What happened
On 26 November 2002, a Boeing 737-229, registration G-CEAI, was performing a positioning flight from Bournemouth International Airport to London Gatwick. During the initial taxi, the crew noted several system failures, including the weather radar, the first officer's vertical speed indicator, and an anti-skid warning. After engineers replaced the faulty components and repaired a loose connection, the aircraft departed again.
While climbing through FL70, the crew attempted to adjust the autopilot altitude selection, but the system failed to respond as expected. Shortly thereafter, the anti-skid warning illuminated brightly despite the panel being set to dim. As the crew discussed returning to Bournemouth, several additional warning lights illuminated, accompanied by a noticeable smell of electrical burning. The crew declared a MAYDAY, donned emergency oxygen masks, and initiated an immediate descent.
Following a safe landing at Bournemouth, the aircraft was evacuated via a fire ladder. Although no fire was detected by emergency services, both pilots required oxygen due to experiencing light-headedness and tingling in their fingers.
The investigation
Investigators examined the aircraft's electrical systems, including the battery, APU, and engines. While no smoke was present during the initial checks, testing the Altitude Alert Unit caused the Zone 6 circuit breaker to trip. Further inspection of the co-pilot's annunciator revealed evidence of burning within the connector.
To identify the root cause, engineers performed water leak tests on the flight deck. They discovered that water was entering the cockpit through a leak in the co-pilot's windscreen and a significant leak in the windscreen wiper sealing boot. It was determined that this water ingress had reached the electrical components.
Findings
- Water was leaking into the flight deck through the windscreen and the wiper sealing boot.
- This moisture caused electrical burning in the co-pilot's annunciator connector.
- The failure of the vertical speed indicator was likely also a result of water entering the unit through its cooling holes.