Closing a LEGS Discontinuity
PremiumbeginnerThe 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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Everything about this scenario is on this page, steps included — reading it costs nothing and always will. What Premium adds is doing it: the CDU validates every entry as you make it, tells you when an entry was accepted but wrong, and saves your progress across devices.
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.
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.
The rest of Flight Plans & Routing
London Heathrow to Paris CDG
beginnerLondon to Paris on the 737: RTE, PERF INIT and TAKEOFF REF, the complete short-haul setup in one sitting.
📍 EGLL → LFPG⏱ ~5 min📊 9 stepsSID & STAR: KPHX → KLAS
intermediatePhoenix to Las Vegas: enter the route, then program a departure and an arrival from the DEP/ARR page.
📍 KPHX → KLAS — SID/STAR practice⏱ ~6 min📊 5 stepsRTE Airway Entry & the "NOT IN DATABASE" Fix
PremiumintermediateTwo error messages every 737 pilot meets on the RTE page — NOT IN DATABASE and INVALID ENTRY — and what each one is actually telling you.
📍 KLAX → … → LAS (via J146)⏱ ~4 min📊 8 stepsRoute Correction — Fix a Mistyped Destination
intermediateThe destination is wrong on the RTE page. Overwriting a field you already filled is a distinct skill from filling it.
📍 KJFK → EGLL (initially mistyped as EGKK)⏱ ~3 min📊 5 steps
Background reading
- GuideWhy LNAV Doesn't Work When You Turn On the Autopilot
LNAV engaged but the aircraft flies straight past the magenta line in MSFS 2024? The 6 most common causes on the PMDG 737 and Airbus A320 — and the fix.
- GuideThe Boeing 737 CDU / FMC Explained: Every Page
What is the 737 CDU and FMC? A plain-English guide to every Boeing 737 CDU page plus the EXEC/MOD workflow. Practice free in our CDU trainer.
- GuidePMDG 737 FMC Tutorial: Program Your First Flight Plan Step by Step
Program the PMDG 737 FMC from scratch: IDENT, POS INIT, ROUTE, PERF INIT, takeoff speeds, and VNAV setup. Every CDU page explained with the exact inputs.