As anyone who has flown has heard, using a cellular telephone aboard an aeroplane is dangerous. American Airlines warns passengers that mobile phones "may interfere with the aircraft's communication and navigation systems." Similar warnings come from Delta, United and Continental. British Airways links cellular interference to potential problems with compasses and even cabin pressure. What the airlines don't tell passengers is that there is no scientific evidence to support these claims. What concerns there are about cellular phones in aeroplanes dwell in the realm of anecdote and theory -- and to some extent in that of plain finance. There is money to be earned or lost by cell-phone companies and airlines if cell phones are used in-flight. A 1996 study commissioned by the U.S. Federal Aviation Administration looked at thousands of flight records and failed to find a single instance in which equipment was affected by a wireless phone. The study was conducted by RTCA, a nonprofit organisation that sets industry standards for aeroplane electronics. Plane makers Boeing and Airbus Industrie have bombarded their aircraft with cell-phone frequencies and discovered no interference with communication, navigation or other systems. One likely reason that no problems were found: cellular phones don't operate on any of the frequencies used by aeroplane systems. "The airlines are misleading the travelling public," says John Sheehan, who headed the RTCA study and says he has often used his own cell phone in the sky. "There is no real connection between cell-phone frequencies and the frequencies of the navigation" or communications systems. Using cell phones aloft on commercial and private aircraft is banned not by the FAA but by the Federal Communications Commission, which regulates telephone use. In prohibiting airborne use in 1991, the FCC was mainly concerned about cell phones' potential to interfere with ground-to-ground cellular transmission. The FAA has never outlawed cell-phone use in aeroplanes. But the agency supports the FCC ban "for reasons of potential interference," according to an FAA advisory. Despite the findings of the 1996 RTCA study, the FAA remains concerned about anecdotal evidence of cell-phone interference in flight records, says an FAA spokeswoman. The FAA isn't the only party still concerned. Boeing continues to advise airlines against cell-phone use in the sky. That's because the electrical charge from the batteries in most handsets exceeds the plane maker's standards. Although Boeing's tests have never shown this to be a problem, in theory the electricity emanating from the device could create interference with aeroplane systems. The airlines and telecommunications companies also have an economic incentive to keep cell phones turned off in the air. The carriers receive a cut of the revenues from the telephones installed onboard. The two main providers of this air-phone service, GTE and AT&T charge about $6 (£3.65) for a one-minute call, more than 20 times typical cell-phone rates. These in-flight telephones also operate on cellular technology -- using a single aeroplane antenna to which the onboard phones are typically wired. AT&T and GTE, which recently agreed to sell its Airfone service, decline to discuss air-phone financial arrangements, as do several airlines. But Sheehan says airlines pocket about 15 percent of all air-phone revenue generated on their planes. GTE declines to discuss Airfone revenues, but analysts estimate the unit's annual revenues at $150m. Some airlines also restrict cell-phone use on the ground, which isn't covered by the FCC ban, and which the FAA leaves to the airlines' discretion. Sheehan says he believes air carriers have resisted allowing cell-phone use on the ground because it "detracts from the revenue they get from the air phone." Airlines deny this, and say the bans are for the benefit of the passengers. "We don't believe it's a good safety issue" to allow normal cell phones, says Andy Plews, spokesman for UAL's United Airlines. "We'd like people to use the air phones."