When sharing an airplane, what liability does the owner bear? What liabilities apply to the owner of an aircraft who allows other pilots to use his aircraft for not-for-profit and non-commercial use?

Example: An aircraft owner has a friend that wants to use his aircraft on a semi-regular basis (20-40 hours/year). His friend is willing to pay a fair share of maintenance, hangar, fuel, etc.

(Liabilities not limited to insurance, but also regulation, maintenance, etc)

Most of the liabilities (but not all) fall under the insurance of the aircraft. Your insurance policy is your largest liability. This is important not just for protection of your aircraft, but if your insurance ends up void in an incident, you could have a lawsuit on your hands. Don't assume that aviation insurance is similar to car insurance, either.

Insurance

Pilot (on plan)- Aviation insurance has 3 catagories: Pilot, Named Individuals, Other (explained later). The owner pays insurance based on their qualifications and those specifically named. If you are going to be borrowing on a regular basis, weigh the cost of adding you to the plan.
Pilot (off plan) - Insurance specifies whether or not you may borrow the aircraft to others, but also what qualifications that pilot must have. You may have only have 100 hours and are insured, but the insurance company may ask for 500 hours of someone you borrow the aircraft to.
Aircraft - Regardless of the pilot, if the aircraft is out of compliance on maintenance or airworthiness, it is not being flown legally and therefore voids the insurance. As the owner (primary on the insurance), this is your responsibility as an owner and the pilot's responsibility as PIC. Both of you can be hit for this one.
Reimbursement - Just because it is non-commercial use, your insurance may still specify what you can be compensated for. This may be only the actual expenses of the flight (fuel, oil, airport fees) and may include limits on hourly charges for maintenance. Keep in mind that this does not authorize you to break regulation on commercial use. You cannot make a profit.
Legal flight - Above all else, the flight must be legal. The flight needs to comply with all FAA regulations.
Non-commercial use - I keep coming back to this, but this is very important. The pilot may be a commercial pilot, but this doesn't authorize them to use the aircraft for hire. With many insurance plans, you can have a CFI train you since you are the owner, but a CFI cannot train someone else and you cannot train someone else. This makes and aircraft for hire and requires a 100-hour inspection.
Other - Pilots who do not own an aircraft may have their own (renters) policy. While this may cover liabilities of those in the aircraft, don't assume that this also covers your aircraft. A pilot paying $200/year won't likely have coverage for your shiny new Cirrus.

Other

As the owner, you have liability in accurate record keeping. If you fudge documents on your aircraft that make it appear FAA compliant to the PIC, then you can be held liable.

Notes

These are only the items that come to mind. Feel free to add.


Is it the tower's purview to recall an aircraft from the runway for loading the wrong cargo? On August 17, Ryanair flight FR 11 entered the runway after already being delayed for 1.5 hours, only to exit at the first available opportunity. After being jokingly welcomed to Stansted from where we had very nearly departed, we proceeded to wait for at least 1.5 hours more while the wrong cargo was being unloaded to make way for the correct one.

The crew told us the delay was due to having loaded the wrong cargo, and made it very clear to us that this wasn't their fault, which I have no reason to doubt. The pilot made it sound like the tower told them to abort. We were already on the runway and turned back at the very last minute.

My question is this: Does the tower have the authority to tell an aircraft to abort take-off for having loaded the wrong cargo?

My impression is that the tower is a traffic cop and safety officer rolled into one, and operational questions such as which plane does this cargo belong are outside their normal range of concerns.

Yes, the tower can tell an aircraft to abort take off and instruct it to return to the gate, although it probably never got that far in this case, I'm guessing that they got to the runway or were asked to line up when the request came in. This was probably at the direction of Ryanair, since the tower would otherwise not have any information on the wrong cargo being loaded, nor would it really care.

I'm guessing at this part....

The back-and-forth probably went like this:

  Tower: Ryanair 1234 line up and wait
  Ryanair: Line up and wait, Ryanair 1234
  Tower: Ryanair 1234 taxi on runway to Delta, call company
  Ryanair: Taxi delta, call company

The tower probably didn't relate too much information about what is wrong, the pilots have the ability to talk to the dispatcher for Ryanair to get more information.

Regardless, a tower controller can tell an aircraft that a take-off clearance is rejected, even after the take-off run begins. It is up to the pilot to determine if they can safely reject the take-off and to follow those instructions.

The probability though that the tower knew that the incorrect cargo was loaded is probably zero, at least until somebody from Ryanair called them and had them stop the take off. I'm guessing that they didn't actually get take-off clearance (line up and wait is not take-off clearance) and either they were asked to line up, or they were waiting to enter the runway before they were told to go back (and taxiing on the runway is the easiest way to turn around).

Typically Tower will not call a RTO unless there is a safety concern like an incursion or they notice a wing fell off. Once clearance is given it is rarely rejected. If they had gotten clearance Tower would probably have advised them to call company "once in the air" rather than reject a take off.

If you want to know what it sounds like for Tower to reject a take-off, listen to this YouTube about a near miss regarding similar call signs and two aircraft taking off at the same time.


What is the effect of earth's speed on flight? What happens if an object (plane or space ship) travels at a speed less than that of earth rotation?

The stationary plane and the atmosphere were already moving at the speed of Earth's rotation. Because of this, the Earth's rotational speed really has very little effect on aircraft speed. To answer the second part of your question, almost all passenger jets fly slower than the Earth's rotation. The Earth rotates at 1675 km/hr at the equator; average cruise speed is 878-926 km/hr.


Do any aircraft windshields have anti-reflective coatings? Anti-reflective coatings are used in many applications to reduce glare and prevent double images.  Double images in particular are a concern because of the several layers involved in a cockpit window.  More high-tech optical coatings can also have other desirable properties such as reducing UV and IR light transmission. There is some evidence of them being used in industry, like this patent for a windscreen. On a related note, cockpit instruments have a limit for maximum reflectivity of their glass/plastic surface and sometimes use anti-reflective coatings.

I started thinking about this because of a recent question on why windscreens aren't more reflective.

However, I can't find many examples of anti-reflective coatings (possibly because of the harsh environmental conditions?).  UV and IR protection seem to be mostly through tinting instead of coating.

Do any production aircraft use anti-reflective or other coatings?  Why or why not?

An antireflective coating for the F/A-18's canopy was successfully tested sometime in the 1990s; no word on whether it made it into production, though.

Wikipedia claims that "[n]early all glass windows in large aircraft have an anti-reflective coating", but provides no source for this assertion.


Is it possible to upgrade old Mirage-III and Mirage-V variants with thrust vectoring engines? Is it possible to upgrade old Mirage-III and Mirage-V variants with thrust-vectoring engines so that they can match the maneuverability of F-16s?

Given enough tinkering, it is technically possible. But so many other modifications would be needed - strengthening, wing revisions to maintain the relationship between centers of lift and gravity, trim authority, managing the changes in stability, never mind the engine mounting and avionics refit, etc. etc. that really you would just be cannibalizing an old airframe to make an entirely new plane.
Somebody might conceivably do it as a short-life proof-of-concept prototype, if a new plane were proposed, the Mirage airframe was a good enough match, and a sophisticated digital control system was not necessary for these early tests. But in practice there is nobody in the world with all of the need, the industrial capability, the Mirages and the budget to embark on a new fighter programme that would require such a prototype.
Dassault did experiment with adding separate lift engines in the fuselage, in the Balzac V (which was just such a hacked-about Mirage III airframe) and the Mirage IIIV (which was twice the size), but it all went nowhere.


Where can one buy old 737 parts? I'm trying to build a home cockpit for the PMDG 737-800. I'm looking for some landing gear position indicator lights but I don't know where to get them from. I've tried ebay and amazon, but it returned no promising results. The other alternative is of course to make them. Does anyone know where I could have something like this made?

Airliner parts have a tiny market outside of the airline and maintenance businesses (in the sense thst the general public generates no demand) which means that there probably aren't many places to shop for unairworthy (read: cheap) spares.  I would place "want to buy" ads on Barnstormers and maybe cruise the site from time to time clicking through relevent categories.  I did see one ad offering 737-200 parts but you'll have to contact the seller to see exactly what they have and how much they want.

You could certainly buy a new part but you will not like the price tag.  I imagine that an unairworthy spare for an -800 series will be hard to find compared to a -200, for example, because of the quantity of inoperative junk parts generated thus far by the respective fleets.

Good luck!


What does it mean to move a single flight control to its full deflection? It is the speed below which you can move a single flight control one time, to its full deflection, for one axis of airplane rotation only

This is part of the explanation of maneuvering speed.
What does 'full deflection' mean?
Is applying full rudder full deflection?
Is turning the yoke 90 degrees full deflection?
What about in the pitch axis?

Yes, that is correct. Moving any control surface until you hit the stops is considered "full deflection". For the purpose of maneuvering speed, this is the speed at which any single control surface can be fully deflected without risking damage to the control surface or structure.

Pitch is full back/forward on the yoke or stick, which is where the elevator stops movement. This isn't always 90 degrees for ailerons, especially in aircraft that have stick controls versus yokes.


If engines are closer to the wing (or in the wing) does it increase efficiency If you were to make an engine in the wing or at least closer to it in commercial airliners such as the 747, 737 and A380 etc. Would it increase efficiency?

Yes, mounting jet engines in the wing root increases aerodynamic efficiency due to:

No drag from engine pods
Smaller yawing moment upon engine fail, and resulting smaller vertical tail.

But that is about it. There is a host of disadvantages to mounting the engines in the wing root, one of them being that high lift devices cannot be used and the wing is bigger, causing more drag.

Three huge advantages of mounting the engines in pods, underneath the wing, in front of the aeroelasitc axes are:

High bypass engines can be mounted, vastly improving fuel efficiency.
The engines are within easy reach for maintenance.
The weight of the engines provides both bending relief, and reduces structural weight to prevent flutter.

Also see this question.


How close does a plane have to come to the ground before triggering the ground collision alert? The recent AF953 near-miss incident as reported by the BBC describes the ground collision alert warning triggering, preventing a CFIT by the plane.

  An initial incident report published by BEA said an automated warning,
  saying "Pull up", was activated. The plane then ascended quickly from
  9,000ft (2,743m) to 13,000ft (3,962m) to avoid crashing.

  BEA did not say how close the plane came to Mount Cameroon.

How close does a plane have to come within the ground (as measured by the radar) before the warning triggers? Does this number vary significantly between aircraft models, or is there some legislated fixed value for the distance?

There are 7 modes of warning

Mode 1. Large descent rate near terrain. Gear and flap position not a factor in the warning.

Mode 2. Large closure rate with terrain.

Mode 2A. Occurs for a large closure rate if the flaps are not in the
landing configuration and glideslope deviation is more than 2
dots.

Mode 2B. Alerts for a large closure rate if the flaps are in
the landing configuration.

Mode 3. Alerts for a large altitude loss during takeoff or
during a missed approach with the flaps not in the landing
configuration or landing gear not down.

Mode 4. Alerts when the airplane is too close to the
terrain and the landing gear or flaps are not in the landing
configuration

Mode 4A alert when the landing gear is not
down and gives the aural messages TOO LOW GEAR at
low airspeeds, or TOO LOW TERRAIN at high airspeeds.

Mode 4B alerts when the landing gear is down and the flaps are not in landing configuration and gives the aural messages TOO LOW FLAPS if the airplane is at low airspeed, or TOO LOW TERRAIN at high airspeeds.

Mode 5 alerts when the airplane goes below
the glideslope during approach if the landing gear is down.

Mode 6 gives aural callouts when the airplane descends through set altitudes with the landing gear down. It gives callouts like MINIMUMS or DECISION HEIGHT.

Mode 7 gives windshear warnings.

Every one of these modes is altitude, speed and aircraft configuration dependent.

In the Enhanced GPWS, these triggers are also changed based on the position of the aircraft.

Regulatory requirement 14 CFR 121.354

Technical requirements TSO-151C


(How) do pilots round the time? I'm wondering what's the time for pilots.

Given the case the watch says 10:00:20 - is it basically 10:00?
And when does 10:01 start? Is it past 10:00:30 or only as the seconds move from 10:00:59 to 10:01:00?

The roundings I know of in the context of US general aviation aren't strictly defined or enforced because they don't need to be (with the exception of racing; see below). The military might need to be more precise, but I have no idea about that.

Anyway, here are the main roundings a typical GA pilot might encounter:

Logging: to the nearest 6 minutes

Pilots usually log their flight time in tenths of an hour, i.e. they round to the nearest 6 minutes. There's no regulation on it and some people prefer to log in hours and minutes (maybe it's even required in some countries), but tenths seems to be most common.

ATC instructions and clearances: to the nearest minute

See this question for information on timing under IFR (some of it applies to VFR too); instructions, clearances and reports are given to the nearest minute. It is required to have a clock with a second hand but that's to make it possible to do timed turns, not because ATC gives instructions down to the second.

Air racing: to the second or less

In air racing, there are at least two ways of using time. In a speed race - like the Red Bull Air Race - you measure the elapsed time over the course. The timing for Red Bull is to one thousandth of a second.

In precision navigation air racing, you plan your flight over a specific course and calculate the time (and sometimes even the fuel) you'll require to fly it. When you fly the course, any deviation from the numbers you submitted beforehand - under or over - is penalized. Timing in those competitions is to the second.


At what exact point in time should we be giving the 10 minutes to diversion call to ATC? I wanted to get some clear information if it’s available as to which exact point should the 10 mins to diversion call be given to ATC with respect to our FOB. The way I have been doing it currently on the A320 is by looking at the FUEL PRED page and when that shows a time value equivalent to 10 mins I give the call to ATC. But this is not a very reliable way to do it while in a HOLD. As the FMGC only updates the extra fuel and time value after crossing the waypoint/PPOS/pseudo waypoint at which the hold has been inserted.
This 10 mins to diversion call is mainly given while flying in India to let ATC know that we only have 10 mins worth of extra fuel left for holding beyond which a diversion will have to be initiated to our filed destination alternate.
Is there any precise way to calculate when we will exactly be at 10 mins?

(I just came up with this, so it may not be the official solution - but it should be fairly simple this way)
You first need to know the fuel you need for the diversion with all safety requirements included. This is computed like normal using the MCDU, EFB, etc.. I'll call this your "bingo" fuel. You just need to add the fuel required a 10 minute period for your current flight situation. Once your FOB on the upper ECAM reaches that final result, call ATC.
The easiest way to compute the fuel required for the next 10 minutes is with the fuel flow indication.
On the engine display you have the fuel flow indication, which should be nearly constant during a hold at constant speed and constant altitude.

Add all fuel flow values together
Round up to the nearest 20 kg/hr or so to be safe
Then divide the total fuel flow by 6 (10 min = 1/6 of an hour) to give you fuel required for a 10 minute hold at the current conditions.

Example:
Let's say each of the two engines shows a fuel flow of 1000 kg/hr. Add together to get 2000. Then divide 2000 kg/hr by 6 to get roughly 340 kg fuel consumption for the next 10 minutes. Add this to your bingo fuel.
( 1000 + 1000 ) / 6 = 333 -> 340 ... (+ bingo fuel)
Quicker Alternative:
You could also take the higher of the two engine fuel flow values and divide by 3. Then round up:
1000 kg/hr divided by 3 = 333 kg -> 340 ... (+ bingo fuel)
Note: This works the same if you have fuel flow in pound per hour. The result will be in pounds of fuel required for 10 minutes.


Why do wind tunnels not have oval turns? Why do wind tunnel not have oval turns to reduce energy which is lost in sharp 90 degree turns and decrease turbulence?
Wind tunnel with oval turns:

Wind tunnel with 90 degree turns:

Some closed wind tunnels do have rounded corners, for instance the inner circuit ("thermal test chamber") of the Jules Verne climatic wind tunnel in Nantes.
This plan view is from fig. 1 of "Numerical modeling of the flow conditions in a closed-circuit low-speed wind tunnel" by Moonen et al, J. Wind Eng. & Industrial Aero. 94(10):699-723.

More diagrams of this wind tunnel are in "Benefit of Wind Tunnels with Large Test Sections for Wind Engineering Applications" by Flamand et al, Mathematical Modelling in Civil Engineering 15(2).


How does the power requirement change when weight and speed change? Suppose I have an airplane with a certain weight M, which uses a certain amount of power P, to fly at a certain speed V.

If I use the same airplane, but quadruple the weight (4xM) and double the speed (2xV), what is the resulting need in power?

I would like to understand the physics of this, for an ideal system. We can ignore how efficiencies change with velocities, etc.

Here is my reasoning: the airplane needs four times the lift to fly in the same mode, which will be provided since the speed has doubled. The only force the engine needs to overcome is the drag, which is four times higher, i.e. the required power is 4xP, four times higher.

Short answer: No. $ P = F \cdot v$ and since $ D = \frac{1}{2}C_D\rho v^2S $ it follows that $P\propto v^3$

Let's take a more simple case: a block moves over a rough, horizontal surface in a vacuum and a force is applied such that its velocity $v$ stays constant. This force is always the same, since the "drag force" is given by the friction coefficient times the normal force. Here, the kind of relationship that you're thinking of would hold: if we're moving at $2v$, we need twice as much power ($P=F\cdot v$).

Look at a fixed distance along that trajectory: we do the same work in both cases ($v$ and $2v$), so we put in the same energy which dissipates through friction, i.e. heat. However, we now need to do the same work in half the time, so the power is double. In a certain way, it's "more difficult".

Now for an aircraft, the drag force itself is proportional to the square of the velocity. So, in conclusion, there are "two effects" which "add up": a quadrupling related to the increased force and a doubling due to the increase in velocity. Hence, the power is multiplied by 8.

Edit (in response to comments below):

Ok, let's do another thought experiment: we place a jet engine in a wind tunnel and keep it at constant thrust and constant mass flow (choked nozzle), which results in a constant velocity increment across the engine. The power contained in the jet (the airflow exiting the engine at higher speed $v_j$) is $\frac{1}{2} \dot m v_j^2$  and the power contained in the incoming flow is $\frac{1}{2} \dot m v_0^2$. The difference between the two is called the "propulsive jet power"; it's the power which is "available" from the acceleration of the flow. Now, you can see that because both terms are squared, it is not just the velocity increment which counts (as it was the case for the thrust), but also the magnitude of the two terms.

So alternatively to the "thrust power" given by
$$P_{thrust} = \dot m (v_j-v_0) v_0 = T v_0 = D v_0$$

we can also consider the "propulsive jet power"

$$P_{prop} = \frac{1}{2} \dot m (v_j^2-v_0^2)$$

where the proportionality to $v_0$ at constant thrust is maybe more clear. The difference between these two quantities is called the power loss ($P_{loss}=\frac{1}{2} \dot m (v_j-v_0)^2$) and is a measure of how much of the orginal chemical energy contained in the fuel is lost to kinetic energy in the jet.

You can see that if the velocity differential is constant, the power loss is constant. So if we have shown that the propulsive jet power is proportional to $v_0$ at constant thrust, the thrust power (which is the power that "actually moves you") must also be proportional in this way. And this proportionality is related to the fact that the kinetic energy of a mass is proportional to the square of its velocity.


Why isn't Vy equal to Vmd? $V_{md}$ is the speed where the rate of sink is the lowest. So if you add full power, it should be the speed with greatest rate of climb, namely $V_y$.

But we also know $V_{md}<V_{bg}$ according to this speed polar graph as a general rule

If I connect the inequalities, I have $V_y=V_{md}<V_{bg}$. In the particular Cherokee I'm trained in, $V_y$ is 75 knots and $V_{bg}$ is 69 knots. So where in the reasoning am I wrong? Why isn't $V_y$ equal to $V_{md}$?

Vy includes engine/prop efficiency factors, so it can not be directly compared to glide data.  Vbg is the best speed for DISTANCE, which is what you want to make your emergency landing site if you have an "engine out". Vmd will give you a lower sink rate, but you will not glide as far.

Interestingly, if you have to glide INTO the wind, best glide speed will be FASTER than Vbg to make up for loss of groundspeed.  Best glide speed there for is the optimal ratio of altitude given for ground gained.


If my parachute fails to open, why should I aim for land rather than water? I was watching these 'what if' parachute failure videos on YouTube and everywhere they say do not choose water over land. Why should we not choose water and what happens if we land in water?

One such video:

If your parachute fails to open entirely you are almost certainly extremely dead no matter where you land, however occasionally people do survive. A partial failure of a chute is much more survivable. You will be coming down much faster than you would with a good chute, and you are going to get injured, probably badly. Think broken legs and arms with neck or back trauma. You may sustain a concussion as well, and you will certainly go into shock.

If you come down in the water you will be injured and unable to keep yourself afloat, and it will be much harder to rescue you. On land help will be much closer and, and as @RonBeyer says in comments, you can't drown on land.

Land has features which may help you survive: trees, vegetation, hay bales, crops, snow, and many other things may cushion your impact enough to make the difference between life and death. Water is the same wherever you go, and doesn't have nearly enough give to cushion an impact.


How to calculate variable pitch propeller parameters? I'm carrying out a project on Variable Pitch Propellers for UAVs and especially self-adjusting or Passive Variable Pitch Propellers that change pitch due to aerodynamic forces and Pitching Moments.

At the moment I'm working on a single element (sectional airfoil) using blade element theory for three different stages of the aircraft: Take-off (with a given high RPM,e.g 2000), Cruise (medium RPM,e.g 1000) and landing (low RPM e.g 500) and corresponding free stream velocities 0 , 80, 20 m/s respectively.

However while I'm trying to calculate first Lift and Drag and then Pitching Moments, I realized that there are unknown factors needed for the calculations such as lift and drag coefficients as well as Angle of Attack on that sectional blade element.

The purpose is to use at first a symmetric aerofoil with no camber and I was wondering whether I have to use a commercial aerofoil such as NACA and if so, how to get the Cl and Cd from this aerofoil.

My questions are:

How to calculate the angle of attack or pitch angle while I know their subtraction?
Do I need to give some values for a new parameter that will help to find  these calculations or RPM, Vinf, and r of the propeller is enough?
What about the chord length; do I need to give a random value there?

I am sorry if some of the questions are silly but I'm coming from a different background and I'm fresh in the aerodynamic field.

There are a couple of key parameters in propeller design, the main ones being the power to drive the propeller, and the thrust that the propeller delivers.

A blade element of a propeller looks like the figure above, from E. Torenbeek, Synthesis Of Subsonic Aircraft Design, which shows the characteristic angles:

The angle between blade chord and propeller plane is the geometric pitch (blade angle $\beta$)
Angle of attack of the blade element $\alpha = \beta - \Phi$
The effective pitch angle is $\Phi = arctan (V / \omega*r)$. This is often given relative to rotational speed $n$ = rounds per second of the propeller, and the propeller diameter R:
$$ \Phi = arctan \frac {V}{n \cdot D } * \frac{1}{\pi \cdot r / R}$$

The advance ratio J of the propeller is

$$ J = \frac {V}{n \cdot D}$$

Induced velocity should be constant over the blade, implying that $\beta$ decreases linearly with increasing r: the propeller blade twist. Because the twist changes linearly, one point on the blade can be taken as the representative blade $\beta$, and this is usually taken at either 70% or 75% of the radial distance.

It can be shown that for a given blade geometry, the power and thrust coefficients $C_P$ and $C_T$ of the propeller are determined only by J and $\beta_{0.75}$. If tip speed is below speed of sound and the blades are not stalled, Mach and Reynolds number effects are negligible.
$$ C_P = \frac {P}{\rho \cdot n^3 \cdot D^5}$$
$$ C_T = \frac {T}{\rho \cdot n^2 \cdot D^4}$$

We can now look up $C_P$ as function of J in for instance NACA reports from the NACA server (check NACA WR 286). Unfortunately these diagrams are not given in metric units, so would need to be converted. But there are many other sources, such as underneath from a question on this forum.

Regarding the profiles: propeller design is pretty specialistic but old fashioned NACA profiles are still being used in helicopter blades for both main rotor and tail rotor. For a symmetrical profile NACA 0012 would be a representative choice, again available from the NASA report server, for instance NACA TN 4357. A google search on NACA 0012 data results in lots of hits. A reference book on this is Theory Of Wing Sections by Abbott & Von Doenhoff.

For the chord length, you may want to have a look at existing propeller designs and their aspect ratio (propeller radius/avg chord). You could start with a value of 15 -20.

There is quite some more info on propeller design for the pre-design phase that you are in, in Torenbeek's book referenced above, including on design for variable incidence props.


Why would a flight from Frankfurt to Madrid fly over London? While enjoying the sunshine at the bank of the river Main in Frankfurt (Germany) this evening, I saw a LATAM B787-9 depart over the city center towards the north. This was slightly  surprising, as the only LATAM flight from Frankfurt is LAN705 to Madrid (Spain) (and then to Santigo, Chile) which is to the southwest. I have taken this flight quite a few times before and I don't remember it ever flew a northerly departure.

Even more surprising was that continued for a while to north, until I lost sight of it and had to take out the Flight Radar 24 App to track it. The flight continued towards the northwest, into the Netherlands and across the North Sea towards the UK.

By now I highly doubted the flight was going to Madrid. The only reason I could think of for this routing is that either LATAM changed their flight schedule and flies via another city to Chile or the aircraft was being ferried to Boeing in the US for maintenance (LATAM has many problems with the B789 Rolls Royce engines).

Coming back home, I just checked again where the flight was (fully expecting it to be mid-Atlantic) and I see this:

source: Flight Radar 24

source: Flight Aware

The flight departed Frankfurt, turn north, then northeast over Belgium and the Netherlands, into the UK. Then went over London, west-southwest over Lands-end, then over the Celtic sea it finally turns southbound toward Spain. It now entered Spain and seems to be heading to Madrid, but the flight will be over 2500 km, while a straight line between Frankfurt and Madrid airports is about 1420 km.

What is going on? What is the reason for flying over 2500 kilometers, while it could be only 1500?

I can't be completely sure but the most likely reason is a planned strike by ATC in south-east France. It was planned from the 30th of June to the 1st or 2nd (depending on the source) of July, meaning that ATC services would be unavailable or at least seriously reduced in that area, presumably with knock-on effects in other parts of the country. Some airlines had cancelled flights completely although in the end it looks like the strike was called off.

Strikes are notoriously common in France (there's even a website dedicated to tracking them all) and especially in the summer, so an airline might just plan to avoid French airspace altogether to avoid the uncertainty. Of course that costs more money, as you said, but a missed connection to Chile and stranded passengers in Madrid would also cost money.


Can aircraft ferry engines for other types of aircraft? There have been a number of questions that highlighted the fact that the 747 could ferry an engine on an underwing pod.
Was the 747 only capable of ferrying 747 engines? Or was it possible to ferry engines for other types of aircraft?
If it can only ferry 747 engines, is there, or has there ever been, an aircraft that can, and has, ferried engines of other aircraft types?
Edit: Following the comments and the one answer posted to date - ideally, I'm looking for cases of aircraft that can ferry engines externally for other types of aircraft. However, if no such case exists (and I suspect this to be the case), it would be nice to see more comprehensive details of how engines are ferried around the world (I'm sure i.e. Qantas won't always  use sea cargo, nor an An-124 to)

Yes.
The Antonov 124 regularly ferries even the largest engine, the GE9X, internally. Нема проблем!

GE9X being loaded into the An-124 (picture source)


What is the meaning of the crosshatch marking on altimeters? What does the crosshatched area on an altimeter tell you?  It's only visible when below 10,000 ft indicated altitude.  Can't be terrain awareness, a lot of terrain around here is over 10,000 feet.

This sample altimeter display from the FAA manual on flight instruments shows the crosshatched area:

Here's the accompanying text from the manual where it points out the crosshatched area:

  Crosshatch flag: A crosshatched area appears on some altimeters when displaying an altitude below 10,000 feet MSL.

It looks like the crosshatch (or barber pole) indicator doesn't have a consistent meaning (a similar marking is used on some airspeed indicators, by the way). I found several different examples of when it can appear:

Below 10,000' (FAA PHAK; Army RC-12H manual)
Below 15,000' (instrument service catalogue)
Below 16,000' (another instrument catalogue)
Above 0', i.e. above sea level (instrument repair information)

The only source that gave any explanation is the last one:

  When the altitude is below sea level the barber pole is no longer
  visible. This is provided to avoid the error of reading –1,000 Ft. as
  being +10,000 Ft.

After following a bunch of links and skimming some discussion forums, my conclusion is that @RalphJ is probably right: manufacturers added the flag to highlight that the aircraft is at a relatively low altitude, in a way that's much easier and quicker for the pilot than 'parsing' multiple hands on the altimeter face. And the fact that different sources give different altitudes highlights an important point: it's always a good idea to read the handbook or manual for all your instruments because you can't assume that everything works the same way in every aircraft, even something as 'simple' as an altimeter.


What would happen if I exceeded mach 1 over a regulated area? What would happen if I exceeded mach 1 over a regulated area? Are there any penalties in place for this violation?

Would ATC actually know I’m flying supersonic or would they have to go out of their way to check my groundspeed and work out my approximate air speed?

It would be a violation of 14 CFR 91.817

  § 91.817 Civil aircraft sonic boom. (a) No person may operate a civil
  aircraft in the United States at a true flight Mach number greater
  than 1 except in compliance with conditions and limitations in an
  authorization to exceed Mach 1 issued to the operator under appendix B
  of this part.

It would be extremely easy to track such an aircraft, and the pilot would be dealt with upon landing.


Why do propellors have white markings on their tips? Why do propellors have white markings on their tips?
Here are a couple of examples:

It is so you will see a circle when they are turning.  Without contrasting bright tips turning propellers are invisible, and pose a grave risk for someone who is deaf, wearing heavy hearing protection, or distracted by other noises.


What would cause an RPM drop and engine cutoff after starting a Cessna 152? My flight school's Cessna 152 is 30 years old. After refueling, the engine RPM went down and down. I tried putting full throttle in but it just stopped. Was it air in the fuel line or something more serious?

Gradual power loss would not be caused by "air in the fuel lines".

The most likely cause is blocked vents. There are pipes that allow air into the tanks so the fuel will flow out. If they are blocked for any reason, fuel will stop flowing, often in a gradual manner.

Another possibility is that there is corrosion or sediment which is blocking the fuel line. Are you checking drain valves on every preflight and after refueling?


Transition level has to be readback and what about transition altitude? is it also required to be readback by the pilot? These are the readback requirements, source by https://www.skybrary.aero/index.php/Read-back_or_Hear-back
a) ATC route clearances;
b) clearances and instructions to enter, land on, take off from, hold short of, cross or backtrack on any runway; and
c) runway-in-use, altimeter settings, SSR codes, level instructions, heading and speed instructions and, whether issued by the controller or contained in Automatic Terminal Information Service (ATIS) broadcasts, transition levels.

The word “level” is used to refer to Flight Level (or Pressure Altitude). It can (and probably is in this case) also infer the word Altitude (or True Altitude). They are sometimes used interchangeably. Though, by strict definition, they are not the same.

In the US, QNH (Transition Altitude), QNE (Transition Level), and QFE (Transition Height) are not widely used much outside of the military. Instead, the term Lowest Usable Flight Level is substituted. The word “Transition” itself is commonly used to refer to passage through an area. The process to transition the Transition Layer between the Transition Altitude and the Transition Level is standardized for all flights and airspaces.

The Flight Level is the altitude standardized at an altimeter barometric pressure setting of 29.92 inches of mercury. That allows all aircraft to fly at a standardized altimeter adjustment. In theory, this should avoid confusion caused by aircraft flying at different Indicated Altitudes but the same actual True Altitude due to different Kollsman Window settings. All aircraft in the US are to adjust their altimeters and fly using Flight Levels at and above 18,000 feet MSL. Above this Flight Level, precise Absolute Altitude is not necessary.

The following was taken from the FAA-H-8083-16B, Instrument Procedures Handbook, 2017 (IPH)


How can I identify airliner types that don't have ACARS? Is there a way to identify what commercial airliners are still flying without ACARS today? Most modern commercial airliners are equipped, but I know some 737s still fly without ACARS, for example.

ACARS is a operator specific (optional) system. It depends on the operators requirements if an aircraft is equipped with ACARS or not. You can't tie it to an aircraft model or variant. As a rule of thumb: The older the aircraft, the less likely it is ACARS equipped.


Can an engine that is extinguished by foam be returned to service? During the Qantas 32 accident, one of the engines could not be shut down as the control cable from the cockpit to the engine is cut. The fire crew eventually shut down the engine by shooting fire-fighting foam into the engine.

In such cases, can the engine be returned to service following an inspection? Or, does foam damage the engine to the extent that it is more economical to simply buy a new engine?

A foamed healthy engine
(Original)

Inaugural Virgin Atlantic flight to Atlanta cancelled after 'water cannon salute' is swapped for foam and clogs engines
The de-clogging work was carried out as the pilot and flight crew went on a diplomatic mission to explain exactly what had gone wrong.

If it's only foam, the non-faulty engine can be de-clogged, checked, and returned to service.

For QF32, all the engines were replaced. There were issues with the Rolls-Royce Trent 900 engines. The rest of the Qantas fleet either received new engines or modifications.

Video for an engine inspection.
Video for an engine wash.


How does sideslip indicator react during crosswind? I will basically consider two types of sideslip indicator: the yaw string and the marble/ball inside a curved tube. I don't know the exact aeronautical term for both and I don't know if other sideslip indicator exists.

The pilot should maintain no sideslip, i.e. keeping the string centered. Let's say the pilot is able to do it. What happen in case of cross wind? Considering the air mass as frame of reference, the aircraft fly straight forward, but considering the earth as frame of reference, the aircraft is flying a bit sideways (crabbing). Thus the yaw string and the marble/ball may disagree. Am I right?

Going further, if the aircraft fly straight forward in an airmass (no sideslipping) with winds and gusts, the aircraft will experience sideways accelerations. How will the marble react? Will its indication still be reliable?

EDIT: Given comments and answers, I need to provide an extreme situation: the constantly increasing crosswind. The air mass is no longer an inertial frame of reference. Keeping no side slip result in an increasing crabbing. My understanding is that the ball should react to this increasing crabbing (sideway acceleration relative to an inertial frame of reference) but not the string (no side slip relative to the air mass)

1. Answer to original OP

In a stable airmass, the amount of sideslip/skid indication is irrespective of the wind strength and orientation; it is a function of the relative velocity of the airplane against the airflow only (stable winds are referenced to ground). That is, with all engines operating and rudder centered (assuming no inherent asymmetry), then the sideslip indication will be zero, whether you have winds or not.

In gusts, the winds are transitory against both the ground and the airplane frame of reference, which will induce change in airspeed, AOA and sideslip. This will result in change in aero forces and moments, until the inherent stability of the airplane stabilizes the airplane or the gusts die down.

In a pure lateral gust, this translates to a sideslip (and an increase in airspeed) measurable on the yaw string, and generates a side force measurable on the skid indicator.  Both would concur in measurement (opposite in visual direction).

2. Addendum to address the modified OP

  The air mass is no longer an inertial frame of reference.

No one ever uses airflow as a frame of reference unless you're doing fluid mechanics, which is not very relevant here. Airflow matters to airplane only because the latter responds to airflow changes (via aerodynamic forces).

The ball does not measure the crab angle; it's dumb and doesn't know what the airplane heading is or where it's going. The ball only measures the side force relative to the aircraft, much like a bubble level. With all engines operating (or no engine operating) and in flight, only aerodynamics can generate side force, most directly via sideslip (control surfaces can, too; but the magnitude pales in comparison).

In your scenario of a linearly increasing crosswind, sideslip increases with crosswind, measurable by the string. Increasing sideslip induces side force, measurable by the ball, whilst pushing the airplane off course, further increasing crab angle. At the same time, the airplane yaw stability responds by yawing the nose toward the crosswind. After some time, without pilot correction, the airplane has yawed sufficiently that the crosswind becomes a head wind.


What does one do when the weather report has a bogus altimeter setting? I saw a METAR for a nearby airport that had A2248 ... SLPNO in it with no maintenance indicator.  Is the SLPNO a sign that the altimeter setting in the METAR is untrustworthy as well?  How else does one get the altimeter setting (assuming ATIS is drawing from the same, messed up data source that the METAR is -- this is an RMK AO2A METAR), or is one grounded until the AWOS reading is fixed?

We might speculate why the METAR is giving innaccurate data, but the question asks, "What should one do..."
I will take the liberty of inferring the question to be "what should one do if one is unable to obtain an altimeter setting for the airport from which he or she is preparing to depart?" Note that this question could be asked due to an airport with a faulty weather reporting system (as in the question above), an airport without weather reporting at all (quite common, for example: D42 or S68), or an aircraft that is not radio equipped.
I will further take the liberty of answering from what I know: the FAA regulations.
I will point out that it is the pilot's responsibility to be familiar with all available information and this should include a thorough weather briefing whenever possible.
Under the FAA's jurisdiction, the relevant regulation is 14 CFR 91.121 (a) (1):

(a) Each person operating an aircraft shall maintain the cruising altitude or flight level of that aircraft, as the case may be, by reference to an altimeter that is set, when operating—
(1) Below 18,000 feet MSL, to—
(i) The current reported altimeter setting of a station along the route and within 100 nautical miles of the aircraft;
(ii) If there is no station within the area prescribed in paragraph (a)(1)(i) of this section, the current reported altimeter setting of an appropriate available station; or
(iii) In the case of an aircraft not equipped with a radio, the elevation of the departure airport or an appropriate altimeter setting available before departure

(Since all airports and terrain in the US are below 18,000 MSL, this applies to takeoff; the rules governing flight above 18,000—14 CFR 91.121 (a) (2)—do not apply to our question. "Operate" is defined in 14 CFR 1.1.)
We can go through this like a flowchart:
If "the current reported altimeter setting of a station along the route and within 100 nautical miles of the aircraft" is available, use that.
(Note that this could include current reported altimeter settings obtained by radio (ATC, flight service, or a nearby airport), phone call (briefer or AWOS), internet, or any other method that would procure the needed data.)
If no such station is available, then use "the current reported altimeter setting of an appropriate available station."
Now, if the aircraft in question is not equipped with a radio, then (and I would read technically only then) the pilot is permitted to set the altimeter to "the elevation of the departure airport or an appropriate altimeter setting available before departure."
Now, that being said, if a pilot is in the middle of the Idaho backcountry at Wilson Bar (C48), or on the face of an Alaskan glacier, with no cell reception and no real way to acquire a current reported altimeter setting despite being in a radio equipped aircraft, then actual practice will be to set the altimeter to field elevation and get on with it. However, the requirement to use a current reported altimeter setting remains, and should be acquired at the first available opportunity.


Do airplanes or helicopters ever fly at night using only white navigation lights? I've been trying to figure out if airplanes or helicopters ever fly at night with only white Nav Lights.

See this UAV video taken on July 12, 2015, 05:38 AM Pacific time in San Francisco, CA, USA.

There are no such things as white navigation lights - nav light, by definition, are red/green/white.

However, we're talking about filming lights at night. And the problem here is that the light source, no matter what colour, is a concentrated source of light in major contrast to the lack of light around it. And so the light will often 'burn in' on a camera and appear to be a lot of light, ie white, where the human eye will pick up the colours.


Do Federal Regulations really require compliance with all crewmember instructions? On commercial flights, we're frequently told that all passengers are

  "required to comply with lighted signs and placards, and all crew member instructions".

But when it comes to the CFRs, the closest I can find is 14 CFR 121.571

  (a) Each certificate holder operating a passenger-carrying airplane shall insure that all passengers are orally briefed by the appropriate crewmember as follows:
    (1) Before each takeoff, on each of the following:
      (i)Smoking. Each passenger shall be briefed [...] This briefing shall include a statement that the Federal Aviation Regulations require passenger compliance with the lighted passenger information signs, posted placards, areas designated for safety purposes as no smoking areas, and crewmember instructions with regard to these items.
      (iii) The use of safety belts, [...] This briefing shall include a statement that the Federal Aviation Regulations require passenger compliance with lighted passenger information signs and crewmember instructions concerning the use of safety belts.

These rules seem to say passengers only have to follow instructions regarding Smoking, and Seatbelts.

Is there some specific, Federal Rule1, other than catch-all 91.3, that requires general compliance with all instructions?

What if I want to leave my tray table down?  Or keep my electronics on? Or put my feet in the aisle?

1Not a contract of carriage with the airline

I found an overview from a lawyer on what "interfering" actually means. The short version is that there's no regulation or law that says "you must obey all instructions". Instead, they say "you must not interfere" and that's interpreted very broadly.

She based the article on one regulation and one law:

14 CFR 121.580 (see also 91.11 and 135.120)

  No person may assault, threaten, intimidate, or interfere with a crewmember in the performance of the crewmember's duties aboard an aircraft being operated under this part.

49 USC 46504

  An individual on an aircraft in the special aircraft jurisdiction of the United States who, by assaulting or intimidating a flight crew member or flight attendant of the aircraft, interferes with the performance of the duties of the member or attendant or lessens the ability of the member or attendant to perform those duties, or attempts or conspires to do such an act, shall be fined under title 18, imprisoned for not more than 20 years, or both. However, if a dangerous weapon is used in assaulting or intimidating the member or attendant, the individual shall be imprisoned for any term of years or for life.

The lawyer's analysis is that not following an instruction would be a violation of the regulations (civil penalty). If you go further and physically interfere with a crewmember that would be assault (criminal penalty).

When it comes to instructions specifically, she says that "interference" can be interpreted very broadly (emphasis mine):

  Almost any offensive or disruptive behavior that distracts the crew
  can be considered interference, such as:

  [...]

  disobeying repeated requests to sit down, return to your seat, or turn off an electronic device

Her basic advice is to follow all instructions if possible and never touch a crewmember:

  Any time you disobey a crewmember’s instructions, you run the risk of violating federal law. But civil penalties and criminal prosecutions usually result only when passengers repeatedly ignore, argue with, or disobey flight attendants; or when they act out in a way that is dangerous.

You can see the FAA's enforcement statistics on this, by the way. Interestingly, it seems that the number of cases fell sharply in 2005 and has been trending down since then.


Is it Ok for co-pilots to snooze? What are the policies on long flights and taking shifts sleeping between 2 pilots?
Could a pilot set the cruise control and alarm during any flight?

Yes, it is called in-seat-rest.

  This is the process whereby pilots may take short periods of sleep (naps), while temporarily relieved of operational duties in accordance with carefully prescribed ‘controlled rest’ procedures, when part of a two-man operating crew of an in-flight aeroplane.

Source: skybrary


What is the FAA's definition of "for hire"? (note: references to specific FAA documents or cases are always appreciated!)

This is a general question and might best be considered as open-ended.

It is, however, posed for clarification of a few specific regulations:

91.205(c)(4) "if the aircraft is operated for hire, one electric landing light..."

91.409(b) "...no person may operate an aircraft carrying any person for hire .... unless within the preceding 100 hours..."

91.205(b)(12) "if the aircraft is operated for hire over water and beyond vliding distance from shore, approved flotation devices..."

There may be others regulations that reference the term in Pt 91.

The point is: do these regulations imply that neither landing lights, life jackets, nor any current 100 hr inspection are required for e.g. (1) a solo rental, (2) a rental with $\leq$ pro-rata contributing friends, (3) a checkride ? It is my understanding that none of these are not "for hire" flights but perhaps there is clarification otherwise.

i.e.1) what flight scenarios/activities are conducted under for hire circumstances and therefore must comply with the above regs?

i.e. 2) Does the term for hire describe the airplane or the pilot?

Without combing through the regulations, and the vast collection of LOI on the subject, I will give a general answer.

For hire for the purposes of 91.409 applies to the carriage of persons. So the carriage of a student pilot, with a flight instructor is "for hire."  The carriage of people receiving a scenic tour that they paid for, is "for hire."

If you belong to a flying club, essentially operated as a partnership, then the aircraft is not for hire, as the fixed and variable expenses are shared among the members of the club.  Essentially, you are paying your share for your aircraft, assuming the club or partnership owns the aircraft.  In that case the landing light and 100 hour examples do not apply.

In general, if operating part 91, and the pilot is paid in furtherance of the part 91 entity, then for hire does not apply. This would include for example a corporate pilot, paid by the owning entity of the plane.

In general, flight instruction is not considered a for hire operation, when the person receiving instruction is one of the "owners" of the plane, as in a partnership or flying club.

So, you have some examples for your second to last question.

For the last question, it covers the operation, and the purpose of the operation. So your paid pilot flying your TBM taking your family to your private island, is not for hire.

There are no less than 20 FAA Letters of Interpretation which have been written on this topic. The interpretations have even impacted states and how they charge sales and use tax.

Addendum #1: For the FAA, under 91.409, the term "for hire" applies to the carriage of the aircraft.  So if a flight school owns a plane, and a student gets instruction in the aircraft, it is subject to 100 hour inspections, because the student is being carried by the plane and CFI which the flight school is providing.


Does a large simulator bay have standard public address announcements? We are simulating a "simulator bay" (with simulators) within a simulation and were wondering if there were any announcements made over the loudspeaker audible to people inside or outside the simulators.  Thinking of these large 5-simulator buildings like FlightSafety operates so anyone that spends a lot of time there (instructors, airline new-hires, someone headed there for recurrent training)... any help is appreciated. Thinking of things like:

Bay 5 is now available, bay 5 is now available
James Mahan, please report to bay 4 for your check-ride
Maintenance crew to bay 1, maintenance crew to bay 1

My experience with the military is that loudspeaker announcements like this do NOT occur in simulator buildings.  They would be both distracting and largely unnecessary when there are land lines and published schedules available.

Besides the simulators themselves there are briefing and debriefing rooms where crews and instructors are discussing conduct of the events.  Frequent loudspeaker announcements would interrupt these discussions and negatively impact the learning process.

A simulator building is much closer to the environment of a classroom or library than it is to a busy airport, train station, or other public area where you would hear announcements of the type you describe.

Depending on how much realism you are striving for there are some distinctive sounds you will hear in a room full of full motion simulators. These range from the low frequency industrial hum of HVAC and equip cooling fans, to the higher pitched whine of hydraulic pumps, to the swoosh and thump sounds of actuating cylinders moving and hitting bump stops during more aggressive motions associated with unusual attitude or spin recoveries. Up at the control console it will be a bit more muted, but you will hear the crew ICS and radio communications over a speaker, stall warning horns, GPWS alerts, etc.


Where can I find the transition altitude in different countries/regions? Where can I find the transition altitude in different countries or regions?

Hoping to cover as many places as possible. Links to the applicable regulations would be nice.

See also:

What is the difference between "flight level" and "altitude"?

Where can I find regulations in different countries for flight level rules based on direction of flight?

EDIT: Instead of request a list, now the question asks for how to find in general. A few examples should still be given though.

Asia

Hong Kong: 9,000 feet

Africa

South Africa: fixed and to be at least 1000 ft above the highest ground within 25nm radius of an airfield (AIC 20.2)

Europe

Belgium: 4,500 feet (AIP Part 2,, ENR 1.7.2)
Netherlands: 3,000 feet (IFR) / 3,500 feet (VFR) (AIP Part 2, ENR 1.7.1.1)

Oceania

Australia: 10,000 feet MSL (AIP ENR 1.7 (2.1.1))
New Zealand: 13,000 feet AMSL (CAA CAR 91.425)

North America

Canada: 18,000 feet (SOR/96-433 602.34)
United States: 18,000 feet MSL (14 CFR §91.121)

South America

Argentina: Depends on terminal airspace
Brasil: Depends on terminal airspace
Chile: Depends on terminal airspace
Colombia: 18,000 ft (SAM/IG/12-NE/23)
Ecuador: Depends on terminal airspace
Paraguay: 3000 ft (SAM/IG/12-NE/23)
Perú: Depends on terminal airspace
Uruguay: 3000 ft (SAM/IG/12-NE/23)
Venezuela: Depends on terminal airspace


Why does this chart seem to show airports with magnetic variation up to 90 degrees? I am trying to understand some plots of the earth's magnetic variation (declination) by trying to correlate what I see in a figure like the one below with the variation reported for some airports. I'm particularly interested in those locations where the figure seems to show an extreme variation of up to 90 degrees. But when I look up nearby airports, I don't see any variations values outside of 22 degrees.

Some locations I have checked are

SOG, Sorong, Indonesia (1.47 degrees)
BWN, Brunei (0.13 degrees)
CMB, Bandaranaike International Airport , Sri Lanka (2.15 degrees)
REC, Recife, Brazil (22.12 degrees)
IVA, Ambanja, Madagascar (9.8 degrees)

Am I misreading the figure? What are some airports with very large magnetic variations? I'd like to compare them with this figure and try to understand what I am missing.

Source: Wikipedia

How do you think "the figure seems to show an extreme variation of up to 90 degrees"?

I don't think you're understanding that diagram correctly.

There is nothing close to a variation of 90° anywhere that is populated.

The angle of the curve has nothing to do with how strong the variation is.
All points on a single curve share the same variation.  If you find a 10° variation on a curve, and trace that curve a thousand miles away, that point will also have a 10° variation.

Look at about Washington DC, with -10° variation.  Follow that curve all the way down to the middle of South America.  That point also has -10° variation.

Look near Denver, CO, where it is +10°.  Follow that curve out to near eastern Hawaii.  The curve there is almost due East-and-West, but that doesn't matter.  That area also has +10° variation, because it is on the same curve.

The bold curves are at 10°, 20°, 30° etc variation.
The red curves have a positive (east) variation.
The blue curves have a negative (west) variation.

All of this information is in the legend at the bottom of the map.


How does ICAO differentiate between Commercial and Private flights? How is the difference between Commercial and Private/General Aviation flights defined by ICAO?

Here in the US, we use 91 flights -vs- 135 flights, but from what I've heard they use a different definition, particularly concerning business aviation flights that fall under Part 91.

In other words, what determines whether ICAO Annex 6 Part 1 or Part 2 applies to a particular flight?

According to ICAO Annex 6 Part 1 (PDF):

  The Standards and Recommended Practices contained in Annex 6, Part I,
  shall be applicable to the operation of aeroplanes by operators
  authorized to conduct international commercial air transport
  operations.

According to ICAO Annex 6 Part 2 (PDF):

  International general aviation operations with:

  a) aeroplanes with a maximum certificated take-off mass exceeding
  5,700 kg; or

  b) aeroplanes equipped with one or more turbojets engines.

The International Flight Plan form also requires flight type to be declared, but I am not sure that this would be ICAO's sole requirement.

Type of flight—Indicate which one:

S—Scheduled air transport
N—Non-scheduled air transport
G—General aviation
M—Military
X—Other

For a simpler explanation, this PDF lists civil aviation activities:

This page contains a table (at the bottom) classifying each type.


How much lift is created by a half-sized rotor? I wonder if lift is directly proportional to the diameter of the rotor disc.

For example, if an engine spinning a 26 ft diameter rotor can lift 635 kg, will a 13 ft diameter rotor using the same engine lift 318 kg? The power absorbed by the rotor is the same in both cases.

For the hovering rotor, the stationary case, it can be safely assumed that the lift $L$ is a function of the input power $P$, the diameter $D$ of the rotor and the air density $\rho$.

Thus, $L = f(P,D,\rho)$

where $f$ is a function to be determined.

From dimensional analysis, the lift $L$ can be easily derived:

The variables are Lift $L$, dimensions $MLT^{–2}$; Power $P$, dimensions  $ML^2T^{–3}$; Rotor diameter $D$, dimensions $L$ and air density $\rho$, dimensions $ML^{–3}$

The variables form a non-dimensional product $k$

$k = L^a\cdot P^b\cdot D^c\cdot \rho^d$  where $a,b,c,d$ are numbers to be determined.

Let’s form now a parallel product $k^*$ with the dimensions:

$k^* = (MLT^{–2})^a  (ML^2T^{–3})^b  (L)^c (ML^{–3})^d$

Clearly, $k^* = M^0 L^0 T^0$... We now take the exponents for each dimension:

$a  + b + d = 0 \\
a + 2b + c – 3d = 0 \\
–2a – 3b = 0$

We make $a = 1$, since $L$ is the variable we’re going to solve for.

$b = –2/3 \\
d = –1/3 \\
c = –2/3$

Then,

$k = L^a\cdot P^b\cdot D^c\cdot \rho^d \rightarrow  k = L\cdot P^{–2/3}\cdot D^{–2/3}\cdot \rho^{–1/3}$

Solving for $L$

$L = k\cdot P^{2/3}\cdot D^{2/3}\cdot \rho^{1/3}$

where $k$ is a constant

Hence, for rotor diameters $D_1$ and $D_2$, and for the same power and air density, the corresponding lifts $L_1$ and $L_2$ are:

$L_1/L_2 = (D_1/D_2)^{2/3}$

For the case of $D_1 = 13 ft$ and $D_2 = 26 ft$, $L_1/L_2 = (13/26)^{2/3} = 0,63$

In other words, the smaller (13 ft) rotor gives you, for the same power and air density, just 63% of the lift attained with the larger (26 ft) rotor.

That's for the hover. For the climb, you'll need extra power. In order to move 635 kg vertically upwards at 1200 ft/min (6,09 m/s) you would need $635 \cdot 9,8 \cdot 6,09 m/s = 37,9 kW...$


Is there any benefit of using a swept wings at low-subsonic speeds? Is there any benefit to using swept wings for speeds up to 250km/h ?
(By the way..Why do some fast birds like falcons, swifts and most migratory birds use swept wings or raked wingtips since they fly at low subsonic speeds?)
Peregrine falcon - swept wings

Alpine swift - swept wings

Albatros - straight wings with raked wingtips

Common swift - straight wings with raked wingtips

Birds adjust wing area by pulling in their humerus and ulna so the wing sweeps first forward, and to compensate for that, have to sweep their outer wing backwards which is accomplished by folding the digits backwards. Instead of adjusting lift coefficient, they adjust wing area to not expose more surface to air friction than necessary.
The picture below shows how the wing is opened for maximum lift at low speed. As Zeiss Ikon correctly observes, folding the digits back and fore is the bird's way of pitch control, so the sweep is a consequence of pitch control, not aerodynamics.

Red-tailed Hawk, captive bird, Bacara, Santa Barbara, California. By Steve Jurvetson / CC BY (https://creativecommons.org/licenses/by/2.0)


Are passengers more likely to see other airliners from a window seat on the right or on the left? From the window of an airline, one is more likely to see a plane travelling in the opposite direction, than one flying in the same direction.

Are planes travelling in the opposite direction more likely to be found on the right or the left? That is, are opposite airways given a left/right arrangement?

Your best bet is flying across the Atlantic, where the Strategic Lateral Offset Procedure (SLOP) is used.

This is where the pilot flies a offset to the route, either 1 or 2 NM, always to the right, chosen randomly (or based on surrounding traffic/TCAS), and is kept from the ATC (no radar coverage, although that is changing with satellite ADS-B).

As shown below, you have a chance to see another plane above/below from either side depending on the pilots' flip of coins.

Source; adapted

Regarding two-way airways elsewhere and with the navigation paradox, where flying accurately is ever increasing, it will be a slim chance except for a crossing traffic (not same airway).

If in a holding pattern, the standard turn is right turns, so the right side gives a better look "inside the track," unless the pattern is non-standard to the left.

Source: Mid-Atlantic Race!!! Virgin Atlantic A340 vs Norwegian 787 Dreamliner


Why are airplanes sometimes led by a vehicle? Airplanes are sometimes led by a vehicle on taxiways until they reach the runway or stand, why is this?

"Follow-me cars", as they are known, make it easier for pilots to get to the correct spot by the correct route.

This could be the runway before take-off, or the final stand after landing.

They're useful at airports with complex routes and relieve the pilots of some additional navigation burden; their function is one both of safety and convenience.

They might be used as a rule at a particular airport, or on request (if perhaps the pilot is unfamiliar with the taxiways, or there's an urgent need to get to a gate very quickly), or because of local conditions (signage or lighting problems, re-routing because of ground maintenance work).

However, the presence of cars around aircraft and runways is not without its own risks, and follow-me cars in fact add an extra layer of complication to airfield management.

(And sometimes, they have a rather odd marketing role... at Bologna (BLQ) one of the follow-me cars is a Lamborghini. Make of that what you will.)


Do reduction drives increase tach time quicker than direct drive engines? For example, the Rotax 912ULS engine has a 2.4:1 reduction. Does this mean that the Time Between Overhaul (TBO) will be reached 2.4x faster than in a direct drive engine or is tach time reduced by 2.4:1 as well?

The relationship between tach time and RPM varies based on the engine. As mentioned in the linked answer, it is difficult to find this relationship based on engine specifications, but if you find a tachometer certified for the engine it will have the correct ratio built in.
If the 912ULS needed TBO based on tach times, the tachometer would likely count tach time at 1:1 somewhere below 5500 RPM, since that is the maximum continuous speed for this engine. However, Rotax actually uses Hobbs time to determine when an engine needs a TBO, and specifically notes that a "mechanical hour meter" may give different times. Therefore, the TBO of 2000 hours is 2000 hours of real-world time.
From the 912 Series Maintenance Manual: (p. 3)

All of the maintenance intervals, such as the 100 hr. inspection and the engine TBO, relate to the number of operating hours of the engine.
The operating hours are defined as follows in order to prevent misunderstandings and to ensure safety:

All time during which the engine is running is counted towards the total number of operating hours.

The time is counted irrespective of the load factor of the engine, such as idling or take-off power.

NOTES: A mechanical hour meter is directly coupled to the engine speed, the readings may deviate considerably from those given by electronic remitters (e.g. TCU, FlyDat). Maintenance and overhaul intervals are always dictated by the readings of the electronic hour meter.


Did the Soviet Union build any commercial jetliners? I read about a Wikipedia article regarding the Soviet Union's Tupolev 114 which was comparable to the time's jetliners, and brought up the question: Did the Soviet Union build any jetliners for commercial use (i.e, airlines, firefighting).

Of course. Probably more than any other single country except the US.

Tupolev Tu-104, one of the first jetliners. For a short time, when the Comet was grounded, it was the only jet airliner flying in the world.

Tu-124, its development, the world's first jetliner with turbofan engines.

Tu-134, further development; claimed to be the world's most produced twin jet of this class (80-pax).

Tu-154, a trijet, USSR's most popular airliner. (Similar to B727 or Trident).

Tu-204 was developed in the 1980s and is essentially 'Soviet', but never saw service in the USSR.

Tu-144, of course. Although it first flew a few months before Concorde, it had extremely limited commercial (or better say, 'civil') service.

Ilyushin Il-62, a long-range jet, a relatively rare example of a quad-jet with all engines on the back (similar to Vickers VC10).

Il-86, USSR's first commercial widebody, and its later development Il-96.

Yakovlev Yak-40, a small (30-40 pax) regional trijet, the world's first in its class.

Yak-42, the first Soviet jet with high-bypass turbofan; a trijet smaller than Tu-154.

All these jets (except Tu-144 and 204) were in wide use in the USSR, and most were exported at least to the Eastern Bloc countries and beyond. Of them, only Il-96 and Yak-42 remain in limited commercial use, although more are operated by air forces and government bodies.


How does ATC ask a pilot to resume flying a published SID / STAR? Let's say that you are cleared for and are flying the ZZOOO2 departure out of KSAN and you get vectored off of the departure before you complete it.

At some point, ATC wants to put you back onto the departure.

How would ATC ask you to get back on the ZZOOO2 departure, adhering to all altitude and speed restrictions along the way? Is there standard phraseology that exists?

More than likely they give you vectors to fly to get you back on course, or have you proceed directly to the next waypoint. Really depends on where you got turned away and your intended direction of flight afterwards.


How is the runway pavement chosen? Wikipedia list a number of runway pavement. For specific airports, the choice is obvious (e.g. ice, snow). But there are few pavement I cannot figure out how to decide which one use.

Why an airport would choose asphalt over concrete or bitumeous asphalt for the runway pavement? What are the reason for the choice?

I am neither builder nor airport manager and can't provide details, but as there is no answer yet, I'll try to summarize the main differences between the materials:

Concrete can support highest loads and lasts long. It is however most expensive to build and when it eventually gets damaged, it can't be patched and has to be rebuilt in large slabs. It also requires stable base, because it is rigid. It is built in slabs with spaces between them that are required due to account for thermal expansions and give somewhat bouncy ride.
Asphalt. There is a big variation between different kinds of asphalt concrete, but it is generally cheaper than concrete, but can't support as big loads and is susceptible to damage by frost. Asphalt is never completely rigid and slowly deforms under load. This allows using it on unstable base, but it means bumps form by prolonged use reducing the total lifetime. It also easily cracks and when water freezes in the cracks, potholes quickly form. They can however be relatively easily patched. It does not require any thermal expansion gaps and provides smooth ride.
Asphalt and concrete can be combined. A concrete with a layer of asphalt on top provides the strength of concrete and smooth ride of asphalt. The asphalt also protects the concrete base from freezing water while itself can be easily patched. This is lately common on roads (where concrete alone is really uncomfortable), not sure how much on runways.
Gravel is simple to build; all you need is some stone from nearby quarry and a steamroller. The main disadvantages are risk of foreign object damage by loose stones thrown up by the wheels and need of regular maintenance. So it is used mainly in remote cold places where concrete would be too expensive and asphalt would not last long in prevailing weather conditions.
Rolled earth with or without grass is even simpler then gravel, used for small airports where big loads are not needed.
When runway is listed as grass, it means it is not even rolled. Usually enough for small airplanes and obviously cheapest.
Other surfaces usually as provided by local conditions (snow, salt etc.)

For every particular airport, the investor evaluates the available technologies whether they can handle the expected traffic (to handle large jets like 747 or A380 concrete or concrete-asphalt combination is preferred), whether they are buildable in given location (the ground has to support the weight too) and how much building them will cost and choose the option that satisfies the requirements and they have money for.


How far can a Super Hornet fly at max supersonic speed in a clean configuration? How far can a Super Hornet fly at its top (supersonic) speed, assuming it's in a clean configuration, i.e. no external tanks and no ordnance?

An estimate:

An F-18E Rhino can only obtain its maximum speed of Mach 1.8 only at 36,000 ft in ideal air conditions and only in a clean configuration i.e. no external stores.

At 36000 ft at STP, Mach 1.8 translates to approximately 550 m/s or   1069 KTAS using the equations:

v = M*SQRT(g * R * T)

Where v is the velocity of the jet in m/s, M is the Mach number, g is the ratio of specific heats, R is the Ideal Gas Const. and T is temperature in Kelvin (use OAT at -40 C, 233 K).

Now an F-18E holds 14,400 lbs of fuel internally.  The GE F414 engine burns about 36,000 lbs/hr of fuel in full afterburner, so a total fuel burn of 72,000 lb/hr would be typical for this power output.  Assuming the Rhino could start at these altitudes and at Mach 1.8 with the tanks topped off, that gives us 14400/72000 = 0.2 hr or 12 minutes of flight time.  Ignoring upper level winds this would give us a maximum range of 213.8 NM.

This might be enough to put warheads on foreheads, then run like hell back to friendly airspace and a tanker or a carrier but it's not very good for a long range cruise profile.


How common are missile defense systems on civil airplanes? I've just stumbled upon Why don't El Al 747's stay at the gates at JFK? in the hot topics.
I kind of get why El Al decided to put missile defense systems in their planes but I was wondering: How common is this? Are there other airlines who do this?

It's far from being common. The following carriers use missile defense on all or part of their fleet:

El Al
Israir and Arkia "can be assumed" to use the laser-type from Elbit (flightglobal.com, 2009).

  While Elbit has not identified any of the carriers likely to receive the equipment, a source familiar with the agreement says "it can be assumed" that all three major Israeli airlines - El Al, Israir and Arkia - are in line to be fitted.

FedEx uses Northrop Grumman Guardian (Wikipedia).

Missile defense systems are also reportedly installed on some VIP business jets.

Further reading:

Flight Guard
Directional Infrared Counter Measures
Civil Aircraft Missile Protection System

Related: How does the Guardian counter-MANPADS system work?


Why don't NK-12 powered contra-rotating propellers turn in equal rpm? The Kuznetsov NK-12 is a Soviet turboprop engine of the 1950s, designed by the Kuznetsov design bureau. The NK-12 drives two large four-bladed contra-rotating propellers, 5.6 m (18 ft) diameter (NK-12MA), and 6.2 m (20 ft) diameter (NK-12MV). It is the most powerful turboprop engine to enter service.
   (Wikipedia: Kuznetsov NK-12)

Tupolev TU-95 (NATO codename Bear) and Antonov AN-22 (NATO codename Cock) use the NK-12 turboprop engine, and as can easily be seen in many videos available on the internet, their propellers are not geared in a way that would make them turn in equal rpm.

TU-95 propellers turning ratio is such, that for each full turn of the front propeller, the one behind turns a little more than half a turn: TU-95 engine start on Youtube.

The two frames above show the aft propeller's lag after the forward propeller moved a quarter-turn.

For AN-22 the ratio is one full turn for front prop, and about 3/4 for the rear one: AN-22 engine start on Youtube.

Why such ratios? What is the reason the front propeller has significantly higher rpm compared to the one behind it? The air that has gone through the disk of the first propeller has obviously been accelerated, and has gained a swirl momentum. This would have to be taken into consideration somehow.

My own guess would be that this has to do with the aforementioned flow changes, load distribution between propellers, and/or something with vibrations and resonances of the powertrain.

Please do not hesitate to prove me wrong on this one, but for that to fly, I'd require some hard evidence concerning the NK-12 powertrain.

The NK-12 engine uses a differential epicyclic (planetary) gearbox. Unlike the typical planetary gearboxes where one of the gears is fixed, here all three parts are moving: the turbine drives the sun, the planet pinion drives the front propeller, and the ring drives the rear propeller.

The gear ratios are the same between the front and rear, but the torque is not. The difference you see has to do with this torque difference, as well as with different rotating masses and friction during start. The torque is what is actually important.

If you hold one part, then the other two will be in the expected sync. Say, if you jam one prop and start the turbine, it will simply spin the other prop twice as fast. (But if you jam the turbine, things will be less intuitive: the props should turn in the same direction if you move one by hand). Now if you allow the turbine to free spin, you'll have two degrees of freedom, and the eventual relative RPM in such system will depend on the torque on each prop.

In the explanation below, I use this work by S. Falaleyev et al (pdf in Russian). Here is the diagram of the gearbox:

If we assume the torque balance such that the RPMs are the same (except the sign), the gear ratio will be (I skip the derivation):

$$i = 1 + 2(\frac{z_2 \cdot z_4}{z_1 \cdot z_3})$$

($z$ are the teeth numbers according to the diagram). For the given numbers we get $11.33$ (or rather, it should be $1/11.33 = 0.088$ reduction). The nominal speed of the props was just 735 RPM!

At the same time, the torque ratio (front to rear) is

$$\frac{M_f}{M_r} = -(1 + \frac{z_1 \cdot z_3}{z_2 \cdot z_4}) \approx -1.19$$

Thus, the front prop gets almost 20% more torque than the rear prop. This is why it spins up faster on start-up. (It is also lighter than the rear one).

At the nominal speed, the governor automatically sets a finer blade pitch on the rear prop to equalise the speeds. (Why not just accept higher speed/thrust from the front prop? Apparently, because the prop is limited by the tips speed, and we don't want the front one go supersonic while the rear one is relaxed).


What are these aircraft at the Naval Air Weapons Station China Lake? What are these 12 aircraft at the co-ordinates 35.677323, -117.676903?

I recognize the F-111 and C-130, but not the others.

Source: google maps

This answer is a Community Wiki.
Community edits are welcome.

The aircraft in the photo appear to be the following, or varients therof:

Douglas R4D-8 (Super DC-3)

source, photographer: Brian Lockett

North American T-39 Sabreliner
Boeing C-97 Stratofreighter or KC-97 tanker varient
McDonnell Douglas F-4 Phantom II

source, photographer: Brian Lockett

Vought F-8 Crusader
North American T-39 Sabreliner
North American RA-5C Vigilante tail number 156640

source, photographer: Brian Lockett

Boeing B-29 Superfortress

source, photographer: Brian Lockett

General Dynamics F-111B tail number 152715

source, photographer: Brian Lockett

Grumann A-6 Intruder
Douglas R4D-8 (Super DC-3)
Grumann A-6 Intruder

No C-130 appears in the photo included in the question, though one may appear elsewhere near the coordinates linked.

Notes:

1 and 11 are not C-47 Skytrain. They are longer and have a square tail which is typically for the Super DC3 / R4D-8
3 is longer from the wing root to the nose than the same measurement on 8, supporting that 3 is not a B-29.
Moreover, in the photo of 7,  aircraft 3 appears in the background which clearly shows engine nacelles like those of the B-50 or C-97 series.
The same photo shows a fuselage rising higher above the wing than a B-50, which is consistent with a C-97 variant.
5 measures nearer the 54 ft in length of an F-8, versus the 46 ft in length of an F-7, supporting that it is an F-8


What provides a higher lift to drag ratio, a thick laminar wing or a thinner wing with a vortex generator Recent readings have shown the benefits of "boundary layer tripping devices" or vortex generators for reducing drag and maintaining attached air flow at higher angles of attack.

It seems these generators effectively increase the width of the wing by creating an energized
"fence", which further deflects the free air stream, almost acting as a lubricant to reduce friction drag on the surface of the wing and increasing the "bend" of the airstream, thereby increasing lift.

But generating these vortices comes at the expense of increased drag to create them, though they (and slats) could be made to be retractable.

Is there a clear advantage to either a thick laminar wing or a thinner one with vortex generators?  The Reynolds number for the application would be around 200,000 to 500,000.

You confuse turbolators with vortex generators. And then you misunderstand the function of vortex generators.

Turbolators trip the boundary layer and change laminar to turbulent flow in order to enable the boundary layer to remain attached over a steep pressure rise. That is definitely required at the Reynolds numbers mentioned (200,000 to 500,000) in order to maximize performance.

Vortex generators work in a fully turbulent boundary layer and re-energize it by mixing more of the outer flow into it.

Both increase the boundary layer drag but help to prevent early separation. Only over the range of that prevented separation they reduce drag.

The answer to your question depends on the Reynolds number. At 1,000,000 and above, the laminar wing should be better and if you add boundary layer tripping at the right chord station, it also wins down to 200,000.


What should you do if you are forced to enter restricted or prohibited airspace? Apparently, even if you have declared an urgent situation or an emergency, you cannot enter restricted or prohibited airspace, and ATC may not be able to clear you into the area. So, my question:

If you had a serious condition onboard an aircraft (say severe icing) and high terrain (I know, you probably haven't planned) in all directions except for restricted or prohibited airspace, what should you do to try and make sure that people are aware of your situation and can try and minimise the risk of doing so?

Even if ATC can't legally give you permission to enter prohibited airspace (they sometimes can for restricted airspace by the way), if you declare an emergency they will still coordinate with the controlling agency which will help to keep you safe.

Squawk 7700, fly as high as possible, turn on all your lights, broadcast your intentions on 121.5 if you aren't talking to ATC, and don't make any sudden dives toward the ground/buildings, etc.  This will all help to show that you aren't trying to "sneak in under the radar" and can only improve your situation.

Don't be surprised if you get intercepted though, and make sure that you talk to them on 121.5 if you do.  Know your intercept procedures (section 5-6-2) and follow them exactly.  They will most likely try to help you as much as possible.  You will still face a lot of questions and probably even certificate action when you get on the ground though because, as you said, poor planning is no excuse....

Also, this applies in the US. Some other countries (possibly Cuba, Iran, etc.) tend to be far more sensitive to issues like this and may "shoot first and ask questions later".


Do commercial airlines have procedures for evacuating pets from the cabin in an emergency? I recently got into an offsite discussion about flying with pets in the cabin and what would happen in an emergency on board. I brought up the fact that United Airlines' pet policy states, "In the event of an emergency, oxygen service will not be available for pets." Southwest has a similar phrase in their pet policy, though I couldn't find anything similar in American Airlines' or Delta's pet policies.

Beyond oxygen service, I was arguing that if an evacuation took place, I wasn't aware of any procedures for getting a carryon pet evacuated safely. I said that the human lives of the other passengers on board would take precedence over a pet. To my knowledge, pets in carriers are essentially treated as a carryon bags in the cabin. I also hypothesized that letting a pet out during an emergency could be extremely dangerous and would likely invoke the ire of the crew, if not get you into legal trouble.

TL/DR: The discussion left me wondering are there actually any procedures for getting carryon pets out of the cabin in an emergency? This is mostly directed at pets that would be in a small kennel and stowed for the duration of the flight under the seat in front of you as a personal item. Are they considered carryon luggage, which should then remain stowed (as awful as that sounds), or treated like infants? Have there been instances of serious accidents where carryon pets were also evacuated in the past?

There are no policies because pets are technically cargo/baggage as you note in the question, and treated as such. In other words in case of an emergency evacuating the passengers is a priority and as you would be instructed to leave your baggage behind you would more than likely be instructed to leave any pets behind.

  The Federal Aviation Administration (FAA) allows each airline to
  decide if they will allow you to travel with your pet in the passenger
  cabin. If an airline does allow you to bring your pet into the cabin,
  we consider your pet container to be carry-on baggage and you must
  follow all carry on baggage rules (14 CFR part 121, section
  121.589)

The exception to this is service animals which are not treated as pets.

  Service animals are not pets. They are working animals that assist persons with disabilities.


What are the curved centerlines at taxiway intersections for? At all airports where I've looked out the window during taxi, the taxiway intersections have been painted with nice circular arcs connecting (the centerline of) each entry to the intersection with each exit.

I've always assumed these lines were there to guide planes through the turns -- that is, if the nose wheel follows the curved line exactly then you're guaranteed that there'll be pavement under the path the main gear will follow, and there will be sufficient wingtip clearances for the aircraft types allowed to use the intersection, etc.

However, in this question it comes to light that pilots can't even see the curved centerlines while they're turning and rely on spatial intuition instead. Very well, that seems to work fine in practice.

Which leads to the question: Who are the curved centerlines for, then? What would go wrong if they weren't there and instead just the straight centerlines of the intersecting taxiways continued through?

Possibly the pilots of smaller aircraft than the intersection is built for (say, a Dash-8 in an intersection that can take A380s?) can see the curved lines ahead of them while they turn. But in that case they should have plenty of room to do the turn by intuition too, right?

The line along taxiway centreline is meant to provide guidance before initiating the turn. You're right that in a large aircraft they would lose sight of the centreline when they're over it, but it gives the pilot an idea of where they need to keep the nose in order to keep the outer main wheels behind them on the taxiway surface.
For example, in Canada, Transport Canada document TP312 (Aerodrome Standards and Recommended Practices) define how you need to place the centreline in order for the outer main wheels to remain at least a certain distance from the taxiway edge. (I believe the United States would have very similar rules; I just don't know the names of the appropriate documents)

Taxiway Curves
3.4.1.6 Recommendation - Changes in direction of taxiways should be as few and small as possible. The radii of the curves should be compatible with the manoeuvring capability and normal taxiing speeds of the aeroplanes for which the taxiway is intended. The design of the curve should be such that, when the cockpit of the aeroplane remains over the taxiway centre line markings, the clearance distance between the outer main wheels of the aeroplane and the edge of the taxiway should not be less than those specified in 3.4.1.3.

Think about when a large aircraft has to make a big turn (>90degrees). If there was no indication on the surface about how to take the turn in a large aircraft, you could inadvertently take it too early and run the main wheels off the pavement.
Here's an example of an aircraft following the centreline with its nose as best as possible in order to keep the main wheels in the right spot.


How can a crew be warned of "Clear Air Turbulence" during the flight? Is there any piece of equipment, on board, to assist the crew to identify and avoid clear air turbulence (CAT)?

Not on board the aircraft as of yet, no (unless, as @Gerry said, you count the radio that lets you hear about the person ahead of you running into CAT).

There are ways of detecting CAT, however, which could become standard aircraft equipment in the future.  LIDAR is one possibility, as both @Gerry and @mins mentioned; see also here and here.  Interestingly, the idea of using LIDAR to detect clear-air turbulence has been around since at least the late 60s, but, when it was first proposed, they couldn't get the sensitivity high enough; later, when this obstacle was overcome, they faced a different problem, which was that the equipment required was far too heavy, bulky, and power-hungry to be practical, which remained the case until very recently.  N-slit interferometry can also be used to detect clear-air turbulence (see here, for instance), and an interferometric (I think; the patent description is remarkable in its unintelligibility) CAT detector was patented in 1978, but the equipment required is far too large to fit on an aircraft; as a result, most work with interferometric CAT detection seems to have been in the area of developing a ground-based clear-air turbulence detector that could monitor runway approaches for dangerous CAT.


What are the spiral marks in the center of the engines? When I get a chance to peek inside an engine, I noticed that many have spiral marks painted in the center. What is the purpose of these?

Pictures:

The benefit of these white markings inside the engine is twofold:

The ground crew can tell if the engine is running or not.
When an airplane is at the gate before departure, there are several personal attending the airplane. They would be loading luggage, refueling, removing waste products, loading food, etc. etc.. Not all of these people are in constant contact with the pilots and they can come very close to the engines. Although there are several precautionary measures taken before the pilots turn on the engines, it is a very clear visual indication by looking at the spirals to tell if the engines are running or not. Rotor or prop blades sometimes have their tips painted for the same reason.
The birds keep away.
During flight, when the engine is running, birds see the spinning spiral as opposed to just a black hole, and hence they stay away from it. It is like an anti-bird signal because if there was nothing in the middle it just looks like a cave or black void that birds tend to fly toward. Even though it looks like a blur to us, birds can see motion better. Wikipedia mentions this also:

  ... it appears as a white circle which discourages birds from flying into the engine.

Rolls-Royce brochure (PDF) quoted the following (identified by user BowlOfRed in comment below):

  Our aerospace engines have swirls painted onto their spinners in order
  to indicate when the engine is rotating while on the ground. In flight
  these swirls flicker as the engine rotates at high speed, scaring
  birds and allowing them to fly clear of the engine.


What is the Extent of the Emergency Authority of the Pilot-in-Command (FAA)? When writing an answer to this question regarding flight deck access during flight, I mentioned the authority of the PIC to deviate from regs as necessary for safety purposes, citing the usual FAR, 14 CFR 91.3(b), which says:

  In an in-flight emergency requiring immediate action, the pilot in command may deviate from any rule of this part to the extent required to meet that emergency.

However, the exact wording used here made me wonder exactly how far this reg has been interpreted to go. In particular, it grants the PIC emergency ability to deviate from "Any rule of this part", which would be 14 CFR Part 91. So, does this authority extend only to Part 91 of the FARs or does it extend to all of the FARs? The exact wording seems to suggest it only applies to Part 91 and not, say, Part 121, which was the part under consideration in the other question.

So, the major things I'm wondering are:

What is the extent of the authority granted by 14 CFR 91.3(b)? and
If 14 CFR 91.3(b) indeed only applies to Part 91, is there a similar explicit emergency PIC authority exception to the other Parts of the FARs?

Obviously, the PIC is the one in charge and will deviate where necessary for safety regardless of regs and worry about the explaining later after he's ensured he'll be alive to explain, but I'm curious about the regulatory basis for deviation from FARs.

The scope of the part 91 exception is exactly what it says: deviate from any rule of this part, i.e. part 91. But remember that part 91 applies to all flights anyway; part 121 (or 135 etc.) regulations are in addition to part 91, not instead of them.

But that still means that 91.3(b) doesn't give any authority to deviate from other parts, like 135 or 121 regulations. 14 CFR 135.19(b) is the equivalent authority for part 135 operations:

  In an emergency involving the safety of persons or property, the pilot
  in command may deviate from the rules of this part to the extent
  required to meet that emergency.

Note that the language is again of this part. So in an emergency, the PIC of a part 135 flight can deviate from part 91 rules per 91.3 and from part 135 rules per 135.19.

Part 121 is a little different. 14 CFR 121.557(a) says:

  In an emergency situation that requires immediate decision and action
  the pilot in command may take any action that he considers necessary
  under the circumstances. In such a case he may deviate from prescribed
  operations procedures and methods, weather minimums, and this chapter,
  to the extent required in the interests of safety.

Here, the regulation allows for deviations from any part of this chapter, not this part as in the part 91 and 135 regulations. Chapter I of title 14 is at a higher level: it includes all parts of the FAA's aviation regulations, so under part 121 emergency authority the PIC can deviate from any aviation regulation, not just the ones in part 91 or 135.