# If left stationary at a specific height, would altimeter change the displayed value over time?

I have been reading a lot about how altimeters work, and from what I understood, most common altimeters are based of atmospheric pressure to calculate the altitude.

The question that I still have though is: If left stationary at a specific height, would altimeter change the displayed value over time? Something tell me there would be slight variation, because air pressure changes almost daily, or am I totally off with this one?

• Yes, an altimeter based on atmospheric pressure will change over time. If you, say, park your airplane at an airport with an elevation of 400 feet msl and your altimeter is reading 400 feet, when you come back a day later, for example, it will read differently. If a low pressure system has come in, it will read higher. If a high pressure system has come in, it will read lower. And that is why there's a knob on the altimeter to allow you to change what it reads, and also why you should change the setting to match local pressure when you're enroute and below 18,000 feet in the U.S. – Terry Jul 15 '15 at 0:20
• So it also takes an input of local pressure at a known height? – Alexus Jul 15 '15 at 0:23
• The variation is not necessarily slight, but can be several hundred feet or more. – jamesqf Jul 15 '15 at 2:36
• @Alexus You don't have to know the height of the local station you're passing because the setting they give is adjusted to mean sea level. For example, if the station reports 30.00 inches, that's what you would set in the Kollsman window. If the station were at 2,000 ft msl, the actual pressure there would be approximately 28 inches given the standard pressure lapse rate of approx 1 inch per 1,000 ft. However, a barometer at the station would typically read 30.00 inches because it would be adjusted to read relative to mean sea level. – Terry Jul 15 '15 at 4:30
• @Alexus the inputs to the altimeter are actual local pressure and a reference pressure. The difference between the two pressures determines the altitude of the local pressure level (approximately, it does not take temperature into account). An altimeter actually tracks constant pressure levels, not true altitudes. – casey Jul 15 '15 at 12:44

You are correct in both regards -- as Terry's comment points out, an undisturbed pressure altimeter's reading (imagine one sitting on a bench outside the local hangar) will change with the local barometric pressure. TSO'ed "sensitive altimeters", i.e. ones with a hundreds pointer, not just a thousands, are required to have a knob and window, called a Kollsman window, for setting the local barometric pressure, or standard pressure (29.92) when above the transition altitude (18,000' in the US, check with the local aviation regulator for where you are in the world). "Glass cockpit" systems have a knob/function for the same job -- whether it be a dedicated knob, or a mode on a multi-function knob.

• Can you define TSO'ed "sensitive altimeters"? Not all of us have been through flight school. – FreeMan Jul 15 '15 at 2:49
• @FreeMan "A TSO is a minimum performance standard for specified materials, parts, and appliances used on civil aircraft. When authorized to manufacture a material, part, or appliances to a TSO standard, this is referred to as TSO authorization. Receiving a TSO authorization is both design and production approval." That's from faa.gov/aircraft/air_cert/design_approvals/tso, and TSO stands for Technical Standard Orders. – Terry Jul 15 '15 at 4:37
• @Terry -- perhaps we should have a Q&A here explaining sensitive vs. non-sensitive altimeters? (I suspect that many people who've been through flight school did so without ever seeing a non-sensitive altimeter!) – UnrecognizedFallingObject Jul 15 '15 at 4:38
• @UnrecognizedFallingObject Be my guest. I'm a lazy old man. LOL. One might also ask about the additional altimetry requirements and checks for using 1,000 foot separation in the North Atlantic MNPS. Again, somebody else should ask it as my experience is from the 1990s and I'm not sure it's applicable anymore. – Terry Jul 15 '15 at 4:46
• @UnrecognizedFallingObject Ask and ye shall receive! (or someone else will ask - either way really) – voretaq7 Jul 15 '15 at 19:21

Yes - as noted above, an Aneroid Altimeter will indeed show different altitudes if you come back the next day.

and No, a Radar Altimeter will not change its reading as it is measuring the altitude by "listening" to return echos from its transmitter.

• Would a radar one change altitude based on what's below the plane (mountains, buildings, etc.) – Alexus Jul 15 '15 at 16:44
• @Alexus - Yes, this is one reason they exist, to provide very accurate "AGL" altitudes for pilots flying at low altitude or through high terrain. It was initially developed by the military as "terrain-following radar" for low-altitude bombers and strike fighters like the B-1B and F-15E, and a simplified version found its way into commercial airframes as the "Terrain Avoidance Warning System" or TAWS. – KeithS Jul 15 '15 at 18:15
• You mean on airliners there would be 2 systems side by side? – Alexus Jul 15 '15 at 18:25
• @Alexus -- yes, airliners carry both barometric and radar altimeters -- the latter provides increased precision during instrument approaches in low visibility as well as helps with terrain avoidance. – UnrecognizedFallingObject Jul 15 '15 at 22:31