B737 PMC (CL) vs EEC (NG)

[DC-8]

Does anybody know the difference between engine PMC's installed in Classic series and EEC's installed in NG's? A technician told me they are completely different. Any book where I can find a detailed description of both systems?

Thanks in advance!

[barit1]

The classic uses a basic hydromechanical fuel control for N2 governing, accel/decel fuel scheduling, and VBV and VSV servo control. The engine will run fine on pure hydromechanical systems; the electronic PMC fine-tunes this to achieve N1 control.

The 737NG has fully electronic control - all the above functions are handled by FADEC.

[DC-8]

Thanks a lot for your reply!

But according to the FCOM, NG's have a HMU instead of a MEC as installed in the CL's. So whats the difference?

[barit1]

NG's have a HMU instead of a MEC

The HMU is simply the fuel metering valve w/associated plumbing. All the "brains" are in the EEC.

(Well, that may not be quite true. I think there's an overspeed governor in the HMU - in case all else fails )

[IFixPlanes]

Originally Posted by barit1

I think there's an overspeed governor in the HMU - in case all else fails

Quoting my Training Manual
Overspeed Governor
A mechanical overspeed governor can prevent an N2 overspeed condition.
When the overspeed governor senses an N2 overspeed condition, it causes
the bypass valve to open more. When the bypass valve opens more, less fuel
goes to the FMV. This causes the N2 speed to decrease. This is an alternative
N2 overspeed protection from the EEC. The EEC monitors the condition of the
overspeed governor.