Weak ice-panel bonds led to Austrian Fokker field-landing

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German investigators have concluded that insufficient bonding of ice-protection panels in the engines of an Austrian Airlines Fokker 70 led to their breaking away, impeding thrust and forcing the jet to make an emergency landing in a field outside Munich.

The aircraft had been operating a service from Vienna to Munich on 5 January last year, with 28 passengers and four crew members, when it encountered icing conditions while descending to flight level 100 on approach to the German airport.

German air accident investigation agency BFU says that during the Fokker 70’s prolonged operation in moderate icing conditions and with low engine thrust, ice developed on the low-pressure compressor rotors of both Rolls-Royce Tay 620 powerplants.

Ice-impact panels on the engine – bonded into the intake of the engine and located between the low-pressure compressor rotor and the outlet guide vanes – are designed to protect the engine casing from ice falling from the rotor.

But BFU says that shedding ice, as well as ice-induced vibration of the powerplants, caused the bonded joints of these ice-impact panels to fail. It states: “Investigation of the ice-impact panels and the associated bonding surfaces on the engines clearly showed an adhesion failure.

“This means that neither the adhesive nor the materials to be bonded failed, but that the bonding between the adhesive and the materials was insufficient. In this case the reason for it could only be an insufficient preparation of the bonding surfaces.”

BFU says that the engine fan case was “not prepared correctly” prior to the adhesive being applied and that evidence shows that the bonded joints had separated some time before. But it stresses that instructions for maintaining and repairing the ice panels were unclear and difficult to follow.

Once the ice-impact panels in the Fokker 70’s engines had broken away, they became trapped in front of the low-pressure compressor vanes, affecting the air-flow in the bypass duct, and resulting in the engines only being able to generate low thrust.

As the aircraft descended towards Munich, the engine pressure ratio of the left-hand powerplant fell from 1.5 to 1.0 and, within 30sec, that of the right-hand engine also started falling, dropping from 1.4 to 1.0 over a four-minute period – a reduction which, say the investigators, was “independent of the thrust setting”.

By this time the crew, concerned about earlier engine vibrations, had declared an emergency and requested an immediate landing. But no warnings accompanied the loss of thrust from the engines and the seriousness of situation did not become fully apparent until the aircraft had descended to about 3,500ft, when the crew noticed that the engine pressure ratio did not increase as the throttle levers were pushed forward.

Realising that the aircraft was unable to maintain the glideslope, the crew told air traffic controllers that they would not reach the runway. The jet touched down at about 08:16, some 2.5nm short of Munich’s runway 26L, with the landing-gear partially extended. It slid for 220m before coming to rest, severely damaged, although none of the occupants was badly injured.

BFU has advised German civil aviation regulators that the installation and repair procedures for ice-impact panels in the Tay engines should be clarified, and ensure that these installations are such that a failure will not lead to a significant loss of thrust from the powerplant.