Unstable Cruise

It seems that this topic has invariably been covered ad nauseum but does anyone else suffer with this problem (which by all accounts doesn’t affect the entire range of aircraft).

Once established in the cruise I find the aircraft (B737-800) in this example seems to be porpoising.

I’ve selected FL360, M.78 and yet the vertical speed moves up +/- 400fpm.

I’ve tried the following to rectify the issue:
-Disconnecting AP and hand flying/trimming to 0
-Selecting VS to 0

The altitude and speed I’ve selected is suitable for the weight of the aircraft as I use the cruise chart from the 737 FCOM.

Does anyone have any solutions how to resolve this problem?

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Set the V/S to 0 - it should sort it out

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Could be a few thing but you may just be going to high for your weight.

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@BennyBoy_Alpha, I think you skipped over this part.

@coffinpilot, does this happen all the time? How much cargo, fuel and PAX do you have onboard. Typically porpoising is due too heavy overall weight and too high of an initial cruise altitude.

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So I’ve tried step climbing to resolve the potential weight issue and it still occurs.
Average load is about 165pax and about 1.5ton of freight.
I tend to fly with a forward CG for the sake of stability and I’ve tried playing around with the cargo position to try and cure the issue; to no avail.

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Once you get above FL280, have you tried reducing your ascent VS to a value below 1200, I would try a less aggressive VS rate once you start reaching the low end cruise altitudes.

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Typically once I pass FL290 and into RVSM airspace I tend to reduce to 1500fpm to avoid busting through a level.

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I doubt that would help unless the last bit of the climb is less than 400fpm. Otherwise it would just get stuck at ±400

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What was your speed? The plane tends to pitch up and down continuously when you are flying too fast.

Also what aircraft were you using?

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Boeing 737-800, cruising at M.78 @ FL360 so by no means caning it

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Yea that shall be alright, I’m out of ideas sorry!

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And I think this belongs in #support

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Going to throw a similar issue I encountered recently out there: In an A321 with full fuel but no passengers, I climbed to 350 (I think, maybe was 370), and the plane was rocking side to side up to 5-10 degrees, possibly due to turbulence. I was doing LFBO-VHHH

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@coffinpilot this normally happens when your flying heavy and your flying at a high altitude you should fly at al sower cruise altitude for the first few hours of the flight until your lighter than you can climb to higher altitude
flying at high altitude when your aircraft is light improves fuel effieciency

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Slow airspeed means less control authority no matter your Mach number such as 0.78. The more dynamic pressure (KIAS) over the control surfaces the more authority they have.

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This is almost certainly an autopilot bug not a control issue.

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You are a bit heavy. 165 would almost fill an all economy config.

I have done fuel tests, and in IF with M79 cruise, the optimal altitude only goes above 34000 at about 55000kg weight. Maybe try cruising at a lower altitude next time because it seems to save fuel anyways (and it’s faster)

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I’ve found the more control authority autopilot has the steadier it is and less over exaggerated control movements made by the system. Similar to pilot induced oscillations

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An excerpt from the 737-800 FCOM, which I use to calculate cruise performance.
On the occasion I’m referring to the TOW was roughly 63,000kg and the direction required an odd FL.
As you can see at 65,000kg, FL350-FL370 is highlighted as the optimum cruise level for the given weight.

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I don’t want to go too much off-topic, but this is just not true. A B737-800 all-economy can easily have 189 pax.

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