If an impulse coupling spring breaks in some models of aircraft engines, the engine loses power. How should the aircraft mechanic inspect springs? When should you replace them?
Because there are many different types of springs carrying out many different functions, the answer is not always easy. Normally it is not necessary to NDT springs (non-destructive testing such as a magnetic particle inspection) since the time between crack initiation and breakage is extremely short - sometimes explosively short! NDT assumes that cracks can exist benignly until the next inspection interval allowing one to find the crack before the part breaks in half.The following inspection and replacement criteria is suggested as a general aid to be used when the maintenance manual is lacking:
If the spring is inexpensive - then replace it.
If the spring works hard and hot - then replace it during major maintenance. For example, valve springs.
If the spring is in a corrosive environment - the replace it.
Springs exist in tension and the slightest amount of pitting will cause the spring to break. This is usually what fails impulse coupling springs and garage door springs.
If the spring serves some critical or safety function - then replace it.
Impulse coupling springs on Lycoming engines equipped with dual drive magnetos (D-2000 or D-3000), failure of the impulse coupling spring will cause the engine to lose power. Impulse coupling springs operate in a corrosive environment as they collect condensation. So these springs meet two of our replacement criteria: 1. corrosive environment, and 2. critical safety function.
So how do we inspect springs?
inspect spring free length,
inspect spring compression force,
inspect for bends,
inspect for pitting,
inspect coils for rubbing.
Remember to mark rejected springs as "condemned" to prevent their accidental reuse.
