What happened
On 19 November 2016, a Boeing 777-300ER, registered A7-BAO, was operating a scheduled ultra-long-range flight from Miami to Doha. While cruising at FL 350 near Basel, the flight crew detected a strong electrical odor within the cockpit. The commander, feeling unwell due to the fumes, donned an oxygen mask, and both pilots subsequently utilized oxygen masks as the smell persisted.
Recognizing the potential for smoke, the crew declared a 'Mayday' and requested a diversion to Zurich. To facilitate a safe landing, the crew initiated fuel dumping procedures to reduce the aircraft's weight. During this process, the crew experienced intermittent technical issues with the fuel nozzle valves, but eventually stabilized the weight reduction, dumping approximately 8 tonnes of fuel. The aircraft performed an overweight landing at Zurich, which was approximately 6 tonnes above the maximum landing weight, using a standard flap configuration.
The investigation
Investigators examined the cockpit voice recorder, flight data, and the aircraft's cooling systems. The investigation focused on the source of the odor and the crew's management of the emergency, including the fuel dumping process and the decision-making regarding landing configurations.
Findings
- The primary cause of the odor was a jammed rotor in the right equipment cooling fan, located in the forward cargo compartment. This malfunction caused the electrical coils within the fan to burn, releasing an electrical smell and slight smoke into the cockpit via the ventilation system.
- The flight crew's decision to use oxygen masks and divert to the nearest suitable airport was considered systematic and safety-conscious.
- While the second crew (Crew B) provided helpful communication support, the investigation noted a lack of sufficient oversight during the fuel dumping checklist, which led to inconsistent information being provided to air traffic control.
- The crew's decision to perform an overweight landing was appropriate and compliant with operating procedures given the circumstances.