How is take-off power defined? In a recent post it could be read that by definition, turbofans do not produce power in the static case (at velocity zero). The statement could be a bit confusing since turbofans and propeller engines are at their highest power setting just before the take-off roll commences.

What is the referenced definition that results in zero power for turbofans at zero speed, regardless of the throttle setting?

You're asking about this answer by Peter Kämpf, in which he writes (emphasis added):

  Now for turbofans. Here we have thrust which needs to be converted into power first by multiplying it with flight speed. It would be nonsensical to compare the static case – here by definition turbofans do not produce power.

You ask, "What is the referenced definition that results in zero power for turbofans at zero speed, regardless of the throttle setting?" The referenced definition is the sentence in bold above. Peter is defining power as thrust multiplied by flight speed.

Now, let's back up a little bit. Why would Peter define power like that?

For propeller engines (and car engines), defining the "output power" of an engine is easy: it's the power being transmitted through the drive shaft (which is the product of the torque the engine is exerting on the drive shaft, and its rate of rotation). I'm going to call this definition "shaft power."

A jet engine, however, doesn't have a drive shaft, so we are unable to use this definition. We're forced to choose a different definition of power, and so Peter chooses to use the product of thrust and flight speed. I'm going to call this definition "ultimate power". (This generalizes to any vehicle; we can define "ultimate power" as the forward force on the vehicle produced by its engine, times the vehicle's forward speed. Equivalently, "ultimate power" is the rate at which the engine adds kinetic energy to the vehicle.)

However, Peter also points that power as defined this way (ultimate power) can't be directly compared with the other definition (shaft power), at least not when the vehicle is stationary. This is because the "ultimate power" for a stationary vehicle is always zero, whereas the "shaft power" for a stationary vehicle is generally much greater than zero.

It may seem counterintuitive that the "ultimate power" for a stationary vehicle is always zero, but it is. Kinetic energy is proportional to the square of speed, which means that if an object is traveling extremely slowly (as in the first few nanoseconds after it starts moving), it gains kinetic energy at an extremely slow rate (on the order of nanowatts).


Does it make sense towing airplanes to the head of airstrip by external (eg electric) means? Does it make sense towing airplanes to the head of airstrip by electric means, whether internal or external?

Taxiing from the gate to the runway on jet power seems like a waste of fuel. Is it possible and it would make sense to tow the airplane to the start of take-off run by a motorized towing unit or perhaps with electric motors in the airplane's landing gear? Was this ever tested or assessed?

Was [towing unit or motors in landing gear] ever tested or assessed?

Yes.
2014 Main Gear

Using the Auxiliary Power Unit (APU) generator to power motors on the main wheels, the EGTS taxiing system allows aircraft to push back without a tug and then taxi without requiring the use of the main aircraft engines. One wheel on each main gear is equipped with an electric motor to drive the aircraft

From EGTS

Airbus signs MoU with Honeywell and Safran to develop electric taxiing solution for the A320 Family

From Airbus press release

Lufthansa Technik has proven with a technology demonstrator that aircraft can be driven on ground by electric motors.

From Lufthansa Technik (with video)

2015 Towing Unit

As of 2015 Taxibot, a semi-robotic towbarless tractor which meets and connects to aircraft, is the only alternative taxiing system certified and currently in use by airlines in the market.

From Wikipedia

2016 Nose Gear

Honeywell and Safran have pulled the plug on their jointly conceived electric taxiing system, while rival developer WheelTug aims to enter service with its system in 2018.
"After thorough market analysis", the US supplier says, the two partners "agreed to stop work on the Electric Green Taxiing System due to dramatically lower oil prices and the current aviation industry's economic environment".
EGTS had been scheduled to enter service this year, with Honeywell and Safran having planned to produce a system for line- and retrofit on A320-family and Boeing 737 jets.
Meanwhile, Gibraltar-headquartered WheelTug plans to enter service with its nose gear-mounted electric drive system in 2018.
... initially on 737s and, at a later stage, on A320-family ...
Neither Airbus nor Boeing are supporting the programme

From FlightGlobal


What does the crosshatching mean on this VOR-DME? I noticed that the frequency of this VOR-DME is covered by crosshatching.

The FAA chart guide says this "indicates shutdown status."

What does this mean? Is this navaid being decommissioned?

You're right. From FAA Aeronautical Chart User’s Guide,

  NAVAIDS in a shutdown status have the frequency and channel number  crosshatched. Use of the NAVAID status “shutdown” is only used when a facility has been decommissioned but cannot be published as such because of pending airspace actions.

Image from FAA Aeronautical Chart User’s Guide, 12th Ed.

FAA is in the process of reducing the VOR based navigation network to minimum levels so as to provide only an alternate means of navigation in case of Global Navigation Satellite System (GNSS) outage. This VOR Minimum Operational Network (MON) is expected to be in place by the year 2020, which will permit non-GNSS guidance to an airport within 100 nautical miles (nm) that has an ILS or VOR approach procedure.


What was the lift equation used by the Wright brothers? NOTICE: The modern lift equation and the lift equation used by the
  Wright brothers in 1900 are slightly different. The lift coefficient
  of the modern equation is referenced to the dynamic pressure of the
  flow, while the lift coefficient of the earlier times was referenced
  to the drag of an equivalent flat plate. So the value of these two
  coefficients would be different even for the same wing and the same
  set of flow conditions.

Above is a quote from a NASA website (emphasis mine). It's new for me that the Wright brothers applied different calculations to their plane. So, what was the equation used by the two brothers?

The Wright brothers' equation is
$$L=kV^2AC_L.$$
where $C_L$ is the ratio of the $L/D$ of a specific wing (AoA, span, chord, camber, thickness all set and fixed) to a perpendicular board into the wind of the same $A$, $k$ is experimentally obtained, and is specific only to the medium (air or water).

In a sense, the Wright's lift equation is seeing lift from a momentum transfer perspective, i.e. the $kV^2$ term corresponds to the momentum flux of the incoming air, and the term $AC_L$ describes how much downward discharge is produced at the price of how much drag.

While the modern lift equation is
$$L=\frac{1}{2}\rho V^2AC_L.$$
This is derived from the Kutta-Joukowski theorem:
$$L=\rho V\Gamma,$$
where $\Gamma=\int_C{\bf v}\,\mathrm{d}{\bf s}$ is a line integral of the second type around a close and simple route around the wing. $\Gamma$ is proportional to $V$. And $C_L$ is defined as
$$C_L=\frac{L}{\frac{1}{2}\rho V^2A}.$$ $C_L$ is not calculated through analytics, but derived through $A,\rho,V$ as obtained in experiments.


Aerospike nose cones for supersonic jets? Not to be confused with aerospike nozzles. This is about the big pin-like rod in front of ICBM nose cones.

This answer (from Space) about aerospike nose cones notes that one of the potential advantages of the aerospike nose cone is to considerably decrease noise from the supersonic boom. In this comment the author evokes projections for the Concorde, dropping noise from 110 dB to 60-70 dB, that is 15 to 30 times quieter.

Noise from the supersonic boom, as the story goes, is what ultimately killed the Concorde: most nations forbid overland flight because of it, limiting it to ocean flights. So there would be clearly benefits from this.

Have there been any supersonic planes using this design that flew? Are there projects to do so that are currently active? Otherwise, why not?

There are no historical aircraft with an aerospike nose, at least, none that I remember, and none that I could find after 30 mins of research. I'm not sure if there are any planned projects using it, but I doubt there are, given the purpose of aerospike noses - aerospike noses really only have one main purpose - to streamline a blunt body. This is commonly used in missiles and such (especially in SLBMs), where height/length is a constraint - a slender, sharp nose is very long, so to save height, those missiles are fitted with an aerospike that telescopes out. The flat plate at the end of the aerospace forms a detached bow shock, which makes the airflow "see" the blunt body as having a different, more streamlined shape, reducing drag. Here's a visual of that happening:

Image source - https://iopscience.iop.org/article/10.1088/1742-6596/1276/1/012031/pdf

However, the bow shock generated by a drag reducing aerospike causes more drag and a heating than the oblique shock generated by a slender nose of the same length. For SLBMs, a long, sharp nose is obviously out of question (well, you could have a telescoping sharp nose, but that would be heavy), but for aircraft, length really isn't a concern, you can make the nose as long as needed. Therefore, it doesn't really makes sense for an aircraft to have an aerospike nose.


How is the weight of an airplane measured? Many very large airplanes have specific information of their empty weight. For instance the weight of an empty, regular 747-400 is 393,263 lb (178,756 kg) according to Wikipedia. The Concorde is 78,700 kg.

How is the weight of these airplanes measured so precisely (note that in the 747-400 example is precise to the unit's place!). Is the sum of the weights of all the parts used? Also, is Zero Fuel Weight (ZFW) the same as empty weight?

The empty weight of airplanes can only be given to single-pound precision for one specific airplane. Minute differences in metal sheet thickness, equipment and paint will make sure that there is a difference of many pounds between airplanes of the same type and furnishing. Add to this the difference in seating, galleys and instrumentation between airplanes of the same type which are operated by different airlines, and a precise number as in your question becomes almost meaningless. A better way would be to give a range in which most airplanes will fall, or to round the number like the Concorde weight of your question.

Weighing is quite straightforward: All tires of the landing gear are placed on scales and the weights are summed up. If the attitude of the airplane does not change, the weight on the tires can even be measured consecutively, so only one scale is needed. Small floatplanes can be suspended from a crane with a scale in the line holding the aircraft.

In Ernst Heinkel's biography he remembers an aircraft deal with the Soviet Union in the late 1920s. He had to deliver five floatplanes of a specified weight, and the Russian approval inspectors insisted on all planes having the exact same weight. Of course the planes were not identically heavy, but his advantage was that the scale in his factory was indicating the weight by printing it on cards. So the evening before the inspectors came, he let one aircraft be weighed five times, put the cards back into the machine and had the printing mechanism disabled. The next day he closed the business successfully with his very happy Russian guests.


Is having the tailplane at the wings backwash a design flaw? Most airliners have their tailplane below or above the backwash of the wings, so is placing the tail control surfaces directly in the backwash  of the wing a disadvantage?

I would say not really. The main impact would be on what incidence angle to set for the horizontal stabilizer because of the difference in local flow angle in or out of the downwash, and on the trim changes to expect if there are wing flaps, larger trim changes in the downwash, less if out.

For your ultralight, I would say the priority is simplicity and lightness, so just stick the horizontal tail at the most structurally convenient location, which is more or less where it is in the picture, like 90% of the other airplanes out there.

Size it based on the standard formula, make an educated guess (er... calculation) on the incidence angle to set, but make an attachment for the stab leading edge with multiple holes that allows you to change the incidence after flight testing.

And remember that when you test fly your ultralight you are effectively an Experimental Test Pilot taking the risks that go with that job description.


Why does the A320 have an anti skid system and how does it work? Can somebody explain why do A320 aircraft have an anti skid system? What is its purpose? And a brief explanation on how it works.

It's very similar to Anti-Lock-Brakes on cars.

The main reason aircraft have it is that a locked tyre will heat up, wear excessively and can burst (aircraft tyres are typically run at much higher pressure than cars or even HGVs).
If you lock a wheel under braking, it no longer provides any steering. Aircraft might not be swerving around obstacles, but staying on the runway is more important than minimising stopping distance.
A locked wheel generates slightly less than maximum stopping force. An automated system can hold the brakes at the point where it's not quite skidding, and brake harder than a human trying not to skid.

They work by monitoring the speed of each wheel, and releasing the brake on any wheel that slows down significantly more than the rest.

I believe that airliners are tested to check that locked wheel won't wear to the point where it bursts, but you can only do that once!


How do pilots do a preflight inspection of tail-mounted engines? The recently released first part of an Air Clips documentary of a Lufthansa Cargo MD-11 FRA-NBO flight shows in some detail the pilot carrying out the preflight visual inspection of the aircraft (from ~ 6:20 min). As in many other pilots' accounts of these 'walkarounds', the captain states one important aspect is the condition of the engine, including e.g. fan blades, intakes, checks for any foreign objects.

I understand that a major point of this visual inspection is to have the pilot-in-command getting a full picture of the aircraft condition before assuming the final responsibility for the flight. Obviously, the PIC cannot inspect a center tail-mounted engine like on the MD-11 by himself during the walkaround – so when or how is that inspection done for the center engine? I imagine that an inspection of the center engine is just as important as for the wing-mounted ones, prior to each takeoff.

In personal experience, I often look for a step ladder to inspect ground inaccessible engines and control surfaces and I have seen others look at their aircraft from a second story window.

My personal practice is to walk around the aircraft prior to every flight, not just at the first flight of the day, but I often had to make time for that. It may not be common practice. But that is an aside.

Here's the reality. If you are talking about the walk-around that the pilot or flight engineer might do, there are not up close inspections of the elevated engines and control surfaces. The walk-around will look for leaking oil, missing $1500 fasteners or whatever, but one is not checking the dipstick prior to engine start.

How are things inspected?  The manufacturer factors that into the design.

From an operational standpoint, the pilot(s), FE will look at performance during engine start, and make sure that the profile does not deviate from the expected temps, flow, etc.

In summary, if you can't get to it, it is not likely on the walk-around checklist.

Addendum.  At least one company is seeking FAA approval to utilize drones in aircraft maintenance inspection (not pre-flight) processes, which would reduce the cost and risks associated with scaffolding and lifts. As part of their application to FAA, there will be demonstration periods where aircraft are inspected with both methods, and results are shared with the FAA.


Who has the final authority over sidesticks? I’m wondering what would happen if - for some reason - the pilot and his copilot were fighting over the control of an Airbus with their respective sidesticks.
Does the pilot gets the final word ? Does it works like in a traditional system, where the strongest one takes it all, but through force feedback ?

If they were fighting, intentionally attempting to gain control over the stick, the pilot pressing the last would win. This differs from the situation in a cockpit with coupled controls, where the pilot who pushes hardest wins. So in an Airbus, a quick pilot wins from a strong pilot..
From the FCTM:

When the Pilot Flying (PF) makes an input on the sidestick, an order (an electrical signal) is sent to the fly-by-wire computer. If the Pilot Not Flying (PNF) also acts on the stick, then both signals/orders are added.
Therefore, as on any other aircraft type, PF and PNF must not act on their sidesticks at the same time. If the PNF (or Training Captain) needs to take over, the PNF must press the sidestick priority pushbutton, and announce: "I have control".
If a flight crewmember falls on a sidestick, or a mechanical failure leads to a jammed stick (there is no associate ECAM caution), the "failed" sidestick order is added to the "non failed" sidestick order.
In this case, the other not affected flight crewmember must press the sidestick priority pushbutton for at least 40 seconds, in order to deactivate the "failed" sidestick.
A pilot can at any time reactivate a deactivated stick by momentarily pressing the takeover push button on either stick.

Flight International featured an article on the A320 on 30 August 1986. It discusses the sidesticks, which were new for civil aviation at the time, and an Airbus representative makes an interesting point:

"If sidesticks were the normal way for flying aircraft, no one would even consider changing to a control column that makes it difficult for the pilot to get into his seat and which blocks his view of the instrument panel when he's there.."

It is really hard to find an unassociated person though, who is of opinion that uncoupled sticks are better than coupled sticks.


Why don't American passenger airlines operate dedicated cargo flights any more? I was reading recently that Northwest Airlines was the last American passenger carrier to operate dedicated Cargo aircraft. Why do none of the large US airlines operate dedicated cargo flights anymore? This seems to be common among international Airlines (Lufthansa, China Airlines, Qatar Airways, EVA, etc).

Presumably UPS and Fedex's extensive networks have something to do with it? I'm surprised that none of them find it economical, however.

Alaska Airlines, Hawaiian Airlines still have dedicated freighters.

American, Delta, Southwest and United all have thriving cargo shipping operations, but as far as I can tell, no longer have freighters.

https://www.aacargo.com
https://www.alaskaair.com/content/cargo/general
https://www.deltacargo.com
https://www.hawaiianaircargo.com
https://www.swacargo.com

Alaska used to run 737 Combi to ship cargo and passengers in the main cabin on their “Milk Run” up the Alaskan panhandle. They now have dedicated freighters to handle the cargo.

https://thepointsguy.com/2017/10/final-flight-alaska-airlines-737-400-combi/

Good luck shipping your parcel.


What is the expiration period for a 3rd class medical certificate? I got my 3rd Class Medical for a Private Pilot certificate in May 2013 at the age of 39.  I'm now 41.  Does the 60 month rule apply to my certificate or do I need to retest under the 24 month rule for over 40 year olds?

According to the table in 14 CFR 61.23 (d), the expiration date of your medical is based on your age "on the date of examination for your most recent medical certificate."
So, if you were 39 on the date of the examination, you're good for 60 months.

There is a possibility that the requirement to renew 3rd class medicals may go away completely soon. On Dec 15, 2015, the U.S. Senate passed the "Pilot's Bill of Rights 2". The bill would still need to be passed by the House and signed by the President before it would take effect. As of Dec 15, it had 152 co-sponsors of both parties in the House. According to AOPA:

Under the medical reforms of Pilot's Bill of Rights 2, most pilots who have held a valid third class medical, either regular or special issuance, within 10 years of the legislation’s enactment would never need to get another FAA medical exam. The rule would apply to pilots flying VFR or IFR in aircraft weighing up to 6,000 pounds and carrying up to five passengers at altitudes below 18,000 feet and speeds up to 250 knots.
Pilots who develop certain medical conditions, including a small list of specific cardiac, mental health, or neurological conditions, will have to get an FAA special issuance medical one time only, significantly reducing the time and money spent navigating the FAA’s medical bureaucracy.
For pilots who have not had a valid medical in the past 10 years and those who have never applied for and received a medical certificate, a one-time third class medical certification by an aviation medical examiner will be required. After a pilot has been medically certified once, either through the regular or special-issuance processes, he or she will also be able to fly indefinitely without needing to go through the FAA medical certification process again.


What is the relation between geodesic airframe and geodesic curves? That is certainly a beginner question but I cannot find the answer anywhere.

The Wikipedia page on geodesics says that they are the shortest path between two points on a geometric object (surface, manifold,...). It also  mentions geodesic airframes as a real-life application. However, the linked page about geodesic airframes does not seem to deal with geodesic curves. It is mostly about about a structure made by triangles or other polygons. A related question explaining why geodesic frames are no longer in use is answered only with this point of view.

Hence, my question: are geodesic frames related to geodesic curves? If yes, in what way?

My intuition is that geodesic curves are used to design the shape of the "perfect" frame (with respect to some constraints), and that the polygonal structure is an approximation of this perfect shape.

Before answering the questions, let's try to see how simple is the notion of a geodesic. In planar construction, we use latices to rigidify a structure, e.g. a truss:

The structure is made of straight segments that don't deform when axial forces are applied. If we switch from the plane to an arbitrary curved surface, something similar can be done, but geodesics take the place of the straight lines (actually the line is the geodesic for the plane).

The idea behind a geodesic construction is that its rigidity is obtained from segments on the surface of the truss, and the inner volume is left empty and available for other uses.

What is the relation between geodesic airframe and geodesic curves?

Are geodesic frames related to geodesic curves? If yes, in what way?

Geodesic airframes are indeed constructions made of geodesics, a technique first applied to aviation by Barnes Wallis. From the Barnes Wallis Foundation, and article Geodetic Aircraft Design:

  A geodetic (or geodesic) airframe makes use of a space frame formed
  from a spirally crossing basket-weave of load-bearing members. By
  having the geodesic curves form two helices at right angles to one
  another, the members became mutually supporting in a manner that the
  torsional load on each cancels out that on the other. In addition to
  being comparatively light and strong, the fact that the geodetic
  structure was all in the outer part of the airframe meant that the
  centre was a large empty space, ready to take payload or fuel.

A close-up view of the geodetic structure designed by Barnes Wallis (source)

Vickers Wellington's under construction (source)

[The wikipedia article] does not seem to deal with  geodesic curves. It is mostly about about a structure made by triangles or other polygons

Geodesics are curved lines. Using 2 or 3 geodesics on a closed surface creates repetitive and symmetrical curved polygons, e.g. curved triangles and curved lozenges. Providing the supporting geodesics are angled so that "the torsional load on each cancels out that on the other", the resulting geometry belongs to the family of geodesic modeling.

Whether the actual construction is an approximation of the perfect model is a different engineering/costing problem. This is related to the actual cancellation of the torsional loads after approximation and the effect of the residual loads on the structure. This residual load could potentially be absorbed by additional engineering device if appropriate. This dome is made of straight segments only, the triangles are planar.

The geodesic dome at Amundsen Scott South Pole Station by Ernie Mastroianni (source)


Can an amateur-built aircraft ever be used for commercial activities? There are some non-recreational activities, such as aerial photography, wildlife observation, environmental monitoring, etc., that seem well-suited to certain amateur-built aircraft.  The open-cockpit Lockwood AirCam, which I understand was specifically designed with back-country nature photography and videography in mind, is just one example.  Yet FAA regulations relating to airworthiness certificates appear to make no allowance whatsoever for an amateur-built experimental aircraft to be used for non-recreational/non-educational purposes.  Am I misreading the regulations?  Is there a legal way to use a homebuilt airplane for, say, commercial aerial photography?

According to the Experimental Aircraft Association (EAA), aircraft with Experimental/Amateur-Built airworthiness certificates cannot be used for commercial purposes. However, I cannot find an FAA reg that specifically says that. (Anybody else know of a reg that says that?)

There are other grey areas as well. For example, what if your insurance is requiring dual instruction in the type before they will issue you a policy? The CFI would be using your aircraft for commercial purposes, even if you can legally claim you're the Pilot-in-Command.

However, the FAA talks about what things you cannot do without a commercial pilot license -- fly passengers or cargo for hire. My suggestion is that if you want to use your amateur-built aircraft for commercial photography, call your local FSDO and ask about that. Be nice to them and you'll be impressed how nice they can be to you.


Is one-sided thrust reverser deactivation still allowable? While browsing the major accidents in the Aviation Security Network's database, I noticed that a couple of accidents seem to at least be aggravated by one-sided deactivation of thrust reversers. One-sided thrust reverser deactivation seemed to be permissible according to the respective Minimum Equipment Lists, yet the reports indicate that the crews have been caught off guard by the resulting asymmetric thrust after reverser activation during landing.

As some of the mentioned accidents resulted in a high number of fatalities, I wonder if the procedures and requirements regarding thrust reverser deactivation have ever been changed. How common is it for an airliner on a commercial flight to have thrust reversers deactivated asymmetrically nowadays?

Operating with a deactivated thrust reverser is relatively common on normal line ops.

There are normally type specific operating restrictions e.g. full power take-off, no short or narrow runway ops.

On B757/76 there is a clue that you a have a reverser locked out - you can't select it.


Is it fact or fiction that Ashgabat control tower is misplaced? It's often written that the construction of Ashgabat International Airport was sabotaged by the aesthetic whims of the country's late leader, Saparmurat Niyazov.  The story goes that Niyazov, presented with the architectural plans for the airport, insisted that the air traffic control tower be moved to the "wrong side of the runway".  He supposedly overruled the better judgment of the airport planners because he thought the airport would look better that way.

In the West, Niyazov is widely considered to have been an eccentric and megalomaniacal dictator, thanks in part to stories like this.  While many of these stories are undoubtedly true, others have been shown to be exaggerations or misinterpretations.  I'd like to know whether there's any truth to the story of Ashgabat's air traffic control tower, or if it's just another one of the false but plausible-sounding rumours cooked up to further mock Niyazov.  So my questions are basically as follows:

Is there, in fact, any significant problem with the placement of the air traffic control tower at Ashgabat International Airport?
If the tower is poorly placed, what specific problems does this placement cause for air traffic controllers and airplane pilots, and how do they work around these problems?
If the tower is poorly placed, is there any reliable evidence that this was a result of Niyazov's direct personal involvement?  If there isn't, what's the earliest known appearance of this claim?

It would seem your question ends with the first part. Having checked out the airport online, checking images, maps, etc, I can't see anything wrong with the placement of the tower. It actually seems nearly optimal... It's very near the center of the airport with an unobstructed view of the whole facility, and between the 2 main runways....  Parts 2 and 3 of your question become non-operative, so yes, this is a valid question here, and need not be sent to a skeptics group.

What was interesting to me though was that the image in google maps satellite view seems to show the parking spaces around the tower completely empty... No controllers on duty the day the image was taken?


What does a pilot have to do in order to fly as a second in command in a private jet? If a pilot has the opportunity to fly as an SIC in a private jet, what FAA requirements must they comply with in order to fly with passengers?

Don't know if the same rules still apply, but back in the 70s and 80s I used to fly legally as the second pilot on corporate jets with just a commercial license with IFR and multi-engine ratings. At the time I had no type ratings.


Would an accurate weight of a loaded aircraft be helpful? If a commercial aircraft's actual weight at takeoff is accurately known (instead of being estimated), can this help with use of thrust, optimal altitude, etc and therefore fuel savings, emissions etc.?

A friend of mine was FO on a regional jet with 50 passengers. During the take-off, the PIC could not rotate the plane at the calculated VR, but only much later. They reported the incident during the flight. The luggage containers were re-weighted at the destination, and it turned out that one of the containers had a grossly wrong weight recorded at the origin.

Weighing mistakes can end in a disaster, especially for smaller jets. Systematic weighing of ready to launch planes would eliminate this risk. I am not aware of an accident caused by this kind of weighing mistake, but surely there have been accidents due to overloading or wrong load distribution.

The technology to build load sensors into the undercarriage is available and it appears that a few planes are equipped with it.

On the other hand side there must always be a risk assessment with a trade-off between costs, likelihood and severity.


How is the static pressure in the pitot static system reconciled when there are two outside static pressure ports? On airplanes where there are two static pressure sources (ports) on each side of the fuselage what is the result of the static pressure as compared to aircraft with a single port. (I'm not talking about the alternate static port which is usually inside away from freezing weather).

For example, from the diagrams I've looked at it appears that the dual static ports are connected to a common line such that the pressures from each port are averaged out, such that if one port has 14 psi and the other 10 psi the result will be 12 psi. Is this how it works?

Another example to help illustrate my question, imagine an aircraft with two static ports on each side exactly symmetrically opposed is in a forward slip attitude, (ignore ram air pressure for this example), does the static pressure from each side when mixed together cancel out the error caused by the slip attitude as compared to an aircraft with a single static port in the same attitude? Does dual symmetrical static ports correct error of a slip to give you the actual static pressure as if the aircraft was flying perfectly coordinated? (Again, this question is just about the static pressure, not ram air).

If anyone has a more concise way to ask this question please chime in.

I work on corporate jet aircraft and perform troubleshooting, testing, and certification on pitot-static systems.

The reason aircraft have static ports for each pitot static system is to equalize or average out the small differences in static pressure caused by crosswinds when the aircraft is in flight. The difference in pressure caused by crosswind components on the static systems is not in the psi range, rather it is in the millibar or 1/10 of inches of mercury (inHg) range.

14CFR, Part 43, Appendix E, Table IV "Pressure-Altitude Difference" lists several baro-scale test points and associated altimeter readings used when certifying the altimeters. From the chart you can get an idea of how a difference of 1/10 in-HG  will case a change of 90 to 100 ft in the altimeter reading (you can Google this table yourself).

An aircraft could easily have a difference in barometric pressure between the up-wind and down-wind sides of the aircraft when flying in a crosswind and this would cause an error of 100 ft or more on the altimeter reading if there was only a single static port located on one side of the aircraft.

On large aircraft which require 2 pilots, there are two completely separate pitot and static systems for each of the flight crew. Each pilot's instrument panel has its own altimeter and airspeed indicator (or air data computer) and display of altitude and airspeed information. There is a single pitot tube for each crewmember's instrumentation located on his side of the aircraft, but each crewmember's static system has two ports, one  on each side of the fuselage which reduces the error effects of crosswind on the altimeter (static) systems (total of 4 static ports).
For peace of mind and safety, the flight crews continually compare (cross-check) each other's altimeter readings and if they see an error of 80 ft or more between the two systems they have an uneasy uncertainty about which altitude is correct and they will bring it to the mechanic's attention.

A blocked static port is a funny thing to troubleshoot, because a flight crew will say they saw a big altitude difference on one leg of the trip, then the next couple legs it was fine, then it had a split again. Next crew will fly and say there were no problems, followed by another crew complaining about splits. If you question the crews and ask if they were experiencing heavy crosswinds when they had the split they will say, "hmm, now that you mention it... yeah, we had some heavy crosswinds".
We then connect our pitot-static tester to each side's pitot tube and one of each side's static port, block off the opposite static port, and bring the airspeed and altitude up on the tester. What we are looking for when we connect the tester to the obstructed static port is a split between the captain's and first officer's altitudes. (Actually the obstructed side will start to wind the Airspeed indicator up really fast and can damage it if you don't stop the test quick enough).

One side is obstructed and the difference in static pressure caused by the crosswind is throwing off the altitude reading on one of the altimeters.
Two ports per system is to reduce errors in the altitude indication.


Does this formula hold true for a source sheet covering a closed body of arbitrary shape? Let me explain the picture above: We have a uniform flow and a source sheet cover the the surface of an arbitrary closed body. Let s be the distance measured along the source sheet, λ=λ(s) is the source strength per unit length along s. Assume that we already have the λ=λ(s) along the source sheet that when we superimpose the uniform flow with this source sheet, we obtain the streamline over surface of the body. Could we have this formula:
$$\oint_{body} \lambda(s).ds = 0$$  ?? If yes, is there any way of thinking to pop out this formula right away before using math to prove it? An idea is that: "the body itself couldn't be adding or absorbing mass from the flow" but I can't relate this state with the formula above.

If a streamline is closed, then there's no mass flow that can go throught it. That means that inside the streamline the net mass flow produced is null. You can have for example sinks and sources inside a closed streamline, but they must compensate each other to make the streamline remain closed.

Another important aspect about source sheets is that they actually don't match with the border of the body, i.e, with the closed streamline. If they did, then some flow produced by the sources would go out of the source sheet, and some other flow would be in. The source sheet needs to be inside the border of the body. Anyway you could find a source distribution that makes the source sheet to almost match the border of the body.

What I wanted to say, is that if the streamline is closed, then the sum of sources and sinks inside the body is null. If the sinks and sources are the source sheet itself, then this proposition is equivalent to:

$$\oint_{Source\hspace{1pt}sheet} \lambda(s)ds=0\hspace{40pt}(1)$$

Note that each infinitesimal element of sheet, $ds$, produces an infinitesimal source strength of $\lambda(s)ds$ in the $s$ position. The relation $(1)$ means that the sum of all infinitesimal source strengths along the source sheet is null, or, in other words, that the net source strength is null.


Do aircraft fitted with both a trimmable horizontal stabilizer and a trim tab exist? This question is restricted to trim on the pitch axis.

Both trim tab and THS (trimmable horizontal stabiliser, a whole moving horizontal stabilizer whose movements are used as trim) are used to trim an aircraft. Aircraft having a larger speed range (e.g. jet airliners) tend to be fitted with THS whereas aircraft flying slowlier (e.g. GA aircrafts) tend to be fitted with trim tab.

Is there any aircraft type that use both? (Maybe one being a backup device)

Trimmable horizontal stabilizer (THS) and trim tab serve the similar purpose for two entirely different control architectures:

Trim tab neutralizes the force on the pitch control for a reversible control system.
THS neutralizes the pitch position (and, in the process, the force) for an irreversible control system (e.g. hydraulically/electromechanically actuated, analog or digitally commanded).

In the presence of an irreversible actuator, the actual hinge moment on the elevator no longer matters since the elevator position is being closed-loop controlled by the actuator (provided the actuator is sized correctly, of course); but more importantly, it is no longer felt directly by the pilot. Without this direct feel, a trim tab would be more than useless.

On the other hand, in the case of reversible control with direct elevator-to-stick linkage, deflecting the stabilizer would not allow the dual objective of maintaining trim and neutralizing hinge moment to be accomplished, since a TE up stabilizer would impart even more nose down stick force to the stick. The THS is equally useless for a reversible control for this purpose.

For this reason, I would be very surprised to see an aircraft fitted with both. But maybe there are oddballs?


Where is the camera in the A320? On the A320 family there are circuit breaker panels in the cabin between the exits. On the 2000VU panel there is a "camera" circuit breaker (C5 position):

Top: Source -- Bottom: A320 CCOM p165 -- Right: A320 CCOM p166

Which camera is it related to?

There's an option for a tail mounted camera for the passenger entertainment system (PES). It's the same one they use on widebody jets, but it's not hooked up to the cockpit for taxi purposes.

Airbus press release says

  Designed for a private customer, this cabin seats just over 30 passengers and includes such features as touch-screen technology, advanced passenger entertainment and communications, plus a tail-mounted camera.


Why does the Convair 990 produce this howling sound during landing? Watching this video of a landing Convair 990, one can hear a whooshing howl at about 0:11-0:12, as it flares just prior to touchdown (not to be confused with the run-of-the-mill howling that you can hear from 0:13 on as the engines spool down after touchdown); what makes this sound?  Does the CJ805-23 engine have a different sound profile than other engines?

What you hear at 0:11-0:12 is just the sound of the engines being reduced to idle thrust, and the doppler effect as the aircraft passes your position.

I have heard that sound hundreds(maybe thousands) of times. It is very normal, and sounds just like all jets of that generation.

Here are more examples:

Skymaster DC-8-63F Landing

707 ! Touch & Go, Depature, Landing.

Boeing 707 Landing - Smokey!


Could the reversers of a PW4000 on a 777 be deployed in-flight? Could the thrust reversers accidentally be deployed during a flight? Here, I’m referring to the PW4000 series on a B777, where the engine is similar to the type involved in the crash of Lauda Air Flight 004.

A basic requirement of certification is the probability of any single point of failure that causes loss of the airframe (hazard level = catastrophic) must be better than 10 -9, or one in a billion.  A reverser deployment in flight would meet that level of hazard so system design has to meet that probability level.

In the case of Lauda Air, I would have to assume Boeing made a mistake in the system architecture that allowed a one in a billion event to happen "prematurely", and in fact they introduced a design change to get back to the required probability level.  In the case of the 777, or any other airliner, you have to assume the homework was done properly and the architecture meets the requirement until an event indicates otherwise.

Most of the time, gaps in design redundancy turn up during in-service events that are not as nasty, but that set off alarm bells at the OEM as potentially something more serious, and after a long process of analysis of risk and design, changes are implemented in the field by Airworthiness Directive.

It seems cold and calculating, but the whole thing is a numbers game where people with pointy hats with stars and crescents on them work out probabilities, of risk of death basically, using arcane methods like weibull analysis, to decide what to do and how soon to do it when design flaws turn up.  The transportation system couldn't function without it though.

Anyway, it's safe to say the risk of a reverser deployment in flight on a 777 is microscopic, but it's not zero, as nothing is in aviation.


Can air-to-air refueling be done in compliance with the FAR's? I am wondering if any FAR's would prohibit the air-to-air refueling of two civilian aircraft in the US. In our use case, the receiving aircraft would be custom-built, and it would receive fuel pumped from a portable tank temporarily installed in a Quest Kodiak turboprop with the cargo door taken off.
There's the umbrella "don't be reckless" rule, but if it's done in a controlled way, what laws might stop us?

I can't provide a definite yes or no, but I can discuss some of the FARs that will need to be considered. In most cases you have to consider three groups of FARs; covering aircraft, aircrew, and operations.
For the aircraft, they have to have an airworthiness certificate.
The Kodiak is certificated, but you have to address operating without the cargo door (may be approved) and if the temporary installation of your refueling equipment needs any approvals. Many temporary installations don't but since your refueling equipment will connect to another aircraft's fuel system it may need a 337 or STC.
You say the receiving aircraft will be custom built. That implies an experimental certificate. There are two, maybe three, different experimental certificates that you could theoretically apply. The first is Experimental-Amateur Built (E-AB). This could work if you are not a business and this effort is for personal educational purposes. It's the least restrictive, but is in a gray area of applicability. The second is Experimental-R&D. It would be applicable, but it's the most restrictive certificate as it limits you to just R&D flights and crew proficiency flights and it requires annual renewal. Experimental-Exhibition might apply, but it's usually limited exhibition flying of existing, non-certificated (typically ex-military) aircraft. In any of these, I would expect that you would need to include aerial refueling within the operating limitations and that means getting an FAA inspector to buy off on it.
For the aircrew, I expect the one potential issue is one of compensation. If everyone holds at least a Commercial certificate that would avoid any gotchas. The other issue may be meeting the FAA training guidelines for formation flying.
Flight operations would be subject to Part 91 and the operating limitations of the aircraft. I would expect that refueling operations would be limited to airspace 'not over populated areas.'
So, I expect it would be possible but it could be difficult getting the necessary approvals.


Can an FAA Pilot Examiner accept BasicMed in place of a medical certificate from a checkride applicant? This has been a subject of debate recently. §61.23 states that you can use BasicMed to apply for some certificates or ratings but limits this to gliders, practical examination in a simulator or flight training device and balloon categories. For other ratings, unless you were a former pilot with the armed forces and applying for a conversion of your military ratings, you must present at least a valid third class medical certificate to a pilot examiner when applying for a rating.

I talked with the FAA Medical Certification Division and they seemed to think one could apply for a airman certificate using only BasicMed.  does anyone out there know if Pilot Examiners in the field are accepting BasicMed with a valid driver’s license from checkride applicants?

I think you may be confusing Basic Med with No Medical At All, some operations do not require any sort of medical exam

  (b)Operations not requiring a medical certificate. A person is not
  required to hold a medical certificate -

  (1) When exercising the privileges of a student pilot certificate
  while seeking -

  (i) A sport pilot certificate with glider or balloon privileges; or

  (ii) A pilot certificate with a glider category rating or balloon
  class rating;

  ...

  (8) When taking a practical test or a proficiency check for a
  certificate, rating, authorization or operating privilege conducted in
  a glider, balloon, flight simulator, or flight training device; or

Basic med can be accepted if you comply with the Basic Med regulations. However you need at least one valid medical (that can be expired) to be eligible for basic med, since a Class III medical is valid for 5 years (under the age of 40) its more than likely your training will be accomplished before it expires and thus you will have a valid medical when you sit for your check ride.

The FAA address this in this Q/A document

  Q17: Do student pilots need to hold a medical certificate when they go
  for their private pilot checkride?

  A: Individuals who are new to
  aviation will need to get one medical certificate if they want to fly
  under BasicMed. After that, they can fly under BasicMed forever, as
  long as they fly a BasicMed-compliant aircraft and never fly for
  compensation or hire. Most student pilots will still hold their first
  medical certificate when they apply for their private pilot
  certificate.

To trace the legislation exactly:

As you note for some student pilot operations as per 61.23

  (c)Operations requiring either a medical certificate or U.S. driver's license.

  ...

  (v) Exercising the privileges of a student,
  recreational or private pilot certificate if the flight is conducted
  under the conditions and limitations set forth in §61.113(i); or

This would potentially allow it but sends you over to 61.113(i) for more info

  61.113 Private pilot privileges and limitations: Pilot in command.

  ...

  (i) A private pilot may act as pilot in command of an aircraft without
  holding a medical certificate issued under part 67 of this chapter
  provided the pilot holds a valid U.S. driver's license, meets the
  requirements of §61.23(c)(3), and complies with this section and all
  of the following conditions and limitations:

which just bounces you right back to §61.23(c)(3) one section down from where we were earlier

  ...

  (3) A person using a U.S. driver's license to meet the requirements of
  paragraph (c) while operating under the conditions and limitations of
  § 61.113(i) must meet the following requirements -

  (i) The person must -

  (A) Comply with all medical requirements or restrictions associated
  with his or her U.S. driver's license;

  (B) At any point after July 14, 2006, have held a medical certificate
  issued under part 67 of this chapter;

So at some point you will need to get at least one medical. Most students do this along with either student pilot certificate (allowing them to fly solo) furthermore most complete their training within 5 years of that date although there are other cases. For example, you started your training in early 2006, got you class III medical/student pilot on July 15th of 2006 which is good for 5 years. Life gets in the way and you stop training for a while. Your medical expires in 2011 but you now decide to get back into the left seat. Since you never lost your medical and once held a valid medical you are legal to fly and train under basic med (and even sit for a check ride).


What type is this aircraft, appearing in the movie "Kingsman"? In a scene of the movie Kingsman: The Secret Service the young aspiring agents have to skydive from an airplane. I didn't recognise it: which airplane is it (assuming it actually exists)?

I could only find a picture from inside and one from below.

That distinctly squarish shape looks like a Short SC-7 Skyvan 3-100, especially since the registration on the photo from below is visible: G-BEOL.
 -

(Image Source: WikiPedia - Author: Adrian Pingstone)


How is US Class B airspace enforced in Canada? Detroit, MI is the center of Class B airspace, however a good portion of that airspace within the inverted wedding cake is in Canada.  What are the mechanisms that allow US to define Class B airspace force and effect in Canada?

The VFR Terminal chart for reference:
https://aeronav.faa.gov/content/aeronav/tac_files/PDFs/Detroit_TAC_91_P.pdf

The portion over Canada is not designated US Class B but Canadian Class C.

Note that Canadian Class C airspace definition is similar to US Class B.  In particular,  VFR aircraft must obtain clearance prior to entry.  A mode C transponder is required.

To add: airspace over Canada is generally managed by NAV CANADA.  Definition for the Canadian side of the Detroit Terminal Control Area (TCA) can be found in NAV CANADA's Designated Airspace Handbook, portions excerpted below (from page 78):

We can see that these definitions matches the eastern portions of the Detroit TCA exactly, e.g, the outer area altitudes from 6,000' to 10,000' bounded 30nm from the Detroit VOR (DXO).

The Detroit TCA airspace was modified a few years ago, which required coordination between the FAA and Nav Canada.  The following discussion from the US Federal Register (page 3307) may be of further interest:

  Also, as noted in the NPRM, the
  eastern portion of the DTW Class B
  airspace area extends into Canadian
  airspace. The equivalent Canadian
  airspace to Class B airspace, as
  designated in the United States, is Class
  C airspace. NAV CANADA, the
  Canadian air service navigation
  provider, generally designates Class C
  airspace with a 12,500 feet MSL ceiling,
  however, has advised the FAA of its
  willingness to establish corresponding
  Canadian Class C airspace adjoining the
  FAA’s DTW Class B airspace with a
  ceiling of 10,000 feet MSL.
  Additionally, NAV CANADA advised it
  would make the Canadian Class C
  airspace action effective to match the
  effective date of this DTW Class B
  airspace modification action.


Which air transport aircraft have a lavatory included inside the cockpit? Another answer on SE Aviation (Why pilots would leave the cockpit in a short flight?
) had this bit of information in it:

  The cockpit of some airliners lacks a lavatory.

Isn't is more the other way around: i.e. A Lavatory inside a cockpit is a rarity?

Which airliner cockpits have an entire Lavatory inside them?

I always thought the Flight Crew came out to use the forward-most Business / First class lavatory.

As far as I know, some Boeing 747-400s (BA, Air France) had lavatories inside the cockpit. Apparently, Boeing 747-8 Intercontinental has a lavatory behind the cockpit door:

  ... Lufthansa Boeing 747-8 Intercontinental is unique in that the flight deck crew have their own lavatory, ... behind the locked cockpit door, allowing for extra security and a non-disruptive cabin service involving trolleys blocking the door.


Fighter jet with two rudders, the deflection angle it is not identical. One have more angle of deflection. Why? If an aircraft have two vertical tails then pilot push in the  rudder  pedal the rudders will not be deployed at the same angle.It is used throughout the all envelope of flight or is it possible only for low speed? Why and which ones  is  more deflected?

The jet in the top picture is turning slightly left using nose wheel steering, which is generally the same control as the rudder. I'm not aware of any fighter jet that has a tiller. Therefore the computer has commanded the rudders to deflect left. This is likely combined with a "toe in" (a characteristic of some fighters that aids in stability) of the rudders so it looks like only one has moved.

In the bottom picture, the flight control computer has decided that the best way to maintain the pilot's commanded attitude is to asymmetrically deflect the rudder. Flight control computers move surfaces in fly-by-wire systems, not pilots. Pilots just ask the computer if it is okay to do that. The computer decides if it is okay and how. It will use a combo of all the available surfaces to achieve the desired attitude within the programmed flight envelope and it doesn't have to do it symmetrically.


Why do propellers have less theoretical range (i.e. range ignoring impatient passengers wanting to go fast) than jets, despite greater efficiency? I was playing around with the breguet range equation recently, and came across a suprising result, that propellers have less range than jets, despite having greater efficiency.

The range equation for a prop is:

R = 550 * np * ((L/D)/(C / 3600)) * ln(Wi / Wi+1)

where C is the SFC, in lb/(hp * s), and R is the range, in feet. Note that the 550 here coincidentally is the same as the speed in the next calculation, but the 550 here is actually a conversion factor from hp to lb-f/s

According to wikipedia, the most efficient turboprop engine is the NK-12, used on the Tu-95 bomber (and Tu-114 airliner), and that engine gets a SFC of 0.36 lb/(hp*h). Assume a propeller efficiency of 0.8, which is typical.

R = 550 * 0.8 * ((L/D)/(0.36 / 3600)) * ln(Wi / Wi+1)

R = 550 * 0.8 * ((L/D)/0.0001) * ln(Wi / Wi+1)

R = 550 * 0.8 * ((L/D)/0.0001) * ln(Wi / Wi+1)

R = 4400000 * (L/D) * ln(Wi / Wi+1)

For jets:

R = (V / (C/3600)) * (L/D) * ln(Wi / Wi+1)

a SFC of 0.5 lb/(lbf*s) in cruise is considered pretty good for jets, and let's assume a 550 mph cruise speed (or 806.667 ft/s). Range is now:

R = (806.667 / (0.5/3600)) * (L/D) * ln(Wi / Wi+51)

R = 5808002.4 * (L/D) * ln(Wi / Wi+1)

The range expressions for jets and props is identical, except for the coefficient, and 5808002.4 is greater than 4400000, so therefore, jets have better range, even on paper, than props, assuming same L/D ratio and fuel ratio. This is kinda suprising, given that props are quoted to have higher efficiency than jets. I've heard people say props have less range, but I've always heard it backed up with the evidence that "props are slow, and people like to go fast". Are my calculations and data correct? If they're correct, why, intuitively, do props end up having less range?

Clarification: user3528438 has pointed out that some turboprop engines provide both a power specific fuel consumption number, as well as thrust specific. However, if you plug the thrust specific fuel consumption number for that turboprop into the jet range equation (essentially treating the prop as a jet), the resulting range is very different than the range you'll get if you plug the power specific fuel consumption number into the propeller equation -- so which one is right? I doubt my formula for propeller range is wrong (though I might be using it wrong - could someone please check) because the same formula (but without the conversion factor, because they're using metric) is found here - https://nptel.ac.in/content/storage2/courses/101104007/Module2/Lec8.pdf.

I also have my derivation of the formula on hand too, if anyone wants to check that.

For more background, this question is about the usage of Breguet Range Equation (I find this explanation better suited for this question)

There are many problems with your calculation:

Time unit of speed should be the same as the time unit of SFC. For your first calculation, since you used mph for speed and lb/(hp*h) for SFC, your SFC term shouldn't have 1/3600. For your second calculation, since your speed is fps and your SFC is also in lb/(lbf*s), there's no need to have 1/3600 either.
For both calculations, end result should be the same unit of length. Your first calculation uses mph and the second uses fps, so the the end result would be in miles and feet so not comparable. But since you made the mistake #1 you end with numbers still on the same order of magnitude.
For the propeller case you are using PSFC (Power specific fuel consumption) rather than TSFC (Thrust-specific fuel consumption). Breguet Range Equation requires TSFC to work.

If you look at the Breguet Range Equation you see the only difference between prop case and jet case is TSFC. So to compare relative range differences, you only need to compare TSFC and avoid all other complications.

Take Cruise TSFC from some modern examples:

GE90: 0.545 lb/lbf/h
Europrop TP400: 0.38 lb/lbf/h

(1/0.38)/(1/0.545) - 1 = 43% so with everything else being the same, prop should have about 40% more range than jet.


Why should one not slip a Cessna 172 with flaps extended? The POH strongly recommends not slipping the AC with full flaps...why?

It seems natural in a crosswind approach to
allow the horizontal lift component to cancel the airmass movement across the centerline.

The basic problem is that sideslip induces a cross flow over the fuselage that can increase the local negative AOA and flow disruptions beyond that already caused by the turbulence and downwash being generated by the flaps.  The stabilizer surface on the "lee" side of this cross flow may get enough flow disruption to be felt in the elevator, maybe accompanied by a bit of pitch bobbing at some extreme condition that maximizes the tail loading, like side slipping at forward C of G with large flap angles.  In spite of its very powerful flaps, the 172's issue is fairly benign (or there would be a prohibition against using slips with flap as opposed to an advisory).

This can be a significantly worse problem with stabilator or all-flying tails that operate at larger negative AOAs than fixed stabilizer tails, as Cessna Cardinal owners are well familiar with.  The Cardinal required slots to be added to the stabilator inboard leading edge to fend off tail root stall during landings, and resulting broken nose gears, during the first year of production, and that was just doing regular landings with maybe a bit of slip due to crosswind (the '68 Cardinal I once owned had the field mod slots added under the "Cardinal Rule" service bulletin - Cessna sent teams out to modify every aircraft pre production cut-in in 1968, somewhere around 500 a/c, and you can slip them with flap all you want).

The stabilator equipped Thorp T-18 homebuilt had to have a flap limitation added due to stabilator stall in sideslip (John Thorp did some testing after an incident in the 60s, discovering that the stabilator could stall with full flap, forward CG and sideslip, causing a nasty pitch over, and advised owners not to use full flap for landing).


What happened to the blades in this picture? Here is the picture

Source:(brians-travels.com)

This looks like a photoshopped picture, but I believe it isn't. Is this some kind of strange occurrence that occurs only with the human eye? The blade on the top looks bent downwards. I can't figure out what happened in this picture. And will your picture look like this every time you take a picture of a fast moving propeller?

It's an artifact of how a digital camera downloads captures the image from the sensor called rolling shutter.

Essentially the image is captured a single line at a time and the fast moving propeller changes location between each line captured.

Wikipedia has a nice diagram showing what happens step by step:

On our sister site photography.SE has a question asking about it.


Do I need FAA permission to install tundra tires on a Piper PA-28 Cherokee 140? I do a lot of back county flying  and a lot of times the landing area is not in the best possible shape. I would like to install 29" tundra tires on it. Due to the change in wheel and tire size do I need to get FAA permission to make the modification and if so what would be the process?

Any time you make a modification to a certificated aircraft you need FAA approval. Even what appear to be minor things, like replacing you sun visors with better ones requires an STC. Putting in shoulder harnesses requires and STC. I could go on forever, but you get the idea.

Putting tundra tires on your Cherokee is a major change. If you look in the parts manual, your Cherokee probably came with 4 ply 6.00x6 tires so to change them you need an STC. You would also need to get an STC for the wheels and brakes. You’d probably need a new nosewheel fork as well.

I have never heard of anyone putting tundra tires on a Cherokee, but stranger things have happened.


Can construction of an airport lead to more foggy weather? The following paragraph is from Wikipedia "Airport" page with no citation.

  The construction of airports has been known to change local weather patterns. For example, because they often flatten out large areas, they can be susceptible to fog in areas where fog rarely forms. In addition, they generally replace trees and grass with pavement, they often change drainage patterns in agricultural areas, leading to more flooding, run-off and erosion in the surrounding land

Is that bold statement true?

The statement is apparently from the Introduction to Airports Design and Operations of
Center for Air Transportation Systems Research at George Mason University.

One form of fog that is formed in airports usually is radiation (or ground) fog. As the flat open surfaces in airport are prone to cooling overnight due to thermal radiation, the air close to the surface is also cooled to the dew point. This reduces the ability of the air to hold moisture, allowing condensation and fog to occur.

One reason for the statement may be that open areas in the airport give up their heat much quicker than urban heat islands, which does not have flat surfaces prone to thermal radiation.


New Research: VGs increase lift to drag ratio up to 880%. Does it work for flaps too? Aerodynamic Free Lunch: VGs increase L/D ratio up to 880%. Does it work for flaps too?

I understand this research was for a wing without flaps.

I understand this reduction in drag with VGs works only for the alpha range past the max Cl point of the wing without VGs, eg. 8-18 deg, increasing the L/D by 880% at 18 deg AOA as shown in the table and graph below.

Questions:

Would a wing with 20% chord NON-SLOTTED flaps see a similar 84% reduction in drag for each flap setting eg 10,20,30 deg?
Would a wing with 20% chord SLOTTED flaps see a similar 84% reduction in drag for each flap setting eg 10,20,30 deg?
Why does the drag almost stay the same even thought the lift goes up? My intuition is that the VG wing drag value stays close to the 8 deg drag value w/o VGs, as that is the local AOA the wing is seeing due to the VGs, instead of the drag at 18 deg, even though the lift is increasing.  Is that correct?

Article:

Experimental and Numerical Analysis of the Effect of Vortex Generator Height on Vortex Characteristics and Airfoil Aerodynamic Performance
Xinkai Li 1,2,3,* , Ke Yang 1,2,3,4 and Xiaodong Wang 5
1 2 3 4 5
102206, China; wangxd@ncepu.edu.cn
* Correspondence: lixinkai@iet.cn; Tel.: +86-010-8254-3038
Received: 29 January 2019; Accepted: 7 March 2019; Published: 12 March 2019
 Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, China; yangke@iet.cn Key Laboratory of Wind Energy Utilization, Chinese Academy of Sciences, Beijing 100190, China
Dalian National Laboratory for Clean Energy, CAS, Beijing 100190, China
Graduate School, University of Chinese Academy of Sciences, Beijing 100049, China
College of Energy, Power and Mechanical Engineering, North China Electric Power University, Beijing
 Abstract: To explore the effect of the height of vortex generators (VGs) on the control effect of boundary-layer flow, the vortex characteristics of a plate and the aerodynamic characteristics of an airfoil for VGs were studied by both wind tunnel experiments and numerical methods. Firstly, the ratio of VG height (H) to boundary layer thickness (δ) was studied on a flat plate boundary layer; the values of H are 0.1δ, 0.2δ, 0.5δ, 1.0δ, 1.5δ, and 2.0δ. Results show that the concentrated vortex intensity and VG height present a logarithmic relationship, and vortex intensity is proportional to the average kinetic energy of the fluid in the height range of the VG. Secondly, the effects of height on the aerodynamic performance of airfoils were studied in a wind tunnel using three VGs with H = 0.66δ, 1.0δ, and 1.33δ. The stall angle of the airfoil with and without VGs is 18◦ and 8◦, respectively, so the VGs increase the stall angle by 10◦. The maximum lift coefficient of the airfoil with VGs increases by 48.7% compared with the airfoil without VGs, and the drag coefficient of the airfoil with VGs is 84.9% lower than that of the airfoil without VGs at an angle of attack of 18◦. The maximum lift–drag ratio of the airfoil with VGs is lower than that of the airfoil without VGs, so the VGs do not affect the maximum lift–drag ratio of the airfoil. However, a VG does increase the angle of attack of the best lift–drag ratio.

Source: www.researchgate.net › publication › 331712096

Graph C tells the story.  Better to go with slats and flaps.  Vortex Generators remain valuable to alter the airflow characteristics of a section of the airfoil or control surface.

Drag producing and lift destroying chaotic turbulence on the upper rear of the wing appears to be reduced by the Vortex Generators at higher Angle of Attack.  Notice turbulence starts to form before the wing is fully stalled on a "clean" wing, resulting a peak Lift/Drag  of around 6 degrees.  By energizing the air flow and keeping it attached, more lift can be created at higher AOA as seen in Graph A.

However, anything that cannot be retracted will carry a drag penalty at optimum L/D, but, as with many larger radius leading edges, Vortex Generators, or VGs, can make the L/D curve much smoother and predictable rather than a sharp break from a very narrow peak.

VGs have found application in elevators, but a flap beyond a given angle of deflection is actually more useful stalled and draggy.  Lower degrees of flap deflection, along with slats, serve to increase camber of the wing to create more lift at lower speeds, also with a drag penalty compared with "clean".

Note the airfoil used in this study is cambered.  It starts to generate lift a -5 degrees AOA, and drag with VGs is actually greater than "clean" at lower AOA typically found in cruise (Graph B), with little difference in lift produced.


How does a non US pilot buy a N reg plane? I wish to buy a single engine light aircraft. I have a FAA ppc. I wish to buy an N reg plane and use it outside the US. I believe I need to have a trust set up any info would be appreciated. I am not a US citizen

The ownership requirements for registering an N-reg aircraft are discussed in this question. The current FAA information says that the only options open to a non-resident foreigner are:

  A corporation or association:

  organized under the laws of the U.S. or a State, the District of Columbia, or a U.S. territory or possession,
  of which the president and at least two-thirds of the board of directors and other managing officers are U.S citizens, and
  in which at least 75% of the voting interest is owned or controlled by persons that are U.S. citizens,

  A non-U.S. citizen corporation organized and doing business under the laws of the U.S. or one of the States as long as the aircraft is
  based and primarily used in the U.S. (60% of all flight hours must be
  from flights starting and ending within the U.S.)

So you either need a corporation that's majority controlled by US citizens, in which case there are no restrictions on flight activities; or, you can use a non-US citizen corporation but then most of the flight hours have to be in (or to/from) the US.

The ideal way to go about this would be to find someone near you who's already done it. Many flight schools and individuals outside the US operate N-registered aircraft so you might be able to find someone who can tell you how they did it. You'll need to do this research anyway, because if you're basing an N-reg aircraft outside the US then you'll need to find an FAA mechanic near you who can legally perform maintenance on it. If your country has some kind of GA or pilot's association then you could ask them too.

You can also look for companies (usually law firms) who specialize in doing this and get their advice and/or retain them to actually set up the ownership structure that you need. That's going to cost some money - and many firms are focused on business jets, not light singles - but it could be a minor cost compared to the aircraft and maintenance, depending on what you plan to buy. I found a few by Googling but a personal reference from someone near you would be a lot more useful.


Where can I get AMM for Boeing 737NG? I am not connected to airlines .I am just an aviation enthusiasts .while studying the 737 FCOM I realized to study AMM also .but in Google I cant find it anywhere .so can anyone tell me how to get access to AMM ?

Buy it from Boeing.
Documents like the FCOM and the AMM are copyrighted and proprietary documents and aren't meant to be freely available.  Sometimes you'll find one on the internet that somebody posted, but they did so in violation of a license.
If you do find an AMM (or excerpts from it -- as far as I know the entire document is enormous) on the internet, changes are good that it's well out-of-date.
That said, there are better resources than the AMM to learn about the operational side of the 737.  As a pilot who flies it, the AMM isn't a document that I study or even have in the electronic flight bag.  It's for mechanics, and in some cases its limitations are significantly different than the limitations in the flight crew manuals (the company equivalent of the FCOM).  There are things that the mechanics can do on an as-needed basis that they don't want the pilots doing during routine ops, and so our limitation, for example, was in the 737-300, only one pack run off of APU bleed air for cooling.  The mechanics could run both packs for cooling, but when they did that it wasn't in order to cool the aircraft, it was to load up the APU and see how it responded to a high bleed air demand.  The first time I saw them do that was a "wait, you can't do that!" kind of moment, but their manual allows it.
Boeing, however, does NOT want pilots performing hybrid procedures, or "I saw a mechanic do this..." fixes.  Things that may work fine on the ground at the gate, may be a really bad idea to try in flight.  So Boeing has pilot procedures & limits for the pilots, maintenance procedures & limits for the mechanics, and just like in Ghostbusters, "don't cross the streams... it would be bad."


(B737s) Why is the automatic activation range of the AUTO mode in flight limited to "between idle and 5%"? This is the explanation on the Engine Start Switch in the 737 series FCOM. What might be the reason why the automatic activation range of the AUTO (or OFF) mode in flight limited to "between idle and 5%," rather than simply to "below idle" as in the CONT mode? Is there any good systemic reason for the limitation?

The reason to go automatically to ignition below idle N2 is to try to avoid a flameout. If the engine has slowed down to be at or below 5% N2, it is way too slow for combustion to happen - you don’t add fuel to a normal start until about 25% N2, by comparison. So “below idle in flight” almost is a valid characterization of when AUTO will turn on the igniters, it is just that the bottom of that range stops not at zero but at a slightly higher value. Ignition isn’t going to accomplish anything between 0% and 5% N2 anyway. (Probably can’t accomplish much at 6% or 7% or etc either, but those are probably seen only briefly on the way to, or from, near zero, so the engineers picked 5% as their cutoff.)

Note also that some switches are

GRD-OFF-CONT-FLT

While others are

GRD-AUTO-CONT-FLT

So, for the switch with an “AUTO” position, there is no other “OFF” position, so there needs to be a cue to the system to turn the igniters off, rather than to let them run continuously on a shut-down motor. And rather than saying simply that OFF happens at 0.0% N2, they’ve specified a range, so that the igniters can be turned off for a shut-down but still slowly turning (windmilling N1) motor.


Is it still "landing" in a seaplane? This probably sounds like a silly question, but I was watching floatplanes land on Lake Union, Seattle, and it got me wondering whether a plane that touches down onto water is still talked about as "landing" in aviation parlance.

Is it?

Yes. One 'official' example is from the FAA's test standards for land and seaplanes, which includes these tasks for seaplanes:

  Task H: Glassy Water Approach and Landing
  [...]
  Task J: Rough Water
  Approach and Landing

The FAA's Seaplane, Skiplane, and Float/Ski Equipped Helicopter Operations Handbook also uses it; Chapter 6 is called "Seaplane Operations - Landings".

Interestingly, the word "landing" itself isn't defined in the FAA's glossary or in 14 CFR 1.1 (the US air regulations general definitions). Presumably it's considered to be too 'obvious' to require a definition.


Why doesn't G-force knock this pilot out twice? In this video an F-16 goes into a dive after the pilot loses consciousness. The Auto-GCAS corrects to avoid a crash, and the pilot regains consciousness and pulls up even harder. Why doesn't he pass out a second time?

According to this article and the G-force meter (at the top left) the pilot:

Goes into turn and loses consciousness at about 8g's.
The Auto-GCAS rolls and pulls up to a limit of 5g's.
The pilot recovers consciousness and pulls up to 9.1g's.
Pilot responds to radio calls to cancel the exercise and gain altitude.

It seems he held the recovery for quite a bit longer and at higher G-forces than the turn that made him pass out. I assume he also didn't have time to do anything breathing exercises or preparations for a high g maneuver. He is also experiencing the G-forces in the same direction both times (pulling back on the stick).

So why didn't the pilot pass out the second time?

Your body responds to a lack of oxygen to the brain with physiological responses that can take up to ten seconds to be effective.

...The brain is highly sensitive to cellular hypoxia (lack of oxygen), so rapid impaired function and almost unconsciousness follows (A-LOC), and then ultimately total unconsciousness (G-LOC). To avoid this, your body will reflexively make adjustments in heart output and blood pressure to increase blood flow to the brain (by parasympathetic response–an adrenaline surge), but it takes 6-9 seconds for this reflex to initiate.  By this time, unconsciousness has likely occurred, which obviously means no one is conscious to fly the aircraft.

Raptor 9.0g turn

In addition, the pilot likely made greater effort to avoid a second blackout.

Breathing: Rapid (< 1 sec) exhalation/inspiration cycles every 3 seconds.  This maintains oxygen content and decreases carbon dioxide in blood, while also relieving increased pressure of chest, and allowing the heart to refill with deoxygenated blood from the rest of the body.

Isometric Contraction:  Flexion of skeletal muscles of legs and abdomen.  When muscles contract, the small blood vessels in them constrict, making them smaller with less room for blood.  This step increases pressure in chest and displaces blood away from these contracted muscles into the upper body and brain.

Together, some combination of these measures would prevent a repeat blackout.


Was the "Air Berlin" lap of honor over Düsseldorf problematic? A few days ago, flight AB 7001 (Air Berlin, New York - Düsseldorf) went for a "lap of honor" around the airport.
One video can be found here.
No emergency or technical issue, it just was that airlines last ever flight on that route (they're bankrupt), and the pilot wanted to give a nice gooodbye.

Now, of course comment-sections on news regarding this are full of "experts" saying if it was ok or not. Here the question for the ACTUAL experts I can reach:
Did the pilot do anything "bad" or even dangerous, or should a quick circling of the airport be just fine as long as the pilot can actually, well, fly his airplane properly?

UPDATE:
The Bundesaufsichtsamt für Flugsicherung (BAF) (German bureau for flight-safety) agrees with this page, the manoeuvre was ruled to have been legal and safe.

The maneuver was safe.
A spokesperson of the DFS (German Air Traffic Control) told the Berliner Morgenpost that the pilot requested and was given clearance to fly a left curve in case of a go-around. The necessary distance to other aircraft was ensured and a specific height for the maneuver was not determined.
The spokesperson also said that the DFS believes that this maneuver was safe.
However the LBA (Federal Aviation Office) started examining the case and requested Air Berlin to submit a statement which provides a reason for the go-around. The editorial staff of that newspaper believes this to be a routine procedure.
source (in German)
A rough translation of the relevant part in english:

Christian Hoppe, spokesperson of the German Air Traffic Control confirmed that: "The pilot previously requested if he is allowed to fly a left-curve in case of a go-around and was given clearance." The necessary distance to other aircraft was ensured, the specific height of the maneuver was not determined. "From the DSF's point of view the maneuver was safe." The Federal Office for Air Navigation corrected earlier information, which claimed that the pilot had reported problems with the landing-gear. "That was our assumption."
The Federal Aviation Office will nonetheless examine the case, Air Berlin should submit a statement on the reason of the go-around. Our editorial staff believes, after multiple requests, that this is most likely routine operation.


What is the difference between a business jet and a regional jet? At first glance, business jets and regional jets seem virtually identical - both are small jets, usually low-winged twinjets (unless they're by Dassault, in which case they're trijets) with tail-mounted engines and T-tails.

What differentiates, say, a Legacy 600 or a Falcon 9 or a G650 or a Learjet 75 from a DC-9 or a CRJ-100 or an F-28?  Is it merely who operates it?

I would say its not quite who operates but how its operated. A business jet is typically outfitted to carry a small number of people in a very luxurious way while a regional jet is outfitted to carry a lot of people a short distance.

No one even says business jets have to be small or of a select list of airframes, you can order a 747 business jet if you have the cash. Similarly no one says a regional jet has to be small, we are now seeing 747SR's doing short haul flights in Japan.

From a legal standpoint, the difference may be in the operation type as well. Scheduled regional flights here in the US will occur under part 121 while charter business operations will generally occur under part 135 if you own the plane the operation may even occur under part 91. This mainly has differences on inspection intervals for the airframe and some other operational related things. Assuming you can land the plane on the runway and depart again safely these don't limit where you can go.


Why are traditional knobs not replaced by touchscreens in cockpits? Looking at this picture of an A380 cockpit I see that there are still a lot of traditional switches (meaning manual click on and off or knobs). Being technology so advanced in these days, why is it that traditional switches and knobs are used instead of having a couple of touch screens on which the various controls are placed?

Wouldn't this make it more economic as there would be savings on material and wiring? They could also be less susceptible to failures as traditional switches and knobs still rely on mechanical actions.

Another advantage I could think of is that every pilot could customise positioning of controls and switches based on his/her way of piloting. This could then be reset to a default state every time a new pilot takes over.

Adding a pilot's perspective to what others have said:

Look at the switches and knobs in an airplane up close some time. You'll notice that nearly all of them have different textures,heights,sizes or shapes. That's intentional. Pilots train and train and train and train on checklists, especially the emergency ones. Muscle memory is a big part of quickly and correctly executing an emergency checklist.

As an example, flap levers/switches normally have a cap or top that is flat and parallel to the wings. It's easy to identify solely by touch.

Checklist item: "flaps up"
Action: Hand to flap lever (automatic after doing it a hundred times in training), verify feel of lever, move, look to verify

Without the tactile element, every motion would have to be verified by looking before it's made. With the tactile element, the movement can be made and then verified visually while the hand is moving to the next item. In an emergency, every second counts and the savings from tactile + verify matters.

Beyond emergency procedures, they're simply safer. The plane I fly the most has a touch screen display and several non touch devices. In turbulence, the touch screen is basically useless. But, the important devices all have buttons and knobs. I can be banging my head on the ceiling (literally) and still adjust the auto-pilot or radio. In that kind of turbulence, the touch screen device is completely useless.


What is the minimum size of a ramjet? Having recently learned that 155 mm ramjet artillery shells are now a thing, I wonder how much ramjet engines can be miniaturized. Could we see ramjet tank shells? Man-portable recoilless guns? Shotgun shells? Rifle bullets? For less martial applications, could we see micro-ramjet engines for small drones, similarly to micro-jet engines? (Whether or not it would actually sell if built is out of the scope of the question, let's simply imagine that there is a market for ramjet-powered microdrones.)

Given current (or near-future) manufacturing techniques, what would be the limiting factor for ramjet miniaturization? What would be the performances for the smallest engines, and how difficult/expensive would it be to make them? Are there theoretical physical limits to the minimal size of a ramjet engine?

Note: this ramjet-assisted shell is probably using solid fuel, but limitations for liquid fueled ramjet specifically are also relevant.

Ramjets could be miniaturized, but at some point, air viscosity and boundary layer thickness will make it impossible for them to work. Simply put, the intake hole will get so small that air will find it easier to flow around the whole thing and ignore all of its complicated inner structure.

In more technical terms, as the intake shrinks, intake velocity will approach an asymptotic limit regardless of flight velocity due to boundary layer friction, and for very small intakes, the air mass flow will be insufficient to sustain fuel combustion. That point seems to be somewhere in the centimeter range, but an engine becomes increasingly less useful as it approaches this point. Manufacturing parts this size isn't a problem even now.

Ramjet shells can help long-range artillery, but ramjet tank shells would not be effective in combat.

The figure of merit for tank rounds is their high-density penetrator's sectional energy at the target. Discarded sabot rounds are more effective at delivering this than any kind of engine internal to the projectile. Combat ranges are short enough to preserve the velocity.

If you had to use some sort of engine for a tank projectile, it would be a rocket. The reason is easiest to explain in numbers. At a range of 7.5 km with average 1.5 km/s velocity and a 10 kg round, to get 500 m/s of delta-V, you need 10 g = 1 kN of thrust (engine), plus 50 seconds of impulse (fuel). We'll ignore engine mass for the first approximation.

A rocket with TWR=50 and ISP=250 seconds will weigh 100 kg / 50 = 2 kg for the engine + 10 kg * 50/250 = 2 kg for the fuel, for 4 kg total.
A ramjet with TWR=5 and ISP=1000 seconds will weigh 100/5 = 20 kg for the engine + 10 kg * 50/1000 = 0.5 kg for the fuel, for 20.5 kg total.

Simply put, when flight time is this short, a heavy engine like a ramjet doesn't pay for the minute amount of fuel (or rather oxidizer) mass it saves vs a light and powerful rocket.

This applies to all combat applications within visual range: rockets are better, and guns are better still if you can afford the launcher weight. Beyond visual range, rockets or hybrid gun/rockets take over, and past 100 miles, air-breathing engines become viable.


What is the cause of the whiny noises in cabin of the Boeing 787? What are the various whine noises coming from the Boeing 787 cabin? Especially near the wing. I have seen tons of YouTube videos in which it has a noise during takeoff, or when flaps are being lowered or raised?

Other people have complained about this too. Boeing should rename the type to the "Whineliner".

The noise is caused by a hydraulic pump which is situated directly below the cabin. It mostly runs on takeoff and landing, but it can also run automatically at other times.

Dogs have been reported to need psychiatric treatment after being transported on a 787.


What is the stress tolerance of landing gear of commercial aircraft? Context: The landing skills of commercial airline pilots vary widely -- ranging from the ability to routinely make feather-touch landings and taxi and almost imperceptible deceleration on the one hand, to habitually landing with a medium-heavy thud and decelerating abruptly, even under normal flying conditions. One guesses that the landing forces on various parts of the landing gear (and also other parts) would momentarily vary by a factor of three or four, or even greater, on some key parts.

Questions:

1) Please elaborate on the stress tolerances built into various parts of the landing gear of common commercial aircraft, such as the Airbus A320 and Boeing 737?

2) What kind of stress testing is done on crucial parts of the landing gear, such as tyres, wheels, axles and hydraulic parts?

3) What are the backups and fail-safe measures designed into such landing gear?

First off, the thud and deceleration may be more related to the conditions as well as the runway length than pilot skill, but lets discuss some of your questions.

  1) Please elaborate on the stress tolerances built into various parts of the landing gear of common commercial aircraft, such as the Airbus
  A320 and Boeing 737?

This is a complex question to answer simply as airplane gear sees lost of loads and some compromises are made for simplicity of operation. For one the rubber used on the tires is quite thick since some of it does wear away on every landing since the gear is not spinning when the plane touches down (you can read more on why not here) The tradeoff here is weight (you need to haul the tires around with you) but they can be used for many cycles. The vertical load is generally handled by an Oleo Strut these allow for a margin of error but they (or the gear system) can be bottomed out and a hard enough landing can result in a bounce. The last serious load that gear sees is usually side loads when a crosswind landing is made. These are tough to mitigate and its generally the job of the pilot to reduce these loads they will manifest them selves as tire wear and mechanical wear. Perhaps the coolest solution to this was the B-52's full rotating gear.

(Source)

One of the more interesting non Oleo designs for GA gear is the rubber disks used on Mooneys. These are said to require less maintenance and are easier to replace but are far less forgiving than Oleo struts.

(source)

Some of the vertical force is absorbed by the tire its self when the plane touches down the tire does ballon out a bit. As such it is of vital importance that the tire pressure is within a small margin since low pressure can cause excessive ballooning which would lead to side wear depending on gear design.

  2) What kind of stress testing is done on crucial parts of the landing
  gear, such as tyres, wheels, axles and hydraulic parts?

These parts are generally made by different manufactures and are designed to comply with the aircrafts specs as set fourth by the builder (Boeing etc). Like everything else on a plane these parts have a cycle life (TO/LD) and are replaced when necessary. Since they are high impact I would assume the life is on the shorter side generally speaking. Here is some info from Goodyear on how to care and maintain tires for aircrafts. Heres some good info on Oleo struts as well.

  3) What are the backups and fail-safe measures designed into such
  landing gear?

This depends on the plane, bigger planes generally need to spread the load out over more axles/tires which gives them greater redundancy a plane like a 747 has 2 sets of twin axle main gear on each side of the plane so you could lose an axle and still be ok (relatively speaking)

(source)

compare this with a 737 and you will see that the main gear is less redundant as its a single axle only system. In this case losing one of two tires would be less catastrophic than losing all the gear.

(source)

Compare this all to a smaller GA plane where you generally have a single wheel situation

(source)

The one thing all these planes have in common is that the nose wheel is the most delicate part of the landing gear. Hard landings on the nose wheel can collapse the gear or cause porpoising both of which are bad. This is what happens if the gear does not rotate properly in other words that nose gear saw complete side loads. Planes have be successfully landed with collapsed nose gear or nose gear that wont come down. Its a sight but generally speaking its not the worst thing to happen.

Sensors: The only sensor I know of on the gear (and this is plane dependent) is the squat sensor which is a simple on off for being on the ground or not. The FDR will record 3-axis acceleration but that will be of the airframe which is behind the Oleo struts so the gear its self may see higher loads.


How is the emergency locator transmitter of the A320 fixed, activated and carried away? I've some closely related questions about the emergency locator transmitter, ELT, the "distress beacon", on the Airbus A320. From this question about antennas on the A320:

The ELT antenna seems to be located aft of the cabin.

However the ELT might need to be activated manually in case the acceleration sensor doesn't work. More importantly the ELT must be taken away if the crew/passengers decide to move away from the aircraft wreckage, e.g. after a ditching.

It could be interesting to know some details before being the only survivor of a A320 crash in the middle of the Amazon rainforest:

Were is the ELT located and how is it fixed?
How is it manually powered?
How is it connected to the fuselage antenna and to the overhead panel?
Is it possible to remove it from the aircraft and take it away?

It's a self-contained unit with its own battery, GPS transmitter and antenna, with a port that can be plugged into the antenna connector of the aircraft. Just disconnect, stick out the whip antenna, and swim downstream the Amazon :)

Image source

Here is Australia boats are required to bring a similar device on board called an EPIRB if going off-shore for more than a couple of miles, also a really good idea if driving from Darwin to Adelaide with a detour through the desert.


What are the slash markings on Gatwick's 08R/26L? Google Earth @ 51°08'52.24" N   0°11'35.71" W

The markings are at 100 m intervals, to one side only of the centerline. They're also lighted as seen below.

They are missing from the ICAO aerodrome chart, and are not mentioned in CAP 637 (Visual Aids Handbook).

I measured from each threshold, but the distances to the first in each set don't match. They also don't seem to be new (judging by older satellite imagery).

YouTube

They are "Rapid Exit Taxiway Indicator Lights" as described in section 9.6 of CAP 637.

9.6 Rapid Exit Taxiway Indicator Lights
9.6.1 Rapid  exit  taxiway  indicator  lights  (RETILs)  provide  pilots  with  distance  to  go information to the nearest rapid exit taxiway on the runway, to enhance situational awareness  in  low  visibility  conditions  and  enable  pilots  to  apply  braking  action  for more efficient roll-out and runway exit speeds.
9.6.2 RETILs consist of six yellow lights adjacent to the runway centreline and configured in a three/two/one pattern spaced 100 m apart; the single light is 100 m from the start of the turn for the rapid exit taxiway, see Figure 1.6.


What is this fighter aircraft parked in the South African desert? What aircraft is parked at 27°50'24.0"S 21°35'54.4"E in the South African desert?

According to the unofficial South African Air Force forum it is a fake Mirage F1, intended as a decoy. It seems it works as intended.

According to that forum, approximately 30 were built. They were constructed of metal tubing, metal plates and glass fibre. According to one post they even contained heating elements to give infrared sensors on satellites the impression that the "exhaust" was hot.


How precisely must ATC speed instructions be adhered to? Is there any speed tolerance in regards of ATC speed control instructions during the landing approach?

You don't say what country, but I'll base this on the U.S... There is no "rule" here, however you can expected to be held to the Airmen Certification Standards. These standards are used by your designated pilot examiner (DPE) to determine if you are able to perform according to published standards.

For example, the Commercial Pilot ACS, depending on what phase of flight you are in, says that you should be able to maintain a specific speed +/- 5 knots. A lot of places in the ACS say "maintain manufacturers recommended speed".

So the FAA expects you to be able to reasonably maintain an assigned speed given that you were provided your license in accordance with ACS. That being said, if ATC assigns you a speed (N1234 Maintain 200kts until established), you should be OK if you are doing 190kts, or 210kts, but if you are blowing through there at 250kts, ATC will chime in and ask you to slow down.