American Airlines grounded 45 regional flights at Sky Harbor Airport in Phoenix Tuesday as temperatures soared near 120 degrees.
A veteran pilot in Dallas agreed to explain why a simple math problem rules whether those planes get to take off at all.
"It's not unusual at all," Russell Julian said of the flight cancellations. The CEO of Aerospace Training and Support Services is a 22-year Army veteran with flight time in helicopters and airplanes, spent years as a commercial airline pilot, and has taught at least 400 flight students.
And a little something every student learns is a formula for "Density Altitude." In math speak, it is "pressure altitude plus the sum of 120 times the difference between the outside air temperature and standard temperature” or “density altitude = pressure altitude + (120 x (OAT - ISA Temp)).”
In Phoenix Tuesday, with the temperature at 119 degrees, the density altitude measured 5,232 feet. What that means is that the density of the air, at an airport physically at 1,000 feet elevation, is measuring much thinner as if it is above 5,000 feet.
"And if it's thin air, it's obviously going to create less lift," explained Julian. "You've got to use more runway to take off, which you have to calculate. And it's going to take you longer to get in the air.”
Regional jets grounded on Tuesday have a temperature limitation of approximately 117 degrees.
And this can be a helicopter problem too. Thin air is not a helicopter's best friend.
"Basically, the factory saying this is what the limitation is on the aircraft at your current conditions. If you exceed that, 'Hey you're taking a huge safety risk,'" said Phoenix-area pilot Steven Watkins.
So our experts say be glad some flights on smaller regional jets with lower temperature limitations are being cancelled, and that pilots know how to compute that confusing formula. The right numbers might mean that a plane needs to stay on the ground.
"That's like there's gambling in a casino. That's not unusual at all," said Julian.
Because in the air, who wants to gamble at all?
Larger commercial jets like Boeing and Airbus aircraft are not as readily affected. Their temperature limitations are 127 and 128 degrees respectively.