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pcubine

Winds Aloft/Step Climbs

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I think I have run across an issue for which there is no readily apparent solution. I am completing a EDDF-KLAX MD-11 fight. Based upon the strong headwinds in the forecast I entered 300/320/340/360/380 in cruise levels on F-PLN INIT page 1 of 3. Normally I would not enter FL380 but entering an average headwind on the F-PLN INIT showed a climb to FL380 for about 40 minutes before the descent started. I usually don’t use averages but enter the winds by waypoint on the VERT REV page 2 of 2. Now this page has space for four altitudes but I have five between 300 and 380. So on the ground in Frankfurt I enter the climb forecast and the winds by waypoint and altitude up to and including FL340. I will enter the remaining waypoints and altitude during the climb. I enter waypoints, winds thru the VERT REV page for FL360. Now the four altitudes have been used on the VERT REV page and I still must enter 4 waypoints for FL380. I am almost certain that one time in the distant past I entered FL380 over FL300 and the FL380 waypoints were correct and FL300 remained unchanged from what was originally entered. The next time I tried that FL380 was fine but FL300 was screwed up. It’s been so long ago, I don’t remember what was wrong with FL300. So on this flight I thought I would enter FL380 over FL300 and see what happens. The four waypoints for 380 were what I had entered. The waypoints for 300 had changed from individual entries to 299/40 for all the 300 waypoints remaining. Where did the 299/40 come from? I don’t really have a definite idea. Maybe out of thin air or an average of direction/speed for the remaining waypoints at FL300. Who knows? I certainly don’t. Does anyone know. And ideas?
 
Thank you
Michael Cubine

 

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And ideas?

 

Not having the raw data from the flight, I couldn't tell you.  As always, I'll mention that too much data can be a bad thing.  While it's nice to kill time by having something to do at cruise, the cruise winds are seldom exactly as they do on the wind aloft reports anyway (especially because the various tools and models have sampling points that are hundred of miles from each other).  In fact, this is a current discussion within the airlines and the FAA.  The wind data, while it's nice, is very sparse and often not as accurate as it should be.  As such, there's discussion in using the aircraft as sensors, and somehow sending this from the aircraft to a central FAA location, for use by NAS users.  Even the real world is looking for better data.

 

As for the issue of the waypoints changing, I'd assume this:

If I'm programming an FMS (in the literal sense of me writing the code, not in the simmer sense of just plugging numbers into it), I'm going to write it with bounded expectations.  Essentially, I'm going to tell it that the first altitude it sees is going to be the altitude it bases wind off of.  Why?  Just in case I'm not entering other altitudes, it needs to know to use that as a generic wind plan.  As I enter higher altitudes, it can expect those, but only after the step climbs to the relevant (or closest) altitude.

 

Here's the tricky part, though:

You told it FL300 first, and entered winds at those altitudes.  As such, the FMC took those as the base forecast, and augmented the higher altitudes only as appropriate.  When you changed that baseline altitude, it overwrote the rest of them because that's how it's programmed to work (or how I assume that it is).

 

Here's the solution:

You're getting too granular.  As you seem to post primarily about entering and interpreting winds, I see that this is very much your primary way of seeking efficiency.  While I understand your sentiment, I believe that it's turned into somewhat of a fixation.  Step back for a moment and watch the wind-blown contrails.  You're entering a lot more data than is necessary.  I'm not sure where you're getting your wind data, but in the end, it always comes from here (Stateside, anyway).  If you look there, for the altitudes you were flying during your last flight, there are only three relevant data altitudes: FL300, FL340 and FL390.  The reason the FMC only has a few altitudes available is because there really isn't a need to throw in tons and tons of data.

 

Throw in FL300, FL340 and FL390 for the various waypoints, and let the FMS do the rest of the math.  After all, the rest of the data at the intermediate altitudes is just your own weather program's interpretation.  The FMS is capable of making its own, without all of the extra work on your behalf.

 

Don't lose the big picture by running around with a microscope staring at pixels all the time.

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Don't lose the big picture by running around with a microscope staring at pixels all the time.

Kyle

Kind of akin to "how many angels can dance on the head of a pin"?

 

Michael Cubine

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Kind of akin to "how many angels can dance on the head of a pin"?

 

Precisely.

 

I will say, however, that your attention to detail is laudable.  I wish I had that attention to detail, and want for precision in some of the things I do.

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