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VNAV Question

Started by Will, Sun, 3 Apr 2016 20:55


I was climbing to an initial height of FL310, followed very shortly by a step climb to FL350. It was a standard VNAV climb, with THR REF | LNAV | VNAV SPD, with a command speed of 329 kts. The VNAV page on the FMC showed ECON SPD 329/.846. The mach number was, of course, increasing, having started out much lower.

When I hit FL310, the aircraft started stabilizing, and then it was time for the stop climb. I put 350 into the FMC and pressed the altitude selector, and the aircraft went back into VNAV SPD for the climb.

Just prior to this point, the command speed changed from 329 kts to 0.846. The actual aircraft mach number was a bit higher at the transition, perhaps 0.849, and so when back in VNAV SPD, the aircraft pitched up to slow down to the command speed. That led to a rate of climb that was pretty fantastic, almost reaching 4000 fpm, and the cabin rate of climb exceeded 800 fpm, so the pressurization controls appeared on the primary EICAS.

Here's my question. Would you ever see the speed mismatch, and then proactively choose a different climb mode, such as vertical speed, to let the cabin pressure change more slowly? I know the passengers can't perceive the difference between 1000 fpm and 4000 fpm (although they can perceive the acceleration), and I also know that the physiological perception of pressure change is more uncomfortable when descending than when climbing. Still, in the interest of giving the folks in the back as smooth a ride as possible, I'm wondering if the crew ever avoids VNAV SPD when actual airspeed is greater than command speed, in order to keep the rate of climb constant.

Will /Chicago /USA


Follow-up to the original question. Here is a situ:

You're all set up for a step climb to FL390. Load the situ, and then press the altitude selector to initiate the climb in VNAV SPD. You'll see the aircraft pitch up a little, and apply power. Then the actual mach number goes beyond the commanded mach number, and the nose pitches up even more, from 3º to 10º or more. The vertical speed exceeds 4500 fpm, the cabin rate of climb exceeds 1300 fpm, and the delta P exceeds 8.8, meaning it's triggered to change from white to amber on the ECS synoptic.

Also, for what it's worth, the cruise EPR changes from 1.46 to a CRZ CLB target EPR of 1.50, seen on the VNAV page. However, the actual EPR goes up to 1.77. Is this an issue with the throttles exceeding the target? Am I doing something wrong?

Anyway, it seems like a fairly aggressive climb, so if this is normal behavior, then I'm wondering if real-world 744 pilots might use V/S to climb more gently, instead of VNAV SPD?

Thanks again.
Will /Chicago /USA



I think the main reason for this is because you are very light. Your GW is only 265.8t (you have only 20t of fuel), you are at FL350 while your MAX ALT is FL413 (I'm not talking about OPT ALT because of the next reason), you are only 130nm from TOD (you shouldn't make any step climb at this point anyway) so giving all of that the aircraft can climb like a rocket (or close to it) so everything is a result of it.

By the way, when I pressed the ALT knob the trust reference did change to CLB and set to 1.76. The real EPR never crossed it.

Avi Adin


Thanks for the input, Avi. The original step climb (from 310 to 350) made more practical sense than the last climb (from 350 to 390), but I saw the same behavior there. I did the last climb as more of a demonstration than a practical maneuver. Still, your points about a light aircraft are well taken. Perhaps pilots avoid climbing in VNAV SPD when the aircraft is very light?
Will /Chicago /USA


Also, Avi, thanks agains for your help so far. Perhaps you could take a look at this screen shot and tell me what I'm not understanding.

In the FMC, line 3L, the target thrust in ECON CRZ CLB is 1.51 EPR, and yet the auto throttle has commanded 1.77 EPR. Note the corresponding high actual speed (mach .855, when target is .845), the climb rate (4550 fpm), and the excessive cabin climb rate (+1350 fpm) and the excessive cabin delta-P (8.9 psi).

Thanks for your perspectives on this. I've never flown the actual aircraft, so I'm eager to learn.
Will /Chicago /USA


Interesting situ. It sure does take off on the climb rate. I saw 8.9" on the cabin diff. I'm not familiar with EPR settings for climb. I don't recall ever seeing such a high climb rate on the CF6 at that altitude, but VNAV is going to set THR REF and the elevator is going to pitch for speed. So, if the EPR is correct, I guess that's what you get unless the FCC decides to intervene because it exceeded one of its parameters.

The ACT ECON CRZ CLB appears on the CRZ page when you do a cruise climb. The EPR you see in 3L is the power it will take to maintain the cruise speed at FL390. Thanks for the interesting situ, Will.

Hardy Heinlin

This effect now occurs because I've increased the power of the RB211 a bit in one of the last PSX updates.

It does not occur with GE or PW engines, and not in PSX 10.0.0 with RR engines.

I guess the RR just has too much power now in that specific area.

Sigh ... the fine-tuning will never end ... -- This part is really "rocket science". Not trivial. One more hair here, and lots of side effects there ...





First of all, I'm not a pilot, never was (unlike you) and never will be.
Secondly, I never was in the cockpit of a B747 (in the Beta forum I told once what happened to me the last time I tried to enter. I didn't recover from it yet).

This is what the PSX manual says about 3L:
QuoteIndicates thrust required to maintain command speed at cruise altitude.

As I understand it, this is your EPR to maintain speed while cruising. However you are not cruising, you are climbing. For climb you get max (allowable) thrust which is 1.78 EPR.

As I said: you are light, you climb when normally pilots don't climb and Hardy added the RR engines produce too much power at this altitude.

Avi Adin


Thanks, Avi. I think that explains it pretty well.
Will /Chicago /USA


I've seen some guys will use V/S for step climb so the engines don't spool up too high and pax comfort, I don't think it makes a big difference from using Vnav, and remember V/S is a Very Special mode!


FWIW, even with "comfort mode" FLCH and VNAV CLB, I have always used SPD VS for small changes (step climbs) in the interest of imperceptibility... less spilled Martinis...

VS requires the pilot to be aware of the performance envelope as there is no "protection", however, it is a useful tool in the kit bag, often being a viable (and dare I say it, a better) choice.


Best- C
Carl Avari-Cooper, KTPA

Hardy Heinlin

RB211 has been fine-tuned again in PSX 10.0.9-beta2: