Skyfox HEDARO CA25N, VH-PSM, 1 May 1999

As the Skyfox Gazelle climbed to approximately 250 ft above
ground level following takeoff, engine power reduced significantly
and engine vibration increased. The pilot landed the aircraft on
the runway distance remaining. An investigation carried out by the
aircraft's maintenance organisation revealed no defect that could
have led to the loss of power. Two days later, the aircraft
experienced similar engine problems, and the pilot conducted
another precautionary landing. This was followed by a third similar
incident 3 days after the second event. In both of those subsequent
incidents, investigation revealed no defect that could have led to
the loss of power. Although the reason for the engine power losses
could not be determined, it was found during the investigation that
the air intake system on the aircraft incorporated two lengths of
wire reinforced impregnated cloth hose, commonly known as "SCAT"
hose to transfer air from the air intake box underneath the engine,
via the rear of the engine, to the two carburettors on the top of
the engine. SCAT hose was approved by the Civil Aviation Safety
Authority as a part of the intake system for the Rotax 912 engine
on the Skyfox aircraft. The Civil Aviation Safety Authority has
subsequently issued advice on SCAT hoses in an information leaflet
entitled "SCAT/SCEET Hoses". The leaflet stated, in part, that
"SCAT and SCEET hoses manufactured by Thermoid RHD industries and
marketed as Aeroduct flexible ducts, are not approved by the
manufacturer for use as a piston-engine intake duct/hose. Contrary
to popular belief, Aeroduct "SCAT" and "SCEET" hoses, as detailed
in certain aviation parts supplies catalogs, are not approved by
the hose manufacturer for use in negative pressure applications.
This effectively prevents the use of Aeroduct SCAT and SCEET hoses
as engine intake hoses." A negative pressure application is one in
which the air pressure outside the hose is greater than the air
pressure inside the hose. In an engine intake application, air is
drawn through the hose by the engine past an air filter. In this
case, the air pressure outside the hose is greater than the air
pressure inside the hose. The hose is not designed to withstand
such a pressure difference, and it is possible that the hose could
collapse, causing an obstruction to the airflow to the engine.
Investigation revealed no indication that the SCAT hose had
collapsed during the occurrence.