Christmas tree
A panel of indicator lights in an aircraft or a submarine.
Christmas tree: submarine's status panel at a glance
In submarine operations, the Christmas tree is the control room's primary indicator panel showing the status of hull openings and critical systems. It consists of a grid of red and green indicator lights, typically mounted on the ballast control panel or nearby console. Green lights mean a valve or door is shut and secure; red lights signal an opening or problem. When preparing to dive, operators scan the board until every light glows green, earning the panel its festive nickname from the colored lights display.
The panel accounts for each sea valve, torpedo tube door, engine room hatch, and other penetrations through the hull that must be sealed before submergence. On modern submarines, a typical Christmas tree may monitor 50 to 100 individual closure points. The lights are wired directly to mechanical indicators on the actual valves and hatches, so they reflect true physical position rather than command status, making them reliable for safety-critical decisions.
Operation and Variants
Before a dive, the diving officer or ballast control operator works through a printed checklist, opening and closing specific valves in sequence while confirming each light change on the panel. Any light remaining red after closure procedures triggers investigation; a stuck valve indicator or an actual open valve can delay departure or abort the dive. On diesel-electric submarines, the Christmas tree is especially critical since these boats surface frequently and cycle through dive preparations multiple times per patrol.
Aircraft carry analogous indicator panels for landing gear, cabin pressure, and other systems, and the term occasionally transfers to those displays in naval aviation. However, the term is most entrenched in submarine culture, where the ritual of clearing the board before diving has persisted for decades and is considered fundamental seamanship. The name reflects both the visual appearance and the moment of relief when all systems show safe, as if the crew has received a gift before heading into the depths.