Transponder Failure Under a Class B Shelf

First, tell ATC. You are already in contact, so state plainly that the transponder and ADS-B Out have failed, give your position and altitude, and ask what they want you to do. Under the equipment-failure provisions for the ADS-B rule, you notify ATC and may continue only if ATC authorizes it; if ATC does not authorize, you must not operate in the airspace that requires the equipment.

Second, understand what you lost access to. Both Class B and Class C airspace require a functioning transponder with altitude reporting, and both are within the ADS-B Out requirement. Your clearance into that airspace - even the implied one that comes with flight following - is no longer something you can assume.

Third, consider whether it is actually a failure. Check the breaker, the master/avionics switch, and any squat switch or air/ground logic. A popped breaker that resets and holds is a different situation from a dead unit; a breaker that pops again means do not keep resetting it.

Fourth, do not squawk 1200 and proceed. Losing your beacon code and continuing into the shelf is exactly the kind of unannounced deviation that generates a Brasher warning and a report.

Plan on not being cleared. Request the shelf transition; if Center or Approach approves, fly it. If approval is not forthcoming - and plan for that - the flight is still completable below the shelf.

• Altitude: the Class B floor is 3,000 ft MSL, so remain below it. Terrain reaches 2,000 ft MSL, so 2,500–2,900 ft MSL gives 500–900 ft of terrain clearance and puts you under the floor. Crucially, the cloud bases at 4,500 ft MSL mean you have 1,600 ft of VFR clearance below the deck in Class E airspace, where the requirement is 3 SM visibility, 500 ft below, 1,000 ft above, and 2,000 ft horizontally from clouds.
• The destination is Class C, which also requires the failed equipment. Do not fly 140 nm and negotiate at the boundary. Request authorization now, or plan a diversion to a non-ADS-B-required airport and arrange ground transportation.
• Airspace boundaries are fixed; your options are not. The judgment call is to resolve the destination question with ATC early, while you still have fuel and daylight to divert, rather than arriving at the Class C boundary with a decision to make.

Preflight: a quick check of the equipment list against the route would have shown that this flight depends entirely on a transponder and ADS-B Out. That makes a backup plan part of the flight plan, not an afterthought. A 140 nm leg with 3.2 hours of fuel and no alternate is a fragile plan for any airplane; it becomes indefensible when the route threads through airspace the airplane might not be allowed to enter.

ADM:

• Set a decision point in advance - e.g., "if the equipment fails before the shelf, I divert to the non-ADS-B airport 25 nm south." A pre-briefed trigger beats an in-flight negotiation.
• Recognize the plan-continuation bias. The destination is close, the weather is beautiful, and the airplane flies fine - all of which push toward pressing on. None of those facts changes what the regulation requires.
• Distinguish "can" from "may." The airplane is perfectly capable of the flight; the airspace rules are what constrain you. Confusing the two is how pilots end up talking to a controller who has already violated them.
• Communicate early and specifically. "Unable ADS-B, request deviation" is a sentence ATC can act on; silence followed by an incursion is not.