Narrative:

During VNAV climb to FL230 with autopilot on; center issued a heading vector 15 degrees left and requested speed reduction to 250 KTS for in-trail spacing behind the preceding aircraft. This clearance occurred as I was climbing through approximately 21;800 ft MSL at 300 KTS. After setting the new heading I selected vertical speed climb mode to prevent the autopilot from zooming through the assigned altitude due to the speed reduction (the autopilot uses pitch to reduce the speed during climb and would begin a high rate climb otherwise). Unfortunately; I failed to notice that the aircraft had already begun capture of the altitude in VNAV mode and the selection of vertical speed mode during the VNAV capture phase nullifies the capture of the altitude (the altitude preselector was selected to the correct altitude of FL230 throughout the event). The vertical speed was set about 1;300 FPM. The aircraft did not capture FL230 and at FL232 I disconnected the autopilot and hand flew the aircraft back to FL230. The altitude peaked at 23;300 ft before I returned to the assigned FL230. Center gave no indication that they had observed an altitude deviation; nor was any conflict noted via ATC or aircraft system. The deviation was caused by my failure to note the altitude armed mode deactivate as I selected vertical speed mode to prevent the autopilot from overshooting the altitude due to the speed reduction. In attempting to avoid one undesirable outcome known to the autopilot system I induced another. As the ATC clearance occurred during the normal 1;000 ft prior call to confirm the armed status of the altitude preselector system was missed. Additionally; it was busy during this time changing the course and speed of the aircraft so attention was drawn away to verify the altitude armed mode status. This is a problem I am well aware of (selection of vs during altitude capture negating the capture mode) and should have noted the loss of the armed mode. Due to the speed reduction and my attempt to avert the pitch zoom I failed to notice the armed altitude mode drop off. It would have been preferable to allow the aircraft to capture the altitude and then reduce the airspeed; even though this would delay compliance with the clearance slightly. Another method would have been to reprogram the FMS climb speed in the computer which would take even longer to comply with the clearance but may have averted the altitude deviation.

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Original NASA ASRS Text

Title: MD88 First Officer reports overshooting the assigned altitude while attempting to reduced airspeed at ATC request and prevent a zoom climb by selecting vertical speed. Altitude capture is negated in the attempt resulting in a 300 FT overshoot.

Narrative: During VNAV climb to FL230 with autopilot on; Center issued a heading vector 15 degrees left and requested speed reduction to 250 KTS for in-trail spacing behind the preceding aircraft. This clearance occurred as I was climbing through approximately 21;800 FT MSL at 300 KTS. After setting the new heading I selected vertical speed climb mode to prevent the autopilot from zooming through the assigned altitude due to the speed reduction (the autopilot uses pitch to reduce the speed during climb and would begin a high rate climb otherwise). Unfortunately; I failed to notice that the aircraft had already begun capture of the altitude in VNAV mode and the selection of vertical speed mode during the VNAV capture phase nullifies the capture of the altitude (the altitude preselector was selected to the correct altitude of FL230 throughout the event). The vertical speed was set about 1;300 FPM. The aircraft did not capture FL230 and at FL232 I disconnected the autopilot and hand flew the aircraft back to FL230. The altitude peaked at 23;300 FT before I returned to the assigned FL230. Center gave no indication that they had observed an altitude deviation; nor was any conflict noted via ATC or aircraft system. The deviation was caused by my failure to note the ALT Armed mode deactivate as I selected vertical speed mode to prevent the autopilot from overshooting the altitude due to the speed reduction. In attempting to avoid one undesirable outcome known to the autopilot system I induced another. As the ATC clearance occurred during the normal 1;000 FT prior call to confirm the armed status of the altitude preselector system was missed. Additionally; it was busy during this time changing the course and speed of the aircraft so attention was drawn away to verify the ALT armed mode status. This is a problem I am well aware of (selection of VS during altitude capture negating the capture mode) and should have noted the loss of the armed mode. Due to the speed reduction and my attempt to avert the pitch zoom I failed to notice the armed altitude mode drop off. It would have been preferable to allow the aircraft to capture the altitude and then reduce the airspeed; even though this would delay compliance with the clearance slightly. Another method would have been to reprogram the FMS climb speed in the computer which would take even longer to comply with the clearance but may have averted the altitude deviation.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.