What does it take to turn a 747 around 180 degrees? Assume that a 747 is flying at cruising altitude and speed. If the pilot decided (for a presumably good reason), to turn the plane 180 degrees,

How quickly could the turn be completed?
How wide would the turn radius be?
How much altitude would be lost, if we're not assuming a level turn?

EDIT: I learned something new from both of the great answers below. I accepted the one that was easier to follow. Since both answers assumed a level flight, I have added a #3.

How long, and how wide of a turn depends entirely on three things:

What is the true airspeed.
What is the bank angle of the turn?
Is level flight maintained during the turn.

Since it's easiest to calculate, and presumably what you intended, we'll assume #3 is true (level flight). However, if you really needed to get turned around fast, turning in a descent will give you a tight turn without the extra g-forces.

Cruise Speed for a 747 depends on altitude, generation, company guidelines, etc, but we'll go with Mach 0.85 for this example, which seems to be in the ballpark. If we say it's cruising at 35,000 feet in standard atmospheric conditions, that comes out to 490 knots TAS.

Next, pick a bank angle. A reasonable bank angle for that airplane with passengers on board is 25°, and you could probably get away with 30°. Anything beyond that will cause passengers to complain. Of course, if it's an emergency you might consider something higher. Just remember, the higher the bank angle, the stronger the g-forces which will be felt by the people inside, and the aircraft itself, which has structural limitations. The G-force can be calculated using:

$$
gforce = \frac{1}{\cos(bank)}
$$

For an example, let's select 25° bank since that's the most realistic.

Radius

Calculate the radius of the turn using this formula, slightly modified from Wikipedia to give nmi instead of feet:

$$
Radius\ of\ turn\ in\ nautical\ miles= \frac{velocity^2}{68579 \times \tan(bank)}
$$

Which gives us:

$$
7.51nmi = \frac{490^2}{68579 \times \tan(25°)}
$$

So the turn itself would be about 15 nautical miles wide (≈ 91,000 feet) not accounting for wind.

Distance Traveled

Using basic geometry, the distance traveled for a 180° turn is $d = r\pi$ (half the circumference of a circle), so:

$$
23.59= 7.51\pi
$$

Giving us 23.59 nautical miles traveled.

Duration

$$
time\ in\ minutes = \frac{distance \times 60}{TAS}
$$

Therefore, at 490 knots (nautical miles per hour) and 25° bank, it would take about 2 minutes, 53 seconds to complete the turn.

$$
2.89 = \frac{23.59 \times 60}{490}
$$

You could go through and calculate the results for any speed and bank angle you want. This is true regardless of the type of airplane in question.

Turning in a Descent

Your edited question asks how much altitude would be lost if it were not a level turn. There is also no single answer to this question because it depends entirely on how you perform the maneuver.

One reason you may have to descend in a turn is because the increased lift results un increased induced drag, and the aircraft engines may not have enough power to compensate, resulting in the decreased airspeed, and possibly a stall. In this case, the equations for calculating the turn radius are exactly the same as in level flight. The amount of altitude lost will depend on the descent rate required in order maintain airspeed, which will vary depending on the engine power available, the weight of the aircraft, and the drag curve at the given speed and AoA. Peter Kämpf's answer gives an example of what this might look like for a 747 in a 1.5g turn.

Another version of a turning descent would be an accelerated descent. The advantage in this case it that it would allow turns to be made without additional g-force. The disadvantage is that you'll be accelerating downwards instead of descending at a constant rate. This can get out of hand very quickly and would only be good for very short duration turns.

To illustrate this disastrous option, let's look at what would happen in a 1g turn at the same 25° and 490 knots TAS. Since the math is more complicated, it'll be easier to talk about this portion in metric units. Here's a conversion table:

1 nautical mile = 1852 meters
1 knot = 0.514444 meters per second
1 foot = 0.3048 meters

First, in order to maintain a constant 1g, the lift vector is simply rotated in the turn (instead of rotated and increased in order to maintain altitude). Therefore, the magnitude of our lift vector will be equal to the acceleration due to gravity near the Earth's surface. We'll round that to 9.8 m/s/s.

Inward Acceleration

Our acceleration towards the center of the turn $a_c$ (the portion of our lift vector pointed inwards instead of up) can be determined using the following equation where $\theta$ is the bank angle:

$$
\sin(\theta) = \frac{a_c}{g}
$$

Solving for $a_c$:

$$
a_c = g\sin(\theta)
$$

Therefore

$$
4.142 = 9.8\sin(25°)
$$

Radius

So 4.142 meters per second per second is how fast, at 25° of bank, and 1g, we'll be accelerating towards the center of the turn. With that, information, along with our known velocity, we can calculate the radius of the turn using this equation where $v$ is our velocity in meters per second, and $R$ is the radius in meters:

$$
R = \frac{v^2}{a_c}
$$

Plug in our numbers:

$$
15341 = \frac{(490 \times 0.514444)^2}{4.142}
$$

This comes out to a radius of 15341 meters (8.28 nautical miles).

Distance Traveled

Now that we have our radius, we can calculate how long it will take us to turn 180°. This part is the same equation as before.

$$
48195 = 15341\pi
$$

Giving us 48195 meters (26 nautical miles).

Duration

Distance divided by velocity gives us duration.

$$
t = \frac{d}{v}
$$

In our case:

$$
191.2\ seconds = \frac{48195}{490 \times 0.514444}
$$

So, it'll take 3 minutes 11 seconds to complete the turn.

Downward Acceleration

The last value we need before we can calculate the altitude lost in the turn is to calculate how fast we're going to be accelerating towards the ground in this turn. Lets first calculate the upward portion of our lift vector:

$$
a_u = g\cos(\theta)
$$

Therefore

$$
8.882 = 9.8\cos(25°)
$$

Our lift vector is accelerating us upwards at 8.882 m/s/s, while gravity is trying to pull us down at -9.8 m/s/s. That comes out to a net vector of -0.918 m/s/s.

Altitude Lost

This part requires a little bit of calculus because our descent rate is accelerating. The integral of acceleration is velocity ($v = at$), and the integral of velocity is distance:

$$
d = \frac{at^2}{2}
$$

So, lets calculate the change in vertical distance (altitude):

$$
-16780 = \frac{-0.918 \times 191.2^2}{2}
$$

So, theoretically, our airplane lost 16,780 meters of altitude (55,052 feet). Of course, since we were only 35,000 feet in the air to begin with, this is rather bad news for us. Besides hitting the ground, you would also come close to risking structural damage to the airframe because in this example, 490 knots is considered the speed over the ground, but the total velocity would be higher (about 596 knots) by the end of the turn due to the sink rate.

You'll also notice that, assuming we had the altitude to lose in the first place, the turn took longer to complete than in level flight. This is because the magnitude of the lift vector was smaller.

You can feel free to experiment with other speeds and bank angles, and you could also experiment with higher g turns (simply replace $g$ in the equations with $2g$ or similar). In some cases higher than 1g, you may actually gain altitude, though it's unlikely a 747 would be able to sustain a high-g maneuver which gains altitude.

As a quick second example, consider a 2g turn with 80° of bank at 400 knots:

$$
a_c = 19.302 = 2g\sin(80°) \\
R = 2193.8 = \frac{400 \times 0.514444}{a_c} \\
d = 6892 = R\pi \\
t = 33.5 seconds \\
a_u = 3.404 = 2g\cos(80°)
$$

Giving a total altitude lost of 3589 meters (11,775 feet) over 33.5 seconds. However, at the end of the turn, you would have a sink rate of 214 meters (703 feet) per second. That's over 42,000 feet per minute. It would likely be possible to recover from that in the altitude remaining, but it would not be pleasant.

if you are unconcerned with spilling the drinks in Business class you could do a hammerhead or a split-s, although the hammerhead might be a bit rough on the cabin baggage during the rollover (the FA should NOT occupy the rear bulkhead seat directly off the aisle). The radius of either turn would be zero, time would probably be about half a minute with a drop of 10-20,000 feet.


Can you fly an airplane at a 90° roll angle without losing altitude? I've seen this answer to the question about flying upside-down, but tilted by 90°, there should be no surface creating lift (except maybe the vertical stabilizer).

It depends. As always. If thrust is high enough, why not?
Knife edge flight is a regular part of aerobatic performances, and the fuselage is producing almost* all the needed lift. This requires

Enough speed,
A low ratio of aircraft mass to fuselage side area, and
Enough rudder deflection to trim the aircraft in a sufficiently high sidelip angle.

For horizontal flight your installed thrust must be sufficient to compensate for the high drag in this attitude. This should be no problem with powerful propeller aircraft if their fuel system keeps them supplied at this attitude. Jets must make sure that the sideslip angle can be tolerated by the intake.

The smoke from the exhaust indicates that this biplane flies horizontally. Yes, it is a large model airplane (not man-carrying, but almost big enough for that). But the laws of physics don't change with scale. For an example of a full-size aircraft doing a knife edge, see this video. Thanks go to @romkyns for pointing this out to me!
Note the rudder deflection to keep it at this angle of sideslip. Direction is controlled with the elevator. At this rate of sideslip the propeller thrust contributes to overall lift as well, however, the vertical adds a substantial downforce in order to trim the aircraft and the fuselage has to supply the remaining lift.
* Almost means that the propeller will carry some of the weight, but the large downforce on the vertical needs to be compensated, too. In sum, the lift contributed by the fuselage is very close to the weight of the plane.

Only if there is a force vector component pointing up.

Model planes tend to have a large rudder which provides lift when turned 90 degrees.  Fighter jets have vectored nozzles that can turn to provide lift in a 90 degree maneuver.

Most passenger airplanes cannot perform such maneuvers.


Why don't most fighters have thrust reversers? With the exception of the Panavia Tornado, fighters do not have thrust reversers. Why is this? Why does the Panavia Tornado have them?

Thrust reversal is a complex system (equals money) and it uses significant amount of fuel (also equals money). It is not a very efficient means to reduce the speed of a landing aircraft either, and other methods (e.g. brakes or drogue parachutes) are much better suited.

The biggest advantage of thrust reversal is that it cancels out the idle forward thrust working against the aircraft. Canceling out idle thrust isn't an issue on most fighters, as they have an engine nozzle that is open at ground idle to prevent any thrust from being produced. This nozzle probably also makes it a bit difficult to integrate Thrust reversals, but the Panavia Tornado proves it can be done.

It is because they already have several ways of braking, so they don't need reverse exhaust.link This should explain to you the braking systems used by the f-16 instead of reverse exhaust. This jet you are describing may not have as many braking systems as a normal jet does so it uses reverse exhaust.


What makes a bomber able to deliver nuclear bombs? Currently the German Luftwaffe has Panavia Tornados to deliver American nuclear bombs in case of war. The bombers are getting older and are currently modernized. The German Luftwaffe also has Eurofighter Typhoons which are said to not be able to deliver nuclear weapons. Why is that so? (Same goes for the F-35 and the F-22)

AFAIK all that's needed for an aircraft to deliver nuclear bombs is

The ability to actually carry the bombs, which probably conform to some standard "form factor".
Some special electronics for arming the bombs, which must be installed into the bomber - this probably also follows some standards.

Is that all? I seem to remember some special aerobatic maneuver the aircraft should be able to perform to allow the survival of the crew. Does that exist and is it a problem for modern fighter/bomber aircraft?

You are right in saying that's all that's needed, with the addition of a strategic or tactical need.

The need for a "special evasion" technique is only needed for aircraft dropping megaton range weapons in an air burst.  Kiloton range weapons could be dropped from altitude with no special evasion required except a lot of speed.  Sub-sonic aircraft would need to be "frisky" in their escape. Read about Tsar Bomba for a good example which demonstrates that even with the biggest bomb ever dropped, a relatively slow, lumbering aircraft can escape.

Even helicopters can drop nukes.

Most aircraft are only tasked to drop tactical, low yield weapons.  The real madness is assigned to subs and missiles.

As the comment above says, I suspect that most crews dropping strategic weapons will not care much whether they survive or not.

US Army had a special type of nuclear explosive, they called: 'Demolition weapon',  or something like, its power was in the range of kilotons, had a weight of around 60 kg (133 pounds), and was discontinued by 1950. So, you can drop a nuclear weapon, if you have it, from any two seat ultralight airplane, and escape if you fly high enough. Plutonium can be obtained from a homemade reactor not bigger than a metal grain silo, they had one like this in the Barcelona University.
Fighting airplanes' technology is to ultralight aircraft what Formula 1 cars technology is to mopeds, it's hard to imagine that the aerial nuclear war procedures and capabilities are known to anybody out some restricted military audiences, and the features and performances of airplanes able to carry and launch nuclear weaponry can be watched at air shows and specialized publications, the USSR said that most of the technology information they got from the free world came from open access and purchase publications, probably, no government will sell a military technology to a third party if they don't have in reserve at home something better, it would be silly selling a technology that someone could use against you, as the experience is that big changes in government's lines are a true possibility


How do you request a 3D block of airspace to to freely maneuver your privately owned military jet? Referring to this How can you fly your own military jet in terms of altitude and acrobatics? how can you request a block of airspace to freely maneuver your private military jet (barrel rolls, vertical climb, pulling Gs, etc..).
If that kind of block of airspace is indeed possible to be permitted, what are your limits flying your jet in it?

As @jamessqf points out, once the aircraft is no longer owned by the government1 there wouldn't be any regulatory difference between you requesting air work and, say, a Gulfstream acceptance or post-maintenance flight that needs to fly around at odd altitudes and attitudes to check out various systems. That said...
Below FL180 you can fly VFR as you like. Of course you still have see-and-avoid responsibility, which might be helped by any onboard targeting radar on your military jet but then again might not be.
If you want to go higher (and have a smaller chance of turning someone's Cessna into a hood ornament) you need an IFR flight plan. Ideally you'd ask someone for permission to enter a Military Operations Area (example) and/or Air Traffic Control Assigned Airspace (which is the same concept as a MOA but exists in Class A airspace). FAA Order 7400.10 will tell you who the controlling agency and using agency are for each MOA; if you wanted you could try cold-calling Albuquerque ARTCC and asking if they'd take the Mt. Dora MOA hot for you, but the Air Force's 27th Special Operations Wing is the using agency and the line controller probably won't help you unless you run it by the military beforehand.
Assuming you can't get clearance into special-use airspace your only other option is to request a quadrant clearance in conjunction with a block altitude assignment.
A quadrant clearance (7110.65 4–1–1i) allows you to fly between two specified radials of a navaid and within two defined radii (DME arcs). Depending on the performance characteristics of your jet you might need a significant amount of distance, e.g. between the 10-mile radius and 40-mile radius, in order to get a large enough slice of airspace to work in.
A block altitude assignment (7110.65 4–5–7g) is more commonplace and allows you to operate anywhere between the two indicated altitudes. Again you'd probably want a good eight-to-ten thousand feet or more.
For both the vertical and lateral considerations you'd probably have the most luck over the Western states (ZLC, western ZMP, ZAB, etc). You could also try going offshore as long as you checked to make sure the warning areas weren't hot.

1Or operated on behalf of the government; I've seen a couple N-numbered British/European fighters operate using tactical callsigns as aggressor aircraft for military training exercises.

The simple answer is that you don't get your own block of airspace to do aerobatics in your private turbojet.
You could fly into a military operations area, but even if authorized to use that airspace (and some private owners are) for that purpose, anyone flying VFR may transit that airspace without talking to a soul...which means you don't own it.  You just use it...which is the same as being anywhere else.
Military aircraft maneuvering in mock engagements or aerobatics are operating under VFR.  Civilian aircraft maneuvering are doing the same thing.  They are responsible for their own traffic separation.  Participating traffic may be alerted or the aircraft doing aerobatics may be alerted, but VFR traffic separation is not guaranteed.  It's the responsibility of the pilot performing aerobatics to clear the airspace, and to ensure he or she doesn't create a hazard or a collision.


Why don't modern heavy bombers have gun turrets? I know that for a period of time the B-52 had a tailgunner, but nowadays many heavy bombers lack any turret defense.Why?

There are really two reasons. First, fighter tactics have evolved over time, and second, the role of the B-52 has evolved over time.

Fighter tactics

Fighters used to depend (heavily) upon getting fairly close to an enemy, and shooting it down with a machine gun. Over the years, guns have become less and less dominant, and instead fighters tend to use missiles from outside a machine gun's effective range.

Role Change

The B-52 was designed from the beginning to carry nuclear weapons. Nonetheless, it's never been used to actually bomb an enemy with a nuclear weapon.

Early in its life, the B-52 was used to drop conventional bombs on enemies though. During the Vietnam conflict (in particular) B-52s were used to drop thousands of conventional 500 pound bombs on various targets.

The Linebacker II campaign was, perhaps, especially notable. During Linebacker II, 15 B-52s were lost--all of them to surface to air missiles (SAMs) not enemy fighters.

Linebacker II did see some limited success from tail guns as well: two B-52s shot down two MiGs. No B-52 (nor any other aircraft of similar size) has shot down an enemy fighter since.

Now, B-52s act as an aerial platform for launching stand-off weapons. Its normal operation is to fly in low (where it's difficult to detect on radar), and launch from a substantial distance from the target. Since it can be a substantial distance from the target, an enemy would need to defend a much larger perimeter (e.g., a large number of fighters basically blanketing a large area) to stand a decent chance of intercepting it.

Even if we assume an enemy fighter were to intercept the B-52, we get back to the situation with fighter tactics outlined above--they'd be unlikely to get within range of its tail gun anyway.

I believe that another reason is to do with the speed of modern bombers. At transsonic speeds old-style gun turrets would be ripped off the plane.  On helicopters and other non-transsonic aircraft one can still find modern equivalents of the gun-turret. Have a look at e.g. the  Mil Mi-24 https://en.wikipedia.org/wiki/Mil_Mi-24


How can ADS-B replace primary radar when FlightRadar24, using ADS-B, is so inaccurate? I read the following on Flightradar24.com's 'How it works' page.

The percentage of aircraft equipped with ADS-B receivers is steadily
increasing though, as they will become mandatory for most aircraft
around the world by 2020. When mandatory, ADS-B will replace primary
radar as the primary surveillance method used by ATC.

Sure, that sounds lovely, but it it seems to me that a large portion of aircraft do not actually use GPS, but instead rely on inertial navigation systems or similar. Of course, the ATC doesn't need millimeter precision, or even an accurate position within a kilometer, but whenever I check flightradar24, there is quite a large bunch of airplanes reporting their position with offsets up to a few kilometers - if I were to judge by the ADS-B data, I would report a major emergency every few minutes.
How will ADS-B actually reliably replace primary radar? Will GPS have become mandatory by 2020? Or is Flightradar24.com just being optimistic?
(Side note: I know that redundancy is very popular in the aviation world, but I don't think you can call something 'replaced' when primary radar is still used for a large portion of the flights)
Edit: some parts of some answers seem to be concentrating on update rate / transmission losses and errors. For clarification, I included a screenshot (which I could literally take the second I looked at fr24.com, indicating how often this happens).

At the time this picture was taken, the 36R runway was in use for landing ('yellow' aircraft top right just landed). The selected aircraft is however happily taxiing in a field (pattern matching 36R's adjacent taxiways). This is not caused by update rate or transmission losses (i.e., this can't be old information, the plane was never in that field). Transmission error also seems out of the question, since the entire track is a smooth line (errors sometimes do happen, but are not 'hidden' on fr24 - a large 'jump' in the track can be seen sometimes). So either the plane is reporting a 'wrong' position (how is this handled by ATC?) or fr24 somehow messes up the data (how is that possible?

TLDR

How will ADS-B actually reliably replace primary radar?

It will not.

Will GPS have become mandatory by 2020?

Strictly speaking no, effectively yes.

There are so many problems with that quote from FR24 that I don't know where to start.
Let's take it apart and analyse it sentence by sentence.

The percentage of aircraft equipped with ADS-B receivers is steadily increasing though

Totally irrelevant. An ADS-B receiver allows reception of ADS-B signals from other aircraft. These can be used for example to create a Cockpit Display of Traffic Information (CDTI) or augment TCAS. The number of aircraft with this so called ADS-B IN is increasing, but the equipage percentage is probably below 2% at the moment.
If you are concerned with using ADS-B operationally for ATC then airborne receivers are not of interest. Instead you should be looking at the transmitter side.

[ADS-B receivers ...] will become mandatory for most aircraft around the world by
2020.

ADS-B receivers are not mandatory in any foreseen regulation yet. ADS-B transmitters are / will become mandatory in many parts of the world. The EU and USA rules mandate ADS-B in 2020.

When mandatory, ADS-B will replace primary radar

ADS-B is not intended to replace primary radar. As a new addition to the surveillance technology mix, it will not replace any other technology in the short term.
ADS-B is a form of cooperative surveillance, meaning that the aircraft or vehicle that is supposed to be tracked needs to cooperate by transmitting the ADS-B signal. Primary Surveillance Radar (PSR) on the other hand is a non-cooperative surveillance technology; it works by detecting signals reflecting off the target of interest. PSR can detect aircraft that do not transmit any signals. Since PSR and ADS-B are fundamentally different in their working, they have fundamentally different use cases. ADS-B will not replace primary radar and never was intended to.

will replace primary radar as the primary surveillance method used by ATC

Primary Surveillance Radar is not the primary surveillance method used by ATC in many parts of the world. Since PSR only gives azimuth and range measurements essential information like aircraft identification and altitude is missing. Therefore Secondary Surveillance Radar (SSR) is the primary surveillance method used for ATC since it provides altitude and identity in addition to azimuth and range measurements.
Secondary radar, like ADS-B is a form of cooperative surveillance; it relies on the aircraft replying to an interrogation from the radar. ADS-B can and will replace a number of secondary radars, but not all of them.

Now for the rest of your question.

it seems to me that a large portion of aircraft do not actually use GPS, but instead rely on inertial navigation systems or similar.

Indeed a large number of aircraft do transmit their position on ADS-B that is based on a non-GPS source, usually from the INS. INS can not provide the accuracy and integrity that GPS can provide and therefore you see large position errors on FR24. The fact that the position is not to be trusted is reported in the ADS-B message in the quality indicators, but unfortunately FR24 doesn't use this information.
All the ADS-B mandates require that the system is able to provide high integrity data, which effectively means that GPS is required as position source, although theoretically other means are possible as well. Therefore in 2020 all aircraft subject to the various ADS-B regulations will have to be GPS equipped.

Of course, the ATC doesn't need millimeter precision, or even an
accurate position within a kilometer,

ATC does not need millimeter precision (GPS wouldn't provide that anyway) but they do need position accuracy within a kilometre and sometimes within about 10 meters (e.g. runway entry stop bar monitoring). ADS-B  with appropriate GPS source will be able to provide that.

What people are missing here is FR24 is NOT for ATC operation but secondary data source to be used as 'non critical use' by anyone interested. Technology for ADS-B in/out itself is perfectly accurate when operated under certification guideline of aviation authority. In reality FR24 web service or other crowdsourcing data provider never 'guarantee' 100% accuracy for their data. Also FR24 website show intentionally delayed data by couple of minutes.Website user  access level also have different accuracy on different time of day between FREE and Paid account. Also if you carefully look on 'settings' of FR24 web interface you will find option for 'Estimations value in minutes' which sometime show aircraft position entirely by estimation of schedule, last received data etc in case of no real data received for any particular scheduled aircraft.

There are other limitation for FR24 data viewing which might give wrong impression. But if ever ADS-B become primary for ATC operation, they will have their own setup for that purpose but ATC operator will never guide aircraft based on screen what you and me are watching on open Internet. TCP data like HTTP/Web traffic will never use on time-critical operation. Switching mode data network is there for time critical network. Even your VoIP traffic normally avoid TCP for voice transmission between two party.

ADS-B is NOT Flightradar24.COM, but Flightrader24.com use ADS-B for sourcing their data. Deciding ADS-B technology by watching Flightrader24.Com is like trying getting experience of traveling on Airbus A380 while traveling on Boeing DC-10:))

Edit adding more info: In nutshell if we compare all other technology available for avation industries for accuring position data at this moment ADS-B is on top of list. If you don't want to beliave FR24 website, try FAA document issue 2010 on this PDF Document. By using FR24 site you got wrong impression about ADS-B. Actually this is very much accurate in operation when use as per regulation guideline and equipment setup both on aircraft and control tower is proffational grade. Just think of it, people now preparing for riding driverless on highways runing hundrudes of kilomiter/hour with much proximity than aircraft would never come close (Even on a parking space). Both position data source is GPS sattelite indeed. Current operating prosidure used guiding allmost all avation traffic is product of Warld War II era. Equipment might become sophisticated but structure is same. You can have more details on ADS-B here in Wikipedia Article.


Where can I find an image of the Thales FMS220 RMS page? I'm looking for photo/color image showing Thales FMS220 RMS (Radio Management System) page. Also COMMUNICATION and XPDR pages are highly welcome.

The answer is on the official website, here is a direct link:

https://fr.scribd.com/doc/204747124/Fms-User-Guide

You need to go to the website yourself because the document content is not to be disclosed without official approval.


Is it theoretically possible to create an aircraft like this? I was browsing through some funny airplane pictures when I came across this. An airplane with the engines replaced with the cockpit and the cockpit replaced with a huge turbofan.

I was intrigued by this idea, but then it also brought up the question: Is it theoretically possible to create an aircraft like this and be possible for the aircraft to take flight? (as in fly in the air for prolonged time)

Source: YouTube

The idea has been tried. This WWII aircraft is built around the idea very much like yours: engine in the main fuselage, pilots/passengers in the nacelle.

It must be reasons to choose this design. The design requirements were:

One engine.
Crew one or two
Best possible view into to all sides
(reconnaissance aircraft).

For the passenger aircraft as pictured it is probably just not optimal.

There inst a jet engine that size, and this is very unfathomable when it comes to reliability, safety, and efficiency. But, if you are asking if we can design a plane with the power plant as the main fuselage/airframe, then probably yes, a very bad one.


How would an airplane land if the on-board radio breaks down? Is there a special protocol for a situation where the on-board radio communication equipment suddenly breaks down? Would the pilot be able to land on a busy towered airfield or would they be forced to look for a small landing strip in the middle of nowhere?

It is important to remember that it is often difficult to diagnose radio malfunction during flight. Therefore it is hard to know whether you have a malfunctioning transmitter, a malfunctioning receiver, or perhaps both. You also have the point-of-failure that is the headset you are using. For this reason it is quite usual to carry a spare headset and the first piece of diagnostics is to swap headsets. Many General aviation aircraft also have a handheld microphone and speakers integrated into the radio stack.
Once you've ascertained that its the actual transmitter and/or receiver that has failed it is common to set the transponder to 7600. For landing, the relevant detail for the procedure to follow is contained within Chapter 4, Section 2-13 of the AIM:
Receiver Inoperative

Remain outside class D airspace until the direction of air traffic can be ascertained
Notify the tower your type, position, altitude, intention to land and request you are controlled with light signals
At 3 -5 miles, advise the tower of your position and join the traffic pattern
Watch the tower for light signals
Continue to advise of position (downwind, turning base, etc)

Transmitter inoperative

Remain outside class D airspace until the direction of air traffic can be ascertained
Monitor the primary local control frequency as depicted on sectional charts for landing or traffic info.
Look out for light signals that may be directed to your aircraft
During hours of daylight, acknowledge tower transmissions or light signals by rocking your wings
At night, acknowledge by blinking landing of navigation lights

Transmitter & Receiver inoperative

Remain outside class D airspace until the direction of air traffic can be ascertained
Maintain visual contact with the tower and acknowledge light signals as above.

The light signals used for an aircraft in flight are as followssource:
Steady green - Cleared to land
Flashing green - Cleared to approach airport, or return to land
Steady red - Continue circling, give way to other aircraft
Flashing red - Airport unsafe, do not land
Alternating red and green - Exercise extreme caution
They can be seen in this video.

If your radio was broken during a flight, you could use a light or come up with a sign of help that a helicopter or other airplanes can see so that they can contact the radio people and rescue them to safety.


Why is "runway behind you" useless? I was recently answering this question and for some reason it made me think of this fairly famous aviation saying

  The three most useless things to a pilot are the altitude above you, runway behind you and fuel on the ground

Now, I get altitude above you - you may as well use some of it lest you meet a mountain. I also get fuel on the ground - if in any doubt fuel up before you go.

But I'm not sure I understand why "runway behind you" is useless in this context

Could someone explain?

The runway ahead of you is useful to accelerate and - if needed - to decelerate.

If you enter (or touch down) the runway in the middle, you will not be able to benefit from the part of the runway behind your entering point. It is the same as if that part would not exist. Therefore it is useless.

I've always known the saying as "The three most useless things to a pilot are the altitude above you, runway behind you, and the fuel you had"

What the saying is really talking about, is give up your margin grudgingly, especially when you don't need to.

This works for all aspects of life.


Can someone/something other than the pilot trigger the ejection seat? Let's say there was a two-person jet (take the F-15 for example) that was involved in some sort of accident in the air that incapacitated the pilot (unconscious but still alive) and left the aircraft unflyable. Could the copilot eject his fellow pilot in an attempt to save his/her life from the inevitable crash that would eventually take place?

Is there some sort of system in the aircraft that would do this automatically?

Seems like an messy problem to have. On one hand you don't want to be blasted out the windshield accidentally by your copilot or faulty computer system but on the other hand it sure would be nice to survive in the off chance you are unable to exit the plane yourself.

In a word, yes.

There is a switch which controls this and the default position is "both".  If any handle is pulled (pull down over the face or the one between your legs), then both seats fire.  If solo, the switch is turned to "solo" so that the rear seat is not ejected and the front seat gets to leave a little bit quicker.  If the switch is left on solo, and both seats are occupied, then each crew member can eject on their own.

You would also have if set to "solo" with an untrained passenger in the back.  You wouldn't want someone getting a bit excited and firing them both out of a perfectly working jet.

See this question about what happens, and this one to understand why you won't hit the canopy.

As far as I know, there is no computer controlled ejection system.  I don't think anyone would trust it to know better than the crew when it's time to bail out.

Added: This answer was based on advice I received that the SU27 had auto-ejection capability.

In a comment xxx states that the SU27 does not have auto-eject capability.
I have not yet obtained supporting factual references for either point of view. I will update this answer when I know more.

If the SU27 does NOT have auto eject capability then the crew members were extremely brave, regardless of what responsibility they did or did not have for the crash. I viewed a video of the crash and did a frame by frame examination from 1st contact with the ground by a wing until ejection.
The video speed was 25 frames per second.

The video was never "super clear" and became increasingly less so as the crash progressed (apparently due to the videographer attempting to not die).
From the time the wing 1st touched ground 4 frames (0.16S) elapsed until certain canopy motion was observable. It may have been starting to move for 1 to 2 frames before this.
The canopy was well clear by frame 8 (0.32 seconds post contact) and one crew member ejected at frame 11 (0.44 s post contact). The aircraft was moving out of frame but it appeared essentially certain that a prior ejection had not occurred so the 2nd ejection probably occurred some time after frame 11 (after 0.44s post contact).

While ejection initiation would have occurred at some time before frame 4 when canopy motion was first noticeable it would almost certainly have been a very short period prior, so it is very likely that the aircraft wing had touched the ground at or before ejection initiation.

That's brave.
Also stupid, if you want to live.
But very brave.

_______    End of addition.    ______________________________________

Yes.

This is a frame grab from a video about a Ukrainian airshow disaster when an SU27 crashed into the crowd with many fatalities. Both crew members ejected essentially "on impact" and I'm told that the ejections were automatically triggered. Both survived.

The ejection shown here obviously had to be preceded by a trigger, canopy eject, fire, ... sequence which I'd expect to last at least some "tenths of a second", so the actual decision point is uncertain.


What factors affect the minimum time to go from point A to point B? I've created this trajectory for a small RC aircraft to go from 15m to 61m as quickly as possible (e.g. quickest time to get from point A to point B). I'm trying to perform a sanity check for why the aircraft would go to the lowest allowed altitude to maximize velocity before beginning it's climb. Can anyone provide an explanation for why this would be most time-efficient for this problem?
The reason that the aircraft does not fly lower than 5m is because I set 5m as the lowest possible constraint. If that constraint was not in place, it would try to fly at 0m.
Edit:
The reference listed for this type of problem is given as 'J. Betts. SOCS Release 6.5.0, 2007.'. This is an inappropriate place to ask for this, but I have tried searching for whatever this paper/journal this may be referencing and cannot find it. If anyone has any tips/tricks on how to actually find this source, please let me know

It looks to me like your airplane, starting with a very low velocity, is trying to climb a height of 46m (61m - 15m).

Because it has a low initial velocity, your basic choices are:

Start climbing immediately, demanding engine power both for velocity and climb rate, and climbing very slowly.
Fly level for a period of time to build up velocity, then begin the climb.
Use gravity and engine power together to get more velocity quickly, then begin climbing at a much faster rate.

The velocity you need to achieve is called $V_y$, or best rate of climb.
Based on the Yellow color on the main segment of the climb, it looks like this plane's $V_y$ is about 26 $m\over s$.

It looks like your software package chose #3.  While it makes intuitive sense, without the math behind it, I cannot tell you the details.

That's an example of an optimization problem, the mathematics of which is a significant field in itself. Your case is similar to the renowned "Brachistochrone curve" problem (see Wikipedia).


How similar/different are the roles performed by Military and Civil pilots? Broadly, Military and Civil are the only two categories in which the Pilots can be classified on the basis of the roles performed by them. What are the similarities and differences in tasks performed by them?

The biggest difference is that most everything in commercial aviation is considered administrative in the military.  The tactical employment of our aircraft, the non-administrative portions of flight, is what separates military sorties from commercial flights.  This emphasis on tactical flying has also led to significantly different cultures between commercial and military aviation.  There isn't a commercial pilot I would stand a chance against in a game of FAR/AIM trivia, but I also doubt they'd fare much better if the tables were turned and the questions were related to carrier operations.  We are two separate organizations with two unique mindsets to flying.

Basic difference is training. A military pilot is trained to achieve assigned mission even if he has to endanger his life or aircraft, where as Civil pilot is trained to keep safety of his aircraft paramount as it has many other lives with him. Converting from Military flying to Civil Commercial flying, pilot finds it most difficult to adhere to follow strict safety regulations.


Can you accept a LAHSO on your private pilot checkride? Suppose you're on your private pilot checkride and the tower gives you a land and hold short clearance.

Are you able to accept that clearance? I know you're expected to fly as if you were a private pilot, and you're allowed to carry the examiner even though they're not providing you dual instruction...

Are you allowed to accept a clearance on your private pilot checkride that you couldn't accept as a student but could as a private pilot?

First, let's see whether it's legal. FAA Order 7110.118 notes that:

  7. EXPLANATION OF CHANGES.

  The following changes apply to all LAHSO.

  a. ...
  b. Solo student pilots will not conduct LAHSO.

However, you could interpret that as not applicable to your situation, as you are not the sole occupant of the aircraft:

  14 CFR §61.87

  (a) General. ... The term "solo flight" as used in this subpart means that flight time during which a student pilot is the sole occupant of the aircraft...

Though it might be allowed, note it does not say must:

  AIM 4-3-11(b)

  ...
  Student pilots or pilots not familiar with LAHSO should not participate in the program.

I'd recommend against doing so because LAHSO can add to your workload, especially if you're a bit nervous on your checkride. Also, the AIM note on discouraging LAHSO is rather valid and if done improperly, can affect safety. Having said that, the examiner is likely to ask questions on LAHSO during your practical examination, especially since LAHSO is a special emphasis area in the PTS. This may include:

Requirements to accept a LAHSO clearance
ATC pharseology for LAHSO
Pilot responsibilities while carrying out LAHSO

In any case, always remember that you do not need to accept a LAHSO clearance -- if you do not feel comfortable with doing so, ATC will provide arrangements to land full-length on the same runway or another one. You should not accept a LAHSO clearance if you believe doing so would compromise safety.

Note: this does not necessarily apply to all situations where a student pilot is prohibited from doing something that a private pilot would be able to do.

Strictly speaking you are technically flying as a private pilot and pilot in command on your check ride, you are not flying as a student pilot, the DPE doesn't even need to have a current medical certificate. It is a strange sounding thing but the check-flight is the one flight where you are not required to hold the certificate for that type of flight being conducted. The flight will be logged as a private pilot, it will not be logged as a student pilot.

To put it another way, you are essentially considered a qualified private pilot when your instructor endorses your for the check-ride. The check-ride is just a confirmation and it is as much about double checking your instructor's judgment as it is about your skills.

So yes, you can do anything a private pilot can do during your check ride. If you want a belt and suspenders answer then just ask the examiner during the question and answer portion before the exam officially starts, in the end it is their interpretation that counts.(unless they are way out of line and you need to appeal to the FAA) It will not count against you as it is something unique to the exam environment not to normal flying, and it is not a test item in the ACS.


What does "inertial" refer to in "inertial reference"? An ADIRU makes use of air data reference and inertial reference. Inertial reference calculates the heading, position, ground speed and attitude. I was wondering why is it called inertial. Is it related to the term inertia/mass?

It is called inertial, because it works by measuring—and integrating—inertial forces¹, that is forces due to acceleration of the reference frame (i.e. the aircraft). Gyroscopic effect used to measure rotation is also due to inertial forces.
The measured forces include gravity², which can't be easily separated, but this is done by observing that the speed is limited, so the average acceleration must be zero and therefore the long term average equals gravity.

¹ In general relativity the inertial forces are usually considered as real as any other, so the term “fictitious” is not really appropriate.
² In general relativity gravitational force is considered an inertial force. In standard English terminology, gravity also includes centrifugal force due to rotation of Earth.

https://www.ion.org/publications/online-tutorial-intertial.cfm has my online tutorial for GPS/GNSS and inertial navigation, free to Institute of Navigation members.  This basic material is also available at low cost to nonmembers.
An inertial measuring unit "IMU" typically has a triad of gyros and a triad of accelerometers.  These sensors DIRECTLY provide angular rate and "specific force" (defined shortly), and INDIRECTLY (by propagating the effects fo those direct measurements) provide velocity, position, and attitude.  Gyros sense absolute (i.e., total) angular rate with respect to an inertial coordinate frame (i.e., one that doesn't rotate and doesn't accelerate).  Specific force is the total NONgravitational force; if you dropped an accelerometer it would read close to zero (e.g., a drag effect), not 32 ft/sec/sec down -- and in a cruising aircraft a vertically aligned accelerometer would show the 1-g lift.


Can pilots tell if a cloud or fog is made out of water droplets or ice crystals when flying through it? I understand that clouds can be made out of water droplets or ice crystals. And that fog is nothing else than low-lying clouds.

However, I've experienced fog in a wide ranges of temperatures. I like mountaineering and you often get fog high in the mountains. I've been lucky enough to visit the high mountains of Antarctica and I've also experienced fog there at very low temperatures.

This answer (in the comments) and the fog wikipedia article mention that ice fog happens only at extremely low temperatures (-35 or -40 C). But I think I've experienced fog at those temperatures too. Some times, with the right light I've seen the air filled with tiny crystals, but I wouldn't call that "fog", because it is very thin.

Then, as far as I can tell, all thick fogs/clouds feel and look the same. No matter if they are made out of ice crystals or water droplets. Is that right? Or is there a way to differentiate between ice/water fogs?

Or maybe contrary to what I think, I've never experienced frozen fog.

But, would I notice the difference if I were to ever experience it? How would they feel/look different?

Other sources, like the UK meteorological office say that water always remain in liquid state in fog, no matter how low the temperature is. Is that true? Is it true for clouds too? In that case there would be no such thing as "ice clouds".

One possibility is that ice or water clouds would give rise to different optical phenomena, like sun dogs that are produced by ice crystals, but usually in conditions were they are not dense enough to be considered fog, like in this image:

(Image from Wikipedia page for sun dogs)

So I wonder what's the experience of pilots, as they go through clouds?. When they do so, they know also the external air temperature, which is a plus. Do they see sun dogs or other optical phenomena that differentiate water from ice clouds?

Boeing provides some decent notes on this in their powerpoint which you can find in full here. Their sections on pilot reports and the summary section have some good notes for identifying ice crystals in flight/clouds/fog

  Pilots have reported rain, sometimes ‘heavy rain’ on the windscreen, Sometimes at impossibly cold temperatures. This is
  believed to be the result of these small ice crystals melting on
  impact with the heated windscreen
  No observations of significant airframe icing
  Turning on landing lights at night and seeing reflective precipitation .It is likely they were seeing the larger particles even
  though they make up a smaller fraction of the population
  Hearing a different sound from rain. Ice crystals hitting the airframe sounded different than rain or hail
  Light to moderate turbulence
  No significant airframe icing
  TAT probe frozen
  Ice detector not detecting ice (when installed)
  Appearance of rain on the windshield

The same powerpoint also discusses that satellite imagery can be used to see ice crystals. A lot of planes even small stuff have the ability to receive feeds of this data which can be used to identify sections of ice crystals in flight.

According to various sources the threat is particularly dangerous due to the fact that there is no hard and vast way to visually identify the crystals and in many cases the effects are similar to other events and may lead the pilot to belive something else is happening.

For pilots we can only go by look, unless you are flying slow in a 172 you cant exactly pop the window open at altitude and feel the cloud. Simply put pilots can not reasonably tell if a cloud is vapor or crystal prior to flying into it and observing the above noted actions.

The telltale sign is observation of ice buildup as supercooled water droplets strike and freeze onto leading edge surfaces.  The probability of this happening increases as one approaches + 32 F (or 0 C), and decreases in colder air ( - 15 C). Freezing rain (rain falling from a warmer layer of air into a freezing layer of air) will also cause ice build up. Yes, it can rain 10 degrees below freezing.

In doing a pre-flight weather check, one can only determine probabilities, as weather (fronts, freeze lines, precipitation) cannot be pin pointed to the nearest mile and are always changing.  However, certain weather patterns such as warm fronts overriding cold air, relative humidity, and near freezing temperatures do provide vital clues.  One would build in a safety margin and avoid if at all possible.

Unfortunately, it is very difficult to tell exactly what is in a cloud until the ice actually starts to form.  Vertical air currents could melt, supercool, and refreeze the same drop of water and a plane flying at the same altitude could experience a variety of "weather" as it passes through. Under these conditions constant observation is a must, as well as a "Plan B" to get out of there if icing compromises safety.


Have B-25's and B-26's been converted for private business use? In the movie Cash McCall, the main character, McCall, owns an aircraft that looks remarkably similar to a B-25 or B-26, but is painted yellow and has no turrets.

Have these aircraft been converted for private business use or were there business aircraft based on the design of WWII bombers?

IMDB:

  McCall's plane is a demilitarized Douglas A-26 Invader, a type used in WWII.

Since the film was made in 1960, I imagine there were plenty of them around.

(The A-26 was known a the B-26 from 1948 to 1965, according to Wikipedia. The B-25 is easily distinguished because it has the twin tail.)

The question leaves it unclear as to whether the "B-26" reference was meant to apply to the Martin B-26 Marauder, or the Douglas A-26 Invader, later renamed B--26 after the retirement of the Marauder.  Both were converted for use as executive transports (see https://en.m.wikipedia.org/wiki/Martin_B-26_Marauder , and https://en.m.wikipedia.org/wiki/On_Mark_Marksman ), as was the B-25 (see http://www.joebaugher.com/usaf_bombers/b25_31.html ).

So, regardless of details, the answer to the title question is "yes".


How long is a clearance void time valid? You're picking up your IFR clearance on the ground and you get a 10 minute void time from now.

What phase of flight is that 10 minute void time clearance good until?

..your wheels are up?
..a certain altitude?
..on course?
..your initial call to ATC?
..or is it from when ATC gives your position report and says "radar contact"?

Since you didn't state jurisdiction, I'm assuming US.

From the AIM:

  1. Clearance Void Times. A pilot may receive a clearance, when operating from an airport without a control tower, which contains a
  provision for the clearance to be void if not airborne by a specific
  time. A pilot who does not depart prior to the clearance void time
  must advise ATC as soon as possible of their intentions. ATC will
  normally advise the pilot of the time allotted to notify ATC that the
  aircraft did not depart prior to the clearance void time. This time
  cannot exceed 30 minutes. Failure of an aircraft to contact ATC within
  30 minutes after the clearance void time will result in the aircraft
  being considered overdue and search and rescue procedures initiated.

The requirement is to be airborne prior to the clearance void time. You need to contact ATC within 30 minutes of the void time either in the air or to notify them that you did not depart.

That would be contact after takeoff. The 10 minutes is your interval to get yourself into the controller's airspace and report yourself as such.


How to say the name of this approach clearance: COPTER RNAV (GPS) 027° JFK has this approach: https://aeronav.faa.gov/d-tpp/2006/00610COPTERR027.PDF

How is this approach said on frequency? Is it the "copter RNAV zero two seven approach" or the "copter RNAV zero two seven degree approach" or the "copter RNAV heading zero two seven approach" or something else?

Based on the 7110.65 2–1–17h, "degrees" is never used. Based on Note 9 under 4–8–1a, the text in paranthesis is omitted. So, ATC would say:

Cleared Copter RNAV zero two seven Approach

or

Cleared Copter RNAV heading zero two seven Approach

If you were requesting it, you'd say "Requesting Copter RNAV zero two seven approach."
However, since it is the only Copter approach at JFK, you could probably just request "the copter approach" and they'd know exactly what you're talking about. But ATC is required to use the phraseology as indicated in the Order linked above.


Is it possible to design a helicopter with fixed rotor blades only (no blade angle/flap/pitch)? I am not familiar with helicopter rotor design but I wonder if it is possible to build a working/flying helicopter with fixed rotors that cannot change the blade angle, flap, pitch or adjust blades otherwise?

I am curious whether the changes in rotation speed would suffice for controlling the aircraft. I guess the cheap RC helicopter models may use such simple design, but is it possible to scale that up to 1:1?

I mean propellers that are solid state and cannot be adjusted in any way except changing their rotation speed, like plastic props used in RC modelling. I do not mind if the whole helicopter design is a traditional main rotor + tail rotor or a coaxial rotor design.

I am interested in 1:1 scale size (at least the Mosquito personal helicopter size). Has anyone tried such simple design?

Also I consider electric propulsion where the RPM changes may be quicker that with internal combustion engines.

Scaling laws are your enemy here.

Model helicopters can be controlled with changes in rotational speed, but for full-sized helicopters the energy needed to quickly change the speed of their rotors relative to the energy needed for lift creation is far too high. In detail:

The moment of inertia of a scaled-up rotor changes with the fifth power of length. The mass of the rotor changes with the cube of the linear scale, and the moment of inertia adds another factor proportional to the square of the linear scale.

A bigger object also needs slower changes, but time scales only with the square root of the linear scale. Next, the engine power available for speed changes will scale with the 3.5th power of scale*.

This still leaves a deficit of one power in the engine power available for speed changes when the helicopter is scaled up.

Lift asymmetries in forward flight can be dealt with by adding a second, counter-rotating rotor, but scaling laws can not be designed out. As Jan Hudec points out, you need to repeat the same trick for forward-backward lift shifting, so four rotors would be the minimum to control this type of helicopter in all directions.

* Proof: Thrust must exceed weight, so thrust goes up with the third power of scale. Rotor disk area scales only with the second power of scale, so a higher acceleration through the rotor disk is required to create more lift per rotor area. If we look at the power required in the static case
$$P_{min} = \frac{\eta_{Prop}}{T_0}\cdot\sqrt{\frac{\frac{4\cdot T_0}{\pi\cdot d^2}\cdot g}{2\cdot\rho}}$$
and insert the scaling factor in all scaleable variables, a power of 3.5 remains for the minimum power $P_{min}$. This is more than the increase in static thrust because the efficiency of the rotors declines with higher disk loading.

Thinking outside the box:

If you can't effectively brake the rotors could you break the airflow that applies the lift?

Given that the amount of lift is controlled by the air flow I see no reason why it wouldn't be possible to interfere with that using separate structures above and/or below the blades. By raising or lowering this interference you could adjust the amount of lift by increasing/reducing the distance between the interfering structure and the blades themselves. You'd no longer need to tilt the blades themselves, which could improve the structural integrity. The distance of the interfering structures on the right and left could be differentiated to compensate for forward (or reverse) motion.

Unfortunately I have no way to establish whether this could be implemented without risking the blades hitting the interfering structures or whether it could apply enough of a difference to the lift. I'm no engineer, but there would obviously be other risks to at least discount, particularly as I'm unsure whether it would be possible to make the interference continuous, otherwise resulting in stress on the blades as they bend slightly each time they move into/out of the interference, which could also result in vibration build up.


Why do most radial engines use an odd number of cylinders? To help suppress vibration, most 'V' and 'horizontal' engines use an even number of cylinders as closely opposed as possible. Radial engines are well known for excessive vibration. For some reason they nearly always use a staggered odd number of cylinders that would seem to promote vibration, why?

An odd number of cylinders is required by the combination of the single-crank radial design, the four-stroke (Otto) work cycle, and the desire to keep the power strokes evenly spaced in time.
To keep the design simple and lightweight, a single-bank radial airplane engine has one crank, which means that the pistons must reach the top of their travel in rotation order. But the four-stroke cycle requires that a piston must reach the top of its travel twice for each power stroke. The only way to promote evenly timed power strokes is to fire every other cylinder in rotation order.
With an even number of cylinders this would require a hesitation or skip in the firing sequence on every rotation as the engine switched between the odd and even cylinders. With an odd number of cylinders the timing is quite naturally smooth. For example, the firing order of an eight-cylinder radial would be
1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8
*   *   *   *     *   *   *   * *   *   *   *     *   *   *   *

while the firing order of a nine-cylinder radial is
1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9
*   *   *   *   *   *   *   *   *   *   *   *   *   *

If you could watch a radial airplane engine in slow motion you would see that when a cylinder is in its compression stroke, the cylinders on either side of it are in their exhaust strokes, and when a cylinder is beginning its power stroke, the cylinders on either side of it are near to beginning their intake strokes.
Two-stroke radial engines do not need to have an odd number of cylinders.

A even number of cylinders in one bank of a radial engine just does not work, purely for the fact that each cylinder has to fire once in every two rotations of the engine, as their is only one crank it is impossible to achieve a firing sequence that would work.

The firing sequence on a nine cylinder engine is 1 3 5 7 9 2 4 6 8, this provides a smooth production of power

Go to YouTube and look up “How a radial engine works”


Why is it harder to maintain laminar flow on larger aircraft? I am told that on larger aircraft, suction will be needed to maintain laminar flow because of the larger leading edge radius. Why so? Is it because the air has to travel over a longer distance on the skin, so slows more, and thus piles up toward the trailing edge, causing adverse pressure?

Sort of interesting that very slow, lower Reynolds number airfoils, such as models, actually have to trip the airflow to form a boundary layer.

Surface friction causes the air near the wing surface to move at the wings speed.  In a wind tunnel this is slower than the free stream, in the air, the moving wing speeds up the air around it.  In relative terms, the effect is the same.

Boundary layers are draggier than laminar flow, but much less draggier than turbulent flow.  The boundary layer is like "grease", helping reduce drag and create lift.

At higher subsonic Mach numbers, early efforts at a "laminar flow wing"  for long range escort fighters  resulted in something resembling an airfoil flown backwards, with a sharp leading edge, a very gently increasing thickness (to keep laminar flow), maximum thickness further back towards the trailing edge, and a much shorter taper to the trailing edge.
This is reminiscent of the blunt "boat tail" approach being tried these days, also found in bullets.

Though successful, low speed handling and stall characteristics led to some needed modifications.

But chord length, and airspeed, as well as airfoil type and surface smoothness, are very important parameters in predicting airflow around a wing.  Airfoil Tools, on the web, is a great place to begin to explore airfoil types for various applications.  Particularly of interest are Coefficient of Lift vs AOA, and Lift/Drag ratio.


What is the slowest fixed-wing airplane? It's easy to find information about the fastest airplanes, in different categories (e.g. X-15, SR-71, the Concorde etc), but what is the slowest one? Which powered, manned airplane is capable of sustained level flight at lowest velocity?

The Gossamer Albatross is a human-powered plane with a top speed of 29 km/h (18mph).

It was used to cross the English Channel and seems to meet the criteria of the question.

It is the shock ultra by savage.  Stalls at 18 mph.    2016 savage produced it and it by far out flies all other planes by a mile in the low and slow cathegory


How can someone without a pilot's license arrange an initial test flight for a homebuilt aircraft? I think this has more to do with culture and expectations than actual rules, so maybe this might attract opinionated answers, but I still want to know what people think:

If a person were to construct their own homebuilt, like a Kitfox or RV, without first obtaining their PPL, how can they go about completing the 25 hour initial test phase required for the new aircraft?

This encompasses a subset of questions:

Is it customary to pay an experienced pilot to do it, acknowledging the risk involved with the activity?
Or is free use of an aircraft and loggable time/experience compensation enough?
Is it generally hard to find someone willing to do this for you (maybe do this with you is a better phrase since you will be part of the ground crew)?

Even licensed, experienced pilots often hire other pilots who have experience in test flying and hopefully time in the same model to perform the initial flights. Expect to pay for the service; test pilots do not do the work to build time. How hard it will be to find someone to do it depends on many factors such as the type of aircraft, the reputation of the design, your location and your willingness to allow the testing to take place at a safe field, the quality of your work, if you worked with an EAA chapter and at least one advisor, the quality of your build logs and photos, the quality of your flight test program docs, etc., etc. The EAA can further advise you on this, the membership fee is well worth it.

You don't need any kind of licensing for the pilot or the plane so long as you are flying an ultralight as defined under "FAA part 103."

If you Google "FAA 103" you can get the specifications straight from the source.  Some of the requirements are:

Weight of less than 254lbs empty. (Safety equipment does not count against this.)

Single seaters only.

Fuel tank no larger than 5 gallons.

Must be slower than 55 MPH in straight level flight.

Must have a stall speed slower than 24 MPH.

There are some other requirements that can't recall off hand.


What RA will be generated if intruder traffic is at our level in TCAS II? If the other traffic is at the same level, and neither climbing or descending. Will the RA say "CLIMB" or will it say "DESCEND"? How will it be decided and what are the possibilities?
Assume:

Mode S is not available
Only one aircraft has TCAS

Will it be random?

The TCAS algorithms are specified in RTCA/DO-185B (not freely available).
In case of an encounter at same altitude with a non TCAS equipped aircraft, assuming that neither climb nor descent are inhibited, the TCAS will produce a descent advisory (part 2, sec. 2.2.4.1.2.1.5 - sense selection).
This is a consequence of the somewhat subtle signs used in the specification (<= for descend, > for climb).

A non mode-S intruder will broadcast altitude with a 100ft resolution, and a TCAS equipped aircraft will normally use barometric altitude with 25 ft resolution, so the two aircraft do not need to be at exactly the same altitude - in the worst case, a difference of up to 62 ft will be interpreted as "same altitude".

There are also provisions for climb and/or descent inhibits. In particular descend RAs are inhibited below 1000ft radio altitude.
Climb RAs are inhibited when the aircraft performance are not compatible with a 1500 fpm climb (for example, one engine out). The exact mechanism through which this information is provided to the TCAS computer is aircraft dependent.
In your case of same-altitude encounter, if descend RAs are inhibited, a climb RA will be generated.

EDIT
As requested, more clarification.
A simplified description of RTCA-DO185B initial sense selection follows. This description assumes that the intruder is not TCAS equipped, and that the firmness (confidence in intruder altitude and position) is sufficiently high.
First, choose the direction (climb or descent) that will produce the highest separation at CPA (see below).
If equal, select descent (see 3.52 Noncrossing_Biased_Climb and 3.53 Noncrossing_Biased_Descent).
Denote this direction with D

If a RA in the D direction does not result in a crossing (see below), then issue an RA in the D direction.
Otherwise, if the opposite direction still provides at least ALIM separation, then issue an RA in the direction opposite to D.
Otherwise, issue an RA in the D direction.

Separation: The TCAS algorithm models a RA response as follows:

5 seconds of reaction time (pilot maintains current airspeed and rate-of-climb)
A segment with a constant acceleration of 8 ft/s^2 in the direction of the RA
A final segment at constant rate of climb (usually + or - 1500 fpm).

To select the sense, the TCAS algorithm considers:

Distance from intruder
Time derivative of distance from intruder (closure rate)
Time derivative of the closure rate
Ownship altitude and rate of climb
Intruder altitude and rate of climb

With these 5 parameters, the vertical separation at the CPA (Closest Point of Approach) can be computed.
In this scenario, the relative altitude is zero, and the rate of climb of both intruder and ownship are 0. Due to the symmetry of the model, the separation will be the same for both climb and descent.

Crossing
Crossing is defined as either:

A climb RA, with ownship at least 100 ft below the intruder
A descent RA, with ownship at least 100 ft above the intruder

In this scenario, ownship and intruder altitude are equal, so neither direction will result in a crossing RA.

The original poster's question assumes that mode S is not available.  The original poster did not stipulate that only one aircraft lacks mode S, therefore, neither aircraft has mode S.
Mode S is necessary for TCAS II, and therefore one cannot have a resolution advisory without a mode S transponder.
Under the conditions stipulated, no mode S and only one aircraft with TCAS, that aircraft can only have TCAS I, which will provide a traffic advisory, but not a resolution advisory.  With no mode S and no RA, there will be no direction given for a climb or descend, only a traffic alert.  It will be up to one or both pilots to see and avoid and to take necessary action to avoid a collision.


What is the difference between Boeing 777-31H and Boeing 777-31H(ER)? What exactly is the difference between a B777-31H and B777-31H(ER)? Is ER the latest version of 777-31H?

The 777-31H is a standard Boeing 777-300 manufactured for Emirates (customer code 1H).

The 777-31H(ER) is a Boeing 777-300ER (Extended Range) again for Emirates.

Compared to the standard 777-300, the 777-300ER has:

A longer range due to the extra fuel it can carry (145538 kg compared to 135880 kg for the -300 baseline)
A higher Operating Empty Weight (167829 kg compared to 159570 kg for the -300 baseline)
A higher Maximum Take-Off Weight (351535 kg compared to 299370 kg for the -300 baseline)
A higher maximum structural payload weight (69853 kg compared to 64960 kg for the -300 baseline)
A wider wingspan (64.80 compared to 60.93 m)
A different engine delivering more thrust

The Boeing 777-300ER is always equipped with General Electric GE90-115BL1 engines, while the baseline -300 can be ordered with four different engine models from General Electric, Pratt& Whitney or Rolls Royce. The Emirates Boeing 777-300 aircraft have the Rolls Royce RR Trent 892 engines installed. They deliver 415 kN of thrust, almost 100 kN less than the GE90-115BL1 engines.

B777-31H(ER) can fly further than B777-31H.


Why evacuate upwind? Evacuation procedures after a forced landing recommend directing passengers upwind of the aircraft.

What is the reason why evacuating upwind is better than other directions?

There are two problems linked to the wind after accidents:

Inflating the slides.
Running away from the aircraft when on the ground

@DavidRicherby listed the reasons related to running away upwind to try to avoid the effects of flames and fumes (visibility, heat, toxicity). This is part of the IATA guidelines for post-evacuation:

  Post-evacuation. Once outside the aircraft, the cabin crew is
  responsible for the passengers until relieved by the authorities or
  emergency services. Until this help arrives, the cabin crew should:
  - Direct passengers upwind and away from the aircraft;
  - Assemble passengers;
  - Direct passengers away from fuel, fire and vehicles;

Source

While it appears running upwind is a good choice, deploying slides upwind has not always been a good idea.

A study was conducted by the National Aerospace Laboratory (NLR) in Netherlands, about slides issues during evacuation. It was published by the Flight Safety Foundation in 2005:

Analysis of Evacuation-slide Problems
Calls Attention to Recurrent Issues

The scope of the search was the period 1970–2003 and covered aircraft operations worldwide. NLR researchers identified 150 accidents in which slides were used. A total of 89 slide problems were identified:

One problem was slides not inflating due to the wind:

  Wind had an adverse effect on the use of slides in 11 slide problems
  (12.4 percent) analyzed by NLR researchers. Typically, wind blew the
  slide against the side of the aircraft, preventing its use.

To allow using the upwind side of the cabin for slide deployment, today, aircraft certification include this requirement for slides:

  4.20 Wind. The device must be shown, in 25-knot winds directed from the most critical angle, to deploy and, with the assistance of only
  one person who has evacuated down the device, to remain usable after
  full deployment to evacuate occupants safely to the ground.

Source

See also:

IATA - Cabin Operations Safety - Best Practices Guide
EASA ETSO-C69, Emergency evacuation slides, ramps, ramp/slides, and slide/rafts

The evacuation at least 500 meters upwind is to prevent from being intoxicated with toxic fumes, gases from burning materials and alike


Why does An-225 have anhedral wings though it is a cargo aircraft? What are the rationale behind this anhedral high mounted wing configuration? Is that usual for very large carge aircraft?

Anhedral wings will induce roll instability and improve roll maneuverability.

In a large/heavy airplane with a high-wing configuration there is usually excess roll stability, so this type of wings can be pretty common.

Both the high wing configuration and wing sweep contribute a negative sideslip-induced rolling moment, and anhedral is necessary to limit this moment. If sideslip causes strong rolling, the aircraft will have a tendency to dutch roll.

If I am not mistaken, previous answers state that a high center of mass destabilizes the aircraft, which can be circumvented by the anhedral desing of the wings.

In such situation, it must be reminded that the An-225 was designed to carry the russian space shuttle buran !


Can I become a pilot if I am under treatment for Attention Deficit Disorder? I know that before you even begin training in flight school, you are required to get a physical examination from a doctor to determine if you are fit to fly. Online articles stressed the importance of being completely honest with the doctor about any existing conditions.

I was just recently diagnosed with ADD (or rather, I described my issues to my regular physician and simply started receiving medication to treat it) and this is something I would naturally disclose at the physical examination.

Does FAA disqualify someone from getting their pilot's license on the basis of ADD?

I take medication for this once a day and it resolves any issues I face from it.

I'm going to assume you're asking about US/FAA requirements - If not what's below may not be particularly helpful.

ADD is not a disqualifying condition, however if you have a formal diagnosis of ADD or ADHD you may need to undergo additional testing in order to receive a medical certificate.
Also some of the medications used to treat ADD/ADHD may be disqualifying (I know that Adderall, Focalin, and Ritalin were all on the FAA's "No Fly" list - I'm not sure if they're still an absolute NO or if you can get approval for them).

AOPA has a pretty decent drug list that you can search to see if what you've been prescribed is permitted - it's a little easier to search/cross-reference than the official FAA "Do Not Issue/Do Not Fly" list and they do a good job keeping it up to date, and a brief article on ADD/ADHD & medical certification.
They also have a service to assist you with getting your medical (as part of Pilot Protection Services) which can be helpful.

I am in a very similar situation as you. If you have ADD, my initial advice is "good luck!" My process went as follows:

First, I had to go to my doctor and asked to be officially taken off of the medication. Next, I went through a 90 day waiting period. During that 90 day waiting period, I met with my AME. Your AME must be HIMS certified. He gave me my physical and all other aspects of my health checked out. Out of pocket cost for physical and using him as my case manager was \$600.

After the 90 days, I took my aviation neuro-psychological exam. This exam usually lasts 7-9 hours and insurance does not cover it. My out of pocket cost was \$2500. Within 24 hours of my neuro-psychological exam, I had to have a urine drug test done. This test must be done under direct observation; otherwise it is not valid. This only cost \$100.

About a week and a half later, I met with a psychiatrist. This was about a three hour meeting that cost me \$550 out of pocket. This test was to determine my personality, check for alcoholism, depression, etc.

Unfortunately, I was still not done after that. The way the neuro-psychological exam is set up is that if you don't do at least average on one section, then you need cognitive therapy. My scores were superior and high average on all but one section and I got the therapy recommendation. They gave me an IQ test in which I scored a 138. That's an exceptional IQ, but somehow I cannot pass this test.

Right now I'm waiting for my AME's recommendation and my the psychiatrist results. Pretty sure I'll need therapy. I've been in flight school since July and my CFI has been ready to let me solo for months now. The whole process is pretty screwed up. Looking back, I should have kept my mouth shut.


How much fuel do the new A320-Winglets burn vs. the B737-800? Now that the new A320-Winglets have established themselves, I'm wondering if they have better fuel consumption than the B737-800?

I was told that Boeing used to be marginally better. Is that still the case?

Airbus says this about their new blended winglets (aka "Sharklets"), being fitted to new A320s, and retrofitted:

  Airbus’ next step is the large Sharklets™ wingtip devices, representing another element in the ongoing continuous improvement programme for its best-selling A320 Family. These devices improve aerodynamics, reducing fuel burn by up to 4 per cent – which amounts to annual savings of more than 900 tonnes of CO2 per aircraft.

Given that an A320's fuel burn rate is approximately 5.13 gallons per seat per hour, reducing this by 4% gives you 4.92 gallons/seat/hr.

Comparatively, a 737-800 burns about 4.88 gallons/seat/hr.

Source:

  A 737-800 burns 4.88 gallons of fuel per seat per hour, compared with the comparable A320's burn of 5.13 gallons per seat per hour, according to The Airline Monitor, an industry publication.

So they're near enough the same.

The 737 MAX is more fuel efficient than the A320ceo. The A320neo, however, has marginally better range and fuel efficiency than the 737 MAX, but it is not in production yet.


Why do airplanes use MAYDAY when in danger but ships send SOS? I thought SOS means "save our souls",  but apparently it doesn't.

But ships send SOS when in danger and they used Morse code.  Why do airplanes use MAYDAY?

The difference here isn't between ships and aircraft: it's between Morse code and voice.

The SOS signal is only for Morse code. It's short, easy to send, and easy to recognise. But it's not as convenient to say. It doesn't actually mean "save our souls". The letters were chosen just to form the simple Morse pattern, and "save our souls" is a backformation: it was made up by sailors later, partly as a joke, partly as a mnemonic.

"Mayday" is an English-looking spelling of French m'aidez, "help me". Spoken out loud, it's short, easy to send, and easy to recognise. It doesn't have any sounds that some nationalities can't say (such as r, th, or v). It's a good signal to use as voice, but would be much worse as Morse code, because it's too long.

Back in the days when aircraft were equipped with Morse code transmitters, they would have sent SOS as a distress call, just like a ship. And a ship with voice radio would send "Mayday" instead of SOS. Now that Morse code has fallen out of use, SOS is also disused. You only hear it in movie plots where the plucky hero doesn't have a working radio but can somehow improvise a way of signalling Morse code (usually by holding two wires together on a broken radio).

MAYDAY is from the French "m'aidez" meaning Aid Me, or HELP. Try it in a French to (your language) translator and it becomes clear. Hard to believe no one came up with the right response.


Does object mass have a significant bearing on wind interference? I'm learning to fly RC quadcopters, and before I go outside with an expensive, possibly dangerous (if it were out of control and hit something) large unit, I've decided to practice flying for a while on a smaller entry level "palm" drone (if I lose/break it, I won't be out more than $30).

While trying out the palm drone outside, I've noticed that a very tiny gust of wind will knock this thing out of the sky, or at least into a very different direction then intended. Intuitively I want to assume the larger drone will do better against fair-weather Kansas winds; however I've also learned not to assume that my ground based experience translates well into aviation experience. So

TL;DR

Will a larger, heavier drone be less impacted by wind gusts than a smaller palm drone?

Scaling laws are your friend here.

While the area which can be attacked by a gust grows with the square of the length increase, the mass of the drone will grow with the cube of length. This translates into higher disc loading of the props, which in turn requires them to operate at a higher dynamic pressure. The same wind gust will change forces relatively less on a bigger drone. It is the same with aircraft: Light indoor models have the same problems outside that you experienced with the small drone.

It gets even better when you look at the capability to resist rotation: Here the moment of inertia is the deciding factor which grows with the product of mass and length squared. This means the bigger drone will better resist attitude changes by the length increase to the fifth power. To be fair, gust moments will increase with the cube of length increase (area times lever arm), leaving you still with an advantage of length squared. This also means that all changes will happen more slowly, which improves the chances that the control system (the MEMS sensors onboard as well as your control inputs) will keep the situation under control.

I would heavily look into the forces of flight: Lift, which counteracts weight.  Thrust which counteracts drag.  I would also look into what software and guidance system you are flying on the small vs the big drone.

I have been a drone pilot for the military and a an owner of a drone company for years.  I can confidently state that the smaller palm drones typically don't have a way to adjust for winds and GPS positioning(other than manual pilotage), however a drone like a phantom for example, can....and they are really good at it I might add.


Do helicopters need to use yaw pedals to fly a coordinated turn? I recall years ago, when I took a helicopter tour, I noted the pilot simply moved the cyclic left or right to bank the helicopter and initiate a turn. No pedal input was made whatsoever, and the ride seems completely fine.

In many fixed wing aircrafts, if rudder is not applied during the entry of a turn, the airplane will have a tendency to yaw to the opposite direction due to the drag induced by the ailerons.

Is pedal (rudder pedal? what are they called in a helicopter?) input necessary for turns in a helicopter?

Many helicopters have different flight characteristics, but for general information you can always look in the FAA Helicopter Flying Handbook, this is from Chapter 9:

This is from a section on the four fundamentals of flight (straight-and-level, turns, climbs, and descents):

  Anticipate the following characteristics during
  aggressive maneuvering flight, and adjust or lead with
  collective as necessary to maintain trim and torque:

  Left turns, torque increases (more antitorque).
  This applies to most helicopters, but not all.
  Right turns, torque decreases (less antitorque).
  This applies to most helicopters, but not all.

So some (most) helicopters do require some torque pedal movement for aggressive maneuvering, but the pilot may have anticipated this and added the correction prior to beginning the turn. You may not have noticed as much pedal movement as you would in a coordinated turn for an airplane, or the helicopter you were riding in had different flight characteristics and was not maneuvering aggressively enough to need it.

Many helicopters "wind vane" pretty well, that is that they stay aligned with the relative wind at high speed. It could be that the helicopter you were in was travelling at a high enough rate of speed that a large (or any) correction was not needed to maintain proper alignment.

For an example of how speed cancels out the need for anti-torque control, see this article about a helicopter pilot that lost the tail rotor and continued to fly for half an hour to a proper landing site.

Short version:
Yes, helicopters require anti-torque correction to achieve co-ordinated flight.
However, co-ordinated flight is usually not necessary/desirable


What is the difference between "Standard Metric Level" and "Altitude" in ICAO flight plans? What is the difference between "S" and "M" in flight plan altitudes?

The FAA's ICAO flight plan documentation says:

  Standard Metric Level in tens of meters, expressed as S followed by 4
  figures (for example, S1130), or

  [...]

  Altitude in tens of meters, expressed as M followed by 4 figures (for
  example, M0840)

I don't know what "Standard Metric Level in tens of meters" means exactly? How is that different from "Altitude"?

Standard Metric Level is the metric equivalent of the foot-based Flight Level. It is what you get when the altimeter is set to QNE, i.e. above the Transition Level, and read in meters.

Metric Altitude is the metric equivalent to the foot-based Altitude. It is what you get when the altimeter is set to QNH, i.e. below the Transition Altitude, and read in meters.

These are actually the official ICAO systems, with the foot-based systems declared a "temporary" alternative.

In the FAA's ICAO flight plan documentation, it states the following:

  Cruising level (maximum 5 characters)
  ENTER the planned cruising level for the first or the whole portion of the route to be flown, in terms of:
  Flight level, expressed as F followed by 3 figures (for example, F085; F330), or
  *Standard Metric Level in tens of meters, expressed as S followed by 4 figures (for example, S1130), or
  Altitude in hundreds of feet, expressed as A followed by 3 figures (for example, A045; A100), or
  Altitude in tens of meters, expressed as M followed by 4 figures (for example, M0840), or
  for uncontrolled VFR flights, the letters VFR.

Flight Level and Standard Metric Level are synonymous terms for the same value. Flight Level is measured in imperial feet. Standard Metric Level is measured in metric meters. Either, whether represented in feet or meters, is the distance at which an aircraft flies above the atmospheric level of standard pressure (Pressure Altitude). This is the Indicated Altitude shown on your altimeter when the Kollsman window is set to 29.92 inches of mercury (or the metric equivalent). In the US, Flight Level is what you would use at and above FL180.

Altitude, whether represented in feet or meters, is the distance at which an aircraft flies above the standard datum line of the average level of the sea (Mean Sea Level). This is the Indicated Altitude shown on your altimeter when the Kollsman window is set to the local atmospheric pressure (if it is accurate). In the US, altitude is what you would use below 18,000 feet MSL.

Depending on atmospheric pressure, there will be a transition space between 18,000 feet MSL and FL180.


Are there any known maintenance issues with older Mooney wooden wings? Are there any maintenance issues or ADs for the older Mooneys with wooden wings? I'm looking at a 1958 M20A (always hangared) and I'd like to know if there are specific points to check.

You may want to look around here or here or even here but the short answer is they are a wooden wing airplane that comes with all the issues a wooden wing airplane does. One of the major things to look out for is the M20A's that had a wooden tail, those should generally be passed up on. The later planes should require some inspection of the joints where the tail meets the rest of the airframe as it is a stress point (if memory serves). For what its worth you are better off with a metal mooney. With a wood plane you need to inspect for rot the way you would for corosion on a metal plane.

Like any wood aircarft the M20A's can suffer from delamination which would need to be checked for. unfortunatly the list of people that still work on wooden planes is dwindling with time so it may be hard to repair this. It would be well worth the effort to seek out a mechanic that has experience with the A model and can work on a PPI for you.

You can find all the AD's for the M20A here.

I have some experience with the M20F aircraft; however this is an all metal Mooney.

This gentleman may have some good advice for pre-purchase inspections to conduct if you are hunting for a M20A.

http://www.n6030x.com/


What are the landing stall speeds of a Cessna 172? I understood from Why is stall speed listed in a POH? that the Cessna Skyhawk POH lists stall speeds for specific conditions due to a lack of AoA indicator. However, what's unclear to me is what the stall speeds actually are.

Page ii

In page ii of the Cessna 172 POH it lists 48 KCAS for the Flaps Down, Power Off-configuration:

Page 2-5

Then on page 2-5 it lists the white arc spanning 40 - 85 KIAS, with Lower limit is maximum weight Vs0 in landing configuration.:

According the V Speeds Wikipedia page Vs0 is Stall speed or minimum flight speed in landing configuration. I understand that KCAS is not the same as KIAS, but this is quite a significant difference.

Question: what is the actual stall speed for a Cessna 172 while in landing condition and what do these two data points in the POH mean?

Short answer

At low speed, near the stall, 40 KIAS (the low end of the white arc) and 48 KCAS (Vs0 in the POH) refer to the same actual speed. While there is a limited divergence (0 to 2%) between CAS and IAS at medium and high speed, this difference can be as much as 20% at low speed, when the aircraft pitch is large.

Details

In your question, you mention:

The POH states a Vs0 of 48 KCAS.
The low end of the airspeed indicator white arc is 40 KIAS.

The end of the white arc is also Vs0. That's a bit confusing indeed as two types of measures are used, but in fact they refer in practical to the same actual airspeed.

Indicated airspeed

Unless computations are done, airspeed values displayed to the pilot IAS (indicated airspeed).

(Source)

Errors affecting IAS

IAS reflects directly the difference between the total air pressure in the pitot tube, and the (mean) static air pressure at different static air pressure ports (on the side of the aircraft and in the pitot). The difference (the dynamic pressure) is subject to possible errors:

Total air pressure is wrong it the probe axis is not parallel to the airflow. Obviously the pitot tube angle changes between low and high speeds. For a given speed the pitot pitch also varies with mass and balance, and the status of drag devices such as flaps and landing gear.
Static air pressure at a pressure port is likely affected by pressure variations created by the aircraft fuselage. This error greatly depends on the port location and its design, but also on the yaw angle.
Airspeed indicator itself.

Obtaining CAS from IAS

All V-speeds are defined in term of "calibrated" airspeed (CAS). The aircraft manufacturer provides tables to find the CAS from the IAS read on the airspeed indicator. These tables defined for different drag devices status (e.g. flaps values) are valid at sea level and 15°C. They are included in the POH. I found this C172N one online:

(Source: POH C172N 1978)

For a 40° full flaps the table says that 40 KIAS are actually 47 KCAS. This is close to the 48 KCAS in your POH.

As you realized, the difference between KCAS and KIAS becomes larger as the air speed decreases, because the angle of attack and the errors in pressure measurement increase.

Multiple ways to express airspeed

TAS depends on parameters and need to be computed. However it can be approximated by the indicated airspeed. IAS is close to the TAS in favorable atmospheric conditions (sea level air density), and at low angles of attack.

The IAS is easy to display to the pilot using the an aneroid cell. For higher angles of attack, the calibrated airspeed (CAS) corrects for instrument and probe placement errors. CAS can be obtained using manufacturer tables. CAS, as IAS, is valid at sea level and 15°C.

When the air temperature or pressure are not close to the ideal ones (15° and 1013 hPa), then CAS differs from TAS.

The ASI usually has a window (see above) for reading the TAS after the outside temperature and altitude pressure have been selected.

(At speed higher than 200 KCAS or altitude higher than 10,000 ft, the compressibility of air becomes significant, and the equivalent airspeed (EAS) is better suited to describe the aerodynamic effects.)

Vs0 is determined assuming the maximum landing weight and the most unfavorable CG location. It is expressed in KCAS. If the Vs0 CAS were unknown, the pilot wouldn't be able to correct it to include the effect of weight and balance changes.

The ASI, because no correction is done, can display only IAS. So the IAS value for the lower speed of the white arc (Vs0) is in KIAS.

The design stall speed for a Cessna 172 at maximum weight with flaps fully extended is 40 knots indicated air speed.


Are pilots expected to keep an ear out for conflicting authorizations? I just watched the Mayday episode Cleared for Disaster, a dramatization of USAir Flight 1493. The episode covered how the controller lost situational awareness and gave a landing clearance to 1493 while SkyWest 5569 was still on the runway, as well as how the runway lights obscured 5569's lights. They did not, however, talk much about 5569. This was at least partly because 5569 was not carrying a CVR.

The tower would have given 1493 the clearance to land on runway 24L on the same frequency that 5569 should have been on. In other words, 5569 would have been sitting on 24L and hearing tower give landing clearance for 24L, which would be a big red flag. Are they expected to be listening and thinking about landing clearances given to other aircraft? If they noticed, would they be expected to speak up, or to shut up and trust the controller?

The short answer is yes, they are definitely expected to listen and act accordingly. This is one example of situational awareness in aviation, and pilots are expected to use all useful information sources to maintain awareness of what's going on around them.

A nice example is this one where the crew of a US Airways aircraft avoided an accident. There was heavy fog at the airport, and while the US Airways flight was waiting for takeoff clearance a United aircraft ended up on a runway instead of a taxiway. The controller couldn't see the runways because of the fog and she believed the United aircraft was on a taxiway, so she gave the US Airways flight clearance to take off even though the United pilot said several times that they were on a runway.

The US Airways pilot had heard the discussion (actually, almost a debate) between the controller and the United pilot, and he refused the takeoff clearance because he wasn't confident that the situation was under control. It's well worth watching the whole thing, but the key exchange is this:

  ATC: [...] USAir 2998, runway 5R, fly runway heading, cleared for
  takeoff

  US2998: Uh, tower, USAir 2998, 'til we figure out what's going on down
  there, we're just gonna stay clear of all runways

This level of awareness is expected of all pilots, including light aircraft pilots at uncontrolled fields. Although in that situation there's the additional complication that there may be aircraft around that don't even have radios!

Short answer, never trust the controller.  If the worst happens all the controller will have is paperwork to do and lots of meetings.  But your family will have your funeral to tend to.  As PIC you are the sole authority as to the safety of your flight.  As for listening, IMHO, it's more important to listen carefully at takeoff and landings.

We train ourselves, while in the air to listen for the 1st word which is the call-sign of the aircraft and if it's ours then it has our attention and if there are other passengers on my plane my hand goes up for them to keep quiet.

But during landing, approaches, etc, we must familiarize ourselves not with just the approach or runway we are landing on but we should know the configuration of the airport in general.  In single runway airports, it's not as hard.  You just have to know which runway is landing and taking off and if you hear other wise then you speak up.    In multiple runway airports, especially with low visibility if you are still high enough, do not be afraid to key up and ask a question.  Just as you would key up even without a call-sign and ask 'Wind Check'.   The controllers then broadcast the current winds in the blind.

But if you are on short final and are still unsure, there is only one thing that is the safest. Abort the landing, and do a go-around as you are informing the tower you are going around because of unclear instructions.

If they give you a number to call because you messed up their nice string of pearls they were setting up, giving them more workload, or messed with their approaches and departures, that's fine.  Like I said above, I'd rather be on the ground talking to, explaining to or writing up documents (especially a NASA form) than having someone crawling over the mangled mess of my airplane trying to figure out which part was part of me or someone else.