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there 1s some known ba51s 18 the condltlon of whlrl mode. The probablllty that thlS acoldent was so caused is supported by the followlng
1. So far as 1s known, the alrcraft was 1n stralght and level fllght and at a normal cruise speed wlth no serlous mechanlcal problems.
2. A sound 1dent1f1ed as a supersonlc or hlgh Speed propeller occurred 30 seconds prior to fuel 1gnitlon (ulng fallure).
3. There was structural damage ev1dence compatlble w1th ceclllatory motion of the No. l QEC and the left slug.
4. Flrst stage compressor blades of No. l englne rubbed the air inlet hou31ng supports.
5. The probable cause of a 51mllar acc1dent of another Electra was due to whlrl mode.
If prior damage is a requlrement for the necessary reductlon in stiffness, it must be assumed that the ev1dence of such damage was elther obllterated.1n the crash or never existed in a dlscernlble form.
Probable Cause The Board determines that the probable cause of this scoldent was structural
fallure of the left wing resulting from forces generated by undampened propeller thrl mode.
BY THE CIVIL AERONAUTICS BOARD:
/s/ ALAN s. BOYD Chalrman
/s/ ROBERT T. MURPHY Vice Chalrman
/s/ CHAN GURNEY Member
/s/ G. JOSEPH MINEI‘TI Member
/s/ WHITNEY GILLILLAND Member
