Btv controller reported that an aircraft cleared for the RNAV (GPS) Z runway 33 approach descended b...

Narrative:

Aircraft X was cleared for the RNAV Z runway 33 approach. When the aircraft turned from the 'T' fix dongy to join the final approach course at janud; the last time I saw his mode C it was at 5;600 ft estimated and then gave me a short la then the target went coast. I issued the low altitude alert and the MVA and told the pilot he was radar contact lost. The MVA in this area is 5;400 ft and the minimum altitude on that segment of the approach is the same. A couple of sweeps after this I picked up the primary and then shortly after the beacon which showed an altitude of 4;100 ft estimated. What this means is that the aircraft was below this altitude before the radar began to pick him up again. This is an incredibly dangerous situation because he was below the level of nearby mountain peaks in mountainous terrain. The closest mountain peak that is depicted was less than three miles away with a height of 4;088 ft MSL. When I issued the report; the pilot said something along the lines of 'it pushed us through.' then when informed that radar contact was lost he said; 'we're climbing.' the radar picked up the target again within a couple of sweeps. I was sure that it was the same target because of the position and did not re-identify the aircraft. The aircraft went out for the approach again and landed without further incident. While on the ground; during a conversation with the pilot on the ground control frequency; the pilot informed that the GPS never 'locked in correctly' or something along those lines. This implies to me that the waas altitude information was not going through correctly and the pilot was under the impression he could descend lower than 5;400 ft. Waas approaches are still fairly new to many pilots and I think some may not fully understand some of the issues at hand. This event was scary because I was able to alert the pilot to something that his equipment may have been telling him was safe; but was most definitely not. This approach is available when the tower is closed and the center would not normally have maintained communications with the aircraft long enough to inform him of the unsafe situation. I believe making this approach not applicable when the btv TRACON is closed is one option; but really the problem in my mind has to do with the reliability of the waas equipment.

Original NASA ASRS Text

Title: BTV Controller reported that an aircraft cleared for the RNAV (GPS) Z Runway 33 approach descended below depicted altitudes. The pilot indicated that aircraft equipment was not properly receiving navigational information.

Narrative: Aircraft X was cleared for the RNAV Z Runway 33 approach. When the aircraft turned from the 'T' fix DONGY to join the final approach course at JANUD; the last time I saw his Mode C it was at 5;600 FT estimated and then gave me a short LA then the target went coast. I issued the low altitude alert and the MVA and told the pilot he was RADAR contact lost. The MVA in this area is 5;400 FT and the minimum altitude on that segment of the approach is the same. A couple of sweeps after this I picked up the primary and then shortly after the beacon which showed an altitude of 4;100 FT estimated. What this means is that the aircraft was below this altitude before the RADAR began to pick him up again. This is an incredibly dangerous situation because he was below the level of nearby mountain peaks in mountainous terrain. The closest mountain peak that is depicted was less than three miles away with a height of 4;088 FT MSL. When I issued the report; the pilot said something along the lines of 'it pushed us through.' Then when informed that RADAR contact was lost he said; 'We're climbing.' The RADAR picked up the target again within a couple of sweeps. I was sure that it was the same target because of the position and did not re-identify the aircraft. The aircraft went out for the approach again and landed without further incident. While on the ground; during a conversation with the pilot on the Ground Control frequency; the pilot informed that the GPS never 'locked in correctly' or something along those lines. This implies to me that the WAAS altitude information was not going through correctly and the pilot was under the impression he could descend lower than 5;400 FT. WAAS approaches are still fairly new to many pilots and I think some may not fully understand some of the issues at hand. This event was scary because I was able to alert the pilot to something that his equipment may have been telling him was safe; but was most definitely not. This approach is available when the Tower is closed and the Center would not normally have maintained communications with the aircraft long enough to inform him of the unsafe situation. I believe making this approach not applicable when the BTV TRACON is closed is one option; but really the problem in my mind has to do with the reliability of the WAAS equipment.

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.

Attributes
ACN	985624
Time
Date	201112
Local Time Of Day	1801-2400
Place
Locale Reference	BTV.TRACON
State Reference	VT
Aircraft 1
Make Model Name	British Aerospace Undifferentiated or Other Model
Flight Phase	Final Approach
Route In Use	Other RNAV Z Runway 33
Flight Plan	IFR
Person 1
Function	Approach Departure
Qualification	Air Traffic Control Fully Certified
Events
Anomaly	Deviation - Procedural Published Material / Policy Inflight Event / Encounter CFTT / CFIT

Btv controller reported that an aircraft cleared for the RNAV (GPS) Z runway 33 approach descended below depicted altitudes. The pilot indicated that aircraft equipment was not properly receiving navigational information.