A line-replaceable unit (LRU) is a modular unit of an airplane, ship or spacecraft that is designed to be replaced quickly at an operating location. By simply opening and closing fasteners and connectors, this ‘black box’ hardware can be exchanged in a short time without removing the host system from service.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is an LRU (line replaceable unit) in an aircraft?
An LRU, or Line Replaceable Unit, is a modular unit of an aircraft that can be exchanged for a replacement part in a relatively short time by only opening and closing fasteners and connectors. Typically a sealed ‘black box’ like a radio, transformer-rectifier unit, GPS receiver, or VHF radio, it is removed and replaced at the field level to restore the end item to an operational ready condition. Avionics LRUs, including integrated modular avionics racks and standalone GPS receivers, can be swapped on the ramp without disassembling other systems. Engine and powerplant LRUs - propellers, certain turbofan modules, actuators, and other sealed auxiliary hardware - follow the same rule, being stocked and replaced quickly from on-site inventories. Landing-gear actuators, flap and rudder trim actuators, and elevator servo-control units likewise qualify as LRUs, allowing army and civil maintenance crews to perform gate-level line maintenance without hangar support.
What are examples of line replaceable units in aircraft?
Examples of line-replaceable units in aircraft cover avionics, powerplant, electrical and mechanical domains. Avionics LRUs include the Flight Management Computer, VHF Radio, GPS Receiver and transponder. Most aircraft carry both a primary and a back-up radio to satisfy equipment redundancy. Powerplant LRUs embrace the Power Transfer Unit and hydraulic pumps that pressurise flight-control and utility systems. Electrical LRUs consist of the Transformer Rectifier Unit and Static Inverter that convert and condition on-board power. Mechanical LRUs include the Elevator servo control unit, Rudder trim actuator, flap actuators and landing-gear actuators that move primary and secondary surfaces. Environmental and fuel-system LRUs comprise the Cabin Pressure Controller, Air Cycle Machine, Fuel Quantity Processor Unit and Fuel Boost Pump.
What is the difference between a line replaceable unit and shop replaceable unit?
The difference between a line replaceable unit and shop replaceable unit is that a Line-replaceable unit (LRU) can be taken out and installed while the airplane is on the flight line. Mechanical access, simple tooling, and an aircraft maintenance manual are all that the line-maintenance crew needs and the exchange is done in minutes. Because the failed LRU is turned into the supply system, its purchase price plus the supply procurement surcharge becomes the Depot Level Repairable (DLR) exchange price charged to the organizational-level maintenance budget.
A shop-replaceable unit (SRU) is a specific part inside an LRU - typically a circuit-card assembly, power supply, or valve module - that a backshop certified repair/overhaul facility removes only after the larger LRU has been pulled from the vehicle. Replacement must take place at a suitable workshop; the aircraft does not remain in use. Failed SRUs are returned to the depot where they are repaired. The repair cost factor for an SRU is usually ten to sixty percent of the LRU acquisition unit cost.
What is the difference between a line replaceable unit and a line replaceable module?
The difference between a line replaceable unit and a line replaceable module is that a line-replaceable module is more generic than a line-replaceable unit. While an LRU is a function-specific assembly, an LRM is a generic building block that can be re-used across many avionics functions.
Because the LRM is generic, one module type replaces several different LRUs, so the number of replacement parts that need to be kept on stock is reduced. After removal at the flight-line (O-level) the same LRM is returned directly to its original manufacturer for repair, simplifying the supply chain. Integrated modular avionics works with these LRMs. The introduction of LRMs has reformed the logistics tail and the infrastructure of electronics maintenance.
