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Can the FPV be used to determine AOA
In smooth air or turbulent air?
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If you combine the FPV and attitude, then technically yes. However, you will also need to know what reference the attitude is taken from (one would assume the body angle) and compare this with the angel of incidence. So it's a bit complex.
Several accident reports have mentioned that an angle of attack indication would have been useful to the unfortunate crew and I gather it is an option on the A320 family. But no customer seems to fit it. I have flown aircraft with AoA gauges (all military) and they are very useful. |
I'd be very careful, especially if you are dealing with a "non-normal" and above all check what your type's FCOM has to say on the subject.
Certainly on some Boeings there's an Air Data input to the FPV. As a result there is a warning in the 777 QRH that for at least one Flight Instrument non-normal that you must not use the FPV. |
On the Ejets not only can it be used but it is the primary indication of alpha margin in conjunction with the PLI.
The non-normal caveat already stated also applies. |
To quote forum member OPEN DES directly from another thread regarding the A320:
"FPV is actually a derivative of Vzbi. Vzbi is baro-inertial vertical speed, i.e.: the one we see indicated in normal conditions. FPA is derived using GS and Vzbi. In conditions where Vzbi is not available FPA is derived from the difference between pitch and AoA, this is however a degraded FPA, relative to the air only" |
It is not so much a question of smooth or turbulent air, it is a matter of the (constant) motion of the surrounding air. In a windshear situation (in a downburst) when you need it most it will have the greatest error. In calm air you do not need it.
Given the fact that any large transport aircraft does have an AoA sensor, it is just a matter of installing the indicator (or to implement the indication into your PFD). So what do you win if you calculate AoA from other data? The AoA data is there. |
Originally Posted by Volume
(Post 8609485)
In a windshear situation (in a downburst) when you need it most it will have the greatest error. In calm air you do not need it.
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In no wind yes...your angle of attack is the difference between your pitch (angle of incidence) and FPV angle.
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In no wind, with no air data malfunction, not climbing nor descending. Easier to ask your airline to pay for the AoA indication. It is available as customer option after all.
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From an airbus/boeing doc discussing about loss of control,upset recovery...
The flight path angle is the angle between the flight path vector and the horizon. This is also the climb (or descent angle). On the newest generation jet transports, this angle can be displayed on the primary flight display (PFD), as depicted in Figure 18. Flight path angle can also be inferred from the vertical speed indicator (VSI) or altimeter, if the ground speed is known. Many standard instrument departures require knowledge of flight path angle in order to ensure obstacle clearance. Angle of attack is also the difference between the pitch attitude and the flight path angle with no vertical wind component. The angle of attack determines whether the aerodynamic surfaces on the airplane are stalled or not. |
On A320 (and probably on other Airbus types too) the AoA value is available in Alpha Call up MCDU maintenance pages. However it displays only the values for ADR's AoA #1 and #2 if I recall, the SBY (#3) can't be shown.
I remember some A320 in Air Inter (absorbed by AF in the late 80's or early 90's) had an AoA "analog" gauge displayed just below the PFD/ND control panel, but only on the captain's side. |
Boeing
I've never seen any Boeing with an AOA display.
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If you combine the FPV and attitude, then technically yes. However, you will also need to know what reference the attitude is taken from (one would assume the body angle) and compare this with the angel of incidence. So it's a bit complex. |
I've never seen any Boeing with an AOA display. Photos: Boeing 737-823 Aircraft Pictures | Airliners.net Photos: Boeing 777-223/ER Aircraft Pictures | Airliners.net |
AoA gauge
Here's a good write-up from Boeing on their AoA gauge.
Aero 12 - Angle of Attack It includes a nice diagram. http://www.boeing.com/commercial/aer...tack_fig13.jpg http://www.boeing.com/commercial/aer...tack_fig13.jpg I couldn't find any information on an Airbus AoA gauge. |
So in smooth air with no malfunctions, is it
Originally Posted by Denti
(Post 8609692)
In no wind, with no air data malfunction, not climbing nor descending.
Originally Posted by Kefuddle
(Post 8609986)
Have I glossed over something? I reckon the simple difference between the FPV and PFD attitude in all phases is the angle of attack (within the limitation of the FPV capabilities of course).
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AOA
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The difference between Pitch and FPA certainly gives a rough guide to AoA, particularly changes of AoA. As has been said by others, it should be correct in still air, but becomes increasingly inaccurate as the headwind or tailwind component increases. That's because FPA is in relation to the earth, not the airmass. (So vertical movement of the airmass, particularly a microburst, will also cause inaccuracy.)
The wind-component problem is perhaps best illustrated by considering the extreme case where headwind equals TAS, in which case any rate of climb or descent would produce (unreadable) FPAs of +90 and -90 respectively. |
As posted above the FPV in boeing and airbus is not completely inertial, it receives air data input as well. Boeing used to promote its use in some non normal scenarios but have since removed those remarks or put some in that warn pilots not to use the FPV.
For AoA indication boeing offers an AoA gauge on the PFD as customer option which takes the place of the circular RA gauge above the AI, RA moves to the bottom of the AI as digital output only (737, might be different on other types). |
Like Dan said, it's not really Alpha as that is defined, aerodynamically, as the angle between the chord of the wing and the freestream airflow.
As the angle of incidence varies along the span of the wing so does Alpha. |
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