Airfield temperature vs runway temperature

[Porrohman]

Yesterday, in my very sheltered and paved back garden, the temperature was 22C in the shade but the temperature at nearby EDI was 15C. The difference was no doubt caused by the effect of the strong sun heating the paving etc. together with the very sheltered situation of my back garden.

Later, I watched a recording of the Spanish Grand Prix. The air temperature at the circuit was 23C and the track temperature was in the high 30's.

When there is prolonged strong sunlight, the same effect must happen on airfield runways i.e. the local temperature close above the runway could be quite a bit higher than the temperature given by the local met report.

Is this effect something that pilots need to take into account when calculating take-off performance or is some allowance for this effect already included in take-off performance tables?

On a related subject, how does runway temperature affect landings? I can remember, in my younger years, watching Tridents landing at EDI on hot days and they would sometimes have to use reverse thrust when still airborne due to "wing in ground effect" prolonging the flare for too long. Was this phenomenon also to do with runway temperature or was it just a consequence of the wing design in the Trident?

[HEATHROW DIRECTOR]

One point - the official "met" temperature is measured in a Stevenson Screen so it is really the "shade" temperature - that's the temp quoted in Metars and weather forecasts. I'm not aware that measurements of the surface temperature in the sun are available.

Stand by for more detailed responses from more knowledgeable souls.....

[Atreyu]

There's no calculation involved to compensate for runway temperature, I couldn't tell you whether it's already built into the figures or not but considering the wings and engines would be above any air that is locally heated above air temperature, I imagine it isn't considered. Air temperature is the key because it affects wing performance and engine thrust.

Regarding the trident, to my knowledge reverse thrust was always used on landing; it was once described as the only aircraft that was landed by the person not flying it (as reverse was selected by the non handling pilot)

Standing to be corrected!

Atreyu:ok:

[Porrohman]

AFAIK, standard practice in the Trident was to engage reverse thrust just before touch down using flight idle setting. Where necessary, additional reverse thrust could be initiated by the aircrew during the flare. This usually resulted in a pretty firm touchdown but shortened the unusually long flare (compared to other aircraft) that was often a feature of Trident landings. This effect seemed to get worse on hot days when I would often observe significant reverse thrust being applied when the a/c was still airborne. I wondered if this was connected to the runway temperature enhancing the wing in ground effect phenonenon?

[Nearly There]

Just to throw something else into the mix for you, its very possible to get a temperature inversion, i.e as you climb temp increases, this is important to know as the aircraft performance will decrease for a short while until you pass through it, rare but it happens:ok:

This talk of warm temps is depressing as its been bloody miserable for far to long now, roll on the summer and beer gardens again:cool:

[Porrohman]

Ateyu said;
There's no calculation involved to compensate for runway temperature, I couldn't tell you whether it's already built into the figures or not but considering the wings and engines would be above any air that is locally heated above air temperature, I imagine it isn't considered. Air temperature is the key because it affects wing performance and engine thrust.





Wind direction and strength as well as sun strength I assume will determine the extent to which the air temperature above the runway might be different from the met data for the airfield. If the breeze is very light and straight down the runway or if there is no wind at all then the heating effect would be greater than if there is a good breeze and/or a slight crosswind. Also some aircraft types might be more effected than others by any additional heat close to the runway surface e.g. 737 intakes are much closer to the runway than some other types such as VC10/727/DC9/C5.

[Atreyu]

Well in my experience of ATPL theory exams and day to day performance calculations, runway temperature isn't accounted for so it obviously isn't considered to be a major factor. I grant you the engines on certain types (and perhaps even the wings of some bizjets) may be in this 'heated' air but considering performance calculations are factored, any degredation of performance caused by this would already be accounted for anyway. (Net vs Gross performance)

Atreyu:ok:

[Porrohman]

Thanks for your reply PKPF68-77.

My thermometer was situated in the shade but was certainly picking up the effect of the warm air rising from the solar heated patio and heat coming off the walls of my house. It was, I'm sure, an accurate measurement of the air temperature in the localised conditions on my patio but would not qualify as a true measure of air temperature in met terms as it wasn't measured within a Stevenson Screen and wasn't measured in a location that was well away from buildings, trees, etc. Having said that, all I was interested in finding out was how much warmer than the reported met conditions the air was in the shade close to where I was sitting and the answer was that it was significantly warmer.

Similar microclimates must be able to exist above a runway in certain circumstances. If the tarmac reaches say 50C and the air temperature at the airfield's weather station is 23C then on a calm sunny day the air temperature above the runway is likely to be higher than 23C.

Looking at the chart posted by PKPF68-77, it would be interesting to know what the very localised air temperature was at, say, 2ft, 3ft, 4ft 5ft and 6ft above a hot runway, just to see how the runway temperature affected the air temperature.

I suppose that, if I was a manufacturer and was calculating T/O performance figures, I would assume a worst case scenario (i.e. strong sunshine heating the runway and the surrounding air), then on cloudy days with a cool runway the performance would be better than expected and on sunny days performance would still meet the specifications. That would then explain why, as Atreyu says, it's not something that pilots need to take into account when calculating T/O performance.

Regarding the Trident, was the wing in ground effect phenomenon worse, better or the same on hot days? Are the landing characteristics of any other aircraft affected by hot runways?

[corsair]

Track temperatures at Grand prix circuits is important for purposes of tyre grip. Not really a huge consideration for aircraft. As others have pointed out. I suspect the higher temperature at the runway is factored in. In any case reported temperatures are always shade temperatures. Inevitably it will be higher on the baking runway. There is always a built in safety buffer in any performance figures produced by the manufacturers. There has to be, after all, there will inevitably be differences between actual pressures and temperatures at the moment of take off or landing and the figures used when the everything is worked out pre flight.