B757-200
Flight Crew Air Transport Pilot (ATP)
Human-Machine Interface; Confusion
Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Other / Unknown
General Maintenance Action
Procedure; Human Factors
After block-in and shutdown; I went back to open the entry door. I first went to the R1 door and looked out the window; to confirm that the stairs were not present and then disarmed the door slide. Then I went to the L1 door and looked out the window and confirmed the stairs were on the left side. The stairs were not abeam the door; but there was a set of 757 stairs forward and left of the nose and angled towards the airplane. I disarmed the slide and rotated the handle far enough to unlock and crack the door. Then I went back the cockpit to complete post flight duties.After completing post flight duties and a debrief; I went back to remove my bag from the storage bin and remove our crew meals from the cooler. At some point in doing that; I realized that it had been quite a while since I had cracked L1 and no one had come with the stairs. I went over to the left door and looked out and realized there were no longer any stairs on that side. I went back to R1 and looked out the window and saw that the stairs had been moved and positioned on that side and were already up against the aircraft. At that point; I rotated the handle far enough to unlock the door and crack it open. To the best of my recollection I had confirmed that the door was disarmed; and I have no memory that the knuckle knocker was still protruding from the door. Since I wasn't sure if the stairs were fully parked and secured; I left the right entry door and went over to the left side and reclosed the L1 door. When I came back to the R1 door I looked out around the left side of the partially open door and saw a member of the ramp crew coming up the stairs. At that point; I grabbed the handle and rotated it while pushing to open the door. After opening a little more; the door started opening on its own. After a moment I realized that the pneumatic opener had activated and let go of the door. When it was fully open; the slide had come most of the way out of the storage bustle and was resting on the stairs; while the girt bar was still attached to the door threshold. The pins were still in the slide pack; so it did not open. I warned the Ramper on the stairs to go back down; as I wasn't sure if the slide might subsequently inflate. A mechanic came up the stairs to deal with the slide. He said that the slide handle; which I could not see because I was still on the plane; was armed. The captain; who was not present when I opened the door; wrote up an AML entry and then we departed the aircraft.I should emphasize that the above description is based on my best effort to recall what happened. It seems probable that some element of it is incorrect as it is likely that I either failed to disarm the R1 slide when I initially went back to open the L1 door; or unintentionally (or accidentally) rearmed it prior to opening the door. I would allow for the remote possibility that there was some problem in the rigging of the slide arming linkage; but that is purely speculative. After the incident I had a clear memory of having disarmed it when I first went back; which would seem to indicate that when I went to open it later; I actually rearmed the slide; perhaps from an ingrained habit of moving arming handle prior to cracking the door. However; that is also a pretty speculative guess at what happened. As a final point; I should note that the ramp personnel's repositioning of the stairs to the right side after I had cracked the left door; complicated things and likely contributed in some way to what I have to accept was most likely caused by distraction or inattention on my part.
B757 First Officer described the events leading up to the opening of the R1 door with the slide armed. He believes that he initially disarmed both doors and cracked the L1; but later closed the L1 and opened the R1 when the stairs were noted to be in position at that door. He may have rearmed the door just prior to opening. The slide dropped onto the stairs; but did not inflate.