Definition
A long, fore-and-aft structural member running along the bottom centerline of an airship, flying boat hull, or seaplane float. The keel forms the main backbone of the structure and provides the primary attachment point for frames, bulkheads, and other load-bearing components.
Plain English
The main spine that runs front to back along the bottom of an airship or water-landing aircraft, holding the rest of the structure together.
Context Anchor
Seen in aircraft structure discussions, especially with seaplanes, flying boats, floats, and lower fuselage construction.
Derivation
From Old Norse 'kjölr', the central timber along the bottom of a ship's hull. Aviation borrowed the term directly from shipbuilding because early flying boats and airships were built using similar structural principles -- a strong central backbone with frames attached to it.
Why Pilots Care
On seaplanes and flying boats, the keel takes the impact loads of water landings. On airships, it provides the structural backbone that the gas cells and gondola attach to. Damage to the keel is a major structural concern.
Intuition Check
A keel on an aircraft does not usually mean a heavy underwater fin like on a sailboat. In this context, it means a lower centerline structural member that helps support and shape the aircraft body, hull, or float.
Example Sentence 1
The flying boat's keel was inspected after the hard water landing to check for cracks or deformation.
Example Sentence 2
The fuselage skin panels attach directly to the keel and the upper longerons.