Hacker News Reader

Comment by dreamcompiler | Hacker News Reader

I'm amazed that the autopilot kicked off just because the pitot tubes froze. The A330 has an inertial reference unit as well as (almost certainly) GPS, both of which could have provided ground speed. Air speed could be extrapolated from previous air speed and inferred from ground speed, engine thrust, and known weather. IRU and GPS could also have provided altitude and thus climb rate. And we know the plane had a barometer, although it had to be corrected for airspeed according to TFA. I'm guessing the plane also had a radar altimeter, but maybe not.

With all these sources of information, the autopilot could have said "Hey the pitot tubes are giving me weird data, but no worries. Still flying the plane. Carry on."

Why didn't that happen?

View on HN

> Air speed could be extrapolated from previous air speed and inferred from ground speed, engine thrust, and known weather.

I think this is probably the main sticking point. I don't think you can do this accurately without some direct measurement. If the plane thinks its air speed is fine but it's actually approaching its stall speed because it guessed its airspeed incorrectly, that'd be really bad.

Stall is determined solely by angle-of-attack, which TFA points out. Airspeed doesn't factor in to whether the computer thinks it's about to stall.

Here's the relevant sentence: "Crews involved in similar incidents reported that they assumed the brief stall warnings were generated by the erroneous airspeed readings, a not unreasonable interpretation which breaks down only when one learns that the stall warning calculations are based on angle of attack and do not incorporate any airspeed data."

Except that AoA vanes are not perfect and are speed sensitive.

> From the A330/A340 FCTM (Flight Crew Training Manual) section 8.110.4:

> The ADRs provide a number of outputs to many systems and a blockage of the pitot and/or static systems may also lead to the following:

> …

> Alpha floor activation (because AOA outputs from the sensors are corrected by speed inputs)

From what I can tell, these systems are all pretty complicated and have undergone significant real world testing to ensure that the expected protections actually work, assuming the flight computer is receiving accurate data. Which is precisely why they have Alternate Laws - if they can’t guarantee that these protections can work, then they won’t.

Reply on news.ycombinator.com