Why is air traffic control done manually by humans? I'm not a pilot but have a software developer background, so forgive my aviation naivety.

From what I understand there have been numerous issues and almost-issues relating to human air traffic controllers. Not to mention there is considerable cost and complexity required for this delicate system to work perfectly all the time.

My question is simply why do humans manually attempt to guide all aircraft in real time? Has computerized air traffic control ever been suggested, and maybe even attempted? What was the problem?

Before anyone says that air traffic control is computerized; that's not what I mean. I mean fully computerized, with digital communication between the aircraft and a central computer on the ground which continuously updates the aircraft FMS, OANS (On-Board Airport Navigation System) or similar, and autopilot with all the required information and directs aircraft as needed with optimal real time sorting and queuing.

As for more simple aircraft without FMS, OANS or other suitable systems, perhaps they are better suited for having human guidance for the foreseeable future. A speech to text interface to a computerized tower would probably be disastrous.

It seems, (probably naively) to me, that this would eliminate a lot of latency, a lot of cost, and a lot of complexity. As an aside, this appears would also give the pilot contextual awareness if desired (I cannot imagine this would be useful during normal operations), as the OANS would be able to display everything the tower can see.

Having it explained to me briefly what exactly the tower does that cannot be handled by such a system would probably enlighten me somewhat.

My own reasoning for why this isn't a good idea:

The only thing I can think of is, I can imagine that sometimes information is relayed to the tower that requires some use of context and best judgment that would be hard to implement in full in software. Perhaps there is an emergency, and the pilot needs information about advice for some ad hoc emergency landing.

However, if this is the problem, the extreme cases where a human is preferred, why not keep a few humans around, similarly to how aircraft without OANS would perhaps need humans, just in case one is needed. The vast majority of the mundane work that seems (again probably naively) incredibly better suited for a computer to deal with, be handled by, a computer?

Having it explained to me briefly what exactly the tower does that cannot be handled by such a system would probably enlighten me somewhat.

Fair warning, that's my job you're trying to replace with a computer, so I'm probably biased. Still though, hear me out:

What do we do?

Handle a large amount of non-standard requests and communication between the cockpit, the airport, the airlines, flight schools, fire and rescue, authorities and many other parties
Deal with vehicles or even individual persons moving on the maneuvering area, where the only available communication is an (analogue) handheld radio
Identify obstructions or animals (such as birds) on and around the airport
Deal with smaller general aviation aircraft with no sort of digital communication, only a radio. Sport aircraft, gliders, hot air balloons.
Deal with individual aircraft without transponder (not visible on radar). A digital system would have no way of knowing where the aircraft are.
Handle traffic at entire airports without radar coverage. A digital system would have no way of knowing where any aircraft are.
Deal with aircraft with no flightplan (only way to know what the pilot wants is to talk on the radio)
Make decisions not only based on what is said, but also how it is said. It's quite easy to notice an inexperienced trainee pilot on the radio, and they need a bit more space and might not always do what they're told
Setting the runway lights to an appropriate level based on a number of factors (darkness, visibility), some of which are not easily measured digitally
See, visibly, when an aircraft enters or leaves the runway (most airports don't have ground surveillance, so the only way to determine the position of aircraft is to look out the window)

Could these things be solved/handled by a computer? In theory, sure. You could:

Invent some sort of artificial intelligence that intuitively knows exactly what someone means as they say it, and know who to forward the message to.
Replace all aircraft vehicles worldwide with fully autonomous, self-driving Teslas that can communicate with a central control system.
Install a number of high definition cameras on every airport worldwide, and use artificial intelligence to classify visible objects. Then alert the self-driving Teslas to go and scare the birds away.
Force millions of hobby pilots and flight schools worldwide to invest thousands of dollars in new equipment that can communicate with a new digital ATC system (before that: actually invent that equipment, and get it certified worldwide)
Force millions of hobby pilots and flights schools worldwide to invest thousands of dollars in transponders. Invent new light-weight batteries that enable the installation of transponders in light weight aircraft, gliders and hot air balloons.
Install thousands of new radar stations worldwide. Invent a way to deal with garbling and FRUIT caused by all the new radars.
Force all pilots to file a flightplan prior to flying. Change the flightplan format so that it is much more accurate than it is today, and teach all pilots worldwide the new format. Force all small, single-engine planes to always be flown by two pilots: one will fly the plane, the other will be busy on the iPad, changing the flightplan in real time as the flight progresses, since a short message over the radio is not an option (a digital system wouldn't understand it)
Assume the worst case scenario for all flights. Add extra separation between all aircraft. Reduce capacity at many airports by 50%, thus increasing delay by millions of hours worldwide.
Install a light control panel in the cockpit of every aircraft in the world. Teach all the pilots to use it, install redundancy systems in case it fails.
Since we've already made everyone buy transponders, why not install ground radar in every airport in the world? Will probably only cost a few million dollars per airport.

Last but not least, invent the system that actually processes all this data, to replace the decision making that is today done by air traffic controllers. Have it work in every single edge case, in every airport, in every type of climate and weather conditions, with every aircraft type, with every type of flight - and have it certified by aviation authorities worldwide.

Anyway, I think you see where I'm going with this. Is training air traffic controllers expensive? Yes. Do humans make mistakes? Yes. Would creating and getting certified a new digital system be cheaper? Absolutely not. Would it be safer? In the long term, maybe marginally. In the short term - most likely the opposite.


How to calculate the amount of thrust of a autogyro during auto rotation? When a autogyro/gyrocopter is in a horizontal steady flight the rotor should be able to provide enough thrust using the auto rotation principle if the horizontal speed can be regulated by the vertical propeller.

I was wondering how the amount of thrust provided by the main rotor of a autogyro during auto rotation can be calculated. All the characteristics of the autogyro and his blades are known.

The horizontal speed of the gyrocopter can be regulated by additional engines and they do not need a specification.

In auto-rotation, either flying level, or with a constant sink speed, the vertical component of the thrust provided by the rotor is exactly the same as the weight of the gyro...

In Wikipedia  https://en.wikipedia.org/wiki/Disk_loading#Momentum_theory  you can find the derivation of the following expression:

$T = 2v^2· A·  rho$

where T is the thrust of the rotor, v the sink speed in vertical auto-rotation, A the 'disk area' (that swept by the blades) and rho the air density.


Does Global Entry give you any benefits when flying GA? If I fly GA into Canada from the US or into the US from Canada, does having Global Entry give me any advantage? Can I taxi straight to the FBO or my tie down spot, or do I still have to taxi over to customs? Is there any ability to fly direct to a destination and not have to stop at a port of entry airport?

What is the difference between Pre clearance and Global Entry? Can you get pre clearance without having Global Entry?

CBP has a lot of programs. Government programs are confusing. Regarding the two you mention...

Global Entry basically expedites the Customs process for pre-approved travelers, making use of an automated kiosk. The service is available at approved locations, generally major aiports. ClearlyNormally, this program is meant for the commercial traveler. However, large international GA destinations can have this service as well. Example: FLL, one of the nation's busiest international GA airports. They have a global entry kiosk at their GA facility.

Preclearance means that traveler processing/aircraft inspection are handled before the flight, rather than upon arrival in the US. But, this isn't very widespread yet. Private aircraft preclearance processing is only available at Shannon, Ireland and Oranjestad, Aruba. And the list of airports you can fly to, from what I have seen, is smaller than the list of Ports of Entry.

So no, neither of these programs really helps you out as a Canada/US travelling GA pilot. They aren't for small airports. You have to deal with Customs, APIS, and Ports of Entry, and everything else :)


Are airlines responsible for Airport security or Airports concerned or third-parties on terminal-side? In Indian Airports, all sort of terminal-side security is done by CISF . They do all the frisking and baggage screening etc. I am concerned more for the terminal-side security rather than airside where it comes under the airlines domain.

Is it similar in the rest of the world or are airlines responsible? Some notable incidents I recall are the recent Brussels Airport attack, and the 2015 Sabiha Gökçen Airport bombing. I would suspect most airports now are under some sort of integrated command and control center modeled on the American system after 9/11 rather than have disparate silo-based security which may have holes that both terrorists and criminals may be able to get through.

I am talking from port of departure, where the procedure is, show tickets, passport at the main gate, go through security check, get boarding pass and board the plane.

Similarly, while de-planing, you go through customs and immigration, security check, baggage inspection etc.

At both places, security check is apparently done. My question is who is responsible, either the Airport Operator or the Government or a multitude of authorities ?

I am trying to be as generic as possible so as to know if there are variances or are most Airports following some set conventions ?
There have been many articles about TSA high-handedness but guess that's to be expected.

In the United States of America passenger and baggage screening is handled by the Transportation Security Administration (TSA). It is a government run agency and deals with the pat downs, X-Ray machines and frisks. This will get you through "security" and to the terminal area and ultimately on to the plane.

If you are coming in from another country you will need to pass through customs which is managed by its own agents and falls under the jurisdiction of U.S. Customs and Border Patrol who are also a federal agency.

At some of the really busy airports here in the US its not uncommon to see local police officers helping out with things like traffic flow and passenger management of those entering and exiting the facility and security in general.


How to scale the data of a wing aspect ratio to correspond to the case of another aspect ratio? I am reading "Fundamental of Aerodynamics " by J.D.Anderson Fifth edition. If you got the book, go to chapter 5, page 440 and 441. If you don't have the book then here you go: https://avionicsengineering.files.wordpress.com/2016/11/john-d-anderson-jr-fundamentals-of-aerodynamics.pdf  (page 465 and 466 of the pdf file).

I don't know how to insert the respective values of $C_{D,2}$ and $AR_2$ from figure 5.21 into Equation (5.66). How can we obtain the graph in figure 5.22 ? I am stuck at this point, can you help me out ?

For reference, here is figure 5.21, which is originally from NACA report 116 (1921):

Each curve represents a wing with an aspect ratio as labeled. In modern terms, $C_a=100C_\mathrm{L}$ and $C_w=100C_\mathrm{D}$, where $C_\mathrm{L}$ is lift coefficient and $C_\mathrm{D}$ is drag coefficient.

Anderson derives eq. (5.66), which scales the drag coefficient of a given wing (subscript 2) to that of a wing with aspect ratio of 5 (subscript 1):
\begin{equation}
C_{D,1}=C_{D,2}+\frac{C_{L}^2}{\pi e}\Bigg(\frac{1}{5}-\frac{1}{\mathrm{AR}_2}\Bigg)
\end{equation}

The original NACA report states that this formula has "been found to hold for distributions of lift [that] do not deviate too much from elliptical ones, although strictly speaking [it applies] only to the latter." I'm not sure why Anderson doesn't include this important point, but the implication is that we can safely take $e=1$.

Now proceed as follows: Select a curve from figure 5.21 and plug the corresponding aspect ratio into eq. (5.66). Then pick any $(C_w,C_a)$ pair off the curve and plug those into the equation. Plot the resultant $C_{w,1}$ on a figure against the same $C_{a}$. Continue with as many coordinate pairs as you want from the same curve; plot them all with the same symbol. Then select another curve and repeat but using a different symbol. Include a legend. The result should look something like figure 5.22:


How wide would an A330's wingtip vortices be around a minute after landing? I recently watched MenTour Pilot's new video about the Royal Air Maroc 737 takeoff from Frankfurt several years ago that made the news at the time because it rotated, failed to fly away at high AoA for several seconds, lowered pitch and settled back to the runway, accelerated some more, and then took off normally.
The original video of the incident is here:

In his video, MenTour Pilot suggests that the 737 was prevented from taking off because of the sudden drop in airspeed from encountering the left wingtip vortex of the A330 that had recently landed on a nearly perpendicular runway. However, the original video seems to show the RAM 737 at high AoA for around 11 seconds after its first rotation attempt. At Vr for a 737, that seems like it should burn through 2,000 to 3,000 feet of runway, depending on weight. (135 kt = 228 ft/s, for example)
So, my question is, how wide is each wingtip vortex of a landing A330 around 1 minute after landing? The conditions appear to be almost calm wind. Would each vortex really be thousands of feet wide by that point?

Vortices of large aircraft often move laterally at speeds of 2-3 knots when they sink to within 100-200 feet of the ground.
In calm wind conditions, vortices created by large aircraft during landing operations can remain for many minutes over the normal touchdown area. They can also slowly drift from one runway to a nearby parallel or crossing runway. (NASA; emphasis mine)

and

Source: FAA

Using the two pieces of information above, the vortices start moving laterally at say T+15'. At T+60' (45 seconds later) they would have covered a lateral distance of 150–225 feet. In windier conditions they would travel farther downwind but dissipate quicker.
The day the video was taken (July 23rd, 2016) there were winds in daytime but almost all the reports having the direction variable, e.g.:

EDDF 231150Z 03005KT 330V110 9999 BKN023 SCT080 24/18 Q1017 NOSIG=

Assuming for the sake of simplification that the variable wind lessens the travel/dissipation, and plotting 650 feet (225 each direction + ~200 feet wingspan) on Google Earth on the affected runway, this result is shown below.

The core is surrounded by an outer region of the vortex, as large as 30 metres in diameter, with air moving at speeds that decrease as the distance from the core increases. (CAA New Zealand via skybrary.aero; PDF)

For the core diameter above, that would be a radius of ~50 feet at each end for a large plane.

About 3,200 feet down runway 18


Which aircraft is shown in this TV show? Its registration appears to be VH-AES but a search of the Australian database shows it as being a DC-3??

It looks like a Bristol Type 170 Freighter: (All images are from the linked Wikipedia article).

The more I look at the images you posted the more it looks like a CGI than an actual aircraft.

Regarding the registration:

To me it looks more like VH-AEN
The images you posted seem like they are from a movie. Movies often use fake registration numbers.
Next time it might be beneficial to mention the name of the movie.


Why is 0 ft a valid target altitude for an autopilot? Inspired by this story;

  The pilot of a Flybe plane that dropped 500 ft (152m) in 18 seconds had
  selected the wrong autopilot setting shortly after take-off, an
  inquiry has found...

  ...The plane climbed to 1,500 ft, but then pitched and "descended
  rapidly" because the autopilot was set with a target altitude of 0 ft.

Why is 0 ft a valid target value for the autopilot? Is there any practical use for being able to set it so low and are there safety features that would prevent the plane just hitting the deck?

Quite simply, it's because sometimes you fly below sea level. There's a couple stories out there of aircraft navigation systems acting a bit odd due to their flight below sea level. For example, there is this one, which involves a C-130 landing on an airfield that is 1,210 feet below sea level.

There are numerous areas in the world that are below sea level. For example, flying at sealevel above Laguna Salada, Mexico means you are still 30 feet above the ground! It's added as an additional option to cover a small set of scenarios. A pilot is certainly not expected to select the incorrect altitude setting, as this is something that you should always doublecheck. Beyond that, he should not have enabled his autopilot so soon after takeoff (unless 1,500 was his cruising altitude).


What does the ACARS Weight and Balance Request page look like? In the ATSU AOC Preflight menu of an Airbus (maybe Boeing too), there is an option for a weight and balance request. The selection name varies by airline but it may say "W/B Req" "Send LoadSheet" "LoadSheet REQ & Confirm" "Weight Manifest Request" "Loadsheet Request" "Load Data Report" or "W/T Balance".
What does this page look like? What information does the pilot have to enter in this page?

As you said, it will vary by airline. Mine has a few screens, and the pilots enter in number adults and children in each seating zone, number of total infants, number of standard and heavy items in cargo, any additional weight in cargo (shipping cargo, mail, ballast, etc). Also the desired runways to get data for and their condition (dry, wet, snow, etc).
Many, sometimes all, of these items are auto-populated when the gate agent closes the flight, except for the desired runways.
When the response is received, it contains weights and center of gravity numbers, and then each runway will have specific thrust (temperature, ECS/bleeds, flex, and target resulting thrust) and flap settings, as well as the V speeds.


How is ATC handed for private rural airstrips? In rural Australia there are farms that have private airstrips. My question is when an aircraft wishes to land at such a strip (for example a flying doctor) who handles the landing aspects of ATC.  Things such as reporting on wind speed and direction, air pressure, ensuring that the strip is clear, advising on glide slope?

I can’t speak for Australia, but in the United States there are thousands of uncontrolled airports throughout the country, both privately and publicly operated.  Larger, better equipped public uncontrolled airports have good facilities such as paved runways and taxiways, instrument approaches, automated weather reporting stations, pilot controlled airport lighting, FBOs, fuel and maintenance depots, etc. but many uncontrolled airports are simply grass, gravel or dirt strips with little or no facilities, particularly in back country or mountainous areas.

Typically the decision to go into these kinds of airports is at the pilot’s discretion and the pilot alone must make the final call as to whether they can safely take off and land at these locations.  Bush pilots departing from said locations will typically carry small handheld weather stations used by mountaineers for determining local atmospheric conditions, winds, etc.  Remote airports will use a standard left hand traffic pattern flown at 1000’ AGL, but many do not due to nearby terrain and are one way in/out.  If a CTAF is not published for the airport, pilots will use the MULTICOM frequency of 122.9 MHz to announce their position and intentions.

Condition of the airport and runway are often not monitored and it is up to the pilot to determine if they can be used.  Arriving traffic will often make a low, slow pass over the airfield to determine runway condition and look for obstacles which could impede landing.

All types of aircraft can and do use such airports but most often the aircraft operating from them are specially build bush aircraft with good STOL flight characteristics plus beefy landing gear and tires for operating from unpaved strips or off field operations.


For the 737 Combi, How did changing the proportions of the passenger vs cargo areas work? I am very interested in the 737 combi models and want to learn more about how they worked in practice. For the 737 Combi, How did changing the proportions of the passenger vs cargo areas work?

I have read that regulations require passenger areas to be maintained
at higher pressures than cargo areas in order to prevent potential
fumes from reaching the passengers- is that true? If so, how does
that work in terms of the air pack design and other aspects of the cabin air design?
Could operators change
back and forth the proportion of floorspace that they dedicated to
cargo vs passengers, or was it semi-permanently fixed? If it could be
changed in normal operations, about how long did it take or how many
man hours?
Could the bulkhead between the passenger and cargo areas
be located at any point inside the aircraft ("infinitely variable
bulkhead location" you might say) or were there a certain number of
fixed points that the bulkhead could be positioned?
For 737 combis
operated in a 100% passenger configuration, what did the cabin look
like next to the door? Was the cargo door basically fully finished as
though it was any other part of the cabin, or was it more industrial
looking/louder/colder?

The Boeing 737QC (Quick Change) is a Boeing 737-200 which featured a 134 in × 87 in (340 cm × 221 cm) freight door just behind the cockpit. The floor was strengthened and allowed for palletized seating for faster configuration changes between cargo and passenger flights. A switch using a system designed by Falcon Aviation in Sweden allowed the seats to be removed in 20 minutes. Each seat row would roll forward to the cargo door, then slide out into a container for storage. The container would be rolled away and cargo loaded into the space where the seats used to occupy.

The video below shows how the conversion system created by Falcon worked.

Good luck creating a toy hauler.


Why does the radar dome on the E-3 have a white stripe down the middle? Every picture I can find of the E-3 has a blue/black radar dome on top with a fairly thick white stripe down the middle but I can't figure out why.  Is it related to the position of the radar unit inside perhaps?  why is it marked like this?

An example E-3 aircraft, credit: wikipedia

The white strip is the metallic part which houses the electronics and serves as support structure for the antenna. The black part is basically a composite (fiberglass) shell, which covers the antenna. According to RAF:

  The radar is actually the white part of the disc, the black being there to balance and streamline the radar as it rotates

According to this page:

  Each AWACS 707 plane has two fiber glass radomes, joined by a 6 ft. (1.82 m) wide metal spine to form a 30 ft. (9.14 m) diameter radome.

This can be seen in this Northrop Grumman AN/APY-1/2 AWACS multi-mode radar inside the E-3D rotodome.

The Northrop Grumman AN/APY-1/2 AWACS multi-mode radar inside the E-3D rotodome. (Credit: Scott Carson. Twitter.). Image taken from aviation.stackexchange.com

The fiberglass shell is painted over in black as black was one color that did not require metallic particles in its paint composition (which would interfere with radar waves) as discussed in another question in aviation.stackexchange.


How are engine pylon designed? Currently I'm working on my master's thesis on the topological optimization method of aircraft structures. I'm gonna to explore this method of design on the example of the pylon. Make something like this:

But firstly I need to provide information about conventional methods. Moreover I'll evidently need to compare old and new methods.

That's weird, but I've managed to find much more information about new method, although it is not broadly implemented yet. But seems that it's too few info on the internet about conventional techniques.

To be more specific I'm interested in that red caisson or frame on the picture below. How exactly it (or similar) is designed? Could you suggest some information (books, manuals, tutorials, abstracts... everything) about that. Russian text is also OK.

Like all complex artifacts, aircraft primary structure design is an iterative process.  It begins with aerodynamicists designing the Master Surfaces of the Aerodynamic Envelope.  Structures designers then cope with the space requirements thereafter, although compromises between multiple design teams will often be needed.

A few things will be known.  We will know the intended operating envelope of the aircraft with some intended design limitations in airspeed, maximum Mach no., service ceiling, etc of the aircraft so we can calculate the aerodynamic loads on the pylon.  We also can obtain performance numbers on the engine, so we can determine the thrust loads and moments about the mount.  We also have the design load factors for the aircraft as well as crash and emergency design loads based upon aircraft certification requirements, factors of safety, etc.  From there these external loads can be computed and used in the design requirement as external loads to a 'black box', that is an as of yet determined structural assembly for the pylon.

The exact design of the pylon and associated mounts are going to depend on these figures.  Pylon design uses similar structures from OEM to OEM and its based on their years of experience designing these thing based on both expected mathematical models as well as flight test data.  Unfortunatly the exact details of how and why it's designed the way it is remain a proprietary trade secret of the OEMs and they are not likely to talk about it.  Boeing, like KFC, has a secret recipe for its wings and is not freely released to the public.  The same is true for its pylons, empennage, etc.

The basics are most new pylons will be based upon this proprietary structural arrangement used in previous designs.  The design loads and basic configuration of the parts an assemblies going into the pylon will have been calculated by a stress engineering group into sizing for the geometry of the parts to be designed.  The first pass at the assembly design will be done by the pylon group, then used as a basis by stress for sizing and this iterative process will be repeated until an acceptable design solution is found.

Ground and flight tests will be the ultimate proof of the design and also usually leads to a weight reduction process to reduce part geometry of excess material once the actual flight loads have been verified.

Your question is an interesting one and it may be more beneficial to consult with either a retired employee from an aircraft design company who worked on these structures or a scholar in aircraft design.  Your school could probably forward you on to someone like that and you can consult with them about this.

After doing so, it might be beneficial to take a trip out to an aircraft boneyard and examine an existing engine pylon on an airplane like a 737, 747, DC-10, etc.  Heck, if the airplane is decrepit enough or about to be sold for scrap, ask the manager how much it would cost to amputate an engine pylon (minus the engine!) and take it back with you for the Master's project.  You could examine it and build CAD and FEA models of the unit, then test these using expected loads for the aircraft and engine in question.  While this might not answer your immediate questions, it will provide your project with a treasure trove of information about how and why an engine pylon is designed the way it is.  You can then examine other OEM pylon design and dissect and analyze them very accurately based on this previous experience.

Hope that helps!


Why do zero gravity flights not stall? Airbus' (A300 / A310) are being used by French companies (i.e. Novespace) that offer zero G flights.

Now the airbus seems to risk a stall at a 40 degree angle of attack (i.e.Air France 447).

In a zero g flight, they climb at 45 degrees and then nose dive at the same angle.

Why does the climb not stall the aircraft?

The aero-dynamics of a flight in reduced gravity are different than a flight operating in normal conditions. When an aircraft is experiencing zero gravity, it does not stall because the wings are not generating any lift, because the aircraft is in a state of weightlessness.

In the paper for the requirements for zero gravity flights, it is mentioned (PDF):

  ... zero G flights stalling conditions are different as compared to
  conventional flight maneuvers, because the plane is weightless and is,
  therefore, in no need of aerodynamical support. On the other hand, a
  minimum speed is required to execute the delicate control through the
  parabola. For this reason the stalling speed is taken as the minimum
  aircraft velocity required for effective control.


Who applies an airworthiness directive when the airframe is newer than the AD? For example, an AD was issued in 1990 while the aircraft was manufactured in 2002? Who applies the AD, the manufacturer or the owner?

I am interpreting your question as asking whether an owner of an aircraft is responsible for complying with ADs that were issued before the aircraft was manufactured.

Short answer: yes.

There are several types of ADs that affect your aircraft. Some of them apply to all aircraft of a specific type, some apply to specific serial numbers or model numbers, and some apply to specific parts that are installed on your aircraft.

Some are one-time and others are recurring. The one-time items would usually have been checked before the airplane left the factory. The exception would be if they were required after a certain number of airframe hours. Recurring items can still apply to all aircraft—even those made after the AD was issued.

The two ADs listed below require the owner to check for excessive wear in the listed parts. I just replaced the Bendix switch in my Cherokee because it failed the required test in the AD.

Here’s one that was issued in 2011 for all Cessna aircraft AD 2011-10-09:

  We are superseding an existing airworthiness directive (AD) for Cessna
  Aircraft Company (Cessna) 150, 152, 170, 172, 175, 177, 180, 182, 185,
  188, 190, 195, 206, 207, 210, T303, 336, and 337 series airplanes.
  That AD currently requires repetitive inspections and replacement of
  parts, if necessary, of the seat rail and seat rail holes;

Here’s one that was issued in 1976 that applies to all aircraft that have Bendix ignition switches AD 76-07-12:

  Applies to all aircraft employing magnetos and using Bendix ignition
  switches listed in the table below except switches identified by four
  digit date code (new) adjacent to the model number or a white dot
  (modified) on the support plate adjacent to the Bendix logo.

Your A&P should have software that allows them to easily check which ADs apply to your aircraft and then create an AD Compliance Report for you. It will list all of the ADs and the date that they were complied with. Most reports also list ADs that apply to your aircraft model, but are excluded by serial number.


How do people suffering from hypoxia not realize they aren't getting enough oxygen? I've seen reports of flight passengers/crew suffering hypoxia, and many times without knowing it.

My question is, how could you not be aware of it? For example, at above 10000ft, if your body is suffering from a lack of oxygen, doesn't it make you breathe faster and harder? (which in turn, makes you aware that something is wrong in the air you're breathing)

Second question: if you're dealing with air containing half as much oxygen, is it not enough (sustainable) to inhale (faster) twice as much air when breathing (without oxygen mask of course) ?

The key point here is that your body does not measure blood oxygen levels. Instead, the urge to breathe is caused by a surplus of carbon dioxide in your blood. At sea level, this is fine: the only way to get so much carbon dioxide in your blood is if you used up all the oxygen.

At higher altitudes, the pressure is lower, and consequently the oxygen partial pressure. Since the rate of diffusion in your lungs is a function of pressure, this means that oxygen will diffuse less readily into your blood. At the same time, carbon dioxide diffuses more readily out of your blood due to the low pressure. This reduces your urge to breathe; you do not feel 'out of breath'. See this Wikipedia page.

At even higher altitudes, the pressure is actually so low that breathing will cause oxygen to diffuse out of your blood; you can breathe as much as you want but the partial pressure of oxygen in your blood will never be higher than the partial oxygen pressure in the air. This is why oxygen masks work (even if they're not airtight): they replace all air near your nose and mouth with oxygen, so that the partial pressure of oxygen equals the total atmospheric pressure rather than the usual ~21%. See this and this Wikipedia page, or for example this YouTube video.

Somewhat related: at sea level, you can also run out of oxygen before you feel the urge to breathe. You might think that breathing deeply before having to hold your breath for a long time increases the amount of oxygen in your blood, increasing your endurance. This is only partially true: it also artificially reduces your carbon dioxide levels, reducing your urge to breathe. This way, you can black out before feeling out of breath, which can be dangerous under water!


Can you continue an ILS approach when weather falls below minimum? If, after passing the final approach fix, weather deteriorates so that the reported values are now below required minima, is it allowed to continue the approach?

You didn't list a jurisdiction so Ill answer for the FAA: Yes you can continue;

Under part 91 you can both start and fly an approach if the conditions are below minimum.

If you are flying under part 121 then you can't start the approach but § 121.651 Takeoff and landing weather minimums: IFR: All certificate holders. is going to be the regulation to reference and allows you to continue if you are passed the FAF.

  (c) A pilot who has begun the final approach segment of an instrument
  approach procedure in accordance with paragraph (b) of this section,
  and after that receives a later weather report indicating
  below-minimum conditions, may continue the approach to DA/DH or MDA.
  Upon reaching DA/DH or at MDA, and at any time before the missed
  approach point, the pilot may continue the approach below DA/DH or MDA
  if either the requirements of § 91.176 of this chapter, or the
  following requirements are met:


Why does the wing root of the A330 have a pronounced twist? just wondering what the purpose of the pronounced twist on the Airbus A330 wing root is,  I've noticed this on the 380 as well.

Short answer:
To create a funnel for all the air streaming towards the fuselage.
Slightly longer answer:
Air approaching a swept wing will be accelerated towards the area with the lowest pressure and, therefore, will be sucked towards the wing's center. The center itself will show a markedly different pressure distribution over chord than the two-dimensional flow over the same airfoil would because here the acceleration is much lower while the deceleration length at the end of the airfoil is compressed. If the swept wing has no twist and the same airfoil over span, a flow pattern similar to that pictured below will result:

The blue lines on the wing surface are lines of equal pressure (isobars) and in the plots denote the pressure coefficient on the upper side of the wing section. The red lines show the pressure coefficient on the lower side.
In order to maximize the achievable lift at transsonic speed it is desirable to modify this wing such that the isobars will run along lines of equal chord percentage over the full span. To achieve this, several tricks must be combined:

Make the root airfoil thicker at the front and thinner at the rear part.
Do the reverse for the tip airfoil: It must be made thinner at the front and thicker at the rear part.
Use negative camber at the wing root.
In order to compensate for the much higher zero-lift angle of attack of the modified root airfoil, increase its incidence.

The last step is what causes the kink in the trailing edge. Of course, increasing the wing chord at the center increases this effect. In case of the A330 and A340, the incidence changes from +4.5° at the root to 0° between 30% and 70% span and -2° at the tip when on the ground and -2° at 70% span and -5° at the tip in flight.
Thanks to @ymb1 who found a very illustrative picture of the A330 (shown below, source) the root twist becomes obvious: The root airfoil has visibly much less forward camber (negative camber, actually) and, consequently, a higher zero-lift angle. In order to produce a similar lift coefficient, the center wing needs a higher incidence to compensate for this change in airfoil shape.

Another find by @ymb1 is this picture of Airbus root airfoils (source). The thick nose is easy to spot; it is needed to shift the suction area on the swept wing root forward so that the pressure distribution becomes more similar to the one of the airfoil at mid-span. Also, while the top has a long straight stretch, the bottom is rounded which in combination produces negative camber. The rounded bottom helps to maximize relative thickness. Since the chord at the center wing is increased, the local lift coefficient can be smaller than on the mid and outer wing and still carry the same circulation for low induced drag. What is saved in lift is added in thickness for structural efficiency and for stowing the landing gear. The lack of rear loading (= high local camber in the rear airfoil section) helps to gain the needed volume for the landing gear bays.

Here is another one, this time an English Electric Lightning F.1. The reflections on the leading edge show nicely how it is pulled down at the root with plenty of nose camber there. This being a design from the early Fifties, this nose camber is used to give the wing better stall behavior and isobar shaping was not yet practised. Note that the local incidence has not been increased at the root.

EE Lightning F.1, Imperial War Museum, Duxford (Edited), CC BY-SA 2.0 (picture by Roland Turner from Birmingham, Great Britain; source)


Did the FAA order the installation of "Cooper vanes?" On November 24, 1971, a man hijacked a Boeing 727 and parachuted away, never to be found.

According to Wikipedia, this prompted (together with 2 other hijackings) the installation of "Cooper vanes" devices that prevented the opening of the 727 ventral airstair in flight.

  In the United States, following three hijackings in 1972, the Federal Aviation Administration ordered that Boeing 727 aircraft be fitted with Cooper vanes.

Wikipedia cites "Beyond Fear: Thinking Sensibly About Security in an Uncertain World," which I don't have access to. I can find the same claim here and here.

I couldn't find any Federal Aviation Regulations specifically mentioning "727." I also couldn't find any Airworthiness Directives specifically mentioning "727" relevant to the situation. There is this one. Furthermore, the policy change isn't mentioned here.

Did the FAA order the installation of "Cooper vanes" on Boeing 727 aircraft?
Was the "order" in 1972?

I am asking this question because of a recent April 2 answer on Aviation.SE:

  Boeing did have a small regional jet called the Boeing 727. This plane was designed to operate at smaller airports, with independence from ground facilities as a selling feature. The best example is that the 727 had built-in stairs in the rear underbelly of the aircraft. This could be opened in flight until some hijacker known as DB Cooper jumped out to make his escape.

Apologies if the background research is poorly conducted and irrelevant. I am not familiar with the best way to research historic and current FAA "orders."

The answer to both of your questions is "yes". The FAA ordered, in effect, the installation of "Cooper Vanes" and did do so in 1972.

The reason you could not find the answer may have been that the modifications were not called "Cooper Vanes" by the FAA and nor was the FAA targeting the 727 specifically. Instead they addressed ventral and tailcone exits on any aircraft.

CFR 14, Part 25 (Airworthiness Standards: Transport Category Airplanes), Subpart D (Design and Construction, Emergency Provisions) contains, since 31st of December 1972, the following text (emphasis mine):

  Sec. 25.809

  Emergency exit arrangement.

  ...

  (j) When required by the operating rules for any large passenger-carrying turbojet powered airplane, each ventral exit and tailcone exit must be--

  (1) Designed and constructed so that it cannot be opened during flight; and

  (2) Marked with a placard readable from a distance of 30 inches and installed at a conspicuous location near the means of opening the exit, stating that the exit has been designed and constructed so that it cannot be opened during flight

This rule was announced in the Notice of Proposed Rulemaking. Notice No. 72-15, Issued on June 6th, 1972.

It clearly references the hijackings as the reason for this change.

  ....

  In spite of concerted efforts made by the FAA and the air carriers,
  incidents continue to occur wherein the safety of the flight of
  aircraft engaged in passenger-carrying operations under Part 121 of
  the Federal Aviation Regulations has been jeopardized by persons
  intending to harm the crew or take command of the airplane. On a
  number of occasions in recent hijackings, the ventral exit of an
  airplane has been opened and a hijacker aboard has parachuted from the
  airplane through that exit. The agency recognizes that every possible
  step must be taken to deter persons from boarding aircraft for such a
  hijacking purpose. Accordingly, the FAA deems it appropriate to
  propose certain amendments to Parts 25 and 121.

  Specifically, it is proposed to amend Sec 25.809 to provide that, when
  required by the operating rules, for any large passenger carrying
  turbojet powered air-plane an approved means must be provided so that:

  (1) takeoff cannot be started if either the ventral exit or tail cone
  exit is not locked; and

  (2) neither the ventral exit nor the tail cone
  exit can be opened in flight.

  A similar amendment is proposed to be made to Sec 121.310, to become
  effective with respect to persons conducting operations under Part 121
  six months following its adoption.

  However, it is to be noted that to achieve compliance with the
  proposed regulation both the ventral exit and tail cone exit would
  have to continue to meet all of the requirements applicable to their
  approval as emergency exits. Specifically, to achieve compliance, the
  conditions that would have to be met to obtain approval of
  modification to the locking mechanisms of these two exits are as
  follows:

  (1) The mechanism must be locked while the airplane is
  aloft;

  (2) Takeoff of the airplane cannot be started if either ventral
  or tail cone is not locked; and

  (3) The exit must be available for use
  in the event of an emergency.

Since Part 25 is for airworthiness certification of new aircraft, an additional change was proposed to Part 121 so that operator of existing aircraft were given 6 months to apply the new rules to already certified aircraft.

Somehow the change to Sec 121.310 cannot be found in the FAA Regulatory and Guidance Library (at least not dated 1972), even though the final rule of 30th November 1972  includes it.

A revision of 121.310 from 1997 includes the text.

Special thanks to the FAA for making the pre-internet era rulemaking archives easily accessible through the internet.


How do I fill the PBN\ part in a Flight Plan Form? Flight Plan Form Section 10 is the "Equipment" section, where one input whatever navigation or communication equipment on board. In addition, Section 18 is the "Other information" section, where you can add any supplemental information, some of it in relation to the aforementioned "Equipment" section.

My aircraft has every RNAV\RNP capabilities with indicators 1 through 10 (RNAV/RNP 10, RNAV 5, RNP 4, RNAV 2, RNAV/RNP 1). I need to use the "PBN\" indicator to detail PBN equipment capabilities. My question is how exactly?

In my case, should I file PBN\A1B2B4C2D2L1O2S1? Is it over the character limit? Is it actually necessary? Does it make sense?

You have 8 items selected, and the max allowed is 8. There is guidance material from ICAO with steps to follow on how to get down to the 8 (in case you exceed it in the future). Right now you're fine.

For example:

  If having applied the guidance provided in Step 3 there are still more than 8 PBN codes remaining:

    Identify the capabilities considered to be the least relevant to the flight;
    Insert them under Item 18 within the NAV/ element;
    Insert the letter 'Z' in Item 10a.

  For example, the codes relating to long range Oceanic capabilities (A1, L1) will not be a priority if the flight will take place entirely within European continental airspace. Inclusion of an RNP APCH capability will not be a priority if none of the destination or alternate aerodromes provide such a procedure.


How does Kalman filtering work in inertial navigation systems? As far as I understood it correctly most INS-systems use a Kalman filter to compensate measurement errors.
I would like to know:

Does the Kalman filter compensate the errors from all the onboard sensors?
IIRC the Kalman filter is a tracker, that predicts future computation values. How is this useful here?
Is this used on IRS as well? Or only on older INS?

Thank you

A Kalman filter is used on all kinds of IRU's and INS's, even with airspeed information as in an ADIRU.  A Kalman filter is just a general method (and a very useful one) for state estimation and sensor fusion, which is exactly what's going on in an INS system.  I wouldn't call this "compensation". Inertial naviagation requires us to read in sensor values like angular acceleration, direction of magnetic north, longitudinal acceleration, and even airspeed, then convert those into more meaningful values like ground speed.  If done correctly, these processes (called sensor fusion and state estimation) will minimize errors on all the sensors you feed into it.   That's the quick version.  For a longer explanation of how it works, read on.

Kalman filters are used for sensor fusion and/or state estimation.  State estimation is when the basic physics parameters we want to know like speed and position (called the state of the system), are different from the sensor values we're getting, like magnetic compass readings and gyroscope values.  State estimation problems attempt to estimate these values like speed even in tricky situations like when errors are present or when the state can't be directly computed from the sensor values.  State estimation in Kalman filters can also take into account inputs to the dynamical system like airplane thrust and aileron position to try to improve the accuracy of the model, but this is not necessary and I'm skeptical about how often this is used based on my experience.

Sensor fusion, on the other hand, is when we want to combine several sensors to try and get a sensor that has the best of both worlds.  Sensor fusion and state estimation can overlap. A classic example is GPS and accelerometer sensor fusion.  GPS gives accurate position, but only down to a few meters and may not update at the rate desired. Accelerometers give accurate accelaration values even at small scales, but their position estimate tends to drift over longer periods of time.  By combining the two we can get the precision of INS but with no drift. One advantage of a Kalman filter is that the different sensors being combined don't need to be measuring the same values as long as we have equations for how the sensor values and the internal state are related.  For example, we can combine magnetic compass and gyroscope values even though the two aren't directly analogous. Sensor fusion can also be done with other methods like a complementary filter or Bayesian/Markov methods.

How does a Kalman filter work?  First a Kalman filter estimates the state (say, position and speed) using a system model to predict what the new position should be based on previous values.  This is the new predicted state.  The filter then looks at the sensor values to see what the new position should be based on those measurements.  It then reconciles the predicted and measured position to get a new position estimate based on the fact that both of those values may be inaccurate. The filter does this all including weights based on which errors in measurement and estimation are largest and which errors tend to be correlated with each other.  Note that this includes errors in both estimation and measurement: outside forces and inaccuracies in our model can throw off our prediction step just like bad sensors can affect our measurement step.

Technically the six degree-of-freedom equations often used for inertial navigation are non-linear (which is kind of like saying we can't scale, add, and reorder these transformations and rotations).   A regular Kalman filter will not work in this scenario and the Kalman filter must be a non-linear filter like an extended or unscented Kalman filter.

Kalman filters have several advantages over other filters like a complementary filter.  In elementary situations they can arrive at an identical solution to a similar complementary filter or averaging filter.  You mention tracking while predicting future values, and I'll explain why a Kalman filter helps.  If our last three positions were 1.0, 1.5, 1.7, and 1.8, a simple filter might average 1.7 and 1.8 or similarly try combine past and current values.  This would produce a lag behind the current value.  Kalman filters (and some simple filters) extrapolate previous values to predict the current value more accurately.  Kalman filters really begin to shine as the system we're trying to track changes in complex but predictable ways.

Simple filters like complementary filters struggle more than Kalman filters when errors are correlated or have different weights.  Kalman filters use weighting in their estimate to compensate for correlation in errors and also to give the most bias to the best sensor values.  For example, if angle of attack and airspeed tend to have the same inaccuracies at the same time, or if the gyroscope is much more accurate than the magnetic compass, the Kalman filter will give the sensors weights to compensate for that. If you're in a situation where you can model the equations of motion, or if you know something about the different types of noise in your system, the Kalman filter is probably more accurate than simpler alternatives.

Despite the accuracy of Kalman filters, I've seen a few places where simpler devices like complementary filters were used instead of Kalman filters.  Why?  The main reason is complexity, which I won't elaborate on too much at this point.


What scale is the speed trend line on a PFD's ASI? A primary flight display (PFD) in a glass cockpit has an airspeed indicator (ASI) that doesn't just display the speed, but also a vertical arrow or bar that shows the "speed trend" i.e. the acceleration. But there's no extra "acceleration scale" shown behind it like there is for speed. What's the scale of this trend arrow and how do you read it?

I found some training material on Airbus PFDs which explains:

  A vertical arrow, called the speed trend arrow, indicates the aircraft's
  predicted speed in 10 seconds if the current acceleration or deceleration rate remains constant.

Put another way, if you measure the acceleration in knots per second, and then multiply it by ten, it's on the same scale as the speed tape.

If you want to reach a particular speed in ten seconds' time, adjust pitch or power to put the head of the arrow on that tick on the speed tape, then keep adjusting to keep it on the same tick as it approaches.


Are military jets always scrambled when communication to a commercial aircraft is lost? I've heard that in case a commercial flight does not respond to Air Traffic Control, military jets would always be scrambled to check the state of the flight.

The benefits of scrambling jets are clear. My question is, are they always scrambled in every case?

Over Europe and many other parts of the world, the general rule is that if an aircraft in controlled airspace does not reply to instructions from Air Traffic Control, military jets will be scrambled to intercept the aircraft.

Basically you are asking if there are exceptions to that rule.

I think there are a few. First of all, ATC will not inform the military of every instruction that is not reacted to; there wouldn't be enough jets. Pilots do miss instructions occasionally, sometimes aircraft are handed of to the wrong frequency by ATC, wrong frequencies are entered into the radio etc. All these things usually sort themselves out in matter of minutes and it would be a waste of resources to scramble. If no contact is established after a certain amount of time, only then the military will be involved. However if an aircraft starts to deviate from earlier acknowledged instructions without reacting on the radio, it is safe to assume that the time to alert the military is considerably shorter.

And then of course the aircraft must be in range of military jets. An aircraft halfway across the Atlantic is simply too far out for most interceptors so until the aircraft is getting closer to shore I don't think there is much reason to scramble jets.

Some more information about loss of communications, causes and prevention  can be found in this Skybrary briefing note. For obvious reasons the exact procedures for scrambling military aircraft are not publicly available.


How is the steepness of turn bank properly defined? In aviation we often refer to the bank of a turn as shallow, medium  or steep. I remember reading somewhere that the definition is not universal as to how many degrees it is, but it has to do with the airplane and whether the bank tends to roll back to level, remain where it is or continue to steepen. Does anyone know if this is a correct definition or have any references to share?

I don't know if this is only definition out there, but according to the FAA's Airplane Flying Handbook:

  Shallow turns are those in which the bank (less than approximately
  20°) is so shallow that the inherent lateral stability of the airplane
  is acting to level the wings unless some aileron is applied to
  maintain the bank.

  Medium turns are those resulting from a degree of bank (approximately
  20° to 45°) at which the air- plane remains at a constant bank.

  Steep turns are those resulting from a degree of bank (45° or more) at
  which the “overbanking tendency” of an airplane overcomes stability,
  and the bank increases unless aileron is applied to prevent it.


Was the Cessna 208 developed to meet Fedex's requirements? I remember hearing or reading somewhere that the Cessna Caravan was developed because Fedex had issued an RFP for a light aircraft to carry cargo on shorter routes and to unimproved airstrips. According to whatever I heard/read, that made it unique as the only mass-produced civilian aircraft to be developed based on a customer's request, at a time when manufacturers built aircraft according to their own opinion of what was needed in the market and then tried to sell them.

Is there any truth in this?

Evidently, the idea for the Cessna 208 existed before FedEx got involved. Cessna started with a widened and stretched 206 fuselage. It was designed to replace the aging De Havilland Canada Otters, Beavers and Beech 18 aircraft in use with bush operators.

  Marketing surveys from the late seventies concluded that a modern, large capacity, rugged single engine aircraft for sale could compete successfully...

They completed the first flying prototype and were expecting orders mainly from the target market of bush operators. However, the CEO of FedEx saw it on the cover of Flying Magazine and sensed opportunity. (Whether this is true or not, FedEx was looking for a small aircraft to replace trucks on some routes, and the Caravan came along at just that time.) Cessna made some changes such as adding a cargo pod to create the Cargomaster. The big orders from FedEx helped to drive the popularity of the Caravan.

So FedEx was not the reason for designing the Caravan in the first place, but it was a big part of the success of the design and did influence some aspects of it.


How are the runways operated at Toronto Pearson? I have a sort of technical question regarding the operations of runway systems at larger airports, specifically at Toronto Pearson (satellite view here). As far as I can see, the airport has 5 runways: 05, 15L/R, and 06L/R.

(Public domain via Wikimedia Commons)

Since RW05 and RW15L/R intersect it is clearly infeasible to operate RW05 at the same time as RW15L / RW15R. As for the rest, there are several ICAO guidelines regarding parallel or near-parallel runways. What do those restrictions imply regarding simultaneous operations?

Is it possible to operate RW06L and RW06R simultaneously, maybe in a segregated mode of operation? What about RW15L/R?
Similarly, can runways 15 and 06 be operated simultaneously?

According to Toronto Pearson's official website, the airport operates in four main configurations, this is what they look like:

Also judging by the satellite imagery, CYYZ is LAHSO enabled, that is the intersecting runways (05 and 33L, for example) can be used simultaneously. (It resulted in a near-miss back in 2002.)


Is repeatedly pitching an aircraft up and down to help it lose its energy quicker a viable landing technique? Say a pilot "bobs" his aircraft's nose up and down. The rapidly changing AOA should generate more drag than if the aircraft were to fly straight at a constant pitch where the AOA shouldn't be significantly large, right? Is the extra drag significant enough for this technique to be used in a practical landing approach?

No, that kind of landing technique is too risky and dangerous.

You’re most likely to just exchange kinetic energy for potential energy, gain altitude, then slam into the ground with the aircraft exhausted of airspeed.

Also it’s superfluous, considering that a properly managed approach on glide path and airspeed will accomplish pretty much what you intended to do with your proposed landing technique.  A stabilized approach and entry into the roundout will result in the aircraft exhausted of energy and settling onto the runway once the power is pulled to idle, with little float.


Do jet aircraft have an emergency propeller? I once heard, or misheard, this:

  There is an emergency propeller that comes down in an emergency of jet-engine aircraft. It is behind the main landing gears.

Is this true or a story somebody made up?

I think you might have heard about the Ram Air Turbine, which is deployed in case of some aircraft in case of loss of main electrical power supply. From A320 Systems briefing:

  In case of total loss of all main generators, the RAT is automatically extended and drives the emergency generator via a hydraulic motor.

The location of the ram air turbine varies with different aircraft and some of them (like the popular A320) have them near the landing gear. The following image is from @mins' excellent answer to another question.

Location of Ram Air Turbines in different aircraft, image from aviation.stackexchange.com

Bombardier BD700-1A11 Global 5000 (source)
Airbus A320 Flight test (source)
Bombardier CRJ700 (source)
US Navy T-45A (source)
Airbus A330 (source)
Boeing B747-8 (picture by Gregor Schläger, source)
Boeing B777, RAT extension switch (source)


What is the call for when an enemy air-to-air missile has been fired? In a scenario where a large multi-crewed aircraft was being attacked by a fighter that launched an air-to-air missile what would the call be that would alert the pilot that a missile was in the air and potentially tracking them.  I know the term 'Rifle' but I always assumed that meant a friendly missile in the air, is there a specific call for an enemy missile?

Also is there a call for a SAM launch?

From an older NATO book on brevity words that I have, a hostile guided missile is VAMPIRE.

A hostile surface to air missile is, indeed, SAM.

Edit.
From the better list of brevity words

VAMPIRE  Hostile anti-ship missile

So VAMPIRE is not an air to air missile.
My old list did not specify the type of the hostile missile.


To what requirements / assumptions RSAs & EMAS designed? I've several questions regarding runway safety area (RSA) and engineered materials arresting system (EMAS):

Is RSA/EMAS designed for impact loading at the point of touchdown and to absorb energy of the moving aircraft for deceleration?
Assuming that the landing gear is down: Can the load be assumed to be distributed equally through all wheels?
Are the brakes applied to assist deceleration over the EMAS?
What happens in case of belly landing and when there are burst tyres?
Is there any other loading case?
What are the max design loads and velocities?

I will try to answer as much of this as I can, but there may be some holes to fill:

  Is RSA/EMAS designed for impact loading at the point of touchdown and to absorb energy of the moving aircraft for deceleration?

No, the arresting materials is only there for a runway overrun condition. It is not designed to absorb an aircraft impact, nor is it designed for an aircraft to touchdown on it, crash or normal landing. EMAS area's are very specifically marked:

(Source: BusinessInsider.com)

As you can see, the area is actually very small, and the markings denote that this is not a touchdown zone. It would be very hard for a commercial airliner to hit that small patch on touchdown, purposely or not.

  Assuming that the landing gear is down: Can the load be assumed to be distributed equally through all wheels?

Not always, because this depends on the loading of each aircraft gear. The load can be thought of as being distributed evenly probably at the same ratio as the weight on each gear, but only when all the gear are inside the EMAS area. This wouldn't be true, for example, as the aircraft enters the EMAS and the EMAS is acting only on the nose gear until the main gear exit the runway and into the EMAS.

  Are the brakes applied to assist deceleration over the EMAS?

YES! The pilot is going to be braking hard to try to stay out of the EMAS, it is unlikely that they would release the brakes as they enter the EMAS. Remember that EMAS is basically very weak concrete. The pilot is probably going to be using all available braking (wheels, aerodynamic, thrust reversers, etc) to get the aircraft to stop. Once they enter the EMAS they've destroyed the EMAS pad, and have done significant damage to the aircraft.

  What happens in case of belly landing and when there are burst tyres?

If a belly landing made it all the way to the EMAS, which is unlikely, I doubt that the EMAS will have a significant effect on stopping the aircraft. The EMAS is specifically designed to grab onto things that dig into it. By the time the aircraft slides onto the EMAS, whatever is "hanging" under the aircraft has probably been broken away. I don't know of any case where EMAS encountered a belly landing aircraft (other than one's who have broke off the nose gear), but I'm guessing that the aircraft would slide right over it.

Burst tires on the other hand would probably not reduce the effectiveness of the EMAS since it will still be dragging through the gear.

  Is there any other loading case?

Not that I know of. The system is designed to arrest aircraft overruns. Specifically this means a normal landing but unable to stop on the runway such as in ice, snow, or water. It is not designed as a crash safety system.

  What are the max design loads and velocities?

If you want to read the FAA's Advisory Circular on Engineered Materials for Arresting Systems (EMAS) for Aircraft Overruns it has quite a bit of information:

  The design
  (or critical) aircraft is defined as that aircraft using
  the associated runway that imposes the greatest
  demand upon the EMAS. This is usually, but not
  always, the heaviest/largest aircraft that regularly
  uses the runway. EMAS performance is dependent
  not only on aircraft weight, but landing gear
  configuration and tire pressure. In general, use the
  maximum take-off weight (MTOW) for the design
  aircraft. However, there may be instances where less
  than the MTOW will require a longer EMAS. All
  configurations should be considered in optimizing the
  EMAS design. To the extent practicable, however,
  the EMAS design should consider both the aircraft
  that imposes the greatest demand upon the EMAS
  and the range of aircraft expected to operate on the
  runway.

FAA Advisory Circular AC150-5220 22b, 9.c

And this one:

  Entrance Speed. To the maximum extent
  possible, the EMAS must be designed to decelerate
  the design aircraft expected to use the runway at exit
  speeds of 70 knots without imposing loads that
  exceed the aircraft’s design limits, causing major
  structural damage to the aircraft or imposing
  excessive forces on its occupants.

9.g from the FAA Circular, emphasis mine.


What are the advantages, disadvantages and current state of affairs for text-based ATC? This answer has a rather interesting (and quite lengthy) discussion, on a concept of using text-based rather than voice-based communication between ATC and the cockpit. As it turns out, the FAA has some plans for that already as mentioned by @reirab. Being a simmer rather than a pilot, I've always found text-based ATC quite convenient (it provides a direct reference to your own instructions as well as instructions to others on your frequency), but the discussion in the linked answer showed some pilots opposed to such a system. My question is twofold:

What are the advantages and disadvantages of such a system?
What is the current state of affairs of this concept? Are there specific methods of implementation across various instances - e.g., FAA, EASA, ICAO or other regulatory bodies - and do aircraft (systems) manufacturers anticipate such systems, or would perhaps a software update suffice on modern aircraft?

Edit: Perhaps it's better to concentrate mainly on IFR traffic, since their procedures are more formal (spacing, landing sequence etc. are decided by ATC) and things like voice call-outs (e.g., the nice voice that calls you a retard in an Airbus also calls for a go-around) are available at no extra cost.

I'm a newbie pilot, but I wanted to give the obvious example that crossed my mind when I read your question. In the UK GA world we have to, for example, call up to say we're on final approach. Finals can be a pretty busy time for the pilot, and at any kind of busy GA airport you're likely to have something along the lines of the following discussion:
(Note in my example, my airport has an "Information Service" rather than full blown ATC, but nonetheless - the example stands)

Me: "G-AB Finals, Runway 16"
Tower: "G-AB, Runway Occupied"
Me: "Continuing, G-AB"

This is me telling the controller that I'm aware I can't actually land, but I'll continue flying my approach path down towards some kind of decision height - somewhere around 300'. If I hear nothing again, I'll go around.

Tower: "G-AB, Land At Your Discretion"
Me: "Landing, G-AB"

I now continue my landing, except I'm not happy with my approach so I now declare a go around

Me: "G-AB, Going Around"
Tower: "G-AB, Roger"

Remember that all of the above is going on while I'm setting flaps, keeping on the glideslope, watching the runway and maintaining my airspeed.
Now also, imagine the alternative:

Tower: "G-AB, Land At Your Discretion"
Me: "Landing, G-AB"

So I now have clearance to land. I carry on, there's a bit of a crosswind and I'm working hard. I cross the airport boundary and I'm concentrating on the runway.. Unbeknown to me, a small dog/child/whatever has broken loose and is charging across the grass to the runway:

Tower: "G-AB, Go Around, I say again, Go Around"
Me: "Going Around, G-AB"

In less than 2 seconds the danger is gone, and the tower know now that I've understood and acknowledged. Now in no world am I ever going to be looking down at a screen during that moment - even if a message came through and beeped/bonged or whatever - my concentration will be on landing. Now you might argue for a specific "Go Around" call/button or whatever - but how many of those different things will we end up with. I simply can't imagine a situation where I won't need my headset on and radio tuned.
I'm not saying any of this stuff will never come in, but pressing a button to talk and listen is much more favourable to typing or reading something during critical phases of flight. At best, I can imagine it complementing some of the more mundane messages and perhaps Ground Controllers.
An ATIS text system might be nice, and I imagine routing and similar will be useful - but I can't imagine it taking over voice completely. Sometimes you just can't beat talking - there's a reason people still e-mail the line "I'll call you".
Beyond that - during those radio comms every aircraft in the vicinity now knows that G-AB is on Finals. This is incredibly important to GA traffic operating in VFR because pilots work hard to keep a mental picture of whose where - not to the degree the tower does, but if someone is following me but doesn't have visual (Very common - planes are surprisingly hard to spot), or loses visual, they know exactly where I am and what I'm doing.
The aircraft that's just touched the threshold also knows that control are aware the runway is occupied and hasn't just cleared an aircraft to land.


Can a safety pilot acting as PIC also log cross country time? I am working on my instrument rating and I need to build up both my instrument time and Cross Country time to meet the requirements.  I am planning on making most of that time by flying with a friend who I will switch off between hood time and safety pilot.  We are planning on doing lots of cross country hood time to meet our hours.  Also the safety pilot will be designated PIC so that we can claim PIC time as safety pilot.

We need to build 50 hours PIC cross country time so my question is can the safety pilot, if they are designated PIC for the flight, claim cross-country time even if they are not touching the controls for that flight?

The safety pilot cannot log cross-country time.

The FAA has issued a Legal Interpretation (Gebhart) that clarifies the FAR.

  Section 61.65(d) contemplates that only the pilot conducting the
  entire flight, including takeoff, landing, and en route flight, as a
  required flight crewmember may log cross-country flight time. Because
  a safety pilot does not conduct the entire flight, a person acting as
  a safety pilot for a portion of the flight may not log any
  cross-country flight time for the flight.


What kind of drone might be able to fly so high that it "operate(s) outside national boundaries"? In the question What is a propellant burner trailer? I mention that

CNN and NPR have pointed out that the US president recently tweeted a surveillance photo of an explosion of a rocket on a launch pad.

The NPR article linked above speculates on the source of the tweeted surveillance photo, mentioning that it might not be a satellite product. At one point it says:

It was not entirely clear where the president's photo came from. Panda believes it was most likely taken by a classified U.S. satellite. But Melissa Hanham, deputy director of the Open Nuclear Network at the One Earth Foundation, believes that the resolution is so high, it may be beyond the physical limits at which satellites can operate. "The atmosphere is thick enough that after somewhere around 11 to 9 centimeters, things get wonky," she says.
That could mean it was taken by a drone or spy plane, though such a vehicle would be violating Iranian airspace. Hanham also says that the European company Airbus has been experimenting with drones that fly so high, they are technically outside the atmosphere and thus operating outside national boundaries. But she says she doesn't know whether the U.S. has such a system.

This comment suggests that answers to Is there a height limit to national airspace? address the height required to operate outside national boundaries, so I have modified my question to ask only the following.
Question: What kind of drone, Airbus or otherwise, might be able to do this?

Normally, controlled airspace ends at 60,000 ft (Flight Level 600). Airplanes flying higher than that would be operating outside of controlled airspace. Currently, there is no internationally agreed definition where national airspace ends. However, aircraft flying higher than 60,000 ft (18,288 m) could claim to fly outside of national boundaries.

There are several airplane designs capable of flying above that height:

The venerable Lockheed U-2 should be first on this list, even though it is sometimes loaded with sensors such that it can climb above FL600 only at the end of its mission.
Next would be the Lockheed SR-71, but none are still flying.
Both the Myasishchev M-17 and the larger M-55 also qualify.
But most likely the photo has been taken by a RQ-4 Global Hawk which officially flies up to 60.000 ft high.

Even though those designs fly at a height where 98% of the atmosphere is below them, they are still firmly flying within it, technically or otherwise.

There are several more which can fly much higher, but those are research aircraft which do not take photo trips into Iran. Of course, there can still be a classified design of which we don't know yet.

EDIT: Now it is reported that satellite tracking has identified the source of the photo as USA 224, a spy satellite launched in 2011. Using the shadows in the picture as a sun dial, the exact time could be determined and the corresponding satellite was traced down by amateur satellite hunters.


Lapsed Advanced Ground Instructor certificate, can I take a FIRC and and restore my privileges? I hold an AGI certificate that I haven't exercised the privileges for in quite some time (more than 12 calendar months).

According to 14 CFR 61.217, can I simply take a Flight Instructor Refresher Course (online or in person) and restore my privileges?

  § 61.217 Recent experience requirements. The holder of a ground
  instructor certificate may not perform the duties of a ground
  instructor unless the person can show that one of the following
  occurred during the preceding 12 calendar months:

  (a) Employment or activity as a ground instructor giving pilot, flight
  instructor, or ground instructor training;

  (b) Employment or activity as a flight instructor giving pilot, flight
  instructor, or ground instructor ground or flight training;

  (c) Completion of an approved flight instructor refresher course and
  receipt of a graduation certificate for that course; or

  (d) An endorsement from an authorized instructor certifying that the
  person has demonstrated knowledge in the subject areas prescribed
  under § 61.213(a)(3) and (a)(4), as appropriate.

It appears so, but I guess at the root of my question is, is it really that simple or did I miss a regulation or sign-off requirement somewhere (I know I could get signed off in leiu of the FIRC).

Then to keep current, what constitutes activity as a ground instructor giving pilot...training?  Just teaching and logging that I've been teaching ground school, or would I actually have had to endorse some people to take the written exam during those 12 calendar months?

First, as the regulation says, you can indeed take an FIRC to restore your privileges.

Second, I couldn't find any FAA clarification on exactly what "employment or activity as a ground instructor" means, but the language is fairly clear: you just have to be able to show that you were providing instruction as a ground instructor. That could be via training records, invoices, employment contracts, written test endorsements etc.

As a general rule, there's no need to look for extra requirements that aren't stated in the regulations.


Could a variable sweep aircraft also use negative angle? Variable sweep on fighter aircraft is a bit out of fashion, but seems to a worthwhile option to adjust the wing to the current speed and needed lift. All this is bought with complexity and weight of the system.

Would it be possible to build a variable sweep aircraft that goes low speed and in a dogfight into a forward swept wing configuration like the Su-47 and high speed in a rearward swept wing?

Image of Su-47 credit to Wikipedia (Leonid Faerberg)

Yes it could, but don't hold your breath.

Forward sweep means that the aeroelastic forces will twist the wing at high lift into an increased angle of attack, so such a wing needs to be stiff in order to avoid divergence (PDF!). Also, aileron effectiveness is excellent, so a forward-swept configuration is a good choice for dogfighting.

Since backward sweep is mostly helpful at trans- and supersonic speed, swing wings were the right answer when very high top speeds (Mach 2+) needed to be combined with high loads at take-off and short field length requirements. When flying ever faster went out of fashion in the 1970s, the attractiveness of variable sweep disappeared.

Forward sweep helps both in air combat and in transsonic flight, so there is little advantage in a variable-sweep configuration with forward sweep. An aircraft optimised for air combat has little need to fly Mach 2+, so the moderate sweep angle is a good choice for most of its flight time. Also, the aeroelastic instability increases with sweep angle, so more than moderate forward sweep needs a very stiff and heavy wing.


When cleared for an approach and not on vectors, is a depicted course reversal always required? At least where I fly, most instrument approaches now use a Hold In Lieu of Procedure Turn (HILO). The "full procedure" now consists of a turn in the hold before proceeding inbound.

In the example above (click to embiggen), if a pilot is northeast of EYWOK and receives the following clearance from Seattle Approach:

  Cessna 123AB, proceed direct EYWOK, maintain 3000 until established, cleared for the ILS Yankee approach at Paine Field.

Are they required to fly the course reversal, or can they proceed inbound on the final approach course immediately upon reaching EYWOK and fly direct to ITIPE?  If not, what instruction must be received in order to skip the HILO procedure?

Yes, with this procedure you are required to fly the procedure turn unless you are flying the CVV transition (which is notated as NoPT), are receiving vectors to final, or receive a clearance for a "straight-in approach" over EYWOK.  If you are cleared direct to the final approach fix, none of these apply (but remember that you can always request a straight-in approach if you don't want to do the course reversal)!

FAR 91.175

  (j) Limitation on procedure turns. In the case of a radar vector to a
  final approach course or fix, a timed approach from a holding fix, or
  an approach for which the procedure specifies "No PT," no pilot may
  make a procedure turn unless cleared to do so by ATC.

The AIM states:

  5-4-6. Approach Clearance

  4. If proceeding to an IAF with a published course reversal (procedure turn or holdinlieu of PT pattern), except when cleared for a
  straight in approach by ATC, the pilot must execute the procedure
  turn/holdinlieu of PT, and complete the approach.

  5. If cleared to an IAF/IF via a NoPT route, or no procedure turn/holdinlieu of PT is published, continue with the published
  approach.

  6. In addition to the above, RNAV aircraft may be issued a clearance direct to the IAF/IF at intercept angles not greater than 90 degrees
  for both conventional and RNAV instrument approaches. Controllers may
  issue a heading or a course direct to a fix between the IF and FAF at
  intercept angles not greater than 30 degrees for both conventional and
  RNAV instrument approaches. In all cases, controllers will assign
  altitudes that ensure obstacle clearance and will permit a normal
  descent to the FAF. When clearing aircraft direct to the IF, ATC will
  radar monitor the aircraft until the IF and will advise the pilot to
  expect clearance direct to the IF at least 5 miles from the fix. ATC
  must issue a straightin approach clearance when clearing an aircraft
  direct to an IAF/IF with a procedure turn or hold-in-lieu of a
  procedure turn, and ATC does not want the aircraft to execute the
  course reversal.

  7. RNAV aircraft may be issued a clearance direct to the FAF that is also charted as an IAF, in which case the pilot is expected to execute
  the depicted procedure turn or holdinlieu of procedure turn.  ATC
  will not issue a straightin approach clearance.  If the pilot desires
  a straightin approach, they must request vectors to the final
  approach course outside of the FAF or fly a published “NoPT” route.
  When visual approaches are in use, ATC may clear an aircraft direct to
  the FAF.


Are piezoelectric energy harvesters too expensive to be used commercially as a power source in drones or small aircraft? Piezoelectric energy harvesters can draw energy from the mechanical loads and the vibrations in the fuselage/wings/blades of aircraft.

What are the disadvantages of such a system being used as an auxiliary power source if not the main one? Is it not worth the money?

What are the disadvantages of such a system being used as an auxiliary power source if not the main one? Is it not worth the money?

It's not worth the weight.

The power that can be extracted from piezoelectronics is on the order of microwatts (100 uW in the above article). In contrast, light aircraft have their power measured in kilowatts and commercial aircraft reach a megawatt of electrical power.

In other words, it would take 8-16 million of piezoelectronic harvesters to replace the alternator on a small aircraft, and 3-10 billion on a modern airliner. Drones use more power than small aircraft as they've got a lot of electronics.

Piezo harvesting is being developed to power completely unattended tiny sensors that won't have access to any other power source for years. All aircraft, including drones, already have their own power source - the engine - with fuel tanks that get refilled each flight. So power's cheap and available.