What is a Finger Doubler?

"The finger doubler repair is, in the opinion of the author, the best compromise for a permanent repair to a basic fuselage structure."

T.Swift, Repairs to Damage Tolerant Aircraft", FAA-AIR-90-01

A finger doubler is a type of skin lap splice that has a number of advantages:

1. Does not hide cracks. Easier to inspect from the outside. The critical locations for future fatigue cracks are in the basic skin at the first attachment row in the doubler. A finger doubler does not degrade the inspectability of the basic structure because a crack propagating in the skin at this first row will be externally detectable.
2. The purpose of the fingers is to reduce the first fastener peak load stress and thereby increase fatigue life. The fingers at the end of the inner skin are flexible and softens the load transfer through the rivets from one skin to the other at the end of the splice.
3. Reduces skin bending stress by increasing the distance between rivets.
4. Increased distance between rivets permits any crack to grow longer and thereby easier to detect.
5. Avoids the need of drag-producing protruding-head rivets to reduce bearing stress. Allows the use of countersunk rivets because of lower peak load stress.

References:

Patent US5297760

Repairs to Damage Tolerant Aircraft by T.Swift, FAA-AIR-90-01