How are turbine blades overhauled? How are turbine blades overhauled? What techniques are commonly used and how does it affect the integrity of the blade?

Discs, blisks, and blades are all very expensive components so if they can be salvaged through repair and overhaul programs they usually are. According to this article overhauling a high pressure turbine blade from the CFM56-7BE is 80% cheaper than producing a new one, which is roughly inline with my experience. The actual overhaul process will vary depending on why the item was taken out of service and what condition it is in. There are many companies out there that are built around overhauling various components of gas turbines and many OEMs like RR & Siemens have programs or side businesses for it.
Compressor blades
For a compressor blade a full inspection is required to check it still meets the original specification. This would include at a minimum:

Visual inspection for nicks, scratches, or any other damage
CMM inspection (or equivalent) for shape and size
Weight and balance measurements

If there is any damage to blade it may be possible to repair them through welding or other approved methods depending on the location and severity, but I've never actually come across any company doing so for compressor blades, they're normally discarded.
Turbine blades
All of the above also applies to turbine blades, except that they often can be welded and repaired if they're cracked. They're much thicker and sturdier than compressor blades. If the blade is coated, (pretty much all modern blades are) the coating will have to be removed, replaced and re-inspected for thickness. There will also be a check that all the cooling holes are clear and functional.


Are there specific regulations for airports near water bodies? Are there any regulations that requires that international airports maintain a navigation channel deep enough to have access in case of an emergency when the airport runway ends near a body of water?

Yes, there are ICAO regulations requiring water rescue facilities. For example, from New marine rescue fleet launched at Auckland Airport:

  Airports which have a significant proportion of aircraft movements over water are required by the International Civil Aviation Organisation (ICAO) to provide a specialist rescue facility, capable of an effective response in a marine area around the airport. Marine rescue craft are required to be able to transport sufficient flotation equipment to an accident site to support the number of people carried in the largest aircraft.

You can find this regulation in the ICAO Aerodrome Standards document:

  Emergencies in difficult environments

  9.1.14 The plan shall include the ready availability of and coordination with appropriate specialist rescue services
  to be able to respond to emergencies where an aerodrome is located close to water and/or swampy areas and where a significant
  portion of approach or departure operations takes place over these areas.

  9.1.15 At those aerodromes located close to water and/or swampy areas, or difficult terrain, the aerodrome
  emergency plan shall include the establishment, testing and assessment at regular intervals of pre-determined response for the
  specialist rescue services.

Note that this does not necessarily require a navigation channel, as rescue operations can be primarily provided by hovercraft (as in Auckland).


North Atlantic Tracks entry and exit points Are the actual locations of the North Atlantic Tracks entry and exit points published anywhere? I'm looking for definition (latitude, longitude) of the entry and exit points for North Atlantic Tracks.

You can find the routes them selves here, the start and end points are noted like other airway intersections by phonetic names. You can look up the names here (use world hi maps to see them) for exact locations. For example you can see a route start at TUDEP and if you take a look on SkyVector you will find (lat/long in the top corner):


How do I plan TOC/TOD in an IFR flight plan that includes SIDs and STARs? For a checkride, I'm being asked to plan an IFR cross-country from Miami Executive (KTMB) to Tampa International (KTPA), using SIDs, STARs and approaches.

I'm going to use MIA5.MIA (Miami Five Departure) to depart KTMB, then get radar vectors to the WINCO transition, then fly via V97 to LBV (La-Belle VOR) and then use LBV.BRDGE8 STAR (LBV, ROGAN, BRDGE, JSTRM, PIE), and then get radar vectors to the Final Approach Course and perform the approach at KTPA.

My question is, when choosing the fixes for the flight log how should I include Top Of Climb (TOC) and Top of Descent (TOD)?

For the SID, which one of these options should I use:

1) Write KTMB, TOC, DHP, WINCO
2) Write KTMB, TOC1, DHP, TOC2, WINCO

Option #2 has two TOCs because the SID states that I should maintain 2,000 or assigned higher altitude, and then expect my filed altitude 10 minutes after departure.

And then on the STAR, where should I place the TOD: after the LBV VOR or after ROGAN? And what altitudes should I write after LBV if I don't get a descend via clearance?

I'm not sure if there's a standard, 'correct' way to do this, but for planning purposes I'd just assume that your climb and descent aren't affected by the SID and STAR. Calculate them as if it were a VFR flight, in other words, based on the route you have.

The issue here is that you don't know what ATC is going to do. The SID says "maintain 2000 or assigned higher altitude" so it's possible that they just clear you directly to your cruising altitude. Or they might keep you lower for a while for traffic, or have you climb in multiple steps, or whatever. Unless you know the local ATC practices very well you can't plan accurately for that and even if you do know what altitude they assign 99% of the time, your flight might still end up in the 1% that gets other instructions. Ultimately, you have to deal with whatever ATC gives you.

It would be worth asking your instructor about this too. First, he may have a better answer than I do! Second, he may know what the local ATC procedures are in that area. And finally, since you mentioned a checkride he might have some insider knowledge on what the examiner expects or likes to see. However you end up planning it, you should be able to explain to the examiner how ATC's instructions might affect your time/fuel calculations.


What security regulations apply to US private airstrips? What kind of security regulations govern the 15,000 private airstrips in the U.S.?  Do they fall under TSA jurisdiction?

I'm a freelance writer working on a story and wondering how senior Al Qaeda operative, Adnan Shukrijumah, was able to fly unhindered from Mexico to Cielo Dorado in Anthony, New Mexico.

Is this kind of laxity uncommon or par for the course at all private airstrips?

Private airports have very little security and light aircraft flying within a country typically aren't required to file a flightplan and aren't required to have transponders for secondary radar. You can get in a private aircraft and fly off pretty much as easily as you can get in a car and drive off.

Private aircraft crossing international borders are subject to much stricter requirements. In the case of flights from mexico to the USA they will be required to have a transponder, to file details of the flight with the authorities in advance, to land at the first available airport of entry and to maintain a certain altitude so they remain visible on radar. At the airport of entry CBP (customs and border protection) will gather will check passports/visas, gather customs declarations and may inspect the plane. If you want to know the gory details take a look at http://www.cadds.com/bcpilots/docs/CrossBorderManual.pdf .

There is a section of airspace surrounding the USA and Canada called the North American ADIZ. All aircraft in that zone are required to identify themselves. If an aircraft is spotted in the ADIZ cannot be identified then it may be intercepted by fighter jets.

But a small plane flying low with the transponder turned off is pretty hard to keep track of on radar and if the criminal can slip undetected through the ADIZ he can easily find a quiet private airstrip to land at.


Why would the cabin crew spray the cabin before landing? Once when I was on a plane that was about to land, the cabin crew came around and they were spraying these small canisters. I've been on many flights but it was that particular flight where they did the spray. Can someone please tell me what those spray's are?

The sprays are (usually) insecticides used to prevent the spread of invasive species and diseases.

  The process of ‘disinsection’ is required under the International Health Regulations of the World Health Organization (WHO) on flights to and from certain destinations to prevent infectious and contagious diseases carried by insects and other volatile bodies. Rules established by the International Civil Aviation Organization (ICAO) require that disinsection does not injure or cause discomfort to passengers or crew. These rules permit the use of certain insecticides, which have the approval of and are recommended by the WHO based on their effectiveness and safety.

From the US dept. of Transportation


Is it possible to use US-made engines in Russian made air-frames? Suppose, someone buys 300 Pratt & Whitney engines from the USA and 150 Mig-35 air frames from Russia. Then, they use their in-house expertise to fit those engines to those air-frames.
What challenges would have to be faced by the user?

Re-engine an existing airframe is such a complex undertaking, in a way that so much R&D is required that the entire aircraft ends up being reverse engineered to make it work. Or on some occasions, the airframe is first licensed or reverse engineered, after which, experience and expertise is gained to enable the re-engine effort.

China is notable in this area due their Russian lineage of airframes and a long lasting desire for western engines, and a mixed relationship with both. India is in a similar position but I'm not aware of any such attempts.

Notable examples:

Y-8-F600(Antonov An-12 clone) with PW engine, failed
Sabre II(Mig-21 derivative) with GE or PW engines), failed.
Shenyang J-11(license production Su-27 derivative) with Shenyang WS-10 engine (reverse engineered CFM56 derivative), successful.


Why are the positions of an aircraft's angle-of-attack vanes dependent on airspeed? According to this answer to an earlier question of mine, the positions of the angle-of-attack sensor vanes on the A330/A340 are dependent not only on AoA, but also on airspeed (and, according to @ymb1's comment, this is true for all aircraft, not just the A330/A340):

1. AoA vanes are speed sensitive
An AoA vane is in essence a weather vane, but due to various aerodynamic interactions it's not perfect. From flight testing they would have worked out how speed affects the position, and based on that the sensed position is corrected by speed input.

This doesn't make sense - the airflow over the wing is qualitatively the same for a given angle of attack, no matter what the airspeed (as long as you stay below the first critical mach number, at which point you get shockwave formation and all sorts of other goodness), so how can the airspeed affect the readings from the AoA sensors?  Shouldn't the airspeed merely affect how long it takes for them to weathervane in the direction of the local airflow?

It's because the vane isn't weathervaning into the freestream, it's weathervaning into the local flow a few inches away from the skin, and because of the curvature of the nose, this flow changes with body AOA (and therefore speed) with some variation relative to the change in freestream AOA.  The Stall Protection Computer software will have calibration tables derived from development flight testing to compensate for most of this.


Are walkalong gliders based on the principles of ground effect? Does a walkalong glider stay aloft because of ground effect (caused by the paddle), or for some other reason?

No the paddle acts as an inverted wing and pushes air up, this in turn will push the glider up.

Even with ground effect a non-powered glider cannot maintain height and speed.


How is the centre of lift and centre of gravity misalignment problem solved in Delta wing aircraft? Since Delta winged aircraft don't have horizontal stabilizers to produce down force, how is that problem solved ?

The effect of a horizontal tail can be built into a tailless aeroplane in two ways:

By integrating into the wing profile: a horizontal s-shape with the trailing edge turned up.
By combining positive sweep with negative twist.


What is the minimal landing area required for an EC135 air medical services helicopter? The Netherlands has a few Eurocopter EC135s for air medical services. (See Wikipedia.) How much clear landing space is required (either theoretically, practically or regulatory) for these?

I don't think there's a hard and fast rule for this, but it depends on a number of factors:

Wind/ Weather conditions requiring safety margins.
Effect of nearby objects/structures. If they are big I think they might cause unpredictable wind effects, again requiring compensation and space.

(source: flugzeug-bild.de)
That said, I'm no helicopter pilot, so somebody else might be able to give a more detailed answer. YouTube is a pretty decent place to find some tight landings though.


Why are aircraft parts built in different places and assembled in one? Airbus has its manufacturing facilities in Spain, France, Great Britain and Germany. Each country produces different parts of the aircraft and is finally assembled in one place.

Why is this? Wouldn't it be more economically sound that everything was built and assembled in one place? Is it because all of the countries in Airbus group have to have some work? Is it because you can't get all those workers in just one country? I don't understand economical and logistical logic behind it (if there is one).

I believe Boeing has the same practice which puzzles me even more, since USA is more of a one country than EU. Is it really cheaper to manufacture something big as a wing overseas and transport it to one facility?

You can only hire so many people at one place, and also only find so much land there, so when the whole process no longer fits, there is no other option than start building components at other places and move them to the final assembly place.

In part, it is also historical. Airbus, but to an extent Boeing too, was formed by mergers of smaller companies as building a plane became more complex over time. So the companies got already working facilities with already qualified workers. Closing them down would lose them those workers, and cause quite a bit of bad publicity too (and at least in case of Airbus some political pressure), so they got used for whatever part of the process fits in them.

And some factories simply don't have any unused land around them, so the companies can't grow them even if they want to and have to open new ones instead. This is probably worse in Europe, which is more densely populated, so building a really big factory is basically out of question anywhere.


Are there any examples (real life cases) of invalid FMS data affecting flight operations? Do you know of any real life cases, when invalid FMS data affected somehow the flight operations?

By invalid FMS data I mean a case, when the procedure coding does not reflect properly the original procedure as published by state's AIP service.

Related question: What happens with RNAV procedures on the way from AIP publication to FMS database

Here is some old material used in Europe to prepare for the transition from AIS to AIM: AIS to AIM: Enabling the Net-Centric Information Environment by Roderick van Dam, Head of Eurocontrol Legal Service. The document references two legal cases where navigational data and hardware were involved.

Pakistan International Airlines

  Bejon v. Pakistan International  Airlines (Toulouse Court of
  Appeal, 1998) Liability of Jeppesen raised.  Jeppesen argued that
  approach map  was based on map provided by States.  Court concluded
  that map was  technically acceptable This case confirmed  the risk
  related to  the publication of Aeronautical data.

More: PIA Flight 268.

American Airlines

  American Airlines v. Jeppesen- Anderson and Honeywell (US Federal
  Court, 2000) The Accident Report had concluded: the probable causes
  of the accident were  related to failures of  the flight crew;
  contributing factor was related to FMS-generated navigational
  information; AA blamed the Jeppesen software in the  Honeywell
  computer; alleged that products  helped caused the crash; A Federal
  Jury found  that Jeppesen was  17% at fault and Honeywell 8 % at
  fault.

More: American Airlines Flight 965.


What is the minimum separation for aircraft operating on or intersecting the same runway? How capturing of this photo was ever possible:

(photo courtesy of jetphotos.com)

Shouldn't there be a bigger time / distance separation of three aircrafts using, operating or intersecting the same runway?

Shouldn't pictured situation be considered as a bit dangerous in terms of ground operations?

As both aircraft are still on a taxiway, turning into two different intersections it's absolutely normal. Each intersection has a so called "holding point" where all aircraft must hold until a clearance to enter the runway is given.

In this particular situation the tower controller will ensure that only one aircraft enters the runway and waits until departure separation minima are given before clearing the second aircraft for takeoff.

In this way maximum efficiency is reached by using different intersections as the following aircraft can line up (and hold on the runway; only after receiving a clearance to do so of course) immediately behind the first aircraft.


Is there any combination of two airports that are connected via taxiways? Are there any 2 (or more) airports that are ground-connected so that e.g. aircraft can taxi between them? Would that be even possible from a definition point of view, having a common area with two airports?

I thought of airport being a facility with an ICAO identifier, but maybe a different definition suits better here?

I could imagine e.g. having an international one and a GA / regional airport being so close together that they were connected at some point. Does anything like that exist?

Update: I'm also curious about airports that are not directly connected in a physical way but e.g. in an operational way: an airside bus takes passengers from the gate in a terminal in airport A and brings them to their flight in airport B or other situations like that.

There are numerous joint use (commercial/military) airports with separate terminals/ramp areas. There aren't many where there are two connected fields. The only one I'm aware of is Naval Air Station Whiting Field near Milton FL that has two operational fields with separate identifiers.

There is NAS Whiting Field - North. ICAO: KNSE and
NAS Whiting Field - South. ICAO: KNDZ

The two fields have a central support base area between the two fields. There is a single taxiway that connects the two fields around the east side of the support area.


What could be the reason for a feeling of speed oscillation after takeoff in a 747? Took off from LHR in a B747-400 on Sunday and I experienced something new that is puzzling me.

After take off as the aircraft was climbing it seemed to continually speed up and slow down at the rate of once every one or two seconds. Kind of like an oscillation in the direction of travel.

After a few minutes of this it stopped.

What caused this to happen?

There are a whole bunch of possible reasons.

The first thing to note is that, from inside the cabin it can be very difficult to tell the difference between speeding up/slowing down, pitching up/down, and the rise and fall from turbulence (particularly when in a climb). And one I wasn't aware of but as pointed out by Sami, the air conditioning pressure varying because of a malfunction in the engine bleed or pack valve.

So why could it be happening?

Turbulence causing the aircraft to gentle rise/fall, which, in a climb, is fairly akin to small acceleration/decelerations - both to our inner ear, and in terms of what's physically happening to the aircraft
The air conditioning pressure rising and falling more than usual rather than maintaining a constant pressure
Slight pitch up/down manoeuvres, possibly from a relatively inexperienced pilot flying the aircraft
The auto-throttle throwing a hissy fit or, if climbing at 250 knots under 10,000 ft, oscillating slightly in order to stay at/below the 250 knot speed limit
The pilots adjusting thrust manually, possibly for the same reason as above
A gusting head/tailwind. Particularly noticeable near the ground and when in a climb, and can cause a feeling of acceleration/deceleration.
A malfunction in one of the engines causing the power to vary slightly. It's possible this was only noticeable at takeoff/climb thrust levels, and disappeared when throttling back

Typically you'd expect it to be due to either turbulence or a gusting wind, but there are several systems which could develop a small, fairly benign fault and have this symptom.

Edit: As noted by user Jamiec, Storm Katie (a reasonably strong storm) was over the UK at the time, which would certainly back up turbulence/gusting winds being the most likely culprit


Why do pilots have the ability to turn off the transponder in-flight? These days, when reading news about missing Malaysia Airlines Boeing 777-200, I keep coming across a scenario where pilot might have deliberately turned off the transponder which is used for the communication of flight with ATC.

When there is a possibility that any bad thing can happen when pilot turn off transponder, why would one give the ability of turning off the transponder to a pilot when he/she usually depends on instructions from ATC or flight control. Is there anyway that ATC can turn on transponder back from ground?

Basically everything that consumes power on an aircraft can potentially cause interference, short-circuits, or otherwise jeopardize the safety of flight and therefore must be switchable. Sometimes the switch is in the form a button, otherwise by a fuse.

There are several particular reasons that the transponder can be turned off.

If the transponder malfunctions, it may cause interruptions to all ATC surveillance in an area. There have been occurrences in the past that due to a fault the transponder it was basically acting as a radar jammer.

In one particular incident it took a while before the aircraft that caused it was identified and after requesting the pilot to switch of the transponder, secondary surveillance was restored.

Another reason is that when the aircraft is at the gate, the transponder is switched of to reduce the amount of radio transmissions. 100 aircraft on the surface of a large airport can produce a massive Radio Frequency noise, which negatively affect radar systems. When taxiing, radar replies are useful for aircraft identification, hence the transponder is switched on at push-back or engine start.

For an example to see how such requirements are worded in aircraft certification regulations, see rbp's answer.

The focus here and in the media1 on the transponder in relation to hijacks is a red herring. If an aircraft is hijacked, there are way more important things to worry about than an active radar transponder.
If hijackers turn off the transponder it means that they have access to the cockpit already.

1This answer was written shortly after flight MH370 went missing while en-route from Kuala Lumpur to Beijing. Initial speculation focussed on a possible hijack scenario.


Why EU/US airline checked baggage allowance limited at 32kg? For most full services on EU/US airlines, the permitted weight of each of the checked baggage allowance is 32kg. It is similar for the routes operated by non-EU/US airlines, even though their overall checked baggage allowance is more than that. What are the reasons behind that?

To add to Antzi's answer it is most likely about worker safety. Your question is where do the 50lbs/23kg and 70lbs/32kg numbers come from. The current answer for US based airports would be the recommendations of the Occupational Safety and Health Administration (OSHA). Here is a link to their current lifting standards. There is a neat worksheet down at the bottom for calculating the weight a worker should lift based on the types of movements they have to perform, distances traveled, angles bent. It is a system based on multipliers whose base limit on a normal lift is 51lbs/23.4kg. Then 70lbs/32kg would amount to the multipliers all working out to 1.4 which is again a nice round number.

I would also speculate that at some points these numbers made it into union contracts for ground workers but I have no actual information on that.

What I couldn't find were the OSHA regulations that were in force when IATA first enshrined 23 and 32kg into their rules. It is perhaps a bit disappointing that the current OSHA regulations to not have a "max lift" number but their interest is in reducing injury and a single very heavy lift is apparently less damaging than repetitive above average lifts.


Is $24.01 million for 15 used A320 reasonable? A few hours ago the insolvent Austrian airline Niki was bought by the owner of British Airways, IAG for a whopping \$24.01 million (plus a further \$19.08 million to provide liquidity to the company). This was the highest offer available.

The acquired assets include 15 Airbus A320 and landing rights for some European airports.

So far this was basically a summary of the Reuters article.

Given that Niki kept their business up until December 14th, 2017 it can be assumed that the aircraft are well maintained and properly working.

So summing up and getting to the question, the price seems rather low for 15 airworthy A320 aircraft in my opinion.

Did IAG get those aircraft for unbelievably low cost, or is the amount in the ballpark for such a deal?

Edit: The article also states that the landing rights are the most valuable asset of the deal, thus further devaluating the aircraft.

Update: As many comments point out, the aircraft enclosed in the deal were not Niki's own, but leased. To date I have only found German-language sources that indicate this.

Update 2: This source (as hinted by @Eike Pierstorff) clearly states, that the aircraft themselves are not included in the deal. They are in possession of Lufthansa, which in turn is bound by this document (section 5 paragraph (40)) to lease or sell the aircraft to any possible acquirer of Niki (IAG in this case) at market terms.

Short Answer: If the 24 Million was for just 15 airworthy A320s, then yes, its a steal because $24 Million is barely enough to buy 2 or 3 A320 engines. However, Niki Airline likely has hundreds of Millions of dollars in debt that British Airways is also "buying", that is why the final price seems so low.

Longer Answer:
There is so much that goes into the value of an aircraft that this cannot be definitively answered. Even a properly maintained aircraft may be of little value if it has too many cycles (landings and take-offs) and total hours. For example, if those 15 A320s all need engine overhauls then its going to cost a lot more than $24 Million to get those done, not to mention the downtime.

Additionally, and adding even more uncertainty to your question (sorry), is that the $24M was for the airline. As you probably know, the airline includes other assets, such as the landing rights that you mentioned, but it also includes other liabilities, such as debts to previous creditors (i.e. vendors, debt service, etc).

So, just as a simple illustration, lets say that the 15 A320 are collectively worth 150M and the landing rights are worth 200M for a total combined value of $ 350M, if the company has liabilities (debts) of 300M, then the net value would be 50M for the whole company, and paying 24M would be a steal!  But on the other hand if the company had liabilities of 400M then the company would be worth -50M. Usually, when a company is insolvent it is in this situation where it may have some valuable assets, but it has more liabilities than the assets or cash flow it generates. British Airways is likely hoping to streamline the business and turn it around, or just scrap it and take the landing rights for its own fleet.


What’s the speed for lowest sink rate on a DA20? In the DA20, the POH lists for glide speed (flaps up) only one speed giving the best glide angle / furthest distance (73 KIAS).

But what about, if I don’t need that much distance to glide (e.g. because I’m high above an airport) and rather want to optimize for the maximum time airborne, i.e. the lowest sink rate. Which speed would I need to fly for that? I couldn’t find any L/D diagram in the POH either where I could find it from.

Admittedly, my prop performance theory is a tad rusty, so maybe someone else can provide proof here.

I seem to recall that - assuming a quadratic drag polar which might not hold true for your aircraft - minimum sink speed has a theoretical value of .76 x best glide speed, so in your case that would be around 56 KIAS.

If you’re comfortable and consider it safe to do so, why don’t you (at a safe training altitude) trim for 73 KIAS and establish a stable idle descent, note vertical speed descending through a predetermined altitude, reposition to initial altitude and repeat in maybe 5kts decrements down to stall speed or some safe margin above - the former of course only if flying right down to stall is still trained and allowed in small aircraft. This will give you at least a rough idea of where minimum sink speed lies and what rates to expect.

The DA 20 Approved Flight Manual doesn’t help much, by the way.


How many US airports have 4 or more parallel runways? Which airports in the US have four or more parallel runways?

How many are planning additions in the next 5 years?

A few years back, 10 years, Charlotte opened their 3rd parallel runway and it was said to be only one in seven in the country. Just wondered if there is an updated number for 4+ runways. I have searched and some sites have great information but I am not sure it is up to date. Not trying to offput just looking for information that might not be posted to the public.

Based on my current navigation database (AIRAC 1909, valid from 2019-08-15), I found 7 US airports with 4 (or more) parallel runways:

KATL (Hartsfield–Jackson Atlanta International Airport):
5 parallel runways: 08L/26R, 08R/26L, 09L/27R, 09R/27L, 10/28
KDEN (Denver International Airport):
4 parallel runways: 16L/34R, 16R/34L, 17L/35R, 17R/35L
and 2 more parallel runways: 07/25, 08/26
KDFW (Dallas/Fort Worth International Airport):
5 parallel runways: 17L/35R, 17C/35C, 17R/35L, 18L/36R, 18R/36L
and 2 more parallel runways: 13L/31R, 13R/31L
KDTW (Detroit Metropolitan Airport):
4 parallel runways: 03L/21R, 03R/21L, 04L/22R, 04R/22L
and 2 more parallel runways: 09L/27R, 09R/27L
KLAX (Los Angeles International Airport):
4 parallel runways: 06L/24R, 06R/24L, 07L/25R, 07R/25L
KMCO (Orlando International Airport):
4 parallel runways: 17L/35R, 17R/35L, 18L/36R, 18R/36L
KORD (Chicago O'Hare International Airport):
5 parallel runways: 09L/27R, 09R/27L, 10L/28R, 10C/28C, 10R/28L
and 2 more parallel runways: 04L/22R, 04R/22L

I do not know how many are planning additions, but I linked the Wikipedia article for each of the airports, maybe that will help you find more information on that.

Trivia: There are only 3 airports outside the US with 4 parallel runways: LFPG (Paris Charles de Gaulle Airport), LFTM (Istanbul Airport) and ZSPD (Shanghai Pudong International Airport).


Does FAA recommends specific maneuvers for complex endorsements? I was pretty excited for a 5 hour flight in a piper arrow in a new school this coming weekend but my instructor that I have not flown with before texted me saying that he will not sign me off after one flight and would like to narrow down the time to 2 hours only. Although I was not expecting to get signed off after one flight, I canceled the flight as I didn't want to fly with an instructor with a definitive mindset of not signing off. I would have liked to fly with an instructor who'd judge me based on my performance and learning capability during the flight and ground training and then decides whether to endorse me or recommend further training. Not only I have had bad experience with one of my "time builder" instructor as a student pilot, I have heard from other pilots that some instructors can give us hard time and not be fair.

So I wanted to ask two questions.

Is there a FAA recommended list of maneuvers/training material for each endorsement? So that if a student fulfills all the requirements and fly all the maneuvers safely, he'd get endorsed?
How do we keep instructors/DPE in check? Obviously they are experienced and safer pilots but it is quite possible that in one out of hundred scenarios they are wrong and pilots receiving training are right?

To answer question #1:
A complex endorsement is just that. It’s an endorsement. There are not any prescribed maneuvers to receive the endorsement. But, the instructor is holding himself liable for your performance after receiving the endorsement. Primarily, the areas where that liability may prove hazardous to the instructor, their certificate and career is on every single one of your takeoffs, landings, and go-arounds after the instructor is no longer in the aircraft. The flows and checklists for that aircraft have to be consistent and second-nature to you even without the instructor’s prompting.

A 2 hobbs-hour flight from start-up to shut-down is typically the limit of learning for most people. Two 2-hour flights along with a couple of hours of ground instruction seems reasonable for satisfying your instructors confidence in your ability. That is all conditional to your actual performance. It is also a very subjective assessment.

To answer question #2:
As far as correcting someone with more knowledge and experience than yourself, that can and should be done if (big if) they are wrong. Many things in life and aviation are subjective. There can be multiple right answers. In the case that an instructor gives you a wrong answer, be courteous and cordial in your correction. But, your answer must be backed up with fact. You should question your own answer until it can be: backed up with a regulation or recommendations out of the FAR/AIM; documented from an FAA handbook, publication, advisory circular, or other official document; referenced from the POH, AFM, owner manual, or other manufacturer source; cited from a reputable source.

If the question can not be answered objectively, the answer may only be a matter of opinion, style, or personal technique. Or, it could be a quirk of that particular aircraft or environment. Either way, it would probably be best to tailor your training to the criteria of your evaluator.

Remember, your instructor is human. They are fallible. But, so are you. And, they have been doing this aviation thing safely way longer than you. But, regulations do change. Take a slice of humble pie before you have to eat crow. You both may learn something new or improve upon something old. And remember, a good pilot is always learning.


What is the function of the protruding outlet on the Trent XWB? Original by Bernd Sieker from Germany (Trent XWB) [CC BY-SA 2.0], via Wikimedia Commons

Above is a photo of the Rolls-Royce Trent XWB highlighting the aft facing outlet in question.

In a video for the A350 you can see condensation coming out of it. Since I haven't noticed such protrusion on other jet engines before, it got me thinking, can it be a pressure relief outlet to allow higher fan RPM without choking the burner?

In other words, the engine would bite off more than it can burn to gain higher bypass thrust, and yet still be able to vent the extra air when not needed, at idle power for example.

Or is it just an oil drain for the accessory gearbox? That wouldn't explain the condensation though. Educated guesses are welcomed. I couldn't find anything on it.

That is the drain mast and breather outlet combined. There's some discussion about this in airliners.net (not the same engine, but I sure they are similar):

The exhaust from the air/oil separator goes out here. Usually hot air, but when the engine is running at idle you can see smoke coming out. This is engine oil that has got past the seals. The smoke stops as the engine speeds up, the air pressure increases, and the seals work properly.
Also in the main body of the mast are four holes. These are tell tale drains from various parts of the gearbox. They drain out the cavity between the accessories (starter, fuel pump, hyd pump etc.) and the gearbox.

You can see the drain in the cutaway of Trent 500 here. Here's a closeup of the part in question:

Trent XWB drain; image from a380.boards.net; credited to Part of the Trent XWB by JKKW, on Flickr


Why are turbine-powered aircraft "considered ready for takeoff"? In an answer given to How can I politely remind ATC that I'm waiting for instructions or clearance? by @Mark, he quoted part of AIM regs:

4-3-14. Communications
a. Pilots of departing aircraft should communicate with the control tower on the appropriate ground control/clearance delivery frequency prior to starting engines to receive engine start time, taxi and/or clearance information. Unless otherwise advised by the tower, remain on that frequency during taxiing and runup, then change to local control frequency when ready to request takeoff clearance.
NOTE- Pilots are encouraged to monitor the local tower frequency as soon as practical consistent with other ATC requirements.
REFERENCE- AIM, Automatic Terminal Information Service (ATIS), Paragraph 4-1-13.
b. The tower controller will consider that pilots of turbine-powered aircraft are ready for takeoff when they reach the runway or warm-up block unless advised otherwise.

I have bolded paragraph b because that is what I am asking about here. Why is the tower controller to consider turbine-powered aircraft to be "ready for takeoff when they reach the runway"?
Why are non-turbine engines (piston?) not ready?
What does this paragraph cause the tower to not have to do?

Piston aircraft require a run up, in which the pilot sets the power to a specified RPM, in order to check proper functioning of a variety of systems, and to ensure that the oil is warmed up to a specified temperature. Among the systems to be tested are carborator heat (for normally-aspirated engines), magneto operation, alternate air (for fuel-injected engines), hydraulic systems, and propeller governor (for variable-pitch props). In an aircraft with a hydraulically-activated variable pitch prop, the prop is "exercised" by varying the pitch, which allows the cold motor oil in the prop governor to mix with the warm oil in the engine sump, to bring the entire lubrication system's oil temperature up into the green arc. During the runup, the vacuum system is checked to ensure that enough vacuum pressure is provided to critical instruments. Lastly, a grounding check is made with the magnetos, to make sure that the magneto electrical system is properly grounded.

Performing a run up at the ramp would prove dangerous to people and aircraft if not tied down and due to FOD.

Generally turbine warmup occurs pretty quickly after startup and during taxi. They are usually ready to go at the ramp.


How is fuel drawn from drop tanks in WW2 airplanes? Low-wing tanks typically have fuel supply lines at the bottom. For external drop tanks, I thought they came built with internal lines that mate with the plane.

That appears to be the case for more modern tanks – though I'm not sure yet – based on a mid-50s patent. Earlier patents (image shown) that correspond with WW2, as far as I've seen, don't mention any relevant internal structures. And two Wikipedia articles weren't helpful:

Aircraft fuel system
Drop tank

Most I could find was an F-86 manual on Google Books. And it mentions the jet engine providing compressed air into the tank to draw fuel (the mechanism isn't yet clear to me, but I'll pursue that later).

For now, WW2 aircraft. For example: Where electric motor compressors used instead of bleed air? Were there internal tank structures that assisted?

If there are differences between implementations, then the most common/typical would do or the typical American-fighter implementation (to assist with references since they're typically much easier to find).

Like the F-86, air is supplied into the tank so the fuel can be pushed up. On the P-51, the source of the air was the vacuum pump (the one used for the flight instruments), providing 5 psi of pressure.
Knowing this, the two connectors, pressure and fuel, can be seen in many photos.

Source: P-51 manual

And the two connectors:

Source: ww2aircraft.net


Why was the 737 main rudder servo valve’s vulnerability to causing uncommanded reversals never discovered in testing? As was discovered during the NTSB investigation into the crash of USAir Flight 427, the design of the servo valve inside the main rudder power control unit (PCU) formerly used on 737 Original- and Classic-series aircraft was vulnerable to causing uncommanded rudder reversals if the valve’s secondary slide jammed to the valve housing at a position sufficiently far from neutral (the precise threshold varies depending on the valve and whether the secondary slide jammed in the extend or the retract position).

The valve was designed so that a secondary-slide-to-valve-housing jam would not cause a reversal:

(Image source, page 83 of report/page 107 of PDF file)

However, if the pilot applied a sufficiently fast, abrupt rudder input, this would cause the valve’s primary slide to overtravel past the secondary slide, porting hydraulic fluid in the wrong direction and causing the rudder to move in the direction opposite from that commanded, all the way to its blowdown limit:

(Image source, page 84/108)

Given the potential catastrophic results of an unexpected rudder reversal (especially at lower airspeeds), why wasn’t the valve tested to make sure that it would, in fact, react to a jam in the manner in which it was designed to do so, and, if so, why wasn’t this failure mode discovered in testing?

After the accident the NTSB did very extensive testing on the PCU to determine the cause of the failure. In addition to the flight 427 PCU they tested new units, units that had been removed from other aircraft, and the units from three other crashes (SGR517, UAL585 and SLK185) as well as one selected by Boeing that had minimum tolerances.

They subjected the units to numerous tests. They could not get any of them to fail under any conditions expected during flight. The only test they could get a failure on involved an extreme temperature differential of injecting very hot hydraulic fluid suddenly into a very cold PCU. The report states "The extreme temperature differential produced during this test condition would not be expected during normal flight operations." (NTSB report page 79)

All of the PCUs passed the testing procedures for certification even after the failure testing. The temperatures of the PCU and hydraulic fluid on a 737 during flight were tested by Boeing and did not match those used for the testing.

So the PCU can only be made to fail under specific, fairly unrealistic conditions that would not be expected during flight. There was no evidence that those temperature conditions were even present during any of the flights where PCU failure was suspected. There was no reason for testing the part during design or certification in conditions that would not be expected to occur.


What is that pitot-like device under the left wing of the Cessna 152? Source: tkaviation.net

I suspect it's related to the fuel system, but it's only found on the left wing (see the yellow circle, not the pitot-tube). I haven't been able to find the right keywords to find anything about it.

I'm hoping to know its function and how it does it.

It's the fuel tank vent. It prevents forming a vacuum in the gas tank as it's drained, which would prevent further fuel flow to the engine.


How much do a DC-9's wings weigh? How much do the wings weigh?  How much does the landing gear weigh? How much fuel does it take to fill the tanks? We, at Confederation College are trying to donate a DC-9 for training purposes and require information to facilitate this.

From this answer, for the DC9-10 variant:

the wing group weighs 9470 lbs
the landing gear weighs 3660 lbs

Wikipedia mentions the fuel weight as 24,743 lbs.

For other variants, scale the wing and landing gear weights up with the ratio of Empty Weights found on Wikipedia.


Did the airfoil choice on the P51D affect its maneuverability? Did the wing design on the P51D, which used a laminar airfoil, affect its maneuverability? If so, how? To what extent?

Yes it did because the stall behaviour of the Mustang, due to the airfoil, was very abrupt and it was much easier to get into an accelerated (high speed) stall pulling high G than it was with an airplane such as a P-38, which had wonderfully benign stall behaviour and could be manhandled all over the sky in relative safety.  At the beginning of a trip in the P-51 this was made worse by the aft fuel tank's effects on CofG which made the stick forces very light so you wanted to avoid radical maneuvering until you burned off the fuselage tank fuel.

The later versions of the P-38 with hydraulic ailerons and dive flaps were just about the best propeller driven aircraft in the sky in 1944, because at high speed they could roll as fast as SE fighters and had the benefits of crazy rate of climb and no engine torque effects, on top of the very gentle stall behaviour.   One reason the Mustang dominated, even though the Lightning's technical bugs were worked out by that time, was the P-51's performance and range was great AND it was only about 2/3rds the unit cost of a P-38.


Why don't hyperbolic navigation transmitters on the surface (e.g. Loran-C) allow altitude measurement? I have been reading the following article about radio navigation in general and GPS / GLONASS receiver design:

A homemade receiver for GPS & GLONASS satellites

in which the author states the following about hyperbolic radionavigation:

  Hyperbolic navigation systems were first implemented as ground-based navigation systems [...] Since the transmitters are located on the Earth's surface, the geometry of the problem does not allow a three-dimensional navigation. [...] To measure the altitude, one of the transmitters should be located above or below the user's receiver or at least out of the user's horizon plane.

I do not seem to be able to comprehend why this is the case. Even if the transmitters are located on the surface (e.g. Loran-C), the differences between reception times of signals from 4 transmitters would generate 3 rotational hyperboloids in 3D space, the intersection of which could be above (or below) surface.

You are correct, the time difference generates hyperboloids in 3D space and as such can be used to find the 3D position. However, if the transmitters are far apart and/or in the same plane, the geometry of the intersecting hyperboloids is such that the accuracy in the vertical axis is very poor.

When the receiver is in the same horizontal plane as the transmitters, all intersections almost form a vertical line. The slightest inaccuracy in time difference, would cause only a slight error in the horizontal position, but a significant error in the vertical position.

The concept is called Geometric Dilution of Precision (GDOP). A good start for more detail can be found on Wikipedia


How is the JFS manual hand pump operated on an F-16? The JFS (Jet Fuel Starter) on an F-16 allows for two attempts to start the engine.

https://www.quora.com/How-do-you-start-the-engine-on-an-F-16-Fighting-Falcon

However, if this fails, there is a manual pump that can be operated:

Where is this pump? Inside the cockpit? Hidden in some hatch outside? How is it operated? How long does it take to build up enough pressure for one more attempt to start the engine?

As a bonus question: Surely the JFS system can be charged with external equipment rather than this hand pump? What is that procedure called?

I used to be a crew chief on F-16s in the Air Force. Our pilots usually jump both bottles at once during a start up to ensure a proper start, if it fails it's most likely because there wasn't enough pressure accumulated. It's part of our launch procedures before they take off to check to see if it's within limits, if not you have to get the "T handle" and go manually pump it up to pressure. This is done in the left hand wheel well there's a big slot for the handle to be inserted and pinned into to keep it in there while you pump. There's a gauge on the opposite side under a quick access panel to check the gauge. Usually if we do have to pump it you'll pump for awhile and take a look at the gauge and keep going back and forth to check it. We rarely hook anything up to automatically charge it, it's manual 9/10. Actually, it usually takes 15 min to charge however some people are slower or even faster, it's more on the person to determine how fast it charges. It's very tiring to do it.


Why is the detcord on canopies wavy? We know that the wavy lines on the canopy are detcord, to shatter the glass in case of ejection.

Why are they wavy?

Possibly to shatter the canopy into smaller fragments.  Straight dets might just detach a large proportion, possibly leading to unwanted severe secondary strikes on ejecting aircrew.


What noise reduction measures were taken in the time between hush kits and chevrons? In older jets such as 727 hush kits are used in the engines to reduce noise. In very modern jets such as 787 or 747-8F, chevrons are used for the same purpose. What happened in between? The switch to high bypass ratio turbofans alone is sufficient an improvement in engine noise that no special noise reduction device is required? (Not even hush kits?)

A hush kit:

A chevron:

Generally speaking one is an add on and the other is integrated into the design. Over the years the FAA has implemented progressively more stringent noise regulations both for aircraft that are flying as well as to the full certification process. The changes came in stages and have been progressively altered for the past 30 or so years take a look at §36.103   Noise limits for the current regulation .  You can find some of the FAA legislation here and the full text of the relevant FAR's here. As such newer engines will incorporate noise mitigation directly into the design as they need to pass the certification process. However in cases relating to older designs it may be cost effective to design an attachment (like a hush kit) that can be added to an existing engine to comply with regulations since the engine was certified at a time that the regulations were far more relaxed. Due to the letter of the law planes can be grounded for being non compliant so a hush kit may be cheaper than a new plane. For example this was the case with the Lear 23 which had required hush kits.

Interestingly at least one patent for a hush kit stresses that it is for an existing engine design.

  BACKGROUND OF THE INVENTION

  The present invention relates generally to
  jet aircraft turbine engines and, more specifically, to a hush kit for
  use on an existing engine and airframe.

You can similarly find the patent for the chevron here. Which describes it more as part of the engine than as an add on.


Which country has the lowest flight time requirement for the issuance of a pilot certificate? Which country requires the fewest number of flight hours for pilot certification?

Here in the US you can get your sport pilot in 20 hours but that is a pretty heavily limited license, your basic VFR PPL will require at least 40 hours here in the US. You should note that this is flight time which is generally tracked on the hobbs meter (positive oil pressure or master switch on depending on configuration). There is a lot of other time in ground school, flight planing, pre-flight checks, and waiting for weather to pass that you will undoubtedly run into.

Looks like Canada requires 45 hours and thats day only. The UK also requires 45 hours.

These hours are for VFR only. An instrument rating, which is not a separate license but an additional rating on the pilot certificate, will take more hours. Some countries limit night flying privileges without an instrument rating, and if you intend on doing any serious flying its a good idea to have.

As mentioned by Ron in the comments, not all licenses are valid everywhere. Some are recognized with limitations, and some can be transferred depending on what you are trying to do. Some privileges may be recognized if you are flying to an area to visit etc. Other situations may require more training or filing different types of flight plans. Almost everyone is on ICAO format these days.


Does the DC-3 passenger airliner have slats? The DC-3 is a very popular and widely used aircraft, yet has low speed tip stalling risks due
to the design of its wing, which at the time of its development represented the leading edge in drag reduction for larger and faster mono-wing aircraft in the 1930s.

Specifically, the swept and tapered wingtips, at high angles of attack, would stall first.

Did the original design, or later modifications, include slats to remedy this issue?
Is there any way to reduce this risk other than retractable slats?

Although it's technically possible, you aren't going to find slats retrofitted to a DC-3 (nobody would ever pay for it).

But yes, in theory you could improve the DC-3's stall behaviour and single engine performance very cheaply with Vortex Generators which can be thought of as a "poor man's slat".  VGs increase stalling AOA by between 1/3 to 1/2 of what you get with a slat (say, about 18-20 deg, vs 25 with slats) depending on the airfoil.

VGs have huge benefits on light twins, reducing stall speed, taming stall behaviour, improving aileron response, and often reducing Vmc to the stall when installed on the fin.  They are quite miraculous, although they can be a headache because they are usually glued on and if the kid you pay 20 bucks to wash your plane knocks off too many, you are grounded until you fix them.

I can't find anybody that sells a VG kit for the '3 although it does seem to be possible to install them on a wing with boots. Possibly the DC-3's airfoil requires VG placement far enough forward that the boots are in the way (each airfoil has a chord wise sweet spot and if they are too far aft they don't do anything).  Or maybe nobody has thought of doing it yet.


What kind of filters are being used to filter the bleed air? Bleed air is using by airplanes for various purposes including deicing and also to feed air conditioning units that maintain temperature inside cabin. What kind of filters does bleed air pass through before reaching the cabin?

Although typical HEPA filters are what's used (speaking from Boeing experience only), it's important to understand how bleed air is used in a typical aircraft environmental control system (ECS). Bleed air is taken from the compressor section of the engine before the fuel is introduced, so it's still relatively clean ambient air. As such, special filtering is not really required, not any more than say a cabin air filter on a car.

Because taking bleed air from an engine robs it of power, commercial airliners will generally use a mix of engine bleed air and recirculated cabin air for the ECS. This reduces the amount of bleed air taken from the engines, and improves their efficiency.

Where the HEPA filters become important is for filtering of the recirculated cabin air. If you were to look under the floor of a commercial airliner that's been in service for a few years, there is a lot of collected dust, dirt, hair, and grime from all the people who've sat for hours on end. Since a significant amount of the air you're breathing onboard is recirculated, the HEPA filters are important for filtering out the typical air impurities you get from having a confined space with lots of people, not necessarily from the engine bleed air.


Why do new helicopters have rotor blades with swept tip? Many new helicopers have rotor blades with swept tip. Why? Acoustic? e.g.

Helicopter rotor design is, among many other things, a balancing act of retreating blade stall and transonic effects on the advancing blade.
Helicopter blade tip devices usually serve to reduce transonic effects on the advancing blade. The Ka-50's blade tip is rather simple (only swept) as the coaxial design is affected less from these transonic effects, as they happen on both sides. For the same reason, retreating blade stall is also a lesser problem, so the designer can get away with a slower blade tip speed.
Helicopters with a single main rotor don't have this luxury, so many of them have complex blade tip devices with sweep, larger chord and sometimes different airfoils. I believe a good example is the Westland (now merged, AgustaWestland) Lynx.

Blade leading edge is top of picture. Sweep, increased chord and droop on end are evident.
For my Bachelors thesis, I was looking for blade tips with the highest drag divergence Mach number, the Lynx blade tip's was M 0,91. I believe this is an important reason why the Lynx is the fastest conventional helicopter.


How do large airport refuelling systems work? As I am sitting on board my plane and have been informed that there may be a three-hour delay due to a pressure problem in the fuelling system at the destination, it seems like an appropriate moment to ask: how do these systems at large airports (in this case, Amsterdam Schiphol) work?

Apparently, it is an airport-wide problem affecting all flights that need refuelling.

How and where is fuel stored?
How is it delivered to the aircraft?
What sort of technical problem with pressure could affect an entire airport the size of Schiphol?

To complement the other answers with pictures:

(Source)

Both systems shown above work well for big airports. The system depicted at the bottom is called a Hydrant Fueling System.

(Source)


Do some airplanes have weighing scales on the landing gear? It seems to me that takeoff weight is an important piece of data, and if one had weighing scales between the wheels and the aircraft body, one could precisely determine it. However, searching the internet, there seem to be no scales and cargo/fuel/passenger weight is estimated by adding up what is brought on board. Why aren't scales common? And are there any commercial airplanes that have them?

Yes, such system exists, for example the Honeywell WBS.* It was developed during the sixties and installed as an option on the Boeing 707-300 freight aircraft. Other aircraft that had a similar (optional) system include the L1011 and Boeing 747.

It is mostly found on freight aircraft because they have a less predictable load distribution (centre of gravity) than passenger aircraft.

The reason they are not common is that they add cost in the form of installation, weight and maintenance. While quite accurate when in good condition, the reliability of the weighing systems used to be poor, which probably contributed to the fact they never became wide spread.

Because crew are commonly operating on several aircraft types these days that have take-off weights of more than 100 metric tonne difference (e.g. A330 and A340, B747-400 and B747-8) there is an increase in take-off weight confusion related incidents. This, combined with the fact that systems are becoming more reliable, makes it likely that these weighing systems will be used more in the future.

* An advert for it in a 1987 Flight issue.


How can an ATC choose the right flight level and heading for airplanes? I don't understand how air traffic controllers can choose the right flight level and heading for airplane when they are in the sky or they're approaching the airport to land.

During the enroute portion of the flight at cruising altitude, pilots will have filed a cruise altitude and route based on airplane performance and weather. ATC will not need to change this unless the pilots ask to deviate or there would be a separation issue. Separation will depend on the local regulations and type of airspace and aircraft involved. For airliners this is usually 3 miles laterally or 1,000 feet vertically.

Pilots may request deviations from their filed route due to weather or other reasons. ATC will look at other traffic in the area and determine what changes can safely be made. ATC may also need to give altitude changes to keep them clear of other traffic. If two planes are converging at the same altitude, at least one of them would need to climb or descend.

When arriving at the destination, many airports have stardard terminal arrival (STAR) procedures that pilots use in their route and may give route and altitude instructions. ATC will also have standard procedures for routes and altitudes that will allow arriving aircraft to descend without interfering with departures or other airspace in the area.


How much humidity is being maintained in cabin and cockpit of B787? I think in general there is less than 10% humidity is being maintained in most of the commercial aircraft but I think they have improved that in the 787. I am not sure how much that is and if the cabin and cockpit have same amount of that?

These sources say that most airliners maintain a humidity of 4-7%, while the 787 maintains 10-15%. Of course these values can vary with conditions, and other sources put them lower or higher.

The cabin and cockpit will not have very different air qualities. While some cockpits may humidify the cockpit air slightly, the cockpit tends to have a greater amount of circulation, leading to drier air than the rest of the cabin.


Can weather groups contain multiple descriptors in a METAR? I understand that a weather group in a METAR is composed of qualifiers (intensity, descriptor) and the weather phenomenon: precipitation, obscuration and other phenomena.

My question is can weather phenomena have multiple descriptors? Eg: Are TSSH or TSSHRA valid in a METAR?

No, a METAR descriptor can only appear once in a METAR or line of a TAF. That is because METARs and TAFs report weather phenomena in the format of

[Intensity][Proximity][Descriptor][Precipitation][Obstructions to Visibility].

Thunderstorms, for example, are one descriptor; therefore, thunderstorms (TS) will only appear once.

Intensity

Applies only to the first type of precipitation reported. A “−” denotes light, no symbol denotes moderate, and a “+” denotes heavy.
Proximity

Applies to and reported only for weather occurring in the vicinity of the airport (between 5 and 10 miles of the point(s) of observation). It is denoted by the letters “VC.” (Intensity and “VC” will not appear together in the weather group.)
Descriptor

These eight descriptors apply to the precipitation or obstructions to visibility:

TS = Thunderstorm
DR = Low drifting
SH = Showers
MI = Shallow
FZ = Freezing
BC = Patches
BL = Blowing
PR = Partial

Precipitation

There are nine types of precipitation in the METAR code

RA = Rain
DZ = Drizzle
SN = Snow
GR = Hail (1/4" or greater)
GS = Small hail
PL = Ice pellets
SG = Snow grains
IC = Ice crystals
UP = Unknown precipitation

Obstructions to Visibility
There are eight types of obscuration phenomena in the METAR code

FG = Fog
HZ = Haze
FU = Smoke
PY = Spray
BR = Mist
SA = Sand
DU = Dust
VA = Volcanic ash


What is the difference between AIM, AIP and AIS in the US? I know that in the US there exist documents which are published by the FAA such as the Aeronautical Information Manual (AIM) and the Aeronautical Information Publication (AIP). What is the difference of each one? What is their relationship with the Aeronautical Information Service (AIS)?

From the beginning section of the Aeronautical Information Manual (AIM).

This manual is designed to provide the aviation community with basic flight information and ATC procedures for use in the National Airspace System (NAS) of the United States. An international version called the Aeronautical Information Publication contains parallel information, as well as specific information on the international airports for use by the international community.
This manual contains the fundamentals required in order to fly in the United States NAS. It also contains items of interest to pilots concerning health and medical facts, factors affecting flight safety, a pilot/controller glossary of terms used in the ATC System, and information on safety, accident, and hazard reporting.

Aeronautical Information Publication front section

2.1 The AIP is prepared in accordance with the Standards and Recommended Practices (SARP) of Annex 15 to the Convention on International Civil Aviation and the Aeronautical Information Services Manual (ICAO Doc 8126). Charts contained in the AIP are produced in accordance with Annex 4 to the Convention on International Civil Aviation and the Aeronautical Chart Manual (ICAO Doc 8697). Differences from ICAO Standards, Recommended Practices and Procedures are given in subsection GEN 1.7.

Aeronautical Information Services

Is the authoritative government source for collecting, validating,
storing, maintaining, and disseminating aeronautical data for the U.
S. and its territories.
Develops and maintains all public instrument flight procedures and airways.
Serves as the FAA's aeronautical charting authority for the development, publication, and dissemination of aeronautical charts and products to support aviation and to meet demand for increased capacity, efficiency, and predictability in the airspace, routes, and airports of the NAS.


Apart from Kenya, what are the current dangerous / unsafe airspaces? Notams are regularly released to warn about unsafe areas, e.g. Kenya:

  A0040/15
  - SECURITY .. UNITED STATES OF AMERICA ADVISORY TO U.S. OPERATORS
  CONCERNING POSSIBLE ATTACKS WITH MAN - PORTABLE AIR DEFENSE SYSTEMS
  (MANPADS)

  ATTENTION U.S. OPERATORS: INFORMATION INDICATES THE POTENTIAL FOR
  TERRORIST ATTACK AGAINST U.S. AND WESTERN INTERESTS IN KENYA.
  WHILE SPECIFIC DETAILS ARE NOT AVAILABLE, ONE POSSIBLE TACTIC WOULD
  BE AN ATTACK USING MAN - PORTABLE AIR DEFENSE SYSTEMS (MANPADS),
  SUCH AS THOSE USED AGAINST AN ISRAELI AIR CARRIER DEPARTING FROM MOMBASA,
  KENYA ON NOVEMBER 28, 2002.
  THE DEPARTMENT OF STATE HAS ISSUED A TRAVEL WARNING FOR KENYA REGARDING
  TERRORIST ATTACKS.

Is there a list of these hazardous airspaces, or a site than can be checked without searching in Notams from around the world? Is there a ranking of the hazards?

I feel there is more to say than currently answered, feel free to reuse part or all of this content to build an improved answer which could be selected.

There are recaps of flight restrictions, e.g. French DGAC publishes maps and plain text files recapitulating areas, in addition of individual NOTAMS:

Source: DGAC

ICAO announced in 2015 the creation of a specific member-collaborative tool CZRI, conflict zones risk information (currently very poor).

EASA also publishes a list of conflict zones bulletins that can be exported as pdf, and a map:

There are also other initiatives, e.g. SafeAirspace with text summary and maps. "SafeAirspace is managed by Flight Service Bureau, with contributions from The Airline Cooperative (225 international airlines), OpsGroup (1500 members), International Ops Bulletin feedback (40,000 readers), and data from the German BMVI, the UK DFT, the FAA, DGAC France, ICAO, and EASA."


Is solar power used (as auxiliary power) on commercial aircraft? Are there any commercial aircraft that use solar energy in terms of auxiliary (not as main power source, as of now) power source ?

If Yes, In what forms is the solar power used?

If No, Why isn't solar energy used in commercial aircraft as an auxiliary power source?

The sun's energy is a max of about 1000 W/m2 at sea level (sun directly overhead, clear day). A solar power installation may have around 15% efficiency, yielding a max of 150 W/m2. Let's just see roughly what that gives us for a 737, to see if it's even that much.

The wing area of a 737 is 124 m^2, and the fuselage is 42 m long and 3.76 m wide. This gives a total area of 280 m2 if the sun is directly overhead. Covering all of this with solar panels gives a total (extremely optimistic) output of 42,375 W. The fuselage tapers at both ends though,  and needs to have openings for the windows, antennas, etc. The wings will probably only have the fixed panels covered in solar panels (not flaps, slats, etc) so you would be looking at significantly less for a realistic installation. The Solar Impulse 1 was in the same ballpark (200 m2 for a peak of 45 kW).

The APU on a 737 is rated at 90 kVA. Of course you will usually use less than that, but we are looking at solar only providing maybe 20% of the capacity for any considerable part of the day. Since solar is not always available, you cannot even downsize the APU.

Another important function of the APU is to provide bleed air. This can be used to run the air conditioning packs and to start the engines. Photovoltaic panels would be a very inefficient way to provide bleed air.

An airplane will probably look strange being covered in solar panels, but covering so much area also causes some issues. Inspecting the aircraft for damage will be difficult. This means the solar installation should be easily removable (also for repairing damage to the solar cells), and the structure may need to be more damage tolerant. The panels would also need to be attached well so that they would not depart the plane even if damaged. This all adds more weight.

You would be adding a lot of weight to install solar that only helps a little bit sometimes. You may get a small benefit by reducing the generator load on the engines, but probably not nearly enough to make up for the increase in weight.

Airlines are big on reducing fuel usage, and are willing to add weight to save fuel overall (such as with winglets), but we haven't seen solar put to use on any airliners yet. The most efficient way overall to provide regular power is to extract it from the engines that are already running. For auxiliary power, a small turbine generator, which can provide both bleed air and electricity. And for emergencies, a RAT.


What is this trio of fins at the back of a T-38? On a T-38 Talon on static display at a museum, I spotted a trio of aerodynamically shaped devices located aft of the vertical tail and between the engine aftbodies fore of the nozzles, as shown in the photos.  If these are vortex generators (VGs), what in the world are they doing there where there's no aerodynamic surface aft of them for the energized vortices to flow over? Could they be some other sort of protuberance, such as antennae for some system on the aircraft?

There are a couple of possibilities, both out of the T-38 Flight Operations Manual.

It's a nozzle for a smoke generator.  This was an option on the T-38 airplane to generate a smoke trail behind the aircraft for training purposes.

If possible, inquire if that aircraft was an AT-38B.  Those aircraft apparently had an optional 'ballast box' in that region, which I'm assuming was some sort of a counterweight placed there in conjunction with the bomb racks for practice bombs and other range equipment.

As to the vortex generators?  Who knows!


What is the "true static pressure"? What is true static pressure? How do we define it? Is it same as corrected static pressure for installation and errors?
ARINC 429P1-18 defines Static pressure as corrected and uncorrected where in few places I have heard this term "True static pressure".

From what I can see, the "true" static pressure is the measured static pressure corrected for airspeed & position (Static Source Error Correction).

In my copy of ARINC 429 (May 2004) I don't see a label definition that fits this "true static pressure". Label 171 can be used for manufacturer defined data, so I would see if you can get a better answer from the manufacturer of your data source.


How do aircraft carriers treat foreign Flight Information Regions? Aircraft carriers have the right to be anywhere as long as they stick to international waters. But as soon as a plane leaves the deck, that's somebody's FIR.

Granted they can fly low, or even know where the radars are pointed. But that's not good for exercises and other missions. Do they coordinate, ask permission, pay a certain fee, or just simply show up. What about clandestine missions?

I've also seen before on Jeppesen en-route charts rectangular areas marked as air refueling corridors, if my memory serves me well, such example was near Malta. Same question, how does that work?

(wikimedia.org)

Sovereign airspace begins at the 12-mile nautical limit.

In international airspace noone can legally interfere with a flight except the authorities of the country in which the aircraft is registered (see What restrictions apply in international airspace?).


Why should the ball be centered during steep turns? I was taught to make steep turns with the ball centered, but this seems wrong to me. Shouldn’t the ball actually be held slightly to the inside of the turn?

My thought is that by using some cross-controlled rudder, the elevator pressure can be reduced, reducing the angle of attack and making the turn safer.

Essentially, in a 45 degree turn for example, a standard tail aircraft is turned into a V-tail. Pulling on the elevator is only half effective in keeping the nose up, the other half of the force goes to turning faster.

A knife edge is the extreme form of this, where the rudder becomes the elevator completely and the fuselage the lifting surface. No elevator is used (to keep the nose up. It’s actually used as a rudder instead).

So, if keeping the ball to the inside reduces G-force and angle of attack, keeping the plane further from the stall, why isn’t it commonly taught?

Coordinated flight, even in turns, will minimize drag.

Therefore, there is no efficiency gain by uncoordinated flight, in turns or not.

Therefore, rudder input should be as required to center the ball.


How will the lack of ground stations affect navigation? VOR's and NDB beacons are getting closed day by day and they were used for making and then referencing the holding patterns and the entry procedures and how they were named.

So

How will we make holding patterns and entries from now on? What will be base them from now and how will we name them?
Will the GPS approaches replace other approaches? (Which will make GPS mandatory at least for Class B airspace I Guess)
If GPS approaches doesnt replace them... Will the old names be kept for holding patterns and entries? Will we still say RW29 VOR approach and missed entry procedures?

GPS/WAAS will replace all but ILS in the end. Even now, with graphic displays, smart-lookup from databases, and a line to follow presented to the pilot, VOR use is rapidly falling by the wayside, and point to point navigation will become the norm (and that was occurring in 1996 even, I recall getting cleared direct to a waypoint that was off my paper chart and having to ask for a heading to put in the autopilot to fly the plane while I found it on the chart).  In my airplane, my AVIDYNE IFD-540 does all the work, and I have ILS for precision landings and VOR if needed. Nearby VORs are going away.  DME and ADF were pulled out of my plane years ago.


On ATC's radar, does an aircraft that loses its transponder simply disappear? If you look at shows like Air Crash Investigation, which surely take some creative license in how events are portrayed, they often portray the disappearance of an aircraft, or specifically loss of its transponder return, as the radar return simply disappearing from ATC's radar scopes.

My question is: is that actually the only thing that happens when ATC secondary radar loses an aircraft's transponder return, when the aircraft is not actually on approach to land (and thus losing line of sight to the radar would be normal and expected, and/or that radar return is no longer relevant for air traffic control purposes)?

In general, humans are better at picking out things that are there than they are at picking out things that aren't there, even if they were there previously. It's also very rare for an aircraft to lose its transponder, let alone crash. Therefore, some kind of positive notification that a transponder return was lost when it shouldn't have been, possibly after a short delay (to account for those aircraft where the transponder is set to standby while a new code is being selected) would seem helpful.

Yes, it's the controller's job to keep track of the aircraft they are in charge of, but especially with modern, digital radar systems, it seems like something like flashing a distinct symbol where the aircraft was last seen, or sounding an alert chime, or just about anything, could make their job a lot easier the very few times that an aircraft transponder return disappears when it shouldn't have.

It depends on the system. Assuming only secondary radar is available, in some systems, the track will indeed just disappear when radar contact is lost. Other systems will put the track in "coasted" mode, continuing to show the track (but with another track symbol), waiting to pick up the transponder again. The track will then be terminated after a specified time limit.

Of course, if primary radar coverage is available - which works independently of aircraft equipment, such as transponders - the system would still be able to track the aircraft, but automatic flightplan correlation would most probably be lost, along with mode C data (altitude). So all flight information would vanish from the screen and the controller would have to re-identify the flight using proper procedures.

Producing an alert when a track is lost could be useful, but it could also potentially result in a lot of nuisance alerts, particularly in areas with spotty radar coverage. So alerts should only be shown for areas and levels where radar coverage is expected to be perfect, otherwise the controllers would likely develop a habit of ignoring such alerts. Also remember that radars lose contact with flights all the time - as they move out of the range of the radar. No ATC station has global radar coverage. Showing an alert every time an aircraft leaves the area of radar coverage would also be pointless, and the range of a radar can vary from day to day depending on atmospheric conditions, so defining a precise area in which alerts should be suppressed might not be a trivial task.


What is the name of the movement which is carried out by plane beginning from the runway and ending at takeoff? What is the name of the movement which begins after the plane lines up on the runway and takeoff clearance is received, and ends when the plane lifts off from the runway? Is it also called as 'taxiing'? I know that roll-out is used for plane's decelerating movement on the runway after landing; but what is the name of vice-versa?

Takeoff Roll

As per the FAA handbook (page 5-2)

  Takeoff roll (ground roll) is the portion of the takeoff procedure
  during which the airplane is accelerated from a standstill to an
  airspeed that provides sufficient lift for it to become airborne.

And after that

  Lift-off is when the wings are lifting the weight of the airplane
  off the surface. In most airplanes, this is the result of the pilot
  rotating the nose up to increase the angle of attack (AOA). >
  The
  initial climb begins when the airplane leaves the surface and a climb
  pitch attitude has been established. Normally, it is considered
  complete when the airplane has reached a safe maneuvering altitude or
  an en route climb has been established.


What's the theoretical background of the critical angle of attack? The critical angle of attack seems to be at all (most?) airfoils around 15-20°. Why is that? Why is it in this range and not lower or higher? Is it just the result of optimizing airfoils? Or is it some inherent property of the air an airfoil is moving through that determines it? (Let's limit the question to subsonic flight.)

PS: I read the answer to "Does airspeed affect the critical angle of attack on an airfoil?" and don't think that this is a duplicate since the other one was more generally about "what are the factors that determine the critical AoA" while I'm asking why the critical AoA seems to be always in this range for most airfoils on planes flying nowadays.

Both the airfoil shape and inherent properties of the air contribute to the stall angle of attack. You asked for a theoretical background, but I will list the factors that influence stall because there is no simple formula for it.
The most important factor is the suction peak which develops right behind the stagnation point on the upper side of the leading edge. High suction means high speed and that in turn means high friction, so the air loses energy which it needs further downstream to regain its pressure. If too much energy is lost, the flow separates. Enough separation and lift suffers, so here you have the most immediate reason for a stall.
What can be done to shift that point to a higher angle of attack?

Pitch up faster. That way, the flow over the rear part of the wing has a boundary layer from lower angles of attack and will not separate when the leading edge passes through the angle of attack at wich it stalls in normal conditions. This can shift the stall angle of attack up by 50%. But that works only temporarily and the same mechanism will delay recovery once the stall has occurred.
Increase the leading edge radius. This spreads out the suction peak and makes it less pointy. A blunt leading edge is especially helpful with higher wing loadings when Mach effects play into the stall angle of attack mechanics. Once the local suction peak at the nose of an airfoil reaches a local Mach number of slightly less than 1.6, no lift increase could be observed in experiments.
Increase wing camber, either by nose and/or trailing edge flaps or by cambering the airfoil. This helps to reach high lift coefficients already at low angles of attack, and especially nose devices (slats, Krüger flaps) shift the stall angle of attack up as well.
Use a well-designed airfoil with a long laminar run and a Stratford pressure distribution past the turbulent transition point on the upper side. This helps to reduce boundary layer losses and maximizes energy reserves for the steepest possible pressure rise. But you need a clean, smooth and well built wing for that to really happen. And the right Reynolds number range: Gliders use this effect extensively but airliners cannot use it at all.
Increase wing loading. This will shift the stall speed to a higher Reynolds number where friction losses are smaller relative to the inertial energy of the air. Of course this will increase stall speed, but is will also shift the stall angle of attack higher. A bit.
Increase wing chord (while maintaining the same area). This has actually two effects: The smaller one is again from the increase in Reynolds number, but the more powerful is from the reduction of the wing's aspect ratio. At a smaller aspect ratio the lift curve slope is flatter, so the same lift coefficient (and suction peak) is reached at a higher angle of attack.
Increase wing sweep. The pressure changes over the wing are now proportional to the cosine of the sweep angle, so all effects are shifted to higher angles of attack accordingly. But beware of a combination of high sweep and high aspect ratio: Stall will become outright nasty. If you combine 6 and 7, you will at some point arrive at a delta wing which flies well even with fully separated upper side flow (vortex lift). Now your limit angle of attack will be defined either by the vortex bursting or loss of directional stability.
Fly in hotter, less dense air. This also helps to increase the Reynolds number because you need to move faster for the same dynamic pressure. However, much of that advantage is eaten up by the increase of the air's viscosity with temperature.


Do negative angles-of-attack create lift? (wikimedia.org) A typical curve showing section lift coefficient versus angle of attack for a cambered airfoil.

Based on the graph above, do negative angle-of-attack values create lift? How?

A symmetric airfoil will generate no lift at no angle of attack, and negative lift at a negative angle of attack. However, cambered airfoils are curved such that they will generate lift at small negative angles of attack.


Are there any studies on the safety of glass versus steam instruments? Has anyone come across any academic study on the effects of information provided by "glass" instrumentation that affects, favorably or unfavorably, situational awareness, skill and performance, and decision making?

Related question: Where does the term 'steam cockpit' come from?

The FAA released a 70 page report back in 2003 titled General Aviation Technically Advanced Aircraft FAA – Industry Safety Study which basically says that the overall accident rate of the two types of airplanes are almost the same.

They found that the "available safety" of Technically Advanced Aircraft (TAA) is greater than non-TAA aircraft, however in emergency situations pilots often were not aware of the resources available. To offset this, they recommend (among other things) more training for pilots before flying TAA's so that they actually have the safety related aspects available to them.

Here were their official findings:

  Team Findings:

  The safety problems found in the accidents studied by the Team are  typical of problems that occurred after previous introductions of new
  aircraft technology and all also reflect typical GA pilot judgment
  errors  found in analysis of non-TAA accidents
  Previous safety problems similar to those identified in this Study have  been remedied through a combination of improved training and, in
  the  case of new aircraft capabilities, pilot screening (i.e.,
  additional insurance  company requirements of pilot experience).
  The predominant TAA-system-specific finding is that the steps required to  call up information and program an approach in
  IFR-certified GPS  navigators are numerous, and during high workload
  situations they can  distract from the primary pilot duty of flying
  the aircraft. MFDs in the  accident aircraft did not appear to present
  a complexity problem. The  Team also believes that PFDs, while not
  installed in any of the accident  aircraft and just now becoming
  available in TAAs, similarly are not likely to  present a complexity
  problem.
  TAAs provide increased “available safety”, i.e., a potential for increased  safety. However, to actually obtain this available safety,
  pilots must  receive additional training in the specific TAA systems
  in their aircraft that  will enable them to exploit the opportunities
  and operate within the  limitations inherent in their TAA systems.
  The template for securing this increased safety exists from the  experiences with previous new technology introductions –the current
  aircraft model-specific training and insurance requirements applicable
  to  high-performance single and multi engine small airplanes. However,
  the  existing training infrastructure currently is not able to provide
  the needed  training in TAAs.
  Effective and feasible interventions have been identified, mostly  recommending improvements in training, and effective implementation
  mechanisms for the recommended interventions exist. Therefore, TAA
  safety problems can be addressed, and the additional available safety
  of  TAAs to address traditional causes of GA accidents can be realized
  as  well.

APOA has a nice summary here.


Time Tracking: Does Turboshaft count towards Turbine time? another edit
To clarify - I'm programming an application for logbook tracking and want to automate turbine time calculations to help people fill in insurance firms easily.

edit: I guess a more general way to ask this is do you consider turbo shaft to count towards turbine time? Do insurance companies have a standard opinion?

original question:
For insurance forms, when you are asked your turbine time, do you include all of the following in that hour calculation:

Turboprop
Turbojet
Turboshaft

I want to make sure I'm not short changing my time, but don't want to put some helicopter specific turbine in my time if that isn't correct. As far as I'm concerned they should count, right? It is, after all, only asking about the source of thrust not category of aircraft...

Thanks!

Turbine is turbine.  If you burn fuel to spin a turbine that drives a compressor, it's turbine.

I counted all turboshaft time as "turbine" when reviewing resumes for pilot applicants.  Understanding the theory behind turbine engine operation is what everyone cares about, whether is was a turbo-this or a turbo-that is not as important.

Contact a few insurance brokers just to be sure.  Falcon Insurance, AOPA, TrueCourse just to name a few.


May pilots receive compensation for performing preventive maintenance under US regulations? Under FAA regulations, I know that at least a private pilot certificate must be held in order for a pilot to perform preventive maintenance on an aircraft.
May a pilot also get paid by someone else for performing that maintenance, and does the grade of pilot certificate make a difference? For example, is there a requirement to hold at least a commercial certificate to be compensated for the performance of preventive maintenance, or is preventive maintenance solely to be considered an incidental, non-compensatory privilege of any pilot certificate holder of private or higher grade regardless of that grade?
This hypothetical scenario is with the assumption that this is freelance and has no oversight by a certified mechanic. An example could be a fellow pilot casually offering to change your oil or replace a lightbulb for some monetary labor compensation.

First, it's a little too broad to say that a private pilot can perform preventive maintenance. The full wording in 14 CFR 43.3(g) is (emphasis mine):

Except for holders of a sport pilot certificate, the holder of a pilot
certificate issued under part 61 may perform preventive maintenance
on any aircraft owned or operated by that pilot which is not used under part 121, 129, or 135 of this chapter.

In other words, a pilot can't perform preventive maintenance on just any aircraft. The definition of "operate" in 1.1 could be stretched, I suppose, but the pilot has to have some degree of operational control over the aircraft. Another pilot without that control couldn't do preventive maintenance on your aircraft anyway, whether or not you were paying them.
Beyond that, the word "compensation" doesn't appear in part 43 so it seems not to be a factor in who can perform maintenance, preventive or otherwise. And common sense says that a pilot can be compensated because otherwise a part 91 ferry pilot wouldn't be allowed to top off hydraulic fluid or replace a landing light bulb (for example).


Is precession a problem for two-bladed propellers? The vast majority of wind turbines are a three-bladed design. Although two blades would be more efficient, two-bladed designs are more likely to suffer from precession. Do two-bladed propellers suffer from precession? If not, why would it be a problem for wind turbines, but not aircraft props?

Wind turbines suffer not from precession per se, but from precession 'wobbling', when the gyroscopic force varies dramatically over a revolution. Normally, a 3-bladed prop would be heavier than a 2-bladed one, and will have greater gyroscopic effect (at the same RPM). However, it will be better balanced across a revolution. The closer to a solid disk, the better.

(In fact, a 3-bladed prop appears to be perfectly balanced in extreme positions. Considering blades slender rods or length $r$, the moment of inertia (MOI) of the propeller disk for turning across the spin axis (e.g. yawing) will be $1/3\cdot mr^2 + 2\times 1/3\cdot m(r\cos60°)^2 = 1/2\cdot mr^2$ in one extreme position (like −<), and $2\times 1/3\cdot m(r\cos30°)^2 = 1/2\cdot mr^2$ in the Y position. Compare this with variation from almost 0 to $2\times 1/3\cdot mr^2$ for a 2-bladed prop).

For a 2-bladed prop, this wobbling occurs at double the frequency of rotation. For wind turbines with their ~20 RPM this is < 1 Hz, a frequency that can interfere with normal yawing and even damage the structure. But aircraft props turn at much higher speed; a typical 2400 RPM will give you an 80 Hz hum. May be unpleasant, but far away from the natural frequencies of manoeuvring and of the airframe.

Also important to note that there are two effects at play here: precession (gyroscopic effect) exerts a force perpendicular to the applied rotation, but there is also simply a moment of inertia of the prop/rotor across its spin axis (as calculated above) that needs to be overcome. This latter one applies even if the rotor is stopped, but both wobble when it spins. Now, for airplanes with their fast spinning but relatively small props, the gyroscopic effect may be noticeable (see below), but just MOI is totally negligible (the mass of the blades is nothing compared to the rest). Yet for wind turbines the opposite must be true: the rotor mass/MOI is (probably) the dominant mass of the whole thing.

As for precession itself, i.e. the tendency to turn perpendicular to where you want to turn, most aircraft don't 'suffer' from it either. Aircraft simply don't turn (manoeuvre) too fast for it to be a problem, and the aerodynamic forces outweigh the gyroscopic ones by orders of magnitude. On normal propeller aircraft, only if you want to notice it, you might notice it when doing a manoeuvre.

In some circumstances, however, precession can be quite significant. This involves the cases when angular velocities are high (aerobatics), the aerodynamic forces are low or very finely balanced (hovercraft), or both (spin).


Comparison of pitching moment at different locations I've conducted some numerical simulations of a pitching airfoil at three different locations of the pitching axis along the airfoil. The locations (from the leading edge of the airfoil) are: $0.25c$, $0.4c$, and $0.75c$. where  $c$ is the chord length of the airfoil.

Each simulation computes the pitching moment coefficient using the pitching axis itself as the reference point. That is, In, the first simulation the pitching moment is computed about $0.25c$ axis, and in the second simulation the pitching moment coefficient is calculated about $0.4c$, etc.
My questions are:

Is it OK to compare all three moment coefficient together given that the reference points are different?
Do I have to change the reference point to $0.25c$ for the two last results? if so, how can I achieve that without restarting the simulations and specifying the $0.25c$ as reference point?

Your help is greatly appreciated.

Is it OK to compare all three moment coefficient together given that the reference points are different?

No, it's not ok. The pitching moment will be different at different reference locations, unless lift is 0.

Do I have to change the reference point to 0.25c for the two last results? if so, how can I achieve that without restarting the simulations and specifying the 0.25c as reference point?

The industry standard for reporting and comparing pitching moment is quarter chord, because the classic Thin Airfoil Theory concludes that infinitely thin airfoils in subsonic flow have their aerodynamic centres at quarter chord.
That being said, there is absolutely nothing wrong gathering the pitching moment data about any other point, as long as you stay consistent when comparing to other data (or more importantly other people's data). You can convert the pitching moment from one point (say $x_0$) to any other point ($x$) via:
$$M(x) = (L\cos\alpha+D\sin\alpha)(x-x_0) + M_0 \approx L(x-x_0) + M_0$$
where $L$ is the measured lift, $D$ is the measured drag, and $M_0$ is the measured pitching moment at $x_0$. The approximation works great at small AOA ($\alpha$).