Group:
I have recently started my IR training so, as you might expect, I'm paying much more attention to my VOR receiver and CDI. In the past, I have suspected it might not be completely accurate, but yesterday I paid particular attention to it so as to explain it to an avionics guy, but thought I'd run it past the group first.
Equipment: KX-170B, KI-208 (the one without GS) and '68 original 'whisker' VOR antenna with balen on the tail.
Yesterday, took off and tuned in a nearby VOR. I was able to center the CDI but at one point, I noticed the needle actually swinging from left to right as I was flying straight and level. Deflection was about 3 or 4 dots and slowly swinging to either side.
At other points in the flight, it seemed to indicate correctly.
Also, I have noticed that the needle will lag when adjusting the OBS. In general, I have to turn the OBS slowly to allow the needle to 'catch up' with the OBS's indicated heading. What I mean is that the needle may be fully deflected, I turn the OBS to the appropriate heading to center the needle, about 2 seconds pass, THEN the needle centers.
From a systems perspective, I see 4 possible sources for this issue (especially the 'swinging' issue observed yesterday):
1) Bad CDI
2) Bad KX-177B
3) Bad cables/connectors
4) Bad antenna
I bought the CDI from Flying Fish who has a good reputation, but it WAS bought without a 337.
The KX-177B is original to the plane as is the antenna.
As background, last year, I spoke to an avionics guy at Pacific Coast Avionics when I described to him another issue I had noticed: when using the #1 CDI and having it properly tuned in and centered, when I turned on the #2 Nav to tune and compare the #2 CDI, the #1 CDI would deflect! Damnedest thing.
His explanation was the use of the Balen in 1968 for the antenna: that when turning on the #2 Nav, the voltage on the antenna wire had to 'build up' again (The #1 WOULD come back after a few moments and center) thus causing the #1 CDI to 'lose' the voltage and have the needle deflect.
Because of this background, I am suspicious that the cause of the CDI's weird indications now is the antenna/balen and NOT the KX-177B or CDI. But what do I know?
So, I'd like to throw it to the group and see what the consensus would be regarding the CDI swinging left/right. If it DOES appear to be the antenna, when I look at Aircraft Spruce, there are a million VOR antennas with varying costs. Any recommendations? And is it necessary to replace the old original coax when replacing the antenna?
Alternatively, any thoughts on how else to figure out what is causing the issue?
I have recently started my IR training so, as you might expect, I'm paying much more attention to my VOR receiver and CDI. In the past, I have suspected it might not be completely accurate, but yesterday I paid particular attention to it so as to explain it to an avionics guy, but thought I'd run it past the group first.
Equipment: KX-170B, KI-208 (the one without GS) and '68 original 'whisker' VOR antenna with balen on the tail.
Yesterday, took off and tuned in a nearby VOR. I was able to center the CDI but at one point, I noticed the needle actually swinging from left to right as I was flying straight and level. Deflection was about 3 or 4 dots and slowly swinging to either side.
At other points in the flight, it seemed to indicate correctly.
Also, I have noticed that the needle will lag when adjusting the OBS. In general, I have to turn the OBS slowly to allow the needle to 'catch up' with the OBS's indicated heading. What I mean is that the needle may be fully deflected, I turn the OBS to the appropriate heading to center the needle, about 2 seconds pass, THEN the needle centers.
From a systems perspective, I see 4 possible sources for this issue (especially the 'swinging' issue observed yesterday):
1) Bad CDI
2) Bad KX-177B
3) Bad cables/connectors
4) Bad antenna
I bought the CDI from Flying Fish who has a good reputation, but it WAS bought without a 337.
The KX-177B is original to the plane as is the antenna.
As background, last year, I spoke to an avionics guy at Pacific Coast Avionics when I described to him another issue I had noticed: when using the #1 CDI and having it properly tuned in and centered, when I turned on the #2 Nav to tune and compare the #2 CDI, the #1 CDI would deflect! Damnedest thing.
His explanation was the use of the Balen in 1968 for the antenna: that when turning on the #2 Nav, the voltage on the antenna wire had to 'build up' again (The #1 WOULD come back after a few moments and center) thus causing the #1 CDI to 'lose' the voltage and have the needle deflect.
Because of this background, I am suspicious that the cause of the CDI's weird indications now is the antenna/balen and NOT the KX-177B or CDI. But what do I know?
So, I'd like to throw it to the group and see what the consensus would be regarding the CDI swinging left/right. If it DOES appear to be the antenna, when I look at Aircraft Spruce, there are a million VOR antennas with varying costs. Any recommendations? And is it necessary to replace the old original coax when replacing the antenna?
Alternatively, any thoughts on how else to figure out what is causing the issue?
