Emergency Warning Systems: Airplane and Airline FAQ

DARK COCKPIT

In the cockpit all lights, including individual system lights and warning alert lights, are designed using a dark cockpit concept. This means that if all systems are turned on and working normally, there are no lights visible. The only lights on during a normal flight are the few green lights, such as the green gear down and locked lights, indicating that system is functioning normally. Switch positions are uniformly designed also. If up is on for one switch, it’s the same for all the switches. On newer aircraft using a push-button type switch, in is always on and out is off.

LIGHT BULBS

Every light on the airplane contains a minimum of two bulbs. If a light bulb burns out, there is a backup. If the backup were to burn out as well, it can be replaced with one from the complete set of replacement bulbs carried in the cockpit.

SYSTEM MONITORS

On some of the early jet aircraft, the third pilot was the system monitor. He alerted the captain and co-pilot if a warning light appeared on the flight engineer’s panel. On newer aircraft, the advisories, cautions, and warnings are grouped together on annunciator panels and can be scanned at a glance. The annunciator panel, which in flight should be completely blank, highlights a specific abnormal, directing you to the appropriate system and its redundant amber light. Just in case the pilots did not see the annunciator panel light, or the system light, in front of them are glare shield master warning and caution lights.

The latest system, called EICAS—Engine Indicating and Crew Alert System—goes a step further. A dual computer monitor warning system, it detects malfunctions, displays the specific problem on a CRT (mini TV) screen and prioritizes the degree of urgency. Whereas the older systems had a light to indicate part of a system was not as it should be, the new EICAS system tells the pilots exactly what part. Furthermore, the computers are able to differentiate between malfunctions that can be fixed in the air and problems that require ground maintenance. If both of the two independent EICAS computers failed, there are still the basic alerts.

Alerts come in varying degrees of severity. A warning requires immediate corrective action, so is always indicated with a red light, plus a siren, bell, computer-generated voice, or beeper. Examples of warnings would be an engine fire, or rapid depressurization. On EICAS equipped air planes, amber alert lights are divided into two categories: cautions and advisories. Cautions indicate some kind of system fault. Timely corrective action is required, but not necessarily urgently. Some cautions have aural warnings, but the less urgent ones do not. An example of a caution would be an engine overheat. Advisories are the least urgent. They usually indicate a component of a system has malfunctioned, but the system is still operating. Corrective action can be taken as time permits. An example of an advisory is a faulty air-conditioning valve, or an electrical problem that has already been isolated or bypassed by the automatic backups.

FIRE WARNING

Everyone is concerned about fires. Each engine, the APU, the wheel well areas (hot brakes), and the cargo compartments have fire detection and prevention systems. Engines are mounted on struts to put distance between them and the rest of the aircraft. Even tail engines are isolated and shielded. Each engine has overheat detection systems and multiple fire loop sensors. An overheat may require no more than retarding the power slightly to reduce the internal temperature. An engine fire is more involved.

Both fire loops—the sensors loop around the entire engine—must detect a fire before the fire bells, alarms, and red warning lights are triggered. This prevents an inadvertent engine shutdown due to one faulty fire loop. If only one system does go off, a fire test system checks each fire detection system and distinguishes the reliable from the faulty. If there’s no reliable test of either fire alarm system, then you have to assume a fire and take the required corrective action.

TAKEOFF CONFIGURATION WARNING

Takeoff is obviously a critical part of any flight. As a double-check to the flight crew taxi checks and their verified completion on a written checklist, there are both a takeoff configuration warning horn and red warning lights. If any one of the essential takeoff components are improperly set, a horn will sound and red lights will flash. On some airlines this horn is tested on taxi-out each flight and can be heard in the cabin. Takeoff flaps and slats set, ground spoilers completely stowed, and the elevator stabilizer (for pitch control) in the takeoff range, must all be completed, or the horn will blare at initial power application.

LANDING CONFIGURATION WARNING

Like takeoff, specific parameters have to be met when a plane is landing. Obviously, the landing gear must be lowered prior to touchdown. But as a redundant check, there is a warning system that would aurally and visually warn the pilots were the gear not down and locked. The flaps and slats are also wired into the landing configuration warning system. Though a plane can be landed with partially extended flaps and slats, or even with them fully retracted, a warning is sounded if the flap/slat setting is unintentionally abnormal. The speed brakes are regularly used on an approach to descend and slow down at the same time. But because they increase the descent rate, they are never used flying close to the ground. When descending through approximately 800 feet AGL as measured by the radio altimeters, all three of the landing configuration warnings are active.

OVERSPEED WARNING

All airplanes, like your car, have maximum speeds. The maximum speed your automobile engine can accelerate your car up a hill or on level ground is well below the structural limits of the car body itself. However, take that same car and drive it at full throttle down a steep incline, and that car may be a little less stable. Airplanes are much the same during climb, cruise, and descent. A red line on the airspeed indicator, which varies with altitude, indicates the maximum, never-exceed speed. There are both a warning horn and red lights to prevent exceeding this limit. All the built-in protections against flutter, vibration, and general structural strength are based on speeds 25 percent above the never-exceed speed. So if the horn goes off at the airspeed red line, it is time to slow down before accelerating any closer to those critical limits. Slowing an airplane is a simple procedure. Retarding the power, deploying the speed brakes to increase the aerodynamic drag, or simply shallowing out the descent is all that is required.

CABIN ALTITUDE WARNING

Normally in cruise, the interior cabin altitude is kept at the 6000 to 7000 foot level. If the cabin pressure ever decreased, causing the cabin altitude to rise above 10,000 feet, a warning would sound. With a sudden change in cabin pressures, one possibly caused by a rapid depressurization, the pressure change felt in your ears, the gauges and the red lights would alert the crew. If it didn’t, then the horn would alert them to don their oxygen masks until the cabin pressure could be controlled. Above 14,000 feet cabin altitude, supplemental oxygen becomes available to the passengers from the automatically deployed overhead masks. When the system is restored or the plane descends to a breathable altitude, supplemental oxygen is no longer needed. Slow leaks do not cause pressurization problems because the outflow valve is able to compensate for the leak by closing slightly more than normal.

STALL WARNING

As noted earlier, though called a stall warning, it has nothing to do with the engines. It’s a term often misunderstood, having to do with the performance of the wings. A commercial airline, which can fly with all the engines shut down like a glider, could stall with full power if it tried to climb straight up. As discussed in the basic aerodynamics, a plane stalls when not enough air is passing over and under the wings, resulting in insufficient lift. An airplane’s stall speed varies with flap and slat configuration, weight, altitude, and angle of bank. Below the stall speed, an airplane will not fly. Pilots never fly anywhere close to this minimum speed. All airspeeds for slow speed flight during takeoff and landing are calculated on a minimum margin of safety of 30 percent or greater. If the plane inadvertently slowed to within 5 percent of the stall speed, lights and horns alert the pilots before the stall occurs. A “stickshaker” will even vibrate the control column to further warn you. If, by freak chance, the warnings were ignored and the plane stalled, full recovery could be made almost instantaneously by increasing the airspeed with power or lowering the pitch angle, or both. Stall recognition and stall recovery are studied and practiced at the required pilot recurrent training every six months.

GROUND PROXIMITY WARNING (GPWS)

The Ground Proximity Warning System is really a multiple warning system in one. Tied to the radio altimeters, the system is only active within 2500 feet of the ground. Four conditions will activate the warning that includes lights and a computer-generated voice alert: a descent immediately after takeoff, flying into rapidly rising terrain, excessive descent rates, and getting too low on the electronic glide slope on the approach to land.

WINDSHEAR WARNING

Windshear alert systems are currently being installed on new aircraft and retrofitted on planes already in service. Windshear, a common but little understood term, involves a very extreme change in the wind direction and velocity over a very short distance. Hence the name shear. Certain known conditions give rise to a possible windshear—such as nearby thunderstorm activity—and are avoided. In cruise, windshear causes turbulence but no other great hazard. Near the ground, the temporary decrease in lift caused by a very severe shear must be overcome. On approach, pilots are trained to begin windshear recovery techniques if any of the following occurs: the airspeed unexpectedly fluctuates plus or minus 15 knots, the vertical speed increases or decreases 500 feet per minute for no apparent reason, or if it is taking unusual pitch or power to fly the approach. Though the criteria is very specific, multiple red warning lights—one located right on the attitude indicator directly in front of each pilot—and a computer-generated voice stating “Windshear, Wind- shear,” will alarm to further alert the pilots to initiate the safety procedures. Full power and the maximum rate of climb will be used to fly away from the shear.

AUTOPILOT DISCONNECT WARNING

If the autopilot isn’t working properly or disconnects, red warning lights and horns sound. In cruise only one autopilot is employed. If the disconnect was for a faulty autopilot, another one can be engaged. Most airliners are equipped with at least two, many with three. If the disconnect was intentional, the alerts act as a confirmation that the disconnect was complete even though it is easily felt in the flight control yoke. Since the autopilot is capable of trimming the airplane to an inherently stable flight condition—if you are climbing, it trims for a steady rate of climb; straight and level, it trims to remain in steady cruise—an unintentional disconnect may not cause a change in the flight attitude. Without a warning or light, the disconnect could go unnoticed for a few seconds or so.

When flying on autopilot in cruise, holding the flight control wheel is not necessary. During an autopilot approach, the rules change. The pilot operating the autopilot will also keep his hand on the control wheel in order to sense an unintentional disconnect immediately and be in a position to take over manually.

AUTOTHROTTLE DISCONNECT CAUTION

Just like the autopilot disconnect, if auto-throttles are installed, there will be a disconnect alert. Whether or not the throttles are intentionally or unintentionally disconnected, the caution beeper and lights will alarm. A second push of the disconnect button will silence the beeper.

SPEED BRAKE EXTENDED WARNING

Speed brakes (spoilers on the ground), the large panels on the top of the wings, create a significant amount of drag when raised from their flush position. This is precisely what they are designed to do. However, there are certain times during flight, specifically takeoff, initial climb, and on the final approach to land, that their deployment would be inopportune. There is sufficient power to overcome their added drag, but a speed brake extended warning below 800 feet alerts the flight crew to the extra drag and the higher than normal descent rate that will be the result. Some airplanes do not allow the speed brakes to be used with the flaps and slats extended more than a certain amount, regardless of altitude.

FUEL CAUTION

Not surprisingly, some passengers express concern about the aircraft running low or even out of fuel. The amount of fuel carried on each flight far exceeds the amount required, and that amount is verified to be in the fuel tanks with double- and triple-checks. Extra fuel is boarded for all known and unanticipated delays. Extra fuel is also boarded in case the headwinds are stronger than forecast. Pilots, dispatchers, and fuelers are a conservative, cautious group by nature. Still, low-fuel warning lights are installed. The fuel burn per engine per hour is different for each type of airplane, but when the fuel quantity gets down to where there is only 40 to 50 minutes’ worth left, the low-fuel lights will indicate it is time to land. Most pilots have never seen this light outside of practice simulators.

ALTITUDE DEVIATION CAUTION

All commercial airliners have altitude alert systems. When Air Traffic Control clears you to a new altitude, whether climbing or descending, the new clearance altitude is entered into the alert window. When approaching the clearance limit altitude, an altitude light and beeper serve as a reminder. Once steady at that altitude, any deviation of more than 250 feet will also cause the alert.

TRAFFIC ALERT AND COLLISION AVOIDANCE

Collision avoidance is one of the primary reasons for Air Traffic Control. Even if a pilot were constantly looking out the window, the incredible airspeed of the modern jets makes seeing other planes and then avoiding them an inefficient safeguard. ATC keeps all planes separated by lesser amounts near the airport, where the speeds are slower and when the pilots can see the other planes, and greater amounts at the higher altitudes and faster speeds. ATC does a fantastic job. Pilots would not fly if they did not completely trust the Air Traffic Controllers and their ground-based radar equipment.

The ATC computers are programmed for collision avoidance. If two aircraft were to get too close, within 2000 feet vertically and 10 miles laterally at the higher cruise altitudes, a conflict alert would signal the controller and he would ask one of the planes to turn or change altitudes. As an added safeguard the FAA has mandated, no later than the end of 1993, a Traffic Alert and Collision Avoidance System (TCAS) be installed on all commercial airplanes.

All airliners have transponders with encoding altimeters. When a distinct four-digit code is entered into them, they send the plane’s exact location and altitude to ATC. ATC uses this transponder information to track all the flights. With TCAS, the individual planes will be able to receive and interpret the transponder information from other nearby aircraft just like ATC. If two aircraft are converging, the two TCAS systems can “talk” with each other and alert both crews which way to turn to avoid a collision. This by no means is a substitute for ATC, but it does act as an additional safeguard, and has significant promise.

MISCELLANEOUS CAUTIONS AND ADVISORIES

There are numerous other cautions and advisories, be cause each and every system has some kind of alert to notify the flight crew of any small problem before it becomes something bigger. But as we have continually stressed, they are cautions and advisories, since a component failure may not render a system inoperative. If it does, there are multiple redundant systems and emergency backups for the backups. For instance: flaps that do not extend hydraulically can be lowered electrically. If the electric system does not work, you simply land with the flaps up. The loss of electrical power from an engine generator can be replaced with power from another engine generator, or the APU generator, or the standby battery. A fuel pump failure is not a problem, another fuel pump will do the job. If the automatic pressurization system is working erratically, it can be substituted with another system that is good or with the manual backup. For the most part, though, things just don’t break.

What we should conclude from this section is that an airplane is designed to be user friendly. Using horns, buzzers, lights, and even computer-generated voices, the plane can talk to the flight crew. Pilots are thoroughly trained and retrained semiannually, continuously checked and rechecked. Though pilots individually and as a team are expected to be perfect and correct in every decision 100 percent of the time, installing these myriad emergency warning systems is an inexpensive insurance policy. Except for the routine warnings like the autopilot disconnect, the other warnings are heard only in the aircraft simulator during practice. Still, if they are only needed one time, they are a welcome added safety feature.

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