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Each engine has an AC generator. The constant-speed drive unit (or other system) is the link between the generator and the engine. The drive unit has its own oil system for cooling and lubrication and should be checked in the sight glass before flight. This oil system is independent of the engine oil system.

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CSD - Constant Speed Drive

Check oil quantity in sight glass, note different levels for left and right engine as unit is same for both sides.

The reset handle (red) is visible at the bottom.

Supply capacity is 45KVA.


VSCF - Variable Speed Constant Frequency

Check oil level in the sight glass, note different levels for left and right engine as unit is same for both sides.

The photograph shows an old style VSCF (two LED's). Notice "Fault" and "Open Phase" LED's. Press "Indicate" button (top left) with cockpit lights set bright for indication. If any LED's illuminate, cross check the generator diagnostics panel & report to engineer. Button on top right is the LED lamp test.

In 2001 Boeing & Hamilton Sundstrand introduced modifications to the VSCF to improve its reliability which had been approx 1/3 that of CSD's. The failure rate was just over 1 every 2000hrs. You can identify a new VSCF by the three LED's rather than two on the unit. The instruction plate for the buttons & LED's is located just below the LED's.

Due to poor reliability, if either generator is a VSCF there is a (UK CAA) limitation to remain within 45mins flying time of a suitable airfield.


IDG - Integrated Drive Generator (NG's only)

Check oil level in the sight glass, note different levels for left and right engine as unit is same for both sides.

IDG's will auto-disconnect with a high oil temperature, thus no IDG oil temperature gauges on the gen drive panel. They are cooled by both fan air and a fuel-oil heat exchanger.

Supply capacity 90KVA, with options available for up to 180KVA eg for AEW&C aircraft.

Notice the three coloured plugs at the top right. These are the three phase cables and are colour coded as follows:

A = Red

B = Yellow

C = Blue




To correctly check the oil level the CSD/VSCF/IDG must first be vented for 15 seconds to release pressure. Caution: a spray of hot oil can emit from the vent during this process. The vent on the IDG is at the top of the sight glass

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