738: VNAV-SPD INTV not working in highSpd descend

[Guest Stefan Bergert]

Hi everyone!Very happy with the 737NG in general, but flying online yesterday I noticed a strange thing (which is maybe! correct...don't know)...I was cleared for a high-speed descend with restriction of 240kts at FL100 as usual. Descending in VNAV PTH from FL330, only slight winds, speed is close to the target of ~300kts.Because being cleared for high-speed, I engage SPD intervention and set SPD to the current 300kts.Now, approaching FL110, the window goes blank and speed cursor down to 240kts, I re-select SPD-intervention (think now in VNAV SPD-mode) and still, the window goes blank again and speed cursor down to 240kts.Is this real behaviour? I thought with SPD-intervention, VNAV would take MY speed and continue on path or SPD-descend...Best regards,Stefan Bergert

[horace 3]

I also experienced this and MCP inputs should always override the figures in the FMC (like Speed restrictions for example) because of that I usually use LVL CHG or Vert Spd when ATC gives me "no speed".

[crvm 0]

I am not sure if it is correct for the NG but as a side note, if you do not desire to hold the restriction why not simply DElete the restriction?[h4]Randy J. Smith[/h4]AMD 64 4000+|ASUS K8V DELUXE|SAPPHIRE ATI X800XT PE|MUNCHKIN 3200|80 gig SATA|DELL 1905FP 19" LCD|TRACKir PRO|PFC JEPPESEN MOONEY YOKE|CH PRO PEDALS|

[horace 3]

yep, this is also a solution ;p

[Guest Stefan Bergert]

Hi again :-)In controst to real life I'm not looking for solutions but looking for a reply that confirms, a) as said, the MCP should override and VNAV shouldn't blank the window again and reduce on it's own or:( sbdy saying: "But this is how it works in real life" (though I can't believe it's like that in real B737NG up to now) solutions are always there...I could also switch to pilot mode where I have control over throttle levers and steering ;-)

[Guest tmetzinger]

I can confirm that if you have SPD INTV active for a leg, when the waypoint is reached the SPD INTV goes away and you resume the path descent with the FMC programmed speed. I _THINK_ this is how it works on the real bird, but won't be back in a sim to check for a month or so.Workarounds: - VS/SPD mode for descents using the DES page as a guide. - This in many ways gives the smoothest ride to the pax. - LVL CHG Mode - Reset the speed in the DES page, and if you want to resume it, just reset it back to ECON. (this is guessing as I'm not at the sim at the moment)Best Wishes.

[Guest Stefan Bergert]

Hi!But I didn't reach a waypoint or was close to one when I selected SDP INTV and therefor followed a VNAV pth descend with 300kts dialed in the MCP manually. But approaching the donut for SPD decel, the SPD-window turned black and SPD target went to 240 - still being on PTH descend.

[Guest tmetzinger]

that "donut" is a waypoint, at least to the autoflight system, it's a programmed point where the autoflight system will dial the speed back for your descent - in this case it could be the coded speed restriction for the next waypoint, or the 240 below 10000 default speed restriction if you didn't change it.In reality, if ATC ever asks you for a specific speed or to "keep your speed up", you should strongly consider exiting the VNAV PATH mode and using a mode where speed is one of the target parameters, either VNAV SPD, LVL CHG, or SPD/VS.Hope this helps,

[Guest Stefan Bergert]

Hi Timothy!Have to quote me:"I'm not looking for solutions [for such situations]" but want some pilot/engineer/other person who is really sure that the PMDG AutoFlight-System is behaving like the real NG would.Just to clarify: In real life, in VNAV SPD or PTH and SPD INTV with 300kts, VNAV will blank the display again and reduce to 240 (if this restriction is entered), although you had SPD INTV active ?;)Anybody?Stefan

[Guest tmetzinger]

I think you had better phrase the question:In the real airplane, in VNAV PATH descent with SPD INTV selected, what events (if any) will trigger a resumption of the normal VNAV PATH descent and cancellation of the SPD INTV mode?Since, as I said, the likely event that triggered this was you reaching the FMC-computed speed reduction point.

[Guest Stefan Bergert]

Hi Timothy!You're right...now this is clear to me, so the PMDG 737NG equals real life 737NG with cancelling my SPD INTV...always thought it would be like Hendrik said "MCP inputs should always override the figures in the FMC", but then it's not.Think this woud've been a great remark in Bill Bulfer's guide... it's like Airbus changing from HDG select to NAV managed when making a direct to in the FMGC...Really thanks a lot,Stefan

[Guest tmetzinger]

Well, I can't say that the real NG would cancel the SPD INTV, at least not until I get into the real Aircraft or Sim again. But since your speed intervention was cancelled in the PMDG at an FMC computed waypoint, it seems likely that that was the trigger.Whether the real airplane behaves that way, we'll have to wait until a real operator chimes in.Bulfer is silent on when/if the programmed route will override the speed intervention.