ZFW controller described low altitude alert event when aircraft conducting a RNAV/GPS Runway 18 approach to PSN was observed starting a descent below published altitude; reporter questioning the absence of a MSAW alert.
Synopsis
ZFW controller described low altitude alert event when aircraft conducting a RNAV/GPS Runway 18 approach to PSN was observed starting a descent below published altitude; reporter questioning the absence of a MSAW alert.
Narrative
I was training on the radar position at ZFW Donie Low sector. This sector's altitude stratum is 000-FL230 and is primarily the low altitude sequencing sector to aircraft inbound to DFW and surrounding satellite airports. It contains roughly 8 uncontrolled airports; three of which have published instrument approaches. The volume was average but RADAR/NEXRAD weather depictions on the RADAR scope made normal traffic more complex. A flight was inbound to one of these airports and was issued the RADAR derived weather and given a clearance to deviate around it after all necessary coordination had been accomplished. The aircraft was asked what type of approach he was requesting and upon receiving that information; he was given a descent to 3000 feet until established and cleared for an RNAV GPS Runway 18 PSN. After scanning the sector and communicating control instructions to other aircraft my instructor and I noticed that Aircraft X was below the MEA. My instructor seized the frequencies and questioned him if he was established on the arrival and given a response of no. The aircraft said he was having some problems with his GPS and was trying to correct it when he was issued a low altitude alert and told to climb to 3000 immediately. Recommendation; it is my understanding that MSAW alerts won't work in the near vicinity of an airport if an aircraft is inbound to that airport. An effective scan potentially saved a flight from inadvertently striking the ground. Perhaps enabling EMSAW alerts and allowing controllers to suppress them would be advantageous when the volume and complexity of a session doesn't allow a scan rate that could prevent an accident below the MEA/MVA.
Second reporter narrative
We were training on the radar position. Aircraft X was approaching the PSN Airport from the east and was deviating south of a direct route to DUCAl which would be the appropriate fix to shoot the RNAV/GPS 18 approach which he requested. Approximately 15 miles east of the airport; the trainee cleared Aircraft X for the approach and to maintain 3000 until established. Trainee was issuing weather to other aircraft and sequencing to the DFW terminal area. I was watching Aircraft X descend and observed him descend below 3000 but was not concerned because this approach has a 2500 foot Terminal Arrival Area associated with this quadrant. When I observed him break 2500 I seized the frequencies and queried him if he was established on the approach or what fix he was navigating to for the approach. At this point he told me he was having trouble with his GPS and I observed him descending below 2000 at about 1000 FPM. I issued a low altitude alert and an immediate climb to 3000. He stated that he was climbing back up and I observed the mode C bottom out at 1500. I next told him that the minimum safe altitude in the sector of the approach was 2500. As I observed the mode C start to go back up; he asked me for a vector to the airport where he might be able to get a visual. Since I was satisfied that his altitude was sufficient and he was climbing rapidly; I told him to 'proceed direct the airport and maintain 2500' which at this point was 12 o'clock and about 7 miles. As he read back this clearance his mode C showed 1900 and as soon as he finished the read back the next hit showed 2400; which actually is the MEA. Technically he was below the MEA when I told him to 'proceed direct the airport'; but he was climbing rapidly and leveled a few seconds after the read back. It is obvious that this pilot was having difficulty getting his GPS set up and had lost situational awareness of his position and altitude. Another minute and he would have gotten a ground proximity warning or worse.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.