For some of the world's biggest airlines, the sweetest sound will be the roar of jet engines returning over the north Atlantic.
Six months into the coronavirus crisis, the skies remain quiet as the threat grows to the large carriers that depend on the transatlantic airways — among the busiest and most lucrative in the world.
Europe's British Airways and Virgin Atlantic and US groups such as American Airlines and United Airlines rely heavily on the income from the London to New York route as well as to other US destinations.
They have been hit hard by the closure of the US border and the introduction of UK quarantine rules, forcing passengers to self-isolate for two weeks on arrival, and are only running skeleton services between the two countries.
"Quarantine effectively cuts us off from the United States, our most important economic partner," Virgin Atlantic's chief executive Shai Weiss told the Financial Times.
"Over 500,000 jobs in the UK are supported by our industry and at risk without air travel at scale to key markets like the United States."
The transatlantic routes are worth an estimated US$9 billion ($13.4b) in revenue to US and UK carriers — vital for an aviation industry that is heading for global losses of more than US$80b this year.
But the prospects for recovery are grim with a full revival in aviation not expected until the middle of the decade, with long-haul travel expected to be the last part of the market to return to pre-pandemic levels.
Up until this year the number of transatlantic flights had grown steadily over the past decade, with airlines competing to carry business travellers in highly profitable premium seats.
For Virgin and BA, the north Atlantic is crucial. It makes up 70 per cent of Virgin's route network, and the airline's financial modelling is based on the transatlantic airlanes reopening in the first quarter of next year.
BA is the largest long-haul carrier into the region. Flying to 33 destinations in North America last year, the routes over the Atlantic are about a third of its flight capacity.
Last year it completed a £50 million ($96.1m) refit of lounges at JFK airport including automatically cooled and oxidised wine and a bartender to mix New York-themed cocktails. On board, it is rolling out upgraded premium-class cabins to ensure it stays competitive.
"There is a lot at stake here, much more than in Europe," one former executive at the airline said, adding that new routes to cities in the US would often turn profitable within a year.
"BA talked a good game on expanding in China, but when you looked at the nuts and bolts they always pointed west," he said.
The US airlines are less reliant on the transatlantic routes than their European rivals. They accounted for just over 15 per cent of capacity for American and United in 2019.
But they are still hurting. United chief executive Scott Kirby noted this month that the company's international business was particularly suffering. Borders "are closed around the world, and that's 50 per cent of our revenue. And until borders open up, that's not coming back."
He reiterated his view that "demand is not coming back" until late 2021, once a vaccine is available and distributed. It means the airlines are facing a tough end to the year with the crisis likely to deepen rather than ease.
International travel has been savaged at other US carriers as well. Delta Air Lines forecast international capacity would drop 80 per cent in the third quarter compared with the same period last year, while domestic would only fall 50 per cent.
Yet, despite the gloomy numbers, the political will towards reopening the skies may be weakening as coronavirus cases rise in Europe, alarming policymakers.
Indeed, politicians have so far been impervious to the fierce lobbying from the airlines for at least a tentative start to a market reopening.
Heathrow airport's state-of the art testing centre is a physical sign of the lack of conviction for a return to the skies, standing empty nearly a month after it was finished.
Even relatively modest calls for an air corridor between London and New York, backed by airport testing, that might kickstart transatlantic travel have not been met with much enthusiasm from politicians.
This was one of the measures BA chief executive Alex Cruz and Virgin's Weiss were demanding in a letter to UK prime minister Boris Johnson in September.
The executives also said they hoped "real-world data" gathered on a corridor between London and New York would give ministers "reassurance" to slowly open up other routes.
Weiss added he hoped regional corridors and testing would be a first step to "restoring transatlantic flying at scale".
In the US, the lobbying is just as intense. Industry figures have written to the secretaries of health, transport and commerce requesting the government help develop an international framework for airport testing to open up long-haul travel.
"Governments on both sides are looking at how links could be reopened in a phased way, beginning with cities like New York. We urge them to do this as soon as is feasibly possible," BA parent company IAG said in a statement.
Other executives, however, say there are grounds for optimism, with moves by UK ministers toward eventual testing of passengers for coronavirus when they arrive at British airports.
This month the UK transport secretary Grant Shapps said the government was assessing how testing might work at the UK's borders, although he warned it would not eliminate the need for people to quarantine.
In America, United's Kirby said the Trump administration and Congress had been "very supportive" of the airline industry, including by exploring ways to open borders through increased testing.
Sharon Pinkerton, senior vice-president for policy at trade group Airlines for America, also thinks the industry has some "momentum" behind it in reopening some international routes.
"The international market is still barely limping along, so we think it is critical to start to restore it," she said.
In the meantime, carriers are looking elsewhere, attempting to capture business from expatriates visiting family overseas and guided by the needs of their cargo businesses.
BA and Virgin have both announced new routes to Pakistan, while United is adding new Atlantic-crossing routes for destinations outside Europe. This month the airline announced new long-haul routes from the US to South Africa, Nigeria, Ghana and India.
"We're really focused on rethinking the network," said Patrick Quayle, vice-president of international network and alliances at United. "We're not just looking at adding everything back prior to the way it was before coronavirus."
However, despite the lobbying of governments and moves to introduce new routes, one of the biggest unknowns and potential threats to the airlines is the possible change in behaviour of passengers post-Covid.
Whether people will be more inclined to travel less as they shift to remote working and videoconferencing is the big, unanswered question.
Traveller skittishness, not regulatory barriers and corporate travel bans, is the large hurdle to the recovery of demand, Delta executives told JPMorgan Chase analysts, with the chances of next summer's transatlantic season improving compared with 2020 a possible but optimistic hope.
One lawyer at a top London firm summed up the demand worries, forecasting fewer business trips in future.
"We used to often buy business-class round-trip tickets on the day, whatever the cost. But that may change," he said, in a sign of a switch in behaviour that may prove costly to the world's big airlines.
Written by: Philip Georgiadis and Claire Bushey
© Financial Times