2.6 Schedule freeze Windows & change control

An optimized supply chain relies on stability. If the production schedule changes daily, or if Engineering modifies part numbers while procurement is actively managing the existing Bill of Materials (BOM), the result is “System Nervousness.” This forces the Material Requirements Planning (MRP) engine to issue conflicting expedite and de-expedite signals, reducing the clarity of communication with the supply base. To stabilize the purchasing signal, it is essential to implement and respect Time Fences.

The time fence architecture

The planning horizon must be divided into distinct zones based on the impact and cost of making changes within that timeframe.

Zone 1: the frozen zone (e.g. 0–2 weeks)

• The Definition: The physical “Clear to Build” (CTB) window. Raw materials are being staged, production lines are scheduled, and machines are programmed.
• The Protocol:
  • Schedule Changes (Quantity or delivery date) are generally not permitted.
  • Engineering Change Orders (ECOs) should not cut into active, frozen orders.
• The Exception: Breaking the freeze should require Operations Leadership approval, with the understanding that the requesting department may need to absorb expedited freight or scrap costs.

Zone 2: the slush zone (e.g. 2–6 weeks)

• The Definition: The “Material Drive” window. Purchase Orders are actively placed, and physical materials are either in transit or secured.
• The Protocol:
  • Moderate end-item volume changes (e.g. ±10%) are often permissible.
  • Schedule date shifts are allowed if the necessary physical material is already available to support a pull-in.
  • Engineering Change Orders (ECOs) are allowed if the engineering “Effectivity Date” aligns with the component lead time, avoiding disruption to already-ordered material.

Zone 3: the liquid zone (e.g. >6 weeks)

• The Definition: The “Unconstrained Forecast” window.
• The Protocol:
  • Planning can adjust product mix and volume to align with long-term Sales demand, allowing the MRP system to calculate appropriate future supply signals.

Engineering change control (ECO) discipline

An Engineering Change Order (ECO) must be synchronized with procurement reality to avoid creating excess inventory or shortages. Define clearly how the physical change will enter the production system.

The cut-in strategy

• Running Change (The “Use-Up” Transition):
  • The Logic: The existing stock of the Old Revision should be consumed before transitioning to the New Revision.
  • The Impact: Minimizes scrap and operational risk. This is the preferred method when the old revision remains functionally acceptable.
• Hard Cut (The Immediate Transition):
  • The Logic: Using the Old Revision must be stopped immediately. Only the New Revision must be built moving forward.
  • The Impact: Creates immediate obsolescence of the Old Revision inventory.
  • The Protocol: This should require formal financial “Scrap Authorization” from leadership, especially if the change occurs within the Frozen or Slush zones.

The blocked stock protocol

• The Trigger: When a Hard Cut ECO is formally released.
• The Action: All existing physical inventory of the affected part must be placed on Quality Hold in the WMS.
• The Goal: This prevents the warehouse from inadvertently picking obsolete parts for a new setup while the change is being processed.

Inventory disposition

When an engineering change causes obsolescence, the old material becomes a financial liability. A formal disposition must be assigned quickly to clear the warehouse and update financial records.

If the ECO dictates a Hard Cut:

Gate 1: Physical Rework

• The Question: Can the Old Revision be modified (e.g. firmware flash, adding a jumper wire) to meet the New Revision specification?
• The Decision: If Yes, issue a formal Rework Order. If No, proceed to Gate 2.

Gate 2: Return to Vendor (“RTV”)

• The Question: Will the supplier accept the unopened reels back for credit?
• The Decision: If Yes, execute the RTV (recovery will be partial, less restocking fees). If No, proceed to Gate 3.

Gate 3: Open Market Liquidation

• The Question: Is there commercial market demand for this component?
• The Decision: If Yes, and there are no legal restrictions (e.g. proprietary ASICs or ITAR parts), execute an Asset Recovery Sale. If No, proceed to Gate 4.

Gate 4: Scrap (The Write-Off)

• The Action: Finance must formally write off the asset value. It must be ensured to properly dispose of the parts to prevent them from re-entering the supply chain.

The change request form

Standardizing the change request process helps stakeholders understand the financial impact of schedule or engineering instability.

The CRF structure

• Requestor: [Sales / Engineering / Product]
• Change Type: [Schedule Pull-in / Push-out / Engineering Change Order (ECO) Hard Cut]
• Constraint Warning: Identify if the request falls inside the Frozen Zone.

Impact analysis (conducted by supply chain):

1. Direct Material Cost: Estimated costs for Broker Expedites or Premium Freight.
2. Scrap/Obsolescence Cost: Estimated value of inventory to be written off.
3. Line Down Risk: Assessment of operational impact.
4. Physical Feasibility: Can the supply chain quantitatively support the change?

The approval matrix

The required signature level should correspond to the Time Zone impacted and the Financial Cost.

Time Horizon	Change Type	Typical Financial Impact	Recommended Sign-off Level
Liquid (>6 Wks)	Any Change	Minimal	Master Scheduler
Slush (2-6 Wks)	Moderate	Low to Medium	Supply Chain Manager
Slush (2-6 Wks)	ECO Hard Cut	Medium to High	Plant Manager + SCM Director
Frozen (<2 Wks)	ANY CHANGE	HIGH	VP Operations + Plant Manager

Final Checkout: Schedule freeze windows & change control

Control Point	Engineering Requirement	Target Goal
The Freeze	Frozen Zone duration defined in system.	Respected consistently
ECO Phase-In	’Use-Up’ logic applied whenever possible.	Default practice
Disposition	Obsolete Inventory is quarantined.	Promptly upon ECO release
The Authority	Approvals inside Freeze Window are restricted.	Escalate to leadership
Financial Auth	Expedite/Scrap costs are documented.	Visibility for decision-makers
MRP Integrity	Invalid POs are formally cancelled in ERP.	Executed promptly upon ECO cut-in