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ALL ú ú C03 Page 25 ú Mar 15/97
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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CFM56 ENGINES
737-300/400/500/ /MAINTENANCE MANUAL ////////////////////////
1. The CIT sensor changes the CIT into a hydraulic
_ signal pressure to the MEC. This pressure actuates a servo system that sets the 3D cam axially. A decrease in temperature (cold shift) will decrease the pressure signal to the MEC. The affect is to decrease speed and position the 3D cam to make sure the VSVs will track on the open side of the schedule. An incorrect CIT (cold shift) input could cause a compressor stall. A full (cold shift) failure of the CIT sensor will cause a large drop in hydraulic signal pressure and will move the VSV schedule to its fail-safe position. The result will be a power loss but no stall.
b) N2 Core Engine Speed
1.
ENGINES WITHOUT THE HPTCC TIMER;
_ N2 core engine speed is sensed by the governor and tachometer systems of the MEC. Flyweights in these systems are moved by a shaft and a gear coupling through the fuel pump and the engine accessory drive system. This supplies a control input that is a function of N2. The governor system uses this signal input as feedback to null the power demand input as described in speed governing above. The tachometer system utilizes this signal input to command the position of the turbine clearance valves (TC1 or TC2) and the rotational position of the 3D cam. While N2 increases, the 3D cam starts the opening of the VSV's and establishes the fuel flow acceleration schedule for the engine.
2.
ENGINES WITH THE HPTCC TIMER;
_ N2 core engine speed is sensed by the governor and tachometer systems of the MEC. Flyweights in these systems are moved by a shaft and a gear coupling through the fuel pump and the engine accessory drive system. This provides a control input that is a function of N2. The governor system uses this signal input as feedback to null the power demand input as described in speed governing above. The tachometer system utilizes this signal input to command the position of the turbine clearance valves (TC1, TC2 and TC3 [on engines with HPTCC timer]) and the rotational position of the 3D cam. While N2 increases, the 3D cam starts the opening of the VSV's and establishes the fuel flow acceleration schedule for the engine.
c) Compressor Discharge Pressure (CDP)
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73-21-00
ALL ú ú C02 Page 26 ú Nov 12/00
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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CFM56 ENGINES
737-300/400/500/ /MAINTENANCE MANUAL ////////////////////////
1.
CDP or P3, is piped to the MEC from a port in the
_ compressor case. The CDP sensor, in the MEC, changes the pneumatic (P3) pressure to a hydraulic pressure and through the CDP servo system turns the CDP cam. The CDP cam, 3D cam and their computer linkage system makes a fuel flow schedule. As CDP increases, the fuel flow schedule is increased. A low CDP anomaly such as a leak or failure of the CDP tube or a blockage or failure of the CDP sensor bellows would result in a reduction of the fuel flow schedule which would cause the engine to lose power or prevent an acceleration of the engine to a desired power level.
2.
The CDP input is biased by a CBP measurement to
_ provide automatic resetting of the acceleration schedule to maintain rapid acceleration times when airplane bleed demands are large and provide adequate stall margins when bleed demands are lower. The CBP signal is obtained from a venturi sensor put on the CDP extraction port. The pressure measurement from the venturi, which is a function of the flow rate, is transmitted by tubing to the control CBP sensor and emposes a bias on the CDP sensor which changes CDP cam positioning to allow for a higher acceleration rate when airplane bleed rates are higher.
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