B777 flight crew reported descending through the altitude that was programmed into the FMC after the aircraft; in VNAV; thought it was high and pitched down aggressively.
Synopsis
B777 flight crew reported descending through the altitude that was programmed into the FMC after the aircraft; in VNAV; thought it was high and pitched down aggressively.
Narrative
We were cleared direct ZZZ ZZZ1 ZZZ2 as filed to join the ZZZZZ.ZZZ1 arrival. We were level at FL380. We were given a clearance to cross 10 NM north of ZZZ at FL280. The PF inserted the waypoint and restriction into the box. A Top of Descent (T/D) point was calculated by the FMC; well ahead of the airplane. As we began our descent; with 280 in the Mode Control Panel (MCP); everything appeared normal except that it kicked out of VNAV Path into VNAV Speed. It seemed to show below path followed by above path. The PF used both speedbrakes and thrust to try and regain the path. He was successful. VNAV PATH was annunciated.However; as we got into the low 30s while in VNAV PATH; it showed us high and the airplane pitched down aggressively and approached red line. The PF used speedbrakes to try and control the speed. As the PM; I called 29 for 28" and at that time the PF said; "It's not going to level." He disconnected the autopilot early and aggressively pulled up. Even starting as early as he did; we descended both through the MCP altitude as well as the altitude programmed into the FMC. We "bottomed out" at 27;700 feet. We immediately climbed back to FL280 and reengaged autoflight.The rest of the arrival was uneventful. This is not an anomaly; it seems to be happening more and more frequently! In fact; this has happened to me 3 times in the last 3 months on this arrival as well as the ZZZZZ1 Arrival. It is also happening to other pilots. Below; please find a previous write-up regarding the ZZZZZ1 Arrival.ZZZ3 ZZZ4 B777. We were assigned ZZZZZ2 at FL270 (different from the chart); then descend via; except own speed until ZZZZZ3. ZZZZZ2 was set a hard FL270 in the FMC; and we set 6000 in the MCP. Even though we pre-briefed it as a possible threat (because it has happened to one of the relief pilots as well); we were being vigilant; and as well as being in VNAV PATH; the plane dipped below FL270 before reaching ZZZZZ2. As the PF; I really thought VNAV was going to honor the restriction. Within a mile of ZZZZZ2; the airplane pitched down aggressively and accelerated and we crossed it approximately 400 feet low. It happened so quickly; I thought it would be a greater threat to disconnect the autoflight than continue."
Second reporter narrative
FO was PF. Descending to FL280 in VNAV PATH. 320 KIAS. At FL290; the FO realized the aircraft was still descending at 3500 fpm. He disengaged the autopilot and manually initiated a level-off. We bottomed out at FL277 and climbed back up to FL280 immediately. ATC did not say anything.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.