B737 flight crew reported receiving a cruise altitude hold using FMC recommended holding speed in knots; resulting in a stall warning and altitude deviation. The flgiht crew regained control of the aircraft then received clearance to continue the flight.
Synopsis
B737 flight crew reported receiving a cruise altitude hold using FMC recommended holding speed in knots; resulting in a stall warning and altitude deviation. The flgiht crew regained control of the aircraft then received clearance to continue the flight.
Narrative
While approaching ZZZ we were assigned holding at ZZZZZ waypoint at FL370 due to thunderstorms in ZZZ. Captain and First Officer (FO) set up the holding in the FMC with the hold altitude of FL370. Two minutes prior to entering the holding; the aircraft slowed down from approximately M0.75 on VNAV to the prescribed holding speed of 229 KIAS. This was observed and discussed to be somewhat close to the yellow speed band by the Captain and FO and identified as a threat. Approaching the holding fix; ZZZ Center instructed us to descend and maintain FL360 for holding. We used a VNAV descent to FL360 and the throttles came back significantly to both continue to slow the aircraft to holding speed and descend to the commanded altitude. Upon reaching FL360; the auto throttles were very slow to increase to catch the airspeed and provide enough thrust for the aircraft to level off at FL360. It was observed by both the Captain as pilot monitoring (PM) and FO as pilot flying (PF) that the jet was beginning to sink slowly below FL360 to FL359; this also happened as the jet crossed the holding fix and began a right hand turn. FO as PF manually pushed the throttles forward and assumed a guarded position on the yoke. Despite the throttles being pushed forward the engines still slowly spooled up to their commanded setting. Captain and FO then encountered a 'TRAFFIC' TCAS call as there was an aircraft 1000' immediately below us established in holding. The TA quickly became an RA that instructed us to climb. FO as PF disconnected Autopilot and auto throttles and checked the throttles to the max position while pulling back on the yoke. In doing so; the increase in AOA; while in a turn; without commensurate thrust brought us immediately but briefly into stick shaker. Captain as PM called out 'UPSET' and the FO as PF completed the steps of the upset recovery maneuver (UPRT). FO released back yoke pressure; eased the angle of bank; and ensured the throttles were at max and we immediately stabilized. FO continued to hand fly the aircraft to straight and level flight on the outbound holding leg and reengaged Autopilot and auto throttles once level. It was observed that at no point was there a descent below FL359. While established in holding with auto flight on; Captain and FO debriefed the event. We determined that we were caught off guard by how far back the throttles had gone in the descent and how slowly they were to spool back up. We also agreed that we had been distracted by checking the holding and continually evaluating fuel state and time in holding for a possible divert. The training to immediately respond to a TCAS RA had also been ingrained in us and our reactions were commensurate with an aircraft at a lower altitude and higher airspeed. Simultaneously; we were also being commanded by the RA to maneuver the aircraft in a way that would have exacerbated the situation of converging aircraft. The roll portion of the UPRT created the best divergent vector away from the possible traffic. Captain as PM did not make a 'TCAS RA' call on Center frequency. This was also debriefed and discussed that the far greater threat was the UAS (Undesired Aircraft State) which commanded all of the crew's attention vice a radio call. Captain also determined that such a call; well after the event; would add significant confusion on a frequency that was very full of aircraft holding and diverting. A report has been submitted for the TCAS RA.
Second reporter narrative
Approaching top of descent for ZZZ; ATC assigned holding instructions at the ZZZZZ waypoint on the arrival at FL370 due to thunderstorms in the vicinity of ZZZ. As the pilot flying (PF); I built the hold in the FMC and the Captain verified the input prior to execution. Approaching the hold pattern; the aircraft began to slow to the calculated hold speed of 229 KIAS. The slow down was acknowledged by both Captain and First Officer (FO) as well as the proximity of the speed to the yellow band due to the altitude and identified as a threat we needed to keep an eye on. As we approached ZZZZZ; ZZZ Center directed a descent to FL360. The Captain changed the altitude on the Mode Control Panel (MCP) and the FO changed the altitude in the FMC and executed after it was verified with the Captain. With VNAV engaged; the aircraft began a descent to FL360 while still slowing to hold speed and it was observed that the throttle reduction showed RETARD initially for a short time and then FMC SPD. Upon level off at FL360; auto throttles slowly increased to catch the speed for holding. Both Captain and FO observed the slow auto throttle input and a corresponding slow descent below FL360 to FL359. The level off occurred at the hold point of ZZZZZ and the aircraft entered a right turn as directed by the FMC. As PF; I advanced the throttles to arrest the small descent and assist the auto throttles in maintaining the directed hold speed. Due to the high altitude and almost idle position when we leveled off; the engines were slow to spool up and provide the necessary thrust to maintain the directed altitude and airspeed for the turn. At this point; the TCAS system produced a 'TRAFFIC' call for the aircraft established in holding at FL350. The TA quickly switched to a Resolution Advisory and directed a climb. I as the PF disconnected Autopilot and auto throttles while advancing the throttles and pulling back on the yoke at the same time to remain in the safe area of the RA. Due to this action and the current flight envelope; the AOA increased due to the slow spool up of the engines and lack of corresponding speed increase. The stick shaker activated briefly with the Captain calling UPSET and the FO completing the necessary UPRT steps. By releasing back pressure; rolling out of the turn; and having time for the engines to spool up fully; aircraft speed increased and the aircraft stabilized immediately. I continued to hand fly the aircraft through the remainder of the turn and re-engaged the Autopilot and auto throttles after rolling out on the outbound leg of the hold pattern. The Captain and FO discussed the parameters of the what happened and highlighted that neither saw the aircraft descend below FL359. We both highlighted the large reduction in throttle position as the aircraft slowed and descended as well as the proximity of the directed speed to the stall window. And as we identified it as a threat; we both acknowledged our focus was on fuel calculations for a potential divert scenario. To mitigate any further speed problems; the MCP speed window as opened and a higher speed than was directed in the FMC was selected (I believe it was 240 KIAS). We also discussed that the Captain as the pilot monitoring (PM) did not call TCAS RA on the Center frequency as we were focused on the higher priority of the upset recovery and believed it would have caused confusion on a very busy Center frequency by the time it was made following both events. Following the debrief; we remained in holding until we were vectored to the ZZZZZ1 arrival and continued to ZZZ without further incident.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.