Extensive use of pushbutton switch-lights has been made in cockpit design. Switch-lights control the system as well as show a system message. Switch-tights can be either alternate action or momentary action.
The alternate action switch activates the associated component and the switch contacts remain engaged. When pushed again, the contacts disengage and the controlled component returns to its deactivated state.
Alternate action switch-lights are divided into halves. One half uses a mechanical display to show the command switch position. The display is a descriptive word, symbol or combination of the two, such as a main tank fuel pump switch, which displays ON and a bar. As the switch is pushed in a mechanical shutter retracts, exposing the word/symbol behind it. The shutter closes when the switch is deactivated.
The other half of the alternate action switch-light has an indicator light that illuminates to indicate system response to switch movement, or a system message, as appropriate. In the example below the amber PRESS light indicates fuel pump output pressure is low.
The momentary action switch contacts are only engaged, while the switch is actuated. When released, the switch contacts disengage.
Momentary action switch-lights are also divided into halves. Both halves may contain indicator lights that can illuminate to indicate a system message. If only one half has an indicator light, the other half has a dashed line to show there is no message in that half of the switch. A Generator Disconnect switch is an example. The DRIVE light illuminates to indicate certain system malfunctions. When the switch is pushed, the generator and drive are signaled to disconnect from the engine, which is a momentary, irreversible signal.
Peculiarities of other type switches on the airplane control panels are addressed in the systems chapters, as necessary.
Note: Maintenance personnel should be contacted for all relamping operations. Unintentional system operation can result from improper relamping.