Maintenance Airframe; Maintenance Powerplant
Situational Awareness
Deviation / Discrepancy - Procedural Published Material / Policy; Ground Event / Encounter Other / Unknown
Company Policy; Equipment / Tooling; Procedure
A787-800 was being prepared for a tow operation out of a hangat to the taxiway due to hangar construction and equipment congestion. The tow bar was attached to [the] 787-800 and a Tow Tractor GT350. The pavement was dry and the B787 had 30;000 pounds of fuel. The wheel chocks were pulled; TTWS active; and entry stair removed. The Tow Tractor parking brake was set and was shutoff. I exited the Tow Tractor to establish cockpit communication. After establishing cockpit communication; the wingwalkers; guideman and I; met at the Tow Tractor. We discussed clearance issues due to hangar construction and that we would not follow the lead out line because of construction equipment. Prior to entering the Tow Tractor; I instructed the taxi crew to release the brakes. Immediately upon release of brakes; the B787 began rolling backwards pulling the Tug with it. A Maintenance Technician noticed the B787 and Tow Tractor moving; entered the Tow Tractor; and brought it to a stop. The aircraft and tug rolled 3 feet before coming to a stop.I strongly feel Company has over rated the GT350's braking capability for wide body aircraft. The mechanical brake on the GT350 may have sufficient holding strength on a flat dry surface; but has proven to fail on a surface with a grade and is not able to hold the aircraft weight. I assume that a Tow Tractor would be able to under any situation provide sufficient braking pressure and prevent an aircraft from rolling.[Procedures Manual] states 'If you get out of the Tow Tractor set the parking brake and turn off the ignition.' Is the intent of that statement only apply when not attached to an aircraft? I feel any Tow Tractor that I use should be capable of stopping an aircraft from moving. The applicability and rating of the GT350 breaking force and stopping force on a grade needs to be reevaluated and updated.
Maintenance Technician reported an unmanned Tow Tractor; coupled to a B787; began rolling on the ramp even though the brakes on the Tow Tractor were set.