I noticed that United says it gives NO mileage credit for flights booked on Air Canada in "W". Since I fly a lot between SFO-YVR, I've often ended up flying Air Canada codeshares. Since United's information (found at http://www.united.com/page/article/0,,1180,00.html) was unclear about whether "W" fares booked with a United flight number but on Air Canada metal count as miles, I emailed the question to United. Actually, at the time I didn't see the ambiguity and just read it was saying Air Canada metal booked in W never gave miles.

I emailed United asking whether I could earn miles when on a code share on AC metal. Their response: "The contact agreement between United and Air Canada only allows the following classes of service to be eligible for mileage accrual with Mileage Plus: C, J, Z, A, B, E, G, H, I, K, L, M, N, O, P, Q, R, S, T, U, V, X, or Y class. You can review this online at www.united.com."

From the time I got their email response, I've avoided ever ending up in W on Air Canada metal, even if I booked with a United flight number. However, last month it became unavoidable and I flew a short segment in W on AC metal. To my surprise, I got miles recorded as Air Canada "A" class.

So either United's customer service got it wrong or I got 500 miles I shouldn't have. I don't really care either way about 500 miles, but I very much care about whether I need to continue to avoid booking any UAL codeshare on AC metal.

More experienced fliers -- do you have an answer?

I guess the side-question is this: Does it ever make sense to book a flight that is 100% UAL metal as an Air Canada codeshare in order to earn the 500 mile minimum? Most booking classes on AC earn 500 miles minimum and 100% mileage credit on UAL (see http://www.united.com/page/article/0,,1180,00.html).