Not very, according to this article from The Register (based in the UK). I’d like to pass this along as a central theme of L’avant Poste is travel.
Apparently it takes more than just mixing two liquids together in the bathroom of a plane in mid flight for a bang. It takes a cocktail of items, several hours of work, and the terrorists have to be carful they don’t blow themselves up in the taxi on the way to the airport.

After a few hours – assuming, by some miracle, that the fumes haven’t overcome you or alerted passengers or the flight crew to your activities – you’ll have a quantity of TATP with which to carry out your mission. Now all you need to do is dry it for an hour or two.

And these guys aren’t too bright anyway. Attempting to attack the most difficult mode of transporation on the most scrutinized routes between two cities on constant high-terror alert.

Hope you haven’t changed your travel plans because of it. This might make you feel better after all.

Plus planes can fly despite the worst of conditions, take the Aloha flight: