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'[OT] Re: RF Noise'
1998\02\05@185547 by Sean Breheny

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At 08:56 AM 2/5/98 -0500, you wrote:
{Quote hidden}

<big snip including summary of incident>

I don't understand why the directional gyros are affected and not the nav
gear!!!!?????? AFAIK, the directional gyros are inertial (by definition)
and might be slaved to magnetic compass to make sure they roughly agree,
however, I don't see any way they could be affected by RFI. Especially
considering that the radio nav gear was functioning at least two of the
cases.

Sean

+--------------------------------+
| Sean Breheny                   |
| Amateur Radio Callsign: KA3YXM |
| Electrical Engineering Student |
+--------------------------------+
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1998\02\05@214655 by Lee Jones

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>>>  Umm, yeah.  Names and other verifiable details conveniently
>>> omitted (even to the type of "navigation gear" that was
>>> affected).  Pilots have several

>> This description sounds somewhat similar to the situation described
>> earlier.  This came from the ASRS Database Report Sets Index

> <big snip including summary of incident>

> I don't understand why the directional gyros are affected and not
> the nav gear!!!!?????? AFAIK, the directional gyros are inertial
> (by definition) and might be slaved to magnetic compass to make
> sure they roughly agree, however, I don't see any way they could
> be affected by RFI. Especially considering that the radio nav
> gear was functioning at least two of the cases.

All gyros precess.  The attitude indicator gyro is corrected
by gravity and ingenious mechanics since most aircraft spend
most of the time in level flight.

In light aircraft, directional gyros are manually corrected
to the magnetic compass using a small knob on the face of the
instrument.  It's needed every 5 to 20 minutes.

In large transport category aircraft, this is automatic.  I
believe that the magnetic sensor is a fluxgate and is commonly
mounted on a wingtip.  It needs to be away from large ferrous
objects like the landing gear.  The connection, obviously, is
a wire between the two.  How sensitive this is to any EMI/RFI...


From a pilot's point of view, I don't understand one thing.
Transport category aircraft are almost always on IFR flight
plans.  They are flying airways defined by VOR radials (the
"nav gear" in 108-118 MHz).

Your heading, as read from the directional gyro & magnetic
compass, should roughly agree.  But if you drift off the
airway, you turn back toward it.  This is quite common when
correcting for wind (cross wind pushing you off track).

Now if interference displaced the directional gyro _and_
the magnetic compass (possible if on same sensor), then they
would not agree with the VOR/airway data.  Your DG heading
would be _way_ off and should raise an obvious red flag to
the pilot.  But you'd be able to track the airway just fine.

                                               Lee Jones

1998\02\06@015647 by Ron Fial

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My 2 bits, an a guess, for what its worth:

At a college where I taught many years ago, we had a blind student who
would get his computer 'output' via an FM radio on the console, with the
program set to loop at various frequencies for a dot and a dash, and he
could listen to the output in morse code (he was also a ham radio operator).

The FM radios are the main culprit in aircraft, since most directional
headings are based on VOR signals that occupy many MHz just above the FM
band, where the local oscillator of the FM radio sits, typically 10.7 MHz
above the station being received.  The signal only has to be nearby in
frequency to mess up the VOR signal.  The VOR XMTR is not high powered (20
to 50 watts), and may be 50 miles away, whereas the FM receiver is maybe
only 1 milliwatt but 50 feet away!  The direction signal is transmitted as
a phase difference between a low frequency audio tone and the same
frequency audio tone modulated onto a 10 KHz carrier.  By comparing the
phase difference, you can tell the direction in degrees from North that you
are from the transmitter.  It is real easy for an FM radio local oscillator
hetrodyne to mess this up.

When the pilot in an airliner refers to his Directional Gyro, he is really
refering to a more modern and complex device called an HSI (horizontal
situation indicator).  This instrument shows you gyro heading alright, but
it also shows you (with a needle) how much you are to the right or left of
course, and the pilot may simply have calculated a 20 degree or more
correction in order to get the needle to center.  Keeping the needle
centered, he thinks he is just correcting for wind, while his actual
heading is quite wrong.  I am guessing that this is why he refered to the
DG the way he did in the report. (yep, i'm a pilot, fly my own plane).

In our FCC testing for class A/B, we keep an old Toshiba 286 plasma panel
laptop around with a metal (I think magnesium) case  -- it is super quiet
in the RFI department, and we can test our boards, external adapters, etc.
in an otherwise quiet RF environment.


Regards,
 Ron Fial

1998\02\06@034623 by Lee Jones

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> When the pilot in an airliner refers to his Directional Gyro,
> he is really refering to a more modern and complex device
> called an HSI (horizontal situation indicator).

Maybe, maybe not.  Depends on the equipment fit.  Probably
true for large scheduled carriers flying large transports.
Now smaller feeder flying turboprops...

> This instrument shows you gyro heading alright, but it also
> shows you (with a needle) how much you are to the right or
> left of course, and the pilot may simply have calculated a
> 20 degree or more correction in order to get the needle to
> center.  Keeping the needle centered, he thinks he is just
> correcting for wind, while his actual heading is quite wrong.
> I am guessing that this is why he refered to the DG the way
> he did in the report. (yep, i'm a pilot, fly my own plane).

As long as the aircraft is flying the correct ground track,
it will have obstacle clearance.  The heading is immaterial.

For example, assume a news helicopter is tracking a parade on
an east-west road.  Parade is going east at 3 knots.  Wind is
out of the north at 30 knots.  Helicopter needs to fly at 30.1
knots airspeed on a heading of 006 to stay on a ground track
of 090.  That's 84 degrees of crab -- but the helicopter will
stay over the floats.  This is a exagerated example, but the
same thing works on a airway.

If the DG shows a value that's grossly at odds with the VOR
heading when corrected for expected/reasonable wind, then the
flight crew should suspect an equipment malfunction.  And use
all other available navigation gear (ADF, DME, GPS, and/or
Loran on board; radar track on ground [i.e. ask controller]) to
verify their location and pinpoint which instrument(s) failed.

                                               Lee Jones

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