KLN 94

Tristar

Tristar

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Apr 7, 2005
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Tristar
I would really appreciate some clarification on the OBS mode on the KLN 94.

The OBS mode I understand you can use it as a radial like a VOR. If you're flying to a checkpoint, you can push the OBS mode and dial in a radial using the CDI and intercept it or use it to hold. I know it's main function is to keep the GPS from sequencing off to the next checkpoint.

What I don't get is I tuned in the OBS and dialed in a radial and it gave me something I didn't expect. For example, today I told the GPS to go direct to the VOR and then put it in OBS mode but it gave me a radial 100 degrees off from where I was and no matter what I set at the top, it always set in a radial 100 degrees off. I thought I could dial in a radial at the top, intercept it and fly to the checkpoint. What I'm mainly wanting to understand is how to use it in a hold though. Could someone talk me through what I'm doing wrong/missing?
 
Well..Lets start with the basics. Does it work properly on the self-test when it boots?

When you boot a KLN-94 it will bring up a blue internal test screen. On the right of this screen it will say OBS with a degrees value next to it. This value should be whatever the CDI is set to. If this value is always 100 degrees off of what the CDI has set then something isn't calibrated right and needs to be fixed.

It looks like this:
kln-boot.png

In that case the pilot has the CDI set to 180 degrees, the pilot should see a half-scale deflection to the right on his/her CDI, and the CDI should have a FROM indication.

It amazes me on instrument proficiency checks how often I see pilots completely ignore the test screen of their GPS. Not only do they ignore it, they've never been taught to pay any attention to it...

So to sum this up...does the value on this test screen match what you have set on the CDI?
 
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Tristan,

Assuming the KLN94 is coupled to a CDI properly, when you enter direct to a waypoint, then put the KLN94 in OBS mode, the CDI OBS knob will drive the KLN94 to choose the "radial".
Selecting a "Radial" using the GPS unit only works when the KLN94 is NOT coupled to an external CDI.
So I bet you have a NAV/GPS switch and it's in GPS mode. You need to turn the OBS knob on the CDI and you should see a corresponding change on the KLN94. If you don't then I'd suspect the CDI and KLN94 are not communicating properly.
 
Yep, it sounds like an OBS to GPS communication problem. I had the same thing happen on my Garmin right after it was installed. The GPS read more than 30 degrees off at some OBS settings, and right on at others. It turned out the avionics shop had accidently grounded a pin on the nav head and the Garmin calibration procedure only uses one position on the OBS, so they didn't catch it.
 
You're right, I didn't even think to look at the Test page. I'll look at that.

Tim - We do have a GPS/NAV button. That's what I was twisting in GPS and OBS mode. When I twisted the CDI, there would be a corresponding change on the OBS radial reading on the GPS but it was always really far off from anything I selected. So I didn't know if I was doing something wrong. Even while trying to fly a hold, I would fly directly towards Ayare (IAF waypoint) which is north/southish radial and would get a 076 instead.

I'll take a look at that test page for starters though.
 
Tristar said:
You're right, I didn't even think to look at the Test page. I'll look at that.

Tim - We do have a GPS/NAV button. That's what I was twisting in GPS and OBS mode. When I twisted the CDI, there would be a corresponding change on the OBS radial reading on the GPS but it was always really far off from anything I selected. So I didn't know if I was doing something wrong. Even while trying to fly a hold, I would fly directly towards Ayare (IAF waypoint) which is north/southish radial and would get a 076 instead.

I'll take a look at that test page for starters though.
Click to expand...


Quick Test:

OK, when you're on the ground, put the NAV/GPS button in GPS, and select DIRECT to your nearest VOR on the KLN-94. You should get a centered CDI and a message to set the course on the CDI to a certain value. Go ahead and set the CDI to that value.

Then, press the OBS button on the KLN94. All that should happen is that the KLN94 should show the same number on it's course readout as you have dialed into the CDI. Your needle should still be centered (or close to it). As you twist the CDI knob, the numbers on the KLN94 should change to match the course selected on the CDI and the needle should move.

If it doesn't work this way, then it needs to go to the shop. What's probably happened is the signal that tells the GPS what course is selected on the external CDI is not correct.
 
Tim, I thought the same thing and tried that too. It still gave me 100 degrees off.

Here is what I got on the Test screen vs. VOR. I didn't change anything in between.
 

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Tristar said:
I would really appreciate some clarification on the OBS mode on the KLN 94.

The OBS mode I understand you can use it as a radial like a VOR. If you're flying to a checkpoint, you can push the OBS mode and dial in a radial using the CDI and intercept it or use it to hold. I know it's main function is to keep the GPS from sequencing off to the next checkpoint.

What I don't get is I tuned in the OBS and dialed in a radial and it gave me something I didn't expect. For example, today I told the GPS to go direct to the VOR and then put it in OBS mode but it gave me a radial 100 degrees off from where I was and no matter what I set at the top, it always set in a radial 100 degrees off. I thought I could dial in a radial at the top, intercept it and fly to the checkpoint. What I'm mainly wanting to understand is how to use it in a hold though. Could someone talk me through what I'm doing wrong/missing?
Click to expand...
Has it always been this way or is this a recent change? It sounds like the OBS is miswired to the GPS. There is an OBS calibration function in the maintenance pages and you may be able eliminate the discrepancy with that:
 

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Yep. Definitely not communicating the OBS setting properly to the GPS.
 
Tristar said:
Tim, I thought the same thing and tried that too. It still gave me 100 degrees off.

Here is what I got on the Test screen vs. VOR. I didn't change anything in between.
Click to expand...
Theres your problem.

The test screen can really help one avoid serious confusion in the air. If you're going to be IFR consider it as important as making sure that you have oil.
 
My KI-202 just went TU that was attached to my KLN89B (I had to tap the indicator to get a correct reading) If it turns out to be the CDI, SEA Aerospace is running good sale on KI-202s right now, and the rectilinear movement is nice over the pendulum movement.
 
Thanks for the idea. It should have dawned on me to check that but thats why I ask. We're having mx take a look at it. Supposedly it's been messed up for a while but nobody bothered to look into it I guess because it doesn't fly in OBS mode that often. I'm the only one with an instrument student at the moment so had to learn it during a flight...little embarrassing that I couldn't get it to work.
 
MX found a calibration error. It's been fixed so we'll see if it works this time on the next instrument lesson. It's always a good feeling when the system was actually broke instead of second guessing how it worked in the first place!
 
Tristar said:
MX found a calibration error. It's been fixed so we'll see if it works this time on the next instrument lesson. It's always a good feeling when the system was actually broke instead of second guessing how it worked in the first place!
Click to expand...
Haha that is true. My first suspicion when things go wrong is that it is operator error so I'm always glad to find out that was not the case.
 
Everskyward said:
Haha that is true. My first suspicion when things go wrong is that it is operator error so I'm always glad to find out that was not the case.
Click to expand...

Yeah, the hard part is being able to spot that things went wrong and not blindly following a needle into the side of a hill.
 
jason said:
Yeah, the hard part is being able to spot that things went wrong and not blindly following a needle into the side of a hill.
Click to expand...

the good news is that in nebraska you'd probably run out of gas first
 
Okay, it works like a charm!

Yea you're not going to hit much in nebraska.
 
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