This article about the weekend heat wave said something I’ve never heard before in any discussion about high temps.
Too hot to fly in a plane? Why would the heat affect planes to the point where it sounds like they would be canceling flights? How hot is too hot? Is there a too hot to drive temp too? I’ve driven in 110+ degree heat and, while uncomfortable even with the AC up on full power, it was actually cooler in the car than in the house
Air is less dense at higher temperatures. This means the wings of an aircraft generate less lift, so allowable weights are lower and required runway is longer. In some cases, perhaps, it’s not safe for some airplanes to fly at all.
One would assume that it might be conditions where the plane can’t generate enough lift. Or possibly the actual surface of the runways softens to the point where the planes can’t safely use them, especially landing.
Basically (and overly-simplified), as the air gets hotter, it gets less dense, too. The plane has fewer air molecules to work with and cannot perform (think take-off and climb) as well as it can in cold, dense air. The performance of piston-engine planes suffers also because they now have less Oxygen to mix with fuel and burn, so their engines are producing less power, too.
Wow, never knew that! So what would be a temperature that’s too hot to fly? Since its colder the higher the altitude, I’m assuming that the typical 110 degrees we can get in the summer is not the real danger temperature, but the relatively cooler temps higher up are dangerous?
In the information manuals used by pilots are charts for every major airport showing what is termed “density altitude” for given temperatures. The nominal density altitude is calculated at, if memory serves, 68 degrees. The density altitude goes up as the temperature rises.
Since needed takeoff distance is calculated as a function of density altitude, a high enough temperature could increase that needed distance by enough that the needed margin of safety was no longer there, and that particular aircraft wouldn’t, following safety rules, take off. A sea level airport at 100 degrees F turns into Quito, Ecuador at 65 degrees (not sure of this calculation).
Many private pilots in small planes have started their takeoff runs on a hot day and found out to their surprise that they can’t get off the ground, or only lift a few feet and can’t climb.
I remember back in 1990, they had to cancel flights in Phoenix when the air temperature reached 122 degrees.
I remember getting delayed three hours on a flight to Lubbock because it was “too hot to land in Lubbock”.
It probably ties into things like weight restrictions and runway length as well. If the air is less dense, you have to be going faster to stay up, so you’re going to be going faster the moment you land and will need more distance to come to a stop. If the runway is already plenty long, it’s no problem, but if it only has the minimum safety factor it is.
My entire knowledge on this subject comes from watching “Air Crash Investigations” so I don’t know if this is a reliable cite, but on one show they did mention that excessive temperatures were a factor in causing a tire to blow out and catch fire during takeoff. When the gear came up, the burning tire retracted into the wheel well and continued to burn, damaging hydraulic control systems and eventually bringing down the plane.
The incident happened in Saudi Arabia, where temperatures are a bit hotter than in most parts of the world. Poor maintenance was also a factor. In addition to the hot conditions, the tires were under-inflated, which further contributed to the failure. The plane was also delayed, and sat on the runway in the heat for a while.
This. Lower density means you need to be going faster to generate the required amount of lift, which means you need a longer takeoff roll. And as noted, if you have a piston-engine, lower density means less power available, which means an even longer takeoff roll is required to achieve the needed airspeed.
here’s what can happen. This is onboard video from a piston-engine plane taking off from a grass strip in Idaho. Note the extremely long takeoff roll, and in fact - unable to climb - they set down again on the runway to continue their takeoff roll before lifting off again. For several minutes after liftoff the plane struggles to gain altitude; there’s just not enough power available. Finally the plane settles into the trees and goes CRUNCH. No fatalities, though the pilot got banged up pretty good.
An article here about that crash; NTSB findings are here. Temperature at the time of the crash was only 80F, but they were already at a real altitude of 6370 feet, so it doesn’t take much temperature to create an unacceptably dangerous density altitude.
Nitpick: Should probably add “at a given true airspeed and angle of attack”.
You can get the lift you need by flying faster and/or with a larger AoA - but that leads to the problems that you and others have noted.
I just came from Phoenix this morning and the natives mentioned the cancellations but supposed since I was flying out at night I should be OK. They’re expecting temps around 118 today. I thought that sounded bad but when I got back to DC I found it 80 and muggy. Which is worse than 95 in Phoenix (Motto: “But it’s a *dry *heat”).
Correction accepted, thanks.
Being an airfoil itself, the propeller suffers as well. So there’s a triple-whammy of wing, engine, and prop all losing effectiveness.
Why only piston engines? The compressor of a turbine engine is also a volumetric device - its maximum power output is reduced in proportion to the density of the air.
The Weather Network in Canada has an article describing the heat wave. Mention of temperatures of 50C was what got my attention, but the article also mention problems with planes:
So it was software not knowing how to deal with the external conditions, then?
Density altitude. The hotter is is the thinner the air, and the less weight the plane can carry.
The charts stop at 120F, so when it it 122F they had stop flying.
In addition to the problems with air density, there’s the problem that the runway can soften, which allows the heavy planes to sink into it, which is probably bad in some way example: http://travel.ninemsn.com/blog.aspx?blogentryid=1024840&showcomments=true
this will be a wag I hope a commercial pilot can verify but you can push the turbine engine faster to compensate. Like all engines it’s a heat pump but you can vary the rpm to maintain X amount of thrust. You’re just trying to keep it in a specific temperature range and you only need to push the engines on takeoff. Air cools at 3 degrees per 1000 feet so it’s just a function of getting up to cruise altitude.
Yes and no. Up at cruise altitude the air is a lot less dense as well as colder so the engines need to be operating much closer to their limiting temperatures and RPMs to get the required thrust. On a hot day it affects your take-off performance and it also affects the maximum altitude you can climb to due to both wing and engine performance.
And this is the correct answer. There is nothing about 122º per se that prevents an aeroplane from safely flying, but aeroplanes come with performance data that is used to calculate take-off, climb/cruise, and landing performance. If the highest temperature on the take-off performance chart is 120ºF then you can not calculate take-off performance at 121ºF and therefore it is illegal to take-off at the higher temperature. If you do a lot of flying in hot climates you can sometimes pay the aircraft manufacturer to provide performance data that covers higher temperatures, but it is an extra expense to the airline, if it is available at all.