Why Are Planes Not Allowed to Take Off in Very Hot Days?

It is a common misconception that planes cannot take off on very hot days. While it is true that high temperatures can pose challenges, the reality is more nuanced. This article delves into the factors that affect plane takeoff on hot days and explains why some runways may require longer distances for takeoff.

The Impact of Temperature on Engine Performance

When an engine's temperature rises, its efficiency decreases. This directly impacts the plane's ability to generate sufficient thrust to lift off the ground. On a hot day, the engine may not produce enough power to provide the necessary acceleration, especially if the runway is not long enough to sustain the plane to its takeoff speed.

However, with extremely long runways, planes can effectively generate the speed they need. Therefore, it is not an absolute prohibition but a matter of having the appropriate runway length to accommodate the increased takeoff requirements.

The Role of Ambient Temperature in Jet Engine Efficiency

The efficiency of jet engines is directly influenced by ambient temperatures. While planes can technically take off in extreme heat, they may require significantly longer runways to achieve the necessary takeoff speed. The core principle is that the operational performance charts must be in accordance with the manufacturer's testing under specific temperature conditions.

The Physics Behind Lift and Temperature

For a plane to take off, it needs to generate lift. Lift is a function of the temperature and density of the air. As the temperature increases, the air density decreases, reducing the lift generated by the wings. Consequently, the plane requires a higher ground speed to generate sufficient lift to take off, which can be challenging in very hot conditions.

Hot air is less dense and thinner, which reduces both engine power and the lift potential from the wings. This is why taking off from a hot or high-altitude airport can increase the required takeoff speed, possibly reaching the limits of the tire speed and requiring a longer runway. This is crucial as all runways have a finite length.

Practical Considerations: Certification and Cost

Hot conditions can severely impact the maximum load a plane can carry. However, they usually do not outright prevent an aircraft from flying. The real challenge lies in certified performance data. Many airplane manufacturers only test their planes up to a certain temperature and publish the necessary performance charts accordingly. In some cases, airlines need to purchase additional high-temperature performance charts as an extra-cost option. If these are not available, the aircraft cannot legally fly under those conditions.

For example, in conditions like 120°F (49°C) or slightly below (117°F or 47°C), different airlines with identical equipment may or may not be able to fly due to the absence of the high-temperature performance charts or the airline’s decision to not purchase them. This can lead to varying situations depending on the specific airline and weather conditions at the time.

At high elevations, the air is already thinner, and the heat exacerbates this. Some planes cannot fly at all under these conditions, while others may barely manage to take off early in the morning but need to be grounded due to the intense heat during the day.

Despite these challenges, planes do fly in hot conditions. The key is having the appropriate runway length and the necessary certification data to ensure safe and legal flight operations.