Flying in Hot Weather
3 RULES-OF-THUMB FOR FLYING IN HOT WEATHER
When the weather gets hot, these rules-of-thumb can help.
1) DENSITY ALTITUDE INCREASES OR DECREASES ABOUT 120 FEET FOR EACH 1°C FROM ISA
If the temperature rises 1 degree from ISA (ISA=15°C at sea level), your density altitude goes up about 120 feet. So if it’s 30°C at sea level, your density altitude is going to be about 1,800′. (30°C-15°C = 15°C above ISA, 15 X 120 = 1,800)
But keep in mind, ISA decreases 2°C per thousand feet. This is important for airports that aren’t at sea level.
Let’s take Denver Centennial, for example, at 5,885′ field elevation. To make the math easy, we’ll round up to 6,000′.
At 6,000′, ISA isn’t 15°C, it’s actually 3°C.
So if it’s 30°C at Denver Centennial, you’re 27°C above ISA, and the density altitude is roughly 9,240′. That’s an increase of more than 3,200′ over field elevation.
The higher your airport, the bigger the difference a hot day makes.
2) TAKEOFF ROLL INCREASES ABOUT 10% FOR EVERY ADDITIONAL 1,000 FEET OF DENSITY ALTITUDE
For most normally-aspirated GA airplanes, you’ll add about 10% of takeoff roll for every 1,000′ of DA.
If we stick with the Denver example from above, with an increase of 3,200′ of density altitude, we’ll increase our takeoff roll by about 32%.
So if we have a 1,500′ takeoff roll on a standard day in Denver (3°C), we’ll increase that roll to almost 2,000′ on a 30°C day.
3) TRUE AIRSPEED INCREASES ABOUT 2% PER THOUSAND FEET OF DENSITY ALTITUDE
In a 172S, your landing speed at 50 feet (roughly the threshold) is 61 KIAS. And while your indicated speed doesn’t change based on DA, your true airspeed does.
On a standard day at sea level, your indicated and true airspeed are going to basically be the same, 61 knots.
But say you’re in Denver on a 30°C day. With a density altitude of 9,240′, your true airspeed is going up, a lot.
If we round to 9,000′ DA to make the math easy, your landing true airspeed at 50 feet is going to be 72 knots true. (again, your airspeed indicator will read 61 knots, but you’re actually going 72 knots through the air)
And that extra 11 knots can make a big difference on landing. Both on landing distance, and possibly even more importantly, controllability.
When you’re landing on 8 inch tires, going faster means your plane is less controllable.
Hot weather has a significant impact on your plane, in multiple ways. But if you know what to expect, you can mitigate the risk.
You should (of course) always use your POH to calculate performance. But with some simple rules-of-thumb, you can get a quick idea of how your plane is going to perform, even before you open up your aircraft book.