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With ice on the stabilizers, the increased negatived angle of attack caused by flap extension to 30° could have been sufficient to destroy tail load. This would induce a serious nosedown pitching moment. If the ensuing aircraft rotation is severe enough, or for sufficient duration, recovery at low altitude would be impossible.
Considering all the factors involved the Board concludes:
- That the aircraft collected ice, particularly on its tail, during the flight to Boston and especially during the approach.
- That 15° of flap extension was used from the outer marker inbound and that "abeam the stacks" the flaps were further extended to 30°.
- That the increased downwash resulting from the latter flap extension changed the stabilizer angle of attack to a position which, coupled with ice formation, destroyed the tail lift, thereby disrupting the aircraft's necessary balancing tail loads.
- That the resultant pitchover was too severe at the aircraft's altitude for the crew to effect recovery.
Probable Cause
The Board determines the probable cause of this accident was loss of balancing forces on the horizontal surface of the aircraft's empennage, due to ice secretion, causing the aircraft to pitch nosedown at an altitude too low to effect recovery.
BY THE CIVIL AERONAUTICS BOARD
/s/ ALAN S. BOYD
Chairman
/s/ ROBERT T. MURPHY
Vice Chairman
/s/ G. JOSEPH MINETTI
Member
/s/ WHITNEY GILLILLAND
Member
GURNEY, Member, did not take part in the adoption of this report.
