Turbine Engine Starting Systems

5-6 Adjusting stud Starter switch Starter drive pinion Return spring Starter lever Flexible starter control rod 9/16” 1/16" clearance The engine selector switch must be positioned and the starter switch and the safety switch—wired in series—must be closed before the starter can be energized. Current is supplied to the starter control circuit through a circuit breaker labeled “Starter, Primer, and Induction Vibrator.” [Figure 5-7] When the engine selector switch is in position for the engine start, closing the starter energizes the starter relay located in the engine nacelle area. Energizing the starter relay completes the power circuit to the starter motor. The current necessary for this heavy load is taken directly from the master bus through the starter bus cable.

All starting systems have operating time limits because of the high energy used during cranking or rotation of the engine. These limits are referred to as starter limits and must be observed or overheating and damage of the starter occurs. After energizing the starter for 1 minute, it should be allowed to cool for at least 1 minute. After a second or subsequent cranking period of 1 minute, it should cool for 5 minutes. Direct Cranking Electric Starting System for Small Aircraft Most small, reciprocating engine aircraft employ a direct cranking electric starting system. Some of these systems are automatically engaged starting systems, while others are manually engaged.

Manually engaged starting systems used on many older, small aircraft employ a manually operated overrunning clutch drive pinion to transmit power from an electric starter motor to a crankshaft starter drive gear. [Figure 5-8] A knob or handle on the instrument panel is connected by a flexible control to a lever on the starter. This lever shifts the starter drive pinion into the engaged position and closes the starter switch contacts when the starter knob or handle is pulled. The starter lever is attached to a return spring that returns the lever and the flexible control to the off position. When the engine starts, the overrunning action of the clutch protects the starter drive pinion until the shift lever can be released to disengage the pinion. For the typical unit, there is a specified length of travel for the starter gear pinion. [Figure 5-8] It is important that the starter lever move the starter pinion gear this proper distance before the adjustable lever stud contacts the starter switch.

The automatic, or remote solenoid engaged, starting systems employ an electric starter mounted on an engine adapter. A starter solenoid is activated by either a push button or turning the ignition key on the instrument panel. When the solenoid is activated, its contacts close, and electrical energy energizes the starter motor. Initial rotation of the starter motor engages the starter through an overrunning clutch in the starter adapter, which incorporates worm reduction gears. Some engines incorporate an automatic starting system that employs an electric starter motor mounted on a right angle drive adapter. As the starter motor is electrically energized, the adapter worm shaft and gear engage the starter shaft gear by means of a spring and clutch assembly. The shaft gear, in turn, rotates the crankshaft. When the engine begins to turn on its own power, the clutch spring disengages from the shaft gear. The starter adapter uses a worm drive gear shaft and worm gear to transfer torque from the starter motor to the clutch assembly. [Figure 5-9] As the worm gear rotates the worm wheel and clutch spring, the clutch spring is tightened around the drum of the starter shaft gear. As the shaft gear turns, torque is transmitted directly to the crankshaft gear.

Other engines use a starter that drives a ring gear mounted to the propeller hub. [Figure 5-10] It uses an electric motor and a drive gear that engages as the motor is energized and spins the gear, which moves out and engages the ring gear on the propeller hub cranking the engine for start. [Figure 5-11] As the engine starts, the starter drive gear is spun back by the engine turning, which disengages the drive gear. [Figure 5-12] The starter motors on small aircraft also have operational limits with cool down times that should be observed. Reciprocating Engine Starting System Maintenance Practices Most starting system maintenance practices include replacing the starter motor brushes and brush springs, cleaning dirty commutators, and turning down burned or out-of-round 5-7 Starter motor Starter adapter housing Bearing Worm gear Starter shaft gear Worm wheel Clutch spring Propeller Starter ring gear mounted on propeller hub plate Electrical connectionStarter drive gear Mounting bolts and alignment pin connector.

starter commutators. As a rule, starter brushes should be replaced when worn down to approximately one-half the original length. Brush spring tension should be sufficient to give brushes a good firm contact with the commutator. Brush leads should be unbroken and lead terminal screws tight. A glazed or dirty starter commutator can be cleaned by holding a strip of double-0 sandpaper or a brush seating stone against the commutator as it is turned. The sandpaper or stone should be moved back and forth across the commutator to avoid wearing a groove. Emery paper or carborundum should never be used for this purpose because of their possible shorting action.

Roughness, out-of-roundness, or high-mica conditions are reasons for turning down the commutator. In the case of a high-mica condition, the mica should be undercut after the turning operation is accomplished. Refer to FAA-H-8083-30, Aviation Maintenance Technician—General for a review of high-mica commutators in motors. The drive gear should be checked for wear along with the ring gear. The electrical connections should be checked for looseness and corrosion. Also, check the security of the mounting of the housing of the starter. Troubleshooting Small Aircraft Starting Systems The troubleshooting procedures listed in Figure 5-13 are typical of those used to isolate malfunctions in small aircraft starting systems.

5-8 Starter ring gear mounted on propeller hub Starter gear Small Aircraft Troubleshooting Procedures Starter motor runs but does not turn crankshaft Starter drags Starter excessively noisy Probable Cause Isolation Procedure Remedy Starter will not operate • Repair circuit. • Replace switch or wires. • Adjust starter lever in accordance with manufacturer’s instructions. • Remove and repair or replace starter. • Adjust starter lever in accordance with manufacturer’s instructions. • Replace defective parts. • Charge or replace battery. • Replace with serviceable unit. • Repair or replace starter.

• Turn down commutator. • Replace starter drive. • Replace crankshaft gear. • Check master circuit. • Check switch circuit continuity. • Check starter lever adjustment. • Check through items above. If another cause is not apparent, starter is defective. • Check starter lever adjustment. • Remove starter and check starter drive and overrunning clutch. • Remove and check pinion gear and crankshaft gear. • Check battery. • Check contacts. • Check starter brushes, brush spring tension for solder thrown on brush cover. • Clean and check visually. • Remove and examine pinion. • Remove starter and turn over engine by hand to examine crankshaft gear.

• Defective master switch or circuit. • Defective starter switch or switch circuit. • Starter lever does not activate switch. • Defective starter. • Starter lever adjusted to activate switch without engaging pinion with crankshaft gear. • Defective overrunning clutch or drive. • Damaged starter pinion gear or crankshaft gear. • Low battery. • Starter switch or relay contacts burned or dirty. • Defective starter. • Dirty, worn commutator. • Worn starter pinion. • Worn or broken teeth on crankshaft gears. Gas Turbine Engine Starters Gas turbine engines are started by rotating the high-pressure compressor. On dual-spool, axial flow engines, the high- pressure compressor and N1 turbine system is only rotated by the starter. To start a gas turbine engine, it is necessary to accelerate the compressor to provide sufficient air to support combustion in the combustion section, or burners.

Once ignition and fuel have been introduced and the lite-off has occurred, the starter must continue to assist the engine until the engine reaches a self-sustaining speed. The torque supplied by the starter must be in excess of the torque required to overcome compressor inertia and the friction loads of the engine’s compressor. turbine engine, regardless of the type of starter employed. As soon as the starter has accelerated the compressor sufficiently to establish airflow through the engine, the ignition is turned on followed by the fuel. The exact sequence of the starting procedure is important since there must be sufficient airflow through the engine to support combustion before the air-fuel mixture is ignited. At low engine cranking speeds, the fuel flow rate is not sufficient to enable the engine to accelerate; for this reason, the starter continues to crank the engine until after self-accelerating speed has been attained. If assistance from the starter were cut off below the self-accelerating speed, the engine would either fail to accelerate to idle speed or might even decelerate because it could not produce sufficient energy to sustain rotation or to accelerate during the initial phase of the starting cycle. The starter must continue to assist the engine considerably above the self-accelerating speed to avoid a delay in the starting cycle, which would result in a hot or hung false start or a combination of both.

At the proper points in the sequence, the starter and ignition 5-9 Time (seconds) Compressor rpm Exhaust gas temperature (°C) Idle EGT Peak starting ECT Exhaust gas temperature versus time Compressor rpm versus time Starter cuts out Ignition off Engine attains self-accelerating speed Ignition onFuel on Engine lights up EGT rise indicates light up Engine attains idle rpm Relief valve Turbine exhaust ring Overrunning sprag clutch Hot gas nozzles Turbine rotor Gear shaft Compressed air inlet Spline shaft Exhaust from turbine and fan Breech capBreech handle Switch Flyweight Cartridge Switch actuating rod are automatically cut off. The basic types of starters that are in current use for gas turbine engines are direct current (DC) electric motor, starter/generators, and the air turbine type of starters.

Many types of turbine starters have included several different methods for turning the engine for starting. Several methods have been used but most of these have given way to electric or air turbine starters. An air impingement starting system, which is sometimes used on small engines, consists of jets of compressed air piped to the inside of the compressor or turbine case so that the jet air blast is directed onto the compressor or turbine rotor blades, causing them to rotate. A typical cartridge/pneumatic turbine engine starter may be operated as an ordinary air turbine starter from a ground- operated air supply or an engine cross-bleed source. It may also be operated as a cartridge starter. [Figure 5-15] To accomplish a cartridge start, a cartridge is first placed in the breech cap. The breech is then closed on the breech chamber by means of the breech handle and then rotated a partial turn to engage the lugs between the two breech sections. The cartridge is ignited by applying voltage through the connector at the end of the breech handle. Upon ignition, the cartridge begins to generate gas. The gas is forced out of the breech to the hot gas nozzles that are directed toward the buckets on the turbine rotor, and rotation is produced via the overboard exhaust collector. Before reaching the nozzle, the hot gas passes an outlet leading to the relief valve. This valve directs hot gas to the turbine, bypassing the hot gas nozzle, as the pressure rises above the preset maximum. Thus, the pressure of the gas within the hot gas circuit is maintained at the optimum level.

The air-fuel combustion starter was used to start gas turbine 5-10 Gear ratio approximately 435 to 1Cooling air To voltage regulator Output To generator paralleling and protective circuitry Shunt Series D B + C +A +E Commutating Compensated field Shunt field C field engines by using the combustion energy of jet A fuel and compressed air. The starter consists of a turbine-driven power unit and auxiliary fuel, air, and ignition systems. Operation of this type starter is, in most installations, fully automatic; actuation of a single switch causes the starter to fire and accelerate the engine from rest to starter cutoff speed.

Hydraulic pumps and motors have also been used for some smaller engines. Many of these systems are not often used on modern commercial aircraft because of the high power demands required to turn the large turbofan engines during the starting cycle on transport aircraft. Electric Starting Systems & Starter Generator Starting System Electric starting systems for gas turbine aircraft are of two general types: direct cranking electrical systems and starter generator systems. Direct cranking electric starting systems are used mostly on small turbine engines, such as Auxiliary Power Units (APUs), and some small turboshaft engines. Many gas turbine aircraft are equipped with starter generator systems. Starter generator starting systems are also similar to direct cranking electrical systems except that after functioning as a starter, they contain a second series of windings that allow it to switch to a generator after the engine has reached a self-sustaining speed. This saves weight and space on the engine.

The starter generator is permanently engaged with the engine shaft through the necessary drive gears, while the direct cranking starter must employ some means of disengaging the starter from the shaft after the engine has started. The starter generator unit is basically a shunt generator with an additional heavy series winding. [Figure 5-16] This series winding is electrically connected to produce a strong field and a resulting high torque for starting. Starter generator units are desirable from an economic standpoint, since one unit performs the functions of both starter and generator. Additionally, the total weight of starting system components is reduced and fewer spare parts are required.

The starter generator internal circuit has four field windings: a series field (C field), a shunt field, a compensating field, and an interpole or commutating winding. [Figure 5-17] During starting, the C field, compensating, and commutating windings are used. The unit is similar to a direct cranking starter since all of the windings used during starting are in series with the source. While acting as a starter, the unit makes no practical use of its shunt field. A source of 24 volts and 1,500 peak amperes is usually required for starting. When operating as a generator, the shunt, compensating, and commutating windings are used. The C field is used only for starting purposes. The shunt field is connected in the conventional voltage control circuit for the generator.

Compensating and commutating or interpole windings provide almost sparkless commutation from no load to full load. Figure 5-18 illustrates the external circuit of a starter generator with an undercurrent controller. This unit controls the starter generator when it is used as a starter. Its purpose is 5-11 To external power receptacle B G C M C B A E Advance Retard To fuel pumps On Off To fuel valve Battery on Start Emergency stop Engine master switch Battery and start switch Battery cutout relay Throttle switch Throttle relay Motor relay Under current relay Throttle relay lock Ignition relay Ignition vibrator Starter generator B-4 relay starter motor would damage the battery. Closing the motor relay allows a very high current to flow to the motor. Since this current flows through the coil of the undercurrent relay, it closes. Closing the undercurrent relay completes a circuit from the positive bus to the motor relay coil, ignition relay coil, and battery cutout relay coil. The start switch is allowed to return to its normal off position, and all units continue to operate.

As the motor builds up speed, the current draw of the motor begins to decrease. As it decreases to less than 200 amps, the undercurrent relay opens. This action breaks the circuit from the positive bus to the coils of the motor, ignition, and battery cutout relays. The de-energizing of these relay coils halts the start operation. After these procedures are completed, the engine should be operating efficiently, and ignition should be self-sustaining. If, however, the engine fails to reach sufficient speed to halt the starter operation, the stop switch may be used to break the circuit from the positive bus to the main contacts of the undercurrent relay.

to assure positive action of the starter and to keep it operating until the engine is rotating fast enough to sustain combustion. The control block of the undercurrent controller contains two relays. One is the motor relay that controls the input to the starter; the other, the undercurrent relay, controls the operation of the motor relay. The sequence of operation for the starting system is discussed in the following paragraphs. [Figure 5-18] To start an engine equipped with an undercurrent relay, it is first necessary to close the engine master switch. This completes the circuit from the aircraft’s bus to the start switch, to the fuel valves, and to the throttle relay. Energizing the throttle relay starts the fuel pumps and completing the fuel valve circuit gives the necessary fuel pressure for starting the engine. As the battery and start switch is turned on, three relays close: the motor relay, ignition relay, and battery cutout relay. The motor relay closes the circuit from the power source to the starter motor; the ignition relay closes the circuit to the ignition units; the battery cutout relay disconnects the battery. Opening the battery circuit is necessary because the heavy drain of the