What happened
On 25 January 2007, a Bell UH-1H/B205 helicopter, registration ZU-UHI, was conducting firefighting operations near de Doorns, South Africa. The aircraft had successfully completed four water drops and was approaching a farm dam at the Orchards farm to collect a fifth load of water.
As the pilot transitioned into a hover, an unusual noise and significant vibrations began emanating from the rear of the aircraft. Shortly after, smoke was observed near the engine area. The helicopter immediately experienced a leftward yaw, and the low rotor RPM warning light activated. Unable to maintain altitude, the pilot was forced to execute an emergency landing, ditching the aircraft into the dam. The pilot was able to shut down the engine, apply the rotor brake, and exit the cockpit, swimming to shore without injury.
The investigation
SACAA AIID investigators examined the wreckage after it was recovered from the dam by a heavy-lift helicopter. A teardown inspection of the Honeywell-Lycoming T53-L-13B engine was conducted under supervision. The investigation focused on the engine's history, noting that the unit had been in long-term storage for approximately 15 years before being installed on the aircraft in October 2006. Records also indicated that just three days prior to the accident, the engine oil pressure had been noted as running high, which was subsequently adjusted.
Findings
- The engine suffered an internal bearing failure caused by the failure of a carbon seal.
- The carbon seal had perished due to excessive heat exposure, allowing hot combustion gases to enter the bearing cavity.
- This intrusion caused the oil within the cavity to overheat, leading to the seizure of two roller bearings.
- The prolonged 15-year storage period of the engine likely compromised the integrity of the carbon seal.
- The pilot's position over the dam during the water-collection maneuver left no opportunity for evasive action once the failure occurred.
Safety action
- It is recommended that South African maintenance organizations perform rigorous assessments of any components that have been in inhibited storage for more than 12 months before installing them on an aircraft.
- A recommendation was made for the SACAA Airworthiness department to review maintenance requirements for this aircraft type to ensure safety during specialized firefighting operations.