Cessna 172sp KAP 140 altitude tolerance
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Joined: May 2005
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From: dk
Cessna 172sp KAP 140 altitude tolerance
Hi
I'm renting a 2000 cessna 172sp. It is a really nice airplane.
I have now gotten the hang of the main functions of the autopilot, a KAP 140, but I don't know what to expect from it.
When I start the avionics the KAP 140 is asking for the BARO setting. I input the same as on my Altitude indicator.
Later when trying to use it in ALT mode it is holding the altitude but often with an offset of a few hundred feet. I feel that tolerance as being somewhat large. What tolerance can I expect from an autopilot like this ?
ps. I know that of course the system will have an offset if the baro settings from the different systems do not match. I have not tried to change the baro setting of the gps a kln94, but I don't thinkt that has any to do with it.
I'm renting a 2000 cessna 172sp. It is a really nice airplane.
I have now gotten the hang of the main functions of the autopilot, a KAP 140, but I don't know what to expect from it.
When I start the avionics the KAP 140 is asking for the BARO setting. I input the same as on my Altitude indicator.
Later when trying to use it in ALT mode it is holding the altitude but often with an offset of a few hundred feet. I feel that tolerance as being somewhat large. What tolerance can I expect from an autopilot like this ?
ps. I know that of course the system will have an offset if the baro settings from the different systems do not match. I have not tried to change the baro setting of the gps a kln94, but I don't thinkt that has any to do with it.
Joined: May 2001
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From: UK
In my experience in reasonable conditions the KAP140 is capable of maintaining the set altitude to within 50 feet.
However, it does seem to be the single feature of the KAP140 most prone to not work properly for a range of reasons, including any blockage or water in the pipes, and the tolerance of the software set to wide.
However, it does seem to be the single feature of the KAP140 most prone to not work properly for a range of reasons, including any blockage or water in the pipes, and the tolerance of the software set to wide.
Joined: Dec 2001
Aviation Qualifications: ATPL
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From: GA, USA
The KAP 140 is capable of maintaining altitude to within 50 feet.
However, on the intercept of an altitude it is very common for it to level of +/- 200 feet.
The trick is to press alt when VS appears set the vertical speed for +200fpm or -200fpm and press alt again for it to capture the desired altitude and maintain it.
Or another trick is to start reducing the arte of climb or descent as you are approaching the target altitude.
Reasons:
It controls pitch through the electric trim, it will start levelling off when it reaches the preset altitude but it does not anticipate the level-off.
It therefore usually ends up a little high or low, deviation depending on the rate of climb that was used. A high rate of climb or descend will lead to a larger altitude deviation.
The KAP 140 does not use the same static port as the altimeter, therefore small pressure variations can affect the altitude level-off.
Windows open or vents?
Software inaccuracies?
Cable slack in the flight controls?
Auto-pilot servo's need adjusting?
However, on the intercept of an altitude it is very common for it to level of +/- 200 feet.
The trick is to press alt when VS appears set the vertical speed for +200fpm or -200fpm and press alt again for it to capture the desired altitude and maintain it.
Or another trick is to start reducing the arte of climb or descent as you are approaching the target altitude.
Reasons:
It controls pitch through the electric trim, it will start levelling off when it reaches the preset altitude but it does not anticipate the level-off.
It therefore usually ends up a little high or low, deviation depending on the rate of climb that was used. A high rate of climb or descend will lead to a larger altitude deviation.
The KAP 140 does not use the same static port as the altimeter, therefore small pressure variations can affect the altitude level-off.
Windows open or vents?
Software inaccuracies?
Cable slack in the flight controls?
Auto-pilot servo's need adjusting?
Joined: Jun 2003
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From: EuroGA.org
Generally, an autopilot will use the gray-code output from an encoding altimeter to capture the preset altitude, and afterwards will use an internal barometric sensor to hold that altitude.
There are thus two separate issues.
If the preset alt is not being accurately captured, that is down to the altimeter, the link to it, or something subtle in the AP.
If the preset alt is captured OK but is not held thereafter, the baro sensor in the AP system is probably duff. This is not unusual.
There are thus two separate issues.
If the preset alt is not being accurately captured, that is down to the altimeter, the link to it, or something subtle in the AP.
If the preset alt is captured OK but is not held thereafter, the baro sensor in the AP system is probably duff. This is not unusual.
Thread Starter
Joined: May 2005
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From: dk
It is indeed the preset altitude that is wrong.
When the altitude is captured it is maintained within a small tolerance.
I have been suspecting the transponder to show an altitude also off by some hundred feets. Is the grey-code information coming from here?
When the altitude is captured it is maintained within a small tolerance.
I have been suspecting the transponder to show an altitude also off by some hundred feets. Is the grey-code information coming from here?
Joined: Jun 2003
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From: EuroGA.org
The source of the altitude capture/preset data could be an encoding altimeter, or some other source e.g. a stand alone altitude encoder which might also be feeding a Mode C transponder.
You can get your Mode C XP checked by ATC next time you fly - ask them what you are returning. Unless it is one of the modern ones e.g. a KT76C which shows the actual returned flight level on the front panel.
It is not unusual for one of the dozen or so gray code wires to have a bad connection, which manifests itself as all kinds of weird things...
You can get your Mode C XP checked by ATC next time you fly - ask them what you are returning. Unless it is one of the modern ones e.g. a KT76C which shows the actual returned flight level on the front panel.
It is not unusual for one of the dozen or so gray code wires to have a bad connection, which manifests itself as all kinds of weird things...
