The accessory section of the AH-64 main transmission drives the aircraft’s generators and hydraulic pumps. It is connected to the rest of the drive train by two sprag clutches. The primary clutch is attached to an eighty-five tooth gear, while the secondary clutch is attached to an eighty-four tooth gear. Under normal operation, the primary clutch is engaged and the secondary clutch is slowly overrunning. If the primary clutch fails, the secondary clutch will engage, and the accessory section will turn at a slightly slower speed. Historically, there has been no means of determining which clutch was engaged. One accident caused by a double sprag clutch failure on an Israeli AH-64 led to the loss of life and aircraft.

The current HUMS has the capability to compare the signal from the tachometer on the main transmission to the frequency of the AC power produced by the generators. The resulting ratio indicates which clutch is engaged. Recently, RMCI implemented and began monitoring this condition indicator. Thus far, three transmissions have indicated that the secondary clutch was engaged.

Teardown analysis of these transmissions showed that the primary clutch had not failed, but rather the transmission was incorrectly assembled. Rather than having one eighty-four-tooth gear and one eighty-five-tooth gear, it had two eighty-four-tooth gears. This arrangement caused both clutches to act like secondary clutches, resulting in the indication from the HUMS. The teardown also showed that having two identical gears led to the clutches wearing more rapidly than they would in a correctly-assembled transmission.

RMCI’s discovery of these faults has led to the institution of additional quality-control measures in transmission manufacturing and overhaul processes. It has also provided additional impetus for quickly equipping more AH-64s with HUMS systems. The result is a safer, more reliable aircraft for the soldier as well as the cost savings of preventing potential Class-A accidents.