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121 derived alt mins question

bgaviator

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flight going to PHL......listed alternate as MDT.

Question arises from pilot about if MDT is valid. The forecasted ceiling at ETA was 800 bkn, greater than 6.

Captain calls and says he needs a different alternate due to the whole 1 navaid/2 navaid thing.
I was out of the room, and came into the conversation late as another dispatcher was on the radio with the capt trying to figure this out. When I came back I just obliged and gave him a different alt to get him out quicker without having to argue. MDT has an ILS 13 and 31, as well as a VOR 31. The ILS freq is the same from each direction.....the question I have is.....is it considered two different navaids if the Morse Code ident is different? I thought I remembered back in school that even if the ILS freq was the same, if the Morse code was different for each approach, it was considered two separate? Am I right or wrong on this?

Would have adding 200- 1/2 or 400 - 1 have been appropriate, and in either case does that get added to the mins for the VOR, or only the ILS if the ILS is the approach you plan on using?

I think I was right on this issue, and I hate second guessing myself.....but I just want to be sure.

thanks!
 

WarnerNo13

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I've never heard anything about morse code, they need to be 2 different freqs for each runway.

As for what to add to the mins, I've worked at places where one's DOM said to add the mins to the most restrictive runway, whereas another's language said to use the expected runway.

Not knowing the mins for MDT, I would've added 400-1.
 

AntiJedi

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Simply put, you are right, and he was not.

You are correct about the different idents. They are two different transmitters, and the tower can switch between the two on the fly. Two seperate approaches, to two different runways---even if they are the same piece of concrete.

As far as the alternate mins, you would have been fine using 1-nav or 2-nav for either of the ILS.
 

bgaviator

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Warner is saying they have to be two different freqs to be considered separate, and AntiJedi is saying they are two different transmitters, though they share the same freq.

The derived mins chart makes the choice between 1 or 2 "operational navigation facilities."........So in this case where there is just 1 freq, but 2 ILS from opposite ends of the runway, with their own unique morse code ident.....is that considered 1 or 2 "operational navigation facilities"?

We need a tie breaker
 

WarnerNo13

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Two different freqs...the pilots don't use morse code to fly the approach, they use the ILS. It's the whole purpose to allow lower mins for 2 navaids. Two different freqs gives the pilots two different approaches ie more options therefore the lower mins.
 

AntiJedi

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Two different freqs...the pilots don't use morse code to fly the approach, they use the ILS.

You've never flown a plane before, have you? Pilots must IDENTIFY nav sources before they use them, On top of that, they must KEEP identifying that nav source to ensure that they are still receiving that nav. The more advanced aircraft make it easy by putting the identifier right on the HSI, but older, and simpler, aircraft must identify via the morse code.
 

WalterSobchak

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Warner is saying they have to be two different freqs to be considered separate, and AntiJedi is saying they are two different transmitters, though they share the same freq.

The derived mins chart makes the choice between 1 or 2 "operational navigation facilities."........So in this case where there is just 1 freq, but 2 ILS from opposite ends of the runway, with their own unique morse code ident.....is that considered 1 or 2 "operational navigation facilities"?

We need a tie breaker
As long as the navaids have separate identifiers, which is what I think you mean when you refer to the "morse code", you're all good for use of the two navaid rule. In this case, the localizers at KMDT are I-HQA and I-MDT, making them two separate navaids.

Consider your tie broken...

Just an experienced sugguestion: in general, the two navaid rule should be used as a very, very last resort. If you can't get by with adding (400-1), search for another alternate until you've exhausted all of your other options.
 

WarnerNo13

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You've never flown a plane before, have you? Pilots must IDENTIFY nav sources before they use them, On top of that, they must KEEP identifying that nav source to ensure that they are still receiving that nav. The more advanced aircraft make it easy by putting the identifier right on the HSI, but older, and simpler, aircraft must identify via the morse code.

Easy there champ, we're talking about a 121 carrier, not grandpa's old duster.
 

homerjdispatch

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Sorry Warner, you should know better... Remember SAN? I know its pounded into people at OO....It used to have two seperate idents with the same freq. It was counted as two seperate navaids, just as this example in MDT states... Two separate idents, same freq. two navaid rule....
 

bgaviator

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But there are two different approaches.....ILS 13 and ILS 31.....they just happen to use the same freq of 110.9.......there are two different Morse code idents so the pilots can determine which ILS approach they are on.....so I think it all comes down to the definition of "operational navigation facilities." Is the unit that broadcasts the ILS 1 or 2 "facilities"? AntiJedi says they are two different transmitters that share the same freq, but can be switched by the tower......

oh well....not that big of a deal....I'm not going to beat myself up over it.....delay still goes on the crew in my opinion :)
 

OneBadLT123

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I know I am late to the party, but yes 2 different idents are considered two different approaches.
 

dispatchguy

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From the FAA 8900.1

Question: “Does the FAA consider an ILS facility that contains a single transmitter frequency for an ILS, but with two different ILS identifications (depending on which runway is used), as one or two navigational facilities?”

1) The words “two operational facilities” mean that in the event there is a single failure of one facility, the other would be operational. In the situation where both instrument landing system (ILS) facilities share a single transmitter, they are “one operational navigational facility” because both ILSs would become inoperative in the event of a single transmitter failure.


2) The two ILS identifiers would have to be different even though the ILS transmitter frequency is the same for both. The instrument approach charts indicate to the pilot whether there is one frequency or two. Thus, one or two navigational facilities.

Two idents, two approaches - the pilot was wrong (I love to say that)
 
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