Uld loadmaster — National Airlines 102, refused at the manifest

uld_loadmaster.cell

// ============================================================================
// ULD LOADMASTER — National Airlines 102, refused at the manifest
// ============================================================================
//
// On 29 April 2013, National Airlines Flight 102 lifted off from Bagram
// with five armored vehicles strapped to its main deck. At rotation, at
// least one 12-tonne MRAP broke loose, slid aft, smashed the jackscrew
// of the horizontal stabilizer — and the 747 stalled onto the runway,
// killing all seven crew. The NTSB found the cargo was restrained by a
// scheme the operator had invented, far below the required tie-down
// strength, and the load was never properly verified.
//
// This loadmaster makes both failures structural: a unit load device's
// weight ledger refuses any write past the position's certified limit,
// and the pallet lifecycle has no path onto the aircraft that skips
// `restraints_verified`. An unverified pallet cannot board — the edge
// does not exist.
// ============================================================================

cell UldLoadmaster {
    face {
        signal add_cargo(uld: String, kg: Int) -> Map
        signal verify_restraints(uld: String) -> Map
        signal board_aircraft(uld: String) -> Map
        signal uld_weight(uld: String) -> Int
    }

    memory {
        // PMC pallet on a 747 main-deck position: 6,804 kg max gross
        weight_kg: Map<String, Int> [persistent, consistent]
        invariant weight_kg >= 0 && weight_kg <= 6804
    }

    // building -> restraints_verified -> aboard. No shortcut to aboard;
    // touching the load voids the verification (back to building).
    state pallet {
        initial: building
        building            -> restraints_verified
        restraints_verified -> building
        restraints_verified -> aboard
    }

    on add_cargo(uld: String, kg: Int) {
        if kg <= 0 { return map("ok", false, "reason", "cargo weight must be positive") }
        if get_status(uld) == "aboard" {
            return map("ok", false, "reason", "ULD is aboard — load is closed")
        }
        // adding cargo to a verified pallet voids the verification
        if get_status(uld) == "restraints_verified" { transition(uld, "building") }
        let cur = weight_kg.get(uld) ?? 0
        let w = try { weight_kg.set(uld, cur + kg) }
        if w.error != () {
            return map("ok", false, "reason", "REFUSED: {cur} + {kg} kg exceeds 6804 kg position limit")
        }
        return map("ok", true, "uld", uld, "weight_kg", cur + kg)
    }

    on verify_restraints(uld: String) {
        if (weight_kg.get(uld) ?? 0) == 0 {
            return map("ok", false, "reason", "empty ULD — nothing to verify")
        }
        let r = try { transition(uld, "restraints_verified") }
        if r.error != () { return map("ok", false, "reason", "{r.error}") }
        return map("ok", true, "state", "restraints_verified")
    }

    on board_aircraft(uld: String) {
        let r = try { transition(uld, "aboard") }
        if r.error != () {
            return map("ok", false, "reason", "restraints not verified: {r.error}")
        }
        return map("ok", true, "state", "aboard", "weight_kg", weight_kg.get(uld))
    }

    on uld_weight(uld: String) { return weight_kg.get(uld) ?? 0 }
}

cell test Bagram102 {
    rules {
        // legal build-up
        assert add_cargo("PMC-1", 4000).ok == true
        assert add_cargo("PMC-1", 2500).ok == true
        assert uld_weight("PMC-1") == 6500

        // overweight for the position: the ledger refuses
        assert add_cargo("PMC-1", 1000).ok == false
        assert uld_weight("PMC-1") == 6500
        assert_fails weight_kg.set("PMC-1", 12000)

        // THE BAGRAM PATH: boarding without restraint verification.
        // building -> aboard does not exist.
        assert board_aircraft("PMC-1").ok == false
        assert_fails transition("PMC-1", "aboard")

        // verified, it boards — and the load is then closed
        assert verify_restraints("PMC-1").ok == true
        assert board_aircraft("PMC-1").ok == true
        assert add_cargo("PMC-1", 10).ok == false
        assert_fails transition("PMC-1", "building")

        // touching the load AFTER verification voids it
        assert add_cargo("PMC-2", 3000).ok == true
        assert verify_restraints("PMC-2").ok == true
        assert add_cargo("PMC-2", 500).ok == true
        assert board_aircraft("PMC-2").ok == false

        // an empty pallet cannot be "verified" pro forma
        assert verify_restraints("PMC-3").ok == false
    }
}