What happened
On the night of October 3, 1995, a Beechcraft Travelair D95, registration HB-GDH, was performing an instrument approach to runway 16 at Zurich Airport. The flight, arriving from Vienna, was carrying the pilot and three passengers. During the final approach phase, the aircraft descended below the glideslope, triggering the Minimum Safe Altitude Warning (MSAW) system in the air traffic control center.
Despite receiving instructions from the air traffic controller to climb back to the correct glide path, the pilot continued the descent. The aircraft eventually struck the treetops with its right wing before the nose impacted the ground approximately 3.4 km from the runway threshold. The impact caused the aircraft to catch fire, and all four fatalities occurred at the scene. The aircraft was destroyed.
The investigation
The investigation examined the aircraft's recent maintenance history and the pilot's flight experience. It was noted that the aircraft had undergone significant avionics upgrades between August and September 1995. While the pilot had previously reported intermittent issues with the autopilot's altitude hold function, recent tests had shown the system working correctly.
Investigators also reviewed the cockpit instrumentation. The Horizontal Situation Indicator (HSI) used for navigation was not equipped with a glide path error flag. Furthermore, the aircraft was only equipped with a single glide path receiver, preventing the pilot from cross-checking the signal accuracy. The investigation also looked into the weather conditions, which included patches of fog and mist, reducing visibility to approximately 2500 meters.
Findings
- The primary cause of the accident was an erroneous assessment of the meteorological conditions, leading to a loss of control during the instrument approach.
- The pilot failed to respond to the air traffic controller's instruction to climb immediately after the MSAW alarm was triggered.
- Pilot fatigue may have been a contributing factor to the error.
- A malfunction of the onboard navigation equipment, specifically the glide path receiver, could not be entirely ruled out.
- The lack of a second glide path receiver and the absence of a glide path warning flag on the HSI increased the pilot's workload and difficulty in identifying the deviation.
Safety action
Based on the findings, the following safety recommendations were established:
- Aircraft approved for Instrument Flight Rules (IFR) should be equipped with at least two glide path receivers.
- Minimum IFR equipment should include an autopilot capable of performing coupled approaches.
- Primary navigation indicators, such as the HSI, should be equipped with a glide path alarm flag to alert the pilot of signal loss or deviations.