experience over the last four years, during which no spline coupling failures or malfunctions were reported. There, also, have been no reported failures or malfunctions of the bolted brake cage units for more than ten years. The above maintenance bulletin and installation of new bolted brake cage units should prevent repetition of brake failures as experienced by Standard Airways.
With respect to your recommendation that installation of mechanical low pitch stops be made mandatory, we do not believe there is sufficient justification to require them in view of the corrective action already taken as a result of the Seaboard and Western accident and the corrective action being applied as a result of the Standard Airways accident. Apart from the fact that there is reason to believe the brake failures on the Standard Airways aircraft were due to insufficient tightening of the brake cages to the brake housings, it is noted that the service record on Curtiss propellers has been highly satisfactory over a great number of years and no serious control difficulties have been reported other than the Seaboard and Western and Standard Airways cases. Since the causes of these malfunctions have now been corrected and in view of the fact that the propeller incorporates an electrical low pitch stop actuated by blade angle to prevent unwanted travel below the stop, there ls no evidence to support a mandatory requirement to back up the electrical low pitch stop with a mechanical low pitch stop, which, while contributing little to safety, would impose a considerable financial penalty on the affected operators.
We believe that closer surveillance of the threaded type brake cage units and replacement of the threaded units with bolted configurations will preclude the possibility of further Curtiss propeller brake failures.
Sincerely yours,
/s/ Harold W. Grant
HAROLD W. GRANT
Acting Administrator
Honorable Alan S. Boyd
Chairman
Civil Aeronautics Board
Washington 25, D. C.
Enclosure
(page 5 of 6)
