What happened
On 5 April 2007, a Piper PA32S-300 Cherokee 6, registration ZK-DOJ, was conducting a local charter flight near Queenstown for Glenorchy Air. The flight, carrying a pilot and two passengers, included a planned stopover at the Elfin Bay airstrip on the shores of Lake Wakatipu.
Upon approaching the grass airstrip, the pilot noted calm conditions with no visible wind. The aircraft touched down near the intended point, but the landing was described as firm. In an effort to reduce lift and increase braking effectiveness, the pilot retracted the flaps and immediately applied the brakes. As the aircraft traveled down the strip, the pilot realized the plane was not decelerating sufficiently to stop before the end of the runway. To avoid overrunning the end of the strip, the pilot executed an intentional ground loop, applying full left rudder and a burst of power to steer the aircraft off the side of the airstrip. The aircraft subsequently struck a fence, causing moderate damage to the engine, propeller, and undercarriage, though all occupants escaped without injury.
The investigation
The investigation focused on why the aircraft failed to stop within the available distance. Investigators examined the condition of the Elfin Bay airstrip and found that while the surface grass appeared dry, heavy overnight dew had left the underlying foliage and clover damp. This moisture created a slick contact surface that significantly reduced braking effectiveness.
Analysis of the wheel marks revealed that the brakes locked up approximately 40 meters after application and remained locked until the aircraft exited the airstrip. The investigation also noted that the pilot's continuous application of heavy braking pressure prevented the wheels from rotating, which would have been a more effective technique for maintaining traction on the damp surface. Furthermore, the absence of a functional windsock at the remote site prevented the pilot from accurately assessing the possibility of a tailwind component during the approach.
Findings
- The primary cause of the excursion was the continuous application of braking pressure, which caused the wheels to lock up on the damp grass, thereby reducing braking effectiveness.
- The moisture retained beneath the surface of the grass, due to heavy overnight dew, created a slick surface for the tires.
- The pilot's decision to steer the aircraft off the side of the strip was a deliberate attempt to avoid a more serious overrun.
- The lack of a serviceable windsock prevented an accurate assessment of wind conditions during the landing phase.