THE LOADED BOX · VSSI MONTHLY

April 2026 · Vessel Schedule Stability Index

The median window held its shape across the horizon. A near-flat aggregate is not a settled plan - it is movement that netted out.

Where in the planning horizon the window actually moved - and what that means for commitments still open.

Published June 22, 2026 · Framework TradeLanes VSSI Monthly v1.1 · Companion publication Safe-to-Gate Monthly April 2026

1. The Reading

The median cargo receiving window held between 137 and 138 hours at every snapshot from T-4w through to the final - near-flat across the planning horizon, with only a slight late expansion at the end. On aggregate the window barely moved.

Volume tells the operational story. The month produced 4,462 CRW revision events across 621 distinct vessel-voyages - quieter per voyage than March (roughly 7.2 revisions per voyage against 9.4), even as the voyage count held near its quarter level, and broad-based across carriers and ports rather than isolated to one alliance. Just under half - 48.0% - of those events landed beyond T-7, outside the 72-hour zone where exporters, terminals, and carriers usually fight over the window.

A near-flat median is the trap. It does not mean schedules sat still; it means per-voyage movement netted out in aggregate while individual voyages kept moving mid-horizon. The window's instability did not leave in April - it averaged away on the chart.

2. The Mechanism

Sustained Churn pattern · Terminal anchor

The mechanism in April 2026 is paired ERD and CY Cut activity. 42.7% of voyages with movement showed both fields revising together (co-moving) - the largest class in the dual-signal breakdown, and among the operational cohort that carries an Earliest Return Date the paired share reaches 45.8%. CY Cut activity alone was the next-largest class at 20.8%; only 3.9% showed the two fields moving in opposite directions. The dominant mode stayed paired and sustained, not divergent.

USSAV Savannah carried the month's heaviest revision load and its top balanced-activity voyages - the terminal anchor consistent across the quarter.

The median at T-4w sat at 137 hours and finished near 138 hours. The shape on the chart is essentially flat. Where the movement sat - mid-horizon, per voyage - is what shows the mechanism the aggregate hides.

The three-element read

Contradiction. The median window barely moved - and 621 voyages produced 4,462 revision events. The aggregate hides where the mechanism operates: not in the median, but in the concentration.

Operator implication. Plans committed at T-4w faced revision activity arriving between T-14 and T-3, inside the same horizon where rail commits lock and dispatch chains warm. The information arrived before the plan closed, but not before the commit chain opened.

Failure mode. Plan committed at T-4w. Mid-horizon revisions arrive in paired moves at the terminal. The window reshapes as a unit. By T-3, the window the operator committed against no longer exists. CMA CGM AMBITION MRKQE at USSAV Garden City (departure April 5, 2026) recorded 23 ERD and 22 CY Cut revisions - the receiving window redrawn as a single unit, over and over, through the planning horizon.

3. Key Metrics

Two metrics reinforce the same finding. Revision events are concentrated mid-horizon. Late-stage share - events landing inside 72h of ERD - sits below the mid-horizon mass.

The breakdown matters more than the aggregate. Beyond T-14: 26.0%. T-14 to T-7: 22.0%. T-7 to T-3: 17.8%. T-3 to T-1: 6.2%. Final 24h: 3.0%. Post-ERD: 11.5%. The mass sits between T-14 and T-7 - the zone where rail commits and drayage dispatch are being finalized.

4. VSSI-P - Port-Level Snapshot

Where the mechanism operated

Per-port observed patterns:

• Savannah (USSAV) - High. 134 distinct vessel-voyages, 1,479 CRW revisions - the month's heaviest load and terminal anchor. Late-stage (inside 72h) on 17.1% of events; longer-horizon stability movement (>7d) on 65.5%. The mechanism lives here: paired mid-horizon revision, cross-carrier, at one terminal.
• Houston (USHOU) - Elevated. 120 vessel-voyages, 540 CRW revisions. Late-stage on 18.5% of events; longer-horizon movement (>7d) on 53.3%.
• Norfolk (USORF) - Elevated. 108 vessel-voyages, 599 CRW revisions. Late-stage on 20.9% of events; longer-horizon movement (>7d) on 61.4%. See the note below.
• Oakland (USOAK) - Elevated. 70 vessel-voyages, 371 CRW revisions. Late-stage on 31.5% of events; longer-horizon movement (>7d) on 18.1%. The activity here skews late-stage rather than mid-horizon.
• Los Angeles (USLAX) - Elevated. 52 vessel-voyages, 716 CRW revisions - the largest U.S. gateway by throughput. Late-stage on only 12.7% of events; longer-horizon movement (>7d) on 23.6%. The churn concentrates well before the compression zone, and the contrast with Savannah locates April's instability at Garden City, not at the largest gateway.
• New York (USNYC) - Moderate. 47 vessel-voyages, 205 CRW revisions. Late-stage on 17.6% of events; longer-horizon movement (>7d) on 22.9%.

Long Beach (39 vessels), Newark (35 vessels), and trace volume at smaller gateways fell below the eligibility threshold for primary ranking; observations are available for completeness.

Note on Norfolk. The Norfolk band reflects port-level voyage distribution. A subset of MAEU voyages at USORF shows a CY Cut revision pattern under data pipeline review (documented in Safe-to-Gate Monthly Dec 2025, Section 4 methodology note). If confirmed as a pipeline artifact, the corrected band will be reissued in a subsequent edition. The month's operational concentration sits at USSAV Garden City, not Norfolk.

5. Representative Vessel-Level Movements

Supporting sample · curated by the month's pattern · not a ranking

The following vessels illustrate how receiving windows moved in practice. Each card shows the voyage's identification, departure, revision activity, and the dominant pattern in the dual-signal space. The mix reflects the month's distribution - Sustained Churn voyages at Garden City predominate.

CMA CGM AMBITION · USSAV · CMDU / MRKQE Departure: April 5, 2026 CRW revisions: 45 (23 ERD + 22 CY Cut) Pattern: Sustained Churn - paired ERD+CY revisions, the window redrawn as a single unit See this vessel in Safe-to-Gate Weekly →

JAMAICA · USSAV · ZIMU / 124E Departure: April 6, 2026 CRW revisions: 35 (21 ERD + 14 CY Cut) Pattern: Sustained Churn - paired ERD+CY revisions See this vessel in Safe-to-Gate Weekly →

YM TRILLION · USSAV · YMLU / 018W Departure: April 5, 2026 CRW revisions: 28 (15 ERD + 13 CY Cut) Pattern: Sustained Churn - paired ERD+CY revisions See this vessel in Safe-to-Gate Weekly →

6. What This Means for Planning

The decision horizon changes under this pattern.

When late-stage volatility dominates, exporters can buffer. Commit at T-4w, recheck at T-3, absorb the last-72-hour shift through reserved equipment and pre-booked appointments. The system is designed for that decision sequence.

When mid-horizon volatility dominates - and April 2026 sits at 48.0% of events arriving beyond T-7 - the buffer moves inside the commit window. The plan is moving while it is being committed. Rail commits, labor scheduling, and drayage reservations are placed against a window the schedule has not finished drawing.

A near-flat median is the trap this month. It reads as calm. But the per-voyage movement that netted out in aggregate is exactly the movement an individual exporter feels on an individual booking. For a voyage already showing mid-horizon activity, the plan committed at T-4w will not be the plan that executes - whatever the network median says.

Cost is created when action is taken against a window that hasn't stopped moving. If the same voyage is showing mid-horizon activity, every dollar priced against that T-4w window is priced against a plan that will be rewritten.

7. Bottom Line

April 2026's median window held its shape across the planning horizon while the mass of revision activity sat mid-horizon, before the commit chain closed. The near-flat aggregate is the headline trap: it averages away the per-voyage movement operators actually experience.

The mechanism is mid-horizon revision activity, concentrated at USSAV Garden City. Volume eased per voyage from March but stayed broad-based - the mode stayed paired and the anchor stayed at Garden City.

Through the middle of Q2 2026, the operator's problem was not too many vessels at once - it was no stable point at which to act. The window moves as a unit, mid-horizon, at Garden City, and the exporter who commits early commits against a shape already being overtaken.

TRADELANES · VSSI MONTHLY · APRIL 2026

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Appendix A - Measurement Layer

Band vocabulary. Stable - window held through execution. Moderate - window moved within planning tolerance. Elevated - window structure under stress. High - window not reliable for commitment. Used consistently across Safe-to-Gate Weekly, Safe-to-Gate Monthly, and VSSI Monthly.

Outcome classification. Compressed, Mixed, Late expansion, Moderate drift, and Stable describe how each window moved between its first and final snapshot. The exact classification thresholds live in the internal methodology appendix and are not published.

Sustained Churn. Voyage-level volatility pattern describing windows that keep moving through the planning horizon without settling - paired ERD and CY Cut revision activity with weak directional consistency. The classification thresholds live in the internal methodology appendix and are not published.

Late-stage qualifier. Inside 72h, inside 24h, or outside 72h, measured as the time between the latest revision event and ERD_final.

Methodology note. Analysis covers April 2026 U.S. departures with observable schedule revisions: 4,462 events across 621 distinct vessel-voyages. Voyages are counted as distinct vessel-voyages (normalized vessel name plus terminal, split into distinct sailings), not booking or schedule-version rows. VSSI-P bands computed from voyage-level distribution using the current methodology; classification thresholds live in the internal methodology appendix and are not published.

Carrier-terminal suppression carry-forward. A pipeline artifact in CY Cut revisions for Maersk vessels at Norfolk's primary container terminal - first identified in Safe-to-Gate Monthly Dec 2025 - remains active. MAEU/USORF voyages appearing in residual top-activity lists with CY Cut activity that does not reflect underlying terminal behavior are excluded from concentration and repeat-offender findings in this edition. A fuller treatment will appear once engineering review resolves the underlying data.