What are the RVSM Critical Area markings? I couldn't find any image to show you what I'm talking about but on many aircraft there are markings around the static ports and the pitot probes that say RVSM Critical Area. What is this?

Aircraft need to know their altitudes to avoid bumping into each other in the vertical dimension. The default instrument for this is a pressure altimeter. A pressure altimeter needs access to a static port.

RVSM stands for Reduced Vertical Separation Minima. Basically, aircraft above a certain altitude (29,000 ft) used to be kept further apart from each other vertically (2000 ft) since the accuracy of the pressure altimeters degrades with altitude.

Over the years, as traffic density increased there was a need to pack aircraft closer vertically (1000 ft). This is possible if you have a special more accurate altimeter system in all aircraft and these ops are RVSM (late 1990's and early 2000's).

Now, in order to guarantee that this system works as designed several points need to be satisfied one of which is the integrity of that static port I mentioned earlier. Static ports can be sensitive to external air flow and hence any variation / disturbance / abnormality in the area immediately adjacent to them on the fuselage must be avoided whenever you want to use the aircraft for RVSM operations.

And hence the markings to serve as a guide to all involved in aircraft ops about which is the area to worry about. e.g. Any imperfection in that region will have to be dealt with immediately. And worst case the aircraft would not be capable of flying in RVSM operations till any problems are fixed.


What criteria does ATC apply to determine if an aircraft is established on the localizer if the pilot does not report it? How close in time or distance will ATC let a pilot fly to the localizer, before cancelling the approach clearance, where the pilot has not reported being established? Is it the controller's call when to cancel the clearance?

Each instrument approach consists of a lateral part and a vertical part, in the case of the ILS approach these are: localizer and glideslope.

Aircraft are usually vectored to intercept the localizer approximately 1-2nm before the Final Approach Fix (FAF), where the glideslope is captured. Aircraft can be vectored onto the localizer as far as 20-25nm out, depending on terrain, localizer equipment and many other factors. If an aircraft is not established on the localizer, there is no point in continuing the approach, so it will be cancelled and the aircraft revectored.

If the aircraft is established on the localizer, it will try to intercept the glideslope, which is the vertical guidance system in the ILS approach. For VOR/DME, LOC/DME, NDB/DME or RNAV approaches, the vertical guidance is either achieved through correlation of location to a specific altitude in the chart or technical means which do not require a glideslope.

If an aircraft passes the Final Approach Fix and does not seem to descend, it will be considered as not established and the approach clearance will be cancelled, instructing the aircraft to either follow the missed approach procedure or manually vectoring the aircraft for another approach.


What powers the air required for pneumatic brakes in aircraft? What powers the air required for pneumatic brakes in large airliner aircraft? Is it bled from the engines, or do they have a separate motor and pump?

For a medium size turboprop, the F-27: two separate compressors, driven by the turboprop engines.
From this article

Fairchild’s F-27 is the first American-made plane to use a complete pneumatic system to operate landing gear, wheel brakes, nose wheel steering, propeller brakes, and passenger door operation. It also has an emergency air supply that will lower the landing gears and operate the brakes.
Compressed air for the system is supplied by two 2-cfm, 3300-psi compressors located in each of the engine nacelles. The compressors are driven through a gearbox by Rolls Royce Dart engines, which power the aircraft. They carry their own individual dehydration and filtration equipment to supply clean, dry air at a pressure of 3300 psi

Most present day airliners use the hydraulic system for the landing gear, including wheel braking. For instance on the B737:

The normal brake system and autobrakes are powered by hydraulic system B. If brake pressure drops below 1500psi, hydraulic system A automatically provides alternate brakes which are manual only (ie no autobrake) and the brake pressure returns to 3000psi.

The hydraulic systems are each powered by an Engine Driven Pump and an Electric Motor Driven Pump.


How does a Hawk jet avoid pitching down when its air brake is applied Some aircraft have air brakes in an aerodynamically neutral position with respect to rotational forces on the airframe (e.g. F-15)...

source

... more often, others have brakes that are aerodynamically neutral by being opposing (lots of examples, F-16, Space Shuttle, here's a Buccaneer)

source

... but the Hawk has an air brake that, because of its position at the tail end of the aircraft, and without any opposing force, I would have expected to generate a severe downward pitch as it is deployed, in much the same way as the elevator.

[source - Ministry of Defence]

It obviously doesn't, so the question is - why not?

It did generate a nose down pitch moment, but the tail had enough control authority to deal with it in most cases. Airbrake extension at greater than 450kt produced an unacceptable trim change.

Efforts to increase control authority to counter "phantom dive" caused by tail stall during simultaneous flap and gear extension helped. The solution was to affix a leading edge extension to the tail, but since the tail was all moving it was felt that the structure would not be able to handle the extra load. Instead, the tail LEX was affixed to the fuselage and still gave acceptable results.

A later version, the Goshawk, developed as a military trainer for the US Navy had side mounted airbrakes, though this was done to facilitate the installation of an arrestor hook. This photo shows the LEX just above the side mounted airbrakes.

Hawk mk100 with ventral fins, center airbrake and tail canard LEX.


What information on a specific flight can be retrieved? I would like to know where and what to search in order to have information on a specific flight (i.e. scheduled departure time, real departure time, technical failures reported, etc.)

I'm guessing that this information can be found :
in the airplane's log
at the control tower (or the aeronautical uthorities)

Can you please tell me more about all this, especially the technical name of the different documents and the information contained within?

flightradar24 takes as input mainly ADS-B data, as explained here. It seems to be enough to know scheduled departure and arrival time, departure and arrival airport, registration number and some extra pieces of data you may not need.

For technical failure, you may see the aviation herald website (I don't know how to access ACARS messages)


What is the best major to study for a career in propulsion systems design? I want to be an aerospace engineer but am not sure what to major in. I really want to design propulsion systems. What would be the best major for this and what could I do to increase my chances of being hired?

While the question is certainly subjective, I wanted to help you out as much as possible. I spoke to a friend who works as an electrical engineer at NASA and asked him your question. Based on his advice and some of my own research on your behalf, the most direct route would be to pursue an aerospace engineering degree in a good university program - CIT, MIT, UM Ann Arbor, etc. (there are plenty of schools to choose from). Most aerospace engineering students will choose a specialty in either an area of design or even in a specific type of vehicle - many universities will allow you to specialize in propulsion - essentially tailoring your electives to include subjects like fluid dynamics, multivariate and non-linear controls, combustive thermodynamics, and turbomachinery. That said, my friend indicated that the field can be very competitive, and that many propulsion system designers probably did not begin their career doing that kind of work. Be prepared to be flexible and plan on playing the long game when it comes to reaching your dream job. Also, as is the case in any competitive industry, networking can be key. Be sure to find opportunities to meet people in your field, ask questions, and get to know others who can give you the inside track on openings and opportunities.
Hope this helps.


Why do airfoil's trailing edges point downward? The trailing edge of and airfoil is mostly pointing downward. Is there any reason for that? Down wash increases drag and down wash is due to the direction of air pointing down. If the trailing edge is made horizontal (or maybe pointing upward), they can produce less down wash and Kutta Condition can still be satisfied.

What are the specific reasons for this shape?

You are right, the trailing edge does not need to point down. Take symmetric airfoils - here the trailing edge runs parallel to the airfoil chord. Or take reflex airfoils (like the HQ 34 of the SB-13 tailless glider): Here the trailing edge is indeed pointing upwards, and still this aircraft flies.

But to create lift with as little drag as possible, it helps that the trailing edge points slightly down. Why? Because then it will be least in the way of the desired local flow. Lift is created by accelerating air downwards. The wing deflects the air flowing over it, and the trailing edge should reflect this deflection angle.

But - as always - too much is not good: The Eppler 417 airfoil of the SB-7 glider was an extreme layout with too much rear camber. It is said that pilots could still wipe off raindrops near the trailing edge after landing when they had crossed a shower an hour earlier. This means that airflow separated well ahead of the trailing edge on the upper surface, and the raindrops would not be blown away in the separated flow. Flow separation increases drag, and this effect shows that the shape of the Eppler 417 had too much downward camber at the trailing edge.

If the plane is large and heavy, its airfoil's rear camber can be higher - an extreme case is shown below. This is an early supercritical airfoil designed by McDonnell-Douglas, and the highly cambered rear part allows it to integrate very effective Fowler flaps. Flaps help because they allow to change the direction in which the trailing edge is pointing: Low lift coefficients require no flap deflection, or even negative flap angles in gliders, and the higher the lift requirement grows, the more the flaps will be extended, pointing more and more downwards.

The same is true for control surfaces: Depending on the desired lift change, their trailing edge will point up- or downwards. See below for an example where I plotted the pressure distribution for three flap deflections in one graph. Upper and lower surface pressure are shown by color-coded lines, and lower lines belong to the lower surface. Dashed lines show the inviscid pressure, and solid lines the pressure distribution with friction effects added. The wider two lines of the same color are apart, the more lift is created. Note the contour plot below, which follows the color scheme of the pressure plots.

If there are slots to re-energize the flow, extreme trailing edge angles are possible and help to create lots of lift at low speed, which helps airliners to get into small airfields. See below the triple-slotted flaps of the Boeing 727, which was designed for regional traffic from and to small airports.


Is a civilian pilot allowed to fly under part 91 rules with Night Vision Goggles? I asked a related question: How hard is it to fly with night vision goggles?

It appears from the answers so far that there are differing opinions, and I guess it might depend on the quality/age/capability of the equipment.

If someone was flying non-commercially (Part 91), Private Pilot (or higher) and it was VFR flight, if the pilot owned a pair of The ANVIS 9, Aviator Night Vision Goggles, Model M949, for example, could they legally fly around with them in their own airplane at night (and be acting as Pilot-In-Command)?

(Assume that they are a safe, prudent aviator, and took efforts to properly train themselves on the equipment and flew with a safety pilot until they were comfortable in the air with the equipment).

Would it be any different if they were flying IFR (appropriately-rated and on an instrument flight plan)?

Theoretically? Yes. In practice? Maybe.

The FARs provide for civilian operations under NVGs, but the story is a bit more complicated.

NVG operations requirements for pilots are covered in 61.31(k) for obtaining an endorsement and 61.57(f) for currency. Reading over the wording, the FAA appears to take the same general approach that they do with tailwheels: i.e. get trained and endorsed and then stay current.

The problem is with the aircraft rather, as 91.205(h) requires, in addition to day and night VFR equipment, a gyroscopic pitch and bank indicator (not typically an issue), a generator or alternator as per IFR reqs (again, not generally an issue), interior and exterior lighting appropriate for NVG ops (may be an issue), and a radar altimeter (definitely an issue in almost all light, and some not-so-light, aircraft).

Not sure about the goggles themselves, maybe there's a TSO out there giving tech specifications for aviation-specific NVGs?


What's the north boundary of the KINS Class D? (Or: does a SUA designation take precedence over controlled airspace?) This is inspired by the question regarding aerobatics over a surface class D area.

KINS (Creech AFB) sits right outside the south boundary of the Nellis Range Complex (R4806W to be precise).  Considering this, and considering that it has surface Class D around it whenever the tower's open as per the A/FD...does that class D airspace abruptly end where R4806W begins just north of the airbase, or does it continue all the way around, leaving a portion of airspace that's both a Class D (sfc) area and SUA (i.e. restricted airspace)?

According to the sectional chart, Class D airspace does not extend into the restricted area.

SkyVector KINS Sectional Chart

The restricted airspace starts at the surface to an unlimited altitude.  According the JO 7400_11A.pdf, Class D does not enter the restricted area.

Class E airspace is the same.


Does pilot monitoring have authority to go around in exceptional circumstances? Let's say the captain is acting as the pilot flying(PF) and the first officer the pilot monitoring(PM) in a commercial jet. They are shooting an ILS to their destination airport but the approach is so unstable that the first officer shouts "go around, go around" after passing minimum. But the captain ignores him and wouldn't balk the landing.

In this situation, does the first officer have the authority to take the controls from the captain by shouting "my controls" and go around? What about if their roles are switched? That is, what if the captain was the PM and the first officer the PF? Is the captain allowed to overide the first officer's decision to continue and initiate a go-around? I think this question can be better answered by commercial or airline pilots because I believe this is determined by company SOPs rather than FAR or ICAO rules. If so, are there any airlines that render the PM the authority to go around in defiance of the PF's decision under certain circumstances, like when the PM firmly believes the plane will very likely crash if an immediate balked landing is not initiated?

The captain is the captain, so when he says "go around" then the PF needs to initiate the go-around.  If the FO doesn't initiate the go-around as directed, then the captain can (at that time as at essentially any other time he sees it necessary) take over control of the aircraft.  That's the captain's authority and it's pretty clear.

The other case is slightly less clear, but only slightly.

Most operations probably have a directive along the lines that the PF is required to go around when the PM so directs.  So if the captain is flying and the FO directs a go-around, what the captain/PF should do is perfectly clear.  If he doesn't do so, the next step is probably spelled out in the operations manual; probably something along the lines of, repeat the directive once, then assume incapacitation and take over the controls to execute the go-around.

The really bad case would be, the captain is intent on landing regardless and the FO believes that they're about to crash.  At that point, the question (really for both pilots) becomes, are you sufficiently sure of your position that fighting for the controls close to the ground is less dangerous than doing what the other pilot wants?  If we're somehow down to our last minutes of fuel & the copilot wants to go around because a dog ran onto the runway, a reasonable captain would probably overrule him and land anyway.  If the captain is so intent on landing that doesn't see the aircraft on the runway that he's about to  land on top of, a reasonable copilot might push the throttles forward like his life depends on it.

If you've reached the point where "go around" "go around" "I have the aircraft, go around thrust" hasn't worked, you've left the realm of most manuals.  The implicit assumption is that an incapacitated PF isn't going to have the capacity to fight you for the controls; I doubt very many operations manuals devote much space to the nightmare case of wrestling for the controls against a maniac determined to do something awful.  Thankfully, such scenarios are almost entirely the stuff of imagination -- commercial flight is essentially the safest form of transportation ever known to mankind.  That remains true even though the one in (very literally) millions of flights where something does go horribly wrong (Germanwings crash into the Alps, etc) gets extraordinary publicity.


How to leverage Lynx NGT-9000's integrated GPS/WAAS? I’m inheriting an IFR capable 1964 Cherokee 235 that is not equipped with a GPS. I’m looking at installing a Lynx NGT-9000 to get ADS-B compliant and gain the traffic and weather information.

Is there any way to leverage its integrated GPS/WAAS to enable LP/LPV approaches and file IFR direct? I don’t want to over panel this plane.

Sorry, but windmills approach certified GPS systems do not work that way: A GPS approved for IFR navigation and approaches, like a Garmin 430W or GTN650 or an Avidyne IFD440, are more than just an approved position source such as what's included in the NGT-9000 transponder. They have an approved navigation database and interface with (or provide the equivalent of) a Course Deviation Indicator (CDI).

You can't just suck position data out of the NGT-9000 and fly around using it because while that device has an approved high-quality position source it doesn't have an IFR-approved navigation database, so there's no way to take that GPS data (your position as a point in space) and relate it to known waypoints (VORs, NDBs, airports, intersections, etc.) in a way the FAA considers satisfactory as a primary means of navigation.

You can take the GPS data from it and interface it with an iPad - Foreflight speaks the Lynx L3 language - to use for enhanced situational awareness, which is a huge win, but that's not enough to get you to file GPS flight plans: You would be limited to filing flight plans based on your other installed primary navigation instruments (VOR, ADF, DME).


Does Flight Following automatically include permission to fly through class C or D airspace? The situation is as following - I requested flight following at approach, got a squawk and instructed to maintain VFR with heading and altitude at my discretion. My route is through class D and class C airspace for the altitude I chose. The question is - should I establish additional communications or report to the approach controller about flying through the class C and D airspace, or I am already permitted to do this?

Lets back up a minute and discuss what is required to enter C and D airspace.
For C and D airspace you must establish 2 way radio communications with tower or approach controller. Let's first discuss how this works if you are not taking Flight Following. As you approach Class C airspace you hail them and wait for a response, for example,

You: "Allentown Approach Piper 1234J @ 3000 en route to Queen City Municipal"
Controller: "1234J Squawk 1-3-5-2 and Ident"

Since the controller responded with your tail number you may enter the Charlie airspace (Delta has the same requirements).
Now when you are under Flight Following, ATC will coordinate a handoff to the Approach or Tower for the airspace you are about to enter, or ATC will say "radar services terminated," and you must contact the controller/tower yourself before entering.
You SHOULD NOT change frequencies until you are told to do so, since ATC may be trying to give you a traffic advisory. You can however tune your second radio (if you have one) to the approach/tower you are approaching, and begin to monitor traffic.


Are there turbofan/turbojet analogues to a centrifugal-compressing jet engine? Axial-compressor jet engines fall into two categories (and only two as far as I'm aware): turbofan and turbojet. I'm wondering if centrifugal-compressor jets also have such a distinction or something similar.

I cannot imagine how a big rotating fan-like object could both compress air centrifugally and bypass it out the rear, but I would not be surprised if some engineer found a clever way of doing it. And anyway, a centrifugal jet could be separated into two broad types based on something slightly different.

Yes, but the classification is not a matrix.

A turbofan by definition needs a fan, but at its core it can have a centrifugal compressor. A good example is the Pratt & Whitney Canada PW600 series. This allows a compact design, at the expense of compression ratio. It works well for low-thrust applications such as the Cessna Mustang. The advantages are the engine being small, lightweight, and having fewer parts.

(Google Patents) The arrows point at the two-stage compressor. The first stage is a mixed flow compressor rotor, and the second stage is a centrifugal compressor. Label 9 is the fan up ahead.

The PW617F provides a whopping 7.18 kN (1,615 lbf) of thrust and weighs under 180 kg. (The fan is slightly bigger than a human hand.)

You can watch a moving cutaway model for the JT15D turbofan with a centrifugal HPC here.


What is the average number of daily flights for all types of aircraft in use worldwide? I'd like to know how many flights occur on average every day worldwide. This includes all commercial passenger and cargo aircraft, all non-commercial aircraft, private aircraft and military aircraft. A flight consists of a planned take-off and landing.

As mentioned above there are a significant number of aircraft that have no way of being recorded. However, we can make an educated guess. According to the FAA, they support; "Every day, the FAA's Air Traffic Organization (ATO) provides service to more than 42,000 flights" - https://www.faa.gov/air_traffic/by_the_numbers/. These numbers would be aircraft with flight plans, controlled airport landing & takeoffs, flight forwarding, vectoring, etc.

Probably (a guess) at least as many general aviation aircraft do training, VFR flights, helicopters (i.e. ranch cattle herding), balloons, scientific, skiing, hunting, sightseeing, gliders, ultralights and maintenance flights without filing a flight plan or interacting with ATC.

So that puts the US at about 80,000 flights per day. The US is approximately 5% of the world population and if we use that as a basis for calculating flights, then there are about 1.6 to perhaps 2 billion flights per day, including all forms of aircraft.

This is interesting - we might at first conclude 1/4 of the earths population is flying on any given day. This would be a wrong assumption because many passengers embark on several flights to reach a destination each day. For an example, a sightseeing cross country trip might include 3 or 4 stops that include re-fueling and eating, or restroom breaks. But, this is still a staggering number of people flying each day. Conservatively, perhaps 1 out of every 50 to 100 people are traveling by air each day!!!


What type of commercial aircrafts generate strong Wake Turbulence? I know that 757, 737 generate strong Wake Turbulence and as pilots, we should always be aware of that and caution for the wake turbulence while landing and departing.

I would like to know if is there like a list of commercial aircraft based on their Wake turbulence generation?  Which one generate the most and which one generate the least? Let's assume they are all heavy, clean, slow!

Big ones.

Wake turbulence is primarily proportional to the aircraft's weight, since this is generally directly related to the size of the aircraft and the amount of air the wings need to move to provide enough lift. Aircraft are categorized with respect to wake turbulence based on their maximum certificated takeoff weights. FAA AC 90-23G describes classes for ATC purposes as follows:

Super - Interim class for the A380 and AN225
Heavy - 300,000 lb or greater
Large - 41,000 - 300,000 lb
Small - 41,000 lb or less

Heavy and Super category aircraft are required to add their category to their callsign when contacting ATC, e.g., "Speedbird 12 Super".

Aircraft in higher categories have larger separation requirements when in front on aircraft in smaller categories, such as between takeoffs or landings, and ATC will issue caution messages cautionary advisories when required (Section 2-1-20).

The AC linked above discusses further on wake turbulence, such as how it propagates and how it should be avoided or escaped.

The FAA is also working on a 6 category system (A to F), which is in effect at MEM, SDF, CVG, and ATL. There are plans to expand this to the Metroplexes of Houston, New York, Charlotte, Chicago, and Northern California in 2015.

  This approval was based on years of joint research and development by
  the FAA, Eurocontrol, scientific experts in wake, and experts in
  safety and risk analysis. Categories are now based on weight,
  certificated approach speeds, wing characteristics, along with special
  consideration given to aircraft with limited ability to counteract
  adverse rolls.


Relationship between wing aspect ratio and speed For some reason, I cannot find a relatively straightforward source online which discusses the relationship of wing aspect ratio and how it affects speeds of an aircraft.

Specifically, is there a decrease in efficiency to fly at a faster cruise speed as the aspect ratio of the aircraft's wing increases? Could an aircraft with a high aspect ratio - similar to those of gliders - travel at the same cruise speed as a jet?

Thanks in advance!

It's really the other way around, speed of the aircraft affects aspect ratio.

Hawks fold their wings to dive at high speeds, and have them straight out for lower speeds.
At high speeds the main concern is drag.  The less drag, the faster you can go.  At lower speeds, the main concern is lift.  Here a higher aspect wing is more efficient.

  Why not use a smaller high aspect wing at higher speeds, and just vary the sweep?

This is done, but along with higher speed comes greater aerodynamic forces.  This is one reason why modern fighters feature low aspect wings and deltas.  These are simply much more able to withstand higher G forces.

The variable sweep wing concept was developed in the 1950s and 1960s, but generally carries a weight penalty.  Variable camber is widely seen in airliners today.
Some military aircraft, such as the B-1 bomber, use variable sweep to expand their useful
flight envelope, giving them the ability to "get low and fast".

Cruise missiles, optimized for one speed only with little abrupt maneuvering, can be found with very simple high aspect swept wings, which offer an improvement in drag over a lower aspect wing, thereby increasing fuel efficiency and extending range

There is no direct relationship with speed and aspect ratio, indeed, high subsonic aircraft such as airliners, that strive for maximum efficiency, are seen with increasingly higher aspect ratios in recent years.

Structural strength is the main limitation which continues to challenge engineers today.


Windshield to prevent laser strikes Is it possible to prevent/avoid laser beams with new technologies on windscreem?

Only if you know the laser wavelength.

If the windshield uses a color filter, it can attenuate the light intensity at one wavelength while being transparent for other wavelengths, so the pilot can still see what is going on outside. The colors will look strange, but laser beams of the filter wavelength would have less intensity when viewed through this screen. This only works, however, if the laser wavelength is known beforehand, so the right filter can be used. All other light signals, approach lights and what not would need to use sufficiently different wavelengths so they are still visible.

Think of an approach in fog, rain or haze. Sometimes all the pilot sees is a faint glow - if the filter takes this away, the pilot will notice the lights only later, when they are closer and there is less time to react.

The other alternative is synthetic vision. The cockpit has a row of screens which show the images of cameras in the nose of the aircraft. This was already investigated by NASA for supersonic passenger aircraft, but met resistance from pilots. The reduced contrast of screens will make lasers merely a nuisance, but not harm the pilot. Clever software could even filter out too bright light sources.


What’s the difference between a tail number and a Manufacturer's Serial Number? What’s the difference between a tail number and a Manufacturer's Serial Number (MSN)? How and when are they generated?

The tail number is the civil registration, and the manufacturer serial number (MSN) is a build number used by the manufacturer.

It is similar to the licence plate on your car, and the vehicle identification number applied by the builder of the car.


Why didn't the U-2 spyplane have any chaff? The U-2 spyplane was shot down over Russia in 1960 and over Cuba in 1962, both times by a radar-tracking missile. Why wasn't it equipped with any chaff to throw off the missile?

Chaff is just a cloud of aluminum particles that return a big radar blur, masking the real craft so it can get away. The missile would home in on the chaff instead since it's a big juicy radar target. Chaff can be stored in very small cylinders near the tail of the plane, and the radar lock warning would let the pilot use it at the right time.

Chaff was invented in World War 2, giving plenty of time all the way to the sixties. So why wasn't this airplane equipped with chaff?

There are two main reasons for this:

USAF and CIA believed that the Russians (or anyone else) wouldn't be able to shoot down aircraft operating at (>60000ft).
Most of the aircraft operating during that time did not have chaff dispensers to deploy chaff as needed. For that to work, the pilot has to either visually acquire the missile or have a RWR, which became widely used only in the later stages of Vietnam War.

Another important reason is tactics. The chaff was not used in WWII in a tactical capacity i.e. to protect individual fighters, but to provide blanket coverage to bomber fleets. This practice continued well into the Vietnam War. For example, specialized aircraft (usually fighters) will dump chaff to provide a 'corridor' for following bombers. For an aircraft on reconnaissance mission, this is as good as advertising its presence.

A lot of aircraft in that era went without any countermeasures. For example, the English Electric Lightning carried no chaffs till its retirement in 80's.


4 Boeing 707 for $50,000. Is that possible? I'm helping a friend with an article on some issues in my country, mostly related to corruption. His English isn't very good, so here I go

One of the cases he's analyzing is the sale of 4 Boeing-707 airplanes for $50,000 . These airplanes couldn't fly but according to public information, they had their avionics, each of them has 4 working (not suitable for flying) Pratt & Whitney JT3D engines and one of them had a working intelligence/surveillance system (this was a very complex subject and now government says they'll try to keep this for training). Airplanes were for military use, both passengers and cargo.

Even considering selling them for scrap or replacement parts, is $50,000 a realistic price? For example, is there a place I can find prices for the parts, eg engines, landing gear and such?

EDIT: this is in Argentina, newspaper article (in Spanish).

Some data translated from article:

Approximate cruising speed: 977 km / h
Service ceiling: 11,000 meters
Approximate range: 8690 km
Empty weight: 62,500 kg
Load capacity: 94,000 pounds
Maximum operating weight: 335,600 pounds
It could carry up to 180 passengers

They are four-engine jet aircraft, Boeing brand.

Primary function: Transport of passengers and cargo, as the case may be.
Wing type: low implantation. Undercarriage: type retractable tricycle with 10 armed wheels. They have four Pratt & Whitney engines (turbines).
Length: 44 meters. Span: 47 meters. Tail height: 13 meters.

3 of the airplanes were bought by an US company specialized in airplanes' instrumental and parts, 1 was bought by a local party.

Its surely possibly and there are a few things I can think of off hand,

Missing Log Books: generally speaking if an aircraft is missing its log books you need to do quite a bit of tear down and rework on it to make it airworthy once again. In many cases the cost of this far out weighs the cost of a logbook/airworth plane so not always done. As such this can severely drive down the price of the aircraft making it close to worthless. In line with this, without the log books you can not sell any of the parts for replacement either. With out the ability to properly tag the part they are only worth their weight in scrap. Considering they seem unflyable and are fairly large to chop up and ship scrapping it can also become a costly maneuver.
The tax man...: Airplanes (and all manner of vehicles) often change hands for strange dollar amounts due to various tax regulations. That does not look like the case here but it does happen.
Simply Outdated: keep in mind the 707 is a fairly old plane by anyones standards these days. While it may have avionics chances are they are no newer than when it was parked there to collect dust, like it or not the best LORAN C unit in the world may have cost a fortune to install back then but is worth next to nothing now.

These things dont exactly go up for sale all that often but here is a pretty nice one listed for 6.2 Million and here is reference to one listed for only 700K. Please keep in mind that there are countless variables for this kind of thing that could drive the price both up and down drastically.


Since speed is related to dynamic pressure, then why is static pressure used instead of dynamic pressure in ASI to measure speed? Since speed is related to dynamic pressure, then why is static pressure used instead of dynamic pressure in ASI to measure speed?

Why is dynamic (or pitot) pressure not sufficient?

Since speed is related to dynamic pressure, then why is static pressure used instead of dynamic pressure in ASI to measure speed?

It's not. The reading on the airspeed indicator is directly determined by the dynamic pressure. The ASI uses the static pressure for the sole purpose of determining the dynamic pressure.

  Why is dynamic (or pitot) pressure not sufficient?

Dynamic pressure is sufficient, but the pitot tube doesn't give you the dynamic pressure; it gives you the total pressure, which is the sum of static pressure and dynamic pressure. In order to determine the dynamic pressure, an airspeed indicator has to find the difference between static pressure (from the static tube) and total pressure (from the pitot tube).


How the antenna on the B737 Max fuselage doesn't disrupt the airflow? As the title already say, how does the antenna on the Boeing 737 Max fuselage not disrupt the airflow?

(Boeing) 737 MAX antenna.

Anything new attached to the outside of the airplane will change the airflow in someway; that is unavoidable.

The only question is if it will disrupt the airflow in a way that has a significant negative impact on the plane.

You'll recognize that the antenna is installed very close to the CG, so the new air resistance has virtually no pitching moment.

It is far from any control surface, so those all work the same.

The increased air resistance affects fuel economy a very tiny amount, but paying customers have enough preference for inflight wifi that its worth it.


Is there a list of airports with Category 3 ILS systems? Is there a list of airports with category 3 Instrument Landing Systems?

According to my current navigation database (AIRAC 1912), the following 228 airports have a category III ILS approach to at least one of their runways:

CYVR (VANCOUVER INTL) on runways: 08L, 08R, 26L, 26R
CYYC (YYC CALGARY INTL) on runways: 17L, 35R
CYYT (ST JOHNS INTL) on runways: 11, 29
CYYZ (LESTER B PEARSON INT) on runways: 05, 06L
EBBR (BRUSSELS NATIONAL) on runways: 25L, 25R
EBCI (BRUSSELS SOUTH) on runway: 24
EBLG (LIEGE) on runways: 04R, 22L
EDDB (SCHOENEFELD) on ALL runways: 07L, 25R
EDDC (DRESDEN) on runway: 22
EDDE (ERFURT-WEIMAR) on runway: 28
EDDF (FRANKFURT/MAIN) on runways: 07C, 07L, 07R, 25C, 25L, 25R
EDDG (MUENSTER/OSNABRUECK) on runway: 25
EDDH (HAMBURG) on runway: 23
EDDK (COLOGNE-BONN) on runways: 14L, 32R
EDDL (DUESSELDORF) on runways: 05R, 23L, 23R
EDDM (MUNICH) on ALL runways: 08L, 08R, 26L, 26R
EDDN (NUERNBERG) on runway: 28
EDDP (LEIPZIG-HALLE) on ALL runways: 08L, 08R, 26L, 26R
EDDS (STUTTGART) on ALL runways: 07, 25
EDDT (TEGEL) on runways: 08L, 26R
EDDV (HANNOVER) on runways: 09L, 27R
EDDW (BREMEN) on runways: 09, 27
EDFH (FRANKFURT-HAHN) on runway: 21
EDLV (NIEDERRHEIN) on runway: 27
EDNY (FRIEDRICHSHAFEN) on runway: 24
EDSB (KARLSRUHE/BADEN-BADE) on runway: 21
EDVK (KASSEL-CALDEN) on runway: 27
EFHK (VANTAA) on runway: 22R
EGAA (ALDERGROVE) on runway: 25
EGBB (BIRMINGHAM) on ALL runways: 15, 33
EGCC (MANCHESTER) on runways: 05L, 23R
EGCN (DONCASTER SHEFFIELD) on runway: 20
EGGD (BRISTOL) on runway: 27
EGGP (LIVERPOOL) on runway: 27
EGGW (LUTON) on ALL runways: 08, 26
EGHH (BOURNEMOUTH) on runway: 26
EGHQ (NEWQUAY) on runway: 30
EGKK (GATWICK) on runways: 08R, 26L
EGLL (HEATHROW) on ALL runways: 09L, 09R, 27L, 27R
EGNM (LEEDS BRADFORD) on runway: 32
EGNT (NEWCASTLE) on ALL runways: 07, 25
EGNX (EAST MIDLANDS) on runway: 27
EGPF (GLASGOW) on ALL runways: 05, 23
EGPH (EDINBURGH) on ALL runways: 06, 24
EGSS (STANSTED) on ALL runways: 04, 22
EHAM (SCHIPHOL) on runways: 06, 18C, 18R, 27, 36C, 36R
EHBK (MAASTRICHT-AACHEN) on runway: 21
EIDW (DUBLIN INTL) on runways: 10, 28
EKBI (BILLUND) on ALL runways: 09, 27
EKCH (KASTRUP) on runway: 22L
EKYT (AALBORG) on runway: 26R
ELLX (LUXEMBOURG) on runway: 24
ENGM (GARDERMOEN) on ALL runways: 01L, 01R, 19L, 19R
EPWA (CHOPIN) on runway: 33
ESGG (LANDVETTER) on ALL runways: 03, 21
ESSA (ARLANDA) on runways: 01L, 01R, 19L
ETAR (RAMSTEIN AB) on runway: 26
GMMN (MOHAMMED V INTL) on runways: 35L, 35R
KADW (JOINT BASE ANDREWS) on runways: 01L, 19R
KAFW (FT WORTH ALLIANCE) on runway: 16L
KATL (HARTSFIELD - JACKSON) on runways: 08L, 09R, 10
KAUS (AUSTIN-BERGSTROM INT) on runway: 17L
KBDL (BRADLEY INTL) on runway: 06
KBGR (BANGOR INTL) on runway: 15
KBNA (NASHVILLE INTL) on runways: 02L, 02R
KBOI (BOISE AIR TERMINAL/G) on runway: 10R
KBOS (LOGAN INTL) on runways: 04R, 33L
KBWI (WASHINGTON INTL/MARS) on runway: 10
KCAE (COLUMBIA METRO) on runway: 11
KCLE (CLEVELAND-HOPKINS IN) on runways: 06L, 24R
KCLT (CHARLOTTE/DOUGLAS IN) on runways: 18R, 36C, 36L, 36R
KCVG (CINCINNATI/NORTHERN ) on runways: 36C, 36R
KDAY (COX-DAYTON INTL) on runway: 06L
KDEN (DENVER INTL) on runways: 34L, 34R, 35L, 35R
KDFW (DALLAS-FT WORTH INTL) on runways: 17C, 17L, 18R, 35C, 35R
KDSM (DES MOINES INTL) on runway: 31
KDTW (DETROIT METRO WAYNE ) on runways: 03R, 04L, 04R
KEUG (MAHLON SWEET) on runway: 16R
KEWR (NEWARK LIBERTY INTL) on runways: 04R, 22L
KFAT (FRESNO YOSEMITE INTL) on runway: 29R
KGEG (SPOKANE INTL) on runways: 03, 21
KGSO (PIEDMONT TRIAD INTL) on runway: 05L
KGSP (GREENVILLE SPARTANBU) on runway: 04
KGTF (GREAT FALLS INTL) on runway: 03
KHOU (HOBBY) on runway: 04
KIAD (WASHINGTON DULLES IN) on runways: 01L, 01R, 19C, 19R
KIAH (GEORGE BUSH INTERCON) on runways: 08L, 26L, 26R, 27
KILN (WILMINGTON AIR PARK) on runway: 22R
KIND (INDIANAPOLIS INTL) on runways: 05L, 05R
KJAN (EVERS INTL) on runway: 16L
KJAX (JACKSONVILLE INTL) on runway: 08
KJFK (KENNEDY INTL) on runways: 04R, 22L
KLAX (LOS ANGELES INTL) on runways: 24R, 25L
KLIT (CLINTON NATL/ADAMS) on runway: 22R
KMCI (KANSAS CITY INTL) on runways: 01R, 19R
KMCO (ORLANDO INTL) on runways: 35L, 36R
KMDT (HARRISBURG INTL) on runway: 13
KMEM (MEMPHIS INTL) on runways: 36C, 36L, 36R
KMHT (MANCHESTER) on runway: 35
KMKE (GEN MITCHELL INTL) on runway: 01L
KMSN (DANE CO REGL-TRUAX) on runway: 36
KMSP (MINNEAPOLIS-ST PAUL ) on runways: 12L, 12R, 35
KMSY (ARMSTRONG NEW ORLEAN) on runway: 11
KOAK (METRO OAKLAND INTL) on runway: 30
KOMA (EPPLEY) on runways: 14R, 32R
KONT (ONTARIO INTL) on runway: 26L
KORD (CHICAGO OHARE INTL) on runways: 09L, 10C, 10L, 10R, 27L, 27R, 28C, 28L, 28R
KORH (WORCESTER REGL) on runway: 11
KPDX (PORTLAND INTL) on runway: 10R
KPHL (PHILADELPHIA INTL) on runway: 09R
KPIT (PITTSBURGH INTL) on runways: 10L, 10R
KPVD (GREEN STATE) on runway: 05
KPWM (PORTLAND INTL) on runway: 11
KRDU (RALEIGH-DURHAM INTL) on runway: 23R
KRFD (CHICAGO/ROCKFORD INT) on runway: 07
KRIC (RICHMOND INTL) on runway: 34
KSDF (LOUISVILLE MUHAMMAD ) on runways: 35L, 35R
KSEA (SEATTLE-TACOMA INTL) on runways: 16C, 16L, 16R
KSFO (SAN FRANCISCO INTL) on runway: 28R
KSLC (SALT LAKE CITY INTL) on runways: 16L, 16R, 34L, 34R
KSMF (SACRAMENTO INTL) on runway: 16R
KSTL (ST LOUIS LAMBERT INT) on runways: 11, 12L, 30R
KSWF (NEW YORK STEWART INT) on runway: 09
KTPA (TAMPA INTL) on runway: 01L
LBSF (SOFIA) on runway: 27
LDZA (FRANJO TUDMAN) on runway: 05
LEAS (ASTURIAS) on runway: 29
LEBL (JOSEP TARRADELLAS/EL) on runways: 07L, 07R, 25L, 25R
LEGE (GIRONA) on runway: 20
LEMD (ADOLFO SUAREZ MADRID) on runways: 18L, 18R, 32L, 32R
LEPA (PALMA DE MALLORCA) on runway: 24L
LEVD (VILLANUBLA) on runway: 23
LEVX (VIGO) on runway: 19
LEZG (ZARAGOZA) on runway: 30R
LFBD (MERIGNAC) on runway: 23
LFBL (BELLEGARDE) on runway: 21
LFBO (BLAGNAC) on runway: 14R
LFBP (PAU/PYRENEES) on runway: 31
LFJL (METZ-NANCY/LORRAINE) on runway: 22
LFLC (CLERMONT-FERRAND/AUV) on runway: 26
LFLL (SAINT EXUPERY) on runways: 35L, 35R
LFML (MARSEILLE/PROVENCE) on runway: 13L
LFOB (TILLE) on runway: 12
LFOK (VATRY) on runway: 10
LFPG (CHARLES-DE-GAULLE) on ALL runways: 08L, 08R, 09L, 09R, 26L, 26R, 27L, 27R
LFPO (ORLY) on runways: 06, 24, 25
LFQQ (LESQUIN) on runway: 26
LFRB (BRETAGNE) on runway: 25L
LFRS (NANTES/ATLANTIQUE) on runway: 03
LFSB (BASLE-MULHOUSE) on runway: 15
LFST (ENTZHEIM) on runway: 23
LHBP (LISZT FERENC INTL) on runways: 13L, 13R, 31R
LIMC (MALPENSA) on runways: 35L, 35R
LIME (ORIO AL SERIO) on runway: 28
LIMF (CASELLE) on runway: 36
LIML (LINATE) on runway: 36
LIPE (BORGO PANIGALE) on runway: 12
LIPO (MONTICHIARI) on runway: 32
LIPX (VILLAFRANCA) on runway: 04
LIPZ (TESSERA) on runway: 04R
LIRF (FIUMICINO) on runway: 16R
LJLJ (BRNIK) on runway: 30
LKMT (MOSNOV) on runway: 22
LKPR (RUZYNE) on runway: 24
LOWG (GRAZ) on runway: 35C
LOWK (KLAGENFURT) on runway: 28R
LOWL (LINZ) on runway: 26
LOWS (SALZBURG) on runway: 15
LOWW (SCHWECHAT) on ALL runways: 11, 16, 29, 34
LPPT (LISBON) on runways: 03, 21
LROP (HENRI COANDA) on runways: 08L, 08R
LSGG (GENEVA) on runway: 22
LSZH (ZURICH) on runways: 14, 16
LTAC (ESENBOGA INTL) on runway: 03R
LTBA (ATATURK INTL) on runway: 05
LTFM (ISTANBUL) on runways: 16R, 17L, 34L, 35R
LYBE (NIKOLA TESLA) on runway: 12
LZIB (M.R. STEFANIK) on runway: 31
MMTO (LIC ADOLFO LOPEZ MAT) on runway: 15
NZAA (AUCKLAND INTL) on runway: 23L
OMAA (ABU DHABI INTL) on runways: 13L, 31R
OMDB (DUBAI INTL) on ALL runways: 12L, 12R, 30L, 30R
OMDW (AL MAKTOUM INTL) on runways: 12, 30
OPLA (ALLAMA IQBAL INTL) on runway: 36R
OTBD (DOHA INTL) on runway: 33
OTHH (HAMAD INTL) on ALL runways: 16L, 16R, 34L, 34R
PAFA (FAIRBANKS INTL) on runway: 02L
PANC (STEVENS ANCHORAGE IN) on runway: 07R
RJAA (NARITA INTL) on runway: 16R
RJCC (NEW CHITOSE) on runway: 19R
RJCK (KUSHIRO) on runway: 17
RJFT (KUMAMOTO) on runway: 07
RJGG (CHUBU CENTRAIR INTL) on runway: 36
RJOA (HIROSHIMA) on runway: 10
RJSA (AOMORI) on runway: 24
RJTT (TOKYO HANEDA INTL) on runway: 34R
RKSI (INCHEON INTL) on ALL runways: 15L, 15R, 16, 33L, 33R, 34
RKSS (GIMPO INTL) on runway: 14R
SAEZ (EZEIZA INTL MINISTRO) on runway: 11
SBGR (GUARULHOS-GOV ANDRE ) on runway: 09R
SCEL (ARTURO MERINO BENITE) on runway: 17L
SCIE (CARRIEL SUR INTL) on runway: 02
SCQP (LA ARAUCANIA) on runway: 01
SKBO (EL DORADO INTL) on runway: 13R
SPJC (JORGE CHAVEZ INTL) on runway: 15
UAAA (ALMATY) on runways: 23L, 23R
UACC (NURSULTAN NAZARBAYEV) on ALL runways: 04, 22
UBBB (HEYDAR ALIYEV INTL) on ALL runways: 16, 17, 34, 35
UKBB (BORYSPIL INTL) on runway: 36R
ULLI (PULKOVO) on runways: 10L, 28R
UMMS (MINSK-2) on runway: 13L
URWW (GUMRAK) on runway: 06
UTAA (ASHGABAT) on runways: 12R, 30L
UUDD (DOMODEDOVO) on runway: 14R
UUEE (SHEREMETYEVO) on runways: 06L, 06R, 24C, 24R
UWWW (KURUMOCH) on runway: 15
VECC (NETAJI SUBASH CHANDR) on runway: 01R
VHHH (HONG KONG INTL) on runway: 25R
VIAR (SRI GURU RAM DASS JE) on runway: 34
VIDP (INDIRA GANDHI INTL) on runways: 11, 28, 29
VIJP (JAIPUR) on runway: 27
VILK (CHAUDHARY CHARAN SIN) on runway: 27
YMML (MELBOURNE INTL) on runway: 16
YPPH (PERTH INTL) on runway: 21
ZBAA (BEIJING CAPITAL) on runway: 36R
ZBAD (DAXING) on runways: 01L, 35L
ZHHH (TIANHE) on runway: 04R
ZSPD (PUDONG) on runway: 34L

Trivia: only about 19% of all airports in the database have some ILS at all and only about 9% of them have a Cat. III ILS:

And this is the geographic distribution:

Map generated by the Great Circle Mapper


Is trimming for constant speed equal to trimming for constant angle of attack? My understanding of trim:
When you say you have trimmed an aircraft for a constant speed, say 100 mph, you are actually trimming the horizontal stabilizer so that there are no forces on stick (or tail down force is balanced with nose weight) at that speed. To do that you either control the horizontal stabilizer/trim tabs until no force is felt on the stick.

My question is, (if my understanding is right), can I say that we are trimming for constant angle of attack instead of speed (and the constant angle of attack is represented in form of speed). By trimming the aircraft, aren't we making the aircraft fly  at constant angle of attack?

If so, can I make these following statements about trim: (ignoring the things like propeller blast etc..,)

When you are cruising in a trimmed condition and decrease( or increase) the engine power, the aircraft will pitch down (or up) until it finds the old angle of attack where all the forces are balanced and the trim is reestablished.
When you are cruising and closed (or opened) the throttle completely, the aircraft will descent (or climb) at the same trimmed angle of attack.

Short answer: Yes

Generally you are right. You are trimming the aircraft for a point on its polar, and that point is reached at a specific angle of attack.

You say yourself we should neglect secondary effects like the propeller blast, and under this condition you are right. Trimming means to set the distribution of lift between wing and tail surface, and this directly sets the angle of attack.

Now the more elaborate part of the answer

You trim the aircraft for a specific dynamic pressure, that is the product of air density and flight speed. If you slightly change your question from "trimming for constant speed" to "trimming for constant indicated speed", you gain precision and the answer is still yes. With a change in altitude you will also affect the Reynolds number the aircraft flies at, and the same polar point is reached at a slightly higher angle of attack the lower the Reynolds number gets when the aircraft climbs into colder air.

In turns the lift demand is increased. Not changing trim during a turn will let the aircraft speed up to increase lift at an unchanged angle of attack. That is why you pull more when turning - you want to increase the angle of attack, so speed can stay constant. When speeding up, you again change the Reynolds number and indirectly also the angle of attack, but this effect is very small.

When you change the power setting, you add energy to the aircraft at a different rate. Thrust becomes bigger and is counteracted by the sum of drag and the component of weight which acts in parallel to the thrust vector. Now the vertical component of weight is reduced, and so is your need for lift - but ever so slightly. This effect by itself means you will fly at a slightly smaller speed which will also very slightly affect the angle of attack. But since you did not change trim, the distribution of lift between wing and tail is unchanged, so the angle of attack is the same, save for a minuscule Reynolds number effect.

Increasing thrust will also increase the prop blast over the tail surfaces, making them more effective and slightly changing the balance of the aircraft. Depending on the direction of tail lift, the result is an increase or a decrease in angle of attack, but this is precisely the effect you wanted us to disregard.

Another secondary effect is a constant loss of weight as the engine burns fuel. This makes the aircraft lighter, so it requires less lift and a smaller angle of attack over time to remain at the same speed. Since you trimmed a specific polar point, the aircraft would need to slow down in order to stay at this polar point at its lower weight. But the lower weight also creates less drag, so instead of slowing down the aircraft will climb. Now the reduced density will require more angle of attack and will reduce engine output so in the end the same angle of attack is maintained at a higher altitude but a slightly lower indicated airspeed.


Is English sufficient for international aviation? Is it possible to fly internationally understanding only English for radio communications? Are there exceptions?

ATC and pilots must be able to communicate in English on international flights involving ICAO member states. There are 191 countries that are members of the ICAO, covering almost all of the world.

While communications may be performed in the local language, English must be used upon request on international flights. Controllers and flight crew engaged in communications for international flights should be proficient in spoken English in addition to English aviation phraseology.

Some domestic airways and airports may require communications in the local language instead of English.

So English is sufficient if you stick to international airports and airways, but be sure to check the language requirements before you venture into foreign domestic airspace and airports.


Is the passenger capacity of the A350 really only 250 pax? I read online that the capacity of an A350 was 250 passengers, but I took the max cabin width and length of an A350-900XWB, and I ended up with a 400 passenger capacity in a 4-class layout.

Economy was a 3-3-3 configuration, with 2.48 feet seat pitch
Premium was a 2-4-2 config. with a 3 feet seat pitch
Business was a 2-2-2 config. with a full 6 feet seat pitch
First class had a wide 1-2-1 configuration with a full lie flat bed to make for a 7 feet seat pitch

Yet, I still got a capacity way higher than 250 of 234 economy seats, 54 premium seats, 42 buisness seats, and 16 first class seats, and the total for 346 passengers + 8 crew! Yes, I included washrooms for all classes, emergency exits, and the galleys. What does the capacity of 250 really mean?

Airbus themselves give max passenger seating 440, and typical seating 3-class 300-350, for the A350-900. https://www.airbus.com/aircraft/passenger-aircraft/a350xwb-family/a350-900.html

The undeveloped A350-800 is listed with 276 passengers. Maybe that's where you got your 250 number.


Are there any air navigation uses for the HF frequencies? I'm busy revising for a license conversion... Out of curiosity, does anyone know of a radio navigational aid that used the frequencies we call HF (2-25 MHz)?

Current airborne radionavigation systems use MF (NDB), VHF (VOR/LOC, marker beacons), UHF (G/S), and L-band microwave  (GPS/GNSS, DME) frequencies.  (MLS operates in the C-band, and airborne radar in the X-band, but those are niche applications.)

Historically, the only system that used HF was the UK Gee system.  LORAN-A had options for the use of HF frequencies, but these were never used, as MF operation just below the 2 MHz mark gave much more consistent results in both reception range and performance, and LORAN-C migrated into the LF band at 180kHz, partially to avoid the difficulties of being co-located with the amateur radio 160m band.  Decca also used LF frequencies, and the Omega system operated at VLF to provide (imprecise) global coverage.

Furthermore, the FCC frequency table shows no allocations for the Radionavigation service anywhere from 2 to 30MHz, in any ITU region.


What is the difference between published and unpublished waypoints? While reading some articles about free route airspaces I found reference to two types of waypoints:

  Free route airspace (FRA) is a specified airspace within which users
  can freely plan a route between a defined entry point and a defined
  exit point, with the possibility of routeing via intermediate
  (published or unpublished) waypoints ...

My question is the following: What is the difference between published and unpublished waypoints? How are they created? When are they used?

Could you please provide me documentation/citations as well in order to have a complete overview of the topic?

Published waypoints are waypoints that appear on the charts and in the published navigation databases used by GPS navigators or FMSes.  They are referenced using the published identifier for the waypoint. Examples: TPA, ITAWT.

Unpublished waypoints are waypoints created by the pilot.  FMS allow for the creation/entry of a waypoint by the pilot.  The waypoint is defined by one of the following:

Latitude/Longitude
Place/Bearing/Distance
Place-Bearing/Place-Bearing

The pilot-defined waypoint can then be added to the flight plan.

Here's a snip of how a waypoint is defined in an A320:

As to when you would use it, the short answer is when you need to or want to.  A common example would be to deviate a short distance off an airway (maybe 5 or 10 nm) to avoid a temporary flight restriction.  You would just want to avoid the restricted airspace and using a published waypoint or waypoints would take you farther off your route than necessary.


How do linear actuators for flight controls work in fighter jets? I've built some rc planes and know that in order to control an aileron in a rc plane, that you need a servo, a control horn(sticking out of the aileron), and a pushrod. However, I was wondering how fighter jets control their ailerons. I was researching and I found that it control is via linear actuators. I was wondering how exactly it worked and tried to look for a video or an animation. However, I couldn't find anything that explains exactly how a linear actuator moves an aileron. Can you please help me understand this with a video or something like this. Thanks so much.
This is a picture of an f22 raptor someone sent me when I asked this question on Flite Test...they didnt respond after this this. You can see orange actuators in the picture but it doesn't really help me besides that.                     https://conceptbunny.com/lockheed-martin-f-22-raptor/

The working principle is surprisingly similar between the RC as you've described and most actuator powered aircraft. The linear actuator extends/retracts a pushrod, which is connected to a horn attached to the pivot point on the control surface.
Below is the zoomed-in picture from the OP. I've taken the liberty of highlighting the two rudder actuators and the left stabilator actuator. The one attached to the stabilator is the most revealing: the pushrod essentially connects to a horn, which translates the linear motion to a rotational one.

Ref: https://conceptbunny.com/lockheed-martin-f-22-raptor/
Due to aerodynamic and stealth reasons, this mechanism is hidden under a smooth fairing for the F-22. But for other airplanes, you can see small blisters under their control surfaces that fair the actuator. Below is a picture of the Boeing 787 aileron. The blisters have been highlighted.

Ref: https://commons.wikimedia.org/wiki/File:Low-speed_aileron_of_Boeing_787.jpg
The linear actuator itself is almost always energized via hydraulic fluid, whether it comes from a central hydraulic system or an self-contained hydraulic systems (EHA). The following diagram illustrates the working principle:

Ref: https://www.machinedesign.com/mechanical-motion-systems/linear-motion/article/21832047/whats-the-difference-between-pneumatic-hydraulic-and-electrical-actuators


Can a change in altimeter setting make you change the airspace? If you are at 17999 MSL VFR, you twist in 29.92 into the altimeter, and it raises your altitude to above 18000, did you bust the class A?

Then theoretically, could you be in class A airspace with a local altimeter setting if there is a very low pressure?

No, fiddling with your altimeter setting doesn't change the airspace you are flying in. So you can't bust the class A above 18 000 ft by changing the setting. The top and bottom boundaries of the airspace are defined by applying the altimeter setting local to that airspace.

Conversely, and perhaps more important, you can't weasel out of a busted airspace by quickly changing your altimeter setting.

Your transponder sends your altitude as if your altimeter was set to 1013.25 hPA / 29.92 inHg, regardless of what your actual setting is. The ATC system will do the conversion according to their setting, so you can't fool them.

Note that the altitude resolution of your transponder is either 100 feet (Mode C) or 25 feet (Mode S with appropriate altimeter encoder), so 1 feet difference doesn't really matter.


Why is the flight duration of the same flight different in different months? For example:

CX880 (HKG TO LAX) takes 13h40m on the 2nd of August, but only takes 12h50m in September.
CI008 (TPE TO LAX) is the same; it takes half an hour less in September than in August.

What are the factors that cause this difference?

Jet stream is a stream of air flowing around the Earth from West to East at high altitude. With wind speed averaging more than 100+ km/h, it has a tremendous effect on a plane's ground speed.

The pattern, strength and location of the stream is constantly changing. For example, during the (Northern Hemisphere) winter the stream generally strengthens, and during the summer it generally weakens. With modern technology such as weather satellites and data fusion from thousands of weather stations around the world, the stream is fairly predictable.

The effect is more pronounced on long flights because these flights stay in high altitude much longer.


What is this white rectangle painted on runway 23 at KCRW? (Related to this question.)

Runway 23 at Charleston, WV (KCRW) appears to have a (roughly) 500' x 50' white rectangle painted on the touchdown zone (I've added red arrows pointing at each end of the box):

Source: Google Maps

What is it used for?

Seems to be touchdown zone markings for assault landing practice by C-17's. For assault landings, the standard is to touch down in the first 500' of the Landing Zone (LZ), and to go around if your mains aren't on the ground by the 500' point. On a dirt LZ, there will typically be panels set out to mark the threshold, the 500' point, and the far threshold -- that way the combat control team which is marking the LZ doesn't have to put out lots of panels, but there are enough for the crew to get what they need. The 500' markings on the main runway would give enough information to the crew to simulate landing on an LZ, even though the actual runway is far longer & wider.

This frame below shows examples of C-130's practicing assault landings, and you can see orange panels at the threshold and 500' points in them:

Source: YouTube

Some places, KLRF (Little Rock Air Force Base) for example, had an actual 3500' long paved strip for crews to use for assault landing practice, with permanent markings at the threshold and 500' points (2013 imagery shown below):

Source: Google Earth

Most airports don't have the budget to create a separate runway for this specialized requirement, so some sort of markings on the main runway are used instead.


On a larger airplane, how do they manage balance? I have flown a bunch of times on a Bombardier Dash-8 Q400 on a couple of different airlines, and several times I have flown a shuttle flight that is basically empty.

I have been instructed to change seats on these flights, reportadly to balance the plane better.

It appears as though the request comes from the cockpit relayed back to the cabin staff.

The plane appears to be static in place, so how do they determine that they need to balance the plane?  Does the landing gear report this sort of info?

A Dash-8 is a reasonably 'large' airplane that it seems sort of surprising that the balance is so delicate that the weight of a few people can make the difference.

Before the main cabin door is closed, the ramp personnel will deliver a cargo sheet that lists

number of checked bags
number of gate checked bags
number of heavy checked bags
pounds of commercial cargo
pounds of company cargo

With numbers broken down by cargo bin where accurate.

The flight attendant will deliver a sheet with passenger count in zones.

The FMS or EICAS screens can tell you how much fuel is on board.

From there, some airline have ACARS programs that you plug those numbers into and it will give you weight information (to turn #bags and #people into pounds). Other have worksheets to calculate the weights.  In the EMB-145 we then had to spin a special wheel with the weight information to get the balance info.  This can vary by operator.

Some operators will have the W&B calculated by dispatch and all of the numbers I mention above are put into the computer system by various people and the dispatcher will deliver the W&B and takeoff reference data to the pilots via ACARS.

What if there is a problem?

Problems can be overweight overall, overweight in the cabin, overweight in the bag compartments or out of balance.

For overweight, remove people or bags as necessary.  If fuel has not arrived and you can do without some, call dispatch to revise the numbers and load less fuel.

If you are out of balance, you move people or bags around.  This was rarely an issue and if it does happen you just ask a passenger from zone X to sit in zone Y for takeoff.


What are some examples of holding? What are some examples of holding instructions and how to enter them? I understand entry with direct, teardrop, and parallel. Just having a hard time coming up with formulating the procedure. Example; C172 hold south of ABC VORTAC. Does that mean race track is the 360 radial and holding south of the vortac?

Remember, radials come from a fix.  You cannot hold south of a VOR on the 360 radial in the same way that you could not hold east on a 270 radial using the VOR as the holding fix.
You could hold south on a 360 radial, but the fix would have to be somewhere away from the VOR.  For example, if you're told to hold south of the 10DME on the 360 radial, that could be done.
What that would mean is that your fix, for the purposes of the inbound course, would be the 360 radial of the VOR at the 10 DME distance.  So your racetrack pattern would be south of the point 10 miles north of the VOR (per the compass rose's north).
Examples:


What are the checklist items for a runaway stab trim on a B737 MAX 8 and MAX 9? This is a follow up question to this question.

This question is not directly related to the MCAS system in the new B737 MAX aircraft, instead, my question is about the checklist for a runaway stab trim. What are the items on the Boeing B737 Max checklist to deal with a runaway stab trim? (Better yet, a picture of the actual checklist would be awesome!).

Here is a screenshot of a B737 Max 8 Runaway Stabilizer Checklist.  There might be some small variations in the checklist when comparing each individual airline.

Additionally, here is the Boeing "Uncommanded Nose Down Stabilizer Trim" bulletin that was issued to all B737 Max 8 operators after the Lion Air crash.


How can I correctly calculate fuel for a Cessna 152? I'm learning to fly in a C152 and I've found the fuel calculation to be a little ambiguous. Based on information from my instructor, Internet and the Cessna 152 Information Manual (1980), I've home cooked the following formula and wanted to run it by the community for some verification.

I hope this is an appropriate use of this forum and please excuse my newb-ness. Here's my formula:

Fuel Required =
    5.6L unusable +
    5L run-up and taxi +
    24L/hour flight (rich) +
    18L (45 minute) reserve

Based on this formula, a one hour lesson conducted in the training area and below 3,000 feet AMSL requires a minimum of 52.6 litres in the tanks, calculated as follows:

5.6L unusable + 5L run-up & taxi + 24L flight + 18L reserve = 52.6L total

Or 26.3L in each tank. Since we're measuring fuel by the surprising method of dipping a stick in the tank, I'm realistically looking for around 25L per tank, and maybe topping it up to 30 per tank to be on the safe side.

Is this correct and reasonable?

The short answer is: Yes, your calculation is reasonable.

Fuel consumption values are not an exact science, they are average values and rounded in many cases or based on pilot experience. Using 24L/hr in your example for traffic patterns is the right assumption, as you will most likely remain in rich configuration. Even if you lean a bit within the pattern, you would not gain enough fuel efficiency to offset these 24L/hr.

Your 5L run-up and taxi obviously depends on the airport you are based at or use the formula at. If this value was your calculation for a smaller airfield, you might need to adapt your formula when flying to larger regional airports or airports with high-traffic, where your taxi time could be significantly longer.

Be reasonable however with topping up your values. Going from 52.6L to 60L is the equivalent of 5.3KG, which you will need to consider in your weight and balance calculation.


What are the US Customs requirements upon returning to the US after pre-clearing customs in the US Virgin Islands? I have heard conflicting information regarding pre-clearing customs in the USVI.  On one hand, I have heard that once you clear customs you are done and don't need to do anything special on arrival.  But the I've also heard that you need to land at an Airport of Entry upon arrival and notify local customs of your arrival.  I have even called Customs and Border Protection and gotten conflicting answers!  What is actually required?

You're required to notify and clear customs if you land in the 50 U.S. states or Puerto Rico from the U.S. Virgin Islands, even though the U.S. Virgin Islands is part of the U.S.

"All private aircraft arriving in the United States via the Gulf of Mexico or the Atlantic
Coast from a foreign location in the Western Hemisphere south of 30 degrees north
latitude, or from any place in Mexico or from the U.S. Virgin Islands, shall also furnish a
notice of intended arrival. Such aircraft must then land for inspection at the nearest
designated airport unless an overflight exemption has been granted under Section
122.25 of the Customs Regulations."


What is an augmentor wing? What is an augmentor wing in the context of STOL aircrafts? and is it still used today? This seems to be a term that I can only find in NASA aerodynamics (specifically powered-lift) research papers in the 70s and 80s. Example reference paper: https://apps.dtic.mil/dtic/tr/fulltext/u2/a177750.pdf

In a blown wing, such as blown flaps, air is driven by the engines through ducts in the wing and out through various orifices to boost the airflow over the wing. This helps increase lift, especially to give good STOL performance at low forward speeds.
The augmentor wing takes the sophistication a stage further by arranging the air outlets so that the flow draws in or entrains ambient air into the wing flow, further increasing or augmenting mass flow and the efficiency of the system.
A typical example was the DHC-5 modified with an augmentor wing; a little bit about it is given on Wikipedia. The Rockwell XFV-12 was a supersonic VTOL fighter prototype, which blew the air directly downwards through vertical slots in the wing, however it did not entrain as much additional air as expected and lift augmentation levels were disappointing.
The system has never reached operational use, mainly because the weight, complexity and cost have never been justified by the performance gains.


What is the behaviour of ILS when there is an unmodulated interfering carrier signal? Let us assume we have a localizer (identifier I-STK) transmitting at 110.10MHz and a matching glideslope at 334.40MHz.  If a device aboard an aircraft tracking the I-STK ILS were to emit an unmodulated interfering carrier on either of those frequencies that is then picked up by the ILS receiver's antennae, with enough power to render the LOC or G/S signal unreceivable, would the pilots see a consistent wrong indication, erratic/wildly changing indications, or a FAIL flag?  Would the behavior under this circumstance differ between glass and round-dial aircraft cockpits?  Also, would the ILS Morse identifier be rendered unreceivable before or after the cockpit indications fail?

If an invalid signal is received, then it must be modulated since it is only by measuring the modulation, that the receiver knows if the signal is valid and where it is relative to the localiser and glideslope. An unmodulated carrier on the same frequency is additive.

An unmodulated carrier on a different frequency will not affect the receiver (ignoring possible harmonics of course).

I suppose that if you could increase the power enough, an unmodulated signal might cause the signal to noise ratio to drop so low that the failure flag would show.


Do IFR flight plans ever get rejected for operational reasons? I'm not talking about errors or omissions. Does a FP ever get rejected for example if there are too many flights scheduled into an airport or it conflicts with other flights or your FP as filed would take you through an active TFR area? How much scrutiny does a FP get when it's filed?

From this question I see that VFR plans get pretty much just filed away in case you need S&R. Does someone at ATC go through and verify that an IFR plan doesn't have conflicts?

Generally speaking, the only reasons for which a flight plan would be actually rejected would fall under what you referred to as "errors or omissions". But, the operational reasons you cite might lead to an amendment to your flight plan (which the pilot would most likely receive as an amended clearance).

IFR flight plans filed electronically are rejected by the flight plan processing computer if the filer files an impossible flight plan. I would presume that a flight plan filed verbally or in person through ATC or FSS would also be rejected for the same reasons, probably by the same computer, but with the rejection relayed to you through the third party. See these rejection examples that AOPA lays out in this article:

  If you plan a "direct" flight, point-to-point, follow the rules and
  enter the correct equipment. For example, if "/G" (or "slash Golf,"
  meaning that your airplane is equipped with an IFR-approved GPS and a
  Mode C transponder) or "/I" (you have loran or VOR/DME and a Mode C
  transponder) are not entered for these kinds of flights, the air
  traffic control computer will reject your flight plan because that
  route can't be flown with less capable equipment.

and,

  File fixes that are actually on
  the airways that you file in your IFR flight plan. Pilot error causes
  flight plans to be rejected when the filed points are not on the
  routes that are filed.

Depending on how you file, other possible rejection reasons might include formatting errors, a non-existent clearance limit, or might depend on the specific filing means; I am certainly not familiar with all means of filing. I have filed in person with a sheet of paper when standing in the office at the bottom of a control tower, over the phone, and online, and over the aircraft radio; I've never had an IFR flight plan actually rejected.

Beyond such reasons, a flight plan will typically not be rejected as such, but may well be amended. The reasons you propose—traffic density, route issues, etc—are usually negotiable, or are circumvented by amendments to your clearance. Certainly, these issues may present challenges, but they can usually be worked around.

These flight plans do receive scrutiny; IFR clearances are all about traffic separation, and this is ATC's primary task. Understand that to file an IFR flight plan is, essentially, to apply for a specific routing for specific time frame; you may or may not be given your requested routing when you receive your actual clearance. Receiving a clearance different from the route filed, or receiving a clearance amendment while en-route are both common. Note that an amendment need not be ATC initiated: the pilot can request an amendment, either before or during a flight.

One final note on TFRs: you can sometimes (often?) fly through these, even under VFR; you just have to get approval. For example, I've flown through the Grand Forks TFR while flying VFR with ATC approval.


How does a pilot know if he has headwind, tailwind or crosswind? While in cruise, without GPS how does a pilot know the wind direction? The airspeed sensors only register forward speed. Is it a matter of reaching waypoints sooner or later? And if flying on a compass heading how do they know what their ground track is and whether or not they're being blown off course? Especially with no visual reference.

With no visual or instrument references you don't know if you have a headwind, tailwind, or crosswind.
You also don't particularly care because you're flying a plane with no way to navigate and therefore have more imperative concerns.

With visual references it's something you can determine with a little thinking and some intentional oversimplification:

Headwinds mean your ground speed is lower than your airspeed.
If your airspeed is 100 miles per hour and you're flying a 25 mile leg it should take you 15 minutes. If it takes you 18 minutes you know you've got a headwind.
Tailwnds mean your gound speed is higher than your airspeed.
If you're flying the same 25 mile leg at 100 miles per hour and it only takes you 12 minutes you have a tailwind.
Crosswinds are a bit more difficult: The easiest way to tell you have a crosswind is if you know what your magnetic heading should be in order to fly toward a given point and you have to point the nose at a different compass heading in order to maintain the desired ground track you have a crosswind.

Note that you almost never have a direct headwind, tailwind, or crosswind, but rather some wind at an angle to your direction of travel which will have both a crosswind component and a headwind/tailwind component. The wind speed and direction can be computed using an E6B flight computer with relatively little effort.

If you have instrument references the problem gets easier:

GPS is the ultimate solution -- it can display your ground speed and ground track, from which you can derive the winds. GPS systems that know your airspeed can even calculate the winds for you so you don't need the E6B anymore (unless the GPS breaks).
VORs provide a magnetic heading reference, which simplifies the crosswind calculation (and largely eliminates the need to do one to figure out your crosswind correction angle: Just keep the needle centered and you have your desired crosswind correction).
It doesn't give you the wind speed and direction without some additional data (how long it takes you to cross some waypoint or fix), because you still need that information in order to determine how much of the wind speed is acting as a headwind, tailwind, or crosswind component.
DME systems can give you your groundspeed. In combination with the VOR this gives you the same information as the GPS and makes determining the wind speed pretty simple.
ADFs can be used for this kind of calculation in the same way as you would for a visual waypoint: If you expect to be flying due north to head toward an ADF beacon and you're flying 010 degrees to keep the pointer needle where you want it then you know you're dealing with some crosswind component.


What material is used to make the hot sections of jet engines? What metal alloy is used for the hot section of a jet engine? Specifically the exhaust nozzle? I have found mention of a titanium alloy and nickel alloy but nothing with that much detail. What sort of metals does one have to use to create something that can withstand temperatures of 650 degrees Celsius? Thanks

Short answer

Fan: Aluminum, titanium, or stainless steel
Compressor: Nickel-, cobalt-, or iron-based alloys. Additive are aluminum and/or titanium, chromium, as well as rare earth elements like yttrium.
Combustion chamber: Superalloys with refractory metals such as tungsten, molybdenum, niobium, tantalum. Ceramics and ceramic-metal mixes.
Turbine: Nickel-based superalloy, outside air is circulated through channels inside of the turbine blades. For lower pressure turbine blades iron-based superalloy or even stainless steel. The metals used for turbine blades are often grown as a single crystal.
Exhaust nozzle: Nickel Inconel and stainless steel alloys.
Casing: Aluminum or polymer matrix materials.

The high-pressure turbine is, with the combustion chamber, very hot (more than the exhaust nozzle). You may read this Wikipedia article which has a short list of materials used for turbine blades (with alloys names and cooling techniques).

Engine structure (turbofan)

(source: Wikipedia)

An actual engine looks like that (CFM56 turbofan). It's easy to map the detailed elements to the simplified schematic. The fan pushes about one ton of cold air in the engine every second in high power turbofans. This is why temperatures and pressures can be so large after compression and after combustion.

All quotes are from Nasa Guide To Engines, 2007.

  In 1903, the Wright brothers built an aluminum
  block engine because of its light weight compared to cast iron. Its
  melting point of 660 °C was well above the engine’s operating
  temperature, so for them it was a good choice. However, aluminum could
  not be used in the hotter parts of a turbine engine where temperatures
  reach of 1800 °C or more because it would melt.

Fan

  This typically does
  not get very hot (<150 °C) so aluminum, titanium, or stainless steel
  are all suitable for the fan blades. Most engines use titanium because
  it has a high strength-to-weight ratio, is corrosion and fatigue
  resistant, and would be able to withstand the impact of a bird strike.

Fan example (with a fan blade missing after the engine blew apart).

Compressor section

  The pressure of the air can be raised up to 30 times and the
  temperature, depending on the number of stages in the compressor, can
  rise to 1000 °C. Here the materials must have high strength at high
  temperatures; must resist fatigue, cracking, and oxidation; and also
  must resist “creep.” Creep is the tendency of a material to slowly
  change shape when stressed at high temperature. Since no single metal
  would have all the desired properties, an alloy (a mixture of metals)
  is used. Very high- temperature alloys are called superalloys and are
  generally nickel-, cobalt-, or iron-based alloys. Aluminum and/or
  titanium are added for strength, and chromium, as well as rare earth
  elements like yttrium, are added to improve corrosion resistance.

Compressor example (high pressure rotors)

Combustion chamber

  Temperatures can exceed 1800 °C and again superalloys are used, but
  without the titanium or aluminum for strength because there are no
  moving parts. Instead, refractory metals are often added to a
  superalloy. These are metals of unusually high resistance to heat,
  corrosion, and wear such as tungsten, molybdenum, niobium, tantalum,
  and rhenium. They are used in alloys and not as pure metals because
  they are among the densest of all the elements, a negative property
  when it comes to aircraft that need to keep weight to a minimum.
  Ceramics and ceramic-metal mixes are also used here because of their
  high heat resistance. We are familiar with pottery, tile, crucibles,
  and fire bricks as types of ceramics. They have very high melting
  points and don’t require the cooling systems like those needed to keep
  metals from melting so they make for lighter, less complicated engine
  parts. The down side is that they tend to fracture under stress, so
  engineers seek to create new ceramics composites that incorporate
  other materials to improve properties.

Combustion chamber example

Turbine

High-pressure turbine blade (source: Wikipedia)

  The first set of turbine blades are in the highest pressure, hottest
  part of the gas flow and are generally made of nickel-based superalloy
  or ceramic blades. Unheated outside air is circulated through channels
  inside of the turbine blades to keep them from melting in this extreme
  environment. Further down the engine lower pressure turbine blades
  often sit. Since the gases have somewhat cooled by this point, the
  blades can be made of iron-based superalloy or even stainless steel.
  It is interesting to note that for strength, the metals used for
  turbine blades are often grown as a single crystal. A close look at
  most metals and alloys show that they are composed of crystals (also
  called “grains”), and the places where the crystals meet are called
  grain boundaries. A material is weaker at the grain boundaries than
  within the grains—especially at high temperatures— so turbine blades
  fashioned from metal formed as a single grain (no boundaries) are
  stronger.

See single crystal material (known as monocrystalline material in electronics) .

Turbine example (high pressure stator)

Exhaust

Exhaust from Boeing 787 engine (GEnx, one of the world's largest jet engine - Source)

From FAA Aviation Maintenance Technician Handbook - FAA-8083-30:

  Inconel and stainless steel alloys. The Inconel [nickel-chromium-iron]
  alloys are frequently used in turbine engines because of their ability
  to maintain their strength and corrosion resistance under extremely
  high temperature conditions.

See Inconel details.

Casing

  Although it need not withstand high temperatures like the core of the
  turbine, the materials here need to be strong enough that if a blade
  were to break off it would be contained within the casing and not
  enter the wing or cabin of the aircraft and cause further damage.
  Aluminum or some polymer matrix materials are used as engine casings.


Why do turbofan engines have low pressure compressors? In my understanding turbofan engines usually have 2 stages of compressors:

one low pressure compressor
followed by a high pressure compressor

But it looks like the high pressure compressor cancels the job done by the low pressure compressor, doesn't it?

What's the point of decreasing pressure in the first compressor if you then increase it in the second compressor?  Why not using only the high pressure compressor?

The low pressure compressor does not reduce pressure. It increases it, but operates at a lower pressure than the high pressure compressor.  Some engines also have an intermediate pressure (IP) stage.

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

https://www.grc.nasa.gov/www/k-12/airplane/compress.html

Each stage increases the pressure as the air is forced back into the flame tubes.


Why do some A380 main wheels have no brakes? The A380 has 20 main and 2 nose wheels. Apparently the nose wheels have no brakes (compare with Are there any aircraft with a nose wheel braking system?).

While researching for this question (How are brakes cooled on heavy aircraft?), I assumed that all of the 20 main wheels are braked. I looked at a few pictures of A380 landing gears. As far as I have seen, all main wheels are equipped with brake cooling fans (BCF), except the four rear wheels in the inner 6-wheeled  undercarriages. (e.g. here).

This picture below clearly shows that there is no brake unit inside the rim:

There is another picture here whithout BCFs, but the same four wheels apparently have no brakes.

What is the reason for omitting the brakes at these four wheel positions?

Is this a common configuration, or quite unique to the A380?

Because they are not needed (for the passenger variant). Addition of brakes increases the weight and complexity of the system, which is undesirable. Why add something that is not required?. If it is required, additional brakes can always be added, like in the (proposed) freighter version. According to flightglobal.com:

  Sixteen of the A380-800's 20 main wheels are braked, with the two aft wheels on each body landing gear of the passenger version not having brakes (unlike the freighter variant, where all 20 main wheels are braked due to its increased weights).

Also, Aft Axle of BLG is steerable to ensure better maneuverability for small radius turn (Body Wheel Steering); braking a steerable axle would require special linkages, increasing weight and complexity. This is not insurmountable, however- Boeing 777's landing gear is similar, but has brakes on all its wheels.


Why would Emirates fly over Iraq, avoiding Iran? For the last few years flying with Emirates from the UK to Dubai, I've noticed it avoids Iraq and fly over Iran to the north. For example, EK40 from BHX to DXB

From Flightradar24

However, flying this month on EK40 I noticed the exact opposite, the flight avoided Iran and flew over Iraq to the south.

I know that the conflict in Iraq has mellowed recently, but that doesn't explain this: the first picture is of EK40 on Feb 7th 2018, while the second picture is of EK40 on Feb 6th 2018.

So, what could be the reason for avoiding Iran so explicitly when the next day the same flight avoids Iraq?

EK 040 on Flightradar24

Since December 2017 the Iraqi airspace is used by Emirates again, after Iraq declared the victory over the Islamic State.

Now there are basically two routes to choose between when flying from Turkey to Dubai.

One route passes through Iran  and make it look like the flight is explicitly avoiding Iraq. The route crosses the Ankara FIR / Tehran FIR border just north of Iraq, before turning south towards Dubai, such that it just stays outside Iraqi airspace.

The other route (UM688) passes through Iraq and makes it look like the flight is explicitly avoiding Iran. The route structure in Iraq is such that the north-south routes follow the Iran border in order to avoid the military activities near the Syrian border.

The map below shows the route structure with two routes through Iraq highlighted (UM688, Southbound and UM860, Northbound).

source: Flight Service Bureau

It is likely that the choice of route is made based on financial reasons. From looking at recent flights, Emirates seems to have a preference for flying over Iraq, but occasionally flies over Iran.

The ATS overflight fees of Iraq are at \$3751 (approx. \$0.38 per km) considerably cheaper than those of Iran. Iran charges \$0.004061 per 1000 kg Maximum Take-off Mass (MTOM), per KM, with a 30% reduction for the MTOM in excess of 150 000 kg. For an A380, the cost of crossing Iran would be approximately \$2800 (approx. \$1.80 per km). The slightly longer flight over Iraq, Kuwait (~\$1351 ATS fees) and Bahrain (~\$2101 ATS fees) would consume more fuel but will still be cheaper than crossing Iran.

A reason for crossing Iran occasionally might be that Iraq's airspace is very congested since the re-opening of the airspace. To avoid delays, sub-optimal altitudes or adverse meteorological conditions it may be more attractive to cross over Iran in certain situations.

1sources of the ATS Fees: AIP's of the respective states, section GEN 4.2. As high volume users, Emirates may have negotiated better rates.


Can anyone identify this 1910s era biplane? I tried a number of searches with absolutely no luck. I remember a few that looked sort of similar without that smooth aerodynamic nose and body. I did try at least.

If this helps, the individual standing before the plane is believed to be of French origin or descent.

It isn't German or even European, not with an OX-5 engine on the front - unfortunately there were hundreds of similar designs in the post-ww1 period, however the fact that it has a two bay wing and the struts are wire braced instead of having a diagonal narrows it down, and lo and behold, the Pitcairn PA-3 Orowing appears to be a match.


Can the phrases "carbon fibre reinforced polymer composites" (CFRP) and "carbon fibre-epoxy composites" be used interchangeably? Does the "polymer" in CFRP imply the use of an epoxy resin?

No.
Epoxy resin is more specific than the terms polymer or plastic from CFRP. CFRP does also include carbon fibre composites with other duroplastic matrix materials (e.g. phenolic resin) or even thermoplastic matrices.


How does angle of attack vary during climbing flight? I'm a little confused by the angle of attack of an aircraft during climbing flight. Suppose you have an aircraft in level flight which then rotates to some angle x. Just after rotation, I would assume that the angle of attack is equal to x, but after this I get confused. The aircraft continues to climb at the angle x, but does the angle of attack vary with time spent in climbing flight? Or once the rotation is over does the angle of attack go back to whatever it is for steady level flight?

Similarly, if you have an aircraft travelling at steady level flight with net lift = 0N, which then rotates to some angle x where for an angle of attack x the net lift is 50N. Does the aircraft only experience this extra 50N of lift for a brief moment during/after rotation, and following that the net lift acting on the wings is again 0N?

Angle of attack is the angle between relative wind, a.k.a. flight path, and the reference datum (chord line in the below image, but might be just given relative to longitudinal axis of the aircraft especially since many aircraft have wing twist, so the chord line varies along span).

(figure from How It Flies section 2.4)

Now to you cases

  Just after rotation, I would assume that the angle of attack is equal to x, but after this I get confused.

As the pilot pulls on the control column and the elevators deflect up, the aircraft starts to pitch up. As it pitches up, the angle of attack increases. That increases lift, which is proportional to angle of attack (in the non-stalled region).

As the lift increases, the aircraft will start accelerating upwards, which means its flight path angle will increase. The pitch angle still increases at the same rate, but now the flight path angle also does, so their difference—angle of attack—increase will slow down until it stabilises at some value where the flight path angle rate catches up with the pitch rate.

If the pilot then pushes the control column back and neutralizes the elevator deflection, the aircraft will stop pitching up and the angle of attack will decrease again until the flight path rate stops as well.

In fact the angle of attack at which the aircraft will stabilise is proportional to the elevator deflection (for longitudinally stable aircraft), so if the pilot lets the control column return to its trimmed position, the aircraft will assume the original angle of attack again.

I'll refer you to the mentioned How It Flies chapter 2. Angle of Attack Awareness and Angle of Attack Management for further details.


What are the requirements for a PPL to upgrade from SEL to MEL? Reading the requirements for MEL it is confusing which things must be done in a multi-engine, and which things carry over from SEL. If I already have SEL do I need to do a cross country in a multi-engine? Assuming I can meet the PTS for MEL, what other requirements are there?

The appropriate paragraph of Part 61 of CFR 14 is 61.63 (emphasis mine, the 'unless' part does not apply in your case):

  (c) Additional aircraft class rating. A person who applies for an
  additional class rating on a pilot certificate:

  (1) Must have a logbook or training record endorsement from an
  authorized instructor attesting that the person was found competent in
  the appropriate aeronautical knowledge areas and proficient in the
  appropriate areas of  operation.

  (2) Must pass the practical test.

  (3) Need not meet the specified training time requirements
  prescribed by  this part that apply to the pilot certificate for the
  aircraft class rating sought; unless, the person only holds a
  lighter-than-air category rating with a balloon class rating and is
  seeking an airship class rating, then that person must receive the
  specified training time requirements and  possess the appropriate
  aeronautical experience.

  (4) Need not take an additional knowledge test, provided the applicant
  holds an airplane, rotorcraft, powered-lift, weight-shift-control
  aircraft, powered parachute, or airship rating at that pilot
  certificate level.

So, to answer your question, no, you do not need a cross-country, you do not need any solo time in a multiengine airplane, either. All you need is a sign off from your instructor.


When does an airliner switch from using the tiller to the rudder? When does an airliner switch from using the tiller to steer to the rudder on takeoff? Same for landing- when does the pilot switch from using the rudder to the tiller? What is this process? Once a pilot switches to the rudder on takeoff, is there a lock for the tiller? I'd imagine that it would be necessary to have some sort of system to lock the tiller so the pilot cannot accidentally hit it at high speed on the ground. If there is a locking mechanism, does it actually lock the tiller or does it simply disconnect the tiller from the nose wheel leaving the wheel free spinning so it can move as the rudder makes slight corrections while taking off/landing? Please expand as much as possible! Thanks.

I flew for two 747 carriers that never bought new aircraft, and thus this answer applies to 747-100/200 aircraft as originally manufactured for a number of different airlines.

The tiller is active when the aircraft is on the ground and the nose gear is compressed regardless of groundspeed as I remember.

I've never heard of a pilot accidentally hitting it at high speed on the ground, but if they did it would scrub the front tires. It wouldn't appreciably turn the airplane.

Pilots did have to be wary of having their hand within the travel range of the tiller when a tug is being hooked up. Ground handlers would sometimes work the nose gear back and forth a bit, and that action is reflected in tiller movement. If they do it quickly, the tiller will snap quickly and a hand in the way will get hurt.

The nose gear is relatively weak compared to the rudder, the brakes, or the engines insofar as its ability to turn the aircraft. To make turns off a runway onto a taxiway that is 90° to the runway, you usually aim for a 10 knots or less groundspeed. You can do a little more depending on the runway surface and what your center of gravity is, but sometimes 10 knots can be too much. I once skidded the nose gear badly at Jeddah, Saudi Arabia at midday in the summer when I failed to account for the fact that hot asphalt is slippery.

To illustrate how poor tires are in preventing sideways scrubbing, consider the 747 body gear. 747s have body gear steering for tight turns, but you can dispatch the aircraft with inoperative body gear steering. You can still make tight turns, but not quite as tight, but you do it with engine power and just scrub the body gear around, all 8 tires with a lot of weight on them. You might have difficult doing that, though, if you c.g. is greater than 26.6% mac. That's where the wing gear is, and if you're aft of that (limit is 33% to 37% typically) you're got a lot more weight on the body gear. Body gear steering is disarmed for takeoff and landing though.

Barring a strong crosswind, the usual takeoff procedure was to not use the tiller unless you were doing a rolling takeoff, starting the power up before being aligned with the runway. Once aligned and moving the rudder will do it. Even in a strong crosswind, you can forget the tiller very soon in the takeoff roll.

On landing, no use of the tiller until down to taxi speed except maybe a little use for a strong crosswind just before reaching taxi speed.

Boeing offered a rudder pedal-nose gear steering link, but almost all 747s I flew did not have that. I flew a couple of airplanes that had it. It helped in a crosswind.