Aircraft Maintenance – Line, Base and Defects

In general, aircraft maintenance seems simple enough – a plane brakes and mechanics need to fix it, right? Well, nothing is all that easy in aviation!

In most general terms, aircraft maintenance can be divided into three categories (as defined by law and by common sense, if you think about it):

  • Defect rectification (which is the most obvious part – the “fixing” of aircraft)
  • Line maintenance
  • Base maintenance (or heavy maintenance or hangar maintenance)

In this generic article, I will try to go through the main differences in those three and explain why we need them, who and where can perform them and why do we care in the first place.

Line and Base Maintenance

Line Maintenance

Most aircraft (although there are exceptions, like many business jets for example) require line maintenance tasks to be performed quite frequently. In many aircraft types, typical line maintenance tasks would include a daily check (performed anywhere from every 24 to every 48 hours) and a weekly check (every 7-8 days). Apart from that, there may be several OOP (out of phase) maintenance tasks which can be considered to be line maintenance and carried out by a line maintenance provider.

So how can we define line maintenance? The Part 145 regulation defines line maintenance as any maintenance tasks which can be performed outside of a hangar (under open skies) except for situations in which the weather deteriorates to such conditions, that a hangar becomes mandatory. Are you confused? No wonder – a typical example of a legal definition which means precisely nothing. Technically, one could do any maintenance task under the open sky in the Caribbean assuming the weather is nice and warm and there’s no wind.

So what can we do? Many airline professionals have a good “feel” for the line / base distinction and know by heart which is which, especially if they have years of practical experience on a given aircraft type. However, if you’re new to the business there’s still hope J

First of all, for some aircraft, the scope of line maintenance is specified in the MPD or MS (Maintenance Planning Document or Maintenance Schedule). Those documents may either bluntly tell you that line maintenance is for every check up to and including the 500 HR A-check, for instance. Or for any A-check. On top of that, the same documentation will specify components which are known as LRU – Line Replaceable Unit. This implies that the replacement of such components can be done during line maintenance. Be careful with oversimplification though – an engine is generally considered to be an LRU and as much as you’re allowed to replace it during line maintenance (for instance, if you get FOD – Foreign Object Damage – such as a bird ingestion, at an airport where no hangar facility is available) you should probably stick to a base maintenance facility for this job if there is one nearby.

On the other hand, most modern aircraft tend to define their maintenance programs as stand-alone tasks rather than complete work packages. In this case, you will not get a definition of an A-check or C-check, and therefore also no distinction as to which one of them can be assumed line maintenance. Then, the definition needs to be created by you and approved by your local aviation authority. In other words:

  • If you’re running an airline or an independent airworthiness management organization, you need to define a Maintenance Program specific to the given aircraft in your fleet. This maintenance program should define which tasks you consider to be line maintenance. There are various ways to define such tasks. The easiest one (although also the one with least common sense) is to define line maintenance as maintenance tasks limited by a certain interval (for instance all tasks with an interval of up to 500 flight hours, 500 flight cycles and 1 calendar year may be considered line maintenance).
  • If you’re running an MRO, you will need to specify your so-called scope of approval, which defines what maintenance you’re actually allowed to do. In your internal documentation (the MOE – Maintenance Organization Exposition – chapter 1.9) you will need to define whether you do line or both line and base (I have yet to see someone who does only base, although it’s possible) and also what that means to you. Of course, your local authority needs to approve your ideas.

Another important thing to note regarding line maintenance is that it is fairly easy to set up from an airline perspective. Airlines operate to and from various airports around the world and they need maintenance support for their fleet. Furthermore, some operators (especially charter and ACMI providers) fly to airports on an ad-hoc basis, so establishing steady relationships with maintenance providers may prove difficult. In those cases, line maintenance providers come in handy because a you can hire those companies often via one email and one work order for a simple check on your aircraft and they will be there to assist.

Base maintenance

The definition of base maintenance is simple – it is all maintenance which does not fall under the line maintenance category. In practice, this will be mainly heavy checks such as C and D checks. During those checks major and minor aircraft systems are being evaluated together with complex and time consuming tasks such as corrosion prevention, structural work, replacement of major components, interior refurbishment, etc. Of course, all this work needs to be done in a hangar and requires quite a bit of planning and a significant dose of cooperation between the airline and the MRO.

The arrangement of base maintenance is not as simple as with line maintenance. Operators are required (by law) to have base maintenance contracts in place for all aircraft they operate. Such contracts need to fulfill the Part 145 contracting / subcontracting requirements and need to be approved by the respective aviation authority.

The way this is generally done is that major MRO’s have so-called GTA’s – General Terms Agreements – which can be signed by an operator. A GTA does not warrant anything really. It does not ensure that there will be an available slot for your aircraft when you need one, neither does it guarantee any kind of technical support. However, it does present a general framework of the cooperation between the airline and the MRO once a heavy aircraft check is being ordered. It can also be approved by the authority, which means that it satisfies the Part 145 requirements mentioned above.

Once you agree on a check date and price with the MRO of your choice, all you need to do is sign an attachment (or appendix or side letter – whatever you wish to call it) to the GTA. All the attachment does is specify the work scope, induction and extraction dates and pricing. All other terms and conditions have already been agreed upon in the GTA, so there is no sense in repeating those. Last but not least – the attachment does not need to be approved by the authority, as it is not an agreement by itself. This allows for more flexibility as the operator is not limited by the time it usually takes to approve documents at the CAA.

Defect rectification

This part can be tricky, although it may seem obvious at first. Of course, if a defect arises on your aircraft, it needs to be rectified as soon as possible – there is no question about that. However, how do you now determine if the rectification of the defect constitutes line or base maintenance? Can your local line maintenance provider sign off for the job, or are you in trouble because you need to get a base maintenance facility to look at your aircraft?

There is no easy answer to this question and a flow chart would probably best describe the decision process with respect to complex details. Hopefully, one day I will have the time to actually create one. For now, let’s take a look at things to consider.

Just one more thing before we start – keep in mind that being an operator (or an independent airworthiness management company) it is your responsibility to have your maintenance performed by the right people and companies. In other words – just because your line maintenance provider claims that they can do the job does not, on its own, allow you to order them to do so. It is your line maintenance definition, your engineering knowledge and your common sense that are in charge here.

Now, a quick method for the evaluation of defects:

  • In most cases, your line maintenance provider will be the first to get the word of the defect (most often through an appropriate entry in the TLB – Technical Log Book – made by the crew or through the results of an inspection like a daily check).
  • There is no problem in them initially evaluating the defect. Keep in mind that the appropriate Part 145 approvals are needed for performing work on the aircraft (i.e. removing a defect) and not for finding and describing one.
  • For the sake of this exercise, let’s assume that a line maintenance mechanic discovered a fuselage dent on your aircraft. The dent has not yet been described in the dent and buckle chart, which implies that it must have just recently occurred.
  • Your engineering team (perhaps with the assistance of the line maintenance MRO) should check your SRM – Structure Repair Manual – for allowable defects in the area in question. You checked and there are certain limits within which the defect is acceptable and can be easily released to service (after an appropriate entry in the dent and buckle chart has been made), other limits allow the defect to be released but only for a certain amount of time (say 500 flight hours) by which a permanent repair needs to be done, and everything outside those limits is a “no go” item and requires immediate rectification before next flight.
  • Here is your first decision – the size of the dent needs to be accurately measured. The measurements you take will define whether your aircraft is airworthy or not, so they need to be really accurate. Mistakes can either endanger flight safety or empty your wallet. In either case, you need to be sure. Can you order your line maintenance provider to do the measurements or should you call a structure expert from a base maintenance facility to do so? There is no definite answer. Ask your line maintenance provider if they feel comfortable with the task – if they have decency, they will give you an honest answer. Furthermore, if you’ve worked with them for a while you will have your own idea on their abilities and skills. It’s really a judgment call – if you’re in doubt, have an expert fly in.
  • The measurements have been made. Bad luck – the dent needs to be fixed immediately. Your line maintenance provider claims to have a good structure engineer on site who can do some of the sheet metal work for you provided you give him instructions from the SRM or obtained from the aircraft manufacturer if necessary. Can you do that? This one is easy – no, you should not allow that. You should check the Part 145 certificate of your maintenance provider and if you’re still in doubt – chapter 1.9 of their MOE. If they are only approved for line maintenance and minor repairs (as is generally the case) they should not be permitted to do the work. You will need to call a base maintenance organization and ask for support. If you’re in luck – there will be one at the airport at which your aircraft has been grounded. If not – they need to fly in and do it on site away from base (they most often have their own procedures which allow them to do that).

Having said all that you may ask what will happen if there is no base maintenance provider on site, and perhaps even also no hangar in which such a complex repair can be performed. There are a few options still out there. The most obvious one is to apply to EASA (or your local authority) for a PtF (Permit to Fly) which is a document permitting repositioning flights of aircraft which are considered not airworthy for some reason. Of course, not every aircraft can be flown like that but in many cases you will get your permission. Then you can reposition your aircraft to a facility of your choice.

Of course, there are several exceptions to the generics which I wrote about above. However, those are common guidelines and definitions for aircraft maintenance. Please leave comments and questions below if you wish.

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Posted in Maintenance, Maintenance of Aircraft
15 comments on “Aircraft Maintenance – Line, Base and Defects
  1. Steve says:

    Excellent oversight and humour 🙂

    • Mike says:

      Thank you, Steve. I’m very glad you found the article useful. And I’m especially happy that you liked the humour 🙂 🙂 🙂

  2. Skid Rowe says:


    Like the style , but if I could make one observation from many years of reading peoples help pages like this, and hearing people state something specific .

    When you identify, a specific detail or statement under the Basic Regulation, it would actually give it more credibilty if you were to state the actual ref that could then be checked upon .

    for example , your page about Base and Line and the definition you are quoting ,

    “The Part 145 regulation defines line maintenance as any maintenance tasks which can be performed outside of a hangar (under open skies) except for situations in which the weather deteriorates to such conditions, that a hangar becomes mandatory”.

    This would be so helpful and not enable any doubt as to the actual validity of your statement.

    Other than that observation, your site is fabulous and so helpful , many thanks .


    • Mike says:

      Hi Skid!

      Thank you very much for kind words and your input. Indeed, this would probably be more specific for the readers, although it would also position me almost like a lawyer 🙂 But I agree, this would be beneficial to the readers, and I promise to look into that.

      Once again, thank you for reading my articles and all the best!

  3. Ram says:


    Is there any information what percentage of aircraft landings arrives with a Defect. How many of these landings with defects causes Delays and Cancellations. Also I feel that it does take some time to record the damage since this involves measurement of all reference dimensions with respect to nearby structural features like spars, ribs, fasteners , bulkhead etc.How long does this take ? does the operator would appreciate automation of this process

    • Mike says:

      Hi Ram,

      Typically, there’s a reliability program in place in each airline. It shows the percentage of flights arriving with a defect, it also specifies which system has a defect (by ATA chapter) and provides additional information as well. It wouldn’t, however, include information about delays and cancellations – this is the duty of the operations department. The purpose of the reliablity program is to make changes to the AMP (approved maintenance program) which would reflect the actual problems you may have in your fleet. And as you say, damages need to be measured and verified agains the SRM, which is being done by mechanics afer every flight if there is a damage.

  4. Nitin Deshpande says:


    Most of the time airline maintenance is taken as a base but a business jet flies to various locations and out pf phase tasks can be performed without hangar by sending a mechanic. How does MRO or regulation consider these activity and do we require to take reg approval ?

  5. Mike says:

    Hi Nitin, great questions!

    Regulations are always most applicable to regular airlines and business jet operators are orphans… sorry to say that, but that’s the truth. If a business jet operator has its own MRO, then the MOE would define procedures allowing mechanics to go to external stations and perform out of phase tasks on location. That MOE procedure would need to approved by the CAA. That’s the only way. Creating an AMP which calls for several out of phase tasks is a different story – tat may also require some “fighting” with the local CAA to get approved.

  6. TINAKICHE says:

    I am an aircraft engineer , I work in planning departement and i want to know how can I reduce the size of the file check ‘ base maintenance’ and if we have to print all job cards and task cards and sing them specially in the C CHECK it become huge.
    Thank to you sir

    • Andrew says:

      No need to print job cards. Print the work order(one page). Let the mechanics go find the reference to do the job. This is assuming they have iPads

  7. Tim says:

    Hi Mike, do you know of any policy or legislation in place that allows aircraft engineers/technicians access to aircraft for AOG work in foreign countries when they don’t hold a valid ‘work’ permit or VISA?


    What is the difference between Line Base check and Base Maintenance checks

  9. Thanks for sharing this blog. Starting with aircraft maintenance facility management and managing MRO throughout the supply chain, the use of advanced digital technologies is imperative to establish high-efficiency operations. With the advancement in AI and analytics, MRO companies are progressing from centralized maintenance diagnostic operations, through real-time condition monitoring services, to prescriptive maintenance operations

  10. The smooth operation of air travel hinges on a meticulous system of aircraft maintenance. This system revolves around three key pillars: line maintenance, quick yet essential checks and repairs performed at airports; base maintenance, thorough hangar-based interventions for bigger issues; and defect management, efficiently addressing discrepancies that arise.

    These elements work in tandem. Line maintenance catches and fixes minor problems, while major defects trigger deeper dives in base maintenance. By adhering to strict regulations and prioritizing both preventative and reactive care, aircraft technicians keep our skies safe and flights running smoothly.

    Read More-

  11. Paul says:


    Interesting read, how about defects noted post base check release? For example MRO issues block CRS, crew arrive to collect aircraft and upon power up and preparation a low APU oil level is indicated. Can an MRO without LINE maint approval perform maintenance and release in the Tech Log? Aircraft has not operated post base maint?

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