121Dispatch said:
We had an RJ captain call dispatch a few weeks ago regarding his ability to shoot an approach. The aircraft had the marker beacons deferred, and he questioned how he could shoot an ILS without the audio/visual beacon in the cockpit. I don't think GSO was the airport, but it had the same circumstance...Could he not be on the LOC/GS, and have LIB dialed into Nav 2 to identify Pagan...then while on the LOC/GS go missed at 200' AGL if runway environment not in sight...and as a back up to that use 5.1 and .4 DME respectively?
I guess the overall question is: Are the marker beacons required to shoot an ILS approach?
First Disclaimer - your ops specs are controlling.
Second Disclaimer - this is not legal advice but is instead an educational discussion. I make no warranty as to the correctness of the information given.
Once upon a time all ILS approaches took a 50' hit on the DA when the OM or MM was inop. This has not been the case for about 15 years. In other words, if you have a marker beacon on the approach and it is not operative, then unless othewise stated, there is no penalty applied to the descent minima. It would therefore follow, that if the MB receiver in the aircraft is inop then there is also no penalty. BUT the aircraft MEL will be controlling - if it says something like - one installed - may be inoperative - placard required - no operational restrictions, then you are good to go as far as your aircraft is concerned if the MB receiver is inop.
You asked if he could use the Liberty 306 radial to identify Pagan - yes he could.
You asked if he could use 5.1 DME to identify PAGAN - no he cannot. PAGAN can be identified one of three ways - OM/Interesection/Radar. The distance on the profile view (5.1 nm) is the distance from the OM to the MM and is not a measurement from any navaid or fix. For an example of an ILS that uses DME to identify fixes see e.g. ORD ILS 14L where LEAMA is identified either by the OM or by the LOC 7.1 DME fix. But note, that this is a LOC that actually has an associated DME station, and there are just not that many of those. You'll also note that the distance between the OM and the MM is just 4.7nm, and the total OM - runway threshold is 5.2 miles. The DME at the runway threshold is 1.8, b/c the LOC transmitter is at the opposite end of the runway and 14L is nearly 2 miles long. (Note, this does not mean that if the GSO VOR has a DME I wouldn't have it dialed in, I absolutely would, as a situational awareness tool, and not as an approach aid.)
Also you suggest that the missed approach point is 200' agl. You are essentially correct, but there is a significant gloss on this that must be understood. The missed approach point on an ILS is the Decsion Altitude, not an altitude AGL. It just happens that for many (most) Category I ILS approaches the DA is 200' above the TDZ elevation. But this is not always the case, poor approach light systems and/or obstacles in the missed approach path, and/or g/s instability near the threshold can all lead to DA that is above the 200' AGL "normal" for Cat I ILS approaches. You must also recall that the cockpit contains no instrument that can reliably show your AGL except the radar altimeter. But CAT I DA's are based on MSL, not AGL and the radar altimeter is primarily used as an input to the GPWS in normal CAT I operations. It is only when you get to CAT II approaches that the chart starts to reference Radar Altitudes (RA) See ORD ILS 14L (Cat II) approach. Also note, that by the time you get CAT III (see ORD 4L Cat III) approaches there is no DA and Radar Altitude is irrelevant since the whole idea of Cat III is to land without any discernable ceiling - e.g. the old WOXOF - celing indefinite sky obscured visibility 0 in fog. Generally followed by some obnoxious RVR like 800.
Does this help?
