Drift Down Alternate

[Whiskey Tango]

A friend of mine had an interview and they asked him the maximum distance the drift down alternate can be from the course (i guess) for method 2. I don't know of any max and i could not find one. Does anybody know the answer to this question or what the story is?

 

[mar]

More info please

To be perfectly frank, I'm not sure I get your question.

I think you're talking about an enroute alternate.

The term 'drift down' refers to the *altitude* one will maintain after the loss of an engine. In other words, the 'service ceiling' for a given condition (i.e. one engine, two engines or three engines).

Under Part 121, if you lose an engine, you must divert to the nearest suitable airport anyway regardless of what altitude you can maintain.

In mountainous terrain your operations may require the designation of a specific airport as an enroute alternate (as opposed to the destination alternate).

In these cases I'm not aware of any max distance and I'm not familiar with the term "Method 2".

I don't think you're asking about ETOPs (Extended Twin-engine Operation Procedures) but just for your information there are certain limitations on being so far from enroute alternates BUT they're based on time.

I'm sure you'll get a better answer if you can help clarify what you're searching for.

 

[Singlecoil]

Mar, I guess you 4 engine boys don't worry about this one, but method one is the zero net gradient method of enroute planning, method two is driftdown. Meaning your enroute course must be such that you can maintain 1000 feet above the terrain on one engine for the entire enroute portion (method 1), or maintain 2000 feet above the highest obstacle within 5(?) sm of your course utilizing a decision point where you will either turn around or continue to some alternate airport (method 2). Method 2 is used when you have one unusually high obstacle (Mt. Shasta) on your course. Rather than having to limit your takeoff weight just because of one bump in the road, you can plan to have a decision point (right near the mountain) where you will simply turn around and go somewhere else if you lose one. It is called driftdown, because you are allowed to account for fuel burn (weight loss) and time to get down to your max single engine altitude. It should be noted that the method 1 altitude isn't single engine service ceiling. That would be the zero gross altitude, the zero net altitude takes into account a safety margin which is 3500 feet lower in my twin jet.

I too am not aware of any lateral requirement, but maybe there is a one hour rule or other time requirement?

 

[mar]

Awesome!

Did you just type that off the top of your head?

Thanks for clearing that up.

 

[Singlecoil]

Actually I did. Then I almost deleted it becaused it was confusing to me and I could just imagine how it would look to a reader. I guess I'm glad I didn't. Congrats on walking through the schoolhouse exit door, by the way! I'm going to be going through the entrance door for a two month stint sometime this winter. Not sure when exactly. Hopefully I'll get Xmas off out of the deal.

 

[minitour]

uh....is this something that I'll pick up....

...or should I know this by now??

Seriously,
Some of the stuff I hear being asked on interviews....please tell me its stuff they (someone) teach you and I don't just have to know it like...now...

-mini

 

[Whiskey Tango]

Singlecoil I certainly could not have explained it any better.

As to the interview stuff, I want to say that this was an interview at ASA but that might not be right. My friend had previous 121 time and is the type of guy that really knows his stuff. I believe that the interviewer told him 130 nm. I don't know if that is in someone’s ops specs, this guy was just making it up to see how my friend would deal with it or if this guy has no idea what he is talking about. These are the only possibilities I could come up with.

 

[mar]

Upgrade *already*?

I'm going to be going through the entrance door for a two month stint sometime this winter.

Or jumping ship to the Boeing? Either way, I suppose wintertime is a good time to be in the schoolhouse. I was a little disappointed to spend half my summer in a classroom but I digress...

In fact I never should've responded to this post as I obviously don't know what the hell I'm talking about. I just happen to fancy myself as a little bit of a performance nerd but as you point out, this is something that I just don't use.

And maybe that'll answer minitour's question <<...is this something that I'll pick up...?>>

The answer is a 'definite maybe'. It depends on your background I guess. And that's why I maintain the best pilots come from diverse operational backgrounds.

Singlecoil for example is an authority not just on the Stratocaster. He's flown the bush, turboprops and jets.

 

[Andy Neill]

A friend of mine had an interview and they asked him the maximum distance the drift down alternate can be from the course (i guess) for method 2. I don't know of any max and i could not find one. Does anybody know the answer to this question or what the story is?

In the for what it's worth category, Aerodata uses 130 miles as the max distance off route that the drift down alternate will be. As was previously stated, your takeoff weight will guarantee that you clear all obstacles by 2000' after an engine failure at cruise altitude as you descend to your single engine altitude along your planned route of flight. Further, it provides the same obstacle clearance from your planned route of flight at the point abeam of your divert airport to that airport.

With that in mind, as was previously stated, you would still have the reponsibility to land at the nearest suitable airport in point of time after the engine failure.

 

[146guy]

yep 130 miles at a right angle from airway or route to airport