Wed, Jun 30, 2010
Says "Concentrically-Centered Controls" Can Lead To Mistuning,
Inadvertant Changes
Many of the newest PFDs and MFDs
have a lot of functions that are accessed by what are known as
"Concentrically-Centered Controls", nested tuning knobs that often
have different functions depending on which direction they are
turned and which is turned first. The FAA Tuesday issued an
Information for Operators (InFO) document to alert operators of the
potential for inadvertently changing existing, correct selections
in avionics and other systems so equipped.
According to the InFO, manufacturers have increasingly equipped
avionics and other systems with concentrically-centered controls.
Their objective has been to conserve limited space in the
instrument and control panels of aircraft. In one reported
incident, rotating the navigation course select knob resulted in an
unwanted change to the barometric altitude setting. Such unwanted
selections can be caused by mechanical interference between two
concentrically-centered knobs, or by a pilot’s accidentally
rotating two knobs at once. Reports from manufacturers and from
pilots indicate that these incidents are occurring with some
regularity.
The most familiar implementation of concentrically-centered
control is probably in navigation and communication tuning heads,
in which the smaller, outer knob is used for selection of kilohertz
and the larger, inner knob for megahertz. But more complex
variations exist, in which multiple and mixed functions are
assigned to concentrically-centered controls, or controls with
different functions are located near each other. A change to one
function might affect another, totally unrelated function without
being detected by the crew. The FAA says that operators should
report instances of mechanical interference to the appropriate
equipment or system manufacturer; or they may submit reports
through the FAA Service Difficulty Reporting using a Malfunction or
Defect Report Submission Form.
The FAA recommends that directors of safety, directors of
operations, fractional ownership program managers, training
managers, flight instructors, and aircraft owners and pilots should
be aware of the potential for inadvertently changing existing,
correct selections in avionics and other systems equipped with
concentrically-centered controls (knobs). They should collaborate
to address that potential in operations and training of flightcrew,
stressing diligence in operating concentric controls, and in
checking for unwanted changes to unrelated systems.
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