What happens if one or more rotors are stopped during autorotation on a quadrotor? I get the basics of autorotation when it comes to a helicopter in unpowered flight, but it's a bit unclear to me when it comes to quadrotors.

My objective is to cover some directed horizontal distance while in freefall and resume control before crashing. I want to do this to save battery, rather than starting controlled flight from the start. If I get horizontally closer to a target location without using much any battery, this is optimal.

Let's say the quad has stable attitude and is in free fall. Then I brake one motor, causing the prop to stop. A naive freebody diagram says the quad will pitch since there's no longer any lift on that side.

My question is what happens after that. Does it reach some equilibrium position? Does the decreased upward lift yield horizontal acceleration relative to the ground, or will it just spin in a circle? Do the rotation directions of the remaining props (2CW/1CCW or 2CCW/1CW) affect what happens next? What if I cut off 2 motors, either next to each other or across from each other?

Thank you.

After watching the video the problem he was having was in keeping it horizontal. It appears that the software isn't able to control the unpowered motors.

I know absolutely nothing about quads but I'm assuming with a fixed rotor pitch they must use variable voltage to the motor to control everything such as translational movement. I'm also assuming they have an accelerometer to determine pitch. The first question is how accurate the accelerometer is at low g's.

Second, when in free fall if a rheostat that is controlled by the computer could be connected across the sides of the motor then your problem could be solved. The freewheeling motors will create a voltage just like a generator so a rheostat would be able to control the current and thus the mechanical resistance to the motor. The rheostat will have an inverse relationship with the mechanical resistance (more electrical resistance less mechanical resistance). How linear that relationship is you'll have to experiment with. If you have the software switch to controlling that resistance it should be able to direct the freefall in the same way it does everything under power, only it will be more limited In its authority. If you do it right you might even be able to use the power generated to recover a little bit of the energy from the fall to charge the batteries like a hybrid car does.

Like I said, I know absolutely zero about quadecopters so I can't fill in any details, but it seems feasible.


What are runway Aiming Point markers used for? I'm interested in knowing what aiming points on runways are used for:

Why do some runways have aim points and some do not?
We say that pilots must use the maximum length of the runway. Therefore, why don't we touch down on or near the threshold lines as our aiming point?

I'll try to answer just the first portion. The aiming point marking basically serves as a visual aiming point for a landing aircraft.

Image from code7700.com
They are not found in all runways because the requirement is based on the aerodrome code number and they are not required in all cases. Aerodrome Reference Code from ICAO Annex 14 Volume I:

Table from ICAO Annex 14 Vol I
From the Same reference:

5.2.5.1 An aiming point marking shall be provided at each approach end of a paved instrument runway where the code number is 2, 3 or 4.
5.2.5.2 Recommendation.— An aiming point marking should be provided at each approach end of:
a) a paved non-instrument runway where the code number is 3 or 4;
b) a paved instrument runway where the code number is 1;
when additional conspicuity of the aiming point is desirable.


Would the front rudder of the Wright brothers really have helped in recovering the glider from a stall? I have found a paragraph in the 1904 British patent of the Wright brothers (see the citation below) and it seems, from what they explain, that the two inventors believed the front rudder could have recovered the plane from a stall in any situation.

The question is, why were they so sure that after a stall the plane would have started to fall nose down, would have accelerated and the front rudder would have been able again to control the pitch of the flying machine?

If the main wings (and also the front rudder) had stalled their glider/airplane could have started to fall sideways or tail first.

  "Contrary to the usual custom, we place the horizontal rudder in front
  of the main surfaces or “wings” at a negative angle, and use no
  horizontal tail at all. By this arrangement we obtain a forward
  surface which is almost free from pressure under ordinary conditions
  of flight, but which, even if not moved at all, becomes an efficient
  lifting surface whenever the speed of the machine is accidentally
  reduced very much below the normal, and thus largely counteracts that
  backward travel of the centre of pressure on the main surfaces or
  wings which has frequently been productive of serious injuries by
  causing the machine to turn downward and strike the ground head on."
   Wright brothers, “British Patent No. 6732, A.D. 1904 – Improvements in Aeronautical Machines”, Date of Application: 19th Mar., 1904, Accepted: 12th May, 1904.

  Wright glider

No, it would not have helped by itself.

What could prevent a stall or recover the aircraft from an incipient stall would be a counteracting movement of that canard surface, but that would need to be commanded by the pilot. It makes little difference if that control surface is ahead or behind the wing as long as it has enough lift potential left to produce the desired pitch down moment.

The way we understand stability today is that the aircraft will correct a deviation from the desired pitch attitude without pilot intervention. Using this definition, the Wright Flyers were unstable. Only by active and permanent corrections could straight flight be maintained. This is rather stressful and makes longer flights very tiring. Flying faster also can be ruled out because the speed of deviations goes up with flight speed, such that the human reaction time delays corrections too much and can even make them counterproductive.

The Wrights were correct that an unloaded surface gives them the highest control authority and, when moved in a correct and timely manner, helps to prevent a stall. However, placing that surface ahead of the wing comes with several disadvantages, loss of stability being only one of several. On the other hand, placing that lightly loaded surface at the rear results in a stable aircraft with better performance.


What airport fees are charged to airlines in the case of operations following an unscheduled landing? A plane may need make an unscheduled landing in the case of a diversion due to mechanical difficulties, unexpected weather, a medical emergency or other reasons (this could even be at an airport with which the operator has no prior agreements).

Airport operators agree various fees with the airlines that use them for their scheduled operations:

runway/landing fees
fees for stairs, buses to the terminal building, or airbridges
passenger charges, should the passengers need to disembark
parking fees
...amongst others

Which of these charges are typically levied in the case of an unscheduled landing caused by unforeseen circumstances?

A while back I was on my way to Frankfurt on Delta for a business meeting when a frail passenger (who quite honestly should have never been allowed to board in the first place) passed away before getting to the midpoint of the trip. The captain turned the aircraft around and landed at Gander International in Canada, where we spent the night before continuing on our way.

Because I've seen this in the news once in a while, I decided to check the airport's web site to see what is their fee structure. Lo and behold, they publish a comprehensive list of fees that covers everything related to an occurrence of this type.

The fee structure even covers "air rage" incidents under the category of "Emergency" and is charged at so-many-dollars per 1000 kg!

  Emergency
  Including medical, mechanical, air rage, etc.
  $12.57 per 1000kg


What are common airline crew titles and positions? I realize that airlines can operate with different numbers of Pilots and Flight Attendants, depending on the route, aircraft and length of flight (EET). Which are the types of crew members that an airline can have, with the correct titles? I've heard about captains, master captains and cruise captains, but how can I identify them during the flights?

As the EET is variable from flight to flight you should better say that it is dependent on specific kinds of operation or routes, which is quite the same but includes also the time before and after the flight. Flight duty time starts at check in and ends at check out.

There are general requirements based on the maximum allowed duty time of a crew member, which are again based on the regulations of the aviation authority of the operators country. Finally it differs from airline to airline and is regulated by the operations manual, which can of course be not less restrictive than the authority, but there are some general titles you often hear:

Captain The Captain is the PIC (pilot in command) of this flight and responsible for a safe operation. He must be on station for departure and arrival.
First Officer The F/O is also found on every airliner operation. On short or medium ranges he is the 'Copilot', some airlines fly with two F/Os on long haul flights and in some portions of this flight most probably the two F/Os are in the cockpit and the captain takes a nap.
Second Officer When starting at an airline, you sometimes begin as a S/O and are only alloweed to do specific tasks. So for example handling the radios during departure and arrival, or having the controls during the cruise phase.
Cruise Captain Well, read the name ;) He might be a Captain and only in the function of a Cruise Captain for a specific flight, but once again, it depends on the operator.


Why do we lean the mixture in a turbocharged airplane (C182T)? An increase in Altitude will result in a decrease of atmospheric pressure -1 in. Hg. / 1000ft (ISA conditions), but with a turbocharged airplane it will keep the pressure constant (+/- MSL pressure) up to the critical altitude? Why do we have to lean the mixture although pressure is kept constant or has it do with the fact that density decreases and thus reduce the amount of molecules for a given fuel molecule? What is the relationship between pressure and density? Thanks :-)

Turbo or not, you lean mostly because the mixture required for takeoff is far richer than necessary for lower power settings, even at sea level.  Aircraft engines are partly air cooled and partly liquid cooled - the full-time liquid cooling is oil, and on takeoff, there is part time liquid cooling in the form of excess gasoline from the full rich mixture.
The reduction in air pressure with altitude, or density with heat, means this effect becomes more extreme the higher you go, but you should be leaning, even at sea level, once power is reduced from takeoff (you should be leaning on the ground as well as soon as the engine is running smoothly after start, until just before takeoff, if you want to best mitigate the effects of the lead in the avgas).
The turbocharger just fools the engine into thinking its altitude is lower than it really is. A cruise power setting of 22" of manifold pressure is 22" inches of manifold pressure to the engine, whether it's 22" of natural atmospheric pressure achieved at wide open throttle at 8000 ft on a non-turbo engine, or 22" of artificially produced manifold pressure at 15000 ft in the turbocharged one.
The density of the compressed air intake charge is reduced by compression heating of the charge by the turbo, which reduces its efficiency (you'll need a slightly wider throttle opening to get the same MP with a hot charge) and complicates cooling (the hot air reduces detonation safety margins). Intake intercoolers are used to reduce the temperature of the intake charge, without reducing the pressure, to mitigate this problem.
In all cases, leaning is done in reference to exhaust gas temperature, usually some value on the lean side of peak EGT to get best fuel economy and relatively cool cylinders, with an eye on cylinder head temperatures.
With a turbo, the main effect of altitude that has to be kept in mind is cylinder cooling efficiency.  The turbo gives the engine an artificial ability to make low altitude power at high altitude. The problem is although the intake charge is "thick", being compressed air, the air coming in to cool the cylinders is "thin" and the heat rejection rate via the cooling fins declines, so leaning and cowl flap settings need to take this into account in managing cylinder head temperatures.
Exactly how that is to be done depends on the engine/airframe, per the POH and you should be leaning at all times based on POH procedures.
Cooling efficiency is critical to cylinder life with turbocharged engines, because they tend to operate a lot closer to temperature margins because of the demands of cooling in thin air. Good engine baffling is key.  Badly installed baffles or degraded baffle seals, that wouldn't be a problem in a non-turbo installation, can cause high cylinder head temperatures with a turbo at higher altitude.  Even if the cylinder head temperatures are good, bad baffles can lead to hot spots not picked up by the CHT probes and can ruin a cylinder with no obvious outward signs until it's too late.


Is it possible to determine the mass flow rate through the engine of MiG-21 during its various cruise Mach no. operations? Is it possible to obtain the mass flow through the Tumansky R-25 turbojet engine mounted on a MiG-21 during its various standard operating (cruise) Mach numbers?
Note: standard as in older planes where there were just 3 positions of the inlet cone - upto Mach 1.5, between 1.5-1.9 and above 1.9.

Also, what should be the cruise condition for this plane, given that it is capable of flying at various Mach numbers and thus speeds?

The mass flow is approximately $20 \ldots 100~kg/s$.
You can try to calculate it yourself using thermodynamic equations (see for example NASA Resources, MIT Resources or dedicated books).

Although the (idealized) calculations are not very difficult, please note that you will have to make a lot of assumptions concerning the efficiency of the turbojet components (particularly intake, compressor, combustion chamber, turbine and nozzle), since this data is not published by engine/airplane manufacturers. Some variables that are usually available are the thrust, the total compressor pressure ratio and maybe the (approximate) geometry of the intake and the nozzle.
Also, you are asking for supersonic calculations, which are more complex than the subsonic ones -- including the assumptions.


Is it possible for squawk to change automatically in emergency situations? It might be a silly question, but is it possible for an airliner to automatically switch the squawk code to emergency (7700) during a flight? (For example in case of losing all engines)

Thank You In Advance :)

Whether or not an airlines is in an emergency situation is not a black or white question. There are millions of different internal and external factors that come into play, any combination of which may or may not be considered an emergency. No computer can be programmed to accurately and consistently calculate whether or not a flight is having an emergency situation - in fact, that's one of the main reasons we still have real, live pilots seated up front.

Could you program a computer to detect a few obvious emergencies (total loss of power, rapid decompression, fire etc.)? Sure. But it wouldn't be able to detect everything.

What's more, setting squawk 7700 is not always the best cause of action in an emergency. In fact, most of the time a flight will already be in contact with ATC and have an assigned discreet transponder code - in such a situation, setting squawk 7700 will often have little or no effect, since you can talk directly to ATC on the radio and explain what's going on. You might even risk ATC losing identification if you start changing your squawk code out of the blue, which could add additional risk. In other cases, squawking 7700 can be beneficial, for example if you are not talking to ATC - or maybe the frequency is very busy and you can't get through - or maybe you don't have time to call ATC, but still need their attention (rapid decompression). ATC might also request that you squawk 7700, to reduce workload on their end.

So to summarize, no, a computer would not be able to necessarily detect when a flight is having an emergency situation. And even if it could, setting squawk 7700 might not even be useful.


Why do aircraft with turboprop engine have black painted anti-icing system? Everyday as I watch aircraft with turboprop engines, a question pops up: Why do aircraft with turboprop engines have black painted anti-icing system?
For instance:

These are rubber boot deicing systems. When ice forms on the leading edge of a wing, pressurized air is used to inflate the boot so the ice will pop off. Normally, this inflation is not permanent but the air is pulsed. Activation is done by the pilot, so this system is normally switched off. Operation needs some care because if used too late (with too much ice) the boot may become impossible to be inflated.
Historically, it was believed that activation with too little ice would also render the boot ineffective, with ice building up around the inflated boot (ice bridging). This seems to have been a myth.

Deicer boots in operation (source: Wikipedia)
Other deicing systems use heat or constantly seeping deicing fluid. The first method is normally used on jets with bleed air or electricity as the heat source. The second system was for example used on the Beech Starship which had a titanium leading edge with millions of tiny, laser-cut holes for the application of deicing fluid.
Alternatively, surfaces are designed large enough to still be effective under icing conditions.
Fast jets do not need a deicing system - their stagnation point temperature is sufficient to remove ice. However, ice can still be a problem: When the supersonic B-1A was converted into the more stealthy, but subsonic B-1B, ice buildup inside the intake (which now had obstructions added so the engine compressor would be hidden from radar) would result in large chunks of ice building up, then breaking loose and damaging the compressor blades.


What are these green "bags" on the SSJ100? I've seen some green "bags" on the SSJ100 when they were building it. What is the purpose of these?

That's cabin insulation - probably fiberglass.

It's not always in green bags (blankets), and not always fiberglass, but some kind of insulation is necessary to help control temperature and reduce cabin noise (air moving over the fuselage is REALLY loud).

Here are some other photos of different types of insulation:

Basic fiberglass with a plastic backing, cut to fit.
Fiberglass with either plastic or foil backings is commonly used because it is both light and inherently flame-retardant (as long as the backing is fireproof it will not burn).

Soundex - a sandiwch of acoustic foam, thermal insulating foam and aluminum foil - is a popular choice on propeller aircraft: Slightly heaver than fiberglass, but better at dampening sound.

Foam padding (similar to the foam used under carpets) does a decent job at both sound and thermal insulation. In this case it's green & has no covering, but colors and coverings vary.

Prefabricated insulation blankets on a Boeing 777.
(I believe these are fiberglass sealed in polyethylene, they're manufactured to fit each bay without cutting which avoids loose insulation material floating around).


Chocks - why the wide variances in price? In a comment to a question about setting the parking brake, @rbp said to just get a set of wheel chocks and use those instead. I was piqued - it never occurred to me you'd need your own, I thought they'd be littered around the tarmac for common use - and did a quick Google Shopping search for "aircraft chocks". I was surprised to see a wide price variance, even within a subset of relatively small-size chocks.

For instance, here is a pair of 10" rubber chocks with 3ft rope, about 15 lbs, for \$15. Here is what looks like exactly the same thing, for \$98. Tigerchocks make considerably lighter ones, but they are \$120-ish per chock (over \$200 for any roped pair) while other lightweight designs are as cheap as \$30 for about the same size. Other designs seem to vary between $20-60 for a roped pair, and have noticeable differences in size and design, but c'mon, they're a wedge you put under the tire to stop the plane rolling.

How much of this variance is "you get what you pay for" (more money equals a better product in some way; lighter, grippier, more durable etc) and how much of this is "caveat emptor" (the aviation crowd are used to paying a lot for their gear, so some yahoo with an MBA says "let's see how much we can milk them for a pair of rubber blocks tied with a rope")?

The point of wheel chocks is to prevent the tire from rolling (and thus the aircraft from winding up "somewhere other than where we parked it). The aircraft doesn't know how much we spent on the chocks, and if it did it probably wouldn't care. (OK, it might laugh at you if you buy solid gold bars to chock a Cessna 150.)

Aside from pretty obvious quality-related things (a $5 set of chocks made out of cheap rubber that fall apart when you squeeze them in your hand)
the big reasons for price differences come down to a few things:

Size

Just like with chocking a car or truck there are various sizes of chocks available, and they vary in length and height:

For very large aircraft very large chocks are used (wider and taller):

…and for smaller aircraft smaller chocks are used (shorter and narrower):

Big chocks require more material, and thus cost more to make than small chocks, so logically they cost more.

Nice Features & Laziness

I have a set of $20 aluminum wheel chocks. They have a bungee cord that keeps them tight against the tire (so there's no room for the tire to roll), and they weigh just over 1 pound (they're light and small enough to throw in the plane without thinking about it).

The guy two planes down from me has a wheel chock made of three sections of 2-by-4 inch lumber, screwed together to be just slightly wider than his tires and painted safety-orange. It probably weighs less than 5 pounds and cost him $10 and a few minutes with a power drill & spraypaint.

Both work equally well, but I was willing to pay for someone else to cut and bend some aluminum that I can fold up into a small space easily rather than finding a chunk of lumber I didn't need and making my own chock.

"Standard Aviation Markup"

There's a joke that when trying to determine the price of a component for an aircraft you take the price of that component in a non-aviation application and add some zeros to the end of it.
That's not a joke in this case: Those two rubber chocks you posted are, for all practical purposes, identical - with the exception of their price.


How does Mach number of Jet engine vary with respect to Compression Ratio? How does the performance of jet engine vary with Compression ratio.
What are the variables that affect the Mach number of Jet engine.

The performance and exhaust Mach numbers depend on very many variables, to name a few:

Inlet shape.
Compression ratio.
Combustion inlet velocity of the airflow.
Throttle setting
Turbine entry temperature.
Exhaust shape.

A multi-dimensional matrix which to fully describe takes a book like this one. My edition has 491 pages, and the scope of the answer would be very broad indeed.

The exhaust Mach number however, is mainly determined by the shape of the exhaust. If the exhaust is convergent only, maximum Mach number of the exhaust stream is M = 1. In order to reach supersonic exhaust velocities, the shape must be converging then diverging. If converging only then depending on the operating state the exhaust can be choked, at a higher pressure than the environment. The higher pressure will still contribute to thrust.

Figure above shows an example for a supersonic exhaust for a low bypass engine.


What is the final approach fix altitude for this localizer approach? As I'm looking at this approach chart, I see that the glideslope intercept altitude is at 1400 feet MSL. However, for the localizer approach, I believe we stay at 1400 feet MSL until RIZER, which we can then descend to our MDA. Is that correct?

Coram is not the FAF for a non precision approach.  The FAF for localizer only is Rizer.  You must protect 1400 till Rizer then you may descend to the applicable MDA.


What is "Operational Control"? 14 CFR 121 and 135 certificate holders are required to maintain operational control of their flight operations:

§135.77   Responsibility for operational control.
Each certificate holder is responsible for operational control and shall list, in the manual required by §135.21, the name and title of each person authorized by it to exercise operational control.

121.533, 121.535, and 121.537 contain similar requirements for 121 carriers.

What exactly is "Operational Control"?

Operational control is defined in 8900.1 Volume 3, Chapter 25 as:

  Operational control functions include, but are not limited to:

  Preflight planning;
  For part 121 operations, preparation and dissemination of dispatch/flight releases;
  Canceling a flight due to potential hazardous or unsafe conditions;
  Ensuring that only those operations authorized by the OpSpecs are conducted;
  Ensuring that only crewmembers who are trained and qualified in accordance with the applicable regulations are assigned to conduct a flight;
  For part 121 operations, ensuring that no flight is started unless it has been authorized by a dispatcher or person authorized to exercise operational control in accordance with §§ 121.593, 121.595, or 121.597, as applicable;
  Ensuring that crewmembers are in compliance with flight and duty time limitations and rest requirements prior to departing on a flight;
  Designating a pilot in command (PIC) and, where applicable, a second in command (SIC) for each flight;
  Providing the personnel who perform operational control functions (PIC, dispatcher, etc.) with access to all necessary information for the safe conduct of the flight (for example, weather, Notices to Airmen (NOTAM), airport aeronautical data (analysis), and inoperable instruments and equipment);
  Specifying the conditions under which a flight may be dispatched or released (weather minimums, flight planning, airworthiness of aircraft, aircraft loading, and fuel requirements);
  For part 121 operations, ensuring that when the conditions, limitations, and authorizations specified in a dispatch or flight release cannot be met, the flight is either cancelled, delayed, rerouted, or diverted;
  For part 121 operations, ensuring that each flight is conducted in accordance with the conditions and limitations of the dispatch or flight release;
  Ensuring that each flight is operated in accordance with the limitations of the MEL and CDL;
  For part 121 operations, monitoring the progress of each flight and initiating timely actions when the flight cannot be completed as planned, including diverting or terminating a flight;
  For part 135 operations, locating a flight for which a Federal Aviation Administration (FAA) flight plan (FP) has not been filed;
  For part 121 operations, ensuring rapid and reliable communications in accordance with §§ 121.99 and 121.122, as applicable;
  Restricting or suspending operations in accordance with §§ 121.551, 121.553, or 135.69 as applicable; and
  Exercising the emergency authority described in §§ 121.557, 121.559, and 135.19, as applicable.

These functions are normally performed by a variety of people:

Dispatch
Crew Scheduling
Maintenance
Chief Pilots
Director of Operations
The PIC of the flight
Manager of SOCC (System Ops Control Center)
Director of Training

Operational control is implemented by the above people and must hit all of the bullets in the definition above.  This means that to exercise operational control you must always know where your planes are, you must determine who flies them, plan those flights, ensure the pilots and aircraft are legal, ensure everyone is trained for the operations performed and to always be able to communicate with your aircraft.

There is also mention of Operational Control Systems, which require that any flight operated by the carrier is dispatched, followed and can be located at all times.  These functions are normally carried out by the Dispatcher assigned to your flight.

Further reading beyond the 8900.1 can be found in AC 120-101 for 121 ops.


If jet engines did leave chemtrails, wouldn't the chemicals cause adverse reactions in the engines? Please note, this question does not call into question whether or not the conspiracy theory is true or untrue, but the feasibility of certain chemicals being used as fuel additives in order to make the theory viable.

Chemtrail conspiracy theorists believe that there are certain additives being put into jet fuel to spew out [nano-]particles that rain down on us, for means that are being kept secret.  The hypothesized reasons behind 'chemtrailing' range anywhere from weather modification, population reduction, and sunlight blocking to reduce the earths temperature hence combatting global warming.

Some popular agents that are called into speculation:

Aluminium based aerosols
Mono-atomic gold
Barium

My question is, wouldn't these additives, chemicals, etc, cause adverse reactions inside the jet engines, if the turbine is the actual catalyst supposedly spraying these things?

Jet fuel is seldom if ever pure hydrocarbon molecules. It's got a lot of other stuff mixed into it to improve the properties. I'm going to quote Shell for the sake of examples:

  Antioxidants (gum inhibitors) must be used in Avgas to prevent the
  formation of gum and other antioxidation products. Jet fuels, which
  are inherently more stable than gasolines, may contain them, depending
  on the treatment process used during manufacture.

  A metal deactivator - now rarely used - may be added to nullify the
  effects of dissolved trace metals, especially copper, which can impair
  the thermal stability of jet fuels.

  Corrosion inhibitors can be used to reduce corrosion in fuel system
  and improve the lubricity - lubricating properties - of jet fuels.

  Fuel system icing inhibitors reduce the freezing point of any water
  that may be in the fuel system and prevent the formation of ice
  crystals that could restrict fuel flow. They are mandatory in military
  Jet but are not used in civil aircraft that have fuel heaters. They
  may be added to either Jet or Avgas during the fuelling of small
  aircraft.

  Static dissipater additives minimise the hazardous effects of static
  charges that build up during movement of jet fuels. They are sometimes
  used in Avgas as well. Source

Further analysis

No practical application:

  Aluminium based aerosols, Mono-atomic gold, Nano aluminum-coated fiberglass, Radioactive thorium, Cadmium, Chromium, Nickel, Desiccated blood, Mold spores, Yellow fungal mycotoxins, Polymer fibers, Barium.

Useful additives from your list:

  Ethylene dibromide- Anti-Knock agent in aviation fuel.
  Alumium: Derivatives containing it have been used.

Many of the materials you list might have the very real potential to (seriously) damage jet turbines and piston engines if mixed in during extended use.


What are the implications to consider when starting a university Flight Club? I'm currently a University student in the United States, and finishing my PPL within the next two weeks (assuming I pass my checkride!)

Unfortunately, when I return to University for the Fall semester, there are zero flying resources. No local clubs, or anything of that nature. I am told by many of the pilots around my out of state flight school about their student flying clubs back in college, and it sounds amazing.

My University encourages students to start clubs, offering a maximum funding of $10,000 - $50,000. I presume because clubs increase the schools admission, it is worth the money for them? Being a large engineering school, I could see it being a selling point in tours.

My question is in three parts.

What are the implications of starting a student Flight/Aviators Club, and what benefits could it provide, outside of meeting new aviation friends? (Implications referring to expected safety precautions a school might want to impose, any FAA regulations pertinent to a club, and associations/partnerships we could create to show legitimacy to the school)
What would be a good way to find pilots within the University? (I'm sure out of 30,000 students there are some)
Would this be percieved by the general aviation community as "not my place" to start a student flying club, as I am a new pilot?

What are the implications of starting a student Flight/Aviators Club,
  and what benefits could it provide, outside of meeting new aviation
  friends? (Implications referring to expected safety precautions a
  school might want to impose, any FAA regulations pertinent to a club,
  and associations/partnerships we could create to show legitimacy to
  the school)

I was in college not all that long ago and I can speak to this a bit as I was involved in some clubs that built cars (Formula SAE) while not the same I see a lot of parallels. Generally speaking, and this is very university dependent but I have seen it apply across the board, if a school puts money towards a club the club must abide by the school's rules but more importantly it typically falls under their insurance policy. For something like a poetry club there is not that much risk, but when we were racing cars the university would inspect them all the time and there always had to be a faculty member present when we were running tests. We were lucky in that we had good faculty backing which is key, my advice here is to first find a professor (you are generally going to need an advisor anyway) that flies and get them excited about it and onboard as this will go a long way to help your case.

What gets tricky here is you flying on the university's dime. If you are not looking to buy a plane and you simply want to use the funds for rental that is an interesting predicament. An aviation lawyer may be able to answer better for you, but you need to confirm what the deal is with using club/org/university funding to rent and fly planes. Technically you are not getting compensated to fly but you are also not paying to fly while someone is paying for the aircraft (you are in turn paying 0 which is below your pro-rata share). If the students are splitting the rental costs, renting from a local FBO and are all checked out in the airframe you don't really need to even get the university involved.

There are universities like Embry-Riddle that are very aviation focused and offer training as a part of the curriculum so there are places that do it.

Getting a local flight school or FBO involved may help if you have a place to rent planes from and can guarantee business they may offer a small price break.

I would reach out to the National Intercollegiate Flying Association and see if they can offer any advice or help.

  What would be a good way to find pilots within the University? (I'm
  sure out of 30,000 students there are some)

The airmen registry here in the US is public, you could always run it up against your school directory... I would ask around, post fliers etc. keep in mind that flying is not the cheapest hobby and college kids tend to not be all that cash liquid... I sure couldn't start flying until I had my first job.

  Would this be percieved by the general aviation community as "not my
  place" to start a student flying club, as I am a new pilot?

IMO pilots are very receptive of other pilots. Old pilots love to inspire young pilots and you will most likely find a lot of support from them. GA awareness is never a bad thing either!


Why are there fewer stages in the turbine than in the compressor of a turbine engine? I'm a student interested in aviation. I'd like to understand why the turbine section has fewer stages than the compressor section in a jet engine.

What would be the effects of adding stages to the turbine, or to the compressor? How is the optimal number of stages determined?

Very simply, a turbine has fewer stages than the compressor, because the flow in the turbine is going in the direction of reducing pressure - the air wants to go that way naturally. But a compressor has to make the air flow in the direction of increasing pressure. If this is done too aggressively (attempting too much pressure rise per stage), the air flow on the blade seperates, and stalls. In severe situations, the flow can completely reverse direction and exit the intake.


Are there any situations where having high lift but low lift to drag ratio would be beneficial? From my understanding, high lift to drag ratio could make an aircraft efficient during cruising conditions. This is when the aircraft is in equilibrium, lift is equal to weight, and thrust is equal to drag, and since there's less drag, less thrust is required. Is this correct and are there situations where having higher lift but higher drag as a consequence would be beneficial (e.g. higher camber has higher lift, but drag increases more leading to a lower lift to drag ratio)?
For example, if lift generated were to be around 10000N with 700N of drag (14.29 l/d) compared to less lift and drag, 8000N of lift with 600N of drag (13.33 l/d), would less drag at the expense of less lift be worth it in the context of cruising conditions (where thrust would need to equal drag to maintain constant velocity)?
Sidenote: I'm mainly thinking about this in terms of cruising conditions, not necessarily during climb or descent, but if you're happy to share any info on that, I would appreciate it too!
EDIT: Just to clarify (thanks for the comment quiet flyer), this is lift and drag generated by the wings.

Landing phase would benefit from high lift but low lift-to-drag ratio. At most phase of flight you need about the same amount of lift to keep the plane in the air. However during landing you need to slow down to landing speed. Hence you lower lift-to-drag ratio by keep the same amount of lift but increase amount of drag. This is usually accomplish by selection of large amount of flaps.


How do SIGWX chart updates work? AWS says the SIGWX charts are issued at 0000, 0600, 1200, and 1800 UTC, which I assume matches the VT times on the charts themselves. What confuses me is AWS' parenthesized statement based on the current ETA model available (http://aviationweather.gov/products/swl/info).

Let's say it's 1100Z on Monday morning, and I'm trying to use the SIGWX charts to predict weather as far in the future as possible using SIGWX charts available to me at this time (again 1100Z on Monday morning), and only SIGWX charts. I know there are other information sources, but this question pertains only to SIGWX charts and their timing.

1) Which SIGWX charts are relevant for future weather developments (at 1100Z Monday)? The only two updates scheduled at 1200 and 1800 at this time?

2) I'm assuming that today's (Monday's) 0000Z and 0600Z SIGWX charts will be updated using the current ETA model available at the time the 1200Z chart is published. So is it fair to say that "all" SIGWX charts are updated when a SIGWX chart is published (at 0000, 0600, 1200 and 1800)?

Thanks for any clarification.

First, an explanation of "the ETA model"
The science of weather prediction is the science of data analysis.  Meteorologists receive data from weather reporting stations worldwide, and from that data, they attempt to predict the future, using the laws of physics as well as assumptions and educated guesses.
Educated guesses?!  Say what?!
That's right.  Weather is one of the original chaotic systems, and although the weather obeys physical laws, we can't gather enough information about the current state of the atmosphere to predict what will happen in the long term with any degree of accuracy.
Bummer.  So what are all the computers for?
Well, we've found that supercomputers can predict the weather out to a few days ahead with a reasonable degree of accuracy, even without perfect information.  They do this by applying educated guesses and rules of thumb.  Using different assumptions gives a different forecast model.  Some of the more well-known models are the GFS, the NAM, and the EMCWF.  The ETA is one of the lesser-used models.
Great.  So what does "based on the current ETA model" mean?
Simple.  It means that the SIGWX chart gets created based on the most recent data that the ETA supercomputer churned out.  If the chart is supposed to come out at 1800 UTC, but the ETA model run doesn't complete until 1845, then that new forecast data doesn't make it into that chart.
Enough.  Just answer my questions.
Okay.

which SIGWX charts are relevant for future weather developments (at
1100Z Monday)?

All the SIGWX charts are relevant, but the newer charts will have the newer forecast data.  That doesn't necessarily mean that they're going to be more accurate, but they probably will be.
To use your example, it's 1100Z on Monday morning.  The 0000 chart says "IFR" but the 0600 chart says "No IFR".  There might still be IFR out your window - nothing says that the supercomputer will get it right.  Maybe the older one was more correct than the newer one.  Doesn't happen often, but is it possible?  Sure.

So is it fair to say that "all" SIGWX charts are updated when a SIGWX
chart is published (at 0000, 0600, 1200 and 1800)?

No, as soon as a chart comes out, it becomes part of history.  Charts are not updated with new weather information; they remain as a historical record of what was predicted to occur, given the best information possible at the time (which, for the forecasters, can be just as valuable if not more valuable, to see where they went wrong in the past).


What would it take to have a new large civil aircraft manufacturer? What would it take to have a new large civil aircraft manufacturer besides Boeing and Airbus, that is not state sponsored (C919, MC-21)

In other words, what are the chances a new company would break into the market? What would it take?

If you look at the US past aircrafts, there is an abundance of companies producing large civil aircraft: Boeing, Convair, Douglas, Lockheed, Martin. Then there were the French, the British and the Russian manufacturers. Now there is Boeing and Airbus. Embraer and Bombardier are producing rather small planes for today's standards, even if they are impressive compared to historic planes.

The regional jet market is a lot more competitive, with the CRJs and ERJs under pressure from MRJ, SJ-100 and ARJ. Could one of those companies successfully grow into Boeing and Airbus market?

What are the chances a start-up, with no state money, will come to market with a new 100+ seats plane in the next 20-30 years? Could a SpaceX type of company be successful in the airplane manufacturing industry?

What are the chances a start-up, with no state money, will come to market with a new 100+ seats plane in the next 20-30 years?

If you count Bombardier, the chances are roughly 100%. The Bombardier C-Series, which will seat 140 (160 max,) is scheduled to enter service within this year (2015.) However, Bombardier is hardly a 'start-up' in the normal sense.

Bombardier C-Series Mock-Up (Source: Wikipedia)

For a real start-up (e.g. a SpaceX-style company,) the chances will be much more slim. It takes billions of dollars worth of investments to design a modern commercial airliner before you will ever see the first dime of revenue. Start-ups generally won't have access to that kind of money until they have had time to mature and developed a steady revenue stream from other successful products (which is how Bombardier is doing it.)

  Could a SpaceX type of company be successful in the airplane manufacturing industry?

That depends greatly on the type of airplanes in question. Undoubtedly, this is possible in the light general aviation market. Several such companies have come and gone over the years. One interesting start-up that is planning to enter production soon is Terrafugia, a start-up from MIT, which is building aircraft that fold up into street-legal cars that can just drive away from the airport and park in your garage. However, the larger airliner market is a completely different ballgame, involving development costs that are orders of magnitude higher. Start-ups have a difficult time getting started in markets with such huge barriers to entry.


How can seagulls slide in the wind without flapping their wings? There are a lot of seagulls in a park near where I live. Sometimes I can see them somehow "sliding" forward or backward diagonally when flying against the wind, however, their wings aren't flapping. What are the flight mechanics behind this?

Can gliders mimic this style of flight?

I've never heard of other birds than albatrosses do dynamic soaring, but since this is over land, it can be simpler ridge soaring—as the sea breezes hits the shore (and obstacles on it), it turns upwards and the birds simply fly in this updraft. They are flying against the wind and gliding down relative to it, but when you add the speed of the wind, they may be stationary relative to the ground—or zig-zag from side to side if the wind speed is a bit less than the forward speed the birds need to efficiently glide.

Gliders can do the same, but since they are bigger, they need a mountain range where the seagulls can do with a several feet embankment.


What are ICAO English language requirements? As a supplement to this SE question; "What are the requirements to become a language proficiency examiner?" (something I never heard of...) What are  ICAO English language requirements?

ICAO defined a framework of language proficiency levels for aviation. It ranges from level 1 ("Pre-elementary") to level 6 ("Expert" / native speaker). Level 4, "operational", is the minimum required for flight crews. Many airlines require at least level 5 from their pilots.

Native speakers can get level 6 with a simple proof of that they're a native speaker (I heard recently that e.g. for a German license the authorities see it as sufficient if the person visited an English speaking school). Non-native speakers do assessments to certify their language proficiency. An assessment for level 4 has to be repeated every 3 years, level 5 lasts 6 years and level 6 is life-long.

For languages other than English, there are assessments / language proficiency certifications that are regulated by country-specific authorities and it's practice in at least some countries that native speakers can get their native language put into the license as level 6.

There are detailed requirements for each level (e.g. pronunciation, vocabulary etc.), they are usually presented in a big table:


Why do the wing tips on the Boeing 777X fold up instead of down? The outer 11.4 feet (3.8 meters) of the wings of the Boeing 777X are designed to fold up when on the ground. Why aren't they designed to fold down?

It seems to me that folding down would be more "fail safe" if the locking mechanism failed in the air. The wings are generating lift, so they would hold themselves up and continue to contribute to keeping the plane aloft.

Up-folding wings could lift themselves if the locking mechanism failed in flight and destabilize the plane. The folding bit is less than 12 feet while the diameter of the engine nacelle is greater than 14 feet, so it doesn't seem like proximity to the ground is the issue.

What are the biggest reasons for folding up instead of down?

The FAA has decided/proposed (Nov 2017) that the folding mechanism must comply with certain standards, of which:

The wingtips must have means to safeguard against unlocking from the extended, flight-deployed position in flight, as a result of failures, including the failure of any single structural element.
All sources of airplane power that could initiate unlocking of the wingtips must be automatically isolated from the wingtip-fold operating system (including the latching and locking system) prior to flight, and it must not be possible to restore power to the system during flight.
The wingtip latching and locking mechanisms must be designed so that, under all airplane flight-load conditions, no force or torque can unlatch or unlock the mechanisms. The latching system must include a means to secure the latches in the latched position, independent of the locking system.
It must not be possible to position the lock in the locked position if the latches and the latching mechanisms are not in the latched position, and it must not be possible to unlatch the latches with the locks in the locked position.

Source: federalregister.gov (broke it down into paragraphs for easy reading)
As to why they chose up and not down, only the responsible engineering team at Boeing can answer that. My guess is that they trust their design over the drivers around the plane when it's parked.
This is echoed by airinsight.com:

(...) Boeing could point out that the upward folding wing offers considerably less opportunity for "ramp rash".  It is, unfortunately, true that vehicle drivers around aircraft are often clumsy and hit aircraft.

Interestingly, the folding wingtip was envisioned 28 years ago for the 767-X (what eventually became the 777):

(flightglobal.com)


What is the range and accuracy of ATC radar systems? What is the average range, accuracy, and update frequency of radar systems used to monitor airspace in the USA?

I'm primarily interested in capabilities for tracking smaller general aviation aircraft without enhanced return data channels like ADS-B, etc.

Please include sources, if possible. As an example, by looking at TRACON/ASR data exports (CDR), these systems appear to update an aircraft echo at about a 4.7 second frequency, give or take a few tenths. However, I not been been able to source or verify my findings or similar statements made by expert witnesses in litigation cases.

This is a general answer, not specific applicable to the situation in the USA.

There are several types of radars in use to monitor airspace.

Basically you can distinguish two classes of radar:

Primary radar does not require the aircraft to carry a transponder, it is a form of non-cooperative surveillance.
Secondary radar on the other hand requires the aircraft to carry a transponder; the transponder replies to interrogations from the radar. This is called cooperative surveillance.

Radars operate basically by line-of-sight, however due to atmospheric effects the radio waves follow the curvature of the earth to some degree. Aircraft flying low are quickly below the horizon, but aircraft at FL450 can be detected on secondary radar at ranges up to 300NM if the radar is operated for such range. Typically the range is less.

In general, the further the range that a radar needs to look, the slower it rotates. An airport surface radar may rotate as fast as twice per second, long range en-route radars can go as slow as once per 12 seconds. For typical TMA / TRACON use, 5 seconds would be in the right ballpark, for en-route 8 seconds will be about right.

The accuracy depends on the type of antenna, whether it is primary or secondary radar, and the distance of the aircraft from the radar head. The accuracy of the distance measurement are not so much affected by the range, and varies from about 5 meters to 300 meters. However, atmospheric circumstances can degrade these figures, especially at long range. The across beam accuracy is very much dependent on the distance from the antenna due to limited angular accuracy of the radar. The further you get, the worse the across beam accuracy becomes.


Can you accept a LAHSO on your private pilot checkride? Suppose you're on your private pilot checkride and the tower gives you a land and hold short clearance.

Are you able to accept that clearance? I know you're expected to fly as if you were a private pilot, and you're allowed to carry the examiner even though they're not providing you dual instruction...

Are you allowed to accept a clearance on your private pilot checkride that you couldn't accept as a student but could as a private pilot?

First, let's see whether it's legal. FAA Order 7110.118 notes that:

  7. EXPLANATION OF CHANGES.

  The following changes apply to all LAHSO.

  a. ...
  b. Solo student pilots will not conduct LAHSO.

However, you could interpret that as not applicable to your situation, as you are not the sole occupant of the aircraft:

  14 CFR §61.87

  (a) General. ... The term "solo flight" as used in this subpart means that flight time during which a student pilot is the sole occupant of the aircraft...

Though it might be allowed, note it does not say must:

  AIM 4-3-11(b)

  ...
  Student pilots or pilots not familiar with LAHSO should not participate in the program.

I'd recommend against doing so because LAHSO can add to your workload, especially if you're a bit nervous on your checkride. Also, the AIM note on discouraging LAHSO is rather valid and if done improperly, can affect safety. Having said that, the examiner is likely to ask questions on LAHSO during your practical examination, especially since LAHSO is a special emphasis area in the PTS. This may include:

Requirements to accept a LAHSO clearance
ATC pharseology for LAHSO
Pilot responsibilities while carrying out LAHSO

In any case, always remember that you do not need to accept a LAHSO clearance -- if you do not feel comfortable with doing so, ATC will provide arrangements to land full-length on the same runway or another one. You should not accept a LAHSO clearance if you believe doing so would compromise safety.

Note: this does not necessarily apply to all situations where a student pilot is prohibited from doing something that a private pilot would be able to do.


Why are the wings of some modern gliders tadpole shaped? Why are the wings of some modern gliders tadpole shaped, rather than teardrop shaped? An example is the Schleicher ASG 29. Or did they just add a flat plate at the trailing edge for the flaps and ailerons?

Source: flickr.com

The flaps and ailerons are "reflexed" on this glider. They have been raised to a setting above the normal zero position, above the airfoil's normal chord line. A number of flapped gliders have this feature.

Two main benefits are a reduction in pitching moment as the pressure distribution on the wing is moved forward, so less downforce work for the tail, reducing trim drag, and lower induced drag because with the trailing edge partially unloaded it has an effect like reducing wing area and moves the best L/D speed up a bit.  So overall, best glide angle is a bit better, and the speed it's achieved is a bit higher.

You use reflex for "penetration", going as flat as possible as fast as possible, for whatever reason, like getting through an area of sink in a hurry.


Is it possible to drive a prop directly from a jet engine without a gearbox? I'm studying turbojets, turbofans, and turboprops. I can see that, for the latter engine configuration, a gearbox is necessary so that the propeller rotates at a lower velocity than the main compressor. Here is an image:

For the experts in the aerospace field, my question might be regarded as a bit "odd", but here I go: is there any reason why we can't use a second turbine to move the propeller with coupled shafts, instead of relying on a gearbox? The idea would be to use a low pressure turbine that moved the propeller.

I think this might be unfeasible, because the added weight of a second turbine would be higher than the weight of the gearbox itself, but I don't know if there are additional considerations to this.

But on the positive side, we would have the following (and please, correct me if I'm wrong):

Less maintenance work.
Propeller rotation shaft aligned with the main engine core (does this even represent an aerodynamic advantage?).

EDIT 1

The idea would be using two turbines. A high-pressure turbine (HP) that moved the compressor, and a low-pressure (LP) turbine that moved the turboprop. My idea was taken from the turbofan engine config.

EDIT 2

As the user mins pointed out, we would need a very slow turbine or even a gearbox. But we could design the LP turbine with few stages (say 1, 2 or 3) so that its rotational velocity would be low enough as to move the propeller without having aerodynamic losses in the tips of the prop. And as this same user pointed out, there already exists such a configuration (see here). The question then is, why isn't this configuration preferred over the "conventional" one? (i.e. the one that uses a gearbox).

As a side comment, there is a type of engine researched in the 80's/90's, but never marketed, where an unducted fan is driven directly by the turbine shaft without a gearbox: The propfan. It's close to your idea, albeit the unducted fan is not exactly a propeller. Safran has restarted researching on this design, end of 2017, for an operational use after 2030.

Two counter-rotating unducted fans on a free turbine engine (source)

Having discarded the propfan, let's go to the actual answer!

Short answer

A propeller rotational speed doesn't exceed 1,000 or 1,500 RPM (it depends on its size too). The slowest shaft of a jet engine spins in the range of 5,000 to 15,000 RPM, sometimes more. A reduction gear is used to compensate for the difference.

A fan may spin at 5,000 RPM, however the use of a reduction gear is also desirable, though it is often avoided by limiting the speed of the core, at the expense of a loss of efficiency. New aircraft like the A320 NEO can be fitted with geared turbofan engines, for better performance.

The reason the turbine cannot spin at a lower speed is that a low speed means low torque, a problem common to many air-breathing engines (maybe except the free turbine).

Dotted line: Theoretical torque at low speed. Plain: Actual torque. You may compare with trying to pull away in a hill in fourth gear.

Low torque also means increased spool up time, a drawbacks that is often a no-go factor.

Details

Jet engines must rotate at high speeds by principle, as a lot of air must be compressed to burn a lot of fuel in a small amount of time. On the other hand the fan or the propeller are more efficient at low RPM.

Let's see how engineers have dealt with these two opposite needs in different engine types.

Single-spool turbofan

The simplest design is the single spool engine (all discs rotating at the same speed). While this is possible, the output shaft is at a high rotational speed. This is usable for a fan which also rotates at high speed. The core compressor is not as efficient as it could if it rotated faster.

The Snecma M53-P2, used on the military jet Mirage 2000, has a rotational speed of 10,600 RPM.

Better efficiency can be achieved by increasing the compressor speed and reducing the speed of the fan. It requires the use of a distinct second spool.

Two-spool turbofan

Such configuration is used in many turbofans (Rolls-Royce being a notable exception with a third spool). A low-speed / low-pressure turbine drives the fan and usually the first stages of the compressor.

From this EASA certificate, the CFM56-7B used on all B737 NG has a maximum HP shaft speed of about 15,000 RPM and a LP shaft speed of about 5,400 RPM.

Increased efficiency needs to further increase the fan diameter but also to slow down the fan to keep the linear speed of the tips at a reasonable value. A slower fan will not be directly driven by the turbine.

Geared turbofan

Slowing the LP turbine adds complexity and also creates a problem: The output torque also decreases (which is a phenomena in all mechanical engines). This leads to increased reaction times after thrust requests, with also means less safe engines. A gearbox is used to prevent sacrificing the precious torque. This unusual configuration is an option on the A320 NEO.

PW1000G (formerly known as project GTF) has a HP shaft at 20,000 RPM, a LP shaft at 15,000 RPM, and the fan connected by a 3:1 reduction gearbox rotates at 5,000 RPM.

planetary reduction gearbox 3:1 (source)

If the altitude is sacrificed for other benefits and the engine is used in denser air, then a turbofan is not anymore an efficient solution.

Turboprop

When the fan is replaced by a plain old propeller working at a much slower rotational speed, the use of a speed reduction gearbox with a larger ratio is a required all the more for the same reasons.

The Dash 8 / Q400 which has STOL performances, is equipped with two PW150A turboprops. The PW150A is a two-spool core with a free power turbine (two discs not linked to the gas generation section). The free turbine allows for better propeller speed management, allowing an engine start with the propeller stopped.

From its EASA certificate, the HP shaft runs at 31,150 RPM, the LP shaft at 27,000 and the maximum output shaft speed doesn't exceed 1,020 RPM.

PW150A (source)
Note: this schematic is not accurate, but it illustrates the principle. Look with an accurate (but less readable) schematic of the engine sections.

Additional experimentation: Propfan

Interestingly there is an intermediate solution between the fan and the propeller: The unducted fan. It combines the advantages of a high rotational speed and the lack of duct, the drawback being the noise generated by unducted blades close to Mach 1 airflow at the tip.

Testing was conducted by several engine manufacturers but no engine was launched on the commercial market for reasons that appear to be non-technical. An example studied by GE and Nasa combined a double unducted fan (UDF) and a free turbine in a new type of engine known as propfan, in a pusher configuration.

(Source)

The two counter-rotating fans were mounted directly on the free turbine output shaft without a gearbox.


Elliptical wing planform and elliptical lift distribution I know that there are similar topics already here, but after reading them I still do not understand everything. From what I know elliptical lift distribution gives the least amount of induced drag. That is because there is the least amount of lift at the wingtips compared to other parts of the span - is it true what I wrote? And know question with elliptical wing planform. Is  this planform generating the least amount of induced drag? And I read somewhere that this wing planform is most efficient among other planforms in subsonic flight. Is it true and if yes why? And does elliptical wing has elliptical wing distribution? Because I heard lot of versions.

Lift force over the wing is said to be of elliptical distribution if the lift per span ($L'$) along the wing span is of the following:

$$L'(y) = \rho_\infty V_\infty \Gamma_0 \sqrt{1-(\frac{2y}{b})^2}$$

where $\rho_\infty$ is density of the airflow, $V_\infty$ is the free-stream airspeed, $b$ is length of wing span, $y$ is lateral coordinate along the wing (0 being wing root), and $\Gamma_0$ is the circulation at wing root.

The above is essentially an equation of an ellipse, and hence the name elliptical lift distribution.

In low Mach flight, it can be theoretically demonstrated that an elliptical lift distribution produces the least induced drag for a given flat span. Induced drag of a lifting surface in incompressible flow can be expressed as (Ref Anderson, Fundamentals of Aerodynamics):

$$C_{D_i}=\frac{C_L}{\pi e A}$$

where $C_L$ is the lift coefficient, $A$ is aspect ratio and $e$ is the span efficiency factor.

For a flat span, $e$ must be smaller or equal to 1, and it's only equal to 1 when the lifting surface has an elliptical lift distribution.

When the wing has zero twist, an elliptical planform produces an elliptical lift distribution. However, elliptical lift distribution can also be coaxed by judiciously twisting the wing for a planform that is not geometrically elliptical.

Addendum on non-flat span:

In the case where the span is not flat, either because it has dihedral or is curved, it has been theoretically demonstrated that a non-flat span can achieve lower induced drag than a flat wing of equal projected span. This forms the theoretical basis behind winglets, end plates, spiroids, etc.

However, if we compare the non-flat wing to a flat wing whose length is that of the unfurled non-flat wing, the flat wing always has lower induced drag. That is, if there is no limit on the wing span length, elongating it is always better from an induced drag perspective.

Below is a graph comparing the induced drag of an optimal non-flat (circularly cambered wing) wing to that of an elliptically loaded flat wing. A value of 1 indicates no change. A value greater than 1 indicates more induced drag and a value less than 1 indicates less. $\beta$ is the span camber factor.

Data and figure compiled from https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19630006412.pdf


In WW2, were self-sealing fuel tanks really cost-beneficial? I just read this question and answer, which seems to confirm my fears that an incendiary bullet will still destroy you if it hits your fuel. In other words, a self-sealing fuel tank will not protect against incendiary bullets. The wing will be punctured, and the fuel will either explode, or catch fire and remain on fire. (Correct me if I'm wrong.)

So the question is, why have them at all? It seems so easy to defeat the system by just using incendiary bullets, which must be a whole lot cheaper than the rubbery fuel tank.

I know there is accident protection and flak fragmentation protection. However, if you're taking flak, it seems to me you've got many problem-areas under the aircraft to worry about. I remember reading about a lot of bomber pilots in WW2, who took hits in the legs from flak, and some of them started sitting on their flak jackets all the time.

I realize this is a complex question, because cost-benefit analysis is a complex thing. But maybe someone can explain the rationale held at the time. Self-sealing fuel tanks added cost and weight, yet seem easily defeatable with incendiary bullets. Maybe I am missing something?

Your description of a self-sealing fuel tank remaining on fire when lit is the exact situation of the sealing mechanism failing. As soon as the tank is punctured, regardless of a fire or not, the rubber in the middle layer will react with the fuel and swell until the hole is reasonably obstructed. This both extinguishes the fire and seals the leak. Consider a case study: in the Pacific theater, Japanese fighter designs did without self sealing tanks for the cost and weight saving benefits. American naval designs such as the Hellcat however, had every possible damage-control system of the day installed everywhere possible to make the planes as rugged as they could. In the end, Japan mainly lost control of their own airspace due to the fact that they simply ran out of materials to build new planes from. The rate at which they lost planes was incredibly high due to both the small .50 caliber rounds' ability to set the unprotected tanks on fire, and the poor training of Japanese pilots towards the end of the war. If you look at statistics, per battle the IJAAF and the IJN lost more planes per squadron and battle group than any other country's air service in the war. In fact, many of the IJAAF's main fighters including the Mistubishi A6M Zero and the Kawasaki Ki-61 Hien were infamous among pilots of both sides of their ability to catch on fire from what should have been inconsequential hits and particularly their inability to stop burning. Another point of interest is the design of the self sealing fuel tank:

This all goes to prove strongly that NO, self sealing fuel tanks are a definite must in combat airplanes, and a very important safety feature in other aircraft types. I'm confused however as to why your question switches from bullets (incendiary?) to flak as anti-aircraft guns such as the Flakvierling88 were not meant to ignite fuel tanks, they were supposed to destroy the whole plane, fuel tanks and all. If you want some more numerical data backing up my position, Wikipedia references many sites that give a bit more data on why self sealing tanks are a must for fighter planes. The Defense Systems Information Analysis Center also provides some interesting information on the design and how they weren't all too much of a cost/weight penalty later in the war. Hope this answers most of what you're asking.
In addition, there is a specific part of the article of my second link I would like to draw attention to:

This simple composition and design, which are inexpensive, lightweight, and easily produced in large volumes, confronts the deceptively complex ballistic dynamics of a bullet perforating a fuel tank.  When a normally oriented bullet pierces a fuel tank, it leaves a small residual hole less than the diameter of the projectile.  But this small hole is just the beginning of the challenge.  The bullet, travelling at 3,000 ft/s, begins to unload its tremendous kinetic energy on the fuel inside.  This unloading generates a high-velocity wave of pressure in the fuel that is reflected back on the wall milliseconds after the bullet tears through.  This pressure wave, known as hydrodynamic ram, blasts a jet of fuel back through the entrance wound.  The bullet, still moving at a high velocity, and now followed by the same pressure wave, begins to tumble as it travels through the fuel.  When it reaches the back wall of the tank, the bullet erupts sideways through the material and leaves an elongated gash that is often torn further by the trailing wake of hydrodynamic ram.
Initial fuel tank prototypes tested by Navy engineers were discouraging failures.  Engineers had not anticipated the challenging dynamics, and often the entire back walls of prototype tanks were blown out by the hydrodynamic pressure wave.  However, these early development tests shed some light on the mechanisms that brought down so many pilots and aircraft during the World War I.  The Navy and rubber company engineers persisted and established remarkably effective design features and manufacturing processes that ultimately saved untold lives not just in World War II but in each of the armed conflicts since then.

This statement strongly backs up the cost/weight penalty not being an issue position, if this clears anything up.


What happen if an aircraft is more than 10 hours overdue for its 100-hour inspection? Let's assume an aircraft for hire's 100-hour inspection is about 20 hrs past due (120 hrs since the last 100-hour inspection). Can I just say that the aircraft is not airworthy and the aircraft cannot be operated?
Or, if I'm the owner, can I operate it just for personal flights? Is this illegal?

FAR 91.409(b) says:

  Except as provided in paragraph (c) of this section, no person may operate an aircraft carrying any person (other than a crewmember) for hire, and no person may give flight instruction for hire in an aircraft which that person provides, unless within the preceding 100 hours of time in service the aircraft has received an annual or 100-hour inspection and been approved for return to service ...

(Emphasis mine)

According to a literal reading of the regulation, it only applies to operation "for hire" and not to the owner(s) flying it for personal use. So, in theory, what you propose is legal.

However, if an FAA inspector ever reviews the log books and sees that you went over 100 hours between successive 100-hour inspections, you would need to provide evidence that every such flight was not for hire, which could cost you significant time and effort.

So, while it is technically legal, in practice it is unwise. If the plane is regularly used for hire, get it inspected every 100 hours regardless of the nature of specific flights.


What alternatives to pitot static systems are available or in development? Air France 447 is a powerful example of what a serious situation unreliable air data indications can be if not handled properly by a crew. Still, whether crews are properly trained or use that training, unreliable air data presents a dangerous situation. Pitot tubes are fallible devices susceptible to errors caused by environmental factors such as ice crystals. With this in mind, what technical alternatives to pitot static systems exist or might be developed that are more accurate and less prone to interruption?

A number of systems are under development- most are optical systems- determining airspeed using LiDAR. NLR, for example has developed an Optical Air Data System for this purpose:

  The  developed  system  applies  the  LiDAR technique to measure air speed in four different directions. The aircraft’s TAS (True Air Speed),  AOA  (Angle  Of  Attack)  and  SSA  (Side  Slip  Angle)  are  calculated  from  these  speed  measurements.

Other systems are in prototype stage- including a photonic system that is based on back scattering of light, but none of them are yet operational.


Are there any future regulations planned for electric multi-motor LSAs? It seems with electric control of the multi engine configuration, such as with NASA's new leap tech technology, that there should be a way to use multiple motors in the LSA category. Is there anything in the works regarding regulations for this type of aircraft in the LSA category?

Light-Sport is a category for pilot certification and flight rules. The aircraft themselves are simply part 23 normal category risk classification level one, low speed.

Part 23 was fully rewritten in 2016 it now describes the desired safety outcome but lets manufacturers develop new methods of complying, nothing is bias against electric multi-motor planes in the new part 23.

The issue is that the pilot must be rated for multi-engine operation and you must have at least a private pilot certificate for the multi-engine rating. Most multi engine training is actually about operating with an engine failure and the associated aerodynamics.


What aircraft or airline differences would make the Sukhoi Superjet unsuitable for US regional airline operations? What differences in the "stock" production SSJ-100 (as compared to CRJ-900s or EMB-190s), or in US regional airline operations (vs Interjet's success-so-far with the SSJ-100 in intra-Mexico and Mexico-to-US ops) would put the Sukhoi Superjet out of consideration for US regional airline operations?  Would post-sale support be the killer, or would the only thing in the way be the US government not liking it (either by the FAA shooting down the idea, or by way of an exorbitant trade tariff making it more expensive than other options)?

This is some speculation, but one potential reason is that U.S. carriers already have fleets of Embraer and Bombardier jets in this basic size (the Su-100 competes directly with the Embraer 190/195 and Bombardier's larger CRJs and upcoming C-series). The companies making these jets are located in pretty firmly U.S.-allied countries that are unlikely to see any economic restrictions that might hamper the ability of U.S. regional carriers to procure new airframes or parts.

The same cannot be said for Russia, which from 1946 until 1991 was pretty much embargoed by NATO signatories, and the relationship between Russia and the West since then has been testy. While the U.S. currently does not have a trade embargo with Russian civilian jet production operations (current sanctions attempt to specifically target military R&D and direct investment), that can change in a heartbeat if Russia continues to throw its weight around in neighboring countries.

Second, the seating capacity of this jet is a bit of an odd duck, as it blurs the line between the needs of regional vs. national U.S. carriers. The basic problem is that regionals, by their very nature, operate out of smaller cities with fewer potential passengers, at distances from major hubs that makes driving to that larger airport and flying from there more palatable for passengers. These considerations cause passenger count on regionals to drop off dramatically compared to longer "national" routes. The largest thing American Eagle currently flies is a Bombardier CRJ-900 (75 passengers in 2-class config), with the most commonly operated jet in AE's fleet being the Embraer ERJ145 (50 seats all-economy). By comparison the smallest plane operated under the full American badge is the A319 (124 seats in 2-class config).

There's an obvious gap between the two subsidiaries' aircraft capacities, but not much interest (by American at least) in filling it with a 100-seat jet like the SSJ100 (or E195 or C100). The primary reason is the collective bargaining agreements that pilots have with the airlines; they include, among other things, a requirement segregating the aircraft used by regional versus national subsidiaries. If you want to fly a plane with more than 80-100 seats (depending on exact terms of the agreement) on the route, you're paying a national airline flight crew the extra bucks to fly it; it's a violation of the CBA to put a 737 or A319 in the hands of a regional airline pilot because that takes the job away from the more senior pilots.

A secondary reason is that U.S. domestic airlines like American tend to favor more, smaller flights over fewer larger ones. This allows the airline to "rightsize" the route schedule (and its fleet) to passenger demand more closely and with fewer disruptions. If on a given day of the week, 200 passengers need to get between a regional airport and a larger hub, the airline could do that with four 50-place jets, or 3 75-place jets, or two 100-place jets. Theoretically, if demand were constant, it could save money the fewer flights it made, as the cost of the 50-seat Embraer's two-man flight crew would be roughly the same as the Sukhoi's, but with the Sukhoi you spread that cost over twice the passengers so your gross margin is higher.

However, an empty seat is a big fat goose-egg on the revenue for that seat for the flight, so if passenger demand varies (and it does, from day to day and seasonally), running larger jets either forces you to turn away some customers that you can't fit into the reduced schedule, or add flights that run mostly empty. Neither of these are palatable options for the airline; one pushes potential customers into the arms of their competitors, while the other has you running at least one flight that day costing tens of thousands of dollars for a handful of people with tickets whose regional component of the price might not even cover the pilot's salary for the duration of the flight (regionals are often "connecting" flights; DFW to PHX costs about \$320, ACT to DFW about \$300, but ACT to PHX via DFW only costs about \$360, so depending on how you part it out, the ACT-DFW leg of ACT-PHX might only really make you \$40 a seat, which might not break even if the plane's full, much less if it's half empty).

So, assuming any plane an airline would buy or lease is profitable when it's fully loaded with paying customers, American is more likely to successfully operate a route, and its entire network, using more smaller planes and scheduling a different number of interchangeable airframes and flight crews each day to maximize utilization, than it would be trying to also minimize the number of flights by using a variety of airframe sizes, which involves ensuring those airframes and the flight crews to run them are all in the right places at the right times (and deadheading - flying an empty plane from where it is right now to where it will be needed later - is bad no matter what the plane size, but typically more palatable with a smaller jet that has lower total operating costs).

Now, American isn't the only carrier with a regional subsidiary. US Airways Express and JetBlue, as well as several other New World regional carriers outside the U.S., regularly operate 100-seat Bombardier and Embraer airframes on regional connectors, and it would stand to reason that Sukhoi could get a piece of that action. It could also "stretch" the Superjet and try to compete in the national domestic market with the 125-seat A319 and 737-700 which are becoming more and more popular as U.S. domestic flights get "rightsized". However, the delicate nature of Russian-Western relations will probably continue to make Sukhoi a risky proposition for U.S. carriers that need a reliable source of airframes and parts on a 20-25 year timeline.


What is the aerodynamic effect of a panorama (skydeck) near the tail section of a commercial airline? A company called Windspeed Technologies posted the following concept video of a panorama seating option aboard a commercial plane.

From the limited understanding I have of the phenomena called flow separation, such a structure on the fuselage may induce separation.

The sphere example here is just an approximate representation for the semi-ellipsoid.

The reason I am asking this question, is because the ellipsoid is relatively near the tail section of the plane. It does not seem like a good idea to have a turbulence inducing structure that close to the tail.

How will this addition to the plane fuselage effect the air flow ? Worst case, can it cause navigation problems due to turbulence near the tail section ?

You're correct that the structure may induce separation under some conditions. However, the effects are unlikely to be significant given the (publicly) available data on the concept. From the available images, the deck is quite small, with a seating capacity of only two.

Image from gizmag.com

Also, it is located at some distance in front of the vertical fin and appears shaped to reduce drag.

Image from technology.org

The company says that it has over come many design hurdles including

  ... structural modification, structural integrity of the canopy to withstand a bird strike and flight loads, condensation, noise levels, UV protection, aerodynamic drag, potential disruption to the vertical tail's performance, safety, ingress and egress requirements per the FAA requirements.

Though this should be taken with a grain of salt (it from the company after all), there is nothing insurmountable in the engineering sense. For example, the canopies of fighter jets are of similar size in smaller aircraft and they are operated without problems.

AWACS aircraft routinely carry huge antenna on top of their fuselages; they have no problem flying, albeit with reduced performance. For example, the following image shows the E-767, the AWACS version of Boeing 767 operated by the JASDF.

"E-767 Japan AWACS 112010" by Jerry Gunner - AWACS Boeing E-767 of Hiko Keikai KanseitaUploaded by Altair78. Licensed under CC BY 2.0 via Commons.

However, it is another matter if this is going to enter into service. My take would be that this is not going to happen in current time for a number of reasons including:

The cost quoted is pretty high and downtime, too long.
cutting up pressurized fuselage is not a good idea.
The concept is going to take up seats and increase weight for questionable benefits. What are you going to see up there? the same clouds that you are going to see from normal windows anyway.


Are strategic bombers still useful, or are fighters taking over this role? During the mid 20th century and cold war, armies were having clear distinction between those aircraft designed as strategic bombers and those designed to be fighters for aerial combat. My understanding was that there were few exceptions to this rule.

However, the fifth generation of jet fighters include models designed normally for aerial combat but also used for strategic bombing. Old designs of strategic bombers are still being used, but it is clear that jet aircraft are being significantly improved by new developments compared to obsolete bombers.

Why is this happening? Is current warfare no longer requiring strategic bombers? Are designs of jet fighters so flexible that they can perform different missions? (usually it is the other way around, as time passes more specialization is seen).

The reason that small aircraft are used in the bombing role is flexibility and cost. If you load up a B-52, it can only be in one place at one time, travel at subsonic speeds, and be expensive to operate, risking more crew and equipment. Using smaller attack jets allows reaching targets across a wider geographical area faster. And while past conflicts saw massive bombing campaigns that favored the use of large bombers, more recent conflicts have seen much less of this tactic. Smaller, more targeted strikes are more typical, where a large bomber would be overkill.

Take a look at the cost per flight hour for various aircraft. A B-52 will cost \$70,000 per hour to operate, and a B-1B \$60,000. Compare this to an F-16C for \$23,000 or an F-15C for \$42,000. The A-10 costs \$18,000. For stealth, the costs go up to \$170,000 for a B-2 and \$70,000 for an F-22. So why send a B-52, when a few F-16's would cost the same amount? The fighters will also be able to defend themselves, and still be able to carry out a mission if an aircraft drops out. The same argument can be made for the stealthy class.

Strategic bombers still have their uses, but the bomber role is much less demanding than the fighter role. With proper maintenance and multiple equipment upgrades, old bombers can still fly bombs to their targets just fine. Fighter jets have much more complicated requirements, such as stealth, speed, and maneuverability, where advances in technology will justify the costs of continued development of new designs.

Aircraft are becoming more generalized because it is more efficient. In some areas such as naval aviation, this is even more critical. Why have separate planes for electronic warfare, attack, and air defense? You would be limited to a certain number of each type, with little or no cross capability. But with more generalized aircraft, you can have more aircraft with a certain capability across the fleet. If a certain capability becomes less important, you don't end up with a whole group of aircraft becoming useless. Aircraft can fulfill multiple roles in a single mission or deployment.


Do pilots ever intentionally takeoff or land downwind? Is there a reason a pilot would takeoff downwind or land downwind? Is it legal to do this?

It's perfectly legal, but check the Chart Supplement (or AFD, or instrument approaches) to see if there are special exceptions for that airport.

If it is a non-towered airport and you aren't assigned a runway, you should of course try to use the prevailing runway that is already in use by other traffic in the pattern when it makes sense.

Some airports have obstructions in one direction of their runways that make it necessary (or safer) to always land or depart in a certain direction.

Some runways have a steep incline in one direction, making it preferable to land upslope and depart downslope.  (For example Lukla, Nepal: YouTube)

It's also common for larger airports to have preferred directions for arrival or departures in order to keep the flight volume high, even when the wind prefers other runways.

However, that usually relates to operating with intentional crosswinds at an airport that has parallel runways in a primary direction (allowing them to conduct more simultaneous flight operations), but not in the intersecting direction.


Is there other option than a Chinook to transport 2 cars simultaneously? If a civilian wants to fly between homes several hundred miles apart AND carry 2 cars with him what are the options other than a chinook?

You could use an Mi-26 to transport your two cars, as seen here:

With a payload of up to 20 tons, it should be able to lift many useful combinations of two vehicles.


What is the best word for lateral translational movement in a rotorcraft? I'm talking about movement left and right without changing attitude. This is a motion that can be done with a quadcopter and to a limited extent a helicopter (I presume).

On a helicopter a left/right movement of the cyclic will command a roll motion from the rotor, but in a hover the aircraft would only roll slightly then begin to move sideways. Does the pilot just call this a roll or is there another, more appropriate word?

I thought of slip or crab, but those describe specific attitudes associated with forward flight. They don't seem to fit the bill here.

The term I preferred was "slide"

In our NATOPS manuals, and in our trained procedures in both Search and Rescue ops, and cargo ops, in the Navy the crewman would call for us to "slide left" or "slide right" as he positioned us over the pick up point.  (Easy Left/Easy Right as we got closer to the spot).

The FAA term "sideward hovering" suffices, but it is two words.  If you want one word, slide is about right.


How is the battery power on the Airbus A340's electric system switched? On the electric panel of the Airbus A340 there are three battery buttons (BAT 1, BAT 2 and APU BAT, indicated by (3) and (4) in the diagram) controlling battery charge limiters. When the aircraft is on the ground, below 50kts and batteries are the only power sources (as FCOM states) batteries 1 and 2 should be connected to the DC-Battery bus (DC_BAT_BUS) and the DC-Essential bus (DC_ESS_BUS).

source: Airbus A340 FCOM 1.24.20 P1 REV08

Should both of battery buttons 1 and 2 be pressed to Auto mode to have these connections? Or it is possible to connect battery 1 separately from 2 to DC_BAT_BUS? Seems that both buttons should be pressed to have such connection to DC_ESS_BUS, but for DC_BAT_BUS it is not so obvious.

From A340 FCOM 1.24.20 P2 (REV 8):

  BAT 1 (2) pb sw

  Controls the operation of the corresponding Battery Charge Limiter

  AUTO:

  The Battery Charge Limiter controls automatically the connection
  and the disconnection of the corresponding battery tot the DC BAT BUS
  by closing and opening of the battery line contactor.

  The batteries
  are connected to the DC BAT BUS in the following cases:

  Battery voltage below 26.5 Volts (battery charge). The charging cycle ends when battery charge current goes below 4 Amperes (for 10 seconds on ground 30 minutes is flight)
  On the
  ground (with speed below 50 knots) when batteries only are supplying
  the aircraft.

My understanding is that for the DC BAT BUS this means when a button is set to AUTO the corresponding battery is connected when that battery needs to be charged or:

The aircraft has a speed below 50 knots AND
The only power source is batteries (i.e. no generators, APU or external power supply are connected to the electrical network)

A note further down mentions that the batteries are disconnected  when the voltage drops below 23 Volts for more than 16 seconds, in order to protect the battery from discharging completely.

For the DC ESS BUS it states:

  The batteries are connected to the DC ESS BUS when batteries only are
  supplying

  in flight
  on the ground (speed below 50 knots) provided they are
  both selected auto

That clearly states that both push button switches need to be set to AUTO in order for the DC ESS BUS to be powered from the batteries on the ground.


Do lights on wingtips change color at different times of the day? I have gone through this post which explained me why there are red lights turned on the plane but I doubt with the color code whether it's only during the environment specific conditions or its indication that it is relaying coordinates?

Every time one thing caught my attention during my flight journey. During the specific time of the day the lights on the wings changed with color.

On one flight it had a white constant blinking. This flight was during the day. The blinking was slow and repeated up to thrice.

On the second flight, there was a blinking green light along with the blinking white light. This blinking was fast so I couldn't get a clearer shot and this is during sunset.

Note:-In the above two images both are of the Airbus A320 Sharklet. I believe it has something to do with tracking and relaying the coordinates to the ATC.

Every plane has green and white lights on the right (starboard) wing, as in this photo, and red and white on the left (port) wing. These lights are called "navigation lights," and are the same as are used in boating.

The green and red lights are always on, no matter what, and are used to help other aircraft know which direction the aircraft is going, based on whether the other pilot sees red, green or both lights.

The white lights are also on all the time, and flash constantly. These lights are called "anti-collision lights."

(Image Source: Wikipedia)

What you are actually seeing is the green light constantly on, and the white flashing - but because the white is so much brighter and is flashing, it's more apparent than the green (which, especially in the day, is fairly dim)

At night, the green becomes more obvious and the flashing white becomes more visible. The flashing white over-powers the green, which gives an optical illusion that the Green is flashing. The reality is that the Green is always on, and the white is flashing.


What certifications must one have to repair/alter propellers and instruments? According to 14 CFR 65 Subpart D

  A certificated mechanic may perform or supervise the maintenance, preventive maintenance or alteration of an aircraft or appliance, or a part thereof, for which he is rated (but excluding major repairs to, and major alterations of, propellers, and any repair to, or alteration of, instruments), and may perform additional duties in accordance with §§65.85, 65.87, and 65.95.

(Emphasis mine)

What certifications are necessary to affect the repairs or alterations specifically excluded from a "mechanic" certificate?

I believe you need to be an FAA Certified Repair Station approved to work on instrumentation, propellors etc. as noted here

  The FAA lists and approves ratings on a form called operations
  specifications. The operating specifications will also be displayed in
  the maintenance facility, usually right next to the certificate. Like
  other businesses, most maintenance facilities advertise on the Web and
  in trade magazines, so you have many ways to research what rating a
  particular repair station may have. There are basically six ratings
  that pertain to a repair station:

  •  Airframe

  •  Powerplant

  •  Propeller

  •  Radio

  •  Instrument

  • Accessory

To be an approved Repair Station the employees must conform to

  § 145.157 Personnel authorized to approve an article for return to
  service.

  (a) A certificated repair station located inside the United States
  must ensure each person authorized to approve an article for return to
  service under the repair station certificate and operations
  specifications is certificated under part 65. (b) A certificated
  repair station located outside the United States must ensure each
  person authorized to approve an article for return to service under
  the repair station certificate and operations specifications is—

  (1) Trained in or has 18 months practical experience with the methods,
  techniques, practices, aids, equipment, and tools used to perform the
  maintenance, preventive maintenance, or alterations; and

  (2) Thoroughly familiar with the applicable regulations in this
  chapter and proficient in the use of the various inspection methods,
  techniques, practices, aids, equipment, and tools appropriate for the
  work being performed and approved for return to service. (c) A
  certificated repair station must ensure each person authorized to
  approve an article for return to service understands, reads, and
  writes English.

They also must meet the requirements set fourth in:

  (b) Meets the eligibility requirements of §65.101.

which you can find here.

You can find the full regulations pertinent to becoming a repair station here


How are aircraft separated in a non-radar environment? In remote areas that have no radar (oceanic airspace, etc.) how is aircraft separation maintained so that airplanes don't get too close?

What do areas that normally have radar do when there is a sudden radar outage?  There are airplanes going everywhere, not necessarily on standard airways, and a lot of them are being given radar vectors.  What happens when the screens go dark?

For oceanic airspace, flights proceed along specified tracks and altitudes specified in their filed flight plan. From there, once they leave radar-controlled airspace, radio reports are made to ATC, who ensure that aircraft are appropriately separated if a flight needs to change its track or altitude. See the wiki article the North Atlantic tracks, which give a good overview on the track system. Specifically:

  Prior to departure, airline flight dispatchers/flight operations officers will determine the best track based on destination, aircraft weight, aircraft type, prevailing winds and Air Traffic Control route charges. The aircraft will then contact the Oceanic Center controller before entering oceanic airspace and request the track giving the estimated time of arrival at the entry point. The Oceanic Controllers then calculate the required separation distances between aircraft and issue clearances to the pilots. It may be that the track is not available at that altitude or time so an alternate track or altitude will be assigned. Planes cannot change assigned course or altitude without permission.

and

  Contingency plans exist within the North Atlantic Track system to account for any operational issues that occur. For example, if an aircraft can no longer maintain the speed or altitude it was assigned, the aircraft can move off the track route and fly parallel to its track, but well away from other aircraft. Also, pilots on North Atlantic Tracks are required to inform air traffic control of any deviations in altitude or speed necessitated by avoiding weather, such as thunderstorms or turbulence.

I'm going to leave the "what happens when things break" to someone else.