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Author Topic: Navigating the LOP with the offset method.  (Read 167517 times)

Mark Petersen

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Navigating the LOP with the offset method.
« on: June 25, 2010, 04:48:42 PM »

The more I've thought about navigating using an advanced LOP, the more it seems completely logical to employ the offset method (http://tighar.org/Projects/Earhart/forum/FAQs/offset.htm).  If a person doesn't use an offset method then they arrive at some point on the advanced LOP and then have to make a decision to go north or south on the line.  Choosing incorrectly means flying for some time in the wrong direction before eventually giving up and then doubling back to get back to one's starting point, in short not a great option for a long flight with limited fuel reserves.  Choosing incorrectly represents a 50-50 chance which seems pretty risky. 

With that in mind, what was the norm for LOP navigation back in AEs day?  Did FN typically employ the offset method or was it's use more sporadic?  I realize that the goal was to eventually get within radio range, but you would think that if FN had a preference for the offset method he would have employed it during the lost flight.

Let's assume for a moment that FN had employed this method, wouldn't it make sense to come in north of Howland and then traverse down the LOP knowing that if Howland could not be found they might reach landfall at the other islands (like Gardner)?  If they used the offset method how much offset would they need to use to correct for the large dead reckoning errors that would occur during such a long over water flight?  Would they have enough fuel reserves to reach Gardner or the other islands if they used a large offset and come in from the North?   If the answer is no then this might be one reason why the offset method would not have been used (insufficient fuel reserves).

The FAQ that I linked to says that there is no evidence that shows that offset was used, but some evidence to suggest an offset wasn't used.  What evidence is this?
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Ric Gillespie

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Re: Navigating the LOP with the offset method.
« Reply #1 on: June 26, 2010, 07:30:04 AM »

what was the norm for LOP navigation back in AEs day?  Did FN typically employ the offset method or was it's use more sporadic?  I realize that the goal was to eventually get within radio range, but you would think that if FN had a preference for the offset method he would have employed it during the lost flight.

Back in AE's days nobody routinely tried to find islands by LOP navigation.  Pan Am was the only airline flying the Pacific and they used Radio Direction Finding (RDF) to navigate to their island destinations. For example, when a clipper flight departed California for Hawaii if tracked outbound on a bearing from the Alameda station until it was out of radio range.  The navigator then kept the flight more or less on course until it came within range of the Mokapu station which guided it in. Earhart and Noonan planned to find Howland by the same method.

For Noonan to have used the offset method to find Howland he would have had to know that RDF was not going to work when they were still far enough out to make the offset.  Earhart was still trying to use RDF as late as 1930Z (0800 local) after they had already reached the advanced LOP.

The FAQ that I linked to says that there is no evidence that shows that offset was used, but some evidence to suggest an offset wasn't used.  What evidence is this?

At 1912Z (0742 local time) Earhart said, "We must be on you but cannot see you...".  If you are using the offset method there is no moment when you can say that you "must be" at your destination. Earhart probably meant that, according to the clock, they had now reached the advanced LOP and no island was in sight.
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Mike Piner

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Re: Navigating the LOP with the offset method.
« Reply #2 on: June 26, 2010, 10:20:30 AM »

Mike Piner
We have little or no indication of sunrise, from the radio logs, how did she know she was "about 200 miles out", and "must be on you".
Time alone wont guarantee you are at Howland.  what is your best supposition?  Mike piner LTM
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Mark Petersen

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Re: Navigating the LOP with the offset method.
« Reply #3 on: June 26, 2010, 08:18:46 PM »

what was the norm for LOP navigation back in AEs day?  Did FN typically employ the offset method or was it's use more sporadic?  I realize that the goal was to eventually get within radio range, but you would think that if FN had a preference for the offset method he would have employed it during the lost flight.

Back in AE's days nobody routinely tried to find islands by LOP navigation.  Pan Am was the only airline flying the Pacific and they used Radio Direction Finding (RDF) to navigate to their island destinations. For example, when a clipper flight departed California for Hawaii if tracked outbound on a bearing from the Alameda station until it was out of radio range.  The navigator then kept the flight more or less on course until it came within range of the Mokapu station which guided it in. Earhart and Noonan planned to find Howland by the same method.

Thanks for the info.  It sure seems like a leap of faith to fly out over the open ocean and put all of your trust into the reliability of the RDF gear.  But from what you're saying that was the norm.  It really brings home the phrase "on a wing and a prayer".  Ultimately it was this misplaced trust that was the undoing of AE and FN. 

In your book you mentioned that AE made one last RDF test flight at New Guinea which ended in a failure and yet they still attempted the flight.   I find this pretty shocking if RDF was their primary navigation method.  But then again AE assumed that the Coast Guard would be able to take a bearing on them if the RDF in the Elektra failed.  If anything this goes to show that putting ones safety entirely in the untested hands of another person or entity (the coast guard in this case) is a very bad idea. 

Quote from: Ric Gillespie
For Noonan to have used the offset method to find Howland he would have had to know that RDF was not going to work when they were still far enough out to make the offset.  Earhart was still trying to use RDF as late as 1930Z (0800 local) after they had already reached the advanced LOP.

Makes sense.  Also since the flight was so long and at night (when it would be difficult to get a bearing on the wind direction and speed), it would have meant using a large offset and perhaps cutting into the fuel reserves that much more. 

Quote
At 1912Z (0742 local time) Earhart said, "We must be on you but cannot see you...".  If you are using the offset method there is no moment when you can say that you "must be" at your destination. Earhart probably meant that, according to the clock, they had now reached the advanced LOP and no island was in sight.

Yup that makes sense as well.  Thanks for the history lesson :)
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Mark Petersen

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Re: Navigating the LOP with the offset method.
« Reply #4 on: June 26, 2010, 08:33:49 PM »

Mike Piner
We have little or no indication of sunrise, from the radio logs, how did she know she was "about 200 miles out", and "must be on you".
Time alone wont guarantee you are at Howland.  what is your best supposition?  Mike piner LTM

Yes that makes sense.  I don't think a person would say that if they employed the offset method. 

Now that I think about it the other drawback to the offset method comes when there is a large offset.  With a large offset wind drift will come into play as one travels down the advanced LOP.  Was it typical back then to estimate wind drift and fly on a wind corrected LOP?  If not a person could be well off the LOP by the time they reach the desired intersecting point. 
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Ric Gillespie

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Re: Navigating the LOP with the offset method.
« Reply #5 on: June 26, 2010, 08:49:53 PM »

how did she know she was "about 200 miles out", and "must be on you".

Supposition of course, but "about 200 miles out" was probably a sunrise position.  Noonan then could have advanced the sunrise LOP through Howland and estimated how long it would take them to reach that line. He probably then gave AE a note saying something like "ETA Howland at 1900 hours" (they were using Greenwich time). Or maybe it was 1906 or 1910.  In any case, at 1912 Earhart told Itasca "We must be on you but cannot see you..." .  Of course, Noonan knew that they would only see Howland at the estimated time if they hit the line at the right spot. All Earhart knew was that the time had arrived and no island was in sight.  Remember, communication between Earhart and Noonan was carried out by written notes even if they were sitting side by side.  The noise level in the 10E was horrendous and by the 19th hour of flight they were both almost certainly nearly deaf.
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Ric Gillespie

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Re: Navigating the LOP with the offset method.
« Reply #6 on: June 26, 2010, 08:54:58 PM »

Was it typical back then to estimate wind drift and fly on a wind corrected LOP?  If not a person could be well off the LOP by the time they reach the desired intersecting point. 

A navigator always corrected for wind. The problem was figuring out what the wind was doing.  During the night, the flight appears to have experience overcast conditions, thus preventing Noonan from correcting their course with star sightings.  After sunup he could estimate the wind by observing the surface of the ocean using a drift sight.
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Ted G Campbell

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Re: Navigating the LOP with the offset method.
« Reply #7 on: June 28, 2010, 07:35:59 PM »

Ric,
If AE said we are 200 miles out and then later said we must be on you but cannot see you, does this fall in line with flying an offset course?

If you are flying an offset course when you reach the LOP you are not going to be “on” the final target.  You will be, by design, somewhere either right or left of your target.  Only after flying the distance of the offset will you then be “on” the target.  If one could estimate the time between the “200 miles out” comment and the “we must be on you” comment couldn’t it be estimated how much of an offset was built into the flight plan?
Ted Campbell
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Ric Gillespie

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Re: Navigating the LOP with the offset method.
« Reply #8 on: June 28, 2010, 08:05:34 PM »

Earhart also is reported to have said that she was 100 miles out (although I can make an argument that she never said that). Earhart's announced plan was to use RDF to make a straight-in approach to Howland.  She kept trying to use RDF until she was where she thought Howland ought to be.  I know of no evidence that she used an offset, but we'll never know and, ultimately, it doesn't matter.
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Mark Petersen

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Re: Navigating the LOP with the offset method.
« Reply #9 on: June 29, 2010, 12:55:40 PM »

Was it typical back then to estimate wind drift and fly on a wind corrected LOP?  If not a person could be well off the LOP by the time they reach the desired intersecting point. 

A navigator always corrected for wind. The problem was figuring out what the wind was doing.  During the night, the flight appears to have experience overcast conditions, thus preventing Noonan from correcting their course with star sightings.  After sunup he could estimate the wind by observing the surface of the ocean using a drift sight.

So they were probably flying a wind corrected 157/337 then correct?  In other words their compass heading probably deviated a few degrees from their true heading correct?

Quote
I know of no evidence that she used an offset, but we'll never know and, ultimately, it doesn't matter.

You're right we'll probably never know.  But if we did know it would probably matter as it most likely affected their fuel reserves. 

It's also fun to play "what if" games :)    My guess is if they had planned on using the offset method they would have to decide in advance if they wanted to intersect with the advanced LOP north or south of Howland.  With that in mind the only logical sense is to come in north of Howland so that if they didn't find the island they could continue on the LOP and hope to find other islands in the group.  To do otherwise (come in south of Howland) means flying out into a vast expanse of open ocean.  With the straight in RDF method they could also come to the same conclusion to fly south on the LOP, but it means making this decision during a time of high stress after flying all night.  Both are logical outcomes but one is possibly a little higher probability than the other...


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Ric Gillespie

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Re: Navigating the LOP with the offset method.
« Reply #10 on: June 29, 2010, 01:14:03 PM »

So they were probably flying a wind corrected 157/337 then correct?  In other words their compass heading probably deviated a few degrees from their true heading correct?
Quote

Correct
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Martin X. Moleski, SJ

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Re: Navigating the LOP with the offset method.
« Reply #11 on: June 30, 2010, 11:10:48 AM »

...  I know of no evidence that she used an offset ...

To me, the fact that the last recorded transmission said, "We are flying the line north and south" makes it pretty clear that they were not using the offset method.  If they had used that method, they would have 'known' (i.e., had good reasons to believe that they knew) which way to turn toward Howland.
LTM,

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Mark Petersen

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Re: Navigating the LOP with the offset method.
« Reply #12 on: June 30, 2010, 11:39:47 AM »

...  I know of no evidence that she used an offset ...

To me, the fact that the last recorded transmission said, "We are flying the line north and south" makes it pretty clear that they were not using the offset method.  If they had used that method, they would have 'known' (i.e., had good reasons to believe that they knew) which way to turn toward Howland.

That is a good argument, but it depends on how literally a person wants to give the exact wording of the logs from the Itasca radio operators.  I think Ric's book mentioned the possibility that the actual transmission could easily have been "on the line from north to south".  It's amazing what a difference recording "and" vs "to" makes.  In this case it would have completely changed the entire direction of the post-loss search activities and the USS Lexington would have thoroughly searched Niku. 
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Erik

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Re: Navigating the LOP with the offset method.
« Reply #13 on: June 30, 2010, 11:49:46 AM »


It's amazing what a difference recording "and" vs "to" makes. 

Your not kidding.  The worst aviation accident in history was partly a result of one pilot hearing "at" the runway and the other pilot hearing "on" the runway.  Moments later two 747's would be consumed in a huge fireball.

Sometimes details do matter....  to bad we never know until after the fact....
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Martin X. Moleski, SJ

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Re: Navigating the LOP with the offset method.
« Reply #14 on: June 30, 2010, 11:51:40 AM »

That is a good argument, but it depends on how literally a person wants to give the exact wording of the logs from the Itasca radio operators.  I think Ric's book mentioned the possibility that the actual transmission could easily have been "on the line from north to south".  It's amazing what a difference recording "and" vs "to" makes.  In this case it would have completely changed the entire direction of the post-loss search activities and the USS Lexington would have thoroughly searched Niku. 

See "Last Words" research bulletin for a detailed analysis of what can and cannot be surmised about the last recorded transmission.

Finding Amelia, p. 100:

"The log is a mess, but it appears that Earhart made a transmission at her regularly scheduled time. Those who heard the message understood her to say, “KHAQQ to Itasca. We are on the line 157 337. We will repeat message. We will repeat this on 6210 kilocycles. Wait.” A minute or so later she added, “We are running on line north and south.”

Pp. 101-102:

"Her statement that she was 'on the line 157 337' was presumed to be a reference to a line of position—a navigational line heading south-southeast (157 degrees) in one direction and north-northwest (337 degrees) in the other—but the line was meaningless without a reference point. She probably meant she was on a line of position that passed through Howland. 'Running on line north and south' would make sense as a means of locating the island given the failure of radio direction finding."
LTM,

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