Did Kai Tak have a straight-in approach to runway 13 for higher-performance aircraft? Runway 13 at Hong Kong’s Kai Tak Airport (closed in 1998 and replaced by the current airport at Chek Lap Kok) had a hideously difficult approach in which pilots would fly in from the west, following an instrument guidance system (IGS)1 to the approach’s middle marker (also helpfully marked by a giant checkerboard painted on a hillside, which, if not spotted in time, required a missed approach), and then immediately hand-fly a steep right turn2 at very low altitude directly over the heart of Kowloon City to line up, seconds before landing, with the runway, using only the Mk. I eyeball for guidance, with no room at all for error.

For obvious reasons, only the best of the best pilots were allowed to shoot the bent approach to runway 13, and only after extensive simulator training for that specific approach, and even they had considerable trouble with it, even in good weather.

The bent approach was necessitated by the presence of an inconsiderately-placed mountain in what would otherwise have been the runway 13 approach path, which, for a straight-in approach, would have forced the use of a glideslope too steep for a widebody jetliner to safely fly.  Not all aircraft (or even all airliners) are widebody jets, though, and smaller, higher-performance aircraft (including virtually all STOL aircraft) can frequently shoot approaches much (much) steeper than what would be safe for a 747 or A330.

Given the difficulty and hazard of the standard bent approach, did Kai Tak have a straight-in approach to runway 13 for use by steep-approach-certified aircraft?

1: Like an ILS, except that following the IGS all the way down would kill you.

2: I don’t know if the Kai Tak checkerboard turn would technically qualify as a pylon turn, but it must surely have come close!

did Kai Tak have a straight-in approach to runway 13 for use by steep-approach-certified aircraft?

During the 1990s I regularly flew the Hong Kong IGS approach in 747s. At that time, to the best of my knowledge, there was no straight-in approach to runway 13.

  only the best of the best pilots were allowed to shoot the bent approach to runway 13, and only after extensive simulator training for that specific approach, and even they had considerable trouble with it, even in good weather.

While the above might have been true for some carriers, it was not true (Wikipedia to the contrary) for the two 747 carriers I worked for. Cathay Pacific might have had some such requirement as that was their home base.

Go to https://terryliittschwager.com/talking-of-flying.php#kaitak for an account of my first time in.

Wikipedia has another error or at least a poor choice of words IMHO when they say that upon reaching the checkerboard hill you started the turn toward the runway. if you look at the approach plate, you'll see the turn starts well before the checkerboard. The rule was that you had to have the checkerboard in sight ahead before starting the turn. If you didn't it was missed approach time. Fortunately I never had to do that.

  to the eternal gratitude of non-masochist pilots everywhere) had a hideously difficult and dangerous approach

I never considered the approach dangerous. Challenging, yes, but not dangerous. And I'm really not a masochist or at least I didn't think of myself as such. After you got used to it, it was fun!


What is the exact meaning of "manoeuvring" when said by ATC in reference to traffic? In the UK, when receiving a basic service, sometimes the ATC unit passes traffic information. They may say it like this:

“Traffic to your three o'clock, similar level, C172 manoeuvring.”

What exactly is the word “manoeuvring” conveying? Is there a defined point where an aircraft is manoeuvring and when it is not?

To add to @HiddenWindshield's answer, it's explained in UK's CAA Radiotelephony Manual CAP 413:

Traffic Information and Avoiding Action Phraseology
5.21 Relative movement should be described by using one of the following terms as applicable:
(...)
5. 'manoeuvring' where the conflicting traffic’s flight path and/or level information is unpredictable and/or showing significant variation.


Question about flying around hurricane I'm supposed to fly from NYC to Jacksonville on Friday afternoon. If the forecast for Hurricane Florence holds, the storm should be somewhere over the Carolinas at that point. With neither NYC or Jax being directly impacted, can commercial airliners fly around the hurricane? Would it be likely that our flight would be cancelled/delayed because of a major hurricane being in the flight path? Thanks

Hurricanes move en masse pretty slow compared to a jet airliner.
Heck even compared to a small plane. It is not a problem to fly a path around them to avoid them.  Eventually the winds and rain will impact local travel, but the hurricane must be 'pretty close' for that - how close will depend on the aircraft size.  Wind/Rain/ceilings will impact small aircraft sooner than large jet airlines that fly hundreds of miles faster.


Radar Control; is it possible for ATC to instruct track instead of heading? Is it possible to use the track instead of headings in the aim to apply a radar separation?

Radar controllers take wind into consideration when issuing headings - "Fly heading one six zero".

Issuing a track would not be practical. This puts the burden of figuring out what course to fly on the pilot. Not all aircraft have a glass cockpit or instrumentation necessary to easily, quickly, and accurately compute a heading to fly to stay on course. The radar controller knows real-time information, knows the 'big picture', and has practice issuing headings.

Actual position is used to provide separation. Heading doesn't matter for separation.


How did aviation industry come to know about the drastic difference in air pressure in high altitude? After watching many documentaries about air plane crashes, it seems that aviation industry learned many of its lessons from disasters. For example, from the 2 mid-air explosion of de Havilland DH 106 Comet in 1950s they learned a lot about metal fatigue that can cause Explosive decompression. So, I'm curious to know how did the industry come to know about the drastic difference in air pressure that occurs in high altitude? Was the knowledge already there before an airplane was even invented?

The entire reason why the Comet broke up in midair is because they pressurized the plane in the first place.

It was known for a long time that air at high altitude was thinner. This can be experimentally verified by taking a barometer up to a mountain. This was first done in 1648 (and subject to hot debate regarding the nature of air) more than 250 years before the Wright brothers first flew.


Why is the flight going north just to go south? I am tracking my kid's flight from LAX to EWR. It seems like a pretty straight West to East kind of flight but the flight path has the plane moving somewhat north through Ohio and then angling back to the south

Why wouldn't the flight just keep going East?

Aside from the storms causing an earlier re-routing, that jog north is to join the arrival into EWR.  A common fix in NE Ohio on flight plans into EWR is the DJB VOR which is in Loarine county (I think, somewhat near Cleveland). and then routing over Slate Run and then Williamsport, PA on the way into Newark.  If you are a jet coming from the west, this is literally the only way into EWR.  The other two ways in are from the south joining somewhere on a line between Atlanta and Baltimore (which you can transition to from the west by going over Beckley, WV) or from the north coming in over Albany, NY.  The primary reason for the strict routing starting at around the OH/PA border is the need for multiple arrival descents for both jets and turboprops into the various NY area airports (EWR, JFK, LGA, TEB, etc), the westbound departures out of NY center and the letters of agreement between Cleveland and NY centers to manage these traffic flows.

See this image:

This represents a route of ..J29.DORET.J584.SLT.FQM3.  This is not the exact route a LAX originating flight would normally get as they would end up a bit further north (going over Detroit instead of Cleveland), but they would still converge on J584.SLT.FQM3 (the portion east of the OH/PA border).  No matter your position west of the OH/PA border, Cleveland center is going find a way to put you on J584 before you get very far into PA.  This is why the flight couldn't just go direct to EWR and had to first fly north.


Why don't pilots parachute from small planes that are in distress? There was another question that asked why commercial flights don't have parachutes.  The almost ubiquitous response was that the parachutes would be useless because:

Most accidents with commercial airliners happen on take off and landing, and there is no time to parachute.
In order to get to a position where 100+ people can successfully jump out, you'd most likely need to descend some 20,000 ft and then maintain straight and level for a good 3 to 5 minutes once you got past 12,000 (so people have oxygen to breathe when they jump).  And if you can descend and maintain level flight, you might as well land.

But what about in a light, single engine plane (think Cessna 172 or Piper Cherokee)?  Engine failures in small aircraft, for example, seem to be more common, so you have more accidents that start high above the ground.  Thus, you usually have a few minutes before you're going to hit the ground and there's often only 1 or 2 passengers (rather than 100). Plus, you're usually already at an altitude where you don't need oxygen to bail out.

With that in mind, couldn't you put the plane into a shallow dive to keep it from stalling, trim it to keep it going straight and then bail out?  It seems like a somewhat practical solution, yet I have never heard of anyone doing it.

Why do pilots often try to find a road to land on or a lake to ditch in when trouble strikes instead of just bringing a parachute and bailing out?

Mainly because in the situation that you describe, the airplane is perfectly capable of flying.  You don't need an engine to fly as airplanes are designed to glide without it. Part of every pilots training is how to land the airplane when this happens.

Many of the same issues also apply in the smaller airplanes.  Unless the pilot and the passengers fly around with their parachutes on, it would be quite difficult to put them on in the confined space, in a high stress situation, and with the very limited time available.  Even if they did, untrained people are going to be hurt (probably quite badly) during the landing even if everything else with the jump goes well. (See my answer on your linked question for some of the things that can go wrong during the jump.)

You are also creating a hazard with the aircraft not being piloted and crashing into a random place on the ground.  All of this when the airplane could have just glided in for a landing in a field or on a road.  Most of these types of emergencies end quite well, and even a lot of the accidents will have fewer injuries and deaths than if people were jumping out all of the time. As an added bonus, the airplane can even be used again!

On the other hand, a major structural failure that makes the aircraft incapable of flight could be a reason to jump.  This is extremely rare however, and even if one has a parachute in this situation it may be impossible to exit the airplane safely because of high G forces and the possibility of hitting the airplane as you are both tumbling through the sky.

Some manufacturers are now building ballistic parachutes into the airplane which can bring the entire airplane down safely in these situations.  This is much more safer, reliable, and does  have a positive impact on safety in these extremely bad situations.  For just an engine failure though, I doubt that most pilots would even fire the ballistic parachute if there were suitable landing spots within gliding distance.

My view is that you would have a far greater safety impact by requiring everybody in a moving car to wear a helmet.  Would it improve safety?  Absolutely.  Would people want to do it?  A few would (and perhaps a few do), but the general population doesn't want to be inconvenienced by something that is only a remote possibility.  The odds of a parachute helping you in a typical GA airplane is far more remote than the odds of the helmet helping you in a car.


What is a biplane propeller and how efficient is it? Lazair ultralight series II used biplane props to absorb more power from the engine....so my question is what really is a biplane propeller and are they efficient in comparison to a four bladed prop?

It is essentially two propellers stack on top of each other. You can see it in this picture of a Lazair.

As for why they were chosen for the Lazair,

  Many have asked why Ultraflight opted for double propellers stacked in
  a biplane fashion. This is a very unique setup and draws lots of
  attention. Apparently when Ultraflight changed over to the Rotax 185
  they tried several different propellers. The problem was the 185’s had
  a woodruff key slot in the crankshaft and would snap the crank
  whenever the engine coughed or kicked back on start up. This was quite
  a serious problem. After much deliberation it was determined that the
  rotational inertia of the propellers was too much for the crank to
  handle.

  Dale could not locate a two bladed prop light enough to fit the need.
  However, since they had been producing a lot of the early lazairs with
  Pioneer engines fitted with a single plastic prop, he had a ton left
  over that fit the pioneers.  He found that these could be “doubled up”
  and help to produce the extra thrust that the Rotax 185 was capable of
  producing over the 5.5 hp Pioneer engines.

It appears to be a quick way to gain more thrust from the props without increasing prop mass too much.


Does shutting down the engine affect the rate of rotation during autorotation? If a helicopter's tail rotor shears off while flying then the helicopter's fuselage will start rotating in the direction opposite to the main rotor's rotation direction. During autorotation, if I switch off the engines of the helicopter, then will it affect the rate of rotation of helicopter?

with the engine disconnected from the main rotor, no torque is being applied to the rotor by the engine so there is no torque reaction trying to rotate the fuselage backwards. In this case, the only torque being applied to the fuselage is the frictional drag in the main rotor bearing, which will be small. If a tail rotor failure occurred and the fuselage began rotating because of this before the engine failure, then the fuselage will remain in rotation after the engine failure and will slowly lose rotational speed as the main rotor bearing friction acts on it.


In the UK, which ATS unit would be responsible for CTR entry clearances? In the UK, which ATS unit would be responsible for CTR entry clearances? Would the approach controller issue them, or would the approach controller need to co-ordinate a clearance from the aerodrome controller? If the former, I assume the approach controller would pre-note the aerodrome controller?

The answer depends on which aerodrome you are talking about, and can be found in the UK AIP: http://www.nats-uk.ead-it.com/public/index.php%3Foption=com_content&task=blogcategory&id=165&Itemid=3.html

Click eAIP to open the electronic AIP. In the left menu, open the AD 2 Aerodromes section and navigate to the aerodrome in question. Then open the AD 2.17 section of that aerodrome.

In AD 2.17, airspaces related to the specific aerodrome are listed. Column 1 contains the airspace designator and lateral limits. Column 4 contains the responsible ATS unit.

As an example - the Birmingham (EGBB) CTR is controlled by Birmingham Approach.


How is a glider recovered and returned to an airport after landing in a field? If a glider has a safe normal landing in a cow pasture or other farm field and has no damage, how is it typically recovered to be returned to an airport for a future flight?  Are the wings designed for quick/easy removal and the wings and fuselage are just hand-carried to a long trailer waiting on a nearby road?  That's kind of how it's done (I believe) with balloon landings, but not sure what the process is for gliders.

All gliders can be disassembled to take off wings and elevator.

For each glider type there is a car trailer into which the glider fits after disassembling.

Here you see the “reverse” process (assembly):


What is this NASA UAV, and what is its function? For the question What's up with the “What's Up at Wallops” app? I was reviewing the What's up at Wallops website and drifted to the launch photos page where I saw this aircraft. I did not find a caption, and a quick reverse image search in google did not return any helpful results.

I am fairly confident that it is not a "spaceplane".

What is this NASA UAV, and what is its function?

This is a Northrop Grumman RQ-4 Global Hawk. NASA has done some research on civilian applications for drones and this one appears to be for storm research. You can find more info on the project here.


Are the A380 engines interchangeable (given they are not all equipped with reverse)? I have noticed that the A380 only uses the two engines closest to the fuselage to reverse thrust to brake. Are all four engines the same (i.e.: could they theoretically swap engine 1 and engine 2) or are the inside ones different from the outside ones?

In general, would the outside ones have reverse capabilities if they were installed on the inside or are they different types of engines?

The Airbus A380 comes with two engine choices:

Rolls-Royce Trent 900
Engine Alliance GP7000

The thrust reversers are not part of the core engines. The reversers are developed by the company which produces the engine nacelle, in this case Safran Nacelles. They developed nacelles for both engine variants for the A380, including the first electrically actuated reversers:

  Safran Nacelles nacelle systems equip the Airbus A380 powered by four Rolls-Royce Trent 900 engines or four Engine Alliance GP7200 engines. [...] they are the first in the world to have a thrust reverser with an Electric Thrust Reverser Actuation System (ETRAS®).

(source: safran-nacelles.com)

(image source: safran-group.com)

Are the nacelles interchangeable?

No, the nacelles for the outer engines are different from the inner ones to save weight. This article says:

  Four nacelles were developed for the A380: one inboard nacelle (with a thrust reverser) and one outboard nacelle (without), for each of the type's two engines, the Rolls-Royce Trent 900 and Engine Alliance GP7200.

As ymb1 pointed out in the comments, this is confirmed by the type certificate for the Trent 900:

  At engine certification, the engine has been approved for use with Aircelle Thrust Reverser Unit
  (TRU) at the inboard engine positions (part numbers ASE 0010-XX-0 for the left hand installation
  and ASE 0050-XX-0 for the right hand installation) and for a Fixed Fan Duct (FFD) in the outboard
  engine positions (part numbers ASE 5010-XX-0 for the left hand installation and ASE 5050-XX-0
  for the right hand installation).
  The TRU and FFD do not form part of the engine type design and must be certified as part of the
  aircraft type design.

and also for the GP7000:

  This engine is approved for use with Airbus thrust reverser system P/Ns (left and right)
  L78DR130000xxx / L78DR230000xxx and with Airbus fixed fan duct P/Ns (left and right)
  L78DR140000xxx / L78DR240000xxx.


Do any other nations have an equivalent of the "Air Force One" call sign? Air Force One is the official air traffic control call sign for a United States Air Force aircraft carrying the President of the United States. - Wikipedia

Do any other nations have an equivalent call sign for an aircraft carrying their head of state?

It is all in your link (in the "See Also"), E.g.:

Brazil: https://en.wikipedia.org/wiki/Brazilian_Air_Force_One
France: https://en.wikipedia.org/wiki/Cotam_001
India: https://en.wikipedia.org/wiki/Air_India_One
Japan: https://en.wikipedia.org/wiki/Japanese_Air_Force_One
Vatican: "Shepherd One", or just in Italy "Volo Papale #" (https://en.wikipedia.org/wiki/Air_transports_of_heads_of_state_and_government)

Note: Often it is better to disguise who the airplane is transporting (especially when flying above a lot of different countries), and often the president has not so much power ("nuclear football"), so a hired jet it is enough, no need to communicate home.


Would the twin engine Boeing Y3 be able to compete better than the Airbus A380? The low sales performance of the quad engine Airbus A380 and declining sales of the Boeing 747-800 have largely been attributed to lesser fuel economy of quad engine aircraft when compared to their newer more fuel efficient twin engine counterparts.

The image rendering of the future Boeing Y3 aircraft shows a jumbo size twin engine twin deck aircraft the size of an Airbus A380.

Whether or not a prototype was ever made, in light of fuel economy technology improvements more prevalent and effective in twin engine configurations like the GE9X, would the twin engine Boeing Y3 be able to compete better than the quad Super jumbos and Jumbo respectively the Airbus A380 and the Boeing 747-8?

The answer is quite simply, no, at least, not if it is or was intended to compete as an airliner of roughly the same size.

That's because your question is based on a false premise, that the struggles of the A380 in the current market are due to its fuel economy.

The A380 is indeed uneconomic to operate except on few routes, but that is because of its size and capacity - which on most routes, can't be filled.

The market has changed since the A380 was conceived and launched; airlines are using more and smaller aircraft with more connections, between more minor airports.

To a lesser extent, new regulation has made a difference too. It has allowed smaller aircraft with twin engines - which are more flexible in an airline's operations - to take on routes that previously were considered suitable only for four-engined aeroplanes.

The technology and fuel consumption of the A380 are perfectly adequate to serve for decades into the future. It has been overtaken not by technological advances in its rivals, but by forces that change much faster than technology can: market conditions, and regulation.

Aircraft that will be selected instead of the A380 by airlines won't be chosen because they have better fuel economy (and even less so, marginally better fuel economy because they have two engines rather than four) but because they fit market needs better.


Do propellers have any lead/lag mechanism? The fan blades in a fanjet are mounted loosely in a fir-tree shaped mount so they are free to lead or lag and balance themselves. The blades on fully articulated and semi-rigid helicopter rotor heads are mounted with a drag hinge that allows them to lead/lag as needed. Do any propeller aircraft have a method to allow the blades to lead/lag, or is this for some reason unnecessary?

The two things are completely different and exist for different reasons:

Fan and turbine blades are mounted loosely, so they can self-balance. This is because at the very high RPM fans and turbines spin at they are extremely sensitive to imbalance. On the other hand, the self-balancing only works at high RPM. It also requires the blades to be connected at the tips with the “shroud”.
Rotors have lead-lag hinges because the rotor spins parallel to the flow so in forward flight the advancing blade  meets the air at significantly higher speed than the retreating blade.

Propellers spin slower, so they are not as sensitive to imbalance, and they don't have the shroud, so they can't be mounted loosely anyway. They spin perpendicular to the airflow, so all blades meet the air at similar speed and angle of attack, so they don't need to deal with as big differences in the aerodynamic forces. And they are smaller and more rigid.


What are the primary and secondary flight controls for the SEPECAT Jaguar? What lift augmentation devices are used in the SEPECAT Jaguar? I couldn't find any information.

According to  airvectors.net, the flight controls of the SEPECAT Jaguar are:

The Jaguar A had a high-mounted wing with a sweep of 40 degrees and an anhedral droop of 3 degrees, with relatively small wing area for a smoother ride at low altitude. There was a leading-edge flap along the outboard section of each wing, and large, double slotted trailing edge flaps on both the outboard and inboard sections of each wing. There was a "dogtooth" between the outboard and inboard sections of the wing.
There were no ailerons, roll control being provided by a two-section spoiler on top of each wing, just forward of the outboard flaps. There was a prominent "fence" between the inboard and outboard sections of the wing. Along with a conventional tail assembly, with all-moving one-piece tailplanes, there were two ventral fins. There were twin hydraulically-operated perforated airbrakes ahead of the ventral fins, under the wings. The hydraulic system was dual-redundant.

(Emphasis mine)
The omission of ailerons from a ground attack fighter might seem odd, as one might presume this makes the plane less  maneuverable. But, as described in the first sentence of the quote, the Jaguars main mission was to (or at least it often had mission critical sections with) flying fast and low, which often translates into a very choppy ride. This was mitigated by the use of a smaller wing = higher wing loading. To achieve low enough approach and landing speed with such a small wing, efficient and large flaps were needed. As sufficient and consistent roll control is somewhat hard to achieve with flaperons (or at least it was when Jaguar was designed), the Jaguar had to make do with full span flaps and use spoilers for roll control.


What is Lower Traffic Area (LTA) airspace in France? In France, AIP ENR-2.1 subsection 2.1.2 defines airspaces of type CTA (control area), UTA (upper traffic area) and LTA (lower traffic area).

I'm not familiar with LTA which appears not to exist outside of France and isn't part of AIXM neither. Can anybody fill me in on how LTA differs from CTA and how it could be coded in AIXM?

@mins Has answered the question in his comment above:

  LTA: FL 115 to 195, UTA: FL195 to 660" (Wikipedia). Created in 1992, a LTA is a class-D CTA (E over Alpes and Pyrénées to allow VFR gliders) between and excluding other CTA (airways and TMA), and up to 12 NM of the shoreline. I've highlighted a class-E LTA nearby Perpignan TMA and the Spanish border on this SIA map. Synoptics of the French airspace (source).


How do you use a flexible drive shaft? Saw it on a 747 freighter nose door, see it again on an F16's M61A1 gun assembly.  How do you design with a flexible shaft? In what situation do you use it to replace a bunch of rigid shafts and gears? Does it provide the strict angular synchronization of normal rigid driveshafts? Or is the synchronization only approximate, and can only be used in situations like either the 747 nose door or M61A1 gun-to-drum drive where there is only a requirement for a rough 1-1 translation of motion and power and no real demand to perfectly synchronize? Can you guarantee that if there is a constant speed revolution in one side you would get a constant speed revolution in another?

Flexshafts are used on a lot of flap systems to interconnect an array of screwjacks.  They are more or less aerospace versions of weedeater shafts, inside a hydraulic line style casing (typically stainless braid over a teflon liner).  They are used because the envelope and clearance requirements are much easier to meet than with solid shafts with angle drives and gearboxes.  You can route them through very tight spaces and around gentle corners without having to maintain clearances to adjacent parts; you just have to keep the flexshaft housings from chafing on adjacent parts, like hydraulic lines.

Flexshafts aren't meant to take high torque like solid drive shafts and are normally used in high RPM low torque applications.  Or better to say that if you want to use a flexshaft, you design your drive system to run at high speed/low torque.  A drive line might be designed to run at 500-1000 rpm with solid shafts and a moderate gear reduction at the actuators, whereas a flexshaft will run at 2 or 3000 rpm at much less torque, but with a much higher gear reduction in the actuator to get the same output force result.

Flap system flexshafts as used on CRJ200s and Embraer 145s will only have to sustain a little less than 100 inch pounds of torque at operating load, while producing several thousand pounds of force at the end of each screw jack thanks to the gear reductions in the actuators.  You get some torsional displacement, or wind-up twist, from one end of a drive line to the other, maybe 1/3rd of a turn, but with the high speed and large number of revolutions (say 800 to 1000 turns) to cover an operating range, a third of a turn of wind-up doesn't have a major effect on position sensing, so synchronization isn't a problem.

It'd be the same deal on a 747 nose door.  The flexshaft connecting the door actuators runs at a high enough speed so that synchronization down to a fraction of a turn is not really necessary because each turn is a very small part of the total motion.  Cascade thrust reversers on the CF-34 engine are also run by screwjacks interconnected by flexshafts; same thing there.

The big downside to flexshafts if used in exposed areas is sensitivity to contamination through imperfectly sealed connections.  Also the carbon steel wire windings tend to suffer from fatigue where they are crimped at the terminals.  There is a lot of back and forth flex as they spin.  They usually start to break down as individual strands fracture, and the bundle swells out and jams in the casing or sometimes lets go completely.


why are there no production canard GA aircraft? You see a lot of experimental canard GA aircraft, and their performance characteristics seem great. Why don't we see any production canard designs?

EDIT

For clarification. I'm asking about small airplanes, competitors to the Cessna 172 or Piper Archer in size. I'm also asking about airplanes that you can purchase from a manufacturer, like the above aircraft. An example of a company attempting to eventually make a small canard style production GA aircraft is Raptor Aircraft

Why would you fly with a deployed spoiler when you can have a clean aircraft? Sure, there are many canard kit plane designs, but when you look closer at their performance, its is not improved by the location of the elevator. Burt Rutan would always choose a higher wing loading than required to fulfill the FAR part 23 requirements for low speed, so his designs could cruise faster. When asked about this, his usual response was "**ck the FAA!".

When you look closely, canards will give you:

With natural pitch stability a higher lift coefficient on the forward wing which also needs to have some margin for control surface deflections. Consequently, the big, rear wing is operating at a fraction of its potential when the aircraft stalls.
The vorticity from the front wing creates a downwash at the mid section of the rear wing and an upwash at the outer sections. The strength of these induced speeds varies with speed, so the main wing can produce an elliptic circulation at only one angle of attack. At other angles, its induced drag is higher than what is possible with a conventional configuration.
The center of gravity is ahead of the main wing, so the landing gear needs to retract into the fuselage, taking space away from the cockpit. To move the wing center section forward, so it can accommodate the landing gear, the wing needs to be swept (which creates a host of problems of its own).
There is no good position to put a vertical surface, so again sweep is used to shift some part of the airframe backwards and combine the vertical with winglets. Those need to point upwards, so they do not hit the ground during take-off and landing when the better winglet would point downwards.

Canards start to make sense when they are unstable, but this is not a good approach for a GA aircraft.


Why use jet engines instead of propellers when designing new airplanes? I'm looking at the 100 people project and I am looking at clean airplanes. Why did we stop using a clean source of lift, the propeller?

Look into Propulsive efficiency (and maybe also this one).  Turboprops have a speed of peak efficiency, which is around mach 0.6 or so.  If you try to go faster, it gets significantly less efficient.  A high bypass turbofan, on the other hand, is most efficient at a higher speed, maybe mach 0.8 - 0.85.  So the simple answer is: we switched from props to jets because people wanted to fly faster but didn't want to burn a lot of extra fuel to do that.


Why do aircraft use hydraulics instead of pneumatics? I was reading about how the F-35 is having trouble with the "fueldraulic" system. This made me wonder:

Why don't aircraft use pneumatic systems instead of hydraulic/fueldraulic systems?

There are several advantages of pneumatic (compressed air) over hydraulic (pressurised liquid) or fueldraulic (pressurised fuel) systems:

No need to carry hydraulic liquid. That saves weight and maintenance cost.
Half as much piping, as there is no need to return hydraulic liquid to the pump - compressed air can be released after use. That saves more weight.
Leaks are less dangerous. There is no danger of running out of hydraulic liquid or of leaking fueldraulic liquid catching fire.

There are disadvantages too: less pressure -- which means less power per unit of gas/liquid -- and less precision, as gas is compressible. But I feel like these problems should be solvable in a modern computer-controlled aircraft. So what's the problem?

The big disadvantage here is the loss of precision due to the high compressibility of gas compared to liquid. Because gases are highly compressible, they provide a buffer to changes in pressure commanded by the operator to move the piston in the cylinder. That poses two problems; first, it means that the pneumatic cylinder doesn't respond instantly to pressure differentials, because the differential must first overcome the cylinder gasket's static friction. Second, it means that the movement of the cylinder is more easily opposed as long as whatever force opposes the gas pressure can overcome said pressure without causing whatever the pneumatic system is controlling to fail.

To overcome these shortcomings, most pneumatic systems run at very high pressures, so that the pressure differential between the two halves of the cylinder readily overcomes static friction and any other opposing forces. However, that creates another precision problem; high-pressure pneumatic cylinders are essentially two-state systems; the piston or actuator is typically at one or the other of its extremes of movement, and transitions between them very quickly as gas pressure is applied to one side or other of the cylinder.

None of these behaviors are desirable for aircraft controls; instructors labor daily to teach their students not to ham-fist the controls, instead using a bit of finesse to get the plane to do what they want in a smooth, controlled fashion. Why then, would you want to undo all that finesse with a control system that can only move the surface to the extremes of its travel?

Hydraulics, by contrast, allow a much higher degree of finesse. Because liquids don't readily change density, the pressure changes within a hydraulic cylinder require much more force to oppose, but by the same token, as the volume changes the pressure on the side being supplied with fluid decreases rapidly. This allows a hydraulic cylinder to be positioned much more accurately, regardless of any external forces acting on the system. The disadvantage is hauling a fairly heavy liquid up into the air, and having only limited capacity to replace it if any of it leaks.

Electrical actuators are a common solution to that disadvantage, especiually in light aircraft. Electrical actuators use an electric motor or servo to provide the mechanical action. These actuators can be controlled with a high degree of precision, and their "supply system" is just an electrical circuit, no heavy and complex hydraulic lines and cylinders. Their disadvantages are a tradeoff between speed of movement and maximum applied force while moving; you can either make an actuator that moves very quickly, or an actuator that will move no matter how much force is opposing the movement, but you really can't do both. They're still useful in light aircraft to control flaps (with a cable system used for the main surfaces), because they allow for precise amounts of extension or retraction, and don't have to instantly respond to input like the primary control surfaces do.

There is something on the horizon that could make pneumatics feasible for aircraft. Hydraulics systems were recently improved with the development of the electrohydraulic servo valve. This system uses a variable electrical potential (voltage) to move a hydraulic cylinder by a prescribed amount proportional to the voltage applied. Pure electrical servos have been around for decades, but the maximum amount of force available from a servo is insufficient for large airliners, while for smaller aircraft the servo motor's relatively high weight compared to simple cable controls is a disadvantage. The electrohydraulic servovalve concept is used in newer large aircraft to replace pure hydraulic or cable/hydraulic hybrid control systems, because the hydraulic system can now be controlled by an electrical circuit instead of hydraulic lines or tensioned cables coupled to the control column. This allows for "fly-by-wire" aircraft such as most Airbus airliners as well as most fighter jet designs of the last 40 years.

A similar concept is under development for pneumatics, allowing the precise placement of an actuator using pressurized gas in response to an electrical voltage. This would provide all the advantages of an electrohydraulic system, with considerably lighter weight and faster response, but still having the disadvantage that a significant opposing force could prevent movement of the actuator especially as it approaches the desired position. Whether that will be an issue in a large aircraft remains to be seen, and the weight savings of losing the hydraulic fluid might not be worth it, but if the tradeoff is acceptable, it would further increase range or payload of the next generation of passenger aircraft, with the added safety/reliability feature of being able to compensate for a slow leak in a pneumatic system by simply adding more air with a compressor pump.


How is airbrake function performed on the Dassault Rafale? At the end of airshow display (just before landing), the dassault rafale seems to slow down very quickly as it reach the top a final loop. It seems as if the pilot apply full airbrakes, but it seems there is no special flight control surface deployed to perform this function. How is this feature performed? Is it a special configuration of the split elevons? A combination of elevons and canard?

The Dassault Rafale has no dedicated airbrakes. By having two means of pitch control, namely wing elevons and canards, in combination with relaxed stability and an advanced FCS, it can move both into extreme deflection angles without sacrificing pitch control. This site states:

  When landing, both canards and trailling-edge control surfaces can be
  used for braking, and Rafale may be able to use canards for braking
  even while in flight.

Also, having little load to carry helps. The display aircraft was most likely flying close to its minimum wing loading (which is just 276 kg/m²), and by cutting back thrust could employ its regular aerodynamic drag for braking.


What is the main reason why cruise flight in the transition layer is prohibited? What is the main reason why cruise flight in the transition layer is prohibited? For example if we have a separation of 1000 ft between transition altitude and transition level, why should we not maintain a level in this layer of 1000 ft?

I know about transition altitude and transition level but what is the cognitive reason of this rule?

The main reason for the transition layer is to assure separation between the highest usable altitude (TA) and the lowest usable FL (TRL). That means it has to be at least 1000ft thick.
If the QNH is 1014 or above the transition level is always just 1000ft above the TA (5000ft and FL60) because the whole FL system is shifting upwards above the MSL. This means for each hPa above standard QNH 1013 your transition layer will be about 30ft thicker.

If QNH drops to 1013 or below the opposite happens: The base of the FL system falls below MSL, so FL000 is at -30ft MSL per 1hPa below 1013.

If QNH is 1003, the difference between 5000ft and FL60 is only 700ft because 1000ft-(10hPa*30ft)=700ft. In this case the TRL above TA 5000ft has to be FL70 to assure the separation between highest altitude and lowest FL is at least 1000ft.
So now we have a transition layer with one spare FL - FL60. If one would now fly at 6000ft we only have the above mentioned 700ft to FL70; if one would fly at FL60 only 700ft to 5000ft. This is why the transition layer may not be used for cruise flight


Do most commercial jets have a negative lifting body? I took a look at some example airfoils and noticed they all seem to have a downward hook at the end. I believe this draws the air downwards, creating a reaction force upwards: lift.

Now here is a typical wide-body jet profile:

Notice how the fuselage points upwards at the tail. I'm not talking about the vertical stabilizer, not the horizontal stabilizers either. I'm talking about the fuselage getting narrower and narrow at the tail. It tapers to a point, but the taper flows upwards.

I'm sure this shape has to do with avoiding tail strikes. But is seems to me like it also incurs negative lift. Does the tail draw air upwards, creating a reaction force in the downwards direction and thus negative lift?

You are right, the upsweep at the tailcone allows the pilot to rotate the aircraft during take-off and landing without touching the ground with the rear fuselage. When such a tail strike  happens, the aircraft needs to be inspected and repaired, and incorrect repairs after a tail strike have already caused loss of life.

While most Boeing fuselages show some downslope of the upper tailcone contour, this effect is even more pronounced with all Airbus aircraft except the A380. There the upper contour of the tailcone is straight. Military transports with a rear ramp show even more upsweep to keep the heavy ramp short, and most need additional strakes to limit the area of separated flow there at low speed.

This upsweep adds a small downforce and, since it happens away from the c.g., a pitch-up moment. First, at the transition from the cylindrical section to the tailcone you find a strong suction peak at the high curvature region on the lower fuselage. Also, the lower side of the tailcone has a lower pressure than the upper side. Both create the downforce and the pitch-up moment. However, a three-dimensional body of a very low aspect ratio like the fuselage has a very low lift curve slope, so even the strong upsweep creates a small moment when compared to the contribution of the horizontal tail surface.

The local flow at the tail is dominated by the wing wake. To create lift, the wing has to deflect the flow downwards, and this produces a local flow direction at the tailcone which aggravates the situation created by the upsweep. The best indication for the local direction of flow is the rain diverter above the rear fuselage door. See below the rear door of the A320: To create the lowest possible drag, the rain strip is aligned with the local direction of flow during cruise. When flying at lower speed, the downwash is even stronger.

A320 rear fuselage (own work, based on this source)


How is altitude correlated with true airspeed? I am trying to understand the BADA 3.0 dataset (specifically I am looking at Performance Summary Tables):

E.g.

What is not obvious to me is why those tables mention only single TAS for specific FL.

Is it true that for a given specific aircraft there is a single specific TAS that aircraft can travel on given FL (due to wings lift forces produced, etc)?

I thought aircraft are capable of traveling with a range of speeds at given altitude depending on Angle of Attack (AOA).

Or maybe what I am looking at is just a summary table that does not cover all possible values, however, those are the only one FL+TAS pairs that makes sense in practice (as flying otherwise would be highly inefficient).

(Airbus via SKYbrary)

Is it true that for a given specific aircraft there is a single specific TAS that aircraft can travel on given FL (due to wings lift forces produced, etc)?

Above shows a common climb profile, and is the best illustration to the main title question. As you can see, what the plane would typically be programmed to target is IAS or Mach.

Related to that point and graph:

How is the airspeed-Mach number transition handled in modern airliners?
What is the reason for changing the speed reference (IAS or Mach number) with altitude?
How did ICAO SARPS come to include the 250 knots speed limit for aircraft below 10,000 feet?

I thought aircraft are capable of traveling with a range of speeds at given altitude depending on Angle of Attack (AOA).

There are ranges of speeds, but the economic range (see: What is Cost Index?) is not that big (approx. ±20 knots), and the dataset you have simply shows the typical value ATM can expect, for simulation purposes for example as the website says:

  The BADA APM is designed for simulation and prediction of aircraft trajectories for purposes of ATM research and operations. Aircraft performance parameters and trajectories can be calculated using information and data contained in BADA.

The filed flight plan would also include the planned TAS for the different flight levels, and as long as the pilots don't deviate by more than ±5%, they don't need to notify ATC.

Or maybe what I am looking at (...) the only one FL+TAS pairs that makes sense in practice (as flying otherwise would be highly inefficient).

In essence, yes. Also worth pointing out is that a lightly loaded jet-liner would typically aim to fly higher if permitted and if possible. The same is true even for short-haul flights (What should be the minimum time spent in cruise (for e.g. a B737)?). So basically optimization is solved by aiming for the optimum FL.


If unable to procure Helium, are there any regulations against filling lighter than air craft with Hydrogen instead? The price of helium is going up again, and it is very difficult to find suppliers without established contracts.  If all possible safety measures are taken, are there still regulations by FAA or other that would prohibit use of hydrogen instead for buoyancy in lighter than air non-thermal aircraft?

The causes of the Hindenberg fire are well documented and studied so lessons learned such as sudden grounding when aircraft surface has accumulated significant electrostatic charge can be avoided through measurement and continuous static dissipation techniques used on current aircraft.

The Hindenburg utilized aluminum flake in a nitrocellulose impregnated fabric.  Materials have improved since then.

In the case of He, diffusion is inhibited by polar surfaces.  For the He to diffuse through the membrane in a balloon, it has to disolve into the fabric layers prior to a barrier such as aluminum. I understand that lithium and beryllium foils have higher impermeability.

In the case of He, poly(ethylene terephthalate) is a reasonable barrier, and PET is commonly available in films (and soda bottles).  It is rather polar, and serves to reduce solubility.

H is a different bird, in that it has low polarity, and different materials will aid in reduction of solubility. Butadiene-acrylonitride co-polymers have low polarity and may perform better with a low polarity gas such as H2.

The gas envelope questions are really materials science questions, and require a fair understanding of not only the mechanical properties, but even more so of the permeability, solubility and diffusivity of the copolymer you might consider.

The legality of using H2 rather than He2 may not be an issue, but even if it were, I am sure that waivers are possible.

A quick check of regs on gas balloons indicates that hydrogen may be used if the aircraft manufacturer approves the gas.  So it appears there is no hard regulatory bar.

Checking with a source at NWS, who indicates their balloons are launched with H2.


Do Air Traffic Controllers have to remember stall speeds for different aircraft? While dealing with Takeoffs and Landings controllers provide speeds to pilots for different phases of the flight, specially during a landing approach.

How do they know which speeds are safe for all the different models operating to and from their airport? Are they provided with that data daily or they have to remember those speeds?

If a general answer is not possible I would be more interested in knowing about stall speeds during a landing approach. Obviously they will be different for different aircraft in different conditions.

No because aircraft are categorized by their speed at the runway threshold (1.3 times stall speed).

VAT —Speed at threshold used by ICAO (1.3 times stall speed in the landing configuration at maximum certificated landing mass)

By knowing the category, ATC is able to use appropriate speeds. The category is not actually listed anywhere, so the controller will have to know the approach category by heart for each aircraft type. E.g. if the controller sees "B738" as the aircraft type in the flightplan, they have to know that the corresponding approach category is C.

More:

ICAO Doc 8168 PANS-OPS Vol 1
14 CFR Part 97.3 (for the US)


Do some light sport aircraft have clipped wings to lower their cruise speed? Light sport aircraft may have a max airspeed in level flight at MCP of 120kts. I have heard that there are some light sport aircraft that have had their wings clipped in order to have a lower cruise speed.

But why do they clip the wings to reduce the cruise speed? Wouldn't that require a higher speed as less lift is produced at the same speed with less wing area?
Also, thinking about gliders having a big wing span and fighter jets having a short wing span I can not see why you would clip the wings? What am I missing?

You must have heard wrong. Clipping wings is done to reach higher maximum speed.

You mention light sport aircraft, so I assume they have propellers driven by piston engines. This means their power output is constant over speed and the thrust is inverse with speed. To calculate their optimum cruise speed, you can use the venerable Breguet equation which shows that the aircraft has to fly at its optimum L/D for maximum range. At this speed, induced drag equals zero-lift drag, and span helps to cut down the former. Clipping the wing will shift the optimum cruise speed up, not down.

With clipped wings the aircraft has less surface area, which will reduce zero-lift drag. This is the drag component which determines maximum speed. Induced drag at a given speed will go up, but it is insignificant at maximum speed, so the aircraft will be able to fly slightly faster.


Do I understand the rules of logging cross-country time as a Safety Pilot correctly? Let's say there is a private pilot ("primary pilot") practicing instrument training under the hood and a private pilot who is a Safety Pilot, and the flight involves a landing at an airport more than 50 nm away in VFR conditions.  I want to see if I have this right:

The primary pilot can log instrument time and cross country time, but not PIC time.
The Safety Pilot can log PIC time, but not cross country time or instrument time.
If flying a high-performance and complex airplane, both the primary pilot and Safety Pilot must have a complex and high-performance rating.
The symbiotic benefit of this system is that the primary pilot gets instrument time and the Safety Pilot gets PIC time, both at half price if they split the cost of flying.

According to FAA regulations, do I have this right?

As background, see this question for a general explanation of how safety pilots can log time. Also have a look at the Gebhart (2009) interpretation for the cross-country part (and see this question too).
Note that the two key things to keep in mind in these logging scenarios are:

Acting as PIC and logging PIC time are two different things
You have to be very clear about who is the acting PIC for the flight

With all that in mind (and referring to 14 CFR 61.51):
The primary pilot can log instrument time and cross country time, but not PIC time
The primary pilot can log all three. Whether they're the acting PIC or not, they can still log PIC time as the sole manipulator of the controls (61.51(e)(i)). They can also log instrument time while they're under the hood (61.51(g)) and cross country time.
The Safety Pilot can log PIC time, but not cross country time or instrument time
If the safety pilot is acting as PIC then that's correct. They can log PIC time because they're acting as PIC "of an aircraft for which more than one pilot is required under [...] the regulations under which the flight is conducted" (61.51(e)(iii)). If the safety pilot is not acting as PIC then they can log SIC time.
But either way, the safety pilot may not log cross country time (Gebhart). They also can't log instrument time because they were never operating "the aircraft solely by reference to instruments under actual or simulated instrument flight conditions" (61.51(g)).
If flying a high-performance and complex airplane, both the primary pilot and Safety Pilot must have a complex and high-performance rating
Only the acting PIC requires those endorsements (not ratings), per 61.31(e) and (f). So if only one of the two pilots on board has them, that's the only pilot who can act as PIC.
If only the primary pilot has the endorsements the flight can still go ahead because a safety pilot isn't required to have them. However, the safety pilot couldn't act as PIC and could only log SIC time.
If only the safety pilot has the endorsements the primary pilot can still log PIC time because you don't need the endorsements to log PIC time. But the safety pilot must act as PIC.
The symbiotic benefit of this system is that the primary pilot gets instrument time and the Safety Pilot gets PIC time, both at half price if they split the cost of flying
Essentially, yes. Typical practice is for the pilots to agree that the safety pilot will act as PIC, and they split the costs 50/50 (61.113(c)). That way they can both log PIC time - which is valuable for further ratings - while only paying 50% of the costs. This question has more information on cost-sharing.
And thanks to JScarry for pointing out that if the safety pilot has BasicMed rather than a regular medical, they must act as PIC. See this question for more details.
Whatever you do please be sure that both pilots do know and understand who is acting PIC for the flight. That person has the final responsibility if something goes wrong. Knowing who's acting PIC is important to prevent things like a safety pilot without a complex endorsement agreeing to act as PIC in a complex aircraft, because "I'm just a safety pilot". You might also have insurance or flying club rules on who can act as PIC.


How did Airbus A380 overcome corrosion issue in which aluminium wiring used? The corrosion of aluminium is generally recognized to be a major problem. What new technologies or technics might be used in aluminium wiring to get away from the corrosion issue?.

The mechanisms by which aluminum corrodes are well-know and the measures needed to prevent it are similarly known to design engineers.

Aluminum corrosion via direct attack is an issue where the aluminum is exposed to air and the air contains moisture, and is made worse when salts are present in the environment and at certain pH ranges. It is prevented by encapsulating the aluminum in either an impervious coat of paint or plastic or a coating of intentionally-grown aluminum oxide. In an airplane, the insulation on the wire will prevent corrosion by direct attack.

Aluminum corrosion by galvanic action occurs when the aluminum is bonded directly to or in contact with a piece of metal that has a different position in the galvanic series, and the joint is wet or at least moist with water. You prevent it by excluding water from the joint by sealing the joint with glue or enclosing it in a housing that is airtight, and by coating the joint with a grease or oil that excludes water. In an airplane's wiring system, this would be done with a plastic plug-and-receptacle for the joint that keeps water out.

Electrical fires in wiring system connectors that contain aluminum wire joined with copper wire occur when the physical joint between them is vibrated or worked back and forth, which scrapes the native oxide off the aluminum in the joint, promoting more oxide growth which is scraped off, etc. which eventually leads to loading the joint with aluminum oxide and creating high contact resistance between the aluminum and the copper wire at the joint. I-squared-R heating at the degraded joint then sets the insulation on fire. This can also happen by temperature-cycling the joint which will cause the joint surfaces to be worked back and forth and once the joint begins to show resistance, then the heating occurs right inside the joint and the process accelerates itself.

In airplanes, houses, and cars where aluminum and copper wire must be joined, a special connector is used where the metals meet in which the crimp joint is exceptionally tight and the joint is loaded with a grease or chemical paste containing inhibitors that halt the oxidation of the aluminum.


What is that hole on the front portion of the empennage of a CRJ-1000? I've noticed that there is a hole on the front tip of the "dorsal fin" of the CRJ-1000. I've been thinking that it would be logical if that's the intake for its APU unit. But I'm not really sure.

Image source: https://en.wikipedia.org/wiki/File:Garuda_Indonesia_Bombardier_CRJ1000_@BPN.jpg

It is the Ram Air Inlet Duct for the cabin air system. It normally acts as cooling air for the packs but can also be used as an alternate vent source in an emergency.

  The cooling efficiency of each air conditioning pack is enhanced by
  ram air cooling via a heat exchanger. Ram air enters through an inlet
  at the abse of the vertical stabilizer and passes through the pack
  heat exchangers and then exits overboard. In case of failure of both
  packs, ram air can be used as the sole source of ventilation. This is
  accomplished by opening the ram-air valve via the RAM AIR switch. Ram
  air then enters the occupied areas through the normal vents.

(source)


How can two planes be on the same taxiway facing opposite to each other? Curious to know how two planes can be on the same taxiway facing opposite to each other at the same time? Please have a look at the satellite view of the following coordinates in Google Maps:

(-27.369843, 153.126482)

The taxiway is being used for parking. Specifically, parking of two MD-80s that have attracted press attention—The Australian: Clive Palmer’s $10m in tax-haven jets sitting idle:

  TWO 155-seat jets owned by Clive Palmer and branded with the name of
  his controversial and longtime loss-making company Mineralogy sit
  mothballed in a remote corner of Brisbane airport.

  The McDonnell Douglas MD-82 jets, worth roughly $5 million each, are
  registered in the Cayman Islands tax haven and have sat mothballed for
  months, possibly years.

  ...

  A spokeswoman from Brisbane airport confirmed the planes were owned by Mr Palmer and had sat idle for “at least several months”, but was unable to confirm exactly how long. “The space is leased from Brisbane airport and the owner pays leasing fees to have them there,” she said.

That article from 2014 shows them parked head-to-tail, but presumably they've been moved since then (the Google maps imagery is from 2017).

If you're wondering how I found this, I zoomed in on the satellite image and used 3D mode to get a better view of the side of the aircraft well enough to sort of make out the "Mineralogy" label on the side, and Googled from there.


Is it possible to perform the Lomcovak manoeuvre in a C152 aerobat? Has anyone performed a Lomcovak manoeuvre in a C152 aerobat, is it even possible, or would that likely overstress the engine?

The list of approved maneuvers is in the POH, and I'm sure the Lomcovak is not listed. If you are willing to do unapproved maneuvers, the issue is that because the aircraft tumbles rather than flies in this maneuver you have a few things to worry about: it starts with an outside maneuver, it is inverted, and the exit requires a high-speed dive which will exceed Vne or the +6g the 152 Aerobat is rated for.


Do planes turn while climbing? Is this because of air characteristics variation? I've developed the impression that often commercial planes will bank and turn as they climb across thin cloud layers that appears to be stratus or stratocumulus.

I think these clouds indicate a separation between different air masses. So my question is two-fold:

Are planes actually doing this or is it a coincidence?
If not a coincidence, what is the reason? and what would happen if they didn't?

Welcome to Aviation SE.

There is no need or reason to turn when going through clouds.  Aircraft frequently climb (or descend) and change direction at the same time.  If it happens in cloud, it's just a co-incidence.

As an example, on an approach, ATC often issue instructions such as:

  Turn left heading 300 degrees, descend and maintain 4000 feet.

The pilot will start turning towards heading 300 and start descending at the same time.

  Resume own navigation, climb flight level 370.

Following a previous instruction to deviate from the heading on the flight plan, the pilot is now cleared to turn back onto the planned track and climb to the planned flight level.

There is a high probability that you will see aircraft turning in clouds since arrival and departure routes, which is when most of the turning, climbing and descending is done, are when you are most likely to see aircraft in and out of clouds and manoeuvring.

In cruise, when an aircraft turns, it is also common to descend or climb since the altitude flown according to the heading is determined by the quadrantal and hemispheric rules.


Are General Aviation tires filled with Nitrogen? Long story short, I'm working on a concept for a book.
We all know why commercial aircraft have nitrogen tires, but my question is if GA aircraft have nitrogen tires, or are they just filled with helium, oxygen or any other gas? My theory is that commercial aircraft are heavier, so probably need nitrogen tires, whereas general aviation aircraft are much lighter, so wouldn't be needed to withstand as much surface friction, meaning it may not need a more heat-prone gas for the tires, like nitrogen.

Although I can’t speak for every GA pilot, the aircraft that I have flown have had their tires inflated from an ordinary air compressor. We have used handheld battery operated compressors, floor standing compressors, small-tank mobile compressors, and hand carried compressed air storage tanks.
The max speed that we actually use the tires is under 100 mph. The max weight that we carry on them is under 6000 pounds.


Why do brushless motors heat up? I would like to know what makes a brushless motor heat up. I'm aware of the problems generating the high temperature such as wire contact problems or overloaded motors. I want to know what makes a normal motor in normal conditions warm up. Is it related to the Joule effect?

The motor is converting electrical energy in mechanical energy, and unfortunately all energy conversions lose some energy to heat. This site states the cause of the heat losses as:

Friction losses due to bearings etc.
Windage losses due to air resistance.
Iron losses caused by magnetic paths of the motor (hysteresis losses and eddy current losses).
Ohmic losses from current flow.
Stray losses: other


Why are early wing planforms mostly rectangular? I am looking at the history of wing planforms, and I am having trouble finding reasons for why the wright flier and other early aircraft were built with rectangular wings. I thought it was due to being more structurally secure, but I can't find any sources that explicitly say that, or describe the thought process of early designers.

Is my reasoning correct, and are there any other reasons for the decision to use a rectangular wing (vs a tapered wing)?

The benefit of tapered wing lies in its proximity to the elliptical lift distribution while retaining much of the structural benefit of a rectangular wing. But we owe this knowledge to a few things:

The Kutta-Joukowski Theorem: published in 1906 by Nikolai Y. Joukowski and influenced a great deal by Martin W. Kutta.
The Lifting Line Theory: published in 1919 by Ludwig Prandtl and inspired by the work of Frederick Lanchester. The term induced drag was coined by Max Munk, a colleague of Prandtl in 1918. Although Lanchester had published some results on aspect ratio and finite-wing aerodynamics as early as 1907, they weren't taken seriously by his compatriots and had very little impact as a result.

The Wright Flyer flew in 1903. By all accounts, the Wright brothers designed it through trial and error. As a result, the insight of tapered wing couldn't have been known to them during the Flyer's design and through much of the First World War.

In any case, most of the early airplanes were bi/triplanes with external struts being their structural members. The drag from the struts would've overwhelmed any benefit gained from tapered wings.


Does the pilot still have final control authority on modern Boeing airplanes? In this answer it is said that Boeing (contrary to Airbus) gives final authority to pilots, but the example is about a 747-100(SP). This answer states, that modern Boeings have have a lot of fly-by-wire systems too. After reading over 10 (I think) Aviation.SE questions about the B737Max and MCAS it got me wondering.

How much authority does the pilot have or can the pilot get in modern Boeing planes? (as in how much does fly-by-wire/other electronics involved ignore/adjust/overrule pilot input (or independent commands) in each mode or law.)
Is there a difference between models? (for example 747-8i vs 787)
And how do the laws switch on their on vs pilot forced?

Disclaimers:

I am not interested what you think is better or are used to, or design philosophy, or stick vs control column.
I appreciate data about 737NG too but consider it an "old" model.
I am interested in 777X too but unsure how much definitive data there is.

Correct me if I got my facts wrong and please be gentle, I am a noob.

I think you fell for a common misunderstanding:

Fly by wire ≠ flight control augmentation

It is - in the first place - another method to get the pilots input out to the control surface just like cables or pushrods. Just not mechanical but electric/digital.

If you want to alter the pilot input for whatever reason it is way easier with digital signals then with a mechanical input.

Example: The speed brake in an A320 vs in a B747. If you rapidly extend the speed brake in an A320 aircraft, the extension rate is limited by a computer making for a smooth movement of the control surface. If you do the same in a B747 the system tries to match the lever position as fast as possible with all available hydraulic power (3000 psi). So rip on the speed brake lever in a B747 and the heavy aircraft will jolt quite a bit.

Concerning flight control augmentation it really boils down to the design philosophy. In a B747 you can do basically whatever you want (even stalling) but in the B747-8 you have some systems augmenting pilots input. Tail strike prevention on rotation and the nose down trim when flying a flaps 30 approach come to mind. I can't speak for the B787 but the B747-8 is, despite its largely fly-by-wire architecture, still a conventional airplane with (almost) full control authority by the pilots.

The B777 family brings flight control augmentation to the table where the computer shows you the limits via the control forces generated in the control column. The computer shows you where it thinks you should stop but if you decide to press on you just have to double the force exerted on the control column and the aircraft will do what you want.

The Skybrary has a good primer article on flight control laws for both Airbus and Boeing designs. It gives a good overview about the similarities and the differences in design and philosophy. Shoutout to user Simon Opit who brought it up.


What is “Weather and Chaff” information provided to pilots by ATC? This one appears in an entry under "Additional Services" in the FAA Pilot/Controller Glossary and claims that ATC provides a wide range of additional services to pilots, including "Weather and Chaff" information, which seems like an unusual entry.  What does "chaff" mean in this context?
Originally chaff was the remains of a wheat plant cut down and with the grains in the head thrashed out, which is then discarded.  It's also a biblical reference to the wicked condemned by God to hell i.e. "the chaff, which the wind driveth away".  In modern times, it is associated with a physical countermeasure to radar whereby thousands of metallic strips are ejected out of an aircraft to confuse enemy radars or break the track of radar guided anti aircraft weapons.  I though the phrase in the P/CG might have been a reference to some sort of anachronistic term about the environment which was termed "chaff" but I don't know.  I did locate this story from NOAA's website where their weather radar in Louisville, KY, was spoofed by military chaff which had been dispersed during nearby training exercises.  This gives though to another risk of an aircraft flying through a cloud of chaff and ingesting it into the engines, or causing some other kind of system damage.  This would seem to be a remote possibility but who knows.  Anybody know what the P/CG meant by chaff in this reference.

In the Pilot/Controller Glossary, this is the definition of "Chaff" and would be its meaning in the context of "Additional Services" referenced in your question.

CHAFF- Thin, narrow metallic reflectors of various lengths and
frequency responses, used to reflect radar energy. These reflectors,
when dropped from aircraft and allowed to drift downward, result in
large targets on the radar display.

On some radar equipment chaff may appear similar to areas of precipitation.


Are red obstacle lights near an airport required to be lit during the day? What are the FAA Beacon Light Requirements for 80 foot tall High Mast Poles Near an Airport?  Specifically, do the RED beacon lights need to be illuminated 24 hours per day?  Or, do they only  need to be illuminated at night?

Here are examples of Holophane high mast pole installations around the country.

https://www.facebook.com/media/set/?set=a.236398096375560.78767.131284353553602&type=3

https://www.facebook.com/media/set/?set=a.194847107197326.56223.131284353553602&type=3

It's unclear exactly which lights you're referring to (none of the example pictures you linked to are red), but the FAA AC on obstruction lighting says that only structures over 200ft are required to be lighted:

  Any temporary or permanent structure, including all  appurtenances,
  that exceeds an overall height of 200  feet (61m) above ground level
  (AGL) or exceeds any  obstruction standard contained in 14 CFR part
  77,  should normally be marked and/or lighted.

However, there are exceptions:

  The FAA may also recommend marking and/or  lighting a structure that
  does not exceed 200 (61m)  feet AGL or 14 CFR part 77 standards
  because of its  particular location.

The same document describes the use of red lights:

  Aviation Red Obstruction Lights. Use flashing beacons and/or steady
  burning lights during  nighttime

The rest of the document suggests that red lights are for night use and white for day use. The FAA also has a dedicated portal for obstruction evaluation and reporting if you're involved in building or managing potential obstacles.


Do "water bomber" aircraft pilots have more relaxed rules? I have no exposure to aviation regulations, but I'm curious if some rules are different for emergency aircraft pilots.  Similar to ambulance drivers and police officers being allowed to speed or run red lights for the sake of emergencies.

Specifically, this video shows a water bomber very close the ground and bystanders during an approach:

My gut tells me two things:

A) that this plane is dangerously close to the ground, and

B) that this would be a serious incident, representing some sort of strike against the pilot's record.

Please correct me if I'm wrong about either point.  If either are true, though, is the situation treated differently for emergency pilots?  Are they trained to go that close to the ground, like police officers are trained to handle a car at higher speeds?  And are their rules different for that reason?

I'm not sure how much the answer would vary country to country, aircraft to aircraft, or even situation to situation.  So if the question is too broad, assume the United States or Canada even though that's not where the video is from, and assume a water bomber pilot in a situation just like this.

All aircrews have to comply with laws and regulations, unless they have a waiver that makes them exempt for certain regulations. Aerial Fire Fighting need to fly lower than the minimum safety altitude in order to pick up water, so they will have a written permanent waiver to do so.

This is similar to ATO's conducting training for student pilots. They can be allowed to fly below the minimum safety altitude (in Germany: 500ft) for the purpose of training engine failures and attempted emergency landings in fields.

In all honesty, it does not look like recklessness to me, it looks like the pilot miscalculated his approach and came in too low and with not enough speed, as he bounces off the water surfaces instead of doing a proper flare.


How to determine the maximum servo torque required for a nose gear? It's pretty much it. I know how to calculate the friction force on the nose gear for a three-point landing condition using the method set forth in Structural Loads Analysis for Commercial Transport Aircraft Theory and Practice, by Lomax. To calculate the torque I am looking for, would I just take the cross product between the friction and the radius of the nose gear?

Assuming that we are designing a steering system, for a single steering wheel aircraft.

The servo is yawing the wheel to a position. The position causes side force on the wheel. And that generates a yawing moment for the aircraft. (The aircraft can be moving in various directions, but assume you know the max side force required.)

This side force, multiplied with the distance of the wheel contact point and the wheel yawing axis is one major contributor to the servo torque. (Torque1)

Another contributor is the rotational friction of the wheel. Which is the resistance to yawing further. (Torque2)

Torque_total = torque1 + torque2.

This is the output torque for the servo mechanism. The mechanism will affect the torque on the servo shaft.


Why would an airport have a beacon light on when there's no traffic? I live in a small town (Lone Pine, CA) with a tiny airport that sees a couple planes a week at best... busy-ish days when a major fire is happening. Over the past month the airport beacon light has been running all night long, never stops and I have seen one plane land. This never happened in the past. This just started. The lights shines in my window all night and I am trying to understand the reasoning behind it.

I get airports need to have it on when planes are approaching but the question is why all night with no air traffic? Some say its broken and its supposed to be triggered by oncoming aircraft while other says its FAA regulation to have it on dawn till dusk.

The Chart Supplement says "Arpt bcn OTS indef."
("Airport Beacon is Out of Service, indefinitely")

It also says "ACTIVATE MIRL Rwy 16–34 and bcn—CTAF"
("Activate the Medium Intensity Runway Lights(MIRL) for Runway 16/34 and the Becaon, by using the CTAF")

Based on this, it is unclear if the beacon is out of service, or pilot-activated.
But it is clear that it should not be on all night.

A phone number is listed on that page for the airport manager, and I might start there.

After that, consider contacting the local Flight Standards District Office (FSDO).


Who was the first black person (in the world) to go airborne in an aircraft? Who was the first black person in the world to go airborne in an aircraft?

Surprisingly, google fails me in this so I don't think it falls under the closeable category of trivial trivia.

(I do not use the term "African American" because the answer might not be an American. I don't use "African" either because there are white Arab Africans. I suppose Sub-saharan African? But then there are black Africans on the north coast of Africa too. So I don't know what the term is that's both accurate and politically correct.)

Closest I've found so far:

Bessie Coleman was the first black woman to hold a civil pilot license, but I could not find a date for her very first airborne event, either as a passenger or pilot.
Eugene Bullard was one of the first black pilots, as was Ahmet Ali Çelikten and Emory C. Malick according to wikipedia. I could not find dates for their very first airborne events either.

My question is about any such person to be airborne in an aircraft. Obviously this could be a passenger, not a pilot. Edit: and it could be an employee too. I don't know how the personnel was organized back then, but maybe some cargo handler went along with some flights.

BTW the definition I'm using for "aircraft" is any heavier-than-air thing flies via lift. So no hot-air balloons and no dirigibles. But rotary-aircraft and gliders are okay. I think they had experimental ones very early in aviation history.

The earliest example I could find is Emory Malick, a African American Aviator who received his pilots license in 1912 but stopped flying after a series of crashes. For the first pilot with a major career, every source I could find points to Eugene Bullard, an American who studied in France and fought in WWI, receiving his license in 1917.

Specific searches to look for British, German, or Italian pilots of African descent came up with nothing from that time period, so I think it's fairly safe to say that these two were the first.

Outside of the US, Ahmet Ali Çelikten flew for the Ottoman Empire beginning in 1914, and is widely regarded as the second black pilot following Malick.

As for passengers, I doubt you'll find much unless logs from the time included race on tickets for aviation.


Are helicopters aerodynamically stable? On a fixed wing aircraft, if the aircraft's attitude is disrupted by a small angle and the controls are let go, it will slowly right itself to straight and level, due to built-in aerodynamic stability (on most aircraft).

If a helicopter is banked or pitched a small angle, will it right itself with the horizon if the controls are let go?

It's complicated ;)

There are two types of stability; dynamic and static.

If an aircraft is disturbed by, say, a gust of wind, it will deviate from its attitude but then will immediately and without control inputs return to its original attitude.  This is positive static stability.  If it remains in the disturbed attitude unless corrected, it has neutral static stability.  If it continues to deviate, it has negative static stability.

If the aircraft returns to its original attitude, overshoots a bit, then corrects back, overshoots a little bit less and so on, eventually returning to its original attitude, then it has positive dynamic stability.  If these oscillations continue and do not "damp" down until the original attitude is regained, then it has neutral dynamic stability.  If the oscillations grow increasingly large, then it has negative dynamic stability.

For helicopters, I will ignore those with autopilots, hydraulic controls and stability augmentation systems.

Imagine hovering.  A gust of wind hits the helicopter.  The disk will "flap" away from the gust and the helicopter will move with the gust.  When the gust stops, the disk now experiences airflow from the opposite direction, since it is now moving sideways.  The disk will flap back the other way, returning to neutral but the fuselage acts as a pendulum hanging under the disk and its momentum will cause it to swing further out, tilting the disk more and causing the helicopter to move faster in the opposite direction to the gust than is required to return to the original attitude.  Eventually, the airflow now comes from the direction the helicopter is moving in so the disk flaps away, the fuselage swings too far and......  Unless corrected, it will continue to swing backwards and forwards, rolling, yawing and pitching.  It is statically stable (because it tends to return to the original attitude) but dynamically unstable since it continues to swing backwards and forwards from the original attitude.

In forward flight (and unless you want to know, I'll skip the details which include what direction the gust comes from, the tendency of the fuselage to weathercock and "blowback" since the details all about which way the disk flaps and which way the fuselage weathercocks and are a lot of words!), a helicopter is statically stable and dynamically stable in yaw and is statically stable and dynamically unstable in pitch and roll.

You can take your feet of the pedals and the aircraft will be stable in yaw.  You can take your hand off the collective (although you should never do this - if the engine quits, you need that lever down fast) and it will stay where it is, ignoring perhaps vibration making it eventually move up or down.

You cannot take your hand off the cyclic since this is the control which determines pitch and roll and corrects for dynamic instability on a constant basis.

On a light helicopter, you would be out of control within a second or two.  On a heavy, maybe three or four seconds.

In summary, a helicopter is statically stable and dynamically unstable in all three axes in hover and forward flight except yaw in forward flight where it is dynamically stable.


Where can I find airport diagrams from airports all over the world? I am trying to build a database of airport diagrams (PDF documents). For the US, the FAA seems to be the best source. Are there any other free sources for Europe, Asia, etc...

Also the data needs to be reliable.

For Europe, the European AIS database (EAD) would be the best source.


What could the fuel pump failure have been that my flight experienced? I was on an Easyjet flight (I think an A320), when mid-flight, I overheard a member of cabin crew tell another passenger there was a 'technical' issue with the aircraft, that might make us slightly behind schedule.

Naturally, I imposed on their conversation to basically ask her to repeat the issue to me in detail. She said that the 'mechanism' that transfers fuel between the two tanks had failed, and so only one tank was available.

Obviously, the flight completed without any noticeable events, but could someone explain to me what happened as I do not understand.

The conversation may have been about fuel pumps or fuel transfer valves but the flight attendant was probably told that as a simple explanation - it would not have been the real cause.

Aircraft have multiple systems to pump fuel, transfer, and pipe it so no single failure will affect normal operation.

For example, pumps have automatic bypass valves so that other pumps will continue the normal flow past the failed pump. No single transfer valve obstructs fuel flow. There are multiple paths to get the fuel to the engines; either directly or by transferring through a completely different set of pipes to another tank then on to the engines.

What likely happened was there was one or more FILTERS that became obstructed. Both ice particles and algae often obstruct filters. But once again all the filters have automatic emergency bypass valves and pipes. However, it is not unheard of for polluted jet fuel containing large amounts of algae to overwhelm the filters.

[ In my 10 years with Continental Airlines, we had one aircraft engine obstructed by ice particles, but numerous occurrences of algae contamination. ]

  If the filters and/or bypasses either obstructed engine filters, made
  one or more tanks unusable, or reduced the fuel flow, the Captain
  would have chosen an altitude and speed that gave best fuel economy;
  and would have arrived late.

It is worth noting that algae contamination is quite severe in some parts of the world such as Asia and Pacific Island nations. It is common practice for American airliners to carry more fuel than necessary when headed abroad to help "dilute" the bad fuel in other parts of the world.

Simplified Airbus diagram of a typical fuel system showing multiple pumps, transfer valves, and paths to each engine.

B737 diagram (one of the simplest airliners) showing multiple pumps, transfer valves, and paths to each engine.


How can the A320 receive 3 DME signals if it has only 2 DME receivers? How can the A320 receive simultaneously 3 DME signals (VOR1 + VOR2 + ILS) if it has only 2 DME receivers (and antennas)?
Does it multiplex them in some way? If yes, does it mean if one DME receiver fails I can still get simultaneously 3 DME values?
There is a video that shows 3 DMEs tuned simultaneously: Airbus Cockpit Views | Episode #6 | PFD View Landing STN RWY22

VOR1 (DME1) being automatically re-tuned, followed shortly by VOR2 (DME2), plus the already tuned ILS

Mighty interesting! There is a clue in the FCOM:
Four frequencies per FMGC
If you check FCOM Chapter 22 § 20-20-30 you'll note it says:

Each FMGC [Flight Management and Guidance Computer] automatically uses its four DME frequencies as follows:

One DME frequency for display. It is possible to tune it manually or automatically. This DME frequency is also used for VOR/DME position
computation.

Two DME frequencies in DME/DME mode for calculating the radio position of the aircraft. The FMGC automatically tunes these as a
function of their best accuracy. The flight crew does not receive any
indication that this process is happening.

One DME frequency is connected to the ILS/DME.

Note my bold emphasis above. Similarly for the A380, in clearer language:

In normal operation, each FMS can tune four DME s at the same time. (...)

Scanning DME
Wikipedia says:

Scanning DME (distance measuring equipment) that check the distances from five different DME stations simultaneously in order to determine one position every 10 seconds [emphasis mine]
—Wikipedia: Flight management system § Position determination

The patent for this scanning-type DME was filed in 1982:

(...) a scanning type DME unit which is rapidly sequenced to interrogate said multiple ground stations successively and determine distance thereto for all stations within range, and a computer to validate and process these distance measurements in sequence to provide an accurate computation and display of aircraft position and wind information. [emphasis mine]
—Meyer, Donald H. "Accurate DME-based airborne navigation system." U.S. Patent No. 4,583,177. 15 Apr. 1986.

Failure scenario
Regarding the failure scenario, each FMGC can tune both its interrogator, and the other interrogator if the other FMGC failed.[1] Each FMGC is still limited to four frequencies though. So, if one DME fails (antenna or its interrogator), you'll have two frequencies which you can control (one for display, and one for ILS), and two the FMGC will use for DME/DME position, but you can't have two different DMEs for display.
1: Airbus Single Aisle Training Manual ATA 34


Does the phrase ".78/280/250" have a standard meaning to an airline pilot, in the context of a long descent? Does the phrase ".78/280/250" have a standard meaning to an airline pilot, in the context of a long descent?

Specific context-- the table in this answer-- Is the descent performance of the PMDG 737-800 accurate?

It means the profile is based on descending at 0.78 Mach until the IAS increases to 280 knots, at which point that speed is maintained to 10,000', at which point the aircraft slows to 250 knots for the remainder of the descent until it's time to slow & configure for landing. That 0.78/280/250 is a very standard profile for descent planning in the 737, although variations such as 0.80/320/250 or 0.76/250 are certainly possible.


How could a PA-28 be made to fly higher? How would I have to modify the Piper PA-28 to fly at 25,000 feet (same as PA-34 and PA-46)? Could it fly higher?

Larger wings, other wingform, bigger motor something else? Do you have to change the whole construction of the plane?

The Turbo Dakota (PA-28-201T) is very close to what you seem to be describing and is likely much less expensive to buy outright vs. the time and materials required to modify a lowly PA-28-1xx.  The published ceiling is FL200 with its TSIO-360 but at light weights* I'm sure a determined pilot could coax it to FL230 or FL250.

So, the answer is: a big TSIO engine.  The 260hp TSIO-470-B (only TSIO-470 made to my knowledge) would probably do the trick.  A TSIO-520 has taken me to FL250 in a T210 so I know it would probably take a Cherokee at least that high assuming that you successfully attach the engine to the plane (approvals, engineering, mechanical, W&B, etc).

*The Dakota is one of the few small single engine planes with a gross weight of more than twice its empty weight leaving plenty of wiggle room for a pilot seeking to extract better than book performance from the plane.

**Answer assumes you have an O2 system that will do the job at FL250.


How different is ADFX/ARINC 664 router from normal Router? ADFX or ARINC 664 is a protocol that is derived from the Ethernet protocol. What I understand it is

Point to Point (no routing of data)
Deterministic
used VLAN for addressing

I just wonder do we have to get specialised router that cater to ADFX? Can the conventional ethernet router/switch(for eg used in home, office) be used instead?
Regards
EDIT: Sorry based on answer below given, I would like to edit my question to being instead of routers to switch

The point of ADFX/ARINC 664 is that it is real time. That means the switch guarantees each type of message will be delivered in specified time according to its priority and allocated bandwidth.
For example the control computers probably need to know the inertial reference (attitude and speed) readings every 10 ms, so the switch can be configured to deliver one message from each inertial reference unit every 10 ms with at most, say, 2 ms delay (based on number of packet types with higher priority). And the switch guarantees that barring hardware failure it will deliver the messages within that delay always. That is a lot of logic on top of normal ethernet that just delivers packets on a best effort basis.
Additionally the logic also has to guarantee that if one of the units goes haywire and starts sending the packets faster than the allocated bandwidth, or starts sending jumbled packets, these errors won't interrupt any other communication going through the switch. If one inertial reference packet should be delivered every 10 ms, only one will be delivered even if the unit sends hundred of them.
And last there has to be a lot of integrity checking of the switch itself so if hardware does fail, the communication fails over to the backup network.
Since it is ethernet on the physical and data link layers, you can use plain ethernet switch for testing a component. But not for production and not for system test where you have to verify the traffic policing is set up correctly.


Why is tethered hover testing performed on helicopters? The KGUC website notes that they have a tether for helicopter testing. From what I can tell this testing has something to do with hot and high conditions, but I don't know what the purpose is of testing with a tether. Here is a video of testing being performed. What is tether testing for?

The cable is attached to the helicopter with a tensionometer.  This can be used to vary the effective weight of the helicopter by varying the tension applied to the cable.

The cable length can also be varied from 0 height, to hovering in ground effect and  hovering out of ground effect.

This test enables data to be gathered quickly for the helicopter performance from minimum to maximum weight on the ground, IGE hover and OGE hover without the pilot having to control height (since the cable does this) and with quick and easy changes to the effective weight without having to add ballast to the helicopter for each weight.

This test is a simple and quick, albeit risky, way to gather data on hover and climb performance throughout the weight range.


How does the target indicator on a jet fighter work? How does the target indicator in Jet Fighter Aircraft work?  How does it stay on target? Is this queued from the radar, from infrared or is this image classification? What does the x in the target indicator mean? Are the other symbols around it part of the target indicator or related to something else?

There are two main questions here, mainly how does a fighter jet track a flying target, and questions about the symbols used on the Heads Up Display.
How does a fighter jet track targets
A fighter jet tracks its target(s) using its radar, which (as you are all aware) works by emitting EM waves and detecting the reflected waves bounded off by the target. The onboard computer processes this data and translates it to target information.
There are two modes of radar operation*. The first is a scan mode: the radar makes a sweep across a wide angle to look for targets. In this mode, the radar scope is wide, but target update rate is slow, and target information is coarse. The second is targeted mode: the radar focuses a narrow beam of energy at the target, which provides accurate readings with a high update rate. On the HUD, you can see "200Vc", meaning the jet is closing in on the target at a relative speed of 200 knots. Below that is "1000 FT", the distance to target.
* There are many modes actually, but to keep it simple, let's just focus on two.
A fighter jet does not identify a target optically nor using infrared. Neither of these two methods can provide accurate position information of the target. This may change in the future, but it is what the technology is now.
About the HUD
The HUD is from a F-18 (I am not sure which variant). Let's go over the symbols top to bottom and left to right:
Left column

On the upper left is a black square. It indicates that the trigger is pressed at this moment. It is displayed only on the recorded tape and is not visible to the pilot.
the boxed number is the indicated airspeed. It is reading 179 knots.
Going down is the angle of attack, or "alpha". It is reading 20.3 degrees.
Mach speed is 0.36.
G-loading is 1.7.
The max g-loading on this flight is 7.6 G.
Below that is the time in UTC.

Center column

On the upper center, the arrow pointing upwards "^" is the needle of the heading indicator. Above it is the ruler marked with heading numbers, which is apparently cropped off in this image.
The cross sign below "+" is gun cross. The cross is always displayed at the gun inclination angle as a backup.
The "w" sign is the attitude indicator. In the background we can see the numbers: the aircraft is pitched 26 degrees down and banked approximately 70 degrees to the right.
"GUN" indicates machine gun is the selected weapon. The cross indicate it cannot be fired, most likely because this is a practice mission and master-arm is not on.
"0" is the number of bullets left.
Near the bottom edge of the image, a bit to the right of center line, is the flight path indicator, a circle with three bars extending from its left, top and right respectively. It shows where the aircraft is travelling to, and it is useful for landing or flying low near terrain.

Right column

Current altitude is 15180 feet MSL.
Target closure rate is 200 knots.
Range to target is 1000 feet. If the range is longer than 1 nautical miles, it will display in nautical miles instead, up to the nearest 0.1 nautical mile (e.g. 2.3)

Symbol around the target

This particular circle is the target reticle for gun mode. A bigger circle is displayed if it is in missiles mode.
The center dot shows where the bullet will hit if the gun is pressed at this very moment. The circle moves around the HUD as the jet is maneuvered. For example, if the pilot pulls on the stick, the circle will move towards the bottom of the HUD because that is where the bullets are going.
Targets are identified by a square box on the HUD. The square is not displayed when the gun reticle overlaps with the target, which is the scenario in this image.
"IN LAR" means In Launch Acceptability Region. "IN LAR" is displayed instead of "SHOOT" because the target is designated as friendly. If the target is designated as hostile, "SHOOT" will be displayed instead.
Below the letters "LA", we can see a bold portion from 12'o clock to 1'o clock on the circle, and a mark at 1'o clock. This is the range to target. The mark rotates clockwise as target distance increases. For example, if the bold portion is instead from 12'o clock to 3'o clock, the target range will be 3000 feet.
There is a similar mark on the circle slightly pass 6'o clock, but the mark is outside the circle. This is the maximum range of the currently selected weapon. By comparing the bold portion of the circle to this mark, a pilot can tell whether the target is within range of the current weapon.
I have no idea what the "x" means.