Can hours accumulated in a glider count towards the 1500 hours for an ATP certificate? If you have ratings in Glider, ASEL, AMEL do all your PIC hours in all categories count towards an ATP certificate, or must the Glider hours be kept separate?

According to my reading of 14 CFR 61.159, glider hours do count toward the 1,500 hour total time requirement, but not several of the other requirements. From 14 CFR 61.159 (parts in bold are where I've highlighted those requirements that must be met in an airplane category, i.e. not a glider).

Presumably you're also not logging much instrument flight time in gliders, so I didn't highlight that.

  § 61.159 Aeronautical experience: Airplane category rating.

  (a) Except as provided in paragraphs (b), (c), and (d) of this section, a person who is applying for an airline transport pilot certificate with an airplane category and class rating must have at least 1,500 hours of total time as a pilot that includes at least:

    (1) 500 hours of cross-country flight time.

    (2) 100 hours of night flight time.

    (3) 50 hours of flight time in the class of airplane for the rating sought. A maximum of 25 hours of training in a full flight simulator representing the class of airplane for the rating sought may be credited toward the flight time requirement of this paragraph if the training was accomplished as part of an approved training course in parts 121, 135, 141, or 142 of this chapter. A flight training device or aviation training device may not be used to satisfy this requirement.

    (4) 75 hours of instrument flight time, in actual or simulated instrument conditions, subject to the following:

      (i) Except as provided in paragraph (a)(4)(ii) of this section, an applicant may not receive credit for more than a total of 25 hours of simulated instrument time in a flight simulator or flight training device.

      (ii) A maximum of 50 hours of training in a flight simulator or flight training device may be credited toward the instrument flight time requirements of paragraph (a)(4) of this section if the training was accomplished in a course conducted by a training center certificated under part 142 of this chapter.

      (iii) Training in a flight simulator or flight training device must be accomplished in a flight simulator or flight training device, representing an airplane.

    (5) 250 hours of flight time in an airplane as a pilot in command, or as second in command performing the duties of pilot in command while under the supervision of a pilot in command, or any combination thereof, which includes at least -

      (i) 100 hours of cross-country flight time; and

      (ii) 25 hours of night flight time.

      (6) Not more than 100 hours of the total aeronautical experience requirements of paragraph (a) of this section or § 61.160 may be obtained in a full flight simulator or flight training device provided the device represents an airplane and the aeronautical experience was accomplished as part of an approved training course in parts 121, 135, 141, or 142 of this chapter.

  (b) A person who has performed at least 20 night takeoffs and landings to a full stop may substitute each additional night takeoff and landing to a full stop for 1 hour of night flight time to satisfy the requirements of paragraph (a)(2) of this section; however, not more than 25 hours of night flight time may be credited in this manner.

  (c) A commercial pilot may credit the following second-in-command flight time or flight-engineer flight time toward the 1,500 hours of total time as a pilot required by paragraph (a) of this section:

    (1) Second-in-command time, provided the time is acquired in an airplane -

      (i) Required to have more than one pilot flight crewmember by the airplane's flight manual, type certificate, or the regulations under which the flight is being conducted;

      (ii) Engaged in operations under subpart K of part 91, part 121, or part 135 of this chapter for which a second in command is required; or

      (iii) That is required by the operating rules of this chapter to have more than one pilot flight crewmember.

    (2) Flight-engineer time, provided the time -

      (i) Is acquired in an airplane required to have a flight engineer by the airplane's flight manual or type certificate;

      (ii) Is acquired while engaged in operations under part 121 of this chapter for which a flight engineer is required;

      (iii) Is acquired while the person is participating in a pilot training program approved under part 121 of this chapter; and

      (iv) Does not exceed more than 1 hour for each 3 hours of flight engineer flight time for a total credited time of no more than 500 hours.

    (3) Flight-engineer time, provided the flight time -

      (i) Is acquired as a U.S. Armed Forces' flight engineer crewmember in an airplane that requires a flight engineer crewmember by the flight manual;

      (ii) Is acquired while the person is participating in a flight engineer crewmember training program for the U.S. Armed Forces; and

      (iii) Does not exceed 1 hour for each 3 hours of flight engineer flight time for a total credited time of no more than 500 hours.

  (d) An applicant is issued an airline transport pilot certificate with the limitation, “Holder does not meet the pilot in command aeronautical experience requirements of ICAO,” as prescribed under Article 39 of the Convention on International Civil Aviation, if the applicant does not meet the ICAO requirements contained in Annex 1 “Personnel Licensing” to the Convention on International Civil Aviation, but otherwise meets the aeronautical experience requirements of this section.

  (e) An applicant is entitled to an airline transport pilot certificate without the ICAO limitation specified under paragraph (d) of this section when the applicant presents satisfactory evidence of having met the ICAO requirements under paragraph (d) of this section and otherwise meets the aeronautical experience requirements of this section.


Why can't you have sail-powered airplanes? So, sailing boats take the kinetic energy of a crosswind and turn it into thrust by using their sails as lifting surfaces, and the discussion I had in the comment section of this question made me wonder why nobody has ever built any (working) sail-powered airplanes.
Basically, imagine a glider, but with sails on it (maybe on both top and bottom of the aircraft, to avoid imbalances in the center of thrust tipping it over) to catch the crosswind and provide forward thrust. Since these vehicles don't exist, there are presumably problems with them that would prevent them from working.
What are these problems? Do the aerodynamics of the sails interfere with the aerodynamics of the wings or something? Even if sails don't provide enough thrust to get the plane off the ground, that wouldn't be a problem for a glider that can't get off the ground by itself, right?

High performance sailboats can sail faster than wind speed in certain conditions. Therefore, your question is not as absurd as it might seem at first glance.
However, the technique that enables sailboats to do so is not available to airplanes, at least not in flight. Sailboats use the speed difference between water and wind. Their keel keeps them from being blown with the wind. A glider in the air has no such second medium to resist being blown away by the wind.
Historically, the Andrée ballon used for artic exploration in 1897 tried to "sail" by combining sails and trailing ropes. However, this did not work in practice.
The only way to extract energy from the wind is when this wind changes. Wind gradients allow albatrosses to soar dynamically, and changing updrafts can be exploited by changing the load factor.


What are these shaded areas? What are these shaded areas? I looked in the Jeppesen legend, but couldn't find the answer. Thanks

It means that no Procedure Turn is required/expected if arriving within the sector defined by the 297 and 020 radials (inclusive) of DBL. The grey bars are a way of denoting the boundaries of the No PT sector (the entire pie section between the radials) without obstructing too much of the other details. That's why the bars are bordered on the north side of the two airways.
The Jepps wording seems to generate quite a lot of confusion because of the practice of including the words "Via Airway".  I found a message thread here that might help a bit.  Just remember that if confronted with something like this on a check ride and are confused, express your confusion to ATC and ask for clarification rather than blunder into a wrong move.


Can I land a plane in a field in the UK and Republic of Ireland? So I already saw this question here for the US, but I live in the UK and quite close to the border with the Republic of Ireland (ROI). We have property over the border in the ROI and I was wondering if I could fly back and forth by using the fields to take off and land in a small plane like the SD-1?

No, you can't shuttle back and forth over the border from one field to another between the UK and Republic of Ireland (RoI) because of regulations. The RoI requires you go through an airport of entry when entering and leaving the country, see the IAA AIP Gen 1.2-1:

  Aircraft landing in the territory of Ireland must first land at an
  International aerodrome or at a licensed aerodrome where customs and
  immigration facilities are available. Aircraft departing must finally
  depart from an International Aerodrome or from an aerodrome where
  customs and immigration facilities are available.

The UK has no requirement to take off and land from a customs airfield, you can go and land from anywhere (from a customs perspective), which is handy. You do have to notify Special Branch at least 12 hours ahead of your flight if you are going to and from the RoI, channel islands or Isle of Man and give them details on who is going and where you are going from and to. You must do this even if you plan to go to and from a customs airport on the UK side.

In the UK you can use a field up to 28 days a year without getting planning permission from the council, provided you own the land (or have permission from the owner) and it is safe to do so. If you plan to do more than 28 days you'd need to talk to the local council and the CAA. I don't know about the RoI though.


Is LPV considered a precision approach? WAAS LPV approaches can have decision altitudes as low as 200 AGL, the same as many Cat I ILS approaches. Previous opinions on the Internet have quoted the FAA as saying LPV is considered a non-precision approach, but many of these entries were made more than seven years ago, when the technology was much less mature. Does anyone know if this is still the FAA's position, and are there any RECENT references to their position?

LPV, LNAV/VNAV, and Baro VNAV are considered to be an 'Approach with Vertical Guidance (APV)'. These types of approaches are differentiated from 'Precision' approaches (ILS, PAR, etc.) in the FAA AIM (Section 5-4-5, Paragraph 7):

  (b) Approach with Vertical Guidance (APV). An instrument approach based on a navigation system that is not required to meet the precision approach standards of ICAO Annex 10 but provides course and glidepath deviation information. For example, Baro−VNAV, LDA with glidepath, LNAV/VNAV and LPV are APV approaches.


To what extent is remuneration under a PPL enforced in the UK? I came across this, How do PPL, CPL, and ATPL compare? and was intrigued by the fact that a PPL cannot fly for any payment whatsoever.

To what extent is this enforced?  An example scenario would suggest that, for example, I fly my employer to an exhibition on the continent, which he then claims from the company and privately reimburses me for the hire and landing fees. Alternatively, I receive some kind of company bonus to cover the cost, or even further, he blatantly pays on a company card.

Is this illegal? How could this kind of activity be stopped?

The rules are as follows:

  Privileges and limitations

  A holder of a PPL may, for compensation or hire, act as a crewmember of an aircraft in connection with any business or
  employment if—

  a. the flight is only incidental to that business or employment; and
  b. the aircraft does not carry passengers or property for compensation
  or hire.

"Compensation or hire" means income, not expenses. You can always be reimbursed for expenses such as fuels. Only income is forbidden under certain circumstances (see below).

Concerning getting income, you can be paid income if both of conditions (a) and (b) above true. That means your business cannot be anything to do with flight. If your company is Boeing or any air-travel related business, you cannot receive ANY income (even your salary) for piloting such a flight.

The aircraft cannot be used to carry paying passengers or cargo.

So, basically what this means is that if you fly anybody from your company, they can pay only the expenses of the aircraft and nothing else. You can be paid your salary, only if flying is not your job in any way. As long as you have a regular job and it is fully incidental to flying, then you can fly clients and fellow employees and receive expense reimbursement only. If you are flying clients, they cannot in any way be paying money for the flight other than direct expenses like fuel.

The usual consequence of violating these regulations is that your license to fly can be suspended or revoked. Technically speaking, flying for hire without a license is a minor criminal offense (see below), however prosecutions in the past have only been given out for egregious violations (like working for a charter company without a commercial license).

The way these regulations are enforced is the following:

(1) If you are flying passengers or cargo for hire, and somebody rats you out, complains to the CAA, or it otherwise comes to their attention, then they will investigate and revoke your license if they determine you are flying for hire.

(2) The CAA has inspectors and they may do surprise inspections of aircraft. If you start flying passengers regularly, you may get inspected. If the inspector finds you are flying a bunch of passengers with a PPL, they will closely question your passengers about what money those people are paying. If the inspector determines they are paying money other than incidental expenses to be flown, you will likely have your PPL license revoked.

EXAMPLE:

You are flying 3 colleagues to a business meeting. A CAA inspector directs you to park on the ramp and inspects you and finds out you have a PPL. He will then question your passengers. How much are you paying for this flight? Colleagues answer:

(1) "nothing". You are ok.

(2) "18 pounds". You are probably ok.

(3) "200 pounds". You will lose your license.

If he see cargo on the aircraft, then there are two possibilities:

(1) He determines that the cargo is incidental to the flight and it belongs to you. (Like scientific equipment and you are a professor flying to an archeological site). You are ok.

(4) He sees cargo on the aircraft that does NOT belong to you (like 8 cases of computer chips being delivered to an electronics company). You will lose your license because you are flying cargo for hire.

Criminal Issues

Note that technically, flying for hire is a minor criminal offense in the UK and can be punished with a fine up to 2500 pounds in a magistrate court. However, it is very rare for this kind of offense to be criminally prosecuted. In the linked news story it was found there was only a single prosecution over the course of a year, and in that case it was a pilot working for a charter company and doing many commercial passenger flights, which was a blatant violation of the rules.


Does anyone have photos of internal parts of ELAC, FAC, SEC or Boeing 777 flight computers? I'm an electrical engineer and I'm interested in civil aviation, more specifically in fly-by-wire planes like the Airbus A320, Boeing 777 and others. But I fell in love with the Airbus A320 as it is the first civil fly-by-wire plane. I read a lot of its flight computers (FAC, SEC, ELAC), I saw its photos. But I want to have a look at their internal parts.

Does someone have photos of these internal parts? I want to have a look at those computer's internal boards. At last, I would appreciate any deep technical overview of how they work.

This is an indirect answer at best, but I think you'll get something good out of it.

I would definitely recommend you have look at CuriousMarc's series on YouTube where he and a couple other SMEs restore an original Apollo Guidance Computer to working order. You might think "that's not civil aviation, why?", but the AGC was the first digital fly-by-wire system. It was revolutionary both in terms of it's use of silicon integrated circuits and it's effects on software engineering. the AGC was also the basis for the first fly-by-wire systems installed on aircraft for atmospheric flight.

The other reason I'd recommend this is because it has all of the electrical cutting and shutting that you seem to be expecting. They spend a lot of time looking over original schematics and performing diagnostics with Marc's extensive personal collection of equipment in order to find individual failed circuits which require repair. They also had to find ways to interface the computer and emulate missing components, which involved a lot of bespoke design.

I'll be honest, I think there is a possibility you might find modern computers quite boring in comparison if your interest is limited to the circuits and does not include the software (not easy to tell from the wording of the question). This is for two reasons:

The AGC represents a time where the atomic components of the digital computer could still be seen with the eyes and cut/soldered out by hand. That is almost certainly not the case with modern computers, and I would also expect that the vast majority of people who work with the aircraft directly have never cracked open one of the computers for this reason. They are sealed (maybe even tamper-proofed), and are simply replaced as a unit and shipped back to the OEM when any invasive work is required.
A cursory search informed me that the original A320 computers actually used derivatives of the Intel 8086. If you're interested in early x86 micro-architecture (especially in fault-tolerant configurations), this is interesting. But, if you're expecting to open the computer and find the flight control laws to be implemented in hardware, you'll be a bit disappointed.


For jetliners, how is a swept wing justified over a thin wing? For a given speed, a swept wing is a fatter alternative to a straight, thin wing. But why? Does not the sweep lengthen the wing and make it heavier? Is the swept wing chosen for the extra fuel tank volume?
Edit
Could you quantify the justification? Like, let's say we have this beam of depth x............

Yes, sweep makes a wing heavier and less efficient. But making the airfoil of a straight wing thinner runs into diminishing returns and also will drive up mass.
Since a thin wing needs to create the same lift as a thicker one, only the local speed increase from thickness can be tackled by making the airfoil thinner. The speed increase necessary for lift creation needs to be maintained, so the thickness contribution becomes a minor factor for the wings. However, a thinner wing requires a heavier spar to carry the same lift loads, so more lift needs to be created by the thinner wing to carry that extra weight. Now further thinning will drive up the speed increase from lift which will eat up the gains from thinning the wing.
If you look at parametric weight equations for airliner wings, the relative thickness at the wing root $\delta$ is part of them and generally occurs in a factor like
$$m_{Wing}\propto\frac{1}{\delta^{\,0.36}}$$
Now it is important to stress that this is valid for the range of thicknesses from which that relation had been derived, something between $\delta$ = 14% and 16%. If you drop below 14%, the exponent will most likely go up since the mass of the spar alone should be proportional to $\frac{1}{\delta}$ and becomes more dominant the thinner the wing becomes. But if root thickness is decreased from 15% to 14%, that means we stay within the limits of the statistical model, the wing mass increase should be 2.5%.
The same model gives the influence of sweep on wing mass as
$$m_{Wing}\propto\frac{1}{cos\varphi^{\,0.57}}$$
Based on 40° wing sweep, every decrease in spar thickness of 1% requires a sweep decrease of about 2.9° in order to keep wing weight constant, based on that relation. If that 1% thickness decrease allows you to desweep the wing by 3° or more, you come out ahead.
As always, a compromise is the best solution. Airliners of the propeller era had thicker wings than jet airliners. Some sweep and some thickness reduction will allow to push the critical Mach number to the highest value. Tank volume is indeed an important factor and is responsible for the comparatively low aspect ratios of early jet airliners.


What defines the end of a "cross-country flight" for ppl aeronautical experience purposes and for general logbook purposes? Assume "pilotage" is used for all flights other than circuits around the pattern.
Assume, in the flights that follow, at any point where required, an instructor travels to the departure point and approves the flight.
At what point in the following sequences must the pilot stop logging the accumulating flight time as one single cross-country flight--

For general logbook purposes
For meeting the cross-country aeronautical experience requirements for a private pilot's license under part 61 in the USA? (NOT the "long" cross-country requirement.)
To meet the 200-hour cross-country experience requirement for the ATP rating under part 61?
To meet the 100-hour cross-country experience requirement to act as PIC under part 135?

Please explain your answers.
Let's say a pilot does the following:
Case 1:

a) Takes off
b) Flies to a point 60 miles away, full stop landing
c) Flies back to start, full stop landing
d) Flies to a point 60 miles away, full stop landing.

In case 1, would it change the answer in any way if I revealed that the pilot got out of the airplane and slept overnight between steps b and c only?
How about if she got out of the airplane and slept overnight between steps c and d only?
Case 2:

a) Takes off
b) Lands at a point 60 miles away, full stop landing.
c) Flies to a point 10 more miles away, full stop landing.
d) Gets out of the airplane at eats lunch
e) Flies to a point 10 more miles away, full stop landing.
f) Gets out of the airplane and stretches
g) Makes 10 circuits around the pattern without using pilotage, with full stop landings.
h) Flies to another point 60 miles away, full stop landing.
i) Gets out of the airplane, takes a nap.
j) Gets back in airplane, flies to a point 60 miles away from both the latest takeoff point and the original start point, full stop landing.
k) Rents a motel room and stays over night.
l) Flies on to another place 100 miles away.
m) Stays with a friend for a month, while making 10 short flights around the airport without using pilotage.
n) Flies back to the original starting point.
o) Makes 10 more short flights over the course of the next month without using pilotage.
p) Flies to a point 60 miles away.

In case 2, would it change the answer in any way if I revealed the pilot was on a long vacation and the aircraft and pilot were actually based at the point that the last flight "p" ended up at, and he had departed from there two days previous to the first flight in the list (a)?
Relevant: http://www.boldmethod.com/learn-to-fly/regulations/logging-cross-country-flight-time/

It is not in the interests of the FAA to provide a specific definition of how long an aircraft can spend on the ground before a flight "ends".
Pilots are expected to use their judgment to deal with weather, fuel planning, air traffic control, etc.  Pilots are also expected to use their judgment when deciding what should and should not be logged as a single cross country flight.
This is good for pilots. The FAA trusts us to do the right thing.
Ok, but what about my questions?

For general logbook purposes you can do anything you want. Exactly nobody cares what's in your logbook except you (and maybe the person interviewing you for a job in your future).

Private, commercial, and ATP experience require hours, not flights. Therefore you may log case 1 as one flight, two flights, or three flights, depending on which is most reasonable in your judgment as a pilot and based on the unique circumstances of the flight.  For the most part this simply means you need to decide whether you want to fill out one, two, or three lines in your logbook.

Similarly, you may log all of case 2 on one enormous line in your logbook, if for some reason the unique circumstances of the flight indicate that that's the best way to do it. (Maybe you're going to the south pole, and the month-long break is because you had mechanical trouble along the way.) Similarly, you may choose to log each leg or even each landing as a separate line in your logbook (for a total of 38 lines), if that makes more sense in your judgment.

The longer the stop, the more difficult is it to argue that it should be considered one flight.  I think it would be exceptionally difficult (though not impossible, see above) to make the case that two hours worth of flying separated by a month of not flying should be considered one flight.

As guidance, remember that your logbook is a reflection of you as a pilot.  If there's uncertainty, ask yourself "How would I like to be perceived, if someone were to audit this logbook?"  If there's still uncertainty, ask an instructor.  If there's still uncertainty, ask an FAA inspector.  They're there to help.


Why are the fan blades mounted loosely? In this question it is mentioned that the fan, compressor and turbine blades are mounted loosely in the disc. This is well demonstrated by this video posted by RedGrittyBrick.

What is the reason for this?

The blades are loose in their "Fir Tree" blade mounts so that they can self balance. They are called "fir tree" because they are v shaped.

As the rotation speed and centrifugal force increases, they move in the mounts and establish individual lead/lag positions to achieve a balanced disc.

  It will be understood that turbine wheels rotate at very high speeds
  of rotation, thereby generating large centirfugal forces tending to
  throw the blades outwardly from the wheel. By providing contact
  surfaces of equal and opposite slope with respect to the longitudinal
  axis of the root on opposite sides of root, the force components of
  the centrifugal force balance each other out. Periodic and other
  vibrations which might otherwise be generated are damped out and
  greatly minimized

Source: Fir tree blade mount Patent


Could a plane land vertically in a strong headwind? I know there are VTOL craft meant to take off and land with a vertical trajectory. But, under the perfect wind conditions, could a normal airplane ever land (safely) with a vertical or nearly vertical trajectory?

Yes.

I assume that you do not have a good understanding of aerodynamics.  I apologise if that is not the case.

The way to think about these sort of questions is to ask "does the aircraft know that it's moving in relation to the Earth".  The answer to this is "no".

The only thing that matters, and the only thing that the aircraft cares about is airflow (wind) over the wings.

Ignore all sorts of little side effects and imagine an aircraft needs 70 kph of wind over the wings to generate lift and stay afloat, straight and level.

Now, imagine the aircraft is flying along and you could somehow make the Earth disappear but leave behind gravity and the atmosphere.  What would happen to the plane?  Nothing!  It still has 70 kph of wind over the wings and it keeps flying.

Now put the Earth back.  It still keeps flying.

If the pilot reduces the power to 69 kph ("thrust X"), the force generated by lift pushing the aircraft up is a little less than that generated by gravity pulling the aircraft down and the craft will start to descend.  Eventually, it will land (assuming that you remembered to put the Earth back!).

So two scenarios.  First, one where the air is perfectly still and the thrust from the engine ("thrust Y") is pushing the aircraft forward at 70 kph.  Second, where the wind is blowing at 70 kph and thrust Y is such that the aircraft is stationary with respect to the Earth.  Again, the aircraft does not know, or care, that it is stationary with respect to the Earth, just that it has 70 kph of wind over the wings.

Modify the second scenario slightly so that the wind is now 69 kph. The aircraft moves forward at 1 kph. Again, the aircraft does not know, or care, that it is going fowards with respect to the Earth. The wind over the wings is still 70 kph (speed of wind, 69, plus speed of aircraft 1).

Reduce the thrust to X again and the aircraft will start to descend, and also become stationary.  Eventually, it will land with no forward or backward speed.

In reality, this would be almost impossible since the wind is never of a constant speed from a constant direction so little movements would always be taking place.

I have seen a video, which I can no longer find, of a bi-plane which took off into a strong wind, flew backwards down the runway and landed on a spot before the one it took off from.

This comes very close.  With just a little more wind, a vertical landing would have been possible.


How is the total drag variation with velocity related to altitude? Here in the question given I am confused how to relate the altitude with the given plot of total drag and velocity.

At higher altitude, density is lower. For the same true airspeed and the same drag coefficient, higher altitude will have less total drag.

However, there are factors affecting drag coefficient itself:

Due to the lower density, the lift coefficient must increase for the same TAS. This results in increased induced drag.
Decreasing density also decreases the Reynolds number, which increases the skin friction drag, especially for speeds as low as the ones shown in your graph.

Therefore, it can be expected that the trend will invert below a critical speed.

The answer is h2 > h1 from the above rationale.


What does it mean to "firewall" an aircraft engine? I (think) I understand what a firewall is (at least, in a single engine aircraft where the engine is at the front of the fuselage), but what does it mean "to firewall" an aircraft's engines, as described in this incident report: is it just applying full thrust? What is the connection with the physical firewall?

"To firewall" is a phrase meaning to go to full power. Most aircraft throttle controls provide full power when moved to their furthest forward position - the direction towards the firewall separating the nose mounted engine from the cockpit in aircraft in the past. The phrase is still used, just as we "dial" a telephone even though the telephone dial is no longer used, either. A similar one for automobile driving is "pedal to the metal".


How is a variable sweep wing constructed? How is a variable sweep wing able to change its sweep?

The easiest way to sweep a wing is an oblique flying wing. This is essentially a straight wing with pivoting rudders and engine pylons which flies at an increasing angle of sideslip where a swing wing aircraft would fold its wings back.

Oblique flying wing (picture source)

If you want to know more, I suggest to read this website. Excerpt:

  Although the oblique flying wing concept was described by Jones in
  1958 and a drawing by G. H. Lee was published in 1962, the concept
  received little attention until the 1990's when active digital flight
  control systems became more capable and widespread. The oblique
  all-wing configuration was studied in detail by researchers at NASA,
  Stanford University, McDonnell-Douglas, and Boeing in the 1990's as
  part of a NASA program.

If the wing is attached to a fuselage, only the wing should move. Again, this can be achieved by pivoting a straight wing, as done in the NASA AD-1 research aircraft, or by folding the wings back with a huge hinge at the wing root, as done on swing wing aircraft.

NASA AD-1 in flight (picture source)

The first aircraft with a variable wing sweep was the Messerschmitt P-1101 which was almost completed in 1945 when WW II ended. It was supposed to fly as a fighter-bomber with reduced wing sweep and as a pure fighter with increased wing sweep, and sweep could only be changed on the ground. On the basis of this design Bell built the X-5 research aircraft, the first to change sweep in flight.

P-1101 under construction (picture source)

The mechanics of a swing wing is rather straightforward: All you need is a big hinge connecting the wing spar at the root and a big hydraulic cylinder or spindle drive for adjusting the wing's sweep angle. And a flexible fairing at the wing root. And something to compensate for the shifting aerodynamic center.

Swing wing mechanism of the F-14 (picture source)


What is an Airworthiness Directive? Aircraft which have been in service for a long time often have a lot of entries in their logbooks for Airworthiness Directives (ADs). What are they, and why are they important?

Straight from the FAA:

  Airworthiness Directives (ADs) are legally enforceable rules issued by the FAA in accordance with 14 CFR part 39 to correct an unsafe condition in a product. 14 CFR part 39 defines a product as an aircraft, aircraft engine, propeller, or appliance.

When an aircraft or other component is found to have flaws or other issues, the FAA will issue an AD specifying a solution to be applied to the item. The AD must be complied with, often by a date or total time on the airframe. The action required could simply be an inspection (often recurring after, say, 100 hours of time in service), while in other cases requires modification, strengthening, or replacement of parts.

One of the more prominent cases requiring ADs to be issued was the V-tail Bonanza, which had issues with the tail's structural integrity under high loads.

The cost of complying with an AD falls on the owner/operator of the aircraft. Failing to comply with an AD renders the aircraft unairworthy. It also could incur FAA penalties, but more importantly, it could result in structural damage or failure, and in some cases ultimately cause a fatal accident.


What is the difference between fuel control unit and fuel flow meter (transmitter)? Hello can someone explain the difference between the fcu and fuel flow transmitter and how they work together alongside the fuel pump?

The Fuel Control Unit controls the flow of fuel to a turbine, it is in effect the mechanism by which the throttle works.

A fuel flow meter is simply a device that measures fluid flow (in this case the flow of fuel) through a line. The location, quantity and operation of flow meters varies by aircraft and configuration. Broadly speaking they are used to calculate actual fuel burn as well as to make sure the proper amount of fuel is flowing to an engine.

The fuel pump is the device that drives fuel to the engine/FCU from the tank. Again systems vary heavily but broadly pumps are used to move fuel from the lowest point in the tanks to an engine that may be above that point as well as to build sufficient fuel pressure to allow injectors/delivery systems to work.

In the simplest of systems a fuel pump sends fuel over a fuel line to an FCU. Somewhere on that line is a flow meter so that you can measure fuel flow out of the tank, into the engine.


Does the dew point affect density altitude? I am looking for the most accurate formula to calculate the Density Altitude (DA) to use it for calculating aircraft performances during landing.
Now I found the following formula:

DA = PA + [120*(OAT - ISA temperatures at given altitude)]

PA = ((standard pressure - given pressure)*30) + field elevation

My question is: Why is the dew point not included in this formula and what influence does it have on the density altitude? What is the correct formula with dew point included to calculate the DA?

Density altitude is the air density expressed as an altitude above MSL.  Both temperature and humidity affect air density; however, temperature has a far greater impact.

Electronic E6Bs account for the effect of humidity by using both temperature and dew point temperature, the latter of which provides a measure of the air's water vapor content (technically it measures the temperature to which the air would need to be cooled for saturation to occur).

In contrast, the simple rule of thumb provided above only addresses the impact of non-standard temperature on density altitude. However, the effect of humidity can be "roughly" approximated by multiplying the dew point temperature (in degrees Celsius) by 20 and reading the results in feet.

This correction is then added to the above temperature correction rule of thumb for density altitude.  Note the correction for humidity is highly non-linear and so it provides only a rough approximation.  It should only be used in the dew point temperature range of 5-30 degrees Celsius.

Details can be found in Guinn, T. A., & Barry, R. J. (2016). Quantifying the Effects of Humidity on Density Altitude Calculations for Professional Aviation Education. International Journal of Aviation, Aeronautics, and Aerospace, 3(3). http://dx.doi.org/10.15394/ijaaa.2016.1124


Is the 737NG hydraulically controlled or electronically controlled? I know that the 737NG is not fly-by-wire, but are the control surfaces controlled with hydraulic systems that transfer the input from the pilot to the control surfaces via fluid (hydraulics), or is the pilots' input transferred to the control surfaces' electronic actuators via electric signals?

Of course it's hydraulic.

Electric actuators or power-by-wire is actually a far more exotic technology than fly-by-wire, and the latter is a hard prerequisite for the former.

Normal fly-by-wire is still based on hydraulic systems, with the electrical FBW system controlling the valves. And even in power-by-wire systems, the final actuator is typically a small enclosed hydraulic unit.

The newer 737 MAX has a couple FBW elements (spoilers), but is still a manual system, well short of FBW or PBW.


How to program an FMC if the database is missing the arrival airport? Let's say you are in a 737 flying to Geita, Tanzania, as you can see this little airport/airstrip has an IATA code, but not an assigned ICAO code. What should you enter into the FMC if I want to fly to that airport?

And in general, how to navigate to an airstrip that has only an IATA code?

If the arrival is missing:

If the departure is missing:

And here are the affected items:

Source: 787 FCOM.


Is there a solution to radio interference? I was reading about the Tenerife Disaster and came across what seemed to be the  probable cause behind the accident.

  A simultaneous radio call from the Pan Am crew caused mutual interference on the radio frequency, which was audible in the KLM cockpit as a three-second-long whistling sound (or heterodyne). This caused the KLM crew to miss the crucial latter portion of the tower's response. The Pan Am crew's transmission was "We're still taxiing down the runway, the Clipper 1736!". This message was also blocked by the interference and inaudible to the KLM crew. Either message, if heard in the KLM cockpit, would have alerted the crew to the situation and given them time to abort the takeoff attempt.

https://en.wikipedia.org/wiki/Tenerife_airport_disaster

I understand that the science behind radio waves is the cause for this interference, but is there a solution (non-radio if need be) for when ATC and other aircraft talk over each other?

edit1: By solution I mean a way of only allowing 1 party to speak on a specific frequency at a time , be it technical or not. I would just want the possibility of pilots/ATCs talking over each other to be close to 0.

edit2: I know the Tenerife disaster was caused primarily by the pilot's inability to wait for clearance, and then report it back, but this question is about radio interference and if a solution exists, not if fixing radio interference is a worthwhile task to begin with.

Firstly, it is worth noting that the blocked radio transmission is only one point on a very long list of factors that caused the horrible accident on Tenerife. There is enough redundancy in aviation systems and procedures that a single fault somewhere in the system will [almost] never be enough to cause an accident. Weather, stress, a terrorist attack, company policy, poor CRM and many other things were also contributing factors.

Communication between air traffic control (ATC) and pilots is mostly based on simplex radio. Simplex radio only allows one station per channel to transmit at the same time; if two stations transmit at the same time, the signal will be blocked. Sometimes, if one signal is stronger, it will be possible to hear that signal but not the other. The main problem is that the two stations transmitting will have no way of knowing that they are blocking eachother. This limitation regularily results in various incidents - for example, when there are two aircraft with similar sounding radio callsigns on an ATC channel, one crew might mistake an instruction for the other flight, thinking it is actually meant for them. If both flights read back the instruction at the same time, neither of them will hear the readback from the other aircraft and realise the mistake, and ATC might hear just one of the readbacks, and not realise that both flights are following the instruction meant for just one of them.

So why isn't there work being done to phase out simplex radio in aviation?

There is. It's called CPDLC, or Controller-Pilot Datalink Communications. (Other, similar systems exist as well, with different names. In the following I will just discuss CPDLC).

CPDLC is a digital system that allows ATC and pilots to communicate via text. It is already widely deployed in many areas, one example being Maastricht UAC, one of the busiest area control centres in Europe. Multiple stations can transmit at the same time, without the risk of blocking other transmissions.

So why do we still have simplex radio?

So far, CPDLC is only used for area control. In areas where ATC instructions are time critical, such as an approach or aerodrome environment, we still rely on simplex radio, because it is guarenteed to be instantaneous - and pilots are used to acting immediately to a voice instruction. However, it should only be a matter of change of procedures and appropriate training before CPDLC can be used in all phases of flight.

More importantly, aircraft equipment is expensive. Replacing a simplex radio with CPDLC equipment involves a significant cost per aircraft. While this may be feasible for large airlines, remember that they only make up a part of flights. There are thousands and thousands of private planes, balloons, gliders and whatnot, and private pilots or flying clubs may simply not have the funds to implement CPDLC. And obviously, ATC can't turn off their simplex radio until everyone has equipment to replace it.

Another disadvantage of CPDLC, or other communication equipment that allows direct communication between cockpit and controller working position, is that it reduces situational awareness. A big advantage with simplex radio is that everyone has to listen to what everyone else is saying, since you can't talk when someone else is talking. This gives pilots the possibility to establish a mental picture of the traffic around them, which ultimately increases safety. As we saw with Tenerife, the lack of such a mental picture can result in tragic outcomes. However, simplex radio has probably prevented many other accidents, because everyone on the frequency knew what was going on around them. The Providence incident of December 1999 comes to mind as one example among many.

So, is there a solution to radio interference? Yes, there is. Is it easy to implement? No, it is both technically and financially difficult. Will it necessarily prevent accidents such as the one on Tenerife? No; a thorough safety and risk analysis must be carried out before deploying it everywhere.


Alphanumeric flight identification in flight plan Why do some airlines use alphanumeric callsigns in the flightplan when the flight number printed on the ticket consists of only numbers?

For example, the flight TK871 uses callsign THY4JT.

when flight delays cause a duplication of a flight number- there is a suffix added.  For example- flight 1423 DFW/OKC is also operating as OKC to DFW. Normally a turn of equipment- however, DFW/OKC delayed significantly- equipment and crew is available to operate 1423 OKC/DFW.  These 2 could be in route simultaneously- one is usually designated as a suffix- 1423J  Call sign American 1 4 2 3 Juliette


For pilots, are the procedures different between airframes with different engine options? Lets use the B757 as an example. Engine options are PW2000 and the RB211. As I'm sure they are both FADEC engines we don't have to worry about and mechanical linkages and such. But each engine although comparable in thrust ranges have different requirements from the aircraft.

As a B757 pilot are the procedures different between the airframes with different engine options?

I can only speak for the Boeing B787, but the 787 Flight Crew Operating Manual (FCOM) indicates that there are slight differences between different engine options. Three examples:

autostart modes (different failures can be automatically detected and corrected for during engine autostart),
warm-up times before take-off and cool-down times after landing,
take-off thrust setting ("GE engines: Advance the thrust levers to approximately 40% N1" vs "RR engines: Advance the thrust levers to approximately 20 TPR" - this also indicates that the primary engine indications are slightly different: N1 and EGT vs. TPR, N1 and EGT).

I don't know though how much these slight differences translate to different procedures, as I am not a pilot.


Will the pilot in a plane be able to tell if people are jumping up and down? If someone is jumping up and down in the bedroom of a private jet will the pilot be able to tell without listening? In other words, will the plane noticeably move?

If an aircraft is large enough to have a “bedroom”, it will be too large for the movement of an average sized person in the bedroom to be noticed in the cockpit.


How is a lead acid battery and a Ni-Cd battery charged on an aircraft? How is a lead acid battery and a Ni-Cd battery charged on an aircraft, such as an A320?

Is it constant current or voltage?

Airliners like the Airbus A320 use Nickel Cadmium batteries; in case of A320, a pair of main batteries with 23 Ah capacity is used.

While on the aircraft, the batteries are charged using a constant voltage method. As this article notes:

  ... are "constant-voltage chargers" that apply a fixed voltage to the battery as it charges, similar to what the aircraft electrical system does in flight. With a constant-voltage charger, the charging current starts out relatively high and gradually tapers down toward zero as the battery becomes fully charged.

FAA Aircraft Electrical System Handbook also notes:

  The battery charging system in an airplane is of the constant
  voltage type.


Would a homemade lawn chair balloon be visible on ATC and collision avoidance radar? I am not a pilot, but have a question about radar detection of improvised lawn chair balloons that I thought might be on-topic here.

This BBC news article describes a marketing stunt gone awry involving a lawn chair, a large number of multicolored helium balloons, and the owner/operator/parachutist

According to the article, he was charged with "mischief causing danger to life".

A similar stunt by American truck driver Lawrence Richard Walters, nicknamed "Lawnchair Larry" or the "Lawn Chair Pilot"  also resulted in fines and legal action.  In that case, the government dropped the fine for "operating a civil aircraft for which there is not currently in effect an airworthiness certificate" because it was not applicable to that type of aircraft. The article also mentions some other attempts to fly lawn chair balloons, including a fatality from crashing at sea.

My question is whether such an improvised manned balloon would be visible on typical airport radar or the independent radar/collision-avoidance systems operated on board large modern passenger aircraft, and thus avoidable.

A 1970s military research paper on radar supporting the need for secondary transponders in detecting light aircraft also mentions detecting doppler from propellers (p.4) in addition to the primary radar reflection, and I would expect the cross section of a lawn chair to perhaps be within an order of magnitude of that of a propeller.   A 2m x 0.5m x 0.5m sparsely filled aluminum chair frame is also still larger than typical microwave radar wavelengths (1Ghz = 30cm wavelength, most radars are going to be above 1Ghz), so it should reflect the radar but of course is a fairly small object compared to large aircraft and therefore the detection range could be lower.

Would a homemade lawn chair balloon be visible on ATC and collision avoidance radar?

It might be visible to ATC, but, at best, only as one of several inconspicuous unidentified red dots.

It wouldn't appear on aircraft collision avoidance displays (TCAS) and would not trigger any TCAS alerts.

Radar cross section

Balloons, and clusters of balloons are mostly air (or similarly insubstantial gas). There is a thin layer of plastic but many plastic materials (e.g. mylar) are transparent to radar, or only weakly reflective.

It is for this reason that aerostats and high altitude balloons carry radar-reflectors when they need to be detected by radar.

Whether the chair itself has a detectable radar cross section depends partly on what materials it is constructed from. I suspect most available lawn chairs are not constructed from solid sheets of metal fixed at right angles but, at best, from a relatively tiny amount of relatively small-gauge metal tubing.

I'd be surprised if any improvised lawn-chair balloon had the same radar cross-section as a glider - and those are allegedly hard to see on primary radar.

Some helium balloons are made from metallised mylar (because it keeps the helium in for longer). These may be much more reflective to radar. The Calgary aviator pictured was said to have used "oversized party balloons" - they don't look like the metallised balloons that are commonplace where I live.

A perfectly conducting metal sphere of 1.15m diameter has a radar cross section of 1 m²

Some other radar cross section values:

  Human: 1 m²
  small combat aircraft: 2–3 m²
  Cargo aircraft: up to 100 m²

This suggests that military radar might display a return to an operator. A civilian primary radar might be less likely to display a return to an operator.

Primary radar

So far as I can tell, most civilian ATC and powered aircraft (TCAS etc) don't really make much use of primary radar to detect other airborne vehicles. They rely on transponder returns (i.e. secondary radar)

Even a large metal object like a Boeing 777 can disappear from the view of ATC and other aircraft when it turns off its transponder.

The Guardian newspaper reported

  Last month, the Swedish government complained that a Russian military aircraft had been flying near its airspace with its transponders turned off to avoid being spotted by civilian radar, and nearly collided with a passenger jet.

I'd be surprised if an improvised lawn-chair balloon was detectable where a Tu-95 was not.

What ATC see

This is from what is said to be a radar recording of flight 77 (at 4:18) probably at Dulles after controllers were aware of the prior 9/11 attacks.

Flight 77 had turned off its transponder, it is one of the small red dots at the left. Here's a closer look at that

Flight 77 is the red dot about 20% in and up from bottom left. There are other red dots that come and go but this one tracks steadily from left to right. I'd guess an especially alert controller who is not very busy would notice this sort of dot.  Even if the radar display doesn't filter it out,  unannounced slow-moving lawn-chair balloons appearing from the suburbs might not always be noticed (though see James' comment to Peter's answer which suggests they often do notice and act on such targets).

Transponders

Radios and transponders designed for hot-air balloons are available (example) and I've read that

  Transponders are mandatory for flights in some types of airspace in many parts of Europe from March 2008 onwards. This includes balloons and other light / recreational aviation and aircraft.

So I suspect that authorities will expect aviators in lighter-than air vehicles will take appropriate measures to be in communication with ATC/other aircraft and perhaps to be visible to secondary radar in airspace where they may come into conflict with other aircraft.

Kevin Walsh

Heres a news report from The Milwaukee Sentinel - Jul 19, 1984

Snopes reports

  He had wrapped his tether lines in foil in the hope that they would show up on radar, and, sure enough, he was picked up on the screens of controllers at Boston's Logan airport

So you probably need to take specific measures to make yourself more likely to show up on primary radar.

Conclusion

When attaching balloons to your lawn chair, first scrutinize the intertubes for designs for large improvised radar reflectors you can dangle from your vehicle. Hope that nearby on-duty ATC controllers have their displays set to show primary radar returns and not just secondary radar returns.


What are the rules for logging PIC time on an IFR flight plan in mostly VFR conditions with a safety pilot? I act as safety pilot for a student who is building hours towards his instrument rating. I have my IR but am not a CFI. We typically fly VFR with him under the hood as the "sole manipulator of controls" so we can both log PIC hours.

On an upcoming long flight, I'd like to file an IFR flight plan. We expect the conditions to be VMC, but we need to plan for the possibility being routed through some patches of light IMC. Are we permitted to both log the PIC time during which we are in VMC, and then just switch off "actual PIC" duties when we enter patches of IMC (during which time I would log "actual IMC" time, and he would log nothing)? In reality, it will be the autopilot doing most of the flying.

The FAA has a legal interpretation that addresses exactly this issue, and that's what I used for this answer. We also have this question and this one that cover very similar but slightly different scenarios.

First, PIC time. Since you're instrument rated and the other pilot isn't, only you can act as PIC under IFR. So, with that in mind, when can you both log PIC time per 61.51? You didn't mention the aircraft, but I assume it's a light single that only requires one crew member.

The key point here is that operating in simulated instrument conditions is an operation that requires more than one crew member. But operating in actual instrument conditions is not such an operation.

As long as you're in VMC and assuming that your friend is under the hood, you can both log PIC time. You're acting as PIC during an operation requiring more than one crew member (61.51(e)(iii)), he's the sole manipulator of the controls (61.51(e)(i)).

But when you enter IMC, your friend continues to log PIC time as sole manipulator, however you cannot log PIC time because you're not a required crew member while operating in IMC (see the penultimate paragraph of the legal interpretation above).

Second, instrument time. The only way to log instrument time (in your scenario) is described in 61.51(g)(1) (and see this question):

  (g) Logging instrument time. (1) A person may log instrument time only
  for that flight time when the person operates the aircraft solely by
  reference to instruments under actual or simulated instrument flight
  conditions.

Assuming that only your friend controls the aircraft (manually or using the autopilot) then only he can log instrument time. If you take the controls in actual IMC then you would log instrument time but your friend wouldn't.

Third, putting it all together. Assuming that only your friend controls the aircraft throughout the entire flight and he stays under the hood while in VMC then you would log time as follows:

VMC. You: PIC (acting PIC while more than one crewmember is required). Your friend: PIC (sole manipulator), instrument (simulated IMC).
IMC. You: nothing. Your friend: PIC (sole manipulator), instrument (actual IMC)


How can a NACA2312 airfoil or similar have a camber location from 30 to 39% without a change in airfoil name? How can a NACA2312 airfoil or similar have a camber location from 30 to 39% without a change in airfoil name?

Was fooling around in javafoil, and noticed I could go from 30% camber to 39% camber, without the NACA number of the airfoil changing!! What the heck?

What name is used to differentiate NACA2312 with 30% camber location and NACA2312 with 39% camber location?

The NACA airfoil numbers are coded as follows:

$$ \mathrm{NACA} \; M P XX $$

  $M$ is the maximum camber divided by 100. In the example M=2 so the camber is 0.02 or 2% of the chord
  $P$ is the position of the maximum camber divided by 10. In the example P=4 so the maximum camber is at 0.4 or 40% of the chord.
  $XX$ is the thickness divided by 100. In the example XX=12 so the thiickness is 0.12 or 12% of the chord.

source: airfoiltools.com

In your case you changed the position of the maximum camber ($P$) from 30% to 39%. Both result in $ P = 30 / 10 = 3 $ and $ P = 39 / 10 = 3 $ (when using integers, no rounding is applied). The airfoil name therefore only tells you the position of maximum camber with a 10% accuracy.


I have an FAA ATPL based on a foreign ATPL, can I convert it to an EASA ATPL? I have an FAA ATPL based on my Foreign ATPL. Am I now able to convert my new FAA ATPL to EASA ATPL? Of course with me doing all the exams etc..

or am I only allowed to convert my original foreign ATPL to EASA?

Do you actually have an ATPL or a frozen ATPL?

Essentially, EASA requires you to sit all the ATPLs as well as the individual IR exam and your practical flight tests for the CPL, ME and IR. That is conditional on you having completed an "approved course of training" and whether your FAA stuff counts I don't know; I doubt it as it isn't EASA.
Some stuff (CPL, ME & IR) can be converted depending on experience but from what I understand from a friend, many UK schools are very reluctant to do this at all. I can't speak for other EASA nations.

So essentially, plan on doing your ATPL again.


Can someone actually "fly under the radar"? It is quite a common phrase, usually without having anything to do with radar or aviation. But. Is there such a thing as 'flying under the radar'? If so, why is this (and how low is that)?

In many cases, yes you can fly under the radar because typical civilian radar is line of sight, meaning that it has to have a straight unobstructed path to an object in order to "see" it.

This is because it works by sending a radar signal out and waiting for it to reflect off of something.  If it reflects off of another object, like the ground, a mountain, a building, a thunderstorm, etc. then it won't see what is behind it.

Since the earth is round, flying "under" the radar is flying beneath the coverage area that the radar can "see" directly from where it is, and the height of radar coverage depends on the distance from the site as well as the terrain.

An example is here:

The top half shows how the curvature of the earth affects it and the bottom half shows how other objects can shadow radar (even though is is using aircraft radar, the same principle applies).

That being said, there are other types of radar and other ways of utilizing radar that minimize this problem and make it nearly impossible to fly under it.

One common type in use is the Tethered Aerostat Radar System:

This is downward looking radar attached to a 25,000 ft. tether.  From 15,000 ft. it can detect aircraft and even vessels all of the way down to the surface of the ocean out to 200 miles.

Another type is Over-The-Horizon Radar which can see further by reflecting radar off of the ionisphere like this:


How to read this step climb profile? FL STEPS CYUL/0330/TUDEP/0340/NETKI/0350 How to read this step climb profile?

FL STEPS CYUL/0330/TUDEP/0340/NETKI/0350

I know CYUL is departing point but what do 0330 represents? The altitude in Feet?

FL330 is the initial cruising altitude after departing CYUL. At TUDEP, a climb to FL340 is planned, and at NETKI, there is another climb to FL350.


Why should some piston engines not be run lean of peak? Leaving aside the claimed pros and cons of lean-of-peak (LOP) vs. rich-of-peak (ROP), the C172S Lycoming IO-360-L2A handbook I have says "Continuous operation at mixture settings lean of peak EGT is prohibited", and indeed I was always taught to run the engine ROP.

Obviously you should always follow the POH for your aircraft, but considering piston engines in general, why would running LOP be prohibited?

Considering piston engines in general there's a few reasons not to run "lean of peak", but the arguments I'm aware of all fall into three broad categories:

Poor Instrumentation
Most folks advocating Lean-of-Peak engine operation will also tell you that you need a good engine monitor (with Exhaust Gas Temperature (EGT) and Cylinder Head Temperature (CHT) probes for each cylinder) so you can ensure that all your cylinders are operating within safe temperature limits.
Manufacturer CHT/EGT systems are often "single-probe". On the IO-360 in your Cessna example that leaves you with a 3 cylinder blind spot in your monitoring, and the potential risk of damaging your engine based on the incomplete information you have.
Uneven mixture distribution
Some engines simply can't run lean of peak because of uneven mixture distribution: Either the engine will get rough as some cylinders "miss", or you'll have some cylinders lean of peak EGT and at a safe cylinder head temperature while others are redlining their CHT and potentially causing damage to your engine.
This sort of uneven mixture distribution happens frequently in carbureted engines, but there are many injected engines which also have poor fuel distribution & significant variation in mixture between cylinders (This is where GAMIJectors makes their money - by providing balanced injectors to even out fuel distribution).
"In Case Something Breaks"
Hypothetical scenario: you are cruising at 8000 feet, you've leaned for cruise, and your mixture control suddenly fails in such a way that your engine is stuck at the current mixture setting.  All the airports within your endurance range are roughly at sea level.
An engine running lean of peak may begin to run rough as you descend for landing (the mixture becomes too lean to support combustion).  An engine running rich of peak may continue to run acceptably through the descent. (The mixture may transition into "the red box zone" where operation at high power settings are dangerous, but you'll theoretically be reducing power for your descent, so the chance of doing damage to your engine isn't that high unless you have to perform a go-around or other high-power maneuver.

A prohibition on Lean-of-Peak operation in your POH is probably historically based on #1 or #2 above -- the manufacturer doesn't want to advise you to do something that might damage your engine.  Running Rich-of-Peak is conservative operating guidance, where the downside is slightly increased fuel consumption and the possibility of fouling a spark plug (which is not likely at normal operating power settings) versus the downside of potential engine damage from improper Lean-of-Peak operation.


Why would the runway separation for parallel triple simultaneous precision approaches be dependent on airport elevation? Based on FAA advisory circular AC 150/5300-13A, Airport Design, when operating triple simultaneous precision instrument approaches the parallel runway separation could be reduced from 5000 feet to 4300 feet if the airport elevation is above than 1000 feet:

  Triple simultaneous precision instrument approaches for airports below
  1,000 feet (305 m) elevation normally require parallel runway
  centerline separation of 5,000 feet (1524 m) between adjacent runways.
  Triple simultaneous precision instrument approaches for airport
  elevations at and above 1,000 feet (305 m) and reduction in separation
  are currently under study by the FAA. In the interim, the FAA will, on
  a case-by-case basis, consider proposals utilizing separations down to
  a minimum of 4,300 feet (1311 m) where a 5,000-foot (1524 m)
  separation is impractical or the airport elevation is at or above
  1,000 feet (305 m). Reduction of separation may require special radar,
  monitoring equipment, etc.

What is the theory behind the advice?

What the text means is for triple simultaneous approaches for airports above 1,000 feet MSL, the FAA needs to conduct a case-by-case study. I.e. there is no automatic approval, yet.

Also, but not related, for airports below 1,000 feet MSL, reduction in separation from 5,000 feet also requires a case-by-case study.

  ... impractical or the...

I do think the paragraph you quoted can benefit from better punctuation.

Runway-to-runway separation does increase with elevation due to increased ground-speed and reduced reaction time.

The case-by-case study is usually done via numerous trials.

As for why 5000 feet, I was stuck in a loop between Annex 14 Volume I, PANS-ATM (Doc 4444), and the AC 150/5300-13A. Each quoting the other. Literally.

The Manual On Simultaneous Operations On Parallel Or Near-Parallel Instrument Runways does indeed discuss reaction times.

So, it's fair to deduce the increased separation is to give more time for ATC to notice any deviation and have time to react.

The FAA AIM does confirm the OFZ (Obstacle Free Zone) increases by 20 feet per 1000 feet elevation. The OFZ is in turn quoted as the reason for the runway separation increase the higher the airport is in the AC you quoted.

In short, increased elevation/ground speed requires case-by-case study to approve or bring down the separation. It's down to the reduced reaction time and airport equipment—e.g. radar accuracy, refresh rate, and so on.

The values themselves seem to be derived from trials plus added factor of safety plus rounding up. I can't find information on the trials.

Separation is not reduced with increased elevation, it's the opposite—if the airports (low and high) have the same exact equipment.


In the ARINC specification, does "sig bits" include the sign bit? In the ARINC specification, there are sections where all of the different labels (and their pay-loads) are described. In each of the tables where the labels are defined, there is a column called sig bits. For where the labels are designated as BNR, do the sig bits include the sign bit?

In A429 BNR words bit 29 is the sign bit.  Individual label descriptions are not entirely consistent in that many will identify bit 29 as part of the sig bits field.
In any case it's important to look at the details of a specific label, especially the range, MSB, and LSB values.  Some are always positive (such as airspeed) and so the sign bit is assumed to be positive and bit 29 can be the MSB.
One common factor is that the majority of BNR labels are coded in 2's-complement binary.  In this case, the MSB is half the range and each of the following bits is half the value of the previous. Example: For a "heading" range of 0 to 359, the MSB (bit 29) is 180, the next is 90, followed by 45, 22.5, 11.25, etc. Alternately, you can declare bit 29 the sign bit and the range becomes -180 to +179.
So you'll note that in 2's-complement encoding, it doesn't matter if you consider bit 29 as an MSB or a sign bit.  The difference is that if you call it MSB, the range is 0 to 359.  If you call it a sign bit, the range is -180 to +179.
You can prove this to yourself by declaring bit 29 as a sign bit and code -135 degrees.  Then declare bit 29 a MSB value of 180 and code 225 degrees. The coding should be the same.


What are the reasons for using an instrument approach on a clear day / CAVOK? As a new VFR pilot, I was wondering why anyone would fly (or be assigned) an ILS (or other) approach in clear weather?

I ask this because I watched Vienna - Tokyo (Narita) on PilotsEye.tv where the F.O. flew the approach looking at the display all the way until the flare. The visibility was good. Runway 34R.

There are actually quite a few reasons to fly an instrument approach, especially one with vertical guidance (like an ILS), even if the weather doesn't require it:

It serves as a backup to the visual approach.  There are various visual illusions that can cause a pilot to fly an approach too high or too low and monitoring the vertical guidance can help to mitigate those.
It helps to ensure that you land on the correct runway, and even at the correct airport.  (Unlike this airplane.)
ATC uses it when pilots do not have the airport or runway in sight.  This happens quite frequently at large airports where they have to line many aircraft up on final.   The last guy is a long distance from the airport so may not be able to see it but can fly the approach.
It can be useful if it is hazy or the sun is making it hard for the pilots to find the airport.
Some parallel runways are approved for simultaneous approaches (as is the case at Narita for runways 34L & 34R) so ATC will clear two airplanes for the respective ILS's.  This keeps them tracking the centerline of the approach instead of maneuvering visually where they might drift over into the way of the other airplane.
Flying the approach on the autopilot leaves more time for the pilot to focus on other things and possibly catch something that might have been missed while hand flying a visual approach.
Sometimes instrument approaches are used for noise abatement reasons.

If I had to guess about the particular approach that you mention, I would say that it is probably company SOP (Standard Operating Procedures) to fly the ILS whenever possible for the safety benefits, and that Narita allows simultaneous parallel landings to the two runways so ATC would require it as well.


What exactly are those openings in the bellies of many jetliners? I have circled the areas (in yellow) under the belly of the 737.

What exactly are those?

(airliners.net)

That's an air inlet -- technically a "Ram Air" inlet -- for the air conditioning packs.  There is a "left" and "right" pack, which supply air to the forward and aft part of the aircraft respectively (although either pack can supply enough air to keep the aircraft pressurized to its max altitude).  They use high temperature/high pressure bleed air from the engines or APU, but they also get airflow from the ambient air which is used to cool the bleed air.  The inlet noted in the question is where the ambient air enters, and there are exhaust vents for this air on the bottom of the aircraft.  THIS air isn't used for the pressurization per se (i.e. it doesn't enter the cabin), but just for cooling the air that will enter the cabin (which came from the compressor in the engines/APU, upstream of the combustion chamber).

On the ground, there are louvers that close to keep water and debris out of these ducts, and fans are used to pull in air in that configuration.  In-flight, the louvers open and airflow is generally sufficient to move the air through the system (although the fans still provide some assist).

While there is some amount of drag here, it's fairly minimal, and the ability to keep the cabin pressurized and cool is sufficiently important that the little bit of drag that's caused here is considered acceptable.

Close-up photo of the inlet door (Next-Gen 737):

Close-up photo of the inlet door (Classic 737):

Photo of exhaust louvers:

source page for all of the above (with technical discussion of the air conditioning & pressurization system): http://www.b737.org.uk/airconditioning.htm

Even more technical description of the system -- text & schematics but no photos


Are the panels in this vertical stabilizer part of the flight controls? I've noticed some sort of "flight controls" on an A319 vertical stabilizer.

Here's the original photo

What are those things? Are they used for controlling the movement of the aircraft?
Source: Airliners.net (modified to indicate the segments of the vertical stab).

The part I have outlined below is the rudder. That is the only control surface on the vertical stabilizer.

The segments on the leading edge are just the leading edge surfaces. One may include an antenna, but they are only segmented for easier manufacturing and assembly.

The ones just forward of the rudder are panels that cover the rudder attachment and actuating systems. They can be removed for inspection or maintenance purposes.

The rest of the surface, in the middle, is the main structure of the vertical stabilizer. There is a spar on the forward and aft sides, and ribs that run between them. This is all covered by one piece of skin. This creates a structure that is both strong and light weight.

Below is a cutaway showing the internal structure.


What are the advantages of a Pelikan tail compared to a V-tail? The Pelikan tail was originally proposed for the X-32.  Why was the Pelikan tail proposed rather than a V-tail?  What advantages and disadvantages does the Pelikan tail have compared to a V-tail or a conventional tail?

The X-32 was Boeing's entry for the JSF contest, which was supposed to serve USAF, USMC and USN. JSF called for an affordable VTOL capable aircraft operated by all three services. Boeing decided to go for a tailless single piece delta with a direct lift system, similar to the Harrier.

Image from militaryfactory.com

The adoption of direct lift system and other considerations meant that X-32 had weight issues (They actually had to strip off some parts for VTOL). However, midway through the program, USN changed its requirements, requiring improved performance and payload capabilities. This caused serious problems for Boeing as its present design was expected to fall short of the requirements.

In order to overcome this, Boeing decided to change the wing and tail design of X-32. For tail design, two designs were proposed- a conventional four poster tail (like Lockheed's X-35) and a Pelikan tail (it was conceived by Ralph Pelikan, who worked for McDonnell Douglas and now for Boeing after their merger). The main points put forward in favor of Pelikan tail were,

It caused less drag compared to conventional tail
Offer pitch control at high AoA.
It had better stealth characteristics (two tails would be better than four).
The design would look different from the Lockheed concept.

However, the problem was that nobody has flown with this design so far. After some discussion, Boeing decided to go with the Pelikan tail. However, after a few days, the decision was reversed as the design was found to be heavier by a considerable amount as the larger control surfaces required beefier hydraulics and Boeing decided to go with a conventional tail, which turned out to be around 1500 lb lighter compared to the original design.

However, the prototype was not actually modified due to lack of time. The modified aircraft would've looked like this:

Image from aerospaceweb.org

Saab is testing a (sub-scale) concept with Pelikan tail called the FS2020, which has Pelikan tail.

Image from gripen4canada.blogspot.in


What is the reference used when measuring flight speed? I'm a physicist who was amused by the argument in the comments of this question about gravity.  The correct answer about the affect of the accelerating reference frame of the plane on the apparent gravity depends on how the airliner's speed is measured.

I know very little about practical aviation.

When I'm on a trans-Atlantic flight moving 900 km/hr, with respect to what am I moving?  Is that my speed relative to the ground (which is also moving)? relative to the local atmosphere (which could be additionally moving due to wind)? relative to some imaginary, absolute grid hovering above the Earth's surface? relative to the "fixed stars"?

This is probably a duplicate, but I can't seem to find the right search terms...

Speed of a plane is actually measured in a number of different ways, and relative to different things. Here is a summary of the different types:

Indicated Airspeed (IAS). This is the number shown on the instrument that measures airspeed, and isn't really relative to anything.
Rectified Airspeed (RAS) or Calibrated Air Speed (CAS) This is IAS corrected for known errors in the instrument and the measurement system.
True Air Speed This is the speed relative to the air around the aircraft; usually found by correcting the RAS for pressure and some other errors.
Ground Speed This is the speed relative to the ground, or a fixed point on the earth ('imaginary grid). Its the vector addition of the True Airspeed (and direction) plus the wind speed (and direction). In olden days this was estimated from the Airspeed and the wind velocity, or through dead reckoning navigation, but now it is trivially found from GPS or other navigation aids.

Planes use different ones of these for different purposes. Measurements relative to the fixed stars are not generally useful until you are in the realm of space travel.


Is it possible to change/repair an aircraft tire while in flight? Consider the following scenario: A crew experiences a flat/burst tyre above V1.

Is it possible, in some type of aircraft, to change it while in flight?

This might include, but not limited to:

access the landing gear compartment
access hydraulic operated equipment
changing the damaged tyre

Accessing hydraulic lines & resevoirs is perfectly straightforward in plenty of aircraft -- the C-130 as one example.  With crew chiefs & their tools & spare parts on board, a temp repair on a hydraulic line would be possible. Highly unusual, but possible. Spare fluid is common enough to have on-board to replenish the resevoirs.

However, access to the tires... Nope.  I can't think of anything where you could do that, and it would have to be a designed-in capability. Typically, crew-accessible areas are pressurized, but where the wheels retract, aren't.  You have things like access to unpressurized bomb bays, and in unpressurized aircraft that's not a barrier, but still... If there are designs where the crew can access the tires in flight, I'm not picturing them.

As noted elsewhere, landing with a flat tire isn't typically a big deal.


Do ultralight engines need a thrust bearing? Do ultralight engines need a thrust bearing?

Most gas engine applications only have a radial load ( perpendicular to the shaft).

Aircraft, not only have a radial load but also an axial prop thrust load pushing/pulling the shaft out/into the engine depending on whether the prop is a tractor or pusher prop, which usually requires a different kind of bearing able to take a thrust load.

So...do ultralights need a thrust bearing?

If so...

I assume the bearing of the output shaft of a reduction drive would have to be a thrust bearing.

However, we see 1/2 VW engines being used in direct drive applications, and I don't think they change the final bearing into a thrust bearing.

The reason I'm asking is, that I'm thinking of using a $400 15hp 4 stroke industrial engine for an ultralight application, just as we see in Rutan's quickie(?), ultralights using the vanguard brigs and stratton engines, and M Columbine's new design ( designer of the Cri Cri). Yes they are heavy, but they are cheap, 4 stroke, a TBO of 2,000 hrs, and are rated for continuous duty.

I plan to run at 75% power, which is about 2,500 rpm so I really don't need a reduction drive.

If I need a thrust bearing,  I could have a reduction drive with something with a 1.1 reduction. I could also have just a support thrust bearing bolted directly to the engine.

If I don't need a thrust bearing, then I would be off to the races.

I don't think the Lazair had a thrust bearing or a reduction drive when they used the non rotax engines.

So... do GA airplane engines need a thrust bearing, in the 15 to 30 hp range?

Any aircraft engine that spins a prop to generate thrust will necessarily contain a thrust bearing to prevent the propeller thrust force from pulling the crankshaft out of the block.  But since a bearing designed for mostly radial loads will also be capable of resisting a certain amount of thrust load, some automobile engines converted for aircraft use can get by without having a thrust bearing added to their crankshaft supports.


Passenger hypoxia in a small plane at 11,000 feet: what is the pilot's responsibility? An acquaintance of mine visiting western U.S. was recently a guest passenger in a small plane flying from Santa Fe, New Mexico to Colorado reaching an altitude of 11,000 feet. She related that she started experiencing hypoxia and there wasn't any supplemental oxygen available. Apparently they couldn't reduce altitude (mountains). She said that since then she has had trouble breathing, has gone for medical tests, etc.

I didn't want to be "that guy", so I didn't ask, but it occurred to me to wonder if the pilot was irresponsible in not having supplemental oxygen available. Apparently the pilot and his other passenger were from the area and acclimated to the altitude.

Since one can't just "ask" someone if they will be OK at that altitude, should the plane (pilot) have had oxygen available? In other words, is this an area of pilot responsibility?

There is no regulatory requirement for oxygen to be available for anyone on board an aircraft so long as it remains at altitudes of 12,500' MSL or less. And only the required minimum flight crew is required to have oxygen if they are flying above 12,500' up to and including 14,000 MSL' IF they will be at those altitudes for more than 30 minutes. CFR 91.211  If they were going above 15,000' MSL that would be another story.
So the short of it is, No, that pilot was not irresponsible, perhaps, maybe slightly inconsiderate, but not irresponsible.

The reason for this is the lower density of the air isn't low enough to harm the majority of passengers.  Except for extreme cases, such as those with severe and uncontrolled asthma or heavy smokers, the worst thing that would happen would be the passengers would just get sleepy and fall asleep.  This is natural, and because the body does not need as much oxygen when asleep.  For passengers there generally won't be any harm done from a mild case altitude induced hypoxic hypoxia.

And YES! You can ask someone if they will be 'OK' at higher altitudes!  Just as it is not only 'OK' but required to ask their weight for weight and balance calculations. (And quite frankly I believe most people find that more intrusive.)
In fact you should take 5 minutes to tell your passengers that the flight will be in excess of, say 11,000', that way they can be prepared for some unusual effects and not have to worry about them.  To take it a step further, remember you are the Pilot in Command and you should be aware of your passenger's limitations as that may trigger any self imposed safety limitations you have for the safety of the flight.

Now, if you just want passengers to be comfortable and/or they absolutely insist on not taking a little nap, then it is entirely up to you if you want to carry a portable aviation oxygen canister with you for your passengers.

For further information on hypoxia I would recommend the FAA's page on it and the differing effects on pilots, and by extension passengers.

Hope this helps!


Will occupants usually survive a decent autorotation? When I learned to fly helicopters, I of course spent significant time learning about and practicing autorotations.

The CFI at my school, who had around 15,000 hrs (that's right, fifteen thousand!) said a few times that practice, knowledge and currency are vital — but as long as you got the entry right (following which you can fly to the ground) and executed at least a decent attempt at the flare and cushion, you would survive. Is this a correct take on the survivability of an autorotation? Do fatalities arise primarily from errors during the autorotation entry or flare?

While you might not get to use the machine again, and you might spend some time in hospital, you would live to fly another day.  I am assuming a reasonable place on dry land is available to finally come to rest and no post-crash fire.

If the pilot flubs the flare and landing, the result is frequently survivable.  The reason is that even a flubbed autorotation will reduce the vertical speed.

Student and instructor walked away from this one:

http://www.nydailynews.com/news/national/helicopter-crashes-westchester-county-airport-article-1.1560222


What is compressibility drag? I don't mean wave drag here. What is compressibility drag, which is understood to be a form of miscellaneous drag?

Let's do a Gedankenexperiment:
Think of air streaming around a body as flowing inside a stack of flexible tubes. The walls of the tubes are impenetrable, infinitesimally thin and follow the local streamlines faithfully. When the body approaches at subsonic speed, the air in the tubes near that body makes way for it by speeding up: This reduces the needed cross section and lowers static pressure, so the total pressure will stay constant. On the back side of the body the air slows down again and the tubes regain their old cross section and static pressure. Bernoulli in action.
When the speed nears the speed of sound, however, the speeding up is joined by a drop in density. Still, the air near the body speeds up but that will not change the cross section as much as before, because now this speed increase is coupled with a loss of density. The cross section still drops, but not by as much as before. More tubes have to bend away from the body and need the air in them to speed up so the body can squeeze through. More general: A change in body thickness (more precisely: The second derivative of its cross section) will work on more tubes, so its effects do not die down as quickly as in subsonic speed.
At the speed of sound the cross section decrease due to speed changes is exactly balanced by the drop in density, so the same mass of air needs more volume and eats up all the gain from increased speed. Now there is a wall of air which cannot yield facing the approaching body. That is the sound barrier. In reality, the speed around that body does not reach the speed of sound at the same station in all tubes, so there are mildly sub- and supersonic sections which will allow it to squeeze through. Still, drag is much increased and depends heavily on details in the body contour.
At supersonic speed density changes more than speed, so in order to reduce its cross section, the air in the tubes will slow down in order to make way for the body. Since it has no advance warning of the approaching body, it does so in a shock. As a consequence, the cross section of the stream tube can now be reduced because density increases in that slower air past the shock. Static pressure increases also so total pressure can stay constant again. The drag coefficient drops with further increasing Mach number because the density change becomes dominant, allowing the body to squeeze through the air more easily.
This thought experiment was explained in 1951 to researchers at NACA Langley by Adolf Busemann. One person in the audience, a young fellow named Richard Whitcomb, used the insight he gained to formulate the area rule a few weeks later.


Do we have a unit to measure turbulence? Do airplanes have any way to quantify the turbulence like if shakes are there or the some kind of air drift is happening every parameter of these have some units but do collectively there is any such unit which can quantify the turbulence?

Turbulence can be quantitatively expressed as Eddy Dissipation Rate (EDR). EDR can be calculated from available onboard measurements, however it is not yet common for aircraft to have software to calculate the EDR.

Originally it was thought to have EDR reported over ADS-B, but it did not become a part of the current specifications. It might be in the next ADS-B version.

NASA Langley paper

World Meteorological Organization (WMO) presentation


Does the tailwind component always have the same absolute value as the headwind component? If we are on course 360 and we have crosswind from 020 at 20 kts...does the amount of head wind on this leg equal the amount of tailwind on the reciprocal course (i.e. 180)?

To amplify the question with an example:

TAS:   260kts
Wind velocity: 310/35 kts

Given the above information, calculate ground speed on courses 240T and 060T.

When I use e6b+ aviation calculators, I get the following answers:

On a course of 240, the ground speed is 246kts. This is 14kts slower than the true airspeed.
On a course of 060, the ground speed is 270kts. This is 10kts faster
than the true airspeed.

Since I believe the headwind and tailwind components should be equal, the question is: why is there a difference of 4kts between the computed headwind component and the computed tailwind component

YES, If the "amount of headwind" was 10 knots due to the crosswind, then the "amount of tailwind" would also be 10 knots on the reciprocal course.

BUT, because the 10 knot disadvantage with a head wind will last for a longer period of time than the 10 knot advantage with a tailwind, the entire trip will take longer and use more fuel than a trip in still air.

EXAMPLE: The headwind leg is 15 minutes longer, but the tailwind leg is only 10 minutes shorter, so you have a net loss of time.


When calculating the lift coefficient, what area is used? Lets say I had this rectangular double curve wing. Orange represents the chord length, red represents the airfoil outline length below the chord line, green represents the airfoil outline length above the chord line, and blue represents the wing span.

If I were to calculate its coefficient of lift, I would need to know the area. Which area does it refer to? Could you explain how to calculate the desired area using this diagram?

Could it be the length of green multiplied by the length of blue? Could it be the length of green and red multiplied by the length of blue? Or something else?

Correct me if I am wrong, but in this case, I think the planform area would be the length of orange multiplied by the length of blue.

Reference area for the aerodynamic coefficients (lift, drag, moment) is the planform area. Your picture shows only one wing, while the reference area S would also take into account the fuselage and the other wing: orange times twice blue and then some.

In its most generic case, the equation for wing area S is $$\int_{-b/2}^{b/2} c \cdot dy$$ with b = wing span = distance between the wing tips, c = local wing chord (orange line), y is the coordinate in direction $Y_m$.

For clarity in reference, the picture above from my old uni book by prof. Gerlach on stability & control, which shows how the definition works when sweepback and dihedral angles must be accounted for. Note how the wing/fuselage intersection is defined for the reference wing area, by extending the local leading/trailing edges of the wings.


Is the F-117 fleet at Tonopah Test Range being mothballed? Are the F-117s based at Tonopah Test Range still in service? I heard from some reputable sources they are still flying around and being maintained, but someone else told me they are going away permanently. Does anyone know what's up with the F-117s right now?

Some F-117s under storage (not in active service) were still flying sometime back because their storage type required for it. Attributed to USAF by http://intercepts.defensenews.com/2014/11/we-now-know-why-the-f-117-is-still-flying/:

  Per Congressional direction within the FY07 National Defense Authorization Act the aircraft were placed in Type 1000, flyable storage for potential recall to future service.  In order to confirm the effectiveness of the flyable storage program, some F-117 aircraft are occasionally flown.

The same article gives some details about the type 1000 storage:

  Aircraft in Type 1000 storage are to be maintained until recalled to active service, should the need arise. Type 1000 aircraft are termed inviolate; meaning they have a high potential to return to flying status and no parts may be removed from them. These aircraft are ‘re-preserved’ every four years.

However, this may soon change as the House Armed Services Committee’s version of the National Defense Authorization Act repeals the 2007 provision for storage:

  This section would amend section 136 of the John Warner National Defense
  Authorization Act for Fiscal Year 2007 (Public Law 109-364) by striking subsection (b), which would remove the requirement that certain F-117 aircraft be maintained in a condition that would allow recall of those aircraft to future service.

So, while F-117s are not in USAF service, some were still flying (until recently) though they are probably going away for good soon.


How quickly should a maneuver be started when ATC gives new heading? This is going to be a no-brainer for a pilot, I am sure. I am not one, however, only an enthusiast.

Scenario is: I am flying under the directions of ATC, into a relatively busy Class-B airport and am given a change of heading by them.

  Piper N1234A, fly heading 235 ...

The radio is full of communications, and I cannot seem to be able to squeeze in to confirm receipt of heading change. I assume I would start the standard-rate turn immediately, and acknowledge as soon as possible, even if I've already completed the course change?

You are correct, you should turn to the assigned heading immediately unless you aren't sure of the instruction (was that 100 assignment a heading or a speed?).

When ATC gives you a change, it is based on a continuously changing environment and they want you comply right away, as long as it was clear. Usually, doing the wrong thing is worse than doing nothing at all.


Difference between ETA and On-Block time? I see that the definition of the ETA (Estimated Time of Arrival) according to Eurocontrol is:

(SESAR) the time computed by the FMS for the flight arriving at a point related to the destination airport
(ICAO) For IFR flights, the time at which it is estimated that the aircraft will arrive over that designated point, defined by reference to navigation aids, from which it is intended that an instrument approach procedure will be commenced, or, if no navigation aid is associated with the aerodrome, the time at which the aircraft will arrive over the aerodrome.

For VFR flights, the time at which it is estimated that the aircraft will arrive over the aerodrome.

However I do not understand what is the difference with the Estimated On-Block time.
Would you be able to explain the difference please?

The Eurocontrol Lexicon can help here as well. The Estimated In-Block Time is defined as:

  The estimated time that an aircraft will arrive in-block.

(Eurocontrol)

This is not very helpful, but the definition for Actual In-Block Time is:

  The actual date and time when the parking brakes have been engaged at the parking position.

(Eurocontrol)

This time is relevant for gate availability because the aircraft is expected to be at its gate (or remote parking position) at this time.

The Estimated Time of Arrival is, as your quote says, a time where the aircraft is still in the air. This is relevant for Air Traffic Control because the number of aircraft that can fly an instrument approach to the destination airport at the same time is limited. If too many aircraft have a similar estimated time of arrival, some will have to hold in a pattern or are delayed on-route already.


Is it allowed to fly a right traffic pattern when a left traffic pattern is published (IFR)? Allentown Queen City airport (XLL) has a VOR-B Circling approach procedure from the west.  Circling south of runways 7/25 is not authorized ("NA").  If the wind is favoring runway 25 is it legal to circle north under IFR using "right" hand traffic and land on runway 25 considering the following?:

XLL does not have a control tower and is in Class G airspace (uncontrolled airspace).
It is published that runway 25 uses "left" traffic.
FAR 91.126(b)(1) requires that all turns be made to the left unless airport displays (light signals or visual markings) show turns should be to the right.  This is NOT the case for XLL.

VOR-B approach:

Map of XLL in Class B Airspace:

Traffic Pattern Info for Rwy 25:

FAR 91.126: (full regulation here)

Yes, it is legal to circle north under IFR using "right" hand traffic and land on Runway 25.

"Left traffic" refers to VFR traffic. Traffic patterns are generally flown at 1000' AGL. IFR aircraft don't fly traffic patterns unless they cancel IFR or are doing a visual approach under direction of an active control tower.

An IFR aircraft on a circling approach can maneuver at any altitude (above the published minimum altitude), and in any direction, in order to align itself with the landing runway. .

When you are issued an IFR clearance at an uncontrolled airport, the IFR airspace around the airport belongs to the IFR aircraft. I think FAR 91.126 (a) General. "Unless otherwise authorized or required" would supersede the left traffic requirement.

In this case it is mandatory to circle right because Circling south of runways 7/25 is not authorized. ("NA")

If the weather was above VFR limits, and there was also a VFR aircraft in the area, they would have to fly a left traffic pattern and communicate to avoid conflicts.