Engine failure and ETOPS

From:         vac@air16.larc.nasa.gov (Victor A. Carreno)
Organization: NASA Langley Research Center
Date:         21 Aug 94 15:35:10 
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In the 8 AUG 94 Aviation Week & Space Tech. (page 57) Mr. Karl J. Krapek,
president of Pratt & Whitney, is quoted as saying that PW4000s powered 767s
experiance an inflight shutdown every 71,000 engine flight hours. The GE
CF6-80C2s have demonstrated only a shutdown evry 250,000 hours.

A few calculations of interest:

Assumptions:

A1. A flight from New York to London is 6 hours with no airports in the
way. (pesimistic)

A2: The speed with to engines is essentially the same as one engine.
(optimistic) 

A3: Engines under ETOPS certification maintenance fail at the same rate
that other engines in the fleet. (pesimistic)

A4: Engine failure is statistically independent. (optimistic)

Probability of engine shutdown during NY-LON (GE CF6-80C2)

     (6 hours)(2 engines)
----------------------------
       250,000

Probability of both engine shutdown (loss of plain)


     (6 hours)(2 engines)         (1.5 hours) -- average distance to landing
---------------------------- * -------------------
       250,000                   250,000


2.88 x 10^(-10) probability of airplane loss per flight

A5: 300 flights a day across the atlantic.

109,500 flights a year.

We sould loss a plane evry 31,709 years due to engine failure!

With PW engines it should be every 2,557 years.

Hope you enjoy the numbers.

Victor.