Could higher aspect ratio actually reduce gliding efficiency of a smaller glider?

With again thanks to other writers who have inputted valuable information, it has been found that excessive aspect ratio, by way of Reynolds Number, can effect gliding efficiency
of a smaller, slower glider negatively as follows:

Reynolds number = Vc/v.  Slower speed and shorter chord reduce Reynolds number. Kinematic viscosity of air v does not change.

Using Airfoil Tools, Coefficient of Lift/Coefficient of Drag at various Alpha was evaluated using a range of Reynolds Numbers from 50,000 to 1,000,000 for the Bleriot Eiffel 428 (thin undercambered)  and the Wortmann FX 60 126 (thicker, less under camber).

It was found that Clift/Cdrag is significantly greater, at higher Reynolds number, particularly in a "sweet spot" around 6 degrees AOA.  In this region, drag seems to "plateau", while Clift continues to rise.

At very low Reynolds Numbers, the Eiffel 428 showed a slightly better Clift/Cdrag ratio, but in ALL cases higher Reynolds number improved Clift/Cdrag ratio up to around 500,000 (will vary depending on air foil type), after which the polars seemed much more consistant.


Does a "climb and maintain" instruction override a "climb via the SID" one?

If the controller issues a "climb via" SID with an interim altitude of 8,000 and then subsequently amends the altitude and states:

"Climb via SID and maintain Flight Level 240" then the altitude crossing restriction to cross INLAND at or above 14,000 still applies.

However, if the controller does not use the words "Climb Via," and instead states:

Climb and maintain Flight Level 240" then the altitude crossing restriction to cross INLAND at or above 14,000 is cancelled.

The words "Climb Via" are essentially the same as "comply with all SID restrictions."  If a new altitude is issued without the "Climb Via" verbiage, then (unless the altitude crossing restrictions are restated) climb unrestricted to the new altitude is expected.

From FAA JO Order 7110.65W, para. 4-5-6 and 4-5-7 (Air Traffic Controller's handbook):

  6.
  When an aircraft has been issued an interim
  altitude  and  after  departure  ATC  can  subsequently
  clear the aircraft to climb to the original top altitude
  published  in  the  SID  instruct  aircraft  to  “climb  via
  SID.” When issuing a new altitude and compliance
  with  published  restrictions  is  still  required  instruct
  aircraft to
  “climb via SID
  except maintain (altitude).”

  PHRASEOLOGY
  −
  CLIMB VIA SID.
  CLIMB VIA SID except maintain (altitude).

  EXAMPLE
  −
  1. (An aircraft was issued the T
  eddd One departure, “climb
  via  SID”  in  the  IFR  departure  clearance.  An  interim
  altitude of 10,000 was issued instead of the published top
  altitude of FL
  230; after departure ATC is able to issue the
  published top altitude): “Climb via SID.”
  NOTE
  −
  In Example 1, the air
  craft will track laterally and vertically
  on the Teddd One departure and initially climb to 10,000;
  Once  re-issued  the  “climb  via”  clearance  the  interim
  altitude is
  cancelled aircraft
   will continue climb to FL230
  while complying with published restrictions.
  EXAMPLE
  −
  2. (Using Example 1, after departure ATC is able to issue
  an altitude higher than the published top altitude):  “Climb
  via SID except maintain flight level two six zero.”
  NOTE
  −
  In Example 2, the air
  craft will track laterally and vertically
  on the Teddd One departure and initially climb to 10,000;
  once issued “climb via” clearance to FL260 the aircraft
  will  continue  climb  while  complying  with  published
  restrictions.

  NOTE

  1.
  Considering the principle that the last ATC clearance
  issued has precedence over the previous, the phraseology
  “maintain  (altitude)”  alone  cancels  previously  issued
  altitude  restrictions,  including  SID/STAR  altitude
  restrictions  unless  they  are  restated  or  modified,  and
  authorizes  an  unrestricted  climb  or  descent.  Speed
  restrictions r
  emain in effect unless
  the controller explicitly
  cancels the speed restrictions.
  2.
  Restate
  “climb/descend via” and then use “except” or
  “except  maintain”  phraseology  to  modify  published
  restrictions  or  assign  a  new  top/bottom  altitude.  Use
  “resume” phraseology with “maintain” to rejoin a route
  and assign a new altitude
  where
  compliance with published
  altitude restrictions is not required.

[all preceding emphasis is mine]

Also see AIM para. 5-2-8.

Lastly, here is some good guidance regarding "Climb/Descend Via"
Climb/Descend Via FAQ


Why don't aircraft (commercial and military) use reflective visor windshields?

Reflective materials cut down the amount of light let into the cockpit, which is useful in very bright sunlight, but would be undesirable at night. At night these materials could potentially blind the pilot, so you would have to make these materials retractable somehow, which would add complexity and weight. Over time these coating would be damaged by airborne debris, smashed bugs, and regular wear and tear. Add this that these coatings are expensive - outfitting them would be a significant cost.

Add to this that ever pilot would have different tinting needs - some people with less sensitive eyesight would need less tinting and others more. It's far cheaper and more effective for people to simply buy sunglasses.


What is the appropriate fuel tank selection to land a 172?

You should check your POH but it may just be that the instructor in question was recently flying a lot of Pipers and it was force of [bad] habit.

This POH for the 172S is in agreement with you.

  BEFORE LANDING

  ....

  Fuel Selector Valve -- BOTH.

You should consult the checklist in the POH that is with the airplane in question to be sure.


How does wing aspect ratio influence lift and drag?

Your suspicion is leading you in the right direction. A lot of what you read on the Internet about induced drag is at least misleading if not blatantly wrong. But it is repeated by sloppy authors who don't ask the right questions.

If you have read stuff like "high AR creates stronger vortices at the tip of wings, which reduce lift", you hit such a completely wrong statement. The misleading version of it is high AR creates weaker vortices. For a hopefully better explanation, please turn to this answer.

Now to your questions:

  How does the aspect ratio influence induced drag, why is the AR in denominator?

The surprising answer: It is the lift per unit of span and the inverse of the square of flight speed which determine induced drag. Another influence is the distribution of lift over span, but aspect ratio does not play a role. See this excellent answer by @DeltaLima for an explanation, or this one for a longer one.

  How does it influence lift?

Indirectly, by increasing the lift curve slope.

  What really happens at the wing tips?

On the top side, suction turns the airflow inwards so the air gets a rotational component and some of it flows around the tip from bottom to top. Due to the small wingtip curvature, this vortex has low pressure at its core (and lets air condense early), but is overall weak and rather insignificant. The real trailing vortex results from a rollup of the wake, but even this does not create drag.

  What happens in the wind tunnel on a wing that spans its whole width?

Please read the answers to this question.


Is the APU classified as a component of Airframe or Powerplant?

I was a tech writer in another life.  Under ATA100/2100, the Air Transport Association system of categorization of systems for maintenance manuals, the APU is Chapter 49 (Airborne Auxiliary Power).
Power Plant chapters are 70 and up. (Some OEMs also use a equivalent numbering convention for engineering drawings to align them to ATA100/2100, where, say, if Flight Controls are Chapter 27, flight controls related drawings would have "27" at the start of the drawing number with the applicable sub-system numbering as appropriate - makes finding drawings in a database quick and easy).
Airframe systems are Chapters 20 thru 50.  So from that standpoint, its Chapter 49 designation makes the APU an airframe system, not a Powerplant (which only refers to propulsion engines per ATA), as far as the industry is concerned, and I think you are pretty safe to go on that.


Have there ever been forward-swept horizontal stabilizers, and what would be its advantages?

Forward sweep is chosen when the designers value resistance against tip stall and a rear location of the wing spar higher than low mass. None of those advantages of a forward swept wing has any weight in the design of a horizontal tail, so a forward swept tail has never been selected.


Is there a checklist for lightning strike?

For the aircrew, at least in the aircraft that I've flown, there is no defined checklist.  You scan and then deal with anything that occurs (inoperative equipment, pressurization issues, whatever you get) with whatever checklists apply: if the lightning knocked out a hydraulic pump, run the checklist for the inop pump, etc.

Once you're on the ground, maintenance has a HUGE checklist to work through, essentially checking every surface of the aircraft for damage and verifying that every piece of equipment still works.  The more that the crew can tell them (i.e. radios & instruments unaffected, no equipment stopped working, pressurization normal, etc), the more of it they can sign off from the crew observation.  But even then, it's still pretty lengthy.


Hydro-pneumatic fuel controls and hydro-mechanical fuel controls

They are actually the same thing. Hydro mechanical fuel controls often include the use of pneumatic pressure to regulate the fuel flow. Here’s an example of a simple one: http://blog.covingtonaircraft.com/wp-content/uploads/2015/08/fuel-control-unit-pt6a.gif

The mechanical part is the input of rpm by a shaft connected to the compressor spool. The pneumatic part is the P3 (compressor exit pressure) input.


How does a vertically higher center of gravity affect handling?

As long as flight is coordinated, the vertical position of the center of gravity (cg) is of minor importance. It will show in issues like thrust changes, when acceleration will cause higher trim changes because of the increased lever arm between the inertial forces and the thrust forces.

This will only change when flight is no longer coordinated and the aircraft sideslips. Now the big cargo volume is more important than the cg location because it will contribute a considerable side force. Also, it will need additional means of lateral stabilisation because the big fairing will destabilise the aircraft in yaw. Still, the lever arm between the cg and the lift force vector will be much, much smaller than the lever arm of the ailerons, and a little aileron deflection will be enough to stabilise the aircraft.

Maneuvering will look funny to crews used to fly the unmodified version of the same airplane, because now a roll manoeuver will induce a lateral motion at the location of the cockpit which was not there before. Same goes for eigenmodes like the Dutch roll which will be more noticeable for the crew because their cockpit sways laterally.

"Flipping over" needs deliberate commands from the pilot; this is no different in aircraft with unusual vertical cg positions. The most extreme is probably the VM-T "Atlant" version of the Russian M-4 Molot bomber.

M-4 Molot version VM-T Atlant with Buran booster tank (picture source)


How high does a jet have to be to hear its radio call?

The formula you're using is correct, but don't forget your own altitude.

$$d = 1,23 × (\sqrt{h_1} + \sqrt{h_2})$$

So in your case:

$$123 = 1.23 × (\sqrt x + \sqrt{1000})$$
$$100 = \sqrt x + 31.623$$
$$\sqrt x = 68.377$$
$$x = 4675\ \mathrm{ft}$$


The "DESCEND VIA" clearance is described in FAA order 7110.65U (pdf) Section 4-5-7 paragraph h, which defines:

  h. Instructions to vertically navigate on a STAR/RNAV STAR/FMSP with published >restrictions.

  PHRASEOLOGY-
  DESCEND VIA (STAR/RNAV STAR/FMSP name and number)
  TERMINAL: DESCEND VIA (STAR/RNAV STAR/FMSP name and number and runway number).

  EXAMPLE-
  “Descend via the Mudde One Arrival.”
  “Cross JCT at flight level two four zero, then descend via the Coast Two >Arrival.”
  TERMINAL: “Descend via the Lendy One Arrival, Runway 22 left.”

This instruction authorizes the pilot to navigate the STAR laterally as published and to navigate vertically to meet all published altitude restrictions.

  Clearance to “descend via” authorizes pilots:

  To vertically and laterally navigate on a STAR/RNAV STAR/FMSP.
  When cleared to a waypoint depicted on a STAR/RNAV STAR/FMSP, to descend from a previously assigned altitude at pilot's discretion to the altitude depicted for that waypoint, and once established on the depicted arrival, to navigate laterally and vertically to meet all published restrictions. ATC is responsible for obstacle clearance when issuing a “descend via” clearance from a previously assigned altitude.

Though speed restrictions are not specifically addressed in the above paragraph, it does note that you will meet "all published restrictions" to include speed restrictions.  This is supported by a later note:

  EXAMPLE-
  “United 454 descend via the Haris One Arrival, except after Bruno, maintain one zero thousand.”

  NOTE-
  The aircraft should track laterally and vertically on the Haris One Arrival and should descend so as to comply with all speed and altitude restrictions until reaching Bruno and then maintain 10,000. Upon reaching 10,000, aircraft should maintain 10,000 until cleared by ATC to continue to descend.

This example and note pertain to modifications to a published altitude, but the note establishes that you comply with all published altitudes and speeds when flying the procedure as published.

Furthermore, InFO 12014 explicitly states:

  Published speed restrictions are always mandatory. Published speed must be complied with regardless of the use of “Climb/Descend Via” clearance.

For the example of the JAIKE 3 Arrival, the clearance

  N1234, Descend VIA the JAIKE THREE ARRIVAL

will be issued if you are already established on the arrival laterally and authorizes you do follow the published route and altitudes.  Assuming the FAK transition is your cleared route when you receive the descend via clearance, you would fly the lateral route

  FAK QBEES VAALI PALEO PEEDS FUBRR BUZIE SPNCR JAIKE ILENE WACKI REGLE SBJ

and you would descend at your discretion to be at 13,000 ft at JAIKE, at 13,000 ft at ILENE, at 11,000 ft at WACKI (and at 250 kts) and 7000 ft at REGLE.  You descend to and cross the fixes at these altitudes because you have been authorized to vertically navigate the STAR via the "DESCEND VIA" clearance.

NOTE: This arrival contains an "EXPECT" altitude and the controller should not issue a DESCEND VIA clearance until you are past this fix unless you gives you specific instructions as in:

  N1234 cross PEEDS at FL240 then descend via the JAIKE 3 arrival

If the controller at any time issues an instruction e.g.

  N1234, Descend and maintain 7000

then:

In the USA

then you would continue to navigate laterally on the STAR but would descend to 7000 ft, disregarding any published altitudes on the STAR.  You are no longer authorized to navigate vertically on the STAR.  If the controller requires you to still cross certain fixes at certain altitudes, they will specify those crossing restrictions in the descent clerance, e.g.

  N1234 Cross WACKI at 11000 250 kts then descend and maintain 7000 ft.

In Cadanda / ICAO rules

you shall continue to meet all published crossing altitudes on your descent to your cleared altitude.  For reference on this difference, see FAA InFO 12003


It's less about the engine and more about the combination of engine and plane. Theoretically, all that is needed is for the engine(s)' thrust without afterburners to be able to overcome the plane's inherent drag at a speed greater than Mach 1. Simple, right?

Well, to do that, you're up against some pretty heavy physics. First and simplest, given a drag coefficient for the aircraft in a "clean" configuration, the drag force increases on the square of velocity. So, to go twice as fast, you need four times the thrust with all other things being equal. Jet engines simply don't scale that way; you need a bigger engine to provide more max thrust, and then all things aren't equal anymore.

Second, planes aren't built like cars, where in many cases there's enough room under the hood to drop in a bigger engine block and a turbocharger to double power for negligible additional weight. Fighter designs have to essentially wrap an aerodynamic skin as tightly as possible around the pilot, engine, avionics and fuel storage. For the longest time, engine efficiency was pretty static, therefore more thrust meant more engine. A bigger engine means a bigger plane, which means more drag, which offsets the thrust gains and thus the speed increase of the larger engine.

Thirdly, there are some complex aerodynamic changes involved as a plane transitions the sound barrier. Airflow into the engines increases at higher speeds, which reduces the compression ratio of the engine; the intake turbines become less and less efficient as speed increases, and thus you get less and less benefit from the "bypass" airflow of the turbofan. Afterburners are more or less a simple way to continue to get more thrust after the intake turbines are no longer significantly compressing air, by taking that air, dumping a bunch of extra fuel in it and riding the prolonged explosion. This, however, uses ten to fifty times the aircraft's cruising fuel flow rate.

The F-22 achieves supercruise in part by extending the efficient operating range of the engines with "variable bypass". At lower airspeeds, engine bypass is increased to allow the turbofans to increase thrust with minimum fuel. At higher airspeeds when the turbofan effect decreases, compressor bypass is reduced to put more air into the combustion chamber (along with more fuel), which increases thrust beyond the normal compression limit more efficiently than simply burning the bypass airflow with an afterburner.

Finally, there is the simple requirement of supercruise in an aircraft's mission profile. Not many things in air combat occur at supersonic speed; you can't dogfight effectively at Mach 1 because your sustainable turn rate is measured in miles, you often can't drop gravity ordinance (bombs) because they won't fall away from the aircraft in a stable or predictable way, and in the case of the Raptor you have to open payload doors to deploy your weapons and that will increase your drag right there.

Supercruise has a singular purpose; to increase the speed of the aircraft's transit to and from the target zone without sacrificing range by dumping fuel into the afterburners at ten to fifty times "military power". For an air superiority fighter (combination interceptor/dogfighter), that's very important, but the F-22 was the first U.S. fighter developed from the ground up to take advantage of the latest technological progress in aircraft design such that it could be achieved. The last time U.S. fighter designers got an opportunity to design a "cost-is-no-object" superfighter was almost 40 years ago with the F-15, and the two major fighter designs to enter U.S. service in the interim, the F-16 and F/A-18, were the result of a design competition specifically intended to create a lower-cost multirole fighter to serve as the bulk of the air fleet.

Other supercruise-capable fighters designed by NATO allies reached production first; the English Electric Lightning interceptor is considered the first supercruise-capable production fighter (predating the coining of that term), and the Dassault Rafale is reported to be supercruise-capable in certain mission-ready configurations up to Mach 1.3, and beat the Raptor to market by about 2 years, though the Raptor has a substantally higher top speed under full military power (100% non-afterburning throttle).


To be more general, a vortex generator is a small angled plate installed on an outer surface of an aerodynamic body. The angle of the plate causes the air to swirl, creating a vortex behind it. This effect allows the air flow to remain "attached" to the surface even at points where the flow without a vortex would separate from the surface.

One of the most common uses of vortex generators is on aircraft wings forward of the ailerons. When the aircraft wing stalls, the flow detaches from the wings. This means that the flow will detach before it reaches the ailerons, making them ineffective. The use of vortex generators helps the ailerons to provide control even if the rest of the wing is stalled.

Source

Of course, these also add drag, even when the wing is not stalling, so they are only added where needed.

They are also used in other places, such as the engine, as in Farhan's comment above, and also on the tail of the 737.


How does wing sweep increase aircraft stability?

Directional stability

When a swept wing is flying in a sideslip, the windward side behaves like a wing with less effective sweep $\varphi_{eff}$ and the leeward side like one with more effective sweep. Wing sweep  causes a flattening of the lift curve slope for two reasons:

The effective angle of attack is reduced by the cosine of the sweep angle.
Only the component of speed normal to the quarter-chord line of the wing is creating lift, so a swept wing creates less lift per area than a straight wing.

The increased lift rolls the aircraft, but also produces increased lift-induced drag which pulls it back to straight flight. The sketch above shows this for the flying wing glider SB-13. This effect is so strong that swept, high-wing configurations need anhedral to keep the sideslip-induced rolling moment down.

For completeness, also the side force $Y$ of fuselage and winglets is added and shows that the winglets help a lot to create directional stability. This is needed in case of the SB-13 because it has a nearly elliptical lift distribution. Using a triangular distribution (N9-M) or even a bell-shaped distribution (Horten flying wings) avoids the need for winglets, but causes higher induced drag in straight flight. Another downside is a low directional stability at high speed, because this sweep effect increases with the lift coefficient on the outer wing.

Longitudinal stability

Wing sweep also helps in longitudinal stability by stretching out the wing lengthwise. This is important for flying wings which lack a separate tail surface. By changing flap angles at the center or the wing tips, the lift at the most forward or most backward sections can be changed for pitch control, and more sweep increases the lever arm of these changes. Also, in swept flying wings natural static stability can be achieved without the use of reflex airfoils, but by applying washout. Again, the higher the sweep angle, the less washout is required.

Too much sweep?

Easily! Sweeping a wing creates lots of problems:

Sweep reduces the lift curve slope and the maximum lift of a wing. The maximum landing attitude with a highly swept, slender wing is severely limited by wing tip clearance, so swept wings need powerful high-lift devices.
Sweep causes the boundary layer to be washed outboard, which will cause nasty stall behavior once a specific ratio of wing aspect ratio and sweep has been exceeded. This can be somewhat limited by wing fences, but should better be avoided altogether.
Sweep changes mean that bending moments will partially get converted to torsion moments, requiring a torsional stiffening of the wing.
For flying wings, sweep will let the aircraft center pitch up and down when the wing flexes. This creates a powerful interaction between the fast period mode (which is only moderately damped in flying wings) with the wing bending mode, resulting in flutter.

Why sweep a wing at all?

Generally, an aircraft designer will allow only as much sweep as necessary.
Wing sweep reduces drag when the aircraft flies with transonic or supersonic speed. Now the Mach effects depend only on the normal speed component, so they are proportional to the cosine of the sweep angle. For the N9-M this was not a factor, however, the B-2 benefits from it with a higher drag divergence Mach number.


Is it okay to take off with weight more than the POH max gross weight?

Can you take off over gross weight? Yes, it's possible on any airplane, dependent on how far over the weight limit you are, the density altitude, how much runway you have, what obstacles are in the area and other factors. It's very easy to go above gross in a Cessna 152 with full fuel and 2 passengers, and probably happens more often than people realize or own up to. That doesn't make it a good idea, which is why pilots are trained to do weight and balance plus performance calculations before every flight.

Besides the safety angle, taking off over gross weight could land you in trouble with the FAA even if nothing goes wrong, you can lose your ticket for that. If there was an incident your insurance may not cover damage or litigation costs.


Here's the history, at least for the US.
When I hired on, way back in 1985, the FAA had printers based on IBM Selectric mechanisms. They had a two-color ribbon, red and black, and a typeball with capital letters in two sizes, plus a few special symbols.
When a strip was printed, color could be selected by the computer for certain items. Callsigns on eastbound aircraft, for instance, were printed in red.
A red route appeared when an aircraft filed a non-preferred route to or from certain airports.
In the US, red is used for a PLANNED route or restriction - something the controller SHOULD do, but hasn't accomplished yet. As an example, if you filed KBFF..KDEN in your C650, the strip would reflect your filed route, but the computer would include a route over LANDR, like this - "KBFF.LANDR1.KDEN" (routing would be over AALLE, today)
That route would be printed in red, to alert the controller, and was the forwarded route. In other words, all subsequent positions would show that route in black, with no reference to the original filed route.
The controller issuing the clearance is expected to use the red coordinated route. If they choose not to (such as for a MEDEVAC), they must update the flight plan to force processing of the original route.
Today, there are no "red routes" per se, if you observe a preplanned route in EDST, it's highlighted in blue. Our strips no longer have two-color capability, so "red" text is now printed in black with a grayscale highlight.
The jargon, however, lives on!
To further address your example, if both a red route and black route exist in the system for a particular aircraft, that means the preferred route has NOT been issued, and some action must be taken. The controller with responsibility for meeting the restriction for that route will issue it, or force the computer to override the red route. Either way, it ensures that subsequent sectors get the right information.


There was a belief that activating de-icing boots too early could result in an ice bridge forming over the boot and rendering it ineffective. This led to a recommendation that de-icing boots should not be activated until a significant build-up of ice had occurred.

(Source: FAA)

Recent testing has established that modern de-icing boots are not susceptible to this problem, and advice from the FAA has been changed.

In InFO9007 (dated 5th May 2009) the specific advice given is:

  Pilots should follow the boot activation and operating procedures
  specified in the AFM or FAA approved operator’s manual for the
  airplane they are flying

And further:

  If the AFM does not have specific boot operation procedures or if ice
  thickness is difficult to judge, pilots should not be reluctant to
  cycle modern boots at the first sign of ice accretion and then as
  needed.

A 'modern boot' is defined in the same publication as having

  less than 1.75 inch diameter tubes, operating pressures of 15 psig or
  greater, and fast inflation/deflation times

It also suggest that any aircraft built later than 1960 is likely to have modern boots.

Essentially, for most cases that you might encounter the video clip is correct, and your class instructor is wrong.

That said, the definitive guide should always be the Pilot's Operating Handbook for the aircraft you are flying.


Can another pilot act as my safety pilot for only part of a flight?

Yes. This is all perfectly in accordance with the regulations.
You personally conducted the IFR flying through IMC, as an instrument rated pilot on a filed IFR flight plan. Check.
You cancelled the IFR flight plan midflight with a plan to continue VFR. Check.
In VMC, your VFR-rated pilot friend assumes the responsibility of acting as safety pilot while you undertake practice IFR approaches in simulated IFR. Check.
When logging the time, you would log the total time as PIC for yourself. Your friend was acting as a necessary crew member, but NOT necessarily as PIC. Nevertheless, his name and certificate number should be logged in your book as well. You would record the portion of your flight that was through IMC as ‘actual IFR time’ and your time under the hood as ‘simulated IFR time’. You would record your practice approaches and count them toward your required six-in-six as long as they were performed according to the required standards.
I found this article at Pilot Workshops that can provide you with more detail.
Clear skies!


What is the reason Light Sport Aircraft (LSA) minimum speed is limited?

The FAA mandates a maximum to the stall speed, not a minimum. The minimum speed must not be higher than a given value.
The minimum speed is limited in order to reduce the risk and severity of accidents, especially during landing. The same was used in FAR 23 where a speed limit of 61 knots was mandated for singe-engined aircraft and multi-engined aircraft of 6000 lbs MTOW or less (§23.49 (6)c).
A slower stall speed translates into a slower approach speed and shorter landing distance. Both help to prevent accidents by giving the pilot more time to get the landing right and come to a stop without crashing. Also, the energy involved is smaller so if something goes wrong the accident is more survivable.


How is Kool-Aid used as a visual cue to create contrast for an off-airport landing?

Kool-Aid is usually brightly colored due to added dye. It comes in powdered form, and you would normally add water to make a solution.

If you have some in your aircraft in a suitable container (perhaps a paper bag), you can drop it on the ground during an overflight of an off-airport landing location. Hopefully the bag will burst on impact. When you return for an actual landing, the brightly colored powder can be a useful visual reference. This is especially important for landing with undistinguished surface cover such as snow.

Doing this successfully will take some practice, because you have to drop the right amount, in such a way that it breaks and spreads on the ground, at a suitable location, from some height that maintains your margins of safety. And if you're doing an off-airport landing in the first place, you probably have some other emergency going on at the same time.


When should a PPL student start learning navigation?

As early as possible.

They say "aviate, navigate, communicate", which puts navigation into a pretty high priority - just behind flying the plane. It is more important than talking on radios.

I cannot see any downside for learning navigation too early beside the natural "memory decay" for acquiring any new material & knowledge. Even basic navigation techniques will benefit your first day of flying (unless you're absolutely not interested in knowing where you are). You can even discuss the route with your instructor during the pre-flight, even if flight planning is not the lesson today. You will not get to know the details, but you will be introduced to aerial navigation.

That, of course, assume that you are learning at a comfortable pace and you are not struggling with the present material.


Why do high-speed water impacts / ditching usually result in structural loss of the airframe?

Plywood sheets can penetrate tree trunks unscathed

The plywood isn't unscathed by the process

  Why wouldn't an airplane [be unscathed by] impacting ocean water at a steep angle at 300-500 mph?

The aircraft will not travel at that same speed underwater. That means that a considerable force is applied to the aircraft to decelerate it in a short distance. That force greatly exceeds what the structure was designed for.


Why is the 737’s aileron/spoiler authority reduced at low flap settings?

Okay, turns out this wasn't so different from the crossover-airspeed question after all!
To quote @PeterKämpf's answer to that question:

how can [an airplane’s ailerons and spoilers] lateral control authority surpass that of the rudder past a certain speed?

This depends on the wing's lift coefficient. At a higher lift coefficient the lower aileron can not add the same amount of lift that it could at a lower lift coefficient. While the raised aileron on the opposite side will still reduce lift locally, the lowered aileron becomes less effective in raising lift as the lift coefficient increases. A higher lift coefficient causes a higher suction peak near the leading edge and puts more stress on the boundary layer, and adding more of the same will become harder as the wing approaches stall conditions.
Another factor is adverse yaw which increases with lift coefficient. This adverse yaw will add to the yawing moment of the hard-over rudder and will increase sideslip, which in turn will produce more rolling moment from dihedral against the aileron effect. As adverse yaw dies down with lower lift coefficient, so does sideslip angle and the ailerons gain control power.

Which also explains why the 737's crossover airspeed is lower with flaps down; when the flaps are up, the aircraft has to fly at a higher angle of attack in order to generate the same amount of lift at the same speed.  This increases the lift coefficient over the entire wing, including the outboard portion where the ailerons are mounted, which makes the downwards-deflected aileron, to use Peter Kämpf's terminology, "less effective in raising lift", and increases the adverse yaw generated by the deflected ailerons.  All of which decreases the roll authority of the ailerons, and, since the spoilers' roll authority can't increase to compensate, decreases the roll authority of the aircraft's lateral controls as a whole.
With the flaps down, on the other hand, the lift coefficient of the inboard wing increases, which means that the rest of the wing doesn't have to generate as much lift, which means that the aircraft can fly at a lower angle of attack for a given speed, which decreases the lift coefficient of the outer, flapless, aileron-containing section of the wing, which increases the effectiveness of the lowered aileron and decreases the adverse yaw generated by the ailerons, which increases the aircraft's roll authority.


Why does the F-111 have a capsule ejection system rather than a conventional system?

The reason the F-111 has an ejection capsule is pretty weird in itself- its there because USN wanted it.

In 1960, USAF asked for an aircraft capable of supersonic flight for 400 miles, and capable of operating from short/unprepared fields and capable of crossing Atlantic unrefuelled through Specific Operational Requirement (SOR) 183. At the same time, USN was trying to develop a fleet defender for its carrier groups which had high loiter time (> 6hrs) and can detect/launch missiles at incoming bombers and sea skimming missiles at over 100 miles. To this extent, the Douglas F6D Missileer was proposed, which got cancelled in 1961.

Unfortunately for USAF (and lot of others), US SecDef Robert McNamara decided that both these requirements could be met by the same aircraft- actually, the only things they agreed about were that the aircraft would have swing-wing, have two seats and have two engines.

The USAF wanted tandem seating, while the USN wanted a side-by-side seating for improving crew co-ordination sharing the large radar display to launch missiles (like the A-6 Intruder, which had side by side seating, but conventional ejection seats). The reason the ejection capsule was included was that USN believed that it would give better chances of survival in case of ejection over sea (where it was to function as a life raft) and in high speed ejection. The USAF hated this as it increased the weight by over 500 lb, affecting performance.

In fact, the Boeing proposal included conventional ejections seats, which was required to be changed into a capsule (and was selected before McNamara intervened and selected the other one). In the end, USN escaped from the project (ironically due to weight and performance issues), while the USAF and others got stuck with the ejection capsule, which they never wanted in the first place.


How do you find the difference in degrees between two headings?

Figured it out.

LH turn: [origin hdg] - [destination hdg] (if less than 0, add 360)

RH turn: [destination hdg] - [origin hdg] (if less than 0, add 360)

Edit: for the acute angle between two headings you would just take the lower number between these two values.   There is no different formula.   This is alluded to in this SO post.

By the way, this is not really useful for normal pilots because you can just visualize a compass rose and figure out the different angles pretty easily but I am making a program that will make flying holding patterns a lot easier.   You input the inbound leg course (not radial), the wind (direction, velocity), your true airspeed, and whether you are making a left or right hand turn.   The program calculates what heading you should turn to on the outbound leg and the heading you should then turn to on the inbound leg (assuming standard rate turn).   It also gives you the time to fly on the outbound leg after you reach your heading for that leg.   It even takes into account turning from/to different crab angles (which would make the turns more or less than 180 degrees).


what is the mechanism of hydro-mechanical system in power control unit that uses follow up feedback?

There is a position control loop in there that opens the servo valve until the required position is reached, then closes it. The loop uses position of the actuator as a feedback signal, measured by a position transducer, usually an LVDT. Open loop servo valves just generate an actuator velocity, not very desirable when flying a plane.

A more robust system used on aircraft has the hydraulic actuator moving together with the flight control surface. The pilot control stick is connected to the servo valve, and when he deflects the stick he opens the valve and oil starts to flow. This makes the flight control surface deflect. The actuator moves with it, in doing so closing the valve again.

You mention half powered controls in a comment - I assume you mean boosted flight controls, where the pilot directly deflects the control surface but hydraulics help in overcoming the aerodynamic forces. These work in the same way as described above, with a follow up mechanism that moves the actuator until the valve closes, but the mechanical linkage is different.

Here is a schematic diagram of a hydraulic actuation system with a mechanical servo valve:

You can see that the cylinder is anchored to the airplane structure. The diagram is a bit primitive, but the gist of it is:

The elevator is at rest, not moving.
The pilot gives a control input, which opens the servo valve (the little box on top of the cylinder).
The piston deflects because the servo valve is open.
The pilot input linkage is attached to the piston as well. Because of the movement of the piston, the servo valve closes again.

This mechanical feedback system is robust because there are very few electronics that can fail. In this set-up, elevator deflection velocity is proportional to stick deflection. Other transfer functions are possible bit programmed feedback loops and transducers.

This system is fully irreversible: airforces on the elevator push against the cylinder piston and are not fed back to the mechanical linkage at all.


Why are aircraft turbine blades made of nickel alloys?

Not all turbine blades are made of nickel alloy, some blades are made of such family of alloys but clearly not all. Some blades are actually made of ceramic materials.

The turbine is always located after combustion chamber and the temperature which the first stage of turbine blades is able to resist is a proof of high technology engine. Those blades are exposed over to 2000ºC; the melting point of steel is around 1100ºC, and nickel can go much further up to 1700ºC. Ceramic can go even further. Note that there is also a significant difference between external flow temperature and surface temperature.

Using a high melting point material is one way to solve the problem, there are also ways to reduce the effective temperature over the surface of the metal like using film cooling.

In the figure I introduced above you can see holes in the blade, where "cooling air" (maybe at 400ºC) flows out of the blade creating effectively a film that goes around the blade protecting (cooling) it from external temperature.

However a metal exposed at a high temperature (below the melting point) is not behaving in the same way as at normal temperature. There are some phenomena affecting the materials like creep. When a material is exposed during long term to high temperatures close to the melting point and high stress at the same time (this is important) it starts suffering deformations that lead to a failure of the metal.

So nickel alloys, apart for having a higher melting point are also having one of the best behaviours in creep environments.

The last stages of the turbine rotors, and more easily stators (less demanded in stress) are less exposed to creep conditions and are not needed to be manufactured in a nickel base alloy.

Just to add as well, in order to reduce creep another approach followed (combined with previous) is to solidify the metal ensuring having a single crystal metal (instead of being a polycrystalline metal). This is utmost art of material engineering (from my point of view).


Help reading a FAA departure procedure chart

You have a good grasp on departures so far.
This particular SID advises the pilot departing Runway 3/21 to turn heading 335° and expect Radar Vectors to the GBN VORTAC. This is normal procedure considering the fact that so many airports in the same area share SIDs. Not all airports will have a co-located Navaid from which to start a SID. In this case, ATC would vector the aircraft back around the Phoenix airspace to GBN (Southwest) since the original 335° heading would be taking the aircraft away from GBN (due True North). Depending on traffic and ATC availability, ATC might vector you to the 249 Radial between GBN and JUDTH.
On other SIDs, there will be multiple fixes from which to leave the SID and transition to the en route portion of your flight. You, and/or ATC would file/assign for the fix most directly on your route of flight. In this case, there is only one, MOHAK.


How can I get flight destination from ADS-B?

If you have the call sign, or Air Carrier flight number, you can track flights with http://flightaware.com. Air Carriers are required to file IFR flight plans so the destination is in the system and trackable. Part 91 (general aviation) and Part 135 (non-scheduled for hire) IFR flights also file a flight plan and their origin and destination are trackable. Most Part 135 flights are on an IFR flight plan but most Part 91 flights are not. However, Operators can block the release of information so FlightAware may not have the info for some aircraft. The information is also available if Part 91 fliers are receiving flight following. If a Part 91 operator is not in contact with ATC the information will not appear in FlightAware.

FlightAware gets the info from the FAA, so there must be an API for it but I don’t know anything about it. “Flightradar24 has a network of more than 3,000 ADS-B receivers around the world that receives plane and flight information from aircraft with ADS-B transponders and sends this information to our servers.” It also gets data directly from the FAA radar with a 5 minute delay.


Where can I find documentation that state sales tax was paid in the 1980s when an aircraft was purchased?

This is a state level issue (here in the US) and you will have to contact the state in question. While aircraft are registered federally the sales tax is state imposed (and collected). If the family does not have the documentation you may have to go through some hoops to get it. Its also important to add what state you are in as that may change the answer to the question.

Furthermore depending on how you are setting up the new partnership there are tax implications to that as well. You should talk to a lawyer about the various ways this can impact things. Unfortunately in the eyes of the state you had no stake in the plane (despite your informal ownership, the state is only worried about formal ownership).


If engine rotation was in reverse would it result in some kind of thrust reverser?

The propellers are like airfoils. Just as the airfoils do not produce useful lift if flown backwards, the propellers don't produce thrust if spun in opposite direction.

Source: www.quora.com

That being said, it is possible to obtain reverse thrust from propellers by setting a negative pitch, similar to the airfoil being flown upside down.

Source: 12charlie.com


What does a fuselage frame structure of a non-pressurized transport aircraft look like?

A pressurized aircraft structure differs from non-pressurised in that the fuselage center area, including the cabin and baggage compartments, are built as a pressure vessel.

Except for:

strengthened frame and windows
forward and aft pressure bulkheads
pressurization systems

the aircraft structure and design not related to the pressure vessel (like the floor) remain unchanged.

There are many examples of non-transport civil aircraft that have pressurised and non-pressurized versions.  For example, a Beechcraft "P-Baron" or a Cessna P210 Pressurized Centurion has smaller windows and different cabin doors you can easily observe.  Yet, the design principles of the aircraft structure, besides beefing up the pressurized areas remain the same.

Most non-pressurized transport aircraft are utility or military in nature, as is the DeHavilland aircraft you mentioned.  Design considerations for these aircraft, like the floor you mentioned, are related to their mission, not pressurization.  That is, unless the floor itself is a pressure vessel wall.


Can UHMWP be used as composites for aircraft impact susceptible areas?

Short answer: Yes, when used properly. It does have its limits (low melting point, low adhesion with epoxy), but if you design around those limits it helps to raise energy absorption considerably.
Long answer: I've used Spectra for improving the crashworthiness of glider cockpits in a study. That was back in 1987-88 when that stuff was brand new.
I molded models of forward fuselages in glassfiber-epoxy as a baseline and then added more models using graphite, aramide and Spectra woven fibers. UHMWP is great in tension but poor in compression, so using it all by itself for the fuselage cannot be recommended. I used 100% Spectra layups only for the seat pan of the models but covered it on both sides with glassfiber or graphite fiber for improved compression and shear strength when using it for the fuselage shell.
Next, I set up a dynamic and a static test. The dynamic test used a swing to let the models crash into an angled, greased steel plate while the static test used a hydraulic testing machine which slowly lowered a steel plate on those poor, unsuspecting models until they were completely crushed. Using a video camera and strain gages, I could prove that the quasi-static test delivered equivalent results to the dynamic test (since the speed of sound in the material is much higher than the impact speed), so the much easier to measure static test could be used for the majority of the tests.

Left: Quasi-static test in progress, right: Model mounted on the swing. Own work.
While the glassfiber and carbon baselines produced a strong force spike at the beginning of the impact and then developed cracks which greatly reduced the stiffness and impact energy absorption, the Spectra samples would not crack in the same way: The UHMWP fibers prevented those cracks. Instead, the epoxy matrix crumbled over large areas and the cover layer delaminated, producing a nice, continuously high force over the duration of the impact. I could improve energy absorption by 200%.

Lineup of some models (after the test). On the far right is a graphite shell with an aramide seat pan.
Those strain gages you see in the pictures were used to calibrate a FEM model using ADINA, a nonlinear code which allows for large displacements. Running the FEM model on a Microvax took days but produced very convincing results after I had measured the material properties of the materials employed in the models. Never use manufacturer-supplied data but gather your own!
The results were published in the Journal of Aircraft.


Why would a C-17 Globemaster be painted in Qatar Airways livery?

It is a military C-17 of the Qatar Air Force, one of the four in operation.

It is in this livery as it is being operated by the Qatar Amiri Flight, a VIP airline owned and operated by the government of Qatar. The vast majority of the Qatar Amiri Flight fleet is painted in the standard livery of Qatar Airways, like this A340 below.

The aircraft is only operated by the Qatar Amiri Flight, not owned by it. The picture below shows the Qatar Air Force roundel on the underside of the wing.

Source: www.youtube.com


What is the purpose of a reverse-flow orientation for turbo-prop aircraft?

Are you talking about the reverse flow of the combustor or the reverse flow of a PT-6 installed in a tractor configuration (puller)?

The reverse flow design of a PT-6 using a power turbine separate from the gas core offers several advantages:

simplified gas core and power turbine without the additional complexities of nested spools.

modular gas core, power turbine, propeller gearbox and accessory gearbox for ease of maintenance.
design facilitates ease of installation in a pusher configuration e.g. Beech Starship, Piaggio P180 Avanti without complicated and heavy propeller drive trains.
Engine exhaust from a pusher configuration additionally provides propeller deicing without additional electrical or chemical deicing boots.
the reverse flow of the tractor layout allows for additional FOD protection vis a vis an intertial separator prior to air entering the first stage of the compressor.  This makes the engine more adept to operating from unpaved airstrips or ones covered with rocks, ice or other debris.
separate gas core and power sections reduce spool inertia in the gas core, making the core - and the engine - more responsive to throttle inputs from the flight crew or autothrottles.


How dissimilar are redundant flight control computers?

As far as Airbus is concerned:

Each unit is composed of two dissimilar boards, one driving the output and the other checking it. Dissimilar means both different CPUs and chipsets (A320 uses i386 (Intel) and m68k (Motorola); newer models use different combinations, basically whatever was widely used at the time they were designed) and software written by two independent teams.
There are fail-overs, two or three depending on the system (IIRC the unit reading the side-sticks is the only one with four copies).
The two main axes, pitch and roll, are controlled by two different systems. ELAC controls elevator and ailerons, SEC controls horizontal stabilizer and spoilers. This is two completely independent chains including different actual control surfaces except for the side-sticks.
A320 has (hydro)mechanical backup for pitch via the trim wheel and yaw via the pedals, utilizing yaw-roll coupling for roll. This works even with complete electrical failure. IIRC the backups on newer models don't though (because complete electrical failure has never happened).


Is this plane safe to fly on? The turbine blades look damaged

Yes, it is safe. Prior to every flight, ground technicians and one of the pilots will complete a pre-flight walk around of the aircraft, which includes checking the engines thoroughly. Any issues with the turbine blades would be noticed at this point and the flight would not depart.


From page 5-1-7, contractions in the AIM:

  RVRT . . . . . . . . . RVR Touchdown

The full list is:

  RVR . . . . . . . . . . Runway Visual Range
  RVRM . . . . . . . .  RVR Midpoint
  RVRR . . . . . . . . . RVR Rollout
  RVRT . . . . . . . . . RVR Touchdown

On some runways there are multiple RVR sensors and different points along the runway, and these are the names given to identify the different locations.


Does Daylight Saving Time affect the flight time?

It will affect scheduled flight times the same way it will affect your favorite TV show's air time.

i.e. it won't*

It will affect apparent flight duration if the departure and arrival airports switch to DST on different days of the year, but actual time in the air will remain the same.

* The caveat for that is if you happen to be in an area that A) does not switch to DST and B) your favorite TV show happens to be broadcast from an area that does switch to DST. (This situation happened every year in Indiana for many years until IN joined the 19th** century and switched to DST in the summer.)

** Yes I know that DST was a 20th century invention, it's just that Indiana is a bit behind the times...


Do ramjets use variable geometry supersonic inlets?

Why is that the ramjets do not require [variable geometry] to maximize pressure recovery?

Turbofans, in aircraft that have variable geometry intakes, such as in an F-111 or F15, operate from standstill, at the start of the runway, to about Mach 2.5. That means, they go from having no shock, to having quite a large (strong) shock. The intake needs to provide the about the same mass flow to the engine, regardless of the forward speed of the aircraft. And, at high speed, it doesn’t want to spill excess air, because that creates spillage drag, especially when the intake has sharp lips, need to reduce form drag at high speed. But, at low forward speed, it needs a bigger intake area, as the forward motion of the aircraft isn’t providing a lot of air. So, the intake has conflicting constraints. It needs a small area at high speed, but a large area at low speed. It also needs to form an oblique shock at high speed, as an oblique shock has less total pressure loss than a normal shock wave. The oblique shock wave needs to be kept out of the intake, hence typically a spike protrudes forward of the intake. The spike moves forward as the aircraft speed increases, and the geometry is made such that this spike reduces the intake area as it translates forward.

Now, consider a RAM jet. It typically operates from Mach 3 to 6, say. So, it’s going from a speed at which shocks exist, to a speed at which shocks still exist. It might still be an operating range of 6-3=3 Mach, compared with 2.5-0=2.5 Mach, so not much different. But, in terms of the change in fluid behaviour of the air, 0 to Mach 2.5 is a much much larger change in behaviour than Mach 3 to 6.

For example, the angle of an oblique shock at Mach 2.5 is 23.6 degrees. Mach 3 is 19.5 degrees and Mach 6 is 9.6. (See formula here at NASA). So, between zero speed an Mach 2.5, the shock changes from not being there at all, to starting as a normal shock of 90 degrees, to having an inside angle of 23.6 degrees (a change of 90-23.6=66.4 degrees). In comparison, for the RAM jet, the shock only changes angle by 19.5-9.6=9.9 degrees. It’s pretty easy to accommodate a change in shock angle of only 9.9 degrees with the same geometry.


Is the FAA expected to issue more regulations soon on drone usage in rural areas?

I'm not sure if this is really an aviation question because you're asking about how and when Federal agencies make regulations, and you might get a better answer to that on law.SE.
But having said that, agencies are required to publish their rulemaking plans twice a year. You can find them in the "unified agenda" for that agency. If you look at the Department of Transportation agenda there are currently six FAA rules that include "unmanned aircraft" in their title. Four are already final, the other two are:

Safe and Secure Operations of Small Unmanned Aircraft Systems
Prohibit or Restrict the Operation of an Unmanned Aircraft in Close Proximity to a Fixed Site Facility

I haven't read them and I have no idea about the timeline for them or if they include anything about rural operations specifically.


The problem with working together on a global scale or even trans-Atlantic is the long time and distance between. Flow control at least in the U.S. (and I'm sure elsewhere) is more short-term (1-2 hours out), mainly based around airport acceptance rates (AAR) which can change hourly or even more frequently for example because of runway configuration changes, or weather or visibility changes. There is also short-term projected sector saturation issues, and short-term sudden localized things like weather buildups that can come and go.

That means for trans-oceanic flights you can't effectively implement departure metering (the main tool in the U.S. for flow control) or ground stops because you can't predict 5-10 hours in advance the traffic level and acceptance rate at the destination airport. It's much more effective to wait until they're back in radar coverage and closer time-wise to the destination when flow control calculations will be more accurate, and controllers can achieve miles-in-trail spacing requirements with quick radar vectoring. Over the ocean, outside of radar coverage, significantly adjusting in-trail spacing is more difficult because you really only have speed adjustments which are limited because of the narrow aircraft envelopes at high altitudes.

For situations where there will need to be long-term flow control (such as, the only other runway is suddenly closed and will be indefinitely), we already do have a manual process to implement flow restrictions trans-oceanic, by each facility passing restrictions to adjacent facilities, on and on up the chain.  For example, Shanwick can tell Gander we need 10 minutes all aircraft all tracks landing EGLL, Gander tells Boston 12 minutes in trail, Boston coordinates with U.S. command center, etc to ensure Boston doesn't get overwhelmed with EGLL-bound aircraft. Those kinds of things happen very rarely though, so it's probably not worth connecting our different automation/flow control systems together and instead just keep handling it manually on a case by case basis.

(Source: I'm a former FAA controller and worked in flow control/Traffic Management Unit for several years).


For the 747 it does exist. It's called body gear steering. The two center bogies/trucks rotate when the nose landing gear exceeds 20° and the speed "decreases through 15 knots."

Both conditions would not exist during takeoff, and thus the system is locked. The 777 also employs body gear steering, as do many large aircraft. Such as the C-5 Galaxy. As for the An-225, someone says the aft 3 axles, but I couldn't corroborate it, but it is likely.

  Body gear steering operates when the nose wheel steering angle exceeds 20 degrees. This reduces tire scrubbing and lets the airplane turn in a minimum radius. Body gear steering is activated when ground speed decreases through 15 knots. As speed increases through 20 knots, the body gear is centered and body gear steering is deactivated (747-400 FCOM).

The system reduces the asymmetric thrust needed for tight turns, and reduces tire scrubbing.

Here's the 777's via YouTube:


Why does the Piper Cherokee (PA-28-140) engine have such low horsepower despite the very large displacement?

Aviation engines run at near max RPM through out the flight. A car on the other hand doesn't use the full RPM spectrum except in bursts.1

If a car engine was utilized the same way an aviation engine is, it won't last long.

So an aviation engine is sturdier, heavier, and weaker (hp) for the same displacement, but also provides higher torque (big cylinders).2

Formula One engines have small displacement, very light weight, yet deliver near a 1,000 hp. But they don't last long either. Something like 15 hours of racing and practice sessions.

Further reading: Do Car Engines Make Good Airplane Engines?

1 Piston planes run at near max RPM because they don't have and don't need gearboxes.

2 Same era alloys and technology.


Can a CFI whose instrument rating isn't valid legally offer instrument instruction?

if a CFII is not IFR current, they can still offer training on a simulator, or offer training in IFR simulated conditions (e.g. hood time) while the CFII is flying in VFR conditions


Can anyone tell what kind or even which airplane this is?

It may be an A318 with cabine plan like this one for Avianca:

My guesses are:

the emergency light line stops around row 6, with the (red?) curtains around row 4.
the seats arrangement is approximately 3 rows of business class then economy class.
best guess would be 2x3 seats per row, with central screen every three row.
cockpit door is slightly not centered to the left

So it can fit with this type of plane.

Sorry,  I can't guess the company. The curtains seems red, but the cabine does not fit with AirFrance (and Hop) neither British Airways.

EDIT

According to the video posted by @Florian in comments, Avianca is a good guess for the company.
Following the comment of @jean, and considering the shadows on the seats, it may be taken during daylight (in flight or on the ground) but for sure not during night.


Should I put yesterday's date for destination date/time on a flight plan if the flight is to go east bound across Zulu +-12 time zone?

When you file a flight plan, you don't enter an arrival date or time. You just enter proposed UTC departure time and estimated length of time enroute. For aircraft or pilot logbooks we only use UTC date and time for departure, and UTC date and time for arrival. The local arrival date or time does not matter. The international date line has no bearing on UTC date or time.


Is there a technical name for a several mile stretch of flight before landing?

The best term to describe it would be the 'final approach segment', which is defined by ICAO as:

  This is the segment in which alignment and descent for landing are made. Final approach may be made to a runway for a straight-in landing, or to an aerodrome for a visual manoeuvre.

The FAA defines the same as:

  The FAS begins at the PFAF and ends at the landing threshold point (LTP).  The FAS is typically aligned with the runway centerline extended. The segment OEA is divided into the FAS OCS and the visual segment obstacle identification surface (OIS).

Note: Both Eurocontrol (which refers to the ICAO document) and Jeppesen define it only for instrument approaches.


In a piston aircraft engine, how much heat must be removed by the oil cooler?

According to Fundamentals of Powerplants for Aircraft, quoted in Mike Busch on Engines (chapter 2), "Other thermal losses" account for 12.2% of fuel energy:

Conduction to air: 7.2%
Conduction to oil: 1.6%
Radiation and misc: 3.4%

The other two areas of loss are exhaust (51.6%) and mechanical (36.2%).

I don't know how "cylinder heat dissipation" would fit into this classification (some combination of air conduction and friction losses?), but looking only at conduction losses gives a ratio of air to oil of 4.5:1.


Why 360 degrees is in the top instead of 90 degrees in aviation?

Conventionally, geographical maps are shown with the North up and North is also by convention 0 degrees (or 360), which is equal to the compass heading of North. The angle increases clockwise, with 90 degrees being East. This is not specific to aviation; in fact during flight the preferred presentation is to have the map oriented such that the direction of flight is upward.

The mathematical plane you seem to describe on the left has the primary axis as the angular datum (also by convention, more info here and here). So the positive x-axis is 0 degrees. The angle increases counterclockwise, so the positive y-axis is 90 degrees.


I just flew into class B airspace without a clearance, what should I do now?

While still in the airspace, you should contact the controller if you can, since it may be important to safety.

After landing, you may get the dreaded phone number from a controller (probably tower or ground) which you're supposed to call and speak with someone from the FAA. You should not volunteer any information about the incident during this call without talking to a lawyer or AOPA legal services first.

Then, most importantly, fill out a report in the Aviation Safety Reporting System. Any information you submit will not be used against you in an enforcement action by the FAA, and in many cases, people who volunteer information through the system receive much softer treatment, and sometimes forgo enforcement all together.

The reason the FAA incentivizes this self-reporting is so they can better collect information about accidents which inevitably happen. If they know why you accidentally strayed into the bravo, maybe they can improve procedures or charts to help keep other pilots from making the same mistakes.


Why is mist abbreviated BR on METARs and TAFs?

According to a CFI's lesson plan on the internet, it comes from the French word for mist, brume.


What is allowed with the new EASA SPL license?

I know very little about this, but it looks like you can fly commercially with an SPL if you pass an additional checkride. This is from the UK CAA, and the same wording is quoted on a bunch of European gliding organization sites:

  You cannot operate commercially, or receive any kind of payment, until
  you:

  Are 18 years of age or over
  Have completed 75 hours of flight time or 200 launches, after your licence has been issued
  Have passed a proficiency check with an examiner

EASA's Part-FCL guidance defines what the additional checkride includes:

  AMC1 FCL.205.S(b) SPL — Privileges and conditions

  CONTENTS OF THE PROFICIENCY CHECK FOR THE EXTENSION OF SPL PRIVILEGES TO EXERCISE COMMERCIAL PRIVILEGES ON A SAILPLANE

Note that it calls it an "extension" of SPL privileges, which strongly implies that an SPL by itself doesn't allow commercial privileges. There is one exception, which is that an instructor can be paid to instruct with just an SPL. The CAA site says:

  The exception to this is for holders of a SPL with instructor or
  examiner privileges, who can be paid for the following:

  Flight instruction
  Conducting skill tests and proficiency checks

  for the LAPL (S) or SPL and any associated ratings and endorsements.


Would cone shapes grills in front of jet engines prevent damage in case of a bird strike?

No, that wouldn't work.

Kinetic energy is measured using the formula $\tfrac12mv^2$ . This means that the kinetic energy of a 4.5kg goose at 170 knots is about the same as a motorcycle at 30mph. At 400 knots that goose's kinetic energy is the same as a small car at 30mph. Remember that velocity in this case is not the speed of the goose but the speed of the airplane hitting it, any grill would have to be immensely strong to survive that kind of impact, meaning it would have to be thick and heavy. This will block airflow to the engine, which is a big problem. If the heavy, hard metal screen fails it will get sucked into the engine and cause far more damage than a soft, squishy bird.

One method for the mass production of french fries is to shoot potatoes at a mesh screen at high speed using a water cannon, when the potato hits the screen its speed forces it through, slicing the potato perfectly in a fraction of a second. A similar thing is going to happen to a bird which hits a screen in front of the engine, rather than being deflected it will simply be sliced and ingested into the engine anyway, or it could potentially be caught on the mesh of the grill and impede airflow further.


Can a U.S. President have a non-government owned aircraft that would routinely be given the call sign AF1?

Air Force One is the call sign of whatever USAF aircraft (see @slookabill's comment below) the US President travels in. Once he boards another aircraft, the call sign AF1 is transferred to that aircraft (for a dramatic example of this, see the climax of Air Force One movie).

If a president were to fly in their own aircraft (or commercially) the appropriate ATC call sign would be Executive One.

As far as fitting out a personal aircraft, the VC-25s are USAF assets (operated by 89 AW) and are highly customised and specifically configured for the job of transporting the president (and support staff), costing over $300 million. I doubt any president is going to get the option of flying their favorite aircraft on a regular basis instead of the state aircraft.

It is not likely that the government would pay to configure another (non-USAF) aircraft for the president to the same specifications as the VC-25s serving as Air Force One. After all no US President's personal car has been configured as the Presidential State Car so far.


Is GLS (GBAS landing system) an RNAV approach?

Is GLS an RNAV approach? Short answer is no. It is also not in the framework of PBN.
RNAV approach
Out of confusion the naming was not standardized, which led to more confusion. This will be fixed, from 1 Dec 2022 only the term RNP will be permitted for the minima you mentioned. (ICAO Circular 336, starts at page 19)
RNAV?
It stands for area navigation, which is the ability to fly any course in an area, not bound by VOR radials for example.
GLS
GLS uses a ground augmented signal, anchor point, course, and slope, so it does just the one thing (like an ILS).
PA, APV, and NPA
Another reclassification will start to impact the older terms precision approach, approach with vertical guidance, and non-precision approach. There will be only two main types, A and B. Here's a Eurocontrol slide that breaks them down, and also shows that GLS is not in the PBN scope:

(eurocontrol.int)
Last but not least in case any of the above is confusing, this big Airbus presentation (icao.int, Sep 2017) covers the history from conventional navigation to PBN and beyond, and also talks about the GLS.

Addendum based on comments:

A 'conventional procedure' in the slide above includes NDB, LOC, LDA, VOR, ILS, with or without DME, and MLS approaches. ILS and MLS would fall under PA / Type B.
For how the GLS plots the approach, see: How do GLS avionics plot an entire accurate approach path?
One of the things that sets the GLS apart from RNAV is the need for a ground component at the airport, the VHF Data Broadcast (VDB) transmitter.


What caused the Belgian aircraft mechanic who stole an F-16 to crash?

Unauthorized flights of military jets by mechanics have happened in the past.  Some of the aircraft landed successfully after the ordeal others resulted in fatal accidents.  The two successful cases which I’m aware of, the mechanics had previous flight training.  We still cannot determine whether this F-16 went down as a result of inability of the pilot to control the aircraft or whether it was an intentional suicide by airplane.

There is nothing outrageous or unusual about an F-16, save that it is a high performance fighter, with a high thrust to weight ratio, (relatively) high wing loading and low aspect ratio wings and handles accordingly.  The sidestick controller combined with the fly by wire flight controls and nimble handling requires a light touch on the controls - some pilots describe it as almost a ‘kissing’ sensation - to properly fly, which possibly could get away from a neophyte.  Like most high performance airplanes, it needs to be flown by the numbers and can be demanding to land, though not as difficult as some other military jets eg T-38, etc.

The physical act of flying seminal maneuvers is very easy in these jets with a little flying knowledge.  Most fighter pilots describe their jets as easier to fly than say a Cessna.  Flight control laws eliminate a lot of the babysitting and grunt work one has to do in an aircraft with conventional controls.  The pilot has an unobstructed view from the canopy of the environments, offering very good situational awareness.  All primary flight data is displayed prominently in the HUD and easy to decipher.  More complex flying will start to become demanding and potentially lead to trouble.

Aviation has a lot of scenarios which seem benign but turn into death traps if you don’t fully understand what’s happening.  There are plenty of sad tales such as stall spin accidents, low level aerobatics, etc. where people thought they could handle a situation but their reach quickly exceeded their grasp.  This is compounded by untrained human psychological responses to stress and fear.  Some of the lethal gotchas I’ve seen are inability to understand aerodynamic concepts like stall speed to load factor, density altitude, aircraft system interactions, emergency procedures, operation in adverse weather or IMC, jet engine thrust response time to throttle commands, etc.  You also have the unusual quirks specific to each aircraft.  The F-16, for instance, can enter a deep stalled condition lethal at low altitudes if not properly flown.  These kinds of phenomena exist on other fighters with neutral or negative dynamic stability as well.

The exact cause of the accident in question is not totally clear, so anything specifics would be speculative.  If someone can find the Norwegian AF accident investigation report on this, we can read it and see what their conclusions were.


How many commercial jetliners are equipped with ADS–B?

The majority of commercial airliners are currently equipped. I don't have the latest statistics, but for Europe over 80% of the airliners were equipped with 1090 Extended Squitter ADS-B two years ago. The percentage in the USA was similar but a bit lower.

That doesn't mean that all those aircraft are compliant with the European rules that mandate ADS-B for these aircraft from 2020 onwards. To be compliant, the aircraft need to have ADS-B version 2 (EUROCAE ED-102A / RTCA DO-260B compliant transmitters) and the installation in the aircraft has to meet certain criteria. At this moment (May 2016) I estimate the percentage of rule compliant airliners to be less than 20%. I haven't run statistics on my own dataset for a while, I may update this figure over the summer.

In Europe, all aircraft heavier than 5700 kg maximum take-off mass or a maximum cruise speed exceeding 250 knots true airspeed are subject to the ADS-B rule.
Therefore private jets and some small propeller aircraft will need to equip with ADS-B as well. In addition many people voluntary equip their aircraft with ADS-B; it is not uncommon to see ADS-B equipped gliders in Europe.

The situation in the US is a little different. The FAA mandate requires aircraft flying in certain airspaces to equip with ADS-B. In my view this makes more sense since it will prevent a mix of equipped and non-equipped flights in one airspace.

In the USA, two types of ADS-B are allowed. The 1090 ES is required above 18 000 ft, below that altitude Universal Access Transceiver is also allowed. This causes all kinds of interoperability problems that can only be fixed at high cost.

The FAA mandates aircraft in the 48 contiguous states (and D.C.) that operate in class A, B, or C airspace or class E airspace above 10 000ft (class E airspace below 2500 AGL excluded) to be ADS-B OUT equipped by January 1st, 2020. The mandate itself can be found here and the associated Advisory Circular (AC90-114) can be found here.

This means many more aircraft are subject to the rule. I have no recent information on what the implementation status is.


What's that in front of the overhead panel?

That's where the analogue/backup compass is stowed. The compartment can be opened downward if you ever need to use the compass, as seen in this image:


What are the advantages/disadvantages of in-fuselage engines (eg most fighter jets) vs wing/fuselage mounted engines?

The answer is roll inertia.

Fighter aircraft need to maneuver quickly, and one of the most important parameters is roll speed. In order to achieve a high roll rate quickly, low moment of inertia in roll is helpful. Placing the engines on the wing would increase roll inertia without much benefit in air combat.

Wing-mounted engines can only be found in designs which were optimized for raw speed, such as the P-38 or the YF-12. Even the Me-262 was initially designed with fuselage-mounted engines, hence its triangular fuselage cross section. Only when the engines grew in diameter during development had they to be relocated to the wing.

Disadvantages of fuselage-mounted engines:

Worse accessibility for maintenance.
Variants using a different engine cost much more to develop.
Engine fires or loss of a turbomachinery blade pose a much bigger threat to the structure.
No bending moment relief or flutter damping for the wing.
Complex intake geometry with boundary layer splitter plate, unless the intake is in the fuselage nose. The air requirements of high bypass ratio engines cause high losses with side intakes.

Note that at supersonic speed the engine can actually benefit from the proximity of a fuselage: The precompression of the forward fuselage of the F-16 or the Dassault Rafale help to increase thrust, especially when it is most needed at high angles of attack (well, those relatively high angles in supersonic turns, that is).

Advantages of fuselage-mounted engines:

Low moment of inertia in roll.
Lower overall surface area, thus lower friction drag, when compared to podded engines.
Precompression of the forward fuselage can be used to increase supersonic thrust.


What liability does an aviation inspector have for errors & omissions?

I only can speak for Canada but we use much of the American regulations in Canada as part of our regulations. In Canada, all mission critical systems Ie flight control systems etc ... have to be signed off by 2 inspectors for a total of 3 people working on the system. The maintenance engineer will sign of there own work and the inspector will sign off the inspection. A second inspector will sign off the secondary inspection. So typically if there is an error one person is typically not solely responsible for it. Additionally all inspectors and maintenance engineers licenses are only valid while working for a maintenance organization AMO. The organisation's insurance typically covers the employees working for them. When I went through my licensing I was told that we could still be held liable for criminal prosecution if we were willfully negligent. It was explained to us this only incurred once previously.  If memory serves me correctly it was the mechanic responsible for a plane that went down in the Florida everglades.


What are the "channels" on the A320 Rudder Travel Limit Unit?

The problem is that you're using the Flight Operations Manual, and not the Schematic Manuals. Quite frankly, FOM's pretty much suck in terms of technical details, but that's not their purpose either.

In regards to your question about what the channels are:

Channels are really just a euphemism for system redundancy. The FAC's are two completely separate entities (with crosstalk capabilities) - kind of like flight crews. The PIC (pilot-in-command) has primary control of the aircraft, while the SIC (second-in-command) has the same independent authority. The PIC and SIC can talk to one another, and monitor what each is doing. If the PIC falls ill, becomes incapacitated, etc.. the SIC can take over the functions of the PIC.

The FAC's work the same way. They both receive the same information, and have full capability of performing the work of one another. Take note of this schematic, courtesy of Airbus (found on the web), and which I took the liberty of highlighting:

Basically, what this schematic is saying is that the ELAC's, under normal law, provide turn coordination and dutch roll damping to the FAC's. They do this by receiving inputs from the rudder pedals, sidesticks, FMGC's, ADIRU's, SFCC, LGCIU's, and hyd pressure. Based on those inputs, it computes the proper yaw commands and sends that to the FAC's. It should be noted that both FAC's receive the same information on, you guessed it, separate channels. What does that mean? The data from ELAC 1 is sent to BOTH FAC's on separate sets of wires. Inside the FAC's, those data paths are split as well (Channel A & Channel B) and a separate processor computes the data on Channel A & the data on Channel B. Never the two shall meet. Likewise, the data from ELAC 2 is sent to BOTH FAC's on separate sets of wires, and again, those two data paths shall never meet.

At the same time, both FAC's are receiving the same FMGC, ADIRU, SFCC, LGCIU data that the ELAC is. Again... Separate wires, separate outputs, and computed with two different processors inside the boxes. The FAC's are also receiving hydraulic pressure, commanded rudder trim position, and SEC fault data. Guess what? All on different channels.

Now, take note of the highlighted portion in red. Ordinarily, FAC 1 is in control of the servo loop for the RTL Motors. This essentially means that based on ALL of the inputs received from the ELAC's and other independent systems, the FAC 1 sends a signal to the rudder travel motor and says "Move here." The motor moves, and a feedback signal from the RTLU circuitry is sent to the FAC saying "Here I am." When the commanded position of the surface position minus the actual position of the surface is approximately ~0, the surface is nulled. This is a basic feedback loop/servoloop.

The FAC 2 has the exact same capabilities as FAC 1. The two are in constant communication, making sure that neither has failed. If FAC 1 reports a self-failure, or FAC 2 detects a problem with FAC 1 and/or its servo loop, FAC 2 can immediately take over.

Notice that FAC 2 is responsible for the M2 (second motor) of the rudder travel limit system, and FAC 1 is responsible for the M1 (primary motor) of the rudder travel limit system. The wiring for the motors and its associated servo loop are completely separate. Again, the idea is complete system isolation. As such, any one failure cannot take out the entirety of a system.

All of the above is under the presumption of NORMAL LAW. It appears that cracked solder joints, emphases on the plural form, caused failures of Channel A & Channel B of the RTLU (see page 66 of accident report). Basically, like I mentioned previously, FAC 2 was monitoring FAC 1 for faults with it or its associated servo loop. When RTL channel A failed (the servo loop feeding data back to FAC 1; associated with Motor 1), the FAC 2 picked up on it, and took over on its own separate channel. A cracked solder joint on RTL channel B caused FAC 2 to trigger a failure. At some point during these failures, the flight crew pulled the circuit breakers, presumably to reset the faults. The loss of both FAC's put the aircraft into alternate law.

Once the aircraft was in alternate law, it seemed that there was a lot of confusion as to what was happening. From there, things went from bad to disastrous.


If, one day, we have an electric airplane, will it be easier to control than a fuelled airplane?

There are a few electric aircraft now, and because batteries hold far less energy than fuel, they're low-performance and short-range compared to normal aircraft.

If we imagine that at some point in the future we'll have batteries that can match fuel, the answer would be: Only slightly.

Back in the 60's and 70's, airliners regularly carried a flight engineer, who's full-time job was to monitor the engines and tweak various controls to keep them running properly. Now that's almost entirely automated. Pilots still have a checklist to follow to start the engines, and things to monitor during the flight, but it's a small part of their responsibilities.

Even if switching to electric propulsion completely removes this, it won't make a huge difference to the pilot. (and that's assuming that the electric system has nothing to monitor or adjust)

The aircraft handling is unlikely to be affected significantly, although there are be minor effects:

Fuel gets used up, so electric planes will have to be designed with stronger landing gear.
Electric motors are easier to scale, so it's easier for small aircraft to have multiple, counter-rotating motors to eliminate torque swing.
The battery is the heaviest part of an electric power system (and for one with hours of endurance, by an order of magnitude), so fitting a motor with excess power has a relatively smaller weight penalty. Electric power systems are good for providing short bursts of high power between long periods of cruising. Unfortunately this is more useful for a car than an aeroplane.
It would be much easier to design an electric power system for an aerobatic plane. Supplying fuel and lubrication while flicking between positive and negative G takes careful design.
Electric motors are quiet, but they are still transferring lots of power into the air, which is inherently noisy.

Ultimately these are all minor effects compared to the elephant in the room, which is that batteries are far heavier than fuel.


What radio equipment do pilots use to communicate with ATC?

https://en.wikipedia.org/wiki/Airband
emphasis added

As of 2012, most countries divide the upper 19 MHz into 760 channels for amplitude modulation voice transmissions, on frequencies from 118–136.975 MHz, in steps of 25 kHz.

A typical plane often has two communication radios, referred to as COM1 and COM2.
Each radio has an active and standby frequency. The pilot can listen to either radio, or both radios at the same time, but can only broadcast on a single radio.
There are often 2 navigation radios as well, NAV1 and NAV2, which can be tuned to VORs or other radio beacons, and the pilot can listen on those radios (typically for Morse Code IDs, Recorded Voice broadcasts, and occasionally, Voice-over-VOR ATC)


Do airlines prefer younger pilots after a cpl?

29 is not too late for them. It might be too late for you, depending on where you are in your life now.

First off, a CPL won't cut it to fly for most Part 121 or Part 135 outfits, including major airlines, cargo haulers etc. As of 2013, the requirements to get on the flight deck of a commercial airliner include an ATPL, which requires a minimum of 1500 hours (1000 if you attend a four-year flight school).

A CPL, with instrument rating and an endorsement to ferry passengers for hire, would allow you to fly for a company like NetJets, or for a corporate or government jet fleet, though you'll face stiff competition for those jobs; they're fairly cushy and well-paid for the required experience, and you'll find a lot of ex-military and ex-airline pilots beating down the door. You'd also be able to fly foreign-registered aircraft for a non-US airline, as long as the jurisdiction hasn't adopted a similar rule as the FAA's.

When you get your ATPL, with only the 1500 hours and no other airline experience, you'll likely land your first gig as First Officer on a regional route. This is your toehold into a career in flying, and to be perfectly honest if you don't absolutely love to fly I would tell you to do anything else, especially if you've already started a life (married, kids, mortgage). The average starting salary for a regional FO is about \$22,500 per year, which for a 40 hour work week is a meager \$10.75 an hour. You would literally make more working full-time as a barista in Sea-Tac Airport serving coffee to the pilots than flying the puddle-jump between there and Portland. And that's just the average; you can expect half the currently flying pilots with less than 1 year experience to make less than that. You are virtually guaranteed to make more money doing anything else that requires more than a high school diploma.

It gets better, but it takes a career to do it. Senior captains flying the big widebodies on major transcontinental and intercontinental routes make six figures, in excess of \$175k in some cases. Most of this is the airlines trying to retain their most experienced pilots for just a little longer by giving them the most profitable routes for which they can pay the pilots more. The other part is making up for the peanuts they pay you in your early years.

For the majority of your career, your entire life will fit in a carryon and/or a garment bag; flying regionals, you'll make the same flight five to ten times a day and have a good chance to get back to your home city, but on regional pay you'll be hard-pressed to afford more than a single-bedroom flat. As your experience grows, your promotions will naturally lead to longer flights on bigger jets, with the best-paying routes being intercontinental flights where you'll sleep on the plane two nights out of three. Many pilots have month-to-month arrangements with their landlords to rent a single room to crash in and store anything that doesn't fit in their flight bag. This will be your life for 20-30 years.

It is extremely common for newly retired pilots and their spouses to invest in a "pilot's house"; while most retirees are downsizing from the house they raised kids in, most pilots are finally able to settle down in one location for the first time in their lives, and are buying McMansions in cash at about that same age, with savings from the last few years of their flying career.

All in all, flying sounds like a glamorous, "see the world" job, but you have to be sure you can live with the downsides. Your spouse will essentially be a single parent (if you have kids), you'll be living in a small apartment for most of your life and will move several times, requiring your spouse to have a career that allows easy relocation, and you might only be home every other day if not less often, depending on the airline's scheduling model; it's fairly common especially among LCCs to run "end-to-end" routes instead of "hub and spoke", and that could mean you'll start in New York, run hops through Chicago, Atlanta and Dallas with your eight hours allowable flight time per day, then sleep in Dallas and fly the reverse route the next day back to New York.

Just to make a direct comparison, my wife has a cosmetology license (haircuts, nails, spa treatments) in Texas. To get that license required a 2-year course of study and 1500 hours total instruction and guided practice. Her first year with that license, she made about $33000, almost 50% more than a first-year airline pilot. The economics of the airline industry (and the regulations in the cosmetology industry) are bass-ackwards.


What can happen if a bird strikes the engine during landing?

A range of things can happen.
For an airliner with a modern turbofan engine, the mostly common outcome is, absolutely nothing happens to the aircraft (although the bird's day is pretty thoroughly ruined). A sparrow can be chopped up by the fan blades & blown out the back, and the crew will never notice it.  The engine won't hiccup, and until somebody sees a trace of blood and feathers on the fan blade during a walk-around, nobody will have any idea it happened.
Next case is a small bird gets ingested into the core. Chances are good the engine will keep running (remember, small bird we're talking about here), although the crew may notice a fluctuation in the engine instruments and there may be the smell of cooked (charred) bird in the air conditioning system.
In both of these cases, the engine will be inspected; the case where the bird went into the core takes longer, because maintenance will have to use a bore scope to look inside the core to inspect for damage.
The worse case than either of these is a big bird (or several) going into the engine.  Depending on the specifics, the engine may chop them up & spit them out without damage, or the fan blades may be damaged, and/or the core may be damaged, and in the bad case the engine loses power, and in the worst case, it quits.
Losing one engine to a birdstrike is quite rare and is a pretty big deal, but most certainly not unrecoverable. An airliner can fly fine with one engine shut down.
If the birdstrike happens on short final, the crew might simply use more power on the operating engine and continue on to land. If it happens farther out, they might continue or they might discontinue the approach, go around, shut down the damaged engine, and then come back and land.
The really, really bad case is where you get enough bird strikes that they shut down all engines, and at that point you're a glider.  If that happens at 50' over the runway, your only choice (land straight ahead) still gets you where you wanted to be anyway. If it happens farther out, well, that's where somebody gets to try to be Sully 2.0.  That sort of a scenario is extremely rare – like one in tens (if not hundreds) of millions of cases rare.
You're far more likely to get hit by a semi truck driving home from the airport than having your day ruined by a bird going through an engine.


What makes it safe for the DC-8 to use reverse thrust in flight?

Juan is correct, inflight reversing was a bit noisy; however, it was a MEL requirement to have the inboard reversers operational to operate above FL350. The procedure was as follows: For a steep or emergency decent all throttles are retarded to idle and the inboard thrust brake levers are put into "Reverse Detent Thrust" and when the "Eng Thrust Brake" indicating lights come on the throttles are moved to the reverse power stop. The outboard engines will be at flight idle as reverse in engines 1 & 4 is not possible until the landing gear is extended. Additionally, the use of reverse thrust was to be discontinued below 190K. Sorry Juan, no speed brakes on the DC-8, but yes on the DC-9. The spoilers on the DC-8 were used to augment flight control (three outboard on each side) but only when the gear was down. This made handling the aircraft in gusty weather much easier giving a quick roll response. On landing the spoilers(two inboard each side and three outboard) would deploy when armed by a signal from rear wheel spin-up or, as a back-up, by nose gear compression. The crew could also deploy the spoilers manually by pulling the spoiler lever back and up.


What is the difference between side-by-side vs under-cockpit engine intakes?

I wouldn't call the Su-27 small, but that is another issue.

The main difference is the wing planform. The Su-27 wing is optimised to work over all angles of attack up to 120° with very little pitch moment change. This requires the addition of strakes which create vortex lift at higher angles of attack. With this planform the straight side intakes would be impossible. But there is more to it.

At high angles of attack and high transsonic or supersonic speed, when thrust is needed most, the precompression below the strakes increases the efficiency of the intake.

While the F-15 was optimized for speed and transsonic agility within the usual angle of attack range, the Su-27 (and MiG-29, which uses the same basic design) are optimized to be flyable over all possible angles of attack which might be encountered in air-to-air combat. The F-15 designers stopped when the planform and intakes were good enough. TsAGI only stopped until they could not improve the design anymore.

Now to your many detail questions:

  What is the main difference between these two engine configurations?

There isn't one, it comes down to details. Both designs support long, afterburning engines, show little yaw when one engine fails and the intakes allow Mach 2+ speeds. The details are less sideslip susceptibility in case of the F-15, because the fuselage helps to adjust airflow, and better high angle of attack performance in case of the Su-27.

  What are the differences between planes that have the intakes below
  the cockpit as compared to those who have them on the sides?

Again, the main difference is in off-design performance.

  What are the aerodynamic differences?

Now I run the risk of repeating myself. One is better in yaw, the other in pitch. You may decide for yourself wich is more helpful in air combat.

  How does this affect the speed/radar signature/maneuverability od the
  aircraft?

This are actually three questions, wrapped into one. In speed the F-15 intake has a small advantage if the greater freedom in adjusting intake geometry to Mach number is taken advantage of. This means that the upper lip can be adjusted, not only the lower lip and the ramps inside, as is the case for the Su-27 intake.

For radar signature, both designs are a poor choice. They are basically corner reflectors, just like the vertical tails. You can see that stealth was no consideration in both designs.

For manoeuvrability, it is the wing planform that counts most. Here the Su-27 wins hands down.


What other materials apart from conventional nickel and steel superalloys can be used as a first stage turbine blade material?

Turbine blades must withstand high tensile stresses at their blade roots and be resistant to cyclic stresses from fluctuating fluid dynamic loads- all of this at very high temperatures, and over great numbers of thermal cycles. This argues in favor of metallic blades and against ceramics in this application, because they perform better against fracture in tension and fatigue crack propagation.