Rollercoaster ride on autopilot approach

[Paughco]

My recently purchased Cessna 182 is equipped with a RXP GTN 750. I tried a RNAV (GPS) RWY 34 approach at Arlington Airport (KAWO), and she flew the outbound, teardrop reversal, and the approach just fine. I hit the APR button on the autopilot and started slowing the airplane down, checking fuel, mixture, prop, etc. The glideslope pointers started coming down, and... YES! The nose dropped as the glideslope was captured! Woah, Nelly, we're picking up some speed here - time to select 10 degrees of flaps. That's when the rollercoaster ride began. The airplane began to hunt for the glideslope, and the up and down action wasn't dampening out at all. I turned off the autopilot and hand flew her in, referring to the PAPI lights and the glideslope pointers which were still available on the HSI even with the autopilot off.

I figured that the oscillations were due to a combination of low pitch authority of the autopilot and a significant trim change in the 182 when even 10 degree flaps are selected. I'm going to play around with this some more, but I read something by Scott that you guys are going to be doing more real-word testing, and thought maybe you could check this out in a real 182 sometime.

Maybe I should select 10 degree flaps before the glideslope is captured? Or maybe plan on no flap landings in IFR conditions? Yeah, I know - real men don't fly IFR by the autopilot, but I enjoy exploring the gadgetry aspect sometimes.

Thank you
ATB

[bobsk8]

What was your speed when you intercepted the glideslope?

[Scott - A2A]

I also want to add this should to happen with a normal approach but keep in mind dropping flaps creates a significant nose up movement and the autopilot can have difficulty adjusting fast enough.

Scott

[Paughco]

I tried RNAV (GPS) RWY 34 approach at Arlington Airport (KAWO) again last evening. Kept speed around 120 knots on inbound. Autopilot on. Hit APR as the glideslope markers showed in the HSI. Slope captured, and before the airplane got over 130, I selected first notch flaps. As the flaps started down, and the nose started coming up, I rolled in a bit of down elevator trim. I think I got that right, as she stabilized and continued down. I selected the second notch of flaps, and tried the trim correction again. This time I heard the autopilot disconnect beep, and I was flying the airplane. Stabilized with two notches of flaps, I got below the glideslope, hit the AP button, waited a few seconds, then the NAV and ALT buttons, then the APR button. I'm going to practice this a few more times to get it smoother. Then I'll try it under true IFR conditions.

Enjoying the 182, along with the rest of my A2A aircraft! It's a nice day, so I think I'll get on my BMW S1000XR and take a turn around the Snoqualmie Valley.

Seeya
ATB

[Scott - A2A]

ATB, we will test this on our end too.

Scott

[AKar]

It often makes sense to anticipate the flap "ballooning effect" by selecting a step of flaps when something like half to one dot below the glide slope, depending on how close you are capturing it. Timed right, you'll "balloon into the glide slope", making the compensation of the flaps extension and the capturing of the G/S a one continuous maneuver.

Some GA autopilot systems include an input for flaps discrete. I am not sure what it exactly does, but best I've understood it is that it either introduces some open-loop pitch command, or temporarily modifies the gains. Not sure.

-Esa