IFR Alternate Minimums

[MooseTrax]

I am trying to get an answer on the purpose of alternate minimums. FAR 91 says that inlieu of alternate minimums they shall be 600-2 precision and 800-2 non-precision. The first question is why higher alternate minimums than these and also why at a field with both precision and non-precision approaches, the ILS has higher alternate minimums than the VOR or TACAN to a different runway. Thanks.

 

[avbug]

The idea is to have a viable option to land. If you've provided legal IFR reserves have already missed at your destination and have arrived at your alternate, you need to be able to get in. That's all.

 

[MooseTrax]

Avbug,

I understand that fact, my question is why would an ILS at airport X have an alternate minimum of 900-2 to runway 17 and the VOR at airport X 800-2 for runway 29. It just doesn't make sense to me how a precision approach has a higher alternate minimum than a non-precision approach. I imagine it has to be something in the mystical TERPS criteria. Just trying to find out what it is. Thanks.

 

[midlifeflyer]

Moose,

Take a look at a chart where this is the case. Chances are there's a clue to the reason. Take a look at such things as the circling minimums, not just the straight-in ones.

 

[Swerpipe]

Another factor in alternate mins

Avbug,

I understand that fact, my question is why would an ILS at airport X have an alternate minimum of 900-2 to runway 17 and the VOR at airport X 800-2 for runway 29. It just doesn't make sense to me how a precision approach has a higher alternate minimum than a non-precision approach. I imagine it has to be something in the mystical TERPS criteria. Just trying to find out what it is. Thanks.

Hey Moose,

The alternate minimums are affected by 3 other factors:

1)Terrain around the applicable airport which varies between airports
2)1 or more independent NAV facilities at the airport and whether or not they are available to different suitable runways (runway 12 and 30 are not different suitable runways in this example).
3)Some airports are not legal to designate as alternates period even when they have ILS approaches due to the unavailability of altimeter settings or the monitoring of its nav facitlities.

 

[JAFI]

Moose,

Dont forget terrain. I know an ILS that has a DH of 3000 feet AGL because it is in a box canyon and the end of the runway is a 2000 foot cliff stright up. There is a missed approach but if you do the missed at 200 agl you will eat rock in about 10 seconds. Unless you can fly stright up.

JAFI

 

[MooseTrax]

Thanks for all the replies!

 

[dispatchguy]

Jafi

Is that particular airport even approved as an alternate, or does the back of the Jepp plate say "NA"

There is also a proposal, for those under FAA OpSpecs, to amend the OpSpecs for 121/135 operators to also allow a Charted Visual Flight Procedure to be considered for alternate minimums; I think the additive is 500' and a mile.

But, its only a proposal; you can find it at www.opspecs.com