Monday Accidents & Lessons Learned: Watch It Like It’s Hot

A B737 crew was caught off-guard during descent. The threat was real and had been previously known. The crew did not realize that the aircraft’s vertical navigation had reverted to a mode less capable than VNAV PATH.

From the Captain’s Report:
“While descending on the DANDD arrival into Denver, we were told to descend via. We re-cruised the current altitude while setting the bottom altitude in the altitude window. Somewhere close to DANDD intersection, the aircraft dropped out of its vertical mode and, before we realized it, we descended below the 17,000 foot assigned altitude at DANDD intersection to an altitude of nearly 16,000 feet. At once, I kicked off the autopilot and began to climb back to 17,000 feet, which we did before crossing the DANDD intersection. Reviewing the incident, we still don’t know what happened. We had it dialed in, and the vertical mode reverted to CWS PITCH (CWS P).

“Since our software is not the best and we have no aural warnings of VNAV SPD or CWS P, alas, we must watch it ever more closely—like a hawk.”

From the First Officer’s Report:
“It would be nice to have better software—the aircraft constantly goes out of VNAV PATH and into VNAV SPEED for no reason, and sometimes the VNAV disconnects for no reason, like it did to us today.”

We encourage you to use the TapRooT® System to find and fix problems. Attend one of our courses. We offer a basic 2-Day Course and an advanced 5-Day Course. You may also contact us about having a course at your site.We encourage you to use the TapRooT® System to find and fix problems. Attend one of our courses. We offer a basic 2-Day Course and an advanced 5-Day Course. You may also contact us about having a course at your site.