Class D Transit with a Failing Radio
You are cruising at 1800 ft AMSL toward a Class D control zone whose floor is surface and ceiling 3000 ft. The ATZ is active and the tower has just cleared you to cross the zone, reporting traffic on a three-mile final for runway 27. Two minutes later, while still inside the zone, your radio transmits but you hear only carrier noise - the receiver appears to have failed. You are 4 NM from the far boundary, visibility is 8 km, and there is scattered cloud at 2500 ft. Your transponder is Mode C capable and squawking assigned. The aerodrome has a published light-signal procedure and you have the relevant signals memorized.
1. What are your immediate priorities and what should you do about the clearance you were given?
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First, fly the aircraft and maintain terrain and traffic separation - the clearance does not override see-and-avoid. Reduce speed slightly to buy decision time and keep a sharp lookout for the traffic on final.
The clearance was predicated on two-way communication. With the receiver failed you can no longer be sure the clearance remains valid, and ATC cannot reach you to cancel or amend it. The safest course is to continue on the cleared route to exit the zone by the shortest practical path, since you are already inside and turning back would increase time in conflict with known traffic. Squawk 7600 to signal radio failure, keep the assigned code if the transponder is serviceable and the POH/ATS procedure says to change, and watch for light signals from the tower.
If a light signal is received, comply: steady green means cleared to land, flashing green cleared to taxi, red means stop/give way. Do not assume; if no signal is seen, exit the zone and land at a suitable aerodrome to repair.
2. How would your plan change if the cloud base lowered to 1200 ft before you exited?
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At 1800 ft with cloud at 1200 ft you would be in cloud, which is not permitted and not survivable VFR. You must descend or divert before entering the cloud, but descending inside a Class D zone without a clearance is itself a problem.
Options in order of preference: (1) request a lower level - impossible without radio, so (2) turn immediately toward the nearest boundary and exit the zone at the lowest safe level consistent with terrain and obstacle clearance, accepting that you are already in a communication-failure situation that ATS anticipates; (3) if the tower is visible and signals are being given, follow them.
A 180-degree turn back toward the approach path is the worst option because it puts you head-on with inbound traffic. The key judgement is that cloud avoidance and terrain clearance outrank the letter of the clearance; declare the situation by squawk and light signal and get clear of controlled airspace and cloud as directly as possible.