B777-200
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 180; Flight Crew Total 31000; Flight Crew Type 1800
Human-Machine Interface; Situational Awareness
Aircraft Equipment Problem Less Severe
Flight Crew Rejected Takeoff
Aircraft; Human Factors; Airport
After being cleared for Takeoff from being instructed to 'Line Up and Wait' - I ran the power up to 55% N1 holding brakes due to an extremely heavyweight Takeoff (T/O) at 764;000 with a Stop Margin of: 648 feet. I released the brakes at the same time I depressed the GA switches to engage the Autothrottles but they did not engage until After I depressed the Autothrottles Disconnect Switch and then recycled the switches from OFF to ON. At which time I pulled the power to idle to 'Reject' the T/O at low speed. The aircraft was moving from the initial start T/O position during this time and I did not want to invalidate the Accelerate-Stop distance calculation of 648 FT by applying power a Takeoff Power further down the runway than normal so I decided to taxi off the runway at the next exit and make another Takeoff from the beginning of the runway. Aircraft speed was never greater than 30 to 40 knots during taxi down the runway to exit the runway as instructed by Tower.
When the B777's autothrottles failed to engage promptly on a MTOGW takeoff with minimum stopping distance; the Captain elected to reject the takeoff. A subsequent attempt proved successful with normal autothrottle response.