When might a pilot hand-fly a jet at cruise altitudes, and is it difficult? How difficult is it to hand-fly a jet at cruise altitudes? Other than autopilot failure and just doing it for the fun of it, when might you want to hand-fly at those altitudes?

I suspect the answer to the first question varies with different aircraft. I have an answer for the 747-100 and -200 series below.

For 747-100 and -200 aircraft at 35,000 ft and above, you can do it, but it's hard to keep the airplane within 100 feet of the assigned altitude, and you typically can't do that (or at least I couldn't) without practice. 200 foot altitude excursions were the norm when I first took control if I had not done it for a while. For the few first officers that accepted an invitation to try it, an initial 300 foot altitude excursion was typical when they first tried it, especially immediately after disengaging the autopilot.

I found the concentration required was very tiring. Ten minutes was typically enough to satisfy the urge.

Insofar as when you might want to do it, I found hand-flying produced a smoother ride in heavy to severe turbulence than the autopilot's turbulence mode, which tended to try to keep the assigned altitude and aircraft pitch too slavishly.


Is turning off engines before landing "Normal"? I was traveling by Airbus A380 recently and I was sitting near the wing.
Throughout the flight, I could hear the airplane noise even with headphones on.

Suddenly before landing, everything went silent. There was no noise at all.  There was a strange silence throughout the aircraft.  Everyone was scared and looking at each other, some even started praying.

But we landed perfectly fine. It was like routine landing.

So, I want to ask, whether it was normal? Or the pilot avoided telling us something went wrong?

P.S. Before starting the landing process, the pilot informed about the weather being fine and said "there's a safety manual in your seat pocket."

There's virtually no chance of the pilot deliberately switching off all 4 engines, and even less chance of all 4 engines failing at the same time. It's not entirely impossible for it to happen, but it definitely wouldn't be quietly ignored within the aviation community. I promise you that you would've heard about it afterwards.

That said, the A380's engines are modern, quiet, and actually mounted quite a long way out on the wings, this makes them fairly quiet when idling, even when compared to other aircraft. This is particularly noticeable after a long flight where they've been running at fairly high power for 8 or more hours, and is even more the case if you're not used to the A380, eg if you usually fly on common twins such as the A320, 737, 777 which are much louder within the cabin.

I suspect, but can't be sure, that the engines were simply pulled back to idle (minimum power/thrust) and it sounded much quieter than you expected.

But no, the engines were almost certainly not simply turned off, that just doesn't happen. The only thing I can think of that would cause that would be a flame-out, and you'd definitely notice that!


How can I ensure that I have a mode C transponder? How can I make sure my aircraft is equipped with a mode C transponder?

======================

If a transponder has an ALT button does that mean that the aircraft is equipped with MODE C?

For any aircraft you should check the accompanying POH for a full equipment list of what the plane has been outfitted with. Likewise you should be alert for any place cards noting defective or inoperative equipment. You should also check the aircrafts logs to make sure the transponder has been properly inspected according to § 91.413 ATC transponder tests and inspections and that it is operable. Just because you have one does not mean its legal to fly with. A transponder capable of Mode-C (altitude encoding) needs to either be connected to an altitude encoding altimeter or a blind encoder to report altitude. You should be able to find this out in the POH, if its not connected to one of these two units it will behave like a Mode-A transponder. You can find a  discussion on the various setups here. If you are really curious about how Mode-A/Mode-C communication works this is a pretty good read.

Specific to the Garmin unit you have pictured you can get it to display the aircrafts altitude by pressing the "func" button (pressure altitude that is) and compare with your altimeter. If the unit has a readout it then an pressure unit can be assumed to be connected. NOTE: This is not an official test by any means.


How do you open and get a IFR clearance while in the air? I once had a traffic controller give me a hard time about how I requested IFR clearance once in the air. I had previously filed an IFR flight plan, and took off from my untowered home airport. On approach control's frequency, I said:

  Tampa Approach, Cirrus 123AB, 5 miles southeast of Tampa Exec at 1000 feet, IFR to Ft. Lauderdale Exec

The approach controller responded, annoyed, saying something like "Well do you have an IFR flight plan or are you reporting IFR??"

I had always used that phraseology because it seems the least wordy way to get the info across, which can be helpful when the freq is busy. What is the technically correct way to get an IFR clearance on an existing IFR flight plan?

There is one example in the AIM used for transitioning from VFR to IFR, and it how I've always done it:

  AIM 5-2-5. Abbreviated IFR Departure Clearance (Cleared … as Filed) Procedures

  EXAMPLE-
  "Los Angeles center, Apache Six One Papa, VFR estimating Paso Robles VOR at three two, one thousand five hundred,
  request IFR to Bakersfield."

Reference: 5−2−5. Abbreviated IFR Departure Clearance (Cleared … as Filed) Procedures (13 MB PDF)

Notice that they specifically say "VFR" right before their current location and also use the word "request" to make it clear what they are looking for.  I would change your request to:

  Tampa Approach, Cirrus 123AB, VFR 5 miles southeast of Tampa Exec
  at 1000 feet, request IFR to Ft. Lauderdale Exec


Which FAA certifications may be used as a name suffix and how are they properly used? You often see pilots using some of their certifications as name suffixes. eg. John Smith, CFI or John Smith, ATP CFII. The Wikipedia page for ATP mentions that the ATP may be used as a name suffix but provides no reference. Does anyone know which certifications may be used as a name suffix and how they are properly used?

I don't know that there's any kind of official rule about what name suffixes a person may use. I suppose as long as he is not being subversive then there's no problem.

These would fall under the "professional" suffix category and like many suffixes may only be recognized by people familiar with a certain industry. For example, CISSP or CISM may only be recognized, ie. they know what it stands for, by those who work in information technology and other related fields.

If it isn't a professional FAA certification, then obviously, it shouldn't be used as a suffix.  I have seen the following used as a suffix or listed as a desirable qualification:

CFI, CFI-I, CFI-G and MEI
AGI, BGI, IGI
ATP, ATP-R
DPE

There is, however, a point that shouldn't be missed;  you won't see pilots going around signing documents and adding suffixes at the end of their name like a doctor, but it is usually a way of conveying to an audience what that person's qualification or source of legitimacy is such as on a business card, advertisement, or flight school staff page.

A commercial pilot would not use that qualification like a suffix because it is not commonly abbreviated, and without any other qualification that would imply that a commercial certificate is held, they are extremely limited on what they can actually get paid to fly for. Therefore, the advertising aspect is irrelevant because there is virtually no public target audience.

The lack of a public advertising target also goes for certificated flight dispatchers.


What is the difference between low energy warning and alpha floor in A320? I am always confused between both of these two. I know both signals low speed or high AOA

Low energy warning is a function of Flight Augmentation Computer (FAC). It monitors aircraft’s flight path and warns if speed is about to decay below the threshold where power is needed in order to maintain positive (ie. climbing) flight path with pitch control.

Alpha floor is provided by auto thrust (A/THR) - (commanded by FAC), which sets full thrust (TOGA) regardless of pilot input if angle of attack rises above pre-determined angle. The threshold depends on aircraft configuration.


What kind of aircraft may land on iced areas, like Antarctica? I know some aircraft may land on iced surfaces. what are the difficulties associated and is this limited to specially designed aircraft.

Is there some conditions necessary for the landing and the next takeoff.
Where are the main iced airfields currently active?

I've spent 6 summers in Antarctica (Mt. Erebus), travelling through McMurdo, where the busiest airstrips on the continent are. Most flights to McMurdo are on C-17s or C-130 Hercules. Some of the Hercules are LC-130 -- the L  indicates that they have skis. The skis enable them to land and take off on some including ungroomed or poorly groomed snow / ice surfaces. Sometimes they need JATO for takeoff, though, if the conditions aren't great.

McMurdo has three runways which open and close operations during the year: Willy Field, Pegasus, and the Ice Runway. The Ice Runway is on sea ice and the other two are on the Ross Ice Shelf with Pegasus built on blue ice and Willy Field on snow. The Ice Runway breaks up each year during the height of the summer so it is carefully monitored. It bends downwards each time a C-17 lands. That deformation is measured and is one of the parameters that's used to decide when to stop using the Ice Runway each year.

Sometimes, planes designed for non-military use are flown to McMurdo; the New Zealand Antarctic program often flies an Airbus A319 there. As far as I know, a well-groomed snow / ice runway can accommodate any plane in good weather. A Boeing 757 flight to McMurdo from Christchurch had a scary incident recently where it was forced to land in low visibility because it didn't have enough fuel to "boomerang" back to New Zealand. The C-17s often boomerang due to bad weather. The Hercules, like the 757, don't carry enough fuel to boomerang, but they are better at landing in inclement conditions so it doesn't seem to be an issue.

For flights within-continent, Twin Otters and Baslers are in common use.

No, there's no military presence with offensive capability. I don't really understand this (recently added) part of the question -- did you think they'd hit you with antiaircraft??? There's a bunch of air force guys who operate the flights, and a single US Marshall in McMurdo.


Did the 1500 hour rule expire? So the 1500 hour legislation was set to expire on the 30th of September 2015. Anyone know the outcome of the expiration. Did congress extend the rule, or is it no longer in effect. I tried searching the web but couldn't find any info.

The 1500 hour rule is still in effect.

The rule wasn't set to expire on the 30th of September; technically the entire FAA was going to expire. Congress votes on the authorization for the FAA to regulate air transportation; that authorization was set to expire 30 September, 2015.

The House voted to authorize funding on 28 September while the Senate passed a 6-month extension on 29 September.

Expect to see similar discussions in 6 months.


Does distributed propulsion create unwanted lift at cruise speeds, requiring a negative angle of attack? Does distributed propulsion create unwanted lift at cruise speeds?

The great thing about slats and slotted flaps is that you can "turn them on" when you need high lift at takeoff, and "turn them off" when you don't need high lift during cruise.

So what about distributed propulsion by using 4 engines with props covering the majority of the wingspan? You can't turn off the extra lift off if you're trying to cruise as fast as possible.

I understand distributed propulsion adds a clmax of about 3.

Typically, you only need a Cl of 0.5 during cruise, but most GA planes cruise at about 75% power, almost at full power as on takeoff, so wouldn't a cl total of say 1-2 be generated with distributed propulsion?  That's 200 to 400% higher lift than needed.

To get rid of this extra lift and drag, wouldn't you need to lower the nose until the wings produce a " negative induced cl" of about -.5 to -1.5, adding the "distributed propulsion cl" of 1-2, for a total desired cL of about 0.5?

Is this correct, at least conceptually? Or does lift due to distributed propulsion trend to zero at a zero AOA? i.e. it's just additive if the wing is producing lift above a zero AOA

The fundamental way distributed propulsion adds extra lift is by blowing additional air over the wings. This is a direct thrust-to-lift conversion.

If you produce less thrust, you produce less lift with this arrangement.

So, no, you don't go to a negative AoA to counteract the excess lift produced by running all of your motors at full power. You never produce that excess lift in the first place. This involves running your wing propulsors at reduced power, and if you have lift-assist propulsors, such as blown flaps, turning them off altogether.


Can variable incidence wings replace ailerons? If a wing can change its incidence angle, can aileron be omitted in the design?

Since aileron are used to increase lift and decrease it. so is the incidence angle.

How would the two solutions compare?

So did the Wrights in their 'Flyer'. It had no ailerons, but the wings could be slightly deformed to roll the plane.

Possibly for constructive reasons, ailerons are almost always used today for roll control (spoilers are sometimes used in very light planes) but the Wrights' method was more elegant...


How does a U.S. military air traffic controller become a civilian air traffic controller? If a person is working as a United States military air traffic controller and then they get discharged, do they have to go through additional training and tests to work as a United States civilian air traffic controller? or do they just get hired, without any additional training and tests?

Wasabi's answer is not correct, or at least not fully correct. You actually ask two questions: does a military controller need to take additional tests and training to get hired, and do they need additional training to work as a civilian controller. The answers, respectively, are "no" and "yes."
The hiring practices change every so often, but here's how it is now:
"Off-the-street" FAA controllers receive training at the FAA Academy in Oklahoma City. This training starts with Air Traffic Basics and continues to Initial Tower Cab (for Terminal-track new hires) or Initial Enroute (for Enroute hires). New hires do not go directly to standalone TRACONs anymore. People who graduated schools in the Collegiate Training Initiative do not have to attend Basics but do attend Initial.
In order to get to Basics, new hires must take the ATSA aptitude test, as well as undergo medical and security investigations.
After graduating the Academy, new trainees are sent to their field facilities and go through classroom and simulator training before eventually progressing to On-The-Job instruction (plugged in working live traffic alongside a trainer who retains ultimate responsibility for the control position). Any controller, even one who has been previously certified at an FAA facility, must go through this facility-specific classroom/sim/OJT progression any time they transfer to a new facility.

Prior-experience new hires (who mostly come from the military) may elect to go through the off-the-street hiring process if there is a bid open at the time they are discharged. But usually they apply though a separate bid that is only open to prior-experience controllers. This bid allows them to be placed directly at a facility that should, in theory, correspond to the type and level of their prior experience; this is almost always a Terminal facility (tower, up/down, or TRACON) but an Enroute facility may be offered very rarely.
If necessary these new hires may be sent to the Academy for training, for example if they only worked radar before but were assigned a tower facility, but for the most part they enter the training process at their new facility immediately. This process is not waived just because they have previous experience. They do benefit by not having to take and pass the ATSA aptitude test, however.

As far as "tests" go, in the FAA, there are annual or biennial "refresher" simulator problems and "skills evaluations" where a supervisor does a limited checkride with the controller to ensure they don't need additional training in any area. But this is based on when the controller certified at their facility and has nothing to do with any tests they might have done in the military.


Why haven't quadcopters been scaled up yet? Why are quadcopters not flown by human pilots yet?

Wouldn't they be more stable and easier to control than helicopters?

Wouldn't they be more stable and easier to control than helicopters?

No, they would not.

Quadcopters don't have any special inherent stability. When you increase power of one of the rotors to pitch, the increasing pitch will not do anything to the power difference and therefore the pitching moment.

The advantage of quadcopters is that the rotors can be fixed pitch while single (or double) rotor helicopter needs complex control mechanism. While this is huge advantage for the small scale devices where each rotor can be powered by its own simple electric motor, the complexity of either additional engines or long transmission shafts would outweigh any advantage from the simpler rotors in full-scale vehicle.

And why can't full-scale helicopters use electric motors like the small ones? The reason is that when you scale an airfoil up, the lift it produces increases with its area, which grows with the second power of size, but its weight increases with volume, which grows with the third power of size. Therefore models have much more lift for weight and can afford simple but relatively heavy batteries while full-size aircraft need propulsion systems with higher power density.

And then there is also the factor of safety. In case of power failure, helicopters can still glide to the ground and still land vertically using autorotation. But since the rotor rotating speed can't be changed without power, controlling the helicopter during such manoeuvre requires variable pitch rotor. So there goes the main advantage of quadcopters.


What does this NOTAM about a STAR mean? When planning an IFR Flight into KTPA using the BRDGE 8 STAR, I saw this NOTAM:

  !FDC 7/5458 (KTPA A1015/17) TPA STAR TAMPA INTL, TAMPA, FL BRDGE EIGHT
  ARRIVAL... BRDGE STAR UNUSEABLE BELOW 5000FT PIE VORTAC RADIAL 138
  UNUSABLE BELOW 5000' 1705151500-PERM

Would that mean that I can't use the LBV.BRDGE8 transition below 5,000, so then I would need to be higher on the en-route portion?

What does the NOTAM actually mean when I fly the STAR?

Check for additional NOTAMS concerning the PIE VORTAC.  It appears that, for whatever reason that VORTAC radial is no longer functioning well at lower altitudes and, for whatever reason, the FAA is opting to not repair it.  As it's a needed navigational aid for that STAR, the additional NOTAM applies.  I'd guess the Orlando FSDO is in the process of decommissioning that STAR and issued NOTAMS until then.

UPDATE: It appears that this VORTAC out in Clearwater, FL has been malfunctioning, causing aircraft to stray off course and overfly noise sensitive areas at low altitudes, causing the Tampa airport management to receive a great deal of complaints.  The FAA has up to now been faced with the repair tasks on that navaid.  But it appears that, with the gradual decommissioning of VOR stations throughout the country combined with the preeminence of GNSS navigation, they have elected not to make the repairs and just leave the VORTAC as is for the foreseeable future.


Why stall on 2D faster than 3D? why cl on 2D is better than 3D. and why cl 2D stall faster than 3D?
what happens when CP continues to move forward on the wing. and what happens when CP continues to move forward on the elevator.

The 3D-wing allows for air to flow over the wingtip into the low pressure zone over the wing, thus reducing the overall pressure differential and reducing overall lift.

This explains the stall at higher AoA because the additional airflow over the wing surface "energises" the air. Flow seperation happens when the kinetic energy is equal to the pressure differential that the airstream flows into, so if you can increase the kinetic energy, you can decrease the tendency of the airstream to separate.


Is there a boomless sonic technology in any state at this time? I've heard rumors about airplanes being on the horizon with "boomless sonic" technology, or the ability to fly supersonic without creating that hazardous shockwave.

This would be very valuable for business travelers who want to cover populated areas at high speed. These aircraft would presumably be certified to fly at supersonic speeds where it is now forbidden due to the damage of sonic booms.

Does this technology exist? Is anyone on the trail to making this work? Is it even possible?

Shock wave is a physical phenomenon and as such is unavoidable. The best we can do is to reduce their intensity and reduce their effects on ground.

A method to eliminate the shock wave (theoretically) was proposed by Adolf Busemann. His airframe, called the Busemann's Biplane which consists of two triangular cross-section plates a certain distance apart (so that choking is prevented) with the flat sides parallel to the fluid flow.

Image from Supersonic biplane—A review by Kazuhiro Kusunose et al.

The idea is that the shock waves are formed internally between the two (triangular) 'airfoils' and cancel each other out at design condition, shown in the first case.

Image from Supersonic biplane—A review by Kazuhiro Kusunose et al.

At off design conditions, shock waves are produced. However, this doesn't produce any lift at zero angle of attack, as is obvious. As researchers explain,

  ... the design lacks lift: The two wings create a very narrow channel through which only a limited amount of air can flow. When transitioning to supersonic speeds, the channel ... could essentially “choke,” creating incredible drag. While the design could work beautifully at supersonic speeds, it can’t overcome the drag to reach those speeds.

Research is being carried out to overcome this issue- One method proposed is to have the wings 'morph' shapes- so that they are efficient at all speeds. Another group is working to optimize the shape of the airfoils in Busemann's Biplane so that the shock-wave is reduced at all stages of supersonic flight. However, all these studies are in conceptual stage and no experiments have been carried out so far.

NASA carried out the Shaped Sonic Boom Demonstration(SSBD), in which the fuselage of a Northrop -5E was modified (kind of like pelican's beak) so as to change the pattern of shock waves formed in supersonic flight.

"F-5E Shaped Sonic Boom Demonstration aircraft" by NASA/Carla Thomas - http://www.dfrc.nasa.gov/Gallery/Photo/SSBD/HTML/EC03-0210-1.html. Licensed under Public Domain via Commons.

NASA reported that the modifications were able to reduce the intensity of the shock wave by a considerable amount, as shown in the image below (the 'flat' pressure line is from the SSB demonstrator).

Image from Quieting the Boom- The Shaped Sonic Boom demonstrator and the Quest for Quiet Supersonic Flight by NASA

Active research is still being carried out by various organizations in this field.


Can a fixed-wing plane jump up by blowing air over its wings? NASA's X-57 electric plane has 12 wing-mounted propellors which drive air past the wing.

This video describes how the resulting increased air velocity causes an increase in the amount of lift the wing generates.

Suppose the propellors could generate enough airspeed over the wing to lift the entire plane -- could the plane then jump into the air without needing a runway?

This would require that sufficient lift is generated before the propellor thrust (minus wing drag) exceeds the craft's friction with the ground -- otherwise the propellor thrust would push the craft forward some before a liftoff could occur. And that leads to my secondary question: Is blowing air over a wing a more efficient means of achieving lift than would be directing those propellors downward rather than laterally over the wing?

As a partial answer to your question, air moving over a stationary (relative to the ground) wing will generate lift. There have been several weather related incidences where aircraft were picked up off the ground by significantly strong winds. Videos of these incidents showed that it was lift and not force from form drag that lifted the aircraft off the ground. In the case that the planes were tied down, they would come back down to earth in place. Unfortunately, the force of the decent, in some cases, was still enough to damage the planes.

I am uncertain about what you mean by “This would require that the lift generated would exceed ground friction reacting the propellor thrust...“. Lift would be acting roughly perpendicular to the wings chord line, and the plane’s longitudinal, and lateral axes. On the other hand, any friction created by the relative wind would be acting roughly parallel to the relative wind and the plane’s longitudinal axes. The only ground friction would be that caused by the wheels touching the ground.

Thrust is a function of Newton’s Laws of Motion. The propellers turn chemical energy (in this case electrical energy) into thermal and mechanical energy. The mechanical energy is used to accelerate a mass of air. The mass of the air is accelerated by the propellers creating force. This force acts on the mass of the airplane to accelerate the airplane. The relative wind created by the propellers would act upon the airplane in the form of thrust and upon the wings to create lift regardless of how close to the ground the airplane was.

P.S.
Edited to include a new link pertinent to the discussion. Here is a new VTOL aircraft soon to be on the market.

Blackfly


Where to find US airport weather Service Levels? Where can I find an airport's level of service for weather services?
In other words, how do I know if a given airport's Service Standard is a categorized as Service Level A, B, C and D?

For background, the FAA has determined Service Standards to categorize the levels of detail in weather observation sites. See the following from Appendix D of FAA JO 7900.5C (p. 152):

The term Service Standards refers to four levels of detail in weather observations at sites where there is a commissioned ASOS/AWSS. The first category, known as Service Level D, is completely automated service in which the ASOS/AWSS observation constitutes the entire observation, that is, no additional weather information is added by a human observer.
The second category, known as Service Level C, consists of all the elements of Service Level D, in addition to a human observer, who adds information to the automated observation. This is referred to as "augmentation." [...] Service Level C is provided at all airports with a properly sited, fully qualified Federal facility during facility hours of operation. During hours that the facility is closed, the airport reverts to stand-alone ASOS or Service Level D as described above.
The highest category, referred to as Service Level A, includes major aviation hubs and high traffic volume airports with average or worse weather. The remaining group of airports (smaller hubs or special airports in other ways, that have worse than average bad weather operations for thunderstorms and/or
freezing/frozen precipitation, and/or that are remote airports) are referred to as Service Level B airports

The last of the service standards issued by the FAA was a 2014 report and ONLY available via a FOI request.

According to AJT-21, the yearly updates have been discontinued (without giving any reason that we are aware of).  Last "published" report was 2013.

If you wish to see it, please email @ wxobservers@yahoo.com


Is a 40-year-old person too old to start an ATC career? My wife:

is 40 years old
is a mother of two (able to do five things simultaneously / in the same time)
has been working as an English and French teacher for her whole career (19+ last years)
has very little to do with airline industry (being a passenger and low-level enthusiast)

I think that this is enough (especially point 2. and 3.) and I'd like her to at least try to become an ATC (i.e. enroll for a preliminary English test).

She is reluctant saying that she:

is too old
have very little or no technical background / education / experience

to pass even first tests, not mentioning becoming a professional ATC.

Can anyone judge which one of us is right or at least provide some source of information, based on which such judgement can be done? I.e. to formulate an answer (similar to this one) on unique challenges and limitations for starting a career as an ATC at 40 years old?

It is possible in the country where we live (Poland). The original limitation (26 years old) was raised to 35 years old some 10-15 years ago and then 40 years at most. And now the age barrier is lifted completely.

Some years ago (in the 80s) I tried out for ATC in Canada and took the preliminary screening test, which was a series of 25 diagrams representing radar displays with targets moving around different airways, crisscrossing each other in various directions, and you had to answer 2 questions for each diagram, whether or not targets at different speeds and altitudes would conflict with each other (there was a table with all the targets' speeds, tracks and altitudes).  It was a timed test and I wasn't paying attention so I was at question 45 when time was called and you had to drop your pencil right away.  So I didn't answer the last 5.

I got one or two wrong, giving me a score of 43, a "pass" for the test, but there so many applicants they were only taking candidates with scores over 45 for the next stage.  Probably would have made it if I'd just answered the remaining 5 at random.

Anyway, actual background is not that important. They are looking for a very special set of mental skills, and someone has them or they don't.  Mainly, a very high cognitive and spatial IQ, and a very powerful memory.  To the extent that educational background matters, it would mostly be as a sign of intelligence.

I used to know a centre controller who told me that once trained and experienced, and having the required mental abilities in the first place, and the fact that the high stress periods don't last an entire shift (there are "rush hours" where things are intense, then relatively quiet) the job wasn't nearly as tough as the public perception.

She should just go ahead and apply and start the screening process, as I don't believe there are any specific professional or educational prerequisites, and there is no harm in trying.  She will probably find out pretty quickly whether there is a chance.  One thing going for her is the overall personnel supply situation which is affecting ATC to some degree as much as other parts of aviation (shortages of applicants with what it takes).


Why are oxygen requirements based on pressure altitude? The FAA's oxygen rules in §91.211 are based on pressure altitude. Assuming our lungs are like our engines and wings and dependent on density altitude, why are the oxygen requirements not based on density altitude?

If the answer is practicality, then would it not be wise to be more conservative with these requirements on hot days and more lenient on cold days?

Living in Colorado, the oxygen requirements are almost exactly what is usually required to transit the front range of the Rockies without an oxygen bottle, which leads me to believe practicality played a part. If density altitude were used instead, on a hot day you'd be unlikely to remain within the regulations and still transit the area without hitting anything made of rocks.

The air in the lungs is in pressure equilibrium with the cabin, but not temperature equilibrium. It's always at body temperature. So the only variable that can affect how much oxygen is available to the body is the pressure of the outside air - thus pressure altitude, not density altitude, is physiologically relevant.

That said, if the density altitude is high it may mean you are hot and dehydrated, and the deleterious effects of those factors may be additive with those of hypoxia.


What is the standard procedure in case of inconsistent sensor readings? After reading the answers I now know there are three independent sensors. I will leave the question as-is, to not take the answers out of context:

I do remember reading that airliners have two independent sensors and channels to deliver readings like airspeed and altitude to the cockpit (pitot-static system).
  If this is not the case my question will not make sense.

If the two systems show conflicting information, what is the standard procedure? If one of them is 'overspeed' or close to stall speed should the pilots assume this is the one to focus on? Or do the pilots reason based on each situation independently and all the other factors at play?

The procedure used would depend on which information is conflicting/unreliable (and of course the aircraft type).

With one or more unreliable airspeed indications (which is one of the more critical ones), the initial action is NOT to start troubleshooting/figuring out which indicator is "the correct one", but to ensure safe speed/flight path by other means. The initial actions on Boeing aircraft is to disconnect automatics (auto throttle, autopilot, flight director) and control the aircraft given memorized combinations of pitch and thrust. With the aircraft under control, and when directed by the checklist, you start analyzing the situation, to see if you can rectify/isolate the faulty source. If a reliable source can not be determined, the aircraft continues to be flown by pitch/thrust settings given in the manual (based on altitude, if known, weight, flaps/gear, desired vertical profile etc).

In the case of unreliable altitude, in case you can't determine a reliable source, you can typically use radio altitude below 2'500 ft. A big caveat is that if the transponder is set to use an unreliable static pressure source, the altitude that ATC sees on secondary radar will also be unreliable. An example of this was seen in the Aeroperú 603 accident, where the aircraft took off, with the static ports taped over, and ATC was asked to assist with altitude and speed information: CVR transcript - Wikipedia

Erroneous airspeed/altitude indications often imply each other, as an unreliable static pressure source will affect both.


Why are fighter jets rated differently than passenger jets? Recently I watched a video on the economics of running an airline. They mentioned that passenger planes have a life span rated in cycles. That got me thinking because as far as I know, fighter jets are rated by flight time hours.

My answer to this was that the only thing that gets pressurized is the cockpit as opposed to basically the entire fuselage. But surely that would still limit the fighters cycling? Or can they simply change the canopy because it is the softest part and absorbs all the flexion?

The philosophy of fighter cabin pressurization is different from that of the passenger aircraft. In case of airliners, the cabin pressure decreases while the aircraft climbs upto a certain altitude, after which the value is kept constant (usually at 6000 - 8000 ft cabin altitude).

In case of combat aircraft, the principle is different. The cockpit pressure is maintained at certain value to a particular altitude (usually 8k feet); then the pressure is maintained (at 8k ft) till the aircraft reaches another altitude (~23k feet); after that a pressure differential (~5 psi) is maintained between the cockpit pressure and the ambient pressure. This reduces stress on the airframe and is safer in case of rapid decompression during combat. So, a fighter cockpit at 40k feet is actually stressed less (due to pressurization) compared to an airliner at that altitude.

There is another thing to be noted- a combat aircraft is designed for much higher loads compared to a civilian airliner; so, the structure is capable of taking more stresses. So, pressurization is not the limiting factor in case of combat aircraft.

One way of calculating the life of fighter aircraft is to apply loads on the airframe depending on the expected mission profile (like TO- cruise- loiter- cruise- landing), then calculate the theoretical life based on how long the airframe survived. Of course, this means that the life will change if the mission changes- like what happened when some aircraft were shifted from high level bombing to low level penetration roles.


Could a shrouded fan blade with long chord length be efficient? I am wondering if it would work to have a fan with two or three blades which have short-ish span and very long chord length, like half a rotation for each blade. Would a shroud (that is molded on the tip of the blades) create/solve any problems?
I am also thinking about tandem blades, but that would only work with shorter chord lengths, correct?

The main objective is to keep noise to a minimum.

I have no background in aerodynamics, just been reading up about it for a while. So please try to dumb it down where possible.

Edit: this is what I came up with, taking all the advice I've received so far.
https://www.thingiverse.com/thing:4069048
Any ideas on improvements?

The main objective of the engine is to generate required power $\text{P}$, which is:

$$\text{F} \times \frac{D}{T}$$

where $F$ is force, $D$ is distance, and $T$ is time.

Power generated by modern jets through continuous combustion of fuel dwarfs their piston predecessors.

Next up is efficiency. As in wings, longer, thinner blades do a better job converting this power. A typical measurement is the thrust-specific fuel consumption:

$$\frac{g}{(s \cdot kN)}$$

which is fuel flow (fuel over time) per thrust.

Piston engines went from 2 to 3 to 4 blades, enabling more thrust output, but the "wake", or turbulence generated from one blade lowers the efficiency of the next one following it. 1 blade is most efficient, but to get more thrust from the more powerful engines, some efficiency is sacrificed. Fan jets follow this trend with many blades.

But short blades with a very long chord would be hopelessly inefficient for generating thrust in air. A very good example of this thinking is the modern windmill design, with fewer, longer blades than the old style mills. The faster you turn, the more you want to be like the modern mill. The turboprop makes the most of propeller efficiency and turbine power but:

All types of props or ducted fans reach a speed limit where their own drag makes them less efficient than low bypass turbojets. Propellers reach this limit before fans do, which is why we see fans on longer range airliners.

So we did not get to noise reduction with this concept, which is relegated to desk top fans.

Modern jets do have some success with noise reduction by controlling their fan blade tip speeds, and by improving the design of their containment housings (nacelles).


What determines the noise an aircraft's undercarriage makes at the moment it leaves the ground? In an Embraer E170 at take off, there is only the faintest clue from the undercarriage that the wheels have left the ground.

In a Fokker F70, there is by contrast a loud mechanical "clonk" that can be heard and felt very distinctly.

What's different in the design of the undercarriage in these aircraft? And what's going on in the F70 that some component of the machinery is walloping around enough to produce such a noise?

Without hearing it I would imagine that what you are hearing is the landing gear reaching full extension and hitting it's mechanical limit. Think of it like one of those bicycle air pumps, and the sound it would make if you pull that handle all he way up. It's the same basic mechanism, except upside down, and the landing gear should be full of hydrolic fluid to support the force of the aircraft landing on it.

One aircraft has a mechanism that has a hard stop at the end of the stroke, and so you hear the extra forces being translated as that sound. The other aircraft has a softer end, so maybe there is a spring or rubber bumper pice or something that softens the end of the extension.


How can I triangulate a position using two DMEs? What is the formula for determining an aircraft's position (latitude, longitude) using barometric altitude and the ranges to two DMEs?

We can find the exact method to use online, after all this is a common 3D trigonometry problem. I'll use Michael Geyer
US DoT report: Earth-Referenced Aircraft Navigation and Surveillance Analysis. The principle is:

The two DME distances determine two spheres which intersection is a circle. The aircraft is on this circle.
The aircraft altitude determines a surface (depending on how altitude is measured, it can be a quasi-plane or a surface of constant pressure) which intersects the circle at two points. The coordinates of the points can be computed from the two DME distances and the altitude.
As a matter of fact DME signals at the two points are identical, therefore one must be eliminated by some method.

Intersect two spheres to get a circle

The slant distance determined with DME tells us the aircraft is on the surface of a sphere centered at the DME station, with radius equal to the slant distance. By interrogating two DMEs, two intersecting spheres are determined. The intersection is a circle (pink circle in the figure below). The closer the aircraft is to the line joining the two DME stations (the baseline), the smaller the circle, down to a single point when the aircraft is exactly on the baseline.

Intersect the circle with a surface to get two points

The aircraft is flying in a quasi horizontal plane. This plane intersects the circle at two locations (green points in the figure above) which are symmetrical to the baseline and where the aircraft must be located. By using the direct information we have (altitude and slant ranges), we can compute the coordinates of the two points.

Eliminate the ghost location

At a given time, the aircraft could be located at either points and still receives the same DME signals. In the mathematical solution, this will appear when using a function like $\arcsin(x)$, which gives two angles for the same x value, e.g. 40° (90°-50°) and 140° (90°+50°).

There are several methods to eliminate the virtual position:

Monitor the progress of the aircraft: If the aircraft flies north, the top point moves north while the bottom point moves south. We can detect such illogical trajectory (e.g. using a predictive Kalman filter).
Use a third DME: The third range sphere will intersect the other two at only one point.
Determine aircraft bearing using a VOR, preferably collocated with one of the DME.

In large aircraft, position is determined using multiple means including inertial and GNSS, so it's not difficult to know where the aircraft is approximately.

Determination of the points coordinates

The DME/DME/Elevation case boils down to:

  Two DME stations $\small U$ and $\small S$ with known latitudes, longitudes and altitudes ($\small L_U$, $\small \lambda_U$, $\small h_U$ and $\small L_S$, $\small \lambda_S$, $\small h_S$).
  An aircraft A with known altitude ($\small h_A$).
  Two measured slant distances ($\small d_{UA}$ and $\small d_{SA}$)

$\small C$ is earth center (earth radius is $\small R_e$. Following the method mentioned previously, we execute these steps:

  Step 0: Convert slant-ranges to angular distance
  Step 1: Solve the spherical triangle for each station
  Step 2: Confirm inputs are consistent and a solution exists
  Step 3: Solve the spherical triangle USA
  Step 4: Compute aircraft latitude and longitude

Step 0: Convert slant-ranges to angular distance

With altitudes and slant ranges known, it is possible to calculate angles $\small \theta_{SA}$ ($\small \widehat{SCA}$) and $\theta_{UA}$ ($\small \widehat{UCA}$) between DME and aircraft. For each DME $\small x$ ($\small x$ being either $\small U$ or $\small S$):

$$\theta_{xA} = 2\space \arcsin \left( \frac{1}{2} \sqrt{\frac{(d_{xA}-h_A+h_x)(d_{xA}+h_A-h_x)}{(R_e+h_x)(R_e+h_A)}} \right)$$

By convention we need to name $\small U$ the station which is westward. This allows us to refer at the aircraft position like "the one south of the baseline".

Step 1: Solve the spherical triangle for each station

The first actual step is to get the angle $\small \theta_{US}$ formed by the two DME stations and earth center:

$$\sin \left( \frac{1}{2}\theta_{US} \right) = \sqrt {\sin^2 \left( \frac{1}{2}(L_S-L_U) \right) + \cos(L_S) \cos(L_U) \sin^2 \left( \frac{1}{2}(\lambda_S-\lambda_U) \right)}$$

where $\small L_U$ and $\small L_S$ are latitudes of $\small U$ and $\small S$ and $\small \lambda_U$ and $\small \lambda_S$ are longitudes of $\small U$ and $\small S$.

We will need to know one of the azimuth at the extremities of the baseline between $\small U$ and $\small S$:

$$ \tan(\psi_{S/U}) = \frac {\cos(L_S) \sin(\lambda_S - \lambda_U)} {\sin(L_S) \cos(L_U) - \cos(L_S) \sin(L_U) \cos(\lambda_S - \lambda_U)} $$

Step 2: Confirm inputs are consistent and a solution exists

If the slant ranges are 10 NM and 15 NM, and the known distance between stations is 30 NM, then there is no actual solution, something must be wrong. This step checks whether the two DME range spheres intersect. We have already computed angles $\small \theta_{UA}$ and $\small \theta_{SA}$ and the one between stations $\small \theta_{US}$:

If $\small \theta_{UA} + \theta_{SA} < \theta_{US}$, then the spheres don't intersect (centers too remote)
If $\small \left| \theta_{UA} - \theta_{SA} \right| > \theta_{US}$, then spheres are concentric, they don't intersect.

Step 3: Solve the spherical triangle USA

Now we know the three sides of the triangle $\small \widehat{USA}$, we can determine any of its angles. We need only the angle with its apex at one station:

$$\cos(\beta_U) = \frac {\cos(\theta_{SA}) - \cos(\theta_{US}) cos(\theta_{UA})} {\sin(\theta_{US}) \sin(\theta_{UA})}$$

Step 4: With all data now available, compute aircraft latitude and longitude

It's time to decide which of the two aircraft positions we want by selecting the corresponding azimuth angle at station $\small U$ (it could be at $\small S$, but we did the previous step for $\small U$):

If $\small A$ is south of $\small US$ baseline (assuming $\small U$ is west of $\small S$): $\small \psi_{A/U} = \psi_{S/U} + \beta_U$
If $\small A$ is north of $\small US$ baseline: $\small \psi_{A/U} = \psi_{S/U} - \beta_U$

Aircraft latitude $\small L_A$:

$$\sin(L_A) = \sin(L_U) \cos(\theta_{UA}) + \cos(L_U) \sin(\theta_{UA}) \cos(\psi_{A/U})$$

Aircraft longitude $\small \lambda_A$:

$$\tan(\lambda_A - \lambda_U) = \frac {\sin(\psi_{A/U}) \sin(\theta_{UA})} {\cos(L_U) \cos(\theta_{UA}) - \sin(L_U) \sin(\theta_{UA}) \cos(\psi_{A/U})}$$

I posted a Python implementation on Stack Overflow.


Does the P&W F100 turbofan engine of the F-16 really produce this much power? I've been trying to calculate the power a P&W F100 engine on a F-16 is producing using some of the available data on thrust and velocity and I ended up with a number that I want to confirm. Here's how I made the calculations.

First of all, I don't want my calculations to involve any complications due to the altitude and its effect on the engine thrust, so I just chose to make my calculations at sea level. Wikipedia states that the maximum speed of an F-16 at sea level is 1,470 $km/h $ which is ~$408$ $m/s$.

Secondly, I found out that jet engines in general produce different thrust at different speeds. I did some research and found that turbojets produce somewhat  constant thrust at subsonic speeds and gain extra thrust at supersonic speeds, while turbofans suffer degrading thrust with speed with a degree depending on the bypass ratio of the turbofan. I found that high bypass ratio turbofans suffer much more thrust degradation at high speeds compared to low bypass ratio turbofans. which means that low BPR turbofans fall somewhere in between the turbojet and high BPR turbofans according to their BPR.

I found that P&W F100 Wiki gives a bypass ratio of $0.36:1$, then looking at this thrust vs speed graph, which confirms what I concluded above, I could determine that the F100 engine with 0.36 BPR can have no more than 10% thrust degradation at mach 1.2. I also think that using the afterburner shifts the curve further up since more air is now entering the core than before without increasing the fan flow and therefore the engine behaves even more like a turbojet than a turbofan. Overall, I think it would be safe to say that the afterburner thrust at Mach 1.2 at sea level will not degrade more than 10% and so the F100 engine thrust will become 0.9×127 kN = $114.3$ $kN$

Now the power is simply $P = F . v$ = $(114.3 × 10^3) × 408$ = $46.6 × 10^6$ $W$ or about $62,000$ $hp$.

So my question now is: Are my calculations acceptable to a certain degree or are they completely off and I didn't take into account another significant factor?

EDIT: I have another question. I'm assuming here that the engine is producing full thrust at the sea level to counter the air drag and achieve the quoted speed. But is this a correct assumption? In other words, could an F-16 fly at its maximum speed at sea level while not using the maximum thrust?

https://en.wikipedia.org/wiki/Pratt_%26_Whitney_F100 gives a maximum afterburning thrust for the F100-PW-220 of 105.7 kN, so you were pretty close there.  Power is not listed on wikipedia, but you can get there.  It lists "Specific fuel consumption: Military thrust: (0.73 lb/(lbf·h))", so all you need to know is the lower heating value of jet fuel, and you can calculate the power. Off the top of my head, 46 MW seems a little high for that size engine, but it's certainly on that order of magnitude.


What exactly is the purpose of a pilot keeping a logbook? More specifically: in Canada what are reasons and protocols of keeping a logbook?

Almost everything in flying is measured in hours, including your experience.

To start with, in most places you need to have trained a certain number of hours before taking your flying skills test, regardless of how good you are.

And then it continues, want to do an instrument rating, you need x hours. Want to be an instructor, you need y hours. Want to be a Captain on a 747, well you'll need z hours as First Officer first and to be First Officer, you'll need....guess what!

Beyond that, many countries place restrictions on what you can and can't do based on your recent experience. For example, in the UK, to take passengers you must have performed three landings on type within 90 days.

And then it carries on outside of legislation, too - if you want to rent a plane most places will place some sort of hour requirement, whether it be your total hours or hours on type etc.

And so it follows that you need somewhere to log this. As a pilot your log book is sacrosanct - its a legal document forming a factual record of your flying experience.


Can you identify this aircraft? This photo, taken circa 1939, was taken of my uncle's aircraft at Roosevelt Field on Long Island, New York.  Lotar Munch and his partner operated "Munch & Romeo Flying Service" out of Hanger 32. I'm trying to learn more about his business and his aircraft as well.  Note the registration number "NC966V."  I have learned that the "C" indicated that it was a commercial registration. Any help would be much appreciated!

As some have written in the comments, it is a Westbrook W-5-B Sportster.
Westbrook started as a company in 1929 and failed in 1931. Approximately five W-5 sportsters have been built:

[NC9N] c/n 501 destroyed in a 1931 crash
[853W] c/n 502,
[NC966V] c/n 503 lost in a 1938 accident
[NC92V] c/n 504,
possibly unlicensed one,  c/n 505,

source / further info: aerofiles.com / secretprojects.co.uk / all-aero.com

Munch & Romeo, Inc
The aircraft was shown on the business card of Munch and Romeo, which also shows the prices they charged for flight instruction:

I found evidence that one of the aircraft of Munch and Romeo was  involved in a mid-air collision in 1936:

Negligence -Collision -Insufficient Allegation of Agency.
-[NewYork]  In an action for damages arising out of a collision between
the plaintiff's airplane with that of the defendant as both were about
to land,the complaint alleged that the defendant's plane was being
operated by one Donahue with the consent and permission of the
defendant, and that the collision was caused solely by Donahue's
negligence. On motion to dismiss the complaint for failure to state a
cause of action, held: complaint dismissed, with leave to plaintiff
to serve an amended complaint within ten days.There were no
allegations that Donahue was operating the defendant's airplane as
his agent, servant or employee, or in the defendant's business
and within the scope of his employment. None will be inferred from the
allegation that the defendant was the owner of and in control of the
airplane.An allegation that Donahue was operating the vehicle with
defendant's con-sent and permission would be sufficient to charge the
latter in a case arising out of an automobile collision on a public
highway under §59 of the Vehicle and Traffic law, but the statute is
not applicable here. Consequently the common law rule governs, by which
it is settled in New York that the owner of a vehicle will not be held
liable for negligence of its operator unless at the time it was being
used in the owner's business by the operator within the scope of his
employment. Munch & Romeo, Inc. v. Caton, 96 N. Y. L. J.876, 235 C. C.
H. 1213 (County Court, Nassau County, New York, Sept. 25,1936).

source: Journal of Air Law and Commerce, Volume 8|Issue 1, Article  8 scholar.smu.edu

The company owned other aircraft as well; by 1936 they seemed to be active in aircraft trading as they tried to sell several aircraft in advertisements in Aero Digest, Volume 29 of July 1936

AERONCA C-3: New September 1935; 150 hours  total time ; just top
overhauled, A-l condition  throughout, SI. 425. Munch & Romeo, Inc.,
Hangar  7, Roosevelt Field, Mineola, New York.

1936 TAYLOR CUB: 2 months old, 35 hours total  time, \$1,195. Gee Bee,
Kinner K-5: 45 hours since  overhaul, \$875. Avian Cirrus, \$600. Also
Great Lakes,  Birds, Cessnas, Monocoupes. Munch & Romeo, Inc.,  Hangar
7, Roosevelt Field 1 , Mineola, New York.


Why has the maximum service ceiling of Boeing and Airbus products remained about the same for 30 years? When Boeing introduced the 747-100 in 1969, its maximum ceiling was 45,100 feet; half a century later, when Boeing introduced the 777x, its maximum ceiling was 43,100 feet.

Similarly, the maximum ceiling of the Airbus A300, introduced in 1972, is 40,000 feet, while the ceiling of the A350, introduced 2 years ago, is 43,100 feet.

Given the all the other advances in aircraft design between the past and current generation of airliners, why has the ceiling remained unchanged?

Mainly, the optimum cruise altitude is where thrust and lift requirements for both take-off and cruise balance well. An additional benefit is the colder air which increases the efficiency of heat engines. Since this helpful drop in temperature ceases once the aircraft climbs above the tropopause, the benefits of flying higher increase most below the tropopause.

With increasing flight altitude, the airliner needs:

Bigger engines to create the needed thrust in thinner air
Bigger wings to create the needed lift

With the wings, the size of the tailplanes will also grow; this effect alone likely will weigh more than the beefing up of the fuselage structure for the increased cabin pressure. Flying higher will make almost all parts bigger and heavier.

Note that Mach 0.85 is a hard limit for efficient flight; airliners cannot compensate for lower density by flying faster. The only way to allow higher flight levels is to attach bigger wings and tails.

Another consideration is Breguet's formula: Jet aircraft have their optimum cruise lift coefficient at a value of $c_L = \sqrt{0.6\cdot c_{D0}\cdot\pi\cdot AR\cdot\epsilon}$, if we assume the thrust of high-bypass-ratio engines to vary with speed proportional to $v^{-0.5}$, which is a reasonable assumption. This means the airliner cannot fly higher by flying at a higher lift coefficient: This would decrease efficiency.

(Nomenclature: $c_{D0}$ = zero-lift drag, $AR$ = wing aspect ratio, $\epsilon$ = span efficiency)

With the wing size and the engines needed for flight at Mach 0.82 in the tropopause (Mach 0.85 is really not as efficient; follow the link to find out why this is the quoted cruise speed for long-range airliners), the take-off distance is quite reasonable and approximately matches the airports which had been defined by NATO during the cold war. Flying any higher into the stratosphere would increase the aircraft's mass due to bigger engines and wings, but would not incur the efficiency gains of increasing cruise altitude in the troposphere, where temperature drops with altitude.

Conversely, picking a lower design cruise altitude would allow to make both wings and engines smaller, but this would translate into:

Higher take-off and landing speeds, and critical speeds during take-off due to the smaller wing,
Lower take-off acceleration due to smaller engines,
For twins: Not enough thrust during take-off when one engine fails,
Lower climb speeds, so it would take longer to reach cruise altitude, and
Not fully taking advantage of the cold air up in the tropopause.

Designing for a lower cruise altitude would translate into much longer runways and less efficient flight overall.

Designing for cruise in the tropopause is simply the sweet spot for airliner designers where all conditions match well and produce a balanced outcome.


Why choose a seaplane for crew rotation at sea, supply of spares to ships, long-range search and rescue and surveillance? The Indian government has been discussing with Japan since on buying Shinmaywa US-2 aircraft which would be used for various roles such as crew rotation at sea, supply of spares to ships, long-range search and rescue and surveillance according to Flight Global.
The partnership is strategic for Japan that seeks to contain the rise of China and India that can not get an alliance with Pakistan, but that, we do not care. It is also the return of Japan to the club of countries selling international aircraft.

Yet, my question is : why choosing such an atypical airplane for such missions ? In France I know that SAR missions are done by Falcon 50 and 200, which are long-range business jets, or helicopters. Neither of them are able to land at sea but jets are faster and helicopters have divers to be to take care of the SAR mission.

For crew rotations, it is true that the US-2 can carry up to 20 people. And if this kind of missions only concerns large boats where my lack of knowledge in the Indian Navy suggests to me that they still have helipads, the NH-90 carries as many troops as the US-2 and can even land directly on it if it has one. Furthermore it can land anywhere, but the sea indeed.

Thus why choose a seaplane for search and rescue, crew rotation at sea, supply of spares to ships, long-range search and rescue and surveillance ?

To my mind, their is lots of positive aspects using this aircraft as far as it would enable them to be faster in crew rotation at sea, supply of spares to ships but to be slower for SAR and surveillance, which seems to me to be the most important mission. Is it a difference of philosophy between French and Indians ? Or is their a deeper reason to prefer seaplane ?

The US-2 is an amphibious airplane not a pure seaplane, it can land on water and land. MTOW is a bit higher on land as well.

Purely logistics alone means that a single craft type is better.

It is better to have a jack of all trades than a bunch of specialists when building out your fleet unless you have immediate need for those specialists.

Each aircraft type that gets added to the fleet has its own maintenance handbook, maintenance schedule, set of spares, their own type rating, etc.

Helicopters have smaller tanks and can't stay in the air as long or get out as far away from base. They also cannot carry as much stuff as the US-2 can. They would be handy for search and rescue and supplying helipad-equipped vessels. However not all ships will have a helipad.

Jets cannot land out at see so they can only do the search part of search and rescue. They cannot do any supplying out at sea that doesn't involve putting the packages on a parachute.


Can Airbus A400M take-off from the beach fully loaded? I've recently saw this footage. Airbus A400M tactical capabilities on natural surfaces by performing take-off, landing and taxiing manoeuvres. I wonder if she could take-off fully loaded from that surface ?

The surface of the beach will need a certain California Bearing Ratio to be allowed to have the aircraft land there. This will be measured by the ground support prior to the arrival of the aircraft.
This will allows the aircraft to operate largely normally when on the ground roll and taxiing, however the crew would need to be mindful about brownout when using reverse thrust and to be gentle when turning so as not to bury the nose wheel.
The aircraft will have a maximum mass for operations at the given CBR. There will also be a max mass based on the aircraft performance versus the field conditions and parameters with margins for safety (though not the same as standard Engine-Out performance or Balanced Field) which is unlikely to be its normal maximum takeoff mass.
EDITED for scope.


What would be a technical or slang term for 'in the air'? I'm writing a sci-fi story involving aircraft and space craft. I'm struggling to get the terminology right for referring to a pilot being 'in the air' either on a mission or a training exercise. I have a feeling there is a phrase for this but I'm struggling to think of it.

The context would be military, as in the following sentence:

  She kept the Squadron Leader updated as much as she needed to, but she preferred to keep comms chatter to a minimum when she was in the air.

She kept the Squadron Leader updated as much as she needed to, but she
preferred to keep comms chatter to a minimum when she was...

Military-related terms:

...on patrol
...on station
...on a sortie
...on a shout [British English, specifically used for search and rescue]
...on recce/on recon [British and American English, respectively]

General terms:

...in the air
...airborne
...at the controls
...flying
...in flight
...on a mission
...doing (her) rounds
...aloft
...wheels up

There might be other, more technical terms in use in the military for specific operations or missions but I have no experience there.


What is the the origin of 12,500 pounds in the FAA's definition of large aircraft? According to the FAA's definitions in 14 CFR 1.1, a Large aircraft is an "aircraft of more than 12,500 pounds, maximum certificated takeoff weight." I have heard this threshold was set based on the DC-3, but that makes no sense to me. Also, there doesn't seem to be any common-sense/international connection to kilograms (i.e. 12,500 pounds is roughly 5670 kg, not a nicely rounded-off 5700 kg).

What is the origin of 12,500 pounds being the cutoff between Large and Small aircraft?

The answer by pithblot is more or less correct. The 12,500 lb. limit was chosen as an approximate halving of the DC-3 gross takeoff weight.
Consider the following quote:

As noted earlier, when first promulgated, the 12,500 lb. weight limit
could be understood in light of the general use of the DC-3, an
aircraft with a maximum gross takeoff weight of 25,000 lbs. and
carrying from 21 to 25 passengers in airline service... The imposition
of a 12,500 lb. gross weight maximum, by effectively limiting the
number of passengers that could be carried as well, did indeed protect
the local service carriers' interests.

(U.S. House of Representatives, 94th Congress, 2nd Session, 1976, pg. 48)
References:
U.S. House of Representatives, 94th Congress, 2nd Session. (1976). The future of aviation: Hearings before the subcommittee of aviation and transportation R. & D. of the committee on science and technology [no. 82]. Washington, D.C.: U.S. Government Printing Office. Retrieved from Google Books.


gear down during takeoff for extended period of time Normally when an aircraft departs, the landing gear is retracted shortly after the aircraft is airborne. While watching aircraft departing from an airport in my city, I noticed, that occasionally the pilots leave landing gear down for many miles (10NM or possibly more). What is the reason for this, are they trying to cool down something?

It's usually for one of two reasons:

Brake temps are displayed in the cockpit and if the temps were elevated from a lot of, or excessive, brake use during taxiing, the capt may delay retraction until the indications come down, mainly to avoid gear bay overheat indications. This will be more common on airplanes that have more idle thrust than necessary to roll at normal taxi speed, which can cause a tendency to ride the brakes.  CRJ200s, which can get going pretty fast on idle thrust with both engines running, are really bad for this.

Also in winter on a departure from a slushy runway, retraction may be delayed to try to blow off as much slush as possible since once the slush freezes, it can immobilize the wheels, preventing normal spinup on the next landing (if it's still cold at destination).


What do the white dumb-bell signals indicate? From ICAO Annex 2:

  4.2.3.1 A horizontal white dumb-bell (Figure A1-4) when displayed in a signal area indicates that aircraft are required to land, take off and taxi on runways and taxiways only.

  4.2.3.2 The same horizontal white dumb-bell as in 4.2.3.1 but with a black bar placed perpendicular to the shaft across each circular portion of the dumb-bell (Figure A1-5) when displayed in a signal area indicates that aircraft are required to land and take off on runways only, but other manoeuvres need not be confined to runways and taxiways.

The text from Annex 2 is clear. For the first one, aircraft are required to land, take off and taxi on runways and taxiways only. But when is this not the case? Why would you need a signal to specify this - isn't it normally the case that you need to land, take off and taxi on runways and taxiways? Same question for the second one, basically.

Please try to elaborate on the text from Annex 2, and give examples of practical use of these two signals.

I keep thinking of some of the big fly ins (like Air Venture in Oshkosh) where pilots are allowed to park on the grass.

During the fly in they might use that second symbol (dumbbell with stripes) to let people know it's okay to park on the grass, but you still need to take off, land, and taxi on official taxiways and runways (as defined by the event rules).  Thus significantly increasing capacity during the event.

But during other weeks that same airport probably doesn't want anyone taxing onto, or parking on, the grass.  So they would display the first symbol (dumbbell with no stripes) to remind everyone that the exception is gone, and the rules have changed back to what you would normally expect at most airports.  Thus people would be aware they need to stay off the grass and use official tie down areas for parking.

In essence, I think the first symbol exists mostly because of the second one.  At airports where there are occasionally exceptions along the lines of those described above, they may also want a symbol to help pilots know when the exceptions are not in effect.


What prevents aircraft with a tailcone APU from using tail-mounted airbrakes? The BAe 146 and Fokker F-28, which are virtually alone among jetliners in lacking thrust reversers, compensate by using large, tail-mounted airbrakes to help slow the aircraft during descent and landing:

(Image by Adrian Pingstone at Wikimedia Commons.)

According to this answer to a previous question about why we don’t see this on more (and bigger) jetliners, the BAe 146 and F-28 are only able to use tail-mounted airbrakes because, unlike most jetliners, they lack an APU in the tailcone:

  ...Air brakes such as the RJ100 have only upsides, since they influence drag only, not lift. They're not used everywhere because the bit of fuselage where they are mounted usually houses an APU.

(Emphasis added.)

But I don’t see why having an APU in the tailcone would preclude the use of tail-mounted airbrakes; the APU is situated in the middle of the tailcone and exhausts out the tip of the tail,1 while tail-mounted airbrakes are mounted on the sides of the tail and deploy to the left and right:

(Image by dtom_cro at Photobucket.)

As such, having a tailcone APU shouldn’t necessarily preclude having airbrakes on the sides of the tail; in the 146, the space between the airbrakes is basically empty, with only a light truss supporting the rear collision beacon, but it'd be easy to have something more substantial - such as, for instance, an APU-and-other-machinery-housing structure - in between the petals:

What am I missing here?

1: As @ymb1 points out in their comment, the BAe 146 does have an APU, but its APU obviously does not exhaust out the tip of the tail; that statement regarding APU exhaust placement refers to its location in the vast majority of jetliners in general.2

2: That last statement obviously does not apply for jetliners where that real estate is taken up by a full-blown engine (727, Trident, Tu-154, Yak-40, L-1011, Yak-42, etc.).

The airlines prevent them or: the airlines won't buy them if they were offered

The photos you have already show an APU in the tail section (exhaust on the starboard side).

Adding air brakes requires increased structural integrity (weight; money) and additional system connections (weight; money).

When they are of no use, they're simply not added. Why are they of no use?

Well, pure air brakes (as opposed to speed-brakes/spoilers) have 1–2 functions (for jetliners):

Compensating for the lack of thrust reversers
Performing steep approaches when the thrust-to-weight ratio is high (small plane that has too much power). As I noted here: The thrust-to-weight ratio for the RJ100 is 0.28:1, compared to 0.16:1 for the comparable Boeing 717. It's even higher for the smaller RJ's.

Neither affect most jetliners (note that the babybus (A318) can be certified for steep approaches, and it doesn't need such an air brake).

The airlines would rather carry payload (paying load) and not bother paying for maintenance to regularly check and fix a novel item – not all airports are London City: high thrust-to-weight ratio is good for taking off from short runways and performing steep climbs from in-city airports for noise reduction.


Why isn't landing in the water a good option in an emergency? Some people say that landing on water should be considered as a last resort because it is dangerous. But why exactly is it dangerous to land an airplane in the water? Wouldn't it be safer since theoretically, water would reduce the impact?

Note: I am referring to all planes (small and large).

For one you can not "land on water" you can ditch an aircraft in a body of water but you would be hard pressed to call it a landing. You can however, "land in a grass field" or "land on a runway nearby" or "land on a highway" all of which are substantially more controllable then trying to stall a few feet above the ocean.

The result of catching an edge which can dig into the water is the same or worse than belly landing on grass. As others have mentioned at those speeds water is not so forgiving. but historically it tends to end poorly. While the miracle on the Hudson ended well Ethiopian Airlines Flight 961 did not have the same luck. You can see the outcome of contacting asymmetrically is pretty devastating.

The secondary concern is that even in a successful ditching still puts you in a sinking airplane with potentially injured people making rescue all that much harder.

Although Aviation Saftey Magazine paints an interesting counter point.


Is a holding pattern required on the Twentynine Palms VOR for RWY 26? I am an instrument student and confused about whether or not you have to fly the holding pattern charted on the VOR RWY 26 at Twentynine Palms. At Jehli (Intermediate Fix), it says "No PT".

This chart provides two ways to begin the approach. The first is at the TNP VOR, which is labeled as an IAF (Initial Approach Fix). Since a holding pattern is charted, you are required to fly that holding pattern as part of the procedure if you begin at TNP.

The other way to begin the approach is at JEHLI, which is labeled as an IF (Intermediate Fix). The "NoPT" instruction only applies if you start the procedure at JEHLI, as this establishes you on the 255 inbound radial to TNP and no hold is needed.

If a hold or procedure turn is charted, you need to fly that maneuver unless you are coming from an IAF or IF with the "NoPT" instruction (or if ATC clears you "straight in").


Is the September 11th the only time a plane was crashed into skyscrapers? I wonder if there has been other plane crashes with skyscrapers in history, except the one of the World Trade Center.

Intentionally with a large aircraft, yes. There was however an instance in 1945, where a B-25 Mitchell collided with the Empire State building. This however was an accident.

There have been several suicides by airplane where the aircraft was deliberately crashed into buildings, such as the C172 which was crashed into a Tampa FL high rise in 2002 or the Piper Dakota crash in Austin, TX.  There was also a case of an accidental crash of an SR-20 into a NY high rise apartment building in 2007, killing baseball player Cory Lidle.


What do jet pilots say during the takeoff as they are speeding down the runway? I was watching a documentary on youtube and the pilots always say something that sounds like "B1E" when they have enough speed to take off. What exactly are they saying, and why do they say it?

What you are hearing is "V-One", written as V1.
It actually is said when they can no longer safely abort the takeoff with the remaining runway, but they still are not quite ready to takeoff.
As they get a little faster, there should then be a second callout of "Rotate" when they have achieved the required takeoff speed, and that is when they actually takeoff.
To answer the "why" part, it is said by the pilot who isn't actually flying the airplane since he has more time to monitor the speed and can say it at exactly the right time.  It alerts the other pilot that it is no longer safe to abort the takeoff and that pretty much no matter what happens now, they should continue and come back to land if needed.
Edit:
In my answer above, I simplified somewhat by only mentioning the available runway.  There are other factors that can come into play even with a runway that is longer than needed.  One example would be the speed after which the brakes can no longer stop the airplane because they would over-heat and become ineffective.  This is known as the brake energy limit, and it is not safe to abort above this speed, even if you have "plenty" of runway.
V1 must be calculated or looked up prior to every takeoff, along with the other performance numbers.  They vary based on the weight of the aircraft, the configuration of the aircraft, the altitude of the runway, and the weather (outside temperature, wind, etc.).  Sometimes this is done via a computer and sometimes manually via lookup tables or even graphs.
For an example of the graphs, and more information about how temperature and altitude affect takeoff performance, see my previous answer here:
Does temperature affect takeoff performance?
There are also times when a pilot may increase or decrease V1 intentionally, even when it is not required.  For instance, if there is wet ice on the runway it may not be safe to abort even at a very low speed, so you would be forced to continue no matter what (note that I'm NOT saying that this is a good idea). Using a lower power setting to extend engine life is another example, and you can read about it in my question here:
Why would an airplane pilot choose to intentionally use more runway than required for a takeoff?
For way more information than you probably care about takeoff performance, see my other answer here:
Minimum Runway Length and V1
In closing, aircraft performance planning is a rather complicated subject and it is only one of many subjects that we must master in order to safely fly an airplane.  This only just scratches the surface of it, and there is a lot more that must be considered (such as "hey, will we be able to out-climb that tree off the end of the runway?").


Why isn't the Transition Altitude depicted on SID charts? Why isn't the Transition Altitude depicted on SID charts?

I would assume it would be a relevant information to be presented on this type of chart.

FAA

Because it is a fixed value at 18,000 feet.

ICAO-style and Jeppesen

They show the TA (and sometimes the TL) on SID charts.

ICAO Annex 4 confirms the depiction on SID charts (chapter 9):

  9.9.4.1 The components of the established relevant air
  traffic services system shall be shown.

  (...)

  5) transition altitude/height to the nearest higher 300 m or
  1 000 ft;


What is the difference between mist and haze in terms of relative humidity? I am an aviation meteorologist and I struggle to distinguish the two of them.

The term haze does not say anything about the humidity of the air because it is by definition caused by dry particles, so not by water. If you have mist (or fog), then water droplets are suspended in the air, which usually comes with 100% humidity.
Here is a set of definitions by the UK's Met Office:

Fog
In our meteorological glossary fog is defined as 'obscurity in the surface layers of the atmosphere, which is caused by a suspension of water droplets'.
By international agreement (particularly for aviation purposes) fog is the name given to resulting visibility less than 1 km. However, in forecasts for the public, this generally refers to visibility less than 180 m.
Mist
Mist is defined as 'when there is such obscurity and the associated visibility is equal to or exceeds 1000 m.' Like fog, mist is still the result of the suspension of water droplets, but simply at a lower density.
Mist typically is quicker to dissipate and can rapidly disappear with even slight winds, it's also what you see when you can see your breath on a cold day.
Haze
A third term you might also hear mentioned is haze. This is a slightly different phenomenon which is a suspension of extremely small, dry particles in the air, not water droplets. These particles are invisible to the naked eye, but sufficient to give the air an opalescent appearance.

(metoffice.gov.uk)
Also see: What is the difference between mist and fog?


Is lift/drag the same as coefficient of lift/coefficient of drag? I see it being used interchangeably (for example by Airfoil Tools Fusion360 AddIn), so can I just pick an airfoil from the airfoiltools database with a certain CL/CD for my aircraft and assume L/D is the same?
An example airfoil from the database: clarky-il
(I know there is induced drag as well, which is not calculated in the databse)
I know the formulas, so it makes sense to me. Just want to verify there is not something I am not aware of, in case I want to calculate the lift depending on the angle of attack.

Yes. Lift and Drag are both obtained by multiplying the CL & CD by the same factors (1/2 rho V^2 S); the ratio L/D cancels out those factors, leaving CL/CD.


How is true airspeed measured? Now, indicated airspeed is pretty much the dynamic pressure that air is creating.
What about true airspeed? How do the calculations go about factoring out all the wind patterns and finding the speed?
Also, for ground speed, how can we factor in the head, tail and crosswinds to determine the ground speed?

You don't measure TAS. You measure IAS (or CAS), then correct for altitude and nonstandard temperature to get TAS. As a rough rule of thumb, add 2% to the IAS per 1000ft of density altitude.
With modern avionics, you know your GS from GPS or similar systems, so the difference between TAS and GS tells you the head/tailwind component and course vs heading tells you the crosswind component, which will be combined to display the direction and speed of the actual (not forecast) wind.
With older (or no) avionics, you get a winds aloft forecast before departing, and you do the reverse of those same calculations with an E6B to find out what heading to fly to get the desired course.


Where can I find the German GAFOR online? Where can I access the GAFOR for Germany for free online (or at least something close)?

You can find all the free aviation weather services of the DWD here:

https://www.dwd.de/DE/fachnutzer/luftfahrt/teaser/luftsportberichte/luftsportberichte_tabelle.html


How much thrust do you lose by putting chevrons on a engine? The chevrons on a 787 are there exactly to decrease a type of drag that makes the engine louder, but the chevrons form vortices causing more drag. How much thrust is lost from the vortices on the chevrons?

The main purpose of the Chevrons is the reduction of engine noise. During research into this, NASA tested a number of configurations under simulated flying conditions and measured the nozzle thrust coefficient given by,

$C_{T_{r}} = \frac{F^{d}_{g}}{F^{i}_{g}}$

where,

$F^{d}_{g}$ is the measure gross thrust and

$F^{i}_{g}$ is the ideal gross thrust

We have,

$F^{i}_{g}$ = .{m}$_{c} v_{i,c}$ + .{m}$_{f} v_{i,f}$

where .{m}$_{c}$ is mass flow rate of core, .{m}$_{f}$ is mass flow rate of fan and $v_{i,c}$ and $v_{i,f}$ are core and fan ideally expanded jet velocities respectively.

The reduction in nozzle thrust coefficient was compared with baseline (no modifications) configuration.

Source: NASA/TM—2000-209948

The configuration used in engine (Chevrons in the fan and nothing in the nozzle produced the lowest loss of thrust coefficient, 0.18.

However, the noise reduction was lesser in this configuration compared to the others. Further modifications have been carried out in the chevrons used in production engines (they are rounded, for one). So, the noise reduction has been obviously improved, though the minimal loss of thrust would've been maintained.

Note: All data from Acoustics and Thrust of Separate-Flow Exhaust Nozzles With Mixing Devices for High-Bypass-Ratio Engines, Naseem H. Saiyed, Kevin L. Mikkelsen and James E. Bridges, Prepared for Prepared for the
Sixth Aeroacoustics Conference and Exhibit, Lahaina, Hawaii, June 12–14, 2000


What is high altitude testing? When new engines are designed for aircraft (military or civilian) they undergo high altitude testing (especially the military ones).

What is high altitude testing of engines? How is that important?

High altitude means low air density. Ignition is harder to achieve with lower density, and the size of the combustor determines up to which altitude and throttle setting an engine will stay lit. Testing that it really works at those low densities before flying is a really good idea.


What effect does tropopause entry have on the FMGC? In my company it’s a procedure to enter the highest tropopause level into the INIT A page. Could someone please explain the effect it has.

It adjusts the FMGC temperature model to account for non-standard temperature. This will have the effect of improving performance predictions by having a more accurate temperature model.

The FMGC uses a temperature model based on the International Standard Atmosphere (ISA). The ISA is based on a sea-level temperature of 15 deg C. In the model, temperature decreases with altitude at a rate of 1.98 deg C/1000 feet up to the tropopause. Above the Tropopause, the temperature is a constant -56.5 deg C.

Updating the tropopause as well as entering the cruise altitude/temperature will adjust the temperature model closer to actual and thus improve predictions. Not entering either value will result in predictions using the ISA model which will reduce the accuracy of predictions (except in the off chance the actual weather matches the ISA.)


In modern commercial aircraft, is it possible for pilot inputs to exceed the tolerable structural loads? It seems that modern fly by wire systems should / could limit pilot inputs to prevent structural damage.

For example, American Airlines 587:

  From the NTSB report of the accident:

  The National Transportation Safety Board determines that the probable
  cause of this accident was the in-flight separation of the vertical
  stabilizer as a result of the loads beyond ultimate design that were
  created by the first officer’s unnecessary and excessive rudder pedal
  inputs. Contributing to these rudder pedal inputs were characteristics
  of the Airbus A300-600 rudder system design and elements of the
  American Airlines Advanced Aircraft Maneuvering Program.

Airbus fly-by-wire aircraft have what is called envelope protection which is to prevent pilot inputs from exceeding structural limits as well as keep them from getting the aircraft into an upset flight regime (stall, overbank, etc.). The philosophy behind it is that, if the pilots need to perform an extreme maneuver they can be as aggressive with the controls as they want, confident that they won't break the plane.

Boeing generally disagrees with this approach saying that, in an emergency, the pilot needs to have all possible force at his disposal, even if it breaks the plane. See the question Federico linked to in comments for more on this.

Of course, the Airbus system can only provide these protections when systems are all functioning properly. If part of the system is not working the protections are removed. Examples: AF447, XL Airways 888T, Indonesia AirAsia 8501,

As far as AA587, the A-300 is not a fly-by-wire aircraft and doesn't have envelope protections. It's debatable, though, if envelope protection would have helped in that situation. None of the rudder inputs by itself was outside of the structural limitations of the aircraft so the computer would probably not have prevented them. The failure was due to the pilot making alternating inputs in quick succession which caused the sideslip angle to build up to twice the certified design strength of the vertical stabilizer. The A300 system was designed to limit the rudder travel available as speed increased, which is probably fairly similar to what the fly-by-wire computer is programed to do in other Airbus planes. But it would be difficult to determine what complex combinations of inputs might exceed the limits and prevent them.


Is an "atmospheric geostationary satellite" feasible with current technology? Meaning by "atmospheric geostationary satellite" a vehicle capable of hovering 30 km above Earth surface, hence insde atmosphere, for unlimited time, making use of air propellers, solar panels and batteries.

How much energy is needed to keep an object steady in air at 30 km above surface?

There are two possibilities: a plane-like vehicle, flying in circle, using wings to stay up; an helicopter-like vehicle, hovering.

Short answer: No.

Long answer: Your "unlimited time" condition makes the short answer easy. If you need to go up for a limited time, and you have the liberty to choose the time of ascent and descent, the answer is: Possibly.

First, there are two ways of going up there: A balloon and an aircraft. A helicopter would be much less efficient, the tether of a balloon will be too heavy to lift, and you will see that even a glider-like aircraft will struggle to stay for some time at 30 km.

Next, atmospheric pressure at 30 km is 12 mbar, just 1.2% of what it is on the ground. So your aircraft has to go at some speed just to create enough lift. If we assume that you fly at Mach 0.5 to 0.6 (where high-lift airfoils and propellers can still operate), your dynamic pressure is that of a modern glider at low speed. The flight speed actually is between 150 and 180 m/s, however.

Now let's assume your aircraft has an L/D of 50. This translates into a power requirement just to stay aloft of 3 to 3.6 kW per ton of aircraft mass. Your propeller will have no more than 85% efficiency, and your engine will create some loss as well, so your installed power needs to be at least 4 kW per ton of aircraft mass. Since you fly at a glider's dynamic pressure, you will also have a glider-like wing loading of 30 kg/m². Per ton, your wing area is 33.3 m². To operate an air breathing engine at this altitude is a challenge, so I will assume solar electric propulsion. Let's be optimistic and we assume a solar constant of 1.4 kW/m² at this altitude, and you fly at moderate latitudes (say, 45°), so your solar panels can create (20% efficiency) 200 W per m² or 5.3 kW per ton of aircraft mass (assuming 80% of the wing is covered, the rest will have too much curvature). Of course, this is all valid only around noon, so if you want to stay overnight, even in summer there is not enough energy to keep the aircraft up for a longer time, even with very optimistic assumptions. You might want to assume batteries which can provide the power for the climb phase, so you take off in the middle of the night and arrive (with empty batteries) at altitude in the late morning. Then you might really stay there until 2 or 3 pm, when the lowering sun means that your solar panels will not provide enough power to stay up, let alone charge the batteries. And I have not even started to subtract the power for operating any equipment (which would be the reason to go up there in the first place, right?).

To go with a lower wing loading means you will have a very delicate structure which cannot cope with the winds at 30 km and gust loading at lower altitude.

This wind will also be a problem for any ballon. 20% of winds at 30 km are above 76 m/s (Mil Std. 210C), 5% are even above 98 m/s. One cubic meter of helium gives you 10 N of lift at sea level, but only 0.156 N at 30 km. To lift one ton of balloon up to 30 km, you will need 63,000 m³ of volume. This is a sphere with a diameter of 50 m! The wind forces to keep this thing tethered will need a strong and heavy tether, so without going into the details I think that a tethered balloon will be a challenge, to put it mildly.

The "Facebook drone" articles are not serious, they are meant to create some P.R., and don't hold your breath for Internet delivered by drones anytime soon.


Can active flight plan be sent for the aircraft in the air? We are processing some aviation data and I wonder if it is possible that flight plan message can be sent for an active flight (aircraft in the air)?

In the documentation we have the sentence:
"Active flight plan, aircraft is in the air. For ARTS III indicates an overflight"

Your question is short on details, but in general, yes this is possible. A flight could be coming from anywhere, for example:

Another jurisdiction (FIR)
Uncontrolled airspace
A beach somewhere

In these sorts of cases, you may see the first FPL message after the aircraft has departed. The departure aerodrome, if known, may appear in the ADEP field (field 13a), or it may be ZZZZ for an unknown aerodrome, or it may be AFIL for an "air filed" flight plan.


Why can't Centre of pressure and Centre of Gravity coincide? I understand that normally CG is placed ahead of COP so that a tail down force can make the aircraft stable. But why can't the COP and CG be placed in the same position so that there needn't be a tail down force to stabilize the aircraft.

I read an answer in Quora saying that "When the CG and CP coincides, the pitching moment will be 0 as the distance between them is 0" What does that mean?

Centre of pressure (CP), by definition, is the point at which the aerodynamic moment of something is zero.

If you are talking about the CP of the whole airplane, then at trim condition, the CG must always coincide with its CP. Period. This is achieved for any operational speed through well designed pitch control that varies in flight. For example, stabilizer/elevator for tailed airplanes, elevons for flying wing.

If you are talking about the CP of a single aerodynamic surface, like a wing or tail, then it gets more nuanced. Let's restrict the discussion to pitching moment only. First of all, CP is not a very useful measure of stability: it always moves around for a cambered airfoil. In fact, at an AOA where the lift is zero for a positively cambered airfoil, it moves to infinity!

Instead, use aerodynamic centre (AC) for stability analysis. For pitch stability, you would like the CG to be ahead of the AC of the entire airplane (this is called the neutral point).


Why is an air intake located on the root of the tail on some aircraft? There is a small air intake on the root of the tail of some aircraft, especially those with engines near the end of the aircraft. What is it and why is it located there?

That is the Ram Air Inlet for the air conditioning system:

Ram air, taken from the air scoop in the base of the vertical fin, is used as the cooling medium for  the  heat  exchangers.  Ram  air  passes  across  the  precooler  and  the  dual  heat  exchangers, extracting heat from the bleed air used by the air-conditioning packs. The ram air is discharged through exhaust louvers on the upper left and right sides of the aft fuselage.

During unpressurized flight, ram air may be used to ventilate the cockpit and cabin.

(Bombardier Challenger 605 - Air Conditioning & Pressurization)
The reason why it is located there is obvious from the picture above: the two air conditioning units (ACUs) are located just below so this provides the shortest duct. For tail mounted engines, this is the optimal location for the ACUs (also called PACKs) because the bleed air ducts from the engines (or APU) can be short this way. The ram air can also be used for ventilation when the pressurization has failed.
For wing mounted engines, the PACKs are usually located in the belly. The inlets are then found on the underside of the aircraft:
What exactly are those openings in the bellies of many jetliners?
On some turboprops you can also find them on the side:
What's this hole on the ATR-72?


Can an American citizen schooling abroad join the airforce? I am a citizen of the U.S who is studying abroad from the age of 9.
If I am to Apply to become a test pilot will there be any impediments?
Due to security issues as it deals with classified aircrafts.
Aviation is something I really want to get into and I plan on taking aerospace for my college course, becoming a test pilot is an overlap of both worlds and is something I always wanted to be. I feel such reasons could be a hindrance.
Any links and answers will be much appreciated and If this question is not within guidelines a point in the right direction would awesome.

In which countries have you been studying? It probably won’t make any difference. Also, you do realize that there is no way to apply to be a test pilot ar entry into the military. You would have to serve as a pilot first before getting the opportunity to be a test pilot.
It is very possible for you to become a military test pilot. Follow these steps. Move back to the US. Get into a university. Join AFROTC. Graduate from both. Pass the FBI background check. Join the military. Become an officer or warrant officer (for helicopters). Become a military pilot. Serve and gain experience as a military pilot. Apply for test pilot school. By the time you apply to test pilot school, it won’t make any difference that you spent 10 years living abroad.
You may still be able to become a military test pilot without moving back to the states or graduating ROTC before your background check. It just makes the process more difficult. Unless, of course, you were overseas because your parents were serving a diplomatic or military mission. Living in an allied country or a community of American expats would help the process of the FBI background check as well. Read about Andy Thomas, an astronaut from Australia that did not even become a US citizen until he was 30.


How to determine if flow transitions or separates from a Cp plot? I came across this image:

My question is: given a general Cp plot, how does one tell from the shape of the curve if flow transitions or separates? Specifically, what features on the plot do I look out for that can tell me if flow transitions or separates (e.g. inflection points, plateaus, etc)

I know that a strong adverse pressure gradient causes separation which shows as a steep downward slope on the graph. What I don't know is where exactly on the graph transition or separation happens.

Separation happens at the separation point, when the steep pressure gradient suddenly changes to a plateau, and transition happens at the transition point when the plateau changes back to a steep pressure gradient. Reattachment is a little harder to discern: Here the slope of the pressure gradient, which is quite steep right past the transition point, changes back to something close to its inviscid value.

It helps to plot the inviscid pressure distribution (dashed lines) together with the "real", viscous one (solid lines), as shown below for an Eppler 502 at 3° angle of attack:

The blue line shows the pressure coefficient on the upper side and the red one on the lower side. On both sides there are laminar separation bubbles, but the one on the bottom is much bigger, so I will focus on this one. In a way, the plot lines, or more precisely the difference between inviscid and viscous pressures, mirror the behaviour of the boundary layer: With the separation the boundary layer thickens as the outer flow lines move away from the wing contour. With the transition the flow lines bend towards the contour again and at the reattachment they continue to follow the contour.

Detail view of the plot above.