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About Autopilot

Contrary to popular belief, Autopilot does not operate on the premise of magic and Witchery; hard to believe, but it's true. In fact, Autopilot is quite simple in its functionality, and certainly one of the handiest instruments in the cockpit.

First thing we need to understand about Autopilot is this: Never, ever tell Autopilot to do something we ourselves cannot do. You couldn't horse a 230-MPH Merlin from 8000 feet onto a runway that's two miles away in the opposite direction...and neither can Autopilot. Again, there's no magic involved...Autopilot simply adjusts the flight controls the same as you, the real pilot, would do.

Autopilot systems perform a variety of tasks, but under the hood it all boils down to two jobs: (a) Climb or descend to maintain a desired altitude, and (b) Turn the aircraft to maintain a desired Heading. Okay, so let's define those mystical buttons now.

Autopilot Power - Turns power to the Autopilot on or off.

Adjust Altitude - Desired altitude for the Autopilot to reach and maintain.

Adjust Vertical Speed - Desired rate of climb or descent to altitude as set above.

Autopilot Lock - Engages/Disengages Autopilot functionality.

Heading Hold - When AP is engaged, this finds and maintains the Heading set in the HSI.

Nav Hold - Allows Autopilot to find & follow an active VOR-1 radial

ILS Approach Hold - When in range of an ILS station, this maintains Localizer & Glideslope.

Back-Course Hold - Locks Autopilot onto the Backside heading of an ILS runway.

Altitude Hold - In conjunction with Vertical Speed setting, this maintains desired altitude.

Nav/GPS Selector - For use when flying in Autopilot with the GPS system...explained below.

Basically, the Autopilot is a pretty straightforward little device that doesn't need a whole lot explaining...Heading-Hold maintains the Heading, Altitude-Hold maintains the altitude, and so on. But the Autopilot also performs a few other interesting chores, and we'll explain them now.

One: By using Nav Hold (don't confuse this with the Nav/GPS Switch, explained below), Autopilot will find (within reason) a VOR-1 radial and follow it. A big caution here, though, is to be sure you've selected the correct radial to follow. Remember from VOR training that (for example) the 90 degree radial-signal is actually on the East side of the VOR station, which makes perfect sense...I'm coming FROM the East and flying TOWARD the West, so I need the 270-radial signal on the East side where I can receive it and fly TO it. Another caution is that when you pass over the VOR station, Autopilot turns Nav Hold off, because the "TO" signal for the 270-radial dissapears. Smart little puppy, ain't it??

Two: Autopilot also works in conjunction with the GPS system, assuming a Flight Plan has been established. You activate this function by Selecting GPS on the Nav-GPS Switch and also Nav-Hold.

(It's noteworthy that the Nav/GPS Switch happens to be located on the Autopilot in this aircraft, but most often it is found elsewhere on the instrument panel.) The reason you need to select Nav-Hold is because Autopilot treats each leg of a Flight Plan (whether one or many legs are involved) as an individual VOR radial. As we know, a Flight Plan often has many legs running in various directions. An oddity of this feature is this: At the destination, Autopilot will not shut down as it does with a single VOR leg...it just keeps you flying around in circles over the destination until the airplane rots away to dust, or until you finally come home from the bar, sober enough to kick it out of Autopilot and land the plane.

Using Autopilot To Manage An ILS Approach

There's no limit to the controversy and conflict over how to use the Autopilot to manage an ILS approach, and that stems from a misunderstanding of how things work and what's going on. Many of us view Autopilot as a "magic box"...it doesn't matter where I am, what heading I'm on or what altitude; hit that little AP button and sit back. Well, dudes & dudettes, that simply isn't so... we gotta be nice to Autopilot if we expect it to be nice to us in return. How often we hear, "Hey, this thing isn't taking me down!" or "Man, I'm wobbling all over the place up here!" The problem is US, not the Autopilot, so let's see if we can untangle some of this yarn. We'll start by remembering a few of the Golden Rules concerning an ILS approach.

Rule #1: SLOW DOWN! That's where the "wobbling" comes from: too much speed has you shooting past the Localizer, with Autopilot correcting and shooting you past it the other way...and thus the "oscillation" because Autopilot is always correcting.

Rule #2: We approach an ILS Localizer at a reasonable, practical angle that is not in severe contrast to the runway heading. A rule of thumb is not to exceed 60-65 degrees, and we'll use Wilburville ILS for an example. The ILS runway Heading is 155 degrees, so if I'm approaching from the East (or left) I'd want to be on a heading angle not greater than 220 degrees (155+65). Conversely, approaching from the other (right) side, I'd subtract the 65 degrees, and I'd want to be at a heading angle not less than 90 degrees (155-65). NOTE: These numbers are nothing more than guidelines that will help Autopilot perform efficiently; they're not something you have to follow explicitly... Common sense will work just as well.

Rule 3: We want to be about stay somewhere around 3000 feet or so above ground level to intercept the Glideslope...generally, that means 5000-6000 above sea level for most places in our region. If we're flying the ILS manually (no AP), we can intercept the Glideslope wherever we want, but Autopilot works best if you intercept the Glideslope from below.

Think about what I just said: Regardless what our present flight configuration (Heading/altitude) might be, we're in the air and we have control of the plane...can you think of any justifiable reason not to set the plane up so that Autopilot doesn't have to struggle trying to figure out what you want it to do?

Rule #4: Remember the basic layout for any ILS system, as shown in the picture below, which steers us to Golden Rule #5...we don't ask Autopilot to start managing an ILS approach on top of the airport; start your approach from a distance that gives AP plenty of time to adjust and settle down. Ideally, we grab the Localizer first and fly along it until we intercept the Glideslope, but often we'll intercept Localizer and Glideslope at the same time. Nothing wrong with that, as long as we're not too close to the airport, and not too far away from the Glideslope, vertically.

Okay, let's summarize what we know:

1. Don't expect Autopilot to perform miracles.

2. Keep the airspeed down to avoid Autopilot "over-correcting".

3. Localizer works from 25-30 miles out, Glideslope from 15-20 miles out.

4. Try to hit the Localizer at 25 miles or so from the airport.

5. Hit the Localizer at an angle not to exceed 65 degrees in conflict with the runway heading.

6. Hit the Localizer with some distance from either side...not on top of it.

7. Establish and maintain not more than 3000-4000 feet above ground level to intercept Glideslope from

below.

The Perfect Autopilot- ILS Approach

What a brag this is, huh? Well, eating Crow is not my favorite sport, so you can safely assume this works. Granted, most of us develop our own way of doing things, and there's nothing wrong with that if it agrees with aircraft and instrument capabilities. Anyway, this is my own method, and it works with any airplane, any airport, any ILS system...when I have a sloppy or failed ILS approach, it's because I tried to hot-dog my way around procedures that I know work every single time.

1. I fly Autopilot Heading-Hold and position myself around 10 miles off to the side of the Localizer, and I

don't really care which side.

2. I fly along until I'm 25 miles or so away from the approach-end of the ILS runway, then I drop

airspeed to maybe 70% or so and turn toward the Localizer at an angle that's maybe 40-50 degrees

away from the runway heading, something like that, and I'm not real fussy about exact numbers. Why

the distance and the odd approach angle? Because I want to establish the Localizer well before the

airport, not on top of it!

3. I drop down to about 3000 feet above ground level and maintain that altitude.

4. When the Localizer is established (not before!), I kick Heading-Hold off and kick Nav-Hold on.

Remember, Autopilot treats the Localizer as a VOR radial...when the Localizer finally comes up,

Autopilot will continue to fly toward the Localizer until we begin to intercept it, and then make a very

gentle, easy turn to follow it inbound.

5. So Autopilot has me flying comfortably along the Localizer, headed for the runway centerline, and I'm

far enough out that I haven't grabbed the Glideslope yet. But when I do see the Glideslope

Indicators, I simply kick Nav-Hold off and I kick APR-Hold on.

THAT'S IT...Nothing else to do but radio ahead to make sure the beer is cold, and disable Autopilot before touchdown. I don't really care what you do that may or may not work some of the time, or always, or never...this method guarantees you a successful, enjoyable Autopilot-ILS approach every single time.

Autopilot Practice Flights
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Practice Flight-2	Download
Practice Flight-3	Download