Mythbusters Episode and FMS

Saw a repeat episode of Mythbusters for the first time last night about
the use of cell phones on an airplane and interference with cockpit
instruments. I know that this was mentioned in a November 2006 thread
briefly but the short of it was that they concluded cell phones really
CAN interfere with the VOR signals.

One of the tests however, was on a Hawker jet with a glass cockpit
(Honeywell I believe) with a FMS. They could not get any frequencies or
amplitude of cellphone signals to budge the instruments like they were
able to on a 1970's era NAV system. From what I understand, the FMS
relies on a number of different inputs to drive its nav systems. If
that's the case, wouldn't it switch to another source if one seemed
unreliable? If it does, I imagine it would be a fundamental flaw in
their experiment. The FBO manager and avionics guy being interviewed
however, gave the nod to the robust wire shielding.

Marco

Keep in mind that there is a difference between "valid" and "correct".

For example, a computer program may require that a number be entered in a
specific field.

Any number that is entered would be "valid", but it would not necessarily be
"correct".

A given electronic device might cause a reading to be 10 degrees off. The
nav system might well interpret the data as "valid" even though it would not
be correct.





"Marco Leon" wrote in message
oups.com...
Saw a repeat episode of Mythbusters for the first time last night about
the use of cell phones on an airplane and interference with cockpit
instruments. I know that this was mentioned in a November 2006 thread
briefly but the short of it was that they concluded cell phones really
CAN interfere with the VOR signals.

One of the tests however, was on a Hawker jet with a glass cockpit
(Honeywell I believe) with a FMS. They could not get any frequencies or
amplitude of cellphone signals to budge the instruments like they were
able to on a 1970's era NAV system. From what I understand, the FMS
relies on a number of different inputs to drive its nav systems. If
that's the case, wouldn't it switch to another source if one seemed
unreliable? If it does, I imagine it would be a fundamental flaw in
their experiment. The FBO manager and avionics guy being interviewed
however, gave the nod to the robust wire shielding.

Marco

On Jan 25, 11:47 am, "Bill Denton" wrote:
Keep in mind that there is a difference between "valid" and "correct".

For example, a computer program may require that a number be entered in a
specific field.

Any number that is entered would be "valid", but it would not necessarily be
"correct".

A given electronic device might cause a reading to be 10 degrees off. The
nav system might well interpret the data as "valid" even though it would not
be correct.

Agreed. However, having a choice of signals to "believe," the FMS
should have an integrity check in its logic, right? If the VOR-NAV is
showing you 3 miles to the left of the localizer and the GPS and INS
are showing you on it, then the FMS should disregard the VOR-NAV.

Also, there may be other ways of verifying integrity such as the
presence of wild fluctuations in a given period of time.

Marco

Marco Leon writes:

Agreed. However, having a choice of signals to "believe," the FMS
should have an integrity check in its logic, right? If the VOR-NAV is
showing you 3 miles to the left of the localizer and the GPS and INS
are showing you on it, then the FMS should disregard the VOR-NAV.

Yes, it should have such checks, and it does.

--
Transpose mxsmanic and gmail to reach me by e-mail.

Marco Leon writes:

Saw a repeat episode of Mythbusters for the first time last night about
the use of cell phones on an airplane and interference with cockpit
instruments. I know that this was mentioned in a November 2006 thread
briefly but the short of it was that they concluded cell phones really
CAN interfere with the VOR signals.

At very close range and in certain positions, yes. But that doesn't
mean that using a cell phone in a seat in the middle of the aircraft
will do anything. There's already a lot of RF noise in the air around
aircraft.

One of the tests however, was on a Hawker jet with a glass cockpit
(Honeywell I believe) with a FMS. They could not get any frequencies or
amplitude of cellphone signals to budge the instruments like they were
able to on a 1970's era NAV system. From what I understand, the FMS
relies on a number of different inputs to drive its nav systems. If
that's the case, wouldn't it switch to another source if one seemed
unreliable?

Yes, within limits.

If it does, I imagine it would be a fundamental flaw in
their experiment. The FBO manager and avionics guy being interviewed
however, gave the nod to the robust wire shielding.

I suspect the shielding is actually protecting the avionics. They
have to be shielded just to prevent interference with each other, much
less interference from cell phones.

--
Transpose mxsmanic and gmail to reach me by e-mail.

On Thu, 25 Jan 2007 08:15:54 -0800, in
.com, Marco Leon wrote:

snip

Although they are not that bad of a show to watch sometimes, I've noticed
some of their experiments that were not exactly right... They did one on
exiting a car underwater recently... Their experiments methods came to the
conclusion that it was not possible to roll the window down on a car once
it was submerged due to the pressure on the window... To simulate this,
they put some weights on the window of a car door held in a horizontal
position... If you look closely at it, it seems pretty obvious that the
weights window was cracking open up to the point where the weights started
hitting the bottom edge of the window... I suspect that the results
from water pressure being distributed across the face of the window
would not act the same... At the very least, even the small amount that
the window would open would allow the water to enter the vehicle quicker
and as such equalize pressure quicker and thus allow you to open the door
quicker...

With regards to the cell phone usage aboard an aircraft, they also said
that they were prevented from doing it in the air because FAA rules made
it illegal... In fact, it is FCC rules and it is because of concerns of
the phones contacting multiple cell phone towers at the same time... The
way I understand it, the FAA rules concern airliners, so with regards to
that, they could have made the test on a VFR flight without any FAA
consequences...

Now, this doesn't address the issue of whether you can even get a cell
phone signal while in the air... My experience with Verizon is that above
about 500 ft or so, if a signal is available, it doesn't last long enough
for me to make a call... On the other hand, the old analog cell phones
would work at 10K ft... Of course, this was back in the old bag phone days
with a real handset, so you had to cut the engine back to idle slow down
as much as possible to reduce the engine and wind noise so that you could
actually hear what was said on the phone... Let's just say that it was not
conducive towards long conversations...

"Grumman-581" wrote in message
news
On Thu, 25 Jan 2007 08:15:54 -0800, in
.com, Marco Leon wrote:

snip

Although they are not that bad of a show to watch sometimes, I've noticed
some of their experiments that were not exactly right... They did one on
exiting a car underwater recently... Their experiments methods came to the
conclusion that it was not possible to roll the window down on a car once
it was submerged due to the pressure on the window... To simulate this,
they put some weights on the window of a car door held in a horizontal
position... If you look closely at it, it seems pretty obvious that the
weights window was cracking open up to the point where the weights started
hitting the bottom edge of the window... I suspect that the results
from water pressure being distributed across the face of the window
would not act the same... At the very least, even the small amount that
the window would open would allow the water to enter the vehicle quicker
and as such equalize pressure quicker and thus allow you to open the door
quicker...


I seen the exact same thing last night. Why they just didn't submerge the
car and let Jamie mess with it with his scuba gear on was a little dumb, it
would have taken 2 minutes.

-----------------------------------
DW

No, all their tests were done with localizers, no VORs at all. Probably
no back up of that.
However, the show failed to mention that its the FCC rule that states,
no cell phones on in planes ever. The FAA rules are less strict.

-Robert


On Jan 25, 8:15 am, "Marco Leon" wrote:
Saw a repeat episode of Mythbusters for the first time last night about
the use of cell phones on an airplane and interference with cockpit
instruments. I know that this was mentioned in a November 2006 thread
briefly but the short of it was that they concluded cell phones really
CAN interfere with the VOR signals.

One of the tests however, was on a Hawker jet with a glass cockpit
(Honeywell I believe) with a FMS. They could not get any frequencies or
amplitude of cellphone signals to budge the instruments like they were
able to on a 1970's era NAV system. From what I understand, the FMS
relies on a number of different inputs to drive its nav systems. If
that's the case, wouldn't it switch to another source if one seemed
unreliable? If it does, I imagine it would be a fundamental flaw in
their experiment. The FBO manager and avionics guy being interviewed
however, gave the nod to the robust wire shielding.

Marco

On Thu, 25 Jan 2007 13:06:37 -0800, in
.com, Robert M. Gary
wrote:
However, the show failed to mention that its the FCC rule that states,
no cell phones on in planes ever. The FAA rules are less strict.

Correct... And since they're in CA, they could have flown far enough
offshore to be out of the jurisdiction of the FCC... They would also have
been far enough out that the cell phone signal would not have interfered
with any of the cell phone towers (assuming that is even still an issue
with the digital phones of today)...

On Jan 25, 1:59 pm, Grumman-581
wrote:
On Thu, 25 Jan 2007 13:06:37 -0800, in
.com, Robert M. Gary
wrote:

However, the show failed to mention that its the FCC rule that states,
no cell phones on in planes ever. The FAA rules are less strict.Correct... And since they're in CA, they could have flown far enough
offshore to be out of the jurisdiction of the FCC... They would also have
been far enough out that the cell phone signal would not have interfered
with any of the cell phone towers (assuming that is even still an issue
with the digital phones of today)...

It would have been more interesting if they talked about the FCC's
reasons for their ban and if current technology could still cause
problems with towers.

-Robert