Blackened
Topic Author
Posts: 535
Joined: Mon Mar 12, 2001 4:19 am

### Take-off Distance - "hot And High"?

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?
Bring it back, bring it back, bring it back

SQ325
Posts: 1287
Joined: Wed Jul 18, 2001 7:54 pm

### RE: Take-off Distance - "hot And High"?

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

peterba69
Posts: 147
Joined: Fri Mar 01, 2002 9:31 am

### RE: Take-off Distance - "hot And High"?

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.
Peter

timz
Posts: 6318
Joined: Fri Sep 17, 1999 7:43 am

### Maybe You're Confused...

...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.

prebennorholm
Posts: 6603
Joined: Tue Mar 21, 2000 6:25 am

### RE: Take-off Distance - "hot And High"?

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

cedarjet
Posts: 8210
Joined: Mon May 24, 1999 1:12 am

### RE: Take-off Distance - "hot And High"?

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

Av8trxx
Posts: 632
Joined: Sun Nov 11, 2001 7:08 am

### A/C Performace & Density Altitude

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"
http://avstop.com/AC/4-10.html

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