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C.  Gearbox
(1)  The gearbox consists of a spur gear train, which reduces the ac power unit input RPM (nominal 965 RPM) by 8 to 1 ratio to a nominal output of 120 RPM. (See figure 2.) The gearbox is equipped with an external 11/16 inch hexagon drive to permit manual operation. The maximum torque required to rotate the manual drive is approximately 180 inch-pounds measured at the hexagon drive.
CAUTION:  THE TORQUE APPLIED TO THE MANUAL DRIVE MUST NOT EXCEED 250 INCH-POUNDS OR DAMAGE TO THE AIRCRAFT STRUCTURE AND/OR AIRSTAIR MECHANISM MAY RESULT.
D.  Transmission Unit
(1)  The transmission units are directly coupled between their respective gearboxes and aft airstair hinges. Each transmission unit reduces the transmission input RPM (nominal 120 RPM) by 250 to 1 ratio, thereby providing a positive and controlled force to extend or retract the aft airstair. In addition, each transmission unit contains a plunger-type limit switch to deactivate the ac power, unit when the aft airstairs reach a predetermined point during the retraction or extension cycles. (See figure 2.)
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E.  Centrifugal Governor
(1)  The purpose of the centrifugal governor is to prevent excessive rate of aft airstair movement during an emergency extend cycle or in the event of loss of electrical power during a normal extension. The governor consists of two spring-loaded brake shoes which contact the walls of the governor housing to provide braking action when the input speed exceeds 2200 RPM (transfer shaft speed 550 RPM). (See figure 2.) In normal operation, the ac power unit clutch will be energized on the extend cycle, and the rate of descent will be controlled by the magnetic field in the ac motor. This limits the power unit output to a maximum of 1090 RPM. In emergency extend, the power unit clutch is de-energized; therefore, the centrifugal governor limits the rate of extension.
5.  Controls
A.  The aft airstair and entry door are controlled by the interior console control panel, or by the external controls. The external and interior controls perform identical functions and differ only in their physical characteristics and location.
B.  Interior Controls
(1)  The interior console control panel is located in the cabin adjacent to the aft entry door, and consists of a control handle, and a guarded, momentary-type lever lock three-position switch. The switch is located on top of the console and is marked EXTEND, OFF, and RETRACT. The switch is spring-loaded to the OFF position. (See figure 3.) The control handle has three positions, DOOR UNLATCHED, DOOR LATCHED, and EMERGENCY EXTENSION. (See figure 1.)
C.  External Controls
(1)  The external controls consist of an external handle and a momentary three-position switch. The external handle is connected directly to the interior control handle by a teleflex cable. The cable contains a controlled shear joint at the console end assuring interior operation if the external handle becomes jammed. The operation of the external handle and the unguarded three-position switch is identical in function and operation as the interior controls. The external control components are located near the forward lower corner of the entry door.
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6.  Operation
A.  The aft airstairs may be operated in three separate modes: electrical, manual, and/or
emergency.

NOTE:  For maintenance requirements, the manual mode should be used to extend the aft airstairs whenever ac and dc electrical power is not readily available.
CAUTION:  PRIOR TO EXTENDING THE AFT AIRSTAIR IN THE MANUAL MODE, THE AFT ENTRY DOOR MUST BE UNLATCHED OR DAMAGE TO THE AIRCRAFT STRUCTURE OR AFT AIRSTAIR MECHANISMS WILL OCCUR.
B.  Electrical Mode
(1)  
The operation of the aft airstairs in the electrical mode requires both 28 volt dc and 115 (3-phase) volt ac power. The power unit operates on ac power, and the associated control circuits axe controlled by dc power. (See figure 3.)

(2)  
To extend the airstairs, place the console control handle in the DOOR UNLATCHED position. DC power is now simultaneously available to the external or internal airstair electrical control switch. By indexing either control switch to the extend position, the aft airstairs will begin to extend. When the airstairs have extended approximately 128 degrees the extend limit switch in the transmission unit will de-energize the power unit. The aft airstairs will continue to free fall to the ground level.
　
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