B737-700
Flight Crew Air Transport Pilot (ATP)
Situational Awareness; Human-Machine Interface; Fatigue; Workload
Aircraft Equipment Problem Critical; Conflict Airborne Conflict; Deviation - Altitude Crossing Restriction Not Met; Deviation - Altitude Undershoot; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued New Clearance; Air Traffic Control Issued Advisory / Alert; Flight Crew Returned To Clearance; Flight Crew FLC Overrode Automation; Flight Crew Became Reoriented; Flight Crew Requested ATC Assistance / Clarification
Human Factors; Aircraft
The aircraft was in cruise approximately five minutes prior to the Top of Descent (TOD). The aircraft was in LNAV and VNAV with the autopilot engaged. At that time; Center cleared us to descend now and maintain FL330; cross ZZZZZ at FL290. I set FL330 in the altitude select window. I entered FL290 into the Flight Management Computer (FMC) on the LEGS page at ZZZZZ and executed the change. I then selected DESCEND NOW from the FMC DESCENT page and executed the command. The path information displayed on the Primary Navigation Display (PND) showed the aircraft approaching path from below. It appeared that it would capture FL330 prior to intercepting the path. I planned to capture FL330 prior to resetting the altitude to FL290. The airplane captured FL330 and I set FL290; but; the airplane did not descend. The path guidance disappeared from the PND and the FMC did not prompt us to reset altitude. I looked at the Flight Management Annunciator (FMA) and it displayed ALT HOLD. I immediately selected LVL CHG. Shortly thereafter--almost simultaneously--Center asked if we were going make ZZZZZ at FL290. Based upon our distance to ZZZZZ and the remaining altitude to lose; I quickly told the Captain that we were going to need relief. He relayed that to Center and the Controller then cancelled our FL290 clearance and instructed us to maintain FL320 informing us of traffic at FL310. We had just barely passed through FL320 with the autopilot still in LVL CHG. I immediately selected ALT HOLD at FL320 or very close to FL320. Center told us to report level; FL320. The autopilot was not arresting the descent and climbing back to FL320 quickly enough. So; we punched-off the autopilot; arrested the descent and initiated a manual climb; but not before dipping to approximately FL314. Simultaneously; Center queried our altitude and told us with apparent anxiety that she needed us to 'expedite' to FL320 and to report level FL320. We reported to her that we were in the climb and expediting. I overshot by about 120 FT but leveled quickly at FL320. We reported level and Center asked; 'What happened there?' We responded that we had stayed at FL330 too long. Nothing further was said; Center then handed-us-off to the next sector. My failure to set FL290 soon enough was due to my lack of altitude planning and awareness other than reliance upon VNAV graphical path depiction. Unlike normally; I didn't back-up VNAV with mental math in this particular occasion. Consequently; I wasn't aware of the inability to meet the restriction until too late. Normally; I would have used VERTICAL SPEED or LEVEL CHG down to FL330. This would have kept me more aware of my distance to ZZZZZ and whether I was high or not. It would have forced me to do the mental math. Upon reaching FL330; I would have re-set a cruise altitude on the cruise alt page; checked the cruise and descent speeds and then re-engaged VNAV. But; this time I elected to stay in VNAV because of the many crossing restrictions and speed limitations on the upcoming complex RNAV STAR. It was supposed to be easier. In retrospect; I should have just set FL290 just after leaving FL370 for FL330. But; I just was not thinking fast enough. Although I did not consider myself unsafe and I believe that I started the day fit for duty; my fatigue increased as the day wore on. At the time of the incident; I believe that my mental ability was less than 100%. I felt as if my mental processes were slowed. I believe that this was a factor in my reliance upon VNAV and a factor in my delayed altitude awareness.
Fatigue played a part when a B737-700 First Officer failed to back up his autoflight mode selections with raw data based mental calculations with respect to a two phase descent clearance from ATC. A traffic conflict resulted requiring the crew to manually override their expedited descent and climb in order to avoid the crossing traffic.