What are the most important factors to consider when calibrating an aircraft's altimeter?

1 Pressure variations

One of the main factors that affect the altimeter reading is the atmospheric pressure, which changes with altitude, temperature, and weather conditions. The altimeter works by comparing the static pressure outside the aircraft with a standard pressure inside the instrument. The standard pressure is usually set to 29.92 inches of mercury (inHg) or 1013.25 hectopascals (hPa), which corresponds to the average sea level pressure. However, this standard pressure may not match the actual pressure at a given location or altitude, resulting in an altimeter error. To correct this error, you need to adjust the altimeter setting to the local pressure reported by the nearest airport or weather station. This will ensure that your altimeter reads the same altitude as other aircraft in the area and matches the flight charts.

2 Temperature variations

Another factor that influences the altimeter reading is the air temperature, which affects the density and pressure of the air. The altimeter assumes a standard temperature lapse rate, which means that the temperature decreases by 2 degrees Celsius (3.6 degrees Fahrenheit) for every 1000 feet of altitude increase. However, this assumption may not hold true in reality, especially in extreme hot or cold conditions. For example, if the air is warmer than the standard temperature, the air density will be lower and the pressure will drop faster with altitude, making the altimeter read higher than the actual altitude. Conversely, if the air is colder than the standard temperature, the air density will be higher and the pressure will drop slower with altitude, making the altimeter read lower than the actual altitude. To account for this effect, you need to apply a temperature correction factor to your altimeter reading, which can be calculated using a formula or a table.

3 Instrument errors

A third factor that can affect the altimeter reading is the instrument error, which refers to the mechanical or electrical faults that may occur within the altimeter itself. These errors can be caused by wear and tear, dirt, moisture, vibration, or electrical interference. Some common instrument errors are hysteresis, lag, and scale error. Hysteresis is the tendency of the altimeter to show a different reading for the same altitude depending on whether the aircraft is ascending or descending. Lag is the delay of the altimeter to respond to a change in altitude or pressure. Scale error is the deviation of the altimeter from the true value due to a faulty or misaligned scale. To detect and correct these errors, you need to perform a preflight check of your altimeter, which involves comparing it with a known reference altitude and observing its behavior during climb and descent.

4 Calibration procedure

The calibration procedure for an altimeter is a systematic process that involves setting, adjusting, and verifying the altimeter reading according to the factors mentioned above. To begin, set the altimeter to the current local pressure and check that it reads the field elevation of the airport. After starting the engine, observe any changes in the altimeter and note if it has moved more than 75 feet. During taxiing, make sure that the altimeter changes smoothly and consistently with the terrain. During takeoff and climb, monitor the altimeter and compare it with the altitude reported by air traffic control or GPS. If there is a difference of more than 200 feet, it may indicate a pressure or temperature error or an instrument error. During cruise and descent, adjust the altimeter setting to the current local pressure of your destination or en route airport. Finally, before landing, set the altimeter to the current local pressure of the airport and check that it reads the field elevation of the airport. By following these steps, you can ensure that your altimeter is calibrated correctly and that you can fly safely and efficiently.

5 Here’s what else to consider

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