Malév Boeing 737-800 Cabin Pressurization Incident in Budapest FIR (HA-LOK)

Incident Overview and Investigation Authority On 23 November 2011, a serious aviation incident occurred involving a Boeing 737-800 operated by Malév Zrt. The aircraft, registered HA-LOK, was conducting scheduled flight MAH102/103 between Budapest and Moscow. The Transportation Safety Bureau of Hungary (TSB) investigated the event under applicable European and Hungarian aviation regulations, including Regulation (EU) No 996/2010 and Act CLXXXIV of 2005. The investigation focused on technical causes and circumstances without apportioning blame or liability. ## Sequence of Events The flight departed Budapest at 16:21 local time. During the pre-takeoff checklist, the crew declared the air conditioning packs in AUTO and bleeds ON, indicating the system was believed to be operational. After takeoff, the after-takeoff checklist was completed, again confirming the air conditioning switches were in the appropriate position. However, the pack switches were actually in the off position, preventing air from being vented into the cockpit and cabin. During climb, the cabin altitude horn sounded at 14,600 feet, indicating excessive cabin altitude above 10,000 feet. The commander silenced the horn after 30 seconds and decided to abort the climb one minute later. The aircraft reached a maximum altitude of 17,250 feet before beginning descent. The commander reported a technical trouble to air traffic control and descended to FL120. The cabin altitude continued to increase at an average rate of 2,700 feet per minute. Ninety seconds after the horn sounded, the cabin altitude reached 14,000 feet, triggering automatic deployment of oxygen masks in the cabin. During descent, the commander read the AUTO FAIL or Unscheduled Pressurization Change Non-Normal Checklist and discovered the air conditioning pack switches were off. He turned them on and continued the descent. ## Return to Budapest and Landing The commander informed passengers of the return to Budapest due to a technical problem. The aircraft turned back toward VOR Tápiósáp and descended to FL100. With air traffic control permission, the crew entered a holding pattern to consume fuel surplus before executing an automatic ILS approach and manual landing on runway 31R at Budapest Liszt Ferenc International Airport. The before-landing checklist was initiated 18 seconds before reaching decision height and completed 4 seconds before that point. ## Personnel and Aircraft Condition The flight crew consisted of two cockpit crew members and four cabin crew members, with 70 passengers on board. No injuries were reported, and the aircraft sustained no damage. The commander was a 56-year-old Hungarian male with an ATPL license and 17,482 total flight hours, including 2,072 hours on the Boeing 737-300/900. He was serving as pilot monitoring during the incident. The first officer performed the takeoff. ## Investigation Findings and Recommendations The TSB determined that the failure to activate the air conditioning pack switches during pre-takeoff procedures led to the cabin pressurization failure. The crew committed several errors during the turnaround, descent, approach, and landing, which may have been influenced by the preceding period of oxygen deficiency. The TSB formulated recommendations to improve the prevention and handling of similar events in the future.