Background
Prior to taxiing at Chicago, the pilot observed that the aft airstair light on the second officer's panel was illuminated. The pilot requested the ramp agent to check the rear airstair. The ramp agent cycled the airstair and reported it appeared closed, but the light remained on. Referring to the Minimum Equipment List, the pilot noted that a visual check of the airstair was required if the warning system was inoperative. The airstair was visually checked and found apparently closed.
Incident Description
During taxi and takeoff, the airstair handle moved, causing the airstair to open approximately 1 foot inflight. The crew shut off water in the rear of the aircraft to prevent possible ice buildup and continued the flight to Denver. Upon landing, as the aircraft slowed on the runway, the airstair opened fully to the down position.
Findings
After landing, the external airstair handle was found in the full up position, which corresponds to the airstair being open. Investigation revealed that the handle could be pulled down partially without being locked, giving the airstair the appearance of being up and locked even though the uplocks were not engaged. Additionally, residual hydraulic pressure could keep the airstair in the closed position long enough to provide a false locked indication.