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I was watching a tutorial about the FMS and the autopilot in the 737, and I am consfused. I come from a Cessna background, where there is no FMS, there is only the AP (autopilot).

So, is the AP a separate entity being fed info by the FMS and easily overridden by the values you input into the AP (heading, V/S, altitude)? Or is the AP under the FMS, and the FMS has full authority over the AP?

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    $\begingroup$ >I come from a Cessna background, which there is no FMS, there is only the AP(autopilot). I recently added the Avidyne IFD540 to my Cessna Cardinal, which bills itself as an FMS, and appears quite capable. My autopilot is connected to the DG/HSI, the IFD540 controls the autopilot thru the DG/HSI. $\endgroup$
    – CrossRoads
    Aug 23, 2018 at 18:09
  • $\begingroup$ @CrossRoads Yeah and i fly on sims without an FMS on $\endgroup$
    – lpydawa
    Aug 23, 2018 at 18:27
  • $\begingroup$ People often say or write "FMS" when they mean "FMC". In the video you watched, they are probably used with specific, and distinct, meanings. Boeing will tell you that the A/P is part of the FMS, and it can (optionally) be coupled to the FMC. The FMC doesn't have "authority over" the A/P -- it's just one of several modes you can select. However, the A/P and the FMC both are components of the Flight Management System, or FMS. See my answer for more detail. $\endgroup$
    – Ralph J ♦
    Aug 24, 2018 at 14:05

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In Boeing terminology, the FMS, Flight Management System, is comprised of:

  • Flight Management Computer System (FMCS, which is essentially two FMC's, at least on the 737... other jets may have one FMC, or three)
  • Autopilot
  • AFDS (Autopilot/Flight Director System - includes the Mode Control Panel used to select autopilot modes, heading, speed, altitude, etc)
  • Autothrottle
  • GPS(s)
  • Inertial Reference Systems (IRS)
  • Two (in the 737, three in some other jets) CDU's, which are the "Control and Display Units" with a small screen display and alphanumeric keyboards, used by pilots to view, enter, and edit things such as flight routes & performance data. Many such units are now "Multifunction CDU's", or MCDU's, adding ACARS and/or CPDLC capability.

So in their terminology, the FMS is very broad and includes plenty of things, including the autopilot. The FMC, on the other hand, is the computer that stores your route, calculates the aircraft's position (based on inputs from the GPS receiver(s) + the IRS + possibly other sensors), and does all the navigation tasks. One can couple the autopilot to the FMC guidance - select LNAV and/or VNAV - or not.

It's not uncommon to hear or read "FMC" and "FMS" used interchangably, but they are in fact two separate things. The FMS (the system) includes the FMC (the computer) and the autopilot; the autopilot can be coupled to the FMC, but it is always a component of the FMS.

Sometimes you'll see a CDU referred to as "the FMC (or FMS)" since the CDU is the pilot's main interface with the FMC. But they are separate things; you can have a CDU failure with all FMC's still working, and vice versa.

Clear as mud?

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  • $\begingroup$ Nice answer - maybe worth adding the "CDU" (Control, or Computer, Display Unit) to the list of FMS system components. This is the box with the screen and the buttons, often located on the center console, which pilots sometimes casually refer to as "the FMS" but really just another component of the system as this answer explains well. $\endgroup$
    – TypeIA
    Aug 28, 2018 at 18:25
  • $\begingroup$ Related: aviation.stackexchange.com/a/37794 -- also includes an illustration I made that visualizes what you wrote :) $\endgroup$
    – user14897
    Aug 28, 2018 at 19:17
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They are separate systems, but the autopilot can be set to get guidance from the FMC (Flight Management Computer).

For lateral navigation, the autopilot can be set to follow a heading, or a VOR/LOC. It can also be set in LNAV mode, which receives guidance from the FMC.

For vertical navigation, the autopilot can be set to hold an altitude, LVL CHG based on the set speed, or follow the glideslope in APP mode. It can also be set in VNAV mode, which receives guidance from the FMC.

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  • $\begingroup$ So there are multiple modes within VNAV? $\endgroup$
    – lpydawa
    Aug 23, 2018 at 17:18
  • $\begingroup$ @lpydawa that's probably a better topic for your other question $\endgroup$
    – fooot
    Aug 23, 2018 at 17:19
  • $\begingroup$ What is the ALT INTV on a 737 $\endgroup$
    – lpydawa
    Aug 25, 2018 at 8:41
  • $\begingroup$ @lpydawa you could ask a separate question about that $\endgroup$
    – fooot
    Aug 26, 2018 at 0:47
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They are two different systems. The autopilot receives selected inputs from the pilots, or managed inputs from the FMS. Total failure of the FMS has no impact on the autopilot which keeps following selected inputs. Total failure of the autopilot has no impacts on FMS which keeps computing the numerous data he is supposed to do such as time of arrival, fuel at destination, also the navigation data such as point to point required track, heading, optimum altitude for least fuel consumption.... As a summary they are just complementary systems.

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The FMS can be thought of as an electronic equivalent to a human Navigator. The AutoPilot is an electronic equivalent to a human Pilot.

The human Pilot can listen to, or not, the human Navigator. The human Pilot can listen to, or not, the electronic Navigator. And the electronic pilot can listen to (call L/VNav), or not (called Hdg) the electronic Navigator.

Your flight, your choice.

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  • $\begingroup$ But I heard that while the AP is "listening" to the FMS, one can change the altitude and vertical speed of the AP. Is that true? How does it interact with the FMS then? $\endgroup$
    – lpydawa
    Aug 24, 2018 at 12:09
  • $\begingroup$ This answer uses "FMS" to mean "FMC". Everything described here is an accurate description of the interactions of the autopilot with the FMC. But, as my answer describes, both are part of the FMS (Flight Management System). $\endgroup$
    – Ralph J ♦
    Aug 24, 2018 at 13:56
  • $\begingroup$ @lpydawa Lateral and vertical guidance are separate parts of the autopilot that can be set to different inputs, e.g. FMC to navigate along a route but manual selection of altitude. $\endgroup$
    – StephenS
    May 25, 2019 at 6:11

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