What happened
On 10 March 1997, an Air Zimbabwe Boeing 707-330B, registration Z-WKU, departed Shannon Airport for Harare, Zimbabwe, following a refueling stop. Shortly after takeoff, the crew received fire warnings for the No. 3 engine. The captain proceeded to 1,0/00 feet before shutting down the affected engine and deploying fire extinguishers.
Due to the aircraft's high takeoff weight, the crew attempted to dump fuel to reach a safe landing weight. However, they were restricted to using only the left-side dump chute because the aircraft's fuel system prevented transferring fuel from the right wing to the left. During the process, the fuel dump chute failed to retract, which prevented the deployment of full flaps. This forced the crew to perform an overshoot of the runway.
Further complications arose during the subsequent approach when the nose undercarriage failed to lock down. After a period of holding to address the gear and an unrelated ILS display failure, the crew successfully landed the aircraft at Shannon. The aircraft touched down approximately 30 tonnes above its maximum landing weight, causing the nose wheel shock absorber to collapse upon arrival. There were no injuries among the 41 passengers or 12 crew members.
The investigation
The AAIU examined the engine damage, which included burnt-through cowlings and a melted fire detection harness. The investigation focused on the origin of the fire and the sequence of mechanical failures. Investigators discovered that the No. 3 engine had experienced a similar fire only 11 days prior.
Technical analysis of the hydraulic system revealed that shutting down the No. 3 engine reduced the available hydraulic pumps from two to one. This reduction in pressure, combined with high demand from simultaneous flap and gear deployment, likely contributed to the nose gear malfunction. Additionally, the investigation found that the nose gear actuator linkage suffered from insufficient lubrication, making the gear stiff and difficult to lock.
Findings
- The fire was caused by a poorly executed welding repair on the engine diffuser casing, which distorted the mounting boss.
- This distortion created an air/fuel leak that eventually led to the ignition of the fuel/air mixture.
- Maintenance pressure to keep the aircraft available for a VIP tour likely prevented a thorough investigation of the previous fire 11 days earlier.
- The decision to land above the maximum landing weight was necessary due to the deteriorating weather and restricted fuel dumping capabilities.
Safety action
Following the investigation, safety recommendations were issued to the operator regarding the following:
- Improving the diagnostic skills of line maintenance personnel through enhanced training.
- Ensuring operational pressures do not interfere with the proper analysis and rectification of aircraft defects.
- Implementing quality audits in maintenance workshops to ensure welding repairs meet manufacturer dimensional standards.