Maintenance
Line, base and heavy checks — the world of MRO explained.
Every aircraft you board is flying inside a carefully engineered maintenance envelope. Nothing about it is left to chance: each component has defined inspection intervals, life limits and repair procedures, and the whole programme is approved by the national regulator before the aircraft ever carries passengers.
Maintenance, Repair and Overhaul (MRO) is the industry built around that promise. It spans everything from a 20-minute transit check on the ramp to multi-week structural overhauls in a hangar, supported by planners, engineers, certifying staff and a global supply chain of parts and tooling.
Key areas
Line Maintenance
Daily and transit checks done at the gate or overnight — fluids, tyres, defect rectification and quick troubleshooting.
Base Maintenance
Scheduled hangar inputs (A and C checks) where panels open up and systems get deep inspection.
Heavy / Structural Checks
The most extensive inputs — structural inspection, corrosion control and cabin refurbishment, often lasting weeks.
Engine & Component Shops
Engines, landing gear, APUs and avionics overhauled off-wing in specialised shops, then returned to the pool.
Maintenance Planning
Turning the approved maintenance programme into executable work packages without grounding the fleet.
Quality & Safety
Certifying staff, independent inspection, human-factors programmes and audit trails behind every release to service.
The maintenance check ladder
Modern maintenance programmes are usage-based: tasks come due by flight hours, flight cycles or calendar time, whichever hits first. Airlines group these tasks into familiar 'letter checks', although modern programmes increasingly use equalised task packaging rather than rigid blocks.
As a rule of thumb, the deeper the check, the rarer and longer it is — and the more it costs. An airline's ability to keep aircraft flying between deep checks is a major driver of fleet profitability.
- Transit / Daily check
- Visual walk-around, fluid levels, tyre and brake condition — done in minutes between flights or overnight.
- A check
- Roughly every 400–800 flight hours. Light hangar or overnight input: filters, lubrication, detailed zonal inspections.
- C check
- Every 18–24 months. Aircraft out of service for one to two weeks; extensive system and zonal inspection.
- Heavy check (legacy 'D check')
- Every 6–12 years. Structure opened, corrosion prevention, major modifications; can take a month or more.
Who does the work: the MRO ecosystem
Some airlines keep maintenance in-house (often as separate business units that also sell services), while others outsource almost everything. India's fleet growth has made it one of the fastest-growing MRO markets, with line stations at every major airport and expanding base capability.
Behind every hangar visit sits a paperwork trail as important as the physical work: task cards, non-routine findings, component release certificates (like the EASA Form 1 or FAA 8130-3) and finally the Certificate of Release to Service signed by licensed engineers.
The rules that make it trustworthy
Maintenance is regulated at every layer. Organisations must be approved (for example under DGCA CAR-145, EASA Part-145 or FAA Part 145), maintenance programmes must be approved per aircraft, and the people certifying work must hold licences with specific aircraft type ratings.
Continuing airworthiness management — deciding what gets done when, tracking airworthiness directives and service bulletins, and keeping records for the life of the aircraft — is its own regulated discipline (CAMO). It is why a 20-year-old aircraft with complete records is a tradeable asset, and one with gaps is nearly worthless.
Frequently asked questions
What is the difference between line and base maintenance?
Line maintenance covers light, quick-turnaround work done at the gate or overnight — daily checks, defect fixes, fluid and tyre servicing. Base maintenance is deeper scheduled work done in a hangar with the aircraft out of service, such as C checks and structural inspections.
How often does an aircraft get a heavy check?
Heavy structural checks typically come due every 6 to 12 years depending on aircraft type and utilisation. They can keep an aircraft in the hangar for several weeks and are the most expensive scheduled events in its life.
Who is allowed to sign off aircraft maintenance?
Only appropriately licensed engineers — for example DGCA CAR-66 or EASA Part-66 licence holders with the correct category and aircraft type rating — may issue the Certificate of Release to Service after maintenance.
What does MRO stand for?
Maintenance, Repair and Overhaul — the industry covering scheduled checks, defect repair, engine and component overhaul, and the engineering and logistics that support them.