1-2-3 rule is +/- 1 hour of ETA if weather reports and forecasts ceiling and visibility is less than 2,000 ft or 3 sm visibility, you require an alternate. METAR trend and TAF are both controlling. Both indicate the possibility of the ceiling and visibility being less than 2,000 ft/3sm. Airlines encourage you to dispatch solely based off the TAF since it tends to be more generous and saves fuel money. METARs are weather reports and by regulation this means the METAR trend is controlling as well though this is mainly important to consider for shorter flights more than longer ones where METAR trend may change considerably.
As a dispatcher, you aren't required to file an alternate on the flight plan. When required or thought needed by the dispatcher or pilot, an alternate is designated or selected on the dispatch release and the computer dispatch system calculates the fuel needed to legally get to that alternate.
In your question 1, one hour before your scheduled arrival the main body before 02z shows BKN010 which would require the alternate.
In question 2, the TEMPO is also controlling. If you were to arrive +/- 1 hour from the start to the end of the TEMPO, it would require an alternate be designated on the release. All phrases are controlling on a TAF.
The difference comes when determining whether an alternate is required versus when a flight is legal to launch for the destination. Landing minimums and alternate minimums are based on ETA whereas requiring one is based on +/- 1 hour. Many airline pilots confuse this and every dispatcher gets phone calls about this from time to time.
For landing minimums, visibility is almost always controlling unless the approach plate calls for a ceiling requirement.
For a takeoff alternate, if the departure station is below landing minimums you need a takeoff alternate that needs to be within 1 hour in still air with one engine inoperative of the departure station. A dispatcher has a high workload. Average of 60 releases or flights to be released and flight followed. Because of this, airlines use mileage for how long that time is that way it makes it easier for the dispatcher. Since takeoff alternate is based on an emergency situation, you can consider your 45 min FAR
reserve as usable and thus for almost all probable circumstances you wont need extra fuel to get to your takeoff alternate. RVR is controlling for takeoff alternate so legally this can be changed straight up until the takeoff roll.
Remember also, the first line of the TAF is not the main body for the whole TAF. This is a common misconception. The main body in the first line is valid as the main body until the next FM
phrase begins at which point that FM
is the new main body. There is no common prevailing conditions as many incorrectly believe.
Conditional phrases are all considered controlling for the purposes of weather decision making. The only caveat to this is if the flight is less than 1 hour flight time, you can launch a flight where the forecast is below minimums based on the METAR. Thus you can have a situation where the TAF requires an alternate and is not legal to launch and the METAR can be above landing and 1-2-3 minimums and you are launching the flight but of course with the required alternate.
If the flight is in the air and the weather drops below 1-2-3 limits, you still need to find an alternate inside the fuel range of the aircraft. If you cannot find a legal alternate within fuel range, you can legally continue if you and the captain agree it would be safe to do so and you have a method of escape should things deteriorate and make landing not legal. If you are in flight and your alternate drops below alternate minimums, you need to find another alternate with fuel range. If this is not possible, you either need to divert for more fuel or the captain can continue based on his emergency authority. If a flight has a TAF the drops below landing minimums in the TAF, the flight can continue to the destination and land if it turns out to be above minimums when he arrives. This is known as having the ability to have a "look and see" before diverting. The FAA does not like to see flights diverting once they are in the air and tries its best to limit times when a flight must divert.
The joint responsibility does not end when a flight blocks out. You are not legal from start to finish. Things can change and your responsibility remains the same. You and the captain MUST agree on the release from the weather requirements to the NOTAMs to the MELs and everything else that needs to be considered and any amendments thereafter.
As a dispatcher, your judgment is also to be used. Your releases are due out 1-3 hours before departure and as much as 4-5 hours before arrival time. Depending on the airline, they may want you to send the releases farther out. Pilots and station agents want the releases as early as possible. The funny part is they all get upset when you need to change the release. Its a bit funny because none of them seem to realize the weather is not a constant variable, fuel is not cheap, MELs get added and subtracted and yet they all expect perfection out of a release sent long before the weather may even be forecast or occurring on radar.
It is interesting that people and the FAA focuses so much on pilot workload and stress but ignore dispatcher workload and stress. The FAA at every airline gets upset and starts threatening violations and licenses when dispatchers fail to respond to ACARS messages or send release amendments to flights diverting to airports not listed on the original or amended release as alternate airports but seems to think 60-130 flights are an acceptable amount for one person to handle. When you have 20-30 flights going to a hub at one time and the airline fuel policy and aircraft weight limits dictate only a few alternates can be used and when it comes time to divert and other places are better suited, you are stuck in bad situation of having 20-30 flights legally needing and wanting something from you but you can only calculate the required fuel and do all the documentation for changing the releases in flight for one flight at one time. Makes for an almost impossible situation.
We as dispatchers legally have joint responsibility for the flights we release and flight follow. Just remember that whenever you dispatch that airline management can fire you, write you up and counsel you on fuel usage but they cannot take your license away from you. If flights need to go out late, they go out late. Protect your license.
I am a current Part 121 for American Eagle at their DFW
[Edited 2013-07-14 17:55:43]