Can 'neutral static stability' only be achieved by constantly augmenting the control surfaces? I watched a flight mechanics lecture that mentioned that some combat aircraft like the Hornet has a 'mode' where the on-board computer simultaneously and automatically adjusts all the control surfaces to make the plane fly with Statically neutral stability which to my understanding means that the lift moment is constantly aligned with the plane's center of gravity.

The result is that the pilot is able to pull back on the controls and pitch up and release the input and have the plane continue to fly in a new direction without any tendency to diverge or return to equilibrium (the trim position).

Is this true and is this stability only possible with a constant re-balance of the control surfaces of the plane?

I know this is old, but for the people who are still looking for information regarding this subject, this explanation is not complete and partially inaccurate.
First, there are 2 types of stability. Static stability and dynamic stability. It is true that stability comes from the current situation attitude (angle of attack), but it has different effects for different aircraft. And it is possible to have positive static stability but with dynamic instability. Case in point, 727's deep stall where it's attitude effectively locks the tail in a 'wind shadow.' It causes oscillating moments that don't exist outside of this flight profile.
Static stability has to do with the short period moments of the aircraft. An aircraft with positive static stability is said to have a forces acting to try and return the aircraft to trimmed flight at its trimmed alpha. A neutrally static aircraft will desire to hold the current alpha even after disturbed. An aircraft with relaxed static or unstable static stability will continue to increase its alpha in the direction opposite of the flight path. In other words, neutral static stability will not remain at 2 degrees of pitch attitude (theta) once the stick is released. Instead, it will maintain the pitch rate (theta dot) that it had when the stick was released. Static stability will never have a description of oscillating moments, instead it will describe what happens right here, right now if a momentary force acts upon the aircraft.
Dynamic stability is the stability of the aircraft in the long term. It describes what happens after a certain amount of time. A dynamically stable aircraft will have no or little oscillating moments and will dampen oscillations over time. It will follow a trend that is predictable. An aircraft with neutral dynamic stability will maintain its oscillations until acted upon, and then after, continue whatever oscillations remain. But an aircraft with negative dynamic stability will continue to have diverging oscillations, increasing in amplitude from its original orientation.

The following 9 conditions describe, by example, what will occur with static and dynamic stability are both taken into consideration:

Positive static stability with positive dynamic stability: Aircraft will immediately attempt to return to its trim condition and
will have oscillations that dampen out as the forces return to
equilibrium.

Neutral static stability with positive dynamic stability: Aircraft will hold its current alpha and its pitch rate will remain
constant with regard to its current alpha and the oscillations from
the disturbance will slowly dampen out and return to equillibrium.

Negative static stability with positive dynamic stability (bit of a misnomer, explained below):
Aircraft will continue to
increase alpha and its pitch rate will increase. The misnomer is that
there will be no oscillations because there are no forces acting to
return the aircraft to its previous state in the long term. So this
technically can't exist. Airspeed will continue to decrease and drag
will continue to increase until it is more of a brick than a plane.
This is undesirable in fighter aircraft as if it enters a spin, any oscillations that could help exit the spin will quickly
dampen out and recovery will be nearly impossible. (with regards to
the lateral axis only)

Positive static stability with neutral dynamic stability: Aircraft will want to return to normal flight, its AoA will decrease,
and its pitch rate will reverse. But the aircraft will continue this
oscillation indefinitely if no corrective measures are taken.

Neutral static stability with neutral dynamic stability: Aircraft will want to continue to maintain it's current angle of
attack and any disturbances and oscillations for correcting forces
will continue to oscillate as they did before. It have epitrochoidal
motion.

Negative static stability with neutral dynamic stability (yes, this exists, and is what is common in some fighter aircraft):
Without corrective inputs in a timely manner, this will have an
oscillating period and will also continue to diverge from normal flight. This can be a good thing when trying to recover a
fighter from a spin as the oscillations will continue and the aircraft can be "rocked out." The F-16's deep stall behavior is very much described like
this, and can be described as a falling leaf. I have a video on this,
from a simulator called DCS World, here. In fact, in the video, you can also see that the longitudinal axis also suffers from neutral dynamic stability in this flight envelope. The switch I press at 1 min and 45 seconds overrides the computers maximum control surface deflection window and I can then "rock the plane out" by increasing the amplitude of the oscillations until the airflow over the control surfaces allows the computer to recover from the deep stall.

Positive static stability with negative dynamic stability: This is what you want when you want to make passenger planes into
vomit planes. The corrective forces acting on the plane cause it to
overshoot and the oscillations increase in amplitude. But the plane
is still trying to right itself in the short term.

Neutral static stability with negative dynamic stability: At this point, I don't know if there is a point in describing the
behavior because I think you might be able to get the gist. The plane will want to maintain its current alpha, but will then reverse, causing larger amplitudes of oscillation on each reversal. It might have 1 or 2 cycles before it completely
diverges from controllable flight. Not the characteristics you really
want to have in an aircraft.

Negative static stability with negative dynamic stability: Just pure chaos. Basically any orientation along the lateral
axis will have a 'butterfly effect' response and unpredictable
behavior. Forces put in to try and correct it will have an
oscillatory response that will increase in amplitude on the next
cycle, causing the need for a larger input to correct and the pattern
repeats.

Currently studying flight dynamics as a hobbyist and with 2 years of engineering school. Current project is for model aircraft with a flight control computer to emulate an aircraft with negative static stability and neutral dynamic stability.
And, for references sake, so you can figure this out on your own in a sort of mental simulation:
https://www.boldmethod.com/learn-to-fly/aerodynamics/3-types-of-static-and-dynamic-stability-in-aircraft/


Can you track drones online? You can tracks planes, automobiles and boats on Internet, but does tracking technology exist for drones and if so, can you follow them online?

As far as the tracking on aviation websites goes, no you cannot, at least not with the drones or UAVs available for leisure activities.

In order to track these devices they would either need to be equipped with FLARM or a transponder with Mode-S, which again would require a registration.

Unless you have fitted a GPS transceiver to your UAV which offers online tracking via the vendor's website, you will be unable to track UAVs.

Related:
How do you track and visualize aircraft from home?
Why do helicopters not appear in flight trackers?


Is a physical lapel pin awarded for FAA Safety Wings? I have recently got my PPL, and started the FAA Safety Wings program. After completion of each phase, are we awarded a physical lapel pin?

Yes, the FAA currently offers a lapel pin at the completion of each phase of the WINGS program.

This "reward" is not automatically awarded or delivered, the WINGS participant must request it. Upon completion of each of a phase, for example Basic, Advanced, or Master, the FAA Safety Team will include the following notice:

  If you have not already done so, I encourage you to go to your My WINGS page at FAASafety.gov and claim your Team Member Reward -- a beautiful set of wings for your lapel.

The pins are sponsored and distributed by AVEMCO, in partnership with the FAA.

The pins are available in at least three finishes: bronze, silver, and gold, and a special sea-plane version is apparently available.

Source: own work


How much air is recirculated (vs. bleed air injected) in modern airliners cabin? Air in the cabin of airliners is a mix between:

new air (engine bleed air or ram air in some aircraft) and
recycled air pumped out of the cabin, filtered and re-injected.

How much air is recirculated? What are the reasons for recycling air instead of using bleed air only?

From an own work report, based on public web presentations for type rating and on The Boeing 737 Technical Guide by Chris Brady:

  A recirculation system reduces bleed air requirements and pack loads, by filtering cabin air and re-introducing it to the mix manifold. Approximately 25% of the cabin air is recirculated. The recirculation fan will switch off if either pack is in HIGH flow, causing a net reduction in ventilation rate of about 15%. Total ventilation rate on a 737-800 is approx 1 m3/sec

  A portion of the conditioned air from the left pack flows directly into the flight deck. The remainder of the left pack air, all of the right pack air, and air from the recirculation system is combined into the mix manifold. Mix manifold air is distributed to the passenger cabin through different zones, via the side-wall risers on the left and right sides of the aircraft. On ground, pre-conditioned air can be pumped directly into the mix manifold

The B737 Technical Guide further mentions that on the B757 and B767, up to 50% of cabin air is recirculated, and on the MD80 none.


Does aircraft/ATC get any information directly from a weather balloon? We have been launching radiosonde (a meteorological instrument containing GPS and other sensors to measure meteorological variables attached to a balloon filled with Hydrogen) to obtain meteorological data of the atmosphere.
This radiosonde transmits the information to a ground station we have in a frequency close to 400 MHz (well, this is what we uses, not sure about others). We frequently update the vertical height, direction, bearing position etc to our nearest ATC.
I was told that ATC and aircraft get information from the radiosonde directly (I am not sure what sort of information they get) other than what we give to the ATC.
So, I want to know whether ATC/aircraft get information directly from a radiosonde and if they do, what sort of information do they get?
I am not concerned about the radiosonde's meteorological data going into the weather models for forecast products that is available to aviation industry. What I need to know is if ATC/aircraft can know the position of the balloon/radiosonde directly by the information the radiosonde transmits?
In essence, can ATC/aircraft track a radiosonde without inputs from the agency that launches/manages the radiosonde?

No, neither ATC nor aircraft receive any information directly from a radiosonde balloon. However, in many cases the meteorological agency operating the radiosondes does use them to produce forecasts that are made available to ATC and pilots.


What could these possibly incorrectly translated numbers for aircraft speed have been? Yesterday I was watching a show about Air Crash Investigation, the
Air France flight 447 case involving an Airbus A330-200.

I was watching it with Spanish voice dubbing and the weird part is that the narrator said the plane...

reached a final speed of 13,000 km/h
started falling at 7,000 km/h
crashed at 200 km/h

The only speed value that makes sense to me is the last one: 200 km/h.

My guess is that something was missed in translation. What is usually the speed measure for aircraft in the US?

I also thought about the 7,000 figure, and perhaps instead of speed in km/h it meant altitude in feet?

I think there is some information lost in translation.

The speeds of 13000 kmph are associated with reentry vehicles. The aircraft won't have had time to accelerate to that speeds in the first place, even if there is no atmosphere.

The (forward) speed is usually measured in mph in US (to the best of my knowledge). The vertical speed is given usually in feet per minute. Note that the aircraft speed is usually given in knots (or its Mach number) by convention.

Now, the numbers you've quoted:

7000 When the autopilot disconnected, the roll angle increased in two seconds from 0 to +8.4 degrees. In response, the Pilot Flying (PF) applied nose-up sidestick inputs, which the Investigation termed as abrupt and excessive. As a result, the aircraft started to climb at 7000 ft/min.
200 The (vertical) speed at impact was around 108 knots (10,912 ft/m), which is around 200 kmph.
13000 I'm not sure about this one. The maximum altitude reached was less than this value (~11500m actually), so doesn't make sense. The only thing that is related to this number for that flight was the depth at which the wreckage was found- 3,980 metres or 13,060 ft.

Note: Most of the data is from the Final accident report of Air France 447.


Is there a "Quid Pro Quo" of flight following? I like to use flight following whenever possible because who wouldn't want traffic pointed out to them for free?

But just as the controllers are doing me a favor, they sometimes ask me to do them a favor.

For example, depending on a given runway configuration, they might vector me way far out from the airspace. During times like these I end up just wanting to terminate. I'm far clear of the class C, and they've added 20 minutes to my flight.

How is it that they move me so far away when plenty of aircraft who didn't even bother to call up approach are flying so freely without incident? To anticipate objection, this is while staying out of the charlie itself.

Is there anything I can do to ask if I can change altitude to avoid rerouting? I'd hate to call up, have them assign me a squawk, and then say "like to terminate" moments later.

There is indeed a Quid Pro Quo with flight following: you're adding to the controller's workload, and they may ask you to do things to help alleviate that workload (like not flying through a busy approach/departure corridor).
Some controllers will also vector you around a hot MOA (even though you can fly through it VFR their normal operating mode is for IFR traffic) while others will simply alert you that the MOA is hot and leave it up to you to decide how to handle that.

Because there's no language in the ATC manual for "Hey could you please fly around this area so you don't make my job harder?" when controllers want to ask you to do something they use the language they do have, which takes the form of instructions (vectors).

As a pilot you should remember that vectors are the opening round in a negotiation: If a controller gives you a vector you don't like you can always ask them why, and often you can negotiate for something better.
This is true for IFR flights as well - The only exception is when a controller uses the word "Immediate" or any variation thereof (then you do what they say and ask your questions later).

If you know you'll be flying through an arrival or departure path you can offer to change your altitude to stay out of the way. Your handy VFR Sectional or Terminal Area Chart shows some of the arrival and departure paths for busier airports:

If you offer a climb or descent controllers will usually tell you what altitude they need you at so you aren't in the way, and if they're not forthcoming you can ask ("What am I being vectored around? Can I go through at a different altitude?").

Finally remember that as a VFR flight you are under no obligation to accept their vectors in Class E airspace: If you don't like what the controller is asking you to do and can't negotiate something better simply say We'd like to cancel flight following at this time and continue on your own navigation (keep listening to the frequency to see what they do and maybe you'll come up with better alternatives for your next flight).

The controllers may grumble about it, particularly if they just put you into the system and you immediately cancel services, but that isn't your problem.


How does the stick pusher prevent an aircraft from stalling? The stick shakers warns the pilot of stall conditions and some fixed winged aircraft have a stick pusher which tries to avoid the stall. What is the working methodology of stick pushers?

A simple explanation is that a stick pusher is a:

  ... device that pushes forward on the elevator control system
  whenever the aircraft’s angle of attack reaches the pre-determined
  value, and then ceases to push when the angle of attack falls
  sufficiently.

A detailed explanation is:

  A stick pusher system incorporates both a passive monitoring component
  and an active intervention component that consists of an hydraulic or
  electro-mechanical pusher mechanism. The monitoring component
  constantly evaluates critical flight parameters such as airspeed, load
  factor and flap setting against the aircraft angle of attack. If the
  angle of attack reaches a predetermined value, the intervention
  component of the system is activated and pushes forward on the
  elevator control system until the angle of attack is reduced to an
  acceptable value. The system operates autonomously and without any
  required action by the pilots.

FAA has an advisory circular (AC) for stick pusher training.

For Embraer 145, this video shows a demo of stick shaker, stick pusher and the warning systems.


What are the relative contributions of the body and wings to missile lift? Slender bodies like arrow shafts, airliner fuselages, and submarine hulls are all capable of generating lift, albeit much less efficiently than a high aspect ratio wing. They also generate little drag, which is their main advantage. I would like to know how their lifting properties compare to that of a wing; to this end I would like to refer to a use case where the slender body makes up most of the airframe: guided missiles.

Consider a missile with a long slender body and short lifting surfaces, with a "conventional" air to air missile aspect ratio like that of the R-27 or AIM-120. The exact wing and control surface layout is not relevant, only the form factor. This means we should purposefully exclude munitions equipped with a moderate aspect ratio wing such as the TLAM or JSOW: I am interested in low aspect ratio BVR designs:

To avoid complications due to the engine or absence thereof, consider it flying in the coasting phase. The lift needed to stay aloft or even climb is then provided by a combination of the wings and the slender main body (where the guidance systems, warhead and sometimes engine are housed).

The question is, what is the proportion of lift coming from the wings compared to that of the fuselage?

NOTES:

A good answer should use a real or realistic design to base their calculations on.
I have purposefully left the choice of design open, so people can pick one they are familiar with or have data on. All that matters to me is that the general shape must be similar to that of modern AAMs.
Feel free to elaborate on how the lift proportion changes at different flight conditions (subsonic vs supersonic); it is quite clear that higher speeds lead to smaller wings.
I am aware that short range WVR missiles like the R-60, R-73 or AIM-9 are designed to reach their target before engine cutoff or shortly after; this has no bearing on the question.
This answer below presents a good extreme where most of the lift has to come from the body, because the only fins are too far away from the center of mass. In such cases, where the fins act as a stabilizer, their lifting force can still be calculated and compared to the total lift required, if we can assume the location of the CG.

What is the proportion of lift coming from the wings compared to that of the fuselage?

It can be almost zero.  The ASRAAM's only fins or wings or lifting surfaces or whatever you call 'ems are at its very tail.  Any "lift" they impart, that far from the missile's center of mass, would pitch (or yaw) the missile instead of counteracting gravity.  So almost all the lift must come from the body having a slight nose-up attitude.

Another missile with a similar planform is the (ground vehicle launched) LOSAT.


Are the details of flight control laws the same for every fly-by-wire Airbus aircraft or are they only the same within the specific family? I was wondering if someone learns the details of every single flight control law for the A320 family, does that knowledge still apply to all the other Airbus aircrafts? If not, what are the differences?

From a pilot point of view they are the same, for all families.

Technically, they are differently implemented, from a family to another one, but they are implemented the same way in the same family.

The basic principle for all Airbus FBW are as follows in normal law:

in pitch normal law the side stick send a «load factor» order that would become limited when protections are triggered such as AOA protection.
in roll the side stick gives a «roll rate» that would become limited when protections are triggered such as bank angle exceeding a threshold.
When sensors redundancy is degraded the laws become degraded and we enter alternate laws or direct law.
On ground we are in direct simulated relation ship between the side stick and the related surfaces.

The differences:
They don’t affect the flying technique from the pilot point of view, they concern the protections mostly those that concern the structure of the aircraft for exemple
The Maneuver Load Alleviation function that distributes the loads on the huge wing of the 330, this protection is specific to the 330/340


What is the best way to keep logbook entries backed up? What is the best way to keep logbook entries backed up?

Should a photocopied or scanned image on a computer suffice? Can an Excel spreadsheet or other software/apps be used? In case a logbook is lost or (unfortunately) destroyed, should all entries from the backup be rewritten in a new logbook?

Also, when one has 100s or 1000s of hours, spreading over several logbooks, how much importance the older logbooks will have? Would someone go over every page to make sure that this individual actually have 1375.2 hours (as an example)?

The FAA requirements are for the information to be logged, without specifying the details of how it is logged.

It's OK to use electronic log books, and multiple log books. Scanned images are as good as photocopies. Just print them if you ever need them on paper. Some items are required to be signed, so scanned signatures and endorsements may be necessary in an electronic logbook.

The older the information, the less its importance. Log books are normally examined to see if what you're doing is legal, checking ratings, currency, training and reviews, etc. If you are flying without required night or IFR currency, for example, it is conceivable that they'd want to check previous flights to see how often that has happened in the past.

An insurance company might want to make sure you've got the hours you claim, but the FAA's interest is to see if you are legal.


What advantages come with the wing on the Boeing Bird of Prey? I noticed this picture yesterday of the Bird of Prey and wondered about the wing design and what advantages would come with this unique wing design compared to other aircraft?

Source:(www.wired.com)

The Boeing Bird of Prey is indeed a unique aircraft, wahich was an internal project rather than an X-Project funded by the government. It was primarily used as a technology demonstrator for low observable technologies.

The Bird of Prey is rather unusual for a wing only aircraft (in the sense that it had no stabilizer) that it was statically stable and required no correcting inputs from a computer. This stability was achieved by the lift generated by the chines in front of the fuselage.

Here is  closer look at the chines.

Source: www.freerepublic.com

The chines produce lift by generating vortices (similar to the delta wing). Basically, the chines produce seperated vortices on either side of the fuselage, which produces lift. According to adg.stanford.edu,

  Rather than reducing the lift of the wing, the leading edge vortices, increase the wing lift in a nonlinear manner. The vortex can be viewed as reducing the upper surface pressures by inducing higher velocities on the upper surface

Image from Effects of Vertical Tail and Inlet/Strake Geometry on Stability
of a Diamond-Wing Fighter Configuration by Mitchell E. Fossum et al., accessed through http://enu.kz/

To compare how the chine generates lift similar to a delta wing, it is instructive to see the vortex flow patterns on a slender (cropped) delta wing with a chine forebody.

Image from On the Nonlinear Aerodynamic and Stability Characteristics of
a Generic Chine-Forebody Slender-Wing Fighter - Configuration by Gary E. Erickson and Jay M. Brandon, NASA Technical Memorandum 89447, labeling mine.

An example of a delta wing with a chine forebody shows the two vortices being generated from the chines and the highly sweptback wing, similar to the Bird of Prey.

Image from Aerodynamic Analysis of a Generic Fighter with a Chine
Fuselage/Delta Wing Configuration Using Delayed Detached-Eddy Simulation by Tiger L. Jeans et al, accessed through calpoly.edu

The forces acting on the longitudinal axis of the aircraft look something like this (I'm not sure of the exact values or locations), leading to a statically stable aircraft.

Profile from bj-o23.deviantart.com, others own work.

The chines also help in reducing the radar cross section, according to dictionary.sensagent.com (attributed to The Radar Game by Rebecca Grant):

  A standard leading edge extension joins to the fuselage at an angle while a chine is an extension of the curvature of the fuselage. Therefore chines avoid presenting corner reflectors or vertical sides to radars.

This design has since become standard in (Boeing) UAVs like X-45 etc.

Another feature is that the fuselage sides are parallel to each other, helping in reducing the Radar Cross section by reflecting the radar waves away from source.

Source: gizmodo.com

The wing design is rather unusual, a polyhedral section with dihedral (upwardly angled) inboard sections and anhedral (downwardly angled) outboard sections, which helped in stealth and stability.

Source: soldiersystems.net

The wing has a rather high sweep angle (~65 degrees) for a low speed aircraft, in order to improve stealth.


What is the relation between the EASA Light Aircraft Pilot Licences and the Private/Sailplane/Balloon Pilot Licences? The EASA licence and ratings requirements pages list:

PPL(A) - for aeroplanes
PPL(H) - for helicopters
SPL - for gliders
BPL - for balloons

which match:

LAPL(A) - for aeroplanes
LAPL(H) - for helicopters
LAPL(S) - for gliders
LAPL(B) - for balloons

It appears that you can upgrade an LAPL to its matching licence; for example, an LAPL(A) to a PPL(A) by taking further instruction and tests (except in the case of the sailplane licence, which seems not to require this, and the airship licence PPL(As), for which their exists no LAPL counterpart).

It looks like the LAPLs are generally easier to obtain (for example, requiring fewer flying hours).

What's the purpose of this apparent duplication of licensing? Are the LAPLs intended to be a pathway to the other licenses, or a destination in themselves?

LAPL (Light Aircraft Pilot License) can be a pathway to a PPL (Privat Pilot License) but can also be just kept by it's own, as it's somewhat cheaper to get and maintain than a PPL.

LAPL has stricter limitations than the PPL, for example they are only valid in EASA countries and not in all ICAO states. Also, LAPL is limited to a maximum take-off weight (MTOW) of 2000kg and does not have the options of upgrading to IFR or similar (Night VFR is an option though) and a maximum of 4 people on board (3 passengers + you).

Therefore: If you are fine with the mentioned limits, you're good to go for an LAPL. If at some point you want to upgrade to PPL, thats not a big issue as it just requires the additional training hours (theory is the same for LAPL and PPL).


Why is the nose of the Mig-21 shaped so differently compared to other fighter aircraft? If you look at this picture below, you will notice that the nose portion of the Mig-21 has a unique conical shape which is completely different from the nose of the F-16.

(Source)

(Source)

So what explains the conical design of the nose of the Mig-21?

The nose design of aircraft, like any other part, is a result of optimization in response to a number of factors. The Mikoyan-Gurevich MiG-21 'Fishbed' is a supersonic, second/third generation Soviet fighter/interceptor.

The first jet aircraft to enter serice during the WWII, the Messerschmitt Me 262 'Schwalbe' and Gloster Meteor had twin engines in their wings. In these cases, the nose was designed to carry the cannons, which were the primary armament at the time.

"Messerschmitt Me 262A at the National Museum of the USAF" by USAF museum - http://www.nationalmuseum.af.mil/shared/media/photodb/photos/040820-F-1234P-081.jpg. Licensed under Public Domain via Commons.

The first generation jet fighters, like the F-86 and Mig-15 were subsonic and had engines buried into the fuselage to reduce drag. The nose, again carried the cannons (in case of Mig-25, in the lower fuselage).

"F86Sabre" by Kowloonese at the English language Wikipedia is licensed under CC BY-SA 3.0

However, the second generation jet aircraft were the first operational jets to go supersonic. The Mig-21 was designed in response to a Kremlin specification, which called for Mach 2 level speed (at altitude), while carrying a simple radar and option for future air-to-air missiles.

As the jet engines require airflow to be subsonic at entry, the flow has to be decelerated before reaching the engine. Early in the design (from Ye-50-3), Mig decided to use a conical centerbody for this purpose, as it was decided that the aircraft would be single engined with engine in the fuselage (the engine was supposed to be the Mikulin AM-9B, however, the aircraft ended up using Tumansky R-11).

This nose design decelerated the flow using two oblique shocks and a normal shock, giving improved pressure recovery at high supersonic Mach numbers. The center-body (cone) moved axially according to the Mach number. Also, the center body housed the radar and associated equipment.

The first generation Mig 21s only had a range finder, and the intake diameter had a smaller cone.

"Mig21F13web" by Newresid stephanelhernault@yahoo.fr - Own work work vector drawig travail personnel dessinn vectoriel. Licensed under CC BY-SA 3.0 via Commons. (Image cropped)

The following image shows the range finder used in early models of Mig 21.

"MiG-21F-13 radar Keski-Suomen ilmailumuseo" by MKFI - Own work. Licensed under Public Domain via Wikimedia Commons.

The photo you've shown are from the later versions (Mig-21 bis from IAF), which had the radars from the "Sapphire" family. To fit the radar antenna into the cone, the cone had to be made bigger, therefore the whole front fuselage had to be redesigned, the diameter of intake got more than 200 mm higher, and the cone itself got longer, wider and moved forwards.

"Mig21fishbedHfamilyweb" by Newresid stephanelhernault@yahoo.fr - Own work work vector drawing travail personnel dessin vectoriel. Licensed under CC BY-SA 3.0 via Commons. (Image cropped)

The following figure shows the enlarged nose cone section housing the radar. Interestingly, the nose cone seems to be made out of (metal tipped) wood to prevent interference.

Source: toad-design.com

A number of aircraft from that time had conical body inlets, like the British Electric Lightning.

"Lightning.inflight.arp.750pix" by Arpingstone - Own work. Licensed under Public Domain via Commons.

The F-16, on the other hand, has a simple normal shock diffuser. By the time of F-16, the radars had become bigger and more important (AAMs have become the primary combat weapon, replacing guns), and needed more space in the nose. As a result, the inlets of most of the aircraft had moved to the sides/downwards. The following figure shows the AN/APG-68 radar in F-16.

Source: defenseindustrydaily.com


How do contrails form? For my pre-scientific work I now need some general information about contrails. I searched for websites that explain the process of contrails being formed in detail but couldn't find any so far.

I realized this question hasn't been asked on Aviation.SE so far, so I decided to ask it now.

I would like to know, how the whole process works and what it needs for it to form a contrail.

What temperatures are needed?
How wet does the air have to be?
What exactly is a contrail then? Only condensation, ice, ..?
Is the contrail made from humidity from outside-air or from the vapor of the engine?
How does a contrail vanish again? What makes the difference between a 20 second contrail and a 2 hour contrail?
Everything that you know and is important to know.

Some of the questions may sound strange, but I found this topic is really complicated and also confusing and I want things to really understand now.

I (and probably many other people googling for this question in the future) would really appreciate a nice, informative, in detail answer, thanks!

Contrails are formed from the condensation of jet exhaust.

When a hydrocarbon, such as jet fuel, is burned, it produces water vapor and carbon dioxide along with some impurities/unburned fuel. High altitude jets are in a much colder environment than ground temperature. Assuming a non-cloudy day, we can expect an approximate decrease of 5.4 degrees Fahrenheit for every 1,000 feet of elevation. Commercial jets fly around 28-35,000ft which translates to a roughly 150-160 degree decrease in the outside temperature. I'm not going to go into all of the physics behind it, but this is the most important point. It is much, much colder up in the air than down on the ground. And it is because of that coldness these contrails exist.

From here the lower temperature and outside pressure of the surrounding air create a ripe environment for condensation, as soon as a particle from one of the impurities in the fuel touch the vapor it condenses forming tiny water droplets and ice crystals. It's the same physical action that occurs when our breath produces a small 'fog' in the cold, and the same one that causes natural clouds to form in the first place.

You can also use contrails to tell the weather, high-humidity causes contrails to be thick and long-lasting which is a sign that a storm may be soon to come whereas a thin contrail or one that doesn't last long is a sign up low humidity.

To answer the rest of your questions:

  What temperatures are needed?

That depends on a lot of factors, but in general they form when the air is colder than -34 degrees Fahrenheit.

  How wet does the air have to be?

The air could be bone dry, the exhaust fumes themselves provide the water; though some of the latent humidity in the air is also heated and re-condensed adding to the effect.

  What exactly is a contrail then? Only condensation, ice, ..?

A contrail is a mixture of condensation, ice, and trapped impurities from the exhaust.

  Is the contrail made from humidity from outside-air or from the vapor of the engine?

The contrail is made from varying degrees of both.

  How does a contrail vanish again? What makes the difference between a 20 second contrail and a 2 hour contrail?

A contrail is basically a cloud and vanishes the same way that clouds dissipate. Namely, temperature increase, mixing with drier air, or air sinking. All three will cause evaporation of the water droplets and dissipation of the cloud.

What makes the difference is primarily the external humidity. The more humid it is, the less a cloud or contrail can evaporate before reaching a state of equilibrium with the surrounding air

  Everything that you know and is important to know.

The disturbance of the wingtips through humid air can kick off the same reaction, even near the ground. Though much less pronounced.

https://commons.wikimedia.org/wiki/File:C27_Spartan_making_condensation_spirals.jpg


Would an A320 override the autothrottle due to an erroneous radio-altimeter? The pilots of Turkish Airlines Flight 1951 had multiple issues with the auto-throttle setting itself to idle during approach: despite manually adding throttle, the plane stalled and crashed before reaching the runway, from a sequence of events initiated by the reaction of the auto-land system to one faulty transducer.

The fault was masked because:

The PF was the first officer, his radio altimeter was working correctly.
Autothrottle in the 737 only looked (looks?) at the captain's side.
The glide path was intercepted from above and throttle reduction was expected.

The F/O was an ex military pilot with 4000 flight hours receiving line training, when the stick shaker activated he immediately gave full throttle. The captain took over command, during which the autothrottle set back to idle again. The plane then stalled and re-applying full thrust was ineffective. Full investigation report pdf here.

The plane was a Boeing 737-800. Had it been an A320, would the autothrottle have reacted in the same way, and set the throttle to idle upon detecting zero radio altitude?

The Airbus thrust levers can only be moved manually. The levers do not move on their own to reflect the auto-thrust setting.

There are detents for the different modes of auto-thrust operation. Regarding the RETARD mode whereby the thrust auto-idles, Airbus doesn't use that (except in autoland).* Hence the callout, "RETARD, RETARD," for the pilots to retard the levers before touchdown.

In a normal uneventful flight, the third detent from the top (CL) is where the thrust levers stay throughout the whole flight from thrust reduction after takeoff, to just before touchdown.

(A320 manual)

* Airbus released an Operations Engineering Bulletin (no. 38) in Mar 2011 (also see the Airbus brochure on the matter). What happened to the Turkish 737 could happen to an A320 due to an "erroneous radio altimeter height indication".

Still, in the scenario given once the pilot flying had applied full power (TO/GA detent) it would have stayed there (unlike the Turkish 737).

Thanks to @user40476 for bringing up this autoland feature, which made me look up further details.


Are there any Part 135 (On-Demand Passenger Charter) companies out there that use Aviation Bio-Fuel? I was recently approached by a client asking if I can provide charters on environmentally conscious aircraft that run on Aviation Bio-Fuel.

Anyone have any insight on what is required for an aircraft to run on bio-fuel?

Who is implementing the use of Aviation Bio-Fuel in their fleet?

How do I find out which airports actually have Bio-Fueling capabilities?

Biofuel has already been considered and tested on some aircrafts. Here you can find an article about tests.It states at least six of them

  At least six airlines, including KLM, Lufthansa and Finnair, have now
  used biofuel on flights carrying passengers.

However there is still a long way to go.On wikipedia it says

  The sustainable aviation fuels certification and production pace seems
  insufficient to meet the IATA target of halving the CO2 emissions by
  2050

But airports do not supply bio-fuels for now since you need tons, even for one flight.

And for the list that used biofuel on their commercial flight you can check this list out. On that list i know KLM, interjet,Hainan offers charter flight. So it is safe to say those companies environmentally conscious, if you want.


Did the contemporaries of the F-14 also have automatic swing wings? The F-14's Central Air Data Computer was a innovative design, using microprocessors for its implementation. This computer was used to control the swing wings of the F-14.

How did the contemporary aircraft to the F-14 achieve the same functionality? There were many variable-sweep wing aircraft constructed in the 60's and 70's such as the Mig-23 and the Su-17.

References:

https://en.wikipedia.org/wiki/Central_Air_Data_Computer
http://firstmicroprocessor.com/documents/ap1-26-97.pdf

MiG-23
For the MiG-23, it was manual.

[The] MiG-23 had a completely manual wing sweep control. But there was no gauge to indicate optimum wing position for the surrounding conditions. "You had to manually put the wings into position to safely fly the airplane," (...) "They had a very nice gauge that showed where the wings were commanded to, where the wings actually were if they were still in motion, and what the Mach limit was associated with that wingsweep position."
— Red Eagles: America’s Secret MiGs p.176

From Wikipedia we learn the sweep steps were 16°, 45°, and 72° – for one of the variants at least – which makes it easy to find that gauge. Here it is magnified below with the limiting Mach number (M2.35) and Velocity (1400 km/h indicated; 756 KIAS) for the current selected max sweep of 72°.

(worldwarwings.com)
Su-17/20/22
It started as a two-position wing, then it was upgraded1 so that the wing can stop anywhere between 30° and 63°. This improved the flight range.
Below, КРЫЛО means wing. And ОТКЛ means off, or deactivate (ОТКЛЮЧИТЬ). This leads me to conclude it too was manual despite the advanced avionics the Su-17 family carried.

(primeportal.net) The wing control switch and position indicator are on the left glareshield.

1 Source: Polish Wikipedia
Bonus: YouTube video showing the pilot using that switch


Is it allowed to log actual instrument under IFR in VMC when VFR would be prohibited? Flying with an IFR clearance near IMC but with visual references where VFR would be prohibited I have the following questions:

Are you technically even in VMC if you are that close to a cloud? Technically flight is not strictly by reference to instruments, but VFR would be illegal.
Can an instructor log instrument time? (a PPL would not be allowed to fly through tiny holes in a cloud)
Does an instrument student need foggles or can it be logged as "actual"?
Are you still required to see and avoid (obviously still a good idea)

Example: you are 100ft above a cloud and 1000ft horizontal in class E airspace under IFR. You can technically see but need the IFR clearance to be legal.

14 CFR 61.51(g) says (my emphasis):

  (g) Logging instrument time. (1) A person may log instrument time
  only for that flight time when the person operates the aircraft
  solely by reference to instruments under actual or simulated
  instrument flight conditions.

  (2) An authorized instructor may log instrument time when conducting
  instrument flight instruction in actual instrument flight
  conditions.

Based on that, the answers to your questions are:

Are you technically even in VMC if you are that close to a cloud? Technically flight is not strictly by reference to instruments, but VFR would be illegal. You're in IMC (i.e. you're operating below the visual minimums in 91.155) but as you said, that doesn't mean you're flying "solely by reference to instruments"
Can an instructor log instrument time? (a PPL would not be allowed to fly through tiny holes in a cloud) If the conditions are IMC, yes (61.51 doesn't require an instructor to operate the aircraft using only instruments)
Does an instrument student need foggles or can it be logged as "actual"? Whether you're wearing foggles or not, you can log instrument time only when you're operating solely on instruments. Practically speaking, using foggles guarantees that you're doing that and can therefore log the time. Without them, some or all of the time might be loggable, i.e. only the part that you really spend flying only on instruments.
Are you still required to see and avoid (obviously still a good idea) Yes, absolutely. According to 91.113(b):

  When weather conditions permit, regardless of whether an operation is
  conducted under instrument flight rules or visual flight rules,
  vigilance shall be maintained by each person operating an aircraft so
  as to see and avoid other aircraft.

It's very common to fly cross country under IFR without logging any instrument time, or logging very little, simply because you're never solely on instruments.


Advantages and disadvantages of air intakes above fuselage/wings in jet fighters? I found some jet fighter designs that never went into production. All of them have air intakes above the fuselage or wings (albeit not in the same manner).

I'm trying to assess if there'd be any point in making planes like those.

There are almost no real-life examples of small planes with similar placement of air intakes, so maybe it would be too disadvantageous? I know only of A-10 and F-117, but they are both ground attack aircraft. Some say those designs would be prone to engine failure while doing sharp maneuvers with high angle-of-attack in a dogfight. On the other hand, isn't air travelling faster above wing/fuselage?

What do you think? Would there be any point in manufacturing planes like this in real life?

Advantages of overwing intakes:

Little danger of foreign object damage (FOD). Remember that F-16 bases need to keep their runways spotless in order to avoid FOD. The low intake will hoover up all loose items around it while the overwing intake mainly sucks in air from above.

Disadvatages of overwing intakes:

Lower pressure recovery at high speeds. The air above the wing is first accelerated in order to create lift and then this accelerated air must be decelerated for maximum pressure recovery. This is less direct and incurs more losses than low intakes which operate in pre-compressed air.
Operation in low density air in supersonic flight. Especially when thrust is needed most, at high angle of attack, an overwing intake will feed the engine with lower pressure and lower density air, reducing the available thrust.
Noise: The North American F-107 was said to be the loudest aircraft from the pilot's perspective, and cockpit noise was one reason why it did not enter production. Granted, here the intake was not over the wings but on the upper side of the fuselage, right aft of the cockpit, but I thought I include this item here nonetheless.

Early stage of F-107 development (picture source), at this time called the F-100 B, but the low intake interfered with weapons release from a central belly bay …

… and that is how it turned out eventually with the re-located intake (picture source)


How do I get certified to fly under FAA 333? tl;dr: How do I go from non-airman to commercial UAS pilot under an FAA 333 exemption?

I understand that the rules are imminently changing, but waiting for the formal implementation of the NPRM could mean a very long wait before even getting started, and there is already a backlog of section 333 petitions.

Say I have a small (under 55 pounds) quadrotor aircraft, and I want to fly it for "civil operations" (commercial). As a non-airman, what do I need to do before I can operate the craft in this manner?

As near as I can determine, the process is something like this:

Obtain a pilot license
File a petition for part 333 exemption
Acquire either a SAC or Waiver/CoA

Do I really need to obtain a pilot license of some sort for real aircraft (either PPL, sport, or recreational) for this purpose, or can I legally operate the craft using some other short-cut? Is there a difference if I plan to fly a craft with a SAC rather than a craft which has none?

The FAA's FAQ says:

  Is an FAA-issued pilot certificate required to operate a civil UAS under an experimental airworthiness certificate or a grant of
  exemption under Section 333?

  If the aircraft is issued an
  airworthiness certificate, a pilot certificate is required.5

  Pilot
  certification requirements for petitions for exemption under Section
  333 are evaluated on a case-by-case basis.

The footnote links to this FAA order called Airworthiness Certification of Unmanned Aircraft Systems and Optionally Piloted Aircraft, which says:

  b. UA PIC Certification and Ratings Requirements.

  (1) The PIC must hold and be in possession of, at a minimum, an FAA private pilot
  certificate, with either an airplane, rotorcraft, or powered-lift
  category; with single- or multiengine class ratings, appropriate to
  the type of UA being operated.

  (2) The PIC must have and be in
  possession of a valid second-class (or higher) airman medical
  certificate issued under 14 CFR part 67, Medical Standards and
  Certification.

So, it looks like there are two possibilities:

Your UAS has an airworthiness certificate, therefore you need a pilot's license per the Order (private at least, not sport or recreational)
Your UAS doesn't have an airworthiness certificate, therefore your section 333 petition will be evaluated individually and the FAA will tell you if you need a license or not


What is that building in Suvarnabhumi Airport? In Suvarnabhumi Airport (Bangkok) there is a building in aerodrome, next to the main terminal (Circle by red colour) . What is that?

On page 5 of current airport chart states it is Transfer Baggage Terminal

It was officially opened on 1 November 2014 and reduce baggage transfer time from 75 minutes to 60 minutes. It was constructed above South Airside Tunnel between gate C, D and E and has usable space of about 12,550 sq.m.

Source (Sorry source is in Thai. My own translation.)
Another source in English. Similar to my original source.


What are those things on the B747 engine pylons? I noticed some dots and lines on the Boeing 747 engine pylons. What are they?

The above image is as seen in this video at 2:18.

These are latches holding the engine support pylon panel in closed. Behind this panel is the diagonal brace.

This area was modified ~1992 due to the El Al Flight 1862 and China Airlines Flight 358 crashes.

See FAA Lessons Learned (via archive.is).


How does aileron reversal work? I want to know how aileron reversal takes place when control reversal velocity is higher than divergence speed  and operating speed is in between them (for conventional wing)

As I understand it, aileron reversal can occur in two regimes of flight, high speed and near stall.

Near stall, the wing is very near it's critical angle of attack. When the pilot inputs let's say a left turn, the right aileron will deflect downward. This downward deflection increases the AOA of the outboard portion of the right wing such that the lift generated by that outboard portion is lower (drag higher as well) than the lift generated on the opposite wing.

You can kind-of visualize this by thinking of a wing operating on the backside of the lift curve.

Aileron reversal at high speed, (again, as I understand it) can occur when the wing isn't stiff enough tortionally to prevent twisting about the aircraft's lateral axis when a control input is made. Some old warplanes had this characteristic at high speeds.

For example, the pilot makes a control input to the left. The right aileron deflects downward, and instead of raising the overall AOA of the wing, it lowers it because the aileron acts like a trim tab, twisting the wing's leading edge downward.

Your question is asking: If the control reversal velocity is calculated to be higher than the divergence velocity, (When the wing breaks apart), how then can control reversal take place in flight?

The answer would be when it occurs near a stall.


When is deicing required? For airliners, under which meteorological conditions is de-icing required?

What factors (temperature, humidity, dew point, precipitation, etc.) influence the decision to de-ice?

Is decision to de-ice down to regulations, the airline, or the captain?

There are a few stages where ice prevention or mitigation is relevant. The FAA publishes a nice handbook on it here if you want more info. Very broadly speaking De-Icing activated when airframe ice is noticed. Anti-Ice is activated when a Pilot believes they may enter icing conditions. Ground based De-Ice is applied when icing conditions prevail in the area.

Before Takeoff

Most, if not all commercial airports offer de-ice facilities if geographically relevant. After the crash of Air Florida 90 the FAA took a bit of a harder stance on De-Ice and now stresses the need for mobile De-Ice units to allow planes to be De-Iced without leaving the taxi queue as well as permanent stations as they have always had. A lot of their suggestions are covered in this AC as well as this one. In this case both airline procedure and FAA regulations will dictate the need for De-Ice. Strictly speaking according to FAR 121.629 a dispatcher can hold an aircraft if they feel it is not correctly De-Iced. However by the same law a pilot can hold their own departure for the same reason.

En Route

Once in the air there are a few ways things happen, but generally its ultimately up to the pilot to activate the De-Ice system. On one hand the aircraft POH will more than likely have guidance on when to do this, however, airlines may implement further (and more strict) procedures. Some aircraft are equipped with systems that automatically activate in the presence of ice build up.

Once in route there are effectually two kinds of systems,

Anti Ice: prevents the build up of ice in the first place, heated leading edges etc. These are activated before entering icing conditions.
De-Ice systems: like boots these systems remove ice once its built up. In this case you need to actually enter the conditions and allow the ice to build up first.

Ice can form in a variety of conditions and can be dangerous or potentially fatal in some aircraft. The temperature depends on the cloud type but generally speaking low temperature and higher moisture content will create worse issues.

(source)

Pilots will be alerted to possible icing conditions via an AIRMET ZULU. These are often quite broad area AIRMET's. If a pilot believes they are potentially flying into icing conditions they should keep an eye on the airframe for structural icing build up. Many airplanes have icing lights to help at night with this. A pilot can activate the De-Ice system at any point if they feel it is needed.


Where to find the callsigns of the en route controllers? I am looking for the callsigns of ATC upper information region (UIR) controllers on a flight from Frankfurt EDDF to Seattle KSEA. According to the flight plan, the UIRs crossed are:

EDDV (Hannover)
EHAA (Amsterdam)
EGTT (London)
EGPX (Scottish)
EGGX (Shanwick)
BIRD (Reykjavik)
CZEG (Edmonton)
CZVR (Vancouver) and
KZSE (Seattle).

First, where can I find charts and information on the sectors within a UIR?

Second, does each sector have a different callsign, or are callsigns coupled to the UIR? For example, EGPX Shanwick will probably have multiple sectors, will the callsign be always Shanwick control? If they are UIR-dependent, what are the callsigns of the above-mentioned UIRs?

You can look in each country's AIP, specifically section ENR 2.1. Below is for the Netherlands:

Note the name/call-sign 'Maastricht Radar' for at or above FL 245. Maastricht UAC is...

  ... an international non-profit air navigation service provider, operated by EUROCONTROL on behalf of four States – Belgium, Germany, Luxembourg and the Netherlands.

Also from the AIP's you can find the division of the FIR's, example:


What happened to Widereøe flight WF0597? WF0597 was a scheduled flight from Bergen Airport Flesland (ENBR) to Kristiansand Kjevik (ENCN). The Embraer E190-E2 immediately started circling Flesland and continued for about 1 hour and 40 minutes, before making a stop at Flesland.

What could have happened to this flight such that it would behave as it did?

The aircraft never changed its squawk, indicating that it did not file an emergency.
Multiple aircraft landed while WF0597 was circling, including an E190-E2 also operated by Widerøe, indicating that nothing was wrong with systems/runway at Flesland.

A pilot friend suggested landing rounds, but this means that the flight was cancelled at some point, even though the websites of Widerøe and Flesland says it had departed as scheduled.

Is this normal? If not, any theories as to what might have happened?

Your screenshot shows the path for flight WF9480. Flightradar24 shows 2 occurrences of such flight one on the 5th August and one on the 6th August, both are training flights for landing practice. From your screenshot, you were looking at the 6th August flight.

All occurrences of WF597 show an aircraft regularly departing Bergen and landing in Kristiansand. All have been operated by a Dash-8, none by an Embraer.

You have probably mixed up two aircraft that were close in the air.


Where does the "APU Auto Shutdown" message appear? I'm trying to find out if and where the "APU Auto Shutdown" message appears on the ECAM.

Both the "APU AUTO SHUT DOWN" and "APU EMER SHUT DOWN" appear on the lower-left section of the E/WD (the upper display of the ECAM) as amber messages.

In the procedures section of the FCOM, the title is on a white background. The introduction of that section, "FCOM Procedure Layout", explains how the title formatting corresponds to how the actual message appears.

The APU SD display will also be called. And the word "APU" will appear under "INOP SYS" once the procedure is completed.

From a CBT video on YouTube, here's what it looks like:


Why are commercial flights not (publicly) trackable in some locations? Flying BA15 from London to Sydney, I noticed that the flight is not tracked over Turkmenistan and Afghanistan, except for a short ping.

Is this for security reasons? Do planes disable their ADS-B, or is the data somehow restricted? Since position tracking is a safety feature, is this regulated or an airline decision?

The reason is that there is no member of the general public in those areas with an ADS-B receiver sharing live data with the public tracking sites.

Websites like Flight Radar 24 rely largely on volunteers installing ADS-B receivers and sharing their received data. They have a good coverage, but some areas are still not covered.


Other than a handful of airlines, do any small operators take PPLs and pay to train them to commercial? Are there operators out there who take SEL private pilots and train them to commercial?  Do small operators like banner tow, agriculture, skydiving do that at all?  Is it way too presumptuous to ask?  To specify, I'm particularly interested in the US, especially California.

With the airlines that do offer such programs, the cost is offset by having a pool of new pilots who they can train to their liking from the start rather than having to retrain existing pilots. They also tend to be large operations for whom the cost of having some trainees on staff isn't a major factor.

For a small operator it's typically too expensive to carry the burden of your training cost as well as employing you while you can't fly commercially (which you won't be able to unless you've completed that training.

So unless they have an opening for a non-flying job you can do, with training provided as part of your payment for that job, they won't be able to employ you.

As most jobs tend to require training and certification that involve aircraft, such a job would almost certainly be something like a desk job, answering phones, cleaning, stuff like that, not something someone interested in a flying job is likely to greatly enjoy, increasing the risk to the company that the candidate is going to quit on them.


How do flight crew train/demonstrate ditching evacuation? In case of evacuation, flight crew can train and demonstrate their ability with a real plane and test passengers.

However; how can one be trained in case of ditching ?

Obviously, it would be quite problematic to ditch an aircraft just for training/demonstration purpose...

Flight crew training facilities have a big pool with a mock aircraft hull with slides and rafts.

Real aircraft are rarely used for training. It's much cheaper to build a fake aircraft interior into the building and use that than to tie up an aircraft that should be making money. It also lets the instructors simulate more dangerous scenarios, like smoke in the cabin, safely.


Which of the pilots flies when it gets dangerous? Who flies when it gets dangerous ?

In other words: Which pilot takes control of a passenger aircraft as Pilot Flying (PF) in an emergency (aircraft upset, loss of control, mechanical failure, etc.) and why ?

the highest-ranking pilot present in the cockpit given his experience
the SIC (second in command), for the reasons quoted below
whoever was PF when the incident began continues flying
or no specific rules, the pilots must make a joint decision

  Just beware that either pilot may be at the controls during a
  particular incident. In fact, while protocols are different [from] carrier to
  carrier, it's not unusual during emergencies or other abnormal
  situations, for the captain to delegate hands-on flying duties to the
  copilot, so that the captain can concentrate on communications,
  troubleshooting, coordinating the checklists, etc. (source)

Case in point:
In the AF447 crash, the captain was not in the cockpit when the incident began. When returning to the cockpit, he did not take over control but sat down behind the 2 First Officers.

Please mention in your answer
- which person (if any) is designated by the regulations, airline rules and best practices
- for what reason
- and whether these rules/recommendations have often been not followed in crashes

In the end it's all up to the Captain.  If the FO is PF, which with most airlines alternates with each leg of a block, and something exciting happens, the capt may let the FO continue flying if the capt feels the FO has things under control.  Or the capt may say "I have control" and take over.  Depends.

The Pilot In Command is pilot in command.  He/she commands, but with modern CRM rules, there is expected to be some mutual consultation and decision making, so the command authority has to take all that into account.

AF447 was an unusual situation where the capt didn't realize early enough what a clusterfark the two FOs sitting in the front seats had gotten themselves into, who were both mentally overloaded into a tunnel vision state, and especially the right seater, who appeared to lack basic fully internalized flying skills, sitting in panic mode with the stick full aft for almost the entire ride down.


Why does wake turbulence seem to move upward on touchdown? Source of the first photo

I saw this photo of business jet touching down (first picture) and the smoke of tires forming wake turbulence . as I have provided other pictures (second and third) of "what we have seen and know about the theory of wake turbulence" , you can see this strange upside down form of wake turbulence . simply what is the reason ?

(YouTube)

This is not wake turbulence. This is a hard landing. The smoke from the tires is hot, it rises, hits the fuselage, curls away from it, and then curls back on itself due to the Rayleigh–Taylor instability.

(YouTube) A more persistent example. Note the still rising column of smoke.


Missing shear pin for tow-bar / front landing gear Sitting on the runway about to takeoff, and the captain comes over the intercom saying that we need to return to the gate.

From the quick announcement, it sounded like he said they found a 'shear pin' at the gate, or with the tow-bar, and we needed to have maintenance inspect the front landing gear.

The aircraft is an A321.

I can't imagine the shear pin was a part of the physical landing gear, but was probably piece of the tow-bar itself? I found the manual for the a320's tow-bar which shows the shear pin in the diagram on PDF page #11, item #6, within the bar assembly itself.

Why would our aircraft need to return to the gate for inspection, wouldn't they need to inspect the tow-bar instead? That says with the tug, does it not?

The Capt would have just repeated or paraphrased whatever Maintenance Control told him.  If a push back is done with the nose wheel scissors connected, it is critical for the tug driver to respect the steering angle limits of the nosewheel to avoid bottoming the steering actuators and stressing the parts.

Although the shear pin protects (hopefully) from major damage it still puts significant loads on the scissors and nosewheel steering system, beyond what those components would normally see.   If they find the towbar shear pin damaged or loose or sheared completely (although that should have been obvious while connected) after a push back, Maintenance will have to assume there was a steering angle exceedence event and there is likely to be a maintenance requirement to inspect the steering system, hence the return to gate.

My guess is that when the tug got back to the gate, someone noticed the pin was deformed, or some other abnormality, and they brought the airplane back for the inspection to play it safe.


How to calculate aerodrome elevation given QNH and QFE Would someone be able to explain how to get the answer to the following question?

13) The QFE for the aerodrome is currently 985 hectopascals. The QNH you have been given is 1031 hectopascals. Approximately what would you expect the aerodrome elevation to be?
A 1380 ft
B 690 ft
C 2663 ft
D 875 ft

Correct answer is A.

Thanks

QNH is the pressure setting that will show the aerodrome's true altitude at field level, QFE is the pressure setting where the altimeter will show zero feet at field level. The rule of thumb is that pressure changes 1 hpa per 30 feet, so the formula you would use is (QNH - QFE) * 30feet/hpa = field height. In this case that is (1031hpa - 985hpa) * 30feet/hpa = 46hpa * 30feet/hpa = 1380ft.

If the QFE is greater than QNH you would get a negative number, which would mean the airfield is below sea level. This is pretty rare, but they do exist, so don't mess with the QNH - QFE order.


Does a turbojet aircraft restriction on a SID or STAR also apply to turbofan aircraft? Just wondering since I've never seen turbofan aircraft mentioned on SID or STAR charts, and many airliners are turbofan nowadays.

The term "turbojet" in this context is used a bit more loosely. The pilot-controller glossary document from the FAA defines it as follows:

  TURBOJET AIRCRAFT− An aircraft having a jet engine in which the energy
  of the jet operates a turbine which in turn operates the air
  compressor.

You can see this is general enough to include turbofan aircraft as well. The main distinction is vs. a turboprop, where the turbine drives the propeller.


Why do aerobatic planes have symmetrical wings? Besides the fact that symmetrical wings fly the same inverted as upright are there any other reasons why aerobatic planes prefer symmetrical wings?  Can symmetrical wings achieve a higher critical angle of attack?  Does the large ailerons on aerobatic aircraft have anything to do with it?

One goal for most aerobatic planes is neutral pitch stability, which eliminates any pitch response related to air speed, which allows for clean straight lines during dives or climbs without requiring elevator adjustment for speed.

Cambered airfoils produce a pitch down torque which increases with speed, and this would interfere with the goal of neutral pitch stability. This would be another reason for using symmetrical airfoils on aerobatic aircraft.


How to Recover from a Spin in Cessna 172? How to recover from a spin in a Cessna 172 ? What are the different maneuvers that a pilot would have to use to to do so?

The memory aid is PARE:

POWER to idle
AILERONS neutral
RUDDER on the floor opposite the direction of spin
ELEVATOR nose-down to break the stall

You will want to read the full explanation in the Pilots Operating Handbook or POH for an understanding of the steps and why you need to take them. Commit the sequence above to memory and better yet get spin training from a qualified instructor.

The POH for the 172N model reads (caps and boldface in original):

  Should an inadvertent spin occur, the following recovery procedure should be used:

  RETARD THROTTLE TO IDLE POSITION.
  PLACE AILERONS IN NEUTRAL POSITION.
  APPLY AND HOLD FULL RUDDER OPPOSITE TO THE DIRECTION OF ROTATION.
  JUST AFTER THE RUDDER REACHES THE STOP, MOVE THE CONTROL WHEEL BRISKLY FORWARD FAR ENOUGH TO BREAK THE STALL. Full down elevator may be required at aft center of gravity loadings to assure optimum recoveries.
  HOLD THESE CONTROL INPUTS UNTIL ROTATION STOPS. Premature relaxation of the control inputs may extend the recovery.
  AS ROTATION STOPS, NEUTRALIZE RUDDER, AND MAKE A SMOOTH RECOVERY FROM THE RESULTING DIVE.

    NOTE

    If disorientation precludes a visual determination of the direction of rotation, the symbolic airplane in the turn coordinator may be referred to for this information.

Other Spin Recovery Methods

The methods below may or may not recover from all spins. Seek out a qualified instructor for spin training — or, better yet, avoid spins altogether!

Finagin

  Finagin advocates a four-step recovery technique for unintentional spins and departures from controlled flight that pilots can put to use without hesitation — and without fear of making the situation worse — even when they’re confused or disoriented by the unusual attitudes and strange sensations that can accompany spins.

  Rip the power to idle.
  Forcefully neutralize the stick and rudder pedals.
  Look at the airspeed indicator, and once it shows 100 knots or more (roughly 1.4 times the airplane’s stall speed) …
  Pull out of the dive.

Source: AOPA

Beggs-Mueller

  Power – Off.
  Remove your hand from the stick.
  Apply full opposite rudder until rotation stops.
  Neutralize rudder and recover to level flight.

Source: Spins in the Pitts Special by Gene Beggs

Hands In Your Lap

Some advocate the “hands in your lap” method of pulling power, taking your hands (and feet) off the flight controls, and waiting for the airplane to fly itself out of the spin. This may work for certain airframes but not all. Catherine Cavagnaro of Ace Aerobatic School says she likes to perform spin training in a Cessna 152 because that airplane will not self-recover from a spin.


What happens if a language other than English is used over the radio? In many non-English speaking countries, communication with ATC may be conducted either in English or the native language. Famous examples are China, Russia, France and possibly Germany.

When a pilot who is a non-speaker of the native language hears such communication, which is he to do? Just sit back with no idea of what is going on?

During the 1990s I flew into countries on every continent except Antarctica. As an American English speaker flying U.S. registered airplanes, I never had a complaint that a controller had trouble understanding me, and I was always addressed in English by the controllers. Sometimes, though, I would have trouble understanding the controller's accent and would have to ask them to repeat their instructions.

As you mentioned, in some countries the controllers will address the pilots of that country in their native language. Personally, I always felt that compromised the smooth functioning of the system to a degree, because it denied a good bit of the overall picture of what was happening to the pilots who didn't speak that language. For example, if two airplanes preceding you are being rerouted for some reason, but you can't understand what they've been given (and which you're probably going to be given), you're not as prepared for the reroute when it comes through to you in English.

Even if controllers spoke to you in English, they would typically pronounce place names and fixes as they are pronounced in the native language. My first flights in France were a real problem in this respect. The French controllers were speaking good and understandable English, but the French pronunciation of the place names did not match our English rendition of the names we were seeing (and had never seen or heard before) on the IFR maps (no glass cockpits back then for us). In a couple of instances, we had to request that they spell the names, which didn't make us popular. On one occasion, when we made such a request, another pilot came on frequency and said, "He means ...." and gave us an English transliteration if you will of what the controller was saying.

Interestingly, the worst time I ever had understanding a clearance was one from a nominally English speaking controller. We were talking to Scottish control, and received a reroute into Amsterdam from a controller with a very pronounced accent (Glaswegian?). All I was able to get was that at the end he was turning us over to Dutch control and the frequency. I went over to the Dutch, and asked for them to restate what our new clearance was, and I apologized, explaining that I had been unable to understand Scottish control. His first words were, "That's okay, we can't understand them either."


Is is possible to use non-hydraulic fluids in a hydraulic system in an emergency? In this answer, Ralph wrote:

Spare fluid is common enough to have on-board to replenish the resevoirs.

After reading that, I recalled an incident I was told about nearly twenty years ago about a hydraulic failure on an Aeroflot flight, where Lemonade (a Russian soft drink of sorts) was used in a pinch. I've always wondered about the authenticity of this story.

Is it true? Is it plausible? Have there been other times where some non-ideal fluid was used in place of hydraulic fluid?

Is it possible? Sure. Theoretically. Once.

I can't speak to your Aeroflot scenario without more details, but broadly speaking any liquid will work as "hydraulic fluid" within certain limits: At the most basic level all that is required is that it be incompressible and flow at an acceptable rate (i.e. It is a liquid with a viscosity that falls within a certain range).  Engine oil is frequently used as a hydraulic fluid for certain applications (constant-speed propellers on small aircraft), and special-purpose hydraulic oil for others (the brakes on small aircraft).

There are some challenges to using any old liquid as "hydraulic fluid" though, which are why I say this is only theoretically possible (and probably only once). The two big concerns are the fluid's operating temperature range - its freezing and boiling points - and its corrosion potential.

Let's consider MIL-PRF-5606H (a typical hydraulic oil) and ordinary lemonade as examples:

Operating Temperature Range

In terms of temperature 5606 hydraulic oil has a much wider operating range than lemonade: It's "pour point" is below -60°C and its boiling point over 175°C, while lemonade's operating temperature range is probably close to water: It will freeze somewhere close to 0°C and boil somewhere close to 100°C (the exact numbers will depend on the chemical composition of the lemonade).
If we use both of these fluids in a brake system there is a chance the lemonade will boil when the brakes are applied (creating a gas pocket that's compressible, and reducing brake effectiveness), or that it will freeze in flight and the brakes will fail to apply because the ice is blocking the lines.

Corrosion Potential

As far as corrosion potential 5606 hydraulic oil is a mineral oil with some red dye in it: Since it doesn't contain dissolved water or electrolytes it's generally non-corrosive. Lemonade on the other hand is nothing but water and dissolved electrolytes (mainly citric acid to make it taste like lemon) plus sugar and a few other things. If we ran hydraulic actuators on this stuff we should expect them to corrode (rust) and eventually fail.

So it's possible that under the right circumstances an airplane could complete a flight with lemonade, water, etc. as "hydraulic fluid", but having done so the maintenance department will have to purge the "creative" fluid from the system, and probably follow some steps to remove any corrosive residue from the system (possibly up to and including replacing major system components). Long term operation in such a condition would probably damage components to the point where many of the system's moving parts (actuators, valves, etc.) would need to be replaced.


How are wind tunnels used with scale models? This answer introduced me to the problem of using scaled models in a wind tunnel.

How are wind-tunnels used in practice (for modelling the performance of subsonic airplanes)? In particular:

Do they used increased air pressure, or another mechanism, to compensate the Reynolds number?
What scale model is used? A scale of e.g. 70:1 appears to be difficult, because you can't pressurize air that much before it liquifies. So do they use a scale like 10:1? More or less?
What are the forces (lift and drag) generated on the model? The weight (therefore the lift) of an A380 is about 500 tons. Apparently the lift on a 10:1 scale model would be 50 tons. Is that real? Are they making 10:1 models which cope with 50-ton forces?
This picture (from Wikipedia's Wind Tunnel article) shows what seems to me to be a 10:1 scale model of a supersonic plane in a subsonic tunnel. The tunnel doesn't seem to me to be built to handle multi-ton forces. What kind of air-speed will they be using, what would they be measuring with that kind of test?

During development, a lot of different wind tunnels are used. Scales range from full scale to less than 1/100, and sometimes not only the geometric, but also the dynamic and elastic properties of the aircraft to be tested must be modelled correctly.

Today, preliminary work is mostly done in numerical simulations, but before the availability of computers, early wind tunnel models would be between 1/50 to 1/16 scale (depending on the size of the real article and the available wind tunnel), often using modular models on which different engine placements or tail geometries could be tried out. Since wind tunnel testing costs real money, early tests are restricted to small, low-cost tunnels which run at ambient temperature and density. Differences in the Reynolds number between test article and the real aircraft are covered with correction factors and experience.

Only when a design is moving ahead during development, bigger and more expensive tunnels are used. Again, matching the Reynolds number is not possible and not even necessary for some tests: For a spin tunnel the test article must be scaled dynamically (so it has correct moments of inertia), but separated flow is less affected by the Reynolds number, so the precision of a 1/20 scale model is sufficient. Also, a free-falling model in a spin tunnel needs quite a bit of space to move around, so even small models already need test sections with a diameter of several meters.

To reduce cost and avoid the tight schedules of wind tunnels, free-flying or radio controlled models are also used. Watch how Dornier engineers tested the dynamic behavior of the then-new tricycle gear of the Do-335 in this video (starting at 2:00') or ditching characteristics (starting at 2:13').

Only when both Mach and Reynolds number need to be matched, cryogenic and pressurised wind tunnels are used. Due to their energy demand and availability, such tests need to be scheduled years in advance and meticulously planned. Models can run into millions of dollars or Euros, so such tests will mostly narrow down parameters which had been measured previously in simpler tests. Large tunnels like the 9.5x9.5m LLF in Marknesse (the Netherlands) are mostly used for low-speed and high-lift characteristics testing since its 12.6 MW engine can only support a speed of 62 m/s at full power. On the other end of the scale are hypersonic blow-down tubes which support a testing section of 0.5m diameter in which Mach 6 can be achieved for a fraction of a second.

Full-scale testing in a wind tunnel is only possible with small aircraft - see the answers to this question for examples. Even then, the speeds do not match the top speed of the aircraft, so the loads are a fraction of the real loads.

Jan Roskam observes in his book "Roskam's airplane war stories":

  It certainly is an unusual luxury for an engineer to have full
  Reynolds Number tunnel data available.


What's the point of having ILS on runway signs? Runway signs show the ILS category next to the runway designation. But if you flew and didn't drive to the airport, you already know (hopefully). Either way, you are taking off, so you don't really much care. One might say that you will have to land again in case of emergency, but ATC might not give you the same runway. So what's the reason of showing ILS category on runway signs?

Note: I changed "markings" to "signs" in this question since markings are painted on the ground, signs are the red ones I am asking for.

EDIT: The linked question is not exactly the same:

What is the purpose of several mandatory stops (RWY AHEAD) at busy airports?

What are mandatory stops? How are they several? (I never noticed that). I didn't know all these until I asked about the "runway signs". The other question presumes that you are familiar with some terminology. And it doesn't mention ILS.

Also, it looks like a similar case. I vote for reopening.

These markings designate the hold-short point during different runway operations.

Runway hold-short points are marked by white text on a red background. They designate a point which any aircraft must not cross without an explicit clearance from ATC. Note that in the below picture, there are two markings:

Cat II or III ILS is used during low visibility operations. In the case of an auto-land, the autopilot literally lands the plane on the runway. To do this, the aircraft systems need a very precise signal from the ILS antenna. Any aircraft on the ground crossing the line of sight between the ILS antenna and the landing aircraft will interfere the signal, sending the landing plane off course.

When a plane is landing on Cat II or III, other planes on the ground need to stay further back the runway. This has an undesirable side effect - the airport capacity is reduced. If somebody is doing a visual approach on a clear day, you can move a bit closer.

This is what happened to a Singapore Airlines Boeing 777 when it used auto-land without informing ATC. Since no one is aware they're using auto-land, the standard hold-short procedure (rather than the low visibility procedure) applies. They drifted off the runway pretty much like a racecar!


How can you identify an Airbus or Boeing from the inside? How can you recognize a plane from the inside, when you are already sitting as passenger? In particular, is there a way to understand if you are inside a Boeing or an Airbus?

Look at the safety card in the seat pocket in front of you:

As a side note, it will be beneficial to read the safety card as well.


Can we apply landing minimum with ALS when ALS is available with low intensity only? Can we apply landing minimums with ALS when the ALS is available with "low intensity" only?

For example, the following NOTAM is issued at KABQ:

  !ABQ 05/270 ABQ RWY 03 ALS OUT OF SERVICE EXC LOW INTST ON CONS
  1705302145-1706151800EST

In this case, is the applied landing minimum for ILS RWY03 VIS 1/2 or VIS 3/4?

I'm talking about part 121 operator.

The ILS to RWY 3 at ABQ (ILS RWY 3 ABQ plate) shows a required RVR of 1800  (not vis in sm) with everything working.  Using the inop component table (no special provisions for this in Operations Specs) shows a required increase to 4000 RVR with the MALSR (ALS) inop. (look at table)

The fact is that the "LOW INTST ON CONS" (low intensity on continuously) does not provide any benefit since the MALSR (ALS) is shown (by NOTAM) as being out of service.

This is the way I understand the issue.


What is the standard date format used in aviation? As English is the standard language used in aviation and Zulu/UTC time is used as standard too or at least between pilots to calculate flight time and fuel. Is there any standard date format used in the industry like ISO 8601 (YYYY-MM-DD)?
https://en.wikipedia.org/wiki/ISO_8601

I'm not sure if there is really an industry wide standard for the date format, but at least the ICAO (International Civil Aviation Organization) uses the YYMMDD date format for flight plans:

DOF/ The date of flight departure in a six figure format (YYMMDD, where YY equals the year, MM equals the month and DD equals the day).

(FAA on ICAO flight plans)
Other flight plans may use different formats however. See e.g. the following example of a LIDO flight plan (Lufthansa Systems format):

(image source: SimBrief.com)


Why does the A340 have 4 engines instead of 2? I have noticed for a while the A340 has four engines and the Boeing 777 only has two engines and they are about the same size. So why can't an A340 have two engines instead of four?

A340

. B777

On a jumbo jet like the B747-8 four engines are a requirement.

The A340 was designed at a time when ETOPS (Extended-range Twin-engine Operational Performance Standards) had not been developed. Some airlines preferred two engines which reduced operational costs, while others preferred four engines with increased reliability at an additional cost.

Airbus decided to split the development into distinct aircraft having the same wing and airframe - A330 with two engines and A340 with four engines.

However, as time has passed, ETOPS has become the norm with improved engine reliability, and A340 production has been stopped. Almost all the civil airliners under development now have two engines.

The Boeing 777, on the other hand, was developed with twin engine operation in mind.


Has wall to wall airfoil induced drag? Walls in wind tunnel stop air leakage at outboard parts of airfoil,pressure distribution at center section is same as at outboard sections(if we neglect interferenece with walls).
-Does that mean that induced drag is zero?
BUT
Brilinat words from memeber @JanHudec:
"The infinite span has nothing at all to do with having no ends.
The ratio of lift to induced drag increases with span. If you fix the lift, and vary span, the induced drag will tend to zero as the span tends to infinity. That means in limit a wing of infinite span generating finite lift (and therefore flying at zero coefficient of lift!) will have zero induced drag."
That mean wall to wall airfoil is not theoretical 2d airfoil,so it has induced drag but smaller than 3d wing BUT REMBER FOR SAME LIFT.

A wall-to-wall wing in a windtunnel should best be compared to one in ground effect and not as a segment of an infinite wing. Both are similar since the infinite wing will be infinitely close to the ground, relative to its span.
The opinion that an infinite wing has no induced drag is misleading. When we plot the amount of induced drag over aspect ratio, induced drag will tend to zero for an infinite aspect ratio. But what does that mean in practical terms? That infinite wing also produces infinite lift by pushing an infinite amount of air downwards by a finite amount. Induced drag is still there but becomes insignificant relative to lift.
Better to look at what happens in reality. It is worth to show the picture of the linked answer again here:

Induced drag is the consequence of lift. Lift is the consequence of air flowing around the obstacle that the airfoil poses to the air. While less than in free flight, some downward acceleration of air still happens but is soon stopped by the ground. Therefore, while less than in free flight, some induced drag is still there and will only disappear when the airfoil merges with the bottom of the tunnel.
Now it is crucial to know what induced drag really is. Air is accelerated downwards while flowing over the wing and the reaction force to this acceleration is lift. Since that does not happen instantly but gradually, some of the lift is created in an already downward-bent flow, so the reaction force to this, being perpendicular to the local flow, is pointing slightly backwards. This backward component is induced drag.
The ideal of the infinite span wing produces lift instantly because its chord is vanishingly small relative to span. The same cannot be said of the real wing in ground effect.
There is no free lunch. As long as lift is produced, induced drag will be present.


What is a weight-shift-control category aircraft? Aircraft categories include:

Airplane
Rotorcraft
Glider
Lighter than air
Powered lift
Powered parachute
Weight-shift-control

I'm familiar with all of these except for weight-shift-control.  What are they??

Basically, it's a powered hang glider.

  Weight-shift control (WSC) aircraft means a powered aircraft with a framed pivoting wing and a fuselage controllable only in pitch and roll by the pilot’s ability to change the aircraft’s center of gravity (CG) with respect to the wing. Flight control of the aircraft depends on the wing’s ability to deform flexibly rather than on the use of control surfaces.

— FAA H-8083-5 Weight Shift Control Aircraft Flying Handbook

They're commonly called "trikes" and are literally big delta-wing hang gliders with a lot more structure and an engine. Check out a cockpit video of one in flight!


Is the purpose of variable pitch propellers to maintain an optimal angle of attack? I recently had a discussion with an aviation expert, who works as a Flight Instructor, about constant speed propellers - variable pitch propellers - on light piston aircraft.

I have always known that these kind of propellers are capable of changing their pitch angle in flight in order to keep propeller blades at their optimum angle of attack across a wide speed range, while maintaining constant RPM. Therefore, they are very efficient over a wider airspeed range compared to fixed pitch propellers. A change in TAS in flight, for example, would lead to a change in pitch, in order to restore the optimum angle of attack of the propeller blades.

To my surprise, the instructor told me that what I knew about variable pitch propellers was not true: he explained to me that a change in TAS during the flight would lead to a change in the angle of attack of the blades, but that doesn’t mean that the resulting pitch change would lead to the optimum angle of attack.

His answer really surprised me, so I am now wondering what is the purpose of variable pitch propellers then. I understand that there may be a point where the PCU and CSU won’t be able to adjust the pitch to the most optimum angle of attack, but I thought that during normal operating speeds the purpose of this kind of propellers was to provide the blades with the most efficient angle of attack. Am I wrong?

What you know about variable pitch propellers is generally correct. If you make a slight change, your flight instructor would not have had any reason to object: They keep propeller blades near their optimum angle of attack across a wide speed range.

As the distance from the hub increases, the blade section gains circumferential speed while being in the same forward speed as all other sections. Therefore, the twist of the propeller blade should change linearly from the hub to the tip. If you twist the full blade at its root, you add a constant change to each cross section, so at high speed the root of the blade sees too high an angle of attack and the tip has too little angle of attack. Normally, the section at approximately 75% of propeller span is kept at its ideal angle for the best overall compromise. That is good enough for practical use.

Ideally, the propeller would spin faster with increasing flight speed, but that would require a gearbox and switching gears with the limited speed ranges of combustion engines, would deliver little thrust at the beginning of the take-off run and at high speed you would run into compressibility problems at the tips much sooner - practical high speed propellers spin more slowly than typical low-speed propellers.

If you look at the efficiency chart of a three-bladed variable pitch propeller below, you will see that efficiency peaks for one speed (given as an advance ratio in the plot below) and one pitch angle. This is when the full blade is flown at the optimum angle of attack for each section. This optimum would shift to higher speeds for less twist and lower speeds for more twist. Operating the propeller away from its design point incurs small losses since not all of the blade is exactly at its optimum angle of attack, but if you compare the overall efficiency with that of a single pitch setting, the advantage of variable pitch becomes obvious.

Efficiency chart of a variable pitch propeller. Source: McCormick B.W. Aerodynamics, Aeronautics &Flight Mechanics. John Wiley & Sons, Inc., 1979.


How long should I wait after I've gotten off my OCD medication to consult an aviation medical examiner? Im getting off my OCD medicine in a few weeks and wanted to know how long I should wait til i should consult a aviation medical examiner, any advice on which aviation medical examiner I should consult?

Timing probably matters very little as far as talking to the AME. He'll need to consider not just the medication you aren't on any longer (and he can tell you what timeframe has been "long enough" there), but also the nature of your condition without the medication. The time needed to evaluate how you're doing without the medication is probably longer than the timeframe required to "have the medicine out of your system before you go fly."

In cases like cold medicine, the latter is some multiple of the dose interval, and it's pretty clear that the initial condition (the cold) is gone & won't cause you any further problems. I am not an AME, but my expectation would be that OCD is enough different that the timeframe will be much longer.

It will take an AME to tell you what the path from where you are to where you want to be (having your medical issued) looks like and how long it will take.  There are too many variables in these sorts of things for much of a "one size fits all" answer.

As far as which AME, I'd suggest calling a few in your area & asking if they have experience with patients with OCD, and if not, whom they'd recommend.  It may take a bit of research, but it should be possible to find one who had done this before.  Ideally, "local" is better than far distant, although if may be possible for the right AME, even if he's some distance away, to work with your local doctor to gather what's needed so he can send it to the FAA & get approval, so you might only need 1 in-person visit with him.  I'd definitely want somebody familiar with more than the routine "check eyes & BP and sign the paperwork" procedures!


How does a gyro compass indicate true heading? From what I've researched, a gyro compass is usually calibrated with with reference to a magnetic compass. Once you've engaged the gimbal locks, and start to rotate the compass to a magnetic heading; will it automatically compensate for true heading or will you not even reference a magnetic compass when calibrating?

Or are there cards used to indicate what's your position from true north, based on your magnetic indication?

The directional gyro (DG) does not know about heading. Its gyroscope will hold its position (e.g. the heading you set from the magnetic compass) in space. The plane is turning "around it", but DG is holding its position, so you can read the heading from the scale fixed to the airplane while the gyroscope holds its position in space. Only friction and other forces (acceleration forced upon the aircraft) force the gyroscope to wander from its position.
E.g. the gyro shows a magnetic heading if set using a magnetic compass and it shows a true heading if set using a GPS (or some map feature like a road with a heading visible on a map).


How to calculate the location of C.G. A.C. & N.P. in static longitudinal stability? We are in the conceptual stage of design and, using Raymer & Nicolai as references, I am doing a rough calculation of "power off" neutral point (N.P.) to find the optimum c.g. and a.c. positions.

As seen in this photo, which is also reflected in Nicolai, when doing a static longitudinal stability analysis, the moments are summed assuming the c.g. is aft of the wing/body a.c. and the tail provides a negative moment (nose down) while the wing provides a positive moment. It assumes both tail and wing provide positive lift. I understand that in this configuration, the positive tail lift would shift the overall a.c. (neutral point) aft and the c.g. would just need to be in front of the neutral point for static stability.

However, what is confusing for me is that conventional stability usually involves placing the c.g. in front of the a.c. and n.p. so that the wing provides a negative moment (pitch down) and the tail stabilizes this by providing a negative lift (down force) to pitch the nose up. Why assume the c.g. is aft of the a.c. and the tail is providing positive lift? Why not sum the moments assuming the c.g. is in front of the a.c. for calculating neutral point?

Any help would be much appreciated, thanks!

Why assume the c.g. is aft of the a.c. and the tail is providing positive lift?

Maybe the two authors were sick and tired of that old meme that positive longitudinal stability requires negative lift on the tail. This is simply not true and never was, still it is believed like some religion.

What is true is that more static stability needs less tail lift, and many aircraft have so much stability built in that their tail lift is always negative. But with modern flight control systems this negative lift means more drag than necessary. Back in the old days, when the pilot wanted to let go of the stick or yoke in order to read a map or take a leak, a tail downforce was the best way to make sure the aircraft would behave. Still, low and sometimes missing roll stability meant that the pilot could never take his eyes off the horizon (real or artificial) for more than a short while.

  Why not sum the moments assuming the c.g. is in front of the a.c. for calculating neutral point?

The position of the center of gravity relative to the wing's aerodynamic center has no significance for stability — all what counts is the center of gravity position relative to the overall aerodynamic center, fuselage and tail effects included. A smaller tail means that for the same stability the tail lift has to be lower, and most aircraft have the smallest tail surfaces that their designers could fit.