Jens Flottau Frankfurt and Guy Norris Los Angeles

That the very large aircraft market has its limits has long been recognized by Boeing and Airbus. But five years after entry into service of the Airbus A380 and two years after the Boeing 747-8F was first delivered, even their existing backlogs appear to be partially under threat.

Boeing has listed 108 firm orders and 37 deliveries for the 747-8 and -8F, leaving it with a backlog of just 71 aircraft. The situation looks more comfortable for Airbus, which still has 165 outstanding deliveries for the A380. But, in spite of the wing-rib-feet redesign slowing production next year, its output is still much higher.

One factor also cannot be disregarded in the A380 figures: More than one-third, 62 aircraft, are still to be delivered to the single largest operator of the aircraft, Emirates, which has left no doubt that it will take the outstanding jets as part of its huge expansion plans. But of the 103 other orders, there are several commitments for 23 aircraft with big question marks.

Most immediately, an order for 10 aircraft placed by Hong Kong Airlines will likely be converted very soon to much smaller A330s. The carrier has indicated it plans to pull out of the long-haul market and focus on regional flying, for which it appears the A380 will not be the right-sized aircraft. There have been allegations that the order conversion might be linked to China's opposition to the European Union's Emissions Trading System (EU ETS), much like another commitment for 35 A330s that are to be allocated to several Chinese airlines. The fact that simple market concerns are driving Hong Kong Airlines away from the A380 gives Airbus little comfort.

Another dubious order is Kingfisher Airlines' commitment for five aircraft. The airline has been grounded since early October, after flight crews went on strike for not having received salaries for months. Part of its current fleet is impounded. Even if efforts to resurrect Kingfisher succeed eventually—which would likely involve a new foreign minority shareholder such as Etihad Airways—it is doubtful if Kingfisher could support operations of five A380s, even in the long term.

Much less dramatic cases give Airbus reason to be worried, too. Virgin Atlantic's outgoing chief executive, Steve Ridgway, says the airline will not take its six A380s on order before 2017, if at all, and he questions whether Virgin could ever viably operate such a small sub-fleet. It is too early to tell what influence Delta Air Lines' new 49% stake in the British carrier will have. While the planned transatlantic joint venture is likely to drive more passengers into Virgin's aircraft, Delta is not particularly keen to operate very large aircraft. It inherited a fleet of Boeing 747-400s in its merger with Northwest Airlines, but they do not seem likely to be replaced by a similarly sized or even larger aircraft type.

Qantas has eight more A380s to be delivered and has not indicated that it might not take them all, despite suffering huge losses in its long-haul operations. However, it has deferred to 2016 and 2017 two that were due to be delivered in 2013, and it has pushed back beyond 2019 the remaining six, which also has freed up earlier production slots in Toulouse.