undercarriage (aircraft structure). The landing gear of an aircraft.
undercast (meteorology). A cloud layer of ten-tenths (1.0) coverage, as viewed from an observation point above the layer.
undercompounded generator (electrical generator). A condition in an electrical generator which has both a series and a shunt field in which the effect of the shunt field is greater than that of the series field. The output voltage tends to drop as the electrical load increases.
undercurrent relay (electrical protection device). An electrical circuit protection device that opens a control circuit when the current drops to a specified low value.
underpowered (aircraft condition). An aircraft is underpowered when there is not enough power delivered to its propeller or rotor to give it the required performance.
undershoot (aircraft landing). A condition in an aircraft landing in which the aircraft touches the ground before it reaches the end of the runway or the landing strip. A pilot can correct an undershot approach by applying engine power as soon as it is evident the aircraft will not reach the spot of intended touchdown.
underslung rotor (helicopter rotor). A helicopter rotor with its center of gravity below the point at which the rotor is attached to the mast.
underspeed condition (propeller governor condition). A propeller governor is in its underspeed condition when the engine is operating at a speed lower than the governor calls for. The speeder spring holds the pilot valve down and the flyweights in. With the pilot valve down, the governor directs oil in the proper direction to decrease the pitch of the propeller blades. When the blade pitch decreases, the RPM increases, the governor flyweights spin out to an on-speed condition, and oil is trapped in the propeller to hold the pitch.
undervoltage relay (electrical circuit protection device). A type of electrical circuit protection device that opens the alternator field circuit when the output circuit voltage drops below a specified low value.
underwing fueling (aircraft fuel system). The method of fueling an aircraft in which fuel is pumped into the aircraft tanks through a single-point fueling port located below the wing. Underwing fueling differs from overwing fueling, in which the fuel is pumped directly into the tanks through openings in the tops of the tanks.
Underwriters’ Laboratories. Independent laboratories recognized in the electrical industry as setting safety standards for electrical equipment and components. Before a piece of electrical equipment is granted the right to carry the UL symbol, it must pass the tests prescribed by the laboratories for that type of equipment.
undrainable oil (aircraft specification). Oil that cannot be drained from an engine lubricating system when the aircraft is in the normal ground attitude and the drain valve is left open. The weight of the undrainable oil is included in the empty weight of the aircraft.