the speed and altitude. Select "Lift" on the right side and see how the lift changes with speed.

If you divide the lift by the gravitational acceleration of the selected planet (yes, you can run Foilsim on Venus or Mars, too - on earth that would be 9.806 m/s²), you get the mass of the aircraft that will stall at the selected speed.

However, since this is a very simple simulation that is not meant for actual aircraft design, and the airfoil shape cannot really be selected beyond camber and angle of attack, I advise you to not build an aircraft with those results. The fact that the stall angle of attack is always 10°, regardless of the airfoil contour used, should be warning enough.


Is an ultralight aircraft allowed to fly near Class B airspace? May an ultralight vehicle operating under FAR part 103 operate within Class G and E airspace while within the mode C veil surrounding a Class B airport?

Quoting directly from Title 14 CFR 91.1 Applicability

  FAR 91.1(e) This part does not apply to any aircraft or vehicle governed by part 103 of this chapter, or subparts B, C, or D of part 101 of this chapter.

Part 103 addresses airspace in this way
103.17 Operations in certain airspace

  No person may operate an ultralight vehicle within Class A, Class B, Class C, or Class D airspace or within the lateral boundaries of the surface area of Class E airspace designated for an airport unless that person has prior authorization from the ATC facility having jurisdiction over that airspace.

Where can I find a restriction that applies to ultralight operation within the mode C veil but still in Class E or G airspace?

Yes, an ultralight aircraft operated under 14 CFR 103 can be operated within the "Mode C Veil" within the limitations of §103.

As you note, 14 CFR 91—including §91.215, ATC transponder and altitude reporting equipment and use—does not apply to ultralights governed by §103.

The limitations of §103 do generally prohibit: operations over congested areas; operations in certain types of airspace; operations in certain types of airspace.

  §103.15   Operations over congested areas.

  No person may operate an ultralight vehicle over any congested area of a city, town, or settlement, or over any open air assembly of persons.

  §103.17   Operations in certain airspace.
  No person may operate an ultralight vehicle within Class A, Class B, Class C, or Class D airspace or within the lateral boundaries of the surface area of Class E airspace designated for an airport unless that person has prior authorization from the ATC facility having jurisdiction over that airspace.

  §103.19   Operations in prohibited or restricted areas.
  No person may operate an ultralight vehicle in prohibited or restricted areas unless that person has permission from the using or controlling agency, as appropriate.

Let's look at an example, the MSP Class B area:

Point A, near A R S Sports Strip (7Y7), would be an area where operations under §103 would be allowable. It is not a congested area, and is not a type of airspace for which operations under §103 are prohibited.

Points B, C, and D, however, would be areas where operations under §103 are generally prohibited. Point B is arguably over congested areas, and by that determination would therefore be prohibited under §103.15. Points C and D are in Class D and B airspace, respectively, and are therefore prohibited under §103.17. The exception would be where the operation has received prior authorization as specified in §103.17 above.

For what it is worth, I have personally witnessed ultralight type aircraft flying in the area of Point A.


How to log pattern work time during X-country? I am working on my IFR part 61 and must have 50 hours of X-country as PIC.

Lets say my instructor and I fly to an airport that is 55 miles from my take off airport - it takes us 0.8 hours from take off to land.
Then, on our way back, we do pattern work at another airport - 0.4 hours to fly and 0.7 hours for pattern work.
Then, final leg of flying - 0.3 hours.

Total flight time - 0.8 + 0.4 + 0.7 + 0.3 = 2.2 hours

What is the best and legal way to log this 1.5 hours of X-country, in my log book? If I could, I would like to take advantage of the 0.3 + 0.4 hours during my return flight as X-country time.

You can log it all as XC. This is quite common. XC is all flight time to an airport more than 50 NM away from origin, even if you don't spend it flying in a straight line.


What differentiates the entry procedures for spins and snap rolls? Both a spin and a snap roll are aerobatic manoeuvres involving stalling one of an airplane's wings earlier and more deeply than the other.  Both are entered by pitching up sharply to stall the airplane, and applying a hard rudder input to yaw the airplane sideways and ensure that one wing stalls before the other, causing that wing to drop and the airplane to roll off to the side.  What happens next differs between the two:

In a spin, the increase in drag on the wing that stalls first causes the airplane to enter an extremely fast yaw (rotation about a vertical axis1) in the direction of said wing, keeping it more deeply stalled than the other wing; the high yaw rate centrifuges the airplane's mass away from the spin axis, preventing the airplane from rolling beyond the bank angle induced by the initial spin-entry yaw input.
In a snap roll, it's the decrease in lift on the first-to-stall wing which is important (rather than the increase in drag), causing the airplane to enter an extremely fast roll (rotation about the aircraft's longitudinal axis) into the first-stalled wing; unlike with a spin, the airplane's inherent directional stability prevents the initiating yaw input of a snap roll from producing any persistent yaw.

Thus, the same type of entry procedure can apparently result in two wildly different outcomes: a spin, with an extremely-high yaw rate but no significant roll rate, or a snap roll, with an extremely-high roll rate but no significant yaw rate.
How does one differentiate a spin-entry procedure from a snap-roll-entry procedure?  What determines whether the sudden pitch-up and hard rudder input produces a spin, or whether it generates a snap roll instead?

1: This axis usually is not coincident with the vertical axis of the airplane as a whole (that passing through the aircraft's center of mass), although it can be.

A snap roll isn't a roll.  It's an "enhanced" spin, done horizontally.
A normal spin entered from level flight at 1G stall speed has an initial roll component along with the yaw component as the wing drops at the stall.
The same thing happens in a snap roll, except that the spin is entered with an accelerated stall, well above level flight stall speed and with power, with both the roll and yaw initiation enhanced with aggressive rudder and aileron input to get the horizontal auto-rotation roll-yaw going as fast as possible.
Because the snap is done in an accelerated stall, the stall can be broken immediately with forward stick, and aggressive opposite control inputs have to be used to overcome inertia to stop the snap roll.  You initiate the left snap with the hard stick back/left and left rudder, and recover with hard stick forward/right and right rudder when about 3/4s of the way around.
You don't actually need aileron; airplane can snap roll quite nicely with no aileron, just rudder, it'll just be a bit slower.  A pilot can enter a snap roll accidently just from mishandling, say pulling too hard in a turn and inducing an accelerated stall with the ball off center, and if the airplane has a sharp stall break, around you go, and before you've figured out what happened you're already right side up again.
If you hold the control inputs and keep the snap-roll going, you eventually arc down until you're snap-rolling vertically, or spinning the old fashioned way.


What is the difference between EFAS and FSS? What services do each provide and on what frequencies?

EFAS - Inflight Weather Advisory Services (aka Flight Watch) is a special frequency in the United States on 122.0.  It uses remote communication outlets (RCO) and is typically collocated with navigational equipment.

When you talk with a FSS weather briefer on EFAS you are talking with a more experience weather briefer who can provide a greater degree of accuracy with changing weather conditions.  When I took a tour or the Prescott FSS station after Lockheed Martin took over, the tour guide state the EFAS station is manned by a Level 2 weather briefer who is more knowledgeable than a Level 1 briefer who you might talk too over a regular FSS frequency.

FSS personnel can talk to pilots over RCO and the frequency is typically found along the top of the VOR dialog box on sectional charts as well as 122.2 and 122.4.  These briefers are available to open and close flight plans, accept PIREPs and provide basic weather services.

The picture below shows that you can talk with McAlester FSS on 122.1 and receive their response over the VOR 115.2.


How can I plan a VFR flight through controlled airspace, without knowing what instructions ATC will give me? I'm currently planning a VFR flight from KFRG to KDOV. Due to airspace restrictions in this area, I plan to contact NY Approach and request flight following. If all goes well, then I believe I will get transferred from center to center and eventually be handed off to KDOV tower where I will land. Happy Days.

What I'm worried about is that somewhere along the way the controller will drop me with the dreaded words "radar services terminated". Say this happens between KEWR and KPHL, what do I do? Fall back on GPS/VOR? How does one create a VFR navlog for such eventualities?

Short answer: make your flight plan as usual but if something changes then just deal with it. After all, as a private (?) pilot you've been trained to handle that situation.

Even an IFR flight plan is only 'plan A' and on any flight you may have to deal with changes because of ATC, weather or other issues. That's one reason why the private ACS includes a diversion task to test whether you can re-plan in the air. Note the wording of that task (emphasis mine):

  Make a reasonable estimate of heading, groundspeed, arrival time, and fuel consumption to the divert airport.

It's helpful to think about general options in advance (e.g. "I'll go west around the KPHL class B if I don't get a clearance"), but you can't predict every possible situation. Always having that "reasonable estimate" for where you're going is more important than trying to anticipate and plan for every eventuality.

The good news is that as part of your original flight planning you should already have reviewed all the information you need to make decisions in the air: winds aloft, fronts, airspace, TFRs etc. So if you do have to re-plan you already have a good overview of the big picture and what your options are. "Plans are useless, but planning is essential", as someone once said.

And if you don't already have a flight app like Foreflight or Garmin Pilot then you should look into it. They make planning, diversions and replanning much simpler, which removes a lot of stress from the situation. They're also much easier to work with than paper and pencil, especially if you're single-pilot without an autopilot.

Finally, don't forget the magic word "unable". ATC can't see what you see, and they might drop you in an awkward situation, e.g. you're VFR between cloud layers and the only way to stay in VMC is to enter class B. If that happens, don't hesitate to tell them that you need a clearance, or vectors, or whatever. If they don't cooperate, you can always declare an emergency if the situation is becoming unsafe (but don't abuse the system for your own convenience).


Why a higher compressor pressure ratio results in higher Thrust and lower TSFC? My professor usually asks us in our exams to plot the Thrust-Pressure ratio and TSFC - Pressure Ratio charts and justify the tendencies observed.

(TSFC = Thrust Specific Fuel Consumption).

We all know that, as the compressor pressure ratio ($\pi_c$) increases, the higher the Thrust ($F$) and the lower the $TSFC$. Here is a graph (which was taken from the book Fundamentals of Jet Propulsion with Applications, by Ronald D. Flack).

This chart was made after taking the final expressions for $F$ and the $TSFC$ (which are huge) and plotting them against $\pi_c$ using a computer. Obviously we don't have computers in the exam, nor we have enough time to derive such complex formulas. Here is an example for the (non-dimensionalized) $TSFC$:

Instead, I want to impress him with a convincing, non-mathematical, qualitative explanation of this behaviour.

Therefore my question reduces to: why, physically speaking, does the thrust increase with $\pi_c$? (The answer to "why does the TSFC decrease with $\pi_c$ can be answered saying that $F$ and $TSFC$ behave inversely to one another).

This is my attempt:
As the compressor pressure ratio increases, the flow will be mixed and burned more efficiently in the combustion chamber, therefore increasing the energy available for the nozzle to accelerate the flow. Since the jet thrust increases with the exhaust speed, we can rest assured that a higher pressure ratio will translate into a higher thrust.
The $(TSFC-\pi_c)$ behaviour will be inversely proportional to $(F-\pi_c)$, since $TSFC=\dot{m}_f/F$

NOTE: My explanation of the process, I think is a bit incomplete. For example, it doesn't explain why there is a $\pi_c$ for which $F$ will actually start decreasing (in the graph, this $\pi_c$ value is around 15).

Because the constant pressure lines are diverging with increasing enthalpy.

Have a look at the right most diagram.
The two lines are constant pressure lines, with the higher line indicating the higher pressure, and as you can see, they are diverging.

1 is the beginning of the cycle, thus at ambient temperature

From 1 to 2, we increase the temperature, and go to a higher pressure in the compressor. This will cost some energy, which is delivered by the turbine.

From 2 to 3 we increase the Temperature at the the same pressure by adding energy.

From 3 to 4, we decrease the temperature and pressure in the turbine (and use some of the extracted energy to drive the turbine)

From 4 to 1, we use the energy in the flow to propel us, and we go back to ambient temperature.

In principle we could also skip the higher pressure, and simply go from 1 to 4 and back to 1 (which is just heating air, and propelling yourself by the heat.)

But by moving to a higher pressure, we can take advantage of the divergent pressure lines. This yields us a benefit because the distance 3-4 is larger than 2-1. This means that if we use the turbine to drive the compressor, we gain some 'free' energy.

Source

Following OSUZorba's comment:

To illustrate the point of the 'free energy', look at the image below, here two extra stations are added the inlet, which isn't that important for now, and the nozzle. Note that the vertical distances of the compressor and the turbine are equal (they have to be, because one drives the other), thus
$$ \begin{aligned}P_{comp}  = P_{turb}  &\Rightarrow \Delta T_{comp} = \Delta T_{turb} \\ &\Rightarrow (T_3 - T_2) = (T_4 - T_5) \end{aligned}$$

But because of the divergent nature of the lines, we have some extra energy left. We use the nozzle to optimally expand the high pressure, high temperature flow and extract the 'extra' energy.

Source


What are the protocols of flying over conflict areas? Apparently Malaysia Airlines Flight 17 used a general airway over the east of Ukraine, which currently is 'in conflict'.

What are the 'normal' procedures regarding (not) flying over conflict areas? Who is responsible for an up-to-date risk assessment? Who advises? Who makes the (final) decision?

This is horrendous Incident, and my heart goes out to the victims Families who lost their loved ones.

The FAA filed a special Warning flying over Crimea back in April but apparently not where the airplane went down.

The procedure is to avoid flying over (Dead/No fly Zones) which Aviation Authorities are responsible in providing to the Airline companies so they can adjust their flight plans.

FAA Warning over flying over Crimea.

Planes Flight Path.


What is the altitude record for the F/A-18 Hornet? Does anyone know how high (above MSL) the F/A-18 ever went? Its service ceiling is 50,000 ft, but when going up steeply, is it able to surpass 100,000 ft altitude?

I can tell you my experience during my stay in Zaragoza Air Base in northeastern Spain, in 1998. After heavy maintenance work, the twin-seater was ready to fly. In order to check all work done, we did a parabolic flight in clean configuration (no loads under wings or fuselage). It was a flight test so we decided to push the limits a little and see the real capability of the General Electric F404 engine. After acceleration and trying to maintain the best angle, we were able to climb up to 63,000 ft.
It was quite difficult to fly the plane when crossing 45 or 48 thousands (I don't remember well), so we decided to connect the auto-pilot. The auto-pilot can handle the plane easily at those altitudes with such low air density.
I recall the last portion of the climb was a nice parabolic trajectory in zero-g (you cannot imagine how dirty the cockpit was, I could see a pencil and some screws from maintenance floating in front of me, among some other things) until we regained positive g's in the descent. It was an interesting flight, but as stated by others, not practical at all from the combat viewpoint.


What is the physics of crosswind landings for large airplanes? This always confuses me from a physics perspective. Upon touchdown, what is the effect of the wind on a large airplane/airliner (Boeing 737 and above)? Below are three cases I put together to make it easier to explain my confusion:

No decrab, the wind acts on the tall fin yawing to plane further into the wind at touchdown, but the pilot anticipates it and counters this by pushing the rudder downwind (video).

The pilot decrabs, overcooks it (rudder remains deflected), and the correction is rudder upwind initially (video). Based on the previous analysis, the wind would have helped to correct it.

You see in the video the second time they are facing upwind, the pilot lets the wind correct it before settling on rudder downwind like the previous example.

So I have two cases that say eventually it's rudder downwind, and this case in particular shows the yaw effect on the fin (with and without using the rudder).

What I normally read: the wind pushes the aircraft, and the correction is rudder upwind. This AOPA article for instance, shows a drawing with the wind force pushing a [small-er] plane downwind.

Apologies if I butchered the crosswind landings. To summarize:

When the wind acts on the tall fin upon touchdown, is there a yaw effect, or does the wind push the airplane downwind? And what is the rudder correction to be anticipated?

When the wind acts on the tall fin upon touchdown, is there a yaw effect, or does the wind push the airplane downwind? And what is the rudder correction to be anticipated?

Let's assume a strong crosswind at 90° from the right with a 747 on a dry, uncontaminated runway for illustration purposes.

Once the airplane is firmly on the runway, the wind is acting on the whole airplane, with the crosswind pushing it downwind, and given the greater side force aft of the c.g. (especially because of the tailplane) than the side force forward of the c.g., there is at the same time a net yaw effect wanting to rotate the airplane about the c.g. into the wind.

Perhaps its best to think of the rudder pedal displacement not just as where you want the rudder, but also what you want the nose gear to do. While there were some 747s that had no connection between the nose gear steering and the rudder pedals, most did (at least the ones I flew). Now think of what you have to do to get the airplane where you want it, pointed in the direction you want, and actually traveling in the direction you want.

What you want to wind up with is the airplane's longitudinal axis on the runway center line, nose pointed straight down the center line, and the airplane actually moving straight down the center line. Let's say you've succeeded in getting the airplane to that state. To keep it, in our strong 90° right crosswind,you would use right aileron to keep the wind from getting under the upwind wing so to speak, and some left rudder. How much would depend on whether you had rudder pedals that connected to the nose gear steering, your speed, your weight and when you started using the tiller for steering. I forget the details of the connection between the nose gear steering and the rudder pedals, but it's not linear, and as I remember you can have the nose gear tracking straight ahead even though you are using some rudder.

On a 747, once you've slowed down to tiller speed, you would use it, and on a dry, uncontaminated runway, the main and wing gear will keep you from moving sideways and the nosegear will control your direction of movement, at least in the strongest of winds I operated in, say 40-45 knots or so.

Chances are, once you are firmly on the runway after touchdown, you will not be exactly on the center line nor pointed precisely down the runway. Your first and second illustrations are dealing with getting there from a less than ideal touchdown, and if I understand them, they are correct. If to the right of the center line, you can allow a drift leftward. If to the left, you don't have a lot of room to spare so you need positive action to get to the center line.