Originally Posted by
73qanda
Am I correct in thinking that the Navy displays ‘units’ instead of degrees in order to create an appropriate scale?
Yes. Because actual degrees of AOA really is meaningless, unless you're a "boffin" interested in things like measuring the field of view from the design eye position (more nose-up means less view), or for extreme cases the off-boresight capabilities of a missile attached to a launch rail, or things like that.
Admittedly off-topic for this MCAS problem, but still maybe of interest to some: Because a carrier-landing aircraft must land within a very narrow (~2 degree) range of angles relative to the flight deck to maximize its chances of capturing the arresting cable, and to make certain it's at an appropriate speed for not breaking itself or the cable, AOA is incredibly critical for carrier operations. However, the pilot still doesn't care what the actual values are. He needs an easy-to-remember, easy-to-read, and eminently predictable value as reference. So to provide nice round numbers to look at on displays, and to memorize for safety, all Naval aircraft AOAs are usually remapped to units with a rather arbitrary scale. And it varies with configuration; flap position, for example, significantly changes actual aerodynamic AOA at a given airspeed, but the airplane still has to land at the same nose-to-deck relative angle. So that remapping is variable. And it's typically indexed to 15 (plus or minus) for convenience and memorization. There may be simultaneously four or more different airplane types on a carrier (F-18E/F, F-18G, EA-6B, C-2, E-2, T-45, JSF), but all the pilots know "15 units" is the default landing AOA, more or less. The landing signals officer (LSO) handles all the types in a given day, and can't be bothered to remember 12.5 deg for this one, and 9.3 for that one, and.... Just make them all 15ish.
Similarly, the stall AOA varies with configuration and weight and other things... but keeping mental track of stall speed variations with weight and configuration is beyond the brainpower available. So it is also convenient to map stall speed to a fixed number - 30 units becomes convenient. And for a fighter pilot, stall AOA is incredibly important, because in a combat situation he may touch or even exceed that angle routinely (and safely).
At the low end, near-zero AOA in units generally corresponds to zero lift. Not really relevant much of the time, except during combat maximum pushovers, but still a useful metric.
Thus, the pilot always knows with a glance where he lies on the stall/landing/zero-lift continuum. It may have nothing to do with the actual AOA in degrees, but to his brain, that matters not at all.
To bring it back to MCAS and 737, adding true AOA - or even units AOA - to the flight deck of a commercial airliner is sort of irrelevant. They should NEVER approach stall; they better not be doing zero-lift pushovers; they don't have any extreme limits on AOA for touchdown or liftoff (other than perhaps avoiding tailstrike - but for that, pitch angle is the relevant data, not AOA.
Thus, from my point of view, adding AOA to commercial aircraft displays is "interesting but irrelevant." I challenge any pilot to tell me whether 12.3 degrees (for example) is meaningful by itself. It is only interesting if you also know that stall at your current weight and flap and gear setting is 19.7 deg, and the minimum load factor happens to be at 4.8 degrees, and if you also have the other AOA unit's value to compare to make sure they both agree within a reasonable limit. But making sense of all that data is just distraction from the real task of flying, and has no practical value. The only data you really need is the stall speed indication on the zipper, which does all that thinking for you.