What is the preferred way to slow down an airliner on a long runway? EDIT: Another way to phrase the question is, given an extra long runway, does it make sense to substitute mechanical braking (which is the primary way to slow down in a typical situation) partially or completely with something else, to reduce brake wear / roll down the runway faster?

Consider an airliner landing on a very long runway (e.g. Boeing 737 landing on 12,000 feet+). The pilots are planning to vacate the runway near the end, so there is a lot of distance to spare.

Ignore noise procedures, and assume there is no traffic behind to suggest an early exit. What would be the preferred way to slow down the aircraft in this scenario?

Light application of reverse thrust only, no brakes?
Light application of brakes only, no reverse thrust?
Moderate braking until below a certain speed (e.g. 80 knots), then "coast" the airplane down the runway?

(This question addresses the primary means to slow down after a typical landing, while this question is about an idealistic case where there is plenty of braking capacity to spare.)

There are several ways to slow down an airliner:

aerodynamic drag
friction in the wheel bearings
reverse thrust
wheel brakes

and a few even used a brake chute, but that went out of fashion a long time ago. If the runway is inclined, landing uphill also will slow down the aircraft.

Friction and drag you get for free, so I would rely on those first. Since they are highest when the speed is high, use them first until the aircraft is so far down the runway that it would overshoot without additional application of wheel brakes. Use spoilers and leave the flaps down to maximise the deceleration. Only at the end add wheel brakes so you have slowed down to taxi speed when the exit of the runway is reached.

So the best answer is none of the above but:

Coast along with flaps and spoilers out until you need to brake more to avoid an overshoot. Then apply wheel brakes.

The exact way how brakes should be applied depends on their type.


How long does it take for a contrail to hit the ground? I'm not even sure if they end up falling to earth, or if they accumulate and form/join other clouds or whatnot, which is part of the reason I'm asking this question.

In contrails, heavier particles fall faster than the light ones, so as the contrail disperses it spreads vertically as well as horizontally.

How long does it take for a contrail to fall back to earth if at all?  If they do make it to the ground, is it then water vapor that touches down?

A contrail is a cirrus cloud composed from water vapor nucleating on soot in the engine exhaust and the contrail itself doesn't fall to the ground.  You will occasionally see cirrus uncinus "mare's tails" form from contrails, which are heavier ice particles in the contrail precipitating from the contrail.  These particles evaporate long before reaching the ground.

(image courtesy of Wikipedia, CC licensed)

The fate of the contrail cloud itself is death via entrainment.  Watch some over time as they distort from the winds aloft and then mix with the surrounding dry air and evaporate.

Once the contrail is gone and only vapor remains it moves with the air it resides in.  For synoptic scale flows, vertical motion of the atmosphere is on the scale of centimeters per second and as such until that vapor finds its way into subsiding air (e.g. a high pressure system, the dry conveyor belt of a midlatitude cyclone), it will tend to remain aloft.


How long does flight planning take / when is it done / how is it done? When I left an A320 today, the fuel truck was already there and refueling the aircraft. (At least, the hose was connected.) Also, a FA said they will fly back within 40 minutes.

This isn't much time, and I wonder if they start to refuel the aircraft before actually calculating the required amount. (Since it's a 4h flight, they'll need quite some fuel.)

So I'd like to know how much time is needed for flight planning of an airliner. Maybe some planning for the next flight can already be done in advance during the current flight? Maybe some part can already be done by others?  Or does the entire process take just a few minutes today?

It's also not clear to me what is done during flight planning. Of course planning the route, including possible ways to divert the aircraft, and amount of fuel. What else?

EDIT: To make it more clear: I'm asking about the work the pilots have to do to prepare for the next flight

The pilots don't do the flight planning. It is done 1-2 hours before the flight by dispatchers using computer software.

On a quick turnaround, one pilot goes to a printer to retrieve the flight plan. The other stays with the aircraft and prepares for the next flight.

Sometimes someone will bring the flight plan directly to the aircraft. Some airlines have stopped using paper flight plans and instead download to an iPad.

The pilots review the flight plan to ensure it meets all requirements, and can request a reroute, or additional fuel it they feel it is needed.


How do flaps help an aircraft take off at a lower speed, yet cause drag at the same time? Wouldn’t the drag caused by the flaps just decrease the acceleration, so, although they can lift off at a lower speed, wouldn’t it be faster to just use no flaps and rotate at the higher speed that is required?

Increasing the flaps does increase the drag, but not by that much initially. For the first stages of flaps you gain more by reducing required takeoff speed. If you would increase the flaps more and more, eventually the drag would become too much and you would lose takeoff distance again.

Flap setting has an affect on the wing’s lift coefficient and on the aerodynamic drag. Increasing flap angle increases the lift coefficient, and therefore reduces stalling speed and the required takeoff speed (the same lift will be created at smaller air speed due to greater lift coefficient). This reduces the takeoff distance. In the same time increased flap angle increases drag, reduces acceleration, and increases the takeoff distance.
The net effect is that takeoff distance will decrease with increase of flap angle initially, but above a certain flap angle the takeoff distance will increase again. An optimum takeoff setting can be determined for each type of aircraft and any deviation from this setting will give an increase in the takeoff distance.
The flap setting also affects the climb gradient. Increasing the flap angle increases the drag, and so reduces the climb gradient for a given aircraft mass. If there are obstacles to be considered in the takeoff flight path, the flap setting that gives the shortest takeoff distance may not give the required climb gradient for obstacle clearance.

(skybrary.aero)
Note that the climb gradient is also affected:

More flaps allows taking off from shorter runways, but reduces obstacle clearance capabilities.


What are the effects of a rejected takeoff? When a pilot is abandoning a take-off, what are the effects on the aircraft performance? Can a pilot vacate the runway normally or is extra time on the runway needed? Is immediate assistance provided by the airport?

If an airplane is close to V1, the brakes - which have to absorb kinetic energy and dissipate heat - can get hot to the point where the tires automatically deflate, to prevent them from exploding. For a 747 this involves a lot of smoke and glowing brake disks.


What is the difference between Tilt-rotor aircraft and Tilt-wing aircraft? I've  heard more about tilt-rotor aircraft like the Bell Boeing V-22 Osprey
more than a tilt-wing aircraft like the XC-142
What are the differences (other than one changing the degree of wing or rotor), pros and cons of the two?

I would say that the complexities of tilting the entire wing are what compelled Bell to choose just the nacelles.

By complexities, I am thinking 2 major systems. The first is the flights controls. Based on the how old that image looks, I suspect that aircraft want designed with a fly-by-wire flight controls system if it was even an option. That would be, in my head, the only simple way of managing the flight control sister's. Otherwise the designers would need to get VERY creative with the pushrods or cables & pulley systems. That would be an engineering nightmare!

The other system would be fuel. Engineers are going want to put fuel in the wings, as opposed to under the floor in the fuselage. That way cargo space is not used, bit more importantly, that much weight centered on the shortest works reduce the ability of the aircraft top perform missions for to the Zero Fuel Weight considerations.

Again a tilting wing creates some engineering challenges they may just want to avoid. The wing tanks would shift weight as they tilted back, and they would need to ensure fuel could still get to the engines. That's not even the hard part either. Think about trying to run the cross-feed lines through a rotating wing root! No thanks!!

Hope that helps, that's all I've got!!


What equipment is required to operate in Class "C" airspace? I'm nearing my pre-solo test and I need to know this answer.

A radio and a transponder.

As a pre-solo student, you really DO need to know this. If it is not familiar to you, I strongly recommend you read up on your airspace classes.


How do I know if a runway has REIL? I'm designing an airport of a flight simulator. One of the runway settings is Runway End Identifier Lights (REIL). Is there a way I can see if a runway at a US airport has REIL via the internet, and whether they are Omnidirectional or Unidirectional REIL?

You can find this information in the Airport Facility Directory which is published in print by the FAA and is available for free from them online here. REIL will be listed for the specific runway for the airport in question. For example I fly out of KDYL and if we take a look at the page in the AFD for KDYL you will see REIL listed for RWY 23

(source)


What is the APU actually used for? What are the actual uses of the APU? When is it used? When does it go off?

Using 747-100/200 aircraft at the two 747 carriers I flew for in the 1990s as an example, the specific uses (not in any particular order) were:

pneumatic power to start the engines
pneumatic power to run the air conditioning packs when the engines were not running.
electrical power on the ground, available regardless of whether the engines were running or not. Many aircraft can use APU electrical power inflight, 747-100/200s do not.
hydraulic power on the ground when the engines were not running via the pneumatic duct. I can't remember whether that could be done inflight.

Assuming that electrial and air was not available from a ground cart, the APU on a cold airplane would be turned on when the airplane was opened up and turned off after the engines were going.

Prior to landing, the APU would usually be turned on a few minutes before landing. It would be left on until a ground cart (if any) was hooked up or until the aircraft was vacated and closed up.

Other aircraft types and operators employ their own procedures, and their APU uses may differ.


Tailplane/Equilibrium conditions In regards to achieving equilibrium conditions, what is the relation between airspeed and the tailplane?

To be statically stable in pitch, the tail has to generate a downforce that opposes a continuous nose down pitching moment.  Everything else being the same, more speed increases downforce, less speed decreases it.  The equilibrium state is when the opposing forces are in balance in steady state flight, hands off the controls, and the airplane is said to be "in trim".  This is achieved at a given airspeed for a given set of "configuration of forces" you might say (pitching moments, thrust lines, C of G positions etc) and is called "trim speed".

Trim speed is the speed at which tail downforce balances the nose down pitching moments to achieve equilibrium (not accelerating or decelerating) for a given set of physical conditions.  FAR 25 static stability requirements require the plane to positively seek this speed by pitching uphill or downhill when disturbed from that speed in an attempt to regain equilibrium.

So the relation to the tailplane is that speed in excess of trim speed causes the nose to pitch up due to excessive downforce, which makes it go uphill and slow down, trying to get back to that equilibrium state.  Vice versa when speed is less than trim speed; it pitches down trying to get to the equilibrium state.  In an actual airplane it overshoots its equilibrium point several times trying to get there and eventually zeros in (the Phugoid oscillation).

Some other configuration change, a power change, a C of G change, a dragging item change, a flap sticking down, or elevator incidence change, will alter the speed at which equilibrium is achieved or trim speed. Once stabilized at a new trim speed, it will now seek the revised trim speed as before, if disturbed, as the tail responds to downforce changes with airspeed as described.


Why is the font size of runway numbers in Canada small? I've noticed that on runways at Canadian airports, runway numbers are painted noticeably smaller than runway numbers at airports pretty much anywhere else. For instance, here's the beginning of runway 32 at Ottawa Airport in Canada, and here's the beginning of runway 32 at Buffalo Airport in New York. Or the beginning of runway 32 (left) at Madrid airport in Spain.

Does anyone know why the runway numbers are printed smaller? Wouldn't it be best for them to be nice and large so as to be readable from the air?

(Click either to view)

Short answer: Canada paints them by the book. They're not small.

Ottawa airport conforms to the standards set by ICAO Annex 14 for airport design. Those standards guarantee the legibility of the numbers.

  5.2.2.6 The numbers and letters shall be in the form and proportion shown in Figure 5-3. The dimensions shall be not less than those shown in Figure 5-3, but where the numbers are incorporated in the threshold marking, larger dimensions shall be used in order to fill adequately the gap between the stripes of the threshold marking.

Member states can deviate and then notify ICAO they have deviated, as long as the height of the characters does not go below 9 meters (30 feet).

There's no answer to why they are smaller than the rest, checking how +250 countries paint the numbers to compare with Canada is unattainable. Canada and many others (Crete, Athens, Cairo, Lisbon, ...) paint them like that, and that's the norm per ICAO.

Measurement from Google Earth for Ottawa.


Why do aircraft need a vertical tailfin, but birds don't? (and lots of fish do?) I don't know of any bird that has a vertical tailfin, but apparently aircraft need them for lateral stability. Why is this?

I did realize, however, that most fish have a vertical tailfin, or a vertical dorsal fin, or both. What exactly is going on there? Can't see how marine creatures would need lateral stability at low speeds in a thick liquid (compared to air).

If that second paragraph sounds off-topic, let me put it another way. What is it about the air, as opposed to water, that requires this? They are both fluids. One is thicker/more viscous than the other, which seems to imply that the thicker one would require less lateral stability. Yet it seems to be the opposite. I hope this will be taken as a valid on-topic aerodynamics question.

(P.S. I'm leaving out stuff like the B-2 Spirit because it uses active dynamic adjustments to stabilize. Its form is inherently unstable.)

IMPORTANT EDIT: I should have pointed out the large difference in speed. Birds of course are slow. So were the early Wright flyers. Modern airliners fly much faster, around Mach 0.85. I haven't yet looked up Reynold's Numbers, but it certainly could be the case that the faster ones need lateral stability while the slower one's don't.

1 more clarification: When I say "stability", I'm not exactly talking about yaw control. My impression is, just when the plane is flying perfectly straight, there are pressure fluctuations that will cause slips unless there is a large vertical surface area to balance out those fluctuations. I could be wrong, and ultimately I'm trying to find out why most planes seem to need this large vertical area.

1 more: Here's an example model that seems synonymous: Let's say our airplane achieves yaw with butterfly flaps at wingtips, which are aligned with the center of mass (the flaps and CoM have the same height) so there's no side-effect torques along another axis other than the yaw axis (z axis). Let's assume no backup yaw control is necessary. This craft is meant to cruise at Mach 0.85. Is a vertical fin necessary to achieve passive directional stability about the yaw axis? We should probably consider cross-winds too, even at takeoff/landing.

Not all of them need a fin:

This is the Horten IV, a flying wing glider that did not need a fin (picture source). Instead, it used spoilers at the wingtips to create yawing moments, and the swept wing helped in improving its weak directional stability. It could afford to do so because it was a glider. The second prototype of a jet-powered flying wing, the Horten IX V2, suffered a crash when one engine failed and the pilot could not end the resulting spiral dive.

Fins on airplanes

Birds have two advantages over airplanes which make a fin superfluous:

A variable geometry wing, where wing span, sweep and incidence can be controlled for each side independently.
Propulsion is integrated into the wing, instead of separate power plants which might fail independently.

An extra advantage of birds over older airplane designs lacking a computerized control system is that the bird's brain is wired to adjust the wing on each side subconsciously and continuously. While a fin provides an aircraft with weathervane stability, a bird is inherently indifferent (neither stable nor unstable) and needs continuous tweaking of its wings for directional control.

If a bird wants to control yaw, it simply adjusts its wingtips such that the drag difference between both creates the desired moment. It can do this by reducing wingspan on one side, which also creates a rolling moment for a coordinated turn. Airplanes unfortunately cannot do the same.

Airplanes need positive directional stability because humans cannot adjust the controls continuously - they need to check maps or work the radio, or maybe just want to relax for a moment. Also, multi-engined designs need a stability margin to compensate the asymmetric thrust resulting from a failed engine. With a fin, the mass of the structure behind the center of gravity now needs a compensating mass ahead of it, and the side area of this forward fuselage is de-stabilizing the aircraft in yaw, requiring yet more area at the vertical tail. This is another problem that birds do not face.

The B-52H 61-023 suffered a structural failure of the fin on January 10, 1964, but managed to limp home with almost no lateral stability left. Had it suffered an additional engine failure, it would had crashed (picture source).

Fins on fish

The dorsal fin of fish helps in their propulsion by balancing the areas above and below their vertebral column. By wagging the tail, they create a side force which creates a sideslip condition at the dorsal fin, which in turn produces a correcting side force so the body does not roll while swimming. In this it can be compared to the keel of a sailboat. The hydrodynamic forces on a keel counteract the side force from the sail in crosswind conditions. Its main function is to stabilize the fish from the yawing motion caused by their tail fins, and this stabilization in turn improves the efficiency of the motion of the tail fin.

An additional function is as a roll sensor. Being located at the maximum girth, it gives the best readings of local sideways motion due to roll. This is the main function in marine mammals where the tail fins are horizontally oriented. In fish which have an adipose fin (located aft of the main fin), this fin is most likely their roll sensor.

It is not used for roll control (except as a sensor in a feedback loop), and cannot because it is not adjustable. Roll control is achieved by the adjustable pelvic fins. It also cannot provide roll stability - this would require it to roll the fish back to its initial attitude after a disturbance. All it can do for the roll movement is to dampen it a little and prevent it when the fin is moved sideways.


Can someone identify this crash? I watched an episode of Air Crash Investigation or something else a while ago,  I want to know which one this was:

What I know is that during takeoff, the aircraft received a warning saying some part of the autopilot wasn't functional however they continued the takeoff as it wasn’t a big deal. They reset the flight computer, but since the aircraft was old and the flap settings and everything was set to normal, the aircraft began banking to the right and it crashed.

You might be referring to LAPA Flight 3142 (Episode 9 of Season 17). An alarm sounded during takeoff because the flaps were not set, but the pilots ignored the warnings. The aircraft failed to lift off the ground and crashed at the end of the runway.

If that's not the one, this list on Wikipedia should help you locate the episode.


In wind tunnel testing "same Mach number" is not "same airspeed"? Wikipedia:
"Mach number: the ratio of the airspeed to the speed of sound should be identical for the scaled model and the actual object (having identical Mach number in a wind tunnel and around the actual object is -not- equal to having identical airspeeds)"
source
Why identical Mach number in a wind tunnel and around the actual object is -not- equal to having identical airspeeds?
If plane fly at 0.5 Mach(617km/h) that mean airflow speed for test model must also be 617km/h ,to has same compresibility effects?
Isnt it?

Why identical Mach number in a wind tunnel and around the actual object is -not- equal to having identical airspeeds?

The speed of sound depends on temperature, humidity, and pressure. It is highly unlikely that temperature, humidity, and pressure in the wind tunnel are exactly the same as the temperature, humidity, and pressure at the altitude the aircraft will be flying at.

If plane fly at 0.5 Mach(617km/h) that mean airflow speed for test model must also be 617km/h ,to has same compresibility effects? Isnt it?

Which altitude are you flying at? From the looks of it, you are talking about a car, not a plane!
At typical cruising altitudes for jets, 0.5 Mach is not 617 km∕h. At 10000 m, 0.5 Mach is 540 km∕h.


How much does a D-check cost compared to a new aircraft? As I understand from those videos, a D-check basically consists in disassembling the whole aircraft, inspecting every little piece, and then reassembling it. For me, the last part seems to be more or less the job of aircraft builders, and thus its costs may be comparable to the lasts steps of building an aircraft. Thus the cost of a few D-checks may be the same as building an aircraft.

Counted in D-checks cost, how many D-checks does a newly built aircraft cost?

Let's focus on airliners. If needed, the question may be narrowed down to best sellers (lets say B737 and A320).

Keep in mind that during a D check, the parts on the plane are already owned. There will of course be a certain amount of parts replaced, but it will be much less than the manufacturer's cost to buy/manufacture all of the parts. There will be thousands of hours of labor, which will be most of the cost of a D check. The cost of maintenance space and taking the plane out of revenue service may also be considered.

According to this financial reference:

  For a 15-year old Boeing 737-300, a D check would likely cost \$1.25-\$1.5 million, representing about 8-10% of the cost of the aircraft.

This book is now 15 years old along with the newest 737-300's, so in today's dollars this would be \$1.73-\$2.08 million, not accounting for changes in the cost of maintenance. Larger aircraft will have larger costs. This seems to put the cost in the same range as other sources.


Is it possible for an airliner to cruise at engine full thrust? What would happen if the pilot of a modern airliner like the boeing 747-800 or Airbus A380 left the throttle at full thrust at cruise altitude for an extended period of time e.g 4 hours if the aircraft had enough fuel to reach the next destination?

Full take-off thrust time is limited by the engine producer: bad things like overheating and meltdowns may happen. If you mean set the thrust to maximum allowable cruise setting: nothing much would happen other than consuming a lot more fuel than when using optimum cruise setting: fly fast enough, and the drag rises a lot without icrease in lift, due to compressibility effect. So the aircraft does not need to fly higher when going faster, it only burns more fuel.


Does the autopilot attempt to recover from a mechanical failure of a single control surface? I'm asking this question having watched a documentary about Northwest Flight 85 in 2002, during which a 747-400 experienced a lower rudder hardover at an angle of 17 degrees to the left.

The pilots were able to recover by taking over manual control and using the ailerons, the upper rudder and asymmetrical engine thrust to oppose the forces being generated from the lower rudder hardover.

During the doco, I believe the pilot in control immediately turned off the autopilot as he realised it wasn't going to be able to recover from the initial roll.

I just wondered if an autopilot would be able to recover from such an event - i.e. is it technically possible? It seems like the sort of thing an autopilot could achieve easier than a pilot, assuming it was programmed correctly.

Normal autopilots don't. They expect all systems to work correctly, and in case of  failure of a single control surface they will stubbornly command actions which are no longer possible. For them, thrust is only for controlling the vertical speed, and the application of asymmetric thrust for yaw control is not possible.

There are experimental autopilots which compare the result of their actions with a number of pre-programmed scenarios and select for their next commands the scenario which agrees best with reality. These really do what you assumed, but the specific scenario must have been considered before so the specific control laws are part of the code. Computers small enough to be carried on airplanes became powerful enough to do this in real time maybe 20 to 25 years ago, but I am not aware that regular autopilots of airliners have this capability even today. It is, however, used for unmanned vehicles.


Why do aviation careers have age limits? Many aviation careers have mandatory retirement ages.

Airline Pilot (licensed with FAA): 65 (raised from age 60 a few years ago)
Controller

FAA: 56
NATS: 60

Why do these age limits exist? Is it a matter of increasing risk (even when one is healthy) of becoming incapacitated while on the job (heart-atack, etc.)?

I'm no expert, but I can think of two general reasons: risk of incapacitation, and general physical limitations (reaction speed, stamina, the ability to maintain concentration for extended periods etc.). The age 65 limit only applies to part 121 ATP pilots, by the way; you can still be a commercial or private pilot for as long as you can keep getting a valid medical.

Older people are simply at greater risk of heart attacks and many other medical conditions, and they're also much more likely to require medication that may not be approved by the FAA or other national authorities. Of course that doesn't mean that every older pilot is the same but I guess that the FAA has decided that since the risks increase with age, at some point they become unacceptable. To understand the specific reasons for that decision you would probably need to dig up the original NPRM or transcripts of the public debate/hearings, but I have no idea if that's possible or not.

Then the other aspect is general physical and mental condition. They both deteriorate with age, and the NTSB sometimes mentions that as a potential factor in accidents:

  On the day of the accident, the pilot had flown several formation
  sorties. The day was hot, and the formation activities were demanding.
  The accident flight was relatively long, and the co-owner of the
  airplane guessed it was about 45 minutes long. Most formation sorties
  were 30 minutes or less. The accident pilot was in his early 70s and
  was by far the oldest member of the flight. Due to the pilot’s age,
  the demanding schedule, and the length of the formation sorties,
  fatigue could have been a factor, although it could not be
  conclusively determined.

Of course, any discussion like this is a generalization and I'm sure it's easy to find individual 70-year old pilots or controllers who are 'sharper' than younger ones (after all, experience counts for a lot too). But there's no escaping the overall pattern that older people in general are more prone to medical issues and less physically/mentally capable than younger ones, and at least one US court has ruled that there's no effective way to determine individual risk so a general rule is not discriminatory:

  The court also concluded that ExxonMobil had established, through the
  testimony of several medical experts, that the risk of sudden
  incapacitation in flight significantly increased with age, and that
  there are no adequate means of individually testing each pilot to
  determine whether a pilot was at risk to suffer such an
  incapacitation.


What‘s the thrust of two stacked propellers? Let‘s say that you have two identical propellers and each is powered by their own motor and together they produce a thrust of 1 kN. How does the thrust change if instead of operating them side on side I stack them one over/behind the other? I‘m sure they wouldn't still produce 1 kN but how much would it approximately be?

Furthermore, what if I added another propeller of this type and so on?

There's a really interesting paper by Manikandan Ramasamy (Hover Performance Measurements Toward Understanding
Aerodynamic Interference in Coaxial, Tandem, and Tilt Rotors, Journal of the American Helicopter Society, Vol. 60, 2015) that talks about this question, which is of great interest for multirotor UAS (which, for rotorcraft, is a substantial number of aircraft): how does stacking two rotor change their efficiency and thrust output?

His test apparatus looked like this: two rotors located exactly one on top of the other, and his apparatus allowed him to measure both torque and thrust produced by each one independently.  He could also vary the distance between the two rotors.  For reference, the rotors are ~4.3 ft in diameter.  Also, just to explain the graphs I'll throw up from his paper, he used blades both with and without twist.

The reason why he has torque sensors is going to come into play with the results.  For a coaxial rotor helicopter (not necessarily for a coaxial prop airplane -- I am not sure which you care about), it is desirable to have a coaxial rotor setup wherein each rotor produces the same amount of torque.  Multicopter UAS then disturb that torque balance to control themselves in yaw.  For the tests Ramasamy performed, they were all done when the rotors were torque balanced, so each rotor produced a torque equal and opposite to the other rotor.

His results are pretty striking, as shown below.  This is a plot of how "interference factor" for each rotor changed with respect to how close the rotors were to each other.  "z/D" refers to the vertical separation of the rotors, z, as a fraction of the rotor diameter.  The interference factor is a measure of the extra power required to turn the upper and lower rotors, ($\kappa_u$ and $\kappa_l$, respectively) as compared to an isolated rotor.  An interference factor of 1 means the rotor is equivalent to an isolated rotor.  Greater than 1 means that it is less efficient than an isolated rotor and vice versa.

As you see, as the rotors get further apart, the upper rotor acts as if it is all by itself ($\kappa_u=1$).  However, the lower rotor is always more inefficient than an isolated rotor due to downwash from the upper rotor.  That said, as the two rotors get closer, the effects get a little more wild.

As the lower rotor approaches the upper rotor, it helps pull air through the upper rotor, so you might think that the angle of the air hitting the blades is getting steeper (also called a higher inflow angle).   This also leads to a lower angle of attack on the blades, leading to the following effects for the upper rotor: decreased thrust production (reliant on high angle of attack to produce more lift) and increases torque input (reliant on inflow angle).  Consequently, $\kappa_u$ increases.

However, since the upper rotor is producing less thrust, the downwash on the lower rotor decreases...and, as you might imagine, it sees the opposite effect that the upper rotor sees: higher thrust production, lower torque input, and $\kappa_l$ actually decreases.  Due to this, you now need to either slow down the upper rotor or speed up the lower rotor in order to achieve a torque balance.

Hence, the first answer to your question is...it depends, but it looks like you actually can get a more thrust out of the coaxial configuration (as opposed to two isolated rotors) if you can space the rotors out well.  This is due, at least in part, to some "swirl recovery" performed by the lower, counter-rotating rotor.  In other words, since a rotor not only pushes air down but also forces it to spin (like a tornado), the lower, counter-rotating rotor sees faster moving air and can take advantage of this to operate at a lower angle of attack: producing the same amount of thrust with lower drag penalties.  However, the effectiveness of this, as said, varies with the separation of the two rotors.

And, again, the larger issue, I think (from a performance perspective) in coaxial setups is not thrust production but power requirements.  One of the two rotors (if not both) is always going to be less efficient than an isolated rotor producing the same thrust.  This is shown more clearly below in a comparison of Figure of Merit (Fig. of merit = (Idealized power requirement)/(Actual power requirement), so a measure of rotor efficiency) versus coefficient of thrust (analogous to the amount of thrust produced by the rotor) for the rotors with untwisted blades.

In short, while you can get more thrust out of a pair of coaxial, counter-rotating rotors, there is no free lunch.  And the lunch can get pretty expensive, at least from a performance point of view (not to mention the extra structure, etc. required for larger axial separation distances).


Why did Boeing not create a new 737 instead of just upgrading the old model? This documentary claims that

James McNerney - Boeing's first CEO without a background in aviation, made the decision to upgrade the 737 series to 737 MAX instead of developing a new model.

Is this true? If so, why did Boeing make the decision not to make a new model instead of upgrading a 50 year old one?

Other sources confirm the statement from your documentary that James "Jim" McNerney made the decision to build the 737 MAX rather than a new model:

When Airbus was about to land American Airlines with a huge order for the A320 family, both the ceo and neo, Boeing’s hand was forced. Within 48 hours, Jim McNerney, Albaugh’s boss, made the decision to go forward with what would become the MAX.

(Leeham News, emphasis mine)
The 737 MAX program was eventually launched as a response to the Airbus A320neo, the new engine option. If Boeing had started development of a new model immediately when Airbus announced the A320neo in 2010, they might have been able to come up with a competitive product in a similar time frame. But Jim Albaugh, who was head of the 737 program at the time, underestimated the A320neo and said:

I think Airbus will find re-engining the A320 more challenging than they think it will be. [...] When they get done, they will have an airplane that might be as good as the Next Generation 737. We think we can continue to make incremental improvements to the 737 to make sure that it is a more capable airplane than even the re-engined A320.

(Leeham News)
Boeing wasted valuable time here while Airbus was working on the A320neo. Boeing also struggled with the 787 at the same time:

Boeing was indecisive about which direction to choose.
The ambiguity was driven in no small part during this era by the continued 787 debacle, which at the time of this interview, had not entered service.

(Leeham News)
The final decision to re-engine the 737 instead of developing a new aircraft, came when American Airlines decided to purchase a large number of Airbus aircraft. Boeing realized they had to act quickly at this point:

Boeing faced an unthinkable defection in the spring of 2011. American Airlines, an exclusive Boeing customer for more than a
decade, was ready to place an order for hundreds of new,
fuel-efficient jets from the world’s other major aircraft
manufacturer, Airbus.
The chief executive of American called Boeing’s leader, W. James
McNerney Jr., to say a deal was close. If Boeing wanted the business,
it would need to move aggressively, the airline executive, Gerard
Arpey, told Mr. McNerney.
To win over American, Boeing ditched the idea of developing a new
passenger plane, which would take a decade. Instead, it decided to
update its workhorse 737, promising the plane would be done in six
years.

(New York Times, emphasis mine)


Can aircraft depart/arrive after another aircraft has sprayed snow on the runway? Does snow spray affect runway surface friction? The weather was bitterly cold on March 18th, 2018 and Birmingham airport was covered with snow. As you can see in this video, an A380 sprays snow on the runway when arriving and departing.

(Frame from video)

Aircraft can land in snowy weather without a problem, the fact that there is a little bit of snow on the runway does not change that. The amount of snow can cause problems if it too deep and will interfere with the engines/landing gear struts etc.

The snow sprayed on the runway by the A380 is just a small film and no problem for aircraft to take-off and land on. The runway can become a bit more slippery decreasing the braking performance of the aircraft.


Why was this helicopter denied entry to Class B airspace? This question is somewhat related to this other one.

I listened to this exchange between a helicopter and Newark.

The helicopter wants to land at Newark. The controller tells the helicopter to remain clear of the Class B. I'm aware that the controllers must give clearance to operate in certain classes of airspace, and the helicopter wasn't granted clearance to do so.

Why was the helicopter denied (as far as can be deduced)?

What should the pilot have done differently, either to get clearance to land at Newark or to anticipate not being able to?

According to the post at LiveATC, Newark was closed, apparently due to smoke in the tower. With the airport closed, nobody could be granted clearance to take off or land.

Outside of declaring an emergency, there's nothing the pilot(s) could have done to get clearance at Newark other than just waiting.

In an emergency situation like smoke in the tower, there wouldn't be anything published beforehand for the pilots to look up, but otherwise, pilots look at NOTAMS during their flight planning for planned closures or outages. Even in good weather, proper flight planning would also involve an alternate airport, anticipating the possibility that the intended airport might be closed for an emergency like this.

Eight miles south of Newark is Linden Airport, a small uncontrolled field just outside the Class B surface area. If I had passengers in my ship who were trying to fly out of Newark, I would have radioed operations and asked them to send a limo to Linden to pick up my passengers.

[There is one part of the recording that says "[inaudible]". The pilot is announcing his position at the Statue of Liberty, a reporting point for copters landing EWR.]


Do FAA knowledge test endorsements have an implicit expiration date? If not, can I add one explicitly? Many things in aviation have a 60-day window. For example, a graduation certificate from an approved part 141 course is valid for the purposes of a checkride for 60 days after the date of graduation.

Suppose I endorse an (otherwise fully qualified) student to take a private pilot knowledge test, but the student doesn't take the test for an extended period (because of illness, military deployment, or whatever).  Some amount of time later, the student appears at a testing center with the endorsement in hand.  Is there a particular time limit beyond which a knowledge test endorsement is no longer valid?

If not, could I write the endorsement so that it includes a void date?  Something like:

  I certify that Mr. Orville Wright has met the required training of blah blah and is prepared to take the private pilot knowledge exam. This endorsement is valid until October 31 2017.

The last that I looked into it, there were no expiration dates on written test endorsements by CFI and GI.  However, the FAA has made it clear many times that instructors are free to add conditions to endorsements.

For example, when I endorse a student for solo, I will put limitations like:
1.Prior phone call to instructor, 2. surface winds less than 14K, and within 45 deg of runway heading, 3.Five miles vis with VFR conditions.

Of course solo endorsements have implied expiration dates. But you could endorse J Pilot for a instrument written, and add "Endorsement expires Jan 1, 2018."

Since these things are vetted regionally, if you have any concerns, you can always call the FSDO and talk with an Ops Inspector there.

I did that once when I signed a guy off for a Flight Review, and wanted a condition on the sign-off. The pilot agreed with the condition, and I ran it past the Safety Program Manager who was an Ops Inspector, and his words were, "Absolutely! Any restriction you think is appropriate is fine to add to the endorsement."  That was sometime ago, but I have no indication that the policy/latitude has changed.


Where can I find detailed dimensions of an aircraft engine compressor blade? I am currently working on the modeling of blade/casing interactions in aircraft engines. The work is carried out in partnership with a company, therefore, there is a limited amount of it that could be published openly.

Are there any OpenSource compressor or turbine blade designs available (e.g. NACA airfoil profiles for wings)? Where could I find detailed dimensions and material properties?

The idea would be to use it for publication purposes, thus displaying relevant characteristics and realistic behaviors, while keeping all the confidential data of the company internally.

Since nobody's answered this yet, I'll take a stab at it.
I tried to find some dimensions for you online, but struggled.
Here's a thought though, could you just buy one?
For example - £35:

This is a Rolls Royce Olympus 593 Turbojet engine High Pressure Compressor Vane. This is a flown item that powered BAs Concorde fleet and the blade shows signs of use.
It was manufactured by Rolls Royce and has the Olympus 593 part number B430542.
The dimensions of this item in centimetres are approximately 15.5 x 4 x 4 and it weighs approximately 400g.

There are lots more on eBay too.
You asked about material — one listing says the Tornado's Rolls Royce RB199 engine is made from titanium.


Are there any programs to build a hydrogen powered airplane? So called "renewable energy" is a buzzword these days. Auto manufacturers are starting to roll out hydrogen powered cars. Hydrogen may not currently be the most efficient or economic fuel for jet engines, but that doesn't mean there are companies working on this issue or even a new kind of engine.

Are there any such programs in existence today?

Aviation is very energy-hungry. While hydrogen is quite efficient in terms of energy per mass (141.8 MJ/kg), it is awful in terms of energy per volume. Aircraft today use their wings for fuel storage, which helps to reduce bending loads and uses a volume unsuitable to transport payload. This will not work for hydrogen.

The biggest problem is storage, and none of the solutions is really attractive:

Cryogenic: Storing the hydrogen at a temperature where it is liquid (below 20°K) greatly reduces volume demands, but now you need to add insulation, and even then the energy per volume is only a quarter compared to hydrocarbons. This is used in rockets, because they can rely on extensive ground support before launch and use up the stuff in a few minutes. Airliners, however, need to store their fuel over tens of hours, so their hydrogen tanks will be much heavier and bulkier than those of rockets.
Compressed: This avoids the energy input for liquefaction, but now you need a strong and heavy pressure vessel. Current gaseous hydrogen storage uses up to 700 bar pressure, but even then the volume demand is six times higher than that of kerosene for the same energy content.
Hydrides: Some metals like magnesium or sodium can bind hydrogen and release it when heated. However, even in the most efficient compounds like sodium tetrahydroaluminate, only 7.4% of mass is useable hydrogen, making this exquisitely unsuitable for mass-critical applications like aviation.

Currently a mixture of cryogenic and compressed storage looks the most attractive and has been tested widely.

The first (partially) hydrogen powered aircraft was the LZ-127 Zeppelin which used a mixture of gasses of equal mass per volume as air (Blaugas) to power its engines.

Tupolev designed and flew the Tu-155, the first hydrogen-powered heavier-than-air aircraft, in 1988. The results showed that the principle worked, but could not compete with kerosene.

Currently, several demonstrators are in operation, but none with the hope of widespread application - as it stands, binding hydrogen chemically to carbon will still give the best way of storage (liquid and unpressurized) for aviation.

Hydrogen becomes the best source of energy only when combustion speed is critical, such as in supersonic combustion ramjets (scramjets).


Is there an ATC radio backup? I was thinking, what would happen if the radio systems for an ATC tower fail due to power loss or faulty systems?

Is there any kind of backup radio that would be used to contact the planes and/or any procedures that the planes and ATC would follow?

Edit: And if there is a backup radio, what happens if that fails too?

There is always a backup radio system as well as a several backup systems like for example a backup CWP (controller worker position) in case the primary fails. PANS-ATM (ICAO doc 4444) in chapter 2.5 SAFETY REVIEWS mentions that

  Operational and technical issues to ensure that: [...] communications, navigation, surveillance and other safety significant systems and equipment [...] include adequate backup facilities and/or procedures in the event of a system failure or degradation.

Emphasis is mine.

Also there is a standby generator (or even more than one) that has to instantly take over in case of power supply failure but I can't find any regulations at the moment.

As for the "what happens if the backup fails as well" question, we need to see the fuller picture that the document provides:

  d) communications, navigation, surveillance and other safety significant systems and equipment:

  1) are tested for normal operations on a routine basis;

  2) meet the required level of reliability and availability as defined by the appropriate authority;

  3) provide for the timely and appropriate detection and warning of system failures and degradations;

  4) include documentation on the consequences of system, subsystem and equipment failures and degradations;

  5) include measures to control the probability of failures and degradations; and

  6) include adequate backup facilities and/or procedures in the event of a system failure or degradation;

Emphasis is again mine

As you can see backup is the last line of defense. An ATS unit should ensure that equipment is regularly maintained, it's monitored, it's tested and any failures should be prevented if possible and their impact being documented.

If everything else fails, then you use the backup. And again the "include adequate backup" phrase can be translated as "if one thinks that a second backup is needed they should install one". But the previous 5 clauses are there to eliminate that case.


What is the difference between a Stand and Gate in Airports? How do they differ ? I have seen Aircrafts having been assigned a Stand and Gate, So wanted to know whether the stand and gate are same physical location of airport or they are different but have link between them.

An aircraft stand is a place where an aircraft can be parked.

A gate is a designated area inside the terminal building where passengers will assemble and wait prior to boarding. It includes facilities for checking boarding passes, last-minute customer service such as seating changes and so forth.

In the typical setup most passengers meet, each gate serves just one stand located right outside it. But this is not always the case: airports often have remote stands too, and there will be a smaller number of gates from where passengers are bused (or walk across the apron!) to the appropriate stands. Such a gate is not dedicated to a particular aircraft stand.

In the former case, where a stand is served by a dedicated gate, there is little point in distinguishing between the two. They will often (but not always) share one identifying number, and people happily speak about, for example, taxiing to the gate after landing. But for remote stands a distinction needs to be made.


How to find out what engine a plane had in the past? I want to find out what engine a specific airplane had in the past.
For example for the Airbus A320. The sources which I've found tell me there were different engines. CFM56-5A, CFM56-5B, IAE V2500 etc. Let's say I flew in 1999 with Lufthansa. What engine did I most likely fly with?

Airlines are not free to use any engine that they can shoehorn into an airframe. Engine choices are constrained by national regulatory authorities that grant approvals to use specific engine variants in given airframes.

A Google search will usually identify which engine models are used in a specific airplane model. For an authoritative answer though you would have to consult official records e.g. civil aviation records for a specific aircraft or the national regulatory authority or airline with respect to a specific aircraft model.

So for certainty you should consult the German civil aviation authority, the Luftfahrt Bundesamt or Lufthansa. For practical purposes though a cursory Google search combined with good judgement suggests that your 1999 Lufthansa flight would have certainly employed a pair of CFM56 engines.


Has a Russian Tu-95 ever had to make an emergency landing in Europe? Has a Tu-95 Bear bomber ever made an emergency landing in a European country while the flight was on a mission to test the country's defences?

Not that anyone knows of. A couple of Bears have crashed within Russia's borders.

They usually fly over international waters only, so if one did go down there would be a chance it would sink without a trace.


For a Part 61 IFR rating, how much can be done alone on a simulator? Per Wikipedia:

Requirements for Instrument Rating in the United States are listed in section 61.65 of the Federal Aviation Regulation[2] are:

50 hours of Pilot in Command cross country
40 hours of simulated or
actual instrument time
15 hours of flight instruction towards
Instrument Rating

Does this mean that 40 hours could be done on a simulator alone or does that overlap with the 15 hours with the instructor?

The short answer: It depends.  Either 10, 20 or 30 hours of simulator/FTD/ATD time may be used, but all of it must be with an approved instructor.  None of it may be solo.

None of the requirements you posted are related to simulator time.

  50 hours of Pilot in Command cross country

This can be done solo, as you are a rated pilot and thus PIC.  This need not be solo XC and can be done with a passenger or CFI.

  40 hours of simulated or actual instrument time

In this quote, simulated instrument time refers to time spent in the airplane during flight in visual meteorological conditions (VMC) that you are wearing a view-limiting device to simulate IMC.  While wearing a view-limiting device you must be accompanied by another pilot who acts as your safety pilot.  Thus, none of this time may be accomplished solo.  You will always need another pilot either in simulated or actual instrument conditions.

  15 hours of flight instruction towards Instrument Rating

This time may overlap with the above (actually, must overlap), the regulation reads (14 CFR 61.65 (d)(2)):

  (2) Forty hours of actual or simulated instrument time in the areas of operation listed in paragraph (c) of this section, of which 15 hours must have been received from an authorized instructor who holds an instrument-airplane rating...

Now back to your question, what about simulator time?  For this we look further down to 14 CFR 61.65 (h) and (i)

  (h) Use of flight simulators or flight training devices. If the instrument time was provided by an authorized instructor in a flight simulator or flight training device—

  (1) A maximum of 30 hours may be performed in that flight simulator or flight training device if the instrument time was completed in accordance with part 142 of this chapter; or

  (2) A maximum of 20 hours may be performed in that flight simulator or flight training device if the instrument time was not completed in accordance with part 142 of this chapter.

and

  (i) Use of an aviation training device. A maximum of 10 hours of instrument time received in an aviation training device may be credited for the instrument time requirements of this section if—

  (1) The device is approved and authorized by the FAA;

  (2) An authorized instructor provides the instrument time in the device;

  (3) No more than 10 hours of instrument time in a flight simulator or flight training device was credited for the instrument time requirements of this section;

  (4) A view-limiting device was worn by the applicant when logging instrument time in the device; and

  (5) The FAA approved the instrument training and instrument tasks performed in the device.

The use of simulators and flight training devices is covered by (h) above, and the limit of 20 or 30 hours depends on how the training is conducted.  In either case, note that time must be "provided by an authorized instructor".  Section (i) deals with lesser training devices that are limited to 10 hours but is also required to be conducted by an authorized instructor.

Reprising the short answer, any time spent in a simulator, flight training device or aviation training device where you are solo or without an authorized instructor does not count toward anything.  Sim time is only valid when conducted by an authorized instructor.


How is the change of moment handled in a Weight and Balance calculation with a swept wing aircraft? I'm writing a weight and balance calculator for a charter operation.

Every aircraft I've ever flight planned for has all the tanks on the same moment arm, so I don't know how you calculate the W&B for an aircraft that has swept wings. Do they have one tank on each wing whose moment changes as it drains, or do they have multiple tanks? And if they have multiple tanks, so you have to calculate for 'drain the forward tanks first' and 'drain the rearward tanks first' and plot both of those on envelope?

In supplement to @Dave's answer and as he said depending on the airplane, the complexity of calculating the c.g. can vary greatly. The manufacturer of every large aircraft I've programmed weight & balance for has had tables for each tank. In Boeing's case for the 747-400, each table line is a tank volume in gallons and the tank c.g. at that volume in inches. For example, and using JSON notation since the OP is JavaScript fluent:

{"vol":3200,"ba":1453.7},
{"vol":3250,"ba":1454.8},
{"vol":3300,"ba":1456.0},
{"vol":3350,"ba":1457.2},
{"vol":3400,"ba":1458.4},
{"vol":3450,"ba":1459.7},

"vol" is the fuel volume in the tank in gallons

"ba" is the balance arm at that volume in inches

Since fuel is loaded by weight, one has to use the fuel density to convert to volume.

Typically, at least for large aircraft, you have to satisfy the c.g. envelope for:

zero fuel weight
ramp weight after fueling
takeoff weight
landing weight

Also there has to be provision for carrying ballast fuel if that is necessary.


Why does the F-111 have a capsule ejection system rather than a conventional system? Following on from the questions F-111 emergency landing with no wheel; what is done to reduce risks when loosing a wheel? and
Are there any known incidents of ejections by capsule? What are the survival chances?

I wondered, what is the reason for the ejection capsule on the F111?

It doesn't fly particularly high or fast (compared to aircraft with conventional ejection seats). The only reason I can think of is that the pilots sit almost side-side and it would presumably be tricky to fire the first seat without seriously inconveniencing the remaining crew member.

Are there any side-side aircraft with ejection seats?

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

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

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

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

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


Why do the intake of some helicopters have a bump body? Not all but the intake of many helicopters have a bump body. Why? e.g.

I don't know of the Apache, but the Russian helicopters have a dust separator there. This link shows how the dust protection system works on the Mi-17. The Mi-28 and Ka-52 have the same engines, so the system probably works similarly.

The system works by making the air follow a curved path from entry to the first stage of the compressor. Due to inertia, heavier-than-air particles are forced to the outside of the curve, which is the center of the intake protrusion. Air in the separator is forced through baffles to separate the dust from the air, then through a channel that leads overboard. A bleed air nozzle at the end of the channel creates vacuum at the baffle section to aid with removing dust.
The system reduces engine performance as it reduces the compression ratio with bleed air.


What happens when a pilot has no Instrument Rating and visibility drops? Note: I'm not a pilot, so I don't have much basic knowledge of how the pilot licence ratings work

If a pilot doesn't have an Instrument Rating, and the weather conditions drop below VMC; what do they do?

Presumably you can't instantly land, but nor can you safely continue to fly without sufficient visibility?

I know the basic licence includes some instrument training, but is this enough to fly and land without visibility? I was under the impression that was the entire point of gaining an Instrument Rating... To train the pilot to fly on instruments alone

It is very rare for weather to go IMC unexpectedly.

A responsible pilot should see that weather is deteriorating, and begin planning for the nearest available landing.   If they cannot find a landing, they should contact ATC while still in VMC, declare an emergency, and begin working with ATC to get down safely.

In some cases, a pilot may inadvertently enter IMC unexpectedly.   The standard procedure is to execute a standard rate 180° turn, and fly equal time in the opposite direction.  That should get them back to VMC conditions.

If that doesn't work, unfortunately inadvertent flight into IMC is a leading cause of GA accidents, especially CFIT (Controlled Flight into Terrain - i.e. 'aircraft flew into the ground').   Because of that, even basic private pilot training includes more basic instructions on instrument flying (BAI) than it used to.


Position update rate of GPS GPS satellites send data at the rate of 50 bits per second. One GPS subframe is 6 seconds long. How is it possible to update one's position faster than once in 30 seconds when we require the whole frame (5 subframes adding up to 30 seconds) to extract the NavBits for the pseudorange calculation?

Position interpolation between readings based on last positions & speeds calculated?  Plenty of 10Hz update GPS units exist, so it doesn't seem be rocket science to get there.

Units can track 12 or more satellites.  If calculations are being made from several sets of readings, then multiple positions can be calcuted at any one time as well.


Why is Iceland an efficient connection point between London and Boston/NYC? Air connections thru Iceland (Keflavik) have been increasing explosively in recent years. Keflavik is used as a transfer point on trans-Atlantic itineraries.

The highest-traffic city pair involving Keflavik is Keflavik-London, so Keflavik is evidently being used as a way station between London and North America.

Also, NYC and Boston are the top North American routes for Keflavik.

However, Iceland is quite out of the way of the US northeast-London direct path. Why is it economical to stop over in Iceland, despite it being off-course?

One of the benefits of having a stop halfway is that the aircraft can refuel.

It therefor doesn't have to carry the fuel for the second half of the flight during the first half, which saves fuel.

The uplift of fuel costs approximately 4% of its weight per hour of flight. That means the fuel you consume in the 4th hour of flight has burned approximately 11.5% of its original weight, just to carry itself. The fuel burned in the 7th hour of flight has burned 25% of its original weight , just to be carried for the previous 6 hours. (which means you have to load 133% of the fuel used in the 7th hour)

So a 10% detour in distance, with an extra stop, does not increase the total fuel consumption much. It may even reduce the total fuel consumption of the flight.

And it also means that the aircraft is lighter, so it is more efficient to use a smaller aircraft. Typically you see mostly Boeing 777, 747, 767 and Airbus A380 and A330 across the Atlantic, but if you look at the traffic in Keflavik, you'll note that most aircraft are B757 and A321.

These aircraft perform well at mid-ranges, but on longer ranges they have to reduce their payload for fuel uptake for the long flight. For longer flights the bigger (and more expensive) aircraft are more efficient.

So the effect of the extra stop is that you reduce the fuel consumption, so that you can use smaller aircraft with lower fuel capacity which in turn means lower capital expenses. This makes the Iceland option economically attractive.


What would be logged as the destination for an emergency landing in a field? When logging flights you have a departure point and an arrival point, how would you log the arrival point if you were to have to perform an emergency off field landing in a field or on an interstate?

I would just write "off fld" in the destination box, if there is one, and describe the location in Remarks.  "Emerg ldng on I-90 2 mi S of Whereverville"  or "Pracaut lndg field 2 mi W of Middleofnowheretown".

You aren't going to get audited over that kind of stuff so don't worry about hyper precision.  If anybody ever looks at your logbook it's just going to be to validate flying times for the purpose of a license or rating.  The other stuff is mostly for your own historical record.


What exactly is tower enroute control? Is this similar to 'preferred routes'? I am reading about 'Tower Enroute Control' and I saw this on wikipedia "Tower en-route control is a collection of published low-altitude, short-distance IFR routes through large metropolitan areas that require no level of air traffic control higher than approach-control facilities" I also see that you can get these out of the A/FD.

My question is when do you use tower en-route control? Is it commonly used or rarely used? How is an IFR flight different when on a tower en-route control flight then on say, a preferred route?

Tower enroute control is similar to the preferred route in that both are designed to improve the system effeciency. FAA defines the Tower En  Route Control (TEC) (in AIM section 4-1-19) as:

  TEC is an ATC program to provide a service to aircraft proceeding to and from metropolitan areas. It links designated Approach Control Areas by a network of identified routes made up of the existing airway structure of the National Airspace System.

Preferred routes, on the other hand (from Instrument Procedures Handbook) are defined as:

  Preferred IFR routes are established between busier airports to increase system efficiency and capacity. They normally extend through one or more ARTCC areas and are designed to  achieve  balanced  traffic  flows  among  high  density terminals.

This outlines one of the major differences between the two: in TEC route, you never 'talk' to the ARTCC. As flyingmag notes:

  TEC flights are intended to be relatively short — generally two hours or less, and are conducted below 10,000 feet. Throughout the route, communications are managed through tower controllers, and departure and approach controllers at a TRACON facility. You will never talk to ARTCC.

Another important thing is that the TEC is essentially equivalent of getting a pop-up IFR clearance, as already noted (from AIM section 4-1-19):

  There are no unique requirements upon pilots to use the TEC program. Normal flight plan filing procedures will ensure proper flight plan processing. Pilots should include the acronym “TEC” in the remarks section of the flight plan when requesting tower en route control.

The TEC is established only in areas with Radar coverage. US DoT WP AT 7110.5A specifically notes:

  TEC should be established/expanded only within areas of radar coverage. In areas without radar coverage ... an equivalent level of safety is maintained, TEC may be established and/or expanded.

TECs basically are published preferred IFR routes that are entirely in TRACON airspace. As a result, most of the TECs are in the northeast and the west coast (which can be found here), where the airspaces are close enough that one can fly with Radar coverage the whole way.


What is the angle of attack of horizontal stabilizer? It 'appears' that in most flights the horizontal stabilizer is set for positive angle of attack.

So does this produce an 'up force' (lifting force) on tail thus making the aircraft nose-heavy?

Or does the horizontal stablizer presents itself to the relative air at a different angle of attack than the wing does (which creates down force on tail)?

You are right, the horizontal tail of a conventional airplane appears to have a higher incidence, but the actual angle of attack is smaller than that of the wing.

The wing, flying ahead of the tail, produces downwash, so the flow at the tail location has a distinct downward component. The downwash angle can be calculated from the lift coefficient and the geometry of the aircraft: To simplify things, let's assume the wing is just acting on the air with the density $\rho$ flowing with the speed $v$ through a circle with a diameter equal to the span $b$ of the wing. If we just look at this stream tube, the mass flow is $$\frac{dm}{dt} = \frac{b^2}{4}\cdot\pi\cdot\rho\cdot v$$

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

$S$ is the wing area and $c_L$ the overall lift coefficient. If we now solve for the vertical air speed, we get $$v_z = \frac{S\cdot c_L\cdot\frac{v^2}{2}\cdot\rho}{\frac{b^2}{4}\cdot\pi\cdot\rho\cdot v} = \frac{2\cdot c_L\cdot v}{\pi\cdot AR}$$
with $AR = \frac{b^2}{S}$ the aspect ratio of the wing. Now we can divide the vertical speed by the air speed to calculate the angle by which the air has been deflected by the wing. Let's call it $\alpha_w$: $$\alpha_w = arctan\left(\frac{v_z}{v}\right) = arctan \left(\frac{2\cdot c_L}{\pi\cdot AR}\right)$$
A typical airliner cruise lift coefficient is 0.4, and a typical aspect ratio is around 8: This results in a downwash angle of nearly 2° if the lift distribution over span is elliptical. In reality, it is more triangular-shaped, so the downwash angle is larger near the center of the aircraft. Note that the engine nacelles of the DC-9 and the MD-80 range are tilted 3° up to align them with the local flow.

The resulting angle of attack is lower by those 3°, and if the angle of attack difference between wing and tail is less than that, the tail surface will appear angled upward. To achieve static stability, the tail will have to fly at a slightly lower angle of attack than the wing.


Is Electronic Flight Bag(EFB) any different from our day-to-day tablet? Other than the fact that EFB must be installed with appropiate charts and other manuals, is there any difference between EFB and normal tablet? Can I use the same tablet I play Plant VS Zombie as EFB if I load appropiate charts?

The FAA (unsurprisingly) has definitions of different types of Electronic Flight Bag (EFB) systems, which you can find in AC 120-76C.  It's targeted at airlines, but the concepts are generally applicable.
They break them down into Class 1, Class 2, and Class 3 EFBs, and the answer is a little different for the different classes:

Class 1 EFB Hardware.
This is what most of us General Aviation Folks use: A portable computer or tablet considered "portable electronic devices" with no specific FAA approvals.
Per the FAA a Class 1 EFB can't be "mounted to the aircraft" or connected to the aircraft systems or a dedicated power supply (i.e. they can't be permanently wired into anything), but you can plug them in to an "existing aircraft power supply" (like the cigarette lighter port) for battery recharging.

This is what most of us GA folks are using - an iPad with Foreflight and Stratus is technically a "Class 1 EFB".
This is just a regular computer, so it can in fact be the same device you play Plants vs Zombies on (though if you're using it in an airline environment they've probably locked it down and restricted what you can do: Plants vs Zombies is probably not available in the cockpit of your Southwest flight).

Class 2 EFB Hardware.
Class 2 EFBs are essentially Class 1 EFBs that get mounted to the aircraft, but can be easily removed by the flight crew (for example a GPS or iPad in a panel-mounted cradle) and may connect to aircraft data systems via cables or wireless connections (for example Garmin's Flightstream system).
Class 2 EFBs have the same battery-charging provision as Class 1 EFBs, but can't be directly wired into aircraft power. As far as the FAA is concerned they're still "portable electronic devices".

This is what the more advanced bits of GA are using - An iPad with Garmin Pilot communicating with your GTN stack via flightstream to get its GPS position & transfer flight plans.
Again this can be the same device you play Plants vs Zombies on, but the operator may have locked it down to keep their crews from flying past their destination while playing with their laptops.

Class 3 EFB Hardware.
Class 3 EFBs are installed systems with FAA approvals (for type-certificated aircraft).
The chart data stored on e.g. a GTN650 or GMX200 with charts and plates loaded on it would qualify here, as would flight management systems on larger aircraft.

Generally you won't be playing Plants vs Zombies on Class 3 EFB hardware because the approvals for it wouldn't include that capability.
The exception here is the Experimental Aircraft community: Such aircraft may have a consumer tablet installed in the panel and wired into ship's power in such a way as to qualify for all practical purposes as a "Class 3 EFB", and it's possible you could play Plants vs Zombies on your installed iPad in such an aircraft.


Why were DHL611 and Bashkirian 2937 on a collision course? This is about the crash over Lake Constanz/Überlingen on July 1, 2002. The planes were on a collision course, the automated TCAS told one to climb and the other to descend, but air traffic control gave the opposite order to the Russian plane and they crashed.

My question is, why were they on a collision course? I have not seen anything indicating they were off course. Don't planes flying different directions fly at different altitudes? And aren't there paths in the sky that planes are supposed to stay in?

What about flight plans? Does anyone check to see if two planes are on a collision course?

The Airways that the planes fly on criss-cross the sky and there are many, many intersections where there can be a potential for 2 aircraft to meet.

So yes, it is normal that aircraft are routinely flight planned to be on "collision courses". The flight plans don't keep aircraft separated. They are used so that Air Traffic Controllers can know the exact routing and plan where any conflicts may occur. It is normal and safe as ATC can then keep the aircraft separated using radar.

Generally aircraft flying eastbound fly at odd altitudes and westbound aircraft fly at even altitudes. This works quite well but we can still have 2 westbound aircraft both flying at 36,000' on a "collision course" because westbound can mean any track from 180 degrees to 359 degrees.

In this case the 2 aircraft were crossing paths at about 90 degrees to each other at the same altitude.

Normally they would be constantly monitored and the ATC controller would simply have one aircraft climb or descend to avoid the other aircraft. Another common practice is to tell one aircraft to turn slightly off course to avoid the other aircraft.

On this day the ATC system and the controller were both overloaded and a number of factors contributed to the controller not seeing the conflict until it was almost too late. At the last minute he told one aircraft to descend to avoid the other. This action alone would have been fine but the order was given far too late.

The Traffic Collision Avoidance System on the aircraft also saw the conflict and automatically issued an instruction for one pilot to climb while instructing the other aircraft to descend.

The controller did not know that the TCAS had also spotted the conflict and his  command to descend was contrary to the TCAS climb command. The pilot chose to follow the controller's instruction instead of following the TCAS climb instruction. The other aircraft was also following his TCAS instruction to descend so both aircraft were now descending while still on a conflicting flight path routing.

TCAS is there as a safeguard when the ATC system fails. It should always take priority over an ATC instruction.  It is clear that the ATC system failed these pilots and TCAS operated as it should.

As pilots we spend so much time following ATC instructions that I can see how it would be difficult to ignore the controller and instead follow the TCAS instruction.


Why aren't leading edge root extensions used on airliners? Why aren't leading-edge root extensions used on airliners?

Wouldn't they result in shorter wings and smaller slats?

Image source: Wikimedia

Leading edge extensions like slats are used in airliners. Most of these are controlled ones, so as to improve low speed characteristics.

By Adrian Pingstone (Arpingstone) - Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=4761104

The dog tooth extensions were used in some older airliners like the Ilyushin Il-62 and the Vickers Super VC-10, he reason being the requirement to reduce drag.

By Sergey Lysenko - File:Air-to-air with a Russian Air Force Ilyushin Il-62M.jpg on Wikimedia Commons, originally published on http://russianplanes.net/id164205, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=40618026

The other leading edge extensions like LERX (and even dog tooth) are mostly found in high performance combat aircraft as these increase the stall angle and improve maneuverability; these are not the typical concerns in the commercial airliners. In airliners, vortex generators are used for preventing separation at low speed (high aoa).


Is it permitted to load a full cargo container onto an aircraft without a roller track loading/locking system? Aircraft with roller track cargo loading systems transport containers/pallets in the cargo compartments. When an aircraft doesn't consist of a certain system but just a flat floor (so for example a small bulk cargo aircraft), is it allowed (regulation wise), and if the dimensions are no problem, to place one or more (loaded) containers in the aircraft?

(I just saw you asked about Europe right as I posted this, if this is not applicable any more I will remove it)

If Im reading this correctly the question is asking "how is cargo secured if not loaded through the use of a Unit Load Device (ULD)"

Here in the US under the FAA there are some guide lines for tying down cargo if no ULD is present. You can read them here (SAFO 13008) and here (SAFO 13005). Much of the over arching information can be found in this Advisory Circular On Air Cargo Operations (AC 120-85).

In short,

  ...The manual must state the policy and provide procedures to determine
  the intended reaction load for each tie down to ensure sufficient
  special load restraint for flight loads...

You can tie it down as long as you conform to the above linked documents and ensure the straps and tie downs are sufficient to hold the cargo.


Can I get a third-class medical if I have sleep apnea? Some year-and-a-half ago my physician recommended that I take a sleep study. Some preliminary exam (overnight finger-carried monitor) indicated that a few times per night my oxygen level dips below 91% (and one time below 81%). He indicated that this might be caused by sleep apnea. I ignored that recommendation as my sleeping had improved and I didn't want to spend money and time on the full-blown sleep study. Six months after that I started my PPL training. As it was not an issue, I didn't even think about it when I got my 3rd Class med certificate.

My soon-to-be-ex wife had recently found my old correspondence with my doctor and, out of pure vindictiveness, denounced me to the FAA. I received a letter giving me 60 days for addressing this issue. Here are my questions:

Does anyone at this forum has a mild case of sleep apnea and is still flying (i.e. has a valid third class med)?
Is the dip in the oxygen intake an 'sure proof' of sleep apnea?
Are there any other types of SA than Obstructive SA that are not viewed as dangerous by the authorities (the regs specifically mention OSA)?
Is there any recommended treatment that may still make the med cert possible even if I have a mild case of SA?

The short answer to your question is yes, you can get a third-class medical with sleep apnea in some cases:

  If your OSA is treatable, you can maintain your airman medical
  certificate and continue to enjoy your aviation career.

Everything else in your question is very personal and specific, and the only way to get an answer for your individual case is to consult an AME, preferably one who's handled similar cases before.


What is a whip stall? I was reading 14 CFR 23.3, which discusses airplane categories. In normal and commuter categories, stalls (except whip stalls) are allowed. The regulation, however does not define "whip stalls." What is meant by this term?

The clearest definition I could find is from an AOPA quiz (of all places):

  The airplane is pitched up into a vertical or near-vertical climb. It
  then pauses, slips downward momentarily (tail first), and then whips
  into a steep, nose-down attitude.

The tail-first slip backwards seems to be the defining characteristic of a whip stall. Interestingly, Whip Stall and Tumble Awareness is included in the FAA's Powered Parachute and Weight Shift Control PTS, but they aren't defined there either.


Does any country issue daytime-only commercial licenses? I got my PPL last year (EASA, UK) and I've always been interested in the commercial route. However, due to my red-green colour blindness I've failed the Ishihara, Farnsworth D15 and CAD vision tests which are accepted by the UK CAA to obtain an unrestricted medical. By now I've accepted that going all the way to ATPL might not happen for me, but I'm still interested in any other style of commercial flying that could remain an option.

My question is - can I theoretically get a CPL which would allow me to fly only during the day? Is there any such thing as a Class 1 medical which is restricted to daytime only, like my current Class 2? Can I get an IR with colour deficiency, or just not get one at all?

Of course getting a job is another matter, but I just want to know if I could theoretically fly for a living (or even just a side job) while accepting my colour deficiency as opposed to continually trying to fight it. I'm originally from the UK, but I'm interested in opportunities all around the world - in fact, I'm planning on spending the next few years in Australia, New Zealand and possibly Canada using the temporary visas they offer to UK citizens.

In Australia you can get a class 1 medical whilst being colour-blind, limited to day VFR. Night flying over here is a separate rating and not incorporated into the PPL/CPL so it is optional - it might limit your job prospects but you can absolutely fly on a CPL here in daytime only.


What is the difference between aluminum tape and speed tape? What is the difference between aluminum tape and Speed tape? Are these same or not?

Speed tape is aluminum tape that is much thicker with stronger adhesive than the aluminum tape you get at Home Depot, that is approved for temporary application on external surfaces of airplanes.


Why would the Speedbrake be required for such a long time on approach? As far as I was aware, the Speedbrake is designed to slow the aircraft down. On landing, I do understand why they are required. Once the aircraft has hit the tarmac, they are deployed to 1) slow the aircraft and 2) prevent the aircraft from bouncing up and down. The airflow over the wing increases doesn’t it? To ‘push’ the aircraft down, as I say, preventing it from jumping up and down.

I have (7 hours ago) just landed on a B757, but the Speedbrake was up pretty much for 70% of the descent and approach.

Thinking back to my previous introductory paragraph, the reasoning must have been excessive speed, no? Or a need for the aircraft to slow down dramatically. Is this the case, or am I missing something obvious?

I know Air France flight 447 considered deploying the Speedbrake when their Airbus gave them faulty warnings of speed and altitude; it was advised then that this would have been completely the wrong thing to do.

I have never seen it before, as much as I did today on descent, and I wonder why the Speedbrake was required for near on 70% of our approach?

There are two reasons I can think of off the bat:

Its possible the controller asked them to hold a slow speed due to increased traffic ahead. The brakes may have been deployed to match the speed requested. I have heard this called on occasion over the radio in the terminal area I fly.
The controller cleared them for a steeper decent than usual this would cause them to potentially deploy the speedbrakes to make the descent rate.

This article discusses it in reference to small planes but idea is the same. And another article by a CFI here.


An appropriate way to practice ATC communications I'm a PPL trainee and I'm looking for a way to improve my radio communication.

https://www.liveatc.net/ is a great way to practice but I can't fully understand the conversation between the unit and the aerodrome. If it just had subtitles or general documentation about it, would be great.

Live ATC is an excellent way to listen and get familiar with your local airports procedures. I used to pick out a tail number and then pretended to be that aircraft. When ATC gave instructions, I would say back what I think the response should be.

If there are terms or phrases you dont understand, you should be able to look them up in the Pilot/Controller Glossary here:
https://www.faa.gov/air_traffic/publications/media/pcg_10-12-17.pdf

ATC and pilots should do their best to use the official phraseology in the glossary. However, the goal of communication on the radio should be clarity. If there is something you want to say but dont know how, just say it plainly without worrying about the 'official' wording.


Comparing the Lilienthal and Wright gliders, which one had a better glide ratio? Where can I find reliable figures about the best glides of Lilienthal and the Wright Brothers (1902 - 1903)?

From the sources below it would appear that Lilienthal's most advanced glider was far superior to that built by the Wright brothers in 1902 as the German apparatus was able to fly 1148 ft while the machine made by the Wrights only 600 ft.

It seems like the wind tunnel tests done by the Wright brothers in 1901-1902 to find optimal wings did not lead to improvements in lowering the glide angle and so increasing the flight distance.

1)"Finally Lilienthal become so skilled that he could make his glides in air velocities of 33 feet per second ... Lilienthal carried out more than 1,000 glides and attained distances of 1,148 ft. at a gliding angle of 4 degree from the hill."

Source: John H. D. Blanke, "Pioneers in Gliding", Popular Aviation, pag. 74 - 77, May 1928

2) "World Record Holders
The improved performance of the 1902 glider finally enabled the Wright brothers to gain extensive practice in the air. During September and October, they made between 700 and 1,000 glides. Flights of 500 feet were common, and a few topped 600 feet. Orville enthusiastically wrote home of their success, “we now hold all records!”"

Source: Inventing a Flying Machine

I'm asking about which glider that had better glide ratio because the Lift/Drag ratio is essential: the higher it is the longer the glide and the less powerful and lighter an engine is needed to make the glider fly horizontally.

Otto Lilienthal saw flying as something similar to bicycling: A sport and an opportunity to spend time out in Mother nature. Therefore, he designed all his airplanes such that they would fit through the door of a standard Reichsbahn freight wagon. He used a central truss (Gestellkreuz) to which willow stakes could be bolted such that they would fold backwards. He covered this structure with waxed cotton cloth and used wooden ribs to shape the airfoil of these wings. Consequently, the wingspan of his designs was limited.

The next limitation was his way of controlling his aircraft only by weight shifting. Since his hands were already busy with holding on to the aircraft, he could not use them to move control surfaces. His drawings and his correspondence show that he considered control surfaces and even ailerons, but he did not incorporate them into his designs.

Only his type 18 glider had a rectanguar inner wing which could be pulled close using a scissor link spar. However, with the bigger wing his weight shifting control showed its limits, as this did not lend itself to controlling gliders with bigger wingspans.

Another limit was his insistence that his aircraft would not need a runway or a launching device. Instead, he would grab handles in the Gestellkreuz, stand upright and use his legs as a landing gear. This limited not only the size, but also the wing loading of his designs.

The Wrigths, on the other hand, strived for the mechanically simplest and most lightweight designs and arrived at a biplane of constant chord and much higher aspect ratio. The 1902 glider could already be flown in a prone position and the pilot could control it with control surfaces.

Drag area of the human body in different poses, from Sieghart Hoerner's Fluid Dynamic Drag. The pilot of the Wright flyer caused only one fifth of the drag of the pilot in Lilienthal's designs.

Because the wingspan of the Wright glider was much higher (9.6 m versus 6.7 m for Lilienthal's Normalsegelapparat) and the much higher drag of the pilot's position in case of the Lilienthal design, the Wright glider certainly had a much better glide ratio.


What is the time information on the G1000 MFD map during a hold? I just completed an ILS approach and went missed.  I followed the published missed approach and entered the hold.  When I entered the hold, on the MFD map a timer-like display was on top of the "aircraft" e.g. 00:00

The timer display did not increase after being in the hold for about 5-10 minutes.  I even started the timer from the TMR/REF softkey on the PFD to see if it would change the display on the MFD, and it still remained at 00:00.  Does anyone know what this timer display on the MFD during a hold is?

It's most likely an Expect Further Clearance (EFC) timer. You didn't mention the aircraft type but there are several references to it online: Garmin service advisory; Seneca V impressions; Archer manual quoted in a forum. My G1000 experience is in a C172 and I've never seen it there (I can't find it in the manual either) but the features are different depending on the aircraft, so it's possible that EFC timing just isn't in the C172 version.


Why aren't "radioactive items" allowed on flights? I've done physics, I completely understand what nuclear radiation is and why it is harmful. What I can't seem to figure out are the rules surrounding radioactive emitters/items on flights. Surely it's not just any radioactive items, because everything is at least slightly radioactive.

You get about 12 times the sieverts per hour at 35,000 feet than you generally do at sea level (3 μSv/hr in the air, 0.25 μSv/hr at sea level). And even then it's essentially harmless.

What if I had a strong radiation source, but encased in thick lead shielding? Or a strong emitter of only alpha particles? Do they measure it in Sieverts per hour, or Becquerels? Or Gray? All of it makes a difference.

Something that measures high in Gray or Becquerels could be damn near close to undetectable in Sieverts per hour - arguably the only relevant unit in terms of radiation effects on human health.

There's not enough info available to nerds like me, so insight would be greatly appreciated.

Radioactive substances are potentially hazardous to people.

It is correct that many radioactive items would be perfectly safe to carry onboard, e.g. a key hanger with an alpha or beta particle emitter cast in acrylic, or a radioactive source for scientific purpose, if that source is properly packaged.

Glowing key hanger containing tritium. Image source: Nite GlowRing

Travelers usually do not carry radioactive items with them. If there is the need or wish to bring a radioactive item along, there is normally no need to have it in the hand luggage. This makes radioactive items different from lithium ion batteries. While potentially hazardous, there is a strong demand for lithium ion batteries on board. Banning them would essentially ban mobile phones and laptops.

Airport security staff as well as airline ground and cabin crew are neither qualified nor equipped to inspect and judge the safety of radioactive items. Opening the safe packaging of radioactive items might be dangerous. An item that cannot be opened cannot be checked if something else is hidden inside.

Finally, radioactivity stirs an irrational fear in many (if not most) people.

Given the need to restrict the transport of radioactive material somehow, and the difficulties in controlling it in a more sophisticated manner, the ban of such material from hand luggage seems comprehensible.


Does ATC direct flights to fly eastbound at odd levels and westbound at even ones? Do commercial airlines routinely travel at altitudes specified by the ATC based on the direction they are heading? Example: eastbound flights travel at odd levels and westbound flights travel at even ones?

In addition to ymb1's excellent answer, here are a few thoughts from the controller perspective.

Various rules and regulations for cruising level allocations exist. For flightplanning purposes, the semicircular rules are very notable. As pointed out by ymb1, the exact rules vary from place to place. In some regions, flights flying on a westbound track shall plan to fly at even levels while flights flying eastbound shall plan to fly at odd levels. Note that this is in relation to the magnetic track of the aircraft, except in areas near the poles. In some countries it is the other way around, in others, the rule is north/south instead of east/west.

Example of a general east-odd/west-even rule (excerpt from AIP Denmark)

You can also have specific ATS routes, such as airways, that require you to flightplan certain levels that deviate from the general rule.

Example of ATS route which enforces even levels for eastbound flights (excerpt from AIP Germany)

It is important to note that rules for level allocations are mainly used for planning purposes. When filing a flightplan, the planned levels must obey any such rules - and you should always stick to your filed flightplan when in uncontrolled airspace.

However, when being provided with air traffic control service, the controller may deviate from the general rules if so desired (unless the local operating procedures do not allow it). If I have an upcoming conflict that can be solved by climbing a westbound flight to FL370, then I will be more than happy to do so - especially because establishing vertical separation is often the easiest way of solving a conflict. As long as the flight is within my sector, then I make the rules, so to say.

Example of ATC radar screen with eastbound traffic (SAS528) flying at an even level (FL340) for separation

When transferring a flight to the next sector (or another unit), the controller is responsible for making sure the flight is at an appropriate level in accordance with the local letters of agreement. Principally, this means that we have to go back to the published rules (east odd, west even or specific airway rules). In practise, though, if it is desired to transfer a flight on a non-standard level, all it takes is a phone call to the next sector to check if they can accept it. If the next sector has no conflicting traffic, then there is nothing wrong with keeping the flight at a non-standard level.

So to sum it up, when planning a flight, you have to obey the published level allocation rules, which vary from place to place. A large part of the world uses the east-odd, west-even rule. But once under ATC control, there is no need to worry if you are assigned a non-standard cruising level - if that happens, it is because it is allowed and has been coordinated.


Where can I find electrical pinout of C152 microphone jack and DC bias? Would anyone have the pinout and/or specs of the C152 passenger/pilot microphone jack? I'd also like to know what DC bias voltage, if any, the plane provides to the microphone.

Here's what I have so far, which may or may not be accurate:

              Headphone side     Plane Side
Sleeve             GND              GND
Ring         MIC OUT/BIAS IN       ???(?V)
Tip              UNUSED?            PTT?

I've dug through some Bendix/King nav/com radio manuals, but they don't tell you about what to expect on the plane side of things.

Thanks!

There are some great resources on webpages mostly aimed at home builders and experimental aviation enthusiasts as well as some electrical engineering forums.

Most of these aviation radios are set-up for use with a carbon microphone. They are fairly simple pieces of kit. If my memory serves me correctly the acceptable forward bias voltage range is set by the RTCA (https://www.rtca.org/) however I can't find a solid reference for this.

As far as I know, so called "GA plugs" (The 5.25mm/6.35mm style plugs) are wired as follows:

Sleeve             GND                           GND
Ring         MIC OUT/BIAS IN                    8-16v (Short circuit current ~150ma)
Tip       PTT  (Switch shorts to GND)            PTT

The reasoning behind the PTT switch still being assigned is for handheld backup microphones.
Many newer headsets don't use carbon microphones and use the electret capsule style microphones instead and therefore require some complex electrical engineering wizardry to drive.