Air carrier flight crew reported a momentary loss of aircraft control when the aircraft pitched down suddenly at top of descent. First Officer regained aircraft control and continued until aircraft did a G-snap" roll to intercept the approach localizer requiring pilots to disconnect autopilot to regain control."
Synopsis
Air carrier flight crew reported a momentary loss of aircraft control when the aircraft pitched down suddenly at top of descent. First Officer regained aircraft control and continued until aircraft did a G-snap" roll to intercept the approach localizer requiring pilots to disconnect autopilot to regain control."
Narrative
We were at FL280 and was instructed to descend via the DSNEE5 Arrival into Orange County. I had put in 8;000 ft. for the bottom of the arrival in the MCP window; and we were armed in LNAV VNAV PATH. At the Top of Descent; all of a sudden; the aircraft pitched over into a step nose-down dive at 5;000 fpm. I quickly took control of the aircraft and used Vertical Speed; to help reduce the pitch angle and help me bring the nose back up towards a normal descent rate. At the same time; I extended flight spoilers; because the aircraft wanted to go to VMO overspeed condition. At the same time; my Captain was talking to the people in the back on a PA; and I had the radios and the flight controls; I had ATC calling me for a frequency change at the same time when I was trying to get control of the aircraft. I told the Controller to standby; I had to fly the aircraft. When the Captain came back; I informed him that something was wrong with the automation of the aircraft; and that it pitched over to 5;000 FPM; and that I was still in recovery mode and recovering the aircraft. ATC had given us a frequency change but I was not able to check on. By the time I was able to get the aircraft back under a stable descent rate and the airspeed in the normal airspeed parameter we had already gone through the 25;000 ft. and above restriction at waypoint guitar on the DSNEE5 Arrival. I was no more than 1;000 ft. below the 25;000 ft. restriction when we flew over the guitar fix. We checked on the new Controller and told us the level out at 23;000 ft. so we stopped at 23;000 ft. Also; on final approach into Orange County; as we intercepted the localizer; the aircraft in LVAV did a high G-snap roll to intercept the LOC and glideslope to the point where I disconnected the autopilot to recover that and keeping the aircraft from going into a stall.Suggestions: We need better descent software for LNAV/VNAV PATH especially coming in on the DSNEE5 Arrival into Orange County. That arrival tends to be difficult for the automation; which I have noticed. As my own skills; I relied on my training on flying the aircraft first. It took me a second to analyze the problem to figure out the correct action that I needed to do; and I flew the aircraft first; navigated second; and communicated third. There was a lot that happened extremely fast; and I had to make quick educated decisions on how to recover the aircraft from a steep dive. My job was maintaining aircraft control at all costs; which I did; but in doing that; it caused me to go through a crossing restriction. I relied on my training; and I believe I did everything I could possibly do to maintain aircraft control. I will always do my best to comply with crossing restrictions on arrivals; but when I am in a steep; dive 5000 FPM; my focus will always be to fly the aircraft first and get it back flying safely. I was upset that I went through the crossing restriction; but my emphasis needs to be on recovering the aircraft first.
Second reporter narrative
As the Captain; PM; I notified the very proficient First Officer; FO; that I was making my descent PA to the Passengers. While I was making my PA the aircraft dumped the nose at Top of Descent and immediately went into a probable overspeed situation. The PF did an excellent job recovering the aircraft from the nose over (approximately 5;000 fpm). By the time the aircraft was back in a normal state we had descended below the 'At or above FL250' (FL240ish) restriction at LRSON on the DSNEE5 Arrival. As an aside; the aircraft made a VERY aggressive join up on the SNA localizer when it was in a position (angle and airspeed) to make a smooth transition to the localizer.Suggestions: As PM; I will NOT choose to make a PA at the Top of Descent. I will wait to make sure the aircraft is in a steady state before diverting my attention elsewhere.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.