Cockpit hardware and software use radio transmissions for a number of tasks. Whether transmitting, receiving or simply sitting idle, cellphones are able to garble these signals. As you might expect, aircraft electronics are designed and shielded with this interference in mind. This should mitigate any ill effects, and to date there are no proven cases where a cellphone has adversely affected the outcome of a flight. But you never know, and in some situations -- for instance, in the presence of old or faulty shielding -- it's possible that a telephone could bring about some sort of anomaly.
Notice that I say "anomaly" and not "flaming wreckage." You imagine some hapless passenger hitting the send button when suddenly the airplane explodes, flips over or nose-dives into the ground. In reality, should it occur, interference is liable to be subtle, transient and, in the end, harmless. People have a hard time grasping that every in-flight problem is not an impending catastrophe, and this is no exception. The electronic architecture of a modern jetliner is vast, to say the least, and most irregularities aren't exactly heart-stoppers -- a warning flag that flickers for a moment and then goes away, a course line that briefly goes askew. Or something unseen. I'm occasionally asked if I have ever personally witnessed cellular interference in a cockpit. Not to my knowledge, but I can't say for sure. Planes are large and complicated; minor, temporary malfunctions of this or that component aren't uncommon. Nine times in 10, what brought about that fleeting glitch is never known.
Having said that, cellphones may have had a role in at least two serious incidents. Some blame a phone for the unsolved crash of a Crossair regional plane in Switzerland seven years ago, claiming spurious transmissions confused the plane's autopilot. In another case, a regional jet was forced to make an emergency landing after a fire alarm was triggered by a ringing phone in the luggage compartment. There have been other, anecdotal reports of varying seriousness, but none can be definitively linked to telephones.
Even if not actively connected, a cellphone's power-on mode dispatches bursts of potentially harmful energy. For this reason, all phones must be placed in the proverbial "off position" prior to taxiing. This is usually requested at the beginning of each flight as part of the never tedious pre-takeoff safety briefing.
The policy is clearly stated, but unenforced. We assume the risks are minimal, or else the airlines would collect phones rather than relying on the honor system. I would venture to guess that at least half of all cellular phones, whether inadvertently or out of laziness, are left on during flight. That's about a million phones on about 10,000 flights every day, just in the United States. If indeed this were a recipe for disaster, I think we'd have more evidence by now.
I tend to agree. I mean, a million phones, in the air a day, half of them on (OK, 500,000), would equal roughly 175 million chances for interference (or worse) each year. And yet, the thing is, interference or worse from cell phone signals hasn't happened yet.
Now, I am going to pose a couple of questions to you, my flying, cell-using readers: