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In pilot jargon, the one bolt on an aircraft which mythically holds the whole contraption together. May also refer to any single fastener which actually does hold together an entire subassembly. It doubtless gets its name from what the pilot would find him- or herself shouting if it should fail.

Most aircraft do have fixtures which must be inspected during the preflight to see that they have not exceeded some stress tolerance, and the Jesus bolt is often one such item. These usually take the form of a nut and bolt with a pin or safety wire inserted through both, which would bend or break if overstressed.

On T-tail Piper airplanes, for instance, the part most often called the "Jesus bolt" is located just forward of the elevator where it intersects the rudder. Some other aircraft even have a little Lexan window through which the Jesus bolt, or its equivalent, may be viewed.

In the world of helicopters the term has another sense: for them, it is a fastener that holds on the rotor blade – or even, in a radio controlled model helicopter, the entire rotor assembly. According to this interpretation, they call it a Jesus bolt because the operator must have faith in it. (thanks to Alexa for this tidbit)

gliders, having large wingspan (typically 15' to 25') are designed to be easily disassembled for transport in trailers. The wings are generally two pieces (one per side) and fitted together in some interesting fashion inside the fuselage; a "Jesus Bolt" used to be common but is no longer favoured.

For example, a fork/tongue mechanism with a single vertical bolt holding them together:

front view:
             ---
-------------| |--++-------------
             |J|  ||
         +---|e|--+|
         |+--|s|---+
         ||  |u|
         |+--|s|---+
         +---| |--+|
             |B|  ||
-------------| |--++-------------
             ---

This method is no longer favoured due to the stress on the bolt applied over a short lever-arm (the height of the bolt). More recent aircraft use the whole width of the fuselage as the lever length and attach each wing to the other's root with pins:

top view:

 wing            jesus             wing
 root             bolt             root
  ||               --               ||
  ||---------------||--------------=||
  ||               ||              ==|
  ||---------------||--------------=||
  ||               ||               ||
  ||=--------------||---------------||
  |==pin           ||               ||
  ||=--------------||---------------||
  ||               --               ||

There is still a jesus bolt in the middle of the construction, but it takes no gravity/lift load. Its only purpose is to keep the wings together and attached, so takes only centrifugal and (to some extent) drag loads. With the wings both attached, the pins on each engage the sockets in the other's wing root. The lever length is the width of the aircraft, thus reducing stress.

Such a design does require heavier wing roots, to strongly couple the load from one wing's pin to the other's spar. The original jesus-bolt-under-load configuration had the bolt going directly through the spar.

Gliders have been known to fly without said bolt (in latter configuration), but discovery upon landings generally leads to shaky hands and red faces.

The jesus bolt holding the wings on/together is by no means the only. As mentioned above, there are other single points of failure holding on things like the tailplane, the loss of which would be catastrophic.

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