Under this program, recently launched by Sherpa Pet Group and American Airlines, if your dog fits in one of three Sherpa carriers approved by American and you correctly fill out the Guaranteed On Board form “you can be sure that your pet and its carrier are compliant with American’s rules and regulations. Should you be denied boarding due to your carrier, Sherpa Pet Group will refund the cost of your American Airlines flight and your pet travel expenses – GUARANTEED!”

It’s meant to take the guesswork out of choosing an in-cabin pet carrier, and that’s a nice idea. Am I the only one to notice that two of the three approved carriers don’t actually comply with American’s posted maximum measurements for in-cabin carriers (19″ long x 13″ wide x 9″ high)? The new American Airlines-branded medium Duffle is only a half inch taller than the stated maximum, but the medium Sherpa Original Deluxe is 1.5″ taller than American’s maximum allowed height. The point is really moot, because the carriers are flexible enough that the discrepancy doesn’t make a practical difference, but it goes to show that even a move like this doesn’t remove all the guesswork.

However, according to Sherpa, similar agreements exist with Airtran, Alaska, Continental, Delta, Northwest and Southwest. The company emphasizes that “only NWA accepts carriers in small, medium, and large; all other airlines accept only small and medium sizes.” First on the list of “airline-compliant” Sherpa carriers is Sherpa’s Original Deluxe Bag, with the following measurements: 16″L x 9.5″H x 10″W (Small), 18″L x 10.5″H x 11″W (Medium), and 20″L x 11.5″H x 11.7″W (Large).

Now take a look at Dog Jaunt’s handy guide to the major U.S. airlines’ in-cabin pet policies. Even the small-sized Sherpa carrier is officially too large for all of the listed airlines except Delta, which will tell you the maximum allowed dimensions for your particular flight only when you call Reservations. [Southwest doesn’t publish maximum dimensions, but its published underseat measurements are 19″L x 14″W x 8.25″H (window seat), 19″L x 19″W x 8.25″H (middle seat) and 19″L x 14″W x 8.25″H (aisle seat). At 9.5″ high, the small Original Deluxe carrier is too tall for Southwest.]

Can it really be that all of these airlines also guarantee boarding with the medium-sized Original Deluxe Bag? And that Northwest accepts the large size? Officially, the large-sized Original Deluxe Bag is 3 inches longer and 3.5 inches taller than Northwest allows. Based on personal experience, moreover, I can tell you that any carrier over 19″ long will not fit under an airplane seat.

Does this mean that someone with an equally non-compliant carrier can point to these guaranteed-okay dimensions and argue successfully that their carrier should also be allowed on board? Does the Guaranteed On Board program, in short, actually remove the guesswork — or raise more questions?