High to Low: Look Out for Krazy Kollsman!

Imagine a balloon inflated at the bottom of a swimming pool to a particular size. Let’s imagine that the pressure inside the balloon is the same as the pressure of the water surrounding it (Image A). Now, if we allow the balloon to rise closer to the surface of the pool where the water pressure is less (Image B), then, because the pressure inside the balloon has remained constant, it will expand. The altimeter works in exactly the same way: the pressure inside the balloon/aneroid capsule remains constant and expands or contracts as the pressure of the surrounding air allows.

Let’s look at the scenario in Figure 1: a sea-level airport on a day when the barometric pressure is 29.92” Hg. It just happens to be that one day of the century when we have a standard pressure day! The pressure in the altimeter box matches the pressure in the aneroid balloon and the needle says we’re at an altitude of “0” ft above sea level (ASL). If our pilot flies to and lands at a mountaintop field, with an aerodrome elevation of 4 000 ft ASL, the outside barometric pressure, and hence the pressure in the altimeter box, will drop. In this instance, as shown in Figure 2, the pressure has dropped 4” of mercury to 25.92” Hg, corresponding to our standard lapse rate of 1” Hg for every 1 000 ft. Naturally, the aneroid balloon will expand and, linked to the dial, will give an indication of 4 000 ft ASL.

Figure 3

What we see in Figure 3 is that the barometric pressure at the airport—often referred to as Station Pressure—is now 2” Hg less than Standard Pressure: 27.92” Hg. 2” equals 2 000 ft, and so when the aneroid balloon expands, the needle indicates 2 000 ft. Which makes sense, since lower pressure—fewer air molecules—is higher. “Except,” cogitates our pilot, “I’m NOT higher! I’m still at sea level!” And this is where Mr. Paul Kollsman comes in! He invented the sensitive altimeter in 1928.

Turning the knob and moving the scale in the Kollsman window actually rotates the entire inside mechanism of the altimeter. In this instance, rotating the dial until the MSL of 27.92” Hg shows in the subscale will turn the needle back to “0,” which is the correct altitude, as seen in Figure 4.

Although this is not a practice we use in aviation, another way to explain how the altimeter works is to ask the students what the Kollsman window would display if, no matter at what altitude an airport was located, we set the needle to “0”? In this instance, we are comparing the pressure in the aneroid balloon—29.92” Hg—to the pressure in the box, which is the outside air pressure as measured with a barometer. The scenario is the same as in Figure 4: the subscale would indicate the outside barometric pressure, or Station Pressure. This is sometimes referred to as QFE, “FE” an abbreviation for “field elevation.”

Figure 4

Now, if we were to do this—set the altimeter to “0”—at our mountain airport in Figure 2, we would read the Station Pressure of 25.92” Hg. But that pressure is idiosyncratic to that airport; it wouldn’t help much if we were taking off from here and landing at an airport with a different altitude. So, to keep us all on the same page, as it were, we need to set our altimeters with reference to a different datum–sea level, which is common to all aircraft flying in the area. This will keep us from bumping into each other. To find out what this sea level datum is, we simply turn the knob until the needle indicates the airport elevation. In other words, like Paul Kollsman in 1928, we are seeking a way to adjust the altimeter to account for changes in barometric pressure due to airport elevation. Adding 4 000 ft of airport elevation adds 4” to the indicated pressure in the subscale, and we get our sea level pressure of 29.92” Hg as in Figure 5. This is referred to as QNH, “NH” an abbreviation for “nautical height.” We usually refer to this as the “altimeter setting,” and it’s the number we read in a METAR weather report. It’s useful to know this relationship between station pressure and airport elevation; when flying out of an airport where no altimeter setting is readily available from a local weather station, simply setting the altimeter to the field elevation will give the QNH in the Kollsman window. Essentially, converting the field elevation ASL into inches of mercury will give the difference between the Station Pressure QFE and the Altimeter Setting QNH. In Figure 5, the difference is 4”; the field elevation is 4 000 ft. True altitude equals indicated altitude.

Figure 5

Figure 6
Image with permission of KindPNG

Let’s consider the scenario presented in Figure 6. Albert is at a TRUE altitude of 6 000 ft ASL; the QNH altimeter setting is 28.92” Hg; the outside barometric pressure is 22.92” Hg. The difference between the barometric pressure and the QNH is 6 inches; the needle registers 6 000 ft and Albert is flying at 6 000 ft ASL. Everything is accurate! TRUE altitude equals INDICATED altitude.

But let’s attend to the consequences if Albert, in his flight from Brantford to Ottawa, flies to a region of higher pressure and neglects to update his altimeter setting with a revised QNH. In Figure 7, we see that the higher barometric pressure—entering the altimeter through the static line—is now 2” greater than it was at departure: 24.92” Hg. The increased pressure causes the aneroid balloon to shrink, and the needle goes down.

If Albert had checked with air traffic control (ATC) for a current QNH, he would have found it to be likewise 2” higher: 30.92” Hg. But Albert didn’t make the adjustment and, as we can see, the difference between the outside barometric pressure and the altimeter setting is 4”; the altimeter underreads by 2 000 ft: there’s the error of two inches! So, Albert the Altimeter “thinks” he’s flying at 4 000 ft ASL and climbs 2 000 ft until he reaches his desired altitude of 6 000 ft indicated. I say “indicated” because that’s only what’s indicated! It is NOT Albert’s TRUE altitude which is now 8 000 ft ASL. TRUE altitude does NOT equal INDICATED altitude. In flying from a low-pressure region to a high-pressure region—without making an adjustment—the altimeter will underread, and the pilot will compensate by flying too high.

Figure 7

Figure 8

While this is bad enough, the reverse scenario—depicted in Figure 8—can be deadly; flying from a high-pressure region to a low-pressure region: the static pressure in the box diminishes and the aneroid balloon expands. In failing to adjust the altimeter setting QNH to the lower pressure—26.92” Hg—the altimeter now overreads: the difference between the number in the Kollsman window and the actual barometric pressure entering the altimeter through the static port is now 8”: 28.92” minus 20.92”. The needle indicates 8 000 ft, which is 2 000 ft higher than Albert wants to fly. Consequently, Albert descends to an indicated altitude of 6 000 ft when his true altitude is now 4 000 ft ASL.

As one can easily imagine, these altimeter setting errors can have serious consequences for joining a circuit and landing: an overshoot or an undershoot are on the horizon! The second scenario—when pressure changes from high to low—is particularly dangerous for obvious reasons, especially when flying in Instrument Meteorological Conditions (IMC–IFR). Flying too low can lead to CFIT: controlled flight into terrain.

The change from high to low not only pertains to changes in barometric pressure. The change from high temperature to low temperature is also a factor in altimeter setting error. Pressure levels change more quickly in cold weather; air molecules are closer together and the air has a higher density.

Figure 9