Chris, HazelNuts39;
Originally Posted by HazelNuts39
Ambient conditions: close to sealevel, ISA, still air (initially)
Airplane: A330, 205t, cg as QF72, airspeed 200 kt CAS (= TAS), AoA = 5.5 degrees
Set thrust to achieve FPA = -3 degrees (down), pitch attitude 2.5 deg up, rate of descent 1061 fpm, maintaining 200 kt.
Next, throw in 20 kt headwind, leaving everything else unchanged, and FPA increases to –3.3 degrees.
To return to FPA –3, increase thrust to achieve rate of descent of 955 fpm, and pitch angle will increase to 2.8 deg up. AoA remains 5.5 throughout because airspeed remains 200 kt.
and,
Originally Posted by Chris Scott
It is flying straight and level at a TAS of 50 kts into a 50-knot headwind (GS 0), pitch +3, FPA 0, Alpha +3. Because the glider is stationary in relation to the earth, an FPA of zero would only be possible if the pilot maintains the VS precisely at zero. The tiniest rate of descent would presumably result in FPA -90 (vertical descent), suggesting an Alpha of +93. Similarly, a tiny rate of climb would give FPA +90 (vertical climb), suggesting Alpha -87. But, in fact, Alpha remains at +3.
For me, these two examples make the matter quite clear; thank you both.
BOAC;
Certainly trying to work out my AoA from PJ's picture, whilst wrestling with conflicting IAS inputs and possible IRU problems would have significantly challenged me!
Absolutely agree with you. I think it would challenge anyone primarily because, though in now-understood cases where it may work, the FPV symbol was not intended as an AoA indicator. Throw in degrading systems as described and the task would be very difficult indeed.
I have asked myself in the face of many expressed dispositions towards AoA displays (
almost exclusively from those who have flown military vice civilian equipment and thus have experience with direct AoA displays) why the industry has not moved in a similar direction given the other significant computing, system and display capabilities now available. In his post #540,
Machinbird suggested examining accidents in which the presence of an AoA indication may have prevented the accident through better awareness of AoA. I sought a possible answer, albeit informally, in my response #541 citing six relatively recent fatal accidents which resulted from stalling the aircraft. I thought that an AoA indication may possibly have assisted the crew of only one accident, the USAirways B737 near Pittsburgh. Unless associated with sufficient and unmistakble warning, (we are into complexity and computers again), it appears as though the availability of an AoA indication would not have assisted, for example, the THY B737 crew. The Colgan case is similar in this regard as again, airspeed was permitted to bleed off and it is difficult to know if the either crew would have responded to another, different indication. It is clear to all that an AoA indication would not have prevented accidents in which the stall resulted from a contaminated/damaged wing.
It is interesting that, from the discussion anyway, only military aircraft appear to have AoA displays as standard. Would this point to the fact that military aircraft are operated much closer to aerodynamic extremes than airliners and that the benefits of having such information do not outweigh the cost? I can only speak from my own airline experience and the need for a direct AoA display was never apparent.
I am certainly not arguing against the display - the more information the better providing it is unequivocably clear to the crew and they are trained to use the information effectively; Given the many comments however, I am trying to understand why the display has never been installed in civilian airliners.
Whether such information would have assisted the crew of AF447 is of course speculation. However, given the large speed difference between a M0.80 cruise CAS and VLS (about 60kts and another 20+kts or so to the stall) it is difficult to comprehend a situation, even in most turbulence (unless direct entry into a cell), in which airspeed is lost that, as per QRH memory items, setting power, flying pitch in alternate law, (not difficult) would not in and of itself have stabilized the situation. I can't help but wonder what else went on that so severely destabilized the airplane.
PJ2