Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 150; Flight Crew Total 20000; Flight Crew Type 10000
Distraction; Human-Machine Interface
Aircraft Equipment Problem Less Severe; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Issued Advisory / Alert; Flight Crew Returned To Clearance; Flight Crew Took Evasive Action
Aircraft; Human Factors
On the PURBL transition to the ILS 17; the PF notice that the aircraft was descending below the approach profile altitude from BLSSM to BLUUM. The PF arrested the descent and climbed back to a safe altitude. ATC then called and reminded us of the minimum safe altitude. PM notice that the altitude at BLUUM in the FMC had magically changed to field elevation. We were navigating in LNAV/VNAV; speed intervene activated with the autopilot engaged. An uneventful; stable approach and landing was performed. Altitudes and fixes were reviewed by both pilots during the approach brief. Cause of deviation in the FMC; from what we can determine is PFM. No explanation. One can prevent this from happening by believing nothing; ever. 'Constant Monitoring' would have prevented this event.
Air carrier flight crew on the ILS Runway 17 to MHT noticed the aircraft had descended below the minimum safe altitude (MSA) prior to BLUUM with LNAV/VNAV and SPD INT active. The crew noticed field elevation had replaced the BLUUM altitude in the FMC and climbed back to the MSA as ATC alerted.