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VSI delay

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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.

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"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>

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

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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>

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

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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

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"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)

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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.

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>"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

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"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>

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

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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.

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IRS's are not needed for an IVSI. This link gives a good explanation of an IVSIhttp://www.allstar.fiu.edu/aero/PSI.htmThe IVSI has accelerometers inside the case to reduce the lag. They have been around for a long time and are not as fancy as you would think.C McCarthy

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>"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 areI was just reading an article in Business & Commercial Aviation about Embraer's new corporate jet model (the "Lineage") currently under development. It will have the pitot and static pressure transducers integrated directly into the pitot mast, with an ARINC 429 data bus feeding the ADC's - thus eliminating all traditional pitot/static plumbing in the aircraft. It should greatly simplify maintenance on the air data systems, and I'm sure will be the wave of the future...Jim Barrett

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"It will have the pitot and static pressure transducers integrated directly into the pitot mast, with an ARINC 429 data bus feeding the ADC's - thus eliminating all traditional pitot/static plumbing in the aircraft."This is the case with all newer generation aircraft (B777's, 737NG's, Airbuses, etc), although you may find traditional plumbing being used for the standby instrumentation.Cheers.Q>

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Ye but it would still be no use in a cessna (or similar aircraft).Such small and light planes are very sensitive for even the smallest air turbulences, therefore you mostly get a bumpy ride on it. Thus, the acclerometer would drive the VSI crazy.Just my 2 cents, plz correct me if I´m wrong.

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Ever been in a 737 Classic in even quite moderate turbulence? They shake around a lot too. Doesn't stop them using an IVSI. The electronics within the instrument filters out higher frequencies from turbulence and keeps the sustained accelerations.Accelerometers sound high-tech, but in this case they aren't. It's basically a mass on a stiff spring. Nothing fancy. Also, there is no such thing as an Inertial Lead System. "Inertial Lead" is simply the predictive term from the accelerometer in the instrument added to the heavily lagged baro VSI to produce IVSI.The main reason you might not bother with an IVSI in a Cessna is because it's not really worth it. The Mark I Vertical Accelerometer (better known as "the seat of your pants") does a much better job of telling you that you are deviating from your altitude than an IVSI ever could. :)Kevin

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I don´t think you could really compare the turbulence behavior of a 737 and a Cessna.And those electronics within the instrument smoothing the IVSI you are talking about would already be too much for a C172.Btw, this seems to be drifting too far away from PMDG related topics, still interesting to know:-roll

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