What happened
On 4 December 2019, a Beechcraft King Air B200, registration ZS-MIN, was operating a domestic charter flight from Virginia Aerodrome to Hoedspruit Aerodrome. The aircraft was carrying two crew members and nine passengers under Instrument Flight Rules (IFR).
While cruising at flight level 250, the crew approached cloud cover. In an attempt to prevent ice accumulation, the crew deployed the engine ice vanes. However, shortly after this action, the number 2 engine experienced a compressor stall. This event triggered an Inter-Turbine Temperature (ITT) exceedance, with temperatures reaching 950°C, significantly surpassing the maximum allowable limit of 850°C. Following established emergency procedures, the crew reduced power and shut down the affected engine. The flight was diverted to King Shaka International Aerodrome, where the aircraft landed safely with no injuries to the 11 people on board.
The investigation
SACAA AIID investigators examined the flight sequence and the mechanical state of the engines. The investigation focused on the timing of anti-ice measures and the physical condition of the engine components. Technical analysis of the engine logs and borescope inspections revealed that the aircraft was operating in ambient temperatures of -15°C, well below the +5°C threshold where ice vane activation is required.
Post-incident inspections of the right-side engine via borescope identified open cracks on the compressor turbine (CT) blade tips. Investigators also reviewed the maintenance history, noting that while the aircraft had undergone recent inspections, the compressor stall was directly linked to the engine's internal environment during the cruise phase.
Findings
- The primary cause of the incident was that the engine ice vanes were not activated in time during operations in temperatures below +5°C.
- This delay allowed ice particles to enter the engine, which disrupted the airflow within the compressor section.
- The resulting airflow disturbance led to the compressor stall and the subsequent ITT exceedance.
- A contributing factor may have been the presence of pre-existing cracks on the compressor turbine blades, which likely exacerbated the temperature spike upon the ingestion of ice particles.
- The crew's failure to adhere to standard operating procedures regarding anti-ice activation was noted as a contributing factor.