B737 flight crew reported auto slat fail messages at the gate and experiencing PFD (Primary Flight Display) errors and erroneous stick shaker activation in flight. Flight continued to destination and landed safely.
Synopsis
B737 flight crew reported auto slat fail messages at the gate and experiencing PFD (Primary Flight Display) errors and erroneous stick shaker activation in flight. Flight continued to destination and landed safely.
Narrative
After we pushed off the gate in ZZZZ and initiated the After Start Checklist an AUTO SLAT FAIL light was found to be on. The light extinguished with a master caution reset indicating a single channel failure. The QRH was actioned which indicated a single stall management/yaw damper computer had failed and ELB (Electronic Log Book) entry required. We sent the ELB and called Maintenance Control who verified with me over the phone that the AUTO SLAT FAIL light extinguished once the Master Caution was pressed. Maintenance Control deferred this AUTO SLAT FAIL and while I don't have the paperwork anymore I believe it was under XXXXX. After receiving and reviewing the new maintenance release I called again and asked both Maintenance Control and the Chief Pilot if maintenance had additional steps to accomplish. I wasn't sure due to the Maintenance section indicating they must verify normal operation of the remaining autoslat system and the additional notes indicating 'the inoperative autoslat system can be deactivated by pulling (open)...' circuit breakers. I had no indication of which system failed so this would require maintenance; but the MEL says a breaker 'can be' pulled; but no must be. Maintenance Control and the Chief Pilot also reviewed this and stated it was deferrable over the phone with no further action required of maintenance. We filed the maintenance release and continued to the runway with no other issues and no other erroneous indications were present.I was the pilot flying and shortly after takeoff from ZZZZ I noticed my flight director pitch command disappear and reappear multiple times. Additionally; once airborne the SPD LIM failure flag displayed on the captain's PFD (Primary Flight Display). We noted the anomaly while comparing both PFDs with the standby and concluded speed indication was reliable on all of them. After passing 10;000' I transferred controls and normal ATC comms to the first officer along with switching the flight director and transponder data to his side. My side still depicted SPD LIM; but once the flaps were retracted to UP the pitch command stayed on. The closest QRH action I could find in that moment for this abnormal was 'Airspeed Unreliable;' but we both agreed all three were reliable.I initiated a call with dispatch and was patched through. I spoke to dispatch and Maintenance Control about the original Auto Slat Fail deferral and subsequent SPD LIM failure flag after takeoff with the FD (Flight Director) pitch command issues on the captains PFD. Maintenance Control agreed the only QRH somewhat covering this was 'Airspeed Unreliable;' but speed was determined reliable. They advised there was nothing else to try and to just call back if anything else came up. We concluded the radio patch and a few minutes later the stick shaker activated for 2-3 seconds in association with the min speed bar filling up the speed tape on the captain's PFD. We called Maintenance Control back and relayed this info and they believed it to something (possibly AOA) feeding the captain's side bad data; thus; activating the stick shaker erroneously. The aircraft was flying normal otherwise though and the FO's PFD had no issues. After discussing with dispatch; Maintenance Control; and the First Officer we decided to continue to ZZZ with the First Officer made pilot flying for the remainder of the flight. Through the flight the stick shaker activated in 1 - 3 second bursts about a dozen times until the first selection of flap extension was made descending into ZZZ. Once the flaps were extended the captain's PFD FD pitch commands disappeared and reappeared at random again; but erroneous stick shaker activations stopped. Once parked at the gate in ZZZ we gave the maintenance team a recap and their hunch was a bad AOA vane and/or SMYD (Stall Management/Yaw Damper) computer.
Second reporter narrative
While running the After Start Checklist in ZZZ prior to taxi; the FLT CTRL master caution light illuminated during recall. The AUTO SLAT FAIL light was found to be associated with the master caution light. The AUTO SLAT FAIL light extinguished with a master caution reset. We referenced the QRH which indicated that it was a single stall management/yaw damper (SMYD) computer failure and required an ELB (Electronic Logbook) entry. We submitted an ELB and the captain called Maintenance Control who verified over the phone that the AUTO SLAT FAIL light was extinguished after master caution reset. Maintenance Control applied a deferral under MEL XXXXX. After receiving the new release and the associated MEL; we both agreed we wanted to verify with Maintenance Control that the MEL had been applied appropriately due to a flight crew action stating; 'MAINTENANCE IS REQUIRED TO VERIFY THAT REMAINING AUTO SLAT SYSTEM OPERATES NORMALLY ON INITIAL DEFERRAL.' The captain called Maintenance Control and the Chief Pilot to ask if maintenance needed to physically verify anything in order to meet this requirement and both stated that maintenance was able to verify the remaining auto slat system was operating normally over the phone via the 'AUTO SLAT FAIL' light extinguishing when we pressed the master caution reset. We filed the release and taxied out to the runway without further incident for departure.The captain was the pilot flying and briefly after takeoff he mentioned that his flight director's pitch commands were beginning to disappear and reappear multiple times. As the pitch commands were beginning to malfunction; a SPD LIM flag also appeared on the captain's PFD (Primary Flight Display). The captain and I both took note of the anomaly; cross checked both PFDs and the standby airspeed indicator; and concluded that all three were providing reliable airspeed information. After climbing through around 10;000 feet; the captain transferred controls to me and we swapped the flight directors and transponder data to reflect that as well. The captain's side continued to display the SPD LIM flag; however once the flaps were fully retracted his pitch command bars returned. We attempted to find a QRH procedure relating to the abnormality. The closest QRH we could find was 'Airspeed Unreliable' but we agreed that all three were providing accurate information. After transferring both the flight controls and normal radio calls to me; the captain initiated a call with dispatch and began communicating with dispatch and Maintenance Control. Maintenance Control agreed that the only QRH remotely addressing the issue was 'Airspeed Unreliable;' but we determined we had reliable airspeed indications. We were told there was nothing else that could be done. They recommended we'd continue to ZZZ and to reach back out if it got any worse. Shortly after finishing the call with Maintenance Control /Dispatch; the stick shaker activated momentarily and the captain's min speed bar was filling the entire airspeed indicator. The captain called Maintenance Control again and passed along this new information. Maintenance Control believed it to be something providing the captain's side with improper data (possibly a faulty AOA indicator) and causing an erroneous stick shaker. The aircraft was flying normally and my PFD had all normal indications. Me; the captain; Maintenance Control; and dispatch decided the best course of action was to continue on to ZZZ with me as the pilot flying. Throughout the flight the stick shaker continued to activate for only a few seconds about a dozen times; until the flaps were extended on the descent into ZZZ. Again; with the flaps extended; the captain's pitch commands bars began to disappear and reappear periodically. We made a normal approach into ZZZ and landed without any further indecent.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.