VSI delay

[Guest s_L_Y_F_a]

Since the vertical speed indicator is a barometric device it of course has an indication delay of a few sec. But is this the the case in a real 744 as well or did those Boeing engineers find some way to make the VSI show you your vertical speed in real time? I´m just curious because the VSI in the PMDG 744 seems pretty fast in displaying my current VS to my mind.

[Qavion 19]

"But is this the the case in a real 744 as well or did those Boeing engineers find some way to make the VSI show you your vertical speed in real time? "I don't know if (specifically) Boeing engineers found a way, but "IVSI's" have been fitted to commercial airliners for maybe 20 years. The additional "I" stands for "instantaneous" and uses the IRS's (as well as baro for smoothing).Both air data from the Air Data Computers and acceleration data from the IRS's are required for operation.Some time after the 744 was introduced, Air Data Computers were modified (not sure if ALL were). Because there is also a small lag in the pitot/static system, the airspeed indication on the PFD is now also enhanced electronically Some sort of predictive computation is used.Cheers.Q>

[Guest s_L_Y_F_a]

Very interesting, never thought they would use IRS to determine VS.Many thx for the reply

[Check_Airman 3]

So Q,Should we be able to get VS info (from the PS system) with the IRS off?

[Qavion 19]

No.Both are needed. The P-S (ADC) systems feed the IRU's which then feed the PFD's.Normally, the Left ADC feeds the Left IRU which feeds the Captain's ND. The Right ADC normally feeds the Right IRU which feeds the F/O's PFD. However, there is autoswitching of IRU's if they fail (or are switched off). There is also manual switching using the Instrument Source Select Switches outboard of the displays.Cheers.Q>

[Check_Airman 3]

Thanks Q. Does the 763 operate in the same manner?

[Qavion 19]

Pretty much the same. The ADC's feed the IRU's which feed the IVSI's.No autoswitching of sources, however.Cheers.Q>

[kevinh 409]

Baro VSIs are lagged because they can only measure pressure rate if the rate is controlled. Static pressure is leaked to another static chamber through a small hole so rate can be measured in the form of a pressure difference. Hence the lag. Note, a baro altimeter does not lag. The original IVSIs used an inertial lead system to provide "instantaneous" VS. Sometimes these were known as ILVSIs because of this. The inertial lead came from internal bobweights and springs. As the aircraft pitches up to climb the inertia of the bobweight pushes against the spring, proportional to pitch rate. This rate information is added to the raw baro VSI to lead the indication and give a reading which looks more instant. It had no connection to IRS (many older aircraft had no such thing). Nowadays the vertical speed comes from the ADC, so true instantaneous altitude rate can be derived. It's still lagged a little though, so pilots see what they are used to seeing. If the VSI was truly instantaneous it would be bouncing around too much to be useful. AFAIK there's no direct input from the IRS to the IVSI.Hope this background information is helpful.Kevin

[Qavion 19]

"AFAIK there's no direct input from the IRS to the IVSI."On these aforementioned aircraft there is no direct input from the ADC's to the IVSI's. It is as I described it. For V/S, the ADC feeds "baro rate" to the IRS which feeds the PFD with vertical speed.I'm not allowed to post copyrighted Boeing documents on this site, but you CAN take my word for it.Of course, on aircraft with ADIRU's (combined ADC's and IRU's), as found on 777's, there will be a direct link from the box to the instrument.Rgds.Q> (Boeing Maintenance Engineer)

[Guest s_L_Y_F_a]

So I assume the vertical speed indicator in the PMDG 744 (which does not seem to have an obviously noticable indication delay) is simulated true to real.

[kevinh 409]

>"AFAIK there's no direct input from the IRS to the IVSI.">>On these aforementioned aircraft there is no direct input from>the ADC's to the IVSI's. It is as I described it. For V/S, the>ADC feeds "baro rate" to the IRS which feeds the PFD with>vertical speed.>>I'm not allowed to post copyrighted Boeing documents on this>site, but you CAN take my word for it.>>Of course, on aircraft with ADIRU's (combined ADC's and>IRU's), as found on 777's, there will be a direct link from>the box to the instrument.>>Rgds.>Q> (Boeing Maintenance Engineer)>>My apologies, I should have been clearer that I was meaning analogue aircraft with self contained IVSIs, not integrated with PFD as per 747-400. Trying to fill in a bit of historical background on VSIs, IVSIs, etc. Obviously not very well. :) Certainly don't doubt what you say. Interesting that the 767 vertical speed is derived in a similar way, would never have suspected that.Baro rate from the ADC is pretty much instantaneous VS, so does the IRS still add some pitch rate lead as with the old style IVSIs? Or is it just a convenient way to collect the data to the DUs?Kevin

[Qavion 19]

"Baro rate from the ADC is pretty much instantaneous VS, so does the IRS still add some pitch rate lead as with the old style IVSIs? Or is it just a convenient way to collect the data to the DUs?"The ADC's are still sensing altitude change by measuring air pressure changes, so it won't be as fast as the IRS's. The plumbing to the ADC's from the probes is quite long, so I'd imagine that there would be some kind of delay. The IRS's are used for the instantaneous part, but as you suggest, the VSI would probably go crazy with only IRS info. The ADC data is used to smooth the indications. I'd imagine it would be like the magnetic heading reference system on a Classic 747, which uses gyros for keeping the HSI compasses pointing in the right direation during rapid turns, but also uses magnetic flux gates in the wingtips for smoothing and correction of errors. IRS's and gyros accumulate errors in the long term which need to be corrected.Hope this makes sense.Cheers.Q>

[Guest s_L_Y_F_a]

>Hope this makes sense.>Cheers.>Q>It surely does

[wongben4 0]

Hi, Is C172 equiped with IVSI? or it can only be equiped on the aircraft which has IRS..benthanks

[Guest s_L_Y_F_a]

IRS or an inertial lead system is absolutely necessary for an IVSI to work, as far as I understood and I cannot imagine a C172 having any of them.Besides, I fly the C152 and I can tell you that the VSI there is 100% barometric and has a very noticable delay and I bet the C172 is no different in that aspect.