Fleet commonality

The A330-300 and A340-300 share the same fuselage and wings, however with the A340-300 featuring an extra bogie of landing gears under the belly, and powered by four engines instead of two.

Compatibility with the 757/767 family means that operators can gain better access to the demanding medium-size market (200-300 seats) by matching the appropriate airplane to variable requirements.

They can also improve operating efficiency through lower training requirements, greater flexibility in assigning flight crews, and reduced spares inventories.

[6] The new 767-400ER flight deck makes the airplane easier to maintain and provides flexibility for operators to tailor the flight-deck equipment to their training needs.

On the instrument panel, the most notable change on the 767-400ER is the use of six large liquid-crystal displays in the same arrangement as the Boeing 777 and Next-Generation 737 flight decks.

Southwest Airlines has operated a pure Boeing 737 fleet since 1971, operating nearly every variant of the type.
An Air France Airbus A300 in 1974.
The first client of Airbus is among the companies who have (or have had) all the Airbus models (A300, A310, A318, A319, A320, A321, A330, A340, A350 and A380). According to Air France, the subsequent commonality within the fleet has a "significant economic impact" [ 1 ]