LEGS Constraints: Crossing Restrictions

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📍 Route: KORD → KCLE via J60 — crossing restriction⏱ ~6 minutes📊 8 steps🎓 Enroute Operations

"Cross DJB at or below one-zero thousand, two-four-zero knots." That clearance becomes a LEGS entry with a B suffix.

What this scenario covers

Altitude and speed restrictions are entered on the LEGS page against the waypoint they apply to, in a combined speed/altitude field. The suffix carries the qualifier: A for at-or-above, B for at-or-below, and a bare altitude for an at constraint.

Entering it stages a modification; executing it rebuilds the VNAV path so the descent actually plans to meet the restriction rather than merely displaying it.

How it applies in flight ops

Crossing restrictions are how arrival sequencing is imposed, and getting one into the box is what allows VNAV to fly it. Typed correctly, the aircraft plans for it from top of descent; typed as the wrong qualifier, it plans for something the controller did not ask for.

If the FMC indicates it cannot make the restriction, that is information for ATC. The professional response is a call, not a fudge — a modern jet is not good at losing altitude and speed simultaneously, and the box knows it.

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Step-by-step: LEGS Constraints: Crossing Restrictions

  1. Clearance: Chicago O'Hare to Cleveland Hopkins via the J60 jet route. Press the RTE page key to start building the route.

    Crossing restrictions are entered on the LEGS page — but LEGS can only show waypoints once a route exists. A quick RTE + airway setup first gives us a real fix to constrain.

  2. Type KORD and press 1L to set the ORIGIN.

    KORD = Chicago O'Hare. The FMC anchors the entire route to the origin/destination pair, so this always comes first.

  3. Type KCLE and press 1R to set the DEST.

    KCLE = Cleveland Hopkins. A short hop like this barely levels off in cruise — the descent clearance and its crossing restriction come early, which is exactly the workload this drill trains.

  4. Press 6L to open RTE 2, then type J60 and press 1L to stage the airway in the VIA column.

    J60 is a real FAA jet route; its eastern segment runs from JOT (Joliet, near Chicago) towards Cleveland. On the 737, airways are entered in the RTE page's VIA/TO columns — never on LEGS.

  5. Type DJB and press 1R to set the TO waypoint — where you leave the airway.

    DJB is the Dryer VOR/DME (113.6) west of Cleveland — the last fix on J60 and a natural feed into the Hopkins arrival. The TO entry tells the FMC how far along the airway to follow.

  6. Press EXEC* (6R) to commit the route modification.

    Until you execute, the change is only staged (MOD). Executing makes it part of the active route — DJB now appears as a waypoint row on the LEGS page.

  7. ATC: "Cross DJB at or below 10,000 feet, speed 240 knots." Press LEGS, then type 240/10000B and press 2R — the right LSK on the DJB row.

    Suffix rules: A = at or above, B = at or below, no suffix = mandatory AT. Type the pair WITH the slash — "250 10000" without it is INVALID ENTRY. And watch the font: your entry renders LARGE, while FMC predictions render small. Large font is how a 737 pilot tells a real restriction from a prediction at a glance.

  8. The title now reads MOD RTE 1 LEGS. Press EXEC* (6R) to make the constraint active.

    Every route change on the Boeing follows the same rhythm: stage, verify, execute. The title returning to ACT RTE 1 LEGS confirms VNAV is now planning a descent that crosses DJB at or below 10,000 feet at 240 knots.

What to take away

  • Constraints go on LEGS against their waypoint, in the speed/altitude field.
  • A = at or above, B = at or below; a bare figure is an at constraint.
  • The entry reshapes the VNAV path once executed.
  • An unachievable restriction is a radio call, not a rounding error.

The rest of Enroute Operations

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