What happened
On 23 November 2011, a Malév Zrt. Boeing 737-800, registration HA-LOK, was performing a scheduled flight from Budapest to Moscow via Budapest. Following engine start, the crew failed to switch the air conditioning "Pack" switches to the "AUTO" position, resulting in a lack of air ventilation into the cockpit and cabin.
During the climb, as the aircraft reached approximately 14,600 feet, the cabin altitude horn activated, indicating that the cabin altitude had exceeded 10,000 feet due to insufficient pressurization. The commander silenced the alarm but did not immediately initiate an emergency descent or rectify the air conditioning settings. As the aircraft continued to climb, the cabin altitude reached 14,000 feet, triggering the automatic deployment of oxygen masks for the passengers.
The crew eventually decided to abort the climb and began a descent. During the descent, while performing the "AUTO FAIL or Unscheduled Pressurization Change" checklist, the commander identified that the air conditioning pack switches were in the "OFF" position and corrected them. The aircraft returned to Budapest, landing at Liszt Ferenc International Airport after a period of flight in an oxygen-deficient environment.
The investigation
The Hungarian Transportation Safety Bureau (KBSZ) investigated the incident by reviewing flight data recorder and cockpit voice recorder information, radio communications, and radar data. The investigators also interviewed the flight and cabin crews and reviewed medical records. The investigation examined the crew's performance during the descent and landing, noting several procedural errors that occurred after the period of hypoxia.
Findings
- The primary cause was the crew's failure to turn the air conditioning pack switches to the "AUTO" position after engine start.
- The crew failed to recognize this error during the pre-takeoff and after-takeoff checklists.
- Upon hearing the cabin altitude horn, the crew did not immediately identify or rectify the cause of the pressure loss.
- The crew did not follow the emergency checklist procedures for a cabin altitude warning, nor did they perform an emergency descent despite the rising cabin altitude.
- Hypoxia experienced by the pilots during the period of low pressure likely contributed to subsequent errors during the return, descent, and approach.
- The commander had been involved in a previous serious incident involving cabin pressure, yet no unscheduled medical examination had been conducted following that event.