What biplane is this a model of? What biplane is this a model of?

The plane is probably modelled after both a Sopwith Snipe and a Sopwith Camel. Look at the cutout of the wing above the pilot's seat. It's rectangular as on the Snipe, not a semicircle as on the Camel. The engine looks like a Camel's however.

So, this model is possibly a hybrid of two famous World War I British fighter planes.

It could also be a specially modified Camel according to @Michael Tracy (except the painting scheme does not match the one on the picture he provides).


What is the minimum distance allowed between planes in flight, in Europe? What is the minimum vertical and horizontal distance allowed between aircraft operating in European airspace?

(This linked question is the US-specific equivalent of what I am asking about European regulations for ease of reference)

Separation minima in Europe are pretty much identical to the rest of the world, since almost all countries base their aviation regulations on ICAO recommendations.

To summarize:

The minimum vertical separation is 1000 ft. In theory, above FL290, the minimum is 2000 ft, but since all European countries have implemented reduced vertical separation minima (RVSM), the 1000 ft. rule applies all the way up to FL410.

When horizontal separation is based on surveillance (such as radar), the minimum separation generally is 5 nautical miles (NM). Around larger airports with excellent radar coverage and at lower altitudes (where planes tend to fly slower), the minimum is routinely reduced to 3 NM. On final approach path to some runways, it can be reduced further, typically to 2.5 NM. They can get even closer in case of parallel runway operations, but that's a bit too complex to cover here. Special rules apply to mitigate the risk of wake turbulence. For example, the separation between a light aircraft flying behind a medium aircraft can never be reduced below 5 NM.

Horizontal separation based on position reports (no radar coverage) is pretty rare, but still in use in some areas. Here, the minimum horizontal separation is typically not defined as a distance, but instead can be based on a navigation aid (for example, two aircraft on VOR radials diverging by at least 15 degrees, and one of them is at least 15 NM from the VOR, are separated). Another option is lateral separation, which is basically when aircraft operating in different areas are defined to be separated, regardless of the actual distance between them (for example, an aircraft flying along airway A is separated from an aircraft flying along airway B, since airway A and B never cross). Rules for lateral separation are defined locally.

Finally, in the vicinity of aerodromes, separation can be reduced further (often referred to as "visual separation"). Essentially, if the tower controller has two aircraft in sight, and can determine visually that there is no risk of collision, they are separated (regardless of the actual distance between them).

Remember that all the separation minima above only apply in areas where ATC is required to provide separation. If flying in class D airspace, for example, (used in many control zones and terminal areas) ATC is not required to separate VFR traffic from IFR traffic. Instead, ATC will pass traffic information, allowing the pilots to visually recognise other traffic on their path, and take appropriate action to avoid a risk of collision in accordance with the rules of the air.


Why is IGN SYSTEM A FAULT dispatch a NO GO on the A320 for ETOPS? In the A320 MEL if Ignition system A is under MEL the aircraft can't be dispatched for ETOPS. Why is it specifically restricted for ETOPS because even while conducting non ETOPS
 if we go into emergency electrical configuration we will still lose ignition system A.

Your assumption is incorrect.

In ELEC EMER CONFIG (see your QRH - ELEC EMER CONFIG SYS REMAINING), Ignition A is the only ignition available and hence that would be your reason why IGN A is needed for EDTO / ETOPS.


Wouldn't a Snow Globe prevent pilots from getting disoriented? Several times in aviation history a passenger aircraft has crashed just because the pilots were disoriented. They simply just did not understand what was the position of the aircraft that moment.

Wouldn't a Snow Globe prevent this?

There are 'earth' and 'snow'. Their position inside the glass bubble would give better clue of the position of the aircraft than current mechanisms.

One could also design a specified version for aircrafts. Maybe just bubble of glass filled with pebbles.

I think it would work brilliant as artificial horizon.

Has there ever been an experiment with 'snow bubble' as artificial horizon?

This doesn't work. A plane can be in arbitrary orientations while the the forces felt by everybody in the plane are directed straight at the floor. A famous example is Bob Hoover pouring tea while rolling a plane. If Bob Hoover's tea can't tell that it's being poured upwards, nor can your snowglobe.  Indeed, this is precisely why pilots become disoriented.


Why do planes overfly Moffett Field before landing? I live near Moffett Field, and I noticed that some airplanes go over the field once before landing, along the runway (not across), even when the tower is open. I thought that was a procedure for non-towered airports only?

Moffett Field is a federal airport (requires permission) owned by NASA and has a wide ranging number of traffic patterns/approaches available.  In answer to your question, you may be seeing what is called an "Overhead" approach.  This is a VFR maneuver quite common at military bases and usually consists of the following:

Airplane approaches the runway in use at an altitude well above the airport (2000 ft AGL or higher).  Usually this initial altitude is higher than the downwind altitude.
At some point when over the runway (over the numbers, mid-field, departure end, etc.), the pilot will break left or right and enter a descending turn that will include a modified downwind length based on the point at which the overhead break occurred.
The pilot will then continue to a base leg and ultimately a final approach (often very short) to a landing or touch and go.
This maneuver is done under VFR as coordinated with, and cleared by, the Tower.

This type of maneuver could explain, based on your question, what you are seeing at Moffett Field.  See the pictures below.

From the Moffett Airfield Operations Manual:

Example Overhead approach: (just an example, many variations are possible)


Curvature of Earth - Airplane One of my co-workers is a FLAT Earth believer. I have tried all kinds of logic and facts to disprove his belief.  He is not the sharpest knife in the drawer so trying to explain Gravity, atmosphere moving with Earth, etc is difficult.  I have read the threads trying to explain if pilots make adjustments for curvature. Answers/explanations are all over the place...

What I need is a very uncomplicated explanation as to how a plane tracks along with the curvature of the Earth WITHOUT pilot making adjustments.

Nothing you say can convince this person that the Earth is round.  You need to show him.

Find a hill that is a few hundred feet high.  One of you stands at the top, the other at the bottom and observe the sunset.  You will, of course, be accompanied by a trusted flat-earth friend of his to validate your observations.

Each person will note the time that the sun disappears below the horizon.  Your times will be different.  You can even use math to calculate the expected time delta if you really want to drive your point home.  The higher the mountain, the greater the time delta.  Also, if you're at the top, you can flash him with a mirror to prove that you can still view the sun.  Be careful though...this might blow his mind.

This is a direct observation of the curvature of the Earth.  A rational person will accept this as proof of a curved surface.  An irrational bozo will continue to offer absurd theories.  Please let us know how your friend responds.


Do stall strips mean that wing washout is not required? The Wikipedia page on stall strips states that AA-1 Yankee airplanes used stall strips to avoid tip stalls (and spins) because wing washout couldn't be used.

So my question is: can two stall strips placed on each tapered wing (tapered wings love tip stalling) eliminate the need for a washout?

Secondly, what does it mean for a wing to have a washout of 6°? Does it mean the angle of incidence at the root is 6x greater than the tip?

Rectangular wings can get away without washout because that planform tends to have favourable root to tip stall progression as a characteristic of the rectangular shape.

The stall strips aren't really to change the root to tip stall behaviour as a substitute for washout; their job is to generate a local flow separation, just prior to the natural stall, who's turbulent flow can be felt through the airframe and at the elevator while the rest of the wing is still unstalled.  That's why they are just short sections in front of the outer parts of the horizontal tail, so that the burble they produce will be felt through the controls well before the rest of the wing lets go.

The Yankee had a NACA 64-416 laminar airfoil which had an abrupt stall and very little natural pre-stall buffet - helped by the strips.  Later variants got a revised airfoil with a drooped nose to make the stall itself more gentle, but they still retained the strips to provide the pre-stall buffet.

Many years ago I flew a Yankee that was in a flying club I was in, and it was considered a demanding airplane.  The stall strips made it a bit less dangerous than without, but it still took careful speed control when landing.  If you got too slow it would start to sink and pulling a bit more pitch had no effect.  It had a reputation as a very "hot" airplane for something with only 108 HP.

On a tapered wing, you would be adding stall strips for the same reason as the Yankee's wing, near the root forward of the outer end of the horizontal tail, to provide a distinct pre-stall warning buffet.  If your tapered wing had no washout but had the strips, all that would happen is you would get a pre-stall buffet that wouldn't be there without them, but then when you get to the wing's stalling AOA the whole thing goes at once because it's still a tapered wing with no washout.

Some designers get some of the benefit of washout without twisting the wing by varying the airfoil section itself from root to tip.


How does a large airport switch flow? Airplanes prefer to land into the wind, and airports adjust for changing wind direction by changing the active runway.

I've been flying a small plane in the pattern when the tower asked me to do a 180° turn to reverse the pattern.

But at a large airport with traffic continuously coming-n-going, with fast moving jets that fly enormous patterns, how do they switch directions?   It seems logistically complicated.

It's not too logistically complicated on its own.  You just stop takeoffs, start taxiing the conga line of waiting airplanes to the new runway, let airplanes currently on final land, and start vectoring the other airborne flights toward the new approaches once the last of takeoff traffic is away.  Once the new landing flow is established, start releasing the takeoffs.  This will require coordination between tower and approach and may involve the center if flights need to be put in holding during the transition (depends on how busy the airspace is).  In certain busy airspace this may also require coordination with other nearby airports.

With that said, some airports that take off and land on different runways can have impacts on arrival rates in different configurations, and this adds to the logistic complication.  One example is KIAH, which in normal flow is taking off on the 15's and landing on the 26's and 27.  It isn't too much trouble to switch landing traffic to the 8's and 9, but if takeoffs have to be switched to 33 then the flow rates are negatively impacted.  It gets even worse if landing and takeoff traffic have to be put on the same runways.  How the arrival and departure flows are impacted (if at all) from a runway switch will depend strongly on the specific geometry of the runways.

Anecdotally, the logistic impact of reduced arrival and departure flow is probably the largest impact.  At KIAH they will run their normal flow until the wind gets so bad that pilots start refusing clearances for tailwind limitations, and only then will they switch the departures to northbound.  Similarly, as soon as the tailwind is legal, they'll switch back to southbound departures.


Is there any disadvantage to slipping as a way to lose altitude? I learned how to slip during my powered training as a way to lose altitude when too high and, of course, a way to track the runway center-line during a crosswind landing.

Quite a bit more emphasis was put on slipping during my glider training.  I guess the reason for this is pretty much common sense.  There is no concept of a "go-around" when landing a glider...you only get one shot.  That said, I don't practice slips often enough when soaring because purposefully losing altitude in a glider is counter-productive, except when too high in the pattern.

For this reason, I've now gotten into the habit of using a slip to descend when flying powered.  This allows me to practice slipping with the knowledge that I am guaranteed by virtue of having an engine to have the option of regaining that altitude.  Note: I usually only do this while flying solo since slipping can be a comfort issue for passengers.

This got me to wondering...

Questions:

Putting aside the comfort of passengers, Is there any disadvantage to always using a slip to descend when flying powered?
If not, why does it seem that this method of losing altitude is not used/taught more frequently?

Update after reviewing answers

I wanted to give this some time to stew before I checked an answer.  I came into this fairly (but not absolutely) certain that the short answer was no.  I think the responses confirm this.  I understand the need some have to "warn" against stalling, but I don't really see how this warning couldn't be applied to any maneuver and really has nothing to do with the fact that you are slipping,  short of making a mistake...like skidding instead.

The conclusion I'm taking from the answers is:

Slipping can be uncomfortable to passengers, pretty much for the same reason that healing in a sailboat can be perceived as uncomfortable.
Slipping is aerodynamically inefficient.  Since the objective is to lose altitude, this would appear to be a benefit, rather than a detractor.
That some planes, in some configurations, have restrictions on when slipping is allowed/recommended/optimal.
That slipping will get you down faster, thus making it preferred if getting down faster is the objective.

To address your questions: First, slips are perfectly reasonable for controlling descents. In some aircraft, for example, many Cessna 172s, are placarded warning against extended slips with full flaps.  In the 172 case, there may be some buffeting.

Years ago, slips were emphasized more than they are today. Of course there were more primary aircraft without flaps then.  So today the vast majority of primary trainers have flaps, and with time instructors have simply avoided emphasizing slips as much.  Consequently, the forward slip becomes more of a saving maneuver than a routine maneuver.

Your mileage will vary. There are some instructors who put more emphasis on slips for energy management. Most instructors will emphasize them for more "urgent" maneuvers like making a field with a simulated engine out.

Addendum #1
Being the best pilot you can means constantly developing skill and adding new tools to your bag.  Slips are normally covered when doing simulated engine outs.  I also cover them first with pre-solo students when we cover other emergencies such as flap failure (asymmetric flap extension) or electrical failures with electric flaps. Slips are also helpful when there is a cabin fire, and help clear smoke faster (it's never really fast enough) in planes with bilateral windows.

Addendum #2
Slips have an advantage over flaps, in a temporal sense. You can slip, and then take it out, then put it back in.  As much or as little as you want.  Since flap changes normally mean trim changes, and in the case of the 172 there is a flap motor limiting application and removal rates, the same cannot be said for using flaps. As an example, landing during very turbulent and gusty conditions, I might only use partial flaps, and use slips for glide path management, particularly if going to some mountain top strip with all kinds of up and down drafts influencing the path. Just another thought, for someone as they consider enhancing their proficiency with slips.


What should I consider when storing a light aircraft outdoors for several months? I would like to store a Cessna outside for a period of time, close to six months. The temperature here will not exceed 30°C and will not go below 0°C.

With tie downs and a tarp, how long can I store a Cessna outside for in these conditions? Should I invest in a shed of some sort? I would prefer a specific period of time please.

Are you in a wet or coastal environment?  You can store a "Cessna type" plane outside for a few months and be ok.  Most flight school planes live outside for years and do just fine.

Don't use a tarp.  You might damage antennas, windows, or paint.  Custom fitted covers are good but they're also expensive.

Don't just start the plane and let it idle.  That hurts the engine more than it helps.  To do any good, the engine must reach and sustain normal operating temperature until the moisture boils off.  A quick flight can accomplish this.


Will the plane take off? First, I was unsure where to ask, here on on Physics SE. Second, I have a strong feeling it might have been asked already somewhere but I didn't find it - so if it was, please link as a duplicate.

Classic "plane on a treadmill"

Suppose we have an aircraft standing on a runway and it turns on engines in an attempt to take off. But that's a special runway - it acts like a treadmill moving backwards at the same speed as the aircraft is going forward (dynamically adjusting that speed if necessary).

What I did is I looked through the web on a classic "plane on a treadmill" problem. One of the best hit is xkcd which kind of states .. "yes" and then goes into different options explaining that "there is no plausible way we could build the treadmill matching its speed to the aircraft" (or did I get it wrong?)

Let's suppose we could

That is - regardless of whether it's possible or not to build such a runway, for the purpose of theoretical exercise let's just take is as granted - we have such a runway.

Will the plane still take off? What will happen?

All the treadmill does is increase the RPM of the tires at rotation.  Airliner tires are generally spinning at somewhere around 800-1000 rpm at, say, 140 kt depending on the diameter.  If the treadmill is running at 140 kt in the opposite direction, the tire would have to be spinning at double whatever it is normally when the airplane achieves liftoff speed.

The question becomes:

Can the airplane's thrust overcome the tire's increased rolling, bearing and brake drag (from casual brushing contact of the brake pads) friction when spinning at double its normal RPM?
Can the tire survive running at double RPM, well above its speed rating, without blowing up?

For:

Probably yes, but it would extend the takeoff distance quite a bit, but...
I would say probably not.  An airliner tire will have a speed rating that has a good margin above the normal operating speed, in the low 200 kt range, with probably some fudge factor above that. So it could probably survive up to, say 230 kt.  Taking it to double its normal operating speed, an RPM equivalent to 280-300 kt, is probably too much and there is a pretty good chance at least one of them would blow up.  Almost certainly if they were recaps, which most airliner tires are.


In Airbus flight control laws what does it mean that "control sensitivity depends on airspeed"? Direct Flight Law:

What does the following characteristics exactly mean regarding sidestick input at a very low airspeed (let us assume near stall speed).

'control sensitivity depends on airspeed'

Btw. is direct flight law only achievable with gear down and both FACs off?

In aircraft with mechanical controls, the position of the control column corresponds to position of the control surfaces and the force on the control column corresponds to the force on the control surfaces. At higher speeds, the same deflection generates larger force and correspondingly stronger response from the aircraft.

However in Airbus the forces are not carried and the side-stick is spring-loaded. In normal law the side-stick deflection commands specific wing loading (which mostly corresponds to pitch rate) in pitch and roll rate in roll. So you get the same rate or pitch/roll with the same deflection independent of speed.

But in direct law, the position of the side-stick corresponds to the position of the control surfaces. Because the same deflection causes larger aerodynamic forces, it leads to higher pitch/roll rate at higher speed. Significantly higher, because the control forces grow with square of speed.


Why does RPM for a fixed-pitch propeller change with an aircraft's pitch? Talking bout a C-152 -

If I pitch the aircraft down, and without adjusting the throttle, the RPM would show an increase. Pitch up, and without adjusting the throttle, the RPM shows a decrease.

I've been told that by pitching down you increase the amount of airflow your engine is receiving while pitching up decreases the amount of airflow, or that the propeller at different pitch attitudes either forces more air or acts against the air entering the intake ports, or that it's because of the propeller angle of attack.

The driving variable here is airspeed, not pitch attitude per se.  Your airplane naturally goes faster when you put the stick forward and decrease the wing's angle-of-attack, and this changes the prop rpm.

Your pitch input is changing the wing's angle-of-attack which leads to an airspeed change.  Even with the engine switched off, the aircraft would fly (glide) at a higher airspeed with the stick forward than with the stick aft.  And for a given throttle position, a fixed-pitch prop turns faster at a high airspeed than at a low airspeed.

Just as a wind turbine does.

Your tachometer can be used as an airspeed indicator.

The key factor is the speed of the air flowing through the propeller disk-- with a fixed-pitch prop, this dwarfs any effect related to the amount of air being forced into the engine.

And yes, this effect can be explained in terms of the angle-of-attack of the propeller blades.  Increasing the airspeed decreases the angle-of-attack of the propeller blades, "unloading" them, or in extreme cases subjecting them to negative loading so that the prop is driving the motor rather than vice versa.

In a fast dive at a low power setting, the wind (airflow) is making the prop move, not the other way around, and you'd pick up more airspeed if the prop fell off.  But even at a lower airspeed and higher power setting, the speed of the oncoming airflow has big influence on the speed of the prop's rotation.


How to calculate the radius of wing tip vortices? Is it possible to find the radius of wing tip vortices?

A vortex in free air does not have any sharp boundary, so it does not have an obvious definition of radius.

To get some general idea, consider how the vortex is formed: the wing accelerates the air downward, over all of its span—that is how it generates lift. Since the air outside the span is not moving down, the boundary becomes a vortex line and it starts at the wingtips. This is the wingtip vortex.

So the two wingtip vortices “touch” from the start, and therefore have radius of half span. As the air below gets pushed out, the outside part of the vortices starts moving up in width comparable to the inner part, though not as fast—the vortex as a whole moves down and a little bit inward.

This upward flow on the outside is used by many large migratory birds to save a bit of effort by flying in the V formations. The wing vortex of each follower partially cancels with the wing vortex of the predecessor, saving them a bit of energy. For the leader it is neither advantage nor disadvantage, but when it gets tired, it can fall back to the end of the flock and the next one becomes a leader.

Internal friction of air makes the vortex slow down over time, but also pulls in more air, so the affected area grows in size. There is no specific limit, it grows until it can't be detected anymore.

It is possible to calculate something precisely—so long as you can decide what to calculate (and what is it useful for so it is worth calculating)—but it requires powerful computer as the equations describing fluid motion, Navier–Stokes equations, don't have analytical solution.


Why haven't airlines added safety pads to their carts? I chanced upon this question at 5:11 of this YouTube video. Their answer is "no idea".
A cynic would answer profit motive, for it costs money to add safety pads to all these carts and airlines aren't profiting especially in this COVID19 pandemic.

Because an inch or two of rubber isn’t going to stop a 300lb cart from breaking your foot if it goes out of control down an aisle. If your elbow is in the wrong position, you could hit it with a 300lb roll of memory foam and cause a compound fracture.
The maker of this video is just looking for things to complain about. Keep your body parts out of the aisle and you’ll be fine.


Is it possible to sail a seaplane? Say a seaplane has ditched onto ocean or a very large lake due to engine failure. Is it possible to sail the plane to the shore using wind alone, similar to sailing a boat?

You can sail a seaplane, but there are a lot of considerations. For starters, it makes a big difference as to whether you're talking about a seaplane where the hull is in the water or a floatplane. A floatplane sitting up on its pontoons (floats) is more up in the wind, but it's also more susceptible to rough water because of its higher center of gravity.

My personal experience with sailing a floatplane was with a J-3 cub with an 85 horse engine on rivers and lakes. If we wanted to back onto the shore, we had to sail it since there was no reverse pitch propeller.

For purposes of explanation on how to do it, let's say that there's no wind, but we're on a river with enough current that the movement of the floatplane with the current when the engine is off gives the floatplane a wind relative to the water. Further let's say the river is flowing from east to west, and we want to back onto a beach on the north shore. We land and stop the engine upstream from the beach. As the floatplane drifts it will weathervane into the wind. Thus, with the controls neutralized, we'll be facing west and moving downstream in mid-river with the current, but not as fast the current because of the drag of the wind on the aircraft.

The rule of thumb is that you point the tail towards the shore you wish to get to. We do that by applying left rudder and right aileron. Thus we have increased the drag on the left wing, which will swing the nose left, and the left rudder will also swing the nose left. Thus the nose is now pointed a few degrees toward the left shore (south), the tail toward the right shore (north). The airplane will then sail to the right shore (north) relative to the direction the aircraft nose is pointing because the the aircraft is moving backwards relative to the water.

The trick in this instance is vary the control input to arrive at the downstream point you want. For practice, I used to have students sail to alternate sides of the river on Oregon's Willamette River downstream from the city of Eugene.


Why is Emirates not considering buying or leasing used A380s? Airlines such as Korean Air are retiring their A380s. They are struggling to find buyers/lessors. In such a scenario, Emirates could easily obtain used A380s at cheap prices.

Is Emirates not leasing used A380s because of the costs involved in re-configuring of the aircraft? Since the A380 is an expensive aircraft, I feel that even with added costs in changing seating layout and maintenance of aged aircraft, it might be economical to consider used planes.

We have to also consider that since its the same aircraft, customers won't notice any difference in service or comfort.

(Source: IATA)
Used airplanes are costlier to operate and maintain. Take Ryanair for example, they have 420 Boeing 737's with an average young age of 6.8 years. That age is very important.
The graph above shows the direct maintenance cost of an A320 for the airframe, similar graphs are available for the other systems. Note when the first period ends.

First-Run is the initial operating years, often referred to as the honeymoon period and generally
considered the first 4-6 years of in-service operation. The structure, systems, and components
are new; and there is less non-routine maintenance and material scrap rate.

The following serves as both a proof and an answer: Airbus is buying back the older A380's in the new deal with Emirates for new A380's.

Airbus (AIR.PA) will have to buy back or find new homes for some of the older A380s currently operated by Dubai’s Emirates as it finalises a deal to sell new superjumbos to the Gulf carrier.
(...)
The decision by Airbus to buy back aircraft to facilitate the anticipated deal comes as the aviation market struggles to absorb some of the first A380s, which entered service a decade ago.

The Reuters article is from November 12, 2017. It's no wonder you see older Boeing 777's next to 727's:

Something Airbus is wary of, they don't want to see an A380 in the boneyard so soon.

Airbus is seen as keen to support the second-hand market to avoid having to break up one of the planes, a potential public relations setback as it tries to breathe new life into the iconic European programme following recent production cuts.


Advantages in using a flight sim to prep for a private license? I'm starting to save up to get my pilots license and I was thinking I might be able to do some amount of prep work by using a flight simulator.  That being said, I realize there must be some huge limitations to what I can actually learn, on my own, on a flight sim.

So, out of curiosity, are there good things to try practicing on my own in a flight sim?  There's got to be a few basic things that would make it worth my time...  Otherwise I'll just go back to flying around aimlessly like I usually do :).

I'd advise against "flying around aimlessly".

Even if it is a simple, routine flight between two nearby airports, fly it fully, and try to fly it as professionally as possible (can you keep your heading to within +/- 3 degrees? can you keep your altitude spot-on?)

Another good exercise is to do all your navigation planning in detail (each heading for a certain number of minutes), then try to take off, and using nothing but the compass and stopwatch, find out how close you get.  (and when you end up off-course, figure out where you went wrong!)


Would a sharper leading edge increase lift and reduce drag at lower speeds? A recent scratch built glider kept turning to the right even after painstaking efforts to match the wings perfectly.  Extensive addition of fin area to the tail failed to correct it.  Closer examination showed the left leading edge had been carved to a sharper point, angled more sharply than the curvature of the wing.

After carving the right LE to a point the glider flew perfectly straight.

This was not expected at the 5-7 mph the glider flies at, and the effect seemed to be stronger at lower speeds.

Could sharper leading edges help even at very low speeds?

The leading edge is round to allow a wider range of angles of attack. If the airfoil is operated at one angle of attack only, the leading edge can be sharp. See the leading edges of compressor blades or fowler flaps for example - the stator rsp. the wing ahead of them directs airflow such that the angle of attack variation on the airfoil is small, in case of flaps especially if the flap has only one extended position.

This means that your sharp leading edge works well because (by chance) it is exactly where the stagnation point in trimmed flight goes. Re-trim the aircraft for a different speed and the right-turning tendency will be reversed. Unfortunately, now you made both sides similar, so you cannot run this test directly.


Why is the B787 one of the few aircraft with electro-thermal wing anti-/de-icing? While I was searching for a listing of aircraft with different anti-/de-icing methods - especially for electro-thermal wing anti-ice - I couldn't find any information about other aircraft except the B787.
Is there any reason, why other methods like TKS or using bleed air were prefered in the past?
Are there any studies related to the pro's and con's of the different anti-/de-icing systems?

Partially because 787 has no bleed air available for any purpose. Its engines do not draw bleed air traditionally. Any pneumatic system draws air from other pumps. Hence electro-thermal wing anti-ice system is needed since no bleed air is available.


What is the longest distance at which an aircraft can be visually identified? In the IL-2 flight simulator, when you turn on "markers" for planes you can see a belligerent side, the aircraft belongs to, at a distance up to 6km (3.75 miles) as well as its type with a tail number.

But in some books I've read mentions that pilots were able to see an aircraft number, its specific painting/livery etc. of other aircraft they encountered.

So I wonder: in real life, at what distance can such things can be distinguished by a pilot with good sight (I understand that this depends on the size of an aircraft)?

The key piece of information here is the "Angular Resolution" of the naked eye.

Is called in this way because the key parameter is the perceived angle between two points when looking at them. Let's take 2 points that are distant 1 meter one from the other.

Let's then put them at a distance from your eyes, the angle you see between them can be computed as follows:

Image source

$angle\_in\_degrees = 2*atan(\frac{distance\_between\_them}{2*distance\_from\_you})*\frac{180}{\pi} $

if they are 10 meters from you, they will appear about 5.7° apart
at 100 meters from you, they will appear 0.57° apart
1000 meters, 0.057° apart

Note that two dots 10 meters apart and 100 meters from you will appear with the same 5.7° of the dots 1 meter apart and 10 meters from you: this is why, if you do not have other references, is nearly impossible to understand the distance from you of an object.

Now you are interested in reading something, in particular the aircraft number.

The naked eye cannot distinguish two points that appear less than 0.0128° apart (bright day, with no haze nor warm air distortion)

From what we see above, if the lettering is 1m tall, you could distinguish it at about 4 km.
To actually read it, though, you will have to pick up the details, that will be much smaller: thik of distinguishing a "O" from a "D", you will have to notice that the left side is straight (or not), a detail much smaller than the total height of the letter and this will greatly reduce you maximum distance at which you can read.

Recognizing the livery instead is quite easier, as full portion of the aircraft will be painted, and in this case you only have to pick up the colours, not details of the shapes.

You could also distinguish the shape of on aircraft from a larger distance, given the much larger dimension of a full aircraft, but as for the letters, smaller details will not be recognizable until you get close enough, where "enough" depends on time of day, weather conditions and size of what you want to observe.


How is bleed air used to start a jet engine? I know that most jet engines on commercial jets are started either with bleed air from an APU or bleed air from the other engine...  But I'm not exactly sure how bleed air actually starts the engine?

Is the bleed air just being blown into the compression chamber?  Or is it being used in some other way?

As a starting point, this question here has an answer that describes the entire process of starting a jet engine on a high level.  It includes a short section on bleed air:

  Through the opening of bleed air valves, bleed air is sent to an air turbine starter. These devices typically use the high pressure bleed air to spin and engage a centrifugal clutch connected to the engines accessory drive. This in turn causes the N2 shaft within the engine to spin.

But I'm hoping for a more detailed explanation on what the bleed air is doing, how (and where) the clutch is applied (is there a transmission as well?).  Problems that may arise, and how they are dealt with.  Etc etc...  I just would like more detail if it's possible.

(Source) Image shows how the shaft is connected to the gearbox.
The engine shaft is connected to an accessory gearbox. The spinning shaft spins the gearbox, which drives the generator, the engine-driven fuel pump, and other accessories.
The same gearbox can be spun using high-pressure air—from the APU or a start-cart. This high-pressure air spins a turbine (air turbine starter), which drives the gearbox.
(Source) Air start location.
Once the shaft is spinning fast enough to bring in compressed air that would guarantee a successful and sustainable burn, the fuel is introduced, and the engine starts and speeds up further. The turbine then disconnects. Just like how a Bendix drive1 in most car engines disengages the starter motor once the engine is started.
In a twin-spool (two-shaft) jet engine, the gearbox is usually connected to the high-pressure shaft—the same shaft for the compressor/turbine stages closest to the burners.
What can go wrong?
Apart from starter- or gearbox-related malfunctions, not enough air pressure to spin the shaft—which would be an issue with the APU, start-cart, or piping.
If the jet engine is sitting in fast-enough tailwind entering from the rear (the jet pipe), the engine will be windmilling in the wrong direction, it could result in too much force for the gearbox to overcome. In this scenario, the pilot would select the engine reversers—by opening the reverse doors / engaging the buckets, less wind will be allowed to spin the shaft the wrong direction.

An exceptionally strong tailwind (in excess of approximately 20 knots) may prevent positive N1 rotation at a max motoring RPM of 15% to 20% N2. In this condition the Captain should do one of the following:

Have the aircraft towed into a position (headwind or crosswind) in which the N1 rotation stops or is in the correct direction.

Have the ground crew verify that both engines are clear of personnel and equipment, then deploy both engine reversers. After approximately two minutes, have the ground crew verify zero N1 rotation. Prior to starting an engine, verify the reverser is stowed and latched.

(MD-80 manual)

(Source) Reverse buckets.

1 Same Bendix we know, that's how he made his initial fortune.


What constitute "known icing conditions" and when can you fly VFR while they exist? For flight operations conducted under 14 CFR part 91(other than subpart F) in VMC, what conditions constitute "known icing conditions"? Under what circumstances (e.g., altitude difference or no visible moisture), if any, could a VFR flight operation (other than in an aircraft certified for flight into known icing) be legally conducted despite an Area Forecast, AIRMET Zulu, Pirep or other possible indicator of known icing conditions?

The short answer is that it's up to you as PIC to make that determination; the FAA has no clear definition apart from what a "reasonable and prudent" pilot would do.

The FAA issued a legal interpretation on exactly this question in 2009, here's the closest thing to a definition that they give (my emphasis throughout):

  "Known icing conditions" involve instead circumstances where a
  reasonable pilot would expect a substantial likelihood of ice
  formation on the aircraft based upon all information available to that
  pilot. While "known icing conditions" are not defined by regulation,
  the term has been used in legal proceedings involving violations of
  FAA safety regulations that relate to in- flight icing. The National
  Transportation Safety Board (NTSB) has held on a number of occasions
  that known icing conditions exist when a pilot knows or reasonably
  should know about weather reports in which icing conditions are
  reported or forecast

They go on to say that the PIC is responsible for determining the likelihood of icing conditions:

  Any assessment of known icing conditions is necessarily fact-specific.
  Permutations on the type, combination, and strength of meteorological
  elements that signify or negate the presence of known icing conditions
  are too numerous to describe exhaustively in this letter. Whether a
  pilot has operated into known icing conditions contrary to any
  limitation will depend upon the total information available to the
  pilot, and his or her proper analysis of that information in
  evaluating the risk of encountering known icing conditions during a
  particular operation. The pilot should consider factors such as the
  route of flight, flight altitude, and time of flight when making such
  an evaluation

Note that even visible moisture and freezing temps aren't automatically considered to be known icing conditions:

  The FAA does not necessarily consider the mere presence of clouds
  (which may only contain ice crystals) or other forms of visible
  moisture at temperatures at or below freezing to be conducive to the
  formation of known ice or to constitute known icing conditions.

The specific regulations mentioned in the interpretation are 14 CFR 91.9 (compliance with the operating manual), 91.13 (no careless or reckless operation) and 91.103 (become familiar with all information relevant to the flight). Those regulations would be the basis for any hypothetical enforcement action:

  If the composite information indicates to a reasonable and prudent
  pilot that he or she will be operating the aircraft under conditions
  that will cause ice to adhere to the aircraft along the proposed route
  and altitude of flight, then known icing conditions likely exist. If
  the pilot operates the aircraft in known icing conditions contrary to
  the requirements of § 91.9(a), the FAA may take enforcement action.

So the bottom line is: you're PIC and it's your decision whether it's safe to fly or not because there are far too many variables for anyone else to decide. But personally I think it would be difficult for any pilot to say that it was "reasonable and prudent" to fly a non-FIKI aircraft in an area where forecasts, AIRMETs or PIREPS indicated icing.


To what extent are (non-urgent) PIREPs used by meteorologists? It's inevitable. At some point during your weather briefing, some underpaid, underappreciated briefer will say "Pilot reports are requested." and you'll say "Certainly, my good sir or madam! Why, the only thing that gets me out of bed in the morning is the chance to help my fellow aviators!" and then you won't make one.

But let's say that you do make a non-urgent pilot report.  Briefers will give that information to pilots, but what about the meteorologists who actually make the forecasts?  Supposedly, they're interested in the information as well...

  "NBAA is excited about the new electronic submission form that should streamline the PIREP process, and allow the information to get to the FAA air traffic controllers and the NWS Aviation Weather Center (AWC) forecasters sooner," noted John Kosak, program manager, weather, NBAA Air Traffic Services.  [1]

but what are they used for once they're submitted?

This NOAA newsletter lists a number of specific ways that PIREPs are used by meteorologists:

To "assess the strength and position of the jet stream"
For thunderstorm and snow level forecasting
For forecast discussions of regional weather phenomena, like "downslope wind storms in the lee of the Sierra Nevada"
To "determine the likelihood of clouds thinning or burning off"

Their summary is:

  Better reports and observations, not only from PIREPS but also storm
  spotters, Webcams and road weather networks help improve the picture
  of what’s going on in the atmosphere. Automated temperature, wind and
  humidity reports from MDCRS and the Aircraft Communications Addressing
  and Reporting System in commercial aircraft also get incorporated into computer models. All of these observations lead to improved forecasts and warnings.

There are a few very generic references out there too. A 2014 edition of FAA Safety Briefing says:

  The information will then be added to the distribution system to brief
  other pilots, help generate inflight advisories like AIRMETs and
  SIGMETs, as well as updates to the Current and Forecast Icing Products
  (CIP/FIP), and improve overall forecast accuracy.

And an AOPA article has a brief mention:

  when the Aviation Weather Center (AWC) receives a pirep,
  meteorologists use it to verify that their weather forecast is correct
  or amend it if the pirep contradicts the forecast.

Finally, PIREPs (and AIREPs) are shown on the NOAA website, although that probably isn't what you had in mind by meteorologists 'using' the information.


Is the weather radar on and active during the whole flight? The title says it all: Do airline pilots leave the aircraft's weather radar on at all times during the flight and display the data on one or both of the Navigation Displays, or only if clouds/ weather are visible / expected?
I know it should not be active on the ground due to possible radiation harm, so my question relates to the airborne phases climb, cruise, descent.

Not necessarily; it is the pilot's prerogative to use the weather radar as required. If the weather is clear and the pilot decides that the weather radar display is going to take up valuable space in display, he/she can fly without the weather radar on. Of course, in cases like night flying, it is better to have it on during the entire flight, but it is the pilot's decision.

The regulatory requirements are for the weather radar to be operational only if potentially dangerous conditions are expected en route or for night flying. From CFR §135.175   Airborne weather radar equipment requirements:

(b) No person may begin a flight under IFR or night VFR conditions when current weather reports indicate that thunderstorms, or other potentially hazardous weather conditions that can be detected with airborne weather radar equipment, may reasonably be expected along the route to be flown, unless the airborne weather radar equipment required by paragraph (a) of this section is in satisfactory operating condition.

European regulations are also similar. From JAR-OPS 1.670 Airborne weather radar equipment:

(a) An operator shall not operate:
(i) (1) A pressurised aeroplane; ...
...
unless it is equipped with airborne weather radar equipment whenever  such an aeroplane is being operated at night or in instrument meteorological  conditions in areas where thunderstorms or other potentially hazardous  weather conditions, regarded as detectable with airborne weather radar,  may be expected to exist along the route.


Was the intended purpose of the F-117 Nighthawk originally a stealth-fighter? Based on this question, I learned that the F and A-Letters in the naming of (military) aircraft reflects what their intended purpose is.
A: Attack (Air-to-ground)
F: Fighter (Air-to-air)

their names mostly reflect what their intended purpose is rather than their actual purpose.
It is to be noted that the naming convention is followed only loosely in some cases. A good example is the F-117 Nighthawk, which is not a fighter aircraft in any sense of the word.

from aeroalias answer from this question.
The F117 isn't capable of attacking Air-Targets, it's only used as an attack aircraft. But the name of the Nighthawk is F-117 (Fighter).
Question:

Was the idea behind the Nighthawk originally a stealth-fighter
(Air-to-Air) aircraft but was later changed to a stealth-attack
aircraft? That would explain the name 'F-117'.
Are there other attack-aircraft with the F in the name?
Are there fighter-aircraft with an A in the name?

The short of it is that the "A" designation for a ground-attack aircraft has historically been a Navy distinction. The USAF/USAAF/USAAC has not traditionally differentiated between an aircraft designed for tactical ground strikes versus a dogfighter/interceptor. Most WWII-era Air Force planes were capable of both, and would be called "multirole fighters" in today's parlance (including the famous P-38, P-40, P-47 and P-51). Between WWII and the adoption of the 1962 Tri-Service Aircraft Designation System, the USAF used "F" almost exclusively for any one- or two-man aircraft performing either the air-to-air or tactical air-to-ground role (using "B" for aircraft intended for larger-scale strategic bombing). Thus, the "Century Series" aircraft, many of which were primary ground attack aircraft, all got the "Fighter" designation even when their mission profile was exclusively air-to-ground, such as the F-105 and F-111.

The Tri-Service Aircraft Designation System was patterned most closely to the Air Force's pre-existing system, but resetting all the counters (the Navy's system involved much lower numbers, many of which were grandfathered into the new system) and adding mission designations common in the other branches, including the Navy's Attack designation. The Air Force has more or less ignored many of the finer points of this system when designating its own aircraft; the A-10, and its A-X competitor the YA-9, are the sole airframes designed primarily for the USAF to carry the Attack designation (which is fitting as both planes were designed from the ground up to kill tanks and other armor). The other A-designated aircraft to enter service with the Air Force were Navy planes first (including the A-1 Skyraider and A-7 Corsair II).

The F-15E should, by tri-service rules, be designated AF-15E (emphasizing its modified mission of tactical strikes over its air-superiority pedigree). "F/A" as used for the Hornet is also non-compliant, and stems from an original desire to have separate fighter and attack variants of the Hornet which ended up being rolled into one plane. "FA" or "AF" would suffice not only for the Hornet, but also the F-16 Falcon and the F-35 Lightning II (yet another aberration, this time in the numbering, keeping the number from the X-35 tech demonstrator instead of being the F-24 as expected).

The F-117 is a very special case. The airframe received its designation after 1962 and so should have been designated either A-14 (as its mission has always been tactical ground strikes) or F-19 (keeping with the USAF's tendency to label any light offensive aircraft a "fighter"). Development of the F-117 from the Have Blue project was an open secret, and it is widely believed that the Nighthawk is the missing "F-19" in the U.S. numbering system (the F-20, a variant of the F-5 powered by a single larger turbofan, having been announced and demonstrated long before the F-117's existence was officially acknowledged), with the pre-1962 designation intended as misinformation.


What is the difference between IAN and LNAV/VNAV? Is there a real difference or benefit in using Integrated Approach Navigation (IAN) instead of LNAV/VNAV for a non-precision approach (NPA)?
Both give you lateral and vertical guidance during an NPA. Is IAN more precise?

A Non Precision Approach (NPA) flown with Integrated Approach Navigation (IAN) is not more precise than the same approach flown with LNAV/VNAV. The advantage of IAN is that the instrumentation and procedures are very similar to ILS (Cat I), which is the most common approach type flown for most airline pilots. Having a common set of indications on the PFD and procedures to follow improves flight safety.
The Boeing 737 Flight Crew Training Manual (FCTM) says:

IAN approach techniques are
similar to ILS approach techniques [...]
All IAN approaches provide the functions,
indications and alerting features similar to an ILS approach. Although non-ILS
approaches using LNAV and VNAV can still be performed, IAN is normally used
in place of LNAV and VNAV because of improved approach displays, alerts and
standardized procedures.

(Boeing 737 NG FCTM 5.60 - Approach and Missed Approach, emphasis mine)
The article about IAN on flaps2approach goes into a bit more detail:

Advantages of Using IAN
There are multiple advantages to an airline using IAN, the foremost
being flight safety. Unstable approaches contribute to many aircraft
accidents, and flight crews strive to always establish a stabilised
approach profile for all instrument and visual approaches.
The Global Position System has enabled stabilized approaches at many
airports and advanced features such as IAN take advantage of this
technology to provide consistent, intuitive displays that support
stabilized approaches.
18 Approaches Types to 1
Through the use of IAN, the number of approach types has been reduced
from 18 to 1, greatly simplifying the approach procedure and
minimizing the amount of time an airline needs to train pilots in
numerous approach types.  Time is money and utilizing advanced
technology such as IAN can increase airline productivity.
Additional Data - Increased Awareness
The distance to runway threshold, approach guidance information, and
vertical and lateral deviation markers are displayed when IAN is in
range of a designated runway.  Whether IAN is used or not, this
information provides additional guidance when executing an approach.
For example, when executing a VOR approach, this information has been
displayed on the Navigation Display (ND) as the distance to the actual
NAVAID (VOR) - which may or may not be aligned with the threshold of
the runway.  IAN will by default, display the lateral and vertical
deviation, and distance to the runway threshold, allowing for greater
precision during a non-automated approach.
These are but a few of the advantages to using the Integrated Approach
Navigation system.

(flaps2approach.com - Integrated Approach Navigation (IAN) - Review and Procedures)


Is there a way to avoid exhaust fumes when landing? I've taken a few hours towards my private pilot's license and each time we land I've smelled this plume of engine exhaust that we seem to fly/roll through just as we're touching down.  I must be sensitive to the fumes because I usually end up with a slight headache after a few minutes.  I haven't asked my instructor yet (because he may think I'm nuts) but is there any way to avoid this short of holding my breath?  I'm flying in a Cessna 152 for my training.

Smelling exhaust in the cockpit is not normal.  Tell your instructor and squawk the problem.  Bonus - turn it into a learning experience by arranging to be with the A&P when he decowls and inspects.  You can learn a lot about the engine that way.

If you are getting a headache from this you may have mild CO poisoning.  This does need to be checked out.


Who has the higher authority, the pilot in command or ATC? I'm sure many people have heard about the communication between a Delta flight 2244 and Atlanta tower being on news today.

When the Controller informs the Pilot he is on a wrong taxi-way, things get a little heated up.

Regardless of who was right who was wrong, there were many lives involved (i'm sure they were not put in danger) but then who takes precedence in such a situation?

Can the controller tell the aircraft to stop dead in its tracks or turn back to the hangar for violating his instructions because there are other planes coming in to land and this can become a little dangerous?

Note: I am fully aware that no one was close to danger and lives were not at risk, but still pilot did worry that there is another plane coming down.

Was the pilot right in saying

  We'll taxi out there any way we want when you tell us to.

For those who missed. Here is the whole story

Note

Please note that this reporting on that article is misleading. It is about taxi clearance and not landing on a wrong runway, However I have no control of what reporters write in their articles. The transcript itself is clear.

Hear the original ATC audio here

It isn't a black and white issue of who has higher authority.  A pilot in command (PIC) is the ultimate authority for the safe operation of his airplane.  An air traffic controller is the authority for the block of airspace or pavement he controls.

When you are operating under ATC, it is your responsibility to comply with their instructions as long as they fall within the regulations, they are safe and you are able to comply,  If ATC issues an instruction contrary to regulation or something that would compromise safety, that is when you exercise your PIC authority to deviate from their instruction.  If you are unable to comply, you exercise your authority to deviate and explain why you are deviating.  When you cannot comply with an ATC instruction, for whatever reason, it is the job of ATC to accommodate your and move other airplanes out of your way.

The regulatory basis for the PIC authority and ability to deviate from ATC instructions is in 14 CFR 91.3

  §91.3   Responsibility and authority of the pilot in command.

  (a) The pilot in command of an aircraft is directly responsible for, and is the final authority as to, the operation of that aircraft.

  (b) In an in-flight emergency requiring immediate action, the pilot in command may deviate from any rule of this part to the extent required to meet that emergency.

  (c) Each pilot in command who deviates from a rule under paragraph (b) of this section shall, upon the request of the Administrator, send a written report of that deviation to the Administrator.

The basis for ATC's authority is 14 CFR 91.123

  §91.123   Compliance with ATC clearances and instructions.

  (a) When an ATC clearance has been obtained, no pilot in command may deviate from that clearance unless an amended clearance is obtained, an emergency exists, or the deviation is in response to a traffic alert and collision avoidance system resolution advisory. However, except in Class A airspace, a pilot may cancel an IFR flight plan if the operation is being conducted in VFR weather conditions. When a pilot is uncertain of an ATC clearance, that pilot shall immediately request clarification from ATC.

  (b) Except in an emergency, no person may operate an aircraft contrary to an ATC instruction in an area in which air traffic control is exercised.

  (c) Each pilot in command who, in an emergency, or in response to a traffic alert and collision avoidance system resolution advisory, deviates from an ATC clearance or instruction shall notify ATC of that deviation as soon as possible.

  (d) Each pilot in command who (though not deviating from a rule of this subpart) is given priority by ATC in an emergency, shall submit a detailed report of that emergency within 48 hours to the manager of that ATC facility, if requested by ATC.

  (e) Unless otherwise authorized by ATC, no person operating an aircraft may operate that aircraft according to any clearance or instruction that has been issued to the pilot of another aircraft for radar air traffic control purposes.

Absent an emergency, an urgent need* or an inability to comply with ATC, a pilot is required to obey an ATC clearance.  Given this relationship and its exceptions, its is hard to quantify who has a "higher" authority.  Both are the ultimate authorities of their domains, but who overrules who is dependent on the situation.  In general however, ATC will be issuing proper clearances that a pilot is bound by regulation to follow, though the pilot must exercise his authority to execute that instruction in a safe manner.

*Urgent needs include things like TCAS RA events and things like weather deviations.  In most cases ATC will help you out with a deviation, but if a controller refuses to allow a deviation, I would happily exercise my PIC authority to safely operate my airplane and turn left/right a few degrees rather than penetrate a thunderstorm updraft.

In the particular case above, the audio exchange between DL2422 and Atlanta ground is

  DL2422: Good morning Atlanta ground, delta 2422 approaching 3 South taxi Quebec

DL2422 is at the southern exit of ramp 3 and requesting taxi with ATIS information Q.

  Ground: Delta 2422, behind the MD88 from your right, runway 27R, taxi via Mike and hold short of Dixie.

ATC has instructed DL2422 to let an MD88 pass from right to left, to taxi on taxiway M to runway 27R but to hold short for taxiway D (in ATL D is "Dixie" due to the larger number of "Delta" airplanes).  To follow this instruction, an airplane should taxi south from spot 3S on taxiway L5, cross L and then turn left on M, then taxi on M, stopping short of D and awaiting further instruction.

  DL2422: OK behind the 88 from the right, 27R Mike short of Dixie, DL2422

Here, DL2422 acknowledges this correctly.

  Ground: DL2422 they won't let you go to Charlotte until 57 minutes after the hour, time now 29 after

  DL2422: OK copy 57, DL2422

DL2422 has been issued a wheels-up time, but their taxi clearance has not been amended.  They are still cleared to taxi via M short of D.  Sequencing to meet this time will happen east of D in further clearances.

  Ground: DL2422, you are supposed to be on M

  DL2422: Yes sir, we're going out there right now.

  Ground: Looks like you joined L

  DL2422: You know what, we'll taxi out there any way we want unless you tell us to, I don't like your attitude.

  Ground: I don't have an attitude sir, I'm just saying it looks like you joined L instead of M and I'm just trying to correct you before you stay on L.

  DL2422: Ok, cause my god, there's another plane out there, its like 6 miles away.  Your attitude is really something sir, we're out here on M. Good morning.

  Ground: Good morning.  There was no attitude, I was just trying to correct you, that's my job to correct you if you mess up.  I'm just trying to make sure everyone is doing what I ask them for certain reasons.

  DL2422: You make a mistake every 2-3 minutes, but my attitude is not like yours, we're out on M and you didn't tell us how to get there so next time you can try doing that.

The DL captain is making a scene and a very roundabout argument that the taxi route did not specify how he was supposed to transition from spot 3S to taxiway M.  I agree that this is the case, but do not understand why any assumption would be made other than to taxi straight out and turn left on M.

This is a good example of a minor deviation that had DL2422 just turned right and then left and said "oops, fixing that, joining M, thanks!", then that would have been the end of it.  This captain's unprofessional display will instead net him disciplinary action and classify his deviation as "willful" rather than "accidental".

For a clearer visualization of the event, see this image:

In this image, the green circle is the starting point of DL2422 and the blue line is the cleared taxi route with the assumed transition to M via L5.  The short red line is the initial taxi route DL2422 took.  It is not clear where they transitioned to M, so I did not extend the red route past the first possible transition.

There appears to be some controversy over the verbiage of "behind" used in the ATC taxi clearance.  The only reference I can find to this word in a cursory search is in FAA order 7110.65 (ATC procedures) in section 3-7-2 that gives example taxi clearances.  This does not define "behind" but does give distinct examples that clearly differentiate "behind" from "follow".  Given that I cannot find a better reference, all I can offer instead is years of experience in a 121 cockpit based at busy airports like IAH and EWR and plenty of visits to others like ORD, ATL, LAX, DTW, IAD, etc where clearances involving the words "behind" and "follow" are quite common.

Behind Use of this word means that you are waiting for an aircraft to pass.  You are giving right of way or yielding to the traffic in question.  This clearance is often given when holding short of an intersection and waiting for perpendicular traffic to cross the intersection before you continue through it.  Much like waiting at a stop sign in a car.  You are not expected to follow the airplane in question and even if you are assigned a turn to taxi along the same route as the airplane, you have your own clearance and are not following that aircraft.

Examples:

"Continental 1234 taxi to runway 22R at W via K hold short of S", followed by "Continental 1234 you'll see a B767 passing right to left on S, behind him continue to runway 22R at W via K R hold short of W"

In this case it is quite explicit that an airplane will be passing across your route of taxi and after it passes you are to continue taxiing straight ahead -- not to follow that airplane.  Consider an alternate setup:

"Continental 1234 taxi to runway 22R at W via K hold short of S", followed by "Continental 1234 you'll see a Citation from the right at S, behind him taxi via S W hold short of R.

In this case, you are waiting for traffic from the right again, but this time you are turning left after it passes.  You will be taxiing behind it, but you are not following it.  The Citation has a clearance to taxi S, cross runway 29, to parking.  Your clearance is S turn right on W -- to follow the Citation would be wrong.
Follow Use of this word means you are going to fall in behind this airplane and follow it wherever it goes.  You clearance is no longer a specific taxi route, but simply to follow the airplane.

Examples:

"Jetlink 1234, your sequence is the third airbus to your right, the one painted purple.  Follow that airplane to runway 22R at W.  Once behind him, monitor tower, you are number 28 for departure.", which will be followed by
"United 3412, hold short of J, let the EMB-145 get in line then follow him to runway 22R at W.  Monitor tower passing L, you are number 29 for departure"

These clearances are used to get airplanes in line in a specific sequence and simplify giving repeated complex taxi instructions when you can just follow to conga line to the runway.


Does thin airfoil theory work at very high angles of attack? Thin airfoil theory says that lift coefficient is directly proportional to the angle of attack in radians. I haven't been able to find any limit, short of stall, for applying this theory. It would give us a huge lift coefficient at 90 degrees, where lift is actually zero, and double even that at 180 degrees, where again the lift is zero. This is basic reasoning. So, what am I missing?

Of course not. Thin Airfoil Theory produces a beautiful result that relates the zero-AOA lift to the mean camber of the airfoil, as well as the lift slope vs AOA of any thin airfoil is $2\pi$. But that's assuming:

Airfoil is asymptotically thin: such that the mean camber line represents the flow path
Camber is close to the chord: such that the vortex sheet is assumed to lie on the chord while resulting in the flow path that lies on the camber line
AOA and camber are small: such that all angles are linearized ($\sin\theta \approx \tan\theta \approx \theta$); obviously at large angles, this assumption breaks down and the results are worthless
Potential and incompressible flow: by the virtue of irrotationality, stall will not be modeled.

So in all, the result is only valid for small range of AOA on an idealized thin airfoil. For engineering purposes on general airfoils, you'd want to use panel method + boundary layer equations. But even then, extrapolating results beyond stall lead to ridiculous results (certainly at 90deg or 180deg).

More details on Thin Airfoil Theory can be found in Drela, Flight Vehicle Aerodynamics or Anderson, Fundamentals of Aerodynamics.


How can I calculate effective wingspan of a high aspect ratio model airplane? I've heard of people talking of wingspan and effective wingspan.

Considering:

L = Lift Coefficient . 1/2 Rho . Velocity ^ 2 . Surface Area,

I understand that tip vortices increase drag, but do they reduce the effective wingspan of a wing?

How can this be calculated?

I think the term "effective wingspan" should better be replaced by "effective aspect ratio". There is no change in wingspan when the wing flies closer to the ground, but it behaves like an equivalent one with a higher aspect ratio. The basic formulas can stay unchanged, only aspect ratio should be adjusted.

Think of it this way: If there was an increase in span, would wing area grow with it? Would that mean that wing loading goes down? No, it is better to assume that the wing near to the ground behaves like one of the same area, but of a higher aspect ratio with an increased span and a proportionally shortened chord.

Sighard Hörner gives an approximation on page 7-13 in his book Fluid Dynamic Lift for the increment by which the effective aspect ratio is increased over the geometric aspect ratio:
$$\frac{∆AR}{AR} = 0.09\cdot\frac{h}{b}$$

b is wingspan and h is the height of the aerodynamic center over ground.

He also has graphs for the change in effective aspect ratios of vertical tails with the height at which a horizontal tail is attached, or the effective aspect ratio of a biplane wing:
Plot of effective aspect ratio over geometric aspect ratio of a vertical tail depending on the vertical location of the horizontal tail (picture source)

The case where an effective span makes sense is in the calculation of the induced drag and vortex location of a rounded wingtip. Here is Hörner's list of wingtip shapes from the same book:

Wing-tip shape and tip-vortex location for a family of wings (picture source)

And lastly, no, wingtip vortices don't cause induced drag. It doesn't become true just because it is repeated so often. Please look here or here for a better explanation.


What is the reference point for AGL? I am about to start my PPL soon (still flying in FSX) and I do not understand one thing. As per FAR 91.159 if my altitude is above 3000' AGL I must maintain certain altitude depending on my course, BUT what would be my reference point to know that I am above 3000' AGL? Departing airport? MEF from chart? or just look out my window? Thanks!

FAR 91.159 speaks for itself.  The reference point is directly below your current location.

There are other regulations that use "AGL" (sometimes stated as "Above the surface") as a reference (see FAR 91.155 or 91.119).

As a side note, FAR 91.159 is used to mitigate the risk of midair collisions but not at the expense of prohibiting necessary close to the surface operations (e.g., helicopter EMS, telephone line patrol, crop dusting).


Why does the radome on the AWACS rotate? I'm not sure I understand why the radome on the AWACS aircraft is so large, and could somebody explain the design and why it has to rotate?

Basically, the radar antenna of the AWACS system is just like this (source):

Rotating waveguide antenna (picture source)

The round shape is only an aerodynamic fairing. Proof:

Boeing E-3A AWACS cutaway (picture source)

A surveillance antenna can also be fixed. Electronic beam steering helps to view a sector of approximately ±60°, but in order to get a 360° sweep the airplane needs to fly circles. Beam steering is also used on the rotating antenna to rapidly switch between several targets, so this is not unique to a fixed antenna:

Pakistani AWACS based on EMB-145 (picture source)


How can I tell apart an Airbus from a Boeing? When looking at an airplane, what are the main external features I should look at if I want to determine if it is an Airbus or a Boeing (shape of the tail, fuselage, winglets, wings, etc.)? Same questions from a passenger point of view, quietly seated inside?

I like to train being able to determine the precise model of an aircraft.
I first try to tell if it's a Boeing or Airbus, and then I look at other details to determine the model.

The idea is to learn several differences :

cockpit windows shape (Boeing has an very specific angle) ;
nose shape (pointy nose on Boeing, "fat nose" on Airbus) ;
wingtips (winglets Boeing, fence Airbus) ;
number of engines (A340, B747, A380 have 4 engines, all others have 2) ;
gear.

Since there are already nice answers describing most of these points with pictures, I will focus on the gear to add a new item to the list.

Landing Gear

Looking at the gear adds a new hint. Some landing gears are unique, which means that only looking at them will be enough to determine the precise aircraft type. These very specific gears are :

B777
B747
A340 (2 different kinds for this aircraft only)
A380

It's not worth to look at the nose wheels, because there are always 2. But the main landing gear is helpful.

I made a gear cheat sheet to help me learning them.

Once these differences have been learned, the key is to use several of them to crosscheck and ease and speed up the recognition.
For example if I see the gear

   2
2*2 2*2

I will then check the cockpit windows to determine whether Boeing or Airbus.
If it's Airbus, I will know it's very likely to be an A330! If it's Boeing, it will be harder since there are still 3 different aircrafts. If I want to be able to distinguish between them, I can learn new specific hints for this purpose.


Do coaxial rotors provide twice the amount of lift? Do coaxial rotors provide twice the amount of lift given same diameter and RPM?

What about the engine? Even if engine power was doubled, the coaxial rotor won't produce double the lift.
Airfoils don't like stacking neighbors.
(Source)
This double wing above (biplane) produced about 120% lift, not 200%.
Using the biplane above to illustrate a point, between the wings the pressure will neither be high enough for the upper wing, nor low enough for the lower wing.

In a biplane aircraft, two wings are placed one above the other. Each provides part of the lift, although they are not able to produce twice as much lift as a single wing of similar size and shape because the upper and the lower are working on nearly the same portion of the atmosphere and thus interfere with each other's behavior.

For the same engine output, we're looking at ~10% increase in lift.

Because the coaxial rotor does not require an anti-torque device such as a tail rotor, Fenestron, or Notar, a coaxial rotor helicopter requires about 10% less power than a single rotor/tail rotor helicopter.
For the same rotor diameter and power the coax will lift approximately 11% more weight. This equates to 30% greater useful load, which means more payload or fuel. The coax helicopter can hover at a 6000 ft higher altitude, or at a higher temperature (avxaircraft.com).


Do any airlines hire "pilots" with no previous flying experience? I've previously looked online for pilots jobs at airlines, from budget airlines to cargo carriers; and they always want you to have a ppl.

Do any airlines currently run programmes for applicants with no prior flying experience?

Lufthansa does. They actually prefer people they can train themselves, so they never learned things which Lufthansa considers bad practice.

This becomes impossible if the airline grows quickly. Therefore, all the new carriers (Etihad or Vueling, for example) need to rely on hiring certified pilots.

See Lufthansa-Pilot.DE (only in German but speaking German is a requirement anyway).

Thanks, Danny, for the link. Lufthansa has now adopted English as their corporate language, so I wonder how firm the German language requirement still is.


Is there a minimum amount of time an airline has to leave between using two identical callsigns? Does anyone know if there is a minimum time that an airline has to leave before reusing the same callsign?

I have noticed that airlines reuse callsigns, they usually leave a day or two between reusing them. I'm just wondering is there any regulation on the minimum amount of time that must pass before they can reuse the same call sign again?

Airlines work to make sure there is no duplicate callsigns in the same area at the same time. But there is no regulation for time between the reuse of a callsign.
They achieve this by either a callsign being used only once per day or with enough time between flights that there is no chance for both flights in the air at the same time.
The issue comes if two aircraft are in the air with the same callsign. The air traffic control systems will not be able to tell an ATS message (e.g. DEP, EST, CNG) for one aircraft from an ATS message from the other aircraft with the same callsign. Plus many other issues.
So if an airline needs to use the same callsign twice, like if the first flight is really delayed, they tend to append a letter to the end of the callsign, for example UAE212 becomes UAE212C.
Not asked but for completeness; there is also a risk if the callsigns are close but not the same, for example United 222 and United 232, there could be confusion between the aircraft over the radio. In these situations the air traffic controller can change the callsign to make sure there is no confusion. This could be while they are in the same airspace or for the entire flight.
The risk of callsign confusion has been documented many times in many incidents around the world. e.g. May 10, 2004 near Julian, California Callsign Confusion


How do Airbus and Boeing aircraft handle Dutch roll? I have some homework about Dutch roll filters. Dutch roll is something that is corrected by yaw damper systems, but I would like to know the different ways Boeing and Airbus deal with the issue of Dutch roll.

Is there a separate yaw damper system or is it part of the wider auto pilot system?

This is quite similar to the other question you asked.

From my knowledge as a flight simulator enthusiast I can summarize the following:
Boeing aircraft in the past used to be controlled pretty much direct pilot with cables running through the aircraft to control hydraulic actuator valves that power the flight control surfaces. The rudder is usually augmented with a yaw damper system which measures the yaw rate and roll rate, the rudder input and aileron input can “calculates” the desired rudder deflection to dampen the yaw rate and dutch roll. This can be as simple as performing a weighted sum on the yaw rate, roll rate, rudder input and aileron inputs with some factors that may depend on airspeed. The yaw damper(s) can be turned off with a switch in the overhead panel and are relatively simple in design. Easy to test fully and certify easily.
When the autopilot is on it manipulates the flight controls directly, i.e. moves the yoke with mechanical actuators. Thus, the yaw damper doesn't even have to know about the autopilot state in this case.

On Airbus planes and more modern fly-by-wire Boeing aircraft the flight control computers are fed with the pilot inputs and the computers then send signals to the actuators electronically by wire. The controls laws within these computers can be quite complex and usually also incorporate the autopilot control modes.

With full control over all flight surfaces the computers can manipulate individual control surfaces perfectly to remove any undesired motions. E.g. in the A320 the aileron and elevator computers constantly evaluate the current pitch and roll rate against the target rates. Any deviations (e.g. caused by a yaw rate) are immediately countered by aileron, spoiler or elevator inputs. With that alone the Dutch roll already can no longer exist. The roll is already stopped because of the roll feedback loop.

But it doesn't stop there. The flight augmentation computers receive information about the deflections of ailerons and spoilers for example and then can perform internal flight model computations to figure out how much rudder they need to compensate the adverse yaw for example. Or how much aileron will be needed when to stop the Dutch roll as we add rudder deflection. The computers also take the desired turn rate into account and also have inputs such as the balance indicator to fully coordinate the turn to create a slip free turn with close to no side forces on passengers and pretty much zero Dutch roll.

The flight control systems can be almost arbitrarily complex as you add more and more features into it. E.g. when you have an engine failure the computers could also receive inputs from the engine N1 differences and compute the necessary rudder deflection to prevent any yaw from asymmetric thrust. The A380 even goes as far as to use split ailerons on the opposing side of an engine failure to create artificial drag on one side to increase the counter torque. But it can also use ailerons strategically to dampen any oscillations within the wing to create a smoother ride experience. Now you're not only controlling a rigid aircraft, you start to take wing flex into account.

To answer the second question: The yaw damper can be a separate system with parts like asymmetric thrust compensation or other higher level functions just being turned off in case of a computer failure. This way the yaw damper can be used in a more basic control law even if there are mayor failures in other systems which helps control the aircraft in abnormal situations.

Since the yaw damping itself is sort of the last instance before the rudder actuator it doesn't need to be an integrated part of the wider system but the higher level functions will need to be computed somewhere within the control computers. Both implementations probably exist.

... sorry for the long post :)


What are the B737 MAX 8's OEW and max payload? I have Googled it, but have not found official info on that.

The type certificate of the Boeing 737 provides the following table for the MAX:


What differences are introduced between ICAO Annex 10 volume IV amendment 85 and amendment 89? What changes have been made to ICAO Annex 10 volume IV (Surveillance Radar and Collision Avoidance Systems) between amendment 85 and amendment 89?

I only have access to amendment 89, but I have some material referring to amendment 85. What is the difference between these versions, what has been changed to ICAO Annex 10, volume IV by amendments 86 - 89?

With help of Gerry and Mins, it became clear that Amdts 86,87 and 88 did not introduce any changes to Volume IV of Annex 10.

Therefore any differences between Annex 10 Vol. IV Amdt 85 and Amdt 89 would be introduced by Amendment 89. From this documentPDF it can be seen that the changes where distributed in state letter 2014/21.

Digging a bit further I found the
minutes of an ICAO Communication Navigation and Surveillance  Sub-Group (CNS SG)PDF  meeting which included the state letter in Appendix B. You can find it from page 56 to 91.

Amendment 89 contains over 30 pages of changes to Volume IV of Annex 10 and that was more than I had hoped for; needless to say I am not copying all that information into this answer. A summary of changes can be found below.

General changes

Various recommendations and clarifying notes added.
Various editorial changes

Chapter 3, Mode S Surveillance

Revoking of the Mode A/C/S all-call interrogations after 1 January
2020.
Introduction of new data parity overlay ("Modified AA") in
transponder and Overlay Control (OVC) bit in Mode S uplink protocol.
Changes to Type Control (TCS)

Chapter 4. Airborne Collision Avoidance System

No significant changes

Chapter 5. Extended Squitter (ADS-B)

Introduction of ADS-B 1090ES Version 2

Chapter 6. Multilateration

Spectrum protection requirements


Can the maximum thrust of a turbofan engine be estimated by its intake diameter? I have no clue in aeronautics. How to estimate maximum thrust (at any rated condition, such as altitude, speed, rotation velocity- any given condition) by intake diameter?

No.

You need to know one more parameter: How much fuel will be consumed per unit of time. In the end, thrust is created from heating some of the air entering that engine, and only if you know what proportion of air will be used for combustion can you make valid estimates of the thrust than can be generated.

What you can calculate from the intake diameter is how much air flows through the engine per time. The flow speed at the engine face is about Mach 0.5, almost regardless of aircraft speed, so the mass flow is strongly correlated with engine size. The plot below is from NASA TM-X 73,199 which lists statistical data on weight and dimensions of turbofan engines. Unfortunately, it is now more than 40 years old, but the laws of physics have not changed since then.

Correlation of intake diameter with the square root of mass flow from NASA TM-X 73,199. Wa is the air mass flow and h/t is the hub-to-tip ratio of the fan. $\left(1-\left(\frac{h}{t}\right)^2\right)$ is proportional to the effective fan area.

Now this uses fan diameter, not intake diameter, as the basis for the correlation, but those two dimensions are closely correlated, too. For a first order estimate, you can assume $D_F\cdot\sqrt{1-\left(\frac{h}{t}\right)^2}$ to be equal to the intake diameter.

The next step is to get a grasp on the ratio between core flow (which is available for combustion) and secondary flow (which only goes through the fan and exits the engine without being heated by combustion). This ratio is called bypass ratio (BPR) and has grown over time from 0.25 for the RR Conway, the first operational turbofan from the late Fifties, to 11 for the newest version of the CFM LEAP-1A engine on the A320 NEO. Core flow is Wa/BPR.

The fuel-to-air ratio of turbofans is roughly constant at 0.03. Given the bypass ratio you can now calculate the fuel flow and then continue to the thrust by using the specific thrust (thrust per unit of fuel) from sources like Wikibooks or statistical analysis. Note that specific thrust changes with aircraft speed, altitude and bypass ratio!

Thrust specific fuel consumption over Mach for different engine types. Page 29 from Jack Mattingly: Elements of Gas Turbine Propulsion. McGraw-Hill, 1996


Why does the B777 have no winglets? Are there any drawbacks to having winglets? I have googled and noticed that almost every Boeing 777 is not equipped with winglets. Does anybody know the reasons why Boeing did not equip B777s with winglets? Are there any drawbacks to having winglets?

As in anything in aviation design, the use of wingtips has both advantages and disadvantages. In the end, the decision to use (or not) a winglet is based on the cost-benefit analysis of the winglet to be used in that particular aircraft.

All winglets incur a drag and weight penalty- the question is whether they conserve fuel more than they increase it in the first place.
One problem in winglet addition is that they are prone to flutter.
Also, an equal span extension is sometimes more effective and is less risky.
Another thing to add is that most of the present aircraft using winglets (A320s and B737s) were not designed with winglets to begin with. They were retrofits. Most of the new aircraft are designed with plenty of aerodynamic experience and the engineers can take a call based on the data available as to whether to have 'conventional' winglet or not (See A350XWB Vs B787).

Coming to B777, it should be noted that the -300ER, -200LR, and -200F have wingtip devices after all- raked wingtips. These act in the same manner as the winglets to reduce drag.

Boeing 777 raked wingtip; image from Boeing.com

According to Boeing,

  Each wing on the Boeing 777-300ER (extended range) is being extended by 6.5 feet, and raked wingtips are being added to improve the overall aerodynamic efficiency. The raked wingtips help reduce takeoff field length, increase climb performance and reduce fuel burn.

So basically, these act in the same manner as the conventional wingtips. Also, it is crucial to consider the tradeoff. B777 was designed when the advantages of the wingtips were well known, yet they decided to go without one anyway. From airspacemag:

  Because winglets are a tradeoff: In the highly visible case of the 777, an airplane with exceptionally long range, the wings grew so long that folding wingtips were offered to get into tight airport gates.

Finally nobody ordered the folding wingtips when they were first offered, though that has not stopped Boeing from trying. Note that one main reason for offering folding wings is to fit into existing gates- else the wingspan would've become too great.

Boeing considered all the options before deciding to go with this option:

  Dave Akiyama, manager of aerodynamics engineering in Boeing product development, points out that designing winglets can be tricky-they have a tendency to flutter, for example. "We find that it really doesn't matter what kind of wingtip device you use-they're all like span," he says. "The devil is in the details. Span extensions are the easiest and least risky."

And finally, winglets were considered for B777, especially for the earlier versions, though nothing came out of that- so most probably they were of not much use in improving fuel burn. Note that 787 also has a similar 'raked' wingtip instead of winglet.

Note: The fuel burn can be reduced by other means than wingtip devices- Boeing did exactly the same through Performance Improvement Package (PIP) for the Boeing 777, improving fuel burn by 1%:

  ... for long-range 777s, with United, Continental, and Delta investing in
  modifying the outboard aileron droop and the environmental control system, making ram air improvements, and installing smaller wing vortex generators to yield a total of 1 percent fuel burn improvement.


What happens to the ejection seat after a pilot ejects? When a pilot ejects, what happens to the ejection seat? Does the pilot stay in the seat until he lands, or does it get disconnected from the pilot somehow?

It depends on the seat. Most modern seats, including the ACES II system common in US Fighter Jets, fall away from the pilot at a pre-determined altitude. [src]

  Once out of the plane, a drogue gun in the seat fires a metal slug that pulls a small parachute, called a drogue parachute, out of the top of the chair. This slows the person's rate of descent and stabilizes the seat's altitude and trajectory. After a specified amount of time, an altitude sensor causes the drogue parachute to pull the main parachute from the pilot's chute pack. At this point, a seat-man-separator motor fires and the seat falls away from the crewmember. The person then falls back to Earth as with any parachute landing.

Generally, according to that link, this occurs at just over 100-200 feet above ejection height. However, if you eject at high altitude, such as Captain Scott O'Grady did (26,000 ft), the seat will fall away on a sensor (either 10,000 or 18,000 feet, I don't recall which). The chute may open automatically, or if it is not set to at that height, you can then pull the chute manually, or wait until its set height to open.


Is a ground station radio license required for UAS operations in the US? When conducting UAS operations would a PIC or Spotter be considered a ground station or an air station?  The communications would concern a sUAS in the air, but the radio is physically on the ground.

I'm asking in the context of licensing within the United States.  As far as I can tell from reading the FCC's website, Air stations do not require a station license to operate, while Ground stations do.

Air -> Air, or Ground -> Air describes the ultimate path of the message. It doesn't describe what's being discussed. So:

  The communications would concern a sUAS in the air, but the radio is physically on the ground.

If the radio endpoints are both on the ground, then that's ground -> ground transmission. Put it another way - if a controller talks to a fireman over the radio, about an aircraft in the air, it doesn't magically become a Ground -> Air transmissions.

The definition matters primarily for technical and practical reasons (E.g.., Air -> Air radio will have a better range than Ground -> Ground because of nothing being in the way). If you're on the ground, you're a ground station - it's as simple as that.


Did the Space Shuttle really wait until 50 feet above the ground before they put the gear down? I saw a sign at the Smithsonian about the Space Shuttle where it said that their gear was not extended until 50 feet above the ground.  I've searched and searched for something definitive that had the actual procedures used, but can't find anything.

To put this in perspective, the policy for every jet that I have flown has been to have the gear down and locked between 1,500 ft. and 500 ft. (at the very lowest).  50 feet seems very very low! What happens if it doesn't come down on the first try and they need time to troubleshoot?

If you're looking for a definitive source, how about the Shuttle Crew Operations Manual. It's essentially the POH for the Space Shuttle. Secton 2.14-2 says:

  The landing gear is deployed at 300 ± 100 feet and at a maximum of 312 knots equivalent airspeed (KEAS).

Although, the targeted deploy speed was 288 KEAS according to the Normal Procedures section 5.4-7, which matches up with approximately what you see in HUD videos.

The manual says the gear may take up to 10 seconds to fully deploy, but based on HUD videos (measuring the time it takes for //GR// to change to GR-DN), this process pretty consistently took about 6 seconds and was completed by about 70-130 feet AGL depending on the deployment altitude.

The reason they deployed so late is because the shuttle was a very non-aerodynamic "airplane" to begin with, and lowering the gear would reduce its lift-to-drag ratio even further. However, there were non-standard procedures in place (see section 7.4-25) where, if the speed brake (a split rudder – not a wing surface) was stuck closed, they would have lowered the gear earlier in order to act as a speed brake. Note that the hydraulic system was incapable of raising the gear in-flight, so once down, it stays down.

Because the shuttle was a "glider" (a generous use of that term) at landing, with no chance of going around, the gear absolutely must work on the first try. A shuttle belly landing would have ended very bad due to its high speed and high angle of attack. In order to assure the gear would extend, there were several redundant systems in place:

The doors had a bungee-assist system which exerted 2000 pounds of force (~9 kN) on the nose wheel doors, and 5000 pounds (~22 kN) on each of the main wheel doors.
The nose wheel had a pyro-assist system which fired every time the gear was deployed, and helped assure it would lock into place.
The gear is normally deployed through a combination of "springs, hydraulic actuators, aerodynamic forces, and gravity."
However, if all else fails, and the gear does not begin to move within 1 second of issuing the command, a pyrotechnic initiator cuts the locks and forces the gear down.

So they were pretty confident it was going to work.

Interestingly, the original procedure for deciding when to deploy the gear was based on airspeed, not altitude. However, this resulted in an inconsistent safety margin. This quote from the Flight Procedures Handbook: Approach, Landing and Rollout explains it better (sorry, I don't have a link to a doc, I probably downloaded it from a nasaspaceflight.com forum a few years ago with a paid membership):

  The landing gear deploy cue for STS-1 through STS-4 occurred when the velocity decelerated through 270 KEAS. This corresponded to an altitude of 200 ft on the nominal energy trajectory. Flight STS-1 followed the velocity cue procedure which, due to its much higher than expected energy, did not occur until an altitude of 85 ft. Had the gear been deployed at 200 ft, some of that excess energy would have been dissipated. Flight STS-2 was very low on energy, reaching a maximum velocity of only 274 KEAS at 1100 ft altitude. The 270 KEAS cue occurred at 600 ft, but the actual deploy occurred at 400 ft, adding to the already existing low energy condition. Lower energy occurred since the nominal altitude for gear deploy would have occurred at 200 ft on a nominal trajectory. Flight STS-3 was high on energy, not decelerating through 270 KEAS until an altitude of 87 ft. T/D occurred earlier than expected on STS-3 and the gear was actually down and locked only a couple of seconds before first wheel contact. It was after STS-3 that altitude was selected as the gear deploy cue because it would compensate for off-nominal energy conditions, not make them worse, and still satisfy safety concerns. Downrange was also considered as a gear deploy cue, and it too, had advantages over velocity.

As far as the sign you are referring to, I see three possibilities:

They were referring to the very early missions which sometimes saw very late gear deployment,
They were rounding down for dramatic effect, or
They simply made it up on the spot as if it were a statistic.