What happened
On March 26, 2008, at 14:39 UTC, a Ukraine International Airlines Boeing 737-300, registration UR-GAQ, was being radar vectored toward Helsinki-Vantaa airport. The aircraft had been cleared to descend to 2,300 feet. However, the aircraft descended to a minimum altitude of approximately 1,400 feet, breaching the required terrain clearance for a telecommunications mast located near its flight path. The air traffic controller observed the deviation and immediately ordered the crew to climb back to 2,300 feet.
The investigation
The investigation by the Finnish Safety Investigation Authority (OTKES) established that the aircraft's altimeters were displaying an erroneous altitude of more than 1,000 feet. This error was caused by an incorrect barometric pressure setting. The investigation found that the flight crew had deviated from Standard Operating Procedures (SOP) and had failed to transition from QNH to QNE pressure settings at the appropriate phase of flight. Additionally, the crew had not reviewed the descent-approach checklist prior to the incident.
Detecting the hazard was delayed by two technical factors: the aircraft's Enhanced Ground Proximity Warning System (EGPWS) did not provide a warning for the specific telecommunications mast, and the Minimum Safe Altitude Warning (MSAW) feature in the Helsinki air traffic control radar system was not being utilized for alerting altitude infringements. Furthermore, the investigation revealed that the pilots did not report the incident to the airline or aviation authorities, contrary to company regulations.
Findings
- The direct cause of the altitude infringement was an incorrect altimeter pressure setting.
- Contributing factors included failures in Crew Resource Management (CRM) and the crew's deviation from Standard Operating Procedures.
- The incident was classified as a serious incident, though the air traffic controller initially reported it simply as an incident.
- The lack of active MSAW functionality and the failure of the EGPWS to warn of the mast hindered timely detection of the risk.