B737 Captain reports missing crossing restrictions during the KEPEC arrival to LAS after selecting ALT INTV to smooth the descent.
Synopsis
B737 Captain reports missing crossing restrictions during the KEPEC arrival to LAS after selecting ALT INTV to smooth the descent.
Narrative
On KEPEC Arrival into LAS FL280; cleared to 'descend via the KEPEC Arrival. MCP altitude set to 8000 FT (lowest altitude on arrival). VNAV and autothrottles were engaged as well as LNAV. Aircraft followed VNAV PATH to cross MISEN at FL240. Pushed ALT INTV button for smooth descent to cross the remainder of fixes. Aircraft descended 200-300 FT and then climbed back up to FL240 and leveled off. Pushed ALT INTV again and aircraft descended and climbed as before. Second push of ALT INTV apparently deleted the altitude restriction at CLARR and I did not catch it. Aircraft began VNAV PATH descent for KEPEC at 12;000 FT. Missed CLARR altitude by approximately 2500 FT. Descended rapidly to attain appropriate flight path but then descended 500' below next altitude constraint of 12;000 FT. Informed ATC we were high at CLARR but were correcting and ATC said; 'thanks.' Elected not to use VNAV for remainder of arrival. All systems were working well on aircraft. I 'Won't use the ALT INTV or SPD INTV buttons ever again.'
Second reporter narrative
We were cleared to descend via the KEPEC 2 Arrival into LAS. We verified the waypoints; altitudes; and airspeeds were all properly programmed in the FMC. We had VNAV and autothrottles on and set the MCP altitude to 8000 FT which is the lowest altitude on the arrival.The A/C began a descent and crossed MISEN intersection as required. The aircraft leveled and during this time I looked away to look at a chart. The Captain at some point decided he wanted to begin an early descent to CLARR which has a 250/13;000' restriction. Unknown to me he pressed ALT INTV more than once. The aircraft began to descend and then tried to recapture VPATH at 24;000'. I assume at this point he pressed it a second time. I tried to explain why the aircraft didn't want to make a 1000 FPM descent. I missed the fact that multiple presses had erased the CLARR altitude restriction. As we crossed CLARR I realized we were high by 2500-3000 FT. I reported to LAS Approach we were high and the Controller instructed us to expedite through 14;000 FT. At this point the Captain used control wheel pitch to start an extremely fast descent. I could see we were going to accelerate rapidly and I called out the airspeed which was approaching 300 KTS indicated. I mentioned we were going to approach VMO if we kept this rate of descent. The autopilot went into ALT Acquire and as it pitched up we exceeded 344 KTS indicated and passed through 12;000 FT which is the clearance limit at KEPEC. We finally leveled at 11;500'. The Captain disconnected the A/P and climbed back to 12;000' where the autopilot was reengaged. During the descent I reset the MCP altitude window to 12;000' for KEPEC but at some point the Captain reset it to some other value before resetting it to 12;000' again. I'm not sure if this led or the excessive rate of descent led to us going below 12;000 feet. I considered assuming control of the aircraft but was concerned that it might cause a struggle.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.