TopTup,
Yes, we're on the same page regarding "heavy," though stating that "heavy" may be omitted after first contact and, "should be used below 18,000 feet" is a contradiction. I remember reading in the AIM what you have written, though the new version of the AIM does not state the same thing.
The AIM states:
5. Air carriers and commuter air carriers having FAA authorized call signs should identify themselves by stating the complete call sign (using group form for the numbers) and the word "heavy" if appropriate.
It doesn't specify "first contact," "terminal area" or "enroute." So, we then look to the instructions for the controllers.
This is from FAA Order 7110.65T-Air Traffic Control (edited):
b. The word “heavy” shall be used as part of the identification of heavy jet aircraft as follows:
TERMINAL. In all communications with or about heavy jet aircraft.
EN ROUTE. The use of the word heavy may be omitted
EXAMPLE-
“United Fifty-Eight Heavy.”
NOTE-
Most airlines will use the word “heavy” following the company prefix and flight number when establishing communications or when changing frequencies within a terminal facility's area.
It seems to me that ATC must always use "heavy" in the terminal area and, according to the AIM, pilots must also use the term "heavy." It's not clear that pilots need not use the term, or should not use the term "heavy" in enroute airspace though it IS clear that ATC need not.
Clear as a bell?