It has been said that the chemical limit of the refining process yields a maximum of about 1/3 jet fuel. Delta is pushing the refinery to the chemical limit.
Note that DL is continuing w/ the transaction despite the 15-20% fall in crude prices over the past month or so. If DL wanted out of the deal, they certainly could have reason to do so... but it says they are continuing w/ the transaction because it is aimed at reducing the difference in jet fuel relative to the cost of gasoline - which is pushed up because more and more refining capacity is being shut down. Given that most refineries in the US produce 12-15% jet fuel with the greatest output in gasoline, jet fuel production continues to fall at a rate that DL feels puts the availability of jet fuel at risk based on decreased refinery capacity in the US and Europe, which reflects those economies' current weakness but also the increased fuel efficiency that is occurring in the US.
There is alot of information on the EIA website I cited above that shows that there is indeed a possibility that left to market forces, jet fuel availability will fall and prices will rise relative to other petroleum products. DL's move is intended to keep the market in balance for DL's needs. It is important to note that there is no assurance that other refiners will produce the same amount of jet fuel they currently do since they could reduce production of jet fuel to force the price of their jet fuel up to offset the jet fuel Trainer will produce, although it can be used only by DL.
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Also, DL has said that the refinery deal will free up cash which it and other airlines have to use to buy jet fuel in advance including with the use of hedges which have not been shown to deliver a long term cost reduction. By buying jet fuel as it is needed, DL could be freeing up $1B or more in cash for use for other uses including executing strategic objectives, or reduce the amount of cash it must keep on hand.