744 Improved climb

[TRey]

a simple question for you Improved Climb brainiacs:

Why might one only use I.C. at airports above 4000 MSL on the 744? Is there no benefit below 4000 on the -400? The reason I ask is that my company limits I.C. to airports above 4000 msl but they provide no explanation, maybe they don't know either!:hmm:

any guesses?

[parabellum]

This is no answer, just an aside: "Improved Climb" I always found to be a complete misnomer as it was often used to get more weight off the ground and still maintain the required minimum climb performance.

"Improved Take-off weight" would be a more accurate title!

Still thinking about the 4000' rule.

[mcdhu]

Used to be called 'Increased V2' - perhaps that was better.

mcdhu

[Meikleour]

On the B707 out of Nairobi my then company used a procedure developed by PanAm where the engines were run to an `over EPR` situation (ie. more than normal max power ) and an increased V2 was also employed. The usual limiting factor was the JT3D max N1 of 110.8
This allowed about a 6,000lb increase in runway weight plus an extra 15,000lbs on the climb limit weight. It was only certified for NBO and the performance was, as you might imagine, underwhelming until the aircraft was clean. Brakes release to unstick - 1 min 50 secs!

[mustafagander]

On the big Boeings V2 is well on the back of the drag curve. This means that if you go faster you have more excess energy to climb. Hence "improved climb" which is simply adjusting scheduled speeds (V1, Vr & V2) to stay on the runway longer and naturally faster to reduce drag in the segmented climbs after T/O.

Put simply, you "spend" the excess runway length to go faster rather than reduce thrust as much as is possible. This almost invariably improves 2nd segment climb limit weights (sorry, mass for you Poms!!) which tend to be the limiting ones enabling a greater payload to be lifted out of the port. Some operators are approved to use "Automatic Performance Reserve" too, which is simply a throttle push in the event of EFATO.

On the B744 the gain is trivial until the density altitude is quite high, but the theory holds water. Look at the old B747 classic specific charts out of, say, Harare.

Hello JT, please clarify my poor muddied explanations!! :ok:

[john_tullamarine]

Your explanation seems reasonable.

I don't have any 744 manuals to hand so can't comment on the specifics. Mutt would be in a better position to do so ...

[SuperRanger]

Let me give it a shot!

For a 4-eng airplane, it is almost always field length limited at lower density altitude. Since there isn't any excess runway avail in a field length limited scenario, therefore, an improved climb is out of the question.

As density altitude increases, 2nd segment climb perf deteriorates. Takeoff weights need to be reduced to meet the climb gradient resulting in 'excess' runway length?!

As such, an improved climb becomes avail. Perhaps, your technical dept has determined, any significant 'benefits' from improved climb is above 4,000'??

As always, I stand to be corrected.

SR