It's for use when it's subtle and unobserved and it finally dawns on you that "it's bad" and you are already exploring the edge of the envelope but your instruments are apparently supplying confusing "valid" data.
It's simply a ball park figure to give you time to get the QRH out and find the (performance inflight) real figures! It's not a permanent fix! The plane may climb/descend/accelerate/slow down but it won't get out of control in the time it takes to as said find the real figures and use them.
Read some unreliable airspeed etc accident reports most are very subtle and the aircraft is either nose high low thrust because they "think" they are overspeeding or nose low high thrust as the "think" they are stalling, either case when you apply the "memory items" that will slow/stop the situation escalating in the wrong direction.
If it's a simple immediate loss of data, don't change pitch and power and go straight to the inflight perf as it's obvious.
Last edited by SMOC; 26th March 2014 at 05:49.