diff --git a/src/shared/state/crossSectionNavigation.test.ts b/src/shared/state/crossSectionNavigation.test.ts
new file mode 100644
index 00000000..49e1b4be
--- /dev/null
+++ b/src/shared/state/crossSectionNavigation.test.ts
@@ -0,0 +1,290 @@
+import { describe, expect, it, beforeEach, vi, afterEach } from "vitest";
+import {
+  setBoardFilter, getBoardFilter,
+  setScheduleFilter, getScheduleFilter,
+  setMapFilter, getMapFilter,
+  projectBoardToSchedule, projectScheduleToBoard,
+  resetCrossSectionStore,
+  type BoardFilterSnapshot, type ScheduleFilterSnapshot, type MapFilterSnapshot,
+} from "./crossSectionNavigation.js";
+
+beforeEach(() => {
+  resetCrossSectionStore();
+  vi.useFakeTimers();
+  vi.setSystemTime(new Date(2026, 4, 15, 12, 0, 0)); // Fri 2026-05-15
+});
+afterEach(() => vi.useRealTimers());
+
+describe("4.1.8: store + retrieval", () => {
+  it("stores and retrieves board filter", () => {
+    const snap: BoardFilterSnapshot = {
+      mode: "route", departure: "MOW", arrival: "LED",
+      date: "20260515", timeFrom: "0000", timeTo: "2400",
+      searchExecuted: true,
+    };
+    setBoardFilter(snap);
+    expect(getBoardFilter()).toEqual(snap);
+  });
+
+  it("stores and retrieves schedule filter", () => {
+    const snap: ScheduleFilterSnapshot = {
+      mode: "route", departure: "MOW", arrival: "LED",
+      dateFrom: "20260511", dateTo: "20260517",
+      timeFrom: "0600", timeTo: "2200",
+      onlyDirect: false, showReturn: false,
+      searchExecuted: true,
+    };
+    setScheduleFilter(snap);
+    expect(getScheduleFilter()).toEqual(snap);
+  });
+
+  it("stores and retrieves map filter", () => {
+    const snap: MapFilterSnapshot = {
+      departure: "MOW", arrival: "LED", date: "20260515",
+      showInternal: true, showInternational: true, showTransfers: false,
+    };
+    setMapFilter(snap);
+    expect(getMapFilter()).toEqual(snap);
+  });
+
+  it("getBoardFilter returns null before any set", () => {
+    expect(getBoardFilter()).toBeNull();
+  });
+
+  it("getScheduleFilter returns null before any set", () => {
+    expect(getScheduleFilter()).toBeNull();
+  });
+
+  it("getMapFilter returns null before any set", () => {
+    expect(getMapFilter()).toBeNull();
+  });
+
+  it("resetCrossSectionStore clears all three", () => {
+    setBoardFilter({
+      mode: "route", departure: "MOW", arrival: "LED",
+      date: "20260515", timeFrom: "0000", timeTo: "2400",
+      searchExecuted: true,
+    });
+    setMapFilter({
+      departure: "MOW", arrival: null, date: "20260515",
+      showInternal: true, showInternational: true, showTransfers: false,
+    });
+    setScheduleFilter({
+      mode: "route", departure: "MOW", arrival: "LED",
+      dateFrom: "20260511", dateTo: "20260517",
+      timeFrom: "0000", timeTo: "2400",
+      onlyDirect: false, showReturn: false,
+      searchExecuted: false,
+    });
+    resetCrossSectionStore();
+    expect(getBoardFilter()).toBeNull();
+    expect(getScheduleFilter()).toBeNull();
+    expect(getMapFilter()).toBeNull();
+  });
+
+  it("overwrites previous board filter with new value", () => {
+    const snap1: BoardFilterSnapshot = {
+      mode: "route", departure: "MOW", arrival: "LED",
+      date: "20260515", timeFrom: "0000", timeTo: "2400",
+      searchExecuted: false,
+    };
+    const snap2: BoardFilterSnapshot = {
+      mode: "flight-number", flightNumber: "1234",
+      date: "20260516", timeFrom: "0600", timeTo: "2200",
+      searchExecuted: true,
+    };
+    setBoardFilter(snap1);
+    setBoardFilter(snap2);
+    expect(getBoardFilter()).toEqual(snap2);
+  });
+});
+
+describe("4.1.8-R1/R2: projectBoardToSchedule", () => {
+  it("carries departure + arrival", () => {
+    const board: BoardFilterSnapshot = {
+      mode: "route", departure: "MOW", arrival: "LED",
+      date: "20260515", timeFrom: "0600", timeTo: "2200",
+      searchExecuted: true,
+    };
+    const p = projectBoardToSchedule(board);
+    expect(p.departure).toBe("MOW");
+    expect(p.arrival).toBe("LED");
+  });
+
+  it("sets dateFrom/dateTo to current week (Mon-Sun of board.date's week)", () => {
+    const board: BoardFilterSnapshot = {
+      mode: "route", departure: "MOW", arrival: "LED",
+      date: "20260515", // Friday
+      timeFrom: "0600", timeTo: "2200", searchExecuted: true,
+    };
+    const p = projectBoardToSchedule(board);
+    // Week containing 2026-05-15 (Fri) = 2026-05-11 (Mon) to 2026-05-17 (Sun)
+    expect(p.dateFrom).toBe("20260511");
+    expect(p.dateTo).toBe("20260517");
+  });
+
+  it("clears time/toggles to Schedule defaults", () => {
+    const p = projectBoardToSchedule({
+      mode: "route", departure: "MOW", arrival: "LED",
+      date: "20260515", timeFrom: "0600", timeTo: "2200",
+      searchExecuted: true,
+    });
+    expect(p.timeFrom).toBe("0000");
+    expect(p.timeTo).toBe("2400");
+    expect(p.onlyDirect).toBe(false);
+    expect(p.showReturn).toBe(false);
+    expect(p.searchExecuted).toBe(false);
+  });
+
+  it("uses current date for week bounds when board.date is malformed", () => {
+    const p = projectBoardToSchedule({
+      mode: "route", departure: "MOW", arrival: "LED",
+      date: "", timeFrom: "0000", timeTo: "2400", searchExecuted: false,
+    });
+    // Clock frozen at 2026-05-15 (Fri), so week = 2026-05-11 to 2026-05-17
+    expect(p.dateFrom).toBe("20260511");
+    expect(p.dateTo).toBe("20260517");
+  });
+
+  it("does not carry optional cities when board has only departure", () => {
+    const p = projectBoardToSchedule({
+      mode: "departure", departure: "MOW",
+      date: "20260515", timeFrom: "0000", timeTo: "2400", searchExecuted: false,
+    });
+    expect(p.departure).toBe("MOW");
+    expect(p.arrival).toBeUndefined();
+  });
+
+  it("does not carry optional cities when board has only arrival", () => {
+    const p = projectBoardToSchedule({
+      mode: "arrival", arrival: "LED",
+      date: "20260515", timeFrom: "0000", timeTo: "2400", searchExecuted: false,
+    });
+    expect(p.departure).toBeUndefined();
+    expect(p.arrival).toBe("LED");
+  });
+
+  it("week bounds for a Monday give Mon=Mon+6=Sun", () => {
+    // 2026-05-11 is a Monday
+    const p = projectBoardToSchedule({
+      mode: "route", departure: "MOW", arrival: "LED",
+      date: "20260511", timeFrom: "0000", timeTo: "2400", searchExecuted: false,
+    });
+    expect(p.dateFrom).toBe("20260511");
+    expect(p.dateTo).toBe("20260517");
+  });
+
+  it("week bounds for a Sunday give Mon-6..Sun", () => {
+    // 2026-05-17 is a Sunday
+    const p = projectBoardToSchedule({
+      mode: "route", departure: "MOW", arrival: "LED",
+      date: "20260517", timeFrom: "0000", timeTo: "2400", searchExecuted: false,
+    });
+    expect(p.dateFrom).toBe("20260511");
+    expect(p.dateTo).toBe("20260517");
+  });
+});
+
+describe("4.1.8-R1/R2: projectScheduleToBoard", () => {
+  it("carries departure + arrival, sets date=today, clears time", () => {
+    const sched: ScheduleFilterSnapshot = {
+      mode: "route", departure: "MOW", arrival: "LED",
+      dateFrom: "20260511", dateTo: "20260517",
+      timeFrom: "0600", timeTo: "2200",
+      onlyDirect: true, showReturn: false,
+      searchExecuted: true,
+    };
+    const p = projectScheduleToBoard(sched);
+    expect(p.departure).toBe("MOW");
+    expect(p.arrival).toBe("LED");
+    expect(p.date).toBe("20260515"); // today (frozen clock)
+    expect(p.timeFrom).toBe("0000");
+    expect(p.timeTo).toBe("2400");
+    expect(p.searchExecuted).toBe(false);
+  });
+
+  it("does not carry optional departure when schedule has only arrival", () => {
+    const p = projectScheduleToBoard({
+      mode: "route", arrival: "LED",
+      dateFrom: "20260511", dateTo: "20260517",
+      timeFrom: "0000", timeTo: "2400",
+      onlyDirect: false, showReturn: false,
+      searchExecuted: false,
+    });
+    expect(p.departure).toBeUndefined();
+    expect(p.arrival).toBe("LED");
+  });
+
+  it("sets mode to route regardless of schedule mode", () => {
+    const p = projectScheduleToBoard({
+      mode: "route", departure: "MOW", arrival: "LED",
+      dateFrom: "20260511", dateTo: "20260517",
+      timeFrom: "0000", timeTo: "2400",
+      onlyDirect: false, showReturn: false,
+      searchExecuted: false,
+    });
+    expect(p.mode).toBe("route");
+  });
+});
+
+describe("4.1.1-R26 / 4.1.8-R3: map filter is independent of board/schedule", () => {
+  it("setBoardFilter does not affect map", () => {
+    const mapSnap: MapFilterSnapshot = {
+      departure: "MOW", arrival: null, date: "20260515",
+      showInternal: true, showInternational: true, showTransfers: false,
+    };
+    setMapFilter(mapSnap);
+    setBoardFilter({
+      mode: "route", departure: "LED", arrival: "KGD",
+      date: "20260601", timeFrom: "0000", timeTo: "2400",
+      searchExecuted: false,
+    });
+    expect(getMapFilter()).toEqual(mapSnap);
+  });
+
+  it("setScheduleFilter does not affect map", () => {
+    const mapSnap: MapFilterSnapshot = {
+      departure: "SVO", arrival: "LED", date: "20260601",
+      showInternal: false, showInternational: true, showTransfers: true,
+    };
+    setMapFilter(mapSnap);
+    setScheduleFilter({
+      mode: "route", departure: "KZN", arrival: "AER",
+      dateFrom: "20260601", dateTo: "20260607",
+      timeFrom: "0000", timeTo: "2400",
+      onlyDirect: false, showReturn: false,
+      searchExecuted: true,
+    });
+    expect(getMapFilter()).toEqual(mapSnap);
+  });
+
+  it("setMapFilter does not affect board", () => {
+    const boardSnap: BoardFilterSnapshot = {
+      mode: "route", departure: "MOW", arrival: "LED",
+      date: "20260515", timeFrom: "0000", timeTo: "2400",
+      searchExecuted: false,
+    };
+    setBoardFilter(boardSnap);
+    setMapFilter({
+      departure: "SVO", arrival: null, date: null,
+      showInternal: false, showInternational: false, showTransfers: false,
+    });
+    expect(getBoardFilter()).toEqual(boardSnap);
+  });
+
+  it("setMapFilter does not affect schedule", () => {
+    const schedSnap: ScheduleFilterSnapshot = {
+      mode: "route", departure: "MOW", arrival: "LED",
+      dateFrom: "20260511", dateTo: "20260517",
+      timeFrom: "0000", timeTo: "2400",
+      onlyDirect: true, showReturn: false,
+      searchExecuted: false,
+    };
+    setScheduleFilter(schedSnap);
+    setMapFilter({
+      departure: "SVO", arrival: null, date: null,
+      showInternal: false, showInternational: false, showTransfers: false,
+    });
+    expect(getScheduleFilter()).toEqual(schedSnap);
+  });
+});
diff --git a/src/shared/state/crossSectionNavigation.ts b/src/shared/state/crossSectionNavigation.ts
new file mode 100644
index 00000000..e1a83400
--- /dev/null
+++ b/src/shared/state/crossSectionNavigation.ts
@@ -0,0 +1,111 @@
+/**
+ * Session-scoped store for cross-section filter state (TZ §4.1.8 Table 10).
+ * In-memory only — cleared on page reload. Projection functions implement
+ * the exact field mappings from Table 10 when a user navigates between
+ * Online-Board ↔ Schedule. Map filter is stored separately and is NEVER
+ * projected from / to Board or Schedule (per TZ §4.1.1 ¶12).
+ */
+
+export interface BoardFilterSnapshot {
+  mode: "route" | "flight-number" | "departure" | "arrival";
+  departure?: string;
+  arrival?: string;
+  flightNumber?: string;
+  date: string;      // yyyyMMdd
+  timeFrom: string;  // HHmm
+  timeTo: string;    // HHmm
+  searchExecuted: boolean;
+}
+
+export interface ScheduleFilterSnapshot {
+  mode: "route";
+  departure?: string;
+  arrival?: string;
+  dateFrom: string;  // yyyyMMdd
+  dateTo: string;
+  timeFrom: string;
+  timeTo: string;
+  onlyDirect: boolean;
+  showReturn: boolean;
+  returnDateFrom?: string;
+  returnDateTo?: string;
+  returnTimeFrom?: string;
+  returnTimeTo?: string;
+  searchExecuted: boolean;
+}
+
+export interface MapFilterSnapshot {
+  departure: string | null;
+  arrival: string | null;
+  date: string | null;
+  showInternal: boolean;
+  showInternational: boolean;
+  showTransfers: boolean;
+}
+
+// Module-level in-memory state
+let board: BoardFilterSnapshot | null = null;
+let schedule: ScheduleFilterSnapshot | null = null;
+let map: MapFilterSnapshot | null = null;
+
+export function setBoardFilter(s: BoardFilterSnapshot): void { board = s; }
+export function getBoardFilter(): BoardFilterSnapshot | null { return board; }
+export function setScheduleFilter(s: ScheduleFilterSnapshot): void { schedule = s; }
+export function getScheduleFilter(): ScheduleFilterSnapshot | null { return schedule; }
+export function setMapFilter(s: MapFilterSnapshot): void { map = s; }
+export function getMapFilter(): MapFilterSnapshot | null { return map; }
+
+export function resetCrossSectionStore(): void {
+  board = null; schedule = null; map = null;
+}
+
+function currentWeekBounds(base: Date): { start: string; end: string } {
+  const d = new Date(base);
+  d.setHours(0, 0, 0, 0);
+  const dow = (d.getDay() + 6) % 7; // 0 = Mon .. 6 = Sun
+  d.setDate(d.getDate() - dow);
+  const start = new Date(d);
+  const end = new Date(d);
+  end.setDate(end.getDate() + 6);
+  const y = (x: Date) =>
+    `${x.getFullYear()}${String(x.getMonth() + 1).padStart(2, "0")}${String(x.getDate()).padStart(2, "0")}`;
+  return { start: y(start), end: y(end) };
+}
+
+function todayYyyymmdd(): string {
+  const d = new Date();
+  return `${d.getFullYear()}${String(d.getMonth() + 1).padStart(2, "0")}${String(d.getDate()).padStart(2, "0")}`;
+}
+
+/** TZ Table 10: Board → Schedule projection. Preserves cities + dateFrom=current-week. */
+export function projectBoardToSchedule(b: BoardFilterSnapshot): ScheduleFilterSnapshot {
+  const base = b.date && /^\d{8}$/.test(b.date)
+    ? new Date(Number(b.date.slice(0, 4)), Number(b.date.slice(4, 6)) - 1, Number(b.date.slice(6, 8)))
+    : new Date();
+  const week = currentWeekBounds(base);
+  return {
+    mode: "route",
+    ...(b.departure ? { departure: b.departure } : {}),
+    ...(b.arrival ? { arrival: b.arrival } : {}),
+    dateFrom: week.start,
+    dateTo: week.end,
+    timeFrom: "0000",
+    timeTo: "2400",
+    onlyDirect: false,
+    showReturn: false,
+    searchExecuted: false,
+  };
+}
+
+/** TZ Table 10: Schedule → Board projection. Preserves cities + date=today. */
+export function projectScheduleToBoard(s: ScheduleFilterSnapshot): BoardFilterSnapshot {
+  return {
+    mode: "route",
+    ...(s.departure ? { departure: s.departure } : {}),
+    ...(s.arrival ? { arrival: s.arrival } : {}),
+    date: todayYyyymmdd(),
+    timeFrom: "0000",
+    timeTo: "2400",
+    searchExecuted: false,
+  };
+}