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Take-off Distance - "hot And High"?  
User currently offlineBlackened From , joined Dec 1969, posts, RR:
Posted (14 years 5 months 2 weeks 6 days 17 hours ago) and read 4898 times:

Can anybody please explain how it comes that aircraft need a longer distance for take-off when the airport has a higher altitude or when it's hot?
When it's higher the air is thinner but why is hot air thinner? Doesn't hot air have a higher pressure than cool air?

6 replies: All unread, jump to last
User currently offlineSQ325 From Germany, joined Jul 2001, 1470 posts, RR: 7
Reply 1, posted (14 years 5 months 2 weeks 6 days 17 hours ago) and read 4866 times:

Hot air is thinner because an molecule of air geting hotter it is moving faster and needs more space.
So hot air is thinner than cold air because of the movement of the air molecules.

regards SQ

User currently offlinePeterba69 From United States of America, joined Mar 2002, 70 posts, RR: 0
Reply 2, posted (14 years 5 months 2 weeks 6 days 16 hours ago) and read 4845 times:

Warm air is less dense, thus has fewer molecules/unit, requiring more distance for T/O. Add altitude to it and you come up w/ a combo called density altitude. There's a formula that pilots use to calc. T/O length that factors this, and some A/C might even be temporarily grounded at high alt. airports 'til it cools down. Hope this helps.

User currently offlineTimz From United States of America, joined Sep 1999, 7212 posts, RR: 7
Reply 3, posted (14 years 5 months 2 weeks 6 days 16 hours ago) and read 4830 times:

...about the difference between pressure and density.

You said hotter air is higher pressure, meaning that if you heat up air in a sealed container the pressure increases. Sure enough. But at a given pressure outdoors, hotter air is less dense-- and density is what matters to the pilot.

User currently offlinePrebennorholm From Denmark, joined Mar 2000, 7114 posts, RR: 53
Reply 4, posted (14 years 5 months 2 weeks 6 days 14 hours ago) and read 4822 times:

Also keep in mind that hot air reduces engine performance.

So hot air works double against take-off performance. On the wings, and inside the engines.

Always keep your number of landings equal to your number of take-offs
User currently offlineCedarjet From United Kingdom, joined May 1999, 8634 posts, RR: 53
Reply 5, posted (14 years 5 months 2 weeks 2 days 20 hours ago) and read 4754 times:

That's why a lot of flights in the ME are at night, it's safer. I have been grounded at Dubai (which is almost at sea level) cos it was so hot. We left at 3am in the end.

fly Saha Air 707s daily from Tehran's downtown Mehrabad to Mashhad, Kish Island and Ahwaz
User currently offlineAv8trxx From United States of America, joined Nov 2001, 657 posts, RR: 6
Reply 6, posted (14 years 5 months 2 weeks 2 days 18 hours ago) and read 4746 times:

In addition to the posts already offered, this page provides helpful explainations of how air temperature and density affect a aircraft performance. While it refers to props, the many of the same principles apply.

"Aircraft Performace, Chapter 4"

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