I dug up this thread through the search function because I wanted some guidance for a briefing I was to do on DME Arcs on a 737 sim session.
Traditionally my training section has taught that a 1% of airspeed lead-in for arc intercept was used. That meant that at 210kts (737 min clean speed) the turn (rate 1....30deg bank) would start at 2.1NM from the desired arc.
However, your suggestion was based on the formula of TAS/200. And for 210kts this would equate to a 1.05NM lead in.
Today I went into the sim and tried this formula.
Sorry guys, but it doesn't work. Sounds good, but it's way wrong.
I set the 737-400 sim up at 45 tonnes. 210kts, clean, inbound to a VOR, to intercept a 20DME arc, flying at 3,000 feet, zero wind.
Using the TAS/200 formula the turn was comenced at 21.5DME (allowing an extra .5NM for the time required to roll into a 30degree bank).
The result was we overshot the arc by .5NM....in other words went through the arc due to insufficient lead in.
The exercise was then repeated using the 1% TAS formula (i.e. 2NM lead), and the arc was intercepted perfectly, without significant over/undershoot.
So how do you explain that? Your fancy formulas don't appear to work in the real world!