Aviation is a relatively small amount of global carbon dioxide emissions (2.5% or so). However, there is some evidence that their emissions are much worse than they seem on paper due to NOx and water vapour emissions high in the troposphere. Even if coconut oil was a remotely viable alternative it would do nothing to address these other issues.

There's more to the environment than the greenhouse effect. Coconut plantations would compete with rainforest in much the same way palm oil ones do, since they have more or less identical growing climates.

If you want to use biofuels, use a plant that grows in low-value areas like open fields. Something like corn or sugarcane.