The older term "air taxi" tends to be used for piston-engined or small turboprop aircraft, although the ********s of an air taxi and a business jet are essentially identical; in fact, some airfields have runways unsuited to jet operations and may therefore be more usable by slower aircraft. Depending on the passengers' destination, the overall journey time could then be shorter with a slower aircraft. Generally, jets tend to have a taller passenger cabin and more advanced avionics, which may be advantageous in terms of safety, comfort and resilience to extreme weather conditions. A company may also wish to project its status through the type of aircraft in which its personnel travel. Almost all production business jets, such as Grumman Aerospace's Gulfstream and the Gates Lear Jet (now built by Bombardier), have had two or three engines, though the Jetstar, an early business jet, had four. Advances in engine efficiency and power have rendered four-engine designs obsolete, and only Dassault Aviation still builds three-engine models (in the Falcon line). The emerging market for so-called "very light jets" and "personal jets" has seen the introduction (at least on paper) of several single-engine designs as well. Airliners are sometimes converted into luxury business jets. Such converted aircraft are often used by celebrities with a large entourage or press corps, or by sports teams, but airliners often face operational restrictions based on runway length or local noise restrictions. A focus of development is at the low end of the market with small models, many far cheaper than existing business jets. Many of these fall into the very light jet (VLJ) category. Cessna is developing the Mustang, a six-place twinjet (2 crew + 4 passengers) planned to be available for $2.55 million USD at the end of 2006. A number of smaller manufacturers have planned even cheaper jets; the first is the Eclipse 500 which has become available at around 1.5 million USD. It remains to be seen whether the new jet manufacturers will complete their designs, or find the market required to sell their jets at the low prices planned.