Turning on airplane mode was never really about keeping the plane in the air, a phone has almost no chance of downing an aircraft, and the rule exists largely because a cabin full of phones frantically pinging towers annoys ground networks and, at worst, buzzes in the pilots' headsets

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Almost everybody who has flown commercially in the past two decades has heard some version of the same instruction. Before pushback, before the safety demonstration, before the aircraft has begun to move at all, a member of the cabin crew asks everyone on board to switch their phones and other electronic devices to airplane mode. Almost every passenger complies, mostly because it is the rule, and mostly under a background assumption they have carried since childhood that leaving the phone on somehow poses a genuine risk to the flight. The image, absorbed from decades of news coverage, cabin announcements, and popular film, is of a phone signal quietly interfering with the aircraft’s navigation instruments, and of an unfortunate someday when the accumulated risk finally produces a real accident.

The regulatory record on this, when you read it directly, does not support that background assumption. There has, on the peer-reviewed and government evidence, never been a documented case of an air accident caused by a passenger’s phone. There are two separate rules requiring airplane mode, from two separate agencies, and neither of them, as currently written, is primarily concerned with keeping the aircraft in the sky.

What the rule actually is

According to the official text of the United States Code of Federal Regulations at 47 CFR §22.925, published by the Federal Communications Commission and maintained in the electronic Code of Federal Regulations, cellular telephones installed in or carried aboard aircraft must not be operated while such aircraft are airborne. The rule is short. It has been in effect since 1991. It applies whenever a phone is off the ground. And the reason the FCC created it, on the primary-source record of the original 1991 order, has nothing to do with the aircraft. It has to do with what happens to cellular networks on the ground when a phone starts transmitting from several thousand feet in the air.

The cellular network protocol was designed on an assumption that has held true for almost every phone that has ever existed. That assumption is that a given handset is located inside the ordinary geographic coverage area of a small number of specific cell towers, typically one or two at most, which the network can then use to route the call correctly and to charge the customer accurately. When a phone is at ground level, this assumption is trivially met. The nearest tower is a few kilometres away. Other towers are further, and their signals arrive too faint to matter.

When a phone is at thirty thousand feet, the assumption breaks. From that altitude, the phone can suddenly see hundreds of ground-based cell towers simultaneously, all of them at roughly the same distance and roughly the same signal strength. The network protocol was never designed to handle that configuration. The result, if a large number of phones on an aircraft were all transmitting from cruising altitude at once, would be a small but genuinely disruptive burden on the cellular networks below, which would have to allocate resources to serve handsets that were about to move over the horizon within seconds. The FCC’s 1991 rule, on the primary regulatory record, exists to prevent that congestion.

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The second rule, run by a different agency, has separate origins. According to the Federal Aviation Administration’s Advisory Circular AC 91.21-1D, "Use of Portable Electronic Devices Aboard Aircraft," most recently updated in October 2017, the FAA has maintained a separate concern about portable electronic devices dating back to a 1961 finding that portable FM radio receivers could interfere with aircraft navigation and communication equipment. That concern was broadened over the decades to cover all portable electronic devices, and it is the reason the FAA’s regulations require passengers to keep certain devices off during particular phases of flight. But the FAA’s own current guidance is explicit that the airplane mode requirement itself is primarily a mechanism for keeping passengers in compliance with the FCC’s separate cellular rule. The FAA’s Advisory Circular notes that requiring airplane mode during device operation "will help prevent violation of these regulations," meaning the FCC’s ground-network rule.

The two agencies, in other words, are both worried about interference. But the modern interference the airplane mode switch is designed to prevent is not primarily interference with the aircraft. It is interference with the cellular network on the ground.

What actually happens if you leave the phone on

If you have ever forgotten to switch a phone to airplane mode before...