How did the Thrust Reverser Deploy in Lauda Air Flight 004 crash? How did the electronically controlled thrust reverser in Lauda Air Flight 004 (which crashed) get deployed in mid air without the pilot command. Was there a fault in the electronic controller, a hardware or software fault?

In his book Air Disaster Volume 2, the aviation writer Macarthur Job wrote

  had that Boeing 767 been of an earlier version of the type, fitted with engines that were mechanically rather than electronically controlled, then that accident could not have happened.

How could that incident not have happened in a mechanically controlled engine?

The investigation into Lauda Air Flight 004 determined that the probable cause was the deployment of the left engine thrust reverser (TR) during climb. An unsuspecting crew would not be able to recover control of the aircraft after such an event. Damage to the components involved in the crash prevented identifying a definite cause of the TR deployment.
TL;DR: The hydraulic system that opens and closes the TR includes two separate valves. The investigation found certain electrical and hydraulic failures that could result in both of these valves malfunctioning, resulting in uncommanded deployment of the TR. 767 aircraft with different engine models used a mechanical system to control the TR valves, which were not susceptible to those faults.

Information here summarized and quoted from the accident investigation report, you may refer to it for more detail.
Two important components involved in the investigation were the Hydraulic Isolation Valve (HIV), which determines whether hydraulic fluid can flow into the TR system, and the Directional Control Valve (DCV), which directs hydraulic pressure from the HIV to either deploy or stow the TR. In normal cruise flight, the HIV should be closed, and the DCV should be commanding the TR to be stowed, which provides redundant control of the hydraulic side of the system. The report analyzes electrical and hydraulic failures that could have resulted in both of these valves being in the wrong position, allowing uncommanded deployment of the TR.
Both valves are controlled electronically based on the position of the reverse thrust lever in the cockpit. The HIV normally will not open without the landing gear sensing the airplane is on the ground, but it is also controlled by an "auto-restow" system.

The auto-restow circuit design was intended to provide for restowing the thrust reversers after sensing the thrust reverser cowls out of agreement with the commanded position. The auto-restow circuit powers the HIV to open regardless of indications from the air/ground circuit.

If the aircraft senses that the HIV opens while in the air, a REV ISLN (reverser isolation) message is given to the crew. About 6 minutes after takeoff the crew started to discuss an intermittent appearance of this message, and after referencing their manuals determined that no further action was necessary. The TR opened about 9 minutes later.

That an electrical wiring anomaly could explain the illumination of the "REV ISLN" indication is supported by the known occurrence of wiring anomalies on other B 767 airplanes.

With the HIV open, hydraulic power would be available to the TR system, but the DCV should still command the TR to be stowed. The DCV was also electronically controlled.

The investigation of the accident disclosed that certain hot short conditions involving the electrical system could potentially command the DCV to move to the deploy position in conjunction with an auto restow command, for a maximum of one second which would cause the thrust reversers to move.
...
Boeing could not provide test data or analysis to determine the extent of thrust reverser movement in response to a momentary hot-short with a voltage greater than 8.2 Vdc or the ability of the thrust reverser to return to the stowed position after tripping of the circuit breaker associated with the source of the hot-short.

So the investigation determined that it was possible for electrical issues to cause the TR to deploy in flight, but they could not determine whether the electrical system on the accident plane had any issues.

The degree of destruction of the Lauda airplane negated efforts to identify an electrical system malfunction. No wiring or electrical system component malfunction was positively observed or identified as the cause of uncommanded thrust reverser deployment on the accident airplane.

Electrical issues were not the only reason the DCV could malfunction. Contamination in the hydraulic system could also cause the DCV to command TR deployment. But the investigation was not able to rule out contamination of the DCV in question.

The condition of the left engine DCV which was recovered approximately 9 months after the accident, indicated that it was partially disassembled and reassembled by persons not associated with the accident investigation.

Without knowing the original condition of the DCV and aircraft wiring, the investigators could not identify a definite cause. Some of the issues found in the investigation were addressed by airworthiness directives (AD's) from the FAA, and by design changes by Boeing and the equipment manufacturers. This consisted of making the wiring less susceptible to faults and the valves less susceptible to malfunction from contamination.
The electrical failures were dependent on the configuration of the aircraft electrical system, and the hydraulic failures were dependent on the design of the valves. The report includes an FAA letter analyzing whether a similar failure could occur on other Boeing aircraft.
The accident airplane was a 767 with PW4000 engines. Once the investigation identified the above issues, the TR's on these planes were deactivated until a fix could be implemented. 767 aircraft with JT9D or CF6-80A engines were excluded at the time from any corrective action:

Since these thrust reverser systems employ mechanically actuated directional control valves, it is felt that they do not possess the same potential for inflight reversal as those systems listed above.

757 aircraft with PW2000 engines were very similar in design to the 767/PW4000 system, so system checks and valve replacement were mandated.
747-400 aircraft had similar systems to the 767, but were not immediately affected, because aerodynamic differences meant that a TR deployment in flight would not be as serious. The same fixes were to be implemented though.
767 aircraft with CF6-80C2 and RB211-524 engines had a different system, but inspections and tests were anticipated until design changes were made or determined unnecessary.
757 aircraft with RB211-535 engines had a different hydraulic design that wasn't susceptible to contamination failure, but the electrical system was analyzed for potential issues.
737 aircraft with CFM56-3 engines were not found to have the same issues.


How are runways numbered? I'm not a professional pilot, just an enthusiast, so this might seem obvious to others; but I noticed that airports don't use sequential numbering for runways, starting from 1.

Liverpool (for example) has runway 9/27; so how are these numbers assigned? Also, what do Left and Right signify?

Runways are usually numbered according to their direction, more precisely called runway magnetic bearing or QFU (see Q codes).

Consider a plane flying toward the runway on final approach in a day without any wind. Divide its magnetic heading by 10, round it to the nearest whole number and you'll usually get the runway number. For example, if the magnetic heading is 345° then 345/10=34,5, so the runway number will be 35 which will be a runway used for landings (and takeoffs) to the north.

Opposite ends of the same runway have different numbers, 18 (which represents 180 degrees) apart. A runway with 35  for landings to the north will have runway 17 for landings to the south. Even though these are the same strip of concrete, they are treated as separate runways by pilots and controllers.

If there are two airports near one another with runways at the same angle, sometimes one of the airports will add or subtract one from the runway number to help planes differentiate between the airports.

Occasionally a runway number will change when the magnetic declination angle changes in such amount, making the runway magnetic bearing divided by 10 and rounded to the nearest whole number increase or decrease.

Some runways in areas of large magnetic declination use true instead of magnetic headings for the runway numbers. This is not unusual in northern Canada and Greenland.

When there is more than one parallel runway at an airport, L, R, or C may be appended to the runway number for Left, Right, or Center. These are based on the approach direction, so, for example, the runway 35L would be called 17R from the opposite direction.


How are avionics updates delivered? I'm idly curious to know what type of physical storage medium (e.g. is it a "mini-CD", USB key, a little hard drive, maybe a tape) is used for delivering avionics updates, for an as-recent-as-possible aircraft?

For big (e.g. commercial airliner) and/or military?

I guess you're not downloading updates from the internet?

Could you (optionally) also paint a vague public-domain picture of how the "chain of trust" works? Or is the delivery route (media passing hand to hand) not especially secure, and instead all the chain of trust is in built-in digital signatures?

Different kinds of Software

We need to distinguish between several categories of software.

Field-loadable software (FLS) is, loosely defined, any software which can be reconfigured, updated, or uploaded by technicians and manufacturers, like the avionics software.
Aeronautical databases are not categorized as field-loadable software and are treated separately in regulations. These aeronautical databases may be a terrain database, a navigation database, an obstacle database, or an airport map database.
Factory loaded software, which is assumed not to change during without replacing the device it's on.  To change factory-loaded software you often have to break a seal and flash the memory.  It may also be on read-only memory like an EEPROM.

Thanks to advances in FLS practices, field-loadable software is being used in many places where factory-loaded software had been used previously and frequent software updates are becoming common on most systems. Software on virtually any part can be field-loadable if it follows safety and integrity measures.

Aeronautical databases are treated differently in part due to the difference in update frequency. An aeronautical database must change frequently (often more than once a month), while avionics updates are less frequent and may be less than once a year (especially because the entire aircraft configuration, software included, must be certified).

What steps are taken to update software?

A patent by Airbus explains the process excellently, even with diagrams in French:

  The first step is managed by the manufacturer of the aircraft. The latter decides upon a required modification for the aircraft it has constructed. It develops the modification for the solution found and certifies it with the certification authorities. The manufacturer thus shows that the new configuration proposed is compatible with the environment and with the configuration of the aircraft for which the modification is destined.
  Once the solution has been developed and validated, the manufacturer prepares a pack, referred to as service bulletin, containing a description of the operations to conduct to perform the modification of the aircraft and also a physical element containing the software to change and furthermore containing corresponding documentation. The physical element containing the software depends on the size of the software and it may for example be a USB key (USB standing for “Universal Serial Bus”), a CD/DVD, etc. This pack is delivered physically to the company concerned that operates the aircraft. This delivery may also be performed electronically.

  The second step is carried out under the responsibility of the operator of the aircraft, for example an airline company, or, when the aircraft is under heavy maintenance under the responsibility of the maintenance organization authorized for that. Similarly, this step may be performed by an authorized MRO (standing for Maintenance Repair Organization) when the aircraft is transformed for a change of operator. This is typically of the case for an aircraft hire company.
  The operator receives the service bulletin and transfers it to a technical center (FIG. 1) in order for the operator to allocate and verify the compatibility of the service bulletin received with the environment of the aircraft of its fleet. Once the verifications have been performed and the service bulletin validated, a work request is sent to the maintenance management department of the operator's fleet. That department then defines the times (stopover, heavy maintenance visit, etc.) at which the aircraft may undergo that update and provides the workshop with the software element to install on the aircraft (USB key, DVD, etc.).

  A maintenance workshop of the airline company or of a maintenance repair organization performs the requested task and downloads the software which is in the DVD or USB key (or other medium). The action is recorded and the configuration repository of the aircraft is updated. Of course, the technical center (and possibly other departments concerned) is kept informed of the upgrade which has just been carried out.

Navigational database updates were traditionally updated using a similar process according to this Honeywell patent:

  ... the information contained in the navigation database changes on a very frequent basis as new navigation aids are created, old navigation aids are retired, airports add or retire runways, or the like. Accordingly, government agencies such as the United States Federal Aviation Administration (FAA) typically require that aircraft update navigation databases on a regular basis, such as every twenty-eight days. Other components (such as global positioning systems (GPS)) may also make use of periodic data upgrades.

  Conventional techniques of updating databases have been cumbersome and time consuming. Typically, a customer (such as an airline) obtains a diskette containing the upgrade for a particular aircraft type from a database or component vendor. The customer then duplicates the diskette and distributes copied diskettes to service technicians, who then go to individual aircraft and manually load the data update using a specialized data loader...

See also https://www.google.com/patents/US20030208579 for an example of how in-flight entertainment systems are traditionally updated.

What media is used?

A variety of media has been used over time since storage media changes quickly. Physical media like CD's are quickly being replaced by high integrity network updates due to the cost benefits involved.

A patent for a "Method for controlling customer-implemented data updates" explains how this process was done historically:

  Originally, airline maintenance personnel upgraded flight management
  computers by using an ARINC standard 603 portable tape upload device,
  but such tape loaders were clumsy and slow. Manufacturers then
  progressed to data loading computers that were based on ARINC standard
  615 (Data Loader standard), which is in essence a software protocol
  layered onto an ARINC standard 429 data bus. ARINC 615 data loaders
  abandoned the tape format of ARINC 603 in favor of a 3.5-inch floppy
  diskette medium for transferring data and software... The
  software or data for these updates has grown increasingly more complex
  as the systems in aircraft provide more functionality to the cockpit
  crew and traffic control.

In more recent history, "The media used to load FLS changes with time. In the early 1990's, 3.5 inch diskettes were used.  For example, the original Boeing 777 carried around binders full of floppy disks.  The technology has moved on to compact disks (Cds), digital video disks (DVD's) thumb drives, mass storage devices, and even local area networks (LANs)" source

Another patent discusses in detail the way that software can be loaded over LAN.  The image below is from the patent.  The principles are much the same:

The software must be part of a certified aircraft configuration
Aircraft in the fleet are checked over the network to see if their configuration is out of date or incorrect
Aircraft are updated to a new configuration when and only when the operator initiates the update
A reliable integrity check like a CRC is in place to make sure the update over LAN was successful

What safety and integrity measures are taken?

Fun fact: Field loadable software is considered a part, and therefore the configuration management that applies to parts also applies to software updates.  The updated software must be certified for the aircraft configuration it's on, must have a part number which can be verified, and must appear on the bill of materials for the new aircraft.  Sometimes an entire device will have a new part number when its software is updated, or sometimes the software has a separate part number that's updated while the hardware retains its own part number. Any modifications to the software must go through a change-impact analysis and recertification.

The field loadable software must be certified for the device and aircraft it's being loaded on and there must be checks on the software loading process, the software itself, the software part  number.  DO-178C specifically specifies that:

there must be protection from inadvertent loading (which also means that unlike Windows, it won't force you to update at the worst
possible time)
there must be detection of failed, partial, or corrupted loads
you must be able to reliably determine that the currently loaded software and all its files are correct (for example, by using a CRC)
if a display mechanism is used to display the software load, the display must be accurate and reliable

Integrity checks must have a level of reliability, similar to other reliability check in aircraft.  This often means that the software loading must have a 10^-9 chance of corrupting software or incorrectly indicating the software load is correct.  You can instead qualify the tool used to load the software, but this is less common than showing that the integrity checks are reliable.

Chain of Trust for Aeronatuical Databases

For aeronautical databases, safety levels and error checking are also seriously considered.  Each particpant in aeronautical database creation and transmission must have a compliance plan, configuration management, error checking, a method to ensure unauthorized changes don't occur, and quality management.  As outlined in AC 20-153, The FAA will provide a Letter of Acceptance (LOA) to each particpant in this process indicating their compliance with regulations like DO-200A.  Aeronautical data must meet various standards as applicable like ARINC 424 and DO-291B, but a discussion of those is out of scope of the question.

For more information:

Chapter 5 of FAA Order 8110.49 specifically addresses approval of field-loadable software.  RTCA's DO-200A is the standard for preparing and transmitting Aeronatuical databases from a regulated data provider to the operator of the aircraft.  See also FAA AC 20-153 for certifying compliance and granting letters of Acceptance to DO-200A.
I have heavily referenced "Developing Safety-Critical Software" by Leanna Rierson.  I suggest you reference this or another book.

Disclaimer:  I am not a DER or engineer for the data loading process and so this information is for reference only. For technical work please consult the source documents and a DER instead of relying on this information.

Updating A380 navigation using flash drives


Why don't some airlines assign seats to passengers? Why don't some airlines (for example Southwest in US) assign seats to passengers? How does it help them? Doesn't it delay the boarding (passengers going back and forth inside the plane to grab their favorite seats)?

Are there any studies to show that this is a better method? Or is there any technical reason why some airlines does this while others do not?

It is mainly used by low-cost single-class cabin airline companies to make the reservation process simpler.


What is that letter in the top of the tail of that Jumbo? Aristotle Socrates Onasis, after marrying Ja. Kennedy, bought, from Boeing, a 742, the Olympic Eagle - now left in Hellikon Airport to rot, after the Olympic Airlines went public ..

At the top of its tail the character 'B' appears, what does it mean?

To identify the aircraft from a distance.. eg if you are driving on the ramp and need to get to SX-OAB and there are several Olympic jumbos parked up, and from where you are you can't read the registration on the fuselage. Not too bad if they're in the same general direction but if some are on your right and some are on your left it may help if you could get a hint from the tail. They only had one letter as OA never had that many widebodies.. I think they never had more than 3 747s at the same time and some A300s. Lufthansa on the other had uses the last two letters. Probably because they had 400s (pax and combis) and 200s at the same time. The pax 400s started at D-ABVA, the 400 combis started D-ABTA and the 200s were D-ABZ(x)(Freighters) or D-ABY(x)(pax).

More airlines put the registration on the nosewheel doors, this saves the ramp staff from trying to get a glimpse of the tail (not easy in bad weather). It is also common to see the last one or two digits/alphabets on the front end, most commonly on the nosewheel doors but also above the cockpit windows or just behind the radome.


Why do aircraft piston engines consume so much oil? This question is related to single/twin piston engine aircraft. After flying in a Cessna 152 and a Piper Dakota I have noticed that they both consume a lot of oil. Oil check is an integral part of the checklist. Pilots carry extra bottles of oil in the back. Why is this so? Especially compared to car engines that are 'similar' and do not require such regular top offs?

Automobile engines are not similar. They are liquid-cooled and therefore can be built to much tighter tolerances with regard to thermal expansion and contraction.

Air-cooled aircraft engines must deal with a large range of operating temperatures and oil is consumed due to the relatively looser fit of the piston rings.


Is there any free source for current airline schedule data? I am looking for a free database with complete and accurate airline schedules, especially for major European carriers.

I know there is https://openflights.org/data.html but I find the accuracy of the data very low or at least heavily outdated.

Can you point me to some other, free sources?
It can even be a website one could copy content from, not necessarily a csv or an API.

This is for academic use only. Maybe some paid sources have other policy for non commercial use?

I am currently developing a plugin for virtual airlines to pull live data, and I use the FlightAware's API which is really good in my opinion. With a little bit of php magic you can get all the schedules for your airlines for free (if you sign up for the free account) or you can pay to get access to the getAirlineSchedule query which just basically does what it says.

https://uk.flightaware.com/commercial/flightxml/

There is the website if you're interested, to get access to the free area, sign up and click the 'get prices' area and the very left option should be the free pack ;)
If you're interested in my code which I am developing, you can check it out at my github;

https://github.com/RAZERZ/PullFlightData

Good luck ;)

EDIT: You get the data in the form of json, so you can pull strings of data from there :)


Does the downwash created by induced drag increase or decrease lift? According to the theory I know, lift is produced by accelerating air downwards at the trailing edge (downwash). Hence, as downwash increases so does lift. Induced drag is due to vortices at the wing tips and they also create downwash. So according to the above theory induced drag should also increase lift.

But, the books say that induced drag increases downwash and reduces the effective AOA, which subsequently reduces the lift. Does the downwash from induced drag increase or decrease lift?

It is not "this causes that" - all is happening together. Let me explain:

For me to understand aerodynamics, it helped to disregard all that talk of vortices and induction, but focus on the pressure field around a wing. When the theory of flight was developed, electricity was new and exciting, and it just happened that electric induction could be transferred to lift. Now every author still copies the explanations from a century ago, but they are totally unintuitive.

Every air molecule is in a dynamic equilibrium between inertial, pressure and viscous effects. Inertial means that the mass of the particle wants to travel on as before and needs force to be convinced otherwise. Pressure means that air particles oscillate all the time and bounce into other air particles. The more bouncing, the more force they experience. Viscosity means that air molecules, because of this oscillation, tend to assume the speed and direction of their neighbors.

Now to the airflow: When a wing approaches at subsonic speed, the low pressure area over its upper surface will suck in air ahead of it. See it this way: Above and downstream of a packet of air we have less bouncing of molecules (= less pressure), and now the undiminished bouncing of the air below and upstream of that packet will push its air molecules upwards and towards that wing. The packet of air will rise and accelerate towards the wing and be sucked into that low pressure area. Once there, it will "see" that the wing below it curves away from its path of travel, and if that path would remain unchanged, a vacuum between the wing and our packet of air would form. Reluctantly, the packet will change course and follow the wing's contour, but not without spreading out (= pressure loss). Spreading happens in flow direction - the packet is distorted and stretched lengthwise, but contracts in the direction orthogonally to the flow. This fast-flowing, low-pressure air will in turn suck in new air ahead and below of it, will go on to decelerate and regain its old pressure over the rear half of the wing, and will flow off with its new flow direction.

A packet of air which ends up below the wing will experience less uplift and acceleration, and in the convex part of highly cambered airfoils it will experience a compression. It also has to change its flow path, because the cambered and/or inclined wing will push the air below it downwards, creating more pressure and more bouncing from above for our packet below the wing. When both packets arrive at the trailing edge, they will have picked up some downward speed.

Behind the wing, both packets will continue along their downward path for a while due to inertia and push other air below them down and sideways. Above them, this air, having been pushed sideways before, will now fill the space above our two packets. Macroscopically, this looks like two big vortices. But the air in these vortices cannot act on the wing anymore, so it will not affect drag or lift. See here for more on that effect, including pretty pictures.

What is lift?

Following the picture of a pressure field outlined above, lift is the difference of pressure between upper and lower surface of the wing. The molecules will bounce against the wing skin more at the lower side than at the upper side, and the difference is lift.

Or you look at the macroscopic picture: A certain mass of air has been accelerated downwards by the wing, and this required a force to act on that air. This force is what keeps the aircraft up in the air: Lift.

Either way, you will arrive at the same result. By the way: Most of the directional change happens in the forward part of the airfoil, not at the trailing edge!

Induced drag

The misconception about those "wingtip vortices" and induced drag is hard to eradicate. Most authors copy what has been written before without clearly understanding the issue. Therefore I repeat it here again: Induced drag is the backward-pointing part of the pressure force vector. The vortices are only a consequence of downwash, which in turn is a consequence of lift creation. At the same speed, more induced drag is indeed linked to more lift, but the causality is different: Lift and induced drag are both part of the pressures acting on the wing. If you add up all the pressure forces acting on a wing, their resulting vector will point slightly backwards. The streamwise component is drag, and the component orthogonal to the direction of movement is lift. This is just a defininion, made for simplicity.


Is there information available about an aircraft incident from New York to LHR in 2004? I am attempting to find some form of incident log regarding a flight I took i 2004 (I believe May/June).  I believe it was a British Airways flight but it may have been operated by another airline.

I was on a flight from New York (I think JFK) to LHR when we had to divert to a runway in Bangor, Maine due to a cockpit fire, I seem to recall being caused by an Air Conditioning Unit.

I can't seem to find any details online about this incident.  Does anyone know of any resources I could use to locate this?  It was in the Bangor Daily News at the time but I've searched their site and can't find any info.

It seems to me that this wasn't "major" enough of an incident to be on any main logs, etc.

Here it is: British Airways flight diverted to Bangor.

  At about 8 p.m. British Airways Flight 112, traveling from John F. Kennedy International Airport in New York City to Heathrow Airport in London, diverted to Bangor after smoke was discovered in the cockpit.

Story was published on June 10th, 2004. Article says Wednesday night, so it should have happened on June 9th.

Both the AAIB and the NTSB don't have any reports. It must have been a minor issue. Just a BA 777 on the same day, fuel leak on departing Heathrow.


Why are some airfields not on any sectional chart nor listed in the directory? I would have thought this was a safety issue.  Case in point - Hearst Castle Airfield.  This is located beneath William Randolph Hearst's Castle in California, located at  35°39'35.54"N  121°11'35.64"W.

It is very obviously a well maintained runway, although private;  yet is simply does not exist if you were to believe the VFR sectionals.  It did appear in older sectionals, as discussed on the forgotten airfield site (http://www.airfields-freeman.com/CA/Airfields_CA_SanLuisObispo.htm#hearst2), but now not.

If you look at skyvector (https://skyvector.com/?ll=35.658914169287556,-121.18517303105294&chart=301&zoom=1&fpl=3539N12110W), the runway should be right smack under the crosshairs inthe center, where the map is marked "ranch".  But it's not there.  How can this be allowed by the FAA?  Even secret CIA runways are listed on sectionals.

I assume that the Hearst Castle administrators requested removal of the field from the map.  Isn't this a safety issue - in the event of an in-flight emergency, this would be a perfectly good field to land on, but being unmarked, there is no way to know it is there.  How can this be?

There are lots of airfields not listed on the charts or in the directory. For what its worth there is nothing that requires private airstrips to be listed. As mentioned in the comments sometimes its not worth the hassle. As you mention in the original question it is a nice gesture to list them for other pilots in an emergency. That is covered nicely in this question. They may not have listed it because they don't want people doing touch and go's there, or landing there and knocking the door of their house or they did not want to up-keep it etc. etc.


How many flying hours does one need to become a commercial pilot in the Philippines? I'm trying to calculate all the expenses to become a pilot in the Philippines.

While CAAP seems to be pretty (or even very) vague on the topic, this is what I've been able to find:

To start with the private pilots license:

  You must also complete the required minimum of 40 hours of widespread flight training. The hours of flight time consists of at least 20 hours of dual flight instruction, at least 10 hours of solo flight, and 10 hours of which can be solo flights or dual.

Then you will need to build on this to get a commercial pilots license:

  You will need about 220 hours of total flight time to get your CPL.

* Another source suggests you need a minimum of 150 hours. Be aware however that this is not an exact science: Even for the PPL it can take 60 hours to 'get the feeling for it', so it is difficult to estimate exactly how much you will need. While I'm not a pilot myself I think it's a good idea to be on the conservative side and overestimate the hours you will need for a PPL and CPL.

Source

This is probably unfortunately what you have also been able to find. It is however a very good idea to go past the flight school in person and talk to them about this topic, since it's uncommon, and they can probably give you a better estimate. Be aware that there can be several more costs that are not visible at first sight (checkrides for instance) that can add to the tally so even the ones that appear the cheapest are not.


What are the length requirement for US aircraft N-Numbers / registrations? Many small aircraft registered in the US seem to have a registration that follows the pattern of N, 4 numbers, and ending in a letter. Sometimes they only have 4 characters, or less. And then there's the FAA Gulfstream N1.

Who decides on what the N-Number will be, and are there requirements for how long it should be?

It's not just small aircraft in the US - all aircraft with US registration follow the same rules. The FAA has guidelines on how an N number must be formed. Basically, there are relatively few limits - you can have N + up to 5 characters, up to 2 of which can be letters (at the end).

Here's what the FAA has to say:

  U.S. registration numbers may not exceed five characters in addition to the standard U.S. registration prefix letter N. These characters may be:

  One to five numbers (N12345)
  One to four numbers followed by one letter (N1234Z)
  One to three numbers followed by two letters (N123AZ)

  To avoid confusion with the numbers one and zero, the letters I and O are not to be used.

  Other Requirements

  An N-Number may not begin with zero. You must precede the first zero in an N-Number with any number 1 through 9. For example, N01Z is not valid.
  Registration numbers N1 through N99 are strictly reserved for FAA internal use.

When an owner registers an aircraft, they can request an N number, provided it's not already registered.


What does a helicopter "gas prod" gauge measure? The beginning scenes of the movie "San Andreas" partly take place in a rescue helicopter. One of the panel gauges read "GAS PROD" above it. I googled it but didn't find much. Can someone explain what it does, and how it measures what's it measures? Thanks!

The instrument shows the gas generator rpm in the engine. It think you are referring to this scene (screenshot from youtube):

The Bell 412 in the movie uses a pair of Pratt & Whitney Canada PT6T-3BE Twin-Pac turboshafts, which are free turbine engines. The turboshafts used in helicopters consist of,

Compressor

Combustion chamber

turbine
and other components.

The gas producer (from which 'GAS PROD' came) forms part of the turbine. The gas generator forms the first stage that runs the compressor and makes the engine self sustaining. It also runs the oil and fuel pumps. As it is a free turbine, the gas generator stage is not mechanically connected to the next (power turbine) stage. From faasafety.gov:

The turbine section consists of a series of turbine wheels that are used to drive the compressor section and the rotor system. The first stage, which is usually referred to as the gas producer or N1 may consist of one or more turbine wheels. This stage drives the components necessary to complete the turbine cycle making the engine self-sustaining. Common components driven by the N1 stage are the compressor, oil pump, and fuel pump. The second stage, which may also consist of one or more wheels, is dedicated to driving the main rotor system and accessories from the engine gearbox. This is referred to as the power turbine (N2 or Nr).

In some cases, NG is used in place of N1. The gas generator speed is measured in the PT6 engines using a tachometer-generator mounted  on the engine.
From United turbine training Manual:

The tachometer-generator produces an electric current which is used in conjunction with a tachometer to indicate gas generator RPM. The tachometer-generator drive and mount pad are located at the 5 o’clock position on the accessory gearbox and is driven form the internal scavenge pump.


Who was the air traffic controller voice? I recall reading somewhere that the 'Air traffic controller' voice (you've heard it on TV on many occasions) started with one specific person and his mannerisms have been copied by many air traffic controllers since, particularly in the US.  The article mentioned his name but I can't find the article and can't remember his name.

What was the name of this chap?

Edit: I think that I read this in Fate Is The Hunter, an autobiography of a pilot working in the '30s to '60s.  Unfortunately I don't still have my copy so I can't look it up.

You are thinking of a reference to Charles ("Chuck") Yeager.  He was certainly not an air traffic controller, but there has definitely been a statement published in print saying that his manner of speaking over the radio has been the model that has been copied numerous other times (in movies and by other pilots in real life, as I recall the statement saying.)

The reference was in the book "The Right Stuff"-- pages 33-34 and possibly a few more (not shown) in this ebook edition.


Why do the flap maneuvering speed margins to stick shaker depend on altitude? Why do the flap maneuvering margins to stick shaker reduce with increasing altitude?

Ref: It is written in FCTM of the Boeing 737-800:

It's because the air is thinner, meaning the dynamic pressures are reduced for a given velocity through space, but the inertial mass of the air frame is the same as at sea level. Or you could flip it over and say the inertial effects (true airspeed) are higher for the same indicated airspeed (dynamic pressure). In other words, the dynamic forces have less influence over the inertial forces for a given velocity and mass. As well, the thrust margins to accelerate out of a stall are also less.

Best illustration is to take it to an extreme.  Say you are at 45000 ft, flying near stall at 150kt indicated.  That is, the dynamic pressures of the air are equivalent to 150kt at sea level. However, your actual velocity through space is roughly 300kt, so all of the inertial forces are effectively doubled relative to the aerodynamic forces, and on top of the that, you have more or less zero thrust reserve. You now have WAY less stick shaker margin, and a stall at that altitude requires extreme responses (aggressive dives to gain speed).

In your case, the effects are a lot more subtle, but still significant, especially approaching 20000 ft.


How to hold the heading on the Boeing 777 autopilot? I have X-Plane 10 with Boeing 777 Worldliner Pro- Extended Pack, which is designed to model the real-world operation of the 777.

Why does the AP never go to the heading hold direction even if I press the hold button?

Pressing the heading hold button will assume the current heading.
Pressing heading select will follow the course as selected in the display; this can be set by pressing the rotary encoder. It is also visible on the heading bug.

The Boeing 777 primary flight display will display HDG SEL and HDG HLD for each selection respectively.


Can an airplane on an SVFR clearance land when visibility is less than 1 mile? Let's say I'm on an SVFR clearance and the flight and reported ground visibility deteriorates less than 1 mile, what happens?

If the pilot is on an SVFR clearance, the weather inside the surface area deteriorates below 1 mile visibility, and the pilot has the airport in sight, he can inform the controller that he has the field in sight and will be cleared to land. The pilot is responsible for continuing to the airport or exiting the surface area in order to remain visual.

  NOTE− Clear an aircraft to land at an airport with an operating
  control tower, traffic permitting, if the pilot reports the airport in
  sight. The pilot is responsible to continue to the airport or exit the
  surface area. 14 CFR Section 91.157 prohibits VFR aircraft (other than
  helicopters) from landing at any airport within a surface area when
  flight visibility is less than 1 mile. A pilot could inadvertently
  encounter conditions that are below SVFR minimums after entering a
  surface area due to rapidly changing weather. The pilot is best suited
  to determine the action to be taken since pilots operating under SVFR
  between sunrise and sunset are not required to be instrument rated,
  and the possibility exists that flight visibility may not be the same
  as ground visibility. 14 CFR Section 91.3 authorizes a pilot
  encountering an inflight emergency requiring immediate action to
  deviate from any rule of 14 CFR Part 91 to the extent required to meet
  that emergency. Flight into adverse weather conditions may require the
  pilot to execute the emergency authority granted in 14 CFR Section
  91.3 and continue inbound to land. Air Traffic Control Handbook, 7−5−4


How reliable is the Airbus FAC calculated gross-weight? I've noticed some variation between loadsheet weights and FAC (Flight Augmentation Computer) calculated weight. That's of course normal, since loadsheet is calculated with standard weights and FAC calculates the weight from alpha and speed (at least as far as I know).

Question: Does anyone have any information about the correctness and reliability of the FAC calculated gross-weight?

I'm also interested in how exactly, and with what parameters, the FAC calculates it.

so after having done some more research this is what I have found.The A320 has two independent gross weight calculations. When the aircraft is on the ground the FMS does the GW calculation. Prior to engine start, the current GW and CG are not defined.; predictions are computed based on the pilot entered ZFW , ZFWCG, blockfuel and the FM calculates the TOW. The GW is calculated  by adding FOB to the pilot entered ZFW (GW=ZFW+FOB)  However I don't know what this iteration rate would be.
When the A320 is airborne, the FACs independently determine the aircraft gross weight (GW) by using the aircraft angle of attack sensors. If a significant difference occurs between the FAC and FMS gross weighs (delta > 7 tons) , the  message "CHECK GW" will be displayed on the scratchpad.


How high are you when you are three miles away from the threshold of the landing RWY and get 3 white and 1 red on PAPI? Let's say you were shooting a visual approach and you were precisely on a three mile final. You gave a glance at the PAPI, you find it giving three white and one red. How high you are from the 3 degree glide path at the moment?

Anybody can do math for me?

According to AIM 2-1-4, three white one red would equal 3.2 degrees above the horizontal. Doing some math, and assuming you are 3 NM from the TDZ (ie the PAPI) and ignoring slant range error, 3 NM would put you 18,228 ft from the threshold. Assuming you were on the 3 degree PAPI glideslope, you would then be 955.3 ft above TDZE. If .2 degrees high, or 3 white one red, you would be at 1019.1 ft above TDZE, or about 64 ft above the correct glideslope.

Math if you care: [tan(3.2)*18,228]-[tan(3)*18,228].


What are torque events and are they very common when flying a rotorcraft? On rotorcraft like the MD500 the POH suggests that torque events be recorded in the aircraft maintenance logbook.

What are these events, are they common and how many are typically logged during the average flight?

Most western made helicopters have limitations on the amount of engine torque to which the transmission and drive dynamic components may be subjected. Exceeding the rated torque either transiently or for a specific length of time can trigger the need for special inspections or worse, an actual maintenance evolution. Depending on make and model, short transient excursions may be allowable under certain conditions. Many, if not all, Russian made equipment have well overbuilt transmissions / dynamic components and I have heard that some don't even have torque indicating instruments. Just keep pulling collective till max engine temp or engine droop. This comes at the expense of heavier transmissions though. Hope this helps.


Where can I find nice animations of flutter? Where can I find nice flutter animations/videos (other than YouTube) to add to a presentation without violating Copyright regulations ? It can either be for wings as well as blade arrays.

Are you aware of any OpenSource database on this topic ?

YouTube is really your best bet - is there any reason you wouldn't be able to use one of the many NASA videos?

If you need a release for legal reasons NASA is famously liberal about granting rights to their research/educational videos - just contact then and you'll almost certainly get an official "Sure, you can use it!"


Can windshield washer fluid be used to deice small airplanes? This question is related to small single/twin engine general aviation airplanes with aluminium construction. I understand that it is generally not recommended to use windshield washer liquid to deice an airplane.Based on your personal experience can someone validate this eitherways? I am looking for answers like:

  Used it once and my wing fell off

or

  Been doing this for 15 years with no side effects whatsoever.

As far as I know there is nothing in the POH for the planes that I'm currently flying ( C152 and Piper Dakota ). Also there is a significant price difference in the price of common windshield washer fluid and stuff like TKS. Also the question is limited to deicing the metal parts like the wings and the stabilator, not the plexiglass.

The short answer is, you should not use windshield wiper fluid.

TKS fluid and Windshield wiper fluid are very different things first and most importantly windshield wiper fluid has no FAA approval for this kind of operation (hence the price difference). You can find a full list of de-ice fluid for ground deicing of small aircraft in table 1 of AC 135-17. AC 135-17 also provides the FAA's suggestions on small craft ground de-icing in general.

TKS fluid is not for deicing aircraft on the ground it is for in-flight fluid based FIKI systems and is pumped out of various ports and flows over the wings/surfaces. It prevents the build up of ice in flight. For ground deicing, you use FPD/ADF fluid which is a thick viscus fluid that is often sprayed on the aircraft. Ground based deicing is intended to last from the aircraft from the gate (or other static position) until airborne and thus the FIKI system can be used. it also may prevent the build up of things like snow and ice while taxing.

The reason your POH says little if anything about de-icing is because neither the Dakota or the Cessna you mention are certified for flight into known icing, thus if icing conditions prevail on the ground they more than likely prevail in the air and you are not going anywhere anyway. The typical use case of de-icing a small aircraft is getting it out for the day after outdoor storage over night during snow, freezing rain, or enough frost build up to be an issue. If the temp is warming and icing no longer prevails the aircraft may still be contaminated. In this case you would use FDS liquid to clear the surface (and possibly a brush for any heavy snow build up). Many airports will have this on hand at the FBO.

You should avoid warm water as it can freeze again if the temperature is low enough.

This question is worth reading over as well.


What is the unit for the oil quantity display in the Airbus A320 ECAM? Is this image showing 18.5 liters or quart of engine oil?

The answer is actually in the image: Below "OIL", it says "QT", which according to the "Abbreviations" table under "General Information" in the A320 FCOM, means

  Quart (US)


Are aviation accident investigation reports always made public? How is the dissemination of aviation accident investigation reports regulated? Are the reports public by default, or can they be kept secret? Are there any known rules on this?

If the reports are not public, do the people involved in the accident have a right to access the evidence and/or the reports? Can they find out what the verdict (if there is one) was?

This surely depends very much on the country. Please indicate the regional scope of your answers. (My interest is specific but not limited to France.)

Unless there is some risk of national security to releasing a public report of the accident, the reports are released. You can search the public ones (almost all are public) at the BEA website. The same applies for the other aviation accident and incident reports and you can find the available reports on the websites of the group that has jurisdiction over the accident/incident; NTSB for the US, AAIB for the UK, and many other ones.


How much data do FDR and CVR generate on commercial jets? I'm wondering about how much CVR and FDR data an A320 (for example) would produce and at what intervals.

I believe that there are 4 or 5 voice channel and several sensor data things. I'm trying to get some data on commercial jets. Is there any place I could get some data on this?

The requirements for Flight Data Recorders (FDRs) on airliners like the A320 is in 14 CFR 121. For a good summary of the requirements for flight recorders in various types of aircraft, see the NTSB FDR Handbook.

Depending on the certification date of the aircraft (14 CFR 121.343), the FDR must record between 6 and 17 parameters. Since engine parameters are included, this could be up to 23 different parameters on 4-engine aircraft. Newer Digital FDRs (DFDRs) must record at least 88 items (14 CFR 121.344). Some of these parameters are conditional, and some require multiple parameters, so the total will probably be more than 88 parameters. The NTSB report on the US Airways 1549 accident notes that the FDR recorded 178 parameters during the flight.

According to this page, tape-based DFDRs can record at 64 12-bit data words per second. Newer DFDRs using solid state recording can store up to 256 12-bit data words per second. This 256 words per second is also specified in the information on this FDR. It says it can record >100 hours at 64 words/second, which translates to about 35 MB total. The parameters are per DO-160C standard, which you have to pay to access.

Accident reports will generally include readouts from relevant parameters, to give an idea of the items that are recorded. There is a table here showing some parameters, the measurement range, and recording intervals. The interval is anywhere from 0.125 seconds to 4 seconds depending on the parameter.

Aircraft may also have a Quick Access Recorder (QAR), which records many parameters (over 2000). However, aircraft are not required to have one and the QAR is not required to survive an accident. Airlines use them for data to improve operations, and they can also be used in less severe crashes for analysis.


Can autopilot navigate automatically? When the pilots enter their flight plan into the FMGS, the route will be shown on the ND. Can the autopilot automatically turn when needed with the given plan, or the pilots have to manually adjust the heading everytime?
And if possible how do they can manually turn the aircraft within autopilot when needed like: landing, etc.

Generally speaking (not specific to Airbus), what the layman refers to as "autopilot" is actually three logical systems:

The Flight Management System (FMS), which  figures out where the plane is in relation to the supplied flight plan.
The Flight Director (FD), which figures out how to navigate the plane to where the FMS wants it or according to manual inputs.
The Autopilot (AP), which manipulates the flight controls to follow the FD.

Generally, the FMS and FD will be engaged for the entire flight, but the AP is not engaged until after takeoff and is disengaged shortly before landing.

While the flight plan is issued before takeoff, it is frequently amended by ATC during flight. When this happens, the pilot just enters the new data into the FMS and lets the FD and AP adjust as needed.

However, sometimes ATC will issue "vectors" to aircraft, which is an instruction to fly a particular heading or track. To comply, the pilot dials that setting into the FD control knob and changes modes so it follows that manual input instead of the FMS. When ATC puts them back on course, the pilots simply switch the FD back to following the FMS.


Is more cargo loaded if there is less passengers than expected? When pre-flight weigh in is done, if the plane weighs less than expected because people don't turn up, is more cargo added?

Conversely - if the plane is over the estimated weight - does cargo get removed?  Passenger bags removed?  Passengers removed?

Is it within the airline's strategy to achieve 100% seat capacity utilisation?

You pose several scenarios.

  Is more cargo added if people do not show up?

No. By the time a person does not show up, the cargo is usually already delivered and loaded on the aircraft.

  If the plane is over the estimated weight - does cargo get removed? Passenger bags removed? Passengers removed?

Yes, although passengers almost always have priority. On heavy cargo flights, the cargo department will send a priority list if the load planners need to bump cargo.

  Is it within the airline's strategy to achieve 100% seat capacity utilisation?

Absolutely, although on payload restricted flights, non-revenue passengers may not always priority over cargo. This depends on the route. Many long-haul flights, if payload restricted, a scenario could happen where a staff member traveling as a non-revenue passenger will not be accepted because accepting them would exceed the max payload for the flight.


Is it possible for an airplane to produce thrust from compress air without using fuel? Is it possible for an airplane to fly as it compress the atmospheric air during flight?

First of all, where does the compressed air come from? Is it stored in big tanks, or do you compress it as you fly?

Assuming you use the second way (the first would be quite inefficient), you need to power the Air Compressor, which means you need to generate energy.

If you do not want to use fuel, then it has to be powered by electricity, i.e. an electric plane. This is basically how the Airbus E-Fan works, using two ducted fans powered by electricity (as @Zach Lipton said in the comments)

So to answer your question, yes, your theory works. There are already a few prototypes using it.


Difference between multi slats and three slats on B737 ? (leading edge devices) What is the difference between multi slats and three slats on a B737 ?

Prior to the NG, the total number of slats is 6, that is 3 slats per wing

Thus there are 6 slats numbered from 1 to 6, that is slats 1 and 6 are the outer slat

The NG's have an extra outboard slat on each wing giving 8 in total. They have the same sequencing as the classic

Thus a 3 slats per wing refers to 737 prior to the NG

more precisely

Leading edge devices are comprised of 4 Krueger flaps inboard of the engines and 6 slats outboard of the engines. The LE flaps are extended whenever the TE flaps are not up. The slats extend from 1 to 5 and fully extend when beyond 5. Slat numbers 1 & 6 (the outboard slats) move a few degrees less than slats 2 to 5 when at full extend, causing the leading edge to look slightly disjointed in this configuration, this is normal.


Can you broadcast a very low power nav vor signal in air? Are there any FCC restrictions on broadcasting a test nav signal on 108.0 in the air? Or perhaps on other nav frequencies that can be received within the navigation band?

The use of radio transmitters is strictly controlled almost everywhere in the world. FCC would be the authority in charge if you are asking for the USA.

The International Telecommunications Union (ITU) is in charge of allocating frequency bands for different applications in the radio spectrum. You are only allowed to transmit if you have a frequency assignment, if your equipment is certified, and you are holding the required radio license.

There are use cases where non-certified equipment may be used (e.g. amateur aka HAM radio), or no license is required (e.g. wireless consumer devices like cell phones, remote controls, etc).

108.0 MHz is at the upper edge of the frequency band allocated to FM radio broadcasting, and the lower edge of civil aviation frequencies. Therefore 108.0 MHz is not available for experimental navigation signals. Your usage would be illegal. The same will be the case for any other frequency that can be used with certified navigation receivers.


What are Mode C requirements with ADS-B 978? ADS-B out is available on the 1090 (Mode S) band, and can even be implemented as a firmware update to some Mode S transponders.

In the case of 978 ADS-B Out, I believe that it is still necessary to keep operating a Mode C transponder for operations where Mode C is currently required, as noted in This video at 16:25, in order to work with TCAS systems.  Continuing on to 41:24, there is a recommendation stating that if a Mode C transponder fails, it should be replaced with an ADS-B "out" transponder.  Here are my questions:

Is there such thing as a 978 transponder which will meet ads-b out requirements and eliminate the need for Mode C, or does this recommendation essentially mean that you move to a 1090ES transponder?
Perhaps this recommendation is assuming that there was no ads-b out at the time.  Given an aircraft with a failed mode-c transponder, what equipment options which meet the mandates are worth considering for GA aircraft?  Apparently John Zimmerman does not recommend Mode C + 978 out.  It appears he recommends 1090ES.  Either one of these would seem to benefit from having 978 in.  I can see no advantage to 1090ES in, as it appears to offer only a subset of the 978 services, unless it was desired to be able to receive 1090ES air-to-air or our operate outside the US.

For Class A,B and C, you'll need a Mode C transponder as per the present regulations (unless cleared by ATC). From 14 CFR §91.215 ATC transponder and altitude reporting equipment and use:

§91.215   ATC transponder and altitude reporting equipment and use.
(b) All airspace. Unless otherwise authorized or directed by ATC, no person may operate an aircraft in the airspace described in paragraphs (b)(1) through (b)(5) of this section, unless that aircraft is equipped with an operable coded radar beacon transponder ... and that aircraft is equipped with automatic pressure altitude reporting equipment having a Mode C capability that automatically replies to Mode C interrogations by transmitting pressure altitude information in 100-foot increments. This requirement applies—
(1) All aircraft. In Class A, Class B, and Class C airspace areas;

FAA also addresses this in its FAQs regarding ADS-B:

Will Mode C transponders be required indefinitely?
The FAA is considering additional changes in the national airspace system, such as for TCAS. These changes, may at some future date, reduce or eliminate the need for transponder equipage.

So, at present, you'll need Mode C anyway.
Also, you'll need 1090ES in Class A. From FAA FAQs:

Aircraft that fly internationally and aircraft that require TCAS (Traffic Collision Avoidance System) are already operating with Mode S transponders and many have older versions of ADS-B Out on 1090ES MHz. Aircraft flying in Class A airspace must operate on the 1090ES MHz frequency. General aviation users who choose the UAT (978 MHz) link may take advantage of the ADS-B traffic, weather, and aeronautical services that are transmitted on the UAT frequency at no charge but must retain their current ATCRBS transponder. Mode S, 1090-ES users can operate in all airspace, but cannot receive FIS-B services.

I agree with you that there are a number of services (through FIS-B) for 978 UAT. In that case, (if you are not going above 18,000 ft) I suggest you check the availability of UAT receivers before taking a decision.


How does an ETOPS APU start in-flight? An ETOPS APU has to be on-demand, i.e. must be able to start at any altitude. If it can't, then it's not shut down for the entire flight.

Compared to a normal APU, what are the design differences that would allow a start in a wide range of density altitudes and in the ram air of the cruise?

My guess is a combination of movable doors in the inlet and maybe special compressor stages.

Some aircraft may have a special "ETOPS APU" that can be purchased, but many aircraft simply have THE APU that they come with. It is maintained to the standards required in the approved ETOPS program, and is either run during the ETOPS portion of the flight, or else has some means to ensure that it will reliably start even when cold-soaked. (For instance, at least one ETOPS program requires that the APU be started shortly before the top-of-descent on enough flights to "prove" that it does, in fact, start up reliably even in a worst-case cold-soaked scenario.)

Most (perhaps all) APU's in aircraft currently in production have the general capability of being started in flight, so that particular aspect of things isn't unique to the ETOPS world.  Documenting the reliability of the APU is a uniquely ETOPS thing, but so much of ETOPS is about exactly that: demonstrating and documenting just how reliable the hardware is.

It often isn't different hardware, but the same equipment as in a non-ETOPS aircraft, but maintained & tracked to higher standards.


Why aren't thermal analysis of propeller done? I have searched for articles on Thermal Analysis of Propeller but couldn't find one. Can I assume the aircraft company don't perform heat transfer simulation and analysis of high speed propeller? Isn't that important? What kind of simulation can be done of propeller besides the flow simulation of propeller obtaining drag, thrust and efficiency of it? Thank you.

As a propeller turns through the air at subsonic speeds, the dynamics of creating the pressure distributions in the air surrounding the propeller blade can be accurately modeled as being isothermal (no significant and persistent temperature gradients are created) and adiabatic (no significant net heat transfer from one location to another occurs within the flow field).
This means that the thermodynamic state of the air coming into the propeller disc is almost exactly the same as its state when exiting it, and for this reason there is no significant thermodynamic analysis needed to successfully account for the behavior of a propeller as long as it is subsonic.


Are there any aids (like a padded "belt") that can ease hip discomfort during aerobatics? When flying aerobatics, particularly maneuvers such as a slow roll, I frequently experience rather high discomfort on my jutting hip bones.  I've googled "padded belts" but I have not found anything that would seem to help.  Is such a thing an actual product, and if so, is there a proper term I could search for?

Basically, I'm looking for a garment/belt I could wear that would provide sufficient padding around my hip bones to mitigate this discomfort.  (The discomfort is caused by the tight seatbelt combined with negative G's -- the belt itself is standard issue on a Pitts Special.)

You can try out a padded harness which may help, a variety of places mention the stuff offered by Hooker Harnesses and may be what you are looking for.

(source)

If you currently only have a lap belt installed in the aircraft in question you may want to consider 5 point harnesses as they can help by moving some of the load to your shoulders.


What are these aircraft at AMARG? Why are they dismantled this way? At the AMARG boneyard (Tucson desert, Arizona), there are tons of aircraft, but a limited number of models are dismantled according a special way: wings or tail or both separated from the fuselage:

Image from Google Maps

Source, reframed

I doubt all tails will suddenly be used as spare parts...

What are those military aircraft? why are they systematically dismantled this way?

(Not sure the question is on topic.)

Some B-52s and other aircraft were put out of commission as part of the START treaties.  They were dismantled in such a way that the Russians could verify this with satellite reconnaissance, just as in the photo above.


Is there a reason that commercial jets do not have a “dead man’s switch”? Following high-profile cases of disappearances such as MH370, as well as the lower-profile but no doubt common radar infringement and level / airspace busts that we don’t hear about often, is there a reason that commercial aircraft do not have a dead man’s switch?
Although I have no personal experience in aviation, this would have seemed a very simple way to answer the question of “was it pilot incapacitation?” Either it was, in which case the dead man’s switch triggers some sort of alert to ATC / on the FDR, or it was a  case of outside interference in which case either they a) fake incapacitation by triggering a dead-man’s alert, which the scrambling of jets would soon disprove or b) they don’t trigger a dead-man’s, and so at least pilot incapacitation is ruled out in the subsequent investigation.
Of course there is the question of how a dead man’s switch could be implemented, however given the number of SOPs already taught, this hardly seems a limiting factor.

Very intriguing question. I do wonder what it actually solves, though. Does it only solve the mystery of whether the very rare instance of incapacitation of the flight crew occurred? The Cockpit Voice Recorder does that if it is recovered. In the rare event that the CVR is unrecoverable, the dead man’s switch would only answer if pilot input occurred prior to a crash. How would it save lives? The pilot should already be in periodic verbal communication with Air Traffic Control.
On the other hand, a global monitoring system of the aircraft would make more sense. Each commercial aircraft already has transponders. In the US, each aircraft has ADS-b. This gives a nearly real-time position of each aircraft that can be tracked and archived. This would give searchers a better idea of where to search for survivors. Incorporating a 406 MHz PLB-style beacon into the Flight Data Recorder and the CVR would be even more advantageous in solving the mystery of what happened. Even an automated system that alerts ATC of when contact is lost with the transponder or ADS-b, or when the CVR or FDR 406 beacons were activated, or when the aircraft deviated too far off its filed flight plan, would be better than a dead man’s switch.
After all, in today’s era of modern telemetry and aviation security, would scrambling fighters be of any benefit to the passengers and crew on board while the aircraft is in flight? They would have no way of intervening to prevent a crash.
It’s only benefit would be to alert ATC of a terrorist highjacking in the event that the target were close to the airport. And, the overtaking of the cockpit happened in the last phases of flight. Now, the security of modern cockpits and heightened alertness makes the notification of ATC of a more likely.


Do the cabin announcements follow a common protocol? Until recently, my local airline made all their pre-flight and pre-landing announcements in Swedish and using an informal language. A few months ago they changed this and are now using all the standard phrases (Cabin crew ten minutes to landing, Arm slides, cross check and report, etc.)
Is such a change dictated by some authority or is it the choice of the company? Are the actual phrases defined somewhere, or is it convention?

I can only speak for carriers following FAA regulations. But, It was probably a company policy change. I have a copy of the 2017 FAR/AIM. I don’t remember there being a change since then.

One reason for an air carrier, whether large or small, to use a standard format and phrases is to make sure that every flight and cabin crew completes all of the required elements of a briefing without forgetting items. In the litigious sense, this sort of scripting is much more defendable than simply requiring the crew to mention the required information in their own way.

757toga has listed that guidelines are spelled out in AC 121-24C for US carriers. I would like to also include the locations of the regulations spelling out the requirements for passenger briefings. Other than the required information content contained in the regulations (FAR), and the guidelines in the Advisory Circular and other FAA publications, there is no further guidance on how the briefing is to be delivered. The regulations are in the following Title 14 CFR parts:

91.519
121.571
135.117

Each has a version of the verbiage included below.

In addition to this, the FAA requires all aircraft PICs brief (or ensure briefings are done with all passengers) the operation of safety/seat belts per Part 91.107. The FAA Safety Team recommends following the S.A.F.E.T.Y. format when Part 91 General Aviation aircraft PICs brief their passengers. The acronym stands for the following:

Seatbelts
Airvents & windows
Fire Extinguishers
Exits, Emergencies, & Equipment
Traffic & Talking
Your Questions

This example guidance can be printed out at FAAST SAFETY Briefing

The example verbiage of the actual regulations is below:

  §91.519 Passenger briefing.
  (a) Before each takeoff the pilot in command of an airplane carrying passengers shall ensure that all passengers have been orally briefed on—

  (1) Smoking. Each passenger shall be briefed on when, where, and under what conditions smoking is prohibited. This briefing shall include a statement, as appropriate, that the Federal Aviation Regulations require passenger compliance with lighted passenger information signs and no smoking placards, prohibit smoking in the lavatories, and require compliance with crewmember instructions with regard to these items;

  (2) Use of safety belts and shoulder harnesses. Each passenger shall be briefed on when, where, and under what conditions it is necessary to have his or her safety belt and, if installed, his or her shoulder harness fastened about him or her. The briefing shall include a statement, as appropriate, that Federal Aviation Regulations require passenger compliance with the lighted passenger sign and/or crewmember instructions with regard to these items;

  (3) Location and means for opening the passenger entry door and emergency exits;

  (4) Location of survival equipment;

  (5) Ditching procedures and the use of flotation equipment required under §91.509 for a flight over water; and

  (6) The normal and emergency use of oxygen equipment installed on the airplane.

  (b) The oral briefing required by paragraph (a) of this section shall be given by the pilot in command or a member of the crew, but need not be given when the pilot in command determines that the passengers are familiar with the contents of the briefing. It may be supplemented by printed cards for the use of each passenger containing—

  (1) A diagram of, and methods of operating, the emergency exits; and

  (2) Other instructions necessary for use of emergency equipment.

  (c) Each card used under paragraph (b) must be carried in convenient locations on the airplane for the use of each passenger and must contain information that is pertinent only to the type and model airplane on which it is used.

  (d) For operations under subpart K of this part, the passenger briefing requirements of §91.1035 apply, instead of the requirements of paragraphs (a) through (c) of this section.

  [Docket No. 18334, 54 FR 34314, Aug. 18, 1989; as amended by Amdt. 91–231, 57 FR 42672, Sept. 15, 1992; Amdt. 91–280, 68 FR 54561, Sept. 17, 2003]


How is a confusion possible between Western and Russian Attitude Indicators? Yesterday I watched a documentary on Crossair Flight 498 accident. One of the factors that might have contributed to the accident was the confusion of the pilot on interpreting the attitude indicator; he was initially trained with Russian model (right) instead of Western model (left):

But looking at both type of indicators, could anybody tell how the confusion is possible? In both models, if you see the indicators turning your head right (as if you were sat on the plane's seat), both show that the right wing is "touching" ground. That means that you are clearly rolling right. How can a pilot interpret that the plane is rolling left?

It's so easy to sit in our comfy lounge chairs and wonder how on Earth pilots get things wrong.

The error didn't happen without context. It was dark. There was little to no external visual reference due to cloud. The pilots were manually flying in a busy phase of flight. And the captain was probably adversely influenced by a medication/opioid.

Even without the artificial horizon mixup, the conditions were ripe for spatial disorientation.

This image is taken from this short video (yellow arrows were added). It demonstrates how a left turn on the Soviet display is similar to a right turn in a Western display.

The captain was used to the left display, where if the main line of focus was angled to the left, that would mean he needs to make a right control input to go back to wings level. In the accident flight, he saw that the most predominant line - the horizon on the Western display - was angled to the left. Due to the context I've mentioned, he went back to his basic knowledge - that a left angle means I need to make a right control input.

Of course, if he was on top of his game, he would have remembered that even on the Soviet indicator, the artificial horizon should have been level. But by this point he was already disoriented and confused, with the first officer's instructions ambiguous and worsening his dilemma.


No available data on the 727-200C For a group project in college, it was required to find the data of some randomly asigned aircraft. My group received the Boeing 727-200C, of which only one was produced. Due to this, no one in my group can find any data on this aircraft, only on the 727-200 Basic or Advanced types.

Mainly we have to look for dimensions, weight, in flight performance (cruising altitude and speed, endurance and range), take off and landing distances and which engine it used.

Where should I look for data? If anyone has this data available, could you share it?

So far as the FAA is concerned there is no 727-200C at least not that ever carried a type certificate. The 727 series has a common type cert which you can find here and it has no listing of a 727-200C type. There are however 727-100C and a 727C types listed and some other numberings that use the C nomenclature but none that use -200C. The image that @JuanJimenez links from wikipedia is incorrectly labeled. You can see it carries Canadian registration C-GXFA which is a 727-233 you can find more pictures of it here. Also note that the wikipedia section states only one was built and provides no citation at that.

Interestingly the Smithsonian has a model of one...


What's the standard print size for a sectional chart? I'm checking out the Dallas sectional in digital form for my own amusement. The image, in TIFF format, is massive. I wonder, if you buy one of these in printed form from the FAA (or your local GA office), what would be the sheet size of the map?

The New York Sectional (the only paper one I have on hand right now) is 20 3/4 X 59 1/4 inches. You are correct, they are large and printed on nice thick paper.


Why did Emirates order so many A380s? According to the Wikipedia entry on the Airbus A380:

  As of September 2018, Airbus had received 331 firm orders and delivered 230 aircraft; Emirates is the biggest A380 customer with 162 ordered of which 105 have been delivered.

Emirates ordered nearly half of all the A380s ordered. Why so many? Why aren’t other airlines ordering as much and adding the A380 to their fleets?

Given that most of us don't sit in airline boardrooms it's difficult to say for sure why an airline favors one particular aircraft over another. But I'll have a go anyway. The A380 was initially marketed as having unbeatable costs per passenger compared to similar aircraft, i.e. the 747, so if you wanted to carry a lot of people a long way it was a logical choice.

In reality, those unbeatable costs only come if you can actually fill the seats. The introduction of the A380 coincided with rising fuel costs and the Global Financial Crisis. Less passengers and more expensive fuel helped the business case for heavy twins such as the 777, 787 and now the A350, and most airlines have preferred those types to the A380.

Emirates love the A380 because of some fairly unique circumstances:

Emirates operates a hub and spoke model in an excellent geographic location. It has its main base in Dubai and most of its business comes from connecting passengers through this one city - i.e. passengers travelling between Asia, Africa, Australia and Europe.
Emirates carried 58 million passengers in FY2017-18. How do you carry this many people through one airport, without running out of space? The answer is in bigger aircraft (and soon a second, bigger airport). Other airlines with such passenger numbers usually have multiple bases, and more point-to-point operations.
Finally, Emirates is less sensitive to extra operating expenses than other airlines due to its low cost base. This is largely thanks to favorable government conditions (such as labor and tax laws). Some even argue that they receive outright subsidies from their government.


Wind speed over which a Canadair fire fighting plane cannot safely fly? We recently had some forest fires in Athens, Greece while the weather was very windy. The news said it had 8 beaufort of wind speed (34-40 knots, 38–46 mph) and in these conditions it's dangerous to fly the "canadairs" (I think we have Canadair CL-215 and Bombardier 415).

Is this true? What is a wind speed over which one wouldn't want to fly such a plane?

The problem is not the wind speed per se, but the combination of factors.

Let's start from the data; this is the METAR for Athens on the 17th of July, the day of the fires:

  METAR LGAV 171220Z 03026G38KT CAVOK 32/10 Q1016 NOSIG=

I have highlighted the wind speed data. It says that the wind at Athens airport was on average at 26 kts, with gusts up to 38 kts

26 kts of wind at high altitude would not be a problem for a CL-215
26 kts of wind near ground (to drop the water it has to do low passes) and at low speed (it has to go as slow as possible to be precise) can be highly dangerous, as proximity with the ground increases turbulence. Let's also consider the mountainous terrain around Athen (and the fires were on the side of a mountain), windshear and turbulence were probably quite strong. (See also this question: When crossing a mountain ridge at low height above terrain what consideration is given to turbulence? )
26 kts was the average wind speed, gusts were up to 38 kts, increasing the risks of such operations.

In addition to the above, let's remember that these aircrafts have to fly over fires, that will create additional turbulence through the rising hot air column that they generate. (thanks to David Richerby)

I do not think that a hard limit for such operation exists, as said above is the combination of windspeed, terrain configuration and mission that determines if an aircraft can perform it or not.


Where can I find an accurate list of IATA meal codes? I'm trying to "compile" single list of IATA meal codes. By using different airlines and data sources I have found many meal codes. However, I have also found problems with codes that are used to mean different things, or different codes used to mean the same thing. For example:

On Cathay Pacific's special meal page I have found an information, that both Non-Beef Meal and Liquid Diet Meal are using the same code of SPML. These meal types seems to be far different. How to understand this? A bug on CP's page? To make things more complicated, another source uses SPML code for Special Meal or Specify Meal.
Cathay Pacific uses VGML for Vegetarian Vegan Meal (Strict vegetarian food with high protein, rich iron and high calcium). Other source uses VVML code for this meal type and have VGML code reserved for Vegetarian Non-Dairy Meal. Is there any difference between these two meal types and if yes, then which code is correct / official according to IATA.

Help with the above two examples would be appreciated. However, my main question is:
Where can I find official IATA codes for food?

If there is any "ultimate source", that we can / should use for this purpose (ignoring airline's specific listings, as in case of Cathay Pacific)? If so, then please, provide a link to it.

EDIT: Please, consider above paragraph. This question asks official IATA meal list, not about airline-specific regulations.

SPML is Special Meal, differs from airline to airline.
VGML is vegan, i.e. not even dairy products are used. Unlike vegetarian were animal products can be used, say eggs or milk.
Source: International Air Transport Association. "Cabin Operations Safety Best Practices Guide." (2017).


More than one runway with the same heading. How are they labelled? Given that runways are labelled by magnetic heading. What happens in airports that have more than one in the same direction?

How are they labelled?

Are there any such airports?

Left, right and centre.

For example, Heathrow has 27 left (27L) and right and 09 left (09L) and right.

See here for the airfield chart.

You can see the numbers close to the end of the runways.

Here's an example for an airfield with a 34 left runway.

If there are more than three runways more or less parallel, one or more runway numbers will be shifted by 10 degrees, e.g. 08L, 08C, 08R, 09R.

From the Wikipedia article on runway numbering:

  At large airports with four or more parallel runways (for example, at
  Los Angeles, Detroit Metropolitan Wayne County, Hartsfield-Jackson
  Atlanta, Denver, Dallas-Fort Worth and Orlando) some runway
  identifiers are shifted by 10 degrees to avoid the ambiguity that
  would result with more than three parallel runways.

  For example, in Los Angeles, this system results in runways 6L, 6R,
  7L, and 7R, even though all four runways are actually parallel
  (approximately 69 degrees). At Dallas/Fort Worth International
  Airport, there are five parallel runways, named 17L, 17C, 17R, 18L,
  and 18R, all oriented at a heading of 175.4 degrees.

  Occasionally, an airport with only 3 parallel runways may use
  different runway identifiers, for example when a third parallel runway
  was opened at Phoenix Sky Harbor International Airport in 2000 to the
  south of existing 8R/26L, rather than confusingly becoming the "new"
  8R/26L it was instead designated 7R/25L, with the former 8R/26L
  becoming 7L/25R and 8L/26R becoming 8/26.


Does shrouding a propeller minimize induced drag by equalizing the downwash velocity along its blades? EDIT: It's not a duplicate of Are ducted fans more efficient? That question and the answers doesn't address the reason for the higher theoretical efficiency, it is more about efficiency in practice (drag on the duct, weight, etc) and hence why they aren't used despite the higher theoretical efficiency

I was told shrouded propellers are more efficient becuase tip vorticies are eliminated by the wall which would imply no induced drag but apparently that is wrong Do ducted fans eliminate induced drag? therefore I've been trying to figure out why they are still more efficient than an open propeller despite still having induced drag. It must have less induced drag.

The vortex around an unshrouded propeller:

At first I thought the wall somehow increases the effective wingspan, moving the vorticies to the top of the wall much like winglets do and thus reducing the induced drag that way

However unlike winglets the walls don't have a pressure difference on either side (it's not an airfoil) therefore the vortex can't be there.

So the vortex must be around the whole wall because above the wall the pressure is low and below it the pressure is high.

But this doesn't change the effective wingspan so why does it have less induced drag?

My explanation is that the existence of the wall causes the inside of the vortex to "straighten out" with the flow through the duct which makes the downwash velocity constant along the length of the blades (since the flow is irrotational). This means that this condition is satisfied:

from page 7 of http://naca.central.cranfield.ac.uk/reports/1923/naca-report-121.pdf

Therefore induced drag is minimized. Is this correct?

Basically, yes. The difference between shrouded and unshrouded propeller is that the shrouded one can produce uniform thrust across the diameter, while for the unshrouded one the thrust decreases near the tips.

That way a shrouded propeller accelerates more air than an unshrouded one of the same diameter. This air therefore needs to be accelerated to lower speed, and therefore carries away less kinetic energy, requiring less induced power¹.

However, diameter can be varied, so the efficiency comparison is not that straightforward. When the propeller spins relatively slowly, making it larger is better, similarly to how increasing wing span is better, aerodynamically, than adding winglets.

However increasing the speed of the tips increases parasite drag, especially if it becomes supersonic. And since increasing size while maintaining angular speed increases the orbital speed of the tips, increasing size only helps to a certain point. That's when shrouds become useful.

¹ In propellers and rotors it is called induced power rather than induced drag, because it counts directly against the engine power. It also describes the physics better, since in both cases it is the work that is done on the air by the reaction to the generated lift/thrust.


Why is there such a low level of automation in the aerospace industry (compared to other industries such as automobile)? I have been watching some episodes of the documentaries made by National Geographic lately and I have noticed something that I can't find an explanation for.

In this episode of Megafactories - Eurofighter Typhoon, starting at 17:21, the amount of automation in the factory is apparently very low and the workers seem to be doing everything with their hands.

On the contrary, in this episode about the Volkswagen factory in Wolfsburg, starting at 24:05, the process is much much more automated and there is almost no one around doing anything by hand.

I have always thought that the aerospace industry is one of the most sensitive industries and the highest levels of accuracy, precision and consistency are required. But those documentaries show the opposite to this.

So is there a reason as to why this Eurofighter Typhoon factory is so much less automated than this Volkswagen one? Is there something I might be missing that would make a less automated factory a better choice for the aerospace industry?

There are two big factors at play here: scale and throughput.

First, automation is very good at repetitive tasks. The Wolfsburg plant produces over 800,000 cars per year. It's taken 25 years to make almost 600 Typhoons. That means yearly production is different by four orders of magnitude. It's a lot easier to justify the cost of buying, installing, and programming a robot to save a human from doing the task 800,000 times in a year than just 25 times. This also means that a robot can completely replace a person with a job at one place on the assembly line doing only a few tasks over and over. On the Typhoon line, even if a person only works one line position, they may have many more tasks all over the airplane.

The scale of the Typhoon is also different. It's easy for a few standard industrial robots to access an entire car chassis. It's much harder to access everything on a fighter jet. Larger robots will be even more expensive. Mobile robots are much harder to use with high accuracy. And where you might be able to have multiple humans doing various tasks in the same area, the whole area would have to be blocked off for safety to allow maybe a couple robots to work there.

There is also the factor of processes. A lot of the assembly work on cars is done with welding, which is fairly simple for a robot to do. The Typhoon is mostly assembled with nuts and bolts, which is a much more complex process for a robot to accomplish.

If you break things down to the component level, the balance changes. Drilling thousands of holes in the various components is often done with robotics and some smaller assembly work can be done as well. It's where all of that is assembled into a large structure that humans become the cheaper option.

This also feeds back into the design process. Cars are designed from the start to be easily assembled robotically, because it's much cheaper that way. Not having to design all assembly processes around robot access allows aircraft designers to make things smaller and lighter, a much larger factor for airplanes than cars.


What exactly are "suitcase handles" in a cockpit? I was watching Seconds From Disaster about Alaska Airlines 261 (the one with the horizontal stabilizer issue) and at one point the copilot tells the pilot to "try the suitcase handles". Is that pilot slang for something or did I maybe hear it wrong? (Seconds From Disaster uses actual cockpit recordings for the recreations, so I don't think it's made up or anything.)

From the NTSB report

  "Suitcase handles" is a colloquial term for the longitudinal trim handles located on the center control
  pedestal. "Pickle switches" is a colloquial term for the trim switches located on the outboard side of each of
  the control wheels.

The MD-80 cockpit is shown at this page, with the trim handles on the left side of the console (item 9).

Under normal operation, the pilot will use the switches on the control column to change the trim.  If there's a problem with the thumb switches, the handles on the console are available.


How to calculate the DC-4 landing distance with this graph? Can someone give me a clear explanation on how to calculate the landing distance of the Douglas DC-4 with the following graph? And why is the Temperature variable not in the graph, because temperature also has an influence on the LDR?
Thanks in advance.

Follow the arrows in the graph.

You start with the gross weight on the right X-axis and move straight upward until you intersect the correct field elevation. In the graph the line starts at 58,000 lbs and moves up to a field elevation of approx. 1700 ft.

From there move left into the left side of the graph. There, the X-axis is the effective headwind speed. Note the remark above the graph: This is 50% of the reported speed when the data for this graph was collected. Now you need to find the intersection of the line from the right half with the line vertically up from the headwind. Where both cross you take an interpolation of the landing distance lines. In the example the headwind is 18 knots, so the landing distance becomes 2000 ft.

Yes, temperature would have an effect, but for the landing distance that effect is small. A colder air is denser and reduces all speeds, so a shorter landing distance will be possible in colder air. But compared to a take-off where air density has a much stronger influence on engine performance, this effect can be neglected for a landing chart.


How do I paint an airplane with an electrostatic paint gun? I have a Cessna 150 which needs a paint job. As I have experience in painting objects with a spray can, and also in stripping airplanes, I am thinking to paint the Cessna myself.

What are the considerations required for this job? I would enjoy the experience and I'm able to get it prepped and ready. What would be a good electrostatic paint machine for this job? And how can I practice before painting the aircraft?

Note, I know the paints are different, I've heard of something called Imron, as well as Epoxy paints that harden and set.  Suggestions on that would be welcome too.

I am a retired aircraft painter.

For electrostatic painting: ensure that the aircraft is grounded; ensure that you are grounded to the gun; use compatible electrostatic paint, this is a must.

My suggestion is, unless you are greatly concern about overspraying, to use a normal system on small aircraft, such as suction fed or gravity fed, as it will do the same job. Application is the same for all systems.

Hope this helps


Where are the locations of the elevators or horizontal stabilizers in a Concorde? I don't find the horizontal stabilizers or the elevators of a Concorde at the tail section like other aircrafts. Are they fitted beside the ailerons in the wings?

There is no "horizontal stabilizer" on the Concorde like other commercial aircraft, the wing performs that duty because it is a delta-wing. The elevators for the Concorde are combined with the ailerons into something called "elevons" which perform both functions (Think stereotype paper airplane).

Note: Elevators and Horizontal stabilizers are not the same thing. Most aircraft have their elevators located on their horizontal stabilizers, only a few have the whole stabilizer as an elevator.

Source here


Are modern commercial airliners flyable without operational computer systems? Do modern commercial airliners have manual control capability to fly and land if electric power remains on but all computer systems and digital communication/guidance fails?

For a non-fly-by-wire airliner, yes.  It's just another airplane in the end and as long as there is hydraulic power you can fly it.  If the weather is clear you don't need any electronic aids at all, if you know how to fly in the first place.

For FBW, where computers control the controls and the pilot is just telling the computers what he wants to do, they are dependent on multiple levels of electronic redundancy to account for those sorts of breakdowns.


If we enter and exit through the emergency exit, then how come when the door is opened for the air bridge the slide does not inflate? When we enter through the door it's an emergency door. Right? But if we enter through the emergency door then how come when we open it for the air bridge the slide doesn't inflate?

Take a look at the evacuation slide wiki page for full details.

As a part of the preparations for takeoff the flight attendants will arm the doors. This usually means moving a lever on the door.

If the door is armed when it is opened the slide deploys. If it's unarmed it doesn't.


I just watched at least 20 airplanes flying northeast over Washington, DC every 15-20 seconds, what were they? I just watched at least 20 airplanes flying northeast every 15-20 seconds over Washington, DC. They must have been military aircraft since flying very high. I just saw their lights (not blinking). They actually looked like satellites but I doubt so many in the same orbit every 15 seconds.  What is going on?

This is very likely the "train" of satellites from Launch 5 of SpaceX's Starlink constellation. They have about that spacing at the moment, there are 60 of them, and the timing is about right. I saw them myself last night over Nevada.


Do helicopters have an auto hover system? I'm not a helicopter pilot. The way I understand it, helicopter pilots have to constantly make small adjustments to make a helicopter hover steadily at a certain point in space, just like fixed wing pilots flying in close formations.

Are there any systems which, when engaged, will measure the helicopter's movement with gyroscopes and accelerometers, and automatically compensate for small deviations?

Yes, advanced flight directors and autopilots for helicopters can control the aircraft in all axes, and often they have modes for hovering built in, including maintain altitude, lateral veloctiy hold, hover, and/or autolevel.

Hovering is difficult because it requires more complex controls than maintaining altitude in an airplane.  Helicopters have a different set of control axes than fixed-wing aircraft.  Fixed wing aircraft are controlled in pitch, roll, and yaw, with thrust as an extra variable.  On a helicopter the flight path is controlled through cyclic (roll and pitch), collective, and throttle, with the anti-torque pedals as well to control yaw.  Hovering the aircraft requires controlling of the cyclic to keep the aircraft over its target with no lateral movement, and collective and throttle to maintain constant altitude, although the exact procedures are a little more complex.

While not present on low-end helicopters due to a combination of complexity and lack of necessity, advanced autopilots for helicopters do exist.  Like on other aircraft, there is a wide variety in autopilots between simple functions to maintain attitude (often called one or two-cue guidance depending on the number of axes used) to complex FMS-integrated three-axis modes (which would be "three-cue" guidance). The FAA Helicopter Flying Handbook states, for example, that  "The most advanced autopilots can fly an instrument approach to a hover without
any additional pilot input once the initial functions have
been selected."

On the Agusta AW139 helicopter that I'm familiar with (although I'm not a pilot), they have several modes that provide this functionality:

an altitude mode to maintain a given altitude
an altitude acquire mode to smoothly reach a desired altitude
an autolevel mode to level off at the end of an approach segment
a lateral velocity hold mode to maintain slow lateral motion or no lateral motion
more exotic search and rescue modes providing capabilites to transition to a hover, mark-on-target descents to predefined altitutdes, etc.

A traditional hover would be achieved by combining a low lateral velocity mode with a zero vertical speed mode like altitude or autolevel.