Most of the major airlines of the world do their own maintenance and at the same time contract out maintenance because they are unable to keep up with the need.
What happens is airlines end up expanding their fleet quicker than they can expand their maintenance capabilities.
This can be said about Air India (I'm sure B747-437B can elaborate more) which I believe contracts some of its D Check maintenance to Singapore Airlines.
Air Canada contracts a lot of its 747 maintenance aswell.
There is a major maintenance contractor in Hong Kong partially owned by Cathay Pacific, I forgot what it is called. But they are one of the leading when it comes to conducting D Checks on 747s. D Check is the most intensive type of aircraft maintenance for airliners.
Most of the small airlines do their own minor maintenance as well mostly line maintenance, but when it comes to some heavier, more intense maintenance, most do not have the capability and contract it out to other airlines.
Then there are some small airlines that specialize in maintenance. A good example that I know of is Lebanon's national carrier, MEA
which has 2 747 sized hangers and 2 smaller hangers. They own an excellent maintenance provider called MidEast Aircraft Services Company (MASCO) which is JAR 145 approved which means it get carry on maintenance on European-registered aircraft.
Cyprus Airways has a 3-year contract from 2002 to 2005 to do 27 C Checks on their A320 family. MASCO upgraded 1 of the 747 hangers for aircraft painting and as a result repainted most of Cyprus Airways' A320s in the old livery into the new livery.
I posted this cover a while ago:
You can see the Cyprus Airways aircraft inside the hanger.
The contract is being extended and will include maintaining Cyprus Airways' A330-200s as well.
Cyprus Airways is not a big airline, but they are bigger than MEA
. However from what I have heard, they only have a small hanger capable of doing line maintenance on its fleet, but all main maintenance is contracted out to other carriers, MEA
's MASCO being their biggest one.
It brings a lot of extra revenue for MEA
. So any airline that is capable of contracting out heavy maintenance and is high quality at doing so can recieve a great boost in income.
Hope this helps.
"Generosity is giving more than you can, and pride is taking less than you need." - Khalil Gibran