Investigation Summary
The incident occurred during a scheduled flight from London to Gander. While cruising at 35,000 feet at Mach 0.82, Captain Waldo Lynch left the flight deck to attend to business in the passenger cabin, leaving Captain Samuel Peters (acting as First Officer) at the controls. During this time, the Bendix PB-20 autopilot disengaged for reasons that could not be definitively determined, though investigators noted the system had a history of intermittent disconnections. Because the autopilot disengage warning light was in the fully dimmed position, the copilot was not alerted to the failure. He remained focused on a 'how goes it' fuel and progress chart on his lap, unaware that the aircraft had entered a slow, smooth right-hand spiral. By the time the copilot noticed the unusual attitude, the aircraft was in a steep dive, and the resulting G-forces pinned him to his seat, preventing effective recovery. Captain Lynch struggled back into the cockpit and, with the assistance of the flight engineer and navigator, managed to level the wings and pull the aircraft out of the dive just 6,000 feet above the ocean. The flight engineer had to manually override the stabilizer trim, which had run away to a full nose-down position during the descent.
Final Conclusions
The Civil Aeronautics Board (CAB) determined the probable cause was the inattention of the copilot to the progress of the flight during the captain's absence, following the involuntary disengagement of the autopilot. Contributing factors included the autopilot disengage warning light being in the dim position and the Mach trim switch being in the 'off' position, which prevented the aircraft from automatically correcting its pitch as speed increased.
Photographic Evidence (1)
Video Analysis
How A Broken Bulb Almost Killed 129 People | Pan Am 115 — A detailed look at the 1959 Pan Am Flight 115 incident, where a Boeing 707 plummeted 29,000 feet over the Atlantic due to a combination of autopilot failure and pilot distraction.
If playback is blocked, the owner has disabled embedding — use the link above.
Airframe & Maintenance
Crew Experience
Captain
Waldo Lynch (Pilot-in-Command)
Captain
Samuel Peters (Acting First Officer)
Systems & Failure Modes
Bendix PB-20 Autopilot
The primary flight control automation which failed or disengaged involuntarily, leading to the upset when not monitored.
Mach Trim System
A system designed to automatically apply nose-up trim as the aircraft approaches its Mach limit to counteract 'Mach tuck'; it was switched off during the incident.
Stabilizer Trim
The horizontal stabilizer moved to a full 3-degree nose-down position during the dive, requiring the flight engineer to manually crank it back to a neutral position.
Interesting Facts
- 01The aircraft descended 29,000 feet in a matter of minutes before recovery.
- 02The Boeing 707 was less than four months old at the time of the incident.
- 03The autopilot disengage warning light was found in the fully dimmed position, making it invisible to the crew in the cockpit lighting conditions.
- 04The aircraft sustained substantial structural damage due to high G-forces but was later repaired and returned to service.
- 05The Mach trim system, designed to prevent high-speed pitch-down, was not engaged during the cruise.
Safety Actions & Advisories
Autopilot Monitoring Procedures
The incident highlighted the critical need for constant pilot monitoring of flight instruments even when the autopilot is engaged, especially during the absence of one pilot from the flight deck.
Mach Trim Engagement Policy
The investigation emphasized the importance of ensuring the Mach trim system is engaged during high-speed cruise to provide longitudinal stability and prevent 'Mach tuck' in the event of an autopilot disengagement.
Known Controversies & Unanswered Questions
Autopilot Reliability vs. Crew Negligence
Reference ↗While the CAB primarily blamed the copilot's inattention, the Bendix PB-20 autopilot had a documented history of intermittent disconnections and uncommanded pitch changes on previous flights, leading to debate over whether the equipment failure or the human monitoring failure was the root cause.
Location
APPROXIMATE POSITION · AIRPORT LEVEL · Coordinates are the estimate published in the official investigation report. The ring shows the accuracy the record supports, not the point of impact.