Australian safety investigators have issued an international advisory after fatigue cracking was found in the engine of Regional Express aircraft which lost a propeller while on approach to Sydney Airport last month.
The propeller flew off the right engine of the Saab 340B turboprop aircraft as the crew shut it down about 55 nautical miles southwest of Sydney Airport on March 17. The crew calmly continued on to land safely at Sydney after declaring a PAN-PAN, an alert a step below declaring an emergency
The propeller was recovered a few days later in bush about 8nm south-west of the airport.
Australian Transport Safety Bureau investigators found the propeller shaft had fractured after a fatigue crack had started in a mounting flange connected to the gear box.
The crack originated in a dowel hole which was corroded in parts and corrosion pitting was found near the fracture.
“Further work is ongoing to ascertain whether the corrosion or other factors contributed to the fracture initiation,’’ investigators said.
This was the first known critical failure of this type in this part of GE Aviation CT7-9B engine and investigators noted there are currently no maintenance requirements for routine inspection of the dowel pin holes.
“Any corrosion or cracking within the bore may go undetected until it progresses to the surface of the flange,’’ they said “Other than a visual inspection of the flange during propeller removal, inspection for surface defects (via magnetic particle inspection or dye penetrant inspection) only occurs when the (propeller gearbox) is disassembled for maintenance at a workshop specifically approved by the engine manufacturer.’’
Engine-maker GE Aviation has been actively involved in the investigation and the broken flange has been taken to the company’s laboratories in Cincinnati in the US for further metallurgical analysis.
GE is inspecting other propeller gear boxes and is expected to release additional maintenance and inspection procedures if they are deemed necessary.
Regional Express also quarantined all propeller gearboxes with propeller shafts of the same series and the ATSB has issued a safety advisory notice alerting other operators to the incident.
The report said the pilots noticed a problem with the engine on approach to Sydney and began to go through a checklist.
They noticed minor vibrations which worsened as the checklist progressed and started shutdown procedures.
It was while they were doing this that the propeller separated.
Fatigue cracking is often difficult to detect and can be due to corrosion or defects in the part. The only other instance of a propeller flying off a Saab aircraft occurred in 1991 on a plane operated by US carrier Comair.
In that case, the aircraft also landed safely after experiencing similar problems to the Australian crew. The failure was eventually traced to a sub-surface flaw introduced when an ingot was melted to make the part.
The ATSB investigation is continuing and will look at areas such as the way in which the failed assembly was manufactured, what corrosion protection was on the part’s surface and propeller gearbox maintenance procedures.
It will also look for “opportunities” for crack detection.
