Summary
This document proposes the adoption of a new airworthiness directive (AD) that is applicable to certain Learjet Model 55 Series Airplanes and Model 60 Airplanes. This proposal would require replacement of the brake valve adjustment screw with a new improved screw, and for certain airplanes, it would also require installation of a new brake valve lever stop. This action is necessary to prevent bottoming of the valve components before contact of the brake valve lever with the stop, which could result in loss of all hydraulic fluid and consequent loss of normal braking. This action is intended to address the identified unsafe condition.