Wrong sort of sun

[newswatcher]

With the exceptionally hot weather hitting the UK this week, once again stories of travel disruption abound. Railway lines are buckling, roads are melting.....Yet no word of disruption to air travel yet.

I remember BA having trouble with engine re-starts on a 747, at Kuwait during the 80s, when tarmac temperatures were well into three figures.

But does very hot weather have any other direct effect on air travel?

[newswatcher]

Well thanks for all the responses guys, did not realise this topic was that boring! :zzz:

Rumour has it well press speculation that, since the air becomes slightly less dense as the temperature rises, aircraft will use up more fuel taking off, due to reduction in effective power of jet engines, and wings generating less lift.

[ft]

Engines heat air. The more they can heat the air, the more power they produce. If the air is hot already, they won't be able to heat it as much and the power output will suffer. Hot air is also less dense, meaning there's less mass of air into the engines again meaning less power.

Less dense air will mean that the actual height above ground will be higher at a given indicated altitude. The altimeter really indicates the weight of the air you have between you and your reference level (which is sea level with QNH set on the altimeter). This weight off your shoulders is what causes the pressure reduction. Less dense air will mean a larger volume of air is needed to achieve the same weight.

Wings only care about dynamic pressure, part of which is the air density. For a given dynamic pressure the equivalent airspeed will be the same. In the lower speed range, before compressibility becomes a factor, this will mean that the indicated airspeed is also approximately the same.

IAS (or EAS) is what you see on the ASI. Thus, with a given airspeed displayed, the equivalent pressure will be the same and as will the generated lift. In other words, at a given pre-compression airspeed on the clocks in the front office the wings are just as happy in summer or winter. It will mean a higher ground speed on a hot day though!

Look up density altitudes...

:confused: :8 :bored:

Where's the "I hope I did not get all that way too much ass-backwards" smiley?

Cheers,
Fred

[newswatcher]

Thanks ft! :ok: