Dash-8Q400 nose pitching up and down, autopilot

Yesterday on approach to KASE over KEGE (14 000 ft, speed 150-200 KIAS) I experienced that the nose on the Dash started to pitch up and down. I tried to adjust trim, speed and flaps, but no change.
On base rwy 15 when I disconnected AP, everything returned to normal.

This is not the first time I fly the Q400.
What did I do wrong this time?

(I used the search function, but couldn’t find any matching topic)

Thank you!

André, Norway
Callsign: Jetspeed100

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Cruise speed at 14,000 ft in the Q400 is more like 270-280 knots. 150knots IAS is way too slow without any flaps, the AP will pitch up to try and maintain altitude at that slow speed.

The Q400 has a bit of a ‘sweet spot’ for different situations.

On approach, don’t use anymore than 15 flaps (unless on a very short field runway). Your initial approach speed would be more likely 150-160 knots IAS, and your best Approach speed for short final at flaps 15 is about 107 knots IAS in IF I’ve found. 107 knots allows about 1 degree pitch up and a good descent rate.

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OK, thank you.

I was on approach, controller vectored me over KEGE and instructed to maintain present speed. I had flaps 5 at 170 KIAS.

Regarding approach/landing: I know :-)

André

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It wasn’t just strong winds was it? IF exaggerates wind gusts massively - it fluctuates so quickly and throws the aircraft around harshly, making it bob up and down?

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No, I checked winds. Around 8-10 knots from 270 I think. And I was on a 150-200 heading.

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I’m reproducing the issue. I think I get the same movement of ups and downs. It looks like the plane just wants to drop because it doesn’t have enough speed. The autopilot then reacts with applying more power. But the autopilot overreacts so the plane goes too high. And so on…

The issue goes away when you go faster, at least 190 knots (airspeed) is fine. You’re most likely too slow for the autopilot to work properly.

When on approach, apply flaps. When you get the up and down movement again, just disengage the autopilot and fly manual.

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I had a similar problem with the 787 a few days back. I was on approach with AP engaged at about 180 knots with altitude AP set at 2500 ft and VS 0. The auto-pilot would make the nose go up and down to try and maintain that altitude at that speed. A bit like a see-saw. I tried different flap and trim settings but to no avail.

This was not happening at higher speeds, more than 200 knots.

Finally I turned off the Altitude auto-pilot and maintained 2500 ft with only VS set to 0 on AP and Speed set to 180 knots.

The up and down movement of the nose stopped as the AP was no longer chasing an altitude.

Stable flight at desired speed and altitude :)

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Ok, thank you! As you and other say. Most likely too low speed for autopilot to handle.
I will try to reproduce it myself too.

Again, thanks!

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No clue man but sounds quite funny and I feel you when you said ‘this time’ meaning you know you always screw up.

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Haha ;-) not always

I see it this way: learning something new every day :-)

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I not sure if this is the solution, but today I was in KSAN doing touch n goes. When I was turning base, I turned autopilot off and the nose just pitched up. I tried pushing the yoke down but it didn’t work.
So I just put the altitude and VS autopilot on, calibrated and came back it’s back to normal.

I was in the Q400

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Autopilot tuning is tough:)

It sometimes works well in some conditions but not in others. High altitude approaches are one of the corner cases. That’s part of the reason why I put an altitude restriction for the APPR mode. It’s tuned for a higher air density, and an approach at 9,000ft is tough on it.

I’ll make this better over time, my plan is to one day get smarter and use some sort of AI that would make the plane fly in all conditions…

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Thank you for good info! That makes sense.

I guess i’ll be handflying vectors in this region. :-I

@Andre_S… See “Pilot Induced Oscillation” & “Porpoising” in Wiki. Max

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Wilco!

André
Jetspeed100

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@Andre_S. Also see the Dash “Pilots Operating Handbook” in Tutorial’s . Max

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