I was just browsing flight aware and was surprised to see some flights in the US with altitudes of as low as 13,000 ft.

 

For example

 

SWA824 - KMDW to KTPA - 2hr flight but only 13,000

 

Then SWA1255 - KDFW to KDEN - 2hr flight but 39,000

 

Just curious as to what factors determine such a big difference?

In Europe, one has Upper and Lower airways, and the same is true in the U.S though the lower airways are referred to as Victor airways and they exist between MSL (Mean Sea Level) and FL180.  There are also special military airways at various levels, usually the lower ones.

 

Aircraft will use Victor Airways for two primary reasons, aircraft type (ability to climb above FL180) and route length.  A flight from Philadelphia to New York City (about 200 miles, if that far) would probably use a Victor airway.

 

There is another reason for using a Victor airway... once on a flight from Philadelphia to Boston, our pilot was unable to get clearance for a regular higher altitude jetway, and asked the dispatcher for a Victor airway routing.  We burned more fuel, but kept our schedule. I can't tell you why we were denied a higher altitude flight, but we flew it at a lower level and had a much more scenic flight.

 

Hope this answers your question.

 

 

SWA824 - KMDW to KTPA - 2hr flight but only 13,000

It actually flew @ FL410

http://flightaware.com/live/flight/SWA824/history/20140519/0135Z/KMDW/KTPA/tracklog

The airspace in the Chicago area is highly congested, and there are no RNAV SIDS serving Midway airport. All departing aircraft will be radar vectored to their first enroute waypoints after takeoff. In those cases, 13,000 feet may be the maximum altitude that Chicago departure can clear a departing aircraft to on certain headings. o they would file that as their initial cruise altitude in their FP.

 

The aircraft would be cleared to their actual cruise altitude by Chicago center, once they are clear of the Chicago terminal area. I've seen a similar type of flight plan on Flightaware for northbound flights out of the Orlando area - where the filed initial cruise altitude is less than 14,000 feet for most departures.

 

Another possibility is that Florida-bound flights from northern cities tend to be highly popular, with a good chance that all seats will be filled on any given trip. In this case, Soutwest's dispatchers may not know the actual cruise altitude that will be used until shortly before departure, when the zero fuel weight and fuel load is known. In that instance, they may use 13,000 feet as a placeholder altitude in the initial FP, which is typically filed many hours before the actual departure.