As has been stipulated earlier ,as the aircraft acc'the nose has a tendency to pitch slowly down.Two types in mind which demonstrated these tendencie- the DC8 and the Learjet.
On the DC8 above the speed regime of .825 the Pitch Trim Compenstor used to extend(an electric motor would drive the elevator up to offset the down pitch).The aircraft was unstable slippery) in these conditions,and the PTC was to prclude upsets.
Once one had the PTC activate one just reduced thrust until the aircraft had stabalized.This is described indetail in the NTSB reports of DC8 upsets(Eastern,AirCanada)..
The Lear was limited to M.78,but operators pulled the restrictive C/breaker and cruised faster(82?).This led to some upsets,as the Auto pilot fought the trim.Once the auto pilot let go the a/craft was out of trim for an immediate recovery.Again covered by the NTSB reports
