FMC channel on a Boeing 737 CDU identifies the left or right flight management computer. Learn which setting to use and why displays may differ.
On a Boeing 737 CDU, FMC channel identifies the flight management computer—left or right—whose data the control display unit is accessing. It is not a radio frequency or autopilot channel. In a dual-FMC installation, the computers normally exchange data, so both sides usually show the same active flight plan.
In general flight-simulator use, “channel” is often an add-on or hardware-interface term. Boeing cockpit terminology more commonly refers to the left FMC, right FMC or FMC source. The CDU is the keyboard and display used to communicate with the computer; our explanation of how the CDU fits into the wider flight management system covers those roles in more detail.
What do the left and right FMC channels do?
The left and right channels represent two independent flight management computers fitted to a dual-FMC 737. Each calculates navigation, position, route and performance information, while cross-talk keeps their operational data synchronised during normal flight.
The captain’s side is normally associated with the left FMC and the first officer’s side with the right. Not every 737 variant or simulator add-on models two fully independent computers; some provide one functional FMC with two mirrored CDUs.
Which FMC channel should I select?
Use the normal, automatic or on-side selection unless a simulated failure procedure directs otherwise. Changing channels is not required for ordinary route entry.
| Selection | Meaning | Normal use |
|---|---|---|
| NORMAL or on-side | Each side uses its associated FMC | Standard operation |
| BOTH ON L or FMC L | The left FMC becomes the source for both sides | Reversion after a right-FMC failure |
| BOTH ON R or FMC R | The right FMC becomes the source for both sides | Reversion after a left-FMC failure |
The exact wording and the displays or guidance systems affected vary between 737 generations and simulator implementations. Do not select BOTH ON L or BOTH ON R simply to make the CDU screens match; each CDU can display a different page while sharing the same flight plan.
Can you programme the flight from either CDU?
In normal dual-FMC operation, either CDU can be used for most route and performance entries. Accepted data is normally synchronised, although a modification must still be reviewed and executed before it becomes active. For the actual entry sequence, see our 737 CDU and FMC programming walkthrough.
Why are the two CDU displays different?
Different CDU pages are normal; different active route data is not. One pilot can view LEGS while the other checks PROGRESS without placing the FMCs on separate channels.
- Compare the underlying data. Check the active route, active waypoint and performance entries rather than expecting identical pages.
- Look for a pending modification. Review any MOD route, then execute it or erase it as appropriate.
- Check the FMC source. Return the selector to NORMAL unless following a failure checklist.
- Check for failures. A CDU FAIL indication, FMC status message or deliberately injected simulator failure may leave one channel unavailable.
- Allow for add-on limitations. Some aircraft mirror one CDU, simulate only one FMC or do not synchronise both sides completely.
Is FMC channel the same as autopilot CMD A or B?
No. FMC L/R identifies the computers that calculate navigation and performance guidance; CMD A/B identifies separate autopilot channels. LNAV and VNAV can use FMC guidance, but the FMC does not become an autopilot channel. Our guide to the difference between the FMC and autopilot explains that division.
What does FMC channel mean in a remote-CDU setting?
In a remote CDU, cockpit-hardware driver or add-on configuration screen, “channel” may be only a software connection number. Channel 1 often maps to the captain’s CDU and Channel 2 to the first officer’s CDU, but that numbering is not a universal Boeing standard. Choose the endpoint assigned by the particular aircraft rather than treating it as a cockpit source-selection procedure.