REFLECTOR: AHARS Problems.... reprinted from my EAA newsletter...

[Matt Bucko via Reflector]

*AHARS PROBLEMS*There continues to be a growing number ofreports 
regarding heading errors andanomalies in heading reference systems dueto 
standard maintenance practices and theuse of standard tools while 
maintaining thesesystems.These errors are occurring in both type 
certificated aircraft and aircraft not type certificated but operated 
under anexperimental airworthiness certificate. Theproblem is due to the 
sensitive nature of thesensors used to provide heading data to 
thesystem. The sensitive nature of the sensorsmay be substantially 
affected by ferrousmaterials, whether moving or stationary andclose 
proximity, electro-magnetic effects,electric motors in close proximity, 
or improperinstallation procedures. Improper use ofmagnetic tools during 
maintenance orremoval of wing inspection covers in theproximity of these 
heading sensor locationsand replacing the panels using incorrecthardware 
can substantially affect properoperation of the sensor.

*Recommendations*The FAA recommends that you follow allmanufacturer 
installation / maintenanceinstructions, warnings, and 
troubleshootingprocedures per their instructions for 
continuedairworthiness or maintenance manuals.These usually require the 
magnetometer tobe mounted away from other electrical ormechanical 
systems on the aircraft.A simple test for possible interference in 
theproposed installation location is to use amagnetic compass and slowly 
move aroundthe area of the sensor looking for anydeflections of the 
compass needle. This testcan reveal interference from moving 
landinggear, flap motors, air conditioning systems,etc. Excessive needle 
movement can beevidence of ferrous material or sources ofmagnetic 
interference.Due to reports of flight control systemcomponents 
interfering with heading sensoroperations, we also recommend that 
youoperate control systems from stop to stopwhile observing a magnetic 
compass placednear the sensor. The compass needle shouldnot deflect more 
than a couple of degrees. Ifit does, you may have to 
degauss(demagnetize) the flight control systemcomponents in the 
proximity of the sensor.Additionally, the following information 
mayassist an installer or maintainer in identifyingpotential 
interference on the heading sensorfrom existing systems:

• Avoid use of magnetic screwdrivers in thearea of the sensors. Magnetic 
fields canpermanently affect the sensor operation andaccuracy and can 
leave residual magnetismon some materials.
• Ferromagnetic materials can becomemagnetized and cause magnetic 
interference.It is important to always use nonmagnetichardware such as 
stainless steel or brass, inthe vicinity of sensors. Always 
followmanufacturer’s recommendation on hardwaretype and distance between 
sensors andferrous materials. Never substitute ferroushardware such as 
screws, washers,nutplates, etc. for non-ferrous hardware in thevicinity 
of the sensors.
• Follow manufacturer’s recommendations onminimum distance from magnetic 
sourcessuch as electric motors, flight control cables,and permanent 
magnets in devices such asspeakers.• Check all wiring runs and ensure 
they areadequately spaced from the sensor locations.Be cautious of any 
new installed wiring in thegeneral area and ensure that it has 
beenrouted properly to not affect the properoperation of the sensor.
• Aircraft structure can become magnetizedby the accumulation of static 
energy on thestructure due to inadequate or improperbonding of grounding 
straps and/or staticwicks. Always ensure the integrity of thebonding 
systems using the manufacturer’srecommendations. Newly painted 
aircraftmay be a likely candidate for improperbonding systems.• Degauss 
any magnetized structure near thesensor location(s). Always follow 
themanufacturer’s instructions on use ofdegaussing equipment. Experience 
hasshown that degaussing is not always 100percent successful. Always 
check aftercompleting the degaussing process for anyremaining magnetism. 
In some rare cases,replacement of structure may be necessary.

For Further Information Contact BarryBallenger, Aerospace Engineer, 901 
LocustStreet, Room 301, Kansas City, MO, 64106;phone: (816) 329-4152; 
fax:(816) 329-4090;e-mail: barry.ballenger at faa.gov.
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