As Airbus and Boeing go head to head again in the short-haul market, the two are already exchanging words about the competitiveness of their two new products - the A320neo and 737 Max.
Boeing claims each member of the Max family will be 10-12% more fuel efficient than their equivalent variants in the current 737 range, and will also have a advantage over the A320neo.
However, Airbus counters by pointing out that the new Boeing's specification is still fluid - despite having secured deposits for almost 500 commitments - as numerous key design decisions that will affect performance have yet to be made.
"We believe we'll be able to hold a 7% cash-operating-cost advantage over the competition - it's about 8% today," said Randy Tinseth, vice-president marketing of Boeing Commercial Airplanes.
"A 737 'Max 8', compared with an A320neo, will have about a 4% [fuel burn per] seat advantage," he added.
However, Airbus chief salesman John Leahy said he was unable to assess Boeing's claims because so much of the new aircraft's specification is undefined.
"I don't quite know what it is right now. There is no fan diameter size, engine thrust size, take-off weight or empty weight, or take-off performance," he said.
"As soon as they write a spec and decide what the airplane is, then I'll have some comments on it. But I can't comment on something that doesn't exist. And that's another reason why they're having trouble getting anybody to commit to it."
While acknowledging the new 737 will have increased operating weights and greater range over the current variants, Tinseth concedes that details are still being finalised.
However, he expects the firm configuration will be achieved in "the next weeks and months to come. We want to make sure we do this in a methodical, thoughtful way through our gated process".
Tinseth said Boeing has been able to secure commitments for an aircraft that is relatively undefined "because our customers understand the aircraft today, they understand the work statement we're talking about and they know where we're going".