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Take Half Moon Bay Airport (KHAF) as an example.

  • Under VFR, the traffic pattern is right pattern for RWY 30 and left pattern for RWY 12. This implies the downwind leg is to the EAST of the RWY 12-30, and therefore we should fly over the land at TPA 999'.
  • However, looking at the IFR plates, both RNAV (GPS) Y Runway 30 and RNAV (GPS) Y Runway 12 indicate we are required to circle WEST over the water at 800', since that "Circling NA east of Rwy 12/30".

I understand it is likely that we have more vertical room over the water since there are no obstacles there. However, I don't get the reason to require VFR traffic over the land. Is it for VFR/IFR traffic separation?

Some reasons that I came up with

  • Favors the water, against the land: Terrain/Obstacles, Noise Abatement.

  • Favors the land, against the water: Marine Sanctuary Protection.

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It looks like you have a few different questions here. As a caveat, I know nothing about Half Moon Bay so these are (educated) guesses to some extent. Someone with personal experience at KHAF could probably give better answers.

Why is circling always west of the runway at KHAF?

Circling areas are planned according to the TERPS criteria (see section 2-7-1); the formulas are in the document, but the basic idea is to make sure that there are no obstructions to circling within a certain distance of the runway. It looks very likely that at KHAF the terrain east of the field rises too sharply to allow safe circling there: the minimum allowed in the TERPS is 1.3nm/1.5sm/2.4km, and Google Maps shows that there's definitely rising terrain within that distance.

Why is the pattern always east of the runway at KHAF?

I don't know for sure. Noise abatement is obviously a concern at KHAF but then as you said, why not put the pattern to the west, over the ocean? The most obvious reason is that Half Moon Bay is in the Monterey Bay National Marine Sanctuary and flying over national parks below 2000' AGL is discouraged. Another possible reason is to avoid directing light, single-engine aircraft to fly over water unnecessarily.

Why - in general - would a certain area be used for the pattern but not for circling?

There's no reason why one area should always be suitable for both needs. In particular, circling requires guaranteed obstacle clearance within a certain distance from the runway. VFR pilots should be able to see and avoid, but obviously that assumption isn't possible for IFR. If you fly a circling approach at minimums then you might even go back into cloud while circling (and then go missed); in that scenario you really don't want to have to worry about terrain or obstacles being anywhere nearby.

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    $\begingroup$ On the [map provided by NOAA(montereybay.noaa.gov/materials/maps.html#overflight) of the Monterey Marine Sanctuary KHAF is not in the overflight restriction zone. And the noise abatement link you provided shows "extremely noise sensitive ares" within the designated pattern area, not to mention the hospital you're supposed to avoid. $\endgroup$
    – TomMcW
    Commented Jun 27, 2016 at 21:09
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It’s because of the marine sanctuary. 99% of the time it’s Runway 30, and there are noise abatement procedures (pdf) for the east side traffic pattern as well.

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  • $\begingroup$ Although it's in the sanctuary zone, according to their map it's not in the overflight restriction zone $\endgroup$
    – TomMcW
    Commented Jun 27, 2016 at 21:00
  • $\begingroup$ Just because it's not in the zone that doesn't mean they want traffic over it $\endgroup$
    – rbp
    Commented Jun 27, 2016 at 21:52
  • $\begingroup$ The restriction zones are printed on the sectional as magenta lines with dots inside them. Whereas the sanctuary boundaries themselves are not depicted.There are several airports located on the sanctuary but outside the restriction zones. The MRY class C even has a portion over the restriction zone. It looks like the circling area has an MDA of less than 1000 feet. (Let me know if I'm wrong about that. I'm not good at reading app plates) Not saying the sanctuary is a factor, but I don't see any evidence that that is the case $\endgroup$
    – TomMcW
    Commented Jun 27, 2016 at 22:23
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I live in Moss Beach next to KHAF. Specifically, they want the VFR traffic over land because of the Fitzgerald Marine Reserve to the NW of 30. There's a huge pack of seals w/ cubs on the beach, so they'd rather keep the noise over the residential areas. You wouldn't want to send IFR traffic over that land though, not with Montara Mountain to the N and it getting steep pretty quickly to the NE.

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