EMB-170 crew reported a loss of control after releasing parking brake for push-back. Crew misheard push-back crew who was using a wireless headset resulting in garbled communications to pilots.
Synopsis
EMB-170 crew reported a loss of control after releasing parking brake for push-back. Crew misheard push-back crew who was using a wireless headset resulting in garbled communications to pilots.
Narrative
Preflight was complete; cabin and flightdeck doors closed; and all data entered for takeoff. Before Start Flow and Checklist complete. I attempted to contact the ramp tug driver over headset. When I did; a somewhat garbled and broken voice came back with what sounded like the standard verification that they were prepared for brake release. I asked him to repeat a couple more times and the tug driver still came in very broken. However; I thought I heard 'release brake.'I released the emergency/parking brake; then stated 'clear to capture.' After a couple of seconds I noticed we were slowly starting to move backwards. At first I thought the pushback driver had started the push off the gate; so I instructed him to halt the push since we hadn't gotten a clearance. The response was again somewhat broken. I repeated the instruction once more; then realized that we were rolling backward freely. I immediately applied the emergency/parking brake and we came to a stop.A few seconds later; the tug driver came over and hooked up to the airplane. I asked how far we had rolled and whether anyone or anything had been harmed or damaged. He replied that he estimated we had rolled about five feet back; and that he did not see any damage or injury. Also; when queried; he stated we were still within the gate's parking envelope; and no part of the plane had crossed the roadway.I checked with the flight attendants since the stop was somewhat firm. They confirmed that they and all the passengers were okay. After the event; the tug driver hooked up the pushback tug. I asked him why his communications were so broken beforehand; and was told that he was using a wireless headset for communication. He then told me that he was stating 'DON'T release brake'; not 'release brake'.At this point we all determined that it was safe to proceed. After receiving a pushback clearance; we completed the push from the gate without further incident.Cause:--Obviously the main cause of the incident was my failure to fully verify the instructions given to me; in spite of weak; broken communication.I clearly should have kept the brake on until I knew for certain that I was being instructed to release it. There was also an element of self-induced pressure not to be delayed; so I might have heard what I wanted to hear regarding brake release. In retrospect; the lead on the ground crew mentioned that he was coordinating with three different gates simultaneously; hence the use of the remote wireless headset he said he was using. It likely would have been better if he had ignored my initial query completely and not said anything at all until they were fully prepared to commence push; then I certainly wouldn't have released the brake. On a related note; if we were not connected to a tug at the time this began; there really should have been wheel chocks installed.Suggestions:--I'm normally very scrupulous about establishing clear headset comms and making sure everything is well understood before releasing the brake. After this incident I will redouble my efforts on that front.I also don't feel that it's very safe or effective for a ground crew lead to be handling multiple gates at the same time. If the lead's attention was divided he shouldn't have been trying to communicate with us at all. To me at least; silence speaks volumes i.e. they're not ready for pushback yet.Also; as I mentioned above; there really should have been at least a nose wheel chock installed until completely ready to connect the tug. Most of the time I can't even visualize the tug from the flightdeck; so I'm operating mainly on headset communication for cues on when it's safe to release the brake.Finally; I want to say how relieved I am that there was no damage or injury from my action. I realize this could have turned out much worse.
Second reporter narrative
On Day 0 at approximately XA:28; Flight ABCD was parked at Gate 1. At approximately XA:27; the ground pushback personnel established communication via the interphone. The communication was intermittent but readable; and both the Captain and I clearly heard the instruction to release the parking brake.At XA:28; the parking brake was released; and the aircraft began to move backward. The Captain attempted to communicate with the ground crew; however; communication could not be re-established despite three call attempts. As a result; the Captain stopped the aircraft and set the parking brake.Subsequently; the ground crew re-established communication and advised that the aircraft was not connected to the pushback equipment. The flight crew (the Captain and I) verified that the aircraft remained within the gate envelope and confirmed with the cabin crew that all were safe and no injuries or damage had occurred.After coordinating with all parties involved and confirming there was no incident; the flight continued as normal. Flight ABCD was completed without any further issues.Cause:--not having 100% reliable communication established with all parties involved.Suggestions:--Before taking any action; ensure that 100% reliable communication is established with all parties involved.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.