Can anyone explain this?

[Guest dirkvanderlaan]

Here's a picture of a VNAV PATH descent into EGLL 09L. As you can see the aircraft's well below the computed path and flying level to intercept the path. So far, nothing extraordinary. But then, look at the speed: 20 kts below what it should be. And it stayed like that until intercepting the path.Why is the FMC flying the airplane at a lower speed than it should?

[downscc 1,327]

Did you provide wind forecasts for the descent?I notice the 27kt left nose wind, could easily explain the vertical path deviation. As for speed, the FMC will add power only when required during the descent and it is common to see IAS drop below the descent target speed. Think of the VNAV descent path as a modified FLT CHG where instead of descending at whatever rate it takes to maintain a speed you are descending at whatever rate it takes to reach an altitude at a fix with minumum use of fuel.

[zubart 5]

First, your not that far off the computed Vnav Path, about 600 feet low. That's not bad and it appears to be correcting. I see the thrust levers are up so there is thrust to correct the situation. Also the airplane is passing 10,000 feet and the new FMC speed will be a modified 250kts. The airplane is behind programmed FMC speed probably from the headwind during the descent. If the current descent winds differ from the FMC descent forecast winds, the airplane will be off target FMC speed. A headwind is going to shallow the airplanes rate of descent to stay on the Vnav Path and a tail wind will increase the airplane rate of descent to stay on the Vnav Path. Remember during a Vnav Path descent FMC target speed is just a target for the known wind conditions. The airplane will always pitch to stay on the Vnav Path and disregard FMC target speed until a limitation is met. I see you have modified your FMC Vnav Path descent speed below 10,000 to 250 kts. The Boeing recommendation is 240 kts and there is good reason of this. First it keeps a 10 kt buffer from the FAA 250 below 10,000 rule. Also it also provides a buffer to prevent Vnav Disconnecting when forecasted descent wind are off and the airplane keeps pitching down to stay on the path. As soon as IAS hit's 255 kts, you'll get a Vnav disconnect.So try to stay away from 250 kts when in Vnav Path descent mode. Also, If you want to avoid Vnav disconnects during a Vnav Path descents below 10,000 feet, then change the descent speed below to 230 kts and that will provide plenty of buffer.Good luck,John Floyd

[Guest dirkvanderlaan]

Thanks guys for your replies.>Remember during a Vnav Path descent FMC target speed is just a>target for the known wind conditions. The airplane will always>pitch to stay on the Vnav Path and disregard FMC target speed>until a limitation is met. So is the 230 kts in this case the most economic speed to fly level until reaching the path? I always presumed the most economic speed to fly level below (or above) fl100 to be higher than 250 kts (why else choose an ATC speedrestriction of 250 kts below fl100).btw flying lower than the VNav path for the arrival on 09L came by because I changed the runway from 27R to 09L halfway the descent. I expected the VNav to disengage, but the level flight at 230 kts was what happened

[zubart 5]

First, if you can master Vnav climbs and Path Descents then you have mastered this airplane. Vnav mode is the highest and most economical automation level on the NG

[Guest dirkvanderlaan]

>A more economical speed would>probably be more than 250 kts below 10,000 feet but FAR's>limit the speed to 250 kts for safety. Economics at this point>don

[Qavion 19]

"BTW research has indicated that only 20 percent of the pilots use the VNAV path in the descent and 5 percent in the approach."Let's not get carried away with these statistics.... The report was based on a major US airline. i.e. One airline in hundreds, if not thousands. I don't know if the airline was named, but what aircraft types did they have? Note also that Honeywell was involved in the report (737NG's use Smith's FMC's). "That's debatable. I got the 250 kts from a KLM-pilot who wrote a manual for use of the FMC in his actual flight operations."Does this captain fly 737NG's?Thanks.Cheers.Q> イアン

[Guest dirkvanderlaan]

>Let's not get carried away with these statistics.... The>report was based on a major US airline. i.e. One airline in>hundreds, if not thousands. I don't know if the airline was>named, but what aircraft types did they have? Note also that>Honeywell was involved in the report (737NG's use Smith's>FMC's). The report is from researchers at RAND/Honeywell, Boeing, NASA and the university of Colorado and isn't about one major US airline, but has an extensive literature which encompasses many, many airlines within and outside the US and many, many aircrafttypes including the 737NG. In fact one of the most interesting finding is that VNAV (or Airbus PROF) seems to be one of

[Mats_J 2]

Gents,Please be careful when throwing other people's figures around. And be critical when reading reports. The report is referencing a lot of other material on this subject and without knowing what scope this other material has we need to be careful in our examination of the text. >>Let's not get carried away with these statistics.... The>>report was based on a major US airline. i.e. One airline in>>hundreds, if not thousands. I don't know if the airline was>>named, but what aircraft types did they have? Note also that>>Honeywell was involved in the report (737NG's use Smith's>>FMC's). >>The report is from researchers at RAND/Honeywell, Boeing, NASA>and the university of Colorado and isn't about one major US>airline, but has an extensive literature which encompasses>many, many airlines within and outside the US and many, many>aircrafttypes including the 737NG.The initial number of 20% using VNAV *is* in fact from a major U.S. carrier as the following quote from the section "ISSUES WITH VNAV OPERATION" shows:"Pilots generally use the VNAV function during the climb and cruise phases of flight. In a survey of 203 pilots at a major U.S. airline, McCrobie et al., (1997) found that 73% of pilots used VNAV in climb phase, while only 20% used the function in descent and 5% use the function in approach.">In fact one of the most interesting finding is that VNAV (or Airbus >PROF) seems to be one of

[Guest dirkvanderlaan]

You're right about not throwing statistics around. It was not my intention to argue about percentages, but to illustrate the fact that the use of VNAV in descent and approach has raised serious issues in real life aviation. In the sim-reality many of us have encountered these issues also as contributions in this forum have showed. To read about these issues in this report is interesting stuff and instructive to us all. Period.Btw I am still interested in an explanation for the 230 kts IAS in the case I put forward here.

[zubart 5]

Like I said before, it's probably due to the slight headwind during the descent. When IAS is more than 10kts off FMC target speed, the FMC should add thrust to correct IAS. In your screen shot the thrust levers are up so it appears the FMC is preventing anymore IAS decrease. Also, the FMC knows at this point that airplane is going to intercept the Vnav Path very soon so it would not add any more thrust and waste fuel to accelerate to 250kts when it can just pitch down to get back to 250 kts after intercepting the Vnav Path. I would agree that maybe it should have been more aggressive in correcting to the FMC target speed but I can tell that the real NG does the same thing pretty much and would only correct the IAS to within 10 kts below FMC target. The FMC just doesn't want to add a lot of needless thrust and waste fuel. BTW, many airlines are requiring Vnav as the primary means of vertical navigation when available at airports. In other words, A RNAV/GPS approach with a Vnav Path descent is preferable over the old Dive and Drive method. Much safer.Good luck,John Floyd

[rocketfs 2]

I know for a fact that low cost airlines are more and more implementing VNAV approaches. One thing has to do with fuel prices...However, I don't think the PMDG 737NG is 100% correct set up for VNAV approaches. (flaps 15 by 15nm for NPA..)

[zubart 5]

In my opinion the PMDG 737 does an excellent Lnav/Vnav appraoch but only on the first flight to a destination. I have found that if you try to do multiple Rnav approches it starts displaying inaccurate Vnav information. Not sure what you mean by (flaps 15 by 15nm for NPA..)?Good luck,John Floyd

[Guest dirkvanderlaan]

>I know for a fact that low cost airlines are more and more>implementing VNAV approaches. >One thing has to do with fuel prices...Might be useful when there are only non-precision approaches (VOR/DME, NDB) around, but as a replacement of precision approaches? I doubt it. At least not until the precision is at least that of the ILS (might be in combination with GPS).>However, I don't think the PMDG 737NG is 100% correct set up>for VNAV approaches. (flaps 15 by 15nm for NPA..)Like John I don't know what you mean exactly, but I have to say I agree partly. With the approaches I've done with VNAV PATH the aircraft only in rare occasions followed the ILS GP (loc was OK). However, for replacement of VOR/DME approaches, I think that the PMDG 737NG is sufficiently accurate.Thanks John, for your explanation of the 230 kts level flight. It does make more sense now.:-)

[michal 44]

>Might be useful when there are only non-precision approaches>(VOR/DME, NDB) around, but as a replacement of precision>approaches? I doubt it. He probably meant the latest RNP approaches. They definitely fit into the category of 'precision approaches'. But you have to have a special FMC certified for such an approach (in real life). We can assume that the fidelity of PMDG's FMC doesn't match requirements of the RNP approach. This subject had been extensively investigated on this forum (not sure whether for the PMDG 747 or the 737) by someone who knows a lot about RNP approaches, he performed number of tests and had some interesting results. You would have to search through this forum however but I suspect that for higher fidelity FMC you have to turn to PMDG's latest 747.Michael J.http://img142.imageshack.us/img142/9320/apollo17vf7.jpg