Too hot to fly — why?

Extreme heat alters the density of air, making it thinner. Thin air prevents generation of required ‘lift’, and makes it more difficult for aircraft to take off. Thus, as it gets hotter, planes need progressively longer runways and greater engine power to reach the speeds needed to become airborne.

Written by Sunny Verma | New Delhi | Published:June 21, 2017 1:19 am
heat wave, hot temperature, flight cancelled, heat wave airplanes, american airlines, Heat haze at Sydney airport, November 2006. (Reuters/File)

American Airlines, the world’s largest carrier by fleet size, revenues and destinations served, cancelled nearly 50 regional flights out of Sky Harbor International Airport in Phoenix, Arizona, as the day temperature was forecast to touch 120 degrees Fahrenheit (nearly 49 degrees Celsius) on Tuesday.
American media organisations quoted a release from American Airlines saying the Canadian-built Bombardier CRJ aircraft used by its American Eagle regional services were not certified to fly in temperatures higher than 118°F (48°C).

The cancelled flights — due to be operated by Skywest and Compass airlines under the American Eagle brand — were mostly scheduled to leave or land in Phoenix between 3 pm and 6 pm, when temperatures are the highest.

The US National Weather Service has warned of “excessive heat” across south-western USA Monday through Wednesday, and Phoenix is in the middle of what local media are calling a “hellscape” heatwave.

Extreme heat alters the density of air, making it thinner. Thin air prevents generation of required ‘lift’, and makes it more difficult for aircraft to take off. Thus, as it gets hotter, planes need progressively longer runways and greater engine power to reach the speeds needed to become airborne. In these situations, airlines often put restrictions on onboard weight, and offload cargo and fuel to become lighter.

Back in 2013, Business Insider quoted pilot and author Patrick Smith explaining how this works: “Hot air is less dense. This affects the output of the engines as well as aerodynamic capabilities, increasing the required runway distance and reducing climb performance. Therefore the amount of passengers and cargo a plane can carry are often restricted when temps are very high. How much so depends on the temperature, airport elevation and the length of the available runways. And getting off the ground is only part of it: once airborne, planes have to meet specific, engine-out climb criterion, so nearby obstructions like hills and towers are another complication.”

Larger jets, with more powerful engines, have higher maximum operating temperatures. Boeings can operate at temperatures up to 126°F (52°C), and Airbuses, 127°F, or 53°C, Phoenix-based The Arizona Republic quoted American Airlines as saying in its release. Even so, larger jetliners too are affected — China’s Hainan Airlines recently changed the departure time of its Las Vegas-Beijing flight to the middle of the night for the duration of the summer, so it doesn’t have to cut down on its load.

Former Director General of Civil Aviation Kanu Gohain said temperature is a very important parameter to consider while calculating the take-off weight of an aircraft. “If the temperature is high, the runway length required will be longer. This happens because air density is low when temperature is high, leading to lower power generation,” he said.

“We usually calculate weight against the particular length for a smooth take-off. Delhi airport sometimes registers high temperature — this is when flights are cancelled or the weight is adjusted. Airlines typically use ‘WAT’ aircraft performance cards, which stands for Weight, Altitude and Temperature, to help aircraft operate smoothly,” Gohain said.

An official with a private airline in India told The Indian Express that every aircraft has performance limitations that also depend on factors other than the weather. For example, carriers catering to Patna and some other airports, where the runway is short, fill their planes only up to 80% of capacity to enable them to take off without problems.

Aircraft with single engines, or even smaller, double-engine planes, face performance issues in hot weather, officials said. “When the temperature is very high, it affects the engine performance, especially at the time of take-off. That is the basic problem,” an official in Air India’s engineering department said. High temperatures also increase the risk of tyre bursts, the officials said.

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