Cheers,
I see some discussion of Altitude Hold phase of Flight control
management. While I am in no position to question builders/fliers on their
experience, I feel it imcumbent to mention it if those unaccustomed to
auto-control are contemplating its installation.
The one phase of 'altitude hold' deserves careful consideration.
In modern transport aircraft, this phase by itself is inherently unsafe,
unless Airspeed Control is also imposed. While it's obvious that we don't
want to hold altititude 'til the airspeed loss induces a cruise wing stall -
it's happened - it IS possible that in this phase other distractions can
make it happen.
Even in modern transports (the L-1011 was most advanced at the
time), the failure to monitor airspeed and guard against stall can be
induced without warning. In the case of the TriStar, very often the aircraft
was forced to climb higher for overriding reasons than ideal, which put it
within so many knots of stall, and of Mach limits. Unforecast turbulence
knocked the auto-pilot down a notch to permit it to smooth outcorrections in
airspeed and height to play each against the other for a smoother flight.
However, it knocked the auto-throttles off as well - so we were particularly
sensitive to stall and Mach. An onboard diversion occurred which (only for
seconds) took attention of one away from the 'watch' and the aircraft slowed
to below minimum airspeed. Immediately, the descent in pre-satll was
arrested, but the recovery was reallly delicate, and height was sacrificed
for speed during power application. This cautionary manoeuvre was not best
described in the manual. It was changed .........
While over 35000 approaching Iceland westbound, and facing
200knot winds (mechanical trubulence at that height and oscillating up- and
down-drafts exceeded 2000 fpm) are rare events, it proves that not all
conditions are covered - and altitude-hold can be a fool's paradise under
rare circumstances.
Thought I should share the idea.............
Cheers, Ferg
PS ....guess what I mean is - if the alt hold is 'on', make sure the hand is
on the trhottle, the eye on the speed..........
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