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October 27, 2007
Subject: The altimeter is a tool
What this discussion is leading to is the realization that the altimeter is a tool with limitations. The pilot must understand how to use it. The difference between AGL and MSL must be clear in your head. Going cross country, then setting the Kollsman window to the local barometric pressure is appropriate. Working in close to the airport or just planning to land back at the point of origin, then setting the altimeter to zero may be right for you.
There are places where due to airport elevation the zero setting is not possible due to the limitations of adjustment. Extreme temperature/pressure variations have restricted flying in Canada and Alaska at certain times because of the inaccuracies induced into the altimeter or limitations on adjustments of barometric pressure in the Kollsman window.
Each inch of change in barometric pressure equals 1000' of change in altitude displayed. We know that because of a lack of vibration in the glider the altimeter indication lags slightly during climbs and descents. The internal gears offer a bit of resistance to movement that is not seen in powered aircraft because the engine provides enough vibration to keep things moving smoothly. In most jets an altimeter vibrator taps on the altimeter case because of the low vibration of the jet engine. The next time you are waiting for a tow, take some time to play with the altimeter adjustment knob. Steve.
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