What US airports have approaches with LAAS? LAAS (Local Area Augmentation System) is a ground-based GPS augmentation system, intended to be placed at airports to get position accuracy down to around 3m (~10ft) from WAAS's 10m.

I've read that it has been installed at a few airports, but can't track them down; I've also heard, anecdotally, that installation has ceased as WAAS is more than sufficient.

Is LAAS still being installed, and if so, where can one fly a LAAS approach?

The Local Area Augmentation System was the FAA term for the Ground Based Augmentation System (GBAS), which is an International Civil Aviation Organization (ICAO) standardized system for local area differential systems.

The FAA has migrated to the ICAO term GBAS, though Several historical FAA documents still retain the LAAS terminology.

According to the FAA GBAS FAQ page,

  Currently, two U.S. locations have obtained operational approval for public GBAS use and support revenue airline traffic. These stations are located at Newark Liberty International Airport (EWR) and Houston George Bush Intercontinental Airport (IAH).

According to FAA, development of GBAS continues and,

  The FAA and other States are in the process of validating proposed ICAO GBAS standards to support Category III (CAT-III) precision approach operations. The FAA expects ICAO will approve GAST-D Standards and Recommended Practices in 2015

Also,

  The FAA continues to validate the GBAS CAT-III requirements as part of the Next Generation Air Transportation System (commonly known as NextGen). The FAA has currently deferred any Federal acquisition decision on GBAS.

Also, though GBAS and WAAS are mutually exclusive at present, some future aircraft like A350 are expected to support both GBAS and WAAS.


How does the MD 900 accomplish auto rotation with the NOTAR system? The MD-900 is a helicopter which seems to be quite popular with law enforcement agencies.

As you can see, instead of an anti-torque tail rotor, a fan exhaust is directed out slots in the tail boom. I was wondering if this works in regards to auto rotation, should the aircraft lose its engines.

According to https://en.wikipedia.org/wiki/NOTAR the NOTAR fan is driven by the main rotor transmission. This will ensure maneuverability during autorotation.


Is there any order of loading containers into aircraft instead of weight balance? Assuming there is some container which have to load into the airliners.

All of them are stored into same size ULD and same weight.

The difference is that some are exclusively passenger luggage, while some are cargo, in which some are more dangerous (eg: more flammable) or less durable.

So, in practice, is there any order of loading those containers into airliners?

Yes, there is.

Premium luggage (business and first class) is loaded such that it is the first to be retrieved.
HAZMAT (HAZardous MATerials and items, if applicable) is loaded differently; and placed in different parts of the cargo hold.

There are some areas of the cargo hold (in passenger airplanes) that cannot be used to store ULD, only loose cargo or other special items (such as the transportation of pets).

Some airlines offer temperature controlled ULDs, which require their own special handling.

Of course, transportation of human remains is handled differently.

Strictly for cargo operations; as these are normally multi-stop trips - the ULDs are loaded in the order in which they are to be unloaded.

However, keep in mind that it is very rare to have perfectly uniformly loaded ULDs or crates. So often the loadmaster has to make sure the CG of the aircraft is taken into account when loading and unloading.


Do carrier-less militaries retain the equipment for carrier operations? "How do jet fighters get to war?" made me wonder if aircraft used by the U.S. Navy and U.S. Marine Corps for carrier operations retain their carrier equipment when sold to foreign powers who do not operate carriers?

Pros would be the reduction in weight, repair/maintenance complexity, and cost (why bother paying for equipment you don't use?).

Cons would be engineering complexity (if they're not designed as optional components) or airworthiness (if the components have secondary functions beyond carrier operations).

The initial thought was prompted by a CATOBAR aircraft, but if something like the Harrier has different equipment, that would be relevant, too. What's the reality?

Taking the RAAF F/A-18's for example:

  The Australian Hornets were very similar to the standard US Navy variants, but incorporated a number of minor modifications [including] the deletion of all equipment used to launch the aircraft using catapults.

Even though they retain the tailhook, if they landed on a carrier, they won't be able to takeoff.

The tailhook serves the same function as on the land-based F-15 and F-16, provide arrested landing capability for emergency situations.

(Source) F-16 arrested landing.

As for the Harrier, all the foreign operators were navies. So, export Harriers were used on carriers. (Rhyming unintentional.)

The carrier-borne Rafale fighter jet will retain its carrier capability when sold to the Indian Navy.


What are different types of weights of an aircraft? When looking at different aircraft's specifications or in POHs/AFMs, there are several different weights of the aircraft mentioned (e.g. maximum take-off weight, maximum landing weight, operating empty weight).

What do they mean and how they are different?

The different terms represent different weights or masses1. For each flight, the weights are taken into account for several reasons.

A brief description about these is below:

Manufacturer's empty weight (MEW)
Also called Manufacturer's Weight Empty (MWE) or Licensed Empty Weight

It is the weight of the aircraft "as built" and includes the weight of the structure, power plant, furnishings, installations, systems and other equipment that are considered an integral part of an aircraft.

This excludes any baggage, passengers, or usable fuel2.

Zero-fuel weight (ZFW)

This is the total weight of the airplane and all its contents (including unusable fuel), but excluding the total weight of the usable fuel on board.

As a flight progresses and fuel is consumed, the total weight of the airplane reduces, but the ZFW remains constant.

Maximum zero fuel weight (MZFW) is the maximum weight allowed before usable fuel and other specified usable agents (engine injection fluid, and other consumable propulsion agents) are loaded.

Operating empty weight (OEW) (Roughly equivalent to basic empty weight on light aircraft)

It is the basic weight of an aircraft including the crew, all fluids necessary for operation such as engine oil, engine coolant, water, unusable fuel and all operator items and equipment required for flight but excluding usable fuel and the payload.

Payload

It is the carrying capacity of an aircraft. It includes cargo, people, extra fuel. In the case of a commercial airliner, it may refer only to revenue-generating cargo or paying passengers.

Maximum takeoff weight (MTOW)

This is the maximum weight at which the pilot of the aircraft is allowed to attempt to take off3.

Regulated takeoff weight (RTOW)

Depending on different factors (e.g. flap setting, altitude, air temperature, length of runway), RTOW or maximum permissible takeoff weight varies for each takeoff. It can never be higher than MTOW. More information is on this answer.

Maximum landing weight (MLW)

This maximum weight at which an aircraft is permitted to land3.

The following image depicts takeoff weight components.

Image Source

Maximum ramp weight (MRW)
also called maximum taxi weight (MTW)

It is the maximum weight authorized for maneuvering (taxiing or towing) an aircraft on the ground.

Aircraft gross weight

It is the total aircraft weight at any moment during the flight or ground operation. This decreases during flight due to fuel and oil consumption.

1: As mentioned (below in comments by SentryRaven), several recent references use the term mass instead of weight. Having a Physics background, I tend to agree and know the difference between weight and mass. I did not use mass as most of the references I included referred mass as weight. However, using correct terminology, all the weights mean mass. Thanks SentryRaven for pointing it out.

2: What is the different between usable fuel and unusable fuel?
Usable fuel is the fuel on board an aircraft that can actually be used by its engines. The small amount of fuel that cannot be drained from the tanks is the unusable fuel. For calculation of range, usable fuel is used. For weight and balance total fuel (usable + unusable) is used.

3: This restriction is due to structural, design or operational limitations.


Pressure Distribution Pattern around Wing it is clear that the overall integrated pressure on the top of the wing is less than the overall pressure at the bottom of a wing in order to produce a net nonzero lift force. Based on the attached figure below (AoA going from small to large), it seems that, for low and medium angles of attack, the pressure both at the top and bottom surfaces of the wing is lower than the free stream atmospheric pressure. The arrows' directions, from what I understand, are pressure gradients pointing toward higher pressure area, correct?
When the angle of attack is sufficiently large, the pressure on the wing's bottom surface is higher than the free stream pressure (positive pressure coefficient)...Is that correct?
Whenever the local pressure is lower than atmospheric pressure, I think air would rush to that area so air would be moving from far above the wing towards the wing top surface (same for bottom wing surface when the pressure coefficient is negative). Is that what happens?
Thanks.

Last question first.  Yes, air does try to "rush" from high to low pressure, but the movement of the wing creates a condition where the net movement of the air relative to the wing will be downwards and backwards, and lower pressure remains above the wing. This is why airspeed is so important.

Understanding the arrows in the diagrams is a bit like reading music or another language.  They are simply pointing from lower to higher pressures.  Isobars on the weather map show the same thing as pressure gradients.

Also important is to know that you are looking at a fully symmetrical airfoil.  There will be equally opposing lower pressure areas top and bottom at 0 angle of attack.  This is why many airfoils have a flat or even concave bottom to help create higher pressure underneath the wing.  But even a fully symmetrical airfoil will have a pressure differential as AoA increases.


Can an airline rewrite a flight's origin airport for the day, following an earlier diversion? Suppose a domestic US civilian carrier regularly flies flight N directly from airport A to airport B, and flies back flight M from B to A.
Now, one day, the aircraft flying N is diverted to airport C, and the flight ends there (it doesn't continue to B).
Is it possible for the airline to have the aircraft fly from C to B and call that flight M? Even though that's not the route of flight M?
(By "possible" I mean "known to happen", "conceivable" rather than whether it's legal under FAA regulations, although that's interesting too.)

No.  They've sold tickets for people to get from A to B. They've not sold tickets to anybody looking to get from C to B.
The aircraft would be ferried into position empty, then they'd operate the schedule that has been advertised & sold.
The scenario as presented doesn't really work, though... you don't get to just strand your passengers in C who bought tickets to A & tell them "sorry, good luck." Either you bus them the rest of the way, or you use AN aircraft and A crew to get them where they need to be. Whether it's the same aircraft and/or the same crew as originally planned depends on various things, but as a matter of flight numbers, you fly what you've been selling tickets for (as much of that schedule as you can) and you ferry planes & crews into position when you have to.
But rewriting things so that "flight #X now starts in C instead of A, today" doesn't happen.


When will Europe allow phones (to communicate) on planes? Currently USA has allowed phones to be used in commercial flights, is there a similar ruling coming to Europe or will my mid air Bluetooth battles have to wait?

There is a similar ruling since last week.

The use of electronic equipment as long as they are in "flight mode" during all phases of flight was already allowed in Europe by EASA since November 2013.

Since last week (26 November 2014) EASA also allows phones to be on and connected throughout the flight. This allows for WiFi services, phone services and Bluetooth battles during the flight.

Before allowing the use of these electronics in flight, airlines have to assess the impact of the signals transmitted by the mobile devices on the aircraft, ensuring aircraft systems are not affected. For this reason mobile communication will not be allowed by all airlines on all flights at once.


How should a private pilot renew their part 107 certificate? I got a Part 107 (Commercial Drone) Certificate by passing the test more than two years ago, so it's expired now. I just became a private pilot, and I know that private pilots can get a Part 107 Certificate if they take a few online courses from the FAA.

But since I have the Part 107 from passing a test and now want to use my private pilot certificate to renew my expired Part 107 certificate, how do I do this?

It seems a little convoluted because it's a renewal, not an original certificate.

As a holder of an FAA private pilot certificate, you can take a free course to “renew” a Part 107 certificate. You will first have to create and register an account on the FAA Safety Team website. You should do this anyway to earn WINGS credit toward your next flight review.

The site states..

  The Part 107 Small Unmanned Aircraft Systems (UAS) Recurrent course describes the operational requirements to continue to operate small UAS in the National Airspace System (NAS) under Title 14 of the Code of Federal Regulations (14 CFR) part 107, small Unmanned Aircraft Systems. ***Note: This course may be taken by remote pilots who qualify for relief under Special Federal Aviation Regulation (SFAR) No. 118 to satisfy the aeronautical knowledge recency requirements of 14 CFR 107.65(a) or (b) for a period of six calendar months. FOR PART 61 PILOT CERTIFICATE HOLDERS WITH A CURRENT FLIGHT REVIEW (IN ACCORDANCE WITH 14 CFR PART 61.56), SUCCESSFUL COMPLETION OF THIS ONLINE COURSE SATISFIES THE RECURRENT TRAINING REQUIREMENT TO REMAIN CURRENT AS A PART 107 REMOTE PILOT WITH A SMALL UAS RATING. All other interested individuals may complete this online course as a self-study resource.

A Part 107 certificate does not have an expiration date. You lose the privileges associated with it if you do not renew your currency by taking a refresher course.

The FAR/AIM states...

  §107.65 Aeronautical knowledge recency.
  A person may not operate a small unmanned aircraft system unless that person has completed one of the following, within the previous 24 calendar months:

  (a) Passed an initial aeronautical knowledge test covering the areas of knowledge specified in §107.73(a);

  (b) Passed a recurrent aeronautical knowledge test covering the areas of knowledge specified in §107.73(b); or

  (c) If a person holds a pilot certificate (other than a student pilot certificate) issued under part 61 of this chapter and meets the flight review requirements specified in §§61.56, passed either an initial or recurrent training course covering the areas of knowledge specified in §107.74(a) or (b) in a manner acceptable to the Administrator.


Is there any civil aircraft that decelerates with a parachute? Some previous generations military aircraft decelerate with a parachute to decrease landing distance:

Israel Aircraft Industries Kfir landing with a parachute (source)

It this something found on any civil aircraft today?

(airliners.net)

Some early civilian jets had drogue parachutes such as the Caravelle and the Tupolev Tu-104.

While the reverse systems are heavier than the parachutes, they are more practical (no stowing and running after rogue parachutes).

Regarding the edit for today's planes: they went out of fashion for the civilian jets.

For newly built jets, there aren't any as far as I know. Older jets that may still be airworthy, and military aircraft that may have been converted to civilian use, are not included in my assessment.

The weight saving makes sense for fighters which are range limited due to the high performance engines and the placement (and type) of the nozzle. The weight of a reverse system is better substituted with fuel or ordnance. The Saab 37 does have reverse though.

(Source) Parachute used for an attitude recovery system. It's not meant for decelerating though.

(Twitter) Norwegian F-35's will also have parachute brakes (related to the note on fighter generations).

Related and/or duplicate, you decide: Do any commercial airline models have a parachute at rear to help in landings? (Answers are more thorough and include gliders.)


What is the material that is used for turbine blades that has the highest melting point? What is the material that is used for turbine blades that has the highest melting point ?

Gases temperature on the turbine blades can reach 1700-1800K. There is no material which can withstand such a temperature in conjunction with enormous centrifugal forces.

That's why blades are not only made of nickel superalloys, coated with ceramics, but also are "film-cooled".

They have radial micro-passages where compressed air from high pressure compressor is fed. Most advanced designs also have micro-holes along that passages on the leading and trailing edge of the blade, which are the hottest points on the blade. This enables cooling blades down with air (air from HP compressor heats up to 400-500K, while blades work at 600-700K) and also creates a film of protective air over blade's surface, which prevents direct contact of blade and exhaust gases. That's because most of the heat transfer occurs by thermal conductivity at direct contact, and much less - by heat radiation.


What is the name of this airfoil? Is there a general name for this type of airfoil? I'm having trouble finding any information on it so if anyone knows anything about it please let me know!

That's a reflexed trailing edge airfoil. Its center of pressure moves forward with decreasing AOA giving it an almost constant pitching moment. It is often used for flying wings.


Do we artificially add a smell to jet fuel? Many dangerous liquids have odorants added so that you can know if something dangerous is leaking.  Is anything added to jet fuel (such as Jet A-1) to give it a distinctive smell?

Aviation fuels are all hydrocarbons that have fairly strong natural odors, so no additives are necessary to give them a noticeable smell. Jet-A1 is refined kerosene, which has a distinctive smell which some find unpleasant and others not. Aviation Gasoline (AVGAS) has many "aromatic" hydrocarbons to boost octane which are called aromatic because of their distinctive odors.

So you can smell a fuel spill or leak pretty easily, for at least AVGAS I know this from experience. I once had a fuel leak which filled the cockpit with avgas fumes, I got a lungful before I managed to ventilate the cockpit and it was very unpleasant!


Can I compute the lift coefficient based on the NACA airfoil? I would like to know whether it is possible to compute the lift coefficient $C_l$ based on the NACA airfoil? Usually I look up the values on airfoiltools.com.

Eg: If I have a 2412NACA airfoil and would like to know the lift coefficient for an angle of attack $\alpha$ = 3°. What would be the corresponding lift coefficient?

Thanks

Yes it is possible.
Here is what I did, maybe it answers your question.

1) I went to airfoiltools.com into the comparison section.
2) Looked up the NACA-airfoil 2412

3) scroll down to choose the speed (Reynoldsnumber). I took the minimum and maximum to get an idea of the spread.

4) scrolled further for the polars, at sketched the $\alpha = 3^\circ$ into it

But somehow I do wonder if this is really your question.

Or are you asking for xfoil and the commands to compute it?

In case you want to compute the values without visiting airfoiltools (which most-likely use xfoil themselves) you need a tools to numerically solve the governing equations.
You can do this in many ways. Those ways differ in the way the flow is approximated. So you could go with XFoil (very simple approximation of the aerodynamics) or maybe OPENFOAM (more complex description of aerodynamics).

There is however no way to calculate these coefficients in an easy way.


Why are cockpit alarms still so distracting and unable to be silenced? I'm watching video RV 10 N783V My 1st Emergency
At 8:45 a stall alarm goes off and is a horrible piercing whistle. Why? Surely the pilot has enough distractions without also being deafened while trying to talk to ATC.
Isn't there a better way of indicating a continuing alert, and why can't pilots switch the the thing off when they need to?

EDIT: The video shows that it is a false-positive warning. The pilot eventually manages to switch it off by resetting all his electronics.

A stall condition needs to be handled now, and once it's handled, the alarm will go away. What you don't want to happen is you are in an approaching stall condition, the pilot hits a "silence" button while the situation gets worse without the plane telling them.
An emergency is "AVIATE, NAVIGATE, COMMUNICATE" in that order. Fly the plane first, point it where you want to go second, and third, only if you have time/resources, is to talk to ATC about it. The pilot should be fixing the problem first, focus on flying the plane, ATC can wait.
They can't be turned off for that reason. Deal with the problem and the alarm goes away, you don't want to silence the alarm and forget about it. There is some merit to "overloading", where multiple alarms are happening at the same time, or when the pilot focuses on a low priority item and forgets to fly the airplane (Eastern Air 401).


Are children safe when seated in lap during a flight? We would never consider putting an infant or toddler into a car without a carseat, but we don't hesitate to do exactly this when getting onto a large airplane.

If you aren't aware of the concept of a lap child, this is a child, typically under 2 years of age, who can board an airplane without an assigned seat and must be held by a parent/guardian for the duration of the flight (including the takeoff and landing). The only safety consideration is that the lap child must be seated in a row with adequate O2 masks in case of depressurization (some rows have more masks than seats).

I have always considered this to be an unsafe practice, but have never had data to back that up. It seems to me that holding on to a 20 lbs child when subject to strong turbulence or unexpected maneuvering would be difficult. While not common, adults are occasionally severely injured in these events but I've never noted a lap child being hurt.

Does anyone know of instances where a lap child was injured during a flight? I'm not interested in instances where everyone is hurt but rather instances that having been secured in a car seat would reasonably have prevented the injury.

Your instincts are correct, or at least the NTSB agrees with you - they don't like the idea of "Lap Children" on aircraft, and they've come out and said as much in this Safety Alert.

They reiterated that position again in a 2010 letter to the FAA which includes a number of incidents from NTSB investiations, including:

  … on July 19, 1989, United Airlines flight 232 crashed during an attempted emergency landing at Sioux City, Iowa, after the fragmentation and separation of an engine fan disk. Of the 296 airplane occupants, 111 were killed, 47 received serious injuries, 125 received minor injuries, and 13 were not injured. Four children, ages 11 to 26 months,17 were aboard the airplane and were being held by adults. A 23-month-old child was killed, and the other three children received minor injuries. The parents of the four lap-held children were instructed to place their children on the cabin floor and hold them in that position while the adults assumed the protective brace position. After the accident, three of the parents reported that they were unable to hold onto their children during the accident sequence.

and

  … USAir flight 1016 in Charlotte, North Carolina. The flight crew decided to continue an approach into severe convective activity and was executing a missed approach when the airplane collided with trees and a private residence near the airport. Of the 57 airplane occupants, 37 were killed, 16 received serious injuries, and 4 received minor injuries. Among those occupants who were killed was a 9-month-old child who was held by her mother on her lap. The child’s mother survived the accident but was unable to hold onto her child during the impact sequence, and the child struck several seats. The NTSB believed that the child might not have sustained fatal injuries if she had been properly restrained in a child restraint system.

Granted both of these were pretty severe accidents, but the NTSB investigators found that the lack of restraints was potentially a major contributing factor to the injuries of children on these flights.

In that same letter the NTSB also notes that unrestrained children are not just a hazard to themselves in a crash:

  Passengers are required to securely stow all carry-on baggage during takeoff and landing because of the potential risk of injury to other passengers in the event of an unexpected hazardous encounter. However, passengers are permitted to hold a child of equal size and weight in their lap. When children under 2 years of age are not required to be restrained for their own safety, the safety of their fellow passengers also becomes an issue.

See here for more NTSB pronouncements on the subject of child safety in transportation.
They even have some excellent videos and simulations showing restrained versus unrestrained children.


Why is a reduction in lift called 'drag'? Induced drag is high pressure air 'leaking' to the upper part of the wing (low pressure zone) creating a vortex which means the area of the wing tip with the vortex can't create lift. Drag on the other hand is the opposition to the flow of air. Why is induced drag called a drag when it doesn't oppose oncoming air but instead reduces wing lift?

Same thing applies to interference drag, high pressure air from the top wing of a biplane leaking to the low pressure of the wing underneath it hence reducing lift, yet it's called drag when it actually reduces lift.

So my question again is why is interference drag called a drag when in reality, it reduces lift and does not oppose oncoming air?

Induced drag is unrelated to leaking and vortex creation. It exists even in a wind tunnel where the wingtips touch the walls and thus prevent leaking and vortices.

Then what is induced drag? To create lift, the wing needs to be at some angle with respect to airflow (or be asymmetric), so the force vector points mostly upwards but also somewhat aft. Forces that point aft are generally called drag. Induced drag is thus a 'real' drag, not a reduction in lift.


Why does the MiG-25 have silver-coated panels? This is about the Mig-25. This documentary on youtube, @10:03, says:

  Special insulating panels had to be fitted to protect the area around the engines from overheating. The panels were silver-coated. Five kilograms of silver were used in their manufacture.

My question is, why? What does silver-coating do exactly? AFAIK you still need some kind of cooling, either air or liquid, or else the heat flux will steadily raise the temperature to the failing point.

Wikipedia lists the thermal conductivity for silver and copper as 429 W/(m·K) and 401 W/(m·K) respectively. It is only a slight difference and I can't imagine why you would absolutely need that slight difference for a massive increase in cost. Plus I'm pretty sure silver (and gold) are a lot easier to melt than copper.

The only "special" thing about silver, that I know, is that it's highly reflective and therefore is used in the mirrors of superindustrial strength telescopes. I cannot imagine you would need that inside an engine, as virtually all the heat comes from conduction via the hot engine gasses, not radiation.

Edit: The silver could be part of some silver alloy. It would still have to be a coating of this alloy.

As you have pointed out yourself, the Silver is there because it is highly reflective. The panels around the engines were coated with silver to prevent the heating of the fuselage around it. According to this document,

  The engine bays each incorporated heat shields coated with a 30-micron layer of silver (5 kg/11 lb per  aircraft), which absorbed 5 percent of the heat and represented a more cost-effective solution than gold or rhodium, though these  metals were tested.

The panels basically trapped the heat within the engine bay, which is then cooled using boundary layer bleed air from drain slots between the air intake sides and airframe sides.


What procedures are in place at London City Airport for emergencies after V1? Given the challenging operating environment at EGLC, what procedures are in place for emergencies after V1. Is it possible to return to the airport?

London City certainly has its challenges, with a steep approach and high rise buildings in close vicinity to the short runway, at the same time you have all sorts of places to divert to: Southend to the east, Stanstead and Luton to the North, Heathrow to the west, Farnborough, Gatwick, the list goes on. All of these places have longer runways with less constrained areas and fields all around. North Weald and Biggin Hill could also be used in an emergency. By the time you have returned to LYC you could have reached one of those.

It depends on the severity of the situation; if you have a mechanical problem but can land safely then going back to LCY is convenient for your passengers, if you're on fire or have control problems you're better off an another airport.

It is certainly possible to return to the airport in an emergency, and it has been done at least once. In 2014 an Avro RJ85 returned to the airport after a part of a nacelle came off. The headline is misleading, it says part of the wing fell off but it was really a nacelle. This is sort of "emergency-lite" as the flight could not continue, but the condition of the airplane was good enough that a normal approach could be made and the passengers moved to another flight.

The same year an Avro RJ100 diverted to Stanstead after an engine fire.

So the wisdom of returning to London City in an emergency is entirely dependent on context like the condition of the aircraft, weather, etc.


What are the biggest challenges for GA airports? I'm doing a little research into General Aviation airports and wasn't able to find any airport management forums or the like to pose this question, so I'm hopeful someone here will have some helpful insights.  (Note: I did find a community associated with AAAE, but it didn't seem to have much of a community in it)

So I'm curious to hear from GA pilots, Airport Managers, and others on what the biggest daily challenges for GA Airports are?

I'm not talking about macro issues - federal/state/local funding, scarcity of land, governmental entities run wild (see Santa Monica Airport), etc - I'm interested in the daily operational issues of the local 5000' paved GA airport.

On the outside, these airports and FBOs see to be fairly straight forward enterprises, whether run by a municipality or privately operated.  But I imagine it's not that simple.

Note: I am a private pilot and aircraft owner, primarily looking to expand my knowledge of the business of aviation - particularly GA.  This question is intentionally broad, but I imagine will yield some fairly insightful specific answers.

Edit:

I'm primarily concerned with answers pertaining to the overall business success of an airport or FBO. I understand that this might yield answers with a number of variables, but should overall have the common theme of reasons for business success or failure of the operator which is a key component of the continued operation of the airport facility itself.

One set of challenges that comes to mind are the types of things you find at an airport that is on that cusp between having an operating control tower and not.  I fly out of KJYO, Leesburg, VA and it very much fits into that catagory.  It frankly just gets too busy sometimes to support operations managed only by pilots coordinating on CTAF.  Examples of this include:

Wind conditions causing a conflict over whether Runway 35 or 17 should be in use.
Pilots doing straight in instrument approaches, while VFR pilots do pattern practice.
Conflicting planes on the single taxiway.

Indeed, I think the FAA understands this, since they are using it as the first US test site for a remote control tower.  The several times I've flown when this was in operation where a breath of fresh air!


Aerospace Engineer program in EU? I am looking for an Aerospace Engineering program in the EU. I heard the Aerospace Engineering Department in Delft University in the Netherlands is one of the best programmes.

I prefer English speaking countries, such as the Netherlands, Scotland, and Scandinavian countries.

To be more specific I would like to study about the operation of airlines, airports, maintenance, how to do aircraft take-off performance calculations, and regulations relating to the operation of EFB applications.

The topics you describe are certainly part of aerospace engineering, however they are somewhat specialized topics. I am not trying to discourage you, but Aerospace engineering involves a lot of mathematics and physics. If these fields do not appeal to you, you will be having a hard time to get through to the sweeter parts.

Some universities only offer a Master programme for Aerospace engineering, which typically follows on to their Bachelor programme Mechanical engineering. If you are really interested in aviation, I would recommend to select a full (Bachelor and Master) Aerospace programme.

Aerospace engineering programmes generally involve a thorough basis of mathematics (linear algebra, calculus) , physics (mechanics, thermodynamics), material sciences before introducing more specialized subjects like aerodynamics, aircraft performance, and space flight. At bachelor level, you will get an introduction to a diverse set of aerospace related subjects. At master level you will go deeper into an area, although you will have to accept that you won't master all subjects you mention.

I had a good time at Delft University, it certainly has a good Aerospace Engineering programme which I would recommend.
In addition I had the pleasure to work with a graduate from Bristol university, she is very knowledgeable. I'd certainly look into their programme as well.
For Scandinavia I would look into aerospace programmes in Sweden. Sweden has quite an active Aerospace industry. Linköping is their centre of Aerospace Engineering.


How many tendencies are affected by Gyroscopic precession for single propeller airplane? I have heard about LEFT turning tendency affected by gyroscopic precession of propeller (which is turning clockwise or counter clockwise if seen from front side). But I just watched a video on Youtube explains the tendencies affected by the gyroscopic precession, that the airplane is pitch up (nose up or tail down), which means, there are two tendencies.

Then my question is, is that explanation true? Is the second tendency true?

Both are wrong. (Or rather, both are partly right, which makes them fundamentally wrong).

Precession happens when there is an 'input' rotation (moment) and not by itself. When you balance the airplane to fly straight and level, there is no gyroscopic moment.

The gyroscopic moment is perpendicular to the external rotation, so it can be directed anywhere across the spin axis.

On a typical airplane with the CCW prop (or turbine for that matter), if you yaw (or turn) LEFT, the airplane will want to pitch UP. (See the diagram on your picture). If you pitch UP, the airplane wants to turn RIGHT. If you pitch DOWN, the airplane wants to turn LEFT.1 And so on. The quicker you maneuver, the stronger the effect is. (And of course, the quicker and heavier the prop is, the stronger the moment, so it's most noticeable at full power).

In normal flying with GA airplanes, the gyroscopic moments are rather minor. The typical 'left turning tendency' is caused by aerodynamics. Gyroscopic moments manifest themselves when rotation is relatively fast, e.g. in aerobatics or spin. But they can be observed with quick impulse inputs on the pedals or yoke, if carefully executed.

1This latter is particularly noticeable on taildraggers during takeoff when the tail is lifted too quickly: the aerodynamic forces are still weak, and the power is full.


Can I fly a glider in Europe on a Canadian Glider License? I currently live in Canada as a Permanent Resident and I recently received my Canadian Glider Pilot License. I will be going to Europe soon so I was wondering what I have to do to be able to fly a glider there. I will be going to the Netherlands if this makes any difference and I still have my Dutch Citizenship. Do I have to convert my license or just do some extra training?

Also would it help if I got a FAA glider certificate/endorsement on top of my TCCA license?

In general, it is difficult to convert to EASA. You can find more information here however I suspect the easiest method will simply be for you to phone them up.

I think the bureaucracy will probably be more hassle than it's worth. It's not like Transport Canada where you can just get a FLVC (Foreign Licence Validation Certificate) or whatever (that was a pleasent suprise after our CAA in the UK!).


How does the hinge mechanism of plug-type pressure doors work? Many pressurized aircraft have the type of plug doors that open by first swinging inside,  rotating about at least two axes, then moving through the opening and ending up outside parallel with the fuselage.

I'm intrigued by the mechanism that makes this possible, but I have never been able to really see how these intricate hinges work. As a passenger it's hard to even witness the actual opening or closing of the door. Some drawings are to be found on the web, but none that I know of give a clear understanding of its workings. Does anybody know of a video or an animation that gives some insight?

This video has a clear shot of how a classic plug door (in this case of a B737) works (or search for flight attendant training videos).

You can see the door pops in (tipping a bit downwards) and then rotates on a axis in the center of the opening and then moves outside sideways. This is possible because of how the geometry works out, moving sideways negates the boor being wider than the opening and tipping down lets it fit height-wise.


How do I interpret these values in NASA's C-MAPSS dataset files? I was studying a dual-speed, high-bypass ratio turbofan engine dataset which I happened to obtain from NASA's website. This dataset was generated from C-MAPSS simulator where dataset has nominal and fault files. I studied the user guide doc and some other documents related to the same but I failed to understand the difference between these two files (ie. at the value/ parameter level).

I have following questions, if anyone has used/aware of this dataset please help me:

difference between nominal and fault file
There are 27 parameteres used, among these which parameters will affect fan, HPC, HPT, LPC, LPT
what is the threshold value for these parameters or when do we say that the parameter has reached its max value and above which it would lead to fault condition.

Here is the link where I got the dataset:
c3.nasa.gov/dashlink/resources/140/

difference between nominal and fault file

It is found inside the .tar.gz file. From the README file contained in the page you linked:

  the Simulation_Info.txt contains a summary of the number of flights,
  the fault type that was simulated, if it is nominal or not, and if not
  what flight and time sample the fault was introduced.

  There are 27 parameteres used, among these which parameters will affect fan, HPC, HPT, LPC, LPT

I assume you are asking what are those 27 parameters.
From the README file that is found in the page you linked:

  what is the threshold value for these parameters or when do we say that the parameter has reached its max value and above which it would lead to fault condition.

A detailed answer is out of scope for this site, it heavily depends on the failure simulated. The documentation of the various tests should have enough details to understand.

Additionally, hard absolute values are definitely of no use here, different engines will have different characteristics: the numbers valid for the engine simulated here will be applicable to basically no real engine.


Has a jet "engine" ever been made that only uses the thrust from the bypass air and doesn't have a combustion chamber? I recently have been looking at different types of jet engines and came to this thread wherein they talk about how most of the thrust from airline turbofan engines (high bypass) comes from the bypass air and not the combusted air coming out of the internals of the engine.

So my question is, has there ever been a sort of fan engine used that only uses bypass air and the fan is run by a separate engine? Would this even be viable?

Edit:
Basically a turbofan that just uses the power of the increasing speed of the air through the smaller space, no combustion (other than the standard piston engine/electric motor/whatever).

Well what you are describing is a ducted fan.  They are common on RC aircraft with the fan driven by either a gasoline engine or electric motor.

On large scale applications like an airliner, you have to find a source of power for the fan that is 1) compact 2) light weight 3) delivers a very high specific power and 4) draws energy from a reservoir with a high energy density, all while remaining 5) extremely reliable and 6) safe.  For these kinds of applications, you really can’t currently do better for this than use a kerosene fueled gas turbine engine to drive the fan.  And that's all a high bypass turbofan really is - just a gas turbine powered ducted fan.


Why doesn't a radar-equipped Tower say "radar contact"? Most/all of the control towers in my area have radar displays, and I know from visits that it shows all the same things as in TRACON miles away--even aircraft on the ground.  I hear them occasionally tell both VFR and IFR folks to reset incorrect codes, so they're clearly looking at (and presumably using) the radar display, even in VMC.

So why don't they say "radar contact" like TRACON does?  It seems a little odd that you get a squawk from Clearance Delivery, and Tower checks the literal radar contact on their screen to make sure you got it right, but then you don't hear "radar contact" until talking to Departure.

Also, some busy airports will assign squawk codes to inbound VFR aircraft coming in directly rather than via Approach, but they don't say "radar contact" then either.  This seems even weirder since in that case nobody else ever will--yet you're obviously a radar contact on their screen, otherwise what was the point of giving you the code?

From the ATC Orders 3-1-9(b)(4):

  Unless otherwise authorized, tower radar displays are intended to be
  an aid to local controllers in meeting their responsibilities to the
  aircraft operating on the runways or within the surface area. They are
  not intended to provide radar benefits to pilots except for those
  accrued through a more efficient and effective local control position.
  In addition, local controllers at nonapproach control towers must
  devote the majority of their time to visually scanning the runways and
  local area; an assurance of continued positive radar identification
  could place distracting and operationally inefficient requirements
  upon the local controller. Therefore, since the requirements of para
  5−3−1, Application, cannot be assured, the radar functions prescribed
  above are not considered to be radar services and pilots should not be
  advised of being in “radar contact.”


Where does the factor 1.3 for Vapp come from? The approach speed Vapp is usually given as 1.3 * Vs0, or 1.3 times the stall speed in landing configuration.

Why 1.3? What created a consensus of 1.3 * Vs0 as the generally accepted textbook approach speed?

1.3 Vso is not a universal rule even for the same plane.  First is extra weight, which requires higher AOA for adequate lift at the same speed, or greater speed at the same AOA. Approach a little faster if heavy.  Next is CG.  If CG is near limits, a little extra speed will help aerodynamic trim as you slow down.  Finally, there is weather. Gusting winds or wind gradient (head wind dropping as you near the ground) should make the pilot consider adding a few knots to approach speed as well.

The best policy is to do what is safe for your plane.  A very slow plane that lands at 25 knots may be more like 1.5 or higher, especially on a windy day.

1.3 Vso is a fairly good approximation for a plane that stalls around 50 knots, approaching
at 65 knots.

Another consideration in approach is safely banking in the pattern.  Stall speed increases with bank angle as follows.

Load factor G = 1/(cos bank angle)
Stall Speed (Accelerated) = Vs x Square Root G

For 30 degree bank:  G = 1/.8662 = 1.155  (thought I had it here for a second!)
               Accelerated stall speed = Vs x 1.07

For 45 degree bank:  G = 1/.7071 = 1.414
               Accelerated stall speed = Vs x 1.18

For 60 degree bank:  G = 1/.5000 = 2.000
              Accelerated stall speed = Vs x 1.414

So we avoid steep turns in landing patterns as well!  Yet another reason to add safety margin
to approach speed.


What is a mushing glide? Source: Wolfgang Langewiesche - Stick and Rudder; An Explanation of the Art of Flying

Why is it called a mushing glide?
In this glide, does the aircraft point its nose less (or more?) steeply?
How does it steepen the descent?

I find the explanation confusing. At one point it seems that a mushing glide is caused by a high AOA and at another point it seems that a low AOA (pointing slightly above horizon) causes it.

Why is it called a mushing glide?

The aircraft is not cleanly cutting through the air. It is "mushing" through the air at a high angle of attack creating lots of drag and a slow forward airspeed.

  In this glide, does the aircraft points its nose less (or more?) steeply?

The aircraft has its nose pointed up (less steeply) than a normal glide.

  How does it steepen the descent?

The low forward airspeed and high drag creates a high rate of sink relative to the forward motion. Low forward speed and relatively high sink rate means the descent will be very steep. A fully stalled condition would be an extreme example of a "mushing" glide.

  low AOA (pointing slightly above horizon) causes it.

pointing the nose slightly above the horizon is still a High Angle of Attack, not low AOA.


Do concurrent operations on a single runway occur anywhere else besides AirVenture? At the EAA's AirVenture fly-in in Oshkosh (KOSH), landing and sometimes takeoff operations are very often done by dots which are painted on the runway, making it so that multiple operations (a landing or a takeoff) are done at the same time on a single runway.

A diagram of those dots for those who don't know what I am talking about:

Image Source

Does this kind of runway operation (i.e. concurrent landings or takeoffs on a single runway) happen anywhere else or is this special to AirVenture?

ICAO defined procedures for Reduced runway separation which to some extend permits concurrent operations on a single runway. ICAO does not mention anything about coloured dots, but the ICAO procedures do open up for doing something similar to what you are describing.

The following is from Doc 4444/PANS-ATM:

  7.11.4    For the purpose of reduced runway separation, aircraft shall be classified as follows:

  Category 1 aircraft: single-engine propeller aircraft with a maximum certificated take-off mass of 2 000 kg or less;
  Category 2 aircraft: single-engine propeller aircraft with a maximum certificated take-off mass of more than 2 000 kg but less than 7 000 kg; and twin-engine propeller aircraft with a maximum certificated take-off mass of less than 7 000 kg;
  Category 3 aircraft: all other aircraft.

[...]

  7.11.7    Reduced runway separation minima which may be applied at an aerodrome shall be determined for each separate runway. The separation to be applied shall in no case be less than the following minima:

  landing aircraft:

  a succeeding landing Category 1 aircraft may cross the runway threshold when the preceding aircraft is a Category 1 or 2 aircraft which either:

  has landed and has passed a point at least 600 m from the threshold of the runway, is in motion and will vacate the runway without backtracking; or
  is airborne and has passed a point at least 600 m from the threshold of the runway;
  a succeeding landing Category 2 aircraft may cross the runway threshold when the preceding aircraft is a Category 1 or 2 aircraft which either:
  has landed and has passed a point at least 1 500 m from the threshold of the runway, is in motion and will vacate the runway without backtracking; or
  is airborne and has passed a point at least 1 500 m from the threshold of the runway;
  a succeeding landing aircraft may cross the runway threshold when a preceding Category 3 aircraft:
  has landed and has passed a point at least 2 400 m from the threshold of the runway, is in motion and will vacate the runway without backtracking; or
  is airborne and has passed a point at least 2 400 m from the threshold of the runway;

  departing aircraft:

  a Category 1 aircraft may be cleared for take-off when the preceding departing aircraft is a Category 1 or 2 aircraft which is airborne and has passed a point at least 600 m from the position of the succeeding aircraft;

  a Category 2 aircraft may be cleared for take-off when the preceding departing aircraft is a Category 1 or 2 aircraft which is airborne and has passed a point at least 1 500 m from the position of the succeeding aircraft; and
  an aircraft may be cleared for take-off when a preceding departing Category 3 aircraft is airborne and has passed a point at least 2 400 m from the position of the succeeding aircraft.

(note: I have changed formatting slightly to better fit this format)

These general rules are interpreted different by different countries, but some different forms of reduced runway separation are used in different places of the world.


What is the relation between airspeed and rate of turn? I am reading about aircraft maneuvering and the end of section check contains this question:

  During a turn, if you reduce the airspeed and maintain the angle of bank, the result is a _____________ rate of turn.

The correct answer is "higher" which is confusing me.

If I reduce the airspeed, won't it take longer to complete the turn, thus "lower" rate of turn?
I can understand that the turn might be smoother and more coordinated if I reduce the airspeed, but how can the rate of turn be higher?
What am I missing here?

Independent of airspeed, angle of bank gives you the "G" force in a level coordinated turn. A 60 degree bank = 2 G's, at any speed.

At low speed, a given G force will give you a greater heading change than it would at a high speed. Driving your car at 10 mph you can complete a U-turn, 180 degrees of heading change, in a couple seconds without pinning passengers against the door, but at highway speeds, even a hard turn that does impose big side-loads will take much longer to give you that much heading change.

Related to all this is turn radius: at low speed, your turn radius for any given loading (bank angle) is smaller than it would be at a higher speed with that same loading/bank angle. Thus, even though you're going slower, you have less distance (circumference of the circle) to cover, and so you are able to complete the circle (or one 360th of it) more quickly.

((For the pedantic ones, everything here assumes level, coordinated turns. Change those assumptions enough, things can be different, but those aren't particularly instructive cases until the base case is understood.))


How to deal with both a speed and an altitude crossing restriction while in VNAV PTH descent (Boeing 737NG)? Suppose you are in VNAV PTH descent and are approaching the IAF for your Approach, The IAF has both an altitude crossing restriction and a speed restriction.
Assume that your VNAV PTH descent is at 240 kt at that moment and the speed restriction is 210 kt.

Will VNAV automatically "see" the speed restriction and reduce the descent rate (pitch up) to meet the lower speed? If yes, won't it then miss the altitude crossing restriction, as the planned path is modified?

If the PIC uses SPD INTV to set the limiting speed, won't VNAV switch to VNAV SPD and thus once again automatically pitch up to reduce the speed? Again, this may result in not meeting the altitude restriction, as you will end up higher than your intended path.

So, the question is, what should you do in a situation like this?
My guess is, revert to V/S and MCP SPD, use Speed Brakes and arrange to be over the IAF at the correct speed and altitude, and then re-engage VNAV.

With both restrictions entered on the LEGS page, VNAV will build a deceleration segment immediately prior to the point to reduce speed from 240 to 210 knots in (almost) level flight. There is no need, typically, to come out of VNAV, use the speed brakes, or do anything else like that in order to have VNAV meet both speed & altitude constraints.
Thus, the Top of Descent point will be about 3 miles sooner for a restriction to "cross XYZ at 9,000' & 210 knots" than it would be to "cross XYZ at 9,000' & 240 knots". It takes about 1 NM to slow 10 knots in level flight at idle.
The exception to not needing to come out of VNAV is if you end up with excess energy - perhaps unexpected tailwinds. In that case, you have to lose the energy, which is most easily done with the speedbrake. Switching between modes doesn't bleed energy, although it may affect the tradeoff between kinetic & potential energy. (If you have enough distance, using Speed Intervention to increase speed CAN bleed off excess energy by increasing drag. BUT, you then have a longer decel segment... so to gain from this you either have to spend several minutes at the higher speed, or be descending to a point with only an altitude restriction, but no associated speed.)


What pieces of evidence do investigators have access to in the MH17 crash investigation? In September 2014, Dutch investigators published a preliminary report for the MH17 crash. I understand they based there report on radar records, CVR, FDR and photos and thus can only say the flight goes as expected until multiple impacts from unidentified projectiles came from outside the plane.

I thought it was easy to identify those projectiles based on fire traces and chemical analysis on remaining wreckage, but it seems they were not able to do such analysis.

So my question is: what evidence do they have to perform their investigation? If they have wreckage, how can these be pieces coming from the aircraft? Based on the pieces they recovered (and ignoring the political background due to the Ukrainian situation), what kind of conclusion can be expected?

Air Accident Investigation

Evidence available to the air accident investigators can include

Flight data recorder.
Cockpit voice recorder.
Aircraft wreckage.

residues, burn marks, punctures, tears and evidence of stresses.

ATC communication recordings and controller statements.
Airline records.

Aircraft maintenance records
Pilot schedules, dates of health checks, training, hours on type etc.

Radar and ADS-B data (position, speed, altitude etc).
Technical data transmitted in flight to airline, engine maker etc.
Photographs, measurements and other reports from investigators at crash site.
Eye-witness accounts from people on the ground.
Relevant NOTAMs.
Data from other aircraft in the vicinity at the time and in previous and following days (e.g. typical tracks and variations over eastern Ukraine).

In the case of MH17 the situation is obviously made more difficult by the armed conflict at the crash site and the wider political conflicts. Investigators have to take into account that some evidence may have been tampered with, destroyed or fabricated.

As others have noted, the purpose of an air-accident investigation is not to determine legal liability. Where needed, there is an entirely separate criminal investigation for that purpose.

Until they publish their report, we won't know whether the evidence available to them is sufficient for investigators to make any recommendations for changes by manufacturers, operators or regulators to reduce the number of future future occurrences of this type of incident (e.g. recommending airlines don't fly at any height over areas where there is armed conflict).

Other Investigations

Evidence relevant to other types of investigation might include

US records of alleged missile launches detected by strategic/tactical missile detection satellites.
Ukrainian alleged intercepts of separatist radio communications.
Online material allegedly published by Ukrainian separatists.
Online photos and videos alleged to be of Russian missile launchers in transit through the Ukraine-Russia border regions.

It is difficult to see how the conditions could ever be conducive to a satisfactory judicial investigation of this material acceptable to all parties with an interest in the incident.


What are these grilles in the passenger cabin of this 787? Below the windows, ahead of the seat, there are small grilles. Look:

Source: Airliners.net

What are those for?

The answers to this question indicate that these are the exhaust vents for the cabin air circulation. I think they appear to be much too large for just that purpose. I don't have a technical reference for this, but I believe those vents also prevent the cabin floor buckling in the event of a sudden pressurization failure.

The cargo hold is beneath the passenger cabin, and both are pressurized because the cylindrical hull can resist pressure better than the flat cabin floor. If the cabin suddenly loses pressure the air in the hold could buckle the floor upward, or if the hold loses pressure the air in the cabin could buckle the floor downward. The floor must be generously vented to prevent this.

US patents # 6129312 and # 5137231 claim systems for combining the cabin air circulation vents with the depressurization relief vents. From the '9312 patent:

  An air return grille mounted below the passenger cabin sidewall
  comprises openings to allow return airflow from the air conditioning
  system. The grille openings are covered by a single thin baffle with
  cutouts within its periphery that slip over and are held in place by
  fasteners on the molded grille. During normal operation, the thin
  baffle restricts flow through a small opening in the grille. During a
  sudden decompression of the cargo compartment, the baffle is dislodged
  from the fasteners. This allows the airflow to pass through all of the
  molded grille openings in order to achieve rapid pressure
  equalization.


How can a flying wing style airplane recover from a spin without a vertical stabilizer? I fly a Cessna C-172, and if I have any bank when stalling, I have to use rudder to level the airplane. If I use aileron, the bank angle will increase and bring the airplane into a spin.

My question is, if there is no rudder or vertical stabilizer, like a B-2 bomber for example, how could a plane recover from a stall?

This question is different from others because this is not focus on the stability or control of 6 freedom. I would like more discussion on the recovery of spin and stall.

The answer depends on the type of flying wing aircraft.

Drag rudders

The B2 for example is an example of a flying wing aircraft with rudders. It's just that it doesn't use conventional boat style rudders attached to vertical stabilisers since it doesn't have any vertical stabilisers. Instead it uses drag rudders.

There are several different types of drag rudders. The B2 uses one of the most effective designs as proven by countless experiments: the split elevon (sometimes called duckerons since they look sort of like duck bills). If you look at pictures of B2 landing you can clearly see them open at the wingtips.

Regardless of the type of drag rudders, they work via 2 principles:

The simple to understand one whereby if you increase the drag on one wing you make it advance slower than the other wing thus inducing a yaw.
The less obvious principle of shifting the center of drag backwards. This is the same principle of how badminton shuttlecocks work. Indeed, this is the same principle of how vertical stabilizers work. If the center of gravity is in front of the center of drag you get a weathervaning effect where the draggy part would swivel to the rear. The B2 achieves this by opening both drag rudders. In effect the B2 is creating a less efficient, more draggy, virtual vertical stabilizer. In practice you'd only need this extra stability when flying slow. At cruise speeds the drag rudders would be mostly closed (especially if you have them gyro stabilized, if you don't have computer controls and gyros you'd keep them partially open for stability).

Wing twist/washout

Some flying wings don't have drag rudders. Instead the wing is twisted to achieve a lift distribution that would cause the wing to have proverse yaw. The Dunne series of flying wings were promoted as inherently stable aircraft because they didn't experience adverse yaw. If you have proverse yaw you can simply use your ailerons to turn the plane without rudders.

The big disadvantage designs that don't use drag rudders or any vertical stabilizers is that you depend on banking in order to yaw. This makes landing on straight runways impractical so most such planes land on grass fields which allows the pilot to choose landing direction so that crosswind is not an issue. A lot of Dunne flying wings were seaplanes.


Why do airplanes have rounded windows? Why are the windows of an airplane usually rounded at the edges?

A pressurized fuselage needs rounded corners at all cut-outs to avoid the accumulation of stress in corners. If the corners were not rounded, the fuselage skin would need to be locally reinforced, or it would begin to crack there soon.

This was learned the hard way by de Havilland, when their Comet jet airliner began to plunge out of the sky for unknown reasons. In one of the first major crash investigations in aviation history, the wreck of one was lifted from the seafloor off Elba and re-assembled. Then it was learned that the pressurized cabin had burst from cracks emanating from the window corners.

de Havilland Comet I. Note the square windows. The window which caused the destruction of G-ALYP was on top of the fuselage and cannot be seen in this picture. (Picture source)

Finite element simulation of corner stresses in a Comet fuselage skin.  Red denotes the highest stresses. (Picture source)

Other aircraft companies had built pressurized cockpits twenty years earlier and did it right the first time …


Do any Waco aircraft have model names? Many aircraft manufacturers give names to the aircraft models. Piper had the Cub, Super Cub, and all the Comanche/Cherokee/Navajo/etc. names used in branding. Cessna has the Skyhawk, Skylane, etc.

Did Waco aircraft use anything other than the three-letter names for branding?

Edit: I am wondering about the original WACO aircraft, not the company that produces reproductions today.

Waco Aircraft was originally called "Weaver Aircraft Company of Ohio" in 1919. The first Weaver Aircraft Company aircraft was the 1920 "Cootie" but was it also called the "Waco Cootie". It was a monoplane that crashed on it's first flight and then rebuilt as a biplane.

The aircraft designs were called "Waco" as in "Waco 9" "Waco 10" even though the company became "Advance Aircraft Company" in 1923. In 1929 the name was formally changed to "Waco Aircraft Company".

The last Waco production aircraft design was the E Series "Aristocrat" biplane. There was also one prototype Model W "Aristocraft" which was a pusher prop, monoplane.  All the other Waco types had model designations consisting of letters and numbers, or military designations.


When do pilots hold behind ILS Critical area? I understand the puropse of the ILS critical area. But I have never operated at an airport having an ILS critical area. So I do not know when pilots hold outside the critical area but not at the RWY holding position marking.

Do pilots always hold behind ILS holding position marking?

Otherwise, pilots hold behind RWY holding marking and with a particular ATC instruction pilots hold behind ILS holding making?

If so, how will a controller instruct the pilots?
I want to know controller’s comments.

In the US (and you're looking at an FAA manual, so that implies US rules), this is covered in JO 7110.65, the FAA order covering air traffic control procedures. Flip to page 135 of that PDF (note that the linked version is not yet the currently effective version) for Section 3-7-5:

  3−7−5.  PRECISION APPROACH CRITICALAREA a.ILS  critical  area
  dimensions  are  described  in FAA  Order  6750.16,  Siting  Criteria
  for  Instrument Landing Systems. Aircraft and vehicle access to the
  ILS  critical  area  must  be  controlled  to  ensure  the integrity
  of ILS course signals whenever the official weather observation is a
  ceiling of less than 800 feet or  visibility  less  than  2  miles.
  Do  not  authorize vehicles/aircraft to operate in or over the
  critical area, except  as  specified  in  subpara  a1,  whenever  an
  arriving aircraft is inside the ILS outer marker (OM)or the fix used
  in lieu of the OM unless the arriving aircraft has reported the runway
  in sight or is circling to land on another runway.

  PHRASEOLOGY−HOLD SHORT OF (runway) ILS CRITICAL AREA

It goes on from there to describe further details, including instructions on when to keep the critical area clear and instructions to warn aircraft conducting autoland approaches if the critical area will not be protected.

In short, pilots hold short of the critical area if they're instructed to do so, and controllers tell pilots to do that by simply saying "HOLD SHORT OF (runway) ILS CRITICAL AREA."


Why do older airplanes have a wing on both the top and the bottom of the aircraft? The first airplane by the Wright brothers had a double wing. This concept continued through World War 1 and into the 1930s - why was this concept popular?

Mainly for strength. Up to about 1920, wings used very thin airfoils and were mainly made of fabric stretched over a wooden frame. The main spar was too thin and would have been prone to bending if it had had to support the whole weight of the aircraft in a single span. Only by using two wings, the upper as the compression member and the other as the tension member of a truss, the needed strength was possible.

Airfoil thickness comparison by D.R. Kirk, Florida Institute of Technology (picture source)

In your photo, you can just about see the wires running from the top wing (near the outboard struts) to where the bottom wing joins the fuselage. These wires take most of the strain in flight, spreading the load on the wings. It's the same principle as using triangles in the construction of electricity pylons or bridges.

Monoplanes of that time needed even more wire bracing. Look at the picture of a replica of the Etrich Taube, a very popular plane of the pre-WW I period.

Etrich Taube in flight. Note the truss below the wing and the many wires keeping it in shape (picture source).

The shorter wingspan of a biplane also reduces the load on the wings. Biplanes tend to have a lot of lift (for their size) because of the large wing area, but also a lot of drag, so they're quite inefficient overall. There were also triplanes, with three mainplanes for even more lift in the same wingspan. WW I pilots demanded the highest rate of roll, which could best be achieved with biplanes.

In the time between the World Wars, the use of high-strength aluminium for aircraft and the monocoque airframe both allowed airframes to become stronger. This meant that monoplane designs became more practical, even though the older aviators didn't believe a monoplane could be strong enough. As soon as monoplanes were practical, the improvements in efficiency and flight performance saw them replace bi- and tri-planes.


What would prevent the installation of windows on the roof of an airliner? What prevents airline manufacturers from putting a strip of glass along the roof of the aircraft? It would be a good addition in terms of cabin view, light inside the cabin, etc.

Would it compromise the structural integrity too much?

Edit: The image is for representational purposes only. My question does not pertain to a installation similar to this image

There are a few factors involved in this and you should check out this answer that touches on window size (same issue really). For what its worth the ceiling is prime real estate on an aircraft. Modern airliners have the over head baggage compartments there (which provide a remarkable amount of space)

The area is also used to run cabling (electronics mostly these days) fore and aft as well as power to the reading lights etc. However it should be noted that these things could most likely be routed along the floor just as well.

The real issue comes back to windows being a weak point and glass being heavy. When it comes to planes you want them to be as light as can be so excess windows will only reduce useful load and increase potential failure points. On top of all that you will have more ambient light in the plane which may bother those trying to sleep on late night flights.

If you want a better view fly an unpressurized plane!

While on the topic of small bubble canopy planes, anyone that flies GA planes (especially like the ones shown above) with out air conditioning will tell you how hot it gets on the ground (and even at lower altitudes flying) in the summer. The canopies make the inside of plane behave like a green house and really heat up. This would make the inside of a jumbo jet really bake in the summer while taxiing (which can be a long process at big airports).


how to know about designed airways above my house Newbie here.
I hear quite a lot airplanes and helicopters and all kind of weird flying things above my head, at least one every 2 or 3 minutes, so I started tracking them with ADS-B.
There's a commercial airport in the region (MXP), not extremely close, but it seems that my house sits on some kind of airway (? not sure about the correct word) because everything leaving and approaching that airport to/from east just passes over my head making a lot of noise and I do not notice the same thing when driving nearby.
So my question is: is there a publicly available information about these routes? Are they static? I tried to check skyvector but I can't understand those.
Is there some kind of altitude limitation flights must comply? Just for fun, I'd like to compare those with ADS-B received data.

Edit: screenshot attached, on the left there's the airport, on the right two planes and an helicopter that just passed above my head.
Highlighted one is almost above my house.

Airways is a term that is used for the sort of "highways in the sky" that a lot of commercial air traffic use.

However, as you are reasonably close to an airport what you're most likely referring to is SIDS (Standard instrument departures) or STARS (Standard terminal arrivals). Both of these can be found quite easily by searching for the airport name/identifier along with those 2 terms

One such example is here: https://v1aviation.co.uk/wp-content/uploads/2016/10/LIMC.pdf

It is a long document with a lot of information, however around page 19 you start to see some example departure routes from one of the northerly runways, which do indeed turn east and west depending on the route the aircraft is taking. You might recognise some of the local topography and be able to estimate where your house is based on local knowledge.

Edited to add: The screenshot you have added looks similar to the easterly departure from 35L.

However, as pointed out in comments, the actual departure of that flight was from 35R, so the SID followed is likely to be the one later on in the linked PDF (pp 23-onwards).


Do the Mt. Everest rescue helicopters have modified engines to operate at high altitudes? There are so many climbers on Mt. Everest today that there is now a helicopter rescue service there. They operate rescue up to 21,000 + ft. Because this is essentially stationary service it seems that engine modifications like a larger compressor section would be warranted.

In stories on the internet regarding these helicopters, none mention engines.

My employer had two Rolls-Royce RB211s operating at about 7,000 ft as stationary power and the power rating was cut in half for the altitude, so operation at 21,000 ft is impressive.

No, the helicopters are standard production versions. The Eurocopter AS350 is a common model used for these operations.

In 2005, Didier Delsalle landed a Eurocopter AS350 B3 on the summit of Mt. Everest at 29,029 feet (8848m) (twice). The only changes he made to the standard version were removing a few things like extra seats to reduce the weight, and of course flying solo. It also took careful planning and the use of updrafts to get there.

The B3 version of the AS350 (now called the H125) is marketed as high performance, designed for operation in hot and high conditions. It also holds the record for the highest long-line rescue mission at 25,590 feet (7800m) (in the Himalayas).

Although the official ceiling for hovering out of ground effect is 11150 feet (3400m), the flight ceiling is 23,000 feet (7000m), and rescues at 20,000 feet (6000m) are typically limited to 2 passengers rather than the full capacity of 6.

The Safran Arriel 2D engine used in the B3 version is rated up to 952 Shp on takeoff and operation up to 29,500 feet (9000m).


Can a fighter jet be hit by guns? Can a fighter jet travelling at fast speed be brought down by a weapon like an M134 Minigun or any machine gun?

They're called SPAAG's -- Self Propelled Anti-Aircraft Guns. Older systems tended to fire 23 mm shells. Newer systems tend to fire 30 mm shells augmented by short range missiles.

They are equipped with radars and high-speed elevation and traverse motors for the turreted guns, so they can react quickly to fast a/c flying low ahead. They're also useful against attack helicopters, which also hug the ground (see below).

These are short range weapon systems, typically only effective under 10,000 ft, the guns at much less than that.

They're still useful even today. Medium and long-range SAM's have forced a/c to fly low, below their search and fire control radars, but within range of the guns.

This was standard NATO doctrine during the late Cold War---low level penetration---and gave rise to the F-111 (for example), which was designed to fly fast and low, hugging the bumpy terrain of Central Europe. This would have worked very well had the Soviets not fielded SPAAG's (basically modern AAA) and developed their own MANPADS (shoulder-fired SAM's). Soviet AWACS and look-down shoot-down radar also contributed to the decline of this tactic. These systems denied the use of low-level airspace so effectively that during Desert Storm, the Coalition air forces basically gave up low-level operations within the first week and switched to bombing from medium altitude exclusively (above the reach of AAA and MANPADS). Not even the A-10 with its titanium armor could long survive in that environment. Afterward, they did occasionally venture below 10,000 ft, but typically with special permission and enthusiastic support. Even after the first week, when Iraqi air defenses were ineffective, few a/c still flew below 10,000 ft because the AAA and MANPADS were too numerous to be eliminated entirely.

NATO air forces are getting around this problem by buying stealth fighters, namely the F-35A, which can hide from SAM radars, allowing it to fly at medium/high altitudes, far above MANPADS and AAA. But that's for another question/answer.

Here's an older Shilka (Vitaly V. Kuzmin - http://vitalykuzmin.net/?q=node/306):

A later Tunguska (Leonid Dzhepko / Л.П. Джепко):

And a modern Pantsir (Vitaly V. Kuzmin - vitalykuzmin.net):

(All images taken from their respective wikipedia pages.)


What will happen to the IDG disconnection mechanism if we press and hold it for more than 3 secs? I read in the Elec systems chapter that if we press and hold the IDG pushbutton for more than 3 secs it’ll damage the disconnection mechanism. What exactly happens? Also it says that not to disconnect it if the engine is not running as if we do and then start the engine it’ll damage the IDG. How does this happen?

(A320 training manual)

Pressing the IDG disconnect sends a current to a solenoid. A solenoid is "a cylindrical coil of wire acting as a magnet when carrying electric current."

When activated it pulls a pin that is holding another spring loaded pin that engages the disconnect mechanism.

Holding it for +3 seconds will overheat and thus destroy the solenoid and will require replacement to be able to use it again.

The disconnect mechanism requires a running engine. See here: What happens when disengaging the electricity generators on a B747?

So if it is not running, the next start may also damage the disconnecting mechanism.


What does the last 'B' in 'GEnx-2B67B' stand for? What does the last 'B' in GEnx-2B67B stand for?

I guess it stands for Boeing, but the GEnx-1B series doesn't have it.

Based on the below (in chronological order), it very much looks like the (or a) difference between GEnx-2B67 and GEnx-2B67B is a booster anti-ice (BAI) air duct. And this suggests that the 'B' stands for 'BAI' or 'booster anti-ice'.

  We propose to adopt a new airworthiness directive (AD) for all General Electric Company (GE) model GEnx-2B67 and GEnx-2B67B turbofan engines with booster anti-ice (BAI) air duct, part number (P/N) 2469M32G01, and support bracket, P/N 2469M46G01, installed.

https://www.federalregister.gov/documents/2013/04/11/2013-08447/airworthiness-directives-general-electric-company-turbofan-engines

  The Boeing Company and GE asked that the GEnx-2B67 turbofan engine be removed from the Applicability section of this AD. The commenters noted that this engine does not have BAI hardware, therefore, inspection and modification of the BAI does not apply.

https://www.federalregister.gov/documents/2013/08/19/2013-20097/airworthiness-directives-general-electric-company-turbofan-engines

NB: Another hypothesis, that the 'B' is some kind of alphabetic numbering, seems refuted by the lack of an 'A' or 'C' model. Google will find very, very few (1 or 2) hits for 'D' and 'E', which could be typos.


When is turbulence most dangerous to an aircraft? In what general situations or flight scenarios is turbulence most dangerous to aircraft? When could it be bad enough to bring down an aircraft (even up to something as big as an A380)?

Wake turbulence can affect aircraft in the most critical phases of flight such as landing approach.

For example, consider this AOPA report

  On June 12, 2006, while on visual approach at Kansas City International Airport in Kansas City, Mo., the pilot of a Piper Saratoga crossed below the flight path of a Boeing 737 that was landing ahead on a parallel runway. The Saratoga encountered wake turbulence so violent that it tore apart the aircraft in flight. The pilot and his passenger were killed.

The effects are most severe for small aircraft entering the wake of a large aircraft but large aircraft can be affected too.

Skybrary lists several examples of incidents involving wake turbulance.


How do you design a NACA intake? What are the construction guidelines to geometrically build a NACA intake (submerged intake) for aircrafts?

The relationships between the aerodynamic performance parameters (ram recovery, drag, and mass flow rate) with the geometric parameters (width of the intake, height, length of ramp, ramp angle, inlet radius and angle, divergent wall angles, divergent wall curve radius, etc...) would provide a guide to begin the construction process for a base model, which could then be refined with further investigation for a specific aircraft.

There are even tools to construct the curve of the NACA intakes. However, the background of the tool as its aerodynamic justification isn't available or at least not found.

I would therefore greatly appreciate any material on the correct geometric construction of NACA intakes.

NACA itself published quite a few papers on their submerged duct design. As a starter have a look at:

Mossman, Emmet A., et al. NACA-RM-A7I30 "An experimental investigation of the design variables for NACA submerged duct entrances", 08. January 1948
Frick, Charles W., et al. NACA-ACR-5I20 "An Experimental Investigation of NACA Submerged-Duct Entrances", 01. October 1945

The NASA Technical Reports Server as well as the UK archive of digitised NACA Reports offer a lot more material for further investigation.


What are the potential threats of having the landing gear dirt from mud in a rectractable landing gear airplane? Recently I have been flying an aircraft with the landing gear very dirt with mud due to the parking area of the airfield on grass and the meteo conditions in recent days. I wonder what are the threats to have the landing gear with mud on it especially with retractable landing gear system as I had that day. Just for info I was flying a PA28R-201.

The main issues would be mud interfering with the uplock/downlock mechanisms, the switches that control indication of the function of those mechanisms, the brakes, and wheel balance.   Mud + freezing temperatures = worse.  Gears won't lock down, or they lock down but you don't get an indication, or the wheels freeze and skid on landing and blow tires, or mud in the wheel rim makes the wheel vibrate all to hell on takeoff.

There is usually enough robustness in the design that it can tolerate some contamination (obviously, it has to have some ability to take on mud on takeoffs from dirt runways), so as with most things, it depends how much when you look at it.  A little bit of mud spray here and there, not too big a deal.  Mud caked on on the wheels, brake calipers, any of the mechanical bits in the wheel well, that could interfere with moving parts, it should be removed before flying by hosing it off.

The mechanic who maintains the airplane you are renting would be a good person to ask about how to judge when mud contamination is excessive.


Need to add elevator downforce to total lift when sizing wing area? Do I need to add elevator downforce to total lift when sizing wing area?

Say a small airplane weighs 1,000 lbs, and both the wing and elevator have a max Cl of 1.5, and my elevator is 20% of my wing area, so max elevator downforce is roughly 20% of 1,000 lbs or 200 lbs in a classic configuration.

Doesn't the main wing then have to carry 1,000lbs + 200 lbs negative elevator
lift, for a total lift of 1,200 lbs?

If the plane is in a tandem configuration, the main wing would only have to carry 800 lbs, as the elevator has 200 lbs of positive lift,for a total lift of 1,000 lbs.

1,200/800 lbs is 150%!!, so if I use the tandem configuration, my main wing can be 50% smaller!!! Is this correct?

No.

First, your horizontal tail will need some control margin. A trimmed state where it flies close to c$_{l_{min}}$ is a grave design error. Next, your horizontal tail sees the same angle of attack change as the wing, reduced by downwash. This means that the strongest downforce is trimmed in fast flight, when the angle of attack is small. Fly slower and angle of attack rises not only on the wing, but also on the tail. Chances are that your tail produces positive lift in slow flight.

Flaps will change this picture slightly. Setting flaps will need you to re-trim the aircraft because now the center of the wing's lift has moved slightly backwards, so the load on the tail becomes more negative.

Fun fact: If you have a symmetrical wing airfoil, the trimmed tail load will be constant over the whole envelope. In order to achieve the best control power, this tail load should ideally be zero.

Now for the tandem wing: You assume that now both surfaces create lift, so overall drag should be lowest. But adding area on the tail will reduce the needed wing area. Given that the planform shape stays constant, less area means less wing span. However, induced drag is proportional to the span loading, and a lower span will mean more induced drag. Therefore, a conventional design with an unloaded tail of minimum size will have the lowest drag.


What is this British bomber? From previous answers this is probably at RAF Duxford in the mid-1930s.

Source: wikimedia.org

I would say an Armstrong Whitworth Whitley.

Somehow the uncommon nose section looks like a special variant, maybe a radar-equipped submarine hunter?


What is the maximal altitude at which an emergency slide can be deployed? If I understand correctly, the mechanism to inflate an emergency slide relies on a tank of inert gas and on an "aspirator" that sucks ambient air. This aspirator works thanks to depression causes by the stored inert gas velocity (Venturi effect) while it inflate the slide.
This implies that the slide requires enough outside air to inflate correctly. I'm confident such mechanism are certified to work at high altitude international airport (I'm thinking of Lima but there may be other international airport at high altitude), but an aircraft can still crash in high altitude mountains, higher than any airport.
I wonder what is the minimal outside pressure required to inflate an emergency slide. Maybe air density is more relevant than pressure. Given the standard atmosphere model, both can be converted in altitude (either using pressure-altitude or density-altitude).
What is the maximal altitude at which the mechanism to inflate an emergency slide can work as expected (i.e. suck enough outside air to inflate the slide)?

In the US, TSO-C69c details the FAA's technical requirements and testing protocols for evacuation slides. I can't find anything in it that talks about a maximum altitude or minimum air pressure. The closest thing I can see is this requirement in 4.2 to function correctly in a given temperature range:

The device, including its inflation system, must be demonstrated to be
capable of fully functioning when subjected to temperatures from -40
to +160 degrees F. If the device is intended for installation outside
the pressurized cabin, the device must be capable of functioning after
being stowed at -65 degrees F. The function of the device must be
demonstrated in accordance with the hot and cold soak test procedures
described in paragraph
5.9 of this appendix.

Sections 4.11 on inflation operation and 4.15 on the inflation system don't mention anything about ambient atmospheric pressure.
And the testing protocols in section 5 of the document only mention temperature, not pressure. For example, section 5.9.3 is part of the cold soak test protocol:

Deploy the device into ambient temperature conditions (typically
defined as between 65 and 85 degrees F) from the appropriate airplane
door or a suitable airplane door mock-up or module, within 10 minutes
after removal from the conditioning chamber.

4.15.2 has the only reference in the document to an aspirator system and it only requires a filter to avoid ingesting small objects.
So, unless I've missed something in the TSO - which I might have! - maximum altitude/elevation or minimum ambient atmospheric pressure aren't part of the required certification, at least in the US.


What are the differences between radar flight following and traffic advisories? In typical usage, the terms seem to be used interchangeably. However, aircraft must be transponder- or encoder-equipped for traffic advisories to be possible, for example.

What information does each provide to pilots that the other does not?

Depends on the ANSP (Air Navigation Service Provider) to what each term means to them. In most ANSPs they mean exactly the same thing or their individual regulations only have one of these terms/procedures available.

A traffic advisory technically doesn't always require a transponder especially if it is of a snapshot of traffic. The pilot requesting the traffic advisory gives position and a one time traffic picture is provided to the pilot. This is mainly relevant in areas where there is not Secondary Surveillance coverage and/or no surveillance coverage at all.

However Flight Following does require being able to be identified for continual coverage. Some ANSPs also provide additional reporting services with those receiving flight following like SAR services and Hazard Alerting services.

Some ANSPs only provide these services at the discretion of the controller based on their own perceived workload.

Check the local regulations for the ANSP your flying in to get exactly what you will be provided


Can a super land without flaps/spoilers? Can the Airbus A380 or Boeing 787 land safely without flaps/slats/spoilers or thrust reversers?

Even the largest commercial airliners are able to land without flaps, since flap failures do happen occasionally. See a report here where an A380 landed with no flaps. This was at the Auckland, New Zealand airport, where the runway is 3,635 m (11,926 ft) long.

The pilots have checklists to follow in the event of issues with flaps, which include information about what speeds they should fly with what amounts of flaps. They simply land at a higher than normal speed. The brakes can end up getting hotter than normal, so they may have to stop and let them cool or have them inspected by emergency services. The tires are designed to deflate with fusible plugs before high temperatures would cause them to blow.

Aircraft are also tested to make sure they can reject a takeoff at high weights (higher than normal landing weights) using no thrust reversers. Here is a video of the 747-8 doing this test.


How can I get 'lead point' for aircraft to intercept final course? As controller, I always wonder when should I instruct aircraft to turn. The aircraft does not turn at right angles as soon as instructed, but rather has turning radius. So, I tried to find the formula but failed.

If speed of aircraft is 300 kts, and he is instructed to make 90 degree, how long distance the aircraft need to turn? I make image, and I wonder the formula for 'x'

I was taught the following rule of thumb for providing a final vector to final, in the USA, using the STARS radar system which (when using ADS-B inputs) has a one-second update time:
Set the Predicted Track Line to 30 seconds. Vector the aircraft to a 90º base-to-final. When the PTL reaches the extended runway final, issue the Position-Turn-Altitude-Clearance. The aircraft should roll out on a good heading to intercept final approach course.
This is obviously at low altitudes and relatively low speeds, 230ish knots or below. Theoretically (because the method uses the aircraft's speed instead of a fixed distance) this method should work for any aircraft going any speed, but that may not always pan out. In particular, some military aircraft will be flying rather quickly on the base leg and slow down as soon as they get their dogleg turn-to-final, which could end up leaving them too far from final for a legal intercept.


Is there a formula to calculate how big a ventral fin should be? I've been doing some research into vertical stabilizer designs for a university assignment and have come across ventral fins. I've found a fair bit of information on why they're fitted but can't find any formula or rules of thumb to determine how big a ventral fin should be.

I was wondering if anyone knows any of the maths behind ventral fin sizing and the stability it provides?

There is no single formula - the number of factors to consider are too numerous and the load cases are too diverse. Here is an incomplete list:

Directional stability: Depending on fuselage size and shape, the tail must be sized to pull the aircraft into the wind direction. The fuselage is destabilising in yaw and the tail is needed to counteract that. Depending on factors like wing sweep, some of that stabilisation can be contributed by other parts of the airframe.
In multi-engined aircraft: Yaw from asymmetric thrust. If one engine fails, the fin must be large enough to create an equal but opposite yawing moment to that of the still running engine on the other side. Multi-engined aircraft have a minimum speed below which directional control with one engine inoperative is lost.
In high aspect ratio designs: Counteract adverse yaw. When ailerons are deflected and the aircraft rolls, the direction of lift on the side with the trailing-edge-up aileron tilts forward and vice versa. This horizontal lift component creates an undesired yawing moment that the tail needs to neutralize. This effect varies with speed, so if you want good low-speed maneuverability, this load case determines vertical tail size (mostly of gliders).
Dutch roll damping: Sometimes static stability in yaw is not sufficient and dynamic stability sizes the tail.
Maximum flight Mach number: At high Mach, the upper side of the plane flies in less dense air and a traditional vertical tail loses much of its efficiency. Here a ventral tailfin can help to reduce tail size considerably.

Add to this the fact that the destabilizing effect of the fuselage increases with sideslip angle and the fact that the vertical tail as the higher aspect ratio surface of the two will reach its maximum lift first and suffer a noticeable decrease in effectiveness at larger sideslip angles. Now details like a highly swept fillet are needed to maintain tail effectiveness at high sideslip angles.

Finally, it depends which of the load cases is sizing your tail, and that can be determined by factors like fuselage length. Tail effectiveness for directional control grows linearly with fuselage length and for yaw damping with the square of fuselage length. If you want to fly the same tail on fuselages of different length (like most airliner models do), your tail might be sized by a different load case on a different aircraft than the one it is mounted on.

If you want to fly low and fast, it might be that the loads on the vertical deform the structure so much that the efficiency of the vertical tail is greatly reduced. In that case it does not help to make the tail bigger, instead ventral fins must be added. Now you need not only to look at the best tail size but at the best overall tail configuration as well.


What exactly is a QRH? In this question about aircraft dumping fuel I was told that there may possible be guidance in the QRH.  I know that there are a lot of guidelines given to pilots, but I've never heard of this one.  Would anyone care to explain, from a pilots perspective, what exactly a QRH is and how it's used in modern aircraft operation?

A QRH is a Quick Reference Handbook and is an approved (by some overseeing entity) document specific to an airplane and operator that contains checklists for just about anything the airplane can experience.  At my airline we had copies in the airplane and also in our company flight manual (CFM).

The entry point into a QRH checklist is one of a few things

Memory items
EICAS message
An abnormal event

For option 1, there are various checklists that must be memorized and performed at a moments notice with no reference to the checklist.  Some of these are things like trim runaways, engine fire/severe damage, emergency descent.  Once the memory part of the checklist is complete you refer to the printed version of the checklist in the QRH to verify and perform additional steps beyond the part you were required to memorize.

For the other options, any time anything abnormal happens or an EICAS message is presented, there are indices by procedure name and also by EICAS message into a specific procedure in the QRH to follow.  These procedures may reference other procedures.

For example, If I were to experience a dual engine failure, I would pick up my QRH and find the procedure "DUAL ENGINE FAILURE", this procedure says I should have the master warning light illuminated (and an aural triple chime) and I should see the EICAS message (in red) "ENG 1-2 OUT".  After verifying this presentation and verifying I am in the right procedure, I would perform:

Airspeed....................MINIMUM 260 KIAS
Altitude....................MAXIMUM FL250
Oxygen Mask (if required)...DON
APU.........................START
ENGINE AIRSTART procedure...PERFORM  (note, this is another QRH procedure)

CAUTION: IF THE APU HAS BEEN USED TO START THE ENGINES,
         DO NOT ALTERNATE THE FADECs AFTER START

Note: If the APU is not available, only equipment supplied by the
      Essential DC Bus 1 and 2 will be available.  Engine windmilling
      should drive engine driven hydraulic pumps and supply hydraulic
      pressure.

IF neither engine can be restarted:
    DITCHING or FORCED LANDING procedure......PERFORM


What's a stepdown fix in a non-precision approach procedure? Could someone explain to me what's a stepdown fix in non-precision approaches?

From the FAA's Pilot/Controller Glossary:

  STEPDOWN FIX − A fix permitting additional descent within a segment of an instrument approach procedure by identifying a point at
  which a controlling obstacle has been safely overflown.

In other words, a step down fix tells you "you've passed an obstacle, so you can now descend further".


Is this BA15X flight a logistic flight for British Airways? Right now (26/11/2015, UTC 17:16), British Airways is flying over the Australian Outback with two Boeing 777-336(ER)s, christened flight BA15 (http://www.flightradar24.com/BAW15/8169bb5) and BA15X, which is slightly ahead (http://www.flightradar24.com/BAW15X/8169237).

Why is this? Is flight BA15X a logistic one? And if not, why did they use an "X" to distinguish between the two instead of another number?

EDIT

I am also wondering: is this going to cost extra money to BA, owing to the new slot they had to purchase or rent for operating a contemporary, second aircraft at SYD?

The Wednesday flight was indeed delayed for a full day as was suggested in a comment.