We get calls all the time from pilots who want to remove the vacuum system from their airplanes and want to know the difference between analog and digital autopilots. But what they should be asking about is the difference between an attitude- and a rate-based autopilot. (Seems that somehow they’re getting attitude confused with analog…)
Attitude vs. Rate-based Autopilots
An attitude-based autopilot is going to get its information directly from the aircraft’s mechanical attitude gyro. And since these instruments are vacuum driven, basically the attitude-based autopilot can be considered to be “vacuum driven” too.
As you know, vacuum pumps are legendary for their unreliability – who doesn’t know a pilot that has had a vacuum pump go south in IFR conditions? – so, it’s no wonder pilots want to replace anything vacuum driven with much more reliable electronic instruments.
Of course, when the vacuum pump is working, there’s nothing in the world wrong with an attitude-based autopilot. There are thousands of them in all kinds of airplanes. And if you do want to make the switch from a vacuum to an electrically driven panel, there are a variety ways to make an attitude-based autopilot work with an electric attitude indicator.
If you’re looking for an all-electric autopilot, S-TEC’s family of rate-based autopilots gets their attitude information from the electric turn-coordinator. So if the vacuum pump or gyro fails, that has no affect on the autopilot’s continued functionality. As you might guess, the MTBF (Mean Time Between Failure) rate on an electric turn coordinator is light years ahead of even the best vacuum pump.
The newest generation autopilots like our Digital Flight Control System, DFCS-1950, -2100, and the S-TEC 5000 Digital Autopilot get all of their reference information from the aircraft’s air data and attitude, heading reference system (ADAHRS). That’s a 100% all-digital unit and provides the highest possible level of accuracy and reliability. Because of their higher performance, and cost, these units are found in higher-performance turboprops and jets.
So what’s the benefit in knowing which type of autopilot you have in your airplane? Well, aside from knowing what you might have to change or add should you want to convert from vacuum to an all-electric panel, you need to have a clear understanding of what impact specific equipment failures will have on your autopilot’s ability to keep your wings level – at night and in IFR conditions. It’s a simple fact of flying: nothing ever fails when you don’t need it.
If you have an attitude-based unit ask yourself this: does your airplane have a standby vacuum or manifold pressure backup pump? If so, when was the last time you tested it?
If you have a rate-based autopilot: how much reserve battery power do you have to operate your avionics and autopilot should your alternator fail?
Knowing the answers to these key questions now will go a long way to reducing your stress level should you encounter a problem.
Of course, like any safety conscious pilot, no matter what systems are in your airplane your best course of action is to understand those components now and know what your out plan is before things start going south.
Until next time, fly safely,