Incident Details
On 27 June 2011, an Embraer 195 (registration G-FBEJ) operated a commercial passenger flight from Southampton Airport. The aircraft had departed with only one air conditioning pack operational, as permitted under the Minimum Equipment List. While passing Flight Level 100, the flight crew noticed smoke and a strong sulphurous smell in the flight deck. They promptly donned oxygen masks and declared a PAN, electing to return to Southampton. Approximately five minutes later, the smoke and smell dissipated, and the aircraft landed without further incident. There were 5 crew and 24 passengers on board; no injuries were reported. The only damage was to the air conditioning pack.
Investigation Findings
Subsequent investigation revealed that the operable air conditioning pack had failed in flight. The unit was returned to the manufacturer for strip inspection, which identified that the second stage turbine rotor had failed, causing seizure of the rotor. This failure mode is known to be caused by a resonance condition in the second stage turbine. The affected pack had not been modified in accordance with Service Bulletin SB 190-21-0029, which was issued on 26 April 2010 to incorporate a modified turbine with more nozzle vanes to eliminate the damaging resonance. No modified packs have experienced a second stage rotor failure.
Service Bulletin
Service Bulletin SB 190-21-0029 addresses the resonance issue by introducing a redesigned turbine. Since the bulletin is already in place to prevent such failures, no additional safety action was proposed by the AAIB.