Do airline pilots practice stalls in actual aircraft? PPL pilots are taught stall recovery by experiencing it first hand with an instructor.

Do airline pilots intentionally stall an aircraft during training? Or is it all simulator practice because it's too risky?

In 727 and 747-100/200 aircraft and their simulators for the two airlines I flew those aircraft for up to my retirement in 1999, all stall training was done in simulators, never in the actual aircraft.

Stall training in the sim consisted of slowing (or otherwise loading up the wing) until the stick shaker started. Recovery was accomplished by unloading the wing and then adding power in that order. Whether unloading the wing was accomplished by just relaxing back pressure or by positive forward movement of the control column (or by rolling out of a bank if you were in one) was a judgement call.

Typically the sim instructor wanted a significant stick shaker warning, not just a quick blip that you immediately stopped. Also, it was considered bad form if, once you stopped the stick shaker, it started again. Obviously that meant you had loaded up the wing again and were moving toward a secondary stall, which was a definite no-no. As I remember, that was more likely to happen if you made the mistake of leading with power rather than first reducing the angle of attack.

Every once in awhile someone would want to play a bit and see how the sim reacted to a full stall. As I remember, the sim would just bounce around with a very high sink rate. I never felt its response to a full stall was at all representative of what the actual airplane would do.

I experienced the stick shaker in flight in a 747 three times that I remember: once during a test flight in which that was part of the testing, once with a check airman who liked to play, and once on the line.

The instance on the line was when I was still an f.o., and it was the captain's leg. He had extended the speed brakes for the descent. When he brought the power up for the final approach, he forgot about the speed brakes. The stick shaker came on immediately. He realized what he had done, and retracted them, and the stick shaker stopped.


Could leading-edge dynamic stall occur for a tripped airfoil? As far as I know leading-edge dynamic stall is characterized by the formation of a Leading edge vortex. This leading edge vortex forms due to the 'bursting' of the LSB (Laminar separation bubble). However, if the airfoil is tripped on the upper surface to make the boundary layer fully turbulent, does that suppress the leading edge vortex? if so, what will be the process leading to the dynamic stall in this case?
Your help is greatly appreciated.

if the airfoil is tripped on the upper surface to make the boundary layer fully turbulent, does that suppress the √leading edge vortex?

I would say no, because supercritical wings that have that LSB problem, and so suffer from leading edge stall, aren't improved by leading edge contamination, like frost coatings or heavy bug accumulations, which can be presumed to destroying laminar flow well forward of the normal transition point.
On the CRJ200, which has a particularly nasty unrecoverable LE stall due to LSB formation at the leading edge, contamination only serves to reduce the stalling AOA to below shaker/pusher onset, without making the stall any more benign, as a number of frost contamination crashes have shown.  The bubble is still there even with the rough surface forward of it, and so the wing still lets go all at once, just at 9 degrees instead of 15.
The LSB formation is particularly bad on supercritical wings because the profile has an odd kind of "corner" or discontinuity in the curvature because of the extra thickness forward, which gives a kind of squared off profile at the LE.


Are there reference speeds like V1, Vr or V2 for a touch and go procedure? Are there any reference speeds like V1, Vr and V2 on a touch and go procedure?

During takeoff, these are used to mark certain states of the takeoff, including the speed at which the aircraft starts pitching up (Vr -> rotate speed).

But during touch and go, are those predetermined values as well?

While for touch and go there isn’t a speed that carries the same guarantees as the takeoff speeds (you will not hit anything nor will you exit the runway on ground at undue speeds), for procedure reasons there are indeed predefined speeds at which to rotate and at which to climb. On the 777, for example, rotation is called at VREF (standard manual approach speed for selected flap setting) on touch and go, and climbout is flown at VREF+15 to VREF+25. .

In any case, the touch and go manoeuvre is only a training manoeuvre and is not in any way considered in performance calculations. Since it’s not calculated, it is also not a manoeuvre for which sufficient runway distance and/or obstacle clearance is assured by either calculation or regulation, neither on all engines nor following an engine failure, other than by flight crews’ experience and prudence. The actual speeds for touch and go are therefore somewhat more arbitrary than on takeoff.

The closest manoeuvres that are considered in performance calculations are a normal takeoff and a go-around from minimums. The relevant speeds for the former you know, for the latter it is the predetermined approach speed.


Is it possible to access the baggage compartment in flight? In this incident the cabin crew entered baggage compartment to find and secure a smoking suitcase. But that was in ATR-72 which has the baggage compartments on the main deck behind cockpit and aft of cabin and passage between cockpit and cabin goes through the (larger) forward compartment.

In this incident it was clearly an advantage to be able to get to the cargo. Would it also be possible in some larger planes? Is there any regulation or guideline explaining why it should or shouldn't be possible?

There are 4 classes of cargo compartments listed in 14 CFR 25.857 for transport category airplanes. The main concern with in-flight access is in regards to fire safety.

Class A: Crew will easily discover fire from their station (and therefore be able to suppress it as well)
Class B: There is a fire/smoke detection system, and access for crewmember to use a hand fire extinguisher
Class C: There is a fire/smoke detection system to alert the crew, and a fire extinguishing/suppressing system installed
Class E: For cargo-only aircraft (no passengers): There is a fire/smoke detector, and a way to shut off air flow to the compartment

So basically, there has to be some way to both detect and address a cargo fire. Class A compartments are those such as in the cockpit where a detection system is not necessary. It sounds like the ATR example would be Class B, since they had a fire indication, and the crew had access to extinguish the fire themselves.

In larger planes with cargo in the lower hold, the compartments are usually Class C, so they have a detection system and a suppression system, and in-flight access is not required. As David Richerby's answer notes, the compartment may be accessible, but that doesn't mean that crew could effectively locate or fight a fire in those areas. Especially for large cargo holds, a fire suppression system installed througout the hold is much more effective than crew members gaining access at certain locations. This also provides the benefit of isolating the main cabin from the fire extinguishing agents.

Besides fires, there are even rarer instances where cargo compartment access could be helpful. But even in that case, it was better for the plane to just come back and land to access the cargo compartment.


Would a pilot know that one of their crew is armed? The Federal Flight Deck Officer page on Wikipedia says this:

  Under the FFDO program, flight crew members are authorized to use firearms. A flight crew member may be a pilot, flight engineer or navigator assigned to the flight.

To me, it seems like this would be crucial information for the PIC to know, if their flight engineer (for example) was armed; but on the flip-side of this, the engineer might want to keep that to himself if he's with a crew he hasn't flown with before.

Is there a guideline on whether an FFDO should inform the crew that he's armed?

The answer to this question is protected as Sensitive Security Information (SSI) and can't legally be disclosed here.  Anyone who does answer the question either doesn't know what they are talking about, or faces steep legal consequences.

Many, even most, questions relating to airline security (in the US at least ) are similarly handicapped.


What might the business model be for a hypersonic jet? According to this video, NASA and Boeing are doing research into a new generation of hypersonic passenger planes that can travel at Mach 5. The designs look really cool and I would love to see them in the air someday.

What I am curious about is the business model for such a plane. Are they planning on targeting high-end business travelers alone, or could the ability to make multiple flights in a very short time span bring the cost down enough for the rest of us to be able to afford a seat? Or is this another bid for the high-end market alone?

As a side note, whatever the market is for superfast travel, Elon Musk also seems to think that that there is a business opportunity there.

Well it’s cool but it’s nothing new.  Hypersonic transports and runway to orbit vehicles have been proposed since the late 1970s but never materialized into flyable hardware for a number of reasons, mainly engineers have not been able to solve the technical challenges associated with structures, propulsion, mission profiles, heat transfer and protection, operating costs for skittish airline operators, etc.

The new Boeing hypersonic transport may be the quantum leap forward for air and space travel but it could easily join the heap of fantastic ‘paper airplanes’ that never flew or possibly flew as clandestine prototypes such as:

Rockwell National Aerospace Plane - “Orient Express” from Reagan’s
1986 State of the Union Address.
Lockheed SR-91 “Aurora” reconnaissance aircraft.
High Speed Civil Transport concept from the
1990s
Lockheed Martin X-33 / VentureStar SSTO concept
German Sänger concept aircraft
Lockheed SR-72 Spy plane of the 2010s

As far as the passenger world is concerned, the big market are the transpacific routes, where the high-speed would make a major difference in travel times for both the public as well as business travel. If you could design a hypersonic aircraft that has a 6000 - 7000 mile range with operating costs in the neighborhood of 30 to 40% higher than existing subsonic airframes, you would have a game changer. But that’s always been the challenge.

Who knows, maybe the technology now exists to attempt a successful hypersonic manned aircraft.  I for one would welcome it as I find current aircraft are becoming insufferably boring.  OEMs churn out one bland subsonic twin jet after another, just wringing a few more knots out of a few less gallons of gas with new avionics etc.  I was supposed to be living on one of those cool Gerry O’Neill space colonies at this point!


Could simple fly-by-wire controls be installed in a prototype as quickly as a mechanical system with an autopilot? It seems that fly-by-wire systems can take longer to get certified as the testing requirements are much more extensive. A prototype aircraft wouldn't have to pass these tests.

How much more time or complexity might it be to implement a fly-by-wire system for just one set each of ailerons, rudders, and elevators? An equivalent level of functionality in the mechanical system would require at least an autopilot.

Any links to regulations or example systems would be greatly appreciated.

Early civilian FBW systems were prototyped by taking a conventional cable aircraft and feeding the control surface commands into the existing autopilot system and servos.

The earliest was with NASA Langley's 737 which had an aft "research" flight deck. Various instrumentation and control schemes were tested. The airplane's normal flight deck remained in the front with safety pilots.

(NASA Photo)

When Airbus developed the FBW system for the A320, they took a test A300 (F-BUAD), installed the sidestick on the left seat and rigged the FBW system to give commands via the autopilot system.

(Source, Article)

This was the last time this was done; no subsequent Airbus aircraft ever had cable prototypes and to my knowledge Boeing never did. FBW systems are mature enough that there does not need to be cable backup testing. If you need to prototype some new control law or scheme, you can simply modify the software in an existing FBW aircraft.

A fundamental issue is that the intent of testing is to build a new aircraft type. It takes quite a bit of engineering work to design a cable system that would only be used for early stage FBW testing, and would ultimately be wasted. In a modern FBW system, everything is completely tailored to electronic control, from the stick all the way to the hydraulic actuators.


Can the need for an engine overhaul affect the ETOPS rating? I read this news:

http://edition.cnn.com/2016/04/23/us/boeing-dreamliner-engine-fix/index.html

So seemingly the reliability of the 787 engines made by General Electric is not as high as expected and need to be overhauled to increase reliability. If one engine fails due to icing there should be also increased chances that the second one may fail as well.
Does it mean that - even temporarily - this may have an effect on the plane's ETOPS rating?

Every time an engine is shut down in flight, it is tracked.

Engine reliability does affect ETOPS certification. In addition to the aircraft being certified, each airline is also ETOPS certified. An aircraft or an airline with a poor record could have its ETOPS certification downgraded, or even potentially revoked.

Wikipedia ETOPS Approval

  ETOPS approval is a two-step process. First, the airframe and engine combination must satisfy the basic ETOPS requirements during its type certification. This is called "ETOPS type approval". Such tests may include shutting down an engine and flying the remaining engine during the complete diversion time. Often such tests are performed in the middle of the ocean. It must be demonstrated that, during the diversion flight, the flight crew is not unduly burdened by extra workload due to the lost engine and that the probability of the remaining engine failing is extremely remote. For example, if an aircraft is rated for ETOPS-180, it means that it should be able to fly with full load and just one engine for 3 hours.

  Second, an operator who conducts ETOPS flights must satisfy their own
  country's aviation regulators about their ability to conduct ETOPS
  flights. This is called "ETOPS operational certification" and involves
  compliance with additional special engineering and flight crew
  procedures in addition to the normal engineering and flight
  procedures. Pilots and engineering staff must be qualified and trained
  for ETOPS. An airline with extensive experience operating long
  distance flights may be awarded ETOPS operational approval
  immediately, others may need to demonstrate ability through a series
  of ETOPS proving flights.

  Regulators closely watch the ETOPS performance of both type
  certificate holders and their affiliated airlines. Any technical
  incidents during an ETOPS flight must be recorded. From the data
  collected, the reliability of the particular airframe-engine
  combination is measured and statistics published. The figures must be
  within limits of type certifications. Of course, the figures required
  for ETOPS-180 will always be more stringent than ETOPS-120.
  Unsatisfactory figures would lead to a downgrade, or worse, suspension
  of ETOPS capabilities either for the type certificate holder or the
  airline.


What were those multiple flights leaving London night of 29th Aug 2019? First time post here, please let me know if more detail needed (and which tag would suit it best).

I live in Essex UK, in the evening normally see lights of planes on approach to the London airports.

This evening (29 August 2019) from 21:00 BST / 20:00 UTC I have seen lights of planes flying away from London in large numbers (at least 30 when I started paying attention).

They were in groups of about 5, then another group following not far behind. They appeared to be heading in an easterly direction away from London.

Could someone help to shed some light on this? Have lived in this area for 15 years and never seen anything like this.

Nothing stands out on Flightradar24 for that time period by using the playback function. So most likely they're not civilian.

What I found was a military-related NOTAM via notaminfo.com/ukmap:

  MIL EXER. MIL PARACHUTING AND HELICOPTER HOIST OPERATIONS WI 5NM
  RADIUS 515900N 0012500E (BAWDSEY, SUFFOLK). FOR INFO 01638 545012.
  2019-08-0649/AS3

  FROM: 29 Aug 2019 18:30 GMT (19:30 BST) TO: 30 Aug 2019 00:30 GMT (01:30 BST)

The time matches if we account for padding, and the drop area would make them fly west to east to arrive at Bawdsey, depending on their base and your location.

If too concerned, you can trying calling the number quoted above.


What are the requirements for a small jet type rating in the UK? Just out of curiosity, how would one go about getting a type rating for a small, twoish seater jet (for example the Viperjet Mk2) in the UK?

I've had a brief look but cannot find anywhere that offers such a thing.

As far as I know, you can't get a type rating for a homebuilt jet in the UK. You can do so in the US but you have to find an instructor and examiner rated by the FAA to do that. I would suggest you talk to the manufacturer of the kit about this. They are the most likely to know the answer, but I would not hold out much hope. The UK CAA has historically demonstrated they are not too keen to let people fly homebuilt jets.


How altitude changes with changed QNH? How the AGL altitude changes with changed QNH? I'd like only to confirm or correct my intuition..

Airplane is maintaining certain FL with some QNH, its instruments are showing some AMSL. Now the airplane enters area with lower air pressure, but keeping the same set QNH. How the real AGL altitude changes?

My intuition is that altimeter is effectively showing certain pressure. If the atmospheric pressure drops (e.g. from 1013 to 990), to show the same measured pressure (altitude) the airplane has to descend.  Then the airplane is having lower real AMSL than displayed on instruments.

Is my intuition correct?

Yes. There's an old saying: when flying from high to low, look out below! https://sabushpilot.com/high-to-low-look-out-below/ has nice properly descriptive image.

  Airplane is maintaining certain FL with some QNH

Note that if you were at a FL (flight level), you wouldn't be using QNH, you'd be using QNE. You use QNH up to the transition altitude and clearances will be given to maintain a certain number of feet (or meters in some countries). Above the transition altitude, your clearance would be to maintain a certain FL and you would be using QNE, 1013.2 hPa.


What type are these planes that are flying in formation near Culver City, CA? My son in law took these images a few days ago in Culver City California. We would like to know what the vintage (WWII ?) planes are, and in the last image, what would cause such a contrail. The last image is a composite of several shots. (Something landing at Edwards was my guess). His guess for the aircraft are that they are all the DB Dauntless.

My (current) guess is that it may be a

Nanchang CJ-6

  The Nanchang CJ-6 is an aircraft designed and built in China for use
  by the People's Liberation Army Air Force (PLAAF) as a basic trainer.

Round nose.
Sufficient number airworthy.
Rectangular wing shape.
Main gear folds inwards and the nosegear backwards which seems to fit when I zoom in your pictures.

and they do airshows:

(source: riversideca.gov)

Coincidentally, the upper-left aircraft #43 also appears in your pictures (first picture, rightmost aircraft). Another below of her below:

update: as Cardinal said above, "This is likely Tiger Squadron, perhaps flying for the Israel Independence Day Festival over Rancho Park in mid May, as detailed on their Facebook page."


What is the broadcast rate of an ADS-B message? From what I know, ADS-B (DF=17) message structure is 112 bits long. In terms of type of messages it transmits, there are aircraft identification, surface position, airborne position with baro altitude or GNSS height, airborne velocities, aircraft status, etc. However, assuming an aircraft is on final approach and it broadcasts the necessary information to ground stations then to ATC, what is its transmission rate in seconds? Is it really 1 second per message or does it differ from the type of messages? What factors affect the transmission rate? Thank you.

It depends on the Message Format TYPE Code and the aircraft state. In your example, being on final approach would have no effect as the primary state criteria is being airborne (vs. surface). The rates are 'nominal' rates in that there is a programmed 'dither' to the nominal rate to avoid synchronous garble.
The nominal rate for TYPE codes 1 through 4 (ID and Type) is 5.0 sec airborne and 10.0 sec surface.
TYPE Codes 5 through 8 (Surface Position) rate is 0.5 sec in motion and 5.0 sec when stationary.
TYPE Codes 9 through 18 and 20 through 22 (Airborne Position) rate is 0.5 sec.
Type Code 19 (Airborne Velocity) rate is 0.5 sec.
Type Code 28 (Aircraft Status, e.g. emergency/priority) rate is 1.0 sec.
Type Code 31 (Aircraft Operational Status) rate is 1.7 sec.


What is the fuel mass flow rate at stationary full power for the CFM56 5C2/3C1 engines? Does anyone know what is the value of fuel mass flow rate when stationary at full power for CFM56-5C2 or CFM65-3C1 turbofan engines? I am doing a work on engine performance and it seems that only information I can't find is the fuel flow rate at conditions above. I will be very grateful to someone who can help me.

If you know the TSFC at conditions above I can calculate the fuel mass flow and would resolve the issue.

From this page, the CFM56-5C2 used on the A340-211/-311 delivers 31,200 lbf and its SFC is 0.320 lb/lbf hr. The cruise thrust and SFC are 6,910 and 0.545 respectively.


How did aircraft communicate during early air travel? During the initial era of air travel how do the pilots/Navigation officers would communicate with ground stations?

Do they use radio communication? Or Telegraph was used?

Radio was not widely used until the 1930s, before that, mostly light signals were used. Early ground support aircraft in WW I could drop little notes which were pencilled by the observer, stuffed in a small capsule to which a streamer was fitted, and that was dropped as closely as possible to the troops which should get that message. This method was still used in WW II, when most aircraft had radio, because only specially equipped ground troops had compatible receivers.

The first regular flights by night were 1921 for the air mail connection between Omaha and Chicago, and electric flood lights were positioned along the flight path.
When Lufthansa opened the first night connection between the Reich and East Prussia in 1926, they posted burning oil drums along the way to mark where the pilot had to fly. Communication the other way (air to ground) was not possible on those flights.

The first air-to-ground radio communication used morse signals, and this is the origin for these strange three-letter names for things like local air pressure at the ground (QFE) or pressure corrected to sea level conditions (QNH). There was a whole lot more of those, including some to order coffee before landing, but they are no longer necessary. Read all about it here.


Why is the Antonov 225 (almost) always chocked at the third wheel? Recently I was looking over pictures where the An-225 is parked (because that's a normal thing for a normal person to spend their time doing). I noticed that the chocks are almost always placed around the third wheel from the front in the undercarriage:

(Thanks to ratchet freak for the better picture).

Presumably that's because the centre of mass (COM) is closest to that wheel? But why would the undercarriage not be centered around the COM?

Bonus question: Very occasionally, I've seen the chocks placed around the entire undercarriage, in front of the first wheel and behind the last wheel. Why the occasional difference?

It's a matter of procedure.  Chocking a center wheel provides the greatest defence in the unlikely case the chock is jumped.  Other wheels in the truck must also contend.  If an outside wheel is chocked and that chock is jumped away, all is lost.

No matter tipping or rocking the aircraft in extraordinary winds or loading, the center of a wheel truck is the recommended place to chock.  A chock should be snug against both sides of a single tire to prevent any sway.

Most chocks are paired together by a rope or chain which would prohibit chocking around a large truck of wheels as described in your bonus question.  What's more, chocking at either end of a truck allows more elasticity and movement between the chocks than a single wheel chocked snug.


What is the difference between SMOH and SFOH? When looking at aircraft for purchase online, a lot of acronyms are used.  In particular, the SMOH and SFOH acronyms are thrown around a lot for used aircraft.  What are the differences between these acronyms, and what are their purpose for a potential airplane buyer?

The life of an aircraft engine is measured in hours of operation since some major service was done to it. When looking at ads for planes there are three regulatory terms that you'll see in engine descriptions:

New (TSN)
This one is pretty self explanatory - A brand new engine, produced with all brand new parts that comes to you right from the factory (or an authorized dealer).

  New engines have a "zero-time" logbook and no operating history because they've never been installed on an aircraft. Usually there's a "test cell run" entry in the logbook, if anything.

Rebuilt (There's not a common abbreviation here - I've seen SFRB or SFRM)
An engine that went back to the factory and was rebuilt to the same production limits as a new engine. Some of the parts in the engine will be new, some may be used, but all will meet the same standards and specifications  as a new engine.

  Only the original manufacturer can rebuild an engine, and it comes to you with a zero time logbook and no operating history (because a rebuilt engine is done by the factory and is to new-engine limits and may be made up of components from a bunch of other engines there's no way to give it a history, so it's treated the same way new engines are).

Overhauled (SMOH or SFOH)
An engine that has been completely disassembled (the crank case is split and all the parts pulled out), extensively inspected, and reassembled to either service limits or new limits. An overhaul usually includes some used parts (frequently from the same engine), and new parts as necessary to meet limits.
An overhaul can be done by the factory ("SFOH" - Since Factory Overhaul), or by a specialty engine shop, or even by your local A&P mechanic (both "SMOH" - Since Major Overhaul).

  Overhauled engines retain their operating history, and should have a complete set of logbooks since the day the engine was manufactured. Even if the only part that gets reused is the engine case an overhaul is still "the same engine" in the FAA's eyes, so make sure you can trace the full operating history of that engine.

You will sometimes see another term - "since top overhaul" (STOP or STOH), but this one is not actually defined in the regulations. Generally it means a shop removed and replaced all the cylinders (the "top end"), but did not open the crank case and thus may not have inspected the "bottom end" (crankshaft, camshaft, bearings, etc.)

The terms "overhaul" and "rebuild" are defined in 14 CFR 43.2, and further expounded upon in AC 20-62E:

  §43.2   Records of overhaul and rebuilding. (a) No person may describe
  in any required maintenance entry or form an aircraft, airframe,
  aircraft engine, propeller, appliance, or component part as being
  overhauled unless—

  (1) Using methods, techniques, and practices acceptable to the
  Administrator, it has been disassembled, cleaned, inspected, repaired
  as necessary, and reassembled; and

  (2) It has been tested in accordance with approved standards and
  technical data, or in accordance with current standards and technical
  data acceptable to the Administrator, which have been developed and
  documented by the holder of the type certificate, supplemental type
  certificate, or a material, part, process, or appliance approval under
  part 21 of this chapter.

  (b) No person may describe in any required maintenance entry or form
  an aircraft, airframe, aircraft engine, propeller, appliance, or
  component part as being rebuilt unless it has been disassembled,
  cleaned, inspected, repaired as necessary, reassembled, and tested to
  the same tolerances and limits as a new item, using either new parts
  or used parts that either conform to new part tolerances and limits or
  to approved oversized or undersized dimensions.

There is no difference between a "factory" overhaul and someone else's "major" overhaul - at least according to the regulations: The standards for what constitutes an overhaul are the same.  What is different is who you talk to if the engine breaks (for a factory overhaul you would go back to the factory, for an overhaul by an engine shop you would go to the shop, and for one by your local mechanic you would go talk to the mechanic).

If you're looking at a plane which has had an engine overhaul (the vast majority of the used piston GA fleet) you should pay attention to where it was done and the reputation of the overhauler, but given two engines, one overhauled at the Lycoming factory and the other at a reputable shop like Penn Yan Aero, they would generally be considered equivalent.


Is an area of localized reduced visibility considered to be a cloud? If there an area containing visible moisture in an otherwise clear sky and that area of visible moisture is transparent is it considered a "thin cloud" or simply a "localized area of reduced visibility"?

What about areas of mist and haze with no definite border below an overcast layer that present in the sky as large areas of reduced visibility still above VFR visibility limitations for aircraft within them?

This might be an answer that works: In meteorology, the definition of fog is less than 1000 m visiblity and mist/haze more than 1000 m visibility.  I think you could reasonably infer the same definition to apply to higher cloud as well.  To the extent you are looking for a go/no/go definition, I think that might be all there is.


Can commercial air traffic operate from uncontrolled airport, airspace? Are there any airports with scheduled services where big jets (say 737 or A320) or wide bodies regularly operate without control tower?

Are there any rules (EASA FAA) for this?

If controllers go on strike can commercial air transport continue to operate?

I will answer your question with what we can see in Brazilian Airspace and its rules.

In Brazil, there are some airports - non towered, operated by radio stations - which give information such as wind direction/intensity and temperature and QNH. Based on this information, the pilot decides on which runway they will land or take off, and relays this message to the radio operator. The operator will only relay these pieces of information to pilots monitoring the radio frequency.

Those airports are not controlled, then, if there is any strike, flying visually, no control will be effective.

For example, ARU/SBAU is a non towered airport, and TAM Airlines operates the A320F there.


Why don't we see business jets with wingtip fuel tanks anymore? There is something I miss from the Learjet 35 (one from FS95/98) that I don't see any more:
Fuel tanks on the wingtips!
What happened to the wingtip integrated fuel tanks in the business jet industry?

One of the main reasons wing tip tanks were used was to counteract some of the forces of wing bending by adding mass to the end of the wings. As time has progressed and our understanding of materials has grown as well as our ability to design structures better the need for tricks like this has gone away.

As our general understanding of aerodynamics has gotten better so has the understanding of the impact of wing tip shape on reducing drag. As such wing tip tanks have been traded out for wing tip devices.

In some cases the tip tanks are an option available as a later add-on or at purchase time. Generally this increases the range of the aircraft at the cost of useful load. As such, it may just be that most people who order planes these days prefer the higher useful load and the reduced range, rather than the extended range flying with a lighter load.

You can find some more discussion on it here.


How to calculate the vertical profile for a LOC only approach? I am kind of stuck for calculating a descent path on the ILS RWY 7 approach into KORL. Assuming the glide slope is not available and you have to use the localizer only, what is the best way to calculate the descent and Decision to Land Point?

As you can see in the approach chart, the MDA is 660 feet for the localizer only approach. Assuming a glide path of 3°, I calculated a descent of 318 feet per Nautical Mile. However, that would not correlate to the 1919 feet at the Outer Marker.

What would be the best way to calculate / plan the approach in this example?

The approach shows that the glideslope (if it were working) is a three degree descent angle.   You can calculate your own visual descent point (VDP), since one isn't provided for you, by taking the height above touchdown (600 ft. in this case) and dividing it by 300 ft/NM.  This gives you 2.0 miles from the runway.  Since the chart shows the runway threshold at 0.2 DME, your VDP will be at 2.2 DME.

Assuming that your navigation equipment does not provide a pseudo-glideslope, your best bet is to descend at your normal rate (in a light airplane I would say 800-900 fpm) until you level at 660 ft. (the MDA), then fly to your calculated VDP (2.2 DME) and if you have the required visual cues then descend to the runway knowing that it will be a standard 3 degree descent (300 ft./NM).  If you don't have it at this point, then you can start an early climb and fly the missed approach.

Put another way, first you calculate your VDP:

$$\frac{Height~Above~Touchdown}{Descent~Rate}=VDP~Distance~From~Runway$$
$$\frac{600ft}{300 ft/NM}=2.0NM$$
$$VDP=Runway~Threshold+VDP~Distance~From~Runway$$
$$0.2DME+2.0NM=2.2 DME$$

Then you fly the approach:

Cross HERNY at 2,000 ft.
Descend to minimums (660 ft.) at 800-900 fpm.
Fly to 2.2 DME at 660 ft.

At 2.2 DME, if you see the runway environment, start your descent and land
If you don't see the runway, start your climb and fly the published missed approach


Is it possible for a modern commercial airliner to stop being tracked without substantial mechanical failure? I was speaking to someone about the MH370 incident, and they suggested that a hijacker could turn off the device that sends the plane's current position, resulting in the appearance of a catastrophic midair failure.

I assume that such functionality could not be turned off, but I could be wrong. Is there any way that this could be done? Could an aircraft's internal tracking systems be "turned off" midair, and also not be located by external tracking like radar?

The device you are talking about is called a transponder.  This is a device that listens for a signal (an interrogation) and responds with information about the airplane including an ATC assigned code, altitude information and additional aircraft info for certain equipment.

There are 2 ways ATC watches airplanes:

Primary Radar
Secondary Surveillance Radar

The transponder is interrogated by the Secondary radar and the response is listened for.  Even with no transponder, the aircraft can still be tracked by primary radar.  This is just a normal radar that is good at seeing airplane sized objects and not so great at seeing weather and can occasionally spot a large flock of birds.  The primary radar is what provides the "blip" on radar and tracks the airplane.  The datablock ATC has on the airplane comes from the secondary, so what ATC will see is a blip with no information.  Turning off the transponder is as simple as setting it to OFF or pulling a circuit breaker and yes, it can be selected OFF in flight, it is just a switch.

The problem with radar is that it only works so far from a radar transmitter and the further away you are the higher you must be to be seen by the radar.  Over the ocean away from land, you are going to be on the fringe of radar or out of radar contact completely and thus unable to be tracked directly by ATC (at this point, non-radar procedures such as position reporting and ETAs can be used to track positions).

The flight in question was apparently visible by a military radar before it disappeared from the scopes (it allegedly observed what appeared to be a turn back toward its origin).


What was the purpose of the "tail prop" on the Ilyushin Il-62 and why was it necessary? I read somewhere that the Ilyushin Il-62 had a "tail prop".  What was this device and why was it needed on this aircraft?

Are you asking about the rear support wheel? It is for supporting the aircraft and avoiding tilt on the ground.

Image from wikimedia commons. Autor:M Radzi Desa

As the aircraft had four engines in the aft, the cg would've been aft too; so they added a tail prop to prevent tilt on ground.

Interestingly, this doesn't seen to be unique for Il-62. Another aircraft with rear mounted engine, Sud Aviation Caravelle, has something similar to this.

Image from wikimedia commons. Author: Eduard Marmet


Can I use a LAPL/PPL issued by the UK CAA to fly outside Europe? Will the Light Aircraft Pilot License (LAPL) or Private Pilot License (PPL) be enough to to be able to fly to other countries outside Europe?

The EASA issued LAPL(A) is limited to 2to MTOM, 3 passengers max and only valid in European countries. A PPL(A) issued by an EASA country does not have these restrictions, but can be subject to local restrictions in the non-European country you want to flyin, e.g. you cannot use the EASA PPL(A) to fly in the US without further paperwork.
For more information, see the UK CAA's page on the LAPL.

Your LAPL(A) is a lifetime licence that contains SEP(land) and/or TMG privileges restricted to non-complex EASA aeroplanes with a maximum take off mass of 2 metric tonnes and a maximum of four people on board.
You can fly more complex aeroplanes within the privileges if you have 'differences training' signed off in your log book by a suitable instructor. The LAPL is also valid on non-EASA UK registered aircraft in the UK.
The LAPL requires an in-date EASA Part-Med Medical Certificate, at LAPL level, but Class One or Class Two certificates can also be used. A valid JAA Class One or Two medical certificate is also acceptable. But you cannot use your LAPL with an NPPL medical declaration.
The SEP(land) or TMG endorsement on this type of licence is not issued as a rating. So there is no rating expiry date. Instead you must follow self-validity checks on your own flying history for the past 2 years to ensure currency on the day of your planned flight.

Further Reading from the UK CAA:
EASA Light Aircraft Pilot License Aeroplane - LAPL(A)
EASA Private Pilots Licence Aeroplane - PPL(A)


What is the difference between Marginal VFR (MVFR) and Special VFR (SVFR)? I would like to know the difference between marginal VFR (MVFR) and Special VFR (SVFR), particularly when flying in the US airspace. What does each of them mean exactly?

Marginal VFR
Straight out of the Aviation Weather Services advisory circular (AC 00-45G with changes 1 and 2):

Marginal Visual Flight Rules (MVFR) indicated on the Weather Depiction
Chart represents ceiling 1,000 to 3,000 feet and/or visibility 3 to 5
statute miles and VFR operations can take place. MVFR areas are
outlined with a solid line, but the area is not shaded. MVFR areas are
typically shaded blue in colorized versions of the chart.

For all regulatory purposes, this has no rule changes from VFR.  I believe the reason it exists is for pilots to more easily identify areas where the weather conditions are approaching IMC.
Special VFR in the US
Special VFR allows a pilot to fly within the lateral boundaries of controlled airspace to the ground with at least 1SM visibility and clear of clouds.
Special VFR is something that can be granted by ATC upon request by a pilot.  It may only be granted within the lateral boundaries of controlled airspace that goes to the ground.
If the special VFR request is being made at night, the pilot and airplane must be IFR legal.
JO 7110.65Y Section Regarding Special VFR
There is no requirement that the weather conditions being reported are below basic VFR.

The Difference
MVFR is an advisory term.
Special VFR is an, on request by pilot only, "required weather minimums" change that may be approved by ATC.
For the purposes of special VFR in the US, here is the FAA regulation. (14 CFR 91.157)


Would putting several small propellers on a wing be efficient? I saw this video that NASA posted, which presents the idea of putting several small propellers on a wing:

They claim that by doing this you can force air to blow over the wing more quickly. This would make the wings act as if the aircraft was flying faster than it really was, resulting in shorter takeoffs and more control on landings.

However while watching the video I kept thinking, wouldn't the turbulence and slipstreams caused by the propellers actually make the wings perform worse? Or at least neutralize the proposed added lift and control?

Having no fresh airflow over the wings seems like a problem. Have any aircraft ever tried this concept before with successful results?

It would provide pretty good STOL performance at low speed because there is a significant increase in lift with the wing in the slipstream of a propeller, especially with flaps down.  One of the biggest differences between a jet and a turboprop while flying a landing approach is that power settings have a direct effect on sink rate with a turboprop where a quarter or a third of the wing span is in the propeller slipstream; not so much on a jet.

Then when you speed up, the inefficiency of all those draggy nacelles and relatively inefficient smaller propellers, the weight, the maintenance nightmare, the fuel burn.  Yikes.  That's why it's not done.

There's a reason that airplanes are all built to a few near-universal configurations.  They are the arrangements that hit a sweet spot, in the balance of compromises game, for the airplane's mission, and they are timeless until some major new paradigm comes along.  That's not one of them.


What is the meaning of ACAI and ATCI in Air Cadets? For my staff cadet course, I need to know what ACAI and ACTI stand for but I can't find it anywhere. Would anybody be able to notify me of what this means?

Those four letters can be abbreviations for many things related to aviation. Taking into account that this is for a staff cadet course, I think the most likely candidates are:

Air Cadet Administrative
Instructions
Air Cadet Training Instructions

Source


How can I keep my IR currency outside the USA? I am an FAA PPL/IR holder living in Spain.

My IR license was issued in May 13 of 2016, it has been over 6 months so now I need to log my 6HIT with a safety pilot.

Is it possible to complete the currency requirements in a simulator even though I have gone past the 6 month mark without flying? Is it possible to complete the currency requirements in the EU with an instructor?

Thanks for any help, my flight school is being really unhelpful as they seem to actually know far less than I do about FAA regulations, this is my last hope!

The easiest option is to find a US CFI/AI holder in the country you are in, to sign off on your approaches.  It might be allot easier if s/he is also a Spanish CFI particularly if you are flying in a Spanish registered aircraft.  You do not indicate if you hold a certificate from Spain.

It is not uncommon to find instructors with US FAA CFI ratings in other countries.

There is no prohibition of a US CFI giving instruction in another country.  There may be of a given country, or operational limitations imposed by aircraft registration, host country operating rules, and insurance limitations, as well as owner / lease limitations.

However, I routinely sign off US pilots for approaches made at Canadian airports, in fulfillment of their US instrument recency requirements.


Why bother tracking TSN after an overhaul? I've noticed that mechanics often like to keep track of "time-since-new" (TSN) on components that have already been overhauled.  What would TSN matter when the component is tracked by TSO for maintenance purposes?

No, if the part is not subject to a life limit of its root component.

In the airline business, there are actually two terms used in the industry - "overhaul" and "restoration" which can generally be defined as follows:

If a part doesn't have a structural life limit, then the part can be rebuilt to new specifications an indefinite number of times, and each time, it is equivalent to a new part and will generally be called "overhauled" or "zero timed".  Mechanics can track "time since new" but they don't really have to and if parts move around it probably won't be anyway.  Piston aircraft cylinders are like this.  If you buy a rebuilt cylinder, nobody will know how many hours are on it since it was first made.

However a lot of components, especially components that operate under stress on transport aircraft, can have structural life limits applied as a result of endurance tests (where, say, a series of housings under endurance test will start to develop cracks at X thousand cycles or hours, and a life limit will be applied at 1/3rd of the endurance result, which is the safety factor applied to structural cracks that develop in test).

When these parts get rebuilt, they aren't called "overhauled", they are called "restored".  A structural actuator may get a restoration interval applied where it has to be rebuilt (because parts wore out during endurance testing) but once the root component like the basic housing gets to the life limit, the entire part is scrap.  In these cases the rebuild is not an overhaul but a restoration.  The hours or cycles have to be tracked since new since once it hits the life limit it is scrap even if it was rebuilt (restored) the previous month (which an airline won't let happen).


Russian-built and NATO (USN, European) probe-and-drogue -- are they interoperable? Considering the current diversity of coalition air fleets in some parts of the world (the Horn of Africa situation comes to mind), can Russian-built tankers refuel NATO probe-and-drogue receivers, and vice versa? (Say, a US Navy F/A-18E getting a helping hand from an Indian Air Force Il-78MKI, or an Ethiopian Air Force Su-27 filling its tanks from a Royal Australian Air Force A330 MRTT?)

The drogue system was standardized in the 50s and reverse engineered by the USSR in that time so nearly all aircraft that use it are compatible with each other.


May an instrument instructor log instrument approaches that a student flies in simulated conditions as approaches for currency purposes? There's rumor that a former DPE claimed that a CFII who is providing training to a student can log that student's simulated instrument approaches as the instructor's approaches for currency purposes.
That didn't sound like it could be true, so I asked him if he could provide a written source to back it, but he could not.

So, is it true?  If so, can you please provide a source to prove it in your answer.
Common sense would tell me this is not true because the instructor is neither flying the plane nor logging instrument time.
Does it make any difference if the approach is simulated instrument in a flight simulator or FTD in which neither the instructor nor student have reference to anything but instruments?

It depends on the conditions.

For actual conditions:

The FAA addresses this in this letter of interpretation: (see full letter for the whole explanation)

  QUESTION: Am I correct in understating that a CFII may log approaches
  that a student flies when those approaches are conducted in actual
  [IMC]?

  ANSWER: Ref 61.51 Yes, a CFII may log approaches that a student flies
  when those approaches are in actual [IMC].

  ...

  The Chief Counsel's office agrees that the earlier guidance reflects
  the appropriate interpretation of the regulation.

For Simulated VFR conditions:

If the student is flying under the hood in VFR conditions the Instructor may not log in the time also based on 61.51. Instructors may only log instrument time in actual conditions.

  (g)Logging instrument time.

  (1) A person may log instrument time only for that flight time when
  the person operates the aircraft solely by reference to instruments
  under actual or simulated instrument flight conditions.

  (2) An authorized instructor may log instrument time when conducting
  instrument flight instruction in actual instrument flight conditions.

  (3) For the purposes of logging instrument time to meet the recent
  instrument experience requirements of § 61.57(c) of this part, the
  following information must be recorded in the person's logbook -

  (i) The location and type of each instrument approach accomplished;
  and

  (ii) The name of the safety pilot, if required.

Most GA sims I have encountered can only be used by one person at a time. The CFII is there to observe and advise but cant really "take over"

  (4) A person may use time in a full flight simulator, flight training
  device, or aviation training device for acquiring instrument
  aeronautical experience for a pilot certificate or rating provided an
  authorized instructor is present to observe that time and signs the
  person's logbook or training record to verify the time and the content
  of the training session.


Why do cabin crew also man check-in desks? When flying, I sometimes notice that the person behind the check-in desk also turns out to be the flight attendant on my flight.

Is this always the case, or did I just notice the exceptions?
Why do airports use cabin crew for the (comparatively) simple job of checking bags, which could in theory be done by (presumably) cheaper lower-skilled employees? Or am I underestimating the difficulty of checking bags?

It probably depends on the airline and airport, because on the flights I have been on, the checking-in is managed by "check-in staff" who are not flight attendants. Larger airlines do the arrangement that I just describe while smaller ones might manage everything with the flight crew.


Why are civilians not allowed to fly demilitarized US fighter jets? I have seen it is possible to fly demilitarized Russian fighter jets in the US. Why is it impossible to fly American demilitarized fighter jets?

That's because US military is particularly concerned about chopping up its assets before civilians can lay their hands on them. The procedures are covered in US DoD Manual 4160.28 Defense Demilitarization: Procedural Guidance.
The procedures for 'demilitarizing' military aircraft are particularly brutal:

Military Aircraft
(a)  Aircraft (Figure 29 and Figure 30) that are specifically designed for military purposes shall be demilitarized as follows:  at a minimum, cut or break completely through at least one lobe of the fuselage trifurcated horizontal and vertical stabilizer spar attachment fittings, on both the right and the left hand sides of the stabilizer carry-through spar assemblies.  This demarcation of the prescribed DEMIL procedures is to ensure the aircraft is rendered unfit for flight.

(emphasis mine).

Demilitarization Procedure; image from Popular Mechanics

(b)  Helicopters (Figure 31) specifically designed for military purposes shall be demilitarized by crushing, shredding, or smelting the entire airframe and fuselage, ensuring that the transmission mounts and supporting structural beams, engine deck in area of mounts, wing attaching mounts and support beam structure, and fuselage to tail boom attaching mounts and tail rotor gear ox mounts have been destroyed.

So basically, it boils down to the fact that the US military makes sure that the airframes won't fly again. Of course, there are ways around this- you can assemble the aircraft from parts or some aircraft will simply escape the procedure and is avialble:

In a letter to the chair of the House Armed Services Committee, AOPA President Phil Boyer ... stressed that certain aircraft (possibly aircraft not demilitarized) had been carefully maintained over the years and were now serving useful purposes.

but these are quite rare.


Why is 121.5 called "guard"? What is a "guarded" frequency and how does it differ from a frequency that's unguarded?

It is called guard because everybody is supposed to listen/guard the frequency just in case someone has a problem.

Guard definitions:

  a state of caution, vigilance, or preparedness against adverse
  circumstances
  watch over in order to protect or control.

The AIM 6-3-1(h)(1) says (emphasis mine):

  121.5 MHz and 243.0 MHz. Both have a range generally limited to line of sight. 121.5 MHz is guarded by direction finding stations and
  some military and civil aircraft. 243.0 MHz is guarded by military
  aircraft. Both 121.5 MHz and 243.0 MHz are guarded by military
  towers, most civil towers, FSSs, and radar facilities. Normally ARTCC
  emergency frequency capability does not extend to radar coverage
  limits. If an ARTCC does not respond when called on 121.5 MHz or 243.0
  MHz, call the nearest tower or FSS.


How is airspace in the Persian Gulf determined? In the wake of the Qatar diplomatic crisis, I saw this image from Business Insider:

How is airspace in the Persian Gulf determined? Why, for instance, does Bahrain have such a disproportionately large amount of airspace for its size? And why is Qatar's airspace predominately to the east of the country? Is this related to air traffic routes in and out of the territory (e.g., lots of Qatari flights fly over Das island, despite it being part of UAE territory)?

How is airspace in the Persian Gulf determined?

The map you see is for Flight Information Regions (FIR's), that's different from sovereign airspaces.

Sovereign airspace by international law "corresponds with the maritime definition of territorial waters as being 12 nautical miles out from a nation's coastline. Airspace not within any country's territorial limit is considered international, analogous to the "high seas" in maritime law."

Can military planes fly across this "international airspace"? Yes. An example is Russian war planes flying very close to the UK airspace, passing through many FIR's.

An example for a disagreement is between Greece and Turkey, because they have so many islands between them. According to some sources just last month Turkey violated Greek airspace 141 times in one day. The point is, there are disputes over sovereign airspace just like with land borders.

  Why, for instance, does Bahrain have such a disproportionately large amount of airspace [FIR] for its size?

It's a matter of "administrative convenience". Both Bahrain and Qatar gained their independence from the UK in 1971. It's quite likely the current FIR shapes were carried over from before and based on where the radars were initially installed.

  [...] Qatari flights fly over Das island, despite it being part of UAE territory.

Checking the official Qatari aeronautical chart above, the Emirati islands are not part of Qatari airspace.

See below the official FIR chart for Bahrain, despite covering the area north of Qatar, airplanes in and out of Qatar still fly north.

To illustrate the operational and administrative convenience I've split the commonly traveled east-west airways between Saudi Arabia, Bahrain, Qatar, and UAE. Now you can imagine the impact it will have on ATC, flight planning, flying, etc.

To further illustrate the region, abeam the four countries the distance is a mere 150 NM, or 20 minutes in cruise. If the FIR is split, a single flight would need to contact 4 different frequencies in such a short time. And if that flight is descending/climbing to/from one of the four countries, multiplied by the amount of flights, then you can imagine the substantially higher workload on both ATC and pilots with regard to coordination and hand-offs.

A similar oversized FIR example is Singapore FIR (shown below).

(Source)


Do gliders make a better pilot? I've often heard that learning to fly gliders makes you a better airplane pilot, in person and in answers here and here. Is this true, and if so, what specifically is it about gliders—or at least glider training—that makes this true?

Note: I understand that age, health and/or cost can make gliders more practical or accessible to some folks, but for the purposes of this question, assume those aren't a factor.

I may be biased because I learned in gliders, but, concerning how soaring biases students to be better "stick and rudder" pilots:

The obvious one that has been covered in the linked answer: gliders don't have an engine.  The result of this is that flying a glider is an exercise in energy management and a more direct interaction with your drag polar for a given configuration--because you don't have any way to add more energy to the equation.

In other words, your duration aloft, your ability to land in a particular location (of particular importance in a glider, since you only have 1 shot at landing and it could be unexpected due to variations in ridge lift, etc.) -- everything relies on flying the aircraft so as to manage the kinetic and potential energy of the aircraft to achieve a certain end -- and you don't have any "liquid" energy (e.g., fuel) to add to the vehicle.  Generally, you're trying to fly the aircraft such that it creates as little drag as possible, so turn coordination becomes more important (also can lead to spins, depending on condition), speed to fly for prevailing conditions and flight mode becomes really important.

The quintessential example of this (at least, the one that I have a video of) is from a student at my undergrad.  He was learning to fly one of the Grob 109b motorgliders the school owned.  As part of the class, the students demonstrated their skill at managing the aircraft's energy by starting, with the prop off, 1000' above ground and maybe 600' from the end of the runway (horizontal distance).  Within 15 seconds, the aircraft would be on the ground just a little way beyond the runway numbers.  And you have to do it without exceeding the aircraft's maneuvering speed during the vertical dive, conserving enough energy to make the runway, but not too much energy (else you'd just float the length of the runway).

Another facet of the above: increased adverse yaw effect.

In a glider, keeping the yaw string straight (e.g., maintaining coordinated flight) is constantly drilled into you as one of the most important things you will do (besides flying the airplane).  I remember flying one of my last instructional rides before my checkride and my instructor walked up with a sheet, taped it over what instruments the glider did have, and said, "Let's go flying".  The only instrument I could see was the yaw string, and the point was that that was "all" I needed (I had a feel for airspeed based on pitch).

What the yaw string does is tell you whether or not you are flying straight (i.e., what direction the relative wind is going relative to the aircraft).  As noted before, this is quite important, because, if the relative wind isn't parallel to the longitudinal axis of the aircraft, your drag is going up.  Gliders are, typically, more prone to slipping or skidding (i.e., flying at angles to the relative wind, particularly in turns) due to their long wings, which, when they have an aileron deflected, can induce a greater yawing moment than your typical general-purpose aircraft with a much lower aspect ratio wing (i.e., a more shorter, stubbier span, whereas gliders are made with long, skinny wings to decrease induced drag).  This makes the aircraft's yawing moment more sensitive to aileron deflections and  more prone to fly in an uncoordinated fashion--hence instructors make it a point to train glider pilots to pay more attention to how coordinated their vehicle is at a given point in time.

This is of particular importance, not just for drag, but also for flight safety.  If you have a glider that's low, relatively slow, and making a steep turn while landing (read: flying at a high load factor, thus increasing stall speed), an uncoordinated turn can do this, or stall, spin, and crash due to insufficient altitude for a recovery.  It's a really big deal, and, again, you don't have the ability to just "add power" when you feel like you're getting slow.

Situational awareness

One trick with gliders is that you are very dependent on the atmosphere.  Say you are trying to get from point A to point B, say for a race or what have you, and you planned out your flight to run a ridge based on forecasted winds.  You're up, running the ridge at 200' (or less) above ground level, and the wind peters out and dies -- you're stuck.  When I learned to fly ridges, the mantra was to always have a place picked out such that, if the wind died right now, you'd have a place to land.  200' was relatively high among the folks I knew flying the ridge (most guys would be flying at least half that, and I heard stories about folks flying at 20 feet above the trees), and that doesn't give you a lot of time to figure out where you are going to land...and landing out (i.e., not at an airport) is not uncommon.  Glider pilots train for it, it's part of our ground school, and it's beat into us while flying.  If you're taking off via aerotow (i.e., being towed to altitude by another aircraft), the question during training is always, "If the rope breaks right NOW, where do you land?" Because you're getting energy from the towplane burning fuel on the other end of the rope, and, as soon as it's broken, you have not much time, some speed, and usually very little altitude to get safely to the ground in.

So, in the end, I'd think that glider pilots would have some edge in powered pilots in that regard.  Yes, powered pilots train for engine outs, but, in a glider, every flight is an engine out situation and you're flying at the mercy of the weather, how well you planned ahead for it, and how well you continue to plan while you're flying.  As quoted from an AOPA article:

  U.S. Air Force Capt. Danny Sorenson, who instructs in F-16s, is a glider pilot. He stated, “As a result of my glider training, I’m always thinking, ‘Where can I land this thing?’” He also noted that during his F-16 training, simulated flame-outs were never a problem for him, “It’s instinctive,” he said. “I’d just fly my pattern and glide in.”

Summing it up:

Gliders are much "simpler" flying: there's not a VOR in a glider, no ILS, no autopilot...all you have to do is fly the airplane, and do it well.  And, if you do it well, the airplane flies well.  Do it poorly and you might die (if it's that bad, since you don't have 2nd chances in important things, like landing) or you just might not be in the air for very long.  It's a much more direct connection between the pilot's stick/rudder inputs, the vehicle's performance as an aerodynamic vehicle, and how the pilot wants to use the aircraft, though, and I think that's the key...and all of these skills translate to other aircraft (even rotary wing, as I found out when flying those once).

Sources:

--My experience (PPL in gliders)

--https://www.aopa.org/news-and-media/all-news/2002/may/flight-training-magazine/gliding-your-way-to-a-pilot-certificate

-- https://www.aopa.org/news-and-media/all-news/2009/january/22/training-for-that-moment-when-every-second-counts


Do existing large commecial jets make use of differential thrust to control yaw? Does existing Avionics / Control Systems on large Commercial Jets make use of differential thrust of engines much?

e.g. I was contemplating applications of this sort: Normally the thrust provided by each of multiple jet engines will be different due to difference in age, wear, build etc. and this is usually compensated by rudder trim.

But instead, would it be feasible for a controller to detect the slight yaw moment and modulate the throttle setting to compensate thereby negating the use of rudder trim?

What about a more general strategy in which the use of rudder trim, or more extremely rudder itself is reduced via automated differential thrust modulation of jet engines? i.e. Using differential thrust not just yaw compensation but yaw creation as well?

Do such systems already exist? Or is there a fundamental flaw in my reasoning that make these ideas not viable?

No, differential thrust is used in normal operations only when maneuvering on the ground, and then manually, not driven by a computerized control system.

It is preferable to run all engines at the same speed. Not only would you add additional frequencies, but also the beat frequency between pairs of two engines if speeds differ. This makes it harder to check with your ear that all engines are humming away nicely and generally creates more noise than when the engines run in sync. The answers to this question explain what the beat frequency is and how synchronization is achieved.

Next, the lag in engine response makes the control loop prone to oscillations. A rudder command is executed quickly and precisely, and when the deflections are below maybe 10°, no profile drag penalty is incurred. Induced drag due to lateral trim should be very small, especially at high speed. Trim deflections are normally much below that value, so an efficiency benefit from trimming with differential engine speeds is unlikely.

During take-off and initial climb both engines run at maximum thrust, and any trim requirement would reduce total thrust. This reduction is small, but sometimes every bit counts. During descent, the opposite happens and one engine might need to run at a higher thrust level than ideal. To decouple lateral trim and control from thrust makes sense in those cases.


What measures were taken to keep Soviet airline pilots in the Soviet Union? How did the Soviet Union keep airline pilots from leaving the country?

It seems like it would have been pretty easy to do since the pilots are already in the air. And I would guess at least some flew close to the borders.

Civilian pilots had a quite comfortable life. Flying high-tech gear has an attraction in itself, and the propaganda did what it could to strengthen patriotic spirits. There were not many reasons to defect if you did not mind the nonsense in the papers but appreciated to be part of a privileged class. And everybody knew that defection would have dire consequences for all relatives left behind.

The same is true for military pilots, and on top the Russian air defense was organized quite differently from NATO's, thus military pilots flew almost by remote control. This went as far as using radios which had only a dozen of pre-set frequencies. The pilots did not know the frequency, they just pushed the button with the number they were told by ground control.


What concerns do the FAA and EASA have in 2014 with "own-ship position display" on EFBs? I've read a couple of times that "own-ship position display" is not authorized for use on Class 1 or 2 EFBs (by the FAA), and that both the FAA and EASA are cautioning against that feature's use on EFBs in general. Own-ship position is an aircraft's GPS position displayed against georeferenced charts and diagrams.

Obviously the use of a portable device - iPad or otherwise - should not be enough for primary navigation, but what safety concerns do the authorities have with own-ship display?

Specific accident reports or official citations would be great.

Display of ownship's position has a very strong effect on the mental model that the pilot has of it's navigation. When the EFB displayed position is wrong, the change that the pilot will detect that by comparing to other instruments or outside visual are very limited. This is a safety concern.

To have the EFB approved to display ownship's position, the most straightforward way would have been to certify it under TSO C129a "Airborne Supplemental Navigation Equipment using the Global Positioning System (GPS)" TSO-C129a has now been superseded by TSO-C196 which has slightly different requirements.

Often class I or class II EFB will have a GPS receiver that technically could be used as the source of ownship's position. The GPS receivers chips used in are not designed and certified against the "Minimum Operational Performance Standards for Airborne Supplemental Navigation Equipment Using Global Positioning System (GPS)” (RTCA document DO-208), which is a requirement of TSO-C129a or "Minimum Operational Performance Standards for Global Positioning System/Aircraft Base Augmentation System" (RTCA document DO-316) which is a requirement for TSO-C196.

The major technical difference between simple commercially available GPS chips and aviation grade GPS chips is that the latter include a Receiver Autonomous Integrity Monitoring (RAIM) function which does Fault Detection (and Exclusion).

If a GPS satellite would drift out of it's orbit or GPS data gets corrupted in the receiver, an ordinary GPS chip would most likely display a position error, while an aviation GPS chip would either indicate a failure state (fault detection) or still display the correct position (fault exclusion). This protects the pilot from being mislead by a map shift.


Why is there an altitude limit if we have an IDG failure on the A320? In my company’s MEL there is an altitude restriction if we have an IDG failure of 33500 feet why is that?

Short answer: This is the highest altitude where the APU can deliver 90 kVA at ISA temperature.

Details

In normal conditions, electrical power is delivered by two engine integrated drive generators (IDG), one per engine.

IDG used both on a IAE V2500 and CFM56-5B engines. source

Each IDG is able to deliver 90 kVA, a power which is sufficient to feed all systems in the aircraft. As electrical power is vital, a second source is provided and each source feeds half of the systems, but is ready to take over and feed all systems in case of failure of the counterpart.

When one IDG is off, the auxiliary power unit (APU) is used as a backup for the second source. The APU delivers both hydraulic power and electrical power.

Honeywell 131-9A APU. Source

At ISA temperature and below 33,500 ft, the APU electrical generator has the same 90 kVA generation capability.

Source: FCOM (3.1.49)

After 33,500 the APU efficiency for electric power generation starts decreasing. If the aircraft was above 33,500 ft and the second IDG failed, this would leave the plane with a single generator (the APU) unable to deliver 90 kVA required by all the systems. So the altitude limit in the SOP.


Will Lockheed Martin Flight Services send me ACAS, EasyActivate™, and other text messages if I file my flight plan through ForeFlight Mobile? ForeFlight Mobile users who file flight plans without being authenticated to CSC DUATS will effectively file through Lockheed Martin Flight Services (LMFS).
Considering that the LMFS route does not require my personal  1800wxbrief.com credentials (unlike filing through CSC DUATS), will additional services connected to my LMFS profile such as Adverse Condition Alerting Service (ACAS), EasyActivate™/EasyClose™, Close Reminders, ATC Notices, and Surveillance Enhanced Search and Rescue (SE-SAR) be accessible through my mobile phone?
In other words, will Lockheed associate my ForeFlight flight plan with my LMFS profile in order to send me text messages with information about TFRs, links to open and close flight plans, and so on?

Based on my personal experience, the answer is NO (as of the time of writing). I use both systems regularly, and have discovered the two do not share flight plans at the user account level. ForeFlight uses its own account (and not individual user accounts) to interact with Lockheed Martin Flight Services. Because of this, it appears that LMFS has no way of knowing which user account is associated with a flight plan filed through ForeFlight.

Identifying data within the flight plan itself (i.e., name and phone) are not processed by the filing system, merely stored for later retrieval if needed. I mention this because LMFS can (upon request) associate tail numbers with a phone number, so that when you call them on 800-WX-BRIEF your profile comes up on their screen automatically (though I have found this to be less useful than one might think).

In order to use the services you have identified, you will need to file your flight plan directly on the LMFS web site (and have your notification settings configured properly).

With that said, all of these functions (with the exception SE-SAR), have been available directly within ForeFlight for some time:

ACAS/ATC notices - if configured by the user, sent via push notification (http://blog.foreflight.com/2014/11/24/6-5-brings-flight-notifications-and-notam-advisor/)

EasyActivate/EasyClose - flight plans can be amended/canceled/activated/closed within the File & Brief window (http://blog.foreflight.com/tag/flight-plan-filing/#attachment_4045)

Close reminder - sent via push notification (https://support.foreflight.com/hc/en-us/articles/205052928-What-are-push-notifications-)

Over the past couple of years, ForeFlight has been aggressively working towards tighter integration with LMFS, so I would expect additional functionality to be forthcoming (purely speculative, since they are very tight-lipped about upcoming enhancements).

UPDATE:

It looks like LMFS now offers integrated services with a number of providers (but not ForeFlight). My guess is ForeFlight doesn't allow it because it was already offering the same services and now needs to figure out a way to ensure users don't get duplicate messages:


What does the statement painted on this P-51 mean? In this video, there is a shot of the side of a P-51 Mustang.

The side of the plane is painted with:

  SERVICE THIS AIRPLANE WITH GRADE 100/150 FUEL. IF NOT AVAILABLE T.O. 06-5-1 WILL BE CONSULTED FOR EMERGENCY ACTION
  SUITABLE FOR AROMATIC FUEL

What does this mean?

They are instructions for a compatible fuel substitute for 100/150 octane fuel per Technical Order 06-5-1 if AvGas is unavailable to the service crews in an emergency.


What is the difference between targeted airspeed and selected airspeed? Are Targeted Airspeed and selected Airspeed the same thing?

And can someone explain in what cases managed airspeed or pilot selected airspeed would be used, i.e. for different flight phases/situations etc.?

These terms obviously stem from an Airbus flight deck.

Anything selected represents pilot's discretion. If you want to keep a certain speed, HDG or altitude, you would pull the corresponding FCU (Flight Control Unit = Autopilot Panel) knob. This means in Airbus philosophy: I have control.

Anything managed represents a flight management calculation. If you want to fly, e.g. in the most economic way, you push the corresponding FCU knob and therewith "handover the controls to the aircraft".

The calculations for managed autopilot options are derived from e.g. gross weight entries on the INIT page or current gross weight, Angle of Attack, Air Data Computers, Cost Index, ... depending on the flight phase.

So, you always prefer to follow the economically calculated way and use managed modes. Whenever ATC requests you to keep a certain speed, sink/climb rate, heading whatever, you pull the knobs and select the target values.


Has there been any attempt to use an underground cable system, a la a cable car, to tow aircraft along taxiways? I've been looking at some proposals to try to reduce the time that an airliner's main engines are run at idle or near idle when taxiing to and from runways--a situation that uses non-trivial amounts of fuel and greatly increases local emissions.
I've seen proposals for fleets of tugs and onboard wheel motors, but I wonder if there might not be a simpler way. Why not use a continuously moving cable system, such as that used by San Francisco cable cars? Simply run a cable in a trench in the center of a taxiway, and have a means of attaching to and detaching from this cable as a way to tow aircraft down a taxiway and to the end of the runway. I don't know the details, but I do know cable car designers figured it out more than a century ago, so surely it can be done.
Similarly, one might also utilize a roundhouse system to rotate aircraft to the proper position to pick up the cable. Again, roundhouses have been used with trains for well over a hundred years, and locomotives are quite heavy. Clearly one would need ways to slow the cable speed at termini, but this is done on high-speed ski lifts. Some means of "clutching" to buffer a sudden application of force would also be in order--but again, think of the cable cars, or steam catapults/deceleration cables on aircraft carriers. It's been done. Surely it can be done in this context.
What's not to like? The benefits of reduced fuel consumption and emissions are obvious, and it beats a flotilla of tugs, by eliminating empty return trips, logistical hassle, and large numbers of staff. Also likely beats an onboard motor system, on weight, cost, and complexity. There are tons of reasons to do it.
Seems obvious to me. What are the arguments against it?

Way to go on thinking outside of the box. Actually, it might work. Although the logistics of it are above my pay-grade. Maybe, some smart group out there could make it work.
But, your question doesn’t ask for ways to make it works. It specifically asked for reasons it might not works. So, here it goes:

cable car feasibility over all does not have a great track record. If we were to make a Ben Franklin style decision graph of cable car systems that worked versus those that proved ineffective, Wikipedia lists a lot more defunct systems than it does working systems. All of which run on tracks.
If you get more than two runways at the same airport, the taxiway system become increasingly complex. An aircraft would need an alternate means of locomotion when switching from one runway/taxiway to another. The use of one continuous loop cable would be rendered impossible.
As Ron Beyer has pointed out, departing aircraft would still need time to warm up their engines prior to take off. Currently, that time is spent taxiing. Arriving aircraft would still need to power climate control systems. It would be a cheaper and more efficient use of time and resources to design a system that eliminates wait time between start up and takeoff. Keeping the traffic flow moving at all times without having to retrofit airports or entire airline fleets would benefit all.
Most cable car systems operate with cable cars of lighter weight and similar mass to each other compared to commercial aircraft. How would airports be able to account for the sheer mass differences between a single pilot C208 after a long cross country versus an A380 taking off at MTOW.
Power failure.
Power in night time IMC with four or five airliners about to break out at minimums for GPS approaches on parallel runways.
In the case of a constantly moving cable, an aircraft the size of an A320 could cause a lot of damage if it were not able to unhook itself from the cable at the appropriate time,
According to Wikipedia, the efficiency of a large cable car system is questionable. “Approximately 95% of the tractive effort in the San Francisco system is expended in simply moving the four cables at 9.5 miles per hour.“


What is an Enhanced surveillance system? What is the purpose of enhanced surveillance system and what are the benefits of installing it? And what information is available from this system?

Enhanced Surveillance (EHS) is a variant of Mode S transponder protocol that includes Downlink Aircraft Parameters (DAP) in the information returned by the airborne Mode S transponder.

Mode S can be configured as Mode S Elementary Surveillance (ELS) or Enhanced Surveillance. EHS is the concept behind ELS, ELS being a limited implementation. ELS is deemed analog avionics aircraft oriented while aircraft with digital avionics should use EHS.

While ELS allows for selective interrogation of a Mode S transponder to obtain barometric altitude, EHS allows also to request DAP. These data are intended to be used by ATC to face the rapid traffic increase observed in Europe. With no surprise this initiative is driven by Eurocontrol.

The use of DAP is expected to limit voice communication between ATC and crews. DAP includes:

Source: Eurocontrol

In general, ATC automation tends to get all parameters providing information on current position, but also to anticipate short-term position. This is why roll angle or vertical rate are part of the DAP.

Mode S EHS takes benefit of Extended Squitter (1090 ES).

Related:

Transponder
Transponder interrogation modes
Mode S
ELS and EHS


Does the wifi uplink antenna affect aerodynamics or efficiency? The wifi uplink antenna on airliners seems like a quite aerodynamically inefficient hump, just sticking up into the airflow above the main cabin. If it added even 0.1% more drag, I don't see how it would be financially viable.

(Photo by me, released to public domain)

How much does that big antenna drag? Why not either smooth it out forward and aft or build it into the body of the plane?

Any kind of antenna like that is creating an increase in the flat plate area of the aircraft and thus increasing the parasite drag on the airplane.  Fortunately newer antenna designs, such as the KU/KA band satellite internet systems can be arranged in a manner which greatly reduces the frontal area of the antenna.

The ideal solution, which will inevitably come about on future airliners as these systems increase in popularity with the travelling public, is the creation of an integral compartment on the dorsal side of the fuselage aft of the rear pressure bulkhead and fitted with a dielectric which is flush with the aerodynamic envelope, thereby eliminating parasite and interference drag as well as affecting airflow over the vertical fin.  However for earlier aircraft or those currently in service, this is not going to be possible for several reasons.

First, an installation like that would require major modifications to aircraft structure and pressure vessel of the fuselage and cost big, big bucks in order to get an STC or approval from the airline's engineering department.  Current designs are arranged to as they are to be minimally invasive; all that's required for a typical installation is the attachment of a series of brackets and longerons to the cabin frames which can then be attached to brackets outside of the aerodynamic envelope with a minimum series of fasteners penetrating the fuselage.  This arrangement is preferred by the airlines as their engineering departments and the FAA or other regulatory authority can sign off on the installs without requiring the OEM's involvement.

Secondly, many jets are not owned by the airlines themselves but are leased from financial brokerage firms and any kind of system which minimizes alterations to the airframe is preferable to major - and irreversible - alterations to the aircraft.

As for the economics of these systems, even if you did have an increase in fuel burn by 0.1% you could easily compensate for the loss if installed on a large enough aircraft.  Let's assume on a 747-400 flight from LAX to Narita Intl in Tokyo, you will typically burn about 290,000 lbs or about 44,000 gal of JET-A.  A 0.1% increase results in a mere 44 gal difference in fuel consumption at about $220.00 if you price the fuel at 5 USD/gal.  For a long haul fight like that, lets say that 25% of the passengers use this service at a cost of 44.00 USD each.  With a typ. 747 seating of 440 people, that's 100 passengers buying WiFi service totaling 4,400 USD in income for the carrier.  It's well worth it for them!


How safe are commercial airliners from gallium infiltration? I recently saw a video on gallium infiltration on YouTube.  Gallium is a metal with a melting point somewhere between standard room temperature and human body temperature, meaning if you pick up a block of it sitting on the table, it will melt in your hand.  If this melted metal comes into contact with solid aluminum, it will seep into it and alloy with it, decreasing the strength of the aluminum to somewhere around that of tissue paper.
When I saw this, I immediately had a horrifying thought: aren't airplanes made of aluminum?  And when I've flown in the past, I've seen a few places where there's exposed metal.  This makes me wonder if a malicious person with no regards for their own safety wouldn't be able to bring down a plane with a novelty for children that anyone can buy for a few dollars.
If someone tried to do this, is there anything in the design of modern commercial airliners that would stop them?

Gallium does not interact strongly with steel so it is unlikely to damage hydraulic pipes (these are made from steel, not aluminium). Same source says it also does not interact much with polymers so unlikely to damage the insulated wiring or rubber hoses or epoxy floor panels. It may damage some sensitive point where important device is attached to the aluminium frame, but hopefully most of such places are not single points of failure.
All it can do is to make a hole in the fuselage under the very rare circumstances. Then the aircraft will be depressurized, descend and land. It is probably still not a good idea to allow gallium in the passenger compartment.


Why are pitot-static measurements preferred over other velocimetry techniques? I believe hot wire anemometers (HWA, frequently used in wind tunnels to measure flow velocity) or other sensitive techniques could be used to determine the airspeed. Are there specific reasons for why the aircraft industry continues to use pitot-static instruments? Or is it just tradition?
A nice link on HWA's: The hot wire anemometer

The linked article says that an HWA (hot-wire anenometer) is

fragile and requires complex equations to relate the sensed value to the fluid speed as well as correct for inherent, unavoidable nonlinearities as well as external factors such as fluid type, ambient temperature, humidity, and angle between the flow and the sensor.

Conditions in a wind tunnel or other lab equipment are far more predictable, and change much more slowly and much less, than those encountered during an outdoor flight through variable weather.
Although industrial variants of the HWA don't expose the fragile sensing wire to damage or dirt accumulation (the dirt as thermal insulation causes the sensor to report incorrectly; rain or ice would still affect an aircraft-mounted one), they still need to correct for density, humidity, temperature, and flow angle.  A four-dimensional lookup table takes a lot of work to create and store.  Outside the lab the pitot tube is more reliable, even if it is more "primitive."
HWAs have been flown, but only as test equipment, not as sensors for primary flight instruments.  For example, this set of HWAs flew on an F-15B to characterize flows in the boundary layer, sampled many thousands of times per second, far more sensitive than a pitot tube.  But between each 30-second measurement interval, the HWAs were retracted inside the airplane for protection.


Why do some multi-engine aircraft have accelerate-go distance in their performance charts and some don’t? I'm curious about why some airplanes have accelerate-stop distance (ASD) and accelerate-go distance (AGD) in their performance charts and some don’t include that. Does anyone know why?

It depends on the aircraft in question.  This isn’t required for the certification of a light twin and is solely up to the OEM’s discretion to publish said information.


How does aircraft navigation work? Is it possible to briefly summarize how a commercial plane navigation system operates to a layperson (e.g. a passenger)?

The aircraft takes off, reaches a certain altitude, and then lands perfect on a runway somewhere else.

How does the navigation system affect this?

How do pilots know where to go and where to land?

How do you navigate in your car?

Before you depart, you have a general understanding of which direction you're going.
(the supermarket is to the east of my house, about 3 miles.  I believe it will take me about 5 minutes to get there).

When you start driving, you keep your eyes open looking for information and clues.
(The yellow lines on the road denote a two-way street.  Certain landmarks are familiar and expected.)

As you proceed, you may refer to your car dashboard to make sure things are as expected
(My speed is 43MPH.   I had planned for 45MPH... pretty close.  My engine is running at 2,500 RPM, which is appropriate)

You may have advanced instruments as well
(The GPS in my car is telling me to take the next right turn, and go 1/2 mile)

Its very much the same with flying.

Pilots have pre-flight preparation where they go over a general plan.(we're going to fly north, on a heading of about 025 for 3 hours.  We'll be passing over a large lake, and landing at our destination with a city off to our left)

As they move towards the runway, they look for other airplanes, read the signs around the  runway, and refer to their maps.
(I see a sign indicating I'm on taxi-way Bravo, approaching Runway 31.  I see yellow markings on the pavement indicating I should get permission before I cross)

Once they take off, they look at their instruments to verify they're going at the expected speed, altitude and direction.
(Altitude is 35,000 feet, as planned.  Engines are at the right pressure, and speed is 440 knots, as expected)

They can look out the window for large landmarks, lakes, mountains, cities, etc.
(I'm passing by the mountain I saw on the map.  I see the river valley below me, and a large city ahead of me, just like I planned)

They may use advanced services like GPS Navigation, Radar and radio signals to help navigate.
(My GPS is showing my position and direction.  It says to turn a little to the right to stay on course.   The navigation radio says I'm 25 Nautical Miles away from the radio transmitter)

And they can talk to Air Traffic Control who can observe them on radar, and advise them to make adjustments to their course for navigation, and to stay separated from other airplanes.

If they cannot see out the window, navigation does become a bit more complicated, and reliant on Instruments, GPS, Radar and Radio.But given a very careful plan of "Fly for exactly 31 minutes, at exactly a heading of 027, at exactly a speed of 130Kts", it is pretty remarkable how close the airplane will end up to the intended destination.
That process is called Dead Reckoning.  It requires some detailed advanced planning, but isn't that difficult.   Combining Dead Reckoning with Instrument flying, and pilots can go through zero-visibility and still come out at the intended location.


Fabric-covered Clark Y Max Coefficient of Lift I'm looking at the Aeronca C-3 Collegian, which had a Clark Y airfoil, and I'm finding the following:

Wing area: 142 ft²
Empty weight: 466 lbs
Payload: 470 lbs
Stall speed: 38 mph

When I do the math for that wing area, a gross weight of 936 lbs, and a stall speed of 38 mph, I get a coefficient of lift of of about 1.8; that's higher than I see from either simulations or wind tunnel tests.

I'm inclined to trust these figures, since they line up with pilot reports on the C-3, and the airspeed range would be very narrow if the stall speed were higher; could it be that distortion of the fabric results in a later stall?  The little I could find on the subject doesn't suggest any such effect.  What's going on here?

One contributor with a lightweight airplane like the C-3 is thrust.  At a high angle of attack, a significant fraction of the engine's thrust is directed downward, lightening the loading that the wing's lift is working against.  Even at idle, this allows slowing further before the angle of attack becomes critical -- making the coefficient of lift appear higher than it is.


Distance of downwind leg from runway in glider landing pattern? Is there guidance from the FAA or AGA on the distance of the runway from the downwind leg of a landing pattern for a glider?

For powered aircraft, the FAA recommends in its flying handbook (FAA-H-8083-3B) that the downwind leg be flown from 2500 to 5000 feet out from the runway, a pretty wide variance.

I was taught (by various publications and compenent CFIs) that angles are a better thing to focus on than distances.  For example, this publication suggests:

  You  should  turn  from  your  crosswind  leg  (or  from  your  45  entry)  onto  your downwind leg when the runway centerline, or an extension of it, is 25-30° below the horizon. Using angles automatically compensates for variations in altitude.

For me, this makes sense for several reasons:

Judging angles is much easier than judging distances.
The AGL pattern altitude might differ depending on the airport/field you are landing at.
If you are landing out, you should ALWAYS fly a normal pattern if possible.  This may force you to fly an abbreviated pattern closer to the ground.

Obvously, adjustments to this will need to be made for a crosswind.


What is the historical reasons to start Belgians registrations with "OO-"? For many countries, the registration number begins with a obvious letter (F for France, D for German (Deutschland I guess), C for Canada,...). For other, historical reasons have already been covered here (e.g. Where did the US 'N' registration letter come from?).

But for Belgium, I find it weird to begin with OO. Is there an historical reason for that? if so, what is it?

Since nearly all aircraft have radios, the simplest solution was to make aircraft tail numbers the same as their radio callsigns. Belgium was allocated radio callsigns with the prefixes ON through OT by the ITU, though for whatever reason, they use only prefix OO for aircraft.


Why do carburetors tend to produce richer mixture at higher altitude? The pressure drop in the venturi is proportional to air density and the fuel is at ambient pressure in the float chamber, so I would expect the fuel flow to reduce proportionally with density, and that response to preserve the fuel-air ratio over changing altitude.

But in practice that does not seem to be the case. Proper response to altitude requires additional modification that most carburetors don't have, so the pilot usually has to lean out the engine as they climb. What am I missing here?

(Update) More specifically, I would expect that at the same RPM, the volume flow rate will be the same—because the engine pulls in its displacement per revolution. Now velocity in the venturi $v$ is just

$$ v = \frac{\dot V}{A} $$

Where $\dot V$ is the volume flow rate and $A$ is the cross-section of the venturi. So it will also be the same independent of altitude. Since dynamic pressure

$$ p_d = \frac 1 2 \rho v^2 $$

And that is also the pressure that pulls in the fuel (when the float chamber is open to ambient pressure). Substituting mass flow

$$ \dot m = \rho v $$
$$ p_d = \dot m \frac 1 2 v $$

and as long as $v$ is mostly constant,

$$ p_d \sim \dot m $$

This still leaves open the dependence of fuel flow on $p_d$. If the relation is reasonably close to linear, it should mean the venturi mixes properly by mass. I can see a reason why higher pressure should cause less than linear increase in fuel flow, but not much why it should cause more than linear increase in fuel flow—but that is what the actual behaviour would need.

Note: Did you notice the extra $v$ conspicuously missing from the last equation? That suggests the mixture should become richer with throttle opening—which is actually the reason there are extra fuel jets after the throttle that pull in extra fuel when the throttle is at idle and slightly above. It might also explan what is going on here, but would require explaining why the speed should increase when the density is lower.

The simple reason is that high altitude changes the density of air, but not the density of fuel.

Most simple carburetors use a simple venturi design that mixes air and fuel by VOLUME.

But, the correct air/fuel mixture must be based on MASS, not Volume.

If the air/fuel Volume is correct at sea level, it will be too rich at altitude. This is because a given volume of air will have less Mass at high altitude, while a given volume of fuel will have the same Mass at high altitude.

Aircraft carburetors have a mixture control to "lean" the volume of fuel to the volume of air, and are then able to maintain the correct air/fuel ratio at all altitudes.


Is the Suntoucher CG too far aft? I acknowledge my very limited understanding of flight mechanics, but at first sight something tells me that the CG of the Suntoucher solar aircraft concept lies too far before the main wing and the forewing is too small to attain satisfactory stability and maneuverability.
This geometry is not OK at all.
Question:
Did I miss anything?

The Suntoucher Solar Aircraft concept:

References:

wordlessTech
designboom

Good catch! You are right, the center of gravity will be too much forward for a canard. If they put the prop on the sting behind the wing and the cockpit a bit ahead of the wing, things would look more credible.

The size of the foreplane is not obviously wrong, but the booms at the tips of the foreplane look strange. They serve no obvious purpose, like the sting, and the tailbooms look too thin for transmitting the loads from the vertical surfaces into the wing.

One third thing that looks odd are the control surfaces: The ailerons look too small to allow maneuvering in gusty weather at low speed. I guess the flaps on the inboard wing are for pitch control. Why are they not on the canard, where they would be more effective? Maybe because the center of gravity is so close to the foreplane, the designer decided to put them on the wing.

I agree with your assessment: This thing will remain an artist concept.

For comparison, see below a picture of a solar aircraft that actually flies and got most things right: The Icaré II:

Icaré II in flight (picture source)