Originally Posted by
Dick Smith
The aircraft in the Benalla accident was actually coming from Ulladulla, and clearly the lowest safe altitude for that route was over 7,000 feet.
Dick, LHRT,
It's been a week and lots of ADSB techo discussion has since passed, but I wanted to revisit this point.
At what point should TAAATS go DING with a MSAW alert for an aircraft on RADAR?
Firstly, as previously advised, the a/c must be on RADAR, as ADSB is not QNH corrected and the level readout is plain WRONG at altitudes (except for the trivial case where local QNH happens to equal 1013). The machine must then allow for MODE-C tolerance of 200'. So before we even start working out what the "lowest safe altitude" is for the aircraft's present position, TAAATS should NOT DING until the a/c is 300' below whatever we come up with.
Dick, as you rightly point out, the GRID LSALT coming from UDA to BLA is over 7000'. To be precise, from abeam CRG to abeam AY, it's 7900, then 7100 to BLA. However, it would be simplistic for TAAATS to just look at grid LSALTS, as many spurious alerts would be generated for aircraft legally flying in IMC below those levels within those grids.
How you ask - they could be established on the AY-CRG track with a track LSALT of 5700. Or airworking within 25NM CRG, remaining wholly in the western quadrant with a MSA of 6200. etc etc.
I have drawn up a diagram showing a DCT track from UDA to BLA (in red), overlayed with various circles. As you can see, the route remains wholly contained within the overlap of BLA/WGT/AY/CRG MSA circles.
Adding some numbers.
The AY 25NM MSA to the south is 5400'
WGT 25NM to the east is 6300', 10NM is 3000'
BLA 25NM is 3000' to the north, 5000' to the south and 3500' within 10NM
So, from the time the track enters the 25NM AY ring, 5400' is a justifiable LSALT until it leaves the AY ring at approx 40NM BLA. The next 10NM or so until it enters the 10NM ring from WGT, the best MSA is 6300'. After that, you have 3000' within 10NM WGT followed by 3500' within 10NM BLA.
Thus far, the machine SHOULD NOT DING unless a radar paint is at 5100' within 25 AY, or 6000' for the 10NM only covered by 25 WGT, or a miserly 3200' (well below radar coverage) once west of 10NM WGT.
Not finshed yet though... The dumb computer must also take into account published route LSALTS. "But he's not on one" I hear you saying. True. However, for the approx 10NM the track is ONLY covered by the WGT 25NM MSA (6300'), it IS within the procedural tolerances of either/or of the AY-ELW and AY-SBG tracks (6.9 degree NDB splay from AY and tracks are 12 degrees apart). So, this section covered only by WGT 25 MSA can be further reduced to 5300' worst case being the AY-ELW track (both these tracks highlighted in blue).
In summary, the dumb computer, assuming knowledge of grid, route, and MSA LSALTS would have NO cause to go DING on any aircraft indicating a MODE C level of 5100' or above.
