With 10 passengers and one pilot, a Fairchild Industries Inc. SA226-TC (Metro II) lifted off from the runway at Perth (Australia) at 07.00 lt (local time) on this day in 2010. The charter flight was operated under instrument flight rules (IFR) and had Fortnam Mine (Australia) as its destination. Cruise flight was planned on flight level 210. {Level of constant atmospheric pressure related to the datum of 1013.25 hPa, expressed in hundreds of feet. FL 210 equated to 21,000 ft above mean sea level (AMSL).}
The missing right-hand cockpit side window (© ATSB)
65 nm (120 km) from Perth, while climbing through FL 205, a loud bang was heard, immediately followed by wind noise. A quick check revealed the cockpit side window (right-hand side) had failed and was missing, and that the carbon was depressurised.
Re The pilot put on his oxygen mask and activated the oxygen supply for the passengers. Subsequently, the power levers were reduced to flight idle, an emergency descent was initiated and ATC was informed by means of a distress call.
Remnants of the window (© ATSB)
Using the passenger address system and gestures the pilot informed the passengers to put on their oxygen masks. When reaching 9000 feet the aircraft was levelled off and the pilot and passengers removed their oxygen masks. A diversion back to Perth was initiated and completed uneventfully. The aircraft was fitted with single-pane acrylic cockpit side windows, the failed window had been installed in July 2006. Dual-pane acrylic cockpit side windows were available as an optional installation on the Metro II. In that configuration, the inner pane was installed to provide a quieter cabin and the outer pane was sealed and stressed to retain cabin pressure. The inner pane could also be fitted to the aircraft as an aftermarket addition, supplementing the original single-pane installation. The aircraft did not have the optional inner panes fitted.
The window part number identifies it as an inner pane,
which is not designed to load bearing. (© ATSB)
An investigation was launched by the ATSB (Australian Transport Safety Bureau). they discovered that the failed window was an inner pane, that was installed as a single outer pressurisation-load-carrying window. Contributing factors were;
The right cockpit side window was an inner pane that was not designed for use as an outer, pressurisation-load carrying window.
The incorrect installation of the right cockpit side window introduced cracks and stress raisers that weakened the window’s structural integrity.
The structural integrity of the right cockpit side window was significantly weakened by cracks that propagated from the retainer holes to the upper edge of the pane, before combining with additional lateral cracking between those holes.
The full ATSB report, which was the basis for this blog, can be accessed by clicking on the .pdf file below;