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A hot tarmac runway / where is airport OAT measured?

I have often found the plane won’t settle and just keeps floating, or even lifts up a bit.

I put this down to hot air rising up from the tarmac. Is that true? If so, the air must be getting sucked up from the sides of the runway.

In a similar way one can get a bump when crossing a motorway on a hot day and I have seen this at 1000-2000ft. Or at least I think it was that…

Administrator
Shoreham EGKA, United Kingdom

Absolutely particularly if you are a bit quick over the threshold with a hot runway float can be surprisingly resilient.

EGTK Oxford

Not to worry.
Sticky Tarmac will bring you to a rapid stop…..

Egnm, United Kingdom

Peter wrote:

I put this down to hot air rising up from the tarmac. Is that true?

Yes.

Peter wrote:

I put this down to hot air rising up from the tarmac. Is that true? If so, the air must be getting sucked up from the sides of the runway.

In a similar way one can get a bump when crossing a motorway on a hot day and I have seen this at 1000-2000ft. Or at least I think it was that…

Ask any glider pilot!

ESKC (Uppsala/Sundbro), Sweden

Peter wrote:

If so, the air must be getting sucked up from the sides of the runway.

And from the threshold, so you might have a sink a bit before it

I just played around with a Density Altitude Calculator. Yesterday we a had an OAT of 34 degrees around Munich but of course the air above a concrete runway is much hotter on such days. When I helped get the damaged Aquila off the runway yesterday it felt mor like 50 degrees on the runway. And isn’t OAT always measured in the shade anyway?

What influence do these facts have on DA calculations, take-off performance etc? How do you deal with it?

Edit: Why do airports not measure temperature near the runway and in the sun?

Last Edited by at 22 Jun 09:09

Airports are trying to measure the air temperature. In the sun, your skin (and anything struck by the sun) will be a lot hotter than the air because of infrared heating. The air itself, even close to the object (and especially if there is any wind) will be a lot cooler than the object itself, and it’s the air temperature that matters. Near a runway, you’ll have even more IR heating of objects (but the air not so much) because the concrete is reflecting a lot of the IR energy onto things.

Last Edited by alioth at 22 Jun 09:55
Andreas IOM

In the meantime I have found out that airports measure OAT in 2 m above the runway and in the shade! So the information should be correct. It’s just the littel fields that don’t have that possibility.

In the meantime I have found out that airports measure OAT in 2 m above the runway and in the shade!

How is that achieved, without a plane hitting the sensor?

I don’t think there is a way to measure the temperature of a piece of air at a distance. You can measure the velocity of it (doppler effect on a laser and gating the return to pick off the return with a specific time delay, and that method relies on their being dust or vapour in the air) but not AFAIK temperature.

Administrator
Shoreham EGKA, United Kingdom
26 Posts
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