Dual pump fuel flow system for a gas turbine engine and method of controlling

1. A fuel flow system for a gas turbine engine, the system comprising:

a first pump in communication with a fuel source and a nozzle;

a second pump in communication with the fuel source and the nozzle;

an actuator in communication with the first pump;

a metering valve disposed downstream of the first and second pumps;

a bypass loop connected to the first pump and the second pump and configured to recycle excess fuel from the first pump and excess fuel from the second pump back to inlets of the first and second pumps;

a dual window valve comprising:

a housing having a high pressure side, a low pressure side, a first inlet connected to the first pump, a first outlet connected to the bypass loop, a first flow passage connected between the first inlet and the first outlet, a second inlet connected to the second pump, a second outlet connected to the bypass loop, and a second flow passage connected between the second inlet and the second outlet;

a spool slidably received within the housing and having a high pressure end proximate the high pressure side of the housing, a low pressure end proximate the low pressure side of the housing, a first metering edge, and a second metering edge;

wherein the first inlet of the housing and the first metering edge of the spool define a first window;

wherein the second inlet of the housing and the second metering edge of the spool define a second window; and

a spring disposed between the low pressure side of the housing and the low pressure end of the spool;

a first port formed in the high pressure side of the housing and configured to receive a pressure signal from upstream of the metering valve; and

a second port formed in the low pressure side of the housing and configured to receive a pressure signal from downstream of the metering valve; and

wherein the metering valve is in communication with the first pump, and wherein the metering valve is in communication with the second pump when the second window is restricted.

2. The fuel flow system of claim 1 , the system further comprising:

a main fuel flow path formed between the first pump and the metering valve,

wherein the second pump is connected to the main fuel flow path in parallel with the first pump, and wherein the second pump is configured to supplement fuel flow from the first pump under certain conditions; and

a heat exchanger disposed upstream from the first pump and the second pump,

wherein the bypass loop is configured to recycle excess fuel from the first pump and excess fuel from the second pump back to inlets of the first and second pumps and downstream from the heat exchanger.

3. The fuel flow system of claim 1 , wherein the first pump has a smaller volume than the second pump.

4. The fuel flow system of claim 1 , wherein the first pump operates at a higher pressure than the second pump.

5. The fuel flow system of claim 1 , wherein the first pump is configured to satisfy maximum cruise conditions plus actuator slew requirements.

6. The fuel flow system of claim 1 , wherein the first pump and the second pump are sized to together satisfy maximum fuel flow conditions.

7. The fuel flow system of claim 1 , and further comprising:

a check valve connected between the second pump and the metering valve, wherein the check valve opens when pressure from the second pump is equal to or greater than pressure from the first pump.

8. The fuel flow system of claim 1 , wherein the position of the spool is adjusted to maintain a substantially constant pressure drop across the metering valve.

9. The fuel flow system of claim 1 , wherein the first window allows fuel from the first pump to enter the bypass loop and wherein the second window allows fuel from the second pump to enter the bypass loop.