What happened
On 27 October 2017, a Saab 2000, registration G-LGNS, was taxiing to its stand at Sumburgh Airport after a routine commercial passenger flight. During the taxiing phase, cabin crew alerted the flight crew to the presence of smoke at the rear of the aircraft. Due to high winds exceeding 40 knots, the commander proceeded to the stand to park the aircraft into the wind.
As the aircraft approached the stand, a second report was received indicating smoke had reached the centre of the cabin. The crew noted that the smoke became visible immediately after landing and the cabin lights were activated. The flight crew promptly shut down the engines, and the commander assessed the situation by opening the flight deck door. Passengers were disembarked through the forward main door in approximately 30 seconds. The airport fire service arrived shortly after, and following their guidance, the aircraft batteries were disconnected and the rear door was opened to ventilate the cabin. No injuries were reported to the 18 passengers or 4 crew members on board, and no damage was sustained by the aircraft.
The investigation
The investigation focused on the source of the smoke and the aircraft's air conditioning systems. The aircraft's configuration allows two air conditioning packs to receive compressed air from either the engines or the APU. Under normal ground operations, the APU supplies air to the right-hand pack. The investigation established that the reports of smoke coincided with the activation of the APU following the aircraft's landing.
Engineering inspections conducted by the operator's team involved operating both air conditioning packs on the ground. This process revealed a strong smell of oil emanating from the right-hand pack, along with visible blue, acrid smoke in the cockpit area. The inspection also identified that the right-hand heat exchanger was heavily soiled.
Findings
- The smoke was caused by oil contamination of the air bleed from the APU.
- The right-hand heat exchanger was found to be extremely dirty.
- The timing of the smoke appearance was linked to the starting of the APU after landing.