Piper PA-23-160 crashes into Moel Hebog during post-maintenance test flight

Casualties unknown • Moel Hebog, Snowdonia, GB

A Piper PA-23-160 crashed into a mountainside in North Wales during an engine starter motor test flight, resulting in the death of the pilot.

What happened

On 2 August 1999, a Piper PA-23-160, registration G-ARTD, departed Caernarfon Airport to perform an airborne functional test of a recently repaired right engine starter motor. Shortly after takeoff, the pilot notified RAF Valley Air Traffic Control of his intention to climb to 4,000 feet amsl to conduct the test, which would have required shutting down and restarting the right engine.

Radar data tracked the aircraft climbing through the intended altitude, but instead of maintaining level flight for the test, the aircraft began a continuous descent. By 1324:01 hrs, the aircraft was tracking north towards Moel Hebog. Witnesses near Llyn Cwmystradllyn observed the aircraft flying in straight and level flight with a normal engine sound before it struck a vertical rocky crag on the mountainside at approximately 1,850 feet amsl. The impact resulted in 1 fatality and the total destruction of the aircraft.

The investigation

The AAIB investigation examined the wreckage, which was extremely fragmented due to the high-speed impact. Investigators analyzed radar tracks from St Annes and Clee Hill, which showed the aircraft's flight path and the descent prior to impact. The investigation also included a review of the pilot's medical history and toxicology, as well as an examination of the aircraft's engines and flight controls.

Engine analysis revealed that both propellers showed damage consistent with being at roughly cruise power at the time of impact. No mechanical anomalies were found in the flight controls, airframe, or engines. Additionally, the aircraft's Janitrol heater was found to have been inactive, ruling out carbon monoxide poisoning as a factor.

Findings

  • The aircraft climbed through the intended 4,000 feet altitude and immediately entered a continuous descent.
  • The proposed engine starter motor test was not performed.
  • There was no evidence of any technical defect in the engines, structure, or flight controls.
  • The pilot failed to deviate from the flight path or respond to the descent, and no radio transmissions were made to report any technical difficulties.
  • Witnesses observed the aircraft in stable, level flight with normal engine noise immediately prior to the collision.

Probable cause

The investigation could not identify a specific technical cause for the descent; however, in the absence of mechanical failure, the descent into the mountainside suggests a human factor error.

Frequently asked questions

What happened in the 1999-08-02 PIPER PA-23-160 accident near Moel Hebog, Snowdonia, GB?

A Piper PA-23-160 crashed into a mountainside in North Wales during an engine starter motor test flight, resulting in the death of the pilot.

What aircraft was involved and where did it happen?

The accident on 1999-08-02 involved a PIPER PA-23-160, registration G-ARTD, at Moel Hebog, Snowdonia, GB.

What was the probable cause of the accident?

The investigation could not identify a specific technical cause for the descent; however, in the absence of mechanical failure, the descent into the mountainside suggests a human factor error.

Loading the flight search…