Date: 201911
Local time of day: 0001-0600
Location: ZZZ.Airport, US
Flight conditions: VMC
Weather elements / visibility: Turbulence
Aircraft: B757-200
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Cargo / Freight / Delivery
Flight phase: Initial Approach
ATC / advisory: Center ZZZ
Aircraft component: Autoflight System
Component problem: Failed
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine; Flight Crew Instrument
Human factors: Situational Awareness; Human-Machine Interface; Distraction
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Aircraft Equipment Problem Critical; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Overcame Equipment Problem; Flight Crew Became Reoriented; Flight Crew Regained Aircraft Control; Flight Crew Returned To Clearance
Contributing factors: Aircraft
Primary problem: Aircraft

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.


Second narrative: We had deviated to the west of flight path around thunderstorms; and had just selected ZZZZZ; then LNAV; VNAV. We were eyes out looking at lighting below; behind and to our left. We came eyes back inside; the PF (Pilot Flying) announced we were on PATH; and I felt the jet pitch nose down suddenly. I immediately looked at VVI (Vertical Velocity Indicator); saw 8;000 feet down; looked up to airspeed and saw it rapidly accelerating. I called watch airspeed at the same time the PF disconnected A/P (Auto Pilot) and arrested the rate of descent and stabilized our airspeed. Redline was 345 kts and I saw 346 kts briefly. My best guess is it was 3-5 seconds at most from when we noticed the jet pitch down to when PF recovered. Then we were puzzled because even though we were recovered the Horizontal Flight Director was still commanding nose down - we both saw it commanding 10 degrees nose down - even though we were just below redline speed. PF directed me to reset F/D and then we continued the approach. PF reengaged LNAV/VNAV and we were on PATH when the A/T (Auto Throttle) would not maintain appropriate speed. PF hand flew the rest of the approach.Cause: Unknown. The aircraft pitched down for unknown reasons. PF recovered by disconnecting AP and A/T and hand flying. We then engaged LNAV/VNAV again; and the A/T was unable to maintain airspeed. PF hand flew the rest of the flight; maybe 10 minutes.


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