CDI and 2-blade prop

Hello all! First time posting here, and so far I’ve enjoyed perusing your forums.

I would like to know if you folks have an official answer from Cirrus regarding a warning in the SR20 Information Manual about 2-blade props and localizer/SDF course guidance. Cirrus warns against operating the engine above 2600rpm on either of these types of approach procedures because of possible “CDI needle oscillation”. Can anyone shed some light on why this potential danger exists?

Thank you in advance!

Hi Ben. I can’t answer your question, but I suggest that you post it in one of the member Forums. Most members don’t even look at the Public Forums, and if you post in one of the Member Forums I’ll bet you get an answer very quickly!

Thank you for the advice, Gordon. Unfortunately I’m merely on a free guest membership, which (to my knowledge) only allows me access to the public forum. I was hoping someone on the public forums might have an answer, but that’s alright. I’ve also emailed my question to the folks at Cirrus in the hope they may forward me some information.

I’ll keep checking back to see if anyone happens to spot my inquiry out here!


I can’t answer your question either but I have a two-blade SR-20 that I’ve flown for over 500 hours. I’m based at a Bravo-airspace airport and shoot many ILS approaches, some in IMC. As a practical matter, I’ve never experienced CDI needle oscillation phenomena. A “normal” approach for me is to intercept the localizer at 23" of MP then 17-18" at a dot-and-a-half above the GS (50% flaps) then 13" at the DA if the runway is in sight (100% flaps). I have to admit, with the automatic prop speed governor I’ve never looked at the prop speed on an approach but I’m guessing that the prop speed should be below 2,600 RPM at the MP’s I use. The only time the prop speed should exceed 2,600 RPM on the approach would be going missed. Once I got the plane configured for the missed I’d fly the GPS to the published hold or follow ATC’s instructions and wouldn’t leave the GPS on the “LOC” setting.

Thanks for your question. I hadn’t thought about the RPM issue before. I’m going to check it out the next time I fly a localizer…



I concur with Ron. I use similar MAP settings with my SR20 two blade and I do not ever recall having a prop speed anywhere near 2600 RPM at those MAP settings during approach. I likewise switch to GPS for the missed approach segment so the LOC does not come into play.


Needle oscillation due to harmonics of 2X order or blade rate passage on the fuselage/engine mount…and out of balance props…


Great information, guys! Thanks for taking the time to pop in and help me out.

I’m especially interested in the electronic phenomena that occur during these operating conditions (I’m assuming there is some sort of electronic issue with the harmonics, etc.). I would like to be able to describe the situation in detail if I ever have a curious student who would like to know more. If anyone has more in-depth technical resources which explain the CDI behavior I would be most interested.

Thank you again for the responses!