Considerations for summer flying’s unique set of challenges.
Planes routinely fly on scorchingly hot days. Places like Dubai, Phoenix, and Las Vegas as a few examples of large cities with summer temperatures that exceed 40 °C or 104 °F that have international airports. Like other extreme weather conditions, flying during exceedingly hot days comes with extra considerations. Let’s explore a few.
The most significant penalty incurred when flying on hot days is an increase in density altitude. Density altitude is a measure of air pressure corrected for non-standard temperature. On a hot day, pilots and other weather-focused professionals identify a higher-than-standard density altitude. The “standard temperature” at sea level is 15 °C (59 °F). Any temperature at sea level warmer than this will result in a density altitude higher than zero feet. An example of a “standard” day might be 15 °C at Los Angeles Int’l Airport, right at sea level. In this example, the density altitude of zero is the same as the true altitude. On a 40 °C day at LAX, the density altitude would spike to nearly 3,000 feet, though the true altitude remains unchanged.
Photo: Kate Scott I Shutterstock
Understanding density altitude is necessary because it directly impacts the aerodynamics of flight. Aircraft performance degrades as density altitude increases due to lower air density at hotter temperatures. Wings do not generate as much lift in less dense air, and engines produce less thrust compared to operating in relatively denser air. This degradation in performance results in longer takeoff and landing distances due to faster airspeeds required to generate sufficient lift. This is why “hot and high” airports like Bole Int’l in Addis Ababa or Denver Int’l in Colorado feature runways longer than 12,000 feet.
Along with increased runway distances, the load-carrying capacities of aircraft are appreciably altered by extreme heat. This does not solely apply to general aviation aircraft but commercial airliners as well. As discussed in previous articles, airline operations require aircraft to meet rather strict standards on departure and initial climb. Most significantly, the plane has to reach V1 speed, experience an engine failure, and be able to continue accelerating and climb out safely on one engine. On a hot day, a higher density altitude requires higher airspeeds. Engines produce less thrust due to decreased air density available to mix with the fuel. Fan blades also produce less propulsion in the “thinner” air. Consequently, the maximum weight a plane can carry and with this reduced amount of thrust must decrease accordingly.
A friend once recounted a story about a flight from Phoenix to Boston on a rather hot summer afternoon. They had to make a fuel stop somewhere along the way, perhaps in Kansas City or Tulsa. They were confused because they had made that same flight multiple times before on the same plane without needing a fuel stop. The difference on the day in question was the weight that could be carried for takeoff performance numbers to work. The pilots (and their company) had options: take enough fuel for the nonstop and leave folks behind, or take everyone and make a stop. They opted to bring everybody along that time.
Perhaps you have experienced flying on a hot day, or from a high-altitude airport, and had to make an intermittent stop. Maybe your flight was delayed until after sundown, or passengers rebooked to a later departure. Hopefully this article has given insight into why this happens and how performance is affected on hot summer days.