I think most people know that cell phone signals can interfere with airliner avionics; that's why airplane mode exists. Is this an issue with small GA planes?
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5$\begingroup$ What research have you done to find out? $\endgroup$– JimSep 25, 2022 at 5:33
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You are mistaken. Cell phones are not banned onboard because they do interfere with airliner avionics. They are banned for two reasons:
because their use while airborne interferes with ground-based communications. This is an FCC rule, not an FAA rule, and has nothing to do with the aircraft and everything to do with the consequences on the ground. See FAA AC 91.21-1D, at 9.1 "Restricting Airborne Cellular Telephone Use," and 47 CFR §22.925.
because it is possible that certain portable electronic devices could interfere with aircraft navigation or communication systems. The FAA permits the operator of the aircraft to allow any portable electronic devices once they have determine that they in fact do not interfere with these systems. This is in CFR 14 §91.21 for Part 91 operations, and there are similar rules in Part 121, 125, and 135 operations. This same rule applies to most electronics- even in airplane mode and even if they do not intentionally transmit radio waves, the default rule is that they are not allowed. In practice, many operators allow them during at least some phases of the flight.
In short: any portable electronic device has the "potential" for interfering with aircraft systems. It appears that most of them, at least most of the time, do not do so significantly. Occasional reports of possible PED interference have proven rare and difficult to replicate. See for example this and appendix I in this document.
Regardless of any interference with aircraft systems, cell phones in particular are forbidden by the FCC any time you are airborne.
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$\begingroup$ The one phenomenon I've observed flying when an active cell phone is nearby (like my own phone) is that distinctive "D-D-D-D-D-D-D-D-D" sound that comes over the comms audio, which I believe happens when the phone is in communication with a tower. Mildly annoying but otherwise no big deal. $\endgroup$– John KSep 25, 2022 at 21:49
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$\begingroup$ @JohnK That is mildly annoying but usually not dangerous. More concerning is interference with, say, VOR or ILS, which may just give wrong information. Particularly 2G communications radiate a lot at around 217 Hz, which is uncomfortably close to the 90 and 150 Hz sideband frequencies used in the ILS localizer. Newer phones make do with much less total power radiated so presumably the interference problems are smaller. $\endgroup$– ChrisSep 25, 2022 at 22:15
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$\begingroup$ I've noticed that my Garmin Etrex handheld GPS makes an audible noise over the speaker of a handheld aviation radio when they are close together. $\endgroup$ Sep 26, 2022 at 1:55
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$\begingroup$ As I understand it, the use of radio transmitting devices (that aren't permanently installed) is generally prohibited in commercial aircraft because the crew have very little control over them. In the case of a small GA aircraft this would be far less of a problem, as the pilot could easily ask their passenger to desist using the device if there were a problem. $\endgroup$– FrogSep 26, 2022 at 4:26
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1$\begingroup$ @Frog Radio-transmitting devices are very commonly allowed. Wifi and bluetooth devices, for instance. On a commercial flight, the crew can (and has) ask passengers to turn off their devices if required. $\endgroup$– ChrisSep 26, 2022 at 5:22