AD 58-26-02

58-26-02 BELL: Applies to All Model 47 Series Helicopters Equipped With Marvel- Schebler Automatic Altitude Compensating Carburetor Model MA4-5AA.

Compliance required within next 25 hours of operation.

With a Marvel-Schebler Model MA4-5AA carburetor installed, it is possible to start and run the engine with the carburetor mixture control in the idle cutoff position if the throttle is partially or fully opened. However, as soon as the throttle is fully closed, the idle cutoff will operate and shut off the engine. Therefore, it would be possible to start the engine, takeoff, and fly with the mixture in idle cutoff as long as the throttle is never closed. The first time the throttle is closed, however, as in autorotation, an immediate engine stoppage would occur.

To prevent inadvertent engine stoppage in flight due to failure to place the carburetor mixture control in the "auto" position before takeoff, the following must be accomplished:

(a) For Models 47D1, 47G, 47G2, 47H1, and 47J, spring load the cockpit mixture control to the rich or "auto" position by installing a tension spring (Bell P/N 47-631-226-1 or equivalent) between the mixture control cable support bracket (P/N 47-631-125-18) adjacent to the control quadrant and the control cable end of the quadrant mixture control lever. On Models 47D1 and 47H1, attach the spring to the mixture control lever using existing cotter pin hole through flexible control attaching clevis pin. On Models 47G, 47G2, and 47J, attach spring to mixture control lever through existing lower hole in lever. On Model 47D1, remove corners of jog in quadrant mixture control slot. Check that mixture control will return to "auto" position when pulled to "idle cut off" position and released.

(b) For Model 47D, remove the quadrant mixture control lever and push-pull mixture control. Place the carburetor mixture control arm in the rich or "auto" position and secure with lockwire. This procedure may be used in lieu of that outlined in (a) for Models 47D1, 47G, 47G2, 47H1, and 47J if preferred.

(This same subject is covered in Bell Service Bulletins No. 124SB for Models 47D1, 47G, and 47H1, No. 124SB Supplement 1 for Model 47D, and Service Instruction 257SI for Models 47G2 and 47J.)