Why do GPS Copter Approaches require a slower final approach speed than conventional Copter Approaches?

Question

AIM 10-1-2 says helicopter instrument approaches are designed with a maximum speed of 90 KIAS. However, it specifically exempts the final segment on GPS helicopter approaches, saying the maximum speed there is 70 KIAS. Here is the explanation in the AIM:

Obstruction clearance surfaces are based on the aircraft speed and have been designed on these approaches for 70 knots. If the helicopter is flown at higher speeds, it may fly outside of protected airspace.

Why are GPS copter approaches designed with a different obstacle clearance surface that requires a different airspeed on final? Also, if a helicopter flew faster forward with the same descent rate, wouldn't it get farther from the obstacle clearance surface (i.e. making faster speeds safer)?

Answer 1

Obstacle clearance surfaces are 3 dimensional. The traditional non-GPS approaches are designed around category-A aircraft[90KIAS] approach engineering and the routing options available from the nav equipment was more limiting than airspeed in many cases. GPS allows many new potential approach paths and 70kias helps take advantage of this routing reduced visual distance and turn radius while maintaining reaction time and buffer area.

From 8260.42

4. BACKGROUND. The foundation of these criteria are studies of GPS data from simulation and flight tests. A significant difference exists between approach procedures to runways and approach procedures to heliports. Approaches to runways terminate in relatively obstacle free landing environments. Approaches to heliports commonly terminate in areas of dense obstacle populations where executing a missed approach requires higher than average demands on pilot reaction and performance. Speed limitations incorporated in these criteria take advantage of the unique, slow speed capability of helicopters. These speed limitations allow construction of small obstacle clearance areas and yield the lowest possible minimums.

Which is not a super satisfying answer, but probably the best you can expect from a 25-year-old standard set by now-retired un-named bureaucrats of a subgroup of an office of a department. I do know that GPS receivers were not trusted back then, and they were not augmented so subject to more signal error, and they calculated much slower introducing more inaccuracy. (Location updated once per second = 150ft at 90kts.) Many 70kt approaches can be flown by military at 90kts.