by PhotoSpin

_A reader inquires: On a flight recently, I left my cellphone on by accident,
and yet the plane did not fall out of the sky. Aren’t “personal electronic
devices” supposed to be a danger to cockpit controls?

It’s unlikely, but electromagnetic interference from your personal electronic device, or PED, could disrupt the plane’s avionics in a number of ways. This is because the internal circuits of PEDs generate small magnetic fields, and these changing magnetic fields could create a current — however small — in nearby wires. Now imagine that one of those wires is a cable that sends electric signals from the rudder to the cockpit. When you turn your portable CD player on and off, its magnetic field changes, thereby inducing additional current in the cable and altering the rudder signal. If you’re at cruising altitude, this might not be much of a problem, but if you’re coming in for a landing, it could be disastrous.

In addition, some PEDs — cellphones, laptops with wireless Internet cards, etc. — are designed to emit electromagnetic waves. Their signals are set to specific wavelengths, nowhere near the wavelengths reserved for air navigation. But your laptop’s frequency might combine with a neighboring PDA’s emissions to create new, more powerful frequencies that could, worst-case scenario, be on the aviation bandwidth. Presto, your pilot loses a navigational aid.

No accidents have been attributed to PED interference and studies have concluded that the risk is slight, but it is the suspected cause of a small number of bizarre events in the cockpit: an altimeter suddenly becoming useless, an autopilot disconnecting or initiating a roll for no apparent reason, a display blanking out. In some of these incidents, the flight crew has been able to establish a strong correlation between turning the PED off and on and the disappearance and return of the anomaly. But because of the complexity of the variables, engineers cannot reliably duplicate these incidents and pin down their causes. So airlines will likely continue their better-safe-than-really-sorry approach: Devices are allowed at cruising altitude only if deemed safe by individual airlines; takeoff and landing remain sacrosanct electronics-free zones.

Cellphones, a special case, are never allowed on in the air — not because they may interfere with navigation, but because they may disrupt cell service on the ground. The U.S. cellular infrastructure is engineered to handle calls coming from slow-moving ground-based sources. When you place a call from a car, you’re in sight of, say, four cell towers at a time. These towers decide among them which is receiving the strongest signal from you at any given moment, and hand off your call from one to another as you travel. In the air, you’re likely in sight of hundreds of towers at a time. The system is not set up for hundreds of towers to communicate, and the standing army of poor muddled towers may simply drop their ground calls to accept your call. The airlines don’t care about dropped calls on the ground, but the FCC does, and so extends its regulatory power quite literally into the skies.