Are cargo holds pressurised these days? This question is only regarding aircraft that seat more than about 100 people.

Are cargo holds pressurised these days, what is the situation?

Are only some pressurized, most, of every single one? Does it depend on the carrier, operating region or are there other variations that matter?

Related questions:

Are cargo holds heated?
Do they have lights? (Aren't some lit for animals?)
I've noticed random mentions of "some" of the sections being pressurised. Is this correct? It would seem to me that, if indeed, only some are pressurised then, of course, you absolutely could not rely on your luggage being in a pressurised area.
and what about Fedex-type cargo-only aircraft?

To be clear, I ask this question re "today" because I noticed when googling on this, there is a lot of information, but only old information (say, 10 yrs old plus). That is a recipe for confusion and urban myth, so the total facts from you experts would be great.

(Possibly, it would be difficult to secure accurate online references for this - my quick searching anyway only revealed patchy, out-of-date looking stuffs. Note for example, the Wikipedia article on Cabin Pressurization has only one poor, no-referenced sentence on the whole matter!)

Note for example... Spray bottles (pressurized) in the checked luggage?

Aircraft design has not changed that much in the last 10 years. In fact, most aircraft in production 10 years ago are still in production.

The cargo holds in typical airliners are indeed pressurized. Take a look at a cross section of an airliner (A380 here):

The round shape of the fuselage outline is very efficient at withstanding pressure. Because of that, everything within the fuselage shape is pressurized. This includes the cargo hold below. Only cargo holds located behind the aft pressure bulkhead would be unpressurized, and these are mainly found in smaller aircraft.

The floor of the passenger cabin is not designed to withstand that pressure, because the flat surface would need to be much heavier to do so. Decompression events are considered by regulations though, so there are vents that allow the pressure to equalize. In older aircraft without these vents, a decompression event can cause the floor of the cabin to collapse, as in this incident (and because it wasn't addressed, this one too).

  Are cargo holds heated?

Sometimes. This will depend on the aircraft type. There are many threads over on the TechOps forums about this. The temperature can be adjusted depending on the cargo load.

Although the air outside the plane is cold, the walls are insulated (also for fire protection), and being pressurized along with the cabin helps too. Even in unheated cargo holds, the temperature should be above freezing. For example, the cargo bay of a 767 stays above 7˚C but with heat stays above 18˚C.

  Do they have lights? (Aren't some lit for animals?)

They do have lights, mainly for ground crew during loading and unloading. See this Boeing page about fire protection where it mentions that "another safety feature on many newer airplane models prevents the cargo lights from operating in flight".

  and what about Fedex-type cargo-only aircraft?

These aircraft are still based on the airliner models (and sometimes converted from airliners). Therefore, they have the same pressurization system. The cabin is still pressurized, or they would have to put in some type of bulkhead behind the cockpit.

This includes the cargo version of the 747 (both upper and lower areas are pressurized).

The rare exception would be aircraft like the 747 LCF (Dreamlifter). The cargo section is not pressurized, and there is a bulkhead behind the flight deck section.

Yes, they are pressurized and climate controlled, especially on commercial flights where live cargo eg pets, bees, chicks, etc may be loaded aboard in the spaces and would be lethal to the creatures to be in a space at subzero temperatures and without supplemental oxygen encountered during cruise at high altitudes.


How can pilots obtain runway length information for airports that do not appear in the chart supplement? How can pilots obtain official runway length information for airports that do not appear in the chart supplement such as private airports?

If an airport does not appear in the Chart Supplement, the only official information about it seems to be in the relevant Sectional Chart.

In the case of Horseshoe Acres (FA24), the sectional shows that the longest (and only) hard-surface runway is "18", which means 1770 to 1869 feet.

In the case of Brady Ranch (80FD), the sectional shows that the longest (and only) hard-surface runway is "46", which means 4570 to 4669 feet.

Note that you can only guess the runway numbers, the lengths of shorter runways, and the presence of unpaved runways. As @RonBeyer comments, since you have to get permission to land on these fields anyway, it's probably best to ask for such details at the same time. If you get data from the airport operators themselves, that would seem to qualify as official.

However, Airnav and others have lots of consistent data about many airports not in the Chart Supplement, particularly private ones, so it's possible there is some other common source. Since I don't know what that might be, I can't guess how official it is.

Just use the ruler feature for measuring distances down to feet in Google Earth.  Not "official" but pretty accurate.


What is the formula for the bank angle required for a turn in line-abreast formation? Imagine we have a line-abreast formation of aircraft, with differing airspeeds, turning around a central pivot point, like points on the radius of a phonograph record, with an equal amount time per 360 degrees of turn for each aircraft.  What is the formula for the bank angle required of each aircraft, in terms of the variables 1) turn radius and 2) time period per 360 degrees of turn?

Note that essentially the same situation applies when a slow-flying glider-- maybe even a hang glider-- and a faster, heavier glider are thermalling together and trying to stay exactly opposite each other.  Again the turn rate must be the same for each, but the turn radius is not, and the required bank angle for each might be different.  Clearly, the difference in turn radius is much more significant in this case, than in the case of high-speed aircraft turning in tight formation.

Turning Rate

All planes flying a turn in line-abreast formation are turning at the same rate $\omega$.

A standard turn is commonly defined as $\omega_1 = 3°\frac{1}{\mathrm{s}}$.

Full Circle Time Period

If planes are turning at the same rate, the time period to complete a complete circle will be the same as well.

Time period: $$ T(\omega) = \frac{360°}{\omega}$$

The time period to complete a circle at this standard turn rate is $T_1 = \frac{360°}{3°}\mathrm{s} = 120\,\mathrm{s}$.

Air Speed

Since the outer planes have to travel a longer distance to complete their larger circle in the same perios, the outer planes have to fly at a higher airspeed then the inner planes.

Air speed: $$ v = \omega r$$

Bank angle

Turn rate: $$\omega = \frac{v}{r}$$

Banking angle:  $$\tan{\theta} = \frac{v^2}{rg} = \frac{\omega^2 r}{g}$$

Gravitational constant: $$ g = 9.81 \frac{\mathrm{m}}{\mathrm{s^2}}$$

That gives the banking angle as a function of turning radius: $$\theta(r) = \arctan{\frac{\omega^2 r}{g}}$$

See the purple curve in the graph below for the bank angle in a standard rate turn (3°/s):

This curve seems to imply that one can fly a standard turn (or any other given turn rate) at any radius from 0 to infinity. Therefore I added two more curves to indicate the physical limits:

air speed (green) must be in the operating range of the plane
g load (blue) must not exceed the maximum acceptable for plane and occupants. (The correct scale for the g load is the purple scale on the left divided by 10)

And finally, the same plot but for a double rate turn (6°/s):

Practical Relevance

Typical turn radii are in the order of 1000s of meters. Typical distance (wing span) of planes in a tight formation is in the order of a few 10 meters, that is a few percent of the turn radius.

If you compare the banking angles for two planes flying in tight line-abreast formation, the banking angle difference will have a similar relative difference, that is a few percent, or a fraction of 1° in absolute terms.

[I will calculate and insert an example here, when I have the time.]

For the practical purpose of actual formation flying, pilots will give the arcus tangens a break, and fly seemingly identical banking angles.

Using the formulae $F_c = m * \frac{v^2}{r}$ and bank = arctan (Fc / (mg)), we can derive the formula bank = arctan (4*pi^2 *r / (P^2 * g)), where Fc is centripetal force, g is the gravitational constant, and P is the period or the time for one complete turn. The aircraft flying at a larger turn radius must use a larger bank angle than the aircraft flying at a smaller turn radius.
Note that mass or weight does not appear as a  variable in the final equations.
Note that v represents True airspeed-- the relationship between bank angle and turn rate or turn period (time per turn) or turn radius is not independent of density altitude.
Read on only if you want to see the math in more detail:
v=d/t = 2pir/P
bank angle= arctan(horizontal force/ vertical force)
=arctan (centripetal force/weight)
=arctan ((mv^2/r)/(mg))
=arctan ((v^2)/(rmg))
=arctan ((2pir/P)^2/(rg))
=arctan (4pi^2r/(g(P^2)))
Related link--
A link to an aviation website giving a rule of thumb for bank angle for a standard rate turn-- the rule of thumb is that the bank angle in degrees should be roughly .15 * (True airspeed in knots).


Why do passenger jets accept input that will cause the aircraft to perform dangerous maneuvers it was not designed for? Examples:

A bank angle > 45 degrees is considered an “upset,” putting the plane in a position that can lead to a loss of control.
A pitch > 20 degrees can possibly be dangerous and cause the aircraft to stall (depends on many factors or course).

These are just a few examples of limits that most passenger jets have. Now why is it that the plane actually accepts input that will cause the aircraft to go beyond these limits? What possibly use could “diving” straight towards the ground or towards the sky have? Or having a high bank angle that will almost certainly cause stalling and loss of control?

Generally speaking, pilots don't like it when a computer interprets or limits their actions. They want final control. They don't always get their way on this but that's their preference.

If I recall correctly, Boeing tends to stick with the philosophy that "the pilot is the final arbiter." Airbus is more likely to preempt pilot inputs and modify them.

Although the majority of crashes and incidents wind up being pilot error, there is a serious flaw to modifying pilot inputs. That flaw is in the case of systems failure.

By definition, failure modes involve things going wrong.  When things go wrong it's effectively impossible to plan, in automation, for all contingencies. People are much better at responding to the unknown than automation systems are.

Take for instance the rule that "bank angles > 45 degrees are dangerous, and are therefore prohibited". How does the plane know that the bank angle is > 45 degrees? Well it's a sensor of course, but what if the sensor has failed? A failed sensor will either signal to take action when none is needed or fail to signal when action is called for. What if the control surfaces have failed and the plane cannot correct the bank angle?

The usual answer to that is redundant systems, high reliability parts and design, etc.  All those are great of course and certainly help a lot. However we still have incidents and accidents.

In the end the question is: Who do you trust more? A pilot or a machine? And statistics and science only help you part-way here. A person's experience, biases and feelings will have a lot to say about how they answer. And by "person", understand that I'm including the customers, the paying public.

Pilots want to have control over the aircraft. Personally, if I was an ATP I would never fly an automated FBW aircraft like an Airbus for that exact reason.

You don't to be in a spin or death spiral and then find out you can't get out of it because "the computer will not allow that input", and trust me if you are in an aircraft that goes into a spin you most definitely want the pilot to have FULL control over the aircraft.

You may think: if the computer is controlling the aircraft it will never enter a spin. Not so. Some thunderstorms can arise very quickly and unexpectedly and are so violent they can force the aircraft into a spin or other attitude which the computer has zero chance of reacting to correctly. "Good" computers turn themselves off at that point.

The most advanced FBW aircraft have a feature called "automated recovery" which allows them to automatically recover from some stall conditions, but in extreme conditions the changes in attitude will be happening so fast that you need a human to do it.


Is it hard to hover a coaxial rotor helicopter? Convetional rotorcrafts are hard to hover as when we increase collective, even tail rotor pitch needs to be compensated. But in coaxial design, will just increasing the collective do the job as torque doesn't change?

The difficulty in hovering a helicopter lies in the fact that hover is unstable. Any tiny divergence will increase over time, so you have to keep adjusting the controls all the time. And this does not differ between single rotor with anti-torque and dual-rotor.

There is more than just compensating the tail rotor pitch (which I believe is somewhat compensated for anyways mechanically), which makes hovering a single-rotor helicopter challenging. You also need to tilt the main-rotor to compensate for the side thrust of the tail. If the main rotor is vertical you drift sideways.
However, keep in minds rotors are never perfectly identical. So you still need to compensate manually, if even a very small (possibly unconscious) amount.
I don't know if it's hard on a coaxial, but it should definitely be easier than on a single-rotor.


What does the area around the fan blades in the engine nacelle contain? I was intrigued to find this picture below. There was so much space around the engine. So what is inside of that extra space? Or is it just a hollow casing?

Image Source

Details about the picture as provided in the image source:

  [Last Year] A Southwest Airlines Co. flight landed safely Saturday
  morning following a major malfunction of one of its two engines during
  a flight across the Southeastern U.S.

  The Boeing Co. 737-700 was flying from New Orleans to Orlando early
  Saturday operating as flight 3472 when it suffered the failure,
  causing the jet to quickly divert to Pensacola International Airport
  in northern Florida at 9:40 a.m. CDT, according to a statement from
  the airline.

  A Southwest spokesman said its initial reports indicated no injuries
  were reported among the 99 passengers and five crew aboard.

Yes a bit of the engine called the nacelle is missing. This is an aerodynamic structure that slows the air down before it enters the compressor, the pointy bit in your photo. In the drawing underneath it states Inlet Cowl. It has some thickness, because it requires a certain nose shape for its function. Mainly a hollow casing indeed, but with a high tech shape.

Above all, the clearance in the engine pod is to facilitate routine maintenance and quick visual assessment during turnaround cycle of operations. Further, this greatly helps in efficient engine replacement cutting down on 'Downtime' !
This is one of the factors, I feel.


What is the comparative roughness of fabric surfaces? "The Design of the Aeroplane" by Darrol Stinton (1983) lists comparative roughness of aircraft skin material (pg 154). Unfortunately it does not list dope & fabric.

What is the comparative roughness of dope and fabric surfaces?

It varies with the level of finishing effort, and depends mostly on how much the weave of the fabric was filled in.  If the weave shows in the fabric, you can expect the surface roughness to be similar to lacquered wood.  If there are enough dope coats to fully fill the fabric, you'd have something similar to commercial metal painted finish.  But if you really go to town you can get to the filled and polished glass surface finish.  You'll see some award winning homebuilts at Oshkosh with a fabric finish that looks like shiny mylar plastic, but is actually regular fabric and dope.   Many many coats, and many many hours of sanding and rubbing.

Some of the latest finishing systems can produce that finish without zillions of coats and hours of rubbing.

This is all notwithstanding all the surface irregularities you have with fabric due to the use of reinforcing tapes etc., which may make the whole issue of surface smoothness moot anyway.


What material was/is used on the skin of the Blackbird SR71? What material was/is used on the skin of the Blackbird SR71 and how was it cooled.  How was it manufactured to be stronger with age

Titanium. Only the hottest portions of the fuselage (the chines) were cooled, using the fuel as a coolant. As far as I'm aware, there was no treatment to make the material stronger with age.

Titanium is the only suitable material which can withstand those temperatures.
Stainless steel is much heavier. If you want to find out more about this read Ben Rich's book on the Skunk Works.


Why not mount airliner jet engines above the wings? It seems that most airliners put the engines below the wings, i.e. the Boeing 7*7 series and the Airbus A3** series. This requires long (heavy) landing gear, and the engines are close to the ground which may cause damage to the engines (ingest debris) or the ground.

Why not mount the engines above the wings? In addition to the shorter (lighter, stronger) landing gear, the plane would be easier to service and embark / disembark. What are the major disadvantages to having the engines above the wings for civilian airliners?

The question "How does the mounting location of a jet engine affect aircraft performance?" addresses the performance issues affecting engine placement but answers do not cover over-wing placement extensively and focus only on performance and not other operational issues or design advantages.

Maintenance

Having the engines below the wing makes them fairly easy to access, even if some kind of lift is needed. Putting them above the wing would make them much harder to get to. Dropped a tool? Better start on that wing repair too. This includes both general access to the engine and the procedures for removing and installing the engine.

Structure

An engine below the wing is fairly stable, since it is hanging from above. It naturally wants to keep hanging downwards. Mounting the engine above the wing would be less stable. It naturally wants to tip over.

Noise

If an engine is under the wing, it helps to block some of the noise from at least the mid cabin. An engine above the wing would expose much more of the cabin to noise. It could reduce the ground noise while in flight, though.

Aerodynamics

The top of the wing is the most sensitive part aerodynamically. This is why items like fuel tank access panels are placed on the bottom. Having an engine and pylon on top would affect the flow over the top of the wing.

Safety

Engines falling off the wings tends to be a bad thing. But if they were above the wing, they would tend to fall down onto the wing, which would still not be good. It would be harder to have then break away in a crash. Also, in the event of an uncontained failure, the wing would not provide any protection to the fuselage.

Of course there are benefits, such as the reduced noise, less ground clearance required, no interference between the jet blast and the flaps, and less chance of FOD being ingested. As new engines get larger fan diameters, the constraints become more difficult. Some concepts of the blended wing body design put the engines on top of the body. But for the current design of aircraft, the benefits of having the engines below the wing are greater than those of the alternatives.

Below the wing helps lift.
Above the wing hinders lift.
Below allows the air to go over the wing not spoiling the aerodynamics for lift.  Above disturbs the aerodynamics for lift, depending how far back the engine is on the wing. Sure lighter landing gear is good, aircrafts don't use the landing gear when flying, and besides the engines weigh far more then the landing gear. It's better to have the engine under the wing. New models can raise the wings and can decrease the pylon length. However the lift is not yet enough. They need lift, every heavy cargo aircraft has the engines under the wing. They cannot afford to lose any possible lift.


HSI KG 525 PNI non slaved or only slaved? I'm in Germany and currently on a project about the avionics of an older airplane (PA28). Here is a link to a former reddit question.

My Prof. and me want to update the Avionics now - he asked me to do a bit of research - it is quite a good exercise for me and i learn lot this practical way (i'll become an aircraft engineer)!. I do the prework for our maintenance company and they'll supervise my work/planning.

The main idea is to install new:

Garmin GNS430
Garmin GMA340
Insight G4 + Sensorics
Becker AR6203 22
KI - 525A or Garmin G5 DG/HSI
Insight G4 for experimental stuff (UAS courses with our piper)

This avionic stays, but will be reconnected

KN53 (with GS) – goes to KI 204
KI 204
KAP 100 (AP)

Will be removed:

GPS150
KI 203 - Insight G4 will come here instead
KN53 (w/o GS)
KY 196 (2x COM)

Right now i have the DG - KG107 in it, but my professor wants to update it to a HSI..

Now the question is if a KI-525A has a built-in gyro or is there only the possibility to connect it with the KG102 (which we dont have)?
I would like to have the new Garmin G5 DG/HSI but it seems that it doesn't have an EASA STC. Garmin is kinda quiet.  What do you think - should we install the G5 (DG/HSI) and hope for a STC or pick another HSI (maybe Century)

EDIT: Took 3 questions off the topic

The G5/430 combination was approved by EASA back in February! No reason why you can't use it. Some advice for you, learning how to use Google search is an invaluable skill for any aircraft engineer. :)


Is it safe to takeoff with some ice and snow on the wings? I took a look at the post 'What things can a passenger look out for, to indicate an emergency?', and remembered this video I've seen.

Obviously, video uploader claims that this take off was unsafe due to some ice and snow on the wings. Was he right? Were the pilots really breaking any regulations?

It appears that there was a considerable amount of contamination, either snow, ice, frost, or a mix thereof, on the wing when the takeoff roll started, and that is not considered safe under any current standards I'm aware of.

There are markings on the top of the wing for the 737 Next Generation series that define an area where cold-soaked fuel frost is acceptable up to a defined thickness, but the area covered by the contamination in the video extended beyond those markings, so even if it was CSFF (which I doubt, given what's shown late in the video), it was still more than what's allowed.

Late in the video with the aircraft flying, we see that much of the ice is no longer present, which means that it detached from the wing at some point.  If it was dry snow that blew off in the first 10 or 20 knots of the takeoff roll, that's no big deal, but you don't see the ice gone until late in the video -- this isn't simply dry snow.  Plus, you still see significant patches of ice adhering to the wing at the end of the video -- which suggests that it isn't CSFF nor dry snow, but ice.

The current accepted standards of safety in commercial aviation don't permit taking off with any ice adhering to the wing, and from what is shown in the video, it certainly looks like that's what was done here.  (Disclaimer, I'm no expert on watching u-tube videos of ice-covered wings, so there may be more going on here than is apparent on first or second viewing.)  IF that's correct, then I'd have to agree with the title of the video, that this was unsafe.

The standards of safety allow for a lot of margin, so that when mistakes get made they are still likely to be survivable, and it looks like that's how things turned out here.  Things were done wrong but everybody made it out okay.  But... what if one more thing had gone wrong?  What if they'd lost an engine at rotation?  Would the aircraft have been able to climb out with one engine & the wings in this condition?  Or would these guys have been headline news of a crash out of Sarajevo that killed all on board plus how many more on the ground?

No, this looks pretty far out of bounds to me.

The video appears to show frost or ice on the wing of an airliner during takeoff. Normally this is discouraged because even a thin coating of ice can disrupt an airfoil's boundary layer airflow, causing a premature aerodynamic stall and dramatically increased functional drag.

However, extenuating circumstances often intrude upon standard safety margins. There are many reasons why a flight crew might risk flying with an apparently contaminated wing surface:

The flight crew had hand inspected the wing and found it smooth even if discolored.
No de-icing was available.
De-icing was available but the takeoff waiting line was so long that the wing iced up again while taxiing. This was the minimum amount of ice that the flight crew could get the plane to the takeoff point with.
The captain is a right bastid and ignored the tearful pleas and terrorized screams from his first officer.
The discoloration on the wing is not ice or frost, but de-icing fluid.
Anything is safer than hanging about on the ground at Sarajevo Airport.

In any event, you are survived to tell the tale, so by definition the flight crew's judgement was correct.


Is there a list of aircraft types to be used with a callsign? In the U.S., general aviation aircraft use the airplane type with the callsign when talking to ATC, such as "Skyhawk 123CS" or "Citation 321CT". ATC likes to know what type of airplane you're flying so they know how what its performance is.

It gets complicated with airplanes like the Cessna Corvalis, which is a Cessna (formerly Columbia) but it is faster than a Cirrus and can fly an approach at 200 kts if required. So what would the callsign of a Corvalis be? "Corvalis 123TT" or "Columbia 123TT"? I doubt it would be "Cessna 123TT".

Is there an official (or unofficial) list of airplane types to use when talking to controllers?

I couldn't find any specific guidance on this, the AIM 4-2-4(a)(3) just says (emphasis mine):

  Civil aircraft pilots should state the aircraft
  type, model or manufacturer’s name, followed by the
  digits/letters of the registration number. When the
  aircraft manufacturer’s name or model is stated, the
  prefix “N” is dropped; e.g., Aztec Two Four Six Four
  Alpha.

See also the ATC orders 2-4-21 for the controllers' side. It doesn't give any information either on where to find type and model information and the examples suggest that precision isn't very important:

  EXAMPLE−
  “Tri−Pacer.”
  “P A Twenty−Two.”
  “Cessna Four−Oh−One.”
  “Blue and white King Air.”
  “Airliner.”
  “Sikorsky S−Seventy−Six.”

If "airliner" is an acceptable identifier then anything is :-)

As you said, you can comply with the AIM simply by saying "Cessna", whether you're in a 120 or a 750. But what about experimental aircraft, or a type that the controller just doesn't know?

Personally, I think this is a practical issue rather than a formal one. If it was critical for ATC to know the performance of every aircraft then flight plans would be mandatory. As it is, both controllers and pilots rely on their working knowledge of aircraft types and the system just works. The big, fast aircraft are mostly IFR anyway so the controllers already know what to expect; the small, slow ones can all do 80-100 knots in the pattern and the controllers don't need to worry about them much.

In the unusual cases where a specific aircraft type's performance might be an issue, the pilot can always work it out with the controllers:

N12345, Louisville Approach, I need your best forward speed on the approach, can you give me 120 knots?1
Bowman Tower, N12345, we're a warbird, I need a minimum 90 knots in the pattern and request an overhead break for runway 242

1Real example, and no I couldn't
2Also a real example, but sadly not me flying

They can be found in FAA Order JO 7360.1C - Aircraft Type Designators


What are these two holes on the forward fuselage of a Boeing 747? Does anyone know what are the two holes are on the Boeing 747, on the left forward part of the fuselage?

Source: airliners.net

I guess the right hole is the E/E compartment smoke override valve, is that correct?

Another hole under the belly just before pack inlet:

What is the function or name of this one?

The forward opening is the forward outflow valve or forward overboard valve (depending on reference consulted) which opens automatically if airflow in the forward compartment is low. It is independent from the two large aft outflow valves at the lower aft hull used for pressurization.

The second valve is the smoke override valve used in case there is smoke sensed in the the E&E compartment. Basically you try to get rid of the smoke by dumping it overboard. The second function is in case the fans used for ventilation of the E&E compartment fail. Then this valve is opened as well and the differential pressure ventilates the E&E compartment in reverse flow direction. The duct who was supplying the air is now sucking it out of the compartment.

The opening close to the pack inlet on the R/H side is the ground exhaust valve used to dump the air used for E&E cooling on ground overboard in case the temperature is high. This valve is always closed in flight.

Reference:

Boeing 747-400 System Schematic Manual, chapter 21, valves V462 (forward overboard valve), V447 (ground exhaust valve), V448 (smoke override valve)
internal documentation provided by employer of the author

The forward one is exhaust for E&E cooling. The aft one probably is dirt on the camera lens. You can see from the close up that it is a miss shaped circle.


Is a winglet better than an equal span extension? Is there indisputable evidence that a winglet improves performance over an equal span extension? Please note: I am only interested in L/D improvements.

Winglets do improve roll performance, that is not what I am looking for. Also, if span is restricted, winglets improve L/D over straight wings. Again, that is not what I am looking for.

Boeing and Airbus use fancy wingtip designs to demonstrate technological sophistication and make incredible-sounding claims about them. This is not the kind of proof I ask for. Is there theoretical or practical evidence comparing winglets with span extensions of equal wetted surface which show that the winglet produces better L/D values at any point of the polar?

Bonus points for a comparison of net lift to drag comparison, so the structural impact of both wing extension and winglet get subtracted from the generated lift. This should be the fairest way to compare both, but it seems such research is not published at all.

Here is what I think you need to come to your own conclusion. First I will give a very general overview over lift creation, and then I will look at three wings:

An unmodified wing
This wing plus a winglet
This wing plus the winglet, but this time folded down into the plane of the wing.

For each I will plot the lift and bending moment distribution. I will assume an elliptic circulation, fully knowing that this is not what most aircraft use. But I have to pick a distribution to make all three cases comparable, and the elliptic one makes things easier. The conclusions can be generalized for other distributions.

This will be a lengthy post (you should know me by now), so thanks to all who persevere through all of it.

Lift creation and induced drag

This topic had been covered before, and I mention it again to show a very simple and elegant way to explain induced drag that does not need vortices. I want to dispel the myth that induced drag is caused by air flowing around the wingtip, and winglets somehow magically can suppress this flow.

Consider a wing with elliptic circulation over span (think of circulation as the product of the local lift coefficient $c_l$ and local chord; it is basically the lift per spanwise increment). The wing bends the air through which it flows slightly downwards, and creates an opposite upwards force, namely lift (Newton's second law). I choose an elliptic distribution because then the downwash is constant over span, which makes the following calculations easier.

The sheet of air coming off behind the wing looks trough-shaped and moves downwards, thereby pressing other air below out of the way and allowing air above to flow inwards and to fill up the vacated volume. That is how the free vortex is created, and air flowing around the wingtips has only a small part in this.

Induced drag is the consequence of the wing bending the airflow downwards. To simplify things, let's assume the wing is just acting on the air with the density $\rho$ flowing with the speed $v$ through a circle with a diameter equal to the span $b$ of the wing. If we just look at this stream tube, the mass flow is $$\frac{dm}{dt} = \frac{b^2}{4}\cdot\pi\cdot\rho\cdot v$$

Lift $L$ is then the impulse change which is caused by the wing. With the downward air speed $v_z$ imparted by the wing, lift is: $$L = \frac{b^2}{4}\cdot\pi\cdot\rho\cdot v\cdot v_z = S\cdot c_L\cdot\frac{v^2}{2}\cdot\rho$$

$S$ is the wing area and $c_L$ the overall lift coefficient. If we now solve for the vertical air speed, we get $$v_z = \frac{S\cdot c_L\cdot\frac{v^2}{2}\cdot\rho}{\frac{b^2}{4}\cdot\pi\cdot\rho\cdot v} = \frac{2\cdot c_L\cdot v}{\pi\cdot AR}$$
with $AR = \frac{b^2}{S}$ the aspect ratio of the wing. Now we can divide the vertical speed by the air speed to calculate the angle by which the air has been deflected by the wing. Let's call it $\alpha_w$: $$\alpha_w = arctan\left(\frac{v_z}{v}\right) = arctan \left(\frac{2\cdot c_L}{\pi\cdot AR}\right)$$

The deflection happens gradually along the wing chord, so the mean local flow angle along the chord is just $\alpha_w / 2$. Lift acts perpendicularly to this local flow, thus is tilted backwards by $\alpha_w / 2$. In coefficients, lift is $c_L$, and the backwards component is $\alpha_w / 2 \cdot c_L$. Let's call this component $c_{Di}$: $$c_{Di} = arctan \left(\frac{c_L}{\pi\cdot AR}\right)\cdot c_L$$

For small $\alpha_w$s the arcus tangens can be neglected, and we get this familiar looking equation for the backwards-pointing component of the reaction force: $$c_{Di} = \frac{c_L^2}{\pi\cdot AR}$$

If the circulation over span has an elliptic distribution, the local change in circulation times the local amount of circulation is constant, and the induced drag $c_{Di}$ is at its minimum. If this would be different, a higher local $v_z$ causes a quadratic increase in local induced drag, so the whole wing will create its lift less efficiently.

Now we know we can calculate induced drag and we understand why the vortex sheet behind the wing rolls up, producing two counter-rotating vortices, all without looking at the details of the wingtip. What counts is that the wing is of finite span, so the stream tube which is influenced by the wing is of finite diameter as well. Of course, in reality there is no clear boundary between air which is affected by the wing and other air which is not. There is a diffuse transition the more one moves away from the wing.

Comparison of wingtips

First the geometries: Here are three wingtips in top and front views for comparison:

Now let's look at the circulation distribution of the simple wing tip:

Again, I choose the elliptic distribution for simplicity. The corresponding bending moment looks like that:

No surprises so far. Now we add a winglet and make it work as best as possible. This means we have to give it an angle of attack where it carries the circulation from the wing over on the winglet and completes the elliptic tapering of circulation down to 0 at the tip:

The grey dashed line is the circulation of the original wing. I adjusted the circulation such that both wings produce the same lift. $b_{WL}$ is the span at the winglet tip, and for the bending moment plot I have folded the spanwise coordinate down on the y axis:

Now the bending moment starts at the wingtip with a nonzero value. Since the sideways force of the winglet is parallel to the wing spar, this bending moment contribution is constant over span. But there is more: Now also circulation at the old wingtip location is nonzero, and we get a substantial lift increase at the outer wing stations. This effect is what causes the additional lift and gives the better aileron response that winglets make possible. But it also increases the root bending moment, because this additional lift acts with the lever arm of the outer wing.

How can we compare the induced drag of the wing with winglets to the original wing? The circulation gradient is lower, that helps. Also the diameter of that stream tube is larger, but it is hard to say by how much. The sideways force on the winglet is created by pushing the vortex sheet aft of the winglet sideways out, so the trough-shaped area should become wider. Empirical evidence hints at an increase in diameter of 45% of the winglet span (see chapter 6 for a discussion of several papers on the topic).

Just for the heck of it, let's assume that the diameter really increases in line with winglet span. Then let's compare that to the straight wing extension, where the same diameter can be assumed with much more certainty:

Now also the lift on the folded-down winglet acts upwards, so the circulation at the center of the wing can be reduced even further. However, now it adds a linearly increasing part to the bending moment, and the outer wing section creates more lift, as before with the wing with winglet:

Here, the root bending moment is higher than in the winglet case. This is a second advantage of winglets: They allow to increase maximum lift with less bending moment increase than a wing extension. But the wing extension puts all parts towards the creation of lift, and not some to the useless creation of side force. Both the extended and the winglet-wing have the same surface friction and (when we assume the same diameter of the hypothetical stream tube) the same induced drag. But since the winglet creates some side force, the remaining wing needs to fly at a higher lift coefficient. Also, the intersection of wing and winglet might be as well rounded as possible, this is where early separation starts at higher angles of attack. None of this affects the straight wing extension.

Most evidence shows that winglets improve L/D over the original wing, but folding the winglet down will more than double its effectiveness in lowering drag. Even if we assume that the winglet is just as good as an equal span extension, still the span extension comes out ahead in L/D improvement because all its lift contributes to overall lift, whereas the winglet produces a side force instead. If no separation occurs at the wing-winglet intersection, both will create the same induced and profile drag (pressure and friction), because both have the same wetted surface and the same local circulation. Again, this gives winglets the benefit of equally low induced drag, which is not supported by most measurements.

The extended wingtip in the example above has interesting characteristics. It is a sweptback (raked) wingtip, which causes the local lift curve slope to be lower than that of the straight wing. This increases its maximum angle of attack and  - assuming that the local area is bigger than what an elliptic wing shape would dictate - makes it possible to keep a nearly elliptic circulation distribution over a wider angle of attack range. The bigger local area is a sensible precaution against the wing tip stalling first, so a raked wing tip will combine benign stall characteristics and very low induced drag.

Compare this to the winglet, which has to be tailored for one polar point: Since changes in wing angle of attack will not change the incidence of the winglet, it cannot adapt as well to different flow conditions as can the extended wing. In sideslip the winglet will mess up the circulation distribution on the wingtip and will act like a deflected spoiler.

Conclusion

Comparing equal winglets and wing extensions gives these basic characteristics:

Both have the same viscous drag at low angle of attack.
Both can create more maximum lift, and both lower induced drag.
The wing extension can create most lift for the given increase in wetted surface.
The wing extension is more than twice as effective in lowering induced drag.
The wing extension gives a better circulation distribution at off-design angle of attack.
The wing extension produces the highest root bending moment for a given amount of lift.

How much the bending moment increase will drive up structural mass depends on the aspect ratio of the original wing. Low aspect ratio wings will not suffer much, but stretching high aspect ratio wings will drive up spar mass considerably. But please note that the winglet also causes higher root bending moments, and it creates less bending moment than the wing extension because it creates some side force instead of pure, useful lift.

Yes because a winglet reduces the drag caused by the lower part of the wing comes onto the higher part of the wing and rotates forming a vortex called wing tip vortex and the winglet reduces the strength of the vortex reduces the drag also makes it more efficient and has extra range from the winglet. So its better to have a winglet than no winglet


Would a random passenger be allowed to fix a problem with a commercial aircraft? I just read an article about a man (who is alledgedly an airplane mechanic) who was a passenger on an airplane that had some mechanical issues. Because there were no 'local' mechanics around, he decided to take matters into his own hands and fix the problem himself.

Here's the article I read (it's in Dutch though)

Here's one in English

This all seems nice but in this day of age where airplane security and regulation is so tight, I wonder if this is possible at all. It also seems to me that by the airplane company is taking a huge risk in letting this "random" person fix this problem. The article obviously oversimplifies a few things, but regardless of that, here's my question:

Is it possible for a random passenger, who also happens to be an airplane mechanic (working for airline X) to fix an issue on the airplane he happens to be on, given that there are no "local" mechanics around (however unlikely that might be)?

I happen to have personal knowledge of this particular case. I was in that airplane, too, seated two rows away from him. I'm also a former pilot and was naturally very interested in what happened, so I asked him some questions during the flight back.

He had worked as service mechanic for this company before, servicing their planes in Iceland. Their technical staff and the pilot of this plane already knew him, which made things easier.

Tools and parts were available at the airport in Alicante, but no qualified technician certified for this type of aircraft. The technical staff of the airline contacted his boss in Iceland by phone, who agreed to sub-contract him to the airline for the duration of the repair and pay him overtime for the work.
They faxed a copy of his certification documents to Stockholm to cover the EASA regulations that require a certified professional doing the repair.

Part M requires that:

  Maintenance of large aircraft, aircraft used for commercial air transport and components thereof shall be carried out by a Part-145 approved maintenance organisation.

So, I don't really see how a company would let a passenger work on an airplane, even with a proper licence.

Not to mention the passenger would need tools to perform the job and that's not something one carries in his bag on vacation.


Is there a tank priority sequence for using the fuel tanks? Is there a tank priority sequence for using the fuel tanks, or does the pilot decide?

Which fuel tanks are used in order of priority in aircraft?

What you're talking about, in large aircraft, is often referred to as the fuel burn schedule. Light aircraft generally do not have a fuel burn schedule though they may have have minimal requirements.

The fuel burn schedule is dictated by the aircraft design and is thus different for different aircraft designs.

  Is there a sequence of priority?

Yes, and if you don't observe the fuel burn schedule you risk exceeding the structural limitations of the aircraft, and in particular wing bending moments.

  Or the pilot decides?

No, the pilot does not decide usually. While there may be some leeway in the fuel burn schedules of large aircraft, the pilot or a flight engineer (old aircraft) or an automated system does not decide but rather implements a preset protocol.

As an example, here's a general outline of the fuel burn schedule for 747-100/200 aircraft. This is from memory so might not be entirely correct but is sufficiently so for example purposes:

On start, taxi, and takeoff burn tank to engine. In other words, route the fuel from main tank 1 to engine 1, main tank 2 to engine 2, etc.
Once you're established in your cruise climb, all engines burn out of the center tank if there is fuel in the center tank until the center tank fuel is exhausted (but don't burn any fuel in the center tank that is there for ballast).
Burn engines 1 & 2 from main tank 2 and engines 3 & 4 from main tank 3 until the amount of fuel in main tank 2 matches that in main tank 1 and the amount of fuel in main tank 3 matches that in main tank 4. Main tanks 2 & 3 are larger than 1 & 4. When parity is reached, burn tank to engine.
At some point transfer the fuel in reserve tank 1 to main tank 1 and reserve tank 4 to main tank 4. Likewise, if you have them, from reserve tank 2 to main tank 2 and reserve tank 3 to main tank 3. Most 747-100/200 aircraft didn't have reserve tanks 2 & 3. You have to get the fuel out of the reserve tanks before landing.

Be aware that just as there is a fuel burn schedule, there may also be a fuel loading schedule. In the 747-100/200 that schedule dictated that no fuel goes into the center tank until all other tanks are full. As I remember, the largest center tank available held 114999 lbs of fuel at 6.7 lbs/gal fuel density. Center tank size varied as ordered by the airline and changed through the production run of the aircraft.

Yes! When you are filling up the aircraft and you use the centre tanks, it then means that the pilot has to use the centre tanks first before the 1 or 2 wing tanks are selected, and to keep checking to make sure the tanks are balanced according to the manufacturer’s procedure.


Is there an FAA rule or Advisory Circular with regard to blood donation? Is there an FAA rule or Advisory Circular with regard to flightcrew member blood donation?  For example, how many hours or days is required or advised before a pilot may act as a crewmember?

I'm interested in finding the source of legitimate FAA policy or guidance for flightcrew members.  If the answer is, there is no FAA rule or advisory circular, I'd also be interested in specific examples of FAA-approved air carrier policy.

It seems 24 hours is the generally accepted waiting period to recover from possible hypoxia related to a donation.

An air carrier that I have worked for specifically addresses this issue in their General Operations Manual—which is accepted by the FAA. The manual gives the following requirement:

Flight Restrictions After Blood Donations
Flight personnel shall not participate in any capacity as flightcrew members for a period of 24 hours after donating more than one-half pint of blood.

Note that this portion of this manual is accepted by the FAA, not approved.1
One unit of blood is generally 470-500 mL, or one pint. The one-half pint specified above half of a regular unit of blood.  According the the Red Cross, a typical donation is about one pint, or one unit.

The FAA also publishes the Guide for Aviation Medical Examiners. I am not an AME, nor am I fully familiar with the Guide or how to interpret it. However, I understand that the following table shows that outside the listed time-frames, an applicant for an airman's medical certificate could be considered medically fit for flight duty:

1 The terms Approved and Accepted mean specific things to the FAA. See 8400.10, Vol 3, Ch 15, Sec 1:

Approved: When approved is used to describe a document, manual, or checklist, it means that a regulation requires FAA approval and that the FAA has evaluated and specifically approved the document, manual, or checklist.
Accepted: Accepted is used to describe a document, manual, or checklist that does not have, or is not required to have, FAA approval. Only a portion of an operators manuals are required to have FAA approval. The remaining portions are accepted by the FAA. Operators are required to submit the entire general manual to the FAA for review. If the FAA concludes that an accepted section of the general manual is not in compliance, the FAA must formally notify the operator of the deficiency. Upon notification, the operator must take action to resolve the deficiency.

From what I know, for crew members are prohibited from donating blood unless a period of 72 hrs has elapsed from the time of donation until the next scheduled duty flight.
Source : Regulation of the CAA (South African Civil Aviation Authority which follows FAA requirements and regulation)


How do jet fighters get to war? Currently Australia is deploying F/A-18E/F Super Hornets to the UAE to fight in Iraq. Obviously the UAE (and Iraq) is a very long way from Australia and well beyond the typical range of one of these aircraft.

On TV they literally showed the Australian Prime Minister waving goodbye while the jets took off and flew into the distance, which for some reason seemed odd and made me wonder about how fighter jets are typically ferried to a distant war.

I kind of assumed they were loaded onto an aircraft carrier and shipped there, but Australia doesn't have an aircraft carrier so obviously this isn't the case.

I assume they must therefore fly, but:

Do they fly hop to hop within their usual range refuelling at each stop? Where would these stops be? I assume they can't just refuel at a civilian airport in an arbitrary country and would have to be an air force base belonging to a close ally.
Do they fly continuously with aerial refuelling? Does the tanker need to make hops to refuel?
Do they swap their weapons for extended fuel tanks which provide enough range to fly continuously to the UAE?
Are they actually shipped via regular military cargo ship and the "departure" on TV just for show?
If they do fly continuously, it seems like flying non-stop for ~13 hours in a cramped fighter pilot cockpit would be unreasonable (even commercial pilots take shifts), or would they fly supersonic and perhaps reduce this to 5 or 6 hours?

Answers don't necessarily have to be Australia -> UAE specific, I'd also be interested to know how the US deployed to Iraq and Afghanistan for example and if the process is consistent or depends on the aircraft and location.

To answer the list in order:

Do they fly hop to hop?

An F-18 can go 3,000km unloaded (ordnance adds a lot of drag) and that's plenty from Australia. You can get to the Middle East in 4 hops, no air-to-air refuelling required. Although if you do an in-flight top-up you can add more than the regular range because you don't need to burn any fuel getting back up to cruise altitude.

Where would these stops be? Surely not at civilian airports?

Military aircraft often use civilian airports - they run on Jet-A just like everything else. Fighters were not an uncommon sight at the general-aviation terminal where I'm from. Loaded warplanes are a political issue, unloaded ones are a good day's business for the airport's fuel merchants. (note that fighters are almost never completely unloaded - the ammunition for the gun has a significant effect on the airplane's trim. But you can't see it.)

An external fuel tank can be added in place of weapons.

Are they actually shipped via regular military cargo ship?

Fixed-wing aircraft are typically not boxed for surface shipping. Helicopters are - no wings to remove and rotors can be folded up very easily.
Carrier-based aircraft will depart from land - the only way you get a Hornet off of a docked carrier is with a crane, so they typically launch everything when still offshore, and load the aircraft the same way on departure.

The USA has enough carriers that they would not deploy a carrier-based fleet like this. If they are adding / replacing aircraft there are probably enough already in Europe - send those where needed and replace them in the normal course of events.

Would they fly supersonic to reduce travel time?

No, they won't go supersonic to reduce the flight time. > Mach 1 consumes so much fuel you'd never get there. Fighters have supersonic capability so they can get there or run away as needed, but it's never used for very long. The Concorde was designed from the beginning for Mach 2 cruise, so it didn't use afterburners once at Mach 2. Subsonic flight suffered as a result, fighters would not accept the subsonic flight handling issues as that's rather important to them :-)

Bombers are often deployed directly from home bases. All modern strategic bombers have global range with in-flight refuelling. You don't leave North Dakota, blow up something in Longwayfromherestan and get home for dinner, but you don't land anywhere else either. Yes, it's many hours in the same seat but the military crews are used to it.

Generally, ferry flights are done with aux tanks when possible, and no ordnance on the aircraft.  The logistics are influenced by the urgency, the geo political situation, the location of friendly countries, their assets, and airports to land at, and staffing.

Ferry flights are preferred over boats for many aircraft, as once an aircraft is on a boat is is typically not available for a period of time, and needs special equipment to load and off load.

Logistics has to handle overflight permits, etc. and rapidly deployed aircraft will often have over water routing to reduce permit requirements.  Additionally there may be some interest in reduced media coverage. Landing 16 fighters in Dubai to top the tanks is not done except where the visibility is desired, and most stops are done at bases or bases of allied forces.

It is noteworthy that certain flights, for example, B-2 runs, are originated out of mid-continent US, and return to that base afterwards. They are supported by tankers, and in the case of the B-2 it avoids the risk of landing a plane at a remote location.

For the US and allied forces, there is coordination by logistics, so that Navy assets are tasked with providing support in the unlikely event of off shore crew recovery, support, etc.  For example, I know that US Navy has been advised of AUS deployments, and provides offshore services for asset movements.

Such movements are routine.  For example, a NY Guard unit has supported South Pole NSF operations, and the C-130 used is ferried from New York to the South Pole and back after the missions are complete.


Why do turbofan jets fly faster in cold weather? Why do turbofan jets fly faster in cold weather at high altitudes?

I read somewhere that prop planes do it due to cold weather means air is more dense, and the prop therefore produce more power.

But what about jet aircraft at say 36000 feet? I looked at performance chart of a jet fighter, and at ISA-15 it could fly 0.17 mach faster than ISA.

Do not misunderstand I am NOT asking why it flies faster higher up. I am asking why it flies faster at winter than in summer at same (optimal for speed) altitude.

They don't. They fly slower.

Most aerodynamic effects depend on dynamic pressure. More precisely, lift is proportional to dynamic pressure and drag has parasitic component, which is proportional to dynamic pressure, and induced component, which is inversely proportional.
Therefore there is an optimal dynamic pressure at which the aircraft flies most efficiently, and minimal dynamic pressure needed to maintain flight.

Now dynamic pressure is (half of) air density times velocity squared ($q = ½\varrho v^2$) and due to its significance the “indicated airspeed” shown in cockpit actually corresponds to dynamic pressure expressed as equivalent airspeed. All the desired speeds for various phases of flight are referenced to that.

Since cold air is denser, the same dynamic pressure will occur at lower velocity, and because the pilot will maintain the same indicated airspeed, they will actually fly slower.

One more thing: the maximum speed is one of the few exceptions that is not dependent on the dynamic pressure. There are three effects that may limit maximum speed:

Aeroelastic flutter depends on true airspeed (velocity), so the temperature does not, actually, have an effect on it. However because true airspeed is not usually indicated in cockpit, the pilots will use tabulated indicated airspeed values and since these are only tabulated for altitude, they will actually fly slower.
Mach tuck i.e. loss of lift, accompanied with undesirable change in trim, due to exceeding critical Mach number. Since speed of sound increases with temperature, critical Mach number corresponds to lower true airspeed (velocity) in colder weather, so they'll fly slower again.
Engine power. Engines are able to produce more power in colder weather due to being able to achieve higher mass flow with the higher density air. However, engine power is limiting factor mainly for smaller general aviation aircraft, which have low power engines, or fighters at the other end of the performance spectrum, which are designed to fly supersonic and resist flutter. But transport category aircraft generally hit the other two limits first.

Original answer for the version before the reason they fly at higher altitude was explicitly excluded from the question:

There are two, distinct, effects:

Aircraft fly faster at high altitude due to the lower air density. This is because both lift and drag are proportional to air density. So at higher altitude, the plane both have to fly faster to generate enough lift, and can do it because the drag is lower.

More precisely, lift is proportional to dynamic pressure and drag has parasitic component, which is proportional to dynamic pressure, and induced component, which is inversely proportional.

Therefore there is optimal value of dynamic pressure where the drag is lowest. The optimal cruise speed is slightly above this point. Since dynamic pressure is proportional to density and square of speed, the same dynamic pressure corresponds to higher true airspeed at higher altitude.

Dynamic pressure is usually expressed as equivalent airspeed, which is the speed to which current dynamic pressure would correspond at sea level. The indicated airspeed shown on the instruments is approximation of this value.
Heat engines are more efficient at lower temperatures. This is because efficiency of a heat engine (applies equally to both piston and turbine engines and both internal combustion and external combustion (steam) ones) depends on the ratio of temperature, but the combustion increases the temperature by the same (approximately) difference. So if the starting temperature is lower, the ratio is higher and the engines produce slightly more power for the same fuel flow.

Additionally, the maximum temperature is often limited by what the materials can withstand, so at higher altitude the engine can be operated at higher power (well, not really; the power is also proportional to the density, but the engine will again run more efficiently).

It is simple physics. The law of conservation of Momentum (Newton's third law I believe) says that for each action there is an equal and opposite reaction.
The total momentum (Mass times velocity) must remain constant.
So if you throw a cast iron anvil out the back of the ship, the ship will move forward faster than if you throw a feather out the back at the same velocity.

Cold air weighs more than hot air. A jet engine operating at high density altitude (when air is very warm) is sucking in significantly less air by mass than a jet engine operating at a lower density altitude (colder) at the same pressure altitude.  Higher mass of air sucked in means more mass is being shot out the back. So the Thrust is higher.


What is the EASA definition of night time? EASA requires pilots to log separately night time and night landings in the personal logbook. Now, I'm not sure in which period of time a landing is a night landing and flight time is night flight time. I would appreciate your answer, maybe also with a trustworthy source.

According to this handy document from EASA, night is defined as:

  …the period between the end of evening civil twilight and the beginning of morning civil twilight, or such other period between sunset and sunrise as may be prescribed by the appropriate authority, as defined by the Member State.

(The FAA and ICAO use a similar definition.)

You can get the Civil Twilight times from a convenient astronomical calendar, but ignoring the specifics, the same rule of thumb the FAA uses for purposes of night currency will likely work for you too:

Night starts one hour after sunset
(Sunset being when the last of the visible sun disc drops below the horizon.)
Night ends one hour before sunrise
(Sunrise being when the first of the visible sun disk breaks the horizon.)

This actually cheats you out of a few minutes of "night" on either side of the calculation, but it's conservative guidance if you're conservative in what you log you won't accidentally mess up and give yourself more night time than you're really entitled too (i.e. you won't make yourself "night current" when you really aren't).

Note that I'm a US pilot -- since all my flying and training is based on FAA regs in the US if I'm completely off the mark and any European pilots or CFIs want to set me right it would be much appreciated :-)

CAP393 UK In paragraph (5) ‘day’ means the time from half an hour before sunrise until half
an hour after sunset (both times exclusive), sunset and sunrise being
determined at surface level.


Contribution of wing vertical location to roll stability Recently, I was reading about the factors that contribute to roll stability of an aircraft and came across a point stating that the wing's vertical location (Distance of wing from CG) contributes to the roll stability of the aircraft. I cannot comprehend this statement completely and even if it did, How much of an effect does it truly cause when compared to having features like a wing dihedral? Any information and explanations would really help me understand this concept.

An aeroplane in a pure co-ordinated turn has a deflection of both the ailerons and the rudder. This is cumbersome however, much easier to only deflect the wheel, tilt the lift vector, and let the aircraft sway into the turn. The vertical tail will then experience a slip angle, which will point the nose into the direction of the turn at the expense of a small sway velocity: sideways.

So now the question is: how does the aeroplane react to a sideways incoming airstream. What we want is that if the pilot deflected the wheel to the right, he has to keep the wheel to the right to maintain the right hand turn. So upon an un-coordinated turn, without touching the pedals, we want the aircraft to try to level itself again upon an incoming sideways velocity.

If we look at what happens to a low wing aircraft first, we see that the incoming sideways stream sees little resistance when streaming over the fuselage top, but gets caught in the wing/fuselage intersection and builds up pressure there. So the sideways velocity introduces a rolling moment in the direction of the turn, which the pilot has to compensate for by turning the wheel the other way. This is not the behaviour that we want.

In order to correct for that, we can introduce wing dihedral: now the incoming sideways air stream wants to level the aeroplane again, and the pilot needs to keep the wheel deflected into the turn, which is what we want.

And of course, with a high-wing aeroplane the above is valid vice-versa: the sideways stream wants to level the aircraft, so the roll behaviour is naturally how we want it to be already. Depending on the wing/fusleage shape and many other factors, the aeroplane can be too stable and we'll need to implement anhedral, wings pointing down.

High-wing airplanes are usually designed with less dihedral than low-wing airplanes because of the various different ways that a high wing placement enhances roll stability.

Several of these are addressed in this answer-- Why do low wing aircraft have higher dihedral than high wing aircraft?

For any of this to make any sense, it is imperative that we understand that when an aircraft is banked, it tends to turn (i.e. the flight path tends to curve), and in a turn, there tends to be sideslip, unless the pilot is using the rudder to eliminate it.  Sideslip is the fundamental driver behind nearly all the dynamics at play with regard to roll stability.  The actual reasons for the sideslip are not obvious and are related to the fact that in a steady turn, the wingtips are moving at different airspeeds, which tends to create a yaw torque.  Also, if the vertical fin is tangent to the curving flight path through the airmass, then the fuselage centerline at a more forward location on the aircraft cannot be tangent to the curving flight path through the airmass, so there is no way for the fin to completely eliminate sideslip in the absence of a rudder input.  In fact, in some cases the vertical fin can be said to actually be causing sideslip.  (Imagine a very long fuselage and a small radius of curvature of the flight path.)

Therefore a non-level bank attitude causes a turn which causes sideslip which can create a roll torque towards wings-level via dihedral, "pendulum effect", and other aerodynamic effects.

Note that in a high-wing aircraft, the wing's drag vector, acting above the CG of the aircraft, will tend to contribute a stabilizing roll torque (tending to roll the aircraft toward wings-level) in a sideslip even in the absence of any significant aerodynamic sideforce.  This is an example of what is often called a "pendulum effect".

The "pendulum effect" is tremendously strong in paragliders, and plays a significant role in hang gliders as well.  In hang gliders, when the pilot exerts no muscle force, the pilot's weight acts as if it is located where the flexible hang strap connects to the aircraft structure.  Raising or lowering this point of attachment affects the aircraft's pitch and roll stability dynamics.  Some older hang gliders had the "hang point" located several feet below the wing, creating a strong stabilizing "pendulum effect".

Consider the difference in flight characteristics of a model airplane with a heavy weight attached to the end of a thin strut projecting high above the aircraft, versus a model airplane with a heavy weight attached to the end of a thin strut projecting far below the aircraft.  It's clear the flight characteristics would not be identical in each case.  It's also clear that if both aircraft were trimmed for a steady glide, the aircraft would be much more stable in both pitch and roll in the latter case than in the former.

See this answer for a more extended discussion of hang glider dynamics and other issues related to the "pendulum effect" and roll stability: Does "pendulum effect" apply to hang gliders or any aircraft?


What is the difference between azimuth and heading? What is the difference between azimuth and heading?

I understand heading, track, radial, course and bearing but I don't know what azimuth is. I rarely hear it used. The Wikipedia definition is a horizontal angle measured clockwise from a north base line or meridian. How does that differ from heading?

Bonus question: when atc tells you there's a plane at your 2 o'clock, is there a word for that?

A heading (in the general case of moving "forward") is the direction your nose is pointed in. This may not be your course (as discussed here).
Headings are measured from onboard a traveling vehicle or object (e.g. from the cockpit of an aircraft or the bridge of a ship at sea).

An azimuth is a bearing, more precisely a compass bearing from a specific point of observation like a radar station. (The "North" used as a reference may be either magnetic or true depending on the system you're working with, but for purposes of this discussion it doesn't matter.)
Often the point of observation is fixed (ground radar, a control tower, an artillery spotter, etc.), though it need not be as long as the observation can be mapped to a compass bearing somehow.

Bonus Answer: The "O'Clock" positions are an informal type of relative bearing as mins noted: The 360-degree circle is broken up into 12 chunks (30 degrees each) for easier scanning, with "12 O'Clock" being directly in front of you and "6 O'Clock" directly behind.

An important caveat to be aware of is that when ATC provides a "clock bearing" for traffic it's relative to the aircraft's course (12 O'Clock is aligned with the aircraft's track over the ground as shown on the radar). Depending on wind there may be considerable variation between your course (direction of travel) and your heading (direction the nose is pointing), so the called traffic may appear in a different location when you look out your window.

Azimuth is the horizontal angle in degree, with a reference point True North 0°. This reference point is fixed, cannot be changed
While Heading/Bearing is same horizontal angle in degree/mile, with a variable reference point (user can decide it) & it can be as well the True North


How do Class B transitions work in the US? I'm just wondering about VFR requests relating to class bravo airspace in the US:

Is requesting VFR flight following in a class bravo the same thing as requesting a class bravo clearance?
Does requesting a class bravo airspace transition automatically include VFR flight following (radar traffic information service)?
If I am cleared into the bravo airspace and the controller gives me a heading and altitude to fly that clips the airspace of an underlying class C / D airspace, have I satisfied the requirements to enter said airspace (class C - 2-way communication and squawking mode C, class D - 2-way communication)?
Why does ATC provide traffic advisories for VFR aircraft in class bravo airspace at all if they are on the hook for providing separation?

Is requesting VFR flight following in a class bravo the same thing as requesting a class bravo clearance?

No. Flight following just means that the aircraft is receiving traffic advisories. A VFR aircraft needs explicit clearance to enter the Class B. The pilot should net ATC know that they intend to enter the Class B.

Does requesting a class bravo airspace transition automatically include VFR flight following (radar traffic information service)?

Yes, ATC must separate VFR aircraft from VFR/IFR aircraft while in the Class B. (7110.65X Section 7-9-4)

If I am cleared into the bravo airspace and the controller gives me a heading and altitude to fly that clips the airspace of an underlying class C / D airspace, have I satisfied the requirements to enter said airspace (class C - 2-way communication and squawking mode C, class D - 2-way communication)?

If ATC has given specific instructions that take a pilot through other airspace, then ATC should coordinate this with the appropriate controllers. Per 7110.65X Section 7-9-2 the controller should also either vector aircraft to stay in class B or to inform them if they are leaving it. Pilots should not change frequencies on their own unless asked, and the pilot should ask the controller if they are unsure.

Also, note that:

  Assignment of radar headings, routes, or altitudes is based on the provision that a pilot operating in accordance with VFR is expected to advise ATC if compliance will cause violation of any part of the CFR.

Why does ATC provide traffic advisories for VFR aircraft in class bravo airspace at all if they are on the hook for providing separation?

As described above, ATC is required to separate VFR aircraft from other aircraft in the Class B. ATC is instructed by 7110.65X Section 2-1-21 to:

  ...issue traffic advisories to all aircraft (IFR or VFR) on your frequency when, in your judgment, their proximity may diminish to less than the applicable separation minima.

Unless a VFR aircraft has explicit instructions, ATC does not know for sure what they will do. And IFR aircraft may still need to perform unexpected maneuvers like a missed approach. Traffic callouts make it easier to apply visual separation if needed.

If you request VFR flight following that will transition Class Bravo, you will either be explicitly cleared into the Bravo, or not - perhaps your path will take you under/through the arriving/departing active runway that day, and you will be denied Bravo clearance, and can request it again in a few miles, or they may clear you to a high altitude without asking once you will not be in the way.  I've had it go both ways flying in the Boston, MA area.
Yes, sort of - IFR traffic will be warned of you, you may not be told of IFR traffic near you.
Last time I was tracked to overfly an underlying airspace with flight following, the Class B originated flight following cleared me thru the underlying Class C, and I didn't talk to the Class C folks at all.  The flight following should stay with you until you clear the various class boundaries - or may stay with you to your destination. Really depends where you are flying and if you remain high enough for them to 'see' you.  For example, I flew from west of Boston all the way out to Nantucket Island - was handed off between several controllers on the way, including the Class C in Providence, RI, for a few of the miles.
Seperation is for IFR, with VFR on time permitting basis. See #2.


What is the fastest unpressurized aircraft? What is the fastest unpressurized aircraft with oxygen  masks that can carry people?

Messerschmitt Me 163 Komet was unpressurized, rocket powered WWII interceptor.

  The operational ceiling was limited by what the pilot could endure for
  several minutes while breathing oxygen from a mask, without losing
  consciousness. Pilots underwent altitude chamber training to harden
  them.

Special diets were prepared for pilots to prevent gas expansion in the intestinal tract during ascent.

The "never exceed" speed of this aircraft officially was 900 km/h but that has been exceeded anyway, when the test pilot Heini Dittmar reached 1,130 km/h.

About 370 were built.

In the height of 16000 feet, a human can survive on normal atmospheric pressure. Above 16000 oxygen level keep getting down and changes on pressure.
So we are talking about the aircraft which flies till 16000 feet. Cirrus SR22, Beechcraft Bonanza, Cessna 172, Cessna 182 Skylane, Cessna 310 are some fastest unpressurized aircraft.


Is there any form of checklist that a pilot needs to go through before they use their ejection seat? Obviously, the use of an ejection seat is a last resort tactic but is there anything at all that a pilot must consider, any buttons they need to hit or something similar before they may eject?

Obviously not including anything such as arming the ejection seat or removing any covers over the ejection seat handle or anything.

Yes there are checklists but a lot of the training is done by practice exercises so the procedure becomes instinctive during an emergency.

The exact procedures to be used vary based upon airframe, cockpit configuration, and ejection seat make and model.  Here are two examples, one from the F-16A/B aircraft and the other from the F/A-18E/F aircraft.

From the F-16 USAF Flight Manual.

And from the F-18 NATOPS Manual.

Again there are a lot of mitigating factors here, specifically, airspeed, altitude, attitude, aircraft controllability and imminent danger to the flight crew which must be taken into consideration to order to make the decision to eject.  Since an ejection seat is nothing more than a device to provide an additional velocity vector to the aircrew in order to provide time for the main parachute to open and safely lower the occupant to the ground, they have specific altitude, airspeed and attitude envelopes in which they may be operated to ensure maximum chance of aircrew survival.  This should be reviewed and thoroughly understood by the aircrew prior to use.

There have been a few instances where the seat was used outside of the envelope.  One notable instance being Soviet test pilot Anatoli Kobachev when he punched out of a MiG-29 during a low altitude, high AOA pass at the 1989 Paris Air Show.  His delay was to direct the stricken aircraft away from the crowds and very fortunate that he didn’t die.  He walked away from the accident.

The checklists above should be used only IF TIME AND WORKLOAD PERMIT.  Again flight crews practice ejection procedures separate from aircraft operation in order to become uncounsciously competent in the operation of the seat.  They typically order the cockpit and workstations for aircraft egress prior to situation where there is an increased probability of needing to abandon a disabled aircraft.  These include takeoff, landing, and entering a theater of combat, etc.  Checklists will be use here prior to these events and crews will probably brief ejection procedures here as well.  Carrier operations, in partcular will have crews ready to eject at a moments notice and anticipating emergencies such as a cold cat shot or arresting gear failure during launch and recovery operations, both of which will probably require an immediate ejection to save the lives of the aircrew.

Correct cockpit preflight procedures must be done by the aircrews prior to operations to ensure safe ejection as well.  Crew must attach the leg restraints as well as correct attachment of life support umbilicals such as oxygen, anti-G, radio and HMCS data lines to ensure they separate properly during ejection without causing serious injury or death.  Shoulder straps must be attached into the occupant’s parachute harness to connect them with the seat’s recovery parachute (some seats do not have this; the pilot will wear a separate parachute system as part of their life support garb in this case).  Charts, kneeboards and other foreign objects must be placed such that they do not interfere with the operation of the ejection handles or the seat safing handles.

The ejection process again is going to be based upon the cockpit configuration and seat type, but typically if the aircraft commander says EJECT! EJECT! EJECT!, you will pull your legs and extremities clear of the legwells and flight controls, head back against the seat headrest, grasp the ejection handles, disengaging the grip safeties and PULL HARD.  The ejection itself will happen fast and very violent with a BANG! BANG!  The canopies will either break away with a pyrotechnic charge or be fractured with a similar charge sounding like a shotgun blast at close range.  This is followed a split second later with a jolt and explosion as the pneumatic catapult throws you clear of the cockpit and the seat rocket motor ignites, giving you an approx. 20g acceleration clear of the cockpit.  After this, the recovery process becomes pretty automatic.  Air data sensors feeding a small computer in the seat right the occupant using a drogue chute, followed by seat frame separation and deployment of the main recovery parachute.  The pilot may be required th pull an additional handle for deployment of the seat pan and life raft for a water egress as well.

Wind blast will be a major factor and all material suggests that an ejection be intitiated at the lowest airspeed, if possible.  High speed ejections will invariably result in serious injuries as due to wind blast, as this chart from an F-16 flight ops manual suggests.

Ejection at supersonic speeds is usually fatal, with very few exceptions.  This pilot was very fortunate to survive a supersonic ejection, but was seriously injured in the process.

Yes:

If the aircraft has more than 1 crew, they should eject at different times, not together. PILOT COMMANDS NAVIGATOR TO EJECT. Only after the navigator has ejected, should the pilot eject.
It is bad to eject if the aircraft is upside down, so that should be avoided if possible.
The seat harness should be tight, as well as helmet and mask. If any of these are loose, bad things can happen.
Pilot must hold on tight to the ring with both hands. Do not eject with only one hand on the ring.


Why should some piston engines not be run lean of peak? Leaving aside the claimed pros and cons of lean-of-peak (LOP) vs. rich-of-peak (ROP), the C172S Lycoming IO-360-L2A handbook I have says "Continuous operation at mixture settings lean of peak EGT is prohibited", and indeed I was always taught to run the engine ROP.

Obviously you should always follow the POH for your aircraft, but considering piston engines in general, why would running LOP be prohibited?

Considering piston engines in general there's a few reasons not to run "lean of peak", but the arguments I'm aware of all fall into three broad categories:

Poor Instrumentation
Most folks advocating Lean-of-Peak engine operation will also tell you that you need a good engine monitor (with Exhaust Gas Temperature (EGT) and Cylinder Head Temperature (CHT) probes for each cylinder) so you can ensure that all your cylinders are operating within safe temperature limits.
Manufacturer CHT/EGT systems are often "single-probe". On the IO-360 in your Cessna example that leaves you with a 3 cylinder blind spot in your monitoring, and the potential risk of damaging your engine based on the incomplete information you have.
Uneven mixture distribution
Some engines simply can't run lean of peak because of uneven mixture distribution: Either the engine will get rough as some cylinders "miss", or you'll have some cylinders lean of peak EGT and at a safe cylinder head temperature while others are redlining their CHT and potentially causing damage to your engine.
This sort of uneven mixture distribution happens frequently in carbureted engines, but there are many injected engines which also have poor fuel distribution & significant variation in mixture between cylinders (This is where GAMIJectors makes their money - by providing balanced injectors to even out fuel distribution).
"In Case Something Breaks"
Hypothetical scenario: you are cruising at 8000 feet, you've leaned for cruise, and your mixture control suddenly fails in such a way that your engine is stuck at the current mixture setting.  All the airports within your endurance range are roughly at sea level.
An engine running lean of peak may begin to run rough as you descend for landing (the mixture becomes too lean to support combustion).  An engine running rich of peak may continue to run acceptably through the descent. (The mixture may transition into "the red box zone" where operation at high power settings are dangerous, but you'll theoretically be reducing power for your descent, so the chance of doing damage to your engine isn't that high unless you have to perform a go-around or other high-power maneuver.

A prohibition on Lean-of-Peak operation in your POH is probably historically based on #1 or #2 above -- the manufacturer doesn't want to advise you to do something that might damage your engine.  Running Rich-of-Peak is conservative operating guidance, where the downside is slightly increased fuel consumption and the possibility of fouling a spark plug (which is not likely at normal operating power settings) versus the downside of potential engine damage from improper Lean-of-Peak operation.

Running too far lean of peak is bad for the engine. That's easy to do if the mixture isn't precise enough, or if you descend and forget to adjust the mixture. If there's not much leeway in a particular engine or if the engine is susceptible to damage from running a little too far lean of peak, the manufacturer will prohibit the practice.


Are aircraft transferable between companies with different Air Operator Certificates? Can Company A use Company B's aircraft if the two companies are not on the same Air Operator Certificate (AOC)? Company A will provide the crew. This would be a transfer of equipment use with no transfer of ownership.

The aircraft in question happens to be a medical emergency helicopter.

The answer to this will depend on what the two company's Operation Certificates allow and forbid in the Operation Specifications (Ops Specs). All Ops Spec examples provided below represent FAA examples, are examples only, and may differ from those in any specific situations.
Considerations will include:

Aircraft Authorization
Operational Control
Maintenance
Crewmember Requirements

The Ops Specs will specify in A003 which aircraft and configurations are authorized for use, for example:

A003 . Aircraft Authorization
The certificate holder is authorized to conduct operations under the provisions of Title 14 CFR Part 135 using aircraft with the approved configuration and operations described in the following table:
[table omitted]

If the available aircraft is of a type and configuration authorized for use, then it may be possible to use it. This will most likely be the case if the aircraft is of the same type and configuration, and in the same class/category of operation as those aircraft which Company A already operates.
Ops Spec A008 will probably specify that the certificate holder shall not:

Engage in any arrangement with an aircraft owner, lessor or any other person or entity [...] which allows the use of an aircraft for operation under these operations specification without a complete, effective and sustainable transfer of operation control to the certificate holder for all Part 135 operation conducted under these operation specifications.

The lease agreement must provide Company A with complete, effective and sustainable operational control of the aircraft.
Additionally, the aircraft must conform to the extensive maintenance requirements required by Part D of the Ops Specs.
The flight crewmembers must also be trained and approved for the aircraft. However, if the aircraft is of the same type and configuration as those already listed in A003, this requirement may be easily met.

To operate B a/c at A you would either do a dry lease, but the a/c then has to be named in AOC of A so the crew of A can operate the a/c. Or do a wet lease, but then crew of A needs to fly on the AOC of B.


What simulation software do airlines use? What simulation software do airlines use for training purposes? Do they use software like P3D, FSX, X-Plane, or FlightGear? Or is it a custom? What about aircraft? Do they source them from groups like PMDG or is this also custom?

X-Plane offers a "Professional Level" which is mostly about licensing. However X-Plane also has an FAA certified version that if paired with proper controls is legal for certain training/instructional hours. You can find more info on the FAA's approved simulators here and here. The flight school I trained at had a certified sim from the 80's that was pretty miserable by todays standards. It was certified and had no screen, I would say it was "custom built" FWIW.

Many airlines may turn back to the manufacturer for recurrent training. Both Boeing and Airbus run facilities with full motion simulators. I would think they built the software in-house but I don't have any hard evidence on that. As the OEM builder they have access to genuine interior parts to build the sims.

For smaller aircraft places like SIMCOM offer training in full motion sims but do not list their software of choice. SIMCOM uses actual aircraft interiors (most likely from defunct airframes) for their sims.

If your really adventurous you can build a full cockpit sim yourself.

Commercial carriers use hardware simulators, not software. Each carrier has one or more training facilities. For example, American Airlines has one in Fort Worth and one in Charlotte. Pilots have to learn a wide range of different skills which are not only how to fly particular planes but how to deal with specific types of dangerous situations and ancillary needs such as learning how to perform CPR.

Simulators are generally provided by the aircraft manufacturer and come in two basic types: systems simulators and full-motion simulators. A systems simulator has no visuals, just controls. The pilot learns how to push buttons and perform procedures in the right sequence. The full-motion simulator is a large box on hydraulic mounts that has a visual screens and can tilt in any direction. It is used for training maneuvers.


How can an airplane land if a hail storm damages the windshield? I understand hail can be very damaging to an aircraft, in particular to the engines and windshield.

In the event that a windshield becomes cracked so that visibility is impossible, can pilots pop out the windows when at a lower enough altitude in order to see the runway?

In the EMB-145 the abnormal procedure for obscured windscreens is to secure items in the cockpit, open the side window and look through its opening.

There are more aircraft that share this procedure.

Here is the QRH procedure for "Impaired or cracked windshield" in the EMB-145:

Associated Windshield Heating .............................. OFF

IF  only the outer layer (glass) is cracked, no action is
    required

IF  inner layer of glass is cracked, proceed as follows:
    Cockpit Door ......................................... CLOSE
    Maximum Altitude ....................... 10,000 FT., OR MEA,
                                             WHICHEVER IS HIGHER
    Pressurization Manual Controller ........ 1 O'CLOCK POSITION

                    Wait 15 Seconds

    Pressurization Mode Selector ........................... MAN
    Cabin Delta-P ........................... SET EQUAL TO 1 PSI

    Note:  Pressurization Manual Controller must be used to set
           and maintain Cabin Delta-P at 1 psi while descending

    Airspeed .................................... BELOW 250 KIAS
    Smoke Goggles .......................................... DON
    In case both windshields are impaired:
    Cabin (below 10,000 ft) ....................... DEPRESSURIZE
    Airspeed ................................. MAXIMUM 140 KIAS,
                                                    MINIMUM Vref

IF  forward view through both wind screens is impossible, secure
    loose objects in cockpit and proceed:

    Note:  Intercommunication will be impossible with window
           removed.

    Direct Vision Window ................................ REMOVE
    Landing must be made looking through the direct vision
    window

                              * * * *

Commercial aircraft pilots are required to pass an "Instrument Flight Rules" (IFR) rating.  This rating requires the pilot to flight their aircraft using just the instruments in the cockpit.

As to how will the pilot know where to land.  Commercial and military aircraft possess multiple instruments to provide location information.  This includes GPS based instruments.  I believe that this still includes VOR/DME (VHF omnidirectional range/distance measurement equipment) too but I've been out of the industry for a couple of decades and these may no longer be included.

At commercial & military aircraft flight speeds, busting out your windscreen would be a terribly bad idea - probably fatal.  It would certainly make it so you wouldn't be able to land the plane.  A better choice for non-instrument rated pilot would be to have the tower talk you in.

Per this other SE question & answer, although a true zero-zero landing is actually possible and safe to perform (provided ILS equipment is installed in aircraft and airport), there are problems on the ground with rollout and maneuvering on the ground which for practical purposes eliminate it except under extreme circumstances.


Can instruction be given during a flight conducted under 14 CFR 135? Let's suppose that a flight is being conducted under 14 CFR 135. The airplane is certified for single pilot operations, but two pilots who are both rated as PIC on the 135 certificate occupy the pilot seats. One of the pilots is a CFI. Does anything in 14 CFR 135 prevent the CFI from giving dual given time to the PIC while he flies?

It took me a little bit to figure out the underlying reason for asking this question and it has to do with logging flight time.  For example, in a single pilot operation with an autopilot, the second pilot is not required and therefore cannot log the flight time at all even though their company may want them there.  The FARs don't allow the second pilot to log the time.

The regulation that governs logging of flight time is 61.51.

  61.51(h) Logging training time. (1) A person may log training time when that person receives training from an authorized instructor in an aircraft, flight simulator, or flight training device.
  (2) The training time must be logged in a logbook and must:
  (i) Be endorsed in a legible manner by the authorized instructor; and
  (ii) Include a description of the training given, the length of the training lesson, and the authorized instructor's signature, certificate number, and certificate expiration date.

So the question becomes what training was given during the revenue Part 135 flight and were you an authorized instructor?

The term Authorized Instructor is only found in Part 61.

  61.1. Authorized instructor means—
  (i) A person who holds a ground instructor certificate issued under part 61 of this chapter and is in compliance with §61.217, when conducting ground training in accordance with the privileges and limitations of his or her ground instructor certificate;
  (ii) A person who holds a flight instructor certificate issued under part 61 of this chapter and is in compliance with §61.197, when conducting ground training or flight training in accordance with the privileges and limitations of his or her flight instructor certificate; or
  (iii) A person authorized by the Administrator to provide ground training or flight training under part 61, 121, 135, or 142 of this chapter when conducting ground training or flight training in accordance with that authority.

Since it is a Part 135 revenue flight I would believe the CFI would have to comply with section 135.338 and 135.340.

  135.340(a) No certificate holder may use a person nor may any person serve as a flight instructor unless—
  (1) That person has satisfactorily completed initial or transition flight instructor training; and
  (2) Within the preceding 24 calendar months, that person satisfactorily conducts instruction under the observation of an FAA inspector, an operator check airman, or an aircrew designated examiner employed by the operator.

If the authorized instructor has complied with 135.338 and 135.340 they are also bound to the training program for the 135 company.  If that training stipulates you can give dual given during revenue flights than you are more than welcome to do so.  If there is no provision in the training program to provide instruction during revenue flight then it should not be done.

The best option is to request a legal interpretation on the validity of what you are wanting to do.

Short answer: NO

Long answer: In a flight where the CFI is certificated to provide instruction and the two are sitting in pilot seats then the PIC may log any part of that flight as dual instruction.  It does not require the CFI to actually say a word in order for it to count as dual; even if he's merely observing, if something bad happens he'll open his mouth or take controls.

Part 135 is more about requirements for operations; it does not affect flight instruction, which is a part 91 regulated activity.


Do aircraft tyres have inner tubes? Do aircraft tyres have inner tubes? Modern small single-engine turboprops to the largest Jumbos. If not, did they ever have inner tubes? And if so, why were they discontinued?

From this Goodyear document on aircraft tires, both types exist. For the 747, they are all tube-type.

  Tires are tube-type unless otherwise indicated.

The listed 747 civilian variants do not indicate otherwise (page 18).

(Source: wikimedia.org) Cross section of a 747 tire. Full resolution via link is a must see. You can make out the inner tube (flap) from this image.

But that document is from 2002, nowadays tubeless tires are also available for the 747 and many commercial types, for example the 49X19.0-20 Flight Leader by Goodyear (thanks @user71659).

They are tubeless.

Why? for the same reasons we have tubeless tyres on our cars.

Punctures are slower. A nail or similar which penetrates the tyre will itself form a plug.
Tubed tyres can handle high speed. Friction between the tyre and tube will causes failure due to heat.
Low pressure in a tubed tyre will quickly escalate into a flat as tubes only work well when properly inflated.
Tyres can move along the rim. You often see index marks in the rim matched to similar marks on the tyre. Not good on a tubeless tyre but really bad on a tubed tyre as this will stress the valve area.