The pilot of a C-421C received two altitude alert warnings while flying the RNAV (GPS) 17 approach to SEE. Neither warning seemed appropriate to the reporter as the aircraft was in the appropriate vertical and lateral position in each case.
Synopsis
The pilot of a C-421C received two altitude alert warnings while flying the RNAV (GPS) 17 approach to SEE. Neither warning seemed appropriate to the reporter as the aircraft was in the appropriate vertical and lateral position in each case.
Narrative
There was some confusion about what minimum altitude I should be maintaining during the RNAV (GPS) 17 approach to SEE. While inbound between ASIXY and JUGAL I received a low altitude alert from the Tower when I was level at 2;400. The Tower then asked whether I was inside ASIXY. I had crossed ASIXY at 5;000 so I was well inside before reaching 2;400. Then after JUGAL I descended following the virtual glide slope provided by my GPS. At some point I received another low altitude warning from the Tower saying that the 'MDA' was 1;620. This confused me because the MDA (with the Tower operating) is 1;440. The airport came into sight shortly after that and I asked the Tower what MDA they were referring to. After a little conversation (the airspace was quiet at the time) the Tower referred me to the 1;620 restriction at TOMTY.On the FAA charts this restriction is fairly obvious; but on the commercially prepared charts that I was using it is a bit off to the side and looks more like a footnote. Since I was following the glide slope after JUGAL I wasn't focused on the restriction at TOMTY I actually don't think I was and that it was once again some kind of lag on the radar; but my point is that on the commercially prepared charts it is easy to miss that this is an additional crossing restriction. It was only after the Tower pointed it out and I carefully reviewed the chart and the footnotes that I realized that was what the 1;620 on that chart was referring to. This is quite different on the FAA chart.
NASA callback
A lengthy discussion with the reporter resulted in no resolution of why the Tower would have generated low altitude warning between ASIXY and JUGAL. The alert regarding descending below the crossing altitude for TOMTY appears to have been the result of a misunderstanding of the crossing restriction and the MDA. In addition; the reporter discussed several confusing aspects of the commercial aero chart display; for instance: the 1;620 crossing at TOMTY while on the virtual GS is only appropriate with the local altimeter setting. If the Miramar setting is used the restriction is raised to 1;660 yet there is no published change to the 3.45 degree flight path angle from the FAF; thus raising the probability that a flight following the virtual GS will cross TOMTY 40 FT low. Several other confusing chart issues were discussed that were not germane to the reported incident but raise the specter of RNAV Approach charting being insufficiently succinct to avoid the perceived need for pilots to 'interpret' the intent of the chart. The elimination of the need for interpretation of navigation data should be the primary goal of chart makers.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.