Aircraft Instrument System Warnings and Cautions | Aircraft Systems

Aircraft Instrument System Warnings and Cautions

Annunciator Systems

Instruments are installed for two purposes: to display current conditions and to notify of unsatisfactory conditions. Standardized colors are used to differentiate between visual messages. For example, the color green indicates a satisfactory condition. Yellow is used to caution of a serious condition that requires further monitoring. Red is the color for an unsatisfactory condition. Whether part of the instrument face or of a visual warning system, these colors give quick reference information to the pilot.

Most aircraft include annunciator lights that illuminate when an event demanding attention occurs. These use the aforementioned colors in a variety of presentations. Individual lights near the associated cockpit instrument or a collective display of lights for various systems in a central location are common. Words label each light or are part of the light itself to identify any problem quickly and plainly.

On complex aircraft, the status of numerous systems and components must be known and maintained. Centralized warning systems have been developed to annunciate critical messages concerning a multitude of systems and components in a simplified, organized manner. Often, this will be done by locating a single annunciator panel somewhere on the instrument panel. These analog aircraft warning systems may look different in various aircraft, and depend on manufacturer preference and the systems installed. [Figure 1] EFIS provide for annunciation of advisory and warning messages as part of its flight control and monitoring capabilities, as previously described. Usually, the primary display unit is designated as the location to display annunciations.

Aircraft warnings and cautions
Figure 1. The centralized analog annunciator panel has indicator lights from systems and components throughout the aircraft. It is supported by the master caution system

Master caution lights are used to draw the attention of the crew to a critical situation in addition to an annunciator that describes the problem. These master caution lights are centrally wired and illuminate whenever any of the participating systems or components require attention. Once notified, the pilot may cancel the master caution, but a dedicated system or component annunciator light stays illuminated until the situation that caused the warning is rectified. Cancelling resets the master caution lights to warn of a subsequent fault event even before the initial fault is corrected. [Figure 2] Press to test is available for the entire annunciator system, which energizes all warning circuitry and lights to confirm readiness. Often, this test exposes the need to replace the tiny light bulbs that are used in the system.

Aircraft warnings and cautions
Figure 2. A master caution switch removed from the instrument panel.

Aircraft Aural Warning Systems

Aircraft aural warning systems work in conjunction with illuminated annunciator systems. They audibly inform the pilot of a situation requiring attention. Various tones and phrases sound in the cockpit to alert the crew when certain conditions exist. For example, an aircraft with retractable landing gear uses an aural warning system to alert the crew to an unsafe condition. A bell sounds if the throttle is retarded and the landing gear is not in a down and locked condition.

A typical transport category aircraft has an aural warning system that alerts the pilot with audio signals for the following: abnormal takeoff, landing, pressurization, mach airspeed conditions, an engine or wheel well fire, calls from the crew call system, collision avoidance recommendations, and more. Figure 3 shows some of the problems that trigger aural warnings and the action to be taken to correct the situation.

Examples of Aircraft Aural Warnings
Stage of Operation
Warning System
Warning Signal
Cause of Warning Signal Activation
Corrective action

Flight control

Intermittent horn

Throttles are advanced and any of the following conditions exist:
1. Speed brakes are not down
2. Flaps are not in takeoff range
3. Auxiliary power exhaust door is open
4. Stabilizer is not in the takeoff setting
Correct the aircraft to proper takeoff conditions

In flight
Mach warning
Equivalent airspeed or mach number exceeds limits
Decrease aircraft speed
In flight


Intermittent horn

If cabin pressure becomes equal to atmospheric pressure at the specific altitude (altitude at time of occurrence)
Correct the condition

Landing gear

Continuous horn
Landing gear is not down and locked when flaps are less than full up and throttle is retarded to idle
Raise flaps; advance
Any stage
Fire warning
Continuous  bell
Any overheat condition or fire in any engine or nacelle, or main wheel or nose wheel well, APU engine, or any compartment having fire warning system installed

Whenever the fire warning system is tested
1. Lower the heat in the the area where in the F/W was activated

2. Signal may be silenced pushing the F/W bell cutout switch or the APU cutout switch
Any stage
High chime
Any time captain’s call button is pressed at external power panel forward or rearward cabin attendant’s panel
Release button; if button remains locked in, pull button out