Originally Posted by
John Farley
Surely it is about whether it is easier to glance at an AOA indication and at once have a reasonable feel for your margin from the stall or whether you would do better to direct the glance towards the ASI?
Looked at that way it is no contest as far as I am concerned as the ASI reading is no help without in some way taking into account the current weight, bank angle and g loading – the effects of which on the ASI indication at the stall as we all know can be large.
...and if the pilots are briefed properly on where the real limits are, and if the gauge is reliable, that's fine. We routinely flight test with a boom (as does most everyone) and the AoA is displayed prominently in the cockpit, and limits are briefed for various tests. I'm sure our TPs wouldn't fly without it.
BUT, for routine line use, unless you generate the same level of briefing knowledge in line crews, then it's not so useful. The fear - to use that word again - among the design/engineering community is that an AoA gauge will either mislead or be abused or both.
"raw" AoA would be difficult to justify in a line operation - stall angles are too dependent on config and on speed to expect anyone to remember the values. So some form of indexing/normalisation is required. But even that can't be made "right" and sometimes is downright misleading. For example....
Suppose the stall AoA is 10 deg at M0.6, and the zero lift AoA is 0, to make life simple. A plane dawdling along at 8 deg AoA at M0.6 will displane a normalised AoA of 0.80, and would appear to show a stall speed of sqrt(AoA-n)*0.60=M0.54. But as you slow down, let's say by M0.55 the stall AoA has increased to 11 deg; the plane will be somewhere near 9.5AoA at this slower speed, but AoA-n will now be 0.86, and the stall speed will now appear to be sqrt(0.86)*0.55=M0.51.
This Mach-dependency effect is real and confusing, especially to crews who've never seen it before. So predicting what our hypothetical crew might do with this imperfect information is hard. Similar examples can be shown for things like contamination/anti-ice failures, OEI operations, sideslip conditions. There's a lot of compensation going on the stall computers to address all these kinds of things, but since AoA itself isn't being displayed it's possible to be conservative in applying corrections (one example might be deliberately mis-compensating for sideslip to provide more stall protection in sideslip cases); if it were displayed we'd have to make it "right", whatever that means.