Is it mandatory to go missed following full scale CDI deflection?

The point being, once you've reached a full scale deviation in IMC, there's no telling how much beyond it the airplane has gone. Your USA AIM 5-5-5 is quoted here, and point 'b' is sufficient to conclude an immediate Missed Approach:

5-5-5. Missed Approach
a. Pilot.
1. Executes a missed approach when one of the following conditions exist:

(b) Determines that a safe approach or landing is not possible.

With a full scale deviation and the LOC beam narrowing as you approach the RWY, what is the pilot going to steer at low altitude and descending lower in IMC? What is the IFR reference that the pilot would use? A VOR course? An FMC course? A locator beacon (NDB)? Even if these existed, there would have to be an IFR procedure to continue to landing, including revised minimums.
Quoted below are the Elements of a Stabilised Approach, widely accepted as The Gospel by regulators and industry, All manufacturers have incorporated into their procedures, this concept from the Flight Safety Foundation's ALAR 'toolkit':
(ALAR = Approach and Landing Accident Reduction )
(notice too, the applicability to VMC conditions)

Most airlines and other aviation organisations specify minimum acceptable criteria for the continuation of an approach to land. These vary in detail but the following summary published by the Flight Safety Foundation is one view of the important considerations.

Their Approach-and-landing Accident Reduction (ALAR) Briefing Note 7-1 suggests that "all flights must be stabilised by 1000 feet above airport elevation in IMC and 500 feet above airport elevation in VMC. An approach is stabilised when all of the following criteria are met:

The aircraft is on the correct flight path
Only small changes in heading/pitch are necessary to maintain the correct flight path
The airspeed is not more than VREF + 20kts indicated speed and not less than VREF
The aircraft is in the correct landing configuration
Sink rate is no greater than 1000 feet/minute; if an approach requires a sink rate greater than 1000 feet/minute a special briefing should be conducted
Power setting is appropriate for the aircraft configuration and is not below the minimum power for the approach as defined by the operating manual
All briefings and checklists have been conducted
Specific types of approach are stabilized if they also fulfil the following:
ILS approaches must be flown within one dot of the glide-slope and localizer
a Category II or III approach must be flown within the expanded localizer band
during a circling approach wings should be level on final when the aircraft reaches 300 feet above airport elevation; and,
Unique approach conditions or abnormal conditions requiring a deviation from the above elements of a stabilized approach require a special briefing.

An approach that becomes unstabilised below 1000 feet above airport elevation in IMC or 500 feet above airport elevation in VMC requires an immediate go-around.


What is the call sign for the US president's private helicopter?

The callsign of a civil aircraft transporting the President of the USA is Executive One, as you already mentioned yourself. An aircraft owned by a corporation which happens to share the name of the president is not any different. (According to a comment by Dan Sheppard, Vice President Nelson Rockefeller used his personal plane, whose callsign was Executive Two – this would be the closest thing to the situation you ask about that has actually happened.)

However, as was already mentioned in another answer, it is highly unlikely in this day and age that the Secret Service would allow transporting the President on anything other than one of the VC-25s, VH-60Ns, SH-3s, or "The Beast", publicity stunts such as George W. Bushs flight with the Lockheed US-3A notwithstanding. Note that this was the only time so far that the callsign Navy One was used, which tells you something about how often something like this (POTUS using an aircraft other than one of the VC-25s, VH-60Ns, or SH-3s) happens.


What makes inertial guidance unsuitable for auto-land?

Suitability for CAT III autoland requires much more than just accuracy. A big factor is system integrity which involves fault detection and alerting. Fault detection in autoland approved ILS/MLS/GLS receivers dual channel processing (command and monitor) with continuous cross-checking and required detection and alerting response times. In the multi-mode receivers (MMR) I've worked on, the monitor tripped if the two channels disagreed by more than 0.0065 DDM for a period of 350 milliseconds. Total time to alert has to be less than 1 second.

To my knowledge, as a result of that requirement GLS has not been approved for less than CAT I approaches as they have not been able to demonstrate the necessary integrity and time to alarm.

I don't know of any method for monitoring an INS system that would be able to meet the requirements. The FMS I work with is capable of detecting and excluding an IRU from the nav solution, but the tolerances are much larger than what would be required for approach and it requires much more than 1 second.

The best reference for what is needed is AC 120-28D. Section 5 covers the Airborne Equipment requirements.

Ultimately, it not only has to work, you have to be able to get it approved. That is not a trivial or inexpensive process, even with published standards. Doing something new, without industry standards, is significantly more difficult. Can you make a business case that would show a long term savings?


How is flyblackbird.com operating under Part 91K?

Long story short, Blackbird appears to be set up so that the customer leases an aircraft and separately hires a pilot to fly it. That makes it a 'straight' part 91 operation. However, the FAA has stated that pilots using Blackbird are holding out and require part 119/135 certificates (with some technical exceptions, such as 91K).

Blackbird's terms and conditions for pilots specifically mention 91 B but not K and include a few interesting points (emphasis mine):

  Pilot services are offered as an independent contractor to users under a dry lease
  The pilot is not directly employed by the owner of the airplane
  The pilot operating the flight cannot own the aircraft being rented
  Pilot hiring is at the discretion of the passenger; BlackBird will facilitate matching pilots with aircraft based on passenger
  selected routes, pilot qualifications, and location and availability
  of both pilot and aircraft
  Pilot understands that passenger (lessee) renting the aircraft and hiring the pilot accept and maintain operational control of the flight; these operational duties may be delegated to the pilot in command.

In other words, the passenger rents an aircraft from an owner and then separately hires a pilot to fly it, according to the passenger's instructions. Blackbird is saying that they only provide a marketplace, and the customers actually lease and operate the aircraft themselves.

This is from their general term and conditions:

  The Platform provides a marketplace that enables users to (a) connect
  with and lease aircraft from third parties that own, lease, or
  otherwise control aircraft (“Aircraft Owners”) and to find and hire
  flight crews (“Flight Crews”), or (b) join flights with third-party
  operators or other BlackBird users on the Platform (“Third-Party
  Operators”).

And:

  If you wish to lease an aircraft from an Aircraft Owner and / or hire
  qualified Flight Crew, then you can use the Platform to search for
  available aircraft and Flight Crew

Because the owner just rents out the aircraft without a pilot, they aren't operating an air carrier. Because the pilot doesn't provide the aircraft, they aren't holding out. That means the only pilot qualification needed is a commercial certificate, although Blackbird also requires 500hrs, an instrument rating and a background check.

Now, whether or not that business model will survive if the FAA gets interested in it is another question. There was some discussion on the AOPA forums (members only) a while ago about Blackbird and I think the consensus came down to "technically legal, but pushing their luck with the FAA and DOT".

A semi-useful comparison here might be Uber. They've always claimed that they aren't a transportation company but rather a marketplace that just connects independent drivers with customers. That argument has succeeded in some places but not in others, and many countries and cities have decided that Uber is a transportation company and started regulating them. I have no idea whether or not that will happen with Blackbird.

Update: on December 17th, 2019 the FAA made its opinion very clear in a letter to Blackbird's lawyers:

  The information that BlackBird has presented leads us to conclude that
  the pilots participating in BlackBird's platform and using its app are
  holding out and thus are engaged in common carriage.

If you're a pilot who flew via Blackbird, their closing statement isn't reassuring:

  Accordingly, please expect further investigative activity into
  BlackBird's operations, particularly regarding its pilot database. In
  addition, we would be interested in learning of any action you intend
  to take in view of the jeopardy facing pilots who participate in
  BlackBird' s service.

There's a lot more discussion online, especially in Reddit (/r/flying) if you want to read more.


Where can I find the aerofoil geometry of a commercial aircraft so as to design it in CAD software?

All the current manufacturers develop proprietary airfoils combined with wing planform, washout, etc. using CFD for optimum pressure distribution, low drag, high critical Mach number, high maximum CL, benign stall propagation and benign aeroelastic characteristics. That applies to Boeing, Bombardier, Embraer and Airbus.
This type of info is not normally released by the manufacturers.
TCDS's do not have this kind of data.
You can do your own research starting with a standard NACA airfoils like those found in NACA report 640(Summary of Airfoil Data) and modifying them.


Can a flight attendant have only one eye?

Let me refer you to the answer of this other question, that says a pilot may be considered for any class of medical (including First Class) with only one functional eye.

I think its fair to conclude that if a pilot can get a First Class medical with one functional eye, then you can be a flight attendant with one functional eye, assuming that you can fulfill all of the duties required.

Good luck.


On a small/micro jet engine, can the turbine blades and vanes be made out of ceramic?

I can't speak for all engines, but GE has been testing ceramic blades in their LEAP engines.
Here is their report on it.


It's named for one of its founders William E. Boeing which is the American spelling of his father's German surname "Böing".

To answer the question directly: it does not mean anything in particular.


How much does it cost extra to fly with one additional passenger?

I am basing the following calculations on the excellent answer by Peter Kämpf to this question: How much of an improvement would a 1% weight decrease on an airplane be to the industry?.

We can approximate the fuel usage with the Breguet equation

$$ m_\mathrm{TO} = m_\mathrm{Landing} \cdot \exp \left( \frac{R \cdot g \cdot b_f}{v \cdot L/D} \right) , $$
where:

$m_\mathrm{TO}$ is the total mass at takeoff
$m_\mathrm{Landing}$ is the total mass at landing
$R$ is the distance flown
$b_f = 0.000018 \mathrm{kg} / \mathrm{Ns}$ is the fuel burn
$v=250 \mathrm{m}/\mathrm{s}$ (about 900 km/h) is the velocity
$L/D=18$ is the lift to drag ratio

Assuming a fully loaded A320 (OEW $ = 42.6 \, \mathrm{t}$ with $ 19.9 \, \mathrm{t}$ payload and $3 \, \mathrm{t}$ reserve fuel $\Rightarrow m_\mathrm{Landing} = 65.5 \, \mathrm{t}$), we can do the calculation for the fuel ($m_\mathrm{TO} - m_\mathrm{Landing}$) with and without an extra 100 kg and compare for different distances $R$:

Note: this equation become increasingly inaccurate towards small distances because the takeoff, which uses more fuel, is not taken into account here.

Using the given speed, this can be converted to different flight times:

And by multiplying with the current jet fuel price of 616 US Dollars per metric ton, we get the additional cost of a 100 kg passenger per flight time:


Why are several of the Tu-104’s cabin windows oddly positioned?

I found a cutaway diagram of the Tu-104B. It appears that, at least on this model, some of the seats in the middle were raised a bit to perhaps accommodate some engine parts (there appears to be a label on the diagram which I can't quite read). The windows were raised correspondingly to match the seats. The mid-positioned one at the aft may have been placed there to match one in a door on the port side of the aircraft. The three high-positioned ones appear to be windows in the toilets (туале́ты).


Why did wings start to be made with aluminium skin instead of fabric?

Wings and fuselages are subjected to twisting loads as well as bending so they need to be torsionally stiff.  Prior to aluminum, the options were to make the wing/fuse with a frame of spars and ribs with fabric to provide the aerodynamic contours, braced with struts and/or wires to provide the torsional stiffness as well as resistance to bending, or, to do away with the struts and brace wires and make a monocoque structure (like an eggshell) with a plywood skin to provide torsional stiffness (with the spar beam inside for bending).  If you wanted to go fast, you had to do away with all the bracing so plywood monocoque was the way to go.
When "duraluminum" came along after WW1, it was realized that as well as replacing sitka spruce for spar beams and ribs, it could also replace plywood for a stressed skin monocoque structure.  You could build an airplane in the 1930s using plywood stressed skins bonded to wood ribs and spars, or aluminum stressed skins over aluminum spar beams, and have pretty much the same performance.  You could make your egg about of plywood, or aluminum sheet, and it was pretty much the same egg.
There were so many manufacturing and durability advantages to aluminum (no glue, no wood rot etc etc), it was a no brainer to switch, once the metallurgy started to become understood well enough to use it as an engineering material, and plywood skinned wooden airplanes faded quickly.


Is there a modern single-seat turboprop?

Yes. Although most aircraft to find good use for a turboprop need to have more than one seat, some single-seat uses do exist. For example, agricultural aircraft like the Air Tractor AT-802, Thrush models, and the PZL-106 Kruk are all currently in production.


Can I use motor oil in a aircraft piston engine?

By motor oil, I assume that you mean automotive motor oil. In that case, it's a bad idea for a bunch of reasons. First, every airplane POH I've seen has specified that the oil must qualify to a standard - either MIL-L-6082 or MIL-L-22851 (though those have been superseded by an SAE standard). Modern automotive oil doesn't meet that standard, and therefore is effectively forbidden by the POH. More to the point, automotive oil is bad for aviation engines.

Aviation and automotive engines work on the same basic principles, but many of the details are different.1 One significant difference is that aviation engines normally burn some oil as they run, while well-performing automobile engines burn little or none.

Automotive oil contains a number of additives, such as detergents and wear inhibitors, intended for use where the oil doesn't burn off. Some of those additives do not burn completely, but instead stick around in the form of ash, where they can foul the engine and create deposits that lead to pre-ignition. That is why aviation oil is ashless dispersant: it cleans the engine by dispersing combustion byproducts into the oil (to be removed on the next oil change), while being ashless when burned. Lycoming considers it serious enough to put in an all-caps "CAUTION" section in their Lubricating Oil Recommendations document:

  UNDER
  NO CIRCUMSTANCES SHOULD AUTOMOTIVE OIL BE USED. THE USE OF AUTOMOTIVE
  LUBRICANTS IN TEXTRON LYCOMING ENGINES IS NOT RECOMMENDED BECAUSE ITS
  USE COULD CAUSE ENGINE FAILURE.

If all of that wasn't enough, use of an improper oil may void your engine warranty. In short: don't use it.

References:

An editorial summary on Avweb
An aircraft engine overhauler's best practices
Lycoming's Lubricating Oil Recommendations

1) Note that I'm referring to air-cooled aviation engines that burn 100LL - made by, say, Lycoming or Continental. Rotaxes and new aero-diesels are different, and more like their respective automotive counterparts; in fact, Thielert engines are actually based on automotive diesels.


How does flight duration affect the hourly fuel consumption in heavy aircraft?

General Answer

The heavier the airplane, the more fuel it will burn in a given amount of time.  Therefore, if you takeoff with enough fuel to fly further, you will be burning more fuel in each corresponding hour than if you took off at a lighter weight to fly a shorter distance.

Example

I don't have performance information for the 747, but I can give you an example for a G-IV corporate jet.  Instead of comparing a short trip to a longer trip, I planned the same trip with different amounts of fuel in order to make the airplane heavier.  I planned the flight for the following conditions:

Flight Conditions

Fly from Los Angeles to Detroit, using the same route and average Boeing winds for July
Climb to and maintain the optimum altitude, with step climbs where appropriate

Trip 1: Climb initially to FL450 and stay there for the entire flight (the optimum altitude for this weight)
Trip 2: Climb initially to FL410 and then climb to FL450 2:32 into the flight (the optimum altitude the weight initially followed by a climb once the optimum altitude has changed)

Fuel:

Trip 1: Takeoff with enough to land with a 1:30 reserve - 59,000 lb. takeoff weight
Trip 2: Takeoff with max fuel, landing with a 5:39 reserve - 73,000 lb. takeoff weight
Taxi fuel is not included in the first hour burns, but climb fuel is.

Results

          Trip 1      Trip 2
Hour     Fuel Burn    Fuel Burn     Diff
 1        4,212         4,844      +13.7 %
 2        2,536         3,088      +21.8 %
 3        2,411         3,123      +29.5 %
 4        2,163         2,103      -2.8 %
 Total    11,722        13,558     +15.7 %

Summary

As you can see, the fuel burn is significantly higher in the first 3 hours due to the additional fuel that we were carrying (and therefore the higher weight of the aircraft).  Hour 4 is almost the same because of the idle power descent used in the planning software.


Why is the VFR traffic pattern area at some airports different from its IFR circling approach area?

It looks like you have a few different questions here. As a caveat, I know nothing about Half Moon Bay so these are (educated) guesses to some extent. Someone with personal experience at KHAF could probably give better answers.

Why is circling always west of the runway at KHAF?

Circling areas are planned according to the TERPS criteria (see section 2-7-1); the formulas are in the document, but the basic idea is to make sure that there are no obstructions to circling within a certain distance of the runway. It looks very likely that at KHAF the terrain east of the field rises too sharply to allow safe circling there: the minimum allowed in the TERPS is 1.3nm/1.5sm/2.4km, and Google Maps shows that there's definitely rising terrain within that distance.

Why is the pattern always east of the runway at KHAF?

I don't know for sure. Noise abatement is obviously a concern at KHAF but then as you said, why not put the pattern to the west, over the ocean? The most obvious reason is that Half Moon Bay is in the Monterey Bay National Marine Sanctuary and flying over national parks below 2000' AGL is discouraged. Another possible reason is to avoid directing light, single-engine aircraft to fly over water unnecessarily.

Why - in general - would a certain area be used for the pattern but not for circling?

There's no reason why one area should always be suitable for both needs. In particular, circling requires guaranteed obstacle clearance within a certain distance from the runway. VFR pilots should be able to see and avoid, but obviously that assumption isn't possible for IFR. If you fly a circling approach at minimums then you might even go back into cloud while circling (and then go missed); in that scenario you really don't want to have to worry about terrain or obstacles being anywhere nearby.


What restrictions are there for flying near urban areas in the United States?

This varies from country to country but here in the US the Federal Aviation Regulations define the minimum safe altitudes:

  Sec. 91.119 Minimum safe altitudes: General.

  Except when necessary for takeoff or landing, no person may operate an
  aircraft below the following altitudes:

  (a) Anywhere. An altitude
  allowing, if a power unit fails, an emergency landing without undue
  hazard to persons or property on the surface.

  (b) Over congested
  areas. Over any congested area of a city, town, or settlement, or over
  any open air assembly of persons, an altitude of 1,000 feet above the
  highest obstacle within a horizontal radius of 2,000 feet of the
  aircraft.

  (c) Over other than congested areas. An altitude of 500 feet
  above the surface, except over open water or sparsely populated areas.
  In those cases, the aircraft may not be operated closer than 500 feet
  to any person, vessel, vehicle, or structure.

  (d) Helicopters,
  powered parachutes, and weight-shift-control aircraft. If the
  operation is conducted without hazard to persons or property on the
  surface--

      (1) A helicopter may be operated at less than the minimums
  prescribed in paragraph (b) or (c) of this section, provided each
  person operating the helicopter complies with any routes or altitudes
  specifically prescribed for helicopters by the FAA; and

      (2) A powered parachute or weight-shift-control aircraft may be operated at less than the minimums prescribed in paragraph (c) of this section.

In your case, the sea-planes can fly closer than regulated since it falls under the "Except when necessary for takeoff and landing" clause.

These minimums apply in all air spaces however different airspaces surrounding cities (typically Class B and Class C) may prevent you from ever getting within 1000ft (or even a few miles) of the city due to clearance. For example you can't get near a city under a bravo space without ATC clearance and even then you can't fly where you want like in other airspaces.

  but how is that and other private flying entities like news choppers

News choppers and the like still must fly by these rules. They often compensate with bigger camera lenses. The somewhat exception to this rule is generally police, fire, and rescue/medical operation choppers/aircraft. This of course is taken on a case by case basis but ATC usually gives them precedent and clears them for whatever they ask within reason. I actually met a guy that was on the council that designed the Philadelphia Bravo Airspace. The airspace is specially designed so that the medical helicopters can take off and fly under the bravo shelf from any hospital in the city. They can do this and still be a safe distance from buildings as well.

  and ad-banner planes governed?

Banner towing planes, like helicopters must also fly by these rules. Although they are not seen in cities all too often I generally see them at the beach. Although beaches are generally not defined as a congested area and generally the planes are flying over "open water" thus they must be only 500ft from the people on the beach (depending on how you interpret the law).


Are engine casings expected to contain turbofan blade fragments?

We don't yet know how or why the engine casing failed to contain the fan blade on Southwest 1380.

But yes: Not only are the engines designed to contain blade failures at fan red-line speeds, but they are also required by 14 CFR 33.94 to confirm they do, typically via destructive "blade-off testing."
(There are a number of good videos of this testing online: Search "turbine blade fail test" or "blade-out engine test.")

As noted here "fan blade-out" failures in the real world are virtually unseen.  (Turbine blade failures are relatively common and can even go unnoticed until major service teardowns.)


With wind, what will a perfectly circular flight path relative to the surrounding air look like from the ground?

It will be this :

http://www.aeroexperiments.org/images/grndtrck.gif

Pilot's of slow-flying aircraft like hang gliders and paragliders see tracks like these all the time on their GPS displays, as they circle in a thermal updraft in the presence of strong wind.

Depending on the strength of the wind, the "curliques" may become so stretched out that they don't form enclosed loops at all, just "points", i.e. a "scalloped" line.  This happens when the windspeed is greater than the airspeed. That means that the aircraft never makes any progress in the upwind direction at any point in the circle.

Related:
http://www.aeroexperiments.org/nocss.shtml
http://www.aeroexperiments.org/introcircles.shtml
http://www.aeroexperiments.org/circles.shtml


Is there a real risk of ingesting FOD into jet engines when using thrust reversers?

For the numbered items:

(1) and (2) At takeoff there is (typically, but there are birds) nothing solid that's airborne in front of the airplane, whereas thrust reversers can put small but solid airborne material in front of the engines, especially if they're still being used with high power as the airplane slows.

(3) If you're saying that the air is only directed sideways rather that forward, that would cancel the forward thrust from that air, but it wouldn't get you much braking action.

Some other points:

The higher the airplane's speed, the more braking action you get from the reversers, so if you're going to use them, get into them as soon as possible.
If it's late night or early morning, and the airport has a noise problem, consider going into reverse but leaving the engines at idle. You'll still get significant braking proportional to the airplane's speed.
If you've got a long runway, consider using just idle or less than high-power reverse. In other words, use what you need/want, but no more.
Make sure you start out of a high power reverse well before slowing to taxi speed. There are speed guidelines for this, but I forget what they are. Actually, it's best in my opinion to avoid high-power reverse unless you really need it.
Make sure you're completely out of reverse by the time you reach taxi speed. Again, I've forgotten the guideline.
Consider where you are. If you're in, say, Germany, you can expect the runways to be clean, in Spain maybe not as clean, in Zimbabwe definitely not as clean. You get the idea.
Be really careful about using reverse on an icy runway. If your nose is not pointed straight down the runway, the reverse thrust will be working to put you off the side of the runway. An acquaintance of mine put a 747 off the runway at KJFK that way (totaled the airplane, a few people hurt, no fatalities).
Consider the characters of the engines on the airplane. Perhaps this is no longer a consideration, but back in the 1990s, P&W engines on the 747 were relatively tolerant of compressor stalls brought on by using too much reverse power at too slow a speed. GE engines, however, were not so tolerant.

The second 747 carrier I worked for used to have maintenance done at Santa Barbara, CA. When we would pick up the airplane, they would close the relatively narrow runway after our takeoff and sweep it. Not a reverse thrust situation, but it does illustrate the FOD problem.


Where can I get a reliable and a updated Airport API or Airport Database, and I also want to know the charges for the API?

Location Identifiers & Airline Designators data files available with daily updates
The perfect solution to feeding your coding database. These files (more information in right hand column) are available as a 12 month subscription or a one-time web download. For the subscription, you will be granted access to the password protected extranet site upon purchase where you can retrieve the data files update daily.

http://www.iata.org/publications/Pages/coding.aspx


Why do helicopters have such limited range?

Helicopters are very inefficient in forward flight, for several reasons.

Consider Glauert's high speed assumption, i.e. the helicopter can be represented as an aircraft with a circular wing during high speed flight.  Now, if you know anything about basic fixed-wing performance, you'll realize immediately that this implies an aspect ratio just over 1, which is awfully small.  You may recall that for a fixed-wing aircraft, induced drag is inversely proportional aspect ratio--so a very low aspect ratio means very high induced drag!  Induced drag is normally not a big deal for fixed-wing aircraft at high speed because it's proportional to the square of the lift coefficient (which decreases with speed), but...
In reality, the situation for a helicopter is even worse!  Glauert's assumption assumes we have an ideal rotor, i.e. one that has a uniform induced velocity throughout.  In practice, the inflow distribution over the rotor is highly non-uniform, especially at high speeds where the advancing side of the rotor is in a very different aerodynamic environment than the retreating side.  Even on a very simple level, the "working area" swept on the retreating side is much smaller than that on the advancing side. Since lift needs to be balanced, momentum theory implies a higher induced velocity over the smaller area "worked" by the retreating side, and therefore a much higher induced drag at high speeds than an equivalent fixed wing aircraft.

Rotor wake computation from TsAGI RC-VTOL code (Kritsky, B.S).
As the helicopter goes faster, and as the rotor turns more slowly, more of the retreating side blade will stall, which leads to a dramatic increase in drag. This starts on the inboard section of the blades, and moves outwards as the advance ratio (ratio between flight speed and tip speed) increases.  In fact, the most inboard sections of the blade will even be in reversed flow--and airfoils don't work too well backwards.
On the advancing side, things are bad too, but for different reasons. The profile drag of the rotor increases rapidly with speed.  Moreover, as the advance ratio increases, the advancing tip Mach number also increases--and as the blades get into the high transonic regime, local shocks begin to form and lead to even larger increases in drag.  This can be delayed by slowing the rotor down, but that makes the problems on the retreating side even worse!  In general, it's hard to develop airfoils that work well in both regimes.
Outside of the rotor itself, the rotor hub is very draggy, between the main rotor shaft, pitch links, swashplate, hinges and blade grips, etc.  Helicopters usually have pretty "utilitarian" fuselages, as well, which are not so well streamlined.  Plus, you have the tail rotor, which has most of the same problems as the main rotor.
Back to the specific question of range, the Bregruet range equation says that the range of an aircraft is proportional to the lift-to-drag ratio and the speed of the aircraft.  We've already established that the L/D of a helicopter is pretty bad, especially at speed.  So, the cruise speed is going to be pretty low and the L/D is going to be pretty low, so to sum up, the range is going to be really low.

The tradeoff is that the helicopter is very good at hovering--even amongst VTOL platforms, the helicopter is the most efficient. Speed and range are the price you pay for that capability.


Does static longitudinal stability require download on the tail?

No, static longitudinal stability does not necessarily imply a download on the tail.

Static longitudinal stability requires that the Centre of Gravity is in front of the Centre of Lift, indicated as n.p.$_{fixed}$ in the drawing. Only then will an increase in Angle of Attack d$\alpha$ result in an opposing pitching moment: if d$\alpha$ > 0 then $dC_N$ > 0, resulting in a higher lift force. The neutral point must be behind the CoG in order to transform the higher lift into a nose down moment. Note that the drawing shows a longitudinally stable situation with a positive lift on the horizontal tail.


What is the reason for the strips on the nose of the Embraer E175

I believe this is a duplicate of another answer, but they are lightning arresters which conduct electrical charges away from the radome and avionics contained therein to the metal airframe where it can pass through and exit the aircraft without harm.


What education is needed to become a civil pilot?

First, does someone know where I can get my licences for being an civil pilot in Croatia?

I'm afraid nobody here can help you with that: You will need to find a flight instructor or flight school in your area. (Google can probably help you locate candidate schools, and we have some general advice on selecting a school and selecting an instructor that might help you narrow down your choices.)

  Second, about theoretical knowledge (physics, mathematics), could I watch MIT OpenCourseware on that (and if I do, should I watch this or this? I stumbled upon Flying Academy webiste (can't post the link because I'm new here) and it says "Sufficient level of knowledge of Mathematics and Physics".
  What is sufficient and what would I have to learn?

To begin flight training you don't need to know anything about math or physics: Your initial training will be about manipulating the controls of an aircraft to make it do what you want it to do, and for the moment you don't care how or why it works, you just need to learn the physical technique of controlling an airplane.

At some point in your training you will learn the physics of flight - this is generally part of "Ground School" but your flight instructor may introduce the concepts to you as you approach lessons where they will be relevant.
A basic understanding of the laws of motion and air pressure will be helpful, but everything you need to know can be found in the various FAA handbooks (the Pilot's Handbook of Aeronautical Knowledge, the Aeronautical Information Manual, and the Airplane Flying Handbook are good starting places - regulations will probably differ in Croatia, but the basics of flight will be the same, and are explained in fairly simple language in those books).

You will also need to be able to work basic flight planning calculations as you advance in your training, which means you will need some basic mathematical knowledge: The ability to add, subtract, multiply, divide, and do simple algebra will cover nearly everything you need to do as a pilot, and higher math is generally not necessary unless you start getting into the Why of aerodynamic theory.


To what requirements / assumptions RSAs & EMAS designed?

I will try to answer as much of this as I can, but there may be some holes to fill:

  Is RSA/EMAS designed for impact loading at the point of touchdown and to absorb energy of the moving aircraft for deceleration?

No, the arresting materials is only there for a runway overrun condition. It is not designed to absorb an aircraft impact, nor is it designed for an aircraft to touchdown on it, crash or normal landing. EMAS area's are very specifically marked:

(Source: BusinessInsider.com)

As you can see, the area is actually very small, and the markings denote that this is not a touchdown zone. It would be very hard for a commercial airliner to hit that small patch on touchdown, purposely or not.

  Assuming that the landing gear is down: Can the load be assumed to be distributed equally through all wheels?

Not always, because this depends on the loading of each aircraft gear. The load can be thought of as being distributed evenly probably at the same ratio as the weight on each gear, but only when all the gear are inside the EMAS area. This wouldn't be true, for example, as the aircraft enters the EMAS and the EMAS is acting only on the nose gear until the main gear exit the runway and into the EMAS.

  Are the brakes applied to assist deceleration over the EMAS?

YES! The pilot is going to be braking hard to try to stay out of the EMAS, it is unlikely that they would release the brakes as they enter the EMAS. Remember that EMAS is basically very weak concrete. The pilot is probably going to be using all available braking (wheels, aerodynamic, thrust reversers, etc) to get the aircraft to stop. Once they enter the EMAS they've destroyed the EMAS pad, and have done significant damage to the aircraft.

  What happens in case of belly landing and when there are burst tyres?

If a belly landing made it all the way to the EMAS, which is unlikely, I doubt that the EMAS will have a significant effect on stopping the aircraft. The EMAS is specifically designed to grab onto things that dig into it. By the time the aircraft slides onto the EMAS, whatever is "hanging" under the aircraft has probably been broken away. I don't know of any case where EMAS encountered a belly landing aircraft (other than one's who have broke off the nose gear), but I'm guessing that the aircraft would slide right over it.

Burst tires on the other hand would probably not reduce the effectiveness of the EMAS since it will still be dragging through the gear.

  Is there any other loading case?

Not that I know of. The system is designed to arrest aircraft overruns. Specifically this means a normal landing but unable to stop on the runway such as in ice, snow, or water. It is not designed as a crash safety system.

  What are the max design loads and velocities?

If you want to read the FAA's Advisory Circular on Engineered Materials for Arresting Systems (EMAS) for Aircraft Overruns it has quite a bit of information:

  The design
  (or critical) aircraft is defined as that aircraft using
  the associated runway that imposes the greatest
  demand upon the EMAS. This is usually, but not
  always, the heaviest/largest aircraft that regularly
  uses the runway. EMAS performance is dependent
  not only on aircraft weight, but landing gear
  configuration and tire pressure. In general, use the
  maximum take-off weight (MTOW) for the design
  aircraft. However, there may be instances where less
  than the MTOW will require a longer EMAS. All
  configurations should be considered in optimizing the
  EMAS design. To the extent practicable, however,
  the EMAS design should consider both the aircraft
  that imposes the greatest demand upon the EMAS
  and the range of aircraft expected to operate on the
  runway.

FAA Advisory Circular AC150-5220 22b, 9.c

And this one:

  Entrance Speed. To the maximum extent
  possible, the EMAS must be designed to decelerate
  the design aircraft expected to use the runway at exit
  speeds of 70 knots without imposing loads that
  exceed the aircraft’s design limits, causing major
  structural damage to the aircraft or imposing
  excessive forces on its occupants.

9.g from the FAA Circular, emphasis mine.


Why is there a pressure drop in the combustion section of a jet engine?

The air moves¹ in direction of decreasing pressure except where forced by the compressor. So the pressure must be decreasing to maintain desired flow. If the pressure was increasing, the flow would stop and reverse and the engine would stop operating.

In fact, that's exactly what happens if fuel is added too fast: the energy quickly raises, but the still slow-spinning turbine provides too much resistance, so pressure will increase above what the compressor can provide and the compressor will stall, the engine will emit loud bang and some flames from both ends and likely flame out as it runs out of oxygen for a moment.

As already explained, in the normal flow the released energy increases the velocity of the flow instead of pressure.

¹ More precisely accelerates. If the pressure was constant, it would keep moving. But it must not raise. It decreases slightly due to friction.


Can a MEI CFI give a valid flight review in a plane for which he is not endorsed?

The flight instructor is limited to give training in the category and class ratings of their pilot and instructor certificates, not by make and model.

Per §61.193 and §61.195 the flight instructor is authorized to train and issue endorsements for a flight review so long she or he holds a pilot certificate and flight instructor certificate with the applicable category and class rating.

Furthermore, per §61.51(e)(3), the flight instructor may log pilot in command flight time for all flight time while serving as the authorized instructor in an operation if the instructor is rated to act as pilot in command of that aircraft.

The instructor is advised according to Advisory Circular: AC 61-98C,  Section 4-2 Paragraph e. Instructor Qualifications:

  Instructors should also consider their own experience and
  qualifications in a given make and model aircraft prior to giving a review in that model... ...To conduct a flight review in a multiengine
  airplane, the instructor must hold an airplane multiengine rating on their pilot and flight
  instructor certificates. For aircraft in which the CFI is not current or with which he or she is not
  familiar, he or she must obtain recent flight experience or sufficient knowledge of aircraft
  limitations, characteristics, and performance before conducting the review. In any case, the CFI
  must observe the rating limitations of § 61.195.

So, yes.  An appropriately rated flight instructor (whether or not they should) is authorized to conduct the flight review.

What's more, even if the instructor is not qualified to act as PIC (e.g. does not have a high-performance training endorsement), that instructor will still log pilot-in-command time as instructor because he or she is appropriately rated to do so.


Not all commercial jets have autobrakes, and the ones that do usually have a company policy dictating when it will be used.

The criteria for activating the brakes are different depending on the manufacturer and model of aircraft, but are typically some combination of:

System must be armed
A certain takeoff power setting must be set by the throttles
A certain airspeed must be obtained
Thrust levers must be pulled back to idle
Aircraft must still be on the ground

Usually for a rejected takeoff it does use max braking, however the autobrake usually has different settings for landing along the lines of "light/medium/max" braking.  Each setting will use a different amount of runway for landing, much like lightly pressing on the brake pedal in your car versus jamming it to the floor.


Why did the Swiss International Air LX40 (a 777-300ER) emergency land at Iqaluit airport?

Given that the 777-300ER could have easily flown the remainder to LA with one engine -- which means it could have easily diverted to a nearby larger airport -- why didn't it?

Standard operating procedure for engine failure on a twin calls for emergency and landing as soon as possible. That was their closest diversion point at the time, so there they landed.

This is an instance of a more general rule that whenever all redundancy on a critical system is lost, it warrants an emergency and requires landing at nearest suitable airport. Continuing the flight on one engine would most likely considered reckless operation.

Regarding your suggested alternate, “somewhere larger like Nuuk” is definitely not true. Nuuk has runway of mere 3,117 ft, far too short for a 777. The 8,605 ft at Iqaluit is adequate, and it is the standard diversion airport for this area.

Plus it was at least 300 nmi further. You are not risking that when your gliding distance from FL360 is ~120 nmi (and there is no way one engine can keep you up in FL360, so it's more like FL300 and ~90 nmi) and you really can't be sure the problem that killed the one engine is not going to affect the other too.

Also, regarding the complicated repairs, there is no better place nearby. It is just as cold in the whole region and there are no heated hangars where a 777 would fit, so they needed the tent to provide reasonable working environment around the engine in the cold weather and would need it in any other airport in the region. That's why they have it.


What does "Where AOC requirements are redundant to AAC requirements, or more detailed, the AOC requirements will be followed." on Part 7 of CAR

It means that if the AOC is more detailed, or contains the same thing as the AAC, then whatever is written in the AOC should be used.


What is communicated between flight crew and cabin crew during flight?

The FAA at least would seem not to have any requirements of communication between flight and cabin crews. However, airlines must have guidance or standard procedures for such communication. One thing I have always heard--but cannot find in any regulation--is a PA announcement to the effect of "cabin crew, prepare for takeoff;" I can only assume that all airlines have adopted this.

This is not to say that communication does not occur; quite the opposite in fact. Cabin and flight crews communicate regularly about all sorts of issues, from upcoming weather to passenger issues and operational concerns. It is only that such communication is not required by regulations (at least not in the United States).


What exactly does it mean, that plane has diverted due to the needs of a passenger?

The only passenger needs that would result in a non-scheduled landing would be a medical emergency or acts/threats of violence of the level that prompt removal of the passenger is needed (with law enforcement coordination).

A passenger forgetting his wallet would not result in a diversion.  An aircraft deviation to enplane more passengers and proceeding to the scheduled destination would be a highly unusual situation and would not be for passenger needs, it would be for operational needs.


Is there an advantage to combining trim function with yoke?

I think you misunderstand what the purpose of trim is, which is to change the attitude of static stability.
The yoke (or stick) is the primary flight control. Virtually all changes in attitude will originate here, and any temporary changes also end here. If you take your hand off, the plane returns to the original attitude.
When you want to make a permanent change, you set the new attitude using the yoke (or stick) and then trim until the required pressure goes away. When you take your hand off the yoke (or stick), the plane remains in the new attitude.
Some planes do have an elevator trim control on the yoke (or stick) to make things easier on the pilot, but this doesn’t change how it is used. Some planes also have rudder and aileron trim controls; I’ve never seen those on a yoke (or stick), but I’m sure someone, somewhere has tried it.


How does an airspeed indicator look from the inside?

The Indicated Airspeed (IAS) is measured based on the difference between the static air pressure (measured via the static port) and the ram air pressure (measured via the pitot tube). A typical GA airspeed indicator looks like this:

(source: Wikimedia)

The difference in the pressures causes the diaphragm to flex. This flexing is then mechanically translated into a rotation of the indicator. See this question for the full details: How does the mechanical linkage of the airspeed indicator convert dynamic pressure to KIAS?


What are the ways to perform air traffic control on long distance flights?

Direct link vs satellite

  There are three major ways of sending control messages to an aircraft:
  Direct radio link [when] in line of sight, satellite, ground stations,
  e.g., tracon.

The way the question starts brings confusion so the many comments. There are two categories of radio links involved in voice/data communication: Between a ground station and the aircraft and via a relay, e.g. a satellite.

Satellites are merely used today for air traffic control, it's a thing of the future (2 to 3 years for tracking the aircraft using ADS-B). However satellites are sometimes used for aircraft communication with the airline technical staff and/or the manufacturers. However the information exchanged here is not ATC related at all: It can be technical, e.g. live ACARS sent to airline technical staff or to the engine manufacturer, to prepare maintenance at the destination airport. It can also be internal exchange with the airline operations. Satellite communication is also used for passenger services like telephone and Internet access aboard the aircraft.
So ATC uses the other kind of links, the direct ones, which means a transmitter on the ground. However these "ground stations" can serve different purposes because ATC is organized so that ATC facilities are specialized.

Simple ATC organization

There are ATCOs on the site who may add details to this answer, but I'll draw the mainlines. In the US, ATC operational facilities are divided into 4 categories:

ATC Tower. Controllers are located in the airfield tower and they control aircraft on the ground (e.g. at their gates, or on the taxiways), aircraft on the runways (landing or taking off), aircraft approaching the airfield to land, aircraft leaving the airfield and climbing to their cruise altitude. The tower controllers have a good sight on all those aircraft.
Tracon (terminal radar approach control). Controllers maybe located in the tower or at other locations. They are in charge of taking over the aircraft that just took off and are now departing. They continue the control that was started by their colleagues of the Tower. They also take care of the descending aircraft before they are handed over to their colleagues of the Tower for landing. As you see, tracon actually takes aircraft from cruise altitude to near the landing runway, and from near the takeoff runway to cruise altitude.
Next, there is the enroute ATC center for controlling aircraft in cruise, between the departing tracon and the arrival tracon. This facility is also known as air route traffic control center (ARTCC). The enroute controllers can be located anywhere, they don't need to see the sky, and usually they don't see it at all, they work in a dark room using only displays and microphones. There are a small number of centers in the US to control the whole US airspace.

This split is shown on this figure:

that is extracted from this question.

The last type of ATC facility is the oceanic ATC which controls an ocean area, because the procedures and tools used are a bit particular. Oceans are not belonging to a particular country, however all countries have agreed to delegate the control to a particular country for a particular area. In the US, for the East coast, New-York oceanic center will control an area, Canada will control another, UK and France will manage the other side of the pond. These centers control areas named OCA, for oceanic control area. See this map for North Atlantic. When the aircraft progress along its route, it is handed over to the next area controller.

This was the first part: We have clarified who are the controllers and ATC facilities, where they are and what phase of flight they control. Now let's see the radio equipment they use.

Radio links

First let's clarify between radio and radar.

Radar is used to determine the position of the aircraft. This can be done everywhere, except over oceans. The reason is radar has a limited range, and there are no radar stations in the ocean (at least not for civil aviation).
Radar stations are located at selected locations, on hills and other high places, to increase their range. some radar are located on airfields too, but that's not a requirement. Actually, all information collected by all radar stations are gathered and distributed to all control centers as a synthetic picture.
Radio is used to exchange voice messages and data. As I wrote, all radio links have a ground station at the center. This ground station is usually at the same location than the controllers. So some ATC radio equipment is at the airfield, some at the enroute centers, etc. For OCA, the equipment can be remote from the controllers, but this is just a technical detail.

Aviation radio for ATC messages can use two types of frequencies:

VHF (very high frequency) which works mostly in line of sight, so the range is something around 50 km to 200 km depending on propagation conditions and location. VHF is used when the aircraft is flying over lands. Controllers at the towers and tracons use VHF, as they deal with aircraft that are either under they eyes (tower) or at a moderate distance from the airfield (tracon). Enroute use them too as they control the part of the sky around the transmitter.
HF (high frequency) works on long distance, like between continents. HF is used by OCA controllers, enroute ATC can also use them. The particularity of HF links is they work somehow erratically, the links can be broken at any time when an aircraft flies over an ocean. The reason is HF waves are reflected by the high ionized atmosphere and the ground, but the degree of ionization varies with time. OCA facilities have procedures to deal with that.
A special word on ATC communication in OCA: Satellites are not used by ATC today. In OCA, ATC communicates with aircraft using HF transmitters located on the ground. When the distance or propagation conditions make the HF communication impossible, ATC (or the aircraft crew) will ask another aircraft to relay the message. Aircraft equipped with satellite phone can also try to contact the OCA center using telephone. If communication is still impossible, then the crew will continue as already cleared by ATC, and after they will continue as indicated in the flight plan which was filed with ATC before the flight. ATC will clear enough airspace around the assumed position of the aircraft to prevent collisions. This is not very different than when there is a loss of communication over land. See section Communications Failure After Entering NAT Region at the end of this document (NAT is the standard North Atlantic route system often used to cross the Atlantic Ocean).

  For an intercontinental flight, how does the control know which ground
  station to use (since it wouldn't make sense to connect all ground
  stations to each airport) for control messages?

I hope you have got a better understanding, and know now this is not the case. Each ATC facility (tower, tracon, enroute center) uses different radio stations and a particular tower controls only the aircraft at this airport, using the local transmitter. The key is that ATC works with pre-agreed procedures to hand off control to the next facility when the aircraft leaves a given control area. An ATC unit controls aircraft in its area, and uses a ground radio station nearby the aircraft. OCA ATC indeed works with aircraft that are remote, and sometimes very far.

  Are the control links (e.g., control-to-ground-station-to-aircraft) to
  communicate with an aircraft predetermined before take-off?

Yes they are, not on a paper, but by the procedures. When the aircraft starts its engines, the crew is in contact with the "ground controller" who is a controller in the tower, specialized in controlling aircraft which aren't on the runways. When the aircraft is ready for takeoff, the ground controller hands over to someone else in the tower who manages the takeoffs (this is "the tower controller"), then after take off the tower hands over to the nearby tracon, then to enroute (there will be several successive enroute centers), etc.

The current controller hands over to the next controller on the aircraft route, when the aircraft is leaving their area of responsibility, this is part of the ATC organization itself.

"Plates" used by crew indicate how to reach a given control facility. For example, on this chart, the frequency of the Denver ARTCC is mentioned explicitly.

Source

This is in case you are lost because this information is normally given by the previous controller prior to handing you over to Denver enroute center:

And here is how to contact the different ATC units at Paris - Charles De Gaulle:

Source

More: ATC Facilities


Why aren't airports built identically (like aircraft)?

1. Terrain and land use

Several new airports are build on landfill island or coast next to mountains, their shape is influenced by the terrain and their size.

Hong Kong International Airport runways and shape are influenced by the terrain. Its curved east bank looks like the Chek Lap Kok island, which the airport is built on there. Both HKIA runways are parallel to the hill of Lantau island, south of the airport island.

HKIA is on the north of Lantau island (largest green shape), the grey colour stands for reclamation land and the green colour is the originally Chek Lap Kok island.

2. Prevailing wind

Prevailing winds influence the runway heading. Since takeoffs and landings are usually done as close to headwind as possible, wind direction generally determines the active runway. If the prevailing wind is uncertain, multiple runways with different directions will be built, Boston Logan airport is a extreme example.

3. Terminal building and bargaining power of airport management

This factor largely influences the terminal layout. In USA the airport management is largely influenced by the airlines, sometimes the terminal buildings are managed by airlines themselves, like New York JFK. The airlines prefer the terminal building which can minimize the taxiing time, passengers transfer time and maximize the perimeter (and hence the number of gates), Atlanta Hartsfield airport is one good example. Its linear shape can minimize the taxiing time and increase the number of gates available.

On the other hand, for main airports outside USA and Europe, the airport owner has strong marginal power of the airport management. They will choose the terminal building which can best fit for their strategy. Some airports owned by the government prefer a massive size of airport as an image of the country/cities; and some airports prefer large commons which can maximize the shopping experience (and hence rental income). Pictured below is Beijing Capital International Airport.

4. Historical factor

Several airports were built before World War II and some even existed before World War I. At that time the airport was just a new concept. A full development and operation methodology for airport had not been developed. Many full airport layouts, which nowadays seem to be "inefficient", were developed. And since the layout with the buildings were built, it is costly and time-consuming for reconstruction, especially at the busiest airports. Airport managers tends to expand the existing building with new gates and other facilities, rather than total reconstruction. Pictured below is a master plan for O'Hare International Airport from 1948.

Heathrow airport is reconstructing Terminals 1-3, replaced by linear buildings like Terminal 5. It will take 5 years to destroy the old Terminal 2 and phase one was just completed last year. Terminal 3 will be rebuilt after 2019 and the whole project will take more than a decade.

5. Airport operation

Operations of the airport are influencing the airport layout. For airports have more flights for both general aviation and airlines, they tend to have more runways, like Dallas DFW and Amsterdam Schiphol. On the other hand, in most parts of the world there are only 1 or 2 runways even for the most busiest airports where most of the flights are from airliners, like Dubai and Hong Kong, ranking for 6th and 10th busiest airport in terms of total passengers.

6. Reversal: Experience

On the other hand, huge number of airports has been built in the half century, several airport design has been well developed. We can see the latest airport may share similar terminal shape, runway plan and airport layout.

Hong Kong international airport and Hamad International airport

Chongqing airport (Far future) and New Lisbon airport proposal

Conclusion

To sum up, there is not one optimal conclusion of airport layout. The airports are built related to physical limitation, historical constraints, stakeholders opinions, and, of course, the architects' designs. No airport will be identical, regardless the layout, terminal shape and style. On the other hand, the experience from airport planner will create a similar airport.


Is there a public NOTAM archive that's searchable by airport?

On Pilotweb, there's a "Federal NOTAM System (FNS)" section on the right side of the screen. If you select "NOTAM Search" it takes you to another NOTAM search tool. If you then select "Archive" from the drop-down list, you can search by date and location.


How do I get an airworthiness certificate for a prototype aircraft?

You can't legally fly any aircraft heavier than an ultralight unless it is certified. Your prototype will be certified as Experimental Category in one of these classes. The certification is awarded based on inspection during construction, on completion, and after a flight test period. The design is not subject to FAA approval.

If you want to manufacture aircraft for sale, you need approval of the design. This is called a Type Certificate. For smaller general aviation aircraft the appropriate regulation is Part 23 which states the requirements. There are also Advisory Circulars which provide clarification on approved means for satisfying the design requirements.

To avoid the expense of inspecting and flight testing every aircraft as is required for the experimental category, manufacturers instead obtain a production certificate. This reference is for larger part 21 aircraft. The production process is approved rather than inspecting each aircraft as it is produced. Each aircraft is still flight tested, but this may take an hour or less as compared to an Experimental flight test, which is 40 hours.

Reading these four references will give you some limited understanding of what you are asking. Here is an 2012 swag at certification costs. Part 23 was estimated at $$25m for design and testing, then another $25m for certification. This does not get you a production certificate.


Since there are no toilets on smaller aircraft, how do you deal with the inevitable?

The other answers are good, but don't touch on one key method...

Prevention.

Small aircraft like a Cessna 172 have a limited cruise time, about 4 hours maximum. They are small enough that they get relatively uncomfortable after 2-3 hours, and if flying in turbulence or challenging conditions, flights can be relatively short.

The best way I've found to avoid the problem is to make sure to use the facilities as close to departure time as possible and to limit my intake of fluids in the hour or so leading up to a flight.

The other option you have is to land and use the "facilities". Many unmanned airports that have terminals (like KMNM) have a keypad access and facilities inside. Airports are pretty common and you usually aren't more than 30 minutes away from an airport with a bathroom.

Just as a personal account of this, I was doing night training at my local field (Class C) and it was late (10:00 PM or so). One gentleman came on the radio and asked the tower if the FBO was open so he could use the facilities. The controller responded that the FBO was closed but he could taxi him to a quiet corner of the airfield for a few minutes. The pilot gladly took him up on the offer.


Experiences with LiFePO4 starter batteries in light aircraft or motorized gliders?

Well, I'll start by saying that (at least to my knowledge, in the USA) there are no certificated/PMA'd/TSO'd LiFePO4 batteries currently available on the market for use in certificated airframes.
That means if you're using a type-certificated aircraft (or an LSA that didn't ship with one originally and for which there is no manufacturer-approved procedure to install one) you're basically out of luck unless you can convince your local FSDO to approve it as a one-off modification to your airframe.

That said, there are some people using them in homebuilts. This guy, for example has used one on his SQ2000 to replace the starter/cranking battery, and there's a company called AeroLithium that sells aviation lithium batteries (albeit uncertified) which some folks seem to be using in (experimental) Carbon Cubs with a good deal of success.

As to the batteries themselves, my aircraft is certificated, I've got no desire to take it experimental, and my relationship with the local FSDO is not one where I could just walk in and say "I want to stick one of these in my plane. Sign off on the 337 form KTHXBYE!" so this is all just theoretical:

Operationally, LiFePO4 is a relatively safe chemistry.
Unlike conventional lead-acid batteries it's not going to spit sulfuric acid on your plane during charging or hard maneuvering.  Unlike other Lithium-based chemistries (say Lithium-Cobalt of Dreamliner fire fame) it's not as prone to blowing up or catching fire if short-circuited or otherwise abused (though puncturing it is still a Bad Thing).
Note however that as LiFePO4 is a lithium based chemistry water may not be the best extinguishing agent for a ruptured/burning battery. One of the potential decomposition products of a burning or ruptured LiFePO4 battery in the presence of water is Hydrogen Fluoride (HF), which is rather nasty stuff.
Handling and Charging is generally normal - as in you can connect it to your aircraft charging system with appropriately-rated breakers/fuses.
Checking cell balance is recommended periodically, but without adequate information on the use of these batteries in aircraft we can't say what "periodically" means -- Every 10 hours? Every 50? Annually? (If installing one of these in a homebuilt I'd start with every 10 hours and work from there).
Robustness should be superior to lead-acid batteries in all respects (mechanical and electrical).
For use in an aircraft a sturdy housing around the cells would be a Must Have to prevent the battery from being ruptured in the event of an incident, but that could be produced reasonably.
Electrically LiFePO4 has a lower self-discharge rate, better tolerance for overcharging, and better (flatter) discharge curves - all Good Things.  It is also stable over a wide range of temperatures, and at least theoretically will outlast lead-acid batteries on a charge/discharge cycle and calendar-year basis (though again, without real-world utilization data we can't really say anything for sure here).
Cost-Benefit is hard to qualify.
LiFePO4 batteries can be had cheaper than "aircraft batteries", largely owing to the fact that they're not approved for use in certificated aircraft.
Because of their higher energy density they're also substantially lighter for the same amp-hour capacity, which is always attractive when we're talking about aircraft applications.
All of those benefits aren't worth a warm bucket of spit to me because I can't install one in my airplane (though I'd certainly consider it seriously if it were a viable option).  If you can install one in your airplane as a replacement for your main battery it may well be worth considering.


How do pilot and copilot operate and coordinate the radio?

Both yokes have PTT buttons. Generally one person is in charge of the communications. In military aircraft that is often the task of the navigator in twin crew airplanes. I would think it generally falls on the pilot not flying the aircraft although both should be listening to the instructions.

The switch is on the front of the yoke (reverse side from what the pilot sees) and is generally triggered with the left index finger. Here is a pic from a yoke for reference. Note the button on the left upper part labeled "mic".

source

I have never seen it (but then again there are many radios I have not seen) but some radios may have a VOX (voice-operated transmit) option which would bypass the PTT. This however would be very irritating on the channels since there are lots of planes on a single frequency talking to the tower.


What is the difference between axial compressor blades and centrifugal compressor blades?

The blades are shaped completely differently. Typically a centrifugal compressor is one solid piece that looks like a turbocharger and an axial compressor is a flat disk that looks like a fan with individually manufactured blades inserted, though axial compressor disks with integral blades "blisk" is starting to be used. Here are compressor pics that show how they are used:

Two stage radial/centrifugal compressor circled (with a three stage axial turbine following)

Fifteen stage axial compressor

A radial compressor can usually generate higher pressure in a single stage so it's usually simpler, while an axial compressor is easier to stack in multiple stages to get much higher pressure ratios.


Are there any safety hazards when testing the F-16's fire control radar on the ground?

Yes, and aircraft equipped with emitters have their arcs clearly indicated in their manuals, for example, take this diagram for the A-7E (in pink, the wider cone coming off the radome, the other areas are engine intake or turbine blade-off hazards):

There are two main ways in which a radar installation can be harmful.

Radar waves

These are the desired emission of a radar and are emitted from the antenna in a well known arc.

Based on this page by the WHO (emphasis mine):

  Military radars are numerous and vary from very large installations, which have large peak (1 MW or greater) and average powers (kW), to small military fire control radars, typically found on aircraft. Large size radars often evoke concern in communities living around them. However, because its power is radiated over a large surface area, the power densities associated with these systems vary between 10 and 100 W/m2 within the site boundary. Outside the site boundary RF field levels are usually unmeasurable without using sophisticated equipment. However, small military fire control radars on aircraft can be hazardous to ground personnel. These units have relatively high average powers (kW) and small area antennas, making it possible to have power densities up to 10 kW/m2. Members of the general public would not be exposed to these emissions because during ground testing of radars access to these areas by all personnel is prohibited. The military also use most other types of radars described below.

The specific risks are listed as:

  Possible health effects
  Most studies conducted to date examined health effects other than cancer. They probed into physiological and thermoregulatory responses, behavioural changes and effects such as the induction of lens opacities (cataracts) and adverse reproductive outcome following acute exposure to relatively high levels of RF fields. There are also a number of studies that report non-thermal effects, where no appreciable rise in temperature can be measured.

  Cancer-related studies: Many epidemiological studies have addressed possible links between exposure to RF and excess risk of cancer. However, because of differences in the design and execution of these studies, their results are difficult to interpret. A number of national and international peer review groups have concluded that there is no clear evidence of links between RF exposure and excess risk of cancer. WHO has also concluded that there is no convincing scientific evidence that exposure to RF shortens the life span of humans, or that RF is an inducer or promoter of cancer. However, further studies are necessary.

  Thermal effects: RF fields have been studied in animals, including primates. The earliest signs of an adverse health consequence, found in animals as the level of RF fields increased, include reduced endurance, aversion of the field and decreased ability to perform mental tasks. These studies also suggest adverse effects may occur in humans subjected to whole body or localized exposure to RF fields sufficient to increase tissue temperatures by greater than 1°C. Possible effects include the induction of eye cataracts, and various physiological and thermoregulatory responses as body temperature increases. These effects are well established and form the scientific basis for restricting occupational and public exposure to RF fields.

  Non-thermal effects: Exposure to RF levels too low to involve heating, (i.e., very low SARs), has been reported by several groups to alter calcium ion mobility, which is responsible for transmitting information in tissue cells. However, these effects are not sufficiently established to provide a basis for restricting human exposure.

Incidental radiation

@Dohn Joe pointed out that a secondary danger of radars, which is actually common to most high frequency energy sources: incidental radiation in the form of high energy photons (X-rays) generated in parallel to the desired radiation. Basically, the process used to generate high frequency electromagnetic waves is not 100% efficient, meaning some energy goes into generating other stuff, like heat, or X-rays. Note that at these energy levels it is often more common to use particle theories instead of waves, since they are the same.

This phenomenon led to a number of health cases in Germany in the 2000s (link in German), when crews who had worked on radar systems in the '60s and '70s without adequate protection developed health complications. Some also passed them on to their children due to genetic damage.

A report dealing with the dosage rates for technicians can be found here. One conclusion that stands out is that the effects and dosage rates were heavily dependent on the position of the operator, since x-rays attenuate when travelling through any media, and while air is not the best shielding material, it does help.

To obtain a rough estimate of how far would this incidental radiation propagate, we can run some numbers. The F-16 has had a number of radars along its service life, but we will use the AN/APG-68 radar introduced in the F-16C/D Block 25 as it is the one with the most readily available information.

X-ray attenuation follows this exponential formula:

$$
I/I_0 = e^{-\mu \rho x}
$$

Where $I/I_0$ is the ratio of energy after attenuation to the original energy, $\mu$ is the linear attenuation coefficient, $\rho$ is the density of the medium and $x$ is the distance traveled. It is important to know the energy of the emitted particles as $\mu$ is different at different energies. Wikipedia gives the peak operating voltage in the Traveling Wave Tube of the AN/APG-68 as 24kV, which should limit the theoretical energy of any emitted photons to 24keV.

Taking $I/I_0 = 1/e$ we can obtain the distance after which the intensity will have been reduced to about 36.7% of the original. Taking $\mu= 0.56585\,cm^2/g$ (from here, interpolating for 25keV) and $\rho = 0.0012\, g/cm^3$, we obtain $x=1472.7\,cm$.

This means that after about 15 meters the intensity will have dropped by almost two thirds, and it will continue doing so for every 15 meters afterwards. This is a conservative value, the real distance should be less due to moisture in the air and a lower median energy in the emitted beam, meaning that even if the equipment was improperly shielded, the risks should be limited to people working directly on the radar.


Where is the first Boeing 747-8 ever built stored at today?

The first 747-8 built was not a protype, but a production unit. The first delivered aircraft in both the versions (747-8i Intercontinenal and 747-8F Freighter) are in service, the Passenger version with Lufthansa and the cargo version with Cargolux.

The first 747-8 customer was Cargolux for the 747-8F version.

The first 747 ever built, 747 RA001 'City of Everett' is in Museum of Flight in Seattle.


What is the altitude required to bypass Russia's airspace restrictions for airlines?

The Kármán line is considered to be the upper limit of airspace and the lower limit of outer space. According to the Outer Space Treaty of 1967 outer space cannot be appropriated by nations (nations cannot claim ownership of parts of outer space).

In that sense, one would be able to fully legally avoid navigation (and other) fees and bureaucracy imposed by any country one was to fly over if it was done above the Kármán line, but since the stunt would require a spacecraft, it hardly would pay off...


When and how can pilots waive wake turbulence holds in the US?

The AIM 7-3-9 lists the required separations. Behind a heavy aircraft, the departure hold is 3 minutes and it may not be waived. Behind a small aircraft, the hold may be waived on pilot request only:

  3. Additionally, appropriate time or distance intervals are provided to departing aircraft:

  (a) Two minutes or the appropriate 4 or 5 mile radar separation when
  takeoff behind a heavy/B757 jet will be:

  (1) From the same threshold.

  (2) On a crossing runway and projected flight paths will cross.

  (3) From the threshold of a parallel runway when staggered ahead of
  that of the adjacent runway by less than 500 feet and when the runways
  are separated by less than 2,500 feet.

  NOTE- Controllers may not reduce or waive these intervals.

  b. A 3-minute interval will be provided when a small aircraft will
  takeoff:

  1. From an intersection on the same runway (same or opposite direction) behind a departing large aircraft,

  2. In the opposite direction on the same runway behind a large aircraft takeoff or low/missed approach.

  NOTE- This 3-minute interval may be waived upon specific pilot
  request.

  c. A 3-minute interval will be provided for all aircraft taking off
  when the operations are as described in subparagraph b1 and 2 above,
  the preceding aircraft is a heavy/B757 jet, and the operations are on
  either the same runway or parallel runways separated by less than
  2,500 feet. Controllers may not reduce or waive this interval.

EDIT: The AIM 4-3-10(f) (intersection takeoffs) also addresses this question, including suggested pilot phraseology for waiving the hold (emphasis mine):

  Controllers are required to separate small aircraft (12,500 pounds or
  less, maximum certificated takeoff weight) departing (same or opposite
  direction) from an intersection behind a large nonheavy aircraft on
  the same runway, by ensuring that at least a 3-minute interval exists
  between the time the preceding large aircraft has taken off and the
  succeeding small aircraft begins takeoff roll. To inform the pilot of
  the required 3-minute hold, the controller will state, “Hold for wake
  turbulence.” If after considering wake turbulence hazards, the pilot
  feels that a lesser time interval is appropriate, the pilot may
  request a waiver to the 3-minute interval. To initiate such a request,
  simply say “Request waiver to 3-minute interval,” or a similar
  statement. Controllers may then issue a takeoff clearance if other
  traffic permits, since the pilot has accepted the responsibility for
  wake turbulence separation.

Wake turbulence in general is considered a key safety issue by the FAA and there is a large amount of information in the AIM, the ATC guide and elsewhere.


Is smart phone's GPS accurate enough to calculate airplane speed?

That landing speed sounds rather high.  There's a regulatory maximum landing speed for airliner type aircraft, and as I recall, it's typically not far from 150 kt (240 or so km/hr).  Since it's unlikely the airplane you were in was actually flying at more than double normal speed at landing (and 180 kph will seem pretty fast to your own senses) I'm inclined to believe there was an error in your measurement.

The GPS in a tablet, reading from inside the airplane, is subject to a number of error factors.  First, even in good reading conditions, the tablet's position is generally only good to (in my experience) around 50 m; second, from inside a metal fuselage, there may be path-length errors, satellite changes, and other errors on top of the accuracy limitation of your device.

This level of positional error can add up to a large percentage error in calculated speed.

The GPS installed in aircraft have better software and can fix location more accurately, as well as more regular position checks which give more accurate speed calculation.  Those instruments make a good check against the pitot/static airspeed system.  A smartphone or tablet GPS, probably not so much.


What United States state and federal agencies generally participate in aerial weather modification?

The North Dakota State Water Commission runs the North Dakota Cloud Modification Project, which primarily seems to work at reducing hail damage to crops during the thunderstorm season, and has been active (perhaps not continuously) since 1975.

They've got a section on program evaluations; a peer-reviewed article published in 1997 states:

  [...] the expanded 13-year NDCMP data base showed a 45% decrease in losses in the target area (Smith, et al., 1997).

Their website has a ton of information; a company called Weather Modification, Inc. is the contractor for ND State and does a lot of other work as well. Either the state or WMI would probably be able to provide additional data.

It may also be worth noting that Russia does lots of weather modification work as well (perhaps more when it was the Soviet Union). No idea how successful any of it is.


How does Bernoulli's principle contribute in lifting airplane?

If I understand correctly you want to know how to get from the first formula to the second one. If that is the case, the answer is simple: you don't :)

The lift formula is a simplified one, which considers the wing as a whole. It doesn't use velocities on different points of the wing, it uses the speed of the airplane. All the factors such as profile, wing shape, etc are included in the lift coefficient. This formula is useful when you already know the behavior of the wing in different situations (the behavior is determined in wind-tunnel tests or with numerical methods).


Will the blade angle of a constant speed propeller be at its least angle (fine pitch) during full throttle and prop full?

Prop full forward means fine pitch, so when the throttle is pushed full open the engine can spool up to its maximum rated RPM and thereby develop maximum power for takeoff. The fine pitch stop is set so as to prevent the engine from overspeeding at full throttle conditions.
With the throttle still wide open, deepening the pitch angle (pulling back on the prop control) will increase the torque load on the engine and cause it to slow down.
With the engine thus slowed down to a lower RPM setting and the throttle still wide open, pulling back on the throttle will cause the prop governor mechanism to decrease the prop pitch, so as to automatically maintain that RPM setting.


What problems do you start run into when an aircraft is disproportionally long?

The question reminds me of the DC-8 Super 61. I always found its fuselage to be absurdly long (picture below, source).

Friction drag

First of all, your volume to surface ratio will suffer. A more slender fuselage will have proportionally more surface area for the same volume than a more stubby one. The first aerodynamic effect, thus, is a higher friction drag relative to fuselage capacity. Below a fineness ratio of five, increasing flow separation will unduly increase drag. Moving from an A340-style fuselage to something more slender, however, will not affect separation.

While volume would need to be held constant in case of airships, the goal for an airliner should be to maintain seating capacity, which can be assumed to be proportional to floor space (within limits - aisle height should not drop below 6 ft). Therefore, the figure of merit should be volume$^⅔$, and the growth in skin friction drag (and structural weight) should be less severe with increasing slenderness of the fuselage. Also, with a longer tail the tail surfaces can be made proportionally smaller, which reduces friction drag a bit, too.

The graph below is from S. Hoerner's Fluid Dynamic Drag, figure 24 of chapter 13, and actually valid for flying boat fuselages. It shows nicely how the drag coefficient grows with increasing fineness ratio; however, since the drag coefficient is based on frontal area, this is slightly misleading.

Hoerner's comments to this graph include an optimum based on volume$^⅔$:

  The optimum ratio with respect to drag on frontal area, seems to be in
  the vicinity of 5. Based on (volume)⅔, a minimum drag coefficient is
  obtained at l/"d" ≈ 9, corresponding to l/b ≈ 15 in the hull family
  tested

Maximum Cruise Speed

When being operated at speeds approaching Mach 1, the more slender fuselage will show a higher value of the drag divergence Mach number, so the second aerodynamic effect of a more slender aircraft is a slightly higher cruise speed, wing aerodynamics permitting.

Again, a graph from Hoerner's Fluid Dynamic Drag (figure 37 of chapter 15) as proof:

Pitch Damping

Third, since the fuselage length will determine the lever arm of the empennage, a more slender fuselage of the same capacity will exhibit a higher pitch damping relative to its static stability. Pitch damping depends on the square of the tail's lever arm while static stability grows only linearly. Note that the tail size needed for the same stability and pitch authority will change inversely to fuselage length, offsetting some of the increase in zero-lift drag of the longer fuselage.

Take-Off and Landing Speed

If the landing gear remains unchanged, a fourth effect could be a lower rotation angle on take-off and pitch attitude on landing in order to avoid a tail strike. This might result in higher take-off and landing speeds.

This answer would be much longer if aeroelastic and structural considerations would be included. I restricted it to aerodynamic factors as requested.


Where can I find detailed dimensions of aircraft panels?

There appears to be some (free) quality materials accessible through the download section of Open Cockpit's Website, including CAD (.dwg) files and drawings such as this 737 Overhead Panel.

(seem to have at least partially answered my own question, but a clear technical drawing if somebody could provide/suggest a source of one would be greatly appreciated)


How do pilots detect fuel contamination in flight?

If fuel contamination is suspected (because of engine control system fault indications, slow engine responses or rapid engine pressure ratio (EPR) fluctuations), there is an associated checklist procedure to followed which can confirm the contamination:

Fuel contamination checklist as posted by "330 skipper" at Nov 24th 2017 13:47Z on Avherald:

  If ENG 1(2) CTL SYS FAULT alert triggered associated with AVOID RAPID THR CHANGES or ENG 1(2) SLOW RESPONSE, and with rapid and
  continuous EPR fluctuation indications on both engines:
  A/THR.........................................................................................................OFF
  If EPR fluctuations stop on both engines: ECAM PROC..........................................................................................APPLY
  A/THR.........................................................................................KEEP
  OFF
  If EPR fluctuations continue on both engines: FUEL CONTAMINATION CONFIRMED Apply condition: If fuel contamination confirmed
  If ENG 1 CTL SYS FAULT and ENG 2 CTL SYS FAULT alerts triggered with AVOID RAPID THR CHANGES or ENG 1(2) SLOW RESPONSE:
  A/THR.....................................................................................................OFF
  FUEL CONTAMINATION CONFIRMED Apply condition: If fuel contamination
  confirmed
  If fuel contamination confirmed: LAND ASAP

In addition, the Flight Crew Operating Manual (FCOM) has the following procedure to be followed if fuel contamination is confirmed:

Special thanks to @mins for providing the images of the procedure and the FCOM


Why do cargo flight companies reposition their crew via passenger airlines?

Reposition is just what it sounds like, to move a crew from one place to another, generally the move occurs in a plane other than the one they are slated to fly.

Both cargo and passenger airlines reposition crews all the time for various reasons. Some but not all reasons are:

Crews are not always required to live in the place they are based. Some crew members may chose to live elsewhere and can generally fly their carrier for free to get to their starting point of the day. Cargo airlines may have deals with passenger airlines to allow this flexibility for their crews as well.
A crew trained on a given airframe may be needed to fly a different plane than the one they just operated.
Commercial crews have duty time limits and at the end of a "shift" they may need to get home if the cargo airline does not keep them wherever they are. On the same note a crew may be needed elsewhere to start a flight.

A cargo airline may not fly as many back-and-fourth routes as a passenger airline so they may utilize airline flights to fill the movement needs of their crew. It is worth mentioning that some cargo aircraft do have limited seating that can be used to re-position a crew.

This cargo pilot describes it quite nicely in her blog:

  Or you expect to operate a flight, but it is changed to a positioning
  flight. A positioning flight is a flight you do for the company as a
  passenger, to get you to a certain destination from where you will
  operate, either straight away or after a layover, or sometimes you
  position after a duty. Positioning flights can be with the company, or
  booked with another airline. You might anticipate returning home on
  Tuesday, but when your block of working days finishes Friday, big
  chance you will get some more flights and will not return earlier than
  Friday. Due to these roster changes, you also might end up in a
  completely different part of the world. So in my suitcase I pack all
  kind of electric adapters, and even if I go to a subtropical
  destination, I still pack jeans and a jacket. I learned this the hard
  way by ending up in Alaska, while having packed for my trip to high
  summer Colombo.

Note: If commercial planes stop flying a fair bit of cargo stops moving any way.


What IFR clearances may a pilot obtain from ATC directly without a pre-filed flight plan with FSS?

The theoretical answer to your question is that you can always get an IFR clearance directly from ATC over the radio, and filing a flight plan in advance isn't required (but it is strongly advised to avoid delays).

In reality, however, you should file in advance if at all possible. This is from the AIM 5-1-8 (my emphasis):

  Instrument flight plans may be submitted to the nearest FSS or ATCT
  either in person or by telephone (or by radio if no other means are
  available). Pilots should file IFR flight plans at least 30 minutes
  prior to estimated time of departure to preclude possible delay in
  receiving a departure clearance from ATC.

The IFR flight plan system is there for good reasons: it helps ATC manage busy airspace and it avoids the need for lengthy radio discussions. If you don't file a flight plan in advance then you'll probably annoy ATC and you may be delayed because they can't take your request directly.

Pop-up clearances by definition don't have a full flight plan filed in advance, but they do have an abbreviated IFR flight plan:

  An authorization by ATC requiring pilots to submit only that
  information needed for the purpose of ATC. It includes only a small
  portion of the usual IFR flight plan information. In certain
  instances, this may be only aircraft identification, location, and
  pilot request. Other information may be requested if needed by ATC for
  separation/control purposes. It is frequently used by aircraft which
  are airborne and desire an instrument approach or by aircraft which
  are on the ground and desire a climb to VFR-on-top.

Weather isn't predictable and a VFR flight may have to become IFR for safety reasons, so there needs to be some flexibility for aircraft that are actually in the air. A pop-up clearance is just a different, simpler way of requesting an IFR clearance; once you have it, you're IFR and operating in exactly the same way that you would if you had filed in advance.

Descending to VFR conditions is an interesting idea but ATC can't clear you to descend below the minimum IFR altitude for the area. You could request the lowest altitude ATC can give you, then cancel IFR if you break out. But descending without a specific altitude restriction seems very risky: the temptation would be to keep going lower to "have a look", and that could end up as a CFIT incident.

Finally, at least in my experience the 30-minute 'rule' is really a guideline to avoid delays. I've never had an issue filing IFR and departing 15 minutes later, but then I'm usually flying between class C or D airports and the airspace isn't particularly busy. A class B airport is probably a different story.


Why does the N1 on a FJ 44-2A turbofan go to 104.4%?

100% is the designed speed - sometimes it can be proven, that slightly more is still safe to operate on. Now: Change all scales, so that the new full power is once again 100% - or allow more than 100% on the scale? As a matter of fact, this is much easier to do, as the software does not have to be changed a lot, dependencies to other components are still valid, the recertification mostly is also easier, ... This also has been done for the Space Shuttle Main engines, routinely operating at >100% nominal ( = design) thrust.


Are nose wheel tyres designed differently from the main wheel tyres?

Wheels on the 747-400 are the same all round.

One operator I worked with had a mixed fleet of -400 and -200s. They used the -200 wheels on the -400 nosewheel (mainwheels had to be -400).


What, if any, equipment on a commercial airliner is (allowed to be) inoperative most of the time?

If a system goes south while sitting at the gate or the ramp, that brings on a cockpit indication, the crew will break out the MEL/CDL using whatever protocol or procedure they follow and if it's not covered, they are grounded until the system is made serviceable again and will take a significant dispatch delay, or worse, a cancellation.

The OEM will try to get as many items covered under the MEL as possible and will continuously try to expand it, because it has a big effect on overall dispatch reliability.  An unreliable system system or subsystem that has a lot of failures will just be maintenance cost issue (and possibly minor delays) if it has MEL coverage, but if it doesn't, it becomes a cost plus dispatch delay/cancellation problem, much more expensive.


Why are pitot-static measurements preferred over other velocimetry techniques?

The linked article says that an HWA (hot-wire anenometer) is

fragile and requires complex equations to relate the sensed value to the fluid speed as well as correct for inherent, unavoidable nonlinearities as well as external factors such as fluid type, ambient temperature, humidity, and angle between the flow and the sensor.

Conditions in a wind tunnel or other lab equipment are far more predictable, and change much more slowly and much less, than those encountered during an outdoor flight through variable weather.
Although industrial variants of the HWA don't expose the fragile sensing wire to damage or dirt accumulation (the dirt as thermal insulation causes the sensor to report incorrectly; rain or ice would still affect an aircraft-mounted one), they still need to correct for density, humidity, temperature, and flow angle.  A four-dimensional lookup table takes a lot of work to create and store.  Outside the lab the pitot tube is more reliable, even if it is more "primitive."
HWAs have been flown, but only as test equipment, not as sensors for primary flight instruments.  For example, this set of HWAs flew on an F-15B to characterize flows in the boundary layer, sampled many thousands of times per second, far more sensitive than a pitot tube.  But between each 30-second measurement interval, the HWAs were retracted inside the airplane for protection.