On a jet card's pros list is convenience: Users don't have to exchange contracts or make payments each time they arrange a flight. One call and they can be off on an impulse flight in a matter of hours, the cost debited from their card.

But mark-ups can be considerable on some cards, with clients effectively underwriting suppliers' businesses. Customers can also be tied in for a given period.

Air Charter Services recently launched a jet card called the Lindbergh Card, which is different, it says, because there is no small print, though it does require at least £50,000 in upfront payment.

ACS does not own a fleet and is therefore free from aircraft mortgages. Jet cards from fleet-based operators, says its founder and chairman Chris Leach, "tend to be full of restrictions, covering every variable - taking into account empty legs... so by and large their hourly rate is forty percent above cost."

Leach added in an interview that his company had always been the antithesis of the jet card idea, but things changed when some regular customers gave him an ultimatum to create one. ACS eventually did this in association with Erik Lindbergh, an aviation enthusiast whose grandfather Charles was the first man to fly solo from New York to Paris.

According to Adam Twidell, CEO of Private Fly, an online marketplace for private charter aircraft, some corporate customers do prefer to commit funds once a year to a fixed number of flight hours. "Buying a card allows them to ask their board once a year only. They are prepared to take a commercial hit for this convenience."

But, Twidell says, European clients in particular are beginning to ask questions about the extravagant mark-ups on jet cards. "Many card-holders compare their options for every flight... they buy 50 percent less upfront on their card and compare each flight against the charter market."

Clive Jackson, a serial British entrepreneur who set up rival online private jet marketplace Fly Victor is less equivocal. He thinks business customers should buy on the spot because what is on the market today is a reflection of the real-time cost and availability of aircraft.

"You don't do this for limos or taxis, why would you continue to do this for private jets?" he said in a phone interview.

Jackson says that in most cases ad-hoc charter on his website on a like-for-like jet on the same route can save between 20 to 40 percent of the cost of going through traditional air charterers. "Due to the time pressures clients are under, jet card providers thought they could get away with this."

"Brokers are literally calling for my head."

ACS believes they also are creating significant savings with their Lindbergh Card. "Every five or 10 flights we'll send clients a statement comparing the going hourly rate from the fleet-based cards and ACS's - and usually they'll find they saved between ten and 40 percent," says Leach.

Another jet card purveyor Air Partner, who offer a product which they describe as "completely transparent and very flexible", say they are finding that private jet users are increasingly looking for charter solutions.

The company said by email that 30 percent of its new JetCard customers have transferred from fractional ownership schemes, and JetCard membership is at record levels.~

"The question educated buyers are asking themselves," says an Air Partner spokesperson, "is if I want access to a private jet, potentially a depreciating asset, why should I buy it when I can rent it?"

Asked to comment, a NetJets Europe spokesperson wrote that, "Given macro-economic conditions, businesses are keen to avoid the large capital outlay traditionally associated with jet or even fractional jet ownership."

However, they added that despite the used jet market being at its most crowded for some years, their model has proved remarkably resilient, with that many of their customers wanting the financial and tax benefits that come with ownership of an asset.

"The recent NetJets global purchase of aircraft valued at around $9.6bn, the biggest deal in the history of private aviation is testimony to this long term confidence across the industry."

(Editing by Mark Kolmar)

• A Permanent Economic Emergency