121 Alternates --

[BoDEAN]

Trying to read up and learn the 121 alternates, and when they are needed.

In 121.619 it states :: When the weather conditions forecast for the destination and first alternate airport are marginal at least one additional alternate must be designated: However, no alternate airport is required if (123 rule).


Does this say, that if the weather is marginal, but above 2000 ft ceiling, 3 miles vis, that an alternate is required? Seems to conflict with eachother.

I am preparing for an AE interview, and I know they ask about 121 alternates, and when an alternate is needed.

 

[Andy Neill]

First, differentiate between alternate and additional alternate. An alternate is always required UNLESS the 1-2-3 rule is met. If the weather forecasts at the destination AND the alternate are marginal, an additional alternate is required. I have heard different rules among airlines as to what constitues "marginal".

 

[Atccfi]

I have heard different rules among airlines as to what constitues "marginal".

Would that be defined in the ops specs...?

 

[Andy Neill]

Not necessarily.

 

[MaceyHold]

An alternate is required when the forecast +/- 1 hour from ETA is LESS THAN 2000' ceiling, and/or LESS THAN 3SM vis. This really has nothing to do with marginal.

The term "Marginal" is based on approach mins for the destination airport, and alternate mins for the alternate airport.

So, if the destination has a usable ILS with mins 200' 1/2sm, and the forecast/metar are showing better than 200' and 1/2, then it is not considered marginal.
If the destination does have either 200' or 1/2sm, then that IS marginal, and you have to check to see if your alternate is also marginal.

Assuming the carrier in question uses standard ops specs, your alternate is derived by adding 200' and 1/2sm to the highest mins if using the two navaid method, or adding 400' and 1sm using the 1 navaid method.
If the ceiling and/or visibility forecast/metar equals the alt mins, then that is also marginal...in which case a second alternate needs to be planned.

I have worked for two airlines, and this is how marginal was defined for both of them. I have never seen an FAA definition of marginal.

 

[Andy Neill]

That is not the standard at another airline.

Using the published minima from your example the destination would be marginal if the forecast was 100 1/2 or 200 1/2 or 500 1/2 but would be above minimums (not marginal) if the forecast at the destination was 100 3/4 or 200 3/4 or 500 3/4.

The first alternate would be marginal only if the forecast was 400 1 but not 500 1 or 400 1 1/4.

 

[Atccfi]

Not necessarily.

Thanks for clearing that up.

 

[atldc9]

3 airlines, 3 different definitions of marginal. Marginal is left up to the individual operator to define, with POI approval.

 

[AZaviator]

3 airlines, 3 different definitions of marginal. Marginal is left up to the individual operator to define, with POI approval.

That is correct. The main thing the interviewer wants to hear is the 1,2,3 rule. I wouldn't add any more unless they specifically ask for it. I've interviewed at several airlines over the years including Eagle and they all were happy with the "1,2,3 rule" response.

 

[172driver]

Marginal at Comair is defined as below 600' celings OR 2SM vis at ETA.