Controlling lubricant flow in epicyclic gearbox

1. A lubrication system for a turbofan engine comprising:

a pump for driving lubricant through a lubrication circuit, wherein the lubrication circuit includes a first lubricant line for providing a first flow of lubricant to a first portion of a speed change device and a second lubricant line for providing a second flow of lubricant to a second portion of the speed change device;

at least one sensor for generating a signal indicative of an engine operating condition;

a first valve for controlling the first flow of lubricant to the first portion of the speed change device through the lubrication circuit;

a second valve for controlling the second flow of lubricant to the second portion of the speed change device through the lubrication circuit; and

a controller for controlling operation of the first valve and the second valve based on the engine operating condition, wherein the controller is configured to vary each of the first flow of lubricant to the first portion of the speed change device and the second flow of lubricant to the second portion of the speed change device based on a plurality of predefined lubricant flow rates corresponding to the engine operating condition, wherein the plurality of predefined lubricant flow rates vary over a range of engine power settings during operation of the turbofan, the second flow of lubricant to the second portion of the speed change device being varied independent of the variation of the first flow of lubricant to the first portion of the speed change device.

2. The lubrication system as recited in claim 1 , wherein the engine operating condition comprises horsepower generated by at least one turbine section.

3. The lubrication system as recited in claim 1 , wherein the at least one sensor comprises a first sensor for generating a signal indicative of a pressure within a compressor section, a second sensor for generating a signal indicative of rotor speed, and a third sensor for generating a signal indicative of altitude.

4. The lubrication system as recited in claim 3 , wherein the compressor section comprises a high-pressure compressor and a low-pressure compressor and the first sensor generates a signal indicative of a pressure proximate an exit of the high-pressure compressor.

5. The lubrication system as recited in claim 1 , wherein the controller is configured to control the first valve and the second valve to vary the flow of lubricant to the first portion and the second portion of the speed change device for engine conditions between Sea Level Takeoff Thrust and a cruise condition.

6. The lubrication system as recited in claim 4 , wherein the controller is configured to control the first valve and the second valve to vary a lubricant flow parameter of pounds of lubricant flow/hour/horsepower produced by a turbine section driving the speed change device in response to an engine power setting.

7. The lubrication system as recited in claim 1 , wherein the first portion of the speed change device comprises a bearing supporting rotation of a gear of the speed change device and the second portion of the speed control device comprises a plurality of gears of the speed change device.

8. The lubrication system as recited in claim 1 , including a third valve controlled by the controller for varying a third flow of lubricant to lubricated components of the turbofan engine other than the speed change device.

9. The lubrication system as recited in claim 1 , including a control valve disposed between an auxiliary pump and a main oil tank, wherein the control valve provides lubricant flow to a primary lubricant circuit supplying lubricant to the speed change device in response to a detected loss of lubricant pressure in the primary lubricant circuit.