Flight Crew Air Transport Pilot (ATP)
Human-Machine Interface; Workload
Aircraft Equipment Problem Critical; Deviation - Altitude Crossing Restriction Not Met; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Returned To Clearance
Chart Or Publication; Human Factors; Procedure
I flew the GIBBZ RNAV STAR to IAD numerous times this month. Each time; after the route was entered into the FMS; frequent MCDU messages commenced warning that the 9;000 MSL altitude constraint at OTTTO could not be met. It IS a tight constraint. These MCDU messages continue to recur after engine start and during flight. Entering descent winds did not prevent these messages; so I was on my guard; having seen profile information in error previously. Prior to the clearance to descend via the GIBBZ1 STAR; I had checked each speed and altitude constraint against the chart and confirmed all were correct in the flight plan. I remained concerned about the OTTTO constraint as the MCDU messages continued. The autopilot had a hard time managing the descent. The autothrottles were 'fishing'; going from idle to a thrust setting too high for a descent; and back to idle. Numerous times I disconnected the autothrottles as automation was having difficulty managing the descent. Even with full speedbrakes and throttles at idle; the autopilot kept attempting to overspeed the aircraft in order to fly the descent profile which is very demanding performance wise; much more frequently than is normally the case. Close to OTTTO; with full speedbrakes; I had to downgrade the automation to vertical speed in order to keep from overspeeding. This put me above the descent profile. I managed the descent using vertical speed while I attempted to meet the crossing restriction. When OTTTO became the 'TO' waypoint; it turned magenta (normal); the same color as the 9;000 figure. At this point; I cannot remember what blanked out part of the '9' in the crossing restriction. The '9' was touching either the last letter in the fix name (OTTTO) or it was touching a magenta circle around the fix itself. This overlap caused me to think it was an 8;000 MSL crossing restriction which I was battling to meet. As I reached 9;000 the First Officer called out that the MCDU showed 9;000 at OTTTO; vice the 8;000 I was attempting to meet. I immediately disconnected the autopilot and returned to 9;000. I believe my altitude was approximately 8;800 at its lowest. During the event and after; as we looked at the NAV display; we could not tell if the constraint was 8;000 or 9;000. I was able to cross OTTTO very close to; if not at; 9;000.The GIBBZ STAR needs to be examined to determine if the OTTTO altitude constraint is unreasonable considering aircraft performance. Extending the slats has little effect on descent rates in addition to speedbrakes in the MD-10 and 11. Primarily; ensure that critical altitude restrictions are not superimposed over or blanked by other data on the navigation display.
A DC10 flight crew; struggling to comply with the GIBBZ RNAV STAR into IAD descended below the required 9;000 MSL at OTTTO. Masking of the magenta 9 in 9;000 by other data on the NAV moving map display may have contributed to the deviation.