Air carrier flight crew on the ILS runway 17 to mht noticed the aircraft had descended below the min...

Attributes
ACN	1318712
Time
Date	201512
Local Time Of Day	1801-2400
Place
Locale Reference	MHT.Airport
State Reference	NH
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Make Model Name	Widebody Low Wing 2 Turbojet Eng
Operating Under FAR Part	Part 121
Flight Phase	Initial Approach
Flight Plan	IFR
Component
Aircraft Component	FMS/FMC
Person 1
Function	Captain Pilot Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 150 Flight Crew Total 20000 Flight Crew Type 10000
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Altitude Overshoot Deviation - Procedural Clearance Deviation - Procedural Published Material / Policy Inflight Event / Encounter CFTT / CFIT

Narrative:

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.

Original NASA ASRS Text

Title: 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.

Narrative: 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.

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.

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 speed int active. The crew noticed field elevation had replaced the bluum altitude in the FMC and climbed back to the MSA as ATC alerted.