55-16-01 BELL: Applies to All Model 47 Helicopters With 47-641-025-1 or 47-641-057- 1 Tail Rotor Hubs, and All Spares.
Compliance required prior to next flight.
Recent serious accidents have been caused by failure of the tail rotor hubs P/N 47-641- 025-1 and 47-641-057-1. Imperfections such as corrosion, tool marks, scratches, and pitting in the fillet set up stress concentrations which cause failure of the hub spindle. Consequently, immediate and complete inspection of the fillet area of the tail rotor hub must be accomplished as follows:
1. Inspect the inboard fillet area on both spindles of the hub for accuracy of the radius using a 10-power magnifying glass. Fillets must have a true continuous radius all around the spindles and also must have a smooth machine finish free of corrosion and pitting. When inspected under a magnifying glass, tool marks appear greatly exaggerated, and by a smooth machine finish is meant having extremely fine, uniform, closely-spacedand uninterrupted tool marks. No copper plating is allowed in the fillet.
2. Hubs not meeting these requirements must be replaced.
(Bell Service Bulletin No. 96, Revision B dated July 15, 1955, covers this subject.)
(This covers the same inspection as the CAA telegram of July 13, 1955.)