What materials are plane windows made of? I've always wondered, what are those dual-layered (bonus: sometimes they have a little <2mm hole, why is it there?) windows commercial jets put next to their seats made of?

Generally aircraft windows are made of what we colloquially call "plexiglass" of some kind (Lexan polycarbonate is common in light General Aviation aircraft, acrylic plastics are also used).  This material is light, relatively strong (not shatter-proof, but it'll take a moderate beating) and has decent optical properties.
Its major drawback is "crazing" -- hairline cracks on the surface of the material that form over time from dirt, stress, temperature, UV exposure, and the like.

Bonus answer: The little hole serves a couple of purposes, but the biggie is pressure relief/equalization. This gets discussed over on airliners.net a lot and they've covered it pretty thoroughly.

Basically the hole ensures that the cabin pressure is pushing against the outer (primary, usually thicker) sheet of plexiglass, which is plug-wedged into the fuselage structure and can't go anywhere.
Should the outer pane be damaged the hole will also theoretically bleed cabin pressure off at a controlled rate - though how successful that equalization is would depend on your altitude (and thus the pressure differential between the cabin and the outside air) and the degree of damage to the outer pane.


What would the altitude profile for a typical airline flight look like? It's hard to appreciate an aircraft's rates of change in altitude, in particular with respect to its travel through the mass of air, rather than over the ground, since the latter is affected significantly by wind-speed (the distance an aircraft travels with respect to the ground is highly variable, compared to the distance it travels with respect to the air around it).

Then, when climb profiles are plotted, it's always on a graph with a hugely compressed horizontal axis (naturally, because it's hard to discern much information when one axis is a thousand times longer than the other).

Finally, when climb/descent rates are given they are usually in vertical distance/minutes, which is useful from an aviation point of view, but hard to compare with a vertical distance/horizontal distance measurement, or for example the familiar gradient figure that would be used to describe the slope of a road.

So, assuming absolutely still air throughout a flight, what might the altitude profile as plotted against distance look like, for a typical airliner on a flight long enough to be at cruising altitude for a few hours?

And, what sort of gradients would be represented for its climb after take-off and its descent towards its destination?

It is somewhat unclear what you want to achieve.

If you want to know how it looks like, you can search for some papers like this. To keep it short, altitude vs distance curve is pretty much similar to altitude vs time, but climb and descent segments are more curved with larger second derivatives for climb and descend. For details please read the paper.

If you are interested in some analysis, you need two components: flight data itself and data analysis software.

If you are looking for professional flight data analysis systems, for example, flightDataServices, generally they are not provided for free, but even if you have an access, you still need flight data for analysis. It can be more difficult to find it. If that is what you are looking for, just google "Flight Analysis Software" and "Flight Data".

You can do that analysis by yourself though, with a little less accuracy than a professional system. The closest option for getting an accurate flight data is FlightAware.com. They do not provide altitude vs distance out-of-the-box, but they provide plenty of data for further analysis. For example, this flight. You can open track log and see altitude/speed vs time (which is not you are looking for, but useful for a 'big picture'), and in the log itself you can find latitude, longitude, speed and rate of climb for some points. The points are provided with a less-than-a-minute frequency, which is pretty precise. It is not so hard to export them in any spreadsheet or CSV and generate whatever you are interested in: a pretty common data visualization task. It can be easily done with Matlab/Excel/LibreOffice. For example, altitude vs distance travelled, course vs time, rate of climb vs time, rate of climb vs distance etc.

If you are looking for data visualization software specified for flight analysis, I am not sure that something exists that fits all requests. Especially an online tool. The reasons are: flightaware provides both data and the most common plots (i. e. alt vs time and speed vs time); and if you already have data, it's pretty easy to plot whatever you want using Matlab/R/Python/Excel.


Are jets deliberately excluded from the "Complex airplane" definition? A jet aircraft seems for sure very complex as such. But, referring Wikipedia, a "complex aircraft" (as legally defined) has:

A retractable landing gear
Flaps
and A controllable pitch propeller

And there is no propeller in a jet.

What are the reasons of defining "complex aircraft" so that it does not cover jets? Are these reasons other than historical?

All jets require a type rating. Therefore you don't need the complex endorsement directly (other than needing it likely for your commercial checkride, which requires a complex aircraft). The historical training progression is: simple prop, complex single engine prop (requiring the complex endorsement), high performance (200 hp+, though this one is sometimes not gotten), light multi-engine props, and then finally moving into heavier turboprops and then jets which require type ratings.

Note, my answer is FAA specific, and other agencies might have different rules/interpretations for this issue.


What is the purpose of a departure procedure? On many webpages it is said that a departure procedure will keep aircraft away from terrain and obstacles and avoid collisions. Which type of obstacles? Buildings? Towers? What is the purpose of a departure procedure? This is a follow up of this question.

A major advantage is standardization. An aircraft can be given a departure procedure, and assuming everything goes to plan, everyone involved will be on the same page as to what is going to happen. When air traffic control instructs an aircraft to fly a departure procedure, a large volume of information (e.g. climb to this altitude, turn this way, turn that way, etc...) can be communicated just by giving the name of the procedure. This is far more convenient and predictable for both the crew and controllers than giving every instruction individually. This allows controllers to focus on preventing conflicts and dealing with abnormal situations rather than issuing routine instructions, and it reduces the risk those instructions may be miscommunicated or misunderstood.

The procedure itself will have been developed by specialists based on all sorts of criteria, among them:

Obstacle clearance. This can be anything from the huge mountain range near the airport to buildings to towers to anything else. The procedure can be designed with a required minimum rate of climb, and pilots can ensure they are able to meet that requirement to climb over any obstacles. Obstacles might also be virtual, such as restricted areas for military purposes.
Noise abatement. Communities in the vicinity of airports often have noise complaints. Departure procedures can be designed to direct aircraft away from noise sensitive areas.
Traffic flow. Near a busy airport, large numbers of aircraft are maneuvering at low altitudes. Standardizing departure and arrival flows along certain corridors allows air traffic control to deal with aircraft moving in predictable ways and avoids potential conflict between different aircraft. Departure procedures help ensure separation between arriving and departing aircraft.
Regional traffic flows. In congested airspace where several busy airports are located in close proximity, procedures can be developed to ensure that traffic to and from each airport are kept separated. A good example is the busy airspace around New York City, where complicated departure and arrival procedures are used to separate traffic into the region's airports.
Winds. Different runways may be used depending on wind conditions. There may be different departure procedures available for different runway configurations and/or universal departure procedures that have different instructions depending on the runway used.
Navigational aids. While modern RNAV procedures are now available, traditional departure procedures guide aircraft from the runway through routes navigable through NAVAIDS like VORs and NDBs onto established airways.

Of course, an aircraft flying a departure procedure can always be given instructions to deviate from that procedure if it becomes necessary, such as for other traffic. If conditions permit, it's also possible for a controller to give a "shortcut" by circumventing part of the procedure, perhaps by turning early toward the next waypoint on the flight's filed route.


Is it allowed to launch powered paragliders from national parks? I'm new to the sport and can't find a written answer. I've been told by local PPG pilots that it is illegal to launch from state/federal parks.

I live near Johnson's Beach, Gulf Islands National Seashore and was told by the officer at the gate entrance:

  Yeah, go ahead, use the parking area and beach to the west, we have guys that fly out of there regularly.

FAA has the following informational web page for Alaska National Parks:
https://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/systemops/fs/alaskan/advisories/parks/

Which has contact information for parks where flight operations are periodically done.  Several of my pilot friends have used park and forest service airstrips with permission, so it is worth asking.  In Alaska, the parks are generally more open to aircraft, than in the lower 48.

There is also an advisory circular, 91-36C.  https://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/systemops/fs/alaskan/advisories/parks/media/ac91-36c.pdf

Some of the lower 48 have strips where permission may be granted, plus forest service has some strips (usually gravel) which permission may be granted to use.  Again, contact the park in question, because they often have local rules which are permissive.

State parks vary. Some states prohibit aircraft of any kind, others permit operations, usually situationally.

Paragliders are just one class of airman who are impacted by park policies. sUAS pilots are also impacted.

Regardless of what you are told verbally, I would only consider ops in a Congress Designated Wilderness with written permission. Those areas have a prohibition of mechanized devices, and that includes hang glider and sUAS activities.


Would it have been feasible to install 'conformal fuel tanks' on the Mercure? The Dassault Mercure was a short-range twin-jet airliner. It was canceled with sales not reaching the breakeven point. Other similar jetliners like the Boeing 737 had a more convenient longer range.

The structural design features of the Mercure precluded increasing the fuel capacity, but there are jet fighters where conformal fuel tanks were added, same as in the Hypersonic experimental X-15.

Are this type of added fuel tanks a feasible option for increasing the Mercure range?

Adding conformal tanks will add drag and weight. Each on its own will impact performance but both together will definitely have a huge impact and I doubt that it would even get airborne.

The reason that jet fighters have these is for ferry flights eg deployment overseas.. and since they usually leave the bombs/missiles/etc behind on these ferry trips there is not a big increase impact in take-off weight.


Can simultaneous departures be authorized from converging airports? I am an ATC'r stationed overseas where we service two military airfields. The runways are 3 miles apart but converge. Our current regulations require aircraft from one airfield to delay in takeoff position while another aircraft is released or cleared for an approach to the other airfield, in order to protect from the missed approach point or departure route. However, our SIDS are designed to turn aircraft 45 degrees away from the other converging airport, but we are still required to wait.
We are in class G airspace with one airport 24/06 and the other, just south about 3.8 miles, on rwy 29/11. 90% of the time Rwys 24/29 are in use. The closest point of divergence, I'll have to double check for the exact distance however, the 24 departure SID is an immediate right turn off departure end, diverging at least 45 degrees from the missed approach or even departure of the southern airport in contention. Current rules require releases to be held if one or other other airports is departing aircraft however, it seems like the flight path deconflicts initially, making this unnecessary. I'll double check the distance, but our airspace isn't a factor. It's simply cutoff procedures that seem as a fail safe way of preventing an issue should the pilots heavily deviate from the SID (essentially flying rwy heading off 24) but very restrictive during departure phases.
Is there ANY way around this? Is it legal to clear both aircraft for departure with a SIDs that deconflict by 45 degrees from seperate airfields?

If this were in an FAA ARTCC's airspace, they'd handle them as one airport, because of their proximity.
The protected lateral airspace for an aircraft is 4 miles either side of the course to be flown. That eliminates a lot of possibilities.
In a non-radar environment, with only 3.8 miles between the airfields, you need a reduced separation rule, such as the 13/3 DME rule (7110.65 6-2-1, a3). One and two minute rules won't work, because of the convergence.
If you have a NAVAID that the departures fly over, you can use 3 miles between aircraft (faster in front), until their courses diverge.
The divergenge has to be 45 degrees or more, and the NAVAID has to be within 13 DME of both airports to  use the rule.
The 44 knot rule (7110.65 6-4-2) could possibly be used if you can maintain a minimum of five miles between the aircraft. Remember to account for any local speed restrictions (such as 250 knots below 10,000 feet) that may keep you from having 44 knots between your aircraft. Also, the 44 knot rule is based on filed true airspeed, not ground speed. But again, someone has to wait.
If this is a radar environment (I'm assuming you use 3 miles separation), you can release the second aircraft as soon as you get radar on the first (using radar contact within one mile of runway end) as long as you maintain 3 miles.
These rules are usable in either terminal or enroute environments. There may be more in 7110.65 chapter 3, but I don't go there, being a center controller.
If it's possible to post a plan view of the airspace, there's some CTO rated controllers on here that could give more definitive answers.


What do compressor/turbine stages mean, and why we need a different number of stages? I am new to aviation. I realize that LP (Low Pressure) and HP (High Pressure) compressors/turbines have a various number of stages.

What do stages mean?
What is the reason to have a different number of stages for LP and HP?

For axial compressors (what you'd find in a typical turbofan), each stage is:

A rotating disk of blades
Stationary blades that follow.

They're called rotors and stators, respectively. So those two rows of blades are one stage.

The rotors increase the air's velocity, the stators turn this velocity into pressure, and repeat (see image below). The following stage would compress the air more, and so on, incrementally increasing the pressure.

Likewise but in reverse for the turbine stages (the fixed blades also come first).

Source: The Jet Engine (book)


Why is a weight and balance graph lozenge-shaped? (fireair.co.uk)

Why is a weight and balance graph lozenge-shaped? I have tried to find the answer but it seems to be really hard to find. Why is it not a square?

In a strict mathematical sense the graph should be extended into a complete right triangle, not a rectangle. However there are practical limits on how far forward you can fill the compartments so the graph can be cut off at 83 inches in this case. The top is clipped because increasing weight above that point reduces performance and safety margins below design intent and angers bureaucrats.
The reason for a right triangle is because the acceptable location of the center of mass is more critical at increased gross weight. When the plane is very low weight the center of mass can be very far forward and not overwhelm the tail down force and leverage. At high gross weights the tail needs more leverage to counter the nose down moment, so the forward limit of the center of mass must be brought back closer to the center of lift. There is no angle on the aft limit side of the chart because moving the center of mass behind the center of lift, at any loading, will make stall and spin recovery impractical or impossible on a traditional wing and tail layout.
The slight radial flair in the particular chart pictured is simply styling.


Who operates a 707 with Boeing factory livery? I have just seen a 707 depart from HTDA Dar es Salaam that has the tail painted with the Boeing colors. Any idea of who the plane belongs to?

Livery like this one:

Obviously 707 instead of 777 painted on it.

Official airplane of Democratic Republic of Congo

Today, the Democratic Republic of Congo uses a B707-138B, registered 9Q-CLK: LK for Laurent Kabila.

Laurent Kabila has been Congo's President until his assassination (he was succeeded by his son).

This is an airplane with a long history (and some controversy). She has now 707 painted on the tail, but not the Boeing markings. Thanks to our friends at Airliners.net several pictures are available:

(source: Airliners.net, Mark. Szemberski)

Once this a/c had no logo at all, here in 2008:

(source: Airliners.net, Weimeng)

On Youtube too: B707 9Q-CLK .

Qantas in the past

Back in 1959 when it entered service at Qantas as VH-EBG, the first B707 with a 5th engine in a pod-pak:

(source: Qantas Heritage Collection)

More on the curious engine exhaust and the pod-pak.


At what intervals do planes land and take off from Chicago O'Hare? Is there some data available which shows the intervals at which planes take off and land at Chicago O'Hare?

Just by taking a look around the last time I visited ORD couple of months back, it seems to be one almost every 10-15 secs.

As a general average, Chicago O'Hare (KORD) seems to do somewhere between 1.5 and 2 operations per minute (so roughly one takeoff or landing every 30-40 seconds).

I based this on data from the FAA's Air Traffic Activity Data System - Terminal Operations report, and specifically looked at November 23 through November 30 of 2014 (17835 operations, over 8 days works out to 1.55 operations per minute - I poked around randomly and found days as high as 1.7 operations per minute).

One of the limiting factors for number of operations at any airport is separation between aircraft, which is covered here

  Incidentally we were discussing airport/tower activity levels in chat earlier today, and Chicago O'Hare is the busiest airport in the United States overall - a title it recently stole back from ATL (which was #1 from 2004 to 2013).


What are the fonts used in MCDU screens? All MCDUs (and other screens) I've seen share a very similar, if not identical, set of fonts - a monospaced, sans-serif, very clean and easily readable font, often in two vertical sizes at the same horizontal pitch. What font is that? Do different instrument/airplane makers have their own? Do airlines standardize on one across their fleets?
FAA's Advisor Circular 25-11b (https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_25-11B.pdf) makes some recommendations on fonts.
Also, http://www.b737.org.uk/fmc.htm has screen photos where the pixels are discernible, presumably from an early 737 model. There isn't a name, but there is enough unique features for it to be identified, if it has an official name.

My guess is that these are raster (non-scalable) fonts without any special name.

At least initially, the screen resolution wasn't (and needed not be) particularly high, and the characters were the typical 8x8 pixels or only slightly better. Such fonts are drawn by hand (by pixel) and don't have too many variations to speak about. (Although the sales people might disagree).

Later models with higher-resolution screens probably modeled the same terminal-type font to keep commonality. I still doubt MCDUs have an operating system (if any) with scalable font support.


I want to become a commercial pilot, can you do that through the RAF? I want to become a commercial pilot, can you do that through the RAF?

Yes, you can. It means giving a commitment for a fair number of years in service of your country, where you will be trained as a pilot (assuming you get on to that - there are more jobs in the RAF than just pilot!). After your service, one thing you can do with your military flying experience is become a commercial pilot.

When I was young enough to consider it, there were 3 ways to become a commercial pilot

Learn in the airforce
Pay your own way (PPL, CPL, ATPL etc)
Get a scholarship from a big carrier like BA. There were usually about 4-6 given per year to the highest achievers.


Why don't tanker aircraft have ejection seats? Apparently, the KC-135 has no ejection seats (they used to have parachutes to allow the crew to manually bail out, but those were removed in 2008 to save money), and, according to this Quora discussion, that's the case for all tankers (presumably excluding the ones converted from bombers), not just the KC-135.

This seems somewhat illogical to me, as tankers act as potent force multipliers; to quote the linked post:

  It is quite possible that a sophisticated opponent will go after the tankers, as an easy way to disable a lot of aircraft without actually having to shoot them down. Doesn't really matter if they go down from a missile hit or lack of fuel. The math on that one looks very good - shoot down a tanker, and you have probably shot down four or five smaller, agile, hard to hit strike aircraft.

Not only that, but tankers are big, slow (for military aircraft), and generally devoid of any defensive armour or armament, making them easy targets relative to combat aircraft.

Even in peacetime, the role of a tanker aircraft inherently places it at a much greater risk of a midair collision than most aircraft are ever exposed to (example) - and, with most MACs, the only way to survive is to abandon aircraft.

So why don't tanker aircraft have ejection seats for their crew?

When I was flying the KC-135 tankers - A model and Q models - all crew members had parachutes, and all parachute packs had supplemental oxygen bottles that were good for at least 10 minutes - which was enough time to descend to the point where you did not need supplemental oxygen. If you had to bail out, it would be through the crew entry hatch, on the port side just behind the pilot's seat and in front of the boom operator's seat. If you had to pull the handle for bailout, the crew entry hatch would be jettisoned and a metal air dam would be pneumatically deployed about 4 feet down at the leading edge of the hatchway, to provide a wind block to allow crew members to fall free of the aircraft before getting into the slipstream. This prevented crew members from being blown back onto the body of the aircraft.

I have never heard of any tanker crew ever having to bail out of a KC-135, so perhaps this is why they got rid of the parachutes? Of course, I retired more than 20 years ago, and much has changed since then. But the KC-135s are still flying, even though they are older than anyone flying them today.

To answer the original question - both the KC-135 and the KC-46 are modifications of civilian aircraft, which do not have ejection seats; whereas fighters and bombers are designed for their specific mission.

Finally, I'm not aware of any cargo aircraft that have ejection seats, either.


How is windspeed scaled in wind-tunnel tests? Let's say you have a scale model that is 1:10th the size of the real thing. You want to test this in a wind-tunnel.

Do you also scale down the windspeed 10x? Why or why not? I would think it has to be scaled down in order to get the right results for things like propeller tip speed.

Edit: I saw this question. It could be a duplicate, except I did not see an answer to my concern there. I want to know if windspeeds are scaled down in a windtunnel, whereas that one asks about how windtunnels are used in general and doesn't seem to mention any scaling of windspeeds.

In order to achieve the same flow conditions, you need to match several similarity numbers. The most important one expresses the ratio of inertial forces to viscous forces within a fluid and is called Reynolds number (Re):

$$\text{Re} = \frac{v\cdot l\cdot\rho}{\mu}$$
Nomenclature:
$\kern4mm v\kern6mm$flow speed
$\kern4mm l\kern7mm$characteristic length along flow path, like wing chord
$\kern4mm \rho\kern6mm$fluid density
$\kern4mm \mu\kern6mm$dynamic viscosity of the fluid

The next one on the list is the ratio between flow speed and the speed of sound in the fluid and is called Mach number (Ma):
$$\text{Ma} = \frac{v}{a}$$
Nomenclature:
$\kern4mm a\kern6mm$speed of sound
Others depend on the nature of the flow - if oscillating vortex shedding is involved, the Strouhal number needs also be observed, with elastic oscillations the Froude number or with heat transfer the Prandtl number.

In your case of a 1:10 model, the flow speed needs to be increased tenfold in order to match the Reynolds number. However, this will bring you in conflict with the Mach number, because the faster flow will probably already be supersonic. This problem is discussed at length in this and the linked answer, however, knowledge of the influence of flow speed is implicitly presumed.

Normally, in a wind tunnel test the flow speed is too low for correct scaling (which means that the viscous forces are higher in proportion to the inertial forces), and the consequences of this mismatch need to be covered by correction factors.

There is no need to scale forces correctly. Wintunnel models were hung on wires which were connected to scales to measure forces and now are mounted on a sting, the bending of which is captured by strain gages or an internal balance. Only the models used in free-fall tunnels for spin research will create as much lift as they weigh. The forces measured are used to compute coefficients, dimensionless numbers which are normalized for speed and size. When multiplied with the speed and size of the original, the (corrected) coefficients will yield the actual forces and moments.


Why can't commercial aircraft have a wing boundary layer suction system? I've read that NASA's boundary layer suction system on the wings can reduce drag by removing the boundary layer through the suction holes. If this can reduce a lot of drag (hence improving fuel efficiency), then why can't the same idea be used in commercial aircraft?

Boundary layer suction systems have been considered for a very long time, since they provide significant advantages.

However, their main disadvantage, which has never been satisfactorily overcome, is their propensity to clog with dirt and biological matter. The maintenance costs to ensure the system is operational would outweigh the gains in aerodynamic efficiency.

Additionally, this is not the kind of system that could be left to its own devices (good if it works, fine if it doesn't); it would need to operate consistently because the critical angle of attack would depend on it.

Furthermore, this system would require power to operate the pumps, and it is not obvious that the efficiency gains would make up for the power requirements in all scenarios.


How to properly compute the stable vertical path load factor I've been told by someone else that in a stable flight when the vertical path of the aircraft is maintained, the Nz value is equal to cos(pitch)/cos(bank), but I can't seem to figure out how that is derived, the only place on the whole internet where I was able to find that info was here: Inside another stack exchange post, however, it didn't explain how was the formula derived. I do understand that inside a banked turn with 0 disturbance to the vertical path the aircraft's load factor is defined by 1/cos(bank) as explained in the textbook I read: Introduction to Aerospace Engineering with a Flight Test Perspective.
But this doesn't explain the cos(pitch) part of the formula, and nowhere was I able to find anything related to it on the internet. Can someone please explain this to me? Thanks!

Let's look at the extreme case of a 90° pitch attitude. The airplane climbs vertically. Thankfully, in case of an F-16 without loads this is possible (left side of sketch below).

The vertical load factor is defined in the body axis system (blue arrows): $n_z$ is the multiple of gravitational acceleration that presses the pilot into his seat. With $\Theta$ = 90° no gravitational acceleration along the body z axis is left (red arrow), all acts along the body x axis. So here $n_z$ is zero and $n_x$ is -1. Gravitational acceleration presses the pilot into his backrest, but his behind is unloaded.
Since the cosine is 1 for 0° and 0 for 90°, it describes the variation in $n_z$ over pitch. For 180° the cosine becomes -1 and the pilot will hang in his shoulder straps. And so on. The right side of the sketch shows an intermediate case and it should be immediately clear that the component of g in z direction is sin(45°) = 1/√2.
Combine this with the change in $n_z$ for a turn and the formulas
$$n_x = -\frac{sin\Theta}{cos\phi}$$
$$n_z = \frac{cos\Theta}{cos\phi}$$
should make sense. Since the x axis is defined as positive in flight direction, the x component of the load factor is negative with positive pitch attitude, hence the sign of the equation for $n_x$.


What is the difference between SSR and ADS-B? Can someone explain what is the difference between SSR transponder and ADS-B. From what I know ADS-B is a capability that can be incorporated into a transponder, but I do not understand the difference between them, I mean that from my point of view its' function and working principle is almost identical (both broadcast information that can be collected by ATC (and ADS-B info can be collected by other aircraft) and its like one device doing two identical things. Please help me out.

The biggest difference between SSR and ADS-B is how the aircraft position is determined. Secondary Surveillance Radar uses a radar antenna on the ground to send an interrogation signal and then determines the position based on the reply from the aircraft transponder:

The radar antenna rotates (usually at 5-12 rpm) and transmits a pulse which is received by the onboard equipment (transponder). The transponder sends back a reply containing at least a code (if operating in Mode A) but more often this is combined with level (mode C) or other information, e.g. aircraft identification, selected level, etc. (Mode S). The information received depends on the interrogation mode (A, C or S) and the transponder capability. [...]

When the reply is received, aircraft position (range and bearing) is determined. The range is calculated by knowing the time difference between the interrogation and the reply (the speed of propagation is the speed of light). The azimut is taken from the antenna position.

(SKYbrary - Secondary Surveillance Radar (SSR))
An aircraft equipped with ADS-B Out will regularly send signals without an interrogation. The aircraft position can therefore not be determined based on receiving such a signal. It must therefore be encoded in the signal itself, which requires that the aircraft knows its own position from a GNSS source:

ADS-B is a Surveillance technique that relies on aircraft or airport vehicles broadcasting their identity, position and other information derived from on board systems (GNSS etc.). This signal (ADS-B Out) can be captured for surveillance purposes on the ground (ADS-B Out) or on board other aircraft in order to facilitate airborne traffic situational awareness, spacing, separation and self-separation (ADS-B In).

(SKYbrary - Automatic Dependent Surveillance Broadcast (ADS-B))
ADS-B usually uses the same frequency of 1090 MHz to transmit, but it may also use other frequencies (see this answer for details).
Not only ADS-B signals can be received by other aircraft (ADS-B In). The ACAS (Airborne Collision Avoidance System) can also receive replies from SSR:

ACAS II is an aircraft system based on Secondary Surveillance Radar (SSR) transponder signals. ACAS II interrogates the Mode C and Mode S transponders of nearby aircraft (‘intruders’) and from the replies tracks their altitude and range and issues alerts to the pilots, as appropriate. ACAS II will not detect non-transponder-equipped aircraft and will not issue any resolution advice for traffic without altitude reporting transponder.

(SKYbrary - Airborne Collision Avoidance System (ACAS))


How would a pilot apply the information found on a 500 Millibar Vorticity chart to a flight? The U.S. Flight Service website has these weather products listed:

I have circled the 500 Millibar Vorticity Charts.  I've been a pilot for 10+ years and have never heard mention of this chart by anyone in aviation.  I am wondering why is it listed here, but no one seems to use it.

What purpose does the information it provides serve for pilots?

My understanding is that it shows the Isobars of atmospheric pressure at around the 18000 foot MSL altitude. It will show pressure gradients, low pressure and high pressure zones of the atmosphere at the altitude that demarcates where 50% of the atmosphere resides below. It’s main purpose is to indicate horizontal and vertical spin of air mass movement due to ridges and troughs of atmospheric pressure, wind shear, and the corialis force. This gives a good indication of the upward movement of rising air, energy being infested by a thunderstorm, and how much of spinning force is being applied to a storm to increase its risk of growing to dangerous levels.

Typically, other aviation weather tools would point a pilot to a no-go flight decision way before reaching this one. Some pilots may use it for limited applications (like glider pilots using the skew-t/log-p diagram). This chart can be a useful tool for meteorologists and meteorology enthusiasts to come up with reports of broader scope. You will see it mentioned in Area Forecast Discussions.


What is the operational advantage of starting an RNAV SID with a VI-CF leg combination? I am examining Standard Instrument Flight Procedures, mainly from FAA's charts and Coded Instrument Flight Procedures ARINC-424 file (CIFP). I am keen in understanding the operational advantages or other criteria by which a procedure designer would choose a certain leg combination over another.

As far as I could tell by searching through the aforementioned CIFP file, the following leg-combinations are mostly used for the first couple legs of an RNAV departure procedure:

VA (Heading to Altitude) -> DF (Direct to Fix)
VI (Heading to Intercept) -> CF (Course to Fix)

The first leg can also be DF, CF or TF, as it is stated in FAA's 8260.58A Performance Based Navigation manual, in Chapter 5. Departure Procedures, section 5-1-1. General.

For an example, take BRYCC4 Departure and its respective narrative page. Takeoff from runways 16L/R uses a VA-DF leg combination ("Climb heading 173° to 5934, then right turn direct to cross MUGBE at or below 10000..."), whereas takeoff from Runway 17L uses a VI-CF leg combination ("Climb heading 173° to intercept course 127° to cross GISTT at or above 7000...").

I can understand the need for a VA leg before a turn, since the aircraft is required to climb to a certain altitude (eg. for LNAV engagement or for obstacle clearance) before the first turn. However, I have a difficulty grasping the circumstances under which a VI leg is a more preferable choice.

Could someone point me to any source material that describe the criteria on which the decision about the leg combination during takeoff is based?

The PBN manual is the place to start. In Section 5 on departure procedures it includes the following reference:

  b. Leg type limitations. See Order 8260.46, paragraph 3-1-5 for
  permissible leg types.

Also, all the TERPS requirements also apply, except where modified by the PBN manual.

There's usually no one rule that applies universally. It's about finding the best solution that meets the terrain/obstacle avoidance and meets the necessary airspace and air traffic needs.  I can't say with certainty why the procedure writer made the choices they did, but I can point out a few things that would apply in the SID you reference.

The 17L/R departure is primarily constrained by the 7000 ft minimum at GISTT. It's there to keep the path within the Class B airspace.  There's a cutout with a 7000' floor around Front Range airport to the southeast. GISTT is located at the edge of the cutout.  It's far enough south to leave room to reach the minimum altitude at the published minimum climb gradient.  The VI/CF is the solution that gets you to GISTT expeditiously.  It also has an intercept angle of about 50 degrees which works well with the FMS and LNAV steering.

The 16L/R departure has two factors that push it to the VA/DF solution.  First is that it departs over runway 07/25 and there's no intercept (more later), the altitude termination defines when the turn can begin. The reason the DF to MUGBE is used instead of a CF is that there is a 140 degree course change.  You can't have a VI/CF combination with that large of a course change. Thus the VA/DF combo.  If MUGBE (or another convenient waypoint) was better located, a VA/RF leg combo could work but RF legs are relatively 'new' in the RNP FMS world and they aren't very common.


What is this feature on a Piaggio P.180? (wikimedia.org)

What is this feature on a Piaggio P.180?

I believe it's called a dolphin tail, but why is it exactly there and what is it for and why don't all airplanes have this?

The airfoils shown are delta fins.  They provide lateral stability during higher speed flight.  It helps to reduce dutch rolling tendencies and to prevent deep stall characteristics.

So much so that on the Learjets I fly, there is no need for an operable yaw damper.  There is also no stick pusher or stick puller system.  Older Learjets without delta fins are required to have those two systems installed.

https://en.m.wikipedia.org/wiki/Piaggio_P.180_Avanti


Do airline pilots routinely carry out "gross error" checks? Malaysian MH132 seems to have had a bit of a mix up over flight plans.  It appears to be a non-event but it does make me wonder if "gross error checks" are commonly used by airline pilots.

I'm only a PPL(H), but gross error checks are built in to me from training. For example, "after setting off on the first leg, the compass should be pointing NE which means the town should be on my right and the river on my left".

Some commentators have remarked that the captain did a "great job" in recognising the unexpected heading and querying it.  To me, it seems like basic airmanship to recognise and query it and leads me to also wonder if the crew studied the route during pre-flight and after it was loaded into the FMC.

Can any airline pilots add facts to the story?

I agree, Malaysian MH132 was a Non-Event.   If the pilots had any concerns over the slight variation in their routing, they should have mentioned it before they loaded it into the Flight Management System (FMS).

I currently fly long haul B777 for a major airline. I can't speak for others, but at the airline I work for we do many "gross error checks". Some are done in flight planning, some during the loading of the FMC, and some are done during the flight.

While still in the flight planing area, the Captain and First Officer will look at the total routing in regards to wind, temperature, turbulence, volcanic activity, etc.  Often a call is made to the dispatcher who generated the Operational Flight Plan (OFP) to discuss the routing, expected levels of turbulence, and fuel reserves.  About 20% of the time the Captain will request a slight change to the routing or additional fuel be added.  A new OFP is then generated and it will be designated "Release 2".

Both the flight plan route and ICAO flight plan route are printed on the OFP and we compare the two.  I have never found an error in the 17 years I have been doing it, but we still do it every time.

Once the Captain, FO and augment pilots ALL agree, the OFP is officially accepted and we head to the aircraft. (up to 4 pilots on Ultra Long Haul flights)

When we get to the aircraft we run though the basic preflight checks, and then the flight plan is downloaded to the FMS using the Aircraft Communications Addressing and Reporting System (ACARS) digital datalink system. The departure runway and SID is also loaded at this time.

Before we "activate" and "execute" the routing, the Pilot Monitoring (PM) reads the entire FMC routing and the Pilot Flying (PF) compares it to the OFP.

After runway takeoff performance data is loaded we then check the "Legs" page of the FMC for a logical sequence of legs. We generally just check to make sure there are no course reversals on the various legs of the route.  If there are any user defined waypoints (Lat/Long waypoints) we perform bearing and distance checks of  the FMC against the OFP. This is done with the the Heading Reference switched to TRUE heading instead of Magnetic for more accurate numbers.  These bearing and distance checks are not done for "Named" waypoints, only Lat/Long waypoints.

Wind data is then requested and then loaded via ACARS.

Next is a check of the FMC distance compared to both the OFP distance and Great Circle Route distance.  The FMC distance is almost always somewhere between the OFP distance and GCR distance. This check is one sure way to spot a routing entry error.

Finally a check of the FMC and OFP predicted fuel over destination is done. If there is a large difference, it means either the routing or winds were incorrectly entered.

Once airborne there are more checks:

In the old days before GPS we would check the accuracy of the Inertial Navigation System (INS) as we approached the coast for any over water flight. This was usually done using a VOR/DME fix.

Once airborne we again check bearing and distance as we approach each user defined waypoint (Lat/Long waypoint). Once past the waypoint by about 5 mins we then plot our current Lat/Long on a plotting chart to insure we are still on course.


Why do landing gear tires smoke upon touchdown? I have been observing several aircraft landings and have noticed that when the aircraft's landing gear touches down, it precipitates the rising of a cloud of smoke. Why is this?

(Source: airliners.net)

Because the tires are momentarily skidding on the pavement as they rapidly spin up from standstill to the jet’s touchdown speed.  It’s just a puff of burnt rubber but the tire tread itself becomes quite warm, reaching 600-700°F for an instant.  It’s also the reason a pilot does not ride the brakes upon touchdown as this could cause a blowout as the tire is ground up rapidly.


Is it normal for the flaps to move like this? I noticed this particular portion of the flaps moving differently from rest of the wings. In the attached picture, the middle flap is raised. Is this considered normal?

The landing and takeoff in this flight was normal, but the flaps moving like this made me a little (lot) uncomfortable! I've added a video, as well.

Video on Youtube

I believe the engine is mounted in front of that part.

This means that if the part moved along with the rest of the spoiler the jetblast would be deflected down which will create an upward force on the wing. You don't want that while you have full spoiler deployment.

Putting a gap in the spoiler will reduce stress on the full spoiler from the jet blast. This leads to less expensive maintenance as the stresses are reduced mostly to a single panel and its attachment points.

As @Ron points out the flap geometry is such that you can't have the full flaps adjacent to each other without them jamming up when they deflect. Putting an extra panel between them lets it move on top of the other 2 flaps when necessary.


Why simple paper planes fly better than most "realistic" paper plane models? Just a A4 paper sheet folded into a plane seems to fly decently. But a model of actual (big) plane made out of paper is only good at falling...

What is different for paper planes? Are there "modern and improved" designs of paper planes?

The reason is really the size! Air molecules have the same size if they flow around a small paper airplane or a big real airplane, but since their relative size is so much smaller when they flow around the real airplane, friction effects are much smaller for the big airplane. When comparing airplanes of different sizes, a dimensionless number first formulated by Osborne Reynolds is most helpful. It is basically the ratio of inertial to viscous forces in the airflow, and the formula is
$$Re = \frac{l \cdot v}{\nu}$$
Nomenclature:
$Re\:\:\:$ Reynolds number
$l\:\:\:\:\:\:$ characteristic length, for example wing chord
$v\:\:\:\:\:$ airspeed
$\nu\:\:\:\:\:$ kinematic viscosity of air

Flows with the same Reynolds number behave identically, but your scaled-down aircraft flies at a very different Reynolds number than the original (both speed and characteristic length are a lot lower), so the airflow around it will be very different. For it, the air looks more like a syrup when that around the original is like flowing water! Therefore, lift creation needs different means at this scale (wings with more chord, for example), and the paper airplane is better suited to cope with this particular flow condition.

The next difference is wing loading: The paper airplane is very light; its mass per area is low. Therefore, it can fly at a slower speed than your scaled-down real airplane, and it will be easier for you to launch the paper airplane properly into the air. The scaled-down airplane needs to be thrown harder, but still straight, and this might be another reason for it falling down quickly.


Why did the Flightaware groundspeed readout suddenly spike for this flight? My 18yr old son is currently in Barcelona - first time abroad and in a plane. Being an overprotective parent I thought I'd track his flight so I could see when he had landed safely = and save the data for him so he could see his journey.

Anyhow, I noticed weird spike in the flight graph (which I couldn't save). A sudden jump of 200mph in air speed - and equally sudden reduction - at around 15,000ft. No jump in altitude increase though. Also a rapid change in direction.

The flight was on Friday 3rd at around 7.18 am bst leaving from Bristol. Data copied below - any thoughts?

BST Latitude Longitude Course  Direction  KTS MPH feet   Rate
Fri 07:22:11 51.3043   -2.6938 254° West  306 352 10,900 2,338
Fri 07:22:37 51.2927   -2.7573 255° West  322 371 11,900 2,759
Fri 07:23:38 51.2674   -2.8943 360° North 503 579 14,900 2,804
Fri 07:24:24 51.2459   -3.0081 253° West  340 391 16,900 2,365

Looking into one set of data from the the popular Flightradar24 service it is not visible.
I think I found the data you're looking at. Since the heading also suddenly changed to (exactly) 360, I'm thinking this is just an invalid data point. Notice that the receiving station jumped between these measurements and something could have happened with the processing in between.
From the same site:

Aircraft with an ADS-B transponder broadcast their identification and this three dimensional position (latitude, longitude, altitude)

This simply implies that this service tries to interpolate the speed from the time between two positions. So say that the clock at the receiving station or something is off (for instance, just an idea), it will just screw up the reading. It comes with the disclaimer:

I believe I witnessed a traffic conflict, altitude deviation, or some other anomaly. Should I report it?
Please don't. Rather, read the terms of use and understand that this data is for casual observation only and not for any operational purpose. FlightAware technology was not designed with the intent to observe safety or regulation anomalies, so please do not try to use it for that purpose.


Is there any U.S. / FAA regulation that could apply for aviation software? I want to build some software applications applied to the aviation industry and would want to know if any U.S. / FAA regulation could apply in the design of them.

I know there are some regulations when a software holds information related with finance, health or kids.

For simulators you have Part 60 of CFR 14

For airborne software the FAA has published AC 20-115, but the main document that refers to is the FAA/EASA RTCA DO-178/ED-12 currently at the "C" version: Software Considerations in Airborne Systems and Equipment Certification.
If you want to certify (and thus commercialize) your software for flight, this is the main document to read.

Depending on the scope of the software and the design/development process there might be different supplements (see here):

  DO-330 Software Tool Qualification Considerations
  DO-331 Model-Based Development and Verification Supplement to DO-178C and DO-278
  DO-332 Object-Oriented Technology and Related Techniques Supplement to DO-178C and DO-278A
  DO-333 Formal Methods Supplement to DO-178C and DO-278A

It has been prepared by the RTCA/EUROCAE joint committee and approved by both FAA and EASA under different names, but the content is the same.

Note that these documents are not freely available.


What is the maximum (unloaded) range of the Beluga XL? Airbus states that the range of the Beluga XL is 4000 km at maximum payload. Does anyone have a figure (or estimate) for it's maximum range with no payload?

For an estimate, we can use the Brequet range equation:

$range = V\cdot t_f
= \frac{L}{D} \times I_{sp} \times \ln\left(\frac{W_i}{W_f}\right) $

with:

$V$: optimal velocity
$t_f$: flight time
$L$: lift
$D$: drag
$I_{sp}$:  propulsion efficiency
$W_i$: initial weight
$W_f$: final weight

Assuming the lift to drag ratio $\left(\frac{L}{D}\right)$ and the propulsion efficiency will stay constant, the range will depend on the ratio of initial and final weight.

$range = C \times \ln\left(\frac{W_i}{W_f}\right)$

In case the aircraft departs with the maximum payload, the weight is:

$W_i = W_{oe} + W_{payload} + W_{fuel} = W_{mto}$

$W_{oe}$: operating empty weight: 127 500 kgf
$W_{payload}$:  payload weight: 51 000 kgf
$W_{mto}$: maximum take-off weight: 227 000 kgf
$W_{fuel}$: fuel weight =  $W_{mto} - W_{payload}-W_{oe}$ = 48 500 kgf

The final weight at maximum range is then:
$W_f = W_{oe} + W_{payload}$

Filling in the numbers we find:

4000 km = $C \times \ln\left(\frac{\textrm{227 000}}{\textrm{127 500 + 51 000}}\right)$

That means $C \approx \textrm{16 640}$ for the Beluga XL.

If we now remove the payload and take-off with the same amount of fuel, we find:

$W_i$ = 176 000 kgf
$W_f$ = 127 500 kgf
$range \approx \textrm{16 640} \times \ln\left(\frac{\textrm{176 000}}{\textrm{127 500}}\right) \approx 5365$km

But that is just removing the payload without adding fuel. Of course we can now bring more fuel on board without exceeding the maximum take-off weight.

The fuel capacity of the Beluga XL is 73000 kg (source:Aircraft Characteristics
Airport And Maintenance Planning Beluga XL manual). This changes the numbers to:

$W_i$ = 200 500 kgf
$W_f$ = 127 500 kgf
$range \approx \textrm{16 640} \times \ln\left(\frac{\textrm{200 500}}{\textrm{127 500}}\right) \approx 7530$km

So the Beluga can fly approximately 7500 km when empty, enough to cross the Atlantic from Toulouse, even if there is normal headwind.


Downwash distribution on basis of Lift distribution I am new here so I might make some elementary mistakes. Correct me if I am wrong.
I just stumbled upon the downwash distribution for a given lift per unit span distribution along the wingspan. I have attached a photo for your reference.

I was wondering that if we look upon a $dy$, an infinitely small spanwise slice of the wing, then the lift it produces should be proportional to the downwash it created as more downwash equals more distortion which equals more lift. But we can clearly see that at the end where the lift is minimum the downwash is maximum and at the center where the lift is maximum downwash is minimum. Could anyone please explain it?

You are confusing downwash ($w$) with circulation ($\Gamma$). As per Kutta-Joukowski theorem, lift per unit span (in the case of a single lifting line) depends on the distribution of circulation along the wing span:

$$L'(y)=\rho_\infty V_\infty\Gamma(y)$$

If you have an elliptical lift/circulation distribution, the circulation decreases towards zero at the wingtip, therefore lift at the tip is zero. Induced downwash, on the other hand, is constant across the span.

Source: Anderson, Fundamentals of Aerodynamics

You may believe that downwash is the largest at the tip due to the presence of wingtip vortices. However, the vortices are better understood as downwash rollover at the wingtips. The vortices themselves do not contribute to lift generation (but unavoidable for finite span).


How does the Pratt & Whitney Canada PT6 differ from other turboprop engines? As far as I know, all turboprop engines have mechanical continuity between the engine and the propeller except for the Pratt & Whitney Canada PT6 family of engines. What is the correct term for the setup the PT6 uses to power the propellor and how does the PT6's design differ from other turboprop engines?

Split Shaft

Several turboprop engines, including the PT6 turboprop engine, use an epicyclic gearbox only, many other turboprop engines use a multishaft gearbox. The latter usually results in the propeller axis being offset from the turbine axis.

In common with some other engines, the PT6 turbine is also reversed. This facilitates the split-shaft arrangement where the engine and propeller are driven by separate (but in-line) shafts, each connected to their own turbine wheel.

PT6

Image: P&W

Image:12Charlie

A contrasting and common design

Image:12Charlie


What criteria are used for exiting an airplane in an emergency? I often see after a crash that people die unable to exit the aircraft. The news reports and passenger and safety organizations criticize the FAA for not having realistic emergency exit regulations. What determines the criteria for a plane being "safe" in an emergency?

Per 14 CFR 25.803:

  For airplanes having a seating capacity of more than 44 passengers, it
  must be shown that the maximum seating capacity, including the number
  of crewmembers required by the operating rules for which certification
  is requested, can be evacuated from the airplane to the ground under
  simulated emergency conditions within 90 seconds

Appendix J to Part 25 specifies exactly how the evacuation tests should be done. It's too long to quote here, but it includes requirements for lighting levels, number of passengers, minor obstacles, infants on board etc.

I have no idea whether or not those simulated conditions are reasonable and effective, but they at least set a standard.


What regulations surround fitting aftermarket fuel tanks inside the cabin? Watching the first episode of Dangerous Flights, a ferry pilot has aftermarket fuel tanks fitted inside the cabin of an old Merlin IIB:

What are the regulatory requirements to be able to do this? / How can you get permission to fit extra tanks inside a plane?

Is there anything that needs to be specially considered when designing the tanks and valves etc?

Is this commonplace when ferry flying long-haul?

This is covered by 14 CFR 91.203:

  §91.203   Civil aircraft: Certifications required.

  ...

  (c) No person may operate an aircraft with a fuel tank installed
  within the passenger compartment or a baggage compartment unless the
  installation was accomplished pursuant to part 43 of this chapter, and
  a copy of FAA Form 337 authorizing that installation is on board the
  aircraft.

14 CFR 43 covers MAINTENANCE, PREVENTIVE MAINTENANCE, REBUILDING, AND ALTERATION.

Basically, it needs to be an approved installation that is signed off and approved by the FAA using a Form 337 - MAJOR REPAIR AND ALTERATION (Airframe, Powerplant, Propeller, or Appliance).  The maintenance shop will take care of that aspect for you.

It is pretty common for delivering aircraft that need to cross the ocean, and there are even companies that specialize in it.  Really, your only option is to purchase an already approved kit because certification costs are so high.

NBAA actually has a nice article which goes into more details, with lots of helpful links.


Behavior of air over a wing Venturi´s effect states that air over the wing accelerates and static pressure drops. There's a different theory which states that as air is viscous, once it impacts the wing, slows down... Can those two theories coexist?

Yes. In different regions.

A fluid flowing through a constricted section of a tube undergoes a decrease in pressure, which is known as the Venturi effect. This is a consequence of Bernoulli's principle, which says that total pressure is constant. Basically,

$P_{1} - P{2} = \frac{\rho}{2} (v_{2}^{2} - v_{1}^{2})$

along a streamline. But here's the catch- it is applicable only for steady, inviscid, incompressible flows. This condition is satisfied when the flow speed is small (subsonic) and at a certain distance from the body (i.e. wing) and can be used to calculate the velocity or pressure fields at some distance from it.

However, in real world, the fluid (i.e. air) is viscous, though its effects are restricted to a certain region immediately near the body, as seen below:

By F l a n k e r - Own work, CC BY-SA 3.0, Link

You can see that in the immediate region of the immersed body, the fluid is slowed down- there is a velocity gradient between the different layers of the fluid as they are being slowed down. However, after some distance from the body, the fluid behaves as if it is inviscid- there is no velocity gradient.

In the end, we choose the situation that is more useful for us- in case of subsonic flow, for calculating lift etc. the flow can safely be assumed as inviscid, while the boundary layer is very important when we consider cases like heat transfer etc. There is nothing right or wrong here (the air follows the same laws, wherever it is)- it is simply the case of using the theory that makes more sense for easier understanding and computation.


In parallel runway ops, is it better to dedicate a single runway to landings, or is it best to have a mix? Assuming taxiing is not a consideration, in an airport with two parallel runways does the airport gain capacity by:

having a single runway dedicated for landings and the other for take-offs, or
is it best to have both take-offs and landings from both runways?

Would there be other ways to divide the movements (say by class/size of aircraft)?

That would no doubt depend on a lot of factors. Geometry for example. If the parallels are far enough apart there's no chance of a missed approach on one interfering with traffic on the other, you're a lot more flexible than when they're close together.
Maybe you have noise abatement procedures that make the use of one of the runways for either landing or departures illegal at specific times (e.g. Schiphol can't use certain approaches during the morning rush because overflight of parts of the city are not allowed at those times).
Type of traffic is another. If you're getting a lot of inbound heavies at an hour where you have a lot of outgoing smaller aircraft, it soon becomes a good idea to have one runway set aside during that period for landings, one for arrivals (the wake turbulence of the heavies would mean longer waiting times on the runway for the lighter aircraft).
So overall, there's no definite answer. It all depends on traffic, local conditions, etc. etc.


Does propwash increase pitch stability? Here is a polar for a piston single-engine aircraft:

(This is lifted from a Russian manual for Yak-52, so $C_x$ is drag, $C_D$, and $C_y$ is lift, $C_L$).

1 is the 'normal' case without propwash. 2 is nominal, and 3 is the max takeoff power.

It is evident that the whole lift curve slope $C_L^{\alpha}$ is increased significantly (by some 30%) as the engine revs up, with $C_{L_{max}}$ reaching 2.

But what immediately follows, amongst other things, is that the AoA (pitch) stability should increase accordingly: it is proportional to $C_L^{\alpha}$ (as well as to the distance from NP to CG).

However, somehow I've never seen this fact mentioned explicitly, despite it being quite significant. Can anyone confirm it, either from literature of experience flying higher-powered prop aircraft?

(It may not be very obvious to feel the difference because other conditions are rarely the same between idle and full power, but the closest approximation I can think of is glide descent vs full power climb at the same speed: the aircraft should be 'stiffer' and possibly more oscillatory in pitch at full power. However, propwash over tail may mask this effect; perhaps a twin is a better testbed for this...)

There is not an obvious answer to this. I'll outline three effects (among others):

Placement of thrust line. If the thrust line is placed below the CG, it will have a destabilizing effect; the converse is true. That's why wing mounted engines tend to destabilize aircraft as throttles are increased. But that's not the title of your question.
The local increase in flow from prop wash increases the wing lift slope. This tends to decrease the pitch stability. An increase in lift has an associated increase in downwash as a function of AOA. This also tends to decrease stability.
The local increase in flow from prop wash increases the tail lift slope. This tends to increase the pitch stability.

The neutral point contribution is as follows (cited from Etkins, Dynamics of Flight):

$$h_n=h_{n_{wb}}+\frac{a_t}{a}\overline{V_H}(1-\frac{\partial\epsilon}{\partial\alpha})-\frac{1}{a}\frac{\partial C_{m_p}}{\partial\alpha}$$

where $h_n$ is the location of neutral point, $h_{n_{wb}}$ is the wingbody aerodynamic center (AC), $\overline{V_H}$ is the tail volume with respect to the wingbody AC, $a_t$ is tail lift slope, $a$ is the total aircraft lift slope, $\epsilon$ is downwash on the tail, $C_{m_p}$ is the pitch moment contribution from thrust.

Forget about thrust line for a moment, then we have:

$$h_n=h_{n_{wb}}+\frac{a_t}{a}\overline{V_H}(1-\frac{\partial\epsilon}{\partial\alpha})$$

Therefore, increasing the wing lift slope decreases the neutral point, as will an increase in downwash. Increasing the tail lift slope has the opposite effect.

So there is no blanket statement.


If the intake of a running jet engine is blocked for some short duration what would be the sequence of events following it? If the intake of a running jet engine is blocked for some short duration what would be the sequence of events following it? Will it cause any damage to the engine parts?

This explanation is for a subsonic aircraft, a similar process occurs at higher speeds.
Causes
A bird, an object or ice can disturb the flow of air entering the engine. Another source of compressor stall is an excessive engine angle relative to the air flow, when a large pitch or crab is forced, or when entering the wake of a preceding aircraft.
Immediate effect
Regardless of the case, if the flow departs from the operating area at some location in the engine compressor, an aerodynamic stall of the compressor can be experienced exactly like a wing can stall:

Stalled airfoil in a wind tunnel, flow separation and turbulence visible. Source
The air boundary layer detaches from compressor blades surface and becomes unsteady.
Compressor stages are made of rotating blades and stationary vanes delimiting small channels:

Source
The problem develops from a low mass flow region, with disparities increasing until the flow stops, in a process called rotating stall:

A passage is blocked by slow air (detached boundary layer in orange), upstream air tries to use the next passage:

However the next passage is reached under a higher angle of attack, this condition tends to stall this passage too.

This effect propagates rapidly from one blade to the next on the compressor stage.

As passages are blocked by detached air over stalled airfoils, pressure increases in the passages and upstream:

Detached airflow is very turbulent with changing velocity (including negative values).

Stalled cells keep rotating at a fraction of the compressor rotational speed, pressure around the blades varies periodically.

Blade/vane vibrations can start. Vibrations are not good for an engine, buffeting can break a metal part if resonance occurs.
A turbine engine works by reaching a precise equilibrium around the flow of air in the compressor, the combustion chambers and the turbine:

The turbine must receive a given quantity of energy to drive the compressor at the proper speed.

Combustors must receive the exact quantity of fuel and air to deliver the energy to the turbine.

Compressor must provide the required quantity of air to the combustors.

(Let's ignore the fact that turbine engines have most of the time two or three spools, i.e. compressor-turbine assembly with a mechanical linkage, each one turning at its own speed, which have to find this shared equilibrium).
As soon as air velocity or pressure changes for any cause, a new equilibrium must be found, else bad things start to happen. In a compressor stall, sometimes no new equilibrium can be reached. This lead to dramatic consequences.
Subsequent effects
After the rotating stall, a cascade of events is triggered:

Pressure decrease at combustion chamber inlets
The compressor being now less efficient, pressure downstream the compressor decreases in addition of being unstable. The magnitude of the pressure variation is somehow limited by the plenum chamber effect of the compressor diffuser for an axial compressor (by the diffuser stator after the impeller for a centrifugal compressor).

Location of the diffuser, source

Depending on whether a new balance can be found or not, combustion may continue while the rotating stall occurs. In that case it'll be at reduced power due to the less efficient compressor, and a lower fuel flow is required.

It may be necessary to shutdown the engine and restart it to get out of the compressor stall.

Overheating
Less air being now available for combustion, if fuel flow is not reduced, the mixture becomes too rich, combustion temperature increases, overheating occurs in the combustion chambers and downstream in the turbine. As the materials used for the combustion chambers and the turbine are working at their highest sustainable temperature, any unwanted increase can damage them.

This can happen if the fuel control unit (FCU) doesn't limit fuel flow correctly and attempts to maintain previous thrust / fuel flow.

Surge
The compressor diffuser (or plenum in a centrifugal compressor) is upstream of the combustion chambers. The latter is a very high pressure region at hot temperature. If the diffuser pressure is too low compared to turbine inlet pressure, hot gas can start flowing back into the diffuser and the compressor. This is a surge.

The surge creates a loud bang and the pressure wave can damage the fan and the engine inlet.

The surge can be associated with flames getting out of the engine via the exhaust pipe or engine inlet. Flames are due to the ejection of unburnt fuel accumulated in the combustion chambers.

Compressor elements are not designed to sustain hot gas from the combustion chambers they may be damaged if expulsion is repeated.

Again, the engine may recover itself from the surge if there is no damage. The cycle can repeat but at some point damages are inevitable.

More on compressor stall: What exactly is a compressor stall?

Loss of control
The surge also creates an asymmetrical additional thrust leading to a yaw force, for an engine not on the center-line.

If the surge is important and the aircraft in a critical phase (e.g. rotating to take off), the yaw effect can lead to a loss of control.

Stall actual cases

In this accident due to icing, the repeated surges are suspected to be the result of the improper fuel flow management. Both engines were destroyed.

This accident was caused by a compressor stall at large crab angle.

In this takeoff accident, the yaw effect of the surge was actually interpreted as a collision with another aircraft.

Fortunately most compressor stalls are handled safely, like this one near takeoff rotation speed (note the flames at 0:34).
Obviously a compressor stall is a serious problem, so engine designers spend a lot of efforts to prevent it and maintain the engine working point in a safe (and efficient) state. For instance using vanes with variable angle of attack and air escape paths, etc.


Which among the two contributes most of the lift, the upper or lower surface of the airfoil? I see some explanations that lift is due to the suction of low pressure on the top surface of the wing and deflecting down of airflow on the bottom which results to upward force lift. I wonder, which among the two surfaces (top and bottom) contributes most of the lift?

In subsonic flight, the majority of the lift is generated by the lower pressure (compared to ambient) at the upper surface of the airfoil, which produces a net suction. A portion of the lift is also provided by the lower surface that has higher pressure, but is much smaller in comparison.
The graph below shows experimentally derived data on a symmetric airfoil, which breaks down the pressure distribution between upper and lower surfaces. The vertical axis of the graphs shows difference between local static pressure and free-stream static pressure divided by dynamic pressure, which is also called pressure coefficient. Note that a negative number means lower pressure compared to free-stream static pressure, and is suction; a positive number means higher pressure, and is pushing.
As you can see, in the 9deg AOA case, the upper surface suction dwarfs the lower surface push force by observing that the area under the curve is much larger for the former.

Image ref: http://web.mit.edu/13.021/13021_2003/Lifting%20surfaces/lectureC.htm
The next two plots show the stall and post-stall. This airfoil stalls at leading edge, so the entire suction peak is removed once the flow is stalled.


How much energy/prop efficiency does the e-volo copter sacrifice by having a smaller propeller? It seems like on an electric aircraft, it would be very important to have a high energy efficiency. Wouldn't the e-volo copter be much less efficient due to the small radius of the propellers when compared to a traditional helicopter?

What counts is the mass of air which is available for lift/thrust creation. Several smaller props would still offer about the same area for lift creation as a single rotor, so in this respect that design is not at a disadvantage. The smaller props will have to spin faster than a single rotor to have the same average dynamic pressure, and the lift distribution over the whole area covered by propellers will be less asymmetric at forward speed than it is over the disc of a single rotor.

What should reduce the efficiency, however, is the smaller Reynolds number of the blades. Even if the blade airfoil is carefully optimized for their small Reynolds number, I would expect that at least 10% more energy is needed for the same lift compared to a conventional helicopter. The smaller inertia of the blades, however, is a big advantage in control, because the smaller propellers can be spun up or down faster, avoiding the more complex rotor head of conventional helicopters. With a modern FCS the e-volo should be controllable by individually adjusting prop speed alone. The electric motors most likely are capable of short-term torque bursts which far exceed their continuous performance, so control responsiveness should be adequate.

The smaller moment of inertia of the propellers becomes a liability in an emergency, because the e-volo arrangement is less capable of an autorotation landing in case the energy supply fails. That is probably the reason why the e-volo carries a full aircraft parachute. On the other hand, many small engines and propellers certainly improve redundancy over conventional helicopters.

I also have to wonder about the mass of the massive framework which is used to mount the rotors. It is in the way of the propeller flow and should contribute a substantial fraction to the empty mass of the e-volo. I expect the designers use it to store the battery cells, but I could not find much about this on the website - which offers, however, a funny movie which nicely sums up the projected capabilities. Just don't get distracted while the e-volo waits for you in hover mode: The claimed flight time is currently only 20 minutes.

My verdict: It is less efficient than a comparable helicopter, but not by much. The two problems are the lower blade chord, which translates into more viscous friction, and the high mass and drag of the framework.


What is the lighting technology behind the Dreamliner's dynamic interior lighting? The flights that I've been on in the Dreamliner (and also Virgin America), have dynamic overhead cabin lighting that provides nice mood lighting, helps alleviate jet-lag, and is otherwise very pleasing to look at. What is the technology behind this? The closest technology I've seen that can offer dynamic light coloring is phillips hue lightstrips.

This 'mood' lighting is offered by Boeing in 787 and Airbus in A350XWB (as an option) and is achieved by using Light Emitting Diodes (LED). According to Airbus,

  Airbus is applying the latest technology in energy-efficient LED (light-emitting diode) mood-lighting on the A350 XWB. This enables the cabin ambience to be tailored to each airline’s own identity, as well as mimicking natural sunrise and sunset illumination that helps the body adapt to jetlag on long-range flights.

The main advantages of LEDs over conventional lights are,

The LEDs are longer lasting (~50,000 hrs).
They are more energy efficient.
They generate less heat.

Phillips seems to be involved in the development of lighting systems for the(se) aircraft. According to led-professional.com,

  Philips Lumileds provided the LEDs while Diehl Aerospace engineered the overall lighting system for the Boeing and Airbus cabin interiors.

The light intensity and colors are decided by the airliners and are pre-programmable (and can be changed remotely). According to Bloomberg,

  The Finnair A350 cabin has two dozen light settings, aligned with stages of a long-haul flight. It will also feature warmer, amber colors on flights arriving in Asia and cooler “Nordic blue” hues when flying into Finland. ...
  Virgin Atlantic has five primary color periods on its 787 flights: rose-champagne for boarding, purple-pink for drinks, "amber" for dining, another for the pre-sleep period called “work-rest-play,” a silvery glow for overnight sleep, and a waking color.

Similar lighting systems are also offered by third parties for other aircraft.


Is a compressor blade slowing down the airflow or accelerating it? When a jet aircraft flies at maximum speed, are compressor blades actually a barrier to the air flow, or is the airflow accelerated by the compressor?

are compressor blades actually a barrier to the air flow, or is the airflow accelerated by the compressor

These are not exhaustive alternatives. Keep in mind that subsonic fluid flow when constricted accelerates and pressure decreases (Bernoulli's principle).

What the compressor does is increase the pressure. The speed decreases slightly.

  are compressor blades actually a barrier to the air flow

Actually it's complicated. In a sense they are. The compressor always takes the air at around M0.5 irrespective of flight speed. That means at slower speed the engine needs to suck air from wider stream while at higher speeds some air is spilled over the intake lips. So in a sense, the compressor is a barrier.

But then, the compressor must increase the energy of the stream, otherwise the engine could not produce any thrust. Just the energy goes in pressure, not velocity. So in this sense it is definitely not a barrier.

In fact, the thrust acts mainly on the compressor, because that is the most significant aft-facing surface on which the pressure in the combustion chamber can act.

At supersonic speed it works differently. Now pressure changes can't affect the flow upstream, so the air can't be spilled over the intake lips anymore. Instead now the intake must decelerate it by shock waves and then expansion in a diffusor.

This is needed, because turbomachinery only works well in subsonic flow¹. Since pressure increases as the flow speed decreases, this also increases the overall pressure ratio of the engine and thus its efficiency.

Note that the air also heats up, via adiabatic heating, as it slows down and compresses. Somewhere around M5–6, the air becomes so hot from the compression that adding more heat to it by burning fuel is no longer practical, so above that speed, the combustion must occur in supersonic flow. This is the scramjet engine. It does not have any compressor or turbine, because it gets all the compression from constricting the flow and it turbomachinery wouldn't work at that speed anyway.


If the pilots used the brakes upon landing, would the force essentially slam the front wheel down? So if the pilots braked immediately, with only the back wheels down, and nose wheel still up, would the force from braking cause the front wheel to come slamming down?

If you get on the brakes hard after main touchdown, once the anti-skid is active following wheel spin-up (you can't land with brakes applied on any airliner with anti-skid - brakes are depressurized until after the main wheels start to spin and/or you are weight-on-wheels for a minimum time) and don't apply any additional compensating elevator, the nose will come down faster than if just hold the existing elevator input, and will contact a bit harder, but "slamming down" would be a bit extreme unless you also relieved some of the control back pressure as you did so. Then it would be a bit firm and if you were reckless enough you might blow the nose tires, and somebody would have a chat with you.

In practice though, on most jets you land the mains, then the nose in two separate actions and during that "second phase" you are applying elevator to modulate the sink rate of the nose to make it touch down reasonably gently for that "2nd landing".  If you were on the brakes hard at that point you would normally just compensate instinctively with extra elevator input to get a gentle landing of the nose.

You would certainly have enough tail power available to counteract the nose down torque of the braking at high speed.  But you would probably get scolded for getting on the brakes before nose wheel contact anyway, and a lot of pilots will keep their feet low on the pedals to avoid applying brake, then move them up once the nose wheel is down.


How to calculate the best glide speed if there is none on the POH? I am studing the POH of Seneca II and could not find the Best Glide Speed. Where can I get this information from? I thought to multiply Vs clean for 1.4 to get an approximate result but I am not sure if this is the correct way to do so.

According to the FAA, the short answer to your question is approximately "halfway between Vx and Vy."
While there is a lot of physics and calculus that can give a precise answer to this question, there is another answer that uses "pilot math". The FAA has a nice publication titled Best Glide Speed and Distance, which explains that there are two different types of "best" glide speeds.

The glide speed that gives you maximum distance covered for a given altitude (Best Glide Distance); and

The glide speed that gives you the maximum time in the air (Best Glide Time).

Per the FAA document, the glide speed that gives you the most distance covered is approximately halfway between Vx and Vy. For example, on a C172 with Vx at 53 and Vy at 73 Max Glide would be about 65. The speed provided in POHs is usually calculated at Max Gross Weight, so your actual Best Glide Distance speed will be a little lower if your weight is less than max gross.
Alternatively, if your goal is to stay in the air as long as possible, then you'll want a speed that minimizes the descent rate. This is typically slower than Best Glide Distance speed, and can be easily determined by pitching for an airspeed that gives you the lowest vertical descent speed on the VSI. There are times when you don't care how far you glide, and instead want to maximize the time in the air in order to troubleshoot. For example, if you are over an area with no suitable landing location (like over the ocean), you'll want to maximize your time to try to restart your engine. Or if you are over your landing location, but want extra time to finish your emergency checklists.


What are the disadvantages of having vortex generators? I know the advantages of vortex generators (VGs): lower stall speed, lower landing and takeoff speed according to www.stolspeed.com. So what are the major reasons most GA planes and low speed ultralights don't have VGs on their wings?

Cost.  A set of Micro AeroDynamics VGs can run around $1500 plus installation cost.  On my plane they are installed on the top of the wing, the sides of the vertical stabilizer (fin), and the bottom of the stabilator.  I wasn't aware there were other suppliers of STC'd VGs for certificated planes. I installed mine in early 2000s, price may have gone up some since then.

They do work well, lowering stall speed while allowing the controls to be effective the same as at higher speeds. No noticeable loss of airspeed at higher power.


About ramjet efficiency in supersonic speed. Is there an optimal speed? I know there is an optimal speed for ramjet.

This are the results of my research.

The reason the efficiency increases with supersonic speed to an optimal value then decreases is related to the combustion speed.
The burn flame in the combustion chamber propagates at subsonic speed.
When the air is compressed the speed of intake airflow falls subsonic while flowing into combustion chamber. However, when the air speed in the combustion chamber matches the flame advance speed, then the flame remains stable inside the combustion chamber and the top efficiency is achieved.
At higher speeds the airflow still needs adjusted to match the flame propagation speed. This means the compression has to reduce the speed to the airflow to the same subsonic value, while dealing with even stronger effect generated by shockwaves. There is a maximum flow rate through engine that still maintains the maximum subsonic speed of the flame propagation. Anymore air resulted from higher speed will push the front of the flame into the exhaust nozzle which in turn pushes the expanding exhaust gas outside of the nozzle, with loss of thrust.

Well, that would depend on the design parameters, but for example this ideal ramjet would, with stoichiometric combustion process,  provide best thrust per unit mass flow at a speed of approx. 2.4 Mach

  ...the best operating range of a hydrocarbon-fueled ramjet is $ 2 \leq M_0 \leq 4$ . The parameters used are $ \tau_\textrm{max}
= 10$ , $ a_0 \approx 300\textrm{ m/s}$ in the stratosphere, $ f_\textrm{stoich} = 0.067$ for hydrocarbons, such that $ a_0/g
f_\textrm{stoich}
\approx 450\textrm{ s}$ .


Were inclined runways ever under consideration? A long time ago I read an article (in Popular Science, probably) of a plan for an airport runway constructed on a grade, so that one end was ten stories higher than the other. The idea was to save gas: at takeoff, the plane would be going downhill and get a speed boost from gravity. When landing, the plane would be going uphill, and take less engine power to stop because gravity is slowing it down.

Did anyone ever research or actually test this plan? Does it seem practical?

Yes, it does work, but cannot really be called practical.

The first reason is wind: It helps to take off and land with a headwind. If the runway slope is on one end, this has to be the end where the take-off run starts and where the landing run ends. Since both are in the same direction, this scheme now needs twice the runway length, the first half for landings and the second half for take-offs.

The next reason is the magnitude of what can be saved. The energy from the height change $∆h$ can be translated into a speed gain $∆v$ by this formula: $$∆v = \sqrt{2\cdot g\cdot ∆h}$$

Let the ten stories be a height of 40 meters, and your speed gain is only 28 m/s. This was significant in the age of propeller aircraft, but jets need much higher speeds to get airborne - 150 knots or more. In sane units this is 77 m/s, and since the energy is proportional to the speed squared, the 40 m slope will save only 13% of the energy needed for take-off. It is simply not worth it.

Now look at the operational consequences: The landing has to be performed such that the aircraft has slowed down to 28 m/s when it reaches the foot of the slope. If it is still too fast, it will overshoot and roll down the other side, and if it is too slow, it needs to run up the engines in order to climb up the slope, expending the energy it hopes to save at take-off.

No, practical is not what I would call this.


Are any airlines actually flying the Comac ARJ21? The Comac ARJ21, China's answer to Embraer's regional jets, was launched with great fanfare in 2016.  However, if FlightRadar24 is to be believed, launch customer Chengdu Airlines does not appear to be flying any of its aircraft (click open AJ27, then check the aircraft).

Are any airlines actually flying the Comac ARJ21?  If not, when and why did they stop?

You are correct in that the FlightRadar24 site doesn't show any flights for the ARJ by aircraft. But going to the routes tab and searching through the routes there are ARJ assigned to some routes. So it appears that Chengdu Airlines is operating the aircraft at least on a limited basis.

I would also say that while FlightRadar24 data is generally very good, it isn't perfect. They do rely on data from external sources and considering the Chinese government's information and internet policies, it's entirely possible that the data availability is not the most reliable.


Do aircraft carriers always face into the wind? When landing at a normal ground airport, the runway facing the compass point nearest to the heading of the wind is always operated, so that aircraft landing and taking off alway have to use the least distance of runway as necessary.

On an aircraft carrier the whole ship can rotate; so do they typically always face into the wind? How often do they rotate the ship if this is the case?

During flight operations, the aircraft carrier strives to maintain 30 knots of wind down the angle of the flight deck.  Generally speaking, the carrier will adjust its speed and course through the ocean to maintain the desired winds.  However, shifting winds and the sheer magnitude of the carrier may mean that the winds are generally down the angle, but may involve a crosswind component.  Also, the structure of the carrier, because of its enormous size, actually influences the flow of the wind, and can create pockets of low pressure in the approach where high sink rates can develop.


Which design method is used for super critical airfoils? How is an Airfoil categorized as super critical or not super critical? What are the ways to design a super critical airfoils?

Critical speed or Mach number is the speed at which the speed of sound is locally reached somewhere along the flow around an airfoil. A supercritical airfoil can tolerate operating speeds slightly above the critical speed (hence the name), which means that a pocket of supersonic flow will show up, most likely on the upper surface. What qualifies an airfoil as supercritical is that the recompression does not need a shock, but can be done shock-free for a small lift coefficient range by clever shaping.

See this question for more.

Calculation of a supercritical airfoil is best done with a computer program which models the flow conditions around Mach 1 faithfully. Once such program would be TSFOIL. The picture below shows one result for the BACJ airfoil at Mach 0.8, and you can see that the speed on the upper surface reaches Mach 1.3.


How are the dimensions of the empennage determined for jetliners? How is the height of the vertical and span of the horizontal stabilizers determined for different turbofan aircraft like the Airbus A380 and the Boeing 747-8?

What are the maximum forces (in kilograms or Newtons) acting on both the horizontal and vertical stabilizers for aircraft like the Airbus A380 or the Boeing 747-8?

The tail volume coefficients for the horizontal stabilizer ($C_H$) and vertical tail ($C_V$) for jet transports are 1.00 and 0.08 respectively.

For the horizontal stabilizer, for example, with the $C_H$ given, and the lever arm, [main] wing reference area ($S_W$), and MAC known, the area can be calculated.

The shape, sweep angles, aspect ratio, etc., of each surface have their own rules.* Knowing the shape and area, the span or height is calculated.

Example 1: When the lever arm was reduced when the A319 was shortened to become the A318, they had to compensate by increasing the vertical stabilizer area, which meant a higher tail.

Example 2: Boeing designed the 757 and 767 simultaneously, so in order to reduce the fabrication and tooling costs, both shared the same tail, with the 757 tail shortened at the root (Flight, 1979).

(Own work)

* Reference and further reading on the aforementioned rules: Empennage General Design (.pdf), Hamburg University of Applied Sciences.


Do airlines schedule departures based on average environmental conditions of departing airport? I have noticed that at Dar es Salaam International (HTDA, Tanzania, Africa) all major airlines (namely KLM, Swiss, Emirates, Turkish) schedule their departures mostly at night starting from 10:00 PM local time onwards.

Besides the fact that they have to match their flights with ongoing connections, I was wondering if they do this because they fly here with big aircraft (A330-300, B777, B737-900ER) and the airport, even though being at sea level, has very high day temperatures throughout the year (30C and higher).

Maybe this helps during takeoff, which in hot conditions could be risky, especially if fully laden?

In general, high temperatures affect the performance of the aircraft in two major ways:

As the density is lower at higher temperature, the thrust produced by the engine is reduced.
The lift produced is reduced as lift is proportional to the density.

Due to these reasons, an aircraft operating in hot condition will have to either reduce weight for taking of from a fixed runway length or increase the takeoff run for taking off with the same weight. Also the climb performance will be affected at high temperatures.

Due to this, the airlines could've schedule operations in conditions when the temperature is less if they believe that takeoff weight is critical for example, in long range fights or where a set number of passengers is necessary for economical operation.

However, this problem is mostly critical in case of 'hot and high' airports. In this case, this is most probably for facilitating the arrival of passengers in destinations in the morning.

For example, the SWISS 296 leaves Dar Es Salaam at 11:20 PM (its connecting flight lands only at 1005 PM) and reaches Zurich at 06:16 CET.


How much force can the landing gear of a Piper Arrow withstand? I'm currently working on my CFI-A in a Piper Arrow and i was wondering if anyone knows how much force the landing gear can withstand? The reason I ask is because I have never been asked this question and I know there has to be a concrete answer out there. For example, aircraft in the Navy that need to land on a aircraft carrier have to go through a series of landing gear stress tests.

The Arrow is certificated under 14 CFR 23, and the landing gear loading limits are specified in the ground loads and landing gear sections. I don't know much about this subject and the regulations are fairly technical, but 23.473 apparently defines the load limit. I don't really understand it myself, but it looks like the landing gear has to support at least 2.67 times the aircraft's weight in a landing:

  No inertia load factor used for design purposes may be less than 2.67,
  nor may the limit ground reaction load factor be less than 2.0 at
  design maximum weight, unless these lower values will not be exceeded
  in taxiing at speeds up to takeoff speed over terrain as rough as that
  expected in service.

In reality they should be designed to withstand more than this, as there are manufacturing uncertainties which can affect a component's performance.

Someone who's actually an engineer could probably explain it better (or correct me, as the case may be!).

As for testing, my limited understanding of the certification process is that the manufacturer has to prove to the FAA that the requirements in part 23 have been met, but there's no specific way of doing that. And indeed, 23.721(c) says:

  Compliance with the provisions of this section [i.e. the landing gear section] may be shown by
  analysis or tests, or both.


Is the Boeing 777X being certified as a derivative of the 777 Classic? Given the corner cutting Boeing did with the Max certification, is the FAA allowing the 777X to be certified as a derivative (easier) of the 777 Classic or requiring a new Type Certificate? (harder)

The wings are new, the engines are new, the flight deck is new (from the 787 I understand), the weight is higher and the fuselage longer.

Yes, Boeing applied for an amendment to the existing 777 type certificate:

On April 19, 2017 (for the Model 777-8 airplane), and May 12, 2015 (for the 777-9 airplane), Boeing applied for an amendment to Type Certificate (TC) No. T00001SE to include the new Model 777-8 and 777-9 airplanes. These airplanes are constructed with new carbon-fiber-reinforced plastic (CFRP) wings with folding wingtips.
The Model 777-9 airplane, a derivative of the Model 777-300ER airplane currently approved under TC No. T00001SE, is a stretched-fuselage, large, twin-engine airplane with seating for 408 passengers and a maximum takeoff weight of 775,000 pounds.
The Model 777-8 airplane, a shortened-body derivative of the Model 777-9 airplane, is a large, twin-engine airplane with seating for 359 passengers and a maximum takeoff weight of 775,000 pounds.

(federalregister.gov, emphasis mine)


Are composite materials, which are being widely used in newer aircraft, better at handling bird strikes? Do current composite materials handle bird strikes better than their predecessors? Or they are not part of forward-facing components/areas?

For example, the Boeing Dreamliner or the Airbus A350-XWB.

Out of the box, no. They are worse at handling birdstrikes. But with some thought they can be made much better.

All materials have an elastic range of deformation, and if that is not exceeded, they will assume their original shape without damage. Composites fail if this range is exceeded. First the fibers with the highest elongation fail, which enables the other fibers to take more load, but at some point too few fibers remain, and the part fails catastrophically. This is audible: The first small failures cause a faint crackling, and the eventual failure a loud bang.

Metals add a plastic deformation before ultimate failure, and this enables them to absorb a lot of energy in the process. Since energy is force times distance, the small elastic deformation will not absorb much energy. However, the plastic deformation is magnitudes bigger and makes metals the much better energy absorbers. Even a deformed structure can take a sizable fraction of the load it took before deformation. A birdstrike will cause a dent, but the rest of the structure will not see much of the load since the dent-forming could absorb a lot of energy. However, the changed contour can provoke changed aerodynamic loads, so new failure modes might result.

Impact loads on composites have a second disadvantage: The impact creates shockwaves which travel through the structure and cause delamination (separation between fibers and resin matrix) when they hit a boundary. Therefore, the biggest damage occurs on the opposite side of the impact. A birdstrike might not leave a visible dent on the outside, but can shatter the material on the opposite side such that the structure will break at a fraction of the load it would have carried without the bird strike.

Therefore, some composite wings use metallic leading edges to protect the composite structure from birdstrikes. But also the composite structure itself can be improved: If very tough fibers like Kevlar or Spectra are added, the impact energy will not break those fibers, and the load will be distributed over much more of the resin matrix. This will delay delamination, and when it happens, it will be spread over a bigger area and cause the fibers to break at multiple locations. The energy needed for the widespread delamination and the multiple fiber failures will increase the energy absorption dramatically. This structure must be designed as a protective armor, so the load-carrying structure below is not affected. Proper crashworthy design with composites allows to make the structure even tougher than a comparable metallic structure.