Erratic airspeed indications, 241 km north-north-east of Perth Airport, Western Australia, on 7 September 2006, VH-NXI, Boeing 717-200

On 7 September 2006, a Boeing 717-200 aircraft, registered VH-NXI, was being operated on a scheduled service from Perth to Port Hedland, WA. Approximately 10 minutes after takeoff, the crew became aware that both the pilot in command's and copilot's computed airspeed displays had become erratic. The pilot in command's computed airspeed rapidly dropped, while the copilot's airspeed gradually increased. During the event, the pilot in command's displayed airspeed dropped as low as 115 kts, while the copilot's computed airspeed reached a maximum of 348 kts. Both the stall warning and overspeed warning sounded. The crew assessed the accuracy of the Integrated Standby Instrument System (ISIS) and used it for air data information.

The crew carried out the non-normal checklist from the Quick Reference Handbook. As part of the checklist, the crew cycled the air data heat switch. Approximately 15 minutes after the first signs of irregularities, both the pilot in command's and copilot's airspeed displays returned to normal and both airspeeds matched. The aircraft returned to Perth and conducted a normal approach and landing.

Analysis of the recorded data indicated that both the pilot in command's and copilot's pitot probes had iced up, which resulted in erratic airspeed indications and erroneous altitude and Mach numbers. The accuracy of the ISIS could not be determined from the recorded data.

The air data heat switch was removed from the aircraft and examined by the switch manufacturer. The switch manufacturer concluded that the latching mechanism in the switch was damaged, when the lamp capsule was forcibly opened while the switch was in the latched position.

It is possible that a piece of the broken latching mechanism jammed the switch in the OFF position, which resulted in no heat being supplied to the air data sensors, including the pitot probes.