What happened
On 28 February 2000, a BAC 1-11 475 Series, registration VP-CDA, was performing a positioning flight from London Luton Airport to Bournemouth. The aircraft was carrying two pilots and three engineers. During the pushback and engine start sequence, the crew activated the No 2 Engine Driven Pump (EDP) without issue. However, upon selecting the No 1 EDP to the ON position, the aircraft's nose began to sink with a stepping motion. The nose landing gear (NLG) collapsed, causing the aircraft to settle on its doors and tyres. There were no injuries to the five occupants, though the nose landing gear tyres and doors sustained damage.
This incident followed a flight from Brindisi to Luton a few days earlier, during which the crew had observed an intermittent failure of the No 1 EDP and a drop in hydraulic fluid levels. Following that flight, a major leak was discovered in the left main landing gear bay, which was subsequently repaired by replacing a flexible hose on the No 1 auxiliary AC pump.
The investigation
The AAIB investigation established that the hydraulic system had been replenished following the hose replacement, but the reservoir had been nearly empty. While the engineers had used the AC pump to bleed the EDP and exercised the flying controls and steering, they did not perform landing gear retraction and extension because the aircraft was not jacked. Additionally, the co-pilot's windscreen wiper, which is powered by the No 1 system, was not exercised.
Testing of the removed components, including the Steering and Lock Changeover Valve and the up/down lock jack, did not reveal any significant mechanical defects. However, the investigation found that the cockpit hydraulic quantity gauge was unserviceable; a fault in the sender prevented it from registering fluid levels below the green sector, meaning the crew was unaware the reservoir was nearly depleted.
Findings
- The primary cause of the nose landing gear collapse was the presence of air in the hydraulic system, which had not been fully removed after the hose replacement.
- The hydraulic reservoir had been almost entirely depleted during the previous flight, significantly increasing the risk of air entering the system.
- The crew was unable to accurately monitor the fluid level due to a faulty quantity gauge that failed to indicate low levels.
- The maintenance procedure performed did not include the full exercise of the landing gear or the windscreen wiper, which is necessary to ensure all air is bled from the system.
Safety action
- Airbus UK accepted a recommendation to emphasize the importance of exercising the landing gear and windscreen wiper following a complete loss of hydraulic fluid in the No 1 system.