Can you look at these sentences and help to explain what it refers to and what fleet fuel means?

The aviation industry has developed a set of ambitious targets aimed at limiting its climate impact, while enabling it to continue to provide a key vehicle for economic growth. The targets include: improving fleet fuel efficiency by 1.5% per year until 2020; capping net aviation emissions from 2020; and most ambitiously, to halve aviation Co2 emissions by 2050, compared to 2005.

Thank you.
Fuel is the diesel that they put in the planes. The fleet is all the planes so the fleet fuel is all the fuel that the planes use.

improving fleet fuel efficiency

Both 'fleet' and 'fuel' here are used as adjectives here to describe efficiency.

fleet fuel efficiency - this refers to the efficiency by which all the planes in the fleet consume fuel.
eg if they consume a lot of fuel over a short distance, they have poor fuel efficiency.
eg if they can fly a long way and use only a small amount of fuel, they have good fuel efficiency.

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