What happened
On August 19, 2018, a Bombardier DHC-8-400, registration N-404AV, operated by LC BUSRE S.A.C., departed Lima (SPJC) for Ayacucho (SPHO). Prior to departure, the crew noted a red "unlocked" indication for the nose landing gear (NLG). A maintenance technician cleaned a proximity sensor, but this action was not recorded in the technical flight log.
During the approach to Ayacucho, the crew attempted a normal powered gear extension, but the red "not downlocked" indication persisted. Due to unfavorable weather and fuel concerns, the crew decided to return to Lima. Upon approaching Lima, a second attempt at a normal gear extension yielded the same failure. The crew then attempted an alternate gear extension; however, the red indication remained, and the alternate indicator failed to show a green "downlocked" light. Air Traffic Control also observed that the nose gear appeared to be in the up position.
Consequently, the crew performed an emergency landing with the NLG in the UP position. The aircraft came to a stop on the runway, and an emergency evacuation was conducted with the assistance of airport fire services. No injuries were reported.
The investigation
The CIAA investigation focused on the functionality of the landing gear indication system. Investigators found that the bracket supporting proximity sensor S12 had developed free play, likely due to frequent handling during cleaning, which caused the sensor to signal a "FAR" (far) condition. Additionally, sensor S11 had its inductance affected by cleaning tasks performed with improper tools (non-magnetic or non-stainless steel), leading to the failure of the primary gear indication.
Findings
- The proxmity sensors S11 and S12 of the nose landing gear failed, preventing the crew from receiving a "downlocked" indication.
- The failure of these sensors forced the crew to attempt an alternate gear extension that ultimately did not complete successfully, resulting in the emergency landing with the nose gear retracted.
- Limited maintenance experience regarding the DHC-8- and the failure to record the pre-flight sensor cleaning in the technical log contributed to the event.