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Destination Alternate mins- FAR 121.631

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Destination Alternate mins- FAR 121.631

Old 1st Nov 2010, 13:15
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Destination Alternate mins- FAR 121.631

I know I've posted this subject in the past but still looking for some clarifications..

FAR 121.631 states amongst other things that:

"No person may allow a flight to continue to an airport to which it has been dispatched or released unless the weather conditions at an alternate airport that was specified in the dispatch or flight release are forecast to be at or above the alternate minimums specified in the operations specifications for that airport at the time the aircraft would arrive at the alternate airport."

Now, speaking about alternate minimums: I know that once airborne, one can use his alternate down to minimums and doesn't need to have it above the alternate minimums (FAR 400-1/ 200-0.5 1 Navaid 2 Navaid rule).It's also written above. So, how come after taking off it changes?

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Old 1st Nov 2010, 14:10
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From a regulatory standpoint, the threshold is not really "once airborne" at which you can use the published landing minimums, but when you actually divert to the alternate. At that point, it becomes your destination, so published minimums can be used. Before that point, the alternate forecast must remain within the criteria for use as an alternate.

From a probability standpoint, I believe the idea is to account for forecast errors within a pretty wide range of time, i.e., takeoff, en route, arrival, vectoring, miss, holding, decision, etc., while always ensuring a higher than normal probability that a legal landing can be made at the alternate, since it is assumed that no alternative beyond that exists. Once you commit to it as your destination, the time frame narrows considerably (hopefully, although in some parts of the world this is another discussion!), and the probability of a legal landing being precluded by a weather change diminishes quite a bit.
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Old 1st Nov 2010, 14:19
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Now, speaking about alternate minimums: I know that once airborne, one can use his alternate down to minimums and doesn't need to have it above the alternate minimums (FAR 400-1/ 200-0.5 1 Navaid 2 Navaid rule).It's also written above. So, how come after taking off it changes?
The weather at your alternate has to continue to be forecast to remain at, or above, alternate minimums set forth in your ops specs, at all times during the flight. Once arriving at the alternate you may used the lowest published minimums.
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Old 1st Nov 2010, 17:19
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Let's see if I got it right:
It has to stay at or ABOVE alternate weather mins (forecast and/or actual) up till the point I divert there.At that time, it's my (new) destination and I continue treating it as one.Meaning, can fly it down to published minimums.

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Old 1st Nov 2010, 19:40
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600-2, 800-2, 0ne hour before, 200' & 1/2, 400' abv mins, 2 aprch ends - - Oh my


In a generalized big picture view the regulations are trying to avoid a situation where you and I arrive after a flight and cannot land because the weather hads deteriorated and we run out of options and fuel at the same time.
To help preclude that unhappy state we have weather minimums that need to be met. We have destination, alternate and sometimes takeoff alternate minimums. [For the sake of this discussion the example under consideration is a passenger carrying Part 121 flight {Part 91 and flight into uncontrolled airspace is ignored.}

The alternate minimums have to be much “better” (greater visibility and sometimes ceilings as well) than your destination minimums. This is because we do not want to get into a position where we are unable to land at either the destination or alternate. So, to keep this from happening the alternate minimums are set higher. The system is designed to give you and me (and the paying passengers) a safety net.

During the preflight planning or the actual enroute phase your alternate minimums must meet and continue to meet the stated regulatory minimums for alternates. If during flight the forecast for the alternate changes and the alternate no longer meets the required alternate minimums, your dispatcher should/must find you a new alternate within the confines of your actual fuel load. If you don’t have a dispatcher then the entire requirement to monitor your weather at your alternate falls upon you. Remember you want to give yourself a margin of safety and somewhere to go.

If you cannot land at your destination because of poor weather then you proceed to your alternate. (You are not obligated to go to your filed alternate) The intention is to be able to get the airplane safely on the ground. Your alternate now becomes your “new” destination. When you get to the alternate airport the minimums published on the approach chart apply for the approach. Remember the intention is to land the airplane not to force some new artificial higher weather requirement on the airport.

Of course if you arrive at your alternate and the actual weather is below published approach minimums (that should not happen with accurate weather forecasting, tracking, in flight monitoring and proper dispatching) and you are limited on fuel; now you have another problem and it is time to use your emergency authority determine the safest course of action and do whatever you need to and get the airplane safely on a runway before fuel exhaustion.

I hope that is helpful.


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Old 1st Nov 2010, 19:45
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Completely NothBeach,

Thanks for the great explanation and the other ones as well.
It's all straighten out now.

Fly Safe!!
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Old 1st Nov 2010, 23:39
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Interesting variations on individual operators' Ops Manuals obviously exist here.

For example - I may operate my aircraft (including to the United States) without the restriction of needing my alternate's forecast (or actual) to remain at or above alternate planning minima once dispatched (I did need this prior to dispatch - at planning).

I can continue provided I have two airports within range (i.e. with 30' fuel remaining at touchdown) and landing is "assured" (i.e. it could be completed taking into account any forecast Wx deterioration and a plausible "single" failure or equipment such as Cat 3a to Cat 2). Makes sense that one of these two is the planned destination, obviously..., but it doesn't have to be.

So I can continue toward JFK even if it is 0/0, if (say) EWR and PHL have Cat II Wx with Cat IIIa available. As soon as landing at either of these two would involve 30' fuel on landing - then I'd have to go land, and land with >=30'. Not saying that's what I'd do (each real life scenario is different); but that's the limit allowable by my OM.

Not quite the same as requiring alternate minima at an alternate - and all FAA approved as an international operation foreign to the US.

Do US operators require their planning minima to remain at the alternate (FAA planning minima are defined slightly differently to those over here - though they achieve the same thing: a more conservative forecast required for alternates) thoughout flight? Even to a country whose national rules don't requite this (such as mine)?
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Old 2nd Nov 2010, 00:29
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The US rules normally require the US carriers to follow whichever is more restrictive, their rules or the local rules.

So yes, the US 121 rules do not allow us to continue to our destination if the alternate is forecast to be below ALTERNATE minimums at our time of arrival.
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Old 2nd Nov 2010, 00:55
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Just a historical note, the basis for this rule and much of FAR 121 came out of a TWA DC-2 accident in 1935 that killed a senator. The Copeland Committe was formed by the US Senate to investigate air safety, recommendations included forming a separate agency to handle air safety matters, research on pilot fatigue and numerous rule making. The TWA accident was caused on part by rapidly changing weather that closed the destination (KMKC) and the alternate that also closed, the plane crashed in fog out of fuel.

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Old 2nd Nov 2010, 11:22
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That's an interesting thing to know....
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Old 12th Dec 2019, 20:01
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First thing we consider during planning is legality.
During the planning stage, we consider the worst case. Hence the 123 and 1 NAV AID, 2NAV AID rules.
But as soon as you divert, that Alternate, becomes your new destination, hence the Destination minimums applies.
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