CHICAGO, Nov. 24 (Reuters) – The number of air travelers over the US Thanksgiving holiday will be down this year, which means shorter lines and fewer airport hassles, but full planes and new fees may keep passengers grumbling.

''With seat cutbacks in the 6-7 percent range – and on some airlines even higher – the bottom line is planes are going to be as full as ever,'' said Rick Seaney, chief executive of Farecompare.com, on Monday.

''While there may be less people physically at the airport, the planes are going to be packed,'' Seaney said.

The US airline industry, battered for more than a year by economic turmoil, has slashed capacity since 2007 to offset weaker demand and support fares. As a result, airports are less busy but planes are extremely full.

The Air Transport Association (ATA), an industry trade group, said this month that it expects a 4 percent year-over-year decline in the number of passengers on US carriers during the traditionally hectic Thanksgiving holiday.

''It is increasingly apparent that the economic headwinds facing the airlines and their customers are anything but behind us. The recent announcement that US unemployment surpassed 10 percent highlights one of the key factors impacting consumer buying decisions,'' said ATA president James May in a statement on Nov. 9.

Nevertheless, the skies over the United States tend to be more crowded on the days around Thanksgiving than at any other time of the year. The four busiest travel days surrounding Thanksgiving Day this year are expected to be Nov. 20, 25, 29 and 30, the ATA said.

Holiday travelers cringe at the thought of weather delays or air traffic control glitches like the one last week that led to widespread delays and cancellations, particularly in the heavily traveled New York area.