What happened
On the morning of 24 June 2002, an Air Napier Piper PA31-310 Navajo, registered ZK-NPR, was conducting a routine freight flight from Palmerston North to Napier. During the approach to Napier Aerodrome, the pilot attempted to extend the landing gear but discovered that the right undercarriage failed to deploy, leaving only two green lights illuminated on the instrument panel.
After an initial touch-and-go attempt revealed the right wing dropped lower than normal, the pilot sought assistance from ground observers to confirm the gear status. Despite performing various emergency procedures—including cycling the gear through positive and negative-g maneuvers and using the manual hand pump—the right leg remained stuck.
As daylight arrived, the pilot executed an emergency landing on a grass area alongside the sealed runway. While the aircraft initially maintained stability, the right wing eventually dropped during the landing roll, causing the aircraft to skid nearly 18 and a half degrees. The impact caused substantial damage to the aircraft's undercarriage, right engine, propeller, and flaps, though the pilot escaped without injury.
The investigation
Investigators examined the aircraft's landing gear assembly and discovered that the right oleo strut had lost pressure and become compressed. This compression caused internal components, specifically the upper scissor-link and the lower leg casting, to make contact within the wheel bay. This physical interference prevented the uplock hook from releasing the gear leg.
Further analysis revealed that an O-ring seal in the lower bearing had rolled over within its groove, leading to the loss of pressure in the strut. The investigation established that this specific mechanical interaction—where a flat strut prevents the release of the uplock mechanism—was a previously unknown design deficiency in the Piper PA31-310 Navajo.