Warning Systems: Name that tone!

There is an apocryphal story of a pilot who was warned repeatedly on short final that his gear was unsafe. Despite this, he landed gear up. When asked why he disregarded the controller’s warnings, he said he could not hear the tower because of the loud horn sounding inside the aircraft.

Then there was the obstetrician flying his Beechcraft Bonanza who, on a long final, heard a beeping tone. Those were the days when many physicians carried a beeper with no voice or text. This doctor pilot repeatedly pressed the beeper button to silence it. He expected it was a call concerning a woman in labor, and he needed to land. He was unsuccessful in quieting the beeping sound, but did successfully land gear up.

And those are just some of the problems with aural warnings. When the pilot is under duress or when attention is directed elsewhere, extraneous beeps, buzzes, clicks, and horns may be disregarded or mistakenly attributed. Most of us cannot describe each generated sound, but we assume we’ll know what the sound means when we hear it. More confusing are two or more warning tones triggered simultaneously­—for example, a stall warning superimposed upon an unsafe gear warning.

Aircraft have test modes for aural warnings, but no ability to sound all the possible combinations. And although an alert may sound when pushing a test button or cycling through a rotary annunciator test switch, there’s never a complete test of whether an alarm will be triggered when a particular discrepancy really occurs.

Know how the warning and annunciator test modes function in your aircraft. That sounds simplistic, but you may be surprised. In my Daher TBM 900, pressing the overhead alert test button not only produces a combination of tones, but while in flight disconnects the autopilot.

Airframe and aerodynamic tone warnings were first instituted for stalls and unsafe gear conditions. Only as avionics advanced to the computer age were voice warnings instituted. Considering that only a few dollars will buy a greeting card that sings a song, voice technology is inexpensive enough to be in every aircraft. That’s a good thing, because a human voice eliminates a vital mental step—translating a warning into words. However, an increasing vocabulary among different aircraft types raises additional considerations: standardization, priority, repetition, generic annunciation, and the ability or inability to cancel.

Traffic and terrain advisory systems, the most recent avionic cockpit additions, have standardized vocabularies. The list has become so long that the FAA has seen fit to address warning
priorities. Advisory Circular AC 25-23 advises that terrain warnings are favored over traffic advisories. Within the terrain group, wind shear is primary, followed by pull-up warnings. This is followed by terrain closure without gear or flaps. Traffic advisories have a lower priority. However, the FAA does not provide priority guidance concerning other warnings.

An aural warning must be repeated, because a single unexpected click, buzz, or horn may not be consciously recognized and may be disregarded if not repeated. A voice warning has the advantage of being specific and more difficult to disregard, but repetition must still be considered. For example, a stall or gear warning should repeat—and not be cancelable, although some are—until the condition is rectified. An autopilot-disconnect warning is reasonably a one-time call. The FAA considers a cabin altitude warning a one-time warning. See AC 25.1322-1 for the specific wording.

While multiword voice alerts are more specific, they are more likely jumbled and truncated if there are competing alerts. The FAA says that an active aural alert should finish before another aural alert begins—unless interrupted by a higher urgency alert if a delay of the second-higher priority alert will impact timely crew response. If the condition that triggered the first interrupted alert is still active, that alert may be repeated once the second, higher-urgency alert is completed. See FAR 25.1301(a) for details.

If more than one aural alert requires immediate awareness, and interrupting the alerts affects safe operation, the FAA requires an alternative means of presenting the alerts to the flight crew. This is typically a visual alert system. Two levels are standard: warnings indicating a threat that requires immediate action, and cautions requiring immediate awareness, but subsequent corrective action. When an anomaly occurs, the crew is directed to the warning panel or crew alerting display by an illuminated annunciator, which is often accompanied—you got it—by still another aural signal.

Warnings should be audible in the cockpit and also in crew headsets. But cabin speaker volume must be considered relative to ambient noise. When early traffic systems were installed, the “Traffic, Traffic” warnings were so loud they could be heard by passengers.

Current turbine aircraft have several classes of alerts that deal with airframe, aerodynamic, autopilot, terrain, traffic, and other anomalies. These are described in the airplane flight manual and pilot’s operating handbook (POH). However, a comprehensive glossary of all tone and voice warnings is not usually provided. To further complicate matters, the advent of integrated avionics has given rise to a separate avionics POH, which may contain only some of the alerts. There should be only one POH containing everything, and that alerts should not be segregated artificially by the system issuing the warning communication.

The answer to the cacophonous babel of sounds and lights might be a specifically tailored training program. But I am unaware of any audio training in the combination of alert sounds and phrases specific to each aircraft. There are opportunities here. This training could be easily accomplished with an iPhone or computer application. After some practice, a pilot could quickly learn the game of “Name That Tone.”

Let there be no doubt. Alerts can be confusing, but alert systems are also fallible. We ignore more thorough training at our peril. There is the story of a pilotless airplane of the future informing frightened passengers all is under control. “This is your autopilot speaking....This is your autopilot speaking....This is your autopilot speaking....”

Author Dr. Ian Blair Fries is an orthopedic surgeon and TBM 900 owner.