The invisible tug-of-war that doomed a commercial airliner
In short: When the autopilot and human pilots began fighting for control of an Airbus A300, neither realized they were losing. A series of mechanical contradictions turned a routine landing into a fatal aerodynamic stalemate, where the plane's own safety systems became its greatest threat.
On April 26, 1994, China Airlines Flight 140 was approaching Nagoya Airport in Japan. During the final descent at roughly 1,000 feet, the first officer inadvertently engaged the 'go-around' mode. This triggered an automatic thrust increase and a climb above the glide path. While the crew attempted to manually push the nose down to correct this, they also re-engaged the autopilot. Because the aircraft was still in go-around mode, the autopilot began commanding the trimmable horizontal stabilizer (THS) to pitch the nose up.
This created a 'mis-trim' condition: the first officer was using the yoke to move the elevators down, while the autopilot was moving the much larger THS surfaces up. Because the THS has approximately three times the surface area of the elevator, it possessed far greater aerodynamic authority. The pilots were essentially fighting a losing battle against the plane's own hardware. Furthermore, because pitch trim is inhibited during autopilot operation, the first officer’s attempts to use the trim switch had no effect.
By the time the captain took control at 700 feet, the THS was locked at its maximum nose-up limit of -12.3 degrees. As airspeed dropped, an 'alpha floor' protection feature activated, increasing thrust further and forcing a pitch-up to 22 degrees. The aircraft stalled and crashed approximately 340 feet from the runway centerline. This tragedy highlighted how complex automated systems can create counter-intuitive behaviors when human inputs conflict with pre-set flight modes.
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Sources
- Summary by Intellect · source: Airbus 300B4-622R | Federal Aviation Administration
- Also: China Airlines Flight 140
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