Has nothing to do with factory install or not. 'Required equipment' refers to that which is required by the type certificate.
The FARs and CARs set out what engine and flight instruments meet MINIMUM standards, i.e. piston engines are required to have a tach, oil pressure indicator, and oil temp indicator. All aircraft are required to have an airspeed and altimeter. Some type certificates specifically call this out and some may not. Anything required, above these minimums will be called out in the type certificate for EITHER the engine or airframe. A good example is a Cessna 185. In addition to the minimum instrumentation called out by CAR 3, a vertical speed indicator is required and since it has a constant speed prop, a manifold pressure indicator is required. And if you look at the type certificate for the continental 520, a single point CHT is required.
If you wanted to REPLACE any of these instruments, a 'certified primary' instrument is required. Think of the CGR-30P and the JPI 900 series instruments. All engine instruments in one package and certified primary. Problems arise with some of the older styles like the JPI 700 series as not all of the systems in the unit were certified. I've seen them installed in Cessnas, replacing the CHT gauge, and the manual states that the CHT system is not certified primary in that unit. So that unit is not legal for that aircraft unless a certified primary CHT is also installed.
On the flight instrument side, a good example is a vertical speed indicator in a Cub. It was never required to be installed as it is not listed as required equipment in the type certificate. If the owner wants one installed it does not need to be 'certified primary'.
The rule is, to REPLACE a required instrument, a certified primary instrument is needed. To ADD an instrument, above what's required, does not require a certified primary unit.
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