Prop Speed, Fuel Lean Mixture, and Pitching/Stalling

[Xyn_Air]

Would incorrect prop speed and/or fuel lean mixture at higher altitude (say, around FL 100) cause a plane to either wildly pitch up and/or enter a stall, specifically if the prop speed were too low and the fuel lean mixture too high?

I ask, because while flying a Rockwell Commander 650 I failed to adjust my prop speed and fuel lean mixture correctly. Cannot remember the prop rpm, but the setting was probably around 50% when it should have been closer to 65%-75% (which would be about 2500-3000rpm when cruising). The fuel lean mixture was around 50%, which was way too rich. Seems like around 8,000ft an above, lean mixture hovers between 25% and 30% for peak engine power if I go by EGT and engine sound.

So, with these incorrect settings, I would expect to lose power in the engines, thereby losing airspeed, thereby losing lift, and eventually entering a stall that would be either flat or pitching downward. In fact, the aircraft did stall several times, dropping to about 60KIAS. I would correct the stall by pitching down, increasing speed to about 120KIAS and leveling out.

But the weird thing that followed prior to each stall and after leveling out after each stall, is that instead of a flat or pitch down stall, unless I held the flight stick fairly far forward (would would normally pitch the aircraft nose down into a dive), instead of remaining level the aircraft would wildly pitch upwards to 20 degrees nose up or higher. This left uncorrected would, of course, cause another stall.

For the life of me, with more or less insufficient engine power (from the combination of too low prop RPM and too high fuel lean mixture) and low IAS (120KIAS is flyable, but the 650 operates normally while cruising at 220KIAS, and easily between 180-200KIAS while climbing), I cannot figure out why the aircraft kept pitching wildly upward. I would expect it to do the opposite and pitch downward.

As a note, I reset the flight, this time correctly monitoring, setting, and adjusting both the prop speed and lean mixture. As hoped and expected, there was no uncontrollable pitching and the 650 flew along smoothly, engines purring like two extremely large, happy kittens.

[slimcooper2002]

was your auto pilot set for a certain altitude? The aircraft would try to get to that altitude even in a stall, thats why you normally have to disengage the AP if you get into a stall

[Xyn_Air]

was your auto pilot set for a certain altitude? The aircraft would try to get to that altitude even in a stall, thats why you normally have to disengage the AP if you get into a stall

Initially, yes, the AP was engaged, but I disengaged the AP entirely once I started having issues. And, the uncontrolled pitch up occurred both with and without AP on. So, without AP, and the stick dead center, the aircraft would pitch upwards 20 degrees or more. I had to to push the stick forward at least 2/3 to 3/4 pitch down for level flight.

That all said, I am wondering if it could have been something to do with wind conditions or strong updrafts. I do usually fly with real world weather and automatic updates. I was flying a Gippsland GA-8 out of KPSC and into KSEA last night with what seemed like somewhat dodgy wind conditions, and a had a moment or two of upward pitching as well, though not as wild or for as long a duration.

Both the 650 and the GA-8 are not terribly large aircraft, and I could see wind and air currents having a significant influence on flight. Still, it was weird as all get out, and a bit of a mystery.

OH! Idea! I wonder if, when I disengaged the AP, if the elevator trim was left trimmed waaaaaay up! That could be it, couldn't it? So, yes, the engines aren't producing enough power, but the AP is still trying to reach the designated altitude, vainly pitches upward, aircraft stalls, I disengage the AP, but the trim is still left in an upward setting?

[Fozzer]

Whenever you disengage the Autopilot always remember to manually re-trim the aircraft for the desired pitch/straight and level.

Nasty, unexpected things can happen after disengaging the AP.... ....!

The position of that little Trim Wheel is very important!.... ...!

Me?...Trim = Haircut: Short back and sides!.... ....!

Paul.... ...!

[Xyn_Air]

Whenever you disengage the Autopilot always remember to manually re-trim the aircraft for the desired pitch/straight and level.

Nasty, unexpected things can happen after disengaging the AP.... ....!

Yeah. I feel like a derp for letting this one throw me off. I don't usually disengage AP in anything but level flight. So, needing to re-trim immediately is not something I am always thinking about or in the habit of doing.

Well, something else to add to my mental SNAFU emergency procedures manual.

Have I mentioned how rusty I am?

[Fozzer]

Its surprising how one can be caught out by the previous Pilot having used the Autopilot for landing and then shut the system down and disembarked.
The next pilot, not being aware, will experience great difficulty in taking off without checking the AP, and setting the Trim Wheel for take off!

The aircraft will either climb sky-high and stall, or be unable to take-off at all!

The old Pilot's Check List can sometimes come in handy.... ....!

Paul.....Reading the 'structions!.... ...!

[garymbuska]

That is for sure. If you have ever watched the trim wheels while in auto pilot you will notice that they are constantly moving up and down and left and right.
One of the first thing you do in a prop driven aircraft is to set the trim to take off then once you are at altitude you readjust the trim to where you do not need to touch the controls to maintain straight and level flight.
When you disengage the auto pilot you almost always have to readjust the trim unless you get extremely lucky and catch it at the right time, which the odds of doing that are rather high.