Sophisticated on-board warning systems have been installed on every airplane since the Wright Flyer. These highly integrated wonders were known from the beginning simply as — you guessed it — pilots. But it wasn't long before aircraft evolution rubbed up against the inherent limitations of an aviator's eyes, ears, nose, and seat of the pants for monitoring important in-flight events. Impending stalls, loss of cabin pressure, an engine fire not visible from the cockpit — any number of things could be overlooked or misinterpreted by a pilot at a critical time. Thus was born the impetus to develop dedicated on-board warning systems.
The warning systems on early generation turbine aircraft were largely nonintegrated; that is, there was no master system that watched over all of the aircraft's vital signs. Warning lights were situated almost anywhere in the cockpit. Loss of hydraulic fluid, for instance, might be indicated by the illumination of a Low Pressure light on the forward instrument panel, while an electrical fault might cause a different light to illuminate on the overhead panel. This scatter-shot approach got the job done, but it wasn't the most elegant solution to keeping pilots informed. For instance, it didn't always impart information about the relative importance of a particular warning. A fault might represent a serious problem or merely a minor nuisance, something the pilots wouldn't know for sure until troubleshooting was complete.
To instill some sense of order, designers developed the master annunciator panel. This is a collection of clearly labeled annunciator lights, placed together in a single location. If a system fault of some kind is detected, a prominently located master caution or warning light illuminates, which draws the crew's attention to the annunciator panel. There, individually labeled, color-coded lights indicate the exact problem and its relative significance. By convention, a red warning light signifies a serious problem requiring immediate crew action. Amber-colored caution lights, on the other hand, indicate trouble of a less urgent sort that merely requires immediate crew awareness. Blue is usually reserved for lights that are informational only — so-called advisory or agreement lights, depending on the aircraft manufacturer's penchant for naming things. These blue lights let the crew know that something is working as it is supposed to (engine anti-ice or fuel crossfeed valves, for example), and may also serve as a reminder to turn it off when no longer needed.
Since turbine aircraft come with a great deal of built-in system redundancy, it stands to reason that the majority of possible problems and their associated lights would fall into the caution category. A scan of a middle-aged jet cockpit that makes use of an annunciator panel, such as the DC-9, reveals this to be so. Its annunciator panel contains only a handful of red warning lights. Amber lights take up most of the space, with just a few blue advisory lights sprinkled in for good measure.
Late-generation turbine aircraft, such as Boeing's 777, take cockpit warning technology to new levels. Its Crew Alerting System (CAS) generates various kinds of aural, visual, and tactile cues that draw attention to situations that the crew should be aware of. An important part of CAS is the Engine Indication and Crew Alerting System, or EICAS. This system monitors hundreds of engine and aircraft system parameters. Like master annunciator panels, it provides three different levels of information to the crew: advisories, cautions, and warnings.
CAS first gets the crew's attention by illuminating the master caution or warning lights located on either side of the forward instrument panel. Depending upon the situation's urgency, CAS may also emit a beeping tone, sound a bell or siren, or generate a synthetic voice message to ensure that the crew is tuned in to the problem. (Hearing "Engine fail!" during the takeoff roll is especially attention getting.) At the same time, a color-coded text message is sent to the EICAS liquid-crystal display screen, located in the center of the instrument panel. The message describes the existing condition. In the case of a fire in the left engine, for instance, the red text message would read "Fire Engine L." This happens to coincide exactly with the title of the emergency checklist needed to handle the situation. On an adjacent screen, EICAS presents the crew with a list of pertinent checklists (if more than one is required). The crew then clicks on the desired title with a mouse-like controller and the full checklist appears on screen. A system schematic (or synoptic, as Boeing calls it) can also be generated on screen if so desired, to help the crew better understand just what components have been affected.
With so much to monitor, the medium — how warnings are presented — becomes nearly as important as the message. Too much at the wrong time can be a distraction to pilots, more a hindrance than a help. Not enough can lead them down the primrose path towards wrong decision making. Just the right touch, however, and the crew's situational awareness will be maximized, which is what warning systems are really all about. For this reason, some non-critical alerts on the 777 are inhibited at the beginning of the takeoff roll until the aircraft has reached a safe altitude. This is to prevent pilots from initiating high-speed aborted takeoffs for minor problems that could be easily taken care of once safely airborne. The same philosophy of inhibiting certain alerts applies on approach too — again, to help avert pilot errors at a crucial time. Although computer-generated voice warnings are used sparingly on the 777, manufacturers generally consider the tone, volume, and even apparent gender of computer voices, which may vary, depending on the nature of each warning.
What happens if two or more critical conditions are detected simultaneously? CAS will issue alerts in a predetermined priority sequence. Suppose, for instance, you're a 777 captain having a really bad day at work. Shortly after takeoff, GPWS senses that your aircraft has begun to sink toward rising terrain. At the same time, the reactive wind shear warning system detects that the twinjet has encountered a microburst, just as the right engine catches on fire. To make matters worse, another aircraft is approaching on a collision course, something which causes TCAS to generate a resolution advisory (RA), warning of the traffic conflict. Presuming this admittedly unlikely scenario is not some kind of bad dream, what will the sequence of warnings be? According to Boeing spokesman Brian Kelly, voice warnings will sound in quick succession for wind shear, GPWS, and TCAS, in that order. While this is happening, the engine fire will trigger a fire bell and an EICAS text message simultaneously. Meanwhile, the traffic conflict as detected by TCAS will be displayed on the primary navigation display screen. All that remains is for you and your first officer to deal with this flood of information. (Hint: "Make it so, Number One," isn't an acceptable solution.)
All the warnings in the world won't help if they are ignored or misinterpreted by the crew. In the past, many controlled flight into terrain (CFIT) accidents have been attributed to crews failing to heed GPWS warnings until it was too late. Specific training in how to respond to GPWS and wind shear warnings is one way pilots today are learning to avoid this kind of accident scenario. Other sorts of accidents have occurred because pilots were confused about the nature of a system abnormality, causing them to take inappropriate actions in response. Sophisticated crew-alerting systems help to alleviate such confusion by offering pilots a clearer picture of just what is going on.
No discussion of warning systems would be complete without mentioning one with which all veteran airline travelers are familiar, the lavatory smoke detector. For anyone tempted to light up in the privacy of the aircraft lav, be forewarned that it too is hooked into CAS on the 777. Activation of a smoke detector will generate an EICAS advisory message to the crew. When it comes to the latest in aircraft warning systems, Big Brother is most certainly watching.
Vincent Czaplyski, AOPA 690264, holds ATP and CFI certificates. He flies as a Boeing 737 captain for a major U.S. airline.
Here are some of the sounds made by various cockpit warnings.
Engine or APU fire: FARs require that engine and APU fires be indicated by a bell accompanied by red fire warning lights. No other cockpit warning uses the bell sound.
Stall: Approach to stall is indicated by a stick shaker, which physically vibrates both control columns, creating a rattling or shaking sound when aircraft speed is a minimum of 7 percent above the actual stall speed. Some stall warning systems also generate synthetic voice warnings ("Stall!") to indicate an approaching stall.
Overspeed: An overspeed "clacker" sounds when a limiting mach or airspeed is exceeded. Some aircraft also combine clackers with synthetic voice warnings that further clarify what speed is being exceeded (e.g., "Slat overspeed! Flap overspeed!").
Autopilot disconnect: Various kinds of siren, klaxon, or chime sounds, accompanied by red warning lights, signal that the autopilot has disconnected. On some aircraft, warning lights illuminate, but there are no aural warning sounds.
Stabilizer trim movement: On some aircraft continuous beeping or clicking sounds indicate that stabilizer trim is operating. Others, such as the Boeing 757, have no aural indication of trim movement.
Landing gear: A horn sounds and appropriate gear position indicator lights illuminate when an unsafe gear configuration exists. Once landing flaps have been selected, the horn normally cannot be silenced until the landing gear is properly extended.
Altitude alerter: A single chime or other distinctive tone, accompanied by a light, alerts pilots when they are leaving the current altitude or approaching a new one. Some alerter designs omit the tone, utilizing only the light itself.
Configuration warning: An intermittent horn or beeping tone warns when flaps, slats, stabilizer trim, or speed brakes are improperly configured prior to takeoff.
Pressurization: A continuous horn, accompanied by a red warning light in some aircraft, warns of loss of normal cabin pressure.
TCAS: A variety of voice warnings and visual displays warn pilots of traffic conflicts.
GPWS: Various voice warnings and attention-getting "Whoop, Whoop!" sounds warn of potentially dangerous situations, such as descent towards terrain when not in the landing configuration. — VC