Planes do not stop working at a fixed temperature, but heat makes it harder for them to take off, and every aircraft type has a maximum temperature it is approved to operate in. Hot air is less dense. That reduces engine power, propeller thrust and wing lift, so the airplane needs more runway and climbs more slowly. In June 2017, American Airlines cancelled more than 40 regional flights in Phoenix because its Bombardier CRJ aircraft have a maximum operating temperature of 118 degrees Fahrenheit.
Key Takeaways
- Hot, high or humid air has a high density altitude. The FAA says aircraft perform as though they were at the density altitude, whatever their real altitude.
- Thin air reduces engine power, propeller thrust and wing lift.
- Airlines limit operations when the temperature passes an aircraft's approved maximum. Phoenix's 2017 cancellations affected the CRJ, with a stated limit of 118 degrees.
- Small airplane pilots plan for heat with performance charts, lighter loads and early morning takeoffs.
Density Altitude: The Physics of a Hot Day
The FAA's Pilot's Handbook of Aeronautical Knowledge explains that density altitude is pressure altitude corrected for nonstandard temperature. When air density increases, aircraft performance increases. When density falls, performance falls. Air density is affected by altitude, temperature and humidity. The handbook lists high elevations, low pressure, high temperatures and high humidity as the conditions that produce a high density altitude, and says the airplane will perform as though it were operating at an altitude equal to the existing density altitude.
The handbook says thinner air reduces three things:
- Power, because the engine takes in less air
- Thrust, because a propeller is less efficient in thin air
- Lift, because thin air exerts less force on the airfoils
The FAA gives a worked example. With the altimeter set to 29.92 and a pressure altitude of 5,000 feet, an aircraft's ground run might be 790 feet at standard temperature. If the temperature is 20 degrees Celsius above standard, the density level rises above 7,000 feet, and the ground run may be closer to 1,000 feet.
The handbook's density altitude chart gives another sample: at an airport elevation of 5,883 feet with an outside air temperature of 70 degrees Fahrenheit and an altimeter setting of 30.10, the density altitude is about 7,700 feet.
Aircraft Temperature Limits
Separate from performance, each aircraft type has an approved range of operating temperatures. American Airlines said in 2017 that its Bombardier CRJ aircraft have a maximum operating temperature of 118 degrees, and reported higher limits for larger jets: 127 degrees for Airbus aircraft and 126 degrees for Boeing aircraft. The airline cancelled more than 40 American Eagle regional flights at Phoenix Sky Harbor International Airport scheduled between 3 and 6 p.m. on Tuesday, June 20, 2017, the hottest part of the day.
Maximum operating temperatures stated by American Airlines, Phoenix, June 2017| Aircraft | Stated maximum operating temperature |
|---|
| Bombardier CRJ (regional jet) | 118 degrees Fahrenheit |
|---|
| Boeing | 126 degrees Fahrenheit |
|---|
| Airbus | 127 degrees Fahrenheit |
|---|
Those figures are the airline's statement as reported in 2017. Limits depend on the aircraft type and the flight manual, so they are not universal.
What Pilots Do About Heat
- Check the performance charts. The pilot's handbook describes finding the density altitude and then using takeoff charts that account for pressure altitude, temperature, weight, wind and obstacle height.
- Allow more runway. The ground run in the FAA's example grows from 790 to about 1,000 feet with a 20 degree Celsius temperature rise.
- Reduce weight if the numbers do not work. The charts include weight as an input, so a lighter airplane gets a shorter takeoff roll.
- Fly in cooler hours. Airlines in Phoenix cancelled flights in the 3 p.m. to 6 p.m. window, the hottest part of the day.
For more on how speed and airspeed work, see our guide to how fast airplanes fly.
Phoenix and Las Vegas are the hot-weather test cases. Both are VFR for 99.4% of hours on our PHX airport weather and LAS airport weather pages, so heat rather than low cloud is the main weather problem there.
JFK Airport sees about 9 days a year at 90°F or hotter. Our JFK Airport weather by month page has the temperature, wind and storm numbers for every month.
Not every big airport runs hot. At LAX, average daily highs stay between 66°F and 76°F all year, as our LAX Airport weather by month page shows.
Frequently Asked Questions
Why can't planes fly when it is too hot?
Hot air is less dense, which raises the density altitude. The FAA says thinner air reduces engine power, propeller thrust and wing lift, so the airplane needs more runway and climbs more slowly. Aircraft types also have approved maximum operating temperatures.
What temperature is too hot for a plane to fly?
It depends on the aircraft. American Airlines said in 2017 that its Bombardier CRJ aircraft have a maximum operating temperature of 118 degrees Fahrenheit, with 126 degrees for Boeing and 127 degrees for Airbus aircraft.
What is density altitude?
Pressure altitude corrected for nonstandard temperature. An aircraft performs as though it were at the density altitude, regardless of its actual altitude.
Does humidity affect takeoff performance?
Yes. The FAA lists high humidity among the conditions that produce a high density altitude, along with high elevation, low pressure and high temperature.
Why were Phoenix flights cancelled in June 2017?
American Airlines cancelled more than 40 American Eagle regional flights because its Bombardier CRJ aircraft have a maximum operating temperature of 118 degrees, and Phoenix was hotter than that during the afternoon.
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