What happened
On December 28, 2019, a private Cessna 560XL, registration LV-FQD, was conducting a general aviation flight from San Fernando International Airport to Estancia La Venancia in Buenos Aires. While descending through flight level 33,000 feet, the crew experienced a significant increase in Inter-Stage Turbine Temperature (ITT) in the right engine, prompting the pilot to shut it down. Approximately 25 seconds later, the left engine experienced a similar temperature surge and subsequently stopped.
Following the loss of power, the crew declared an emergency with Mar del Plata air traffic control and performed an emergency landing in an unprepared field. While the aircraft sustained significant damage to the airframe and engines, all 9 occupants (two crew members and seven passengers) evacuated the aircraft without injuries.
The investigation
The investigation examined the engine performance data recorded by the Engine Control Units (EEC) and Data Collection Units (DCU). Analysis of the Pratt & Whitney engine diagnostic data revealed that as the aircraft crossed 31,600 feet, the N2 speeds of both engines began to decline simultaneously. In an attempt to maintain target N2 speeds, the EECs automatically increased fuel flow. However, the engines continued to run below target speeds, causing the ITT to rise.
The investigation also looked into the activation of the engine anti-ice systems. Data showed that the air bleed valves were activated on both engines roughly 80 seconds after the initial N2 decline. Following this activation, the N2 speeds dropped rapidly. The investigation focused on the presence of high concentrations of ice crystals within convective cloud layers, a phenomenon known as Ice Crystal Icing (ICI).
Findings
- The primary cause of the engine malfunctions was Ice Crystal Icing (ICI). In convective environments, high concentrations of ice crystals can strike the compressor blades. As these crystals enter the warmer engine environment, they partially melt and then re-freeze, creating ice deposits that obstruct airflow to the combustion chamber.
- This obstruction altered the air-to-fuel ratio, leading to the observed increase in ITT and the subsequent loss of engine power.
- The engines were operating in an area with high liquid water content and ice crystals, typical of convective cloud structures, which are difficult to detect via standard weather radar.
- The activation of the engine anti-ice system, while intended to prevent ice buildup, contributed to the rapid decline in N2 speeds due to the introduction of bleed air into the engine cycle during an already unstable combustion state.