Step Size vertical climb rate - VS or VNAV

[NGXfanatic 85]

Hello. I wanted to know what the commonly used climb rate is for step climbs, especially on the 747. I know I can just input the new cruise alt in FMC and VNAV will fly max climb rate. Is it more realistic to manually use the VS button and climb 500 feet per minute (for passenger comfort)? If so, is the 500 fpm used for 2000ft or smaller step climbs only? What about 4000 ft climbs in non-RVSM territory, are automatic VNAV climbs the norm?

[Guest alex_m]

Not sure about RW. If I am flying passengers, I climb as gently as possible for their comfort. If flying cargo, I let 'er rip with VNAV.

[downscc 1,327]

I use 500 fpm not only for smoothness (which I equate to good piloting) but also for efficiency. I haven't done the math, but I think the default step climb technique of max cruise power is less efficient than an incremental increase in power enough to maintain 500 fpm. The default takes less time but I am assuming that the fuel burn in the default case is higher. It would be interesting to run the calculations or flight tests trials to test that hypothesis. The more I think about it, the less sure I become about by theory. Any others?

[737BBJ 0]

I use 500 fpm not only for smoothness (which I equate to good piloting) but also for efficiency. I haven't done the math, but I think the default step climb technique of max cruise power is less efficient than an incremental increase in power enough to maintain 500 fpm. The default takes less time but I am assuming that the fuel burn in the default case is higher. It would be interesting to run the calculations or flight tests trials to test that hypothesis. The more I think about it, the less sure I become about by theory. Any others?

Dan, I believe Boeing states that when climbing, using max climb thrust will result in maximum fuel economy (probably because the elapsed time of the climb is minimized.) However, using max climb thrust will reduce engine lifespan. Bit of a tradeoff there. Fidel Lopez

[downscc 1,327]

I think step climbs are at max cruise thrust, there is that old age memory thing again. But like I said, I am talking myself into a false hypothesis.

[jon b 2,050]

Generally I use VNAV for RW step climbs, for normal 1-4 000 ft step climbs it behaves fine, it knows it only has a short distance to climb so it doesnt go ballistic.The general advice is if someone is on tcas within 2000ft of you dont climb or descend >1500fpm and intervene with VS if required. You see guys using VS for step climbs but personally I dont like using it at high levels and high weights as its got no speed protection.When climbing right at the top of the envelope you can watch vnav often reduce the climb rate below 100fpm or to zero, accelerate then take a run up at the last 500ft or so. VS isnt that clever it will just keep the rate of climb on and let your speed fall away.My own theory is a short more rapid climb at a higher engine power is going to use as much fuels as a slow protracted climb at 500 ft per min where the engines are at a slightly lower setting but for much longer, its only when you get into very shallow cruise climbs of 1-200fpm you might save fuel.We now climb from after takeoff to cruise alt at cost index 0, im told it saves fuel.cheersjon

[NGXfanatic 85]

Thanks for the replies. I noticed that the vnav climbs to the new step altitudes seem a bit too high, with the engines surging to max climb thrust and upwards of 1500 fpm or more, didn't seem comfortable or realistic. But then one can argue that such behavior is ok for a 4000 ft step climb and that the climb duration is much shorter than using 500fpm VS. On my rw flights on a British Airways 747 or the 767 i would notice the thrust gain but it seemed like a smooth climb, not max climb at all. It's a nitpicky thing to be sure, but I want to do it by the book. On a related note, what is the commonly used cost index for the 747, in this day and age of high oil prices. I use 40 throughout the flight, does that make sense? I only fly the 747 pmdg for British Airways flights, but may take up some United routes in the future. Does anyone know their guidelines for 747 cost index in present times?