Why is the aiming point distance shorter in the US vs. Europe for long runways? In essence, if you compare EGLL and KJFK (but it works for other airports too) you would see that US airports generally adhere to the rule of 1000 ft in distance between threshold and aiming point base. In Europe there usually is an additional (2nd) pair of TDZ zone markings which brings an additional 150 m to a total of 450 m between threshold and aiming point.

What is the source of this diversification and does it change how a pilot would handle final approach in two different settings? Lower flare/harder touchdowns in US?

For simplicity let’s assume we are comparing long (i.e., 3 km or more) runways only.

EGLL example: please notice that even at EGLL there is 420 m at 3 directions but 27R is 470 m.

KJFK example:

Good observation. It's often misquoted that the 1,000 feet is the [international] standard. The standards and recommendations for runway markings can be found in ICAO Annex 14. In particular section 5.2.6 for the touchdown zone marking (diagram shown below).

When a country decides it wants to deviate, it lists the differences in its AIP. In this case indeed the US AIP lists a difference from 5.2.6.4:

  The U.S. standard places the aiming point marking 306 meters from the threshold where it replaces one of the pair of three stripe threshold markings. The 306 meters location is used regardless of runway length.

(Note: 306 m is ~1,000 feet.)

The UK AIP does not list a difference in distance, but the reason why it's >400 m is in a note (missing from the 2016 edition of Annex 14) in the 2017 EASA guidance material, the 400 m is the standard, except:

  Where a PAPI system is provided for the runway, the beginning of the marking should be coincident with the visual approach slope origin.

The AP marker has always been (using Google Earth historic view) where the PAPI lights are at EGLL. Example for EGLL 27R:

When VASI is used instead of PAPI, like the 3-row VASI at OERK, it's 400 m on the mark:

I initially thought maybe it's because EGLL went through various lengths in its long history, but a new runway such as 05R at HECA that was built from scratch, is actually like EGLL 27R. There's more to PAPI than meets the eye.

So there is diversification indeed. As for 'landing short in the US', we are looking at ~100 m difference (306 vs. 400) for the aiming point, but don't forget the extra touchdown zone marking closer to the threshold in the ICAO standard.

Note: the figures apply to runways that are >2,400 m. Shorter runways have the aiming point brought closer to the threshold.

Trivia: Also still in the UK AIP is the unique aiming point, even though they are clearly no longer used at EGLL and EGCC for example (check historic view in Google Earth or current charts).


Is any device installed on airplane to measure wind speed relative to the ground, and its direction? A device or tool which can measure actual wind speed to the ground rather than to the airplane. Also a device which detect the wind direction to the plane. The device I meant is something like this pitot. But unfortunately this pitot is measures the airplane relative to the air around it, not the ground.

The "wind speed relative to the ground" would just be the (vector) difference between ground speed and true air speed:

 (image source)

Most aircraft do not measure ground speed and track directly with instruments installed on the aircraft. It can be measured with a Doppler radar (see Peter Smith's answer), which is only used on military aircraft (mostly helicopters) today.

Ground speed and track can be determined with external tools:

DME: The Distance Measuring Equipment can be used to measure distances to fixed radio beacons equipped with DME (usually VOR/DME or VORTAC). If you know the angle at which you are flying towards/away from/to this station, you can calculate ground speed from the change in distance. Some DME instruments display this:

(image source)

When tracking a VOR, you also know the true track angle required for computing the wind direction.
GNSS: A Global Navigation Satellite System like e.g. GPS allows precise determination of the aircraft's position. The ground speed can be calculated from the change of this position and the true track is simply the direction of change. A more accurate determination is possible by measuring the Doppler shift of the signal (thanks to bogl for pointing this out).
ADIRS: An Air Data Inertial Reference System as installed on airliners knows the aircraft's attitude and position based on integrating data from accelerometers and gyroscopes. It can be updated via GNSS, but would still provide data for ground speed and track without it.

Modern avionics can display the result of these computations directly:

left: Boeing 737 Navigation display (source), right: G1000 (source)


What is the maximum number of planes a controller can control? In a small and un-busy airport, the same controller can be the ground and tower at the same time. If the sector is un-busy (e.g. late at night), he may also be a sector, ground and tower all at the same time. At busy airports of course, ground and tower are two persons. Sometimes even ground is split to "Ground South" and "Ground North".

So there is a threshold where the communication has to be split into two, either because the frequency would be too busy (only one person can talk on the frequency at any time), or there are so many planes that a controller would start making mistakes.

What is that threshold? (Is it a concrete number, or some other conditions which can be objectively determined?)

There is no fix maximum a controller is allowed to have under control, as the other answer already pointed out. The airspace under control of ATC is divided up into sectors containing the airways to and from airports within that sector or in upper sectors, airways just for en-route traffic. (see also: Why do pilots use airways instead of GPS direct?).

Sectors can be merged during off-peak hours, split back into separate sectors during peak hours and in the case of TMA or approach sectors, the controller owning the sector can receive additional help by splitting the approach sector further and using different controllers for different tasks within that sector: Feeder/Pickup - Director - Departure Controller. The thresholds of when a sector is split is dependent on many factors, like weather, runway in use, outside influences like adjacent sectors having more traffic that usual, special occurances within that sector like parachute jumper drops.

Sectors also vary in size and capacity based on their intended use. While TMA/Approach sectors are rather small and need to accomodate arriving traffic into an airfield, an enroute or upper sector can be significantly larger in size, lateral and vertical. Again, this is individual to each sector, because some factors which hold true for TMA sectors do not apply to enroute upper sectors and vice versa. It also really depends on the controller working the sector and his personal limits, favorite procedures and work practice. A few examples:

A TMA controller using vectors to the approach instead of standard arrival routes or transitions will have more workload to monitor and sequence aircraft.
The minimum radar separation in TMA sectors and upper sectors can be either 3NM or 5NM. While upper sectors are larger in size, they have to maintain a greater separation, so in some cases, an upper sector twice or multiple times the size of a TMA sector will only be able to accomodate the same number of aircraft (ceteris paribus) before reaching saturation.
A controller having to work with special situations such as aircraft in distress or student pilots requiring more attention than experienced pilots, will reduce the controllers personal capacity.

In summary, to answer your question: There is a situation and controller based threshold, at which saturation will be reached. Controllers are trained to anticipate this threshold and make their supervisor aware of the situation beforehand, so that measures can be taken. There is however no fixed numerical maximum, unless specified in the ARTCC/ACC's operating procedures.


What type of hydraulic oil is used in the aviation industry? Among the three different types of hydraulic fluid types (mineral based/petroleum based, synthetic esters, naturally occurring vegetable oils) which one is used in the aviation industry and why?

For the aviation industry, flammability is a major concern, and aviation hydraulic systems require Fluids that cannot burn yet don't create corrosion and still lubricate. Skydrol for instance is a phosphate based ester, a nasty substance that irritates the skin, but at least has some flame resistance.

Old hydraulic motion systems on simulators used the stuff, later ones went to mineral oils like Shell Tellus. If there is an oil leak in a pressure hose, a fine mist of fluid droplets appears, flammable if mineral oil but this could be handled in a ground facility with extinguishers everywhere. Not a good circumstance in an aviation accident though, and that is why in aircraft the esters have to be used.

Vegetable oils are not used for hydraulic systems in the aviation industry at this moment as far as I am aware.


Can the HAL Lca Tejas perform Pugachev's Cobra maneuver? Can any variant of the HAL Lca Tejas (either the naval variant, the mark 1, mark 2 or later) perform Pugachev's Cobra?  If not, why?

No.
With that delta wing shape it will have pitch control problems at higher angles of attack. The Cobra maneuver requires a small pitch-down moment over the post-stall region up to 110° angle of attack. I very much doubt that this wing planform will have such pitch characteristics, and without thrust vector control a docile pitch behavior is a prerequisite for the Cobra maneuver.
In the development of the LCA a high lift coefficient without leading edge devices was the main goal of the wing design. It should be able to operate in the vicinity of flocks of birds - birdstrike is a real risk on many Indian Air Force bases. At the same time, the small aircraft should be able to fly supersonically, which lead to the double-delta wing as the best way to achieve both goals, and poor post-stall pitch response had to be accepted.
When HAL switched from MBB to Dassault for technical support, leading edge devices were added in order to fulfil the take-off and landing requirements. Post-stall flight was not seriously considered at any point in the development.


Are “zero-bag fares” really profitable for airlines? Any extra weight costs the airline in fuel. So, if none of the passengers carry any extra luggage (be it checked in or cabin luggage), will it lead to lesser fuel for the airline, thus lesser expenses, which would justify the lower  “zero-bag fares”.

Or, do the airlines have to meet some minimum weight criteria anyway to make any feasible fuel savings.

That's mostly a marketing ploy; people that don't have any bags are more likely to go with a airline that has a no-bag fare.

However not having any checked-in luggage means less overhead in terms of baggage handling as it's a bag less to load and keep track of. It also means that if you got lost and didn't make it to the gate in time they don't have to offload any luggage but can simply pushback as soon as the gate closes.

The freed-up space is then available for non-passenger cargo transport like mail.


HSI KG 525 PNI non slaved or only slaved? I'm in Germany and currently on a project about the avionics of an older airplane (PA28). Here is a link to a former reddit question.

My Prof. and me want to update the Avionics now - he asked me to do a bit of research - it is quite a good exercise for me and i learn lot this practical way (i'll become an aircraft engineer)!. I do the prework for our maintenance company and they'll supervise my work/planning.

The main idea is to install new:

Garmin GNS430
Garmin GMA340
Insight G4 + Sensorics
Becker AR6203 22
KI - 525A or Garmin G5 DG/HSI
Insight G4 for experimental stuff (UAS courses with our piper)

This avionic stays, but will be reconnected

KN53 (with GS) – goes to KI 204
KI 204
KAP 100 (AP)

Will be removed:

GPS150
KI 203 - Insight G4 will come here instead
KN53 (w/o GS)
KY 196 (2x COM)

Right now i have the DG - KG107 in it, but my professor wants to update it to a HSI..

Now the question is if a KI-525A has a built-in gyro or is there only the possibility to connect it with the KG102 (which we dont have)?
I would like to have the new Garmin G5 DG/HSI but it seems that it doesn't have an EASA STC. Garmin is kinda quiet.  What do you think - should we install the G5 (DG/HSI) and hope for a STC or pick another HSI (maybe Century)

EDIT: Took 3 questions off the topic

The G5/430 combination was approved by EASA back in February! No reason why you can't use it. Some advice for you, learning how to use Google search is an invaluable skill for any aircraft engineer. :)


What can trigger the ELT on GA aircraft? Does the Emergency Locator Transmitter (ELT) on smaller aircraft activate automatically in case of an accident? If so, what triggers it? Is it a certain amount of G's that triggers it or are there other parameters taken into account?

There are four different types of ELTs:

Automatic fixed ELT (ELT(AF)). An automatically activated ELT which is permanently attached to an aircraft.
Automatic portable ELT (ELT(AP)). An automatically activated ELT which is rigidly attached to an aircraft but readily removable from
the aircraft.
Automatic deployable ELT (ELT(AD)). An ELT which is rigidly attached to an aircraft and which is automatically deployed and
activated by impact, and, in some cases, also by hydrostatic sensors.
Manual deployment capability is also provided.
Survival ELT (ELT(S)). An ELT which is removable from an aircraft, stowed so as to facilitate its ready use in an emergency,
and manually activated by survivors.

All the aircraft I fly in (all two of 'em) have the third type. If the aircraft is submerged or experiences a high G-force, the ELT will be triggered. There's also a button on the panel to activate it manually.

I'm not sure what G-forces are required to set off the ELT, but they must be fairly significant—I've slammed my plane down pretty hard while misjudging my landings, and never had it go off.

According to https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_91-44A.pdf:

  To meet the "g" force requirements of TSO-C91, automatic fixed-type inertially activated ELT's (except overwater type) must activate at any inertial force, parallel to the longitudinal axis of the aircraft when installed in accordance with the manufacturer's instructions, of 5(+2, -0)g and greater for a time duration of 11 (+5, -0) milliseconds or longer.

So, there you have it. You need 5–7 Gs for 11–16 milliseconds—but only in the direction the plane is pointing. Misjudging your landing flare and stalling the plane 20 feet off the runway won't set it off, as that's not on the axis they're measuring.


Could sprinkling cool water on the runway in hot summer decrease air density? Why is there no such technique used where cool water gets sprinkled on a runway?

Cooling down the air above the airport would decrease air density and therefore lead to a shorter starttime for airplanes.

This image explains exactly what I mean.

(related article: https://www.boldmethod.com/learn-to-fly/performance/prevent-a-density-altitude-crash-on-takeoff/)

There are a few problems with this idea:

The water evaporating would increase the humidity, likely canceling any cooling benefit.
The effect would be localized in a very small region just above the pavement.
The effect would be temporary as any breeze would quickly blow the cooled air away.


How to convert an EASA CPL(H) to FAA CPL Rotorcraft With a Helicopter Class Rating? I just finished my CPL(H) license and was just wondering if it is difficult to convert your license to an FAA (or TCCA) license.

Do you have to do additional exams or check rides?

It's hard to find a step by step guide to do this, or do you recommend to contact a training organization in the US or Canada to perform the conversion?

I just saw somebody try to do this. They had an EASA CPL(H) and a FAA PPL(H). The DPE needed to have a letter, sent from EASA to the FAA in Omaha, Verified, and then a copy sent to the pilot. This letter is EASA confirming the logbook entries attained in Europe were legitimate. This letter is only valid for six months. Despite getting the necessary flight training from a FAA certified CFI, as per the appropriate requirements, without this letter a DPE can’t administer a practical exam.

You need to pass the FAA practical to get any FAA certificate. Written test requirements vary.


Could the Airbus A350 be converted into the role of air tanker / be used for aerial refuelling? Since the A350 is one of the most efficient jets out there, Is it possible that it can be transformed into a fuel tanker for Aerial refuelling?

Yes, it could be done. There is an A330 already working as fuel tanker.

Image Source: Wikipedia

Airbus might build a A350 air re-fueler as mentioned here:

  Airbus is already offering a tanker version of the A330, and could
  (potentially) upgrade their proposal with an A350 air-refueler. The
  A350 would offer a substantial increase in fuel offload from existing
  tankers, as well as the 767.


Why is angle of attack information not displayed in the cockpit? Would not it be useful to have the angle of attack information displayed on the flight display? I know that we have shakers that alert pilot when the critical AOA is exceeded, but still we have cases of pilots stalling the aircraft (sometimes stalling one wing more than other).
I am sure designers have considered this but not chose to display AOA. I am curious what the reason could be.

Many aircraft do include them, because it's very useful.

For example a modern 737:

A Garmin G1000 (one of the more common General Aviation glass cockpits):

Specifically the part that looks a bit like this

And a more traditional analog/mechanical gauge:

The problem is that usually when someone stalls, it's because they've made a mistake - not because they can't see the AoA. Either they're disorientated, or they've made a mistake with airspeed or an obstacle and are exceeding the aircraft's performance envelope.

Typically, the lack of an AoA dial isn't the reason you stall.


Will having a history of depression prevent me from flying? The AOPA website lists four medications as acceptable treatments for depression. I was treated with one of them, but it ended up making things far worse, to the point where I had issues with suicidal ideation. I've since stopped taking it and, while I'm still somewhat depressed, no longer have those thoughts, and don't have any issues getting along with daily life.

My depression is more along the melancholic lines, so I basically just don't find most things enjoyable. Being in the air is one of the few things that helps, so obviously I'm worried about not being able to do so. I don't have any of the symptoms that would seem to have an impact on actually flying, but I know having a history is a problem, so I've been trying to figure out how much this will affect things.

Not sure if aircraft type matters for medical requirements, but I was hoping to either fly helicopters commercially, or in the very likely case that I can't find a helo school, I'd probably end up crop dusting.

This is one of those questions where you really need to talk to an AME, preferably one who has significant experience working through these sorts of issues.  In order to get the Class II medical that you'll need to fly commercially, you'll end up needing to work through the process anyway, so you're probably better off starting with the expert, rather than with a message board like this one.

The only case where you'd save any money by starting here is if someone here were to accurately tell you that there is no need to go to an AME as there is no way anything could possibly work.  I doubt you're going to get that sort of advice, because (a) we don't have any AME's that post here on av.se, as far as anybody has ever self-identified, at least, and (b) this stuff is sufficiently complicated that you don't want to trust a layman's info or opinions, even one who "knew somebody who had a similar situation."

That said, I'd suspect that the chances are pretty good that you may well be able to get a medical certificate, so I would encourage you to pursue that path.  Let a good AME take a look at the specifics of what your history looks like, and listen to his/her advice about what would need to happen (maybe very little, maybe quite a bit) in order to satisfy the FAA.

This thread deals with an entirely different medical condition, but the answers, and the logic behind them, apply here as well.

Be honest with the flight surgeon, and don't let random posters here talk you out of giving yourself every opportunity to pursue your dream of flying commercially.

Best of luck to you!


Why does the Beech Staggerwing have its low wing ahead of the high wing? Why does the Beech Staggerwing have its low wing in ahead of the high wing? What are the aerodynamic ideas behind this? Other biplanes of the era had the opposite, low wing behind the high one.

Notice the retractable landing gear? With a tail-wheel design, the main gear work best at or ahead of the center of gravity. The Staggerwing has a neat and efficient retractable main gear, that folds onto the lower wing. But the lower wing has to be farther forward than usual for this to work.

The two wings on a biplane interfere with each other, but the biplane structure is very strong and stiff for its weight. So the Staggerwing's design gives up some wing efficiency for strength and light weight. Then it gives-up a little more for the reverse stagger that puts the downwash of the lower wing right under what should be the high pressure under the upper wing.

However, the design gets good speed from the retractable gear reducing drag. And both wings, upper and lower, join the fuselage at 90 degrees, with no struts or other sources of drag. Aerodynamically, a very clean design. the Beech Bonanza looked more modern, but the Staggerwing had the same cruising speed, 176 knots, 202 mph. It also had a 450 hp P&W Wasp Junior, while the Bonanza got along with a 300 hp Continental.


What were the rules for logging "simulator" instrument time in 1994 (non-full motion)? Long before the FAA updated the rules of BATD, AATD, FFS, and FTD, what were the rules for logging simulated instrument time in a synthetic trainer?
Specifically, this question came up, an older pilot had a few log book entries for simulated instrument in a Curtiss-Wright P.3 simulator and it was logged as "simulated instrument", "synthetic trainer" and "total flight time." Nowadays, you do NOT log instrument time in a simulator (unless its full motion) as "total fight time", but how was it logged back in 1994? This person's log book entries were logged by an instructor/simulator operator, and the entry was signed off by the instructor. I looked it up and the Curtiss-Wright P.3 was used by airlines to do instrument training, but I couldn't find anything on how that time was appropriately logged back in 1994.
I imagine back then it may have been appropriate to log it as flight time, but I can't find anything that old that says anything one way or the other.
UPDATE: I found AC 120-45A issued in 1992, which described Airplane Flight Training Device Qualifications. Section 14 of this AC said that Previously Approved Flight Training Devices are essentially grandfathered for 5 years from the date of that AC in 1992. So if the Curtis-Wright P.3 was previously approved as a flight training device then, it would be grandfathered. So, if anyone knows if the Curits-Wright P.3 was ever approved as a flight training devices, that would be helpful as well.

This answer is somewhat non-responsive but may be helpful.
In the 70's I utilized training devices, both in preparation for an instrument rating, and for currency. The training device had a plotter and would tract the path flown.  As I recall it was a regulatory requirement that a CFII or an IGI (Instrument Ground Instructor) provide training for all simulated time.  It was logged as simulated instrument time, and instrument instruction received.
Simulator time on the device was not acceptable for any ratings or currency, unless an instructor was present and endorsed the logbook or other record.
As I recall, two inspectors from the FSDO also used this facility.
When I accomplished my instrument airplane rating and my CFII airplane rating, there was no concern either from the DPE who conducted the instrument checkride, nor from the DPE and operations inspector who rode along on my CFI instrument ride.
I will amend this if I am able to dig out the guidance at that time, which as I recall was not an AC, but rather a collection of Letters of Determination from FAA Legal.


Checkerboard painted on fuselage of Boeing test aircraft What is the purpose of the four square block checkerboard painted on the aft fuselage of Boeing's flight test aircraft?  It is either break in markings for fire rescue or some sort of optical measurement for speed, distance associated with flight testing, I believe.

the checkerboard mark allows accurate indication of the position of the plane using cameras as the plane flies by. by tracking the movement of the checkerboards between frames of a series of camera shots, the plane's speed, roll rate, acceleration, etc. can be measured.


Why is cloud base estimation taught in Fahrenheit? I've seen the formula for estimating cloud base consistently presented using Fahrenheit temperature and dewpoint (for example, see PHAK 12-14; Jeppesen also teaches it this way):
$$
\mathrm{cloud\:base} \approx \frac{(\mathrm{temperature\:°F})-(\mathrm{dewpoint\:°F})}{4.4\mathrm{°F}} \times 1000'
$$
However, a METAR always gives temperature and dewpoint in Celsius, and pretty much everything else in aviation uses Celsius — even lapse rate, which is the basis for this formula, is taught as "2°C per 1000 feet". It seems to me that using °F in this formula invites error, since temperature and dewpoint must first be converted to Fahrenheit before using this formula. If the constant on the bottom of the fraction were $2.4 \mathrm{°C}$ then the formula could be used directly with numbers from the METAR.
Is there a specific reason why this formula is given in °F?

Why is cloud base estimation taught in Fahrenheit?

It's not, the source you are looking at is taught in Fahrenheit.

Jeppesen being an American company, I would assume that their teaching material uses units of measurement common in America.
There are many places which have the same formula in Celsius such as the wikipedia entry on Cloud Base among others.
I suspect that pretty much everybody looking to calculate this would understand the difference between Fahrenheit and Celsius and adjust accordingly. That's not to say that errors are impossible, but errors are always possible when calculating anything.


Should I inform ATC about minor deviations to maintain VFR while on practice instrument approaches? When talking to approach control for the purpose of performing practice instrument approaches in VMC with a safety pilot, the pilot (and safety pilot) is required to "maintain VFR." However, the pilot is also required to follow the controller's instructions and vectors.

When maintaining cloud clearances in order to comply with the "maintain VFR" requirement, it is sometimes necessary to maneuver around, under, etc. small clouds. Sometimes the maneuvering involves a lot of small corrections: maybe a few thousand feet off course, or going under a cloud by 500 to 1000 feet.

Do controllers expect to be told every time a VFR aircraft in VMC has to maneuver around small clouds? What phraseology is required? For example, is "maneuvering to maintain VFR" sufficient, or do you have to state each turn descent/climb etc.? At busy approaches it seems like a huge annoyance to controllers to call with every little course correction/adjustment to maintain VFR, but perhaps that is what they expect.

Yes, definitely. Approach is expecting you to fly the approach as published and/or instructed and if you can't do that for any reason then you need to tell them. They may be providing separation between you and other aircraft, and if you start maneuvering without any explanation that could make things difficult for them.

The last time it happened to me I was being vectored when my safety pilot told me that another aircraft was heading towards us. I turned and climbed and then told ATC "maneuvering for traffic, will resume 3000 and heading 150 when able". The controller acknowledged, waited until I was (more or less) back on course and then gave me another vector.

Having said all that, if you can't fly the approach without deviating all over the sky then you should consider if it's really worth it. First, because it may not be very useful or realistic practice, and second because if you put a higher workload on ATC then they may just drop you completely. The ATC orders 4-8-11 say:

  [...] ensure that neither VFR nor IFR practice approaches disrupt the flow
  of other arriving and departing IFR or VFR aircraft. Authorize,
  withdraw authorization, or refuse to authorize practice approaches as
  traffic conditions require.


How to fly to an uncharted airport? I'm learning X-Plane 11 and started to use an add-on called FS Economy.

One of its missions is to fly to small airports that I can't find on skyvector, so I'm unable to plan a direct flight.

I usually fly to the nearest charted airport, and fly using the GPS from there. Not knowing any information like runway length beforehand.

How realistic is it in the real world that an airport would not be charted, and how are flights planned to such airport?

I know nothing of X-plane, but I fly gliders in real life and often land in fields and "uncharted" airports.  Uncharted does not mean the airport does not exist. However, you do need some idea of where it is, GPS coordinates, or some fix from a known point.  Even a general description of its location can get you close enough to find it.  I have often used "Google Earth" to pinpoint a potential landing area and once I find it there, I have what I need to navigate to it.  I check the date of the GE photography some are several years old and things could have changed that make the field unsuitable.
GE also has tools that you can use to measure the length and width of the "runway" and its orientation.  I look for obstacles that might interfere with my approach.  I also check to see if it is fenced because that can complicate getting my glider out of the field.


Why does the FAA SPL require different endorsements depending on cruise speed? Part 61 section §61.327 requires that sport pilots require separate endorsements for aircraft with a VH below 87 KCAS and aircraft above 87 KCAS.

What is the reason for two separate endorsements and what is the significance of 87 kn?

When looking at the rules on ecfr.gov you will find references at the bottom of the paragraph indicating the Amendment, Federal Register notices, and other documents related to the rule.

In this case it leads to the following FAA doc.  That document includes the following explanation for the rule;

  II.B.5. REQUIRE 1 HOUR OF FLIGHT TRAINING ON THE CONTROL AND
  MANEUVERING OF AN AIRPLANE SOLELY BY REFERENCE TO INSTRUMENTS FOR
  STUDENT PILOTS SEEKING A SPORT PILOT CERTIFICATE TO OPERATE AN
  AIRPLANE WITH A V Current regulations require student pilots seeking a
  sport pilot certificate to receive and log flight training in the
  control and maneuvering of an aircraft solely by reference to flight
  instruments. This training must be received before conducting a solo
  cross-country flight or any flight greater than 25 nautical miles from
  the airport from where the flight originated. It also must be received
  prior to making a solo flight and landing at any location other than
  the airport of origination. These requirements are detailed in § 61.93
  and are applicable to persons seeking a student pilot certificate to
  operate any category and class of aircraft. That section, however,
  does not specify any minimum flight training time to meet these
  requirements. In addition, current regulations for the issuance of a
  sport pilot certificate do not require an applicant to receive flight
  training on the control and maneuvering of any aircraft solely by
  reference to instruments. The FAA is concerned that persons exercising
  student or sport pilot privileges in airplanes with a maximum airspeed
  in level flight with maximum continuous power (VH) greater than 87
  knots calibrated airspeed (CAS) may inadvertently encounter conditions
  less than those specified for VFR operations due to their greater
  speed and range. Operators of these aircraft are more likely to
  encounter instrument meteorological conditions than operators of other
  categories of aircraft. In order to enhance the ability of these
  pilots to appropriately react to the possibility of encountering
  instrument meteorological conditions and the potential consequences of
  attempting continued visual flight rule (VFR) flight in instrument
  meteorological conditions, the FAA is proposing to require persons
  operating an airplane with a VH greater than 87 knots CAS to receive
  and log 1 hour of flight training on the control and maneuvering of an
  aircraft solely by reference to instruments. The FAA recognizes that
  persons may currently be authorized to operate aircraft with a VH
  greater than 87 knots CAS. To provide those persons with a reasonable
  period of time to obtain this training, the agency is proposing that
  the training be completed by 1 year after the effective date of the
  final rule. This training would include straight and level flight,
  climbs and descents, turns to a heading, and recovery from unusual
  flight attitudes. Due to the slower speeds and limited capabilities of
  categories and classes of aircraft other than airplanes, the FAA is
  not proposing that this requirement be extended to operators of those
  categories and classes of aircraft and airplanes with a VH less than
  or equal to 87 knots CAS. The FAA notes that for training to be
  conducted solely by reference to instruments in visual meteorological
  conditions, it must be conducted with a view-limiting device.

My take is that earlier rules covered the conditions with VH less than 87 KCAS.  The FAA felt it was necessary to add rules to cover flight at the higher airspeeds.


What is this gigantic dish at Ben Gurion airport? As the title asks, what is this thing for?  Have never seen anything like it at other airports I've been to.

It is just a design feature, and serves no technical purpose, beyond filtering some light and providing a waterfall feature.

  Safdie designed the roof of the rotunda as an inverted dome, an asymmetric dish floating above the piazza that lets sunlight in and filters the daylight with a special white fabric ceiling.
  source: https://www.bdcnetwork.com/ready-takeoff

and

  Rain falling upon the suspended dome drains towards the center and enters the rotunda through an oculus skylight as a waterfall. In the dry season, a continuous flow of water washes the roof, contributing to the rotunda’s cooling and forming a fountain through the oculus.
  source: https://www.safdiearchitects.com/projects/ben-gurion-intl-airport-airside-terminal

Image, because it looks nice (Same source as second quote above)


How do I renew my ability to fly after 40 years? I am 83 years old and it has been over forty years since I piloted an aircraft and would like to go back to flying again. I hold a single engine rating and had flown several hundred hours, but my log book is long gone. Much of my time was ferrying new Cessnas from Kansas to Houston for a dealer. Other than a physical and a checkride what would I realistically need to resume flying again without endangering myself or others?

Much has changed in 40 years, including airspace designations, ATC communications, and new regs.  If I were you I would take an online Private Pilot Ground School to learn about all the new changes.  These courses are not too expensive and it well worth the money.

The AOPA does have a Rusty Pilot Seminar as well (link).

Also it might be worthwhile just to sit in the passenger seat of a plane for a while to re-familiarize yourself with how it all works.


Why did the Avro Vulcan use a delta wing? Why did the Avro Vulcan use a delta wing?

The B52, from the same era, used a swept wing, and had a higher maximum speed.

In the answer to What are the advantages and disadvantages of a delta wing compared to a swept wing?, Peter Kämpf explains that a swept wing is ideal for subsonic airliners.

No one else has built a subsonic delta-wing aircraft since, military or civilian. Why did the designers of the Vulcan make this unusual choice for a subsonic aircraft?

By Ministry of Defence [OGL 3], via Wikimedia Commons

It was obvious to the design team that conventional aircraft could not satisfy the required specification; knowing little about high-speed flight and unable to glean much from the Royal Aircraft Establishment or the US, the team at Avro investigated German Second World War swept wing research.
The team estimated that an otherwise conventional aircraft, with a swept wing of 45°, would have doubled the weight requirement. Realizing that swept wings increase longitudinal stability, the team deleted the tail (empennage) and the supporting fuselage, it thus became a swept-back flying wing with only a rudimentary forward fuselage and a fin (vertical stabilizer) at each wingtip.
The estimated weight was now only 50% over the requirement; a delta shape resulted from reducing the wingspan and maintaining the wing area by filling in the space between the wingtips, which enabled the specification to be met.
— Wikipedia

In short, the team didn't know much about high-speed flight (blame Wikipedia). And to reduce the weight, they eliminated the tail-plane, shortened the span, and filled in the space between the wingtips. Achieving their weight target.
For the same area, a wide wing is lighter to attach, as it requires smaller spread spars.
So, for weight reduction, a delta wing wins against a conventional swept-back wing aircraft. In all, great engineering solution to a big problem, with one downside—
—higher fuel burn at those speeds, limiting the range.
The B-52's were known for loitering in the air for extended durations. Their efficient design requires fewer rendezvous' with the tanker. Unlike the famous Vulcan train.
Some numbers (in kg and km):
        empty weight     MTOW      payload+fuel      range
B-52      83,250        220,000      136,750         7,210
Vulcan    37,144         77,111       39,967         4,171

Maintaining the same wing area; saving weight by reducing the heavy structural elements—the tail-plane, big spars, and wing-span. (Eyeballed the scale, only illustrative.)


At what minimum occupancy ratio can airlines operate profitably? Often, airlines are unable to sell all the seats of a flight and some seats go unsold. So, with what occupancy ratio is it usually profitable for an airline to operate?

For example, Recently, a Turkish Airlines A330, flying from Bangkok to Istanbul (12,000 KM) made an emergency landing after a bomb hoax. The interesting thing was that it was carrying only 142 passengers, against its marked capacity of 250 passengers (occupancy ratio of about 0.6).

And what are the factors which would influence this choice – like probably, it would be cheaper to operate a B787 with 0.6 occupancy ratio compared to a B747.
What are some other factors which influence this?

They can operate with 0 passengers, but will most likely not generate any revenue (unless cargo is carried to generate additional revenue). Revenue and profit on a passenger flight (assuming no paid cargo) will be based upon how many tickets were sold at which price, so the number of passengers required for any given flight to break even will be variable and cannot be answered in a general way.


Why can airliners not fly as fast as fighter jets? Why do passenger aircraft not fly at the speed of fighter jets? What factors stop them from achieving such speed?

There are 2 big reasons:

Speed costs lots of money. Fighter jets can go much faster than passenger jets but they spend most of their time cruising as the same speeds because it is more fuel efficient. Faster airplanes need more exotic materials and would be more expensive to build. The engines would need to be bigger and more powerful and would cost more, and to go very fast uses a lot more fuel as air resistance increases exponentially with speed. Concorde tickets were very expensive not because of any sort of exclusivity, but because that's what it cost to operate it. Newer technology could probably decrease that somewhat, but it would still be very expensive. Air forces can operate fast jets because they have the funding, airlines have to make a profit
Passenger jets fly pretty close to the speed of sound already, to go much faster they would need to break the sound barrier, which creates a very loud sonic boom that disturbs people on the ground and can break windows in some cases. Supersonic travel over land has largely been banned because of that, so unless you are over water you can't go any faster anyway


What is this airplane? Found this old pic of an aircraft that used to service the north. Was wondering if anyone could identify, or had registration information. It would have been in service sometime between 1950-1970.

The letters on top of the tail fin is called a registration or tail number, like a license plate if you will.

In this case searching the Canadian Civil Aircraft Register (historical search) for C-FCGV reveals that it is a Beech model 65-B80 with serial number LD 408 and the owner was "Govt Canada Dept Of Transport".

  Introduced in 1966 the B80 was to be the final production model. The B80 was by far the longest produced Queen Air with production lasting some 12 years. Its major improvement was the increased gross weight to a 8,800 lb (4,000 kg). This gave the B80 a useful load of well over 3,000 lb (1,400 kg). Production ended in 1978.

It was removed from the register on 1988-02-03.


What is the correct holding altitude for the missed approach at Unalaska (PADU)? On this approach plate, the holding pattern shown is depicted for a missed approach:

However, in the notes, it says to

  Descend to 6000 in holding pattern.

even though you should only climb to 4700 feet, according to the missed approach procedure:

  Climb to 3000 via 166° bearing then climbing left turn to 4700 direct DUT NDB/DME and hold.

What does the note actually mean (especially since it seems to be implying you would be higher than 6000 on the missed approach procedure)?

That note actually refers to the holding pattern in general, not to the missed approach procedure. Your reading of the missed is correct.

  Descend to 6000 in holding pattern.

Refers to descent from the MSA or any other enroute altitude. Before transitioning from enroute to the outbound initial segment (departing the IAF) the pilot must descend in the hold to 6000. Once leaving DUT outbound, descend as specified to 4700.


Why are light aircraft engines so noisy? I live near an airport where students spend a lot of time flying circuits. During the base leg, pilots can be heard reducing engine revs prior to the final left/right turn to line up with the runway.

I've often wondered why in this day and age light aircraft engines are not made quieter; the technology seems to be stuck in the 1960's.

Can engines be made quieter or is it so prohibitively expensive/bureaucratic to get an engine through certification that aircraft builders just use off the shelf engines?

What's the quietest light pilot trainer out there (and don't say glider, thank you!)?

Engines could be made quieter by the simple solution of adding a muffler (or silencer as it's called in some places). They are available for many models and can be installed, at an astronomical cost. The parts and labor are actually pretty cheap, it's the certification costs that make it sting so badly. It costs a lot of money to develop and certify parts for airplanes. Mufflers aren't mandatory, and because they are so expensive to install most don't because the costs so outweigh the benefits - it costs far less to buy a decent active noise reduction headset instead.

Propellers are a big source of noise, and there are quieter designs for many aircraft, however they have to be certified with a particular engine and model of airplane and those costs add up.

The current certification rules are there to keep things safe, in some cases they can stifle innovation. It's why we still have carbureted engines with manual mixture controls instead of computerized fuel injection and turbonormalization.

As for what the quietest trainer is I don't think there's a definitive answer as I don't know of a source of light airplane noise measurements, and any airplane can be used as a trainer. Of the certified light airplanes out there that are commonly used for primary training the Diamond DA20 Katana is the quietest I can think of, that's because it is much lighter than the more common Cessnas and Pipers and therefore uses a smaller engine and propeller. Smaller engine and propeller generally means less noise. Turbo-diesel powered airplanes like the Diamond DA-40s are quieter because the turbo muffles the exhaust noise.


Is there a special kind of fuel needed to obtain high efficiency engines? Are there any special kinds of fuels that manufacturers are considering in order to make high performance, high efficiency jet engines?

Surprisingly, it isn't a big factor in GA or commercial aviation.

                            Specific  Energy
                            Energy    Density
                            MJ/Kg     MJ/L
Diesel / Fuel oil           48        35.8
LPG (inc Propane / Butane)  46.4      26
Jet fuel (Kerosene)         46        37.4
Gasoline (petrol)           46.4      34.2
HMX (Rocket Fuel)            5.7      10.8

Even high-performance long-range military aircraft like the U2 and SR-71 use some variant of Kerosene with additives, their fuels are optimized for high flash point, not for energy density or for specific energy.

Generally, availability and infrastructure trump efficiency.

Wikipedia: Energy Density
Aircraft Diesel Engine


what is the disadvantage of blended wing body ,that they not used to achieve more than subsonic speed? Blended wings have more efficiency than today's wing design,high lift to drag ratio,more efficiency,less fuel rate ,it have lot of advantages so why not they used for commercial airplanes as well as military aircraft rather than UAV.

One of the few production designs that approaches a blended-wing-body was Lockheed Sr-71:

Surprise, with maximum Mach 3.2 it is also the fastest production aircraft ever built.

Because blended-wing-body does not have better L/D at low speed. But generating part of lift with the body has some advantage for high-speed aircraft as it allows smaller wings that is better for high speed (and worse for low-speed).


What was 'Lufthansa Express'? I found this picture earlier with a Lufthansa express livery:

It also appears to have been on a few larger aircraft like the A300:

Could somebody enlighten me as to what this operation was or the difference to mainline Lufthansa? I tried to search but couldn't find anything.

I found this forum discussion from a few years ago, which stated:

LH Express was their brand name for domestic services for some years in the mid 90s. I'm not sure when the LH Express name disappeared, but they definitely were not promoting the Express name in 1997 any more. So the aircraft you saw was probably just not repainted yet. LH used to operate B737 of various series and several Airbuses.

One of the guys went on to say it was a low-cost domestic route, set up in competition to Deutsche BA. Unfortunately, it did not succeed enough in competition and was eventually shut down. They did fly some small Boeings like the 737, and the Airbus 300 and 310.
Additionally, a NY Times article from 1992 gives a small blurb about the startup of the line:

Under the new strategy, eight key domestic German routes are to be served by a special spin-off division, Lufthansa Express, that promises passengers a better on-time record, faster check-in and a chance at bargain prices.
...
Another innovation of Lufthansa Express will be bargain prices on late-night and short-notice flights. Passengers get a 55 percent discount savings on flights after 7:30 P.M., even without advance reservation, for example.
...
The most controversial aspect of Lufthansa Express is something passengers are unlikely to hear much about: The starting pay for a crew on Express flights will be about 25 percent less than on traditional Lufthansa flights.

Another thread here has a few minor other details. Interestingly enough, Amazon still has a listing for a model LH Express 737 (out of stock).


Do pilots keep light in the cockpit during long night flights? During long times of the cruise flight, is the internal light normally switched on or off inside the cockpit?
Knowing it is generally dark in the night would make a smartphone-based night time logger easy to write.

White lights are usually off in the cockpit in night flights to protect pilots' night vision. Red lights are sometimes used as these do not impact night vision.
One of the challenges you'll run into is knowing when it's night, as the start of night varies by latitude and the exact time. Phones generally set their time from the mobile network, which may be absent on long flights, especially those over water. A pilot flying a jet over the pacific may may have a phone that is hours off the local time.
Putting my (albeit old and tattered) mobile developer hat on I would suggest you look for ways the phone can tell you whether it thinks it is night or day and use that, plus a manual selection for day/night mode. Look at the Android UIModeManager as a way this could be done native.


Where does an FMS wind information comes from to calculate V approach? To calculate the V approach the FMS is using a wind either taken by the ADIRS or inserted on APPR PERF page. At first I will say that it takes the wind inserted by the crew, but I am not completed sure about this assumption since the MINI FUNCTION on A320 will need as well wind info taken by ADIRS to be able to cope with sudden loss of headwind / increase tailwind.

It uses the wind entered in the PERF APPR page to correct the computed VAPP:

Takeoff, performance and go around speeds
[...]
The FMGC uses the performance model and either the predicted landing weight or the
current gross weight at transition to the approach phase to compute approach speeds
(VLS, VAPP, F, S, Green Dot). On the PERF APPR page, the selected LDG CONF determines
the applicable VLS and VAPP, the latter being updated by the WIND correction that the pilot
enters on the same page.

(A320 FCOM - Autoflight - Flight Management - Performance, emphasis mine)


What has happened to make me experience negative G with the control stick FULL AFT near the top of a loop? I experienced negative G with the control stick FULL AFT near the top of a loop.

My questions:

Aerodynamically, what happened? Was the wing meeting the air at a negative-lift angle-of-attack (creating a skyward lift force) at this instant?
How can I change my technique to prevent this?

You were too slow.

Start the loop with more speed and pull more (if the g limit of your aircraft allows) at the beginning, so you have more speed at the top.

  Aerodynamically, what happened?

Without aerodynamics, you feel -1 g at the top of the loop, simply by having inverted the plane. Since the plane feels the same acceleration, both of you move together - until vertical speed picks up and drag will keep the airplane back. This is when you have the sensation of negative gs. In order to stay with positive gs, you need a centrifugal force to compensate for earth's gravity. This is the product of pitch rotation speed squared and the radius of the loop. If your flight speed drops, the radius will drop as well and leave little centrifugal force, even at a high rate of pitch. Dan Pichelman correctly describes how this kind of loop looks to an outside observer.

So there is little aerodynamics involved here besides drag - low speed will reduce all aerodynamic forces. The angle of attack on the wing is insignificant - what counts is the inertial force of the moving aircraft (besides gravity, of course).

If your airplane places you behind the center of gravity, pulling hard will give you the sensation of negative gs even when the aircraft as a whole is still at positive gs. Any maneuvering will add inertial effects which grow with the pilot's distance from the center of gravity.

  How can I change my technique to prevent this?

Fly faster, and be fast enough at the top to push in order to round out the loop, so it will look more like a circle. But make sure to stay within the limits of your plane. Again, Dan is right: It really sucks when the wings fold up.

Negative gs are completely normal at the top of the loop. When I did my first loop in an open-cockpit aircraft (a Boeing Stearman, no less), I did exactly that: Push near the top. However, this did not last long because my instructor immediately whined "pull, pull!". He had not closed the pockets of his flight jacket and started to lose their contents. So make sure that all equipment is secured before doing aerobatics.


Is it easier to take off against the wind or with it? I was wondering if it would be better to take off into the wind because then you would need less overall acceleration to takeoff, or with the wind behind you - thereby making it easier for the plane to fly when it's in the air?

Almost always into the wind. The ground roll will be substantially decreased because you've already got a certain amount of airflow over the wings.

Taking off with a tailwind is pretty much never advisable unless it's a one-way strip or runway, or the winds are below the tailwind limitations for that aircraft type.


Which altimeter setting should I use? In the Denver metropolitan area (where I fly), we have a large class B area around KDEN, with the associated approach control providing services to the surrounding airports (KAPA, KBJC, KFTG) as well.

Whenever I fly IFR and am in contact with Denver Approach, I am given the Denver altimeter instead of the altimeter of the closet airport which is usually (slightly) different.

I use the Denver setting while in contact with Denver Approach, since I've always assumed everyone else in contact with them are doing the same, and only switched to the destination altimeter after being handed off to the tower controller at my destination.  Is this considered the correct procedure?

Yes, that is correct procedure, except that if you're flying an instrument approach, switching to destination altimeter should be part of your approach setup "chart briefing" (which likely happens before you get handed off to the tower).

From AIM 7-2-2(a)(1) (emphasis mine):

  [Set the altimeter] to the current reported altimeter setting of a station along the route and within 100 NM of the aircraft, or if there is no station within this area, the current reported altimeter setting of an appropriate available station. When an aircraft is en route on an instrument flight plan, air traffic controllers will furnish this information to the pilot at least once while the aircraft is in the controllers area of jurisdiction. In the case of an aircraft not equipped with a radio, set to the elevation of the departure airport or use an appropriate altimeter setting available prior to departure.

Unfortunately the AIM text doesn't directly address destination altimeter settings, but section 4-1-8 alludes to the fact that controllers must verify you have the best destination altimeter setting, either by telling you or by having you report that you have the current ATIS or the AWOS/ASOS broadcast ("the numbers" for the latter):

  a. Numerous approach control facilities have established programs for arriving VFR aircraft to contact approach control for landing information. This information includes: wind, runway, and altimeter setting at the airport of intended landing. This information may be omitted if contained in the Automatic Terminal Information Service (ATIS) broadcast and the pilot states the appropriate ATIS code.

  b. Such information will be furnished upon initial contact with concerned approach control facility. The pilot will be requested to change to the tower frequency at a predetermined time or point, to receive further landing information.

For instrument approaches under IFR, setting and checking the altimeter to the destination's setting should be part of your approach briefing checklist. My personal "checklist" includes the altimeter setting as part of the weather information when I get to the bottom of the chart and check ATIS/ASOS/AWOS numbers against the approach minimums.

Interestly enough, the FAA's Instrument Flying Handbook doesn't have much to say on altimeter settings at all (except for mentioning that keeping the setting up to date is important and that bad things can happen if you don't).


What does it mean when there is no ceiling or "CLR" in an airport METAR? Here's the current METAR at KFLG:

  KFLG 210657Z AUTO 21010KT 3/4SM -SN BR VV010 M07/M09 A2961 RMK AO2
  SLP044 P0003 T10721094 410331178

It doesn't have anything about cloud layers or "CLR", but it shows what the vertical visibility is. Why doesn't it have information about the cloud layers?

Vertical Visibility with a number means that cloud layers cannot be determined because of haze, fog or mist, instead it's a measure of how far up the weather station can see. Because its sensors cannot penetrate further the station cannot tell you whether there's clouds above that level or not, but it does say in this case that there are no clouds below 1000 feet.

You may see VV/// sometimes, this means cloud cannot be measured. If fog, mist or haze and poor visibility is reported as well then it's likely that the fog is so thick the sensors cannot penetrate it, if on the other hand visibility is good with no fog or haze it's likely a sensor malfunction.


What's this part on the wing of BAE Systems Avro 146-RJ100? It was always a question that what's this part on the wing of BAE Systems Avro 146-RJ100:

Is it kinda aerodynamic component?

They appear to be "Stall Strips" and are used to trigger a "premature" flow separation of the section of area of wing affected, that is, stall that section, before the rest of the wing gets to its natural stalling angle of attack.
Usually you see them near the wing roots but for some reason BAE saw fit to put them a little farther outboard, possibly to mitigate an excessive pitching moment if the wing was stalled at the root (due to the wing sweep which would cause the overall center of lift to shift aft when lift is removed from the root), or, possibly to the get the turbulence induced by the tripped zone away from the horizontal tail where it could cause a fair amount of vibration (they do appear to be placed to put their wake just clear of the tail surfaces).
Stall strips are one of the most "painless" of engineering band-aid mods to cure some kind of unpleasant stall behaviour, or other low speed behaviour, found during testing. At normal AOA's, the strip is in the leading edge stagnation zone (the point where the flow splits to go above or below), so there is little to no drag penalty, and all you are giving up is a bit of the potential maximum lift the wing can make, since you are taking out part of the wing's lift a bit before it normally would.
Stall strips are very effective at transforming light aircraft with very bad natural stall behaviour (sudden, without or with little warning) into very docile machines that can be flown around will full aft elevator just bobbing and shaking, all because the flow was tripped prematurely at that spot and which partially unloaded the tail, preventing further increase in AOA.  Sometimes they are just used to create the turbulence itself that impinges on the tail, just enough to create vibration that can be felt through the stick.


Are helicopters expected to follow the lateral guidance of taxiways in a hover taxi or air taxi? If a helicopter is asked by ATC to hover taxi or air taxi, are they still expected to follow the lateral guidance of taxiways on the ground, or do they proceed "as the crow flies", direct to their destination?

Yes, unless given an explicit clearance to hover taxi direct to a location on an airfield.


How high do they cut the grass on airports? I've read that they try to keep it short. But on other sites and documentaries, it is said that they have to keep it long so that the birds don't look for food. Which is correct: short or long?

It depends on the type of bird and mammal species in native to the area and also the type of grass used in turfing; some of them may not survive if mowed very closely. FAA's Wildlife Hazard Management Manual concedes:

  Species-specific responses may be expected.  ...  Airport managers
  need to work with airport wildlife biologists to determine what species of concern in regards to aircraft safety are in their area and what the habitat needs are for those species.

For most airports, FAA recommends a height of 5-8" of grass.

  Considering bird, mammal and vegetation limitations, mowing at least monthly at a target of 5 - 8 inches may work in many airport environments as
  part of a wildlife hazard management program. ... Mowing at 5 - 8 inches should also reduce
  small mammal abundance.


How does the Boeing 787 Dreamliner's glide ratio compare to other airliners? I just saw a documentary on the design of the Boeing 787 "Dreamliner" and one of the designers said that the wings were inspired by gliders. Looking at the production version of the plane, it is obvious that the wings are indeed more glider-like than other airliners.

How does that affect the glide ratio? I can't find any data on this (is there any data on that at all?)

As discussed here, glide ratio is related to the L/D (lift-to-drag) ratio of the plane. When talking about commercial aircraft, "glide ratio" is typically used to mean the ratio with engines inoperative. If the jet engines aren't running, they add a lot of drag, which will decrease the L/D ratio from the typical max value.

This analysis estimates the max L/D of the 787 at 20.8, while this site estimates 21.

Other sources estimate L/D of the A330 and A340 at 20. The 747 is estimated at about 18. So based on these estimates, the 787 is slightly better than other large airliners.

Most of the efficiency gains of the 787 are from the lower weight of composite materials (which allow bigger wings) and more efficient engines.


How do we use the runway condition assessment matrix? Starting on October 1st, 2016 the FAA is moving away from providing Mu values for runway friction reports to a new format showing contamination type and depth for each third of the runway.  Along with the report is a code X/X/X showing runway condition codes for each third of the runway.

How do we, as pilots, use the field condition NOTAMs in calculating takeoff and landing performance?

From the FAA Improves Runway Conditions Reporting article:

  The pilot or dispatcher would then consult the aircraft manufacturer data to determine what kind of stopping performance to expect from the specific airplane they are operating.

The important bit:

  The FAA is advising operators to develop procedures for pilots and dispatchers that address the changes to runway condition reporting procedures.

Here are the corresponding braking coefficients:

From: Takeoff Performance Data for Operations on Contaminated Runways, aimed at manufacturers.


why lift is greater at wing root Is lift greater at the wing root of an horizontal wing,just like it's greater on a tapered wing?

Is lift greater at the wing root of an horizontal wing,just like it's greater on a tapered wing

Well this is sort of definitional.

Generally speaking the idea of any sort of taper or shaping of the wing like the examples you posted above is to deliberately reduce the lift at the tips in order to reduce induced drag. In that case, no, the rectangular wing would be more even across the span - although there's all sorts of other effects that would likely come into play that would adjust that to some degree.

But really what you're asking about is the lift per span. If you really mean "is there more lift at the root" then the answer is "absolutely every time" for the simple reason that the lift force at the root is the lift force of the entire wing, whereas at the tip it's the force of only the tip. That's why the spars get thicker as you move in toward the root.


What happens when auto-pilot fails over the ocean? Let’s assume the current location puts the aircraft 2 hours from the nearest airport, but the destination is 2 1/2 hours away.

Would the pilots hand fly the rest of the way?
Are they allowed to fly to their destination, although it’s just an additional 30 minutes?
And how difficult would that be (For the sake of the conversation, we can say I’m specifically asking about the 787, but I am generally interested in long distance capable aircraft)? Or is there tons of ways to fix the auto pilot?

In short, what I’m asking is: how difficult it may be for two pilots to manually fly for this long, is it a big enough deal to cut the flight short 30 mins to land elsewhere?

I'm not sure under what rules, and not sure how the auto-pilot fails either.

As @Bianfable points out in the comments RVSM airspace is a problem but ignoring that for a minute:

The question of will they continue? depends on the airlines op-specs. Different airlines will handle this differently and it may very well be handled different on different airframes within an airline. A call to base may even occur but there are lots of factors that can play into it such as what other equipment is operable.

Assuming the Op-Specs dictate that the flight continues

  how difficult it may be for two pilots to manually fly for this long

Potentially not too difficult at all, most pilots trained in aircraft with no autopilot and all pilots are qualified to hand fly the aircraft. However after Op-Specs it is ultimately up to the pilot and they will consider many factors. Hand flying an aircraft in IMC can be a tiring task for even the best of pilots, a pilot may chose to divert or land sooner if they feel it would be unsafe to continue even if they are capable of flying the aircraft by hand.


What is this aircraft on the grass in Budapest International Airport? While taxiing after landing in BUD recently, I noticed a widebody aircraft half-hidden in the trees on the grass of the airport. You can see it on Google Maps over here.

What is this aircraft, and why is it positioned like that? There’s the fire brigade building nearby, but they already have a training aircraft a lot nearer.

More 3D views:

It has the distinct shape of the 767, and it is indeed a 767 (registration HA-LHB, ex Malev) as can be seen in these photos:

https://www.airliners.net/photo/Malev-Hungarian-Airlines/Boeing-767-27G-ER/4847571
https://www.airliners.net/photo/Untitled-Malev-Hungarian-Airlines/Boeing-767-27G-ER/4850697
https://www.airliners.net/photo/Untitled-Malev-Hungarian-Airlines/Boeing-767-27G-ER/4808155

By the state of the airframe (and the fact that it is illuminated) it is indeed looks like it is being used as a firefighting/rescue training frame.


Which commercial aircraft can stay in the air the longest? Which commercial aircraft, when fully loaded with fuel (and passengers and luggage up to its max takeoff weight), could stay in the air the longest without landing, and how long could it stay in the air? What conditions would it need to have maximum possible endurance if the plane had no intended destination to land or particular flight path? (This is for a fictional story.)

Note: I was thinking commercial jetliners when I wrote that, and I still am, but I was accidentally unclear about it. That's preferably what I'm looking for, though.

A commercial airship, like the 'Zeppelin NT' can stay airborne for 24 hours.

The record (for a military US-Navy blimp in the 1950s) is 264 hours, 12 min. It covered more than 13,000 km.

  Record flight of 8,216 miles in 264 hours. Lakehurst NJ to west Africa to Key West FL non-stop. (Source)


Where is the weather data for the Garmin G1000 retrieved from? In a few videos I saw the possibility of looking for example at wind data. Now my question would be where does the G1000 gets the data from?

That is coming from a satellite based XM weather service which provides a variety of weather products on demand to pilots using a G1000 system.


Why does this Strikemaster have a textured leading edge? I was at Duxford recently, and spotted a Jet Provost Strikemaster that had a rough texture applied to a portion of the leading edge of the wing.

Why was this applied?

The leading edges are roughened to improve the stall characteristics. Typically vortex generators are used for this purpose nowadays.

  The 5 and the 5a were used by the RAF for pilot training and the nose strakes were fitted to improve the spinning characteristics - the leading edges of the wings had a roughened paint finish to improve the stall characteristics. The 5s with the tip tanks (I'm not sure if 5b is an official or 'unofficial' designation) were used by the RAF for nav training and didn't have the nose strakes (not sure about the roughened wing leading edges) I don't know if it was because the tip tanks themselves improved spin handling or if it was because they weren't flown by studes for spin training.

(source: User "Fritag" at www.britmodeller.com)

  The rough grey coating on the wing of the aircraft was applied in order to break up the smooth airflow and give an early indication of the onset of a stall as the T5's original clean wing design gave the pilot little prior warning.

source: all-aero.com / RAF Museum

  A great deal of work has been put into stalling, spinning and
  stall-warning over the years of Jet Provost and Strikemaster
  development. This has led to all sorts of devices, like
  inverted-aerofoil tailplane, fences under intakes, strakes round the
  nose, roughened outboard leading-edges and little "eyebrow slats" at
  the wing roots. Of all these devices, the Strikemaster requires only
  the "eyebrow slats" to give it that precious smooth spin, a tidy g
  break in the stall and a distinct stall warning.

source: Flight International 12 October 1972 500-501

  At first it was forecast that the T.5 would take the place of the T.4
  on certain roles such as high altitude training. Unfortunately the
  majority of T.4s had to be taken out of RAF service much earlier than
  anticipated so the T.5 had to take on more responsibilities. As a
  result, the RAF decided to update its fleet of Jet Provosts with new
  avionics suites, spin strakes, and roughened leading wing edges in
  1973.

source: Jet provost heaven


Why can't lifting line model be applied to yawed/skewed/oblique straight wing (low M) If all the horseshoe vortices have skewed central bound vortices on the same skewed 1/4 chord line, then the trailing vortices from the downwind half span  of the wing induce less on the windward halfspan and vice versa. So then the windward halfspan of the wing will have increased angle of attack and lift vs unskewed and the leeward have less.
But I gather this is the wrong result vs experiment? Please don't confuse the issue with symmetrical swept back wings. No, I  used Lifting Line to generate results that disagree with experiments for asymmetrical oblique or yawed straight wings? What am I overlooking ?

Lifting Line analysis is not suitable for low aspect ratio wing, highly swept wing or a wing undergoing large sideslip, because each of these would have significant spanwise flow gradient.
To see why, it's worth revisiting the central equation of the Lifting Line, applicable to a planar wing:
$$c_l(y)=a_0 \left[ \alpha - \frac{w(y)}{V_\infty} - \alpha_{L=0} + \theta_t(y) \right]= \frac{2}{V^2_\infty c(y)} \lVert \vec{V}_\infty \times \vec{\Gamma}(y) \rVert$$
where $y$ is the spanwise coordinate of the section, $c_l$ is the 2D airfoil lift coefficient, $a_0$ is the 2D airfoil lift slope, $\alpha$ is the free-stream angle of attack against the wing, $w$ is the downwash generated by the trailing vortices1, $\alpha_{L=0}$ is the 2D airfoil zero-lift angle of attack (or aerodynamic twist), $\theta_t$ is the geometric twist of the section, $c$ is the section chord length, $\vec{V}_\infty$ and $V_\infty$ are the vector and magnitude of the free-stream air velocity against the wing, and $\vec{\Gamma}$ is the bound vortex strength at the section2.
The left hand side of the central equation is the 2D airfoil lift and the right hand side is the lift we expect from the circulation generated by the bound vortex. Assuming the 2D characteristics are known in advance (i.e. the 2D lift slope, zero lift AOA, etc), and the wing geometry is known (chord length at every section), then we should be able to calculate the unknown, $\Gamma$, at every section.
To put it crudely, Lifting Line is cheating: it relies on you knowing the 2D airfoil characteristics apriori, in order to calculate the 3D lift and drag (which is its biggest advantage). The downside is that it inherently assumes the 2D data tells the whole story; all influences from the neighbouring sections are lumped into a single downwash (induced AOA), which remains 2D. In a highly swept or sideslip'ed wing, this assumption breaks down.

1 $w$ can be calculated using Biot-Savart from the trailing vortices (which have constant strength, $d{\Gamma}/dy$, along the filament), but will need to be slightly modified from the typical result shown for the Classical Lifting Line (which is only applicable for straight tapered wing), since the trailing legs are parallel to the freestream, not the bound vortex.
2 Note that I've used the vectorized version of the Kutta-Joukowski Theorem since in a swept or sideslip'ed wing the bound vortex and freestream aren't perpendicular.


How much force does a helicopter's tail rotor counteract? How much force does a helicopters tail rotor counteract?

What are the formulas for determining this? How much rotational torque is produced by spinning the blades?

The lift-to-drag ratio of the main rotor blades is around 10, so the tail roter has to provide a force equal to one tenth of the aircraft's weight.

Of course you have to take into consideration torques and lever arms, but if we assume that most of the lift and drag is created by the outer portion of the main rotor and that the tail rotor's lever arm is approximately the same, we can simply ignore them. On most helicopters the tail boom is a bit longer than the radius of the main rotor, so the force might be 20-50% lower. On the other hand, the tail rotor is often less efficient, especially on some newer models with small, ducted fans, so the tail rotor might consume a bit more than 10% of the power.


Why do the RNAV (GPS) approaches to KAPC not use the same missed approach procedure as the ILS/LOC approach? While looking at the various instrument approaches for Napa Valley (KAPC), I noticed a discrepancy between the RNAV (GPS) Y/Z RWY36L missed approaches and the ILS/LOC RWY36L approach:

Why is it that the ILS approach is able to use a turning missed approach to keep pilot and plane clear of the high ground to the north, but the RNAV approaches to the same runway force you to either have visual contact with the high ground early, or outclimb it?  To me, it'd make more sense to have the RNAV procedures use a similar missed approach to the ILS, as then you could have an all-aircraft RNAV procedure with low minima, vs. one with high minima that works for everyone and one with low minima that requires a significant (more than twice the norm) missed approach climb rate.

As best I can understand from the TERPS, it's because there are certain criteria for RNAV missed approaches that wouldn't be met by copying the ILS missed approach.

Specifically, I found the following (see chapters 7 and 15):

In RNAV missed approaches "turns shall not exceed 120°", but the ILS missed approach requires about a 180° turn. There's some related information here about the FAA testing turns greater than 120° in RNAV instrument procedures, it looks like some avionics can't handle them properly.
RNAV missed approaches are made up of legs between waypoints, and the first leg after the MAP has a required minimum length "to allow the aircraft's stabilization on course immediately after the MAP". Unfortunately I couldn't identify what that length is, because the FAA's PDF quality is abysmal and some information is unreadable, but the ILS missed approach requires a turn after a climb of less than 300' (from 214' to 500'), so the distance traveled in that time would presumably be quite small. If the RNAV minimum leg length is significantly greater than that distance, then it might be necessary to continue straight ahead, as both RNAV approaches do.

I'm certainly not a TERPS expert and there may well be other reasons for not using the ILS missed approach, but those two points seem to show that the criteria for designing missed approaches is different enough between procedure types that you can't just 're-use' them freely.


How are "Owner-Produced Parts" handled under FAA regulations? FAR 21.9 provides the ability to install parts that were "Produced by an owner or operator for maintaining or altering that owner or operator's product". This seems like a pretty broad scope of authority for aircraft owners.

Under what circumstances are owner-produced parts typically used, and what requirements are imposed on the part (and the owner producing it)?

The July/August 2002 edition of FAAviation News (now FAA Safety Briefing) has a detailed article on owner-produced parts, including advice on best practices if you want to make your own parts. It does confirm that owners have very broad authority to do so, provided that they document everything properly.
Typical reasons for producing your own parts are (I've paraphrased them from the article here):

The aircraft has been out of production for years
The aircraft is an orphan without a Type Certificate
There is no technical support because the aircraft is so old
There may be long delays in getting parts from the manufacturer
The price of official parts is extremely high

As for requirements of the part itself, the article list four characteristics that a part must have:

The part must be properly designed
The part must be produced to conform to the design
The part's production must be properly documented
The part must be properly maintained

The owner must be involved in the production but doesn't actually have to do the manufacturing himself:

The aircraft owner must participate in the manufacture of the part
in at least one of five ways for it to be considered an Owner Produced
Part.

The owner provides the manufacturer of the part with the design or performance data.
The owner provides the manufacturer of the part with the materials.
The owner provides the manufacturer with fabrication processes or assembly methods.
The owner provides the manufacturer of the part with quality control procedures.
The owner personally supervises the manufacture of the new part.

More recently, in the September 2014 edition of the Cessna Pilots Association magazine, Mike Busch has an article on replacement parts that calls the current regulations on owner produced parts "extremely liberal":

“Owner-produced parts” do not require a  PMA. These are parts produced
by an aircraft  owner for use on his own aircraft, and not  offered
for sale. There is an FAA letter of  interpretation that sets forth
precisely what  an owner's involvement must be to qualify a  part as
“owner-produced,” and it is extremely  liberal. The owner need not
actually  manufacture the part himself, so long as  he provides
specifications or materials  or supervision or some other meaningful
participation in the part's production.

Unfortunately, he didn't mention exactly which letter of interpretation covers these parts but the FAA magazine article refers to an "FAA Memorandum dated
August 5, 1993" and I guess it's probably the same one. I wasn't able to find it on the FAA site.


Tail Wing Dented—Is it safe to fly? I’m sitting in the lounge waiting to board this flight. Saw a dent in the attachment of tail wing to the body. Is this common?

Also I could be wrong and it is part of the design of the airplane.

Any guidance is appreciated!

The aircraft in question is an Embraer ERJ 175 the "dent" allows the tail trim to operate properly you can see it more clearly in this picture.

(source)


Is a clearance needed to depart a helipad in class B airspace? I have noticed statements like the following on sectionals:

This would seem to suggest that a clearance is needed to "depart" any helipad, even if you are staying under the wedding cake. Is that true?

The "Wedding Cake" literally defines the airspace, so for a the sectional part you listed above, the floor of the Class-B is at 3000 feet in the outer ring and 2000 feet in the inner ring, these heights are MSL.

Anything outside of the wedding cake is not Class B airspace. You may be confusing the Class B airspace with the Mode-C Veil that surrounds 30nm outside of most Class B airports, which only requires that you have an operating transponder, you are still allowed to fly around VFR squawking 1200 below the Class-B without any clearance.

The airspace under the wedding cake, provided you are not within a Class-E airport, is most likely Class-G. So no, you are not required to contact Tower until you want to enter Class-B airspace or if you need a departure clearance for IFR operations.


How do I track my airplane? I am a student of a flying school which has 10 Cessna-152's. Since all the aircraft are not equipped with a transponder, there is no way to track them. I am searching for a way to track them like Flightradar24 does with the maps. This is a project assigned for a bunch of students, so we are looking for an inexpensive but still good quality system.

You could try to get some cheap Car GPS trackers on aliExpress. But they would need to be in range of cell towers. Flying to high will cause connection loss.

A more expensive one, which I found to be working great is this one.

But you can get android based trackers for about 10\$US.


How can you calculate the vertical speed of an helicopter during auto-rotation? How can I calculate velocity of a helicopter when it falls with autorotation when the engine fails, given all the parameters such as mass , height , wing size etc.

It would depend on the pilot's inputs, since autorotation is still controlled flight and the pilot could choose to descend faster or slower. Depending on the helicopter, there may be a trade off between the amount of time it takes to descend and the distance covered. For example, the sink rate of the best ratio of forward distance covered to vertical distance covered (equivalent to glide ratio in a fixed wing aircraft--the engine failed over water and the pilot wants to go as far as possible toward land) could be different than the minimum possible sink rate (the engine failed over a safe place to land and the pilot wants to buy time while trying to restart the engine).

The most important quantity that affects minimum sink rate in either case is disk loading, which is the equivalent of wing loading in fixed wing aircraft. Page 4 of this pdf (heading: "Basic Physics of Autorotation") has the information you need to calculate minimum sink rate during autorotation based on an assortment of parameters.


How do we find the breaking point of an old braced wing? I was curious about how to find the maximum g's an old plane like the Spitfire (c-wing) could endure, given all realistic factors. Since the Spitfire's wing, in particular, was quite thin and braced with wood for most of its variants how much positive and negative force could it withstand?

Not sure what you mean by "old braced wing", the Spitfires were full cantilever wings from the first prototype 300. From this link:

This link references the original Spitfire specification (without any further references to them):

5 Structural Strength
(a) The strength of the main structure when carrying the load specified in paragraph 3, plus 100 lb shall not be less than as specified hereunder ——
Load factor throughout the structure with the centre of pressure in its most forward position 9.0.
Load factor for wing structure with the centre of pressure in its most backward position in horizontal flight 6.0
Load factor in terminal nose dive 1.75
Inverted Flight
(1) Load factor at incidence corresponding to the inverted stall and with C.P. at 1/3 of chord 4.5
(2) Load factor at incidence appropriate to steady horizontal inverted flight and at the maximum speed of horizontal normal flight 4.5

Most WWII planes still flying are not stretched to the limit regarding g's, to keep exceed the fatigue life. I doubt any current owner would like to test what load the wings are up to at present.


Why are military planes painted differently than civil planes? I read Why are planes generally painted white?, and got interested in knowing, why don't military aircraft follow this? Rather than the advantageous white, most of the military aircraft are rather painted pretty darker, green or black or some other color. Why is this done?
Probably, it is not for distinguishing military with civilian aircraft .Becuase, the RADAR knows which aircraft is what.

This wiki article speaks to this somewhat. In modern-day air forces, the paint job will depend on the mission the aircraft is used for. However, with the improvement of radar, visual camouflage is less important than it used to be.

Blacked-out aircraft

Some aircraft are paint black or nearly black for use in night missions. The F-117 (shown above) was painted with a radar-absorbent paint and also primarily flew at night, lessening its probability of detection. Other aircraft, like the special operations MC-130 and AC-130 are painted very dark grays or blacks to lessen their probability of detection since they are primarily used at night in support of covert missions, often at low level where visual detection is the main threat. Modern Examples: B-2, MC-130

Ground camoflauge

Some aircraft are painted to blend in to the ground. Some of these are those designed for low-level attack roles (like the Tornado pictured above), to limit the chance of visual detection by enemy fighters from above since their radar signature is unlikely to be detected so low to the ground. Others, like the now-retired Saab 37 Viggen, may be camouflaged to prevent detection when parked on the ground, as many were dispersed around Sweden, sometimes hidden in forests, to prevent single attacks from taking out many defenses. Many military helicopters are also painted either a patterned or dark green color to blend into forests. Some aircraft have been painted tan to blend into desert environments. Modern Examples: Su-25, Super Tucano

Air camouflage

Many air superiority fighters, and a large group of combat aircraft in general are painted in some way to blend into the sky. I have read that the human eye has difficulty picking a gray plane out against the blue sky (or grayish clouds), and most USAF and NATO planes are painted in some form of "Air Superiority Gray". Additionally, many will have different-colored tops and bottoms to help blend in from above and below. This excellent aviation.se Q&A talks about false cockpits on military jets that contribute to better dogfighting performance. Some aircraft, like the Su-27, may have several different grays and whites painted on it to break the aircraft's shape up. Modern examples: F-22, MIG-29

There may be other aircraft that have unique paint jobs to assist in a specialized mission - a darker color on one area to help with sensors, special coatings to minimize damage from a nuclear flash. Overall, it is not as common as it used to be for aircraft to be green - most US Military aircraft were green in color during the Vietnam War, but as the distance of aerial combat has increased, so have the camouflage schemes. Grays are probably the most common paint jobs now among military aircraft of many countries, with other aircraft having specialized paint jobs for specialized roles.


Is lift in fact a kind of drag? Given the similarities between Lift and drag and their very similar relationships to the density, surface area, and airflow velocity, could one, in theory, describe aerodynamic lift as a kind of drag where most of the force from lift is just directed in the vertical direction where this vertical component of the force opposes gravity?

An 'aerodynamic force' (just one force...) appears when a body is immersed in a fluid stream. By convention, two components are chosen, one of them parallel to the stream direction, called 'drag', and the other one, perpendicular to that 'drag' is termed 'lift'.


Eviation Alice Powerplant Placement I saw an article from the BBC about the Eviation Alice this morning that left me puzzled. The design is a tail wheel tri-motor with the power plants located in a pusher configuration at the wing tips and tail.

The article describes the aircraft thusly:

  "Alice is an unconventional-looking craft: powered by three
  rear-facing pusher-propellers, one in the tail and two
  counter-rotating props at the wingtips to counter the effects of
  drag."

I'm wondering about what drives the configuration of the Alice? What compelling benefits does the configuration offer to balance the challenges it poses:

Controllability in the case of the failure of one of the wing tip power plants - even with the ability to fly (and presumably continue a worst case take off) on only the rear motor - why wouldn't you move the wing mounted engines inboard and have even better "engine" out performance?
Ground clearance and crosswind problems for the tail gear.
Designing a light and efficient wing with tip mounted power plants.

Obviously Eviation thinks that their configuration makes sense, has anybody heard or seen their arguments?

According to a recent article from the travel blog The Points Guy (whose reporters were at the Paris Air Show and toured the aircraft,)

  [W]hile it has three motors, the aircraft can continue to fly with just the rear motor engaged. If one wing-mounted motor fails, the other will be disabled to maintain stability, and additional electric power can be diverted to the rear motor so the plane can continue its flight.


Is the Reynold's number a function of the fluid flow stream or the obstruction? I am slightly confused with this notion of the Reynolds Number. Specifically, I am looking at airfoil performance at very low Reynolds number.

I am reading a lot of papers saying things like because it is a small model aircraft, the Reynolds number will be small (so then it is to do with the shape of the obstruction in the fluid flow?). But then I see something like large turbine performance at low speeds involve low Reynolds numbers (from here I would think it is to do with the 'quality' of the fluid flow itself, whether it is laminar or turbulent, uniform etc).

If it has something to do with the fluid flow, then why do I see Reynolds numbers attributed to specific airfoils etc? Say that an airfoil has $Re=100000$, what does this mean then? If the fluid is flowing at a very low speed with high viscosity, then how can we make use of this number at all?

Do correct me if I'm wrong, I am only beginning to learn these things!

Maybe it helps to see the Reynolds number as the ratio of inertial to viscous forces. High flow speeds mean high inertial forces, and so lead to high Reynolds numbers. Viscous effects are strongest when the airflow hits the obstacle, and become less strong the longer the air flows around a body, so a short flow path has a low Reynolds number.

In the end it does not matter if the length of the flow or its speed drive the Reynolds number up. It is the combination of both. I am neglecting the gas properties here, because I assume we talk about air at moderate temperature and do not want to make the issue more complex.

Airfoils have no Reynolds number per se, flow conditions have. The Reynolds number of a measurement can be doubled by making the chord of the airfoil twice as long at the same airspeed, or by doubling the airspeed at the same chord. The wing of an airplane operates in a range of Reynolds numbers, depending on flight speed. That is why polar diagrams normally contain the results of several test runs rsp. computations, each at a different Reynolds number.

Sometimes you see airfoils which are optimized for a particular Reynolds number. To avoid early separation, the recompression area of an airfoil must become proportionally longer the smaller the Reynolds number is at which the airfoil is supposed to function well. At higher Reynolds numbers such an airfoil will still work, but will produce more drag than one which is optimized for a higher Reynolds number. The Reynolds number allows the aircraft designer to pick an airfoil that works well at the combination of speed and chord length for which that airplane is designed.


How does ATC direct aircraft on ground from the parking to the runway? For medium size airports, ATC will direct aircraft with a single message like this "Taxi to runway 29 via Papa, Sierra".

I was wondering if it was the same thing for bigger airports (like international airports) which have a lot of taxiways.

I know that if the pilots are not very familiar with the airport, they can ask for controller guidance. But does the ATC can give all the taxiways points like for medium size airports?

Thank you

Yes, it's the same. An airport consists of one or several runways, one or several parking areas (aprons) and a number of taxiways connecting them. All runways and taxiways are uniquely named.

When taxiing around the airport - be it from runway to parking, or vice versa, or between parking areas, hangars etc. - ATC instructs the pilot of the exact route to follow by specifying the taxiway names in order. ATC can also instruct the pilot to stop and wait at a certain position, or to follow or give way to specific other aircraft. Some airports, in addition to named taxiways, have named holding points around the airport which can be used to specify where the aircraft should stop and await further instructions.

Pilots who are not familiar with the airport will have no problem following instructions, since they all have a ground chart (essentially a street map) of the airport layout, with all the taxiway and runway names.

Example of aerodrome chart - Copenhagen Airport, reproduced from AIP Denmark

Some modern airliners even have a moving map which shows the aircraft GPS position directly on a digital airport map in the cockpit. In addition, signs are posted at all taxiway and runway intersections, showing the name of the current and any adjacent taxiways and runways.

Taxiway signs

In extreme cases, ATC can make use of turn-by-turn instructions (progressive taxi), but that is very rarely needed. Follow-me/marshall vehicles are also available, but those are usually not to help the pilot find the way, but rather to provide an extra set of eyes when taxiing very big aircraft, to ensure they stay on the pavement during a narrow turn/tight gap etc.


Why do some fighter jets have two pilots while others have only one? Why do some fighter jets have two pilots, while some have only one?

For example the newer fighter jets such as Dassault Rafale, Eurofighter typhoon etc. are mostly with single-seat, but two-seat models of these jets are also available.

All fighter aircraft have one pilot, though some have two crew members.

The backseat crew member is not a pilot, but often either a Radar Intercept Officer (US Navy), Navigator/Weapon Systems Officer (USAF, IDF), or Flight instructor.

Though not fighter aircraft, I believe that the SR-71 backseat crewmember operated the reconnaissance equipment as the single crewmember on the U2 was often overloaded with keeping the thing in the coffin corner and operating the reconnaissance equipment. Jan and Habu mention in the comments that the SR-71 backseat crew member was also the navigator and operated the ECM equipment. Additional comments mention that the B-1B consists of four crew members: two pilots, an OSO (offensive systems operator), and a DSO (defensive systems operator). The B2  has two crew members, both pilots, and while one is flying the other can rest, navigate, or operate the weapons system. Thanks to Jan, Habu, and Joel for comments and clarifications!


Is head-on or trailing wind better? Does a powered aircraft fly faster when in a head wind or with a trailing wind? The question revolves around the head wind should provide 'better' lift, and trailing winds have very little to 'push' against.

A tailwind provides a faster groundspeed.

The aircraft moves through the air so whether the ground is moving relative to the mass of air has no relevance to the aerodynamics of an aircraft at cruising altitude.

  a British Airways passenger jet approached supersonic speed this week as it rode a surging jet stream from New York to London.

from The Telegraph, 10 Jan 2015

They mean groundspeed - so the term supersonic is moot.

  The Boeing 777-200 jet reached a ground speed of 745mph as it rode winds of more than 200mph across the Atlantic. At ground level, the speed of sound is 761mph.

Aircraft often use the predominantly westerly† jetstream when flying from the USA to Europe, From Japan to California or across the USA. Flying the other way they pick an altitude to avoid it.
 Wikipedia

The Jetstream is a high speed, high altitude, wind that blows moderately reliably in one direction.

 Metcheck

† Westerly means "blowing from the west" - towards the east.