Flying the Magenta KLN94 with KAP140

[Jaybird180]

Attached Is the flight plan of my most recent flight. I am glad that I purchased and viewed the King video for my GPS, it was money well spent.

I entered the flight plan based on course planning from Foreflight into the KLN94 and FF gave me ETA updates (will have to view video again to learn how to do that on KLN94). I was hoping for a 2hr (maybe +15min) enroute, but it was not to be. Headwinds put me at about 74kt GPS readout GS, so I decided to get fuel at HNZ. I forgot to mention that I was at full gross @ 37gals on departure from home base.

Side note: At full gross and 10deg flap Normal takeoff, the SP climbs well. I also saw 700+ for an enroute climb at 90kts!

A couple presses of Direct on GPS and were out of the departure airspace on our way efficiently. At ~4500 I engaged the autopilot, first in heading mode, no surprises, since I was already on course to OTT. Then I turned to NAV mode, the wings rocked a little and then it stabilized. Most of the trip TRK and DTRK were within 2deg or right on target.

I think my heading indicator precesses a lot. As I lined it up, the AP wanted to turn the plane. Then I had to move the heading bug to get TRK and DTRK to line up again. I thought the AP would ignore the heading indicator and bug while in NAV mode

How often should I have to realign the Heading Indicator? I've flown some planes that were stable for extended periods of time and others I couldn't trust after 10mins.

 

[TMetzinger]

I need to look at my manuals again to be certain about this, but I believe the KAP140 will keep looking at the signal coming from the DG heading bug even in nav mode, and if it changes more than a certain amount it starts the whole tracking process over again because it thinks that something has changed significantly.

When the AP is tracking the nav course properly, the difference between the heading bug and the 12 o'clock position is the wind correction angle. As long as that angle doesn't change too much in too short a time the AP will stay in a "fine" correction mode and look to the CDI L/R signals for adjustment.

I'll look in manuals to see if I'm remembering this right.

Now, how many degrees does your DG precess in 12 minutes while holding a constant compass heading?

 

[poadeleted20]

Do you have a GPSS roll steering module? If so, you want to stay in HDG on the a/p while selecting GPS on the GPSS in order to have good GPS tracking, I.e., for the GPS to directly "drive" the a/p. If you select NAV on the a/p, the a/p just tries to follow the CDI needle (with, I believe a bias from the heading bug), and the CDI needle is quite insensitive when the GPS is driving it in Enroute mode, resulting in a lot of wandering and overcorrection, especially at turnpoints.

As for the HI, 3 degrees every 15 minutes is not unusual.

 

[spinfire]

The Wikipedia Article for Heading Indicator has some interesting information on the sources of drift.

 

[Jaybird180]

Do you have a GPSS roll steering module? If so, you want to stay in HDG on the a/p while selecting GPS on the GPSS in order to have good GPS tracking, I.e., for the GPS to directly "drive" the a/p. If you select NAV on the a/p, the a/p just tries to follow the CDI needle (with, I believe a bias from the heading bug), and the CDI needle is quite insensitive when the GPS is driving it in Enroute mode, resulting in a lot of wandering and overcorrection, especially at turnpoints.

I suppose I could look that up in the logbook. Not sure. I'd have to get back to you on that one, but it's likely whatever was the 'standard' configuration of the single axis version of the KAP140.

When the AP is tracking the nav course properly, the difference between the heading bug and the 12 o'clock position is the wind correction angle. As long as that angle doesn't change too much in too short a time the AP will stay in a "fine" correction mode and look to the CDI L/R signals for adjustment.

It did a pretty good job with WCA.
Now that you reminded me, I was told that it will reference the CDI and not the GPS directly. So the GPS/NAV button determines what signal is feeding the CDI and the AP is slaved to it, not directly to the GPS like I thought....hmmmmm which confuses me even more as to why changing the heading bug enroute would cause the airplane to turn.....

I'll look in manuals to see if I'm remembering this right.

It's also on my 'to do' list.

Now, how many degrees does your DG precess in 12 minutes while holding a constant compass heading?

I'll have to get back to you on that one.

 

[poadeleted20]

I suppose I could look that up in the logbook. Not sure. I'd have to get back to you on that one, but it's likely whatever was the 'standard' configuration of the single axis version of the KAP140.

You need only look on the panel to see if it's there.

Now that you reminded me, I was told that it will reference the CDI and not the GPS directly. So the GPS/NAV button determines what signal is feeding the CDI and the AP is slaved to it, not directly to the GPS like I thought....hmmmmm which confuses me even more as to why changing the heading bug enroute would cause the airplane to turn.....

As I said -- check to see if there's another switch somewhere which says "GPS/HDG" on it -- that would be the GPSS switch. And if you don't have GPSS, you'll find the GPS tracking isn't all that good due to the low sensitivity of the CDI needle when the GPS is in Enroute mode, not to mention a lot of manual intervention for interception and at turnpoints. If you don't have it, it's typically about $2K to get it added, and you'll be glad you did.

 

[Jaybird180]

Next to the annunciator panel, there are 2 buttons (I should have taken a picture of the panel - on my 'to do list').

#1 Button is NAV/GPS
#2 Button is Approach Arm

What does #2 do? I thought the GPS approach arming was based on meeting pre-determined criteria (RAIM, etc) at the FAF. When would this button be used?

 

[TMetzinger]

Grasshopper, read the supplement in the airplane flight manual. It will explain the approach arm button.

 

[Jaybird180]

Lesson: Do not confess to RTFM on your 'to do list' on PoA.

 

[gismo]

Next to the annunciator panel, there are 2 buttons (I should have taken a picture of the panel - on my 'to do list').

#1 Button is NAV/GPS
#2 Button is Approach Arm

What does #2 do? I thought the GPS approach arming was based on meeting pre-determined criteria (RAIM, etc) at the FAF. When would this button be used?

LIke Tim said, it's important to read the AFMS (and Pilot Guide) for the autopilot to become familiar with all of it's features. This is an area that's commonly short-shifted during pilot training, partly from the old school thinking that you have no business using the autopilot until you've got a lot more experience hand flying, and partly because (for the same reason) many CFIs don't know the autopilots well in the airplanes they use for teaching.

But I'll give you an idea what the Approach Arm does, it prepares the autopilot for following a glideslope once the airplane reaches the point where the glideslope intercepts your level flight path with altitude hold engaged.

WRT your question about the effects of the heading bug when flying in NAV mode, think about how you'd intercept and track a straight course. If the airplane was displaced to the side enough to deflect the needle halfway to the peg, and you didn't happen to know what your heading was you would probably start a turn towards the direction of the deflected needle and you might continue that turn until the needle was nearly centered. That would likely lead to overshooting to the other side of the course where you would turn back in the other direction and hopefully you would eventually settle down on the correct heading to keep the needle centered. Now consider how much better you could do if you knew the airplane's heading relative to the desired course. If you turned to an heading that sent you towards the course center at a shallow angle, say 10-15 degrees beyond the course and held that heading until the needle began to move back towards the center then gradually reduced that 10-15 degree intercept angle such that the intercept angle reached zero at the same instant the needle centered you would have made a much smoother and more precise job of getting back on course.

That's what your autopilot does when you are off course and the result is a smoother job of tracking even when the lateral offsets are small (the improvement from including heading in the operation is much more evident when there is a big error or a big course change but it does help even when the offset is small. The tradeoff is that you should always set the heading bug to the desired course at the same time you tweak the OBS on your CDI. If you had a HSI which combines the function of a CDI and a DG you'd accomplish both at the same time by adjusting the course pointer which gives the autopilot heading information just like the heading bug on a DG when the autopilot is in NAV mode.

 

[TMetzinger]

Lance, not on this airplane, he's talking specifically about a button for the GPS, not the autopilot.

It is both an annunciator that the approach mode is armed (coming to the FAF) and active (at the FAF), as well as a button to take it OUT of approach mode, if I remember correctly. That's why I told him to look it up. - I didn't have a reference handy.

Edit: Looked it up. It's both an annunciator and a APCH mode toggle. Normally it's only used as an annunciator, but if you are inside the FAF and your nav flags (RAIM, loss of signal), using the button to switch OUT of approach mode will normally bring the navigation back and you can use it for the missed.

 

[gismo]

Lance, not on this airplane, he's talking specifically about a button for the GPS, not the autopilot.

It is both an annunciator that the approach mode is armed (coming to the FAF) and active (at the FAF), as well as a button to take it OUT of approach mode, if I remember correctly. That's why I told him to look it up. - I didn't have a reference handy.

Ah, sounds like we're talking about the "ARM" button for IFR GPSs, often found on a remote annunciator/switch. Sorry, my bad for not reading the entire thread. IIRC on the '94 it's not generally necessary to use that button and I think it's there to allow the pilot to force the GPS into "approach armed" mode prior to the GPS doing that automatically by itself. Like you I'd have to go read the PG for more info than that even though there was a time when I had it memorized (back when I owned a KLN94 about 10 years ago).

 

[Jaybird180]

Ah, sounds like we're talking about the "ARM" button for IFR GPSs, often found on a remote annunciator/switch. Sorry, my bad for not reading the entire thread. IIRC on the '94 it's not generally necessary to use that button and I think it's there to allow the pilot to force the GPS into "approach armed" mode prior to the GPS doing that automatically by itself. Like you I'd have to go read the PG for more info than that even though there was a time when I had it memorized (back when I owned a KLN94 about 10 years ago).

Is there such a thing as forcing ANY IFR GPS into approach arm mode?

My understanding is that the programming will not allow you to do so unless specific conditions are met at the FAF.

 

[gismo]

Is there such a thing as forcing ANY IFR GPS into approach arm mode?

My understanding is that the programming will not allow you to do so unless specific conditions are met at the FAF.

Methinks you are confusing Approach Arm with Approach Active. Here's an excerpt from the KLN94 Pilot Guide:

NOTE: If an optional external GPS approach switch is installed it is possible to arm the approach mode at a distance greater than 30 NM from the airport by pressing the external GPS approach switch, but the KLN 94 will not change the CDI scale factor until the aircraft reaches the 30 NM point.
If the external GPS approach switch is pressed while the approach mode is armed, then the KLN 94 will disarm the approach and change back to en route mode. The approach can be re-armed by simply pressing the GPS APR switch again. If the KLN 94 is in the APR ACTV mode, pressing the external GPS APR switch will change the mode to APR ARM. Once past the FAF, it is not possible to return to the approach active mode without conducting a missed approach and flying back to the FAF.

 

[Jaybird180]

You would be correct Sir.

 

[JohnAJohnson]

How often should I have to realign the Heading Indicator? I've flown some planes that were stable for extended periods of time and others I couldn't trust after 10mins.

A DG with perfect bearings, no friction and running at speed should not show any random drift. But it will always show apparent drift. This is the drift that manifests because the gyro points to the same place in space, but the earth turns, so it appears to be drifting. The amount of apparent drift per hour can be calculated by multiplying the sine of the local latitude by 15. For instance, at Leesburg, VA, with a latitude of 39.1°N, you could calculate apparent drift as sin(39.1) x 15 = 14.8, Any drift more or less than 14.8° per hour is random drift (the bad kind). So Ron is about on the money. At your latitude, 3° drift every 15 minutes is about what you can expect from a good gyro.

 

[pericynthion]

sin(39.1 degrees) * 15 deg/hr = 9.4 deg/hr

You were taking the sin of 39.1 radians

 

[TMetzinger]

Damn DRG key strikes again

 

[Jaybird180]

Regardless, it was the first explanation of gyro drift I've understood.