MD80 flight crew resolve compass equipment problem with I80 approach control.

Attributes
ACN	530034
Time
Date	200111
Day	Thu
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : iah.airport
State Reference	TX
Altitude	msl single value : 4000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : i90.tracon
Operator	common carrier : air carrier
Make Model Name	MD-80 Series (DC-9-80) Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	climbout : intermediate altitude
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 200 flight time total : 17000 flight time type : 2000
ASRS Report	530034
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Events
Anomaly	aircraft equipment problem : critical conflict : airborne less severe other spatial deviation
Independent Detector	aircraft equipment other aircraft equipment : radio mag indicatoes other controllera
Resolutory Action	controller : issued new clearance controller : issued advisory flight crew : overrode automation flight crew : returned to intended or assigned course flight crew : overcame equipment problem
Consequence	faa : reviewed incident with flight crew
Supplementary
Problem Areas	Aircraft
Primary Problem	Aircraft

Narrative:

As soon as we started to turn after takeoff, we realized that there was a 30 degree error between our 2 compass system, but we didn't know which one was correct. Departure told us to go direct idu VOR, but the RMI needles were pointing 30 degrees apart also. I picked one (turned out to be the wrong one) and first officer told controller that we had an instrument 'anomaly' and asked if it looked like we were headed to idu. Controller said we needed about 10 degrees right, which didn't help because that was 1/2 way between the RMI needles. A short time later, departure asked our heading. I told him our heading and explained in a little more detail that we had a compass problem. He came back with 'make an immediate right turn,' which of course means that there is a conflict of some kind. We finally got stabilized on a course to the VOR (so we now know which system was correct). I manually slaved the bad system to the good one and it stayed that way for the rest of the flight. In hindsight, you could say we should have noticed the difference before the takeoff roll (if it was present them). But it is not a common problem and when it does occur the comparator always gives a warning. Probably the best thing we could have done was be more specific about the problem with the controllers immediately.

Original NASA ASRS Text

Title: MD80 FLC RESOLVE COMPASS EQUIP PROB WITH I80 APCH CTL.

Narrative: AS SOON AS WE STARTED TO TURN AFTER TKOF, WE REALIZED THAT THERE WAS A 30 DEG ERROR BTWN OUR 2 COMPASS SYS, BUT WE DIDN'T KNOW WHICH ONE WAS CORRECT. DEP TOLD US TO GO DIRECT IDU VOR, BUT THE RMI NEEDLES WERE POINTING 30 DEGS APART ALSO. I PICKED ONE (TURNED OUT TO BE THE WRONG ONE) AND FO TOLD CTLR THAT WE HAD AN INST 'ANOMALY' AND ASKED IF IT LOOKED LIKE WE WERE HEADED TO IDU. CTLR SAID WE NEEDED ABOUT 10 DEGS R, WHICH DIDN'T HELP BECAUSE THAT WAS 1/2 WAY BTWN THE RMI NEEDLES. A SHORT TIME LATER, DEP ASKED OUR HDG. I TOLD HIM OUR HDG AND EXPLAINED IN A LITTLE MORE DETAIL THAT WE HAD A COMPASS PROB. HE CAME BACK WITH 'MAKE AN IMMEDIATE R TURN,' WHICH OF COURSE MEANS THAT THERE IS A CONFLICT OF SOME KIND. WE FINALLY GOT STABILIZED ON A COURSE TO THE VOR (SO WE NOW KNOW WHICH SYS WAS CORRECT). I MANUALLY SLAVED THE BAD SYS TO THE GOOD ONE AND IT STAYED THAT WAY FOR THE REST OF THE FLT. IN HINDSIGHT, YOU COULD SAY WE SHOULD HAVE NOTICED THE DIFFERENCE BEFORE THE TKOF ROLL (IF IT WAS PRESENT THEM). BUT IT IS NOT A COMMON PROB AND WHEN IT DOES OCCUR THE COMPARATOR ALWAYS GIVES A WARNING. PROBABLY THE BEST THING WE COULD HAVE DONE WAS BE MORE SPECIFIC ABOUT THE PROB WITH THE CTLRS IMMEDIATELY.

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.