CRJ900 Control column vibrations

[dcoded]

To all CRJ pilots:

Regarding CRJ900

I have noted slight control column vibrations during descents slightly above idle (N1 30-45%) These vibrations are not present when at idle, or at thust settings above 45%.

Can some one shed a light on this?
Why are there vibrations?

I cannot recall them on the CRJ200.

Thanks in advance!

[Amadis of Gaul]

It's been over 4 years since I've flown a -900, but I can't say I remember such vibrations. Are you experiencing them on every airplane or only on certain ones?

[dcoded]

Thanks for the reply.

I have experienced it on different individuals.

[Intrance]

Only on the control column/yoke or do you simply feel the vibration through the plane?

I can't say I have ever really noticed it. Some of the ones I fly have a slight vibration at idle in certain conditions, but not really any I recall with vibrations above idle. Any indication on the fan vibe gauges?

[dcoded]

Thanks for your input.

There is no vibration in the airframe, just in the yoke itself at powers just above idle, as stated.

I will try to note the speed and N1 settings more carefully next time.

I recall its at speed below 250 to 200.

Will make a note of it next time, otherwise I tend to forget!

[Amadis of Gaul]

Thanks for the reply.

I have experienced it on different individuals.
But there are some on which you have not experienced it?

[john_tullamarine]

Something which should be passed back via the operator to the OEM's field service folk for info to the design organisation.

Sometimes oddball things can surface .. eg, many decades ago, John Blacker (design engineer) and Randy Green (TP), who were working on the early Victa development, found a nuisance vibration (which, as I recall, was picked up through the stick). Eventually, they tracked it down to one of the engine cylinders producing somewhat less power than the others .. been the case with the engine for a long time but didn't surface until the engine was matched with the particular configuration of the Victa ..

Really scratching the memory cells, now - John was an Industry lecturer when I was an undergrad at Sydney and this story surfaced during that particular design course - I (very) vaguely recall that the problem surfaced due to an aspect of the elevator circuit balancing ?

[neilki]

My -900 operator has a note about this on IRS Birds IIRC. Nothing to worry about..
Not present on the AHARS fleet. I'll dig it out and PM you the gist...

[ICT_SLB]

It's a long time since I worked on the CRJ900 Flight Test but we found a fuselage fundamental frequency would effect the IRS Vertical Acceleration (IIRC) term into the Autopilot. A High Pass Filter in the AFCS software around 3 Hz cured it. Might want to get your Maintenance people to check the IRUs are snug in their mounting trays and the mounts themselves. As neilki stated, the AHRS-4000 just isn't so sensitive to pick up the vibration. The same filter was retained for the CRJ1000 but both the 200s & 700s never needed it.

[dcoded]

Wow, very interesting indeed!!

I will certainly go into depth with this a bit more.
So, next time I notice these vibrations, and disconnect the AP, they should go away?

Thanks guys

[Intrance]

Spoke to a colleague about this while checking it in descent (no vibration by the way). He also mentioned there used to be a note about this somewhere in documents from our current or one of his previous operators. Resonance with the IRS and the autopilot or something like that.

I'm going to dig into it a bit as well, as I don't think I have seen it in our documents yet.

[ICT_SLB]

Intrance,
It's been well over 10 years since we did the testing but IIRC you won't see it "up and away" but was only really noticeable when Approach mode is selected owing to the higher gains needed to track the Glideslope. Again the filter should take care of any stick movement in the Production aircraft. It was only ever seen on IRS-equipped aircraft. It was noticed on the prototype during testing as it was also fitted with a HGS (HUD) and thus had the IRUs installed. Unless your aircraft has IRUs fitted, you won't have the AFM Supplement, which may have the note.