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An ILS where the DA is higher than the MDA

Posted by on October 12, 2009 3 Comments Category : Flight Instructor Blog

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.

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.


  1. Paul on Oct 13, 2009

    Yesterday I called the FAA about this approach and they called me back today. Here’s the scoop:

    1) The ILS has already changed as of 22-OCT-09. The new ILS DA is 216 with 3/4 required visibility

    2) The FAA representative that I talked to shed some light on the subject by saying that the previous DA might be related to the CAT D missed approach. The new approach has removed CAT D missed approach procedure. (You can’t shoot this approach in a CAT D aircraft)

    3) He did say, that most likely the higher minimums were due to some ostacle that was penetrating the clearance plane as Eric indicated.

    4) He also said that you can see this same behavior in the new RNAV approaches. There are times where the LPV DA will be higher than the LNAV / VNAV or LNAV only minimums.

  2. Rafael Sierra, CFII on Nov 16, 2009

    Higher or lower visibilty is based on the missed approach distance from the Threshold and higher DA is based on the OCS or Obstacle Clearance Surface requirements. for ILS or LPV, LNAV/VNAV and Cat of aircraft. Order_8260.54A.pdf helps explain all this.

  3. John D. Collins on Jan 24, 2011

    A common reason for the lower MDA verses the DA of a vertically guided approach (ILS, LPV, LNAV/VNAV) has to do with the fact that the glideslope/glidepath is fixed and can’t change along the final approach segment. In the case of a Non Precision Approach with a MDA, if there is an obstacle on final, the procedure designer can introduce a step down after the obstacle is cleared. The same obstacle, if it penetrates the Obstacle Clearance Surface (OCS) which is a sloped surface used to evaluate obstacles on an approach with a glidepath, will force the DA up accordingly. In fact, often the DA will be pushed back further than the obstacle so that the obstacle will appear in the visual segment so the pilot can see it and avoid it.

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