bigbob
The prime reason stabiliser fuel is burned fairly early in long range flight - before most of the centre wing tank fuel or virtually any of the wing tanks fuel – is because of concerns about the aircraft CG following a failure of the stabiliser tank transfer/jettison pumps and a consequent inability to use any of the (now trapped) stabiliser fuel. Secondary reasons would include fuel temperature concerns and aircraft cruising efficiency.
On a fully fuelled B747-436, (in itself a very rare event) the CWT holds 52,167 kg, and the stabiliser tank holds 10,030 kg. Somewhere around 90 minutes after take-off, when the CWT fuel has decreased to 36,470 kg, the Fuel Management System Cards will automatically open the appropriate valves and activate the transfer/jettison pumps and transfer all the stabiliser fuel from the stabiliser tank into the CWT.
All the stabiliser tank fuel should be gone around two and a half to three hours into a 12+ hour flight, and this is done deliberately to avoid a rather nasty potential problem.
If we left the stabiliser tank fuel to the end of our long flight, when we have no fuel in the centre tank and little fuel in the wing tanks, and at that point we discovered that both the transfer/jettison pumps had failed leaving fuel trapped in the stabiliser tank, then, with little or no fuel in the centre or wing tanks, the CG would be far too far aft for the aircraft to land safely, and there would be very little we could do about it.
So, to guard against this problem, if there is to be a failure of both transfer/jettison pumps, we need to know about it early in the flight, so that we can land whilst we still have sufficient fuel in the centre and wing tanks to keep the CG forward of the aft limit for landing.
This scenario is generally covered on conversion courses, and new B744 pilots are often surprised at how awkward a problem this is. Very broadly, you will have to land within 6-7 hours, not the 12 -15 hours you'd planned!
Regards
Bellerophon