Recently in Phoenix, more than 40 flights were canceled due to the extreme heat that approached 50 degrees Celsius. Surprisingly, the cancellations were not a result of the aircraft overheating or the engines malfunctioning. Rather, in hot weather, planes can simply not generate enough lift.
Understanding why requires some basic knowledge of how planes generate lift and how environmental factors can effect how a plane flies.
How a plane produces lift
The single fundamental process that enables planes to fly is the ability to generate sufficient lift. Without lift, there is no flight.
Lift is created when a solid object turns the flow of a gas. The gas flows in one direction, and lift is created in the other direction. The effect is a direct result of Newton's Third Law of action and reaction.
[Image Source: NASA]
In the instance of an airplane, a slanted wing collides with air particles, changing their momentum. The wing forces air molecules down and inevitably, they push right back up, giving the wing lift.
Both the top and the bottom of the wing contribute in redirecting the air to generate lift. The wing alters the velocity of the air which creates the force of lift. In normal flight, the force is typically directed downward to push the plane up.
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 remains lower. As a result, there is a net direction of force downwards which pushes the plane into the sky.
Changing 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.
Generating lift is not difficult; a plane with forward momentum will naturally want to fly under the right conditions. However, in the wrong conditions, a plane will struggle to take off at all.
Low pressure, little push
It is a well-known fact that the upper atmosphere is much thinner than it is at sea level. Higher altitudes have less dense air which makes it difficult for planes to produce sufficient lift. The air molecules are spread much further meaning less air comes into contact with the wing.
At high altitudes, aircraft engines must work harder to produce enough thrust to keep the plane moving at a fast speed so enough air can pass under and over the wing to generate the pressure gradient and create lift.
However, just like in the upper atmosphere, hot weather similarly causes the air density to decrease. As the temperature rises, air molecules spread out, creating a lower air density.
In hot weather conditions and at high altitudes, less dense air means there is less "stuff" for wings to push 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. 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 propeller or 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 quick enough to clear obstacles towards the end of the runway.
Hot weather and dangerous flying conditions
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. If the plane cannot climb fast enough, it will run out of space and may crash into an obstacle.
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 slower climb rate. [Image Source: Wikipedia]
The effect has brought aircraft down before.
Four passengers flying in a Stinson 108-3 165 horsepower single prop plane came crashing into the ground shortly after take off. In the video linked below, it becomes shockingly apparent that the plane was starved of power and could not generate enough thrust to clear the trees lining the end of a runway.
The hot temperatures and high altitude of the airport degraded the plane's ability to maintain sufficient lift. The plane was also operating near its maximum takeoff weight in temperatures hovering around 27 degrees Celsius.
Shortly after take off, the pilot attempted to clear a forest at the end of the runway. However, the plane stalled and crashed into a tree, injuring the pilot and one passenger on board.
A brutal reminder to not fly hot and high
Fortunately no one was killed, however, the video serves as a brutal reminder that flying in hot climates and high altitudes can have drastic impacts on aircraft flying capabilities.
In Phoenix, officials are taking no chances. The recent slew of hot weather combined with Phoenix airport hovering at an altitude of almost 350 meters above sea level made flying conditions extremely dangerous to some aircraft.
Although it is rare, extreme heat can certainly bring down an aircraft. As temperatures continue to climb, perhaps engineers will have to adapt aerospace technologies to keep up with the weather. Until then, it is likely airplanes will continue to be grounded due to extreme heat.
Written by Maverick Baker