What happened
On 7 August 2008, a Raytheon Premier I, registration G-FRYL, was performing a non-revenue commercial flight from Copenhagen to Farnborough. While cruising at 40,000 feet, the aircraft encountered significant turbulence near a thunderstorm. During the subsequent descent, the crew observed a discrepancy in airspeed indications, followed by a rapid and continuous reduction in airspeed across all primary and standby instruments.
In an attempt to manage the anomaly, the commander reconfigured the air data source. However, the airspeed indications continued to drop, eventually reaching zero. This was accompanied by the loss of the Multi-Function Display (MFD) and the Primary Flight Displays (PFTS), leaving the crew to rely on standby instruments to navigate the remainder of the flight. The crew successfully landed the aircraft without further incident.
The investigation
The AAIB investigation focused on the cause of the air data degradation and the subsequent loss of electronic flight instrument system (EFIS) displays. Investigators found that the aircraft had been parked outside in recent days, exposing the pitot probes to the elements. During the examination of the aircraft, approximately 15 ml of water was removed from the right pitot/static system via nitrogen purging.
Testing performed on the aircraft demonstrated that a blockage in the right pitot line, combined with an increase in static pressure, could replicate the loss of airspeed indications and the activation of the stall warning. The investigation also examined the sequence of display failures, noting that the crew's use of the display reversion switches likely contributed to the momentary blanking of the screens.
Findings
- The primary cause of the airspeed loss was a blockage in the right pitot system caused by moisture ingress which subsequently froze.
- The loss of all three EFIS displays was attributed to the crew's reconfiguration of the air data source and the use of reversionary modes.
- The aircraft had been exposed to rain and wind while parked, which allowed moisture to enter the pitot-static system.
- The crew did not consult the abnormal and emergency checklists, which may have helped them prepare for the loss of flight control functions and pressurization control.