When a top of class D ( on the chart below) conflicts with bottom of class C (37) which prevails?
AIM 3-1-3 answers this:
Hierarchy of Overlapping Airspace Designations
When overlapping airspace designations apply to the same airspace, the operating rules associated with the more restrictive airspace designation apply
This is more a “thought exercise” question than one that’s likely to present a real-world quandary. The two controllers are in near constant communication with each other and have probably worked together every day for a long time. The two facilities will have memorandums of understanding about how to handle various situations, that you’ll probably not know of.
As a general rule, if you are taking off you’ll be talking to the tower (Class D) first, and they will know when to hand you off to the Class C facility. If you are arriving, you’ll be talking to the Approach facility (Class C) first, and they will know when to hand you off to tower (Class D). If you were approaching the airport in your chart fro the east, you could contact approach several miles before getting to the east border of the Class B. They usually welcome arriving traffic to contact well beyond the magenta rings. Or, you could contact tower before crossing the east boundary, because if you were landing, you’d most likely already be below the Class C floor well before reaching the arrival airport. So either is perfectly legal.
Perhaps the most common driving factor is whether you need radar navigation. The Class C has radar and the tower may not, so generally the agreement is that the Class C controller will stay with you for as long as you need navigational support. If you can’t find the airport, they won’t hand you off until you do, and they will take responsibility for coordinating with tower until you are handed off. If the Class C controller is saying “Airport 11 o’clock 6 miles”, they’re hinting that they are ready to hand you off. If you say “Airport in sight”, they’ll almost always hand you off right away, even if slightly outside the D because they have radar visibility into anything between you and the airport.
In my experience, the Class D space tends to “win the tie”. That is, a Class C controller will tend to hand you off before you get into the gray area of ambiguity. Or, a Class B or C controller will often tell me I am cleared to enter into an associated Class D, and this clearance does meet the legal requirement for entering the Class D.
Astute controllers or not, there IS still a regulatory 2-way comm requirement on YOU as PIC, so if you feel like you should have been given the frequency change by now, or an explicit clearance, just ask. Often just “Spokane Approach, Skylane 1XX, runway in sight” is a reminder to the Class C controller that it’s time to go.