212man,
In the approach we are discussing, the MAP is not tied to a DME, so the timing is an acceptable substitute. Were the DME used to fix the MAP, it would be stopping you from hitting something beyond the approach that timing might not be avoidable with non-DME timing.
With the ancient method of timing approaches, the error the pilot can assume is worked into the cleared airspace. If you ever saw the TERPS approved airspace for such an approach, it is enormous, and allows for quite gross errors of position, based on the possible errors in the groundspeed and the timing.
If one could see the data used to create the approach, we could see why the MDA lowers when the DME fix is had, because there is an obstruction back there that rises enough to have to be sure to get over before the drop from 440 to 380 can be allowed.
There is an interesting method using probabilities of position that helps create the approach lateral boundaries and the altitudes. The approach is assumed to have a 1 in 10,000,000 safety factor in the primary zone, making the rotor blades less safe than your liklihood of hitting an obstruction.
Last edited by NickLappos; 7th July 2005 at 11:18.