What happened
On February 12, 2021, a Piper PA 34-220T operating as an air ambulance flight from Leticia to Guaymaral, Colombia, experienced a serious incident during landing. While on approach to runway 11, the crew noticed that the green "down and locked" light for the left main landing gear was not illuminated. Following standard emergency procedures, the crew performed a go-around and a subsequent pass to verify the gear configuration. During this second approach, the crew observed three green lights and visually confirmed that the gear was extended.
The aircraft landed normally; however, approximately 100 meters into the landing roll at an airspeed of 40 knots, the left main landing gear suddenly retracted. This caused the aircraft to veer toward the left side of the runway. The pilot attempted to maintain directional control, but the aircraft deviated 4/0 degrees from the runway centerline. To prevent a more severe accident, the crew shut down the engines as the aircraft left the runway and came to a stop in the safety area. All 6 occupants—including the pilot, co-pilot, medical staff, a patient, and an attendant—evacuated the aircraft without injury.
The investigation
The GRIAA investigation focused on the mechanical failure of the landing gear system. Investigators examined the landing gear mechanism and found that the left main gear had not fully locked into the down position due to a lack of freedom of movement in the components. The investigation specifically looked at the retraction arm pins (P/N NAS464P6-18 and P/N NASA64P6-20) and the condition of the linkage system.
Findings
- The primary cause of the runway excursion was the sudden retraction of the left main landing gear during the deceleration phase.
- This retraction was triggered by the jamming of moving safety components in the left landing gear, specifically the retraction arm pins.
- The jamming was caused by increased tolerances in the linkage system and/or the accumulation of grease, dust, or dirt on the components.
- The investigation noted that the mechanism may have been improperly adjusted, causing the bellcrank to endure loads that led to deformation and eventual binding.
- A contributing factor was the operator's maintenance processes, which failed to detect the gradual loss of adjustment and the increasing stiffness in the landing gear components during inspections.