Is a Cessna 172S still airworthy if factory-installed instrumentation not required for day VFR flight is inoperative?

Question

A C172S I occasionally rent had the EGT gauge placarded INOP when it had a JPI EDM-700 engine monitor installed. The placard sits on the gauge so you can no longer see the needle. Probably the single point EGT probe has been removed so the needle wouldn't work anymore. Of course that is fine - the EDM-700 is TSO'd. Now recently the EDM itself is placarded INOP and its circuit breaker collared so it cannot be reset.

So now this 172S has two INOP EGT gauges, and I'm down to leaning by ear. I know the EGT gauge is not required daytime VFR equipment. However, since the airplane was certificated having a factory EGT gauge installed, I'm wondering whether it's airworthy like this. The Plane is N-registered and I'm asking within FAA jurisdiction.

Answer 1

Part 91 does not require an EGT probe for any operations so the next thing to look at is your AFM.

Your C172S has an AFM that includes a Comprehensive Equipment List. It is probably similar to this one that indicates that the EGT probe is standard equipment—not required for certification, this airplane can fly with it placarded.

Answer 2

The 172S has both a Comprehensive Equipment List and a Kinds of Operation List in the POH. Check them both to see if you can operate the aircraft without an EGT. My guess would be yes, since practically, it does not NEED it. You can lean the engine using the Tach and your ear. And, 91.205 does not require an EGT nor a CHT.

Answer 3

It’s airworthy if the gauge reaches here are not required for the operations in question per 91.205 and the procedure outlined in 91.213 is used to label the gauges inoperative AND this problem is addressed and repaired during the next maintenance session.