An accepted rule of thumb for installing bolts in an aircraft structure is to have the bolt head

• A. Up, or in a forward direction
• B. Downward
• C. Aft
• D. Sideways

Answer: (A)

A bolt is best installed with the head on the up side or facing forward so that the joint maintains its preload under flight loads and vibration. When the head is up or forward, the direction of the applied loads tends to keep the fastener tight rather than allow it to back out, helping the nut remain seated and the clamping force maintained. If the head were down or aft, gravity and vibration could promote loosening of the joint as the fastener shifts and the preload drops. This orientation rule is a practical guideline to reduce the chance of a bolt loosening in service.