What happened
On 22 December 1999, a Boeing 747-2B5F, registration HL-7451, was operating a cargo flight from Stansted Airport to Milan Malpensa. During the turnaround, maintenance was performed to address a reported defect regarding the captain's Attitude Director Indicator (ADI), which was noted as unreliable in roll during the previous flight leg.
Following the completion of loading and engine start, the aircraft departed Stansted. Shortly after passing 900 feet, the cockpit crew experienced multiple 'Comparator' warnings, indicating a discrepancy between the pilot's and first officer's attitude instruments. As the aircraft climbed, the flight data recorder showed a continuous left turn and a significant decrease in pitch attitude. At approximately 1,400 feet, the aircraft's trajectory deviated from the expected path, and the rate of descent increased sharply. The aircraft impacted the ground near Hatfield Forest, 1.9 nautical miles from the runway, at a high speed and steep nose-down attitude. The impact resulted in a large fireball and the destruction of the aircraft. All 4 crew members on board were killed.
The investigation
The AAIB investigation examined the wreckage, the flight data recorder (FDR), and the cockpit voice recorder (CVR). Investigators analyzed the maintenance actions taken during the turnaround, specifically the work performed by a local avionics engineer and an operator's ground engineer. The investigation also reviewed the aircraft's inertial navigation systems (INS) and the functionality of the ADI comparator module. Radar data from Stansted Watchman confirmed the aircraft's rapid descent and the high rate of descent exceeding 5,000 feet per minute prior to impact.