This is from Northwest's website. I'm sure a minute of your time will find teh same information from your particular airline.
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Skiing and Snowboarding Equipment
Northwest accepts one item of skiing/snowboarding equipment in lieu of one piece of luggage included in the free luggage allowance. Skiing/snowboarding equipment is accepted as checked luggage only. An item of ski equipment is defined as one of the following:
* One (1) ski bag containing up to two (2) pairs of skis and one (1) boot bag
* One (1) snowboard
* One (1) snowboard bag containing up to two (2) snowboards
If combined weight of ski bag and boot bag exceed 50lbs/23kg an overweight fee applies.
Additional ski and boot bags will be subject to normal excess charges. Northwest recommends hard-shell cases when checking your ski equipment.
Northwest does not accept liability for loss, damage, or delay of skiing/snowboarding equipment. Excess valuation insurance may not be purchased for transport of skiing/snowboarding equipment.