Re: [Phys-L] electronic devices on planes?

[John Denker <jsd@av8n.com>]

On 01/03/2013 12:12 PM, Forinash III, Kyle wrote:
> The following is a link to a New York times article about using 
> electronic devices on planes (ereaders, phones, etc.). They point
> out that pilots now use iPads instead of paper flight manuals and
> this doesn't seem to cause a problem.
>
> I don't understand why this isn't settled by knowing what
> frequencies are being used by the plane instruments and the devices.
> Doesn't the FCC know the frequency bands in question? Do they
> overlap? I would not think so (it is the FCC's job to divvy up the
> spectra so there is not interference).
>
> http://nyti.ms/TqUxzY<http://p.nytimes.com/email/re?location=InCMR7g4BCLYd6XQPeTeK/7pc2TahmQx&user_id=bdb0c35f5f1d6a455a8e0bd5f0ea943b&email_type=eta&task_id=1357232882921939>

That article is complete garbage.

The author makes the point that there is no "proof" that the
devices have ever caused harm to the plane.  Yeah, so what?
That's not how such things are decided ... and it's not how
such things should be decided.

Suppose I invent a new modified design for a magneto.  I do 
*not* get to bolt such a thing to the engine of somebody's 
plane just because there is no "proof" that it is harmful.
The burden on me is to show, by a combination of analysis 
and testing, that the thing will not fail.  The threshold 
is quite strict.  I'm not an expert on the details, but for 
some things it's less than one failure per million flight 
hours.

In my opinion, the regulations are sometimes slightly too
strict.  For example, it would seem to me sufficient to
show that the new magneto design was provably better than
the old design.  However, this is obviously a very mild
criticism, not anything I'm gonna get worked up about.

The FAA is risk-averse, which is fine with me, within 
broad limits.

As for pilot using an ipad in the cockpit, let's get real.
If the pilot's ipad is causing a problem, the pilot will
notice, and can easily shut down his ipad.  Also the pilot
is clever enough to turn off the wifi transmitter on his 
ipad during flight.  In contrast, if there are 400 passengers 
in the back, one of whom is using some new made-in-Elbonia 
device that nobody has ever heard of, it would be a nightmare 
to find it and get it turned off.

Phones, computers, and walkie-talkies that have transmitters
in them are a particular nightmare.  For all the reasons
that a Faraday cage protects you from stuff outside, a
transmitter on the *inside* of a Faraday cage is bad
news.  Yeah, there are frequency bands assigned by the
FCC and ITU;  however (a) a strong transmission from one
device can saturate the receiver frontend of another
device, upstream of whatever filter defines the receiver
band.  Also (b) nowadays all these things have software
defined radios, so who's to say that there will never be
a software bug that leads to splatter across the whole
spectrum, from DC to daylight.

I would be willing to argue that the current regulations on
passenger gizmos is not strict enough.  I have personally
seen RF interference sufficient to trash a navigation signal
coming from the plane's own electric motors, due to worn-
out brushes.  There is supposed to be a capacitor across
the motor circuit, but the capacitor had failed.  I don't 
know if there are any NASA ASRS reports about this, or any 
SDRs about this, but if not it's because it is considered
routine, not because it is considered a non-problem.

I have not done the experiment, but I would be willing 
to bet I could trash the navigation signals using a non-
electronic electric shaver ... even though such things 
are explicitly excepted from regulation by FAR 91.21
  http://www.ecfr.gov/cgi-bin/text-idx?c=ecfr&rgn=div5&view=text&node=14:2.0.1.3.10&idno=14#14:2.0.1.3.10.1.4.11

BTW, note that the FM radio band ends at 108 MHz and
the aviation navigation band begins at 108 HMz ... so 
it's not like interference is a far-fetched conjecture.

Also, I know when my cell phone is going to ring before
it rings, because the wireless signals interfere with
the speakers on my computer ... the *wired* speakers.
If that sort of interference is possible, imagine what
could happen to a radio receiver, which is designed to
handle signals at a vastly lower level.

I spent part of my career getting paid to prevent and
abate RF interference.  It pays well, in part because 
there is nothing easy about it.

When a passenger or a senator says "I left my cell phone
on last time and nothing bad happened" it's strongly
analogous to saying they didn't wear their seat belt
last time they drove their car.  I don't care if nothing
bad happened last time.  The bar is -- and should be -- 
much higher than that.