EMB-170 flight crew reported uncommanded aircraft movement at the gate due to brakes being released with no hydraulic pumps on. Crew reapplied the brakes.
Synopsis
EMB-170 flight crew reported uncommanded aircraft movement at the gate due to brakes being released with no hydraulic pumps on. Crew reapplied the brakes.
Narrative
We were at the gate getting ready to pull the bridge and pushback for our 4th and final flight of the day on what was day 2 of 3. We were running a tad behind because of the plane swap and because this aircraft we just received was late coming in. We had just closed up when the captain commenced the normal procedure of dropping the brake with the brake pedals pushed to start the clock and avoid a delay. When he did this the aircraft started to roll. This coincided with the movement of the jet bridge pulling away which made it extremely difficult to perceive if we were moving. I was heads down at the moment looking at my taxi chart in preparation for our departure briefing when I felt the movement. I immediately looked up but struggled to determine if we were moving or if it was an optical illusion with the jet bridge. I then noticed the ground crew waving their wands and so I promptly slammed on my brake pedals. I did this at the same time that the captain pulled the parking brake which caused the plane to lurch and come to a stop. For a moment I couldn't tell what had happened and whether it was a malfunction of the captains brake pedals; a hydraulic failure; or even a runaway pushback tug. So for a moment I held max pressure on my brake pedals while we sorted out the situation. Immediately we checked on our flight attendants; who were okay. Then we checked in with the ground crew whom were also safe and equally as perplexed about what had just occurred. They confirmed for us that we were not attached to the tug; and that the wheels were chocked; but brought up that we were on a slope and that there was glycol all over the ramp from de-icing a previous aircraft.We briefly considered reattaching the jet bridge and getting maintenance as we were processing what was wrong with the plane. I mentioned that we must not have brake pressure and that's when the captain pulled up his hydraulic page and verified that was true. It was at this point we both looked up to see that the hydraulic pumps were in auto and 3A was selected off. This had all occurred before we had completed the before start to the line" checklist or flows; which is likely why this hadn't been selected on.We had the ground crew inspect the airplane for damage and thankfully none was found. We decided to hookup to the tug and proceed with the flight as planned after the captain talked with operations and the ground crew conferred with their supervisor to ensure nothing else was needed prior to us continuing our departure.What we learned from this is that the brake dropping technique has more pitfalls than we had previously thought and that during winter operations there is hidden risks of the plane sliding on a glycol coated surface; such as gate XX in ZZZ. We also learned that waiting till the jet bridge is completely away and in its safe zone is paramount to protect the aircraft in the event of the slide. We were fortunate that the jet bridge was already detached and at least a foot off of the airplane when the slide occurred. We were also abiding by the recent memo reinforcing waiting till all doors were closed to drop the brake. That memo protected our ground crew and aircraft during this incident.As an FO I will do a better job of briefing this threat to my captains and also ensuring crew communication and situational awareness when performing this procedure. I will add the captain did verbalize that he was going to drop the brake and get us on the clock; which is an industry standard procedure that I have witnessed every single captain at this company do; so it was nothing abnormal for me to hear. It may be worth adding this procedure; and any subsequent remarks or modifications; into the SOP's and FOM and considering a change to the de-ice procedures at gates that have a slope gradient significant enough to cause a slide like this to occur."
Second reporter narrative
It was a plane swap for us. Day 3 for me in challenging; snowy; icy conditions with many delays...leg 4. Arriving aircraft was a little late and we were trying to get out on time. As soon as we were boarded; A FA asked if it was okay to close. I said; 'yes'. The FO and I were briefing and trying to get caught up on checklists. The following all happened over about five seconds. As the jet bridge was moving away and all doors were secured (AND with my feet on the brakes); I dropped the brake to get us on time. Many times I've experienced the illusion of rolling backwards as the jet bridge moves away...in nine years; it's never actually been true! I had that experience in that moment. I think my FO may have asked if we were moving; which made me shift my vision forward. The rampers were waving and; for a second; I still couldn't process why as my feet were mashing on the brakes so we COULDN'T be moving! Well; we were; and a soon as a realized what was going on; I reapplied the emergency brake. The plane jerked a little. My first concern was the flight attendants; but they both said they were fine. Next; I was able to make contact with the ramp crew. I confirmed everyone was safe. The airplane had rolled 5-10 feet but had not made contact with anything. I asked if we were chocked. Ramp said yes; which didn't make sense because we definitely would have felt the airplane jump the chocks. They said the chocks moved with the airplane...they were pushed/slid along the ground because of all the deice fluid on the tires and ground. My FO did say it was very slick outside. I then told them I might need the jet bridge back because I thought I had a maintenance issue with the brakes or hydraulics. That's when I looked up and realized the hydraulic pumps weren't on. We had not yet done the Before Start to the Line checklist. That's why we rolled even with my feet firmly on the brakes. I told them we would not need the jet bridge. They said they could go ahead and hook up there and we could get on our way.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.