FILAMENT TEST – ATA 33-14 Part 4

Editor's note: You can see that even in the documentation we have gone from ATA 33-12 to ATA 33-14... Curious isn't it, as for the fuel tanks and the rows of seats, the number 13 are ignored.

Before starting this article a small explanation on the terms:

- Filament is actually small 5 V light bulbs specially designed for

announcers.

- Announcers are rectangular or square indicator lights on all cockpit panels.

Checking the filaments is a safety operation and makes it possible to detect a defective bulb and replace it to avoid losing a fault that risks going undetected in time.

The filaments of all caption and warning light assemblies fitted on panels in the flight compartments are lit by means of a "LIGHT TEST" push - switch for each group of panels. This operation proves the serviceability of the filaments; it does prove the associated indication circuit’s serviceability.

The "LIGHT TEST" switches, mounted on various panels, are double-pole.

The announcers:

Each caption light assembly comprises a rectangular unit housing the electrical connections, filament contacts and associated blocking diodes; a press - to - release type hinged cover contains two filaments fitted behind a translucent caption cover.

And as can be seen in the photos above, the replacement is easy and will be carried out to prevent the loss of the announcer's second filament from causing the loss of important information by the non-illumination of the said announcer.

To avoid losing important information due to the failure of a bulb, the crew regularly checks the "LIGHT TEST" and tests the filaments. If in an announcer you notice that a bulb is no longer working, under the flight engineer's shelf there is a small drawer that contained a reserve of filaments.

And now by performing a LIGHT TEST we can clearly see that in each announcer above both filaments are in good working order.

And here is what the famous filaments (bulbs) look like…

There are a few hundred small bulbs (filaments) in all the announcers and when you think that the small pocket above costs around 15 $ all the announcers in the SA don't light up...

Power supply:

The filaments controlled by the "LIGHT TEST" switches on the upper centre switch panel and the flight engineer's station panel are powered by the 28V main circuit, direct current coming from the "B" bus via the circuit breakers.

The main horn display panel filaments are fed from the main warning system. All remaining filaments are powered from the 28V main circuit, direct current from the 'A' bus via the circuit breakers.

To be continued…