What am I doing wrong? When I am making my descents with the A/T engaged (usually using FLCH), the nose has a tendency to bounce up & down a great deal (to maintian the correct A/T speed I suppose). I must be doing something wrong because if this were real life, my passengers would be extremely sick from the excessive motion! :-)Can anyone help me? It usually begins to happen when the flaps start to come out a little at a time (according to schedule & necessity).ThanksChris Catalano

Chris,I haven't experienced this in the Fly! II 757, but in the Fly!2K version this oscillation was caused by the autothrottle hunting for the right power setting for the ordered speed in high angle-of-attack flight regimes, such as landing. It's important to deploy the flaps at the right speeds. Here's what works for both versions of the plane:210 KIAS FLAPS1195 KIAS FLAPS5180 KIAS FLAPS15170 KIAS FLAPS20160 KIAS FLAPS25150 KIAS FLAPS30Don't use the flaps as speed brakes. Pull the power and let the speed bleed off to the numbers above before deploying the flaps.I still get some motion on final, but it seems very realistic...the plane's not riding on rails, after all.

I'll try the schedule of flaps that you recommended, but aren't I supposed to use whatever flap schedule is printed on the flip cards on the lower center panel (which is dependent on weight)?Thanks,Chris

FWIW: I've experienced the oscillations referred to by Chris. I've seen it when descending at a speed lower than the default economy descent speeds as I tend to us 300 knots and 0.78m for my descents.The oscillations are most noticeable when using FLCH, if I switch to VS then things are fine. I've seen the same oscillation when climbing out using VNAV. Again - switching to VS smooths things out nicely.-michael

Michael,I've experienced those oscillations on climb and descent, but that didn't seem to be what Chris was referring to. There was another thread about those. As I recall, Lefteris gave a good response as to why that happens.Chris, the numbers in the flap schedule are the minimum airspeeds for each flap setting, as I understand it.

Chris-I've also experienced this problem. Someone said that it's likely to occur on slower machines.I think it is not your fault, you shouldn't think too much about the flap settings. The A/P *should* be able to maintain a steady climb/descent rate (in reality). I assume that it's just a weakness of the software.Since FLCH in the PMDG 757 is not very friendly to the passenger's stomach, I tend to use V/S and I can live with that. I still hope that they are going to fix the oscillation problem and the autothrust problem.regardsmalteEDDH

Chris,I use the V/S the same way that Malte does. ATC seldom clears me for the DP/SID climb profile anyway, so it's just easier to use V/S to manage the climb. I select 2500 fpm below 10,000 feet, and 2000 fpm above it, which seems to keep the engines within parameters for the most common weight range I use flying for America West Virtual. (Under 220,000 lbs) For descent, I plan on about 1500 to 1600 fpm downhill run, which means I start down way before the FMC-calculated TOD. That keeps the use of the speed brakes to a minimum, and means I need to use the V/S mode.

Actually, this IS what I am experiencing. I just can't figure out why. I have one question for you all - If you use V/S to manage the descent, how do you then manage the speed on the descent? I thought that V/S will use whatever speed is necessary to maintain the dialed in V/S - if this is so, how can you predict how fast or slow the KIAS of the plane will be - espedcially if it needs to be less than 250 below 10000 ft?

Chris,Using V/S doesn't mean blind acceptance of the speed the plane chooses to fly. Theoretically, you can make a descent at 300 KIAS and around 30,000 fpm, but your dive angle will be near vertical. Managing airspeed in the descent means balancing drag with accelleration from both gravity and thrust, and you do this by controlling pitch. There is only so much you can point the nose down before reducing thrust stops slowing you down and you have to dirty up the airplane to keep airspeed under control. Exactly where that point is, you'll have to discover for yourself...that's part of being a good pilot. America West uses mach 0.78 and 300 KIAS as descent speed guidelines, so that's what I use. My experience has been that the FMC-calculated TOD tends toward the what I consider an aggressive descent rate, that is, a descent rate that forces me to use speed brakes to hold the airspeed to the guideline, initially something around 2400 fpm. When I follow the FMC descent, I invariably get the DRAG REQUIRED warning, which is a little noisy for the folks who pay for their seats, so I try not to do it unless I absolutely have to.To that end, I opt for a more conservative descent, when I can. I ask ATC for descent at a point where I can reasonably expect to come down around 1500 fpm, so .78M/300KIAS is easily manageable with throttle alone, and in fact, I have to carry a little throttle; that is, the A/T indication on the EADI will read SPD and not IDLE.Getting to your slow-down point on speed and altitude is tricky, and requires some practice. If you've carried some power to maintain 300 KIAS throughout your descent, you can set the A/T to 250 KIAS and the system will reduce power and slow you down some, but you'll also have to reduce your rate of descent at the bottom, too, which will reduce the effect of gravity on your accelleration (you're pitching up, relatively speaking...not up above the horizon, just LESS DOWN) and give you the decelleration you need to get to 250 KIAS (or slower) when you need to. I have no idea if my technique is considered acceptable to real ATP certified pilots, but it works for me, since I seem to make my arrivals on time and I avoid those porpoising descents in VNAV.Hope that helps!

Just noticed this thread. I 'always' experience this bouncing (porpising(sp)) only during climb out to my cruise altitude. In addition, the speed is constantly moving forwards/backwards as the auto-throttles try to keep up.I've never experienced this during any descents.Regards,Bob L

Bob,Use V/S and this'll go away. Depending on your PLD, you may get 2,500 FPM all the way up to your assigned cruise altitude when you use 300KIAS/0.78Mach as your climb speed.

Kurt,I'll give it a shot.It sounds like there's a bug in the FMC logic when using VNAV...Bob L.

Bob,Lefteris gave an excellent explanation in another thread...essentially, it has to do with CPU speed. I've got a fairly fast machine, and mine does it, too...so for now, we just live with it. From what I've read, most r/w pilots use v/s for their climb until they're near the assigned cruise altitude, probably because it gives you better control over your climb rate and allows you to be more responsive to ATC commands. I flew a few flights with pure VNAV when I first bought the new bird, and found that ATC often cleared me to a lower altitude than what was programmed into the FMC. More often than not, when I was OK'd to continue my climb, I was so far below the VNAV path I'd get the "UNABLE NEXT ALT" alert and have to use V/S anyway. Now, it's habit...I use VNAV for takeoff and cleanup, then select V/S and set a comfortable climb. When I see the green top of climb arc I set the next waypoint altitude to the cruise altitude and select VNAV again. Might be more work, but it's probably a valid technique.

Kurt,I have had the same experience except that I usually use FL CH instead of V/S for the Airpseed protection. VNAV works great in cruise and for Step climbs but FL CH and V/S are modes I use for most altitude changes. I asked several captains on the 757 how they used the A/P for Altitude transitions and most use FL CH or V/S due to the restrictions applied by ATC. Once established in cruise they will switch back to VNAV until the A/C hits its descent profile however more often than not, ATC has other ideas about where and when you will come down and you end in V/S or FL CH again. RegardsPaul :-cool