B747-400 crew reported unreliable airspeed during climb in rain and thunderstorms resulting in a stick shaker after previous maintenance on the air data system. The crew recovered the aircraft then diverted to an alternate airport; dumped fuel; and landed.
Synopsis
B747-400 crew reported unreliable airspeed during climb in rain and thunderstorms resulting in a stick shaker after previous maintenance on the air data system. The crew recovered the aircraft then diverted to an alternate airport; dumped fuel; and landed.
Narrative
On Day 0; I was part of a 4 man crew operating from ZZZZ1 to ZZZZ; wait to be unloaded/loaded; then continue on from ZZZZ to ZZZ. In ZZZZ; I assumed the duties of pilot flying. Minutes before pushback; the CA's PFD blanked and then reappeared but showed a failure of airspeed and altitude. Maintenance was called and re entered the aircraft. After multiple tests; as well as switching the ADC's; the test results showed a CA's side ADC fault and Left AOA sensor failure. We timed out and were told a new sensor would be delivered overnight and installed. The next day we arrived back at the aircraft; and at the time the weather was intense thunderstorm with significant rainfall. The mechanic was outside in the rain; uncovered; installing the sensor. When they finished; they ran numerous tests and calibrations and signed the aircraft off as airworthy. At the conclusion of my preflight brief; the CA raised his concern that we should be ready; just in case; if there are any unusual discrepancies in altitude or airspeed; so he asked the 3rd FO to have the QRH open to pitch and power settings for a airspeed disagree scenario. We took off and departed from RWY XXR with no issues. I hand flew the departure to 10;000 feet at which time I engaged the R autopilot. The aircraft continued to climb normally. At around 18;000 feet; ATC instructed us to slow to 280 kts for traffic. I opened the speed window and set 280kts and verified this was above the amber band. We were in IMC conditions at the time. As the aircraft was approaching 280 kts; I noticed the green trend vector continue below 280 to near 260 kts or so. I immediately began adding a little power to stabilize the speed. The aircraft dipped to about 279 kts and at that time the stick shaker engaged. I was initially stunned by this as there were no indications on the PFD of a stall. We were above the amber band and the red stall bars were below that. I immediately throttled up and applied forward yoke and disconnected the autopilot and he CA shadowed my control movements and asked if I had it under control; which I acknowledged. The stick shaker went away. As I began to pitch back up slowly; the stick shaker again engaged for a brief second; so I relaxed the yoke and then began accelerating substantially and pitching up at about 2.5 degrees. At this point the airspeed was around 310kts. We then cleared the flight directors and re automated and the aircraft seemed to be acting as it should. As I received climbing instructions I used V/S the first time and VNAV the next time to watch how the aircraft handled both commands. At this time the CA contacted maintenance and the company via SATCOM. I did not hear the conversation as I was handling the aircraft. At the end of the call the CA informed me that we would be diverting to ZZZZ2 and that we would need to dump fuel. We established the route to ZZZZ2 and the CA asked if I would be comfortable turning off the automation during the arrival and hand flying through the approach using the flight director backed up with pitch and power callouts from the QRH; I said I had no problem with this. We agreed that I would keep the 10-15kts above flap speeds in case of unreliable airspeed. After the fuel dump was completed; I flew vectors to the XYL ILS and had an uneventful landing. During and after the event; there was no deviation from the route.Cause: I can't say for certainty the cause of this issue. I was startled and surprised by the stick shaker as there was no indications of a stall and the aircraft was fully automated at the time. I can only assume there was some sort of fault or issue with the AOA sensor; but again I can't say for certain. Suggestions: Perhaps the aircraft should have been in a hangar for the repair; or wait until the storm had passed. Also; I need to remember to act immediately with the stick shaker; as I felt I hesitated just a moment; as I think I had a moment of disbelief because there were no indications of a stall with the exception of the shaker. Personally; I think I could have been a little more deliberate in my recovery actions. But the CA did a good job of making sure the right actions were taking place.
Second reporter narrative
Original flight was canceled due to left AoA needing to be replaced in ZZZZ. when crew was brought back to the aircraft; ZZZZ was under severe thunderstorm rainfall and maintenance was working on the AoA sensor in heavy rainfall. we delayed many hours while they finished the operation. We were able to have it signed off and we departed ZZZZ with no incident. we were at max gross weight. I was in the observer seat. FO was PF and CA was PM. we were flying through heavy rainfall on climbout; and at around 16;000 feet; ATC gave us a heading and a speed restriction of 280 on climb. this speed was an immediate red flag in my mind as we were at max gross weight and flying through turbulent rainy weather. I was heads down; working on completing the ETA's on the flight plan. at around 18;000 feet I observed the aircraft being pushed nose down. this was from the captain overriding the AP and reducing the AoA. speed was initially decreasing below 280 when I looked at the PFDs. I saw the red speed band climbing and our speed was about to enter the rising red band. captain reduced pressure and the aircraft began to rise back up and this is when the stick shaker was activated; and wing buffeting occurred. The FO as PF disconnected the AP and AT and reduced the AoA to around +2 degrees captain increased thrust to an unknown value to me. captain called for me to read out the climb pitch from the QRH; which we had opened up and ready from being briefed about the possibility of an issue in flight by the captain on the ground. I told the captain to maintain 4 degree pitch. this allowed the aircraft to level off and accelerate. once in positive control and airspeed and altitude increasing the PF reautomated; and we resumed a normal climb. the rest of the flight had no further insident. however; we were all on edge and nervous from having encountered a stall. the captain called dispatch on satcom and I took over comm 1 with ATC. the sat call took around 45 minutes; and the PF and I were left with managing the aircraft. captain decided to divert to ZZZZ2 for safety of flight concerns. this required a complicated reroute from ATC; and fuel jettison of 50;000kgs. after jettison; we flew the ILS XYL and captain wanted approach speeds to be increased to avoid another stall incident. around 1000 ft after AP AT disconnect; we had gains and losses of 10kts from gusty winds; which made the CA micromanage the approach. at 100ft captain saw a decreasing airspeed that was nearing vREF and at 50 feet he pushed up the thrust levers to a very high value causing a long float; and 2 pilots on the controls. at touchdown a large bank angle was observed and the captain got on the yolk and started to input corrections. it made for a very unsettling touchdown that I felt could have been avoided by letting the PF make all necessary speed and bank corrections on his own.Cause: ATC gave speed restriction of 280 on climb. A stall warning and buffeting occurred. Suggestions: Maintenance replaced AOA sensor during very heavy rainstorm and new seal was not properly installed. May have caused AOA error in flight. Suggest not performing sensitive maintenance in heavy thunderstorms. Do not accept speed restrictions when margins are that close; while in heavy CB environments.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.