B737 flight crew reported the aircraft's flight computer lost its VNAV capability. The aircraft was unable to comply with two crossing restrictions until the crew was able to reprogram the FMC and VNAV path.
Synopsis
B737 flight crew reported the aircraft's flight computer lost its VNAV capability. The aircraft was unable to comply with two crossing restrictions until the crew was able to reprogram the FMC and VNAV path.
Narrative
Just after my First Officer briefed the RAVNN 6 Arrival into Baltimore; Washington Center cleared us to descend via the arrival. We were level at flight 28;000 ft.; and ready to go down in a few miles. Suddenly the flight computer said we were 2000 ft. high. Then in a couple seconds later; it said we were too low. Then it said we were right on path. My First Officer started to investigate the problem when Washington Center said again 'Descend on the RAVVN 6' again. But by that time we were too high to make our crossing restriction at FZEEK and then WHHTE. The Controller inquired what was going on; all I could tell him was the flight computer was failing to descend on the arrival. ATC said just to 'make WALKN at 17;000 ft..' We manually started our descent down to 17;000 ft. and made the crossing restriction. We recruised the flight computer at that altitude and it descended perfectly on the rest of the arrival. The remainder of the flight was uneventful. Also; there was no conflict with any other aircraft on the arrival. Monitor; monitor and monitor. You never know when the flight computer is lost it ability to know where it is exactly. You also have to be quick in recovery or you will find yourself high on your arrivals. Always double check your cruise altitude in the flight computer and monitor once again.
Second reporter narrative
Cleared to descend via the RAVVN 6 to BWI at FL 350. The arrival was briefed and all altitudes were checked and correct in the box. We got side tracked briefing the approach and talking about weather and somehow the altitudes for FZEEK and WHHTE were removed. The aircraft then appeared to be 'lost' as it kept jumping between us being 2000 ft. high and then back to on profile several times. I had lost situational awareness to the altitudes required at FZEEK and WHHTE; and when ATC saw we weren't going to make it they said 'Just make the WALKN restriction'; which we did. No ATC conflicts occurred. Both Pilots became distracted at trying to figure out why the system wasn't preforming like we expected and lost situational awareness to where we were on the arrival. We should have been more focused on maintaining situational awareness to where we were on the arrival; and when it became obvious VNAV was not working as expected; quickly transitioned to another mode of automation.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.