Date: 201303
Local time of day: 1201-1800
Light: Dusk
Aircraft: MD-83
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
ATC / advisory: Center ZZZ
Aircraft component: AHRS/ND
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Distraction; Workload; Troubleshooting; Human-Machine Interface
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Flight Crew FLC complied w / Automation / Advisory; Flight Crew Overcame Equipment Problem; General Maintenance Action
Contributing factors: Aircraft; Chart Or Publication; Procedure; Manuals; Company Policy
Primary problem: Ambiguous

Narrative: This particular aircraft has a 3 AHRS NAV system; a Right; a Left; and an Aux. After takeoff we received Heading Comparator flags on our FMAs; and noticed our headings differed by approximately 10 degrees. We also noted there were differences between our left and right Winds Aloft readings. Our QRH does not have an applicable abnormal checklist that I could find. We have a Heading Fail procedure; but our headings had not failed. We very specifically and clearly had a Heading Discrepancy; as noted by the Heading Comparator flags on the FMAs. It is strange to notice that in our systems manual there is mention of a procedure of sorts regarding AHRS realignment on the ground in adverse magnetic environments that cautions not to slew the compasses in flight as this could result in Attitude changes. But of course this was in reference to a Slaved/DG compass set up; which of course we did not have. Was that a relevant caution? Why is it not then in our QRH were normally we turn to for answers to our inflight problems? Should this 'procedure' perhaps be moved to our AOM where some of our less used procedures are found? In the end; after much guessing and reading and attempting to determine the best course of action to address our situation; the most appropriate thing to do seemed to be to select AHARS Revisionary Switch to 'R ON AUX.' This allowed us to remove the incorrect information from our Nav presentations; and fixed the Heading Comparator annunciation. I could not find any procedure in our QRH or our systems manual that addressed the appropriate action to take; much less even the presence of the AHRS Revisionary Switch; or the possible benefits of having this switch selected to anything other than NORM; available to a pilot for their use. What is the correct procedure for a Heading Comparator light? We have various different Navigation Systems installed on our aircraft; it would be appropriate; and seeming essential and required; if whatever procedures we are to follow would take into consideration each and every possible aircraft configuration; because eventually we will run into that un-addressed configuration; and then what? Also; as I was explaining our problems to Maintenance; I noticed that the switching of AHRS Revisionary Switch from NORM to R ON AUX resulted in the brief illumination of a 'blank' annunciator located top right on the OAP. I think that this 'blank' light is likely supposed to be a blue AHRS BASIC annunciator. There are 3 AHRS BASIC annunciators located on the overhead panel; but I seem to recall in addition to those three blue Specific; 1; 2; 3 AHRS lights; there was also a Generic AHRS BASIC light on the OAP. When aircraft are inducted into our fleet; each and every piece their installed equipment needs to be carefully inventoried and compared to an established list of existing equipment configurations. Each and every instance of a new or different configuration needs to be carefully and completely examined and addressed. This would include not only the obvious; 'Do we have the parts; manuals and knowledge to use and keep this system operational?' but also; 'How shall we proceed when this system becomes inoperable or degraded?'




Synopsis: A MD-83 HDG COMP flag annunciated after takeoff but the QRH did not have an applicable procedure so the Captain selected R ON AUX which cleared the fault. The Captain noted that the carrier's diverse MD-83 fleet creates equipment and publication knowledge problems.