Closing a LEGS Discontinuity
beginnerDecode the box-prompt route discontinuity on the 737 LEGS page — what it is, why the FMC inserts it, and how to close the gap and EXEC it active. The Boeing way differs from the Airbus: closing the gap is staged until you press EXEC.
Start this scenario
Open the SimTuts 737 CDU trainer and select “Closing a LEGS Discontinuity” from the scenarios list to walk through it step-by-step on the simulated keypad.
Open trainer →Step-by-step: Closing a LEGS Discontinuity
- Set up a route. Press RTE, type EGLL and press 1L for the origin, then type LFPG and press 1R for the destination.
EGLL is London Heathrow, LFPG is Paris CDG. With an origin and a destination but no airway joining them, the FMC has a gap it cannot connect on its own — it marks the gap with a route discontinuity.
- Press the LEGS key to see the route legs and the discontinuity between them.
On LEGS the discontinuity appears as a box-prompt line (shown here as ---DISCO---) between EGLL and LFPG. The FMC will NOT fly across it — in LNAV the aircraft flies to the last good waypoint then reverts to heading mode. Every discontinuity must be closed before departure. It is not an error; it is the FMC saying "I don't know how to join these — you decide."
- Close the gap. Type DVR into the scratchpad, then press 2L — the line key next to the discontinuity — to insert DVR there.
Line-selecting a fix onto the box-prompt overwrites the discontinuity, joining the route EGLL → DVR → LFPG. But this is the Boeing — the change is only STAGED. The title now reads MOD RTE 1 LEGS and the white EXEC light is lit. The route is not active yet.
- Commit the change. Press 6R EXEC* to make the closed route active.
EXEC commits the modification: the title returns to ACT RTE 1 LEGS, the EXEC light goes out, and the route is now continuous and active. Every route change on the Boeing — closing a discontinuity, a direct-to, an airway — is staged until EXEC. That is the difference from the Airbus, where the change takes effect immediately.