What happened
On 27 March 2019, an Avro 146-RJ85, registration ZS-SSJ, departed O.R. Tambo International Aerodrome for a scheduled flight to Upington. The aircraft was carrying 25 passengers and 4 crew members. Shortly after climbing to 6,500 feet, the crew experienced a loud bang and a sudden rightward yaw.
Flight data indicated that the number 4 engine suffered high vibration levels, a drop in low-pressure compressor speed (N1), increased exhaust gas temperature (E/GT), and a decrease in high-pressure compressor speed (N2). Following established procedures in the Quick Reference Handbook, the pilots shut down the affected engine. The crew declared a "PAN-PAN" emergency and requested an immediate return to O.R. Tambo. The aircraft landed safely at 1349Z with no injuries to any occupants.
The investigation
An investigation by the SACAA AIID examined the engine's maintenance history and flight recorder data. The analysis of the Honeywell D-FDR confirmed that the N1 speed dropped abruptly at the same moment the lateral accelerometer recorded a significant spike in vibration levels.
Investigators reviewed the maintenance records for the number 4 engine, which had been in service for only 230.16 hours since its last shop visit in August 2018. The inspection confirmed that all applicable Airworthiness Directives and Service Bulletins had been performed, and the engine was within its scheduled limits for borescope inspections.
Findings
Technical analysis determined that the engine failure was caused by the failure of a first-stage compressor blade, which had developed a crack at its root. Upon separation, the blade moved toward the rear of the compressor section, causing consequential damage to the low-pressure and high-pressure compressors, as well as various internal engine components. It is believed that the root crack was likely the result of metal fatigue.