Does the auto thrust at takeoff compensate for asymmetric engine characteristics? When a percentage thrust has been determined for take off the auto thrust system (Airbus) or the auto throttle (Boeing) will send the data to the engines FADECs to provide the required thrust.

When the levers are set at FLEX (Airbus) or when the levers push button is pressed (Boeing) the engines will develop the takeoff thrust which is displayed as an N1 or as an EPR.

It seems the FADECs will command the same N1 (EPR) for all engines since the displayed N1 (EPR) are the same for both (all) engines. Am I right?

Considering the engines may have different age and therefore producing different thrusts for the same indicated N1 (EPR), a thrust assymetry will result. Is it true? How this is acceptable at take off?

Considering the engines total running hours (aging of the engine) are known by the operator, is there a way  to enter such a data to the auto thrust or to the auto throttle or to the FMS in order to get at take off really equal thrusts and not just equal N1(EPR)?

@JuanJimenez's comments are correct. N1 (and the accuracy of EPR) are not indicators of a precise thrust force.

So to answer the title question, generally no, moreover the acceleration of each engine may be different, and that is why the pilots go through a stabilization step, where they apply some thrust, make sure they're responding in a sufficiently similar manner, then they proceed with takeoff thrust. The pilots will correct any yaw with the rudder, and any remaining effect could be similar to a very light crosswind.

Also see: How is thrust of engines on the two sides of an airplane kept exactly equal to prevent yawing? The same phenomenon due to wear and tear is discussed. It is trivial for the most part since there is a rudder.

Of course this affects fuel consumption in-flight as there will be a tiny yaw as well that needs to be trimmed out.

But don't take it from me, check the Airbus article, Engine Thrust Management - Thrust Setting at Takeoff.

  Every engine has its own performance level due to manufacturing tolerances. In addition, engine performance evolves with time due to wear and ageing. As a consequence, the acceleration profiles may slightly differ from one engine to another on an aircraft (fig.3), even if fitted with new engines.

  Similarly, the idle thrust can slightly differ from one engine to the other, moving the acceleration profile to the left on the graph (fig.4).

Earlier I said generally no, because there are always exceptions (never say never on the internet). The articles notes that the Trent engines of the A330 and A380 automatically manage the stabilization step.

RE notice: On old aircraft before FADEC pilots were manually adjusting the thrusts to be equal (...)

It's because before FADEC the link between each lever and engine was a cable, and each cable depending on its age and environmental factors, acted differently – different stretching, friction, set torque, etc.

With FADEC, this is no longer a problem. The same electronic command is sent. And depending on the target, e.g., x N1, this is what the engines aim for, regardless of each engine's age and the fact that exact N1 values don't mean the same thrust force. Pilots pre-FADEC, like pilots with FADEC, don't know the thrust that is being applied in lbf/kN.


Do pilots turn off the engines during de-icing? During winter operations and de-icing are the engines (jet engines) usually in operation? If not, what is the reason for that? Also I was wondering if the engine nacelles require de-icing since I have never seen them being spryed in any video footage.

Generally, it's not required to shut the engines down.
Sometimes you're deiced at the gate or just off the gate, and in those cases you've not started the engines yet. Or if it's necessary to remove ice on the bottom of the wings or from the bottom/front of the (extended) flaps, those would be cases when you'd shut the engines down for deicing. But most commonly, you taxi to a de-ice pad (parking area), leave the engines at idle with the bleed valves closed (to avoid getting deice fluid into the air conditioning system - even a trace of it creates kind of a stink), and deice with engines running.
The engine cowl is deiced NOT with fluid but thermally, tapping bleed air (upstream of the bleed shutoff valves, so it's always available) to heat them. Thus the fluid is not directed at the engines, but is intentionally kept away from them.


Can light aircrafts with no retractable landing gear make an emergency water landing? In the suburbs of Lisbon today two people were killed because a pilot of a light aircraft (I imagine without retractable landing gear) landed on one beach full of people (it's Summer and the beaches are crowded).

The question immediately arose on the public opinion: can those light aircrafts, with wheels, land safely on water? This aircraft seems to be a Cessna 152.

The other post "What are the risks in landing on a beach vs. shallow waters?" does not solve this question.

Can light aircrafts with no retractable landing gear make an emergency water landing?

Can they land on water? Yes.

  Most pilots contend that the ideal airplane for ditching is a low-wing aircraft with landing gear retracted. Statistics, however, do not substantiate this. Aircraft geometry and landing gear configuration do not appear to affect survivability appreciably. (Source.)

If you try this Google search, you'll find plenty of high-wing fixed-landing-gear water/sea/ocean landings. The outcomes for the occupants vary.

In Oct 2015:

  The [Cessna 152] was ditched in the [waters off Grand Bahama]  after experiencing a technical failure. Both trainer pilot and trainee remained unhurt.

In Dec 2013:

  The Makani Kai Air Cessna 208B Grand Caravan impacted ocean waters shortly after takeoff from Kalaupapa Airport (PHLU), on the island of Molokai in Hawaii. The airplane sustained substantial damage. The pilot and two passengers were seriously injured, one passenger was fatally injured, and five passengers received minor injuries.

Related: What are the risks in landing on a beach vs. shallow waters?


How do I choose the correct airfoil for my UAV project? I am not able to decide which airfoil to choose for my UAV project as I am confused. I need to lift at least 10kg of extra payload excluding the weight of the fuselage. I can afford a wingspan of 3m with a cruising speed of at least 70kmph. Also I would like to know if there is any software or application to decide the airfoil for me?

There are two methods of aircraft aerodynamic design. One method is through using known airfoils (for wing and tail surfaces) and deciding other geometric parameters iteratively while meeting certain criteria for performance, stability, control, aeroelasticity, manufacturibility, etc.

The other method is deciding roughly on the size and then attempting to come up with an airfoil shape, that (again) meets certain criteria. This second sort of design process is named inverse design. Where you try to get a desired Pressure distribution through computational iterations.

There are open source and free software as well as commercial options for both methods. Search terms: Airfoil design.

Any general RC aircraft design guideline and relevant spreadsheets will speed up the learning process. Search terms: RC aircraft design guide.

For detail information about the theory and practice; please refer to aircraft aerodynamic design textbooks.

Hope this answer sheds some light onto this wonderful and exciting topic. Feel free to shoot specific questions.


At what minimum height above terrain does a vertical structure need to be indicated on a sectional map? I recently drove by what seemed like an unusually tall structure and used Google Maps satellite view, its distance measurement tool, and basic trigonometry to calculate that this building (some kind of grain silo at a poultry processing plant) was 175 feet tall above the ground.

But, it does not show up on the sectional map.  Other nearby structures, mostly 300-350 feet or greater, do indeed show on the map.

At what minimum height above terrain does a vertical structure need to be indicated on a sectional map?

Refer to Aeronautical Chart Users Guide 2017. There are 351,148 obstacles in the database. Generally it is when ”a manmade obstacle is 200' above the highest terrain in the quadrant”. However, “in high density areas like city complexes, only the highest obstacle is represented on the chart”. Then they do the calculations below to show the obstacle on the chart.


Are there any airports which use a metro to transport cargo? Many airports around the world are increasingly being joined by Metro conveniencing people. Majorly, it is people who probably will use the metro. I do have a sneaking suspicion that the metro would also be a pretty inexpensive good carrier. Two things which I think work in its favor -

a. Security on Airport Metros is higher so that much less hassle.
b. Most of these Metros run on specific times so it's easier as it's consistent.

Are there any airports that people know that transport air cargo via metros after being bought by Aircraft ?

DFW Airport's original train system had cars built to carry mail and luggage.  Most of the system was never used because it was slow, and there were worries that it was hard to train employees to interface with it. I would guess that a lot of airports have not tried such as system because their trans are optimized for passengers, so it is faster to just move luggage across the tarmac on trucks.

Edit: Jonathan Walters mentioned that Denver also had a system, but it was buggy so they stopped using it.


What is that Boeing 757 custom cockpit interface? Cropped; flickr.com, 2008

The photo is taken in cruise. The selected display shows takeoff data. There is also a send button (which hints at a datalink capability for communicating values).

I searched for performance calculators and datalink solutions for the 757, but came up empty. Nothing matched that unit.

What are its features? Say compared to Boeing's own ACARS/AOC for datalink (via the standard CDUs).


What is the meaning of "static pressure"? I don't really know what static pressure is. Does the ASI (Airspeed Indicator) use cockpit pressure as static pressure, to find the dynamic or differential pressure?

I just found this statement :

  if the instrument static pressure line becomes disconnected inside a
       pressurized cabin during cruising flight the altimeter and airspeed
       indicator will both read low.

Could anyone help me understand static pressure and this statement?

There is a static pressure port, often on the side of the aircraft, that senses the air pressure at the altitude the aircraft is flying and unaffected by the aircraft's passage through the air.

The pitot tube, facing into the relative wind caused by the aircraft's passage through the air, measures the ram air pressure, which is higher the faster you go.

The indicated airspeed is the difference between the ram air pressure and the static pressure, subject to some adjustment due to sensor positioning.

The altimeter operates off the static air pressure.

So, if the static line is somehow opened to the inside of a pressurized cabin, the system will read that higher pressure and believe the altitude is what it is inside the pressurized cabin, which will be significantly less than the actual altitude.

And the difference between the ram air pressure and the false higher static pressure (the pressure inside the cabin) will be much less and the system will read this reduced different as a much lower speed.

This is a somewhat simplified answer.


Why do some airplanes have simultaneously a straight wing and a swept horizontal stabilizer? I'm sure there are many other examples like this one: Embraer EMB 120 with an almost straight wing and an obviously swept horizontal stabilizer.

Why is there a difference in sweep angle between the main wing and the horizontal stabilizer, since both surfaces fly at the same speed?

The tail sweep helps to extend its range of angles of attack.

For both stability and control, it is very important that the tail doesn't stall before the wing in any circumstances (for a classic design; for canards it may be beneficial). Generally, for high positive angles of attack, the horizontal tail already has an edge because it is set at a lower angle of incidence for statically stable airplanes. Plus, the change of downwash with AoA helps.

Still, to delay stall further, particularly when control surfaces deflection is considered, a higher sweep angle can be used. The lift slope vs AoA decreases with the [cosine of] sweep angle, and the stall angle increases. Note that the maximum lift coefficient also decreases, so the tail may need to be sized larger.

You might notice that even for the airplanes with swept wings, the sweep angle of the tail is nearly always higher (typically by 5°), primarily for this reason.

The leverage advantage (mentioned by John K), may be important in practice only for naturally short-tail airplanes, typically with rear-mounted engines. A classic solution is to put the horizontal tail on top of a swept vertical tail. But in other circumstances the additional lever arm of a higher sweep will likely be offset by the lower effect per AoA and thus larger size (as mentioned above), and heavier design due to additional torsional loads.

And let's not forget looks! It's fashionable to have at least the vertical tail sweep. Compare different versions of C175, for example.


Light aircraft elevator/aileron/direction not working What is the correct procedure if a light aircraft (under1000kg) elevator/aileron/direction stop working? Is it alright to use the ballistic parachute(if the aircraft is equipped with one)?
If not, then in what situation ballistic parachute is used?

If a major flight control surface fails in flight, you'll be lucky if the ballistic parachute can save you.  An aileron, for instance, that goes "hard over", will put even the most docile training aircraft into a tight spiral or a roll that will quickly become a spin (because the airplane doesn't have enough speed for a complete roll and will stall).  A stuck elevator could result in anything from a stall to a steep dive.
In this situation, immediate deployment of a ballistic parachute might prevent fatalities; waiting will not.  Delay will allow airspeed to build, G forces on the pilot to increase (presuming a spiral or spin), and potentially the wings come off due to flutter or G forces after passing Vne -- not to mention likely exceeding the parachute's maximum deployment safety margin.
Fortunately, this kind of failure is exceedingly rare, absent the kind of damage that comes from air combat, ground impact, or extreme weather phenomena.


Do takeoffs and landings in a Pilatus PC-12 count for currency in a Piper Archer? I just read this:

  No person my act as PIC of an aircraft carrying passengers unless, within the preceding 90 days, he/she has made three takeoffs and landings as the sole manipulator of the controls in an aircraft of the same category, class, and type (if required)

Does this mean that you are legal to take passengers in an Archer if you have logged three takeoffs and landings in the past 90 days in a Pilatus PC-12?

Category: Yes, both are "airplanes" as opposed to rotorcraft or gliders.

Class: Yes, both are single engine land planes, as opposed to multi-engined or sea planes.

Type: Yes, neither aircraft requires a type rating (both are under 12,500 lb max takeoff weight, neither is powered by a turbojet)

So the Piper Archer and Pilatus PC-12 qualify as currency for each other per this regulation.


Headset for both plane and heli: buy for plane, use adapter for heli, or vice versa? I fly both planes and helicopters and am about to buy a Bose A20. I'm wondering if I should get the helicopter version with a U-174 plug and a coiled cable, or the plane version with a dual plug and a straight cable. (In both cases of course I'd also get an adapter to go from one plug to the other.) Does someone have recommendations as to which way to go? E.g. is the coiled cable a hassle in the plane, or a feature? Is one of the two plug types better, or more impaired by the adapter?
Thanks!

There's no issues with the adapter either way, as long as it's good quality. The helicopter versions are coiled for a good reason: to keep it out of your way, so I would buy the helicopter version. In most fixed wing airplanes the coil will be absolutely fine, if there's not enough reach you can get extenders pretty easily.


CH-37 Mojave Helicopter decibels What was the decibels level for the Sikorsky CH-37 Mojave Helicopter?   1956 --1959. I need this information for the VA, to process my hearing claim. I was a crew chief in the Army.

Although that specific information may be impossible to get without starting up a Mojave and standing underneath with a sound level meter, it is generally understood that helicopter noise can easily exceed 100db. The CDC indicates that permanent hearing loss can occur after 15 minutes of exposure to this level of sound.
Here is a link to a study on helicopter decibel levels.
Here is a link to the CDC page on hearing loss.
Here is a link to a Purdue University chart, listing a Bell helicopter’s sound level at 100db flying 100 feet overhead.
I hope this helps at the VA. Thank you for your service.


What is the maximum fuel amount that fits into the rare Boeing 737-700ER version? Title is self explanatory: what is the exact maximum fuel amount that fits into the still rare-to-see Boeing 737-700ER version?

According to the Payload/range diagrams from the Boeing Airplane Characteristics for Airport Planning manual (page 95), the fuel capacity is 32 539 kg (71 737 lb)


Do emergency services helicopters fly VFR when leaving base? I've recently been watching an interesting documentary on helicopter paramedic teams. As you all probably know, their job consists of a lot of sitting around doing nothing before a scramble and then a short (or not-so-short) flight out to the middle of nowhere and back again. On the (UK) documentaries I've seen, the aircraft are usually based at reasonably large commercial airports, physically located close to the fixed-wing runways.

In order to find the guy lying injured in the field and help him out of asystole, they have to rapidly launch from a presumably busy airspace, and get there ten minutes ago. Do airports have procedures that allow them to always assume an "exit route" is always okay, under some ceiling? Do they have to get take off clearance as other private aircraft would? I understand that they get given priority ATC clearance when in controlled airspace, but quite often their target call-to-takeoff times are less than two minutes, and it seems unlikely that the nearby 777 on FA would be done that quickly.

What happens if it's not possible to be VFR entirely on route? I have seen cases reported where difficulties are encountered due to fading light halfway through treating a patient --
meaning that the pilot is worried he can't take off again. It seems rather odd that, if a pilot was able to land in a field comfortably with good visibility, he can't ask for a route to be cleared, file a flight plan, and go to hospital IFR.

For that matter, how do police helicopters chase the bad guys at night in zero visibility?

Disclaimer: I've never set foot in a helicopter in my life, but have a single-digit number of hours in light aircraft -- sick people are more my forte.

If the helicopter is operating at an airport, it needs clearance like a regular flight, otherwise it would be a very dangerous situation for the helicopter and any other traffic in the area. Usually the ATC will give priority to the emergency helicopter (as much as possible), so it will leave the area without delay.

Regarding the weather conditions, there are two aspects that need to be considered:

The regulations allow emergency helicopters to fly VFR below VMC minima (this is true in Europe, I have no idea about FAA). So, an emergency helicopter, can continue VFR with visibility down to 1km, or even 800 meters for short periods of time.
If the helicopter is IFR equipped and the pilot(s) IFR rated, it can switch to IFR if necessary. On the way to the accident site this might not be very useful if the weather will not allow safe descent and switch to VFR, but on the way back to the airport/hospital, it can be very useful, provided the landing site has some kind of IFR procedure. Again, ATC will usually offer full cooperation in all this switching of plans and flight rules.


How is the flow of air through a NACA submerged inlet determined? This question pertains to the design and/or selection of NACA submerged inlets. I am trying to understand what kind of airflow would enter a NACA submerged inlet/scoop and am looking for a good reference to use.

Suppose you have a typical NACA submerged inlet , which has a 2.75 inch diameter hose connection (so an area of 5.94 inch2), and you are flying at 100MPH. How much air actually enters the duct?

I've seen the famous NACA 5i20 Report but can't seem to find the answer or don't understand it fully yet. The report routinely compares the inlet air velocity (Vi) to the free stream velocity (Vo), calling it a ratio "Vi/Vo". It then shows pressure losses for various Vi/Vo ratios. But how do you know what Vi your scoop will get?

I've thought about relating the mass flow entering the duct area (the triangular part) to the mass flow through the inlet, but I'm not sure if that is the most accurate method.

The flow through the inlet is determined by the resistance of everything behind the inlet. If the inlet is blocked off internally, there is no flow and $v_i$ = zero.

The mass flow is indeed the way to determine through-flow: mass in = mass out. Which is a function of the pressure build-up (both in and out), cross section area (both in and out), temperatures, etc. A similar process occurs in a jet engine inlet.


Do auto-flaps exist? Slats can be extended automatically depending on dynamic air pressure. Fly-by-wire systems provide flight envelop protection when flaps/slats are extended but I cannot find any reference of fly-by-wire feature such as auto-retractation/extension of flaps/slats depending on angle of attack or air-speed.

Do systems to automatically retract/extend flaps depending on flight conditions (speed, angle of attack,dynamic pressure,...) exist?

It can be much simpler than that.

The aerobatic glider Mü-28 has a symmetric airfoil with 20% flaps, and those flaps are automatically set as a function of the g-load experienced near the center of gravity and the dynamic pressure. Basically, a spring-suspended lever with a mass at the end creates the necessary control input. In straight flight they are deflected downwards which makes the airfoil more suitable for gliding, in a zero-g parabola or in a knife-edge flight they are moved to 0° deflection, and in inverted flight they show a negative deflection angle. If flight speed increases, the deflections become proportionally smaller, thus preventing overloading the airframe (which is dimensioned for ±10g, so overloading it is hard anyway).

Mü-28 in inverted flight (picture source). Thanks to the low sun the rear part of the fixed wing section is illuminated so the negative flap deflection is easy to discern.

From this page:

  This innovative automatic flap system fitted to the Mü28 was developed
  at the Flugwissenschaftliche Vereinigung Aachen under the project
  designation FVA-21, controlling the position of the flaps, depending
  on the speed and the load factor, so that the aircraft flies with
  optimal angle of attack in all flight conditions.

Scheduling the maximum deflection angle with dynamic pressure as an overload protection can thus be done in two ways:

Fully mechanically by adding a spring element in the flap linkage.
In a fly-by-wire control system by hard-coding it into the control laws. For faster airplanes this is preferred as it does not require a spring in the system - any elasticity will at some point cause flutter, this is only a question of speed.


Do some aircraft now have "full aircraft" parachutes? I was intrigued to see this mentioned in passing,

http://robbreport.com/aviation/cirrus-aircrafts-vision-sf50-personal-jet-completes-flight-testing-video

Is it true that some new products now have this?

Do they actually work, are there any demo videos, who pioneered it, in what situations is it actually useful, etc?

Yes, Ballistic Recovery Systems or Ballistic Parachutes (BRS) are now being implemented in newer aircraft where the manufacturer can provide them or where they are required by law. In Germany, Ultralights are required to have an installed BRS to be deemed airworthy.

  In Europe the sporting (FAI) definition limits the maximum take-off weight to 450 kg (992 lb) (472.5 kg (1,042 lb) if a ballistic parachute is installed) and a maximum stalling speed of 65 km/h (40 mph).

The general idea is to allow the airframe to glide down to the surface and being attached to the airframe, the time saving for pilots to otherwise have to jump out and open their own parachutes (if equipped) is substantial.

The first parachutes were used by Comco in 1982:

  In 1982 Comco Ikarus developed the FRS rocket-launched parachute system for its ultralight and hanglider aircraft.2 In 1998 Cirrus Aircraft provided the first ballistic parachutes as standard equipment on their line of type certified aircraft, the Cirrus SR20.

(Image Source: WikiMedia Commons - Author: NASA)

Here is a Video of the Cirrus Airframe Parachute System in action:

(Image Source: YouTube - Author: AIRBOYD)

An entry on www.aviation-safety.net about the Cirrus parachute system:

  The aircraft experienced a deployment of the Cirrus Airframe Parachute System (CAPS) over west Fayetteville, Arkansas. The airplane came to rest on a roadway shoulder sustaining substantial damage. There was 4 occupants on board, at least two people taken to the nearest hospital with minor injuries, according Police Department.
  The plane started having oil pressure issues immediately after take off.


What is the amount of lift produced by friction? As Mr. John D. Anderson states, the aerodynamic forces on a body are due to only two sources: pressure distribution over the body and shear stress distribution.

Shear stress is acting tangential to the surface, so to some amount, friction is also contributing to the lift of a body, not only to drag. But what I most of the time see, is that one is looking at the pressure distribution and integrating this over the surface to get the resulting force.

Do we neglect the part of friction lift because it is so little? But for drag we account for it?

First, let me illustrate how I understand your question. Please comment accordingly if I got you wrong.

Below you see the friction drag computed by XFOIL for an airfoil at 3° angle of attack.

Friction drag over chord for an E502mod airfoil at 3° AoA. Blue: Top surface, Red: Bottom surface. Source: XFOIL 6.97.

And here is a plot of the pressure distribution at that point:

Where the longest arrow pointing at the airfoil contour points to is the stagnation point. So the large blue peak in the first plot is exactly in the region of the nose where its curvature is strongest and the surface is mostly vertical. Integrating all the friction drag over the height of the airfoil should result in a small upwards component from that high local friction around the nose. Lift, from friction, in other words.

Did I understand that correctly?

I've never seen that component of lift being mentioned separately, and even XFOIL does not include it in its calculations (just checked the FORTRAN source I have lying around). You assume correctly that this component is indeed negligibly small, it is about two orders of magnitude smaller than the pressure forces.


How do paper airplanes create lift if their wings are flat? Paper airplanes' wings are completely flat, unlike the droplet shaped curvature of a full sized wing. How does a flat wing generate any lift if both sides have the same air pressure?

Paper airplanes create lift just like any other airplane. Don't let the wing cross section confuse you; what the air "sees" is a different shape.

When a flat plate flows through air at a positive angle of attack, the stagnation point of the flow (where air splits into an upper and a lower flow path) sits slightly below the forward edge of the wing. Now the streamline just above the one which hits the stagnation point has to negotiate the sharp corner of the leading edge, which will cause it to separate from the surface. This creates a small separation bubble which to the streamlines further up looks like a round leading edge, and since the rest of the wing has no curvature, the flow will attach again soon. The bigger the angle of attack gets, the farther back the stagnation point will be and the bigger the separation bubble becomes. At some point, which is promoted by leading edge sweep, the separated flow will not reattach, but produce a vortex, like the vortex which provides lift on delta wings.

The picture above is from a simulation; see the full video here. Note the red color on the forward part of the upper side: This indicates low local pressure, which is needed to make the outer air negotiate the leading edge plus the separation bubble (see here for an explanation on the molecular level). In the separation bubble, low pressure is not accompanied by high flow speed since the separation turbulence has eaten up most of the kinetic energy.

The actual nose radius of the paper wing does not matter, and it might even be zero (like when you fit razor blades to the leading edge): The air will create its own airfoil shape by padding with separated air what cannot be reached with attached flow.

Paper airplanes have a low enough wing loading that their angle of attack is low and the separation does not become too big. All it needs is a center of gravity at the quarter chord of the wing; that is why you need to fold the piece of paper first.

It is interesting to watch a slightly tail-heavy, delta-shaped paper airplane fly: It will gradually reduce speed and will start to rock its wings when it approaches stall. Once one wing is going down, the sudden increase in its local AoA will increase lift so much that the rolling motion reverses, without noticeable yawing. The delta-shaped flat plate has effective roll damping right into the stall. The lack of yaw is mostly due to the high yaw inertia, but indicates also that drag buildup is moderate at best.

Of course, after a few periods of rocking the nose will drop and it will pick up speed like any well-behaved airplane.


How does an airplane owner correct an airplane misidentification showing up on FlightAware? I have a friend in Pennsylvania with a plane in Pennsylvania. FlightAware is currently showing his airplane in the Bahamas, when it is not. He wants to know whom to contact to get this corrected. I'm assuming it's in some database being use by both FlightAware and FlightRadar24.

For the sake of it:
contact flightaware directly
https://flightaware.com/about/contact/


Controllability vs. Stability in PHAK. Is it an error? My understanding on the relationship between stability and controllability has been that greater stability means less controllability and vice versa. This post confirms that. However, I've came across this paragraph from PHAK 5-43 today:

  Generally, an aircraft becomes less controllable, especially at slow
  flight speeds, as the CG is moved further aft (my emphasis). An aircraft that
  cleanly recovers from a prolonged spin with the CG at one position may
  fail completely to respond to normal recovery attempts when the CG is
  moved aft by one or two inches.

Aft CG means less stability, so if I'm not mistaken, the phrase is basically saying less stability leads to less controllability. How should I understand this statement? Should I understand that, although controllability and stability are generally opposed to each other, when it comes to a stall or spin recovery, the opposite is true? Or is it simply an error?

Your understanding of controllability and stability is correct, for the following definitions:

Controllability: the response of an aircraft in steady flight, on pilot control inputs. For instance deflecting the ailerons: a high resulting roll rate means a fast response.
Stability: the tendency of an aircraft air frame to return to trimmed position after a disturbance in air stream. Note that in this definition of stability, the airframe itself does all the work in getting back to neutral position, no pilot input required.

So the above is for steady, trimmed flight. The paragraph that you cite is about how to provide flight control inputs to recover from a spin. Now we need both control deflections and aerodynamic stability to recover. If the CoG is too far aft, there is not enough stability and the airframe may not recover even if the correct control inputs are given.

Use of the word Controllability is correct here, since it refers to controllability of recovery from a spin.


What is this mysterious thing on the vertical stabilizer of the Boeing 707? Have you ever seen this mysterious line-shaped thing on the vertical stabilizer of a Boeing 707 or 720? What purpose does this thing do?

It is the high-frequency shortwave antenna of a 707-321B, specifically Clipper Yankee Ranger N418PA. Also present on the KC-135.

You can read about the Antenna Coupler Program from the people involved in its development.

You might want to look at What radio frequency bands are associated with the long wire antennas, why do some aircraft have them and others don't? for more information.

source

source


Does the rate of turn increase or decrease by turning at a higher speed? It is easy to derive a formula for rate of turn in a steady banked turn as:

$$\dot\psi=\frac{g \cdot tan\phi}{v}$$

which indicates that as speed increases rate of turn decreases. This feels counteruntuitive especially given the identity for circular motion:

$$\omega=\frac{v}{r}$$

which would suggest rate of turn increases linearly as speed increases.

Can someone give me some intuition as to why speed has the opposite effect in a banked turn. Given the above I can only think that speed in a banked turn increases radius at a rate greater than it should increase $\omega$ during normal circular motion.

The first formula is a function of bank angle $\phi$ and the velocity $v$. By varying only $v$, you are keeping $\phi$ constant. As a consequence, to keep the turn "steady" and coordinated (that are the assumptions needed to write that formula) you need to increase the radius of turn $r$ and decrease the turn rate.

The second formula is a function of $r$ and $v$. Once again you change only $v$, $r$ will be constant here (while before was $\phi$). This leads to a turn with a higher $\phi$ (again to mantain valid the assumptions behind the formula) and higher rate of turn.

So, to conclude, assuming that you whish to perform a coordinate turn, whether the turn rate increases or decreases is a function of what you whish to keep constant when you change speed:

if you have a constraint on the bank angle, an increase of the velocity will decrease the turn rate and increase the turn radius
if you have a constraint on the radius, an increase of the velocity will increase both bank angle and turn rate

To complete the set of equations and see this in practice, you would need to flip each equation and apply it after the other. That means:

Case 1:

$$\dot\psi=\frac{g \cdot tan\phi}{v}   \;\;\;,\;\;\; r = \frac{v}{\dot\psi}  \;\;\;,\;\;\; \phi = const.$$

Case 2:

$$\dot\psi=\frac{v}{r} \;\;\; , \;\;\; \phi = atan\left( \frac{v \cdot \dot\psi}{g} \right) \;\;\; , \;\;\; r= const. $$

Note in case 2 how $\phi$ is computed through an $atan()$ function. This means that increasing $v$ will make $\phi$ approach $90^\circ$. The validity of the model used to derive this equations is no more guaranteed at that point.


Can this helicopter/wrecking ball combo remain airborne? I just ran into this meta-answer which includes this image:

My initial guess is that the wrecking ball is far too heavy for the helicopter to lift, but I'd be interested in the numbers to see just by how much.

From this silhouette, can the make/model of helicopter be determined?  (I'm hoping this image is based of some real-life craft)
If so, presumably the vehicle length is known, and the diameter of the wrecking ball may be calculated (spikes may be ignored) according to the scale of this image
Assume the wrecking ball is solid mild steel with a density of 7.85 g/cm³.  The mass of the wrecking ball can then be calculated
If the make/model is known, then presumably the max payload weight is also known, and we can see if staying aloft is a possibility or not.

Close-voters: I asked this question on Aviation.SE because the key piece of information I was missing was the identification of the craft in the cartoon image.  I think aircraft-identification is on-topic here.

EDIT

Huge apologies to one and all, in my first version of this answer I mistyped 184 as 84, and then proceeded to do all my calculations based on that.  This is the corrected version.

Also, huge thanks for all the up-votes and comments. I'm so glad it's not just me that is enjoying this question immensely.

The radius of the ball in the image is 21 pixels. The length of the fuselage is 184 pixels, which is 50' 7.5" (15.43m) according to widely available statistics. Hence, a pixel represents approximately 84mm

So, the radius of the ball is 21 x 84 = 1764mm = 1.764m

And the volume of the ball (if solid) is 23.0 m³.

For it to be liftable by the helicopter, using Pilothead's quote of 9000lb (4082kg), the density would need to be 177.5 kg/m³. Mild steel, at 7,850kg/m³, is way too heavy. Expanded polystyrene has a density of 28-45kg/m³, so that would be feasible.

However, if the ball were not solid, and were made of mild steel, the total volume of steel permissible would be 0.52m³. At an outer radius of 1.764m the inner radius must be at least 1.750m to keep the volume of metal down to 0.52m³, so the ball would only be 14mm thick (9/16") and would only cost about $3,300.

So, let's try Titanium (darling of the aircraft industry). With a density of 4,506kg/m³, we could have a volume of 0.90m³. That would allow a thickness of 24mm (15/16"). However, the price of that much titanium would be $229,500 (today's prices - 6 years ago, it would have been over a million US dollars).

Aluminium, however, has a density of 2,700kg/m³. So now we could have a volume of 1.51m³, allowing a thickness of 40mm (1 9/16"), and would only cost $9,360

Pumice, at 250kg/m³, would allow a volume of 16.33m³, allowing a thickness of nearly 0.6m (23½"). That would be very sturdy, I reckon. I can't find a price for a solid lump of pumice that size though.

Incidentally, the people in the picture are also about 21 pixels tall, making them around 1.76m (5'9") tall.

This has now taken up about 3 hours of my time, which like all other hours up to now, are hours I'm never going to get back.  However, it has been so much fun.

FURTHER EDIT

Following a suggestion from Jules in the comments, I've discovered Lithopore 75-150, which is a ridiculously low density foamed concrete.  This could give us the magical solid sphere that we want, with sufficient strength to squish multiple personnel without significant damage to itself.  At the 75 kg/m³ end of the range, we could possibly even have some spikes, and a non-negligible cable mass too.

More maths and engineering research required.....


1.VRB02KT: Is it varying more or less than 180°? 2.VRB03KT: Is it surely varying more than 180°? Based on the following ICAO Annex 3 criteria for the use of the abbreviation VRB (not FAA, which is different and evident):

when the total variation is 60° or more and less than 180° and the wind speed is 3 kt or more, such directional variations shall be reported as the two extreme directions between which the surface wind has varied

when the total variation is 60° or more and less than 180° and the wind speed is less than 3 kt, the wind direction shall be reported as variable with no mean wind direction; or

when the total variation is 180° or more, the wind direction shall be reported as variable with no mean wind direction"

I bring up the following QUESTIONS

Based on these criteria, is there a way to identify if a variable 2KT wind is varying less or more than 180° when the indication is just VRB02KT?

If it were varying less than 180° but at least 60° or more, would it be possible to measure its mean direction and direction range despite its low intensity of just 2KT? E.g. 15002KT 110V200.
If not, it brings me back to my first question, what is the actual variation of a VRB02KT, less or more than 180°? Could I just answer impossible to specifically tell, but it is surely 60° or higher, otherwise the mean direction would be possible to measured, even at only 2KT. E.g. 15002KT

If the wind intensity following a VRB indication is 3KT or more, can I assume it is varying more than 180°? Since this excessive variation in direction (not the relatively low intensity of 3KT) is the reason why the station can't measure the mean direction precisely.

E.g. VRB03KT would mean varying by more than 180° (not 60° or more, and less than 180°) with an intensity of 3KT (which would be strong enough to provide feasible conditions for measuring the mean direction, if it wasn’t for its excessive variation in direction above 180°).

Based on these criteria, is there a way to identify if a variable 2KT wind is varying less or more than 180° when the indication is just VRB02KT?

No, there is no way to tell. And you don't need to tell. The wind velocity is too small to have any operational impact.

If the wind intensity following a VRB indication is 3KT or more, can I assume it is varying more than 180°? Since this excessive variation in direction (not the relatively low intensity of 3KT) is the reason why the station can't measure the mean direction precisely.

Yes, that is safe to assume, as per the definition you quoted.
Remember the values you see in a METAR are average values over a period of time. If you need the immediate wind, ask the tower.


Can stainless steel replace titanium for Mach 3+ speeds? In the documentary Wings of Russia: MiG-25 and MiG-31, they mentioned both aircraft had to use special stainless steel alloys and arc welding to withstand the Mach 2.83 - 3.2 speeds.

I couldn't help but recall the space pen vs. pencil myth. Could the SR-71 (Mach 3.3) have been built using such stainless steel alloys instead of covertly importing the titanium from Russia?

  The major supplier of the ore was the USSR. Working through Third World countries and bogus operations, they were able to get the rutile ore shipped to the United States to build the SR-71.

Since the SR-71 flew higher, I'd guess both had comparable EAS / dynamic pressures. The titanium used on the SR-71 was an alloy, figuring out which alloy to use and how to shape it caused many challenges.

So gained knowledge and cool factor aside, what other factors are there?

Both the MiG-25 and MiG-31 designs had engines that would overheat and be damaged an anything past Mach 2.83, so both planes were limited by this. Typical speeds were closer to Mach 2.5 to extend the life of the planes. The highest aerodynamic heating occurs at the leading edges. It's possible that the MiG designs used titanium in these areas but opted for steel in most other areas, which was cheaper and easier to work with. This would have allowed the aircraft to reach speeds closer to the SR-71, even if the engines couldn't keep up.

The SR-71 was designed to cruise at up to Mach 3.3 at 80000ft, a similar altitude to the MiGs. Atmospheric heating is related to the cube of the speed. Mach 3.3 generates 10% more heat than Mach 3.2, and the SR-71 reached at least Mach 3.5. It seems the SR-71 flew faster and higher than the MiGs.

Heat is only one consideration though. Another big issue is weight. Steel is generally much heavier than a comparable structure made from titanium. The SR-71 empty weight was about 40% higher than the MiG-31, but the MTOW was 70% higher. This allows much more payload for mission equipment, but also fuel. The MiGs were designed to fly relatively short range interception or reconnaissance missions, having unrefueled ranges of 2000km or less. The MiG-31 could fly 3000km but only with drop tanks. The SR-71 was designed as a long range reconnaissance aircraft, and had a range of 5400km which enabled it to fly long missions over enemy territory.

Another consideration is size. The SR-71 was a larger plane than either the MiG-25 or MiG-31. It had a wingspan of 17m, compared to 14m of the MiGs, and a length of 33m, compared to 20m and 23m for the MiGs. This means higher forces on the aircraft structure.


Which one needs longer runway of an airplane between take off and landing for a constant weight? Consider we have an airplane that the weight will be constant when take off and landing (in fact will be no such common airplane right now as the fuel will be less due to engine consumption. But in an electric airplane, that will be possible, I guess). What I am asking is, which one is required longer runnway from an airplane starts taking off in the edge of the runway until it just leaving the runway (ground), compared to when that airplane is landing, from it start touching the runway until it can stop safely in the runway. How do we calculate the required runway for landing?

Generally, aircraft require longer distance for taking off than landing roll.

The standard definition for take-off phase is from the start of acceleration (stopped aircraft) to the point where aircraft reaches 35 ft above ground. Similarly, the landing phase is from 35 ft to the final stop. What you mentioned is a sub-phase, which is called "ground roll".

When the aircraft is accelerating for the take-off, the engins' thrust must overcome the aerodynamic drag and the ground friction, and the resultant will contribute to the acceleration. But when the aircraft is landing the reversed thrust in addition to an increased aerodynamic drag (due to spoilers and increased flap deflection) and increases ground friction (due to application of brakes) are decelerating the aircraft. This is physical reason.

While there are some engineering methods (such as equations or charts, usually presented in aircraft design books i.e. Roskam or in aircraft performance books), the most accurate method is to writing the force balance acting on the aircraft and computing the acceleration at any time step, and solving the position differential equation for the distance. But this method requires details about aerodynamic and thrust characteristics. This method somewhere was called time marching.


What airfoil simulation software is suitable for a cut trailing edge? I'm looking for a way to simulate airfoils used to reduce turbulence.
airfoiltools.com already does this a long way.

However I have width and length limits, and I'm interested in comparing full profiles with profiles that are virtually longer but cut where the trailing end meets the boundary.

Can CAESES or SimFlow do this without too steep learning curve? (I'm a complete newbie.)

(Specifics: Chord 30 cm, width 25-30%, Reynolds number 1,500,000, NCrit 9, 50-70m/s)

One tool you might want to consider is MIT's XFoil. I'm not sure I fully understand what you'd like to do, but this tool is used in many of MIT undergraduate aerodynamics classes.

If you think it might work for your purposes, here is a course by the program's creator from MIT's OpenCourseWare site. Lecture 3 is a tutorial on how to use XFoil. You might want to browse the OpenCourseWare site for more advanced uses of XFoil, too.

Good luck!


Why can't most jets safely use reverse thrust while airborne? According to Wikipedia:

  Some aircraft are able to safely use reverse thrust in flight, though the majority of these are propeller-driven. Many commercial aircraft cannot use reverse thrust in flight

What are the repercussions to using reverse thrust while airborne?

The problems are two-fold:

The airspeed is much higher in flight than when landing, so the thrust reverser doors would have to be much stronger, and therefore heavier to withstand the additional forces, along with the relative wind making them harder to close.
What happens when it gets stuck in the deployed position?  Now you have to shut down a perfectly good engine and have additional drag on the airplane while in flight.
(Bonus) It isn't really needed on most airplanes with spoilers, flaps, etc. that accomplish the same thing.


How to interpret BECMG and TEMPO in TAF? I'm still confused on the change indicator BECMG. I am sure of the definition of TEMPO being only a temporary fluctuations in the time frame given, but will a TAF with a change of BECMG continue on with the changes in the BECMG or will it revert back to the general forecast in the beginning of the TAF (basically is it permanent change or is the change only applicable in the time frame given just like TEMPO). As it says that once a TEMPO is expected to be have fluctuations of more than 1 hour it will be classified as BECMG.

This is me quoting from WMO document 306, which is also  written in Annex 3 (1.3.3):

  The change groups TTTTT YYGG/YeYeGeGe in the form of BECMG YYGG/YeYeGeGe shall indicate a change in forecast meteorological conditions expected to occur at either a regular or irregular rate at an unspecified time within the period YYGG to YeYeGeGe. The duration of the period YYGG to YeYeGeGe shall normally not exceed two hours and in any case shall not exceed four hours. The change groups shall be followed by a description of all the elements for which a change is forecast. When an element is not described in data groups which follow the change groups, the description of this element for the period between Y1Y1G1G1 and Y2Y2G2G2 shall be considered to remain valid subject to Regulation 51.1.5.

  Note: The conditions described after the groups BECMG YYGG/YeYeGeGe are those expected to
  prevail from YeYeGeGe until Y2Y2G2G2, unless a further change is expected, in which case a further
  set of change groups BECMG YYGG/YeYeGeGe or FMYYGGgg must be used.

  51.8.4 The change groups TTTTT YYGG/YeYeGeGe in the form of TEMPO YYGG/YeYeGeGe shall indicate frequent or infrequent temporary fluctuations in forecast meteorological
  conditions which are expected to last less than one hour in each instance and, in the aggregate cover, less than half of the period indicated by YYGG/YeYeGeGe.

  Notes:

  If the modified forecast condition is expected to last one hour or more, Regulation 51.8.2 or
  51.8.3 applies, i.e. the change groups BECMG YYGG/YeYeGeGe or FMYYGGgg must be used at the beginning and end of the period during which conditions are expected to depart from
  those forecast prior to YYGG or YYGGgg.
  To keep forecasts clear and unambiguous, the use of change indicators should be carefully
  considered and kept to a minimum. In particular, the overlapping of change periods should be avoided. At any time during the period of validity of the TAF, only one possible variation in the prevailing forecast conditions should normally be indicated. The subdivision of the forecast period by FMYYGGgg should be used to avoid too complex forecasts in cases where many significant changes in weather conditions are expected to occur throughout the
  forecast period.

The BECMG group in a TAF describes a gradual change which will take place over a prolonged period of time, but will be the prevailing weather once that change is complete. The time period described in the TAF is the beginning and ending hour during which the gradual change is forecast to be in progress. At the end of the described time the change is expected to be complete and the forecast weather should be the prevailing weather thereafter.

See the description of BECMG from the US Aviation Weather site's TAF decoder page:

  The BECMG group is used when a gradual change in conditions is expected over a longer time period, usually two hours. The time period when the change is expected is a four-digit group with the beginning hour and ending hour of the change period which follows the BECMG indicator. The gradual change will occur at an unspecified time within this time period. Only the conditions are carried over from the previous time group.

Conversely, with the TEMPO group, the described time period is a beginning and ending period after which the weather is forecast revert to the previously prevailing weather.

See the description of TEMPO from the US Aviation Weather site's TAF decoder page:

  The TEMPO group is used for any conditions in wind, visibility, weather, or sky condition which are expected to last for generally less than an hour at a time (occasional), and are expected to occur during less than half the time period. The TEMPO indicator is followed by a four-digit group giving the beginning hour and ending hour of the time period during which the temporary conditions are expected. Only the changing forecast meteorological conditions are included in TEMPO groups. The omitted conditions are carried over from the previous time group.


Has there ever been a centrifugal turbine in an aircraft engine? These days, most jet engines feature axial compressors and turbines. However, there are some older aircraft engines with centrifugal compressors, like the Rolls Royce Nene. But I've never heard of any engine with a centrifugal turbine (radial flow turbine).
Does such a thing exist? Has one ever been built, tested, and flown? I'm interested in turboprops, turbojets, turbofans, even very tiny ones for RC aircraft.
Edit: Okay a radial flow turbine will not produce much of a jet of thrust, so I guess the only possible engine for this would be a turboprop. Edit 2: Not necessarily, a centrifugal compressor can redirect air outwards to another path, then downwards into a centrifugal turbine, which redirects it back to axial direction after all.

The Heinkel Hes 3b, which powered the Heinkel He 178, had a radial turbine. This was the first operational jet engine to power an airplane.


Is there a specific formula or way to calculate flex temperature for an Airbus A320? I have hear of RTOW charts but I haven't seen any calculations with the runway length or slope. What is the formula or equation to finding flex temperature and V-Speeds?

The RTOW (regulatory takeoff weight) charts are runway and configuration specific, so the length and slope (and permanent obstacles) and all taken care of (see red box below).

There is no manual formula, either the charts or a performance calculator such as the Airbus FOVE or the company's proprietary software.

This is what an RTOW chart looks like:

The information needed (example):

  Takeoff from Paris-Orly, Runway 08
  Slat/Flap configuration: 1+F
  Actual TOW = 66 tons
  OAT = 24ºC
  Wind = +20 Kt headwind
  QNH = 1013 hPa
  Air conditioning: Off
  Runway state: Dry

Source: Getting to grips with aircraft performance (Airbus via skybrary.aero)

Here's why there is no equation, from the Boeing book Jet Transport Performance Methods:

  Here’s the complication: for the very first increment of time after engine failure, one second, we
  need to calculate the average acceleration in order to find the airplane speed after one second.

  But
  we can’t find the average acceleration because we don’t know the thrust after one second. And we
  don’t know the thrust after one second because we don’t know the speed after one second. And
  we don’t know the speed after one second because we don’t know the average acceleration.

What they end up doing is iterative calculations and flight tests to come up with the graphs / tables.


Why don't the newest versions of Mi-35Ms have retractable gear? Older versions of Mi-35 Hind helicopters had wheel boxes so that they could retract their wheels while flying.

Why don't the newest versions of Mi-35Ms have wheel boxes?

Quick answer: because Russian engineers don't like fancy stuff ;)

Long answer:

Fixed landing gears are common on helicopters even very recent ones, just look at French helicopter Tigre, Italian mangusta or American black hawk.
It offers a very lightweight and robust solution for landing while allowing easy ground maneuvers and also offers crash absorption.


LPV/LP approach (RNAV with WAAS): What is actually the "channel number"? Curious about the answer to this question: How may I verify WAAS channel number on G1000?, I just found that SBAS/WAAS use in aviation seems to be poorly explained.

In particular the WAAS "channel number" seems to suggest that some radio frequency is associated with a given WAAS approach.

(Image taken from the linked question)

I think the WAAS service is provided by only two geosynchronous satellites, each one broadcasting on the same frequency the corrections for the whole US area. So there is no need to tune the WAAS receiver based on the airport location, the runway number, or the LPV identifier.

So the question: Are these 3 informations strictly redundant (maybe not taking into account version numbers), and they exist only for triple-check purpose?

LPV approach name: RNAV (GPS) Y RWY 36L,
WAAS channel number: WAAS CH 56613,
WAAS approach ID (aka reference path identifier): W36A

Could you confirm that selecting any of these values on the avionics will only change what is retrieved from the local navigation database, and will not impact the GPS/WAAS data received?

Note: I could understand if the approach were LAAS/GBAS (local ground-based) instead of SBAS.

This pdf by Jeppesen provides the meaning:

  WAAS Channel Number, unique to each airport, and  Reference Path
  Indicator (RPI), which is made up of the letter “W” plus two or  three
  characters representing the runway number.  These are shown in both
  the Briefing Strip (chart heading) and plan view.

So the Channel Number provides the airport, and the RPI provides the runway number as well as the specific procedure. The combination of the two of them provides a specific approach at a specific airport, in much the same way that the title of the procedure on the approach plate is only meaningful if you combine it with the airport identifier.

As far as what you need to select in a particular avionics package, that depends on the design, but you are correct that it doesn't affect how it processes the GPS / WAAS signals.


Can glide ratio be increased with bi-wing configuration? We all know that increasing the wingspan of an aircraft tends to increase the glide ratio.  Instead of increasing the wingspan, could an extra wing of the same size or less as in a bi-wing configuration also increase the glide ratio?  Which bi-wing configuration would be most ideal for this purpose?

Glide ratio is determined by L/D. If we can increase lift without increasing drag, or decrease drag without decreasing lift, we have a better glide ratio.

Increasing the wing span (for a given wing area => reduce the chord) helps: the longer wing span spreads out the lift creation over a longer distance, thereby decreasing induced drag. Keeping the flow laminar for as long as possible also helps.

Your question is about adding a second wing to an airplane. The problem with biplanes is that they were mainly constructed decades ago, and we associate them with older construction methods and mainly lots of drag, such as addressed in this question. It seems that the classical biplane has no aerodynamic advantages, and always has a lower L/D ratio than a comparable mono wing.

However there are now Computed Fluid Dynamics. There has been work done on biplane configuration with low drag in recent years - many are behind a pay wall, this one is not. Their work found two configurations with high L/D ratios:

Upper wing AoA = 13º
Upper wing forward sweep = 30º
Lower wing AoA = -2.8º
Lower wing backward sweep = 3.5º
Wing profile of both wings = NACA 4412
Both wings 6 ft span and 6 inch chord
Computed L/D = 32.3

The other configuration they found with a high L/D was a canard configuration: one wing in front of the other. If you insist on using two wings, this may be your best bet: one wing mounted in front of the other, in such a way that the back wing is out of the way of the wake of the front wing, or mounted at the optimum AoA for the disturbed flow. Their canard configuration used two different wings, resulting in an L/D of 35.8:

Note that none of the configurations included a fuselage of any kind, so their CFD work would have to be repeated with the fuselage and with interference computations. Fortunately computing power has increased dramatically in recent years.


Why bother having different engine options on one aircraft? When an airline purchases a new passenger jet, they often have a choice of engines (from different manufacturers). Why is this?

I would expect that one of the engine options is "best" and that all customers would buy that option. Thus it wouldn't be worthwhile to certify and offer the other (unwanted) options. Evidently this is not the case, so the question is why.

I can think of two possible reasons for this:

Different customers may have different requirements (e.g. shorthaul versus longhaul routes, operating in hot or cold environments) and different engine options have differing performance (e.g. better cruise efficiency versus climb efficiency or better performance in hot/cold temperatures). But I can't imagine that the requirements would be significantly different and, if they were very different, they would buy a totally different plane, not just a different engine.
Business/legal considerations: Perhaps airlines have contracts with engine manufacturers separate from aircraft manufacturers. e.g. British Airways may have a special relationship with Rolls Royce (possibly due to trade tariffs). But, again, I would imagine that airlines would always seek the most efficient engines, rather than signing such contracts.

Are either (or both) of these reasons right or is there a totally different reason?

When Lockheed built their L-1011 TriStar, they had one engine option, the Rolls-Royce RB211. RR did not deliver in time, and that has been attributed to the airplane's demise.

Although the TriStar's design schedule closely followed that of its competitor, Douglas beat Lockheed to market by a year due to delays in powerplant development.

So, having more than one engine option lowers the risk of a new airplane launch, and this has the benefit of allowing the airlines to pick their favorite.
Having one engine option on the other hand, like with the Boeing 777-300ER and the 777-200LR/F, allows cheaper certification, but it frustrates the customers:

Even if the 777X programme is successful, the sole-source deal has left some customers frustrated. Boeing argues that it chose the GE90 as the best engine for the 777X programme after its own rigorous competition. But as one airline executive says: "Our response is we'd rather run our own competition."
He adds the suspicion that Boeing decided to take the benefits of the engine choice competition for itself rather than let it accrue to the customer. Also, by eliminating a choice of engines, the company saved the costs of certifying more than one of them, he points out.

Back to your points: Your final conclusion is right, the performance difference is not a factor. For the second point, airlines get maintenance deals, so they like to stick with the manufacturer they already have business with. Getting discounts is easier when there is competition.

Related and interesting: How much would it cost for a large airline to switch engine manufacturer for their planes?


What is meant by empennage height? I know that the empennage is the tail section of the aircraft and includes the horizontal and vertical stabilisers. But what is the empennage height? Is it the height of the horizontal stabiliser from the ground or is it the height of the vertical stabiliser?

Of the two possibilities you mentioned, the most likely is the height of the top of the vertical stabilizer from the ground with the plane parked normally. For conventionally-configured tricycle-gear airplanes, this point is the tallest on the aircraft, and therefore this height is useful in determining the required clearance for covered parking areas and hangar openings.

There are a few designs for which a different height measurement is needed. A tail-dragger's tallest point will usually be either the cockpit or a point in the prop's sweep (more likely the cockpit for civilian designs, more likely the prop for old warbirds). The Predator drone uses an inverted V-tail, and its tallest point is actually its communications antenna. The Beechcraft V35 Bonanza's tail tips are within a couple of inches of the top of the cabin. In all these cases a "parked height" figure is required which will be greater than that of the vertical stabilizer.


What is the maximum number of waypoints one can have in a flight plan? Is there any constraint on how many waypoints may be in a flight plan (any regulation), including pilot defined waypoints?

The only info about this I have been able to find is this quote from the ICAO:

  ATS data systems may impose communications or processing constraints on information in filed flight plans.  Possible constraints may, for example, be limits regarding item length, number of elements in the route item, or total flight plan length.  Significant constraints are documented in the relevant Aeronautical Information Publication.

Which essentially just says that individual countries could impose (due to technical reasons) restrictions, but there are no general rules.

I have no doubt that many ATS systems would probably break if you asked them to process an unreasonably long flightplan. Hopefully the flightplan processing system is set up to reject such flightplans before they make it into the live system.

After playing around a bit with the EUROCONTROL flightplan validator tool (IFPUV), I have noticed that it starts throwing a syntax error when the FPL message is above approximately 985 characters in total. It's hard to say if this is just a limitation of the validator or if it's actually a limitation of the flightplan processing system. The actual error message is "SYN73: SUSPECT INVALID FIELD (F15 GENERATES TOO MANY RENAMED POINTS)", which I haven't been able to find an explination for in any of the usual EUROCONTROL manuals.

So, for practical purposes, you should probably keep your entire flightplan below ~985 characters, at least if you want it accepted in European airspace. But refer to the local AIP (probably ENR 1.10) of whichever country you are flying in to check if any local restrictions are mentioned.


Is this how ailerons are actuated? I was wondering how ailerons work on fighter jets. I came across this after hours of research.

Is that how fighter jet ailerons work? Is their a better picture or image that would be better. If there is please post it to help me understand. I've researched for so long on this subject with no avail except this picture that helped me understand it. Thank so much!

In the lower picture that accompanies the graphic in your link, focus on the elongated figure eight shaped piece.  The lower portion is attached to a piston extending from the actuator, and is free to pivot about the attaching point.
The upper portion of this link is fixed to the aileron and the entire part acts as a lever arm.  So, when the dark gray piston of the actuator moves left and right, the figure eight lever pivots clockwise or counter clockwise centered on the middle of the upper circle.  Since this part of the link is attached to the aileron, it also moves.
The graphic isn’t great, but try to visualize which parts move from my description and let me know if you are still confused.
You typically won't see the actual control horn on full sized aircraft because it will either be covered be a fairing to streamline it, or it will be contained within the thickness of the wing structure.
For example, in small RC models it is common to have an external horn like this:

However, you will generally NOT see a large external actuator like this on a full sized plane:

There simply isn't a need for such a large mechanism on most airplanes, and it would come with a large drag penalty.  If extra leverage is needed and the mechanism protrudes beyond the thickness of the wing it will be covered by a fairing like this:

Instead, most designers will try to keep actuators contained within the wing, as in the photos below:


How is an airline call sign assigned? Commercial aircraft are identified on the radio with an airline call sign and their flight number.

The answer on this related question mentions:

  the telephony designator of the aircraft operating agency

How is this designator assigned? Are there requirements for getting one?

These designators are assigned, on request, by ICAO. The request is made by the applicant through their National Aviation Autority.

There are actually two designators: the ICAO three letter code and the ICAO R/T designator.
All assigned designators are published in ICAO document 8585.

Two things are to be considered:

The three letter code must be unique to the aircraft operator
The R/T designator (callsign) must not cause confusion with other operators flying in the same area,  preferably at most three syllables long and it must be pronounceable in at least one of the following languages; English, French, Spanish or Russian.

Since there are many operators that have "airline" or "airways" in their name, that word is usually avoided in the R/T designator as it easily causes confusion.

For pilots it is not too difficult, they are typically paying attention specifically to their own R/T callsign. For controllers it is more difficult since they have to associate the three letter code on their screen with R/T designators that or not related in an obvious way.

For example:

British Airways has three letter code BAW and R/T designator
"SPEEDBIRD".
Thomas Cook Airlines Ltd has three letter code TCX, and R/T
designator "KESTREL".
Thomas Cook Airlines Belgium has three letter code TCW, and R/T
designator "THOMAS COOK".
Thomson Airways has three letter code TOM, and R/T designator "TOMSON"

Since the application goes through the NAA, there are slight differences in the procedure from country to country. Some examples are listed below:

Netherlands

AIC

USA

FAA Advisory Circular AC 120-26K and
FAA Order JO 7340.2D.
NBAA call-sign application guidance website


What does Alternate NA mean on an approach plate? The ILS Y or LOC Y RWY 3 approach at Raleigh Exec Jetport at Sanford-Lee County Airport (KTTA) in Sanford, North Carolina features the alternate minimums symbol with an NA next to it. Normally this symbol means that the airport has non-standard alternate minimums. What does A NA mean in this case?

That symbology means this approach cannot be considered when deciding if this airport is a suitable alternate.  This is probably due to the lack of weather reporting or the navaid might not have the required monitoring.  This means the availability of the approach cannot be guaranteed and so you can not rely on it as an alternate.

To file TTA as an alternate you must comply with the the non-standard alternate minimums and you must be legal and equipped to fly either the RNAV (GPS) RWY 03 or RNAV (GPS) RWY 21 procedures.  If the ILS Y RWY 03 is available when you get there, you may fly that procedure instead of the RNAV procedures, you just cannot file it as an alternate on the basis of the ILS.

The wording in the Terminal Procedures legend (PDF) is

  Alternate minimums are Not Authorized due to unmonitored facility or absence of weather reporting service


Why were the first airplanes "backwards"? In the question Is the location of an aircraft spoiler really that vital? the accepted answer states "Surfaces behind the CoG act as stabilisers, keeping the nose pointing forward. An aeroplane has vertical and horizontal tail surfaces at the back just for this purpose."

I agree that this seems straightforward, to a layman (me). So why then were so many of the first aircraft built 'backwards'. Taking a look at the Wright Flyer

Image (C) Bay Images

as an example. There are many other examples from the earliest days of aviation. Why did many put the elevators up front, thereby destabilizing the whole thing?

"Backwards" is relative, there are modern aircraft that have forward placed elevator i.e. canard designs that fly quite successfully

(source)

As for why its not more popular you can read up more on that in why are there no production canard GA aircraft? and Why is the Tu-144 the only commercial airplane with canard configuration? as well as in the answers to lots of 'canard' questions on this site.

As for why the Wrights did it this way, NASA offers an explanation

  The placement of the elevators at the front of aircraft is rather
  unique for the Wright flyer. Modern aircraft typically have the
  elevator at the rear, attached to the horizontal stabilizer. The
  Wright's placed their elevator at the front to provide protection to
  the pilot in the event of a crash. (The pilot of this aircraft lies
  next to the engine on the lower wing.) But there is also a static
  performance advantage when the elevator is placed forward. Lifting
  wings have a natural tendency to flip tail over nose because of the
  way the pressure is distributed.

So they were not necessarily backwards so much as different considerations were taken under advisement during the design.


How many layers do the Boeing 787 cabin windows have and what might make them fail? I am interested to know how the Boeing 787 cabin windows are made. Specifically, how many layers do they have and what can make them crack in-flight?

This article about the Boeing 787 Dreamliner states that

  [...] The Dreamliner’s new windows are made of two thin layers of glass with gel wedged between them. [...]

The windows have also a failsafe mechanism in order for the outer pane to crack or fail first, it's this tiny hole that you may have noticed, which serves to regulate the pressure exerted from the cabin to the window.

I couldn't find a more technical article about aircraft windows, but I found an AIAA article about Space Shuttle's cockpit window system, if you are interested about something a little bit more extreme!


What is the visual manoeuvring area (circling approach) radius for PANS-Ops (ICAO)? I'm very confused of the actual radius of the "protected area" in a circling approach for a PAN-OPS approach plate. The ICAO doc 8168 contains information for circling approach at 1 000 ft. But what about airports that are sea level? How do you calculate the radius?

P.s. Not TERPS :)

The 300 m and 1 000 m tables you found are examples. For the actual radius for any aerodrome elevation you need to apply the formulae.

The radius is determined using the formulas in Section 2, Chapter 3, Turn area construction (...) The TAS is based on:
a) altitude: aerodrome elevation + 300 m (1 000 ft); and
b) temperature: ISA + 15°.

First is $R$ (rate of turn) using $V$ (TAS in km/h) and $\alpha$ (bank angle of 20° for circling per Table I-2-3-1). Then plug $R$ in $r$ for the radius.
$$R = (6\;355 \tan \alpha) / \pi V
$$
$$r = V / (20 \pi R)
$$
Now, the $V$ (TAS) depends on the category and altitude (which is already aerodrome + 1 000 ft).
First get the IAS from Table I-4-1-1, then convert the IAS to TAS using the conversion factor in Section 2, Appendix to Chapter 1 (I-2-1-App-1).


Does a glider pilot with a PC canopy need to protect himself from UV radiation? I and several of my glider pilot friends are developing skin cancers.  I am looking for information about the risk.  Canopy tints or polymer types that provide protection would be important to know.

I assume that you mean polycarbonate by "PC". Polycarbonate plastics block nearly all UV radiation. Here is a typical transmission spectrum:

UV is the light below 400nm. As you can see, the curve shows 0% transmission in that range. In general, the risk of damage by radiation is greatly reduced by elimination of sub-400nm wavelengths.

Nevertheless, high energy visible light from 400-500nm should not be ignored and long exposure to such light can be damaging, especially to the eyes and is implicated in macular degeneration, and I know sailplane pilots with high hours who have suffered macular degeneration.

Also, if you are flying, you know that most pilots spend many hours out on the field waiting to launch or doing other chores, so there is plenty of time when you are not protected by a canopy at all.


What activities take place in the warning areas offshore of Washington, D.C.? Looking at the Washington sectional, several areas offshore are designated "Warning" areas: W-386, W-107, W-105 etc. The only notes given are "National Defense Operating Areas. Operations hazardous to the flight of aircraft conducted within these areas".

That would be enough to keep me well ashore, but out of morbid curiosity, what kind of activities are we talking about? Are these areas designated for naval gunnery/missile ranges?

Answering my own question, these areas are designated for a variety of naval training and testing exercises including missile launches and heavy guns firing, which is the primary reason it's hazardous. The airspace can be traversed under guidance from either Washington Center or FACSFAC VACAPES, but entry by VFR aircraft without established comms is not recommended.


Why, after decades of redesign and enhancement, does it still rain inside B737? For as long as I can remember, seats 1A/1B suffer from rain during acceleration/takeoff as the condensation pours down from the heavens, (well, the overhead panel where the reading lights, gaspers, etc. are).  I guess it was, at most, a minor inconvenience until pax started carrying more consumer electronics in their hands. Now it can be costly.

Obviously, I can fix the issue myself by avoiding those seats, but sometimes they're the only extra-legroom seats left, and I'm tall. I try to remember to keep my electronics pocketed until after takeoff, but it's so boring sitting there while 188 other pax try to stuff their inflated bags into straining lockers, and I forget.

I can understand that this rotation rain, or similar aggravation, might happen on a new aircraft design; something undetected until mass operations and not safety-critical anyway, but how is this still happening on an aircraft that first flew decades ago and has had so many enhancements?  I can't be the only one who has had a Seattle soaking, so why has this, possibly costly, annoyance not been fixed?  It can't be that difficult/expensive to cure, surely?

I've never heard of this on a 737. The last rows in a DC-9/MD-80 series is where I seen this the most.

The simplest solution is for the flight crew to set the cabin temperature control to not be full cold on a humid day. Just a little heat will cut down on the condensation being developed while sitting at the gate.

Here's an interesting article on moisture in the airplane, though it is not specific to the gasper vent rain.

http://www.boeing.com/commercial/aeromagazine/aero_05/m/m01/index.html

Some larger aircraft have dehumidifiers, but as the Boeing article states

  Dehumidification systems (ground-based or onboard) are effective at removing moisture but are not cost effective.


What twin engine is this USAF-painted plane? I keep trying to identify a group of aircraft, what bimotor is this plane?

(photos: own work; click to view)

(wikimedia.org) A T1 variant.

It appears to be a British Aerospace C-10A or Jetstream 3M.

  In order to improve sales prospects in the US, the fifth prototype was fitted with the US-built Garrett TPE-331 in place of the French Astazou. Changing to the US-built engine was enough to allow the United States Air Force (USAF) to consider it for cargo use. It eventually placed an order for 11, fitted with a cargo door and accommodation for 12 passengers or six stretcher cases, to be known as the C-10A, or Jetstream 3M. The USAF cancelled its order in October 1969 citing late delivery.

From https://en.wikipedia.org/wiki/Handley_Page_Jetstream

and it sounds like none may have been actually built.

See http://www.aerospace-technology.com/projects/turboprop/ for more info.

In the 1980s I remember them being a successful civilian commuter design. However, they were slow compared to the Fairchild Swearingen SA-227 Metroliner, a major competitor, and suffered from a lack of baggage capacity. They somewhat mitigated that by having an optional belly cargo carrier on them, but that slowed them down even more.

I was flying the SA-227 at the time for Horizon Air. Over the route from Redmond, Oregon to Portland, Oregon, the Jetstream (I can't remember the carrier's name—they eventually ceased operation), would leave a few minutes before us, but we would always overtake them and land in Portland before them.

For a time, over routes they shared with us, they would send the bags they couldn't carry over to us under a reciprocity agreement since we had ample baggage capacity. Eventually Horizon Air required them to pay since it was turning out to be a one way agreement, they never having any spare baggage room.


Why do jet nozzles close at different rates? After recently seeing the trailer for Top Gun 2, I was reminded of something that I always wondered about: why do the nozzles of some jet aircraft seem to close at different rates?

Sure, it's likely that both engines of the aircraft won't spool up at the same rates, but what about the nozzles? Are the nozzles dependent on the engine power output (which would then answer this question), or are they independently controlled? If so, what causes them to close/open at different rates?

Here's an example from the Top Gun 2 trailer:

The nozzle position is automatically adjusted according to the current engine state to maintain optimal exit flow conditions at the end of the nozzle.

The difference in nozzle actuation you are referring to is caused by a slight difference in activation time of the afterburner between the engines.

Please see the following Aviation Stack Exchange question and answer(s) for more information on the purpose of the nozzle:
Why are adjustable area jet engine nozzles mostly limited to military use?


Why do tapered wings not stall at the root first? Why do moderate tapered wings stall first at mid-wing section and high tapered wings stall at tip first? The effective AoA should be always higher close to the root? (considering no geometric or aerodynamic washout)

I will try to make it simple without going into mathematical details.

Source
Here, λ = ctip/croot
The important factors controlling the lift in the Tapered wing.

The ctip being too small affects the Reynolds Number (as the distance travelled is very small). Assuming the constant speed, density and viscosity, Reynolds number only varies with the distance travelled (x) directly. As the tip distance is too small, Reynolds number is not much increased for the boundary  layer and therefore transition from laminar flow to turbulent can not happen. Also due to skin friction, the flow gets slow and becomes separated. The separation causes loss of Lift and thus Wing Tips stall first.

The sweepback effect makes the boundary layer tends to flow spanwise toward the tips and becomes separated near the leading edges of tip.

Increasing Span Efficiency Factor,e as it is higher for tapered wings, it produces more Cl.

There may be other factors too, that I might have missed. But these are the most prominent. You can refer to below image. The Red Line shows seperated flows.

Yes, wingroot has a higher effective AoA. It is easier to understand this way. For a given AoA, the wingtips suffer from downwash and vortices which decreases AoAeff and increases Induced AoA (AoAi). As the wingroots do not suffer from the vortices, their AoAeff is higher.

Figure 2 & 3: FAA Pilot's Handbook of Aeronautical Knowledge
Figure 4: Introduction to Flight, Anderson


What are the pros & cons of Autobrake? What are the advantages/disadvantages of an autobrake system? In particular I am interested in the Fokker 100 system.

Have any studies been done for the F100 autobrakes and the affects that this system has on the landing gears?

Autobrake is a smart system which basically implies that the pressure is applied to the brakes when a set of circumstances are reached, such as when the plane detects that it is on the ground and the thrust is in idle.

There are in principle only advantages with an autobrake system, since it reduces pilot workload during the moments after landings and reduces necessary runway to stop. While autobrake has what I know not been the reason for any accidents, the faulty computer logic of the computers in the autobrake and spoiler system contributed to a Lufthansa accident in 1993.

As I gather, the Fokker 100 autobrake will be activated when the thrust levers are commanded to idle reverse and the system is on. It has 5 settings: RTO (Rejected Takeoff), OFF and LO/MID/HI, the latter 3 representing the amount of automatic brake pressure.

Autobrake is closely related to the spoilers, which are simply a button on the Fokker 70/100. The direct effect of spoilers on the landing gear is that they are pushed harder down onto the runway (since the lift from the wings is disrupted), making the braking more effective early on during landing.

Source (not very good but the best i could find)


Is it legal to build your own plane and fly it? Is it legal to build and fly your own airplane? This essentially leads to two central questions: Will the FAA even let you build and fly your own airplane?, and Will there need to be inspections and testing records to certify it as flightworthy and legal before being able to fly it?
Say I built my own plane, a large plane. 3 Propellers powering the plane. Amphibious, can land and take off on a runway or on the water. It's red, if this makes any real difference. And it is built to support 2 people and is supposed to be able to fly for up to 1 week without refueling. That being said, if the plane crashes it'll cause quite some damage. And to fly for 1 week supporting 2 people and quite a lot of cargo in the form of food and other essentials, the airplane is loaded with fuel.
If I know the plane floats on the water, could I fly it in an unpopulated area in short testing runs, or is that considered illegal? This also brings the question, Is there is a limit to how much fuel a plane can legally carry in a single flight? Any help would be much appreciated.

Yes, it's legal. The FAA has an amateur-built aircraft homepage with links to regulations, guidance etc.
It's also very common. The Experimental Aircraft Association has a lot of resources too.


Are there any accident reports that mention hacked/compromised avionics as a factor? Does anyone know of any documented examples of airline accidents caused by hacked or otherwise deliberately compromised avionics?

Searching the NTSB Accident Database which also covers some international commercial flights for the key words "hack", "hacked" do not provide any results related to hacked avionics. Likewise using the keyword "tampered" does not provide any results related to deliberate tampering with avionics (mostly "tampered" wreckage).

There is one individual, Chris Roberts, who claims to have hacked into the In-Flight Entertainment System (IFE) to cause the aircraft to deviate from its course, however I find it highly suspicious. From a security perspective, I can't of any good reason to have the IFE network on the same network as any avionics, however that isn't to say that there isn't some possible connection. The FBI certainly didn't think it was very funny.

So as of today there have not been any aviation accidents that are attributed to hacked or deliberately tampered-with avionics as a cause of said accident. The closest thing I think you will find is pilots loading incorrect flight plans, outdated (or even pre-dated) navigation data, and other cases of pilots not correctly using the equipment.

I did read your original question and would like to clarify. Conspiracy theories, guesses, and internet speculation are not a replacement for hard evidence. In the case of MH-17 the wreckage was recovered and no evidence has been presented in the favor of hacking, while there is a lot of evidence of the shoot-down. Airliners even at cruising altitude can be shot down by surface to air missiles rather easily.

As for MH-370, until the wreckage is found we may never know (and even if it is we may never know). Black boxes do not have an indefinite life span especially if they are under water. Until then theories of that flight being hacked are just as credible as those saying aliens abducted the entire flight and they are living on Neptune in specially constructed Igloos.


What is the difference between ICAO, JAA, FAA, EASA, and CAA? There seems to be many regulation bodies across the globe, how are they different and how are they alike? Specifically, how is ICAO different from the rest?

ICAO is the International Civil Aviation Organization, a specialized agency of United Nations. It was created after the Convention on International Civil Aviation (the Chicago Convention) of 1944 was ratified in 1947. The purpose of ICAO is according to the convention:

  "WHEREAS the future development of international civil aviation can greatly help to create and preserve friendship and understanding among the nations and peoples of the world, yet its abuse can become a threat to the general security; and

  WHEREAS it is desirable to avoid friction and to promote that co-operation between nations and peoples upon which the peace of the world depends;

  THEREFORE, the undersigned governments having agreed on certain principles and arrangements in order that international civil aviation may be developed in a safe and orderly manner and that international air transport services may be established on the basis of equality of opportunity and operated soundly and economically;

  Have accordingly concluded this Convention to that end."

Within ICAO, the 191 Member States and a number of global aviation organizations work together to develop international Standards and Recommended Practices (SARPs). These SARPs are the reference for states developing their national civil aviation regulations which are legally enforcible. This is an important aspect: ICAO SARPs are not legally binding by themselves, they form the basis of national regulations which have legal status.

In this way civil aviation regulations are harmonized all over the world, with slight differences based on the actual implementation in national regulations. These local differences are then reported back to ICAO and published.

CAA is the Civil Aviation Authority. This is a generic term used in many countries, notably the UK and China. A CAA is a national regulatory body responsible for aviation. The CAA implements the ICAO SARPs in national legislation and is responsible for regulatory oversight.

FAA is the Federal Aviation Administration. As the Civil Aviation Authority of the USA, it is responsible for establishing aviation regulations in the US. These are known as FARs (Federal Aviation Regulations).

In addition to its regulatory role, the FAA is also responsible for Airspace and Air Traffic Management, maintenance of Air Navigation Facilities infrastructure and has an active role in Research and Development of aviation related systems and technologies.

JAA were the Joint Aviation Authorities, a co-operation of most European (EU and non-EU) civil aviation regulatory authorities. It was not a regulatory body itself, the member authorities were responsible for the regulation.

Originally started as the Joint Airworthiness Authorities in 1970 its objectives were to produce common certification standards for large aircraft and aircraft engines to facilitate a European aviation manufacturing industry (Airbus). Over time, the scope was extended to included aircraft operations, maintenance, licensing and certification/design standards for all classes of aircraft.

With the creation of EASA (see below) in 2002, the EU members transferred the airworthiness regulations away from the JAA. Over time, EASA became responsible for operations and licensing as well.  In 2009 the JAA system was disbanded. Only the training organisation (JAA-TO) remains, it mainly provides courses for CAA staff of European countries.

EASA is European Aviation Safety Agency. Created in 2002 by the European Commission, EASA took over the functions of the Joint Aviation Authorities of the EU countries.

The responsibilities of EASA include drafting of aviation safety legislation and providing technical advice to the European Commission and to the EU Member States, airworthiness and type certification of aircraft and aircraft parts for aircraft operating in the EU, approval of aircraft design organisations world-wide and of production and maintenance organisations inside and outside of the EU.

Since its creation, the competences of EASA have gradually expanded. This process is still going on.


How does ATC know if you have been assigned a squawk or not? Departing VFR from class D airspace, either you can request flight following after you leave the airspace or you may ask the airport ground/tower and they assign you a squawk code and setup the flight following for you.

For example, on a short flight from KCNO to KSNA, I usually get my squawk code from tower and after departure I switch to SOCAL and let them know I departed KCNO and usually follows by "ident".

How does ATC know if you already have a squawk code assigned to you or not?

How does initial communication with SOCAL differ from when you've been assigned squawk on ground before take off from when you want to request one?

Let's remember what a transponder code is used for in the first place.

When transmitting a non-discrete code, such as 1200, ATC has no way1 of telling you apart from other aircraft transmitting the same code. On the ATC radar, you will simply show up as something like "1200/A030" where 1200 is the code transmitted and A030 (3000FT) is you mode C altitude.
To provide you with radar service, ATC has to be able to identify you on the radar, which means that they have to be able to distinctly associate your flight with a specific track on the radar. They do this using transponder codes2. An assigned (discrete) transponder code will be unique for a flight within a given area. Say you are instructed to squawk 2174, you will now show up on the screen as "2174/A030", and because ATC knows that you, and no one else in the area, is squawking 2174, they know that they are you looking at you.

Your question is, how does ATC keep track of the assigned transponder codes? Today this is mostly an automated process. A central flight data processing system (or the US equivalent - sorry for my European terminology) contains a database of all active flightplans as well as their assigned transponder codes. When a discrete transponder code is received by the ATM system, it will automatically query this database, asking for any flightplan information associated with this specific transponder code. It will then show the received information on the radar screen. Essentially, the transponder code is a database key, allowing the system to look up information about your flight and show it to the controller.

Now you are no longer shown on the radar with just your transponder code and mode C altitude, because the system has a lot more information about you. You might be shown as something like "N12345/A030/C172" (callsign/altitude/aircraft type). The exact label varies greatly from system to system - the point here being that the system is now able to show specific information from your flightplan rather than just generic information submitted by your transponder.

So how does a controller know if you have been assigned a transponder code or not? If you have, you will be shown on their radar screen as a full label, with callsign etc. If not, your callsign is no where to be found on the radar, and they will have to guess that you are probably one of the (potentially many) 1200-tracks on the screen.

1: They can in fact identify you by other means, such as requesting your exact position and heading, but identification with the help of transponder is a lot simpler.

2: Normal 4-digit transponder codes are slowly being replaced by mode-S transponders, which transmit a unique 24-bit aircraft identifier at all times to the radar system. But that's probably best left for another question.


How do I clear an aircraft to exit a hold? I'm learning ATC and one question is worrying me..

How do I tell a pilot to exit a hold and continue inbound...

Eg: pilot has entered hold and done one orbit.
and started the second,
and I want the aircraft to turn again 180 degree on inbound.

Basically how do I tell an aircraft to break out the stack, and continue flight plan.

This depends on what exactly you want the aircraft to do:

Immediately depart the hold and fly directly to another fix:

  N1234 cleared to 'clearance limit' via direct 'fix' and then as previously filed.

Immediately depart the hold and fly an assigned heading:

  N1234 Cleared to 'clearance limit' via radar vectors. Fly heading 'heading'.

Proceed to another fix after passing the holding point inbound:

  N1234 After 'holding fix', cleared to 'clearance limit' via direct 'fix' and then as previously filed.

Fly the published approach after passing the holding point inbound:

  N1234 Maintain 'altitude' until 'holding fix', cleared 'approach name'.

Altitude

Lastly, add the altitude to any of the above clearances if it is different than the current altitude by adding a climb/descend and maintain  'altitude' clearance.