What happened
A Challenger 604 corporate jet crashed into mountainous terrain near Shahr-e Kurd, Iran, during a flight from Sharjah, UAE, to Istanbul, Turkey. The flight entered the Tehran FIR at approximately 13:26 UTC. While climbing to a requested altitude of FL380, the crew observed a discrepancy between the left and right airspeed indicators, with the difference exceeding 10 knots. This discrepancy triggered an 'EFIS COMP MON' caution message on the EUI.
During the climb, the crew reduced engine thrust to idle. Shortly after, an overspeed warning activated, indicating the Mach number had exceeded 0.85. Rather than following the Quick Reference Handbook (QRH) to identify the reliable Air Data Computer (ADC) and airspeed source, the pilot reacted to the warning by reducing engine power. This action caused the aircraft's actual airspeed to drop into a stall condition. Although the co-pilot attempted to initiate the abnormal procedure for the airspeed discrepancy, the process was interrupted multiple times.
As the aircraft lost speed, stall warnings, stick shakers, and stick pushers activated. The pilot mistakenly reacted to the stall by pulling on the control column, believing the aircraft was in an overspeed situation. This led to significant pitch and roll oscillations. The crew disengaged the autopilot, and the engines eventually flamed out due to the stall. The aircraft then impacted the mountains, resulting in 11 fatalities.
Findings
Investigation into the accident identified several contributing factors related to instrument reliability and crew response:
- The crew failed to follow established QRH procedures to validate and select a reliable airspeed source following the initial discrepancy.
- The pilot's incorrect reaction to the overspeed warning caused the aircraft to enter a stall.
- Environmental factors, including moderate to severe turbulence and icing conditions, may have caused ice crystals to obstruct the left-hand pitot tube.
- The aircraft had been parked in dusty conditions at Sharjah Airport for three days without pitot covers, suggesting that dust accumulation inside the pitot tube was a possible cause for the instrument error.