Speed fluctuation in cruise

[Josh F]

I am a relatively new owner of a 2008 XLS, with about 20 hours in the plane so far and still learning the ropes. I’m having trouble managing speeds, particularly in instrument approaches - hit a patch of benign but rising air and the airspeed tape can shoot up from 100kts to 125kts in the time it takes for me to dial in a new frequency, with an increase in MP and RPM as well. Is this just a really slippery plane or could there be an issue with the throttle or carburetor?

[Colin]

It is a very slippery plane and it is also very easy to dump a bunch of energy on short final. I don't know how many hours you have in it. After about three hundred in mine I was able to be 125kts on final and drop it down to 80kts over the fence and set it on the thousand foot markers (which was necessary at Dulles on one of our trips, I had a lot of faster people behind me).

[Rich]

The big challenge is that the plane takes little pitch force (especially true when CG is aft, as is the case with newer models) and has very little stick movement to produce enough changes in pitch to make more difference than you’d experience in other aircraft types. You need to make good use of trim, so your steady state is zero fore/aft hand force, and a light touch when you do adjust. I fly approaches with my wrist/forearm resting on my thigh and my hand more caressing the stick than gripping it. As Colin mentions, one can easily bleed off speed late in the approach when ATC has some fast-mover following you in. Otherwise setting half flaps fairly early in the descent helps keep speed at bay.

But you mention sudden increase in MP. That I don’t get. On a lengthy descent it is expected to slowly increase with a loss of altitude. Normally one needs to adjust the throttle rearward during the descent to maintain a particular power setting. This is quite pronounced for approaches I encounter all the time around here, where the final approach course can often require 5,000 ft of descent.

[dant]

carburetor

Just as a note, the 2008 XLS is a fuel injected engine - no worries about carb ice!

The only time I have such troubles is when I'm trying to maintain an altitude through some rising air. The plane will speed up very fast in a pitch down attitude, and you'll have to manage the throttle or just accept the altitude adjustments.

The increase in RPM/MP sounds off to me.

Also as a note, I find it pretty tough to get exactly hands-off level with the trim if I do it manually due to the friction mechanism making it hard to do fine tuning - I usually end up +/- maybe 20 fpm. If I have an INOP A/P I'll get as close as I can and then either maintain the slightest pressure (like, brushing against the stick slight) or adjust the throttle a wee bit to fix the remaining vertical speed. Of course the cheater option is to use the A/P to trim it for you.

[Colin]

Good point about the control forces. I fly with my right hand on the throttle and my forearm resting on the trim wheel, which I adjust bit by bit on the approach.

[Boatguy]

Good point about the control forces. I fly with my right hand on the throttle and my forearm resting on the trim wheel, which I adjust bit by bit on the approach.

I do that same thing with my forearm. It's perfectly positioned!

[ZAV]

Put a SD card in the upper MFD slot and save the engine logs. There are free online resources to visualize the data. You can graph the changes you’re worried about and show us. May be able to help. Your description of the airspeed change seems large to me unless you are making big pitch changes. Also seems weird to notice RPM changes on a constant speed prop at that airspeed without RPM control lever changes.

[JanSq]

check the friction wheel on the right side of the console. I have seen it be too loose. May need tightening by a 1/2 turn or so. Experiment with it.

[Josh F]

Thank you everyone for the comments. The plane is rock steady at altitude, so I think it was just me getting used to how the plane handles in rising air while overloaded with training tasks. Not sure why the RPM/MP changes, but I haven't seen them repeat.