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Step-by-Step Buffer Methodology

1.        Reduce task duration estimates by SO percent: Iden­
tify the project's network of activities and paths by
unbuffered time and by resource. Collect activity du­
rations as normal estimates, which are expected to
have a high probability of success. Then estimate the
50 percent probability duration by cutting these es­
timates in half.

2.   Eliminate resource contentions and determine the
critical chain: Determine the critical chain as the long­
est chain of dependent events, task and resource. The
critical chain is the constraint of the project. It is im­
perative that the resource contentions be deconflicted.

3.   Insert a project buffer sized and positioned to aggre­
gate critical chain contingency time (normally 50
percent of the critical chain path length): This step
aims to exploit the constraint.

4.   Size and position the feeding buffers on all paths that
feed the critical chain: Use feeding buffers to protect
the critical chain from accumulation of negative
variations on the feeding chains. The other project
paths are thereby subordinated to the constraint.

5.        Plan scheduled activities to start as late as possible,
protected by buffers: Subordinate further the other
paths to the constraint by allowing the critical chain
to normally start first, with possibly a few other paths.

6.   Resources deliver optimal performance (eliminate
multitasking and the student syndrome): The re­
sources work as quickly as possible, as soon as the
schedule triggers their activation, on their activities
and pass their work on as soon as they complete,
rather than looking to use all of the time available
whether it is necessary or not.

7.   Provide resources with activity durations and esti­
mated start times in the master schedule: Encourage
resources to pass on their work when done. This re­
sults in elevating the constraint.

8.        Use buffer management to control to plan: The project
and feeding buffers provide the information to trig­
ger the master scheduler and other users as to when
to plan for recovery and when to take recovery actions.
It provides visibility when the project or system is in
control and simply experiencing acceptable fluctua­
tion. As the workers performing the tasks report their
progress, it becomes evident where slack and short­
falls are developing. These can be charted and com­
pared to probability durations and remaining available
buffer time. Now take appropriate action as warranted
when part of the project buffer or feeder buffers have
been consumed:


1.        Establish the size and placement of the project and
critical chain feeding buffers.

2.        Determine the daily status of buffers by asking per­
forming activities to project activity completion.

3.   Develop action plans if the buffer penetration ex­
ceeds one-third.

4.        Implement corrective action if buffer penetration
exceeds two-thirds.

Day 4—The overall objective of the critical chain implementation in the H-46 area was established and intermediate objectives (IOs) were identified.

The team determined that the goal would be: By in­creasing throughput, NADEP Cherry Point will remain the preferred H-46 source by producing high quality aircraft, under cost, with a turnaround time of 90 cal­endar days.

Obstacles to achieving this goal were identified by the team in detail. This was a preparatory step for day 4.

Day 5—The obstacles to achieving the project's goal were used by the team to develop intermediate objec­tives that would resolve the obstacles and ensure suc­cess.

Day 6—The network for the overall project was con­structed, using the intermediate objectives. Specific in­dividuals were assigned to be responsible for achieving each identified intermediate objective. The network was fed into a critical chain software tool, and project buffer and feeder buffers were automatically calculated. This was a team activity to create the implementation plan for the project.

The afternoon was dedicated to commencing the network creation for the H-46 repair process. Yellow stickies were used to identify tasks with the following information required in each case:



      Number and type of resources required

      50 percent and 90 percent duration times for each

The intent is to pull one thread at a time out of the total process, study it, and put it in its proper relational position. This effort purposely starts with a clean sheet, as the groupings of activities and related timing may dif­fer from other efforts in the old paradigm. It was deter­mined that the last task in an H-46 repair project is closing out the aircraft logbook. From this point on, the team worked backward to deal with all required tasks. This ef­fort was continued into the second week with the comple­tion of scheduling three aircraft and input as separate projects into the critical chain software. Then the mul­tiple projects were combined into one portfolio stream through the software and schedules developed that were subordinated to die available resources. With this accom­plished, the model was established to continue adding new aircraft items proceeding into repair, until a com­plete transition from the old to the new method of mas­ter scheduling would be complete.

During the following weeks and months after startup, the previously inducted workload was completed and every new H-46 aircraft has been introduced into the CCPM software and related master scheduling process.

To Be Continued

For balance of this article, click on the below link:

Lean Manufacturing Articles and click on Series 12

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