Measuring Thin Air

In the near future using barometric pressure to determine altitude will be a thing of the past, but until then, it will pay to know a few of the altimeter’s tricks.

The Big Question
While taxing out for takeoff you listen to the AWOS and get the current altimeter setting of 30.32. When you turn the knob to adjust for the current air pressure, the indicator hands of the altimeter move as well. You stop turning when you reach 30.32 and the indicator hands also stop. Then you notice that the altimeter is reading 700 feet, but you are taxing at an airport that claims to have an elevation of only 615 feet. What do you do … what do you do?

Dissecting The Problem
I would bet that the field elevation is correct and the error exists because we, so far, have not come up with a better way to measure altitude than by measuring air pressure. The altimeter is old technology. It compares the ever changing outside air pressure with the pressure inside a sealed bellows. The bellows expands when the air around it has less pressure than the pressure inside or contracts when the outside pressure is greater. This expanding and contracting bellows is attached to a series of levers and gears that move the hands on the face of the altimeter. That’s it — no electricity, no lasers, no satellites…

The altimeter is not all that precise and can be fooled with widely varying temperatures and pressures. Plus, the bellows and the mechanism that links the bellows to the indicator hands will stretch and wear and eventually will produce incorrect readings. In our ‘Big Question’ above, the altimeter was 85 feet off. Can you still go flying? Yes. You could set the altimeter, not by the barometric pressure, but to the field elevation — this was how it was always done before the invention of AWOS.

Solutions For The Long-Haul
Setting for field elevation will work fine for a short local flight, but may not be a good plan for a long cross-country flight. The length of a cross-country flight may take you into an area where the air pressure is different, so using the air pressure setting from where you took off will no longer work. Use this method to determine your altimeter’s error:

  1. Before takeoff, turn the altimeter knob so that it reads the field elevation.
  2. Look in the altimeter’s Kollsman window and see what the altimeter setting is.
  3. Compare the AWOS altimeter setting with the altimeter’s setting. The difference is the error.

Example: Let’s say the altimeter reads 30.32, but the AWOS says the current setting should be 30.57. Your altimeter is reading 0.25 inches less than it should be. As the flight progresses you can subtract 0.25 from every altimeter setting that you get along the way, insuring that your altimeter is reading correctly.

The Rules
Altimeters that are used for IFR flight must pass an inspection every 24 calendar months. The altimeter will fail the inspection if the error is greater than 75 feet, therefore it will need calibration before it can go IFR again. However, between inspections a 75-foot error does not ground an IFR flight. Of course, the pilot may elect not to go IFR with a large error, but it is a pilot-in-command decision. Still: Can the altimeter error get so far out of calibration that it would be illegal to fly? No. A pilot can even go IFR using the correction method above.

Inside Information: In the airplane we do not use a barometer with Mercury in it, but nevertheless we use ‘inches of Mercury’ as the units of air pressure. Normally the ‘inches of Mercury’ scale runs from 31.00 (which would be extremely high) to 28.00 (which would be unusually low). Flight is not recommended when the actual air pressure is either higher than 31 inches or lower than 28 inches. Note: A pressure reading of 28 inches is already ‘hurricane’ pressure, so having an altimeter that does read that low would really be the least of your concerns!

Change Is In The Air
The pressure of the air is always changing with the temperature, altitude and humidity. This is why it is vital to update your altimeter setting as you travel. If you forget to adjust the altimeter along the way, the error will increase the farther you go. If you fly from an area of high pressure and/or high temperature, into an area of low pressure and temperature, the airplane will gradually get closer to the surface than the altimeter says you are: ‘Going from high to low – look out below!

Final Thoughts: Why You Need To Stay On Top Of Things
Your Mode C Transponder transmits your altitude — not your altimeter reading. The transponder is always set to the standard pressure of 29.92 inches and the reading is then calibrated for current altimeter setting on the ground at the radar site. Translation: If you accidentally get off your assigned altitude, you cannot fool the Mode C by moving the altimeter knob and making it look like you are at the correct altitude!