What happened
On 30 December 2021, an Air Botswana ATR 72-600, registration A2-ABK, was performing a scheduled commercial flight from O.R. Tambo International Airport (FAOR) to Sir Seretse Khama International Airport (FBSKS) in Botswana. During the climb phase, while passing flight level 87, the flight crew noticed fluctuations in the number 2 engine torque and a significant 40°C rise in the Interstage Turbine Temperature (ITT).
As the aircraft reached flight level 130, the crew observed master caution and engine electronic control (EEC) fault indications. Shortly thereafter, an engine fire warning light illuminated for the right-hand engine. Following established emergency procedures, the crew shut down the engine and requested an immediate return to O.R. Tambo. The aircraft landed safely on Runway 03L without any injuries to the 74 people on board, though the engine sustained minor damage.
The investigation
The South African Civil Aviation Authority (SACAA) AIID investigated the mechanical failures following the incident. Analysis of the engine components and Quick Access Recorder (QAR) data revealed that the engine parameters were fluctuating due to mechanical damage within the engine module. Post-incident inspections of the right engine showed damage to the exhaust pipe, turbine blades, exit vanes, and the handling bleed valve electrical connector, with metal fragments found in the bottom cowling.
Findings
- The primary cause of the incident was the fracture of three PT2 blades in the hot section of the engine.
- These blade fractures led to a cascade of secondary failures, including damage to the stator outer shroud, the P10 connector, and the exhaust duct.
- A significant contributing factor was sulphidation, a chemical process where sulfur compounds in the fuel reacted with the chromium coating on the blades, leading to corrosion and subsequent structural failure.
- Although the engine had been updated according to a previous service bulletin to include chromium coating, this specific coating was found to be susceptible to the corrosive effects of sulphidation under operating conditions.