Try to think about what would be happening in the air.....
If IMC - no 1 priority is to stay 1000' above the terrain. How do you know where the terrain is? LSALT for the route safely gets you to the aid/point but only within the tracking tolernces of the route. If you're within 25nm of the destination you now have additional information about where the terrain is and so you have info on how to stay above it from the chart - this is the answer they're looking for and so 25nm will get you the marks.
Note that there are other ways of avoiding the terrain in practice. Some that come to mind are:
- be 'visual' in the sense that you can legally descend below LSALT (can't do that at night though). (Ignore 'Visual approach's' - they are a completely different kettle of fish - by definition you're not going to make the instrument approach.
- have another LSALT figure to work with - for example you can use the grid lowest safe to ensure that you won't hit any terrain within, say, 100nm of the destination (it may be a higher figure but you can still make it work)
I'm sure there are others but the other question is why would you want to start manouvering from more than 25nm out? At 180kts you're still 8 minutes from the aid and all you'll achieve is the adding of useless track miles.
One other trap to watch out for - if the airways LSALT is lower than the MSA (and it does happen - YMRY I think is an example) then if you're screaming in at LSALT trying to get visual and then want to manouvre inside 25nm then you must CLIMB to that MSA to ensure terrain clearance.
Very easy mistake to make.
Hope this helps,
UTR
Last edited by UnderneathTheRadar; 19th Feb 2007 at 10:43.
Reason: Brain fart - last section is about LSALT LOWER than MSA