Northwest Airlines frequent flyer may also be known as being a WorldPerks member. You can earn miles without even leaving the ground, read more here. Northwest Airlines frequent flyer miles.

Earning Miles on Airline Flights
Three easy steps to earn miles when you fly on Northwest Airlines®, KLM Royal Dutch Airlines and their other airline partners:

1. Provide your WorldPerks® account number when making your travel reservations. Please ensure that your WorldPerks membership card contains your full name as it appears on your passport or photo identification, and always provide the exact name and WorldPerks number shown on your card.

2. After your travel, the correct number of Northwest Airlines frequent flyer miles will be credited automatically to your account based on the nonstop mileage between your origin and destination, as shown on your ticket.

If your trip includes a connecting flight with a change of flight number, you will receive mileage credit for each flight segment.

3. Save your tickets and boarding passes until the correct mileage is reflected on your Mileage Summary statement. Northwest airlines frequent flyer mileage is normally credited to your account within two weeks after your travel date.

For mileage credit on KLM or other WorldPerks airline partners, please allow six to eight weeks. If you haven't received your miles by the time specified, learn how to submit a mileage credit request for any discrepancies using their website

Northwest Airlines Frequent Flyer Review by WebFlyer
Northwest Airlines, which earlier that year dropped the "Orient" from its name after merging with Republic, introduced its WorldPerks program. It was roughly five years behind the curve when it came into existence, but today it stands as one of the best -- and most stable -- programs among American majors.

The full review of the above can be found on the WebFlyer website.