Solenoids and EMI

There are 3 solenoids used on the Airbus flight deck – one for each sidestick, and another for the gear lever panel. The sidesticks are locked in a neutral position by the solenoids when autopilot is engaged. The solenoid in the gear lever panel is activated when the aircraft is airborne (wheel strut pressure logic) and allows the movement of the landing gear lever from the down to the up position.

Airbus sidestick Solenoid; the solenoid activates with autopilot engaged preventing unintended sidestick movement

The 3 solenoids operate on 28 VDC. To simulate the solenoid operations, the power supply is logic controlled by a relay board connected to the avionics software. Interfacing the solenoids is reasonably straight forward, however there was significant electromagnetic interference (EMI) when the solenoids (especially the gear lever) were activated. When the gear lever was pulled to the up or down position, the ECAM (centre main instrument panel) displays would momentarily disconnect. This is not favourable, especially during a critical phase of flight (take off).

Attempts to ground the gear lever to the aircraft MIP chassis did not fix the problem. Using an isolated power supply was also not helpful. After hours of troubleshooting a second look at the aircraft schematic manual (ASM) determine a protection diode was required at the point of supply.

Aircraft Schematic Manual – LGCIU (see right) supplies 28VDC to the gear lever with a protection diode in situ

A protection, or flyback diode prevents inductive loads due to voltage spikes. In the case of the gear panel, pulling the gear lever out induces a voltage spike causing the interference with the display screens. Diodes are a simple and effective solution. By bridging the diode in parallel with the solenoid supply and ground the issue was resolved and EMI was no longer observed.

Diode 1N5822 Schottky 40V 3A DO-27 has been implemented into the gear solenoid voltage supply / gnd

One thought on “Solenoids and EMI

  1. Pingback: Gear Lever Pinouts | Soarbywire

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