What happened
On 21 October 2016, an Airbus A319-131, registration G-DBCB, was performing a non-revenue commercial flight when the crew encountered an unusual odor. The incident began the previous day during a cross-bleed engine start, when the co-pilot detected a brief acidic smell. This scent reappeared just prior to takeoff and twice during the climb phase. While the commander did not initially notice anything unusual, the smell intensified significantly once the aircraft reached the cruise stage.
The odor was most concentrated near the co-pilot's side-stick controller and rudder pedals, though the passenger cabin remained unaffected. Following the Smoke/Fire/Fumes emergency checklist, the crew diverted the aircraft to Bordeaux, landing without further incident.
On the following day, during a repositioning flight to London Gatwick, the avionics blower fan had been inhibited prior to departure. However, as the crew began their descent, the acidic smell returned. In response, the crew donned oxygen masks and declared a PAN-PAN signal.
The investigation
Following the occurrence, the operator conducted an investigation to identify the origin of the fumes. Although several components were replaced during the maintenance process, a definitive source for the odor could not be established. One notable finding during the inspection was a leak in the right brake master cylinder, which was found to be discharging hydraulic oil. Since the initial incident, the aircraft has operated more than 30 flights without any recurrence of the smell.