[SystemSafety] US House Transportation Committee Preliminary Report into Boeing 737 MAX

[Peter Bernard Ladkin]

On 2020-03-16 06:27 , Kinalzyk, Dietmar AVL/DE wrote:
> 
> I did not understand, why the AOA Disagree alert is not safety critical. 

The AoA info feeds in to the airspeed (AS) calibrations. If AoA differs on each side, then AS will
differ and you get a UAS condition. The main indicator of an issue is thus unreliable air speed
(UAS), and there are procedures for coping with UAS. The priority is the UAS handling procedures.
The cause of the UAS (here we are supposing it is AoA sensor problems) is very much secondary to
handling it appropriately.

AoA information also feeds into stall warning. But part of the UAS procedures is to fly on
"pitch+power", that is, to set power and pitch according to a table and the aerodynamics then gives
you the AS you want, irrespective of what is indicated. In fact, AS and pitch are both functional on
power setting if you hold altitude (fly "straight and level"). And you can usually do that, because
the static ports and their data path are physically completely separate from AoA sensing and its
data path, into the ADIRU, so UAS will not generally be correlated with wonky altitude info. (If
your ADIRU is playing up, then that is a different matter.) Pitch will be dependent on wing
configuration, but that is a nominally static parameter.

That is the "classical" approach, and you will find commercial pilots (almost without exception)
agreeing with this. And practicing it in the simulator during review, nowadays. Almost nobody finds
AoA information in commercial flying at all helpful, although there are a few people who advocate
it. There has been sporadic but regular debate for decades in the manufacturers' magazines about
whether showing AoA values to flight crew are of any use.

Navy and marine pilots use it, though, for carrier landings. They don't use AoA itself but rather a
scale. The wings produce lift according to a bell curve and they want to be near the top of that
when landing on a carrier at the lowest speed possible, but not over the top and down the other side!

Commercial pilots want to stay well away from that peak during all phases of flight, and do (except
when they screw up royally).

> Boeing took a sophisticated solution: combination of airspeed and altitude errors, hiding the real root
> cause !!!  

Pilots don't necessarily care about the root cause of a hazard condition. They care about handling
the hazard condition appropriately.

> ... Since 2006, the AOA DISAGREE alert has been installed on all newly manufactured Boeing 737 NG aircraft, and is available as a retrofit for older aircraft.> The AOA DISAGREE alert has not been considered as a safety feature by Boeing (737 MAX), and is not
necessary to safely operate the aircraft.

All that is true of the NG.

It turned out not to be true of the MAX, for reasons which the report laid out. The report made very
clear that, first, the company developed a faulty hazard and risk analysis (which was evident to
some of us from the beginning) and, second, some of its engineers knew that (as one would have
hoped!). The reasons why they weren't paid attention to seem to have nothing to do with engineering.

PBL

Prof. Peter Bernard Ladkin, Bielefeld, Germany
MoreInCommon
Je suis Charlie
Tel+msg +49 (0)521 880 7319  www.rvs-bi.de





-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 833 bytes
Desc: OpenPGP digital signature
URL: <https://lists.techfak.uni-bielefeld.de/pipermail/systemsafety/attachments/20200316/6c288b35/attachment.sig>