With advancements in avionics technology and GPS satellite navigation, it’s a wonder that aircraft still rely on the Earth’s geomagnetic field to calculate aircraft heading. Magnetic compasses provide a reliable, independent backup system that doesn’t require electricity (in the case of a true standby ‘wet’ compass). And since the entire global aviation infrastructure (runways, charts, and air traffic control vectors) is built upon a magnetic reference frame, it’s no wonder it still exists. While GPS calculates direction based on your position over the ground, a magnetic heading tells you exactly where the nose of the aircraft is pointing, regardless of wind conditions or the reliability or validity of satellite signals.
The more popular avionics upgrades often consist of a GPS navigator along with some form of display system. Somewhere in the mix of installation kits, trays, and receivers exists a magnetic sensor of some capacity. Often relegated to the dark, far corners of the aircraft wing tip, tail cone or tailboom, these little devices are responsible for measuring the Earth’s magnetic field.
Unlike older, traditional mechanical compasses or simple two-axis flux valves, it captures full three-axis vector measurements (X, Y and Z) of the magnetic field. This provides the most precise digital indication of magnetic field strength and direction, even during maneuvers.
The internal microprocessor is programmed to compensate for the aircraft’s fixed magnetic interference. During initial setup, the installation technician performs a soft-calibration by executing a 360-degree turn while following on-screen instructions, allowing the unit to map local magnetic interference.
Suffice it to say, their position is critical and they are very sensitive to ferrous metals and electromagnetic interference.
On a recent installation on an Airbus AS350 series helicopter, the components from one serviceable helicopter were transferred over to a newly refurbished airframe. Using the original airframe as a template, parts were moved over and mounted as they had been in the original installation.
During the checkout and configuration process, pitch and roll calibrations were carried out followed by the “soft calibration,” which is more formally known as a magnetometer calibration.
The first calibration failed. This is often due to the aircraft’s proximity to metal buildings, other aircraft, or being on a concrete pad that contains a lot of steel. We moved the aircraft several times and each time the calibration failed. Clearly something was causing interference, to the point where it caused the magnetometer to read erroneously and fail the test.
We took a standby compass and held it to a fixed heading and moved around the magnetometer. It held its heading, which meant there were no problems with the mounting location. Typically, nonmagnetic hardware causes this, or close proximity to control cable or other ferrous structures or hardware.
We became convinced that something more related to EMI (electromagnetic interference) was likely to blame. Given the calibration was done with the engine running, we had to consider that electrical wiring (carrying higher levels of current) could be the cause.
Upon comparing the original airframe and the new one, we noticed the main battery location was different. The original aircraft held the battery in the right-side baggage area, forward of the tailboom. The new airframe had a modification done to move the battery to the tailboom, much closer to the magnetometer and with much longer, heavy gauge wires running through the tailboom. This became our immediate focus.
We relocated the magnetometer temporarily to see if we could create enough physical distance to enable the calibration to pass. This proved unsuccessful. Next, we monitored all aircraft systems much more closely during engine runs, and noted that whenever the oil cooler would cycle, this caused a heading anomaly warning. This was our “ah-ha” moment. The oil cooler draws a high current, and isolating this system — even briefly — caused the warning to cease.
Our long-term solution involved relocating the battery back to its original location and using an overbraid shield wire sleeve over the magnetometer’s wiring to further protect it from EMI. This combination proved successful — the calibration passed on the next try and we were able to resume ground runs and flight tests.
Examples like this always remind me that no matter how much you prepare or plan, something can always find a way to disrupt a project’s flow and stop you dead in your tracks. Thankfully, through a couple of days of trial and error, we found a resolution.
