Narrative:

I flew flight from miami to quito (uio/seqm). We flew the ILS Z rwy 36 approach as published; beginning at the qit VOR. This approach is on page 11-3. This was the first-ever flight into quito for both myself and the first officer (first officer). Although a recent upgrade to captain; I have almost 4;000 hours in the airplane and my first officer has over 5;000 hours in the airplane. We do not believe this was operator error. We both checked the waypoints and constraints on the FMS and there were no last-minute changes. We began the approach in LNAV/VNAV path. As recommended on page 10-7B-2; we constrained qit at 220 KIAS and 18;000 feet. When cleared for the approach; I selected 10;500 feet as the final approach fix altitude; planning to transition to localizer/GS guidance when joining the localizer. When we were near point D253I (the 253 degree radial; at or above 14;200); we were descending through approximately 15;000 feet; flaps 5 and about 185 KIAS; the pitch mode changed from VNAV path to MCP speed all by itself. My sharp first officer noticed this immediately and I reset the altitude selection to 14;200; to comply with the restrictions at D253I and D223I. We then re-selected VNAV; and VNAV path resumed control of speed and descent path. The remainder of the approach was completed normally. With over 9;000 hours of time [in this aircraft] between us; we feel that this anomaly may be [aircraft type] related. As you can see from looking at the approach chart; this is not a place where you want to have VNAV quit unexpectedly. If this scenario happened without the pilots noticing the change to MCP speed; the airplane would have continued descent below the 14;200 feet restrictions; and come dangerously close to the high terrain.

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Original NASA ASRS Text

Title: B737 Captain reported an anomaly with the VNAV function during the approach into a high altitude; high terrain airport at night.

Narrative: I flew flight from Miami to Quito (UIO/SEQM). We flew the ILS Z Rwy 36 approach as published; beginning at the QIT VOR. This approach is on page 11-3. This was the first-ever flight into Quito for both myself and the First Officer (FO). Although a recent upgrade to Captain; I have almost 4;000 hours in the airplane and my FO has over 5;000 hours in the airplane. We do NOT believe this was operator error. We both checked the waypoints and constraints on the FMS and there were no last-minute changes. We began the approach in LNAV/VNAV PATH. As recommended on page 10-7B-2; we constrained QIT at 220 KIAS and 18;000 feet. When cleared for the approach; I selected 10;500 feet as the final approach fix altitude; planning to transition to LOC/GS guidance when joining the localizer. When we were near point D253I (the 253 degree radial; at or above 14;200); we were descending through approximately 15;000 feet; flaps 5 and about 185 KIAS; the pitch mode changed from VNAV PATH to MCP SPD all by itself. My sharp FO noticed this immediately and I reset the altitude selection to 14;200; to comply with the restrictions at D253I and D223I. We then re-selected VNAV; and VNAV PATH resumed control of speed and descent path. The remainder of the approach was completed normally. With over 9;000 hours of time [in this aircraft] between us; we feel that this anomaly may be [aircraft type] related. As you can see from looking at the approach chart; this is not a place where you want to have VNAV quit unexpectedly. If this scenario happened without the pilots noticing the change to MCP SPD; the airplane would have continued descent below the 14;200 feet restrictions; and come dangerously close to the high terrain.

Data retrieved from NASA's ASRS site 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.