Incident Overview
While establishing a 125-foot out-of-ground-effect hover during a mission to search for and recover debris from the Columbia Space Shuttle, the helicopter experienced an unexpected loss of power. The aircraft descended rapidly into a forested area with trees approximately 80 feet tall and impacted the ground. No radio communications or distress calls were reported.
Investigation and Component Examination
Post-accident examination of the engine components revealed anomalies during the rigging procedure for the Hydro-Mechanical Unit (HMU) in an engine test cell. The actual position of the throttle Position Lever Angle (PLA) did not match readings from the Electronic Control Unit (ECU). The Full Authority Digital Electronic Control System (FADEC) controlled engine operated erratically in Auto mode when rigged with its original ECU and HMU. Switching to Manual mode allowed normal engine response to throttle inputs, but ECU readings remained erratic. Installation of a slave ECU produced similar erratic readings. When a slave HMU was installed, the engine operated normally without erratic readings.
Findings on the HMU Potentiometer
Downloading the ECU's nonvolatile memory revealed no faults recorded during the accident flight. Extensive electrical and mechanical testing of the original HMU identified severe signal dropout on the PLA signal. The dropout was random in location, bi-directional, and present during both rotational and stationary operation of the PLA input. The source was traced to the HMU potentiometer. Examination of the PLA potentiometer included mechanical measurements, electrical testing, inspection of conductive epoxy joints, and microscopic examination of the three lead wire connections. The potentiometer was found faulty due to insulation breakdown between the rotor and shaft, causing a single-point failure that induced erratic fuel metering to the engine.
Safety Recommendations
As a result of these findings, the Safety Board issued four safety recommendations (A-03-18 through A-03-21) on May 27, 2003, to the Federal Aviation Administration addressing the PLA potentiometer deficiencies.