Event Overview
On 21 September 2007, a Bombardier DHC-8-400 (serial number 4028, built in 2000) was operating a scheduled flight from Munich to Florence. During the approach to Florence, the flight crew observed that the nose landing gear had not extended. Multiple attempts to extend the gear using the normal system were unsuccessful. The crew reported hearing unusual mechanical noises and decided not to use the emergency extension system. They elected to return to Munich.
Emergency Landing
At Munich, further attempts to extend the nose gear using both normal and emergency systems failed. The aircraft performed an emergency landing with only the main landing gear extended. It touched down on runway 08R at 19:52 local time and came to a stop on the centerline after approximately 465 meters. The passengers evacuated via the forward door stairs. No injuries were reported. The aircraft sustained heavy damage, and airport facilities were also damaged.
Investigation Findings
Examination of the aircraft revealed that the two forward nose gear doors were open and pushed aside. The fuselage skin and the pressure bulkhead behind the nose gear bay had been abraded. The normal extension and retraction systems and the emergency extension mechanism showed no damage. The emergency extension handle had been pulled only 11 cm, compared to the normal 30 cm travel measured on another aircraft of the same type.
A broken coil spring (part number 47844-101) was found jammed between the hydraulic lines of the nose gear control and the lower part of the nose gear drag strut. The spring was a component of the nose gear door actuation mechanism. It was broken at one end, and the fracture surface showed signs of fatigue cracking. After removal of the spring, the nose gear could be extended and retracted without issue.
Metallurgical Analysis
The spring was examined by the Institute for Materials Science at the Technical University of Braunschweig. The analysis confirmed a fatigue fracture initiated on the inner surface of the hook end. The hook surface showed additional damage from fretting corrosion, which reduced the protective compressive stresses from shot peening. The crack propagated over at least 3,000 cycles. The damage resulted from spring actuation during takeoffs and landings, not from in-flight vibrations. The opposite hook also showed fretting corrosion and a developing crack, indicating a general design issue.
Probable Cause
The investigation concluded that the immediate causes of the accident were:
- A coil spring in the nose gear door actuation mechanism fractured due to fatigue cracking from fretting corrosion.
- The spring fragments fell into the closed gear bay near the shock strut and became jammed when the nose gear retracted.
- The jammed spring locked the nose gear retraction mechanism in the retracted and locked position.
- The force required to release the locked mechanism could not be generated by either the normal or emergency extension systems.