Boeing CH-47D Chinook - The Combining Transmission.

The CH-47 Chinook Combining Transmission

A drawing of the component placement for items installed on the CH-47D Chinook Combining Transmission (C-Box).

A left rear view of the CH-47D Chinook Combining Transmission, shown with the oil coolers removed, 9 November 2005. Click-N-Go Here to see a larger version.

Pay particular attention to the blue colored cap installed on the Debris Screen oil return line (shown above is the one from the Number One Engine Transmission - the Number Two Engine Transmission return is on the other side). Often a new C-Box is received through the supply system and it is painted the same color as the rest of the C-Box - either gray or black, making it look like it is a normal part of the transmission. Failure to remove this cap and re-install the return line from the engine transmission has been the cause of several "Transmission Hot" caution lights over the years. In every instance, the return line has been folded back and placed into the torque boxes near the bottom left and right of the transmission. The Flight Engineer, Crew Chief, Technical Inspector, and Maintenance Test Pilot have all missed the improper installation. What happens when this cap is left installed is: The transmission oil pressure reads normal during engine start - because the pressure is read from the transducer located on the bulkhead aft of the C-Box (see oil flow diagram on the Engine Mounted Transmission (EMT) webpage). However, since the oil cannot return to the Combining Transmission, it rapidly over-heats while it sits in the nosebox. About 60 seconds after engine start, the "Transmission Hot" caution light illuminates indicating 190 degrees Celsius has been exceeded on the Nosebox and maintenance has to replace both the C-Box and the Nosebox. There is also the strong possibility that the nosebox will catch fire and explode if engine shutdown is not performed immediately upon illumination of the light.

The CH-47D Chinook Combining Transmission, shown with the oil coolers removed, 9 November 2005. Click-N-Go Here to see a larger version.

The aft side of the CH-47D Chinook Combining Transmission, 11 November 2005. Click-N-Go Here to see a larger version.

The CH-47D Chinook Combining Transmission Oil Flow Diagram. Click-N-Go Here to see a larger version.

Above: A drawing showing the internal components and gears of the D model and later versions of the Chinook helicopter Combining Transmission. The rotor brake is an external attachment to the transmission. The D model version (as well as previous A, B and C models) of this helicopter did not utilize a rotor brake. The brake was provided by the manufacturer on the E and G models per Army requirements. Numbers associated with a "T" indicate the number of teeth on a particular gear. Click-N-Go Here to view a larger image.

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