G'dayA question regarding what is actually happening to the aircraft when spped intervention is engaged.This is brought as i am confused as to what the aircraft is supposed to be doing when it is engaged.OK here is the scenario.We have just reduced speed to 250 kts through 10K.ATC call up and ask for 280 kts to help with traffic flow.OK now is when things at least seem to go astray.When i hit spped intv dial 280 kts the aircraft goes into a gentle descent to accelerate.No thrust is added just a lowering of the attitude.The vnav profile goes off the top of the scale as the aircraft descends below path to accelerate.At all stages the aircraft remains in vnav path.Now i am confused in why this is happening.In all the real aircraft i have flown with a speed intv function the aircraft should remain on profile with the engines to spool up and accelerate to the new selected speed.IE as the manual say's it simply overrides the fmc speed values and is driven to this new selected speed.Is the speed intv different from the 76 and similar aircraft and if so how?If the path is being ignored what is controlling the vertical path of the aircraft remembering the aircraft is still in vnav path mode?If the aircraft descends below profile to accelerate and is not spooling up the engines with vnav path engaged why?To give yourself a more exaggerated effect bring the speed up to 300 kts from 240 and watch as ROD exceed 4000 fpm as the aircraft attempts to accelerate.Manual engagement by use of speed intv is something which is used in many aircraft to help maintain a selected speed and maintain your own profile while being controlled by ATC in the vicinity or airports.My question is is the PMDG 73's autothrottle/afcs operating correctly in this instance.Great product and after talking with JT at the Aussie flightsm expo the other day can't wait to see the 747 when it is finally released!!ThanksDarren

I believe the logic used here is to acheive the goal of accelerating and not missing any altitude constraints in the VNAV profile.The "Cheapest" way to do that is to lower the nose and use pitch to get the airspeed desired, and not burn any more fuel.Rest assured that if your descent brings you up against an altitude constraint (i.e. cross XXX at 5000) that the aircraft will level off, and thrust will increase to maintain the FMC speed.VNAV PTH in the PMDG SIM does not appear to mean rigid adherence to a profile. It means that the airplane is going to stay within certain vertical boundaries created by the altitude constraints for each leg of the flight.

Looking into it Darren. There IS an option (Geometric path) on the NG to keep the profile during RNP VNAV approaches with SPD INTV. Not sure what triggers this OR if it is kept at higher altitudes. Best Wishes,[h4]Randy J. Smith[/h4]http://www.rawbw.com/~bdoolin/shinault/Animation1.gifCaution! Not a real pilot, but do play one on TV ;-)AMD 64 3200+ | ASUS KV8 DELUXE | GFORCE 5700 ULTRA @535/1000 | Maxtor 6Y080M0 SATA 80 GIG | 512 DDR 400 | Windows Xp Pro | Windows Xp Pro 64 |

Hi Tim and Randy.Thanks for taking the time to look into it.The only reason i ask is i tend to use the speed intv function quite a bit as i believe most operators prefer the speed intv fuction over going head down and both crew becoming fixated on changing numbers on the FMC particularly at low level or in vmc.If i am doing something incorrect then please gladly point it out to me.I found the nose over situation and no thrust application on an acceleration to be a little strange given the aircraft remains in Vnav path and it just lets the path go straight off the deviatin scale..Doesn't really help with speed intv being an optional fit and not fitted to many 73's around the world.Anyway thanks for taking a look and i look forward to hearing what i have buggered up!!Thanks.Darren

Hi TimJust a small one on the descents using speed intv.I think it is actually more efficient for the aircraft to throttle up while on the descent as opposed to just nosing over to gain free airspeed.Given that currently as modelled the 73 will descend at up to 6000'fpm to accelerate and descend well off profile it creates some major problems.Firstly.I wouldn't like to be a flight attendant in that aircraft as i would need a safety rope to stop myself hitting the cockpit door as the aircraft noses over to 20=+ deg deck angle.The aircraft has flown well below vnav profile and will now spend far more time in the far denser lower atmosphere.Meaning lower TAS,astronomically higher fuel flows and greater flight time from point A to B.Given the NG flies a softer VNAV profile than other Boeings some deviation would seem ok but going 4000' below profile seems a little strange especially as its combined with the extremely steep(almost an emergency descent body angle) descent.You can fix the problem but it means taking the aircraft out of speed intv and using the FMC speeds to alter speed.SOmething which is frowned upon as i said due to fixation/distraction when your eye's really should be out of the windows.ThanksDarren

From what I've been told (past) SPD INTV is a *dumn* mode - i.e. it does not do anything except follow the speed while remaining in VNAV, I suppose like VNAV SPD. The manuals are silent on this point so will need to talk to a pilot who has this option on his bird! Interesting though ;-)..Best Wishes,[h4]Randy J. Smith[/h4]http://www.rawbw.com/~bdoolin/shinault/Animation1.gifCaution! Not a real pilot, but do play one on TV ;-)AMD 64 3200+ | ASUS KV8 DELUXE | GFORCE 5700 ULTRA @535/1000 | Maxtor 6Y080M0 SATA 80 GIG | 512 DDR 400 | Windows Xp Pro | Windows Xp Pro 64 |

Good thread...If Speed Intervene is a 'dumb' mode, then why not use level change? I guess the only benefit will be the level off at altitude constraints. That doesn't seem horribly efficient to me.

>Good thread...>>If Speed Intervene is a 'dumb' mode, then why not use level>change? I guess the only benefit will be the level off at>altitude constraints. That doesn't seem horribly efficient to>me.>I tried hitting the SPD INTV button last night and noticed that the pitch mode went to VNAV SPD, confirming my impression that the only difference between it and LVL CHG is the respecting of altitude constraints, which can be very handy on a DP or STAR.

I am chasing up a couple of friends who are on the NG with Virgin Blue and will let you know what i hear about speed intv.Only a portion of there aircraft are fitted with it,as we said before its an option.MmmmmDarren