Can a light glider without thermal protection land from the orbit, starting from the orbital speed? I ask about a glider without any special thermal protection (pilot in the space suit), so both answers to the other question do not cover the topic.

When the glider enters the atmosphere, it starts generating lift. Could this lift keep the glider high enough, in low density air, and this way prevent the heat damage? The glider then would fly a long distance, losing speed and altitude slowly, until reaching speed and elevation at which it can safely land. Seems that lots of lift could be generated at speed and air density where the metal burns already.

Is there any analysis ever been done if such landing is possible?

No.

The stagnation point temperature goes up with the square of true air speed.

Temperature dissipation is proportional to true air speed and density.

Lift is proportional to the square of airspeed and density.

The lower wing loading of a glider (compared to the Shuttle, for example) means that all of the reentry will be slower, but that is not necessarily an advantage. Note that the Shuttle needed external cooling to prevent heat stored in the heat shield to dissipate into the Shuttle's structure. If the glider flies a longer reentry, it will be closer to thermal equilibrium. Also, the aerodynamic shape of a glider is a disadvantage here because the lowest heat load is possible with a blunt object - this is why reentry vehicles look like they do.

Just to give you an idea what temperatures are involved:

If we express speed as Mach number, a typical reentry speed would be Mach 25. This makes the reentry temperature at the stagnation point $$T_s = T_{\infty}\cdot \left(1 + \frac{\gamma-1}{2}\cdot Ma^2 \right) = 126\cdot T_{\infty}$$
at Mach 25. If we assume 195K at the edge of space, this comes out to 24,570K - theoretically, because ionization effects will cause the eventual temperature to be lower and the ratio of specific heats $\gamma = c_p/c_v$ is no longer constant. To calculate the real value needs a non-equilibrium gas model since the nose radius of typical glider wings and tail surfaces is in the order of one centimeter. The distance between the detached shock wave and the structure is too small for the gas molecules to reach equilibrium.

On the other hand, the epoxy resins used in glider construction cure at room temperature first and are then tempered at about 60°C. The glass transition temperature $\text{T}_g$ of such resins is at best a couple of degrees above the curing temperature. $\text{T}_g$ marks the onset of weakening of the composite matrix, and heating the glider above it will permanently damage it. Given that the glider will stay for dozens of minutes in air of a temperature of several thousand degrees and has no thermal protection will make sure that all which reaches the ground is a charred lump of material.

For small objects, there is probably be a 'critical aircraft density' below which a return from orbit is possible. A balsa wood model glider would probably land unscathed after a very long re-entry... The same may be expected from paper plane. The small mass will mean that little energy would be dissipated, and the large drag and long re-entry would ensure that its dissipation rate ('power at reentry') is kept low enough...


How to record Intercom / ATC in a small aircraft like a Piper Archer? For making videos it would be nice to have some more sound than the engine sound recorded by my GoPro. What ideas do you have to record the aircraft intercom? It won't be a problem for me to have two files (an audio and a video file).

Edit: I'm looking for a good sound quality comparable with that which you will have while sitting in the aircraft listening to the intercom.

For the GoPro specifically there are audio recording cables you can buy which connect to your intercom jack and provide a pass-thru connection for your headset.
There are other cable options available to support recording on different devices (e.g. iPhone/Android phones and tablets).

As is often the case with "aviation" accessories these can be somewhat expensive, but they're also pretty reliable drop-in solutions. As mah noted if you have a spare headset jack you're not using plugging into that can be much cheaper and just as effective.

Find a small microphone or even an earbud speaker that will plug into your recorder's IN jack.  (The earbud will in most cases work like a microphone).  Then just wear it inside the earcup on your headset.


What are the advantages of single vertical stabilizer over a twin set? Why did the PB4Y patrol aircraft use a single fin while the B-24 it was derived from used twin fins?  I have read that the twin fins were thought to be superior but this was realized to be false.  Why did engineers believe the twin fins to be superior and what caused them to reconsider?

Why did engineers believe the twin fins to be superior?
There doesn't seem to be any single definitive reason for selecting the number of vertical fins on an aircraft design (except, obviously, for the twin-boom designs such as the Lockheed P-38 or Fairchild C-119).  I've seen all of the following cited by one authority or another as advantages of multiple vertical fins.  (Sorry, I didn't save or remember the sources.)

Reduced hangar overhead requirement.  This is the most common reason for including total aircraft height in a purchase specification, and it is a sine qua non requirement for carrier deployment.
Also I remember reading a magazine article that supplied this reason for the triple tail on the Lockheed Constellation.

Reduced cantilever stress at the root.  This may actually save weight over the single-fin design.  It may also help keep a tactical aircraft from disintegrating under the enormous stress of combat manoeuvres.

Reduced parasite drag.  The lower half of a vertical fin can be twice as thick as the upper half.  A twin tail design effectively omits the lower half and uses the upper half twice.

For multi-engine designs, operating the rudders in the engine slipstream affords improved rudder authority at low speed.  Pilots who taxi loaded bombers into takeoff position between other loaded bombers like lots and lots of rudder authority.
Also I notice that practically all the big flying boats of the 1940's Flying Clipper era had multiple vertical fins.  Those craft had some tricky low-speed steering requirements — they not only had to make their way through a busy harbor, but usually docked and undocked without a tugboat.  I'm only guessing at the connection though.

A single vertical fin can be masked by the fuselage wake at high angles of attack.  You can see how this could be a problem for modern fighter aircraft, which look like flying tennis courts.  Traditional bomber designs with their broad wings and heavy aerodynamic loading can also generate quite a troublesome fuselage wake.

Improved position and field of fire for a tail gunner.  I don't see it myself but I've heard the claim so I'm including it here.
I concede that the tail gunner slot in the Avro appears to benefit from the absence of bracing structure.

Reduced rudder-induced roll due to a shorter rudder offset moment.  Fast tactical military aircraft have short wings, and you don't want the rudder to extend further from the roll axis than the ailerons do.  On the other hand, the rudder doesn't need to go all the way to the top of the fin, does it.

Twin fins can be canted to reduce radar return.  They usually are, although I don't understand how the designers know where the hostile radar will be.

On the A10, the vertical fins actually mask the hot engine exhaust over a large angular area.  Thus in order to aim a ground-to-air heat-seeker up the tailpipes of an A10 you have to be directly behind it.  Which means that a few seconds ago you were directly in front of it, so good luck with that.

Twin fins/rudders reduce yawing from landing or taking off in a crosswind as the second one is blanked and the affected side area is smaller. Twin tails are in the prop wash of the engines on multi-engine aircraft and so are slightly more effective especially at low speeds, and can thus be smaller than fins/rudders that are not. Some aircraft used multiple tails to reduce their height so as to fit into smaller hangars such as the Lockheed Constellation. Twin tails used as endplates on the tailplane also increase the tailplane's efficiency so it can be smaller and produce less drag. As mentioned however - they are more complex, and in an engine out situation yaw control may be degraded slightly. The only aircraft where redundancy was claimed was with the A-10 and that claim is dubious as the rear fuselage remains a weak point. Similar aircraft would be highly dependent on how the controls are designed. The B-24 had snaking issues that could have been resolved by lengthening the fuselage slightly but they chose to go for a redesign.


Can you push the throttle forward while using reverse thrusters? Can you push the throttle forward while using reverse thrusters (for turbofans)? Does this have any effect, such as strengthen the "reverse" air blanket? Is it ever used?

In the B757 aircraft (RR engines), for example, the "reverse" thrust levers can only be raised/employed when the aircraft is on the ground and the "forward" thrust levers are in the idle stop position.  During reverse thrust operation, the forward thrust levers cannot be pushed forward.

This is different than many Turboprop airplanes

Yes.  In reverse thrust, at idle, there is no effective thrust.  Advancing the throttle provides beta thrust.


What is the best method to time your round out or flare in Cessna 172? I am a student pilot and I usually round out too high and I am not sure why. Can somebody  guide me how to make decision whether I am too high or not.

The usual cause for flaring too high is that the pilot is looking at the runway in front of the propeller instead of down at the far end of the runway.

Next time you're up with your instructor, have him do the landing, and look at the far end of the runway while using your peripheral vision to judge your height above the surface.

On the next lap, you fly the airplane, and try to get the same sight picture, both in front of you, and in your peripheral vision.

Don't "flare". I think the whole "flare" thing got started in World War II or something. I don't know, but ignore it.

The 172 is a tricycle gear aircraft that is designed for 3-point landings. You should not be "flaring" it. They called it the "land-o-matic" for a reason: you don't need to (and should not) "flare" it.

The error to avoid is landing on the nose wheel, which is why some instructors teach "flaring", but this is really bad practice and an example of overcompensation to a fault. Teaching students to "flare" a 172 to avoid a hard nose-wheel landing is a faulty and improper teaching technique that goes against Cessna's own guidance in the POH and in their flight manuals. The only requirement is that you land mainwheels first. Other than that you should be landing in a level attitude.

When instructors tell you to look "at the end of the runway" or whatever, that advice is just as bad. The majority of runways have a grade of some sort and many have significant gradients. Looking at the end of a runway with a gradient will cause errors. Where you should be looking is at the touchdown point, not the end of the runway.

Watch the touchdown

The whole key to landing is to know EXACTLY where you are going to touchdown. The touchdown point can be recognized because every point before it will be moving downwards in your vision, and every point after it will be moving upwards.

On final, establish your glideslope and adjust your touchdown point to be on the numbers. If you cannot recognize the touchdown point, do go-rounds until you can visualize the touchdown point (also called the "aiming point"). The touchdown point is that point that stays motionless on your windshield.

If your glideslope is anywhere near correct, your attitude will be correct automatically and you will land on the main wheels. There should be ZERO need to be flaring or doing anything weird. You should set that glideslope and land on that same glideslope on the touchdown point. Once you touchdown, ease the nose wheel down. No need to "flare". Good glideslope, good touchdown point, good landing. It's that simple.

Nosewheel Landings

Okay, if you understand the above, you might be asking: "Fine, I understand, but how do nosewheel landings happen then, if that is the bad thing?" Good question. Nosewheel landings happen when the pilot dives--a very bad thing. What happens is that they come in too high and react by pushing the stick hard forward, diving. Obviously if you do this, you are abandoning your glide slope aren't you? If you have a set glideslot and touchdown point, this should NEVER happen EVER. When somebody dives, it is because they do not even know what their touchdown point is, and consequently it turns out to be off the runway. Trying to force a landing when your touchdown point is off the runway is a bad idea, obviously. If you have cognizance of your touchdown point, you will never be in a situation where you feel the need to dive.


Could a Personal Locator Beacon or EPIRB carried by a passenger aid in locating a downed airliner? There've been a lot of questions lately about tracking aircraft, and after a conversation with a friend of a friend I started wondering:

Could a PLB or EPIRB carried by a passenger or crew member aid in locating a downed or missing airliner? There are a dozen or so brands out there, most using similar technology, like this ACR ARTEX unit:

Now, I'm well aware of the FAA and FCC regulations about electronic devices in flight, and that's not what I'm interested in. I also realize that airliners already carry an ELT.

If a paranoid passenger brought one on board, would these devices function at all from inside an airliner cabin? Some feed GPS location data to a 406MHz locator beacon, and those would potentially be less useful (as they'd have to be at hand and probably triggered manually. Others, like the Spot GPS tracker, have the ability to upload GPS track data on the fly to satellite communications networks such as Iridium.

I'm personally skeptical, given that GPS generally needs a clear view of the sky and that a fuselage seems likely to block or reduce a 406Mhz ELT signal, but I'm curious to know if there's any information out there.

Relatively short answer

Cospas-Sarsat: Feasible but...

It's possible, but there are many individual elements involved. It's a matter of probability that most of the time there will be a combination preventing the link to work. If you know what to do and take care of ensuring a good direct line of sight (LOS) you may be able to avoid some of these combinations. It cannot work underwater.

Parameters that can attenuate the signal reaching the satellite receiver are of different origins:

The aircraft design itself (composite materials or aluminum, number of windows, ...)
The location of the accident (canyon, trees, ...)
The transmitter use (position, window proximity, metallic surface or debris interference, body proximity, ...)

I'm not aware of experiment results for using a PLB within a metallic container like a commercial aircraft cabin. It's still possible to evaluate the RF channel loss in different configuration and see if the signal can reach the receiver with the minimum required level.

As explained in the next sections about the Sarsat satellites of the international SAR service on 406 MHz:

The threshold for signal reception by the receiver is 161 dBW.
With an initial power of 7 dBW (5 W), the attenuation must be at most 168 dB
In good conditions we can expect an attenuation of "only" 148.1 dB. Such signal 12.9 dB stronger that the threshold can be detected easily.
In degraded conditions we may experiment an attenuation of 177.5 dB. Being 9.5 dB below the threshold, this signal is not detected.

Indoor transmission put the system at its limits of detection capability (a S-meter unit is 6 dB).

Weak signal on a S-meter, S-unit is 6 dB (source)

Many small attenuation causes that, when summed, can prevent the link to work must be taken into account, hence this answer is a long one.

SPOT LLC

SPOT LLC solution you mention is a commercial messaging initiative working on 1.6 GHz, out of the Cospas-Sarsat system. Higher frequency means higher attenuation. The limited 0.4 W RF output also makes the signal weaker. It's very unlikely that SPOT could work within the cabin.

Details

Introduction: PLB principles

Emergency PLB operate on 406 MHz. The previous 121.5 MHz civil frequency is not monitored anymore by SAR satellites, but can be used (in addition of the actual signal at 406 MHz) as a homing guide for the SAR team. They are informed about whether this homing feature is active or not by the message sent by the PLB.

PLB output power is 5 W (equivalences: 7 dBW 37 dBm) at 406 MHz, in vertical polarization.
From C/S T.004, the performance of the satellite system is:

A "probability better than 95% to provide a valid solution (15 hexa identification provided) for a beacon transmitting with a 37 dBm output power (with a whip  antenna) and for satellites passes with elevation above 5°)"
A "probability better than 95% to provide a Doppler location with an accuracy better than 5 km"

There are no satellites entirely dedicated to the SAR service, rather distress signal receivers are carried as one of the payloads aboard satellites. The system is made of two constellations:

GEOSAR constellation, Geostationary Earth Orbit SAR. These satellites are at fixed positions, 36,000 km above the equator, and are covering the globe except the polar regions.
LEOSAR, Low Earth Orbit SAR. Satellites in near-polar (about 85°). LEOSAR is made of two sub-constellations: Sarsat satellites orbiting at 850 km, and Cospas satellites orbiting at 1,000 km.

(Source)

A third constellation is planned to operate at a higher altitude than LEOSAR (MEOSAR, currently not fully deployed).
While GEOSAR are likely to be first to detect the PLB activation due to their permanent LOS, LEOSAR will provide the best detection due to the shorter distance to the device.

I'll focus on LEOSAR / Sarsat payload which provides the best chances for the detection of a PLB signal partially blocked by RF impermeable obstacles.

Sarsat payloads

Satellites carrying a LEOSAR Sarsat payload are NOAA 15 (Sarsat 7), NOAA 18 (Sarsat 10), NOAA 19 (Sarsat 12). Other are scheduled to be active in the future (some may be already, I haven't checked). They are fitted with a SAR receiver (search and rescue processor, SARP). Sarsat satellites can be tracked on N2YO.com.

According to Cospas-Sarsat LEOSAR Space Segment Commissioning Standard - C/S T.004, a SARP-2 has a sensitivity of -161 dBW, and SARP-3 -164 dBW (that's about 60 attowatts to be compared with a cellular phone cell: -134 dBW, 1,000 times less sensitive).

Can this small quantity be delivered to the SARP from the inside of a cabin?.. This is a matter of knowing the initial power, and determining the loss along the wave path until the signal reaches the receiver.

The power generated by the transmitter amplifier is 5 W, or 7 dBW. 161 dBW on the receiver means receiving a 1 W signal that has been attenuated by 161 dB. If we have already 7 dBW at the source, we can bear a maximum attenuation of 168 dB.

Main loss inventory

Propagation loss in direct view

The most important part of the signal will be lost in the atmosphere between the antenna of the PLB and the antenna of the SARP.

Loss due to distance

Loss is dependent on two factors: distance and wavelength. Both factors limit the area of the receiving antenna compared to the whole sphere of the wave front, and in consequence the portion of energy received is smaller too.

Using the propagation model ITU-R P.525 (free-space propagation).
Distance = 850 km, f = 406 MHz.
Assuming isotropic antennas for the time being.
According to the model, the loss (dB) is:
Lbf = 20 log(4πd/λ)
.   = 32.4 + 20 log(f) + 20 log(d)
.   = 32.4 + 20 log(406) + 20 log(850)
.   = 143 dB

The signal that left the PLB antenna reaches the SARP antenna -143 dB weaker, assuming the best antenna alignment. But this is never the case, because when in a polar orbit, the satellite is unlikely to be exactly above the beacon.

Misalignment of the receiving antenna

There will be an angle between the nadir of the SARP and the beacon. This off-nadir angle, depends on the actual track of the satellite for the current orbit:

Target off-nadir

The gain of the antenna varies according to this angle. Sarsat-TIROS SARP Receive Antenna (UDA) Gain Pattern:

UDA antenna gain at different off-nadir angles (source)

Assuming the polarization is favorable (RHCP), the gain may varies between +2 dB and -10 dB. Note that:

The gain is better when the off-nadir angle is large. This is likely to compensate the higher distance to the beacon.
I'll assume a link length f 850 km, but the actual length is larger when the beacon is not directly at the satellite's nadir.

We should now work with the probability distribution of the misalignment for the LEOSAR antennas to have the full range of possibilities and their likelihood (e.g. Monte-Carlo simulation). To simplify, let's just use a bad case: -4.4 dB (this is not the worst possible). Loss so far: -147.4 dB (-143 - 4.4).

Attenuation due to feeder and duplexer

On the satellite, the SARP is connected to an antenna via a duplexer to isolate the SARP uplink antenna from the SARR downlink signal (SAR repeater used on local mode, not detailed here):

Hardware on the Sarsat satellite (source)

The duplexer + feeder loss is -1.6 dB. Loss so far: -149 dB (-147.4 - 1.6).

Loss margin left

We had a budget for -168 dB loss, we have used -149 dB. The margin until the PLB signal is lost by the SARP is 19 dB, assuming we are in this configuration:

PLB is outdoor.
The LOS is completely clear, no foliage, no roof, etc.
PLB antenna has no gain/loss (0 dB gain).
PLB antenna is vertical, this is the normal use of the SAR device.
PLB antenna and RF output stage impedances match (no SWR).
There is no atmospheric loss (free space)

Additional losses

It's time to re-look at the assumption made until here, and take into account the additional effects encountered when the PLB is used from the cabin of a crashed aircraft.

PLB is outdoor.

Actually the PLB is behind a cabin window.
From Arkema:
"Most formulations of colorless Plexiglas sheet readily transmit standard broadcast, television and most radar waves of the electromagnetic spectrum". It can been assumed that windows (including the internal protection sheet) don't block the PLB signal. Loss = 0 dB.
In the cabin, numerous reflections will occur on the wall, and due to the closed space with only small windows, it's likely the reflected signals will globally interfere in a destructive way (phase mismatch) with the original signal.
If the PLB is displaced, even by a small amount, the reflections are changed. The RF energy actually vary continuously, creating a fading effect. Reflections has been studied in somehow comparable environments, for UHF frequencies. While the frequency was a bit higher, and the environment was a simple one, we can use the results for a rough order of magnitude:

RFID signal (915 MHz) loss due to reflections (source)
The spatial fading is visible, at some distances reflected signals add, at other they subtract. This loss is also known as multipath effect. The signal may be attenuated by 10 dB or more compared to free space.
Each reflection has also the potential to change the polarization. See next point.

PLB antenna is vertical, this is the normal use of the SAR device.

The transmitter and receiver antenna polarization must match or an attenuation occurs. The polarization of an antenna describes the plane in which the electric field is transmitted. For a simple antenna, made of a single active element, the electric field is sent in the plane of the element:

Vertically and horizontally polarized waves (source)
Vertical polarization creates less reflection on the ground, which is usually what is desired, so a PLB creates a vertical polarization, but only if the antenna is kept vertical, else the polarization will follow the antenna orientation.
To cop with both linear vertical and horizontal polarizations, and everything in between, the SARP antenna is designed to be circularly polarized, and is able to receive both in good conditions, but not optimal conditions:

  linearly polarized antennas will work with circularly polarized
  antennas and vice versa. However, there will be up to a 3 dB loss in
  signal strength. In weak signal situations, this loss of signal may
  impair communications (source).

The antenna on the satellite should be like this:

Helical, circularly polarized, antenna (source)

The LOS is completely clear, no foliage, no roof, etc.

Indeed, if there is a metallic obstacle in the LOS and there is no possibility for reflections (no hole in the metallic cabin), no signal is transmitted. This could happen if the PLB were surrounded by metallic debris from the airplane wreckage.
Else there is a volume (the Fresnel zone) between the transmitter and the receiver that must be free of obstacles. This zone is an ellipsoid around the LOS. If obstacles are present there will be diffraction and reflection. Diffraction scatters the signal (no need to have knife-edge corners), and reflection deviates it, which could be a positive effect when there is no clear LOS.
Dense vegetation or heavy foliage on the LOS: There is water in plants, a loss occurs, typically larger than 0.1 dB/m at 406 MHz. We can use models to evaluate the attenuation:

(Source, page 155)
Example: The aircraft crashed in a forest. The aircraft is surrounded by 25 m trees, except in the direction from which it came.
Satellites low on the horizon will not be accessible: GEOSAR satellites over the equator are usually low on the horizon when viewed for medium latitudes, and some orbits of LEOSAR will be hidden by the trees due to the loss exceeding -15 dB as soon as the depth of foliage is 50 m (slant depth). This depth can be quickly encountered, leaving a small free field of view from a window (it would be larger for the windshield though):

Tree trench created by crash-landing in a forest

PLB antenna has no gain/loss (0 dB gain).

This is likely the case for many PLB. PLB antenna seems to be a shortened 1/4 λ whip. Antennas are measured against an hypothetical perfect antenna (isotropic antenna), and the ratio of amplification is expressed in dBi (dB comparing to isotropic). dBi and dB are equivalent in the current discussion as the isotropic antenna has a gain of 0 dB.

PLB antenna and RF output stage impedances match (no SWR).

In order for the current out of the PLB RF amplifier to be radiated entirely by the antenna, PLB output impedance and antenna impedance must be equal. As soon as there is a mismatch, a part of the RF current is not converted into an electromagnetic wave and returns to the amplifier, mixing with the current flowing in the other direction. The result is the appearance of standing waves in the PLB device. The ratio of SW is smartly named SW ratio (SWR). PLB are stated having a SWR less than 1.5 or 2 in their specifications. The corresponding loss is 0.2 and 0.5 dB. Not so much.
The impedance of the antenna varies greatly as soon as the antenna is not in a free space. A common cause is that a part of the body (containing water) is close to the antenna, or touching it. This could easily lead to a loss of 10 dB. The radiative efficiency of the antenna (gain) also suffers: Touching the antenna is like deforming it. Its characteristics are changed. For example is is no more optimized for the signal frequency to radiate. So in addition of SW appearance, the current that reaches the radiator is also poorly converted into waves. In your scenario it is important to use the PLB as intended, the antenna being unobstructed.

There is no atmospheric loss (free space).

This is another effect difficult to estimate, as it varies with time of the day, weather aspects and Sun activity.
Oxygen and water vapor play an important role. From ITU-R P.676-10:

Attenuation in dB per km (source)
It seems for UHF the absorption is rather limited, so let's ignore it.
Other losses are due to sporadic electrical activities in the ionosphere. They are more difficult to estimate, and they translate into noise rather than attenuating the signal received by the SARP. Other elements like the capability of the SARP to reject noise would also need to be considered.
Ionosphere anomalies could potentially introduce errors in the Doppler localization.

Bottom line

Let's recapitulate the losses we have discussed:

 - Impedance mismatch / SWR:                 -0.5 dB
 - Transmitter antenna gain:                 +0 dB
 - Reflection in the cabin:                  -0 to -10 dB (center).
 - Cabin window:                             -0 dB
 - Fresnel obstruction and trees:            -0 to -15 dB
 - Free space loss:                          -143 dB
 - Absorption by atmosphere gases            -0 dB
 - Other propagation losses:                 Not evaluated.
 - Off-nadir angle:                          -0 to -4.4 dB
 - Linear to circular polarization mismatch: -3 dB
 - Satellite duplexer:                       -1.6 dB

                               Acceptable:   -168   dB
                               Best case:    -148.1 dB
                               Worst case:   -177.5 dB

The worst case is encountered when:

The PLB experiments the higher level of reflected waves. This can be eliminated by being very close to the window, and shielding the rest of the cabin with something metallic, but not too close.
The Fresnel ellipsoid is blocked by trees. Not much freedom here. LEOSAT will hopefully have orbits with sufficient high elevation to be out of the obstacles. If the aircraft is really covered by deep foliage, there is no much chance for a successful link.
The aircraft is at the nadir of the satellite. The SARP antenna is optimized for off-nadir locations (no sure off-nadir locations will be better served, due to the increased slant distance, that would need to be checked). Too bad if the aircraft is in a forest.

Note that carrying a personal locator doesn't cause a safety problem, as it doesn't transmit before being activated. Activation will be either manual (usually for PLB) or by sensing a large acceleration (seems limited to ELT). I don't know if the security guys at the airport will let you board with a transmitter. It should be easier to persuade the pilots.

Case of a SPOT device

Compared to the numerous documents that describe governments-financed SAR service on 406 MHz, SPOT is a more confusing solution.

This is not related to SPOT, the observation satellite family built by Astrium. GEOS IERCC (International Emergency Response Coordination Center) has no link with the GEOS satellites family launch by Nasa some decades ago. It is not either GOES, the geostationary satellites that support GEOSAR.

Globalstar is a large satellite phone company owning satellites. SPOT LLC is a subsidiary of Globalstar providing satellite data links and related sets for messaging and voice communications. GEOS-Alliance has launched a privately-owned assistance service based on SPOT capabilities. GEOS-Alliance services are sold online, and by retaillers.

From SPOT website:

  SPOT Gen3 provides location-based messaging and emergency notification
  technology that allows you to communicate from remote locations around
  the globe. It offers custom tracking interval options,
  motion-activated tracking, long-life battery and more.

In short:

Instead of buying a SAR transmitter and then having the service for free, the GEOS-Alliance subscriber receives a transmitter and pays an annual fee.
Instead of having the SAR device connected to the SAR organizations worldwide through the LUTs and MCCs/RCCs, the subscriber is connected to GEOS which calls the local emergency services on your behalf.

The technology behind:

SPOT/Globalstar seems to use LEO spacecraft at 1,414 km instead of 850 km for Sarsat, with a 52° inclination instead of 85°, meaning their coverage could be reduced.
SPOT messengers are limited to 0.4 W instead of 5 W for Sarsat, maybe because of regulations, as they can be used without actually signaling a distress situation.
Globalstar frequency used is 1.6 GHz, instead of 406 MHz for Sarsat.

It seems that these three technical differences are not going to make SPOT devices as efficient as Sarsat:

Using the same ITU-R P.525: Lbf = 20 log(4πd/λ) gives a free space loss of 159 dB upfront, to be compared to the 143 dB for Sarsat (difference: -16 dB).
0.4 W = -4 dBW instead of 7 dBW for Sarsat (difference: -11 dB).
We may add a loss of 1 dB for the atmosphere absorption at this frequency.

If I'm not wrong, that's already a difference of -28 dB at the receiver. It must be very sensitive, or be liked to hi-gain antennas to compete with the governments-operated SAR. That's not going to make it from within the cabin of an aircraft with a limited LOS, and sometimes this seems to even not work well in the open space.

Reference documents

C/S A.003: Cospas-Sarsat System Monitoring and Reporting
C/S G.003: Introduction to the Cospas-Sarsat System
C/S G.004: Cospas-Sarsat Glossary
C/S G.006: Cospas-Sarsat System Assessment
C/S G.008: Operational Requirements For Cospas-Sarsat Second Generation. 406 Mhz Beacons
C/S T.001: Specification for Cospas-Sarsat 406 MHz Distress Beacons
C/S T.003: Description of the Payloads Used in the Cospas-Sarsat LEOSAR System
C/S T.004: Cospas-Sarsat LEOSAR Space Segment Commissioning Standard
C/S T.011: Description of the 406 Mhz Payloads Used in the Cospas-Sarsat GEOSAR System

More Sarsat documentation: Cospas-Sarsat site

No, they couldn't. EPIRB beacons have a very short range, by the time you get a signal you'd also have a signal from the beacons in for example the flight data recorder.
You seem (like a lot of people) under the impression that GPS receivers send their location to the satellite which then sends it somewhere to a data center where it can be retrieved. This is incorrect. The device only reads from the satellite network, calculating its position based on signals from the satellites. And it then uses either a short range radio transmitter or a cellphone network to transmit that location to interested parties. But of course in the middle of nowhere there are no cellphone towers and a short range radio doesn't help if the nearest recipient is far away.
On a ski slope that's no problem, as you're in civilisation and not far from help. In the middle of the ocean it's of no use whatsoever.


Do all B777 family aircraft have the same engines? I think there are multiple variants of engines for the different variants in B777 family but I am not sure but is it GE90-115B in one while Trent and PW in others?

TL;DR: No.

For the true answer refer to the FAA Type Certificate Data Sheet

777-200

2 Pratt and Whitney Turbofan Model:
PW4074, PW4074D, PW4077, PW4077D,PW4090,PW4084D, and PW4090-3
(Engine Type Certificate No. E46NE)

2 General Electric Turbofan Model: GE90-76B, GE90-85B, GE90-90B, GE90-94B
(Engine Type Certificate No. E00049EN)

2 Rolls-Royce Turbofan Model: RB211-Trent 875-17, RB211-Trent, 877-17,
RB211-Trent 884-17, RB211-Trent 892-17, or RB211 Trent 892B-17, RB211 Trent 895-17
(Engine Type Certificate E00050EN)

777-300

2 Rolls-Royce Turbofan Model: RB211-Trent 884-17, RB211-Trent 884B-17,
or RB211-Trent 892-17
(Engine Type Certificate E00050EN)

2 Pratt & Whitney Turbofan Model: PW4090, PW4098
(Engine Type Certificate E46N)

777-300ER

2 General Electric Turbofan Model: GE90-115B
(Engine Type Certificate No. E00049EN)

777-200LR

2 General Electric Turbofan Model: GE90-110B1

2 General Electric Turbofan Model: GE90-115B

(Engine Type Certificate No. E00049EN)

777F

2 General Electric Turbofan Model: GE90-110B1

2 General Electric Turbofan Model: GE90-115B
(Engine Type Certificate No. E00049EN)


Did we get closer to another plane than we were supposed to, or was the pilot just protecting our delicate sensibilities? Yesterday, April 28, 2019, I was flying on Alaska flight 557 from LAX to PDX.

About an hour into the flight (near the California/Oregon border) I was looking out the window and saw a jet travelling the opposite direction from us.  It was travelling exactly the opposite direction of us and I saw it when it was ~45-60 degrees forward of us.

Within a couple of seconds of my seeing it, we banked sharply to the right.  It was not a violent turn; An answer to this question describes true evasive maneuvers as, "those of you not strapped down would have been hurled to the ceiling or slammed to the floor and your stomach would be heaving in a different direction from the rest of you." - and we didn't have anything like that.  I'm not sure I would have even noticed the turn had I not been looking out the window.

However, it was a pretty sharp mid-flight maneuver, and because I was over the wing, I didn't get to see the other airplane at closest approach because the wing blocked my view.  I would estimate that it was about 3-5000 feet away from us.  I felt like he was at pretty close to our altitude, but again, as the linked question says, that's hard to say for sure.

The linked question points out that distances and altitude differences are hard to judge.  That's fair, so I'll offer the following bona fides:

I live about half a mile from a commercial airport, perpendicular to the end of the runway, so I have a good idea of what jets that are half a mile away from me look like.  I would estimate that the other jet was a bit but not a lot further away than that.
If two jets pass by each other at 1/2 mile, and you can see another jet that is 60 degrees in front of or behind you, and he, like us, was travelling 480 mph, he would have been in my field of vision for 6.5 seconds.  That sounds about like what I remember.  At 3 miles, that number turns into 40 seconds.  I don't remember the exact amount of time, but my initial estimate before I did this math was 8 seconds.

I could imagine our pilot banking the plane simply to lessen the odds of the passengers noticing a safe encounter, as even a safe encounter leads to awkward questions on sites like this.

So, my question: Did we have a near miss, or was this a standard close-but-intended encounter?

From the flightradar24 playback, it appears to have been an encounter with Etihad Airways 171.

Your flight (Alaska) was at FL360, while the opposing was at FL370. Therefore, there was 1000 feet of vertical separation between them, which is typically considered safe when operating with RVSM, Reduced Vertical Separation Minima rules (see other answers).

I also looked up the LiveATC recordings archive (Seattle Centre Sector 14 high, 28/04 2000Z), and found a clip that I think corresponds to the event:

  Alaska 557, can we take a heading right of course to avoid some wind?
  Affirmative
  Ok, deviation approved [...]

The Etihad flight was a 777, which is a much larger aircraft, but of almost the same proportions as the 737.  Also living near an airport, I am well familiar with how difficult it is to gauge altitude.  You see airplanes apparently moving at quite different speeds; actually a "slow" plane is moving at a similar speed but is larger and farther away.  It's really a problem in open air, with no reliable visual references for size.

So I suspect you saw the same optical illusion, where the 777 seemed closer simply because of its large size.  That said, it was quite close vertically - 1000 feet separation. The horizontal separation did not matter because the vertical separation was sufficient.

It's quite possible the wake turbulence from that huge 777 was of concern to the Alaska pilot, so he deviated (with permission) to avoid it.


Is "Tally-Ho" used in ATC radio communication, specifically in the UK? This question Are "Tally-ho" and "no joy" acceptable ATC terms for civil operations? concerns the use of the phrase "Tally Ho" to mean "target in sight" in civilian air-traffic-control communication.

It was largely answered (and somewhat asked) in relation to the US/FAA, where it is not acceptable but sometime heard. However "Tally Ho" is/was a British phrase, and my question asks the same specifically of British/NATS/CAA communication. Again, I'm interested as to whether it is actually heard, and whether or not it's generally considered acceptable.

I am aware that some efforts are made to keep English usage international, though most feeds/transcript I've heard from the US have an awful lot of regional idioms and phrasing in them, so I'm not sure the extent to which it is adhered to.

(For people outside the UK, "Tally Ho" is generally considered archaic and comically mannered in general life these days, even within the UK, which is why I'm particularly interested as to whether its usage survives here in aviation).

I have never heard "tally ho" used in civilian aviation and it not a recognised phrase so should not be used.  A civilian ATCO would not think positively about anyone using that phrase.

It used to be used in military comms in combat.  Its usage arose during WW1 when the Royal Flying Corps (and later the Royal Air Force) drew its crews mostly from the "officer classes" who, in general, were also fox hunters or supporters of the same.

"Tally ho" is the cry shouted out by a huntsman when the fox is spotted and the hunt is on.

I served in the RAF from 1976 to 1986. Then as a civilian in the military until 1993 and in that time never heard the phrase.  I suspect that it has died out since it is not standard phraseology and perpetuates stereotypes which have no place in modern, professional organisations.

A non-standard, but common phrase is a simple "visual", but the correct phrase in both civilian and military use is "traffic in sight".

In Iran , in the fghter section of the air force ,  (fighter squadrons ) , we use Tally Ho for ( target insight ) and No Joy for ( target not in sight ) .
I flow F-4D/Es & F-14s in Iranian air force for 25 years & have 8 years war experience with Iraq ,  incuding 5 air kills with Tomcats .


Could a 30 knot direct crosswind make a Learjet roll over 90 degrees? Could a 30 knot direct crosswind really make a Learjet roll over 90 degrees?

A recent incident has prompted this question, but it's not explicitly about that incident (with its ongoing investigation).

It only ever makes sense to talk about cross-wind when the ground is involved. While the aircraft flies, it moves relative to the air and how fast the air is moving relative to ground only affects ground speed and track. If the air is moving smoothly, it can't turn anything over. Only thing that could bank an aircraft over is strong turbulence.

Natural turbulence of a 30 knot wind is unlikely to be strong enough to bank a Learjet 35 significantly, but a wake vortex close behind a heavy aircraft possibly could. The separation minima are there to prevent encountering wake turbulence on approach.

The original question was could the wind roll the Learjet 90 degrees and cause it to crash while landing in gusty winds.

PART 1  THE DANGERS OF WINDY CONDITIONS CAUSING AN AIRCRAFT TO STALL - LEADING TO A ROLL AND CRASH

Ground speed is affected by wind speed and direction,  which can make holding a ground track (landing pattern) in wind problematic.

Gusting wind presents a danger with tailwinds as a gust can lower airspeed and cause a stall.  For example, an aircraft that stalls at 50 knots Indicated Air Speed will approach at around 1.2 - 1.3 V stall, around 60-65 knots IAS.

A 15 knot tailwind gust will put IAS at or below stall in level flight.  An alert pilot will pitch down and add power to avoid stalling.

What compounds the problem is holding the ground track in the wind. A 20 knot tailwind will make 65 knots IAS 85 knots ground speed.  A pilot who tries to hold the track with a steeper banked turn risks an accelerated stall.  Stall speed for a 45 degree turn is 20% higher, now 60 knots.  A 15 knot tailwind gust while turning would disastrously lower IAS to 50 knots.

Note also that a lull in headwind similarly would create a loss of airspeed.

So close monitoring of airspeed is essential in these situations, as well as an awareness of wind speed, direction, and gusts.  While there are steady breezes,
calm air will never gust or change direction.

PART 2  EFFECT OF CROSSWIND ON AIRCRAFT IN FLIGHT

An aircraft in flight is unaffected by any CONSTANT wind as it moves with the wind. The only motion relative to the wind is velocity caused by thrust and control inputs.  Imagine the entire air mass and the plane moving through the sky.  One way of looking at it is the motion of the earth and moon.  The moon orbits the earth, but BOTH share the same motion around the sun.

There for a constant cross wind will not roll an aircraft. However, a GUST or change in wind from the side will put a side force on an aircraft as follows:

Gust strikes side of aircraft, pushing it sideways and rolling it away from wind.
Plane moves sideways at speed equal to gust, or gust begins to pass.  Nose will start to point into gust as a result of change in relative wind.
When gust has passed, plane now has side force on OTHER SIDE due to "push" from gust and slip from roll and rights itself.  Nose will point back to original course.

It is unlikely a 30 knot gust will roll a Learjet 90 degrees while it is in cruising flight, or even when it has reduced speed to land.

But, tragicly, the 90 degree roll is the death knell of a stalled aircraft near the ground.  This is what we work to avoid.

PART 3 WAKE TURBULENCE

Wake turbulence from a large aircraft would also be capable of rolling a Lear Jet if it followed too closely.  The vortex flows, especially when the aircraft is pitched up into a high induced drag configuration (even more so with slats and flaps fully deployed) is very dangerous to smaller aircraft.

PART 4 TURBULENCE CREATED BY TERRAIN OR LARGE BUILDINGS

This one is for mountain flyers, who must beware of rotors that form in the lee of the peaks, as well as strong updrafts on the windward side.  This will turn a smooth wind flow into an unpredictable pattern that can suddenly roll an aircraft.  Lenticular clouds over the mountains warn of the danger.


How can I calculate a pitch/power table? I want to make a pitch and power table for C172s based on calculation, just like I attached above.

I picked up this table on internet, but I'm not sure where it comes from.

Do you have any ideas how to get these values?

I got the answer from cessna company, as below.

The pitch/power table you have provided is not a product of Textron Aviation and has been developed outside of our flight testing and engineering. We have no formula available for calculating required pitch attitudes, The table shown appears to be developed by a flight school based on actual flight test observations.
In general terms, pitch relates to angle of attack. Angle of attack required for certain climb and descent rates are dependent on variables such as gross weight, C.G. location and indicated airspeed. In relation to the quote “pitch + power = performance”, a more accurate approach for developing a quick reference chart assuming the aircraft gross weight and C.G. remains constant is to match indicated airspeed with power settings. Pitch + power can approximate an airspeed, but more accurately power + airspeed = pitch. Flying an aircraft “by the numbers” relies on power settings and airspeeds, which consequently will produce a certain pitch attitude. Conversely, to facilitate a desired climb or descent rate we set power and approximate a pitch attitude to arrive at the desired airspeed.
The interrelation between pitch and airspeed is for the most part interchangeable, but most accurate is thinking of pitch attitude being a consequential product of power and airspeed in the context of normal flight maneuvers.

Well, nothing new here. But I found that there are no exact formula to deal with power and pitch table. I'll try to collect actual data.
Thank you guys.

When you are flying you will be looking at airspeed. Pitch, throttle setting (and trim) will depend on this and whether you are climbing, descending, or in level flight.  Data can be gained from experience, or one can go on a flight dedicated to gathering this information all at once.  Fill in the blanks one at a time!

A chart such as this could be a useful part of the POH  in the event of an emergency, such a malfunction of the airspeed indicator.

A scenario pointed out in comments would be a pitot tube icing event. Rpm settings for pitch would be useful for maintaining straight and level flight, giving valuable time for problem solving. Possible solutions mentioned were use of GPS to get ground speed, then land into the wind so airspeed at least is as much or more than ground speed. Also on the list would be to ask ground radar for your speed and fly a straight in landing pattern factoring in the wind.

But under normal conditions, your eyes belong either out the window or on your instruments.


Why do cargo aircraft still have floors? Even with dedicated cargo aircraft built as cargo aircraft (rather than converted from a passenger-carrying configuration), the cabin is virtually always still divided into an upper and a lower lobe (which would, on a passenger aircraft, be the passenger cabin and the cargo hold, respectively) by a floor located a third to a half of the way from the bottom of the fuselage, drastically reducing the maximum size of the cargo that can be carried and making it much harder to transport round outsize cargo (such as, for instance, large high-bypass turbofan engines as are used on essentially all modern jetliners) by air.  Given that the essential structural members of a large modern jet are

the skin itself;
longitudinal longerons running the length of the fuselage just beneath the skin; and
circumferential frames encircling the fuselage just beneath the skin;

it should be possible to remove the floor, and thus dramatically increase the freighter's ability to carry very large objects, without significantly compromising the fuselage's structural integrity (the middle of the cabin would still be partially occluded at the bottom - or the top, or the middle, depending - by the center portion of the wingbox, but there would, nevertheless, be a considerable increase in the outsize-cargo capacity ahead of and behind the wings).  Why isn't this done?  Is the floor left in to facilitate possible future conversion to a passenger aircraft?

Because the overwhelming amount of cargo is fairly small and floors allow easy loading of both decks at the same time which is more important than the ability to load big cargo on occasion. By having floors you can load standard cargo containers quickly and efficiently. It also gives you the ability to standardize the containers with a reasonable degree of reuse most importantly they can fit into the lower deck of a passenger aircraft with out modification. Passenger airlines make a lot of money moving commercial cargo.

When it comes to flying around big, cumbersome cargo there are simply aircraft designed and built for such a task like the Airbus Beluga

(source)

Employees walking through aren't always the most cautious. From experience working at a cargo airline, it is very easy to misstep and fall through and land on something expensive when the floors are up for maintenance.


What is the proper procedure to change VFR altitude when changing heading from east to west? When you fly from west to east, sometimes your route heads west to avoid obstacles or to clear class B or C airspace.

According to hemispheric rule, let's pretend I selected 7500 feet for VFR to the east. En route to a destination with flight following, when I change heading to the west and the route to the west takes about 30 mins, can I remain the altitude until I get a new instruction from ATC? Or, should I request descent/climb and follow the hemispheric rule?

The destination and the post-take-off route are to the east, but when most of the route is to the west, it makes me confusing to select a proper altitude.

If you are on VFR Flight Following you usually don’t need to ask permission to change altitudes. The exception would be if you are given an altitude to fly from ATC. For example, last week I was flying west from Corona and approaching Burbank’s airspace. While climbing through 4,000' I was given an altitude restriction of “At or below 5,000'”. ATC had me fly over Burbank’s Class C altitude (4,800') but wanted to keep me below the departing traffic. I could see a big jet pass me and once it did I was told to “resume own navigation”. On my initial call I had indicated that I was going to fly home at 6,500' so I climbed to that altitude. I could fly at any appropriate altitude, e.g. 4,500', 6,500', 8,500' etc. but given how busy the airspace is and the expectation of ATC that I would remain at 6,500' I would not change altitude unless I first arranged to do so with ATC.

On the flight into Corona, at around 15 miles from the airport, I was given instructions to fly at or below 3,000'—presumably to avoid traffic departing Ontario. There was no need to inform ATC that I was leaving 3,000' when I had the airport in sight and began my descent.

On the other hand, once I got past Ventura, the traffic volume was much lower so I wouldn’t necessarily talk to ATC if I wanted to climb to go over a mountain or avoid a cloud.

As a general rule, when you begin your descent to land ATC can figure it out. In many cases the beginning of the descent corresponds with picking up the ATIS. Since ATC likes to know that you have the ATIS I will usually combine the descent and ATIS call in one transmission.

In your specific example, since you will be flying at the wrong altitude for 30 minutes, you should either climb or descend—unless your current altitude was assigned by ATC.

FYI, in SoCal when ATC says  “At or below 5,000'” they really mean “at exactly 5,000'”.

First, your question implies you have flight following.  So there would really be no need to avoid class C or B airspace since ATC would simply fly you through them or make their own decision to ask you to go around, over, or under them.

ATC should know to assign a different altitude when you change from east to west or vice versa, but if they do not, yes, you should request an appropriate altitude for the heading that will keep you in VMC conditions.

Keep in mind, that if you are told to "resume own navigation" or "proceed on course", they are allowing you to change your heading and altitude as you please, so you wouldn't need to ask for special permission to change it, and in that case, just change to an appropriate altitude that will keep you in VMC.  No need to say anything; if ATC decides they don't like what you're doing, they would give you a new instruction.


FAA Regulations for Passenger Aircraft Does the FAA have regulations for aircraft that carry passengers over large oceans regarding number of engines and flying range of the aircraft ?

Definitive answer: yes, it does.  Lots & lots of them.


Is it required for pilots to contact ATC while flying VFR? Does a pilot need to communicate with ATC while flying VFR (during departure, flight or departure) or does it depend on type of flight (like private, commercial, etc.)?
Or is it required for pilot to inform ATC only during arrival and departure for runway clearance and altitudes?
If a pilot does not remain in contact with ATC during the flight, how does ATC know what pilot's intentions are?

It depends on which class of airspace you are flying in. For airspace classes A, B, C and D, a continuous two-way radio contact with ATC is required for all VFR flights. In airspace classes E, F and G, there is no such requirement.
The below table shows requirements for both IFR and VFR to fly in different airspace classes (source: ICAO Annex 11, Appendix 4). You should notice the second column from the right, which specified the radio communication requirements for a given airspace class.

These are general rules, that apply to all ICAO member states (basically, every country in the world), unless a specific state has decided to deviate from the ICAO standards. Such deviations will be listed in the national AIP, section GEN 1.7. For example, in some countries, two-way radio communication may be required for VFR traffic within certain airspaces classified G, even though that is generally not the case. A table with rules specific for your country will be available in your national AIP, section ENR 1.4, and will probably look quite similar to the above.
At controlled airports, a control zone will be established, which will be class A, B, C or D (cf. ICAO Annex 11 para 2.6, a control zone cannot be classified E). This means that, at a controlled airport, VFR traffic must establish a two-way radio contact with ATC, just like IFR.
At uncontrolled airports, however, this is generally not required.
During the approach, departure and en-route phases of flight, whether or not you are required to be in contact with ATC depends entirely on where you are flying. Very generally speaking, if flying close to controlled airports or above a certain level, you can expect the airspace to be controlled, and thus, contact with ATC is required. At low altitude, away from major airports, the airspace will often be uncontrolled, and no radio contact required.
The above is a very general description, and you really must refer to local charts and procedures to figure out if you are required to contact ATC in a specific area.
As a point of advice, ATC is there to help you. It is a good idea to contact ATC even if you are not required to do so, in order to receive flight information service and alerting service. This allows you to receive important information about traffic, weather, military activity, etc., and if something happens to you, a search and rescue mission can be initiated without delay.

To give you a complete answer to all your questions:
AFAIK the jurisdiction does apply to all countries in the world (ICAO members). There are probably additional, national rules (which I estimate to not be a lot as aviation is quite international; only regarding the airspaces and mandatory radio-contact).

It only depends on the airspace you are in. As mentioned by others, there are 7 of them, from A to G where as A is the "strictest" and G is the least regulated one. Countries are free to split their airspace into the different classes (which often results in countries only using certain classes but not all). Radio contact is mandatory for VFR traffic in A, B, C and D. Every airspace (except E (has, but only mandatory for IFR), F and G) therefore also has a corresponding radio-frequency to contact. As a simple conclusion, only so-called controlled airfield (airfields inside of an airspace A, B, C or D) have an ATC. Others are "free to land" (there are rules which you should respect, see VAC or similar approach informations, but there is no mandatory radio-contact required; there also is no one having the permission to give you any clearance).
For more information regarding this answer, have a look at an ICAO chart or the airspace list posted by others.
Basically, he is required to contact the ATC on the corresponding frequency before entering the airspace. Additionally, some airports require you do contact them at specific points a certain time (5 minutes) before approaching (so not just before you enter their airspace). This is written on the VAC (Visual Approach Charts) of every airport and is not a "flight-restriction" but a "condition" to get a clearance to enter their airspace. Summarized: There is only obligation to contact an ATC if you want to enter it's airspace. Probably, the obligation is to call it early enough from outside, but this is an additional requirement IF you want to enter it's airspace. Stay outside and you never ever have to contact them.
That's why radio-contact is mandatory inside certain airspaces (A, B, C, D), the others (E, F, G) do not have an "ATC". The only possibility to switch the frequency of your current airspace is on approval by the ATC (which sometimes makes sense, imagine you want to cross a neighboring airspace: you first have to contact that one before you can enter, but you're still in your first one. In this case, the ATC you are in will allow you to change frequency). As the pilot talked to the ATC before, the ATC knows know very well (even dough they are not on the same frequency anymore) what he is going to do.

For all the people complaining about the wrong rules of the airspaces: If you want to know them exactly with all the details, please refer to one of the posted pictures, my answer is regarding to the question of mandatory VFR radio-contact. Nothing more and nothing less.

edit: airspace E does not have mandatory VFR radio-contact.


Why does it feel like a plane accelerates just before touching down? On commercial flights it often feels like the plane gets faster just before touching down. What creates that sensation?

The aircraft flares just before touching down. It descends with a constant velocity, and just before touching down pulls the nose up to reduce the descent. This results in a higher angle of attack, more lift, and a vertical deceleration of the airplane. A passenger perceives this vertical deceleration as a force. Direction of the force is straight down and the aircraft is nose up, you're leaning back, so there is a component of (gravity + vertical deceleration) that pushes you into the back of your seat.

This same effect is used in flight simulators with a motion base. Upon accelerating, the simulator pushes forward like the aircraft does, but also slowly rotates backwards so that the pilot feels sustained seat back pressure. He cannot see the leaning back angle inside the simulator because the horizon of the projected outside visual image does not move. As happens in the aircraft: look forward in the aircraft and your view of the inside of the aircraft is always the same, irrespective of aircraft angle.

So for your eyes there is no apparent tilt, the inner ear senses rearwards force, and there is increased pressure from the seat backrest. The brain translates this into perceived forward acceleration.

It does accelerate just before touching down in case something goes wrong with the landing gear and the plane has to immediately take off again.


Are German, Japanese, and Chinese companies not able to build aircraft jet engines on their own? As far as I know, only 4 countries can build aircraft jet engines on their own:

USA {GE, P&W}
Russia {Klimov, NPO Saturn}
France {Safran}
UK {Rolls-Royce}

All other countries use jet engines bought from companies of these 4 countries.

Is that true?
Are German, Japanese, and Chinese companies not able to build aircraft jet engines on their own?
Why is building jet engines so tough for companies from other countries?

Well Rolls Royce designs and builds jet engines in Germany with their RR Deutschland division.  Rolls Royce Deutschland is a subsidiary of Rolls Royce and is a collaboration between RR and BMW.

The Shenyang Aerospace Corporation and Aero Engine Corporation of China are homegrown Chinese jet engine manufacturers, making their names largely for the manufacture of copies of Russian military jet engines for PLAAF need, but are now manufacturing their own designs for military and civilian applications.

Japan’s Aero Engine Corp is a partnership between Rolls Royce and several Japanese industrial powerhouses, most notably Mitsubishi Heavy Industries, who has begun to develop a line of small to medium thrust high bypass turbofan engines for regional airliners use.

While the basics of a jet engine are fairly well understood - it is, after all just a Brayton Cycle gas turbine - the details of designing and manufacturing modern high efficiency jet engines requires quite a bit of technical talent and capital to accomplish.  A new engine program often takes longer, and is more expensive than, the aircraft program it is being designed to power.  Most likely many entrepreneurs and prospective OEMs are deterred by this as any improvement they are likely to offer can be either bought out by an existing engine OEM eg Pratt and Whitney, etc. or simply crushed by the economies of scale.

The Germans are very capable of designing any sort of complex engines,no doubt they are the pioneer of jet engine technology, but inter political and economic agreement discourage then from doing so,I have to come to realize that the Germans are the best when it comes to highly sophisticated technologies,history has always shown what their engineers are capable of doing, they are also the pioneers of gas turbine technology, the Germans are just too awesome when it comes to advanced engineering.. The US and Russian space programme and technology were made possible by means of abducted German scientist and engineers infact critical observation of the nazi Luftwaffe aerospace engineering technology  at the end of ww2 was what helped the allies nation to establish their own, the Germans are incomparable genius  when it comes to complex innovation and technology,but economic and government boundaries limit them from re establishing their jet engine production once again

JAPAN

Another fantastic nation filled with mouth opening innovations and unbelievable technology these nation is extremely capable of building even the most advanced jet engine in the world no doubt they are the most advanced country in the world right now, the Japanese are just too amazing when it comes to technology, but also the reason why they aren't building or haven't build there own jet engines is not as a result of inability no far from it rather as a result of eco- political reasons coupled with the fact that they don't see much reason to waste resources to establish theirs when they can actually import quality jet engine from others

SUMMARY

Jet engine technology is often related to military application and not civilian application so which means it is only countries that grow interest in investing heavily in their military budget that end up showing great interest in building or accumulating jet engine technology, Germany and Japan those two tech giants are not for now investing heavily or have interest in making their military a world recognized one as before unlike Russia, US,Britain, France china that quest for military supremacy hence they source for means to achieve their quest by building a powerful jet engine to air supremacy.


How is the 737 main landing gear protected from weather? I really like the 737, it is perhaps my favorite airliner. One curiosity of this aircraft (afaik, it is the only commercial airliner) is that the main landing gear has no cover:

Here is another picture from the inside:

In this situation, how does the aircraft protect the gear mechanism from weather/freezing?

Aircraft icing is not caused by ice in clouds- it is formed by supercooled water droplets (Source). They freeze on impact with a surface that has some location with zero flow velocity, most often on the leading edge of the wing.

  Ice accumulates on every exposed frontal surface of the aeroplane – not just on the wings, propeller, and windshield, but also on the antennas, vents, intakes, and cowlings. It builds in flight where no heat or boots can reach it. It can cause antennas to vibrate so severely that they break. In moderate to severe conditions, a light aircraft can become so iced up that continued flight is impossible. The aeroplane may stall at much higher speeds and lower angles of attack than normal. It can roll or pitch uncontrollably, and recovery may be impossible. Source

The gear as such as is not worse off than normal landing gear. The B737 has wheel caps visible in your first photo to cover towards the outside, and is not allowed to fly without them. If icing had been a problem, the same concern would extend onto landing gear doors being frozen shut. Landing gears bays are unpressurised and very cold normally anyways.

Most of the aircrafts have covers (caps) so as to isolate the landing gear from the air around so as to prevent it from icing. Commercial airliners reach altitudes to the proximity of 35000 feet (and maybe more in some cases) and temperatures could drop 80 degrees below zero. In such cases, if the landing gear has caps and is isolated from the free stream, the heat retained in the landing gear bay from the hydraulics and the friction in the tires while takeoff does keep the landing gear warm and hence protect it from icing (source).

  Heat from the plane’s hydraulic lines in the wheel well, as well as
  heat retained in the tires, could have helped the stowaway survive as
  the aircraft climbed to altitudes with sub-zero temperatures, the FAA
  reported.

Now, for airplanes like the one shown above, there is a possibility of the landing gear getting iced. When this happens, it might cause a range of problems. Problems ranging from skidding while landing to jammed landing gear. Usually, before landing when an airplane descents, the rise in temperature is good enough to deice the landing gear, but if that does not happen then pilots need to tackle with the icing problem in flight itself. Source

  When surface temperatures are low enough for frozen deposits to be
  present, consider whether the mechanisms might have become frozen
  during flight as a result of prior extended taxiing through slush or
  wet snow. Flight above the freezing level for prolonged periods may
  result in the landing gear becoming frozen in the retracted position.
  This is less of a problem on most modern commercial transport
  aircraft, where the gear bays are largely enclosed by doors unless
  retraction/extension is in progress, but is a relatively common
  occurrence on aircraft such as the LOCKHEED AC-130 Spectre. If it is
  suspected that taxiing through slush or wet snow may have led to
  deposits adhering to the landing gear assemblies, then it may be
  advisable to cycle the gear after initial post take off retraction to
  try and shed any deposits.


Why was the Vulcan bomber used for the Falklands raid? There is a lot of information about this complex operation. But can anybody explain why they chose to use an aircraft? They could easily shoot cruise missiles from a ship or ballistic missiles from a nuclear submarine. The easiest and quickest option would be just to shoot an ICBM directly from the UK using a conventional warhead. Just put the coordinates in and task done. No risk to anyone! I think the UK had all the needed technology by that time.

This is borderline as an aviation question but I will answer it anyway.

They didn't use those weapons because they did not have them at the time:

Conventional cruise missiles did exist but most were nuclear, and the UK didn't possess any conventional ones.

ICBMs with conventional warheads did not and probably still do not exist because they are not a good idea:

Their capacity is low: A sub launched ICBM has somewhere around 2.5 to 3 metric tons' throw weight, much of that would be taken up by warhead guidance, thermal protection and other necessities so your actual warhead would be maybe 1000kg, or 2200lb. That's not that big a conventional bomb, and it could only launch one. A fighter bomber could drop 2 of those. The Vulcan bombers on the raid carried 21 1000 pound bombs.
They aren't accurate enough: a nuclear weapon can be a bit off and still destroy its target utterly, a conventional weapon needs a great deal of precision. The Polaris missile had an accuracy of 3000ft (900m). That's okay for a nuke, it' still ruin someone's day, but completely inadequate for a 2000lb bomb. More modern ICMBs have more like a 100 meter accuracy, which is a lot closer but still not good enough for a conventional bomb
They are expensive: that's a lot of cash for a 2000lb bomb that probably will miss
Launching one could start WW3: If you launch an ICBM someone is going to see it and if they think it might be directed at them a retaliatory strike could be the answer. The world has come perilously close to this more than once, it simply isn't worth the risk!

As noted in a comment, the UK did have access to the UGM-27 Polaris submarine launched nuclear missile.

Launching a missile from a sub ranged a few hundred miles (or even a few thousand miles) off-shore would have been significantly less risky to UK military personnel and equipment. However...

Launching nuclear warheads against a target that you intend to occupy, as opposed to completely annihilate and make uninhabitable for a century or two (or three), is somewhat counter-productive. It also tends to, um, raise eyebrows in the international community.

Since Hiroshima and Nagasaki, no nuclear weapons have been used against actual foreign targets or people. Doing so would have been absolute political suicide for Thatcher and company, and would have most likely brought extreme backlash and sanctions against the UK as a whole.

Oh, and don't forget the numerous UK citizens who were actually living in the Falklands would have just been killed by their own government.


How is vertical speed managed in Airliners? When an Airplane starts to take off, the Horizontal velocity would generate it the necessary lift, and then Airplane pitches up to further increase the lift. As the Airplane is climbing up, the net vertical force on it should be the difference of lift and its own weight. When there is a resultant force, should not the climb rate be ever increasing? And when the Airplane reaches the desired height, I'm assuming - either the horizontal velocity is decreased or Airplane goes to normal state from Pitch-up position, basically balancing the lift with own weight. At this moment, Airplane has some vertical speed right, So Isn't the Airplane supposed to be climbing up with a constant velocity, even after reaching desired rate, as Objects continue to move in the same velocity when there exists no force on them? How is this managed in Airliners?

Vertical speed is managed, primarily, with controls.

Lift is proportional to square of speed and to angle of attack¹ and angle of attack can be adjusted using the elevators. Pilot, or automation, use those to control the flight path of the aircraft.

As the aircraft accelerates along the runway, at some point the pilot pulls on the control column, which turns the elevators up and the resulting downforce on the tail lifts the nose off the ground². That increases the angle of attack—because the aircraft is still moving horizontally—, lift exceeds weight and the aircraft accelerates upwards.

As it starts to accelerate upwards, the pilot eases the pull on the control column to prevent the aircraft pitching up further. And as the aircraft accelerates upward, the angle between direction it is flying and pitch—that is the angle of attack³—decreases again, until the forces get in balance. Then the aircraft is in steady climb.

When the aircraft reaches the top of climb, the pilot⁴ pushes on the control column. This causes the aircraft to pitch down, which reduces the angle of attack, and thus lift, and the aircraft starts to accelerate downward, that is slow down the climb. At the point the aircraft is moving horizontally, the control column is pushed back to get the forces back in balance.

Now for the details of operating the controls, the longitudinal stability comes into play. Aircraft are normally⁷ designed to be longitudinally stable. That means the aircraft will pitch up as its angle of attack decreases—which increases it again—and pitch down as its angle of attack increases—which decreases it again. Net result is that when the elevators are left alone, the aircraft will maintain specific angle of attack.

As the aircraft climbs, it will tend to slow down unless engine power is increased, because its potential energy increases and it will be taken from kinetic energy if the engines are not providing enough⁸. And as it will slow down, its lift will decrease, so its angle of attack will increase, but that will cause it to pitch down due to stability. The net result is that it will refuse to climb unless enough power is provided.

Similarly when it descends, it will tend to accelerate, because the potential energy will be converted to kinetic⁹ and that will increase lift, which will decrease the angle of attack and the stability will make the plane pitch up. So it will refuse to descent unless power is reduced.

So in the end, vertical speed is actually managed by thrust levers. Which is also much more easily proven from law of conservation of energy.

¹ Up to critical angle of attack, where stall occurs.

² This is called rotation.

³ Angle of attack is properly defined as angle between the relative wind and the chord line of the wing, but aircraft axis is often used in practice. This makes things simpler as you don't have to consider the angle of incidence—angle between the aircraft axis and wing chord—especially since many aircraft have twisted wings where the angle of incidence changes along the span.

⁴ Or more often the autopilot. While take-off is always flown manually, up above FL290⁵ use of autopilot is required as it gets too tiring and unreliable to maintain the altitude by hand with enough precision to ensure separation⁶.

⁵ Flight levels are defined by pressure corresponding to given altitude, in hundreds of feet, on a standard day. So FL290 is 29,000 ft, but more when it is warm and less when it is cold. The reason is that pressure can be measured easily and quite accurately and flying at sufficiently different pressures ensures the altitudes are also different.

⁶ Originally the minimum separation was 2,000 ft above 29,000 ft due to the lower accuracy of altimeters and lower accuracy of flying as the aircraft move faster in the thinner air up there. But because all the aircraft wouldn't fit up there with those separations, 1,000 ft was allowed provided the aircraft is flying on autopilot and has sufficiently accurate altimeter. This is called reduced vertical separation minima

⁷ Some fighters are intentionally designed as unstable, because it allows faster control response. All such aircraft have computerized controls that compensate this, otherwise it would be very tiring to fly.

⁸ Alternatively, consider that the lift vector is tilted aft, and thus has bigger aft component, which is drag. Physics always has multiple ways to analyse a situation.

⁹ The lift vector is tilted forward in descent, so there is some forward component that accelerates the aircraft.

"Accelerate wildly like an 18 wheeler gaining momentum"? Now hold on there, just a second.

The crux of this question is balancing of forces.  In atmosphere drag is what prevents any object from continuously accelerating.  Drag will increase with velocity until its force is the same as the accelerating force.  Now you have balanced forces at constant velocity.

Easiest example is the parachute.  Without one your terminal velocity will be around 120 mph falling prone with arms and legs out.  Even faster if you point straight down (less drag).
The parachute provides much more drag, and requires only 15 mph to balance gravitational
acceleration.

In 18 wheelers, we try to control our speed with dynamic braking (from engine) to avoid heating brakes.  We also know doubling speed will heat brakes 4x faster as kinetic energy is
proportional to the square of velocity.  Also of note is the stopping distance is 4x greater as well.  Speed control also serves one well in landing as the same kinetic energy will square with velocity.  At 70 mph you have 36% more energy than at 60mph.


What lead time is required to change an already submitted flight plan? A government agency of an EU country commissions a private company with a chartered flight from Europe to Africa/Asia/... (e.g. to involuntarily take people to a specific country).

Then it wants to change the time of departure with short notice. How long before the changed departure time do they need to file the new flight plan and what are some obstacles for changing it at the last minute?

The flight plan can be changed by the minute, as long as the two governments and the two airports and the airline agree on this.

As it seems like the government pays the airline to just take off whenever they are allowed to, take the airline out of the equation.
Likewise for the airports, since they are either state-operated or have a concession which states that they have to assist in lawful deportations, emergencies or state affairs.

This keeps two governments in, which have to agree on a time and the who-pays-whom-how-much for the deportation to really take place. In fact, I think this is the only reason why the plane is not yet airborne - because of two governments playing their diplomacy games.

I am the US, but here you can amend a flight plan while in air.  I have done this several times to change the destination from airport X to airport Y.  ATC nicely updated my clearance to the new destination.


Is it sensible to harness wind energy on an airplane? Is it sensible to harness wind energy on an airplane?

Actually the idea is to build an electric aircraft whose power comes from solar and from wind while the plane is landing. instead of using spoilers as airbrakes, these wind turbines separately mounted could add drag and harness energy at the same time . In other words, regenerative braking.

Normally not: Extracting energy from the airflow produces drag, which must be overcome by added thrust. Since every form of energy conversion produces losses, more thrust energy must be added than can be gained from the airflow.

Only when the engines fail and the generators stop running does it make sense to extract energy from the airflow. In airplanes there are two applications which are driven by "wind energy":

Older airplanes use gyros for the artificial horizon which are powered by ram air. This way the gyros work even after an engine failure.
Jets use Ram Air Turbines (RAT), propeller-driven generators which are moved into the airflow when all other means of generating electricity and hydraulic power have failed. Note that this is done in unpowered flight and increases the sink rate.

Deployed RAT (picture source)

Rocket-powered airplanes have no easy way to generate electricity, so the Me-163 B used a small windmill at the tip of the fuselage to drive a generator.

Me-163 B (picture source)

EDIT:
With your question's new focus on electric propulsion the answer will become different. Now you most likely will have propellers which are powered by electric motors. During landing, those could run inverted and charge a battery, which will most likely be empty by the time the aircraft lands. This can be done during final approach until the end of the runout after touchdown. It can be expected that any electric airplane will have a high L/D, so deploying speed brakes to allow for a steeper approach makes sense.

I would be surprised if an additional device would be economical, however. This recharging must be done by the regular propulsion system, or it would add dead weight during most of the flight.

You asked for formulas, but all I can provide here are some back-of-the-envelope calculations. First, it must be said that variable-pitch propellers will be lousy as windmills, because their camber and twist are wrong for windmilling mode. I would expect that their efficiency is around 30%, which means that only 30% of the energy extracted by drag will be converted into mechanical energy which drives the electric motor.

Next, running an electric motor as a generator will again require compromises. Good motors make poor generators, and re-wiring the motor for better generator performance will degrade its efficiency in normal use. You will quickly lose more than you gain from the short flight phase when running the motor inverted makes sense.

Now let's assume that you keep your propulsion system at top efficiency (say, 90%) and accept that it will only convert 10% of the drag energy into electric energy. Let's also assume that your prospective solar airplane has an L/D of 30 which needs to be reduced to 10 for a practical approach. You do this from 1000 ft downwards and use the windmilling propeller also during the runout. Approach speed is $v$, mass is $m$ and the initial energy of the airplane is $305\cdot m\cdot 9.81 + \frac{m}{2} \cdot v^2$. Two thirds of the potential energy go into propeller drag, and to be generous we assume that 100% of the kinetic energy will also go into propeller drag, even though the braking force of the propellers at low speed is really lousy and needs support from wheel brakes.

Now it is important how fast your airplane flies, because this will shift the ratio between potential and kinetic energy. To keep things simple, I will relate both to the energy required for the next flight. 10% of the full kinetic energy will accelerate the aircraft to less than one third of its flight speed - after this, the remaining 91% of the energy to reach $v$ must be added by charging the batteries between flights.

The electric energy taken from the potential energy will help you climb to 60ft or sustain level flight at $v$ for a distance of 1800ft. At an L/D of 30 the airplane will fly a distance of 30,000ft without thrust, and by braking you will extract the energy to cover 20,000ft, which at 10% conversion efficiency (and 90% propulsion efficiency!) will carry you over just 1800 ft.

This will be built-in, and almost mandatory.

Your question has a built-in assumption: regenerating energy is useful.  The only way it would be useful is if the propulsion system (or part of it) was electric. Therefore by definition we must be referring to an aircraft with an electric propulsion.

Also presumably, we are not throwing 1880-era tech into the sky. That means we are using AC motors with flux vector drive, i.e. Large semiconductors synthesizing 3-phase sinewaves from DC.

If the drive frequency is the same as motor speed, that is inert - no current flows and no thrust is made.  If a higher frequency, this applies power.  If higher still, it applies more power.  If slower, it regenerates.   If slowerer, it regens more.

So if you have an AC electric drive, you already have regen.  Done!

It'll essentially be thrust reverse.  Where do you get to use that?   Glide slopes are designed very shallow so you'll usually need power all the way down.  Look at how they fly the C-17, they would need to redesign approaches to be like that, which would also make them exclusive to electrics and thrust-reverse-in-flight machines like the C-17.


Is there a definition of light/medium/heavy aircraft? I'm surprised this hasn't been asked before but I couldn't find it by searching.

For separating arriving aircraft, ICAO PANS-ATM-Doc 4444 Air Traffic Management refers to aircraft as "LIGHT", "MEDIUM" or "HEAVY" but I cannot find any definition of what makes an aircraft fall into any particulate category.

I would of expected a limit on MTOW for each category, does such a definition exist or is it more of an informal definition?

  5.8.2 · Arriving aircraft

  5.8.2.1 Except as provided for in 5.8.l.l a) and b), the following separation minima shall be applied.

  5.8.2.l.l The following minima shall be applied to aircraft landing behind a HEAVY or a MEDIUM aircraft:

  a) MEDIUM aircraft behind HEAVY aircraft- 2 minutes;

  b) LIGHT aircraft behind a HEAVY or MEDIUM aircraft- 3 minutes.

Section 4.9.1.1 of ICAO PANS-ATM-Doc 4444 Air Traffic Management defines LIGHT, MEDIUM & HEAVY aircraft for the purpose of wake separation:

4.9.1 Wake turbulence categories of aircraft
4.9.1.1 Wake turbulence separation minima shall be based on a grouping of aircraft types into three categories according to the maximum certified take-off mass as follows:
a) HEAVY (H) — all aircraft types of 136,000kg or more;
b) MEDIUM (M) — all aircraft types less than 136,000kg but more than 7,000kg; and
c) LIGHT (L) — aircraft types of 7,000kg or less.
4.9.1.2 Helicopters should be kept well clear of light aircraft when hovering or while air taxiing.
Note 1.— Helicopters produce vortices when in flight and there is some evidence that, per kilogram of gross mass, their vortices are more intense than those of fixed-wing aircraft.
Note 2.— The provisions governing wake turbulence minima are set forth in Chapter 5, Section 5.8, and Chapter 8, Section 8.7.3.

please note that there is a "Super" category only for A380 as the wake turbulence can cause issues even for Heavy aircraft if they use the same seperations as "Heavy-Heavy"


Are jet streams a net benefit in time and fuel savings? Suppose a plane has a cruise speed of 400 knots  TAS. In a 4000 nm route we would have a round trip of 20 hours. If there was a jet stream of 100 knots in either direction, we would have 4000/500=8 hours in the tailwind direction and 4000/300=13.3 flying into the jet stream for a total of 21.3 hours. Now clearly, airlines are smarter than this and plan flightpaths, altitudes and speeds that maximize tailwinds and minimize time spent flying against the wind. The question, is to what extent does this strategy help? Does it simply offset the time lost compared to flying at the same speed in both directions? Or does it help even more than that. In other words, would a hypothetical windless world hurt the airline industry?

Yes, Jetstreams are a net benefit on longer flights because it is easy to minimize headwinds and maximize tailwinds for both directions.

Longer flights can detour hundreds of miles in order to take advantage of more favorable winds and still result in a net gain.

I frequently fly from Toronto to Hong Kong. We always fly a polar route YYZ-HKG as it is shorter, but usually take a HKG-YYZ North Pacific route over Alaska to take advantage of the prevailing winds.

This daily flight is always scheduled at 15 hours 40 mins for Toronto to Hong Kong over the pole but only 14 hours 45 mins returning via a longer route over Alaska.

The longer flight routing over Alaska is a net benefit in time and fuel due to the prevailing westerly winds over the North Pacific.  This type of flexibility is not available on shorter flights.

YYZ-HKG 6,787nm non-Stop 15:40

HKG-ANC-YYZ 7,050nm non-Stop 14:45

The strategy helps in term of block fuel.

Case tailwind: at the same True Air Speed (TAS),you fly faster in respect to the ground, Ground Speed GS.
Case headwind: to keep the same TAS now your GS will be slower.

Let's compute the impact of Ground Speed / True Airspeed on Fuel Consumption, from Airbus getting to grips with Aircraft performance, page 131:

  The specific range (SR) is the distance covered per fuel unit.
  Basically speaking, the specific range is equal to:

$SR_{ground} = \frac{GS}{FF}$

Considering Air Distance the specific range is equal to:

$SR_{air} = \frac{TAS}{FF}$

Where FF is the fuel flow [Kg/h], and TAS/GS is given in [NM/h].

So in a no-wind world, $TAS = GS$, otherwise at the same TAS constant, being the GS larger/smaller for a tail-/head- wind situation you will cover more/less distance with the same amount of fuel in respect to the ground.

In practice, operationally speaking, an aircraft tries to flies always at its most efficient speed, how speed effects the specific range can be seen in picture below (always from Aibus getting to grips with Aircraft Performance):


Is it safe to fly on airplane with lithium batteries in cargo hold? Lithium-ion batteries have the tendency to go on fire.

I checked baggage policy of European low cost carriers (easyJet, Ryanair, WizzAir), and while spare batteries are banned from cargo hold, it seems that batteries in devices are allowed (also hand baggage could be placed in hold if there is no place in cabin).

If spare batteries are dangerous, aren't batteries in devices too?

A few incidents happended when lithium-ion battery exploded, and the fire has been extinguished by cabin crew.

Such fire could also happen in cargo hold. What's happens then? I've seen people question the ability of halon fire suppression systems to extinguish lithium battery fire, because it works by blocking oxygen from fire, while lithium-ion batteries do not need oxygen to burn:

  Halon basically works by displacing the oxygen that is feeding the
  fire - and won't work when the material that is burning can provide
  its own oxidizer. Li-O batteries fall into that category, along with
  things like ammonium perchlorate (used to make solid rocket
  propellant), gunpowder, and oxygen generation canisters. Once ignited,
  they will continue to burn until the fuel is exhausted....
  (http://www.pprune.org/australia-new-zealand-pacific/538637-melbourne-airport-737-cargo-hold-fire-poss-due-lithium-ion-battery-2.html)

Also this:
https://batterybro.com/blogs/18650-wholesale-battery-reviews/19673027-plane-safety-systems-are-confirmed-to-not-stop-lithium-ion-battery-fires

On the other hand in comments to this question: What safety differences exist between carrying laptops in checked versus carry-on baggage? people suggest that it will keep fire from spreading until battery extinguishes on its own.

Moreover checked baggage is not always handled gently by airport employees, which can damage electronic devices - and batteries in them, which makes them more likely to catch fire.

For me it seems like any airplane carrying electronic devices could go on fire at any moment, and each flight is as safe as the batteries it carries. On the other hand aviation has stringent regulation about using electronic devices on board out of fear of interfering with airplane systems. I guess that if lithium batteries were so dangerous as I think then they would be banned from cargo hold.

So is airplane crash due to lithium battery fire a real danger, or do I worry too much?
(I guess I'm a little nervous - I'm going to fly a third time in my life so far.)

No, it is not safe to fly on an aircraft that has lithium-ion batteries in the hold.  There is a commentary on FAA Safety Alert for Operators (SAFO) 10017 here, which was issued after the crash of UPS Flight 6.

Admittedly, this was a bulk load of 81,000 batteries on a cargo flight.  However a search for 'battery' on AV Herald shows between one and two incidents each month, of lithium ion batteries/devices igniting in the cabin.  These are only the ones reported on that website, we don't know how many may go unreported

The ICAO also considers Li-ion batteries a risk but as Simon Hradecky points out, there is a conflict between this safety advice and the security requirements that force passengers to check-in laptops, tablets etc on some flights.

Personally, I think that, whatever the ICAO, FAA and airline requirements, there are going to be some of these devices that get into the hold (people will forget that they left a battery bank in a bag and security will not recognise it on the scanner). We know that they regularly catch fire.  There are almost certainly going to be airliners brought down by this, it may have already happened - the EgyptAir MS804 fire hasn't been explained yet.

Li-ion battaries are safe if they isn't counterfeit
Battaries manufactoring is hi-tech process, and there is limited count of companies, that can make it. For example nanotechnology can be used while li-ion bataries manufactoring.
Each battery has own datasheet. This document contains techical information and information about tests.
Battaries testing
Consider typical datasheet of ICR-18650-26F battery, manufactored by Samsung SDI
It is safe to

Heat battery to 130°C for 60 minutes
Short-circuit by less than 50mΩ wire for 3hours
Drop full-charged battery from 1.5m 6 times
Overcharge with 12V and 2.6A for 2.5 hours
Vibrate between 10Hz and 55Hz with amplitude of 1.6mm
Reverse Charge with current 2.6A by –12V for 2.5 hours

Root of ban rules

In my opinion this bans based not on logic and statistics, but on fears and prejudjes.
But anyway if someone is seriously tuned, there is no problem
Counterfeit
This is a problem. There is no quality control on low-quality manufactories. Good news is that well-known electronics manufactorers don't use it.


How often is QFE set on an altimeter during flight? In countries that use QFE rather than QNH (such as China), how often will a pilot set the QFE setting on their altimeter during flight? Is it only when they intend to land at an airfield, or does it get set for the nearest airfield/aerodrome and updated when entering a new airfields's airspace?

I understand that in the US, QFE is primarily used for airshows and aerobatic competitions, which are presumably operating out of a single airfield and therefore wouldn't be changed in flight.

You don't use QFE or QNH in an airspace, you use it for a purpose.

QFE is rarely used in the US, other places in the world it's primarily used for landing, although that's personal preference (I use QNH). You set QNH on the ground at your departure airfield using the local setting then update QNH on route, when approaching an airfield for a landing you would use that airfield's QNH setting as given to you over the radio or from an information broadcast.

The only time I could see setting QFE more than once would be in the case of a diversion, if you needed to fly to another airfield you'd need to go back to QNH, then set the local QFE at the other airfield. This is extra workload and could be a forgotten step, which one of the reasons I prefer QNH for landing - it's one less thing to do.

95% of all airports in China are now "QNH" qualified, except some small military airports, thus we don't use QFE most of the time.