How would gyroscopic effects affect push-pull configuration vs conventional twin engine in the event of engine failure? This answer, and the Wikipedia article on push-pull configuration both claim that aircraft with push-pull configuration are easier to fly in the event of engine failure, because the thrust provided by remaining engine would remain in the centerline.

However, lot of aircraft with push-pull configuration seem to be relatively small ones, so I am wondering how gyroscopic effects would affect the handling of the aircraft in the event of engine failure.

I found this answer comparing WWI era FB19's to F16's. But as I said before, lot of push-pull reconfigured aircraft probably have less mass and much slower than F16, so the calculations in that answer may not apply.

So my question is, is there any noticeable handling difficulties in push-pull configured aircraft, when an engine fails?

Gyro precession effects from a propeller are minimal when at flying speed and making modest pitch and yaw motions, where the precession is mild and there is lots of aerodynamic damping.  You don't really notice it. Propeller slipstream over the rudder has a much bigger effect, which a lot of people may confuse with precession.

You have to make large sudden pitch or yaw motions at low speed, like when taking off a taildragger with a very large propeller. If you raise the tail suddenly at low speed, precession of the propeller will make the airplane swing noticeably.  Or, with aerobatic airplanes that make extreme pitch and yaw motions and where precession will have an effect that has to be taken into account (as well as putting massive stresses on the crank at the prop flange).

For wing mounted engine twins, the effects of offset thrust are the main limitation, with a minimum control speed that is usually well above the stall and below which the airplane will roll over.  Centerline thrust aircraft avoid this problem for obvious reasons.  In any case, I don't believe gyro precession is a factor at all in a Cessna 337, on either engine, or on a regular twin for that matter because the pitch and yaw rates you generate aren't enough to create significant precession effects that could affect controlability.

In the end though, with piston twins, gyro precession is the least of your problems.  The biggest one is the fact that the loss of 50% of thrust leaves you with almost no performance reserve.  Wing mounted engines leave you with a somewhat more dangerous situation due to the ability to lose complete control well above the stall by dropping below minimum control speed, but even with center line thrust, the single engine performance is so marginal that if you are at gross weight and above sea level and it's hot, you are going down anyway. Yes there is no Vmc limit, but you are still faced with having to maintain minimum speed for performance and chances are you are still going to crash land eventually and you may ending up stalling and spinning it the regular way if you panic trying to will the plane to fly, just like a regular twin.

For this reason I don't believe center line thrust aircraft have all that much better a safety record following engine failures than regular twins.


Why do airlines seem to go out of business suddenly? FlyBMI are currently in the news for failing and stranding passengers abroad. This is not the first time an airline has halted operations seemingly overnight and left passengers without a way to get home.

Why is it that airlines stop operating so suddenly, compared to other businesses which seem to go through a more protracted period of profit warnings, administration... and generally more orderly windup?

Mainly because winding down airline business will turn it into a sudden and from there on increasing loss. Airline operation nowadays is calculated in most cases toward a rather high plane utilization. Even more so on low cost enterprises. Sometimes even 10% less passengers may turn a profitable flight into a loss.

Airlines depend on filling up flights - until the last seat and last moment (last minute bookings being, depending on the business plan, even being more profitable). As soon as an airline would announce the intention to close down, new bookings will dry up quick, making next to every flight after this unprofitable.

This all is due the fact that, unlike a brick and mortar store, most cost of an airline are operational cost. There is no valuable inventory to be sold of over time, or much existing machinery to be run until the last moment to convert existing materials in (somewhat) higher valuable products, to turn it into cash. For Airlines there is none (No, collector value of in flight magazines don't really count), so shuting down the flights removes next to all further expenses, stoping any further increase of debt.

Not to mention, that flying after such an announcement would become almost impossible as each and every supplier would demand cash payment from that moment on - with airport fees and fuel payment most prominent - something a money strapped airline hardly can provide. Airlines that tried to go that way usually postponed the shutdown just by days - and often with planes stuck in all corners of the world, unable to return, not able to sell them, but instead adding more debt in airport parking fees.


Instrument Approaches which do not have a FAF Most instrument approaches denote the Final Approach Fix (FAF) as the start of the final approach segment terminating in either the Missed Approach Point (MAP) for non-precision approaches or a Decision Altitude (DA) for a precision approach.  On IAPs the FAF is designated either as a Maltese Cross for non precision approaches or as lightning bolt where the glide slope intercept occurs on a precision approach.  That’s all fine and dandy until I encountered this IAP.

There appears to be no FAF designating the final approach segment.  What does this mean?  Are the FAF, the IAF, and the MAP all coincident?
UPDATE
Someone here suggested that this was because the VOR Rwy 27 has straight in minimums.  However that does not appear to be a general rule.  For instance, take VOR Rwy 01 into KSAV.

This approach has straight in and circling minimums yet clearly has a FAF FREMN defined and located by either a DME or GPS.

The Lakeland approach you show does not have a FAF (Final Approach Fix) because there is no defined point (fix), where you are established inbound and start to descend. In this case, the FAF is given by the FAP (Final Approach Point):

FINAL APPROACH POINT− The point, applicable
only to a nonprecision approach with no depicted
FAF (such as an on airport VOR), where the aircraft
is established inbound on the final approach course
from the procedure turn and where the final approach
descent may be commenced. The FAP serves as the
FAF and identifies the beginning of the final
approach segment.

(FAA Pilot/Controller Glossary)
The chart only says "Remain within 10 NM", so the actual FAP depends on how you fly and cannot be depicted on the chart. Note that the approach does not require DME, so the point AMBAZ cannot be identified by every aircraft (which is why the chart lists minima with and without AMBAZ). It therefore cannot be the FAF. The second approach you show (Savannah) requires DME or TACAN, which means it can have a FAF (FREMN) defined via DME (SAV 7.8).
The MAP (Missed Approach Point) of the Lakeland approach is the LAL VORTAC, which is also the IAF (Initial Approach Fix). The Jeppesen chart for this approach explicitly says "MAP at VOR" and the FAA chart you show has the MAP indication (dotted line with upwards arrow) at the LAL VORTAC.

The Jeppesen chart also shows a point FF27 5.7NM before the runway, where a CDFA (continuous descent final approach) at 2.96° should be started, which is probably the point an FMS would use:


Why don't aircraft carriers have multiple sets of arresting wires? On this answer to this question regarding the merits of angled flightdecks on aircraft carriers, @JanHudec makes this interesting observation:

  You don't really have "two flight decks". The catapults are well distributed, but there is just one set of arresting wires, so while hit to the bow disabling catapults 1 and 2 would leave 3 and 4 available, hit to the landing area still prevents all landings.  [Emphasis added.]

Why don't aircraft carriers have multiple redundant sets of arresting gear, to allow them to land aircraft even if one of the landing runways is disabled by battle damage?

Having a second set of 3-4 wires would need a longer deck. But if one set is damaged, so would this landing area, so we need 2 runways:

Doubling the size of the flight deck would result in a massive ship, and scaling doesn't favor something getting too big.

Consider the 1967 USS Forrestal fire, even if the deck were to be double the size allowing two landing runways, an attack or an explosion would spread easily across a flat surface and render the whole ship out of action when it comes to air ops.

Note that USS Forrestal was already engaged in combat. But since you have tagged it redundancy, the answer would likely be (although not this site's scope) military planning redundancy, not an engineering one, i.e., if engaged in combat, having more than one carrier is the redundancy to continue engaging in said combat (and land the other planes).

Of course not all freak accidents can be anticipated. Let's say something bad happens to the deck during peacetime, what's the worst that can happen? The airborne aviators unable to land will eject, and the rescue helicopters will bring them back, it's a monetary loss, but not loss of airborne lives.


Why do helicopters typically have blades at the top? This question has several great answers showing that it's perfectly possible for a helicopter to fly with its blades underneath the rest of the aircraft.

There are some obvious issues with having a helicopter's blades at the bottom, to do with proximity to the ground and being in the way when someone wants to get in or out of the helicopter. Is the answer that simple, or are there other reasons related to flight dynamics, manufacture, or something else?

Yes the answer is that simple, that mincer needs to be out of the way so it doesn't chop peoples' head or feet off, or slashes into a bit of concrete and shatters into 100 pieces scattered around like bullets. That is the main reason why the light 2-seater R22 has the rotor mounted on a pylon, at 2.7 m it is out of the way of most people.

Image source

Flight dynamics wise, a rotor underneath the helicopter has speed stability in the hover, which makes the helicopter easier to control. The rotor acting as a pusher makes it more efficient; being closer to the ground means more lift in ground effect and softer take-off and landing. Manufacturing:

The rotorhead of an underslung rotor would look pretty similar to the top mounted one, a hinge offset head would line up the fuselage with the rotor disk, top or bottom.
The rotor would have to be situated away from the fuselage to allow for the upwards coning and tilting under flight conditions - for instance mounted on an upside down pylon quite similar to the R22.

So all in all an underslung rotor poses no particularly vexing engineering problems and provides improved stability and better performance - it would just be an immensely impractical machine.


What is ADS-B and who needs it? I read that ADS-B is required by the year 2020. What does this mean? What is ADS-B used for, and who needs it?

To answer your first question, ADS-B stands for Automatic Dependent Surveillance - Broadcast.

Automatic in the sense that it operates without external stimulus (unlike radar)
Dependent in the sense that the surveillance information is derived from on board systems
Surveillance in the sense that it's primary intended to provide surveillance information to other parties
Broadcast because it transmits to all listeners without knowing who those listeners are, there is no two-way communication.

The capability of an aircraft to transmit ADS-B signals is called ADS-B OUT, the capability of an aircraft to receive ADS-B signals is called ADS-B IN.

ADS-B data contains information about the identity, position, altitude and velocity of the aircraft. The position and velocity information is derived from GNSS (GPS).

There are three technologies defined for ADS-B

1090ES is operating on 1090 MHz, the same frequency used for Secondary Surveillance Radar (SSR) and Airborne Collision Avoidance Systems (ACAS/TCAS). Messages are transmitted by the same transponder that also replies to radar interrogations. This will be a Mode-S capable transponder because the data is formatted in a similar way. Since this frequency is the worldwide used standard for aviation, international aviation will use this ADS-B standard.
UAT (Universal Access Transceiver) works on 978 MHz. A special transceiver is required to use this form of ADS-B. The transceiver is able to send and receive data, it will also receive ADS-B transmissions from other aircraft. UAT is only allowed within the USA and China. The frequency 978 MHz is reserved for other purposes (DME/TACAN) in other countries.  To encourage people to equip with ADS-B voluntarily, the FAA provides additional services on UAT. These are Traffic Information Service - Broadcast (TIS-B) which transmits position of aircraft detected by FAA's SSR's and Flight Information Service - Broadcast (FIS-B) which provides the receiver with weather information and operational data such a airspace restrictions and NOTAMS.
VHF Datalink Mode 4 (VLDm4) uses a frequency in the 118-136.975 MHz block, which is internationally reserved for aviation. Normally these frequencies are used for voice communications, so a special radio is required to support VDLm4. Just like UAT, VDL mode 4 allows for services like TIS-B and FIS-B. VDLm4 was tried in Sweden and Russia, but has not been able to gather much mass. It is unlikely that it will be used on a large scale.

Since you mention 2020 as the year by which aircraft should be equipped with ADS-B I am going to assume you are from FAA-land.

The FAA mandates aircraft in the 48 contiguous states (and D.C.) that operate in class A, B, or C airspace or class E airspace above 10000ft (class E airspace below 2500 AGL excluded) to be ADS-B OUT equipped by January 1st, 2020.
The mandate itself can be found here and the associated Advisory Circular (AC90-114) can be found here.

Aircraft operating below 18000 ft can use either UAT or 1090ES. Above 18000ft, 1090ES is to only approved ADS-B technology.

Countries that have ADS-B mandates are (this list is probably not exhaustive)

Australia
Canada
(Hudson bay)
China
Countries in the European Union
Fiji
Hong Kong
India
Indonesia
Seychelles
Singapore
USA
Vietnam


Does anybody recognize these wings? I was on a tour at the University of Maryland, and I saw these wings sitting against the wall:

I asked the guide, and he had no idea. (In case you can't tell these are two wings. The green lines indicate the extent of the two wings.)

What aircraft do these belong to?

Those wings are from an ERCO Ercoupe. The diagonal ribs are very distinctive.


Why are only captains allowed to taxi the aircraft? At most airlines, only the captains are allowed to taxi the aircraft (i.e. Lufthansa). Even if the copilot is pilot flying, the captain will taxi the aircraft to the runway, and later from the runway to the gate. What is the reason for letting only captains taxi the aircraft?

Taxiing can sometimes be one of the more difficult tasks required on a flight, and affords one of the few opportunities to run into something.

Historically, in the earlier days of crewed flying, a captain may have done most or all of the flying. A captain might have let the copilot fly some of the tedious cruise portion of the flight, and a nice captain might have even let a copilot land or takeoff. As CRM developed, the concept of both pilots sharing the flying duties was eventually standardized.

Most larger aircraft are steered in taxi primarily by means of a steering tiller. When installed, a steering tiller is always installed at least on the left side of the cockpit. In the past, aircraft did not offer an option for a second steering tiller on the right side, likely reflecting the norms of the day. Updated aircraft and newer designs commonly include as standard—or offer the option of—a steering tiller on the right side as well. Some companies choose to omit this option as a weight and cost saving measure.

Some companies allow either pilot to taxi the airplane unless the aircraft is not properly equipped to allow this. I do not have personal knowledge of any companies that forbid the first officer from taxiing, but such policies may well still be in place. In many cases it is still not an option since the aircraft is not properly equipped to be taxied from the right seat.


Can I stop crop dusters repeatedly flying over my home? I have a crop-duster spraying the fields around me often. He flies over my house no matter what field he is spraying every few minutes. I think he uses my house as a pivot point. The old pilot rarely flew over my place.

My livestock goes nuts. It is annoying. He has to fly low and is loud. I called the company  awhile back and they said he was new and he would talk to him. He is back today flying over repeatedly and I called the company and the owner told me he would call me back. No call back and it goes to voice mail now when I call him.

Is there anything I can do? I told the owner that I understood they may have to fly over once in awhile but this is over the top.

In a rural area pilots can fly pretty low, and are permitted to fly even lower while carrying out a lawful activity such as crop-dusting. Even if the pilot flew at 500 feet above your house, which is probably the maximum that he needs to stay away, it would still be loud and disturbing to your livestock.

You should try to work it out with the company and/or the pilot in an amicable way.

Unless your house is located in a congested area, the minimum altitude the pilot must maintain is 500 ft. However, if your house is located in a sparsely populated area, the pilot may be able to fly much lower than that. However, while the pilot is in the process of spraying, or turning around to spray he is allowed to fly lower than that minimum. Otherwise, the pilot may have to do dangerous steep climbs and descents.

For example, lets say the pilot is in violation of minimum altitude regulations by flying over your house at some minimal altitude below 500 ft. Lets assume you get the pilot busted and the pilot receives a warning, citation, or temporary suspension. When he comes back he can just fly over at 500 ft legally all day long, and that will probably be almost as noisy and disturbing to your livestock.

Alternatively, you can lease a small part of your land to AT&T to put a 1000ft tall cellular antenna on your property.

But seriously, as a pilot, if I heard that my flying was causing you so many problems, I would try to avoid it as best as possible, so long as it doesn't affect safety.


Contribution of wing vertical location to roll stability Recently, I was reading about the factors that contribute to roll stability of an aircraft and came across a point stating that the wing's vertical location (Distance of wing from CG) contributes to the roll stability of the aircraft. I cannot comprehend this statement completely and even if it did, How much of an effect does it truly cause when compared to having features like a wing dihedral? Any information and explanations would really help me understand this concept.

An aeroplane in a pure co-ordinated turn has a deflection of both the ailerons and the rudder. This is cumbersome however, much easier to only deflect the wheel, tilt the lift vector, and let the aircraft sway into the turn. The vertical tail will then experience a slip angle, which will point the nose into the direction of the turn at the expense of a small sway velocity: sideways.

So now the question is: how does the aeroplane react to a sideways incoming airstream. What we want is that if the pilot deflected the wheel to the right, he has to keep the wheel to the right to maintain the right hand turn. So upon an un-coordinated turn, without touching the pedals, we want the aircraft to try to level itself again upon an incoming sideways velocity.

If we look at what happens to a low wing aircraft first, we see that the incoming sideways stream sees little resistance when streaming over the fuselage top, but gets caught in the wing/fuselage intersection and builds up pressure there. So the sideways velocity introduces a rolling moment in the direction of the turn, which the pilot has to compensate for by turning the wheel the other way. This is not the behaviour that we want.

In order to correct for that, we can introduce wing dihedral: now the incoming sideways air stream wants to level the aeroplane again, and the pilot needs to keep the wheel deflected into the turn, which is what we want.

And of course, with a high-wing aeroplane the above is valid vice-versa: the sideways stream wants to level the aircraft, so the roll behaviour is naturally how we want it to be already. Depending on the wing/fusleage shape and many other factors, the aeroplane can be too stable and we'll need to implement anhedral, wings pointing down.


How often do V1 cuts come up in airline sim training? Just how often do airline pilots practice V1 cuts when in the sim? Is it the kind of thing that turns up practically every sim session, or is it something that's much more sporadic?

During initial and upgrade training you will fly many V1 cuts. At every recurrent check ride you'll experience V1 cuts both as pilot flying and non flying pilot.

In our sim events the V1 cut would generally be a 600 RVR takeoff with 10 kt direct crosswind from the side that would fail. Not always though -- you expected the V1 cut on every takeoff.


Do the captain's and first officer's rudder pedals always move together? I worked on Russian Fighter aircraft where both the Rudder Pedals were mechanically interlinked i.e Captain applies force on his pedal than both pedals (Captain & First Officer) move & vice versa. Single Pedal sensor Unit (of course redundant sensors) senses the position and sends it to the Fly-By-Wire Computer for moving the control surfaces.

I would like to know if this is true for all aircraft (Fighter/Commercial, Boeing/Airbus, etc.) and if not, what are different implementations? Maybe different sensors for Captain/First Officer, different arch etc.

Also on the same lines, how does Pedal Feel operate i.e, does it takes a signal from pilot pedal sensor or from actuators to provide feel to the pilot/copilot?

In the EMB-145 and -135 the rudder controls are mechanically linked in the tail in an assembly that converts torque from the rudder cables to linear rod input into the rudder hydraulic control units.  Up until this assembly the rudders are technically independent and the cables for each rudder are routed differently through the airframe so that a single point of damage does not render the rudder inop.  The autopilot servo is only located on the captains rudder cables.  Through the rear linkage both pilots rudder pedals will move together when either provides input.

Our other controls are a bit more interesting.  The roll input is also linked, but it is linked at the control column,  Each yoke independently controls its own aileron -- the CA yoke only flies the left aileron and the FO yoke the right aileron.  It is only through the control column linkage that each yoke can control the whole airplane.  It is done this way so that if there is a problem with a cable jamming, the control interlock can be disconnected and at least half control regained.


I'm thinking of building a balloon-launched glider, is this idea plausible? So, I know nothing about aviation. I'm a student studying maths and computing and I feel like building a high altitude glider controlled by a Raspberry Pi and Arduino as a personal project.

First things first, I'll be launching using a weather balloon or something to reach a given altitude (references to any information on glide ratios as you get to higher altitudes would be much appreciated). Once it's high enough the balloon will be deflated and released, the glider will then be dropped and start its descent.

Assume I've control of the glider in the form of waypoints I can plot on a map to be put into the onboard computer beforehand (or from signals sent from a computer). How complex an autopilot would be required to follow a set of coordinates (pre-calculated so they fit the descent pattern) and then release a parachute for a safe landing?

I was thinking of purchasing a basic glider structure as to have the maneuverability from the professionally built wings and stuff, so construction shouldn't be a problem. The main concern is how plausible a project like this would be considering I'll be building the flight computer myself and I know basically nothing about flight control.

Any other material you have on flight control, flight computers, personal projects like this would also be much appreciated.

This idea is entirely plausible and has been tried before, and even successfully. But I guess you have already read through all the pages which you can find via Google.

Several things are to be considered:

It is really cold up there (see below), and the ascent will take long enough that you need to heat the batteries and to remove grease which will likely freeze solid up there. It would be a shame if the onboard camera cannot document the flight because its battery was too cold.
Due to the low air density, flight speed will be high, so your sailplane will likely experience flutter if you try to use a regular RC glider. A more sturdy configuration which is less optimized for high glide ratios might serve you better.
Winds can be very high at altitude, especially between 10 and 16 km height. You need to factor wind drift into your planning. Try to launch on a calm day (calm all the way up, that is), or the plane might not reach the waypoints.

Since lower temperature means a lower Reynolds number, viscous effects will be higher at altitude than on the ground. This means that your glider's drag for a given lift will be higher at altitude. How much depends a lot on the choice of wing airfoil. If you go with an existing model, pick one which flies well over a wide speed range. A reasonable glide ratio for a model at altitude would be between 10 to 15, so it will glide 10 to 15 km for each km of altitude.

The maximum altitude in which subsonic flight is possible depends on the wing loading (among other things), so try to keep masses down.


At what point is a turbine powered ducted propeller considered a turbofan? Turboprop Propellers can be ducted for extra thrust.  What are the criteria for determining that the engine is no longer a ducted propeller design, but a turbofan? Does there have to be a type of blade design, a fully separate nacelle/pod, number of blades?  Or are all turbofans technically shrouded propellers with some augmented thrust from the turbine exhaust ?

tl;dr

They're pretty much all considered turbofans with a few exceptions from the '70s such as this experimental Britten-Norman Islander. They key difference is a prop can be considered a separate entity (add-on) to the turbine for a turboprop, but a fan is an integrated component for a turbofan.

Slightly longer answer

After a fair bit of thought and research I hope I've managed to come up with a satisfactory answer, though to start with a disclaimer. The aviation industry is far too fond of merging and creating new words to describe new engine architecture. Take Open Rotor vs. PropFan for example.

To get technical about it, you could look at the similar/reverse argument about the difference between an open rotor turbofan and a turboprop, as addressed by the EASA.

If you look at Appendix 1: Open Rotor Definition (Page 86) of that document they outlined the following key differences

  Open rotor module that cannot be distinguished as a separate entity

However the following was the agreed definition, which is still rather ambiguous.

  A Turbine Engine featuring contra-rotating fan stages not enclosed within
  a casing

I think the first one is more key, the prop can be considered as a separate entity, attached to the front of a turbine and powered by a gearbox. A fan is an integral part of the engine and cannot really be considered a separate entity, as it actually forms part of the low pressure shaft and is considered the first stage of the low pressure compressor.


How are aircraft controlled that have no forward view out of the cockpit? Modern aircraft tend to have quite reasonable cockpit visibility. But throughout history there were some that had literally no visibility directly forward.

The best known example is probably Ryan Spirit of St. Louis:

but there were others like Supermarine S6. Granted, they were all special purpose planes. But still, how did the pilot align with the intended runway and ensure the runway is clear? There was no ILS to help him, so it still had to be done visually. And how did he avoid obstacles and other aircraft?

You don't need a panoramic view to land, and in Lindbergh's time there was no other traffic to avoid. Navigation is easy enough through side windows (in many modern airplanes you can't see the ground out the front unless the nose is pointed down) and you can just yaw the plane left and right for the times you do want to line up with something. Airfields 100 years ago were quite literally fields, so you didn't need to be all that precise about it. Lindbergh had a periscope and was used to flying mail planes with the front seat occupied with the bag, so it wasn't anything new.

Even today, tail draggers have basically zero forward visibility on the ground so they taxi by watching the edge of the pavement with the occasional sharp s-turn to check ahead.


F-16 "training rounds"? From this Wikipedia article

  A fighter pilot based at Andrews Air Force Base, Billy Hutchison, claims that while in the air he spotted Flight 93 on his scope and planned to first fire his training rounds into the engine and cockpit, and then ram the airplane with his own jet.[135][136]

One of the references refer to these as "practice rounds".

What are practice rounds? Are these a special type of ammunition with inferior performance to standard ammunition for the M61?

From the M61 article I found that these rounds appear to exist: "PGU-27 AB training rounds"

Are these the correct rounds the plane was supposedly loaded with?

The article seems to claim they are both cheaper and more effective than standard rounds, but maybe I'm reading it wrong.

Less effective.  20mm projectiles are normally explosive.  Training rounds are non explosive, basically really big bullets, and are coloured blue.


Which airplanes require two (or more) pilot/operators? There is a related question about when regulations require a co-pilot. This question is about which airplanes cannot be physically flown without at least two people operating it.

Are there any airplanes currently in existence with such a requirement?

This can be because there are controls or systems that must (as in fall-from-the-sky-must, not FAA-must) be controlled or monitored by people in different stations. And by "flight," I mean taking off, controlled flight, and landing (in one piece) - and not any mission specific, tactical or navigational requirement.

Related: When does a flight require a copilot?

The question boils down to,

  "Are there any critical flight controls that are out of reach of the
  Pilot In Command?"

Some older aircraft that required a Flight Engineer in the third seat (not the copilot) and controls at that station that were not available to the pilot or copilot. Some older variants of the 747 (pre 400 series, not sure if any are still in service in their original config) had a required 3 man crew. The Concorde (when it flew) had its fuel system controls at the FE station and those had to manned in flight as the fuel had to be moved around during different stages. You may be more likely to find these older airframes/configurations in cargo service these days. For these planes the legal requirement is often 3 people (not 2) but from a pure "get it off the ground, fly it, land it" case you need at least 2 people to operate it.

The FE station looks something like this in most planes (747 in this pic)

(source)

The Concorde had a pretty daunting panel as well

All that being said you would need at least a Pilot and FE to physically fly the plane since the pilot cant reach all the controls.

Modern avionics controls (glass cockpits mainly) have allowed more information to be presented directly to the pilot in a more condensed manner and allows displays to have multiple functions unlike gauges which tend to display only a single datum.


Do Red Bull Air Race aircraft make use of nitrous oxide injection? Microsoft Flight Simulator X contains Red Bull Air Race missions where the player flies an Extra 300S through a race course and makes use of Nitrous Oxide injection for the engine.  While I have heard of NO injection for automobile engines, I have never encountered an aviation engine which made use of this.  Can anybody verify if RBAR teams make use of this?

This is from the 2014 season

http://airrace.redbull.com/en_US/article/lycoming-engines-power-behind-air-race

  In previous years, the race teams were allowed to modify their
  aircraft, but since 2014, the engines and propellers have been
  standardised for all teams. The Lycoming Thunderbolt AEIO-540-EXP
  engine and the Hartzell 3-bladed 7690 structural composite propeller
  have both undergone rigorous testing and have performed to
  specification.

Here's a link to a smaller engine using nitrous to get 300 HP out of a 100 HP engine for record setting purposes

https://books.google.com/books?id=DhLSUHx136QC&pg=PA36&lpg=PA36&dq=lycoming+io-540+nitrous&source=bl&ots=UwecpWRA8S&sig=RgiMHhHnQuHJYph0TASw0Y6bB2o&hl=en&sa=X&ved=0ahUKEwieiZTGqf7aAhVxUd8KHc5uBLoQ6AEIhAEwDA#v=onepage&q=lycoming%20io-540%20nitrous&f=false

and even more HP here

https://en.wikipedia.org/wiki/Bohannon_B-1


Why does a sport certificate have no distance restriction while recreational does? A recreational pilot certificate only allows flights within 50 miles of home airport. A sport certificate has no distance limits even though it's easier to get - no medical certificate, less flying time and less instruction. What's the reason?

Because a sport pilot certificate requires the applicant to undergo at least three hours of cross country flight training whereas a recreational pilot certificate does not have that requirement.

I always got the impression that a recreational pilot certificate was essentially for people who had soloed and did not want to accomplish any other aviation goals save flying around their local airport.  Instruction is restricted to pre-solo aeronautical knowledge and skills.  It’s pretty limited in capability compared with a private pilot certificate, which explains why there are less than 600 recreational pilots registered by the FAA.

And one can expand a recreational pilot certificate to permit PIC on cross country flights over 50 nm, but this will require additional training and logbook endorsements by an instructor


What is the "departure cable" that tower mentions? Sometimes the tower will advise aircraft that the "departure cable" is up or down. What are they referring to?

KPDX is one airport where this can be found.

The "departure cable" refers to an arresting gear cable on the departure end of the runway, similar to the one used for landing on aircraft carriers. Airports that often have fighter jet traffic may have an arresting system installed near each end of the runway.

Fighter jets tend to take off and land at higher speeds than other aircraft, and don't have the same braking power that other jets do. The speed brakes may not provide substantial drag, and the engines typically do not have reverse thrust capability. Since there is more room on a runway than on an aircraft carrier, land-based systems take more distance to stop aircraft.

The system is not used for every landing, but is useful to have in certain situations. For normal landings or aborted takeoffs, the system on the departure end will be used to prevent a runway overrun. If an aircraft needs to make an emergency landing, it will use the system closest to the approach threshold. Some cable systems are able to retract into a recess in the runway when not in use, which prevents issues when aircraft roll over it.

When the system is deployed in normal use, the tower will advise aircraft that the departure end cable is up. The departing aircraft will generally be in the air by the time it reaches the cable, but in case of a late rotation or aborted takeoff, the pilots can anticipate running over the cable.

Arresting gear is indicated on the FAA airport diagram as follows:

Source

The cable is marked by 10-foot diameter yellow circles on the runway.

Source: Google Maps


Why can't helicopters fly at high altitudes? I recently watched a movie and in the movie it stated that the helicopter could not fly to the top of mt.everest to rescue the person stuck on the top. So why can't helicopters fly at such high altitudes?

Don't believe everything you see in the movies. Doubly so if the movie originated in Hollywood.

The world altitude record for a helicopter is 40,814ft (12440m), some 11,000 feet (3353m) higher then Everest, set in 1972 Reference: Wikipedia

In 2005 Didier DelSalle famously touched down on the summit of Everest in an AS350 Squirrel Reference: Wikipedia. See below for a video.

Both feats involved specially prepared aircraft and highly experienced crew.

As has already been pointed out, a helicopter, just like any other aircraft, has a ceiling above which they cannot fly. Since it's clear that it is possible to fly helicopters to the required altitude in certain conditions, this is just a red herring.

The real reason helicopters don't routinely fly to those altitudes is that they're not designed to. As with everything in aviation, performance is a trade off between engineering, aerodynamics and commercial pressures.

There's a big market for helicopters that can fly up to, say, 12-15,000ft (3600-4500m). The market for helicopters capable of rescuing climbers from the top of Everest is, at best, limited, and the technical demands of building a craft routinely capable of such a flight will push the costs up dramatically.

In short, they don't because nobody will pay for it.


Why is KOAK's alternate minimum higher than standard? Why is KOAK's alternate minimum higher than standard?

According to TERPS (FAA Order 8260.3C) section 3-4, alternate minimum for PA is the higher of PA minimum or standard alternate minimum(600'-2).
KOAK ILS 12 minimum is 200'-1/2. Standard alternate minimum is higher than that PA minimum. Thus, alternate minimum should be 600'-2(standard). But actually, alternate minimum for KOAK ILS 12 is 1400'-3(for CAT D aircraft).

I'd like to know how alternate minimum to be determined.

regarding non-standard alternate minimums -

From the US Terminal Publications, SW-2:
OAKLAND, CA
METROPOLITAN
OAKLAND INTL (OAK) ILS or LOC Rwy 12:

ILS, Category C, 700-2; Category D, 1400-3

If you plan the KOAK ILS RWY12 to Circle to another runway (e.g., RWY30, RWY28L/R, etc.) the published "no approach light" (ceiling/vis) minimums for your approach are:

CAT A/B: 600-1 1/4 (meaning Alternate Minimums are standard, i.e., 600-2, because 600-2 is higher than 600-1 1/4, the published ceiling/vis minimum without approach lights).

CAT C: 700-2 (meaning that this becomes the non-standard minimum because it's higher than the standard alternate minimum of 600-2)

CAT D: 1400-3 (meaning that this becomes the non-standard minimum because it's higher than the standard alternate minimum of 600-2).  Since 1400-3 is VMC, no "without approach light" additive (the 1/4 mile) is applied.

NOTES:

See inop component table for visibility additive -which is 1/4 mile vis added- when approach lights are considered inop, as required by TERPS when determining alternate minimums.
From 8260.3C, para. 3-4-1 a. (2): PA. For each approach CAT, select the PA’s ceiling and the PA’s visibility (without approach lights). When more than one line of PA minimums is published on the same chart, use the line with the higher HAT..


Can someone identify these drop tanks? I was wondering if anyone can help me identify either of these drop tanks?  The one in front in the top photo measures about 11' long.

(Source of second photo-- this related ASE question)

I'm going with tip tank for a Lockheed P-80 Shooting Star.


Why do wingtip vortices affect the upwash ahead of the aerofoil? I understand that wingtip vortices increase the downwash aft of the aerofoil and the theory that they will alter the effective airflow angle, reducing effective angle-of-attack, with the total reaction of lift acting at 90 degrees to Effective airflow, not the relative airflow which now has an extra component of induced drag.

But why does this happen? How do the vortices increase the upwash ahead of the airfoil when they occur rearwards of the wing?

This is a fascinating topic, and it really boils down to the mechanics of lift. Physically, upwash and downwash are generated as the air accelerates/decelerates and curves around the LE of the wing. At the wingtips, higher pressure air curves upward and generate the vortices we are familiar with. In fact, there are vortices generated at the trailing edge of the wing, too, just not as noticeably. Obviously, this picture is unsatisfying since it doesn't give us any insight to lift.

According to the potential flow theory (irrotational, incompressible and inviscid), lift is generated whenever there is a net circulation of air. A particularly successful model, the Lifting Line Theory, models the span of the wing as a collection of U-shaped horseshoe vortices: there's a bound vortex at the span location, and two trailing vortex arms extending to the far field.

So there you have it. A tiny downwash is generated by an individual vortex as it turns the flow downwards at that location, and a tiny upwash is generated as it turns the flow upwards. If you proceed to do the math and sum over all the influences at any point on the span, the net effect is a downwash (which changes the effective AOA and gives rise to induced drag as you mentioned).

If you do the same for a location ahead of the LE, you will get a net upwash, and inverse for TE. This seems intuitive: if you zoom out and treat the whole wing as one single circulation, the circulation should be clock-wise (assume air comes from the left), with air moving upward to the left and moving downward to the right.

There are more complex models, like vortex lattice, vortex panels, etc. But this theory is fairly intuitive and introduces the induced drag. By the way, induced drag is a finite span phenomenon. If you have an infinite span, you fully recover the 2D lift coefficient and the induced drag disappears, even though the vortices remain.


What appears on EGT Gauge if the mixture is rich or lean? I have some questions on interpreting Exhaust Gas Temperature (EGT) readings.  Can you give me some input on:

What appears on EGT Gauge if the mixture is lean or rich?
what is the normal range, high range, and low range for EGT?
What is the relationship between the EGT and rich or lean
mixture?

The exact number on an EGT gauge is usually meaningless: The Alcor EGT gauge you'll find on many aircraft has no temperature markings on it - just 25-degree divisions, and a yellow "reference pointer" which the pilot sets to the maximum observed EGT:

The notable exception to this is turbocharged engines which have a Turbine Inlet Temperature (TIT) limit to protect the turbocharger. For engines with exhaust gas / turbine inlet temperature limits these limits will be specified in the aircraft's Pilot Operating Handbook, and usually marked with green/yellow/red arcs on the relevant gauges.

Exhaust Gas Temperature (EGT) provides pilots a way of monitoring the fuel/air mixture in the engine. It uses the stoichiometric mixture (where Fuel and Air are perfectly balanced so that there is no unburned fuel and no unburned oxygen at the end of the combustion event) as a reference: At this mixture the EGT is at its hottest ("Peak EGT"). Making the mixture richer or leaner will reduce the EGT, and all other mixture settings are described in terms of "Degrees Rich of Peak" or "Degrees Lean of Peak".

Refer to the handy chart from Lycoming below for the remainder of this discussion:

EGT and Mixture

When we talk about mixture in piston aircraft engines we're generally interested in one of two things: Best Economy - getting the most power from the engine for the least amount of fuel, or Best Power - Getting the most power from the engine regardless of fuel burn.  We're also interested in something called Detonation Margin, which in simple terms means "how likely is this mixture to explode rather than burn nicely?"

As you can see on the Lycoming chart above the EGT provides us a way of finding those mixtures.

Best Economy Mixture

The "Best Economy" range begins at peak EGT, and continues on the lean side until the engine won't run smoothly anymore (usually the limit is about 100-150 degrees Lean of Peak).
Operating the engine at Best Economy mixture provides the most power for the least amount of fuel, and thus the longest endurance. It comes at the cost of a reduced detonation margin on the fuel/air mixture - operating at high power settings with a best economy mixture increases the chance of detonation.

  Most aircraft/engine manufacturers recommend Best Economy mixture for power settings below 75%, in part to reduce the risk of detonation.

The Red Box - Where you don't want to run your engine.

The area of the chart from peak EGT to about 100 degrees Rich of Peak is often referred to as "The Red Box" -- in this range the fuel/air mixture has a low detonation margin, and the combustion event is producing the most internal stress on the engine components (cylinders, pistons).
As you can see from the Lycoming chart the Cylinder Head Temperature (CHT) peaks in this range, and there is a risk of exceeding the CHT limits and seriously damaging your engine by operating in this range for extended periods of time, particularly at high power settings.

Best Power Mixture

From approximately 100 degrees Rich of Peak to 200 degrees Rich of Peak is the "best power" range - in this mixture range you can safely run the engine at high power settings (from 75% of rated power to full rated power) with limited risk of detonation. The engine is producing the most power it can in this range, which is useful if you need to climb over an obstacle in your flight path.
For simple aircraft with fixed-pitch propellers the "Best Power" mixture can also be found by adjusting the mixture to obtain maximum RPM for a given throttle setting.

  Operating in the "Best Power" range is not without risks - in particular, detonation is still possible at high power settings. The same Lycoming manual that I pulled the chart above from advises monitoring CHTs when using Best Power mixture settings.

Less Interesting Mixtures

There are two ranges on the chart I did not discuss above - they are what I call the "Less Interesting Mixures"

Leaner than Best Economy

As you continue to lean the engine past the "Best Economy" range you will notice a sharp drop-off in power. This drop is so precipitous that you will no longer be saving fuel as you leave the Best Economy range: You will be burning more fuel per unit of horsepower produced than you were in the economy range.
Eventually the engine will run rough as the mixture becomes too lean to burn, and at the very extreme end the engine will quit, starved of fuel.

Richer than Best Power

Mixtures richer than Best Power don't produce the same precipitous drop in horsepower, but they consume substantially more fuel per unit of horsepower produced. They also have other negative side effects, like producing black sooty exhaust that will get all over the belly of the aircraft.
As the mixture gets richer it will eventually become too rich to burn, and the engine will begin to run rough as it did being leaned. At the far extreme the engine will become flooded, and cease to run.

Recommended Reading / Viewing

Your engine manufacturer's operating instructions
Mixture Magic (John Deakin)
EGT, CHT, and Leaning (Mike Busch)
Leaning Basics (EAA/Mike Busch) (video)
Leaning: The Advanced Class (EAA/Mike Busch) (video)


How can a plane fly far longer than designed? The current record for longest commercial flight is 21,602.22km by Boeing 777-200LR.

However the designed maximum range for B777-200LR is 15,800 km. So how is this possible?

The design range is fully loaded.  This flight had 35 passengers, no cargo, full fuel and was run as a project and flown by a test pilot.  It was not a commercial flight in the meaning that most people would recognise.

They had to wait for weeks until the winds aloft, especially the jet stream, were at their most advantageous.


Polars for multi element foils? Where can I find polars for multi element foils?

Unless I missed them, couldn't find any on airfoiltools.com

Is there a family of multi-element foils like Naca,or Selig, etc.?

Your question is too general. It would be more helpful if you specify the flight conditions and the types of aircraft that you are interested in. For instance, give us a range for the Reynolds number.

However, I have a fast answer for both high and low Reynolds. For high Reynolds ($Re \geq 10^6$), NLR-7301 (main and flap) and MDA (slat, main and flap) are airfoils for which you can find published experimental and numerical results (NASA, AGARD, etc.). For low Reynolds numbers ($Re < 10^6$), there are really not many experimental results that I know of. In fact, I was once lucky enough to find an airfoil catalog of F.X. Wortmann (printed format and in German) that had aerodynamic results for some fowler flap airfoils. Also, during my diploma thesis, I worked on an aerodynamic method for the fast analysis of multielement low Reynolds airfoils (based on XFOIL) and thus I used my method to analyze certain multielement low Reynolds number airfoils that I designed (starting from the S1223 airfoil of Selig). The document can be found here but it is written in Greek. Nevertheless, you can check the figures (NLR-7301 is also included in my study and you may trace the relevant references for further info).


Is the turning error of a magnetic compass greatest when tuning through South in the Southern Hemisphere? Question:

The turning error of a magnetic compass in the Southern Hemisphere is greatest when turning through South.
Is the statement true or false?
And why is this/isn't this the case?

“The turning error of a magnetic compass in the Southern Hemisphere is greatest when turning through South. Is the statement true or false?”

True

I am going to answer this question for a second time and delete the first try. It was a mess. Hopefully, this one will make more sense even though it has the same information. Just a better structure.

There are three factors that would explain why turning errors are greatest when turning through North and South. They are specific to the aviation wet, liquid-filled, or whiskey compass. Officially, two of them are magnetic dip and acceleration errors caused by magnetic dip. Unofficially, the final one is the counterintuitive way that you have to read the compass.

The magnetic compass turning errors that we usually identify as UNOS and ANDS while in the Northern hemisphere are caused by a combination of magnetic dip and the off center balancing of magnetic dip. As you have stated in your comment, a compass has the tendency to dip it’s North-seeking element down towards the Earth in the Northern hemisphere. The same is true for the South-seeking element in the Southern Hemisphere. The Northerly turning errors (UNOS) are greatest (up to the same amount of error as your degree of latitude) when turning through North or South.

In the picture below, the black text is the actual direction to which you are turning. The red text is the amount of compass error at 30° latitude.

An aviation, liquid filled, “whiskey” compass works the same way as a handheld lensatic compass. It has a magnetized bar held in place on a low friction bearing, needle or pin point. In the case of the whiskey compass, there is also a card noting the bars direction attached to it that is read counterintuitively.

To better understand this, remember, with a whiskey compass (as opposed to a dry, vertical card, compass) you are reading the directional card from the opposite side of the actual direction that you are facing. You read the North direction from the end of the side facing South, and the East direction from the end of the side facing West. Picture it like the lid of a bottle or can. If you are looking at it from above, you can mark “N” at the edge of the top closest to North. If you are looking at it from the side, you can not see the top. So, you put an “N” on the side furthest away from North because that is the only side you are able to see when facing North. The same goes for the other directions. Therefore, East is physically to the left of North (clockwise when viewed from the top) on the direction card. The numbers of degrees is read in increasing increments from right to left. A good mnemonic memory aid for this is, “If you always run/turn away from the one you want, they will chase you.”

The magnetized bar will always try to align itself or stay aligned with the magnetic flux lines of the Earth. The Earth is a giant magnet. The flux lines Of the Earth exit the Earth at its magnetic South Pole and re-enter it at its magnetic North Pole all the way around the Earth. This places the flux lines comparatively horizontal (parallel to the ground) at roughly the equator, and vertical (perpendicular to the ground) at either Pole. The closer you get to either Pole (the higher the latitude), the more the compass bar will try to align itself vertically.

To counteract this phenomena, compasses are sometimes weighted to move the center of gravity of the bar & card arrangement closer to the side of the bar that would point upward, away from the Earth. In the Northern Hemisphere, the CG would favor the magnetic bar’s Southern pole. In the Southern Hemisphere, the CG would favor the magnetic bar’s Northern pole. This is done so that the bar or anything attached to it will not touch the inside of the compass housing. Any type of contact, friction, or drag could stop the compass from moving and realigning itself freely. When the compass is not weighted to move its CG, a compass that is suspended from its top will move its CG towards the opposite direction of the compass’s magnetic dip on its own.

The CG of the compass’s moving parts is very important in its movement. It creates inertia. The inertia of the offset CG will be apparent whenever there is a change in speed or a change in direction of movement. The CG will always want to remain at the same speed and travel in the same direction as it was. The CG will want to lag behind during acceleration, keep moving forward during deceleration, and keep going straight, swinging to the outside of a turn/curve. Picture it as the same phenomenon that will cause a glass marble in a Petri dish to roll around due to velocity change. Changing speed and/or direction will cause the compass to become misaligned until speed and direction are stabilized. When facing North in the Northern hemisphere, a right hand turn to the East will cause the compass to initially spin clockwise (relative motion to the aircraft) to the West backside indication. Here is a good article on turning errors that explains it much better than I can. Or, this one from AOPA.

In the case of a compass where an off-center CG is not affecting the compass, aircraft banking will cause magnetic dip to swing the directional card out of alignment. When facing North, banking to the right (East) will cause the compass to initially spin clockwise (relative motion to the aircraft) to the West backside indication. Due to this directional indication from the perspective of the backside of the directional card, magnetic dip while in a coordinated bank (where g-forces keep the weight of all objects inside the cockpit acting perpendicular to the floor) would cause the compass to initially indicate a turn in the opposite direction of the actual physical turn of the aircraft if your direction is North in the Northern Hemisphere and South in the Southern Hemisphere. This means that the compass would initially spin clockwise in the above North to East turn. The compass would then lag the actual turn until the bank is brought back to level, or your aircraft reaches an East or West heading. This article, as well as this one here both explain the above phenomenon better.


How can an international MRO apply for FAA approval? I own a small maintenance, repair, and operations (MRO) shop in Indonesia. We are having trouble with our customers that required us to obtain FAA approval in order to get order from them.

I already tried to ask the Indonesian regulator to help us to obtain FAA approval, but no luck. FAA requires an MRO to maintain at least one N-registered aircraft, and we don't have any in here.

Does anyone have same experience? And how can we proceed?

These 14 CFR Part 145 Forms might be the forms you are looking for however there may be other forms for applying from overseas.

This site (FAA Aircraft Repair Stations Homepage) has some good links and here are the pre application regulations you need to comply with.

Here is the letter of the law of the FAA regulations (Starts at Title 14 Section 145.1).

I am no lawyer and cant really comment on this but I have no idea how FAA regulations work outside of the country and how/what foreign entities must comply with. I would advise contacting a local lawyer as well as a US lawyer about this.


Would a Chinook helicopter fly better with contra-rotating along with counter-rotating propellers? Would a Chinook perform better or have any advantage with dual contra-rotating propellers over a Chinook with just normal counter-rotating propellers?

No.  The RC model there is using counter-rotating rotors because the upper rotor with the stabilizer bar has a specific function that makes the model controllable in the first place.

The Chinook already benefits from built-in anti-torque control so that would be a waste of time and oodles of money on the real one.

Fun fact about the Chinook: it can climb better, and fly on less power overall, going sideways. Both rotors are able to bite clean air, achieving translational lift, when going sideways.  When going forward, the rear rotor has to cope with the wake of the front one.


Why does an aircraft as big as the Airbus A400M use turboprops? Why is it that a plane as big as the C-17 (the A400M is slightly smaller though) use turboprops instead of turbofans? Both aircraft seem to have the same purpose and are both capable of landing at short runways. So why does the A400M have turboprops?

I want to know why two very large planes have different propulsion systems.

Both aircraft seem to have the same purpose

Not entirely.

The RAF see the C17 as a strategic airlifter, the A400 is seen as a tactical airlifter replacing the turboprop C130.

Strategic airlifters are more likely to make longer, higher-altitude journeys. Where jets are more fuel efficient. They are more likely to operate between airports with concrete runways where FOD is less of an issue.

Tactical airlifters are more likely to make shorter, lower-altitude journeys of the sort where turboprops are more efficient. They are more likely to operate between smaller airfields where FOD may be more of an issue.

References:

ThinkDefence
Airbus
RAF

Some random internet website alleges the fuel costs in \$ per Nautical mile are 19.63 for A400 and 88.14 for C17. They don't say for what mission profile. At a minimum, this illustrates that there is some motivation for matching engine types to expected mix of mission types.

The similar sized AN-70 also uses turboprop engines, illustrating that this choice is not unusual for this sized aircraft.


Are engines fed differently in reduced gravity aircraft? Reduced gravity aircraft are basically airliners without passengers seats. As airliners, those aircraft were not designed to fly for a long time in zero-G. Specifically engines were not designed to be fed with fuel for several tenth of seconds in reduced gravity conditions. I'm aware that aerobatic aircraft are equipped with devices to feed the engine in negative-G conditions for a limited amount of time, but what about the reduced gravity aircraft?

EDIT: aerobatics aircraft are equipped with header tanks and/or flop tube able to get fuel in tank even if flying inverted (e.g. flop tube's end goes to the bottom of the tank, bottom defined in Earth frame of reference, whatever the attitude of the airplane). This design allow pipes to always be inside the liquid fuel of the fuel tank, and thus pumps can do there job (pump liquid). But when in zero G, liquid is no longer attracted toward one side of the tank (either the bottom or other side) and thus pipes may be fed with a mix of air (or whatever gas or vapor that is inside the fuel tank) and liquid fuel. Pressurized tanks do not change this. Yet, engine must be fed in fuel for several tenth of seconds in those condition, and I doubt fuel contained in pipes can last more than few seconds. Devices to fed a reduced gravity aircraft's engine should be different from the one in aerobatics in at least two way:

aerobatics are designed to operate in inverted flight (not zero-G) and are in zero-G condition for a small amount of time (less than a couple of aerobatic maneuvers before being again in normal or inverted flight)
reduced gravity aircraft are airliners (may be modified, but how?)

Zero 'g' aircraft are basically commercial aircraft.

In general, it is mainly the fluid systems like the fuel and hydraulic systems in the aircraft which are affected by the zero gravity conditions.

In the case of jet engines, the fuel is transferred by pumps operating under pressure and the fuel injection is also under pressure in the combustion chamber. Consequently, the engine fuel system does not need any modification. In a few aircraft, the fuel tanks themselves are pressurized (like C-5) but this is for safety and wouldn't affect operations.

The engine oil and lubrication system is also usually pressurized and can operate briefly under reduced gravity conditions.
Work done by NASA indicates that aircraft can be used in zero gravity conditions without any major modifications to systems.

The zero 'g' aircraft don't operate in weightless environment for more than 20-30 seconds and the engine systems should be able to cope during that time. The main concern appears to be the training levels of the pilot.


Would an aircraft with no ailerons, no elevators and no rudders be safely flyable? With contrarotative propeller (or not, if twin engine), cyclic and collective blade control.

Would such an aircraft be safely flyable, and how efficient would it be, assuming it can have perfectly smooth wings (and tailplane)?

In case of engine stop (allowing axle free spin and control on blades), could it control its glide and land on the runway, with the propeller's blades in an almost feathered autorotation configuration, allowing attitude control and minimum disk drag?

(like autorotating one reversed Kamov on its rotor-head, in a skydiving wind tunnel blowing a bit slower than terminal velocity)

Edit: If it goes twin engine and tailless (and still controlesurfaceless) how active cyclic pitch control would be necessary to allow use of non-reflex wing's airfoil?

Edit2:
Switch from thrust + attitude control mode, to no-thrust + attitude control "reversed autorotation" mode.
Reversed rotation allows most efficient use of blade's airfoil camber.

An interesting thought. Control the aeroplane through its propeller(s), like a helicopter does.

Propeller torque differential would control roll
Propeller cyclic would control both pitch and yaw
Propeller collective would control engine thrust, like already done in constant speed propellers.

The thing that immediately comes to mind is moment arm for pitch and yaw. Helicopter blades are relatively long, and the rotor is mounted about halfway along the fuselage. The prop is limited in its blade length due to ground clearance.

With a configuration like above, longer propeller blades can be mounted so the pitch and yaw moment arms can be extended. As @Sanchises points out, placing of the propeller like this creates a coupling between pitch and thrust - not a bad thing, increasing pitch controllability by controlling thrust. A strong nose wheel might be required for take-off.

Not sure about your reference to the skydiving Kamov, but the aircraft could glide down after engine failure while keeping enough RPM to control cyclic. It would be a bit draggy though, with the drag comparable with a parachute of the same diameter as the propeller. Autorotation works best with a large blade moment of inertia, and the propeller would definitely not have the optimal blade length for that.


How much force are large aircraft brakes able to exert? In a comment left on my answer here Jan Hudec says:

  Note that usually on dry surface the brakes exert much less force than the tires could handle.

This got me wondering how much force large aircraft brakes can provide.

The limiting factor in brake force for a vehicle is the friction between the tire and the surface. On my car, even on dry pavement, the brake force available is always greater than the tire/ground friction, so I can always lock the brakes and set off the ABS. I may be misunderstanding it, but Jan’s comment makes me think that maybe large aircraft don’t have that much brake force available. If you commanded maximum braking on a dry runway would the brakes try to lock? Is the ABS really only necessary on a contaminated runway?

This has been answered before, but in a different context. Therefore, I copied the essential part here:

Emergency procedures at take-off weight will produce the biggest braking loads. I will calculate the highest possible braking loads for a Boeing 747-400 that weighs up to 400 tons on take-off. For this I need this plot of a polynominal for the braking coefficient, which is the ratio between vertical and horizontal forces before the tire skids. I do not know the source; I collected it somewhere in the past and never found a reason to doubt its validity.

Shortly before the aircraft comes to a stop, the highest friction coefficient is reached, and then little lift is produced by the wings, so the vertical tire loads are those of the static case. The braked wheels are the 16 main wheels; I assume that the unbraked noswheels carry 4% of the total weight. At 96% of 400 tons acting on those 16 wheels, this is 24 tons = 235,344 N = 52,907 lbs of downward force per wheel. Since the friction coefficient is 1 at low speed, the same load is transferred horizontally from each wheel to the ground and into the gear struts.

And, yes, brakes will lock on a dry runway if the disk brakes are fully engaged.


If I have multi-engine land, can I learn multi-engine sea directly? Let's say I hold PPL privileges for Multi-Engine Land. Now I want to start my seaplane training.

Can I begin my training directly on a multi-engine seaplane? Or, do I have to get a license for Single-Engine Sea first before I can proceed to multi-engine?

As shown on page 77 of this FAA document, the various tests needed for an AMES (air multi engine sea) rating depend on the ratings you already hold and your area of operation. Depending on your area of operation, holding an AMEL (air multi engine land) rating can allow you to skip most or all of the required tasks for an AMES rating. In many cases, an AMEL rating is actually better for getting an AMES rating than a single engine sea (ASES) rating will be. So in short, you do not need an ASES rating first, but having one can make the process of getting an AMES rating easier in certain areas of operation.


If jets carried pure O2 to use for combustion, would they be more efficient? Air is around 21% Oxygen. The bulk of air is non-flammable Nitrogen. Wouldn't surrounding the jet fuel with only O2 create more efficient and clean combustion?

Vehicles which carry their oxidizer with them are called rockets. It is a good thing that air is not pure $O_2$, because the nitrogen, being employed as a process gas,  helps to get useful work from the combustion process. By being heated and accelerated in the combustor, it does the main work of driving the turbine and turning heat into mechanical work. Engines would be even cleaner if less oxygen is around, because already at 21% $O_2$ they run at fuel-to-air ratios well below the stoichiometric optimum.

If you consider the amount of air flowing through the core of a modern turbofan, you will see that the amount of oxygen which needs to be available will dwarf the amount of fuel, and most of it would not burn but would replace nitrogen in its role as the process gas. It could not be carried with the airplane nor isolated from surrounding air if any meaningful payload and range should remain.

For some background on the combustion process, please see this answer.


What are these fins on the Beechcraft 1900? Here's an image for you to see these little fins. What are they used for?

Source (image has been modified to indicate location of fins).

Well, those fins were put there to increase yaw and pitch stability on the D version because of the change in the centre of gravity due to a higher fuselage from the C version. This D model also had more powerful engines than the C model. To add on, the strakes at the bottom were added for the same reason too.

More info can be found here.


How to calculate the heading correction with a circular flight computer? While flying between 2 points 120 nautical miles apart, after about 40 nautical miles we discover we are 5 nautical miles off to the right of the track.

What heading correction is necessary in order to reach the destination? How do you calculate the correction using a circular flight computer?

1 - First I calculate the track deviation angle:

To find the track angle:

outer scale: distance off track (NM) in this case 5 NM

divided by

inner scale: distance already travelled(NM) in this case 40 NM

To do this I divide 5NM per 40NM as per the picture below, the division is within the red circle and the result is pointed by the red triangle that says '60' on the other red circle with the arrow pointing to it.

the result is 7.5 as you can see on the picture below.

2 - Then I calculate the closing angle for the remainder of the journer:

To find the closing angle:

outer scale: distance off track (NM) in this case 5 NM

divided by

inner scale: distance to go (NM) in this case 80 NM

The remainder of the journey is (120NM - 40NM) = 80NM
The current deviation which is 5 NM.

To do this I divide 5NM per 80NM as per the picture below, the division is within the red circle and the result is pointed by the red triangle that says '60' on the other red circle with the arrow pointing to it.

the result is 3.77 rounded down to 3.7 as you can see on the picture below.

3 - Then I calculate the total heading correction:

total heading correction to reach the destination = track error + closing angle

that is:

7.7 + 3.7 = 11.2 degrees

so the heading must be corrected to 11.2 degrees left.


What is communicated between flight crew and cabin crew during flight? During a typical trans-atlantic flight, can anyone list communication between flight crew and cabin crew?

I am interested in "mandatory", i.e. safety-relevant communication between the flight deck and the cabin, such as the before take-off and before landing notifications from the flight deck to the cabin crew.

What information must be communicated (by law) according to, e.g., ICAO or a major country jurisdiction?

Let's exclude any information regarding passengers, pax health issues, operational delays, meals, anything that is not directly safety relevant.

The FAA at least would seem not to have any requirements of communication between flight and cabin crews. However, airlines must have guidance or standard procedures for such communication. One thing I have always heard--but cannot find in any regulation--is a PA announcement to the effect of "cabin crew, prepare for takeoff;" I can only assume that all airlines have adopted this.

This is not to say that communication does not occur; quite the opposite in fact. Cabin and flight crews communicate regularly about all sorts of issues, from upcoming weather to passenger issues and operational concerns. It is only that such communication is not required by regulations (at least not in the United States).


Who manages International Airspace Traffic? For example, if a plane is flying from Amsterdam to Miami and is in the middle of the ocean, who manages that traffic? The ARTCC? Or the airplane just flies without communicating for some hours until it reaches an ARTCC area?

Global ATC in general
In the civil aviation domain, international rules are proposed by ICAO. The foundations of these laws have been agreed in 1944 under the Chicago Convention, in particular its Annex 2, Rules of the Air. Nearly all countries are signatories of this convention.
ICAO global airspace structure is divided into large Flight Information Regions (FIR -- their upper layer being conventionally named UIR). A FIR over a domestic airspace is under the responsibility of the country below. A FIR over the international airspace (e.g. airspace over international waters) is under the responsibility of a single ICAO member state which has received a delegation from ICAO.
Within a FIR, enroute services are provided by Area Control Centers or ACC (ARTCC is the FAA local name for ICAO ACC). The first services available (and guaranteed) are information and alerting. Control services like separation can be provided, depending on the FIR.
(That's unfortunately a lot of acronyms, and more are coming soon... I give them only for precision in case someone wants to dig further into details, but you don't need to remember them to understand the system.)
Specificity of Oceanic Control
When a FIR provides control services over an ocean area, such area is named Oceanic Control Area or OCA, and the corresponding ACC are named Oceanic Area Control Centers (OAC). OAC and ACC differ by the procedures they can use to provide their services: ACC are able to track aircraft using reliable radars, while OAC mostly work with voice/data position reports from crews over unreliable long distance communications (that's less and less true with modern means like space-based ADS-B and ACARS/CPDLC/DCPC (see Guidance Material for ATS Data Link Services in NAT Airspace here).
North Atlantic case
All flights which want to maximize (eastbound) or minimize (westbound) the effects of high altitude jet streams of the day over the Atlantic Ocean must use the crowded North Atlantic Organized Track System (NAT-OTS, not to be confused with NATS Holdings, the UK ANSP) when flying at or above FL55. They must obtain an oceanic clearance from the first OAC before reaching their entry point into the NAT-OTS. So to answer directly your question:

The crew using the track system will be in contact with the OCA/OAC controllers during the flight between domestic areas. However to bear with possible unreliability of HF radio transmissions, this contact doesn't have to be permanent. Instead it may occur only at intermediate reporting points spaced by 10° longitude.

They will also be in contact with FIR/ACC controllers prior to entering and after leaving the OCA/OAC.

Transition areas (OTA) exist to bridge between ACC and OAC: See areas ending in "OTA" in the Shanwick FIR/OCA, like BOTA for Brest OTA, bridging Shanwick OTA (UK) with Brest FIR/UIR (domestic France). In OTA, traffic is handled by the regular domestic FIR (actually its UIR, given the altitude flown at this stage) rather than the OCA.

North Atlantic ocean is covered by 6 OCA managed by different ICAO member States: UK (Shanwick), Iceland (Reykjavik), Canada (Gander), Norway (Bodø), USA (NY West) and Portugal (Santa Maria).

North Atlantic OCAs (source ICAO, now removed from the site)
A common route for the flight you mention will cross Shanwick and Gander OCA.

(source FlightAware)
FIRs and their boundaries worldwide can be displayed with this ICAO GIS viewer (be patient at the first use, background resources take time to load):

(source: ICAO FIR viewer)


How is varying modulation depth achieved by localizer ground transmitters? An approaching aircraft's navigation receiver uses the difference in depth of modulation between the 90Hz and 150Hz signals which are amplitude modulated onto an RF carrier (on a channel between 108Mhz and 118MHz).

My understanding is that the ground transmitter is formed by several pairs of transmitters forming two interleaved phased arrays to achieved directional beam forming.

My questions is how is the difference in modulation depth achieved by the transmitter system based upon aircraft's position relative to the landing strip's entreline? The beam strength decreases as you move away from it's own centreline, so is it actually that the entire modulated signal strength decreases which when de-modulated is effectively a difference in amplitude modulation depth?

The beam strength decreases as you move away from it's own centreline,
so is it actually that the entire modulated signal strength decreases which
when de-modulated is effectively a difference in amplitude modulation depth?

Your question is a really good one. The depth of modulation is the ratio between the modulation amplitude and the carrier amplitude. Changing the relative strength of a signal changes both amplitudes in the ratio, but keeps the ratio unchanged. No DDM can indeed appear this way.
The 90 and 150 Hz tones are not used directly. The heart of the system creating the DDM is two signals: CSB (carrier and sidebands, see below what is a sideband) and SBO (sidebands only) and surprise! they are containing the same quantity of 90 and 150 Hz tones and are transmitted in equal quantity on both sides of the runway.
Now the key points of ILS:

SBO phase depends on the runway side (+/-90°, CSB is kept unchanged)
Phase of the 150 Hz tone is inverted in SBO (90 Hz is kept unchanged).

As SBO contains only the sidebands when CSB and SBO interfere in space, only the sidebands can actually interfere, the carrier found in CSB is unaltered.
When the sidebands interfere, the way they are constructed (by tweaking phases) makes 150 Hz more prominent on the left of the runway, and less prominent on the right side (details below, drawings will clarify). So:

The carrier amplitude hasn't changed in the process
The sidebands amplitude has been altered.

As the sidebands are the modulation, in effect there is a change in the DM (the ratio between modulation and carrier amplitude), and a DDM appears magically, increasing as we move away from CSB, that is as we move away from the centerline.
Countless oversimplified descriptions put the focus on 90 and 150 Hz, each one allegedly transmitted on a specific side of the runway (cf. image above). They miss the point the ILS is not based on amplitude comparison, but on depth of modulation comparison.
Two last points:

A precise location cannot be determined by simply using received signal strength (RSS) of two or more signals. RSS-based positioning improved by averaging algorithms is commonly found for rough positioning (e.g. in cellular networks) because the cost is marginal, but RSS depends on many factors which cannot be controlled.

On the other hand, the phase of a signal remains unaltered until the signal is reflected, diffracted or enters a material with a different refractive index. By taking appropriate protection measures in the Fresnel zone, the phase-based ILS is therefore very reliable. Phase is used in many sophisticated radio applications. Missing the phase role in the ILS (and conventional VOR) is actually missing the cleverness of these technologies.

If this summary increased your curiosity for more details, just follow on, but for a good understanding, we'll need to first clarify a few notions: Phase, modulation and sidebands.
Phase and phase comparison
When representing a signal, e.g. a sine signal, we can look at different parameters. Often we draw the amplitude as a function of the time (right side):

Two representations of a signal: Phase/amplitude and time/amplitude
We can also imagine a circle with a scale from 0° to 360°, a point traveling on this scale in the time of a signal cycle (left side above).
The phase is the value of the angle. We are usually interested in the phase difference of two signals with the same frequency:

Phase difference
The two signals can be in phase (no difference), in phase opposition (180°), in quadrature (90°) or have any phase difference. The phase difference plays a major role when two signals interfere:

If the signal are in phase, the amplitude of the result is the sum of the individual amplitudes.
If the signals are in phase opposition, the amplitudes subtract.
For any another phase difference, the amplitude of the result is between zero and the sum of individual amplitudes.

In the ILS, basically one signal is sent for reference (CSB) and one signal is sent for comparison (SBO). SBO on one side creates additive interferences with CSB, and substractive interferences on the other side. But actually because 150 Hz is inverted in SBO, on the additive side, at the center of the interference area, 150 Hz cancels itself, 90 Hz doubles its amplitude. On the other side 150 Hz doubles its amplitude, 90 Hz is cancelled. As we move away from the center of the interference area, this additive/subtractive effect decreases until SBO is left unaltered because we are out of CSB area.
Now let's clarify the part related to the modulation and depth of modulation.
Amplitude modulation, depth of modulation, sidebands
Modulation relates to a carrier, which is just a fancy name for a constant (usually sine) RF signal at a given frequency, therefore with all the wave energy concentrated at this frequency. The frequency of the carrier is usually very large compared to the modulation.
The modulating signal (or modulation) is the signal to transmit, it can have any shape, but in our case its shape is a 90 Hz or 150 Hz sine curve.
The amplitude modulation consists in changing the carrier instantaneous amplitude according to the modulation instantaneous amplitude. After it's done, the  carrier outer envelope reflects the modulation outer envelope:

Carrier modulated at 50%
The modulation can imprint itself at a variable depth (taller or smaller peaks). This is measured by the modulation index aka depth of modulation. The index value is an amplitude ratio: modulation amplitude / carrier amplitude.
The depth of modulation used for the LOC signal is only 20% (40% for the G/S signal) to leave some room for identification tones on 1020 Hz.
ICAO definition for ILS modulation depth:

The depth of modulation is the ratio of the amplitude of the modulation of
the 90 Hz or 150 Hz signal to the carrier amplitude. The DDM is the
modulation depth of the stronger signal minus the modulation depth of the
weaker signal.

Let's look at how energy is distributed according to frequency.

Spectrum of a carrier AM modulated by a sine tone
Before  modulation, all energy is concentrated at the carrier frequency (blue bar below). The modulation process creates two variable sidebands: lower (LSB) and upper (USB), depicted in magenta.
If the modulation index is 100%, the carrier contains 66% of the energy and each sideband 17% (mathematical explanation). When a 100 MHz carrier is modulated by a 150 Hz tone, the LSB is created at 100 MHz - 150 Hz, and the USB at 100 MHz + 150 Hz.
The larger the depth of modulation, the larger the energy transferred from the carrier to the  sidebands and the lower the energy remaining at the carrier frequency. Sideband energy and depth of modulation are the two sides of the same coin. In the next  paragraphs I'll talk about energy rather than depth, but it's the same.
Modulating the ILS signal with 90/150 Hz tones
The localizer signal is modulated by two tones, the reference at 90 Hz, the signal to compare at 150 Hz. Therefore each sideband energy is split between two  frequencies:

CSB signal spectrum
This signal, known as CSB (carrier and sidebands) will be used as-is in the localizer. However as explained earlier, the in-phase / out-of-phase element must be sent along with CSB.
From 90/150 Hz modulations to CSB/SBO modulations
The localizer transmitter sends an additional signal with an inverted 150 Hz, known as SBO (sidebands only). For this signal the carrier element is cancelled (DSB-SC modulation):

SBO signal spectrum with 150 Hz modulation inverted
The signal is sent in phase opposition with CSB on the left side of the runway, and in phase with CSB on the right side (this is done using phase shifters at the antenna level, see further down).
To summarize:

The localizer transmits CSB on the left and right sides of the array. CSB contains both 150 Hz and 90 Hz with no phase adjustment, and the carrier.

The localizer also transmits SBO which contains only the sidebands, but where the 150 Hz modulation is phase-shifted by 180° on both sides. The SBO signal itself is sent with a different phase on each half of the antenna array.

How CSB and SBO are sent by the antennas
The actual phase between SBO and CSB is adjusted to compensate for space modulation effects, and SBO is sent with a +90° phase relative to CSB for the antennas on the left of the runway centerline, and with a -90° phase relative to CSB for the antennas of the right of the enterline (the two sides of the array still have a 180° phase difference).
The combination of exact opposite phases for SBO signal cancels the EM field at the center, splitting the SBO radiation pattern in two portions, it seems there are two beams for SBO. CSB is radiated centered on this hole:

CSB + SBO radiation pattern
The elements compositing CSB and SBO must be synchrone. Phases are adjusted carefully to form the radiation pattern (i.e. align the loalizer with the centerline and set its width) and to ensure the linearity of the DDM in the central portion.
When spread in space, the three signals mix by space modulation, the principle the ILS is based on and which is more simply known as wave interference.
Space modulation
One signal (SBO) has its carrier suppressed preventing the carrier energy to be increased during the mix CSB+SBO. In contrary when mixing the sidebands from each signal, the space modulation adds the sidebands energy where the sidebands are in phase, and subtract the  sidebands energy when they are in phase opposition.
SBO is in phase opposition with CSB on the left side of the runway and is in phase with CSB on the right side of the runway.

Along the centerline CSB only is present, with equal energy for 90 Hz (green) and 150 Hz (magenta). The LOC indicator will be centered. In order to create a CSB prominent area along the centerline, SBO power is also reduced on the center antennas.

On the left side, 150 Hz modulation has been shifted by 180° in SBO, and SBO signal has been shifted 180° relative to CSB at antenna level. 150 Hz modulation is therefore in phase for CSB and SBO. Sideband energies are added, while energy for 90 Hz modulation is  decreased because 90 Hz modulation in CSB and SBO are out of phase. So there is more energy for 150 Hz (which means there is a larger depth of modulation). This effect is getting stronger as we move away from the centerline.

On the right side where 150 Hz modulation is out of phase for CSB and SBO, 150 Hz energy is reduced compared to 90 Hz  energy in the sidebands, and the difference is getting stronger as we move away from the centerline.

Past 10° from the centerline, CSB cannot be sensed, only SBO can, the LOC indicator will deviate to full scale left or full scale right depending on the phase of  SBO. Another signal is used to ensure a full scale deviation past this point (see clearance signal further down).
If you want to do some space modulation simulation, here are the details of the signals to mix (see details here):

$E_{CSB} = E_C \space \cos \space 2 \pi f_C t + E_{90} \space \sin \space 2 \pi f_{90} t \space \cos \space 2 \pi f_C t + E_{150} \space \sin \space 2 \pi f_{150} t \space \cos \space 2 \pi F_C t$

$E_{SBO \space 90} = K \space \sin \space 2 \pi f_{90} t \space \cos \space 2 \pi f_C t$

$E_{SBO \space 150} = K \space \sin \space 2 \pi f_{150} t \space \cos \space 2 \pi f_C t$

We talked about the sidebands energy. Regarding the carrier, which is only present in CSB, the space modulation result always contains the energy of the original CSB  carrier until reaching full scale (then the clearance signal provides the carrier because while all modulation information is in the sidebands, the constant carrier  is necessary to synchronize and lock the receiver and to retrieve the coherent CSB and SBO from the modulated radiofrequency resulting signal).
Where the DDM finally appears
Now let wrap up:

The carrier energy is not changed by the space modulation as it is present in only one signal (CSB).
The sidebands energy is increased or decreased in the resulting signal.

The energy of the sidebands varies relatively to the energy of the carrier, but wait... this is the same than saying the DM is varying (without actually changing anything at the transmitter level).
The DDM between 150 and 90 Hz, is a function of the receiver offset from the centerline (in the range -5° to +5° the function is quasi-linear).

The space modulation actually creates a DDM
The alignment offset is determined by sensing the DDM in the resulting signal.
Antenna array
The LOC antenna is an array system, usually made of a large number of individual log periodic antennas, to form a narrow directional beam.

Localizer array at Melbourne airport, source
CSB and SBO are not distributed equally to all individual antennas of the array. Before reaching the antennas, their amplitude is tuned to form the three sub-beams, and the SBO signal is phase-shifted:

Individual amplifier-phase shifter-mixer
We have now all the basic elements to draw a LOC system:

Components of the ILS LOC system
Example with an array of two antennas (and final answer to the question)
Let's imagine two antennas fed with our earlier CSB and SBO signals with SBO phase being shifted by +90° for antenna 1 and by 270° for antenna 2.

Two antennas with phase shift of 90° and -90° (270°)
Let's see what happens when the receiver is on the centerline at P0, and at any point P1. For this simplified explanation to work, distance to LOC array (D) must be much larger than distance between antennas and centerline (d).
Receiver on the centerline
The received signal is the sum of the signals sent by the two antennas. As the distance traveled is the same, the phase at the receiver location is also the same.  The signal at point P0 on the centerline is:
$$S_{P0} = K_1 \cdot (CSB - SBO) + K_2 \cdot (CSB + SBO)$$
With $K_1$ and $K_2$ being the attenuation in free space, as the attenuation is the same for both antennas, this can be simplified:
$$S_{P0} = K_1 \cdot [(CSB - SBO) + (CSB + SBO)] = 2K_1 \cdot CSB$$
In CSB, the 90 Hz depth of modulation is the same than the 150 Hz one (see previous figure) There is no difference in depth of modulation (DDM) between the  tones: DDM = 0%.
Receiver at point P1
The distance traveled is shorter for antenna 1, and longer for antenna 2. A phase lead and a phase lag respectively appear for antenna 1 and antenna 2. The phase  lead is also equal to the phase lag. After some simplification that can be read in the linked thesis:
$$S_{P1} (\beta) = 2K_1 \cdot (\cos \beta \cdot CSB - \sin \beta \cdot SBO)$$
with $\beta$ being the angular distance from the centerline. This formula shows:

when $\beta$ is null (receiver on the centerline), we have the result already seen in the previous case,

when $\beta$ is negative or positive, the SBO signal influence increases and the overall depth of modulation between 150 and 90 Hz tones follows the same trend  until reaching -100% or +100%, which correspond to the absence of CSB signal influence at the receiver side.

Actual principle of the ILS, from this source.
The range of difference of depth modulation obtained must be mapped on the actual standardized values for LOC needle deviation, reducing the full range to a  standardized guidance angle. The LOC specification are found in ICAO annex 10 to the Chicago Convention, volume 1, attachment C:

This setting is obtained by tuning the amplifier of the phase-shifter from each individual antenna (the amplifier is actually common to all antennas). This amplifier  reduces the SBO signal influence on the DDM.
Clearance signal
So far we have assumed the localizer signal is only composed of the course guidance mixing the two original signals CSB and SBO. But it is very difficult to obtain  an actual highly directional beam, like the course beam, without creating unwanted lobes:

Radiation pattern of beam antenna, source
The problem with the secondary lobes is an aircraft approaching aligned with one of them will see it as being the main lobe, and will approach the runway using a  wrong course. There are some techniques to minimize these unwanted lobes, but none is practical for large arrays like a LOC system. Alternatives are to block the  unwanted waves after they are radiated, or to flood them, the solution used for ILS.
A replica of the course, the clearance signal, is sent with the course signal to outshine the secondary lobes.
The clearance signal has a power level of 10 dB less than the course level (a tenth of the power) and is radiated with a less directional pattern (less individual  antennas to form the beam):

Course and clearance
Today's localizers use a dual frequency system, this means the course and clearance signals are sent on separate frequencies.
When an ILS has a published VHF frequency f, the localizer course signal is actually sent on f + 4.75 kHz and the clearance on f - 4.75 kHz. The two signals can use  the same aerials or dedicated antennas in the array. Example of signals send to each of the 14 antennas of a Thales LLZ array (refer to above for what are course,  clearance, CSB and SBO signals):

Thales Dual Frequency – Localizer Antenna

Most of the information about ILS come from the 1983 thesis presented by Capt Dennis M. McCollum, B.S.. Respectful thanks to the author.


Why does pulling the mixture to idle cutoff increase the RPM slightly? When adjusting fuel control on a reciprocating engine, why is there a slight RPM rise (50-RPM rise) when pulling the mixture control to idle cutoff? Why if more than a 50-RPM rise, carburetor is set too rich?

Engine are usually run slightly richer than a stoichiometric mixture, to ensure reliable running and avoid overheating the pistons and valves. When you cut off the fuel, it goes from slightly too much fuel to none - and so it's briefly at a stoichiometric ratio.

The engine runs slightly faster at this ratio because it's not wasting energy heating up unburnt fuel.

If you see more than a 50rpm rise, it was previously running further from stoichiometric than necessary (as determined by the engine manufacturer, and many years of experience), reducing power and wasting fuel.


If no low-speed compromises were made, what would an ideal Mach-3 wing look like? Things like the Concorde and Tu-144 and many other supersonic craft need a wing that has good high-speed and good low-speed performance. After all, it needs to be able to take-off and land. Good low-speed performance is achieved with things like flaps, slats, and the wing shape itself.

But let's just say, hypothetically, you don't need good low-speed performance at all. So no such compromises in the wing shape. What would an ideal Mach-3 wing look like?

The goal is to have a wing with the highest lift-to-drag ratio at Mach 3.

I chose Mach-3 because that seems to be the limit on what jet aircraft have been put into production (MiG-31, SR-71) (which does not count experimental aircraft).

I'm interested in conventional designs, not things like the Blended-Wing-Body. So presumably this ideal aircraft would look similar to the Concorde or Tu-144.

Some things I've noted over the years:

The X-15 had a "wedge" shaped vertical stabilizer with a flat back. You can see it in photos. They said it was because the flat back produces more stability for less drag at high mach. Would a mach-3 wing also have a thick, flat trailing edge?
Ogival Delta seems very popular for large sophisticated supersonic aircraft. Never understood why that is exactly, since the pure delta triangle is easier to manufacture and bear loads easier.
The XB-70 Valkyrie had folding wings to give it anhedral, supposedly to capture the shockwave and thus be a wave-rider, which is more efficient. I've noticed the Concorde also has slightly drooping wings near the tips.

Note: I don't need a complete 3D model of the wing shape. I'm more interested in things like what planeform it has, what anhedral/dihedral if any, etc.

One Mach 3 design which did not care about low speed characteristics was the Lockheed Q-12 / D-21 reconnaissance drone, which used a Marquardt RJ-43 ram jet for propulsion and had a flight Mach number of up to 3.5. It was initially air launched from the back of an A-12 / M-21 and crashed when fuel ran out. The purpose was deep penetration of hostile airspace to perform photographic reconnaissance in pre-satellite days. When later the launch aircraft was switched to the B-52, the D-21 needed an enormous booster to bring it up to its cruising speed of Mach 3.35.
This should be closest to a uncompromising Mach 3+ design; however, it drew massively on the aerodynamic work with the A-12 / SR-71, so probably the solution space was restricted from the start.

D-21 three side view (picture source). The anhedral was needed to compensate for the rolling moment of the high tail in sideslip. An even more uncompromising design would had used a smaller vertical and an equal fin on the lower side and no anhedral. The lower side fin, however, would have made the air launch from the back of the A-12 impossible.
All surfaces had sharp trailing edges. A wedge-like cross section like the tail of the X-15 only starts to make sense at speeds in excess of Mach 5 when using drag for directional stability is more effective than adding more tail surface.