Does a Stalled Wing Exhibit No Circulation If a wing is stalled, will the wing not exhibit any circulation as predicted by the Kutta-Joukowski theorem?  If so how would this affect the pressure distribution between the top and bottom portions of the wings?  Would the top portion of the wing (low pressure) carry most of the lift?

If there is no circulation, there would be no lift. In reality, circulation stops to increase linearly when the angle of attack nears the stall range and drops beyond stall. However, there is plenty of lift left beyond stall.

180° polar for several NACA 4-digit airfoils, from Hoerners Fluid Dynamic Lift.

  how would this affect the pressure distribution […]?

Think of the stalled airfoil as of an entity which comprises the airfoil itself, the boundary layer around it and the separated flow in its wake. This will still have nearly-inviscid flow around it, and the wake will make it "look" to the outer flow like an airfoil with a lower angle of attack and a really drawn-out pressure recovery. There is still a suction peak at the nose, albeit much weaker than for the clean airfoil in attached flow, and constant suction past the separation point. On the bottom, however, flow is almost unchanged except for a region near the trailing edge where the low pressure in the wake speeds up the local flow.

  Would the top portion of the wing (low pressure) carry most of the lift?

The answer to the question on which side more lift is created depends a lot on how lift is interpreted. In the end, lift is the difference between the pressures on the bottom and the top sides, and cannot meaningfully be split into a top or bottom contribution. What can be done is to compare the pressure on one side with static pressure, which will yield a side-specific lift fraction. But then you can arbitrarily compare surface pressures with any pressure level found inside the wing box - it will cancel out once you calculate total lift.


What are the differences between EASA (UK) and CASA flight training? I'm a British citizen living in Australia who is tossing up on whether to train back home or in Australia. Given the sheer amount of complex information out there, I was wondering what the differences are between the PPL, CPL/ME/IR, ATPL theory & MCC/JOC training in Australia versus in the UK (or the EASA region as a whole), in terms of hour requirements, syllabus, technicalities, rules, licencing system, etc...

A comprehensive guide from those who perhaps have experience or knowledge would be great!

I'm an Australian PPL pilot but know some people who have gone through the UK system. Terminology differences aside, the training pathways are pretty comparable. If you choose to train in Australia you can convert your licence to the UK fairly easily by passing a flight test.

The main differences are:

In Australia you need to do 7 CPL exams and then 7 ATPL exams. Once you start each set of 7 you have 2 years to complete the remaining 6. You can self-study if you wish. In the UK there are 14 exams of each (but I think you only must do the ATPLs and CPLs are optional). You must complete all 14 within 18 months and self-study is not an option, you must attend a formal ground school.
Australia does not offer a Multi-Crew Pilot Licence, but still require a Multi-Crew Cooperation Course be completed before the ATPL.
A lot more schools in the UK are tied to an airline, greatly increasing your chances of immediate airline employment, if that is your goal. Not to say that's impossible in Australia but it only happens to a few dozen pilots per year who secure an airline cadetship.
Training in Australia will cost about $120,000 all up. In the UK seems to be about 120,000 pounds!

There are a lot more little differences but ultimately no matter where you train you will come out with an ICAO compliant licence. If you have specific questions you can either edit this one or head to chat.


Do commercial pilots have their own lavatory? I was wondering if commercial pilots had their own lavatory to the two instead of using a passenger lavatory.

In an Airbus A320; pilots share the toilet with the passengers. On AirAsia, a flight attendant blocks access to a toilet when the pilot enters the lavatory.


Could most commercial jet engines today run on ethanol without any problems? Most commercial jet engines today would be those large jet engines on large passenger craft like a Boeing 747. Can these run on straight ethanol without any problems?

Would more maintenance be required? Would the engine lifetime be reduced? Would it be more susceptible to flameouts or other mechanical risks? Would the airplane never make it off the ground? (I imagine thrust reduction will be limit the takeoff weight.)

One reason I'm asking is because eventually petrol will be gone. (I'm not an apocalyptic saying that will happen next year, but somewhere down the line it will certainly be gone.) Will jet engines require a major redesign, or could they just use ethanol for a few years/decades and be fine?

Note: I'm talking about jet engines (turbofans, but turbojets are okay too), but not piston engines and turboprops.

EDIT: A lot of talk has been generated that ethanol contains less energy than kerosene. I assure you I'm aware of this. Ethanol is like 25 MJ/kg and kerosene is more like 40 or 45. I'm not asking this question with energy efficiencies in mind, but rather out of curiosity if the same engine geometries will work well with both fuels.

A rephrasing: Would current jet engine geometries (blade angles, compression ratios, volumetric flow, etc.) work well with ethanol instead of regular jet fuel?

Although turbines could be designed to run on ethanol it's actually a lousy aviation fuel (actually it's a lousy fuel in general). Petroleum-based jet fuel is high grade kerosene which has lubricant properties, which ethanol lacks, and ethanol has different characteristics which mean you can't pour it in and use it as a replacement.

Some problems with ethanol as a fuel are:

It's a solvent which dissolves some types of seals and hoses
It evaporates very easily, and can cause vapor lock
It is hygroscopic, meaning it attracts and mixes with water very readily. Water contamination would not settle to the lowest point, instead it would be suspended with the fuel
It has significantly less stored energy per unit of weight. Jet fuel has an energy density of 46 MJ/kg (megajoules per kilogram) and pure ethanol has 26.4 MJ/kg; that's only 57% of the energy density of jet fuel! Even if a jet was able to burn it at its maximum efficiency it would get just over half the oomph from it.

There are bio-fuel replacements for jet (and diesel) fuels based on oilseed crops like rapeseed (canola), palm oil, etc. These are processed to remove their OH atoms using transesterification to create fuels which are almost identical chemically. Modifying jets to use ethanol would be hugely expensive if it is even possible to do, so there's no point given there's a suitable replacement.

The point is actually moot though because crop-based biofuels are never going to replace petroleum. There simply aren't enough crops grown to feed people and engines at the same time, in fact even if we used all the worlds farmable land to grow bio-fuel crops instead of food crops we still would not have enough of it to replace fossil fuels. We have enough trouble feeding people already and the biofuels craze is not helping that. Bio-fuels based on non-food crops from marginal land may some day become economically viable and partially replace petroleum fuels.

After some searching I found a link to an article I wrote on biofuels for AOPA UK magazine which goes into more detail on biofuel issues.


What does "Raven" [phonetic] means when used by ATC? What is the meaning of "Raven" [phonetic] heard in use at Reagan National Airport?  Most recently heard: The ATC will say "proceed direct raven" or "direct raven maintain" whatever altitude; or "direct raven; raven at 6,000."  I've looked in every ATC/Pilot glossary I can find, but nothing.  Have not heard at other airports, not even Dulles or BWI in the Washington metro area.  A military pilot friend thinks it is an enroute, departure, approach or holding fix, which he says would be specific to one airport.  Can anyone tell me exactly what we are hearing [an acronym?] and if this is the meaning?

RAVNN (Pronounced Raven) is the name  of a waypoint in Maryland. From the FAA Fixes / Waypoints directory

  RAVNN MARYLAND BAL*C*174.00/22.98 OTT*C*070.89/12.17
  ENO*C*250.66/53.40 DCA*D*106.56 38-48-15.9900N 076-31-04.9100W

38°48'15.9900N 76°31'04.9100W(Google Maps link) is east of Washington.


Can 80hp Rotax 912s be upgraded to 100hp? The more recent Rotax 912's are 100 hp engines. Can the earlier 80 hp models be upgraded to 100hp - without a complete engine swap?

It looks like there are several kit upgrade options from third parties, e.g.:

120HP turbo kit
125HP kit
95HP kit

I have no idea about the pros and cons or costs. The 125HP kit "requires machining of the crank case and heads", the others seem to be additional components.


What determines in which direction a runway is used? A nearby airport has a single 05/23 runway (3900m long), where airplanes start and land in one direction during certain periods and in the opposite direction during other periods.

How is the direction determined -- I assume it's air traffic control which decides that but based on what ? (wind, sun ?)

There are several factors:

Prevailing winds
Neighboring airport traffic patterns
Size of inbound aircraft
Where the aircraft is coming from
Final parking space
Congestion
Fuel status
Weather (thunderstorms)
Noise considerations

At small airports, it is usually just the runway that faces into the wind and can accommodate most of the traffic coming in.

At large airports, the wind dictates which flow is being used (like a "north flow" or "south flow"). From there, aircraft usually are assigned to land on which ever side of the airport their arrival procedure brings them to. Sometimes, it is also decided by which gate/terminal they'll be parking at. Lastly, the available approach procedures can change things if the weather is not good


Why couldn’t Boeing just use the stab trim system to do MCAS job? If I’m incorrect in the question, please correct me. From what I’ve read and gathered the stab trim system essentially did the same before hand? It relies on AoA? And if so, did the computer for that take information from one AoA at a time like MCAS did? I’m pretty sure I’m wrong anyway, this is super confusing for me, but that’s my understanding as of now, so basically Boeing needed the trim system to make a further command than the previous system, correct?

No, the 737 NG stabilizer trim system does not feedback from AOA. My source of information comes from this LinkedIn Slide Share and corroborated by this online FCOM.

There are several ways through which the stabilizer can be commanded:

Manually via the trim switches.
Manually via the trim wheels.
Automatically via the Speed Trim System (STS). A helpful explanation of the STS can be found in this answer. STS feeds back from stabilizer position, thrust lever position, airspeed and vertical speed.
Automatically via the Autopilot trim offset. This is only applicable with the relevant autopilot mode engaged.

The Mach Trim function also modifies the trim characteristics. However, it does not actuate the stabilizer and repositions the elevator centering and feel unit instead, which backdrives the column.

MCAS' requirement is completely different from those of the STS or the Mach Trim.


Why shouldn't the pilot change the aircraft's configuration while avoiding windshear during landing? According to the procedures, when going around, due to windshear, pilots shouldn't change the aircraft's configuration, such as flaps position and landing gear extension. If changing, would the aircraft lose the lift? why?

As far as landing is concerned, it is better to leave the landing gear down because if something goes down and the aircraft touches the ground, it is better to do so with the landing gear rather than the aircraft fuselage. Also, the retraction process may increase drag, not what is desired.

During landing, the retraction of flaps (which may be already at their max) can cause loss of lift (as you noted). Reduced airspeed due to windshear causes loss of lift; retracting the flaps in this condition is not desirable.

There is also the point that during the critical moments when the pilots are trying to pilot the aircraft during wind shear, their attention should be in flying the aircraft and not in fiddling with controls.

From FAA Advisory Circular 00-54 Pilot Windshear Guide:

  CONFIGURATION

  Maintain flap and gear position until terrain clearance is assured. Although a small performance increase is available after landing gear retraction, initial performance degradation may occur when landing gear doors open for retraction. While extending flaps during a recovery after liftoff may result
  in a performance benefit, it is not a recommended technique because:

  1) Accidentally retracting flaps (the usual  direction of movement) has a large adverse impact on performance.

  2) If landing gear retraction had been initiated prior to recognition of the encounter, extending flaps beyond a takeoff flap setting might result in a continuous warning horn which distracts the crew.


Do frequency response / Bode plots for an aircraft or helicopter come from modeling or empirical testing? I was intrigued while reading an article by this snippet:

  Examination of the V-22’s frequency response plots (Bode plots) in
  2000 showed that the aircraft failed to meet the basic stability
  requirements mandated for rotorcraft and was indeed PIO-prone.
  Since 2000, the Navy and Bell/Boeing have stated that the frequency
  response of the aircraft has been modified to remove this tendency.
  Updated Bode plots have not been made available to date.

My question: How does one go about obtaining frequency response plots for an aircraft? Is it a modelling exercise or an  empirical one?

http://www.g2mil.com/V-22safety.htm

The answer is both modelling and/or empirical methods are used.

For aircraft with a closed loop Flight Control System then the clearance requirements before flight requires that sufficient gain and phase margins exist for the system. For example with Military Aircraft this was defined in MIL-F-9490D. This would involve building mathematical models of the aircraft and control laws and calculating margins from the frequency response of the model. This allows the engineers to analyse the robustness of the system under various tolerance and failure states before flight.

It is possible to analyse stability margins from flight data, but note even the RTSM software referenced in the comments talks about comparing the flight data calculated margin against an a priori model.

Also note that Handling Qualities requirements, i.e. saying it is PIO prone, are additional requirements over and above the basic stability requirements. This introduces another feedback path with a pilot in the loop. For this there are additional frequency response parameters to meet, for example average phase rate. Again examples in MIL-Spec requirements MIL-F-8785C, these will all be assessed using mathematical models.


Why does exhaust flow inwards through a radial turbocharger? In a radial compressor, the air enters centrally and passes outwards, aided by centrifugal force. But, as the diagram shows, in piston-engine superchargers the driving exhaust gas flows inwards through a radial turbine. This has to fight against centrifugal force instead of leveraging it. So why is it done?

This has to fight against centrifugal force instead of leveraging it.

In fact, the opposite is true. The arrangement shown actually helps to extract energy, perhaps counterintuitively. A typical example of this arrangement is the Francis turbine.
A high flow speed plus high tangential velocity input, fed through to a low flow speed/low tangential velocity exit, will create the maximum energy differential for extraction. And that describes inwards flow.
There are a few ways of looking at this. One is to look at the exit velocity of the fluid. In a compressor, it's okay if the fluid has some leftover velocity as it will by converted to static pressure when slowed down by Bernoulli's law. For a turbine, the exit velocity should be as low as possible, as kinetic energy of the exhaust flow is 'wasted' energy. So it makes sense to place the exit at the inner radius of the turbine, where the radial velocity component is lowest.
Another way of looking at it is pressure. Centrifugal force creates a pressure gradient across the turbine in radial direction. As a 'packet' of air moves inwards, its pressure becomes lower. By conservation of energy, this must mean that it is expending work on its surroundings. The turbine extracts this work done from the packet of air moving inside.
Another intuitive explanation involves an ice skater retracting their arms in a pirouette (or a bored office worker on a swivelling chair). By conservation of angular momentum, their angular velocity increases. A packet of air would 'like' to increase its angular velocity, but this is kept fixed by the turbine rotating at a fixed velocity. The packet of air must thus be slowed down moving inwards, and as a result exerts a pressure on the turbine blade in front of it.
The choice between axial, centrifugal or mixed-flow is done based on the flow conditions, with high-flow low pressure differential favouring axial design and lower flow or higher pressure differential favouring the radial setup as shown. A more compact design means fewer turbine or compressor stages, which results in a higher pressure differential across the single stage. Thus, a radial setup is preferred.


How exactly can an airship lose lift? On this wiki page (under 'Comparison with heavier-than-air aircraft'), the altitude record of the L-55 Airship states:

  The ultimate altitude record for a rigid airship was set in 1917 by the L-55 under the command of Hans-Kurt Flemming when he forced the airship to 7,300 m (24,000 ft) attempting to cross France after the "Silent Raid" on London. The L-55 lost lift during the descent to lower altitudes over Germany and crashed due to loss of lift.

I understand that lift on an airship is created through internal heated air and/or gases, rather than by the movement of air over a wing.

So, how exactly can an airship lose lift? (And how does altitude affect this?)

The reason is a temperature difference between the lift gas and the surrounding air, and probably water uptake by the hull when descending through clouds.

A given mass of hydrogen will create a constant lift force, regardless of pressure or altitude, when at equal pressure and temperature with the surrounding air. Therefore, a change in altitude will not change the lift a rigid airship creates. Ideally.

However, air gets colder the higher you climb. The sun heats the atmosphere from below, by way of heating the ground, and space cools it from the top. Therefore, on many days the temperature gradient is bigger than its adiabatic value - that is the way thermals work! L-55 stayed at altitudes where air is -32°C according to the Standard Atmosphere. When descending, the surrounding air got warmer and also warmed up the lift gas, but only slowly. This means that, depending on the rate of descent, the lift gas lagged behind in temperature relative to the air, and this temperature difference reduced its lift capacity.

Note that adiabatic heating will already heat a gas when being compressed. The lapse rate of the atmosphere must be above the dry adiabatic value for this mechanism to work, which it is on many days. Especially behind a cold front. Note that L-55 encountered strong winds - just what you find in and behind a cold front. So it is safe to conclude that L-55 flew in labile air, and when it descended, that motion became unstable, at least close to the ground.

Kapitän Flemming simply descended too quickly. Slowing things down would have warmed the lift gas more, and less lift would have been lost. But delaying the descent has dangers of its own: The gas bags back then were made from goldbeater skin and had a certain amount of seepage. To compensate, Zeppelins started their trips with several tons of ballast water onboard, which was progressively dropped during the many hours of a normal trip. Taking a detour over France delayed the trip, so he was running short on time.

Normally, a loss of lift can be compensated by dynamic lift. With some angle of attack, an airship can create up to 20% of its weight in dynamic lift - as long as all engines are running. L-55 was blown south during the night after attacking Hull and Birmingham, and found itself far more south than assumed when dawn allowed the crew to get a ground fix. When back over Germany, L-55 ran out of fuel and dynamic lift was no longer available to compensate for the lower lift gas temperature. It made a rough landing in the Thuringian countryside near Tiefenort and had to be written off.

Literature: Heinz Urban, Zeppeline der kaiserlichen Marine 1914 bis 1918


On a Boeing 737, can the APU, BY ITSELF, provide enough thrust to move the plane while on the ground? There are numerous questions and answers about the ability of an APU to provide enough thrust to maintain flight.  In contrast, my question is whether the APU, by itself, provides enough thrust to move the plane on the ground.  This is what motivates this question --
Back in the 1980s I often flew between Bangkok Thailand and Chiang Mai.  It was normally a non-stop.  But on one particular flight, it made a stop at a tiny intermediate airport.  Looking out out the window (passenger window) I saw that the vehicle parking lot abutted the tarmac with no intervening separator.  The parking lot was graveled, and considerable gravel had migrated to the area around the plane.
This seemed to me to be a serious safety hazard for ingestion into the engines, and I wondered whether the APU, by itself, could produce enough thrust to move the plane clear of the gravel.
Thanks.

I can't reference to a document, but I can say with reasonable certainty that the answer is no, because like any turboprop or turboshaft, the APU has nearly all of the energy of its mass flow extracted for bleed air, or for torque production purposes for electricity, so that there is little thrust energy available.  Turboprops typically get maybe 10% of the total energy available from the engine as exhaust thrust.
So using that number for an APU in an airplane the size of a 737 that is making, say, 650 shaft horsepower (just an educated guess; the actual number is surprisingly hard to find), it's producing a thrust equivalent to roughly 65 hp statically out its tailpipe, which equates to perhaps 2-300 lbs of static thrust (I recall that it's something like 4lbs thrust = 1 HP in the static case) at most.  That would be if the tailpipe was even optimized for making thrust in the first place (tapered to accelerate the flow), which they are generally not.
So, figure on a few hundred pounds of push at most, probably quite a bit less, which isn't going to move a 737 unless it's on some kind of near-frictionless surface, or in outer space.


What does "zero offset between two waypoints" mean in ICAO Doc 4444? My question is with reference to the GNSS based separation specified in ICAO Doc 4444. Could someone explain the significance/meaning of the condition "zero offset between two waypoints" criteria?

if an aircraft had deviated, say due to weather, but is able to proceed direct to a particular waypoint, can the GNSS based separation still be applied with respect to another aircraft if we are able to establish that the two GNSS tracks are 15-135 degrees apart (but not on route)? It is this context, that I wanted to know the significance of "zero offset" clause.

GNSS units have the ability to fly an offset, meaning a track parallel to the one drawn between 2 waypoints. For example a left offset of 5 NM means the track is now 5 NM to the left of the original track and running parallel to it.

The lateral separation minima specified in 5.4.1.2.1.2 for two aircraft (both GNSS) on intersecting tracks require that neither be flying such an offset – since the tracks intersect at a common waypoint that is used for separation, an offset would change this common waypoint. Here's a crude representation (blue track is an offset):


How is a "conflict" between aircraft predicted and determined? To answer my question about the prediction and determination of conflicts, we need a clear definition of the term. As far as I know is the term conflict often mistaken with the term loss of separation.

Let me start with the definition of conflict:

  Converging of aircraft in space and time which constitutes a predicted violation of a given set of separation minima. (emphasis mine)

Whereas the definition of a loss of separation is:

  A defined loss of separation between airborne aircraft occurs whenever specified separation minima in Controlled Airspace are breached. Minimum separation standards for airspace are specified by ATS authorities, based on ICAO standards. (emphasis mine)

To summarize the matter, a conflict between two aircraft occurs, not when there is a loss of separation, but when there is an impending loss of separation.

Seems pretty straightforward, right?

The challenge is that little word predicted. Who predicts the converging, when and by which measurements? When does a conflict occur in the eyes of an ATC-controller or a network manager? A related question is At what distance between two aircraft will the air traffic controller get an alarm?

I think this is a rather gray area, where probabilities come into play.

It is a bit of a grey area and the answers you'll get will depend also on which field of expertise you are in. I am involved in developing and standardising ATC surveillance systems, here's my take on it.

Loss of separation is the simplest definition of the two and it has regulatory consequences. But even that one is not black and white. How do you determine the exact position of two aircraft? Suppose you have a 5 NM radar separation minimum and a radar that has an accuracy of 0.2 NM. If the two aircraft are flying parallel 5.1 NM apart at the same altitude there is a high probability that at some point the difference in radar measurements is less than 5 NM. Is that a loss of separation? To make it even more complex, radar tracking systems lag in turn detection; while an aircraft has started to turn 15 seconds ago, the tracking system may see that as radar measurement noise and may stubbornly paint a straight track depending on the tuning parameters.

The definition of conflict you quote has little regulatory meaning but has its roots in workload estimation of aircraft traffic controllers. It is also used in conflict detection systems. Short term conflict detection systems predict the path of aircraft by extrapolating the current position into the future using speed and flight plan, usually augmented by the aircraft traffic controller with cleared altitude, speed, heading etc. If within a predefined time window (approx. two minutes) two aircraft are predicted to be below the separation minimum, an alert is raised. The exact method of prediction varies from system to system. Medium term conflict detection is similar, but depends more on the flight plan than on the current speed vector. The longer term the prediction, the higher probability of a missed conflict or a false alert

The big trick is to design a conflict detection system that has a low missed alert rate (otherwise it isn't effective) and a low false alert rate (otherwise it is a nuisance and will be ignored or turned off) at the same time. It is indeed a trade-off of probabilities.


What’s the grey altitude depicted in the profile view on Jeppesen charts? What’s the altitude depicted in grey in the profile view? Why there are some approaches that this isn’t depicted and some are?

The grey areas are the minimum descent altitude (MDA) for each approach segment. Below those altitudes there are hazards in those segments. When you conduct the approach you should be aware of those altitudes and take action if you get too close to them.


Is it possible to take off on an elevated short field by "falling"? I understand this may be a dumb question to ask, but - in the opening sequence of Golden Eye - James Bond chases after a plane that has rolled off the side of a cliff in his motorcycle, and then somehow (movie magic) climbs into it and pilots it away.

I am not asking about the physics of the motorcycle stunt - except keeping it to the aircraft part:

Is this something that can happen in real life - that is, an elevated airfield so short, that the plane has to "fall" to obtain enough speed to gain lift?

Has something like this ever been attempted, even if simply as an exercise in flight dynamics?

Is it theoretically possible? Sure.
Is it realistic or probable? Absolutely not.

If there is not enough space for the airplane to accelerate to takeoff speed it will be effectively stalled when it reaches the end of the ground. That means that as soon as it runs out of ground it effectively becomes a ballistic projectile: it keeps moving forward, but it also starts to fall because of gravity. You can see this in the Goldeneye clip - the plane adopts a nose-down attitude shortly after it leaves the ground.

Eventually, given enough time, the airplane will reach a flyable airspeed (between gravity accelerating it down and its thrust propelling it forward). How far it needs to fall before that happens depends on a number of factors, but if the ground is sloping downward at an angle such that the aircraft's ballistic trajectory does not intersect the ground before it can reach flying speed you could theoretically make such a takeoff.
The obvious downside is that if things don't go perfectly in the attempt you will intersect the ground in uncontrolled flight ("crash").

In the Goldeneye scenario the aircraft is already in a fairly extreme dive when Bond manages to get aboard, so for our intrepid secret agent it's not so much a matter of getting the aircraft to a flying speed (it's probably there already) but rather of recovering from the dive before hitting the ground and without over-stressing the airframe and causing parts to break off.


Is it a literal hood that's used to simulate IMC? I know this is a basic question, but a lot of Q&As on here talk about being "under the hood" to simulate IMC.

My question is, is this a literal hood? Can someone please explain what this actually means to a non-pilot?

When you read or hear the term "under the hood" as it relates to simulated IMC, it may or may not be an actual hood. The FAA does not require a hood, instead the FAA calls for a "view limiting device." Often the view limiting device used is a hood such as the ones depicted below. However, other view limiting devices can also be used, such as glasses with an opaque coating that limits your view such as the ones depicted in the last picture below the hoods.

Checkout this "superhood" at Sporty's!

This is the type I used for my IFR certificate back in the day.

This is another type of view limiting device that can be used to simulate IMC. It is commonly still referred to as "under the hood" or as "hood-word", but as you can see these are not a hood. There is a name brand of glasses like these called "foggles." And most people refer to these as "foggles", although, these, are a different brand.


Why does V165 have two different MEAs over the same route section? Why does V165 have two distinct MEAs which vary by a large factor (apart from the 180$^{\circ}$ rule of altitudes) considering they would be clearing the same obstacles and receiving signals from the same NAV aids?

The most likely reason is terrain. Here's what the VFR sectional looks like:

You can see that the terrain rises sharply to over 3000' close to WINLO intersection, which is too high to provide IFR obstacle clearance (1000', or 2000' over mountainous areas) at an MEA of 4000'. An aircraft flying south on V165 would need to be well above the terrain before reaching WINLO, whereas one flying north from WINLO can use a lower altitude.


Is the "glory" optical phenomenon frequently seen by aircraft pilots? A glory being this optical phenomenon:

(Source)

It reveals the presence of supercooled water, a hazard for an aircraft (see answers).
Due to the way it can materialize, the center of the rings indicates the position of the observer in the airliner. So this one was not shot from the cockpit.

Does it represent a specific icing hazard?
How frequent is it?

Short Answer:

Yes, pilots frequently see this phenomenon.
No, it does not necessarily present a specific icing hazard.
Yes, in some circumstances it can be in indication of an icing hazard.

Longer Answer:

Speaking for myself, yes, I often see this. I cannot speak for all pilots, but I think many pilots do also see this. The phenomenon depends on several conditions, the primary ones being angle of the aircraft relative to the sun, and the presence of liquid water droplets.

However, I would contest the assertion that it reveals the presence of supercooled water. I see the phenomenon most often in low level fog banks or mist. I do a lot of early morning flying when the conditions are right for the phenomenon. On many of these occasions the air temperature is well above freezing, and all other weather conditions would also indicate that the water droplets are not supercooled. See the following photos from the wikipedia page showing the phenomenon in low level (probably above 0°C) fog banks:

Since the water droplets connected with the phenomenon are not necessarily supercooled, the risk of icing will depend on other factors, primarily outside air temperature. Though the "glory" phenomenon does indicate visible liquid water droplets, it does not necessarily indicate an icing risk. A thorough pre-flight risk assessment for icing conditions should answer that question on a case by case basis.

Now, the conditions in which many airline passengers might see the phenomenon may well be at an altitude high enough that the liquid water droplets may very well be supercooled. In such cases the "glory" phenomenon indicates that the cloud in question is not formed of ice crystals (which pose little icing risk) but of liquid, and possibly supercooled liquid droplets.


Are runways booked by airlines to land their planes? Suppose Indigo lands at Indira Gandhi international airport and the arrival is schedule at 5:30 a.m. Is the runway to land on fixed by the airline.

If not please explain reason behind not to schedule the runway on which the flights will land.

No one can "book" runways. Traffic on a runway is managed by air traffic control, whose job it is to ensure a safe and expeditious flow of air traffic. Generally speaking, a place on the runway is assigned on a first-come first-served basis.

You may want to explore the air traffic control tag to learn more.


Can any land-based plane be made to land on water? Can any land-based plane be converted to a seaplane by installing skids? For example, is it possible to convert a Boeing 747 to one that is capable of landing and taking off on water using pontoons or planing hulls?

Well "any" is an awfully broad statement, but the answer is "Theoretically yes." -- You would just need floats that displace enough water to provide buoyancy for the aircraft at its maximum gross weight.

For a 747 that would be impractical: We're talking about a plane with a maximum takeoff weight in the neighborhood of 700,000-900,000 pounds depending on model and variant, and that would be an ENORMOUS set of pontoons. The form drag alone would be prohibitive, nevermind the structural work required to attach the pontoons in such a way that they won't be ripped off at takeoff speed in the water or blow off the plane in cruise flight.

For smaller aircraft it's certainly do-able: You can take a Piper Cherokee 180, remove the landing gear, and install floats (Piper kit #756-746) to make it into a seaplane:

Note the structural bracing for the floats, which attach both at the main landing gear point and the nose landing gear point to enable them to take the loads imposed by taxiing through water. There are also a bunch of additional seals and blanking plates installed as part of the seaplane kit to prevent the aircraft from taking on water.

Note however that just because something is possible doesn't mean it's a good idea, and the PA-28S is an example of that: The low-wing design of the Cherokee coupled with its single door on the right side makes the seaplane version an exercise in impracticality.


How many parts does the latest-generation 737 share with the original 737? There is an old paradox which states that if a boat sails round the world replacing worn out parts as it goes, once it has replaced all of itself is it still the same boat?

I choose the 737 because it's been around for over 50 years. Specifically, is there anything from the original tooling for the 737-100 that would be a straight out replacement for a 737-MAX?

 737-100 source

 737-MAX-7 source

The cross section and cabin pressure have remained the same. From what I can tell virtually every part is different. Even parts that might have been able to retain the same shape/loading have gotten improved materials and/or corrosion treatments. I am still looking to see whether any window panels could be common as the holes into which they go have remained the same.

Looking at a couple of generations shows major differences, allowing elimination of whole sections of the aircraft. Wings, tail, engines, pylons, landing gear, fuselage center sections, APU, interior and flight deck have all surely changed.

Originally conceived as a 60 seat jet, the first production model was designed for 100 seats.

The Max10 now goes to 230.

The Max also had major improvements to the the cockpit. Even the throttle levers changed. Here is a photo comparison to the original -100 flight deck.

b737-100

b737-max

NG changes, both structural and cosmetic.

larger wing with new flaps slats
larger tail with improved rudder and elevator
simplified higher capacity landing gear
engines
new AC electrical system
new corrosion resistant materials and treatments throughout the plane
new interior seats, panels, bins, coverings
improved avionics


Is it safer to do a regular approach than a short approach? All things equal, from a standard traffic pattern at a towered airport, is it safer to do a short approach when approved by tower, or a normal one? Assume a Cherokee, Cessna 172, or similar small trainer.

My gut is a short approach should be safer, because I stay closer to the airport in case something goes wrong and I can let up on the decent rate if I hit unexpected wind shifts. However, I descend at 1000fpm in a slip, instead of 500fpm with my nose pointing to the runway. The workload also tends to be higher because there is less time.

The safest approach to a normal landing is a stabilized approach, so it's going to depend a lot on you, your aircraft, and the day's conditions. If you're operating in a confined area, then that changes things a lot, and I'm not going to address that.

It also depends on your definition of a normal pattern and approach. The FAA recommends you fly your downwind around ½ mile to 1 mile from the runway centerline, which should keep you fairly close to the airport anyway, and you should be turning base at least ¼ mile away from the runway threshold.

That said, a normal approach is going to be much safer. As you point out in your question, a short approach often requires extreme descent rates or slips (or both), and results in a much shorter final; neither of those situations improve safety. Approaching with a high descent rate just so that you can reduce it if necessary doesn't strike me as a particularly good choice for normal operations, either.


What is the correlation between RNP level and the CDI indication? I got a question regarding the connection between RNP (required navigational performance) accuracy and cdi sensitivity of the HSI.

According to the instrument flying handbook the standard for RNP on route would be 2Nm from the centerline.

So for me this means, that I will be at least 95% of my flight in that corridor (given, if raim  is available all the time)

So let's say I am off by 1 Nm enroute due to inaccuracy.

According to Rnp that would be still ok, however the question now is, what would my CDI show me?

The gps obviously does not know that it is 1Nm off course (if so why would we have such a thing like rnp of 2Nm) so the CDI should be centered even though I am actually 1Nm off the correct course.

If this is true I theoretically could be e.g. 4 Nm off course even though I got only a 2 dot deflection on my HSI given that a full deflection would represent 5 Nm because it is not WAAS capable (2Nm rnp + 2 Nm for the two dots)

Thanks for you help in advance...

The Performance Based Navigation concept allows to optimize the instrument procedure design with the aircraft navigation performance.
This concept is used en route, to reduce aircraft separation, and in terminal area to optimize arrival and departure procedures. The utmost development of Performance Based Navigation for approach, missed approach and departure is known under different names.
FAA initially referred to RNP SAAAR Operations, SAAAR standing for Special Aircraft and Aircrew Authorization Required.
ICAO now refers to RNP Operations with Authorization Required (RNP AR).

The estimation of the aircraft position is performed by the Flight Management System based on different sensors: Inertial (ADIRU), GPS, radio navigation (DME and VOR).
The FMS computes the best aircraft position estimate based on the following hierarchy of navigation modes:

GPS/inertial,
DME/DME/inertial,
VOR/DME/inertial,
Inertial only.

The RNP capability depends on the availability of GPS/IRS integrity.

The GPS integrity mainly depends on the number of received satellites, the FMS will calculate the GPS estimated error as a function of the number of received satellites and which satellites among the most appropriate for the aircraft position.

The ADIRUs are capable to evaluate their own estimated error that they send to the FMS, similarly the FMS itself is able to calculate the ADIRUs  drift error based on previous ILS landing(if any)  as he compares the ADIRU position to the real aircraft position while on the ILS, This error is reduced to a mean average drift per hour of flight.

The FMS therefore is able to calculate an Estimated Position Error (EPE), thus allowing the CDU to display both the Required Precision, and the Estimated precision

With respect to the HSI, it displays a route as well the approach calculated by the FMS, within the estimated precision which normally is better than the required precision, the pilot is supposed to be on the route and not one dot  or 2 dots apart. When the CDI is centered you are within the Estimated Error;  However indeed if the CDI is not centered the error is augmented as you say in the question, but you should not add the RNP but the Estimated NP that shows on the CDU, because the FMS calculations are normally correct.

Of course if your equipments are faulty and mysteriously the multiple faults are not detected you may be neither within the RNP nor within the ENP.

Please refer to the following website for more details:

https://www.theairlinepilots.com/forumarchive/aviation-regulations/rnp-ar.pdf


How much does it cost the airline if an aircraft misses its take-off slot? I was recently watching a TV programme and it said if an aircraft misses its take-off slot it will cost the airline company thousands, roughly how many thousands though???

A slot time is issued for aircraft flying through busy airspaces or to busy airports where the demand (number of flights) exceeds the capacity. If 50 flights are planned to arrive at an airport in 1 hour, but the airport can only handle 40 flights per hour, then the flow of traffic needs to be regulated.

Many years ago, everyone would simply depart, and then traffic would enter airborne holdings (circling in the air) along their route as they approached their destination, until there was room to land. Obviously, this is very inefficient, since planes would use a huge amount of fuel while waiting in the air.

Air traffic flow management was introduced to reduce this. Computer systems now calculate how many flights are planned to arrive at a certain airport in a certain time span. This calculation is done many hours in advance - so early, in fact, that the majority of the affected flights have not left their departure airport yet. This has the advantage that aircraft can wait on the ground, at their departure airport, soaking up the delay they will inevitable face. They will simply not be allowed to takeoff until a time where it can be assured that there is room for them when they reach their destination.

To achieve this, aircraft are issued a takeoff slot time (CTOT - Calculated TakeOff Time), at which time the flight must depart. If the flight misses this slot time, for one reason or the other, they are not allowed to depart - instead, they must request a new slot time. If flying to a very busy airport, the new slot time could be far in the future - sometimes several hours. This means that the plane must hold on the departure airport for several hours before being able to depart.

How these CTOT times are calculated is very complex, and deserves another question if you are interested.

While there are no direct costs from missing a slot time, getting delayed will cause many indirect costs for an airline - passenger compensation for delayed and missed flights, extra pay for crew (on the plane and at the airports) and so on. So the "thousands" it may cost an airline to miss a slot time come from these indirect costs of being delayed. Obviously, the exact amount will vary greatly from flight to flight.


What problems would the early F-16 have if they were to fight during night? From Wikipedia:

  Designed as an air superiority day fighter, it evolved into a successful all-weather multirole aircraft.

There is no citation for this, so I suppose first asking if this is even true should be the first order of business. However, my real questions are the following.

Assuming the quote above is true, what difficulties would early F-16s (F-16A?) have with night operations when encountering enemy aircraft?
What is it with modern F-16s that make them better suited for engaging enemy aircraft during the night? Obviously I know they have more modern avionics, but specifically, what is the difference in this night fighting context?

When the LWF program began, its founders envisioned an aircraft purebred to engage the Soviet threat with machines which more closely matched the majority of Soviet fighters at that time ie lightweight, very maneuverable day fighters ie equipped with minimal avionics, a cannon(s) and short range IR guided missiles.  Pierre Sprey had envisioned an even smaller fighter when he didn’t even have a fire control radar in it, but it was not developed past his proposal.

The prototype and development F-16s were a far cry from a modern Block 60+ F-16 is today in terms of avionics and capabilities, but the first service F-16, the Block 15 F-16A/B, had good day and night air-to-air capabilities with the APG-66 radars.  They were however limited to good weather air-to-ground operations as they were limited in their precision munitions delivery capability.

Upgrades over the next three decades of service led to the current incarnations of the Viper equipped with helmet mounted cueing systems, advanced fire control AESA radar sets, IR night and precision weapons targeting systems, BVR TARH capable air to air missiles, advanced glass cockpit displays, etc.  It must also be noted that, while these system offer addition capabilities, the added weight and complexity diminishes aircraft performance in traditional fighter metrics as well.  Nevertheless the F-16 still remains a fine fighter regardless.

The capability vs cost vs performance debate is always controversial and does not show any signs of ending soon, though the military seems to prefer the most advanced and capable hardware it can buy at the expense of operating costs and reliability.  This has been made painfully clear with the F-35 program.


Transonic buffet - reason for a rapid movements of the shockwave along the wing chord line? I know that transonic aerodynamic buffet is caused by the separated turbulent boundary layer striking the airframe (horizontal stabilizer, wings, fuselage) with considerable force causing a high amplitude vibration, which physically shakes the whole aircraft. And during a buffet, a shockwave will be apparently moving back and forth just like in the video link I provided below:

Since the shockwave is usually stationary on the airfoil, then why it starts suddenly oscillating once we enter a buffet boundary? Any ideas?

Viewing the airfoil as a rotating cylinder, the highest velocity on it's surface occurs at the top of the cylinder (90° using polar coordinates) therefore that's where the shockwave begins and hence where flow separation will start

I know that as the free stream velocity gets higher the point at which the velocity on the surface on the airfoil (cylinder) goes supersonic occurs closer and closer to the leading edge therefore the first tendency of the shockwave is to move "forwards" (towards 0° on the cylinder representation)

I also know that flow separation occurs under an adverse pressure gradient however between 90° and 0° the pressure gradient on a cylinder is favourable (due to geometry), therefore perhaps that is why the flow reattaches when the shockwaves moves <90°


Aerodynamics moment of a wing An old flight book that I am reading discusses the aerodynamic moment about a wing and talks about an upper and lower lifts. I can see how the pressure distribution on the top surface produces an upper lift force while the the pressure distribution on the bottom surface produces also a lower lift force. The upper lift force points upward while the lower lift surface points downward. Both lift forces point away from the respective surfaces.

Wouldn't local forces (whose vector sum produces the upper lift force) due to upper pressure distribution point towards the surface, not away, in the normal direction?

As the angle of attack is increased, the upper surface lift increases while the lower surface lift decreases?

It is said that no change in moment takes place about the point where
the lift change occurs. I am not sure how to interpret that either...

The lower surface lift vector points downwards in the diagram, not upwards.  The nett lift is the sum of the two lift vectors, and since the lower surface lift vector is opposite to the upper, nett lift is upper surface lift minus lower surface lift.

The nett lift points upwards in the top figure, as the upper surface lift is greater than the lower surface lift.

In the middle diagram, both upper and lower surfaces have the same lift magnitude so the nett lift is zero.  Note that the chord line of the symmetric airfoil is parallel to the airflow.  This wing section has zero angle of attack (angle between the airflow and the chord line), so both the upper and lower surfaces have the same angle of attack and generate the same lift, cancelling each other out.  On the cambered airfoil, the angle of attack is slightly negative for zero nett lift.  This is because the camber causes the upper surface to generate more lift than the lower surface for a given AoA.

In the lower diagram, the upper surface has an increased angle of attack, while the lower surface has a decreased angle of attack.  So the lift of the upper surface is increased and that of the lower surface is decreased, and the nett lift is upward.

Note that the terms "up" and "down" have to be used carefully, since we usually use those terms in the context of gravity.  A lift-producing airfoil has no concept of gravity, only of angle of attack.  The airfoils in the diagram would produce the same lift with respect to the angle of attack regardless of the orientation of the airfoil with respect to gravity.  If you turned any of these diagrams upside down, the airfoils would produce exactly the same lift with respect to the surface as they did in the original ("upright") orientation.


Low visibility spins and spirals: difficult-to-understand explanation of false perception I'm not a pilot.
In the following excellent explanatory video I understand everything except for a single point.
The presenter says that in cloud, if the instruments tell the pilot to level the wings when the plane is tilting to the left, the pilot's reaction will be to bank even more steeply to the left, because they will believe they are in a severe right-hand bank.
Spin, Spin, Spin!  2:00  (Video)
Given that, according to the presenter, the inner ear is telling the pilot that they are currently horizontal, why would the pilot assume or feel anything about which way they should correct?  Isn't it 50/50 whether they choose to roll in the right sense or the wrong sense?

Your vestibular system can only recognize acceleration. Your brain combines that with visual inputs to produce spatial orientation. (If the two disagree, you may get motion sickness.)
When you are in a sustained left bank, your vestibular system stabilizes and feels no acceleration. Without a usable visual input, your brain will convince itself that you are instead straight and level. This is spatial disorientation.
If you then look at the instruments and try to correct yourself to actual straight and level flight, the vestibular system feels that change in acceleration as starting a climbing right bank, which now feels wrong. It orders you to bank back to the left even harder to make itself feel “normal” again.
It’s not a 50/50 chance; once disorientation sets in, your body will consistently tell you to do exactly the opposite of what you should do. That’s why it’s so deadly.


In theory, can jetliners remain controllable after losing the entire tail section? The scenario here is a detachment of the tail,one with the whole empennage.
Could a variant of an existing model of swept winged jet airliner be made to remain controllable, using ailerons, differential flaps, differential spoilers and differential thrust, used for pitch, roll and yaw, regulated by an appropriate autopilot program, and with no other modifications?

No - though the vertical stabilizer might be worked around, the same is not true for the horizonal stablizer. Without it you will loose stability in the pitch axis so much that I doubt there will be any chance for a controlled flight on nearly all aircaft (though there is a possibility that a few ones might remain some kind of controllability)


What process do flight attendants and pilots follow to bid for routes? What process do flight attendants and pilots follow to bid for routes and how does seniority play into this?

Is it the same across multiple airlines/does IATA have a preordained process that airlines are required to follow?

It totally depends on the airline and is different from one airline to another. There is no standard way of distributing schedules. Some of the ways that I have personally experienced or have seen it:

Bids by seniority: The crew will bid for flights/rosters based on their seniority. Juniors will cry rivers.
Fair share: The crew will get turns, if you take a specific flight this month, next month you will not get it until everyone else gets it, and so on. Seniors hate this, they will feel unjust and keep complaining that how come a junior took their righteous flights and off.
A combination of fair share and seniority: Crew will get points, the amount of points they get depends on their seniority and other factors, then they use these points to bid for flights or days-off. For example, you can bid 100 points on getting days-off during Christmas and another 100 on getting a Paris flight during new year's eve, while someone else will throw his/her 200 points on the days-off only, and so on.

Almost all systems have some sort of giving the crew the option to at least bid for their desired off. Some other airlines just distribute the rosters randomly and then the crew will trade duties to get their desired flights and days-off, some have a system depends on the crew performance, there isn't really a standard way.

The ultimate target for airlines is to distribute flying hours among the crew equally, to avoid unnecessarily over-time costs and crew legality issues.


How can I check pitot static leak without a test box? More than once on the ramp, I've wished I could check an aeroplane's pitot-static system for leaks, but I didn't have the test box.  I've since seen, in the FAA's handbook for checking  homebuilts, a way of gently pressurizing a pitot system which is great - a length of medical tubing is all you need.

Does anyone know if a similarly simply elegant arrangement exists for the static system?  I've tried a largish hypodermic syringe to check if the port was blocked, but the challenges are:

getting to sit tight on the static port, and
holding suction.

All suggestions welcome.

There are lots of challenges to testing a static system the way you describe.

First, you need something to fit over the static port (not too hard if it's a single fuselage port, slightly more difficult if it's two of them, and if it's on a mast like on some Piper aircraft you just lost a lot of "seal area" and made the task very difficult).
You can get around that with test kits by taking the static line(s) off & screwing the test kit directly to the plumbing, or by using a variety of adapters that come with the kits.

Second, you need to produce a known absolute pressure in the line and hold it for a given duration to check for leaks.
That's most of what those fancy boxes do - they bring the line to a given pressure altitude, and then you watch the altimeter and/or line pressure to see if it leaks air back into the system.

All that said, if you're willing to deal with a really rough test, you can still do this with a syringe and some medical tubing. This RV-12 builder shows you how:

Attach a syringe to some surgical tubing.
Put some air-tight tape over your other static port (if applicable)
Put some modeling clay around the static port.
DON'T get it in the hole - make kind of a cup around the hole.
Put the surgical tubing into the modeling clay and press the clay tight around the tube to seal the chamber over the static port.
Pull on the syringe until you "climb" to a convenient altitude.
Hold or lock the syringe in place and watch the altimeter to see how long it takes for air to leak into the system and lower your altitude.

This is nowhere near as precise as proper calibrated test equipment, but it should help you detect a gross leak in the static system.


What is the suggested position for a canard? In a canard aircraft, what is the suggested position of the canard, with respect to the wing positioning? Considering the effect of down wash of the canard on the wing, what is the suggested position for a canard? Mid canard? High canard? Low canard?

There are two general configurations for canard wings, the long coupled and the short coupled canards. From Torenbeek:

Long coupled canard. This configuration is designed for minimal drag in cruise. The canard wing is placed far forward of the main wing in order to reduce interference drag. Vertical position is higher than the main wing to reduce adverse effects at high angle of attack.
Short coupled canard. The canard wing is again higher than the main wing: the interference is deliberately used to increase maximum lift (at the cost of high drag), achieved:

at large angles of attack;
on surfaces of low aspect ratio with highly swept back leading edges.

Quoting from Torenbeek:

  In short, the canard layout appears to be suitable for transonic or supersonic and highly maneuverable aircraft, in the latter case if sufficient thrust reserve is available


Which tail type is better in order to obtain higher speed? If I want to design an aircraft that can fly with high speed while loaded, which tail type should I choose.

The mission is to design an aircraft that can move a certain distance with high speed while carrying certain load.

Lose the tail.
By Staff Sgt. Bennie J. Davis III [Public domain], via Wikimedia Commons

Split brake-rudders will do the trick.

You can also try and beat NASA and Boeing to the Active Flow Control tails.

Or just settle for a T-tail.

The horizontal tail plate effectively increases the aspect ratio of the fin by virtue of the 'end plate' effect. The T-tail configuration is especially popular on gliders.

(Flight 1977—PDF) Early Boeing 757/767 designs featured T-tails to reduce drag.


When cruising, is any moment that a pilot will reduce his/her airplane speed? Say, a pilot is piloting his/her jet commercial airplane and suddenly see cumolonimbus or a dark cloud. Then, should the pilot reduce his/her airplane's speed to avoid turbulence?

What you're worried about is hitting turbulence. The word "impact" isn't typically used, although hail impacting an airplane has on relatively rare occasions damaged airplanes in flight and even more rarely has brought them down. My experience was that it just makes a lot of noise on the windshield.

Your question title is more inclusive than the body. Taking the narrower question in the body first— yes. If a pilot is expecting or experiencing severe turbulence, he should lower the cruising speed if that speed is above the recommended turbulence penetration speed. And in lesser degrees of turbulence, some might choose to lessen the speed if they believe that doing so will give passengers a less uncomfortable ride.

Now as to "any moment" and interpreting that as "any time", there are a few instances I can think of offhand.

I knew a few pilots that, when they were paid by the hour, would sometimes reduce speed, although I think that they would usually just not bring the aircraft up to the normal cruising speed rather than get that speed and then slow. To me that was unprofessional. I flew with one such captain a few times, and on my legs if I put the airplane (747-100/200) at to mach 0.86 for cruise, he'd ask for it to be slowed.
In specialized and rare instances, it might be desirable to reduce your speed to be a little more fuel efficient. For example, JFK to Tel Aviv in a 747-100 would usually require a redispatch at a fix abeam Athens. To be legal you had to have a certain minimum fuel at the redispatch point before proceeding to Tel Aviv, otherwise you'd have to land at Athens for fuel. So, if approaching the redispatch point — say an hour or two away from it — you might choose to slow to have a little more fuel on arrival at the redispatch point if you thought not doing so would result in a little less fuel than required.
Then there's the instance where you know you're going to be thrown into a hold on arrival at the destination so you may as well slow a little getting there, burn a little less fuel, have to hold a shorter time. You have to be careful with this one though, because it might mean others get into the hold sooner than you would have, and all you had done is put yourself farther back in the line than you would have.
And, of course, ATC may ask for a cruise speed reduction to minimize excessive flow at the destination.


What class is the airspace at KGCN when the tower is closed? I was wondering if someone could help me figure out how the airspace around the Grand Canyon National Park Airport (KGCN) works. The chart supplement here says:

  AIRSPACE: CLASS D svc 1 Jun thru 30 Sep 1300–0300Z, 1 Oct thru 31 May 1400–0200Z other times CLASS G.

... but the Grand Canyon VFR chart here says:

  See NOTAMs/Directory for Class D/E (sfc) eff hrs

The VFR sectional just shows a class D ring around the airport with a class E surface area extension to the southwest:

When the tower is not occupied, how is the airspace around KGCN arranged?

It may be an error on the chart, it should be either Class D or Class G, but it depends...

If the ATIS reverts to ASOS when the tower is closed, the airspace can be Class E. If ATIS/ASOS is not available, then the airport has to be Class-G. I would stick with the chart supplement here:

June 1 through September 30th, the tower is manned from 1300Z to 0300Z and is Class D. October 1 through May 31 the tower is manned from 1400Z to 0200Z and is Class D. All other times it is Class G meaning they do not have ASOS available. (AWOS is available by phone).

So according to the chart supplement, if the tower is not occupied the airspace is Class G.


Redundancy in Integrated modules avionics What kind of hardware redundancy is used in integrated modules avionics based architecture for EFIS displays running on a single processor?

My question is specific to redundancy of display hardware driving three different displays. I'm not sure if such kind of thing is implemented before, but I'm trying to run three different displays with a single processor.

Your question is originally asking for "hardware" redundancy, of which there is none. However, there is operating system and software redundancy built to alleviate hardware issues.

Typical computer based avionics like the EFIS must be FAA-compliant with ARINC 653. Of course, in an experimental aircraft, you could potentially get non-compliant avionics, but there's not much of a market for them.

ARINC-653 requires that there be an real time operating system layer implemented as a form of redundancy. This is a special "round-robin" form of operating system, which in layman's terms, means that the operating system will only spend up to a fixed amount of time on a process before hopping to the next one, meaning that a single process hanging will not halt the rest of the partitions. You might have a navigation process that dies, but the GPS process can continue to operate as it is in a separate partition. It also has partial requirements that partitions are not dependent on each other, but instead retrieve their data directly from sensors. This gives another layer of redundancy in case a dependency crashes.

Running three displays off one processor is ill-advised, and would never be FAA-compliant, but it is technically possible. You would however have to heavily modify the operating system, and the "Guest OS" (ARINC 653) layer.

I'm happy to go far more indepth into this on StackOverflow if you choose to ask a follow up answer there, but the technical specifications are a bit out of scope for the Aviation StackExchange.


What is the recommended pressure for refueling an Airbus A380? What is the recommended pressure on the hose that refuels an Airbus A380 when fueling this aircraft? How many litres per minute does it pump into the tanks. Can it safely be increased to reduce fueling time?

The A380 airport planning manual states (emphasis mine):

  Refuel pressure:
  - Maximum pressure: 50 psi (3.45 bar).

Can it be safely increased? Doubtful. Any increase may damage internal aircraft systems, and not only that, the operators will need to ask the airports to install new refueling systems.

Generally:

  For pressure refueling, a high-pressure hose is attached and fuel is pumped in at 275 kilopascals (40 psi) and a maximum of 310 kilopascals (45 psi) for most commercial aircraft. Pressure for military aircraft, especially fighters, ranges up to 415 kilopascals (60 psi).

As for the rate, from the graphic and the text:

  242 700 l (64 115 US gal) at 40 psig.

242,700 l take 36 minutes, or ~6,700 l/min.


What is Neil Armstrong teaching in this image? It seems to me that in this image Neil Armstrong is teaching something related with aerodynamics forces (lift, drag...), but I don't know exactly what is that "Triangle" that He drew (It doesn't seem to be an aircraft).

1)  The triangle shape represents an aircraft-- imagine something like a supersonic version of a Gee Bee racer.  The thing at the leading edge of the vertical fin is the canopy.

2) Here is a sketch that offers an educated guess about parts of the diagram that are either cropped, or hidden behind Neil Armstrong, in the original photos.  Also, for added clarity, labels have been added to the dashed lines representing the horizon, and the aircraft's longitudinal axis.  Note that for the diagram to make sense, the wings must be attached with zero incidence relative to the longitudinal axis of the vehicle, or the vehicle may be a flat-bottomed wingless lifting body.   Only the green vectors represent actual forces.  "L" is the lift vector, "D" is the drag vector, "W" is the weight vector, and "T" is the thrust vector.  Note that "D" is parallel to the flight path, "L" is perpendicular to the flight path, and "W" is perpendicular to the horizon. "T" is drawn in this sketch as nearly, but not quite, parallel to the aircraft's longitudinal axis-- in the original, the direction of the "T" vector is about midway between the direction of the aircraft's longitudinal axis and the direction of the flight path.  No attempt has been made in this quick sketch to make the force vectors add to zero.  We can't see enough of the original to guess whether the force vectors as drawn there would add up to approximately zero or not.

3) The vector labeled "flight path" in this sketch was labeled "V" (for "velocity") in the original.  It represents the aircraft's instantaneous direction (and speed) of travel through the airmass.

4) In the original, the line representing the orientation of the longitudinal axis of the aircraft was simply an extension of the flat bottom of the aircraft; it ended up getting displaced upward a bit in this sketch. This wasn't intentional but does have the effect of causing the line to pass closer to where the aircraft's CG would likely be. The angles of interest can be represented accurately regardless of whether or not the line representing the orientation of the aircraft's longitudinal axis actually passes through the CG or not-- we just need a reference line that is parallel to the aircraft's longitudinal axis.

5) Another detail that ended up different from the original in this quick sketch is the order of the angles illustrated on the left-hand side of the diagram.  In the original, running from right to left, the order is gamma, alpha (this letter has been rotated about 45 degrees clockwise so it looks almost like an upside-down gamma), theta, while the order in the attached sketch is theta, gamma, alpha.  This has no effect on the meaning of the diagram-- the same greek letters are still associated with the same illustrated angles. A confusing point about the upper photo is that it is cropped in such a way that at first glance the angle labeled "theta" appears to duplicate the angle labeled "gamma", but of course this is not actually the case.

6) It appears that the curved line represents the surface of the earth, the overlined "R" represents the radius of the earth, and "h" represents the aircraft's height above the earth's surface.  Neil is probably explaining that at very high speed, orbital effects start to come into play, reducing the need for lift.


Is a releasable locator beacon (ELT) required for aircraft carrying passengers over a water area? Is an Emergency locator transmitter - ELT required on any type of aircraft carrying passengers over water? Does it need to be fitted with an hydrostatic release unit so that it doesn't sink with the aircraft?

If they don't carry a releasable ELT, how are aircraft planned to be located after an accident over the ocean?

References to regulatory documentation definitely a plus (e.g. for the US, or for island countries like Australia or New Zealand).

Is an ELT required on any type of aircraft carrying passengers over water?

Generally, commercial transport aircraft are not required to carry an ELT.

  how are aircraft planned to be located after an accident over the ocean?

I believe pilots are expected to use radio to inform controllers of any emergency.
If ATC are unable to contact an aircraft for an extended time, they instigate S&R based on the last known location of the aircraft.
The FDR and CVR are required to include an underwater location beacon (ULB), though this is only short range and has limited duration.
It is expected that, sometime in the future, commercial transport aircraft will be required to have automated systems that report their position frequently (perhaps every 15 minutes)

References:

IATA - Aircraft Tracking Task Force (ATTF)
ATTF FAQ
ICAO Normal Aircraft Tracking Implementation Initiative


What is the difference between P208 and P209 base runway materials? Runways are made with P208 base or P209 base does anyone know why there are labeled differently from regular crushed aggregate base? And what the different uses are for?

From the FAA Airports Construction Standards Part 3 ('flexible base courses');

Just a side note that these are just little introductions to the materials. Download the word document from the linked page and just read that if you want.

  P208: This item shall consist of a base course composed of course aggregate bonded with fine aggregate base. It shall be constructed on a prepared subgrade or subbase course per these specifications and shall conform to the dimensions and typical cross-section shown on the plans.
  The aggregate base material shall consist of both fine and coarse aggregate.  Material shall be clean, sound, durable particles and fragments of stone or gravel, crushed stone, [   crushed slag,   ] or crushed gravel mixed or blended with sand, screenings, or other similar materials produced from approved sources. The aggregate shall be free from lumps of clay, organic matter, and other objectionable materials or coatings.

And for P-209

  This item consists of a base course composed of crushed aggregate base constructed on a prepared course in accordance with these specifications and in conformity to the dimensions and typical cross-sections shown on the plans.
  Crushed aggregate shall consist of clean, sound, durable particles of crushed stone, crushed gravel, [   or crushed slag   ] and shall be free from coatings of clay, silt, organic material, or other objectionable materials. Aggregates shall contain no clay lumps or balls. Fine aggregate passing the No. 4 (4.75 mm) sieve shall consist of fines from the coarse aggregate crushing operation. If necessary, fine aggregate may be added to produce the correct gradation. The fine aggregate shall be produced by crushing stone, gravel, [   or slag   ] that meet the coarse aggregate requirements for wear and soundness.

You can download the Word file for more info on this, if you want to.


Can I file a VFR flight plan for a training flight? Let's say you are going to do some maneuvers in a practice area, make a few touch and go landings at a non-towered airport, then return to your home airport...is it possible to file a VFR flight plan for this?

Absolutely. In fact it's not a bad idea to do so, if only to get some practice filing flight plans and using the system.

In the US VFR flight plans are a search-and-rescue tool: They're only used if your aircraft is overdue to give the rescue folks information to help plan the search.

In your particular case I would file a flight plan that lists your home airport as your departure and destination, with the airport you intend to use for your touch-and-go landings as an intermediate waypoint.  If you want to enter information on the nature of the flight (training, touch-and-gos at XXXX, etc.) in the remarks field you can do that too.

Activate the flight plan when you depart, and close it when you return to your home airport. You can optionally also call Flight Service over the radio when you get to the touch-and-go airport to give them a position report and let them know you've reached that waypoint (I would do this on your way out of the area to return home, along with an ETA update if appropriate).

As an alternative (or in addition to the VFR flight plan) you could get flight following from the ATC facility responsible for your airspace. Be aware that they may drop you and ask you to squawk VFR while you shoot your touch-and-gos since you'll probably be going off their radar, but you can always request service again when you start your return leg.


Why doesn't the US have a flag carrier airline? With the recent United Airlines fiasco, I was reading up on their history and the airline industry and to my surprise I found that United Airlines is not the US flag carrier. In fact, the US does not have flag carrying airline at all! All this time I thought UA was the national carrier for the US. This brings me to the question, why? Why doesn't the US have a national carrier? Is this because of some FAA regulation?

The definition of a "Flag Carrier", although somewhat vague and frequently disputed, is generally accepted to mean: an air carrier that is owned by, or subsidized by, the government of the country of registration, especially for the purposes of flying international routes.

Wikipedia:

  A flag carrier is a transportation company, such as an airline or
  shipping company, that, being locally registered in a given state,
  enjoys preferential rights or privileges accorded by the government
  for international operations. The term also refers to any carrier
  that is or was owned by a government, even long after their
  privatization when preferential rights or privileges continue.

Business Dictionary:

  Airline ... registered under the laws of a country whose government gives it partial or total monopoly over international routes.

The US has no carriers in this category. All subsidizing of major domestic and international routes ended with the Airline Deregulation Act of 1978. (Note: there are still some small airlines and routes that are subsidized to serve very small communities... If you want to read more about this program, it's called Essential Air Service.)


How can ADS-B replace primary radar when FlightRadar24, using ADS-B, is so inaccurate? I read the following on Flightradar24.com's 'How it works' page.

The percentage of aircraft equipped with ADS-B receivers is steadily
increasing though, as they will become mandatory for most aircraft
around the world by 2020. When mandatory, ADS-B will replace primary
radar as the primary surveillance method used by ATC.

Sure, that sounds lovely, but it it seems to me that a large portion of aircraft do not actually use GPS, but instead rely on inertial navigation systems or similar. Of course, the ATC doesn't need millimeter precision, or even an accurate position within a kilometer, but whenever I check flightradar24, there is quite a large bunch of airplanes reporting their position with offsets up to a few kilometers - if I were to judge by the ADS-B data, I would report a major emergency every few minutes.
How will ADS-B actually reliably replace primary radar? Will GPS have become mandatory by 2020? Or is Flightradar24.com just being optimistic?
(Side note: I know that redundancy is very popular in the aviation world, but I don't think you can call something 'replaced' when primary radar is still used for a large portion of the flights)
Edit: some parts of some answers seem to be concentrating on update rate / transmission losses and errors. For clarification, I included a screenshot (which I could literally take the second I looked at fr24.com, indicating how often this happens).

At the time this picture was taken, the 36R runway was in use for landing ('yellow' aircraft top right just landed). The selected aircraft is however happily taxiing in a field (pattern matching 36R's adjacent taxiways). This is not caused by update rate or transmission losses (i.e., this can't be old information, the plane was never in that field). Transmission error also seems out of the question, since the entire track is a smooth line (errors sometimes do happen, but are not 'hidden' on fr24 - a large 'jump' in the track can be seen sometimes). So either the plane is reporting a 'wrong' position (how is this handled by ATC?) or fr24 somehow messes up the data (how is that possible?

TLDR

How will ADS-B actually reliably replace primary radar?

It will not.

Will GPS have become mandatory by 2020?

Strictly speaking no, effectively yes.

There are so many problems with that quote from FR24 that I don't know where to start.
Let's take it apart and analyse it sentence by sentence.

The percentage of aircraft equipped with ADS-B receivers is steadily increasing though

Totally irrelevant. An ADS-B receiver allows reception of ADS-B signals from other aircraft. These can be used for example to create a Cockpit Display of Traffic Information (CDTI) or augment TCAS. The number of aircraft with this so called ADS-B IN is increasing, but the equipage percentage is probably below 2% at the moment.
If you are concerned with using ADS-B operationally for ATC then airborne receivers are not of interest. Instead you should be looking at the transmitter side.

[ADS-B receivers ...] will become mandatory for most aircraft around the world by
2020.

ADS-B receivers are not mandatory in any foreseen regulation yet. ADS-B transmitters are / will become mandatory in many parts of the world. The EU and USA rules mandate ADS-B in 2020.

When mandatory, ADS-B will replace primary radar

ADS-B is not intended to replace primary radar. As a new addition to the surveillance technology mix, it will not replace any other technology in the short term.
ADS-B is a form of cooperative surveillance, meaning that the aircraft or vehicle that is supposed to be tracked needs to cooperate by transmitting the ADS-B signal. Primary Surveillance Radar (PSR) on the other hand is a non-cooperative surveillance technology; it works by detecting signals reflecting off the target of interest. PSR can detect aircraft that do not transmit any signals. Since PSR and ADS-B are fundamentally different in their working, they have fundamentally different use cases. ADS-B will not replace primary radar and never was intended to.

will replace primary radar as the primary surveillance method used by ATC

Primary Surveillance Radar is not the primary surveillance method used by ATC in many parts of the world. Since PSR only gives azimuth and range measurements essential information like aircraft identification and altitude is missing. Therefore Secondary Surveillance Radar (SSR) is the primary surveillance method used for ATC since it provides altitude and identity in addition to azimuth and range measurements.
Secondary radar, like ADS-B is a form of cooperative surveillance; it relies on the aircraft replying to an interrogation from the radar. ADS-B can and will replace a number of secondary radars, but not all of them.

Now for the rest of your question.

it seems to me that a large portion of aircraft do not actually use GPS, but instead rely on inertial navigation systems or similar.

Indeed a large number of aircraft do transmit their position on ADS-B that is based on a non-GPS source, usually from the INS. INS can not provide the accuracy and integrity that GPS can provide and therefore you see large position errors on FR24. The fact that the position is not to be trusted is reported in the ADS-B message in the quality indicators, but unfortunately FR24 doesn't use this information.
All the ADS-B mandates require that the system is able to provide high integrity data, which effectively means that GPS is required as position source, although theoretically other means are possible as well. Therefore in 2020 all aircraft subject to the various ADS-B regulations will have to be GPS equipped.

Of course, the ATC doesn't need millimeter precision, or even an
accurate position within a kilometer,

ATC does not need millimeter precision (GPS wouldn't provide that anyway) but they do need position accuracy within a kilometre and sometimes within about 10 meters (e.g. runway entry stop bar monitoring). ADS-B  with appropriate GPS source will be able to provide that.


Where does turbine vane and blade cooling air come from? Some large jet engines have a high combustion temperature to increase maximum thrust. First vanes and blades of the turbine must be cooled to prevent them from melting. One method is to circulate cold air from the compressor inside the element and/or create a coating of air on the element faces using channels and holes.

Trent turbine HP blade with cooling channels, source

How is air from the compressor channeled to the vanes and blades? How is cooling air prevented to escape to the space between rotor disks instead of going to the cooling channels, under turbine gases pressure?

What approximate fraction of core power is used for such cooling?

To build upon a few points in Peter's answer:

This diagram will probably help visualize it (I copied from this question How is the central hub / shaft casing of a two-spool jet engine assembled?):

The seal between the disks is just a labyrinth seal.  Those chambers, however, are pressurized (by similar mechanisms as the cooling air was delivered to the blade). So there is a tiny positive flow of air out of that chambers, which prevents the hot combustion gases from flowing in.  Refering to the diagram, the space in front of the 1st stage disk is pressurized from air coming from the stg 10 compressor through the series of passages colored in orange.  The space between HPT stage 1 and stage 2 is pressurized by air the comes along the inner diameter of the HP rotor, from an impeller between stage 7 & 8 compressor (in blue), and the space aft of the HPT stage 2 is pressurized from air the comes from the stage 4 compressor (by means of pipes on the outside of the engine, light blue color).

Pretty much every cavity/chamber inside the engine is pressurized somehow, to keep air from going in undesired directions.

I don't know exactly how much air is used for cooling. It's on the order of 1% (i.e. I know it's less than 10%, but certainly more than 0.1%)


Are there ground stations at the waypoints a pilot may fly to? How is a waypoint marked? Is there a ground station present at that waypoint? And how are these waypoints named?

Ground based radio navigation (VOR, VORTAC, LORAN, NDB) by necessity have ground stations to handle generation, amplification and broadcasting the signals used for navigation.  Here is an example of what a VOR station looks like:

"Table Rock VOR" by ZabMilenko - Own work. Licensed under CC BY 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Table_Rock_VOR.jpg#/media/File:Table_Rock_VOR.jpg

Waypoints that represent these stations will have ground facilities underneath them, but they are the minority of waypoints.  Most waypoints are defined as a distance along a radial from one of these stations, so there will be no ground station below them.  Other waypoints are lat/lon fixes provided by GPS, so there are no stations below them.

Keep in mind, no matter if there is a ground station or not below a waypoint, their presence is not important to how navigation is performed.  Waypoint passage will be noted by needles, to/from indicators, DME mileage changing, or by a points along a magenta line on a map display.

VOR stations are often named by their location e.g. the VORTAC in San Antonio, TX is named "San Antonio (SAT)".  Others are named for regional things, e.g. "Ranger (FUZ)", "Cowboy (CVE)" and "Maverick (TTT)" all located in Dallas, TX named after their major sports teams (baseball, football and basketball respectively)


What is the minimum and maximum number of jet bridges used on an Airbus A380? What is the minimum and maximum number of jet bridges used for embarking and disembarking on a typical Airbus A380 trip and why?

(Airbus)

The typical is two, one for each level. Three is not uncommon. Any more and the jet bridges will encroach on the adjacent gates, reducing the number of available parking spots at terminal. The right side is busy with other servicing equipment.

Also note the aft-left area being used for catering.

(gulfbusiness.com)

Why do passenger embark on the left side of an aircraft?
Why aren't planes loaded from both ends?
How long does it take to refuel a big jumbo jet?
Airbus patent for detachable cabin (patents.google.com)


Is it possible to upgrade a Boeing 747-8 and Airbus A380's engines for greater speed? Is it possible to upgrade the Boeing 747-8 and Airbus A380's engines (upgrading to a newer engines) for greater speed, if the engines were more efficient and used less fuel at higher speeds relative to distance?

If so, what are the maximum speeds that the airframes or entire aircraft could handle or are designed to handle?

I just wanted to comment on this question since some of my code is on the A380's GE engine.

Those engines are optimized for fuel efficiency at their cruising altitude and speed.  The tradeoff that you fight against when designing the engine for cruising altitude is that it makes the engine hard to start, and the stall margin (the pressure and fuel combo you need to keep the engine from stalling) is razor thin when they are on the ground.

Just to echo what Kevin said, any improvements to allowing the engine to start without stalling would surely go to the fuel efficiency, not to speed.


Can I log PIC time when training for a tailwheel or high performance endorsement? If I hire a CFI to train me for the tailwheel endorsement (this question also applies to the high performance endorsement), can I log PIC time when flying the tailwheel aircraft, or do I have to log dual received?

This question is specifically about FAA rules related to training/logging time, but if there are different rules in different countries, feel free to answer about those too.

Yes and Yes; As long as you are rated for the category, class, and type (if required) of the aircraft that you are receiving the training in you could log both PIC and dual received.

14 CFR 61.51(e)(i) covers the logging of PIC time (which is perfectly legal in the scenario that you describe if you are rated in the aircraft):

  §61.51   Pilot logbooks.

  ...

  (e) Logging pilot-in-command flight time. (1) A sport, recreational,
  private, commercial, or airline transport pilot may log pilot in
  command flight time for flights-

  (i) When the pilot is the sole manipulator of the controls of an
  aircraft for which the pilot is rated, or has sport pilot privileges
  for that category and class of aircraft, if the aircraft class rating
  is appropriate;

Training time is logged per 14 CFR 61.51(h) and you can log that too:

  (h) Logging training time. (1) A person may log training time when
  that person receives training from an authorized instructor in an
  aircraft, flight simulator, or flight training device.

That being said, you may not act as the actual PIC however:

  §61.31   Type rating requirements, additional training, and
  authorization requirements.

  ...

  (i) Additional training required for operating tailwheel airplanes.
  (1) Except as provided in paragraph (i)(2) of this section, no person
  may act as pilot in command of a tailwheel airplane unless that person
  has received and logged flight training from an authorized instructor
  in a tailwheel airplane and received an endorsement in the person's
  logbook from an authorized instructor who found the person proficient
  in the operation of a tailwheel airplane. The flight training must
  include at least the following maneuvers and procedures:

  ...

Notice that the regulations governing acting as PIC are different than those governing logging of PIC time.  This was done intentionally because there were so many pilots that were hired as SIC's at airlines with little to no PIC time and flew for years. When it was time for them to upgrade they didn't have enough PIC time even though they had lots of experience and could easily upgrade safely.


Why is there a need for a Top Altitude in FAA's SIDs? I am examining some of the FAA's SID charts and, as far as I can deduce, each SID provides information concerning the altitude when it is necessary and, if not changed by the controller, each transition/route description will end with something such as "Maintain X altitude etc." which seems that it is the same as the Top Altitude.

Since this information is written in SID's textual description as "Maintain X altitude", why is there a need to exist also separately as Top Altitude?

It is needed for the new climb-via instruction (c. 2014).

Instead of issuing different maintain altitudes to the aircraft based on the SID, direction, etc., a top altitude is charted. From the FAA AIM:

  SIDs will have a "top altitude;" the "top altitude" is the charted "maintain" altitude contained in the procedure description or assigned by ATC.

Related:

Do Jeppesen SIDs have top altitude?
What altitude should you cross DBIGE at on the PALLEY FIVE Departure (SID)?
Are 'climb via a SID' instructions actually used in the US?


When does ATC contact the pilot for a frequency change? Usually, when the aircraft enters a new flight information region for example, ATC will ask the pilot to enter a new frequency. I'm curious about when they do. Is there a rule about it (in an IFR situation)? I assume it may be different between EASA and FAA regulations.

Thank you.

The entire sky and all airports are divided into ATC areas of responsibility. Whenever a flight moves from one area of responsibility to another, the pilot needs to change frequency to talk to the next controller. A flight can only be under the control of one single ATC unit at any one time, which usually equates to being on one single radio frequency. When under ATC control, pilots do not change frequencies unless instructed to do so.

For the en-route phase, airspace is divided into sectors, defined as three dimensional areas. Each sector will typically have its own dedicated frequency, although sectors can be combined in which case several sectors may operate on the same frequency.

On the ground, depending on the size of the airport, there can be multiple different frequencies. It is common to have one tower frequency per runway, and then one frequency for the taxiways - or possible several, if the airport is large and complex.

ATC will instruct the pilot to change to the next frequency at the latest when the flight enters the new area of responsibility, and often a minute or two before that. At that point, the flight will already have been coordinated between the controllers, so the controller of the downstream area of responsibility knows it's coming. Specific rules for transfer of communication is agreed between adjacent ATC units in letters of agreement.

An example from Copenhagen:

AIP Denmark EK AD 2 - EKCH - AREA OF RESPONSIBILITY

Each color depicts an area of responsibility, and (this is a bit simplified but) essentially one specific frequency that pilots should be on when they are within that area. You can see that each runway (roughly) has one frequency, and the apron area where aircraft park has another. When taxiing from one area to another, the controller will instruct the pilot to change to the next frequency.

In the air, it can look something like this:

AIP Denmark EK ENR 6.2 - 3

The dashed lines indicate borders between areas of responsibility, and the bold text indicates the frequency to use in that area. Again, the controller will instruct the pilot to change to the next frequency when crossing from one area to another.

Further reading


Why are some ATP CTP providers not on the FAA's approved list? The FAA has published a list of ATP CTP provider schools. But I can still find other schools not listed in this list, like "ATP flight school".

I'm sure that only approved facilities can provide an ATP CTP course. What's happening here?

The FAA's ATP-CTP Advisory Circular 61-138 does not prohibit a qualified ATP-CTP instructor using an approved training program from delivering the training at any facility that is willing to pay for their services.


MIL-PRF-5606J, What does "J" means? Does it equal to MIL-PRF-5606? MIL-PRF-5606J, What does "J" means? Does it equal to MIL-PRF-5606?
Please let me know it they are same specification or not.

I don't know this specification, but the letter at the end indicates the revision number of the document. J would be the 10th revision/ 11th edition.
Note that there are often also intermediate publications with minor improvements called 'Amendments'

Version J was published in March 2018.
The version before, H, was published in June 2003 with Amendments in 2003, 2005 and 2006, and an additional notice in 2011.
(source)

Referring to MIL-PRF-5606 can thus mean the first edition of the specification, or it can be a generic reference where the edition is omitted.

It depends on the context, but since the original publication must have been ages ago, my bet is on it being a generic reference.