Manufacturers have increasingly equipped avionics and other systems with concentrically centered controls to conserve limited space in the instrument and control panels of aircraft. These are typically designed so that the larger outer knob closest to the face of the panel changes cursor position, selects information category, operating/display mode, or large value changes. The smaller inner knob is used to select among the information content, sub categories of the position selected with the larger outer knob, or fine value changes. The most familiar implementation of concentrically-centered control is probably in navigation and communication tuning heads (e.g., to set the frequency in the standby frequency indicator, turn the frequency selectors to set the frequency). The first digit is always 1. The outer knob sets the second two digits (10 MHz and 1MHz) in 1 MHz increments. The inner knob sets the fourth, fifth, and sixth digits (100 kHz, 10 kHz, and 1 kHz) in 25 kHz increments.
Navigation and Communication Tuning Head example
More complex variations exist, in which multiple and mixed functions are assigned to concentricallycentered 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 flightcrew.
Electronic Flight Instrument System Control Panel example of a complex (outer, middle, inner 3 concentric positions) Barometric knob
Drawbacks and Consequences
A drawback to the concentrical design is that settings/selections can be inadvertently changed. For example, rotating the navigation course selection knob resulted in an unintentional change to the barometric altitude setting. A few examples of common causes of unintentional selections include:
- Mechanical interference between two concentrically-centered knobs;
- Pilot accidentally rotating two knobs at once as a result of finger positioning errors and/or finger slippage;
- Pilot inadvertently selecting the wrong knob and subsequently fails to make appropriate corrections or fails to detect the inadvertent selection.
Reports from manufacturers and from pilots indicate that these incidents are occurring with some regularity. Examples of the possible consequences of inadvertent selection of concentrically centered controls include:
All personnel concerned should be aware of the potential for inadvertently changing existing, correct selections in avionics and other systems equipped with concentrically-centered controls (knobs). The importance of diligence in operating concentric controls as well as checking for unintentional changes to unrelated systems should be explicitly stressed during flight crew training.