It is now considered necessary to postpass VLBA/MarkIV thin tapes after recording. This means winding the tape all the way to the end and back to the beginning without stopping. This avoids irregularities in the tape pack that could otherwise lead to tape damage in handling and shipping. These irregularities are caused by starts and stops. Since tapes cost over $1000 each, this is a serious issue. SCHED will give the necessary commands to request postpasses be done at stations that use VLBA control files.
Unfortunately, a postpass takes between 11 and 22 minutes depending on how far the the tape has to be fast-forwarded to reach the end. This is down from a maximum of 33 minutes prior to late 1997 when it was determined that rewind speed could be used. For two tape stations, this is not a problem. For single tape stations, it is can be serious since it adds this much to the tape change time. Also, if there is an inadequate gap in the schedule, the next tape can be out of the expected position on the first pass which can cause additional losses when the first reverse pass runs out of tape early. SCHED provides a way to avoid postpasses by keeping track of tape motions. If the last pass went from the far end of the tape to the beginning without stopping, a postpass is not required and is not requested by SCHED. Therefore, users with single tape stations (all except the VLBA and VLA) who are using more than 1 thin tape per station are advised to not stop the tape on the last pass before each tape change.