How are pushbacks coordinated between the pilot and the tug? When the pilot (PIC) of a large jet is cleared for pushback and start, how does he signal the tug to start towing the aircraft, and where to go to?

The pilot has to monitor the ground frequency the whole time, so is there a second radio for talking to the tug crew? Or perhaps a wired link between the aircraft and the tug?

Does he just tell them to "take us to start point alpha" or whatever, or does he have to give more detailed instructions?

Are the tug crew listening in to the instructions from the tower?

If the aircraft has to stop unexpectedly (because of an instruction from the tower, or because the pilot has a safety concern), how is this coordinated safely?

Generally speaking1, when an aircraft is allowed/ready to leave (deliberately not using the word depart) the gate, things proceed in the following fashion:

The pilots are in contact with Ground Control, and when the aircraft is ready to leave, Ground approves that they can leave the gate.
The pilots asks groundcrew that the pushback can start.
Pilots are in contact with a designated personal from groundcrew (see picture below).
Pushback tractor/tug will start moving the aircraft away from gates, onto the place where aircraft will be ready to move on its own.
During the pushback, aircraft's engines are started2.
Pilots inform tug driver/designated person from groundcrew which direction the aircraft needs to be facing at the conclusion of pushback.
When aircraft is ready to roll towards its assigned taxi instructions, marshaller signals the pilots that aircraft is in the requested position and pushback tug is about to be detached.
After the tug is disconnected, the designated groundcrew personal communicating with pilots, informs the pilots and disconnects his headset. The marshaller signals (after moving away from the aircraft) the pilots that the aircraft is ready to proceed on its taxi route.
Pilots confirm and acknowledge this last signal, and aircraft starts to taxi.

I have read that different airports have different procedures for the above mentioned activities. Although, I have always seen that a person from groundcrew attaches a headset near the nose gear of the aircraft (picture below) to communicate with the pilot.

Image Source
Please see the cable connecting the headset with the aircraft, near the nose gear. You cannot follow the entire cable though.

1: This is what a typical scenario looks like. Although, there can be great variations.
2: Engines are not started near the gate as there is groundcrew servicing the airplane and to avoid damages to close-by structures.


How is a traffic pattern oriented, entered and exited? If you've received traffic entry instructions from Heathrow Tower (sim) to fly left downwind for runway 9L, does that mean to be to the left of runway 9L (for a right-turning traffic pattern, where downwind is north of the runway) or to enter a left-turning traffic pattern? I ask because the latter would take you over runway 9R/27L and I'm unsure whether this is a problem or whether it's ok because of the difference in altitude between a plane in 9L's downwind and 9R's/27L's final/upwind.

Also, if you've received clearance from runway 9L to depart north, should the departure be done from the traffic pattern? E.g. you take-off, turn crosswind, turn downwind then turn base and depart the airspace? Or should you continue straight until you've left the airspace, then change course?

The instruction to join a "left downwind" is used to clear an aircraft into a standard traffic pattern with lefthand turns. The word "left" could be omitted, as only right turns and righthand traffic patterns are called with "right". In the case of EGLL - London Heathrow the standard pattern would be a northern traffic pattern for runways 09L/09R and a southern traffic pattern for runway 27L/27R.

If you should come to the unlikely situation that you are VFR and want to depart from Heathrow, the clearance to leave the CTR in a northerly direction would be best executed leaving the crosswind, but clarification on frequency never killed anybody:

A: Heathrow Tower, G-ABCD, request leave CTR direct northbound after crosswind 09L.

or

A: Heathrow Tower, G-ABCD, request heading 360 after turning crosswind 09L.

When departing an aerodrome with traffic, it is unwise to leave the CTR along the extended centerline/upwind, as you are impeding other traffic, especially in slower aircraft.


Does CG location affect the stability of the Dassault Falcon 7x while on the ground? Can any one please explain me the basic static stability of Falcon 7x on ground at aft most CG which is 38.5%MAC=12.63498m ... because as per my hand calculations i m getting around 2% load/weight on Nose landing gear... is there something wrong with my calculations or the CG doesn't really affect the aircraft stability...?

I have used following method to determine the load

$$Fn_{min} = \frac{Bm_{min}* W}{B}$$

$Fn_{min} = 4090.817398$, % of load $2.093923332$ where $Bm_{min}$ is distance from Main Landing gear to the aft most cg = $0.201 \mathrm{m}$, $W$ is the design limit weight $=19915 \mathrm{kg}$, $B$ is the wheel base $= 9.5644 \mathrm{m}$.

2% weight on nose wheel sounds plausible.

Almost all weight of an aircraft must be on the main wheels. Otherwise:

the elevator would have to be huge to generate enough force to lift the nose wheel off the ground on take-off and
on landing as the main gear contacts runway first it would create strong moment that would slam the nose into the ground.

See also Why do planes always land on rear wheels instead of the nose wheels?.

I've seen values like 4% weight on the nose wheel, so 2% for CG at aft limit makes sense. It is still enough to keep the aircraft level and leave some efficiency of the steering.

And yes, loading mistake can make an aircraft tip over on the tail:

(copied from this answer; original source)


What are the rules of thumb for intercepting and tracking VOR and LOC courses? Pilots training to become instrument rated must learn to intercept and track VOR and LOC courses. What are the basic rules of thumb all pilots should learn regarding picking a course for intercept and making corrections to maintain the course? How do they vary between VOR and LOC courses?

the rule of thumb for IFR flying, including intercepting VOR and LOC courses, is the 5Ts:

Throttle: turn the throttle to initiate a climb or descent
Turn: to the new inbound or outbound heading
Time: reset the timer to time the
new leg
Twist: the OBS to the new course you're intercepting
Talk:
if you are at a reporting point, you should now use the radio


How much of a plane is replaced over the lifetime of an aircraft? A photo of an OC-135 with (presumably) P&W TF33-P-5 engines made me wonder how many of the components had been replaced, and what was left of the original airframe, engines, and other parts. There are many aircraft operated today that are at least 50 years old (no real significance to that particular number).

Over the lifetime of say, a C-135/707 airframe without engine upgrades, is any of the original engine left? Are body panels replaced? Landing gear? Control surfaces?

In the US an aircraft may be allowed to fly indefinitely so long as it continues to meet the requirements of the definition for "Airworthy" in Court Order 8130 (.2e par 9).

  Note: Aircraft operated by government agencies or the military do not
  have to comply with any rules, laws, or regulations - they may operate
  in any manner the agency desires (as long as they are not used for paying passengers). These aircraft are described in FAR 1.1 as "Public Aircraft". Examples: military fighters and transport, firefighting, research, etc.

There are two possible scenarios for replacing a part.

1) Life Limited: The manufacture or FAA through an AD (airworthiness directive) requires a part to be replaced at a certain limit i.e TIS (time in service) - calendar or operating hours, number of operating cycles (i.e. number of landings or pressurization cycles), etc. The manufacture is required to provide a chapter/section of the POH (Pilots Operating Manual) that list all life limited parts.

2) On Condition: Maintenance personal (often during an inspection) find the part 1) inoperative, 2) near or beyond "Service Limits" published by the manufacture, 3) in the discretion of maintenance personal worn and no-longer safe for operation, deemed not airworthy, etc. These parts are repaired or replaced as needed.

In practice many older aircraft have been completely replaced (technically a "repair"). There is no limit on the amount an aircraft can be "replaced".

War birds are one such example. It is legal to salvage an airplane from the ocean floor, use the corroded metal as a template to make new parts and completely assemble an aircraft from new material. The restored aircraft must be identical to the original aircraft and is legally considered the same aircraft ("all" the parts where repaired!).

There are many examples, such as the P-38 "Glacier Girl" being one of the more recent well document examples. It was salvaged after being crushed under 265ft of glacier ice for over 50 years.


How exactly is the 51% rule applied for amateur-built aircraft? I understand that for an amateur-built that you are required to build at least 51% of the aircraft yourself, but how can there be kits which appear practically completed, just some quick assembly jobs? Could someone explain how this 51% rule works, and what is this percentage based on?

The requirement which states that at least 51% of the tasks needed to make the aircraft airworthy must be accomplished by amateurs is 14 CFR 21.191(g). Presumably this would exclude finishing items like paint, some instruments, avionics, interiors, etc., as those are items which in most cases are not required for airworthiness.

The FAA has several checklists which inspectors use to evaluate the "major portion" requirement. Homebuilders thoroughly document the construction process (or at least, they should!) in a builder's log; a lot of these tend to turn into massive photoblogs. AC 20-27G contains guidance on what this documentation should look like and contain.

The "two weeks to taxi" builder-assist programs that exist out there generally get the basics working, but they certainly aren't "two weeks to flight". Most manufacturers carefully calculate the amount of work they can offer as a kit, based on the number of tasks remaining for the builder to finish.

I'm not a builder, so I can't say for sure, but I understand most of the advantage to these programs is not that somebody is helping you - it's that they can supply tools, jigs, and a lot of behind-the-scenes support like organizing your workflow and providing answers to questions with no turnaround time. They're not there building the plane with you; they're there to make it possible for you to build the plane more quickly... if that distinction makes sense.

EAA also offers a pretty good FAQ for the 51-percent rule, which I admit to paraphrasing here and there!


Where is the airport in Disneyland Paris? (DLP) I just found out that there is an airport in Disneyland Paris with the IATA code DLP. Since I've never heard of it, I wanted to see it on Google Maps, but haven't found it.

Is there actually an airport, and where?

Why does Disneyland has its own, with CDG not far away?

IATA codes don't necessarily correspond to airports. Some non-airports have IATA codes to allow airlines to sell through tickets that might, for example, consist of travelling from A to B by plane and then onward from B to C by train. DLP isn't on Wikipedia's list of railway stations with IATA codes but I imagine something very similar is going on here.


Why is the magneto drop check done at 1700 RPM? In our Cessna 152, we carry out the mag drop check at 1700 RPM, following the POH. My question is why it is done at 1700 RPM? What will happen If I check it at 2000 RPM or less than 1700 RPM? And if I do check it at another RPM setting, will the RPM drop get be acceptable, or will it give an incorrect result?

According to this article from the New Zealand CAA:

  The particular rpm setting at which the check is done is not very
  important for the purposes of the check, but other engine handling
  considerations will usually mean a recommendation from the engine or
  aircraft manufacturer for an rpm setting from the low end of the
  cruise power range. From the magneto checking point of view, the
  important aspect is that the check on any particular aircraft should
  be done consistently at the same rpm, so that any trend can be
  monitored. There is little to be gained, for example, in comparing
  today’s 150 rpm drop at 2000 rpm with yesterday’s 100 rpm drop at 1700
  rpm

And this Lycoming service instruction mentions a range of 1800-2000RPM:

  Aircraft that are equipped with fixed pitch propellers, or not
  equipped with a manifold pressure gage, may check magneto drop-off
  with the engine operating at approximately 1800 RPM (2000 RPM
  maximum).

Obviously you should always follow the procedure and the RPM settings in the aircraft's POH, but those quotes suggest that getting the RPM exactly right isn't essential. However, the comment about consistency implies that the RPM drops will be different depending on your starting RPM, so if you don't use the POH settings then you have no way to know if what you're seeing is acceptable or not.

As for why 1700RPM and not some other value, I assume it's a setting that the engine manufacturer decides is high enough to put a realistic load on the engine but not so high that you could overheat it during the run-up checks. Or, as GdD suggested, it could be to avoid overloading the brakes. But I'm not an engine expert, and someone else might be able to give you a better explanation.


Why Lift-induced Thrust Required(Tr) decreases and Zero-Lift Tr increases with the increase of velocity? As the minimum Tr is at the point where L/D (lift/drag) ratio is the most. And Lift-induced Tr depends on the Coefficient of Lift, (Cl)^2 and Dynamic Pressure (q) which also increases as velocity increases. But why it is decreasing?
And how Zero-Lift Tr depends on velocity and is affected that much when its coefficient is a constant?

This happens because Angle of Attack required to maintain flight at a given weight decreases with the increase of speed.
As for the zero-lift Tr, also known as "parasitic drag", while its coefficient is a constant, its formula is not: it actually has a cube of velocity.


Is the RAT pump on the A320 standard fitment or is it an "option"? Is the RAT pump standard or optional on the A320 aircraft?
What about the B737?

A RAT is never an optional piece of equipment. It is only there because the design of the aircraft systems require it in order for the aircraft to meet airworthiness certification.


Is it allowed to use an ex-military aircraft overseas? We are considering using an ex US military helicopter in our counter poaching operations in the Central African Republic, while keeping it registered as a US aircraft with the US "N" number. We are not exporting it to sell and we will and we will maintain ownership and the US "N" number.

What restrictions are there for taking an aircraft like this out of the US, if any?

I know of a good many former US military helicopters in private and commercial uses in some African countries. And I know that some ex US military helicopters are not "eligible" to export, and some are.

What is the difference?

If it has an N-number and an airworthiness certificate that allows the operations you want to perform with the aircraft, it doesn't matter what it did in a past life, you can operate wherever the country that controls the airspace will allow you to do so. As was mentioned before, the regulations of the country in which you wish to operate the aircraft are more relevant to what you want to do.

If on other other hand you have an ex-military aircraft that has an N-number but an exhibition-only experimental airworthiness certificate for a warbird, to pick an example, or some other type of restricted certificate, you're likely to be required to notify the FAA where and when you intend to fly it, and that may trigger additional oversight and possibly a denial of the permission to do so in a foreign country. The operating limitations document of the airworthiness certificate may also limit you to specific airspace within the US. Check the aircraft documentation and see if the FAA already put limits on the helo's operations.

That's where you will find the answer from the FAA point of view.


What does a blue light by the toilet sign mean? When I was flying with Swiss Air we hit some turbulence. Of course, the seat belt light came on but above the bathroom sign, a blue light came on.

(Source)

Then after a while a second one came on. I'm just wondering if this has something to do with the turbulence or not.It was an Airbus A340 if that helps any.

You don't indicate which aircraft make and model you were flying in. I'm going to answer with some general information regarding Airbus aircraft, since that's the predominant manufacturer of aircraft currently in SwissAir's fleet. There may be subtleties from version to version.

Short answer: a blue light typically indicates a passenger has pressed the "call attendant" button above their seat. It's used to provide a central indication that assistance is required, and may help localise the row the passenger is seated in by only illuminating a blue light in some parts of the aircraft in closest proximity to the relevant row.

Longer answer

The panel with coloured lights to which you refer is the Area Call Panel (ACP). These panels are situated at either end of the cabin, and possibly in the mid cabin or at other locations depending on aircraft type and age.

The ACP typically has multiple colour indications, such as pink, blue, amber and green. Their meaning is as follows:

Pink: communication from cockpit. Normally steady, flashing if an emergency call is taking place (initiated in either direction: cockpit to cabin or cabin to cockpit).
Blue, steady: passenger call using at-seat attendant button in the passenger service unit (PSU -- overhead panel above each seat row). This light may only appear on particular ACPs, which enables the calls to be segregated by cabin and further by seat location.
Amber: assistance request from the lavatory. Steady indicates a passenger in the lavatory has requested assistance from an attendant. Flashing indicates a smoke detector activation.
Green: attendant call, using the cabin interphone to call between interphone stations.

The indication panel is integrated with, and receives event indications from, the Cabin Intercommunication Data System (CIDS). The CIDS is a crucial network on the aircraft responsible for handling crew communications, managing some aspects of the flight attendant displays, operating the speaker system and giving the relevant indications. Calls requiring attention from a flight attendant typically trigger a notification on the attendant loudspeakers and illuminate the relevant light on the ACP (unless already illuminated) in order to draw attention.

Furthermore, for passenger calls, the relevant seat number light in the PSU is illuminated when the attendant button is operated. This allows the request to be localised to the relevant row and side of the aircraft. If an aircraft door is open, the row number flashes -- typically a door open event leads to the PSU row indications to be illuminated to enable embarking passengers to find their seats more easily.

References

Only a small amount of information is publicly available about the ACP, its role in the CIDS and the CIDS more generally. The information here is corroborated by two sources:

A Lufthansa training manual for the A319/320/321 aircraft, published online. See pages 166, et seq.
Jonathan Hurst's site, in particular the excerpts of A320 technical operations manuals, chapter 10 "Cabin".

Any further sources or references, especially for non-A319/A32x models, are gratefully received in the comments.


How heavy are turbofan and turboprop propeller blades? How heavy are the blades from

Turbofan (CFM56-5B, RRTrent800, PW4062A) and
Turboprop (PW127M)

engines?

Is there a general appromixation like
$$ m_\mathrm{Fan} = 0.1 \times m_\mathrm{Turbine} $$
as well?

According to this ATSB Australia Report, the Original Blade Weight of the Trent 800 Blade that failed was 11.6KG. The weight of the blade in grammes is etched into the blade root.
https://www.atsb.gov.au/media/33974/tr200100445_001.pdf Page 9/10.
Fan Blades of the same type all differ (slightly) in weight and are chosen specially to balance the engine when it rotates, or balance weights are added to the blade much like your wheel on your car. This is to avoid vibration.


Can a pilot use glasses to meet the FAA required 20/20 vision? When the FAA said that pilots require 20/20 vision, does that mean that you have to achieve 20/20 vision with glasses or without glasses?

It can be with correction. And note that for third class, distant vision only has to be correctable to 20/40. The near vision standard is 20/40 for all classes. See https://www.faa.gov/about/office_org/headquarters_offices/avs/offices/aam/ame/guide/standards/

As @Pondlife notes, the operative words here are,  "with or without correction."


Are the advantages of a coaxial rotor on a helicopter also applicable to a ducted fan? A coaxial rotor helicopter is connected to the same engine and has the advantage of anti-torque and a +/- 25% increase in lift.
My question is if a coaxial ducted fan connected to the same engine would also have a +/- 25% increase in forward motion ?

Part of the gain of coaxial rotors on a helicopter is not having to power the tail rotor (IIRC it takes ~10%). So for contra-rotating propellers and fans, which don't need a tail rotor, the advantage is smaller, about 6–16% [1].

Now ducted fans (assuming as used in turbofan engines) usually contain stators. These do not stop the rotation of the flow (that takes away the energy) completely, but they do reduce it significantly and with much lower complexity. The remaining possible gain would probably be at most couple of percent and that is likely not worth the increased complexity and weight of additional gears. Don't forget that turbofans run at significantly higher rotation speeds than propellers.


How does the shift in aerodynamic center affect the performance of swing wing aircraft? I am trying to understand how a  variable sweep wing changes the aerodynamic center of a plane, and its effect on the plane's maneuverability. I also would like to know the importance of the pivot point as it somehow affects the center of pressure.

With so many questions I need to structure the answer:

Effect on performance

Moving the wing backwards will also move the aerodynamic center backwards, but the high sweep of a swing wing normally means that the wing's leading edge is still subsonic even at Mach 2. This means that the aerodynamic center of the wing ist still near the quarter chord, so the shift in the center of pressure is limited to the geometric shift from sweeping the wing. Generally, a swing wing needs to be combined with a generously sized horizontal tail to have enough trim authority for all sweep angles.

The first jet capable to change sweep angle in flight, the Bell X-5, therefore shifted the wing forward when sweep was increased. Later designs used fixed pivot points, however.

If no possibility exists to trim the aircraft by shifting weight (mostly fuel), swing wings cause high trim drag when swept back. The biggest source of drag, however, is the increased structural mass for the sweep mechanism which will cause an increase in induced drag. Further, the mechanism takes up valuable volume at the center of the aircraft which now cannot be used for fuel storage. In total, the substantial improvement in wing drag due to variable sweep is almost completely eaten up by the mass increase and the trim drag increase.

A clear benefit of variable sweep is improved low speed performance: Both the reduced sweep and the increase in wing span will reduce low speed drag substantially, so a swing-wring design will show superior loiter performance. Also, take-off and landing distances are reduced because the full wing span can be used for highly effective slotted flaps.

Effect on Maneuverability

Swing wings cannot have ailerons - the high sweep angle of the trailing edge at high speed flight makes control surfaces there almost useless. Therefore, a swing wing aircraft will use differential deflection on the horizontal tail in lieu of ailerons (tailerons). At low speed this reduces the possible roll rate -- the unswept wing contributes high roll damping while the tailerons will have limited effectivity due to their small lever arm. The F-14 uses wing spoilers to improve roll rate at lower sweep angles, but spoiler control is less efficient than ailerons.

At high speed (and sweep angles) the reduced aspect ratio, the increased sweep angle and the much reduced moment of inertia around the roll axis will combine to produce a very agile aircraft.

Pivot position

The further outboard the pivot point, the smaller the shift in the center of pressure. If the wing uses a highly swept root, the pivot can be positioned outboard without loss of efficiency at high speed. Examples are the Sukhoi 17 or the Tupolev Tu-22. An outside pivot position also allows to place external loads on the inner part of the wing (wing glove) and to avoid the complexity of moveable wing stations.

The Grumman F-14 also extends a small, triangular wing extension (glove vane) at the leading edge when the wing is swept back to reduce center of pressure travel. This feature, however, was disabled later to reduce weight and complexity.

The reduced top speed demands on modern fighters mean that variable sweep will no longer be attractive to achieve an optimum design. Variable sweep vanished with the Mach 2+ requirement in the 1980s.


Where can I find a cross sectional area plot for the A320 or other modern airliners? I'm studying to become a pilot and stumbled upon the Raptor Aircraft a while ago.

In one of our subjects we briefly discussed the concept of the area rule but did not get into much detail since our teacher told us that for subsonic airplanes it is not very relevant, especially not for slow flying piston aircraft.

But the designer of the Raptor Aircraft gets into a lot of detail why the Raptor is shaped like it is and what performance benefits it provides.

To me the charts provided on this site make a lot of sense and it seems plausible that there is a drag reduction of 15%.

I get that for airliners 'ease of build' is a major factor and therefore the cabin is a cylinder (reasons described in this question). (Sidenote: as far as I know the A380 has a double bubble design.)

Here comes the question: can someone provide a chart of the cross sectional area like this one for the A320 or another jet airliner?

What interests me is how much performance gain/fuel savings/drag reduction could be achieved if one were to optimise the cross sectional area of an airliner.

There seem to be plenty of drawings available with A320 dimesions.

https://www.google.com/search?q=a320+cross+sectional+area&tbm=isch&source=iu&ictx=1&fir=xqsB2UJmwNJExM%253A%252CZ-goV0XUZnxtFM%252C_&usg=__VvKRKRr0Rto02mNZc1OcLUazPPY%3D&sa=X&ved=0ahUKEwjUpvCm54XbAhWBPZoKHXyTC9gQ9QEILTAB#imgdii=3XaIv6qHo-5doM:&imgrc=IdDuWmIhnnky0M:  I'd suggest pulling the data from several as needed and model the A320 so you can try changes in SolidWorks as the Raptor design team did for their video.

Maybe SolidWorks has a student license you can download & use.

https://www.solidworks.com/


Which is better: a rotor / prop followed by a stator, or contra rotating rotors / props? It is all in the title.

I'm interested in power absorbed/used for a given thrust, or thrust generated for a given power (same diameters of course).

Just thrust and power.  Ignore materials, electric vs turbine, complexity of the  contra-rotating mechanism, etc.

Undisturbed inlet flow provides the highest efficiency.

The single rotor with stator has a straightforward inflow; outflow will have a tangential component which is lost for thrust generation. The stator will bend the outflow back in the axial direction, but there still remain pressure increases with blade passing frequency.
The contra rotating rotor has one of the rotors in the disturbed flow of the first rotor - this second rotor will therefore have lower efficiency in transforming engine power into thrust.


Was there any progress in ramjet-powered helicopters after 1960? It seems in 1950's some countries investigated production of helicopters powered by ramjet engines on rotor tips - examples include American Hiller YH-32 Hornet or Polish JK-1 Trzmiel. The enthusiasm was short-lived - high fuel requirements, a significant load to the power transmission system, noise and high visibility on battlefield were significant disadvantages that ended the research programs which I know of.

Still, that's over half a century ago, and the technology has progressed quite a bit since then so maybe some technical problems of that epoch are now past? - The ramjets are small, lightweight and quite simple in construction, so despite the disadvantages the technology doesn't seem like a total dead-end. Was this line of helicopter propulsion investigated further? Could someone provide a progress report - if there is anything to report? Or is this research branch still dead, with no attempts to revive it?

Tip jet rotors are still being developed, none of them are ramjet powered as far as I know. The Dragonfly DF1 is tip rocket powered, while the Pegasus uses air jets powered by a conventional engine.

Ramjet technology has not improved, so the cases for using them on rotor tips has not changed. They are still too heavy, thirsty and noisy.


Why do the F/A-18 and the F-22 Raptor have horizontal stabilizer as well as canted rudders for pitch control? F/A-18, F-22 Raptor have a horizontal tailplane as well as canted rudders? Why can't the horizontal tailplane done away with (as in the F-117) and have only the ruddervators for pitch control?

The rudders on modern fighters are canted mostly to reduce their radar cross section. A straight vertical tail would produce a corner reflector in combination with the fuselage or the wing and would send radar waves straight back to their source. To reduce the detection radius, such a behavior must be avoided. The first aircraft to use this trick was the SR-71.

On the F-18 an additional reason was to move the tips of the vertical tails into the lateral position of the chine vortex: The wing's big leading edge extension produces a strong vortex at high angle of attack which improves the effectiveness of the tails and gives better yaw control. However, the bursting of the vortex would cause heavy buffeting, and an additional fix was needed to improve the fatigue life of the tails.

The horizontal tail is a necessity for pitch control and to trim the flap moments in the landing configuration. Just look at Boeing's entry to the JSF competition: The X-32 was initially planned as a delta with only vertical tails, but when the Navy refined their carrier operation requirements, Boeing had to add a conventional horizontal tail to achieve the required angle of attack and minimum speed.

Boeing X-32 (left, source) and the planned production version (right, source) with the added horizontal tail. The F-117 was an Air Force plane with very limited agility, so it could get away with a tailless design. Real fighters and carrier airplanes cannot.

Giving the horizontal tail dihedral reduces its effectiveness in two ways:

Angle of attack changes from pitch variations are reduced by the cosine of the dihedral angle, and
the vertical component of the aerodynamic force is also reduced by the cosine of the dihedral angle.

Note that the drag of a V-tail depends on the total lift created, regardless of its direction. Only the horizontal component is effective for pitch control, so a steeply angled V-tail is very ineffective for pitch control. In air combat the high drag will reduce performance when it is needed most.


What holds the overhead lockers? If they carry a lot of weight inside, what avoids them from falling down to the passenger's heads? What holds them? How are they mounted inside the cabin?

The overhead bins are usually mounted to the aircraft structure. For example, in case of Boeing 737, they are mounted to the stingers using tie rods. From an accident report of Boeing 737-400:

  The forward inertial load is reacted by a diagonal tie, mounted at one end to stringer 6 or 7 in the fuselage crown and at the other end to a fitting on the upper surface of the bin. The vertical and lateral loads are reacted by short tie rods attached to the fuselage.

B737-400 overhead bin attachment. Image from the same report.


How are pilots prepared for a go around on a simultaneous converging approach when the "converging chart" is not published? Source: adsbexchange.com

Today I learned that at KPHL (Philadelphia) simultaneous converging approaches are used. (Note that AAL is faster, which narrowed down the time gap by the time it landed.)
Those planes are approaching to runways 35 and 27R.
But, those two runways do not have "converging" approach plates (red paths below). The published ones are for 09R and 17 – ILS V RWY 09R (CONVERGING) and ILS V RWY 17 (CONVERGING) – with missed approach paths that diverge like so (blue paths):

Source: skyvector.com; modified

By checking JO 7210.3BB - Facility Operation and Administration:

SIMULTANEOUS CONVERGING INSTRUMENT APPROACHES
(...)
The requirements for conducting SCIA operations to converging runways are:
(...)
SIAP specifically titled "Converging" and is published in parenthesis after the title of the procedure, for example, ILS V Rwy 17 (Converging).

With those charts not available, how would the pilots anticipate and plan where to go? Are they available but not public, or are the pilots notified by radio on what to expect? How does it work?

The reason you don't see similar charts for the north+west operation (i.e. 35 + 27's) as for those that you pointed out for the south+east (i.e. 17 + 09's) operation has to do with the way the runways are set up, and what ATC has to do to clear aircraft for the approaches in the first place.

In the case of the south+east approaches -- i.e. landing to the south on 17 + to the east on 09R -- the runways shown never intersect, so aircraft can be cleared for the respective ILS approaches without regard for the timing involved.... most of the time, they'll each land, roll out normally, and exit the runway, and they never get anywhere near each other -- even if one or both roll out to the end of their respective runways.

In the case that the weather suddenly goes down & they have to go missed, that's when a deconfliction has to happen, and it's handled by creating the missed approach paths to turn as you showed so that separation is maintained.

So far, so good, and you got all of that already, I suspect.

What differs in the case of the north+west operation that you asked about & illustrated in red is that the paths of the two aircraft absolutely will cross, every single time, and probably at very similar altitudes when landing on 35 + 27L/R.  So ATC has to do a lot more when they clear aircraft for these approaches: they compute (with computer assistance) a separation IN TIME at the point when they will each reach the crossing of the runways.  (And, if the speeds change enough that the separation looks like it won't be maintained, they have to send one or the other around... not common; the controllers there can build in plenty of buffer to avoid that scenario.)

Thus, it doesn't matter if the weather goes down & the aircraft have to go missed... they will still reach that crossing point separated in time.  So they can both go missed straight out.

On the south+east scenario, they don't NEED the "separation in time" constraint (which gets applied to EVERY pair of approaches landing north+west), so they just get a "separation in space" via missed approach routing to keep planes apart in the case of simultaneous missed approaches.

And, to consider the follow-on question, "what about a north+east or south+west scenario"... the south+west scenario (17 + 27's) would probably require separation in time just like the north + west scenario would -- the runways cross (and 17 is probably too short to publish LAHSO), so the timing has to keep the planes apart on roll-out. Which would also keep them apart in the event of go-arounds.  And the north+east option probably isn't used, because if the long runways (i.e. 09L/R) are departing east, aircraft landing to the north (on 35) shut down departures from both the 9's, in a way that doesn't happen when departing to the west.  (West departures don't cross the 35/17 centerline the way that east departures do.)  So in lieu of publishing charts to cover that scenario, ATC simply doesn't use simultaneous converging approaches in that configuration.


What are the advantages of a variable-incidence wing as used in the F-8? What are the advantages of a variable-incidence wing as used in F-8?

Do weight and other issues outweigh the advantages and that's why it's not found in many (any?) other aircraft?

The main reason for having variable incidence wing in the F-8 was that it allowed the production of increased lift due to a greater angle of attack without reducing pilot visibility as the fuselage stayed level.

For aircraft with high angle of attack TO/landings (like delta winged aircraft), pilot visibility is a problem in low speed landings, as the nose will obstruct the runway (or carrier, for that matter) in extreme cases. There are two ways to solve this problem:

Droop the nose, like the Concorde or
Make the wings variable incidence, like the F-8.

The first option is usually preferred (same thing was done in Tu-144) as the weight penalty is (significantly) less. However, this wasn't possible in the F-8 as the engine air intake was in the nose; so the incidence of wings was changed. The variable incidence wing had the added advantage of reducing the length of landing gear required and improved the low speed characteristics.

In case of Martin XB-51, the variable incidence wings were used to reduce take off distance as it was expected to be operated from forward airfields.


Which industry started to develop CFD software? Which industry first started to develop CFD?
(aviation,car-industry,meterology or maybe someone else)

The foundation for CFD is the Navier-Stokes equations, published in 1822. The first application of CFD was in aviation in the 1930s; this analyzed two-dimensional flow, computations were done by hand. Then meteorology in the 1940s. Most of the early work was done in academic and government research circles, the first large-scale user was the Los Alamos National Laboratory (for work related to nuclear weapons). NASA was also an early user.
By 1967, Douglas Aircraft was using and developing CFD. Boeing published some of their work in 1968.


When are flight tracks publicly available? Is there a way to make sure a flight is tracked on flightaware.com or similar sites?
Similarly, is there a way to prevent a flight's track from being available to the public?

Skimming FlightAware's FAQs:

The most consistent way to show up on FlightAware is to have your flight worked by ATC (a squawk code and an IFR flight plan), or to fly an airplane equipped with ADS-B Out.
FlightAware doesn't really track VFR flights unless they have ADS-B Out:

  some VFR aircraft with flight following are available on the position maps but it largely unreliable

  You can block your tail number in a few ways. Both the FAA and FlightAware operate block lists.


What is the correct answer to this Mass and Balance calculation? I was practising some mass and balance questions and I found that my answer is a little bit too far from the listed answers. Can anyone help me to see if I did something wrong?

The question was:

  Given that the Dry Operating Mass (DOM) is 27 372 kg, there are 12 000 kg of
  trip fuel and 4 000 kg of fuel are expected on landing, what is the
  maximum allowable traffic load for the Medium-Range Jet Transport
  (MRJT) aeroplane?

  23,923
  23,528
  19,005
  19,428

My solution was:

  Operating mass = DOM + Usable fuel

  $27 372 - (12 000 - 4 000) = 19 372 kg$

That doesn't match any of the options, so what is the correct answer?

The answer is 19,428, but I could only get there by Googling 'MRJT' to get the missing information:

DOM + all of the fuel is 43,372.

And since MSTOM is 62,800, that leaves you with 19,428 for payload (allowable traffic load).

$$62,800 - (27,372 + 12,000 + 4,000) = 19,428$$

Below you can see the calculations for MTOM, MLM, and MZFM. Since the traffic load for the 'MTOM limited case' is the lowest, the maximum traffic load for the flight is 19,428 kg.

(Table courtesy of DeltaLima.)


What are the operational differences between an airplane equipped with a constant speed propeller and one with a fixed pitch propeller? What makes a constant speed prop different from a fixed pitch prop, and what are the operational differences between airplanes with the two?

A constant speed propeller runs at a specific rpm by changing the blade pitch. When the blades are at a lower pitch (i.e., flatter angle), there is less air resistance, so the propeller puts less drag on the engine and runs faster. Additional pitch causes more drag and slower RPM. A governor (or something similar) will adjust the blade pitch to keep the propeller running at a particular rpm, even when more or less power is produced by the engine.

A constant speed prop is typically found on more powerful engines. Constant speed props are more efficient, but are more complex and heavier than fixed pitch props.

Some planes have a prop speed control for the pilot to adjust the propeller rpm, and some, such has the Cirrus SR22 and Pilatus PC12 have a single prop speed for flight.

The operation of a variable pitch prop depends on the plane. For example, in a Cirrus SR22 you don't even have a propeller pitch control. You operate it just like a fixed pitch prop. With a Pilatus PC12, the only prop pitch adjustments are when you feather the prop on engine shutdown or reverse the prop pitch for landing or ground operations. In a Beech Bonanza A36, you adjust the prop pitch for max RPM (minimum pitch) on takeoff, then add pitch for cruise flight, and max RPM for landing (in case of go-around).


What does "pitch plus power equals performance" really mean? It is often said in aviation circles that "pitch plus power equals performance".
A. What is it supposed to mean?
B. Is it always true?
C. If it isn't always true, why is it a useful concept nonetheless?

A.  It simply means that the airplane will perform according to the combination of pitch and power control inputs that you make.
B.  Yes, why wouldn’t it be?  Aircraft are subject to basic laws of physics.  These laws are consistent.  Every time you pitch nose down and add power you will descend and accelerate.  Every time.
C.  N/A because it is always true, but it is just a saying.  If you find the concept useful to enhance your understanding or explain something, then use it.  If not, then don’t.  Aviation is full of such sayings to help pilots remember things, like “aviate, navigate, communicate”, “ARROW” etc.  Pick the ones you find helpful and don’t overthink or try to discredit the ones that aren’t useful to you.


What was this helicopter photographed on a Chinese aircraft carrier? I've read this twit

Chinese carrier got photographed by a satellite.

On the nose you can find a helicopter and several people around it.

What helicopter is that?

Can't say for sure, but the size and shape (as far as can be determined from the picture) seems to match the Aerospatiale Super Frelon, which is built under license in China and is in use with the Chinese navy for shipboard operations under designation Z-8.

The image is of poor enough quality that a more precise determination isn't feasible, but the size and shape match the type and the type is in Chinese service (unlike similarly sized helicopters).


Can I get official recordings of ATC audio? Is there a procedure in place to request a copy of the audio recording of a specific ATC position over a specific time interval?  How is the request made? What are the limitations of such a request?

For the purpose of this answer, consider that there would be a need for the audio from a controller, which may involve multiple frequencies as well as interphone communication with other controllers.  As such, internet resources that provide recordings of specific frequencies would not be adequate.

In the specific situation of a pilot who is the subject of an FAA investigation, the Pilot's Bill of Rights requires that air traffic data (which includes audio recordings) be provided to the pilot. They would be expected to make the request directly to the FAA. There is a procedure for this:

To ensure that relevant air traffic data you are seeking from a government contractor are obtained, you should provide the following information:
a. Your name and your airman certificate number;
b. other relevant information (aircraft heading, altitude, call sign, transponder code, etc.) that will help the contractor locate relevant data;
c. the name/location of the contractor facility that you believe has air traffic data;
d. the date and local time of day when such data relevant to the your flight operation were generated;
e. the name of the FAA inspector, investigator or other FAA official (e.g., an air traffic controller who advises you about a possible pilot deviation) who notified you that you are under investigation;
f. the airman's phone number or email
The individual should provide that information to the FAA by email to the following address: AirmenDataRequest@faa.gov.

The instructions include a warning about timeliness:

Depending on the type of data, they will normally be stored from 5 to 45 days and then they are destroyed or discarded in the normal course of business

I don't have any information about the general case, though I imagine it depends on the facility and their workload.


How flat does a runway need to be? I always assumed that big runways are flat (or more specifically straight: they might have a slight slope in either direction), but while watching some crosswind landing footage shot at Birmingham Airport I noticed that the runway looks really uneven, and based on the videos it looks like this hinders safe landing especially in adverse weather conditions.

How common are uneven runways like this?
Does an uneven runway make landings more difficult?
Is there any standard on how flat a runway needs to be?

As an anecdotal answer to your first two questions, paved runways used by air carriers that have obviously noticeable vertical curves are a minority but not uncommon.

Unless they've redone the runway at Stansted, north of London, there's a significant hump in the middle. I used to operate through it at night in the early 1990s. As a bit of fun, we'd arrange legs so that a new first officer would have the leg into Stansted. On final at night the hump was not noticeable. However, when you touched down, the runway lights along the last half of the runway would disappear, at which point the captain would observe, "Hmm, not much runway left."

If you define a runway simply as a strip of land regularly used by aircraft, there's a whole lot of them that have vertical curves, and many with horizontal bends as well. It's gone now, but there used to be a strip at a lumber mill in North Fork, California, that had a 10-15 degree bend in the middle. McKenzie Bridge airfield just upriver from my home in Oregon has a steep incline on the east end.

Landings and takeoff on uneven runways are not significantly more complex than on straight runways. If the runway qualifies as an uphill/downhill runway, you generally takeoff downhill and land uphill. Touching down on the downside of a hump can make your landing feel a little softer than otherwise. Touching down on the upside of a hump makes it a little harder than otherwise.

Accommodating a horizontal curve is more challenging. It's worth remembering, though, that seaplanes often use a curved takeoff run.


Can a fighter jet land on a modified airliner? Imagine this scenario:

A modified military 747 is flying at a constant speed and altitude. On the top of the fuselage there is a device like a helipad but with three holes to accommodate the fighter jet landing gear, when pressure is applied the holes close, locking the landing gear in the holes. The fighter is flying at the same speed and slightly above. With proper training, is it possible for the pilot to land on the plane and then shut off the engines?

Technically yes. But I am unaware of any attempt to do this with airliners.

But there were several designs in the past which used a big airplane or a Zeppelin as a mother ship, which took fighters with it for air defense. The B-36 was involved in several such designs.

In all cases the docking was made from below, because this gave the fighter pilot the best field of view and reduced the risk of a tail strike.

While the fighter in the picture above is a Republic F-84, McDonnell even designed one specifically for the purpose of being carried around by a bomber, the XF-85 Goblin. It used a rather complicated trapeze for docking and was then pulled up into the fuselage, as the picture with a B-29 below shows.

Your design proposal with the simple platform on top of the fuselage would carry a high risk of a tail strike if anything goes wrong, so it is unlikely that this will ever be tried for real. But when you change the airliner to a AN-225, the idea looks not totally impossible. The Russians tried to dock a fighter with a mothership as early as the 1930s. The picture below shows a TB-3 docking with a I-Z fighter, the first pair of aircraft in history to dock in flight.

The main reason for docking was the much smaller range of fighters. Today, air refuelling takes care of this deficit, so it is unlikely that anyone will see a benefit strong enough to justify the expense of modifying two aircraft and the risk incurred in the docking process.


What is considered "high altitude"? The AFM for DA40 states in 4A.3.9 CRUISE checklist:

  Electrical fuel pump . . . . . . . . . . . . . . . . . . . ON at high
  altitudes

  CAUTION Operation at high altitudes with the electrical fuel
  pump OFF may cause vapor bubbles, resulting in intermittent low fuel
  pressure indications, sometimes followed by high fuel flow
  indications.

I (sort of) understand the reason for needing to compensate for lower ambient pressure with a mechanical fuel pump, but the problem is I can't find a clear reference to what is considered "high altitudes" for this purpose? It has a demonstrated operating limit of FL164, so we're obviously not talking crazy high.

Is it more of a "if you experience these symptoms, try turning on the fuel pump" type of caution? Or is there some specific altitude above which the pump should always be on?

It seems I've asked a question that has no answer. From the explanation below provided by a Diamond Aircraft representative, it is clarified that the intent of the AFM's caution is not to always run the electric fuel pump at high altitude, but to use it as needed to clear the symptoms - which are more likely to occur as altitude increases.

  The IO 360 with the Precision Fuel Injection System does not have a
  vapor separation and return line to the tank(s). Vapor in the fuel
  system will travel through the fuel system including flow and pressure
  transducers, into the injection system. Vapor can form under higher
  ambient temperatures and low ambient pressures. Fuel gravity fed
  through the system (electric pump off) from the tanks is “lifted” by
  the mechanical pump. The suction of the mechanical pump can cause some
  cavitations (vapor formation), especially as the fuel flows through
  resistances such as elbow fittings. Vapor bubbles will cause momentary
  low pressure indications and may also cause momentary erratic or high
  flow readings as the flow transducer turbine spools up with the loss
  of fluid resistance. These momentary occurrences can be suppressed
  with the electric pump. The electric pump activation increases system
  pressure to compress or return to liquid, the formed fuel vapor
  bubbles.

  [...]

  The intention of the steps for electrical pump selection, above each
  of the Cautions in the AFM is not for continuous operation of the
  pump, as this could potentially mask a defective mechanical pump and
  eliminate the redundancy of the fuel delivery system. If the abnormal
  fuel pressure or flow indications do not stabilize with the electric
  pump on, emergency procedures and maintenance action may be required.


What is the difference between MMEL and MEL? What was the need to introduced MEL if MMEL is there and is less restrictive than MEL ?

The Master Minimum Equipment List is issued by the manufacturer with the approval of the regulator and can be considered a "global" document you might say.  The manufacturer will work hard to build the mathematical case to make the requirements as broad and flexible as it can. It's an ongoing process, and as the type is introduced and matures, the OEM will continue to increase the scope of the MMEL, because each new item added improves Dispatch Reliability (because more more and more components and subsystems can be inop without needing to be fixed right away).
The MMEL is the foundation for the Minimum Equipment List that the airline uses.  The MEL is an in-house document created by the airline and approved by the local authority.  It'll be based on the MMEL but may have additional limitations imposed by the local authority as it deems necessary for that operation.
It's similar to the Maintenance Program. The OEM will issue a Maintenance Program for a given aircraft type, but each airline will have its own variations by negotiation with its local authority.


Can a commercial aircraft theoretically reach 60,000ft? As per this question:

Altitudes above 60,000 feet are class E airspace.
That means that, if you are lucky enough to have an aircraft that flies that high, once you get there you could theoretically call ATC, cancel your flight plan and continue VFR above flight level 600 without ATC supervision.
At that point, you are on your own, and out of the system.
You are literally past the edge of human civilization.

If a commercial airliner, such as an A320 kept ascending, could it reach this altitude? If not, what is physically (ignoring legalities) stopping it from doing so?
Since it is not a legal requirement to use ATC at this altitude, perhaps this could be an airspace future budget airlines occupy to save time and therefore money.

An airliner's service ceiling is the altitude at which its rate of climb drops to less than 100 fpm.  Most commercial airliners can't climb above 40000-45000 on a standard day.  If temperatures are above standard, less than that.  Corporate aircraft can go higher, just above 50000, so they get to avoid pretty much all the traffic because everybody else is below 40.  The Global Express uses low bypass fans and very large wings to do this.

To go toward 60k you need super efficient engines and glider like wings.  P&W developed the U2's engine with virtually no blade tip clearance to be able to operate at 70000ft.

Then you have coffin corner, the spread between critical Mach# and stall.  It gets pretty small at 60000 ft.  In the U2 at 70000 ft the spread between Mcrit and stall is very small, and between Mcrit and the back side of the power curve is just a few knots and they cruise in a tiny speed window, autopilot on in speed mode at all times.  Bit too slow and down you go, bit too fast and mach buffet starts.

At an airliner's service ceiling, you have very little reserve thrust margin, and you may be operating just above min drag speed, not so far from the back side of the power curve in cruise.  If you are at service ceiling and some speed bleeds off because say you hit some down moving mountain wave and the autopilot pitched up to compensate, and you aren't paying close attention, you may have no choice to descend right away as speed decays with no way to stop the decay without resorting to gravity.

Stalls at this altitude take many 1000s of feet to recover from.  In the thin air, a mass has to accelerate over twice the distance through space for the same indicated airspeed increase as down low.  If you get stick shaker at that altitude, you have to push over really aggressively for the recovery, not just lower the nose a bit.  After a fatal Pinnacle Airlines CRJ200 stall incident that started at 41000, airlines started to focus more on training for high altitude stall recovery.

The high altitude world is a dangerous place, even at altitudes in the 30s.


Why does not the lift produced by main wings create pitch moment? Pitch moment is created by lift created by the horizontal stabilizers via the elevators.

But then main wings also create lift, why does not this lift enough to create pitch moment?

Of course it creates a pitching moment! Now we need to define around which reference point this moment should be measured.

If the reference point is the center of gravity, it is even equally strong as the pitching moment of the elevator, it only has the opposite direction.

If you use the aerodynamic center as the reference point, the moment will be less strong. If the wing airfoil is symmetric, the moment will actually be zero (ideally), but with a cambered airfoil there is still a measurable moment left.

A moment is always a combination of a force and a lever arm, measured perpendicular to the direction of the force. Only when the lever arm is zero will the moment disappear. If we use the center of gravity as the reference point, the weight will have no lever arm and will contribute no moment, but the distance between the wing's lift and the center of gravity is large, so this moment is large, too.


How do radio altimeter systems compensate for objects on the ground? My understanding of a radio altimeter is that it can provide more accurate altitude readings below the transition level.

It occurred to me a number of days ago whilst sitting beneath the glide path at my local airport that surely discrepancies must arise when using radio altimeters due to objects such a parked vehicles and trees beneath the flight path of the aircraft.

Are these negligible or are they taken into account by the system or the crew?

Objects on the ground are negligible because the radio altimeter is not designed nor used to such high precision.

There are several uses of the radio altimeter. The first one is for timing the flare during the last portion of the landing. Since the flare maneuver starts after the aircraft has crossed the runway threshold, at this time the aircraft is over flat ground.

The second usage is terrain awareness. The radio altimeter will trigger the GPWS "terrain! terrain! PULL UP" warning if the reading is too low. If you are in a range where the height of cars or trees matters, you are too low anyway.

You may imagine that pilots use the radio altimeter as their main data source for height above ground during the landing phase. That is not true - the barometric altimeter is always the primary instrument to use, until the flare portion at least. That is why pilots need to adjust the altimeter to the local air pressure setting. However, the radio altimeter can provide useful situation awareness. For example, you are landing at an airport with elevation 2,200 feet, you are 10 miles from the runway, and your altimeter reading is 3,800 feet. Suddenly you hear the automated voice "five hundred" in your cockpit. Needless to say something is wrong.

Pilots cross check the barometric altimeter with the radio altimeter during landing. The airport elevation is usually dialed-in somewhere on the cockpit display for quick lookup. When the automated "five hundred" announcement is made (or announced by the pilot not flying / flight engineer on an aircraft without automated announcement system), you quickly glance at the barometric altimeter to see it is 500 feet above airport elevation. It does not have to be exactly 500 - 492 or 518 will do as well. Then you cross check the distance to runway threshold with the DME (if available). Again, you don't have to be exactly 0.8 nautical miles or whatever distance the calculator come up with. As long as you're reasonably close, it is good enough.

If the pilots are using the ILS, then vertical guidance will be provided by the glideslope signal until decision height, at which point they will switch to the barometric altimeter again.


Why don't flight attendants get tired when passengers can, despite sleeping? This Reddit thread refers to "stressors of flight" (many of these stressors are also present in automobiles), sudden altitude adjustment, lower cabin humidity., proximity to strangers.

Yet flight attendants face these stressors too, but they don't appear tired?

I don't think sleep answers my question. I know that on long flights, they are allowed to sleep in crew rest compartments, while passengers sleep too but still can remain tired.

This is bordering on opinion based as its a bit subjective from person to person but generally speaking flight attendants and pilots are more accustomed to the typical stressors of flight and potentially less subject to them after some time as the human body adjusts.

Perhaps more importantly this is also their day job, its what they do. When you get on a flight it may be in the evening, or early in the morning, maybe you are coming off a long work week and really burnt out or that weekend in Vegas really got to you. When you get on a flight you may likely be tired, and small, dry, noisy spaces are not where you want to be. This is not the case for the crew. They are there to do a job, presumably well rested and ready to go.


Calculating the horizontal distance the payload would travel with a parachute (horizontal drift)? When you release a payload with the help of a circular parachute, and you want to find out the range in which it would land. And how would you find the descent gradient of the parachute?

The first thing to figure out is how fast your parachute will fall. From https://www.pcprg.com/rounddes.htm you can compute the falling velocity:
$$ V = \sqrt{\frac{2 W}{\rho C S}} $$
Where

$C$ is the parachute drag coefficient which is approx 0.75 for a parachute without holes or slits cut in the fabric; same value in both Metric and English unit systems

$S$ is the total surface area of the fabric used to build the parachute, plus the areas of the holes and vents cut in the fabric if present. The units of S are in square feet (English) or square meters (Metric). This definition is such that when vents are cut in the fabric, the value of $S$ remains the same but the value of $C$ becomes smaller.

$\rho$ is the air density

near sea level its value is given by
0.00237 sl/ft^3 (English units) and 1.225 Kg/m^3 (Metric)
near 4000 ft or 1219 m above sea level its value is
0.00211 sl/ft^3 (English units) and approx. 1.07 Kg/m^3 (Metric)

$W$ is the weight of the parachute + load, in pounds (English) or Newtons (Metric)

$V$ is the vertical descent velocity, here expressed in ft/sec (English) or m/sec (Metric)

You can either assume a constant speed all the way down, or make it a function of $\rho$.
With starting altitude and speed you can find the time it will take to fall.
Then you add the wind contribution. Multiply the wind speed by the descent time and you get the maximum range the parachute can reach.
The calculations will get a little mode tricky if you have a rho profile by altitude and wind speed profile by altitude.


Based on PANS-OPS, what is the earliest turning point that can be used in a missed approach? I am studying ICAO's PANS-OPS 8168 manual, and I have noticed conflicting guidelines regarding the permitted earliest turning point during a missed approach.

In section 6.2.1.1, it clearly states:

  6.2 Climb Gradient and MOC. 6.2.1 Initial Phase. 6.2.11. The initial phase begins at the earliest missed approach point (MAPt) and ends at the start of climb point
  (SOC). The manoeuvre during this phase requires the concentrated attention of the pilot, especially when establishing
  the climb and the changes in configuration, and it is assumed that guidance equipment is not utilized during these
  manoeuvres. No turns may be specified during this phase.

A few pages later, it also states:

  6.4.7 Turn Specified at the MAPt. Where the turn is specified at the MAPt, which means that the pilot is supposed to establish the aeroplane on a climbing
  path and then to turn, the OCA/H will be taken as the turn altitude/height and the turn initiation area will extend from
  the earliest MAPt to the SOC (see Figures I-4-6-10 and I-4-6-20).

However, in Figure I-4-6-10, a turn less than 15 degrees is depicted to begin immediately at MAPt, rather than at SOC. On the other hand, turns more than 15 degrees seem to begin at SOC as it is intended (Figure I-4-6-20).

Additionally, in section 6.4.5 (Turn initiated at a designated altitude height), it states:

  6.4.5.2.1.1 The point where the designated altitude/height is reached is not fixed. It depends on the climb
  performance of the aircraft and the point from which the missed approach is initiated. The aircraft may reach the
  designated turn altitude/height:

  a) as early as the earliest MAPt when the procedure prohibits turning before the MAPt or as early as the earliest
  FAF when no restrictions are provided

  b) after a climb using the minimum required gradient from the SOC to the point where it reaches the specified
  altitude height. This point is called the Turn Point (TP).

In Figure I-4-6-11, one of the potential turning points is depicted near the MAPt, well before SOC.

All things considered, I can not figure out the earliest point where turning is allowed to be initiated, during a missed approach.

Could someone help me clarify this?

I can not figure out the earliest point where turning is allowed to be initiated, during a missed approach.

It is up to the designer. Nothing in PANS-OPS prohibits turns before MAPt but in that case an additional area extending from the earliest FAF needs to be protected as shown in figure I-4-6-14 PANS-OPS Vol. II.
Because early turns are not always feasible, advisable or convenient, we often disallow turns before MAPt, in which case a note to the effect will be added on the profile view of the approach chart. Any turns >15° will then be executed in the final missed approach area (Vol. II I-4-6-10 6.4.6.3.1).


How big is the TP400-D6 turboprop installed on the A400M? I am trying to re-create the A400M TP400-D6 engine in CAD, mainly the nacelle and propeller, yet I can't find any detailed dimensions for it.

Europrop specifies a 0.92m diameter for the TP400-D6, so I assume the nacelle used on the A400M is roughly 2 meters tall and 1.5 meters wide - is this a reasonable estimate? Or even better, where can I find more information on the actual size of the A400M nacelle so I can accurately model it in CAD?

Thanks a lot.

According to the MTU Aero Engines technical data of the TP400-D6:

  Propeller diameter: 209.8 in

(mtu.de)

That is a propeller diameter of 5.33m. I also could not find more details about the nacelles, but one can estimate the size from a photograph. I used this image from aviationtoday.com, which shows the aircraft from the front. With the known propeller diameter, I estimate the nacelle to be about 1.2m wide and about 1.85m high:

I don't think it is possible to derive more precise numbers due to the inaccuracies resulting from the perspective.


Why is Mode C helpful for ASDE-X systems? Airports that operate an ASDE-X system require aircraft to operate transponders in mode C while moving on the surface.

While I understand the general purpose of operating the transponder to correlate with the other information ingested by the ASDE-X system, why is altitude (encoded in mode C) important for tracking surface movement?

Wouldn't mode A suffice?

Without knowing the altitude, it is very difficult (or impossible) to accurately calculate the position of a transponder by using multilateration (which is what ASDE-X uses). In addition, without knowing the altitude, ASDE-X would be unable to determine whether an aircraft is on the airport's surface, or passing overhead.


How does altimeter reading vary when departing from ISA conditions? I am a soon-to-be student-pilot and am struggling with a basic concept that is altitude. After watching multitude of online videos, I feel that most of the people do not have good teaching skills.

Introduction

The altimeter is a barometer, it reads atmospheric pressure. At 0' altitude, it should measure "this certain pressure that we find at 0'" and indicate 0'. This is obtained by setting the reference pressure in the Kollsman window measured somewhere near the airport. One day it might be 1013 hPa, others it will be 998 hPa or 1036 hPa.

The Scenarios

We are in the aeroplane, at the airport which is at MSL (0' altitude).

SCENARIO 1 (International Standard Atmosphere):

Elevation: 0', temperature: 15 C, pressure: 1013.25 hPa
METAR reads QNH 1013

we set our Kollsman window to 1013 => it correctly indicates 0' altitude
we set it to less than 1013 => it will indicate more than 0' (say 230') LESS than 0'!
we set it to more than 1013 => it will indicate less than 0' (impossible but bear with me) MORE than 0'

SCENARIO 2 (low pressure):

Elevation: 0', temperature: 15 C, pressure: 998 hPa
METAR reads  QNH 998

set our Kollsman window to 998 => it correctly indicates 0' altitude
set it to less than 998 => it will indicate more than 0' (say 230')
set it to more than 998 => it will indicate less than 0'

If I got the above correctly than the following should apply too:

SCENARIO 3 (International Standard Atmosphere):

Elevation: 5000', temperature: 5 C  (as per lapse rate, -2C for every 1000'), pressure: 847 hPa (as per lapse rate, -1hPa every 30')
METAR reads  QNH 1013 (actually our pressure of 847 hPa, adjusted down to MSL)

set our Kollsman window to 1013 => it correctly indicates 5000' altitude
set it to less than 1013 => it will indicate more than 5000' (say 6200')
set it to more than 1013 => it will indicate less than 5000'

My Questions

What happens at this altitude of 5000' if we move out of the ISA?

if the temperature drops further to, say -2 C?
if the pressure goes up to 900 hPa?

EDIT (answer):

My premise of "...we set it to less than 1013 => it will indicate more than 0' (say 230')" was wrong all the time! By setting the pressure in the Kollsman window we are effectively "telling" the altimeter at which pressure the 0' elevation is (or what is the pressure at 0' elevation).

If we "tell it" that the 0' is at 1000 hPa, and it is still sensing 1013 hPa as I initially stated, it will indicate only possible altitude - LESS than 0', not more.

We set it to less than 1013 → it will indicate more than 0'

Above statement is incorrect for all scenarios.

(Source)

Remember, the anticlockwise rotation of the knob to set lower than 1013 also corresponds to anticlockwise rotation of the altimeter hands.

(Source)

Above image is what you're after.

Above the transition altitude (varies by location) you set standard setting regardless of the ISA deviation.

Below the transition altitude (TA) or when descending through the transition level (TL), you set whatever the ATC advises. From a pilot's perspective, it's that simple.

Unless you will be performing an approach in a cold area (usually below 0°C/32°F), then special altimetry equipment may be required. Requirements will be noted on the approach plate.

Related:

Wikipedia: Flight level
Skybrary: Altimeter Temperature Error Correction
How is non-standard OAT accounted for?


What is Alpha protection? This article states:

  An occurrence was reported where an Airbus A321 aeroplane encountered a blockage of two Angle Of Attack (AOA) probes during climb, leading to activation of the Alpha Protection (Alpha Prot) while the Mach number increased. The flight crew managed to regain full control and the flight landed uneventfully.

What is Alpha protection?
Is it part of another major system (like auto-pilot)?
What can it do?
How is it activated?

Alpha is also called "Angle of Attack". It is the angle between the relative wind and the chord line of the wing. When the angle of attack is increased to a certain point, the airflow over the wing is no longer smooth. Turbulence is created after the wing, and it loses lift. This is known as a stall.

"Alpha protection" is a system which prevents the angle of attack from getting too high. This is done by software monitoring the plane's angle of attack sensor. When a high alpha situation is detected, the software lowers the nose of the plane to maintain high but still safe angle of attack. It also commands full thrust to all engines. This system is designed to prevent pilots from stalling the aircraft and to allow them to get the best possible performance in emergency e.g. in a wind-shear.

This is not part of the autopilot. It is integrated into the flight control computer and activated at all times. Autopilot are functions which fly the plane in a certain way to reduce the pilot's workload in normal situations (e.g. climb to 13,000 feet, turn left to heading 340).

Pilots can opt to turn off such protection systems should it malfunction, or a very special occasion exists where these protections should be switched off, such as performing at an airshow.

Notable Accidents

In the Airbus, the A320 was the first "fly-by-wire" aircraft. During a demonstration flight AF296 when showing off the alpha-protection system the pilots took the airplane too low and too slow and disabled the part of the system that should have commanded full thrust. The plane didn't stall but was having too little energy to climb anyway. It crashed gently into the trees.

In an acceptance flight, the pilots were testing the alpha-protection at low altitude. The system should have prevented the plane from pitching too high. However, to the surprise of the crew, it failed. The plane stalled with insufficient altitude to recover. The system did not work because water got into the angle-of-attack probes during previous improper washing procedure, froze during the flight and blocked the probes.

In AF447, the system switched to "alternate law" and no longer provided stall protection, because the pitot tubes (airspeed sensors) froze over. The plane stalled from 38,000 feet to the water after the pilots failed to apply correct procedure for unreliable airspeed.


Reclined seats for take off and landing Normally on commercial flights, passengers are expected to bring their seats to an upright position for take off and landing. There are a few valid reasons for this.

A little while ago, on a couple flights in Upper Class on Virgin Atlantic the pre-flight announcement stated that their seats are certified for take off and landing in any state of recline.  I didn't think of it much at the time (other than, "oh, ok then"), but now thinking back I don't think I've ever encountered this on any other airline.

So my question is two-fold. First, what kind of certification was the announcement referring to? And, second, are there any other airlines that have this "feature"?

Update on response to a comment. The seat belt in that case was a standard lap belt. The seats themselves were positioned somewhat diagonally facing partially forward and were almost like bucket seats in the upright position.

In normal cabin seats, the reason not to recline them for takeoff & landing is so the reclined seat won't impede egress for passengers in the row behind yours.  If the seats in question on VA wouldn't do that (ample space between rows, perhaps), then the usual driver for requiring seats to be fully upright is gone.

Presumably whatever "certification" they referred to considered the ability to get out of one's own seat in case of an evacuation.


Is there an IATA airport code that is unassigned and that can be used for "any airport"? The question is basically up there in the title; I need a default value for an app that won't do anything. A friend suggested XXX, but I'm sure there must be one reserved specifically for that purpose? ;-)

Is there a code that's used in the travel industry to denote "enter city code here" without actually being mapped to a metropolitan area/city/airport? I'm guessing it might be one beginning with Y... :)

Good question, no good answer.

In the ICAO system, the identifier ZZZZ is reserved by that agency to designate on flight plans that the airport does not have an ICAO code assigned (not all airports do; for instance Qualicum Beach Airport in British Columbia is TCCAA registered and has TC and IATA codes but no ICAO code). There is no specific combination of alphanumeric characters reserved to mean "airport not specified"; if the flight plan does not specify an airport at a particular stop, the ICAO code field will be blank (which is valid in many circumstances in general aviation; float planes can set down on any relatively calm body of water, while bush pilots routinely fly out to a dot on the map in the literal middle of nowhere to deliver supplies or people to an established but temporary campsite).

XXXX is an unassigned but unreserved ICAO code that a few websites seem to be using for testing and demo purposes; FlightStats has a page for this code but the data is sparse and nonsensical.

The IATA system labels practically all registered airports (primarily ones of interest to commercial flight, but all FAA-registered airports have a three-character code that have, with few exceptions, been adopted verbatim by IATA). The IATA has no specially reserved code for "not specified" or any other special case. The code ZZZ is currently unassigned, and is used informally by bush pilots to indicate an "unregistered" landing area similar to the ICAO system (though as IATA labels more airstrips especially in North America, this code is used more specifically for unimproved landing areas, which are common in backcountry areas of the Plains States, Northwest and Alaska). But again, that still indicates an entry was made.

If you are writing a computer system for flight planning and you want a string value to indicate that the user has not specified an IATA identifier, the safest are null, the empty string, or three spaces. "000" is probably also safe, as IATA does not and likely will never use three zeroes for any valid identifier, and OOO is also not assigned. The code "ANY" is reserved for Anthony Municipal in Kansas, but the code "NON" is not currently assigned (though with no guarantee it will remain so). XXX, YYY and ZZZ are also unassigned but other repetitive letters are.


How does a CVR / FDR determine when to stop recording? For example, the CVR records only about 2 hours of data.

Overwriting important data must be avoided, and thus the recorders should know if a catastrophe has happened. Then they can stop recording (to protect the already recorded data from overwriting), and start signaling the rescue team.

What is the logic used to stop recording?

The CVR and FDR have impact switches to stop recording when they experience high acceleration. In incidents and accidents that don't involve high enough acceleration, the plane remains intact enough for the boxes to be immediately found and switched off manually. In more serious crashes, the boxes stop recording because of the physical destruction of everything but the memory units (only the memory units are built to survive crashes).


How is TAS affected by wind? Does True Air Speed (TAS) change with wind speed and direction?

My understanding is that if an aircraft were travelling at 470kt TAS, with a hundred knot tailwind, the ground speed would be 570kt. In this case the TAS would remain the same, regardless of the wind, but the ground speed would change.

Correct.  True airspeed refers to the actual speed which the aircraft is moving through the air.  Ground speed is the speed at which the aircraft moves over the ground.  The formula

$v_\text{ground} = v_\text{true} - v_\text{wind} \cdot \cos(\alpha)$

Where

$v_\text{true}$ is the true airspeed
$v_\text{ground}$ is the groundspeed
$v_\text{wind}$ is the magnitude of the winds aloft
$\alpha$ is the angle of the winds aloft relative to the longitudinal axis of the aircraft.


How common is it for military jets to use civilian airports? Today my wife witnessed 3 fighter jets taking off from Teterboro Airport in NJ. She even took a video. How common is it for military jets to use civilian airports?

Fighter jets may fly into local fields for airshows, to re-fuel or for needed maintenance in route etc.

What your wife may have seen may have been related to the VIP TFR's that usually pop up in the summer due to the president (or first family) heading back to their home on weekends.

I fly out of KDYL which is fairly close (in aviation terms) and my local FBO has been warning everyone that the Secret Service has instructed them the VIP TFR is going to be common on weekends this summer.

You can find a map of current TFR's here.


How can I use an ADS-B receiver to make my aircraft visible in Flightradar24? I recently bought a SENTRY ADS-B receiver for the aircraft in order to track the flight.

How could I make Flightradar24 or Flightaware show my aircraft and how to register my aircraft on the website?

ADS-B is broadcasted by many aircraft and contains flight and aircraft information. This can easily be received and decoded, either by dedicated devices or a simple 20 bucks TV USB dongle. All information is then sent to flightradar and other sites by the members of their community. (there are also other sources for their data)

However, once ADS-B is broadcasted and received by any receiver working for flightradar, the aircraft appears on their map. There is no need to register your self, in fact, you have to opt-out if you prefer to be invisible.

Now, you write about a receiver, and also a short visit of the product page shows that your device is a receiver only. Together with the built-in GPS it might just warn you about other near aircraft (which do broadcast ADS-B). You will need an ADS-B sender to find yourself on flightradar.

In the US, ADS-B Out is required to be installed (by Jan 1, 2020), it cannot be a portable unit. ADS-B In can be a portable unit.


Is it possible to reload Electric Aircraft in mid-air from a tanker aircraft? Can this be done military style refueling but with an electric wire boom supplying current from tanker aircraft to electric aircraft mid air in flight? Electricity from the tanker craft can be generated from its engines and/or an on board diesel generator. And utilising fast charging technology for quicker refuel times to recharge the electric aircraft's batteries.

Is a technology demonstrator already available? Or is Google not showing me any results?

This would certainly be possible, but the current status (pun intended) of the battery technology makes it pretty much pointless.

The charging would simply take too long: for example, Eviation Alice has a battery capacity of 900kWh. If we were to charge, say, 75% of this (because you would need to have some reserve capacity in case charging does not work out) it would take about 10 hours to charge if the charge time was comparable to Tesla Superchager (85kWh battery charges in about 75minutes =1.13 kwh/minute). Even if the battery was charged in ten parallel segments, charging would still take an hour.

Alice has an endurance of about two hours, so to double that, you'd have to spend at least an hour, most probably more charging.

Then, there's of course the sillyness of charging an electric aircraft with electricity made from jet fuel (or diesel)... it would be far more efficient to use that fuel to propel the aircraft without the engine-alternator-charger-battery-motor -detour.

For further reading, please see this Aviation Stack Exchange question regarding aerial refueling, many of the points raised in the answers also apply to aerial recharge.


How do I "sightsee" in a class Bravo airspace? There are some good sights to see inside of class Bravo airspace. What is the best way to plan and communicate to ATC that my intention is to fly into the airspace to see some specific landmarks?

Just call up ATC on the approach frequency and request flight following to see the location you're interested in.  They'll assign you a transponder code and any restrictions.  For example:

  N12345: "Houston approach, VFR request for Cessna 12345"

  Houston TRACON: "Cessna 345, say request"

  N12345: "Cessna 12345 is at 1200 ft, 3 miles south of Houston Executive, requesting flight following to overfly the Greenway Plaza area for sightseeing, aircraft type is a Charlie 172 slant Golf."

  Houston TRACON: "Cessna 345, squawk 4123."

  N12345: "squawk 4123 for Cessna 345"

  Houston TRACON: "Cessna 345, radar contact. Cleared into Bravo, maintain at or below 3,500, remain north of the University, advise when your sightseeing is complete."

  N12345: "Cessna 345 is cleared into Bravo. Will remain at or below 3,500 and north of the University. Will advise when complete."

Notice that it is preferred that you don't give your entire VFR request in the initial callup to avoid overwhelming the controllers if they're busy or not yet prepared to copy your entire request.

Just ask for "VFR request" with your callsign and wait for them to ask for the details.


Are aircraft capable of sustained inverted flight at constant altitude? Inverted flight as a part of manouveres, dogfights, acrobatics is fairly common but my question is:

Are any aircraft capable of sustained inverted flight without losing altitude? e.g. Could you fly 10 mins inverted at the same flight level?

To clarify: I'm not wondering about engines, fuel systems, air-frame strength etc. Fundamentally there's no reason why those systems cannot be engineered to be robust against inverted flight.

What I'm curious about is specifically the aerofoil itself.

i.e. As far as I can see, you'd need to balance the gravity vector with lift.

Ergo, are aircraft wings generally capable of providing enough compensating lift in the "wrong" direction to balance gravity? Are all / most fighters / arerobatic certified craft capable of this?

We'd need an aerofoil that can provide a lift vector pointed in the +z direction normally but changable to a large vector pointing in the -z direction in response to a flight surface deflection?

Yes they are! Aerobatic aircraft can have symmetrical wing to improve inverted performance. So with these aircraft there is no any problem at all. Other agile aircraft, gliders can fly inverted as well. Of course is far from optimal but possible. Don't forget that lift depends of angle of attack as well. So if you fly inverted angle of attack and drag will be bigger to compensate for a "wrong" wing profile.


What are the spikes on Ingenuity's blades? I am puzzled by the spikes on Ingenuity's blades (marked by a red box in the image below).

What is their function ?
Are these special to Ingenuity?
Why don't they collide during high RPM?
Don't they induce unwanted drag and/or turbulence?

I have tried searching NASA's website but couldn't get a description of these particular featuers. Answer to any part is appreciated.

In the Mars Helicopter Technology Demonstrator paper these are refered as "Chinese weights" and the purpose stated is

Chinese weights provide a restoring force on the blade moments when
under centrifugal loads thereby reducing the torque requirements on
the swashplate actuators.

Searching for the term leads mostly to RC sites, for example this explanation.
Being placed close to axis of rotation (and presumably heavy but small in volume) the aerodynamic effect should be minimal compared to the rest of blade.
My assumption is that this can be quite specific to Ingenuity (and RC models) because limited torque available in electrical actuator servos compared to control system of full-scale helicopter. (But it is my guess only.)
Edit: I have borrowed an image from https://mars.nasa.gov/resources/25043/mars-ingenuity-helicopter-3d-model/ (go there and rotate the model yourself to get a better idea how counterweights are placed) and tried to sketch how the force balance looks like:
Red arrows are centrifugal forces exerted on counterweight or blade's center of mass respectively. These forces are always perpendicular to the mast (rotational axis). Then, each of them can be broken into two orthogonal components -- parallel with blade pitch axis (green) and perpendicular to it (blue). Note that one blue arrow lays above pitch axis and second under, so they produce opposing torque and can cancel each other.


Where is the toilet smoke detection indication displayed for a B777-200/300? In case of smoke detected in any of aft toilet of a B777-200/300, where is the indication displayed? Is this at a single location, or at different locations?

From the 777 manual:

  Each lavatory has a single smoke detector. If smoke is detected, an aural alert
  sounds in the lavatory and in the cabin. In addition, the lavatory call light
  flashes and the master call light at the associated attendant station illuminates.
  The EICAS advisory message SMOKE LAVATORY indicates smoke is detected in
  one of the lavatories.

The EICAS is a display in the cockpit.

It is treated as a fire, and the cabin and flight crews follow the set procedures to handle it.


What is the average aerodynamic load on a control surface of a commuter-sized airplane? I'm doing a study about aircraft hydraulic pump sizing. In order to do that, I need to know the size of a flight control actuator, then determine the maximum flow and pressure that the actuator needs.

Therefore, I need to know how much force that the flight control surface needs to be moved.

What is the average aerodynamic load on a flight control surface of a commuter-sized airplane (10-20 passengers)?

I think you don't actually want to know the load but rather the hinge moment of control surfaces. The actuator load is the hinge moment divided by the length of the control horn. Below is a rather poor sketch for a typical aileron linkage, but the principle is correct (source):

It already shows one popular way of reducing control forces: The tab, a little auxiliary control surface which moves against the "real" control surface. This reduces effectivity a bit, but forces by a lot. Here, the amount of deflection of the tab is controlled by a spring in its linkage, which is a clever way to adjust its deflection such that the actuation forces become more constant over speed.

Another way of reducing control forces is a horn: An extension of the surface forward of its hinge line, so the aerodynamic loads here balance those on the surface behind the hinge line. The picture below shows the left aileron of the ATR-72 which is moved by mechanical linkage (source)

This way, the lift loads on the control surface are mostly carried by the hinge and only the actuation loads need to be carried by the control rod or actuator. If you think you don't need all those nifty tricks, your actuator and hydraulic system will become much heavier than needed.

Why are two different methods used? The tab reduces the loads from deflection changes while the horn reduces those from angle of attack changes, too. When sized properly, both together will drive the hinge moment close to zero.

Why do I explain all this? It shows that your question does not have a simple answer. Rather, you need to specify exactly how your control surface looks and is moved, and only then can you start to calculate the actuator loads. I also want to show that a subsonic airplane for 10 - 20 passengers will be perfectly flyable with manual controls. The ATR-72 needs hydraulics only for the flaps, the spoilers, the brakes and the landing gear. Avoiding hydraulics for primary flight controls also lets it get away with single redundancy in its hydraulics system.


Why aren't any wingsuits able to sustain flight? I'm aware that under zero power it is a glider, but there are gliders that sustain flight and have enough lift to climb etc (Edit: this is not always true. See below answers for why). So, would it not be an interesting idea to make a wingsuit that you can use to climb and not just fall?

With longer wings could it be done? Or maybe a longer airfoil?

I'm aware that under zero power it is a glider, but there are gliders
  that sustain flight and have enough lift to climb etc.

This statement is a bit incorrect. Gliders don't climb like aircraft as they are constantly falling to the ground unless they catch a thermal, wave lift or ridge lift, in which case they can ride the updraft to gain altitude. In theory, a wingsuit could do that if you could maneuver to stay in the thermal but wingsuits are not maneuverable like a glider and tend not to have nearly the glide ratio of a glider so they sink far faster than a thermal could cary them up.

There is an interesting discussion on it here.


Can thrust reversers be set to automatically activate on touchdown? Is there a switch on the instrument panel that will set the thrust reversers to activate on touchdown like the autobrake? Or is using the thrust levers the only way to activate them?

Can it be done? Yes. Is it done? No. The consequences of unintended operations are serious.
To the best of my knowledge, no aircraft has the option set automatic thrust reverser. Using the levers are the only way to operate them. In fact, most of the design efforts have gone the other way- to ensure that thrust reversers are not deployed inadvertently.

In most aircraft, the Reverse Thrust Levers can be raised only when the Forward Thrust Levers are in the idle position. The thrust reversers have separate levers in front of the engine controls, which are prevented mechanically from operating otherwise.

Source: bloginblack.de

There are sensors installed in the engines which auto-stow the thrust reversers in flight.
The design philosophy of thrust reversers is that it is better to not deploy them when needed rather than deploy them when not needed i.e. they are designed to fail.

The main reason for such a design philosophy is safety. Any uncommanded deployment of thrust reversers in any phase of flight can have serious consequences.

Takeoff- If the trust reversers are deployed in one engine before $V_{1}$, the aircraft will veer off the runway. If it is deployed after $V_{1}$ and before $V_{2}$,the aircraft cannot achieve $V_{r}$ required to initiate rotation.
Flight- Deployment of thrust reversers during flight can lead to controllability issues.
Landing- Deployment of thrust reversers during landing practically eliminates the option of rejected landing.

Because of these reasons and reliability issues with thrust reversers, there is no option for automatic deployment of thrust reversers.


What does "inertial" refer to in "inertial reference"? An ADIRU makes use of air data reference and inertial reference. Inertial reference calculates the heading, position, ground speed and attitude. I was wondering why is it called inertial. Is it related to the term inertia/mass?

It is called inertial, because it works by measuring—and integrating—inertial forces¹, that is forces due to acceleration of the reference frame (i.e. the aircraft). Gyroscopic effect used to measure rotation is also due to inertial forces.
The measured forces include gravity², which can't be easily separated, but this is done by observing that the speed is limited, so the average acceleration must be zero and therefore the long term average equals gravity.

¹ In general relativity the inertial forces are usually considered as real as any other, so the term “fictitious” is not really appropriate.
² In general relativity gravitational force is considered an inertial force. In standard English terminology, gravity also includes centrifugal force due to rotation of Earth.


How are fuel injectors in a jet engine monitored to detect malfunction? As far as I know in a car engine the ECU can adjust fuel mass flow through every injector. I don't know the exact algorithm but probably engine roughness is monitored. Of course this is not possible in a jet engine. What will happen if an injector starts to malfunction? I assume it will be quite difficult to detect and a slight decrease in maximum power will appear. But engines usually are not run at full power? Can this be dangerous when maximum performance is needed (like engine failure at V1)? How are injectors tested in engines with mechanical fuel control? Can FADEC monitor fuel injectors?

Fuel nozzles are overhauled and bench tested to meet the manufacture's operational limits.  They are controlled for both flow (back pressure) and also flow pattern.  The FADAC monitors fuel flow, this along with the compressor vane position are the main input controls for thrust generation.

The fuel nozzles do degrade during engine's operation.  This contributes to the engine running hotter as it ages.  Other contributors are the erosion of the compressor hardware and the burning/oxidation of the combustion chamber and turbine hardware.  In fact a defective fuel nozzles may lead to a "hot spot" that accelerates the burning/oxidation of the combustion chamber.  The FADAC and flight crew will monitor the throttle response and also the Exhaust Gas Temperature (EGT) as the controls to understand the relative health of the fuel nozzles.  If the engine exceeds the EGT limit a mandatory boroscope inspection is required to keep the Aircraft in service, this inspection will visually verify the condition of the fuel nozzles and combustion chamber.