A valve meant to bypass hydraulic fluid from the landing gear upline to the downline when the landing gear is being extended. The weight of the landing gear, when it is released, is so great that it falls very rapidly. Under these conditions, hydraulic fluid cannot fill the space behind the moving piston in the landing-gear actuating cylinder; hence, pressure is increased in the upline and on the opposite side of the piston. By having a cross-flow valve in the system, fluid is permitted to flow from the upside of the piston to the downside and thus allow the landing gear to drop with an even motion and with more ease. Cross-flow valves may be manually or automatically operated. The former is used only during the emergency operation of the landing gear, whereas the latter is used for the normal operation of landing gear.
An Illustrated Dictionary of Aviation Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved