Sliding scale alternate weather

[lostaviator]

Hoping for a little discussion here on the sliding scale options for standard alternate minima (600 and 800).

What is the logic behind this?

My point of view... If I slide my 800-2 to 1000-1 and am traveling at 140 knots on approach, I am going 2.3 knots/min, from the time I see the runway at 1 mile, I have 26 seconds until wheels down. So my descent rate is going to be approx 2300 FPM which puts me out of my SOPS of a stabilized approach.

Thoughts?

[AOW]

The forecast wx has to be 1000/1 or better, not the mins for the approach. In order for the sliding scale to apply, the approach must meet standard alternate minima, that is 300/1 above the published mins is less than or equal to 800/2. Therefore, the highest mins that the approach could have is 500/1. (520/1 technically). However, you'll find that most approaches with an HAT of 520' will have a recommended vis of 11/2 miles, so they won't meet the standard minima. So most likely, the approach has mins in the 350'-400' range, making a descent from 1 mile final perfectly doable in a stabilized approach.

[PositiveRate27]

I'll also throw in that just because you can legally do something, it doesn't automatically make it safe, or a wise decision. From my experience dispatch doesn't always clue in to that notion. The flight crew has the final say and bears the responsibility of the results.


PR

[AOW]

I'll also throw in that just because you can legally do something, it doesn't automatically make it safe, or a wise decision. From my experience dispatch doesn't always clue in to that notion. The flight crew has the final say and bears the responsibility of the results.


PR

+1

[Big Pistons Forever]

Personally I will take vis over ceiling any day, particularly on the non precision approach. When the flight schools trot out the toughy trivia question where the sliding scale magically makes the alternate legal in the example question I always push back with "yah but what is the weather phenomenon that is making the vis so low". In my experience when the vis drops down to a mile it more often then not keeps deteriorating, regardless of what the TAF says.

Choosing alternates is one of the many areas where you want to be taught by somebody who is real world IFR smart, not just IFR book smart.

[PositiveRate27]

+1, BPF, That is my point exactly. Last month during the 2 weeks of epic fog we had out west we were given an alternate which I really didn't agree with. The airport in question had been subject to approach bans all week, and suddenly the TAF offered the opportunity to use it as an alternate for about a 3 hour window before it dropped down again. Despite indicating my objection to both captain and dispatch, based on my gut feeling the window of legal weather wasn't going to materialize, we departed with it as our alternate. Sure enough the weather never improved at that airport and it was still under an approach ban at the time we may have needed it. We also didn't have enough go juice to get us anywhere else.

We never broke any laws, and the flight was carried out in accordance to the CARs. It was a great lesson that legal doesn't guarantee safe or smart. Always remember that dispatchers are typically not pilots. They dont see what's out the window of the cockpit. All they have are prognostic charts, a book of legal limits and ops managers breathing down their necks trying to keep everything on schedule. I will most certainly raise a stronger objection in the future.

PR

[pdw]

Every dispatch would have to understand basics of how approach limitations work in order to be eligible for a job that issues directives to a flight crew.

Wouldn't that be maintaining a great safety-risk otherwise ?

ie: Is it even still allowed for an individual who is void of this simple WX related approach/departure knowledge to interact with a flight crew/pilot over whom they're given a management responsibility ?

[lostaviator]

The forecast wx has to be 1000/1 or better, not the mins for the approach. In order for the sliding scale to apply, the approach must meet standard alternate minima, that is 300/1 above the published mins is less than or equal to 800/2. Therefore, the highest mins that the approach could have is 500/1. (520/1 technically). However, you'll find that most approaches with an HAT of 520' will have a recommended vis of 11/2 miles, so they won't meet the standard minima. So most likely, the approach has mins in the 350'-400' range, making a descent from 1 mile final perfectly doable in a stabilized approach.

Right.. Complete memory fart there. Doing the math with 300+1 makes sense.

And excellent points on using our knowledge to make a decision. I have seen a lot of 1 miles in the TAF lately and have actually ended up in a no movement situation and way worse than forecast (thanks RVOP).

If I have a 100% chance of arriving at my destination given the weather I am comfortable using the "legal" minima. However personally, I think if you start digging for a way to make an alternate work (sliding vis and ceiling) and there is a chance you might actually need to go there... I'll start looking for something else. Granted... I guess landing at your destination is never 100% (incident at airport, loss of navigation equipment power, etc).

[matrix]

As a dispatcher for 10 years of TypeA/B systems, I can assure you that pre-planning an alternate based on sliding scale is almost never done. I don't think I've ever needed to do it. The only time this is used is if enroute, your alternate wx changes and you may need to use the sliding scale to keep your alternate legal. This is done with the utmost caution (high level of confidence that you will land at your destination).

[spaner]

IFR alternate legality is not a requirement of "enroute". Only of "departure".
As stated in the other thread, I always have two,

I have the destination, which the WX I don't really care about. I'm going to go anyway.
The legal alternate, and perhaps a departure alternate (out of landing minima),
meets the requirements as laid out by the regs; I may even make use of the sliding scale. So that it's legal, so that we are legal, so that we can go.

Then, I also have the plan B. Making use of the extra 45 minutes; I usually choose this on the other side of the front, that is generating the system, and I like it when it reads "160/60".

On the missed, I may just go direct to plan B. Depends on the actuals.
FOG, is a tough play though. Plan B had better be in the clear, or it's a no-go.

Approach ban? What's your Ops spec?
If you're talking below commercial spec, or YYZ, YHM center-line...

Why are you even going?

[frozen solid]

Every dispatch would have to understand basics of how approach limitations work in order to be eligible for a job that issues directives to a flight crew.

Wouldn't that be maintaining a great safety-risk otherwise ?

ie: Is it even still allowed for an individual who is void of this simple WX related approach/departure knowledge to interact with a flight crew/pilot over whom they're given a management responsibility ?

Dispatchers don't tend to have management authority over flight crew. They're there to help plan the flight. Genuine licensed dispatchers do know about approach limitations. The decision to dispatch the flight or not rests with the pilot or dispatcher who has the "most conservative" opinion. That is, they both have to approve the flight plan. If either one decides the flight's not going, it ain't going. Neither has authority over the other. Even if the dispatcher wasn't sure about approach weather considerations, the pilots would be. They work together. Dispatchers don't march the crew out to the plane at gun-point. The "safety-risk" still rests with the captain.

[matrix]

IFR alternate legality is not a requirement of "enroute". Only of "departure".

In my case, our COM states we should attempt to maintain validity of the alternate, so not a CARS requirement, but a company one.

[Big Pistons Forever]

I teach that anyone flying IFR should plan on having up to 4 different alternate airports.

1) The legal alternate: That is the one that meets all the legal requirements

2) The commercial alternate: That is where you want to go if you miss at your planned destination because it is the cheapest and/or most convenient place to end up

3) The gold plated alternate: This is reserved for those really crap days with wide spread bad weather and a not particularly warm fuzzy feeling about the weather forecasts and is some place you are confident that you can squeeze into when the weather really craters.

4) The enroute emergency alternate. This is the place (usually the nearest airport to your present position) where you are going to go "if the bad thing happens". This will obviously not be pre-planned but is a natural extension of good situational awareness.

No 1 and 2 are often different and are where the real world renders the pedantry of those "gotcha" alternate questions so favoured by TC and the flight schools, irrelevant.

No 3 can only be developed from hard won experience and local knowledge. Crap weather that is foretasted to improve is a good example. Yes it becomes a legal alternate if the weather at the end of the becoming period is above alternate mins, but by what mechanism is the weather going to get better ? Poor vis due to fog is a good example where the "getting better" time seems to mostly generated by the computer random number generator and wishful thinking on the part of the forecaster. For new guys, this is a good time to talk to the FSS guys in person (Oh how I miss real weathermen at the old airport weather stations ) and get a feel for what is making the weather and then deciding how far I am going to trust the TAF.

[Schteevie]

glad I found this thread.
about to hit the sack though...
- will review tomorrow - this is something I am having trouble understanding.

[CallMeMister]

Every dispatch would have to understand basics of how approach limitations work in order to be eligible for a job that issues directives to a flight crew.

Wouldn't that be maintaining a great safety-risk otherwise ?

ie: Is it even still allowed for an individual who is void of this simple WX related approach/departure knowledge to interact with a flight crew/pilot over whom they're given a management responsibility ?

Dispatchers don't tend to have management authority over flight crew. They're there to help plan the flight. Genuine licensed dispatchers do know about approach limitations. The decision to dispatch the flight or not rests with the pilot or dispatcher who has the "most conservative" opinion. That is, they both have to approve the flight plan. If either one decides the flight's not going, it ain't going. Neither has authority over the other. Even if the dispatcher wasn't sure about approach weather considerations, the pilots would be. They work together. Dispatchers don't march the crew out to the plane at gun-point. The "safety-risk" still rests with the captain.

Amen.