There are two servers, the first one is the central instance and
database, and is sharing /usr/sap/trans using NFS. The other one is an
application server, and imports the /usr/sap/trans from the central
instance using NFS.

The problem is that when the central instance goes down I can't umount
the filesystem /usr/sap/trans because the node who was exporting it is
down, and the filesystem is bussy. I have tried using umount and fuser
commands with no results. I have also tried to kill all processes that
were using /usr/sap/trans.

How could I umount this filesystem?

> How could I umount this filesystem?

Try with Enhanced AutoFS:

http://software.hp.com/portal/swdepot/displayProductInfo.do?productNumber=ENHAUTO

Alain.