The exact layout of what is carried on an engine is decided by the needs of the department. For example, fire departments located in metropolitan areas will carry equipment to mitigate hazardous materials and effect technical rescues, while departments that operate in the wildland-urban interface will need the gear to deal with brush fires. Some fire engines have a fixed deluge gun, also known as a master stream, which directs a heavy stream of water to wherever the operator points it. An additional feature of engines are their preconnected hose lines, commonly referred to as "preconnects". The preconnects are attached to the engine's onboard water supply and allow firefighters to quickly mount an aggressive attack on the fire as soon as they arrive on scene. When the onboard water supply runs out, the engine is connected to more permanent sources such as fire hydrants or water tenders and can also use natural sources such as rivers or reservoirs by drafting water. Aerial appliances allow firefighters to reach high locations. They can provide a high vantage point for spraying water and creating ventilation, an access route for firefighters and an escape route for firefighters and people they have rescued. American fire departments make heavier use of these appliances. In Europe, where narrow streets often limit the size and usefulness of aerial appliances, they are mainly used for rescues. A turntable ladder (TL) is an aerial apparatus with a large ladder mounted on a pivot which resembles a turntable, giving it its name. The key functions of a turntable ladder are allowing access or egress of firefighters and fire victims at height, providing a high-level water point for firefighting (elevated master stream), and providing a platform from which tasks such as ventilation or overhaul can be executed. To increase its length and reach, the ladder is often telescoping. Modern telescopic ladders may be hydraulic or pneumatic. These mechanical features allow the use of ladders which are longer, sturdier, and more stable. They may also have pre-attached hoses or other equipment. The pivot can be mounted at the rear of the chassis or in the middle, just behind the cab. The latter is sometimes called a "mid-ship" arrangement, and it allows a lower travel height for the truck. While the traditional characteristic of a TL was a lack of water pumping or storage, many modern TLs have a water pumping function built in (and some have their own on-board supply reservoir). Some may have piping along the ladder to supply water to firefighters at the top of the ladder, and some of these may also have a monitor installed at the top.