Text 19493, 65 rader
Skriven 2011-05-29 02:29:15 av Ward Dossche (2:292/854)
  Kommentar till text 19470 av Robert Bashe (2:2448/44)
Ärende: Re: In case you didn't read it yet ..
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RB> However, I don't think this will completely explain the crash situation,
RB> since we still have the mystery of why the pilot_s_ raised the nose in
RB> the wake of a stall warning. I've read some speculation that this could
RB> have been due to some malfunction in the system (autopilot kicking in,
RB> for example), since it's inconceiveable that all three experienced pilots
RB> would make the same beginner's error even in a critical situation.
 
I suppose as a scientist you know and understand the forces that keep an
airplane in the sky.
 
You probably know and understand the role of the pitot-tube. They measure
airspeed.
 
However due to way different air-density at, let's say 35.000 feet, the
measured airspeed would be somewhat around 240-250 knots while the actual
groundspeed would be about the double.
 
Supercooled rain on pitot-tubes "might" have the effect of overspeeding, it
would be a faulty reading but nevertheless that reading is there. As a result
the autopilot would put the plane in a slight nose-up attitude to decrease the
airflow over the wings and as a result decrease the speed of the airplane.
 
Remember ... normal airspeed 240 knots, registered faulty airspeed 340 knots.
With enough nose-up attitude that airspeed could drop 100 knots but meaning
that instead of reducing airspeed to 240knots, it gets reduced to 140 knots.
 
Remember, the actual ground speed then is about 400 knots or some 720km/h ...
well enough to keep the plane flying at lower altitudes. But we're at 35.000ft,
with lesser air density and an airspeed of 140 knots, that's 250km/h ...
without flaps nor slats that A330 with nose-up attitude simply stalls and falls
out of the sky caused by the autopilot.
 
To get out of the stall one pushes the stick forward to get a nose-down, gain
speed and recover from the stall.
 
When however the airspeed dramatically dropped below a certain treshold there's
not enough airflow/speed to force the nose-down.
 
Applying power might take care of that, however because of the nose-up attitude
the engine-nacelle opening is at less than 100% due to the angle of attack and
catches less air than that it needs to force the spool-up.
 
Conclusion: unless the flightcrew thoroughly understands what is going on then
the stall is unrecoverable.
 
Contrary to what one might think, the nose-up most likely was not caused by a
pilot but by a computer trying to compensate for a faulty airspeed reading.
 
This is one of those rare cases which will contribute to increased air-safety
by better crew training. Most likely this exact same accident will not occur
again in the future.
 
But what a feat that these black boxes were found. It was a needle in a
haystack.
 
Did you know, by the way, how easy a plane can stall at high altitude? I was
completely unaware, but then it dawned. When at 35.000ft and 240kts of measured
airspeed and lesser density the pilot would make an abrupt turn, one wing goes
up, the other down ... that reduces the surface for lift drastically, the plane
simply stalls and falls out of the sky.
 
 \%/@rd

--- D'Bridge 3.59
 * Origin: Many Glacier / WikiLeaks http://213.251.145.96 / (2:292/854)