Incident Description
On 15 February 2005 at 1323 UTC, a HS 748 Series 2A cargo aircraft, registration G-BVOU, landed at Belfast (Aldergrove) Airport in calm conditions with good visibility. The commander, occupying the right-hand seat, reported that the landing appeared normal with no excessive or heavy braking. After exiting the runway onto the taxiway, steering became difficult and the aircraft could barely be manoeuvred off the runway. Air traffic control reported that the aircraft was moving slowly; when asked if assistance was required, the crew stated that the left engine had stopped. Upon leaving the aircraft, the commander noted that both tyres on the left main landing gear were deflated.
Aircraft and Crew
The aircraft was powered by two Rolls-Royce Dart 534-2 turboprop engines and had been manufactured in 1973. The flight was a public transport cargo operation with two crew members and no passengers. No crew injuries were reported. The commander held an Airline Transport Pilot's Licence, was 58 years old, and had 8,500 total flying hours, of which 4,000 were on type.
Damage and Initial Findings
Photographs taken by airport authorities showed that both left main tyres had deflated, with treads and carcass structures completely worn through locally to 'flat spots'. The left propeller was in a coarse pitch position, at or approaching the feathered setting. The aircraft technical log was annotated with 'Left-hand fire indication on landing; both (undecipherable word) fired'. Subsequent maintenance included replacement of both fire bottles. Analysis of further technical log entries indicated no significant internal damage in the left engine, and ground running was successfully carried out. No problems with wheel braking were reported after the aircraft returned to service.
Subsequent Operating Problems and Component Changes
After the incident, the aircraft experienced a series of engine flame-outs during landing and RPM hunting in flight. A micro-switch within the control console, operated by the flight fine pitch-stop (FFPS) lever, was noted as slightly damaged and suspected of incorrect operation. It was replaced. The plug to the solenoid in the left propeller control unit (PCU) was also replaced, and ultimately the PCU was changed. Rig testing of the PCU initially revealed no functional problems, and a strip examination found no internal defects. However, after reassembly, a functional problem was experienced during a rig test, initially attributed to pre-existing damage to a pressure relief valve ball, but subsequent microscopic examination did not substantiate that theory.
Relevant Aircraft Features
The landing procedure on the HS 748 requires the FFPS lever to be moved to the 'stops-removed' position after touchdown. Operation of the FFPS lever causes micro-switches to supply an electrical signal to each PCU, allowing propeller pitch to reduce below the FFPS setting. The aircraft is equipped with Maxaret anti-skid units that release brake pressure to prevent wheel locking, but they do not prevent locking if a stationary wheel contacts the ground with the brake already applied. Small amounts of grease used to lubricate universal joints in the engine nacelle can be centrifuged away and coat adjacent areas; if the aircraft is brought to a halt with throttles fully aft and low engine rpm, ventilation can be reduced, leading to combustion of the grease deposits and activation of the fire detection loop.
Analysis
The precise sequence of events during this landing is not clear. The anti-skid units normally prevent wheel locking unless there is a system defect or brake pressure is applied as the wheel touches down. The damage to both left tyres, without corresponding damage on the right, cannot readily be accounted for by a system failure. No mechanical reason for the left engine stoppage is evident from the technical log entries covering rectification immediately after the flight. The problems with occasional left engine flame-out on landing and periodic rpm fluctuation in flight ceased after the left auto-coarsen system micro-switch was replaced, although testing of the removed micro-switch failed to reproduce a failure condition. Considerable difficulty was experienced in decoding the flight data recorder, and the limited parameters recorded were of no value; the cockpit voice recorder indicated normal operations during touchdown and initial deceleration.