Airliners magazine (Sept/Oct 2001) has an article about the delivery of 717's from Long Beach to Hawaii. They use a similar system to those mentioned above - additional tanks are fitted in the passenger cabin and feed by gravity (with back-up electric pumps) into the aircraft centre tank. Eight rows of seats are removed to house the tanks. The same tanks are then used to ferry a retiring DC9 back to the mainland, although, where the 717 requires four additional 250 gallon tanks, the DC9-50 requires nine such tanks for the same trip.