Dynon Heated Pitot Ground

Installing a new Dynon heated pitot assembly as part of an HDX upgrade that includes an ACM.

To connect the black ground wire, according to the ACM installation manual, pg127: "Black Ground #14 Locally grounded using ring terminal".

This goes against the Dynon pitot installation manual which specifically states:

Do not connect the Black (Ground) wire to the airframe as a Ground connection.
Doing so will introduce high currents (10A) into the airframe. This can introduce a
significant voltage drop, and potentially cause engine instrumentation, avionics,
and audio system electrical problems.

What's the proper procedure for grounding the assembly?

This is not a subject where you will find agreement among the experts, only different values being emphasized. Using the airframe as a ground is neater, simpler, cheaper, and lighter. Wiring all aircraft electrical systems back to a common ground bus is better electrically, to the detriment of all of the above considerations.

The risk of a voltage drop stated in the pitot tube manual can be mitigated by helping the airframe out where electrical continuity is not guaranteed. For example, with RV aircraft the primary physical contact between the wing and the fuselage is through the spar, which is anodized and non-conductive. Should a pitot tube be grounded locally in such a case, a short 12-14AWG wire should be installed between the wing root and fuselage, as this will allow for better conduction into the fuselage.

Other issues that may arise from using the airframe as a ground can usually be addressed with similar ad-hoc solutions. It is also important to understand that star-grounding your aircraft does not at all guarantee that you will never hear alternator whine in your headset, or suffer erratic engine sensors whose problems can be attributed to electrical noise, etc., though it may lessen the likelihood that these things should occur.

Ultimately, this is a matter to be decided by every aircraft designer/builder/owner/maintainer subject to the values they wish to emphasize.