Boeing 747 declares Mayday after Heathrow rain causes electrical failure

A Boeing 747 cargo aircraft bound for Hong Kong from London Heathrow was forced to declare a Mayday alert mid-flight on 8 February 2024, following significant electrical failures. The incident, detailed in a report by the Air Accidents Investigation Branch (AAIB), occurred after water entered the aircraft's systems due to a cargo door being left open during rainfall prior to departure.

Electrical Failures and False Readings

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Electrical problems began almost immediately after the aircraft took off from Heathrow. Water had entered the main cargo deck because a door was not properly secured while rain was falling. Sealant was also found to be missing from a floor mat near the door. This water ingress led to power loss in two of the plane's main electrical systems, triggering a cascade of failure messages for the crew.

The electrical failures also affected the heating system for the aircraft's airspeed sensors. As the plane ascended, these unheated sensors froze, providing incorrect readings that indicated the aircraft was flying too slowly. With the autopilot engaged, the aircraft responded to these false readings by pitching its nose downwards to increase speed.

Steep Descent and Diversion

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The Boeing 747 entered a steep descent, dropping at a rate of 6,240 feet per minute with its nose angled 7.6 degrees downwards. The situation escalated when a pressurisation issue prompted the crew to initiate a descent through icy atmospheric conditions. Once the aircraft exited the icy air and correct speed readings were restored, the autopilot began to correct the nose-up attitude.

However, the rapid descent caused the aircraft to exceed its maximum permitted speed by 18 knots. It was at this point that the crew declared a Mayday. Despite encountering further issues with the aircraft's flaps, the pilots successfully landed the plane at Amsterdam Schiphol Airport.

Safety Recommendations

The AAIB investigation concluded that the crew faced a highly complex situation beyond normal operational parameters. The report highlighted the crew's training and decision-making as significant factors in preventing a more serious outcome. The AAIB has since issued four safety recommendations aimed at improving inspections of protective equipment, seals, and waterproofing around the cargo floor and electrical systems on Boeing 747 freighters.