Narrative:

We had just resumed direct to zzzzz on the arrival after a deviation west of course to avoid a line of thunderstorms to the east of the flight path. At no time during this portion of flight were we IMC; staying west of the thunderstorms. We were now preceding direct to zzzzz intersection as directed by center and I had just verbalized to the pm (pilot monitoring) that we were on course; LNAV and VNAV engaged and on path. The pm acknowledged my call. I then looked to my left to watch the lightning approximately 10 miles below and behind to the left and felt the aircraft abnormally pitch down. I looked forward and saw the FD (flight director) horizontal bar on the nd (navigation display) indicating a pitch 10 degrees nose low. As I mentioned this to the pm; I looked in the HUD and saw a 'V' indicating an unusual attitude as the aircraft continued to pitch down; now at 10 degrees nose low and airspeed increasing. We were passing 19;700 feet MSL in VNAV and LNAV and airspeed was commanding 325 kts IAS in the FMS (flight management system); but I saw it rapidly exceeding 340 kts IAS as the airplane continued to pitch over. As I said what I was doing to the pm I disconnected the autopilot and the auto throttle and initiated a nose low recovery and observed the flight director horizontal pitch bar command a 10-15 degree nose low during the recovery. During this time I observed the IAS go to 346 kts; the red line being 345 kts. The 'clacker' sounded for approximately 3 seconds as I recovered the airplane to level flight. The flight director; while now in level flight with autopilot off; was still commanding an extreme nose down pitch of 10-15 degrees; so I directed to the pm the FD switches off; then on. This established the FD back to level flight. This all happened very rapidly. The HUD remained reliable throughout the maneuver. As we were still pointing toward zzzzz intersection I told the pm I would hand fly the rest of the way into ZZZ. Enroute I asked for a/T (auto throttle) reengaged and using flch I flew using the HUD guidance. But in a very short amount of time it was evident the a/T was malfunctioning as it lost from 250 kts to 225 kts while in 'path' so I disconnected the auto throttle and hand flew into ZZZ. Upon arrival I wrote up the left autopilot; the 1 kt over speed; and the auto throttle system.I have eight years [of] experience in 757/767 and have never observed this type of behavior before. The only variable to mention is we had just passed an active thunderstorm to the left of our flight path. As mentioned above; we had not entered any form of IMC during this phase of flight and were in sight of ZZZ when the event happened. During our post flight jet walk around we did notice a 10 inch round piece of paint missing from the top of the aircraft above the pilot/copilot front wind screen. Could it have been a lightning strike? We mentioned this to maintenance but they said it was just missing paint; not a lightning strike. My concern is if I had not disengaged the autopilot and auto throttle as quickly as I did; what the aircraft attitude and airspeed would get to. Unknown cause.our action as a crew recovered this airplane by disengaging the autopilot. I have no suggestions as to how to not have a re-occurrence.

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Original NASA ASRS Text

Title: B757-200 flight crew reported a system failure that resulted in an unusual nose down attitude until flight crew recovered and reset system.

Narrative: We had just resumed direct to ZZZZZ on the arrival after a deviation west of course to avoid a line of thunderstorms to the east of the flight path. At no time during this portion of flight were we IMC; staying west of the thunderstorms. We were now preceding direct to ZZZZZ intersection as directed by Center and I had just verbalized to the PM (Pilot Monitoring) that we were on course; LNAV and VNAV engaged and on path. The PM acknowledged my call. I then looked to my left to watch the lightning approximately 10 miles below and behind to the left and felt the aircraft abnormally pitch down. I looked forward and saw the FD (Flight Director) horizontal bar on the ND (Navigation Display) indicating a pitch 10 degrees nose low. As I mentioned this to the PM; I looked in the HUD and saw a 'V' indicating an unusual attitude as the aircraft continued to pitch down; now at 10 degrees nose low and airspeed increasing. We were passing 19;700 feet MSL in VNAV and LNAV and airspeed was commanding 325 kts IAS in the FMS (Flight Management System); but I saw it rapidly exceeding 340 kts IAS as the airplane continued to pitch over. As I said what I was doing to the PM I disconnected the autopilot and the auto throttle and initiated a nose low recovery and observed the flight director horizontal pitch bar command a 10-15 degree nose low during the recovery. During this time I observed the IAS go to 346 kts; the red line being 345 kts. The 'clacker' sounded for approximately 3 seconds as I recovered the airplane to level flight. The flight director; while now in level flight with autopilot off; was still commanding an extreme nose down pitch of 10-15 degrees; so I directed to the PM the FD switches off; then on. This established the FD back to level flight. This all happened very rapidly. The HUD remained reliable throughout the maneuver. As we were still pointing toward ZZZZZ intersection I told the PM I would hand fly the rest of the way into ZZZ. Enroute I asked for A/T (Auto Throttle) reengaged and using FLCH I flew using the HUD guidance. But in a very short amount of time it was evident the A/T was malfunctioning as it lost from 250 kts to 225 kts while in 'PATH' so I disconnected the auto throttle and hand flew into ZZZ. Upon arrival I wrote up the left autopilot; the 1 kt over speed; and the auto throttle system.I have eight years [of] experience in 757/767 and have never observed this type of behavior before. The only variable to mention is we had just passed an active thunderstorm to the left of our flight path. As mentioned above; we had not entered any form of IMC during this phase of flight and were in sight of ZZZ when the event happened. During our post flight jet walk around we did notice a 10 inch round piece of paint missing from the top of the aircraft above the pilot/copilot front wind screen. Could it have been a lightning strike? We mentioned this to Maintenance but they said it was just missing paint; not a lightning strike. My concern is if I had not disengaged the autopilot and auto throttle as quickly as I did; what the aircraft attitude and airspeed would get to. Unknown cause.Our action as a crew recovered this airplane by disengaging the autopilot. I have no suggestions as to how to not have a re-occurrence.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.