Investigation Summary
The investigation by the Swiss Transportation Safety Investigation Board (STSB) relied heavily on passenger-recovered digital media because the 1939-built aircraft was not equipped with flight recorders. Analysis revealed that the pilots were flying at approximately 100 knots, a speed that left little margin for safety in the 'hot and high' conditions of the Swiss Alps that day. As the aircraft entered the basin southwest of Piz Segnas, it encountered a downdraft. The pilots initially reduced engine power and raised the nose to arrest the descent, which increased the angle of attack. When the aircraft subsequently hit an updraft while in a turn, the angle of attack exceeded the critical limit, causing a stall. The left wing dropped, and the aircraft entered a steep, nose-down spiral. Although the crew attempted a recovery maneuver, the proximity to the ground made a successful pull-out impossible. Beyond the immediate flight path, the STSB found that the aircraft was not technically airworthy; the engines were not producing their minimum rated power due to poor maintenance, and the airframe had significant undetected corrosion and cracking. Furthermore, the aircraft's center of gravity was beyond the rear limit, which likely exacerbated the loss of control.
Final Conclusions
The STSB Final Report No. 2370, released on 28 January 2021, concluded the accident was caused by the flight crew's high-risk piloting. The crew navigated into a narrow valley at low altitude and low airspeed with no possibility of an alternative flight path, leading to an unrectifiable stall when turbulence was encountered. Contributory factors included a center of gravity beyond the rear limit and a long-standing company culture that tolerated routine violations of aviation safety rules. The report also noted that the supervisory authority (FOCA) failed to identify or correct these operational shortcomings.
Video Analysis
Explanatory video concerning the accident of a Ju 52 on 4 August 2018 — Official STSB animation and explanation of the flight path, atmospheric conditions, and aerodynamic stall that led to the crash of HB-HOT.
If playback is blocked, the owner has disabled embedding — use the link above.
Airframe & Maintenance
Crew Experience
Captain
Pilot A, aged 63, former Air Force and airline pilot. He was the Pilot Flying (PF) during the accident sequence.
Captain
Pilot B, aged 62, former Air Force and airline pilot. He was the Pilot Monitoring (PM) during the accident sequence.
Systems & Failure Modes
BMW 132A3 Engines
The three radial engines were found to be in a condition where they did not meet the minimum power requirements for airworthiness due to inadequate maintenance.
Flight Recorders
The aircraft was not equipped with a Cockpit Voice Recorder (CVR) or Flight Data Recorder (FDR) as they were not required for historic aircraft of its age.
Main Spar
Post-accident analysis of the wreckage revealed pre-existing cracks and corrosion on the wing's main spar that were not related to the crash but rendered the aircraft unairworthy.
Interesting Facts
- 01The aircraft was 79 years old at the time of the accident, having been built in 1939.
- 02No distress signal was transmitted by the crew prior to the impact.
- 03The investigation found the aircraft's center of gravity was beyond the rear limit during the flight.
- 04Post-crash inspection revealed the engines were not producing their rated power and the airframe had significant pre-existing corrosion.
- 05The two pilots were highly experienced former Swiss Air Force and airline captains with over 20,000 flight hours each.
Safety Actions & Advisories
Operational Suspension
Ju-Air was initially grounded and later had its commercial flight license revoked following the discovery of systemic safety and maintenance failures.
STSB Recommendations
The STSB issued eight safety recommendations to supervisory authorities to improve the oversight of historic aircraft operations.
Location
ACCIDENT SITE · Coordinates taken from the official investigation report.