Yesterday our proprietary airfare processing system detected an unusually large number of domestic U.S. city pairs with an increase of up to $16 roundtrip, a hike, initiated by American Airlines.

Continental and Southwest started matching late last night and hours ago Delta/Northwest, United and US Airways began matching — rounding out the legacy airlines

Most airfare hike attempts occur late in the week and either “stick” or “fizzle” over the weekend as carriers decide whether or not to match — it is unusual to see an airfare hike early in the week which is typically reserved for discounting and sales.

We contacted American Airlines about the airfare hike attempt and they confirmed the mileage based domestic airfare increase at the following levels:

• 0-450 miles – $3ow/$6rt
• 451-750 miles – $5ow/$10rt
• 751+ miles – $8ow/$16r

A spokesperson for Southwest Airlines further notes they have matched with at a lower level than the legacy airlines (with no increase on their cheapest “fun fares”): “Our advertised Fun Fares will not be increased. These
are modest increases varying by length of flight to help offset higher
fuel and other operating cost pressures. This is not a seasonal or
holiday fare increase, and we are not planning an increase for just
certain days of travel.” Here are how the Southwest increases break down:

• 0-450 miles – $2ow/$4rt
• 451-750 miles – $3ow/$6rt
• 751+ miles – $5ow/$10rt

This hike is layered on top a targeted peak holiday surcharge of $10 each way over the past two weeks and appears to be well on its way to “sticking” as the 4th increase of 2009 – all 4 occurring since June (compared to 15 total in 2008 and 17 in 2007).