The weather here over the last two weeks was simply perfect for flying. Blue skies with a few benign clouds in the afternoon, and temperatures reaching the high 20’s (celsius). Air is dry, and it’s not yet the CBs season. Simply cool. Before temperatures get in the 30’s it’s time for the seasonal warning about take-off performance.

You know, the good old “double penalty”. Warm air is less dense, so wings produce less lift, and engine produce less power. Nothing new under the sun, but a reminder never hurts. But what about turbo-diesel engines ? Shall not the turbo be insensitive to air density, at least to a certain level ? Let’s have a look.

The following graph shows the relation between temperature and take-off distance, all other parameters being equal : maximum take-off weight and no wind. If you’re used to the Piper performance graphs, this corresponds to the leftmost part. The Diamond graphs are of the same nature.

The aircraft I chose are the DA40-TDI, and the DA40-180 with fixed pitch propeller. I also added the PA28-181, because all three are used for the same kind of missions. The goal is not to compare the performances, but rather the slope of the curve, which indicates how much the temperature affects the performance.

When I made the first version of this graph, I used only the DA40-TDI and the PA28. I expected this difference in slope, because of the tubro, but it was so different that I also included the DA40-180. So, what does this graph teach us ?

1) Take-off performance degrades with temperature. Changing from 0° to 30° can increase the take-off distance by approximately 40%.

2) Turbo engines are less affected by increase in temperature.

3) As good as they are, turbo engines are not the perfect cure against performance degradation, simply because they don’t compensate for the aerodynamic penalty.

What is also interesting is that the DA40-180 has better performances for temperatures below zero degrees. I don’t have an easy explanation for that, and I also know that the C172 TDI performance is below the classical 172 in some areas of the operational domain, but this is not today’s topic.

When preparing your next flights, if the runway is a bit short, or if you fly with more passengers than usual, have a quick look at the performance charts. You’ll avoid any bad surprise, and have a good summer…