ATA 27

Lift Dump
Scenario:
During a routine “C” inspection, the required lift dump electrical control check is performed per the1124/1124A Aircraft Maintenance Manual (AMM). Both main gear oleo switches are in the ground position and throttles are at idle during the start of the test. Main hydraulic pressure is applied, the lift dump switch is placed to ON, and the lift dumps deploy. The throttles are advanced, and the lift dumps retract because the open throttle micro-switches remove electrical power from the control valve.The throttles are returned to idle, and the lift dumps re-deploy. The final check requires placing the left and right main gear oleo switches to the flight mode (one at a time) to verify the lift dump ground latching(anti-bounce) circuit operation. However, when either main oleo switch is placed to flight, the lift dumps retract.
Note: There are no wiring problems, and the crew reported no faults prior to the inspection. Remember, a“C” check is in progress.
Answer:
Yes, the lift dumps are working as designed; however, during a “C” check inspection, the forward relay panel located under the copilot’s seat is routinely removed for inspection. Because the lift dump “ground-latching” relay (LDR) is located in this panel, the anti-bounce feature is disabled if the forward relay panel electrical connector is removed. Lift dump control valve electrical control power will still flow through the throttle switches and main gear ground contact switches, but the lift dump latching relay is removed from the lift dump control system because the forward relay panel connector is removed. Hook the electrical connector to the forward panel, and the lift dump functional test should now operate “as advertised.”