Diversion — Switch Destination Mid-Flight

intermediate
📍 Route: EGLL → LFPG (originally) → diverted to EBBR⏱ ~3 minutes📊 6 steps🎓 Enroute Operations

Halfway to Paris, ATC closes it for weather. Change the FMC to Brussels the way a line crew would, under time pressure.

What this scenario covers

An in-flight destination change is not just a field edit: the route beyond the current position has to be rebuilt, the arrival is now different, and the predictions on PROGRESS refer to somewhere you are no longer going until you fix it.

The sequence practised here is the realistic one — change the destination, rebuild what follows, verify on LEGS, and commit.

How it applies in flight ops

The decision to divert is made on fuel and weather before the box is touched, and it is usually made while talking to ATC, the company and the cabin. The programming has to be routine enough to happen alongside all that.

Crews brief the diversion airport as if it were the destination, because it now is: approach, minima, runway length, and what happens after landing. The FMC work is the smallest part of it.

Start this scenario

Open the SimTuts 737 CDU trainer and select “Diversion — Switch Destination Mid-Flight” from the scenarios list to walk through it step-by-step on the simulated keypad.

Open trainer →

Step-by-step: Diversion — Switch Destination Mid-Flight

  1. You are airborne, mid-flight to LFPG. ATC has just told you to divert to EBBR (Brussels). Press RTE to open the ROUTE page.

    In flight, the FMC still has the original destination. You need to update DEST so the FMC recalculates fuel, time, and approach data for the new airport.

  2. Type EGLL and press 1L for ORIGIN (the airport you departed from).

    Origin doesn't change on a diversion. The FMC needs it for distance/time calculations from your current position.

  3. Now the diversion: type EBBR and press 1R to set DEST to Brussels.

    Updating DEST tells the FMC to replan everything from your current position to EBBR — fuel burn, ETA, approach charts, alternate selection. Real-world ATC will give you a vector toward the new airport while you do this.

  4. The new destination is a route change — press 6R (ACTIVATE), then the EXEC key to make the diverted route active.

    Any route change — including a diversion — goes through the same staged-then-executed sequence. Under ATC pressure that discipline is what keeps a mistyped destination from instantly becoming the active plan.

  5. On a real diversion you'd also need to load the EBBR approach. For now, press 6R to jump to PERF INIT and check fuel.

    Diversions are fuel-critical. The first thing real pilots do after updating DEST is verify they have enough fuel to reach the new airport plus reserves. PERF INIT is where that calculation happens.

  6. Type 5.0 and press 4L to set RESERVES to 5.0 (in thousands of pounds — this is your final reserve fuel).

    Final reserve is fuel you must land with regardless of conditions — typically 30 min holding fuel for jets. Setting it on PERF INIT lets the FMC warn you if your remaining fuel will breach this minimum at the new destination.

What to take away

  • A destination change invalidates the whole arrival half of the plan.
  • Verify the rebuilt route on LEGS before executing.
  • The fuel and weather decision comes first; the box follows it.
  • Brief the new destination properly — it is a destination, not a waypoint.

The rest of Enroute Operations

Background reading

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