To Yaw Damper
Having suffered five years on the 1-11 with it's attrocious air system, I totally agree with you that the 'every 5000' check would probably have caught the problem. It was drummed into me by 'The Skip' in FR (the greatest Training Pilot I have met in 35 years flying) and I carried it with me to Boeing and all it's advanced types until my dotage and to hell with it if it wasn't a standard call.