It was 110 in Vegas yesterday.....

It was actually pretty cool here in Vegas Yesterday - we had some -TSRA and overcast conditions all day so it hovered around 90F for the most part. I can't wait for those 118F days though.

But the thing is that the less dense the air is the less drag there is on the plane so shouldn`t it have less problems getting to a higher speed? Also how does a jet engine work more efficiently at high altitudes?

The VC10 was designed for exactly this reason! A fully laden Super VC10 at 335,000lbs AUW can easily get off at airports such as Cuzco for example!vololiberista

I thought airports can stand up to about 140, thats nearly how hot it got at haneda last year

Another limiting factor with very high temps is engine temperature at takeoff power. I flew for three months as a flight mechanic on a Gulfstream IV operating out of Nigeria. (Because of a lack of ground maintenance support facilities in Africa, the aircraft ower wanted a mechanic on board for all flights.)We would schedule all departures as early in the morning as possible, before the hottest part of the day - but even so, we had several takeoffs with OAT's at or above 35 degress C. At those temperatures, we were limited in the maximum amount of takeoff thrust that could be set in order to keep from exceeding maximum allowable TGT. (Gas temperature in the turbine section of the engine). With an airfield temperature of 35+, the TGT on takeoff would always be well into the yellow "caution" range, and the pilots would watch those gauges with great attention until the point where the power reduction to climb thrust could be set.During this time, we made a couple of trips north to the UK. (It was winter). Up there, taking off with outside temps around 5 degrees C, the TGT would never get anywhere close to the yellow arc at takeoff thrust.In other words, if it gets hot enough, the power reduction required to prevent exceeding maximum allowable EGT on takeoff will eventually be great enough to prevent the aircraft from being able to safely get off with the available runway length - which is probably why places like KPHX will close if it gets above a certain temperature.Another critical factor when taking off on hot days with high density altitude is "second segment" climb performance. This is the segment of the climb between 35 feet AGL to 400 feet AGL - after "gear up", but before acceleration to flap retraction speed. To meet legal requirements for takeoff, an aircraft needs to be able to continue gaining altitude during the second segment, even if an engine fails - and the ability to meet this requirement becomes much more difficult when it is very hot. The GIV I flew on would normally be able to easily fly non-stop from Nigeria to the UK - but doing so (with adequate reserves upon arrival) would require departing with a near maxiumum fuel load of 29,500 pounds.At the high departure airport temperatures we had to deal with in Africa, there was no possible way to meet second segment climb requirements with full fuel. The only way to do it was to reduce aircraft weight by departing with considerably less than max fuel load, which required an enroute refueling stop in Spain.Going the other direction (UK to Africa), with cool takeoff temps, we could take max fuel and make the trip non-stop.Jim Barrett

If you're looking for hot and high, Johannesburg Intl is not bad - its elevation is close to 6000 ft which when coupled with summer temperatures often of 35C + (95F +) you'll understand the need for long runways (FAJS 03L 14495 x 200 ft). Virgin have started actually making the announcement explaining that the take off run will be longer than usual.I remember watching the old 737-200's use up most of the runway to get airborne on a hot day.When I did my flying out of Lanseria (about 30 Miles away at a lower elevation of about 4500ft) the thing I noticed was the ground speed was much higher for the same IAS - more noticeable on approach and landing as I had done my training out here in the UK out of Biggin Hill which is never really that hot and only 600ft ASL, still on "hot" days you certainly would feel the old PA28 struggling to climb.Also you notice it in an temp inversion (temp increases with alt, normally when a high pressure system has been hanging on for some time, forward vis can drop to 3000m or less in this layer - not fun as its like flying in soup ) the Vert Speed also drops to 150 - 200FPM as you climb until you bust through the inversion layer. Then the view is great as you're in clear cooler air again!Cheers

Another thing to remember for hot days is how much heat is soaked up by the runway. This makes the runway environment significantly hotter than the surrounding air. In Wichita Falls, Texas, in the late 60s to early 70's, the town was consistently one of the hottest in the nation, and the reported temperatures were always significantly higher that expected. It turned out that the official weather observation post for the town was located at the end of Sheppard AFB's main runway, so that B-52s would have accurate performance calculations. IIRC, the station has since been moved.

One thing in terms of max temps to remember (in addition to all the rest), is that the surfaces are weaker at hotter temps. Asphalt will definitely sink at hotter temps with something heavy on it. I'm not sure about concrete, but I know it definitely expands (drive over a bridge in the winter, versus the summer and note the sound of the joints). The impact of the jets on the concrete might be a consideration in the decision.Another reason they may think to close the airport down due to temperature is to keep the employees out of it. It may seem like micromanaging, but they also kick the employees off the ramp when it storms, so the idea isn't too far-fetched. If it's 120 in the air, the radiant heat off of the ramp, plus that of the APUs and engines (and tugs, and trucks, and so on) makes things that much hotter. When I worked on the ramp, if the air was hot, humid and stale, I would actually intentionally walk through the APU blast of the CRJ200s (at a good distance). It was hot, but it was moving air, and it was dry air.

Another limiting factor with very high temps is engine temperature at takeoff power. I flew for three months as a flight mechanic on a Gulfstream IV operating out of Nigeria. (Because of a lack of ground maintenance support facilities in Africa, the aircraft ower wanted a mechanic on board for all flights.)We would schedule all departures as early in the morning as possible, before the hottest part of the day - but even so, we had several takeoffs with OAT's at or above 35 degress C. At those temperatures, we were limited in the maximum amount of takeoff thrust that could be set in order to keep from exceeding maximum allowable TGT. (Gas temperature in the turbine section of the engine). With an airfield temperature of 35+, the TGT on takeoff would always be well into the yellow "caution" range, and the pilots would watch those gauges with great attention until the point where the power reduction to climb thrust could be set.During this time, we made a couple of trips north to the UK. (It was winter). Up there, taking off with outside temps around 5 degrees C, the TGT would never get anywhere close to the yellow arc at takeoff thrust.In other words, if it gets hot enough, the power reduction required to prevent exceeding maximum allowable EGT on takeoff will eventually be great enough to prevent the aircraft from being able to safely get off with the available runway length - which is probably why places like KPHX will close if it gets above a certain temperature.Another critical factor when taking off on hot days with high density altitude is "second segment" climb performance. This is the segment of the climb between 35 feet AGL to 400 feet AGL - after "gear up", but before acceleration to flap retraction speed. To meet legal requirements for takeoff, an aircraft needs to be able to continue gaining altitude during the second segment, even if an engine fails - and the ability to meet this requirement becomes much more difficult when it is very hot. The GIV I flew on would normally be able to easily fly non-stop from Nigeria to the UK - but doing so (with adequate reserves upon arrival) would require departing with a near maxiumum fuel load of 29,500 pounds.At the high departure airport temperatures we had to deal with in Africa, there was no possible way to meet second segment climb requirements with full fuel. The only way to do it was to reduce aircraft weight by departing with considerably less than max fuel load, which required an enroute refueling stop in Spain.Going the other direction (UK to Africa), with cool takeoff temps, we could take max fuel and make the trip non-stop.Jim Barrett

So can the engine not actually produce as much thrust on hot days or is it just the computers limiting the thrust? Here in Japan its crazy, it gets nearly up to 140 on the runway and yet they still do normal operations. Last year at Narita also there were lots of planes taking off nearly or at MTOW and the temperature was really high. Really crazy huh

So can the engine not actually produce as much thrust on hot days or is it just the computers limiting the thrust? Here in Japan its crazy, it gets nearly up to 140 on the runway and yet they still do normal operations. Last year at Narita also there were lots of planes taking off nearly or at MTOW and the temperature was really high. Really crazy huh

Hot air is less dense, so the maximum available thrust in a turbojet engine will be less on a hot day - though jet-powered aircraft rarely takeoff at absolute maximum rated thrust in any case. The limiting factor on a very hot day isn't what the engine could put out, in terms of power, (if no thrust limiting was employed) but how much power it can develop without causing any internal damage. The performance mangagement system will limit fuel flow to prevent the TGT/EGT from exceeding maximum allowable limits.The limits (on internal temperatures) vary depending on the make and model of engine. With a jet engine, in general, the colder and more dense the air is, the more thrust can be generated, and the lower the EGT for a given thrust.Turbocharged piston engines can actually have the opposite problem. If the air temperature is extremely cold, and the density altitude very low, it increases the possibility that the pilot might overboost the engine on takeoff... i.e., produce excessive manifold pressure from compressing (already dense) air in the turbocharger, which can lead to internal damage to the cylinders, pistons etc. When departing under such conditions, the pilot must watch his manifold pressure carefully, and be prepared to use less throttle than he may be used to than when taking of in warmer conditions.Basically, if you have a long enough runway, you can get off the ground safely in a jet aircraft even at very high weights and air temperatures... but getting airborne is only part of the equation. The second segment climb restriction I spoke of before is often more important in determining if a particular takeoff can be made safely, and that is very much dependent on the specific airport and runway, as well as density altitude.Everything else being equal (air temperature and takeoff weight), the second segment restriction at an airport at sea level, like Los Angeles (or Narita) will be less than at an airport at a higher elevation like Las Vegas.Jim Barrett

Hot air is less dense, so the maximum available thrust in a turbojet engine will be less on a hot day - though jet-powered aircraft rarely takeoff at absolute maximum rated thrust in any case. The limiting factor on a very hot day isn't what the engine could put out, in terms of power, (if no thrust limiting was employed) but how much power it can develop without causing any internal damage. The performance mangagement system will limit fuel flow to prevent the TGT/EGT from exceeding maximum allowable limits.

So does that mean when at cruising altitude the engine produces less thrust? E.g, 90N1 on takeoff would have more thrust than 90N1 at 35000 feet. Also about the maximum thrust what does the pilot mean by when he says "max thrust"? Would pushing the throttle all the way be absolute power?

So does that mean when at cruising altitude the engine produces less thrust? E.g, 90N1 on takeoff would have more thrust than 90N1 at 35000 feet. Also about the maximum thrust what does the pilot mean by when he says "max thrust"? Would pushing the throttle all the way be absolute power?

First, the following only applies to Turbo-Fan engines.The amount of thrust produced at altitude isn't determined by N1, it's N2. N1 is for low altitude since this is the large fan at the front of the engine. It's used for power indications at lower altitude because the fan is more efficient and generates up to 80% of the thrust. Think of the fan basically as a propeller; the higher you go, the less air there is for it to grab at and less thrust is produced. N2, or basically the turbine itself is more efficient at higher altitude. At lower altitude, because the air is already dense and it's comparatively warmer, it has to compress it more and since it's already warm, it has to be heated more for it to produce thrust. As it climbs and the outside pressure and temperature drops, its much easier for the compressor to compress the air to a higher pressure than the outside air. That and since the temperature is dropping, the density is in a way increasing. The result is that the turbine produces a majority of the thrust there; 80% at least.This is why you see nearly all modern airliners with Turbofan engines. Unlike Turbojets, you get a large amount of thrust all all altitudes. So if you would imagine at about FL200, the fan is producing about 50% of the thrust and the other 50% is coming from the turbine.

the density is in a way increasing.

does that mean at higher altitudes the air is more dense? I thought the higher the less dense. Thanks for all the explanations, aviation and weather are both interesting. But does this whole density thing, is the effect really big?

Wait, so how about high bypass engines? The engine company says that 80% of the thrust is from the bypass. Is that only at lower altitudes cause I think bypass is N1

I thought I understood this, but I guess I'm now completely confused. In the '80s I was on a "Twin Otter" at 8000 ft. It was the hottest day that summer, about 90F. The plane was pushed back all the way to the end of the runway, the engines were as loud as I've ever heard them. We rolled much longer than usual, went off the end of the mesa, and dropped about 500 feet. (My stomach remained at 8000 ft, though). Somewhere around 600 ft below the mesa we either got enough power or enough density altitude (density pressure? the stuff under the wings) and headed off towards Albuquerque. In many previous trips, we'd never, umm, fallen that far. The ideal gas law says PV=nRT, so P=nRT/V, so as T (temperature) increases, P (pressure) decreases, right? So the reason the DASH-8 went down....why? the air wasn't dense enough, or the engine/propellers/something else/ couldn't spin fast enough?thanks,danp.s. Had we dropped another 200 feet, we would have conveniently landed in the sewage treatment plant at the base of the canyon.

KPHX was nearly shut down yesterday here - we got to 118, they'll shut it down at 120.

Just returned from Palm Springs for the weekend. Saturday it reached 118 there as well. Nearly passed out in my afternoon round of golf . KPSP will shut down at 120 as well. Aircraft just can't get enough lift in that thin, hot, dry air.Brandon Burkley