FAR 121, Subpart U Regulations 121.619a: Alternates: “Marginal”

Regarding the filing requirement of a 2nd (or additional) alternate airport for the destination, what constitutes ‘marginal weather conditions?’

So, we’ve posed this question in the past and I’ve decided to dissect this regulation line-by-line, as needed, over a series of articles. For this 1st article on 121.619, we’ll discuss the term ‘marginal’ only, as seen in 121.619(a), then we’ll move on to bigger and better things within 121.619 and other 121 – Subpart U regulations. These discussions and articles will focus more on regulations and only on OPSPECs’ exemptions, if needed.

Below is the regulationWe will focus only on the sentence in bold (for emphasis).

121.619 Alternate airport for destination: IFR: Domestic operations.

(a) No person may dispatch an airplane under IFR or over-the-top unless he lists at least one alternate airport for each destination airport in the dispatch release. 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 for at least 1 hour before and 1 hour after the estimated time of arrival at the destination airport the appropriate weather reports or forecasts, or any combination of them, indicate—

(1) The ceiling will be at least 2,000 feet above the airport elevation; and

(2) Visibility will be at least 3 miles.

(b) For the purposes of paragraph (a) of this section, the weather conditions at the alternate airport must meet the requirements of §121.625.

(c) No person may dispatch a flight unless he lists each required alternate airport in the dispatch release.

Important notes regarding the highlighted sentence:

  • There is no specified time-period mandated. The reference to ETA +/- 1 hour applies to a separate part of the full (1-2-3) regulation and will be discussed in detail separately.
  • The marginal weather that mandates the filing of a 2nd alternate for destination applies to both the destination and 1st alternate’s forecast, and not METARs or other reported weather sources.
  • There is no 3rd alternate requirement, although the rule allows it (“at least one additional…”) meaning your 2nd alternate can also be ‘marginal.’ Not that you’d want that scenario, especially at multiple alternates in close proximity of each other.
  • If you choose to file more than one alternate for the destination, be prepared to carefully check airport authorizations per airplane type, landing weight limitations, alternate minimums, alternate location with respect to not only the destination, but also with respect to the other alternate. But to also be careful how to apply 121.631b, which I’ve seen misinterpreted many times, and which will be discussed in a separate article. Filing of a 2nd alternate also brings a flawed 121.639b regulation into play. Again, more later.

121.631   Original dispatch or flight release, redispatch or amendment of dispatch or flight release.

(a) A certificate holder may specify any regular, provisional, or refueling airport, authorized for the type of aircraft, as a destination for the purpose of original dispatch or release. 

(b) 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. However, the dispatch or flight release may be amended en route to include any alternate airport that is within the fuel range of the aircraft as specified in §§121.639 through 121.647.

121.639   Fuel supply: All domestic operations.

No person may dispatch or take off an airplane unless it has enough fuel—

(a) To fly to the airport to which it is dispatched; 

(b) Thereafter, to fly to and land at the most distant alternate airport (where required) for the airport to which dispatched; and 

(c) Thereafter, to fly for 45 minutes at normal cruising fuel consumption or, for certificate holders who are authorized to conduct day VFR operations in their operations specifications and who are operating nontransport category airplanes type certificated after December 31, 1964, to fly for 30 minutes at normal cruising fuel consumption for day VFR operations.

Various replies for ‘marginal’ definitions that I’ve heard or read:

“Marginal” means the weather category definition for MVFR. (Sheffield note: we recommend not reading weather category definitions of “MVFR” into this interpretation. ‘Marginal’ is an adjective.)
“Marginal” means IFR weather at destination ETA.
“Marginal” means below C 600 – Vis 2 at destination at ETA.
“Marginal” means below C 800 – Vis 2 at alternate at ETA +/- 1 hour.
“Marginal is at mins or close to mins.” (Which mins? And when? Define ‘close to.’)

Here’s a reasonable set of “marginal” definitions, as permitted per OPSPECS, that were sent to me from one of our ‘Sheffielders’, (esteemed graduates) (Thx, C.A.!) which may not suit every airline:

  • Marginal at Destination: When the forecast destination ceiling (if required) or visibility is at or below Cat I minimums for the intended runway and approach to be used at the ETA. (dispatch can be legal based on SA Cat I, or Cat II or III mins, but if we’re at or below Cat I mins, it is marginal. When no ceiling is required for landing mins – which is most of the time – only the visibility could make the destination marginal.)
  • Marginal at Alternate: When the forecast ceiling or visibility is at alternate mins at the ETA. (One reportable unit above mins for ceiling and visibility is not considered marginal. So, for example, if derived alternate mins are 1 SM and 400′ and the TAF has 1 1/8 SM and 500′, it is not marginal.) also, a very plausible definition for ‘marginal’ at the alternate airport.
  • Destination note: Reported and forecast weather are considered if ETA is within 1 hour of departure. (Point of dispatch is the takeoff roll, so less than 1 hour flying time.) If more than 1 hour flying time, based on forecast only at ETA. (If reported is significantly better than forecast and we believe forecast is incorrect, we can solicit a new forecast from our weather provider.)

Since it could be considered very rare that the alternate’s ETA would be within 1 hour of departure time, it is not far-fetched for an airline to allow the dispatcher, using sound judgement to incorporate reported alternate weather into this equation. But it is important to understand that taking reported trends into account may be smart, it may be within an airline’s OPSPECS, but if these exceptions and special considerations listed above are NOT in OPSPECS, then considering reports, regardless of flight time, is NOT mandatory by regulation. I mention this often in class that priorities need to be established. Wanting (but not requiring) a 2nd alternate to be filed due to a dip in reported weather is NOT above other regulations that apply en route and which are binding and mandatory, as we’ll discuss later regarding 121.631b. Do what you have to do, first. Do what you’d like or prefer to do, thereafter.

I’m only incorporating some OPSPECS into this equation because I’ve seen other alternative definitions that I disagreed with.

Example: saying, “well, if we’re below mins (again, no mention of OPSPECS), then it’s marginal!” I try to play nice and inform them that any claim to require a 2nd alternate for an illegal 1st alternate, and a destination you’re not legal to dispatch to is sort of…..uh…. ________ (you can fill in the blanks.) This statement came from the morphing of various opinions, hearsay, and poor ‘edumication.’

My opinion?

If the destination’s forecast at ETA exceeds the applicable landing minimum and/or the 1st alternate’s forecast at ETA +/- 1 hour is at least 50% greater than the applicable minimums, then no 2nd alternate is required. I’m not a 2-alternate fan in certain conditions and would rather simply pick a better ‘unmarginal’ 1st alternate for simplification. These subjective definitions of ‘marginal’ should always be revisited and adjusted, if needed.

Historical footnote: Decades ago, during the pre-ASD days, with more limited communications, many dispatchers frowned upon multiple alternates because flights would divert and for a time-period the dispatcher did not know the location of his dispatched flight because he/she did not know which alternate was diverted to. That would be a loss of operational control. Fortunately, that should not happen as much nowadays.

It’s ok for the FAA to leave the definition vague, but it is up to the airlines to clearly define what ‘marginal’ is and what the ramifications are once you file 2 alternates. I’ve conducted recurrent training and heard multiple ‘marginal’ interpretations within the same class, some without time frames defined, some without emphasizing the applicable weather: is it reports or forecasts, or combinations, etc..? I’ve heard that ‘you divert to the 1st alternate if you list 2.’ Seriously?! And no…they did not graduate from my school.

Define this term clearly and apply it uniformly in your OCC/NOC, and discuss it again during recurrent training.

Extra food for thought:

One advantage of a 2nd alternate? Less regulatory amendments during inclement weather. We will detail this in a separate article.

Another factor to consider: assume the destination is at landing minimums on a TAF at the ETA – a dispatcher might call it marginal. The alternate forecast near ETA is double the alternate minimums on a TAF – a dispatcher might not call that marginal. So perhaps no 2nd alternate required. But a 2-navaid rule was used to derive alternate minimums, with one approach having an 10-knot tailwind on a runway with standing water over 25% of the length. Now do we have marginal at this alternate?

Should I now consider revising my own opinion of ‘marginal.’ Should your airline do the same or at least discuss it?

More in the next week, hopefully. Thank you for reading this and supporting Sheffield School of Aeronautics! Take care of yourselves and stay safe.

Eric Morris – President – Sheffield School of Aeronautics (est. 1948)

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