What happened
On September 23, 2001, a Piper PA-3able-220T, registration EC-EYV, was performing a private flight from Ibiza to Sabadell Airport. During the landing phase on runway 31, the pilot deployed the landing gear and confirmed via cockpit indications that the gear was down and locked. The landing was initially normal; however, as the nose gear made contact with the runway, it unexpectedly retracted.
The aircraft's forward fuselage impacted the runway surface, causing the plane to slide approximately 150 meters before coming to a stop. Despite the physical retraction of the nose gear, the pilot noted that the cockpit indicators continued to show the gear as down and locked even after the aircraft had stopped. The impact caused damage to the nose section, the nose gear door, and the blades of both propellers. All five occupants (the pilot and four passengers) were uninjured and evacuated the aircraft on their own.
The investigation
The CIAIAC examined several hypotheses to determine why the nose gear retracted upon ground contact. The investigation considered whether a physical failure of the landing gear components had occurred, but structural inspections ruled this out. The investigation also examined whether the hydraulic pump had been inadvertently activated, but the functionality of the "squat switch" (a safety sensor that prevents gear retraction while the aircraft is on the ground) was confirmed to be working correctly.
Investigators also looked into the possibility of a sensor malfunction. While the aircraft had undergone a 100-hour inspection six days prior to the accident, which included checks of the gear locking mechanisms, the investigation focused on whether the nose gear sensor was improperly adjusted. Additionally, the investigation reviewed maintenance procedures regarding the manual free-fall extension valve, specifically whether a failure to follow precautions during the recent maintenance could have led to trapped hydraulic pressure.
Findings
- The investigation identified two primary potential causes for the nose gear retraction: improper adjustment of the nose gear down-and-locked sensor or the accumulation of residual hydraulic pressure within the system.
- It was noted that the aircraft had previously experienced a similar nose gear retraction incident five months earlier due to a misadjusted sensor.
- The possibility of residual pressure was linked to maintenance protocols; if the free-fall valve was not handled according to the manual during the recent 100-hour inspection, enough pressure could have remained in the actuators to unlock the gear upon impact.