DFWHeavy From United States of America, joined Jul 2011, 560 posts, RR: 0 Posted (2 years 3 weeks 1 day 15 hours ago) and read 4570 times:
Just an interesting observation on the takeoff roll lengths of my past 2 747 flights last week.
1. LAX-ICN on OZ 744. Flight time: 12:50 for about a 6,000 mile journey
I was seated in the F cabin so I had a great view of the runway. We hear all the time that the 747 is a dog on long heavy flights, but I was amazed that we lifted off Runway 24L at the "3" marker, which means we had about 3,000 ft of runway left. By doing the math, we only needed about 7,200 ft to take off on this fairly long flight.
2. SYD-LAX on UA 744. Flight Time: 12:48 for about a 7,500 mile journey
Surprisingly, this flight took much more room and at rotation only had about 2,500 ft of runway left on Runway 34L, which is 13,000 ft long.
Now both flights had roughly the same flight time length, load factor, elevation, temperature at takeoff and winds were less than 10 mph on both segments.
Now I know a lot of variables can go into takeoff calculations and whatnot, but I just thought it was an interesting observation that one flight used so much more runway than the other given many similar variables. Granted I do not know anything about the cargo loads. I'm 99% both flights used a de-rated takeoff as well.
Just my observation for my trip. Great flights on both though
KAUSpilot From United States of America, joined Jan 2002, 1958 posts, RR: 35 Reply 2, posted (2 years 3 weeks 1 day 14 hours ago) and read 4312 times:
First off, even though the flight times were close there are still a lot of variables which might explain the difference.
Secondly, thrust reductions are based in large part on runway length. An airplane on a longer runway will generally use a lower takeoff thrust setting to save wear on the engines, making the takeoff roll longer. The 747 often uses "assumed temperature" reduced thrust methods, which give operators a wide range of thrust setting availability up to a 25% de-rate.
In short, if more runway is available, a large jet is usually going to make use of the extra length to save wear on its engines. It's not just a "full thrust" or "reduced thrust" 2 way choice, there are more options than that.
Look up "balanced field length" for more information. There is some calculus involved in runway performance calculations and there are many variables involved (contaminated runways, rising terrain, atmospheric conditions, aircraft center of gravity and weight, engine performance, different flap settings, noise abatement profiles, etc etc).
Notice that you were very close to the same distance remaining each time...since these were flight of similar duration, weights were probably close. Braking distances depend almost entirely on weight (and not at all on thrust selection). In both cases, you rotated with about the same distance remaining and, to respect climb requirements, probably at about the same speed.
The runways were different lengths so it took different thrusts to have the airplane reach rotation speed at the right point on the runway, hence the different length takeoff rolls.
There are tons of other factors involved, as other posters have already pointed out, but the basic scenario is that you want to use all the runway you have available and still be safe.
DFWHeavy From United States of America, joined Jul 2011, 560 posts, RR: 0 Reply 5, posted (2 years 3 weeks 18 hours ago) and read 3744 times:
I appreciate the responses and thank everyone for their input. Some have explained it quite well. It appears both probably uses different de-rate settings to achieve takeoff with about the same amount of runway remaining.
Again, I thought it was just kind of interesting to the the variance in takeoff on a relatively similar flight.
You won't hear that all the time on this forum. As you found out the 747 can be a good performer if it needs to be, but if more runway is available why use more thrust than is necessary to take off safely?
The glass isn't half empty, or half full, it's twice as big as it needs to be.