The Switch Left in MANUAL — Ghost Flight Over the Aegean

After a night flight into Larnaca, the cabin crew wrote up the aft service door — "seal around door freezes & hard bangs are heard during flt." A ground engineer inspected the door and ran a cabin pressurization leak check, pressurizing the aircraft on the ground with the pressurization mode selector set to MAN (manual). The Board concluded the selector then remained in MAN — the leak test was the last known time it was manipulated — and was not returned to AUTO. The aircraft was released at 03:15 and departed for Athens at 06:07. In AUTO the outflow valve closes and the cabin pressurizes itself; in MAN it does only what a human commands, and no one commanded anything. The crew missed the selector position on the Preflight procedure, the Before Start checklist and the After Takeoff checklist. As the 737 climbed, the cabin climbed with it; the warnings that followed were misread, the pilots lost consciousness, and the aircraft flew on autopilot until fuel exhaustion, crashing near Grammatiko. All 121 aboard died.

A single selector position, left over from a routine ground test, defeated four humans in sequence — one engineer who did not restore the configuration and did not record the task he actually performed, and two pilots on three checklist passes. It is the definitive restore-the-configuration lesson: a maintenance test is not finished until every system it touched is back in its flight state and the log says exactly what was done. The AAIASB listed the omission of returning the selector to AUTO after non-scheduled maintenance as a latent cause of the accident.

1. Before a ground pressurization test, record the as-found configuration of every control you will move — the selector, the outflow valve, the toggle.
2. Perform the leak check to the AMM task, by its task number.
3. After the test, restore every control to its flight configuration — for the 737 pressurization panel, selector back to AUTO — and have it verified.
4. Document the work against the specific AMM task reference, not as a one-line summary; the next crew inherits only what the log tells them.
5. Treat any control moved for maintenance as a threat until a second person or a checklist has positively confirmed its restoration.

Frequency of this exact failure is not quantified in public sources — but the AAIASB reviewed previous pressurization incidents in the type as part of this investigation, and configuration-not-restored-after-test sits inside the industry's most persistent recurring-error family, incorrect installation/configuration of components (UK CAA CAP 716, Table 2).

The cabin never pressurized; the crew, misreading the warnings, was incapacitated by hypoxia in the climb. The aircraft flew its programmed route on autopilot for almost three hours, was intercepted by Hellenic Air Force fighters, and after flame-out crashed into hilly terrain near Grammatiko at 09:03. All 115 passengers and 6 crew died; the aircraft was destroyed (AAIASB Final Report).