KAP 140 baro setting glitch

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Rich
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KAP 140 baro setting glitch

Post by Rich »

I've posted this here and there but I thought to see if anyone else has encountered something like this problem: In order to get the AP altitude preset to come to agree with my altimeters I have to put the baro setting 0.11" less than that of the altimeters. The amount needed for agreement changed upon coming out of an IFR/transponder/encoder recert. It has been .06" for years.

Pressure altitude emitted on the transponder appears correct and that was explicitly part of the recert.

Has anyone else seen this kind of thing and more to the point, gotten it fixed? The workaround is OK, sort of, but I'd like to get at the root of the behavior.
2002 DA40: MT, PowerFlow, 530W/430W, KAP140, ext. baggage, 1090 ES out, 2646 MTOW, 40gal., Surefly, Flightstream 210, Orion 600 LED, XeVision, Aspen E5
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Charles K
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Re: KAP 140 baro setting glitch

Post by Charles K »

Never had that issue.
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Re: KAP 140 baro setting glitch

Post by Rich »

I've been trying to discern the interplay in the AP between the static line feed and the Sandia SAE5-35 altitude encoder inputs to come up with how the altitude preselect works. Some things I discovered via various documents/diagrams and experience:

1. The encoder delivers PA data via serial outputs to the transponder and both Garmin GNS units. What the GNSs do with this I don't yet know.
2. The encoder delivers PA data via Gray code (nine bits in as many discrete conductors + 1 common) to the AP.
3. The encoder serial output delivers 10 ft. resolution. Gray code is only 100 ft. resolution.
4. The encoder can be recalibrated to bring it in agreement with the altimeter(s) during encoder recertification. (Ah ha!)
5. The encoder input to the AP is required only for the altitude preselect.
6. Preselect can only be done in 100-ft. increments, as we all know.
7. I can find no configuration provision in the AP installation manual to adjust something like baro bias.

A couple of relevant in-flight observations and factors:

1. The altitude at which the AP will level off is influenced by baro setting variations of ".01", so it resolves down to 10 ft.
2. Altitude alerts (200 ft. and 1,000 ft.) are influenced by the baro setting acting against the altitude set by the pilot. I have found these alerts to be consistent with the discrepancy in the level-off altitude. I'm not 100% sure that they happens to 10-ft. resolution, but it seems to.
3. My plane had a pitot-static and encoder recert done during the same shop visit as the Aspen installation (they were both due).
4. The bias I had to enter was different upon getting the plane back than it was before .
5. PA displayed on the transponder appears accurate but I need to check this against what is in the ADS-B output for better resolution.
6. Altimeter reports from their cert tests show very close agreement between them and with the cert standard. I didn't get such a report for the encoder. I will seek this information, as they must have recorded it.
7. If I adjust AP baro setting intentionally way off the preselect will react to such an incorrect setting accordingly, but still with that additional bias I've noted.

Tentative deductions so far:

1. The technician doing the recert tweaked the encoder calibration during the recert. This would account for the change. It's not improper that this was done and it presumably improved agreement with the altimeter(s).
2. The level-off action (and possibly the alerts) involve some interplay between the AP static line input and the encoder to get that sub-100 ft. target point.
3. Once leveled off the encoder is out of the picture, the AP reverting to the same ALT hold mode available without using preselect.

So now what? For the time being I live with it. I'm trying to identify and get in contact with a technician that knows the ins and outs of this model. Other than this idiosyncrasy it's behaving fine. In fact, with the Aspen it seems to ride better turbulence better than before.
2002 DA40: MT, PowerFlow, 530W/430W, KAP140, ext. baggage, 1090 ES out, 2646 MTOW, 40gal., Surefly, Flightstream 210, Orion 600 LED, XeVision, Aspen E5
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Re: KAP 140 baro setting glitch

Post by Rich »

I'm pretty confident it's the encoder. I hypothesized that the encoder was providing an altitude 100+ ft higher than it should. Note that transponder/encoder tests are done by comparing encoder outputs to the primary altimeter pressure altitude display. The discrepancy is allowed to be as much as 125 ft. My primary (and standby) altimeters have correction records from the recent cert that have them off the standard at 3000 ft. by less than 10 ft. Today I set them at 29.92. They both displayed PA of 3000 ft. I looked at what the transponder showed and it was FL 31 (i.e. 3100 ft.)

More data: altimeter setting this morning was 30.17 and field elevation is 3250 ft. So PA should have been 3000 ft.
2002 DA40: MT, PowerFlow, 530W/430W, KAP140, ext. baggage, 1090 ES out, 2646 MTOW, 40gal., Surefly, Flightstream 210, Orion 600 LED, XeVision, Aspen E5
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Re: KAP 140 baro setting glitch

Post by Rich »

Today I ran an in-flight test and confirmed my hypothesis. First I came up with a different procedure: Enter the AP to the same baro as the altimeters and the altitude preset 100 ft. higher than the desired altitude. In this case, desired was 6500 ft, so I entered the preset as 6600. The idea being to get it to "level off when the pressure altitude from the encoder is consistent with 6600 ft.", which my theory said would amount to 6500 ft.

Bingo, got the altitude alert at 5500 and the plan leveled off right at 6500 ft. :D

Now some more information:
Baro setting was 30.22. After confirming the above I changed the Aspen to 29.92 and got a pressure altitude of 6230. The display on the transponder was bouncing a bit between FL 63 and 64. In Foreflight I then looked to get a more granular look at what PA the transponder was sending. It was bouncing between 6325 and 6350, right at 100 ft high.

So all is confirmed. The good news is I have a workaround. The not so good news is the encoder is at the ragged edge of allowable test limits. :scratch:
2002 DA40: MT, PowerFlow, 530W/430W, KAP140, ext. baggage, 1090 ES out, 2646 MTOW, 40gal., Surefly, Flightstream 210, Orion 600 LED, XeVision, Aspen E5
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