Space reclamaible while Flash tape i staken

Hi All,
How Oracle and it assume backuppiece in FRA as obsolete? when we take the following command:

# Back up flash recovery area to tape
BACKUP RECOVERY AREA;

If the backup of recovery area take a lot of time, then when some backuppieces are already backed in TAPE,
i think that Oracle dont tage it as obsolete while the backup recovery area is not finished?

What we excpected is when Oracle already back one backuppiece, the backuppiece is marked as obsolete(right a way) and the space is reclamaible.

Regards