Are there rules regarding the accuracy of a pilot's delay announcement? Recently in an advertisement, I saw that a pilot gets notified that the plane will be delayed by 30 mins, but he announces that the plane will be delayed by 3 hours. And then he makes another announcement with the update saying that the flight will now be delayed by only 30 mins with the passengers shown cheering. I can understand the human psychology behind it but I think that there must be some rules and regulations about such fake announcements. Can someone tell me about it?

No precise rules that come to mind offhand, but:

these days, the passengers often get text messages with delay info, and if I'm saying "3 hours" as the text comes in saying "30 minutes", that is not good
people who might wait out a 30 minute delay might decide to rebook or cancel their flight for a 3 hour delay
if there are passengers making connections to other flights at the destination, the difference to them between 30 minutes delay and 3 hours is immense, and they can get upset, call the airline reservations people, want off the aircraft to talk to customer service, etc
an actual delay of 3 hours is reason to deplane everybody, now (in most cases); for 30 minutes, not so much. That process & subsequently re-boarding everyone takes time, and it's no fun for anybody.
play the "oh, wait, now the delay is xxxx" card too often, and you look like you don't know what you're doing.

My gameplan is to give the passengers the honest answer. Sometimes, this is along the lines of, "may be a quick fix, like 15 minutes, or it may be a lengthy delay, in the ballpark of 2-3 hours. We expect to know which situation we have in about 15 minutes or so, and we'll let you know when we get the word from Maintenance." I think people appreciate it when you give them the straight scoop, including the times when the crystal ball is cloudy.


What do pilots operating under FAA 14 CFR 91 have to do to fly in any of the Special Flight Rules Areas (SFRAs)? What do pilots operating under FAA 14 CFR 91 have to do to fly in any of the Special Flight Rules Areas (SFRA). Does each SFRA have its own requirements? I know there's a question already about the Washington DC SFRA. Is there a master list of these areas with corresponding requirements?

The "master list" of special flight rules areas (SFRAs) in the United States is FAR 93 (14CFR93)) - aptly named "Special Air Traffic Rules".

It contains a list of areas with "Special Air Traffic Rules", including the requirements for operating in those areas.

Charts with SFRAs on them include a notice directing you to FAR 93 (and often other helpful information about the SFRA):

In addition to "Special Air Traffic Rules" some airspace with "special" rules may be defined by "Special Federal Aviation Regulations" (SFARs), found at the beginning of Part 91.
The only one I see in there currently is the Grand Canyon National Park SFAR.


What's the difference between Stress and Strain? There's a book for Engineers, entitled: 'Roark's Formulas for Stress and Strain', some describe it as 'High level Mathematics'. So what?

Stress is the load divided by the cross section of the material bearing it, and is expressed in units of pressure.

Strain is deformation divided by the original dimension in the direction of the deformation, and therefore is dimensionless.


How close can planes fly to each other over the ocean? While on a recent Air Transat flight from Toronto Pearson to London Gatwick, several hours into the flight and over the Atlantic Ocean, my daughter and I noticed a plane behind ours to the left, flying quite close. Assuming there must be protocol for how close planes can fly to each other, I was more interested than concerned.

It then became apparent that the other plane was flying faster than our plane and coming closer. The plane came close enough for my daughter and me to see all the windows as its path led it to cross in front of our plane. It was only very slightly higher than our plane by mere meters. As it very quickly approached us we lost sight of it as it crossed in front of our path.

My daughter and I figured a crash was imminent. Our plane then altered its path, moving slightly down and to the right.

This was an overnight flight and at the time the cabin lights were turned down and the passengers sleeping. There was no flight crew to speak with at the time and we didn't want to disturb anyone considering we were... well.. still alive.

Upon return from our trip I called Air Transat and spoke with Customer Relations who had the flight records pulled. There was no report of the incident. I cannot believe any regulation could possibly allow for two planes to fly so close to one another under any circumstances. I need to understand what happened here, the distance planes must remain from one another and why such an occurrence would not be logged.

The plane was not marked. It was not a Lear jet but a small, dark, unmarked passenger plane. Thank you for your help. Obviously I am still very disturbed by this event.

It was only very slightly higher than our plane by mere meters.

You think you saw a plane at the exact same vertical level as yours, when in fact it was at least 300 metres (1000 feet) above or below your level. It can be very hard to judge differences in level for the untrained eye, especially over the ocean and at night when there is nothing to reference for scale. Your confusion is understandable, but there was absolutely no safety risk here.

The minimum vertical separation between two aircraft is 1000 feet, and the plane you saw was - with 100% certainty - at least 1000 feet above or below you. This is ensured by air traffic control and a ton of complex regulations and rules governing air traffic.

When over the ocean, where no radar coverage exists, if two planes are flying at the same level, they will either be at least 10 minutes flying time apart (if following the same route), or 25 nautical miles (about 50 kilometres) if flying on parallel routes. Over land where there is radar, two aircraft at the same level will generally be at least 5 nautical miles apart (10 kilometres). But when 1000 feet or more vertical separation exist, aircraft can cross each others paths without any risk. It is perfectly normal for aircraft to fly this close, but passengers seem to take notice more often over the ocean, probably because there is nothing else to look at.

There was no risk of collision during your flight. If, for some inexplicable reason, the standard separation described above was violated, pilots and air traffic controllers would have responded immediately to establish separation again, long before any sort of collision risk would occur. There is very reliable technology in place to warn pilots and air traffic controllers if aircraft are getting too close, and they are trained to react to such warnings without delay.

For more information on separation, please see my answer on "How are aircraft separated at high altitudes?"


How dangerous would it be to start a jet engine with coins inside? With the multiple stories in the news of people throwing coins into a jet engine for good luck, I couldn't help but wonder how dangerous it would be if the act went unnoticed and the crew started up the engine with the coins inside. Would the engine be damaged? Would the damage be noticed before the plane throttled up for takeoff or left the ground? I'm sure the answer is, to some extent, "it depends", but what's a worst case?

As you say "it depends". Modern jet engine front turbines are designed to reject foreign object debris outside the core, which is where the damage would really matter. Jets suck up hard foreign object debris like pebbles often enough and it will ding things a but but hardly cause catastrophic damage. I'm sure coin throwing has gone unnoticed, and it made no difference in the end.

Really, it's not a big risk for the most part, modern high-bypass turbofans have a large bypass duct, when coins are thrown into an engine which is not running they will drop to the bottom of the bypass duct and get blown harmlessly out the back when it's started. However, there's just enough risk that a coin could find its way into the compressors and cause enough damage to need expensive repairs, and that is why they will take the airplane out of service.

Of course it depends on the coin, if you threw this bad boy in it would be a different story:


What is this weird format (R06L/CLRD62) in METAR for runway being cleared? Yesterday, this metar was published:

METAR UUEE 292000Z 07003MPS CAVOK 17/12 Q1013 R06L/CLRD62 R06R/CLRD62 NOSIG=

Can someone explain the meaning of the R06L/CLRD62 R06R/CLRD62 bit? I suspect it means the runway has been cleared but I'm not sure about the details.

More generally, is there a definitive METAR specification that cover these edge cases?

The format follows a standard, although this one is different because of the  R at the front...

The format is the "Runway State Group":

First two digits are the Runway Number, 88 means all runways, 99 means a "repetition of the last message as no new information received".

The third digit is the runway deposits, it tells you what is on the runway:

  0 = Clear and dry
  1 = Damp
  2 = Wet or water patches
  3 = Rime or frost covered (depth normally less than 1mm)
  4 = Dry snow
  5 = Wet snow
  6 = Slush
  7 = Ice
  8 = compacted or rolled snow
  9 = frozen ruts or ridges
  / = type of deposit not reported (e.g. due to rwy clearance in progress)

The Fourth digit is the extent of the contamination:

  1 = 10% or less
  2 = 11% to 25%
  5 = 26% to 50%
  9 = 51% to 100%
  / = not reported (e.g. due to rwy clearance in progress)

The fifth and sixth digits are the depth of deposit:

  00 = less than 1mm
  01 = 1mm etc

to

  90 = 90mm
  91 = not used
  92 = 10cm
  93 = 15cm
  94 = 20cm
  95 = 25cm
  96 = 30cm
  97 = 35cm
  98 = 40cm or more
  99 = Rwy(s) non-operational due to snow, slush, ice, large drifts or runway clearance, but depth not reported.
  // = depth of deposit operationally not significant or measurable.

And the seventh and eighth digits are the braking action:

  28 = friction coefficient 0.28
  38 = friction coefficient 0.38

or

  91 = Braking action: Poor
  92 = Braking action: Medium/Poor
  93 = Braking action: Medium
  94 = Braking action: Medium/Good
  95 = Braking action: Good
  99 = Figures unreliable (e.g. if equipment has been used which does not measure satisfactorily in slush or snow.)
  // = Braking action not reported (e.g. runway not operational; aerodrome closed; etc.)

So we can take your example: R06L/CLRD62

Means

R06L : Runway 06 Left
/ : Deposit not reported (clearance in progress)
CLRD: Contamination ceases to exist, so two groups omitted
62: Friction coefficient 0.62

Source: Skybrary.aero But I can't find one definitive source with all the possible reporting in a METAR, sadly.

It seems that this "Runway State Group" is mostly used in European countries, I can't find an example of it used in the U.S.. Here is one good reference which I think covers 99% of the METAR data for Europe.


Why is a feather check performed on a piston engine and how much RPM drop should be allowed? Light general aviation piston twins have full-feathering props. It's normal to perform feathering checks by pulling the prop level all the way down to feather, briefly, and observing the changes. First, why is the feather check performed and what is the pilot looking for during this check? Also, how far should the RPMs be allowed to drop? Could allowing a large RPM drop cause mechanical damage?

I realize that the specific allowances may vary between Lycoming and Continental engines and between models, but an average number would be helpful for the sake of the question.

The check is performed to make sure the prop will actually feather when pulled past the feather gates.  You'll be in a world of hurt if you are not able to feather a prop if the engine quits.

A couple hundeed RPM drop is acceptable (generally).  So called "deep cycling" props does nothing for you.  I suppose that there are some goofy mechanisms on oddball planes that require a deep cycle but the vast majority only require you to observe a small drop.

Remember, you are checking that the governor can control the prop, not how far the prop can travel.

Edit 1: I should add that you should just do whatever the POH/AFM tells you to do.

Edit 2: a pilot is typically looking for an oil pressure change, manifold pressure rise, and an RPM drop with each prop cycle.


Does opening of emergency doors over wings, lead to automatic lowering of the aircraft's wings? When the emergency doors over the wings are opened do the wings automatically lower themselves, to allow passengers to slide and reach the ground? Or does the pilot have to do it from the cockpit?

Wings are static.

The wings do not move in response to emergency door opening. There is no requirement regarding position of flaps, in most cases involving an emergency evacuation after landing, flaps are likely to be extended, but I don't think this is guaranteed.

Evacuation assistance

On commercial transport aircraft, there are marked routes on the wing for passengers and, if the exit point is more than 6ft from the ground, there are self-inflating slides that allow passengers to reach the ground.

Image from video by "Airline_Alex"

Evacuation on water

The inflated slides also work as life-rafts in the event that the aircraft comes to rest in water and has to be evacuated. Generally this applies only to slides at main exits, over-wing exits have slides that are not designed to be used in this way.

 US 1549

Regulations

The regulations in the USA say

  Each passenger-carrying landplane emergency exit (other than over-the-wing) that is more that 6 feet from the ground with the airplane on the ground and the landing gear extended must have an approved means to assist the occupants in descending to the ground.

The manufacturer must demonstrate that they can evacuate all passengers and crew to the ground within 90 seconds using only half the emergency exits.

This applies to aircraft capable of carrying more than 44 passengers.

Variations in arrangements by aircraft

Not all aircraft need overwing exits and slides. Here are some B757 variants

From CRJ Resets

Boeing 737

As commenters have noted, the 737 does not have self-inflating slides for its over-wing exits.

Pessini

The 737 system does not always work very well

  Passengers climbed onto both wings, which
  were slippery due to the rain, but were able to see
  the markings indicating the direction of movement. It
  was difficult to see the ground in the dark and some
  passengers were not aware that they should slide down
  the flap surfaces. Others expected to find an escape
  slide. On the left wing, some passengers slid down
  onto the ground and assisted others. On the right
  wing, fewer people slid down to the ground, while
  others re‑entered the cabin and exited it using door
  escape slides when it was apparent that there were no
  visible signs of danger.

- from AAIB report


Why does the mass penalty of cantilever vs externally braced wing decrease with aircraft size? From Wikipedia
Cantilever wings require a much heavier spar than would otherwise be needed in cable-stayed designs. However, as the size of an aircraft increases, the additional weight penalty decreases.
Why? The article does not have a citation there.

I think the Wikipedia article is wrong. Scaling the structure has other issues, but spar weight is not one of them.

The correct approach is to compare the scaling-up of a cantilevered monoplane wing with the same scaling-up of the equivalent wire-braced monoplane design.

In both cases, structural weight rises with the square of the linear scaling, for example doubling the length and span will give a fourfold increase in structural weight. The designer's challenge is to maintain the same weight of structure per unit of wing area, called wing loading, at scale.

The real difference comes not with scale but with speed. As the speed rises, the drag of the bracing rises more sharply, while the aerodynamic forces on the wing demand a stronger structure, especially thicker skinning. Eventually you reach a point, at around 200 mph (300 kph), where the drag of the bracing becomes excessive and the strength of the airframe will admit a cantilever construction without excessive additional weight penalty.

This was what triggered the historic change in design habits which the Wikipedia author was trying to explain and is, I suspect, what they would have said had they known better.

I have now substantially reworked that section of the Wikipedia article (current version here) and added a couple of citations.


Do airline pilots use speed stability to cope with unreliable airspeed indications? If not, why not? There have been a couple of airline accidents in which unreliable airspeed indications (or the fear thereof) have played a role:

Birgenair Flight 301 in 1996, where one of the pitot tubes was blocked, causing unreliable airspeed indications. At one point, the captain gave full throttle at low airspeed and high angle of attack, causing the left engine to flame out, and producing a spin.
Aeroperú Flight 603 in 1996, where the static ports were blocked, causing unreliable airspeed and altitude indications. The altimeter read high, and the plane crashed into the ocean.
Air France Flight 447 in 2009, where the pitot tubes iced over, causing unreliable airspeed indications, for about one minute. The aircraft entered a stall and the pilots failed to recover from it. Presumably, the pilots were uncertain about whether or not the airspeed indications were reliable, which contributed to their failure to recover.

Now, correct me if I'm wrong, but if your airspeed indicator fails in, say, a Cessna 172, you can use the aircraft's speed stability to maintain a safe airspeed. Do something like:

Move the trim wheel to the takeoff position.
Keep the nose from rising or falling suddenly (unless you're near a stall), but allow it to rise and fall slowly.
If the pitch stays constant with no elevator input, you're flying at a reasonable speed. If it tends to rise, you're flying faster than that, and if it tends to fall, you're flying slower than that.

Once you get the nose to stay still, you don't have to touch the elevator much unless you change power or configuration.

Yet, I've never heard of airline pilots doing anything like this. Do they do this sometimes, and if not, is there a reason why they don't?

(For that matter, is there a reason why my suggested procedure isn't a good idea in GA aircraft either?)

I don't believe the procedure you've described is formally recommended. Instead there is the unreliable airspeed procedure. The exact parameters vary among aircraft types but the basic principle is that by setting a specific pitch and power combination, you're guaranteed a certain airspeed/performance.

These procedures have existed for many years but perhaps the biggest change to come from the AF447 crash is an increased emphasis on this in recurrent training.

See: What is the correct crew procedure for an unreliable airspeed alert?


Is there a practical use of streaming data received from meteorological sensors installed on aircraft? I am not the practitioner in the field of aviation, and my knowledge is mainly theoretical. Therefore I would be happy to get a feedback from practitioners on the following topic: I was reading about the use of aircraft as meteorological sensors. I wonder how this streaming sensor data is currently used by pilots? Is it validated for logical consistency?

Moreover, I was thinking about possible applicability of such data, and in my opinion it could be used by Air Traffic Controllers for enabling/disabling airspace sectors for free routing (direct flight). Does it make sense from a practical point of view? This data could be shared via System Wide Information Management among different aviation stakeholders.

Yes, such a system would be beneficial to pilots, ATC, and meteorologists.
Pilots are very concerned with meteorological phenomena like icing and turbulence (for safety-of-flight reasons) and winds aloft (for fuel planning purposes). Pilot reports of weather conditions (PIREPs) may be delivered to ATC or Flight Service for local and long-range dissemination. This information helps other pilots avoid dangerous conditions and helps weather forecasters improve their forecasting models. The 7110.65 2–6–2 PIREP Solicitation includes a note:

Routine PIREPs indicating a lack of forecasted weather conditions, for example, a lack of icing or turbulence, are also valuable to aviation weather forecasters and pilots. This is especially true when adverse conditions are expected or forecasted but do not develop or no longer exist.

Live or periodic streaming of sensor information already present on larger airliners (particularly icing status, outside air temperature, and wind velocity, but other sensors could be installed as well) would relieve pilots and controllers of having to discuss routine PIREPs on frequency and could be developed into graphical systems that automatically display hazards. Delta is using vertical accelerometers in some of their aircraft to automatically report turbulence in real time and transmit that information to airborne flights; obviously they see this as a business advantage and are loath to share it, but the system could be expanded.
Whether the costs and complexities of such a system would be worth the real-time and forecasting benefits, I don't know.


How do paraglider controls work? I recently got into paragliding, but I am quite confused on how their controls work.

I am quite aware of how the control surfaces work on fixed-wing aircraft. Let's use for example an aileron. Lowering the aileron, increases the angle of attack of this wing, increasing lift, making the aircraft to roll.

On paragliders, things seem very confusing for me.

The theory

The control acts as a "brake". By pulling one brake there is an asymmetry in drag, causing the wing to yaw. Due to the yawing motion a difference in airspeed in the two sections of the wing causes a difference in lift, having as a side-effect the aircraft to roll too.

Thus pulling the right brake, makes the aircraft yaw to the right, and then roll to the right.

My thoughts

While one of the controls is pulled, the trailing edge gets lowered. This also lowers the aft-portion of the wing cord, increasing the angle of attack. I would expect that this would also increase the lift of this portion of the wing causing the to raise.

Thus pulling the right brake, makes the aircraft roll to the left.

For me, there seems to be nothing special with brakes. For example, in case of emergency (brake line failure), I can use the C-risers to alter the angle of attack of one part of the wing to turn. But then again the same paradox happens. I pull the right C-riser, the right wing has increased angle of attack, but still it dives!

How exactly do paraglider "brakes" work? What makes them behave as a brake, and not as an aileron?

I think the answers so far neglect the curvature of the paraglider sail. Changing the lift at the tips creates an imbalance of the side force caused by the downward pointing tips. This side force will pull the sail sideways and cause the sail-pilot combination to roll.

To illustrate my point, I shamelessly copied this picture and added vectors perpendicular to the sail surface, like this:

Now you will see why I used a photo: The tips of the sail are almost vertical so the side force caused by pulling the local trailing edge down has a sizeable lever arm with the center of gravity (which I assume to be near the pilot's head). @Ken's graphical representation hides this detail and fails to show the lever arm.

This side force will do two things:

It pulls the sail sideways so it will shift away from a position vertical over the center of gravity.
It causes the whole sail-pilot-arrangement to roll, in this case clockwise (from the perspective of the observer).

Next, the increased drag on the pulled-down side will retard this part of the sail, adding a yawing moment and yawing rotation which will now stabilize the bank angle. The whole paraglider turns. With the banked paraglider the side force component of the lift force compensates the centrifugal acceleration (rotating system of reference) rsp. creates the centripetal acceleration (fixed system of reference) which keeps the yaw rotation alive.

Also here the rolling motion is started by a lateral shift of the lift vector, but the vertical component of lift counteracts the desired rolling motion. Only the lateral force, which dominates due to the inclination of the sail, is responsible for the rolling moment.

Exiting the turn is achieved by doing the same, only in reverse.

Now for pitch control: Pulling the trailing edge down on both sides increases camber and lift, so the paraglider rises and the added drag retards the sail. This causes a pitch-up motion around the center of gravity and slows the paraglider down further. Relieving tension on the risers reduces camber, lift and drag so the sail accelerates and the whole paraglider pitches down around its center of gravity.

Therefore, any rotation is around the center of gravity. This does not need any hypothetical pendulum movement or "pendulum effect", which does not exist and is a fallacy anyways.

Oh, and one last word for the believers in the cult of the pendulum effect: I don't care that you downvote what you don't understand. Please try and argue logically and while observing the laws of physics. This, however, will involve the clear danger that you will lose your faith. Apologies for any loss of direction in your lives. That direction was wrong, anyway.


Why are ARINC labels are transmitted LSB first? Why are ARINC labels reversed before transmitting them to destination and the LSB (less significant bit) is transmitted first:

Actual label = 0x58: 0101 1000
Transmitted as label = 0x1A: 0001 1010

Is there any particular advantage over formal transmission and label reversed transmission?

Also all systems are designed for odd parity. Why is that?

Revised answer

The ARINC 429 frame is composed of 4 bytes (32 bits numbered by convention 1 to 32 from right to left). Bits 11 to 29, which are the payload of the frame, can represent data using different formats, e.g. binary or BCD data. The label is used to indicate the content of the data field, and which format is used, and is necessary to convert data bits into useful information.

ARINC 429 frame used in BCD format (source)

The label is usually spelled as an octal quantity (rather than decimal or hexadecimal), e.g. 010101102 will be spelled as 1268 (01 010 110) rather than 5616 (0101 0110) or 8610.

The way bits are numbered somehow makes the label the "least significant byte" and bit 1 the "least significant bit". However:

The format above is for naming and ordering information, but doesn't tell how bits are really stored in components memory.
In digital engineering, bits are usually numbered from 0 to 31, as the number actually indicates the weight of the bit: 0 is 20 (1), 4 is 24 (16). In this convention bit 0 is indeed less significant than bit 31.

The ARINC 429 data bus is composed of 2 wires and the ground reference. When an ARINC frame is transmitted on the bus, it must be serialized, which means sending bits one at a time. The transmission order is:

The label byte transmitted first, "MSB" first: 8-7-6-5-4-3-2-1.
The other bits are then transmitted "LSB" first: 9-10-11 ... 30-31-32.

So, back to your question, why not using the left to right order for the whole frame? Or at least the left to right order for the label if it's sent first?

From Wikipedia:

  Like CAN Protocol Identifier Fields, ARINC 429 label fields are
  transmitted most significant bit first. [...] This notional reversal
  also reflects historical implementation details.

So let's assume there is no reason to reverse the label bits, other than historical implementations that wanted to mimic the behavior of CAN. In the French version of this Wikipedia article, it is stated that this reversal helped to read the label when looking at a frame using an oscilloscope. The MSB arriving first also appears on the left of the display (the signal on an oscilloscope generally enters on the right of the screen and scroll towards the left side as a function of the time):

ARINC 429 bit stream on a datascope (source)

I'm a bit baffled by this explanation, as engineers working with binary representations are used to read them is both directions quite easily after a short time.

Still according to Wikipedia, hardware used to serialize and de-serialize the ARINC word were actually insensitive to the order of the label bits:

  ARINC 429 transceivers have been implemented with 32-bit shift
  registers. Parallel access to that shift register is often
  octet-oriented. As such, the bit order of the octet access is the bit
  order of the accessing device, which is usually LSB 0; and serial
  transmission is arranged such that the least significant bit of each
  octet is transmitted first. So, in common practice, the accessing
  device wrote or  read a "reversed label" (for example, to transmit a
  Label 2138 [or 8B16] the bit-reversed value D116 is written to the
  Label octet). Newer or "enhanced" transceivers may be configured to
  reverse the Label field bit order "in hardware."

About odd parity

It is usual in digital serial communication to use data value changes (0 to 1 or 1 to 0) to derive a clocking clue in order to ensure the receiver samples the line voltage at the right time. This is less expensive than transmitting the clock signal on a separate wire. As each bit lasts the same time, the receiver uses a local clock that is roughly in sync with the transmitting clock. To actually maintain the synchronization in the long term, the receiver uses the raising and falling edges of the signal to detect bit borders and phase-align its clock.

Resynchronization of receiver clock on front and rear edges of the signal

That works well if the delay between edges is short. This means avoiding sending long series of 0 or 1. The parity bit can help. For data containing only 0 bits (or 1 bits) the odd parity bit will be 1 (or 0), preventing long edgeless streams to occur.

In some uses of ARINC 429, the parity bit is not transmitted, and is used for data, but nevertheless it seems superfluous for sync in this case since the link protocol is 3-level with return to zero, ensuring two edges for each bit of data.

Return to zero coding (source)


Why might a moving map Android app consistently record altitude off by 120 feet? I fly Cessna 172 and Cherokee Warrior both older models without a GPS.
I have a very useful freeware navigation app on my android phone which is really good in every regard other than it always reads my altitude a bit higher than were I am.

Best example is when I am taxiing my track and its projection on the ground which should have merged are separate.
The app does not have any setting for adjusting this! I wonder if there is any explanation for this. The app superimposes my 3d coordinates on a moving sectional chart. I re-installed the app which didn't help.
Here is my flight KML on Google Earth on my approach and landing to Catalina airport last Sunday.

While I think it would be unusual in 2016, it is possible that the unit is recording altitude as distance from the ellipsoid rather that distance from geoid (model of MSL on the earth used by the receiver).

If you use this NOAA calculator, you can find the geoid height for a single spot.  At the location you show, it's 37m or 122feet.  That's suspiciously close to the difference you're seeing.

Output from GEOID03
                                         latitude        longitude       N
Station Name                            ddd mm ss.sssss ddd mm ss.sssss  meters
USER LOCATION                            33 24 15.77000 119 24 57.49000  -37.259

If this error is constant over time (multiple recordings), then I think it's more plausible than random error sources.


How does an aft CG affect Vmcg? How does an aft center of gravity affect the minimum control speed on ground (Vmcg)? Does it reduce all V-speeds?

$V_{mcg}$ is the minimum speed on the ground at which control is possible using rudder alone upon failure of critical engine. From FAR Part § 25.149:

  VMCG is the minimum control speed on the ground, and is the calibrated airspeed during the takeoff run at which, when the critical engine is suddenly made inoperative, it is possible to maintain control of the airplane using the rudder control alone (without the use of nosewheel steering), as limited by 150 pounds of force, and using the lateral control to the extent of keeping the wings level to enable the takeoff to be safely continued.

As the center of gravity moves forward, the rudder arm increases, improving rudder effectiveness. As a result a forward cg results in a lower $V_{mcg}$. This is the reason the tests for $V_{mcg}$  is carried out at the rear most possible center of gravity. From Advisory Circular 23-8C: Flight Test Guide for Certification of Part 23 Airplanes:

  For rudder limited airplanes with constant aft c.g. limits, the critical loading for VMC testing is most aft c.g. and minimum weight.  Aft c.g. provides the shortest moment arm relative to the rudder thus the least restoring moments with regard to maintaining directional control.

as,

  There are variable factors affecting the minimum control speed.  Because of this, VMC should represent the highest minimum airspeed normally expected in service.


You are a commercial airline pilot flying between two large US airports. Who do you talk to along the way on the radio and for what purpose? I am a non-pilot who has been listening to various ATC feeds and trying to understand what is going on. I've been able to find surprisingly little information on the various layers that make up the air traffic management system but from listening to SFO atc feeds online, I've come up with the following outline of what radio contacts on a domestic US flight would look like:

ramp control - I know it exists, but have little idea what they do. I would think they manage push back requests, but I hear that on ground frequencies so that cant be strictly correct.
ground control - request pushback, get taxi instructions and hand off to tower when near runway
tower - get takeoff clearance. hand off to departure during climb out
departure - request bearing and altitude, receive vector out of departure airspace in proper direction
center controller (various) - not sure what the relationship is here (pilot requests bearing, altitude, or is it all automated and known to controller) controller maintains separation mins.
approach - there seem to be two stages here? One controller asks for intentions, gives a waypoint and altimeter data. A hand-off is then done to another approach controller who vectors traffic into the pattern for landing, calls base, etc.
tower (arrival) - everything should be all spaced out, so controller just works on fitting departures in. Announce your intention and be granted clearance with initial taxi instructions.
ground (arrival) - get further taxi instructions to gate.

So how accurate is my picture of whats going on? I'd be interested to know exactly how the whole process works from the pilot's perspective. Some specific questions about the process:

How automated are aspects? Do controllers ever have information from filed flight plans or is all info passed over radio?
Whats the deal with the two approach controllers? (BSR2/MOD2 versus landing)

This is a duplicate question, although this is actually asked much better than the other one is.

Generally, your picture of what goes on is pretty good.

As mentioned in a comment, there is Clearance Delivery, which issues your ATC (IFR) clearance.  Except... at most of the larger airports, that function is now automated and the clearance is passed via ACARS, which is essentially a closed e-mail system direct to the aircraft.  So it's actually common to NOT talk to Clearance Delivery now in airline operations.  You still do at smaller airports that don't have the PDC (Pre-Departure Clearance -- the automation that sends the clearance itself over the ACARS network) installed, and you'll call Clearance if you have a question about how you were cleared, and in a few other uncommon situations such as a reroute that's issued after you got your initial clearance.

Ramp Control exists at some airports, and it's essentially a "ground control" function for a separate, isolated area.  So in Atlanta, you actually have several of these, one for Ramp 1, which is the east side of the A terminal and the west side of the B terminal, another for Ramp 2 which is the west B gates and the east C gates, and so on.  They control the "in's" and "out's" of their respective ramps, including coordinating who will push back when.  As the aircraft leaves the Ramp Control area, it's handed off to Ground Control, and things proceed normally.  NOT all airports have this, and not necessarily every terminal (or gate) at a given airport will be controlled by Ramp Control.  Gates that aren't controlled by Ramp will typically call Ground to push.  SFO is like this.

Typically, controllers see most of what you've filed on your flight plan, so they know before a given flight is passed to them that SkyPig 23 is a 757 that departed JFK, is filed for FL 360, and is going to ATL with a routing of whatever set of airways, points, and STAR.  NOT every controller will see everything in the flightplan; I think the TRACON sees remarks but the Center may not, was one example I vaguely remember from a tour a while back.  But essentially they know who you are & what you're doing by the time you switch to their frequency.  Sometimes things get confused or the pilot requests a change (in altitude, routing, or whatever), and sometimes the controller has to change the plan up, but generally you check in at whatever altitude (and climbing/descending to whatever, if that's the case) and the controller acknowledges and that's that until he has a new clearance (further climb or descent, new routing, approach clearance, etc) to issue.

Mostly now, airlines are filed on both a SID (standard instrument departure) and a STAR (standard terminal arrival route) so  "requesting vectors" in pretty uncommon.  Could happen if your FMC (flight management computer) dumps your routing, but that's rare, and usually operator error when it does happen.  A departure or an arrival might INCLUDE vectors (i.e. fly heading 350, climb & maintain 5000', expect radar vectors to JEBBB), but in those cases it's expected.

Multiple approach controllers are pretty common, especially in the larger terminal areas.  Often you'll be switched from Center to one Approach Control frequency, who may be handling all arrivals from the east, with another handling everything from the west.  Then as you get close to the airport, they hand you off to "Final", which is still "XYZ Approach" but his job is sequencing everybody onto the base, dogleg, and final, clearing you for the approach, and then handing you off to tower.  If the first Approach controller owns airport to 30NM out surface to 17,000', the "Final" controller might own 7000' and below within 12 miles, and Tower owns 3000' and 5 miles.  Serious generalization there, but that's the idea... each successive layer of the onion owns a smaller but busier piece of airspace.  Letters of Agreement between the Center and the TRACON and then procedures within the TRACON will spell things out in great detail.

Hope that's helpful.  Good question asked well; hopefully this won't be lost to memory just because it was asked 2nd.


Difference between constant speed and variable pitch propellers? Here is how I currently understand constant speed and variable pitch propellers.

In constant speed propellers, the pilot controls the governor via the prop control knob and the speeder spring, flyweights, and other parts of the system act accordingly to increase or decrease the amount of oil entering the propeller hub. This automatically changes the blade pitch in order to maintain a certain engine RPM, even in an over speeding or under speeding condition. Setting the engine at 2300 RPM will keep the engine at 2300 RPM at all phases of flight even if no adjustments to the blue knob are made.

In a variable pitch propeller, the pilot adjusts the pitch via the prop control knob for different phases of flight. However, the blade pitch doesn't automatically change in order to maintain a certain engine RPM. The pilot would need to adjust the prop control in under speeding or over speeding conditions. Setting the engine at 2300 RPM will not keep it at 2300 RPM if climb or descents are made without adjustments to the blue knob.

Is my understanding of both kinds of propellers correct?
Are the flyweights only found in constant speed propellers? Are there any other parts in the system that can only be found in constant speed propellers and vice versa?

Your understanding is essentially correct.  A constant speed prop is governed.  A variable pitch prop is not.

Constant speed props are also variable pitch.

A constant speed prop will only maintain speed if there is sufficient power from the powerplant.

Most constant speed props will require slight tweaking to maintain the rotational speed during a flight.

Constant speed props require a governor, and the flyweights are a component of a governor.


Does altitude influence the flight range of an engine driven aircraft? I'm reading a chapter about flight performance during the cruise phase and the influence of altitude.

My book (Aerodynamica, prestatieleer en vliegtuigtechniek by Bas Vrijhof on page 155, written in Dutch) describes the following:

  Naarmate de hoogte toeneemt, neemt de luchtdichheid af. De afnemende
  luchtdichtheid heeft een effect op het benodigde vermogen, en dus op
  de prestaties. Om bij een lagere luchtdichtheid te vliegen is meer
  vermogen nodig. Om bij een lagere luchtdichtheid voldoende lift te
  houden moet de TAS toenemen, dat kost vermogen.

Translated to English:

  Airdensity decreases with altitude. A decrement in airdensity influences flight performance. To fly at a lower airdensity more engine power is needed. To maintain enough lift at lower airdensity the TAS needs to increase, this comes with the cost of engine power.

This is totally clear to me. Then the book says.

  Endurance...
  Meer vermogen betekent een hogere brandstofverbruik. De endurance neemt daarom af.

Translated:

  Endurance (amount of fuel per hour), more engine power means a higher fuel consumption. The endurance decreases with an increase in engine power.

Again, this is also clear to me.

  Range...
  De range verandert niet bij toenemende hoogte. De range is niet afhankelijk van de vlieghoogte. Wel neemt de Vmax range toe als de hoogte toeneemt.

Translated:

  Range (maximum flight distance, amount of fuel per km) doesn't change with an increase in altitude. However, the Vmax range increases with an increase in altitude.

The last sentence is confusing me.
Because my instincts say that a higher fuel consumption comes with a shorter flight distance. So my question is:

Is it really true that the altitude doesn't influence the flight range? Does the book has an error?

Your book may not be wrong, but it is misleading, because the answers depend on the the aircraft type, and the type of engine. (piston, turboprop, turbojet, turbofan)

In general:

All aircraft have the best range at higher altitudes.

Piston powered aircraft have the best endurance at low atitude.

Jet powered aircraft have the best endurance at their optimum altitude. (not too low and not too high)


What is this substance on these pipe fittings? In this picture from a different question I noticed some kind of colored substance across many of the fittings. It is in several different colors and looks like it might be sticky. (The pink kinda looks like chewing gum. That's even worse than the duct tape question. ;) I have circled some of them in yellow.

Is it there to mark whether the fittings have been tampered with?

That is called "torque seal" or "torque putty".

It is used by mechanics to mark a part after it has been properly torqued, and thereafter to serve as an indication if loosening has taken place. The torque seal hardens after application and serves as a marking mechanism that will show—by cracking and/or flaking of the putty—if the part has loosened resulting in rotation.

The photo below shows torque seal as initially applied (left), and then with the seal broken following loosening an rotation of the nut (right).

Image courtesy of theleanwarehouse

Just to be clear, the torque seal does nothing to secure the part or otherwise serve any locking function. It serves only as an indication.

Dykem is one manufacturer of torque seal.

Here is a page with additional information about torque seal from a retailer.


How useful is skyscrapers and high buildings lighting to get a visual reference for pilots by night? (not obstruction lights, just general lights) I would like to know how useful is skyscrapers and high buildings lighting to get a visual reference for helicopter and small planes pilots by night in cities like New York. This is different from obstruction lights. I'm talking about shapes and colors.

Buildings and their lights can be very useful to aircraft in VFR conditions at night to orient themselves regardless of the type of aircraft.

Three building lighting examples that I often used were:

When flying Metroliners for a commuter airline that operated to Walla Walla, WA, I would call for a visual approach to the airport when I saw the Washington State Penitentiary. It's distinctive pattern of bright lights, especially those around its perimeter, made it stick out like a sore thumb.
When flying night freight in 727s, from many miles out, we could see the blue neon lighting outlining the Sofitel Hotel just across the road from the southwest corner of Miami International Airport.
Back in the late 1990s, the high rise hotels along the beach at Tel Aviv had a distinctive pattern. Head just to the south of them and you were lined up for Ben Gurion airport when approaching from the west.

Really, if you go into most urban airports regularly at night, you learn to recognize the pattern of the lights and can use them to orient yourself.

Freeways with heavy traffic are also helpful. Interstate 5 running south from Albany, OR to Eugene, OR has a 33 mile straight stretch that produces a line of headlights. Aircraft approaching from the north can aim about 15° right of that line and be headed to the Eugene Airport.


How can I calculate takeoff distance in a generic way? I want to build a generic performance app.

For many of the aircraft the only available method is using the takeoff charts manually (I'm not referring to the more simple-to-calc short field charts but to the charts which consider also the wind components).

Is there a way to convert these charts into numbers that allow me to calculate the output once certain parameters (weight, OAT, PA) are entered? Is this data available somewhere?
Or do pilots do it with chart and pencil for every flight?

(Not a pilot myself.)

example of takeoff chart :

For air carrier type aircraft, the chart you show in your question (sometimes called a "run-around-chart") is generally digitized in some form (performance data printed out on dispatch release, or in a Flight Management Computer on board the aircraft or handheld device provided to crews).

For light aircraft, likely there are many software apps available for the most popular types.

Here is a picture of an Apple APP I noted online for a Cessna 172.


Can a SIGMET last less than initially forecasted? Lets say I have a SIGMET that says VALID 310800/311200.

Is it possible that by 1100 the SIGMET would not be active anymore or it will always be valid until the published end time?

Yes, SIGMETs can be cancelled.

A quick web search uncovered, for example, the ICAO Asia/Pacific Regional SIGMET Guide, 6th edition, May 2017, which on page 3-16 (PDF page 24) says, in part:

  3.5.4. Cancellation of SIGMET

  3.5.4.1. Annex 3, 7.1.2 requires that "SIGMET information shall be cancelled when the phenomena are no longer occurring or are no longer expected to occur in the area”.

  3.5.4.2. As such, it is mandatory for an MWO to cancel any SIGMET that is currently valid but for which the specified phenomena no longer exists or is expected to exist.

It appears that the cancellation is communicated with a new SIGMET, valid until the expiration time of the original SIGMET, specifying "CNL SIGMET XX" where XX is the original SIGMET sequence number. Sections 3.5.4.3 and 3.5.4.4 specify the format of the cancellation SIGMET. MWO is shorthand for Meteorological watch office.

SIGMETs can also be amended or corrected after issuance.

From the same document, the very next section, snipped for brevity:

  3.5.5. Amendment/correction of SIGMET

  3.5.5.1. If it is known that an existing SIGMET no longer accurately describes the existing or expected future evolution of the phenomena a new SIGMET, correctly describing the hazard should be issued, followed immediately by a cancellation of the original, erroneous SIGMET. [...]

So it is entirely possible that on the 31st at 11:00, the SIGMET originally issued with a validity of 310800/311200 is no longer in effect.


What are the typical max range and max flight time (endurance) Mach numbers for the A320? What are these speeds? Average numbers will be fine.

Max range, or Cost Index 0, actually has many Mach numbers depending on the weight, FL, and wind. Its range is from approximately Mach 0.57 to 0.78. Below is part of the relevant table from the FCOM (other intermediate weights are omitted):

The FCOM does not state a max endurance speed, just that in a hold, the FMS will fly the lowest of the "ICAO speed limits, the max endurance speed, or any speed constraint."

So the FMS will take care of calculating it, but like max range, it will vary.


Does the FAA have risk targets for specific activities? Does the FAA have or publish goal values for various risks?

I'm looking for something that says (e. g.) "We rate passenger airline flights as 'safe' if there are fewer than X casualties per million flying hours" or the like.  Even better would be some kind of matrix reflecting "safe", "of concern", "hazardous", "dangerous", etc.

"We rate passenger airline flights as 'safe' if there are fewer than X casualties per million flying hours"

I am not aware of any official FAA document that states such sentence. However, something similar can be deduced from safety requirements related to the development of aviation systems.

ARP4761 is a well-known standard for conducting safety assessments on civil airborne systems and equipment. A probability of failure occurrence associated with the severity classification can be found on page 14, which shows the following table:

As it can be seen, the maximum allowed occurrence of a catastrophic failure is 1.0E-9. Playing a bit with the numbers, this could be translated in:

  Commercial aviation flights are considered as 'safe' if there are fewer than 1 catastrophic condition per billion flight hours

One could assume that a catastrophic condition can lead to fatalities, and this seems reasonable. But I'm afraid that this presumption is not written in official FAA publications (I might be wrong).

P.D.: Now, where these probabilities of occurrence come from is another interesting (and un-official) story, but I think is out of the scope of this question for the moment.


How do airplane designers determine flap deflection angles for flaps of various % wing chords? How do airplane designers determine flap deflection angles for flap chords of various % wing chords? E.g. flap chord of 30%, 40% or 50% of wing chord.
I understand most flaps' chords are about 25% of the wing chord, but I understand Rutan's Grizzly had a flap chord of 55% of wing chord.
I assume you could use theory to figure this out, but that doesn't mean it works like that in the real world.
Is this information readily available in graph or chart form?
I thought I saw a link in a post on StackExchange re this from Peter Kaempf, but I can't find it.

As a rule of thumb, flap effectiveness (that is lift coefficient change per flap deflection angle or c$_{L\eta}$) is proportional to the square root of the flap's percentage. A more precise formula was derived by Hermann Glauert (search for the name and you will find that elusive answer by me)
What also changes is the hinge moment needed per degree of flap deflection; this, however, changes with the square of the flap's percentage. By trading off effectivity versus force, a designer will find the best flap percentage for each control surface.
Next, wind tunnel data is used for finding the effectiveness over deflection, because this value decreases with higher deflection angles and is hard to catch by CFD because viscous effects are strongest near the trailing edge and need lots of effort to model correctly. The desired control response and trim should be achievable with maximum deflections of below ±15° (the useable range is wider for smaller percentages) and the flight test will reveal whether the calculations were correct.


Do helicopters fly the same instrument approaches as fixed-wing aircraft? Do helicopters fly the same instrument approaches as fixed-wing aircraft? If so, which approach category and minimums do they use?

Helicopters can fly the same standard IAPS.  The visibility minimum is the greater of

a) one half the Category A visibility minima,

b) 1/4 statute mile visibility, or

c) 1200 RVR

We use Category A's MDA/DH. See the AIM 10-1-2 for more information.

Also there are Copter IAPs (example here), and the viz/altitude minimums are stated on those plates.


Why are expletives deleted in CVR transcripts? I was reading this when I envisioned this question. In this  age, I don't think expletives can offend anyone. They may not be germane, but oughtn`t authenticity outweigh relevance?

[O]ughtn't authenticity outweigh relevance?

The goal of a CVR transcript is not to give an authentic account of what happened in a cockpit. It is to give a relevant account of what happened in the cockpit.

A typical investigation board (NTSB or equivalent) has a very specific goal: investigation what happened to prevent it from happening in the future. To demand exceedingly intrusive measures into the privacy of a pilot - like installing a number of microphones recording everything they say - safeguards are put in place (remember, in virtually all other sectors, an employer installing microphones in the workplace would rightfully face serious criminal charges).

For example,  the investigation typically explicitly excludes civil and criminal accountability questions. As additional guarantees to the privacy of the pilot, only a selected number of people get access to the original recordings, and they decide on the content of the transcript.

As their goal is to give a relevant account, not an authentic account, they can decide to withhold information that could be harmful to relatives (like the exact language used when their loved ones faced imminent death), if the information does not help in establishing the goal of preventing future accidents. Unless the expletive is pertinent to the investigation (for example, if it could be misheard as a specific command), it is left out.


Why does the DC-9 have engines that are angled slightly upwards? As per subject, I see that the above mentioned aircraft and some others that have engines mounted on the rear of the fuselage, have the engine intakes tilted slightly upwards. Why has this solution been implemented?

Wouldn't it be better having them parallel to ground?

If the airflow of the exhaust is directed slightly towards the ground would't it affect the plane causing it to push the nose downwards?

I believe there this could be traced back to the so-called downwash angle of the air coming off the wing.

Looking at the picture below (and put on some imaginary engines on the back like the DC9/MD80), it becomes clear that by mounting it upwards by a few degrees would make it meet the airstream at the streamline angle:

You would want to that streamline to interact with the aircraft fan blades with the wind at the same angles. I would imagine that if you did not do this, it would result in an uneven force on the fan blades, which would probably be highly undesirable.

Looking at the drawings of the DC9, it might appear that the shorter version has a higher angle, and that the longer version has a smaller angle, as further from the wing the downwash angle is less, as could also be seen above.

For a little further analysis, the following is apparently off the DC-9 Manual, and might shed a little more light on the design, in particular in the last paragraph:

  "The nacelles have been designed to enclose the JT8D engine with a
  shape that will allow the highest installed performance ( thrust minus
  drag ) possible at all flight conditions. The nacelle nose cowling is
  nonsymmetrical to keep the drag to a minimum at the high mach number
  cruising conditions.

  The nonsymmetry is required to maintain an extremely flat nacelle line
  adjacent to the fuselage, such that the nacelle supervelocity
  increments, when added to those from the pylon and fuselage, do not
  lead to super-critical velocities with resultant shockwave losses.

  The nacelle axis and pylon have been set at 3 degrees angle of attack
  relative to the fuselage to align them with the local flow conditions
  prevalent at cruising conditions. This is necessary to eliminate the
  interference and high induced drag that they would otherwise be
  subjected to. The nozzle is then canted upward relative to the engine
  axis, thus eliminating the pitching effects due to thrust."

Following picture appears to show the nozzle angle is in fact more horizontal than the entry angle, with the underside longer than the upper side:

Thus, while it might not aesthetically pleasing maybe to have it off by a few degrees from the fuselage body, but it does bring advantages when seen with the airflow, and resulting concerns about that the thrust is not being vectored along the body axis is corrected by the nozzle.


Are there any publically available recordings of conversations between ATC and pilots? It can be either audio or transcription.
It would be perfect if it also contains aircraft tracks.

LiveATC.net has live and recorded ATC radio for most countries/airports.  There are lots of flight tracking websites, e.g. FlightAware, that have live and recorded radar/ADS-B tracks.  You'll have to correlate the two yourself.


Can the iPad use GPS data from a Garmin aera 660? Can the iPad use GPS data from a Garmin aera 660 portable GPS unit? I assume this would happen via Bluetooth, if it's possible. I would like the more accurate WAAS data from the Garmin for use with CloudAhoy.

Most bluetooth external GPS devices conform to NMEA and are compatible with CloudAhoy. I don't know if that's the case with Garmin Aera 660.

However, if used with Garmin Pilot, then you can always export GP's track log to CloudAhoy. (I'm associated with Cloudahoy)


What is the physical-based/theoretical reason why supersonic area ruling works? Supersonic area ruling is used to reduce the drag rise that occurs as aircraft pass through the transonic regime. While it has been validated experimentally, what is the theoretical/physics-based reasoning why area ruling reduces the drag?

I know that the following answer suggests that it is simply a "rule," but could someone shed more light into WHY the area ruling technique works? (What is the "area rule"?)

That's because Bernoulli's principle breaks down at and above supersonic velocities.

In the subsonic realm, when an atom is sped up, it doesn't have much time to spend pushing around a fixed place (it passes by that place quickly). In doing so, the static pressure drops as the speed increases. And in doing so, the neighboring atoms, who are also not doing as much repelling, can group together.

In Busemann's and Whitcomb's metaphor, the tube of air gets thinner.

In supersonic flow, much like the reason behind shockwaves, the communication between atoms breaks down. They are moving too fast for the air to carry out the effect of moving fast (pressure disturbances cannot propagate), and thus, the tube of air doesn't get smaller. This results in an increase in the drag versus the calculated one; instead of the tubes getting smaller, they remain both big and fast.

Drag increase is highest when there is a sudden cross sectional change. Consider the face of a box compared to a cone pointy-end first.

Applying that, the supersonic area rule aims at keeping the cross sectional size of the whole vehicle constant without abrupt changes. If thinning the fuselage where the wing meets it is not possible, then gradually thickening it before the wing starts is also an option.

The tubes not getting smaller, and considering the cross section of the vehicle as a whole, were the reasoning behind the area rule.

Reference and further reading: Chapter 5 - The Whitcomb Area Rule - From Engineering Science to Big Science, via NASA.


Why does the canopy of the F-22 Raptor have a golden tinge? Why is gold used in the canopy of the F-22 Raptor?

In order to reduce the radar cross section (RCS) of the aircraft, the F22 canopies are coated with a thin layer of Indium Tin Oxide (ITO), which gives it a golden tint.

From the book F-22 Raptor by Bill Sweetman:

  The F22 Raptor canopy ... comprises two sheets of polycarbonate, sandwiched between two layers of optical glass, fusion bonded in an autoclave and drape formed over a canopy blank ... A metallic coating of indium-tin-oxide is added to the canopy to reflect the radar waves, giving it a golden tint.

They are used in stealth aircraft to reduce the RCS. From globalsecurity.org:

  A window member composed of a transparent resin or inorganic glass with a transparent conducting film such as gold or ITO (indium tin oxide) coated thereon, is used as an electromagnetic wave shield window for stealth aircraft. Applying such transparent conducting film enables, while maintaining transparency to visible radiation, both a radio wave stealth property which scatters radio waves in various directions so as not to be detected by radar, and an electromagnetic wave shield property which prevents harmful electromagnetic waves, except for visible radiation, from invasion into an aircraft.

This technique is used in other aircraft like the EA6B Prowler and the F-35.


Why is the critical AoA of most flaps 40° when most airfoils stall at 15°? A plain flap deflected 40 degrees is largely stalled (...)

Flaps stall at 40° according to an answer on Quora, why is this so when typical airfoils stall at 15°?

Flaps are located in the downwash of a wing. Their local angle of attack is not equal to their deflection angle, but equal to the deflection angle minus the angle the downwash leaves the trailing edge of the wing.


How do a narrow airframe and canards keep supersonic shock waves from coalescing? In a follow up to my previous question:  How does the X-59 acheive quiet supersonic flight?

Canards and a long narrow airframe were cited as design choices to prevent shock waves from coalescing. How does this design shape achieve this goal? How does this shape affect the flow field such that the shockwaves do not coalesce?  I want to understand this in terms of the physics and aerodynamics of the shockwaves. It would be useful to see a side-by-side comparison to other supersonic aircraft like the Concorde.

After a lot of digging around, I found this.

  ...the main noise-reduction technique is to redesign the aircraft to
  reshape the acoustic signature. A sonic boom consists of a double bang
  on the ground that is caused by two large pressure pulses. As the
  random shockwaves travel away from the aircraft at the speed of sound,
  they pile up and coalesce. The engineers aim to reshape the acoustic
  signature of the boom by preventing sonic waves piling up.

  “You shape every aspect of the airplane to make the shockwaves as
  close to equal strength as you can. When they’re as evenly distributed
  as possible along the length of the airplane, changes in temperature
  are small and the waves don’t coalesce. When the signal reaches the
  ground, it’s smeared out. Instead of two loud bangs, there’s a
  pressure change and a soft thump,” says Peter Coen, NASA’s commercial
  supersonic technology project manager.

Source: Aerospace Testing International, Dec 2018: Supersonic testing: Boom or bust?

I realize this doesn't answer the question of the physics, but I thought i would stick within my paygrade. Someone else who better understands what the fluid is doing can tackle explaining that.


Can a helicopter be "stalled" by pulling too hard? In the PC game Arma 3, there is a helicopter-race game mode, in which the player tries to manipulate the helicopter to pass a serious of hoops as fast as possible in an AH-6.

In the game, if the player banks the helicopter then make a sudden and hard pull on the cyclic to make a turn while max collective is applied, the low rpm warning will sound, followed by a loss of power and a few seconds later loss of attitude control. The aircraft can be recovered by:

Lowering the collective
Releasing stick pressure
Leveling the aircraft

After which the rpm will increase back to normal again.

My guess explanation is as follows:

When max collective is applied, the blades are working hard by cutting air at a large angle of attack
When the aircraft is banked and aft cyclic is applied, the airflow relative to the aircraft is like when it is in a very high rate descend
The critical angle of attack is exceed on the rotor blades, they stall and cease to produce lift
Due to the greatly increased drag, the main rotor is unable to keep rpm up with its power, therefore rpm decreases
As rpm decreases, less lift is produced and the stall worsens.
By reducing collective, the drag of the blades is reduced, and rotor power is enough to keep rpm at normal range again.

Is my explanation correct? Can this happen in real life?

About High Energy Maneuvers in a Helicopter: not quite like fixed wing flying

  ... if the player banks the helicopter then make a sudden and
  hard pull on the cyclic to make a turn while max collective is
  applied, the low rpm warning will sound, followed by a loss of
  power and a few seconds later loss of attitude control.

I don't understand what you mean by "loss of power" here (engines are red lined and don't produce any more power?) ... but to proceed

  The aircraft can be recovered by:(recovery steps)

While the recovery steps are rational, the point is to not wait that few seconds if you droop Nr.  I am not sure if the simulation is trying to model retreating blade stall or not, but the response to droop in a max performance maneuver cannot be delayed.

With max collective and a maneuver that induces low NR you don't wait: you reduce the load on the rotor head.  You do that because the engines aren't able to keep the rotor turning based on the load you've put on the system.  If you keep the load on you just make the problem worse.  (In that regard, the simulation is more or less faithful to real life ...)

  My guess explanation is as follows:

1. When max collective is applied, the blades are working hard by cutting air at a large angle of attack
Yes.  Higher angle of attack.
2. When the aircraft is banked and aft cyclic is applied, the airflow relative to the aircraft is like when it is in a very high rate descend
No. Not the same.
3. The critical angle of attack is exceed on the rotor blades, they stall and cease to produce lift
While technically true, the reduction of Rotor RPM (Nr) is your cue to Stop Doing That! (whatever "that" is) so that Nr does not decay further.  Note that the retreating blade will tend to stall first, which is a different stall mode than a fixed wing aircraft stalling due to high AoA.
4. Due to the greatly increased drag, the main rotor is unable to keep rpm up with its power, therefore rpm decreases
It's the Engines that can't keep the rotors turning that is causing the Nr to droop ...  and that's due to the loads you are putting on the drive system.
5. As rpm decreases, less lift is produced and the stall worsens.
Less lift is produced and you may approach the stall rpm (somewhere around 70-80% of rated Nr depending on the helicopter) if you don't do something to restore Nr.  That means lessen the severity of the maneuver and stop overloading your engines' ability to keep the rotors turning as soon as the Nr droops.
6. By reducing collective, the drag of the blades is reduced, and rotor power is enough to keep rpm at normal range again.
No, the Engine Power is enough (or becomes enough) to keep the Nr in its proper range, but reducing collective is a key first step (as is leveling the wings) to retain control of your rotor system.
Is my explanation correct?
No
Can this happen in real life?
Flying a helicopter like it's an F-18 in a Blue Angels flight demonstration is a good way to kill yourself.


Can resistive fuel senders/transducers be repaired/reconditioned? I need to replace two fuel senders in each wing of a Socata TB20, and a replacement is almost $500.

I wanted to know whether a shop exists in the US that reconditions these?

After some research, I'm still not sure if reconditioning resistive fuel senders is common practice at all. I only heard anecdotically about it

There are shops that do this and egid gave you a couple of hints on how to find one, but your local mechanic can probably recondition your fuel sender about as well as any of the specialty shops, as long as they're not squeamish about opening up the unit (and as long as the unit can be opened up - I'm not sure about the TB20 senders).

What's basically involved is cleaning the resistor element and wiper contacts, making sure everything is making good contact through its full range of travel, and verifying that the sender is still within a reasonable calibration range when you're done with the cleaning (you can use your other, working sender to determine what's "reasonable", or verify using your fuel gauge).

The Comanche folks have a nice article on reconditioning a (really beat up) fuel sender, and technologically they're similar to automotive fuel senders, so this article about reconditioning Corvette fuel senders may also be useful to get an idea of what you're in for.


Can an instrument procedure be intercepted from the middle and how? While I was reading FAA-H-8083-6 I was wondering if a pilot can be instructed to intercept a procedure from a point somewhere in the middle of it.

Specifically:

A pilot departs from an airport having assigned the FOBAR1 SID. There is RADAR service, hence after departure they are vectored to a position outside of that SID. Can the ATC instruct for any reason the pilot to resume the SID? And if this can be done, how is it done? Simply Resume FOBAR1 SID or more specifically eg Fly heading 125, resume FOBAR1 SID or Proceed direct ABC, resume FOBAR1 SID?

NOTE While I mention an FAA document I am mostly interested for EASA and/or ICAO regulations/procedures.

Answer is yes.

FAA

ATC Section 6. Vectoring:

  When vectoring or approving an aircraft to deviate off of a procedure that includes published altitude restrictions, advise the pilot if you intend on clearing the aircraft to resume the procedure.

  PHRASEOLOGY−

    FLY HEADING (degrees), MAINTAIN (altitude), EXPECT TO RESUME (SID, STAR, etc.).

    DEVIATION (restrictions if necessary) APPROVED, MAINTAIN (altitude) EXPECT TO RESUME (SID, STAR, etc.) AT (NAVAID, fix, waypoint).

  NOTE− After a Climb Via or Descend Via clearance has been issued, a vector/deviation off of a SID/STAR cancels the altitude restrictions on the procedure. The aircraft’s Flight Management System (FMS) may be unable to process crossing altitude restrictions once the aircraft leaves the SID/STAR lateral path. Without an assigned altitude, the aircraft’s FMS may revert to leveling off at the altitude set by the pilot, which may be the SID/STAR’s published top or bottom altitude.

    (Position with respect to course/fix along route), RESUME OWN NAVIGATION, FLY HEADING (degrees).

    WHEN ABLE, PROCEED DIRECT (name of fix), RESUME (SID/STAR/transition/procedure).

ICAO

eurocontrol.int quotes ICAO Doc4444, 15th edition 2007:

12.4.1.4

  RESUME OWN NAVIGATION [DIRECT] (significant point) [MAGNETIC TRACK (three digits) DISTANCE (number) KILOMETRES (or MILES)].

12.3.2.1

  JOIN (specify) AT (significant point) AT (level) [AT (time)].

  Example: LGL123 join upper golf 1 at GORLU at FL330 at time three two.


What was the first plane that allowed walking during flight? What was the first plane that allowed crew members and/or passengers to stand and walk during flight?

P.S. I mean walking inside the cabin, as part of normal operation intended for by the plane designers, not walking outside the plane, on wings and other acrobatic activity.

The first airplane with a cabin big enough for walking around was indeed a Sikorsky. This was the Sikorsky Russky Vityas, also sometimes named "Le Grand", a name which was given to an earlier, two-engined version. Oh, and the cabin included a washroom, too!

Sikorsky Russky Vityas (picture source)

It was the first four-engined airplane in the world and the biggest airplane of its time. It flew first on May 10 (or 13, depending on the source), 1913.

Quote from Wikipedia:

  After the Russky Vityaz's first test flights between 10 and 27 May
  1913 (Old Style dates), it was established that a passenger could even
  walk around the cabins without causing any problems to stability.

Early Le Grand version with two engines (source)


Is it OK to use foam for the ribs of an ultralight's wing? I have seen several YouTube videos of ultralights and even HPAs (Human Powered Aircraft) that use foam as the ribs  of their wings. Are ribs not supposed to be made of rigid, strong, woody and light material i.e pine,balsa etc.?

The foam is used to take small shearloads and support the primary structural element in absorbing tension and compression loads.  The wood rib has the primary compression and tension members in the capstrip, with the diagonals absorbing shear loads, converted to compression and tension loads along the legs of the truss. On a foam rib, if it is a standalone rib not connecting to a stressed skin, you need to have a structural capstrip material, like wood or fibreglass because the foam only has strength when the loads are distributed over a large area.  The foam itself does the job of the diagonal members in holding the caps in place and absorbing shear forces in the rib.  Some ultralights have a foam rib core with an aluminum cap strip bonded to the edges to do this.

If the foam rib is inside a stressed skin wing, the stressed skin is effectively the cap strip and the foam can be bonded directly to it.  Because the foam isn't very strong it requires a large number of foam ribs, closely spaced to properly support the stressed skin.


Is "secondary radar" a misnomer? Radar is based on the principle of transmitting radio waves and measuring the length of time-of-flight, direction, and frequency shift of the returning reflections or echoes.

In aviation, we call this "primary radar" and we use the term "secondary radar" to denote the ATC/transponder system.

Secondary radar uses a transmitter that sends pulses that the transponder picks up and then sends coded pulses of its own back. But these are not based on reflections off the airframe.

Why then call the ATC/transponder system "secondary radar" when the principle is very different from what we might call "real" (aka primary) radar?  Or does the "secondary" here meaning "in addition to" rather than "another form of"?

The terminology is a combination of history and "trustworthiness" (for lack of a better term).

Historically, primary radar was invented first. So it makes sense that this would be called "primary" and another scheme invented later would be called "secondary".

ATC systems like to ensure that the data they are working with is trustworthy. Primary radar systems are based only on the reflected radar signals. If you get a reflection, you know there is something there, whether or not it has a transponder (or indeed, whether or not it is an aircraft). This is a certainly a key point in military radar systems, and ATC radar was derived directly from military radar.

With secondary radar (transponder) operation, there is no guarantee that the transponder isn't lying (whether deliberately or unintentionally). A faulty transponder could certainly report false locations to the ATC system. Or, you can probably imagine scenarios where a false location might be deliberately returned.

ATC systems take both the primary and secondary location information for an aircraft, and correlate them to represent one aircraft in the sky. They don't report the exact same physical location, so there is a degree of fuzziness in the system to account for that. If the two reported locations drift too far apart, you get an uncorrelated track. Sometimes, if two aircraft cross each other's track in certain ways, the ATC system can get confused and associate the wrong primary radar blip with the wrong transponder report. The controllers have procedures to deal with and correct these kinds of problems.

So, primary radar is primary because it (a) came first, and (b) is the most trustworthy source of aircraft location information. Secondary "radar" (which, as you point out, is not really radar) is called such because it is integrated within the primary radar system.

More generally, the various technologies for locating and identifying aircraft in airspace are collectively called "surveillance". Examples of ATC surveillance are primary radar, transponders, multilateration, ACARS position reports, and other technologies.


Wouldn't a triplane be much more efficient than a monoplane of same wing area? Note: not a duplicate of Can biplane or triplane designs be revived with modern materials?

This question compares a high performance triplane with high aspect ratios and high gap, to a monoplane. It does not explore materials engineering.

Wouldn't a triplane be much more efficient than a monoplane of same wing area?

Take a small GA plane, 1,000 lbs, 100sq ft of Wing area, 25' span, 4' chord, a pretty typical LSA/microlight. So, an aspect ratio of roughly 6, and a wing loading of 10psf.

NOTE:  With a gap of 2 chord, I understand the efficiency of multi wing aircraft is almost the same as a monoplane. see graph.

Now make it a triplane, which reduces the chord to roughly 1.3, increasing the aspect ratio from 6 to 18, just like a glider.  That's a 300% increase in efficiency, no?

Instead of a heavy spar for a thin wing, you could just put the stabilator at the wing tips joining the 3 wings: so no extra wing strut interference drag ( expect for a small amount of extra interference drag at 2 wings roots).

So:

less weight( same wing area, but smaller lighter wings),
smaller engine needed ( so could even go with smaller wing area)
less induced drag (due to 3 times the aspect ratio)
a less form drag ( same span, but thinner wings (66% THINNER(??) due to 66% smaller chord)
a little more wing root interference drag,
no extra wing strut interference drag
less induced drag due to 1/3 the wing loading

NOTE: All this could only happen with a gap of 2 chord or more, where I'm told 2 wings act independantly without interfering with each other, just like a main wing and horizontal tail( which is a biplane of sorts)

Is this correct, at least conceptually?

Wouldn't a triplane be much more efficient than a monoplane of same
  wing area?

In strictly forward flight probably, but what are you looking to gain?

You could make the mono wing have a higher aspect ratio and gain the same efficiency, all while being able to use a normal tail stabilizer and predictable flight dynamics.

There's only 2 disadvantages going this route. First the aircraft won't have as fast of a roll response because of the long wing span. Second the long wingspan may be harder to store depending on your ground operations.

But maybe you want to build an efficient aircraft that could land on public roads. So you need the wing span as short as possible while still being efficient, would your idea work?

Probably, but not without it's drawbacks. First how will you pitch stabilize it? If you use a conventional tail, you need to take your "2 cord" rule into effect for this "4th" wing. (I'm assuming for now that the 2 cord distance is correct, but I'll put my own thoughts in on this later).

Mono planes generate lift from 1 spot, pretty easy to predict where (where the wing is) and placing that wing just behind the center of gravity means the stabilizer tail doesn't have to produce much down force to keep the plane in stable flight.

However your center of lift will be moving depending if your the wind hits one of your wings in a different way than the others. You will also have to consider how you're going to make the lift of all 3 wings equal roughly the center of gravity.

Anything off of this mark and would have to provide enough down force to compensate, reducing efficiency.

Now my thoughts on if 2 cords apart is enough for a wing. It probably is in forward flight, but fluid dynamics are hard to predict and when that aircraft starts pitching up and down and the vortexes each wing generate get aimed in different directions your wings will interfere with each-other.

Take a look at the quickie aircraft. It was a "bi-plane" design similar to what you propose. Notice how the wings are not only far apart, but also on separate vertical levels. This is to help dampen pitch response, and give the wings more breathing room in non level flight.

Using this idea for a 3 wing aircraft seems even more difficult, as the more vertical distance your wings take up the less pitch authority you have. A little bit of dampening is okay, but too much and you can't pitch the aircraft.

You'll also notice the quickie doesn't use a normal tail stabilizer (because of the problems discussed above). So differing lift coefficients between the wings are used, but that does make the aircraft harder to fly in general.

Anyway if you're really interested in this idea I'd recommend checking out the quickie project and learning what challenges/breakthroughs they faced/made.


Can you identify this unusual plane by its description? I saw this plane many years ago:

Same shape and size as an commercial airliner.
No windows along the fuselage
Painted flat gray (like a naval ship)
Had a blue circle on its "nose" (Blue painted radome)
Rectangular "hump" with rounded corners on the bottom of the fuselage.

If you can identify this plane, could you please tell me the make and model and what it's used for?

UPDATE: the question has been put on hold so I will try to be more specific. I saw a plane fitting this description flying unusually low over the Chelsea neighborhood of New York City on the morning of September 11, 2001 shortly before 9:00 AM. It was flying south over 8th Avenue. I had just exited the subway station at the 23rd Street and 8th Avenue. I was walking west towards my office on 11th Avenue when I was startled by a very very loud sound directly overhead. I turned and looked up and saw this low flying plane. I have never ever seen an airplane fly that low except near an airport terminal. It was shocking and frightening. I literally thought the plane was going to land in the middle of 8th Avenue. I remember looking for smoke or visible signs of damage but it looked like it was okay. It was actually slowly and gradually descending, but then it climbed back up. There were no markings on the plane apart from the blue "bulls eye" on the "nose" (pardon my lack of knowledge about aviation. I don't know the proper terminology).

Several minutes later I was on the M23 bus when I heard an extremely loud bang, which I would assume was the first plane hitting the WTC. It was so loud that my ears were ringing for a while. I remember trying to convince myself that the sound had nothing to do with the plane because the thought was too awful. At the time, there was a lot of construction going up on west Chelsea (gentrification. high rise luxury condos being erected where transsexual prostitutes used to roam the streets). I tried to convince myself that it was from the construction. The image that came to mind was taking a steel garbage dumpster and throwing it off the top of a skyscraper.

I don't know what this plane was but it had something to do with what happened on 9/11. If only I had had a camera at the time and could have taken a picture. I need to know what this plane was and how it fits into what happened that day. This is not a joke. I realize that this is going to be controversial and some people may be upset. I want to reiterate that this is not a joke of any kind. Please do not flag this post. I need an answer to what has haunted me for 17 years. Thank you.

Sounds like it may have been a Boeing RC-135

(source)

No windows and a flat grey color would lend to either a cargo aircraft or a military aircraft with military being likely considering the color choice.


What is the most efficient way to get your instrument rating? For someone that has recently completed their private pilot SEL, what would be the suggested next steps/process to getting instrument rated as efficiently as possible (both time and money)?

The fastest path to any rating is intensive training.
If you can take time off from work and do an "accelerated instrument training" program you can probably knock out the rating in less than a month. That AOPA article says 8 days, I've seen a lot of two-week courses advertised.

Note however that there's a difference between being instrument rated and proficient for real-world IFR operations in actual IMC. These courses will almost certainly cram all the knowledge and procedures into your head that are required to get you to pass your instrument checkride, but you may have never flown an approach in actual IMC (if suitable weather doesn't pop up for you to fly in your time will all be simulated under the hood), and because everything is compressed and focused on "passing the test" you may come out of the program lacking the confidence to actually go out and use your newfound rating without doing some additional work with a local CFI (this is a point sometimes raised by pilots who have gone through these accelerated training programs).


How can I determine takeoff and landing from an ADS-B feed? An ADS-B feed consists of a series of latitude, longitude and altitude points, without any "start" or "end" markings as such.

Using this data, which can contain arbitrary gaps and errors, what's the most reliable way to determine when an aircraft has landed or taken off at an airport?

Update: I'm looking at preprocessed feeds in VRS's AircraftList format, which on closer inspection has a boolean "Gnd" field.  (Note that there's no guarantee that any but the most basic data fields will actually be available in practice.)

It depends on the type of ADS-B feed you have.

On the lowest level, the messages transmitted from the aircraft, usually contain information about the air/ground status of the aircraft1. When the aircraft is airborne, Airborne Position Message are transmitted (types 9 - 18 or 20 - 22) for position and altitude. Airborne velocity is in message type 19. On the surface, message types 5-8 are used to transmit both surface position and movement. So by looking at the raw messages, the transition between airborne and surface type indicates landing, vice versa in indicates take-off.

This is accurate for most larger aircraft, smaller aircraft and helicopters don't always have a weight-on-wheel switch or similar device that is used to toggle the air/ground status. In these case speed may be used as trigger which is a less accurate indicator.

If you don't have access to the raw messages, you have to find an other way to do it.

Typically ADS-B feeds for air traffic control consist of ASTERIX 021 reports. If you have an ASTERIX category 021 data stream, the air/ground status of the aircraft is included in Item I021/040, first extend, bit 7 - Ground Bit Setting (GBS).

If you have a stream with only latitude, longitude and altitude, then I would try to use the altitude field. On the surface there is no altitude transmitted (it is not in message type 5-8, the bits are used for surface movement), so if the altitude is no longer there, you can expect the aircraft to be on the ground. However, this depends of course on the implementation of the ADS-B decoder; it may just give stale altitude data instead of blanking the altitude on the ground.

If you don't have good detection near the landing zone, you can try to extrapolate the altitude graph from the final approach section and based on that estimate when / where the landing would be. You may want to adjust for the flare if you need to.

1I am assuming 1090ES ADS-B here which is the defacto global standard; UAT and VDLm4 technology varieties of ADS-B also contain air/ground status but in a different manner.


Is everything OK with engine placement depicted in Flight Simulator poster? Does the Airbus A320neo depicted in Microsoft Flight Simulator 2020 box cover photo has the right engine in all the wrong place?
Here is  the snapshot:

Is it only me, who clearly sees that it is way too close to plane's body and actually attached to it?
If I am correct then would that mean that someone has artificially modified the original picture, when preparing promotional photo, and made that modifications completely wrong?

No, Microsoft's image is not doctored.  It looks very much like the image from a 2010 press release for the A320 NEO.
(Dictionaries say that doctored means deliberately altered or manipulated, in the same sense as the original question's "artificially modified".  That would apply equally well to developed photographic film, a software rendering, or even a cocktail napkin sketch.)


What happened to the Boeing Y3 aircraft? What happened to the Boeing Y3 aircraft? Did they ever produce a fully functional prototype? What is the likelihood that it will ever be produced, and when will they start production?

The Y3 project is the third iteration within the Boeing Yellowstone project. Y1 was to be a replacement for the 737, but was put on hold after the decision to move forward with the 737-MAX. Y2 resulted in the 787. Y3 is supposed to address the 350-600+ passenger market, replacing the 777. To my knowledge, no prototype of such an aircraft has been built. Emirates was supposed to be in discussions with Boeing about this, but that was back in 2010, and since then I believe they have ordered the new 777-8X and -9X.


When does the mandatory minimum climb gradient during an Instrument Departure Procedure end? While studying FAA's Instrument Procedure Handbook, I noticed these minimum performance requirements for obstacle clearance, when a SID or ODP is absent for a particular runway:

-- Cross the departure end of the runway (DER) at least 35ft AGL.
-- Climb to 400ft above the DER before turning.
-- Maintain a climb of at least 200ft per nautical mile (FPNM) until reaching minimum IFR altitude.
The minimum IFR altitude is 1000ft above obstacles, or 2000ft in mountainous areas.

I was wondering if the last rule, regarding a constant climb with a minimum climb gradient of 200 ft/NM (if no other is specified) until reaching minimum IFR altitude, applies also in case of SIDs, or it is up to pilot's or ATC's judgement to establish a minimum climb gradient for a particular, arbitrary portion of the SID route.
In other words, if for a particular SID it is mentioned "climb gradient of X ft/NM until Y altitude", then after reaching Y altitude is the pilot allowed to fly with whatever climb gradient he deems necessary, even if it is less than 200 ft/NM?
p.s. I have to give some credit to this question, though it is not a duplicate since it asks if a minimum performance applies in case of the absence of a SID or ODP. I am asking if a minimum performance (especially the last rule regarding the minimum IFR altitude) applies when the pilot is following a SID.

I believe that now I am able to provide an answer to my own question.

I think that the "minimum of 200 ft/NM" rule applies during the whole climb phase. In other words, until the end of the SID, where either the enroute phase begins or the SID itself provides some enroute transitions as a passageway to the enroute phase.

FAA's TERPS manual clearly states:

  Chapter 2, "Sloping OCS" 2-1-4b. The concept of providing obstacle clearance in the climb segment of an IFP is based on the aircraft maintaining a minimum climb gradient. [...] The minimum climb gradient that will provide adequate ROC in the climb segment is 200 ft/nautical mile (NM), unless a higher gradient is specified.

In addition, the rule is also repeated in the PBN manual:

  Chapter 1, "Common Criteria" 1-3-1g(2). Climb Gradient (CG). Obstacle clearance in a climb segment is dependent on the aircraft maintaining a minimum CG to a specified altitude. The minimum CG (also referred to as standard CG) is 200 ft/NM.


How do flight attendants know seat belts are fastened? I'm a simple curious traveller. I always see flight attendants passing by the seats checking whether belts are fastened. They pass quickly and without looking carefully at each single person, to see if the seat belt is fastened.

I stared at them and found them looking at some specific point on the ground that could possibly tell them with a glance, but I couldn't figure out what was that point.

Can somebody explain how do they check the seat belts status?

In addition to Jamiec's correct answer, I would add that you can tell when a seat belt is not fastened even if the buckle is covered by someone's shirt (or belly!).  You can normally see the segment of belt alongside the person's hip and it's easy to tell if that piece of strap is under tension or not.  Some people actually try to disguise the fact that their seat belt isn't fastened by covering up the buckle, hoping to avoid detection.  By looking for the strap alongside their hip, I can avoid asking to see the buckle itself.

Of course, if I can't really tell by a quick glance, I ask them to show it to me.  It's very rare that anyone actually takes offense to this.


Has there ever been an uncommanded lowering of landing gear? Has there ever been an uncommanded lowering of the landing gear on a large commercial jet? I could not find any instance of it in a preliminary google search. Could a simple short-circuit cause such a thing?

I think this would actually be pretty bad at cruising conditions, because the airplane is going much faster up there, than on approach. But the air is also much thinner so I can't be sure.

The answer to your question is, in fact, "yes" - but the circumstances involved demonstrate just how difficult it is to make an aircraft's landing gear deploy without being commanded to do so.

In 1985, CI006, a 747SP, experienced an uncommanded flameout of its #4 (right outboard) engine (something that particular engine was quite prone to doing) while cruising at FL410 en route from TPE to LAX.  An attempted relight failed, and a lack of corrective flight control inputs combined with a poorly-designed autopilot caused the airplane to decelerate and gradually roll into a steep bank despite the autopilot's attempts to counter the thrust asymmetry; the captain eventually noticed this and disconnected the autopilot, but made no flight control inputs of his own.

This, unsurprisingly, resulted in an immediate loss of control, and the aircraft rolled into a steep inverted dive, during which it exceeded mach 1 for an indeterminate period of time, plunging 30 kilofeet before the captain was able to orient himself when the aircraft broke out of the clouds at 11 kft and pull out of the dive, eventually levelling the aircraft at 9,600 feet.  During the dive and subsequent pullout, the aircraft was subjected to aerodynamic loads exceeding +5 Gs,1 which, in addition to causing severe structural damage to the aircraft (including tearing away large parts of the aircraft's horizontal tail), ripped off the body gear doors, broke the support brackets holding the body gear uplock hooks in place, and forced the aircraft's left and right body gear into the down-and-locked position.  (The added drag from the extended body gear [and from the aircraft's 29-kft maximum gear-down altitude] forced the flight to divert to SFO, having insufficient fuel to reach LAX with almost half of the aircraft's landing gear hanging out in the airstream.)

So, yes, it has happened... during an over-5-G pullout from a 30-kft supersonic dive in a 747.

1: The highest normal acceleration recorded on the aircraft's FDR was +5.1 Gs during the pullout from the dive; however, the absolute maximum G-forces encountered by the aircraft are unknown, as the extreme G-forces exerted on the aircraft, and the FDR, caused the latter to fail to record properly for large portions of the dive and pullout.