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Q re attitude-based autopilot (AP-3C) and GPSS
I read a review of the Chelton AP-3C recently, and it has me wondering some
things. It's attitude-based. The article I read seems to work on the presumption that this is better than rate-based, but it does later say that one shouldn't be dependent upon a vacuum-based AI for one's autopilot. So what do people with attitude-based APs typically do? Electric AI? Which electric AIs don't tumble? I'm also wondering about the ability of the AP-3C's ability to use GPS steering in both horizontal (GPSS) and vertical (GPSV) planes. If on a GPS flight plan, what would occur if the AI failed? If the AP is working to keep to the flight plan (in both planes), wouldn't the airplane stay straight and level even if the AP thinks that the plane is in a descending turn (or whatever it thinks from the failed AI)? - Andrew |
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I read a review of the Chelton AP-3C recently, and it has me wondering
some things. It's attitude-based. The article I read seems to work on the presumption that this is better than rate-based, but it does later say that one shouldn't be dependent upon a vacuum-based AI for one's autopilot. The one which is better depends on which kind is made by the person writing the article. There are good and bad autopilots of both types. Vacuum or electric AI for attitude based autopilots doesn't really matter. What matters is that you have enough equipment to cross check so that an instrument failure, especially one the autopilot depends on, can be detected by the pilot. In a spinning mass gyro, failures from bearing wear or other mechanical related problems are as likely occur in air driven as electrically driven instruments. But to compare the sources of power for those, an electrical bus with multiple sources of energy (alternator, battery) is statistically more reliable than a vacuum pump. So what do people with attitude-based APs typically do? Electric AI? Which electric AIs don't tumble? I'm also wondering about the ability of the AP-3C's ability to use GPS steering in both horizontal (GPSS) and vertical (GPSV) planes. If on a GPS flight plan, what would occur if the AI failed? If the AP is working to keep to the flight plan (in both planes), wouldn't the airplane stay straight and level even if the AP thinks that the plane is in a descending turn (or whatever it thinks from the failed AI)? Attitude based autopilots depend on electrical signals from an attitude gyro which may be the instrument in your panel or in EFIS equipped aircraft in a box mounted somewhere in the plane. In addition to the pitch and roll signals, there are validity signals that tell the autopilot whether its ok to use the pitch and roll signals. If the gyro isn't spinning fast enough or doesn't have power at all then the validity signal state will cause the autopilot will stop using that instrument for guidance. Depending on the model, it may disconnect, sound an alert, or both. It's remotely possible for the attitude sensor to fail somehow and the validity signals still indicate its working ok. In this case the autopilot will try to keep flying the plane to the bad pitch and roll with the obvious result. The pilot will have to notice this and disconnect the autopilot if necessary. Normally an autopilot will not keep operating in nav mode if its basic orientation sensors such as the attitude gyro or turn coordinator are broke. GPS H and V steering represent a special case of nav mode because then the FMS or GPS is literally telling the autopilot how much to bank the wings or pitch the nose, which is different from telling the autopilot how much the localizer or glideslope needle is deflected. Answers to your questions about the behavior of GPS H and V steering during attitude sensor failure are autopilot model dependent, and you should ask Chelton about the AP-3C. I think you'll find that in most or all cases, the autopilot will cease to function if the attitude sensor has failed, including roll steering modes. Let us know what Chelton says. This information is very general because autopilot interfaces and behaviors and sensors vary widely. The FAA regulatory guidance for autopilot certification is very lightweight and is basically oriented at ensuring the pilot can detect and recover from an autopilot problem (or underlying autopilot sensor problem) before it can kill him. If any autopilot experts out there think I've said something erroneous, feel free to correct me so we can all learn. |
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Victor wrote:
The one which is better depends on which kind is made by the person writing the article. Well, I was under the impression that the article I'd just read (in Aviation Consumer) was written by a disinterested third party. I could be wrong, of course laugh. There are good and bad autopilots of both types. Vacuum or electric AI for attitude based autopilots doesn't really matter. I don't see that as being the case, for reasoning I explain below. [...] But to compare the sources of power for those, an electrical bus with multiple sources of energy (alternator, battery) is statistically more reliable than a vacuum pump. That almost doesn't matter as I see it. The AP is electrically powered. Lose electricity, and you've lost your AP. In that case, that the AI has failed has no impact upon the AP. On the other hand, for an AP with a vacuum driven AI as a sensor, loss of either electrical power or vacuum disables the AP. So regardless of the relative reliability of vacuum vs. electric, the AP on a vacuum AI is less reliable than the AP on an electric AI (or electric TC if rate-based). No? [...] Attitude based autopilots depend on electrical signals from an attitude [...stuff I didn't know and am happy to learn snipped...] [...] ... and you should ask Chelton about the AP-3C. I think you'll find that in most or all cases, the autopilot will cease to function if the attitude sensor has failed, including roll steering modes. Let us know what Chelton says. Will do. [...] If any autopilot experts out there think I've said something erroneous, feel free to correct me so we can all learn. Please. - Andrew |
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