I noted back on November 22 that US Airways had announced they were bringing back the 100% bonus on purchased miles in December.

At the time I said I thought it was odd that they would post on the purchase miles website that a big bonus would be forthcoming, since it seemed like that would discourage miles sales at full price through the end of November and I tried to come up with a rationalization for it.

Turns out it probably didn’t make sense to announce it, because shortly afterward they pulled the announcement!

But the plan was indeed in the works, as I just received an email from US Airways announcing the 100% bonus. through December 31, 2011.

Of course, last month US Airways raised the price of purchased miles for the second time in a year and a half. That, combined with the award chart bump of two years ago, means that buying an award ticket from the US to Europe in business class through mileage purchase is about 80% more expensive than it was in December 2009. I no longer buy US Airways miles speculatively.

And in fact this new offer limits purchases to 40,000 bonus miles (meaning half off a purchase of 80,000). It’s not possible, this time around, to use the promotion to get all the way from zero to a business class award to Europe with all miles purchased at the discounted price.

However, if you need to top off your account, a 100% bonus (ie 50% discount) is great. And since US Airways allows you to put an award on hold without the needed miles in your account, you can set up an award and then and only then buy the miles you need to top off (which usually post instantly), making this still strategically useful.

If you don’t have an account and you plan to use this offer, open one asap, because as always US Airways mileage accounts have to be open at least 12 days before jumping on the buy miles promotion.