Small aircraft pilot had a track deviation because of erroneous compass readings.

Attributes
ACN	529731
Time
Date	200111
Day	Fri
Local Time Of Day	1201 To 1800
Place
Locale Reference	intersection : trips
State Reference	OH
Altitude	msl single value : 8000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Operator	general aviation : personal
Make Model Name	Small Aircraft, Low Wing, 1 Eng, Fixed Gear
Operating Under FAR Part	Part 91
Flight Phase	cruise : level
Flight Plan	IFR
Person 1
Affiliation	other
Function	flight crew : single pilot
Qualification	pilot : private pilot : instrument
Experience	flight time last 90 days : 12 flight time total : 450 flight time type : 180
ASRS Report	529731
Person 2
Affiliation	government : faa
Function	controller : radar
Events
Anomaly	aircraft equipment problem : critical non adherence : published procedure other anomaly other other spatial deviation
Independent Detector	aircraft equipment other aircraft equipment : compass, dg other controllera
Resolutory Action	controller : issued new clearance controller : issued alert controller : provided flight assist flight crew : returned to intended or assigned course none taken : detected after the fact
Supplementary
Problem Areas	Flight Crew Human Performance
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Operation was a flight with client and spouse as passenger. Departed I69, oh, en route to final destination of shd, va, on an IFR flight plan. (WX was cavu throughout the planned route of flight, but require IFR operation.) received my IFR clearance in the air immediately after departure and ATC issued a vector of 150 degrees to intercept the airway (V128 between cvg and yrk vors). A few mins alter, the controller asked me to check heading, and reported that I had initially turned on to the airway but was now diverging to the right by about 20 degrees. I verified that the directional gyro heading matched the compass heading. However, both my LORAN and handheld GPS concurred with the controller's observation. I corrected my heading as per the LORAN and GPS and reintercepted the airway. A rebuilt directional gyro had been installed in the aircraft just a few days before. I therefore focused my troubleshooting assessment on that instrument. However, within a few mins I was back on course, and all instruments were in agreement before I could isolate the problem. About 10 mins later I checked the directional gyro against the compass and found that they were about 20 degrees out of agreement. Again, I focused my attention on the possibility of trouble with the directional gyro. But within a couple of mins, I realized that since the LORAN and GPS indicated that I was on course, and that the compass indicated a different course, the problem must be with the compass. With this in mind, I soon realized that I had placed my handheld GPS on the dash at closer to the compass than I ever had in the past. I moved the GPS off the dash, and the compass immediately moved approximately 25 degrees, and to the heading in agreement with the LORAN and GPS. I moved the GPS back, and the false reading returned. So the GPS had been interfering with my compass. I felt foolish. I should have figured this out on my own! It was especially insidious because the interference was intermittent, and the recent replacement and the directional gyro had caused me to focus my attention on the directional gyro, the last thing that changed, instead of opening my mind to other potential root causes of the problem.

Original NASA ASRS Text

Title: SMA PLT HAD A TRACK DEV BECAUSE OF ERRONEOUS COMPASS READINGS.

Narrative: OP WAS A FLT WITH CLIENT AND SPOUSE AS PAX. DEPARTED I69, OH, ENRTE TO FINAL DEST OF SHD, VA, ON AN IFR FLT PLAN. (WX WAS CAVU THROUGHOUT THE PLANNED RTE OF FLT, BUT REQUIRE IFR OP.) RECEIVED MY IFR CLRNC IN THE AIR IMMEDIATELY AFTER DEP AND ATC ISSUED A VECTOR OF 150 DEGS TO INTERCEPT THE AIRWAY (V128 BTWN CVG AND YRK VORS). A FEW MINS ALTER, THE CTLR ASKED ME TO CHK HDG, AND RPTED THAT I HAD INITIALLY TURNED ON TO THE AIRWAY BUT WAS NOW DIVERGING TO THE R BY ABOUT 20 DEGS. I VERIFIED THAT THE DIRECTIONAL GYRO HDG MATCHED THE COMPASS HDG. HOWEVER, BOTH MY LORAN AND HANDHELD GPS CONCURRED WITH THE CTLR'S OBSERVATION. I CORRECTED MY HDG AS PER THE LORAN AND GPS AND REINTERCEPTED THE AIRWAY. A REBUILT DIRECTIONAL GYRO HAD BEEN INSTALLED IN THE ACFT JUST A FEW DAYS BEFORE. I THEREFORE FOCUSED MY TROUBLESHOOTING ASSESSMENT ON THAT INST. HOWEVER, WITHIN A FEW MINS I WAS BACK ON COURSE, AND ALL INSTS WERE IN AGREEMENT BEFORE I COULD ISOLATE THE PROB. ABOUT 10 MINS LATER I CHKED THE DIRECTIONAL GYRO AGAINST THE COMPASS AND FOUND THAT THEY WERE ABOUT 20 DEGS OUT OF AGREEMENT. AGAIN, I FOCUSED MY ATTN ON THE POSSIBILITY OF TROUBLE WITH THE DIRECTIONAL GYRO. BUT WITHIN A COUPLE OF MINS, I REALIZED THAT SINCE THE LORAN AND GPS INDICATED THAT I WAS ON COURSE, AND THAT THE COMPASS INDICATED A DIFFERENT COURSE, THE PROB MUST BE WITH THE COMPASS. WITH THIS IN MIND, I SOON REALIZED THAT I HAD PLACED MY HANDHELD GPS ON THE DASH AT CLOSER TO THE COMPASS THAN I EVER HAD IN THE PAST. I MOVED THE GPS OFF THE DASH, AND THE COMPASS IMMEDIATELY MOVED APPROX 25 DEGS, AND TO THE HDG IN AGREEMENT WITH THE LORAN AND GPS. I MOVED THE GPS BACK, AND THE FALSE READING RETURNED. SO THE GPS HAD BEEN INTERFERING WITH MY COMPASS. I FELT FOOLISH. I SHOULD HAVE FIGURED THIS OUT ON MY OWN! IT WAS ESPECIALLY INSIDIOUS BECAUSE THE INTERFERENCE WAS INTERMITTENT, AND THE RECENT REPLACEMENT AND THE DIRECTIONAL GYRO HAD CAUSED ME TO FOCUS MY ATTN ON THE DIRECTIONAL GYRO, THE LAST THING THAT CHANGED, INSTEAD OF OPENING MY MIND TO OTHER POTENTIAL ROOT CAUSES OF THE PROB.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.