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.

FMI: www.faa.gov