What happened
On 4 October 2006, a Boeing 757-236, registration G-CPET, was operating a scheduled passenger flight from Madrid to London Heathrow. Shortly after takeoff, the flight crew noticed a brief oily smell within the cockpit. As the flight progressed into the cruise phase, the situation worsened; both the pilot and co-pilot experienced physical symptoms, including a raw sensation in the throat and an oily taste in the mouth. The commander also developed a headache and felt increasingly confused, necessitating a period of intense concentration to maintain flight duties.
In response to the deteriorating air quality, the crew implemented the 'SMOKE - FUMES AIR CONDITIONING' checklist from the Quick Reference Handbook. The crew donned oxygen masks and declared a 'PAN' emergency. Following the descent, the crew's physical condition improved. The aircraft completed a normal approach and an automatic landing on Runway 2/7R at Heathrow. After the aircraft stopped on the taxiway, the crew assessed the situation and determined it was safe to proceed to the terminal, where the flight concluded without further incident.
The investigation
Investigators performed a strip examination of the aircraft, which revealed evidence of oil leaking from the wiring conduit associated with the left engine's LP compressor speed sensor. The investigation established that the sensor is located at the front roller bearing housing, with wiring routed through a silicone-filled conduit. Over time, the degradation of the conduit's sealant allowed engine oil to escape into the compressor air path.
This oil was subsequently ingested into the bleed air system, which provides the air supply for the aircraft's air-conditioning packs. While initial airline troubleshooting and subsequent inspections on 16 October 2006 were inconclusive, a later incident on 3 November 2006 led to the discovery of oil staining on the left engine's Low Pressure compressor outlet guide vanes. This discovery prompted the removal of the engine for overhaul, where the leak was confirmed.