Jim is working on his Instrument rating and spotted an interesting approach procedure:
Not surprisingly, an ILS approach will have lower minimums that a non-precision approach, such as a localizer. However, I have found at least one airport in my area where the localizer MDA (580′ MSL, 564′ AGL) is lower than the ILS DA (743′ MSL, 727′ AGL). I’d like to understand why this is true and under what circumstances I can anticipate a lower minimum for a localizer than the ILS.
Jim is talking specifically about the ILS or LOC/DME Rwy 24 approach to Bowerman Airport (KHQM) in Hoquiam, Washington. There’s an interesting (if not completely confirmed) story behind this approach in particular, but before I get into that I’ll cover the basics.
Profile view (Jeppesen format) for the ILS or LOC/DME Rwy 24 approach at KHQM.
With this approach, as with any ILS, the FAA must certify that no obstacles penetrate a predetermined gradient. This means that even at full-scale glideslope deflection, there is a built in buffer between your airplane and the ground below. If terrain or an obstacle (buildings, trees, antennas, etc) sticks into the approach path, then the FAA has two options: the minimums (ceiling and visibility) for the ILS will increase, or the glidepath angle (currently 3° at KHQM) must be increased to clear the obstacle.
With the localizer-only version of the approach, you can descend to the MDA of 580′ once past WIMET (D3.0 from the localizer). The additional distance fixes on the localizer approach permit a pilot to descend past the obstacle and land safely. Unfortunately, to answer the second part of Jim’s question, there’s no way to predict the inversion in approach minimums unless you’re intimately familiar with the airport and the surrounding terrain. The real answer is to make sure that you thoroughly brief each approach before flying it.
The specific details about Hoquiam are after the break!
As an active instructor in the Seattle area, I fly out to Hoquiam on an instrument cross country probably once or twice a month.
The ILS at KHQM is a really odd story, in fact. From what I’ve gathered, the Port of Greys Harbor wanted to close the airport and turn it into a seaport facility some years back. The airport sits right on the Pacific coast, which would be great for shipping, and gets socked in almost daily during the winter, so the ILS is kind of necessary for Hoquiam to have a useful airport.
Bowerman Airport was an Army Air Force field in a previous life, and if the port authority closed it they would need to repay all of the FAA grant money they’d ever received, plus the value of the land (or something like that). So, instead of closing the airport, they simply stopped trimming the trees on a hill to the east, which conveniently lies under the ILS approach path. This happened sometime before August, 2007, when the procedure was last revised, and it has been this way ever since.
The glideslope is currently NOTAMed out of service. I have second-hand information which says this is because the FAA is now increasing the glidepath angle to something greater than 3°, which would clear the hill and should bring the minimums back down to 200-1/2 or so by the time winter hits.