Date: 201812
Location: ZZZ.ARTCC, US
Flight conditions: VMC
Aircraft: B737-900
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: INS / IRS / IRU
Component problem: Failed
Reporter organization: Air Carrier
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Human factors: Time Pressure
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Aircraft Equipment Problem Critical
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Flight Crew Overcame Equipment Problem; Flight Crew Requested ATC Assistance / Clarification
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: We were level at FL380 when the First Officer commented that the airplane was turning left while we headed directly to a fix; in LNAV and VNAV. We also noticed that the First Officer's VSI was indicating a climb of around 10;000 FPM while in level flight but mine was indicating correctly at 0 FPM. At that point; BOTH ND's were indicating that our course was approximately 40 degrees to the left. There was a discrepancy on what heading we were actually flying between the map compass vs the PFD compass rose. At that point; the First Officer's attitude indicator was showing a 30-40 degree bank to the right in straight and level flight. I disconnected the autopilot because we didn't know which instruments to trust quite yet and the heading bug was giving us erroneous information; so there was no way to couple the autopilot up to anything. We were in VMC on top of a solid layer of clouds. Shortly after that; the IRS and master caution light illuminated. It indicated an R IRS fault. I told the First Officer that I would fly and he could run the QRH procedure and handle the radios. We ran the QRH procedure for an IRS [Inertial Reference System] fault and decided to attempt to get the First Officer's attitude indicator working. After flying straight and level for more than one minute with the right IRS selector in ATT [Attitude Hold]; his ADI was still inoperative and showing a 45 degree bank to the right now. The IRS fault light went out because the IRS was in ATT now but the problem still remained. We talked about and agreed to put the IRS selector switch into BOTH on L so the First Officer would have reliable instruments instead of looking at an attitude indicator that was showing a 45 degree banked turn to the right. We talked about swapping arrivals and going VOR to VOR but ATC kept trying to give us fixes to fly direct to. At that point; we needed to start our descent and [advised ATC] because ATC was not understanding our inability to fly to fixes and it was getting really busy on the flight deck. They then asked us if we could intercept radials; and we replied yes. We swapped our ND's into VOR mode to navigate and get rid of the erroneous map displays. They gave us a heading to intercept the radial and descents along the way and we did that. I then transferred control of the airplane to the First Officer while I [alerted] the flight attendants. We did not have time to contact [Maintenance] or Dispatch during that time because we'd be on the ground shortly. The approach and landing was uneventful.


Second narrative: During level cruise flight at FL380; I noticed the aircraft begin to turn and saw very abnormal climb rate on VSI and full slip/skid deflection on ADI. We were VMC on top of a layer so I knew the indications were erroneous but I notified the Captain. He selected HDG mode but the aircraft did not respond (unknown if FMA went to HDG from LNAV). He then disengaged AP and returned to last known direction of flight since both of our map DUs and compass roses were giving false information. I notified ATC we were experiencing a navigation problem. At this point; we noticed the FAULT light in the number 2 IRS [Inertial Reference System]. The Captain continued to hand fly while I ran the QRH. We got to the point where we could select ATT [Attitude Hold] to regain the attitude function of the failed IRS. When I selected this; the instrument fully tumbled (full deflection of everything). At this point; we were a little confused as to proceed to the next step of the QRH 'Both on left IRS' because it was expected we would at least regain the ATT function. We still selected Both on Left IRS and the left side ADI was duplicated on my side. During the running of the QRH; BOTH map displays were displaying erroneous and unreliable data. The magenta line was 4 miles left or our course; the heading indicator was at least 30 degrees off and the wind vector was showing 247 k from the wrong direction. We then told ATC we needed at non-RNAV arrival and they responded with two attempts to send us to a waypoint. I don't think they fully understood our limited nav capability. This; along with beginning a descent and the unreliable indications; we decided to [advise ATC]. We broke out into VMC conditions and got a few vectors to a visual approach that was somewhat uneventful. We also gave instructions to the flight attendants and informed passengers of a normal landing. One thing we noticed somewhere on approach was that the heading indications were now appearing to be correct on both maps. We did not take note of any other anomalies pertaining to the nav equipment as we were focused on the landing.


Synopsis: B737NG flight crew reported loss of much of their nav capability following the failure of the right IRS.