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2.24.4 External Source Engine Start. The external start pneumatic port (Figure 2-1) is on the left side of the fuselage. It is the attachment point for a bleed-air line from an external source for engine starting or helicopter heating on the ground. The assembly contains a check valve to prevent engine or APU bleed-air from being vented. The external air source pressurizes the start system up to the engine start control valves, requiring only that electrical power be applied. If an emergency start is made without ac electrical power, No. 1 engine must be started .rst because the No. 2 engine start control valve will not operate without dc primary bus power.
2.25 ENGINE CONTROL SYSTEM.
The engine control system consists of the ECU, engine quadrant, load demand system and speed control system.
2.25.1 Electrical Control Unit (ECU). The electrical control unit controls the electrical functions of the engine and transmits operational information to the cockpit. It is a solid-state device, mounted below the engine compressor casing. The ECU accepts inputs from the alternator, ther-mocouple harness, Np (% RPM 1 and 2) sensor, torque and overspeed sensors, torque signal from opposite engine for load sharing, feedback signals from the HMU for sys-tem stabilization, and a demand speed from the engine speed trim button. The ECU provides signals to the % RPM 1 and 2 indicators, % TRQ meter, TGT TEMP indicator, and history recorder.
NOTE
Phantom torque may be observed on the Pi-lot Display Unit (PDU) torque display of a non-operating engine while the aircraft’s other engine is operating during a ground run. Phantom torque readings of up to 14% have been observed on the PDU of the non-operating engine. During startup of the non-operating engine, its ECU will produce a normal, positive torque signal which dis-plays the correct torque signal on the respec-tive PDU.

a.
In case of an ECU malfunction, the pilot may over-ride the ECU by momentarily advancing the ENG POWER CONT lever to the LOCKOUT stop, then re-tarding it to manually control engine power. To remove the ECU from lockout, the ENG POWER CONT lever must be moved to IDLE.

b.
The torque matching/load sharing system increases power on the lower-torque engine to keep engine torques approximately equal. The system does not allow an engine to reduce power to match a lower power engine. If an en-gine fails to the high side, the good engine will only at-tempt to increase torque upward until its Np is 3% above the reference Np.

c.
The temperature limiting system limits fuel .ow when the requirement is so great that the turbine tempera-ture reaches the limiting value of 837°C to 849°C. Fuel .ow is reduced to hold a constant TGT. It is normal to see

a transient increase above the 850°C TGT TEMP when the pilot demands maximum power (Figure 5-1 transient lim-
its). TGT limiting does not prevent overtemperature during engine starts, compressor stall, or when the engine is oper-ated in LOCKOUT (Paragraph 9.3e).
d. Overspeed protection protects the power turbine from destructive overspeeds. The system is set to trigger at 106%61% RPM 1 or 2 and will result in an initial reduced fuel .ow and will cycle until the cause of the overspeed is removed or % RPM is reduced manually. Two momentary switches marked NO. 1 and NO. 2 ENG OVSP TEST A and TEST B on the upper console (Figure 2-6), are used to check the circuits. Testing individual circuits A and B in-dicates that those systems are complete and performing cor-rectly. Dual closing of A and B serves to check out the actual overspeed system itself, the overspeed solenoid and the POU. This check must be done only on the ground by designated maintenance personnel. The overspeed protec-tion is not deactivated when in LOCKOUT. Power to op-erate the overspeed system is from two independent sources: the engine alternators as the primary source, and the No. 1 and No. 2 ac primary buses as alternate backup source in case of alternator failure. Circuit protection is through circuit breakers marked NO. 1 ENG OVSP and NO. 2 ENG OVSP.
　
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