Is setting elevator to certain angle equal to trimming the horizontal stabilizer? (I was trying to understand the longitudinal stability of aircraft and how it is related to elevator)

If I pull the stick back by, say 2 inches, will it have the same affect as trimming the horizontal stabilizer to make the aircraft fly at that angle of attack?

I mean, does setting the elevator at constant 2 degree will make the aircraft swing its nose up as long as the stick is held or does it trim itself to the new angle of attack and ride steadily in its new position?

Yes, both are equal for small angle changes. Exceptions do apply, especially in transsonic flow.
Both changing the stabilizer incidence and the elevator deflection will change the lift distribution between wing and tail surfaces and will trim the aircraft for a different angle of attack. Changing the incidence gives the empennage a new angle of attack while changing the deflection will change the camber (and with it the zero-lift angle of attack) and the angle of attack of the empennage. For small deflections the effect is the same; however, the change in stabilizer incidence is smaller than the elevator deflection change for the same effect in proportion to the square root of the relative elevator chord.
Say you change the stabilizer nose-down rsp. the elevator trailing edge up. The tail will produce less lift / more downforce and the aircraft pitches up. Now the angle of attack of both the wing and the tail will increase which increases lift on both until the distribution of lift between both is as before - after all, we did not change the position of the center of gravity, and the angle of attack will change until the center of pressure is exactly below or above the center of gravity again. There the aircraft finds a new equilibrium speed and will fly on in its new attitude.
Nevertheless, depending on the initial trim point, the aircraft will now climb or sink because the thrust does not match the new drag at the new trim point. The aircraft will settle at a new vertical speed, too.
Before you think that one of them would be enough: Both are needed because each is a specialist for tasks that the other would handle poorly:

The elevator moves quickly and needs small actuation forces, so it is built for manoeuvring or gust response.
The stabilizer moves slowly but can effect much higher lift changes. It is meant for retrimming during or after changes in configuration. Deflection of the lift devices on the wing will shift the center of pressure backwards, so in order to balance all lift forces, the tail now needs to produce massive downforce, something only the adjustable stabilizer is capable of.

This answer covers these aspects in more detail.


How is Altitude/QNH defined state when passing transition altitude? Let's say an airplane is climbing towards transition altitude (TA = 5000 ft), and it has it's altimeter set to QNH = 1030 hPa. The altimeter is a function of pressure and QNH: altitude = altimeter(pressure, QNH). Let's say P is a pressure at TA = 5000 ft when QNH = 1030 hPa (roughly 857.74 hPa) - so P is the pressure when the airplane reaches the transition altitude.
When it reaches TA, the altimeter setting is changed to 1013.25 hPa so the indicated altitude changes as well. With the same P and different QNH, the altimeter now shows roughly 4545.8 ft. Well, this is way below transition level and transition altitude, so the altimeter should be set back to QNH = 1030 hPa. But now it shows 5000 ft again, so let's set it to 1013.25 hPa. The altitude and QNH oscillates, because the input to the altimeter (QNH) is based on it's output (altitude).
The same thing happens on a horizontal edges of airspaces (changing from REG QNH to METAR QNH) and when descending towards transition level.
I assume that pilots change the altimeter setting to "just something". But how should a software calculating the indicated pressure altitude deal with this? The point of the software is to calculate events where QNH should be changed by a pilot. The software runs after the flight and has access to resources such as airspaces, transition altitudes, METARs, GPS and pressure during the flight, and so on.

In Canada at least, you are supposed to make the change between QNH (alt setting) and QNE (standard pressure) while in QNE airspace, whether climbing or descending; that is, right after passing the transition altitude while climbing, or just before passing transition altitude while descending.  The variable overlap zone you describe resulting from the change in barometric pressure is not critical since you are almost always just passing through on a climb or descent clearance to an altitude a couple thousand feet or more above or below. ATC sees your altitude in 100 ft increments as broadcast by your transponder, computed baro alt (transponder's pressure alt signal corrected for local baro pressure) when below the transition, then pressure alt when above, but they don't know or care what you are seeing on your altimeter while climbing through the transition as long as you level off where you are supposed to be.
So I would expect to see the altimeter setting change shortly after passing transition altitude when climbing, say within the next 500 to 1000 ft, but the pilot doesn't have to work out exactly what QNH altitude to observe to be able to make the switch right at transition alt.  Shortly following is good enough and if you are in a climb to, say, FL100, if you forget to set to 1013.25 until the TA is 8000 ft, it's not that big a deal (nobody's going to know - except for the Flight Data Recorder, and your pilot monitor nanny software that is... lol). All that matters is that you level off at your cleared FL. ATC only sees your FL, or FL corrected internally to baro, on radar so the controller isn't interested in where you switch exactly while in a climb as long as you level at the correct FL, although you are supposed to do it in a timely way.
On a descent, with a TA of FL50, I would expect to see the change from QNE to QNH while still above the TFL, say between FL50 and 60.  Even there, if you forgot to set to QNH until below FL50, if you are descending to a lower altitude, it's not critical as long as you level off at the cleared QNH altitude, monitoring software notwithstanding.
Note that Europe may have variations on this protocol, since in North America the transition level is FL180 everywhere except the high arctic (called the Standard Pressure Region in Canada), where the FLs start at the surface except where near airports.
This is the Canadian regulation on moving between Standard Pressure and Altimeter Setting zones:

Transition – CAR 602.37 – Altimeter Setting and Operating Procedures in Transition between Regions, specifies that except as otherwise authorized by ATC, aircraft progressing from one region to another shall make the change in the altimeter setting while within the standard pressure region prior to entering, or after leaving, the altimeter setting region. If the transition is to be made into the altimeter setting region while in level cruising flight, the pilot should obtain the current altimeter setting from the nearest station along the route of flight as far as practical before reaching the point at which the transition is to be made. When climbing from the altimeter setting region into the standard pressure region, pilots shall set their altimeters to standard pressure (29.92 inches of mercury or 1013.2 mbs) immediately after entering the standard pressure region. When descending into the altimeter setting region, pilots shall set their altimeters to the appropriate station altimeter setting immediately prior to descending into the altimeter setting region. Normally, the pilot will receive the appropriate altimeter setting as part of the ATC clearance prior to descent. If it is not incorporated in the clearance, it should be requested by the pilot.


Could landing gear weighing scales be used to detect lack of lift? There have been several crashes due to ice on wings destroying lift. One of the issues with this is that you can't detect that you don't have sufficient lift until you're already in the air, way too late to abort.
Some aircraft have weighing scales in the landing gear. Could these be used to verify the expected lift, and abort if an anomaly was detected before $V_1$?

Technically one could develop a system that compares the decrease in weight-on-wheels during the take-off run to a theoretical lift model in order to determine whether the wings are performing as expected.

I see a number of reasons why this will not be successful a product:

Limited benefits:
The benefit of such a system is minimal. There are very few crashes due to contaminated wings or wrong flap/slat configurations. Let's assume there are one hundred thousand airline flights per day and one lift related take-off crash every 4 years. That is one in 150 million.
Available alternatives.
For both causes there are viable alternatives to detect them well before V1. Visual inspection of the wing usually detects contamination and many aircraft automatically check for the take-off configuration to be set whenever the thrust increases beyond a pre-defined trigger level.
Increase in number of rejected take-offs: No system is perfect. It will detect some contaminated wing situations and save the day, but it will also cry wolf in cases where there is nothing to abort a take-off for. If the system has a false alert rate of one in ten thousand take-offs (which would be very good and expensive to design and certify) it would cause 15000 aborted take-offs (assuming the same numbers as above) for every crash it prevents (assuming a missed alert rate of 0).

Since lift increases with the square of air speed, it is difficult to detect lack of lift early on during the take-off run. At low speed there is relatively little lift to be expected and it will be in the noise of the measurement. Also the accuracy of the airspeed indicator is relatively low at low speeds, so the theoretical lift model will not give accurate results either. As a consequence the system will only be able to detect lack of lift at high speeds. At those higher speeds, the influence of bumps in the runway start to affect the measurements as well. It is easy to see that detecting lack of lift is no so straight forward. False alerts will be raised and they will be at raised at high speeds.

High speed rejected take-offs are risky.

  Boeing studies indicate that approximately 75 percent of RTOs are
  initiated at speeds less than 80 kt and rarely result in an accident.
  About 2 percent occur at speeds in excess of 120 kt. The overruns and
  incidents that occur invariably stem from these high-speed events.

So effectively such a system will likely increase the number of accidents.


Is cabin air freshened when a plane is taxiing or on the ground? I know that commercial aircraft continuously freshen the cabin air when aloft by compressing and cooling the air.

Is that true when the aircraft is on the ground, at the gate loading or taxiing?

The aircraft when on ground and starts to board passengers is switched to ground power or auxiliary power which runs the air conditioning system. When disconnected, the engines supplies the power and bleed air for the cabin pressurization.

"On the ground, the airplane is unpressurized and the outflow valve is wide open. During preflight, the pilot sets the cruise altitude on a cabin pressure controller. As soon as the weight is off the main wheels at takeoff, the outflow valve begins to close and the cabin starts to pressurize."

Source -
https://www.airspacemag.com/flight-today/how-things-work-cabin-pressure-2870604/


What is "Boost"/"Super Boost" in the PW127M/N? This incident describes rejected take-off due to the "boost" being inoperable. The reason for the RTO is obvious: engine power lower than expected.

But what is this ominous "boost" technology?

"Boost" is an informal name for the ATR72 "Automatic Takeoff Power Configuration System" (ATPCS) which allows for higher than normal takeoff weights.

Higher weights are allowed because the system automatically:

1) senses an engine failure

2) feathers the bad engine

3) applies up to 10% increased power to the good engine.

Theoretically a super sharp pilot could do it all just as well, but the ATPCS does it automatically for an increased level of safety.

Other aircraft manufactures may call it "Automatic Performance Reserve" (APR)

ATR 72-600 Brochure


What is the process to contest a local FAA (FSDO) policy which appears to contradict their guidance? As the title asks, what is the formal process to contest a local FSDO policy which appears to contradict a FAR or FSIMS (8900.1) guidance?

I have already sent an email asking them to reconsider, to which the inspector reviewed the request with his front line manager and said that they were not going to change it (in spite of what I feel is clear guidance to the contrary).

I then asked for the policy to be reviewed by the local FSDO manager who would be in a position to change it, who also said that the policy would not be changed.

What is the next step?  Elevate it to regional/national?  I remember reading about a formal process where you make a request, and if it gets denied you have 30 days in which to ask for it to be elevated to the next level, but I don't know where I saw that.

If you want to force a policy change, write directly to FAA Chief Councel.  If CC writes an opinion that aligns to your position then the FSDO really can't have their own opinion.  If, however, CC disagrees with you then you're sunk unless you want to take it to court.


How should a pilot respond verbally to being cleared for takeoff? When ATC says "cleared for takeoff", what's the correct answer from the pilot?

This is how FAA recommends (PDF):

  A typical takeoff clearance may state, for example, “(Callsign) 123
  RNAV to MPASS, Runway 26L, Cleared for Takeoff”.

  The expected pilot response is, “(Callsign) 123, RNAV to MPASS, Runway
  26L, Cleared for Takeoff”.


What is the Runway Design Code (RDC) for San Francisco airport (KSFO)? I need the Runway Design Code (RDC) for the SFO airport. Can anyone provide a link for this and possibly other airports? I have checked the SFO Master Plan/Development Plan and found nothing.

D-VI. No link. A380 requires this and operates there, so that is minimum.
E is military, VI is a high as it goes.


What (if any) endorsement do I need to give a student who scored 100% on his knowledge test? My student studied his empennage off and got a 100% on his knowledge test.  Usually, when I endorse students, there's a sentence that goes:

  I have given [name] X hours of instruction on the areas in which he/she was found to be deficient on his/her airman knowledge test.

In this case, my student wasn't deficient on any areas of the knowledge test.  Do I need to make that endorsement?

If we're talking about FAA regs I'm not aware of any endorsement required if your student passes the test with a perfect score (they need an endorsement to take the test - I have given the instruction required by..., and they need an endorsement like the one in your question if they fail or don't manage a perfect score).

The endorsement above is intended for use when you need to endorse someone who failed to re-take the exam (similar to what's done if they fail a checkride), or when there's stuff to review from the knowledge test (My instructor combined it in my Checkride endorsement - And has demonstrated satisfactory knowledge of the areas found deficient on the knowledge test.

That said, it is probably a good idea to cover anything the student missed on the exam or doesn't feel 100% confident on, and to log that training - in this case if you do additional ground work you would just log it as you normally do...


What exactly are these unusual flight code names that require ATC priority? (Related to this question)

The FAA's ATC Orders section 2-1-4 lists a number of different flight types that should get priority handling when possible. The obvious candidates are all mentioned: emergency services, VIP flights, search and rescue etc. But these less obvious ones are listed too, what exactly are they?

NIGHT WATCH aircraft
Aircraft using code name FLYNET
Aircraft using code name "Garden Plot"
Special Air Mission / SCOOT flights
OPEN SKIES Treaty flights

NIGHT WATCH / NAOC

Night Watch or Nightwatch is the program identifier for the USAF National Airborne Operations Center (NAOC), including at least certain Boeing E-4 flights.

NAOC (used in flight):

From JO 7110.65W:

  The term “NAOC” will not be a part of the call sign but may
  be used when the aircraft is airborne to indicate a request
  for special handling.

FLYNET

From JO 7110.65W:

  The code name “FLYNET” indicates that an aircraft is
  transporting a nuclear emergency team or a disaster
  control team to the location of a potential or actual nuclear
  accident or an accident involving chemical agents or
  hazardous materials. It is in the public interest that they
  reach their destination as rapidly as possible.

GARDEN PLOT

Classified. Related to operation Garden Plot. Presumably a callsign to be used by aircraft as part of that operation.

SCOOT

Term related to Special Air Mission flights in evacuation of government VIPs.

From JO 7110.65W:

  The term “SCOOT” will not be part of the call sign but may
  be used when the aircraft is airborne to indicate a request
  for special handling.

OPEN SKIES

Callsign for observation aircraft under the Treaty on Open Skies. The USAF aircraft used for this purpose is the OC-135.

From Wikipedia's Treaty on Open Skies:

  Observation/Demonstration flights are conducted
  under rigid guidelines outlined in the Treaty of OPEN
  SKIES that govern sensor usage, maximum flight
  distances, altitudes and priorities.


Can you land IFR at sea? I've heard that some seaplane bases have IFR approaches, but what about landing at sea?

If you flew out into international airspace, could you land at sea and then taxi back into US waters without being subject to US regulations? You wouldn't have to know where the "runway" is because it's everywhere.

(FYI, I'm a SEL pilot, so I'm not considering actually trying this!)

Well there is at least one case where this is true and it's used by the military - A Case 3 instrument approach to an aircraft carrier.

The approach is essentially equivalent to a terrestrial military TACAN approach coupled with a precision microwave landing system on final approach.  See the basic plate below.  Both IMC and night VFR conditions require a Case 3 approach per NATOPS.

As to civilian seaplane bases, the answer is actually, yes, some do.  One example is the GPS RNAV approach into Greenville Seaport (52B) in Greenville, Maine.

As for an instrument approach to land a seaplane at a arbitrary point in the middle of the ocean, no, I am not aware of anything like that.  Maybe one day an advanced version of GPS or some similar satellite based navigation system coupled with an advanced INS could allow a pilot to arbitrarily pick a point then create a custom approach which provided lateral and vertical guidance to the landing site.  Even if you could do this, you still face unknown hazards like surface winds and waves (seaplanes were never made to brave rough open ocean swells).  It would be virtually impossible to safely land on a moonless night.


Why did AF447 never return to normal law? Air France Flight 447 degraded from normal law to alternate-2B law when its pitot tubes were temporarily filled with ice crystals, resulting in a loss of airspeed data.  As a consequence of being in alternate-2B law, most of the aircraft's flight envelope protections were lost, allowing the first officer to stall the aircraft with his counterproductive control inputs, causing the aircraft to experience unplanned high-speed hydrobraking.

However, according to the BEA report on the crash, after only about a minute, the pitot tubes had unfrozen, and all airspeed data was once again valid.  At this point, the aircraft was still climbing, and, although its airspeed was steadily bleeding off, it was still approximately 30 seconds away from stalling (although it was still dangerously close to doing so, and the stall warning horn was sounding); had the aircraft automatically returned to normal law, with all its associated envelope protections, the aircraft would have rapidly recovered itself from the impending stall.

And, yet, despite the clearance of the fault that had caused the aircraft to fall from normal law to alternate-2B law in the first place, it did not transition back into normal law at this point, instead remaining in alternate-2B law all the way up (or down) until impact occurred over three minutes later.1

Why did AF447 remain in alternate law, even after airspeed data from all sources was once again valid?

1: Once the aircraft actually stalled, the airspeed data once again started being invalid (interspersed by periods of validity), but this was due to the aircraft's extreme attack angles during its descent, and would have been avoided had the aircraft not stalled.

If the system detects anomalies for more than about ten seconds, alternate law becomes locked in for the remainder of the flight.

The system detected a change in the median value of the three airspeed sources of more than 30 knots within one second (it actually dropped from 274 to 52 knots within 3 seconds). That started a process where the system monitors the difference for a verification period (about 10 seconds). Alternate Law was triggered, along with limiting the rudder travel limit (which was not annunciated to the crew). The flight control law would have returned to Normal Law if the median speed value was within 50 knots of the original speed prior to the loss, at the end of the verification period. If the speeds remain outside of those parameters, Alternate 2 is locked on for the remainder of the flight, and the rudder travel limit fault is displayed.
emphasis mine

From “Understanding Air France 447” by Bill Palmer


Is there a time limit after the ATPL theory exam to get the license? If I did the ATPL theory exam now, would I have a time limit to finish the ATPL? If so, what is it?

And what about other EASA licenses, CPL, (E-/CB-)IR, etc.?

Generally you have 7 years for your ATPL theoretical knowledge, but actually it depends whether you already have Instrument Rating and Commercial Pilot License. If you do not have CPL and/or IR, you have only 36 months. If you have IR, you have 7 years.

Moreover, according to FCL.025(c), you have to pass your IR in 36 months after the theoretical exam (nevertheless, IR or ATPL), and then, if those exams were ATPL (I believe that you are going to pass ATPL without separate IR and/or CPL theoretical exams), your theory will remain valid 7 years from the date of your IR.

For more information you can refer this document, part FCL.025(c)(1) and FCL.025(c)(2).


How to know if an aircraft is climbing or descending? As a ground observer, how can you tell if an aircraft is climbing or descending?

Listen. Climbing aircraft need more thrust, so the noise they produce is related to their vertical speed. Since most excess performance is available at low speed, a slowly flying aircraft producing a lot of noise will most likely be climbing. Once you gain a little experience, the pitch of the engine and propeller sound of piston-engined aircraft will be a very good indicator of their thrust setting.

This doesn't work for gliders. Here you need to wait to be sure if they are climbing or descending. Only when you see that the glider flies with speed brakes extended you can be sure instantly that it is descending.


Are ETOPS rated planes denoted by a placard or other indicator? A recent AA flight to Hawaii was not ETOPS rated.
(alternate article)

Apparently the failures that led to this mistake occurred on multiple levels, including the schedulers who scheduled this equipment, and the dispatcher who may have prepped the flight plan.

My question is if the crew had a means and responsibility to know if the plane was ETOPS certified?   Would there be a placard for similar indicator in the cockpit stating "ETOPS allowed" (or perhaps "No ETOPS allowed")?

Are parts of the checklist different for ETOPS/non-ETOPS aircraft?
Can the crew reasonably expect to fly whatever plane they're assigned without checking over every piece of the certification, maintenance, and dispatch?

According to FAA §25.1535 Appendix K,

  The airplane flight manual must contain the following information applicable to the ETOPS type design approval:

  (b) Required markings or placards.

  (e) This statement: “The type-design reliability and performance of this airplane-engine combination has been evaluated under 14 CFR 25.1535 and found suitable for (identify maximum approved diversion time) extended operations (ETOPS) when the configuration, maintenance, and procedures standard contained in (identify the CMP document) are met. The actual maximum approved diversion time for this airplane may be less based on its most limiting system time capability. This finding does not constitute operational approval to conduct ETOPS.”

It does not require any identification marking on the aircraft itself, though as @woakley5 noted above, a number of aircraft do have them identifying the aircraft as ETOPS certified, like below.

Source: www.thaiflight.com

I'm not sure about if any indications are there in cockpit (there are no regulatory requirements, till today that is), but won't be surprised if it is available (in some aircraft, at least).

The ETOPS do have some additional checks. According to Appendix G to Part 135,

  (This) This ETOPS maintenance program must include the following elements:

  (b) ETOPS pre-departure service check. The certificate holder must develop a pre-departure check tailored to their specific operation.

  (1) The certificate holder must complete a pre-departure service check immediately before each ETOPS flight.

  (2) At a minimum, this check must:

  (i) Verify the condition of all ETOPS Significant Systems;

  (ii) Verify the overall status of the airplane by reviewing applicable maintenance records; and

  (iii) Include an interior and exterior inspection to include a determination of engine and APU oil levels and consumption rates.

I'm not sure how the AA Flight 31 Flight and ground crew missed these checks.


How was the 'steer' bearing given? From the USAF c. 1955 training film No Sweat, there is a technique called 'steer'. By pressing the PTT button for a few seconds, the tower controller is able to determine the pilot's bearing.

What airport equipment was used to get the bearing of the signal?
Did it work with HF, VHF, and UHF? Or only one of those?
Was this technique available to the civilian/commercial pilots flying to civilian airports?

I've searched for USAF manuals and Flying Magazine issues from that era, but came up empty.

Looks like a wiki answer would be possible, several of us can provide a part of the full answer. I initiate this one... (before saving your edit, copy your text somewhere, e.g. in your clipboard, as in case of simultaneous updates, you may lose it.)

Feel free to edit and complement!

See also:

Wikipedia article on Direction Finding
Wikipedia article on Direction Finder
Wikipedia article on Bellini–Tosi direction finder

Principle: Radiogoniometry, aka direction finding

Available techniques

Determining the bearing of an aircraft from the ground by radio can be done using multiple principles:

With no cooperation from the target: Primary surveillance radar, which means sending a radio wave to the aircraft, and looking for the bearing of the echo returned by the aircraft skin materials.
With automatic cooperation from the target: Secondary surveillance radar, the ground SSR sends an interrogation to the aircraft SSR transponder, which in turn replies by sending pulses which bearing is determined by the ground SSR interrogator.
With manual cooperation from the pilot: The pilot sends a continuous wave using any of the transmitters on-board, e.g. the voice communication transmitter, and the ground station determines the bearing of this signal.

In the documentary video the last technique is used, so let's focus on this one.

Bearing determination techniques

There are several principles which can be used to determine the bearing of the waves from the aircraft:

Peak / null of signal amplitude with a directional antenna privileging reception in one direction. We just turn the antenna until the maximum or minimum signal is found. The maximum gives directly the bearing, the minimum gives the direction +/- 90°. This last option is generally used, as the amplitude rate of change near the null is generally much larger than near the maximum, which increases precision. See How does the ADF null positions find where the NDB is?
Doppler shift: When a radio source moves, the frequency of the source is shifted according to the relative speed between the source and the observer. With some (complex) signal processing, the bearing and trajectory of the source can be extracted.

In 1955, only the first technique was used.

By using two distant stations, two intersecting bearings are available, the distance to the source can also be determined. The technique is known as triangulation (the two stations and the source being the three apexes of the triangle).

Receiver on the ground: Antenna

We know the principle to use: The pilot triggers a continuous wave transmission the time for the ground station to determine the maximum or null direction.

The first generation of direction finders used rotating antennas. Using an antenna with an asymmetrical sensitivity, the voltage at the antenna output is dependent on the direction of the source, according to the antenna radiation pattern (which is also the reception pattern). The antenna can be rotated until the maximum or minimum is found.

A first improvement has been to use the principle of interferometry. With two or more antennas. The same signal reaches the antennas with a difference of phase due to the different distances traveled, unless the source is perpendicular to the antenna array plane. With this technique, the antennas mustn't be directional, they can be more simple: a vertical pole. A system using four antennas, creating two reception directions (North-South and West-East), has been deployed in UK on 1920-1930 (the Adcock antenna network and the Huff-Duff/HD-DF receiver)).

Adcock, Huff-Duff/HD-DF and Bellini-Tosi/BT-DF

The principle of the Huff-Duff was to use the N-S and W-E antenna outputs to drive the electronic beam of an oscilloscope. An oscilloscope is (was) a cathodic ray tube, with a source of electrons and two pairs of deflecting electrodes, one to determine the horizontal position of the electron beam on the front face, and one to determine its vertical position. By ensuring the displacements were a sine/cosine transformation of the Adcock array output, the beam was directed to a spot indicating the bearing of the source.

With this system, it is not required to turn the antennas, and a bearing can be determined instantaneously.

A similar direction finder was also built in the US by Marconi: Bellini–Tosi direction finder (B–T or BTDF) using a loop antenna and two vertical antennas (sense antennas) to remove the front-back ambiguity. The output of the antennas are quite the same than the Adcock system, except being not referenced to the North direction (so technically speaking, the Adcock system provides an azimuth, and the B-T system provides a bearing).

Ground direction finder for US Navy

...

From ground direction finder to airborne direction finder

...

Ground radiogoniometry circa 1955 in the US

...

Time to get a bearing

...

Ground radiogoniometry today

...


What are the advantages of squawking 7700? I was reading this question about how do pilots send distress signals in emergency scenarios, and that got me thinking...

In voretaq7's answer, he states that there are three ways to send distress signals, one of them is by the aircraft's transponder

  Three transponder codes are reserved for unusual/emergency situations:
   - 7700 for general emergencies.
   - 7600 for loss of communication    (radio failure).
   -  7500 for hijacking or other unlawful    interference.

I can understand the reason to squawk both 7500 and 7600 in the appropriate situation, but I'm failing to understand why squawk 7700 in an emergency scenario.

So that's what I'm thinking:

In case of a hijack, the ability to squawk 7500 to silent alert the ATC of the situation, without the hijacker knowing, is an advantage.
In case of a radio failure, where you can't alert the ATC over the radio, the ability to squawk 7600 and make them aware of your situation, is an advantage.
In case of emergency, where the pilots are trying to handle a lot of things (high workload scenario), to stop everything to configure the aircraft's transponder, when they are communicating with the ATC by radio, doesn't seem like a good thing to do.

So, finally, here are my questions:

Is it common for pilots to squawk 7700 in emergency scenarios?
What are the advantages, for the pilots, to squawk 7700 in emergencies situations?

If you make a radio call, unless you are on 121.5 (or 243 military), then only the station you are talking to will initially know about the emergency. Initial calls should always be with the unit you are working with unless you are VFR.

If you squawk 7700, then all stations in transponder range, including possible airborne stations such as AWACS and SAR will know right away that you are in trouble, with position, altitude and possibly aircraft type and flight number.

If VFR, you might not be talking to anyone and may not have the frequencies of nearby units easily to hand.

Squawking 7700 is definitely a good thing.

You mentioned being busy.  The mantra drilled into everyone is "aviate, navigate, communicate".  In most circumstances, fly the plane, fly it where you want it to go, then tell someone about it.  Squawking 7700 will be one of the last actions carried out.


Why do Ryanair pilots get paid such a low wage? As I‘m living in Europe and I‘m interested in a commercial pilot career, I looked up the wages on various websites. (I hope the German screenshots don‘t matter too much).

Example 1 (Source)

Example 2 (Source)

Were/Are there really Co-Pilots who paid for their training themselves (I suppose) and now only get about 2000€ per month?

Before the recent pilot shortage started to really take effect this was fairly common around the globe and is still prevalent in some places. Low Cost Carries like Ryanair pinch pennies across the board to offer cheap flights. This in turn created a segment of the market that the major airlines (at least in the US and presumably elsewhere) took advantage of. Major airlines focused on longer haul international routes flying big metal and left short hop flights to either regional airlines or smaller affiliates which often conform to the low cost model.

However the big planes still needed pilots, so the low cost carriers became feeder programs for their bigger airline counterparts. Part of the reason Low Cost airlines can offer such low wages is that generally pilots don't work there that long. Much like flight instruction or banner towing it has become a way to effectively build hours to fly bigger planes. In some cases Low Cost Carriers also offer cadet/training programs that may undercut the cost to train pilots. Instead of taking it out as debt against the pilot then garnishing wages later they simply pay less.

Recently pilot wages (at least in the states) have seen a healthy increase to attract new people to the profession.


Who has more power? Air Force One PIC or ICAO or FAA? So, ICAO(or FAA) Controls everything in the air, but is Air Force One on top of it, or does Air Force One need to follow the ICAO's (or FAA's) regulations?

First, ICAO has no authority over aircraft movement in and of itself. It's a United Nations agency that, as Wikipedia puts it "codifies the principles and techniques of international air navigation and fosters the planning and development of international air transport to ensure safe and orderly growth." Individual sovereign nations accept whatever they want of what ICAO codifies, and it is the governing aviation authority of the individual nation that has the power.

Now, in the U.S., military aviation is not governed by the FAA, the civilian agency that for the most part complies with ICAO recommendations as do most countries. However, the U.S, government has a common air traffic control system for both military and civilian aircraft. If, for no good reason, an aircraft violates the air traffic rules, the FAA will deal with that for civilian aircraft, the military for military aircraft.

Air Force One is a military aircraft. Further, as @Zach Lipton has pointed out, it is a "state aircraft." Further, whichever airplane is carrying the President of the United States is designated Air Force One (or Marine One or Navy One) only when carrying the President, and the President is the Commander-in Chief of the military. I don't know how all that sorts out, but practical considerations dictate a general adherence to the "rules of the road" so to speak.

And finally, it's accepted practice that the captain of an aircraft is the final authority aboard the aircraft for the operation of the aircraft, regardless of who is aboard.

Better to rule by consensus than by diktat if there's time for consensus.


What is the purpose of anti-skid surge accumulator in a 777-300? What is the purpose of the anti-skid surge accumulator in a Boeing 777-300?

What would happen if it were not present and why is it only meant for the left landing gear wheel assembly?

When an aircraft touches the ground, brakes are applied to decelerate the aircraft. To execute this
on an efficient manner, an anti-skid system is installed. This makes it possible for the aircraft to perform
a stop with maximal deceleration and minimal wear of the tires. In this way the aircraft can
come to a safe stop without unnecessary maintenance.

Further info: https://anoukaviation.files.wordpress.com/2009/07/project-report-landing-gear.pdf (search for skid)

The anti-skid surge accumulator absorbs brake release surges from the left normal anti-skid valve module.
The right normal antiskid valve module does not need an antiskid surge accumulator because the right hydraulic system reservoir is very near to the right normal antiskid valve module.

From: http://www.plane.cc/html/ziliao/jiwu/2011/0426/23491_33.html


What is the meaning of the term "fly back only" in AMM documents? I am reading an AMM document and I don't get the ideas below:

  Examine cracks in area A1 and A3:

  (a) Accept with an inspection each 100 hours, cracks on the inner or
  outer shroud if there is:

  A single hair line crack at the leading edge of the blade, possibly only on the coating on all blades (Area A1).

  (b) Accept for fly back only, cracks on the outer shrouds if they
  are:

  A maximum of 0.08 in. (2.03 mm) long on up to 10 blades (Area A1).

I don't get the whole idea of the sentence (a): why is that: the cracks are on the shrouds then they turn to be on the leading edge of the blade ?
And what is the meaning of the term "fly back only" in the sentence (b) ?

(a) Accept with an inspection each 100 hours, cracks on the inner or outer shroud if there is:
A single hair line crack at the leading edge of the blade, possibly only on the coating on all blades (Area A1).

The statement translated means; Allow to continue in service with 100 hour inspection cycles the following defects.

(b) Accept for fly back only, cracks on the outer shrouds if they are:
A maximum of 0.08 in. (2.03 mm) long on up to 10 blades (Area A1).

Allow for One Time Flight Only, as described in the operators Op Specs and MEL, the following defects


Where is the wing sweep angle measured from? If you look at a 747 planform wing you see something like this:

In several aerodynamics and aircraft design text books they use the wing sweep angle as an important parameter to measure the wing efficiency at transonic velocities, but they don't specify from where they measure that angle. Even I've seen wings with double leading edge angles.

Taking the 747 example you mention, on Wikipedia the wing sweep is stated as 37.5°. Three references are given, checking one of them reveals that measurement was taken at 1/4 chord.

As you have noted, there are many angles, and in aircraft design they are all relevant to the designer.

The 'wing design' chapter from Hamburg Open Online University states:

  The sweep angle $ϕ$ depends on the % line on which it is measured. Normally the sweep angle of the leading edge $ϕ_{LE}$, trailing edge $ϕ_{TE}$, 25% line $ϕ_{25}$ (quarter chord sweep) and 50% line $ϕ_{50}$ are stated.

So normally in design 4 angles will be stated. Otherwise, e.g., when checking Wikipedia, the references used should typically be clear. Below shows two $ϕ_{25}$ angles, for example:


May a Recreational Pilot demonstrate her/his own plane to a prospective buyer? FAR 61.101(e)(12) states a recreational pilot may not

demonstrate that aircraft in flight as an aircraft salesperson to a prospective buyer;`

(Please note use of the world salesperson.)
AOPA's website publishes a question, presumably taken from the FAA knowledge test bank, which reads

Under what conditions, if any, may a recreational pilot demonstrate an aircraft in flight to a prospective buyer?
A) The buyer pays all the operating expenses.
B) The flight is not outside the United States.
C) None

According to AOPA (and other sites which publish the identical question), the answer is C) None.
So we can conclude the FAA knowledge test is saying that a Recreational Pilot may never demonstrate an airplane to a prospective buyer, no matter who is the plane's owner.
Is a Recreational Pilot de facto considered a salesperson when wishing to sell his/her plane? A lay definition of salesperson is someone whose job is to sell something. Certainly a private, non-dealer owner could not be considered to be "someone whose job...".
But perhaps the FAA has defined salesperson more broadly, or the test bank question could be erroneous (it wouldn't be the first time an official answer is subtly wrong).

It looks like a recreational pilot can demo their own aircraft, because the FAA considers "salesperson" to be a profession or job in the lay sense that you mentioned.
This is from the Federal Register:

The FAA maintains that, for sales demonstrations that are not
conducted by an aircraft salesperson, a sport pilot or a recreational
pilot can conduct this activity. Therefore, to ensure that
recreational pilots have at least the same privileges as sport pilots,
the FAA is revising § 61.101(d)(12) to allow a recreational pilot to
conduct sales demonstration flights as long as the pilot is not acting
as an aircraft salesperson.

The original wording of 61.101(d)(12) was:

To demonstrate that aircraft in flight to a prospective buyer

The current wording you quoted is:

to demonstrate that aircraft in flight as an aircraft salesperson to a
prospective buyer

In other words, they added "as an aircraft salesperson" to clarify that the restriction only applies to salespeople, not to all recreational pilots.
That means the question you quoted is apparently wrong. As to why it's wrong, I have no idea. The FAA doesn't actually publish its test question banks so many "FAA questions" out there aren't official at all.


What is a skidding turn (vs slipping turn)? I am trying to understand the concept of

1) What qualifies a turn to be skidding or slipping

2) how we need to give elevator back pressure if we do not want to 'slip' the turn.

While browsing for answer I read that skidding occurs in the absence of back pressure. Why does it occur when there is no elevator back pressure. I understand that when you bank to turn, you reduce the lift component and not giving back pressure might make the aircraft nose low.

But how does that make a skidding turn. Also what causes the turn to slip?

The terms "slip" and "skid" refer to two different types of uncoordinated turn - neither has much to do with the elevator, instead both depend on what the rudder is doing:

Skids

In a skid you have too much rudder input for the turn - the aircraft starts to pivot into the turn.

Because the aircraft is effectively pivoting about its center of lift while flying you can think of a skid as the outside wing advancing (seeing a faster relative wind), while the inside wing is retreating (seeing a slower relative wind). The inside wing thus generates less lift, and the plane starts to bank into the turn.
If you try to correct for that bank using aileron to raise the wing your elevator deflection effectively increases the angle of attack on that low wing, with its reduced relative wind -- do that aggressively enough and the inside (retreating) wing stalls while the outside (advancing) wing is still generating lift, and you enter a spin.

Slips

A slip is the same thing in the opposite direction: You have too little rudder input for the turn, and the aircraft is pivoting out of the turn:

In a slip the inside (low) wing is advancing, and the outside wing is retreating, so the inside wing is moving faster, generating more lift, and trying to level the plane. If you try to hold the inside wing down with aileron input you are increasing the angle of attack on the outside (high) wing to keep it up.  If the outside wing stalls because of this it will drop the aircraft into something closer to a wings-level attitude, which will also bring you out of the turn.
This is why slips are said to be spin-resistant: The stall encourages the aircraft back into a more stable configuration (coordinated flight) rather than a less-stable one (spin).

The images here are taken from BoldMethod's "Why skids are more dangerous than slips" article, which is a great read and also has a better and more comprehensive explanation of the aerodynamics involved.
My explanation above is a vast oversimplification, but the advancing/retreating wings and difference in relative wind is the one that got the idea to stick in my head so maybe it will help you out too.


Is a VHF NAV receiver inherently capable of processing a LOC signal? These last few weeks I have been flying a couple of different aircraft that had inoperative glideslope capabilities, but I was able to fly LOC approaches without trouble. In my career as a pilot and mechanic, I have always had the—possibly inaccurate—understanding that a VHF NAV receiver had the capability of receiving and processing both VOR and LOC signals.

My question is prompted by this answer to a question regarding flight plan equipment codes—and particularly the comment section of that answer. In the comments of that answer it was stated that an ILS capable VHF NAV receiver is necessary to fly a LOC approach. For the purpose of this question let's overlook the inaccuracy of the ILS part of that statement and instead consider the LOC capabilities of a VHF NAV receiver.

I understand that the additional requirements needed for ILS capability include a glideslope antenna (possibly included in the existing NAV antenna), equipment to process the frequency pairing, and a glideslope display. I know that all LOC frequencies fall within the range of VOR frequencies (108.10–111.95 MHz and 108.0–117.95 MHz respectively) and the frequency spacing is compatible. I also understand that the circuitry required for VOR signal processing includes circuitry not required for LOC signal processing (that circuitry which can deteriorate over time requiring the periodic VOR accuracy checks).

Given all this, my questions are:

Is LOC signal processing an inherent capability of a NAV receiver: Will any given VHF NAV radio receiver be inherently capable of processing a LOC signal, or is there additional circuitry required for that capability beyond that required for VOR signal processing?

And if not,

Are there NAV receivers incapable of LOC signal processing: If LOC signal processing is not an inherent capability of a VHF NAV receiver, are there NAV receivers that were built without the LOC signal processing capability? In other words, is it possible that a pilot or mechanic would encounter a NAV receiver that did not have LOC signal processing capability? More precisely—from a pilot's standpoint—might I encounter a NAV radio that I could use to navigate tuned to a VOR station but not a LOC station?

Any VHF radio can receive the carrier signal of a LOC signal but to use it, you need dedicated circuitry.

The LOC transmitter transmits the carrier wave, frequency modulated with two side lobes of 90Hz and 150Hz.  In order to use the LOC signal, circuitry needs to exist which will filter out the 90 and 150Hz then measure the difference between them to determine where in the beam the receiver is located.

This circuitry is dedicated to the ILS receiver.

It's a similar story for GS which simply uses lobes above each other, rather than left and right.

So

  Is LOC signal processing an inherent capability of a NAV receiver

No.

  Are there NAV receivers incapable of LOC signal processing.

Yes.  All of them which do not have an ILS function (e.g. the Rockwell Collins VOR-900).

[ BEGIN Mythbusters Territory]

I've just tuned my hand held airband receiver to the ILS of my local airport.  I think I can make out the beat frequencies from the 90 and 150Hz although I doubt that the low pass filter is letting much 90Hz through.  I guess it's optimised for voice reception so probably cuts off at about 100 or 120Hz.

The next time I drive past the end of the runway, I'm going to test whether I can detect any change in the beat frequencies as I drive through the lobes.  There is a convenient hill about a mile from the threshold which will put me a couple of hundred feet above so I should be in the beam.

I guess it might just be plausible to hear when you are going left and right of the centre line.

I'll report back.

[END Mythbusters Territory]


What method does ATC use to calculate distance between two aircraft? The distance between aircraft must be larger than the minimum separation all the time and is therefore quite important for Air Traffic Control (ATC). As I need to calculate the distance between aircraft for my research, I'm wondering if there are any regulations (in Europe) about how exactly the distance is determined.

How exactly does ATC determine the distance between two aircraft?

I suppose that they are working with latitudinal and longitudinal coordinates and probably the altitude to determine great circle distances. To get the coordinates, they could use either radar or the position which is communicated by the aircraft.

The task to calculate the distance from coordinates is not that trivial as it may sound from the first impression. One method to calculate the distance is the haversine formula. In all methods, the assumed earth radius is rather crucial. If the distance is calculated in that manner:

Which value is used as the earth radius?
Are the different earth radii from the equator to the meridian regarded?
Is the altitude of flights included in the calculation?

The things you mention would matter, if you wanted to know the distance to 6 or more significant digits. But ATC does not need that precision. They need to be able to tell whether it's 4 nmi (too close), 5 nmi (still sufficient, but getting in trouble) or 10 nmi (no worry, but should not be heading directly towards each other). That is one significant digit.

The controller does not use any formula. They just look at the distance of the targets on the radar scope (which is drawn properly to scale). And possibly use the grid that is drawn over it or compares to the leader lines (indicating velocity) or history trail. The controller doesn't need precision, they need to look and see. And they are not getting any precision; on centre station (controller handling en-route traffic) the radar target blob is about a mile wide.

Until recently, aircraft did not broadcast position at all. Many still don't; not all aircraft are equipped with ADS-B. For those that don't, the interrogator may have precision similar to DME (~ 0.1 nmi) in range and a degree or two in angle. Not great precision either. The radar also has single rotating antena that takes couple of seconds to scan the whole circle, so the position of the aircraft is only updated once every few seconds. Jet airliner in cruise moves some 700 m in 3 s.

On such radar, the scope most likely transforms directly from the angle and distance reported by the radar to the screen coordinates. Altitude was usually not taken into account (so higher targets appear further than they really are!) in the older systems.

New radars with ADS-B support (less busy areas don't need it) need to combine the radar data with coordinates reported by ADS-B, which are in WGS84, so WGS84 is usually used as intermediate format for the collected data. But shortcuts can still be taken in the software.

I don't know what which radar system manufacturer uses, but I work on navigations and we tested the precision. By assuming Earth is flat and the coordinate system is orthogonal and using simple Pythagorean formula, you can still get 3-4 significant digits (unless you are working around north or south pole) for points tens of miles apart, which is more than enough for the radar screen. So the system can simply multiply longitude and latitude with appropriate factors (that depend on latitude of the sector centre) and get sufficient precision.


Why is the VOR reference and variable signal modulated at 30 Hz? VOR signals radiated from the ground beacon are modulated at 30 Hertz.
What is the reason behind this?

In VOR early days, the 40s, the stable phase locked loop frequency generator was not available. A common frequency reference was the very stable public AC power grid frequency. The angular velocity of a synchronous motor is locked on the power source frequency, such motor can be used to actuate a variable resistor or capacitor to generate a stable wave which frequency is locked on the power supply frequency.
A VOR relies on two 30Hz signals: Reference signal and variable signal (more on the VOR principle here). The US grid is 60Hz, a synchronous motor with two pairs of poles turns at 1800 rpm, that is 30 rounds per seconds.
The reference modulated the frequency of a 9,960Hz subcarrier (332x30Hz) with a modulation index of 16, meaning the subcarrier swinged between 9,480Hz and 10,440Hz. Here is the description of the mechanical assembly used:

A tone wheel is driven by an 1800 rpm (30 rps) motor; and on this
wheel are 332 teeth which are arranged in a slightly staggered manner
so as to impart a cyclical variation between 9480 Hz and 10,440 Hz
with the rotation of the motor. The frequency modulated signal from
this tone wheel is then used to amplitude modulate the VOR carrier
signal.

(Source: A comparative analysis of area navigation systems in general aviation, by Stephen Malcolm Dodge, NASA -CR-132504, 1973.)
The motor also drove the rotating dipole broadcasting the variable signal:

Early conventional VOR with rotating beacon, source
Here's what the rotating/modulating assembly looked like for a Collins VOR station:

Motor, tonewheel and dipole of a Collins conventional VOR, 1954. Source: Collins
A single motor used to generate both the reference and the variable signal ensured the two signals remained coherent, and therefore allowed a simple phase angles subtraction to get the bearing.
If the VOR had been designed in Europe, the same motor would have turned at 25 rps, and the signals frequency would have been 25Hz instead of 30Hz. There comes @Sanchises question below, about the consequences of the different grids on the VOR compatibility around the world... While the motor angular velocity is the first to be impacted by a different power supply frequency, harmonic interference must also be considered, for example:

In the ILS there are two reference frequencies at 90 and 150Hz, also chosen as multiples of 30 and remote enough from harmonics of line frequency (60, 120, 180Hz, ...). However 50Hz harmonics include 150Hz and interferences may occur on a 50Hz grid.

Modern solutions to solve both the angular velocity problem and the harmonic interferences problem are straightforward, but they were not 60 years ago. It seems designers at this time chose to recreate a 60Hz source locally and dismissed 50Hz AC for the related portion of the equipment. I got this schematics from a 1968 issue of the French aeronautical navigation engineering group bulletin:

Mechanical generation of 90Hz and 150Hz signals in an early ILS. Source: STNA bulletin, page 88
A 1,800 rpm synchronous motor is used to generate mechanically the two frequencies. But France has opted for a 50Hz grid. A static inverter is used as a local 60Hz power supply. As mentioned in the source document, the inverter frequency is stabilized with a 1% accuracy.


How do airlines without assigned seating verify cargo balance? How to approximate the mass of passengers is well explained, but balance of the aircraft is concerned more with where they sit. Other answers have hinted that assigned seating locations are one method for calculating loadsheet balance, but this is not always an option.

Certain passenger airlines (such as Southwest) do not assign seats, allowing passengers to select whatever seat they like during boarding. Since passengers must be accounted for in load balance calculations, how do these airlines know how passenger mass is distributed? Are there strategies besides knowing exact locations to determine whether cargo balance will be within the envelope? Do they simply have flight attendants record where passengers are seated?

The cabin crew simply counts the seated passengers (or alternatively empty seats) by zone (a block of rows), writes it down and hands the slip to the flight crew. It is common to do this on commuter planes even with assigned seating since passenger balance matters more on small aircraft and people may move.

As pointed out by SKYbrary:

  In practice, low cost airlines, which routinely offer free seating, also operate the majority of their flights with a high load factor, so that identifying any unoccupied seats will be a fairly simple matter.


Is there a crosswind that would perfectly cancel out the "torque" of a C172 on takeoff? We all know that on takeoff you need right rudder in order to balance out the "torque" of the propwash hitting the tail.  Is there an exact crosswind that would cancel out this torque?

It is reasonable to believe, (as it is with most hypothetical questions of this sort...) that the answer is yes.  However, there are a few things to consider:

It would need to vary to the degree that the throttle is advanced.

It would need to vary during the acceleration as aerodynamic effects change.

So, if you think you might be able to listen to ATIS and discern that you won't need rudder on the takeoff roll, it isn't that simple.


What are E2 and E4 airspace? We have a number of questions here about E2 vs. E4 airspace (see summary of links in this answer) but what does that mean? As far as I can see, neither the AIM nor the ATC Orders mention E2 or E4 airspace, so what are they? What are the practical differences for pilots and controllers?

According to this website, which directly quotes an internal memorandum of the FAA, the class E airspace can be categorized as below:

E1 – Class E Airspace at and above 14,500 feet MSL
E2 – Class E
airspace areas designated as a surface area for an airport
E3 – Class
E Airspace Areas Designated As An Extension To A Class C Surface Area
E4 – Class E Airspace Areas Designated as an Extension to a Class D
or Class E Surface Area
E5 – Class E Airspace Areas Extending Upward
From 700 Feet Or More Above The Surface of The Earth
E6 – En route
Domestic Airspace Areas
E7 – Offshore Airspace Areas
E8 – Class E
Airspace Areas Designated As Federal Airways

The memorandum is also available as PDF.

Apart from this reference to the FAA internal memo, I could not find any official FAA documents explaining the categories.

For pilots and controllers there seems to be not many differences, but for operators of small unmanned aircraft (i.e. drones) there are.


What are the largest airports in terms of size, gates, and stands? What is the largest airport in terms of:

Size of developed area?
Size of all terminal area?
Total number of gates?
Total number of gates + remote stands?

According to InsiderMonkey DFW (Dallas/Fort Worth International Airport) is the largest at 4360 hectares.
According to Wikipedia DXB (Dubai International Airport) has the largest terminal in terms of floorspace at 1,713,000 m2 (18,440,000 sq ft) with a volume of approximately 10.0 million m³ (360 million cu ft)
According to this forum ATL (Hartsfield-Jackson Atlanta International Airport) has 206 gates
Can't find this one


Why can't pilots manually fly a low visibility approach instead of using auto land? Pilots have instruments that help them land. But in low visibility they are required to use auto land. Why can't they rely on their instruments at those times?

Pilots are required to operate by two separate sets of regulations, protocols, and procedures when flying an aircraft. They are Visual Flight Rules (VFR) and Instrument Flight Rules (IFR). The rules are dependent upon what type of airspace you are flying in as well as cloud ceilings and visibilities outside the aircraft. Most commercial flights, including virtually all scheduled airline services are conducted under IFR.

Landings conducted under VFR must be done by flight under visual references and are subject to specific limitations on cloud ceiling and visibility minimums for the specific airspace surrounding an airport. If the local weather drops below these minimums, you cannot takeoff or land under VFR and the airport can only accommodate IFR traffic in this weather.

IFR operations require pilots to fly specific procedures during departure and approach to an airport. These procedures themselves set weather limitations on the flight depending on aircraft and airport equipment, NavAid availability and GPS reception and the specific regulations in which the flight operation is being conducted under eg. Part 91, 121, 135, etc. If the weather does not meet these minimum requirements, the flight is not permitted under IFR either, which is why airline flights get cancelled in bad weather such as snowstorms, etc.

IFR Instrument Approaches into airports are broadly divided into 2 categories: Non-precision approaches, which provide lateral guidance to either the runway threshold for a straight in approach or to the airport itself for a visual circling approach and landing.  Precision approaches provide the pilot with both lateral guidance to the runway center line and a glide path to the runway touchdown zone. Both types of approaches have minimum ceiling and visibility requirements along with specific minimum altitudes and decision points based on your aircrafts’s approach speed. If you do not have the runway environment in sight when flying these approaches at these points and altitudes you cannot continue the approach to landing under visual conditions and must fly the missed approach procedures specific to this instrument approach.

The most common type of Precision Instrument Approach is the Instrument Landing System or ILS which uses terrestrial radio signals to guide the pilot along an approach path and identify specific points in the approach. ILS approaches fall under 3 different categories, or CATs.  CAT I approaches are standard ILS approaches and can be flown by most IFR Certified aircraft. CATs II and III approaches require specific additional flight crew training, and aircraft equipment.

CAT III approaches are the most precise ILS approaches and have 3 three additional subcategories: IIIa is the most precise approach that still allows the pilot to hand fly an aircraft equipped with an approved HUD. IIIb, and IIIc require the use of a coupled autopilot and allow for even lower weather minimums. CAT IIIc approaches are “zero-zero” meaning they can be flown with ceilings of 0 ft AGL and visibilities less that 1/16 of a mile, though they do require the visibility be great enough to adequately allow the crew to taxi the airplane off the runway environment to parking. Again these are limited to specifically trained flight crews, specifically equipped aircraft and specific airport infrastructure.

While CAT IIIb and IIIc ILS approaches do require a coupled autopilot to perform them, all other Non-precision and Precision Instrument approaches can be flown by hand - and all aircrews should be able to do this in the event of an autopilot failure.


If I've done multiple checkrides, when is my next flight review due? Currently, I hold a Commercial Pilot Certificate, the question is when is my next flight review due?

Certification Dates are as follows:

Private Pilot - 02/05/2017
IRA - 11/10/2017
Commercial Pilot - 02/01/2018

It's supposed that a checkride counts as a Flight Review, so every checkride would add 24 calendar months to the initial?

Or it's just 24 calendar months after the last one?

The due date of your next Flight Review is based on your last check ride (your Commercial Check Ride). You will be required to complete a Flight Review (or equivalent) by the last day of February 2020.

§61.56   Flight review: (emphasis is mine)

  (a) Except as provided in paragraphs (b) and (f) of this section, a flight review consists of a minimum of 1 hour of flight training and 1 hour of ground training. The review must include:

  (1) A review of the current general operating and flight rules of part 91 of this chapter; and

  (2) A review of those maneuvers and procedures that, at the discretion of the person giving the review, are necessary for the pilot to demonstrate the safe exercise of the privileges of the pilot certificate.

  (b) Glider pilots may substitute a minimum of three instructional flights in a glider, each of which includes a flight to traffic pattern altitude, in lieu of the 1 hour of flight training required in paragraph (a) of this section.

  (c) Except as provided in paragraphs (d), (e), and (g) of this section, no person may act as pilot in command of an aircraft unless, since the beginning of the 24th calendar month before the month in which that pilot acts as pilot in command, that person has—

  (1) Accomplished a flight review given in an aircraft for which that pilot is rated by an authorized instructor and

  (2) A logbook endorsed from an authorized instructor who gave the review certifying that the person has satisfactorily completed the review.

  (d) A person who has, within the period specified in paragraph (c) of this section, passed any of the following need not accomplish the flight review required by this section:

  (1) A pilot proficiency check or practical test conducted by an examiner, an approved pilot check airman, or a U.S. Armed Force, for a pilot certificate, rating, or operating privilege.

  (2) A practical test conducted by an examiner for the issuance of a flight instructor certificate, an additional rating on a flight instructor certificate, renewal of a flight instructor certificate, or reinstatement of a flight instructor certificate.


What are the rules of airfoil thickness from root to tip on tapered wings? I'm doing some research on wings and airfoils used in the 30s and 40s, and I'm trying to understand the rules about airfoil proportional thickness changes across the wing (root to tip) on tapered wings.

Many planes of that time used airfoils with thickness ranging from 15% to 9% (root to tip). (Source: https://m-selig.ae.illinois.edu/ads/aircraft.html)

So the question is: what are the rules for dimensioning airfoil thickness from root to tip?

Airfoil thickness at the root was chosen to obtain a good cantilever ratio: the bending moment is highest there, and you need to give it structural height. Also, you want to create most lift at the wing root, that is why chord, thickness and incidence angle are all highest there. All three tapering down towards the tip for structural reasons.

There is a bit more info in this answer.


Surface deflection if loss of hydraulics on combat jet In a typical combat jet from the 1970ties, at loss of hydraulic pressure, how does the deflection surfaces behave?

If it happens on the runway, will the weight of the surfaces make them deflect downwards?

If it happens at speed, will they align with the incoming airstream?

I heard something about a Hydraulic lock, is such a thing common in fighter aircraft and will it make the surfaces 'freeze' at hydraulic loss?

Most of the ones that we measured without hydraulic pressure would just sit there. Moving the stick is possible within the servo valve deadband, but the surface does not move whatsoever, just stays in the last known position.

Hydraulically powered fighter jet surfaces are powered fully redundantly, by separate actuators and separate hydraulic systems. If pressure fails in system 1, opening the servo valve (by moving the stick) just connects pressure to return and its actuator becomes a passive damper that otherwise does not hinder the functionality of the remaining one. Yes the inputs to both servo valves are linked.

Most hydraulic power systems for fighter jets are irreversible, meaning the aerodynamic hinge moment cannot move the stick. But if the hinge moment becomes larger than the total force of the active actuators, the surface cannot be deflected anymore.

Info on military jets don't seem to be not publicly available. I could find this hydraulic schematic for a G450 elevator:

This set-up allows manual reversion: when all hydraulics are off the pilot can manually deflect the elevator, at higher forces than with hydraulics functioning. Note that the pink colour denotes "Hydraulic Lock". It means that when the servo valve is closed, back driving the actuator is not possible due to trying to compress the hydraulic fluid.

Over (a very long) time, gravity may slowly push the hydraulic fluid back through the circuit when unpowered. When all oil in the circuit has seeped back into the reservoir.


Would a slotted "wing" work? The early history of aviation is full of crazy designs, so please allow me this one:

A span wise narrow gap in the wing halfway between the leading edge and the trailing edge.

The door to the gap would be aerodynamically balanced, it would open at slow speeds. The gap cross section would be nozzle shaped to accelerate the air from underneath the wing to the top side. Lowering its pressure.

At the wing root and tip there will be no gap.

The idea is similar to slotted flaps and slats, but with the aim of energizing the aft portion of the wing. Thereby combining slats and flaps into one. Would it work?

In other words, it would split the wing into two smaller wings.

I expect the answer to be no, but would love to know why.

Edit: edges below are meant to be curved.

(Original Image)

No, it will not work in the way you plotted it.

First, the corners of the duct should be nicely rounded. With the sharp corners you will get a lot of vorticity, but little lift increase. Actually, lift will drop and drag will go up.

Next, a gap works best when connected with a contour break, such as that of a slat or a flap. If you round the corners and add a hinge in order to deflect the rear part of the airfoil, then yes, the gap will help to delay flow separation.


What determines the positioning of PAPI lights from the runway threshold? Why are PAPI lights installed 300m from the threshold?
What are the conditions that define the position of the PAPI lights from threshold?

US DoT Order JO 6850.2B contains installation criteria for visual guidance lighting systems. The following is taken from that order.

For siting PAPI along the runway, in case an (ILS) electronic glide slope is present, the lights are located such that the PAPI visual approach path coincides with the one produced electronically as much as possible.

In case of runways without an electronic glide slope, the following criteria are considered for locating the PAPI:

Threshold Crossing Height

  The TCH is the height of the lowest on-course signal at a point directly above the intersection of the runway centerline and the threshold. The minimum allowable TCH varies according to the height group of aircraft that use the runway, ... The PAPI approach path must provide the proper TCH for the most
  demanding height group that uses the runway.

Glide Path Angle

  The visual glide path angle is the center of the on-course zone, and shall normally be 3 degrees when measured from the horizontal.

Obstacle Clearance Surface

  The PAPI obstacle clearance surface is established to provide the pilot with a minimum clearance over obstacles during approach. The PAPI must be positioned and aimed so that no obstacles penetrate this surface.

Image from US DoT JO 6850.2B Visual Guidance Lighting Systems

Basically, in case ILS is there, align the PAPI with that; else locate the PAPI such that it satisfies TCH and OCS.


What is the crank on the ceiling of this aircraft? What is the crank the pilot is operating above his head? Also what type of aircraft is this?

 image source

The pilot is operating the overhead stabilator trim crank in an early model Piper Cherokee. Here's a close up the trim crank:

Piper Cherokee overhead trim crank; image from tnwings.com

In later models, it was replaced with airliner type trim wheels


Are there references for the different maneuver configurations? Since it's helpful to have specifics, lets use a 1978 Cessna 172RG as the reference (it's what I'm most interested in knowing about anyway).

Is there resources that would have the following information (for example, I know you would enter each maneuver differently).

  Maneuver: Rectangular Course
  MP: 15"
  RPM: 2200
  Gear: Down
  Airspeed: 108 Knots

I'm looking for a reference for all private and commercial flight training maneuvers that would have specifics on how to enter the maneuver that would allow the maneuver to be completed without excess airspeed (or not enough). I'm not looking for how to complete the maneuver (the AFH has this information), but how to enter the manuever. Does this exist as a general reference, or is it so specific to each airplane that it couldn't exist in a general form?

My CFI initial students fill out worksheets for each maneuver.  They find the numbers that work, and demo the maneuver with those numbers.

My primary students get a similar worksheet, where I have "grayed in" sample numbers.  I demo with these suggested numbers and as the student practices, they are free to change the numbers.  For more advanced primary students, sometimes I will give them suggested numbers which are off, and let them change the numbers to what they find works for them.

Some maneuvers are highly dependent on load / DENALT and will have varying numbers.  I encourage students to adapt the suggestions to something that works for them.  It helps them understand the performance of the plane, and of themselves.

So to answer your question, there may be a CFI out there who has a little notebook which he may share with you.  Probably 25 years ago there was a series of books, which one could buy with V speeds and maneuvers for various planes.  But I have not seen those in a couple of decades.

Finally, there are all kinds of variants.  As an example, I had a CFI applicant once, who right after takeoff reduced the throttle, and flew around the pattern at final approach speed.  Good thing were were at an rural uncontrolled field!  Eventually I convinced him to fly differently for his CFI initial.  But his worksheets would often be quite different than mine.  It took him a while to get chandelles down.

So if you are a student, I would suggest that you work out your estimate of those numbers, and ask the CFI you are working with.  You could even ask him to assist you in finding the correct numbers for your plane (with both of you in it).

Finally, the actual selection of configurations and numbers is colored by the flying style of you, your instructor, or whoever one is trying to emulate.  What is right or wrong is to a large degree subjective.  Most DPEs will let the applicant select the speed they perform a maneuver at, as long as they do it safely, and consistently.  In other words, while I might think my answer is the right one, there are many others which will work.


How does this test flight escape aparatus work? I was watching this video about the test flight escape systems on a Bombardier flight. In the video, this machine is shown:

The yellow box moves up and down in the video. Does it move up and flip the airman over, mashing their feet? Or does it shoot down out of the airplane and create a hole to jump out of? If it's the latter, why are they sitting instead of standing?

Qualification: I am an FAA master parachute rigger and taught sport skydiving for 10 years.

Most of the comments on the question are correct - the red slide will extend below the airplane and act as a wind buffer so you can clear the fuselage before the wind blast hits you. The yellow part is a cap that would be removed.

No one would clip a static line to the plane in an evacuation - you want to be well clear of the airplane before opening your own chute. The blue packs are the real thing, I've packed plenty of them. There's a 26-foot round parachute inside. Rough landing but usable by anyone with very little training.

How usable this contraption would be is a different question - skydiving is done at about 100 knots, well below the speed of jets. And it also requires a stable aircraft - if that plane loses it's tail and starts to spin no one will be using the escape slide simply because it will be impossible to reach it. The "bail out in the event of emergency" concept is a bit flawed - if the airplane is flying well enough to bail out then you don't need to. There's a very good reason the military spends a fortune on ejection seats.

Side note - the Space Shuttle had a crew escape process, but NASA considered landing a shuttle on anything other than a proper runway to be non-survivable.


Are birds aerodynamically stable? This question is bordering between aviation and biology. I am posting it here, since it is really about aerodynamics.

At first, one would think, that birds should be aerodynamically stable to save themselves the effort of maintaining stable flight.
However, even if humans had rigid joints they are in an unstable configuration other than four-legged animals. Thus, stability does not seem to be a must for biology, at least in certain aspects.

So, are birds aerodynamically stable? Are all of them, or are some species which are and some which aren't?

Birds have integrated active control systems that rely on several sensors to adjust the control surfaces to stabilize the flight. Since it is very difficult (perhaps impossible) to study the aerodynamic properties of the bird independently from its control system it is hard to make any claims on the static or dynamic stability of a bird.

What can be said is that from a control system point of view, most birds are observable, controllable and stabilizable in flight. This does not apply to the class of flightless birds.

If you are interested in the similarities in aerodynamics between insects, birds and aircraft I recommend getting a copy of "The Simple Science of Flight; from insects to jumbo jets" by Henk Tennekes, an  Emeritus Professor of Aerospace Engineering at Pennsylvania State University.

After sitting in the garden on a sunny afternoon, observing various bird species and tinkering a bit more on the subject I actually think that many birds are unstable. Especially the smaller species that fly in the garden through bushes and trees, but also those that hunt for flying insects, are capable of extreme rapid changes in direction of flight; they are very agile.

Stability and extreme manoeuvrability are difficult to combine. In fact modern jet fights are designed to be unstable to improve their manoeuvrability. They then need a computerized control system to stabilize them.

An online search showed that some research has been done on the field of flight stability of birds Petter Krus of the Linköping University in Sweden has published an article on Natural methods for Flight Stability in Birds.

Abstract:

  There is a stark contrast between the behaviour of flying man-made
  artifacts and the effortless elegance of bird flight. In this paper,
  mechanisms are proposed that seems to be at least related to the way
  birds fly when they are gliding (the no propulsion case). It is
  recognized that biological structures and actuator systems (muscles)
  are characterized by a great deal of compliance that produce a
  completely different dynamic behaviour of flight. In this paper it is
  demonstrated how this can be applied on a geometrically unstable
  bird-like configuration. The result is that geometrically unstable
  configurations can be stabilised without the need of rate gyros and/or
  accelerometers. Although the bird might have a fast acting control
  system for flight control, it is unlikely that it is fast enough for
  active stabilisation of the short period oscillation, and this is
  unnecessary with the model described here, which only requires
  stabilisation of the long period oscillation.

In his introduction he also briefly refers to the work of Tennekes, which does not include much about the stability of birds. Also in the introduction he observes that birds have a cambered wing and must have an aft centre of gravity. This combination makes the bird unstable (if it were a rigid body).


Why are three-bladed helicopters relatively rare? I admit that this observation is entirely subjective, but it seems to me that there three-bladed rotors are significantly less popular than two or four.

Factors that I'm guessing contribute -

Storage - two-bladed helicopters take up a lot less space. So you're probably going to stick with two blades until you're compelled to move to four
Simplicity - while an odd number of blades can be perfectly balanced, it may be easier to manufacture systems where the forces counter-balance in opposition rather than having to balance the entire system.
Cost - two slightly longer blades are presumably a lot less expensive to maintain / purchase than three shorter ones.

These are guesses though. My question is - is my perception correct, and if so, why?

Like with propellers, the blade count on a helicopter rotor is a matter of disk loading. Light helicopters get away with fewer blades. Complexity and losses increase with blade count, so helicopter designers try to fit as few blades as possible. However, this would result in huge rotor diameters for heavy helicopters. There are reasons to keep the diameter small:

Blade root loads due to centrifugal forces
Tip speed: The advancing blade will experience transsonic effects earlier if the helicopter flies in cruise the larger the rotor is. This will limit top flight speed.
Blade bending on the ground. The blade needs to be stiff enough to clear the tail when the rotor spins up.

Now let's compare a few helicopters. I used the technical data on the linked Wikipedia page and divided the maximum take-off mass by the area of the rotor disk:

Sud Aviation Aloutette II: 2-bladed rotor. Disk loading 19.6 kg/m². Disk loading per blade: 9.8 kg/m².
Bell 205 / UH-1D. 2-bladed rotor. Disk loading 25.6 kg/m². Disk loading per blade: 12.8 kg/m².
Aérospatiale AS-350 Écureuil: 3-bladed rotor. Disk loading 25.07 kg/m². Disk loading per blade: 8.36 kg/m².
Bell 412: 4-bladed rotor. Disk loading 34.6 kg/m². Disk loading per blade: 8.65 kg/m².
Sikorsky S-65 / CH-53: 6-bladed rotor. Disk loading 50.2 kg/m². Disk loading per blade: 8.37 kg/m².
Mil Mi-26: 8-bladed rotor. Disk loading 69.6 kg/m². Disk loading per blade: 8.7 kg/m².

I guess you see the trend by now. The UH-1D is the odd man out, but it also stands apart due to its unusually high blade chord. Normally, the ideal loading per blade seems to be between 8 and 9 kg/m², and a little more is deemed acceptable if it allows the designer to get away with only 2 blades.

There is no conspiracy to suppress three-bladed rotors. It's the overall compromise between performance and efficiency which determines the number of blades on a helicopter rotor, and the main driver is mass.


How much "power" do ATCs really have? After reading this question about being cleared to go faster than 250kts in airspace that otherwise has a speed limit, I began to wonder how far can Air Traffic Controllers (ATCs) can bend the rules on an aircraft-to-aircraft basis?

Is there a limit to what exceptions ATC can make for someone?  I'm particularly interested for the answer in the USA, but I'm sure information about other countries would also be enlightening.

As an active area radar controller, I completely agree with @rbp answer.
To better explain relationship between power and responsibility, I will try to describe what a controller would usually do: If I need an aircraft, for example, to reduce speed...
I know approx. what would be the minimum convenient speed for a particular type to ask, so I'd take this into account prior asking. But if a pilot tells me that he's unable to comply with the request, I'll find another solution i.e. radar vectoring to loose few miles or in extreme situation, level change. In short, a controller will never force a pilot to action if pilot answers that he can't do that. A controller must plan in advance that pilot may not be able to fulfil his request and have alternative solution. If you, as a controller, put yourself in a situation where you have no alternative, that is usually too late.
There are usually only few things where I can't make exception:

Minimum separation between aircraft
Danger/Prohibited zone
Terrain

Even then, in extreme case, I've never met: Imagine a situation where you have an aircraft which can't change level due to traffic above and below and has a crossing traffic. Then only remaining action I have available is radar vectoring, but what if the pilot advises me that his unable to turn due to weather, (the described situation is usually the controller fault because he should have predicted that earlier). Then controller probably would give the pilot traffic information (tell the pilot what to expect) and advise the minimum distance but it will not force the aircraft to CB.


What are the reasons why we do not have VTOL commercial airliners? I understand that there are noise issues, and that there may be an economic case dependent on stage length, but other than that I'm not quite sure why they haven't happened. Is there something technical I'm missing?

Exhaust gas re-ingestion doesn't seem to be that great an issue, it looks like you just install fences to make sure that exhaust can't get into the inlets.

Commercial airliners are significantly larger, and therefore heavier, than aircraft with use V/STOL. Because of that, it would take more fuel and engine power to run that airplane, driving up the cost of running it. There are also many other airframe stress and balance factors that come into play when doing VTOL.
If you are including turbo-prop airliners in your definition of airliners, they could do V/STOL with a configuration like the V-22 Osprey because they are small enough.


How often should an engine be run, to stay mechanically sound? It's winter time and many light aircraft are sitting idle. This can be detrimental to an engine's health.

What is a good rule of thumb for how often a piston engine should be exercised, and for how long?

If the aircraft cannot be flown, what is an acceptable alternative?

All your plane's systems are happier when you're flying, including the pilot.

Ideally, you'd fly often in order to keep your engine happy (distributing clean oil throughout the system to protect against corrosion and heating it enough to drive off water from the crankcase).
As a rule of thumb Blackstone Labs (the oil analysis folks) consider piston engines "inactive" if they've had less than about 5 hours of flight time per month. Their threshold used to be 10 hours, so I personally try to fly at least 10 hours a month much when practical, but 5 at a minimum.
Blackstone has a LOT of data on aircraft engine wear patterns accumulated, and I generally trust their insight in this regard (after all they're not trying to sell me an engine, just asking me to pay them to play with my used oil).

When weather or other factors mean you can't fly enough to keep your engine "active" you can preserve it in accordance with the manufacturer's instructions, but let's be honest, most of us aren't going to "pickle" our engines for the winter. It's a LOT of work, and it means you can't go fly on those nice winter days we sometimes get.

In situations like this, it may be beneficial to ground-run your engine (say at least on a monthly basis) just to keep things lubricated and happy. Ground runs at a little below runup power or a fast idle are usually the order of the day here. Taxiing around is also probably a good idea, if practical, to keep your wheel bearings and related components moving -- maybe take your plane down to the runup pad and back.

When doing the ground run you want to observe a few guidelines to make sure what you're doing is actually beneficial to the engine:

Make sure you've got clean oil in the plane!
Any time you think the plane is going to be inactive you want to have good, clean oil in the engine (and you want to have done a thorough ground run or flight to spread that clean oil everywhere). This gives you optimal protection against corrosion.
Treat it like a flight.
Preflight the plane like you're taking it up, even if you're not.  You may not be flying much because of the weather, so there's a real chance your plane's systems will be in less-than-excellent shape. That thorough walk-around may catch a potentially expensive problem before it becomes a real issue.
Preheat if necessary.
Hand-in-hand with treating it like a real flight, you want to make sure you're not abusing your engine in the name of protecting it.
Get the engine oil to normal operating temperature.
Part of the point of the ground run is to drive off water that has condensed in your oil -- oil is good for metal parts, and water is bad for them.
If you don't get the engine oil up to normal middle-of-the-green-arc temperatures and keep it there for a good while (20-30 minutes at least) you may not be driving out the water, which can lead to a very unhappy engine.
If you don't get the oil up to operating temperature long enough you may wind up doing more harm than good! Combustion blow-by leaking around the piston rings contains water (and a bunch of other corrosive things your engine doesn't like), and really short ground runs that don't bring the engine to temperature are mainly just causing wear from the startup and dumping these nasty things into your oil. Your engine is not going to like that.
Run long enough to recharge your battery.
Watch your ammeter/load meter after startup, and make sure that your battery has been replenished before you shut down.
This means running your engine at a high enough RPM that the charging system can charge the battery (a fast idle is usually fine), and long enough that the battery can be charged (which isn't a problem, because if you were paying attention to #4 you've noticed we'll be running the engine a good while anyway).
Monitor your cylinder head temperatures.
If equipped with CHT probes, make sure you're not exceeding the CHT limits for your engine during the ground runs.  This goes hand in hand with #7 below.
Lean your mixture to avoid fouling your plugs.
You should be using less than 65% of your engine's max rated power for these ground runs -- in fact, you should probably be somewhere around 50-60%.  Rather than running full rich, pull back on the red knob and let the engine burn cleaner (but keep an eye out for the warning signs of detonation/preignition).
You probably only want to do this if you've got Cylinder Head Temperature readouts you trust -- when in doubt you can always run full rich (for the added cylinder cooling) and just clean your plugs.

Depending on the outside air temperature your ground run may last anywhere from 30 minutes to 90 minutes (I've not personally had one last longer than that, though it's theoretically possible I suppose) -- it all depends on how long #4 and #5 above take.

A final thought: I use ASL CamGuard in my oil. It goes in with every oil change.  Mike Busch (a smarter guy than me) swears by it, and there seems to be ample real-world-anecdotal evidence as well as a good amount of actual materials testing research that indicates it's a pretty decent corrosion inhibitor.
I've not had my plane/engine long enough to know if this treatment is effective or not, but it certainly doesn't seem to be hurting anything, and while it's a little expensive it's not "liquid gold" prices, so it strikes me as cheap enough insurance that you may as well dump it in the sump. This is especially true when you weigh it against the price of a bottom-end overhaul because you got pitted cam lobes or lifters...


How is a near miss defined? With respect to mid-air collisions, how is a near miss calculated, detected, and reported?  Does ATC have radar detection, or is it strictly up to pilots to report a close encounter with another aircraft?

First I'd just like to comment that "near miss" is a bad term (as falstro also noticed). It seems to say that the planes "nearly missed", implying they didn't miss. The FAA uses the terms "Near Midair Collision" (NMAC) for serious incidences, and "loss of separation" for less serious ones. As such, terms like "near miss", "close call", and "narrowly escaped disaster" are not well defined.

ATC does usually have radar. The data is recorded and can be analyzed later. Of course, not all areas have radar coverage. In these cases it is up to pilots to report if they feel safety was compromised. They can use the Aviation Safety Reporting System, which is part of the Aviation Safety Information Analysis and Sharing system.

Especially with commercial flights, Traffic Collision Avoidance System (TCAS) is also used. If TCAS issues a Resolution Advisory (RA), which commands pilots to take action to avoid a collision, the pilots are usually required to report the incident.

A "loss of separation" would be anything less than the required separation.

According the the FAA:

  A near midair collision is defined as an incident associated with the
  operation of an aircraft in which a possibility of collision occurs as
  a result of proximity of less than 500 feet to another aircraft, or a
  report is received from a pilot or a flight crew member stating that a
  collision hazard existed between two or more aircraft.

That and more information about reporting incidents can be found in Section 6 of the AIM.


Why is the generator turned back off for starting the second engine on a King Air? The engine start procedure for the Beechcraft King Air B200 says (emphasis mine):

  Right Ignition and Engine Start Switch – ON [R FUEL PRESS] annunciator – OFF
  Right Condition Lever – LOW IDLE (after N1 rpm stabilizes; 12% minimum)
  ITT and N1 – MONITOR (1000 oC maximum)
  Right Oil Pressure – CHECK
  Right Condition Lever – HIGH IDLE
  Right Ignition and Engine Start Switch – OFF (at 50% N1 or above)
  Right Generator – RESET, then ON. CHARGE BATTERY until loadmeter reads approximately .50, then OFF
  Left Ignition and Engine Start Switch – ON [L FUEL PRESS] annunciator – OFF
  As Left N1 RPM accelerates through 12%:

  Left Condition Lever – LOW IDLE
  Right Generator – RESET, then ON

  ITT and N1 – MONITOR (1000 oC maximum)
  Left Oil Pressure – CHECK
  Left Ignition and Engine Start Switch – OFF (at 50% N1 or above)
  Left Generator – RESET, then ON
  Right N1 – REDUCE TO LOW IDLE

Why is the right engine's generator turned back off before starting the second engine (step 7) and then only turned on again after injecting fuel (step 9)? Doesn't this put unnecessary load on the battery?

Yes it loads up the battery, but the battery is sized to do this. The generator is not necessarily sized to do this, especially on the ground, so this procedure gets you to take the generator out of the system for each start.

On an Alternator, like on your car, there is a phenomenon known as 'forward-stripping', which is where there is such a demand for current out of the generator that it overloads the rectifiers (that convert the created AC electricity into DC for output). So here, during/after the engine start, there may be a large charge request by the battery which might forward-strip the gen's rectifiers.


Are MSN numbers unique Is it possible for two aircraft to have the same MSN number,  I was under the assumption the an MSN was a unique identifier for an aircraft

Each manufacturer has its own conventions. For example, Airbus only use MSN (which is only unique per aircraft type). Boeing on the other hand uses both an MSN and "line number". Boeing's "line number" is unique per aircraft type (like Airbus's MSN) but the MSN is unique across all aircraft types.

Other manufacturers may or may not have the same conventions.


What is oil thermal expansion in the CFM LEAP 1A engine for the A320NEO? While I was going through the FCOM's SOP, normal procedures chapter, preliminary cockpit preparation section I came across the term oil thermal expansion. In the image below it is mentioned in the note section that

  IF THE ENGINES HAVE BEEN SHUT DOWN FOR MORE THAN 60 MINS, DECREASE THE ABOVE QUANTITIES BY 3 QTS TO TAKE INTO ACCOUNT OIL THERMAL EXPANSION.

What exactly is this phenomenon and why does it require us to reduce the oil quantities mentioned when carrying out the engine oil quantity check?

When oil heats up, its volume expands (and its density goes down). This means that if you measure the level of oil in a gearbox when it is hot, the level will read high, and if you come back later and measure it after it has cooled back down to ambient, it will read low.

So if you were to fill a sealed gearbox with cold oil without taking its thermal expansion into account, when it then gets hot it might expand enough to overpressurize the gearbox and blow out its seals.

I had a tractor engine in which overfilling it with cold oil would then cause it to pump the box containing its ignition points full of engine oil once the engine had gotten hot. This would then foul the points and stop the engine.