Closing a LEGS Discontinuity

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📍 Route: EGLL → MID → LFPG⏱ ~4 minutes📊 4 steps🎓 Flight Plans & Routing

The box prompts on the LEGS page are a route discontinuity. Here is what the FMC is asking for and how to close the gap.

What this scenario covers

A discontinuity appears on LEGS as a line of boxes between two waypoints: the FMC has two pieces of route and no instruction about how to join them. It refuses to invent a track, which is the correct behaviour — a guessed leg is worse than an obvious gap.

Closing it means line-selecting the waypoint after the gap onto the boxes, which stages the join as a modification. On the 737 that gap is not closed until EXEC — the Airbus applies it immediately, the Boeing waits.

How it applies in flight ops

LNAV will not fly through a discontinuity. The aircraft tracks to the last waypoint before the gap and then holds heading, which is fine if vectors were expected and a serious surprise if they were not.

Crews scan LEGS after every procedure load specifically looking for these, because a procedure that does not connect to the enroute structure is the most common way one appears.

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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 takes a line of its own between EGLL and LFPG — the aeroplane prints the words ROUTE DISCONTINUITY there, and this trainer shows ---DISCO---. It is not a box prompt: box prompts are the markers the FMC draws where it is waiting for data, and a discontinuity is a statement, not a request. 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 MID into the scratchpad, then press 2L — the line key next to the discontinuity — to insert MID there.
Line-selecting a fix onto the discontinuity line overwrites it, joining the route EGLL → MID → LFPG (MID is Midhurst VOR, where Heathrow's Paris traffic leaves London southbound). 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 the EXEC key 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.

What to take away

Boxes on LEGS mean the FMC cannot join two route segments.
Close the gap by line-selecting the following waypoint onto the boxes.
On the 737 the join is staged — EXEC makes it real.
LNAV stops at a discontinuity and holds heading. Scan for them after every load.

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