Mozart From Luxembourg, joined Aug 2003, 1860 posts, RR: 18 Posted (7 years 4 months 2 weeks 3 days 4 hours ago) and read 3014 times:
This is a bit of a newbie question, but maybe someone has the patience to share their knowledge with me.
I know that airports at higher altitudes such as MEX, UIO, LPB, LXA and so on pose operational limits upon modern jet airliners. I was wondering why and what those limitations are.
I do know that "thinner air" at high altitudes reduce thrust, but why exactly? Also, what else is different when operating from high altitude? I have read about some of the Chinese 757s flying into LXA that they have some modifications - such as what? Lastly, how are high alt airports different in the way they are laid out? I know they require long runways, but what else?
Woodreau From United States of America, joined Sep 2001, 810 posts, RR: 8 Reply 1, posted (7 years 4 months 2 weeks 3 days 3 hours ago) and read 2929 times:
Air is less dense at higher altitudes (and higher temperatures as well), so there is less air to support combustion in the engines, so less performance out of the engine, less air to support the aircraft, so you need more groundspeed to generate the lift required to takeoff (consequently more runway required)
So when you have a hot humid day at a high altitude airport your aircraft performance is no where near the specified performance at a lower altitude.
Aircraft performance published in the aircraft manuals is based on a standard atmospheric day - 29.92in/1013mb, 59F/15C, sea level. As you increase the altitude, standard pressure and standard temperature decrease.
Density altitude is what we use to determine the performance of an aircraft which takes all these factors (temp, altitude, humidity) into account. An airport at 1000ft altitude may have a density altitude of 4300ft on a very hot day. So an aircraft taking off from that airport will perform as if it were at 4300ft. Which means (from a small piston plane standpoint) you have to lean the engines before takeoff, you'll have a longer ground roll, higher groundspeed, etc. However, the indicated airspeeds for when you rotate, liftoff, and climbout will still be the same.
There is an airport here in southern California, which you can easily see the effects of altitude and temp. Its elevation is 6,750' inside a mountain valley with several peaks around the airport topping 11500ft. Standard temperature at 6750ft is around 36F/2C. So a typical afternoon temp is normally around 68-70F/20-21C. So we're already 20C above standard temp. Standard pressure should be around 23.23 inches (don't know the conversion between mb and inHg)
Density altitude for this airport in the afternoon can easily top 9000-10000ft. Most single engine pistons don't climb very well at this altitude, and most light twin aircraft don't climb at all if they had an engine failure and had to fly around on one engine.
So that's the short answer about why "thinner air" at high altitudes reduce thrust. It didn't cover any of the stuff you asked about Chinese 757s, airport layouts, or operational limits on airliners though.
Good judgement comes from experience. Experience comes from surviving bad judgement.
Mozart From Luxembourg, joined Aug 2003, 1860 posts, RR: 18 Reply 2, posted (7 years 4 months 2 weeks 3 days 2 hours ago) and read 2889 times:
Woodreau,
thanks a lot, that was a great explanation! I'll put you on my respected users list...
However, I would like to push the quesiton a bit further, either to you or to whoever else is knowledgeable: why do some airplanes perform better at high altitudes than others? For instance, rumour has it that the 737NG is amazing (Boeing conducted some tests with the -700 in Bhutan I think), whereas others (Airbus 320 series? no idea) less so. Is it as simple as "the one has stronger engines" or is there more to it? What exactly is it in engine design that makes one perform better than another?
Vzlet From United States of America, joined Mar 2004, 803 posts, RR: 0 Reply 3, posted (7 years 4 months 2 weeks 3 days 1 hour ago) and read 2856 times:
Here's what I found to be a fascinating account of the Bhutan evaluation:
http://www.boeing.com/commercial/aeromagazine/aero_23/737-700Bhutan_story.html
"That's so stupid! If they're so secret, why are they out where everyone can see them?" - my kid
Vzlet From United States of America, joined Mar 2004, 803 posts, RR: 0 Reply 4, posted (7 years 4 months 2 weeks 3 days 1 hour ago) and read 2851 times:
SSTjumbo From , joined Dec 1969, posts, RR: Reply 5, posted (7 years 4 months 2 weeks 1 day 2 hours ago) and read 2660 times:
Short answer: Density Altitude.
Basically, the higher you go, the less dense air is. The lower the barometric pressure, the less dense the air is. The hotter the air, the less dense it is. Say I'm in Chicago with an average altitude of around 680' above sea level. If the air is hot enough and the pressure is low enough, the air I'm breathing could simulate equivilant conditions on a 3000' mountain. As my analogy goes, the same fully loaded A380 that would struggle to take off on a 13,000' runway at DFW on a hot day could probably take off from Barrow, AK on their 6,000' runway because of how much denser the air is. That might be somewhat of a far stretch, but the theory is there.
GREASESPOT From Canada, joined Apr 2004, 2993 posts, RR: 27 Reply 6, posted (7 years 4 months 2 weeks 1 day ago) and read 2637 times:
There is more to it than just engine. The wing also plays a part. The less dense the air is the wing also loses efficiency.
Airbus also tested their narrow bodies at high altitude also.
Both had to be able to demonstrate 1 engine out on take off and maintain climb performance. I believe there was a competition between airbus and Boeing for Druk Air and Druk chose airbus. So I am going to go out on a limb and say both have no problem operating at high altitudes.
Greasespot
Sometimes all you can do is look them in the eye and ask " how much did your mom drink when she was pregnant with you?"
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