Alaska confirms source of 737-400 computer malfunction

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Alaska Airlines has confirmed an air-ground sensor malfunctioned and created a chain reaction that earlier today forced a 737-400 to descend quickly and make an emergency landing.

The air-ground sensor, which is mounted in the well of the nose gear, inexplicably notified the 737 flight computer that the aircraft was on the ground, even though it was climbing through 25,000ft about 20min after take-off from Ontario, California, Alaska Airlines says.

The errant message caused the primary cabin pressurisation system on the aircraft to fail, Alaska Airlines says. A backup pressurisation system almost immediately activated, the airline adds, so passengers may not have noticed a change in cabin pressure.

In addition to the pressurisation system malfunction, the flight control also lost auto-throttle capability, which forced them to adjust engine speed manually, Alaska Airlines says.

The flight was en route to Seattle, but was diverted to land in San Jose, California.

The 14-year-old aircraft, registered N797AS, had never experienced a similar issue, the carrier says.