Alternator failure at night in a C172S

Do not use the rated capacity as the plan.

• 24 Ah divided by 15 A gives about 1.6 hours in theory. That figure is useless operationally: lead-acid capacity is rated at a slow discharge, and only roughly half of it is available before the voltage falls below the minimums the avionics, the transponder and the electric turn coordinator need. An ageing battery may deliver less again.
• Working estimate: 45–50 minutes of usable supply at 15 A; plan on 30–40 minutes to keep a margin.
• Shed immediately: landing light (keep it off until short final - it is the largest single load), strobes, pitot heat (no icing risk tonight; switch it off and monitor OAT), the second radio if fitted. Keep: one communication radio, the transponder for as long as it will run so ATC retains your mode C, and the GPS.
• After shedding, the load may fall to roughly 5–7 A, which roughly doubles the endurance. Recompute as you shed: endurance = usable amp-hours divided by the new load, not by the original one.
• Note that shear means the discharge rate falls too: a lower current draw lets the battery deliver a higher proportion of its capacity.

Think in terms of what matters at night, not just what is on the load list.

• The engine continues to run: ignition is by magnetos and the fuel pump is engine-driven. The engine is not the problem.
• Avionics are voltage-sensitive. Expect the GPS and the radio to drop out or reset first as the voltage sags; the transponder and encoder may fail or output an invalid mode C altitude. ATC then loses both your identification and your level information.
• The electric turn coordinator fails. At night, with a poor outside horizon, this removes your only powered roll reference - the vacuum attitude and heading indicators still work, so use them.
• The flap motor will not run (or will run slowly and partially). Brief a flapless landing: a longer, flatter approach, a higher approach speed, and a landing distance to match.
• The landing light dims progressively and then extinguishes. If you need it to see the runway, you must arrive before that happens.
• Fuel gauges will read low or erratic; the stall warning horn (electric) may stop working; the starter will not turn after landing, so do not plan to shut down and restart.
• The lesson for planning: the dominoes that matter are radio, transponder, turn coordinator, flaps and landing light - in that order of consequence for a night arrival.

Turn back.

• 40 NM behind at about 110 kt groundspeed is roughly 22 minutes: you arrive with the battery still alive, the radio and transponder still working, flaps available and the landing light usable at a familiar aerodrome with full lighting and fire cover.
• Pressing on puts you at the destination at about 55 minutes - right at or past the realistic limit of the battery, at an unfamiliar aerodrome, at night, with no flaps, possibly no landing light, no radio and no transponder. You would be committing to a NORDO night arrival with no fuel to go anywhere else afterwards.
• The calculation, not the destination, makes the decision. Because you retained the option to divert while fuel was plentiful, this is a planned diversion, not an emergency.
• Self-brief before you turn: 'No flaps - flapless approach, higher speed, longer runway, expect a longer float and a firm touchdown. Landing light may fail - stay on the runway centreline lighting, use a wider, longer final. No turn coordinator - attitude and heading from the vacuum instruments, airspeed and altimeter as primary. If the radio dies, I will set 7600 if the transponder lives, and watch for light signals.'
• Advise ATC of the diversion and request a priority landing if the electrical state deteriorates.