"Why is your nose bleeding" seemed like a simple question due to the amount of blood on the shop towel. The answer was revealed when the shop towel was taken away from the face and a piece of safety wire could be seen entering the outside of the nose and protruding from the inside of the nose. A trip to the nearest med clinic quickly started the healing process but a tough lesson was learned: don't pull so hard on the safety wire pliers and don't pull them towards your face. This did not happen to me personally but I have had the wire break and the pliers hit me in the face. The pliers are never pulled with my face in the way anymore.
Safety wire? It is a thin, usually stainless steel, wire that the general aviation maintenance technician is trained to use to prevent components vibrating loose on an aircraft. The wire is twisted either by hand or by a special pair of pliers, one end is attached to a non-moving part and the other end is connected to the component that possibly could vibrate loose.
Take an oil filter for example-the filter is very similar externally to a car filter except for 4 small tabs with holes at the wrenching end. A length of safety wire is passed through a hole in the engine, the wire is pulled back on itself, the wire is twisted together until it reaches a tab on the oil filter and then twisted again past the tab. The safety wire is cut off about one inch past the tab and bent back to form a "pigtail". I have never seen an aircraft oil filter come loose in 35 years of general aviation aircraft maintenance. I have seen blood from body parts from improperly secured sharp pigtails.
There are many techniques to prevent unwanted loosening of components on an aircraft. Simple lockwashers used under nuts and bolt heads ("starlock" type lockwashers should never be reused as the locking feature is destroyed as part of its locking design) are often used. A cotter pin passes through a hole in the end of a bolt and through a "castleated" nut to maintain the proper torque. Thin pieces of metal (locktabs) that can be used as a washer can be bent over the bolt head and the other end bent over an unmovable part to prevent rotation. Nuts may have a fiber or reduced diameter metal section that the bolt threads pass through and are squeezed in place stop any movement. There are many more different types of undesired rotation prevention devices, but the important thing to take away from this is the concept of preventing things from coming apart on the general aviation aircraft until a person wants to take it apart intentionally. Race cars use similar techniques to keep everything together during a race.
Every part on a general aviation aircraft must be in compliance with FAA standards. Even a simple flat washer must have approval. Normally hardware will meet a Military Specification. Hardware from the local home center should never be used on an aircraft as it does not meet the strength requirements and size tolerances specified by the FAA. FAR Part 21-Certification of Products and Parts specifies the requirements for all parts on an aircraft. (see faa.gov on the web for more detailed information) During general aviation maintenance over the years, we have seen many parts that did not belong on an airplane: garden hose in the engine compartment for an instrument vacuum system-should be MIL-H-6000 hose; welding machine cabling for starter wires-should be Mil Spec type wire; lawnmower choke cable used for engine controls-should be aircraft manufacturer designed cable and many more examples. I hope these parts were not installed by a certified general aviation maintenance technician but by an uninformed, well meaning aircraft owner.
To keep the general aviation aircraft safe to fly takes a lot of attention to small details and that is part of our job.