What happened
On August 30, 2008, a Cessna 172RG, registration EC-DNG, was performing a local flight instruction session consisting of takeoffs and landings at Madrid-Cuatro Vientos Airport. The flight was being conducted by an instructor and a student pilot under visual flight rules in clear weather conditions.
After completing two successful circuits with the landing gear in the extended position, the instructor decided to perform a third circuit with the landing gear locked down to focus on other instructional aspects. During this third circuit, the pilot had disconnected the electrical power to the landing gear pump by pulling the circuit breaker to prevent the pump from running unnecessarily.
Upon landing, the instructor had to intervene with the controls to correct a premature rotation by the student pilot. The aircraft touched down on the main gear, but as the nose wheel made contact with the runway, intense vibrations occurred until the nose gear finally collapsed. The two occupants were uninjured and evacuated the aircraft safely. The aircraft sustained significant damage, including bent propeller blade tips and abrasions to the nose gear doors and engine exhaust.
The investigation
Investigators examined the hydraulic landing gear system, which is electrically driven and uses a pressure switch to regulate operation between 1,000 and 1,500 psi. The investigation focused on the nose gear locking mechanism and the impact of the landing forces.
Maintenance records revealed that the aircraft had not undergone the specific inspections or part replacements recommended by Cessna Service Bulletin SEB95-20, which addressed potential nose gear extension and locking failures. However, subsequent functional tests of the aircraft on jacks showed that the gear mechanism and warning lights were functioning correctly, and no mechanical anomalies were found in the struts or anti-shimmy dampers.
Findings
- The nose gear collapsed due to an uncommanded unlocking of the nose gear mechanism.
- The pilot's decision to disconnect the landing gear pump circuit breaker prevented the pump from maintaining necessary hydraulic pressure. This loss of pressure likely prevented the actuator from properly holding the locking pin in place.
- The landing was hard due to the instructor's necessary correction of the student's premature rotation. The resulting impact forces acted in a direction that helped release the locking hook from the pin.
- The practice of using a circuit breaker as an on/off switch is improper, as the breaker is designed for overcurrent protection, not manual switching.
- The aircraft had not implemented the inspections or upgrades required by the manufacturer's service bulletin regarding locking pins.