A windmilling engine still produces enough rpm to produce sufficient oil pressure to supply oil to the breaings, and herein lies the problem, ALL 747/340 engines (infact most jet engines) use Labryinth seals at the bearing compartments, these seals utilise Internal Engine Bleed to pressurise the labryinth seals, this helps seal these bearing compartments tightly, thereby decreasing oil consumption radically.
A Jet Engine at windmill speeds cannot produce bleed pressure, this leads to the Labryinth seals being unpressurised, the oil consumption more than doubles because of this, the oil gets dumped overboard via the respective bearing compartment vents, (check the training or ground maintenance manual for precise figures), hence leaving an engine at idle power is far more sensible (if possible), than doing a precautionary shutdown. Doing a precautionary shutdown and then restarting in a few hours time might present you with a mandatory shutdown due to low oil quantity and Low oil pressure light illuminating.
This perhaps explains why there is a windmilling limitation on the 340, small oil tanks/massive endurance, on the classics with either the Pratt & whitney or GE motors there is no windmilling limitation.
If you dont find any info in your ground maintenance manuals let me know and I'll try and paste the Boeing info from their training manual in this thread.
Last edited by leftright; 2nd March 2006 at 12:28.