low-pressure compressor (gas turbine engine component). The first-stage compressor in a dual-spool
gas turbine engine. The low-pressure compressor is called the N1 compressor and its speed is not governed. It seeks its own best speed as the atmospheric conditions change so it can furnish the optimum mass of air to the inlet of the second-stage compressor.
low-pressure oxygen system. A gaseous oxygen system in which the oxygen is carried in steel cylinders under a pressure of approximately 450 psi. Low-pressure oxygen systems were used in military aircraft during World War II, but these systems have been replaced by high-pressure systems or liquid oxygen (LOX) systems.
low-tension ignition system. A type of magneto ignition system designed for reciprocating engines that operate at high altitude. The coil in a low-tension magneto has only one winding, the one that compares with the primary coil in a high-tension magneto. A carbon-brush-type distributor directs the low voltage from the magneto to the proper cylinder.
Each spark plug has its own step-up transformer, to change the low distributor voltage to the high voltage needed to jump the gap in the spark plug. Low-tension ignition systems are not popular today because most aircraft that fly at high altitudes are turbine powered.
low unmetered fuel pressure. The pressure in a Teledyne-Continental fuel injector pump that is adjusted by the relief valve.
LOX. The abbreviation for liquid oxygen. See liquid oxygen.
LR circuit (electrical circuit). An alternating current circuit containing inductance (L) and resistance (R). The total opposition to the current in an LR circuit is impedance (Z), which is the vector sum of the inductive reactance and the resistance.

LRU (line replaceable unit). Components in an aircraft that are designed so that they can be replaced as a unit while the aircraft is on the flight line.
LSB (least significant bit). The bit in a binary number that has the lowest place value. The LSB is the bit on the extreme right.
LSD (least significant digit). The digit in a decimal number that has the lowest place value. The LSD is the digit on the extreme right.
LSI (large scale integration). A method of miniaturization of electronic circuit components, in which 100 or more gates are built into a single integrated circuit (IC) chip. LSI techniques lower the cost per gate of logic components.
lubber line. The reference line used in a magnetic compass or directional gyro, to indicate the nose of the aircraft. The heading of the aircraft is shown as the degrees on the card or dial of the instrument opposite the lubber line.