B737-700
Flight Crew Air Transport Pilot (ATP)
ATC Issue All Types; Aircraft Equipment Problem Critical; Deviation - Altitude Crossing Restriction Not Met; Deviation / Discrepancy - Procedural Clearance
Aircraft Equipment Problem Dissipated; Flight Crew FLC Overrode Automation
Aircraft; Human Factors
We encountered a challenge with our Flight Management Computer on our departure climbout. When cleared out of 14;000 FT; our ALTITUDE SELECTOR DISPLAY on the MODE CONTROL PANEL got stuck and displayed '50;000' while we were trying to set it to the new ATC assigned altitude. However; we determined that we could continue to our destination since we were able to manually maintain any chosen altitude. We advised Departure of our situation and expressed concerns about RVSM. The Controller checked and informed us that we were cleared to continue to our destination in RVSM airspace since we were able to maintain altitude. We also advised Company Dispatch and Maintenance. We endured the aural ALTITUDE ALERT which chimed every 10 seconds. We asked if we could pull the CIRCUIT BREAKER to silence it; but Maintenance would not allow it. Although VERTICAL NAVIGATION MODE worked in cruise; as expected; when we initiated our descent we lost numerous VERTICAL NAVIGATION MODE capabilities. These included normal descent planning tools such as the following: 1. V-NAV path; 2. The Top of Descent symbol (T/D) on the NAV display unit; 3. The Altitude Range Arc on the NAV display unit; and 4. The WAYPOINT/ALTITUDE VERTICAL BEARING display guidance on the CDU unit V-NAV descent page. We descended using VERTICAL/SPEED; our lowest mode of automation. We leveled off at Flight Level 340 as requested by Center. About 60 miles out they asked us to cross a fix on the arrival at 12;000 FT and 250 KTS. We calculated incorrectly and started down at 35 miles away from our crossing fix. This caused us to cross around 10;000 FT too high. Although we informed Approach Control he did not respond but cleared us down to 6;000 FT. Near the end of our descent; at around 14;000 FT; the challenge disappeared and we were able to accomplish a stable uneventful approach and landing. To prevent such an error in the future I would: 1. Silence the Altitude Alerter; 2. Review calculation of 3 degree manual descent profiles.
Climbing through 14;000 FT (clearance level was FL400); the pilot flying was spinning the altitude selector knob to 40;000 with B/AP engaged; the altitude selector inexplicably showed 50;000 (FL500); and was stuck in that position. We were unable to change the altitude selection window from this 50;000 value. We also agreed the cruise ALT set in the FMS would hold the required clearance level of FL400. With this condition however; the altitude alerter sounds every 10 seconds throughout the entire flight; that's approximately 1080 altitude 'bongs' for a three hour flight.While discussing the descent and landing we agreed that by selecting a lower altitude in the FMS; and using Altitude intervention and Vertical Speed Mode; we could effectively descend the airplane. In this mode of operation however; we lost most of our normal tools to control a descent.Descending through 14;000 the situation with the altitude selector corrected itself and a normal landing took place. Contributing factors included a relatively late crossing restriction clearance by ATC; distraction from the altitude alerter every 10 seconds for the previous three hours; and the time of day--nearly midnight.
When the altitude alert display stuck at 50;000 FT on initial climbout the flight crew of a B737-700 decided to continue the flight using FMS cruise altitude programming to maintain altitude in cruise and utilizing Vertical Speed Mode for the descent. When given a crossing altitude restriction on their descent they miscalculated the required Top of Descent point and crossed the restricted fix 10;000 FT high.