Narrative:

I arrived at the airplane to find an over-serviced hydraulic reservoir. Maintenance arrived about twenty minutes prior to departure after boarding had already started. Thinking it would be a quick procedure to remove a small amount of fluid; we continued normal procedures; to be ready as soon as maintenance was complete. About ten minutes prior to departure time we started the APU after telling ground crew to remove the external power; which they did but they also removed external air as well; which was not my intention. Maintenance required an unpressurized hydraulic system; so APU air was unavailable. About ten minutes after departure time; maintenance informed us it would take another thirty minutes to complete their procedure. The flight attendants then told us the cabin was very hot and passengers were beginning to complain. We felt fine in the cockpit; so I was actually surprised. We couldn't see any ground crew around; so we called operations to have ground crew come out and reconnect the external air conditioning. Not sure how long it would take; the first officer went down to the ramp to find someone to help; while I walked through the cabin to assess the situation. The galley areas were fine; but the entire cabin was extremely hot; in fact; I was sweating by the time I reached the aft galley. I was concerned that if we couldn't cool down the cabin quickly; we would have had to deplane the passengers. Normally; I would have waited for the air to be reconnected; but the temp felt so awful I was truly concerned for the passengers. My thought was to open a back door to cross-ventilate the cabin until external air conditioning could be connected. The flight attendant was engaged with a passenger in the aft galley; so in my attempt to help cool the cabin and passengers; I went to open the door myself. The flight attendant jumpseat was down; so I had to walk around it. This made for tight quarters against the door but I started using standard door operating procedures. Standing where I was; I thought I read the word armed. This was confusing to me since the door should have been disarmed but I was convinced I read it as armed. After pushing the lever in; my hand was in the way and I don't remember seeing the word armed after I pushed it in. When I gently pulled on the handle; I didn't see the door armed light come on; possibly because the morning sun was shining brightly through the portal. At the time; I thought I was good to go but in hind sight I realize my error that the blazing sunshine could have masked the light. I was sure I had disarmed the door but as I continued to pull; I was surprised to hear the pneumatic-assist kick in. Immediately; I knew my error. Since passenger loading was finished and the aft cargo door was closed; the area below was clear. After the slide deployed; I pulled the red safety lanyard across the door opening and called maintenance. In an attempt to keep the passengers comfortable and not have to deplane them; I urgently decided to open a door. Even though I know the door should have been disarmed; and door opening procedures followed; I inadvertently deployed a slide and had to deplane the passengers anyway.I should not have started the APU until maintenance was complete.if I asked right away for an estimated completion time (instead of assuming it wouldn't take long) I never would have let them disconnect the external power and air.I should have gone down on the ramp right away to find a ground crew (myself) to help reconnect power and air; even if from a neighboring gate. Even though my first officer took initiative to do it; I felt I needed to do something myself to be proactive; hence my walking through the cabin. A door should never be opened without confirmation by another crewmember; that it is disarmed.if a door must be opened; always double check each procedural step. Think 'lever up; slide stays up' and 'lever down; slide falls down'.except in an emergency; there is never a time a pilot needs to open a cabin door.

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Original NASA ASRS Text

Title: The Captain of a B737-800 reported that in an attempt to keep the passengers comfortable and not have to deplane them; he decided to open a door. The Captain inadvertently deployed a slide and had to deplane the passengers.

Narrative: I arrived at the airplane to find an over-serviced hydraulic reservoir. Maintenance arrived about twenty minutes prior to departure after boarding had already started. Thinking it would be a quick procedure to remove a small amount of fluid; we continued normal procedures; to be ready as soon as Maintenance was complete. About ten minutes prior to departure time we started the APU after telling Ground Crew to remove the external power; which they did but they also removed external air as well; which was not my intention. Maintenance required an unpressurized hydraulic system; so APU Air was unavailable. About ten minutes after departure time; Maintenance informed us it would take another thirty minutes to complete their procedure. The Flight Attendants then told us the cabin was very hot and passengers were beginning to complain. We felt fine in the cockpit; so I was actually surprised. We couldn't see any Ground Crew around; so we called Operations to have Ground Crew come out and reconnect the external air conditioning. Not sure how long it would take; the First Officer went down to the ramp to find someone to help; while I walked through the cabin to assess the situation. The galley areas were fine; but the entire cabin was extremely hot; in fact; I was sweating by the time I reached the aft galley. I was concerned that if we couldn't cool down the cabin quickly; we would have had to deplane the passengers. Normally; I would have waited for the air to be reconnected; but the temp felt so awful I was truly concerned for the passengers. My thought was to open a back door to cross-ventilate the cabin until external air conditioning could be connected. The Flight Attendant was engaged with a passenger in the aft galley; so in my attempt to help cool the cabin and passengers; I went to open the door myself. The Flight Attendant Jumpseat was down; so I had to walk around it. This made for tight quarters against the door but I started using standard door operating procedures. Standing where I was; I thought I read the word ARMED. This was confusing to me since the door should have been disarmed but I was convinced I read it as ARMED. After pushing the lever in; my hand was in the way and I don't remember seeing the word ARMED after I pushed it in. When I gently pulled on the handle; I didn't see the door ARMED light come on; possibly because the morning sun was shining brightly through the portal. At the time; I thought I was good to go but in hind sight I realize my error that the blazing sunshine could have masked the light. I was sure I had disarmed the door but as I continued to pull; I was surprised to hear the pneumatic-assist kick in. Immediately; I knew my error. Since passenger loading was finished and the Aft Cargo door was closed; the area below was clear. After the Slide deployed; I pulled the red safety lanyard across the door opening and called Maintenance. In an attempt to keep the passengers comfortable and not have to deplane them; I urgently decided to open a door. Even though I know the door should have been disarmed; and door opening procedures followed; I inadvertently deployed a slide and had to deplane the passengers anyway.I should NOT have started the APU until Maintenance was complete.If I asked right away for an estimated completion time (instead of assuming it wouldn't take long) I never would have let them disconnect the external power and air.I should have gone down on the ramp right away to find a Ground Crew (myself) to help reconnect power and air; even if from a neighboring gate. Even though my FO took initiative to do it; I felt I needed to do something myself to be proactive; hence my walking through the cabin. A door should never be opened without confirmation by another Crewmember; that it is disarmed.If a door must be opened; always double check each procedural step. Think 'Lever Up; Slide stays up' and 'Lever Down; Slide falls down'.Except in an Emergency; there is never a time a Pilot needs to open a Cabin Door.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.