Weather mins
02-11-2007, 05:00 AM
#1
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Weather mins
Can someone in the 121 or 135 world help me (USN type) understand the rules about IFR takeoff mins, takeoff alternates and/or departure alternates. I think I understand the rules about destination alternate if weather is less than 2K and 3 then the airport needs to have 600-2 for precision and 800-2 for non-precision. The stuff that keeps coming back from internet searches says that it is all dependant on the Airlines Operating Specifications. For takeoff it mentions about having 1/2 mile for more than 2 engines and 1 mile vis for airplanes with 2 engines or less. I am trying to find out how you would look at a TAF/METAR and determine (other than being able to pick out the ceiling and vis) if you need a takeoff alternate and if you have the takeoff mins without knowing the Operating Specs and what type of airplane your are flying. Any info would be much appreciated.
02-11-2007, 06:36 AM
#2
Chief Jeppesen Updater
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Takeoff minimums are going to be directed by each company's individual ops specs. Standard takeoff minimums are 1sm visibility (5000 RVR) for two engines or less, and 1/2sm vis (2400 RVR) for three or more engines. Below that, each individual carrier must apply for and be approved for lower departure conditions.
As a general rule, the lowest that most carriers can depart is usually 600/600/600 (RVR), provided that you have the required visual and lighting cues (runway centerline lights and markings). As stated before, this will vary depending on each company and the authorizations that they have. FAA ops specs C056 and C078 cover the applicable requirements. This section of each company's ops specs will spell out the various conditions (weather and available visual cues and equipment) necessary for lower minimums.
A takeoff alternate is required any time that the weather at the departing airport is below the minimums for a suitable approach that can be used to return to the airport in the event of an emergency (based on current conditions).
As a general rule, the lowest that most carriers can depart is usually 600/600/600 (RVR), provided that you have the required visual and lighting cues (runway centerline lights and markings). As stated before, this will vary depending on each company and the authorizations that they have. FAA ops specs C056 and C078 cover the applicable requirements. This section of each company's ops specs will spell out the various conditions (weather and available visual cues and equipment) necessary for lower minimums.
A takeoff alternate is required any time that the weather at the departing airport is below the minimums for a suitable approach that can be used to return to the airport in the event of an emergency (based on current conditions).
§ 121.617 Alternate airport for departure.
(a) If the weather conditions at the airport of takeoff are below the landing minimums in the certificate holder's operations specifications for that airport, no person may dispatch or release an aircraft from that airport unless the dispatch or flight release specifies an alternate airport located within the following distances from the airport of takeoff:
(1) Aircraft having two engines. Not more than one hour from the departure airport at normal cruising speed in still air with one engine inoperative.
(2) Aircraft having three or more engines. Not more than two hours from the departure airport at normal cruising speed in still air with one engine inoperative.
(b) For the purpose of paragraph (a) of this section, the alternate airport weather conditions must meet the requirements of the certificate holder's operations specifications.
(c) No person may dispatch or release an aircraft from an airport unless he lists each required alternate airport in the dispatch or flight release.
(a) If the weather conditions at the airport of takeoff are below the landing minimums in the certificate holder's operations specifications for that airport, no person may dispatch or release an aircraft from that airport unless the dispatch or flight release specifies an alternate airport located within the following distances from the airport of takeoff:
(1) Aircraft having two engines. Not more than one hour from the departure airport at normal cruising speed in still air with one engine inoperative.
(2) Aircraft having three or more engines. Not more than two hours from the departure airport at normal cruising speed in still air with one engine inoperative.
(b) For the purpose of paragraph (a) of this section, the alternate airport weather conditions must meet the requirements of the certificate holder's operations specifications.
(c) No person may dispatch or release an aircraft from an airport unless he lists each required alternate airport in the dispatch or flight release.
Last edited by FlyerJosh; 02-11-2007 at 06:42 AM.
02-11-2007, 06:52 AM
#3
Chief Jeppesen Updater
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Arrival alternate weather criteria is also much more complex than the basic part 91 criteria. The standard to determine if an alternate airport is required is the same (within 1 hour of ETA, weather forecast better than 3SM/2000' cig).
However determining if an airport is a legal alternate is much more complex. It's based on a series of criteria that include the forecast weather, types of approaches available, number of runways and approaches available. This was called the "method 1/method 2" standard at my previous employer.
Method 1 (basic rundown):
For airports that have at least one approach to a suitable runway (based on winds/weather/notams/notes), the weather must be forecast (at the time of arrival) to be at least 400' and 1sm greater than the published CAT1 minimums for the approach to be flown.
Method 2:
For airports that have at least two different suitable approaches (using different navaids) to two different suitable runways (based on winds/weather/notams/notes), the weather must be forecast (at the time of arrival) to be at least 200' and 1/2sm greater than the published straight in CAT1 minimums for the approaches to be flown.
Confused yet? Probably. But don't worry too much about it. Just remember that if the weather is forecast lower than 3sm and 2000' within one hour of the ETA at the primary destination, then an alternate airport is required.
However determining if an airport is a legal alternate is much more complex. It's based on a series of criteria that include the forecast weather, types of approaches available, number of runways and approaches available. This was called the "method 1/method 2" standard at my previous employer.
Method 1 (basic rundown):
For airports that have at least one approach to a suitable runway (based on winds/weather/notams/notes), the weather must be forecast (at the time of arrival) to be at least 400' and 1sm greater than the published CAT1 minimums for the approach to be flown.
Method 2:
For airports that have at least two different suitable approaches (using different navaids) to two different suitable runways (based on winds/weather/notams/notes), the weather must be forecast (at the time of arrival) to be at least 200' and 1/2sm greater than the published straight in CAT1 minimums for the approaches to be flown.
Confused yet? Probably. But don't worry too much about it. Just remember that if the weather is forecast lower than 3sm and 2000' within one hour of the ETA at the primary destination, then an alternate airport is required.
02-11-2007, 05:37 PM
#5
Chief Jeppesen Updater
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Originally Posted by dbtownley
FlyerJosh,
Thanks for the help. I had never seen or heard of that "method 1/method 2". This is going in my study/gouge pack.
Thanks for the help. I had never seen or heard of that "method 1/method 2". This is going in my study/gouge pack.
I wouldn't worry too much about it. Most airlines aren't going to expect you to know it if you aren't working in an environment where such a method is required. Besides, they will certainly teach you the applicable parts of the op specs and company specific 121 regs when you get hired.
Know when you need an alternate and a takeoff alternate. I'd also know the standard departure minima since that is also in the regulation. Beyond that, whatever you learn is gravy. I'd focus more on the stuff that is general ATP type knowledge.
~J
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