Why Can't Planes Take Off in Extreme Heat?

Did you know it can be too hot for aircraft to takeoff? Here is why.
Christopher McFadden

Back in 2017, more than 40 flights were canceled due to the extreme heat in Phoenix, which saw 122 degrees Fahrenheit (50 degrees Celsius).

Surprisingly, the cancellations were not a result of the aircraft overheating or the engines malfunctioning. Rather, the hot weather prevented the planes from lifting off. But why can't planes take off in extreme heat?

Understanding why requires some basic knowledge of how planes generate lift and how environmental factors can affect how a plane flies. So, let's spend some time going through the fundamentals of flight. 

How a plane produces lift

The single fundamental process that enables planes to fly is the ability to generate sufficient lift. No lift, no flight!

Lift is created when a solid object, like a wing (technically called an airfoil), interferes with the flow of gas (like air). In the case of aircraft, the shape of the wing (in cross-section) deflects air downward, causing the wing to be pushed up. 

But, like anything in life, it is more complex than that. The effect is a direct result of Newton's Third Law of action and reaction.

why planes cannot fly in hot weather lift
Source: Michael32710/Wikimedia

In the instance of an airplane, a slanted wing collides with air particles, changing their direction of flow and relative velocity. 

Both the top and the bottom of the wing surface contribute to redirecting the air to generate lift. In normal flight, the force is typically directed downward, resulting in the plane being pushed upwards.

The shape of a wing directs the flow of air downwards creating a high-pressure point on the underside of the wing. On the upper portion of the wing, the pressure is relatively lower. This is because the air under the wing travels over a shorter distance, over the same time, resulting in it moving slower, and thus having a higher density (relatively speaking). 

As a result, there is a net deflection of air downwards which pushes the plane into the sky. Pretty neat. 

This change of the velocity of a gas is the predominant factor that contributes to the force of lift that keeps a plane suspended in the air. The crucial part in keeping an aircraft airborne is its ability to maintain a high pressure beneath the wing sufficient enough to keep the plane in the air.

how an aircraft wing creates lift
Source: Bernard Spragg. NZ/Flickr

Generating lift is not difficult; a plane with forward momentum will naturally want to fly under the right conditions. However, under the wrong conditions, a plane will struggle to take off altogether. 

Why can't planes take off in extreme heat?

It is a well-known fact that the upper atmosphere is much thinner than it is at sea level. The air at higher altitudes is less dense, which makes it difficult for planes to produce as much lift as at lower altitudes.

The main reason for this is that the air molecules are spread out much further, meaning less air (or amount of molecules) can come into contact with a wing.

To compensate, at higher altitudes, naturally aspirated aircraft engines must work harder to produce enough thrust to keep the plane moving at a fast speed so that enough air can pass under and over the wing to generate the pressure gradient and create lift. Interestingly, however, for jet engines, this can be beneficial (up to a point). 

However, just like in the upper atmosphere, hot weather similarly causes the air density to decrease. As the temperature rises, air molecules move faster and spread out, creating a lower air density.

In hot weather conditions (as with high altitudes) less dense air means there is less "stuff" for wings to push down on and produce lift. If a plane is taking off in such conditions, it must travel much faster before it is able to generate enough lift to take off.

why aircraft stuggle at height
Source: Janusz Jakubowski/Flickr

But that is only part of the story.

Compounding the effect is the lack of oxygen the engine needs for combustion, inhibiting the power output of the engine.

Unfortunately, just as the wings suffer efficiency losses, so do propellers and fan blade engines. The lower pressure systems mean the engine cannot convert as much engine power into thrust.

Essentially, for an aircraft to take off in low-pressure systems, it requires more runway so the plane can build a high enough airspeed for the wings to generate thrust, all while being hindered by a reduction of power output.

As temperatures climb, the effect is compounded until the point where it becomes dangerous for some planes to take off in extreme heat.

Although the planes could take off, given enough runway to build speed, the planes must still be able to climb quickly enough to clear obstacles towards the end of the runway. Not an ideal situation to say the least. 

What happens when planes are not prevented from flying in hot weather?

Should a plane take off at a high altitude, and in an extremely hot climate, the conditions will severely impact a plane's climb rate (as previously discussed). If the plane cannot climb fast enough, it will run out of space and may crash into an obstacle.

This, obviously, could be very dangerous. 

comparison of plane climb rates
Comparison of climb rate between an aircraft taking off at sea level with normal conditions and temperatures versus an aircraft attempting a take-off in hot and high conditions. As indicated by the slope of the line, the hot and high aircraft has a much longer take-off distance, and slower climb rate. Source: Pepe.is.great/Wikimedia

The effect has brought aircraft down before.

In 2012, four passengers flying in a Stinson 108-3, 165 horsepower (123 kW), single-prop plane came crashing into the ground shortly after takeoff. In the footage of the crash, it becomes shockingly apparent that the plane was starved of power and could not generate enough thrust to completely clear the trees lining the end of a runway.

The hot temperatures and high altitude of the airport affected the plane's ability to maintain sufficient lift. The plane was operating well above its maximum density altitude (the pressure altitude corrected for temperature) and was within just 86 pounds (39 kg) of its maximum takeoff weight. In fact, the pilot was ready to abort the takeoff, when a gust of air lifted the plane, and the pilot thought the airplane would remain airborne.

However, when the pilot could not get the airplane to climb as expected, he attempted to locate an open field to land in. The airplane subsequently encountered a downdraft, stalled, and collided with a stand of trees near the end of the runway, injuring the pilot and one passenger on board.

Planes should stay grounded when it is too hot

Fortunately, no one was killed during the crash, but the video serves as a brutal reminder that flying in hot climates and high altitudes can have drastic impacts on aircraft flying capabilities.

In 2017 in Phoenix, officials, quite rightly, took no chances. Although it is rare, extreme heat can, and will, bring down an aircraft.

With global temperatures predicted to rise over time, engineers may be required to adapt aerospace technologies to keep up with the changing climate. Until then, it is likely airplanes will continue to be grounded due to extreme heat.

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