Recently, I took off from the Gongkar Lhasa airport in Tibet at 11,710 feet, and found the Boeing 738 does not accelarate as fast as normal. Normally, I just set trim to 5.8 degrees with 65% fuel, and the 738 will automatically rotate at around 150 knots on a runway less than 13,000 feet long.The Gongkar airport is more than 13,000 feet long, but the aircraft was accelerating slowly on the runway, and did not get to 140 knots at the end of the runway. Moreover, to taxi requires more than 50% throttle - normally 42% percent will get the 738 to taxi.Why is this? How differently do you taxi/take off at high altitude airports?

High density altitude conditions reduce an airplane's performance because: * the engine is now taking in less air to support combustion so power is reduced; * the propellers have less air to move in than under normal conditions so thrust is reduced; * a jet engine has less mass of gases exiting the exhaust end so thrust is reduced; * there are less molecules in the air, the lighter air exerts less force on the wings which result in reduced lift; * reduced thrust and lift means more runway length is needed for takeoff and more clearance area at the end of the runway is needed because of a reduced climb rate. source:http://quest.arc.nasa.gov/aero/virtual/dem...tutorial2f.html

Here's something that may help....High Density Altitude Takeoff TipFlaps... more flaps does not equal more lift.The other day I was at an FBO and I overheard a conversation between two pilots talking about flying to Denver in the summer. The one pilot who had flew to Denver last summer was relaying his experience. He was flying a 182 into Front Range Airport and said he had a heck of a time departing, because he couldn't get the plane off the ground. Finally, after using over 6,000 feet of runway he lifted off was able to just barely get a climb going.This pilot had stated that he couldn't understand how any aircraft could ever fly in or out of an airport like Leadville since he had so much trouble going to Denver.I was intrigued by this conversation and decided to introduce myself as a pilot from Colorado and ask some questions. It turns out the plane was loaded properly, well below gross, the pilot, his wife, and maybe 75-100 pounds of bags. He had taken off into the wind of 7 kts. The temp was 95, and that was a factor to the performance, but as I asked more questions the real problem became evident. This pilot had used 20 degrees of flaps. His thinking was that more was better, and a short field procedure of 10 degrees should be adapted to high altitude procedure of 20 degrees of flaps. WRONG!The reality of the situation is just the opposite. Generally speaking, the first flap setting usually adds more lift than drag, the second, third or even forth flap settings on most aircraft add more drag than lift. The better procedure would have been to use no flaps, or at the most use the 10 degree setting.I explained that the 182 is a very capable machine, he just needed a different procedure. I also explained that I had flown 150hp 172's to and from Leadville (not in 95 degree temps though).So what is a good procedure for takeoff from a high altitude airport? First we'll assume you have evaluated the wind, temp, aircraft performance, weight and balance and have concluded that the attempted takeoff is within the capabilities of the aircraft.If that's the case then the procedure I like to use is to taxi into position using every available foot of runway, lock the brakes, engine to full power, and flaps in the up position (i'll get to using flaps in a second). When you're ready for the takeoff run, release the brakes. Once you get the aircraft in the air keep it close to the ground to use ground effect to accelerate (Ground effect is a reduction in drag caused by being close to the ground in flight). Once airborne in the ground effect you may want to retract flaps if you had used them as well as landing gear. As the plane accelerates past Vy then it will handle and climb better than if it were at or below Vy.As you were doing the takeoff run, if you got to the 1/2 way point of the runway and you're not airborne or close to airborne then you can extend the first setting of flaps. The benefit of doing this during the takeoff run is that the aircraft will initially accelerate faster without flaps.Every aircraft is different, this procedure may not be corect for some aircraft, but this will work well for many small single and multi-engine aircraft.

Here's something that may help....High Density Altitude Takeoff Tip

Thanks for your story and tips :( I shall give them a try when I take off at Gongkar next ...

Thanks for your story and tips :( I shall give them a try when I take off at Gongkar next ...

\You want maximum power at takeoff. Normal procedure to set max power at sea level with a fixed pitch prop is to set mixture to full rich. If you set your mixture to full rich at a high density altitude airport you'll be too rich to develop max power. Under those conditions with a fixed pitch prop, lean to peak RPM to develop max power. You also need to lean on approach so you'll be set to develop max power if a go around is required.I've never checked to see if this behavior is properly modeled in flight sims.Not sure the procedure in a 182 or other aircraft with a constant speed prop.

Flaps... more flaps does not equal more lift.

And that's why. This is basically the same story as the guy on Denver. Pilot performing STOL takeoff, used more flaps than he should have used because he thought it would help him get off the ground faster. WRONG. What happens? Aircraft becomes airborne below Vmc (Minimum control Speed) and flight controls are useless to control the aircraft. Result? Crash.

This pilot had stated that he couldn't understand how any aircraft could ever fly in or out of an airport like Leadville since he had so much trouble going to Denver.I was intrigued by this conversation and decided to introduce myself as a pilot from Colorado and ask some questions. It turns out the plane was loaded properly, well below gross, the pilot, his wife, and maybe 75-100 pounds of bags. He had taken off into the wind of 7 kts. The temp was 95, and that was a factor to the performance, but as I asked more questions the real problem became evident. This pilot had used 20 degrees of flaps. His thinking was that more was better, and a short field procedure of 10 degrees should be adapted to high altitude procedure of 20 degrees of flaps. WRONG!

Very wrong. It's only like what, 5000ft? I flew the Piper Cherokee back in flight school days out of a 7000ft AMSL airport with 0 and 10

I took off from Gongkar again tonight with 70% fuel, OAT was 15 celcius and zero wind - see attached pic.I set the flaps to 10 degrees like last time, but the 738 moved to taxi at 47 thrust this time until reaching the start of the runway where 58 was required ... weird.I released the parking brakes at 94 instead of 70 thrust this time and set trim to 6.4 instead of 6.0 degrees. As you can see in the second pic, the aircraft lifted pretty much at the end of the 13,123 feet runway without manual rotation by myself, but I had to switch to autopilot quickly after lift off. The aircraft began to stall when I retracted the flaps because airspeed was just 190 knots :( So, I re-extended the flaps, and the aircraft was ascending at around 13-15 degrees all the way until reaching FL200 when it lowered to 8-9 degrees. The other abnormality I noticed was the speed tape did not get to the yellow/red marks until close to FL300 at a set speed of 0.77 Mach.All in all, taking off from this airport has been more challenging than landing ... which was not much less challenging :( Thanks for all the advices posted.Cheers.