Do people get less airsick if cabin pressure is at 6000 ft.? This is in relation with a question I asked yesterday . This is part two of the question. Do people feel less discomfort. Whenever I'm traveling on planes, at times I feel strain in back of the head, sometimes my ears get closed, sometimes the throat,nose either get fully dried or they start to blow. Would a lower cabin pressure help in any of those things ?

Boeing supported a study that directly addresses your question:

  On the basis of our findings, we conclude that maintaining a cabin altitude of 6000 ft or lower on long-duration commercial flights will reduce the occurrence of discomfort among passengers.

Note, though, that the lower cabin altitude of the 787 is one of its selling points, and the study was conducted not too long before that type's introduction. However, its methods seem reasonable (at least to someone like me with no background in health sciences) and it was published in an important peer-reviewed medical journal.

The Aerospace Medical Association took a broad look at existing literature on the subject shortly after Boeing's study. That review

  did not find sufficient scientific data to recommend a change in the cabin altitude of transport category aircraft;

however, the authors did not dismiss the notion that a lower altitude may increase comfort, and they recommended further study. Just this month, some results of that further study were released. The results showed

  no adverse effects from an environment pressurized to the equivalent of 8000 ft compared to 6000 ft,

although they did not account rigorously for the subjective "discomfort" that I think you mean by "airsick."

My reading of the current research (based on the above) is that there is no evidence to suggest that a 6000-foot cabin altitude is medically better than a higher altitude (up to 8000 feet), but it might make some folks subjectively feel better.


Is it possible to upgrade an old F-16 to the Block-60/62 standard? Is it possible to upgrade an old F-16A/B Block-15 to the F-16D/E Block-60/62 standard?

What modifications should be made, and how?

F-16D/E Block-60 is an evolutionary upgrade to the F-16A/B Block-15 with the following differences:

Engine: General Electric F110-GE-132 in place of Pratt & Whitney F100-PW-200 turbofan.
Avionics: This is where the majority of upgrades have been carried out. Block 60 has the AN/APG-80 'Agile Beam' AESA radar, AN/ASQ-28 IFTS (Internal FLIR and Targeting System) replacing the earlier external pods, ALQ-165 electronic countermeasures system, improved pilot displays, improved mission computer and ability to fire more weapons like AIM-132 and AGM-154.
The main structural modification is the addition of conformal fuel tanks in the wings, with slight change in dimensions and increase in MTO weight- 46,000lb vs 37,500 lb

F-16 Block 20, Image from f-16.net

F-16 Block 60, Image from f-16.net

Though it is possible to upgrade block 15 to block 60 (albeit with some heavy modifications), the question is why anyone one would do that. Even the youngest Block 15 aircraft is nearly 20 years old (the last aircraft rolled off the assembly line in 1996) and LM has shut down the F-16 production line. The upgrade will be costly (you're changing some of the costliest items in the aircraft- avionics and engines) and will not be cost-effective in an aircraft past half its service life. Also, as far as I know LM doesn't offer any upgrades from Block 15 to 60, indicating consumer indifference.


Where is the formal ICAO flight plan text format specified? I want to write a parser for ICAO format flight plans, like the one shown below.

(FPL-AEE036-IS
-A320/M-SCDE2GHILORTUWY/S
-LGTS1635
-N0471F310 LEKPO UL617 SKP UB1 ABLON
-LGAV0042 LGSA
-PBN/C1B2S1 DOF/160805 REG/SXDVD)

The thing is that everything I come across is related to flight plan filling and not about this format, what to expect, what not to expect, field meanings, possible values etc.

I am looking for something like that but formal, preferably from some ICAO document

You will want to look in PANS-ATM (ICAO Document 4444), specifically, Appendix 2. (reproducing the entire appendix in this answer is not practical; it is 10 pages long). SKYbrary is also a nice resource.

Note that the ICAO model flight plan form actually contains the text format embedded in the form. Where slashes, hyphens, prefix letters etc. are required, they are printed directly on the form. I have highlited a few in the below screenshot for clarification:

<<≡ denotes start/end of message and newline.

➝ denotes a space.


What's wrong in this engine test cell picture from the book 'Ignition'? This is picture taken from 'Ignition' book by John D Clarke.

I am bewildered after seeing picture that I can't find wrong in this picture. I guess this is related to the structure of mach diamonds.

I think there's a mix-up in the labeling of the photo.

Here's the photo and caption from a copy of "Ignition" that I just downloaded:

Here's the photo and caption from the next page:

Seen together, those two photos and captions make sense.

Somebody mixed up the images and the captions in the copy you are reading.


What material is used to make luminescent placard on civil aircraft? I'm an engineer working in the civil aviation industry. Currently my company is preparing to make some placards. I've downloaded the drawings from Airbus. A drawing calls up the material of AD400 TV25/3881/83. I've seen the original OEM placard and I remember it's luminescent (it's located next to the emergency door).

I've searched "AD400" and "TV25/3881/83" as the keyword on Google but found nothing useful. I think it might be the spec of the material, but what it really is and where I can get it.

It will be greatly appreciated if someone can tell me more info on this material. It would be much better if someone can tell me where I can buy this material.

Update:
Below is how the placard looks like (a screenshot taken from the drawing):

The diamond 12 calls up the material of AD400 TV25/3881/83. Also the drawing says the base material is " MAKROFOL-E". I know "MAKROFOL" is a kind of PC film and it's a trademark of Bayer, but I didn't find this material on the website of Bayer either.

I don't know what exactly AD400 TV25/3881/83 is, but if you are talking about a sign like this:

It appears to be plastic and Larchfield sells them. This material AD400 TV25/3881/83 could very well be a type of plastic, but I don't know about it.

In my opinion, you can contact companies which make or sell them (Larchfield or this or this) to get more reliable information.


What are the adverse effects of keeping the PACKs on the A320 running with the LP air conditioning unit connected? I had come across this statement will reading the limitations chapter in the FCOM. But once again there is no reasoning given for it I have also read the air conditioning systems chapter but can't seem to find an explanation.

Can someone explain the reasons?

Latest FCOM, after revision 24 of section 3.01.21, contain the explanation:

  AIR CONDITIONING WITH LP GROUND UNIT

  Do not use conditioned air simultaneously from packs and LP ground unit (to avoid chattering of the non return valves).

The non-return valves prevent air in the mixer unit to return into any source in the case this source delivers air at a lower pressure, or doesn't deliver air. Valves are the one-way separation between the air generation section and the air distribution section:

     Source

If the LP ground connection is used while the packs are ON, pressure will be comparable on both sides of the valves. Valves can start chattering instead of being stable in the open position. While pressures compete, air from the mixer unit can return to the packs.

Damage can occur, as described in this 1991 incident (air conditioning duct and non-return valve damaged).


What is the official FAA definition of the term enroute? I recently saw a ferry permit which included the following limitation:

  Flight in IMC authorized for ENROUTE operations ONLY

Great, IMC is allowed enroute!  But wait, where does the "enroute" portion of the flight start and end?

I did some research and found the following ICAO definition for enroute:

  Instrument Flight Rules (IFR): From completion of Initial Climb
  through cruise altitude and completion of controlled descent to the
  Initial Approach Fix (IAF).

  Visual Flight Rules (VFR): From completion of Initial Climb through
  cruise and controlled descent to the VFR pattern altitude or 1,000
  feet above runway elevation, whichever comes first.

That seems pretty straightforward, but I was looking for something FAA specific and found this in Chapter two of the FAA's Instrument Procedures Handbook:

  The en route phase of flight is defined as that segment of flight from
  the termination point of a departure procedure to the origination
  point of an arrival procedure.

That doesn't match the ICAO definition, but it also isn't from an actual regulation or LOI either.

This leaves arrivals and departures as slightly ambiguous, and they often start high in the flight levels where it can be hard to avoid IMC, even on a nice VFR day at the airport (and also seems contrary to what they are trying to accomplish).  Are arrivals and departures considered enroute or some other phase of flight?

FAA follows the ICAO definition for describing the phases of flight.
FAA is a member of the CAST/ICAO Common Taxonomy Team that formed the Phase of Flight Definitions and Usage Notes in October 2002. From the CAST/ICAO Common Taxonomy Team site:

Federal Aviation Administration (FAA)- FAA has officially adopted the aircraft make model series (ACFT-MMS) and phase of flight taxonomies as an agency-wide data standard for its internal systems.

So, the enroute phase of flight is the same (just repeating from the same document):

Instrument Flight Rules (IFR): From completion of Initial Climb through cruise altitude and completion of controlled descent to the Initial Approach Fix (IAF).
Visual Flight Rules (VFR): From completion of Initial Climb through cruise and controlled descent to the VFR pattern altitude or 1,000 feet above runway elevation, whichever comes first.
This phase of flight includes the following subphases:
• Climb to Cruise: IFR: From completion of Initial Climb to arrival at initial assigned cruise altitude. VFR: From completion of Initial Climb to initial cruise altitude.
• Cruise: Any level flight segment after arrival at initial cruise altitude until the start of descent to the destination.
• Change of Cruise Level: Any climb or descent during cruise after the initial climb to cruise, but before descent to the destination.
• Descent: IFR: Descent from cruise to either Initial Approach Fix (IAF) or VFR pattern entry. VFR: Descent from cruise to the VFR pattern entry or 1,000 feet above the runway elevation, whichever comes first.
• Holding: Execution of a predetermined maneuver (usually an oval racetrack pattern) which keeps the aircraft within a specified airspace while awaiting further clearance. Descent during holding is also covered in this subphase.


Where is Transport Category aircraft defined in CFR 14? Commuter Category is defined rigorously,

https://www.law.cornell.edu/cfr/text/14/23.3

But I was unable to find an equivalent definition for Transport Category in the regulations (not Wikipedia or the FAA website, haha). Nor was I able to find anything like "if requirements not met for Commuter, then the vehicle is in Transport Category."

Could somebody point me in the right direction?

Thank you.

According to the FAA:

  Transport airplanes are airplanes for which a type certificate is
  applied for under part 21 in the transport category and that meet the
  transport category airworthiness requirements. Multi-engine airplanes
  with more than 19 seats or a maximum takeoff weight greater than
  19,000 lbs must be certificated in the transport category.

In other words, there are two possible reasons why an aircraft is transport category: the manufacturer applied for a transport category type certificate, or the aircraft has more than 19 seats or MTOW greater than 19,000lbs.

The first reason refers to 14 CFR 21.21:

  An applicant is entitled to a type certificate for an aircraft in the
  normal, utility, acrobatic, commuter, or transport category, or for a
  manned free balloon, special class of aircraft, or an aircraft engine
  or propeller, if—

  [...]

  (b) The applicant submits the type design, test reports, and
  computations necessary to show that the product to be certificated
  meets the applicable airworthiness, aircraft noise, fuel venting, and
  exhaust emission requirements of this subchapter

The "applicable airworthiness, aircraft noise, fuel venting, and exhaust emission requirements of this subchapter" for transport category aircraft are in part 25, AIRWORTHINESS STANDARDS: TRANSPORT CATEGORY AIRPLANES. If your aircraft meets those requirements and you get a type certificate from the FAA, then you have a transport category aircraft.

The second reason refers to 14 CFR 23.2005(a):

  Certification in the normal category applies to airplanes with a
  passenger-seating configuration of 19 or less and a maximum
  certificated takeoff weight of 19,000 pounds or less.

That means, if your aircraft exceeds 19,000lbs then you can't certify it in the normal category (part 23), which leaves only transport (part 25).

By the way, the 23.3 regulation you quoted doesn't exist any more because part 23 has been completely rewritten.


What are the issues with building modular aircraft? This might make sense as

commercial aircraft basic design has remained the same for the last 50 years or so, but materials used have changed
parts such as wings, landing gear, nose, tail, jet engine, fuselage, avionics and seats all come from different specialized manufacturers
so OEMs like Boeing, Airbus, etc., can build modular planes where each of the above mentioned parts should be replaceable with a better upgraded part
and this can prevent wastage

Designing "better", upgraded part is a lot of work. And then everything has to be thoroughly tested to work together, which is even more work. That's why aircraft manufacturers usually choose to upgrade as few parts as possible and only those where most significant benefit is expected.

Take for example 737. The type was upgraded with new engines several times, because evolution in engine technology brought significant fuel savings. But the wings only got winglets, which was less work than redesigning the wing. And the fuselage was not changed at all, only it is now built longer by adding some frames.

And the situation with A320 is similar. The "NEO" designation for the new series entering production stands for "new engine option", because that's what the upgrade is about. Beyond new engines and winglets there is basically no other change.

And that was just type changes as in new aircraft being built from upgraded parts. Upgrading an existing airframe is another can of worms.

Basically the joins between airframe components have to be very strong and light. The most common joining method that satisfies this is riveting, with gluing gaining popularity for composites and in some cases welding appears (aluminium is difficult to weld, but friction-stir welding seems to work satisfactorily).

Well, none of these methods allows easy disassembly. To make the components replaceable, they would have to be bolted instead. However, bolts have several major disadvantages. They can shake loose, they don't fit tightly (rivets expand to fit) and they are considerably heavier. These things make them totally unsuitable for the high-stress joining, especially for joining fuselage and wings. And drilling out the rivets and riveting in a replacement is too much work and there is a high risk of damaging the parts, which could easily make them unsafe to use.

And then there is a question of the removed parts. So you replace the wings after half of their life, because new, more efficient model is available. But the old wings could have still generated revenue. By discarding them, you've lost those money. So to pay off, the difference in efficiency must make up for that.

Well, it is extremely unlikely to do so. The new engines of 737MAX or A320NEO save about 15% of fuel, but engines are the component where by far largest benefit could be gained. Redesigning any other component can save at most few percent.

Yes, some airframes did get structural upgrades (upgrading avionics or other small components is of course easier and thus done more often), but usually these were military aircraft where the changes provided some other benefit that was worth the effort.

And some more airframes got refitted to different engines, but engines have shorter life, so they have to be designed to be replaceable. And even there it is usually military aircraft and usually only because the airframes are used much longer. For example KC-135 was reengined, but that was about the time its civilian counterpart, B707, was scrapped altogether.

But most of the time, replacing parts of an aircraft would not prevent any wastage but rather create a lot of it.


Who assigns an arriving aircraft a runway and runway exit to use? I recently bought a simulator for tower controllers (Tower! 2011) and have some questions about the responsibilities of the tower controller:

When an aircraft wants to land at 'my' airport I get contacted once they are established on the approach path and already have a runway assigned ("Tower, N1234 with you runway 26L").Unfortunately sometimes it happens that aircrafts are assigned a runway that is actually to short form them to land - which I found out the hard way by getting punished because the plane was not able to stop on the runway.Who is responsible to assign the correct runway in real-life, the tower controller or the approach controller? If if were up to the approach controller that would basically be a bug in the simulator.
Who decides which runway exit an aircraft will use after landing?Again in the simulator I can advise the aircraft to taxi to the terminal via a certain route ("N1234 taxi to terminal via L4 M4 N4") with L4 being the exit from the runway I want this aircraft to take. Unfortunately the aircrafts - in the simulator - decide which exit they are going to use on their own. So it might happen that the aircraft I assigned the route to exits the runway at exit L5 and therefore cannot comply with my assignment.How is that handled in real-life? Does the pilot land the plane and then report back where he is (if I queried the airplane for a position I get a "N1234 is on the ground") or does the tower controller assign an exit that the pilot needs to take - and therefore the simulator would have a bug?

The tower gives the ultimate clearance to land, though the approach controller (if one is available) assigns a specific approach to a runway in the approach clearance. I'm not sure about the game, however in real life if the runway was too short to land on, the pilot would report being "unable" to use that runway and would be assigned another runway instead. It is not the responsibility of ATC to ensure that the runway is long enough to land on.

Unless there is a NOTAM otherwise noting that a particular exit is unavailable for use, the pilot can slow down at whatever rate to vacate at any particular exit. However, once they've slowed down to taxi speed they should vacate as soon as practicable. The tower controller may request that a pilot vacates at a specific exit, but if they are unable to they will go for another exit.
After vacating the runway, the pilot notifies the tower that they are clear of the runway at a particular taxiway, and then will be (depending on the airport) be asked to switch to ground or be given taxi instructions.


What are the rules for racing on airport runways? What are the adverse affects of burnout rubber left on the runways for airplanes?

The adverse effect is once the layer of rubber gets too thick it starts to degrade braking performance, especially wet braking because the rubber is filling in the surface irregularities that help channel water out from under the tire.  Aircraft wheels will hydroplane at much lower speeds and with less water depth.  Airports remove excess rubber from touchdown zones once it gets beyond a specified thickness.

I would expect that the rubber from spinning car tires would be no worse than the rubber that builds up on touchdown zones on heavily used runways.  If an airport is closing a runway to use for drag racing, it can't be a very busy airport, but in theory a really heavy rubber build up could be a concern if infrequent high performance aircraft like business jets use it, you know, when no drag racing is going on.  That would be up to the airport operator what to do about it.


Which is the cable used in AFDX? Avionics Full-Duplex Switched Ethernet (AFDX) is an avionics network patented by Airbus, developed starting with A380 and new standard in all new generation aircraft from Airbus factories.

I know that AFDX is Ethernet technology. But is it used the same Ethernet cable used to connect the computer to a LAN or to the Internet? Or is it used an improved version? Which are the differences between the two cables?

ARINC Specification 664 calls for quadrax cable or fiber optic cable.  Quadrax is a four conductor matched impedance (100 ohm) cable.  Connectors are MIL-DTL-38999.

image source

The primary benefit is improved EMI protection and ruggedized connectors.

The B787 uses fiber optic.  A380 uses the quadrax cable.


Why should the leading edge be blunt on low-speed, subsonic airfoils? I have read that is to help the flow remains attached, but I do not understand the physical principle.

When using an airfoil with a sharp leading edge in subsonic regime, you need to adapt the angle of attack so that the stagnation point occurs right on the sharp edge.

At every flight condition there should be an AoA which achieves this. Higher AoA, and the stagnation point is too low. To low AoA, and the stagnation point is too high. However flight conditions change frequently, so the required AoA changes dynamically as well. Since AoA also drives the lift and lift is the priority in a pilot's mind (he wants to keep flying after all), stagnation point will only occasionally match the leading edge.

During flight the AoA will often fail to match the stagnation point, let's say with too high AoA, separation would occur below the leading edge (on the "intrados").  This is the only good illustration I could find:

The stagnation point is at the tip of the only grey line that stops right on the black plate.
Basically all incoming flow between these leading edge and stagnation point  (unfortunately no line shows this on the picture) will follow this trajectory:

Incoming at high speed onto the airfoil, below leading edge, going towards the stagnation point, but aims just slightly higher up.
The stagnation point is the point of highest pressure, so the incoming flow slows down
Comes a point where the slowdown is sufficient to actually reverse the flow which now shoots up along the surface towards the leading edge, now driven by the pressure from stagnation point
Arriving at the leading edge from below the flow needs to do a U-turn around the leading edge to keep following the wing shape and flow towards the "extrados" and keep on with its life.

Therein lies the problem: when finally arriving at the leading edge, the air has non-zero speed (zero speed only occurs at stagnation point, of which there is only one), but must instantly reverse it. Physics says nope, and you get a large leading edge vortex to smooth the motion out. Unfortunately that is also called a flow separation bubble, which may became a stall.

This is:

bad for performance (drag)
bad for safety (stallable)
bad for comfort (vibration from the eddies)
bad for structure (vibration fatigue)

Why does it only happen in subsonic flight?

At supersonic speeds, information cannot travel back on the flow using the same pressure gradient trick, once the air is on the intrados/extrados, it stays there, and cannot back up the other way. So no 'adaptation' is possible, regardless of AoA.

Why use rounded leading edge to remedy this in subsonic?

With a rounded edge, the U-turn is not so sharp, it is actually manageable for the air. This means you'll not always have flow separation, so you're safe. The rounder, the easier larger the range of AoA stay "adapted". This animated gif shows really well how the stagnation point can move around the rounded tip thus avoiding separation:

Source: UNIVERSITY OF GENOA
FACULTY OF ENGINEERING
DEPARTMENT OF ENVIRONMENTAL ENGINEERING
IRROTATIONAL PLANE FLOWS OF AN INVISCID FLUID

TL;DR It's possible to fly subsonic with sharp leading edge, but less safe and lower performance. The key is AoA adaptation.


Can I use motor oil in a aircraft piston engine? In one episode of the 'reality' show Airplane Repo a pilot topped off an aircraft engine with motor oil; I think it was a Cessna 182 but I may be wrong. What would be the effect on the engine? Is it a practical (and safe) alternative if aviation oil isn't available?

By motor oil, I assume that you mean automotive motor oil. In that case, it's a bad idea for a bunch of reasons. First, every airplane POH I've seen has specified that the oil must qualify to a standard - either MIL-L-6082 or MIL-L-22851 (though those have been superseded by an SAE standard). Modern automotive oil doesn't meet that standard, and therefore is effectively forbidden by the POH. More to the point, automotive oil is bad for aviation engines.

Aviation and automotive engines work on the same basic principles, but many of the details are different.1 One significant difference is that aviation engines normally burn some oil as they run, while well-performing automobile engines burn little or none.

Automotive oil contains a number of additives, such as detergents and wear inhibitors, intended for use where the oil doesn't burn off. Some of those additives do not burn completely, but instead stick around in the form of ash, where they can foul the engine and create deposits that lead to pre-ignition. That is why aviation oil is ashless dispersant: it cleans the engine by dispersing combustion byproducts into the oil (to be removed on the next oil change), while being ashless when burned. Lycoming considers it serious enough to put in an all-caps "CAUTION" section in their Lubricating Oil Recommendations document:

  UNDER
  NO CIRCUMSTANCES SHOULD AUTOMOTIVE OIL BE USED. THE USE OF AUTOMOTIVE
  LUBRICANTS IN TEXTRON LYCOMING ENGINES IS NOT RECOMMENDED BECAUSE ITS
  USE COULD CAUSE ENGINE FAILURE.

If all of that wasn't enough, use of an improper oil may void your engine warranty. In short: don't use it.

References:

An editorial summary on Avweb
An aircraft engine overhauler's best practices
Lycoming's Lubricating Oil Recommendations

1) Note that I'm referring to air-cooled aviation engines that burn 100LL - made by, say, Lycoming or Continental. Rotaxes and new aero-diesels are different, and more like their respective automotive counterparts; in fact, Thielert engines are actually based on automotive diesels.


What are the landing stall speeds of a Cessna 172? I understood from Why is stall speed listed in a POH? that the Cessna Skyhawk POH lists stall speeds for specific conditions due to a lack of AoA indicator. However, what's unclear to me is what the stall speeds actually are.

Page ii

In page ii of the Cessna 172 POH it lists 48 KCAS for the Flaps Down, Power Off-configuration:

Page 2-5

Then on page 2-5 it lists the white arc spanning 40 - 85 KIAS, with Lower limit is maximum weight Vs0 in landing configuration.:

According the V Speeds Wikipedia page Vs0 is Stall speed or minimum flight speed in landing configuration. I understand that KCAS is not the same as KIAS, but this is quite a significant difference.

Question: what is the actual stall speed for a Cessna 172 while in landing condition and what do these two data points in the POH mean?

Short answer

At low speed, near the stall, 40 KIAS (the low end of the white arc) and 48 KCAS (Vs0 in the POH) refer to the same actual speed. While there is a limited divergence (0 to 2%) between CAS and IAS at medium and high speed, this difference can be as much as 20% at low speed, when the aircraft pitch is large.

Details

In your question, you mention:

The POH states a Vs0 of 48 KCAS.
The low end of the airspeed indicator white arc is 40 KIAS.

The end of the white arc is also Vs0. That's a bit confusing indeed as two types of measures are used, but in fact they refer in practical to the same actual airspeed.

Indicated airspeed

Unless computations are done, airspeed values displayed to the pilot IAS (indicated airspeed).

(Source)

Errors affecting IAS

IAS reflects directly the difference between the total air pressure in the pitot tube, and the (mean) static air pressure at different static air pressure ports (on the side of the aircraft and in the pitot). The difference (the dynamic pressure) is subject to possible errors:

Total air pressure is wrong it the probe axis is not parallel to the airflow. Obviously the pitot tube angle changes between low and high speeds. For a given speed the pitot pitch also varies with mass and balance, and the status of drag devices such as flaps and landing gear.
Static air pressure at a pressure port is likely affected by pressure variations created by the aircraft fuselage. This error greatly depends on the port location and its design, but also on the yaw angle.
Airspeed indicator itself.

Obtaining CAS from IAS

All V-speeds are defined in term of "calibrated" airspeed (CAS). The aircraft manufacturer provides tables to find the CAS from the IAS read on the airspeed indicator. These tables defined for different drag devices status (e.g. flaps values) are valid at sea level and 15°C. They are included in the POH. I found this C172N one online:

(Source: POH C172N 1978)

For a 40° full flaps the table says that 40 KIAS are actually 47 KCAS. This is close to the 48 KCAS in your POH.

As you realized, the difference between KCAS and KIAS becomes larger as the air speed decreases, because the angle of attack and the errors in pressure measurement increase.

Multiple ways to express airspeed

TAS depends on parameters and need to be computed. However it can be approximated by the indicated airspeed. IAS is close to the TAS in favorable atmospheric conditions (sea level air density), and at low angles of attack.

The IAS is easy to display to the pilot using the an aneroid cell. For higher angles of attack, the calibrated airspeed (CAS) corrects for instrument and probe placement errors. CAS can be obtained using manufacturer tables. CAS, as IAS, is valid at sea level and 15°C.

When the air temperature or pressure are not close to the ideal ones (15° and 1013 hPa), then CAS differs from TAS.

The ASI usually has a window (see above) for reading the TAS after the outside temperature and altitude pressure have been selected.

(At speed higher than 200 KCAS or altitude higher than 10,000 ft, the compressibility of air becomes significant, and the equivalent airspeed (EAS) is better suited to describe the aerodynamic effects.)

Vs0 is determined assuming the maximum landing weight and the most unfavorable CG location. It is expressed in KCAS. If the Vs0 CAS were unknown, the pilot wouldn't be able to correct it to include the effect of weight and balance changes.

The ASI, because no correction is done, can display only IAS. So the IAS value for the lower speed of the white arc (Vs0) is in KIAS.


Which kind of aircraft have the lowest operating cost? As an owner of an aircraft (either for personal or commercial activities), what kind of aircraft is cheapest to operate? The aircraft must be able to carry at least one passenger not for money for at least 150 miles.

150 Miles is perfectly reasonable distance in most small planes and one passenger is a fairly easy load. There are a lot of options out there but arguable the cheapest would either be the Cessna 150/Cessna 172 Or the Cherokee 140/160/180. The factors that keep the price very low,

Fixed gear: less to service and inspect during an anual and less to go wrong in terms of over all maintenance. You never have to worry about the proverbial "gear up" landing.
Some of the most produced air frames in history by companies that still exist so parts are abundant and not all that hard to come buy when compared to some of the more obscure air frames out there.
Very common engines, decent on fuel burn rates and easy to have serviced, inspected, and generally considered reliable.
Very practical useful loads for carrying one other person and luggage.
Common and for sale in most places, would be easy to find a decent air frame.
Common enough you may be able to find a partner to go in on it and help reduce costs.
Lots of mechanics out there who will happily work on these planes, and are quite knowledgeable on them as well.


What's a CWS button? I've flown a Piper that has a button labeled CWS on the yoke, right next to the "Push-To-Talk" button.  I never got a chance to push it though.  What does it do?

Sounds like that plane has (or once had) an STEC autopilot installed - CWS stands for Control Wheel Steering - basically it allows you to set a pitch attitude / turn rate using the yoke, which the autopilot will then maintain until instructed to do otherwise.

If the autopilot is still installed you also normally use the CWS mode as part of the preflight routine to get the autopilot to activate so you can make sure you can overpower it (details on the preflight as well as using the CWS feature can be found in the autopilot manual, which should be with the POH).

If the autopilot is no longer installed the switch technically should have been removed with the rest of the system and a blanking plate installed in its place.


What is the minimum distance between two civil airplanes parked in a hangar? When parking two airplanes in the hangar, what is minimum safe distance between them?

If you look at some civil-aircraft hangars, you will see that the planes are almost touching each other, as you can see here:

The planes are so close to each other to fit a maximum amount of planes into a hangar.

There are different types of hangars. Some are built just to store aircraft when they aren't used, some are built for aircraft-maintenance-operations and can even contain robots. Hangars can be simple buildings with a big and open space or they can be very complex facilities.

If you want more infos about aircraft hangars take a look here.

You can find the minimum separation distances between aircraft and minimum distances between the aircraft and other obstructions here

Take a look at this table from this document (page 30):

As you can see, the minimum distance between the wings from two parked aircraft is not always the same.

  Dimensions vary for different models and configurations of aircraft.

If you want to find out the min. distance of an A380, you need to take a look at the documents provided by Airbus.

No, there is no basic rule. The safe distance is determined by the aircraft-type, configuration and the hangar (owner).

If the airliners are stored in the hangar, they can be pretty close to each other. However, maintenance of a big airliners takes up a lot of space:


Why did this aircraft fly in this pattern over Southern Italy? Why would an aircraft fly in the pattern shown in the image below? The location is Southern Italy.

That flight pattern is almost certainly aerial surveying / mapping work and it's quite common all over the world.  Companies like Google, Bing, etc pay for low to medium altitude photography and surveying of areas.


Can an airplane flying above 35,000 feet be heard on the ground? Can the sound produced by a commercial aircraft flying at 35000 feet be heard on the ground when the surroundings are quiet?

Yes
It doesn't even has to be very silent around you.
But because of the speed of sound, you will hear the aircraft later. Here is the approximate time, the sound needs to reach you:

Speed of sound at MSL: ~340m/s
35.000 ft in m: ~10660m
10660 / 340 =
~ 31s

That means you will maybe hear the aircraft but you are not going to spot it where the sound comes from.
Note that this is not an exact value, as speed of sound varies with air density and temperature, which both change the higher you are.


Can ATC terminate flight following AND approve (arbitrary) frequency change while still inside Class C airspace? Unusual (for me) event inside Class C. Can ATC terminate flight following inside Class C and approve frequency change.

Background: Requested and received VFR FF at 6500 msl prior to approaching ONT Class C (top is at 5000). Intent was to over-fly ONT and not enter Class C, however after reaching north of the field, ATC said “free to descend altitude at your descretion ” (Point A). My response, “6500 ft decending to 4500”. At about point B, I dropped into Class C and continued to 4500. After leveling off, still in Class C, ATC said “Squawk VFR, frequency change approved.” I responded “Squawking VFR, and immediately departing Class C to the North”. (Point C) His response, “You don’t have to do that as the tower still has you on radar.” I said “departing Class C to the north”

Here is my confusion: First squawking VFR is not an issue, I’ll squawk whatever is requested and maintain my own flight separation but when he said “frequency change approved” I would no longer  be in radio contact in Class C if I actually did as approved. This would mean, to me, I am illegally in their airspace.  What am I missing???

Jim S asks an interesting question.  Can ATC ask you to do something which is contrary to the regulations?  The answer is they can, and just because they do, does not change the regulations.

The practice of advising aircraft to switch to advisory frequency and squawk VFR is common here, in Class C airspace, and I have experienced it in Class B airspace as well.

Discussing this with the ATC supervisor once, it was explained to me this way: The controller knows where you are going, knows your intent, and believes that he can manage the traffic in the area effectively if you leave the  frequency. So for the controller it is an issue of risk and workload management.

With students, this creates a good learning opportunity to have them "monitor" one radio, while using another.  No official guidance on this, but it is common practice. So if you are not familiar with this, have an experienced pilot or CFI demo it to you.

Keep in mind that this is the same process for IFR aircraft making approaches to non-towered airports.

Arguably, ATC can authorize deviations from 91.130, as @DaveCFII points out. Certainly that covers temporary frequency changes to check WX and things like that, but it could be interpreted to have broader scope is that it could be used to terminate communications near to the point where an aircraft leaves Class C.

Having said this, I would prefer that ATC state something like, "communications deviation approved."  Of course that will not happen on a busy frequency.

Having said all this, it is important to remember that the pilot has final authority over a flight, and the FAA in the Granby LOI makes it clear that ATC does not have the requirement to point out to pilots what regulations apply to their operation.  So a pilot who is uncomfortable with this procedure, might monitor ATC on one radio, and broadcast their intents on the advisory frequency. This way they can fully comply with the regulations. Most aircraft have dual COM radios.  If not, delay your frequency change until you are clear of Class C, to remain in strict compliance.

Ref: Granby LOI  https://www.faa.gov/about/office_org/headquarters_offices/agc/practice_areas/regulations/interpretations/data/interps/2006/granby%20-%20(2006)%20legal%20interpretation.pdf


What do the green, white and yellow bands mean on an airspeed indicator? what are these  green and white and yellow bands in the Airspeed Indicator use for ?

The FAA has a nice introduction document on flight instruments, which explains all the standard markings on a typical airspeed indicator (ASI):

  As shown in [the figure above], ASIs on single-engine small aircraft
  include the following standard color-coded markings:

  White arc—commonly referred to as the flap operating
  range since its lower limit represents the full flap stall
  speed and its upper limit provides the maximum flap
  speed. Approaches and landings are usually flown at
  speeds within the white arc.
  Lower limit of white arc (VS0)—the stalling speed
  or the minimum steady flight speed in the landing
  configuration. In small aircraft, this is the power-off
  stall speed at the maximum landing weight in the
  landing configuration (gear and flaps down).
  Upper limit of the white arc (VFE)—the maximum
  speed with the flaps extended.
  Green arc—the normal operating range of the aircraft.
  Most flying occurs within this range.
  Lower limit of green arc (VS1)—the stalling speed
  or the minimum steady flight speed obtained in a
  specified configuration. For most aircraft, this is the
  power-off stall speed at the maximum takeoff weight
  in the clean configuration (gear up, if retractable, and
  flaps up).
  Upper limit of green arc (VN0)—the maximum
  structural cruising speed. Do not exceed this speed
  except in smooth air.
  Yellow arc—caution range. Fly within this range only
  in smooth air and then only with caution.
  Red line (VNE)—never exceed speed. Operating above
  this speed is prohibited since it may result in damage
  or structural failure.

(FAA Handbook - Chapter 8 - Flight Instruments, emphasis mine)


What are the FAA regulations on grass airstrips? What are the FAA regulations on grass airstrips like grass height, lighting, necessary aircraft equipment, and the like?

I went poking around the FAA Regulatory and Guidance Library until I found FAR 139.307, which states:

  Sec. 139.307

  Unpaved areas.

  (a) In a manner authorized by the Administrator, each certificate
  holder [must] maintain and promptly repair the surface of each gravel,
  turf, or other unpaved runway, taxiway, or loading ramp and parking
  area on the airport that is available for air carrier use as follows:

  ...

  (b) FAA Advisory Circulars contain methods and procedures for the maintenance and configuration of unpaved areas that are acceptable to the Administrator.

So I went looking for those advisory circulars.  There are probably more that I haven't found, but AC 150/5300-13A - Airport Design beginning at page 82 seems to have the sort of thing you're looking for.


Can a pilot who is not a CFI give instruction? Can a pilot (other than an ATP) who is not certificated and current as a CFI give instruction in aircraft required for the issuance of a pilot certificate, e.g. a private pilot certificate?

I'm assuming that you're asking about FAA regulations. Anyone can train anyone else - how effective and safe that training is is a different question! - but only an instructor can issue endorsements, including signing off on training time, endorsing solo flight, and accepting a student pilot application (see 14 CFR 61.193 and 61.215).

This is from 14 CFR 61.51:

  (h) Logging training time. (1) A person may log training time when
  that person receives training from an authorized instructor in an
  aircraft, flight simulator, or flight training device.

  (2) The training time must be logged in a logbook and must:

  (i) Be endorsed in a legible manner by the authorized instructor; and

  (ii) Include a description of the training given, the length of the
  training lesson, and the authorized instructor's signature,
  certificate number, and certificate expiration date.

In theory, a non-CFI could train a student to perform as required by the ACS. But, a non-CFI couldn't authorize the student to fly solo, and he couldn't endorse the student's training time in his logbook. The student would end up able to fly but unable to get a certificate because he'd have no official training time and no solo time. So it wouldn't really be helping the student much: he'd have to go and do the required hours over again with a CFI anyway.


Could a person damage a jet airliner - from the outside - with their bare hands? Given just a small hammer, I'm pretty sure I could disable a large airliner with a few well-placed blows.

What about with my bare hands?

In other words, are there any parts of a large aeroplane that are:

exposed
delicate enough that they could be damaged without using tools
critical to the plane's operation?

The only think I can think of are the pitot tubes and angle of attack sensors.

(Throwing oneself into the engines while they are idling does not count.)

Yes, a few possibilities...

Brake hydraulic and electrical sensor lines located on some landing gear.
Putting foreign objects under or between the rotors of brake pads
Covering pitot system sensor holes
Pressing or hitting the pitot tube (on most airliners you will need a ladder)
Letting air or hydraulic fluid out - under inflating tires or cylinders
Bending or mis-adjusting bellcranks/control arms or changing the linkage geometry.
Damaging fuel vents

I am not sure of what your definition of "exposed" is but most airliners allow the E&E (electrical & equipment) bay usually located above or behind the nose gear and gear doors to be opened and closed from external control panels for ground crews. This allows access to most the computers, batteries and thousands of wire bundles. It may also give access to lines, cables, pumps, switches, and sensors.

Opening the gear doors of some airlines exposes the air-packs, generators, converters and aux turbine with it's associated mechanisms. Maintenance and inspection panels, engine cowls and fuel distribution panels expose or control critical systems.


Which aerodromes are in the scope of ICAO recommendations? I'm looking for a clarification of how aerodrome characteristics are regulated in a country which has signed the ICAO Chicago Convention (more on Wikipedia). To guide the answer, I'm providing a list of ICAO statements, which may be ignored if non pertinent. The question follows this list.

ICAO statements

Under this convention, State members participating to international air travel agreed to follow ICAO recommendations and best practices:

  Article 12: Each contracting State undertakes to keep its
  own regulations in these
  respects uniform, to the greatest possible extent, with those
  established from time to time under this Convention. Over the high
  seas, the rules in force shall be those established under this
  Convention. Each contracting State undertakes to insure the
  prosecution of all persons violating the regulations applicable.

The States also agree to provide some infrastructure:

  Article 18: Each contracting State undertakes, so far as it may find
  practicable,
  to: a) Provide, in its territory, airports, radio services,
  meteorological services and other air navigation facilities to
  facilitate international air navigation, in accordance with the
  standards and practices recommended or established from time to time,
  pursuant to this Convention.

Nineteen annexes describe SARPs, standards and recommended practices. Annex 14 (volume 1) describes aerodromes. Applicability is defined in § 1.2:

  1.2.2 The specifications, unless otherwise indicated in a particular context, shall apply to all aerodromes open to public use in
  accordance with the requirements of Article 15 of the Convention.

Article 15:

  Every airport in a contracting State which is open to public use by
  its national aircraft shall likewise, subject to the provisions of
  Article 68, be open under uniform conditions to the aircraft of all
  the other contracting States.

Article 68:

  Each contracting State may, subject to the provisions of this
  Convention, designate the route to be followed within its territory by
  any international air service and the airports which any such service
  may use.

Each State member also publishes aeronautical information (AIP), described in Annex 15 to the convention:

  2.1.1 Each Contracting State shall: a) provide an aeronautical information service (AIS); or b) agree with one or more other
  Contracting State(s) for the provision of a joint service; or c)
  delegate the authority for the provision of the service to a
  non-governmental agency, provided the Standards and Recommended
  Practices of this Annex are adequately met.

Each State must list in the AIP the deviation from ICAO SARPs, in section GEN:

  4.1.1.1 c: a list of significant differences between the national regulations and practices of the State and the related ICAO Standards,
  Recommended Practices and Procedures, given in a form that would
  enable a user to differentiate readily between the requirements of the
  State and the related ICAO provisions;

From the statements listed, it's not clear if all aerodromes open to the public are in the scope of ICAO recommendations and SARPs. This may be only those open to international travel, there is some ambiguity.

Question

How do I know whether such aerodrome in a given country benefit from ICAO recommendations as included in the national regulations. For example, there is this recommendation (annex 14):

  3.9.6 Recommendation.— To facilitate the movement of aeroplanes,
  fillets should be provided at junctions and intersections of taxiways
  with runways, aprons and other taxiways. The design of the fillets
  should ensure that the minimum wheel clearances specified in 3.9.3 are
  maintained when aeroplanes are manoeuvring through the junctions or
  intersections.

How does that apply to aerodromes in Germany or in Canada for instance?

Which aerodromes are in the scope of ICAO recommendations?

Per Annex 14, international airports only, and preferably any public use airport—

  1.3.1 As of 27 November 2003, States shall certify aerodromes used for international operations in accordance with the specifications contained in this Annex as well as other relevant ICAO specifications through an appropriate regulatory framework.

  1.3.2 Recommendation.— States should certify aerodromes open to public use in accordance with these specifications as well as other relevant ICAO specifications through an appropriate regulatory framework.

(Emphasis not mine.)

If for any reason, a country deviates from the standards, it needs to notify ICAO.

How do I know whether such aerodrome in a given country benefit from ICAO recommendations?

Deviations are published by ICAO in Supplements to each Annex. I can't speak for 3.9.6 due to paywalls. However, here are examples for Canada's deviations to Annex 14—

  Canada does not provide runway end safety areas but does provide a 60 m graded strip beyond the runway end. (More on that story.)
  Canada computes runway longitudinal slopes by dividing the difference between the maximum and minimum runway end elevation by the runway length.

Below (may not be current due to paywalls) are countries that fully adhere to Annex 14—

  Contracting States which have notified ICAO that no differences exist:

  Colombia
  Egypt
  Georgia
  Ghana
  Ireland
  Malaysia
  Monaco
  Namibia
  Peru
  Sri Lanka
  United Republic of Tanzania


What aircraft model is this that crashed? Could someone please tell me what aircraft type this is? The crash happened very near to my flight school a few days ago and I want to know if it was one of their fleet that crashed.

That was a Cirrus SR22T, registered in Guernsey as 2-RORO.

The registrar published a statement including the following information:

  The Guernsey Director of Civil Aviation (DCA) and SGI Guernsey Ltd.
  (2-REG) confirm that an accident involving a Cirrus SR22T aircraft,
  registration 2-RORO, occurred on May 12, 2019 between Raglan and
  Abergavenny, Monmouthshire, Wales, United Kingdom. The accident
  aircraft had been registered in Guernsey since August 1, 2018. The
  three occupants of the aircraft were reportedly taken to hospital with
  minor injuries. No further casualties have been reported.

According to the Aviation Safety Network, 2-RORO was owned by Transport Safety Resolutions Ltd. On May 12th, 2019 it departed from Denham (EGLD) and crashed at the A40 in Monmouthshire, Wales, UK:

  The aircraft hit some overhead trainwiring during a forced landing and
  impacting a main road between Raglan and Abergavenny, Monmouthshire,
  Wales. It was consumed by the post-impact fire.

Image of better times by Andrew Goldsmith on Flickr, 09-05-2019

There was already an (only seemingly similar) event on the A40 between Raglan and Abergavenny in June 25th, 2016. A Piper PA-28 (G-SVEA) tried to depart from Abergavenny airfield - a gras strip just next to the A40. It stroke some trees, veered onto the highway and crashed into the central lane separation.


How does the A350/380 operate on Variable Frequency AC generators? To keep a constant frequency of 400 Hz, most aircraft have a CSD (Constant Speed Drive) associated to the AC generator. On most types (e.g. A320, A330, B737, B747, B777) an Integrated Drive Generator (IDG) is used. This unit is simply a CSD and an oil cooled generator inside the same case that provides this fixed frequency output.

Sometimes we even use a variable-speed constant-frequency (VSCF) generator, that provides AC power at constant frequency using a tap converter.

The A380 and A350 have variable-frequency electrical generator, with no CSD or IDG. How can they work without a fixed AC frequency? If the fixed AC frequency is not needed, why it is needed in all the other aircraft types?

According to Civil Avionics Systems, such system is found on the Airbus A350, A380, Boeing 787, and new business jets. But only certain systems use the variable frequency (VF) AC. Namely motors such as in the fuel and hydraulic systems. They are designed to operate at different speeds with tolerances allowed for the slowest modes of operation.

In short, the motors, heaters, etc., are designed to operate on different loads reliably.

From the 787 manual, items such as cockpit displays and instruments use DC power instead. DC fuel pumps, brakes, and igniters, are also used on the 787.

The reason for it not being implemented decades ago is that motors weren't easily made reliable, especially DC motors. Another reason is the cooling requirements for the motor controllers, on the 787 for example, liquid cooling is used. Solid-state relays replaced most of the traditional circuit breakers, enabling automatic load shedding. One reason for that is to further limit the generated heat.

Snippet from the 787 manual.

The above VF system is by far the lightest, and is less complicated. Between it and the IDG system, there was the short-lived variable-speed constant-frequency (VSCF) used on the McDonnell Douglas MD-90. In its early days it proved troublesome to maintain but eventually it achieved high dispatch reliability. As with the VF, the engine generators outputted VF, but then tap converters (a mid-80's invention) were used to produce constant frequency. Below is the MD-90's air-scoop for the cooling of this system.

(airliners.net) Air-scoop for cooling the VSCF on the MD-90.


How effective are modern aircraft at clearing smoke? In reference to this incident by a certain (in)famous artist, i'm wondering how effective modern aircraft are at clearing smoke for single aisle commercial aircraft such as B737/A320 or a business jet like in the story?

Which direction does the airflow in a cabin move? Top to bottom?
How far are people effected by secondhand smoke if somebody lights a cigarette? Would it be within a one-meter range or would people notice across the whole cabin?

The most relevant data I could find was located here, in a study on "In-flight cabin smoke control". In the abstract, they stated that

  Tests on a Boeing 757, modified to allow smoke venting out through the top of the cabin, showed that the buoyant smoke front moved at 0.46m/s (1.5ft/sec) with and 0.27m/sec (0.9ft/sec) against, the axial ventilation airflow. Flight tests in a modified Boeing 727 showed that a ceiling level counterflow of about 0.55m/sec (1.8ft/sec) was required to arrest the forward movement of buoyant smoke. A design goal of 0.61m/s (2ft/sec) axial cabin flow would require a flow rate of 99m3/min (3500ft^3/min) in a furnished Boeing 757. The current maximum fresh air cabin ventilation flow is 78m^3/min (2756 ft^3/min).

In short, during continuous generation of smoke, cabin air will not be recirculated quickly enough to stop smoke from filling the cabin; however, if the smoke production is temporary, then it should dissipate quickly (time of course depends on the size of the cabin and the amount of smoke produced).

On page 2 of this article, a discussion of airflow occurs and contained this image, showing circulation patterns on wide-body airliners. A linked study on that same page noted that an under-floor displacement air distribution system [like the commonly used type picture below] can provide a much better air quality than a mixing air distribution system. The paper goes on to provide more diagrams of airflow and comparisons of various effectiveness if you're interested.

Boeing has a fairly detailed summary of their cabin airflow here. It says:

  Inside the cabin, the air flows in a generally circular patterns and exits through floor grilles on either side of the cabin and on airplanes with overhead recirculation, the air may exit through the overhead. The exiting air goes below the cabin floor into the lower lobe of the fuselage.

So at least on most Boeing planes, air exits at your feet, though it can exit at the ceiling. Airbus also has a page here, but with much less detail. Airbus claims a complete recirculation every 2 to 3 minutes, while Boeing merely states airflow is continuous. The article linked below says in general, airplane recirculation occurs aproximately 20 circulations per hour.

An NBC article on cabin air states that to pick up a sickness, you need to be close, usually within two rows. The article goes on to say that:

  The aircraft is divided into ventilation segments of three to seven rows; you share air only with passengers in your segment.

So presumably if there were smoke, it would be captured in a small area were it not too pervasive.


Can a Greek EASA Part 66 License be converted to a German EASA Part 66 License? I want to convert my Greece Part 66 license to a German Part 66 license. What procedure do I have to follow?

You have to send the request to the Greek Authority.
They will contact the German one, and the German Auth. will probably contact you to fill a Form 19.
(details depend of the Auth. procedures)


What aerodynamic improvements allow variable geometry wings to be omitted on modern fighter aircraft? This sounds like a duplicate of Why aren't variable swept wings being used in newer military aircraft? of which there is at least one existing duplicate, but this question is different.

This question is specifically about what are the aerodynamic improvements that allow the variable geometry wings to be omitted? This seems to be an interesting question, as the aerodynamic design of certain fourth generation fighters such as the Su-27 and F-15, appear to be relatively straightforward. It is certainly not clear to a layperson what enables this.

For this particular question, issues such as stealth, improved engines performance, etc. are not relevant.

It is more a tactical rather than an aerodynamic improvement.

Wing sweep is helpful when the top speed of the aircraft is well above Mach 2 and should be maintained for more than a few seconds. However, studies of past conflicts have shown that this speed capability was never used and almost never helpful even in hypothetical scenarios.

In addition, the lighter and stiffer structures made possible by carbon composite technology lower the maximum sustained temperature those structures can endure. Now the maximum Mach number is not only limited by aerodynamics, but also the stagnation point heating. In case of the F-22, the maximum sustained Mach number was lowered from 1.8 to 1.6 during development.


What are the limiting factors for high altitude planes (e.g: U2 or SR71) preventing them from going higher? I'm curious as to why planes like the U2 Dragon Lady and the SR71 Blackbird couldn't fly higher. What physical constraint set their operational ceiling?

Pilots wore spacesuits, so that wasn't the limiting factor.  Was the air too thin to give enough lift? Was there not enough oxygen for the engines? Some other reason?

EDIT: My question is specifically about planes designed for high-altitude flight, not for general aircraft. To narrow the scope of my question, consider the SR-71 as the prototypical example. What set the operational ceiling of the SR-71?

The limiting factor for subsonic aircraft, including the U-2, is well explained here.

For supersonic aircraft this answer simply says the limit is "a combination of wing loading and maximum speed". If you look at the flight envelope of the SR-71 below, it becomes clear that more altitude can be best bought with more speed.

SR-71 flight envelope (picture source). Tower buzzing at Mach 3 is clearly impossible.

Supersonic speed limits

Intake design: If the kinetic energy of the flow cannot be efficiently converted into pressure in the intake, thrust will suffer and will drop when the flight Mach number is increased beyond the limits of the intake.
Airframe efficiency: If the leading edge sweep of flight surfaces is not high enough to keep those leading edges within the Mach cone, drag will rise and limit the top speed of a design. The desire to reach Mach 2+ speeds was the driver for the many swing wing designs of the 1960s.
Compressed gas temperature: Once the compression heating in the intake brings the gas temperature close to its dissociation temperature, the chemical energy in the fuel cannot be fully converted into heat. This reduces engine efficiency and is the reason for supersonic combustion in designs for speeds in excess of Mach 4 or 5.
Aerodynamic heating: Metals and composites show decreasing strength with increased temperature. Fly fast enough for some time, and the structure cannot tolerate the flight loads, even if dynamic pressure is kept constant.

The order in which I listed those limits ranks them with increasing speed. Once you move beyond Mach 1.6, every consecutive tenth of the top flight Mach number must be bought with increasing expenses and compromises. Going beyond Mach 5 with current technology will only be possible with rockets, so those designs quickly become low-orbit satellites. In the end, it is simply not worth it to push the limit yet further.

EDIT: It seems the answer is not explicit enough. If we try a thought experiment and modify the SR-71 to reach higher altitudes, the possible options are:

Just pull on the stick: This helps in the short term, but flying stationary at lower density would require a higher lift coefficient and a higher angle of attack. This would lower the overall L/D of the plane and slow it down because the engines could not develop sufficient thrust.
Increase engine thrust: This could be tried in flight by advancing the throttles at top speed, and the aircraft would accelerate. This would quickly exceed the limit of the compressor inlet temperature, though, leading to a shorter life time or even damage of the engine's hot section. Next, range would suffer due to the higher fuel consumption. If the engine is improved by using better materials, a moderate increase in cruise Mach and, consequently, flight altitude is possible.
Lower wing loading: A lighter aircraft can cruise at a lower density, all other parameters being equal. At the end of a trip the SR-71 could reach the highest altitude, just like any other plane. Structural changes to lighten the structure beyond removal of all reconnaissance equipment, however, would have only limited potential: The SR-71 was already designed efficiently, so there is very little potential for weight savings without compromising structural strength. And removing the cameras and side-looking radars would strip the plane of its operational value.


Does a plane have more lift while flying in a cloud? (All other things equal...) Inside a cloud, the plane flies in a medium (air + water droplets) with a higher density than 'normal' air. If airspeed and AoA are the same, lift should be higher. It is really so...?

No, higher humidity air is less dense than lower humidity air.  This is because water molecules are lighter than the nitrogen and oxygen they would replace in an airmass.

https://science.howstuffworks.com/nature/climate-weather/atmospheric/is-humid-air-heavier-than-dry-air.htm

  any water vapor that gets added replaces either nitrogen or oxygen in our free-moving air. Nitrogen and oxygen make up the majority of our atmosphere, and they're displaced -- or evaporated -- when water takes their place in the air. And water vapor molecules are lighter than both nitrogen and oxygen. In other words, humid air is going to have less heavy nitrogen and oxygen -- and lighter hydrogen and oxygen -- in its place. Remember that they have the same number of molecules, but the air with water vapor is simply less dense

Humidity, therefore, is also an important factor in density altitude calculations.


What fuels can turbofans burn? I know that Jet-A is the fuel most often used for turbofans, but is there something else that these engines can burn? (ethanol, LPG, methane, butane, 100LL, etc.) Basically, can a turbofan burn anything, or are there limits in terms of temperature, combustibility, and so forth? also, which fuel out of any you can think of would be the most powerful/fuel-efficient?

As explained in Why do jet engines use kerosene rather than gasoline?, turbines can burn almost anything, but the lubricating properties of kerosene make the fuel pump simpler as it is otherwise hard to keep high-pressure pump lubricated.

Temperature limiting is simply a matter of sufficiently lean mixture and cooling air stream along the combustor walls.

And most efficient depends on criteria. For money it is definitely kerosene (Jet-A). Actually, Jet-A is an excellent match overall. It is relatively dense (~0.8 kg/l) while still having good specific energy (~43 MJ/kg; so fuels tanks are not too big), lubricates (good for fuel pumps), has quite high flash point (low risk of fire when handling) and is a large fraction of crude oil (C8–C16 (gasoline is just C6–C8) so it is cheap).


When has China said it will mandate head up displays (HUD)? I tried to google this but could not seem to get a clear answer.  I was wondering because I heard they first mandated all aircraft by some date which was no where near realistic in terms of cost to the airlines.  They therefore either have reconsidered, or will need to reconsider but I am unclear on it all.

2025
Sometime in 2012 CAAC are alleged to have suggested 100% of Chinese airlines should have adopted HUDs by 2025.
It seems the Rockwell Collins Heads-Up Guidance System (HGS) has been trialled and that HGS equipped aircraft are permitted to land at certain airports in low visibility conditions - lower landing minima for HGS equipped aircraft.
Reports mention enhanced vision systems (EFV) and enhanced flight vision systems (AFVS) where the HUD is used to project data, including runway outlines, over the pilot's view of the runway ahead.
The CAAC seem to be actively encouraging this, but I don't find the  word mandatory used in this context.

November 14, 2012, 5:07 PM
Rockwell Collins China managing director Ron Ho believes business for aircraft head-up displays (HUD) in China will soon be looking up. “We have been working with the Civil Aviation Administration of China (CAAC) for the past 10 years on the HUD project, and now it has reached the stage where CAAC will require the Chinese airlines to implement the HUDs into their operations,” he explained.
“This will be done in three phases, I understand,” Ho noted. “During the first phase, 10 percent of all the airlines will be equipped and operating with HUDs. In the [s]econd phase, this will be 50 percent. The third phase will be complete fleet equipage by the year 2025.”

From AINonline - Chinese Authorities Commit To Aircraft Head-Up Display Mandate

FAA has also been coordinating with CAAC. In Aug 2012 FAA hosted a Pacific Rim conference in Long Beach CA identifying key technologies, operations and regulations that could be improved with HUD, advanced PFDs and vision systems. Soon after the conference, the CAAC HUD application roadmap appeared. China has without hesitation laid forth a plan to move the country's civil aviation into a 100% HUD compliant operation by 2025.

From Professional Pilot Magazine - HUD gains favor in Asia Pacific

Start HGS from 2005
End of 2010, 52 B727-NGs
... Primary reference for maneuvering ... is natural vision
Promote airlines using HUD

From CAAC - HUD/EFVS systems in the implement(sic) in China


Could wing tilting be a mechanism for VTOL and high angle of attack for jetliners? My question is about turbofan airliners, could an airplane tilt its wings 90 degrees so that the fuselage horizontal axis is perpendicular to the longitudinal axis of the turbo fan. This way an airliner could be VTOL and at the same time it could have even 90 degrees angle of attack. Am I missing something?

What you describe is called a tiltwing. It has wing-mounted engines which swivel with the wing so their thrust points up for the VTOL mode. Building a tiltwing airliner will face two challenges:

Normally, the static thrust force is only one third of the weight force, so the tiltwing airliner needs to add three times as many engines - at least. Better make this four times as many so there is some excess thrust for hot and high conditions.
In order for the wing to tilt freely, it needs to be straight. A swept wing will hit the ground during tilting unless an absurdly high landing gear is installed. Now the thrust acts along one line - how do you control pitch? You will need another engine at the tail (or nose) which can be quickly throttled in order to balance the aircraft in pitch.

Needless to say that this cuts into payload and makes this design uncompetitive unless VTOL operation is a must.


What problems can be encountered if an airplane does not undergo wind tunnel testing? SpaceShipOne did not use any wind tunnel testing. According to the Scaled Composites FAQs:

  Q: Did you do wind tunnel testing?
  A: No. All design refinements and performance predictions have been derived from Computational Fluid Dynamic tools.

What problems would such an aircraft face (which is designed completely without wind tunnel testing)?

For decades, there was no wind tunnel testing performed. There will be, of course, more unknowns about performance and flight characteristics, but with modern computer simulation technology those can be greatly reduced.


What percent of the world's airline operations are in the USA? What percentage of the world's airline operations are in the USA?  And how does this number compare to other first world countries such as the UK and Canada?  How about places like China and India?  And finally smaller third-world countries?

It may be a bit hard to find this data for many different countries, especially if you want to separate takeoffs and landings.

According to this data for the US, there were 757.1 thousand flights in May 2014, including both domestic and international. Of those, 683.0 thousand were domestic, and 74.1 thousand were international.

Lots of information is available here. This indicates 8.3 million domestic and 1.4 million international flights in the US during 2013. This includes all carriers an all airports, including domestic all-cargo flights.

The best data for Europe I could find is only in passenger count (the more common statistic). It looks like you have to request access to get more detailed information.

However, this collection of data from Eurocontrol showed 9.548 million "IFR Flight Movements" in 2012. This would include all IFR flights, regardless of their type. A graph at the bottom shows figures for 2008. This shows a little over 8 million internal, and a little under 2 million arriving/departing. Since a domestic flight would be two movements (departure and arrival), this is 4 million and 2 million flights. As expected, Europe has a much higher percentage of international flights (about 1:2 international in Europe, versus about 1:9 in the US).

For domestic and international traffic in Australia, there are 702321 domestic flights and 162207 international flights in 2013, for a total of 864528 flights.

Data from the UK show that there are about 800000 domestic and 1000000 international flights per year.

You could also order a year of World Air Transport Statistics for only $799.


What are the drawbacks of a liquid hydrogen jet engine? I am looking into building a jet aircraft utilizing liquid hydrogen as a fuel source. I have already built a few original design pulse jets, and I'm looking for a new project.  Cryogenic fuel jet engines look good, but I feel there may be drawbacks.

Here are a few drawbacks:

Lesser energy density per volume

For the same amount of energy, you need more space, not yet available in current designs

Boil off

The hydrogen will slowly boil off, and vented to the atmosphere. Hydrogen IS dangerous, and this will negatively impact long flights.

Logistics

Fuel is common everywhere in the world and easy to store. We have yet to build a world delivery network for hydrogen.


How can I get General Aviation flight data on smartphone and/or tablet? Is it possible to in-flight receive navigation data on an electronic device (like smartphone and/or tablet) present in the cockpit?
I refer to General Aviation aircraft and navigation data like airspeed (not groundspeed), GPS and AHRS data.

Garmin has a product called Connext that allows you to stream in-flight avionics data to mobile devices. They are pushing that product as a move towards connected components and a "Bring your own device" approach to cockpit automation.

So is it possible in general to receive it? No, not really. Not unless the avionics package was designed to broadcast it out. As other answers have said, a simple GPS and AHRS integration of the sensors in your tablet/smartphone can give a pretty good approximation of the data you are asking for, but things like airspeed, navigation waypoints, and other on-board sensor data are not generally available without a product like Connext.

The other side of it is that if you are a passenger you still probably won't be able to pair with the avionics. Connext allows you to modify the flight plan via a mobile device, and I'm not sure I would want any of my passengers doing that. On top of that, pilots are trained to interpret the data at hand, you don't want a passenger seeing a faulty artificial horizon after a vacuum failure and think that the aircraft is tumbling out of the sky.

There is a lot of security concerns as it is over being able to hack into commercial aircraft (I know you are talking about GA), but I still don't want a passenger to have the ability to connect to my avionics.


Do commercial airliners have the tail number placed somewhere on a placard in the cockpit? I notice, looking at pictures of various light singles, that the tail number of the aircraft is typically prominently displayed somewhere towards the top of the instrument panel. If it were your own plane, you would quickly memorize it given the dozens of times per flight you have to give your callsign, but it's still useful for new pilots, rentals and trainers.

That led me to wonder: are the tail numbers of commercial aircrafts also posted in the cockpit, or is it omitted because commercial airliners more often use their company and flight number as their callsign, while the equipment used for a flight can change every time? I don't see any registration number label in this shot of a 737 cockpit, for instance, but it could be on the overhead panel obscured in this shot.

Yes, on a commercial airliner typically the tail number and the SELCAL (if so equipped) are placarded on the panel.  It's hard to find sometimes in picture because it may be behind the yoke, but it should be there somewhere.


What are these four tubes for in the intake for an Extra 300? The fuel-injected engine has six cylinders, so it's not a "per-cylinder" thing nor is it related to a carburetor.

P.S. That screen is the entirety of the air filter. Apparently it's meant to keep out small birds, and not much else.

Those are the impact tubes for the fuel metering for the RSA-type fuel injection system:

Above is the face of the fuel servo. If you had more light you would have also seen the throttle valve as shown above. When the throttle opens, air rushes in, and the higher pressure in the impact tubes would deliver more fuel. See schematic and description below:

... As a result, the diaphragm moves to the left, opening the ball valve. Contributing to this force is the impact pressure that is picked up by the impact tubes. [Figure 2-32] This pressure differential is referred to as the "air metering force." This force is accomplished by channeling the impact and venturi suction pressures to opposite sides of a diaphragm.

Source: FAA-H-8083-32A. Aviation Maintenance Technician Handbook-Powerplant Volume 1.


How can I read airport maps when choosing a seat as a passenger? When I choose seats on an airplane, I usually go for a window seat since I love the view, especially the views of a city on takeoff/landing.

Sometimes I get good a view of the city and sometimes I don't.

How can I look up information on an airport before a flight to choose a side of the plane that will get the best view of the city?

To give a concrete example, I will be flying into DFW soon from the North. Ideally I would get a view of downtown Dallas (East of the airport) when flying in. From AirNav, and the map it links to, I can see the runways at DFW run North/South. If I pick a seat on the left side of the plane will I be guaranteed a view of downtown Dallas?

It seems you already know to look up the layout of the airport, and how to check a map for which direction from the airport you want to look. The piece of the puzzle you're missing is the wind direction.

Aircraft both take off, and land into wind. So it does not matter that you are "coming from the North", if the wind is blowing from the North an aircraft will go all the way round and land on the Northerly runway. So if you were to check the weather and there is a definite Northerly tendency you would want to sit on the Right side of the aircraft.

This is nice and easy when there is a wind, and it is forecast accurately, and you can select your seat close enough to departure to take advantage of this info. Unfortunately there are many things that can throw a curve ball here

Changing wind patterns
Lack of wind / operational reasons to
Multiple runways

About the best you can do is be aware of the prevailing wind, and hedge your bets on which runway will be in use. You'll win some (most?) and lose some. But you'll certainly get a good view some of the time.


Why did the tower at a Class D airport not set me up with flight following on the ground? On a trip I had a few days ago I had contacted the ground controller at Santa Fe Municipal (KSAF), a Class D airport, to arrange for a VFR departure to the southwest and requested VFR flight following.  The ground controller said he couldn't arrange flight following and that I had to arrange that with Albuquerque Center while in the air.  I was taught (and encouraged) that you could get flight following while on the ground at a towered airport.  The tower was not busy at the time as I was the only plane talking to them and the airport has its own radar which I know the controllers can see in the tower.  In fact I had forgot to reset the transponder to 1200 and they called me out on that after I took off.  Is there any particular reason why they could not set up flight following for me on the ground?

EDIT:  The FAA has a program where they can contract out a control tower at an airport to a private company.  I just looked at this FAA website and Santa Fe is listed as a contract tower.  Could this be the reason why the tower could not arrange VFR flight following?

Some towers have a data terminal that allows them to create and update flight plans*; these are typically the ones busy enough to have a separate Clearance Delivery frequency, but you'll find exceptions both ways.

Other towers do not and would have to call a facility that does, which takes that controller away from actually controlling traffic; these towers tend to be the ones with less traffic and lower staffing (maybe only one person on duty), so they're usually not going to do that for VFR requests.

However, all Approach and Center facilities have the ability to create and update flight plans* at every radar position, so it's usually no problem for them to set you up once you're airborne and talking to them directly.

(* VFR Flight Plans are not accessible by US ATC; when you get Flight Following, it is actually put in the computer as an IFR flight plan with a VFR altitude.)


Are there other examples of human-electric hybrid aircraft? Has there been a human-electric hybrid aircraft after this. A more streamlined design; maybe capable of soaring as well...

Yes. I assume by hybrid you mean that propulsion is provided by a solar-powered electric motor while the control system is powered by a human pilot.

From Paul McCready came Solar Challenger in 1981, certainly less flimsy than the Gossamer Penguin.

Solar Challenger (picture source)

In the same year, Solair I flew for the first time, followed by Solair II in 1998. Both were built by Günther Rochelt, who also built human powered aircraft.

Solair II (picture source)

Rochelt's Musculair 2, the fastest human powered aircraft until today, inspired Eric Raymond to build Sunseeker in 1990. Sunseeker II improved on those designs again, still using the layout pioneered by Musculair 2. It flew first in 2002.

Sunseeker (picture source)

Icaré 2 was based on glider technology and flew first in 1996.

Icaré 2 (picture by Uli Reinhardt, Zeitenspiegel (Zeitenspiegel) [CC BY-SA 2.0 de, GFDL or CC-BY-SA-3.0], via Wikimedia Commons

With Sunseeker Duo the first solar-powered two seater flew in 2014.

For a comprehensive history of solar flight see here (PDF), and for some more background see here.


What is the difference between Full Throttle Height and Critical Altitude? My understanding is that "critical altitude" is the density altitude at which the wastegate on a turbocharged engine is fully closed, so any climb will result in a decrease in manifold pressure.
Is this different to "full throttle height"? I have heard conflicting definitions of that, such as it only applies to superchargers, or that turbocharges have multiple full throttle heights.

Bob Tait explains it perfectly:

Every turbo charged engine has a rated boost which is the maximum manifold pressure permitted for that engine. It is marked as a red line on the manifold pressure gauge. Because the ambient density drops with altitude, there will always be an altitude beyond which it will be impossible to obtain the rated boost. That is called critical altitude.
However the aircraft is still capable of climbing beyond that altitude but the manifold pressure available at full throttle will be less than rated boost. For every value of manifold pressure there will be some other altitude beyond which that particular manifold pressure will no longer be available. So every value of manifold pressure has its own full throttle height. Critical altitude is simply full throttle height for rated boost i.e. the red line on the manifold pressure gauge. (bobtait.com.au)


Is the Batjet Design Physically Feasible? I realize the plane is fictional. However, in the Batman Animated Series by Warner Bros, we see this Batjet.

So this got me thinking (and I'm not a physicist): is this air frame configuration even possible? Has a "rounded", forward-swept wing design like this been attempted before? Or would a design like this never actually fly?
Any real-world examples that are similar in nature?

Yes, rounded wings have been tested in the past. Probably the most famous example is the Vought XF5U "Flying Flapjack". They work perfectly fine. However, the design as pictured does have some flaws.
Efficiency
The lower the aspect ratio (i.e. wider front-to-back in comparison to their length left-to-right), the more inefficient the wing. This is due to there being a lot of area available for the high pressure under the wing to wrap around to the low pressure on top, creating large wingtip vortices. For the Batwing, this would be doubly true, as a large part of the wings have edges on the inside, almost doubling the effect.
Stability
It appears that the engine is either directly behind or directly below the cockpit, putting the center of gravity way too far back compared to the center of lift. This creates an unstable situation, especially at low speed. As the elevator loses effectiveness, the nose of the aircraft will tend to rise, further exacerbating the loss of speed and leading to a stall. Such a stall would be almost guaranteed to be unrecoverable using normal means.
(I say "normal means" because, in the show, the Batwing was shown to have VTOL capability, meaning it has vents in the bottom to redirect thrust down and hover. So, it would be theoretically possible to use engine thrust to rotate the plane nose-down and recover from the stall. If the engine fails, though, this airplane becomes an absolute deathtrap.)
Control Surface Effectiveness
Unfortunately, the animators of the show never animated the control surfaces, so we're left to guess where they are and how big they are. If they're in the standard position in back, that puts them close to the axis of rotation (which goes through the center of gravity, see above), limiting their effectiveness.
The elevators could be moved to the leading edge, but the slipstream on them would cause issues with flutter, and it would take a huge amount of control force to keep them from being pinned at their limits. This also wouldn't help the ailerons (because they'd still be close to their axis of rotation) or the rudder (because there's nowhere to put it other than that big mohawk structure coming off the top of the cockpit).

So, in short, it's possible, but not in any way practical.


Does temperature affect dynamic pressure? Taking off on a cold day (5°C), it can take 10 seconds to reach rotation speed but on a hot day (40°C) degrees it will take longer to reach the same speed.

Is this due to the dynamic pressure? How does temperature affect it?

Yes, temperature affects dynamic pressure by affecting air density: it reduces with increasing temperature.

Dynamic pressure q $ = ½ \cdot \rho \cdot V^2 $

The molar form of the ideal gas law: pressure p = $\rho \cdot R \cdot T$, with R = gas constant and T in degree K

$$\rho = \frac{p}{R \cdot T} \tag{1}$$

Eliminate $\rho$ from dynamic pressure:

$$q = \frac{p}{2R \cdot T} \cdot V^2 \tag{2}$$

From (2) we can see that if temperature increases, dynamic pressure decreases linearly. In a deleted answer @xxavier mentions correctly that Indicated Air Speed is a measure of dynamic pressure, so let’s compare the two situations. 5°C = 292K, 40°C = 327K, TO true airspeed on a cold day = $V_c$, on a hot day = $V_h$

$$ \frac{p}{2R \cdot 292} \cdot V_c^2 = \frac{p}{2R \cdot 327} \cdot V_h^2$$

$$ V_h^2 = \frac{327}{292} \cdot V_c^2 => V_h = \sqrt{\frac{327}{292}} \cdot V_c  $$

$$ V_h = 1.058 V_c \tag{3}$$

So to reach the same IAS, TAS must be 6% higher. But there are more ways that dynamic pressure affects time to reach rotation IAS:

The propeller thrust is proportional to air density: simple momentum theory models thrust as $T = C_T \cdot ½ \rho A \cdot (\Omega R)^2$. Again density $\rho$ which decreases linearly with temperature according to (1). Propeller thrust reduces with factor (292/327), integrated over time until $V_h$ is reached. 11% less thrust to reach a 6% higher velocity.
The engine has less dense air to operate with. From this site:

  When your engine is not equipped with a turbo- or supercharger it will also suffer from the less dense air. Each intake stroke (which is by volume) will contain less air molecules and thus less power can be developed by the engine (due to the fixed fuel / air ratio).

So yes, dynamic pressure plays a role, but the real underlying cause is the less dense air everywhere. Also in the dynamic pressure.

Notes:

All above comparisons at equal static pressure $p$.
$V_c$ and $V_h$ are actual true airspeeds, not indicated speeds.


Is a golf ball surface a good idea for wings or fuselage? I searched for an infamous golf ball question on this site but did not find any, so I guess it's time for one.

Would a dimpled surface like a golf ball somehow improve the aerodynamics of an airplane? (there are many planes for many purposes, so let's just go with my favorite: large commerical passenger planes, Mach 0.85 cruise, several hundred tons loaded weight).

They actually tried this for a car on myth busters and they claimed an 11% improvement in miles-per-gallon, so you can't say the question has no merit. Unfortunately, they did not try it for an airplane.

Golf ball has dimples on its surface because we want a turbulent boundary layer. Golf ball is a bluff body (i.e. the drag is dominated by pressure drag). Hence, the drag on a sphere is dominated by the separation on the rear face. If we could minimize that, the drag would be reduced.

The following figure shows the variation of drag coeffecient of a sphere with Reynolds number.

Image from wfis.uni.lodz.pl, taken from (Hoerner 1965)

As can be seen, the drag reduces at the critical Reynolds number, which is the point where separation takes place. Because of separation, the wake in the rear of the sphere (or ball) is reduced, which reduces drag. So, if the boundary layer of a sphere can be made turbulent at a lower Reynolds number (by some means), then the drag should also go down at that Reynolds number, as a result of the reduced wake. This can be seen in the following picture, where the flow has been made turbulent using a trip wire.

Flow over a sphere: (a) Reynolds number = 15,000; (b) Reynolds number = 30,000, with trip wire. Image from princeton.edu

That is the purpose of having dimples in a golf ball- to reduce drag by inducing turbulence and delaying separation (this can be achieved by increasing the Reynolds number; but we are limited by the ball speed). The resulting wake is much smaller due to the delayed separation.

In case of a streamlined shape (like aircraft wing), the propotion of pressure drag is small, as can be seen below.

Image from pilotfriend.com

In case of streamlined shapes, the pressure drag is small because the wake is small. The net result is that 'tripping' the boundary layer as in bluff bodies is not a good idea (a laminar flow causes less friction drag than a turbulent one).

The aircrafts do use similiar devices, via the vortex generator, which are used mainly to delay flow separation.


How does drag change with Reynolds number? If you want to scale down an airframe, i.e., for wind tunnel testing, you have to adjust for the Reynolds number. I want to ask if in case I forgot to adjust the Reynolds number, would the drag on the airframe be greater or smaller after scaling down than what could be predicted from change in frontal area?

Recall the definition of the Reynolds number:

Where ρ is the density, v is the velocity, μ is the viscosity of air and l is the characteristic length.

The characteristic length is typically the chord length of an airfoil but can also mean the overall length of the airframe. When we scale down a model, we are decreasing l.

Decreasing l while holding all other variables constant (which I assume is what you meant about "forgetting to adjust the Reynolds number"), has the net effect of lowering your Reynolds number.

Looking at airfoil graphs for example, we can see the relationship between the Reynolds number and the drag coefficient of the body:

It is obvious from the graph that the drag coefficient increases when the Reynolds number is lower (higher curve on the graph)

Image source: Airfoil tools


How are bags transported from the terminal to the airplane? I'm curious how bags are transported from the terminal sortation system to the aircraft. I'm mostly trying to understand the "average tarmac distance" a bag will travel. If anyone has direct insight into that metric that might be even more helpful than understanding the process.
I grabbed a screenshot of the United Terminal at DIA to try to map out how this could be implemented. Can anyone help with insight into which process implementation is generally used in the industry?

Each gate has a "sort point" and bags are transported a short distance from that gate sort point to the plane parked at the gate.

One sort point serves "N" number of gates. One baggage handling Cart serves only one plane. (How big is "N" if this method is used?)

One sort point serves "N" number of gates. One baggage handling Cart may serve several planes, detaching a section of the the train at each craft.

Are any of these methods used? Or perhaps could several sort points feed one aircraft? Does this vary airport-to-airport or airline-to-airline? Who is generally responsible for the design of the process - the airport or the airline? Do ULDs have a different process than each-loaded bags? What about smaller regional jets loaded at the far ends of the terminals - are their sort points further away? How does the process look for offloading bags from the aircraft?

From your photo this answer assumes a hub airport.
Some background first: after check-in, the belt conveyor system moves each bag onto its own tray – that's the modern individual carrier system (ICS) which mitigates jams, misreads, and lost baggage. Depending on how early the check-in took place, the tray will either go to a holding area, or directly to the baggage makeup area/space (BMA) where the bags are unloaded onto another belt conveyor system for cart/ULD loading.
BMA is the place you referred to as sort area. There is typically one BMA per airline per pier, and since big airlines usually have their own piers, it's one BMA per pier. Example for San Francisco Intl (see tan colored areas):

— https://www.flysfo.com/about-sfo/sfo-tomorrow/passenger-terminal

Two hours or so before the flight's STD (scheduled time of departure), comes the cart/ULD staging. The cart train length varies; IATA recommends 3 carts per narrow-body and 5–6 per wide-body. Airlines however implement this as best fits them. Easyjet for example stages one cart per narrow-body at Liverpool (internationalairportreview.com).
Regarding how many gates are served per BMA, one of the methods used to estimate the floor area required for a BMA when designing an airport is the EQA, or Equivalent Aircraft factor (see FAA AC No: 150/5360-13 for more).
In short, it's option 2.


What do FAA DERs require regarding low-level requirements in DO-178C? Software developed for aircraft needing TA certification pretty much has to be done using a process following RTCA/DO-178C.  My question regards low-level (sometimes called "derived") requirements:

Low-level requirements either refine high-level requirements, or express design decisions.  I've been told a low-level requirement should be traceable to some statement or small block of code, and at DAL C and above, each piece of code needs to be traceable to a low-level requirement.

Do DERs want to see a test corresponding to each low-level requirement, or is that only so for the high-level requirements?  Would it be OK to do testing at a function level ("unit testing") as low-level requirement testing, and are the DERs happy with that?

One thing I would start with is that low level requirements are not the same as derived requirements, although low level requirements can be derived. Derived requirements are requirements that cannot be traced to a higher level requirement.

For verification of DO-178C Level C software, your SVCP will need to completely cover high level and low level requirements as well as attain 100% statement coverage of your code.

As far as what the DER for your project will approve, there really isn't any way for us to know.


Would a 767-300 be considered FANS-1 capable without SATCOM? It is known that aircraft flying in Europe above FL285 will be mandated to be capable of PM-CPDLC data link communication. Our 767-300 has ACARS CMU, Pegasus FMC, MCDU and VHF radios (three). The ACARS system works fine, but there is no SATCOM.

As indicated on COMMISSION REGULATION (EC) No 29/2009, aircraft with an individual airworthiness certificate issued before 1 January 2014 that are equipped with Future Air Navigation System (FANS-1) are exempt from the requirement of the PM-CPDLC for the lifetime of the aircraft.

My question is, would our 767-300 aircraft with the above avionics be considered FANS-1 capable and qualify for the exemption? Any suggestions on how I can confirm that?

You would ask the maker of your MCDU whether it's FANS-1 capable using just a VHF datalink. Satcom isn't necessarily a requirement, but you would only be able to use it where VDL is provided, for instance in Europe but not on oceanic routes.