What happened
On 8 November 2002, a Boeing 737-436, registration G-DOCH, was climbing through FL240 en route from London Heathrow to Kiev when the flight crew detected electrical crackling and the smell of burning. Shortly after, smoke appeared on the flight deck, prompting the crew to don oxygen masks and initiate the 'Electrical Smoke/Fumes or Fire' checklist.
During the event, the crew experienced significant difficulties communicating with the cabin crew due to the failure of the cabin interphone system. While the pilots were managing the smoke, the cabin crew noticed sparks and flames in a ceiling panel, followed by a continuous stream of water pouring from the overhead area. The cabin crew used towels to prevent water from reaching the avionics bay and eventually informed the commander after he opened the flight deck door to investigate.
Following the assessment that the fire had been extinguished, the commander declared a MAYDAY and requested an immediate diversion back to Heathrow. The aircraft landed safely at 15:27 hrs. Upon inspection by emergency services, a burnt wiring loom and a leaking water supply hose were discovered in the forward cabin ceiling.
The investigation
The AAIB investigation focused on the source of the electrical arcing and the cause of the water leak. Examination of the aircraft revealed that a braided steel water supply hose, intended for the forward galley, had been improperly secured to a wiring loom using a plastic tie-wrap. The investigation found that the hose was excessively long for the installation, necessitating that the extra length be looped through the overhead area.
This arrangement caused the steel braid of the hose to chafe against the electrical wires, leading to insulation failure and subsequent arcing. The arcing was powerful enough to melt the copper conductors and the steel braid, and it caused the inner PTFE lining of the hose to puncture, resulting in the substantial water leak. The investigation also examined the Quick Reference Handbook (QRH) and found that the smoke-related checklists were difficult to locate due to non-intuitive indexing.
Findings
- The primary cause of the incident was electrical arcing between the wiring loom and the steel braided water hose.
- The water hose was of excessive length, which led to the extra material being looped through the overhead wiring area.
- The hose was improperly secured to the electrical wires with a single electrical tie-wrap.
- The arcing caused the failure of approximately 25 circuit breakers and damaged the insulation of multiple conductors.
- The failure of the cabin interphone prevented effective communication between the flight deck and cabin crew during the emergency.