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Rh of all cylinders at ZOO-hour intervals; COIITpI‘BSSlOIl check of all cylinders and rewsed ignition procedures.

The effectiveness of this program has resulted in improved engine per— formance and reliability for AMIGO Airlines.

6. Directing attention to the Pratt 8c Whitnev P/N hh'féh valve, examination of the air carrier‘s records shows that eight engine failures were experienced and reported during the period of July 1, 1963, through December 1963. During this same period, thirteen engines were removed for routine over- haul and four engines were removed, repaired and returned to serVice. Of the eight engines failed, four experienced valve difficulties and involved engines procured from another operator.

The Agency is handling the FEM P/N M4761; valve as an overall industry problem by Airworthiness Directive action. The proposed AD is in our Office of the General Counsel in prep- aration for Notice of Proposed Rulemaking. This AD will require, in essence, replacement of all P/N 1114761; valves during the next engine overhaul. Replacement valves may not be new or reconditioned P/N M7611 Valves.

Reference is made to the Agency‘s AdVisory Circular 121—1. With regard to the transfer of aircraft from operator to operator, we conSider the pro rata time control system described in AC 121-1 to be baSically sound.

The system has functioned quite well in the execution of several hundred transactions during the past four years. Unfortunately, the unusual com- bination of Circumstances which resulted in the overhaul time combination obtained by AAXICO was not ant1c1pated when AC 121-1 was prepared. There- fore, in View of this experience, we are studying a reViSion intended to preclude intentional or inadvertent time accumulation in the transfer transactions.

Our extensvre interest in this matter and the initiation of actions we determined as necessary has accumulated some additional time in our reply.

Sincerely yours,

/s/ C. Schuck

for George S. Moore Director

Flight Standards Service