Aircraft Technical question

[PompeyPaul]

1) If your static port became blocked, you could (in an extreme situation) break the glass of one of the pressure instruments in order to admit static pressure to the system. Which instrument would be most appropriate?
A The ASI (Air Speed Indicator)
B The altimeter
C The VSI (Vertical Speed Indicator)
D The AI (Attitude Indicator)
You chose B, but the correct answer was C.

Well looking at this, I don't know why I've been marked wrong.

A) This measures the difference between static & dynamic pressure and so breaking it will mix these pressures together, I thought
B) This only measures static pressure so seems the one to break
C) This measures a time displaced static pressure, so breaking it will probably only put static pressure into the later chamber
D) This works on gyros

So why would you break the VSI rather than the Altimeter ?

[OpenCirrus619]

Both will have the desired effect - it's just that most people, given the choice, would prefer to have a working Altimeter in preference to a working VSI (difficult to stay about MSA without an Altimeter).

At it's simplest the VSI is simply an Altimeter with a leaky aneroid (OK I know it's usually implemented the other way round).

OC619

[PompeyPaul]

Ah of course!

Thanks, yes obvious. It's the problem with these CAA exams they are inevitably wrapped in trickery and offer several correct answers with one being slightly more correct than the others.

[Russell Gulch]

I think they're all wrong...the glass does not "contain" the pressure.

The pressure bellows (or whatever) is a sealed unit at the back of the instrument.

[Genghis the Engineer]

You'd not break the ASI or the altimeter since those provide vital flight information and won't work after you've hit them with the fire extinguisher.

You'd not break the AI, since it's not connected to the pitot-static system.

So that leaves the VSI - which as it happens has an internal casing connected to static. So, by smashing it, you've turned it into a static port inside the cockpit, which will be at sort-of-ish ambient static.


As a result you'll get a reasonable approximation to static feeding the ASI and altimeter - good enough to get you home (to explain why you've just smashed up a perfectly good VSI).

G

[OpenCirrus619]

Originally Posted by Russel Gulch

I think they're all wrong...the glass does not "contain" the pressure.
The pressure bellows (or whatever) is a sealed unit at the back of the instrument.

5/10....2nd half is correct.



The aneroid is indeed a sealed unit at the back of the instrument. It is surrounded by the air in the instrument case. The case is sealed except for the connection to the static system. Since the glass makes up part of the case it does, indeed, "contain the pressure" (even if the difference between the static pressure and ambient is, usually, very small).

OC619

[BackPacker]

I would still argue that it's not necessarily the case with every static instrument.

In a simple, unpressurized airplane, the official "static" pressure (normally taken from the carefully sited static port) will not differ greatly from the internal cabin pressure. There might be a difference between cabin pressure and static pressure due to differences between inflow and outflow (ventilation) but it will be minor.

However, in a pressurized airplane, the difference between the official static pressure and cabin pressure will be a lot. Fly at 30.000 feet with cabin pressure at 6000 feet is not uncommon for a commercial airliner, but even a modest turboprop can easily go to 15.000 feet with 6000 feet cabin pressure. Have you seen the shape, thickness and general design of the windows on these sort of machines?

I doubt whether the single-milimeter thickness glass on the VSI would be enough to contain this pressure differential, so I assume that there are pressure instruments on the market today that are designed to be installed in pressurized cockpits. These instruments would have two pressure housings: one to hold either the vacuum (ALT) or the pressure from the pitot tube (ASI) and the second one to contain the static pressure. Smashing the glass on these instruments will not work at all - it's the housing that contains the static pressure that you need to smash (and you need to depressurize your cockpit).

Problem is, you can't see from the outside what it looks like on the inside. A VSI designed for a pressurized cockpit will look exactly like one for a non-pressurized cockpit when installed in your panel. And with all the trading in second-hand instruments going on, who's to guarantee that you don't have the pressurized type in your non-pressurized cockpit?

Anybody who has experience in pressurization who can confirm this?

[Genghis the Engineer]

So far as I know, the vertical speed instruments used in pressurised and unpressurised aircraft are substantially the same.

Of-course, if you have a pressurised cabin, all bets are off. But, I'd venture that's likely to be outside of the realms of a PPL exam, and anyway - I've never seen a pressurised aircraft without at-least one backup static system.

G

[bonniejack]

Mind you you wouldn't want to be smashing dials in a PA28 if it has the optional static source valve under the panel. That really wouldn't go down well with them what pays the bills.

[forget]

http://en.wikipedia.org/wiki/Pitot-static_system

[GOLF_BRAVO_ZULU]

If you are ever unfortunate enough to need to smack the glass out of the VSI, remember to pull the pointer off. If you don't, it will confuse you by indicating wrongly