ADS-79D-HDBK Downloaded from https://www.everyspec.com The US Army RDECOM report ―Inserted Fault Vibration Monitoring Tests for a CH- 47D Aft Swashplate Bearing,‖ cited in Section J.2, involves the detection of a swashplate bearing failure in a CH-47D Chinook. This test method offers an acceptable alternative to Reference 21 for obtaining a test specimen for initial SFT. In Reference 22, heavily worn, used components returning from field operation were hand-selected by researchers at the Corpus Christi Army Depot (CCAD). The parts were inspected and selected for their anticipated ability to produce the desired fault condition in the laboratory test stand.
These defective bearings, therefore, provided a natural source alternative to manually degrading a new part. The intention of these referenced articles was to document the methods and results of laboratory seeded fault testing. Therefore, follow-up, on-aircraft seeded fault testing or the need for an AWR was not addressed by the articles in these tests. It would be expected, however, following the example process guidance in Figure H-1, that, in situ, on-aircraft seeded testing be used to validate any laboratory findings before issuing a flight/fielding AWR for a CBM system on US Army aircraft.
The US Army RDECOM report ―Summary of US Army Seeded Fault Tests for Helicopter Bearings,‖ cited in Section J.2, addresses AH-64 SFT for both a main rotor swashplate (MRSP) and a tail rotor hanger bearing. The SFT report communicates the methodologies; emphasizes the necessity of seeded fault testing the US Army employs in pursuit of CBM; outlines SFT applications; and documents how the SFT applications facilitate HUMS validation on actual components. While the MRSP testing is still ongoing, the TRDS results and analyses provide substantiation for an actual US Army approved maintenance credit when using a HUMS device.
This document substantiates a maintenance credit from a 2500 flight hour TBO to 6487 hours for a hanger bearing using a probabilistic assessment. In contrast to the previous two examples cited for the OH-58 and CH-47 aircraft, above, this report provides actual monitored aircraft data along with the SFT bench data. Note also, confidence and reliability play a key role in analyzing the aircraft/bench data, developing statistically significant sample sizes, and establishing a basis for changing from legacy aircraft maintenance practice to CBM for continued airworthiness. 183