Narrative:

Outside the FAF on the btr RNAV runway 4L approach; I asked for the next step down fix and altitude. The non-flying; pilot indicated we were at the FAF and could descend to the MDA. At 1300 ft we cleared a broken layer and appeared to be very close to power lines and towers. I climbed back to our initial altitude and landed visually using the VASI on runway 4L. After landing the non-flying pilot realized he read the DME off the wrong page on the FMS and we were actually outside the FAF when we started our descent to the MDA. We agreed after discussion that the FMS page and corresponding approach plate would be verified by both pilots.callback conversation with reporter revealed the following information: the reporter was the pilot flying and had selected the flight director heading mode because the FMS would only allow the aircraft to proceed to bitac and hold. The non-flying pilot was attempting to remove the hold on the FMS navigation page and while on that page misread the DME and informed the pilot that he could begin the descent. Approaching gocet the aircraft broke out of the clouds at about 1300 ft allowing the reporter to see the power lines. The height of the towers shocked the reporter and at the same time the non-flying pilot realized his mistake and advised the pilot to climb. ATC had previously cleared the aircraft to land and because of that the reporter thought that no low altitude alert message was issued.supplemental info from acn 821328: there are two issues that need to be addressed. The first is that the crew must be aware that the altitude select mode and a change in the altitude pre-select will cause the autopilot to continue to the new altitude. This is a CRM function. Prior to any change in the altitude pre-select; it must be verified that the autopilot mode selector is placed into the altitude hold mode; or a change in altitude will result. This is not unknown; however; coordination of the process during a critical flight segment is necessary to prevent the altitude excursion that resulted. Second; this altitude deviation would probably not have been quite as troubling if it not had been for the tower that was uncomfortably close to the final approach course. Vertical guidance would have made this particular approach much safer; as the presence of a glide slope to that runway would have prevented the altitude deviation; and given safe descent information related to the position of the tower. The btr tower personnel did not advise us of the deviation; as they either did not notice it or we had corrected it before that occurred. The crew is very aware of the complexity of GPS approaches and the need for vigilance in altitude control where no glide path information is available. It was very unsettling to see the tower; at almost our altitude; not more than a half mile to the right of the inbound course. The crew has addressed the need for a change in cockpit procedures when initiating an altitude pre-select function during autopilot use. We feel comfortable that simple verbal verification of the mode selection prior to a change in the altitude pre-select will prevent further such deviations.

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Original NASA ASRS Text

Title: A light transport corporate aircraft crew descended early on a BTR RWY 4L RNAV approach because of FMS programming distractions and the lack of procedure familiarity.

Narrative: Outside the FAF on the BTR RNAV Runway 4L approach; I asked for the next step down fix and altitude. The non-flying; pilot indicated we were at the FAF and could descend to the MDA. At 1300 ft we cleared a broken layer and appeared to be very close to power lines and towers. I climbed back to our initial altitude and landed visually using the VASI on Runway 4L. After landing the non-flying pilot realized he read the DME off the wrong page on the FMS and we were actually outside the FAF when we started our descent to the MDA. We agreed after discussion that the FMS page and corresponding approach plate would be verified by both pilots.Callback conversation with reporter revealed the following information: The reporter was the pilot flying and had selected the flight director heading mode because the FMS would only allow the aircraft to proceed to BITAC and hold. The non-flying pilot was attempting to remove the hold on the FMS NAV page and while on that page misread the DME and informed the pilot that he could begin the descent. Approaching GOCET the aircraft broke out of the clouds at about 1300 ft allowing the reporter to see the power lines. The height of the towers shocked the reporter and at the same time the non-flying pilot realized his mistake and advised the pilot to climb. ATC had previously cleared the aircraft to land and because of that the reporter thought that no low altitude alert message was issued.Supplemental info from ACN 821328: There are two issues that need to be addressed. The first is that the crew must be aware that the altitude select mode and a change in the altitude pre-select will cause the autopilot to continue to the new altitude. This is a CRM function. Prior to any change in the altitude pre-select; it must be verified that the autopilot mode selector is placed into the altitude hold mode; or a change in altitude will result. This is not unknown; however; coordination of the process during a critical flight segment is necessary to prevent the altitude excursion that resulted. Second; this altitude deviation would probably not have been quite as troubling if it not had been for the tower that was uncomfortably close to the final approach course. Vertical guidance would have made this particular approach much safer; as the presence of a glide slope to that runway would have prevented the altitude deviation; and given safe descent information related to the position of the tower. The BTR tower personnel did not advise us of the deviation; as they either did not notice it or we had corrected it before that occurred. The crew is very aware of the complexity of GPS approaches and the need for vigilance in altitude control where no glide path information is available. It was very unsettling to see the tower; at almost our altitude; not more than a half mile to the right of the inbound course. The crew has addressed the need for a change in cockpit procedures when initiating an altitude pre-select function during autopilot use. We feel comfortable that simple verbal verification of the mode selection prior to a change in the altitude pre-select will prevent further such deviations.

Data retrieved from NASA's ASRS site as of April 2012 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.