During VMC conditions in terminal airspace (class B), does the pilots have to strictly follow separation standards issued by ATC? Is it common for pilots to breach separation standards in terminal airspace while the conditions are VMC? I'm interested primarily in commercial aircraft that have scheduled operations in major airports.
I know there are Loss of Separation events reported to FAA - is that a small subset of actual incidents? Are these reports predominantly for during IMC conditions?

There are several different separation standards which ATC attempts to maintain between aircraft. Note that only one of the standards below needs to be maintained; if one is valid that is sufficient. I'm answering for USA-land as you mention the FAA in your question.
Vertical Separation
Aircraft may be separated vertically. If one aircraft is directly above a point on the earth and is at an altitude of 4000 feet, and another aircraft is directly above the same point at an altitude of 6000 feet, they are separated.
IFR aircraft must be separated from other IFR aircraft by at least 1000 feet. At or above FL290 this increases to 2000 feet if either aircraft is not Reduced Vertical Separation Minima-capable (RVSM), and above FL410 the minimum is 2000 feet no matter what (and sometimes more for certain military or supersonic flights).
Within Class B, Class C and TRSA airspace, IFR aircraft are separated from VFR aircraft by at least 500 feet. This also applies to known VFR aircraft operating in the associated Class C outer area.
Within Class B and TRSA airspace, VFR aircraft are also separated from other VFR aircraft by at least 500 feet. VFR aircraft must receive an explicit clearance into Class B airspace, allowing them to be separated from other VFR aircraft. In the case of a TRSA this only applies to aircraft requesting radar services.
Lateral Radar Separation
Vertical separation is not terribly interesting. Let's instead look at lateral separation, where the two aircraft are at the same altitude, so it would not be good if they were also above the same point on the earth.
The absolute bare-bones basic standard of separation is: Did the planes hit each other? This is a highly undesirable situation. For VFR aircraft operating in Class E/G airspace, this is the only minimum that matters, and pilots will apply it themselves using the "see and avoid" principle: They see other aircraft and maneuver to avoid hitting them. This is also the only separation standard that ATC maintains between two VFR aircraft in Class C/D/E airspace, and in fact separation isn't truly lost until the planes trade paint. (Then it's a bad day for everyone.)
At a slightly higher level there is target resolution. This can be applied between VFR-VFR or VFR-IFR aircraft operating in any type of airspace, except in Class Bravo when one aircraft weighs more than 19,000 lb or is a turbojet. Target resolution means the controller issues instructions, traffic advisories, and safety alerts to ensure that the radar targets on their screen are "resolved," which is to say they do not ever touch each other. There can be some error in the positioning of the targets on the screen but if two aircraft are very near each other the difference between the two errors is likely to be minimal—so if the displayed targets never touch, you can be sure the aircraft themselves certainly don't.
Getting into defined radar separation:

In Class Bravo airspace, VFR aircraft are separated from IFR/VFR aircraft weighing more than 19,000 lb by 1.5 nautical miles.
In the terminal environment (i.e. TRACONs, and enroute facilities at FL230 and below where certain requirements are met) IFR aircraft are separated by 3 NM. This can be reduced to 2.5 NM on final approach, subject to various extra rules and procedures.
If the aircraft is more than the prescribed distance from the radar antenna, or if the system indicates a need, or above FL230, IFR aircraft are separated by 5 NM.
At or above FL600 separation increases to 10 NM between IFR aircraft.

Passing or Diverging Separation
Terminal facilities are authorized to discontinue the use of radar separation and fall back on target resolution if aircraft are on tracks which will never meet (either because one target has passed the projected track of another, or if they are opposite-direction because the two targets have passed each other). Enroute facilities can do this as well but have more onerous rules.
Wake Turbulence Separation
Smaller aircraft operating behind heavier aircraft (defined based on maximum takeoff weight) are afforded increased separation, ranging from 4 to 10 NM instead of 3 or less.
Non-Radar Separation
Almost invariably, using non-radar separation will provide a greater actual distance between aircraft than using radar separation, so it is not usually applied in areas of radar coverage. For myself I don't know much about it, having never applied it.
Lateral
Aircraft may be separated by assigning them to fly on different airways, or different radials or arcs of navigational beacons, or other such procedures.
Longitudinal
Aircraft operating along the same airway, or at the same airport, may be separated based on time and speed.
Visual Separation
Airborne separation may be reduced below the applicable minimum if visual separation is used. Some other form of approved separation must exist before and after the application of visual separation, and visual separation is not allowed above FL180 (i.e. in Class A airspace) or if the leading aircraft is a super weight class.
Pilot-applied visual separation may be used if the pilot of one aircraft reports another aircraft in sight and confirms that they will maneuver to maintain their own separation (i.e. they don't hit). Tower-applied visual separation may be used if the tower controller can see both aircraft and issues instructions to ensure they don't hit.
Visual separation is not prohibited if the weather is below VFR minima (IMC) but will likely have limited usefulness because some other form of approved separation must exist once the pilot or controller loses sight of the aircraft. Some facilities have it written in their local procedures that tower-applied visual separation is assumed to be in effect whenever the weather is better than certain minima.
Runway Separation
Not as relevant to your question but provided for completeness. Runway separation is flight-rules agnostic; the controller will apply it regardless if a flight is VFR, SVFR, or IFR. Based on the category of aircraft (category being defined in relation to engine type and number, and for single-engine propeller planes also weight) and type of operation (arrival-arrival, arrival-departure, departure-arrival, departure-departure) ATC will ensure 3000, 4500, 6000 or "the entire runway's worth" feet between aircraft operating on the same runway. They will also ensure an aircraft or vehicle does not cross a runway another aircraft is operating on.

So to finally answer your question: Pilots do not apply "separation standards" in any type of airspace, except that they are required to avoid hitting other aircraft if they see them in time. Instead, ATC applies separation minima which change based on the type of flight rules, type of aircraft, and airspace in which the aircraft is operating. In order to make sure these separation minima are maintained, pilots are required to comply with ATC clearances and instructions, or else have a really good reason why not.


Why are pilots deemed unfit to fly after emergency ejection? There are many instances where pilots need to eject out of the cockpit due to emergency situations, and after such an ejection from some modern aircraft, pilots are deemed unfit to fly airplanes for some years. Why is this done? What is the minimum number of years after which a pilot is deemed fit to fly again?

Ejection Seats are not a free ticket out. They are incredibly violent and rough on your body. This newspaper article has a more chilling quote from an interview:

  About one in three will get a spinal facture, due to the force when the seat is ejected - the gravitational force is 14 to 16 times normal gravity and it might be applied at 200G per second. Bruising and abrasions are typical from the shock of the chute opening or the air blast. In the early days, there were cases where pilots would eject into very-high-speed air and it would whip their arms behind and break them, pop their shoulders out; same thing could happen to the legs. Source

Hence, it is very possible to get back pains and a host of other problems as a result of ejecting. Since these type of things are not that easily reversible, you'd rather take the safe path and remove then from the cockpit than put then back in a work environment that known is pretty hard on your body. Aircrew seem to agree:

  “It was the most violent thing I’ve ever felt in my life,” says one of the B-1 crew members, whom the Air Force asked me to identify as “Captain IROC.” “I lost a full inch in height,” because his spine absorbed such tremendous G-forces. Source

Modern ejection seats are however increasingly intelligent and will gauge the ejection force applied to the conditions, cutting down the number of serious cases.

This paper discusses the symptoms of four individuals who crashed their jets in mid-air. The medical problems encountered with ejection can be classified as follows:

  Injuries from the emergency that causes ejection—fire or collision.
  Canopy jettison: burns from “MDC splatter” and cuts from fragmented plastic. For these reasons, aircrew are always advised to wear their visors down, to protect the face.
  Firing of ejection gun: spinal injuries.
  Entering airflow: wind blast may cause lung damage; seat tumbles at variable speed, which may be as high as 180 rpm. (All seats have a drogue parachute or deployable aerodynamic panels to prevent tumbling); flail injuries to extremities.
  Parachute deployment: snatch injuries.
  Landing: lower limb injuries.


What is that yellow square to the right of the main entry door of this Lufthansa A350? My question is: is it just part of the paint job, or does this yellow square serve a purpose? I haven't seen this square on any other airlines.

It's a yellow square that says Willkommen / Welcome. In one case I have seen that additional text was written into the square with a black (removable?) marker.

Source
This website explains it like this:

A yellow welcome panel painted beside each of the passenger boarding
doors is now visible on each aircraft. The mark is a flat yellow
rectangle, with the word 'welcome' in both German and English,
accompanied by the iconic logo in the corner. What I love so much
about this - is that that bright yellow warmth is presented to every
passenger, right as they board every aircraft. Upon boarding, each
passenger is welcomed, setting the stage for the onboard experience
and hospitality that will follow. The crew can write personalised
messages to give a more human welcome to this otherwise mechanical
setting.


How do passengers board the planes on this airport? I found a quite "rare" layout on Munich airport. It has jetbridges but I don't see any terminal. Look:

How do passengers make it to this point of the airport? By underground?

They are specially designed apron jet bridges. It looks like the normal gates but required to transfer passenger by bus.

A model shows how it runs (credit) :


Can an airliner complete a flight with one pack inoperative? I was travelling as a passenger on a 747-8. Around 5 minutes after we left the gate, the captain said we had to return to the gate due to "technical issues with the air-conditioning". The mechanic troubleshot the problem for around 45 minutes, during which the air-conditioning was completely off (but the fans were still running), and it was getting hotter by every minute. Finally, the announcement came and told us the problem could not be repaired on spot. We had to switch plane to continue the journey.

Most airliners have two "packs": left and right. They provide air pressurization and temperature control to the cabin.

Is it feasible to complete an entire flight with one pack inoperative?
If #1 is true, is it legal to takeoff with one pack known to be inoperative? (i.e. does the Minimum Equipment List allow this?)
If one pack fails shortly after takeoff, would the pilot turn around and land, or continue to the destination?
What alert message did the pilots likely got in their cockpit in my case?

The 747-8 has three packs, and it is feasible to complete the flight with one of them inoperative (in case of aircraft with two packs, cabin altitude can be maintained win any one). The following is taken from the Minimum Equipment list of 747-8:

Screengrab from 747-8 MEL

As can be seen, flight is possible with some limitations. In case of a failure in-flight, the flight crew would follow the applicable checklist and decision will be taken whether to continue the flight or not (as altitude is limited). In case of failure, the appropriate warning message will be displayed (the pack, sensor, valve etc.)


Does RTCA/DO-254 apply to power electronics? DO-254 provides guidance for the development of airborne electronic hardware and it is recognized by the FAA in 2005 via AC 20-152 as a means of compliance for the design of complex electronic hardware.

Is DO-254 only applicable for electronics that perform some kind of 'logic' operations or is it also intended to be used to prove compliance for non-logic electronics, e.g. like power electronics.

Some may argue that power electronics as 'simple' but I believe todays electronic designs have to be considered fairly complex.

The description of AC 20-152 answers your question:

  This advisory circular (AC) applies to manufacturers and installers of products or appliances incorporating complex custom micro-coded components with hardware design assurance levels of A, B, and C. These complex custom micro-coded components include application specific integrated circuits (ASIC), programmable logic devices (PLD), field programmable gate arrays (FPGA), or similar electronic components used in the design of aircraft systems and equipment. This AC provides a means (but not the only means) to gain Federal Aviation Administration (FAA) approval by showing the equipment design is appropriate for its intended function. Further, using this AC will help you satisfy airworthiness requirements when these types of electronic components are implemented

"Power Electronics" is a term so broad as to be nearly meaningless, but it doesn't matter: If the devices in question are designed to be installed on an airplane; have a DAL of A, B, or C (a failure would be Catastrophic, Hazardous, or Major respectively); and incorporate ASICs, FPGAs, or other circuitry with custom microcode then the FAA's guidance indicates you should comply with DO-254.

Note that anything you intend to install on an aircraft needs to demonstrate to the FAA's satisfaction that it is not hazardous to safety of flight in both normal and failure conditions, and that it does what you say it does.
Documented compliance with DO-254's guidance is one acceptable means of demonstrating the first of those elements (by showing robustness in the design, and graceful failure modes), though it is not the only means. (In other words if you're building a panel-mount cell phone charger or something equally ridiculous nothing indicates that you must comply with DO-254, but the FAA may ask you to comply if they feel your device is such that DAL A, B, or C is appropriate.)


Why are 4 blade propellers often in X or + position when the aircraft is parked? Is there a meaning to the position of 4 blade propellers on parked warbirds?
A lot of old pictures have them at X or + positions.

I have operated twin engine aircraft with four bladed propellers. While not warbirds, the following should illustrate some of the reasoning that goes into positioning aircraft propellers. Each type and situation could vary.

In our operation we have two positions in which we place the blades when the aircraft is parked. The positions are the x and + position you mention, and we will choose the most appropriate one dependent on the situation.

The + Position

When we park such an aircraft on a ramp we may want to secure the propellers so that they cannot turn in the wind. One common method of securing the propellers involves placing the propeller in the + position and securing the bottom blade with a cuff that is tied off to the sides with a bungee.

These propellers are bolted to PT6 series engines in which the propeller section (power turbine and gear box) is free-turning. Securing the prop helps ensure that the gear boxes do not turn without proper lubrication, and helps reduce wear.

The x Position

When the aircraft is parked in a hangar or on a ramp with the propellers unsecured, which is more common for our operation, we want to place the propeller blades in the x position.

This is to put the blades in a position which poses the least risk to people moving about the aircraft. The height at which the blades would sit in the + position would typically put the blade tip right at head level, a dangerous position for a relatively sharp and low visibility aircraft part. Conversely, placing the blades in the x position places the upper blade tips out of the way and the lower blade tips down near knee level.


How are lavatory smoke detectors connected to the flight deck? How are the smoke detectors in aircraft lavatories connected to the flight deck so the crew there can see the warning alarm? Do they have wires all the way from the lavatory to the flight deck, or are they connected in some other way?

Data Bus

A quick Google search suggests the general approach is to minimise wiring by using a data-bus approach to carry information from multiple sources along a minimum number of wires

See ARINC 629 Data Bus Standard on Aircrafts

  In earlier analogue avionic systems the number of cables used to transfer information between the various
  system components was considerable. With these systems, at least one pair of wires has been required for each signal
  and so a typical installation requires several pairs of wires. With the equivalent digital systems, all the analogue signals
  are converted into their equivalent and are assigned unique address labels to ensure there are no conflicts. These
  signals are then transmitted down a single pair of wires, which makes up a data bus. Aircraft data bus systems allow a
  wide variety of avionics equipment to communicate with one another and exchange data. The type of language used on
  an aircraft data bus is known as the protocol. There are currently different data bus standards (protocols) that currently
  account for most of the avionics data interchange on today’s aircraft, and these are: ARINC 429, ARINC 629, MILSTD
  1553, MIL-STD 1773, CSDB and ASCB.

I imagine that smoke detectors are just one example of the types of sensor that are integrated into such data-bus systems.

I expect there are several data-bus systems for redundancy and perhaps to isolate different categories of data.

Further reading

ARINC 429 as used on  A310/A320, A330/A340; 727, 737, 747, 757 767; MD-11 etc.
ARINC 629 as used on 777
AFDX as used on A380
Smoke Detectors interfaced by a safety critical aircraft
based CAN-Bus Network


Why are the 747-8 engines placed differently than on the older versions? On the 747-200 the engines were tilted up to "meet the air at a streamline angle":

Source: Wikimedia Commons

Wikimedia Commons

Source: RcGroups.com

But in the 747-8, the engines are not tilted up:

Source: Airways News

Does anybody know why the engines are not tilted up anymore?
Thanks!

On the 747-8 the engines are heavier and mounted farther ahead of the wing. This results in a torsion moment which reduces the incidence of the wing. As a result, even the unloaded wing will have a lower incidence than that of the older 747-200.

Boeing 747-346 of Transaero (picture source). Note that the engines are no longer tilted up (and consequently the wing tip is tilted leading edge down). The lift force twists the wing such that the incidence in flight is lower than when unloaded and on the ground, especially when the high-lift devices are extended.

Also please make sure to read @Lilibete's answer. I don't know who thought it appropriate to downvote it - it contains another reason for the change in inlet angles, namely the difference in avoidance strategies for foreign object damage.


Can instruction be given during a flight conducted under 14 CFR 135? Let's suppose that a flight is being conducted under 14 CFR 135. The airplane is certified for single pilot operations, but two pilots who are both rated as PIC on the 135 certificate occupy the pilot seats. One of the pilots is a CFI. Does anything in 14 CFR 135 prevent the CFI from giving dual given time to the PIC while he flies?

It took me a little bit to figure out the underlying reason for asking this question and it has to do with logging flight time.  For example, in a single pilot operation with an autopilot, the second pilot is not required and therefore cannot log the flight time at all even though their company may want them there.  The FARs don't allow the second pilot to log the time.

The regulation that governs logging of flight time is 61.51.

  61.51(h) Logging training time. (1) A person may log training time when that person receives training from an authorized instructor in an aircraft, flight simulator, or flight training device.
  (2) The training time must be logged in a logbook and must:
  (i) Be endorsed in a legible manner by the authorized instructor; and
  (ii) Include a description of the training given, the length of the training lesson, and the authorized instructor's signature, certificate number, and certificate expiration date.

So the question becomes what training was given during the revenue Part 135 flight and were you an authorized instructor?

The term Authorized Instructor is only found in Part 61.

  61.1. Authorized instructor means—
  (i) A person who holds a ground instructor certificate issued under part 61 of this chapter and is in compliance with §61.217, when conducting ground training in accordance with the privileges and limitations of his or her ground instructor certificate;
  (ii) A person who holds a flight instructor certificate issued under part 61 of this chapter and is in compliance with §61.197, when conducting ground training or flight training in accordance with the privileges and limitations of his or her flight instructor certificate; or
  (iii) A person authorized by the Administrator to provide ground training or flight training under part 61, 121, 135, or 142 of this chapter when conducting ground training or flight training in accordance with that authority.

Since it is a Part 135 revenue flight I would believe the CFI would have to comply with section 135.338 and 135.340.

  135.340(a) No certificate holder may use a person nor may any person serve as a flight instructor unless—
  (1) That person has satisfactorily completed initial or transition flight instructor training; and
  (2) Within the preceding 24 calendar months, that person satisfactorily conducts instruction under the observation of an FAA inspector, an operator check airman, or an aircrew designated examiner employed by the operator.

If the authorized instructor has complied with 135.338 and 135.340 they are also bound to the training program for the 135 company.  If that training stipulates you can give dual given during revenue flights than you are more than welcome to do so.  If there is no provision in the training program to provide instruction during revenue flight then it should not be done.

The best option is to request a legal interpretation on the validity of what you are wanting to do.


Which aircraft type flies the most annual passenger-miles? Which aircraft type flies the most annual passenger-miles?

I understand there are over 4,000 737s in service. However, there are over 1,300 777s in service.

These each carry more passengers, and presumably spend more time in cruise, where miles are clocked up fastest. Is it possible the 777 flies the most annual passenger-miles?

Some heavies (747, 777, A380) fly short- to medium-haul in Asia and the Middle East. You could say they're stealing jobs from 737's and A320's.

Anyway, what I could find was for the US only.

Checking the Revenue Passenger Miles Total by Aircraft Type for 2016 on transtats.bts.gov we find—

All 737 variants: 153.2 billion
All 777 variants: 38.3 billion

Expanded:

Boeing 737-100/200         18,374,213
Boeing 737-200C                58,536
Boeing 737-300          6,609,070,471
Boeing 737-300LR              574,860
Boeing 737-400          1,484,593,783
Boeing 737-500            324,819,083
Boeing 737-700         44,485,736,725
Boeing 737-800         69,014,076,197
Boeing 737-900         18,203,949,655
Boeing 737-900ER       13,058,102,746
Boeing 777-200-        32,808,990,997
Boeing 777-300-         5,466,846,693

Top types with at least 10 billion pax miles:

Boeing 737-800                  69,014,076,197
Airbus Industrie A320-100/200   48,907,277,978
Boeing 737-700/700LR            44,485,736,725
Boeing 777-200ER/200LR/233LR    32,808,990,997
Airbus Industrie A321           28,492,821,887
Boeing 757-200                  26,182,350,221
Boeing 767-300/300ER            24,176,619,545
Airbus Industrie A319           22,087,734,671
Boeing 737-900                  18,203,949,655
DC9 Super 80/MD81/82/83/88      15,379,739,818
Boeing 737-900ER                13,058,102,746
Airbus Industrie A330-200       10,556,831,804

From this top list:

Single-aisle: 285.8 billion
Widebody: 67.5 billion

(Narrow- vs. widebody above is just in case you are wondering. Arbitrarily selecting top-x won't reveal much.)

Boeing's outlook for the future based on past- and current-trends sees the single-aisle market outgrowing the widebody.

I've tried to dig deeper, but it seems some 260 governments don't openly share those statistics, we'd need to hire an analyst firm for a worldwide view.


Could improper deplaning of a flight with full fuel tanks cause the plane to tip? Reading this article regarding an Alaska Airlines 737 Flight that had to turn around back to the gate. There was a following statement:

  At the gate, passengers were unloaded five rows at a time, to minimize the risk that the plane would tip under extra weight of fuel.

My general understanding is that the fuel tanks are usually placed near the COG (wings, cargo tanks). Assuming the tanks are still full, why would unloading of passengers all at one time possibly cause the plane to tip? Or is this bad reporting?

The 737-900ER has (two) auxiliary fuel tanks in the aft cargo compartment, behind the wing (and center of gravity) with ~3400l capacity for increasing range (from 2800nm to 3,200nm).

In usual conditions, unloading of passengers wouldn't be a problem as the fuel would've been used up; However, as this was an overweight landing with full fuel tanks (the auxiliary fuel tanks weigh ~ 3t), the crew are just being cautious to prevent any incidents.


What are these crevices below the wing of B-52 bomber? What are these crevices below the wing of B-52 bomber?

I think that this crevice which you have noticed is the hinge line for its aileron. This might not actually be an aileron hinge line, as later model B-52s don't actually have ailerons - if this is a later model B-52, then this may just be the remnants of what the ailerons once were.
My answer is based upon this diagram of a B-52D.


Why would ASOS have a caution for Low Density Altitude? I did my initial training in a high altitude area, where it got pretty hot in the summer,  so I'm not at all surprised when I hear an ASOS announce:

  Caution: Density Altitude (a few thousand above field elev.)

But recently, I was flying in SE. Kansas when it was extremely cold outside. The ASOS announced:

  Caution: Density Altitude minus 1800 ft

I understand that cold winter air can make the air extremely dense, leading to a negative density altitude. But I don't understand the caution part of it.

As far as I'm aware, low density altitudes mean your engine generates more power, and your wings generate more lift. Everything is better with low density altitudes, right?

What is the caution for?  What should I be looking out for?

Low density altitudes mean your engine generates more power, and your wings generate more lift. Everything is better with low density altitudes, right?

The potential danger in low density altitudes is that your engine generates more power than it can handle. Even without exceeding the maximum rated RPM and manifold pressure, you can exceed the rated power of an engine under low density altitude conditions.

At low temperatures the air becomes denser. At standard sea level conditions air has a temperature of 15 degrees Celcius and a density of 1.225 kg per cubic meter.
When the temperature is -30 degrees Celcius, the density increases to 1.452 kg per cubic meter. The equivalent altitude where this density is found under standard atmosphere conditions would be about -5900 ft. This is called the density altitude.

When air with these properties enters the combustion chamber of your engine and is combusted to the normal exhaust temperature it expands (increases in pressure) far more that air normally would, resulting in excessive power.

This FAA paper on Winter Flying Tips says the following about the risks associated with low density altitudes:

  Do not overboost supercharged engines. This is easy to do because at very low density altitude, the engine "thinks" it is operating as much as 8,000 feet below sea level in certain situations. Care should be exercised in operating normally aspirated engines. Power output increases at about 1% for each ten degrees of temperature below that of standard air. At -40 degree F, an engine will develop 10 percent more than rated power even though RPM and MP limits are not exceeded.


How does a coaxial rotor helicopter achieve yaw? Without a tail rotor, how does a coaxial rotor helicopter achieve yaw control? Does a coax have to bank in order to turn?

The link goes to wikipedia's article, and down a ways it says:

  Rotational maneuvering, yaw control, is accomplished by increasing the collective pitch of one rotor and decreasing the collective pitch on the other. This causes a controlled dissymmetry of torque.

However, I cannot understand what it's saying. AFAIK both rotors always spin at the same RPM because the mechanical engine links are built that way and it's set in stone (correct me if I'm wrong). So I can't see how one rotor makes more torque.

EDIT: related, is this kind of yaw, however it's achieved, slower or faster than yawing in a conventional tail helicopter?

In a conventional helicopter the lift generated by the rotor is controlled by the collective pitch. Raising the collective pitch increases the Angle of Attack of the the rotor blades and consequently the lift, and inevitably increases drag. The drag is overcome by increasing the torque from the engine. Vertical movement is achieved by balancing the collective pitch against the torque while maintaining a relatively constant rotor speed.

The torque from the engine pushing the rotor one way results in a tendency of the fuselage to rotate in the opposite direction. This is conventionally countered by a tail rotor or other similar device, and varying the force generated by this rotor controls the yaw.

So far, so good.

In a helicopter with coaxial rotors there is no tail rotor, but the torque issue remains. Here, it's resolved by applying equal but opposite torque to each rotor.

Lift overall is controlled by changing the Angle of Attack of both rotors. If the same inputs are applied to both rotors then the torque remains balanced.

However, the Angle of Attack of one rotor can be increased while that Angle of Attack of the second is decreased, leaving lift overall unchanged. This will increase the drag on the first rotor, and hence the torque required to maintain rotor speed.

Simultaneously, the lift, drag and torque on the second rotor has been reduced, producing an imbalance in the direction of torque between the two rotors, and a consequent yaw moment around the axis of rotation.

All this can be achieved without a significant change in rotor speed. In fact, the exact rotor speed doesn't play a part in yaw control.


How does compression ratio change, if at all, from sea level to higher altitudes? As I understand it, compression ratio is the max pressure inside the engine divided by the ambient air pressure. This is for turbojets and turbofans.

Sea level pressure is about 100 kPa. An altitude of 10 km would have an ambient air pressure more like 26 kPa.

So you would think the compression ratio must drop 4x, but the thing is, does the turbine spin any faster at high altitudes to compensate?

So how does compression ratio change, if at all, from low to high altitudes?

Edit: Maybe I should have started with hard figures. What are typical compression ratios for turbojets and turbofans?

The compression ratio of the turbo machinery will not change with altitude as long as the speed of the compressor does not change (excluding Reynolds number effects). This will lower the maximum pressure in parallel with the outside pressure.

However, to create the same lift at altitude, the aircraft has to fly faster. In 10 km, where the air density is roughly 1/4 of sea level density, the airplane has to fly twice as fast. Also, air temperature drops with altitude, so the flight Mach number increases even more. And this Mach number determines how much ram compression happens ahead of and in the intake. Flying fast will increase the intake pressure level, which partially compensates for the lower atmospheric pressure. Since the pressure rises with the square of the Mach number, fast airplanes have a clear advantage here.

Take the Concorde: Its Olympus 593 had a compression ratio of measly 12, but the precompression around the intake added another factor of 7 at Mach 2, so the total pressure ratio between the peak pressure in the engine and outside air was over 80! Admittedly, this air pressure was only 76 mbar at the cruising altitude of the Concorde, so the absolute peak pressure in the engine was a much less impressive 6 bar.

Typical compression ratios

To answer the question about typical compression ratios: This correlates strongly with the age of the design: The first operational turbojet, the Junkers Jumo 004 of 1943, had a compression ratio of just 3.2 using 7 compressor stages. The more recent Eurojet EJ200 of the Eurofighter achieves a compression ratio of around 26 with a similar size and weight using 8 compressed stages, but produces almost 7 times more thrust. The main difference is better aerodynamics.

The most recent civilian engines achieve a pressure ratio of around 50 using 15 compressor stages. They can afford to compress the air more because they try to heat and accelerate the air as little as necessary during the combustion process. Since the maximum heat inside the engine is limited by the materials used, a higher compression in military engines would leave less thermal margin for combustion heating. The higher nozzle velocities of supersonic-capable engines are best achieved with less compression and more combustion heating.

Here is a plot of the pressure ratio trends for commercial transport engines (Epstein, 1998)


Why was the DC-9-80/MD-80 so successful despite being obsolete almost from birth? The McDonnell Douglas DC-9-80/MD-80 series of narrowbody jetliners was the second-to-last DC-9 major version produced (from 1979 through 1999), the second-largest, the most-produced (just under half of all DC-9s ever produced were of one of the five DC-9-80 minor versions), and the last to be powered by the Pratt & Whitney JT8D low-bypass turbofan.

By 1985, however, the JT8D was already obsolete, with the first small-to-midsize-narrowbody-sized high-bypass turbofans having entered service in the first half of the 1980s, simultaneously offering greater fuel efficiency and lesser noise than the old, outdated JT8D.  Yet, despite this, the DC-9-80 only got more successful as the 1980s wore on and rolled over into the 1990s, with the single best year being 1991, and the years from 1987 through 1992 accounting for over half of all DC-9-80s produced.

Why and how was the DC-9-80 able to be so successful in the late 1980s and onwards, despite its 1960s-era engines rendering it obsolete just a few years after it entered service?

It's simple.  Cheap (development costs amortized decades before) and reliable.  They made money for airlines.  Or, you know, they wouldn't have bought them.  There's more to operating costs than fuel burn, and in any case, fuel prices in the mid 90s were cheaper than at any time since the 20s in constant dollars.  Fuel was so cheap that Air Canada was bringing L1011s out of mothballs in the mid 90s because they could operate them Toronto to LA profitably.


How can the concept of "intercept geometry" be explained in simple terms? Can someone expand a bit for the layman with emphasis on Intercept geometry the following image depicting the envelope of a SA-2 missile against a moving Target.

Intercept geometry is the bubble of space in which the SA-2 missile can be effective against a specific aircraft. Crucial here is the detection range of the radar system guiding the missile, given here as 40 nm.

Depending on the speed of the target, the missile has only a limited time to ascend to the altitude of the target and to close in. The combination of speed and altitude of the SR-71 (1854 kt), moving at more than 30 nm per minute, will leave only a very small intercept geometry, and it has to overfly the missile launcher almost directly to be vulnerable. Once the target flies above 90.000 ft and Mach 3.5 (or employs effective countermeasures), it will be unassailable by the SA-2.

Contrast that with the fate of an aircraft flying only Mach 1 at 60.000 ft (which equals 573.6 kt): Moving at less than 10 nm per minute, its course can go past the launcher with a 25 nm offset, but it will still be in range of the Fan Song - SA-2 combination.


Why do METARs report wind relative to true north rather than magnetic? Why isn't the wind in a METAR given in magnetic heading, to match the runway numbers? Especially for areas where there is a double-digit magnetic declination.

Secondary/optional question: does the ATIS broadcast in true or magnetic?

Example: BGGH 281550Z 36004KT (an airport in Greenland, var 27.55°W).

Source: Wikipedia (click for full image)

Metars are used by more groups than just aviation.  In the U.S. it is official weather for the National Weather Service.  It is easier to graph winds in relation to true north than have them in shown in magnetic north.  The graph wouldn't make much sense.

Spoken winds are in magnetic because they are specific to aviation and that is what pilots care about.


What criteria are used to classify an airport as a satellite airport? The AIM discusses services provided to satelitte airports of class C airspace.

What determines whether an airport is considered a satellite? Distance, private or public classification,  etc?

The answer is found in §91.130 and AIM 3-2-4.

A satellite airport is any airport under the class C shelf outer are of 20NM.

  §91.130   Operations in Class C airspace.

  (a) General. Unless otherwise authorized by ATC, each aircraft operation in Class C airspace must be conducted in compliance with this section and §91.129. For the purpose of this section, the primary airport is the airport for which the Class C airspace area is designated. A satellite airport is any other airport within the Class C airspace area.

  (b) Traffic patterns. No person may take off or land an aircraft at a satellite airport within a Class C airspace area except in compliance with FAA arrival and departure traffic patterns.

What does Class C airspace area mean?

  AIM 3-2-4 Class C Airspace Sub Paragraph 3 Note 4

  Though not requiring regulatory action, Class C airspace areas have a procedural Outer Area. Normally this area is 20 NM from the primary Class C airspace airport. Its vertical limit extends from the lower limits of radio/radar coverage up to the ceiling of the approach control's delegated airspace, excluding the Class C airspace itself, and other airspace as appropriate. (This outer area is not charted.)

I believe the answer above is correct.  There is also another way of looking at it.

  §71.51   Class C airspace.
  The Class C airspace areas listed in subpart C of FAA Order 7400.11D (incorporated by reference, see §71.1) consist of specified airspace within which all aircraft operators are subject to operating rules and equipment requirements specified in part 91 of this chapter. Each Class C airspace area designated for an airport in subpart C of FAA Order 7400.11D (incorporated by reference, see §71.1) contains at least one primary airport around which the airspace is designated.

That document shows the boundaries of Class C Airspace Area and does not include the outer area.

  ASO SC C  Myrtle Beach, SC Myrtle Beach International Airport   (lat. 33°40'47"N., long. 78°55'42"W.)

  That airspace extending upward from the surface to and including 4,000 feet MSL within a  5-mile radius of the Myrtle Beach International Airport, and that airspace extending upward from 1,200 feet MSL to and including 4,000 feet MSL within a 10-mile radius of the Myrtle Beach International Airport.  This Class C airspace area is effective during the specific dates and times of operation of the Myrtle Beach Approach Control facility, as established in advance by a Notice to Airmen.  The effective date and times will thereafter be continuously published in Airport/Facility Directory.

  AMENDMENTS  03/27/97  62  FR  6698  (Added)

Since the information from the FAA conflicts with one another, I would ask for a legal interpretation.


Who opens an airliner's door first after it lands? I know this is a trivial question, but since I always sit near the back of the plane, I can't see what happens up in the front. Whose responsibility is it to open the cabin door once the aircraft has pulled up to the jet bridge?

It depends on the type of aircraft. But USA today/Ask the Captain gives a nice explanation:

  Question: I was wondering what takes so long to get the exit door open after we land. Could you please go through the steps?

  Answer: Once the airplane arrives at the gate and is properly chocked, the captain shuts down the engines and switches off the “Fasten Seat Belt” light. The anti-collision light (this is the red blinking light on the top and bottom of the airplane) is switched off, informing ground crew that it is safe to approach the airplane.

  Ground crew members then begin to position the jet bridge, aligning it with the door.  Once the jet bridge is mated to the aircraft and the cover is in place, the ground agent either opens the door or taps on it for the flight attendant to open it. This varies depending on the type of airplane to minimize the chance for an inadvertent slide deployment.

  Once the door is initially opened, the ground agent assists in fully opening it and ensuring that it is locked in the open position.

  This is the generic procedure for most jet airliners. Regional or smaller airplanes have a slightly different procedure as their stairs are built into the door.


Can a laser Doppler anemometer be used to measure airspeed? Could this be used in place of or in addition to a pitot-static system?

In place of pitot-static system

No, because it measures different physical quantity.

Laser Doppler anemometer measures velocity of the stream¹. But pitot-static system does not. Pitot-static system measures dynamic pressure and expresses it as airspeed at which that dynamic pressure would occur at sea level on an ISO standard day².

Since lift and control effectiveness depend on dynamic pressure, that is exactly the quantity the pilot needs to see. Laser Doppler anemometer does not provide it, therefore it cannot replace pitot-static system.

In addition to pitot-static system

Anything can be used in addition to what we have. But the question is whether there is a reason to.

For flying the aircraft, true airspeed is not needed and for determining the wind speed by comparing true airspeed and ground speed the airspeed can be estimated well enough from indicated airspeed, pressure and temperature.

However a laser anemometer has the advantage of being able to measure the wind speed ahead of the aircraft. This could be used for load alleviation (the system needs to know about the gusts a bit in advance so it can move the control surfaces in advance) and it could perhaps be used for detecting wind shear, though I am not sure how feasible such anemometer measuring wind speed hundreds of metres ahead is.

¹ A.k.a. true airspeed.

² That quantity is called equivalent airspeed. That is intended quantity, but the measurement is subject to various errors. The raw output of the pitot tube is called indicated airspeed and the value corrected for positional errors (due to the air-stream impacting the pitot tube at an angle) is called calibrated airspeed.


What is the use of heading bug? When I started searching for Heading Bug (other bugs like altitude, speed bugs), I didn't get much clear idea about it. Usually this is displayed on Horizontal Situation Indicator display on the direction of wind (calculated based on wind direction, speed?). This heading bug can be selected to a value by pilots during flight.

What is the use of heading bug?
how heading bug is calculated?
what's the significance of heading bug?

Bugs simply make instruments easier to read with a very quick glance.

If I need to steer exactly 162 degrees, which is easier?  To read where the needle is pointing or just see if the needle is pointing to the bug which I manually set to 162 degrees?

The bugs are set by rotating knobs on the gauges.  Here, the heading bug knob is in the lower right corner.

The way I often use it is to turn onto the next heading required according to my plan, then set the bug to that heading so that I can simply glance at the needle (still pointing to the bug, good) to keep me on course.

Other uses:

If ATC gives you a "next heading", you will read back the instruction, then you can set the bug to that next heading so that a), you don't forget it and need to call back and b) can simply turn onto the bug when it's time to turn.
Set it to the departure runway heading on take off.  To "fly runway heading", just keep the bug at the top of the gauge.
On arrival, set it to the landing runway heading.
As a "gross error" check.  You would not believe how many inexperienced pilots set off in entirely the wrong direction.  As well as setting the heading bug to "first leg of the plan", you check this along with "and the road should be on my left with the river on my right".

On some simple autopilot systems, the autopilot follows the altitude and heading bugs rather than having controls for these settings on the autopilot itself.


What does a typical discovery flight consist of? I've been considering taking up flying as a hobby and have been looking into flight schools around me. One does offer an option of a discovery flight but upon contacting them they really didn't pass along to much information on what it consist of. Could someone here kind of give me an example of what a discovery flight might consist of and help me know what to expect. Thank you!

A typical discovery flight is about a 30 minute on the ground introduction to the airplane and the cockpit controls, followed by a sightseeing flight of 30-minutes to one-hour.  The entire process typically lasts 1 to 2 hours.

During the flight, the instructor will have the controls for takeoff, navigation and landing, but may turn the controls over to the passenger for some gentle, easy maneuvers.

You can expect to ask numerous questions and have them answered, but you won't learn a whole lot about actual flying in a short session.

I would advise you to keep your questions to the flight training process, rather than specifics about flying that need more detailed explanations.

Some good questions may be:

How much ground school does it take to get a license?
How much flight time does it take to get a license?
What are some important milestones along the way to getting a license?
Will I have the same instructor throughout my training?
What supplies do I need for training? How much should I expect to spend?


How is flyblackbird.com operating under Part 91K? The website states they're not a Part 380. Love their service -- how will they avoid the same fate as Flytenow?

Long story short, Blackbird appears to be set up so that the customer leases an aircraft and separately hires a pilot to fly it. That makes it a 'straight' part 91 operation. However, the FAA has stated that pilots using Blackbird are holding out and require part 119/135 certificates (with some technical exceptions, such as 91K).

Blackbird's terms and conditions for pilots specifically mention 91 B but not K and include a few interesting points (emphasis mine):

  Pilot services are offered as an independent contractor to users under a dry lease
  The pilot is not directly employed by the owner of the airplane
  The pilot operating the flight cannot own the aircraft being rented
  Pilot hiring is at the discretion of the passenger; BlackBird will facilitate matching pilots with aircraft based on passenger
  selected routes, pilot qualifications, and location and availability
  of both pilot and aircraft
  Pilot understands that passenger (lessee) renting the aircraft and hiring the pilot accept and maintain operational control of the flight; these operational duties may be delegated to the pilot in command.

In other words, the passenger rents an aircraft from an owner and then separately hires a pilot to fly it, according to the passenger's instructions. Blackbird is saying that they only provide a marketplace, and the customers actually lease and operate the aircraft themselves.

This is from their general term and conditions:

  The Platform provides a marketplace that enables users to (a) connect
  with and lease aircraft from third parties that own, lease, or
  otherwise control aircraft (“Aircraft Owners”) and to find and hire
  flight crews (“Flight Crews”), or (b) join flights with third-party
  operators or other BlackBird users on the Platform (“Third-Party
  Operators”).

And:

  If you wish to lease an aircraft from an Aircraft Owner and / or hire
  qualified Flight Crew, then you can use the Platform to search for
  available aircraft and Flight Crew

Because the owner just rents out the aircraft without a pilot, they aren't operating an air carrier. Because the pilot doesn't provide the aircraft, they aren't holding out. That means the only pilot qualification needed is a commercial certificate, although Blackbird also requires 500hrs, an instrument rating and a background check.

Now, whether or not that business model will survive if the FAA gets interested in it is another question. There was some discussion on the AOPA forums (members only) a while ago about Blackbird and I think the consensus came down to "technically legal, but pushing their luck with the FAA and DOT".

A semi-useful comparison here might be Uber. They've always claimed that they aren't a transportation company but rather a marketplace that just connects independent drivers with customers. That argument has succeeded in some places but not in others, and many countries and cities have decided that Uber is a transportation company and started regulating them. I have no idea whether or not that will happen with Blackbird.

Update: on December 17th, 2019 the FAA made its opinion very clear in a letter to Blackbird's lawyers:

  The information that BlackBird has presented leads us to conclude that
  the pilots participating in BlackBird's platform and using its app are
  holding out and thus are engaged in common carriage.

If you're a pilot who flew via Blackbird, their closing statement isn't reassuring:

  Accordingly, please expect further investigative activity into
  BlackBird's operations, particularly regarding its pilot database. In
  addition, we would be interested in learning of any action you intend
  to take in view of the jeopardy facing pilots who participate in
  BlackBird' s service.

There's a lot more discussion online, especially in Reddit (/r/flying) if you want to read more.


How to reasonably determine minimum safe altitudes over city? (fixed wing) How does one make a determination about what a minimum safe altitude is when over a city. Let's reference FAR 91.119, the two sections that talk about this:

(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.

"(b)" is black and white, no questions there, however "(a)" is much more subjective.
Let's say I'm over a city. For the sake of being specific, I'm not talking about downtown areas with skyscrapers, I'm referring to residential / commercial mixed areas, typical "suburbia". If the city is 15 miles in diameter and densely populated, I would think (erring on the side of caution) I would consider my glide distance when determining a MSA.
If I was directly over the center of this city, that would easily make my MSA 4,000ft AGL (using a Piper Cherokee 140 in this example). Clearly this would not be the determination most others would come to as many GA aircraft fly much lower than this over cities.
What elements come into determining an MSA in a scenario like the one I have described?

What would be considered a reasonable landing spot over a congested area?
What would be considered "undue hazard?"
What other elements come into justifying an MSA?

Let's say I'm over a city. For the sake of being specific, I'm not talking about downtown areas with skyscrapers, I'm referring to residential / commercial mixed areas, typical "suburbia". If the city is 15 miles in diameter and densely populated, I would think (erring on the side of caution) I would consider my glide distance when determining a MSA.

Yes, that's what the FAA expects you to do per 91.119(a): Consider your aircraft's characteristics (glide ratio, landing distance required), your altitude, and the availability of landing areas. Select an altitude that lets you land (ditch, crash, whatever) with minimum risk to persons and property below you.

  If I was directly over the center of this city, that would easily make my MSA 4,000ft AGL (using a Piper Cherokee 140 in this example).

That is not an unreasonable determination if the area you're overflying is heavily congested (streets are full of cars/people, no parking lots, fields, or parks to dead-stick into without hitting a person/car/building).
Note, however, that your emergency landing doesn't have to be a pretty one: Light planes routinely fly through the Hudson River SFRA at low altitudes - well below 4,000 feet - and the West Side Highway or Hudson River Park are not particularly hospitable landing sites (you would be hard-pressed to execute an emergency landing without endangering persons or property). That means your "emergency landing" strategy is going to be the same as US Airways Flight 1549: You're ditching in the river. You can safely do that in a Cherokee or a Cessna from 1000 feet or less most of the time since the surface of the river is not as congested as the streets and parks of Manhattan: You can turn out over the river and be clear of people and vessels.

To address your specific bullet point questions:

  What would be considered a reasonable landing spot over a congested area?

One in which you can set your aircraft down without undue hazard to persons or property. (Yes, I realize that's very close to a circular definition.)
You don't have to make the landing pretty: you may damage the plane in the process, and you might damage some property or even kill a person on the ground in the process - sometimes you just can't foresee all the problems - but you should be looking out for landing areas that minimize the risk of injuring or killing people (priority #1) or damaging other stuff.

  What would be considered "undue hazard?"

"Undue Hazard" is a legal concept, and it will be up to your lawyer and the FAA's lawyers to determine if the hazard posed by an emergency landing you make is "undue" (excessive, disproportionate to what operating at a "normal" altitude would have imposed).
For a common-sense definition look at your operation and imagine you were standing on the ground: If you would be asking yourself "What the heck is that pilot thinking flying so low?!" then there's a good chance your altitude is low enough that an emergency landing would constitute "undue hazard" to the folks on the ground.

  What other elements come into justifying an MSA?

We already talked about the big ones: Aircraft performance & what kind of area you're flying over.
You also need to consider at least two other factors:

Pilot Skill
You know what the best short-field performance of your plane is from
the POH, but can YOU reliably execute a landing and hit those
numbers? Would you bet someone else's life on it?
Weather Conditions
What if your only landing area has a tailwind?
What if you have to fly into a headwind to reach your landing area - will you make it given your best glide speed and glide ratio?


Will I fail a check-ride if I do a go-around for a short-field landing? Let's assume that I'm on a check-ride and my next requirement is to perform a short-field landing.

If I'm coming in for landing and it's obvious I'm not going to hit my mark, can I perform a go-around and try again? Would that be considered a fail?

In general, you get one free go-around.

First of all, any checkride which involves short-field landings probably also involves demonstrating go-arounds.  If you don't do one on your own, your examiner will have to ask to see one later.

Going around when it's obvious you're not going to make your point demonstrates good judgment.  It shows that you're not going to try to force the airplane into an unsafe situation just to hit your mark.  If the runway you're landing on really is a short field, continuing the landing attempt would not only be a bad idea, it would be unsafe, and the examiner would be correct in taking the controls from you, as well as failing you.  If the runway is long and you're simulating a short field landing, the same applies.

If you go around on a short-field landing and then go around on the next one as well, the examiner is going to start to wonder whether you really know what you're doing.  Let's face it, it should not take you three tries to get properly set up for a short field landing, unless perhaps if the day is remarkably turbulent or there is a strong gusty crosswind.  If that's the case, though, why in the world did you decide to take your checkride in those conditions?


What causes this surface texture, and how does it affect flight characteristics? The picture on this answer to "What is this plane seen in Mobile, Alabama?" shows a B-52D Stratofortress with a very irregular surface texture.

(Source)

Were they like this when they were new, or is it just because this is an old aircraft that serves as a display piece? Would a surface like this cause issues with airworthiness?

I believe this is buckling of the fuselage, due to heavy compression loading.
Buckling is the process of out-of-stress-plane movement which happens under compression. The below image shows it well.

Source

More info on buckling here.

Here's another picture, possibly of the same (type of) aircraft, with the description saying:

  It appears that the fuselage contains a very heavy cargo, the weight of
  which is reacted by significant overall shear loading at the axial
  station of the landing gear. The overall shear loading is absorbed by
  the skin of the fuselage, which has developed significant
  post-local-buckling diagonal tension between internal ribs and
  stringers.

This seems plausible to me, it could be that upon a rough landing, there was excessive compressional loading exerted by the landing gear, which caused the buckling.

I'm not an expert on airworthiness, but usually buckling indicates that a part is loaded more than it's supposed to be.

Source

Other aircraft have it too, see  this question with a further explanation about buckling.


If an airline cancels an aircraft order, can a different airline acquire those options? Let's say Airline A placed an order for an aircraft a while back. After a bit of time, Airline A decides it no longer has a need for the aircraft and decides to cancel the order. Meanwhile, Airline B decides it wants to buy said aircraft but doesn't want to go to the back of the delivery line. Is there a process where Airline B can take Airline A's spot in the delivery queue? I vaguely remember seeing something like this happen before, but cannot find any sources.

In general yes, but depending on how close the aircraft was to completion they have to live with a different cabin and cockpit layout than what they are used to or would had ordered themselves.


Are parked airliners always connected to a ground power unit? I'm wondering if commercial passenger planes, like the A320, are always connected to the ground power unit (GPU) when they are at the gate? Even if for a quick turnaround.

Being connected to ground power is the usual case, but there are exceptions.  Typically, the most common reason to not be connected is if the ground power unit at the gate isn't working.  (Because aircraft use 400HZ AC current at some fairly high amperage, you can't just plug in directly to the standard electrical mains -- a dedicated Ground Power Unit is required, which incorporates some safety features so that equipment isn't damaged even if the unit is unexpectedly disconnected.)

Other reasons could be in the case of a quick turn, the pilots signal to the ground crew not to bother.  Also, if high winds prevent the ground crew from connecting the large yellow air conditioning hoses to the aircraft, then the jet's APU will be running anyway to provide air conditioning, so there's no great need to connect & later disconnect the ground power... the pilots just let the APU power the electrical system as well at this point, saving the ground crew a couple of steps & a couple of minutes.

All of this is less common than the alternative case, though, where the ground power and ground air are connected and the APU is shut off.  The air & electricity from the gate are cheaper than running the APU to provide those, so most often we prefer to turn off the APU until we get close to time to push back.


How do I get permission to fly when diagnosed with Sleep Apnea? I was diagnosed with Obstructive Sleep Apnea a few weeks ago.  I'm told that this somehow complicates my ability to get a license (I haven't started on my license yet.)  I'm having a hard time figuring out what the FAA page is trying to tell me (and what the accompanying "helpful chart" means.)

Could someone who speaks FAA style legalese give me a run down of what is being said?  I just want to know if I'm allowed to fly with treated sleep apnea, and what I need to do to make sure my medical is completed properly...

When an Aviation Medical Examiner, AME, issues you a medical certificate, that is the relevant "permission" you need. Having sleep apnea complicates that issuance process, but doesn't necessarily make it impossible. Along with what you read on the AOPA forums, I would suggest making an appointment to talk with a local AME, preferably one who has accomplished this process with other pilots before. He can give you a picture of what the process is likely to look like in your case, which is a better answer than assuming that your journey will look like that of somebody you read about on the internet.


Why does aviation use Zulu time instead of the local time? When reading aviation literature, it's common to find references to Zulu Time.

What is Zulu Time?
Why does aviation use Zulu Time instead of the local time?

Zulu means the letter Z in radio communication. The letter Z designates UTC time.

There is actually a list of time zones for each letter of the alphabet (except J):

Alpha time zone: UTC + 1
Bravo time zone: UTC + 2
...
India time zone: UTC + 9
Kilo time zone: UTC + 10
Lima time zone: UTC + 11
Mike time zone: UTC + 12
November time zone: UTC - 1
...
X-ray time zone: UTC - 11
Yankee time zone: UTC - 12
Zulu time zone: UTC + 0

However, the other time zones are seldom referenced in aviation.

Why do we use UTC time instead of local time?

Because flights often cross time zones.

Imagine this: the time now is 11:59. We will takeoff at 12:30 and fly west. Afterwards, at around 11:48, we will be 15 nautical miles away from the airport.....................Huh?

Even more confusing: We takeoff at 12:30 and land at 13:00 ==> we only need 30 minutes of fuel. Wrong! We actually cross a time zone and the actual flight time is 90 minutes. But we've taken off with 60 minutes less fuel than we need. What do we do?


Can tailwind produce reverse-thrust? I know that the answer will probably be no, and this question might sound silly, but I wanted to think outside of the box.

If engine air intake comes from the front, and is pushed out through the back, what happens if the tailwind is so strong that the opposite happens, and the air is pushed in through the back of the engine? Will it create reverse-thrust? Is this dangerous?

Only when on the ground, and at the inlet end, where a strong wind from  the side and behind could influence the engine's ability to draw air in the front at low power settings.  Some engines have limitations or restrictions on how power is applied during takeoff in strong tail or quartering winds, because of initial flow disruptions at the front from air having to make a sharp corner into the intake, where large thrust increases could cause compressor stall/surge.

You might have a procedural requirement to advance power to some intermediate setting, then let it stabilize there before going the rest of the way to takeoff thrust. The GE CF-34 has limitations like this, where winds above a certain speed and off angle a certain amount require a staged application of thrust.

Once you're in the air, there is no such thing as a tailwind as far as the airplane is concerned except insofar as tailwinds add to ground speed.  For the engine, it's all coming from the front as soon as you are moving with any significant airspeed.


How can I design and build an aeroplane from scratch? Let's say I want to build an aeroplane completely from scratch (not downloading some blueprints on the internet, I mean really designing it myself). The goal is to create a flyable aeroplane from scratch, that can safely fly.

What would I have to take into consideration and where are good resources for it? I don't want to depend on a wrong formula, and have the wing break off the aeroplane in mid-flight, because there was an error in the equation. So what resources are free (or cheap) and trustworthy?

I know, that the aeroplane should be reasonably aerodynamic and the wings should create lift. How do you test it, when you don't have a wind tunnel?

How can you compute the necessary wingspan and wing area and how do you test and analyze your design? They don't have to be that big, as you can see with the "jetman", who has a relatively small wing strapped to his back.

What programs would you use to run simulations? Are there free alternatives, that could be used instead and are they any good?

The design phase shouldn't cost much, except time. However, if there have to be certain precautions taken that cost something, that's OK.

What materials should be used (fiberglass, wood, carbon fiber, metal)?

How do I get the proof of airworthiness (on a design)? After this is done, I might actually go and build it myself.

Take the example of Ed Swearingen: He never went to engineering school, but spent years working with airplanes, gradually moving from handling to repair to improving them. In the end, he was a successful self-employed consultant who helped with a number of airplane designs and created some of his own designs, too.

So here is what I propose to do:

Hang out at a local airport (ideally one with lots of hardware passing through the local repair shops)
Don't shy from getting your hands dirty and from asking questions until you are satisfied with the answer.
Try to find experienced engineers and work closely with them.

Do that for 10 years and you will be hard to beat as an aircraft designer. If you happen to attend University in Germany, be sure to become a member of one of their Akafliegs - that is the best start into an aviation engineering education.

If you have already some idea how your design should look like, build it as a model first. Models can give you valuable lessons in aerodynamics and flight mechanics.


How do I request a touch and go and then remain closed traffic? When I first contact a tower to request touch-and-go, if I want to remain closed traffic after the touch-and-go, can I say
'...tower, call sign, location, inbound for touch-and-go and remain closed traffic'?

I learned touch-and-go yesterday and I'm a student pilot in the US.

Yes you can request that on your call, generally if you don't the tower will ask how you will terminate/proceed at which point you will request closed traffic.


On which part of the descent does a pilot begin to flare? On which part of the landing process does the pilot begin to flare?

This answer is a bit dated since it applies to 747-100 and -200 aircraft in the 1990s. However, the same principles still apply, and I'm sure the same technique would work now on current 747s. Whether or not anybody uses this protocol now I do not know.

We flared the aircraft by the numbers without reference to peripheral vision. The use of outside vision was, of course, critical for runway alignment and any necessary crosswind corrections.

We flared with reference to the radar altimeter. If it was a 'talking' altimeter, it would announce the altitude of the main landing gear above the ground in the following increments: 100, 50, 40, 30, 20, 10, with the increments being in feet. If the the radar altimeter was an old non-talker, the flight engineer would read those increments.

Coming down the typical 3 degree glideslope on a stabilized approach, the nose would be 5 degrees up, and the fuel flow would be about 5,000 lbs per engine. When you heard the 100 foot call, that served simply as an alert. When you heard the 50 foot, you pulled most of the power and raised the nose 2 more degrees. When you heard the 10 foot call, you pulled the rest of the power while raising the nose an additional one degree, maybe a little more if you had the runway length to try for a real greaser at the risk of floating. Then you just waited. Touch down of the main gear would occur within 1 to 3 seconds. If it didn't occur within that time frame, you were floating and would want to reduce your pitch a degree or so.

These 'numbers' would be adjusted if you were making a non-stabilized approach, which we often did back then when conditions permitted because stabilized approaches used more time and fuel. Plus, a non-stabilized approach is more challenging and thus much more fun. As I understand it, they're frowned upon these days even to the point of ops specs requiring a stabilized approach.


What does "ATS climb gradient" mean? On a Jeppesen SID chart, the climb gradient is not standard, because it's greater than 3.3%. What specifically does the ATS (7.4%) climb gradient mean?

Above is the same SID, but from the official chart (aerocivil.gov.co).

ATS means Air Traffic Services. That climb gradient is what the ATS requires. Versus PDG (procedure design gradients), the minimum to clear obstacles.

From the Colombian AIP GEN 2.2 Abbreviations used in AIS publications:

PDG = Gradiente del procedimiento de diseño
ATS = Servicio de tránsito aéreo


When do airline pilots arrive at airport? How much time it takes to prepare a flight, to control the aircraft, before departures?

How much spare time takes the pilots, to reach a foreign airport? I never experienced a delay because of missing pilots because of road traffic (or road accidents), but I assume this is accounted in working time, so airlines will not push pilots to arrive too early.

Airline pilots generally arrive at the airport 2 hours before departure.

Typically, a pilot will gather in a meeting with other pilots flying the same flight (who he/she may have never met before; but that is not a problem as there are Standard Operational Procedures which every pilot follow). They will discuss the flight plan (including expected delays, departure / arrival routes and procedures), weather, review the loading sheet & fuel etc.

After the pilots' meeting, they will also meet the flight crew and have a brief discussion about the flight. Matters such as expected delays, transport of patient (sick passengers) needing medical attention etc. will also be brought out.

Around 30~45 minutes before departure, the pilots will board the plane. One pilot will program the Flight Management Computer in the cockpit while the other will conduct a walk-around.

There are "stand-by" pilots ready to takeover whoever is unable to report to duty at the airport. They can be anywhere and doing anything while on call, however they must be able to arrive at the airport typically 1~2 hours after receiving a call. Also, they must not consume alcohol while on call as this will invalidate their flying status.


Is it normal to be scared of flying before becoming a pilot? Are there any studies that have researched into fear of flying when not in the cockpit? The question arises from the fact that for some reason, I have found myself to be scared when on a plane as a passenger, but when at the commands the situation is totally different. This applies also to a lot of people when driving cars for example. Is this a normal human behaviour?

I think its extremely normal. The fear of the unknown is very powerful. When you are in control, you generally know what is coming and aren't surprised.

Most pilots I know have no desire to sky dive. They love the freedom of flying, but falling uncontrolled is not enjoyable to them.

When I'm not PIC, sudden drops give me that put in the stomach feeling. When I'm in control, that NEVER happens.


Do modern aircraft have the functionality for the pilots to look behind the aircraft? Basically like the functionality of a rear-view mirror in a car. It's made known that pilots have all the information, hence they might not need the feature, but my question is to know if such a feature exists in modern commercial airlines.

Some do, some don't.

It depends by the type of the aircraft and by the need to see behind the aircraft itself.

Just as an example, this is a render of a Boeing F/A-18E "Super-Hornet" cockpit:

On the canopy's frame are located three adjustable mirrors (like the one on the windshield of a car) the pilot can use to see what's happening behind him.

As already stated in comments, aircraft usually don't "power-back" and there is no overtaking like in cars (where cars from behind can overtake you just from left or right), so usually there is no need of mirrors inside a cockpit.

In a military aircraft, mirrors increase your situational awareness, but

with a wide Field-of-View (FoV), details or a distant aircraft or missile cannot be seen
with a narrow FoV, a smaller portion of the sky is visible

Just for reference, the F-16 or the F-22, both featuring a single piece canopy, don't have rear view mirrors. The pilot must turn the head as much as he/she can and look behind.


Why did the pilot in this EC135 crash change the throttle setting after encountering wake turbulence? I’m confused in this accident why the Eurocopter EC135 pilot decided to manipulate the throttle after encountering wake turbulence - I can’t think of a reason for this. Could someone please explain why a pilot would do that?

It's the second sentence on the first page of the report:

He inadvertently rolled the throttles to manual, and never restabilized the
engines or main rotor rpm.

Supplemented by (the next sentence):

In addition, he did not understand the reset procedures for the
engine controls (FADEC), and never returned the engines to FADEC control.


Can I get a pilot's license if my driver's license has been suspended? My driver's license is suspended because I got a bunch of tickets. It was the first time and I've paid the tickets. Can I still go for a private pilot's license?

The FAA has a guide on applying for a medical/student license

https://medxpress.faa.gov/MedXpress/Content/Docs/MedXPressUsersGuide.pdf

Near the end, pages 30-33. They ask for consent to review your National Driver Register.  Read those pages, see if any of it applies to you.


From this cockpit picture I cannot identify this aircraft. Any ideas? It seems a kind of hybrid for testing. The overhead panel looks similar to BAe 146, Any ideas?

That is the cockpit of the Shuttle Training Aircraft, a modified Grumman Gulfstream II. Here is higher resolution picture:

(Wikipedia)

The STA is a "highly modified Gulfstream-2 aircraft," Nickel said. The plane’s cockpit has been adapted to closely resemble a Shuttle's flight deck; the left-hand seat features the same hand controls that a Shuttle commander uses during a mission. Several passenger seats in the back of the aircraft have been replaced by a bank of Shuttle-simulating computers.

(NASA)
They could simulate the glide of the Space Shuttle by flying with the main gear lowered and reverse thrust in flight:

In order to match the descent rate and drag profile of the real Shuttle, at 37,000 feet (11,300 m), the main landing gear was lowered (the nose gear stayed retracted due to wind load constraints) and engine thrust was reversed. Flaps could be deflected upwards to decrease lift as well as downwards to increase lift.

(Texas Air and Space Museum)


Upgrading a Sport license to a Private license - is a new student license necessary to solo non-LSA aircraft? I hold a sport pilot certificate and a valid third class medical. I'm pursuing a private pilot certificate and receiving training in C172's and C182's, though I also have access to a C162. I was wondering if I can be signed off to solo in the non-LSA aircraft without going through the process of obtaining a student pilot certificate, and just getting the logbook endorsement as follows:

  I certify that (First name, MI, Last name) has received the required presolo training in a (make and model aircraft). I have determined he/she has demonstrated the proficiency of FAR 61.87(d) and is proficient to make solo flights in (make and model aircraft).
  4/25/14 J. J. Jones 987654321CFI Exp. 12/31/05

I have several solo missions that could benefit from additional power and useful load provided by the larger aircraft, but with the new student pilot certification process, I'm wondering if it's worth the hassle.

You do not need to get another student pilot certificate.  The endorsement you need is not for student pilots.

70. To act as PIC of an aircraft in solo operations when the pilot does not hold an appropriate category/class rating: § 61.31(d)(2).
I certify that (First name, MI, Last name) has received the training as required by § 61.31(d)(2) to serve as a PIC in a (specific category and class of aircraft). I have determined that he/she is prepared to serve as PIC in that (make and model) aircraft. Limitations: (optional).
/s/ [date] J. J. Jones 987654321CFI Exp. 12-31-19

The answer lies in Advisory Circular 61-65.  Version F is the latest.


Why is the surge line curved? Why is the surge line of an aeroengine curved? Why is it not a straight line?

Simply put, it's supposed to follow a constant flow/fanspeed curve but multiple stages and the mechanics of surging make it a lot more complex in practice.

  The surge line slope on multistage compressors can range from a simple, single-parabolical [sic] relationship to a complex curve containing several break-points or even 'notches.'  The complexity of the surge line shape depends on whether or not the flow limiting stage changes with operating speed from one compression stage to another; in particular, very closely matched stage combinations frequently exhibit complex surge lines... The surge line in many cases has an approximately constant value of Q/N [mass flow/fan speed].  In many multi-stage units, and especially in units which are controlled by variable inlet guide vanes, the surge line curves bend dramatically at higher flows and speeds.Source: "Practical Aspects of Centrifugal Compressor Surge and Surge Control" by  Meherwan P. Boyce (pdf)

So some of the shape comes from the curvature and irregular spacing of the constant speed lines, and some of the curve comes from adding in different stages.

Unfortunately, exact surge conditions are not easy to describe mathematically even if we know the properties of each stage in the compressor.  There are things like multi-blade interactions and boundary layer effect that make things difficult really fast.  These factors are so important because surge is not quite the same as a stall on compressor blades.  Surge is a more complex unstable compressor flow resulting from stalls. Single blade stall, which isn't observed much in practice, will not cause a surge.  Surge happens when the stall propagates across several blades due to blocked incoming air.  There aren't easy,exact, and universal equations for this complex behavior unfortunately.


What is the difference between staged and unstaged fuel manifolds? Also, I would like to know the difference between pilot, primary and secondary fuel flow.

Staged means that fuel is delivered "after launch" instead of immediately.  This terminology is more familiar from staged rockets:

  At engine idle conditions, the electronic engine control (EEC) unit sends a signal to the BSV which then shuts off fuel flow to the ten staged fuel nozzles. As a result, the ten unstaged nozzles receive a higher fuel volume resulting in a stronger flame. [More fuel for those ten and none for the others, encouraging combustion: fewer flames, but bigger ones.]  As the engine accelerates above idle, the EEC signal commands the BSV open, allowing fuel to flow to all nozzles [more than the first ten] so the engine can operate at higher power settings.

This is quite like a campfire started with tinder (unstaged, ignites instantly) and then sustained with thicker wood (staged).

"Pilot fuel flow" is hard to google in an aeronautics context for obvious reasons, but it means the same as "pilot flame" for a household furnace or water heater.  It acts like a continuous low fuel consumption spark,
at start-up and low part load, to encourage more vigorous combustion shortly before that is commanded.


Is it an acceptable practice to take off in a microburst? This video has been posted on Youtube:

Strong winds are blowing at the airfield, pilots are taking off aircraft to prevent them being damaged. However I noticed that gliders have been successfully kept on the ground.

Is that standard practice to take off and fly to try to escape a burst? Is that even allowed and/or sensible?

The airfield in the video is KAFF, the Air Force Academy air base. Everybody you see in the video are air force cadets or retired Air Force pilots (in the tows).

There is a credible thread about this over on Reddit where one commenter says:

  I'm very curious as to how you got this video, i was under the impression it never left the airfield.

  This happened in April 2014, this past year. The weather is absolute crap here, especially for soaring. Well... we get good weather sometimes. Anyway, it's not uncommon for those TG-16A's to go up with a 25 knot gust... But i digress.
  The cadets were pushing in because winds were out of limits and the weather was getting worse... and BAM! Microburst.

  This microburst hit right next to the airfield. The tower spotted it early, gave a verbal warning "look out..." and cadets are trained to do the following: grab a wing (glider) and turn broadside into the wind and put the spoilers out. The tows were not so easy... nor lucky. Their takeoff speed is about 50 knots, and none of them were powered up when they lifted off the ground, to give you an idea of how bad the wind was. Their only maneuver is to face into the wind and pray they dont actually take off. The tows that took off left for COS airport... it took another 30 minutes of holding gliders before the tower let the cadets start moving the gliders.

  As for taking off... 55 kt gusts are the highest the Academy has had in a long time. Considering there were a half-dozen other aircraft within 100 ft of the tows, along with people (i.e. cadets around/in the gliders) if he was moving too far from his position in the queue, the safest action is to get some altitude and try and leave the microburst. Or at the very least put some altitude and distance in between himself and the cadets and aircraft. You saw how slow the tows in the air were moving relative to the ground... those were HARSH winds.
  At about 0:20, you can see a cadet hanging onto the wing of a glider on the bottom of a screen. This wind was scary. I don't know if anyone was up at the time, but full tempo ops can be up to 5 tows and 8 gliders... on a standard afternoon training day 3 tows and 4 or 5 gliders is normal. It looks like they were already pushing the gliders to the hangar...

I believe the commenter is wrong in that the planes were not powered up, but everything else is accurate (relatively so, those Cub (?) tow planes can take off much less than 50kts).

From your question:

  Taking off against the wind seems impossible because where does it get its thrust? And with the wind also seems impossible because all the wind is traveling at the same speed as the airplane, which to me should make the airplane drop like a stone.

Taking off against (into) the wind is the most preferable way to get an airplane into the air. It reduces the amount of ground speed that you need in order to take off. Aircraft work according to the airflow over the wings, if the airflow is great enough and the angle of attack is right, then the aircraft flies.

Taking off with the wind is also possible, but it extends your ground roll since you need to get to the same relative speed with respect to the wind before the airplane will fly.

Look at it this way, lets say you have the wind at your back and you take off on a bike. As you gain speed there is a point at which there will feel like there is no wind since you have the same speed as the wind at your back. As you keep going faster you'll start to feel like the wind is blowing at you, but slower. The amount of wind blowing at you is your air speed, the speed you are actually going is your ground speed.

Airplanes work the same way, taking off down wind requires the aircraft to get a greater ground speed (but the same air speed) to take off. After the airplane starts moving the only thing that really matters is the speed of the relative air flowing over the wings. When somebody says they need 50 kts to take off, they mean 50kts of airspeed, or 50kts of air flowing over the wings.

Now, a little commentary. There are two aircraft that take off in the video, one goes to the left which is up wind and the other goes to the right which is down wind. The upwind pilot has a much easier time controlling the aircraft since they don't need as much ground speed to keep it in control.

The downwind pilot almost bought the farm, and I'm pretty sure when he got back on the ground he needed a change of pants and a new aircraft seat. He's having a lot more difficulty controlling the aircraft because of the low airspeed. It doesn't look like he had much of a choice to go in that direction though, so I'd buy that man a beer if I ever met him just to hear the story.

The thing to take away from this is those pilots are retired Air Force pilots with thousands of hours of experience in high performance aircraft. The average pilot would be a stain on the tarmac in similar conditions.

To answer your edited question:

  Is that standard practice to take off and fly to try to escape a burst? Is that even allowed and/or sensible?

No, it is not standard practice. Larger aircraft would probably not have this issue but the aircraft in the video all weigh less than 1000lbs, a little more with the pilots and fuel probably but not much. Pilots are trained specifically to avoid microbursts if possible because they are very dangerous things. A search of the NTSB accident database for the keyword "microburst" provides 73 accident reports with that mentioned, 21 with fatalities.

Is it allowed? No, airports will halt operations when low level wind shear is detected, however its not to say an aircraft isn't already in its takeoff run when the shear is detected. The highest priority is to stay on the ground. The aircraft in the video however don't have a choice, the wind speed they encountered is greater than their take-off speed, they were probably going to fly or tumbleweed.

CFR 91.3(b) gives a pilot full authority to deviate from any ATC instruction or rule in an emergency. An emergency does not need to be declared first, a pilots responsibility is Aviate, Navigate, Communicate. If all your attention is on flying (or keeping from flying) you can be in an emergency without telling ATC. I'm sure these pilots did not face any disciplinary or regulatory actions as the result of taking off from the parking ramp.


Why are 'ghost flights' necessary? This article details the recent (March 2020) grounding of 90% of Ryanair's fleet due to the COVID-19 outbreak.

  Flight tracking data showed that all bar one of the jets had been flown in recent days: 35 had made loops of the airport, while the remaining 11 aircraft had operated a service at least once every four days.

Why are these flights required? The article states this is to keep the aircraft 'serviceable', but the airport I'm learning at has planes that haven't been in the air for months... if they are booked for a lesson, they get a service and away we go.

Why would a budget airline want to spend so much money on fuel and flight crew for these flights, when this could continue for months, it seems like it would make more sense to keep them grounded, and then service them all when this epidemic is over.

Airliners are both larger and more complex than the small aircraft you're familiar with at your local flight school. For an example of some of the items to consider, see: What do you need to do to bring a 737 Max back in service after 6 months in storage?

A lot of those things won't be improved just by flying the plane periodically, but some items are. Tires can develop flat spots from sitting for too long. Engines and systems need to have their fluids circulated. Getting things warmed up and moving helps to prevent corrosion. Even with proper storage procedures, an aircraft would have to be thoroughly checked for any foreign debris or damage that could have accumulated.

As the article you linked states:

  Planes that have been grounded for a significant period have to be checked over before they are cleared to fly again, a process that keeps them from flying for even longer and costs the airline money.

There is of course a tradeoff between paying to put the aircraft in long-term storage, and later to bring them back into service, versus keeping them in working order. Also, as mentioned in the answer to the linked question, the flight crew need to fly a minimum amount to be considered "current" and allowed to fly passengers. Even aside from regulations, having periodic practice will help to maintain skills for everyone involved.

Having crews do a few laps around the airport helps to keep both the airplane and crew in working order, and avoids having to do more rigorous checks to get them back up to speed when needed, which can save both time and money.


How can I calculate the moment coefficient around the center of gravity? Hi guys I was wondering if you could help me with the following problem:

Wing surface area= 260m2 (sw)
MAC= 6.608m (cw)
horizontal stabilizer surface area=69.5m2 (st)
distance from the cog till the aerodynamic center of the horizontal
stabilizer=30m (lt)
change in downwash angle vs angle of attack de/da=0.5
distance from wing til aerodynamic center (hacw) = 26% of mac angle (hacw)
of the wing is 0.12 per degree (aw)
angle of the stabilizer = 0.07 per degree (at)
downwash stabilizer at an angle of attack of 0 is e0=0
moment coefficient of the wing in the aerodynamic center is
Cmacw=-0.04
CoG=31%

Calculate the moment coefficient around the CoG(Cmcg) of the entire airplane when the angle of attack is 3 degrees and the angle of the stabilizer is 1.5 degrees aircraft nose up.

I know the answer which should be Cmcg=-0.022 but I have no idea how it is calculated.

I am sorry, what i did was the following: I calculated the tail volume coficiency which is 1.213564, the position of the neutral point being it 0.61396. I am trying to use the following formula:

$C_{mcg}=C_{macw} + a_w  a_w (h-h_{acw}-{a_t\over a_w} V_h (1-{\Delta e\over\Delta a})) + a_t V_h (i_t+e_0)$

when filling in I do the following

$-0.04+a_w A_w (0.31-0.26-{a_t\over a_w} 1.213564 (1-0.5) + a_t × 1.213564(i_t+e_0)$

for aw and at I am not sure to use the 0.12 and 0.07 or do it times 3 and 1.5 for the degrees. it seems to be angle of the stabilizer but I am not sure. e0 I think is 0 but again not sure.


Are cirrus clouds stratiform? Cirrus clouds:

Are they warmer than the environment? - Cumuliform.
Or are they cooler? - Stratiform

What differences do cirrus clouds have to the "Cirro" versions of a cumulus or stratus?

All these clouds signal an approaching warm front and potentially a storm.  Warmer, moisture laden air rides over the top of cooler air, forming visible clouds while the ground level front is still many miles away.

Cirrus clouds are composed of ice crystals and form at very high altitudes.  They are the first of the approaching clouds.  Cirrostratus form as more moisture arrives and the front gets closer, followed by status.

Cirrus clouds, being the highest and first to arrive, are stratiform.
Cirrostratus are observed as broken cirrus/blue sky becomes overcast.

Cirrocumulus clouds are the result of high altitude convection and contain supercooled water.  They can transform into status clouds, and are also associated with approaching weather.  The vertical motion that creates cirrocumulus seems to be more of a transient local phenomena, such as turbulence.


How does the raked wingtip of the Boeing 787 work? The wing tips are very different on these airplanes. The wingtip on a Boeing 787 is a sharp triangle, while the wingtip on a Boeing 737-300 is flat.

British Airways Boeing 787-8 G-ZBJC wingtip | by StephenG88 (source)

Why is the wingtip on the Boeing 787 different from that of the Boeing 737?

The raked wingtip of the Boeing 787 is now "le denier cri" in wingtip design, but has been successfully employed by water fowl for many millennia. The use of composite structures has made the three-dimensional shaping of wingtips much easier, and the first airplanes to use increased sweep at the wingtip were indeed composite gliders such as the Schempp-Hirth Discus.

Seagull (left) Schempp-Hirth Discus 2 (right, source)

In the times of metal wings, Dornier introduced their TNT (Tragflügel Neuer Technologie, in English: Wing of new technology) on aircraft like the Do 228, Do 328 and the Dornier ATT flying boat. It used a triangular wingtip with no change in trailing edge sweep, which is easier to manufacture in aluminium.

An Air Alps Dornier 328-110 landing at Fiumicino Airport, Rome, Italy, viewed from Ostia Antica. Picture by Makaristos.

You can think of the raked wingtip as of a winglet which has been folded down. Increasing the wingspan will involve a bigger mass of air into the creation of lift, and the induced drag is reduced. This explains already most of the effect, but the increased sweep adds some more benefits.

Higher sweep lowers the lift curve slope. This helps to reduce the lift contribution (and, consequently, the bending moment!) of the wingtip at high angles of attack. Therefore, raking the wingtip will result in a lower maximum bending moment.
The sweep also shifts the center of pressure of the wingtip backwards, adding a negative torsion moment. This moment lowers the angle of attack over the whole outer wing in gusts and at high load factors, again helping to lower the maximum bending moment at the root.
Especially in gliders, it allows to combine a near-elliptical circulation distribution over a wide range of angles of attack with good stall characteristics. The lower lift curve slope of the wingtip lets it stall at a higher angle of attack and reduces its lift contribution relative to its area at increasing angle of attack.

This aeroelastic tailoring, together with the low fuel consumption of modern high-bypass-ratio engines, allows the Boeing 787 to use a wing of an aspect ratio of 11. Previously, - apart from gliders - only special designs for extreme flying time and range like the B-24 Liberator or the Breguet Atlantique sported such high aspect ratios. Older airliner designs had to use lower aspect ratios (between 6 and 8.5) to have enough wing volume for fuel.