Is it OK to make an approach without any straight-in segment at all? Let's say you are on a 180° heading right of the airport and want to land on runway 36. Abeam 36 you can make a descending turn left, flare and start leveling off at the same time, touch down with the left gear, level off and finish the rollout. No problem in a GA aircraft. Can this be done in an airliner, for example in a circle-to-land approach? What is the minimum straight-in segment required for circle-to-land?

If the aircraft has not established a stabilised approach, a go around is required. According to EUROCONTROL's Skybrary, a stabilised approach is defined as:

  Their Approach-and-landing Accident Reduction (ALAR) Briefing Note 7-1 suggests that "all flights must be stabilised by 1000 feet above airport elevation in IMC and 500 feet above airport elevation in VMC. An approach is stabilised when all of the following criteria are met:

  The aircraft is on the correct flight path
  Only small changes in heading/pitch are necessary to maintain the correct flight path
  The airspeed is not more than VREF + 20kts indicated speed and not less than VREF
  The aircraft is in the correct landing configuration
  Sink rate is no greater than 1000 feet/minute; if an approach requires a sink rate greater than 1000 feet/minute a special briefing should be conducted
  Power setting is appropriate for the aircraft configuration and is not below the minimum power for the approach as defined by the operating manual
  All briefings and checklists have been conducted
  Specific types of approach are stabilized if they also fulfill the following:

  ILS approaches must be flown within one dot of the glide-slope and localizer
  a Category II or III approach must be flown within the expanded localizer band
  during a circling approach wings should be level on final when the aircraft reaches 300 feet above airport elevation; and,

  Unique approach conditions or abnormal conditions requiring a deviation from the above elements of a stabilized approach require a special briefing.

  An approach that becomes unstabilised below 1000 feet above airport elevation in IMC or 500 feet above airport elevation in VMC requires an immediate go-around.

(Source: www.skybrary.com)


Why do ELT panel switches have batteries in them? Does anybody know why ELT remotes (the panel mounted switches with LEDs) have batteries in them?
The ELT itself has a big battery and there's a wire between them that the ELT could use to supply power to the remote.
So - why do ELT remotes have their own, separate, battery (in a place that's often hard to access for replacement)?
Is it some kind of regulatory requirement? Why?

The battery in the remote switch like the ACK 406 ELTs powers the LED in the remote (the ACK ELT has a separate battery for its alarm buzzer).  On some ELTs the remote's battery may also power the alarm buzzer (the one that warns you the ELT is broadcasting in case you missed the red LED) that you install near the remote (my old AmeriKing 406, now a desk paperweight on account of AmeriKing's TSO certification fiasco, had the same remote that ACK uses and it appeared to power the both the LED in the remote and the little alarm buzzer, because the alarm was wired directly into the remote with no other apparent source of power).
Some units like the ARTEX ME406 don't have a battery for the remote switch (the ARTEX unit's buzzer has its own battery), and on those units you have to either connect a remote switch power source wire to the airplane's electrical system, or if in a no-electrics airplane, you have to cobble together a separate battery power supply to power the remote's LED.
I couldn't find an engineering specification with the information, but I dimly recall that all this is because the ELT's internal battery can't have a potential load device, like a light or speaker (buzzer) connected, that could drain the internal battery if the external light/buzzer shorts out, so they require their own power sources.


What are the general post flight sequence checks and do they start automatically? If so, when? What post flight checks are performed? Are they automatically or pilot initiated. If automatic, what are the possible conditions?

For example, one of the conditions to start the postflight checks might be "aircraft wheel speed below 30-40 knots".

All checklists are initiated by either the pilot flying (PF) when between takeoff and touchdown, and the captain (CA) on the ground.  In addition to checklists is the concept of flows, which are a muscle memory sequence of buttons to press / switches to flip / things to touch that are performed by the appropriate pilot before the checklist is called for.  The flows will happen automatically when the pilot responsible knows they should happen and the checklist will be called for when the responsible pilot is ready for it.

For example, when landing, the PF will perform the landing, the deceleration and sometimes the high speed turnoff before control is transferred to the CA for taxi.  Once the CA is taxiing and the airplane is cleared of the active runway, the FO will automatically start the after landing flow and call ground control.  Once taxi is underway, the CA will call for the after landing checklist.

Once parked at the gate, both pilots will do their portion of the parking flow and then the CA will call for the parking checklist.

Finally, if no crew is waiting to receive the aircraft and you are not keeping the aircraft then the FO will perform the postflight inspection and the pilots will perform the terminating checklist.  If you are keeping the plane, the FO instead does a combined post/preflight inspection and begin normal pre-flight actions.

While these actions are all started by humans, the closest thing to automatic actions are the flows.  To summarize the above, with general criteria for starting flows:

Landed, clear of the active runway, CA has the plane -- perform after landing flow
Parked at the gate -- perform parking flow.

Each of these flows are backed up by checklists called for the CA.


Why are there so few carrier capable delta wing aircraft? When looking through various carrier aircraft, I noticed that there seemed to be very few with delta wings. While initially chalking this up to higher stall speeds necessitating higher approach speeds on supersonic deltas, I nevertheless noticed that this seemingly doesn't apply to the Dassault Rafale, nor the HAL Tejas.

Is this due to configuration? Would the addition of canards or conversion to a tailed delta permit carrier operations? Or is this not aerodynamic but more based off cost?

The reason may be change in tactics.  The 4FD was designed to intercept aircraft, and as a point defender and dogfighter, it would have been very successful in this role.

But the WW2 scenario of planes appearing overhead to drop bombs changed to planes launching cruise missiles from farther away.

Carrier aircraft need to go further out to intercept them, carrying as much ordinance and fuel as they can.  Deltas simply do not do this as well as even lower aspect straight wings, because the design does not lift as efficiently for the amount of drag produced.

Lack of stealth would add to their modern-day woes.

The F4D, in its day, possessed many of the advantages the WW 1 triplane did in maneuverability,  this time with cannon and missiles.  Like the triplane, it became obsolete.


Why is there no instrument taxi system for low-visibility operations? In one of the comments on this answer regarding ILS category IIIc operations (or, rather, the lack thereof), it's mentioned that

There is no [radio] guidance for taxiing except the pilot's vision.

And, in an answer to another ILS IIIc question, it is stated:

There is no provision in the ILS system to provide for an auto-taxi or runway vacate. IIIc is just zero-zero and roll-out control, not auto-vacate as the linked AvWeb article seems to suggest. Its a logical extension of the FAA definition, however auto-taxi is not part of the CAT IIIc definition or system requirements. The only requirement is to get the aircraft on the runway and stopped.
...
Right now there is no airport approved for IIIc approaches because the aircraft would just sit on the runway, it has no way of taxiing in. Until an augmentation system like GPS with WAAS or LAAS, or the "Ground-Based Augmentation System" is developed further, automatic taxi is not there.
There is no provision in the existing ILS signals that allow for an aircraft to vacate the runway.

Given that the ability to land is meaningless without the ability to taxi off the runway and to the gate (this is why, even though today's airliners are perfectly capable of IIIc landings, there are, in practice, no such landings, as no airport is equipped to taxi an aircraft in from a IIIc landing), why isn't there an "Instrument Taxiing System" to allow zero-visibility taxi?

If I were an airline executive in charge of determining the configuration of new aircraft being ordered from a manufacturer, and one of the options was an instrument taxiing system to allow so-called zero-visibility taxi, the aggregate cost of that system (purchase, maintenance, training, fuel to carry it) would have to be very low before I would consider it. My initial thought would be to reject it out of hand, but perhaps the ability to help a taxiing pilot navigate the taxiways of an unfamiliar airport would bring me around if the only cost was additional software for existing equipment. I say this as a pilot who on his first trip into Chicago O'Hare succeeded in getting some deservedly unkind words from a ground controller when I didn't turn when I should have and necessitated the rerouting of other aircraft.

In 10 years on the 747 I only once called for a follow-me car because of low visibility, and had the taxiway been better marked or my eye level lower I might not have needed that.

  So it wasn't worth it even for airports where crap weather is common?

Consider the Arcata-Eureka Airport in McKinleyville, CA. The airport was deliberately built during World War II where they could depend on it being very foggy, and in that they succeeded. While the approach and landing were often challenging, there was never any real difficulty in taxiing if you were familiar with the airport.


How can a pilot anticipate cloud formation? I read a few articles and a book about cloud forming, but I didn't really understand it.

Can anybody explain in simple terms how clouds form?
As a pilot, what factors that contribute to cloud formation can I anticipate?

Where do the terms Saturated Adiabatic Lapse Rate (SALR), Dry Adiabatic Lapse Rate  (DALR), Environmental Lapse Rate (ELR) and dewpoint fit into it all?

Clouds can be formed in many different ways. Before we go through them, we need to establish some basic facts.

Clouds are made up of liquid water, or solid ice. Clouds are not water vapour; vapour is a gas and is invisible. Once more, because this is important: clouds are liquid water, not water vapour.

Water droplets in clouds are extremely small. Clouds are visible because light is scattered in these droplets. The water droplets can stay suspended in the sky due to friction and air currents.

In order for water vapour to condense into liquid water droplets, to form clouds, condensation nuclei must be present. This can be tiny particles of dust or other impurities around which water droplets can form. This is where the idea of human cloud generation comes form - by adding condensation nuclei to the atmosphere, clouds are more likely to form.

A mass of air can contain a certain amount of water vapour (=gas). The maximum amount depends on several factors, of which the most important one is temperature. Cold air can contain less water vapour than warm air. If more water vapour is present than the air can contain, the water has to condense (the opposite of evaporate), to form water droplets - clouds.

So, how are clouds formed then?

By adding moisture

Air can only contain a certain amount of water vapour. If we try to add more water vapour to the air, it "won't fit". As a result, the water vapour will condense, forming visible water droplets - clouds. This can happen over a surface of water during a warm day, when surface water evaporates. The air won't be able to absorb the evaporated water, so instead it has to become liquid.

Cooling from below (advection)

Warm air can contain more water vapour than cold air. When a warm, moist airmass moves over a cold surface, the temperature of the air will drop. When reaching a certain temperature, the air becomes saturated - it contains exactly as much water vapour as it possible can. We call this temperature the dew point. If the air is cooled further, some of the water vapour in the air has to condense into water droplets. Imagine squeezing a saturated sponge - the water has to come out.

Adiabatic cooling (convection)

On a sunny day, the sun will heat up the surface of the earth. Because the surface is not uniform, it will heat up at different rates. A sandy field will get much warmer than a wet area, for example. The air directly above such hot areas will also be heated up, by radiation from the surface. Hot air has a lower density than cold air, so at some point it will start to rise ("heat rises"). As this warm bubble of air rises, it will start to expand. This is because pressure drops with altitude. As the air expands, it will cool off. Now we're back at the basic principle that cold air can hold less water vapour than hot air. As our bubble of air rises, and cools down, at some point, it will reach the dew point, and a cloud will form.

The adiabatic lapse rate is the rate at which a bubble of rising air will cool off with altitude. Rising air will cool with approximately 3 degrees C per 1,000 ft of altitude gain (dry adiabatic lapse rate - DALR), simply because it expands as a result of the lower pressure. When air starts to condense, and a cloud forms, this process releases latent heat. Simply put, it took energy to make the water evaporate, and this energy is released again when the water condenses. This means that once our bubble of rising air reaches the dew point, and a cloud starts to form, it will actually cool off slower. The saturated adiabatic lapse rate (SALR) is approximately 1.5 degrees C per 1,000 ft.

Now, for how long will our bubble of air keep rising? It will keep rising as long as the air surrounding it is cooler than itself. The general temperature of the atmosphere at a given location is known as the enviromental lapse rate - ELR or ELT. As long as the DALR is less than the ELR up to the dew point, rising air will form clouds.

Frontal lifting

A warm front is when a mass of warm air moves forwards, displacing a mass of colder air. Since warm air has a lower density than cold air, when the two air masses meet, the warm air will be forced upwards, above the cold air. This causes the air to expand, and cool. As this happens, the air might reach the dew point, and clouds will be formed.

Orographic lift

Orographic lift is quite similar to frontal lift. Instead of being forced up by cold air below, an air mass is forced upwards by terrain (i.e. wind blowing towards a mountain). Then it's the same old story - the air rises, expands, cools and water vapour condenses.

Turbulence and convergence

Mechanical turbulence, for example due to rough terrain, can cause vertical movements in the atmosphere. If these are large enough, air can be forced upwards, which, as we now know, will form a cloud.

Convergence is when two air masses move towards each other, creating a vertical stream of air in the middle. This column of air moves upwards, and clouds are formed.

These are the basics of cloud formation. As you can see, they all depend on the same basic principles that I covered in the beginning of this answer.


What is this pattern on the windshield of aircraft? The image is extracted from this Airbus A350 video (at 3m40s, when taking off)
I circled in purple the strange pattern formed by few black straight lines. I'm quite puzzled as I have no idea of what it is or what purpose it can serve.
EDIT: I would like to sum up the ideas in comments as there is interesting guess and discussions in the comments. It is a strange device which can be:

a kind of gauge useful as the windscreen seems to be a completely new one and the aircraft presented is one of the test aircraft
part of some windscreen heating device

Short answer
These are the three redundant temperature sensors for the windscreen anti icing and defogging system. The design is similar to what is found on the A330 and A340, and most large aircraft. The sensors are used to control the electric current in the heating film(s) sandwiched in the windscreen ply. Usually three temperatures are monitored:

Outer windscreen layer must be above freezing point, e.g. 1.8°C.
Film mustn't overheat, depends on film used.
Inner layer must be below some threshold, e.g. 50°C.

This video shows how windscreen heating works on an ATR42. Extract related to the sensors:

ATR42 windscreen temperature sensors and film bus bars
The heating film(s) is/are used both to prevent icing and failure of the outer panel (air temperature can be as low as -60°C) and fogging on the inner panel. This can be obtain using a single film, but sometimes there are two distinct films:

Structure of the windscreen on a Boeing 787. Source

On the A350
From A-350 Technical Training Manual:

Windows Anti Icing/Defogging Function, Description and Interfaces
The windows anti icing/defogging system is divided into two
sub-systems (F/O and CAPT). Each sub-system is comprised of one
windshield, one FWD lateral window, one AFT lateral window and a
Window Heat Computer (WHC). [...]
The anti icing and defogging function of the cockpit
windshields is ensured by the heating of one resistive/heating ﬁlm. Each
windshield's resistive heating ﬁlm is powered by 230 VAC, provided by
the WHC.
There are three temperature sensors near the heating ﬁlm that
continually send information to the WHC for the heating regulation and
the overheat protection functions. [...]
The WHC regulates the temperature of the cockpit windows (35 degrees C to
42 degrees C) based on the temperature value it receives from one of the three temperature sensors. The other two temperature sensors are
in standby mode. [...]
If the temperature of the cockpit windows reaches a specific
temperature level (+60 degrees C), the WHC will stop supplying the
specific window. [...] For each window, the WHC uses temperature
values from the three temperature sensors to detect an overheat.
The WHCs interface and exchange data via CRDCs with the
AFDX network.The two WHCs are located in the cockpit: WHC1 is located
next to the fourth occupant console behind the Captain. WHC2 is
located next to the coat stowage behind the First Ofﬁcer.

For this test aircraft:

Sensors are larger than usual.
They may be stuck on the glass instead of being built-in.

This setting might be specific for flight test purposes. Someone may bring additional elements.

Another example, on a Pilatus PC12-NG:

Pilatus PC12-NG temperature sensors and bus bars on the windscreen


Where does the Olive Branch (KOLV) class D ceiling meet the Memphis (KMEM) class B floor? I'm having an internal debate on interpreting the Class D ceiling in part of the the Memphis TAC. My understanding was in the case of a Class D under a Class B shelf, the ceiling of the Class D ends at the floor of the Bravo, or in this case: 1,800ft MSL.

However, the [29] box printed inside the area of the Class B/D is causing  confusion. Is this explicitly stating the Class D does extend to 2,900ft MSL or does it end at 1,800ft MSL as I originally thought?

The Class D airspace surrounding the Olive Branch airport extends upward to 2900 feet msl, excluding the Class B airspace.  So, your understanding is  correct.

Here is the official description of this Class D airspace from FAA Order 7400.11,  Airspace Designations and Reporting Points: (emphasis is mine)

  ASO MS D Olive Branch, MS
  Olive Branch Airport, MS
  (lat. 34º58'44"N., long. 89º47'13''W.)

  That airspace extending upward from the surface to and including 2,900 feet MSL within a
  4-mile radius of Olive Branch Airport; excluding that airspace within the Memphis Class B
  airspace area. This Class D airspace area is effective during the specific days and times established in advance by a Notice to Airmen. The effective days and times will thereafter be continuously published in the Airport/Facility Directory.

  AMENDMENTS 04/15/04 69 FR 5009 (Added)


What were the safety recommendations following the Aeroperú Flight 603 crash? I just watched the Mayday Air Crash Investigation about Aeroperú Flight 603 and two questions that weren't answered are:

Who was responsible What caused the ATC to relay false altitude information to the pilots
Has anything changed since then to prevent that from happening again?

who was responsible for ATC relaying false altitude information to the pilots

Not an easy question mostly because it asks (even if not directly) to put the blame on someone. And although there is not an explicit rule for not doing this, I prefer avoiding it in respect to the dead and the ATCOs doing their job as good as they can.

I will focus on the reason behind the "false reports" and not the responsibilities. The RADAR system the ATCOs use relies on aircraft systems for two things: one is to identify the flight (Mode A) and the other is to get its altitude in hundreds of feet (Mode C).

Now if you recall what happened, the static ports were duct taped for some routine cleaning of the fuselage and were not removed. This affected the airplane's altitude indicator and consequently the altitude transmitted by Mode C.

So the ATCO was reporting back to the plane the same erroneous altitude the pilots were seeing in the cockpit.

  has anything changed since then to prevent that from happening again?

I don't know if there was any action taken after the accident but I think that there wasn't anyone needed: there is regulation (need to make some research to find it) which specifies that all "foreign" parts attached to a plane for protecting its vital parts should have a prominent red "REMOVE BEFORE FLIGHT" ribbon.

The duct tape was of silver non reflective color (if I recall correctly) and had no ribbons. When the crew inspected the plane at night with flashlights they couldn't spot them.


How is flow separation related to the chord length of an airfoil? I came across this question while studying stalling of airfoils. How does the chord length of an airfoil affect the position of the separation point of flow?

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

The Wikipedia article on flow separation isn't very clear on what is the relation between location of separation point and Reynolds Number (and thus Chord Length).

The chord length is a minor influence. The main driver for separation is the local pressure gradient.

Make sure to read the first three paragraphs of this answer if you are unsure how the pressure gradient drives separation.

The chord length only comes into play via the local Reynolds number. Higher Reynolds numbers mean steeper pressure gradients can be tolerated, but the effect is rather weak. It shows in the separation point which moves back slightly on the upper side of an airfoil near stall at the same angle of attack when the Reynolds number is increased.


What are the names of the vehicles to provide assistance to aircraft during the turnaround? During the turnaround many vehicles support the aircraft on the gate. What are the name of those "vehicles"and their purpose?

The most common vehicle seen is the tug.

source

source

Tugs are multipurpose tractors that are used to tow around all sorts of things.

Several types of equipment are shown in this diagram:

source

The tow vehicle at the nose is more commonly referred to as a pushback tug. They come in different sizes. Here is an example:

source

The GPU is most often a unit that is towed behind a tug.

source

Belt loaders usually can be driven on their own:

source

The wide loader is used to put cargo containers in the hold:

source

The cabin cleaning truck and galley service truck often have a lift built in:

source

Luggage can be brought in baggage carts

source

or in cargo containers which are brought to the aircraft on dollies

source

Another vehicle often seen is the people mover:

source


Can a wooden/fabric cover design be built of metal? I know this question is somewhat general and could vary from plane to plane so I will elaborate on the plane in question.

The specific plane I am curious about is the Mooney Mite which has a metal center section but wooden wings and wooden rear half and empennage which at the age most of these planes are is prone to issues. It seems like a great little plane.

My question is can one be built out of aluminum to the same specs (maybe with a bigger engine) and still fly properly or will the weight change (CG change) affect the plane too drastically?

Likewise could a Piper Cub or similar be made of all metal (again with a bit more power up front) and still be an effective plane?

For the record I do not currently have plans to build this plane (although it was available as a kit plane at some point in history. I am aware of the certification issues with making alterations like to an existing plane. I was asking more about if it was possible from an aeronautical standpoint.

Ed Swearingen used to say that it made little difference if you build an airplane from wood, aluminum or carbon epoxy - in the end the weight would not be too different if the parts are designed well.

Note that many aircraft in the 1930s used the combination of an aluminum frame and fabric covering. Especially the control surfaces of many aircraft, but also the fuselage, wing and tail surfaces of the Vickers Wellington were built this way. Also, many gliders before the advent of fiber-composite structures used a welded steel-tube frame covered with fabric for their fuselage. Since glueing works better with wood, the fabric cover needs to be sewed to the ribs and stringers, so suction forces will not lift it up over an extensive area.

Key is the limited dynamic pressure: If you do not increase operating speeds, exchanging wood for aluminum will be straightforward. Only the torsion box of the wing would need closer attention, since the low weight per area of plywood cannot be recreated with aluminum sheet - the wall thickness and the resulting buckling strength would be too low. You would be well advised to add more stiffeners and ribs!


What is the relation between an airplane's altitude and the drag it is experiencing? The Reynolds-Number $Re$ is defined as $Re = \frac{c \cdot L \cdot \rho}{\mu} = \frac{c \cdot L}{\nu}$, with

the velocity $c~\left[ \frac{m}{s} \right]$,
the reference length $L~\left[ m \right]$,
the density $\rho~\left[ \frac{kg}{m^3} \right]$

$\rho = \frac{p}{R \cdot T}$ for ideal gases
pressure $p~\left[ Pa \right] = \left[ \frac{kg}{m \cdot s^2} \right]$
temperature $T~\left[ K \right]$
ideal gas constant $R~\left[ \frac{J}{kg \cdot K} \right] = \left[ \frac{m^2}{K \cdot s^2} \right]$

the dynamic viscosity $\mu~\left[ \frac{kg}{m \cdot s} \right]$ and
the kinematic viscosity $\nu~\left[ \frac{m^2}{s} \right]$, $\nu = \frac{\mu}{\rho}$.

From my understanding, drag increases with decreasing Reynolds-Numbers. Hence, drag increases with an increasing kinematic viscosity (see e.g. this book):

Relation between altitude and kinematic viscosity according to the ISA (International Standard Atmosphere)

With an increasing altitude, the density of the air decreases.
The dynamic viscosity decreases with an increasing altitude of up to $11'000~m$, then stays constant to $25'000~m$ and increases from an altitude of more than $25'000~m$.
This is based on Sutherland's Formula for ideal gases, which in turn relies on the air temperature.
According to ISA, air temperature decreases with an increasing altitude of up to $11'000~m$, then stays constant to $25'000~m$ and increases from an altitude of more than $25'000~m$.
Dividing $\mu$ by $\rho$, one can see that the kinematic viscosity increases with an increasing altitude.
See for example here or here for precise data.

In summary, the Reynolds-Number decreases with an increasing altitude, which means that drag increases with an increasing altitude - assuming velocity and reference length are constant.

Does an airplane really experience more drag, the higher its flying altitude is, assuming otherwise constant parameters?

When researching for this question, one often stumbles across the statement that drag decreases with increasing altitude due to the decreasing density. However, no one seems to take the kinematic density into account.
Are there trustworthy charts available indicating drag over altitude?

Yes, it does - skin friction increases with an increasing altitude.

The mechanism is linked more to temperature than to density, but the reasoning in your question is correct. There might be a few cases where viscous drag increases with Reynolds number (like laminar airfoils which lose their laminar bucket when the Reynolds number goes up), but in general the observation is true.

First a plot from Sighard Hoerners book Fluid Dynamic Drag:

Flow properties over altitude, from page 1-11 in Fluid Dynamic Drag, 1965 edition. The ratio of the actual Reynolds number to the Reynolds number at sea level $\frac{R}{R_0}$ shows a clear downward trend over altitude (which is plotted on the x axis). At 60,000 ft the Reynolds number at the same flow speed is only 12% of that on the ground. If you look at the same dynamic pressure, you have to correct that by the square root of the density ratio $\frac{\rho}{\rho_0}$, which is 0.3 at 60,000 ft. In total, the aircraft will fly in 60.000 ft at 40% of the Reynolds number at sea level when the dynamic pressure stays constant. Lift is proportional to dynamic pressure, therefore it is best to keep the dynamic pressure constant for the comparison.

Now for the viscous drag over Reynolds number: From the same book I copied a plot of lots of experimental data which shows the trend nicely:

Skin friction over Reynolds number, from page 2-6 in Fluid Dynamic Drag. Note that both axes are logarithmic to produce a nearly linear trend line. k denotes a correction for a flow which starts laminar but then turns turbulent when the critical Reynolds number is reached (critical = kritisch in German, hence the k).

For even more details I recommend this page from Stanford.

EDIT:

Your comments helped me to understand the source of your doubts. Drag and lift are proportional to dynamic pressure $q$, and this in turn is the product of speed $v$ squared and density $\rho$: $$q = \frac{v^2}{2}\cdot\rho$$
Reduce density, and you reduce drag. But this will also reduce lift. In the end, you want to support the same aircraft mass at lower density, so you need to do something to get lift back to its old level despite the drop in density. You do this by increasing speed. You restore the same dynamic pressure, in other words. Now the drag is also back to its old level, plus some more. Isentropic expansion means that temperature drops when density drops, and the thermodynamic law of isentropic expansion describes what happens to air in the atmosphere (isentropic = no change in entropy). Lower temperature means higher viscosity. Moving at the same dynamic pressure in lower temperature will generally cause more viscous drag.


Is the National Aviation Authority the same as the Civil Aviation Authority? Are the NAA (National aviation authority) and the CAA (Civil aviation authority) the same organisation? According Wikipedia it looks like they are the same organisation.

The terms National Aviation Authority and Civil Aviation Authority are indeed two generic terms to describe the same thing - the government organisation responsible for managing non-military aviation within a country.

It's worth noting that the specifics of how the organisation works and what it regulates is down to each country, so the terms are broad and generic. For example, Iceland doesn't have any kind of specific CAA - rather, those roles are performed by the Icelandic Transport Authority.

In addition, it wouldn't really be right to refer to a countries authority as the National Aviation Authority or the Civil Aviation Authority (Unless that's what they've called themselves).

For example, the FAA is an example of a National Aviation Authority, it isn't known as the National Aviation Authority. The UK's Civil Aviation Authority is the Civil Aviation Authority in the UK and an example of a National Aviation Authority


Can airliners be considered as GA? In a Swiss001 video, he said that the Twin Otter was a general aviation (GA) plane. But then in another video, he said that the Twin Otter was an airliner. Obviously, he's not a credible source, but it does make me wonder: Can airliners be GA aircraft planes at the same time?

(I mean, some airlines do operate King Airs to remote destinations)

GA refers to why you fly your plane, not what kind of plane it is. For example, John Travolta has (had?) his own personal Boeing 707, which is the first generation of jet airliner from Boeing. When he flew it, it was not under scheduled service, so it would be classified as a "general aviation" flight.


Is there ground effect flying below an infinite 2D plane as opposed to above one? People talk about wing tip vortices and say things like the ground acts as an aerodynamic mirror. What (if any) difference would there be for a wing flying underneath a 2D plane? As in the picture below:

The uppermost plane is flying at an altitude well outside ground effect. The middle one is flying in ground effect, and the bottom one in the hypothetical situation with a plane on top of the plane (pun intended).
Source of the picture, edited to suit the question:
http://www.aerospaceweb.org/question/aerodynamics/q0130.shtml

In computational fluid dynamics (CFD), ground effect can be simulated by flying two airplanes / wings in mirror formation (the lower one inverted).
That mirrored wing is only a mathematical trick. By distorting the flowfield in an identical, albeit inverted, way, the effects of the lower wing cancel out those of the upper wing in the plane of symmetry, and vice versa. This way, the flowfield on both sides looks exactly like one where the plane of symmetry cannot be crossed by the flow, just as the ground cannot inhale or expel air.
Now, with the more precise description: Yes, there is also a ground effect for the plane flying below a hypothetical ceiling. Of course, it would help if the vertical tail would not stick up, for minimum distance and maximum effect. Just as the ground stops the downward movement of the wake, a ceiling will prevent air from filling the space left by the downward movement of the wake, so the wake will stick to the ceiling. Call it the ceiling effect, if you will. In both cases the induced drag drops and the center of pressure moves a bit backwards. I wager to predict that the drag reduction over distance relative to wing chord is the same as in "regular" ground effect.
Proof: Fly a model helicopter indoors and get it up to the ceiling. You will notice that it will get stuck there and needs much less power to stay airborne. The rotor blades are like wings, and due to the rotor position on top of the craft, the ground effect is very noticeable. In order to get the helicopter unstuck you must reduce power a lot, such that the helicopter will drop like a stone.
In CFD, this can readily be proven by using the trick described above, only now with both wings / airplanes flying inverted to their original orientation for the ground effect calculation.


Why the ground effect has such a different outcome for wings and cars? I have some confusion concerning the ground effect.
If we take two applications: Aircraft and Cars, they seem to have completely opposite effects.
Aircraft
From Ground effect (aerodynamics), we can read:

During normal flight, the upper wing surface experiences reduced static air pressure and the lower surface comparatively higher static air pressure. Flying close to a surface increases air pressure on the lower wing surface, nicknamed the "ram" or "cushion" effect, and thereby improves the aircraft lift-to-drag ratio. The lower/nearer the wing is vs. the ground, the more pronounced the ground effect becomes.

I can understand this, the air gets 'trapped' between the wing and the surface (and as consequence the pressure on the underside of the wing goes up)
Cars
However, from Ground effect (cars) we read:

Designers shifted their efforts at understanding air flow around the perimeter, body skirts, and undersides of the vehicle to increase downforce with less drag than compared to using a wing.
This kind of ground effect is easily illustrated by taking a tarpaulin out on a windy day and holding it close to the ground: it can be observed that when close enough to the ground the tarp will be drawn towards the ground. This is due to Bernoulli's principle; as the tarp gets closer to the ground, the cross sectional area available for the air passing between it and the ground shrinks. This causes the air to accelerate and as a result pressure under the tarp drops while the pressure on top is unaffected, and together this results in a net downward force. The same principles apply to cars.

The cross sectional area decreases, causing a higher velocity. This higher velocity decreases the pressure, causing the tarpualin to go down.
This confuses me, as it seems that the same situation (a plate in the vicinity of a surface) causes two completely opposite effects.
Both the "trapping air -> pressure goes up"  as well as "trapping air -> velocity goes up -> pressure goes down" explanation seem reasonable to me.
This made me wonder:
Why the ground effect has such a different outcome for wings and cars?
If I have a plate under an angle close to the ground, will the air be trapped, creating an air cushion pushing the plate up, or will the air accelerate, causing the plate to be pushed down? Does it have to do with some degree of area decreasing?

It actually has exactly the same outcome! Well, in reverse.

Wings generate lift by forcing air down (one way or another, let's not go into details). When the wings are close to the ground, the air that is forced down can't get out of the way, resulting in a higher pressure.

Cars generate downforce by forcing air up. When air close to the ground is forced up, it can't be replaced by 'new' air easily. This results in a low pressure, sucking the car down.

The ground effect is thus exactly the same in either case: air that is either pushed out or pulled in, cannot be replenished quickly enough due to the ground being in the way. The difference in outcome is because a wing is attached at a positive angle w.r.t the ground, whereas a car is shaped like a very thick wing at a negative angle.

A comparison could be a hovercraft on the one hand, which is lifted off the ground almost exclusively by ground effect (if you were to drop a hovercraft from a plane, it wouldn't matter much whether it was powered up or not). On the other hand, a vacuum cleaner, which is sucked down to the floor exclusively by ground effect (indeed, Feynman's sprinkler experiment will tell you it is not sucked down because of the upwards acceleration of air - only the 'engine' part of your vacuum is forced down by momentum exchange. Test this by holding the vacuum in the air - you won't feel it 'pull' at all).


How can I land a PA-44 (Seminole)? I’m trying to get my ME endorsement now, and I find I can’t land very well. My instructor told me to forget everything I know about the Cessna, and that this aircraft should be landed with power. He told me to do a really low approach (compared to the Cessna) and keep the power on until about 7-10 ft off the runway, gently round out, level off (about four fingers under the horizon) and start decreasing power, wait for the aircraft to sink, then pull the column to three fingers under the horizon, power to idle, and land.

But I’m really confused:

Where is my aim point? I always do it depending on my feeling, so sometimes it’s unstable. Maybe use the bottom of the compass to match the runway number?
What’s the rate to reduce the power? My instructor told me the best condition was you touch down at the same time your power had been idle. But I don’t know when the aircraft will touchdown, so I always reduce the power to idle before the aircraft starts sinking. Is that OK?
When I was doing it, I didn’t know why the control column was really hard to pull during flare, and I had trimmed the aircraft on final. Maybe use more trim on a short final?

I have some time in a Seminole.  The issue with twins that causes most of the difficulty is the speed brake effect of two propellers when you pull the power off and they go to full fine pitch.  It's like you deployed a couple of drogue chutes, and makes your speed decay very rapidly and you will find yourself sinking quickly and needing large pitch inputs to try to arrest the sink, and with the Seminole's pitch behaviour this can be challenging to master.

The Seminole is not helped by having fairly high stabilator forces in pitch in the flare, and the T tail causes changes in stabilator effectiveness and sensitivity as the surface comes down into the wing's downwash during the pitch up.

You should be trimmed for hands off flight at your final approach speed (you trim to a speed - think of the trim wheel as your hands-off cruise control speed select dial) 90kts if near GW, 80kts when light.  Some pilots feed in a little bit of extra NU trim just before the flare to reduce the pitch effort, but this puts your trim speed even slower, and if you go around, you will be frantically spinning it ND and pushing on the column as you accelerate to blue line - more so than if you were trimmed at Vref.

Your aim point is 3-500 feet down the runway (in big aircraft it's 1000 ft).  The numbers will do fine as a reference. Your target condition is, as you are passing down below 300 ft or so, you should be trimmed with power set so you can let go of the controls and the plane will hold speed and track the glide slope without any input from you.  Then it's little nudges and small power changes to correct things.  The key is trim to hands off on-speed and on-slope so that the airplane works with you, not against you. For your questions:

Don't rely too much on crutches like standard clearances from objects in the cockpit.  Your target is to have the touchdown zone simply staying in the same spot in the windshield as you close in on it, wherever that spot is.  The "two fingers this, three fingers that" varies with loading so can't be used except as a very general reference for starting out.
In a twin like that you carry power through the round out and the start of the flare and you settle into ground effect. For starters, just leave approach power on until you are at about 5 feet then pull it to idle as you are feeling for the runway.  With experience you can work on feathering the power off from a bit higher up, getting to idle just before touchdown, but it's too much to think about early on.  When you want to make small or slow power reductions, it's easier to manage it by walking the levers in little alternating increments by rotating your hand.
The hard pull for the flare is characteristic of the Seminole and is just something you have to get used to.  As I said, you can trim to a slower speed during the round out to reduce how hard you have to pull, but if you do a go around you find yourself trimmed for, say, 65kt and as it wants to accelerate you will be pushing pretty hard on the wheel to keep the nose down to accelerate to blue line while frantically reaching for trim wheel.  Then if an engine quits and you are distracted doing the drill without retrimming, while holding a hard forward push, you will inevitably let it relax, and things go south.

The biggest factor that will help with your flying is mastering the use of trim for managing speed (it gets more complicated later because you pitch to speed when flying visually, but later when doing instrument approaches you pitch to slope because you have to maintain a precise path on a little needle - worry about that later).  You never want to be holding continuous pitch pressures for any more than short transient conditions.  You don't crank the trim wheel to make pitch changes (although you can) you make pitch changes and follow up with trim. This gets more and more important as the planes get heavier and the control forces go up.

Large turboprops are flown with power right up to the touchdown.  Jets come off the power a bit sooner, and jets that are clean in landing config (generally those without slats) come off the power quite early - in the CRJ200, you go to idle at 50ft.


how does the rudder work? What do you know about yaw control and more precisely about the result caused by the rudder (R on picture)?

Below the right pedal of the airplane has been depressed, and the rudder creates a rotation around the yaw axis and generates an aerodynamic result (R on picture) to the left on the vertical tail. Can you develop how this result is create?

https://www.lavionnaire.fr/CelluleGouvernes.php

When the rudder is deflected, it creates a "lift" on the vertical tail due to additional camber. This "lift", when viewed from the airplane perspective, is a side force opposite to the direction of the rudder deflection. Since the centre of pressure of this side force is aft of the centre of gravity, it generates a net yaw moment on the airplane.

  For example, a right rudder deflection generates a left side force, which generates a nose right yawing moment, pulling the aircraft nose right.

Since the side force is relatively small in the big scheme of things, the airplane motion is essentially unchanged. As soon as the airplane nose begins to yaw, the airplane begins to have finite sideslip against the incoming airflow. This sideslip generates a side force that opposes the rudder force (from an increased airflow incidence against the vertical tail), as well as a yawing moment that opposes the yaw moment from rudder (i.e. a restoring moment).

  For example, a right rudder generates nose right yaw motion, which creates a sideslip that produces a nose left yaw moment.

For a finite rudder deflection, the airplane begins to accumulate sideslip, until such a point that the restoring moment balances out the rudder moment.


What are the ground and flight requirements for high performance endorsement? I'm told ground is proficiency based but that in the plane you have to fly up to 25,000 feet! I don't know anything short of a turboprop capable of doing that, let alone have the ability to rent it. I'm finding different answers everywhere I look! Can anyone cite the FAR on this?

You seem to be confusing a high-performance endorsement with a high-altitude one. Both are in 14 CFR 61.31. They're too long to quote here in full, but the high-performance one comes down to this:

  (i) Received and logged ground and flight training from an authorized
  instructor in a high-performance airplane, or in a flight simulator or
  flight training device that is representative of a high-performance
  airplane, and has been found proficient in the operation and systems
  of the airplane; and

The high-altitude one is where your 25,000' number comes from:

  (g) Additional training required for operating pressurized aircraft
  capable of operating at high altitudes. (1) Except as provided in
  paragraph (g)(3) of this section, no person may act as pilot in
  command of a pressurized aircraft (an aircraft that has a service
  ceiling or maximum operating altitude, whichever is lower, above
  25,000 feet MSL), unless that person has received and logged ground
  training from an authorized instructor and obtained an endorsement in
  the person's logbook or training record from an authorized instructor
  who certifies the person has satisfactorily accomplished the ground
  training. The ground training must include at least the following
  subjects: [...]

As well as the ground training, you have to do (at least) the following in an aircraft or approved simulator:

  (i) Normal cruise flight operations while operating above 25,000 feet
  MSL;

  (ii) Proper emergency procedures for simulated rapid decompression
  without actually depressurizing the aircraft; and

  (iii) Emergency descent procedures.


What are the typical facets of ownership costs, for a single engine aircraft? It's easy to go online and look at prices of a Cessna 172, but what are some examples of how to break down the real world costs of ownership?

How much other maintenance should you plan for?
How much does an engine overhaul cost?
Insurance
Hangar
etc..

It would be great to also get some typical costs and ranges, since some element are more predictable than others.  Obviously the costs will vary based on individual aircraft and location, as well as over time, but I'm looking for information that would help someone make the buy/rent decision.

I'm asking for the structure of the costs to start making a plan for cost analysis and diligence. For example, with an IFR aircraft, what costs are involved with keeping it current?

I've heard it said that the break-even point for owning vs renting is 200 hours/year. I fly a lot less than that, but I don't like sharing my toys, so I own anyway.

I make it a point to not actually count my cost of ownership, because it wouldn't be fun any more, but I'll give you some rough numbers.

I bought a 1962 Mooney for \$30,000. That's about as cheap as it gets, and this was reflected in the condition of the plane and the high-time engine.

In the first year had it, I spent \$18,000 on an engine rebuild, new prop, new carb, new magnetos, new oil pump, and all the other firewall-forward upgrades that were required when I got the new engine. Five years later, I spent \$14,000 on new paint. It still needs a new interior, but that can wait. I spent about \$3000 at West Texas Aero installing speed upgrades.

Budget \$10/hour for your next engine rebuild.

Annual inspection typically runs \$3000 to \$6000 depending on what they find wrong. I'm getting a tank resealed this year, so it's going to be an expensive one.

Parking runs typically \$50-\$100/month, depending on where you are.

Insurance for my old crate runs \$750/year. Expect to pay more for a nicer plane or if you have less experience. Note: get a commercial license; it will pay for itself in reduced insurance in the long run.

I think gas will typically run \$5-6/gallon, depending on a lot of factors. My Mooney burns around 9.5gph, and flies 142 kts so it's really economical in that respect (and the main reason I don't upgrade).

Light twins are cheap to buy, but that's because their cost of ownership is through the roof. A low-time pilot can expect to pay \$12,000/year for insurance and the annual inspection will also be a back-breaker.

I used to have money. Now I have an airplane.


Did the Stuka Ju-87 really have a radio altimeter? If so, when? This is what Wikipedia says near the end of its radio altimeter article:

  The radio altimeter first showed up in the German Junkers Ju-87 "Stuka" dive bomber which was equipped with one for automatic pullouts in the dive bomb run which usually consisted of an 80-90 degree dive. The Stuka pilot would set the radio altimeter to 750m which was connected to the bomb release and automatic pull out. The radio altimeter would drop the bombs and pull the Stuka out of the dive at the set altitude to a level flight. This was invented because the pilots would black out on the pull out, usually for 2 to 5 seconds.

No citation, and I would be skeptical anyway. I know the Stuka had an autopilot that pulled out of a dive, but I thought it was activated manually by the pilot dropping his bomb. I figured he would just drop his bomb at whatever altitude he judged by his own eyes, rather than watching some altimeter instead of the target.

I'll also note that Wikipedia is not self-consistent here. If you scroll up just a few paragraphs, it says:

  In 1924, American engineer Lloyd Espenschied invented the radio altimeter. In 1938, Bell Labs put Espenschied's device in a form that was adaptable for aircraft use.[13] In 1938 in co-operation with Bell Labs, United Air Lines fitted a radar type device to some of its airliners as a terrain avoidance device.[14]

Two citations there. So I don't know what the author means exactly when saying it "first showed up" in the Stuka.

I would have thought such a radio altimeter would have been very difficult without the cavity magnetron, invented in Britain and sent to America in the Tizzard Mission of 1940 September.

On the other hand, I have come across some night fighter radars that Germany had, such as the Lichtenstein Radar that did not need cavity magnetrons. That was not used until 1942, though.

So did the Stuka really have some radio altimeter? If so, when? Was it really the B version that saw action in 1939? What frequency and power did it use? How big was it?

It looks like the actual date is 1942. The 'Nachtschlachtgruppen' aircraft were given Ju 87 D aircraft with numerous upgrades including the FuG 101 radio altimeter.

This would make sense because to use the regular contact altimeter the pilot must have knowledge of the height of terrain above sea level and then add to that when preparing his aircraft for dive bombing. A radio altimeter should always give absolute altitude above the terrain. At night trying to estimate your height above terrain is basically impossible. It's either completely black or a sea of lights beneath you.

I found this information on page 58 of "Junkers Ju-87 Stuka", ISBN 978-83-89450-49-4. More information this book and excerpt is found here: http://mmpbooks.biz/ksiazki/46

Some brief information about the Fug 101: https://airandspace.si.edu/collection-objects/radio-altimeter-german-fug-101-1500-meter-type

https://forum.keypublishing.com/showthread.php?132745-FuG-101-Radio-Altimeter-S-101-Transmitter-Unit


Why do cockpit windows often appear to have a rainbow ‘tint’? Why do cockpit windows often appear to have a rainbow ‘tint’? It’s similar to what you would see when looking at a shallow puddle of water with oil on top. Here is an example of a B777 cockpit window with the same rainbow effect:

Thin-film interference

Thin-film interference is a natural phenomenon in which light waves reflected by the upper and lower boundaries of a thin film interfere with one another, either enhancing or reducing the reflected light.

It's the same mechanism behind your oil example. The thin film in this case is the heating coating applied to the windshield, which is intentionally non-uniform (explained below), making the effect even stronger.

(wikimedia.org)

Fringes observed on a windshield due to thin film interference caused by a conductive Indium Tin Oxide (ITO) thin film deposited on the window for electrical defrosting/defogging.
The fringes are strongly visible because the thin film thickness is highly non-uniform on purpose: as the voltage is applied on two opposite points of the window, a uniform conductive film (uniform sheet resistance) would lead to non-uniform current density, and non-uniform defrosting. Some window parts, especially on the sides and far from the electrical contacts, would dissipate less heat, and defrosting would be significantly less efficient here. The ITO thickness pattern is cleverly designed to provide a rather uniform heat dissipation over the whole window.


Can a CFI provide endorsement training to non-US citizens without TSA approval? Does a CFI need TSA approval to provide endorsement training to non-US citizens, such as the complex, high performance and tailwheel endorsements laid out in 14 CFR 61.31?
Recurrent training is exempt from TSA approval, however training for an endorsement isn't recurrent because it doesn't have to happen again. And it includes training in an aircraft, which usually requires TSA approval.
Some places such as AOPA suggest all of 61.31 is exempt:

He/she is seeking recurrent training, such as a flight review,
instrument proficiency check, or flight training listed under 14 CFR
61.31

But the TSA only suggests that 61.31(g) is covered:

High Altitude Training (HAT)  14 CFR Part 61.31(g) - Type rating
requirements, additional training, and authorization requirements.

If only 61.31(g) is exempt then only high altitude endorsement training is exempt, and TSA approval is required for complex, high performance and tailwheel. Is that accurate?
Government references are highly appreciated.

Assuming the crux of this question is about the need for TSA clearance for the training then the answer can be found in the FAQ section of the flight school candidate website under the question;  When is a flight student exempt from the requirement to undergo a TSA security threat assessment?

You will see a table with some exemptions including the 14 CFR §61.31(g) exemption. The table is prefaced with the statement;

  Other training activities that do not require notification to TSA are
  exempt only if the candidate holds an FAA stand-alone Pilot
  certificate

In other words a pre-existing FAA pilots license acts as some what of a verification and you do not need to be re-verified.

Interestingly only 14 CFR §61.31(g) is exempt so presumably only the high altitude sign off can be obtained under this exemption the rest of 14 CFR §61.31 does not fall under it. I would say the AOPA site is incomplete in their information.  A CFI may provide training for 14 CFR §61.31(g) without TSA authorization but if the student is seeking training on any other endorsements under 14 CFR §61.31(g) then they must get cleared by the TSA.


What is the relationship between the terms "airprox" and "near mid-air collision"? Do the terms "Airprox" and "Near mid-air collision" mean the same? If not what is the difference between them?

A Near Mid Air Collision (NMAC) is an Aircraft Proximity (AIRPROX), but an AIRPROX is not necessarily an NMAC. NMAC is used by the FAA (see 7-6-3 in the AIM), while AIRPROX is used by ICAO.

NMAC is empirically defined as coming within "100 feet vertically and 500 feet horizontally" of the other aircraft, whereas AIRPROX is categorized from A to D. The A to D categories are not empirically defined.

  A - Risk of collision.
  B - Safety not assured.
  C - No risk of collision.
  D - Risk not determined.

References:

AIRPROX (skybrary.aero)
NMAC (skybrary.aero)


How do aircraft keep fuel from freezing while in flight? On aircraft which fly above 30,000 feet, the temperature can often can get to temperatures below Jet-A's freezing point.

For example: at 36,000 feet the standard outside air temperature is -56.5°C (-69.7°F) and is well below Jet-A's freezing point of -40°C/F.

What measures are put in place on aircraft to mitigate the risk of the fuel in the tanks freezing and preventing the engines from operating?

Much like water, exposing fuel to below freezing temperatures does not instantly make it freeze.  It takes some amount of time, and as long as the fuel is not exposed to below freezing temperatures long enough to make it freeze, it won't be a problem.

How much time?  That depends on a number of factors like:

Type of fuel

Jet A (common in the US) has a freezing point of -40 C
Jet A1 (common outside the US) has a freezing point of -47 C
Jet B (common for some military aircraft and very cold airports) has a freezing point of -60 C

Initial fuel temperature

If you fuel in the middle east during the middle of summer, the fuel will be a lot warmer than when you fuel in Siberia in the middle of winter.  Warmer fuel will take longer to freeze.

Outside air temperature

Much like trying to freeze water to form an ice cube, the colder the air around it, the faster that it will freeze.

Aircraft speed

The faster that the airplane goes, the more friction there is as the air flows over the wing.  This heats up the air close to the wing and makes it take longer for the fuel to freeze (see previous point).  The temperature at the wing is known as TAT:
For example, at an actual temperature of -56.5 C (SAT) and Mach 0.72 the TAT is -34 C, but at Mach 0.80 the TAT is -29 C.
For more information about SAT -vs- TAT, see my answer about Air Data Computers.

Quantity of fuel

Much like a small pond will freeze before a large lake, in order to get fuel to freeze you need to lower the temperature of all of it.
The more that there is, the longer it will take.

Aircraft design

Some aircraft run hydraulic lines close to the fuel so that the heat will help slow down the freezing.
Some aircraft constantly pump fuel to keep it moving and mixing if it tends to freeze in one part of the tank before the rest
Some aircraft use fuel to cool the engine oil using a heat exchanger, and return the warmed fuel to the tanks
Many other design factors

In general, fuel will not freeze in the typical amount of time that an aircraft is aloft.  If particular routes pose a problem, there are several options that the operator and pilot have:

Use a fuel with a lower freezing point
Fly at a lower altitude where it is not as cold
Choose a route with warmer temperatures
Increase the speed of the airplane
Carry extra fuel
Transfer fuel in a way to keep fuel warmer.  Typically this is done by transferring fuel from the relatively warm fuselage tanks to the wing tanks which are exposed to more of the cold air.


Is the 737 Max's AOA disagreement warning light an actual light? The New York Times article Boeing Believed a 737 Max Warning Light Was Standard. It Wasn’t. includes the following:

When Boeing began delivering its 737 Max to customers in 2017, the company believed that a key cockpit warning light was a standard feature in all of the new jets.
But months after the planes were flying, company engineers realized that the warning light worked only on planes whose customers had bought a different, optional indicator.

later:

The warning light notifies pilots of a disagreement in the sensors that measure which direction the plane is pointed, a potential sign of a malfunction. This light could have provided critical information to the pilots on two flights that crashed shortly after takeoff in recent months.

and later:

Because only 20 percent of customers had purchased the optional indicator, the warning light was not working on most of Boeing’s new jets. Neither Lion Air nor Ethiopian had the indicator.

note: All instances of boldface have been added for emphasis.
My question is asked to clarify the nature and appearance of the warning light itself.
Is this a physical light bulb-based indicator on a physical panel, or is it graphically implemented within a flat panel display screen? In either case, if the option described above was not purchased, is the (now non-functional) warning light still present and visible?
In that case, is there any secondary visual indication that would allow one to know that it's not working?

In the B737MAX, being a full glass cockpit, the light 'AOA Disagree' is indicated by an alert on the PFD (rather than a physical bulb). This is why Boeing was selling it as an option: it is just a software change to show or hide 'AOA Disagree'.

Image taken from B737 MAX FCOM 10.10.33


Do the shock waves from a supersonic plane affect general aviation planes? Are there any considerations to take into account when flying around supersonic aircraft? I know that wake turbulence from large aircraft can pose a threat to smaller planes. Is the same true of the shock waves generated by planes in supersonic flight?

For instance, do fighter pilots need to be aware of the shock waves caused by other fighter planes in the vicinity?

Great question!  I am far from an expert in supersonic flight, but according to this NASA chat transcript, Ed Haering (an aerospace engineer at NASA's Dryden Flight Research Center in Edwards, Ca. who I would consider to be an expert) answers just this question and says that it isn't a problem for other airplanes:

  chetman1020: Do sonic booms ever disrupt other things in the air?

  Ed: Sonic booms can temporarily dissipate or accentuate a "Sun-Dog," the small bit of a rainbow off to either side of the Sun
  caused by high altitude ice crystals. Aircraft are not affected by
  booms from other aircraft.


Are there regulatory bodies for investigating UAV accidents? Is there an international regulatory body for investigating UAV accidents? Does it exist on any national level?

This question is rather broad, and, as worded, does not admit to a good answer beyond "no" to the first part, and "yes" to the second. That said, the rest of this answer might be useful to you.

There is no international body to investigate UAV accidents, just as there is no international body to investigate ordinary aircraft accidents. There are a great many national bodies.

In the United States, the National Transportation Safety Board is responsible for accident investigations, however, in order to get involved they would have to both know about the accident, and consider it worth investigating. The FAA requires that pilots operating under Part 107 report any accidents involving serious injury, or property damage greater than $500 (excluding the UAV) to the FAA. Presumably they would do some form of investigation to assess the remote pilot's continued suitability to hold a license. Hobbyists don't fly under part 107, however.

Other countries have their own investigation groups, such as the Air Accidents Investigation Branch in the United Kingdom. Wikipedia has a list of organizations that investigate aircraft accidents and incidents, but I have no idea how complete it is, and you would have to check each one to know what their particular policies are.


What is the tablet/application being used in this 737? In this video (marker 2:32), you can see that the captain has a tablet mounted to his left with some sort of application open in it. It looks like an iPad (though I could be wrong), but what I'm mostly wondering is what that application is he has open, and what it's being used for?

Just to make peoples lives as bit easier, here's a screen cap from 2:32:

The screens on both electronic flight bags show airspeeds, which would indicate performance data. Specifically, the first officer has a speed card up.

I can't say for certain, but I think the pilot is using the iPad version of the Boeing Onboard Performance Tool. This application is used for weight and balance and performance calculations (which is especially appropriate, since they're flying a Boeing aircraft).


Does the first portion of a trip that was < 50nm count as XC on a long trip? I flew a long trip (> 900nm) but the first leg, where I had a full stop landing to pick someone up, was only 24nm away from my starting point. For the FAA, a cross country trip counts as > 50nm straight line distance from the starting point. How should I log this short leg of the greater XC trip? Just count it all as XC or just subtract the time of the short leg from the XC total in my log book?

The relevant regulation is in 14 CFR 61.1(b)(3):

  Cross-country time means—

  (i) Except as provided in paragraphs (ii) through (vi) of this
  definition, time acquired during flight—

  (A) Conducted by a person who holds a pilot certificate;

  (B) Conducted in an aircraft;

  (C) That includes a landing at a point other than the point of
  departure; and

  (D) That involves the use of dead reckoning, pilotage, electronic
  navigation aids, radio aids, or other navigation systems to navigate
  to the landing point.

  (ii) For the purpose of meeting the aeronautical experience
  requirements (except for a rotorcraft category rating), for a private
  pilot certificate (except for a powered parachute category rating), a
  commercial pilot certificate, or an instrument rating, or for the
  purpose of exercising recreational pilot privileges (except in a
  rotorcraft) under §61.101 (c), time acquired during a flight—

  (A) Conducted in an appropriate aircraft;

  (B) That includes a point of landing that was at least a straight-line
  distance of more than 50 nautical miles from the original point of
  departure; and

  (C) That involves the use of dead reckoning, pilotage, electronic
  navigation aids, radio aids, or other navigation systems to navigate
  to the landing point.

Note that the rule under paragraph (ii)(B) is "a flight" that includes "a" point of landing more than 50nm from the point of departure.  It does not specify that all points of landing must be further than 50nm from the departure point, or even that the first point of landing must be further than 50nm.

Since the overall trip was over 50nm, does the FAA allow you to consider that 24nm leg to be part a single overall flight?  Unfortunately, the FAA doesn't provide a definition of "flight" in this context.  However, it is generally accepted that short stops (to pick up a passenger, refuel, eat lunch, etc.) can be logged as part of a single flight.

Edit to add:  Please also this this previous question, and especially the AOPA article it links to.


What is a Terminal Radar Service Area (TRSA) and what are its operational requirements? Some fairly busy airports that you would expect to have Class B or C airspace have class D airspace at the airport and normal class E and G airspace around that. In addition, they have an area drawn on the chart with similar dimensions to a class B or class C airspace area but they are labeled as Terminal Radar Service Area (TRSA). What is a Terminal Radar Service Area (TRSA) and what are its operational requirements?

A good explanation of a Terminal Radar Service Area (TRSA) can be found in the Aeronautical Information Manual 3-5-6.

  TRSAs were originally established
  as part of the Terminal Radar Program at
  selected airports. TRSAs were never controlled
  airspace from a regulatory standpoint because the
  establishment of TRSAs was never subject to the
  rulemaking process; consequently, TRSAs are not
  contained in 14 CFR Part 71 nor are there any TRSA
  operating rules in 14 CFR Part 91. Part of the Airport
  Radar Service Area (ARSA) program was to
  eventually replace all TRSAs. However, the ARSA
  requirements became relatively stringent and it was
  subsequently decided that TRSAs would have to
  meet ARSA criteria before they would be converted.
  TRSAs do not fit into any of the U.S. airspace classes;
  therefore, they will continue to be non−Part 71
  airspace areas where participating pilots can receive
  additional radar services which have been redefined
  as TRSA Service.

These airports can't be redrawn as class B or class C airspace so they became TRSAs. The airspace at the airport is class D and the airspace in the TRSA is usually class E. The operational requirements are no different than any other class E or class D airspace, but aircraft are encouraged to avail themselves and participate in the TRSA when inside its bounds.

It should also be noted that many TRSAs have their own approach control.


KIBM RNAV (GPS) 10 -- did I find a charting error, or what is the TERPS rationale for this? I was looking at the RNAV (GPS) approach to runway 10 at Kimball, NE (KIBM):

and was left wondering "How on earth could the straight-in visibility minima for approach categories C and D be lower than for approach categories A and B?"  Is this merely a charting error, or is there some strange clause in the TERPS that I'm not aware of that makes this possible?

This is speculation, but in the absence of an obvious technical reason it's at least possible that this is because category C/D pilots have better lobbyists than category A/B ones.

I found an interesting but somewhat dated (1995) document  called Establishing Visibility Minimums; the author used to chair the ALPA TERPS committee. It says:

  [The TERPS concepts] are a combination of science and politics, with a
  lot of undocumented historical precedence thrown in for good measure

And:

  The political horse-trading that led to the 1976 [TERPS] revision [...]

In other words, we shouldn't necessarily assume that rules and regulations are entirely based on well-defined and documented technical criteria.

The (now outdated) 1976 chart in that document shows a default minimum of 1 mile visibility for all categories (with the single exception of an ILS with DH of 200', which is 3/4) or 1/2 with approach lighting, so a 7/8 minimum is a more recent change. So you could ask, why was this change made? One way would be to find the NPRM or other public records that detail the reasons for the change, but in practice it may be easier to contact the FAA directly and ask them (possibly under the FOIA if the information isn't already public).

One theoretical possibility is that the airline industry successfully lobbied to have 'their' minimums reduced in certain circumstances due to better training, equipment or whatever, and this approach is one of the affected ones. Diverting airline flights is enormously expensive, and the industry has a large incentive to make sure that as many instrument approaches succeed as possible. If lowering the minimums by 1/8 allows even a few flights a day to start an approach that would otherwise be illegal, that could add up to a huge financial saving.

But this is entirely guesswork on my part, I just wanted to make the point that sometimes we look for a technical explanation where the real reason is financial or political. For a definite answer, I would ask the FAA directly.


Is it legal to fly under the Golden Gate Bridge? I recently had the opportunity to rent a plane and fly in the San Francisco Bay Area.  While flying in the HWD/OAK/SFO area is challenging enough, when I spoke of my adventures of flying above Alcatraz and the Golden Gate Bridge, some of my fellow pilots asked me if I flew UNDER the Golden Gate Bridge and voiced their desire to do so.  As a relatively new pilot (3 months), while flying is definitely an adventure for me, flying under a suspension bridge is pushing my minimum safety limits and just does not seem lawful.

Others have told me that they have seen GA aircraft fly under the bridge. While I'm sure pilots have done it...I don't see how this can be legal.  According to sources on the internet the bridge is only suspended 270' above the bay.  It doesn't seem enough space to fly through legally.

According to 91.119

Except for takeoff or landing, no person may operate an aircraft below the following altitudes;

a. Anywhere. An altitude allowing, if power unit fails, an emergency landing without undue hazard to persons or property on the ground.

c. Over other than congested areas.  An altitude of 500' feet above the surface, except over open water or sparsely populated areas. In those areas the aircraft may not be operated closer than 500' to any person, vessel, vehicle, or structure.

To me, 270' does not meet the legal requirements set in 91.119(c)  I don't consider the SF Bay or water under the bridge to be "open".

So, is it legal to fly under a suspension bridge like the Golden Gate or Verrazano-Narrows Bridges?

No, it would violate regulations unless it was to meet the needs of an emergency.

Don't forget the catch-all regulation about reckless operation; many regulations may be interpreted very subjectively, and I feel that it is safe to say that flying under the Golden Gate bridge would attract a lot of negative publicity and would ultimately be considered something that the pilot of a fixed-wing aircraft would certainly be reprimanded for.

That's not even considering whether the bridge is 500 feet high, boats that may or may not be passing under the bridge, and your question completely truncated the 91.119(b) that lists information for congested areas, which I am almost certain the Golden Gate bridge would be subjectively qualified as.


How much does a Hundred Dollar Hamburger actually cost? Recently, the Technical Difficulties discussed the \$100 hamburger on their show Citation Needed.
A "\$100 hamburger" refers to the practice of flying to another airport, eating a hamburger there, and flying back again.
The hamburger is used as an "excuse" for flying — the actual reason is just getting flight time to meet minimal flight hours. Presumably, it once cost around \$100 to do this, which is a lot for just getting a burger.
The Wikipedia article mentions that a hundred dollar hamburger likely costs more than \$100 these days, due to increased fuel prices.
Is it feasible to accomplish this mission (in the United States) within a \$100 budget today (2017)?

The flight school I fly out of rents a 152 at 86USD an hour wet. Assuming you need some cash for the burger lets say you save the remaining 14 dollars for that you are right at your 100. With the 152 cruising at just about 95 Kts you need to keep the distance (both ways) under 95nm to say in our budget.

KDYL → Sky Manner (N40) is a 15nm hop and Sky Manner has a nice little restaurant.

If you want to put some actual time on the plane

KDYL → KRDG is a 39nm hop and there is a restaurant there.

Both of these hops keeps the hobbs at under or right at an hour of flying which keeps you under your total of 100USD for the flight and the burger.

That being said, there are bigger, faster, more expensive planes you can fly much farther to get food. I was chatting with a guy who flew his PC-12 from PA to Martha's Vineyard for lunch with some clients... I assume that was far more than 100$.


What type of display is used for cockpit displays? In "glass cockpit" aircraft, what type of display is used for the PFD/MFD system?

Are they normal LCD screens, or are they "special" in some way (temperature tolerance, refresh rates, shatter resistance, etc...)?

Typical PFD/MFD:

Typical raw panel LCD screen:

Modern glass cockpits use LCD screens specially designed to be visible in bright sunlight.  Earlier units used cathode ray tubes.

Both the Boeing PFD screen and the Garmin G1000 PFD shown above use LCD screens.


How does the load factor vary when the aircraft pitches up/down? I know that when an aircraft starts rolling, it's load factor increases. But can anybody explain how does load factor vary when the aircraft pitches up/down? Does it increase/decrease?

Think not about the rate of pitching but rather about the skyward or earthward curvature in the flight path (trajectory).  This curvature is an acceleration, and requires a force to make it happen.

G-load (i.e. load factor) is essentially just the aerodynamic force generated by the aircraft, divided by weight.

In straight-and-level flight the G-load is +1.

In the case where the aircraft is wings-level and in a level upright pitch attitude (not inverted), we can observe--

If the flight path is curving skyward, the G-load is greater than +1

If the flight path is curving earthward, the G-load is less than +1.  It may even be zero or negative.

This answer could be expanded to consider other flight attitudes.  Think about a loop.  At any instant in time, if the flight path is curving, some force is causing that. The main forces that could be contributing to this curvature are gravity, and the wings' lift force.  Of these two forces, only the latter one determines the G-loading, i.e. the load factor.  In the plane's own reference frame, an "upward" aerodynamic force (towards the canopy) creates a positive G-loading, and a "downward" aerodynamic force (toward the belly) creates a negative G-loading.

Knowing this, you can compute the G-load at any instant, as long as you know the aircraft's attitude in space, velocity, mass, and the radius of curvature of the flight path.

Starting from level upright flight, if you pull back on the stick or yoke, you'll feel heavier.  If you push forward on the stick or yoke, you'll feel lighter.

Note: this answer assumes we are measuring the G-force near the center of pitch rotation of the aircraft.  A pilot seated far forward of the center of pitch rotation will feel a greater increase in G-load when pitching up, and a greater shift toward a negative G-load when pitching down, than if he were seated at the center of pitch rotation.  The same is true of the load factor imposed on any part of the aircraft structure.


Regarding the 1902 Wright glider. Why is the airspeed, Va, calculated adding the ground speed, Vg, and wind speed, Vw? Regarding the 1902 Wright glider. Why is the airspeed, Va, calculated adding the ground speed, Vg, and wind speed, Vw?

In August 1903, Octave Chanute published in L'Aerophile a table (see attached) showing the measured quantities:

(1) distance traveled along the ground (col. 2),

(2) flight time (col. 3),

(3) wind speed (col. 5),

(4) glide angle (col. 7),

corresponding to a few flights performed on Oct. 8, 1902. From these parameters and the total mass of the machine and pilot, O. Chanute calculated that the 1902 glider lifted about 62 kg/hp (see the last column).

It is not quite clear for me why Va = Vg + Vw (see col. 6). The airspeed should be constant, independent of the headwind speed, as long as the glide angle is the same because the thrust is the same. It is true Va = Vg + Vw but if Vg added to Vw does not give always a constant Va, for the same glide angle, then something is wrong with the measurements.
The water speed of a boat traveling upstream depends only on its thrust and is independent of the river speed.

The lift in the table (~62 kg/hp) is quite big in comparison to the lift of the best gliders of the time, mentioned by O. Chanute a few lines below the table, that lifted only 45 kg/hp.

The precise value of the airspeed, Va, is essential in calculating the lift. If ones takes it only 1-2 m/s lower, due to miscalculations, then the evaluated lift grows artificially.

Source: http://gallica.bnf.fr/ark:/12148/bpt6k65534693/f190.item.r=wright

Update

There are a few sources of errors that could have altered substantially the results in the table.

(1) O. Chanute wrote, above the table (see the link), the Wrights did not drift, more than 1/4 in a arc of a circle from the straight line, to always have good headwinds. This drift increases the traveled distance.

(2) The glider was first lifted as a kite to a few meters above the ground with the help of two men. This height made the glide angle to be higher than that of the slope, which can be estimated from a picture (see the attached image where the glide angle appears to be 9.01 deg).
If instead of 7 deg 20' the slope angle was 9.01 deg and the 1902 machine started from a few meters above the ground then instead of 58.1 kg / hp it would have lifted less than 114.4 kg / (114.4 kgf * sin(9 deg 20 minutes) * (54.7 m/12 s + 5.68 m/s) ) = 47.4 kg/hp.

(3) The third factor of incertitude is the wind speed that varies with altitude being stronger a few meters in the air than close to the ground.

Wright glider, 1902. Undated picture.

Question: Do you know a modern replica of the 1902 glider that lifts ~62 kg/hp?
If such a thing exists then the 1902 machine could have lifted 62 kg/hp.

The gliders had no instruments of any kind, let alone an airspeed indicator. Thus the only way to measure airspeed was to measure the windspeed and add that to the speed over the ground, itself calculated from distance flown and time aloft for each flight.


Is it possible to use thrust reversers to taxi backwards? Can commercial airliners theoretically taxi backwards using reverse thrust?

If this is possible, why isn't it common?

I can already imagine some safety reasons...

This is called Powerback, most aircraft can do it, but it is not done very often.

In a jet aircraft, the three main problems are:

Reverse thrust tends to throw a lot of debris into the air because the exhaust is deflected to the sides and up and down too. This debris can damage the engine itself, other things on and around the aircraft or injure someone on the ramp. It is less of a problem for aircraft with tail-mounted engines (so DC-9s often used powerback), but that engine configuration is not used as much any more in transport aircraft, as it is less aerodynamically efficient.
A related problem is that the compressor might suck in its own hot exhaust gases. This might lead to temperature-induced damages in the last (high-pressure) stages of the compressor and health issues for passengers, since the air conditioning system works with bleed air from the compressor.
Reverse thrust is rather inefficient on jet engines, so it uses a lot of fuel.
As Casey mentioned in the comment below, the pilot has to be careful using the brakes during powerback: the main wheels are very close behind the centre of gravity, so harder braking can lift the nose wheel, causing the pilot to lose control and even causing damage to the tail if it hits the ground (“tail-tipping”).

So a tug is both cheaper and safer.

In propeller aircraft, reverse is more efficient and does not throw up as much debris, so it is sometimes used. But a tug is still preferred because transport aircraft don't have any rear visibility, so the pilots can't see where they are taxiing. With tug the driver can see behind the aircraft, and a ground marshaller walks along side each wingtip with an intercom  connected to the plane.

Lastly, since aircraft are able to turn almost on the spot using differential braking and thrust and 180-degree nosewheel steering, at airports with few facilities there is generally enough space around the aircraft to permit the pilot to reverse out easily.


Do pilots typically mix roll/yaw when piloting various planes? When flying on GeoFS, a popular flight simulator online, one of the default settings is to mix roll and yaw. Of course, it isn't real life, so I think it might differ from flying in the real world.
Do professional pilots mix roll and yaw? Is it possible that pilots have different settings for different types of planes, such as fighter jets and commercial jets?

Pilots typically make turns in the air by banking the aircraft. The act of banking the aircraft is done by moving ailerons. Since moving one aileron up and the other aileron down produces a roll in the direction of the up aileron, the aircraft will bank (and presumably turn) in the direction of the up aileron.
Induced drag is a byproduct of lift. By putting one aileron up and one aileron down, you are producing more lift on the down aileron side and less lift on the up aileron side. Therefore you are also producing a lot more drag on the down aileron side than you are on the up aileron side. This causes adverse yaw in the direction of the down aileron. So, while the bank is trying to fly the aircraft in one direction, the induced drag is trying to yaw the aircraft in the opposite direction. This is one type of adverse yaw.
While turning is done by banking through rolling instead of simply yawing, yawing must still be done. The main job of the rudder is to combat adverse yaw. Pilots are constantly making judicious use of the rudders to keep the flight of the aircraft coordinated and streamlined. Changes in pitch, roll, and airspeed can all cause adverse yaw.
Some more modern planes will do this to some degree for the pilot through electronics. The mechanism for technology to accomplish this is called the yaw damper. This, however, does not completely eliminate the need for rudder inputs like your “mix” setting does.


Is "takeoff distance" measured over the ground, or to a point in the air? I read that takeoff distance was the distance for which the airplane is on the runway, as well as the distance for which it ascends to 15 meters off of the ground.
Does it get measured as the hypotenuse of the "triangle" the airplane makes, or as the projection of its movement onto the runway, which is a purely horizontal distance?

Takeoff distance is always measured as horizontal distance.
Under 14 CFR Part 23, which apply to aircraft seating 19 passengers or less and a maximum takeoff weight of 19,000 lb or less, the pre-Amdt 64 specifications are:

For multi-engine jets of 6,000 lb or more, or commuter aircraft, 23.59 applies. Takeoff distance is measured as the horizontal distance between start of ground roll to where the aircraft attains 35 ft off the ground.

For others, 23.53 applies. Takeoff distance is measured as the horizontal distance between start of ground roll to where the aircraft attains 50 ft off the ground.

Note that a lot more nuance is involved since the greater of one-engine-inoperative distance and that with 115 percent of all-engine-operating distance goes into the actual performance number.
For Part 25 aircraft (transport category), there is a further distinction between dry and wet runway (25.113):

On a dry runway, it is measured to 35 ft.

On a wet runway, it is measured to 15 ft.

This information is harmonized with other regulatory agencies, such as EASA and TCCA.


Can a pilot still land if the tower gives confusing Information? Recently an airplane crashed at Kathmandu airport in Nepal. The people that owned that plane claim that confusing information was given to the pilot by the control tower, and as a result the plane crashed. Conversation between pilot and control tower given here.

Now my question is: if confusing directions are given by the control tower, what should a pilot do? Should they wait for the exact decision and continue flying, or can they decide to make the plane land safely?

At or near touchdown on the runway? Ignore it. The last thing any pilot should do in the critical period near runway touchdown is try to determine what a confusing or unclear radio transmission actually meant.

On the other hand, if "at the time of landing" includes two minutes prior to touchdown, up on downwind, or 4  nm out on extended final approach, different story.

What the cutoff is between these two extremes is what we pay pilots for. It's called situation awareness and exercise of good judgment.


Is autorotation possible from a 0 rpm start at high altitude? I am working on an enclosure that is to be ejected from a rocket at apogee (10,000') and fall to a target altitude at a descent rate below 20ft/s before releasing its payload. The idea is to have coaxial rotors with collective pitch control deploy and autorotate to slow the descent of the enclosure and payload.

My question is how I can start autorotation with blades that have essential zero angular momentum, and are there special considerations for the use of coaxial rotors?

Go back to Estes rocket days.  There is a whole class of competition rockets known as "heliroc" or helicopter recovery.  These ascend like a conventional rocket, in a vertical(ish) path and fin stabilized; at (or near) apogee, they release a rotor by some means and the rotor spontaneously begins to autorotate, often giving a lower descent rate than a parachute.

Go even further back: look at maple seeds (aka maple keys).  They autorotate so efficiently that they'll enter stable rotation and constant (low) sink rate in as little as one blade length of fall (they do have very low wing loading, however).

So, yes, it's possible for a rotor disk to spontaneously begin autorotation from a dead stopped starting condition.  Mother Nature does it billions of times every autumn, and school children do it thousands of times every year.

Yes, this will work with a coaxial rotor, if you can keep the rotors from interfering with one another during deployment (for instance, by braking them until fully deployed).  However, since there's no reaction torque from freewheeling, autorotating rotors, there's no reason to have coaxial rotors.  It's far simpler to use an ordinary rudder for yaw control than differential collective, and you'll have half as many parts for pitch and roll.


Is the air in aircraft cabin toxic? I just listened to the recent BBC podcast episode Aerotoxicity: the hidden dangers of flying and there were interesting claims there:

Cabin air in modern passenger aircrafts comes directly from engines (it's called bleed air).
This air is unfiltered and during so called fume events it may contain toxins from engine oil and other fluids. These toxins are called Tricresyl phosphate and persons exposed to them for a long time may suffer diseases of nervous systems, from psychological problems like depressions to neurological symptoms, like unexplained pains, severe headaches, etc.

As they said in the podcast, there are allegations that these

  ...fume events have not only made many ill, but they are being blamed for premature deaths

and that there are

  ...hundreds of legal cases going through the court.

I have never heard about this danger before and I consider the BBC to be very reliable source of information, so it surprised me. Are you aware of any study which would prove this danger?

It seems this is a fairly new issue cropping up and I have been able to find some credible research on the topic (i.e. from official aviation authorities and not news outlets or lawyers websites...) but all of it is pretty recent. The CAA seems to be aware of the issue and has some information on it here. The two studies they link were published in 2017 so the topic seems to be recent, you can find the full text here and here.

The Australian authorities via CASA conducted a pretty hefty study on the topic in 2007 and came up with a list of 37 recommendations which includes the development of an anonymous online reporting system.

  That the Australian Government, through CASA and the ATSB, sponsor and
  fund the development of a single, central, internet-based,
  confidential reporting system on cabin air contamination incidents to
  be co-ordinated and operated jointly by CASA and the ATSB. To improve
  the reporting and monitoring of cabin air contamination incidents,
  this system should have web-based forms to facilitate the collection
  and collation of data from all authorities and companies responsible
  for cabin air contamination incidents that would enable the data to be
  tabulated into a de-identified and unalterable uniform document that
  could be accessed and utilized by all stakeholders.

They also are setting some resources aside for studying long and short term effects of an event

  That CASA in association with the Department of Health and Ageing
  (DOHA), the Institute of Health and Welfare or the NHMRC sponsor a
  project to define possible criteria for case definitions for short
  term and longer term health outcomes following episodes of cabin air
  contamination.

Clearly the CASA sees this as a real issue.

The FAA published a safety advisory on the topic in March 2018 and is clearly acknowledging the issue.

Its worth noting that most of these studies and claims refer to an "event" in other words a notable time when there was a fume event implying that this is not a constant occurrence. On the flip side, cabin air is actually pretty clean, as discussed here and researched here. Interestingly this CDC study seems to point to the slightly lower oxygen levels and more importantly tight quarters with a lot of other people to be factors in health issues on aircraft. They did this by comparing flight attendants with teachers who also work in lower circulation, high person density environments.


How can I calculate the angle of attack of an airfoil? Can someone provide me a general algorithm to compute the angle of attack of an airfoil, and some theory along with it?

The angle of attack is the angle between the wing and the direction of flight. The direction of flight is opposite to the airflow, so the angle of attack is also the angle between the wing and the relative wind.

For a more refined definition (thanks @Peter Kämpf): The flight path direction (the velocity vector in the language of aerodynamicists) should be expressed relative to the air mass, not to the ground (as measured by an inertial system), but that's beyond the topic.

How to compute it? Well, it's like airspeed, we don't. It's just measured using a small windvane:

A380 angle of attack sensor (combined with a Pitot-static), source

In the image above, the black angle of attack sensor is combined with a classic Pitot-static which measures airspeed. On airliners, there are usually several AOA sensors. The one in the picture is located on the FO side, air comes from the right side.

Another simple AOA sensor at the bottom, Captain side:

Source (original source not found)

There is a specific AOA sensor found on GA aircraft, to trigger an alarm when approaching the critical angle of attack: The stall sensor:

Source

This strip located at the wing leading edge has two positions and rock abruptly before the critical angle of attack is reached. It triggers an alarm in the cockpit, so that the pilot can reduce the angle of attack.

A final note: The AOA wasn't displayed to the crew in the cockpit, only the pitch angle was. Newer aircraft now display both. I think it was a recommendation of the BEA after AF447 accident, were the crew couldn't figure out the AOA they were at after a Pitot-static froze.


Why do I see moisture coming from the middle of the wing as well as wingtip vortices? A lot of times I see small tube like streams of visible moisture training from the middle of an airliners wing. I assume this is induced drag created by wingtip vortices, when the aircraft is heavy, clean, and slow. However, I figured this would only happen at the “wingtips” and not from the middle section of a wing. I have seen them at the wingtips but a lot of the time it’s coming from the middle. If this is from the wingtip vortices how come they would come from the middle?

They are vortices coming from the flap panel, same as the vortices coming from the wing tip, but much more intense because the angle to the flow is so high.  Because of this, the pressure drop in the core of the flap vortice much more than pressure drop in the flow off the wing tip. This means the temperature drop is also much more, enough to condense the humidity in the air, like a horizontal tornado.