What happened
On 30 October 2001, a BAe 146-300, registration G-UKHP, was performing a scheduled passenger flight from London Stansted to Paris Charles de Gaulle. While on final approach to Runway 2LL, the crew heard a loud mechanical bang and a grinding noise originating from the center-left floor area. This was immediately followed by several warnings on the Central Warning Panel, including a Yellow hydraulic system caution and alerts regarding the AC pump failure and high temperature.
Despite the indications, the commander continued the approach as the aircraft remained stable. During the landing roll, the crew noted that while the Green hydraulic system powered the lift spoilers, the Yellow system spoilers failed to deploy. As the aircraft decelerated, the In-Flight Supervisor alerted the flight deck to the presence of dense smoke in the mid-cabin area. The commander responded by shutting off the air conditioning packs and the APU, eventually bringing the aircraft to a halt on taxiway Y7.
An emergency evacuation was initiated using three slides. While the evacuation was completed quickly with no injuries to the 67 passengers or 6 crew members, several passengers experienced vomiting and choking upon exiting the aircraft due to the smoke exposure.
The investigation
Engineers examining the aircraft discovered that a flexible pipe connected to the Power Transfer Unit (PTU) had ruptured because it had been chafing against a rigid pipe. This rupture caused the Yellow hydraulic system reservoir to empty. The investigation also revealed that the AC hydraulic pump was excessively hot.
A detailed examination of the AC hydraulic pump in the United States showed that several internal bearings were worn and scored, which likely caused the noises heard by the crew. Crucially, the pump's thermostat had failed to open, meaning the thermal protection intended to trigger at 204°C was non-functional.
Findings
- The primary cause of the hydraulic loss was a ruptured flexible pipe caused by chafing against a rigid component.
- The dense black smoke in the cabin was generated by the spray of hydraulic fluid hitting the overheated AC hydraulic pump.
- The AC hydraulic pump continued to run despite the loss of system fluid, causing its temperature to rise rapidly.
- The failure of the pump's thermostat prevented the timely activation of thermal protection.
- The aircraft had not yet received a manufacturer-recommended modification designed to improve the sealing of the hydraulics bay to prevent fluid mist ingress into the cabin.