Investigation Summary
The accident occurred during a scheduled refueling stop. During the descent, the crew misheard or misinterpreted the air traffic controller's instructions regarding the altimeter setting (QNH). The controller provided a setting of 1027 hPa, but the crew set 1010 hPa, leading the aircraft to fly approximately 500 feet lower than indicated. Furthermore, the crew descended to 2,000 feet—an altitude below the minimum sector altitude of 3,000 feet—due to a misunderstanding of the clearance provided by the controller. The cockpit environment was characterized by poor Crew Resource Management (CRM), with the captain and first officer failing to cross-check instruments or challenge the unsafe descent. The aircraft struck the ridge of Pico Alto at an elevation of 1,795 feet while in level flight.
Final Conclusions
The primary cause was the crew's failure to comply with established operational procedures, specifically descending below the minimum sector altitude and using an incorrect altimeter setting. Contributing factors included the air traffic controller's failure to correct the crew's readback of the incorrect altitude and the crew's lack of familiarity with the local terrain and approach charts.
Video Analysis
A Mistake In Plain Sight | The Crash Of Independent Air Flight 1851 — An in-depth look at the sequence of human errors and communication breakdowns that led a Boeing 707 to strike a mountain in the Azores.
Watch on YouTube ↗If playback is blocked, the owner has disabled embedding — use the link above.
Systems & Failure Modes
Altimeter
The crew set the pressure reference to 1010 hPa instead of 1027 hPa, causing the aircraft to fly 507 feet lower than the displayed altitude.
GPWS
The Ground Proximity Warning System alerted the crew 7 seconds before impact, but no corrective action was taken.
Interesting Facts
- 01The crash remains the deadliest aviation accident in Portuguese history.
- 02The crew was using a non-standard altimeter setting (1010 hPa) which they likely confused with a previous weather report.
- 03The air traffic controller was a trainee under supervision, but the supervisor did not intervene during the incorrect readback.
- 04The Boeing 707 was 21 years old at the time of the accident.
- 05The flight was a charter carrying Italian tourists to the Dominican Republic.
Safety Actions & Advisories
ATC Readback Procedures
Reinforcement of the requirement for controllers to listen critically to pilot readbacks and immediately correct any discrepancies in altitude or pressure settings.
CRM Training
Emphasis on Crew Resource Management for charter operators to ensure better communication and cross-checking during critical phases of flight.