The VSI???

[helimutt]

A simple enough instrument I think you'd agree but here is a question I was asked recently. Does the VSI work "exactly" the same at high altitude even though the air density is much less. ?

[Cough]

Just to throw you off the scent for a moment..B737 EFIS ... VSI indications come from the IRS's and are not pressure instuments!

Coming back to your original post, I guess that the instrument has to be corrected for static pressure. How, now that takes an engineer!

[twistedenginestarter]

It's an interesting question.

As you know doubt appreciate the VSI merely responds to the rate of change of pressure in the static line ie what it is now compared to what was a few seconds ago.

The static line is shared with the altimeters so they must be 'calibrated' to hold an ISA model as the altitude rises. The pressure gradient is far from linear. The pressure drops to half in the first 10000 feet.

However the altimeter knows how high it is whereas the VSI just knows whether there is a pressure difference or not across its bleed valve.

My guess is it's incorrect at altitude - under reading.

The reason I make this wild guess is you are generally more bothered about whether it is going up or down rather than by how much, except near the ground where approaches and departures are concerned with real altitude profiles.

[Oz_Pilot]

Just to confuse the issue, an IVSI (where the first "I" is Inertial not Instantaneous as many think) have masses that, as the aeroplane accelerates vertically, cause displacement of air and thus a reading on the dial. Can't tell you much more than that on their operation without digging through the books (unfortunately having a great time working on a tax issue) but I believe this was to counter the altitude related inaccuracy. Most glass aeroplanes grab this info from IRS equipment - far more accurate and not susceptible to variations in the atmosphere.

[QAVION]

"Just to throw you off the scent for a moment..B737 EFIS ... VSI indications come from the IRS's and are not pressure instuments!"

Sorry to hijack the original question, but the 737 IRS's probably get pressure altitude from the ADC's to _assist_ with the vertical speed calculations (as they do on the 747-400 to give long term correction). Try pulling the ADC circuit breakers and see what happens to your 737 IVSI indications.

Regards.
Q.

[DoctorA300]

Is the VSI acurate at Altitude ? Yes, the vsi Basically measures a leak rate difference between two chambers of a bellow, thus not sensitive to altitude.
As Qavion correctly states, the B737 Classic and NG Derives Vertical speed, not from a static source, but from the IRS system, the "pneumatic" altitude rate IS used, but only to Dampen the indication, if you have ever flow on a B737 when an ADC fails, you will see that the onside VSI needle is very erratic.
Hope this can help you
Brgds
Doc

[twistedenginestarter]

<font face="Verdana, Arial, Helvetica" size="2">Is the VSI acurate at Altitude ? Yes, the vsi Basically measures a leak rate difference between two chambers of a bellow, thus not sensitive to altitude.</font>

Sorry Doc but if the pressure difference between the two chambers is only half what was at ground level the needle will only deflect half as much. You haven't explained what compensates for the fact that pressure does not decline at 1 mb per 30 feet at jet altitudes.

At least I don't think it does.

[Blacksheep]

A "classic" VSI is supplied with static pressure which is fed both directly into a capsule and into the instrument case through a calibrated orifice. The pointer deflection is a function of the differential pressure between the capsule and the interior of the instrument case. Thus, the instrument does not measure pressure it measures the Rate of Change of Pressure. The calibrated leak is self-compensated by the change in the viscosity of the air that occurs with the change in source pressure. The cross-sectional area is matched to the length of the orifice so as to obtain a virtually constant characteristic based on the rate of change of pressure regardless of the source pressure. Obviously there are limits to which this tactic can work for, as you approach a vacuum, there is insufficient energy in the gas to support the capsule. There are slight errors but the instrument may be considered accurate enough at the comparitively low altitudes achieved by aircraft using air-breathing engines.

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Through difficulties to the cinema

[This message has been edited by Blacksheep (edited 21 March 2001).]

[twistedenginestarter]

Sorry Blacksheep I don't buy that. It can't compensate by shapes of tubes. It would have to know the absolute pressure like the altimeter does. Whereas, and I agree with you here, it only knows the pressure difference (per second).

I'll do some research. I'm still betting it has errors but at 40000 feet nobody cares.

[The Nr Fairy]

TES :

I'm with BlackSheep on this one - and it's not an IT question !! :)

The Vertical Speed Indicator shows a rate of climb or descent. This can be measured by seeing how quickly the pressure in a chamber changes when allowed to equalise back to ambient through a calibrated leak.

The choke has a capillary and a venturi part, all forming part of the same "hole" into the chamber. Because air viscosity changes with altitude and temperature, this arrangement automatically compensates. My Nav exam notes are being borrowed at the moment, so this is correct from memory.

An "IVSI" uses small dashpots, which induce an immediate change in the inflow or outflow of air from the chamber because they're basically weights on springs which move and push air in our pull air out of the chamber faster than would happen otherwise. They're there for use in aircraft which need an instantaneous ( hence the term ) VSI indication and not one affected by the small lag in display caused by the calibrated leak.

[DoctorA300]

TES,
It´s not as difficult as you make it. As blacksheep and I tried to explain, you are NOT measuring an absolute pressure change, but a pressure difference between two interconnected chamber, therefor the air density is the same on both sides.
Brgds
Doc

[askop]

OZ_PILOT

I've learned that IVSI stands for Instantious VSI. I've used them in a C340 some times, and I'm pretty sure that bird wasnt't eqpd with any inertial systems (if that's what you mean with inertial?)

Please correct me if I'm wrong!

[Ignition Override]

If you lose all normal AC and DC generators, flying on emergency power, does an ADC/IRS get power for up to 30 minutes? I believe that is the case on the 757. When the 737's batteries are dead (and you had better be in VMC or you are....) will the IVSI get useful info from an air pressure source which requires no electric power? That might be a good question for a check airman.

Does your flightbag have a handy flashlight (torch) with good batteries?

[HighSpeed]

ignition override,

if i'm not wrong, on the b737 classics, when on battery power only, the standby instruments, engines indications, left IRS, dome light, some intergral lightings, plus many others will be powered. some airlines even opt for left EFIS to be powered.

but a small torch cant hurt esp. with guys from the old 'jurassics' (dont know how the sys works though). remember a couple of years back, there was a SB from boeing to check all batteries on the b737, apparently some of them dont live up to the 30 mins claim !!

HS

[This message has been edited by HighSpeed (edited 25 March 2001).]

[Blacksheep]

Pitot Static standby instruments on B707/727/737/757/767 as well as B747 'Classics' and Fokker F50 and F100 are "suck and blow" (usually only an ASI and Altimeter though) but you'll need a torch to read them in the dark. Cannot speak for types that I'm not rated on.

In a total power failure its the Standby Horizon that's going to kill you when the battery goes flat. If the Standby Altimeter is unwinding fast, the Standby ASI is reading way too high or your pencils and checklists are stuck to the overhead panel, you'd better believe them!

In simulator tests of 200 pilots, 198 lost control within two minutes of being deprived of any horizon reference in IMC in an otherwise serviceable aircraft. Only one succeeded in landing the simulator. (An ex-naval aviator - perhaps he had better sea legs?)

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Through difficulties to the cinema

[This message has been edited by Blacksheep (edited 27 March 2001).]