The International Cessna^® 170 Association

Why is there a yellow arc on the tach from 2450 to 2700?

Yellow is supposed to be a cautionary range. Are we not supposed to operate continuously from 2450 to 2700?
I can't find anything that says NOT to run in that range but then it seems that the green should go all the way to 2700.

FWIW, I run 2500 all the time unless I'm in the pattern but I haven't checked my tach for accuracy in a while.

Thoughts?

Operating limitatiions are set by the TCDS the AFM and any supplements, STCs and ADs. Markings are or can be set by the same documents as well as those found in the FARs or CAR 3 in our case.

There is only one engine limitation set by any of these previously mentioned documents on a C-145/0-300 powered 170 and that is 2700 RPM. The red line at 2700 mentioned in the TCDS is not required by the TCDS, that is I believe required by the CARs but I'm not digging to find out. The green arc mentioned in the TCDS is not a limitation. The green and yellow arc may or may not be require by CAR 3.

David,
While the C-145/O-300 is certificated to run continuously at 2700 RPM the Operator's Manual says this:According to the Operator's Manual the recommended cruising RPM is 2450. I'd guess that is the reason for the yellow arc.

my 172 Owner's Manual lists the tach markings as:
Normal Operation:
at sea level: 2200-2450 (inner green arc)
at 5000 ft: 2200-2550 (middle green arc)
at 10,000 ft: 2200-2650 (outer green arc)
Max allowable 2700 redline

in the checklist section it says "recommended cruising RPM 2450-2650"

that's with an O300A and a McCauley prop

Per the original Cessna design print, there is not supposed to be a yellow arc on the tach. Some probably assume (incorrectly) that because the Owner's Manual has a maximum recommended cruise RPM, and that the TCDS lists maximum RPM as 2700, therefore anything in between "must" be cautionary? Either way, technically, there is not supposed to be any yellow on the standard-configuration 170 0411087-series tach as specified in the parts catalog. Other installations, anything goes I guess. Incidentally, if anyone wondered, it's supposed to record at 1:1 time at 2426 rpm (at least when it was new, anyway).

I believe the definitive document will be found to be the AFM, which specifies the colored arcs to be "normal range 2200-2450, Green Arc" and " Do not exceed rated engine speed, 2700 RPM (Red Line)"

There is no specified "yellow arc". I suspect some instrument shops merely paint it there for "style".

Personal Note: Notice the Red Line is much faster than the Green Arc.

George, you have no mercy

gahorn wrote:Personal Note: Notice the Red Line is much faster than the Green Arc.

But that'll wear the engine out faster, so those running in the green will be able to keep flying longer while the red one is down for overhaul!

KS170A wrote:

gahorn wrote:Personal Note: Notice the Red Line is much faster than the Green Arc.

But that'll wear the engine out faster, so those running in the green will be able to keep flying longer while the red one is down for overhaul!

Ah, contrere'... 1800 hours is 1800 hours. But 1800 hours at faster speeds means greater miles travelled per hour, which means less hours flown. (If you're a time-builder and going no where however, you might prefer the greenies.)

Seriously, in some circumstances, there may be merit in running more percentage power as it applies to hourly operating costs.
Also, keep in mind that the same cylinders/pistons/rings/etc. run at well over 3,100 RPM in the GO-300 engines. While they have lesser recommended TBOs (largely due to their gearboxes), they are not harmed by the high RPMs..but are frequently hurt by those pilots who run them too s l o w l y.