A desire to meet ATC crossing restrictions and a failure to inform the other pilot of changes in the FMS configuration to do so; resulted in a failure to meet a 12;000 MSL crossing restriction at KADDY for a B737 flight crew.
Synopsis
A desire to meet ATC crossing restrictions and a failure to inform the other pilot of changes in the FMS configuration to do so; resulted in a failure to meet a 12;000 MSL crossing restriction at KADDY for a B737 flight crew.
Narrative
Cleared to cross KADDY at 12;000 and then descend via the TYSSN TWO RNAV Arrival. VNAV was selected with an assigned 250 KT speed in the descent. Eight thousand FT was the selected altitude in the window for the VNAV descent. Center then issued a clearance to fly normal speed until KADDY. I wanted to pick up speed but knew that I would have to get below the depicted glide in order to slow for the 250 KT restriction at KADDY. So without saying a word to the First Officer; I de-selected VNAV and selected vertical descent on the MCP while de-selecting the autothrottle. I descended below the glide and accelerated to about 275 and held that speed as we continued descent towards KADDY. I did not remember to place 12;000 FT in the altitude window. We noticed ice on the wings so I turned on the wing anti-ice and watched as the ice melted off the wings. As I turned off the wing ant-ice; the First Officer noticed I was not leveling off at 12;000 FT as the KADDY restriction requires and he stopped the descent. I was surprised I missed seeing us descending through our altitude and quickly climbed the aircraft back to 12;000 FT. We flew through the altitude by about 500 FT. The rest of the flight continued without further incident. I made several mistakes. The first was that I made an MCP change and did not mention it to the First Officer. Second; I did not remember to replace the altitude alerter to the next restriction altitude as required when de-selecting VNAV. Third; I allowed myself to be distracted at a critical time when I should have had my attention on flying the aircraft. I need to remember that anytime VNAV is deselected; or any other change; I should announce what action is being taken and immediately reselect the next altitude restriction in the window AND FLY THE AIRPLANE.
Second reporter narrative
The Captain then manually pushed the throttles up and started accelerating the aircraft while still in VNAV PATH. We had picked up some light icing during the descent and selected wing anti-ice on. I looked outside to verify that the anti-icing was working; and then looked back inside in time to see that we were descending through 11;700 FT; yet were still 3-5 miles from TYSSN. I then looked to see why we were not leveling off when I noticed that the pitch mode was in CWS.I am disappointed in the 737's lack of smoothness with autopilot corrections in VNAV. In this example; the ideal way to have accomplished what the Captain intended to do would have been to use speed intervention. However; the autopilot is just too aggressive and many of us are hesitant to use it. The only other option is to revert to CWS pitch or vertical speed; but you must remember to make the change in the MCP altitude window as well. There is plenty of documentation out there warning of this exact type of error; but it's still a very easy error to make.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.