Plane & Pilot
Tuesday, October 8, 2013

Your Best Friend Or Worst Enemy

How well do you know your autopilot?

Roll Steering. A simple autopilot system is "reactive," so that turns begin after passing a waypoint with oscillations about the new course until errors "damp out," which may never happen in strong crosswinds. Also called GPSS (for GPS steering), roll steering fixes these problems by using data from the GPS navigator to provide predictive control, so that turns and wind correction are anticipated. Multileg flight plans are followed precisely regardless of the winds, and even hold patterns can be flown perfectly. Roll steering greatly improves the accuracy and smoothness of any AP system.

Lateral Navigation Modes
Here's how AP functions work to get from point A to B on the map.

Roll Mode. When you first engage many autopilots and you don't tell it where to go, it will wake up in "roll mode." In its simplest form, roll mode simply holds the wings level, which is the basis of a simple wing leveler. In more sophisticated systems, roll mode can also hold a constant bank angle (or turn rate depending on how the system is configured). Most APs don't have a specific button for roll mode and will simply revert to this mode when all other navigation modes are canceled. Note that some popular systems can only be engaged in either heading or navigation mode, so there's no "roll mode."

HDG. Heading mode commands the AP to follow the heading bug on the DG or HSI. It's pretty simple—turn the bug in the direction you want to go, and the plane will follow the bug and hold the heading. It's best practice to always "sync" the bug to your current heading before engaging HDG mode. Once the AP is properly engaged, then turn the bug to your desired heading. There are often limits on how closely the heading bug must be to the current heading before the AP will turn to follow the bug, although some systems will allow a range of +/- 179 degrees and more if you engage and then move the bug past 180 degrees. Engaging HDG mode without first synching the bug can result in a surprising and sometimes dangerous turn, so make it a habit to always keep your bug synched to your current heading.

NAV. In simple terms, this mode tells the AP to follow a navigation signal whether it's from a VOR, ILS or GPS. Every AP has built-in rules and limits for acquiring the desired course. In the simplest case, activating a direct course to your next waypoint when you're roughly pointed in the right direction will allow NAV mode to keep the aircraft on the desired track to the next waypoint.

Be aware that simply pressing "Direct To" on your GPS and then "NAV" may initiate an immediate turn toward the next waypoint, so make sure that there's nothing in the way when you press the button. If you're transitioning from HDG to NAV mode, the AP logic will simply "arm" NAV mode (often indicated by a blinking or different-colored indicator) until you're close enough to an active navigation leg to allow the course to be intercepted.

Once intercepted, the NAV mode will go "active," and the NAV annunciator will come on. One "gotcha" to remember is that if you're in NAV mode and you "activate an approach" on a Garmin navigator, the autopilot will command an immediate turn to the initial approach fix—even if it happens to be behind you. That unexpected turn can steer the aircraft into terrain. Be sure you know what the autopilot is going to do before you change anything, monitor it closely, and disconnect it immediately if it doesn't do what you expect it to do.

APC. Approach mode is basically a more sensitive version of NAV mode with a couple of additional features. When you're flying an ILS, APC mode increases the sensitivity of the system so that the course is held more precisely. APC mode also arms glideslope tracking. When flying a GPS approach, the GPS typically controls the course sensitivity, so APC mode simply arms Glide Path tracking (for WAAS-based systems). In order to avoid major turns while on approach, most systems normally impose angular intercept limits (typically around 45 degrees), to intercept a course using APC mode. APC mode normally has the lowest approved altitude limitations—typically 200 feet AGL for Cat I approaches (for virtually all GA aircraft).


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