Flight History
The pilot of Piper PA-24-250 G-BUTL was on a private flight from Bembridge to Blackbushe. While entering the circuit at Blackbushe, he attempted to lower the landing gear using the electrically operated transmission system. The gear did not fully extend, and the electric transmission motor circuit breaker for the landing gear tripped.
The pilot informed Blackbushe Tower that he would fly to the west and attempt to lower the gear using the manual system. After releasing the motor from the transmission screw and trying the manual system, the gear still failed to extend fully. He discussed the problem via radio with an engineer from his maintenance company at Lasham Airfield, but the issue was not resolved. He then decided to divert to Lasham, during which he repeatedly attempted to lower the gear using the manual telescopic lever, without success. The pilot noted that the stiffness of the mechanism increased with each attempt.
The pilot contacted Lasham Air Traffic Control, explained the problem, and stated his intention to perform an emergency landing. He made one fly-past of the airfield for emergency services to assess the situation; the gear was reported to be partially deployed. He then flew an approach for Runway 09 and completed a successful emergency landing. The runway was dry. Fire and ambulance services were on standby but were not needed. The pilot, wearing a lap and diagonal harness, vacated the aircraft safely.
Landing Gear Retraction System
The aircraft has a tricycle-type, fully retractable landing gear operated by an electric motor and transmission assembly. The retraction/extension mechanism comprises an electric motor, transmission assembly, torque tube assembly, push-pull cables for each main landing gear (MLG), and a push-pull tube to the nose gear. Limit switches shut off the motor when the gear is fully extended or retracted and also operate the landing gear indicator lights on the instrument panel.
When the landing gear selector switch is placed in the down position, the motor operates the transmission, which rotates the torque tube assembly, moving the torque arms back. This pushes on the MLG cables and pulls on the nose gear tube to extend the gear. Once fully extended, the down switches stop the motor and illuminate the green gear-down-and-locked indicator.
In case of electric system trouble, an emergency release lever in the cockpit can disconnect the retraction transmission motor from the transmission screw. This allows use of an emergency telescopic extension lever, which is pushed forward to manually lower the gear.
Aircraft Examination
The aircraft was recovered to the maintenance hangar for a full examination. The transmission motor was found to be serviceable. The push-pull control cables and rods were removed and examined. The right MLG outboard sliding sleeve guide had detached from its swaged housing to the fixed sleeve. When tested by hand, the cable operating the right MLG required significant force to move through its limits of travel, while the left MLG cable required considerably less force. The landing gear, once disconnected from the push-pull cables and rods, moved freely and easily swung down into the extended position.
The push-pull cables were taken to the AAIB for further investigation. The inner cables were removed from the fixed sleeves and examined. The right-hand cable had kinks at both ends at points where its limits of travel in each direction would coincide with the position of the swaged joint used to join the fixed sleeve to the sliding sleeve guide. Radiographs of both fixed sleeves showed no evidence of broken strands on the inner surface. The stiffness of the right-hand push-pull cable appeared associated with the kinks, as both left and right cables were in similar condition regarding wear and lubrication. These kinks should not normally occur with the sliding sleeve guide properly in place. The guide sleeve can come free from its swaged housing only with excessive friction on the sliding sleeve guide (which did not appear to be the case) or during maintenance.
Previous Related Incidents
On 24 June 1998, the nose and main landing gear assemblies had been overhauled. The retraction systems were tested and found satisfactory. On 18 September 1998, the landing gear failed to extend, and the pilot used the manual system, which operated satisfactorily, allowing a safe landing. Examination by the maintenance engineer revealed that the landing gear transmission assembly was the source of the problem: the transmission motor screw thread and gear wheel had worn, leading to partial break-up. In hindsight, it was agreed that this occurrence could have been attributed to over-stiffness of the assembly. A new transmission motor and assembly were fitted, and the gear operation was checked and found satisfactory.
Since the main landing gear overhaul, the aircraft had flown 99 flight cycles totaling 59 hours; since the motor replacement, it had flown 17 flights and 10 hours.
Discussion
As stated, the excessive friction experienced during operation of the right MLG push-pull cable appeared associated with kinking of the inner cable. At the outboard end, kinking occurred roughly adjacent to the swaged joint with the MLG down; at the inboard end, kinking occurred adjacent to the swaged joint with the gear retracted. The swaged joints are designed to accommodate a small amount of angular movement of the guide sleeve, but if one separates completely, the unsupported inner cable could kink under compressive forces. Examination of the failed swage indicated it had been subjected to a single lateral load beyond its movement range, possibly during maintenance with the cable assembly disconnected from the MLG. However, it could not be determined when this loading occurred, nor could it be explained how the aircraft operated successfully for 99 flights after the maintenance overhaul.
The engineer acknowledged that, when the motor problem was discovered, he suspected excessive friction in the MLG bushes and bearings and inspected and lubricated several items. He did not specifically check the freedom of the cables, and there was no Maintenance Manual requirement to do so. There is also no provision or requirement to lubricate the push-pull cables, which would require considerable disassembly. As a routine, he had applied lubricant to the more accessible guide sleeves, rods, and pivot pins.