B737 MAX 9 flight crew reported during approach the autothrottle system commanded a climbing turn at the missed approach point without pilot input. Flight crew performed a missed approach and completed a second approach with automation disconnected.
Synopsis
B737 MAX 9 flight crew reported during approach the autothrottle system commanded a climbing turn at the missed approach point without pilot input. Flight crew performed a missed approach and completed a second approach with automation disconnected.
Narrative
RNAV (GPS) RWY XX ZZZ. I was the CA and flying the above approach. It was VFR and I selected this approach for lateral and vertical guidance to the runway as the ILS was out of service.At the depicted VDP I lost vertical guidance; I was a I little confused but I continued visually. Upon reaching ZZZZZ the MAP; the aircraft went into GA mode. That was confusing because now my steering bars were commanding a right climbing turn to the MAP point.I was flying manually; but with auto throttles still engaged. My cross check didn't catch the throttles powering up and I quickly gained 30 kts. I was no longer stable and executed a Go-Around.The FO and I while confused as to what happened; figured out that the FMC went into the go-around mode and we set up for the same approach and we watched the FMAs (Flight Mode Annunciator) and throttle go into go-around. I was expecting this and had all automation turned off and landed safely. My concern; according to 737FM 8.110.10.40:The throttles started to power up at the MAP when the FMC went into Go Around mode.I feel this is a safety issue; that needs an OPS alert; or verbiage on the 10-7 pages for this rare situation.I later discussed this situation with CA XXX and he flew the exact profile in the simulator. Here is his findings:Update ZZZ RNAV (GPS) RWY XX:''737 MAX9 FFS (Full Flight Simulator) Set up RNAV approach in FMS IAW (In Accordance With) SOP. Plan Flaps 40. AFDS (Autopilot and Flight Director System) set with A/P A (Autopilot) and A/T (autothrottle) engaged throughout entire approach and through MA. Legs page looked as expected with one additional conditional waypoint 0.2 NM beyond ZZZZZ (MAP). 1;000 feet set MA alt 2;800 feet. Just prior to FMAs FMC SPD / LNAV / VNAV PTH. At MAP VNAV disconnects and goes into CWS-P and A/T FMA goes from FMC SPD to MCP (Mode Control Panel) SPD. LNAV stays engaged and flies to conditional waypoint and then starts right turn to ZZZZZ1 (first point on MA). Jet continues a slow descent to 620 feet (-100 feet from 707 feet MSL level off on LEGS page). So far; no big surprises; well maybe the descent but the FMAs look right.'' Now the interesting part: After passing ZZZZZ two new waypoints show up on ND (Navigation Display) between ZZZZZ and ZZZZZ1. Green dot (ACCEL and T/C). Wonder what will happen at ZZZZZ2. FMAs still MCP (Mode Control Panel) SPD / LNAV / blank (CWS-P) (Control Wheel Steering) Aircraft staying at 620 feet MSL and LNAVing to ZZZZZ1. At (ZZZZZ2) no FMA changes but throttles go to 80% N1 and we start a 1000/FPM climb. ZZZZZ1 alt 2;800 feet and 2;800 feet set in the MCP at 1;000 feet on the approach. Approaching 2;800 feet FMA pitch mode changes to ALT ACQ and then ALT HOLD. Final FMAs MCP SPD / LNAV / ALT HOLD.
Second reporter narrative
On our flight from ZZZ to ZZZ1 we were advised of the NOTAM on the ILS to runway XX into ZZZ1 which is the primary runway used. Therefore; we proceeded to configure the airplane to perform the RNAV XX. We followed all the steps and briefed. Conditions were VMC at the field during the approach and we didn't have traffic in the vicinity. I was pilot monitoring while the CA had the automation engaged. The FMA's (Flight Mode Annunciators) were FMC Speed; LNAV and VNAV PATH as we continued the approach and the aircraft was performing as expected. At 1;000 feet the CA disengaged the Autopilot but continued with the auto throttle engaged. All the sudden; I noticed the airspeed bug jumping to around 160 kts and the FD (Flight Director) initiated a climbing right turn as the autothrottle commanded the thrust levers to move forward. In between the confusion we got unstable and a go-around was called. We were vectored around and re configured for the approach. During the second approach I payed close attention to the actions of the system and noticed that after the missed approach point that is located 1.9 NM from the runway; the aircraft went automatically into go-around mode; the FD commanded the aircraft to do a climbing right turn and the speed bug jumped to about 160kts again. During the second approach; and as we were in VMS the CA had disconnected all automation prior to this point; and continued a stable approach to land.
More incidents for this aircraft family →
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.