The Extension cycle
Whilst the aircraft is airborne, the pilot may select the GEAR selector switch to the ‘DOWN’ position at any time the aircraft is below 180 KIAS.
This will energise the bypass valve to block flow to the hydraulic reservoir (HYD PRESS ON light illuminated) and energise the gear control valve to allow hydraulic pressure to the sequence actuators and mechanical up locks.
Once the mechanical up locks have been released, hydraulic pressure is then sequenced to the extension side of the gear actuators to extend the landing gear.
Once the landing up locks are unlocked the GEAR UNLOCKED light will illuminate until the down lock micro switches are made with the gear down and locked.
Once the landing gear reaches the extended position, the internal down lock mechanisms in the actuators will engage and the three green landing gear lights will illuminate once the down lock micro switches are made with the gear down and locked.
Once the down lock mechanisms are engaged, the bypass valve will re-open and allow hydraulic fluid to be routed back to the reservoir returning the hydraulic system to the ‘open centre’ condition (HYD PRESS ON light goes out).