SAS B734 at Copenhagen on Jul 23rd 2013, bird strike

Denmark's Havarikommissionen (HCL) released their final report concluding the probable cause was:

The serious incident was caused by a birdstrike to the left engine.

One or more Common Shelducks damaged the left air intake duct and the engine fan which resulted in abnormal vibrations and a fuel leak from the left engine.

A contributing factor to the serious incident was:

As a result of the altitude AGL of the birds, it was not possible for EKCH’s Wildlife Control Unit to observe and act against the flock of migrating Common Shelducks.

The HCL reported that the remains of a Common Shelduck were found on the runway with injuries consistent with an impact with an aircraft. DNA testing confirmed the remains inside the engine of the aircraft were of a Common Shelduck.

The HCL analysed, that the crew had no time to react when they sighted the flock of migratory Common Shelducks. Due to the height at which the birds were flying, the Wildlife Control Unit hunters could not observe the migrating birds, too.

A duck impacted the left hand engine causing fan blade tip damage and engine vibrations caused by the resulting fan disc imbalance. The fan blade tip damage caused further damage to the air intake and increasing vibrations.

The HCL stated: "The AIB DK is of the opinion that in case a birdstrike results in abnormal engine vibrations, it should be considered to shut down the engine to prevent further damage. Obviously considering that the remaining flight – single engine – can safely be performed. The reason for this is that it is unknown how a damaged engine will behave. In this case, the birdstrike caused a fuel leak from the damaged engine. However, the AIB DK finds it appropriate to have the damaged engine running as a safety precaution because except for the vibration level the flight crew did not observe abnormal engine indications."

A flock of Common Shelducks taken in 2009 (Photo: HCL/Lars Ulrik Rostgaard):