1.
General
The purpose of the fuel regulation system is to control the quantity of fuel injected into the combustor as a function of the volume of air passing through the engine, whatever the flight conditions (speed, altitude, pressure, temperature, etc…), while maintaining operational safety.
The system comprises:
• the hydromechanical fuel control unit which processes commands received from the electronic engine computer in the normal mode of operation and responds to the power lever position in manual mode,
• the digital electronic computer,
• the surge bleed valve controlled by the computer.
2.
Description
Refer to fig. 3 and fig. 4
A.
Hydromechanical fuel control unit (FCU)
The unit is installed on the same shaft as the fuel pump. Either component can be replaced separately.
The FCU mainly consists of:
• an overspeed governor driven by the pump drive shaft and measuring the speed of the HP spool,
• the power lever shaft which controls a potentiometer and the fuel shut-off valve,
• a P3 pressure limiter (HP compressor outlet pressure),
• a flow regulator receiving P3 pressure and a signal from the computer,
• a torque motor for fuel flow regulation,
• an operating mode selector electric valve (normal/manual),
• an overspeed fuel shut-off valve,
• a maximum thrust adjustment screw for manual mode,
• a sight window for observation of the graduated FCU quadrant.
B.
Digital electronic computer (N2 control) without SB F900-320-R1
Refer to fig. 6 and fig. 5
engine 2 computer is installed in the rear compartment; engine 1 and 3 computers are in the baggage compartment.
The computers are supplied with 28 V DC power, controlled by the “CMPTR” switch (2EP1)/(2EP2)/(2EP3), for A/C < 179) or (L2EP)/(M2EP)/(R2EP), for A/C ? 179) on the overhead panel:
• engine 1 computer is supplied from primary bus “A” (bus “A1”).
• A standby power supply for engine 1 computer is available from bus “B” (bus “B1”).
• engine 2 computer is supplied from primary bus “B” (bus “B1”).
• engine 3 computer is supplied from primary bus “A” (bus A2″).
When a computer is not in operation, indication is given by a “CMPTR” light on warning panel (2WW).
The “CMPTR” switch has three positions: “AUTO”, “MAN” and “OFF”. The normal position is “AUTO”.
In the event of a failure (illumination of “CMPTR” light on warning panel), the “MAN” position can be selected in order to maintain computer power supply and the N1 and N2 overspeed safety systems in manual mode. In this case, the corresponding “CMPTR” light remains on.
NOTE:
On A/C ? 179, the illumination of the “CMPTR 1”, “CMPTR 2” or “CMPTR 3” light causes the illumination of the “MASTER CAUTION” amber lights (L4WW) and (R4WW) simultaneously.
In the “OFF” position, the power supply to the computer is cut off.
Each computer receives the following data:
• power lever position (supplied by the potentiometer),
• air intake pressure and temperature (Pt2-Tt2),
• LP and HP spool speeds (N1 – N2),
• T5 temperature,
• flight/ground signal,
• “PWR INC” at take-off signal.
In addition, each computer can receive signals from the following optional systems:
• N1 or N2 synchronizer,
• APR (Automatic Power Reserve): power increase at take-off if one engine is inoperative,
• PWS (Power Management System): optimization of engine speed in flight through a cruise computer.
Each computer supplies the following signals:
• fuel schedule,
• manual operating mode electric valve control signal,
• overspeed safety electric valve control signal,
• surge bleed valve control signal (two electric valves),
• “CMPTR” warning light triggering signal,
• end-of-starting phase signal.
The front panel of each computer includes the following components:
• Pt2 air intake pressure tube coupling,
• air filter,
• test connector (J2),
• wiring harness connector (J1),
• 11-position function selector switch,
• 4-digit liquid crystal display,
• a panel listing the 12 failure codes which can be displayed,
• a guarded spring-loaded calibration switch.
C.
Digital electronic computer (N1 control) with SB F900-320-R1
Refer to fig. 7 and fig. 5
engine 2 computer is installed in the rear compartment; engine 1 and 3 computers are installed in the baggage compartment.
The computers are supplied with 28 V DC power, controlled by the “CMPTR” switch (2EP1)/(2EP2)/(2EP3), for A/C < 179) or (L2EP)/(M2EP)/(R2EP), for A/C ? 179) on the overhead panel:
• engine 1 computer is supplied from primary bus “A” (bus “A1”).
• A standby power supply for engine 1 computer is available from bus “B” (bus “B1”).
• engine 2 computer is supplied from primary bus “B” (bus “B1”).
• engine 3 computer is supplied from primary bus “A” (bus A2″).
A computer not in operation is indicated by the illumination of a “CMPTR” light on warning panel (2WW).
NOTE:
On A/C ? 179, the illumination of the “CMPTR 1”, “CMPTR 2” or “CMPTR 3” light causes the illumination of the “MASTER CAUTION” amber lights (L4WW) and (R4WW) simultaneously.
In the “OFF” position, the power supply to the computer is cut off.
Each computer receives the following data:
• power lever position (supplied by the potentiometer),
• air intake pressure and temperature (Pt2-Tt2),
• static pressure (Ps0) connected to engine 1 and 3 computers (which are located in pressurized areas),
• LP and HP spool speeds (N1 – N2),
• T5 temperature,
• flight/ground signal,
• “PWR INC” at take-off signal.
In addition, each computer can receive signals from the following optional systems:
• N1 or N2 synchronizer,
• APR (Automatic Power Reserve): power increase at take-off if one engine is inoperative,
• PWS (Power Management System): optimization of engine speed in flight through a cruise computer.
Each computer supplies the following signals:
• fuel schedule,
• manual operating mode electric valve control signal,
• overspeed safety electric valve control signal,
• surge bleed valve control signal (two electric valves),
• “CMPTR” warning light triggering signal,
• end-of-starting phase signal.
The front panel of each computer includes the following components:
• Pt2 air intake pressure tube coupling,
• Ps0 static pressure,
• test connector (J2),
• wiring harness connector (J1).
D.
Surge bleed valve
Refer to fig. 2
This electropneumatic valve enables air bled from the LP compressor outlet to be discharged into the fan duct.
The valve has three positions: “closed”, “1/3 open” and “fully open”. The position is controlled by two solenoid valves (1/3 open, fully open) located in the fan duct shroud and controlled by the electronic computer.
3.
Operation
Refer to fig. 1
A.
Normal engine operation (N2 control) without SB F900-320-R1
The computer performs the following functions:
• conversion of analog input signals into digital signals in order to calculate output parameters, then conversion of these parameters into analog signals corresponding to fuel schedule and the position of the surge bleed valve,
• during starting, regulation of fuel flow, limitation of T5 temperature and automatic starting sequence interruption,
• holding ground or flight idle, when the power lever is set to “idle”,
• limitation of maximum thrust, when the power lever is set to “full power” (in particular, regulation of the “flat rated” thrust according to Pt2/Tt2 conditions),
• N2 power setting regulation, either directly at take-off or idle, or through a T5 regulation loop at intermediate power settings,
• establishing acceleration and deceleration fuel flowrates or schedules,
• N1 limitation to 100% stabilized with compensation (LP fan/compressor power setting),
• limitation of transient N1 overspeed during slam acceleration,
• protection against compressor surge,
• protection against N1 or N2 overspeed,
• protection against overheating (maximum permissible T5 temperature),
• monitoring of peripherals and computer internal circuits,
• power increase at take-off (at high altitudes or in warm weather conditions).
(1)
General operating principle
The computer determines the N2 power setting to be held for a given power lever position, from idle to full power, and for given Pt2/Tt2 conditions.
(2)
Power lever set to “full power” position
Regulation calculations are made directly in terms of N2 speed. The computer is programmed for maximum N2 as a function of T2 temperature. This basically corresponds to operation at a constant maximum T5 turbine temperature, producing an increasing thrust when the T2 temperature decreases.
Below a “flat-rating” T2 temperature (as a function of P2 pressure), a second N2 program limits the thrust to a constant value.
N2 is only held at the value computed by one of these two programs if the engine is not limited by the maximum N1 speed circuit (100% – with compensation) or the maximum T5 turbine temperature circuit (952°C (1745°F) or 978°C (1792°F) after SB F900-100 ).
(3)
Power lever set to “idle” position
Regulation calculations are made directly in terms of N2 speed. The computer is programmed for N2 power setting as a function of P2 pressure to produce the idle setting necessary to comply with the requirements of 5-second acceleration test bench performance. This program is used during flight. It corresponds to 350 lb test bench thrust.
A second program produces a reduced idle setting, corresponding to 220 lb test bench thrust. This program is used on the ground. Acceleration time from this idle setting is longer by about 1.8 seconds.
The computer automatically selects either idle setting as a function of a ground/flight signal. The ground signal is obtained only when the two main landing gear shock absorbers are compressed.
The power lever has a positive idle stop.
(4)
Power lever set to a position between idle and full power
Reduction of the power lever angle from full power produces a reduced power setting by comparing two calculation channels:
• N2 reduction in relation to full power N2 by means of a differential N2 correction program as a function of the power lever angle,
• T5 reduction in relation to full power T5 by means of a differential T5 correction program as a function of the power lever angle.
For cruise and climb power settings, the T5 loop has priority.
This makes climb possible with constant power lever position and constant T5.
Should the T5 signal fail or there be a short-circuit, the T5 loop is broken and regulation remains in terms of N2.
For power settings lower than cruise, regulation is by the N2 channel.
(5)
Starting
During the starting sequence, the fuel schedule is programmed at a constant fuel ratio of 6, up to a corrected N2 of about 20%, then increases with N2.
In the initial phase, the fuel schedule is increased by a fuel ratio of 5 by an enrichment system, which is automatically inhibited as soon as T5 temperature exceeds 400°F (204°C).
A T5 loop monitors the turbine temperature and reduces the fuel schedule if T5 reaches a maximum of 1350°F (732°C). This loop becomes inoperative as soon as the idle setting is reached.
The computer has an integral circuit enabling automatic interruption of the starting sequence at N2 = 50%.
(6)
Acceleration
Fuel flow during acceleration above the idle power setting is limited so as to avoid excessive T5 temperature. The corresponding program is a function of T2 temperature and N1 speed.
The fuel flow can also be limited by the surge protection system. This system includes a maximum fuel flow program as a function of N1 and N2 speeds with reduction factors for increased altitude or for acceleration following a deceleration.
(7)
Deceleration
The fuel flow reduction corresponding to the power lever position can be limited by the circuit which computes the minimum required flow.
At low power settings, this circuit determines the fuel flow required to avoid lean blow-out as a function of T2 temperature and N2 speed.
Moreover, it also determines a minimum fuel flow as a function of N1 speed and the position of the surge bleed valve.
These two flow calculations are compared with the minimum required fuel flow: the greatest of the three values is selected as the minimum required fuel flow.
(8)
Normal mode protections
(a)
Surge protection
Refer to the above paragraphs (Acceleration – Deceleration).
(b)
Overspeed
N2 regulation limits the HP spool power setting to 100% stabilized or 100.8% after SB F900-100 , although a transient setting of 103% is possible.
The N1 limiter keeps the LP spool power setting at 100% – maximum stabilized compensation, with the possibility of transient settings of 103%.
The overspeed governor (hydromechanical element) prevents the HP spool power setting from exceeding 104% by commanding modification of the fuel flow regulator valve position in response to P3 pressure received.
A computer circuit controlling the overspeed safety fuel shut-off valve can completely cut off fuel supply should the LP spool power setting reach 107% or the HP spool 109%. The control circuit has two branches, one digital, the other analog: any discrepancy between the two prevents action being taken against overspeed.
(c)
Overheating
Maximum T5 temperature limiter (952°C or 978°C (1745°F or 1792°F) after SB F900-100 ) and fuel flow limiter during acceleration (refer to above paragraphs). This limit is extended to 974°C or 996°C (1785°F or 1825°F) after SB F900-100 during a take-off with power increase.
(d)
Internal pressure
The hydromechanical fuel control unit P3 pressure limiter (limiting HP compressor discharge pressure) prevents excess pressure in the combustor by commanding P3 pressure leakage, thereby causing modification of the flow regulator valve position.
(9)
Monitoring
(a)
General
The digital electronic computer incorporates a number of circuits monitoring the principal internal and peripheral functions: these circuits make up the “BITE” (Built-in Test Equipment).
The occurrence of a failure causes a switch to the manual mode of regulation and illumination of the “CMPTR” light in the cockpit.
The computer memorizes transient failures for post-flight analysis if required.
(b)
Monitored circuits
Peripheral circuits
CODE ON COMPUTER FRONT PANEL
CIRCUIT
01
Tt2 sensor
02
Surge bleed valve solenoid A
03
Surge bleed valve solenoid B
04
Torque motor
05
Power lever potentiometer
06
T5 wiring harness
07
Manual mode selector solenoid
08
Overspeed safety valve solenoid
09
N1 monopole
10
N2 monopole
(c)
Internal circuits
Computer internal failures are indicated by code 11.
Settings outside maximum tolerances are indicated by code 12.
B.
Normal engine operation (N1 control) with SB F900-320-R1
The computer performs the following functions:
• conversion of analog input signals into digital signals in order to calculate output parameters, then conversion of these parameters into analog signals corresponding to fuel schedule and the position of the surge bleed valve,
• during starting, regulation of fuel flow, limitation of T5 temperature and automatic starting sequence interruption,
• holding ground or flight idle, when the power lever is set to “idle”,
• limitation of maximum thrust, when the power lever is set to “full power” (in particular, regulation of the “flat rated” thrust according to Pt2/Tt2 conditions),
• N1 power setting regulation, either directly at take-off or idle, or through a T5 regulation loop at intermediate power settings,
• establishing acceleration and deceleration fuel flowrates or schedules,
• N1 limitation to 100% stabilized with compensation (LP fan/compressor power setting),
• limitation of transient N1 overspeed during slam acceleration,
• protection against compressor surge,
• protection against N1 or N2 overspeed,
• protection against overheating (maximum permissible T5 temperature),
• monitoring of peripherals and computer internal circuits,
• power increase at take-off (at high altitudes or in warm weather conditions).
(1)
General operating principle
The computer determines the N1 power setting to be held for a given power lever position, from idle to full power, and for given Pt2/Tt2 conditions.
(2)
Power lever set to “full power” position
Regulation calculations are made directly in terms of N1 speed. The computer is programmed for maximum N1 as a function of T2 temperature. This basically corresponds to operation at a constant maximum T5 turbine temperature, producing an increasing thrust when the T2 temperature decreases.
Below a “flat-rating” T2 temperature (as a function of P2 pressure), a second N1 program limits the thrust to a constant value.
N1 is only held at the value computed by one of these two programs if the engine is not limited by the maximum N1 speed circuit (100% – with compensation) or the maximum T5 turbine temperature circuit (952°C (1745°F) or 978°C (1792°F) after SB F900-100 ).
(3)
Power lever set to “idle” position
Regulation calculations are made directly in terms of N1 speed. The computer is programmed for N1 power setting as a function of P2 pressure to produce the idle setting necessary to comply with the requirements of 5-second acceleration test bench performance. This program is used during flight. It corresponds to 350 lb test bench thrust.
A second program produces a reduced idle setting, corresponding to 220 lb test bench thrust. This program is used on the ground. Acceleration time from this idle setting is longer by about 1.8 seconds.
The computer automatically selects either idle setting as a function of a ground/flight signal. The ground signal is obtained only when the two main landing gear shock absorbers are compressed.
The power lever has a positive idle stop.
(4)
Power lever set to a position between idle and full power
Reduction of the power lever angle from full power produces a reduced power setting by comparing two calculation channels:
• N1 reduction in relation to full power N1 by means of a differential N1 correction program as a function of the power lever angle,
• T5 reduction in relation to full power T5 by means of a differential T5 correction program as a function of the power lever angle.
For cruise and climb power settings, the T5 loop has priority.
This makes climb possible with constant power lever position and constant T5.
Should the T5 signal fail or there be a short-circuit, the T5 loop is broken and regulation remains in terms of N1.
For power settings lower than cruise, regulation is by the N2 channel.
(5)
Starting
During the starting sequence, the fuel schedule is programmed at a constant fuel ratio of 6, up to a corrected N2 of about 20%, then increases with N2.
In the initial phase, the fuel schedule is increased by a fuel ratio of 5 by an enrichment system, which is automatically inhibited as soon as T5 temperature exceeds 400°F (204°C).
A T5 loop monitors the turbine temperature and reduces the fuel schedule if T5 reaches a maximum of 1350°F (732°C). This loop becomes inoperative as soon as the idle setting is reached.
The computer has an integral circuit enabling automatic interruption of the starting sequence at N2 = 50%.
(6)
Acceleration
Fuel flow during acceleration above the idle power setting is limited so as to avoid excessive T5 temperature. The corresponding program is a function of T2 temperature and N1 speed.
The fuel flow can also be limited by the surge protection system. This system includes a maximum fuel flow program as a function of N1 and N2 speeds with reduction factors for increased altitude or for acceleration following a deceleration.
(7)
Deceleration
The fuel flow reduction corresponding to the power lever position can be limited by the circuit which computes the minimum required flow.
At low power settings, this circuit determines the fuel flow required to avoid lean blow-out as a function of T2 temperature and N1 speed.
Moreover, it also determines a minimum fuel flow as a function of N1 speed and the position of the surge bleed valve.
These two flow calculations are compared with the minimum required fuel flow: the greatest of the three values is selected as the minimum required fuel flow.
(8)
Normal mode protections
(a)
Surge protection
Refer to the above paragraphs (Acceleration – Deceleration).
(b)
Overspeed
N2 regulation limits the HP spool power setting to 100% stabilized or 100.8% after SB F900-100 , although a transient setting of 103% is possible.
The N1 limiter keeps the LP spool power setting at 100% – maximum stabilized compensation, with the possibility of transient settings of 103%.
The overspeed governor (hydromechanical element) prevents the HP spool power setting from exceeding 104% by commanding modification of the fuel flow regulator valve position in response to P3 pressure received.
A computer circuit controlling the overspeed safety fuel shut-off valve can completely cut off fuel supply should the LP spool power setting reach 107% or the HP spool 109%. The control circuit has two branches, one digital, the other analog: any discrepancy between the two prevents action being taken against overspeed.
(c)
Overheating
Maximum T5 temperature limiter (952°C or 978°C (1745°F or 1792°F) after SB F900-100 ) and fuel flow limiter during acceleration (refer to above paragraphs). This limit is extended to 974°C or 996°C (1785°F or 1825°F) after SB F900-100 during a take-off with power increase.
(d)
Internal pressure
The hydromechanical fuel control unit P3 pressure limiter (limiting HP compressor discharge pressure) prevents excess pressure in the combustor by commanding P3 pressure leakage, thereby causing modification of the flow regulator valve position.
(9)
Monitoring
The N1 DEEC continuously monitors the necessary parameters and events, and periodically stores them in data buffers located within the N1 DEEC. These data buffers are then down-loaded through the ECTM system for evaluation of engine usage, updating of the engine log-book, and determination of required maintenance actions.
C.
Manual mode operation
(1)
General operating principle
Should the following failures occur:
• computer failure,
• computer power supply failure (voltage dropped below the 12.5 ± 0.5 V alert threshold, and not returned to the 15 ± 0.5 V nominal threshold),
the computer is automatically cut off and the “CMPTR” light illuminates on warning panel (2WW).
When the “CMPTR” light comes on, the computer control switch must be set to “MAN”. This maintains energization of the N1 and N2 overspeed safety circuit, which therefore remains operational as long as the failure is not a power supply failure, an N1 or N2 or overspeed circuit failure.
Should the regulation system function abnormally without the “CMPTR” light coming on, the pilot can also set the switch to “MAN” with the same result as in the previous case. If the switch were set to “OFF”, the overspeed safety circuit would be cut off.
In both cases, manual mode is adopted by closing of a fuel control unit electric valve (normally excited open by the computer). By means of a cam, the power lever drives a variable stop which shifts the overspeed governor threshold (set at 104% N2 in normal mode) and which, in manual mode, will limit N2 to lower values, in accordance with the power lever position.
The engine can be started in manual mode. Since T5 monitoring is no longer provided by the computer, the pilot must pay particular attention to this parameter. Fuel flow during starting in manual mode is greater than during normal starting. Moreover, there is no automatic starting sequence interruption at 50%. The pilot must manually interrupt the sequence by use of the “START STOP” switch located on the overhead panel.
(2)
Protections
Anti-surge: in the event of computer failure, the surge bleed valve adopts the “1/3 open” position. Sharp movements of the power lever should be avoided since the valve cannot open completely.
Maximum fuel flow during acceleration is also less in manual mode, which reduces the risk of surges.
Loss of T5 temperature monitoring: the pilot must pay attention to the T5 temperature indication, particularly in hot weather at high power settings.
Overspeed: as mentioned above, overspeed protection is generally maintained for N1 and N2 (107% and 109%) by setting the computer switch to “MAN”. At the same time, N2 is manually limited by the hydromechanical fuel control unit.
P3 pressure limiter: remains operative.
(3)
Aircraft performance
In manual mode, take-off thrust can be reduced by 20% (in hot weather).
Specific fuel consumption increases (by approximately 3% in average conditions) as a result of surge bleed valve opening.
Idle thrust is generally greater than thrust in normal “AUTO” mode (to be borne in mind when landing).
Figure 1: FUEL REGULATION SYSTEM – NORMAL MODE
Figure 2: SURGE CONTROL – LOCATION OF COMPONENTS – OPERATION
Figure 3: ENGINE COMPUTER – PRINCIPLE DIAGRAM
Figure 4: ENGINE COMPUTER CONTROLS AND INDICATIONS
Figure 5: WIRING DIAGRAM (A/C < 179)
Figure 6: N2 DEEC
Figure 7: N1 DEEC
