Business & GA, Commercial

Avionics System Design: Flight Management Systems

By Walter Shawlee 2 | January 1, 2001
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Written and spoken languages have been with us for a long time, but understanding what is said or written is not always clear. This often is the case in the crowded aircraft cockpit, where labels, displays and controls all compete for panel space, and acronyms and cryptic abbreviations run unchecked.

Once, some standardization of abbreviations and markings existed under Mil standards, which governed everything from visual contrast to type size and font style. This standardization essentially has been eclipsed by whatever the equipment manufacturer now thinks makes sense and fits, which may not be ideal from the user’s viewpoint. So a common complaint among equipment users is that the operation of the equipment can not be readily inferred from looking at the controls and markings. This represents an operational problem, impacting support costs, safety and readiness.

Often, panel and control markings are blocked from the user’s field of view, because they are placed below or behind the projecting control they identify. Most avionics equipment resides away from the crew’s sight line (overhead panels being the one exception). Frequently, this is not fully considered in panel marking. The result: physical controls that are effectively "blank" to the user. Generally, top marking or marking on the control itself is more legible. Alternatively, a bottom marking with a large clear space permits unobstructed viewing of the legend from an angle.

The user’s unfamiliarity with terms and operations specific to some equipment creates still more technical difficulty. I learned this while once looking after a fleet of helicopters. All the high-time commercial pilots told of snags that were really system training issues. They simply didn’t know what certain terms, functions and abbreviations meant in English. What the heck do VOX, ICS, CTCSS, USB, DTMF, RPTR and SIMPLEX mean to them in operational terms? And how do their functions affect what the pilots try to do with a system? I learned an important lesson that has always stayed with me: Absolutely nothing is obvious to the user.

Designers have the bad habit of putting the minimum possible text or an uncommon term on a control. They use two- or three-letter text labels like DTK, XTK, TX, LVL, ID, GPS, ADF, VOR, ILS, A/P, ATC, ALT, AUX, DIR, IFF, ICS, BRG, HDG, and the ever popular VOL which manages to actually get a useful vowel. No doubt VOL (volume) should be changed to VLM to increase aggravation in the cockpit.

Short cockpit labels have a historical origin dating back to early lighted panels. Illuminating lots of text on a transilluminated panel was difficult and expensive. Available room was sparse, but current technology, especially surface mounted LEDs, makes backlighting simpler and far less expensive than old, embedded lamp panels. So, the pressure forcing marking with shortened text has eased considerably.

The marking issue takes a big nosedive towards total incoherence when non-flight personnel–forest fire spotters, paramedics, etc.– have to use the equipment as part of their work. These people likely do not understand many radio functions (like tone-coded subaudible squelch in a duplex repeater system), nor do they have a need or desire to understand them. They just want to talk on the radio, and be understood at the other end.

Few pilots happen to be great radio mavens either. The setup of many communication and navigation systems can easily lose them. I still vividly recall pilots trying to operate a new combined GPS/com system at a recent trade show. They succeeded only in getting stuck in a series of mysterious menu dead ends.

There is an important MSG hiding there for somebody. Fixing this overall situation is complicated, as it requires cosmetic improvements inside the cockpit, system optimizing for better understanding, and very importantly, basic training for potential users.

Years ago, I found it was impossible to mark everything on the panel of tactical FM radios. I wrote a small FM radio guide for customers to explain how the radios worked and what the terms and concepts meant, in language that was not technically oppressive. It was an amazingly popular booklet; we gave away thousands of copies at trade shows over the years. After releasing the booklet, flight problems reduced significantly and, more importantly, people got far better use from, and were more comfortable with, their equipment.

Reading manuals in the cockpit while flying isn’t always practical, so an effort is critical in the early design stage to insure users can understand the operation of a system and its controls. On-screen help for systems with displays would be a huge improvement over a forgotten or lost manual. Alternatively, clear and easy-to-read panel markings, as well as logical system operation of the equipment itself, become essential. Perhaps most dangerous is a system whose function is illogical, and whose correct operation remains an ongoing mystery to the flight crew. Some accidents and incidents described as "pilot error" should more correctly be labeled "designer error."

I have found it sobering to use a piece of specialized equipment from an unrelated field, like a new cell phone, and discover I can’t get it to do anything useful. This inspires me to do better on that "obvious" interface. You may want to try that, too, to help recalibrate your perspective.

Walter Shawlee 2 welcomes reader comments. He can be reached by e-mail at

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