Definition
A threaded fastener with a hexagonal (six-sided) recess in its head, driven by inserting a matching hex-shaped key — known as an Allen wrench or hex key — into the recess and turning it.
Plain English
A screw with a six-sided hole in the top instead of a slot or cross. You turn it with an L-shaped six-sided tool that fits into the hole.
Context Anchor
Seen during aircraft maintenance, especially on panels, clamps, brackets, and small parts that need a compact screw head.
Derivation
Named after the Allen Manufacturing Company of Hartford, Connecticut, which patented and popularized the hex-socket screw in the early 1900s. The name became generic for any screw using this drive style.
Why Pilots Care
Many aircraft inspection panels, avionics mounts, and engine accessories use Allen screws. Knowing the correct hex key size avoids stripping the head, which can turn a five-minute job into a major removal task.
Intuition Check
Do not assume an Allen screw is a special aviation-only fastener. It is identified by the six-sided recessed socket in the head and the tool needed to turn it.
Example Sentence 1
The mechanic loosened the four Allen screws securing the avionics tray before sliding the radio out.
Example Sentence 2
During the annual inspection, all Allen screws on the elevator hinge were checked for proper torque.