6.3 Using and Maintaining the Master Schedule

6.3 Using and Maintaining the Master Schedule

Quick Answer: After creation and RCCP validation, the MPS must be maintained. Frozen / slushy / liquid zones and the demand and planning time fences control who may change what. Continuously update projected available balance (PAB) and ATP so promises and factory priorities stay synchronized.

A perfect MPS on Monday becomes wrong by Thursday if orders surge, a supplier slips, or a machine fails. Domain IV expects you to know how the schedule is steered over time—not only how it is built once.

Time Zones: Frozen, Slushy, and Liquid

Think of the horizon as three management climates:

Exam language varies slightly by Learning System wording, but the management idea is stable: stability near, flexibility far.

Why Frozen Matters

If Week 1 MPS is cut from 200 to 100 after components are already staged, you create:

• Excess component inventory and open POs
• Lost capacity that could have built something else
• ATP promises that may still assume the old 200

Conversely, adding 100 units inside the frozen zone without capacity and material checks recreates the RCCP failure mode—only now with less time to recover.

Demand Time Fence (DTF)

The demand time fence marks where actual demand takes priority over forecast for calculating requirements in the MPS grid.

Common policy (know this pattern):

• Inside the DTF: use customer orders (ignore forecast, or treat forecast as already consumed).
• Outside the DTF: use the greater of forecast and customer orders (or forecast plus unconsumed orders, depending on consumption rules).

Worked DTF Example

DTF = end of Week 3. Starting inventory = 80.

Demand for PAB:

• Weeks 1–3 (inside DTF): 70, 40, 35 (orders only)
• Weeks 4–5 (outside DTF): max(60,10)=60, max(60,0)=60

PAB:

• W1: 80 + 100 - 70 = 110
• W2: 110 + 0 - 40 = 70
• W3: 70 + 50 - 35 = 85
• W4: 85 + 60 - 60 = 85
• W5: 85 + 60 - 60 = 85

If someone mistakenly used forecast inside the DTF in Week 1 (50 instead of 70), PAB would be overstated by 20 and ATP would be dangerously optimistic—classic exam trap.

Planning Time Fence (PTF)

The planning time fence controls MPS supply changes.

• Inside the PTF: MPS receipts are often firm planned orders; planning software will not automatically move them. A human planner (sometimes with manager approval) must authorize changes.
• Outside the PTF: the system may create or adjust MPS proposals as demand and inventory evolve (liquid behavior).

Relationship to DTF: Usually DTF ≤ PTF. Demand becomes order-driven before supply becomes freely replannable. Exact fence lengths are policy: shorter fences increase flexibility and nervousness; longer fences increase stability and risk of protecting the wrong mix.

Maintenance Scenario

PTF = end of Week 4. A large customer wants +40 units in Week 3.

1. Check ATP in the relevant buckets.
2. If ATP is insufficient, evaluate adding MPS inside the PTF—this is a slushy/frozen negotiation, not an automatic MRP suggestion.
3. Run a quick RCCP impact on critical resources for the changed weeks.
4. If approved, update MPS, recompute PAB and ATP, communicate promise date to sales.
5. If denied, offer Week 5/6 promise from liquid-zone supply.

Projected Available Balance in Maintenance Mode

Maintenance is mostly disciplined recalculation. Core identity:

PAB(t) = PAB(t−1) + MPS receipts(t) − Demand(t)

with Demand defined by fence policy, and MPS receipts including only authorized master schedule quantities (not random shop wishes).

Multi-Change Numeric Drill

Start: on-hand 200; DTF through Week 2; PTF through Week 4.

Initial grid:

Demand: W1=100, W2=80 (inside DTF); W3=100, W4=100, W5=110 (max outside).

PAB:
W1: 200+0-100=100
W2: 100+150-80=170
W3: 170+100-100=170
W4: 170+100-100=170
W5: 170+100-110=160

Event A: Week 1 orders rise to 130 (still inside DTF).

New W1 PAB: 200+0-130=70. Downstream balances fall by 30 unless MPS changes. ATP in the first bucket also drops by 30—sales must see the update immediately.

Event B: Planner adds 50 MPS in Week 1 (requires PTF approval).

W1 PAB becomes 200+50-130=120. The +50 restores service but consumes frozen-zone capacity—RCCP and component availability must be checked even for "small" maintenance edits.

Using the MPS Day to Day

Operational uses of a maintained MPS include:

• Order promising via ATP
• Driving MRP for components and raw materials
• Finishing schedules / FAS in assemble-to-order (MPS on options or modules)
• Performance measurement: customer service, MPS stability (schedule changes inside fences), inventory turns at the item level

Stability metrics matter on CPIM-style questions: excessive freeze protects a bad plan; excessive liquidity destroys execution. Good maintenance changes the MPS when reality demands it, with fence discipline and capacity awareness.

Nervousness vs. Rigidity

Closing the Domain IV Loop

Creating the MPS (6.1) sets the build story. RCCP (6.2) proves it can be told on the factory's critical stages. Maintaining the MPS (6.3) keeps the story true as orders, supply, and capacity evolve. When fences, PAB, and ATP are current, MRP and detailed scheduling inherit a master plan worth exploding.