Also
this was posted before, i think it is appropriate to look at this article again.
Some points out of it:
The value of the AoA SW depends
on the Mach number. At high Mach
number, the AoA SW is set at a
value such that the warning occurs
just before encountering the pitch
up effect and the buffeting.
Typically, in cruise at high Mach
number and high altitude, at or
close to the maximum recommended FL, there is a small margin between the actual cruise AoA
and the AoA STALL. Hence, in
ALTERNATE or DIRECT LAW,
the margin with the AoA SW is
even smaller
Equally, in similar high FL cruise
conditions, in particular at turbulence
speed, if the pilot makes significant
longitudinal inputs, it is not unlikely
that it reaches the AoA SW value.
For those reasons, when in ALTERNATE or DIRECT LAW, it is recommended to fly at a cruise flight
level lower than the maximum recommended. A 4,000 ft margin is to
be considered. Then, for the same
cruise Mach number, the IAS will
be higher, the AoA will be lower,
and therefore the AoA margin
towards AoA SW will be significantly increased.
A practical exercise done in flight
in DIRECT LAW on an A340-600
and well reproduced in the simulator consists in performing a low altitude level flight deceleration at idle
until the SW is triggered, and then to
push the THR levers to TOGA while
continuing to pull on the stick in order to maintain the altitude.
The results of such a manoeuvre
are:
q In clean configuration, even if
the pilot reacts immediately to the
SW by commanding TOGA, when
the thrust actually reaches TOGA
(20 seconds later), the aircraft
stalls.
This shows that increasing the
thrust at the SW in order to increase
the speed and hence to decrease the
AOA is not the proper reaction in
many cases (this will be developed
in the following chapter).
In addition, it is to be noticed that,
at high altitude, the effect of the
thrust increase on the speed rise is
very slow, so that the phenomenom
described above for the clean configuration is exacerbated.
Obviously, such a procedure leads
to potentially unrecoverable situations if it is applied once the aircraft has reached the aerodynamic
stall (see next chapter).
Even if the traditional procedure
can work in certain conditions if
the pilot reacts immediately to the
SW, or if he is not too adamant on
keeping the altitude, the major issue comes from the fact that once
the Stall Warning threshold has
been crossed, it is difficult to know
if the aircraft is still approaching to
stall or already stalled. Difference
between an approach to stall and an
actual stall is not easy to determine,
even for specialists.
The AoA decrease may be obtained
indirectly by increasing the speed,
but adding thrust in order to increase
the speed leads to an initial adverse
longitudinal effect, which trends to
increase further the AoA (fig. 4).
It is important to know that if such
a thrust increase was applied when
the aircraft is already stalled, the
longitudinal effect would bring the
aircraft further into the stall, to a
situation possibly unrecoverable
Well, enough said from my point of view, iŽll go back to my armchair, hope i didnŽt produce too much garbage.