Engine Control

Ignition System 

The ignition system operates in the following modes: 

• START IGNITION is available when the IGN switch is selected to NORM. It is activated when the engine start relay closes and automatically cuts off at completion of the start cycle (at start relay opening) 
• AUTOIGNITION is available when the IGN switch is selected to NORM and is activated for at least seven seconds 
• CONTINUOUS IGNITION is active when the IGN switch is selected to CONT or when control power is lost from the related Battery Bus. The respective white IGN light illuminates on the flight status panel anytime the IGN switch is in CONT or when NORM is selected and any of the following occur: engine start cycle, auto relight is activated, control power is lost or the power turbine overspeeds.

IGN system control power is supplied by two circuit breakers: one for the start and continuous modes and one for the autoignition mode. Autoignition is activated and the related IGN light illuminates if the autoignition circuit breaker trips (control power lost).

Digital Electronic Control Unit 

 The DECU increases fuel flow above the HMU base fuel flow as needed for bottom governor function. During ground operations the engine power is minimal while the PLs are in the ground beta range. In this situation, the props rotate slower than the rpm selected by the CLs (under speed). Without additional Ng regulation from the DECU, the props would dwell at the lower end of their rpm range. This results in poor taxi and reverse thrust for ground maneuvering. When the PLs are below flight idle the DECU controls the bottom governing function by increasing flow fuel as necessary to maintain 1,040 rpms throughout the PL taxi thrust range The exception is during maximum reversing prop rpm increases to 1,200 rpm to improve reverse thrust response.

The DECU also increases fuel flow via the HMU torque motor for Constant Torque on Takeoff/CTOT and Automatic Power Reserve/APR functions. The DECU powered by the EES, increases fuel flow using the torque motor to increase fuel above the HMU base schedule as necessary for CTOT/APR and bottom governing functions.
The DECU accomplishes the following functions:
• Np overspeed protection. Fuel flow is automatically recirculated by the DECU if power turbine spool overspeeding occurs due to a fixed-pitch propeller or a power train failure. Fuel flow shutoff occurs at 25000 power turbine rpm (1572 PRPM) and is accomplished by electrically closing the overspeed/drain valve. The valve is re-opened and normal fuel flow returns when power turbine rpm decreases below 25000 rpm. Ignition is also simultaneously activated to accomplish an instant relight when power turbine rpm decreases to within limits
• Autoignition. The DECU detects an imminent engine flameout by comparing Ng rate-of-change to a programmed flameout-detection schedule. When actual Ng decrease exceeds established parameters, the DECU activates the ignition system for at least seven seconds. If Ng decreases below 62% then ignition is cancelled to prevent a relight while below idle Ng
• Bottom Governing. Engine power is minimal when the power lever is in the ground beta range. Consequently, the propeller rotates slower than the speed selected by the condition lever (underspeed condition). Without special Ng regulation, the propeller would dwell at the "bottom" end of its rpm range resulting in poor taxi and reverse thrust response. Bottom governing is active for reverse and taxi thrust control when the power lever is below flight idle, and the DECU assumes this governing function by controlling fuel flow as necessary to maintain 1040 propeller rpm throughout the taxi thrust range. However, during maximum reversing, propeller rpm increases to 1200 to enhance reverse thrusting. On the ground, bottom governing is enabled independent of power lever position when the condition lever is moved into the MIN - MAX range. When the CL is below the MIN position, there is no regulated minimum speed requirement and fuel flow subsequently decreases to idle. With the CL at MIN or above, bottom governing is activated after the propeller rpm increases above 830, and is automatically cancelled if propeller rpm decreases below 1280.

Overspeed/Drain Valve 

Fuel exiting the HMU passes through overspeed/drain valve which controls fuel flow to the fuel nozzles. In case of Np overspeed, the DECU electrically actuates the valve which recirculates the fuel back to the HP pump inlet. After engine shutdown, the drain function permits gravity drain of the fuel lines to their respective fuel collector tanks above the main wheel well. The collector tank must be drained by maintenance and if overfills, will port fuel under the engine nacelles.

Bottom Governing 

The Np bottom governing system provides constant propeller speed during ground handling and in revesere thrust. The governor circuit signals the HMU torque motor to trim fuel flow to match the Np reference signals. The governor circuit signals the HMU torque motor to trim fuel flow to match the Np reference signal. The signal validation circuit receives a power turbine speed signal from the power turbine shaft speed sensor which it validates and sends to the bottoming governor circuit. In the event of a primary signal failure, the signal validation circuit will send an Np speed signal from the power turbine torque and speed sensor.
The bottoming governor circuit is enabled by the DECU when Np is above 60% (830 RPM) and the condition lever is beyond the MIN PROP quadrant position. It is automatically disabled by the DECU if the Np speed is below 20% (277 RPM). This is a safety consideration to prevent engine acceleration on the bottoming governor with a feathered propeller or following an Ng speed signal failure. The system uses an variable resistance potentiometer to adjust the reference speed. This potentiometer is controlled by aircraft power lever setting. In forward propeller pitch operation, the variable Ng bottoming governor maintains a constant minimum reference speed of 751 (1040 RPM) for a quieter, more fuel-efficient taxi. During reverse pitch operation, the bottom governor will increase propeller speed, linearly with propeller pitch, up to 92% (1274 RPM). This increase results in more reverse thrust for braking. The system is designed such that following an open failure of the aircraft Ng reference adjustment circuit the Ng reference signal into the bottoming governor comparator will revert to a constant setting of 82% (1135 RPM).
The bottoming servo torque motor is an electromechanical transducer that converts electrical signals to mechanical movements of a flapper valve arm. The bottoming servo has the authority to vary the fuel flow schedule established by the power lever (PL) as a function of the electrical control unit input to the torque motor. The servo can uptrim Ng a minimum of 35% to a maximum pf 44%, but cannot adjust fuel flow below that set by the PL. Feedback of bottoming servo position is accomplished by a linear variable differential transformer (LVDT) voltage ratio signal to the ECU. The uptrim authority of the bottoming servo performs three functions:

• When the engine is started, it accelerates along the start schedule to the ground idle power level. When the propeller is unfeathered, the bottoming servo is uptrimmed by the ECU to maintain a propeller speed which is higher than the shaft and propeller resonances. 
• When the propeller is reversed by power lever input from the pilot, the ECU trims the bottoming servo to schedule reverse power settings. 
• The bottoming servo may be used by the ECU for the automatic reserve power feature. If one engine loses power on takeoff, the ECU uptrims the bottoming servo to achieve a torque increase on the "good" engine.