Normally, when everything on the plane is fine, nothing.However, in older aircraft, some mass points are often corroded crushing compounds as on shields are no longer quite correct. Maybe at some point in aircraft life there was also some bungling during maintenance.
The components are shielded, but such errors allow the radio waves of the mobile phone to penetrate the system.Screen chains or locking circuits require correct, low-impestate mass points to be effective (there are hundreds of them in a commercial aircraft). Although the frequencies of the mobile phones are much higher than those of the aircraft navigation systems (especially the analogue, such as ILS or BEFORE), oxidized contacts can act like diodes and by mixing two otherwise harmless frequencies, then Mixed products (new frequencies, this effect is exploited in the Super Heterodyne receiver) that can interfere with the devices. This can also happen in oversteered amplifiers (C operation, acoustically noticeable due to distortions) the ILS (instrument landing system) receiver then displays different values than it should, and a switched-on autopilot then follows the wrong signal. This could have serious consequences, especially in the case of blind landings in bad weather.
In addition, these intermodulation products can also penetrate into the analog on-board intercom system (intercom), then one has the typical Bsst-Bsst-Bsst of a mobile phone, which searches a cell, very loud in the headphones.I’ve heard that myself. And that can cause problems if the pilot is currently receiving instructions from air traffic control and does not understand them.
Digital systems are less susceptible, but the distance between the levels of the useful signal and the fault must be large enough that the received device still hears the useful signal.
Especially when the aircraft is in the air, the mobile phone tries to make contact with a cell with high transmitting power.
Aircraft that allow mobile phones have created a so-called picocell, which allows mobile phones to hang with minimal power.The signal is then redirected to a satellite connection (which is why these calls are very expensive). According to EASA guidelines in Europe, however, every aircraft that has installed such a picocell must be individually tested for abnormal behaviour. I was involved in the installation of such a system myself.
Finding errors in grounding or shielding is extremely time-consuming, especially because errors caused by radiation often occur only sporadically or under certain conditions, e.g. a specific configuration of the aircraft or certain humidity and many aircraft systems are now interconnected.
With new aircraft you will have fewer problems than with old people who have long been exposed to the weather (with condensation and dirt in the fuselage).
In addition, if there is no picocell, the phones try to hang themselves in several cells.Since the frequency range of the mobile phone networks is limited, the same frequencies are used by several cells, but normally these cells are spatially separated to such an extent that they do not affect each other for use on the ground. but in an airplane in flight, the phone has a much larger radius of action (the ultra-short waves used in mobile phones behave like light and do not get beyond the horizon). These attempts to log into multiple cells confuse the telephone system. In addition, a phone in an airplane moves very fast, faster than what the ground cells are designed for.
The airlines are, of course, opposed to the bans (as long as they have picocells on board for which they can collect) as they hope for competitive advantages and put pressure on the aviation and telecommunications authorities.
Risk management is carried out: the small risk of an accident happening through a mobile phone (which can then become very expensive) is offset against the potential revenue from the user charges to passengers.
Expensive accident, but very low probability against guaranteed income.
This is only the technical side, I am not talking about the legal or commercial side, nor about courtesy and consideration.I speak for more than 20 years in the maintenance and repair of commercial aircraft, including avionics, and as a licensed radio amateur.