Idle speed set up on PA30

by **AlanBreen** » Tue Dec 15, 2009 4:15 am

Hi Zach,

I agree that it is not set up properly on most twins. Yes it can be a bit fiddly but it's not that hard to set up but I figure most mechanics, or engineers as we call them, probably either don't know about it or can't be bothered to do it. Hence most owners are not aware of the feature.

There does not appear to be anything about the spring loaded idle stop in the Service Manual another reason perhaps why it is not set up correctly on many aircraft.

You are so right about opening the throttle to stop dieselling. So often I see it happen (in other aircraft than PA30) where the engine diesels and the pilot tries to stop it by pulling the throttle closed when opening the throttle is a much better option to destroy the optimum air/fuel ratio.

I don't think the spring loaded idle stop is there expressly for resetting the throttle to the idle position after using the "Cut off" button. While I agree it does help "reset" the idle setting after using the "Cutoff" button, I understand that the main purpose is so that the idle speed can be reduced from ~1500 RPM, that occurs when the throttle is pull back to the normal idle stop during the landing flare, to ~ 1000 RPM, and thus reduce the landing roll.

I don't know if the Twin Comanche is unique with the spring loaded throttle stop but I think it is quite rare and is a product of the constant speed prop and the idle RPM need for a smooth idle on the IO 320.

When you are operating into a shorter fields a correctly set up spring loaded throttle stop can be the difference between a comfortable landing and one which is a bit tight on distance. A correctly set up throttle stop also has a huge impact on reducing brake wear.

If the throttle is held closed against the spring when there is very little airflow thru the props (i.e. at taxy speeds or when the aircraft is stationery) the idle speed is too slow hence the need for the spring to advance the idle speed at this time.

With the throttle adjustment set up to allow RPM reduction by pulling the throttle closed against the spring, the throttle can still be closed further by depressing the "Cutoff" button. There is still a use for the "Cutoff" button.

Alan

by **AlanBreen** » Tue Dec 15, 2009 10:14 am

I don't have a picture of it. Next time I have the cowl panels off I will try to remember to take a picture.

Next time you have the cowl panels off take a look at the throttle arm on the Fuel Control Unit. Make sure you are looking at the throttle arm and not the mixture arm.

You will see that the throttle arm is resting against a stop when the throttle is closed. This stop has a metal button or pin sticking out which the throttle arm rests against. If you get someone to pull back on the throttle you will see the button/pin depress into the body of the stop allowing the throttle to close some more.

by **Zach Grant L1011jock** » Tue Dec 15, 2009 3:04 pm

Alan,
I disagree sligtly with your assumption as to what the spring is there for. If you measure the amount of excess travel the spring stop gives you on the throttle arm, it is slightly less than the additional travel allowed by pushing the cut off button and pulling the throttle lever to the end of the quadrant. This says to me that idle is idle, not some "flight Idle" that can be pulled through when on the ground to run at a lower RPM (remember most of that RPM increase is windmilling/unloaded blades anyway, and that is really a significant source of drag). If there is to high of an idle set yes it can make landings long, but my 320's run well with the idle st at 600 rpm, and don't idle out.

The purpose of the spring is to allow for certification. Cert requirements state that the primary stop must be at the engine end of the control (stop point before getting into the spring) (secondary stop is the cut off stop in the normal position which prevents the spring from compressing). When the button is depressed and the throttle lever is pulled full aft, the only stop is the butterfly valve bottoming out on the walls of the throttle body, as the spring allows this to happen when the secondary "hard" stop is removed by the pilot.

I hope this is a little more thought out explaination than what I sent you previously.

-Zach