Feeling the Heat

August 5, 2024

“Meltdown” is the buzzword used when computer glitches throw air travel into disarray, as happened a couple of weeks ago. But is there a more literal application? How do extreme temperatures impact air travel? The question is important, as heat waves pummel the globe and we keep breaking records for the hottest-ever temperatures.

On the most basic level, hot weather affects air travel the way it affects most things: it makes people uncomfortable and wears them down. Things happen more slowly. With all that nonreflective asphalt and open space, airports become “heat islands,” subjecting people and equipment to sometimes unbearable conditions. Imagine what it’s like slinging suitcases out on the tarmac when the mercury hits triple digits.

Workers are subject to overheating, and so is their equipment: tugs, tractors, cart and lifts — all the myriad moving parts of airport logistics. Airplanes themselves, meanwhile, have a lot of internal machinery, both electronic and mechanical, and much of it runs hot to begin with. Throw in soaring temps and things can break down.

Aerodynamically, warmer air is less dense than cooler air, meaning that in hot weather a wing produces less lift. Jet engines don’t like this low-density air either, producing less thrust. Engines also are subject to internal temperature limits — exhaust gas temps, etc. — beyond which operation isn’t permitted. Together, this dictates higher takeoff and landing speeds — which, in turn, increases the amount of required runway.

What all of that means for travelers is that your flight might be weight-restricted and unable to take a full load of people or cargo. How much so will vary with the temperature and runway length.

Outright grounding of flights is rare, but at a certain point that can happen too. Some aircraft have a hard temperature limit — usually around 50 degrees Celsius — beyond which taking off simply isn’t permitted. We also get a detailed paper printout before each departure that factors in weight, temperature, wind, runway length, and so forth, providing us with the speeds, thrust, and flap settings that we’ll use for takeoff. Sometimes, above certain temperatures, data doesn’t exist.

So, you could say there are hard and soft limits. The aerodynamic limits are soft. That is, you don’t know for sure if a flight can safely depart, or at what weight, until you juggle the numbers. The hard limits are the absolute temperature maximums, set by the manufacturer, that you’re not allowed to exceed.

If your flight is weight-restricted — or canceled — you’re liable to get a dumbed-down synopsis over the gateside loudspeaker or from your pilots. Something along the lines of, “We’re sorry but it’s too hot to fly today.” This makes it sound subjective, or up to the whims of the crew, but in reality it’s more scientific.

Extreme heat is no fun for airlines or their customers. Unfortunately, you can expect more of it as climate change causes more and more extreme weather events, including extreme heat waves.

It’ll be especially interesting to see the effects on those carriers whose hubs are located in what are already the hottest places on earth. The Gulf Carriers, for example (Qatar, Emirates, and Etihad) or Saudia, or Air India. Fortunately for them, many of their long-haul flights arrive and depart in the dead of night, when it’s cooler.

All well and good, but I’ve failed to address the one question you’re lining up to ask: Why are are planes so damn hot when parked at the gate?

Well, a plane’s air conditioning is supplied one of three ways:

1. Via the engines. Air for the “packs” as the AC units are called (it’s an acronym from the words “pneumatic air cycle kit”), is plumbed from the engine compressors. This is the most effective way of cooling or heating the cabin.

2. However, planes can’t run their engines at the gate. And so, during boarding and deplaning, air is supplied to the packs via the plane’s APU, or auxiliary power unit. This is a small jet engine usually located up under the tail. The APU can power the packs as necessary, but they won’t run as robustly as when the main engines are supplying the air.

3. Via an external air supply. You’ll sometimes see fat, caterpillar-like hoses connected to the belly of a plane. These provide hot or cold air as requested from an air conditioning unit attached to the jet bridge. The effectiveness of this method tends to vary plane to plane and gate to gate.

Variations in 2. and 3., above, are the reason the cabin is sometimes sweltering. Or, in winter, freezing cold.

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Landing photo by Victor Forgacs, courtesy of Unsplash.
Emirates photo by the author.