Is it possible or has anyone explored the possibility of capturing static electricity for supplying electrical loads on an aircraft? I am wondering if it's possible to harness static electricity generated on the skin of the aircraft to supply power for system loads? Has anyone ever undertaken a project to achieve this?

One reason the question came to mind is that all aircraft I've ever seen have static dischargers for the purpose of dissipating it.

No, it is not.

To use electricity to do work, you need:

two thigns with stable potential difference,
large amount of charge and
good electrical connection to both terminals

And static electricity does not really give you either:

the amount of static electricity collected depends significantly on atmospheric conditions,
the total charge collected is tiny even though the potential increase can be huge and
one of the terminals is outside air and the electrical connection to it is really poor (if it wasn't, charge couldn't accumulate on the plane).


How does a full fuselage and structure inspection happen in practice ? (for modern Civil airliners) What are the intervals at which a full structure inspection take place ?

How much time does a test commonly take for narrow and wide body aircraft ?

Is every part in the structure manually inspected by the operator in level C Check or some parts only depending on the Structure repair manual (SRM) of the aircraft?

The FAA "Class D" maintenance/inspection of commercial airliners, which must occur every 6 years, is extremely thorough:

The cabin trim is completely removed to inspect the airliner's skin and superstructure from the inside. Seats, safety equipment and control system linkages are also inspected in detail with each individual cable or wire tested.
The plane's exterior paint is removed to inspect the exterior skin, including rivets and seams.
All critical moving parts on the exterior of the plane (engines, gear, ailerons, flaps, spoilers, rudder, elevators, shock booms, etc), are removed from the plane, disassembled and visually inspected for wear or damage, then re-assembled and tested.
All pressurized lines (hydraulic, water, fuel) are removed from the plane and each part individually visually inspected before being re-assembled and pressure-tested in the plane.
The cockpit is stripped, all moving parts in the control column and throttle assemblies visually inspected, and all flight instruments visually inspected and tested.

There are many more maintenance/inspection tasks than I list, I'm just going through the major ones. The plane is more or less stripped to its bare metal frame, every component visually inspected and tested, then the aircraft is reassembled with individual systems tested again on the airframe. The process can take up to two months for each aircraft, and can cost between 5 and 7 million US dollars. Most major airlines have either their own in-house maintenance crews or the manufacturer perform this maintenance, with a team of FAA-employed inspectors overseeing and signing off on all the work.

Most airliners receive 3 Class D inspections in their lifetime; at the fourth, occurring around 24 years of airframe age, the cost of a Class D is typically not considered worth it given the aircraft's remaining value, and the aircraft is usually retired and either sold to smaller non-US airlines operating in jurisdictions with less stringent standards, mothballed and stored for potential replacement parts value to the rest of the fleet, or sold for scrap. Some airframes in service with some airlines have been kept through four Class Ds in order for the airline to have sufficient time to procure replacements; American's remaining MD-80s are largely newer airframes acquired in the TWA merger, but there are a few of American's own deliveries left in the fleet which are past 24 years and are still around mainly to allow American sufficient time to work through the bankruptcy, United merger and procurement of replacements (Boeing's 737 production lines are going full-tilt and American is not the highest-priority customer, so American had to split its order in this class between the 737 and Airbus's A319 and A321).

Because of the level of detail and deconstruction required, including complete cabin trim and paint removal, airlines often use this maintenance interval to perform any desired upgrades or tweaks to their fleet configuration. Common upgrades in recent years have included:

adding winglets for vortex reduction (reducing drag and allowing closer spacing at busy airports),
upgrading avionics and even control systems (some older airframes can be upgraded from direct cable-driven hydraulics to electro-hydraulic controls allowing partial automation of flight with EFDs),
upgrades to cabin trim including re-spacing of passenger seats (more or less legroom or "slant", balancing between class seating),
replacing galley equipment and other secondary systems that may be a different standard from the rest of the fleet,
adding new passenger conveniences (such as satellite-based Wi-Fi, headrest IFE, etc) and
new exterior paint schemes for corporate rebranding (such as American's ongoing rebadging from their decades-old AA silver to the new American white, and Southwest's recent move to the "Heart" scheme from its older "Canyon Blue" and the original red/gold before that).

Aircraft acquired in airline mergers are often the highest priority as the airline seeks to rebadge and standardize the acquired fleet to operate alongside its existing planes (and ensure no corners were cut by a struggling airline that would create liabilities going forward).


A commercial jet with rear tail-mounted engines and 3+3 seat pattern? I have just spotted a cameo in one of the old CSI:NY episodes. External shot showed a commercial jet, rather big aircraft, with rear tail-mounted engines. While internal shots shown 3+3 seat rows configuration and around 150-180 passengers.

Is this combination ever possible or is this an in-movie mistake?

I was always told that rear tail-mounted engines are capable of powering smaller aircraft only (like MD-8x and Canadairs) which are always configured in 2+2 seat rows set and are capable of delivering around 100 passengers at most.

What am I missing (note that I am not talking about trijets)?

If it was a season 5 episode, it was most likely N7264U, a 727-200:1

Season Five
Boeing 727-222
Ep. 5.25
Registration N7264U, c/n 21411/1346 built in 1978 for United. Based at Ontario International Airport (ONT/KONT) in California, where this was most likely filmed.

The 727's three Pratt & Whitney JT8D engines are all mounted on/in its tail, and the plane was large enough to have six-abreast (3+3) seating, as can be seen in this 727-200 seatmap found via a quick google (this particular seatmap is for an EAL 727, but the 3+3 seating was common to all passenger 727s of which I am aware):

(Image by Eastern Airlines, via Darren at Frequently Flying.)
And the 727-200's maximum passenger capacity was 189, according to Wikipedia, although it more usually carried 134 (in a two-class configuration) to 155 (all-economy arrangement), so it fits the 150-180-passenger requirement (and even exceeds it slightly at the upper end).
And yes, I know it's a trijet, but:

the #2 engine is buried within the 727's tailcone and mostly concealed from view, unlike the large, obvious pods holding the #1 and #3 engines (as opposed to, say, a DC-10, whose centerline engine and associated intake duct form a huge conspicuous cylinder above the fuselage at the base of the vertical stabiliser);
as for the centerline engine's tailpipe (which is visible, sticking out the tip of the tail, if you look carefully), the JT8D is a slim low-bypass turbofan, making for a much narrower and less obtrusive tailpipe compared to the chunky high-bypass engines that occupy the tailcone of (say) an L-1011 or Yak-42;

...and, putting both of those together, and considering that it was a (presumably brief) cameo giving no reason, and little opportunity, for you to pay close attention to the tip of the aircraft's tailcone, it's perfectly possible that what you saw was, in fact, a (trijet) 727, and that you simply didn't notice the easy-to-miss-at-a-brief-glance middle engine.

1: Thanx to Gerry for providing the link.


Could sidesticks be linked? A sidestick is spring-loaded to the centre position.

If there were servos under the stick, connected to the other side stick, so instead of being spring-loaded to centre, it was spring-loaded to the position of the opposite stick.
Then each pilot would get feedback on what the opposite pilot was doing, and the application of opposite stick actions could be prevented.

Is that feasible?

EDIT:
As this apparently is existing technology, then is there a reason Airbus is not using it?

Yes. Gulfstream G500 and G600 are the first civilian transport category aircraft to have active sidestick. However, instead of mechanical linkages between the two sticks, the side sticks are coupled electrically. It's much more prevalent in military aircraft.

Source: Aviation Week

So why isn't it being adopted by aircraft manufacturers with side stick controls (e.g. Airbus, Bombardier)? It pretty much all has to do with costs.

Active side sticks introduce more failure modes, such as jam and runaway, which would involve more software and mechanisms to detect and mitigate. These add complexity and introduce failure modes in their own rights, and they all add up to costs.
There is very little certification mandate to make side sticks active. All the objectives, such as stick force/G, stick force/V, have been met by passive side sticks. Airbus introduced envelope protection, most likely not because it was a nice-to-have, but because certifying the combination of lack of aerodynamic feedback on the inceptors and the choice of a pure C* control laws without speed stability demands it.
Certifying the first civilian active side stick is costly (but now it's been done by Gulfstream). There needs to be clear market reasons to do so. For Gulfstream, it may be (personal hypothesis) that the aircraft is being catered to the ultra-rich clientale and a novelty factor. For commercial aviation, the only reason I can think of is safety, which brings us to...
Would active side stick actually increase safety by leaps and bounds? This is a topic under research so the discussion has personal bias. On the A320 and A220, if both PF and CP side stick commands are detected, an aural "Dual Input" warning would sound. Obviously, AF447 crew ignored it. The final BEA accident report didn't mention the lack of tactile feedback as a reason for the crash; rather, crew confusion on stall recovery and lack of airspeed disagreement warning were highlighted.


What's a typical first lesson of glider flying like? What is a typical first lesson for glider flying like? Does it usually include a pre-flight mini lecture on basic theories at all, or if not pre-flight, do instructors usually spend some time on the ground teaching related theories at some point of the lessons?

I understand it may appear as primarily opinion-based to some, but input from instructors or past-students with a similar experience would fall under 'specific expertise' and thus I think insightful answers are possible.

I'm a glider instructor.  The first lesson would have been introductory and if there weren't any soaring conditions, I'm guessing the flight was pretty short, less than 15 minutes, so you probably did/saw a lot less than expected.  It's a challenging instructing environment because of the time limitations on no-lift days.

If you were disappointed with the instructor, note that in glider club operations you will generally fly with all of the club's instructors at different times and you will end up with a couple of favorites.

At least give it enough time to experience a good soaring flight with an instructor to find out what the potential is, and whether you think you can get hooked on the "game" you play with the atmosphere to stay aloft.  Some pilots get into it, some don't and would rather go with power.   Not much different from sail boats vs power boats. Me, I like both but if forced to choose, I'd go with soaring as a fun activity.

Assuming you are into it because you are interested in the sport of soaring, you should hang in until you are solo and get a good soaring flight by yourself.  The first thermaling flight on your own, climbing in many tons of rising air, is an incredible rush, more than first solo in my opinion.  Of course it all hinges on whether you get hooked on the game itself, of working the air to climb.

No two flights are ever the same.  Watch the private owners go out in their high performance machines and disappear for an entire afternoon.  4 hours in a glider is not like 4 hours in a power plane that beats your brains in with noise and vibration; it's like 4 hours of meditation - you return very relaxed and mellow.

Even if you don't think you'll take to the sport, and you plan to learn in power planes at some point, learning to fly gliders first will make you a far better pilot with respect to basic stick and rudder skills, speed control and energy management, so it's worth sticking it out just for that.


If Sweden wants to export JAS39 to a 3rd country, will they have to get permission from the US govt.? Does Sweden manufacture its own jet engine for JAS Gripen, or does it purchase engines from the USA?

JAS39 C/D uses Volvo RM12
JAS39 E/F uses Volvo RM16

I read on the internet that Gripen's engine is a modified American engine by Volvo. Does that mean that they purchase an engine from the USA and then modify it, or, did they purchase the design, and now they are independent for their own manufacturing?
I am curious to know if the USA's 2% export rules apply to these engines. I.e. if Sweden wants to export JAS39 to a 3rd country, they will have to get permission from the USA's govt.

The most correct answer I can give is, It depends. More specifically, it depends on the specific components that are US sourced and it depends on who the ultimate end user is. But the most likely answer is yes.
All technology exports from the US fall under one of two sets of rules. They are the International Traffic in Arms Regulations (ITAR) controlled by the Dept of State and the Export Administration Regulations (EAR) controlled by the Dept of Commerce.
The US engines (F404/F414) that are the source of parts for the Volvo engines are military products controlled by the ITAR. When parts are exported separately from the complete engine, the export controls are set at the part level. So some parts may be controlled differently than others. But, in general any part controlled by the ITAR will require a license from the Dept of State before it can be exported. It is also possible that some components may be EAR controlled and require a license from the Dept of Commerce.
The licenses for export do contain restrictions on use, the end user, and reexport. That is typically spelled out in the individual license. Also, the 'technology' needed to transform the parts into an engine is controlled separately. That includes drawings, software, tooling, etc.
The question of reexport is covered in the ITAR under 22 CFR §123.9 Country of ultimate destination and approval of reexports or retransfers, and 22 CFR §123.10 Non-transfer and use assurances.
Specifically, from §123.9

(a) The country designated as the country of ultimate destination on
an application for an export license, or in an Electronic Export
Information filing where an exemption is claimed under this
subchapter, must be the country of ultimate end-use. The written
approval of the Directorate of Defense Trade Controls must be obtained
before reselling, transferring, reexporting, retransferring,
transshipping, or disposing of a defense article to any end-user,
end-use, or destination other than as stated on the export license, or
in the Electronic Export Information filing in cases where an
exemption is claimed under this subchapter, except in accordance with
the provisions of an exemption under this subchapter that explicitly
authorizes the resell, transfer, reexport, retransfer, transshipment,
or disposition of a defense article without such approval. Exporters
must determine the specific end-user, end-use, and destination prior
to submitting an application to the Directorate of Defense Trade
Controls or claiming an exemption under this subchapter.
(b) The exporter, U.S. or foreign, must inform the end-user and all
consignees that the defense articles being exported are subject to
U.S. export laws and regulations as follows:
(1) The exporter must incorporate the following information as an
integral part of the commercial invoice, whenever defense articles are
to be shipped (exported in tangible form), retransferred (in tangible
form), or reexported (in tangible form) pursuant to a license or other
approval under this subchapter:
(i) The country of ultimate destination;
(ii) The end-user;
(iii) The license or other approval number or exemption citation; and
(iv) The following statement: “These items are controlled by the U.S.
government and authorized for export only to the country of ultimate
destination for use by the ultimate consignee or end-user(s) herein
identified. They may not be resold, transferred, or otherwise disposed
of, to any other country or to any person other than the authorized
ultimate consignee or end-user(s), either in their original form or
after being incorporated into other items, without first obtaining
approval from the U.S. government or as otherwise authorized by U.S.
law and regulations.”


What's the relationship between FANS and CPDLC? I'm confused about Future Air Navigation System (FANS) and Controller–pilot data link communications (CPDLC). Someone said FANS includes CPDLC, another said it does not.

Someone said FANS includes CPDLC, another said it does not.

It depends on the region and/or airplane generation.

There is the ICAO-compliant CPDLC used in Europe (via ATN-B1), and there is the Oceanic/US CPDLC (via ACARS).

A plane fitted with FANS 1/A+ has the Oceanic/US CPDLC, but not the European one.
New long-range planes like the 787 and A350 use FANS 2/B, which has both CPDLCs.$^a$

$^a$ This integration may be offered as a retrofit if it is possible. For example for Airbus – using Airbus' lingo – the A+B package was first introduced on the A350, then became available as a retrofit on the A380 (see slide below by Airbus).

(Airbus via ICAO)

For a brief historical context, see the history section I put together here.


How IATA Code is requested or assigned to an newly built airport ? We have an upcoming airport in our state. An ICAO code is already in place but who and when will the IATA code will get created ?

Airports are assigned IATA codes when an air servicer (airline, cargo operator, etc) requests a code for an airport. The code is requested by filling out a form on the IATA web site.

This is not done by the airport management because it is meant to be for use by a commercial operation, and not the airport itself. IATA codes don't really have a lot of meaning to pilots and other general airport users. They are of use to air travelers and cargo operations as a means to identify airlines, destinations, baggage codes, accounting codes, and traffic documents. It is designed as a standard for industry wide usage, otherwise airlines may use many different codes/abbreviations for the same thing (an airport for example) that don't mix well with other airline systems.

So after your airport is built (or while it is being built), an airline or cargo operation may (or may not) request an IATA code for the airport. Participation in the IATA program is purely voluntary, and it may be that your airport doesn't handle services that conform to IATA.


What Is this on the fuselage of The Fokker F100? What Is  this on the fuselage of  The Fokker F100?

Yes it's the apu exhaust ... there is a post about it:

Where is the inlet for the APU on a Fokker 70/100?


How could I calculate the correct distance from the runway to be able to fly a continuous downwind to final turn? I’m not even sure if this is a safe/allowed procedure at controlled airports so it is purely hypothetical.

Assuming there are calm winds at the field, is there a way to calculate the distance your plane must be from the centerline (see “x”) of the runway while on downwind, so that once you start your turn to base and final thereafter, in a constant rate turn (assume 3 deg./sec.), that you will end up rolling out on the centerline of the runway?

Speed changes (slows) by 27 knots from the beginning of the turn to the end of the turn.

Essentially, if I wanted to execute an 180 degree turn at 3 deg. per second, while slowing the airplane from traffic pattern speed (90 kts) to final approach speed (63 kts), how far away would I need to be from the runway while on downwind before beginning the maneuver? The goal is to be able to use this on any plane given their own specific traffic pattern speed and final approach to landing speed.

Turning at 3 deg/sec, a 180-degree turn will take 60 seconds. If the entire turn is performed at 63 knots, the aircraft will travel 1.05 nautical on a half circle (assuming zero wind). The radius of the circle will be the circumference, 2.10 divided by 2*pi = 0.3342 nm. Therefore, x = 2*0.3342 = 0.6684 nm.
If starting at 90 knots and slowing down to 63 knots, the trajectory will not be a a semi-circle but rather a section of a spiral. The starting radius will be 0.4775 and the final radius 0.3342. In this case, x will be approximately 0.8117 nautical miles. To get an exact value for x, an integration must be performed.


Why does an engine bleed system failure restrict operating elevation? Why are airplanes not permitted to operate to/from airports at over 9,200 ft if Engine Bleed Air Supply System #2 is inoperative?

Without more details on the specific aircraft you are talking about, it isn't possible to give a very specific answer.

In general, if a commercial aircraft is unable to pressurize the cabin fully (usually to <8,000 ft, thanks @jwzumwalt, for both safety and comfort reasons), the operating ceiling is reduced to compensate. The human body doesn't play nicely with high altitudes, so to make sure that passengers and crew don't asphyxiate, the cabin must be pressurized to a safe level.

If one of the engine bleed air systems is inoperative, which among many other things provides for cabin pressurization, the remaining compressed air capacity is diverted to more mission-critical systems on the aircraft. This leaves the cabin pressurization system starved of the compressed air it needs, resulting in it not being able to fully pressurize the cabin to a safe and comfortable level.


Is the lift produced by a wing with a 10° flap equal to the lift created by a wing with 10° AOA but with no flap? After reading this article I would like to know if the lift produced by a wing with a 10° flap is equal to the lift created by a wing with 10° AOA but with no flap?

No, there is a big difference between an entire wing being at an angle of attack and a small part of the wing being at an angle of attack.


What is the lightest jetliner out there? What is the lightest commercial jetliner out there? I was thinking of something like a Embraer E170.

I assume that you want to only compare turbofan-powered aircraft, and exclude aircraft designed to be used as private jets.

In that case, the lightest would be Dornier 328 Jet with a basic operating empty weight of 9200kg, and the next would be Embraer 135 measuring 11402 kg.


Why isn't an LPV SBAS approach considered a precision approach according to ICAO? According to ICAO's classification, LPV SBAS approach is in the APV group, because it doesn't meet some specifications for precision approaches. But what exactly does it not meet?

Glossary:

LPV: Localizer performance with vertical guidance
SBAS: Space-based augmentation system
APV: Approach with vertical guidance

Precision Approaches (PA), as you mention, have specific performance requirements, one of which is how "good" the vertical position measurement must be during an approach. In the case of a PA, ground systems either directly measure the aircraft's vertical position on the glide path (Precision Approach Radar in the case of parallel approaches), or provide a means to follow a precise vertical glide slope via a gauge in the cockpit.

For Approaches with Vertical Guidance (APV), there is vertical guidance, but the aircraft's vertical position is measured onboard the aircraft by either a GPS position source or a barometric sensor. Either of those sensors are subject to measurement errors:

Barometric Measurement Errors: ICAO Annex 10 section 4.4.2.4 defines the error model that is used for barometric altitude reporting systems. The 95% bound of the worst case is +/-282ft.
Barometric Setting Errors: Additional errors are introduced depending on what barometric MSL pressure is selected on the altimeter and how that compares to current (actual) atmospheric pressures.
GPS Measurement Errors: While GPS position sensors provide very good horizontal position, GPS vertical position is generally poor. According to the FAA, the 95% vertical error of GPS systems used in civilian contexts is on the order of 300ft and according to this Advisory Circular (section 5-7), geometric/GPS altitude does not meet requirements for use in air traffic control internationally.

Combining these errors with flight technical errors while flying the approach, the total error you can expect can add up quickly. Compare that to the fact that in the vertical guidance of an ILS improves the closer you get to the ground, you can quickly see why the alternatives of GPS or altimetry don't quite cut it...

So specifically to your question: For an approach to be a precision approach, you need precise vertical position measurements, which an SBAS cannot provide.


Is flying during the day safer than flying at night? Is there any correlation between night or day flight time and rate of accidents?

To me it seems that there should not be a difference. If an airplane needs to execute an emergency landing, it has to do it at an airport which is illuminated at night.

However, what if the pilot needs some ground visuals to navigate, for example, because the instrument are not working similar to the Air France Flight 447.

Edit

I only mean commercial airline flights done by planes such as airbus a320/321/330 or Boeing 737-800.

First, I should reiterate that commercial flights at any time of day are extremely safe.  Saying flights at night are more dangerous is like saying going to buy groceries at night is more dangerous.  It may be true, but it shouldn't be something a traveler worries about.

It seems from the available data that accidents are at least more dangerous at night, even though I can't tell if accidents are more likely. Certain kinds of accidents, like weather-related accidents or disorientation, are also more common at night than during the day.

A great source for this is an incident database that you can download from the NTSB database of aviation incidents (there's a link on the page for downloadable datasets).   Opening the downloadable dataset in Access lets you find statistics for whatever criteria you want. I filtered for records with aircraft operating commercially (not general aviation or commuter) with light conditions (like day vs. night) available.

In this dataset, 1562 incidents occurred during daylight, dawn, or dusk, and 635 occurred at night, or about 29% of all incidents (Incidents include procedural violations and near-misses as well as accidents that cause damage or injury).  To figure out how likely an accident is at night versus the day, we would need to get data on traffic volume that matches the dataset I'm using.  Unfortunately, I don't have that information, but we can look at other statistics to see whether these night incidents are more serious.

Of those incidents, 5.6% of incidents during the day had at least 1 fatality and 8.9% of night incidents had at least 1 fatality.  Also, only 1.6% of incidents during the day had at least ten fatalities compared to 2.5% at night. This confirms that night incidents are slightly more likely to be fatal.  It's also worth noting that among accidents reported as "dark night," 9.7% were fatal, showing only a slight increase from other night conditions.  The percentage of total fatalities that happened at night is a bad statistic because a large amount of those fatalities come from a few large crashes.

In a closely related statistic, 49% of the incidents during the day were not classified as "accidents," compared to 41% at night.  This means the margin between an incident and a full-scale accident is narrower at night.

Some kinds of incidents are also far more common at night than during the day. 27.4% of incidents were marked as involving a weather condition at day versus 38.7% at night (which is surprising considering that thunderstorms and turbulence are less common at night).  Other incident codes that represent a larger share of the incidents at night include "became lost or disoriented," "minimum descent altitude," "crew/group coordination," "airport facilities," "object," and, unsurprisingly, "light condition."  Surprisingly, the following were not unusually common codes for night incidents: "terrain," "anti-ice," and "taxiway lighting" (wait what?).  Some of these codes (like for taxiway lighting related incidents) are so rare that the statistics for them are unreliable.

I looked at the set of flights recorded by the NTSB since 1982 with light condition recorded and FAR part listed as 121, 125, 129, or "NUSC" (I make no guarantees that this dataset is an unbiased representation of all incidents since many crashes are excluded from these criteria).  Dawn and dusk were considered during the day when counting.  I'm counting by plane technically, not by incident.


Why is a small amount of smooth ice over the top of the wing more dangerous than an accumulation on the leading edge? According to a Gleim study guide,

  "Generally speaking, smooth ice on top of the wing is more dangerous than heavy accumulated icing on the leading edge."

Is this true? If this is true, why?

This seems to be from an older Gleim study guide.  It references a now-cancelled AC 91-51A:

  (AC 91-51A) Ice on the leading edge, while seemingly more dangerous, can be removed using deicing systems. Ice on the upper surface is much more difficult to clear from the structure, thus making it more dangerous.

This AC has been replaced by 91-74B, which repeatedly mentions the dangers of ice on nonprotected surfaces, but also points out that:

  Ice on an airfoil can have other effects not depicted in these curves. Even before airfoil stall, there can be changes in the pressure over the airfoil that may affect a control surface at the trailing edge.

and, discussing roll upsets:

  If the flaps are extended, do not retract them unless it can be determined that the upper surface of the airfoil is clear of ice. Retracting the flaps will increase the AOA at a given airspeed.

So current guidance seems to be that ice on the upper surface of the wing is dangerous because of:

The upper surface of the wing generally not featuring an ice protection/removal system, and
The possibility of control surfaces being affected.

but it doesn't go so far as to say it is "more" dangerous.


How do you determine how much "life" an aircraft has left? I am looking to purchase an aircraft, and I've found one described as follows

  Total time is 6300 hours
  Engine time since major overhaul 2100 hours
  Engine runs strong and compression is all in the 70s
  Has VAL Avionics all-in-one VOR/LOC/GS unit

The owner also states that the engine was rebuilt 556 hours ago, but this was never entered in the log books, so we're working based on 2100 hours since overhaul.

We have done a pre-buy inspection through A&P mechanic. On the report was mentioned,

  TACH TIME ;8131.18
  HOBBS ;  7141.9

Can you explain for me what these time figures mean and how I can judge the remaining life left for this engine and airframe?

I'm going out a bit on a limb here, and it isn't specifically an answer to exactly what you are asking (which is what Stack Exchange tends to strive for), but I think this is the answer you need, so at the risk of receiving a couple of downvotes, here goes.

  engine was rebuilt 556 hours ago but was a problem because was never entered in the log books

I think that boldfaced text makes your entire question moot. In other words, the answer to your specific question doesn't really make any difference; the above tells you all you really need to know. Which is:

As Simon said in a comment, if the seller readily admits to not entering an engine rebuild into the log, then what else has not been entered into the log? Especially if you are a beginner, that's the sort of thing I would expect will come back to bite you hard.

Total time is stated as 6300, Hobbs time is stated as 7142, tach time is stated as 8131. And that's by the seller (including the mechanic's report provided by the seller). Does the difference between these figures inspire confidence in the airplane's state, the seller's honesty, and your ability to get an accurate picture of what you are buying?

As has been stated, the only person qualified to answer your question about the state of the engine in a reasonably objective manner is the mechanic who inspected the engine. The odds that this particular mechanic will stumble across this question is already quite low. What are the odds that the mechanic will not only stumble across the question, but also recognize the particular aircraft you are asking about and decide to answer the question with anything that is not already in the inspection report (which you already have received from the seller)?

More generally, with well-serviced aircraft, it isn't as easy as just saying "this aircraft has X number of hours of service left" even based on the current state of a single piece such as the (an) engine, because with good servicing it's perfectly possible to extend the service life of an aircraft for a very long time. A well-treated aircraft is a very different beast compared to perhaps a car (which might be lucky to get a checkup once a year and an oil change every few thousand miles), of course in part because the failure mode is much more serious. (If your car fails on the highway you pull over and call a tow truck; if an airplane fails in mid-flight then what do you do?)

Personally and for the specific situation you describe, I wouldn't go near that particular aircraft if I can avoid it, let alone buy it. At the very least buy something where the trivially obtainable numbers match.


Where can I find climb and cruise performance charts for a specific aircraft? I'm looking for climb and cruise charts for a cirrus sr20 G3, but I can't find them in the POH?  where are these charts typically located?

I don't know much about the Cirrus SR 20 (G3), but here is a manual that has Performance charts in section 5:

[PILOT’S OPERATING HANDBOOK
AND FAA APPROVED
AIRPLANE FLIGHT MANUAL
for the
CIRRUS DESIGN SR20]1

Here is a sample chart:

Hope it helps.


Why is taxiing super slow after an ILS CATIIIB landing in almost zero visibility? I have experienced my very first ILS CAT IIIB automatic landing at my own home airport Lucknow (VILK). I'm super excited but also have myriads of questions related to many unique procedures I saw which I never seen in all my years of flying. Even though I was happy to have experienced such a magical landing, I was afraid how are we gonna park now because it was all white and the taxiing on this flight was the literally the slowest taxiing I have ever seen in my life.
After we landed and I could barely see the taxiway markings and even the runway borderline was hard to see. We crawled to the end of the runway to make a 180 degree and backtrack to vacate via taxiway C. The plane had already slowed down on its initial landing roll and at that time I didn't know about any special procedures or anything like low visibility conditions.
I was wondering why we did not exit via Taxiway B or C or even D or E when we had so many chances to do so and why we had to come all the way to the end of the runway only to go back and exit via C again and park at bay 13?
If any pilots here who have flown in CAT IIIB conditions, I wanna know how hard it is to find the taxiways in such a thick fog?

The reason for using taxiway C is described in the Low Visibility Procedures from the AIP:

4.4.4 At present, TWY ‘C’ and Stand no 11 to 14 are equipped with CAT IIIB lighting system as per short term interim plansubmitted to DGCA by AAI and agreed to by DGCA vide letter no. AV.20015/09/08-AL dated 15-07-2014. Therefore, duringCAT IIIA and CAT IIIB operations, arrivals and departures shall use TWY ‘C’ only.

So the reason you had to taxi via C is because C is the only taxiway with sufficient lighting to support CAT III operations. For CAT III operations, taxiway centreline lights are required. The other taxiways at Lucknow only have taxiway edge lights.
Source: https://aim-india.aai.aero/eaip-v2/eAIP/EC-AD-2.1VILK-en-GB.pdf
The reason you had to backtrack (turn around on the runway and taxi back in the opposite direction) must be that the plane was not slow enough to vacate via C when you passed it the first time. I know you said you had already slowed down, but in thick fog it can be pretty hard to estimate speed, especially when looking out a side window. The pilots obviously have access to precise instruments, and must have determined that the aircraft had not slowed down enough.


What is the pressure on the pressure side in relation to the freestream? The following [edited] images are from a network post:

The top image states on the pressure side, the pressure is greater than the freestream.

On the bottom image, the $C_p$ varies.

According to the pressure coefficient equation:

$$C_p = {p - p_\infty \over \frac{1}{2} \rho_\infty V_{\infty}^2 } = {p - p_\infty \over p_0 - p_\infty }$$

The area marked in blue for the pressure side ($C_p$ in the negative), the pressure is lower than the freestream. In magenta (the leading and trailing areas) it's greater than freestream. (If I'm reading this correctly.)

When all areas are combined for straight-and-level flight, what would the overall pressure be in relation to the freestream? Is the top image still correct with an overall greater pressure?

The question came up in chat and I'm not sure anymore. Of course pressure distribution will vary with speed and airfoil shape, so it's a general question. Any extra information on how speed/shape affect the pressure side is welcome of course.

TL;DR

In most cases the pressure distribution on the pressure-side is higher than on the  suction-side (with respect to the chord-wise location).
There is not general answer to this question since the pressure distribution on the pressure-side is (although mostly close to static ambient pressure) connected to the profile shape.
For low velocities the pressure on the pressure-side is likely to be very close to the ambient static pressure.

Comment on the Figure 2

The depicted airfoil is operating in the transonic-regime. This means the free stream velocity is close to the speed of sound (M=1). Caused by acceleration of the air on the suction side of the profile the air flow will reach supersonic speeds. The rapid increase in $C_p$ at the suction side is a shock.

The following picture shows the Mach-number-distribution around an airfoil in the transonic speed range.

Comment on compressibility

The term $\frac{1}{2}\rho V^2$ is a representation of the dynamic pressure for very low velocities (M<<0.3). Of course this equation can also be used for higher Mach-numbers, however the results (in terms of self-similarity) will contain a large error. Therefore, $p_0 - p_\infty$ should be used to avoid this.

Comment on the pressure distribution on the pressure-side

During level flight (design-speed@design-AoA) there will be a stagnation zone at the leading edge of the airfoil. This results in a pressure higher than the static free-stream pressure. After that the flow will be accelerated resulting in a reduction of the pressure. Often, this reduction will result in a $C_p<0$.
At the trailing edge the pressure of suction- and pressure-side have to be equal, so the pressure rise of the aft-section of the airfoil-pressure side will be adjusted accordingly.

Answer:

For low Mach-numbers (e.i. low free stream velocities) the pressure on the pressure-side is likely to be very close to the ambient static pressure.

Comment on low-pressure-region on the pressure-side

As always with the (elliptic) aerodynamics it's a bit chicken-and-egg. Lets assume (Caution! this is a very simple point of view) there is only one design objective (lift). Then the pressure-distribution on the pressure side needs to fulfil two boundary conditions. 1) high pressure ($C_p$) at the leading edge which is a result of the free stream velocity and 2) the pressure ($C_p$) at the trailing edge which is given by the pressure-distribution on the suction-side. Now, the airfoil-designer needs to find a profile-shape which fulfils both requirements. Whether the $C_p$ becomes negative or not is not of importance, as long as it stays above the $C_p$ of the suction-side and there is no risk of boundary-layer-separation (which is likely to cause missing the pressure at the trailing edge).

  Since there are no simple answers to complex problems, I truly beliefe the best thing would be to install Xfoil (or its successor) and try it out.


Does the "Maintenance Indicator" sign on a METAR imply we can't trust it? As explained in this Q/A:

What does a dollar sign mean in a METAR?

The presence of a Dollar Sign ($) at the end of a METAR is

  included when an ASOS/AWSS detects that maintenance is needed on the
  system.

Does this therefore imply we should not trust this METAR? If so - why do they even continue to publish it? If not, why are pilots informed that the system requires maintenence?

Not necessarily.

Automated stations have a number of sensors for detecting,

Wind speed and direction
Visibility
Obscurations to vision
Cloud coverage and ceiling
Temperature and dew point
Barometric pressure and altimeter setting
Precipitation
Freezing rain (Icing)
Lightning (thunderstorms)

It is likely that some of these are not functioning properly, while the others are working correctly. In such cases, the METAR reports contain sensor status indicators which give information about the sensor which has failed. For example, consider the following METAR:

  KMEM 230853Z AUTO 18014G18KT 10SM CLR 16/M02 A3008 RMK AO2 SLP117
       T01561022 TSNO $

Here, TSNO indicates that the automated station is equipped with a lightning detection system and that sensor is not operating. Following is a list of sensor status indicators:

RVRNO- Runway Visual Range is missing.
PWINO- Present weather identifier sensor (is fitted and) is not operating
PNO- Tipping bucket rain gauge sensor is not operating.
FZRANO- Freezing rain sensor is not operating
TSNO- Lightning detection system sensor is not operating
VISNO_LOC- Secondary visibility sensor is not operating.
CHINO_LOC- Secondary ceiling height indicator and that sensor is not operating.


How often is QFE set on an altimeter during flight? In countries that use QFE rather than QNH (such as China), how often will a pilot set the QFE setting on their altimeter during flight? Is it only when they intend to land at an airfield, or does it get set for the nearest airfield/aerodrome and updated when entering a new airfields's airspace?

I understand that in the US, QFE is primarily used for airshows and aerobatic competitions, which are presumably operating out of a single airfield and therefore wouldn't be changed in flight.

You don't use QFE or QNH in an airspace, you use it for a purpose.

QFE is rarely used in the US, other places in the world it's primarily used for landing, although that's personal preference (I use QNH). You set QNH on the ground at your departure airfield using the local setting then update QNH on route, when approaching an airfield for a landing you would use that airfield's QNH setting as given to you over the radio or from an information broadcast.

The only time I could see setting QFE more than once would be in the case of a diversion, if you needed to fly to another airfield you'd need to go back to QNH, then set the local QFE at the other airfield. This is extra workload and could be a forgotten step, which one of the reasons I prefer QNH for landing - it's one less thing to do.


Could bypass air be used to cool a rocket engine like in turbofans? Rocket engines typically employ a cooling jacket of sorts, usually dual-purposing the fuel as a liquid coolant before it flows into the injectors.

Here is an example turbojet using a bypass. As you can see, the bypass air is far cooler in temperature than the air combustion chamber:

Source

Why are there no designs that use an air bypass (similar to a turbojet) to cool the engine/nozzle? Obviously once the rocket has escaped the atmosphere, a bypass would be of little use. However usually by that stage in a rocket, thrust has been greatly reduced and therefore generated heat as well.

Wouldn't a bypass cooling system make a rocket engine a tad less complex (especially for the first stage)?

Air cooling is not going to contribute much here.

The Shuttle engines were cooled by liquid hydrogen (-253°C) from the fuel supply being pumped up cooling ducts inside the nozzle. Typical nozzle temperature in operation was around 54°C. Yes, fifty-four Celsius. (BBC Engineering Connections - see this Youtube clip)

The top speed of the SR-71 (about Mach 3.2) was limited by the temperature of the air arriving at the engines which, after being compressed in the inlet ducts, reached temperatures around 400°C. (Wikipedia: SR-71)

The shuttle passes Mach 4 about 2 minutes after launch.


What type of aircraft uses this long flashing pattern? 5:00 am every morning I went to Rice-Field Road for a jog. I always see airplanes with flashing lights. All of them have the same flashing patterns except for the one that I just saw yesterday.

The pattern is a long flash. If I'm not mistaken (I counted it), it flash slowly for more or less 10 seconds and then gone at same count.

I was curious and surprised that there's an aircraft that has that kind of flashing patterns. I gazed at it for a minute until it was out of my sight.

I also found this and I'm not satisfied since I'm more concerned of what type of aircraft was it.

What type of aircraft uses this long flashing pattern? References might also help. Thanks in advance :)

UPDATED:

This was the exact location where I saw it.

I'm not so familiar how high or low an aircraft might be for a landing. But I guess the aircraft was too high for a landing and its direction was straight away from a landing area. And yeah, the flash was color white.

I'm assuming your question is : "What type of aircraft was it?".
Actually, we can't answer your question but you could try using Flightradar to identify the aircraft by watching a playback of the regarding time and place.

To hand you some trivia: If it was a red flashing light (the beacon) it is totally normal that the kind of flash varies. Modern aircraft use 360 degrees flashing lights (depending on the manufacturer it flashes faster or slower). Older (and most small aircraft) use rotating beacons which is just a red light shining in one direction which is turned by an motor (turns faster or slower depending on the motor).

If it was a big white light it could possibly have been the landing lights (which flash only on some aircraft) or the strobe light in the wings outer edges. They usually flash 2 times in one second and repeat the two flashes after 1 second of pause.


How can I reduce the Short Period frequency of a Pitch Hold Auto-Pilot while maintaining stability? I am working on a  Pitch Hold Auto-pilot for F-104, which has the following damping characteristics;

which clearly shows that it has degenerated and unstable phugoid mode. well, I made a SAS which consists of 2 loops / Feedbacks.
1) The angle of attack feedback to increase the Cm alpha or static margin of the aircraft
2 Pitch rate feedback to dampen the short period mode.
but it has eventually increased the natural frequency to almost 365 rad/s as shown below,

but this frequency is not according to the handling qualities as it should be less than 3 rad /s.
what feedbacks should I try so that all the states are stable as well as desired handling qualities can be achieved?
states are velocity, angle of attack, pitch, pitch rate.
Simulink Model of SAS:

Root Locus of the outer Loop:

How can i decrease the frequency to the desired range keeping it stable and keeping in mind that it was unstable first.
Also, with not much decrease in damping as it should be greater than 0.5.(short period mode)
Phugoid mode >0.04

You seem to have too high an alpha feedback. For intuitive understanding, alpha feedback mainly increases the pitch stiffness, analogous to increasing the static margin; pitch rate feedback mainly increases the pitch damping. For your purpose, you would need to significantly reduce the alpha feedback.

For example, with your provided state space model,

$$\dot{x}=Ax+Bu$$

using a linear feedback,

$$u=Kx$$

where

$$K=\begin{bmatrix}0 & 0.5 & 0.25 & 0 \end{bmatrix}$$

satisfies your damping ratio requirements. SP frequency is 3.9 rad/s, SP damping is 0.53, Phugoid frequency is 0.023 rad/s, Phugoid damping is 0.14.


How did the service ceiling certification change in the 70s? According to this video (at 4:30-4:45) the "later" Cessna T337 and P337 were limited to 20,000 feet whereas the older models "had no such limitation - a quirk in changes in the certification standards that took place in the 1970s."

What were these standards changes, and why? Did this have something to do with concerns about hypoxia? (For the record, I DO know that pilots use supplemental oxygen at high altitudes in non-pressurized aircraft.)

I'm assuming this video refers to U.S. regulations (i.e. FAA), but are similar laws in place elsewhere in the World?

Also, I have already read this question - it appears the FAA regulation prohibits exceeding the certified ceiling only if the POH says so (according to @pondlife's answer). I did not know that...

I did find a quirk alright. First off the ceiling is either 18,000 or 19,500 feet depending on the source. The video's origin is unclear, but my research hints at the narrator reading an AOPA piece (can be found on superskyrocket.com via web.archive.org):

(...) later T337 and P337 models are only certified to 20,000 feet, whereas the earlier turbo models have no such limitation, a quirk of changes in the certification standards that took place in the 1970s.

The Skymaster's certification falls under Part 23. The relevant amendments I found are 7 and 9, from September 1969 and June 1970, respectively.
Amd 7 (maximum operating altitude):
First limited all Part 23 to 25,000 feet (23.1527). Unless they met a quirky requirement (23.775e). I more than doubled checked it's the right one:

(e) The windshield and side windows forward of the pilot's back when the pilot is seated in the normal flight position must have a luminous transmittance value of not less than 70 percent.

I could not find how the FAA measures the luminous transmittance. But higher pressurization requires stronger windows, it could be as quirky as that. Perhaps the 60s technology couldn't produce clear[er] windows while staying strong.
Amd 9 (oxygen equipment and supply):
Is less quirky, but there is a lot of fine print (23.1447), of which:

(c) If certification for operation above 18,000 feet (MSL) is requested, each oxygen dispensing unit must cover the nose and mouth of the user.

That means proper masks, and for the crew's masks to have built-in microphones connected to the airplane's radio:

(4) If radio equipment is installed, the flightcrew oxygen dispensing units must be designed to allow the use of that equipment and to allow communication with any other required crew member while at their assigned duty station.

That's asking a lot from a light Cessna, and if higher:

(e) If certification for operation above 30,000 feet is requested, the dispensing units for passengers must be automatically presented to each occupant before the cabin pressure altitude exceeds 15,000 feet.

An automatic dispensing system is not something one would find in a Skymaster.

In short, the candidates from the 1970's are the window materials, and the oxygen supply, limiting it first to 25,000, and then to 18,000 feet.


Will landing gear with wheel fairings keep spinning due to airflow? I asked this in a side note to a previous question, but I figured it warranted its own question.

If a fixed gear aircraft is fitted with wheel fairings, even if you tapped the brakes after lift-off, would the wheels start turning again? With typical wheel-fairings I guess about one third to one quarter of the tire sticks out into the oncoming air flow, and would face perhaps 120 kts wind. All aircraft tires I've seen have grooves running along the direction of travel, with nothing going sideways that would be any obvious wind catcher, so I guess they're pretty much slicks in this regard.

Or would any 'normal' amount of friction in the bearings keep them from turning?

This is a very general question. So the answer is:

Sometimes they spin, sometimes they don't.

Have you ever seen a car with a bicycle on a trunk rack, and the front wheel is spinning. Sometimes they spin fast, sometimes slow, sometimes not at all. It just depends on the air flow.

Airplane wheels are the same way, and it all depends on the airflow, the tires and the bearings. The airflow over tires sticking out from wheel pants is going to be turbulent and unpredictable. Sometimes it happens, sometimes it doesn't. I've seen both.


Why are test crashes not made harder on the airframe? This is one of the videos of the crash: 727 Test Crash. It seems it was more of a hard landing without gears. Why wouldn't the experimenters have made it more of a really hard crash if they needed to study the effects of a crash on a plane? Would slamming down the plane really hard (maybe even nose down) given a more realistic scenario and more valuable data rather than making it soft?

If the plane crashes with high vertical speed, the decelerations involved are so insanely huge that there is absolutely no way to make it survivable. That is well explained in
Even after years of research, why are planes unable to keep passengers alive in case of a fiery crash?.

However in many cases the aircraft is still, at least partially, controllable, so a crash-landing can be made. And that is the case where survivability can be improved by careful design, so that is the case they were testing.

This is also relevant for accidents on landing where the aircraft simply does not have that much vertical speed. Together these scenarios cover more accident cases than the out of control scenario.


What is the difference between planned true airspeed and actual true airspeed? My electronic E6B does calculations for both planned and actual true airspeed (yielding different results). I was told the difference has to do with assumptions about temperature, but the manual doesn't give any details. What is the difference between the two?

It's very possible that your flight computer's TAS calculation is expecting calibrated airspeed rather than indicated airspeed; the difference between these two can often be enough to affect your results by a meaningful amount.

Most aircraft do not have a formula for the conversion, or enough information to convert airspeeds other than those specified in the manual. This means that the "CAS" used for TAS calculations is often actually IAS.


Airfoil and Wing Analysis for Transonic Speed (M = 0.85 and Re = 42 million) I am working on a project where I have to design a plane which has to cruise at 0.85 Mach at 35,000 ft. While working on the wing design, I quickly discovered that 6 series airfoils, with a good amount of thickness ratio, have a low Critical Mach Number. So, I started exploring Supercritical Airfoils (SC Airfoils). F or doing analysis of SC airfoils can XFLR5 be used?

Moreover, Aspect Ratio and Sweep also influence Critical Mach Number.

Is there are a relationship that connects these three entities?

Lastly: is there a credible source that compiles the analysis of Transonic Airfoils (Supercritical Airfoil)?  (Similar to Theory of Wing Sections by Abbott and Doenhoff, which contains data for conventional airfoils)

I am working on a project where I have to design a plane which has to
  cruise at 0.85 Mach at 35,000 ft. While working on the wing design, I
  quickly discovered that 6 series airfoils, with a good amount of
  thickness ratio, have a low Critical Mach Number. So, I started
  exploring Supercritical Airfoils (SC Airfoils). For doing analysis of
  SC airfoils can XFLR5 be used?

This is a direct copy-paste from XFOIL manual which is the underlying library for 2D analysis in XFLR5

One should always be wary of trusting solutions which show regions
of supersonic flow.  Such flows can be reliably predicted only with
a truly nonlinear field method (such as the MSES code).  As a rule
of thumb, if the maximum Mach number doesn't exceed 1.05 anywhere,
shock losses will be very small, the Cp distributions will be
reasonably accurate, and the drag predicted by XFOIL is likely
to be accurate.

  Moreover, Aspect Ratio and Sweep also influence Critical Mach Number.
  Is there are a relationship that connects these three entities?

AR does not influence the Critical Mach Number. However, sweep does
A direct copy-paste from old USENET message archive.

The airfoils on a swept wing behave as though they were flying at a
reduced speed, reduced Mach number, and reduced dynamic pressure.

effective speed = V cos(L)
effective Mach  = M cos(L)
effective q     = 0.5 rho V^2 [cos(L)]^2

where L is the sweep angle, and V and M are the airplane's speed and
Mach.

  Lastly: is there a credible source that compiles the analysis of
  Transonic Airfoils (Supercritical Airfoil)? (Similar to Theory of Wing
  Sections by Abbott and Doenhoff, which contains data for conventional
  airfoils)

NASA supercritical airfoils: A matrix of family-related airfoils

  I tried using XFLR5 for the analysis at Re = 42 million and 0.85 Mach
  Number. But, solution diverges

There was 2 issues I could see in the given dat file.

finite TE gap and coarse coordinates.
I fixed this in XFOIL by going to GDES menu and setting the TGAP to 0 with the blending distance 1.
And then used MDES to filter out glitches in the UE distribution.
Mach effects.
Say your plane is flying at CL=0.6 but what happens when the MACH increases with constant alpha is, it improves CL. So the non-converging solution actually refers to a very high CL in the ranges of 3-4 which is not realistic and cp is high above the critical CP. Please refer to the images attached. when you directly use cl or reduce alpha accordingly you can get this sorted as shown on the last pic.


Is there a difference in aerodynamic efficiency between single engine and twin engine airplanes? I've learned a lot of great information about aviation on this website. One insight I learned from a poster here is that, all else equal, for every doubling of engine power that you give an airplane, speeds increase by about 25%.

However you're not always comparing "all-else-equal" airplanes, for example sometimes I want to compare between a single engine airplane a twin engined airplane. Yes the second engine adds weight and drag, but the factor that I have the most trouble accounting for are the difference in aerodynamic requirements between the airplanes.

My understanding is that regulations require all single engine airplanes to have a stall speed of no greater than 61 knots, while a twin-engine plane could have an arbitrarily higher stall speed. The higher stall speed is directly correlated with higher aerodynamic efficiency, meaning that it should be able to achieve higher speeds than a single engine airplane of the same mass, drag, and power, would be able to achieve.

So I am looking for a simple rubric with which to be able to compare single and multi engine airplanes. I think that there are really two parts here: (1) identify realistic stall speeds for the multi-engine airplane and (2) formula for comparing top speeds between planes with X mass and drag and Y power and Z stall speeds.

Any ideas?

Your question really needs further bounding to be able to accurately answer.  I will discuss a few aspects.

First @ratchet freak is right on the money.  What engine mount are we talking about? A podded engine on the single and twin (or multi, since your one of your questions mentions multi) or some different configuration.

In the late 70's and early 80's there were a bunch of academic and not so academic papers and articles written on the topic.  Generally the focus was on the Cessna 336/337, O-2 and Defiant type aircraft.  There were also papers on a loss of engine scenario.  This got rekindled for the Voyager aircraft design and promotion.  The Voyager had a L/D of 27. And it was intended that one engine shut down in cruise flight.

The 61 knot rule for single engine is said to be influenced by crash survivability. I get that, but I will editorialize that several studies have shown that the chance of something bad happening with the loss of an engine is greater in a multi.

Top speeds, stall speeds and mass (weight) are different parameters, subject to different trades.  Furthermore, putting this into the context of existing, production or some arbitrary subset of the universe makes your question hard to answer.

The real answer is that one can design for what you need.  You can have a very slow stall speed in a single or a multi, what would you like?  You can have a nice L/D with any number of engines, what would you like?  And you can haul lots of stuff, although there are not heavy singles around. But to generalize all three parameters, of these non-linear trades, into one rubric, would likely challenge even the most mathamatically talented aerodynamcists that I know.  If you can tighten the bounds on the problem, you might get a better answer.


How are the wings of a mono- bi- or triplane referred to with regard to position? Apologies if this is an elementary question. If I look at a diagram for parts of a plane, they don't distinguish verbally between left/right. If I try Googling for, say, "right wing" or "left wing", well you can guess the result!
Suppose I wish to identify a feature, say, the topmost left-side wing of a triplane, how do I do it?

While boats use port & starboard, aircraft (at least in the U.S.) generally don't.  "Left" and "Right" work well for most things; when talking with flight attendants (on a large aircraft), "captain's side" and "first officer's side" serve the same purpose as "port" and "starboard" (i.e. the port or captain's side is the same side of the vehicle, regardless of whether it's on an individual's left or right at the moment).  For maintenance purposes, the engines and certain other left/right-side components are referred to as the #1 and #2 (and #3 and #4 if you have them), and when talking with a pushback crew (who are in front of the airplane facing it -- so their "left/right" is opposite that of the pilots) it's common to refer to "the #1 side" (i.e. captain's) or "the #2 side" (i.e. F/O's).  When we would back up the C-130, the loadmaster on the cargo ramp looking behind us would direct "turn toward #1" or "turn toward #4", which worked out nicely.
But, I don't think I've ever heard of wings referred to as #1 & #2, or really as anything other than left & right.  (Those who've flown on aircraft carriers may report a different experience while at sea, I've not done that.) I'd say that the most general case would be to refer to a left/right wing, or on a biplane left upper, right lower, etc, and in a triplane, I'd have to assume left middle (or, perhaps less likely, left center) wing.
The Nav lights are generally referred to as left or right, at least in my carrier's operation.  You could refer to them as "the red nav light" or "the green nav light" but I don't think that's particularly common.
Various different organizations will have their specific conventions, but as a general case, plain (or, "plane") English works well & is a good place to start until the preferred local terminology is introduced.


What is required for an EASA pilot to fly an airplane with retractable gear, flaps and variable pitch prop? Here is the FAA's complex airplane definition:

A retractable gear (not necessary for a seaplane).
In-flight adjustable flaps; and.
A controllable pitch propeller.

How is this airplane defined under EASA and what is required to fly it? A pilot with EASA PPL is able to act as a PIC with retractable gear and flaps; or flaps and controllable pitch. Can he fly an airplane that has all three elements without any additional ratings or endorsements? Or are there any other requirements to fly this type of airplanes?

I can't find any equivalent definition for what the FAA defines as a complex aircraft for the purposes of endorsements. EASA defines a complex aircraft completely differently.

“‘complex motor-powered aircraft’ shall mean:
(i) an aeroplane:
with a maximum certificated take-off mass exceeding 5700 kg, or
certificated for a maximum passenger seating configuration of more
than nineteen, or certificated for operation with a minimum crew of at
least two pilots, or equipped with (a) turbojet engine(s) or more than
one turboprop engine, or
(ii) a helicopter certificated:
for a maximum take-off mass exceeding 3175 kg, or for a maximum
passenger seating configuration of more than nine, or for operation
with a minimum crew of at least two pilots, or
(iii) a tilt rotor aircraft;”


what would happen if we use needle shaped nose in blended wing? We are using blunt shaped nose in blended wing,instead of blunt,if we use needle shaped nose what would happen ,it reduce shockwave or it produce bow shock?

I assume you talk about the airfoil's nose, not the fuselage nose.

A pointed nose will cause flow separation in subsonic flow if the angle of attack is only slightly different from the angle of the leading edge. This flow separation will mightily increase drag because now no suction peak at the nose will develop. This suction peak is causing what is sometimes called "leading edge thrust" (PDF!). Without it, the aerodynamic forces will act in a direction orthogonal to the wing's surface and not (almost) orthogonal to the flow direction.

In pure supersonic flow the pointed leading edge is superior, because it causes less wave drag. However, it is preferable to sweep the wing such that the flow around the leading edge will stay subsonic even in supersonic flight, precisely because it still allows leading edge thrust to manifest itself.


Do airplanes with fixed gear usually turn upside down during water landing? During discussion about emergency landing on water in this YouTube video (4:00) the flight instructor said:

  "...the problem with these airplanes with a fixed gear, you're usually
  going to be upside down".

Is it really like this? If so, is there any water landing technique which could minimize the risk?

Depends on the airplane.  I recall reading about a guy flying across from the Aleutians to Japan, in a Piper Super Cub of all things, and he had to ditch it for whatever reason, which he was able to do next to a freighter.

To his surprise, it didn't flip over but kind of submarined, stopped, then popped to the surface nose down but upright. It's possible being a taildragger helped.  I think tricycle gear airplanes would be more prone to flipping right over because the pitching moment from water contact is a little stronger with 3 gears hanging down vs two.

I expect though, that if you surveyed ditchings you would find some do and some don't.  Pitching moment from water contact, elevator effectiveness in resisting the pitching,  amount of up elevator at water contact, speed at water contact, maybe the surface contours of the fuselage could promote it or inhibit it, all kinds of variables.

As xxavier says, it would be prudent to assume you are going to flip over, and if you don't, bonus.


Why are high-bypass turbofans faster than turboprops? The turboprop, and especially the high-blade-count propfan, differs from the high-bypass turbofan principally in throwing away the draggy outer ducting. This makes it easier to obtain a greater diameter, moving more air slower, and hence higher efficiency. Fewer blades further increase aerodynamic efficiency. Tip speeds remain much the same, limited by the speed of sound. So why are turboprops significantly slower than high-bypass turbofans?

Tip speeds do not actually stay the same, turbofan (and turbojet) blade tip speeds can exceed the speed of sound, up to Mach 1.3 (actually stated M1.4 in the link, my experience is M1.3 but also my professor used to mention M1.4) as seen in link. As they are enclosed, the negative effects of the shock are much smaller than if it were an open propeller. Open propellers (and also propfans) are also affected by aeroelastic effects in transonic speeds which are very hard to isolate with their high span-to-chord ratios (I don't like calling it aspect ratio for propellers). Not to mention the NOISE that an open propeller generates at transonic speeds.
Additionally, due to the fan being followed by a stator (which uses the rotational component of velocity added to the airflow by the rotor) and the converging fan duct geometry, turbofans accelerate the flow further than propellers do. The exhaust from the turbine is also faster than the fan and constitutes a greater fraction of the air ingested by the engine, which increases the average exhaust velocity of the engine.
Imagine a turboprop flying at Mach 0.8 and the propeller tip speed being limited to Mach 0.9 . This requires the propeller tip to have a tangential velocity of M0.41 (Pythagorean sum). On the other hand, at the same flight speed, a fan blade tip limited to M1.3 can still have a supersonic tangential velocity of M1.024 . It is obvious that a fan blade turning more than twice as fast can exert greater forces on the airflow.
The thrust is generated by accelerating airflow. We can either accelerate a lot of air by a little bit (think of a helicopter rotor) or a little air by a lot (afterburning turbojet). A relatively slow-moving propeller blade (due to the reasons above) can do little to accelerate airflow going through it, relative to a fan that turns much faster in an enclosure.


Are helicopter with coaxial rotors equiped with blade flapping mechanism? Helicopters with coaxial rotors such as the Ka-27 don't need mechanisms to overcome dissymmetry of lift as there is advancing blades on both sides of the helicopter. A common mechanism to overcome the dissymetry of lift on classic helicopters is the blade flapping as described here.

I see no need to introduce blade flapping on coaxial rotors helicopters, but there can be other reason to keep it I don't know.

Are helicopters with coaxial rotors such as the Ka-27 fitted with blade flapping mechanism?

Yes and no, but mostly yes, especially the Kamov family.

Here's a picture(Wikipedia, CC-SA) that shows the (pair of very complex) flapping mechanism on Ka-27. Even though two rotors together should be able to balance out the asymmetric lift, the rotor hub and rotor for each individual rotor isn't rigid or strong enough for handle its own asymmetry (when it happens accidentally, e.g. the receding blade stalled, the receding blade of the top rotor will bend down and hit the advancing edge of the bottom rotor bending up and cause a catastrophic failure, e.g. Ka-50 had a series of rotor collision accidents).

However, there are rigid rotor helicopters and these don't bend nearly as much and don't need to flap, but none of them are among the Kamov lineage.

This is especially useful for high speed helicopters (e.g. Sikorsky S-97 Raider) because no amount of flapping will generate enough lift before stalling to cancel out the lift asymmetry, for the receding edge is too slow, so it has to let each rotor remain asymmetric and let the two rotors cancel each other.


Why should I request a long landing? I fly in/out of a small international airport occasionally, the runways are quite long. It is not unusual to land half-way or even three-quarters down the runway.

When doing this, pilots request long landings from ATC. I once did not request a long landing and did anyway and got chewed out a bit from ATC.

My question is:

Is it mandatory to request a long landing or is it more of a courtesy? Where is this information documentated?

There's no mention that I can find of "landing long" ("landing deep" is sometimes used outside the US) in the FAA's P/CG, AIM or ATC procedures. So it's fairly safe to say that it's an unofficial instruction (assuming I didn't miss it somewhere).

The main reason for doing it is to avoid a long taxi after landing, but I suppose there could be other reasons too, like coming in too high on a long runway or avoiding wake turbulence. The point of requesting it - and not just doing it - is that it may mess up sequencing at a busy airport because you take more time to clear the runway, with the result that the tower has to instruct the aircraft following you to go around. As my first instructor said, "once you're cleared to land it's your runway".

I've never been asked by ATC to do it but I have been asked by the tower to "land short", to minimize my time on the runway and let an airline flight take off ASAP. However, it was clearly phrased as an informal request, not an instruction; something like this:

  N12345 Clear to land 36L, appreciate a short landing if you can manage it

So I think the conclusion is that landing "short" or "long" is simply an informal way for pilots and ATC to make things work a bit more smoothly.


What do you need to do to bring a 737 Max back in service after 6 months in storage? What kind of maintenance has to be performed on an aircraft if it has been stored for 6 months?

For example, do fluids need to be changed, have the tires become flat, etc.

Bringing any plane back into service will involve more or less the same things but vary in scope and scale depending on the size of the aircraft. It looks like Southwest is moving their 737's to their long term storage facility. Since its in the desert they may wrap/plug the engine inlets to prevent any debris/dust from getting in. This will need to be removed before the aircraft are brought back into service.

Aside from consumables like fuel, oil and hydraulic fluid, etc which are checked before every flight they will need to check all calendar-related maintenance items. Most aircraft maintenance items are tied to hours or cycles which don't really matter when the airplane sits, but some things are tied to the calendar and a sitting plane may lapse on checks and updates it needs. Off the cuff a plane sitting for a while may lapse or need replacement on:

Bi-annual Transponder Checks
Navigation/Terrain Database updates
Fire extinguishers past check/expiration
Expired Oxygen generation canisters
Expired slides/lifejackets (mostly the chemical inflator systems)
Batteries may need tending/replacement if run dead (or allowed to be run dead)

From a non-hardware standpoint, crew currency or type proficiency due to lack of flying hours on the airframe may land a chunk of pilots and FO's in the sim before the planes come back into service.

Jet fuel can grow bacteria in the tanks and a tank flush/purge may be done on aircraft that have been sitting for a long time depending on what they find when they get to the aircraft.

-----EDIT------

Since more information has been released since the original answer was posted. It now appears most airlines are expecting 100 - 150 hours of work to bring the current resting 737's back in to service after the software update gets approved. This only applies to the mechanical aspect, pilot training is still under debate:

  each aircraft will likely require between 100 and 150 hours of preparation before flying

  ....

  The preparations were discussed at a meeting between Boeing and MAX
  customers in Miami earlier this week, and include a list of items
  ranging from fluid changes and engine checks to uploading new 737 MAX
  software. The estimated time frame does not include pilot training,
  they said.


Is it better to shut down the engine(s) or leave it (them) running when a gear up landing is imminent? Is it better to shut down the engine(s) or leave it (them) running when a gear up landing is imminent?

I noticed that the pilot in this video chose to shut down his engine before touching down during a  gear up emergency landing.  However, in Pilot Operating Handbooks, the recommendations seem to be mixed.  For example, the Cessna 210 POH, Page 3-11 says:

  Touchdown -- SLIGHTLY TAIL LOW.
  Mixture - - IDLE CUT-OFF.
  Ignition Switch -- OFF.
  Fuel On-Off Valve -- OFF (pull out).

On the other hand, the POH for the Piper Arrow III PA28R-201, Page 3-14 recommends shutting down the engine before touching down:

  3.lSb Gear Up Emergency Landing (3.Sd)
  When committed to a gear up landing, CLOSE the throttle, move the
  mixture to idle cut-off, and shut OFF the ignition, battery master (BATT
  MASTR), and alternator(ALTR) switches.Turn OFF the fuel selector valve.
  Seat belts and shoulder harness should be tightened. Touchdown should
  normally be made at the lowest possible airspeed with full flaps.

What is the reason for this variation?  Is there some advantage to touching down one way vs. the other?

I've have noticed this question:

  How to execute a gear up landing on aircraft with low propeller?

However, I do not believe this question duplicates that one. While consideration of the propeller would be one factor in deciding whether or not to shut down the engine, it certainly would NOT be the only one.

There's two schools of thought. First is the keep the engine on school because keeping the engine on:

Gives you more control of the airplane
Gives you the option to go around
Keeps the procedure simple and allows the pilot to concentrate on a good touchdown
Keeps the dynamics of the airplane predictable and familiar to the pilot

The shutdown the engine school's view is that shutting the engine down lowers the fire risk by cutting off the fuel and engine ignition spark.

Personally I'm with the keep the engine on school of thought. The last thing I want to be doing when I'm trying to make a smooth landing is reaching for all sorts of knobs and levers, and having to reach the fuel cutoff valve in the inconvenient places that piper likes to put them, and then learning how the airplane behaves without engine power for the first time. As soon as I touch down I am pulling the mixture, cutting off the fuel and killing the mags and the master switch, but until then I'm going to concentrate on making it the smoothest landing in the history of manned flight.


Where do flight sims get their data on aircraft flight characteristics? Flight sims offer very realistic simulations of how a particular aircraft feels, claimed by pilot interviews on youtube.
But how do sims get their data?
First, does all this data even exist, i mean companies have data on how the aircraft changes control for example, due to some wind turbulance, or the slight turn of the steering?
Second, how do companies get this data? Particularly for fighters.
Third, how do they manage to offer such a realistic experience?
Are they in direct communication with the aircraft companies / goverments?
Isn't data like this confidential?

It comes from a few places and it depends on the type of simulator you are talking about, while the data is not necessarily "confidential" I would call it proprietary although much of it is actually out there:
The Public Domain: Home simulators like X-Plane offer the ability to build your own aircraft models and there are many readily available for sale. People may build these planes experimentally or based on the associated POH information. In many cases the airfoil information for light singles like the Cessna 172 is fairly readily available since its based on a specific NACA airfoil.
The aircraft companies: Since a great deal of training occurs in D-Level simulators these days (because its cheaper than burning Jet-A) the aircraft manufacturers may also build simulators for the real aircraft they build in this case they have the engineering data on hand to build the models.

A great deal of the "experience" also has to do with things that have nothing to do with the aircraft flight model. Things like weather simulation, graphics, airport markings, radio chatter, cockpit sounds, button availability, instrument response and avionics functionality. When I fly my sim its just as important to me that I can simulate the same GPS unit (Garmin 430) that I fly with as is the ability for the 172 to fly like it does in real IFR conditions.


Why do some non-precision approaches have stepdown fixes after the final approach fix? Some of the non-precision approaches have a stepdown fix after the final approach fix. My understanding of a stepdown fix with an altitude restriction is that it is to keep you clear of obstacles before continuing your descent right? However, why does it so happen that some stepdown fixes are after the final approach fix rather than let the final approach fix be the "stepdown" fix of an approach?

Take a look at KPRB VOR Rwy 19:

There is a stepdown fix after the final approach fix. Now look at the terrain around the airport. You can see a bunch of small hills around the FAF. In order to provide adequate terrain separation and get you low enough to land, they need a stepdown. You can also see this on lots of non-precision approaches where the approach minimum is lower if you can identify a point past the FAF.

If you are interested in how they decide to include a stepdown fix, check out FAA Order 8260.3C TERPS, Section 2-6. Final Approach. Lots of formulas but the gist of it is they have a required obstacle clearance that must be met and maximum descent angles. Sometimes they need a stepdown fix in order to make the approach work.


How the aircraft sense the sink rate during autoland? Radio altimeter is required for autoland, and the target of autoland flare mode is to reduce the sink rate to an acceptable sink rate at touch down， then how the sink rate is computed by the radio altimeter since radio altimeter just provides the height information rather than the rate?

The radar altimeter doesn't calculate the rate of descent. A radar altimeter only gives height information, but it can take many readings very quickly and feed that data to a computer which will calculate the rate of descent.


Do aircraft owners need to purchase insurance? As far as I know, most (or all) airplane owners purchase insurance that covers liability, if nothing else. Good idea or not, is aircraft insurance legally required in the US, in the way that automobile insurance is required?

Aircraft insurance for personal aircraft is not legally required by any part of the US FARs, nor to my knowledge is it required by any state government, though I'm only familiar with New York in that regard. (As you'll note from the Wikipedia page you linked to, not all states require automobile insurance either, though most do now...)

Aircraft insurance is most directly comparable to marine insurance.
It's generally required by:

Your lender (if you didn't pay cash)
They want to be sure you have hull coverage so if the plane is damaged they still get paid.
Your FBO or Airport Management.
As a condition of having a tiedown/hangar most FBOs require some form of liability insurance in case you taxi into a building or another plane.
Good old-fashioned common sense.
A light piston single is unlikely to take out a building, but if you have an incident costs can rack up fast. Having someone else to step up and pay those big bills is beneficial.


Where can I find airspace maps for Turkey? I am trying to learn more about airspaces. It would be nice if I had the airspace map of my region to look and observe.

skyvector.com shows very few information about Turkey, not useful.

What you need would be named "ICAO 1/500,000 map", this is a VFR map with airspace class, based on a Lambert conformal conic projection. Example in the US (1/250,000):

The best I can find online is SkyVector, but you said it's not sufficient for you needs:

By searching carefully you may find things like that:

(source: University of Texas)

But these are 1/1,000,000th very old maps from US DoD.

An alternative would be to use the online GIS viewer from ICAO, selecting carefully layers and data to display, my trial at that:

It must be improved to suit your needs, but I don't expect it to be better than SkyVector. By clicking on a location, you can display all the information about relevant areas (description, limits, etc).

For enroute, Eurocontrol provide two maps:

High altitude
Low altitude.

Good for intersection points and routes.

On the airport plates side, the AIP for Turkey is not in public access, but several sites provide plates, e.g.:

Turkish Virtual Airlines (static)
RocketRoute (interactive)

AirportNavFinder provides link to other sites like SkyVector.

IVAO Turkey could be a good point of contact to discuss local documentation. They have a section Charts accessible to registered users.  - VACC turkey is another virtual network.

If you can provide additional requirements, me or someone else may provide better material.


Why are the guns on the AC-130 not mounted on the underside of the aircraft? Why must the guns of the AC-130 be on the side forcing the plane to circle the target. Would it not be more practical to have the guns underneath the plane shooting downwards?

By putting the guns on the side, the gunner can direct a continuous stream of bullets at the intended target for as long as the pilot wishes to continue circling it. If the guns were belly-mounted, the pilot would have to make multiple passes at the target zone from different directions, with long pauses between them- long enough for the enemy soldiers to reposition themselves and escape.


Do airlines use a software to calculate center of gravity? How do airlines calculate center of gravity? I have read in several airlines websites that they optimize the center of gravity position, to improve aerodynamic performance and reduce fuel consumption. Do they use some kind of software to do this?

Most airlines of any size use software to calculate the c.g., either their own software or vendor software.

Passenger aircraft weight & balance is relatively trivial compared to cargo aircraft w&b, where such things as floor loading, lateral imbalance, and monocoque limits can come into play.

http://www.champ.aero/ is one provider of that service and software for cargo aircraft for those companies not wishing to program their own.

Fuel efficiency is one advantage of loading to given c.g. An oft-used method is to load to a specified zero fuel weight c.g. For example, on 747 aircraft through the -400 models, a zfw c.g. of 26.6% mac is a typical target.

If you want to see and play with a free, online, open-source w&b system for cargo aircraft, go to http://terryliittschwager.com/WB/index.php.


Am I correctly calculating the density altitude? I came across the following question while studying for the FAA PPL written test:

Determine the density altitude for these conditions:
Altimeter setting: 30.35
Runway temperature: +25°F
Airport elevation: 3,894 ft. MSL

I performed the following steps:
Temperature (°C) $\color{red}{= 5/9 × 25} = 14 $
Pressure altitude (PA) $= 3,894 + (29.92-30.35) × 930 = 3494 $
Density altitude (DA) $= \text{PA} + 120(14 - 15 + .002×\text{PA}) = 4200 $
Yet that answer is not listed, and according to the DA/PA graph provided, DA is 2000ft.
My final formula seems to be correct according to Wikipedia, plus it also matches my intuition of how PA works: According to the question we have 14°C at 4000ft, as opposed to the expected 7°C at 4000ft; seeing as temperature is higher, DA should be higher than PA.
Where am I going wrong?

Check your temperature conversion: 25F is -3.9C...

Should be (25-32)*(5/9).


How much more economical are planes with retractable gears vs. those with fixed gears? Many small planes have fixed gears, some have retractable gears. All things being equal, the plane with retractable gears would have to be more expensive than fixed gears, as the mechanisms involved in retracting them is obviously more complicated than fixed gears. On the other hand, planes with fixed gears should have somewhat more drag than a similar plane with retractable gears when they're retracted. This would imply that a plane with retractable gears would consume less fuel than one with fixed gears. If the difference could be quantified, how much more economical would the plane with retractable gears be vs. one with fixed gears?

Consider the Piper Turbo Arrow IV vs. the Piper Turbo Dakota... Same max gross weight, same engine, same wing, but the Arrow (with retractable gear) is nearly 12% faster at full cruise power.  This would imply that, at the Dakota's cruise speed, the Arrow should burn roughly 14% less fuel (give or take a little; it's 75% vs. 65% power, but at different altitudes).  Given this, the Arrow will cost less to fly for a given trip ($/nm).  The trade-off is that the Arrow costs more to buy, maintain, and insure, while having a smaller useful load.

Note that I chose these models for a reason: The Turbo Dakota is literally a fixed-gear version of the Turbo Arrow IV with no other major changes.  In most cases where a model is available in both fixed and retract, the retract has a more powerful engine.  This makes comparisons much more difficult.  YMMV


What kind of assistance can another aircraft provide to an incapacitated pilot? A recent incident unfolded as follows:

An aircraft overflew the planned destination and did not respond to radio calls on any frequency.
ATC decided the pilot might be incapacitated and asked another pilot whether he could get close to that aircraft to take a look.
The other pilot did that and observed no reaction to signals such as rocking the wings or extending the landing gear, however the incapacitated pilot was changing the course slightly to maintain separation.
Eventually the pilot resumed radio contact (speaking in a slurred voice) and upon insistence from ATC landed at the closest airport without further incident. All this time ATC was asking the other pilot whether they would still stay in the area, which they did until the landing.

There’s a VASAviation video with the entire radio conversation.
How can the other aircraft help beyond what they already did? Why did ATC want them around after getting in touch with the incapacitated pilot? Despite the aircraft belonging to the Royal Flying Doctor Service of Australia, it’s not like they could provide medical help. What else could have happened after radio contact was restored that would have necessitated some kind of action from the other pilot?

This is speculative and I haven't looked at the links you gave, but I can think of a few things:

Having another aircraft there gives ATC a way to gather information that they otherwise couldn't. For example, if the second pilot had observed that the 'incapacitated' pilot was actually awake and alert and there was a second person in the cockpit brandishing a gun, I think it's safe to assume that the response from ATC, law enforcement and the military would have been different.
The worst case is that the incapacitated pilot never recovered and simply crashed. In that case the second pilot could give ATC a precise location for rescuers, which could be important in a more remote area. Especially if ATC had no radar coverage.
The incapacitated pilot could have recovered but still been disorientated and unable to communicate properly. It's possible that the second pilot could have led them to a nearby airfield. That would still require some understanding and communication between them, and the second pilot might not have been comfortable with doing it, but it's an option.
It's possible that the incapacitated pilot recovered but couldn't talk directly with ATC. That could be because they drifted down to an altitude below ATC's radio coverage, or couldn't change frequencies, or whatever. The second pilot could relay calls from ATC in that case, assuming that they could work out which frequency the incapacitated pilot was on. Relaying calls from ATC to another aircraft is fairly common.

I'm sure we could all come up with other ideas, but I think the first point is the most important: when something strange is happening then it's best to gather as much information as possible.


What's the purpose of "over G" penalties in Red Bull Air Race? This post mentions that if a Red Bull Air Race pilot exceeds 10G acceleration limit for longer than 0.6s on their plane then they are unconditionally disqualified with "did not finish" result no matter how they perform otherwise.

What's the purpose of this restriction? Is this to prevent the airplane from falling apart or pilot getting unconscious during the (low altitude) race?

It's a safety rule to protect the limits of the airframe and the pilot.

See the video (also on youtube) from this Red Bull Air Race page which explains the rules. The G-limit is discussed from 3:30.

The following is a quote from the relevant section of the video.

  Maximum load factor, that is how much G's is the pilot pulling. That has
  issues [...] the structure of the
  airplane is designed to withstand a certain amount of G. We need to
  ascertain, to check, that the pilot isn't exceeding that amount of G,
  so we monitor that very carefully. A thousand times a second we're
  monitoring the G inputs that the pilot is putting on the airplane. So
  if you exceed the maximum G's then DNF (Did Not Finish)


Which flap is better suited? Split or Plain? I’m considering which flap to build onto a home built aircraft, but not exactly sure which flap would best fit the aircraft and which one would be more beneficial. The aircraft has a fighterlike look to it, but I’m mostly worried about how the flaps will effect airflow during landing and takeoff.

As usual, the answer is: It depends.

A short comparison would read like this:

Plain flaps have better L/D when deployed at small deflection angles.
Split flaps have a higher maximum lift at the same deflection angle.

Therefore, a plain flap can be used for take-off or slow flight, as well. Split flaps are only good for landing, but in this they are better than plain flaps because the increased drag makes the landing easier.

Airflow on the bottom of the airfoil is very similar, and on the top the plain flap will exhibit flow separation somewhere along the flap chord while the split flap has a well defined separation point at the trailing edge. This should produce a more steady wake and more linear characteristics of the split flap when compared to a plain flap.

Proof: Figure 7 from Chapter 5 of Sighard Hoerner's "Fluid Dynamic Lift":


What are these engine inlet devices used for? While looking at the Boeing Chinook CH-47 for this question, I noticed the screens in front of the engines, and found similar devices in other photos:

Source: left -- right

What are these devices used for?

A recurrent question here is about protecting turbine engines against bird/FOD ingestion. It seems that a screen can be used, at least at low speed.

It is the case? Is it used for this purpose?

The first photo shows the engine air intake screen. Its basically a screen to prevent FOD ingestion, as you noted. From CH-47 Theory of Operations:

Engine Inlet Screens
An engine inlet screen which maximizes foreign object damage (FOD) is installed on each engine. The reduction in engine power available with screens installed is negligible. The engine inlet screens have bypass panels.  These two panels are on the aft end of each screen. Quick release fasteners attach the panels to the screens. They are removed if the ambient temperature
is 4°C or below and there is visible moisture

I'm pretty sure it is supposed to be minimizes foreign object damage.

It is an All Weather Screen (AWS) intended primarily to prevent ingestion of large items that would cause foreign object damage to the compressor blades.

By Mr.Z-man - Own work, CC BY-SA 3.0, Link
The second photo shows the Engine Air Particle Separator(EAPS) unit, which in this case is the  Pall PUREair System

Pall PUREair system; image from Pall aerospace
The EAPS is there primarily to protect against ingestion of sand and dust, which, according to Why an Engine Air Particle Separator (EAPS)? can cause multiple problems like,

erosion of compressor blades

clogging turbine cooling passages

cause sulphidation, corrosion and overheating

As the article notes,

A CH-47 in the desert with only the AWS could achieve only 12 landings before both engines were changed. With EAPS, it did 13 landings with no change in horsepower. in another desert test CH-47 engines with AWS deteriorated to an unacceptable level in only 30 hours. With EAPS, there was no degradation.

In case of desert environments, EAPS is a must if you want your engine to survive any more than double digit hours. This particular type of EAPS belongs to the Vortex tube separators and uses a vortex generator to separate the sand and dust from air.

Contaminated air entering the ... air cleaner tube assembly is given a swirling motion induced by the vortex generator. This swirling motion causes the heavier dirt particle and water droplets to be thrown radially outward by centrifugal force toward the wall of the tube.

Principle of EAPS operation, image from Pall aerospace.
You can find about other types and their operation in A Comparative Study of Helicopter Engine Particle Separators