How IVSI works?

[JABBARA]

If the Inertial refrence system fails, does the IVSI work?

To the best of my knowledge (somehow this is a guess), In the case of IRS failure, it works like a conventional VSI (with lag in the indications up to 6 seconds) without instantaneous trend indication. IRS input is only fed for immediate trend indication but then the system works as relying on the rate of change of ambient pressure.


Well, although I am an SIM instructor on B737 EFIS/NG, there is no option among the snags (such as Total Static Block) which I can try this in SIM. No Boeing Document (including AOMs) give a clear explanation about this either.

What do you guys think all about this?

[Ka8 Flyer]

Hi Jabbara,

don't know 'bout the 37, but on the 767 you need both the IRS and ADC input for the IVSI to fuction. Losing either one will make it inop.

Regards,

Mark

[JABBARA]

Thank you Ka8 Flyer,

It is very likely that B737 IVSI works in the same way.

But I wonder how did you come to this conclusion?

thanks

[Ka8 Flyer]

Jabbara,

it was an instructor who told me. He demonstrated it in the sim.
We were doing the blocked pitot/static scenario.

Regards,

Mark

[alexban]

the VSI takes all the inputs from IRS's .It will function correctly in case of blocked pitot/static sys,and it will be u/s in case of IRS failure.
Try to close the IRS's and see what happens (in the sim ,of course ;) )
Brgds alex

[JABBARA]

Thank you again Mark and Alex.

Alex,

it is smart to switch off IRS and see what happens; I will try it at next session (soon). If I observe otherwse, I will revive this thread again.

ridvan
:ok:

[Ka8 Flyer]

Alex,

I don't think the IVSI will work with only IRS input! Air data is definetely required!

If you only used IRS, the VSI would drift over time (just as GS/position do!)
Using the IRS only for tendency/acceleration info (basicly the I in IVSI)
Without Air data the IRS would fluctuate around 0 as the aircraft climbs/descends. But it won't show a correct value.

Regards,

Mark

[alexban]

Ka8:
" two independent IRSs are installed.........
In their normal navigation mode,the IRSs provide attitude,true and magnetic heading ,acceleration,VERTICAL SPEED,ground speed,track,present position,and wind data to the appropiate airplane systems. ' FCOM 2 11.20.4 NG

also ,14.10.05 ,apr15/88 (older 300 books)
:vertical speed pointer....... indicates zero when IRS vertical speed is unreliable.

I'll try it in a few days in the sim ,and come back with some answer.

Brgds Alex

[Ka8 Flyer]

Hi Alex,

maybe I'm missing something but I'd say you just proved the IVSI needs IRS input to function...
It doesn't say that it ONLY needs IRS input...
Anyway, curious what your sim will show!

Regards,

Mark

[JABBARA]

A blocked or frozen pitot and/or static system may effect the following primary airplane systems ... Vertical Speed Indicator... FROM BOEING B737 400 AOM VOL 2 Page 10.20.1


So one thing is certain: with the blocked static, IVSI will not operate normally. At least this is true for B 737 400.

And if you have time check this (http://www.boeing.com/commercial/aeromagazine/aero_08/erroneous_story.html)

[Ka8 Flyer]

Hi ASFKAP,

"No it needs both"

Read my post again - that's exactly what I was saying aswell!

"IRU receives Altitude, Altitude rate and true heading from its onside DADC"

Heading is an IRS output, not input!
Also, what role does true heading play for the IVSI??

Regards,

Mark

[JABBARA]

ASFKAP,
...this signal is resolved within the IRU and the result is called Inertial vertical speed and is displayed on the IVSI...

"I" in the IVSI stands for "Instantaneous" instead of "Inertial" in older versions of Operations Manual of Boeing. Regardless what it stands for, it implies an immediate indication will be displayed rather than a delay as in the conventional VSI.

In fact, intial querry was due to contradiction of my opinion with my other company collegues who claim that "IVSI works in the case of total static blockage since the V/S indication is derived from IRS input. That sounds illogical to me who has some basic knowledge of physics. That is : IRS can only be capable to measure or calculate "Geometric Height" change not Barometric Height" change. Therefore, to be in conformity with the altimeter which is the measurer of Barometric Altitude, IVSI should be definitely fed with static input. That is the reason I have come up with question "How IVSI works"; whether it works in conventional way without IRS input ? I am more or less sure that IVSI will not work without static input. However, as I mentioned in my previous posts, in SIM (neither EFIS i.e B 737 400 nor NG i.e B 737 800) among the failures which can be practiced, there is no such a failure as "complete static blockage" to confirm this conclusion.

Since my last post, I have directed this question to engineers. After they have gone through the schematics of system in maintenance manual, they have also come up with: "For both types of airplane, IVSI will be unservicable in the case of loss of either input from IRS or static".

Thanks.

[Ka8 Flyer]

...just as I was saying :p

[nixisfix]

From the A310 FCOM: 1-10.50 Flight Instruments

Primary source for IVSI is the IRS. An auto switchover to ADC input occurs if there is a problem with the IRS

Normal Ops: VSI repeats the IVSI data from the IRS, filtered by ADC info for stabilization

If IRS input lost - switched automatically to ADC input only (->slow response like standard VSI)

If ADC input lost the VSI will operate BUT ONLY FOR 30 SECONDS! then the OFF flag appears and the pointer returns to Zero

so I guess everybody is right ;)

[JABBARA]

Nixisfix,

Period. I think all we needed was an Airbus guy who had to put the full stop to the end of subject .
:ok:

I believe this is the general concept in the design of flight instrument system of new jets, regardless who is the manufacturer. But unfortunately no Boeing document is so much pilot friendly, informative and helpul while explaining some issues.

I think it is my time to switch to Airbus as well.

:cool:

Thanks