What happened
On 24 April 2004, a Boeing 747-132, registration N481EV, was conducting a cargo flight from Ramstein, Germany, to New York. While cruising at FL360, the left outboard engine experienced a rundown and failed to restart. The crew subsequently descended to FL210 and declared an emergency, requesting a diversion to London Heathrow Airport.
During the descent, the commander noted that the thrust levers were positioned in a forward "number six" position, yet the engine pressure ratio (EPR) indications remained at idle. As the aircraft continued its descent, the airspeed began to decrease significantly. The crew attempted to manage the aircraft's energy through various maneuvers, including 'S' turns, to maintain a manageable approach path. Despite the lack of familiarity with the local airport layout, the crew utilized visual references to navigate toward the runway. The aircraft eventually touched down at 145 kt CAS within the normal touchdown zone, using available braking and reverse thrust to decelerate.
The investigation
The AAIB investigation examined the engine failure, the subsequent thrust indications, and the air traffic control (ATC) response. Engineers inspected the engines and fuel samples at Heathrow, finding the fuel to be within specification. While a BITE fault was identified in the Engine Instrument Display System (EIDS), the investigation focused on why the remaining three engines appeared to be underperforming.
Simulator evaluations were conducted using a Boeing 74 and confirmed that the rate of airspeed decay experienced by the crew was consistent with the thrust levels recorded. The investigation also reviewed the adequacy of the aeronautical charts carried by the crew and the ATC procedures for managing aircraft in emergency states over densely populated areas.
Findings
- The failure of the No 1 engine to restart was attributed to an inoperative igniter.
- The three remaining engines were not producing the expected thrust required to maintain the selected cruise speed, a phenomenon that was symptomatic of a common issue across the operative engines.
- The commander's decision to perform a visual approach was necessitated by a lack of appropriate approach charts for the diversion airport.
- The aircraft's descent profile was managed through manual handling and visual references due to the lack of established glide performance guidance in the operator's manual.