Say you're flying in IMC (Instrument meteorological conditions = no/low visibility) and the static port of your Pitot-Static System freezes or otherwise gets clogged.

You happen to be in an aircraft without an alternate static port. So what to do now?

Easy thing, break the glass of the VSI! You introduce static air from the cabin into the static system and are good to go again.

But wait! Did this really bring us our static system back? No, it didn't!

Although most books and references (including FAA-H8083-15A The Instrument Flying Handbook p.11-8) recommend that in this situation you should "break the glass on the VSI." to provide a source of static pressure to the altimeter and the Airspeed Indicator (ASI) this may be misleading.

Although the IFH states that "This procedure could cause additional instrument errors" it would be more correct to say it doesn't work.

They never really explain how this source of static pressure to the altimeter and the ASI gets going so let's have a look at what the theory of the "break the glass" method is based on.

The idea is that breaking the glass allows the (static) air from the cabin into the VSI enclosure and from there somehow goes into the pitot-static system and hence to the Altimeter and ASI.

So let's take a look, why that is not working as advertised:

Problem 1:

Since the diaphragm inside the VSI is sealed and connected to the pitot-static system via a tube outside of the casing the only way to get air from the casing into the pitot-static system is via the Calibrated Leak.

As the calibrated leak is designed to exchange the air with a lag of time - guess what - the air we get from the casing is going to be lagged behind the Calibrated leak.

So if we manage to get air to the altimeter and ASI via this way, the indications are going to lag and be inaccurate.

Problem 2:

In cheaper VSI's the Calibrated Leak is not connected to the pitot-static system but just an open end in the back of the casing. So you get the air you take in from the cabin through the broken glass in the front to leave the case in the back - lagged - into the cabin again.

Cheap VSI instrument with the calibrated leak simply venting to the outside, ambient air

Cheap VSI instrument with the calibrated leak connecting cabin air to cabin air after breaking the glass

Guess how great of an improvement that is related to your static blockage issue.

Problem 3:

Ok, let's say you have a more expensive VSI and your Calibrated Leak is actually connected to the pitot-static system. So now you're able to encounter Problem 1 from before and your Altimeter and ASI work but lag.

But what about your VSI? If the hands are still intact it will actually show inverse now!

That is because the Roles of the Case and the Diaphragm changed - now the Case immediately notices Pressure Changes through the open front and the Diaphragm needs time to adjust to the new pressure since it only gets the lagged air through the Calibrated Leak.

Expensive VSI instrument with the calibrated leak actually connecting to the pitot-static system

Expensive VSI instrument with the calibrated leak connecting cabin air to the pitot-static system after breaking the glass

So that could look funny.

So what could we do instead to get a work-around instead of creating new problems?

If you still like the idea of smashing the VSI window you could of course go one step further and smash the Diaphragm. Then you'll have a direct un-lagged flow of static air into the pitot-static system.

The only bad thing is that your VSI is going to be unusable. But since only the Altimeter and ASI are required by the FAR that's just a cosmetic issue.

(Note: I have no idea about how strong or how far back inside the VSI the diaphragm case is, so better bring a tough long knife. If a mechanic reads this and knows more, that would be an awesome comment.)

Another method which leaves all three instruments operable is to break the glass of the ASI. Of course here you better watch what you do, since you want to ideally keep it in operable condition.

If you do so, you'll have a direct static input for the pitot-static system and everything is going to work again.

A little note at the end: I talked to some Pilots, who have been flying IMC and they all told me that if you go into IMC you have an alternate Static Port, so I guess this whole Discussion is somewhat theoretical and in real life you simply switch to your alternate Static Port.

Great Information on this can be found in William R. Kershner's "The Instrument Flight Manual Sixth Edition" ISBN 0-8138-0833-2