Alternate Static Cessna

[NovemberGolf]

G'day,

Why is it that when checking the alternate static source on an aircraft like a C172, a momentary change in indication is noticed in the altimeter and VSI but not the Airspeed indicator? I have so far assumed it's down to the sensitivity threshold of the instruments.

Additionally if this check is done when stationary on the ground then the static pressure should be the same both inside and outside the cockpit correct? why then is a momentary change noticed at all on any instrument when testing the alt static source?

Cheers

[+TSRA]

So think through what types of pressure information each instrument uses.

The altimeter and vertical speed indicator take static pressure only. In essence, they're the same instrument with a different face. One is calibrated to tell you how much pressure change has occurred while the other is calibrated to tell you how quickly the pressure is changing. So yep, any change in static pressure, no matter how small, will be noted on these instruments. Considering they can tell you in generally 20' increments how far you've gone up or down in altitude, you're right - they are very sensitive instruments. Remember that the static port is attempting to sense static pressure from an atmosphere that is not static - it's almost impossible to obtain a completely accurate static pressure because of the way the air flows around the vents. There are normally two static vents installed to help balance out the reading, but things like static port placement and angle to the wind will affect the pressure obtained on the ground. The air inside the cabin is generally static, but is often just ever so slightly less than the outside environment, so that's the jump you are seeing. That's also why when using alternate static air, you'll find some form of calibration card in the POH - to account for this slight pressure difference, among other things.

The airspeed indicator, on the other hand, compares dynamic (pitot) pressure to static pressure. The key thing to remember is that the pitot tube obtains dynamic and static information. This is sent to a diaphragm within the airspeed indicator. Quite separately, the static ports send static information into a sealed case that surrounds the diaphragm. This permits the airspeed indicator to cancel out the static information. Therefore, when you make a change to the static information going to the airspeed indicator on the ground, there is not a massive enough jump as it's still only comparing static to static. It's only once you introduce the dynamic pressure by moving the aircraft fast enough that you'll start to see a change because of the way the gearing within the indicator is designed. It's been a good 15 years since I last flew anything smaller than a Dash-8, and if I remember correctly it's about 20 to 30 knots before the airspeed comes off the peg in a 172, so that's a lot of pressure to get those gears moving.

[NovemberGolf]

Hey +TSRA. Thanks for your thoughtful reply.

I do have some lingering misunderstanding. Specifically considering the aircraft stationary on the ground in a nil wind situation. I'm aware that in flight the cabin static pressure tends to drop in unpressurised aircraft leading to airspeed errors, supposedly due to suction from the faster air outside (like an inside-out venturi), however wouldn't this effect not apply in the case of a stationary aircraft in a nil wind situation? No airflow outside leading to no difference in static pressure between the main and alternate sources? These are the cirumstances that I do the preflight alternate static check. Surely there is something else as the cause. On a different forum I've since been advised that it may be something to do with the way the alternate static is mechanically designed but no details as of yet. It's not a big deal but I'd be interested to know the why.