It’s not unusual – especially during Line Training (instructors beware) for your student to generate an EICAS MAIN GEAR STEERING alert during the initial takeoff run. This results from advancing thrust prior to the main articulated gear achieving a lock during the initial take-off roll.
Practices & Techniques : Main gear steering and Thrust Application
Main gear steering engages when the CM1 tiller is beyond approximately 13° and remains engaged until the CM1’s tiller has returned to approximately 9° – after this point main gear locking takes approximately 5 seconds. Anytime a thrust lever is advanced beyond 60% N1 with the main gear steering unlocked, a CONFIG GEAR STEERING Warning message is generated on EICAS. Note that rudder pedal steering alone generates up to 7° of steering and will not unlock main gear steering.
As crew become more confident with the aircraft, the occasional takeoff configuration warning is generated during thrust application on takeoff, when the main gear steering hasn’t been given sufficient time to align and lock. This is generally best avoided – don’t rush into TOGA when a sharp turn has been required during line up.