Most people have flown in an airplane during their lifetime. Some of us have even had the nerve-wracking moment of seeing another plane in the sky during a flight. Although they sometimes look close, the high accuracy instruments that determine airspeed and altitude are a few of the things that make mid-air collisions highly unlikely. High accuracy pressure measurement and calibration are two factors that make flying more comfortable, safe and efficient.
It takes a lot of calculation and measurement in order to fly, which is where air data comes in. In an air data system, pressure measurements are taken through a pitot tube on the outside of an airplane and are used to determine altitude, airspeed and rate of climb based on static pressure (Ps), impact pressure (Qc) and total pressure (Pt). This information is read by air data indicators in the cockpit.
In the past, flights within the commercial range of 29,000 to 41,000 feet were required to stay within defined corridors every 2,000 feet of altitude. This meant accuracy and uncertainty of altitude, airspeed and pressure didn’t need to meet the high levels of today’s standard. Reduced Vertical Separation Minimum (RVSM) was enacted in 1997 by the International Civil Aviation Organization (ICAO). RVSM space is between 29,000 and 41,000 feet or flight levels 290 and 410. This is the vertical space where most commercial flights travel and where aircraft achieve the highest fuel economy. The purpose of RVSM is to maximize efficiency in travel by allowing planes to fly in altitude corridors every 1,000 feet instead of 2,000 feet. The implementation of RVSM required many changes, including higher accuracy in pressure instruments on an aircraft.
RVSM required air data instruments be within 65 feet of altitude uncertainty. This, in turn, required air data test sets in calibration labs to raise the bar when it came to accuracy and uncertainty.
Calibrating air data instruments isn’t unlike calibrating any other instrument. The procedure still requires a device under test (DUT) and a standard that is at least four times more accurate than the DUT. Air data test sets such as Mensor’s CPA8001 Air Data Test Set and CPA2501 Air Data Test Indicator are well above RVSM calibration requirements, with 0.009% IS-50 accuracy and an altitude uncertainty of approximately six feet at 41,000 feet.
Avionics pressure sensors are configured to measure altitude and airspeed in units of height or speed, respectively. Therefore, their calibration requires these same units in order to control and verify the readings. An air data test set must be able to verify pressure readings at sea level, as well as beyond 41,000 feet, and account for its effect on airspeed, climb rate and altitude readings.
Airspace is only going to gain more traffic as time goes on. The advancements in accuracy on board aircraft and in grounded calibration labs are essential to maintain safety and efficiency in flying.