The scheduled passenger flight departed Derry (EGAE) at 08.10 UTC with 108 passengers and 6 crew for Stansted (EGSS). The Captain who was the pilot flying (PF), elected to perform a “Bleeds Off” take-off for which the First Officer (FO), the pilot not flying (PNF), configured the air conditioning panel for “Bleeds Off” at the pre take-off holding point.
After take-off, the PNF reconfigured the air conditioning panel while performing the after take-off checks and turned the air conditioning packs OFF, in error. The flight then continued up to Flight Level (FL) 270, during which both pilots recall hearing warning horns.
At this level the PF levelled the aircraft, passenger oxygen masks had already deployed, and, in a panel scan, he noticed that the packs were selected to OFF and immediately switched them to ON. Cabin pressurisation returned to normal, aural warnings shortly ceased and the flight continued to Stansted, where it landed without further incident. ATC were not made aware of any onboard problem with this flight.