When the weather gets hot, these rules of thumb can come in handy.
1) Takeoff roll increases by around 10% for every additional 1,000 feet of density altitude
For most normally aspirated GA airplanes, you’ll add approximately 10% of takeoff roll for every 1,000′ of DA.
If we stick with the Denver example from earlier, with a rise of 3,200′ of density altitude, our takeoff roll will increase by around 32%.
So if we have a 1,500′ takeoff roll on a typical day in Denver (3°C), on a 30°C day it’ll increase to almost 2,000′.
2) True airspeed increases by 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 likely to be the same, at 61 knots.
But say you’re in Denver on a 30°C day. With a density altitude of 9,240′, your true airspeed is on the rise, quite a bit.
If we round to 9,000′ DA for simplicity, your landing true airspeed at 50 feet will be 72 knots true. (again, your airspeed indicator will show 61 knots, but you’re actually moving at 72 knots through the air)
And that extra 11 knots can make a big difference when landing. Both in terms of landing distance and, perhaps more importantly, controllability.
When you’re landing on 8 inch tires, going faster means the plane is less easy to control.
3) How To Calculate Windshear
Rule of thumb: the total shear is double the peak wind. If the outflow speed of a microburst is 30 knots, you’ll experience around 60 knots of shear as you cross the microburst. And it can all happen in a very short period of time.
Imagine what would happen to your Cessna 172 if you went from 100 knots to 40 knots in a matter of seconds…
Hot weather has a significant impact on your plane in multiple ways. But if you know what to expect, you can mitigate the risk.
