McCauley Four blade props

[AProfit125]

I operate a King Air C90B with McCauley four bladed props. We were just informed that on a maintenance ground check that our prop idle speed was 1000 RPMs. Since it isn't at 1100 RPMs McCauley is saying that our props are condemned. Has anyone ever heard of this? Thanks

 

[prpjt]

Don't know what they mean by condemned. Sounds like it needs some maint. because they're exceeding a C90 limitation. What's your N1 at low idle?

 

[GravityHater]

Condemned? That's a term for meat or maybe cattle, I think.
Ask them to show you where in the service manual it says that. Ask them how they know the gauges are accurate.

 

[AProfit125]

our N1 idles are set at 58.7% and 58.3%. It doesn't address idle RPM at all in the maintenance manual all it address N1 should be set between 58% and 60%. The POH does state that a 1100 RPM is the minimum.

 

[Kingairrick]

It's because at the lower RPMs the props can develop a "harmonic imbalance." I guess it wears out the hubs or something. I have never heard the term condemmed regarding an airplane part before. It would seem that they could do an inspection on the hubs or something?

 

[avbug]

You may not have heard the term "condemned" before, but you've probably heard "rejected," "our of tolerance," "out of spec," "unacceptable," "does not meet manufacturer criteria," etc.

Absolutely a part may be rejected if it isn't operating within limits, exceeds wear limits, etc. However, your story here is far from complete.

Idle propeller speeds aren't spelled out in the type certificate data sheet for the airplane. The limitation comes from the pilot operating handbook or AFM. Generally these will include a minimum speed which is typically 900-1,100 rpm, except for feathered operations. As these speeds are below the governing range of the propeller governor (next position is feathered), the only two things that can control idle normal range speed are the idle NG and the propeller stop limits. If the limits are set incorrectly such that prop pitch is excessive and unable to achieve minimum idle RPM...then it's out of tolerance and either needs to be adjusted/repaired, or rejected.

You noted that you've looked in the maintenance manual. Did you mean the aircraft maintenance manual, engine maintenance manual, propeller maintenance manual, or any supplemental STC data?