From Wikipedia, the free encyclopedia

The Jesus nut, also called the Jesus pin, is the hexagonal nut that holds the main rotor to the mast of some helicopters, such as the UH-1 Iroquois. It is a slang term first coined by American soldiers in Vietnam; the technical term is main rotor retaining nut.

The origin of the term is the idea that, if the Jesus pin were to fail in flight, the helicopter would detach from the rotors and the only thing left for the crew to do would be to pray to Jesus. Real examples of the Jesus pin failing are few and far between, although in 2002, a civilian helicopter of the Mercy Air Services suffered a "main rotor blade separation", with the death of all those on board. The exact cause was never determined, but it is likely that the Jesus nut failed for reasons unknown 1. Some more recent helicopter systems do not have a Jesus nut.

More recently, it has come to be a generic engineering term, referring to any single component of a system whose failure would cause catastrophic failure of the whole system.

http://en.wikipedia.org/wiki/Jesus_nut

Concerning tandem rotor helicopters it is the part that hold the whole thing together.

"One day the crew chief decided that he had enough of this so he walked out while the Major was starting the aircraft, and in his hand was a Jesus Nut (Which is the one main part that holds the rotor system on the aircraft). The crew chief started flipping the Jesus Nut in the air, and when the Major saw this, he went into emergency shut down procedures, cutting the engine, and securing the aircraft. In a panic, he jumped out and yelled at the crew chief. "How come you were going to let me start this aircraft without a Jesus Nut?" The crew chief replied, " I was not going to do any such thing." The Major who was quite perplexed said, "Isn't that Jesus Nut off of this aircraft Sergeant?" The crew chief replied, "Sir, If you had done a proper preflight, you would have known that it was not." Needless to say, the Major was quite humiliated by all of this, and he was sure to perform his own preflight inspections from then on." 

http://www.helicopterpage.com/html/tandem.html

The Rattler/Firebird Association, a group of chopper veterans, gives out an annual "Jesus Nut Award" to the member who travels the longest distance to its reunions.

Another group of veterans offers this at www.25thaviation.org/: "Enjoy the site and may the Jesus Nut stay tight."