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T.O. 1T-38A-2-7

Engine Start and Ignition Circuit
Description

SECTION VI
ENGINE START AND IGNITION CIRCUIT

DESCRIPTION

6-1. ENGINE START AND IGNITION CIRCUIT GENERAL DESCRIPTION.

6-2. The engine start and ignition control circuit is designed to apply voltages in a controlled sequence for starting the aircraft engine and to terminate the voltages when the starting cycle is completed. (See figure 6-1.) Pushbutton type switches on the left vertical panel in either cockpit initiate the engine start cycle and also energize the starting air diverter valve of the single-point starting system, to direct starting air to the corresponding engine. Once the engine start switch is closed, 28-volt dc will be applied to operate the starting circuit. The time delay in the start and ignition time-delay relay maintains the circuit in operation for approximately 30 seconds. Engine starting can be stopped at any time during the procedures by retarding the throttle lever in either cockpit to OFF. Fingerlifts installed on the front cockpit throttle levers must be lifted to move levers into the OFF position. The rear cockpit throttle levers cannot be moved into the OFF position unless the fingerlifts on the front cockpit throttle levers are lifted. Normal starting procedures utilize external ac power for ground starts. Ignition power for single-engine air starts is supplied by the ac generator which is driven by the operating engine. On Block 20 and later aircraft, if ac power is not available, the engine can be started using the 28-volt dc battery and the static inverter. When only the battery dc power is present on the aircraft, the static inverter converts the dc power into 115-volt ac for engine ignition. AC power for the ignition circuit is normally supplied from the right generator phase B bus. The afterburner circuit, including in the engine start and ignition circuit, is initiated when the throttle lever in either cockpit is moved forward from the military (MIL) position into afterburner (MAX) position. This action applies power to the igniter plugs thru the various components in the circuit. The igniter plugs fire simultaneously when energized. An electronic timing circuit in the time-delay relay maintains the afterburner ignition in operation for approximately 30 seconds. The circuit is then disconnected, making it necessary to retard and again advance the throttle lever to reactivate the circuit.

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6-3. ENGINE START AND IGNITION CIRCUIT FUNCTIONAL DESCRIPTION.

6-4. For a simplified explanation of the engine start and ignition circuit, only the left engine circuit will be covered, the right engine circuit is identical. The circuit uses both ac and dc power to complete its functions.

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6-5. On Blocks 10 and 15 aircraft (see figure 6-1). 28-volt dc power from the ENGINE START & IGNITION CONT. FLOODLIGHT AND ID387/ARN LIGHT TEST circuit breaker on the dc circuit breaker and fuse panel in the forward communication and navigation equipment compartment is fed to the front and rear cockpit left engine start switches. Each start switch is a pushbutton type. When momentarily closed, the switch applies 28-volt dc power to the start and ignition time-delay relay, closing the normally open contacts. The normally open contacts will remain closed for approximately 30 seconds, allowing 28-volt dc power to be applied to start control relays No. 1 and No. 2. When the left start and ignition time-delay relay is actuated, closed contacts of the relay direct dc power thru normally closed contacts of the right start control relay No. 1 to energize the starting air diverter valve, positioning the valve to direct starting air to the left engine. The relays remain energized with contacts closed until the time-delay relay breaks the circuit. When external ac power is applied to the aircraft, the power is fed thru the ENGINE IGNITION circuit breaker on phase B bus to energize the ignition power transfer relay; 115-volt ac is then applied to the engine ignition generator by closed contacts of the ignition power transfer relay, start control relay No. 1, and the throttle switch. Under emergency conditions only, if ac power is not available, engine ignition may be activated with battery dc power by the following method: The battery switch is placed in the ON position; battery power is then supplied to the IGNITION VIBRATOR fuse on the dc bus. From the fuse, dc power is applied to the ignition vibrator by closed contacts of the start control relay contacts of the ignition power transfer relay, start control relay No. 1, and the throttle switch. The throttle switch is closed by manually moving the throttle lever in either cockpit forward out of the OFF position. The
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