AHRS Heading Shift After LED and USB Installation
A light single with an AHRS and a remote magnetometer in the tailcone has just returned from installation of an LED landing light and a new USB charging port. The pilot reports the HSI heading is 040° while the standby magnetic compass reads 012° on the ramp. When the LED landing light is switched on, the HSI swings another 15°; when the USB port is used, it swings 8°. Taxiing 50 m away from the hangar reduces the error to 3° with all lights off. A compass swing was last done 14 months ago; no ferrous cargo is carried. The AME has a compass rose, a milliohm meter, and the AHRS maintenance manual. The aircraft is used for night VFR and IFR training. The magnetometer is mounted in the tailcone near the new LED light wiring.
1. Separate external magnetic interference, aircraft electrical interference, and magnetometer calibration error in this scenario.
Read the worked answer
There are at least three superimposed effects. The change when taxiing away from the hangar is position-dependent and indicates external magnetic interference from the hangar or ramp steel; it is not an aircraft defect by itself. The 15° shift when the LED light is switched on and the 8° shift when the USB port is used are load-dependent and point to aircraft electrical interference, such as current flowing in a poorly grounded or poorly routed wire near the magnetometer. The residual 3° error with all loads off after moving away may be normal compass deviation, residual calibration error, or a small installation effect. The 28° initial error is not explained by calibration alone because it changes with electrical load. The AME should not simply do a compass swing and sign the aircraft off; the load-dependent errors must be isolated first.
2. Describe the tests and corrective actions required before return to service.
Read the worked answer
On the compass rose, with the aircraft in a known heading, switch each electrical load on and off individually and record the HSI change. Confirm the LED light and USB port are grounded to the airframe per the approved installation, check for current returning through the tailcone structure, and inspect the magnetometer wiring for shielding, twisted pairs, and separation from the new light cable. Re-route or repair any wiring that causes a heading shift. Check the magnetometer mounting and connector for corrosion or looseness. Then perform the AHRS magnetometer calibration and a compass swing with engines and avionics operating as required by the manual. Acceptance requires stable heading with all normal loads on and deviation within the approved limits. If the installation cannot be made compatible, the LED or USB circuit must be modified or removed.
3. Is the aircraft airworthy for the next training flight if the heading error is 3° with all loads off but 15° with the LED light on?
Read the worked answer
No. A 3° residual with all loads off may be within compass-swing tolerance, but the 15° shift when the LED light is on is a load-dependent heading error that will affect the HSI during night or IFR training. The pilot would be given misleading heading information whenever the light is used. The aircraft should not be released for the training flight until the electrical interference is corrected and the AHRS/compass system is checked with the light and USB port operating. After repair, perform the compass swing and confirm the heading remains stable across the electrical load range. If the fault cannot be corrected, the affected circuit must be disabled or the aircraft kept out of service for IFR operations.