On August 30, 2008, a Cessna 172RG Cutlass (registration EC-DNG) was involved in an aviation accident near Aeropuerto de Cuatro Vientos (Madrid), ES. No fatalities were reported. Investigators recorded the probable cause as: The incident is considered to have occurred due to the collapse of the nose gear leg, resulting from an uncommanded unlocking during the landing of the third traffic pattern. This summary draws on records from the Spanish Civil Aviation Accident and Incident Investigation Commission (CIAIAC).
A Cessna 172 RG (EC-DNG) suffered a nose gear collapse during a training landing at Madrid-Cuatro Vientos on August 30, 2008. Both occupants were uninjured, and the aircraft sustained significant damage.
Incident Overview and Flight Details On August 30, 2008, at 20:38 local time, a Cessna 172 RG, registration EC-DNG, operated by Aerofan, experienced a nose gear collapse while landing at Madrid-Cuatro Vientos Airport. The aircraft was conducting a local visual training flight with an instructor and a student pilot on board. The flight began at approximately 20:20 local time from runway 28. Weather conditions were CAVOK with daylight, no clouds, and light winds without gusts. The first two traffic patterns were completed without incident. For the third pattern, the instructor decided to keep the landing gear down and locked to focus on other training aspects. The crew confirmed the gear selector was in the 'GEAR DOWN' position and the green gear-down-and-locked light was illuminated. To prevent unnecessary operation of the landing gear pump, the pilot disconnected the electrical power to the pump using the 'GEAR PUMP' circuit breaker. During the landing, the instructor had to correct the maneuver due to a premature flare by the student. The aircraft touched down on the main gear, and when the nose wheel made contact with the runway, a strong vibration occurred until the nose gear leg finally collapsed. Both occupants were uninjured and exited the aircraft on their own after securing it and notifying the tower. The runway was cleared by 20:49 local time. The aircraft sustained significant damage, including bent and scraped propeller blade tips, as well as minor deformations and scrapes on the nose gear doors and the engine exhaust pipe. ## Aircraft and Maintenance Background The aircraft, serial number 172RG-0526, was manufactured in 1980 and registered the following year. As of the end of June 2008, the aircraft and engine had 5,624 and 1,961 flight hours, respectively. Maintenance was performed directly by the operator's personnel at the Madrid-Cuatro Vientos Airport. The Cessna 172-RG features a tricycle, retractable landing gear system with hydraulic actuation. The hydraulic pressure is generated by a pump driven by an electric motor, controlled by a pressure switch that engages the pump below 1,000 psi and disengages it above 1,500 psi. The gear position selector, labeled 'GEAR UP' and 'GEAR DOWN', is located on the left side of the right instrument panel and acts directly on a two-position hydraulic valve. Each gear leg has a separate actuation and locking system. In December 1995, Cessna issued Service Bulletin SEB95-20 with instructions to correct failures in the extension and locking of the nose gear leg, which were caused by defects in the locking pins. The instructions required an inspection of the pins for clearances and movement before 200 hours or one year, whichever came first, and their replacement with a different design if found in poor condition. Interim inspections every 200 hours were required until the new pins were installed. According to the maintenance workshop, these instructions were not followed for this aircraft, and the indicated inspections were not performed. ## Investigation and Technical Findings Maintenance personnel who responded to the scene found the main gear legs extended and the nose gear leg almost retracted. The gear selector was in the 'down' position, the flaps were up, and the circuit breakers for the 'LDG GEAR' and 'GEAR PUMP' electrical circuits were connected. The crew manually lowered the tail of the aircraft to release the nose gear leg and removed the support cradle at the nose. They observed that the nose gear leg extended almost fully under its own weight. When the emergency gear extension was activated, the doors opened, the nose gear leg locked, and the green gear-down-and-locked light illuminated when the master electrical switch was connected. Subsequent inspections and functional tests in the hangar with the aircraft on jacks revealed no additional damage beyond what was reported at the scene. No signs of failure or anomaly were found in the mechanical devices of the landing gear, particularly those related to the nose gear leg, including the doors' bars and springs, and the anti-shimmy damper. Several complete gear actuation cycles were performed without any failure or incident, and no uncommanded unlocking of any leg occurred. It was observed that disconnecting the hydraulic pump breaker caused a small movement in the main wheels, similar to a slight pressure discharge. Repeating the test under these conditions, without locking the wheels, it was confirmed that moving the legs pumped fluid to the nose gear in sufficient quantity to move it in the opposite direction, causing the nose gear leg actuator to extend. According to the gear's operation, this actuator extension movement corresponds to the unlocking of the nose gear leg and its subsequent retraction. The tests also confirmed that the green gear light illuminates when the gear is locked down and the amber light when locked up. Company sources indicated that it was a common practice among pilots to disconnect the pump power once the landing gear was locked, with the intention of not making the pump work unnecessarily, although they noted that it was reconnected before landing. ## Analysis and Conclusions The mechanics of the locking system make it impossible for the nose gear leg to collapse without breaking, and for this to occur, the leg must first be unlocked and the locking pin must release from the hook. The hangar tests indicated that the landing gear was in good condition, despite not having incorporated Service Bulletin SEB95-20 and not having replaced the locking pins or performed the 200-hour inspections as recommended. According to the occupants' testimony, the three landings were performed with the hydraulic pump breaker disconnected. The gear's behavior in the tests without this power supply was different from usual and could result in the actuator not pulling on the locking pin. Although the same procedure was followed in previous landings, the difference in time during which the pump remained without electrical power (a complete traffic pattern) significantly increased the possibility of pressure loss in the hydraulic circuit and, therefore, the inoperability of the actuator on the locking pin. A circuit breaker is a device designed to protect a specific installation from excessive currents and is not intended for use as a switch, making its use as such highly inadvisable. Furthermore, the widespread practice among pilots of disconnecting the pump to prevent it from 'working unnecessarily' actually prevents it from working correctly, as the pump must engage when the hydraulic fluid pressure drops below 1,000 psi to restore pressure to 1,500 psi levels. As a result of the correction of the premature flare, the landing was hard. Even accepting that the nose wheel was not the first to touch the runway, the impact in a hard landing would produce higher loads in the locking mechanism, the harder the landing. Given the horizontal position of the actuator and the locking hook in the nose gear leg, these loads act in the direction of releasing the pin from the hook and opening the lock. Therefore, the incident is considered to have occurred due to the collapse of the nose gear leg, resulting from an uncommanded unlocking during the landing of the third traffic pattern. Although the exact cause of this unlocking could not be determined, the effects of a hard landing, in combination with the circumstance of the gear pump being disconnected during the landing, may have been the origin of the failure.
Probable cause
The incident is considered to have occurred due to the collapse of the nose gear leg, resulting from an uncommanded unlocking during the landing of the third traffic pattern. Although the exact cause of this unlocking could not be determined, the effects of a hard landing, in combination with the circumstance of the gear pump being disconnected during the landing, may have been the origin of the failure.