What happened
On 25 July 2021, an MD 902 helicopter, registration G-LNDN, was performing a commercial air transport mission as an air ambulance. The flight departed from the Royal London Hospital for RAF Northolt. Prior to departure, the aircraft had been parked on a helipad during a period of intense thunderstorms and heavy rainfall.
Shortly after takeoff, the crew experienced several electrical issues. Initially, the commander's flight instrument displays began to fail. As the flight progressed, the crew received fault indications for the Electronic Engine Control (EEC) systems on both engines. While the left engine remained in a functional automatic mode, the right engine experienced a critical fault that required the pilot to use manual throttle control. Despite these complications, the crew managed to complete a safe approach and landing at RAF North and successfully shut down the engines.
The investigation
Investigators examined the aircraft and found evidence of moisture within the fuselage. The rear baggage compartment was damp, and water was observed dripping from the seams of the lining. Specifically, the Symbol Generator and the right engine's EEC showed clear signs of water exposure. The underside of the Symbol Generator was wet, and water was found accumulating within the electrical connectors of the right EEC.
Further inspection of the right engine bay revealed that a drain hole was partially obstructed by debris. Testing with a solvent confirmed that liquid could flow from the engine bay through the deck floor and directly onto the electronic components. The investigation determined that the sealant around the igniter cable passages was insufficient, allowing rainwater to bypass the engine bay and enter the avionics area.
Findings
- Heavy rainfall during the aircraft's time on the helipad caused water to accumulate in the right engine bay.
- A partially blocked drain hole, combined with the high volume of rain, led to water pooling in the engine compartment.
- Inadequate sealing around the igniter cable sheaths allowed this water to leak into the rear baggage compartment.
- This water ingress caused a short circuit in the right EEC and caused corrosion on the Symbol Generator.
- The failure of the right EEC triggered a secondary communication fault with the left engine's EEC, leading to the observed dual-engine fault indications.