Oil mist on face of Oil Pressure gauge

[Robert Eilers]

The problem was the gauge. The Copper line and fittings were pressure tested and determined to be fine. I am replacing the instrument. My mechanic seems to be concerned about replacing the gauge with a similar gauge that has the "Green" range marks on the face of the instrument. In lieu of such a gauge he suggested marking the face of the instrument. I chose to purchase the gauge with the markings, which was a little more expensive. My question is - is it necessary to replace one instrument with a similar instrument just because the original had a "Green" ranged marked on the face of it?

[GAHorn]

It may be argued that no instrument markings are required at all, that only the limit must be observed, since this is not a "limits and markings" airplane, but is an AFM airplane. (Some airplanes do not have approved airplane flight manuals and are allowed to be operated by reference only to instrument markings and placards. The Cessna 170 is not such an airplane, as it has an Approved Airplane Flight Manual which must be aboard for flight.)

The aircraft TCDS specifies operating limit values which have been superceded. In that sense, the aircraft TCDS is in need of revision. The original operating limits were described in that document as being indicated on instruments as green arcs and red lines, but the actual limits are specified numerically.

The latest TCDS for the engine would allow an upper operating oil temp limit of 240-F. It may be applied either on the glass on on the instrument face as a red line. (The original operating instructions ...not approved instructions....for the engine required a red line which was originally at the limit of 225F, no mention was made of a "green arc", but the current Revision 18 increased the limit to 240F when using straight wt. SAE 50 oil.)

I believe it to be true that the limit of 240 must be observed regardless of instrument markings, therefore no colored instrument markings are required.

[lowNslow]

George, CAR3 states:

§ 3.759 Power-plant instruments. All
required power-plant instruments shall be marked
with a red radial line at the maximum and minimum
(if applicable) indications for safe operation. The
normal operating ranges shall be marked with a
green arc which shall not extend beyond the
maximum and minimum limits for continuous
operation. Take-off and precautionary ranges
shall be marred with a yellow arc.

[GAHorn]

Hmmmn. My electronic copy states:
"Required placards and markings for all categories in which the airplane is certified must be furnished in the Airplane Flight Manual."

Since the C-170 AFM doesn't specify colored arcs and red lines.... it would appear none are required... but that does conflict with what CAR3 states as you posted Karl.

Thanks for the reminder...I'll do some more reading....

[lowNslow]

The AFM only discribes airplane specific (make and model) placards that are required in addition to the ones required of ALL aircraft certificated under CAR3. Having said that - I have only applied a red radial for max temp on my new oil gauge, the green arc seems a little superfluous.