B757-200
Maintenance Airframe; Maintenance Powerplant
Confusion; Communication Breakdown
Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy; Ground Event / Encounter Other / Unknown
General Maintenance Action
Human Factors; Procedure; Aircraft
Aircraft was taxiing into gate with a new Marshaller directing this operation. The PIC stopped the aircraft 5 feet short of the stop mark. The Marshaller attempted to get the PIC to move the aircraft forward with his wands but one of the engines was shut down as soon as the aircraft came to a stop. As ground power was being applied; I moved the tow tractor into position and hooked up the tow bar and interphone cable.I attempted many times to contact flight crew interphone and crew call button but was not successful. Exited tractor to make 'eye contact' with flight crew and to motion to release brakes. Once the brakes were released I started to tow the aircraft into the proper location when the plane came to a stop. I thought that I was up against the edge of the concrete parking pad and attempted to keep pulling forward when the shear bolts on the tow bar failed. I immediately stopped the tow tractor and by this time the flight crew was on the interphone wanting to know what the noise was and was anything damaged. I inspected the nose gear and wheel well for damage - none was found. The only damage was to the shear bolts on the tow bar. The PIC stated that he thought the aircraft was rolling on its own and reapplied the brakes. When the flight crew was leaving the plane; the First Officer was the first off the plane and stated that he was having difficulty operating the interphone; (understanding how to use it?) As the Captain was leaving he stated again that he thought the aircraft was rolling away and set the parking brake. I stated to him that I had tried multiple times to contact him with the interphone and crew call button but he said he neither heard the chime nor saw the light. Once the flight crew had left; I function checked the crew call system and found it only worked part of the time. Excessive force was required to get the crew call button to work. Cause: A lack of continuous contact via the interphone with the flight crew during the attempted aircraft move. When I motioned for the PIC to release the parking brake; he thought this was for the down-loading of the aircraft and not for the movement of the aircraft. Possible difficulty of the flight crew was having with the interphone operation. Maybe have the flight crew leave the engines running for a few seconds after Marshaller stops them incase the Marshaller needs to have them move some more. A crew call system that works every time; replace crew call button.
After shutdown; with the beacon off I opened the door and closed out ACARS. I put my headset away and turned on the speaker. While waiting for the stairs to be positioned I decided to use the lavatory. Whole [time] in the lavatory I felt the aircraft start to move followed by a loud snap. Returned to the cockpit and the Captain informed me that the aircraft moved after he released brakes from a Mechanic hand signal; after reapplying brakes the snap sound occurred. I noticed the ground call light was not on and Mechanic was not responding to the Captains inquiries over the headset. I sounded the ground call horn and the Captain started communicating with Mechanic over his headset. I confirmed my speaker was on and the intercom light was green on the panel. I could not hear the communication over the intercom however. I could hear VHF Comm.
Positive communications with B757-200 flight crew was not established; and ground crew attempted to tow aircraft with parking brake set; shearing the bolts on the tow bar.