Summary
The Federal Aviation Administration (FAA) proposes to adopt a new airworthiness directive (AD) that is applicable to Honeywell International, Inc., (formerly AlliedSignal, Inc., and Textron Lycoming) T5311A, T5311B, T5313B, T5317A, T5317B, and former military T53-L-11, T53-L-11A, T53-L-11B, T53-L-11C, T53-L-11D, T53-L-11A S/SA, T53-L-13B, T53-L-13B S/SA, T53-L-13B S/SB, and T53-L-703 series turboshaft engines. This proposal would require initial and repetitive special vibration tests of the engine, and if necessary replacement with a serviceable reduction gearbox assembly, or a serviceable engine before further flight. This proposal is prompted by reports of tachometer drive spur gear failure, resulting in potential engine overspeed, loss of power turbine speed (N2) instrument panel indication, and hard landings. The actions specified in this AD are intended to prevent excessive vibrations produced by the reduction gearbox assembly that could cause failure of the tachometer drive spur gear.