Trench Run Defense (TRD) was an anti-capital ship tactic modeled after the success of Rebel Alliance starfighters against the Death Star known as the trench run, at the Battle of Yavin. Pilots colloquially referred to targets vulnerable to the tactic as being susceptible to "Trench Run Disease".
Pilots facing large starships would fly close to the surface of a ship's outer hull, using trenches to avoid enemy fire. Starfighters could thus deliver their payload with impunity to sensitive ship segments, assuming a successful escort against enemy starfighters. X-wing pilots found particular success using TRD against Imperial-class Star Destroyers and other large capital ships, whose weaponry was primarily designed for combat with other capital ships and thus largely ineffective against small, fast, and agile snubfighters. The tactic was particularly effective when the fighters in question were armed with proton torpedoes, concussion missiles, and other heavy ordnance that could severely damage a larger ship's systems.
The Imperial Navy's response to the successful use of this tactic at Yavin and at other battles was to accelerate the production and deployment of the faster and more maneuverable TIE Interceptor-class fighter, and to develop the Lancer-class frigate, an escort-sized ship specifically designed to eliminate screens of starfighters.
During the Fondor shipyard assault and the Battle of Endor, the Rebels utilized this tactic against an under-construction Executor-class variant possessing cloaking abilities and the Executor, respectively.
After the First Battle of Borleias, Commander Wedge Antilles reflected that, although the actual success rate of snubfighters destroying Imperial Star Destroyers was relatively small, the destruction of the Death Star had made all Imperial commanders fearful of the tactic.
A variation of this tactic was used during the Battle of Thyferra, by Rogue Squadron, who maneuvered their X-Wings with relative impunity around the defenses of Ysanne Isard's Super Star Destroyer Lusankya, using their targeting systems to guide massive salvos of proton torpedoes and concussion missiles launched from nearby freighters.
Trench Run Defense was typically formed in a triangular, three-ship formation. The two starfighters in the back protected the forward fighter for as long as possible with their shields. The forward fighter would usually be carrying powerful weapons to take out a target.
The original "trench run" was employed by Rebel pilots at the Battle of Yavin, and variations were employed in the following years in attacks on planetary bases, or other terrain-based engagement zones. For instance, during the Second Battle of Borleias, Corran Horn successfully piloted an X-Wing on a trench run that destroyed a conduit supplying extra power to the Imperial base's ground defenses.
Another variation of the tactic was pioneered by Wraith Squadron to avoid pursuit in a cityscape, natural trench or valley. By diving into the cover of a trench, the pilots could lose enemy starfighters by weaving an impossibly difficult pattern to follow or by moving deep enough to make overhead pursuit equally difficult.
- The Complete Star Wars Encyclopedia