Originally Posted by
Dai Whirlybird
I hear that the WOG (wheels on ground switch) failed shortly after this airframe returned into service following the undemanded yaw incident - here's a question or two.....
Given that PAC (Power assurance check)s are done when light on the wheels, Is it feasible that the WOG micro-switch can cycle on and off in this configuration or is it isolated by software when conducting the PAC?
The WOG switch undoubtedly feeds it's status into the ADIU (Aircraft Data Interface unit) so is it possible that a cycling WOG switch or an intermittent fault with it could create confusion within the ADIU and generate spurious control inputs? What I don't know is whether the AFCS / AP configuration when conducting a PAC will permit or prevent this.
As a development but not necessarily an answer to my own question, I've just discovered this in the 429 Avionics manual. (8-2)
"The AP will perform a preflight self-test when the AP switch is turned ON
The preflight test requires the following:
• The TRIM switch is ON
• The helicopter is on the ground (weight-on-gear )
• Engine power is available and is set at low torque
• The other AP is not in the process of performing its self-test
During the preflight test, the AP verifies its ability to manipulate the actuators.
So there's a definite link between the WOG status and the AP testing the acuators.
As stated I do not know what state the AP was in at the time, but (to my mind and based on the foregoing) there definitely seems to be potential for a faulty WOG switch to impact on the AP self test of the actuators.
I guess, given the AAIB's lack of involvement that we'll never know.