Boeing Some operators have reported slower than expected nose gear operation during approach. In one instance, a CONFIG warning was triggered because the flaps reached position 30 before the nose gear was down and locked. Also, a GEAR DISAGREE caution message will be displayed on EICAS if any gear is not down and locked within 25 seconds after the gear handle is placed down.
Boeing recommended procedure for gear extension on ILS approach is to extend the gear and call for flaps 20 with initial glideslope movement. This action would normally be initiated at Vref + 40 knots.
It should be noted that gear extension at speeds slower than Vref + 40 knots actually increases nose gear extension time. This is due to the fact that air loads are the primary means for opening the nose gear doors to the point that gear unlock can occur for completion of gear extension. Additionally, the hydraulic door actuator assists door opening, but is limited by a flow restricter to prevent rapid door closure on the ground as a safety measure. Consequently, if gear extension is delayed to speeds lower than Vref + 40 knots, nose gear down and locked indication may take up to 30 seconds.
Following Boeing recommended procedures for ILS, non-precision, visual, and delayed flap approaches will ensure timely nose gear extension.
Boeing Flight Operations Technical Bulletin 757-10, 20th May 1983