Does an increase in air temperature actually improve aircraft performance? Mainly because of what pilots learn about density altitude and aircraft performance, any idea that an increase in temperature could improve performance seems counterintuitive. And indeed it is. Here's what got me thinking about it.

In studying for a written test I ran across a question phrased something like this:


  An aircraft flying at a constant power setting flies from a colder temperature to a warmer temperature. What happens to true airspeed and true altitude?


The correct answer turned out to be that true airspeed and true altitude both increased. I knew immediately on the altitude because I remembered reading how very cold temperatures can cause dangerous errors in altimeters, but the airspeed didn't make sense.

I still don't understand why the increased temperature lead to an increase in true airspeed. Obviously a reciprocating engine doesn't perform better in warmer air. We don't install "interwarmers." Does it really mean that warmer air can actually improve performance? Or does this need a deeper explanation?
Drag (and lift) increases with density. Density decreased and so did drag. So at the same power, you can fly faster.

Now I don't know whether reduction of power of a normally aspirated spark-ignition reciprocating engine at constant throttle setting would be higher or lower than the reduction of drag. But the question says power setting.