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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)?

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  • $\begingroup$ To fly the approach at a higher speed at the same descent rate without overshooting the runway, the helicopter would have to fly a shallower descent profile starting at a lower altitude, placing it closer to (and possibly below) the obstacle-clearance surface. $\endgroup$
    – Vikki
    Feb 15, 2021 at 21:22
  • $\begingroup$ Do the requirements, spell out a different Obstacle Clearance Surface? $\endgroup$
    – skipper44
    Aug 11, 2021 at 6:18
  • $\begingroup$ The max speed of any approach is determined by the required standard turn radius and combined with an assumed combination of navigation errors(Instrument and pilot). Glide slopes must meet certain requirements but they are not directly related to the speed limit. Flying faster forward you can also descend faster because drag is higher, achieving the same slope. The main limit here is distance to shed kinetic energy at the bottom of the slope. $\endgroup$
    – Max Power
    Jan 19 at 23:27

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