What aircraft was involved and where did it happen?

The accident on 2000-02-21 involved a AIRBUS INDUSTRIE A321-111, registration HB-IOA, at KOK VOR, ZH, CH.

What was the probable cause of the accident?

A defective cabin pressure sensor caused a malfunction in the active pressure controller, which triggered an uncontrolled opening of the outflow valve; the redundant controller failed to compensate because the rapid rate of depressurization exceeded the technical limitations of the existing modification standard.

Rapid Cabin Depressurization on Swissair Airbus A321

A serious in-flight incident occurred on a scheduled flight from London to Zurich when a malfunction in the cabin pressure controllers led to a rapid loss of pressure.

What happened

On 21 February 2000, an Airbus A321-111, registration HB-IOA, operated by Swissair, experienced a rapid loss of cabin pressure while cruising at FL 330. The incident occurred during a scheduled flight from London-Heathrow to Zurich-Kloten. Shortly after reaching cruise altitude, the co-pilot noticed a sudden increase in cabin altitude. The aircraft's second cabin pressure controller malfunctioned, causing the outflow valve to open.

As the cabin altitude climbed rapidly, the primary control system failed to intervene because the rate of pressure change exceeded the operational limits of the existing hardware configuration. This caused the outflow valve to remain half open, leading to a significant drop in pressure and visible condensation within the cabin due to adiabatic expansion. Upon reaching a cabin altitude of 9550 ft, an automated warning triggered, prompting the flight crew to initiate an emergency descent. The crew donned oxygen masks and descended to FL 100 at an average rate of 3800 ft/min. Although passenger oxygen masks deployed automatically as the cabin altitude reached 14,000 ft, no passengers or crew members sustained injuries. After stabilizing the cabin via manual pressure control, the flight continued to Zurich.

The investigation

The investigation, handled by the Swiss Aircraft Accident Investigation Bureau after being handed over by Belgian authorities, focused on the failure of the redundant pressure regulation systems. Investigators examined the non-volatile memory of the cabin pressure controllers (CPC) and found that a similar malfunction had occurred on the same aircraft several months prior.

Technical analysis revealed that the aircraft was equipped with two controllers of the STD-8 modification standard. While a service bulletin was in progress to upgrade these to the STD-10 standard to prevent uncontrolled valve openings, the aircraft was in a transitional phase where only one upgraded unit was installed. The investigation also noted that the airline had not initially notified the authorities of the incident, which resulted in incomplete flight data recordings.

Findings

The primary cause was a malfunction of a cabin pressure controller triggered by a defective pressure sensor.
The redundant controller was unable to correct the error because the rate of cabin altitude increase exceeded 2000 ft/min, a threshold that prevented the STD-8 hardware from successfully taking over control.
The outflow valve opened significantly, increasing by 24% within 12 seconds of the initial failure.
The flight crew's rapid response and emergency descent were appropriate, though the high workload prevented them from recalling the specific master warning and caution alerts later.
The airline's internal procedures for reporting serious incidents were found to be incomplete, leading to the delayed notification of the investigation bureau.