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Author Topic: The flight plan, magnetic course, headwinds.  (Read 528080 times)

Harry Howe, Jr.

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Re: The flight plan, magnetic course, headwinds.
« Reply #345 on: February 19, 2012, 02:51:56 PM »


Gary
Thanks for that info
Appears that "drift indicator" wasn't as simple as a dial with a needle to show drift from an intended heading(course).  Bummer.

I assume however that AE would compare her two compasses and the D/G periodically to be sure that the autopilot was holding the set heading.  I understand that the desired course/heading would change to stay on Williams' great circle plan.

It is beyond me why anyone would fly a 16 segment great circle route instead of  the rhumb line just to save what 10 nm (11.5 statute miles) in 2222 nm (2556 statute miles) ?  Boggles the mind
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C.W. Herndon

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Re: The flight plan, magnetic course, headwinds.
« Reply #346 on: February 19, 2012, 03:04:12 PM »


Gary
Thanks for that info
Appears that "drift indicator" wasn't as simple as a dial with a needle to show drift from an intended heading(course).  Bummer.

I assume however that AE would compare her two compasses and the D/G periodically to be sure that the autopilot was holding the set heading.  I understand that the desired course/heading would change to stay on Williams' great circle plan.

It is beyond me why anyone would fly a 16 segment great circle route instead of  the rhumb line just to save what 10 nm (11.5 statute miles) in 2222 nm (2556 statute miles) ?  Boggles the mind

Harry, it appears that the only DG in the Electra was part of the Sperry Autopilot. See cockpit photo and diagram of AP with parts annotated.
Woody (former 3316R)
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C.W. Herndon

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Re: The flight plan, magnetic course, headwinds.
« Reply #347 on: February 19, 2012, 03:19:53 PM »


Gary
My thought re Drift Indicator is that it displays any deviation from the desired course set on the autopilot/DG from say, wind component.
I would be pleased if you would enlighten me, as I am sure you are amply capable of doing.  Thanks in advance.

Harry, here is a picture of AE and FN at the Navigator's station of the Electra with the Drift sight set up on the table. Also find a picture of a Ceramic/aluminum drift bomb for use over water in the day time. The water light was for use over water at night although some things I have read indicate that you might have been able to use it in day light as well. Each made a well defined target to track while computing drift.
Woody (former 3316R)
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Heath Smith

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Re: The flight plan, magnetic course, headwinds.
« Reply #348 on: February 19, 2012, 03:23:13 PM »

Quote
Don't know if it has ever been mentioned before but, has the possibility that they overshot Howland island ever been considered?

That is my current view. I believe they passed to the North as well.
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Heath Smith

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Re: The flight plan, magnetic course, headwinds.
« Reply #349 on: February 19, 2012, 03:53:27 PM »

Quote
I have checked Williams computations of the great circle course and intermediate points and only one of the intermediate points differed by one-tenth of a nautical mile from my calculation done with a calculator, which is amazingly good work on William's part using trig tables.

Have you ever tried to execute his flight plan in Google Earth? I am not seeing anything that precise when attempting to reconstruct the flight. This is a difficult task if you try to perform a segment by segment flight line accurately because for whatever reason when you lay a line down and click "Save" the line jumps like it is snapping to a grid. If you define the points in a KLM file it is much easier to deal with.

I just noticed something that escaped me before. Williams did just subtract 180 degrees for the return trip, that is a no-no for the previously discussed reason that the longitude lines are not parallel. I guess I am a bit surprised by that. Do you suppose Noonan would have caught that mistake?

His magnetic variations seem to be off quite a bit (or a bit too crude) compared to values from this database.

If you had executed the reverse plan precisely from Lae using what I believe is more accurate magnetic variation data that would land you about 18SM North of Howland. Add in a little drift from the head winds and it is not a surprise that they missed Howland. While they might have taken some celestial readings along the way, if they were comparing the readings to a bogus flight line, this would only verify that you are following an incorrect flight line.

The more I look at this flight attempt from the flight plan, the unknown weather conditions, the approach at sunrise, the iffy DF, the more I believe this was a far more risky flight than perhaps they even realized. With a functioning radio their chances would have probably been much improved. The tragedies of circumstances are endless in this non-fiction story.
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C.W. Herndon

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Re: The flight plan, magnetic course, headwinds.
« Reply #350 on: February 19, 2012, 03:58:47 PM »

Heath, you did enter the date of the flight didn't you? Just checking.
Woody (former 3316R)
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Heath Smith

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Re: The flight plan, magnetic course, headwinds.
« Reply #351 on: February 19, 2012, 04:02:55 PM »

Quote
Heath, you did enter the date of the flight didn't you? Just checking.

Yes, sure did. Here are variations for the flight starting at Howland and working all the way to Lae using the coordinates on the strip chart.

9.4833 <- At Howland
9.4833
9.4833
9.5000
9.2000
8.9667
8.6667
8.3333
7.9833
7.6167
7.2500
6.8833
6.5333
6.2000
5.8667
5.4833 <-- At Late

Because Williams just subtracted 180 incorrectly, the variation data is almost meaningless as the magnetic headings are bogus.
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C.W. Herndon

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Re: The flight plan, magnetic course, headwinds.
« Reply #352 on: February 19, 2012, 04:22:58 PM »

Heath, I must be missing something, the variations I see on the strip map that I have are within approximately 1/2 degree of those that you have posted.
Woody (former 3316R)
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Heath Smith

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Re: The flight plan, magnetic course, headwinds.
« Reply #353 on: February 19, 2012, 04:32:21 PM »

Here is the differences from actual:

0.4333
0.4333
0.4333
0.4500
1.1500
0.9167
1.6167
1.2833
0.9333
1.5667
1.2000
0.8333
1.4833
2.1500
1.8167
??? <-- At Lae, not specified in flight plan.

As you see in the actual variations list, it changes quite a bit around Lae. Whether or not Noonan used this same data when they avoided the storm probably cannot be known.

Seemingly small errors do add up. If you were only off by .5 degrees at the end (over 2556SM), that would translate to 22SM. That is quite a distance to be off when searching for your destination.
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C.W. Herndon

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Re: The flight plan, magnetic course, headwinds.
« Reply #354 on: February 19, 2012, 04:45:14 PM »

Heath, speaking as a long time pilot, I can assure you that with the equipment that they had in 1937 they would probably have done well to maintain a compass heading within 2-3 degrees for any period of time. That's one reason why the RDF information was so critical.
Woody (former 3316R)
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« Last Edit: February 19, 2012, 04:48:22 PM by Clarence W. Herndon »
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Heath Smith

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Re: The flight plan, magnetic course, headwinds.
« Reply #355 on: February 19, 2012, 04:49:19 PM »


Yes, you are going to drift all the time, left to right as you go along. This is why you need to obtain fixes along the way to let you know how far you have diverged from your flight line. If you are following an incorrect flight line, there is little that can help you. The flight line must be correct.
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C.W. Herndon

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Re: The flight plan, magnetic course, headwinds.
« Reply #356 on: February 19, 2012, 04:54:08 PM »

You are absolutely correct and that is where FN came in. Something went wrong somewhere but we don't know what it was. FN was a very experenced navigator.
Woody (former 3316R)
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Heath Smith

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Re: The flight plan, magnetic course, headwinds.
« Reply #357 on: February 19, 2012, 05:39:14 PM »

Consider the possibility that FN actually used the original flight plan that was incorrect. I am not 100% sure but I believe that the return trip that was laid out has a true course of 77.07 degrees (257.07 - 180 = 77.07). It seems like a simple enough mistake that could have been made and not caught unless you re-worked the flight plan to verify it. As proof I would ask how many people have looked at this flight plan and not found this simple error previously? I did not spot it after looking at it on a spread sheet for a while.

This would place you at about 106SM North of Howland at 9:12GMT if you were right on the bogus flight line.
« Last Edit: February 19, 2012, 06:08:38 PM by Heath Smith »
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Gary LaPook

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Re: The flight plan, magnetic course, headwinds.
« Reply #358 on: February 19, 2012, 07:11:03 PM »


Gary
Thanks for that info
Appears that "drift indicator" wasn't as simple as a dial with a needle to show drift from an intended heading(course).  Bummer.

I assume however that AE would compare her two compasses and the D/G periodically to be sure that the autopilot was holding the set heading.  I understand that the desired course/heading would change to stay on Williams' great circle plan.

It is beyond me why anyone would fly a 16 segment great circle route instead of  the rhumb line just to save what 10 nm (11.5 statute miles) in 2222 nm (2556 statute miles) ?  Boggles the mind
No, only one-tenth of a statute mile (0.1 SM).

gl
« Last Edit: February 19, 2012, 07:51:03 PM by Gary LaPook »
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Gary LaPook

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Re: The flight plan, magnetic course, headwinds.
« Reply #359 on: February 19, 2012, 07:59:44 PM »

Consider the possibility that FN actually used the original flight plan that was incorrect. I am not 100% sure but I believe that the return trip that was laid out has a true course of 77.07 degrees (257.07 - 180 = 77.07). It seems like a simple enough mistake that could have been made and not caught unless you re-worked the flight plan to verify it. As proof I would ask how many people have looked at this flight plan and not found this simple error previously? I did not spot it after looking at it on a spread sheet for a while.

This would place you at about 106SM North of Howland at 9:12GMT if you were right on the bogus flight line.
First, keep in mind that there is no proof that Noonan used William's plans since Noonan was a more experienced flight navigator than Williams was and most likely did his own computations, not trusting the work of others. I know I would have in his position and I have always done my own computations in the past, I don't rely on anybody else for this type of work.

What mistake are you complaining about?

I guess you do not understand how a navigator plans a great circle course. To actually fly the great circle would require the plane to be constantly turning because the great circle course is never constant and this is impossible to do. So you lay out intermediate points along the great circle spaced at a convenient distance, Williams used 2° 30' of longitude, and you calculate the latitude at which the great circle cuts those longitudes. Then you calculate the RHUMB line courses between adjoining pairs of points, not great circle courses. That way you do not have to be constantly changing your heading, you fly a constant course for the entire segment leg. Then, at the next point, you make a change in course. Since each segment leg is a rhumb line you can reverse the direction of the flight by adding 180° to each leg. This method approximates the great circle and, for the LAE to Howland flight, adds less than one-tenth of a mile (nautical or statute, take your pick) to the flight compared to flying a perfect great circle.

There are so many places where uncertainty enters into the computation of the compass heading to fly it is silly to do these computations to the high level that you are attempting. First you compute the true course and round off to the whole degree thereby introducing a + / - half degree uncertainty into the true course, a total of a full degree of uncertainty. Then you add in magnetic variation, again rounded to the whole degree, and unlikely to be accurate even to that level, which adds an additional full degree of uncertainty into the magnetic course. Then you allow for the wind and the wind correction angle is unlikely to be more accurate than one or two degrees and usually not that good if using forecast winds so we are up to plus and minus 4 degrees at this point. Then you apply the deviation from the compass correction card, again rounded to the whole degree, so we are up to 5 degrees of uncertainty. And the compass correction card itself was determined by swinging the plane and using another compass to determine the heading of the plane on the ground and the bearing read off the testing compass is only read to a whole degree and is probably not that accurate so we are up to 6 degrees of uncertainty in the compass heading. So you can see it is silly to do your computations to the level that you are attempting.  To make this clear by an analogy, let's say you filled your backyard swimming pool with a bucket and then compute how much water is in the pool to the nearest teaspoon. You are doing your computations to the nearest teaspoon.

You are attempting to put too fine a point on these computations.

I posted this before:


Another thing that people get hung up on is about the need to fly the great circle course instead of the rhumb line course. A rhumb line maintains the same true direction for the entire flight while to follow the great circle you must calculate and then make periodic changes in your heading. The great circle is shorter than the rhumb line so that is why people think you must follow the great circle. However this really only makes a difference at higher latitudes but makes virtually no difference when flying near the equator. The great circle distance between the exact coordinates used by Williams for this leg, 06° 47.000' south, 147° 00.000' east for Lae and 00° 49.000' north, 176° 43.000' west for Howland is 2556.1 SM and the rhumb line is 2556.2 SM, exactly one-tenth of a statute mile longer. I can see poor Mr. Williams computing each segment (14 in all) of the great circle between Lae and Howland by hand using logarithmic trig tables only to save 1/10th of a statute mile. Leaving Lae, the initial great circle course is 079.4° true and it changes in steps so that the GC course approaching Howland is 077.6° true. The rhumb line for the entire flight is 078.1° true, only 1.3° difference. And the two course lines lie close to one another, never more than 9 SM apart which is so close that Noonan would not have been able to tell the difference, he would not know if he was on the great circle course line or on the rhumb line course line. Here is a link to Mr. Williams chart. and his data form is attached.

The reason that I specified those coordinates so exactly was so that I could compute the distances to the nearest one-tenth of a statute mile. Williams and all flight navigators would only use coordinates to the nearest one minute of latitude and longitude, one nautical mile of precision. When the input data is only accurate to one nautical mile it is improper to calculate a distance to a greater precision than that of the original data but many people do this and it is not valid. Using the coordinates as Williams did, only good to one minute, would make the distance for the GC course 2556 SM and for the rhumb line also 2556 SM, there is no difference based on the level of precision of the data used by flight navigators.

So when you do your calculations give some thought as to what the numbers actually signify.

gl
« Last Edit: February 22, 2012, 02:41:17 AM by Gary LaPook »
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