Since the accident, a lot more has been learned about the 135 fuel system. Some of you experienced operators commenting here don't realise yet that what you thought was correct is in fact not true. Until recently, I would have agreed with you.
The main fact is about spillage between the main and supply tanks if you pitch the aircraft. Whilst there can be some transfer, the tanks are not like the diagrams, they have gaps between them. Any transfer is a trickle only and is not significant. Therefore pumps need to be on, even if the main is full.
Second, when the supply tanks are full to the brim, they hold more than the gauged 90kg. The excess fuel is added to the main tank indication. The 90kg amount is basically for certification (ish). This affects in 2 ways.
First, when the actual main tank is empty, there will be a small amount of fuel showing on the main gauge.
Second, when carrying out the supply tank drain ground run, after you turn the transfer pumps off, the main tank continues to show a reduction for a few minutes. Then the supply tanks start to reduce. This is that little bit of fuel 'above' the supply tanks burning off. Initially, pilots thought that fuel was still transferring.
This is not my opinion, this is fact