Does line indicate the aircraft is lined up in hangar (!) and maintenance crew can replace it? Or does it mean something else?

Wiki talks about operating location and distances.. What is that?


3 Answers 3


"Line" refers to "Air Line" or "Flight Line". It's the same "line" as when you refer to "line operations" or, for pilots, "line indoctrination". It basically means "in the operational world".

Line Replaceable generally means a system component (not structural) that can be changed anywhere out in the field, even in the middle of nowhere, by persons with the required equipment and paperwork, to replace, rig and test as required for return to service.

An LRU is the smallest component that can be removed from the airplane under the manufacturer's authority, aside from consumables and hardware. All designated LRUs will have a removal/installation procedure in the aircraft maintenance manual.

The next level up from "line" will be a certified repair/overhaul facility, a certified shop at the airline itself (a "back shop"), or the manufacturer of the LRU itself.

An LRU may have sub components that can technically be removed as separate units without removing the LRU from the airplane, but if that sub unit doesn't have its own LRU designation, you can only remove the larger assembly, that has the LRU designation, and any further breakdown of LRU into sub units is a back shop activity that requires a component maintenance manual to perform, with a separate regulatory certification requirement for the back shop.

For example, a hydraulic power control unit (PCU) may have a transducer installed on it that could easily be swapped out on the aircraft without removing the entire assembly, but without an LRU designation for the transducer, you are stuck with removing the entire PCU assembly and sending it to a back shop or offsite to have it repaired.

It's sometimes possible to get a sub component designated an LRU, with its own aircraft maintenance manual procedure, if that sub component doesn't require specialised back shop tooling, or a back shop functional test isn't necessary to validate the installation.

For electronics, you may have a black box, with separate circuit cards internally. Sometimes the individual circuit cards will be designated LRUs and the technician on the ramp can open the box and swap out the card using a maintenance manual based procedure. If the cards don't have LRU designations, the ramp tech can only swap out the box, the smallest component with the LRU designation.

  • 2
    $\begingroup$ I would imagine that "line" here (as in "line indoc," etc) is borrowed from the military. Wiktionary has line 1 sense 17: "(military) The regular infantry of an army, as distinguished from militia, guards, volunteer corps, cavalry, artillery etc." In the FAA there is similarly a distinction made between "line controllers" who actually work traffic and the front-office managers and support staff, or even controllers on support detail, who do not make up "the line." Indeed, until recently the supervisor of a line controller was a "Front-Line Manager"! $\endgroup$
    – randomhead
    Commented May 9, 2023 at 19:13
  • $\begingroup$ Yeah no doubt. It may even have naval roots, or maybe the old British phrase "thin red line". $\endgroup$
    – John K
    Commented May 9, 2023 at 21:33
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    $\begingroup$ Fun fact: for spare parts for the International Space Station, we use a similar nomenclature, except we call them ORUs -- Orbital Replacement Units $\endgroup$
    – Tristan
    Commented May 10, 2023 at 19:43
  • 3
    $\begingroup$ IBM called them FRU's for Field Replacement Unit. The item was freighted to the user for them to do, rather than being hand-carried to site by an engineer who would do the replacement.. $\endgroup$
    – Criggie
    Commented May 10, 2023 at 21:10

It refers to the flight line, in other words the ramp where aircraft are parked. The component is relatively easy to remove and replace.

Other parts require special tooling, jigs, cranes, and so. So a air data computer is a line replaceable unit. A vertical stabilizer is not.


In this context, 'Line' means the opposite of 'Factory': if something is 'line maintenance' it means that it doesn't require a return to the original manufacturer.

  • 4
    $\begingroup$ There are a couple of other levels between line & factory; you can have hanger or overnight maintenance, and then there is depot-level maintenance (which in practice can probably do anything that can be done to the aircraft; the factory itself may or may not do repair work at all). $\endgroup$
    – Ralph J
    Commented May 10, 2023 at 17:02
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    $\begingroup$ @RalphJ The number of levels in between "Front Line" and "Factory" is immaterial, and will vary according to the context anyway, but are still opposite ends of the spectrum in any industry. $\endgroup$
    – MikeB
    Commented May 11, 2023 at 12:07

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