B737 flight crew reported overspeed alert and airspeed indicator disagree resulted in a priority landing. Flight crew also indicated QRH procedure for alternate landing configuration was missed due to workload.
Synopsis
B737 flight crew reported overspeed alert and airspeed indicator disagree resulted in a priority landing. Flight crew also indicated QRH procedure for alternate landing configuration was missed due to workload.
Narrative
Uneventful takeoff out of ZZZZ1. As we approached waypoint ZZZZZ in cruise at FL380 with the autopilot engaged on system B; the clacker for over speed sounded off. The IAS on the FO side showed overspeed on the speed tape. I disconnected the AT (Auto-Throttle) and reduced thrust just slightly before the CAPT (Captain) commented 'Unreliable Airspeed.' As soon as the AT was disconnected; the overspeed clacker stopped and the FO side IAS was back in normal range. I looked over to the CAPT side's IAS and found it to be approximately 10 knots different. CAPT then pulled out the QRC and we completed the immediate action items of disconnecting the AP (Auto Pilot) and turned off the FD (Flight Director). We left the power setting as it were as it provided level flight at the moment. CAPT then proceeded onto the QRH procedures. We examined our IAS differences per the QRH. Initially; it was very difficult to determine which one was reliable as the differences between the two IAS were very close; no more than 7-8 knots difference between either IAS and the standby instrument as well.After stabilizing the airplane in level flight; the IAS DISAGREE flag disappeared and both FO and CAPT side IAS would similar airspeeds. We noticed the IAS DISAGREE flag intermittently came back as we made either heading changes or slight adjustments in trim settings in level flight. After about 5 minutes; we were able to get everything trimmed out and I hand flew the airplane for the next approximately 40 mins.In the meantime; CAPT contacted Dispatch and Maintenance Control and we concluded it was best to proceed towards ZZZZ. There was a big weather system moving through the ZZZZ1 area and we felt it was the safer course of action to go to ZZZZ; where the weather was much better; in the event an unreliable airspeed descent; approach; and landing is required using raw data and the pitch and power settings provided in the QRH.I took my crew rest opportunity at approximately 1 hour 20 mins into the flight. During the crew rest; I studied the QRH procedures and the landing procedures and calculated the landing distances for our condition. All of ZZZZ's runways in use at the time when I left the flight deck would have been satisfactory for our landing.I returned to the flight deck with approximately an hour until landing. I sat down and noticed the AP and FD have been reengaged on the FO side. After discussion with the CAPT and the Relief Pilot; it was their determination that my side was the reliable IAS. The automation was on and worked except the AT. The CAPT also advised that we will be conducting a Flaps 15 landing per the QRH landing distance table. We briefed the approach into ZZZZ and discussed the landing and Go-Around procedures with unreliable airspeed.Eventually; the IAS DISAGREE flag never resurfaced and we did an uneventful landing using Flaps 15 to RWY XXR at ZZZZ. We then taxied to our gate without further incidence.
Second reporter narrative
We departed ZZZZ1 fairly uneventfully. There were reports of turbulence and a couple of cells that we were deviating from; but we were still in between layers of clouds and precipitation. During the climb out; everything was pretty standard for flying in the tropics out of ZZZZ1.As we climbout and level off at cruise; I hop out as the Relief Pilot and go do the first hour break so that the PF can take his 1.5 hour break after mine. I go back to the cabin and about an hour passes and I get the call to come back. As the flying FO steps out; I hear the AP (Auto Pilot) disconnect. I sit down and see that the FD (Flight Director) and FMA (Flight Mode Annunciator) modes are not on the PFD (Primary Flight Display). I look at the airspeed tape and the captain's tape and they're splitting and it says IAS Disagree.The Captain hands me the controls to do administrative tasks and his duties and briefs me on what has happened. We deduce together that the FO side is more reliable in this instance while referencing the standby airspeed indicator. The indicated airspeed seemed to match for both the FO and stand by while the Captains seemed to be frozen.We [requested priority handling] and plan for a Flaps 15 landing and I back the Captain up and we re-run the QRC and QRH (except the performance tables at this point because the captain and FO have calculated it already) as the Captain and the flying FO have already done so prior. Captain told me he didn't call me sooner because he wanted me to rest and decided to brief me when I got back because we would have to raw data hand-fly the entire way for the most part. He briefed me that he and the flying FO decided it was best to just continue to ZZZZ and chose to continue.Captain and I discussed the possibility of going back to ZZZZ1; ZZZZ2; or continuing to ZZZZ. With flaps 15 he believes the best course of action is to go to ZZZZ with a longer runway. With the conditions in ZZZZ1 and ZZZZ2 possibly deteriorating I agree and said it's better to go to a place with better weather. I ask the Captain if we should ask to descend for RVSM incapability and he decides it's safer to stay higher for now because of the weather possibly below us.Flying FO comes back up and as we switch; I just happened to look up and see that both EECs (Electronic Engine Control) are switched to a soft alternate. I tell the guys and ask if they saw this before and they both never saw it and never got a caution light on it. Captain asks me to run the QRH procedure for it. I run the QRH and we switch the EECs to hard alternate.I ran through the QRH again by myself for both situations to make sure we didn't miss anything and this time read the performance tables to make sure calculations for distance and power setting are good. I didn't realize it at the time but I remember flipping the page over from Flaps 15 unreliable airspeed and on the next page glimpse over flaps 30 but at the given time I didn't think deeply about it and continued on to quickly double checking the math for some reason it did not occur to me to not question Flaps 15 at the time with everything else going on. I was trying to note down power settings; pitches; and descent rates for the remainder of the flight and a diversion to ZZZZ3 if need be; a possible go around; and re-briefing the flying FO the deferred items.We land relatively uneventfully with priority to [Runway] XXL taxi to the gate and talk to Maintenance. We did not flip the flaps inhibit and got the too low flaps aural call out. Captain at this point called continue.During a debrief I began running through the series of events that happened and ask the Captain why he chose Flaps 15 and he said that's the only one that was written and he asked the flying FO and I if there was anything else and the flying FO said that there was a flaps 30 option and I mentioned it as well. The Captain said he didn't remember seeing one because of the placement of where it was in relation to the other options. We all verbally debrief and agree that the situation was quite task saturating and it was likely missed because of so much going on. Dodging storms while hand flying; deciding if we should go down out of RVSM but choosing to stay higher to stay out of precipitation. We believe that possibly ice or super cooled liquid had frozen the pitot. While trying to understand the controllers and dealing with a secondary issue with the EECs. We get back to the plane and maintenance says there's nothing wrong with the plane. The flying FO stated that the FD and AP were slaved to his side so that even with unreliable airspeed the plane should not have hit the clacker then. The captain tells the tech ops guys to run more tests and they decide to try a pitot static system test but they don't have the gear to do it and decide to borrow some from Air Carrier Y and Air Carrier Z. We finally run tests on both pitot systems and nothing comes up wrong. Maintenance Control and Dispatch agree we have done everything we could and they decide it may have just been ice that got lodged. We have the Maintenance guys check the drains and they find no liquid and say that the drains are dry. With nothing indicating anything was wrong we decided to continue for the next flight with an extension.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.