Comment by tjohns | Hacker News Reader

Comment by tjohns | Hacker News Reader

I find it interesting how most of the comments focus around maintaining quiet on the aircraft, and very little about the technical details behind this rule (interference with communications radio, interference with ground-based navigation signals, and impact on the cellular network itself).

At least in the US, there's two different rules relevant to in-flight cell phone use:

- 14 CFR 91.21 [1] - FAA rule prohibiting use of any electronics during air-carrier/instrument flight which the aircraft operator hasn't approved, in order to prevent interference with the avionics.

- 47 CFR 22.925 [2] - FCC rule prohibiting airborne use of cellular telephones specifically, in order to prevent phones from talking to too many towers at once and stressing the cellular network. (As altitude increases the radio horizon increases, so at altitude your phone can "see" significantly more towers. The cellular network is built on the principle that a phone can only see a small handful of towers at a time, enabling frequency reuse.) This rule technically only applies in the "Cellular Radiotelephone Service" band, which is one of several different bands a cellphone can use, and it's one of the older ones — but you have no idea which band your cellphone is using at any given time.

The FCC has talked about updating 47 CFR 22.925 and/or harmonizing the rules between cellular bands, but who knows if they'll ever get around to it. [3]

FWIW, as a private pilot, I have first-hand experience with interference when somebody forgot to switch their cell phone off. It was difficult to hear ATC over the "click-click-BUZZ" of GSM interference through the headset. Granted, this was a small airplane, so passengers were much closer to the avionics... but it's not just a hypothetical.

Cell phones tend not to work very well (if at all) above about 5,000 feet anyway, since cell tower antennas are directional and pointed towards the ground.

[1]: https://www.ecfr.gov/current/title-14/chapter-I/subchapter-F...

[2]: https://www.ecfr.gov/current/title-47/chapter-I/subchapter-B...

[3]: https://docs.fcc.gov/public/attachments/FCC-04-288A1.pdf

I'm always amazed that the following things are true at the same time:

a) Cell phones not in airplane mode are supposedly a safety concern for airplanes.

b) Tap water I bring from home is a supposedly a safety concern for airplanes.

c) I'm asked to turn my cell phone off or into airplace mode during take off but nobody really cares.

d) I'm strip searched for tap water before entering the gate.

Your (b) is an especially tendentious phrasing. If the security folks could be sure your bottle contained tap water from home there wouldn't be any reason to exclude it. Instead, your bottle is prohibited because they can't distinguish its contents from bomb ingredients.

Now, I don't think the actual reasoning passes a cost benefit test, but criticizing it on grounds that have nothing to do with the real reason isn't helpful.

> Your (b) is an especially tendentious phrasing.

Yes, you are right, it was maybe a stupid phrasing, I'm particularly annoyed by it and I should have given a better example (but I disagree that you cannot distinguish tap water from bomb ingredients. Of course you can. And I did even talk about the stupid 100ml limit, which is apparently the threshold you need for bombs. And they will lift the limit soonish, because apparently the counter-terrorists have won).

Anyway, let's take a better example: I'm not allowed to bring weapons in the cabin because they are a safety risk. This is an understandable risk, they will enforce it (not by asking but by searching), and if you still end up with a weapon in the plane and disturb the flight, you will for sure end up in jail.

If your cell phone rings loudly during take off, you will politely be asked to turn it off. If it rings silently, nobody will ever care.

It makes a bit more sense when you take into account mitigation strategies for each hazard.

- If somebody has a weapon in an aircraft and is intent on using it, it's unsafe, and there's not a whole lot that can be done. You're all trapped in a sealed pressure vessel, and law enforcement isn't coming until you're on the ground. Your only mitigation strategy is to hope the crew and passengers can fend for themselves.

- If you lose ground-based navigation during landing, it's unsafe, but there are several mitigation strategies. You're not going to land at that moment anymore, but the pilot can abort the landing, switch over to GPS, and fly the missed approach to get back up to altitude. At this point there's time to troubleshoot... They can make an announcement asking passengers to switch off electronics. They could fly an RNAV approach. If the weather's good, they could land visually. Worst case they can fly to a military base and make a ASR approach (I don't think this has ever happened with an airliner).

There's different levels of unsafe. You have to consider what the specific risks of each are.

Was there ever a missed approach due to a cell phone not being in air plane mode? How was the cell phone located? What would happen if the cell phone was in the luggage in the cargo bay?

Sorry, but this sounds all very hypothetical. This this was a real problem there should be other procedures by now.

> Was there ever a missed approach due to a cell phone not being in air plane mode? How was the cell phone located?

Here's a story of an interference event where they went missed:

https://www.quora.com/On-my-last-flight-before-landing-the-p...

A quick search through the NASA ASRS database shows several reports where the ILS or VOR receiver was behaving erratically, sometimes during descent. In some cases the crew was able to land visually, in others the crew crew had enough time to ask passengers to switch off phones and the problem went away.

See: ACN 695049, 321746, 307150, 277118, 200510, 161347, 364464, 467979

https://asrs.arc.nasa.gov/search/database.html

Most of these are older reports, take that as you will. It's entirely possible LTE doesn't interfere with the ILS/VOR. I honestly don't know.

Note also that this only lists reports made through NASAs reporting system. There are other reporting systems used by commercial operators (i.e. ASAP) that I can't easily query.

If I expand the search to include 5G interference, I get 97 recent results. That's a different issue, but at least worth mentioning to show that interference is still a general concern. I'm not going to list all of those, but you're welcome to query for them at the above link.

> What would happen if the cell phone was in the luggage in the cargo bay?

Then they would use one of the other contingency options I already mentioned to land visually or using other sensors. Still not a "safe" situation if this happens when you're close to the ground. If this were to happen during a Cat III approach, it would be extremely bad.

AFAIK 3G and newer don’t have this problem. 3G is more than 14 years old at this point and 2G is largely being phased out in the next few years.

The interference I experienced was just a couple years ago, from an LTE handset. It had fallen back to 2G mode due to the weak signal at altitude.

As I understand it (and someone more versed in cellular backends can correct me), the reason why 2G GSM interference was so noticeable was because the TDMA-based radio would switch on and off at an frequency within the audible range, and once inductively coupled with a nearby audio cable would be strong enough to play sound through speakers.

That doesn't mean newer cellular radios don't interfere. They can, just not audibly. Newer CDMA-style radio modes (used on LTE/5G) aren't switching on and off like the TDMA ones did, but they're still emitting the same amount of power.

In particular, I'd be most worried about the aircraft's navigation radio. The technology used for ILS/VOR navigation is technologically simple, and easy to interfere with. A car driving past the wrong area of the airport can disrupt the signal. (There's "ILS Critical Areas" on airport taxiways that are marked off for this very reason, and ground controllers will prevent aircraft or vehicles from entering those areas when there's an aircraft on an ILS approach.)

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