The jumbo jet, for many years the workhorse of modern air travel, could be close to running out of runway.
Last year, there were zero orders placed by commercial airlines for new Boeing 747s or Airbus A380s, reflecting a fundamental shift in the industry towards smaller, twin-engine planes.
Smaller planes cost less to fly than the stately, four-engine jumbos, which can carry as many as 525 passengers.
The slump in sales of the jets has raised questions over how long manufacturers can sustain production. It has also fuelled internal debate in both companies over the future of the planes, sources said.
The outcome of those discussions will affect the value of existing fleets and thousands of production jobs at the plane makers and their many parts suppliers.
Sales forces at Airbus and Boeing are fighting for potential orders plane by plane as they seek to keep production going beyond the end of the decade.
The aircraft makers are offering discounts of at least 50pc from catalogue prices of around $400 million for a jumbo jet.
Airbus has said it is also considering a revamp to make its 'superjumbo' more attractive to buyers.
Boeing in September plans to slow the pace of production of its latest 747-8 model to an average of 1.3 planes a month from 1.5 currently.
At that rate the orders it already has in hand will only keep the production line going for two-and-a-half years.
The crunch, though, will come earlier because it can take up to two years from ordering the first part to finishing a jet, and no one wants to start the process if it is unclear whether the plane will be completed and delivered to a customer.
"I can see demand for the 747-8 in small numbers, but you have got to ask if they can keep the production line open if they don't get some new orders," said Tony Whitty, chief executive of UK-based aircraft re- marketing firm Cabot Aviation, which trades, manages and leases jets. "You also wonder at what price they are selling."
Use of the 747 has dropped steadily over the last two decades, reflecting the rise of two-engine jets that have come close to matching its range.
Over the same period production of large twin-engined jets like the Boeing 777 has risen seven-fold. Last year, Boeing booked 283 new orders for the 777 and now has a backlog of 547 orders.
Airbus is more upbeat than Boeing about the prospects for jumbo jets but both now agree it has become a niche category.
Airlines still need jumbo jets but only for certain polar flights - where a two-engine jet may be less safe than a four-engine jumbo because of the lack of places for an emergency landing - and busy routes where landing slots are scarce.
The risk is most visible for Boeing, where investors could face a $1bn accounting charge if 747 production is shut down.
Boeing recently received a high-profile boost with a provisional order for two new jets to serve as Air Force One for the US President but the 747's future depends a lot more on sales of the much-less glamorous windowless freight model.
That has a unique hinged nose and can carry very large equipment, such as oil drilling rigs.
So far this year, Boeing has sold three.
Atlas Air Worldwide recently said it plans to order more for its cargo fleet, but wouldn't say when or how many.
The world's biggest 747 freight customer, Cargolux, also says it likes the plane, but has a pending order for only three.
A sustained upturn in air freight traffic could secure the 747 a longer future.
International freight traffic rose 4.8pc last year, but volume has only just recovered from a collapse in 2009 during the financial crisis.