A parked airliner has no air conditioning of its own. Cooling comes from the engines, and with engines off, from a small jet turbine in the tail called the APU that burns about 110 kg of fuel an hour, roughly $90 at today's prices. Airlines track those minutes obsessively, because the APU also racks up maintenance hours toward overhauls that cost hundreds of thousands of dollars. On the ground, cool air is a line item. The physics on the other side of that line item are unforgiving. A resting…