What happened
On 12 May 2000, a Boeing 747-200 operated by UPS departed Dublin Airport for a scheduled conditional test flight following a heavy maintenance "C" check. During the initial flight control checks, the crew noticed a distinct airframe vibration when deploying the spoilers. Although a maintenance inspector on board suggested the flutter was due to low hydraulic flow and deemed it non-critical, the vibration persisted during subsequent ground tests.
Following takeoff, the situation escalated rapidly. Shortly after gear retraction, the aircraft experienced intense vibrations and a wind shear alarm. The crew soon discovered that both airspeed indicators were significantly under-reading, making it impossible to determine the aircraft' and true speed. As the aircraft climbed, the crew faced further complications, including unreliable altitude and vertical speed indications.
To manage the emergency, the commander declared an emergency and navigated the aircraft toward clearer weather. Because the true airspeed was much higher than the indicated speed, attempting to use standard flap settings caused a flap overload warning. The crew eventually opted for a Flap 10 configuration to maintain control. The aircraft landed safely at Dublin Airport, though the high speed during the approach caused structural damage to the flap fairings and trailing edge components.
The investigation
The AAIU investigation focused on why the pitot-static system failed so comprehensively. Investigators examined the maintenance records from the recent "C" check and inspected the Avionics and Electrical (A&E) bay. The inquiry established that the static drain ports, which are essential for maintaining accurate pressure readings, had been left open.
Furthermore, the investigation looked into the maintenance procedures used during the heavy check. It was found that the technicians had utilized a superseded maintenance manual that permitted the use of static drains for leak testing—a practice no longer authorized by the operator. The investigation also scrutinized the handover process between maintenance shifts and the reporting procedures used by the operator following the incident.
Findings
- The primary cause of the instrument failure was the failure to replace the static drain blanking caps after maintenance work was completed.
- Pressurized air from inside the cabin entered the static system through these open drains, causing the airspeed, altitude, and vertical speed indicators to provide false readings.
- The aircraft sustained damage to the flap fairings and trailing edge components due to being flown at an excessive true airspeed while flaps were extended.
- A lack of structure in the weekend shift handover process contributed to the oversight.
- The use of outdated maintenance manuals allowed for the use of unauthorized testing procedures.
Safety action
- The maintenance organization was advised to review its shift handover procedures and training programs for personnel transferring from line maintenance.
- Recommendations were made to ensure only current, approved maintenance manuals are utilized in the hangar.
- The aircraft operator was advised to review weather minima for post-maintenance test flights to prevent aircraft from entering IMC (Instrument Meteorological Conditions) during the early stages of a flight.