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.
• Intersystem interfaces.
• Propulsion-control operations.

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:

• Start and Non-start Automatic Protection Features (these features are disabled for inflight starts). Several new features help prevent engine damage if an abnormal ground engine start occurs.

  -- Wet-start protection stops fuel and ignition if the exhaust-gas temperature (EGT) doesn't increase within 15 seconds after the engine start lever is moved to IDLE.

  -- A hot-start alert blinks the EGT-readout outline if the EGT is too hot for the current N₂ speed.

  -- Hot-start protection stops fuel and ignition if the EGT exceeds the start limit of 725ĄC. The flight crew's engine-start procedures do not change because of these new features; the crew still must sequence the start controls, monitor the engine indications, and act promptly if the start does not proceed normally.

  -- Engine rollback protection (active only on the ground) stops fuel and ignition if the engine, once started, decelerates to less than a sustainable idle speed and the EGT exceeds the start limit.

  -- Flameout protection turns the ignition on if an engine control detects an uncommanded engine deceleration. This must happen in order to relight the engine if it has flamed out but fuel is still available. The control turns ignition off after 30 seconds or when engine speed is less than 50% N₂.

• No-dispatch Alert. An amber ENGINE CONTROL light on the aft-overhead panel (figure 5) illuminates when the airplane is on the ground and an engine-control fault prevents airplane dispatch. The light is suppressed in flight because there is no defined flight crew procedure for this condition. If the ENGINE CONTROL light comes on after landing, the flight crew should notify maintenance personnel immediately, because the associated fault must be fixed before the airplane can be redispatched. If the ENGINE CONTROL light comes on after engine start, takeoff is prohibited.

• Alternate thrust-setting mode. The propulsion controls have two thrust-set modes: normal and alternate. In normal mode, the engine control uses flight condition data from the airplane air data system to compute the command N₁. If valid flight condition data is not available, the engine control switches to alternate mode, which calculates command N₁ from a different thrust-lever-to-N₁ schedule. At the mode change a temporary N₁-speed offset prevents a thrust change. The amber ALTN light in the EEC switch (figure 5) alerts the flight crew that the alternate mode is active. (The EEC switches replace the 737-300/-400/-500 power management control switches, which have a similar function.) To remove thrust-lever stagger that may develop as flight conditions change, the flight crew retards both thrust levers to mid-power and operates both EEC switches. This puts both engines in alternate mode and removes the N₁-speed offset. When in alternate mode, thrust can exceed the certified engine rating at forward thrust-lever positions. To avoid overboost, flight crews should use the flight management computercalculated thrust limit to set thrust for the current flight mode (takeoff, climb, or cruise). This limit is shown as a green "crows-foot" N₁-reference cursor (figure 6).

• Performance-reserve thrust. The CFM56-7 engines on the next-generation 737s can be operated at one of six thrust ratings. Table 1 lists the available engine models, and which engine models can be used on each 737 model.

  Depending on the airplane-engine model combination, extra performance-reserve thrust may be available for emergency use during takeoff and go-around. For example, performance-reserve thrust is available for a 737-700 with -7B22 engines, since the -700 airplane can accept the higher -7B24 thrust. The engine control allows takeoff/go-around thrust up to this rating when the thrust lever is pushed full forward. If the installed engine has the highest rating offered for that 737 model (for instance, a 737-600 with the -7B22 rating), there is no performance-reserve capability. Like the "overboost" thrust of the 737-100/-200/-300/-400/-500, performance-reserve thrust is for emergency use only.

• Engine indication enhancements. The common display system N₁ thrust setting indication is shown in figure 6. Changes to this indication from the 737-300/-400/-500 are:

  -- REV indications (green or amber) replace amber REVERSER UNLOCKED lights.
  -- A command sector is displayed during engine acceleration and deceleration.
  -- A two-color TAI indication shows the engine thermal anti-ice status.
  -- The entire indication turns red if N₁ exceeds the redline speed.
  

  Other engine indication changes include:

• An amber ENG FAIL appears on the EGT indication if N₂ speed decreases to less than idle when the Engine Start Lever is in IDLE.

• A red radial mark indicates the EGT Start Limit when the engine is not running.

• In flight, an amber X-BLD START message appears above the N₂ indication if starter assist is required.

• The EGT and N₂ indications change color to red if the current value is greater than the redline, and the EGT indication becomes amber if the temp-erature is in the amber band range.

• The oil-pressure-indication amber band varies with engine speed.

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.
• Flight Management Computer/Control Display Unit (FMC/CDU) engine-maintenance pages.
• Other airplane systems.
• Engine overspeed and overtemperature.

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

• No dispatch. An ENGINE CONTROL light indicates that the propulsion controls are in a no-dispatch condition.

• Minimum Equipment List (MEL) dispatch. The airplane MEL defines the dispatch requirements if an engine control is in the alternate thrust-setting mode (an ALTN light is on).

• Time-limited dispatch. A time-limited-dispatch condition results from a fault that has no immediate consequences to engine operation. However, the airplane cannot be operated indefinitely this way, as the fault reduces system redundancy, which in turn increases the probability of engine shutdown.

  The U.S. Federal Aviation Administration has defined two time-limited-dispatch intervals for fixing a time-limited-dispatch fault: short-time (typically 150 hours) and long-time (typically 500 hours).

  Because time-limited-dispatch faults are not indicated to the flight crew, maintenance personnel must periodically use the flight management computer/ control display unit (FMC/CDU) maintenance pages to check for them. Each airline must have an inspection and repair policy that ensures that these faults will be found and fixed before the operating time limit expires. Assuming a 10-hour daily airplane utilization, a weekly check allows up to eight days to fix a short-time fault.

• Unlimited dispatch. If no time-limited-dispatch or no-dispatch faults occur and the ALTN light does not show, the propulsion controls are in the unlimited dispatch condition. However, the propulsion controls may still have economic faults; that is, operational equipment faults that do not affect airplane operations. These faults should be repaired when convenient to ensure continued operation of the affected functions.

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.