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Aircraft Schematic Question - Landing Gear Indicating Light

AlphaBob

Mar 1, 2017
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I'm a student working on my Airframe & Powerplant certificate. I'm trying to answer questions on the attached circuit, but can't figure out what type of [instument panel] lights are represented by the red and green lights on the diagram. Does anyone recognize the symbol, and why would the light have three wires (#8, #18, and ground)?
 

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Tha fios agaibh

Aug 11, 2014
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I'm no aviation expert but it appears that wire 8 is the status signal to the light and 18 is always powered for the purpose of testing the indicator lamp.
In machinery this is called a "push to test" function. There is a push switch build into each light for this purpose.
If a lamp was to fail and an indicator light doesn't come on it could spell disaster.

Isn't testing instrument panel lights part of a preflight checklist?
 

AlphaBob

Mar 1, 2017
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Thanks for the quick response - exactly what I needed!

Related to the preflight checklist - It depends on the type of aircraft. Large passenger jets have provision to illuminate all indicator lights as part of the preflight. Smaller planes like Cessna twin engine aircraft, only require push-to-test if there is a suspected lamp failure - i.e. gear extended, but no green light.

Again, thanks for your help - I couldn't find that symbol anywhere!
 
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