Incident Overview
On 20 April 2017, at approximately 9:30 local time, a Reims Aviation F177RG (registration HB-CGM) experienced a failure of the landing gear extension system during approach to Regionalflugplatz Birrfeld (LSZF). The aircraft was operating under visual flight rules (VFR) on a private training flight. The pilot and a flight instructor were on board; both were uninjured. The aircraft sustained light damage, limited to a crack in a hydraulic line of the landing gear mechanism.
Flight Details
The flight departed Birrfeld at 8:45 for a training flight intended for the pilot's license renewal. After conducting a simulated forced landing in the area south of Aarau, the crew returned to Birrfeld approximately half an hour later. Upon entering the west sector, the pilot selected the landing gear down. The green landing gear indicator light did not illuminate. A mirror under the right wing showed that the main landing gear was only partially extended. The crew followed the procedures prescribed in the Aircraft Flight Manual (AFM), including operating the hand pump. The hand pump lever moved without noticeable resistance, indicating a loss of hydraulic pressure in the landing gear system. According to the AFM, the gear could not be extended in this condition.
A ground instructor familiar with Cessna retractable-gear aircraft suggested that the crew pull the main gear legs forward with their feet while the doors were open. This proved impossible because on the F177RG, the gear is located well behind the doors and remained at approximately 45 degrees aft.
After further prolonged operation of the hand pump and positive vertical acceleration maneuvers, the main gear appeared to extend to within about 10 degrees of the vertical locked position. The flight instructor took over flight controls while the pilot, secured by seat belts, opened the right door and attempted to pull the right gear leg forward with his foot but could not apply sufficient force. Using a jumper cable found in the baggage compartment, the pilot looped it around the right gear leg. After several attempts, and with the instructor flying a slow glide with flaps extended and engine at idle, they pulled the gear into the locked position. The left gear leg was then reached by hand and pulled into the locked position with a jerk, causing the green gear indicator to illuminate. The subsequent landing on runway 08 after a total flight time of 1 hour 50 minutes was uneventful.
Technical Investigation
Examination of the hydraulically operated landing gear extension and retraction mechanism revealed a hydraulic leak in the supply line to the nose gear hydraulic cylinder. The crack was located on the inside of a bent aluminum tube. The cross-section of the tube in the bent area was oval rather than circular, indicating that the tube was bent with too tight a radius or using unsuitable tools, or deformed by hand. The tube also exhibited several slight, sharp-edged dents at various points, further indicating improper bending. It is assumed that excessive plastic deformation in the bent area led to the formation of the crack, allowing hydraulic fluid to escape.
Probable Cause
The severe incident was caused by the loss of hydraulic fluid from the landing gear mechanism through a crack in a bent aluminum tube, which prevented the gear from being extended using either the normal gear selector or the emergency hand pump procedure. The crack most likely resulted from improper bending of the aluminum tube.