A few days ago, I took a flight home. Towards the end of the journey, the captain asked us to shut down any electronic device that we had on board, including iPods, CD players, phones and so on, because there was very low visibility, and we were starting an instrument approach.

From what I could understand, an Instrument Landing System (ILS) allows an aircraft to be centred on the approach path, assisted by radio signals delivered by an ILS (a localiser and a glide slope), without any visual reference. These signals are in the 110 MHz (LOC) and 330 MHz (GS) frequency range.

Given that for the take off, visual landing, and the journey itself, shutting off the devices is no longer required (even though there are radio signals being used for communications and navigation), why this extra precaution on a low-visibility ILS approach? What interference could a CD Player possibly produce with the instrumentation to an extent that it's more dangerous than with the normal take off,en route, and landing phases?

On a side note: as you probably understood, my level of expertise is very low, so forgive any dumb mistake or oversight.

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    $\begingroup$ @O.R.Mapper That requirement used to be nearly universal, but it's been dropped in some countries. In the U.S., for instance, the requirement was dropped back in late 2013. $\endgroup$
    – reirab
    Commented Jan 9, 2015 at 15:01
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    $\begingroup$ @NicolaMiotto, reirab: That is good news for anyone wishing to take photos during take-off and landing :) $\endgroup$ Commented Jan 9, 2015 at 15:07
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    $\begingroup$ There is simply no truth to any of it. The idea that electronic devices produce magic "interference" that can cause an airplane to crash has been thoroughly debunked, over and over, at least as far back as the 80s. But if science isn't convincing, how about sociology: it's common knowledge that terrorists exist that want to cause airplanes to crash. If it was as simple as leaving a laptop turned on (safely stowed in a bag where no one could see it) why hasn't it already happened, ever, even once, in the 13+ years since 9/11? $\endgroup$ Commented Jan 10, 2015 at 19:50
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    $\begingroup$ @MasonWheeler Ummm... no. I'm both a pilot and an engineer who designs electronic devices for a living and I can assure you that electronic devices do, in fact, create RF interference. There's nothing magic about it. Anything with a current flowing through it emits an e-m field. Ideally, this EMI should not be within the frequency spectrum of the ILS, but electronics don't always work like they're supposed to. See my answer for more information. Also, the chance of a laptop crashing a plane is not 100% (or even 1% or 0.001%,) which makes it rather ineffective for terrorism purposes. $\endgroup$
    – reirab
    Commented Jan 12, 2015 at 14:55
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    $\begingroup$ @MasonWheeler If you have a better source for your claim that it's been 'debunked,' please write an answer citing that source. Also, please note that just because something is possible doesn't mean it will happen every time or most of the time. Even if it happens once in every 1,000,000 times, that would be an unacceptably high risk when you're flying 300 people around at ~150-200 mph on an approach. $\endgroup$
    – reirab
    Commented Jan 12, 2015 at 15:54

6 Answers 6


Short answer: ILS is rather sensitive to interference and not all electronic devices take much precaution in avoiding the generation of interference. The pilot wants to be sure that the readings he's getting on the localizer and glideslope are accurate, since he can't actually see the runway to verify the final approach path visually.

Longer answer: ILS is actually relatively simplistic as RF signals go. Both the localizer and the glideslope just consist of a pair of AM signals that are broadcast directionally. For both systems, the modulated frequencies are 90 Hz and 150 Hz. For the localizer, 90 Hz is broadcast to the left of the runway centerline and 150 Hz is broadcast to the right of the centerline. When the aircraft is aligned with the runway, it will receive equal strength signals of both frequencies. When it's to the right, it will receive more power from the 150 Hz signal and when it's to the left, it will receive more power from the 90 Hz signal. The glideslope works pretty much the same way, just with the 150 Hz signal transmitted below the glide path and the 90 Hz signal transmitted above it. [1] [2]

Now, suppose some passenger's electronic device is emitting a frequency that it shouldn't be. This is not exactly unheard of among consumer electronic devices. This is more common among devices manufactured in countries with less-strict emissions testing requirements/enforcement, but it's possible due to slight defects in almost any device. [Source: I design RF receivers for a living.] Now suppose that this noise happens to be on the localizer or glideslope frequency for the runway to which the aircraft is flying an approach. Hopefully you're starting to see a problem here... Whatever amplitude changes are happening on this unintended emission are now being received by the AM demodulator of the localizer or glideslope, potentially confusing it and, in low visibility conditions, causing the aircraft to be directed somewhere other than the runway threshold. Needless to say, that's bad.

Boeing recommends the following actions for flight crews flying ILS approaches in order to detect erroneous ILS readings:

  • Crosschecking altitude and DME distance periodically.
  • Crosschecking altitude and flight management system (FMS) threshold distance.
  • Crosschecking altitude and the crossing altitude of the outer marker (or locator, very-high-frequency omnirange [VOR] navigation equipment, or FMS).
  • Crosschecking radio altitude and barometric altitude.
  • Crosschecking ground speed and rate of descent.
  • Questioning air traffic controllers when indications do not appear to be correct.

However, even when following those instructions, slightly erroneous ILS signals may be hard to detect until too late. If a DME is showing that you're half a mile off, you can probably notice that, but a couple hundred feet off might not be noticeable (but is still plenty to be a big problem.)

  • $\begingroup$ so far, this is the only answer I've got with a practical explanation of the possible issue, with very well explained technical details (there were some a bit obscure points in the wikipedia explanation that were perfectly clarified here). At the moment this is the candidate to be accepted. $\endgroup$ Commented Jan 8, 2015 at 17:15
  • $\begingroup$ +1 for the amazing explanation. One question: Is there any checks done for ensuring the localizer is proper? It sounds like checking altitude is good enough for detecting issues with the GS only. $\endgroup$
    – BlueBuddy
    Commented Jan 9, 2015 at 19:10
  • $\begingroup$ Instrument Landing System (ILS) frequences Localizer: 108.1-111.95 MHz Glide Slope: 329.15-335.0 MHz en.wikipedia.org/wiki/… $\endgroup$
    – rbp
    Commented Jan 9, 2015 at 19:12
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    $\begingroup$ I have pruned several comments. Please take any further discussion of citations/credentials/etc. to Meta - comments are really not the place for that sort of thing. $\endgroup$
    – voretaq7
    Commented Jan 9, 2015 at 20:45
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    $\begingroup$ "[Source: I design RF receivers for a living.]" Awesome .. I love the rare occasions you see sound info on the internet! :) $\endgroup$
    – Fattie
    Commented Jun 2, 2015 at 14:28

Having worked as a software engineer on the lateral guidance subsystem of the FMCS (Flight Management and Control System) for the Airbus A310 about 30 years ago I found @reirab's answer fascinating.

I can fill in some gaps as to how the information from the different systems is used and why the ILS information is particularly critical.

On the A310 3 sets of input are used in determining the aircraft's position.

1) Every 200ms the inertial data from the set of 3 laser gyroscopes are used to basically say - "I was there. For 200ms I have been travelling at x knots therefore now I am here".

2) Every 1.5 seconds (in normal flight) data from up to 4 DMEs and VORs are used to accurately calculate the position of the aircraft.

3) During landing when ILS or MLS is available the ILS/MLS data is used in place of DME/VOR data.

There are two significant differences between 2) and 3). Because you are closer to the ground and precise vertical as well as lateral position is critical calculations are performed more frequently than once every 1.5 seconds. Because ILS/MLS data is much more accurate filtering of position adjustment is reduced.

What does that mean?

Well, when I do the radio position fix using DME and VOR I will also have an inertially calculated position. For various reasons beyond just not spilling the passengers' coffee I'm not allowed to correct this position by more than something like 5m (this is different if there has been no radio fix for a prolonged period of time). With ILS/MLS this filter factor is much larger, like 50m or even 100m, although the expectation is that such a large correction will only happen on initially acquiring the ILS/MLS signal.

Because such large corrections of the aircraft calculated position are allowed in Flight Management software when using ILS/MLS data it is critically important that there are no errors due to interference.

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    $\begingroup$ Hello Brian, welcome to Aviation.SE! That's a very insightful answer, I hope to see more of those! $\endgroup$
    – DeltaLima
    Commented Jan 11, 2015 at 12:21
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    $\begingroup$ it's "actual worthwhile information day" on the internet .. my God! :) $\endgroup$
    – Fattie
    Commented Jun 2, 2015 at 14:30

You can read this related question if you want to learn more about the interference between electronic devices and airplanes. An answer there links to a very good document written by NASA on the topic.

Long story short: Electronic devices are complicated. Airliners are complicated. Therefore, we can not predict exactly what the interference between them will be in all situations.

What your captain did happens frequently and is also covered in the relevant FAA FAQ.

Once your airline has shown the FAA its airplanes can safely handle radio interference from portable electronics, they can let you use your devices in airplane mode only most of the time. At certain times — for example, a landing in reduced visibility — the Captain may tell passengers to turn off their devices to make absolutely sure they don't interfere with onboard communications and navigation equipment.

So, the reason is that the captain wants to be absolutely 100% sure that there will not be any interference during such a critical point.

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    $\begingroup$ I'm kind of glad of reirab's answer, as this reads too much like (wavy wavy) it's magic and it might be (wavy) magic magic magic. $\endgroup$
    – CGCampbell
    Commented Jan 9, 2015 at 20:38

Adding to Reirab and Brian Towers' superb answers on this topic:

There are several ways an electronic device can interfere with aircraft operations, but the likelihood depends on age, aircraft design factors, and the nature of the EMI aggression (frequency, modulation type, and power -- there are also broadband EMI sources on aircraft, but we can safely disregard these for the purposes of this discussion, as PEDs generally are narrowband emitters unlike, say, an electric hydraulic pump which lost its interference suppression capacitors, spewing broadband hash as a result).

The typical concern, though, is of interference with an aircraft's sensitive navigation (ILS/LOC, G/S, NDB, GPS/WAAS, radio altimeter) and communication (HF, VHF, SATCOM) receivers (it's possible to interfere with the transponder/TCAS and weather radar signals as well, but those aren't quite the same level of concern as say the glideslope is), from intentionally emitting PEDs -- cell phones (especially GSM phones, as the GSM air interface is more EMI-prone than that a CDMA phone is) and 2.4GHz or 5GHz devices (WLAN, BT) are the main concerns on this front, although unintentional emitters (anything in airplane mode) are also a source of potential trouble.

Furthermore, comms interference takes an immediately noticeable form -- "what's that funny noise in my headset?" (Pilots of small General Aviation aircraft have reported this on a regular basis.) This is in contrast with interference to navigational receivers, which may be subtle but not immediately troublesome (losing a bird in a large visible GNSS constellation), obvious (a fail flag on a VOR or LOC), or insidious (a false glideslope indication).

However, the consequences are high here, especially during coupled or autoland approaches -- a human pilot can see that a full fly-down indication appearing out of the blue at 300' RA during an otherwise stable ILS approach is bogus and whack the TOGA button to get the crew some time to tell the noisemaker to zip their electronic lips, but an autoland computer won't know the difference and will follow it blindly.

A good piece of news is that in the somewhat-more-common case of an unintentional emitter (a device in airplane mode, for instance), the emissions tend to be simple, unmodulated carriers -- I see this as more likely to trigger a FAIL flag than a flight path deviation. (I will put another question up on this topic, even -- it's worth asking about, and perhaps even an experiment or three.)

However, due to the large number of airline flights yearly (66 million according to ACI as per reirab's comment below), something that happens once in every 10^7 landings (Extremely Remote according to AC 25.1309) will happen every other month on average, and even something that happens once in 10^9 landings (Extremely Improbable according to AC 25.1309) is bound to happen every 15 years or so! The reported PED interference instances are not nearly as severe in consequences as the 'worst case' of a full 'fly down' during the late phase of a coupled or autoland approach (or false radio altimeter readings causing a grossly premature mode shift during autoland), though, but the probability of PED EMI with aircraft systems on any given flight is enough to make it a concern -- 75 events were reported worldwide to the IATA from 2003 to 2010, and given this number, we get a probability of roughly 1 in 10^7 for PED interference on a flight.

From this, we can conclude that it would be highly unwise to take the chance of having a device talk all over the ILS signals during a critical phase of flight -- you would have to find a way to reduce the probability of PED interference with aircraft systems to 1 in 10^11 or better (this is 100 times less likely than what is currently allowed for a catastrophic event) in order to have a chance of challenging this wisdom.

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    $\begingroup$ +1 FYI, ACI reports that there were about 66 million airliner flights worldwide (at airports that report to ACI, at least) for the year ended Sept 2014. Since this issue is mainly a concern on landing, that's probably the most relevant figure to use for risk management statistics. $\endgroup$
    – reirab
    Commented Jan 17, 2015 at 0:51
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    $\begingroup$ @reirab -- thanks, I'll rework that section! $\endgroup$ Commented Jan 17, 2015 at 1:48

The only reason electronic devices are demanded to be switched off is because some of them affect the radio (telephones) and it's easier to say "shut them all off" than to argue over which equipment is susceptible to interfere with the radio and ask the crew to carefully inspect whether the device at the other end of your earbuds is actually a telephone or a CD player.

Source: actually asked the question to my aerobatics instructor a few years back. He told me that when the phones weren't switched off it happened that he had to ask Control to repeat 2 to 3 times before understanding them.

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    $\begingroup$ +1 for the [it's easier to say "shut them all off" than to argue over which equipment is susceptible to interfere with the radio] , My sister-in-law witnessed an argument between a steward and a passenger where the passenger asserted "is's a blackberry not a phone" ... If you deal with the public this sort of thing will happen. $\endgroup$
    – Spoon
    Commented Jan 8, 2015 at 12:56
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    $\begingroup$ Oh my god, thanks Spoon for reminding us how stupid some people are. $\endgroup$
    – Mister Mystère
    Commented Jan 8, 2015 at 13:06
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    $\begingroup$ Another +1 for the "all off" rule. But you should also consider that an instrument landing has a greater chance of going wrong than a "normal" landing. If something were to go wrong, be it a crash or a harmless abort, it's important for the crew/captain to communicate with the passengers. If half the people on board have earphones shoved in their ears they're more than likely to miss an key instruction which could save them from injury, confusion, or mass panic. In the worst cases, the cry of "brace, brace, brace" won't get the opportunity to be repeated to those who weren't listening... $\endgroup$
    – chaaarlie2
    Commented Jan 8, 2015 at 15:05
  • $\begingroup$ @chaaarlie2 There's a separate rule about headphones during take-off and landings, and it's easy for the flight crew to check. $\endgroup$ Commented Jan 8, 2015 at 16:30
  • $\begingroup$ faa.gov/news/press_releases/news_story.cfm?newsId=15254 $\endgroup$
    – Scott Seidman
    Commented Jan 11, 2015 at 1:46

Because they don't want to take any chances

Although most electronics have been shown to be unlikely to interfere with most avionics/communications equipment in most common circumstances the simple fact remains that we aren't totally sure yet. As RRR said, airliners and electronics are both very complex, and we can't predict exactly what will happen in every situation.

In a visual landing with instrument assistance, a small error is unlikely to matter too much - if you're a little fast you may get a bumpy landing, or an incorrect descent rate may cause a bounce or a go-around etc, but the landing is unlikely to be off by a significant enough margin to cause a crash. If there's a small deviation in course/speed/altitude it won't matter, the pilot will simply correct it - likely without even noticing he's doing it - or nobody will notice/care. If there's a larger error the pilot will almost certainly notice ("Wait, why is the runway over there?") and fix the problem

In an instrument landing, however, a very small margin of error in course/airspeed etc, present for 20 miles while the pilot has no frame of reference to know that it's wrong, can cause a major issue, because the cumulative error can add up over time and there's no way to know it's wrong.

Chances are, your phone will make absolutely no difference to the plane. But on that one occasion it does cause a problem, 300 people may be killed in a fireball. That makes "asking you to stop playing Angry Birds for 30 minutes" a very small price to pay for a a potential life-saving precaution.

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    $\begingroup$ I don't know that I would say that most electronics have been proven not to interfere. Shown unlikely to interfere would probably be a more accurate statement. Any device can cause interference, especially if it has some slight manufacturing defect (they only do strenuous emissions testing on a sample in the factory, not every device) or something has happened to it since manufacture (which could be as simple as a board or chip getting too hot or cold.) Otherwise, though, good answer. +1 $\endgroup$
    – reirab
    Commented Jan 8, 2015 at 15:46
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    $\begingroup$ Thanks, I've stolen your correction as it's a very valid point $\endgroup$
    – Jon Story
    Commented Jan 8, 2015 at 15:49
  • $\begingroup$ @reirab You are 100% correct. In the early 2000s (well before "tablet computers" were everywhere), I worked for a company specializing in Electronic Flight Bags. Spinning hard drives were the norm since SSDs were new and expensive. A computer can put out an impressive amount of electromagnetic leakage. Most of the systems in the market at the time were not "proven" not to interfere. Gaining that "proof" was incredibly expensive, time intensive and mostly impractical. However, it was up to the individual pilot, as it still is, to demonstrate to himself that the EFB was "unlikely" to interfere. $\endgroup$
    – Shawn
    Commented Jan 9, 2015 at 22:36
  • $\begingroup$ My guess it that the watch on the pilot's hand is more likely to emit EMI than most modern gear is. But since it's still the pilot's responsibility, he's not taking any chances, whether they seem rational to everyone else or not. If he believes that people in blue shirts are a flight safety issue, it's his obligation to refuse to start the flight if anyone is wearing a blue shirt. $\endgroup$
    – Shawn
    Commented Jan 9, 2015 at 22:40
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    $\begingroup$ @JonStory Not necessarily. Rapid braking in and of itself is not necessarily a problem. Also, turbulence happens and regularly projects all kinds of things through the cabin (people included.) $\endgroup$
    – reirab
    Commented Jan 12, 2015 at 14:46

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