Under Pressure: Atmospheric Forces and Altimeters

Most pilots have heard the saying, “high to low, or hot to cold, look out below”, but what does it mean? Additionally, how did a weather-related pressure altimeter error cause a catastrophic instance of controlled flight into terrain?

An altimeter is an aneroid barometer that is calibrated to convert atmospheric pressure to an approximate altitude. In simpler terms, the altimeter is an instrument that displays an aircraft's vertical distance above a fixed level. The altimeter works by taking the air pressure intake from a small opening on the exterior of the aircraft called the static port, which naturally collects air as the plane propels forward through the air. Located inside the altimeter, there is a small enclosed disc called the aneroid wafer. Air that is collected through the static port flows into the same area where the aneroid wafers are located. As the aircraft goes up in altitude, the pressure around the aneroid wafers decreases, and the small device expands like a balloon. As an aircraft goes down in altitude, the pressure around the aneroid wafers increases, and the small devices deflate like a balloon. Pilots must always set the altimeter to the current pressure at an airport so that the altimeter displays the correct altitude while airborne.

An aircraft that flies along a line of constant pressure may change altitude as the air temperature changes. "Without being corrected for the temperature change, a pressure altimeter will continue to read the same elevation..." (Ahrens & Henson, 2018). If an aircraft is flown from an area of high pressure to an area of low pressure without adjustments to the altimeter, the altimeter will only display a constant altitude, but the actual distance of the aircraft above the ground will be lower than the altimeter indications. Hence the saying "from hot to cold, look out below".

On December 25, 2012, an Antonov 72 transport plane crashed because of altimeter problems which led to a controlled flight into terrain (CFIT). En route and during the descent, the flight encountered poor weather with heavy snowfall and limited visibility. During the approach, the captain failed to set the correct pressure for the barometric altimeters. The altitude indicate was therefore off by about 385 meters. The airplane struck the slope of a ravine 21 km short of the runway and broke up ("Altimeter problems caused Fatal Antonov 72 CFIT accident in Kazakhstan", 2013). This goes to show how dangerous it can be if altimeters are not set correctly.

References

Ahrens, C. D., & Henson, R. (2018). Meteorology Today: Introductory Weather Climate & Environment, 12th Edition. Mason, OH: Cengage Learning.

Altimeter problems caused Fatal Antonov 72 CFIT accident in Kazakhstan. (2013, January 24). https://news.aviation-safety.net/2013/01/24/altimeter-problems-caused-fatal-antonov-72-cfit-accident-in-kazakhstan/

Flight Insight. (2019). High to low look out below! FlightInsight. https://youtu.be/EQ--Pf1XWl0