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Guest KenG

Autopilots and vacuum failures.

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A question for all you real world pilots. I've been reading recently about a couple of instances of pilots trying to fly partial panel in bad weather and coming to bad ends. Everytime I hear about something like this, I wonder why the guy didn't just use the autopilot. Then it occured to me that if you've lost vacuum pressure and have no gyro reference, does the autopilot still function.So that's the question. Is the autopilot linked to the Attitude Indicator and thus fails when it fails, or does it have it's own independent system? Or does it vary from type to type?

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The problem is that not all General Aviation airplanes have autopilots and one is not required to fly in IFR weather, although it sure is nice to have one. As far as failure modes are concerned it does depend on the particular type of autopilot installed. Some have their own electric gyro that is independent of the one that drives the attitude indicator and in that case you would be able to use the autopilot but these are usually the more expensive type that are used in high performance (and priced) aircraft.If a pilot practices partial panel enough there is no reason that you can't fly an approach and land succesfully if the gyro instruments fail. That is one use that a good realistic PC sim can help with.Ed Weber a.k.a tallpilot

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As indicated, different autopilots use different systems. However, most GA autopilots do not use the attitude indicator (AI) for determining what the plane is doing. On the S-Tec's, they use the turn coordinator for bank info and solid-state absolute pressure transducer and accelerometer for pitch inputs. These data will be interpreted by a computer and then the correct output data will be sent to the servos controlling the moveable surfaces. The AI and heading indicator do not supply any info as they have too many gyroscopic errors and a tendency to tumble at unusual attitudes. Also, as you indicated, a vacuum failure would cause all kinds of problems for the autopilot.For basic single axis a/p, a wing leveler will be all there is and it is controlled by the turn coordinator. If the a/p has heading hold, the heading info will come from the heading bug. Navigational control (NAV hold, LOC hold, etc.) will come from the NAV gauges. Altitude holds will be coupled to both the altimeter bug on the altimeter or digital input on the a/p gauge and the pressure transducer.At least, that's how I understand it to work from reading up on a/p. Someone let me know if I'm off base on any of this.

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