Medium RPM

[N3322G]

Had the vacuum pumps replaced the other day and in the twin service manual the procedure stated
"10-~:!.. ADJUSTMENT OF VACUUM REGULATOR.
a. With a qualified pilot or other responsible person at the controls. start
engine on same side as regulator to be adjusted and allow time lor warm up.
b. With the engine running at medium RPM. the vacuum llauge should indicate
between 4.8 to 5. I inches of mercury. If the vacuum reading fails to fall within
this range. shut down the engine and adjust relief valve by loosening the jam nut
from valve adjusting screw located on the firewall. Move the valve adjusting
screw clockwise to increase the vacuum and counterclockwise to decrease the
vacuum.
c. Start the engine and repeat the check.
d. When regulator is adjusted properly. tighten jam nut and repeat the check.
e. Repeat the above procedure for the opposite engine."

My question is - what is medium RPM?
Medium on the gauge?
Medium on ground operations?
Medium for cruise operations?

[Tom Veatch]

It's difficult to try to second guess technical service writers, but the run-up check list on my 250 (O-540) has me checking for 5"Hg vacuum at 2000 RPM. Since that's run-up RPM on my engine, I'd vote for somewhere in the vicinity of run-up RPM being the "medium" RPM desired.

[Kristin Winter]

1200-1500 rpm. I would go closer to the bottom so that you make sure that the gyros are getting enough suction to spool while you taxi and run up so that you are taking off with fully operational gyros.

[9089P]

Hi Pat,

I agree with Kris. Mine is set so I get full pressure(4.8") at 1500 which is my run up rpm. Don't have my POH handy here but I'm almost certain that is the rpm spec'd for the run up on the 260.

Good luck, Don

[Tom Veatch]

The POH for my 250 states "The magnetos should be checked at 2000 RPM, the drop not to exceed 125 RPM with manifold pressure of 15"HG."

But regardless of the specific RPM, my point was that the run-up RPM, whatever it might be, is usually intermediate between ground operation/taxi RPM and cruise RPM and should qualify as a "Medium RPM".

[N3322G]

Interesting thoughts. Thanks.

The purpose of this is to set EACH vacuum pump operation individually. Of course, when both engines are running, there is a different number.

1500 is where we check governor operation and 2000 is where we check mags on the twin. I used 2000.

As we taxied today with both engines, we had 4.0 and the second attitude indicator we have has a low vacuum flag. The flag has not deployed even at stationary idle v. pump operation. At 24/2400 cruise we're at 5.3

After flying, we landed and shut one engine down so Ken could see the various suction numbers. 4.8 at 2000 and 5.1 at 2400 rpm.

[Kristin Winter]

IMX, it shouldn't take more than 1500 rpm on one engine to pull the suction into the green. The only exception I have seen is one twin that has three gyro instruments. In that airplane, it takes a bit more rpm to pull the suction into the green.

[N3322G]

Kristin,

I didn't notice Maggie's gauge. Ours doesn't have a green arc, just numbers. In contrast to the last time you saw it when the needle looked still pegged to the left but gyro was still unflagged during taxi, we now have nice suction - we also had all vacuum hoses changed. It wasn't fun but had to be done.

[N3322G]

I was on the phone with Rapco on another subject and asked them what was "medium RPM". The reply was 2100 RPM. He did say 2000 was fine as long as the gauge was at 4.8 vs 5.1