A Garmin setup with autopilot controls on the bezel.
It’s almost certainly happened to you: When you engage the autopilot, the airplane suddenly turns or pitches some way other than what you intended. This “what is it doing now?” moment is more academically known as mode confusion, a disconnect between your automation expectations and what really happens. It’s usually because of a mistake in your inputs into the navigation device or the autopilot controller. An autopilot is a very accurate but very stupid copilot—it will do exactly what you tell it to do, whether that’s what you wanted to do or not. At best, mode confusion is a nuisance. At the wrong time, it can be deadly.
I’ve introduced a lot of pilots to a lot of different types of autopilots in over 30 years teaching in piston twins. What began for me as a building-block teaching technique has evolved in the way I use autopilots myself, beginning with the basics and adding complexity from there. It puts the aircraft in simple automated control right away while giving me time to add advanced modes as desired from there.
Autopilot modes
Autopilots have two basic modes of operation: vertical modes and lateral modes. These are the two axes that make for a two-axis autopilot (to be a three-axis autopilot it must include a yaw damper as well).
Depending on the specific autopilot, vertical modes may include:
- Attitude mode (holding a constant pitch attitude)
- Altitude mode (holding a con-stant altitude)
- Vertical speed mode (holding a constant rate of climb or descent)
- Airspeed mode (holding a constant indicated airspeed). G1000 systems call this Flight Level Change (FLC), but it is really a constant airspeed mode.
- Altitude preselect (holding vertical speed or airspeed mode until reaching a preselected altitude, and then automatically switching to Altitude mode to maintain that altitude)
- Approach mode (following a glideslope or electronic glidepath)
Attitude and Altitude modes are easy. Hit the Altitude button when you’re at your desired altitude—it’s smoother if you manually level off and trim before engaging the autopilot. Or turn on the autopilot without hitting the Altitude button and it will hold your current pitch attitude. Most autopilots default to Attitude mode when engaged unless you select one of the other modes.
The possibility of mode confusion exists when you engage one of the other vertical modes. You must make additional inputs to enter Vertical Speed, Airspeed (FLC), or Approach mode. If you’re delayed inputting all the variables, or if you’re busy and make a mistake doing so, then the airplane may go off in an unexpected vertical direction when you hit the AP (autopilot) button. You’ll have another “what is it doing now?” moment.
Lateral autopilot modes include:
- Wings Level or Roll Control mode (maintaining wings-level flight but not a constant heading)
- Heading mode (maintaining the heading under the heading bug)
- Navigation mode (following input from a navigation source such as GPS or a VOR)
- Approach mode (navigation mode with much tighter tolerances for more accurate tracking during an approach)
- GPS Steering or GPS Roll Control (navigation mode that adds turn anticipation and automatic sequencing, for example, entering and flying a holding pattern depicted by the GPS)
If you’ve not selected any other lateral mode, most autopilots will default to Wings Level (Roll) mode. Assuming your heading bug is set before you choose Heading mode, it will immediately hold that heading (or turn to it and hold, if the bug is not centered). Navigation and approach modes require additional programming of the navigational source, so there’s an added level of complexity and consequently a greater chance of mode confusion.
Simple to complex
Just as I introduce autopilot operation to students, I’ve found it useful in my normal flying to engage the autopilot first in the simple modes. That gets the airplane going in the basic directions (vertically and laterally) I want, reducing my flying workload. I’ll then add complexity. Here’s what I mean:
Autopilot engagement
- Before engaging the autopilot, establish the attitude and direction manually, and trim the aircraft.
- If already trimmed in level flight, push the ALT (altitude hold) button on the autopilot mode controller. The flight director (if equipped) will engage and indicate the level-flight attitude.
- If in a climb or descent, don’t push any vertical mode buttons. The flight director command bars will match the current pitch attitude.
- (Since I consistently use the heading bug even when hand-flying the airplane and therefore it should be centered on my current heading), push the HDG (heading hold) button on the autopilot mode controller. The flight director command bars show a wings-level attitude on the current heading.
- Then push the AP (autopilot) button. The autopilot engages, holding the horizontal and vertical modes already selected.
Confirm each mode as you select it, and that the airplane is on the desired vertical and lateral guidance, before advancing to a higher state of complexity.
There’s very little chance of mode confusion engaging the autopilot this way. Oh, you might not have the heading bug centered and the airplane will turn steeply toward the bug (don’t laugh; I did this on my initial CFI checkride in a Cessna 182RG when the examiner asked me to demonstrate the old Navmatic my instructor had never let me use). But I’m a proponent of always setting the heading bug before a turn even while hand-flying, and even if you forget then following this engagement technique you’ll see that in the flight director command bars (if equipped) and catch it before you turn the autopilot on.
Now that the airplane is on the attitude and heading you want, you can add automation complexity if you wish:
- Confirm the navigation source is programmed correctly and then hit NAV (navigation mode) or APP (approach mode) as required.
- If your airplane is equipped with GPSS (“roll steering”), in all but the latest integrated systems the autopilot must be in the HDG (heading) mode for GPSS to work. Starting—and remaining—in the heading mode is precisely what you need to do to use GPSS in many installations, but it also introduces another mode confusion possibility if your GPS is not already programmed if your GPSS is on. Leave GPSS off, enter heading mode, check the navigation programming, and then turn on the remote GPSS switch.
- Program the vertical mode: vertical speed, airspeed/FLC, and/ or altitude preselect. Then engage the desired vertical mode.
- Confirm the selected modes are engaged and that the airplane is on the vertical and lateral guidance you want.
By design
This technique works at what is probably the highest workload time, a missed approach. In most
installations hitting the Go-Around (GA) button (if you have one) disengages the autopilot but moves the flight director command bars to a wings level (roll control) 7° nose up attitude (attitude mode). The pilot manually flies to match this optimal initial guidance. It’s then an easy two-button task (HDG and AP) to put the autopilot in these modes and confirm they’re working.
Now that the autopilot is on, deal with the navigation of the missed approach procedure, including whether or when to exit GPS Suspend mode. When you’ve exited Suspend mode as applicable and have confirmed the navigation is set for the missed, stay in HDG if your GPSS requires, or turn on NAV if it doesn’t (or you don’t have GPSS). Then set your altitude preselect if you have one and have not already done so.
If you don’t have the GA feature, a go-around would begin with deactivating the autopilot and beginning a straight-ahead climb—again, wings level and 7° nose up in most cases. Hit HDG and AP and the autopilot engages on your current heading at your current pitch attitude. Do all the navigation tasks and transition to more complex autopilot modes afterward.
Some of the most recently certificated autopilots, including the Garmin GFC600 now approved for many light twins and turbines, include the GA feature with a significant difference: Engaging GA mode does not shut off the autopilot. It does, however, move the flight director command bars to a straight-head, 7° nose up attitude—defaulting to the simple roll control and attitude modes—then automatically flying the airplane to that guidance. From there it’s up to the pilot to program, confirm, and add autopilot complexity the same as if you had the GA button. By design, the latest autopilots follow the build complexity technique.
Starting simple and building complexity makes mode confusion unlikely, when otherwise it could have a distracting and potentially disastrous outcome. Using this “start simple, then build complexity” technique eliminates most of the risk of mode confusion. I can truly say I’ve not had any “what is it doing now?” moments since I adopted this as my standard operating practice. It’s a good technique for mastering automation, and the aircraft.
