Most advanced autopilots with a guidance panel allow you to select an altitude and fly to it. It will often switch over to an altitude hold mode if the airplane is close enough to the selected altitude and the flight conditions like current vertical speed are right. My question is about how vigilant the pilot has to be to prevent this altitude capture from happening at the wrong moment.

The Garmin G1000 manual describes the behavior like this:

As the aircraft nears the Selected Altitude, the flight director automatically transitions to Selected Altitude Capture Mode with Altitude Hold Mode armed (Figure 6-7). This automatic transition is indicated by the green ‘ALTS’ annunciation flashing...

At 50 ft from the Selected Altitude, the flight director automatically transitions from Selected Altitude Capture to Altitude Hold Mode and holds the Selected Altitude (shown as the Altitude Reference).

However, this preselected altitude stays on as the flight proceeds, including during most mode changes. There are some mode changes where this old altitude could cause problems if it initiates an altitude capture, like a transition to a go-around mode or an approach mode. (For an example, see "A Sea Level State Of Mind" in this newsletter). The autopilot would try to reach the old altitude once conditions are right (which may require turbulence or other conditions to make the AP think it's trying to fly to the preslected altitude). The plane would be doing what the pilot previously had told it to do, not what he was directing the plane to do at the moment.

From a human factors perspective, having the autopilot follow the old altitude target in a problematic scenario would make the system easier to understand, but would require more mental work from the pilot to maintain mode awareness. Under certain conditions this altitude capture could potentially create a safety issue if the autopilot doesn't otherwise mitigate the sudden motion in an unintended direction.

Having seen a case where the pilot asked "why is that happening" then realized he had mis-managed his altitude selector, I'm wondering about how smart the autopilot is and what the procedures are for managing the altitude selector. Does the pilot usually expect the autopilot or flight director to ignore or clear the preselected altitude when necessary, or does he have to carefully manage the altitude selected to avoid an unintended maneuver?

EDIT: I'm asking if there are any safety features for altitude capture (like not allowing altitude capture while in glideslope mode) or checklist/procedures related to altitude capture. If you think there aren't any of those and that the pilot takes full responsibility for any unintended altitude captures, please answer why you think so. enter image description here

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    $\begingroup$ I'm not sure you are aware, but there is surprisingly little training required to fly advanced cockpits like the G1000, or for auto-pilot systems in general. So what the pilot "expects" to happen is typically based on experience. How "smart" the autopilot is depends on the autopilot, the G1000 being much "smarter" than something like a Gensys System 50. $\endgroup$
    – Ron Beyer
    Aug 17, 2016 at 20:58
  • $\begingroup$ Could you not just ignore the FD bars? From what you quoted it doesn't sound like the plane is going to do anything unless you tell it to $\endgroup$
    – TomMcW
    Aug 17, 2016 at 23:08
  • $\begingroup$ No 'vigilance' required; on a GFC700 AFCS, dial in your desired altitude using the ALT small or large knobs, then select your climb type, be either the vertical speed mode (VS) or a constant airspeed climb (IAS/FLC) mode. Press the nose up/nose down keys to select the desired climb rate or climb airspeed and engage the autopilot. It's that simple. To change to a different altitude, simply repeat the process. $\endgroup$ Aug 17, 2016 at 23:22
  • $\begingroup$ IANAP but from what I understand the flight director is just a visual guide for the pilot and does not control the aircraft. The settings are fed from the AP but unless the AP itself is engaged the pilot can simply ignore anything the FD is showing on screen. $\endgroup$
    – TomMcW
    Aug 17, 2016 at 23:25
  • $\begingroup$ @TomMcW On some autopilots the flight director and the AP are linked so if the autopilot is engaged it tries to follow the flight director. On such airplanes any mitigation for these unintended captures would technically be in the flight director, not the autopilot itself. $\endgroup$ Aug 18, 2016 at 4:12

1 Answer 1


On the Boeing 737 NG:

ALT ACQ is always armed when flight directors and/or autopilot is engaged in a climb/descent mode (V/S, VNAV, LVL CHG, TO/GA, CWS P), it is however, as you suggest, inhibited in G/S and G/P (glide slope, glide path) modes.

On a typical ILS, I would typically set the missed approach altitude (which would be captured during missed approach in TO/GA mode) on glide slope capture. You could also set the missed approach altitude when ALT HLD is engaged, and you know that you will not need to change altitude again until when intercepting the glide slope. (When changing the selected altitude with ALT HLD engaged, the "held" altitude is maintained, until a new vertical mode is engaged, regardless of selected altitude).

HOWEVER, performing a Non-ILS approach in V/S or VNAV, the set altitude WOULD be captured on descent. To avoid this, the approach MDA is usually set on the altitude selector, and kept there until sufficiently (typically 300 ft) below the missed approach altitude. When passing through this altitude, the missed approach altitude would be set (logic is implemented to make sure the altitude is not captured during the "winding up" of the selected altitude).

Lastly, performing a Non-ILS using IAN (Basically Boeing's solution for performing Non-ILS with ILS display and control laws, using FMC data), the final descent would be performed in G/P mode, which behaves roughly like G/S mode, eliminating the use for initially selecting the MDA.

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    $\begingroup$ Out of the practical perspective: It is only the Non-ILS, Non-IAN case where this will be an issue due to "forgetting" setting MDA. Typical SOP: PM makes a callout n miles before the start of the final descent. In response, PF sets the MDA, possibly with verbal confirmation. $\endgroup$
    – Waked
    Aug 22, 2016 at 11:41

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