Within the US: When, and by whom, is the Transition Level determined/published? There are several SE Q&As that deal with the terms Transition Altitude, Transition Layer, and Transition Level.  The very-brief, paraphrased definition of the Transition Level (TL) is:

  The lowest usable flight level that can be assigned, and the point at which aircraft descending out of the flight levels will change their altimeters back to QNH.

Due to atmospheric pressure variations, the TL can vary from day to day and (maybe?) region to region.

My five questions about the Transition Level are thus:

Is the entire US using the same TL at any given point in time?
Or, are individual ARTCCs setting their own TL based on the lowest QNH reading in their geographical area?
Or, are individual sectors within an ARTCC setting their own TL based on the lowest QNH reading in their even-smaller geographical area?

If the individual sector controllers are NOT making the determination "on the fly", I then assume that the TL is determined by a "higher authority" and published to all controllers/sectors/centers under that authority. Which leads to the fourth and fifth questions:

Who (what level, position, etc.) determines the TL?
When, or how often, is this determination made?

The "Lowest Usable Flight Level" ("Transition Level" [TL] - term used outside North America) is FL180 when the (local) altimeter setting within a controller's area of jurisdiction is 29.92 inHG or greater. This is normally, not necessarily always, the province of ARTCC controllers in the U.S.

Your Questions:

  Is the entire US using the same TL at any given point in time?

This is not the case because the local pressure in San Francisco can be (for example) 28.92 and the pressure in Miami could be 30.20.  Therefore, the lowest assignable/usable FL over San Francisco would be FL 190 and the lowest assignable/usable FL over Miami would be FL 180.

From FAA JO 7110.65W, para. 4-5-4: JO 7110.65W

  Or, are individual ARTCCs setting their own TL based on the lowest QNH reading in their geographical area?

Individual areas of jurisdiction within ATC Facilities would be responsible for determining the Lowest Usable Flight Levels based on local QNH/altimeter settings.  Keep in mind that some sectors (areas of jurisdiction a single controller may be responsible for) can be enormous in size (part of Nevada west to near San Francisco, for example).  There can be a significant difference in local QNH settings within that sector that may require adjustments to the Lowest Usable Flight Level.

  Or, are individual sectors within an ARTCC setting their own TL based on the lowest QNH reading in their even-smaller geographical area?

See the answer above.
and 5. In my experience, an individual controller maintains all separation authority and responsibility within his/her assigned sector.  There may be supervisory input or oversight, but the responsibility to separate aircraft in accordance with the JO 7110.65 rests with the assigned controller.  There also may be internal SOP's that dictate how a facility will handle the flow of the information regarding lower than 29.92 local altimeter settings.


Can a helicopter stall? Imagine that a helicopter raises its nose up and then there is a loss in speed. Can it enter a stall? If so, how do you recover from the stall?

Yes and No.

The definition of a stall is that the airfoil stops generating lift, which happens when the critical angle of attack is reached.

In an airplane, this happens during normal flight by pitching up until the critical angle of the airfoil is reached.

In a helicopter, aft cyclic ("pitching up" in an airplane) will only serve to make the helicopter climb and slow down until it reaches equilibrium in the new attitude.

Helicopters, however, can suffer from what is called "retreating blade stall," when the blade is at full pitch on the retreating side of the rotor:

This doesn't happen under normal circumstances, but there are four factors which contribute to retreating blade stall:

fast -- close to Vne
heavy -- at max gross weight
hot -- above 35C
high -- at high altitude

Retreating blade stall is considered an emergency, and can be recovered from by reducing power.


Is there any "instrument take-off system"? Flying close to the ground is really dangerous and yet needed to land and take off. ILS is designed to keep the adequate trajectory close to the ground with (almost) no visibility. Is there a similar system for take off, e.i. a system helping pilots to maintain the adequate trajectory near the ground with almost no visibility at take off?

While there is not a specific physical system dedicated to instrument departures per se, there are procedures that rely upon instrument flight rules (IFR) navigation equipment that ensure obstacle clearance when departing from an airport in instrument meteorological conditions (IMC).

From the Aeronautical Information Manual 5-2-8.

  Instrument departure procedures are preplanned instrument
  flight rule (IFR) procedures which provide
  obstruction clearance from the terminal area to the
  appropriate en route structure. There are two types of
  DPs, Obstacle Departure Procedures (ODPs), printed
  either textually or graphically, and Standard Instrument
  Departures (SIDs), always printed graphically.
  All DPs, either textual or graphic may be designed using
  either conventional or RNAV criteria. RNAV
  procedures will have RNAV printed in the title,
  e.g., SHEAD TWO DEPARTURE (RNAV). ODPs
  provide obstruction clearance via the least onerous
  route from the terminal area to the appropriate en
  route structure. ODPs are recommended for obstruction
  clearance and may be flown without ATC
  clearance unless an alternate departure procedure
  (SID or radar vector) has been specifically assigned
  by ATC. Graphic ODPs will have (OBSTACLE)
  printed in the procedure title, e.g., GEYSR THREE DEPARTURE (OBSTACLE), or, CROWN ONE
  DEPARTURE (RNAV) (OBSTACLE). Standard Instrument
  Departures are air traffic control (ATC)
  procedures printed for pilot/controller use in graphic
  form to provide obstruction clearance and a transition
  from the terminal area to the appropriate en route
  structure. SIDs are primarily designed for system enhancement
  and to reduce pilot/controller workload.
  ATC clearance must be received prior to flying a SID.
  All DPs provide the pilot with a way to depart the airport
  and transition to the en route structure safely.
  Pilots operating under 14 CFR Part 91 are strongly
  encouraged to file and fly a DP at night, during marginal
  Visual Meteorological Conditions (VMC) and
  Instrument Meteorological Conditions (IMC), when
  one is available. The following paragraphs will provide
  an overview of the DP program, why DPs are
  developed, what criteria are used, where to find them,
  how they are to be flown, and finally pilot and ATC
  responsibilities.

There are many details involved in actually using these procedures to make safe departures in IMC. The Instrument Procedures Handbook Chapter 1 is an excellent resource for pilots who want to review these procedures, what they provide and how to use them correctly.

Here is an example of an ODP from my home airport, KCRG. There are some very tall towers less than 10 nautical miles from the airport. They are enormous and easy to avoid under visual conditions, but when IMC prevails these procedures are absolutely life-saving with such large obstructions nearby. They are taken from this takeoff minimums document.

Here is an example of a SID. These are always graphically depicted and will usually be found at larger airports. Here are pages 1 and 2 of the full SID.


Why does the chevron-shaped engine suffer losses in thrust performance? From here I found out a lot about chevrons on the Boeing 787, but not enough to satisfy why it results in performance loss.

My thinking was originally, if they are "cutting triangular holes" in the "bell cone", then some part of the exhaust will expand sooner than others. (I'm more of a rocket guy myself). Or you could think of it as "adding triangular extensions" to the optimal bell-cone, in which case some parts of the exhaust will expand later than usual. Or you could meet in the middle, which is probably the best way, but will still result in suboptimal expansion.

But I find myself wondering about an even simpler analogy: If sound is wasted energy from an engine (that could have gone into thrust), then reducing sound somehow must mean that otherwise wasted energy has actually gone into thrust production and therefor would boost thrust performance?

So which is it really, and why?

The chevrons need a small pressure difference between the fan flow and the surrounding air. Then they will work like small delta wings and create two strong vortices per triangle, which will help to mix fan flow and outer flow. This mixing is responsible for the reduced noise, because it will distribute the kinetic energy of the fan flow over a bigger mass of air.

This mixing by itself uses up some of the energy of the fan flow, and thrust is produced by the conversion of the pressure increase due to the fan into kinetic energy. With the chevrons, a little of this kinetic energy is now spent for mixing and cannot contribute to thrust anymore. Hence the reduced efficiency.

You're right to assume that less sound should mean more thrust. In fact, the sound energy is not evenly distributed over all frequencies, and by adding some noise at frequencies with low noise contribution, the peaks at the lower frequencies which contain most of the noise can be lowered.


What is the aerodynamic benefit of trailing edge sweep? I know that the sweep will allow the flight to fly at higher speed because the perpendicular air flow speed (over wing) is less.

But I didn't understand why there is trailing edge sweep?

We can have a swept leading edge and unswept trailing edge to produce elliptical lift distribution. Also, unswept trailing edge will help to improve flap effectiveness, right?

Could someone please explain this to me.

Thanks in advance

Both leading and trailing edge sweep help to reduce compressibility effects. For simple estimates, quarter chord sweep is used. This is more influenced by the leading edge, but the trailing edge is still involved with 25% of the total value.

If we look at the particular influence of trailing edge sweep, it is mostly found in the effectiveness of flaps and ailerons. Their effectiveness is proportional to the cosine of hinge line sweep. However, flaps at the end of a long wing chord will create a high pitching moment, requiring a big tail. Most delta wings use simpler, less effective flaps with less moment change.
In supersonic aircraft trailing edge sweep helps to reduce sonic boom intensity.

In the end, trailing edge sweep is mostly an outcome of the design process, similar to wing chord. The structural engineer will want to have a deep root airfoil in order to have enough wing height for a stiff spar or the landing gear without increasing relative thickness too much, and the aerodynamicist wants to reduce wetted area, since it will create friction drag and increase wing mass. In the end, all have to agree to a compromise.


Why, on carrier landings, does the Flight Path Marker (FPM) seem to be placed near to the end of the runway? On most military aircraft landings, the FPM seems to be usually lined up with the start of the runway, either around the numbers or the aiming point, I would asume this to be the same on carriers for a higher probability of catching the wires, But it seems in most videos, the FPM is placed almost off the end of the runway.
Ex:

             (Loud noise)

The short answer is that it is because the carrier is moving toward the point that the FPM is indicating; where the ship and aircraft will converge at the moment of landing.

In reviewing my answer to the other question linked in the comments, it struck me that some clarification might be needed to distinguish between landing visually, or by reference to ship based ILS, versus flying a HUD equipped aircraft.  In the first two cases, (visual or ILS) the frame of reference is the ship itself.  Whether the ship is stationary with 30 knots or natural wind across the deck, or steaming at 30 knots to make wind on a calm day, the indications to the pilot are identical.

Contrast that with a HUD equipped aircraft.  The FPM uses the earth as a frame of reference, not the ship.  It shows where the aircraft is going relative to the surface of the sea, its position is based on calculations from inputs of GPS and air data computer, and factors in the wind.  (which itself is calculated…)  It doesn't know or care if the ship you are trying to land on is moving.  Therefore, if the ship is stationary one would expect that the FPM would indicate a touchdown point right in the landing area.  Just as it would a stationary runway on land.  (if the pilot is doing his or her part!)

If the ship is steaming at 30 knots the FPM will still indicate where the aircraft is going relative to the surface of the sea.  However, since the carrier is moving as well, the FPM will be out in front and to the right of the landing area during approach.  (Because the ship hasn’t gotten there yet!)

In the video you can see that the ship has a decent wake behind it, but as the aircraft closes in the FPM moves from up and right, to right in the landing area at touchdown.

Keep in mind also that the FPM is pretty much instantaneous, so that pilot corrections to glideslope deviations will show up as well.  For example, if the pilot rolls out high and right at the start, even if the ship is making all its wind the proper correction might put the FPM down and left until re-correcting.

And finally, the Navy discourages “HUD Cripples”.  While (I have heard) it is a wonderful tool, the pilot should reference the traditional meatball, lineup, and AOA on short final.  So while the pilot is supposed to be aiming for the ship, his or her FPM may actually be aiming just a bit in front!  ;)


What is garbling in the context of radar? Garbling is a problem known to affect secondary surveillance radar (SSR). What is it and why is it a problem?

Garbling occurs when two aircraft are close enough that their replies overlap, as there is no reliable way to sort them out. Usually this occurs when the aircraft are passing above each other as the radar scans the entire vertical column at once, only rotating horizontally.

The problem is that the controller might not be able to check the aircraft are indeed flying their correct assigned altitudes exactly at the moment when it matters most when they are passing above each other.

Quick Googling reveals e.g. this article with some diagrams of the overlapping signals.

A related problem is “fruit”, which is the squawks triggered by other things than the radar beam (e.g. the interrogations from T-CAS on other aircraft) being mistaken for ones that were, and therefore making the aircraft appear at other place than it really is.

Both issues are mitigated by ADS-B, because there the aircraft broadcasts at semi-random points and includes it's own position, so while the messages may still conflict (like in other transmission protocols), the timing is designed to make it unlikely to happen for several following messages.


Is there an FAA rule or Advisory Circular with regard to blood donation? Is there an FAA rule or Advisory Circular with regard to flightcrew member blood donation?  For example, how many hours or days is required or advised before a pilot may act as a crewmember?

I'm interested in finding the source of legitimate FAA policy or guidance for flightcrew members.  If the answer is, there is no FAA rule or advisory circular, I'd also be interested in specific examples of FAA-approved air carrier policy.

It seems 24 hours is the generally accepted waiting period to recover from possible hypoxia related to a donation.

An air carrier that I have worked for specifically addresses this issue in their General Operations Manual—which is accepted by the FAA. The manual gives the following requirement:

Flight Restrictions After Blood Donations
Flight personnel shall not participate in any capacity as flightcrew members for a period of 24 hours after donating more than one-half pint of blood.

Note that this portion of this manual is accepted by the FAA, not approved.1
One unit of blood is generally 470-500 mL, or one pint. The one-half pint specified above half of a regular unit of blood.  According the the Red Cross, a typical donation is about one pint, or one unit.

The FAA also publishes the Guide for Aviation Medical Examiners. I am not an AME, nor am I fully familiar with the Guide or how to interpret it. However, I understand that the following table shows that outside the listed time-frames, an applicant for an airman's medical certificate could be considered medically fit for flight duty:

1 The terms Approved and Accepted mean specific things to the FAA. See 8400.10, Vol 3, Ch 15, Sec 1:

Approved: When approved is used to describe a document, manual, or checklist, it means that a regulation requires FAA approval and that the FAA has evaluated and specifically approved the document, manual, or checklist.
Accepted: Accepted is used to describe a document, manual, or checklist that does not have, or is not required to have, FAA approval. Only a portion of an operators manuals are required to have FAA approval. The remaining portions are accepted by the FAA. Operators are required to submit the entire general manual to the FAA for review. If the FAA concludes that an accepted section of the general manual is not in compliance, the FAA must formally notify the operator of the deficiency. Upon notification, the operator must take action to resolve the deficiency.


What is the standard power supply and voltage supply on modern airliners? I'm looking at reducing the weight footprint of some of the computer systems on a commercial liner. A big consumer of weight seems to be in power supplies and conversion. Being a computer guy, I don't know anything about avionics or electricity on commercial liners.

Is there a standard power supply or voltage supply for modern air liners?

Typically 115 VAC, 400 Hz for most large aircraft, which can be rectified and stepped down for DC powered subsystems.  Smaller GA airplanes typically use 12, 14 or 28 VDC for their electrical systems.


Process for Compensator in Altitude Hold Trying to understand autopilots on a technical level. When making an altitude controller there is a compensator Gc that is used to acquire a Pitchcommand which is passed to an attitude controller (the attitude controller uses a PID loop which I understand). What is the process of creating that compensator? I've read that a PI compensator is less reliable and more complex than a lag compensator. But as a rough approximation (still trying to figure it out) of what needs to be done for a lag compensator you need to have a linearization of the aircraft model, apply root locus, adjust poles, and acquire a transfer function. If anybody could specify what would need to be done (correcting my missing/incorrect understanding) that would be very appreciated.

A lag controller is basically an amplifier whose input is proportional of the present error, that is the difference between the current altitude and the target altitude

With low amplification the trouble is the lack of response as long the error is small.

With great amplification we have an overshoot effect with continuous oscillation around the target altitude.

The best simple low cost compensator consists of an PID

Please refer to this website:

https://en.m.wikipedia.org/wiki/PID_controller

The PID has 3 adjustable inputs:

A proportional input « P » somehow similar to a lag amplifier
An integral input « I » that is proportional to the error but also to the duration of the error, therefore an error as small it may be will be corrected with time and will decay
A derivative input »D » which is proportional to the slope of the error that is the error evolution, therefore it has a predictive value. Therefore the D input will neutralize the tendency of an error to appear or to increase.

The adjustment is done by small steps,

-you start by setting to minima the I and D inputs, and set the P input to get the best non overshooting response.

-In order to reduce the remaining error with the previous adjustment you increase the I input by steps and verifying you don’t get overshoots

-In order to avoid drifting error you act on the D input, increase the input so that you obtain a little overshoot that you correct by reducing the I input.

Trial and error adjustments could be useful


What is the best design for a small flying wing with special flight envelope? I'm designing a small, light and portable flying wing UAV that will launch and land vertically via a Tilt-rotor system. I'm attempting to use the program XflrV5 but am having difficulty understanding the numbers. I'm a design and electronics guy, but a poor engineer.

The craft has a span of 6', root chord of ~8". Since it is a VTOL it does not need to fly slowly - below 15-20 MPH. I would like a top speed of ~100 MPH. the end product will be likely built with carbon fiber so compressibility shouldn't be a problem. After some reading I am tentatively using the s5020 airfoil but I'm not married to it.

My primary priority is forward flight efficiency, second priority is reasonable stability as it for recon use (forest fires, search/rescue military) but this can be mitigated by use of a stabilized camera gimbal. Maneuverability is a low priority if it has to be to accommodate the above items.

My question is with these parameters what type of wing design would work best and would sweep and twist be needed considering the speed envelope?

A UAV with a similar envelope would be the Boeing Scan Eagle as it is catapult launched and flown into a net for retrieval.

-------------------EDIT 1 -----------------------

I have included some pics of possible designs  to help show some ideal designs to meet the portability requirement. Note: the outer 2' (660mm) of the wing will be removable for portability.

It would be a great help to hear your opinions which would be best.

-------------------EDIT 2 -----------------------

A similar model to mine is the Firefly6... mine will have some improvements as well as a different market. I forgot to add that the AUW will be 6 - 7 lbs. (~3Kg)

I know, flying wings are cool. But once you have built and flown one, you get a more nuanced view. To find the right configuration, start with a look at your objectives. You want:

excellent performance and
an observation platform with good stability.

Now let's see what a flying wing will do for you. Apologies for the heavy use of technical terms; please follow the links for more verbose explanations:

It will have little pitch damping, so your airframe will wobble through the air with every gust or control command. You will need wing sweep to get at least a little aerodynamic pitch damping.
Depending on wing sweep, wing spar stiffness and pitch inertia, the frequencies of the wing bending oscillations and the fast period mode might converge within the desired speed range, resulting in flutter.
With a span of 2m and a root chord of 20cm the aspect ratio is above 10. If you sweep the wing, your low speed characteristics become outright nasty. I know, you will use engine power for take-off and landing, but once you fly a tight circle, you will bring the wing close to stall even at higher speed, and then a high aspect ratio swept wing will start to roll on its own and pitch up. In other words: It will become uncontrollable.
A reflex airfoil only produces lift in its forward part. The rear part produces a little downforce at high speed and proportionally much less lift than the forward part at low speed, so the whole wing will become less efficient. It will be better to reduce wing area and to use an airfoil with an excellent ratio of lift to drag.

My advice: Put part of the saved area at the end of the fuselage as a horizontal tail surface. This will give you ample pitch damping (goes up with the square of the tail surface's lever arm) and enables you to use a high-performance airfoil on a very small wing which produces much less drag than a flying wing of the same minimum speed. If you want good high-speed performance it is important to keep the wetted area at a minimum to minimize friction drag.

Now that you have expanded the question and added portability, let me propose something that would combine all three characteristics: The Leading Systems Amber. It was designed to fold to the size of a torpedo to be carried onboard Navy ships by pivoting the wing by 90° and folding the two tail surfaces forward. Combine this with a folding propeller and hand-launch it to avoid conventional take-offs.

Leading Systems Amber in flight (picture source)

Landing was done by deep-stalling the aircraft. To quote from an article about Abe Karem in Air&Space magazine:

  Most ingenious of all, Amber could be recovered by putting it into a
  “deep stall”—a feature used in free flight glider models to escape
  thermal updrafts and stay within flight time limits. The stall would
  bring Amber to “a near-vertical landing so it could be used from small
  ships, submarines or trucks/trailers,”

At least, this is what I would do: Build a scale model of Amber.


How is d(Cl)/d(alpha)@stall and Cl increment to stall defined in Xrotor? I hope there are some people around here with some experience in Xrotor than me. (Xrotor is:"XROTOR is an interactive program for the design and analysis of ducted and free-tip propellers and windmills." Found: here documentation is at the end of the site) For the Blade Section Properties, I need following:

========================================================================
1) Zero-lift alpha (deg):   0.00       7) Minimum Cd           : 0.0070
2) d(Cl)/d(alpha)       :  6.280       8) Cl at minimum Cd     : 0.150
3) d(Cl)/d(alpha)@stall :  0.100       9) d(Cd)/d(Cl**2)       : 0.0040
4) Maximum Cl           :  2.00       10) Reference Re number  : 2000000.
5) Minimum Cl           : -1.50       11) Re scaling exponent  : -0.2000
6) Cl increment to stall:  0.200      12) Cm                   : -0.100
                                      13) Mcrit                :  0.620
========================================================================

Now does anyone know how 3) d(Cl)/d(alpha)@stall and 6) Cl increment to stall is defined?

If I had to guess 3) is just the slop after Maximum Cl. But 6) which is also mentioned in the doc as "delta CL for the stall transition region" gives me some headache.

Any help is appreciated.

Cheers

I interpreted the values as follows:

The angle of attack at which the specified propeller airfoil generates zero lift
The change in lift coefficient per change in $\alpha$ in radians
The same as 2, except that it is this value around the stall angle (it is mostly a safety factor for numerical calculations I think, small but non-zero)
Maximum $C_l$, which is achieved at the theoretical stall angle
Minimum $C_l$, which occurs on the backwards sweep of the propeller
The $C_l$ increment from the last linear part of the $C_l$ polar to the stall point
The $C_d$ at $\alpha=0$
The $C_l$ at $\alpha=0$
Quadratic drag dependence on $C_l$, slope of the $C_d,C_l^2$ curve
The Reynolds number at which these values are obtained
Re scaling exponent as explained in the XRotor documentation
Moment coefficient $C_m$
The critical Mach number.


How to completely track a plane, not a flight? This probably has already an answer somewhere, but probably don't know the proper search keys to find the information.

Out of curiosity today I was trying to track a flight: one of my friend texted me about her flight being a lot late, and for fun I wanted to try to see if it was possible to understand what caused the delay. I knew flightradar24 by fame and found too flightaware by chance; I tried with the flight number (EZY2604...if this is the flight number) and couldn't find many information, 'till I realized that EZY2604 is probably something like the callsign for the route plus the scheduling, so with that I can only find details for this combination. And I can only track a plane which is flying, it seems.

TL;DR: how can I go from a flight number to the full route of the physical plane in the previous hours?

If you want to look for the airplane that flew a certain flight (U22604/EZY42EA for your case), you could go onto flightradar24.com/data and search for that specific flight to find the list of aircraft which flew with the callsign and if you select any one of the planes, you can find out the flight history of that plane for the past few months. You could also search flightaware.com/ for flight history too.


What is the point of making one control surface play the part of multiple control surfaces? For example, flaperons (flaps + ailerons) and elevons (elevator + ailerons), are all individual control surfaces but play the roles of two different control surfaces. What is the point of making a single control surface, undertake the functions of two?

Flaps which only extend over part of the wing will cause lift changes mainly over that part of the wing, which will result in a lift distribution over span which is far from the elliptic ideal once the flap is deflected. Therefore, it makes sense to move the ailerons in the same direction, especially when the inboard flaps are simple camber-changing flaps.

In order to give an aircraft wing docile stall characteristics, the planform should not be elliptic, but be "fuller" (= having more local chord than the ideal ellipse) at the wingtips. With an elliptic planform and an elliptic lift distribution, the local lift coefficient will be identical over the whole wing, and gusts might easily cause a local stall at one outer wing in slow flight. The consequence will be an uncontrollable roll, which can ruin the pilot's day when occurring close to the ground. Therefore, it is better to leave some margin by increasing local chord and adding some washout towards the tips.

Now this wing will have an elliptic distribution only at one angle of attack. If the angle of attack is higher, the lift distribution will be fuller, and if it is lower than the "elliptic" angle of attack, some lift will be missing towards the tips. Both conditions will cause higher induced drag than necessary. Flaps help by adjusting local camber such that if they are set correctly, the spanwise lift distribution will be much closer to the elliptic ideal than without flaps. This requires the flap schedule to make bigger deflection changes towards the root - watch any modern high performance glider change flap settings, and you will see what I mean: The flaperons will change deflection less that the inboard flaps. This has the additional benefit of leaving more margin for aileron deflections.

This is the disadvantage of combined control surfaces: What is needed for one function is no longer available for the other function, because the maximum useable deflection range is limited.

For the nitpickers: Please understand the term "elliptic lift distribution" as "elliptic circulation distribution". Then all is well.


Where can I find GA VFR maps for Australia? Are there any sites similar to SkyVector (US-based) for GA aviation maps in Australia?

I understand that Air Services Australia has miscellaneous single-purposes maps available, but they seem to be mostly airport diagrams and approach procedures. I'm looking for GA VFR maps.

If the only thing available is SkyVector's world maps (of Australia), how accurate are they?

I have both US and Australian PPLs, and having flown VFR in both countries, there are some real differences in the VFR charts and supplemental data available. Mostly the contents are fairly evident but if you are unfamiliar with a specific type of VFR chart, it is important to look over the chart in detail to make sure you understand it.

The US publishes VFR charts known as Sectionals. These cover the entire country, including non-continental areas like Hawaii. IMHO these are excellent although in busy airspace, the 1:500,000 scale can become complex. So pilots learning to fly in the US usually become familiar with the sectional covering their base field, and then with that understanding, can look at sectionals for anywhere else in the US.

Printed sectionals can also be used for long range VFR flight planning as they can be laid out in sequence and routes plotted across adjoining chart boundaries. For online chart examination, SkyVector publishes all US sectionals online, which is a great resource.

Australia does not publish sectionals covering the whole country. It's a good question as to why but essentially the centre of the country is a big desert where few people live. So from a historical perspective, the inner part of the country has been regarded as "remote" and thus less in need of highly detailed maps. In addition, the word 'sectional' isn't even in Australian pilot lexicon, so use the word 'chart' instead.

Australia publishes Visual Terminal Charts (VTC) for each large metropolitan area which are a Mercator projection at 1:250,000. And there are Visual Navigational Charts (VNC) which mostly encapsulate the VTCs and are at 1:500,000 (same as sectionals). The image below is an example of the Brisbane VTC.

As VTC/VNCs cover only largely populated areas, for printed chart long VFR route planning, you will need to revert to the World Aeronautical Charts (WACs), pronounced 'whacks'.

WACs started as an ICAO project after WW2 to transition from the existing US-based international wartime series and so all Allied nations were effectively asked to harmonize their maps with a US design. But in 2015, the FAA stopped WAC production and thus a recently US-trained VFR PPL would never have heard of them(!)

The US WAC coverage looked like this:

WACs are a Lambert projection at 1:1,000,000 and in Australia, printed chart VFR route planning would be done over sequential WAC's, with likely a supplemental VNC chart for the populated areas to be crossed. WAC's do not contain airspace (although supposedly an aeronautical chart!). They are essentially a topo map with features that are easy to spot from the air.

So, to answer the OP's question - no, there are no free online sources for Australian government produced charts. SkyVector shows a very data sparse US Sectional-like view of Australia but I wouldn't use it for anything other than simple distance/leg planning.

However, all of the previously mentioned chart types for Australia are now encapsulated and adjoined in a single app known as OzRunways. I have no association with them, other than being a customer. Another answer has also referred to this as well.

OzRunways works on both iOS and Android and in my experience seems very widely used amongst Australian VFR pilots. For VFR use, they provide a so-called 'Hybrid VFR Map' which simply uses the most detailed chart available. Price at time of writing is about AUD $100 and chart-wise also has New Zealand and Papua New Guinea.

But also of value to pilots flying in Australia, there is comprehensive airfield information available in the ERSA (EnRoute Supplement Australia), which is a free download. It lists technical aspects/refueling contacts/etc. for every sizeable airfield in Australia. In addition to this, AOPA also publish a pilot guide for various airfields. OzRunways contains the ERSA and for an extra amount, they also will bundle the pilot's guide.

I would value any other comments about VFR chart sources for Australia but hopefully this is a good overview.


Does jet engine exhaust velocity relative to itself remain constant despite speed changes? Maybe this is a stupid question.

I seem to have been told, the velocity of the exhaust leaving an airbreathing jet engine relative to the engine, is constant despite changes in airspeed. However, to my understanding:

The ram effect causes air pressure to rise within the engine.
This higher pressure can then be converted right back to velocity with a nozzle.
This velocity adds to the velocity given by the fuel.
For an airbreathing engine with no friction or other losses in the ducts, this means that the exhaust velocity relative to the engine increases with airspeed.
meaning that the reason why we use low-bypass turbofans for supersonic aircraft is to reduce weight and intake losses, NOT propulsive efficiency
and this is what is shown by some attempts to develop low-loss supersonic through-flow turbofans of higher (around 2) bypass ratio.
in short: the statement only makes sense for rockets, which carry their own reaction mass, not airbreathing engines, which ingest reaction mass with any kinetic energy it contains, which cannot simply disappear.

So, what am I missing?

Thrust is created by accelerating a working mass in opposite direction. Net thrust is the difference between the impulse of the air flowing towards the engine and the combined impulse of burnt fuel and the air exiting the engine (and propeller, if one is fitted), derived by time. That impulse is the product of mass and speed.

When flying faster, the entry impulse of a propeller or a fan quickly grows large relative to the exit impulse, so thrust goes down with the inverse of speed. On the other hand, the high exit speed of a turbojet results only in a small increase of the entry impulse relative to the exit impulse while speed increases.

GSP simulation of a turbojet engine for varying altitude and speed, taken from this answer.

But if that were all, even the thrust of a turbojet engine would drop when speed increases. But there is a second effect which helps to let thrust grow with speed. With the square of speed, to be precise. That is the ram effect which helps to precompress the air entering the engine. At subsonic speed, this just about compensates for the loss of thrust: At low speed, the growing entry impulse lets thrust drop a bit but at higher subsonic speed the ram effect becomes larger and raises thrust again, such that a constant thrust becomes a good approximation (see simulation result in the plot above). However, pressure and thermal limits inside the engine will quickly limit the additional compression in the compressor and the amount of heating that can be added in the combustor. What helps is to fly higher, where outside pressure and the intake temperature will be lower, which more than compensates for the pressure and temperature increase from the ram effect. But then thrust will drop in proportion to density.

So far for turbojets.

Turbofans are a different matter. Here, thrust drops with increasing speed similar, but less severe, as it does in propellers. The nozzles of both the cold outer and the hot core flow only allow the exit stream to leave at subsonic speed, so at cruise speed the fan nozzle is critical or close to this (meaning, exit speed is nearly the speed of sound). In both nozzles the speed can not go up indefinitely and when the core engine exit speed at maximum thrust and low flight speed is already close to the speed of sound, the rule that it stays nearly constant is a good approximation of reality.

Uninstalled thrust and SFC for a large turbofan, from "Airplane Aerodynamics and Performance" by Jan Roskam. Found here.
The steeper trend of the sea level thrust line above Mach 0.7 is caused by the pressure limit of the engine.


Does ADS-B In receive track or heading angles? I am reading this: https://mode-s.org/decode/ehs/bds50-track-n-turn.html

I am wondering the Track angle which ADS-B In recieve is really the track angle (derived from line joining the previous and current point) or the heading angle (or its body angle)?

It is the true track angle.

True means with respect to the geographic North (as opposed to the magnetic North)
Track angle means the direction of motion in the horizontal plane (as opposed to heading, which means the direction of the projection of the primary body axis in the horizontal plane).

Track angle can be derived from a line joining successive position points, but that is not typically how its done. GPS commonly uses a combination of Kalman filtered change of position combined with (Kalman filtered) velocity derived from Doppler shifts.

For ADS-B In, the track angle is just what is reported by ADS-B Out. It does not involve processing successive position reports.


What is the typical touchdown vertical speed of a large airliner? What is the vertical speed (range) of a Boeing 787 (or any other large commercial aircraft) that should be achieved just before and during touchdown? I'm not interested in the vertical speed during the approach, my question only targets the last 100 feet before touchdown... this includes the vertical speed after the flare. Obviously a vertical speed of 0 fpm is desired for comfort reasons, but as I understand (see also this question), aiming for 0 fpm makes it difficult for pilots to aim at an optimal touchdown point on the runway.

From an MIT ICAT paper:

  Today's commercial autoland systems utilize the ILS (...) At around 30 ft above the ground, the aircraft enters the flare mode, during which the aircraft pitches up to reduce the vertical speed from its current descent rate to 1-3 ft/s by touchdown.

Which means we're looking at 60-180 FPM touchdown rate.

Kiss landings (near 0 FPM) are undesirable, because they can't be achieved on regular basis without risking long landings. They can also break landing gears (more on that below).

I checked various Boeing/Airbus operating manuals, they do not state the target rate of the flare mode, but from the 747-400 flight crew training manual (page 6.11):

  Do not allow the airplane to float: fly the airplane onto the runway.
  Do not extend the flare by increasing pitch attitude in an attempt to achieve a perfectly smooth touchdown.

Left: a shimmy event "damaged the wheel, tire, and shimmy damper" (Boeing). Right: what a shimmy looks like (YouTube).

  Landing with extremely low sink rates is more likely to experience shimmy than a firmer landing because the torsion links remain in an extended vertical position, where the damper has less mechanical advantage for longer periods of time (Boeing).

Boeing commercial aircraft are designed for 600 FPM landings, with 360 FPM when above the maximum landing weight. Boeing also says pilots are the best evaluators of hard landings, "because of the difficulty in interpreting recorded acceleration values at the CG of the airplane."

Based on pilot reports and data, exceeding 240 FPM is a hard landing, which further confirms the 60-180 FPM to be an ideal firm value whether the landing is flown manually, or with the auto-land (Boeing).


Why would lift coefficent change with weight on swept-wing aircraft? Today I encountered a problem. There is a book saying, "Why does the lift coefficient of a swept-wing airplane decrease with the increase in the weight of the airplane?" I cannot think of the reason. Isn't the lift only coefficient related to the state of the wing?

A swept wing has less torsional strength than a straight wing, ie there's less structure at 90d to the root supporting the tip of a swept wing.
Thus a swept wing twists when it's loaded, which unloads the tips and thus generates less lift at higher weights.


What are the advantages/disadvantages of ultrasonic in aircraft structure inspection? What i mean by structure is (different composite materials and aluminum).

What are the cons in terms of time required to test the full body of the aircraft (B747 for example) in C and D checks ?

Ultrasonic inspection is perhaps the most widely used inspection procedures in aerospace to detect subsurface anomalies.

Portability- A number of portable ultrasonic equipment are available, which are very simple and are widely used.
The detection depth for flaws is far superior compared to most other NDT methods.
It can be used for detecting flaws even if only one surface of the part is accessible (simply use pulse echo method).
The results are more or less instantaneous- Usually there is no waiting time involved as the results are displayed in a screen.
The time required for preparation is very less; usually the part is cleaned and the couplant applied; after the inspection, it can simply be wiped off.

However, just like any other (NDT) method, this too has its own share of disadvantages:

It is very difficult to use this method for parts which are very thin- for e.g. the aircraft skin because of the dead zone, though this can be overcome by using higher frequencies or immersion technique; in most cases of aircraft checks, this may not be an option. Also, it cannot be used for irregular and non-homogeneous parts (At-least, not the portable methods).
The method does not detect flaws that are oriented parallel to the wave propagation.
The material must be characterized prior to inspection. As the returning  signal is affected by attenuation within the material, comparisons can be made only if a similar (defective and good) specimen of same material is characterized first.
Usually, Ultrasonic method requires a trained operator for interpretation of the results.

As far a full aircraft is considered, only some critical parts are tested using ultrasonic (or other NDT) methods. There is no way to check the entire aircraft except by thorough visual inspection.


What are the limiting factors when flying a model UAV plane to high altitude? Setting aside the likely legal factors for a moment, what are the limiting factors when flying a model plane to high altitude?

As I see it these could be:

Battery power consumption - Model planes seem to have a battery life of less than an hour. There is a large distance to be covered in a climb ~20KM.
External forces - Strong winds/turbulence acting on a relatively small and light airframe could easily blow the plane off course. I'm not sure how severe these would be at higher altitude.
Air pressure/oxygen content - I presume this could reduce flying efficiency.
Low air temperatures affecting the operation of the electronics inside and perhaps the control surfaces.

The same rules apply as for "real" airplanes, and the topic has been covered well in previous questions. With model airplanes, scaling laws make the task easier, rsp. the possible maximum altitude higher. I would suggest to hang the plane below a balloon to get it up without draining batteries. NASA has done it before with a remotely controlled glider.

For the limiting factors please see this question.

Here is a question for the maximum altitude of propeller aircraft.

Here is another on the feasibility of geostationary drones.

Depending on the acceptable fragility of the design, a model aircraft should easily go down to a wing loading of 10 $kg/m^2$ and then be flyable at 36 km of altitude. At a true airspeed of 155 m/s it will fly at half the speed of sound and the Reynolds number on the wing with an assumed chord of 0.25 m will be below 20,000. You need to fly it with a lift coefficient of 1.1, which will be a challenge at the low Reynolds number, and the L/D will probably be less than 10, so your propulsion needs to be quite powerful to hold the altitude up there. I haven't done the numbers, but the propeller will be huge. With back-of-the-envelope calculations, my answer for the maximum altitude is 36 km.

Turbulence should be of no concern - all flow patterns up there are of extremely large scale.

The low temperature should also be tolerable if all systems are electric. It is not so cold up there: The standard atmosphere gives -34°C.


How to find the flight paths of an airport? How can I find the flight paths near an airport (e.g., Heathrow or Pearson) including cruising altitude, descending speed (variations)?

Specifically, I need a figure showing the height vs. speed of a plane as it's landing starting from the cruising altitude.

You can find a great deal of approach path information here but that will only give you the path, and perhaps some intercept altitudes for a given approach it will not give you any speed information. Cruising altitudes will also vary heavily by aircraft type.

  Specifically, I need a figure showing the height vs. speed of a plane
  as it's landing starting from the cruising altitude.

This is broadly specific to each airframe as well as the loading of any given aircraft at a given time. But you can find the information if you know the airframe in question, there are also some limits depending on where you are in the world. The FAA (and presumably most legal entities) cant have different rules for everyone so they broadly break down approach speeds into categories. That should give you a fair idea of the approach speeds. Keep in mind these are airspeeds and local wind will alter ground speeds on an individual day basis.

If you start from the cruising altitude there will be a few phases of the decent that will see various speeds. This depends on what ATC clears you for and what the local conditions call for more on that here.


Why are clearing turns supposed to be shallow bank? During my CFI training, my instructor relayed a story in which he had a mid-air collision while teaching (how to teach) clearing turns to two other CFI applicants. The other aircraft was underneath them and may have climbed into them. No one in his aircraft saw the other aircraft. They did not even know they had hit another aircraft until they heard the report of another crash nearby.
My question is (in the title) why is a shallow banked clearing turn considered better than a steep one? I feel like I get a much better view of everything (above, level, and below) in the direction of the turn if it is a steeper turn. The downside is that I'm changing direction more quickly...but I think seeing other aircraft wins over that. Furthermore, large, rocking wing movements tend to make the aircraft I am in easier to see as opposed to gentle, shallow turns.

Clearing turns solve two problems: seeing other aircraft and helping them to see you.
Banking will make you more visible to other aircraft, who may not have noticed you before. A lower bank angle (and thus turn rate) gives them more time to see and avoid you. But a very low bank angle also doesn’t make you as visible in the first place, so it’s a balance.
I was taught 20-30°, and I’ve never had anyone say later that was too much or too little. I also don’t have to worry much about losing altitude at moderate bank, so I can keep my eyes outside scanning for traffic—not checking the altimeter every few seconds like during “real” maneuvers.


What justifies a biplane over a braced monoplane? Biplanes became the first airplanes to enter service due to their structurally efficient design being more suitable for the weak materials then. This efficiency comes from the two wings acting as the chords of a very deep truss.
Another structurally favorable factor was the reduced wingspan and thus moment allowed by distributing lift across stacked wings.
Yet another weight saver was the reduction of wing loading, which enabled thus use of less powerful engines for slower flight.
However, the extra wing only produces 20% more lift with 100% parasitic drag increase. Ironically, this interference drag is even more severe at the lower speeds which biplanes are optimised for.
A braced monoplane has most of the structural advantages of a biplane, with less of the aerodynamic disadvantages. So it would seem to be a more logical design than a biplane, given the same materials and engine.
Yet biplanes were built in large numbers. Why?

Back in the pioneer days both types competed fiercely. The braced monoplane has similar structural depth to the braced biplane but less than twice the total wing area, so many argued in its favour. On the other hand, the shallower angle of its bracing wires introduced greater compressive forces in the wing spars, so they had to be strengthened and that added weight.
The monoplane suffered its first drawback around 1911 when, first in France and soon after in Britain, examples started falling out of the sky and for a time they were banned from military service. The French problem turned out to be a mistaken design assumption leading to over-thin bracing wires, the British to be the Ministry-specified cowling fasteners coming adrift. They were soon flying again, but the damage to their reputation was done.
Nevertheless, a few types were still in service when war broke out, especially with Germany. The Etrich Taube and Fokker Eindecker helped to restore their reputation, especially when Fokker developed interrupter gear for the machine guns, unleashing the Fokker scourge. But this would prove the high point of the braced monoplane's career.
The advantages of the biplane you mention - high strength and short span - proved to give greater manoeuvrability, stiffness and damage tolerance when it mattered, and the monoplane quickly became obsolete.
The biplane remained dominant until engine powers rose sufficiently to take planes beyond 200 mph or so, where the greater weight of the unbraced or cantilever monoplane was a price worth paying for cleaner aerodynamics.


What are the difficulties in building transonic airliners? If the sonic boom is an issue, why don't we build airliners that are capable of flying just below Mach 1? Big airliners fly at Mach 0.85 at most, and the record for current sub-/transonic civilian aircraft is the Cessna Citation X+'s Mach 0.935. Why don't we build airliners that cruise at Mach 0.95? Also, what would happen if the Citation X+ tried to reach or surpass Mach 1, since it's anyway close? There it isn't answered.

That is because as you approach the transonic regime, the drag on the airframe begins to rise and can reach up to ten times the subsonic drag. after you go supersonic, the drag falls down again- and this makes the transonic regime the worst place to operate a plane from a fuel burn standpoint. So you either stay subsonic, operating below the drag rise point, or supersonic, comfortably beyond that point.
Another consideration is this: Even if you are subsonic but nearly supersonic, there will be points on the airframe where the airflow is locally supersonic. You can actually see this in Boeing's long-distance planes, where a cabin seat at roughly mid-chord affords you a view out the window of the normal shock sticking up from the wing at high-speed cruise. If the flow goes supersonic at one of the control surfaces, it may upset the controllability of the plane, which would be undesirable- so better to stay comfortably out of the transonic regime for cruise.


Is there a standard international Minimum Obstacle Clearance Altitude (MOCA)? In Turkey, the Minimum Obstacle Clearance (MOC) in primary areas is 300m; what about other countries and region?

The MOC value you are referring to is related to the en-route phase of an IFR flight, so your statement that it is the lowest altitude a plane can descend to when approaching the runway is wrong. That value depends on the type of approach, aircraft equipment and various other factors. It is not a fixed value shared among different airports. You probably have MOC confused with Minimum Descend Altitude (non-precision approaches) or Decision Altitude (precision approaches).

However, to answer your actual question, 300 metres is an ICAO standard for MOCA (Minimum Obstacle Clearance Altitude) in the primary area for the en-route phase of an IFR flight. So other countries use the same value, unless they have special local rules specifying a different value.

Note than, in mountainous areas, the minimum obstacle clearance applied, depending on terrain elevation, is 450 metres for terrain between 3000 ft and 5000 ft, and 600 metres for terrain higher than 5000 ft.

Minimum Obstacle Clearance Altitude (MOCA) on Skybrary


Was Ludwig Wittgenstein's aircraft propeller ever built? Ludwig Wittgenstein designed (and received a patent for, at the remarkable age of 22) an aeronautical propeller.

Wikipedia describes it:

  Wittgenstein’s design required air and gas to be forced along the propeller arms to combustion chambers on the end of each blade, where it was then compressed by the centrifugal force exerted by the revolving arms and ignited.

and adds:

  Propellers of the time were typically wood, whereas modern blades are made from pressed steel laminates as separate halves, which are then welded together. This gives the blade a hollow interior, and therefore creates an ideal pathway for the air and gas.

I've found some plans for this design, and some discussion of it, but it's not clear to me whether a working model was ever built, or attempted. I'd love to know whether it actually offered any advantage, or even whether it could be made to work with a reasonable degree of success.

Ian Lemco of the Royal Society published a note on this in 2007. He described the invention more as a jet engine than anything, as it featured a central compressor feeding tip-jet nozzles. Burner position varied during development of the design, from central to blade tip. A patent dating from 1910 shows jet combustion at the tip. One central compressor was of the centrifugal type. Thus, it is seen in some ways as foreshadowing the Whittle jet engine.

Lemco found sources to be scarce, mostly anecdotal, and unreliable. Certainly Wittgenstein constructed various experimental devices and tested them, both in the lab and on a railway track. Some may have been rockets. One source remarks in passing on the engine that he constructed, but the reliability and context are far from clear. There is no confirmation that he ever built a complete jet engine.

It seems to have been fundamentally workable, as compressor-driven tipjet rotors were indeed later flown, but it was probably beyond the technology of Wittgenstein's day.

Ian Lemco; "Wittgenstein's Aeronautical Investigation", Notes and Records of the Royal Society of London, Vol. 61, No. 1 (Jan. 22, 2007), pp. 39-51. Stable URL: http://www.jstor.org/stable/20462605


Heaviest Boeing vs Airbus aircraft: Why large (150 ton) difference in Weights? An aircraft tug manufacturer's website had this interesting table:

Just curious, why the heaviest Airbus is a good 150 tons heavier than the heaviest Boeing? Otherwise, these two rivals have close competing aircraft for most specs.

I mean, how do the rivals end up with such different operating points. Obviously, I don't mean its an apples to apples comparison (different payloads) but what I mean is whatever market segment it was that Airbus was trying to serve with an A380, did Boeing not find that an interesting segment? Is this a strategic difference in a large plane / smaller plane sense?

Short Answer:

Difference in corporate mindset between the two companies and incompatible interpretation of market conditions and projections of future market demand.

Long Answer:

@Anilv answers your question to an extent but you should really understand the total scope of the commercial aviation sector. Airbus is a relatively newer company that has a widely different mission goal than Boeing: Airbus is an exclusively EUROPEAN owned multinational company with a majority share under EADS, the European Aeronautic Defence and Space Company. It was primarily brought together to give Europe and its smaller more specialized companies another chance against massive, government backed corporations from America (the big three: McDonnell Douglas, Lockheed Martin, and our big daddy Boeing). This actually caused quite a bit of a political sh*tstorm, with Boeing complaining that Europe's explicit backing of this new Airbus being far too pointed in corporate tie-in with government, but besides for some concessions made to ease relations with America, Airbus was allowed to continue receiving grants and other national backing from its parent countries.

Now to explain specifically why Airbus doesn't compete directly ALL the time with Boeing's aircraft is because frankly, leadership of both companies cuts projects all the time in different sectors because of their individual personas and visage for the companies' futures. I'm glad you mentioned the A380 because except for a few other situations continuing the A3XX (in-house title for A380 prototype) was by and large the most controversial internal decision made by any Airbus executive. The situation was this: Both Boeing and Airbus were looking at the VLA (Very Large Aircraft) concept, an idea becoming popular in the 90's due to burgeoning flight routes and demand for efficient carriers of mass amounts of passengers. Also, as the Gulf War ended Airbus had begun to reap profits with their A330 and A340 series aircraft. However, not too long ago Boeing had released its jumbo-jet 747-100 and -200, which were extremely popular and Airbus leadership was getting worried about their potential loss in this possibly goldmine niche. Alan Boyd, at the time president of Airbus North America reported after a joint study with Boeing of the market that Airbus predicted a market for about 500 to 600 airplanes, while Boeing envisioned a market for only 300 to 350. Interestingly both saw a profit margin of about $15 billion. Boeing saw the risk/reward too high, and at the time (I would argue it still is) was very encumbered with bureaucracy and promises to keep to the American government. It was a large project no matter what, and Boeing to an extent was on America's payroll after all. However, although risks were high for Airbus as well, the company exec at the time, the legendary Jean Pierson saw that the risk was worth taking, not only because of projected efficiency and prototype designs, but also because he needed to protect the rest of Airbus' lineup.

To even comprehend the complexity of the issues facing these men, you MUST understand that decisions to make or scrap a new plane isn't an isolated choice: an airline buying a new plane from one maker is highly unlikely to mix and match to buy a different size plane from another maker just to fit a smaller or larger route i.e. buy the A380 for its long-haul high-demand routes and the 787 for its longer but lower-demand routes. Boeing on the other hand had a relatively fleshed-out spread of planes and due to a HIGHLY incorrect mindset of complacency and EVOlution rather than REVOlution, they did not seize this oppurtunity to expand and improve. This was common knowledge to all business-minded men in the sector who were worth their jumping beans (sadly few at Boeing after , so the most prominent factor in Pierson's mind was probably his defense of the also-new A340 series, a small long-haul aircraft that fills a similar role to the new 787. This, as the chart you provided shows, filled a spot that was projected to be VERY profitable (as Boeing found out too late and rushed in the 787 to fill) but at immediate time was a little too hard to swallow for most medium sized airlines. Keep in mind that Airbus was still trying to prove itself to the world.

So, by Pierson's logic introducing a superior 747 in the form of the A380 would not only draw attention to the plane itself, but also help establish the A340 and start the ball rolling for future development of the wide-body long range medium size airliner. So, that's pretty much why the A380 was a (mostly) no-brainer for the Airbus group, and from the late 80's on, why Boeing and Airbus' line of airliners has considerably diverged. Hope this answers your question, and if you'd like more information or just clarification feel free to comment.

[Edit]
The book Boeing Versus Airbus: The Inside Story of the Greatest International Competition In Business gives a wonderful account of everything I just detailed and more. I've read it multiple times and I never cease to learn something new from something under 300 words. Give it a try if you have time!


Is a powerback allowed by airlines as a safe maneuver? I watched some videos on YouTube where the pilot used the thrust-reverser for push-back instead of the push-back tug. Some examples:

MD80 Powerback

DC9 Ground Reverser Powerback

Is this procedure currently in use? Would an airline today certify a 737, or even turboprop, like a Beechcraft 1900D, to execute such a maneuver?

I've also seen a Boeing 727 video. 727s and MD80s have the engines attached directly to the fuselage, not in the wings. Would that be one of the reasons?

Most modern commercial airliners are prohibited to power back.
Ground operations in aircraft fitted with high-bypass engines are usually restricted to idle and low-idle operations (enough to make the craft start moving, after which momentum enables further movement with idle only).
Aircraft capable of power back are predominantly thus turboprops, several operators of these allow their crews to operate accordingly.

It's not in common use anymore, for the following reasons:

Thrust reversing can kick up a lot of debris, posing a safety hazard. This would generally preclude any ground crew or spotters walking under the jet as is often done during pushbacks.
The previous point's especially true for under-wing jet engine designs, so only jets with fuselage-mounted engines are allowed to do it, and most of the bigger designs (DC-9, MD-80, 727) are being phased out of most fleets.
Pilots in most craft can't see behind them, another possible safety hazard requiring multiple spotters to be in contact with the pilot.
Thrust reversing uses a lot of fuel, so it's usually cheaper for the airlines to use a guy on a tractor for an assisted pushback.
Thrust reversing is loud, and during a powerback that noise is in close proximity to the terminal. The tarmac's a noisy place, but usually not that loud.

If you want to see a powerback performed by a commercial airliner, fly out to L'Esperance in St. Martin. The runway there is very short and has no taxiway leading to the end of the runway from either end, so larger passenger turboprops that service that airport have to turn onto the runway about 2/3 down its length, face their desired takeoff direction, then powerback to the end of the runway in order to have enough to take off. The larger and (relatively) busier Princess Juliana International, on the Netherlands side, has taxiways and turnaround areas at both ends of its runway.


How is the A320neo engine different from the A320ceo engine? I've looked up at internet about the differences. And the only difference they mention is that the new engine has a higher bypass ratio (making it more fuel efficient), quieter and has an improved range. Are there any other differences other than these?

Are the engines lighter than its predecessor (are they made of lighter alloys?)

Do the new engines have the reduction gearbox which lacks in the ceo engines?

And what is TAPS II combustor technology?

It is a very large question and it will be difficult (even with all information) to give a strict answer.

Both LEAP-1A (A320neo family) and CFM56-5B (A320ceo family) are turbofan engines which means they have two cores (HP and LP core) and two flows, and where total airflow is created by a Fan. As you said, LEAP-1A has a larger diameter allowing higher by-pass ratio (efficiency improvement) and lower rotational Fan speed (noise reduction). Besides, OPR (Overall Pressure Ratio) is greater (~40:1 vs. ~33:1) and lead to a performance improvement too.

Nevertheless, even if LEAP-1A has a slightly greater max thrust (~155kN vs. ~140kN), we cannot affirm there is "a range improvement" since they do not operates on same aircraft. If it's true here, it is more appropriate to compare aircraft range than engine range. Moreover, thrust is calculated for an aircraft flight mission (by Airbus, Boeing, etc.) and then generally given to engine manufacturers as input for conception.

Now, to answer more specifically your questions, LEAP-1A service entry was in 2016, more than 20 years after CFM56-5B first flight. Therefore LEAP engines gather a lot of improvements resulting from years of research and maturity tests. So yes, there is others differences between these engines (you can find public characteristics on manufacturers or specialised websites - even wikipedia get a lot of information actually).

1. Are the engines lighter than its predecessor (are they made of lighter alloys?)

Material is probably one of the field where you can find best improvements in aeronautics the last decades, for both aircraft and engines. In particular, many advances have been made with composite materials.

When comparing CFM56-5B and LEAP-1A, the most significant (and literally the most viewable) point is about Fan material. For a CFM56-5, fans are probably made of steel or nickel alloy (I don't know exactly but these are common for old generation engines). In a LEAP engine, composites are used for fans with titanium at leading edge. This allow, amongst others, a lower number of blade (18 blades) and larger chord: fan mass reduction despite larger diameter, aerodynamic efficiency, etc.

But is LEAP-1A really lighter than its predecessor CFM56 ? Not sure, remember that the diameter is larger. A quick search on google give 2900kg for LEAP-1A and 2400kg for CFM56-5B, but I don't know if it was calculating in the same way.

Note that other improvements are made in high temperature parts (like HP turbine) both in cooling and material resistance resulting in a higher combustion temperature (again there is an efficiency improvement).

2. Do the new engines have the reduction gearbox which lacks in the ceo engines?

No, both LEAP-1A and CFM56 do not include gearbox. CFM international has never developed an engine with it. This is a manufacturer choice as the gearbox lead to an extra mass and often extra maintenance costs. In the other hands, gearbox give advantages by decoupling LP turbine / booster and fan speed and remove one constraint to optimise each component.

As @Bianfable said in comments, it is a little bit awkward to talk about ceo and neo engines as Airbus manufacture these aircrafts with 2 different engines. For example, Pratt & Whitney PW1000G turbofan on A320neo include a reduction gearbox..

3. And what is TAPS II combustor technology?

TAPS (Twin Anular Premixing Swirler) is a combustor technology developed by GE aviation. While CFM56-5B used SAC (Single Annular Combustor) or DAC (Double Annular Combustor),  TAPS is a newer type of combustor used for GEnX engine (Boeing 747-8 and 787). The version II is the scaling version for LEAP engine. The main idea is to limit rich combustion zone (mainly responsible for NOx).

Take a look to this FAA report for interesting details.


Why is the Eurofighter's nose gear door shorter than the bay? The Eurofighter Typhoon has an interesting "feature" in that the nose gear door is shorter than the nose gear bay itself by a good few inches, as shown here:

Can anyone shed any light on the reason for this gap?  It isn't a simple case of the aircraft in the photo having a maintenance issue, as this gap is consistent across all aircraft tranches and customers.

Some fighters and as well civil airliners have fans ending in the gear bay. To allow for pressure relief some sort of opening needs to be created to vent to atmosphere. Various designs are available to achieve this, like the well known grills or mesh on carbon or aluminum structure skin. Or, just shorten a nose gear bay door if you have no place to accommodate the mesh and suck it through top part (see holes in the top where cables is routed) If you stick your head into a F18 nose gear bay and it is electrically powered, it will be quite noisy. Seems as the Eurofighters have the same design.


Advances in rivet technology for Aircraft? What advances have there been in the manufacture of rivets which are used for joining for aircraft and rivet technology and materials used?

Rivets need to be harder than the material being joined by them, and the growing use of high-strength alloys has made the use of rivets of the same material as the material being joined problematic. To set a rivet, it needs to have some plasticity, which in turn is greatly reduced when an alloy is optimized for high strength.

Initially, rivets were made from aluminium alloys, but have been replaced by fasteners, especially Hi-Loks (Hi-Lock is also used) which are (mostly) titanium or steel bolts with an aluminium or titanium nut. The nut has two parts; the lower part, the collar, has the internal thread and remains on the Hi-Lok after assembly while the upper part forms a hexagonal head for tightening with a special wrench. Between both is a groove which allows the head to shear off at the right torque. This ensures consistent torque and eliminates the need for torque inspection. The wrench needs a hex bolt at its center which serves as a socket wrench and engages with the shank of the fastener to keep it from rotating with the nut.

Why titanium? Lisi Aerospace says on its website:

  By using titanium nuts rather than steel or stainless steel nuts, we
  can save up to 110kg on a single aisle aircraft and 220 kg on a wide
  body aircraft.

If access is restricted, blind fasteners are also available. Hi-Loks have been standardized and cover a wide range of sizes and shapes.

Hi-Lok fastener (picture source)

Another advance are special bolts for joining composite parts. The head of a regular fastener is too small and runs the risk of being pulled through the composite sheet. Special bolts, washers and nuts for composites spread the load out.

Composite panel fastener (picture by Alcoa Fastening systems, source)


How to escape from an aircraft if the door is not armed? Following the question about arming the door, the link states that doors are usually armed before engine start. What if something goes wrong when passengers are already boading with doors un-armed how to escape from the airplane?

The door the passengers are loading through is obviously one route, back down the jetway or stairs.

The door being "armed" does not affect whether it can be opened or not.  When the door is armed, opening it will also automatically inflate the escape slide.  When the door is disarmed, the slide will not inflate.

Additionally, each door has a manual inflation handle for the escape slide which the crew can use to deploy the slide if needed.

I could not find a non-copyrighted picture to include.

You could start here for more information.


How did old autopilots work? With fly-by-wire and computers between pilot's command and flight control surfaces, it is easy to imagine the implementation of an autopilot inside the computer, but I read somewhere that autopilots existed before WW2. They were made only to maintain altitude and heading, but worked at a that time with the mechanical linked between pilot and control surfaces.

How did these old autopilots work?

It used a set of gyroscopes and pendulums that would correct the plane's attitude when it deviates. They were usually enhanced with pneumatic or hydraulic power to actually move the control surfaces.

For example the V-1 flying bomb (WW2 German UAV) could fly across the Channel and initiate a dive after a preset distance. It remained on course using a pendulum for pitch control and a gyroscope for yaw control. It used its inherent stability for roll control (no ailerons).


Why is a centrifugal compressor in aircraft engine less susceptible to stall and surge? I have came across a similar question here. Although the answer doesn't explain much about why and is somewhat vague. I have read that centrifugal compressor can achieve a higher pressure rise per stage than axial compressor. So technically they are less susceptible to stall and surge.

How is a stall and surge occurring in a centrifugal compressor different from that occurring in axial compressor. What are the scenarios under which a centrifugal compressor stall and/or surge (Is it same as those scenarios in axial compressor. If so I do have a rough idea)? And where does stall/surge first occur in a centrifugal compressor (impeller or diffuser)?

Is it because the rotor blades of centrifugal compressors doesn't act/function like airfoils?

The reason comes down to the principle of operation of the compressors - a centrifugal compressor, as it's name implies, just spins around really fast, and the centrifugal force compresses the air. As you can see in the image below, the vanes of the impeller of a centrifugal compressor just help push the air around in a circle (unlike an airfoil) and centrifugal force does the compressing:
The centrifugal compressor is circled in blue. As you can see, when the impeller rotates, it pushes air to the side.

Image source - https://upload.wikimedia.org/wikipedia/commons/6/6f/Saturn_MD-120_cutaway.jpg

An axial compressor has blades that work like little rotors (which are really just rotating wings) - the rotors accelerate the air, and the stators slow the air down again, using that kinetic energy to compress the gas. Just like wings, these blades can stall and lose function whenever the input flow is disturbed enough (such that the local angle of attack of a portion of the blade is too high for the given local flow conditions). This stall can then potentially spread to other blades, or to the entire engine.

As you said, the vanes of an impeller in a centrifugal compressor don't work like airfoils (instead, they kinda just rotate and push air around, kinda like a turnstile), so if they encounter airflow disturbances, they won't stall like an airfoil would. Of course, if said airflow disturbance is something like severely restricting airflow, the compressor will surge, but this is considered rare in practice.

Therefore, it's much easier to get an axial compressor to surge or stall (just by disturbing the airflow a lot) in comparison to a centrifugal compressor, which requires airflow to be restricted.


Is this the correct sequence of steps to convert from true course to compass heading? I wanted to confirm my understanding of VFR Flight Planning/Dead Reckoning with relation to heading and course calculation. Is the below correct?

Using a plotter, draw a line from departure airport to arrival airport
Using the same plotter, get the true course (TC)
Using an E6B or similar calculator, calculate the Wind Correction Angle (WCA) which is based on Wind Direction and Speed
+ or - the WCA to/from the TC and the result will be True Heading (TH)
Using the chart, see what the Magnetic Declination is and + or - the value (West is Best [add], East is Least [subtract])
The result will be Magnetic Heading (MH)
On the day of flight, check the Compass Deviation Card to + or - the deviation from the Magnetic Heading and this will give us the Compass Heading (CH)

Yep, that's how it's done!
(If you fly the same airplane all the time, one other thing you could do is take a photo of the compass card.  That way, you have the deviation already handled and you save a step at the airport.)


How do airports handle traffic during emergencies? What does ATC do when there is an emergency? This could be a tower or an ARTCC being evacuated or otherwise unusable.

How do they decide whether to close the airport/airspace?
What do they do with the traffic, whether they do or don't close?

On this related question, it turned out that Newark closed because of smoke in the tower.

Another user posted an interesting anecdote about another tower being evacuated, so I thought it warranted a question.

The facility under duress will declare "ATC Zero" and all traffic will be handed off to other controllers while that facility is offline.

In the case of a control tower, this means the airport either acts as a non-towered field or as a closed field (depending on the nature of the emergency).  If it is a TRACON, then the ARTCC will assume control of the TRACONs airpsace.  If it is an ARTCC, that might present bigger problems as continuity of radio communications may be impacted and I'm not sure exactly how this would be handled but assume it would be re-routes around the ARTCC.

I've been inbound to IAH when their tower decided to close (I forget why), and the response was to put everyone inbound to IAH into holds and handle diverts when fuel became an issue.


How to compute thrust to weight ratio of a Cessna 172? Cessna 172 has weight about 1 ton and it's engine produces about 213 Kg of thrust, and it flies great.

So if I make a plane with a weight of 100kg including pilot and fuel so will 15 kg thrust good for the plane, and if not please recommend me engine which is perfect and please also recommend me the best prop size.

What is thrust to weight ratio of Airplanes.
Sorry for bad English. thanks

A good estimate for a personal aircraft thrust requirement would be more like 25% of aircraft weight, and remember, the "ton" of Cessna includes fuel and pilot too!

So: 1 ton x 2000lb/ton x 1kg/2.2 lb = 909 kg Cessna

213 kg thrust/909 kg weight = 23.3 % thrust to weight ratio.

But a lot depends what your plane design is.  You can test thrust requirement for level flight by gliding, then multiplying weight x altitude lost/distance flown = thrust.

So it helps a lot to have a good, strong wing too, and more thrust will be needed to climb.  15 kg may not be enough for your plane, but as long as you are on internet, you may wish to study "ultralight aircraft" to get some better information.


Why isn't airport relocation done gradually? Recently Istanbul airport (IST) was relocated from Atatürk over 2 days. Munich Franz Josef Strauss (MUC) was also moved overnight from Munich Riem. Doesn't it cause chaos, since most employees are new to the place and equipment, work procedures are not well established? E.g. some people do not have badges with correct security clearance.

Why not do it gradually over longer time? Move airline by airline -- smaller first, bigger later.

Gradual relocation essentially mean having to staff and equip nearly two full airports during the transition period. It is also annoying for the travelers that want to transfer planes and need to relocate to the other airport. They would then need to get transported to or from the new location and through security again unless a small short hop flight is established during the transition. In IST's case it's 35 km distance between the old and new location.

Having two busy airports close to each other is also a bigger challenge for air traffic control than a really busy one and a calm one.

The solution to the issues you mention can be solved by thorough preparation. Like making sure all the old badges work (or having the new ones passed out as they come in for their first day at the new location), make sure everyone knows where they need to report for work in the new location. Perhaps having some extra trouble shooters on hand to fix teething issues.


What spacing is needed between aircraft parking stands? I'm doing a project where I have to design an apron for a general aviation airport (ICAO type 2B). The airplanes have to park in angled nose-in parking, or angled nose-out parking.

What is the required separation between two stands? I think that the dimensions of one stand depend on the turning radius of the airplane that is supposed to use the stand, but I don't know where to find this angle for every airplane...

I'm looking for any data or other relevant information.

The Aerodrome Standards manual, which is based on ICAO Annex 14, gives the following values:

Clearance distances on aircraft stands
3.12.6 An aircraft stand shall provide the following minimum clearances between an aircraft using the stand and any adjacent building, aircraft on another stand and other objects:
Code Letter | Clearance
A | 3 m
B | 3 m
C | 4.5 m
D | 7.5 m
E | 7.5 m
F | 7.5 m

A references for the ICAO code letter categories is available here: http://www.skybrary.aero/index.php/ICAO_Aerodrome_Reference_Code


Could higher aspect ratio actually reduce gliding efficiency of a smaller glider? I have been building 1 meter wingspan 100g class free flight mono wing gliders with a chord of around 10 cm and a flying speed of approximately 3 meters per second.  So far glide ratios of around 10:1 have been achieved.  The wing thickness is 1.2 centimeters.  The airfoil
has maximum thickness at 3.5 cm from leading edge.  The bottom is flat, but only covered 6 cm from leading edge, leaving an undercambered area the remaining 4 cm to the trailing edge (this design grew out of thin undercambered wings).

So, is there a maximum aspect ratio  (possibly based on Reynolds number calculations and/or "pressure leakage" due to the small scale or low speed)?  Or do we continue to
go for as high an aspect ratio as structurally possible.

Reynolds number for airfoils = Vc/v where V = 3 m/s, chord = 0.1 meter, kinematic
viscosity at sea level = 1.460 × 10 minus 5th power = 20,548 for my model.

A response by Peter Kampf June 22, 2018 to "Is it best to use a thick airfoil or an undercambered airfoil for for slow speed flight" mentioned the Daedelus airfoils DAE 11,
DAE 21, DAE 31, so it seems to be on the right track.

I am getting ready to start a build, would there be an optimal aspect ratio for this
new aircraft?

With again thanks to other writers who have inputted valuable information, it has been found that excessive aspect ratio, by way of Reynolds Number, can effect gliding efficiency
of a smaller, slower glider negatively as follows:

Reynolds number = Vc/v.  Slower speed and shorter chord reduce Reynolds number. Kinematic viscosity of air v does not change.

Using Airfoil Tools, Coefficient of Lift/Coefficient of Drag at various Alpha was evaluated using a range of Reynolds Numbers from 50,000 to 1,000,000 for the Bleriot Eiffel 428 (thin undercambered)  and the Wortmann FX 60 126 (thicker, less under camber).

It was found that Clift/Cdrag is significantly greater, at higher Reynolds number, particularly in a "sweet spot" around 6 degrees AOA.  In this region, drag seems to "plateau", while Clift continues to rise.

At very low Reynolds Numbers, the Eiffel 428 showed a slightly better Clift/Cdrag ratio, but in ALL cases higher Reynolds number improved Clift/Cdrag ratio up to around 500,000 (will vary depending on air foil type), after which the polars seemed much more consistant.


What is the correct ATC phraseology for tower to clear a practice approach? If you have an aircraft (training flight) that is doing practice instrument approaches and ATC is notified that they will fly the approach and then go missed, what is the correct phraseology for the tower? "Cleared to land" or "cleared missed approach"? Where can I find this information in a document?

If you want to go missed on a practice instrument approach rather than land, the controller will clear you for a "low approach".

See here for more on the legality of low approaches.


When do pilots use NOTAMs? Usually, pilots will check NOTAMs during pre-flight prep. Are there some situations where they use them in a specific flight phase? If yes, in which case would it be useful?

Thank you

Use or check? NOTAMs can give pilots of a wide range of information, so in terms of use each NOTAM is different. For example, they might tell the pilot that a frequency has changed - so that's something they may need to integrate into their flight at the relevant point.

Or, the classic, is that there are standing NOTAMs in the UK that overflying Ukraine and Syria aren't recommended. Not something most pilots need to worry about.

There is no expectation to check NOTAMs after departure, however.


What is the "non-apologizing rule"? I was reading some forum about funny ATC communications and one commentary was

GXXXX: Request status of danger areas X1, 2 and 4
<Pause>
London info: I don't recognise area number x124
GXXXX: My mistake I meant areas x1, x2 and X4
London info (female): Sorry, just had a blonde moment here
(assume she was new and didn't know the non appologising [sic] rule)

(source)
when referring to some thing the ATC said, apologising for not understanding some non-formal phraseology.
What is that rule ?

I suspect there is no such rule. I looked at a few online sources - none mention the word apology (or variants).

ATC communication is expected to be short and follow a standard set of stock phrase forms.

Anything outside that is discouraged unless necessary to clarify instructions etc.

FAA

An FAA document "Radio Communications Phraseology and Techniques" says

  Brevity is important, and contacts should be kept as brief as possible,

So any communication that doesn't follow one of the standard patterns is discouraged. It continues.

  but controllers must know what you want to do before they can properly carry out their control duties. And you, the pilot, must know exactly what the controller wants you to do. Since concise phraseology may not always be adequate, use whatever words are necessary to get your message across.

ICAO

An ICAO document "ICAO Standard Phraseology A Quick Reference Guide for Commercial Air Transport Pilots" says

  Phraseology has evolved over time and has been carefully developed to provide
  maximum clarity and brevity in communications while ensuring that phrases are
  unambiguous.

Again the emphasis is on brevity and standard phraseology. It also continues ina similar way to the FAA document.

  However, while standard phraseology is available to cover most
  routine situations, not every conceivable scenario will be catered for and RTF
  users should be prepared to use plain language when necessary following the
  principle of keeping phrases clear and concise.

Other

For non-pilot rubberneckers like me, a good intro to pilot-ATC communication is a "Say Again" blog post or Two by Don Brown.

  I remember one time when I was a young controller and working a slow, high-altitude sector. I was bored so I got into a conversation with a pilot about something or other. All of a sudden I noticed this F-15 was in a big turn.

 "Peach two one, Atlanta Center, where you goin'?"
 "Atlanta Center, Peach two one is declaring an emergency,
        we've lost an engine and are returning to Dobbins."

  He couldn't get on the frequency because I was having a conversation.


Is there a way to integrate recorded digital media evidence into a pilot report? If I encounter something during flight that I feel other pilots in the area at that time need to be aware of, the standard procedure is to file a pilot report.  If I have taken a photo or video of the phenomenon or incident, is there any way to integrate this digital media evidence file into the pilot report?  Can you type in website(s) link(s) to the digital media?

No. PIREP's follow a strict format that predates that kind of digital media. They must be compatible with distribution via AFTN and ACARS. PIREP's are created from a limited character set, and there is no way to attach any kind of digital media.


What is "Takeoff Minimum: NA" for US Airports as depicted in Terminal Procedures? This question refers to US Air Carrier procedures for determining takeoff minimums, as published in FAR 91.175, 121.651, Carrier OpSpecs, and the US Terminal Procedures Publication booklet.

As nicely answered in this question, we know that low-visibility minimums for takeoff are outlined in a carrier's opspecs, with paragraph C078 granting approval for "lower than standard" visibility requirements, if the airport runway doesn't already have published higher than standard takeoff minimums.

An airport with a published higher-than-standard minimum for one or more runways is indicated on approach charts with the upside-down triangle with the letter T inside (although this may also indicated only an obstacle departure procedure, or both an ODP and a minimum). The published minimums themselves can be found in the US Terminal Procedures Publication.

My question is about runways marked as "Takeoff Minimum: NA". What does "NA" mean? Does it mean that "lower than standard" minimums may not be used.  Or does it mean "all other runways have comments, but no comments are necessary for this runway, so lower-than-standard takeoff minimums published in opspecs are fine and dandy"? There also seems to be different "NA" indications; some with comments such as "obstacle" or "ATC", and some without any comment at all.

I cannot find an answer in the FAA's Instrument Procedures Handbook, the AIM, or elsewhere.

One example is Columbia, CA which has a simple "NA" for runway 35.

Another example is Cloverdale, CA, and RWY 32 is marked as "NA" but has the comment of "Obstacle".

A third example is Merrill Field in Anchorage, Alaska, with the comment of "NA - ATC".

On page D1 of the TPP-Abbreviations

NA.......Not Authorized

Sometimes they give the reason why it is not authorized like in your examples. Other times they don’t. In these cases, you can’t use the runway for IFR departures. (non-Part 91)

N/A.......Not Applicable

I don’t recall seeing this terminology in the departure minimums section, so if you could show me where you found it, maybe I could explain it.


Could an arrow planform be better for a supersonic aircraft than a delta? Why? From this review paper
Page 11:

Strut-bracing for the SST allows very significant reductions in both vortex and wave DDL via an extreme arrow configuration and could be favourable to LFC via reduced chord Reynolds number

All right. Low chord=low Reynolds=more laminar flow. But arrow wing=high sweep=less laminar flow.
Another paper says: Page 20:

The friction drag reduction concept is a world-first technical approach that obtains a natural laminar flow wing with a subsonic leading edge at supersonic speed. An ideal pressure distribution is first designed to delay boundary layer transition even on a highly swept wing, then an

The screen shots figures from the paper show the aircraft, an arrow-winged with a high sweep.

Here is an image found on the web.

As you can see, it has a cranked arrow wing with a nearly straight leading edge. This wouldn't even give much vortex lift, if I understand correctly.
(The aircraft shown is NEXST-1 from JAXA)
All of this is claiming that a highly swept wing can achieve laminar flow. Yet that is not what I am commonly told. So, what am I missing?

The first thing to note is that these papers are from 2003 and 2013 respectively. Since then, they have not set the world on fire.

One concern over their quality arises when I recall that the "Warped NFL wing" is typical of the reflex-cambered supercritical wings first developed around the 1970s. If you check out the inner section of say the Airbus A380 (and I suspect any modern jetliner) you will see a certain amount of that technology incorporated. But laminar flow notoriously vanishes once you fly into a few bugs or some mechanic leaves an oily smear and, despite being touted back then and as far back as the late 1930s, it has only ever produced modest results on any flyable airplane.

Next, what about this braced wing concept. The linked source discusses it on Page 10:

"A possibly viable alternative configuration for the
conventionally sized long-haul CTOL transport mission is the strut-braced wing. Strut-bracing allows
thinner, smaller chord, lower sweep and higher aspect
ratio wings. The smaller chord, leading edge radius
and sweep have a favourable influence upon HLFC,
increasing the amount of wetted area laminarized and
reducing suction mass  ̄ow and roughness sensitivity as
well as increasing attachment line stability. Other
`strut-related’ benefits include large drag due to lift
reductions from the wing tip engines and greater span
enabled by the strut, as well as large wing weight
reductions. Circulation control, powered by the auxiliary power unit, could be utilized on a conventionally
sized tail to work the engine-out problem. All these
benefits produce very large increases in the lift±drag
ratio and range at cruise."

It also notes on Page 5 that:

"More unconventional supersonic applications of [laminar flow control], again by Dr Pfenninger, to a strut-braced high aspect ratio arrow wing configuration indicates phenomenal performance levels, L/D on the order of 15-20 versus the 9-11 typical of advanced conventional configurations (with LFC)."

Now I don't know about you, but when I read of a "phenomenal" 40% drag reduction sought 17 years ago but never realized, I smell a rat. I wonder what software modelling the researchers used - 2D or 3D? At sharp sweep angles it totally matters. Were they naive enough to assume flawless laminar surfaces maintained throughout a transcontinental flight? And engines at the tips of the wings, evidently in a kind of sesquiplane variant of the Busemann biplane? Oh, spare us the FAA's demand for directional stability combined with altitude maintenance and adequate range in the mid-Atlantic one-engine-out condition.

So I think you can see where this is going.


What are the pilot rating requirements for amphibians? I know that both land and sea planes require pilot ratings in order to act as pilot-in-command.

What is the requirement to act as pilot-in-command of an amphibian?

Is there a third rating or just seaplane and land ratings?

What about flights taking off and landing exclusively on either water or land but not both?

According to the Seaplane Pilots Association you can operate an amphibious airplane on hard-surface runways with a land class rating but need a seaplane class rating to take off or land on water, and vice versa.

As far as the regs, it seems that the airplane can simply be considered either land or seaplane class depending on how you are operating it.