Narrative:

Autopilot a on MEL (inoperative for discrepancy of hard uncommanded bank to right; twice; while on cmd a). Planned for and briefed RNAV approach from briefing guide. Cleared for CREDE3 STAR to 16R RNAV rnp Z. Cleared for approach close to if/IAF. I closed up STAR to approach transition once cleared for the RNAV approach; first officer verified. Set 7;000 in MCP (we were at 11;000). Flaps 1 selected; VNAV slowed to make speed at if/IAF. Taken off approach by controller [who] felt that our altitude was too high for safely continuing approach (we were still at 11;000 between cepee and aagee; approach plate altitude 8;600/FMC altitude restriction 8;600A). Vectored for ILS 16R approach. Contributing factors: a VNAV late descent/malfunction; possibly caused by inoperative autopilot a; more detail on the autopilot at the end of this report. Being cleared for the approach very close to the if/IAF. After clearance and close to the if/IAF; closing up the approach forcing VNAV to recalculate the descent profile adding to the challenge of monitoring while VNAV was 'thinking'. Also; there was a challenge in assessing what VNAV was doing since so many things happened very close to the if/IAF and focused our attention elsewhere.more on a/P issue: on 2nd leg with same aircraft (den-ZZZ); VNAV did something that we felt was related to earlier incident. Cleared to cross zzzzz at 270 kts and FL270. We had plenty of time to set it up (20+ nm) to the TOD. At TOD FMA started to descend and displayed a 'retard' on the FMA. Aircraft stopped descent approx 40 ft after starting; added power and leveled off. Then the pdi and VNAV descent page cleared. VNAV recalculated descent and then pdi and descent page reappeared and displayed approximately 800 ft high on path and started a very aggressive descent (approximately 4000 fpm) and sped up significantly due to the descent angle. I selected v/s to correct aggressive and uncomfortable descent and descended via MCP to meet altitude and speed restriction. I believe that this same situation happened on the approach at den; exacerbated by the fact that it was forced to re-calculate very close to the if/IAF. On arrival at gate; I entered a maintenance discrepancy report and also did a face-to-face brief of the issue with the oncoming crew and maintenance. Post-flight learning: common practice on the line is to close up the STAR and RNAV approach after receiving the approach clearance. I will think hard about continuing this practice given what happened here. While I believe there was something not quite right with the VNAV; in analyzing this situation I think that we still put ourselves at a significant disadvantage by closing up the approach and making both VNAV and the crew do so much at the last minute (that is; close to the if/IAF). Without a doubt our situational awareness was degraded (and possibly something was missed) by trying to do and then monitor so much in a short period prior to commencing the approach. Luckily; a vigilant controller saw this developing and took us off the approach giving us vectors off the RF and some extra room to descend. Otherwise we may have ended up in an unstable situation at the FAF. In the future I will consider closing up the STAR and approach transition and briefing the use of heading select at the IAF to discontinue the approach and continue on the vector portion of the STAR after the if/IAF while notifying ATC. Approach clearance earlier in the arrival would help increase awareness and monitoring after closing up the STAR and approach on the FMC.

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Original NASA ASRS Text

Title: B737-800 flight crew reported autoflight anomalies complicated their arrival on the CREDE3 STAR to DEN 16R RNAV RNP Z.

Narrative: Autopilot A on MEL (inoperative for discrepancy of hard uncommanded bank to right; twice; while on CMD A). Planned for and briefed RNAV approach from briefing guide. Cleared for CREDE3 STAR to 16R RNAV RNP Z. Cleared for approach close to IF/IAF. I closed up STAR to Approach transition once cleared for the RNAV approach; First Officer verified. Set 7;000 in MCP (we were at 11;000). Flaps 1 selected; VNAV slowed to make speed at IF/IAF. Taken off approach by Controller [who] felt that our altitude was too high for safely continuing approach (we were still at 11;000 between CEPEE and AAGEE; approach plate altitude 8;600/FMC altitude restriction 8;600A). Vectored for ILS 16R approach. Contributing factors: a VNAV late descent/malfunction; possibly caused by inoperative autopilot A; more detail on the autopilot at the end of this report. Being cleared for the approach very close to the IF/IAF. After clearance and close to the IF/IAF; closing up the approach forcing VNAV to recalculate the descent profile adding to the challenge of monitoring while VNAV was 'thinking'. Also; there was a challenge in assessing what VNAV was doing since so many things happened very close to the IF/IAF and focused our attention elsewhere.More on A/P issue: On 2nd leg with same aircraft (DEN-ZZZ); VNAV did something that we felt was related to earlier incident. Cleared to cross ZZZZZ at 270 kts and FL270. We had plenty of time to set it up (20+ nm) to the TOD. At TOD FMA started to descend and displayed a 'RETARD' on the FMA. Aircraft stopped descent approx 40 ft after starting; added power and leveled off. Then the PDI and VNAV descent page cleared. VNAV recalculated descent and then PDI and descent page reappeared and displayed approximately 800 ft high on path and started a very aggressive descent (approximately 4000 fpm) and sped up significantly due to the descent angle. I selected v/s to correct aggressive and uncomfortable descent and descended via MCP to meet altitude and speed restriction. I believe that this same situation happened on the approach at DEN; exacerbated by the fact that it was forced to re-calculate very close to the IF/IAF. On arrival at gate; I entered a maintenance discrepancy report and also did a face-to-face brief of the issue with the oncoming crew and Maintenance. Post-flight learning: common practice on the line is to close up the STAR and RNAV approach AFTER receiving the approach clearance. I will think hard about continuing this practice given what happened here. While I believe there was something not quite right with the VNAV; in analyzing this situation I think that we still put ourselves at a significant disadvantage by closing up the approach and making both VNAV and the crew do so much at the last minute (that is; close to the IF/IAF). Without a doubt our situational awareness was degraded (and possibly something was missed) by trying to do and then monitor so much in a short period prior to commencing the approach. Luckily; a vigilant controller saw this developing and took us off the approach giving us vectors off the RF and some extra room to descend. Otherwise we may have ended up in an unstable situation at the FAF. In the future I will consider closing up the STAR and approach transition and briefing the use of heading select at the IAF to discontinue the approach and continue on the vector portion of the STAR after the IF/IAF while notifying ATC. Approach clearance earlier in the arrival would help increase awareness and monitoring after closing up the STAR and approach on the FMC.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.