C-GLWS, a Boeing 737-700 aircraft operated by WestJet, was conducting flight WJA1469 from Palm Springs Intl, CA (KPSP) to Calgary Intl, AB (CYYC) with 5 crew members and 120 passengers on board. Just prior to the top of descent inbound to CYYC, the flight crew received an error message on the Flight Management Computer.
It was then determined that the first officer’s Primary Flight Display (PFD) was displaying erroneous altitude and attitude information. The autopilot was also affected by this system fault. After completing the checklist associated with the condition, the flight crew declared a PAN PAN and requested radar vectors for an approach.
Subsequently, an uneventful landing was conducted. Upon inspection, the operator’s maintenance found fault with the right Air Data Inertial Reference Unit (ADIRU), and proceeded to replace the unit.
The aircraft was returned to service.