The Saab 340 B and B+ aircraft are powered by two GE CT7-9b turboprops rated at 1,750 SHP.
General Electric CT7-9B operation
Air comes into the engine via an inlet under the propeller hub where it is regulated into the compressor section via variable inlet guide vanes/IGV. The IGV also operate in conjunction with the engine anti-ice/start bleed valve to restrict or dump engine bleed air as necessary. The compressor section has 5 axial stages and one centrifugal stage driven by a two-stage gas generator turbine. Compressed air enters the combustion chamber where fuel is added via the HMU (fuel pump or "brawn") . Of the total air that enters the engine, about 30% is used for the combustion process. The remainder is used for cooling and other purposes.
A hydromechanical unit/HMU functions in conjunction with the Digital Electronic Control Unit/DECU (computer control or "brain") to regulate the fuel flow to the engines by power lever/PL and condition lever/CL positions. Once the mixture is ignited the process is self sustaining and exhaust gases flowing out then drive the gas generator and power turbines. A 2 stage power turbine drives the prop through the propeller gear box/PGB via a concentric shaft within the gas generator turbine shaft. The gas generator and power turbines are not mechanically connected. Consequently, power turbine rpm (Np) is independent of gas generator rpm (Ng). This is commonly called a "free turbine".
Each power lever/PL is connected to its related Hyrdo Mechanical Unit and Prop Control Unit (governor and HP oil pump). The PLs set Ng by controlling fuel flow via the HMU and also controls prop blade angle via the prop control unit/PCU for taxi and reverse thrust thrust. The PLs also indirectly control the torque motor via the DECU.
The Condition Levers/CLs are connected to their associated HMU shutoff valve for engine shutdown. Each CL is also connected to its related PCU for prop feathering and RPM control. The CL range is from FUEL OFF, to FEATHER (with a start position), UNF (unfeather), MIN-MAX range, to T/M (torque motor lockout).
Automatic Flight Idle Stop
Movement of the PLs below flight idle into the beta range is prohibited by the AFIS. A mechanical stop in the control quadrant aft of FLT IDLE (ground beta range) prevents inadvertent movement of the PLs into this range while airborne. The AFIS is electrically controlled and will engage once airborne and disengages upon touchdown. Once on the ground, the AFIS opens to allow PL movement into the GND IDLE (min thrust) and REV or beta range. In case the AFIS fails to open on touchdown (indicated by the absence of the blue FI STOP light on the flight status panel and the inability to move PLs aft of FLT IDLE), there is a red manual override knob that will release the stop allowing PL movement. If the override was inadvertently pulled, when an attempt to start the left engine is made, a CONFIG warning alert will sound and can't be canceled until the left CL is returned to the FUEL OFF position.
For more on why the AFIS is installed, read this link.
The Engine Electrical System
Each engine accessory gearbox drives an alternator that supplies the EES. Each respective EES powers:
- ignition exciter power supply
- the DECU
- engine Ng indications
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 when the DECU detects any of the following:
a. Engine flameout: including engine shutdown or when the power lever is rapidly retarded
b. Np overspeed; ignition is activated to permit instant relight when the power turbine spool rpm decreases to within limits
- 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.
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.
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.
The Engine Fuel System
Engine Fuel System
Fuel is drawn from the wing tanks via the fuel shutoff valve. All fuel regulation and monitoring accomplished downstream of the airframe fuel shutoff valve is considered to be part of the engine fuel system.
The engine accessory gearbox drives the main pump which draws in fuel from the fuel hopper tank to maintain a pressurized feed to the high pressure fuel pump. A main pump failure triggers activation of the related standby pump which is inside the
respective hopper tank. Main pump failure is indicated by the illumination of its respective amber MAIN PUMP light on the FUEL panel, a master caution chime and a flashing amber FUEL on the CWP.
Fuel filter icing is prevented by an oil-to-fuel heater that
uses PGB oil as its heat source. Heating is regulated by a thermal sensor that controls the flow of oil through the heater. Low fuel temperature triggers illumination of the respective amber FUEL LOW TEMP light on the ENGINE panel, a master caution chime and a flashing amber ENGINE on the CWP.
Before it enters the high pressure pump, the fuel is filtered. If a blockage occurs it triggers illumination of the respective amber FUEL FILTER light on the FUEL panel and master caution alerting with flashing amber FUEL on the CWP. The filter is self-bypassing as necessary to meet engine feed requirements.
Hydro Mechanical Unit
The HMU controls all fuel flow to the engines in conjunction with the DECU. It's integral HP fuel pump is powered by the gas generator accessory gearbox. The HP pump develops the high pressure necessary for proper fuel nozzle operation and operation of the HMU actuator for the inlet guide vanes and start bleed valve. The HP pump inlet is pressurized by the main pump to
prevent the cavitation or air bubbles in the fuel caused by pump operation within the HP pump itself. Fuel cavitation results in decreased lubrication and cooling and increased pump wear. Failure of the HP fuel pump will cause an immediate engine flameout. Fuel shutoff is controlled by the condition lever via a shutoff valve inside the HMU.
The 4 functions of the HMU:
- metered fuel for combustion
- unmetered fuel for inlet guide vane/engine anti-ice start bleed valve operation (hyd actuated with fuel)
- engine shutdown via the condition lever/CL and fuel shut off valve
- overspeed protection (cuts out fuel at 110% Ng)
Overspeed Drain Valve
The fuel metered by the HMU is delivered to the oil cooler and then to the overspeed drain valve. The overspeed drain valve has three functions:
- Provide main fuel flow to the 12 fuel injectors, during engine starting and operation.
- Purge the main fuel manifold when the engine is shutdown. This traps fuel upstream from the ODV during shutdown and thus avoids having to refill the oil cooler with fuel prior to each start.
- Divert fuel flow from being delivered to the engine when the ECU actuates the overspeed solenoid. This 1s accomplished by bypassing all metered fuel through a bypass passage in the Accessory Gearbox.
Constant Torque On Takeoff/CTOT
Automatic Power Reserve/APR functions
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.
The constant torque circuit can only increase the engine torque set by the power lever; it cannot decrease torque below that set by the power lever. Thus, the power lever setting serves as a backup in the event of an DECU failure during takeoff. Also, this authority limit provides the capability to override the pre-set reference torque by advancing the power lever. The constant torque system contains a limiter to avoid exceeding the engine limits while the constant torque system is activated. The CTOT system may be utilized with or without the APR system but if used the APR function increases the good engine torque by 7% in case of an engine failure. The circuit contains a built-in automatic power reserve (APR) that is activated by the aircraft autocoursen computer if the computer senses an engine failure, it will activate a relay which will reset the good engine up 120HP. Neither function can reduce power below the setting selected by the power levers.
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.
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.
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.
The Engine Oil System
The engine oil system supplies oil to lubricate the engine bearings and engine accessory gearbox. The oil tank is integrally constructed within the engine main frame and includes an oil level sight glass for visual oil level checks. Oil is circulated by a scavenge pump assembly which is driven by the engine accessory gearbox. Oil exiting the lubrication pump passes through a bypass type oil filter. Filter blockage is indicated by illumination of the respective amber OIL BYPASS light, a master caution chime and flashing amber ENGINE on the CWP. Oil will bypass the filter as necessary if filter blockage occurs. Oil temperature is monitored by a sensor located at the outlet of the oil filter which supplies a signal to the ENG OIL gauge.
Oil pressure indications come from an oil pressure sensor which monitors the differential pressure between the output of the lubrication pump and the suction of the scavenge pumps. A failure of the pressure sensor is indicated as a rapid drop to zero pressure, or an oil pressure rise to upper limit of the pressure gauge. An oil leak is usually characterized by fluctuating oil pressure followed by a pressure decrease to about 20 psi. In that scenario, oil temperature will remain constant or decrease slightly due to air being introduced into the supply line which causes oil cooling. For alerting purposes, low differential oil pressure (<30 psi) is detected by a separate oil pressure switch which triggers master warning alerting and flashing red ENG OIL PRESS on the CWP. Scavenge oil returning to the oil tank is monitored for metal debris by a magnetic chip detector. The chip detector triggers illumination of the amber CHIP DETECT light, a master caution chime and a flashing amber ENGINE on the CWP. Scavenge oil is cooled by an oil-to-fuel cooler and then directed through the engine inlet frame vanes to aid in inlet anti-icing.
Normal Engine Start/Operation
Engine starting is accomplished with the dual function starter/generator in conjunction with the ignition and fuel control systems. The generator control unit (GCU) provides automatic starter control. There are two ways to start the engines: direct start (detailed below) or motoring start (which unloads the compressor section and extends engine life).
Direct engine starting begins with the:
• Condition lever at START
• Power lever at ground idle
• IGN switch at NORM
At these positions the CL opens the shutoff valve in the HMU which will allow metered fuel to flow to the engine and the PL sets the fuel flow schedule Inside the HMU for start fuel and acceleration to GROUND IDLE. When the START switch is momentarily actuated (two seconds) to initiate the engine starting cycle it activates the related starter and ignition exciter unit, and the IGN light illuminates. Light-off should occur within 20 seconds of starter engagement. At 55% Ng, ignition is terminated simultaneously with starter/generator disengagement and the IGN light extinguishes. The engine will automatically accelerate to GROUND IDLE. The starter/generator is automatically switched to generator mode by its GCU after a 10 second delay if the GEN switch is ON. During cross-generator engine starting, the on-line generator of the running engine assists the batteries throughout the start cycle.
Once GROUND IDLE speed is reached and has stabilized, the CL can be advanced to any position up to MAX PROP SPEED. The PL remains in the GROUND IDLE position. Advancing the CL beyond FEATHER actuates the feathering valve in the Propeller Control Unit (PCU) which will unfeather the propeller and allow it to run at a zero thrust condition. Advancing the CL into the prop speed governing range (between MIN PROP and MAX PROP) closes the torque motor circuit enable switch located in the CL quadrant. Once the propeller has unfeathered, and reached minimum lockout speed, the bottoming governor circuit in the DECU begins comparing the speed signal from the engine to a reference speed signal in the bottoming governor circuit.
When 'dry motoring', the CL is left at the off position, the IGN switch is off and the starter switch is held until Ng reaches approx 20%. The anti-icing/start bleed valve is full open during starting in order to vent fifth stage bleed air overboard and through the anti-icing passages in the air inlet. Unloading the compressor enhances Ng acceleration during starting. In this mode, the engine is motored by the starterfor as long as the START switch is actuated. At 20%, the CL is then moved to the start position and the IGN switch is selected on. After starting, the valve progressively closes as Ng increases and is fully closed at approximately 90% Ng.
During normal aircraft taxi operations, the PL is modulated between GROUND IDLE and FLIGHT IDLE. This action varies propeller pitch mechanically through the PCU. As propeller pitch increases, prop speed starts to decrease due to torque load being applied to the propeller. The bottom governor senses the drop in prop speed and trims fuel flow up to maintain the reference prop speed. When pitch is increased and speed is maintained the prop produces thrust. As the PL is retarded, the pitch on the propeller is reduced and prop speed starts to pick up. This increase in speed is sensed by the bottoming governor and the trim added by the bottoming governor will be reduced to maintain reference prop speed. Movement of CL beyond the MIN PROP SPEED position will set a speed reference within the PCU (constant speed governor) from 75% (1038 RPM) minimum to 100% (1384 RPM) maximum. For takeoff, with CL at its 100% position, PL is advanced beyond flight Idle up to a calculated torque setting. This PL movement will increase propeller speed. As prop speed increases beyond the reference setting, even slightly, the PCU will sense this overspeed and increase propeller pitch to maintain constant propeller speed. This will result in a increase in thrust output from the prop. Likewise, if PL is reduced, resulting in a reduction of engine output power, propeller speed will decrease. The PCU senses even slight underspeeds and will schedule finer pitch to maintain the constant speed set by the CL. Turboprop engines react to increases in forward air speed (ram effect) with a growth in torque and temperature. It is necessary to counteract these changes with a reduction in engine power. To accomplish this, the engine is equipped with a Constant Torque on Takeoff (CTOT) . The pilot sets engine output power intentionally low (15%) in the take off range. Having preset the torque adjustment dial to the required torque setting, (s)he activates the system by turning on the activation switch in the cockpit. The system will compare engine output torque with the required torque for takeoff and trim fuel flow so that they are equal. As ram effect increases engine output, the system continues to monitor engine output and relaxes the trim back to maintain torque and temperature limits. The takeoff torque system will not add fuel flow if built-in torque or temperature limits are reached.
Once the aircraft has departed, the takeoff torque CTOT system is turned off by first turning the adjustment dial down, and then shutting off the activation switch. At this point, the engine is being controlled directly by the pilots modulation of the PLs. Increases in PL position result in increases in thrust and conversely, decreases in PL position result in decreases in thrust. Propeller speed remains constant due to the Constant Speed Governor in the PCU. After landing, the PL is retarded to GROUND IDLE (approximately 72% Ng due to the trim effect of the bottoming governor). If reverse thrust is required during landing rollout, this is accomplished by retarding the PL which, through its connection to the PCU, creates a negative pitch change to brake the aircraft. The DECU through the HMU torque motor, will schedule fuel flow to maintain reference prop speed. To taxi, the PL is modulated between GROUND IDLE and FLIGHT IDLE which manually schedules positive pitch through the PCU. Prior to shutdown, the PL should be at GROUND IDLE which sets the propeller pitch for zero thrust. The CL should then be moved below the MIN PROP position to disable the torque motor circuit. Slowly move the CL to the FEATHER position. This movement of the CL causes the feathering valve in the PCU to feather the propeller. Normal shutdown of the engine is then accomplished by moving the CL to the FUEL OFF position after engine temperatures are stabilized.