737-600/-700/-800/-900 Propulsion Control System


Next-generation 737 airplanes (737-600/-700/-800/-900) feature a new, electronically based propulsion control system that almost completely replaces the hydromechanical systems used in earlier and current-generation 737 models (737-100/-200/-300/-400/-500). One of the principal differences is the addition of the electronic engine control (EEC), which continuously looks for and alerts flight crews to several levels of faults that could affect engine operation. Flight crews will find that the new PCS looks and feels much like the systems in previous models while representing improvements to the operability, capability, reliability, and maintainability of those systems. In addition, maintenance crews will find that many tools useful to them are built into the system.

Electronic engine control is the key feature of the improved propulsion control system (PCS) on all next-generation 737 airplanes. Installed on the CFM56-7 engines of 737-600, 737-700, 737-800, and 737-900 airplanes, this new type of PCS is designed for maximum engine performance, optimum engine operability, and effective integration with other airplane systems.

Full-authority digital-electronic engine controls (FADEC) are not new; the first such system entered commercial service on the Boeing 757 in 1984, and most new jetliners have this capability. The FADEC in the PCS on next-generation 737s replaces the hydromechanical control on 737-100/-200 models, and the electronic-supervisory control on 737-300/-400/-500 models. (The various types of engine-control systems are described in the April-June 1988 issue of Airliner magazine.)

The chief differences between the PCS in the next-generation 737s and earlier 737s fall into three categories:

1. Components and installations.
2. Flight operations.
3. Maintenance operations.

1 Components and Installations
The 737-600/-700/-800/-900 propulsion controls look, feel, and work very much the same way as those of previous 737s, even though many components (and the way they operate) are completely different. For example, thrust-set and engine-fuel on/off control are done electrically, not by mechanical control cables; most interfaces with other airplane systems are now digital; and many of the engine displays in the flight compartment are driven by the engine controls. The following are the major system components and installations of the PCS:

ELECTRONIC ENGINE CONTROL (EEC).
The primary propulsion-control component is the electronic engine control (EEC) (figure 1). An EEC is installed on the fan case of each engine.

The EEC receives inputs from the airplane and engine sensors, computes the desired engine thrust in terms of fan speed (N1), and sends electrical commands to the various engine actuators to make the engine accelerate or decelerate to this desired N1--quickly, accurately, and without surges, rotor-speed overshoots, or other instabilities.

In addition to governing engine operation, the EEC acquires, processes, and outputs data for the flight-compartment displays and for maintenance use; detects and accommodates faults that would otherwise impair engine operation; and can be operated in an interactive maintenance mode.

HYDROMECHANICAL UNIT (HMU).
This unit, as shown in figure 2, is installed on the aft-left side of the accessory gearbox.

The HMU contains the fuel metering valve that controls the fuel sent to the combustor, and other control valves that operate the variable stator vanes, variable intercompressor bleed valve, turbine active-clearance-control system, and fuel-nozzle staging.

The HMU also contains the fuel high-pressure shutoff valve (HPSOV), which closes directly from the flight compartment start lever CUTOFF command.

EEC ALTERNATOR.
The EEC alternator (figure 3) supplies each EEC channel with primary electrical power. It is installed on the forward face of the accessory gearbox.

The EEC alternator powers the EEC at engine speeds greater than 12% N2. At lesser speeds, the EEC uses 115-V ac power from the airplane electrical system. When the engine is shut down, power is turned off.

2 Flight Operations
The new PCS results in several operational differences, though most of these are invisible to the flight crew. They are also similar enough to operations in earlier 737s to allow flight crews of earlier and next-generation 737s to retain the same type rating. The differences are in the following categories:

AISLE-STAND ENGINE CONTROLS.
To the flight crew, the aisle-stand engine controls (figure 4) are unchanged, but the installations inside the aisle stand and beneath the floor have been completely redesigned.

For each engine, a connecting rod transfers the flight crew's thrust-lever command to the auto-throttle assembly, where a double-resolver unit sends an electrical thrust command to each EEC channel. (When the autothrottle is engaged, servo-motors position both resolvers, back-driving the thrust levers through the connecting rods so that the thrust levers reflect the autothrottle command.)

To select reverse thrust after landing, the flight crew lifts the reverse-thrust levers. An electrically operated "balk" blocks each lever at the reverse-idle position until the thrust reversers deploy. Then each balk is removed to allow selection of full-reverse thrust. This electrically operated balk replaces the thrust control cable interlock used on previous 737s.

The engine start levers no longer operate mech-anical cables. A start-lever-operated electrical switch signals a fuel high-pressure shutoff valve (HPSOV) solenoid. Two new ENG VALVE CLOSED indicator lights on the fuel panel show the HPSOV status (open, closed, or in-transit).

INTERSYSTEM INTERFACES.
The propulsion controls have important interfaces with other airplane systems: the common display system, the flight management system, and the autothrottle. ARINC-429 digital databuses transfer data between the EECs and these systems for efficient integrated operation.

PROPULSION-CONTROL OPERATIONS.
Several new PCS features cause some subtle changes in engine operation from earlier 737s. These features are described below:

2 Maintenance Operations
The 737-600/-700/-800/-900 propulsion-control maintenance procedures are significantly different than those of earlier 737s. Specifically, maintenance personnel must know how and when to check the following:

DISPATCH STATUS.
Maintenance personnel must perform periodic checks of the propulsion-control dispatch status. Since EEC logic detects and accommodates many faults, the engine can operate normally when faults exist. For example, a complete failure of one EEC channel has no immediate effect on engine operation because the second channel takes over. The ENGINE CONTROL lights and messages on the FMC/CDU maintenance screens report these non-obvious faults.

The propulsion controls have four basic levels of operational health, listed below in order of improving capability:

FLIGHT MANAGEMENT COMPUTER/CONTROL DISPLAY UNIT (FMC/CDU) ENGINE MAINTENANCE PAGES.
Figure 7 shows the top Engine-1 maintenance page on the FMC/CDU. The CDU menu pages allow maintenance personnel to check for faults in each dispatch category; perform functional tests; check for engine speed or temperature exceedances; monitor EEC input signals; and review the engine control configuration.

OTHER AIRPLANE SYSTEMS.
The propulsion controls have several built-in tests that are accessed through the FMC/CDU maintenance pages. When the engine pages are called up the EEC is automatically powered. Maintenance tests of other airplane systems, such as the autothrottle, require that the propulsion controls be manually switched on so the EECs can communicate with that system. To power an EEC, the flight crew sets the engine start switch to CONT. After the tests, the flight crew places the start switch back to OFF and exits the FMC/CDU engine maintenance pages so that the EEC depowers.

ENGINE OVERSPEED AND OVERTEMPERATURE.
After both engines are shut down, if the readout box for N1, N2, or EGT turns red, an engine overspeed or overtemperature has occurred. The exceedence magnitude and duration is shown on the FMC/CDU exceedences maintenance page. The maintenance manual specifies what maintenance action, if any, is required.

Summary
Boeing and CFMI designed the next-generation 737s with a propulsion control system (PCS) that maximizes engine efficiency and operability. The PCS design of 737-600/-700/-800/-900 airplanes is a full-authority digital-electronic engine control, or FADEC, which is significantly different than the PCS on all earlier 737 models. Though the FADEC-based PCS contains several enhancements, the flight crew will notice few changes from earlier 737s. In addition, maintenance personnel will appreciate the built-in maintainability tools that will help them solve problems quickly.



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