Narrative:

We were on the geela arrival into phx and were cleared from 41;000 ft to FL330. I engaged VNAV and we started descending. Before we got to 33;000 ft; we were cleared off the airway and given a different altitude; then slowed to 250 KTS and told to cross scole at 280 KTS or less. Shortly after; we were cleared to blythe and to descend via the arrival; except for airspeed. Since we were below path at this point; I selected vertical speed 1;000 ft and was going to wait to get back on path before selecting VNAV again. Once we were cleared for the arrival; I selected 15;000 ft in the MCP for protection on the next crossing altitude. We were then cleared again via the arrival and to comply with speeds. As we approached munstr; the restriction was 15;000 ft and I noticed the airplane shallowing out the descent and started to look at the box to see what exactly was happening. I noticed a speed intervention of 250 KTS selected. At that point; the captain said we should cross at 15;000 ft and we noticed the plane was about 500 ft below. I stopped the descent and returned to 15;000 ft. There was no conflict with other aircraft or terrain and that was it. We descended the rest of the arrival on path. I would like to see ATC manage the arrivals better. In a highly complex arrival with multiple altitude and speed changes; it adds complexity to be given different speeds and altitudes and then told to go back to the original arrival profile. We could have verbalized better what the automation was doing when we first sensed confusion.

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Original NASA ASRS Text

Title: A B737-NG on the PHX GEELA arrival was vectored off the STAR routing; given a speed restriction; and when in the SPD and V/S modes; the 15;000 FT MUNSTR constraint dropped out so the aircraft did not level.

Narrative: We were on the GEELA Arrival into PHX and were cleared from 41;000 FT to FL330. I engaged VNAV and we started descending. Before we got to 33;000 FT; we were cleared off the airway and given a different altitude; then slowed to 250 KTS and told to cross SCOLE at 280 KTS or less. Shortly after; we were cleared to BLYTHE and to descend via the arrival; except for airspeed. Since we were below path at this point; I selected vertical speed 1;000 FT and was going to wait to get back on path before selecting VNAV again. Once we were cleared for the arrival; I selected 15;000 FT in the MCP for protection on the next crossing altitude. We were then cleared again via the arrival and to comply with speeds. As we approached MUNSTR; the restriction was 15;000 FT and I noticed the airplane shallowing out the descent and started to look at the box to see what exactly was happening. I noticed a speed intervention of 250 KTS selected. At that point; the Captain said we should cross at 15;000 FT and we noticed the plane was about 500 FT below. I stopped the descent and returned to 15;000 FT. There was no conflict with other aircraft or terrain and that was it. We descended the rest of the arrival on path. I would like to see ATC manage the arrivals better. In a highly complex arrival with multiple altitude and speed changes; it adds complexity to be given different speeds and altitudes and then told to go back to the original arrival profile. We could have verbalized better what the automation was doing when we first sensed confusion.

Data retrieved from NASA's ASRS site as of July 2013 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.