Summary
This document proposes the adoption of a new airworthiness directive (AD) that is applicable to certain Fokker Model F28 Mark 1000 through 4000 series airplanes. This proposal would require replacing certain flexible hydraulic hoses that connect to the UP-port of the actuator of each main landing gear (MLG) with certain new flexible hoses that have built-in restrictor check-valves. This proposal is prompted by results of tests, which indicate that, for airplanes on which restrictor check-valves are not installed, sudden movement of the actuator of the MLG, which could occur under extreme inward sideload conditions (such as touching down at a large crab angle), may pressurize the downlock-actuator and lift the MLG toggle-links. The actions specified by the proposed AD are intended to prevent such pressurization of the downlock-actuator and consequent lifting of the toggle-links, which could result in collapse of the MLG and reduced controllability of the airplane during landing.