What happened
On 4 June 2007, a Boeing 737-500, registration SP-LKA, departed London Heathrow Airport for Warsaw. Shortly after takeoff from Runway 09R, the crew experienced a sudden and total loss of information on both the Electronic Horizontal Situation Indicators (EHSI) and Electronic Attitude Director Indicators (EADI). The flight displays effectively went blank, leaving the co-pilot to navigate using only the standby attitude indicator and a standby magnetic compass.
As the aircraft climbed through 1,500 ft and entered instrument meteorological conditions, the crew struggled to follow radar headings. The commander's communications with Air Traffic Control (ATC) were difficult to understand, leading to several instances where the aircraft deviated significantly from assigned tracks. Despite instructions to maintain specific headings, the aircraft tracked erratically, at one point crossing the localizer course at a 90-degree angle. Eventually, the crew decided to return to Heathrow, and after being vectored by ATC, the aircraft landed safely on Runway 09L.
The investigation
The AAIB investigation examined flight data recorder (FDR) information, cockpit voice recordings, and air traffic control transcripts. The investigation focused on why the primary flight displays lost all attitude and heading data. Investigators analyzed the aircraft's inertial reference systems (IRS) and the Flight Management System (FMS) position data.
Technical analysis by the aircraft manufacturer revealed that the flight displays lost data because the IRS units were likely in an alignment mode rather than navigation mode during takeoff. Furthermore, the investigation looked into a pre-flight error where the co-pilot had entered the aircraft's position using an incorrect longitude, mistakenly entering 'East' instead of 'West' while at the parking stand.
Findings
- The primary cause of the instrument blanking was that the IRS units were likely in ALIGN or OFF mode during the takeoff roll, rather than the required NAV mode.
- A manual entry error during pre-flight preparation resulted in an incorrect longitude being entered into the FMS, placing the aircraft's position approximately 33.5 nm east of its actual location.
- The crew's ability to follow ATC instructions was severely degraded by the loss of primary flight instruments and the high workload of flying in IMC using only standby equipment.
- Communication difficulties between the commander and ATC contributed to the aircraft's erratic flight path and the failure to recognize the severity of the navigation problem early on.