B737-700 flight crew reported abnormal flight control response in pitch during departure phase. They elected to divert to a nearby suitable airport.
Synopsis
B737-700 flight crew reported abnormal flight control response in pitch during departure phase. They elected to divert to a nearby suitable airport.
Narrative
I was the Pilot Flying (PF) during the departure out of ZZZ. I noted that the pitch control characteristics seemed abnormally stiff; despite being an older 700. ATC began vectoring us off the SID and giving us step climbs; so I elected to engage the autopilot to focus on the clearances and to monitor how the autopilot handled. Things seemed okay until shortly thereafter in the climb we encountered wake turbulence from an (other carrier aircraft). The autopilot was reacting slowly to lower the nose. I disengaged the autopilot to help fly it out of the wake and roll the wings level. Despite our higher speed; the pitch control seemed tighter than I would have expected as I flew us out of the wake. I commented this to the Captain at this point.With the autopilot re-engaged; we continued our climb and ATC then gave us normal speed. The autopilot had a hard time matching pitch to speed and began oscillating pitch attitudes; and then started to descend. I intervened. I then informed the Captain of how heavy" the nose feels; and we elected to level off and evaluate. We considered the problem and if there was an appropriate QRH procedure. With each of those events; it was notable that the pitch controls felt different and very firm. Flying an approach in ZZZ1 with gusting winds was not an added risk factor we wanted to accept with an aircraft that needed this much extra pressure to make attitude changes. Additionally; we could not anticipate with certainty; what the autopilot would do or if the condition would not deteriorate further. We both agreed that what we were experiencing was something we were no longer comfortable continuing the flight with. We considered many options; ZZZ2 was nearby and had steady headwinds favoring the long Runway XX. We informed the Cabin Crew. The Captain became the PF. I notified ATC of our plan to initiate a slow descent to divert to ZZZ2. Advised ATC due to our flight control issue/concern and as an added Safety measure upon landing. While the Captain was flying; he confirmed how very stiff the pitch control axis was. We landed uneventfully."
Second reporter narrative
The FO was flying the aircraft. During departure; he noticed the aircraft pitch axis seemed to be stiffer than normal. After engaging the autopilot; the autopilot did not do a good job of maintaining the pitch attitude. We encountered another aircraft wake; and after disengaging the autopilot; he noticed again the pitch axis was very stiff. He reengaged the autopilot; and the autopilot had a tough time changing speed while maintaining a consistent pitch attitude. When we finally leveled off; the First Officer (FO) expressed his concerns to me; and we began to discuss what to do about it.After discussing the problem; and some proposed solutions; we also reviewed the QRH even though there was no procedure for what we were experiencing. We finally decided since we didn't know exactly what was wrong; we could not predict whether or not it would get worse over time and based on that we decided to divert to ZZZ2 and advised ATC. We decided that I would fly the aircraft and the FO would be the Pilot Monitoring. We were very busy; and while I was flying the aircraft; I confirmed the pitch axis was very stiff. We planned for a long final; and slowed down slowly and early taking note of the changes in attitude. We landed uneventfully; and then taxied to the gate.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.