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Author Topic: The flight plan, magnetic course, headwinds.  (Read 439573 times)

Harry Howe, Jr.

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Re: The flight plan, magnetic course, headwinds.
« Reply #435 on: February 29, 2012, 10:30:48 AM »


Heath
It is my opinion that FN , after the 1623  GCT, (0423 AM Howland, 0453 AM Itasca) "fix"(400 SM?out), diverted to the offset, landfall, method and altered his course to intercept the 157/337 LOP at about 67 SM (1/2 hour at 134mph)NNW of Howland (or SSE).  Then he would have known to turn to starboard(or port)and fly to Howland.  In other words, he covered the potential fix and DR error so that he would know which way to turn when they reached the LOP.

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Harry Howe, Jr.

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Re: The flight plan, magnetic course, headwinds.
« Reply #436 on: February 29, 2012, 10:38:39 AM »


Sorry about the bold letters, I'm not shouting, just having problems with the formatting on my Puter   sheesh
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richie conroy

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Re: The flight plan, magnetic course, headwinds.
« Reply #437 on: February 29, 2012, 02:30:04 PM »

have just been reading this on purdue, but if u read down

it says Earhart's flight path took her 20 miles south of  Arorae can that be right ?

as its 137 miles from the Lae to Howland flight path, to Arorae Island

or have i made a mistake ?



http://earchives.lib.purdue.edu/cdm4/document.php?CISOROOT=/earhart&CISOPTR=3119&REC=15
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« Last Edit: February 29, 2012, 02:35:07 PM by richie conroy »
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Heath Smith

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Re: The flight plan, magnetic course, headwinds.
« Reply #438 on: February 29, 2012, 02:51:37 PM »

Quote
It is my opinion that FN , after the 1623  GCT, (0423 AM Howland, 0453 AM Itasca) "fix"(400 SM?out), diverted to the offset, landfall, method and altered his course to intercept the 157/337 LOP at about 67 SM (1/2 hour at 134mph)NNW of Howland (or SSE).  Then he would have known to turn to starboard(or port)and fly to Howland.  In other words, he covered the potential fix and DR error so that he would know which way to turn when they reached the LOP.

The only issue that I see with the idea is that they would have needed to be going faster than 134MPH to do that. If you compute the distance to 67SM North of Howland on the 157/337 line to a point approximately out 422SM out where they should have been at 16:23GMT, this is about 413SM miles from the NW intercept point. This would add up to about 480SM from 16:23GMT to the 19:12GMT ("we must be on you") message. This would require that they were achieving a ground speed of 170MPH. Add in a 23 knot head wind and you could be pushing up in to the 190+ MPH range when it comes to air speed.

That is why I think they came straight in, probably passing Howland to the North, giving up after 16 minutes of straight line travel North of Howland, then proceeding to circle until plan B was devised.
« Last Edit: February 29, 2012, 03:00:19 PM by Heath Smith »
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Heath Smith

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Re: The flight plan, magnetic course, headwinds.
« Reply #439 on: February 29, 2012, 02:56:08 PM »

Quote
it says Earhart's flight path took her 20 miles south of  Arorae can that be right ?

Unless they were in possession of some plan that the world does not know about, it must be a mistake.

What is more troubling to me is the fact that these other radio stations were not utilized as the operator suggested.

Just another coulda-woulda-shoulda for the AE story line.
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richie conroy

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Re: The flight plan, magnetic course, headwinds.
« Reply #440 on: February 29, 2012, 03:19:04 PM »

well i come across this aswell must av only recently been added because aint seen it before

map of flight, notice arc in line from lae to howland



http://earchives.lib.purdue.edu/cdm4/document.php?CISOROOT=/earhart&CISOPTR=3763&REC=1
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Irvine John Donald

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Re: The flight plan, magnetic course, headwinds.
« Reply #441 on: February 29, 2012, 08:34:28 PM »

have just been reading this on purdue, but if u read down

it says Earhart's flight path took her 20 miles south of  Arorae can that be right ?

as its 137 miles from the Lae to Howland flight path, to Arorae Island

or have i made a mistake ?



http://earchives.lib.purdue.edu/cdm4/document.php?CISOROOT=/earhart&CISOPTR=3119&REC=15

It says what you wrote Richie but in the left margin it looks like someone wrote "incorrect".
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Gary LaPook

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Re: The flight plan, magnetic course, headwinds.
« Reply #442 on: March 01, 2012, 01:16:00 AM »


So using your previous example that the last fix was obtained at 16:23GMT (roughly 400NM out). If FN assumed a 15NM error when the fix was obtained, if he was within the average error margin would he have simply ignored any difference between the fix and his calculated DR position or would he always use this new fix and assume the DR calculation was incorrect?
The standard navigation technique is to start a new DR plot from each new fix. In flight navigation you also plot the DR position brought forward from the prior fix for the same time as the new fix and the difference from the DR to the new fix tells you what the wind is so that you can correct for it. Here is a link to an example of how this is all done. We can also see from Noonan's Hawaii flight chart that he used this standard method.
Quote

Assuming that his last fix could at 16:23GMT could have potentially had an error of 15NM, do you then add this value to the DR error around Howland (10% of 400 = 40NM) so that you have a total potential error of 55NM?

The uncertainty in the previous fix is carried forward with the DR. The DR can never be more accurate than the fix it started from. With a 15 NM uncertainty you can draw a circle of 15 NM radius around the fix. You expect to be near the center of the circle but you accept the possibility that you may be near the edge. If you then DR one mile that circle isn't going to get any smaller. So you do add the original fix uncertainty to the additional uncertainty caused by the DR leg, one mile for each ten miles on the DR leg so after DRing for 400 nautical miles the uncertainty in the DR position would be 55 NM and you would allow for this in planning the point to intercept the LOP and for searching along the LOP.
Quote

For the sake of argument let's say he did find a substantial error at 16:32GMT, say 30NM South of the line at 400NM out would he have computed a correction back to the flight line or would he have just computed a new flight line to Howland? When do you switch over from making your way back to a flight line and computing a new course to your destination?

There is nothing magic or important about the original planning course line you drew on the chart, there is no reason to try to get back on it. At each fix you just aim for the destination from that point since this always shorter than trying to get back on the original course line. We can see that this was Noonan's method from his chart for the Oakland to Hawaii flight.

Flying over land it may be important to get back on the original course line if, for instance, the original course approaches the destination down the center of a valley and if approaching from a different direction may cause to smack into a mountain. There are no mountains in the middle of the ocean.
gl
« Last Edit: March 06, 2012, 01:56:29 AM by Gary LaPook »
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Heath Smith

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Re: The flight plan, magnetic course, headwinds.
« Reply #443 on: March 07, 2012, 03:52:19 AM »


I understand what you mean about there being nothing magic about the original flight line but it seems that there are two modes that you would be in during a flight, one to get back to a flight line if you are making small adjustments for ease of computation and another if you are close enough to compute a new flight line without much effort.

I had read an article on the net about how to correct course after obtaining a fix and also adjusting for head winds where you perform a couple of calculations to determine the angle back to the flight line then use another angle once on the line to deal with the future expected effect of the head wind given how far you were off the line initially. Would they be using a similar technique during the night as they approached Howland?

It seems that when you compute a new flight line you would not be including this offset angle that attempts to compensate for the head winds. Could you just use the head wind offset angle that you had computed previously for your new course?

Thanks.
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Erik

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Re: The flight plan, magnetic course, headwinds.
« Reply #444 on: March 07, 2012, 07:26:57 AM »

I just posted an intersting article from Williams on this thread

He basically suggests that a wind correction angle that is not adjusted for a no-wind condition, could put them 180 miles south of Howland.

"If the fliers allowed for a drift of eight degrees, and if the wind died down - so that this correction was not needed at all - they might have gone as far as 180 miles to the south of Howland island."
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Heath Smith

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Re: The flight plan, magnetic course, headwinds.
« Reply #445 on: March 11, 2012, 09:44:01 AM »

Here are the course correction methods that I had mentioned earlier.

What if for example that at around 16:23GMT FN was able to obtain a fix and then used this double-track method to compensate for the head wind and after the track was recovered they would have needed to make the following adjustment:

"When the original track is recovered, the correcting heading must be adjusted: take the heading that produced the drift error, plus or minus the original drift angle".

As they progressed from Lae, the winds where ESE until the Ontario then ENE to Howland. This being the case it seems reasonable that they would have discovered this changing head wind direction by taking a fix that would have told them how far off the line they had drifted and FN would then calculate the drift angle and plot a course correction.

Perhaps they might have doubled the drift angle to get back on course and due to their exhaustion from such a long flight might have forgotten to adjust their heading once the flight track was recovered. If this were the case, they might have ended up North of Howland but within the DR error of 55NM.

See attachment.

To give a more concrete example, lets say that at 14:45GMT FN had a fix. 220SM later at 16:23GMT another fix was obtained and a 15SM error was detected. The drift angle would be about 4 degrees. If they doubled this angle over the next 220 miles, and did not make a correction at the end of the 220 miles, they would have ended up about 30SM North of Howland at about 440SM from where they started the original correction.
« Last Edit: March 11, 2012, 10:42:52 AM by Heath Smith »
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Gary LaPook

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Re: The flight plan, magnetic course, headwinds.
« Reply #446 on: March 14, 2012, 04:24:45 AM »

I just posted an intersting article from Williams on this thread

He basically suggests that a wind correction angle that is not adjusted for a no-wind condition, could put them 180 miles south of Howland.

"If the fliers allowed for a drift of eight degrees, and if the wind died down - so that this correction was not needed at all - they might have gone as far as 180 miles to the south of Howland island."
I don't know where he came up with an eight degree wind correction angle. The plane was cruising at 150 mph and the inbound course to Howland is 078° true and with a wind from ENE at 24 mph the wind correction angle was only 1.6 ° left so the true heading of the plane would have been 076.4° (call it 076°.) With the wind out of the ESE then the wind correction angle would have been 5.2° (call it just 5°) right and the true heading would have been 083°. Neither of these winds would have caused Noonan to use an 8° wind correction angle.

gl
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Gary LaPook

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Re: The flight plan, magnetic course, headwinds.
« Reply #447 on: March 14, 2012, 04:29:06 AM »


I understand what you mean about there being nothing magic about the original flight line but it seems that there are two modes that you would be in during a flight, one to get back to a flight line if you are making small adjustments for ease of computation and another if you are close enough to compute a new flight line without much effort.

I had read an article on the net about how to correct course after obtaining a fix and also adjusting for head winds where you perform a couple of calculations to determine the angle back to the flight line then use another angle once on the line to deal with the future expected effect of the head wind given how far you were off the line initially. Would they be using a similar technique during the night as they approached Howland?

It seems that when you compute a new flight line you would not be including this offset angle that attempts to compensate for the head winds. Could you just use the head wind offset angle that you had computed previously for your new course?

Thanks.
No because the wind correction angle varies with the angle between the course and the wind direction and this angle will change after you change your heading to head towards Howland.

The wind correction angle is used to deal with a cross-wind not a headwind, because it is a cross-wind that blows you off course while a headwind just slows down your ground speed. Since the wind was neither a direct headwind not a direct cross-wind you end up calculating a wind correction angle to compensate for the cross-wind component of the wind.

gl
« Last Edit: March 14, 2012, 04:36:22 AM by Gary LaPook »
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Gary LaPook

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Re: The flight plan, magnetic course, headwinds.
« Reply #448 on: March 14, 2012, 04:56:33 AM »

Here are the course correction methods that I had mentioned earlier.

What if for example that at around 16:23GMT FN was able to obtain a fix and then used this double-track method to compensate for the head wind and after the track was recovered they would have needed to make the following adjustment:

"When the original track is recovered, the correcting heading must be adjusted: take the heading that produced the drift error, plus or minus the original drift angle".

As they progressed from Lae, the winds where ESE until the Ontario then ENE to Howland. This being the case it seems reasonable that they would have discovered this changing head wind direction by taking a fix that would have told them how far off the line they had drifted and FN would then calculate the drift angle and plot a course correction.

Perhaps they might have doubled the drift angle to get back on course and due to their exhaustion from such a long flight might have forgotten to adjust their heading once the flight track was recovered. If this were the case, they might have ended up North of Howland but within the DR error of 55NM.

See attachment.

To give a more concrete example, lets say that at 14:45GMT FN had a fix. 220SM later at 16:23GMT another fix was obtained and a 15SM error was detected. The drift angle would be about 4 degrees. If they doubled this angle over the next 220 miles, and did not make a correction at the end of the 220 miles, they would have ended up about 30SM North of Howland at about 440SM from where they started the original correction.
The method of doubling the drift angle that is shown in the link that you posted is something that we teach student pilots for cross-country navigation by "pilotage" over dry land. This is an approximate solution but works well for getting the student back onto the course line that he drew on his chart. Following the original line drawn on the chart is important for navigation by "pilotage" because the pilot marks visual checkpoints (towns, road junctions, lakes, rivers, railroad tracks, etc.) along that line, measures the distance between them, fills out a planning log with these distances and the estimated time en-route between the checkpoints. In flight the pilot then records the times between the checkpoints to calculate his ground speed so he can figure the time remaining to the destination and decide if he still has sufficient fuel to make it. If a pilot gets too far away from his course line with the planned visual landmarks it is more likely that he will then not recognize features he sees on the ground and then wander even farther off course. You don't use visual check points when flying over the ocean because all the waves look alike. Over the ocean you calculate the wind you have encountered between the fixes or by use of a driftmeter and then use that computed wind to figure the heading that should keep you on the new course line from the latest fix directly to the destination. If you review Noonan's chart for the California to Hawaii flight you will see that this is the method he used.

gl
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Erik

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Re: The flight plan, magnetic course, headwinds.
« Reply #449 on: March 14, 2012, 05:19:51 AM »

I just posted an intersting article from Williams on this thread

He basically suggests that a wind correction angle that is not adjusted for a no-wind condition, could put them 180 miles south of Howland.

"If the fliers allowed for a drift of eight degrees, and if the wind died down - so that this correction was not needed at all - they might have gone as far as 180 miles to the south of Howland island."
I don't know where he came up with an eight degree wind correction angle. The plane was cruising at 150 mph and the inbound course to Howland is 078° true and with a wind from ENE at 24 mph the wind correction angle was only 1.6 ° left so the true heading of the plane would have been 076.4° (call it 076°.) With the wind out of the ESE then the wind correction angle would have been 5.2° (call it just 5°) right and the true heading would have been 083°. Neither of these winds would have caused Noonan to use an 8° wind correction angle.

gl

Unless he made a mistake, or calculated the winds incorrectly.
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