I swear by Lind adapters (www.lindelectronics.com)

it is true that the port on a plane is limited to 75W and some laptops need 90- in that case, the lind adapter forces the laptop to stay in the 'battery mode' thereby reducing the draw to 75W or less. Bad news is that the battery MIGHT not charge, but at least you don't run them down. If you turn the computer off but keep it plugged in, the battery will then charge.

I've used lind adapters with my Dell C6xx, C840, and M70... bullet proof.