What happened
On September 25, 1977, a Grumman AA-5B, registration HB-UCG, departed Munich Airport for a private flight to Basel, Switzerland. The aircraft was carrying three occupants. After departing Munich under Visual Flight Rules (VFR) with a special clearance, the pilot contacted Munich Information approximately one hour later near Kempten to request weather updates for the Lake Constance area. During this communication, the pilot reported that the aircraft had entered clouds. Following this report, radio contact was lost due to distance and altitude limitations.
Between 12:05 and 13:10 local time, witnesses near Rötenbach-Staig observed the aircraft flying at a low altitude before it suddenly pitched forward and struck the ground. The impact resulted in three fatalities and the total destruction of the aircraft.
The investigation
The investigation examined the aircraft's airworthiness, the pilot's qualifications, and the meteorological conditions at the time of the accident. The Grumman AA-5B was found to be airworthy, with all required inspections up to date, and the center of gravity and weight were within limits. The engine was found to be functional, and no mechanical failures were identified in the control linkages or airframe prior to the impact.
Investigators analyzed radio transcripts, which confirmed the pilot was flying at flight level 75. Meteorological analysis indicated that as the aircraft approached a weather front, it likely encountered clouds. Because the temperature was near freezing, the investigation focused on the potential for both airframe and engine icing.
Findings
- The pilot continued the flight under Instrument Meteorological Conditions (IMC) despite being authorized only for VFR operations.
- The aircraft encountered severe icing while flying through clouds at flight level 75.
- It is highly probable that carburetor icing caused engine power fluctuations, as noted by witnesses who observed engine irregularities.
- The accumulation of ice on the wings likely caused a significant loss of aerodynamic performance, leading to the aircraft's nose-down pitch and subsequent stall at low altitude.
Safety action
No specific safety recommendations were recorded in the source material.