RetiredF4;
Further research on the AoA issues raised has yielded some interesting results.
Boeing has produced a document entitled, "
Operational Use of Angle of Attack". In the introduction, Boeing states:
A dedicated AOA indicator shown on the primary flight display (PFD) recently has been developed in cooperation with airline customers. The new indicator is offered as an option on the 737-600/-700/-800/ -900, 767-400, and 777 at this time.
During the development of the new indicator, discussions with airlines, the NTSB, and U.S. Federal Aviation Administration (FAA) pilots and engineers provided a unique opportunity to examine potential uses of AOA and the many existing uses that have evolved in recent decades along with advances in display and indication technology.
This article discusses the following:
1. Basic principles of AOA.
2. Airplane performance and AOA.
3. AOA measurement.
4. AOA indications and flight crew procedures in current Boeing production models.
5. Design and uses of a separate AOA indicator.
The document answers my question:
Boeing and several operators worked together to develop the display format for an optional AOA indicator (fig. 12). The upper right location was chosen as one that can be accomplished without significant rearrangement of the existing PFD or electronic flight display formats. The indicator itself consists of an analog scale and pointer, and digital representation similar to displays of many other parameters throughout the flight deck.
Stall warning AOA is shown with a red tick mark, which will change position as a function of Mach number for those airplanes with Mach dependent stall warning schedules.
A green approach reference band is shown whenever landing flaps are selected. The range of the approach reference band accounts for normally expected variations in CG, thrust, sideslip, and other considerations.
Many AOA indicators used in the past have been of the “normalized” type, where AOA is shown in arbitrary units and scaled so that zero load factor is shown as an AOA of zero and stall is shown as an AOA of one. Normalized AOA on a commercial jetliner would require that Mach number be introduced into the calculation of AOA because stall AOA and buffet margins are a function of Mach number.
The indicator developed shows body AOA in degrees and is not normalized, which is related to the second objective above, that the indicator be useful when pitot or static data, and therefore Mach calculations, are unreliable because of blockage or a fault in the system. The pointer of a normalized indicator in this condition would behave erratically, making the indicator unusable.
With the non-normalized design, the position of the needle is a function only of sensed AOA. The red tick mark for stall warning may behave erratically in a pitot or static failure state, as may stick shaker, PLI, and speed tape amber and red bands. However, the AOA needle and digits will remain stable, and the indicator itself still will be useful as a backup for unreliable airspeed, provided the AOA vanes are undamaged.
I haven't seen a similar document from Airbus yet but will continue research. Essentially however, this is how an AoA indication should function for an airline crew and I think may address
Owain Glyndwr's points as well.