Incident Description
On 21 February 2000, an Airbus A321-111, registered HB-IOA, operated by Swissair as flight SWR 809 from London-Heathrow to Zurich-Kloten, experienced a serious incident during the cruise phase at flight level (FL) 330. At 20:01 UTC, shortly after reaching cruising altitude, the crew detected a rapid increase in cabin altitude due to an uncommanded opening of the outflow valve, resulting in a sudden loss of cabin pressure.
Sequence of Events
The flight had departed London at 19:40 UTC and climbed continuously for about 21 minutes. The co-pilot reported the abnormal cabin altitude rise to the commander, who noted a malfunction on cabin pressure control system 2. The redundant system 1 failed to take over, and both systems subsequently failed, leaving the outflow valve half open. This allowed cabin air to escape rapidly, causing adiabatic cooling and condensation inside the aircraft.
When the cabin altitude exceeded 9,550 feet, the "excess cabin altitude" warning activated. The commander initiated an immediate emergency descent. Both crew members donned oxygen masks, and the co-pilot executed the descent at an average rate of 3,800 ft/min to FL 100. During the descent, the cabin altitude reached 14,000 feet, triggering automatic deployment of passenger oxygen masks. The crew followed the ECAM procedure and received multiple clearances for the rapid descent. After leveling at FL 100, the crew confirmed no adverse health effects among passengers and continued to Zurich at a lower altitude with manual cabin pressure control. The aircraft landed at 21:01 UTC without further incident.
Aircraft and Systems Information
The Airbus A321 was equipped with two cabin pressure controllers (CPC) of modification standard STD-8. These controllers were designed to regulate cabin pressure independently, with one active and the other monitoring in a redundant role. However, a known issue with STD-8 controllers could cause an uncontrolled opening of the outflow valve under certain conditions, leading to rapid pressure loss. A service bulletin (SB VB 15702-21-006) had been issued in 1998 to upgrade CPCs to STD-10, which included software changes to allow the redundant controller to take over even during high rates of climb or descent. At the time of the incident, the upgrade was being phased into Swissair's fleet, and HB-IOA still had two STD-8 controllers installed.
Cause
The serious incident was caused by a known malfunction of a cabin pressure controller, which caused the outflow valve to open. The redundant control system was unable to correct the malfunction. According to the investigation, the malfunction was very probably due to a defective cabin pressure sensor. The same fault had occurred months earlier but was not detected because the non-volatile memory could only be read by the manufacturer.
Findings and Subsequent Actions
The Swiss Aircraft Accident Investigation Bureau (AAIB) found that Swissair did not initially report the incident as required, leading to incomplete flight data recordings. The investigation was transferred from Belgian to Swiss authorities. Temporary emergency procedures for rapid outflow valve opening were in place but were not implemented by the crew due to the speed of events. After the incident, the defective cabin pressure sensor was identified, and the CPC upgrade program continued, with all aircraft using STD-10 or higher by mid-2001.