Engine APU

The DECU controls CTOT and APR operations through regulation of the HMU torque motor. The CTOT system provides for a constant power during takeoff (and missed approach of activated) by counteracting the torque bloom created by inlet ram air during takeoff roll. The DECU holds constant torque until deactivated by opening either one of the two switches or by advancing the power lever to set an engine torque greater than the selected reference torque. The CTOT circuit signals the HMU torque motor to trim fuel flow to maintain engine torque at the preselected value. The signal validation circuit receives two signals, one from the torque computation circuit and the other from the CTOT panel in the cockpit. Both signals are validated and sent to the CTOT circuit. In the event of a torque signal failure, the CTOT circuit will be disabled by the control law in the signal validation circuit and cockpit torque indication will be set to zero. Signal inputs for both functions are monitored by the DECU and controlled by the switch located on the CTOT panel.

HMU Torque Motor Lockout

Torque and propeller rpm signal failures to the DECU, or DECU failure may result in improper control of the torque motor. Such failures may cause an improper Ng increase leading to over temperature, overtorque, or propeller overspeeding. If that occurs, the torque motor must be mechanically disabled by advancing the related condition lever into the T/M lock out position. Thereafter, the condition lever is retarded to match the other propeller rpm. While in T/M lockout, fuel is vented overboard so as to purge the engine fuel system. Because of that, the condition lever cannot be left in the T/M position. Torque motor lockout in flight does not affect propeller rpm governing but bottom governing, CTOT, and APR functions will be lost. The torque motor circuit is normally inhibited via weight-on-wheel sensing while airborne when the power lever is between flight idle and 64° PLA (minimum takeoff power). This prevents asymmetric power if one torque motor fails during approach and landing phases of flight. On the ground, if torque motor lockout has been selected, bottoming governing is not available and taxi/reverse thrust is significantly reduced. If the affected propeller is not feathered then propeller rpm must be manually controlled by the related power lever to avoid the yellow arcs shown on the PROP indicator.

CTOT Operation

A selector knob located on the CTOT panel sets the desired takeoff torque for both engines. After the initial takeoff power setting is made, the CTOT system eliminates the need for "fine tuning" each engine to obtain the target torque for takeoff. This is done via the DECU which regulates fuel flow as necessary above that set by power lever demand. The system requires that the torque be initially set by the power levers to within 15-20% of the value set on the CTOT panel before selecting the switch to ON or APR. Engine torque automatically increases to the selected torque value or until 955°C ITT is reached, whichever occurs first. If the torque is set too high initially, then the target value will be exceeded due to ram air increase in the engine inlet during the takeoff run. Setting the torque within 15-20% also minimizes  asymmetric thrust if the CTOT system subsequently fails. When the system is active during takeoff and go-around, torque can be adjusted via the selector knob but the torque cannot be reduced by the selector knob below the values set by the power levers (the HMU base schedule). Slow movement of the CTOT knob is essential, as the circuit has no compensation for engine acceleration or deceleration. System override is accomplished by advancing the power levers to obtain whatever torque is desired. In case of an aborted takeoff, the system automatically disengages when the power levers are retarded below 64° PLA. The CTOT circuit is controlled by two switches and a variable resistance potentiometer. The arming switch is located in the cockpit quadrant while the activation switch and the potentiometer are located on the pedestal between the pilots on the CTOT panel, directly behind the power levers. In order for the CTOT circuit to activate, both switches must be closed. The arming switch will close when the power lever is advanced to a position beyond the 64° position. The activation switch is controlled by the pilot.

APR Operation

The APR system is an integral function of the CTOT system. If an engine failure occurs, the system provides an automatic 7% torque (120 HP) increase above the selected CTOT setting on the operative engine. The APR torque increase is limited to 107% but can be overridden by advancing the power lever to obtain whatever torque is desired The APR system is armed by APR selection on the CTOT panel and advancing the power levers beyond the 64° power level angle/PLA (marked by a yellow stripe) and is activated by engine failure signals received from the autocoarsen system. The the AUTO COARSEN switch must be ON for APR to work. Individual APR system arming is verified when their respective green APR lights on the flight status panel come on. Once APR is activated, it can be deactivated by moving the CTOT switch from APR or by retarding the affected power lever below 64° PLA. In the latter case, the APR system will be reactivated if the power lever is a moved beyond 64° PLA. Selecting the AUTO COARSEN switch to OFF does not deactivate the system.