The U.S. Army engineers are, like the other combat arms, reducing the number of distinct skill categories (MOS or Military Occupational Specialty) . This is running into some problems. The engineers have more equipment to deal with and smaller crews. While the basic infantry units are a squad (8-11 men), and tanks have a crew of four, many engineer vehicles have a crew of two. The engineer vehicles (self propelled bridges, armored bulldozers and so on) are not, themselves, as complex as a tank, but in use they are. Operators of self-propelled bridges must know a lot about the geography of the streams and gullies they use their bridge to get other vehicles across. While a tank has an experienced vehicle commander to teach the new crewman, a two man engineer vehicle doesn't. This put more of a training load on NCOs in engineer units and will reduce the overall skill levels of engineer units.