Scrap metal, or simply scrap, is a currency in The Lost Frontier. Scrap is a prominent resource in areas visited along the Brink, and has many uses. The primary use is exchanging it for plane weapons and mods, though it was also used once to buy the Gunship from a pirate in the Phantom Blade's hangar. It is also particularly valuable to pirates at Far Drop, who would offer Jak Precursor orbs in exchange for scrap to repair parts of the town after the mission "Defeat the dark mutants".
The most common way to find scrap is by destroying planes or completing side missions, such as from pirate radio towers, Barter's bar brawls, and interestingly even by completing requests for Precursor statues. One minigame also offers scrap as a reward, which is the plane races seen at Brink Island and Sector Zero. It does not appear the Aeropans utilize scrap as a currency, as their Danger Course only yielded Precursor orb awards and it was otherwise never mentioned there.
After exhausting all the above possibilities, the next way to collect scrap is by either Daxterjacking with a completely upgraded jack more scrap mod or by flying around plane-dense locations such as the abandoned research rig with the Armageddon weapon.