“Aircraft engine performance is a function of many things including air temperature,” Glenn Lightsey, an aerospace engineer and colleague of mine at Georgia Tech said. “Hotter days require longer runways and more gradual ascent paths to lift the same weight.” Flight is complex, and it cannot be boiled down to a single number.

The specific aircraft matters, too. American Airlines canceled flights using Canadair Regional Jet (CRJ) equipment. These are the business jets that cover routes between hubs and smaller markets. Larger passenger jets are rated to tolerate higher temperatures, well above those currently being experienced in the American Southwest—after all, planes also fly from Dubai, Riyadh, and Cairo.

The CRJ’s history might play a role in its airworthiness under extreme heat. CRJs are currently made by Bombardier, a multinational transportation manufacturer. Bombardier bought the CRJ line from Canadair, a Canadian state aerospace company. These jets were originally designed for business use, and only later developed to serve the commercial regional jet market. They were not necessarily intended for use in all conditions and markets, nor to be packed full of passengers like they are today. (Bombardier did not immediately respond to a request for comment.)

That circumstance is a consequence of deregulation and consolidation in the American airline market. When regulation demanded that airlines serve all markets, larger jets serviced smaller airports. But as those requirements lifted, and as more airlines merged, even once-thriving hubs like Cincinnati, St. Louis, and Memphis have become minor markets. Airlines began relying on equipment like the CRJ, because they can transport a smaller number of people at a lower cost. Were the affected flights on Boeing large jets instead, there would be no question about their ability to fly.

Speaking of cost, it’s not clear if American itself has issued the CRJ-based flight cancellations, or if they came from the regional partners that actually operate those flights under American-Airlines livery. The business relationships between major carriers and their regional partners are complex. Some are wholly owned subsidiaries, while others—including Mesa and SkyWest, which serve Phoenix on behalf of American—are contracted carriers.

Regional carriers tend to endure financial pressures from their major-carrier partners, some of which might make the effects of high-temperature operation a financial or operational burden. For example, it’s possible that the planes could fly safely above a certain ground temperature, but that the performance data to facilitate that flight is not already available or easily determined. Airlines have to buy the performance charts used to operate flights, and they might determine that it is not worth purchasing them for unlikely or uncommon scenarios.