VNAV at FL310 and climbing fast

[ztimbre 0]

Is it possible that your cruise speed in the FMC was set a fair bit higher than your FMC climb speed? For example cruise set at m .84 and climb set at m .78. Wouldn't the a/c generate a rather high initial ROC as it slows to capture the FMC "climb" speed? Since the altitude change was only 4000 feet you may have arrived at the new altitude before capturing the climb speed and therefore miss out on the a/c reducing to a more typical ROC. (am at work right now so can't test it out and, sad to say, haven't had much time in the Queen lately so can't speak with much authority, just ask questions).Hope you get it sorted.

[Guest SIX]

I just have no clue. I am going to test this in a more controlled environment. I don't have to work until Friday afternoon. So, I will have the time to get the low-down by then. Maybe even first thing in the morning. Everybody please don't give up on this quite yet.Wilson HinesMy Blog: http://www.wilsonhines.com --------------------------------------http://www.precisionmanuals.com/images/forum/747400.jpghttp://online.vatsimindicators.net/837438/3074.png

[Guest D17S]

Ahhh, Better. Now you're good and heavy. (Sorry, I didn't get that the 540 was a ZFW.) This really doesn't sound too bad. As we were discussing, VNAV SPD is a speed on pitch climb mode. Again, if a VNAV SPD climb is selected, the first thing that happens is that thrust just moves up to whatever EICAS limit is in place. That's the thrust mode "THR REF." The airplane overspeeds - just enough for the AFS to see it -- then the AFS reacts by pitching the nose up to control the speed. The more power the AT 'lays into it', the more pitch the AFS is going to need to control the airspeed. A higher pitch will result in a higher ROC. So now it become apparent that the ROC is completely dependant on how hard the AT mashes on the gas peddle when a VNAV SPD climb is commanded. Now I can say: "A higher EICAS thrust limit will provide a greater ROC a in VNAV SPD climb." Also, (just FYI) this the same way the pitch mode FLCH works, except ROC is the target. FLCH activates the thrust mode "THR." With this thrust mode, thrust doesn't just mash up to an EICAS limit. It moves around, actively targeting a ROC (up to an EICAS limit max). FLCH's ROC is determined by how much climb you have selected. Below a ~2000 request, thrust targets 1200 fpm. From ~2000 to about an (~)8000 ft request, thrust targets an increasing ROC. Above ~8000 ft, thrust just goes to the EICAS limit and you get a max available ROC. (Just like VNAV SPD will do for ANY amount of climb selection.) The thrust mode "THR" is the 'smartest' of the thrust modes. FLCH is a good alternatice to a VNAV PATH climb mode. Now you know why it's important to know what EICAS limit you were using for your VNAV SPD climb . . . . and another reason why a lot of these over the shoulder videos show RL ops with an EICAS thrust limit of CLB while in cruise.Careful of V/S mode . . . but only because there's no airspeed protection. If you are reverting to VS to climb, you are almost certainly trading off airspeed for altitude. Are you sure you want to do this? It's OK. Just be aware. IMO, cruise airspeed is close enough to a stall as it is (~V1.3g. That means that if I pull .3 gs, I will stall). There are other ways and I just would not what to mess with it. I love to test fly, but I

[michal 44]

>Is it possible that your cruise speed in the FMC was set a>fair bit higher than your FMC climb speed? For example cruise>set at m .84 and climb set at m .78. Wouldn't the a/c>generate a rather high initial ROC as it slows to capture the>FMC "climb" speed? Since the altitude change was only 4000>feet you may have arrived at the new altitude before capturing>the climbSimple calculations shows it is impossoble because of physics. The difference between 0.84M and 0.78M is only about 20 m/sec - you would bleed this amount of speed in just a few seconds whereas the change of altitude of 4000 ft would take a minute or more.Michael J.http://www.precisionmanuals.com/images/forum/pmdg_744F.jpghttp://sales.hifisim.com/pub-download/asv6-banner-beta.jpg

[Guest D17S]

There on the other hand, ztimbe makes a very astute observation. In the scenario he describes there could indeed be an initial trade off of airspeed for a potentially increased ROC. The principal is entirely correct and the math, far from simple. (On thing though, does airspeed shift for a cruise climb? I

[ztimbre 0]

The speeds listed were conservative because I was at work and could not recall with certainty typical climb and cruise speeds. Regardless, on further testing, my theory turns out to be incorrect.To the OP, I've duplicated your flight conditions re: weight on an ISA day, and get nearly the same max FL of 358. Unfortunately, even if I set the VNAV climb speed artificially low and the VNAV cruise speed artificially high, I still cannot reproduce what you're seeing. If, once stabilized in cruise @ mach .880 and FL310, I select a new cruise level of FL530 via VNAV, VNAV adopts the cruise speed during the "second" climb, and does not revert to the original (slower) climb speed. This results in a climb of approx. 800 fpm.If I manaully roll the speed back via speed intervention (VNAV still engaged -not FLCH), I can momentarily produce a ROC of approx. 3000 fpm. Once speed reaches the revised speed, however, the ROC climb reduces to approx 500-800 fpm. I could not in any case in VNAV produce a ROC of 5000 fpm with the weights you described. Sorry, but good luck to you anyway!regards.