A drain plug in the sump of either a reciprocating engine or a gas turbine engine. Two small permanent magnets built into the drain plug attract and hold ferrous metal chips or shavings that may be in the lubricating oil. Metal on the drain plug is an indication of internal engine failure.

Some magnetic drain plugs in gas turbine engines have the two magnets connected into an electrical warning system. When bits of metal short across them, a circuit is completed, and a warning light is turned on to show that metal particles are loose in the engine.