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Author Topic: The flight plan, magnetic course, headwinds.  (Read 536211 times)

Gary LaPook

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Re: The flight plan, magnetic course, headwinds.
« Reply #360 on: February 19, 2012, 11:10:18 PM »

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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
...
5.4833 <-- At Late

Because Williams just subtracted 180 incorrectly, the variation data is almost meaningless as the magnetic headings are bogus.
You are comparing the variation used by Williams which he got from looking at isogonic lines printed on whatever chart he was using for reference. These are only marked in whole degrees and every navigator knows that these lines are approximate and that variation changes over time. You then compare those with the predictions based on a model of the magnetic field of the earth, these were not measured values, and these predictions are not guaranteed to be accurate. You are going way beyond what the data supports with your computations.

gl
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Heath Smith

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Re: The flight plan, magnetic course, headwinds.
« Reply #361 on: February 20, 2012, 04:41:26 AM »


When you are creating a model, you are not going to intentionally reduce the accuracy of the models as it is pointless and time consuming to do so. For example, with the magnetic variation data, I posted thousandths where the data has an accuracy of .5 degrees. I am not going to re-work the data in Excel and re-format it just so I can post it. It is my data and you need not accept it as your own. Again, this is a tangential conversation. If you feel that data is using excessive precision you can choose to ignore it or re-format it.

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You are comparing the variation used by Williams which he got from looking at isogonic lines printed on whatever chart he was using for reference. These are only marked in whole degrees and every navigator knows that these lines are approximate and that variation changes over time. You then compare those with the predictions based on a model of the magnetic field of the earth, these were not measured values, and these predictions are not guaranteed to be accurate. You are going way beyond what the data supports with your computations.

The variations were critical to establishing the flight plan. As I stated before, I believe that these values are too crude for such an attempt over water where being off by a degree or two over just 1,000 miles could put you out of visual range of such a tiny target as Howland. For most purposes the degree of accuracy was probably fine. My opinion is that flying over such distances over water it was not fine. Variations are important in that if you are unable to obtain a fix to your flight line, and you are using DR, the error in the variations will have a cumulative effect. In this case, an unacceptably high error can accumulate.

Do you have information that suggest that his magnetic variation data was correct by any measure? I would think that the models that claim 30 minutes accuracy might be a bit better than what was available in 1937. I do not claim to understand their mathematical model but I am guessing it is well thought out and models all of this historical data collected over time otherwise it would be entirely pointless to create the models for dates in the past. The point of using this data was for my own flight reconstruction so there is not much point in discussing this model data.

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What mistake are you complaining about?

The assumption in the conversation would have been that FN did not re-work the flight plan and lay down a RHUMB line. While that might have been a great idea it retrospect, do you have any evidence that FN did do this? Do you have any evidence that FN re-worked the flight plan that Williams created? You have mentioned this RHUMB line previously but I have yet to see any evidence that this was indeed the case. Even though FN might have been a great navigator he was also human and perhaps distracted and or lazy, instead using the plan that Williams had created.

As to the point about the strip chart, if you simply subtract 180 from your magnetic headings in the opposite direction, this implies that you are subtracting 180 degrees from your true course heading on the return trip. Is that correct or no? You seem to suggest that this is perfectly valid, I do not believe this is the case.

While you posted that the initial leg from Lae would have a 079.4° true, I believe that Williams assumed a true course of 77.05 that was 180 degrees from the true course to Lae, 257.05. That is why the magnetic heading is 73, which is 253-180. Is that not what is written on the chart for the return flight? Are you saying that this is correct?
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Gary LaPook

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Re: The flight plan, magnetic course, headwinds.
« Reply #362 on: February 20, 2012, 04:49:28 AM »


The variations were critical to establishing the flight plan. As I stated before, I believe that these values are too crude for such an attempt over water where being off by a degree or two over just 1,000 miles could put you out of visual range of such a tiny target as Howland. For most purposes the degree of accuracy was probably fine. My opinion is that flying over such distances over water it was not fine. Variations are important in that if you are unable to obtain a fix to your flight line, and you are using DR, the error in the variations will have a cumulative effect. In this case, an unacceptably high error can accumulate.
Of course you cannot dead reckon for 2556 SM and expect to find a small island since the uncertainty at that point is 256 SM, that is why Noonan was along to obtain fixes along the way. If he couldn't get fixes they could have turned around, as they had planned to do on the abortive Hawaii to Howland flight, prior to the point of no return and tried again another day.
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As to the point about the strip chart, if you simply subtract 180 from your magnetic headings in the opposite direction, this implies that you are subtracting 180 degrees from your true course heading on the return trip. Is that correct or no? You seem to suggest that this is perfectly valid, I do not believe this is the case.

Yes it is after you break the great circle into segments then each segment is a rhumb line.
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While you posted that the initial leg from Lae would have a 079.4° true, I believe that Williams assumed a true course of 77.05 that was 180 degrees from the true course to Lae, 257.05. That is why the magnetic heading is 73, which is 253-180. Is that not what is written on the chart for the return flight? Are you saying that this is correct?
The numbers that you are looking at are not the great circle courses but the rhumb lines to the next intermediate point.

gl
« Last Edit: February 20, 2012, 04:53:46 AM by Gary LaPook »
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Heath Smith

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Re: The flight plan, magnetic course, headwinds.
« Reply #363 on: February 20, 2012, 04:56:05 AM »

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The numbers that you are looking at are not the great circle courses but the rhumb lines to the next intermediate point.

What I see are the magnetic headings from the flight plan toward Lae which are on the other flight plan document verbatim. What I see on the strip chart is the 180 subtraction of the magnetic headings for those instructions. Why list the magnetic headings for the return path if that was not a plan? I see arrows indicating the direction of travel. I think it is clear he made a mistake.

So you are saying that this is correct?

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Gary LaPook

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Re: The flight plan, magnetic course, headwinds.
« Reply #364 on: February 20, 2012, 05:03:58 AM »

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The numbers that you are looking at are not the great circle courses but the rhumb lines to the next intermediate point.

What I see are the magnetic headings from the flight plan toward Lae which are on the other flight plan document verbatim. What I see on the strip chart is the 180 subtraction of the magnetic headings for those instructions. Why list the magnetic headings for the return path if that was not a plan? I see arrows indicating the direction of travel. I think it is clear he made a mistake.

So you are saying that this is correct?
Yes, when you reverse each rhumb line segment between the great circle points they are 180 degrees different.

Are they a plan, sure Williams' plan and if Williams was the navigator on the flight then he might have followed his initial plan, or he might have modified it after considering new information obtained in Lae. But Williams was NOT the navigator and Noonan used completely different methods than Williams.

gl

gl
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Heath Smith

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Re: The flight plan, magnetic course, headwinds.
« Reply #365 on: February 20, 2012, 05:08:57 AM »


Well, I can only say that I believe that you cannot follow the 180 degree reverse instructions for the previously stated reasons.

Since you do not see any issue with this, I would suspect that FN might not have seen the issue either.

I believe that if you consider not the headings, only the 180 degrees reversal of the true course you will see the problem.

Plot a true course of 77.05 in Google Earth from Lae toward Howland and tell me where that lands you.

Are you going to say that this was a mistake yet or no?

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Gary LaPook

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Re: The flight plan, magnetic course, headwinds.
« Reply #366 on: February 20, 2012, 05:54:44 AM »


Well, I can only say that I believe that you cannot follow the 180 degree reverse instructions for the previously stated reasons.

Since you do not see any issue with this, I would suspect that FN might not have seen the issue either.

I believe that if you consider not the headings, only the 180 degrees reversal of the true course you will see the problem.

Plot a true course of 77.05 in Google Earth from Lae toward Howland and tell me where that lands you.

Are you going to say that this was a mistake yet or no?
I still don't know where you are getting your 77.05 from. Look an the strip chart and it shows the initial magnetic course from Lae to Howland is 073° which incorporates the variation of 6° east so the initial true course is 079°. Where I come from, 79 is not the same as 77.05° and is not the reciprocal of 257.05°
Now you go back on Google Earth using the correct value of 079° and see where it takes you.

gl
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Heath Smith

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Re: The flight plan, magnetic course, headwinds.
« Reply #367 on: February 20, 2012, 06:10:39 AM »


If you simply subtract 180 degrees from the magnetic course, as shown on the chart for each segment on the return plan from Lae to Howland,  this infers/implies/requires that you are subtracting 180 degrees from the true course, do you agree with that statement or no? If not, I would like to understand where you are coming from.

Where are you pulling the 79 from? I know where 79.65 degrees ends up. I am looking only at the MC values on the chart.
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C.W. Herndon

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Re: The flight plan, magnetic course, headwinds.
« Reply #368 on: February 20, 2012, 12:12:18 PM »

Heath, I think you may have a slight misunderstanding of some of the terms concerning navigation and use of the compass.

1. True course: this is the line on the map drawn between the point of departure and the destination.
2. Magnetic course: True course corrected for magnetic variation. (this is the course on the lines of the strip map)
3. True heading: True course corrected for winds. (this is used because winds aloft are normally given using true directions)
4. Magnetic heading: True heading corrected for magnetic variation.
5. Compass heading: Magnetic heading corrected for deviation (installation error caused by location of the compass in the aircraft mostly). Deviation is taken from the compass correction card mounted in the aircraft, somewhere near the compass.

The 79 degree course Gary is talking about is the true course between Lae and Howland. It is the reciprocal of the 259 degree TC shown on the lower left hand corner of the strip map under the 15. This is the MC shown on the strip map with the corection for variation removed.
Woody (former 3316R)
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« Last Edit: February 21, 2012, 01:58:06 PM by Clarence W. Herndon »
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Heath Smith

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Re: The flight plan, magnetic course, headwinds.
« Reply #369 on: February 20, 2012, 12:15:12 PM »


Clarence,

I am simply looking at the MC values on the return course. They are 180 degrees out from the corresponding segments used for approach.

I am not reading the 259 in the margin, I did not even see that. I will take a look after work.

Are you suggesting also that the MC value of 73 on the first leg from Lae to Howland is correct?

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C.W. Herndon

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Re: The flight plan, magnetic course, headwinds.
« Reply #370 on: February 20, 2012, 12:19:10 PM »

I'm saying that yes it probably is but do not confuse it with the true course, the straight line on the map between the two points.
Woody (former 3316R)
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Heath Smith

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Re: The flight plan, magnetic course, headwinds.
« Reply #371 on: February 20, 2012, 12:34:05 PM »


Oh I am aware of the true course and the meaning.

I disagree that the return magnetic courses on the strip chart are correct.
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C.W. Herndon

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Re: The flight plan, magnetic course, headwinds.
« Reply #372 on: February 20, 2012, 12:38:48 PM »

Reverse "courses" will always be 180 degrees different. Reverse "headings" will not.
Woody (former 3316R)
"the watcher"
 
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Gary LaPook

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Re: The flight plan, magnetic course, headwinds.
« Reply #373 on: February 20, 2012, 12:39:02 PM »


Clarence,

I am simply looking at the MC values on the return course. They are 180 degrees out from the corresponding segments used for approach.

I am not reading the 259 in the margin, I did not even see that. I will take a look after work.

Are you suggesting also that the MC value of 73 on the first leg from Lae to Howland is correct?
See attached.

gl
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Gary LaPook

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Re: The flight plan, magnetic course, headwinds.
« Reply #374 on: February 20, 2012, 12:45:58 PM »


If you simply subtract 180 degrees from the magnetic course, as shown on the chart for each segment on the return plan from Lae to Howland,  this infers/implies/requires that you are subtracting 180 degrees from the true course, do you agree with that statement or no? If not, I would like to understand where you are coming from.

Where are you pulling the 79 from? I know where 79.65 degrees ends up. I am looking only at the MC values on the chart.
Simple, just back out the variation for each leg and you will find the true course for the rhumb line for that segment of the approximate great circle. The magnetic course shown on the table, line 15, for the first segment from Lae to Howland is 073° the reciprocal of the listed course from Howland to Lae for that segment. Now add the 6° of easterly magnetic variation for that location and you find 079° T.

gl
« Last Edit: February 22, 2012, 02:37:38 AM by Gary LaPook »
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