A B737-700 in a high rate of descent after a late descent clearance lost MCP functioning and allowed the aircraft to go through the cleared altitude. The autopilot had disconnected with no warning; the MCP blanked and returned with a lower altitude.
Synopsis
A B737-700 in a high rate of descent after a late descent clearance lost MCP functioning and allowed the aircraft to go through the cleared altitude. The autopilot had disconnected with no warning; the MCP blanked and returned with a lower altitude.
Narrative
At FL 220 we were cleared to descend to 8000' over our first arrival fix. We had been kept high for traffic. We told ATC we could not make the crossing so they gave us 8000'. We had a high rate of descent with the speedbrakes up. At just before 8000'; we were given a frequency change. When I looked up from the radios we were going through 7600'. The MCP altitude was 5000'. At the same time; the First Officer asked if we had been cleared to 5000'. I answered we had not. He was also trying to disconnect the autopilot and nothing happened. He said; 'It won't let me have it.' He then realized the autopilot had disconnected with no warning at all. The airplane was way out of trim nose down. As he was pulling back on the yoke; he was also trimming the nose up; and we were able to climb back up to 8000' from about 7300'. (All CBs checked okay.) Our highest speed in the descent was 320 knots. We never came near the red line. Our rate of descent was 4300 FPM. Neither of [us] touched the MCP. We never received a Stab Out of Trim light or an autopilot disconnect warning. We have no idea why the altitude changed from 8000' to 5000'. On the downwind after everything settled down; we reconnected the autopilot. Everything worked fine; and when we disconnected it for the approach; we got the disconnect warning. On the ground I called the Company and was told the airplane had no history of problems. It must have been a glitch in the MCP. We agreed to take the aircraft to the next city. We had a normal flight and descent.
Second reporter narrative
Descending; we were told to cross the next intersection at 8;000'. We were given the cross restriction late so our descent rate was 4;200' per minute to comply with the crossing restriction. I was the Pilot Flying. Approaching 8;000'; I observed the aircraft was not beginning to level and immediately applied significant back pressure on the yoke. The aircraft did not respond. I then proceeded to disconnect the autopilot. It was at this point that I realized the autopilot was not engaged. It wasn't until I applied significant force with both hands and re-trimmed that the aircraft began to level. By this point; we had descended to 7;300'. We immediately climbed back up to 8;000' and leveled off. Neither I nor the Captain; at any point; heard the Autopilot Disconnect Warning horn signaling the autopilot had disengaged. We also observed the MCP panel momentarily go blank while I was initially trying to level the aircraft. Furthermore; when the MCP panel regained power it was not showing the information as previously set. (LNAV and VNAV were not selected; autopilot was disengaged; and the altitude window was showing 5;000'.) I reset the MCP panel; engaged the autopilot; and flew the remainder of the flight without incident. Once on the ground we contacted Maintenance Control; advised them of the incident; and were given the go-ahead to continue on.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.