B737-800
Flight Crew Air Transport Pilot (ATP)
Confusion
Aircraft Equipment Problem Critical; Deviation - Track / Heading All Types
Air Traffic Control Provided Assistance; Flight Crew Requested ATC Assistance / Clarification; Flight Crew FLC Overrode Automation; Flight Crew Landed As Precaution
Aircraft
We were at FL370; VMC above the cloud layer; when I noticed the FO's (First Officer) flight director pitch bar commanding a pitch up. It would move rapidly up the attitude indicator and at about 20 degrees nose up disappear and then reappear at level pitch. This was being repeated over and over. At the same time the FO's VSI (Vertical Speed Indicator) would momentarily show a 9;999 foot descent and then indicate nothing. Also noticed the wind on the NAV Display was increasing from 100 up until about 280 knots of tailwind. Ground speed was about 680 knots. Asked ATC what our ground speed was and they replied with something like 540 knots. The FO's attitude indicator then started to show the aircraft was increasing pitch to about 20 degrees nose up. Autopilot was maintaining level flight and on course in LNAV.We got a momentary Unable RNP (Required Navigation Performance) on the NAV display and noticed both attitude indicators were indicating a bank of about 5 degrees. Checked the FMS POS page and noticed the Right IRS was not indicating any position. IRS panel showed no lights and nothing came up on recall. Had FO look thru QRH for a possible procedure; and we found nothing. CA's (Captain) attitude and flight director seemed to be normal other than the bank. LNAV was no longer tracking and when asked ATC said we were off course. Asked ATC for a vector; and switched aircraft roll mode to HDG Select. Called dispatch on AIRINC and told him our issues and told him we were going to divert as our destination was no longer a viable option because of weather. FO was flying and talking to ATC; they told us an airport had good weather. We decided to divert as I knew the airport; and it would be visual approach should we encounter any other instrument anomalies. Had dispatch patch tech in on the phone to see if we could find any solutions or try to figure out what we were missing. They had no suggestions. I then set up for the alternate airfield and resumed flying duties. We both agreed that even though it appeared the right IRS was malfunctioning we didn't know why and that there was no applicable QRH procedure at the time. Didn't want to try and troubleshoot without official guidance for fear of losing the CA's attitude indicator. So thought the safest course of action was to head to alternate. Called FA's (Flight Attendant) and gave them their TEST (Time; Emergency; Signal; Take) briefing and made a PA to the passengers. We declared an emergency since we were still in RVSM airspace and I thought we no longer met those requirements. Did not want to descend into IMC. Prior to that I told ATC I did not think we were RVSM capable as he had asked because of crossing traffic; he gave us a vector.On the descent from 370 the roll mode was no longer flying the heading dialed in. Deselected and tried CWS (Control Wheel Steering). At about this time the autopilot failed and would not re engage; the FO's PFD lost all information as if we were at the gate with IRS in off. And the right IRS fault light illuminated. We set up for the approach and I decided to run the QRH procedure for IRS Fault Light. We tried to realign using ATT (Automatic Attitude Hold) and straight and level for 30 seconds. FO's attitude indicator recovered but was showing the aircraft to be in a 90 degree bank. During this time the aircraft experienced a yaw moment and felt like the YAW damper was off for rest of flight. YAW Damper switch was still on and light was extinguished. Made a normal approach and landing. During the landing rollout and passing about 100 knots we got a stick shaker. Taxied to the gate normally.
Due to erratic flight instrument and flight director indications the flight crew determined it was unsafe to fly into IMC; so they diverted to an airport that was VMC.