I read in the AIM that WAAS vector modes will provide a linear course width of +/-1NM prior to the FAF and then provide a fixed width of +/-0.3NM to the point where an ILS splay would be 0.3NM and then tapers down to 700 ft total width at the runway threshold.

How smooth is the transition from the 1NM segment to the +/-0.3NM segment? Is it time-based, distance-based, or something else? Is it smooth at all, or is it actually an abrupt change?

Are all manufacturers following a common standard on how these scale transitions work?

  • $\begingroup$ I'd like to note that my original missed point was that the transition was "Smooth" over that 2NM period as the question cites the same point in the AIM that I've used as an answer later on with my own improved overall understanding. $\endgroup$ – Ryan Mortensen Feb 8 at 2:58

Beginning 2NM prior to reaching the final approach fix of an activated approach, the GPS will smoothly taper down the CDI scale from 1-NM full-scale deflection (from the center) to 0.3-NM at full-deflection over that 2-NM flight distance and reach a scale of 0.3-NM as the final approach fix is reached.

AIM Page 1-1-25

"When within 2 NM of the Final Approach Waypoint (FAWP) with the approach mode armed, the approach mode will switch to active, which results in RAIM and CDI changing to approach sensitivity. Beginning 2 NM prior to the FAWP, the full scale CDI sensitivity will smoothly change from ±1 NM to ±0.3 NM at the FAWP. As sensitivity changes from ±1 NM to ±0.3 NM approaching the FAWP, with the CDI not centered, the corresponding increase in CDI displacement may give the impression that the aircraft is moving further away from the intended course even though it is on an acceptable intercept heading. Referencing the digital track displacement information (cross track error), if it is available in the approach mode, may help the pilot remain position oriented in this situation. Being established on the final approach course prior to the beginning of the sensitivity change at 2 NM will help prevent problems in interpreting the CDI display during ramp down. Therefore, requesting or accepting vectors which will cause the aircraft to intercept the final approach course within 2 NM of the FAWP is not recommended."


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