53-03-01 NAVION: Applies to All Model B Aircraft Equipped With Hartzell HC-12 X20-8C/9333C-0 Propellers.
Compliance required not later than March 15, 1953.
Vibration tests conducted subsequent to the initial tests of the Hartzell HC-12 X20- 8C/9333C-0 propeller installed on the Lycoming GO-435 engine revealed vibration characteristics of a hazardous nature. Therefore, to prevent possible blade cracks and/or failures, the existing r.p.m. restriction placard should be revised as follows:
"Avoid continuous ground operation between 1675 and 2150 engine r.p.m."