What happened
On September 21, 2007, a Bombardier DHC-8-400 operating a scheduled flight from Munich to Florence discovered during landing that the nose landing gear had failed to extend. Despite multiple attempts to deploy the gear using the normal system, the crew observed that the nose gear remained retracted. Due to unusual mechanical noises, the crew elected not to attempt the emergency extension procedure in Florence, as doing so would have left the nose gear doors open, making a return to Munich impossible.
The crew returned to Munich to perform an emergency landing with only the main landing gear extended. During the landing on runway 08R, the aircraft touched down on the main gear but subsequently slid along the centerline. The aircraft came to a stop approximately 1,550 meters from the runway threshold, resulting in no injuries to the passengers, but causing heavy damage to the aircraft and airport infrastructure.
The investigation
The BFU investigation focused on why the nose gear failed to extend and why the emergency extension system was ineffective. Investigators examined the nose gear bay and discovered that the nose gear doors were forced open and pushed to the side. The inspection of the aircraft revealed that the outer fuselage skin and the pressure bulkhead immediately behind the nose gear bay had been scraped.
Technical analysis of the nose gear mechanism showed no damage to the normal or emergency extension systems themselves. However, investigators found a broken spring (part of the nose gear door actuation mechanism) wedged between the nose gear steering hydraulic lines and the lower part of the nose gear oleo strut. Once the broken pieces of the spring were removed, the nose gear could be extended and retracted without issue.
Findings
The investigation established that the primary cause of the incident was a fatigue failure of a spring within the nose gear door actuation mechanism. The failure was driven by the following factors:
- Fretting corrosion occurred on the inner surfaces of the spring's hook ends, which initiated fatigue cracks.
- The loss of protective compressive stresses due to surface wear further weakened the spring.
- Following the break, fragments of the spring became detached from their mounting and fell into the nose gear bay.
- These fragments became wedged in the nose gear strut, creating enough mechanical resistance to prevent the nose gear from reaching its fully extended and locked position.
- The resulting mechanical blockage was so significant that the force required to unlock the mechanism could not be generated by either the hydraulic system or the manual emergency extension pull.
Following the accident, the aircraft manufacturer implemented design changes, including the installation of protective caps to prevent spring fragments from migrating if a mounting fails.