If ELAC 1 is under MEL why is it a NO GO item when carrying out EDTO on the A320? In the MEL for the A320 it is mentioned that ELAC 1 is a NO GO if carrying out EDTO operations. Why is ELAC 1 specifically for EDTO? In that case shouldn't it be for non EDTO operations also.

This somewhat contradicts ELAC 2 being a NO GO. Could someone please clarify this with an official reference.

Basically, operating ETOPS/EDTO takes into consideration worst-case scenario situations (EMER ELEC CONFIG) where you will only have ELAC1 working.
In addition, its mentioned that ELAC2 in MEL is a NO-GO item due to the logic behind the system operation where it will allow ELAC2 to work first always, and in case ELAC2 fails ELAC1 will take over.
According to :

A320 FCOM ATA (27-10-20) Architecture (Pitch control). In normal operations, ELAC2 controls the elevator and the horizontal stabilizer, if a failure occurs in ELAC2 the system shifts pitch control to ELAC1.
A320 QRH ABN-18 ELEC (ELEC EMER CONFIG SYS REMAINING), In the emergency electric configuration the ELAC2 wouldn't be available when the emergency generator is running and ELAC1 will be the only available.
A320 FCOM ABN PRO (ELEC EMER CONFIG), ELAC2 is written within the INOP SYS table to confirm what happens when the aircraft goes into the emergency electric configuration.

An item may be on the MEL be sure being into for a flight, day ,or other period is an acceptable risk but when giving that approval the authorities always consider the " next worst failure" which most often is the second version of the " failed "item. It was judged that the pilot workload, or reduction on safety margins , when under Erops ( I'm a traditionalist) and potentially up to  3 hours or more from a laudable airfield.

As a general eules, far fewer items are on an erops aircraft rather than an Aircraft released for "normal" service, which assumes a diversion airfield within one hour. (The last MMEL I negotiated was parts of the Airbus A400M avionics, 8 ish years ago)


Why is fuel quantity expressed in flight hours Going through a few aircraft documentaries online, I've noticed the thing that they usually express the fuel quantity in flight hours.
Ex - A 747 burns 4 litres of fuel every second. So with a ton of fuel, it'll stay in air for roughly 3-4 minutes.
Why are flight hours a necessary term while consider fuel quantity? I mean shouldn't they be worried about how much distance they can cover in an emergency to reach an airport, instead of how long they can stay in the air?
Seems a question related to endurance but couldn't find any proper explanation

It’s primarily due to wind.
Aircraft move through the air at a certain speed, which makes it tempting to give range in miles (or kilometers), but the air itself is moving as well, and often at a large enough fraction of the aircraft’s airspeed that it materially affects range across the ground.
For instance, say you have a Cessna 172 with four hours of fuel (plus reserves) and a cruise speed of 110kt. With a 30kt tailwind (140kt ground speed), your range is 560nm. But going the opposite direction with a 30kt headwind (80kt ground speed), your range is only 320nm—just over half as far!
To make it even more complicated, the winds will have different directions and speeds at different altitudes and locations, and all of that changes over time too, which is one of the reasons pilots (or dispatchers) have to carefully plan each flight’s route and fuel load, even for a “fixed” daily route between the same two airports.

Well, there's a simple way to think about it.
A plane with no fuel will do only one thing, fall or just not move (the one leads to the other anyway)
So fuel keeps us up there and without it, we're pretty much doomed.
Given the cruise, climb and descent speeds and the known distance we calculate the time it will take to get to places. If we need to redirect to another destination for some reason we need to know how much longer we can stay in the air which means fuel in liters or gallons isn't useful.
So we rather calculate how long the engine can keep us airborne using fuel flow values in different flight sections and the total fuel to determine this.


Why shouldn't you inflate your life vest until you're outside the airplane? On commercial flights, in part of the pre-flight safety demonstration, you're told in the event of a crash, not to inflate your life vest until you're outside the airplane — why is this?

Ethiopian Airlines Flight 961 was the flight that resulted in this addition to the safety briefing.

After being hijacked, the plane eventually crash-landed in the ocean. Many passengers died because they inflated their life jackets in the cabin, causing them to be trapped inside by the rising water.

Mayday/Air Crash Investigation S03E13 (Ocean Landing) covered the incident. The relevant clip is here (from 23 seconds in):

The main reason not to inflate prior to hitting the water & getting out is that most likely the person would need to swim under water to get out and with Ethiopian Airlines Crash, a lot more people actually survived the impact with the water, but because they inflated their life vests before the crash, the vest pushed the people to the top of the plane, thus making it impossible to swim out. One would think that if someone is fighting for their life and had inflated they would push up on the vest and try to push themselves down to get it off, but I think once inflated, the vest is fairly snug, which may make pulling it off by pushing up & forcing your body down hard to accomplish. Plus with injuries, even harder. I just think a lot of people on planes are older or have children traveling w them or can't swim, which complicates matters even more. All I know is that after watching "Aircrash Disasters" that from now on when I fly, I'm bringing 1) spare life vest (to inflate - not put on) to use as a cushion in the event of impact. 2) leather coat - a mid to long length one (in the event of fire, leather protects the body to a degree - plus sharp debris etc) 3) sit in the back of the plane.  If you look at the majority of survivable plane crashes, most of the survivors are usually seated 1) over the wing, which is the most structurally stable part of the plane (strongest part) - plus, emergency door is usually above the wing, but it's a double edge sword because the fuel is stored in the wings! So fire could get ya! Explosions etc. 2) Back of the airplane / tail section: in quite a few survivable crashes - the only survivors were at the back of the plane - such as the Japan Airlines crash into the mountains of Japan - over 500 people died. Main reason is because Japan refused the help of the us military that had a nearby base & found the crash site 1st. Japan sent a military helicopter & the pilot "assumed" there were no survivors because he didn't see people moving about. Which is nuts! Because after a crash that bad, are people supposed to be jumping up & down? the Japanese military waited until the following day to go to the site (plus they didn't have night vision equipment) and were shocked to find 4 survivors - the survivors were all women all seated in the back of the plane - the tail snapped off & was flug away from the fire/ main crash site. All of the survivors reported that they heard people screaming & crying out in pain and for help. I just can't believe Japan waited until the next day to send anyone to the crash site and let those poor people die. Over what? Not wanting the help of the us military? Everyone else (other countries) seem to gladly take the help of the us. Seems senseless to me. But anyways - back of plane is the safest spot in the plane in my overall opinion. Leather coats, blankets, pillows/anything that can serve as a cushion all can help increase survival/injury rate. The only other thing that can be a huge help is a smoke hood - but we won't see those on planes because it was deemed that smoke hoods would slow people down on evacuating due to the fact people would stop to put the hood on instead of just immediately trying to get out of the plane. Maybe we should just buy our own hoods & in the event of a firery crash throw the hood on while bolting to an exit door! Lol


Is C.G. of wing itself in line with hang point (hang glider)? Where is c.g. of hang glider wing,at hang point ,in front or behind hang point?
Is control bar "neutral" when glider is at rest and there is no wind or flip backward or forward?
At 0:45 control bar  lay at seat,slightly "nose heavy," so  this  wing c.g. is somewhere in front hang point(but to me seems very close to hang point,bar is almost neutral)

here is wing c.g.  excatly at hang point

The centre of gravity must imperatively be located between the nose and the quarter of the mean aerodynamic chord. The quarter of the mean aerodynamic chord is the point where applies the resultant of the forces and moments aerodynamic. One finds of it the definition theoretical low.
If the centre of gravity is placed behind the quarter of the average aerodynamic cord, the wing is unstable and completely impilotable... If the centre of gravity is too ahead, it is hardly less serious, the wing " marsouine " by carrying out oscillations around the axis of the pitching, oscillations which can, at worst if the control surfaces are not sufisament dimensioned to be able to increase the moment to pull up, not diminishing and leading to the crash..
Check out http://aeropic.free.fr/pages/modelisme/theorie/theorieenglish.htm#:~:text=The%20centre%20of%20gravity%20must,of%20the%20mean%20aerodynamic%20chord.

We can view a hang glider (with attached pilot) either as a single rigid system-- in which case we must assume that the pilot is exerting whatever force is necessary to hold himself in a fixed position with his arms-- or as two independent bodies connected by a flexible strap and also by the pilot's arms.  In the latter case, we don't constrain the pilot to be in a fixed position relative to the glider, but instead are interested in the force that the pilot must exert on the control bar with his arms to get the desired result, and the resulting torque exerted on the glider by the pilot. If we are interested in how the glider flies "at trim", then we want to know what happens when the pilot exerts zero force with his arms.  In this case the hang strap can be considered to be the only connection between the glider and the pilot, which means the pilot's weight can be assumed to act as if it is located at the point where the flexible hang strap connects to the glider.
And where is the CG of the glider itself, in relation to the point of connection between the hang strap and the glider?
One way to think about this is to consider the case where the pilot is standing on flat ground with the wind blowing, with the glider producing enough lift to lift all of it's own weight and some of the pilot's weight, so that the hang strap is tight.
If the pilot exerts no "push" or "pull" on the control bar with his hands--if he lets go of the control bar entirely-- does the glider tend to trim to the same angle-of-attack when the wind is just barely blowing hard enough to keep the hang strap tight, so that the glider is barely lifting more than it's own weight, as when the wind is blowing so strong that the pilot has very little weight on his feet?
In other words, does scaling the wind force up and down change the glider's trim angle-of-attack?
Isn't scaling the wind force up and down exactly the same as scaling the glider's weight up and down while keeping the wind force the same, assuming that the glider isn't changing shape under load significantly?  So if the trim angle-of-attack stays the same as the wind speed changes, doesn't this suggest that the hang strap, and the wing's net aerodynamic force vector, are both directly in line with the CG of the glider itself?
My experience is that, standing on flat ground with the hang strap tight as described above, scaling the wing force up or down doesn't significantly change the glider's trim angle-of-attack, suggesting that the hang strap and the wing's net aerodynamic force vector are both approximately in line with the CG of the glider.  If anything, there may be some slight tendency for the glider to pitch up to a higher angle-of-attack when the wind is light, which may suggest that the glider's CG is slightly behind the point of connection with the hang strap.
Note that the hang strap and the net aerodynamic force vector from the glider are not exactly parallel to each other, as the pilot stands on flat ground in the wind with the hang strap tight.  The hang strap's ratio of vertical : horizontal must be equal to (the upward lifting force exerted on the pilot) : (forward force exerted by pilot's feet on glider).  The net aerodynamic force vector created by the glider has a ratio of vertical : horizontal equal to (weight of glider + upward lifting force exerted on pilot) : (drag of glider).  Since the forward force exerted by the pilot's feet on the glider must be equal to the drag of the glider, it follows that net aerodynamic force created by the glider must be closer to vertical, than the line of the hang strap is.  The weaker the wind and the less lifting force the glider applies to the pilot, the more the hang strap must be raked forward, while the direction of the net aerodynamic force created by the glider is fixed by the glider's L/D ratio so long as angle-of-attack remains constant.
But perhaps it is sufficient to observe that many hang gliders have been seen to fly rather well, at least in terms of basic pitch stability, when launched off a hill with no person attached.  Usually this is not done intentionally, but rather as a result of the pilot forgetting to "hook in".  This suggests that the hang point is quite near the CG of the glider.
As a practical matter, consider what would happen if the hang point were far from the CG of the glider.  Imagine that you are running down a hill, connected to the glider by a strap.  If the glider is only balanced nicely when the pilot's full weight is hanging from the strap, that means that when the glider is only lifting part of your weight, it is trying to pitch up, or to pitch down.  This would be dangerous.  But it the hang strap is near the CG of the glider, it will remain balanced to fly at a reasonable angle-of-attack regardless whether it is lifting your full weight yet, or not.  The wing on a powered "trike" with wheels would seem to be somewhat free of this constraint, but it would still seem undesirable for the wing's trim angle-of-attack to change radically as the wing gets "loaded up" during the course of a take-off run, or for the wing's trim angle-of-attack to change radically for different weights of pilots, or as fuel is consumed during the course of a flight.  This can only be avoided by putting the point of connection with the trike "body" close to the CG of the wing.
One more data point: at times I've added some rather heavy masses to the nose and tail of a hang glider.  I always used the point of connection of the hang strap as the assumed CG of the glider for my weight and balance calculations, and the results were satisfactory with no observable change in trim angle-of-attack.  Again this suggests that the hang point is quite near the CG of the glider.
Note that the hang point is adjustable forward and aft over a range of several inches, and some hang gliders even feature an in-flight adjustment of the hang point.  Since a hang glider can be trimmed to fly in stable flight (in the pitch axis) with zero force exerted by the pilot's arms, at higher airspeeds (lower angles-of-attack) as well as at lower airspeeds (higher angles-of-attack) simply by moving the hang point forward or aft, it clearly is not essential for stable flight that the hang strap is located exactly at the CG of the glider.
It is common for hang gliders to tend to fly more nose-high as they get older, because the fabric stretches in a way that increases washout at the wingtips.  Pilots compensate for this by moving the hang strap forward.  Since the actual CG of the glider hasn't moved significantly, this is one more indication that it is not essential for stable flight that the hang strap is located exactly at the CG of the glider.  Similarly, pilots who are heavier than the design target weight for any given glider usually find that the glider tends to fly at an excessively high angle-of-attack, again due to excess washout at the wingtips.  Again, this can be compensated for by moving the hang strap forward.
At present I don't have immediate access to a set-up hang glider on a calm day so am unable to do a direct test by attempting to balance the glider at the hang point--
(Note-- in this answer when we talk about the point where the glider's net aerodynamic force vector is acting, we are using the (arguably outmoded) "Center of Pressure" model, rather than the model of an "Aerodynamic Center" plus a "Pitching Moment".  The "Center of Pressure" moves as the angle-of-attack changes, and need not coincide with any physical point on the actual glider.)


Why/How is the B-52 still in service? The B-52 was introduced in 1955, and is still used by the US Airforce, and is expected to remain in use till the 2040s.

On the other hand, the A-4 Skyhawk was introduced in 1956 and retired in 1998/2003.

Now granted these are completely different aircraft. What I'm wondering, is why one aircraft can remain so useful for so long, while others quickly become obsolete.

Three reasons:

The B-52 stayed useful by taking on "easier" roles as it aged.

It actually started life as a high-performance penetrator, relying on speed and altitude to stay safe. Think Early Cold War, shortly after WWII.
When ground-launched missiles improved, the B-52 was forced into flying low-altitude penetration underneath radars (a role for which it was NOT designed and rapidly wore out the airframe) accompanied by a host of other aircraft to protect it and help it penetrate defenses. Despite their escorts, we lost low-dozens of B-52s in Vietnam.
Later in the Cold War, the B-52 was completely outmatched by air defenses (fighters, SAMs, etc), and had little hope of penetrating defenses (at either low or high altitude) to drop nuclear bombs. Instead, it was relegated to flying in friendly/neutral airspace, lobbing long-range nuclear cruise missiles into the Soviet heartland (a tactic they feared more, as it turned out, than manned low-level penetrators). It was basically a missile bus.
In the Gulf War, the B-52 only flew after air defenses had already been defeated by other aircraft (F-16s, F-15s, F-111s, etc). It was basically a bomb truck.
During ALLIED FORCE (1999), "B-1Bs and other conventional (non-stealthy) aircraft could not survive in the face of double digit SAMs, even when employing advanced decoys and other countermeasures."
The B-1 program, btw, went through a similar progession, starting out as a high-performance penetrator,  then forced into even lower-altitude penetration, then also relegated to bomb/missile trucking, pushed back by AAA, MANPADS, SAMs, look-down shoot-down radar, improved Soviet interceptors, and Soviet AWACS. In fact, the B-1 and B-52 fly largely* the same missions today.

*There are some important differences. The B-1 is no longer nuclear capable (START compliance), leaving that mission to the 20 B-2s and ~40+ ALCM-armed B-52s. The B-1 can also reposition more quickly, lending it to CAS role: it dropped 37% of the then-total ordnance on ISIS (2x that of the A-10)(see source) in OIR before they returning stateside for upgrades and were relieved by B-52s. (And, yes, the B-1 does CAS.)
Overall, the B-1s fly ~330-450 hours/yr, the B-2 ~230-290 hours/yr, and the B-52s ~200-220 hours/yr (2008-2012). The B-1 and B-52 cost roughly the same per hour (the B-1 has higher fixed costs but lower operational CPFH).

In 40 years, the B-52 went from being the US' primary strategic tool against another superpower to being outclassed by small nations.
Bomb/missile trucking is an "easy" job, but far from ideal. You can't bomb your targets until the air defenses have already been defeated---after the "first week" of a war, the most critical phase, when you need them the most. In other words, the B-52 (and B-1) are relatively easy to shoot down, so they're not nearly as useful as they used to be. You can't fly them unless many of the toughest enemies are already dead. So you can't use them as often as you want.
@MSalters phrased it quite nicely: the reason the B-52 (and B-1) hasn't changed is because other aircraft are doing the hard work for them (tearing down enemy defenses), and such aircraft have advanced immensely. But that amount of support is quite resource intensive, so if you wanted to fly B-52s in the "first week" when enemy defenses are still effective, you'll need a lot of help: (Image source: Beyond the "Bomber": The New Long-Range Sensor-Shooter Aircraft and United States National Security - Lieutenant General David A. Deptula, USAF (Ret.), 2015.) The same was true with strike packages during Desert Storm, eg only a third of the aircraft dropped bombs while the rest provided support (jamming, anti-radiation, top cover, etc).

We didn't build enough B-1s and B-2s to replace it, so we kept some B-52s to fill in the gaps (we don't have any other suitable heavy bombers in inventory, this is the only choice).

The 100 B-1Bs were just a stopgap until the B-2 showed up, of which we meant to build 132 (cut to 21 airframes). Had the Cold War continued, the B-2 would have comprised the bulk of today's bomber force.

We were busy buying new fighters and cargo planes.

The official, stated goal is to recapitalize the entire bomber fleet (less the B-2) because it's increasingly obsolete. That's worth repeating: the B-1 and B-52 are near obsolete. They're not nearly capable enough. The next bomber isn't another B-1, B-52, or modded 747 because we don't want to maintain the same capabilities. We want more capability--much more--than what we currently have. We've only waited this long because replacing the airlift and fighter fleets took priority.
The LRS-B (new stealth bomber; formerly NGLRS) will replace the B-52 starting around next decade. The contract is expected to be awarded very soon. The Pentagon briefed a select group earlier this month. Looks very promising---well managed, on budget, very capable, and surprisingly mature.

UPDATE: The B-21 (ex-LRS-B, ex-NGLRS) was indeed awarded 27 Oct 2015, a month and a half after this post was originally written. Very little is known, but it is indeed a stealthy bomber (the official render closely resembles the outer mold line of early B-2 iterations before the redesign). And it is indeed on time and under budget by ~9% (APUC = \$511M vs. the \$550M requirement, FY10\$).

In short, we still fly the B-52 today because we don't have a choice, not because we want to.

Parting thoughts:

I want to emphasize that the B-52 did not remain effective in its intended role (penetrating the USSR on Day 1), upgrades notwithstanding.

Yes, the B-52 survived multiple replacements (eg XB-70, B-1A), but not because it was more capable (it wasn't; the B-70/1A went higher/faster/lower/with more payload), but because those potential replacements couldn't outpace Soviet defenses either and so were cancelled in favor of ballistic missiles and cruise missiles... except the B-2, which could outpace Soviet defenses but was curtailed when those Soviet defenses disappeared. The B-52 survived basically because of luck: against the XB-70 and B-1A, it was the incumbent, already bought and paid for; against the B-2, history smiled.

Also, the B-52 did not "survive" the B-1B. The B-1B was never meant to replace all B-52s in the first place. The B-1B was meant to fill an urgent requirement as a stopgap from ~1985-1995 until the B-2 could enter service. Unlike previous B-52 would-be replacements, B-1B production was not cut short: 100 were ordered, and all 100 were delivered, essentially on time and on budget (though that was actually partly the problem).

Thankfully the B-52's range and payload kept it productive during its long service.

Some background.

The USAF operates ~160 bombers:

76 B-52s
63 B-1s
20 B-2s

The B-52 is slower, less survivable, more expensive, and can't carry anywhere near the payload of a B-1B. In fact, it can only carry 56% the bomb weight of the B-1B.

With the removal of the AGM-129 ACM from service, the B-52 no longer has any capability to deliver nuclear weapons into airspace defended by modern SAM systems.

So why is it in service? Likely political considerations concerning base closures.


How far back from the final approach fix can you intercept the glideslope? I was looking for the point that you engage your auto pilot approach glide slope before the final approach fix in this question. How far back can you go?

From the navaid limitations section of the FAA Order 7110.65 (below), it's 10 miles from the transmitter. For the minimum altitude, it's shown on some charts as a lightning bolt.

Depends upon the approach.  In general for the ILS, the glideslope intercept will be at the approximate position of the outer marker or 4-7 miles from the threshold.


Alphanumeric flight identification in flight plan Why do some airlines use alphanumeric callsigns in the flightplan when the flight number printed on the ticket consists of only numbers?

For example, the flight TK871 uses callsign THY4JT.

when flight delays cause a duplication of a flight number- there is a suffix added.  For example- flight 1423 DFW/OKC is also operating as OKC to DFW. Normally a turn of equipment- however, DFW/OKC delayed significantly- equipment and crew is available to operate 1423 OKC/DFW.  These 2 could be in route simultaneously- one is usually designated as a suffix- 1423J  Call sign American 1 4 2 3 Juliette


Why is the empennage of some aircraft not placed on the centerline? Check the MD80's empennage below.

Can easily tell it's not built on exactly centerline by naked eyes.

Is that because aluminum joint or consider aerodynamics?

I don't think it is manufacturing defects or imprecision.

Some of the small aircraft have the same construction like this.

Couldn't find the similar discussion online though.

The vertical stabilizer is on the centerline.

The line you see is not the centerline, it's the overlap of the aluminum sheets that form the circular fuselage.

(Source)

(Source) Non-aviation example of what I mean by overlap and centerline.

On the other hand, propeller aircraft may employ different methods to counter the left turning tendencies.

  Tail

  The tail group consists of a vertical stabilizer, a horizontal stabilizer, two
  elevators, and a rudder. The vertical stabilizer is mounted on the aft
  fuselage, and the horizontal stabilizer is mounted on the top of the vertical
  stabilizer. The rudder and elevators are mounted on the vertical and
  horizontal stabilizers, respectively.

  The vertical stabilizer is fully cantilevered and sweptback. A scoop for
  cooling the air-conditioning systems is located on the lower leading edge
  section and is heated when the Airfoil Anti-ice is operating.

  – MD-80 Flight Manual

Back to the MD-80 in the photo, from its manual there is no mention of an offset. I searched for multiple terms such as offset, angled, and off center.

Finally, from an official CAD drawing from Boeing's website:

Its my first time seeing this kind of defects. For sure its not a defect caused by manufactures. It could be that this aircraft had gone through a skin change procedures. Its the overlapping of two skins that appear to be not center or could be the dorsal fin's anchor nuts not rivets properly i guess so (refer to my experience working with b737-family).


How is the ATC language structured? Is the content of spoken communications used by air traffic control and pilots defined as part of a rigorous language definition? Such as a morphological or lexical foundational system, or is phraseology the extent of formalization?

And with the advent of text/computer based communications and automation on the horizon so to speak, what is the state of formalizing a language of communication? (If not already answered in the first part of my question.)

References to white papers and other research is fine if the answer requires you to almost write one.

I don't know about a formal structure, but ground to air and air to ground communications usually follow the 3-W or the 4-W rule.

For air to ground communication - use the 4-W rule:

Who the controller is (identification of the controlling agency)
Who you are (Aircraft Identification)
Where you are (what is you position and altitude)
What do you want (state your intentions or request)

Example:

Grand Junction Approach, Cessna one seven two Sierra Papa, five miles southwest of the ABC VORTAC at five thousand, five hundred, inbound for Grand Junction to land with ATIS Kilo.

Here the pilot identifies himself as a Cessna with the identification N172SP, that he is five miles southwest of the ABC VORTAC at 5,500 ft MSL, and that he intends to land at Grand Junction airport and has the latest ATIS weather report for Grand Junction.

ARTCC and other air traffic controllers will generally use the 3-Ws to communicate with you

Who you are (the aircraft they are giving their instructions to)
Who they are (the name of the controlling agency)
What they want you to do (instructions for the aircraft in question to follow)

Example:

Cessna one seven two Sierra Papa, Grand Junction Approach, turn right to 030.  Descent and maintain three thousand two hundred.  Expect vectors for runway three zero left.  Squawk six four one one and ident.

Here the controller (Grand Junction approach) has advised Cessna 172SP to turn to a heading of 030 and to descend to 3,200 ft MSL.  He should also expect further heading instructions for insertion into traffic for runway 30L at Grand Junction airport.  The controller also tells the pilot to set his transponder to the code 6411 and ident, so the controller can verify his position.

As a courtesy, and often requested or implied by controllers to verify the pilot in question heard the instructions and will comply, the pilot will read back the instruction in full to the controller, followed by his/her callsign.

Example:

Right to 030, descend and maintain three thousand two hundred, expect vectors for runway three zero left, squawk six four one one and ident, Cessna one seven two Sierra Papa.

The NATO phonetic alphabet will be used for individual letters to avoid confusion by both parties as many letters can sound similar on a garbled radio transmission. Other oddities include the use of the word 'niner' in place of the number 9; This is to distinguish from the number 5, which is pronouced "fife".

English is also the accepted international language for use in flying and radio communications, but is only required for flights under instrument flight rules (IFR).

Supplementing other answers: there is no formal language for ATC communications, although it has been discussed, and several tasks in contracts have attempted to make recommendations.  The language for ATC has its origins in railroad communications.


Why does the aerodynamic center exist? I am a glider pilot and instructor with a master’s degree in actuarial mathematics. I teach aerodynamics every winter for upcoming glider pilots.

As you surely know, the aerodynamic center is the location where the aerodynamic moment remains constant regardless of the angle of attack – the leverage of the lifting force acting through the center of pressure gets larger for decreasing angles of attack while the force acting through this location gets smaller.

Although I find it easy to understand the mechanisms behind the aerodynamic center, I struggle with understanding how it can exist? How can we be sure that there is such a point in the wing? I know that by definition it is a fixed point, but will it be exactly fixed in practice or just approximately fixed?

Is it not possible to construct an airfoil which will somehow have a discontinuity in center of pressure leverage but delivering the same lifting force in the two situations, yielding non-constant moment around the aerodynamic center?

It exists as a mathematical abstraction for conventionally shaped aerofoils at moderate speeds. In the end it is derived from the centre of pressure. The centre of pressure is the point where the moments of the individual aerodynamic forces over the aerofoil give 0 momentum. This always exist on any aerofoil. Now, the centre of pressure is not at a constant location, it is continually shifting as a function of angle of attack, and it moves quite a lot. However the rate of movement and change in lift force is in proportion to each other, and based on that, the stable point can be found, which for conventional aerofoils at normal speeds is about 25% chord.

Note, that to get this, the proportionality between lift and CP location is a must. You can create any shaped wing cross sections (might not want to call it an aerofoil) which will not have this property and won't necessarily have an aerodynamic centre. Whether it is fit to fly is questionable though. However, they will always have a CP, as it is a simple mathematical property, and it will very likely move with angle of attack. Also note, that the AC does change location with increasing speed, for supersonic speeds it moves significantly backwards.

First it may be helpful to focus on first thought "the aerodynamic moment remains constant regardless of angle of attack".  This is referred to as the "nuetral point" of an aircraft.  Also known as the "parachute point" it has no real value to a pilot as putting CG there would produce no nose down pitching moment in
sink, which is the essence of gliding.

CG at nuetral point would most likely be directionally unstable.  A pilot simply does not use it.  CG belongs at or around the Center of Lift of the wing(s). Easiest way to consider is a flat plate H stab, which has no contribution to lift at 0 AOA, but helps keep TOTAL Clift near CG as AOA increases (on wing Clift tends to move forward as AOA increases).  To flying wing purists, the H stab may seem like training wheels on a bicycle, but it has stood the test of time for millions of years with birds.

Leverage of lifting forces is interesting academicly, but should not overshadow the all important relationship between CG, Clift, and elevator trim.


What radio equipment do pilots use to communicate with ATC? I know of the transponders used for communication by Air Traffic Control, but apart from that, what radio equipment do pilots use to talk to ATC? What modulation method is used?

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

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

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

This is a very broad question just because every type of aircraft has different requirements for its design purpose. As civil aircraft may not need to use UHF frequencies where some military aircraft must have an UHF radio.
And transponders are not used for communication. :)


Why do jet engines get better fuel efficiency at high altitudes? I'm told that this is true, but I can't imagine why.  It seems like the fact that there is less air would make the engines less efficient...  But that probably just shows how little I know about jet engines.

For a quick explanation, you need to know that

Thrust is the difference between the entry impulse of the air entering the engine and the exit impulse of the heated fuel-air mixture leaving the engine. Impulse is mass times velocity, and expressed with a mass flow $\dot m$, thrust T is $$T = \dot m \cdot (v_{exit} - v_{entry})$$
The exit impulse is increased by accelerating the airflow through the engine, and the acceleration is achieved by heating the air.
Each gram of fuel heats up a given mass of air by a certain number of centigrades. The definition of the energy content of fuels is given as the capacity to heat one pound of water by one degree Fahrenheit. The definition of one Calorie is similar but in metric units. Since the heat capacity of both water and air are almost constant at moderate temperatures, the starting temperature makes little difference to the absolute temperature increase when a given amount of energy is added.

Thermal efficiency

Thermal efficiency is the ratio between the mechanical work extracted as thrust and the heat energy spent on heating the air, and it is affected indirectly by the flight altitude. Please see the Wikipedia article on the Carnot cycle. This and similar cycles describe the workings of all combustion engines in thermodynamic terms. Basically, it says that the efficiency of a combustion engine cannot be greater than the temperature ratio between the temperature increase from ambient ($t_{amb}$) to the maximum temperature $t_{max}$ of the process, divided by the maximum temperature. All temperatures must be expressed as total temperatures, where 0° means 0 K or -273.15°C. Operating in colder air makes the ratio bigger and improves efficiency.
$$\eta_t = \frac{t_{max} - t_{amb}}{t_{max}} $$

If $t_{amb}$ is 290 K (16.85°C or 62°F) and the fuel heats up the air to 1400 K (2060°F), the thermal efficiency according to the formula above is 79.3%.

At cruise altitude $t_{amb}$ is only 220 K (-53.15°C or -63.7°F), and the same fuel flow relative to air flow will lift the maximum temperature only to 1320 K (in reality even less; for more precise reasoning see below). Now the thermal efficiency is 83.33%! If the maximum temperature is maintained, both thrust and thermal efficiency will go up; the latter to 84.3%.

In reality, the total efficiency will be lower because we have not included propulsive efficiency, friction effects or power offtake by bleed air, pumps and generators. Propulsive efficiency describes how well the acceleration of air is performed.

Heating the fuel-air mixture

Burning a fuel-air mixture will add thermal energy to it, about 43 MJ for every kilogram of kerosene (if we assume complete combustion). The isobaric heat capacity or specific heat of air (close enough, the mixture has very little fuel but lots of air in it) is 29 J per mol and per K, so those 43 MJ will heat 1000 mol of air by 1483 K. The heat capacity changes slightly with humidity and temperature, but little enough that we can consider it constant for this purpose. If the air starts at 220 K, precompression in the intake will heat it to approx. 232 K, further compression in the engine will heat it up to approx. 600 K if we assume a compression ratio of 25, and this is the temperature at the entry of the combustion chamber.

Those 1000 mol of air weigh about 29 kg, and adding a full kilo of fuel and burning the mixture will heat it to 2083 K. If you want more details about the parameters in a typical jet engine, please see the diagram in this answer. Since the mixture picks up speed as it burns, the fuel mass is also heated and combustion is never complete, the maximum temperature given here will not be reached in reality.

If we start on the ground with an air temperature of 290 K, the temperature in the intake would drop slightly because we will not be flying fast enough for any precompression to happen in the intake. Now the compressor will heat the air to 730 K, and again adding and burning that kilo of kerosene will heat 1000 mol of air to 2213 K. Ideally.

In reality, the engine control will see that the limit temperatures are not exceeded, but here we can play with the numbers as we like. The exact values will certainly be slightly different (more frictional heating in the compressor, heat loss to the outside, slight drift in specific heat with temperature), but the gist of the explanation is correct.

Explanation in layman's terms

Burning the fuel-air mixture heats it and makes the gas expand. This happens at nearly constant pressure and in a restricted volume, so the only way to make room for this expansion is for the gas to flow faster. Nearly constant pressure means that the density of the gas must decrease. The density ratio between the heated and the unburnt gas is proportional to its temperature ratio, measured in absolute temperature.

However, the amount of fuel burnt determines the absolute temperature increase, the difference in degrees between the burnt gas inside the combustion chamber and the unburnt gas at the intake. For a given amount of fuel, the temperature ratio which can be achieved with an absolute temperature increase becomes smaller the higher the temperature of the unburnt gas is. Thus, efficiency decreases with a higher temperature of the intake air.

I'm not even close to an expert. But here it goes.

Air like water is thick. Submarines are slower than boats. Jets are faster than boats. Automobiles are faster than boats.

Space has no friction because there is no matter. But I think jets still work in space. Of course they need oxygen. Just like Superman doesn't need ground friction to run fast, while some other super heroes do. Which is why I find it unrealistic how a super heroes that need ground friction are able to run so fast and make sharp turns without doing serious damage to floors.

So I'm guessing that being that the air is less thick up higher, that the easier it is to travel through. The jets don't depend on friction like propellers do. Superman doesn't need friction like Flash Gordon or Wonder Woman. So in space, Wonder Woman would be helpless because her propellers don't work while Superman's jets will work just fine.

Of course jet's need oxygen. So I'm not sure how all that works.

And something I didn't think of was what was mentioned in another post. Sound needs air. So yeah. Sound can increase friction.


Who bears responsibility when an aircraft takes off from the wrong runway? On occasion, an aircraft is cleared for takeoff on one runway, but actually departs from another runway.  This has resulted in embarrassment as well as catastrophe.  When this occurs, are both the pilot and the air traffic controller at fault?  Or is it only the pilot who is at fault?

It depends. There could be any number of reasons for such a mistake, and it is rarely the mistake of any one single person.

Within aviation, we don't really talk about responsibility the way you seem to imply in your question. In many other industries, when a mistake happens, an investigation  is initiated with the purpose of finding and punishing the responsible party. In aviation, it is believed that no one makes mistakes on purpose; if a plane takes off from a wrong runway, no one meant for it to happen.

So accident and incident investigation in aviation does not focus on finding someone responsible. Instead, the investigation focuses on figuring out how a similar incident/accident can be prevented in the future. That is why ICAO Annex 13 says:

  The sole objective of the investigation of an accident or incident shall be the prevention of accidents and incidents. It is not the purpose of this activity to apportion blame or liability

If someone takes off from another runway than intended, the investigators will not just look at who made the mistake, but rather why the mistake was made. There could be any number of reasons for this. Very often, there are hidden latent threats somewhere in the system.

Maybe the lights at the airport were not sufficient, maybe the signs guiding the aircraft to the runway were misleading, maybe the taxiway layout is confusing, maybe the ground radar was inoperative when it should have been working, maybe the airline had scheduled the crew in a way that imposes fatigue in the cockpit, maybe the procedures are insufficient - I could go on.

The point here is, these potential causes are all things that can be changed in order to prevent the same incident from happening again. You can put up better signs or repair the ground radar, etc. Whereas, if you simple decide that the pilot or controller is to blame and then fire them, without removing the latent threats, then it's more than likely that another pilot or controller will make the same mistake in the future. What are you going to do then - fire them as well?

The whole idea is, instead of focusing on punishing people for their mistakes, if you try to understand what caused them to make those mistakes, you will not only learn much more, but you will also encourage people to report more incidents, that may not otherwise have been reported. We call this Just Culture, and it is all part of the reasons why aviation remains to be one of the safest ways you can possibly travel.

Always the pilot's fault. Why would it be the controller's? If the controller says to depart on 13R and the pilot departs on 13L, then it is pilot's fault.

If a pilot has any question whether they are in the right place, they should ask the tower.


What would happen if an airliner had too much fuel for its next flight? There's an old saying that there's never too much fuel left in your tanks or too much altitude below you.

However, in terms of airline fuel planning, could there be a situation where an airliner would land with more fuel than is needed for its next flight, to the point that this extra fuel would be problematic?

Would this, or other circumstances, necessitate that fuel be removed from an airliner before a flight, for example due to a higher payload on the upcoming flight?

This would be an uncommon situation and the decision on what to do will typically be handled by your dispatcher.

Take the extra gas and restrict passenger/cargo weight as necessary.

This is the easy option but it could mean leaving people behind.
Defuel to the normal amount of fuel needed (e.g. release + taxi + contingency fuel)

This normally means the fuel is considered contaminated and will be disposed of.  There are ways of de-fuelling and re-using that fuel for another plane, but it requires specific operations and equipment that only likely to be available in the hub or perhaps a maintenance station.  De-fuelling can be slow and will delay the flight.

In the end this comes down to either weight restricting the plane or taking a delay and wasting some fuel.  The timing and economics of the decision will depend on many factors and each case would be judged individually.

The people refueling the plane just subtract the amount of fuel in the plane from what the plane needs for the next trip from what I understand your question is. The other thing is that because the saying is true, airplanes come in with just enough fuel and not really enough to go anywhere else.


Why do airlines follow these routes between Barcelona and Düsseldorf? In the wake of GermanWings' terrible incident of flight 9525, I have noticed that the crash location is quite far from the shortest route connecting the two airports.

I then went on FlightRadar24's website and checked that this is the normal route for this flight (and that there is an alternate one that reaches equally far on the other side):

Other companies that fly the same route (Vueling 1894, Airberlin 8947) follow the same flight paths.

Why is this? What limitations are in place that prevent these aircraft from flying over Lyon (and "nearby" region)?

Large parts of France's airspace are designated military training areas. If there is military flight training going on, the choice of routing over France is very limited. The only option is to fly around these areas, which  results in the routes you found.

Higher altitude route map of France courtesy of EUROCONTROL[PDF, 8MB]

Since the PDF seems to be giving problems on some computers, I've combined several screenshots and drew the approximate route of your westerly flight (in yellow), the planned flight of GWI 9525 (blue) and Temporary Segregated Airspaces (TSA's) in red onto the map. The crash location of GWI 9525 is marked by a red/yellow star.

Note that not all the TSA's in the map are actively used at all times and that in some cases crossing is allowed but altitude restrictions apply.

In addition to active TSA's there are other considerations that come into play such as weather and airway capacity & demand. If you have to wait 15 minutes longer for a slot on the shortest route, you might as well fly a slightly longer route if that allows you to depart earlier.

While DeltaLima's answer is very good, it is also due to Air Traffic Control limitations-- you want to spread out where the traffic is across the whole airspace, so routes may dogleg left or right at one point to try to keep traffic flow good (also helps separate aircraft going in opposite directions, but altitude does that too).

Keep in mind that the arrival patterns into certain airports have defined entry points and are designed to line up the plane in a certain way for the airport, and flights may have to go slightly out of their way to join the arrival.


How does the Boeing 787 Dreamliner's glide ratio compare to other airliners? I just saw a documentary on the design of the Boeing 787 "Dreamliner" and one of the designers said that the wings were inspired by gliders. Looking at the production version of the plane, it is obvious that the wings are indeed more glider-like than other airliners.

How does that affect the glide ratio? I can't find any data on this (is there any data on that at all?)

As discussed here, glide ratio is related to the L/D (lift-to-drag) ratio of the plane. When talking about commercial aircraft, "glide ratio" is typically used to mean the ratio with engines inoperative. If the jet engines aren't running, they add a lot of drag, which will decrease the L/D ratio from the typical max value.

This analysis estimates the max L/D of the 787 at 20.8, while this site estimates 21.

Other sources estimate L/D of the A330 and A340 at 20. The 747 is estimated at about 18. So based on these estimates, the 787 is slightly better than other large airliners.

Most of the efficiency gains of the 787 are from the lower weight of composite materials (which allow bigger wings) and more efficient engines.

Total guesswork here, but a typical airline will have a glide ratio of 15-20:1. Gliders  attain 30-60:1.

According to Boeing, the wing has a "highly efficient lift-to-drag ratio", which to me means 20-25:1. So that's my estimate.


Missing shear pin for tow-bar / front landing gear Sitting on the runway about to takeoff, and the captain comes over the intercom saying that we need to return to the gate.

From the quick announcement, it sounded like he said they found a 'shear pin' at the gate, or with the tow-bar, and we needed to have maintenance inspect the front landing gear.

The aircraft is an A321.

I can't imagine the shear pin was a part of the physical landing gear, but was probably piece of the tow-bar itself? I found the manual for the a320's tow-bar which shows the shear pin in the diagram on PDF page #11, item #6, within the bar assembly itself.

Why would our aircraft need to return to the gate for inspection, wouldn't they need to inspect the tow-bar instead? That says with the tug, does it not?

The Capt would have just repeated or paraphrased whatever Maintenance Control told him.  If a push back is done with the nose wheel scissors connected, it is critical for the tug driver to respect the steering angle limits of the nosewheel to avoid bottoming the steering actuators and stressing the parts.

Although the shear pin protects (hopefully) from major damage it still puts significant loads on the scissors and nosewheel steering system, beyond what those components would normally see.   If they find the towbar shear pin damaged or loose or sheared completely (although that should have been obvious while connected) after a push back, Maintenance will have to assume there was a steering angle exceedence event and there is likely to be a maintenance requirement to inspect the steering system, hence the return to gate.

My guess is that when the tug got back to the gate, someone noticed the pin was deformed, or some other abnormality, and they brought the airplane back for the inspection to play it safe.

I’m going to give you a little bit of maintenance advice from a mechanic who has been dealing with stuff like this for 20 years:

Rule No. 1: Never, ever take the pilot’s words at face value. This is not a sleight against pilots but they just don’t speak the same language as we do. They are not taught to troubleshoot, investigate, or do anything remotely close to helping you fix the airplane. If they say “shear pin” it’s about as valuable as them saying “magical bolt.” I don’t mean that to sound nasty, but if you followed the flight crews’ recommendations on how to fix the problem, the plane would just sit forever. Their training and our training are simply not equal.

Rule 2: You are the Doctor, the pilot and his airplane are the patient. This falls somewhat in line with Rule #1. When a patient comes in and says something is wrong with his leg, that’s about all a doctor gets. That’s the level you should keep it at. Ask any doctor and they’ll almost completely agree that having patients diagnose themselves is NOT a good idea. Are they completely incapable? No. However, their depth of knowledge and access to information is simply nowhere near as great as ours is. Don’t let them steer you into the land of outliers.

Rule #3: Occam’s Razor: The simplest explanation is often the best explanation. That is... maybe a nose landing gear valve is stuck and the crew, knowing nothing about the world of aviation maintenance, declared it as a shear pin. The valve is most likely stuck due to mechanical failure or someone left the LOCKOUT pin installed while performing maintenance. Again, refer to rules 1 and 2.

Rule #4: A flight crew who cries wolf is actually right once in a while. This seems contradictory to rules 1 & 2 but it’s not. You see this sometimes with a flight crew member who consistently is incorrect about something. You see their ID number, remember it, and then think “Oh yeah... This is the guy who should be driving a bus, not an airplane.” As a result, you just sign it off. This is a fundamental flaw, like that of a doctor with a hypochondriac patient... you should always investigate on the basis of greatest caution.

Rule # 5: Don’t let anyone ever push you into a diagnosis or an ETIC. If the plane is down for 25 years and growing roots because of your estimated repair time, then so be it. By all means, do everything you can to get the plane up in a safe manner, but don’t cut corners to make a lead or manager happy. They can go to hell. Just investigate and do what’s necessary.

Rule # 6: If you don’t believe me about Rule #1 and #2, get into a conversation with a pilot about how flight crews enter their flight plan into the FMS. You’ll soon find out that you know nothing about it and they know everything about it. Then show them the system schematic of the FMS system and you’ll see their eyes get a befuddled look at what it all means while you stare confidently ahead. We just don’t speak the same language, PERIOD.


Why are two-stroke engines nearly unheard of in aviation? Almost all aircraft piston engines are four-stroke engines, with one power stroke per piston every other crankshaft revolution and a great deal of intricate moving parts which are only too happy to break.  Two-stroke engines, in contrast, have a number of features that would seem to make them excellent choices for aviation use:

They have one power stroke per piston per crankshaft revolution, twice as many as a four-stroke engine, and also have far fewer moving parts, making them smaller and lighter than four-stroke engines of the same piston size and count; these two factors add up to give them a phenomenally high power-to-weight ratio, much better than that of a comparable four-stroke engine.
The same mechanical simplicity that contributes to their high power-to-weight ratio also makes them cheaper to build and sell, and (all else being equal) more reliable than four-strokes.
Most two-stroke engines are lubricated by mixing lubricant into the fuel rather than using a traditional oil sump; this allows them to operate happily in any orientation, including upside down.

The sole disadvantage of two-strokes is their greater fuel consumption and emissions, but:

Good design can reduce this problem to insignificant proportions;
Higher fuel costs could easily be offset by the reduced maintenance costs of a more reliable engine1 and the ability (due to the engine's greater power) to use fewer engines per aircraft, and;
When it comes to piston-engined-aircraft emissions, there are far worse concerns.

Yet, despite these advantages, to the best of my knowledge, only one two-stroke aircraft engine has ever entered mass-production (the highly-unusual Junkers Jumo 204/205/207 series), with a second (the Rolls-Royce Crécy) being cancelled during static-fire testing.

Why are two-strokes essentially nonexistent as aircraft engines?

1: Especially given that maintenance costs are often (though not always) a greater expense than fuel costs to begin with (scroll to page 20); the linked document is discussing jetliners, but the preponderance of maintenance costs would be much more pronounced for piston-engined aircraft, as piston engines are considerably more efficient and considerably less reliable than turbine engines (the latter due mainly to their massive mechanical complexity).

Several issues:

As you mentioned, high specific fuel consumption.  About to that of turboprops, but without the reliability. If you're going to live with that SFC, you might as well as go with turbines.

Poor reliability.  They are very sensitive to tuning and timing.  A 4 stroke piston aircraft engine will run with all kinds of things wrong with it.  Cam lobes worn down to nearly nubs.  Loose valve guides.  Horrible compression.  They can have compression readings that will cause most mechanics to ground them, but they will still make rated horsepower (Continental did testing to find this).  They are very "under-tuned", like a tractor or industrial engine.  You want this in aviation.  About the only thing that will make a 4 stroke aircraft engine blow up in your face is something like a thrown rod from a spun bearing or a cylinder that cracks open, and that requires bad maintenance or criminal neglect in the vast majority of cases.  2 strokes, in the ultralight world where they are widely used, have a reputation for letting go at any time (the only 2 stroke aircraft engine that even comes close to 4 stroke reliability is the Rotax 582 - but only close, and they're only reliable for a few hundred hours).

2 strokes require pressed together cranks with ball bearings because they have to depend on lubrication from oil mixed in with the fuel charge. So you can't have a forged one-piece crank (radials get away with ball main bearings but there is only one or two crank throws, and they still have dedicated oil lubrication).  With the only oil coming from what's diluted in the fuel in a 2 stroke, piston ring lubrication is marginal and ring life is poor.

Which gets us to longevity.  Although it has an official TBO of 2000 hours, if you take decent care of it, a Lycoming O-320 will run 3-4000 hours.  The plain bearings in a 4 stroke only see wear on startup; there is zero wear once they are pressurized because there is no metal to metal contact, and if you never shut it down and changed the oil on the go, the bearings would theoretically last forever.  Ball bearings lubricated with diluted oil have a finite life, and that and the barely lubricated rings limit longevity. You would never dream of a 2 stroke lasting that long in any galaxy in this universe.

2 strokes are popular in the ultralight world almost totally because they are cheap and have a superior power to weight ratio. You almost get a turbine's power to weight, and minimal cost, and that's worth living with the crappy reliability.  Ultralights get away with it because they crash going 30 mph not 70.

Problem is, a lot of the issues are baked into the configuration and can't be designed out.  Combine with with a business that is conservative by necessity, because your life depends on the conservatism, and it's no surprise.

Because 1-cylinder engines aren't powerful enough

2-stroke engines have a thing happen with the cylinder at bottom dead center.  Exhaust gas is pushed out of the cylinder by intake air, which is actively pumped into the cylinder. This is called scavenging.

2-stroke engines, as you imagine them, use the crankcase for an air pump. The piston going down compresses air in the crankcase, and when the intake port is exposed, scavenging occurs.

Guess how many cylinders you can have in a 2-stroke engine. ONE.

Becuase if you have 2 cylinders, then the air volume of the crankcase does not change, and it's not usable as a pump.  (Well if it's exactly two, you can have a 180 degree crank so they both top out at once, but that pretty much defeats all the advantages of two cylinders.)

2+ cylinders require blowing

With 2 or more cylinders, the crankcase is useless as a pump because crankcase volume does not change.  (On the upside, you can use a normal oiling system, and you no longer need to mix 2-cycle oil; of course now more complexity has crept in.)

You must use an air pump of some kind to accomplish scavenging.  So now you've added a bunch of complexity.  And worse, the blower will perform quite poorly at altitude.

"Oh, we know the fix!  We'll just use a turbocharger".  Great kid, but how are you going to start it? The engine needs to be blown at all speeds, even during cranking - so a turbo is useless at low power settings.  You need an overrunning clutch to the engine can mechanically drive the turbocharger at low speeds/start. More complexity.

Further, if the intake and exhaust are both cylinder-side ports, it creates some serious issues with ring wear and carbonization.  Further, the funny shape of the piston (to get air to flow the right way) greatly complexifies fuel burn, and limits maximum compression.  So real soon, as your 2-cycle engine gets bigger, you need to move intake or exhaust to valves, and because of rings and carbonization, exhaust valves make more sense than intake.  So now you're right back where you started with valves in the cylinder head.

However, intake ports have their own problems: the fuel/air mix will come in direct contact with the piston rings, and unevaporated fuel will tend to wash the oil in the rings, increasing ring wear and diluting the oil.  This is not a problem for a diesel, since they direct inject into the cylinder.

So congratulations.  You have an engine powerful enough to sub for a Continental or Lycoming, and you have

An oil pump
Cams and exhaust valves
A blower
Direct injection to solve the ring-wash problem

So that's a simplicity fail.


Why is airspeed different in climbs and descents although the AoA doesn't change? Here are the background facts:
This excercise has the objective to familiarise the student with the slow flight characteristics.
The steps are:
1)Establish straight and level, 90% RPM, 15 units AoA, hands out trimmed. (No specific airspeed, it just happens to be appr. 110 KIAS)

Add 5% RPM (so i set it at 95%) and establish a climb with 15 units AoA for 500 ft climb. No bank, whatsoever. (Again no specific airspeed, it just happens to be appr. 100-105 KIAS)

Reduce 5% RPM so you fly straight and level at the new altitude (Same parameters as step 1)

Reduce 5 % RPM (so i set it at 85%) and establish a descent with 15 units AoA for return to the initial altitude. No bank whatsoever. (No specific aispeed during the descent, it just happens to be appr. 115-120 KIAS).

My question is simple. Why is the airspeed different? The answer might be simple. I simply can't figure it out.
Hope i clarified it as much as possible.

You should see the same airspeed for the same AOA and non-accelerated flight with a mild climb or descent.

Except with a Buckeye?

The Buckeye has its thrustline low enough so that at different power settings you have different pitch trim settings. Add power to climb and the thrust pitches the nose up. You now have to trim nose down. Nose down trim means less down force on the tail, which means less lift required by the wing, which means a slower speed for the same AOA.

Conversely, pull power off and the nose wants to drop, leading to nose up trim, more down force on the tail, more lift required by the wing, higher IAS for the same AOA.

I frankly never noticed this effect, and never taught it. Wish I could go back and check it out!

Consider three airplanes: a normal airplane, one that is heavier than normal and one that is lighter than normal. All of them are travelling at 15 units AoA. You will agree that the lighter one will be the slowest (it needs to go very slow as not to have a smaller AoA) and the heaviest one will be the fastest (it needs to be very fast in order not to have a greater AoA).

In the above example we vary the weight of the airplane, but ultimately the change is due to wing loading. In your example, you do not vary the weight, but the tailplane force.

For example, if increased thrust causes a pitch-up moment on your particular airframe, the tail will have increased lift, reducing the wing loading. To maintain a high AoA, the speed will have to be reduced.

(Edit: Initially, I blamed the effect on the thrust vector, which takes part of the weight in a climb. However, @MikeY correctly identified that the drag vector does the same in a descent so that does not explain the higher speed in a dive)


How did the 9/11 hijackers find their way to New York City? It is quite well known that the airplane hijackers on 9/11 were not professional pilots. So, my question is how some people who didn't have any experience of navigating the airplanes could find their way, for example from Boston to New York City?

I read somewhere that some natural signs like the Hudson River (for example) helped them to find their way through New York City. But I'm still looking to find a more convincing answer if it is available.

Several of the hijackers, including Mohamed Atta, held at least private pilot certificates and had undergone ATP level jet training in DC9 and 737 full motion simulators in December of 2000. Atta himself held a commercial license with instrument and multi engine ratings. They were well versed in aerial navigation techniques and more than capable of navigating the aircraft in question back to New York City and Washington DC after it was hijacked and secured.

A basic scenario that any private or commercial pilot could have done:  the hijackers knew the flights they were booked on and the rough routes they would take to their destinations. A little basic planning amongst themselves would have produced the approximate position the aircraft would be at at the time it was hijacked.

Once seized and the flight crew liquidated, they could quickly determine their positions either from their headings and next waypoints or with a simple VOR fix (any competent private pilot can do this). The autopilots could quickly be disengaged and the airplanes hand flown using basic pilotage (good weather prevailed over the east coast that morning) or radio navaids to return to their targets. The navigation and flying they did that day was relatively simple.

As an update, I know these kinds of questions float around with “9/11 truthers” and other conspiracy fanatics as some sort of proof that the official explanation is incorrect. Quite often they cherry pick quotes and ignore any other evidence that won’t fit their narrative. Combine that with the public that’s largely ignorant about aviation, it allows these kinds of ideas to fester very well without challenge.

The Hudson River will take you straight to Manhattan. On a clear day, such as September 11 was, you could see the WTC for 100 miles or more (I had seen it from 160 miles away at Montauk Point on clear days, but you had to know where to look).

AA77 would have had a problem navigating eastbound with such an amateur as Hani (who most likely never had the certificates reported). Since there aren't any prominent landmarks going east from Falmouth VOR.


How does aircraft weight affect landing distance? I am a private pilot. I posed a question to instructors in the flight school and got differing answers. For a given aircraft, landing at the same speed, with everything the same except for landing weight, and using maximum braking without skidding the tires, which will stop in a shorter distance after the wheels touch down:  the plane at max gross weight, or the same plane lightly loaded?

I submit that because the heavy plane has more energy to dissipate, it would take longer to stop. Some instructors said that because there is more weight on the wheels, the heavier plane would stop in a shorter distance. I think it is a simple energy conversion physics issue.

If you discount aerodynamic effects, and assume the same landing speed, on a theoretical level mass makes no difference. The additional kinetic energy to be dissipated is exactly cancelled by the additional normal force. The kinetic energy is given by the formula:

E = 1/2 mv², where

E is the kinetic energy
m is the vehicle's mass
v is the speed at the start of braking

The work done by braking is given by:

W = μmgd, where

W is work done
μ is the coefficient of friction between the road surface and the tires
g is the gravity of Earth
d is the distance travelled

The braking distance given an initial speed v is then found by putting W = E, from which it follows that

d = v²/2μg

d = minimum stopping distance
v = velocity when brakes were applied
μ = coeff. of friction between wheels and road
g = gravitational constant

Mass is removed from the equation. The kinetic energy is tranferred into heat, so the larger the mass the more heat is generated. But theoretically the stopping distance is unaffected.

In reality, though, the coefficient of friction of tires does not remain constant with changing normal force. As normal force increases the coefficient of friction (μ) of tires decreases.

Since the μ does not remain linear with mass a heavier plane will have an increase in kinetic energy which is higher than the corresponding increase in friction of the tires, thus a longer stopping distance.

All of this assumes that “maximum braking” means you have as much brake power available as the maximum friction force of the tires. Above a certain weight/speed this will not be the case for an airliner. They are designed for a particular amount of deceleration force at MTOW and that will be less than the amount of friction tires can provide on a dry runway. (See this question for more details on that.) That will also cause a heavier plane to roll farther.

One thing is right in your demonstration: braking distance is independent of airplane mass. However, it is the braking force that stop the airplane, not the heat produced by braking. The heat and noise produced during braking only consume a small portion of the kinetic energy of the airplane.

The practice of wheel braking has been in use for over 1,000 years. But as far as literature review can go, It looks like the theory of wheel braking has not been correctly understood by the academics and the industry.


What gas turbines have the following configuration? Is there any jet engines--with published data on the dimension of the turbines and compressors--that are of the configuration of 2 Low Pressure compressor, 8 High Pressure compressor, 2 High Pressure turbine, 5 Low Pressure Turbine? Thank you very much!

I couldn't find any that exactly match. The closest ones I found where
PW1000G-56 : 2-8 and 2-3
PD-14 : 3-8 and 2-6
What is the reason your looking for such specific architecture?


How do head- and tailwinds affect airspeed? I'm sure this was asked somewhere, I just can't find it. So if it's a dupe, let me know.

Anyway, I'm one of those guys that actually looks at the flight information when I'm flying. I fly transatlantic 3-4 times a year and they show it on the personal screens.

They display the groundspeed, headwind/tailwind, temperature etc. I'm wondering how to calculate the actual speed based on that. Is it the groundspeed plus/minus the headwind/tailwind? Meaning, if groundspeed is 500mph, and the headwind is 200mph, does that mean we're only traveling 300mph? Meaning it will take us 10 hours to travel 3000 miles? Or are there more factors involved and more math calculations?

I'm wondering how to calculate the actual speed based on that.

Which actual speed? All of the speeds you mentioned are actual speeds. None of them is in any way artificial, virtual, or otherwise invented.

  Is it the groundspeed plus/minus the headwind/tailwind?

That would be airspeed, i.e. the speed at which the plane moves relative to the air around it.

  Meaning, if groundspeed is 500mph, and the headwind is 200mph, does that mean we're only traveling 300mph?

No. If groundspeed is 500mph into a 200mph headwind, this means that the plane has to fight against the headwind, i.e. its airspeed is 700mph.

Have you ever gone swimming where there is current? Can you imagine riding your bike on a treadmill?

  Meaning it will take us 10 hours to travel 3000 miles?

No. If the groundspeed is 500mph, it will take you 6 hours to travel 3000 miles over ground.

I know nothing about flying an aircraft, but on a recent trip on a Dreamliner to the Carribean I found myself regarding the very helpful maps and flight information accessible through the in flight entertainment system and puzzling over the expressions used such as ground speed, tail wind, head wind and wind speed. Cross winds weren't mentioned but I'm sure they could well have been.

Here's my layman's theory which made sense to me during the flight:

For passengers, the aeroplane's speed relative to the ground is the most important. It is ground cover we want after all, isn't it, to get us from home to our holiday destination and back in the most direct and efficient way possible.

However it seems to me that a moving aeroplane has two states:

a terrestrial state: when it is still in contact with the ground
airborne state: when it has left the ground

When moving forward for take off with its wheels still in contact with the runway a head or tail wind or indeed a side wind or an oblique wind will affect its motion as it would a car or any other terestrial vehicle designed to cover ground while remaining in contact with it.

However, once an aircraft has left the ground i.e. no longer has its undercarriage in contact with the runway, it is entirely at the mercy of the body of air in which it is suspended as it travels though it at several hundred mph. Here an aircraft experiences no more pressure on its nose cone at a constant speed through the air whatever the speed at which and direction in which the air itself may be travelling; head, tail or any other 'winds' surely become irrelevant; although the speed at which the body of air may be travelling is extremely relevant.

For example, if the body of air supporting the aeroplane is travelling at 50 mph in the same direction as the aircraft which is doing 400 mph the ground speed will be cumulative - namely 450 mph but the plane will still only be doing 400 mph relative to the body of air in which it is traveling. If the body of air has a terestrial speed of say 100 mph in the opposite direction to an aircraft travelling at 400 mph, the aircrafts speed relative to the ground - its ground speed - will be subtractive - only 300 mph.

If the body of air in which the aircraft is suspended is moving in a sideways direction' either directly or obliquely, relative to the terestrial course the plane needs to follow in order to describe a relatively direct route from take off to landing the plane won't be 'blown' off course as such but if no corrections are made and if all speeds are steady then the aircraft will arrive at a different destination from the one intended.

For example if an aircraft maintains a steady speed of 500 mph with respect to the ground i.e. has no help or hinderance from the body of air which is supporting it moving in a favourable or unfavourable direction but has a direct sideways motion of say a steady 20 mph left to right and the distance of terestrial travel is 2000 miles, after the plane has travelled for the 4 hours necessary to cover the 2000 miles it will arrive 80 miles to the right of its desired destination.

In these same steady conditions the plane would need to aim at a point 80 miles to the left of its desired destination and then it would arrive at its desired destination. The tail, head and cross winds are a different story though once the (for all intents and purposes) static ground is about to come up and meet the aircraft at considerable speed and as it prepares to leave it's airbourne state and assume its terrestrial state.

It's then, I guess, that the boffins invoke their equations for approach speed relative to the ground, air movement speed - be it head, tail or side wind and runway length in order to make the transition to a smooth landing.

I know this forum is to answer a specific question; please accept this as the offering of a complete layman but it does seem to me to be scientifically sound - indeed is my own answer to a question I have asked myself but I think this would appear to cast some light on the role head or tail winds play in aviation scenarios.


Is there a way to find what types of aircraft have landed at an airport? When I was a child, I recall an aircraft landing at my local airport, CSG, I would like to know if airports keep records of what aircraft land there? If so how long do they keep these records, and are the available to the public?

Speaking for the US, airports only report total movements (landings/takeoffs) and this information is available to the public via the FAA. This information is in aggregate and does not contain the type of aircraft.

Flightaware tracks only the type of aircrafts that land and takeoff at an airport; it also tracks the identifier used. It has very good coverage globally, although some areas have better (more accurate) information than others.

For most GA flights, the identifier is sometimes also the tail number, which can then be used to narrow down on a specific airframe.

Just add the ICAO identifier for your aerodome at the end of this URL: http://flightaware.com/live/airport/ to see stats of a particular aiport; for your airport its http://flightaware.com/live/airport/KCSG

Simple answer... No. You could rule out the types of planes that could land at an airport by an airport's 5010 but there is nothing specific.


Is wheel well overheat detection (not brake temperature sensor) necessary for commercial jet aircraft? Some aircraft do have this detection system (many Boeing aircraft), some do not. Why? What's the main purpose of the wheel well temperature detection system, even though there are brake temperature sensors.

What's the main purpose of this detection system even there are brake temperature sensors.

Far more often than not, the need for brake temperature sensors is after landing to tell you how hot the brakes are. If you had to use heavy braking upon landing, they're going to be hot. How hot they are tells the captain in part how he needs to taxi and whether he should advise ground personnel to stay away from the sides of the tires where the thermal plgus are and possibly to use cooling equipment (typically a huffer) to cool them down.

Wheel well temperature sensors are needed after takeoff. The potential problem here is that you need to keep an eye on how warm the wheel well is getting from the hot tires you have retracted and enclosed in the wheel well.

As an example, let's say you have a 747 freighter loaded to a max takeoff weight of 825,000 lbs or more at Honolulu and taking off on RWY 8R. That's 20,000 feet of taxiing and another 10,000 feet plus of takeoff roll. The flexing of the tires during that taxi and takeoff generates a lot of heat. Once the wheels are in the wells, that heat transfers into the wells and eventually leaves the airplane. However, if for whatever reason, the tires already had a lot of extra heat before the taxi started or a lot of braking was done during the taxi, or the airplane was stuck down low in warmer air after takeoff, whatever, the wheel well temperature might exceed the safe limit.

The solution is usually simple, drop the gear and cool the tires. If you don't you risk a wheel well fire.

  Some aircraft do have this detection system (many Boeing aircraft), some do not. Why?

The short answer is that they have been designed so that they don't need it, which begs the question: why aren't all aircraft designed such that they don't need it. Essentially every aircraft design includes compromises. It's all part of tailoring the design to best accomplish what the aircraft's primary task is to be. Using the 747 example, you could, for example, install a cooling system in the wheel wells to take care of the heat, or you could use heavier tires that didn't flex as much and thus didn't heat up as much, or you could have additional structure able to resist the heat or dissipate it quicker. But whatever means you used, it would mean hauling around more weight and the attendant costs in fuel and loss of payload capability when you didn't need it most of the time. Far better to avoid those high costs, but in the interests of safety install a much lighter system that will alert you to the need to cool off the wheel well by dropping the gear.

Insofar as potential 747 wheel well overheats are concerned, baring mechanical failure, it's generally a problem only when the aircraft is used in a manner that it wasn't designed for. For example, the 747 was designed as a long-haul transport. As such, there would be plenty of time for the wheels to cool down in the wheel wells. As I remember we were told in ground school that 5.5 hours was the cool down time. So, if you have a short leg and then a quick turn with high weights, you might be taking off with tires that have more heat in them than the system was designed to handle.

When JAL decided to use the 747 for high-density, short hauls within Japan, Boeing came up with the 747SR (Short Range) which included a redesign of the undercarriage as well as several other things. The design goal was that that any commuter airliner needs: short legs, frequent takeoffs and landings, and it made sense to optimize the design for that criteria, and that optimization required a decrease in range and payload as I remember.

Cargo, engine and wheel well fire detection is considered essential for many a/c. If a high friction wheel assy. Is taken up into the enclosed environment of a wheel well any ensuing fire would be directly under the pax. Compartment a simple loop detection sys. would enable the crew to lower the gear before catastrophic failure.


Do Red Bull Air Race aircraft make use of nitrous oxide injection? Microsoft Flight Simulator X contains Red Bull Air Race missions where the player flies an Extra 300S through a race course and makes use of Nitrous Oxide injection for the engine.  While I have heard of NO injection for automobile engines, I have never encountered an aviation engine which made use of this.  Can anybody verify if RBAR teams make use of this?

This is from the 2014 season

http://airrace.redbull.com/en_US/article/lycoming-engines-power-behind-air-race

  In previous years, the race teams were allowed to modify their
  aircraft, but since 2014, the engines and propellers have been
  standardised for all teams. The Lycoming Thunderbolt AEIO-540-EXP
  engine and the Hartzell 3-bladed 7690 structural composite propeller
  have both undergone rigorous testing and have performed to
  specification.

Here's a link to a smaller engine using nitrous to get 300 HP out of a 100 HP engine for record setting purposes

https://books.google.com/books?id=DhLSUHx136QC&pg=PA36&lpg=PA36&dq=lycoming+io-540+nitrous&source=bl&ots=UwecpWRA8S&sig=RgiMHhHnQuHJYph0TASw0Y6bB2o&hl=en&sa=X&ved=0ahUKEwieiZTGqf7aAhVxUd8KHc5uBLoQ6AEIhAEwDA#v=onepage&q=lycoming%20io-540%20nitrous&f=false

and even more HP here

https://en.wikipedia.org/wiki/Bohannon_B-1


Why was the P-51 Mustang not adopted by the U.S. Navy? I've heard from many WWII aviation hobbyists and WWII vets that the P-51 was essentially the pinnacle of U.S. piston-engine fighter design; it was fast, maneuverable, long-range, well-armed, allowed superb pilot visibility, etc etc.

So, why did the Navy never adopt it? It seems there were flight tests of the P-51D as a carrier fighter, modified with a tailhook for wire capture landings, but the Navy never pulled the trigger (so to speak). What were the primary reasons behind this decision?

The biggest driver behind the use of the P-51 Mustang in naval operations was the need for B-29 escorts while bombing Japan. No naval fighter at the time had the range to accompany the B-29's so the P-51D was selected to determine if it could be a suitable carrier operator.

The test project was named "Project Seahorse", and one aircraft was selected and specially modified to cope with carrier landings.

Navy Concept Image in Livery (Image Source)

The modifications included:

"Fin Fillet Extension" which enhanced low-speed operation.
Tail hook (for obvious reasons)
Catapult hook
High pressure tires
Higher pressure in the shock absorbers
Reinforced airframe

The modified P-51D started testing in September of 1944 under test pilot Robert Elder. The tests were carried out at Mustin Field in Phillidelphia on a modified runway fitted with a catapult and arresting cables. Throughout September 1944 to October 1944, 150 launches and recoveries were made on this modified runway. One big concern was that the aircraft would need to land around 90mph and the stall speed was 82mph, not much of a margin.

In late October 1944 the next stage of testing started, carrier landings/launches at sea. Lt. Elder made all the carrier landings at 85mph and was pleased with its handling characteristics, but there were issues:

Stall speed margin was extremely low, too low for safety
Rudder control at low speed and high angles of attack was inadequate
Landing attitude had to be very carefully controlled to avoid airframe damage
Go-arounds required slow throttle advancement. The extreme power of the Packard/Merlin engine meant that a high-power fast throttle advancement could put the aircraft into a roll or snap-roll. At low speeds this would prove a fatal mistake.

Only 25 launches/recoveries were made in the suitability trials and Lt. Elder did not believe that the Mustang had a place in carrier operations.

It was a moot point anyway. By early 1945 Okinawa and Iwo Jima were won by the allied forces. Each island had airfields that were taken over and provided a close base of operations for aerial attacks on Japan. In August 1945 Japan announced its surrender and it was official September 2, 1945. The P-51 wasn't required to be used from a naval platform anymore.

The project wasn't forgotten though. North American presented the NAA-133 based on the P-51H, which was a carrier operations "Mustang". It didn't go any further than the design stage though. In 1947 a P-51H was acquired to test new catapult equipment. Either way the war was over by that time and we didn't have to put any more effort into modifying the aircraft for carrier operations.

Here is my main source

The Corsair was a good plane but was used more for ground operations. If anything made the Mustang unnecessary for the Navy it was the F6F Hellcat which replaced the Wildcat as the Navy's mainstay for carrier operations. The Hellcats performance in the Pacific was incredible (4,947 of 6477 enemy planes shot down) and was praised by many pilots. By the time the evolution of the Mustangs would have been good enough for the Navy they already had the Hellcat which started being used in 1943.


Why the number 6 in P6SM in a METAR/TAF? A typical METAR or TAF reads something like this:

VRB04KT P6SM SCT025 BKN040 OVC060

P6SM means "greater than 6 statute miles visibility."  Why in particular is it 6 miles--why not P7SM or P10SM?  Particularly since sometimes METARs and TAFs list actual visibilities greater than 6 statute miles anyway:

28006KT 10SM CLR

Is 6 miles a limitation of commonly used equipment for measuring visibility, or something else?

The METAR format is defined by the World Meteorological Organization's (WMO)  on behalf of ICAO. According to the specification (section 15.6.3), visibility is to be reported in meters. The upper bound for visibility reporting is 9999, which indicates the visibility exceeds 10 kilometers.

In North America, the METARs deviate from the standard format by using non-standard units. Visibility is reported in statue miles. Six statue miles is equivalent to 10 kilometers.

Hence > 6 statue miles is equal to the standard > 10 kilometers.

P6SM - "P" = Plus 6 Statute Miles...in conversational writing we are more used to seeing the plus sign "after" the number, as in 6+, but that's what P6 means. Usually anything less than 7 miles requires an explanation why the "VSBY" (visibility) is less than 7, so there will be an obstruction of some kind included, such as 3SM BR (BR is French, "Brume", mist).


Can private pilots drop skydivers if the pilot pays a pro-rata share? This question was inspired by this other question. If a private pilot was to pay the pro-rata share of a flight for some skydiving friends, could they take those friends up and let them jump out?
If so, what counts as "pro rata share"? Is it expenses for the whole flight from takeoff to landing, or does the pilot assume 100% of the cost after the point where the skydivers leave the plane?

This is not so difficult. Many people like to refer to the regs without actually looking at the regs. This is all taken from §61.113.
Pro rata share is not the same as getting compensated.

(c) A private pilot may not pay less than the pro rata share of the operating expenses of a flight with passengers, provided the expenses involve only fuel, oil, airport expenditures, or rental fees.

In this scenario, the pilot isn’t holding out as a commercial operator; these are his friends.
According to any online legal dictionary, the pro rata share is defined as the total cost divided by the number of shares. In other words, the total cost of fuel, oil, airport costs, and rental fees split evenly between everyone in the plane, including the pilot. As long as it is split evenly, it’s not compensation or hire, it’s cost-sharing.
All that said, NEVER do anything because some guy on stack exchange told you that it was ok. Check the regs for yourself and contact your local FSDO for clarification.

A private pilot may possibly get away with dropping skydivers when there's no question of compensation whatsoever, but the FAA may see that differently as they consider skydiving flights as commercial operations.

Whenever there is compensation for a flight (regardless of whether the
pilot is compensated), that flight is considered a "commercial"
operation. Nearly all jump flights involve compensation and, by FAA
standards, skydiving flights are commercial operations. [...] The FAA
requires pilots to hold at least a current commercial pilot
certificate, along with a current second class medical certificate.

There are some matters to consider for the aircraft used for the operations as well.

When an aircraft is used in a commercial operation, the aircraft is
subject to a 100-hour inspection, as well as an annual inspection.

And:

Federal regulations (14 CFR Part 119) describe the certification
requirements for air carriers and commercial operators, but exempts
skydiving flights operating within 25 miles of the departure airport.
As a result, skydiving flights are conducted in accordance with 14 CFR
Part 91 and 14 CFR Part 105.


Is it illegal for Joe Public to listen to ATC in the UK? I recall that in the UK (unlike in the rest of the world) we have a law forbidding the listening to of air traffic control, dating back a century, to the days of paranoia over spies!

What is this law, exactly? Is it illegal for a member of the public to listen to live ATC in the UK? Are there any exceptions?

It seems like something of a grey area, but some sources claim that listening to a delayed feed (or historical recording) works around this law; is this true?

Yes, it is illegal.

The UK has some very strange ideas about Radio, which bear no resemblance to the reality of physics and how radio actually works. Foremost among those ideas is the idea that you are only legally allowed to listen to transmissions intended for "general reception":

  The services that can be listened to under the definition of general reception are:

  licensed broadcasting stations;
  amateur and citizens' band radio transmissions; and
  weather and navigation transmissions

  Radio scanners should not be used to listen to any other radio services, including illegal radio stations (pirates) (by virtue of the fact that they are not licensed radio stations).

Aviation-band transmissions are not on Ofcom's list, so even though they are broadcast entirely "in the clear" and anyone with an appropriately tuned AM receiver could hear them you are legally prohibited from listening to them.

Providing feeds to LiveATC would also appear to be a violation, though it's not clear to me that listening to LiveATC streams would put the listener in violation of the law.

From the same Ofcom site:

  … it is illegal to listen to anything other than general reception transmissions unless you are either a licensed user of the frequencies in question or have been specifically authorised to do so by a designated person.

and

  … it is also illegal to tell a third party what has been heard in a transmission a person has listened to illegally.

So anyone providing UK aviation band transmissions to LiveATC may be in violation of both of those laws, but I'm not certain what that means for LiveATC listeners (that would be a question to ask Ofcom).

Contact between ATC and Aircraft is purely for Navigation Transmissions and therefore legal. I've never heard them having a general chat other than the odd Merry Christmas Etc.

In my opinion it's general receive.

If it's not illegal to listen to then it's not illegal to pass info on in whatever form, ie web feed.

I've asked ofcom and if I get a reply will post here.

Dave


Why retract flaps at go-around? As I understand the procedure for go-around it includes retracting the flaps.

Why is this?

The decision to make a go-around must sometimes be made quickly, under stressful conditions, like in bad weather. Retracting flaps introduces one more thing to do under sometimes stressful conditions.

Speed is low during landing and by retracting flaps you give away safety margin for lift. (Speed may also slow further since the nose is raised during go-around.)

Retracting flaps may also affect pitch and thereby make control of the aircraft slightly more difficult in a situation which may already be stressful.

For the same reason that you don't takeoff with full flaps. Because the climb performance sucks.

Flaps do add lift, but also a lot of drag. A low flap setting often provides more lift than drag, assisting climbout, whereas full flap offers a whole lot of drag, which is desirable when you want to be slow for landing.

But a go-around necessitates gaining height quickly, which will not work with landing flap selected. The usual procedure, at least in light aircraft, is to wait until the vertical speed is neutral or positive before retracting flap. This minimises some of the risks you've raised about aircraft control.

It is often said that a go-around is a period of heavy workload, and managing the flaps and airspeed is not easy. That is why the go-around is a fundamental procedure taught right from the very beginning, and practiced and tested regularly.

Flaps change the wing properties and thereby changes the lift, the approach attitude and lowers the stall speed (all landings are basically controlled stalls). If you need to get out of trouble quickly in a go-around then you may need all the energy that you can get, retracting the flaps changes the wing back to the efficiency of take off to allow you to climb out of trouble.