Maybe some things that I say do make sense after all...

At present the only tax on passengers taking off from a UK airport is Air Passenger Duty of between £5 (for European flights in economy class) and £40 (long-haul superior classes). On Thursday, the Liberal Democrats unveiled a plan to tax each flight according to the environmental damage caused.

This has the dual benefit of taxing every seat, whether or not it is occupied, and making the business-class cabin proportionately much more expensive. Currently, each of the passengers on the Eos business-class only Boeing 757 from Stansted to New York pays £20 in tax - the same as holidaymakers on a charter aircraft, even though there may four times as many seats on the latter.

Almost as soon as Ed Davey, the Lib Dems' spokesman, had outlined the plans for airline passengers to fund a cut in income tax, the massed ranks of "Nimal" (Not In My Airport Lounge) protesters were lining up to lambaste the plan. What about folk who have bought a place in the sun in France or Spain expecting permanent access to cheap flights? "People who work hard and have lots of savings will benefit from tax cuts. If they choose to spend that on flights, that's up to them," says Davey.

ONE IN four of the seats on scheduled airlines is, on average, empty. This represents a huge waste of capacity. Even on the no-frills airline easyJet, the typical departure has 23 empty seats.

Sir Richard Branson maintains that airlines need no incentive to fill seats, especially at a time of high fuel prices. But Davey has little sympathy for the Virgin Atlantic tycoon: "He should invest in planes that don't do the environment damage." A couple of churlish observations: the only aircraft that leaves no environmental footprint is a recycled paper plane, and Virgin Atlantic is way ahead of British Airways in ordering more efficient aircraft. But the Lib Dems make good sense in seeking to penalise airlines that use thirstier, older aircraft.

THE AIRLINES would be faced with a big tax bill, which they would then pass on to us travellers in the form of higher fares. In unison, they decry any such burden at a time of soaring oil prices and cut-throat competition. They have long described proposals for a Europe-wide tax on aviation fuel as unworkable, pointing out that large airlines will "tanker in" fuel in the tanks of aircraft, to the detriment of the environment. But a tax on every flight taking off from a UK airport would be easy to collect and should not cause such distortions. And I bet we would soon all get used to it.

ONE MORE constituency needs to have its voice heard: the UK tourist industry. By making return flights to Britain more expensive, a tax per flight could persuade some prospective visitors to switch to other countries. But if the present absurdly low cost of leaving the UK were to rise just a little, perhaps more of us might holiday at home more often.