The pressure inside aneroid 1 is constant. As the aircraft climbs, aneroid 1 expands and, through bell-crank lever 2, shifts pilot valve spool 3 to the right, compressing spring 7. When the aircraft reaches a certain altitude, spool 3 admits fluid from passage 5 through passage 6 to the right end of main valve spool 4, shifting this spool rapidly to the left. Then fluid from the pump is delivered through passages 8 and 9 to the speed-changing mechanism. The altitude at which the speeds are changed can be regulated by cam 10 which raises or lowers the aneroid housing.
$4124$EHP,Co$