How Autopilots Decide What To Do Next

[00:00:00]Take a look at this display on the autopilot. Right now, it's flying a heading, but in a few moments, it's going to turn and intercept an approach.

[00:00:08]There's no need to touch a button or move the controls, but the airplane already knows what it's going to do next. And we know this from a few small letters at the top of the screen that many pilots glance at, but never fully understand. Those mode annunciations aren't just telling you what the autopilot is doing right now, they're telling you what's going to happen next.

[00:00:28]In other words, the autopilot has a plan.

[00:00:30]And once you understand how that plan works, you'll stop being surprised by the autopilot and start predicting its next move before it happens.

[00:00:38]Here, we're being vectored for an RNAV approach into runway 34 in Addison, Texas.

[00:00:44]Right now, heading mode is active with HDG in green.

[00:00:49]We get an instruction to turn to 340 until intercepting and get cleared for the approach.

[00:00:55]Twisting the heading bug makes the autopilot turn left to follow it as we're in heading mode. When we intercept the approach course, we want the autopilot to follow that. So, we hit the NAV hard key. Now, GPS appears in the status bar in white next to HDG.

[00:01:11]GPS mode is armed.

[00:01:13]Before we ever intercept the course, the autopilot has already decided what's coming next. When we approach the selected course on the GPS flight plan, the armed GPS mode will go active, and the autopilot will now seek out and intercept the course. Heading mode is no longer active. We could twist the heading bug to anything we want, and it won't affect the present operation of the autopilot. Now, this approach has LNAV minimums, which means it doesn't have an official vertical guidance like you'd see on an ILS approach or a GPS approach with LPV minimums. But, our GPS unit can compute an advisory glide path, so the annunciation we get is LNAV + V, plus vertical advisory guidance. That's what this diamond represents. But, let's say we don't want to follow it and there are good reasons some pilots don't. When we get to the final approach fix, Porter will need to begin a descent. We want our descent to go down to the MDA, 1240.

[00:02:10]So we bug 1300 ft on the altitude selector to give ourselves a buffer. The selected altitude is in light blue. We don't descend right away though, because the current mode in green is ALT, altitude hold mode and it remains 2000 ft, which was previously selected. In order to leave altitude hold mode, we can't just enter a new selected altitude. We have to tell the autopilot how to leave altitude hold. When we get to the final approach fix and want to begin our descent, we do just that by pushing the VS vertical speed hard key and pushing nose down a few times to set a vertical speed.

[00:02:48]Now vertical speed is active in green at a selected rate of 500 ft per minute.

[00:02:53]Notice also that ALT S is in white, meaning selected altitude capture mode is armed.

[00:03:00]The light blue selected altitude of 1300 will be captured. So the autopilot always knows what's next. Green is what it's doing, white is what it's waiting to do and light blue is what target it's working toward. So what'll happen is we'll get close to our target at 1450, 150 ft away, selected altitude capture mode goes active in green. The altitude to be captured is 1300 and now ALT altitude hold mode is armed in white. It levels us off and goes into altitude hold mode at 1300. We're now in GPS mode and altitude hold mode all the way to the missed approach point. We could select a new altitude, 5000 ft, the missed approach altitude and nothing happens.

[00:03:45]It's still holding that old selected 1300. At the missed approach point, we'll only climb when we push the FLC key and set a climb speed. With the approach unsuspended, the autopilot will follow the GPS course of the missed approach as we climb to 5,000. ALTS, selected altitude capture mode, is once again armed in white.

[00:04:07]Let's look at the approach into the opposite runway, the RNAV into 16. This is an LPV approach with official vertical guidance we must fly, not like the advisory glide path which was optional. Once again, we start in heading mode and get it turned to intercept the approach course. So, we twist the heading bug to fly it, but this time we push APR to arm approach mode instead of NAV. GPS is armed as before in white, but we also have GP, glide path mode, armed as well. Again, as before, the autopilot captures the approach course when GPS mode goes active. The minimums for the LPV approach are 944.

[00:04:47]If we set the altitude selector to 1,000, we don't move because we're in altitude hold mode. I wouldn't recommend setting 1,000 like this for this type of approach, and we'll see why in a sec.

[00:05:00]The diamond comes into center, and glide path mode goes active. We start down on the approach.

[00:05:06]On the last approach, the altitude we selected was automatically captured because ALTS was armed. Here, we have no such armed mode on the status bar. So, what happens when we arrive at the bugged altitude of 1,000 ft?

[00:05:21]Nothing. The descent continues. To make matters worse, even arriving at the decision altitude of 944 triggers nothing from the autopilot. We go below 944 and continue on the glide path. We can't go below 944 unless we're going to descend towards the runway. To go missed, it's best to click off the autopilot, start a climb, execute the missed by flying runway heading, and we can bug the missed approach altitude of 3,000, what we should have done originally, and hit FLC and re-engage the autopilot.

[00:05:56]Now, we'll follow the GPS course to the missed and climb at the selected airspeed with selected altitude capture mode armed at 3,000 ft.

[00:06:05]Remember that the autopilot always knows what's next and keeps you in the loop by displaying active and armed modes on the status bar. If you're studying for your IFR checkride, Flight Insight has a great free resource to make your life much easier. It's a 15-page checkride at a glance document with everything your DPE will ask you on the big day. Study it and test yourself as you prepare.

[00:06:30]Download for free at the link here and get started now.