Learn what Boeing 737 dual-channel autopilot means, when CMD A and B are used, how it enables autoland, and why one channel may not engage.
On a Boeing 737, dual-channel autopilot means both independent autopilot channels, CMD A and CMD B, are engaged for the same precision approach. It is used chiefly for an automatic landing, where the second channel supplies the monitoring and redundancy needed to command the flare and, on equipped variants, runway rollout.
What does dual-channel mean on a Boeing 737?
Dual-channel operation couples both flight-control computers to the approach instead of leaving either CMD A or CMD B in control alone. For Aviation & Real-World Flying readers, the practical distinction is that one channel can fly a normal coupled ILS, but both are required for the 737's automatic flare and landing logic.
Only one autopilot is normally engaged during climb, cruise and descent. The 737 permits both CMD channels to operate together in approach mode because each channel can monitor the approach and the other autopilot's performance.
| Configuration | Typical use | Landing capability |
|---|---|---|
| Single channel | Climb, cruise, descent and an ordinary coupled ILS | Pilot disconnects and lands manually; no automatic flare |
| Dual channel | Autoland and approved low-visibility approach procedures | Automatic flare and, when supported, centreline rollout guidance |
The MCP's CWS A and CWS B buttons are not dual-channel autopilot. Dual operation requires the two CMD channels.
When is dual-channel autopilot used?
Dual-channel autopilot is used when the crew intends to perform an autoland or when an approved low-visibility procedure requires that configuration. It is not needed merely because the aircraft is following an ILS localiser and glideslope.
- CAT II or CAT III operations: it may be required when the aircraft, operator, crew, runway and ILS installation have the necessary approvals.
- Autoland practice: crews may use it in better visibility when permitted, and sim pilots can use it to practise the complete sequence.
- Very low cloud or visibility: it allows the autopilot to continue through flare rather than requiring a manual landing at the usual disconnect point.
Not every ILS runway is authorised or suitably protected for low-visibility autoland. In a desktop simulator those regulatory restrictions do not apply, but inaccurate scenery, sloping runways and poorly aligned ILS transmitters can still produce a bad touchdown.
Is dual-channel autopilot the same as autoland?
No. Dual-channel engagement is one part of an autoland; a valid ILS signal, correct approach configuration, radio-altimeter inputs, flare logic and continuous pilot monitoring are also required.
Dual channel does not operate the thrust reversers or taxi the aircraft. Autobrake is a separate system, and the pilots remain responsible for monitoring the touchdown, applying reverse thrust, controlling deceleration and disconnecting the automation when required. Dual channel also does not automatically prove fail-operational capability: the response to an autopilot fault depends on the particular aircraft's approved autoland status.
How do you engage both 737 autopilot channels?
In a capable Boeing 737 simulator, prepare a valid ILS approach, engage one CMD channel, select APP mode and then engage the other CMD channel when the model's approach logic permits it.
- Prepare both sides of the ILS. Confirm matching ILS data and set both course selectors to the published inbound course. Our 737 descent and approach set-up sequence covers the preceding configuration.
- Engage the first autopilot. Either CMD A or CMD B can normally be the controlling channel, subject to the aircraft's procedure and serviceability.
- Select APP mode. Verify that the localiser and glideslope are armed or captured as expected.
- Engage the second CMD channel. On many 737 implementations it can be selected after APP mode is armed and must be coupled before the specified radio-altitude gate. Follow the aircraft or add-on documentation rather than forcing both buttons on early.
- Read the flight-mode annunciator. Confirm the correct dual-channel status and that FLARE, followed by ROLLOUT where provided, arms at the proper stage. The exact words differ between 737 generations and simulator add-ons; our 737 ILS mode and annunciation guide explains what should happen during capture.
- Monitor the landing. If the required status is missing or a warning appears, do not assume the aircraft will flare automatically. Use the appropriate manual-landing or go-around procedure.
For the complete simulator sequence after both channels are coupled, follow our step-by-step Boeing 737 autoland procedure. Real-aircraft operation must always follow the operator's approved manuals and checklists.
Why will the second autopilot channel not engage?
The second channel usually refuses to engage because the aircraft is not in a valid dual-channel approach state or the simulator does not model that function fully.
- APP mode is not selected: both autopilots cannot normally remain engaged together in ordinary heading, LNAV or cruise operation.
- The ILS data do not match: check both navigation receivers, course selectors and the published inbound course.
- The localiser or glideslope is invalid: poor reception, a wrong frequency or an unsuitable intercept can prevent the required approach logic.
- The second channel was selected too late: some models inhibit engagement below a defined radio altitude.
- CWS was selected instead of CMD: only CMD A and CMD B form the dual-channel configuration.
- An aircraft system has failed: a flight-control computer, autopilot, radio altimeter or related input may make autoland unavailable.
- The simulated 737 is simplified: some default or lightweight aircraft illuminate both buttons without modelling genuine dual-channel flare and rollout logic; others allow only one channel.
- A hardware binding is sending a toggle command: a yoke or panel switch may disconnect the active autopilot as the second one is selected. Test the virtual MCP controls directly.
A mistake we see constantly is treating illuminated CMD buttons as proof that autoland is ready. The decisive evidence is the flight-mode annunciation and the expected FLARE and ROLLOUT arming sequence, not the button lights alone.