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3 Rules-Of-Thumb For Flying In Hot Weather

Bose holiday deals: get a free Black SoundLink Micro Bluetooth speaker when you purchase a ProFlight Series 2 Aviation Headset by December 31, 2022.
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When the weather gets hot, these rules-of-thumb can help.

1) The 50/70 Rule

The 50/70 rule is a general rule for GA aircraft that says if you haven't reached 70% of your takeoff speed by the time you've reached 50% of the length of the runway, you should abort your takeoff.

Why do you need 70% of your takeoff speed by 50% of the runway? As you accelerate down the runway during takeoff, you start chewing up more feet of runway for every second you're rolling down the pavement. If you haven't achieved 70% of your takeoff speed by the time you're halfway down the runway, you may not have enough pavement left to get to rotation speed and lift off.

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To use the rule, first, calculate 70% of your takeoff speed. Second, find a visual point that's 50% down the runway. If you've reached your 70% speed before you've reached the visual point 50% down the runway, continue. If not, abort.

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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.


Bose holiday deals: get a free Black SoundLink Micro Bluetooth speaker when you purchase a ProFlight Series 2 Aviation Headset by December 31, 2022.


Colin Cutler

Colin Cutler

Colin is a Boldmethod co-founder and lifelong pilot. He's been a flight instructor at the University of North Dakota, an airline pilot on the CRJ-200, and has directed the development of numerous commercial and military training systems. You can reach him at colin@boldmethod.com.

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