7.3 Changes and Supply Disruptions

Quick Answer: Small MPS or supply changes can cascade through the BOM as MRP nervousness. Planners dampen it with time fences, firm planned orders, and controlled regeneration. Pegging shows which parents and customers are hit; exception messages flag what to fix first. When disruptions hit, revise ATP/promises, re-peg critical shortages, firm recoverable orders, and choose substitutes, splits, or alternate sources with clear customer communication.

Execution rarely matches the last regenerative run. CPIM expects you to manage change deliberately: stabilize what must not thrash, illuminate where a shortage came from, and respond to disruptions with prioritized, pegged actions.

MRP Nervousness

System nervousness is excessive replanning—orders that repeatedly change date or quantity across MRP runs even when the business change is small. Because dependent demand is calculated from parent releases, a one-week shift at the top level can reschedule dozens of component orders below.

Common nervousness drivers:

Damping techniques:

• Respect demand and planning time fences; route near-term MPS changes through master schedulers.
• Convert near-term planned orders to firm planned orders.
• Prefer L4L or controlled POQ in volatile zones if FOQ amplification is the culprit.
• Use net-change MRP carefully, but still review action messages—net change reduces volume of churn, not the need for judgment.
• Stabilize BOMs and avoid unnecessary phantom or engineering changes in the freeze horizon.

Firm Planned Orders

A firm planned order (FPO) is a planned order whose quantity and/or due date the planner has locked. MRP may still explode from it and may still generate messages, but it should not automatically rewrite the firmed fields the way it rewrites ordinary planned orders.

When to firm: inside the planning time fence; after promising a customer from specific supply; when setup sequencing on a bottleneck must not thrash; when a long-lead purchase must stay aligned to a supplier commitment.

When not to firm everything: over-firming freezes bad plans and suppresses legitimate reschedule messages. Firm selectively, then watch exceptions on firmed objects.

Pegging

Pegging links a requirement or supply order to its sources. Single-level pegging shows immediate parents or children; full pegging walks to end items and customer orders (where the system supports it).

Disruption example: Supplier advises that a week-4 receipt of 120 Y will slip to week-6. Pegging shows those 120 units support X releases that cover customer shipments in weeks 5–6. Without pegging, a planner might expedite the wrong item or notify the wrong customer.

Exception and Action Messages

MRP and related planning systems emit exception (action) messages so planners do not scan every item. Typical categories:

Discipline: sort by severity and date proximity; peg before mass rescheduling; never clear messages by blindly accepting every system suggestion—especially across firm orders and supplier-confirmed dates.

Supply Disruption Response Playbook

When yield collapses, a supplier fails, a work center goes down, or inbound freight stalls, use a structured response:

1. Quantify the hole — what quantity is missing in which buckets?
2. Peg the impact — which parents, FAS entries, and customer promises fail?
3. Protect the truth in promising — immediately reduce ATP/CTP; stop sales from booking the ghost supply.
4. Stabilize the near term — firm orders you will actually chase; unfirm thrashing plans that block clear thinking.
5. Recover supply — alternate supplier, partial shipment, premium freight, rework, substitute component, overtime, or alternate routing (CRP must confirm capacity).
6. Rebalance demand — split deliveries, substitute configurations in ATO, prioritize allocations by policy.
7. Replan and communicate — regenerate or net-change, review new exceptions, notify customer service with honest dates.

Worked Mini-Grid: Disruption and Pegging

Recall component Y from section 7.1: planned order receipt of 120 in week 4 (released in week 3). Suppose purchasing reports the supplier can deliver only 70 in week 4; the remaining 50 arrive in week 6.

Pegging from the short 50 units shows coverage was intended for X demand tied to later MPS shipments. Planner actions: firm a 70 receipt in week 4; create a firm 50 in week 6 (or expedite if premium freight recovers earlier); reduce ATP for affected end items; issue reschedule messages only to pegged parents; tell customer service which promises move before sales rebooks.

Change Control versus Firefighting

Not every change is a disruption. Engineering change orders, date shifts inside policy, and forecast consumption adjustments are managed changes. Use formal ECR/ECO effectivity, time-fence policy, and master scheduler ownership. Firefighting starts when uncontrolled edits bypass fences, firming is random, and exception queues are ignored until shortages hit the dock.

Exam Pitfalls

• Calling every reschedule “nervousness” — nervousness is excessive churn, not legitimate response to real demand change.
• Firming the entire horizon so MRP cannot improve anything.
• Expediting without pegging, which wastes premium freight on the wrong order.
• Leaving ATP unchanged after a supplier failure, allowing new orders to steal recovery supply.
• Treating cancel messages as optional noise while inventory and payables absorb unneeded receipts.