How should a glider pilot signal turns to the towplane? When in aerotow in a glider, I thought the correct way to get the towplane to turn right (say) was to move gently to the left. That would gently nudge the towplane’s tail to the left, which would then point the nose to the right and be a good signal that the glider pilot wants to turn right.

The FAA glider flying handbook displays the opposite technique in a diagram:

I don’t understand this. If you move right to ask the towplane to turn right, doesn’t that put the glider on the inside of the turn, and put you in the wrong place to follow the towplane around?

Which is the correct direction?

The glider flying handbook is wrong. The glider signals to the tow pilot by moving to the outside of the turn.

We glider pilots have been trying toget the FAA to fix this and many other errors in the GFH for years. See the newsgroup rec.aviation.soaring

Also, many tow pilots ignore tow steering directions because they think they know better than the glider pilot where the lift is.


How much force can the Dukane DK 100 ULB (Underwater Locator Beacon) widthstand? How much shock (force) can this beacon withstand on impact?

Is there an FAA certification telling us anything about it?

An exact model is too specific to find, but the FAA Advisory Circular 21-10A regarding ULD's from 1983 says:

There is also a patent from 1989 which lists specifications regarding most pingers:

Most pingers are cylindrical in shape. Typical dimensions for flight recorder applications are 1.25 inches in diameter and 3.8 inches long. The flight recorder ULDs can survive crash forces of 6000 g's (0.5 ms) and ocean depths of 20,000 feet.
— Google Patents (1989)

The FAA also mentions the SAE International’s Aerospace Standard (AS) 8045a, Minimum Performance Standard for Underwater Locating Devices, which is behind a paywall. But the aforementioned AC should suffice.


Does sex occur in aircraft cockpits, and is this safe? I've always heard euphemisms about pilots having sex with flight attendants or passengers while flying, but I'm wondering, does this ever happen on commercial flights or in general aviation?

Have there been incidents of sex in the cockpit when the participants got caught or their actions lead to something more disastrous (like a crash) wherein the truth about their actions was clearly exposed?

As jwenting said, it's unlikely to happen for many reasons, and if it does, the participants are even less likely to admit it since doing so could get them in trouble both professionally, and with a government regulatory agency for endangering safety. I seem to remember the FAA taking action against a pilot who posted photographs of himself flying with either naked or bikini clad women at his side, but I can't seem to find a link right now.

There is one NTSB report which is somewhat famous in the aviation community for describing a general aviation crash which was likely caused by sexual activity. However, the summary is very delicately worded, to say the least:

  The private pilot and a pilot rated passenger were going to practice simulated instrument flight. Witnesses observed the airplane's right wing fail in a dive and crash. Examination of the wreckage and bodies revealed that both occupants were partially clothed and the front right seat was in the full aft reclining position. Neither body showed evidence of seatbelts or shoulder harnesses being worn. Examination of the individuals' clothing revealed no evidence of ripping or distress to the zippers and belts.


Straight vs. angled aircraft carrier decks: tradeoffs for rolling takeoff? I have some questions about modern aircraft carriers and the tradeoffs they make when choosing between straight or angled runways when used with modern jet aircraft for rolling takeoffs.

This question is not about old (e.g. WW2) aircraft carriers with straight decks nor is it about propeller planes or jet aircraft with VTOL or STOL.

So let's establish a few things first (obviously please correct me if I'm wrong on anything):

a. Angled runways allow simultaneous launching and recovery.

b. Angled runways allow aircraft to bail out and try again during recovery.

c. Angled decks have a shorter takeoff runways and shorter landing
    runways than straight decks (which can just have a single longer
    runway for both tasks).

d. As a result of (c), assistive devices for landing and takeoff (such
    as catapults) are much more critical with angled decks than straight
    decks. (FALSIFIED because carrier decks are several times shorter than required and a little extra runway length is not going to matter)

So here are my question:

Are modern non-VTOL/STOL jet aircraft still not able to take off from the single longer runway of a straight deck without catapults?
If the following image is correct, on modern straight decks, aircraft stored at the end or on the sides? Because it seems that if the aircraft were stored on the sides, although you couldn't perform simultaneous launch and recovery, you could at least switch between launch and recovery fairly quickly since you would not have to move any planes out of the way except for the one that is on the runway. You could also bail from a bad landing.

How closely does this "fast-switching" between launch and recovery compare against actual simultaneous launch and recovery?

I assume this just wasn't done on WW2 carriers because they were smaller so there was no room to store aircraft on the sides; They had to be stored at the end which meant no landings until every plane ready for takeoff was cleared.
I read that angled decks are much more expensive than straight decks. I imagine this is because angled decks require a runway extension and catapults (since planes would not be able to launch off the shorter takeoff runway of a angled deck without one), plus a nuclear reactor since you need one for a steam catapult. However, if it turns out that modern non-VTOL/STOL aircraft cannot takeoff without catapults from an straight deck, then where is this supposed cost difference coming from? If both straight and angled deck carriers need catapults and nuclear reactors to take off, that would imply this "huge" cost difference is attributed to solely the deck extension which doesn't seem to add up. Although, it may not be true anymore that you need a nuclear reactor for a jet-capable catapult since there is apparently an alternative to steam catapults now with electric catapults, but those don't sound very cheap either even if no reactor is required.

Correct, at the weights and speeds of most carrier based jets they would need several lengths of an aircraft carrier to get airborne.  Catapults became required as the airwings transferred to jets in the 1950s and 1960s.
Aircraft are stored in the hangar below decks, and on the sides.  Anywhere there is free space.  Even during simultaneous launch and recovery.  Look at photos and videos, it can get crowded!
Since we aren't building straight deck and angle deck carriers that are otherwise similar, a fair cost comparison cannot really be made.  (Amphibious ships serving VTOL aircraft lack catapults and arresting gear which would account somewhat for their lower cost.)  Propulsion, radar systems, inflation, and many other factors also account for the increased cost of modern carriers over earlier straight deck ships.  And it is not true that you need a nuclear reactor to generate steam for the catapults.  Steam catapults were around for quite some time before the first nuke boat.

ADDENDUM:   There isn’t enough space in the comments section to refute the multiple inaccuracies in Peter Smith’s response to this question, so I will address them here:

All modern US Navy carriers have 4 catapults, 2 at the bow and 2 at the waist.  (This has been consistent for the last 40-50 years)  The “waist” catapults (cats 3 & 4) are so named because they are amidships, and thus cannot be used during landing operations because they foul the landing area.  The photo in his answer shows the two bow catapults.  The one pictured on the right is NOT a waist catapult, it is cat 1.

Simultaneous launch and recovery is possible however.  Generally the launch starts first while the previous cycle is holding overhead, using all 4 catapults for efficiency.  Once they get to a point where most of the next cycle is airborne and the landing area is clear, they will “wrap” the waist catapults and begin recovering the earlier cycle while they finish launching off the bow.

The only caveat is that longer wingspan aircraft like the Tomcat and E2 will foul the landing area if they are on the left bow catapult (cat 2) with their wings spread.  For this reason they will finish launching them from the right bow catapult (cat 1) if recoveries have begun.  This is not a problem with Hornets, and possibly not an issue on the largest of carriers.

There is almost always some overlap where simultaneous launch and recovery occurs during normal cyclic operations.  And during pilot qual/requal, the deck runs continuously.  You will land, fold your wings, taxi to the bow cat, spread your wings and launch again...  It is a continuous stream of simultaneous launch and recovery operations.

It is also definitely not true that aircraft are not parked anywhere but the bow during recovery.  Any spot outside the landing area foul lines is fair game.  It is a very common sight to see aircraft parked on both sides of the landing area on final approach.  But don’t take my word for it, there are lots of videos and photos on line to verify this…

Finally, the arresting cables are not forward of the “aft lift” you show in your photo.  (Elevator 1 or JBD?)  Again, this photo is of the bow only.  The two waist catapults and the landing area are well aft and left of the perspective this photo was taken from.

BTW, the diagram of the angle deck carrier in the question is not an accurate representation of the current class of US Navy fixed wing CVN Carriers.  Below is a good diagram showing the layout of the 4 catapults in relationship to the landing area.


How can I calculate ambient air pressure from flight data? I am playing around with data collected during a flight. I would like to find out the (real, not ISA) ambient air pressure for each data point, since it is not reported in the spread sheet containing my flight data.
I have the following data for different time points, but do not know how to (if even possible) derive from this the ambient air pressure:

Mach

Density altitude

Pressure altitude

Airspeed ktas and kcas

Static air temperature

Total air temperature

Geometric altitude

Pressure at the ground / surface pressure

I would be thankful for a solution or any tips for literature helpful to solve this.
Under the following link I found a formula I could rearrange but I believe this would return the pressure in the standard atmosphere...
https://www.weather.gov/media/epz/wxcalc/pressureAltitude.pdf

It kind of depends on how accurate you want to be because you could start asking about instrument errors and position errors and stuff like that.
But, assuming you don't want to get into that...
The link you included would indeed show a standard atmosphere pressure but the pressure altitude data you have is also referenced to the same standard atmosphere (it actually depends on the setting of the altimeter... if it is set to 29.92 / 1013 mBar then you're in luck).
Ori


What are the uses of gyroplanes? From my understanding, a gyroplane (autogyro):

Is a rotocraft.
Uses an unpowered rotor for lift generation, instead of wings.
Uses a propeller for horizontal translation like an airplane.
Cannot take off vertically or hover contrary to an helicopter.
Can land vertically like an helicopter using rotor inertia.
Maybe simpler to build and maintain than an helicopter or an airplane.

(Source)

Are gyroplanes a technology that works but without particular application? Or do gyroplanes have well known uses for which they have significant advantages over airplanes and helicopters?

Gyroplanes have quite a few applications, but the problem is that anything they can do, others (helicopters/fixed wing aircraft) can do better.

The main advantage of a rotorcraft is that they can hover at a location; the gyroplanes are unable to do this, which is kind of a deal breaker.
The main advantage of the gyroplane is that they cost less and are easier to maintain, compared to other type of aircraft. However, most of the autogyros available in market are small ones and as the size scales up, this may vanish.
The gyroplanes are used mainly for low speed surveillance operations due to their low stall speed. Of course, helicopters can be used to do this, albeit at a higher cost
So basically, the autogyro can do most of the things the helicopters/small GA aircraft can do at a lower cost. This low operational cost may be its main advantage.

I'm not aware of anything the autogyro can do which the others cannot do though.


Are planes allowed to fly in circles, and is there a minimum legal altitude for takeoff and landing? I live near a small, possibly private, airport. Every day there are small planes flying low over the neighborhood in circles for over half an hour.  They appear to just be flying for recreational purposes. They're also coming in and leaving low enough to be seen through the tree/wire lines and directly over houses.  I have seen the airport's grass runway, and it is quite large, so I don't see why the plane wouldn't be able to just land without circling so long.  Why would a pilot do this and is it legal?  Also, this is not a rural area, it is a highly-populated city.

Most likely there’s a flight school at the nearby airport and they are conducting dual instruction flights over your neighborhood to practice ground reference maneuvers and possibly emergency procedures.  This is perfectly normal and safe.
In the United States, the following requirements for minimum safe altitudes are set for general aviation flight per 14 CFR §91.119:

Except when necessary for takeoff or landing, no person may operate an aircraft below the following altitudes:
(a) Anywhere. An altitude allowing, if a power unit fails, an emergency landing without undue hazard to persons or property on the surface.
(b) Over congested areas. Over any congested area of a city, town, or settlement, or over any open air assembly of persons, an altitude of 1,000 feet above the highest obstacle within a horizontal radius of 2,000 feet of the aircraft.
(c) Over other than congested areas. An altitude of 500 feet above the surface, except over open water or sparsely populated areas. In those cases, the aircraft may not be operated closer than 500 feet to any person, vessel, vehicle, or structure.
(d) Helicopters, powered parachutes, and weight-shift-control aircraft. If the operation is conducted without hazard to persons or property on the surface—
(1) A helicopter may be operated at less than the minimums prescribed in paragraph (b) or (c) of this section, provided each person operating the helicopter complies with any routes or altitudes specifically prescribed for helicopters by the FAA; and
(2) A powered parachute or weight-shift-control aircraft may be operated at less than the minimums prescribed in paragraph (c) of this section.

So unless these guys are dropping below 500 ft AGL, they’re not doing anything unusual.


Calculating distance from VOR station with radial and heading I am currently struggling to understand the following question regarding a VOR station and an aircraft. The aircraft is found in the 30 degree radial from the VOR station at time T(1) and T(2). During the 10 minutes between T(1) and T(2) the aircraft maintains a heading of 120 degrees from magnetic north at 120 knots ground speed. The question goes like so:

  At time t(1) an airborne VOR receiver shows that the aircraft is on the 30deg radial from a VOR station. The pilot maintains a ground track heading of 120deg with respect to magnetic North and 120 knots ground speed. At time t(2) , 10 minutes later than t(1) , the VOR receiver indicates that the aircraft is on the 30deg radial from the same station.
  Assuming no errors, what is the pilot’s distance from the VOR station at time t(2) ? If the pilot had access to airspeed, can the range be determined from the VOR measurements? Can it be determined if the pilot had access to ground speed and ground track heading?

I'm not quite positive of my understanding of the situation. To my understanding the heading indicates the direction the aircraft is pointing in is 120 degrees with respect to magnetic North however its course must be either be 30 degrees with respect to magnetic North or 210 degrees with respect to magnetic North, allowing it to stay in the station's 30 degree radial. Would my understanding of the situation be correct?

Bad news: the question is unanswerable.
Good news: you're right to be confused!

With a groundspeed of 120 knots and a constant ground track, after 10 minutes the airplane will have moved 20 miles in a straight line. There's no getting around this fact.

If the airplane doesn't parallel a radial, and yet it moves, the radial it's on must change.

If it's still indicating the same radial after non-parallel movement, the only option is that the indicator is in error.  But the question removes that possibility.

The only way I can think to make this situation possible is that maybe you've tuned the world's most bizarre VOT. But even still there's no possibility for a valid answer for the question.

The original question:

At time t(1) an airborne VOR receiver shows that the aircraft is on the 30deg radial from a VOR station. The pilot maintains a ground track heading of 120deg with respect to magnetic North and 120 knots ground speed. At time t(2) , 10 minutes later than t(1) , the VOR receiver indicates that the aircraft is on the 30deg radial from the same station. Assuming no errors, what is the pilot’s distance from the VOR station at time t(2) ? If the pilot had access to airspeed, can the range be determined from the VOR measurements? Can it be determined if the pilot had access to ground speed and ground track heading?


Where can I find airport diagrams for the Philippines? I would like to have airport map, just like the one that pilots can see when they are taxiing. Is it possible to get such a map, even if you are not a pilot? Perhaps there are applications which a person can use to simulate taxiing on specific runways?
I am interested in Manila airport (MNL) and Mactan-Cebu airport (CEB).
Something like this:

The easiest way is to type the IATA code into Google along with "airport charts". The PDF you are looking for will be high in the search results. Here are the diagrams for Manila and Cebu.


Does a Second In Command have to be landing and instrument current like a Pilot In Command does? When flying an airplane that requires more than one crew member, does the SIC need to be landing current and instrument current, or is this a requirement only for the PIC?

Does it matter which regulations that you are operating under (Part 91, 121, or 135)?

FAA

This is a compilation of what I could find relevant to the question. Feel free to come up with corrections if I missed anything (I'm an EASA FCL guy)

Co-pilot Landing Currency:

Part 61: 3 takeoff/landings in previous 12 months (61.55)
Part 135: no requirements other than those in part 61 (above)
Part 121: 3 takeoff/landings in previous 90 days (121.439)

Co-pilot Instrument Currency:

Part 61: no requirements for SIC
Part 135: 135.245 requires that SIC's must meet part 61 PIC instrument currency requirements in 61.57(c) (see below)
Part 121: recurrent flight training every 12 months, including at least 1 ILS approach to minimums

Following is the list of relevant FARs :

§ 61.55 Second-in-command qualifications

  (b) Except as provided in paragraph (d) of this section, no person may serve as a second in
  command of an aircraft type certificated for more than one required
  pilot flight crewmember or in operations requiring a second in command
  unless that person has within the previous 12 calendar months:
  (2)
  Except as provided in paragraph (e) of this section, performed and
  logged pilot time in the type of aircraft or in a flight simulator
  that represents the type of aircraft for which second-in-command
  privileges are requested, which includes --
  (i) Three takeoffs and
  three landings to a full stop as the sole manipulator of the flight
  controls;

  (d) This section does not apply to a person who is:[...operating part
  121,125,135...]

  (e) The holder of a commercial or airline transport
  pilot certificate with the appropriate category and class rating is
  not required to meet the requirements of paragraph (b)(2) of this
  section, provided the pilot: [... is not carrying paying
  passengers...]

§ 61.57 Recent flight experience: Pilot in command.

  (a) General experience.

  (1) Except as provided in paragraph (e) of this section, no person may
  act as a pilot in command of an aircraft carrying passengers or of an
  aircraft certificated for more than one pilot flight crewmember unless
  that person has made at least three takeoffs and three landings within
  the preceding 90 days, [...]

  (c) Instrument experience.

  Except as provided in paragraph (e) of this section, no person may act
  as pilot in command under IFR or in weather conditions less than the
  minimums prescribed for VFR, unless within the preceding 6 calendar
  months, that person has: [...6 approaches, holdings, intercepting
  &tracking course...]

§ 135.245 Second in command qualifications

  (a) [...] no certificate holder may use any person, nor may any person
  serve, as second in command of an aircraft unless that person holds at
  least a commercial pilot certificate with appropriate category and
  class ratings and an instrument rating. For flight under IFR, that
  person must meet the recent instrument experience requirements of part
  61 of this chapter.

This is an excerpt from an FAA Legal Interpretation concerning 135.245:

  Next, unlike a PIC, an SIC's qualification to operate an aircraft
  under IFR in part 135 service is not dependent on compliance with §
  135.297.  Rather, § 135.245 sets the qualifications for pilots serving as SIC in part 135 operations.  It states that a pilot must meet the
  instrument experience requirements of part 61 to serve as SIC in
  flight under IFR.  Section 61.57(c) sets out the recent instrument
  flight experience requirements which include, among other things, six
  instrument approaches within the previous six months.  See Legal
  Interpretation to Gerald Naekel, from Donald P. Byrne, Assistant Chief
  Counsel Regulations and Enforcement Division (June 18, 1991) (noting
  that although the section title refers to pilots in command, the
  instrument recency requirements of that section apply to SICs).

§ 135.247 Pilot qualifications: Recent experience.

  (a) No certificate holder may use any person, nor may any person
  serve, as pilot in command of an aircraft carrying passengers unless,
  within the preceding 90 days, that person has—

  (1) Made three takeoffs and three landings as the sole manipulator of
  the flight controls in an aircraft of the same category and class and,
  if a type rating is required, of the same type in which that person is
  to serve; or [...]

§ Part 121.433 Training required

  (c)Recurrent training. (1) No certificate holder may use any person
  nor may any person serve as a required crewmember on an airplane
  unless, within the preceding 12 calendar months --  (i) For flight
  crewmembers, he has satisfactorily completed recurrent ground and
  flight training for that airplane and crewmember position and a flight
  check as applicable;

§ Part 121.439 Pilot qualifications: Recent experience.

  (a) No certificate holder may use any person nor may any person serve
  as a required pilot flight crewmember, unless within the preceding 90
  days, that person has made at least three takeoffs and landings in the
  type airplane in which that person is to serve.

EASA

FCL is very clear about it: PIC and co-pilot both need to be landing current. If holding an IR rating then specific night currency is not required. No instrument currency is required as long as the IR is valid.

FCL.060 Recent experience

  (b) Aeroplanes, helicopters, powered-lift, airships and sailplanes. A
  pilot shall not operate an aircraft in commercial air  transport or
  carrying passengers:

  (1) as PIC or co-pilot unless he/she has carried out, in the preceding
  90 days, at least 3 take-offs, approaches and  landings in an aircraft
  of the same type or class or an FFS representing that type or class.
  The 3 take-offs and landings shall be performed in either multi-pilot
  or single-pilot operations, depending on the privileges held by the
  pilot; and

  (2) as PIC at night unless he/she:

    (i) has carried out in the preceding 90 days at least 1 take-off,
    approach and landing at night as a pilot flying in an aircraft of the
    same type or class or an FFS representing that type or class; or

    (ii) holds an IR;


Why do planes have cross-feed switches or circuits? Why do planes have cross-feed switches or circuits? Are they to move contents of one tank to another (feed the tank) or to supply the engine with fuel from left side to right or vice versa (feed the engine)?

The fuel system in aircraft generally includes a left and right tank within the corresponding wings. Each tank generally provides fuel to the engines on that wing. The crossfeed valve is used to temporarily connect the two sides of the system. This system allows the shortest route for the fuel and keeps the systems separate, so a leak on one side cannot drain the entire system. The crossfeed valve is generally kept shut for this reason. A couple scenarios illustrate when opening the valve might be necessary.

To keep the plane balanced, both wing tanks should have about the same amount of fuel. However, for various reasons, they could become unbalanced. The crossfeed valve allows fuel to flow from the tank with too much fuel to the tank with less fuel.

Another scenario would be the loss of an engine. In a twin-engine airplane, the remaining engine would need to be able to draw fuel from both wing tanks.

Note: especially in larger aircraft, fuel systems can be much more complicated, with multiple tanks in each wing and also in the fuselage. Fuel transfer also becomes more complicated.

Using the 737 as an example, this is the fuel system control panel with the crossfeed switch in the center. With the crossfeed valve open, turning off the fuel pumps on one tank will cause more fuel to be drawn from the tanks on the other side.


What does SWES mean in SNOWTAM? The resources explains that SWEN is the start for Snowtam. (Ref: Snowtam help ) But many snowtams I found starts with SWES which I could not find anything about. What is the difference? Can it have other values? Does snowtam always need to start with one of them?

Example Snowtam from Pilotweb

SWES0701 ESSB 12270841
(SNOWTAM 0701
A) ESSB
B) 12270841 C) 12 F) 4/4/4 G) 1/1/1 H) 5/5/5
N) G1 R S T Y Y1 Y2 Y3
Y4 Y5 YU YW/4/GOOD
R) 1 2 3 4 6 7 EAST NORTH/4/MEDIUM
T) RWY 12 BA 60/48/41. RWY 12 COVER 100 PERCENT. APRON SANDED. ICE
PATCHES ON APRON.)

Snowtams follow the ICAO Format found in ICAO Annex 15, Appendix 2

This says that the abbreviated heading starts with SW** where ** is the ICAO location identifier. The location identifier document, ICAO 7910 is a \$245 document so I can't link to it here, but all the Swedish airports start with location identifers that are "ES", hence any SNOWTAM you find from Sweden will be SWES reports. Reports for the United States typically start with "K" so they would be SWK*. Canada is "C", so SWC*.


What amount of lift would be created in the fallacious "lift from equal transit time" theory? Context

A wrong --but simple to understand-- theory for lift states:

Air molecules that were close before being separated at the leading edge must be close again after they join back at the trailing edge. For this to happen, molecules must travel the two paths in the same time.

As the path is longer on the upper side, due to the camber of the wing, air is accelerated on the upper side. By Bernoulli's equation, static pressure decreases when velocity increases, so the pressure is lower on the upper side, and higher on the lower side.
Lift is the force created by this pressure differential on the wing sides.

(What is wrong is not Bernoulli's equation, but the assumed relationship between path length and speed.)

Such theory is, for instance, presented as correct in this book.

More at Nasa.

Question

We know this theory is wrong and lift created by path length difference would be a very small percentage of the lift actually measured (and even null for symmetrical airfoils).

Let's assume a rectangular wing with a simple cambered airfoil, at the critical angle of attack:

What would be the pressure differential according to equal transit time difference? What percentage of actual lift does it represent?

The theoretical lift coefficient of a NACA 4412 at the critical angle of attack (about 16 degrees) is approximately 1.7.
\begin{equation}
c_\mathrm{l}=1.7
\end{equation}

For our purposes, that same airfoil has its upper "surface" about 1.03 times as long as its lower, which can be seen by using the distance formula on the coordinates or integrating the cambered NACA airfoil equation. Here is the distance-formula method applied to the Airfoil Tools numbers (thanks to ROIMaison for making me check this calculation):

On average, therefore, the fallacy would predict the air moving along the upper surface at about 1.03 times the velocity as along the lower in order to join up properly.

Making all the assumptions from the NASA link, Bernoulli's equation says
\begin{equation}
P_1+\frac{\rho V_1^2}{2}=P_2+\frac{\rho V_2^2}{2}
\end{equation}
Let's represent the lower surface with subscript 1 and the upper with subscript 2. The respective velocities are averages across the surfaces. We have
\begin{align}
P_1-P_2&=\frac{\rho (1.03V_1)^2}{2}-\frac{\rho V_1^2}{2}\\
&=\frac{609}{20000}\rho V_1^2\\
\end{align}
So the average difference in pressure coefficient (i.e., the lift coefficient) is given by
\begin{equation}
\mathrm{avg}(c_\mathrm{p_{ETT}})=\frac{609}{10000}\frac{V_1^2}{V^2}=c_\mathrm{l_{ETT}}
\end{equation}
where $V$ is the freestream velocity and ETT stands for equal transit time.

We have another expression for pressure coefficient that we can apply to get a final value. We end up with
\begin{equation}
c_\mathrm{l_{ETT}}=\frac{609}{10000}\frac{V_1^2}{V^2}=1-\frac{V_1^2}{V^2}
\end{equation}
or
\begin{equation}
\frac{V_1^2}{V^2}=\frac{10000}{10609}
\end{equation}
and therefore
\begin{equation}
c_\mathrm{l_{ETT}}=\frac{609}{10609}\approx.0574 \approx .034c_\mathrm{l}
\end{equation}
which is less than 5% of the actual value. However, it is important to keep in mind that the fallacy predicts no lift change with angle of attack (just one of the many reasons it is silly), and so the relationship between $c_\mathrm{l_{ETT}}$ and $c_\mathrm{l}$ varies with angle of attack for a given airfoil. For the 4412, they meet at just over a -5° AOA. For a symmetric airfoil, they meet at zero AOA. For any other airfoil, the calculation must be redone with the correct path lengths and actual lift coefficients, although magnitude of the result is not likely to change dramatically.


Does the aerodynamic center move when flaps and slats are deployed? Does the aerodynamic center of an aircraft move when flaps and slats are deployed?

I am trying to figure out if the AC of an aicraft moves with landing and take-off configurations. I know Cl and Cm change a lot, but I can't find a reference where the movement of the AC is discussed.

I've checked Roskam, Anderson and Aerodynamic Characteristics of Wing Sections and it seems like the AC position doesn't change only with flaps but they don't show examples of wings with flaps and slats.

The aerodynamic center does not move when plain or camber flaps are used. A plain flap will only change the camber of the wing and, therefore, will only affect its zero-lift pitching moment. Flaps have their own aerodynamic center, and their lift change should be summed up in the flap's aerodynamic center.

Note that the aerodynamic center of the aircraft is used for the lift force change resulting from a change in the angle of attack while the flap aerodynamic center is used for the lift force change from a change in the flap's deflection angle. With flaps deployed, a change in the angle of attack will still cause a lift change which acts on the aircraft's aerodynamic center! Only the constant pitching moment around the aerodynamic center will change with flap deflection, and in order to calculate this moment, assume the lift change from flap deflection to act in the flap aerodynamic center.

The location of the flap aerodynamic center changes with the relative chord of the flap. For a flap approaching a relative chord of 0, the theoretical flap aerodynamic center is at 50% of the wing's chord. With increasing flap chord the flap aerodynamic center moves forward proportionally with the flap's chord increase until it reaches the 25% chord point for a flap of a relative chord of 1 (a full-flying control surface, in other words). See Figure 46a from chapter 5 of Sighard Hörner's Fluid Dynamic Lift for proof.

Flaps which increase the wing's area will of course shift the neutral point in proportion to the change in the wing's chord effected by the flap.

Slats may be viewed like a slat-sized wing followed by a wing-sized flap and an incidence change equal to the slat's deflection angle.


Why would an airliner make a 90° course change? Why would this airliner be doing a 90° course change at cruising altitude? I saw this while waiting for a traffic light to change.

9 Jan 2020 1:42 pm (18:42 UTC) at Virginia Beach, VA, coordinates:
36°46'38.070"N 76°05'18.03"W, my heading: 237 deg.

And from Flightradar24, flying at 37,000 feet:

This answer assumes the manouevring was related to the Military training activities going on off-shore. While plausible, there is another plausible explanition in ymb1's answer.

That was a British Airways Boeing 747 on its way from London to Miami.

Just off the coast there is a military exercise going on (see the LJ35 doing circles in the screenshot from flightradar24.com below; probably a military contractor like the one in this question).

Therefore all flights that would normally fly in that area end up flying over the coast. It seems that when the peak traffic from Europe towards Miami was passing, ATC decided to put everybody onto their original route as soon as they passed the bottleneck in order to reduce the traffic density. The result is quite a few sharp turns.

A few hours later there were at least 4 Learjets playing around off the coast. Note that most military traffic is not shown on FR24, so it may have been quite a bit busier.


Why the pitch rate q is zero while angle of attack is changing and vice versa? In the book Dynamics of flight, the authors present how the pitch rate q and the angle of attack $\alpha _x$ may vary:

Source: Dynamics of flight: Stability and control,
Bernard Etkin, Lloyd Duff Reid, 1996.

Why is the pitch rate q null while the angle of attack changes, and vice versa?

Why the pitch rate q is zero while angle of attack is changing and vice versa?

It generally does not (*). You misunderstood the statement in the Etkin's book.

The image you posted come from the chapter about "the $q$ derivatives" $C_{L_q}$, $C_{M_q}$ and $C_{h_q}$.

The book is quite explicit:

  These derivatives represent the aerodynamic effects that accompany rotation of the airplane about a spanwise axis through the C.G. while $\alpha$ remains zero

It then presents

  Figure (b) shows the general case in which the flight path is arbitrary

(this is the figure you posted)

  This should be contrasted with the situation illustrated in figure (a), where $q=0$ while $\alpha$ is changing.

The book in no way implies that either one case or the other is true, it is only presenting contrasting academic examples to illustrate the concept of what the $q$ derivatives are.

(*): I said generally because the book itself present one case where you CAN have constant $q$ and $\alpha = 0$, the steady pull-up.
Again, this in no way implies that you always have

  q is zero while angle of attack is changing and vice versa


Can we trade some efficiency to curb high-altitude emissions? Considering our problematic issues,

Emissions at high altitude
Contrails
~10,000 premature deaths per year from airline emissions

This is an area of concern where it's worth asking some stupid questions.

What would happen if a hose was attached to the core/hot nozzle of a turbofan, diverted to storage on the body? Could it still be efficient 'enough' (assuming a highest possible bypass ratio) to compress and carry some of that carbon dioxide to the next airport?

There's a fundamental problem. A jet engine performs work by expansion of the combustion products. Compressing the combustion products would require at least as much work as was provided by the expansion of the combustion products. Only an external combustion engine (for example, a steam engine) would work in this case.

A second problem is that the mass flow of the CO2 is more than three times higher than the mass flow of the fuel. Your plane would get heavier as you fly, and since the max landing weight of a typical plane is lower than the max takeoff weight, this will severely limit the aircraft range.

All in all, you can probably manage to make a purpose-built plane that fits the bill but I highly doubt it will have much usable payload if any at all.


Should I monitor the emergency frequency (121.5 Mhz) at all times? When flying on a plane with two or more radios, should I set the second radio to 121.5 at all times? (Assume I'm not using it to listen on other frequencies)

I've heard that sometimes when an intruder penetrates restricted airspace, the military will broadcast on this frequency in an attempt to communicate with that aircraft. Is this a good way to avoid me getting shot?

What about monitoring in case someone around me has an emergency?

Is there a difference in general aviation vs commercial airlines?

EDIT: I'm asking about whether this is a good idea, not about regulations.

Airlines routinely set comm 2 to 121.5 when they don't need it for talking to company or getting ATIS.  But for a light aircraft pilot in busy airspace, the second comm is a really useful tool for reducing workload by being able to preset frequencies, and dedicating the second comm to 121.5 all the time takes that feature away, for the very small chance that you're hear someone's ELT go off or broadcast an emergency that will be heard by ATC anyway.  Very few GA pilots monitor guard frequency regularly.

On the other hand, if you are flying over a remote area, it's not a bad idea at all and you could save someone that crashed with a non-406 ELT.


Can training for an Instrument Rating be applied towards a commercial pilot certificate? Per 14 CFR 61.129, a person who applies for a commercial pilot certificate for airplanes (both single and multiengine) must log at least ten hours of instrument training.

Assuming a pilot obtains their instrument rating prior to training for their commercial pilot certificate, can the instrument time logged during training for an instrument rating be used to satisfy this requirement (assuming class of aircraft requirements are met)?

Short answer: yes, if the training has been logged with the remark that it meets the requirements of both 61.129 and 61.65.

This topic became confusing because of an FAA interpretation that says:

  [...] are the requirements of 14 C.F.R. §61.129(c)(3)(i)
  [commercial certificate] met by the student getting an instrument
  rating or training for an instrument rating? The answer is no. The
  training giving to satisfy the instrument training aeronautical
  experience of §61.129(c)(3)(i) may also be used to count towards the
  aeronautical experience of §61.65(e) [instrument rating], but the
  opposite is not true. The reason for this is that the training
  required under §61.65(e) is general, while the training under
  §61.129(c)(3)(i) lists very specific operations that must be
  accomplished to satisfy the requirements.

But that interpretation was surprising to many people because it implied that a commercial certificate requires additional instrument training, even if you already hold an instrument rating. So AOPA requested a clarification (more information here); you can read the whole thing, but the key statement from the FAA is this:

  We are merely clarifying the requirement that the applicant for a
  commercial pilot certificate provide evidence that they have met the
  requirements of §61.129. There is not an exact equivalence between the
  training required for an instrument rating under §61.65 and the
  aeronautical experience requirements under §61.129.

Practically speaking (as the AOPA article explains) that means that your CFII should explicitly log the training as covering both requirements:

  [AOPA] urges instrument pilot applicants and flight instructors to be sure
  that instrument training is clearly logged to indicate that the
  training given meets the requirements of 14 CFR 61.65 as well as those
  of 14 CFR 61.129. That would avoid questions about the training’s
  applicability should the pilot one day advance to training for a
  commercial pilot certificate.


What are the aerodynamic implications of different conditions with the same Mach number? At a particular Mach number, a fluid behaves similarly, but many other conditions could be different.

For example, an aircraft flying at Mach .85 at a low altitude will be in denser, warmer air, and actually flying faster, than the same aircraft flying at Mach .85 at a high altitude.

What aerodynamic differences (if any) will the aircraft experience between the two conditions?

As far as flight aerodynamics is concerned, which of Mach number, true airspeed and indicated airspeed are most significant, and when?

The Mach number only indicates the behavior of compressibility effects, but does not provide any indication about other effects like inertia and viscosity (read turbulence) or heat transfer.

For high subsonic and supersonic aerodynamics one should typically compare both the Mach number (compressibility) and the Reynolds number (turbulence).

So at the same Mach number but different densities, the Reynolds number will be very different and therefore the turbulence effects will differ highly. Only the compressibility effects will be similar (shock waves).

Note that at low subsonic speeds (Mach<0.3), the compressibility effects become negligible and one can get away by only comparing the Reynolds number.
And at hypersonic speeds (Mach~>5) more similarity parameters are needed.

More information about dimensionless numbers in fluid mechanics on Wikipedia.


Does an airfoil in potential flow produce drag? D'Alembert's paradox states that in a potential flow, the drag force is zero on a body moving with a constant velocity relative to the fluid. But what about pressure drag? Shouldn't there be drag from the pressure distribution around the airfoil? I have heard conflicting answers from multiple sources and I would like to clear up the confusion.

Short answer: D'Alembert's paradox always holds as stated:

  For incompressible and inviscid potential flow, the drag force is zero on a body moving with constant velocity relative to the fluid.

Longer answer: Pressure drag is a viscous phenomenon. Anderson section 3.18 has an extended discussion of experimental results stating that

  the real flow over a circular cylinder is dominated by friction effects, namely, the separation of the flow over the rearward face of the cylinder. In turn, a finite pressure drag is created on the cylinder, and d’Alembert’s paradox is resolved.

Anderson 3.15 and any number of online resources convincingly and rigorously prove that inviscid flow around a cylinder preserves d’Alembert’s paradox (even when the cylinder is spinning and producing lift) because there will always be cancellation of aerodynamic forces due to the symmetry of the flow field across the line concurrent with the lift vector. The easiest way to convince yourself that this is also true for airfoils is the conformal mapping technique explained in this answer.


Does high density altitude affect your landing speed? So I had this question asked in my recent commercial-pilot checkride:

The DPE asked me about high-density-altitude airports; he specifically asked me if I should add airspeed when I'm on landing approach to a high-density-altitude airport.
 My total understanding is that the ground roll and landing roll would be affected, as well as the climb, but I've never heard about landing speed being affected; I've been looking in books and researched on the web but I can't get a solid answer.

Your $v_\mathrm{ref}$ does not depend on density altitude since it is given in Indicated Airspeed, which already accounts for density effects. However, the True Airspeed and therefore also Groundspeed will be higher at a higher density altitude, resulting in more runway required to stop.

The Flight Safety Foundation has a nice summary of factors influencing $v_\mathrm{ref}$:

  Factors Affecting the Final Approach Speed

  The following airspeed corrections usually are not cummulative;
  only the highest airspeed correction should be added to VREF
  (unless otherwise stated in the AOM/QRH):

  Airspeed correction for wind;
  Airspeed correction for ice accretion;
  Airspeed correction for autothrottle speed mode or autoland; or,
  Airspeed correction for forecast turbulence/wind shear conditions.

  Gross Weight

  Because VREF is derived from the stall speed, the VREF value
  depends directly on aircraft gross weight.

  The AOM/QRH usually provides VREF values as a function of
  gross weight in a table or graphical format for normal landings
  and for overweight landings.

(FSF ALAR Briefing Note 8.2 - The Final Approach Speed)

Altitude, Density Altitude or Pressure are not listed as factors affecting $v_\mathrm{ref}$.


Do airline pilots use this high-speed, high-drag technique for losing altitude? This question and another more recent one got me thinking about this.

Years ago on my MEII checkride the DPE showed me a technique for losing altitude on final when the aircraft was high. Typically, I would reduce power, increase drag (flaps, landing gear, etc), hold nose up to lose speed,   and sink like a rock (if necessary slip). However, what he showed me kind of blew my mind. I rarely do it, except when just having fun.

He said, "Do you remember what happens when airspeed doubles?" I said, "Yes, drag quadruples." More precisely the parasite drag quadruples. Drag $= pv^2$, where $p$ is some constant representing parasite drag and $v$ is velocity.

So, we set up high on final, and he dropped the flaps and descended at $v_{FE}$. I was surprised to find that the maneuver used up a lot of the aircraft's energy, and the aircraft was brought down to the glide path and slowed down with plenty of time for a normal landing.

My DPE was a Captain for Southwest back when Southwest was a small airline. My question is whether this is a technique used by airline captains nowadays?

I don't have access to a B737 to try this out, but in a small aircraft like a Cessna 152 you have a $v_{s_{0}}$ of about 35 and $v_{FE}$ of 85.

The idea is you can descend with full flaps at 42.5 knots and power off and it will sink slowly but in a somewhat nose up attitude. Or, you can put the nose down and speed up to 85 knots and quadruple the drag and get on your glide path sooner so that the last bit of the landing is at a proper glide path, attitude, and speed.

Airline pilots certainly do use gear/flaps/spoilers to descend more quickly if needed. However, SOP typically requires an approach to be stable upon reaching a certain altitude on the approach. This means having speed and airplane configuration set for landing, and being lined up with the runway and on the glideslope. Unstable approaches are more likely to result in landing mishaps.

Some situations such as a runway change may require late changes to the approach. It is up to the crew whether they can safely make this change or if a go-around is the better option. S-turns are another way to lose further altitude.

When Air Canada Flight 143 ran out of fuel, the pilot used his glider experience to perform a forward slip to lose altitude. These sorts of maneuvers would be reserved for an emergency on an airliner.


What is the function of Boeing C-17 Globemaster III in Indian military? Why does India need Boeing C-17 Globemaster IIIs?
What purpose do they serve in Indian military?

The C-17 serves the same purpose in the Indian military as it does in other armed forces: it provides large-scale airlift.

India is a large country that has long borders with unfriendly neighbors, so it has a requirement to move troops and supplies quickly to any hotspots.

  "The Indian military needs to do three things: augment its ability to quickly lift larger numbers of troops as it views possible threats on its border with China; strengthen its presence on the Pakistani border; and fight terrorism and low-intensity warfare", said a senior Defence Ministry official. "India needs to triple its lift capacity", said the official.

India spent on the order of $5B buying these aircraft, it's unlikely they'd pay that much for aircraft they don't need.


Do helicopters ever dump fuel before landing? In my answer to this question about fuel dumping I included the following rhetorical question:

  Or if you're dumping fuel from a helicopter (does that ever happen?)
  it could hover over a tank on the ground, pump out the fuel and then
  land.

Two commenters said that helicopters should never need to dump fuel because if they can take off with a full fuel load then they can land again. Makes sense, but this news story refers to a helicopter dumping fuel in order to land, although it's only one article and the general (non-aviation) press is notorious for misreporting aviation incidents.

So, can/do helicopters ever dump fuel as a safety measure before landing?

Yes

07-NOV-1971
Sikorsky CH-124A Sea King (S-61B)
Suffered single engine failure on take-off. Pilot attempted to return while also dumping fuel.

From what I've read, fuel-dump systems are more common on military helicopters.
Sometimes, a helicopter may not have enough power to hover in ground effect for a normal landing or take-off because of loss of an engine, high density altitude, or being overweight. The alternative procedure is to perform a running take-off or running landing, which is an advanced, but not an emergency, maneuver.
But if the copter is landing in a confined area, such as a small pad on a frigate in a stormy ocean, there's no room for a running landing, so its better to dump fuel to allow the copter to hover into a landing.


Why isn't there a "check engine" light in airplanes? Why don't small airplanes (like Cessna or Cirrus) have "Check Engine" lights, like cars do?  For the most part, the people flying these airplanes don't know much more about the plane than they do their car.  Wouldn't a "Check Engine" light be useful to many people flying these planes?

Or do the rigorous maintenance schedules required for airplanes invalidate any use that a "Check Engine" light would have?

With a few exceptions (modern aircraft engines with FADEC systems) a "Check Engine" light on an aircraft would be essentially useless.

In a modern car we have an Engine Control Unit which manages things like mixture and engine timing. The "Check Engine" lamp illuminates when the ECU has detected a fault that requires some sort of attention, but it's not usually specific as to what the fault is or how urgent the problem is.

In most light GA aircraft (like the Cessna or Cirrus in your question) the engines are straight out of the 1950s/1960s: There are no computers managing the engine for you, and the pilot is acting as the ECU.
The pilot should be checking the engine's health regularly (by looking at the engine gauges) and adjusting power, mixture, and propeller pitch as required.

Since there is no computer supervising the operation of the engine in most light GA aircraft there's no system to determine when the light should be on, so there is no "Check Engine" light.

Note that some light aircraft do incorporate warning lights for system status information. For example, the status cluster on newer Piper Warriors:

Top row: Low bus voltage, Pitot Heat off/inop, starter engaged.
Middle row: Vacuum inop, Alternator inop, Oil Pressure.
Bottom row: Not used on this aircraft.

The information provided by these lights is more specific than simply "Check Engine" - directing the pilot's attention to a specific system or condition. The lights are also not a substitute for regularly scanning the engine instruments and managing the powerplant, they're merely a supplement to catch your attention between scans.


Can motorjets be a viable means of propulsion? A motorjet is a very early type of jet engine where intake air passes through a compressor driven by a separate piston engine, and is then mixed with fuel and burned to provide thrust. They were used in a handful of designs around the time of World War II, but fell out of favour when gas turbine metallurgy had advanced to the point where it was feasible to drive the engine compressor from a turbine in the exhaust stream without having to rebuild the engine every few flights; all else being equal, a motorjet, requiring an entire second engine to drive the compressor, is heavier, and, thus, has a lower thrust-to-mass ratio than a turbojet or turbofan. Are there any situations where a motorjet would still be a better choice than a turbojet or turbofan?

I can think of a few maybes:

Aircraft using fuel that produces refractory oxides when burned (such as boron-containing fuels and most organometallics), or operating in air heavily contaminated with particles of same (such as volcanic ash clouds), as these are very hard on turbines unless the exhaust gas temperature (EGT) remains high enough to vapourise these oxides all the way out the tailpipe; a motorjet has no turbine blades in the exhaust stream to suffer ill effects from this (unlike a turbojet or turbofan), does not need an onboard supply of oxidiser (unlike a rocket), and produces reasonable thrust even at zero airspeed (unlike a ramjet).
Aircraft with very severe length restrictions, as a turbojet or turbofan's compressor(s) and turbine(s) all have to be in line with one another, whereas a motorjet's compressor motor could be stacked vertically above the compressor and combustion chamber.
VTOL aircraft, where the torque and gyroscopic effects from all that heavy, rapidly-spinning turbomachinery can cause control problems while hovering or moving at very low speed (due to the control authority of aerodynamic control surfaces being very low at very low airspeeds, and nil at zero airspeed), as pointed out in this comment by Peter Kämpf on an answer to one of my earlier questions.

Internal combustion engines (ICE) have great thermal efficiency, so driving a fan or compressor using an ICE is on paper very appealing indeed.

  It is well known that internal combustion engines are capable of achieving higher thermal efficiency than gas turbine engines.

NASA even looked at it in 1976. The proposal was to substitute a jet engine's combustor with an ICE. Two problems with that (according to Rolls-Royce) are the need for a gearbox and "component aerodynamic mismatching under certain engine operating conditions" due to the single-shaft design.

R-R filed a patent in 1995 that according to them fixes the problems with NASA's engine. So far we know why motorjets will have a hard time making a comeback.

R-R's solution is, more shafts, of course. They propose to have an ICE in place of the combustor, but this time it has its own shaft, fed by its spool's compressor, and the exhaust would drive a turbine (too many parts already) that drives a ducted fan (think turbofan but with ICE thermal efficiency).

  According to the present invention, an aircraft compound cycle propulsion engine comprises a propulsive fan and a core engine to power said fan core engine comprising an air compressor downstream of said fan, at least one rotary internal combustion engine configured to receive compressed air from said air compressor, a power turbine positioned to receive and to be powered by the exhaust efflux of said at least one rotary internal combustion engine, a first shaft drivingly interconnecting said power turbine and said propulsive fan, and a second shaft drivingly interconnecting said at least one rotary internal combustion engine and said air compressor.

Source: https://patents.google.com/patent/US5692372A/en


What does a "hard stand departure" mean? I am reading a table explaining how to plan multiple diversion returns from an individual station:

  widebody gate departures - xx minutes
  narrow body gate departures - xx minutes
  all hard stand departures - xx minutes

I am not a native English speaker and do not understand what a "hard stand departure" is.

Could anybody please explain this term?

A hardstand is a type of parking spot separate from the terminal.  Wide/narrow body gates are associated with the connection to the terminal, where a hardstand is just an area of pavement reinforced for heavy vehicles (or airplanes).


What are the practical impacts of airports in the US being mostly IATA Level 2? IATA classifies airports using levels, which determine how slots are allocated.

Source

Definitions of levels by IATA, from Slot Coordination Switzerland:

Level 1: airports where the capacity of the airport infrastructure is generally adequate to meet the demands of airport users at all times.
Level 2: airports where there is potential for congestion during some periods of the day, week, or season which can be resolved by voluntary cooperation between airlines. A facilitator is appointed to facilitate the planned operations of airlines using or planning to use the airport.

Dublin and Salzburg are Level 2.
Level 3: airports where capacity providers have failed to develop sufficient infrastructure, or where governments have imposed conditions that make it impossible to meet demand. A coordinator is appointed to allocate slots to airlines and other aircraft operators using or planning to use the airport as a means of managing available capacity.

Heathrow, Gatwick, Barajas, Oslo, De Gaulle, Orly, Tokyo, Malpensa, Pearson are Level 3.

Many major airports are level 3, but only one US airport (JFK) has this classification.

Is there a reason US is seemingly deliberately different? What does that mean in term of practical scheduling by airlines? Is the difference apparent to a pilot?

Why aren't there many level 3's in the US?

Let's check the IATA criteria:

  A Level 3 airport is one where:

  a) Demand for airport infrastructure significantly exceeds the airport’s capacity
  during the relevant period;

  b) Expansion of airport infrastructure to meet demand is not possible in the
  short term;

  c) Attempts to resolve the problem through voluntary schedule adjustments
  have failed or are ineffective; and

  d) As a result, a process of slot allocation is required whereby it is necessary
  for all airlines and other aircraft operators to have a slot allocated by a
  coordinator in order to arrive or depart at the airport during the periods when
  slot allocation occurs.

a) Demand exceeding capacity

According to this study, the demand levels on US airports is lower compared to the EU.

  In 2014 the average payload at 11 of the largest US airports was 108 passengers per
  aircraft. At 10 of the largest EU airports, the average was 134 passengers per aircraft.
  This confirms that the structure of traffic is quite different.

b) Infrastructure and expansion

In the US, say apart from New York, expansion is easy. A good comparison would be comparing a major European city with a major US city. The US had the benefit of hindsight when they planned for their cities. Heathrow has been trying to expand since forever, and only recently a third runway was approved.

According to this MIT presentation, for the top 34 airports, the US has 4.12 runways per airport, compared to 2.47 in Europe.

How do airlines manage?

Since a) and b) are usually not met, c) and d) are less likely to happen.

c) and d) Schedule adjustments and slot allocation

14 CFR Part 93. The FAA stepped in a long time ago, and hasn't changed a thing since the deregulation act. All the majors who had all the slots, are hanging on to them.

Any trading that happens is not through an open auction. Only once was an experimental free-market auction held in 1982. So any problem, is evidently usually solved voluntarily.

According to the same MIT presentation, when slots exist in the US, they are "set with reference to VMC capacities". While Europe uses IFR procedures figures. This allows the US to declare higher capacities than Europe. So, it's easier for airlines to schedule flights in the US.

How does it impact the pilots (flights)?

Again, from the MIT presentation, the US faces high delays when IMC (instrument meteorological conditions) sets in, because of using VMC (visual meteorological conditions) capacities. Europe is more reliable in this area. So a pilot flying into a level 2 airport in bad weather in the US is likey to be delayed, whether in the air or on ground.

  Schedule reliability is much lower in the US than in Europe
  Reliability of schedules in US declines over the course of a day, particularly in the presence of poor weather
  In defense, US airlines have been increasing (up to 2007) the advertised flight durations; European airlines have not
  European airports place a premium on predictability and "smoothing" operations, relying on (often too low) declared capacities and IFR separations at all times

They also mention delays are not necessarily due to over capacity. My interpretation: You can have a time like Christmas with many flyers and bad winter weather causing many delays and cancelled flights. But still, airlines can't apply the level 3 options. Especially that they can't resort to c) and d) to move Christmas time to the summer.

All airlines suffer, the passengers expect it, and that's it. However, there are studies for and against changing the way the US handles capacities and slots (messy debates).

So, there are two systems, with no clear winner.


Where can I find more information about my grandfather's crashed B-18a bomber on Mt. Redoubt Alaska? My father and I have chartered a plane for late August to search for the remains of my grandfather's B-18a bomber, which crashed in the summer of 1942 in Mt. Redoubt Alaska.

I am specifically interested in advice from pilots who have flow around Mt. Redoubt:

A Saturday Evening Post article mentions the accident site is at 7500 feet MSL, on the southwest side of the mountain.  The accident report OTH, says the southeast side.  Questions:

What is the terrain/glacier situation at this location?
Is landing possible at this location?
Has anyone seen an accident site at this location?

I've posted additional details and pertinent material in this blog post.

Also, I would think the USAAF would have more detailed records of the rescue, which again, is detailed in the Saturday evening post article.  Perhaps even a dispatch to the rescue crew with a specific location.  Does anyone know where I might find such records?

Update, July 21st, 2017 (One month till departure)

Since I've offered a bounty, I guess I should iterate what I've done so far:

I've posted this to an Alaska bush pilot's forum.  I was surprised to not get a better response from this.
I've ordered (and received) a full sized topo map of the area from mytopo.com. The detail is pretty darn good.
I've scoured over multiple satellite photos.  Note, the obvious photos on google maps/earth are pretty iffy.  I found the photos at zoom.earth to be better.

What is the terrain/glacier situation [on the southeast side of Mt Redoubt]?

The southeast side is all glacier, forming in the valleys formed by previous formative eruptions, which have deepened significantly due to glacier flow. These glaciers do move and churn, so unless the plane crashed on top of one of the prominent ridges then the position of the plane's remains is changed, and they are likely mixed/buried within the glacier.

It's been 75 years though, and while the glaciers sped up during eruptive periods (or melted, as on the north face during the 1989 eruption), the average annual glacier speed is 20 meters per year.  So the wreckage would have moved at least 1.5km down the mountain slope over time, buried under new snow (and occasional ash), and always moving down into the drift glacier.

These reports on Mt. Redoubt's glaciers should provide significant help in determining the location, thickness, and movement of each glacier present:

https://pubs.usgs.gov/wri/1997/4187/report.pdf

https://www.igsoc.org/journal/32/112/igs_journal_vol32_issue112_pg355-362.pdf

In a normal crash if you have the location to within 100 meters it can be found with a simple search, even decades later, even though the plane may have undergone environmental extremes.  In this situation, though, not only will the wreckage have been moved, but it will have been torn apart and spread over a much larger area than the original crash.

  Is landing possible at this location?

You can land on glaciers in general, and these are no exception, but you can't land on the side of Mt. Redoubt any easier than any other mountain.  You may be able to land at the base of the mountain, but you'd have to decide which glacier flow to land on, and as explained below it's unlikely that you will find anything to help you decide which one.

The pilot will have to make the final decision, though, as conditions may vary, not just in the weather, but on the glacier itself.

  Has anyone seen an accident site at this location?

Only the rescuers would have.  Once consumed by the snow and glacier over years it would have remained only as small parts of the whole.  Any pieces remaining visible to the eye would also act as solar collectors - they would melt the snow beneath them, sinking lower into the glacier, and then be covered by another fresh layer of snow.

I commend you and your father for your dedication to this aspect of your family history, and wish you luck!


What would be the stall speed of Solar Impulse? In most of the tables, only the take off (22 mph) and cruise (43) speed of the Solar Impulse are listed. It is an experimental plane, slow, with very huge wing for solar batteries.

The take off speed may not be the slowest speed the aircraft can fly, for instance, when landing. Is it possible to infer somehow the expected slowest speed of this airplane?

Solar Impulse has no flaps and a high aspect ratio wing. Therefore, the c$_{l_{max}}$ of the wing should be no higher than 1.8, most likely 1.6. While a rigid airfoil can be designed with a higher c$_{l_{max}}$, it will not have an attractive L/D and a shorter wing would have a c$_{l_{max}}$ below that of the airfoil. Since no fuel is consumed, mass is constant, which makes it very easy to give a lower bound for the minimum speed.

Solar Impulse 2 has 269.5 m² wing area to carry its 2.3 tons of mass. This is a wing loading of just 8.53 kg/m². If we assume standard day conditions at sea level (density $\rho$ = 1.225 kg/m³), we find vor v$_{min}$:

$$v_{min} = \sqrt{\frac{2\cdot m\cdot g}{c_{l_{max}}\cdot S\cdot \rho}} = 9.24 \;\small{\frac{m}{s}} \;\text{or}\; 33.27\; \small{\frac{km}{h}}$$

For those of you from metrically challenged cultures: This is 18 knots or 20.67 mph.


Can the Mexican presidential aircraft be sold? I have read that the next Mexican president is trying to sell the B787 that serves as presidential transport, and that it was already made an offering to Boeing for it.

As far as I know, the VIP/presidential aircraft are more or less custom made, so it's hard to imagine that anyone would buy it (I'm guessing an airline would retrofit the plane for his own configuration and that would be more expensive than buying a new plane).

But perhaps the manufacturer would use it for leasing purposes.

Then, it is viable to sell this presidential aircraft (or any other)? How much can be paid for it (compared to the initial value)?

Reg: XC-MEX

Not sure about that specific aircraft, but the Dutch government recently sold its transport (a Fokker 70) for the royal family in anticipation of the arrival of its more capable replacement.

I've seen the aircraft many times before it was sold.

I've also seen President's Eisenhower's personal Beechcraft Twin Bonanza, which has been called the very first dedicated Air Force One, which is currently in a private collection in Canada. Not sure when it last flew, but the owner tries to keep his entire collection airworthy and once in a while takes one of them out.


How do the FAR's define a cloud? If you're talking about a big puffy cumulus cloud it's pretty obvious. But from where I am right now, looking up I see grey sky from horizon to horizon. But visibility at the surface is good. All nearby metar's are reading 10SM CLR. It's just a tiny bit hazy, but I can clearly see a water tower that is 8 miles from me. But there is no defined cloud layer. It just seems that the haze gets thicker the higher you go. If there was a vfr plane up there at a few thousand feet I'm sure I could see it but I can't see any of the four airliners that FR24 shows are above me.

My question is, when there's no defined cloud layer, what is considered "cloud" and what isn't? If the minima require you to stay 500 feet below clouds, where exactly is that? With metars showing no ceiling how do you know where it is safe to fly and where there might be an ifr flight descending that can't see you?

My question is, when there's no defined cloud layer, what is
  considered "cloud" and what isn't? If the minima require you to stay
  500 feet below clouds, where exactly is that? With metars showing no
  ceiling how do you know where it is safe to fly and where there might
  be an ifr flight descending that can't see you?

The FAA defines clouds in AC-006B

  A cloud is a visible aggregate of minute water droplets and/or ice
  particles in the atmosphere above the Earth’s surface. Fog differs
  from cloud only in that the base of fog is at the Earth’s surface
  while clouds are above the surface. Clouds are like signposts in the
  sky that provide information on air motion, stability, and moisture.
  Clouds help pilots visualize weather conditions and potential weather
  hazards.

However legally and practically speaking you can fly as high as you like if there are no clouds provided you can maintain situational awareness and proper cloud clearances from any adjacent clouds (including those under you). There are a few practical limits to this. First off Class A airspace starts at FL180 so you cant fly above 18,000 Ft. without being on an IFR flight plan since you cant fly VFR into class A. Further more, since you seem to be asking about small aircraft, you will more than likely be limited by your airframes maximum altitude before hitting the Class A limit. You will also need O2 onboard to fly above 12,500 ft.

The risk of a descending aircraft above you is always a concern be it VFR or IFR flying as they generally cant see below them and you cant always see above you when flying. In many cases the IFR flight will have some help from ATC who will see you on radar even if you are flying VFR and squawking 1200 on your transponder.


At what point of the design process the manufactures build their first prototype? I would like to know when the aircraft manufactures decide to put in practice their designs building the first scaled prototype (or maybe the actual size). Does the design have to accomplish a list of basic parameters or something like that?

There is no definite answer that will fit all manufacturers and projects for the question you're asking.

Simply put, the decision to create a scale model or a first prototype depends on what the goal of the creation is, the underlying technology, cost and market interest. A lot of aircraft, either new models or improved variants of already existing models never make it past the design stage. Other results in scale models to verify aspects of the underlying design or technology, while some may even skip this and go directly to a full-size prototype to be used for testing.

In some cases, a manufacturer will build a prototype regardless of market interest, because it allows them to showcase the technology and show that it actually works, while other manufacturers might not continue to that stage unless they have generated actual orders for the aircraft.

Needless to say these decisions will be based on different input from project to project, but generally speaking the end goal is to lower the risk if you decide to continue a project. If you are designing a very new aircraft, it makes sense to validate aspects of it before committing to production unless you can demonstrate that the risk is already low enough not to matter greatly.


Rings around propellers As a followup to Why do turbofan blade tips exceed the speed of sounds while prop tips should not my question is: why have rings on propellers not been used to overcome this problem?

It seems from this site that they are produced at least for maritime use.

According to this site and this site it seems to have advantages and experiments have already been done as I can see from the picture. (I do not know how reliable it could be though)

Propeller rings (ducted propeller) have been used un aircraft, notably in some early VTOL designs, like the Bell X-22, XV-11 Marvel and also the RFB Fantrainer.

Bell X-22. Image from diseno-art.com

They do have some advantages like,

Better noise reduction and safety compared to 'open' propeller.
For the same thrust, the propeller can be made smaller.
The duct can be designed such that it provides a limited amount of thrust vectoring, useful if we are using it for STOL.

However, they are not widely used because of various other reasons like,

For effecient operation, the tip clearance should be very small. Any increase in tip clearance leads to rapid degradation in effeciency.
The duct and its support structures increase the weight.
The duct increases the drag at cruise speeds (as drag increases as square of velocity).
If we decide to use a lower sized propeller as a result of using the duct, the effeciency suffers- as we are actually accelerating a lower mass of air more (rather than accelerating a higher mass of air less, which is more effecient).
The design of the duct is difficult- the aiflow into the duct comes not only from the front, but is also sucked from the sides from the sides (i.e. the capture area is greater than the duct frontal area). If not designed properly, the flow may seperate, leading to loss of effeciency.

However, ducted fans are used in a number of small UAVs (in RC model jet aircraft, for example, in lieu of miniature jets), electric aircraft like the Airbus E-fan and also some dirigibles.

In case you are asking about connecting the propeller tips like in the maritime propller in the link, the resulting stresses at high speed would tear the whole assembly apart, causing more problems than it solves.


Is a French PAC skydiving license equivalent to an AFF license? I am planning to do a skydiving course and I was thinking of doing the course in France. That means, I would get a stage PAC license course instead of AFF. I want to know how PAC is different from AFF (previously I have only heard of AFF) and can I use the PAC license to skydive anywhere in the world?

If you mean you are learning to skydive and will be taking PAC instead of AFF, then the differences are minor. IIRC, in PAC the last jump is number 8, whereas in AFF and PFF it's jump 7.

There's also the Canadian version, PFF, and I assume other variations as well. Generally, schools do not follow any of these exactly and add their own variations.


Is a 100 hour inspection required for an aircraft to be used in a checkride? Is a 100 hour inspection required when renting an aircraft for a practical test with a DPE, to whom you are paying a fee?

Lots of money is being exchanged, yet the DPE told me that it is not "For Hire" because the money is going to him (i.e. he's not paying me, I'm paying him). How can the direction that the funds are traveling in affect the meaning of the term "for hire"?

The rule is that the pilot may not be "for hire".

He is for hire, but as a DFE, not the PIC (the one flying the plane), so he is not breaking the rule.

You are the one flying the plane (the PIC), but you aren't getting paid (in fact, you are paying someone else), so you aren't "for hire" either.


How do aircraft discern that an ACARS message is being sent to them? This may be a simple question, but how do airplanes know which ACARS messages are meant for them from the ground? I'm not sure how physically close planes would have to be in order for that to be an issue. Do they simply receive all ACARS transmissions in the area and the CMU checks for this information in the header and continues to route it if it has the proper header? I'm assuming the VHF doesn't perform any checks since the data is still an RF signal at that point.

A typical ACARS VHF transmission.

Mode A

Aircraft B-18722

Ack NAK

Block id 2

Flight CI5118 <<< looks like an identifier the receiver can filter on

Label B9

Msg No. L05A

Message /KLAX.TI2/024KLAXA91A1

https://en.wikipedia.org/wiki/ACARS


Why does ATC ask emergency aircraft about fuel on board? From what I hear, it looks like it's a standard procedure that ATC asks about fuel on board whenever an aircraft declares an emergency. Do they need the information for their own planning purpose or just to pass it along to the fire crew so that they get prepared for possible emergency situations like a fire upon landing?

I thought the second was the correct reason, but the fact that some pilots report fuel in remaining time, not in pounds, gets me confused.

It depends on the type of situation. If the aircraft is unable to land for some reason, we'll want to know the remaining fuel to get an idea of how long the aircraft can realistically stay airborne. For an aircraft coming in for an emergency landing - as you have already guessed - the reason we ask for remaining fuel is because the fire and rescue personel needs this information. We will also ask such an aircraft if they have any dangerous goods onboard, since we need to pass this info on to the fire crew as well.

During an emergency, we have a checklist to follow that reminds us which information we need to obtain, and who needs to know what is going on. For a tower position, it may look something like this:

As you can see, fuel and dangerous goods are grouped together, indicating that they are essentially the same thing - something that may cause fire, explode or otherwise cause danger during evacuation of the aircraft.


Do airliner spoilers really "spoil" lift? The two illustrations below show the typical spoiler location and the lift distribution of a wing. For comparison a third picture shows the location of spoilers on a glider.

It appears the airliner spoiler is located so it destroys very little lift but would really only contribute to drag - is this really the primary contribution spoilers have to the aerodynamics of an airliner or is there more happening here?

Typical location and size of airliner spoilers. - source

Typical lift distribution of wing. Red references have been added to show typical airliner spoiler. - source

For comparison, a glider spoiler located at the center of lift - source

You modified the lift distribution diagram by adding spoilers without showing how that modifies the lift of the wing. The trailing edge of a wing controls how much lift is produced, whether you call it flow turning, circulation, or whatever.

Your diagram implies that lift is produced regardless trailing edge position. If this were true ailerons would have no effect since they only occupy the trailing quarter of the typical wing. Look at the camber line of the wing in your diagram with and without spoilers and you'll see. Flow does not turn with spoilers deployed, so you lose lift and gain a lot of drag.

source

  Extending the wing flaps increases the camber or curvature of the wing, raising the maximum lift coefficient or the upper limit to the lift a wing can generate.

A spoiler of the variety shown in the first diagram works like a split flap upside down, reducing camber and maximum lift coefficient.

inverted

source


What is the weight of the fuselage (only) of the Boeing 747? I'm wondering about the weight of the Boeing 747's fuselage.

The fuselage weight for 3/4 of a 747-300 Combi minus engines, fuel tanks, seats, catering, cargo handling, navigation gear, control surfaces, pumps, hose is 70,000 lbs, according to Big Imagination, the people that bought a 747, gutted it and took it to Burning Man. The 70,000 lb figure was used by the Nevada DOT to determine the oversized vehicle permit fee. If you add another 25%, you come up with roughly 93,000 lbs.

In comparison, a A 747-300 dry weight with engines, seats in airworthy condition is about 384,000 lbs.

I know this only partially answers your question, but every other source I have tried features the Operating Empty Weight (OEW), which is a basic working weight for each plane variant.

Additional Resources:

https://www.bizjournals.com/seattle/blog/techflash/2015/02/boeing-747-to-become-largest-ever-burning-man.html
http://www.bigimagination.org/747
https://en.wikipedia.org/wiki/Boeing_747

Good luck finding a better figure. I'd love to know myself.


Do GA IFR flights out of airports w/ SIDs typically use them? What about STARs? Besides the IAPs (which as an IFR flight you're guaranteed to use unless cleared for the visual), there are the STARs and SIDs, and using them in slow GA aircraft doesn't fully make sense to me.

For example, out of RIC there is the Colin Six Departure:

I would imagine that as a 150 KT puttering IFR GA aircraft you would be literally holding up any heavies behind you on a busy day, heading for that COLIN waypoint, that are trying to push the < FL010 250 KT speed limit.

Same for the STARs: your gentle 185 KT descent through 3,000 is a cork for the 747 at 240 KT on the same path but 10 NM back and 1,000 feet higher, getting lower and closer.

Does this simply come down to ATC's ability separate aircraft, and
Are these standard instrument approaches and departures always/often/at all used by small & slow IFR GA aircraft?

I cannot speak for all of General Aviation (GA), but throughout the IFR portion of my flying career I have generally used SIDs wherever available.

NOTE: This choice of using SIDs was not always mine, as ATC will often include a SID in a clearance unless requested otherwise. A pilot has two opportunities to avoid SID's if so desired: firstly, when filing the IFR flight plan the statement "NO SIDS" can be included as a request; secondly, when receiving the IFR clearance, the SID can be declined.

I typically like using most SIDs, and I understand that ATC also likes giving them as part of a clearance. Many SIDs are essentially just radar vectors in normal usage with further instructions in case of lost comms.

My experience with SIDs has included numerous aircraft with typical climbout speeds ranging from 90–280 KIAS. As far as I have been able to tell, my aircraft's speed has never been a problem that ATC couldn't handle.

I have less experience with STARs. In flight training I used them occasionally, and ATC seemed to handle my slow speed without issue. At my current operation—which you may or may not consider to be GA—ATC rarely gives of STARs, but usually just gives us clearances direct or vectors. I personally like filing STARs since it helps me know what to expect.

A note on General Aviation (GA): multiple uses exist for the term, "general aviation" (see my query below your question). All of the definitions I am aware of would include the flights in smaller single and multi-engine aircraft that I have flown during flight training. Some of the definitions I am aware of would include some of the turboprop flying I have done (non-charter §135), some would not. But I think that all of the definitions I am familiar with would include private business aviation under §91—which includes some of the fastest civilian jets out there.


From which rating would it be easier to transition to an Osprey? Fixed-wing or rotary-wing? If the future of flight was to be on aircraft that were a hybrid of fixed-wing and rotary-wing (for example the Osprey), assuming they had similar technical abilities and the same number of flight hours, who would have an easier time flying an Osprey?

Or to put it another way, is the Osprey closer to a plane or to a helicopter?

The Osprey is closer to a helicopter and if I was going to chose from a fixed wing pilot or rotary pilot to train on it I'd choose the rotary pilot.  The rotary wing physical skill set is so much more demanding than the fixed wing one.  A helicopter pilot could master fixed wing cruising flight in the Osprey far easier than a fixed wing pilot could master hovering the Osprey (you can pretty much train a monkey to fly an airplane in cruising flight - airplane physical demands are mostly when you slow down).


How precise is an ILS (Instrument landing system)? I read that an ILS is a precision approach system. It provides a final approach with precision in both horizontal and vertical guidance to the runways. But I don't know how precise it is because there were a number of accidents related to landing operations.

The ILS is precise enough to allow landings in essentially zero visibility, including those performed by autoland.

The ground based bits

The vertical range is 1.4° (0.7° above glideslope, 0.7° below) and the horizontal varies, but is a maximum of 6° wide. What this means in terms of how many feet or meters off the centerline an aircraft is varies depending on where the aircraft is on the approach, but it's generally correct to say that the closer an aircraft gets to the ILS antennae, the more precise the system is.

The specifications for a localizer antenna require that it be no less precise than ±10.5m - this is for a Cat I approach, and it gets correspondingly more precise as the category increases.

There's always the risk of interference with the transmitters; as noted in the comments, accidents have been caused by airport vehicles parked in the ILS critical area, interfering with the signals, and as the equipment ages it can require recalibration.

The airborne bits

It's possible for receivers to have issues that could affect precision; antennas can get loose connections or become covered in ice, while pilots might interpret indications incorrectly, misread reverse sensing, or intercept a 'false glideslope' from above.

Regarding accidents

You're correct - there are many accidents related to landing operations. That said, you may be including those which weren't flying an ILS approach - it's hard to say since you aren't citing anything. Still, it's possible that there's confusion between navigation precision and safety of flight.

Many (not all) of landing accidents involving instrument approaches do center around human error rather than equipment failure; pilots tuning in the wrong frequency or incorrectly setting avionics and instruments (reversing the inbound course on an HSI causes reverse sensing, that sort of thing), or people on the ground screwing up.


What is this star shaped symbol on the tarmac? This symbol is on the tarmac at Ramona, CA airport.  I've seen it at other small GA airports from Google Earth but I don't know what it is.  Can someone please explain?

That is a Compass Rose, painted on a Compass Calibration Pad. It's used to mark a location on the airport surface that is suitable for calibrating the compass of an aircraft.

Here's one in a slightly different style:

and a more basic one from an FAA diagram:

More information on the requirements for the compass rose / compass pad can be found in AC 150/5300-13, in Appendix 6.


How can the same runway have two different gradients? In an unrelated answer rbp linked to this Atlanta airport info page, and runway 8R/26L caught my eye. If I understand correctly, these two runways are the same piece of pavement, but refer to the use of the runway in opposite directions. According to that page, 8R has a gradient of "1.0% DOWN," but 26L has a gradient of "0.4% DOWN." It seems like a physical impossibility - if one is sloped down, the other must sloped up, right? So what am I missing?

The slope of a runway isn't always constant. In some cases, for example, if you stand at one end of the runway, you can't see the opposite end, because the runway goes up then down.

In this case, the ICAO (in the airport design manual) computes the runway slope as follows: the runway is divided into four equal segments. The average slope of each segment is computed and the total slope follows this formula:

[(slope of 1st quarter) + 1.33*(slope of 2nd quarter) + 2.33*(slope of 3rd quarter) + 3.33*(slope of 4th quarter)] / 8

The runway slope has a greater influence when the aircraft has a greater speed. Therefore, the slope at the end of the runway is given a greater weight.

In the case that you mention, if you walk from the west side of the runway to the east side of the runway, you will go up a little, then down, and end up at a lower altitude than when you started.


Flight Level Change or Vertical Speed - When to use it? I've heard that some pilots have this difficulty to determine the correct use of both features. So, which is the difference between using the vertical speed, instead using the flight level change during a descent? When, and how to use it?

FLC mode maintains airspeed during a climb or descent, while VS mode maintains a specific vertical speed.

Often air traffic controllers will request that you "maintain 250 knots in the descent" or something to that effect, which is much easier to achieve when using Flight Level Change. As another answer points out, by maintaining airspeed, climbs are made safer as the autopilot will not attempt to maintain a rate of climb regardless of whether or not the airspeed is safe.

FLC mode is typically only available in advanced autopilots, and as far as I know they need to connect to an Air Data Computer to get the necessary information.

How to use FLC would depend on the equipment available to you:

aircraft with auto throttle will maintain speed and an appropriate rate of descent automatically.
for aircraft without auto throttle you'd set your target altitude, enable FLC, adjust your target speed if needed, and set your throttle for climb or descent to maintain the desired altitude change rate.

I'm referencing the Garmin GFC700 as it's the only autopilot I've used with FLC mode, but I imagine the same steps and rules would apply.


When does the ATC assign runways to aircraft arriving at an airport? I want to know how far from the airport runways are assigned. Approximately how many nautical miles away or typically how many minutes before landing would the pilot know which runway they'd be landing on? I am specifically interested in large busy airports like DFW, ATL, ORD, etc. Does it vary a lot based on weather and traffic conditions?

While exact distances & times vary depending on which airport and what runway configuration you're working with, the most reliable answer to the question is, usually on initial contact with the Approach controller.
As an example, here is the SITTH2 RNAV arrival into Atlanta (pdf link):

Inbound to ATL, you'll be cleared for that arrival, and you'll know that regardless of the runway assignment, you can plan to cross SITTH at or above 14,000' and TIZZY at 12,000'.  But as far as runway assignment, you're making a best guess.  Coming in from the southeast like this, the farthest south runway in ATL, runway 10, would be a fair guess, or the outboard of the 9/27 runways (so, 9R -- since they tend to arrive on the outboards, 9R/27L and 8L/26R, and depart on the inboards) if traffic is light. (Airport Diagram for ATL)  But that's not certain until you're talked to Atlanta Approach.
The approach control is the facility that builds the plan to distribute the inbound aircraft (which they can see hundreds of miles out, along with projections when each one will arrive at the "cornerpost" like TIZZY), and they'll have the plan that Delta 123 will get runway 10, Delta 456 will get 9R, American 789 will get 10, and Spirit 987 will get 8L.  And so on initial contact with Approach, the crew will be told, typically, to "Descend via the SITTH2, runway 9R".  At this point, the crew can update (if necessary) the runway in the FMC, review the approach for that runway, and check that the points in the FMC match what the chart shows  (i.e. after TIZZY, going to FRYES and following points for 9R or 10, or to GAASS etc for 8L).
It's not impossible, but it's uncommon for Approach to subsequently change the runway after this, although a crew can always ask.  (The exception to this is Denver; they seem to rebuild their plans every few minutes.)  If there isn't a lot of traffic, ATC may be able to accommodate a crew's request.  In ATL, there isn't a big difference getting to the gate from the north or the south complex; at DFW, it can be a significantly longer taxi, so there are more requests there for "request east side if we can" to try to get a runway corresponding to the terminal where they're parking.
Once the aircraft is on with Tower, it can be possible to switch runways; this is very dependent on traffic and happens only in visual conditions.  This would mostly be swapping to an inboard from an outboard runway, rather than from a north-complex to the south-complex (or vice-versa).  Atlanta doesn't do this much; in Phoenix, it's pretty common, since the inboard runway on the south side of the airport (25R / 7L) is closer to the south-side gates & doesn't involve crossing a runway to get there, as compared to the outboard runway (25L / 7R, which is typically assigned for inbounds to PHX from the south).  If Tower doesn't have anybody getting close to the inboard runway for takeoff, they may offer it to somebody on final.  And the crew can ask -- although if there's somebody who's approaching the runway for takeoff, the answer is likely to be "no."


How can I use the "control hinge-moment coefficient due to tab" for sizing a trim tab on an elevator? I calculated the control hinge-moment coefficient due to tab by using the dimensions of a tail and its elevator. However, the trim tab dimension parameters in the calculations were approximated values corresponding to similar aircrafts' tail geometries. Thus, what are the next steps to find the precise trim tab dimensions, and what is the role of the control hinge-moment coefficient due to tab in sizing a trim tab?

Notes:

the dimensions of the tail and elevator are precise and could not be changed
hinge-moment coefficient due to elevator is known

The trim tab is sized first, and then the tab hinge moment is computed. The trim tab is sized to the conditions required for maximum trimmed position of the aircraft at:

low speeds;
all relevant CG positions;
all relevant flap settings;

The contribution of the trim tab to the elevator hinge moment is tiny, see the right hand graph which is measured data from the F-27.


Is it possible to use variable wing geometry on a fifth generation stealth fighter? Is there any effect on the stealth features due to variable wing geometry? If not, why have such aircraft not been designed till now? Please explain the limitations using any structural principles if possible.

Of course it is possible. But it does not improve the outcome.

Swing wings were a fad of the Sixties. They never fulfilled their early promises, because the actuation mechanism is heavy. Variable sweep is only justifiable when the aircraft will fly both sub- and supersonically for extended periods. It turned out that the part of most missions which is spent at supersonic speeds is a lot smaller than anticipated in the Sixties. Just take the Rockwell B-1: Designed for supersonic penetration (B-1A) it became a subsonic low-level terrain following strike aircraft (B-1B).

For stealth, edge alignment is an important design principle. Making wing sweep variable is to throw this overboard, compromising the whole strategy of stealth. Also, the best stealthiness is only possible in subsonic flight - airframe heating at supersonic speed will broadcast the aircraft's presence to everyone with a low-cost infrared sensor.