Answer on CEF ILS RWY 23 questions

The USAF terps guy Timothy Lovell sent John Haggerty (FAA terps guy) a
response to his query about the question I asked here, about the odd
step-down on the LOC vs LOC/DME versions of this approach. For those not
playing along at home, the LOC has a MDA of 900 feet, and the LOC/DME has
an MDA of 700 feet with a step-down fix 2.7 DME from the VORTAC.
http://204.108.4.16/d-tpp/0410/00447I23.PDF

The odd thing about the approach is that if you're doing the LOC/DME, then
before the step down, you can't descend to 900 feet like if you were doing
a straight LOC, but only to 1160'. I'll cut and paste his full answer
here, but the executive summary is that they put the step down to "provide
a constant descent rate on the procedure". Weird, eh? I don't see how
that stops you from doing a dive and drive to 1160, then another to 700
myself, but that's just me.

Oh, and the other question people had, about why the missed approach says
"tracking 228" instead of "fly heading 228" or "tracking ILS BC" - that's
just the way the USAF does things.

Ok, here's the response:
The LOC RWY 23 portion of the published KCEF ILS RWY 23 has a step
down fix to avoid terrain (544'MSL) with an adverse assumption of 100'
trees growing on it at N421439.00 W072827.00. This terrain gives you the
MDA for the S-LOC 23 of 544'(terrain)+100'(tree)+250'(Required Obstacle
Clearance)=894'=900'MDA. The published Step Down Fix (SDF) altitude of at
or above 1160' on the S-LOC/DME 23 is not only for terrain avoidance, but
to provide a constant descent rate on the procedure (FAF to SDF
379.81FPM/3.58 degrees, SDF to RWY 366.81 FPM/3.45 degrees). Were the SDF
altitude purely for terrain avoidance the SDF altitude would be 900'.
Having said all of that this procedure will change soon with the
implementation of new AF software and the corresponding criteria changes.
The SDF will move out to CEF 3DME and be raised to 1240', this altitude
raise will ensure that an aircraft executing a circling approach will not
descend below the highest circling MDA inadvertently.

To answer the last part of your question AF criteria requires
departure procedures and missed approach instructions to read track and
not heading. The logic behind this is that an instrument procedure is
built along a specific ground track to be flown and not a heading. The
procedure track does not take in to account wind drift, this
responsibility is placed upon the pilot. The AF trains its pilots very
specifically to fly tracks not headings on departure and or missed
approach.

I hope this answers yours and everyone else's questions.

Tim


Tim Lovell
Air Force Reserve Command, Terminal Instrument Procedures (TERPS) Manager


--
Paul Tomblin http://xcski.com/blogs/pt/
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