Incident
On 28 July 1998, a Boeing 757-236 (registration G-BPEE) operated by an airline as a public transport flight from Heathrow to Belfast City Airport experienced a No 2 engine fire warning while cruising at Flight Level 260 over the Isle of Man. The crew executed the engine fire drill and discharged two fire extinguishant bottles; the warning cancelled after about 30 seconds. An emergency was declared and the aircraft landed safely at Belfast.
Background
On the preceding Manchester to Heathrow flight, the crew had received 'GEN DRIVE' and 'GEN OFF' EICAS warnings for the No 2 engine. They disconnected the Integrated Drive Generator (IDG) using the cockpit switch and used the APU for power. After landing, maintenance messages included 'BPCU BITE', 'DP TRIP', and 'IDG LOW OIL PX'. The IDG oil level was found full, and the engine was run up to idle with the disconnect switch operated again. The aircraft was cleared for the Belfast flight with the No 2 IDG inoperative, per the Dispatch Deviation Manual.
Findings
Post-incident inspection revealed an uncontained failure of the IDG casing, with oil staining and fire damage to the fan cowl inner skin and wiring. The No 2 engine, BPCU, and GCU were replaced. The IDG disconnect switch on the control panel was found to malfunction intermittently during testing, preventing disconnection. The No 1 engine IDG disconnect switch was stiff. The control panel, installed since aircraft delivery in 1991, had switches of an earlier modification standard. No means existed for crew or ground engineers to confirm successful IDG disconnection.
The IDG, fitted on 10 March 1998, was returned to the manufacturer. Strip inspection indicated failure initiated by a worn or failed generator rotor roller bearing; the bearing failure cause was undetermined. If the disconnect system had operated correctly, gross damage would not have occurred. The generator rotor impacted the stator, rupturing the casing and releasing oil and debris. No evidence of fire within the IDG was reported by the manufacturer, but the operator considered a limited fire occurred.
Operator Action
As a result, the operator amended the maintenance schedule to include routine IDG disconnect function checks on 'S' checks, and introduced a modification (EOC-757-024G150) to replace early-standard Korry Switchlights with the more reliable 'D' standard across its Boeing 757 and 767 fleets. Similar switches in safety-related applications are included. Completion was scheduled by 30 September 1999.
Previous Boeing Service Letters
Boeing Service Letters (dated 1994 and 1995) had advised operators of IDG modifications including a case vent system and thermal decoupler, and encouraged retrofit of improved Korry Switchlights. Despite these, the AAIB made no safety recommendations due to existing actions.