History of the Flight
On 5 September 2003, a Cessna F177RG Cardinal (registration G-BFIV) departed Blackbushe Airport for a return flight to Henstridge Airfield. While on the downwind leg at Henstridge, the pilot selected the landing gear down. He heard a mechanical 'graunching' noise from the rear of the aircraft and realized that the main gear had not locked down; it was partially extended and trailing aft in the slipstream, while the nose gear had locked. The pilot cycled the gear up and down, but the main gear remained in trail without any further noise. He decided to return to Blackbushe.
During the return, the pilot attempted to persuade the main gear to lock by maneuvering at high and low 'g' and flying very slowly while operating the emergency hand pump. He felt hydraulic pressure and saw the nose gear extend properly, leading him to conclude that the main gear was jammed. A fly-by of the Blackbushe tower confirmed to the pilot that the main gear was trailing aft.
The pilot realized an abnormal landing was unavoidable. His passenger, also a licensed pilot, requested a diversion to Wycombe Air Park, where the aircraft was maintained, but permission was denied. The pilot then circled north of Blackbushe for about 40 minutes to allow other traffic to return. During this time, they planned the emergency landing. The pilot considered landing with the nose gear extended or retracted. He decided to land on the aircraft's belly with the nose gear retracted to avoid potential large bending loads in the fuselage centre and to reduce the risk of a wing dropping after touchdown.
On final approach at approximately 50 feet above ground level, the pilot and passenger unlatched the doors, turned off the master switch, set the mixture to idle cutoff, turned off the fuel selector, and set the propeller pitch to coarse. The aircraft touched down and slid to a stop without fire. Both occupants exited via the cabin doors.
Landing Gear System Description
The Cessna 177RG has a hydraulically actuated retractable landing gear. The nose gear retracts rearward beneath the engine, while the main gear legs retract rearward into open wheel wells in the aft fuselage. The main gear legs are mechanically linked and moved by a single hydraulic actuator attached via an eye-shaped rod end. The actuator rod extends fully when the gear is retracted and retracts to pull the gear into the locked down position. In hydraulic pressure loss, the gear can free-fall but cannot lock without hydraulic assistance. The aircraft has an emergency hand pump for manual extension.
Aircraft Examination
After the gear-up landing, the aircraft was lifted with air bags. The main gear could be pulled down by hand and locked into position. Maintenance engineers found that the main gear hydraulic actuator rod end had failed, disconnecting the actuator from the gear and preventing extension.
Actuator Rod End Examination
The failed rod end was sent to the AAIB for metallurgical examination. It appeared to have failed first at the greasing hole (surface A), then the remaining strap elongated plastically and failed in tensile overload (surface B). A metallurgist confirmed that the strap fracture was due to overload and noted that surface A did not show classical fatigue but might have involved a very low-cycle, high-stress fatigue mechanism. The metallurgist opined that the rod end's strength had been decreased by gross machining abuse within the greasing hole.
Service Information Letter
On 6 August 1979, the aircraft manufacturer issued a customer care service information letter (SE79-37) about the main gear actuator rod end for Cessna 177RG aircraft with serial numbers 177RG0001 to 177RG1366 and F177RG0001 to F177RG0177. G-BFIV (serial number F177RG0161) was within the affected range. The letter stated that a new high-strength main landing gear actuator rod end was available and recommended replacement prior to or at the next annual inspection. The failed rod end was the older lower-strength type, which lacks a greasing hole (the newer version uses a Teflon bearing) to avoid stress concentrations and potential machining errors.
It is not known why the recommendation was not carried out on G-BFIV. The aircraft was 27 years old, and the rod end may have been the original part. The manufacturer stated it was not aware of any main gear actuator failures on 177RG aircraft with the higher-strength rod end installed.
Analysis
The main gear failed to lock down because the main gear actuator rod end had fractured. The 'graunching' noise heard by the pilot likely indicated the rod end failure. Metallurgical examination revealed that the rod end possibly failed due to a very low-cycle high-stress fatigue mechanism at the greasing hole.