[BC] cell phone hash

[Phil Alexander]

On 3 May 2005 at 10:11, Barry Mishkind wrote:

> At 09:41 AM 5/3/2005, Kirk Harnack wrote
> >And we wonder why we're asked to switch our cell phones OFF on commercial 
> >flights.  This kind of hash can really affect reception of Amplitude 
> >Modulated ILS and VOR signals, no?
> 
>          A very good question.  I hear
>          stories both ways.  I see a lot
>          of cell phones left on ... and
>          no planes have crashed.

In THEORY it can't happen BUT that assumes 100% shielding in the coax
run inside the plane and perfect grounding of the avionics packages.

We, of all people, should know how often that doesn't exactly work out
according to plan. <g>

The problem may be that "well shielded and clean" is incompatible with
"light and small" in the design of cell phones. If they only radiated
within their authorized bands, it wouldn't be a problem because ILS
localizers and VOR operate between 108 to 118 MHz. The glideslope part
of ILS uses 329 to 335 MHz. OTOH, there is a greater possibility of 
interference with DME transponders which operate between 960 to 1215 MHz.

There are some approaches where DME is vital for obstacle clearance.
However, erratic DME operation is rather noticeable and other cross-checks
are available. Still, I don't think it would be wise to operate a cell
when flying in or out of Vegas in IFR conditions, Especially an old, high
powered analog phone.

I can see the NTSB report saying, "Primary cause: Flight crew's failure
to maintain adequate instrument cross-check; Secondary cause: PCS 
operation during approach combined with poorly installed DME antenna 
coaxial connector over cabin area."

Since the dry text of an NTSB report is so final, I've always felt I
should do my part to avoid becoming part of one, but maybe that's 
just me. <g>

Phil Alexander, CSRE 
Broadcast Engineering Services and Technology 
(a Div. of Advanced Parts Corporation) 
Ph. (317) 335-2065   FAX (317) 335-9037




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