KAP140 coupled approach question

[Artiom]

On the flight yesterday KAP140 behaved weird on descends. I was cruising at 6,500', set altitude 4,500' and VS -400ft/min.The plane started descending but actual VS was jumping between +150 to -900. Same happened later on approach when I tried to set next stepdown altitude. Climbs and level flight worked just fine.
Should I notify maintenance or this is know problem?

Was it wandering back and forth betweeh 150 and 900 fpm, sort of bobing up and down? If so, this could be the porpoising issue others have complained about in this forum with the KAP140. I have experienced this in both my previous KAP140-equipped DA40's before. I would hit the CWS button and hold the rate of climb/descent steady for a few seconds, then release CWS and often that would "cure" it - for a while at least. In my case, it would happen on climbs and in level flight, too - but just sometimes... As I recall, theories ranged from moisture in the air to plugged static ports, but no definitive solution was ever found that I can remember.

Rick, what is CWS button?
Yes that was bobbing up and down till I disengaged AP, descended and engaged AP again.

[Artiom]

This is a known problem on the DA40 and is caused by an eddy that forms behind the pitot/static "tube" at certain angles of attack.

Another pilot on our field had the same problem. He had "the fix" and solved the problem.

We finally agreed to implement "the fix" which consists of installing a pair of static ports that replace the static port on the pitot tube. Parts include two static ports (port and starboard just behind the canopy, and lines to the G1000 sensors).

Since then, not even a hint of a problem.

Not an option for me - I'm renting this DA40

[Rick]

This is a known problem on the DA40 and is caused by an eddy that forms behind the pitot/static "tube" at certain angles of attack.

Another pilot on our field had the same problem. He had "the fix" and solved the problem.

We finally agreed to implement "the fix" which consists of installing a pair of static ports that replace the static port on the pitot tube. Parts include two static ports (port and starboard just behind the canopy, and lines to the G1000 sensors).

Since then, not even a hint of a problem.

Both of the DA40's I used to fly had the static port mod, but still porpoised periodically. Some people here have reported that it solved their problem, while others have reported continued porpoising after the mod.

Rick, what is CWS button?

CWS=Control Wheel Steering, that little black button on the top rear of the control stick. It allows you to override the autopilot without disengaging it. Note that when you release the CWS button in VS mode, your current VS become your new setting. I like this method of establishing VS climbs and descents - just press VS, hold the CWS button and put the nose where you want it, then just release the button. No repeated pressing of the up/down keys.

[BlackMammoth]

Use at your own risk: Selecting "Alternate Static" will cause the porpoising to cease if you have the pitot mounted (original design) static source. This will also cause your altimeter to shift, etc.

We have more issues with porpoising when flying near gross weight, with the CG towards the back side of the envelope. Shows up during slow (in VS) AP climbs and descents.

Moving the static port to the fuselage location(s) is a much better fix.

[Rich]

My 2002 DA40 has been prone to oscillate on baro-derived descents. Not always, just sometimes. And less often on climbs. Almost never flying level. And variances are more of the 100-200 FPM variety. But, of course, my 2002 has a more forward CG than the later models.

But on coupled ILS or LPV approaches it does no such thing, as it's not using baro input at all.

The real cause of all this is a mismatch in the response of the A/P and the lag of the static system, combined with some dynamic transient responses in the original static system. The pressure change at a static port is not immediately sensed by the A/P. Note the lag, for example, in your VSI. This is because air is a compressible fluid and before the sensing device can detect the change in pressure at its location, air must flow in or out of the static system (mostly through the teeny weeny static port), which takes some time. And check out the location of the original static port, mounted 15 ft. away on the wing. The air in all that tubing needs to adjust. To get a sense of this, try manually initiating and holding a particular descent by fixating ONLY on the VSI.

Using the alternate static air is an ok technique in some circumstances. But the static pressure in the cabin is unpredictable, depending on whether vents are open, for example. The transition can be a bit exciting. You don't want to be doing this during an approach, especially for real.

[BlackMammoth]

Using the alternate static air is an ok technique in some circumstances. But the static pressure in the cabin is unpredictable, depending on whether vents are open, for example. The transition can be a bit exciting. You don't want to be doing this during an approach, especially for real.

Agreed Rich, I would NEVER use the alternate static on an approach.

[Keith M]

Not to mention that the AFM explicitly bans the use of the KAP140 with alternate static. (See 2.15 LIMITATION PLACARDS.)

[Jean]

Note that when you release the CWS button in VS mode, your current VS become your new setting. I like this method of establishing VS climbs and descents - just press VS, hold the CWS button and put the nose where you want it, then just release the button. No repeated pressing of the up/down keys.

Never figured that out. Will try that. Should be very handy especially during climb when speed drops. I use CWS mostly on approaches when KAP 40 starts swinging on final.
Thanks Rick !

[Artiom]

Out of curiosity - is this issue specific for KAP140 or GFC700 has the same problem?

[Rick]

Out of curiosity - is this issue specific for KAP140 or GFC700 has the same problem?

This problem is specific to KAP140-equipped DA40's. My current DA40 has the GFC700 and is rock-solid, and the reports from other GFC owners are the same.