IRS calculates V/S??
 

Hey,

I am trying to understand a specific part of the instrument stystem of the B737 CL series.
Regarding the VSI.

Its stated that the static source is fed to the ADC whitch in terms feeds data to the IRS?

Now, why would you want the IRS calculate the vertical speed?
On GA aircraft the vertical speed is sensed directly from the static source.
Even IVSI can be fed direclty from the static port if I remember my ATPL correctly.

Clarification would be highly appreciated :)

The IRS eliminates the lag errors associated with a traditional static-source-only based vertical speed indicator. Most transport category aircraft have "instantaneous VSI's" (IVSI). Using the IRS in conjunction with a static air source (through an Air Data Computer most commonly, I would suspect) makes the instrument more accurate. You might recall from your primary training days that it would take several seconds after initiating a maneuver for the VSI in a Cessna or Piper light trainer to begin showing a climb or descent indication. This should not be the case if an IVSI is installed.

The intricacies of the instruments' designs and their failure modes in various aircraft types exceed my knowledge level, however the manual for the aircraft I'm currently studying simply states:

Vertical Speed Indications:

Indicates ADC vertical speed as dampened by the IRS.

Thanks for your answer.

But I still don't see the reason for it.
Is it because a static fed IVSI is less accurate?

The IRS contains accelerometers, they pick up instant rate of climb or decent much quicker than air data can.

OK, so if all statics clog up and the ADC fail, will the IRS continue to provide data to the VSI?

NO.

You would understand when you rotated using VSI, and initially you were not climbing but "descending", for a second or three

Ahhh, does not occur with the L1011, and most did not have an IVSI.
Why?
A superior design, direct from the factory...long ago.:ok:

Yawn.....:rolleyes:

It's a nice way of doeing things
 

When flying with unreliable airspeed using the IRU V/S in combination with the PFA (also IRU) saves the day.

I must however say that the FCOM is a bit contradictious. It states that the IRU V/S is used to dampen the ADC V/S and on another page it states the opposite.

The IRS provides the "instantaneous" part, and the ADC provides longer term correction (i.e. the ADC overcomes accumulated IRU errors). I would guess that a clogged static port would drag the indication towards zero V/S in the longer term.

Rgds
NSEU

On the A320, during rotation you can find a typical example of this.
The IRS supplies the normal indications of VSI, the ADR is used as backup.
During rotation, the VSI points up almost instantly, while the altimeter tape is showing an initial descent (mind you, 20 feet or so...) as the static ports are slightly on the lower side of the fuselage, the aircraft starts to pitch up, and the static ports are exposed slightly more to the relative wind.

Flex