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Author Topic: Navigating the LOP with the offset method.  (Read 161852 times)

Mona Kendrick

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Re: Navigating the LOP with the offset method.
« Reply #30 on: July 04, 2010, 01:40:07 AM »

According to the cockpit photos of the of the Electra and Lockheed literature, the 10E had a heading indicator or directional gyro which allowed  Earhart to fly a true course.  She also had a Sperry gyro-horizon, Pioneer compass, altimeter, airspeed indicator, turn and bank, rate of climb and a Sperry autopilot which held the plane on the course selected. The magnetic compass was a standby compass and had a deviation card which indicated the compass heading to steer a true heading.

?? I don't understand what is meant by the directional gyro allowing her to fly a true course.  Didn't the DG have to be set to match the magnetic compass?  The deviation card for the magnetic compass would only give corrections for magnetic errors unique to the plane, not for magnetic variation.

Mona
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Erik

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Re: Navigating the LOP with the offset method.
« Reply #31 on: July 04, 2010, 06:44:34 AM »


Quote
When Noonan clocked the sunrise he knew according to his nautical almanac where the dawn line that he was on was located and that the angle of that line was, 157/337, for that day. The line was a true heading plotted on his chart.

I would like, if possible to preserve the word "heading" for a single compass direction in which one pilots an aircraft.  A line on a map doesn't have one heading; it has two (e.g., 337/157), and the two are not interchangeable (flying a heading of 157 degrees is about as different as you can get from flying a heading of 337 degrees).

Noon almost certainly kept a track of the plane's estimated position and heading on his charts as he took fixes or did dead reckoning.  From those he would derive a heading for AE to fly.  The original dawn LOP on the chart would have been intersected by the line representing their estimated course made good across the ocean and their current "heading."

Good point.  Some definition clarifications:

Heading = The direction the plane's nose is pointing.  This can obviously be very different from the plane's actual direction of travel, especially with wind correction angles.  Pilots call this phenomenon 'crabbing'.

Bearing = The direction the plane is actually traveling across the ground (or in this case water).  Sometimes referred to as track or course.

* when speaking of these two, it is extremely important to differentiate between TRUE and MAGNETIC *

Magnetic Variation = This is the angle between TRUE north and MAGNETIC north.  Sometimes also called magnetic declination.  This number has an infinite number of possiblilites depending upon the earth's magnetic field and your geographic location.  

Magnetic Deviation = Similar to variation, this is the correction angle to account for localized magnetic disturbances of the metal in the airplane itself.  This number is usually a fixed number and does not change as long as you are always in the same airplane.

« Last Edit: July 04, 2010, 07:06:29 AM by Erik »
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Erik

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Re: Navigating the LOP with the offset method.
« Reply #32 on: July 04, 2010, 07:43:12 AM »

According to the cockpit photos of the of the Electra and Lockheed literature, the 10E had a heading indicator or directional gyro which allowed  Earhart to fly a true course.  She also had a Sperry gyro-horizon, Pioneer compass, altimeter, airspeed indicator, turn and bank, rate of climb and a Sperry autopilot which held the plane on the course selected. The magnetic compass was a standby compass and had a deviation card which indicated the compass heading to steer a true heading.

?? I don't understand what is meant by the directional gyro allowing her to fly a true course.  Didn't the DG have to be set to match the magnetic compass?  The deviation card for the magnetic compass would only give corrections for magnetic errors unique to the plane, not for magnetic variation.

Mona

Correct. 

The DG (directional gyro) would have had to have been manually adjusted for both magnetic variation and magnetic deviation. 

There was not a way back then (or even today) to 'automatically' adjust a DG for these types of variables.  These would have to be constantly adjusted throughout flight. 

Typically today, a deviation card on the compass and isogonic lines on charts are used to make these corrections.  Back then, I would be curious to know what source of information was available for magnetic variation?  And equally important is how accurate that info would have been? 

The earth's magnetic field is very tricky.  Remember, 1 degree of angle equals 1 mile of error over the course of 60 miles.  This would have been especially critical when dealing with flying along the LOP itself, as it is most likely that magnetic compass would have been the primary means of navigation.

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Bill Lloyd

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Re: Navigating the LOP with the offset method.
« Reply #33 on: July 04, 2010, 08:13:38 AM »

I would like, if possible to preserve the word "heading" for a single compass direction in which one pilots an aircraft.  A line on a map doesn't have one heading; it has two (e.g., 337/157), and the two are not interchangeable (flying a heading of 157 degrees is about as different as you can get from flying a heading of 337 degrees).

Noon almost certainly kept a track of the plane's estimated position and heading on his charts as he took fixes or did dead reckoning.  From those he would derive a heading for AE to fly.  The original dawn LOP on the chart would have been intersected by the line representing their estimated course made good across the ocean and their current "heading.

You are correct. When AE reached what Noonan thought was the LOP, if she turned NW to 337 she would be flying a heading of 337, if she turned SE to 157, she would be flying a heading of 157, both would be true courses. The LOP would be a true course line as opposed to a magnetic course line as someone previously wrote.

Noonan certainly would have kept track of his estimated position and heading as he progressed and would give AE a true heading to fly. What he probably did not know was what the wind was doing to their track across the water. Ric has estimated that they arrived at the LOP about 200 miles SE of Howland, that being the case, then there was a very strong crosswind component out of the NE.   

 
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Bill Lloyd

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Re: Navigating the LOP with the offset method.
« Reply #34 on: July 04, 2010, 08:43:08 AM »

According to the cockpit photos of the of the Electra and Lockheed literature, the 10E had a heading indicator or directional gyro which allowed  Earhart to fly a true course.  She also had a Sperry gyro-horizon, Pioneer compass, altimeter, airspeed indicator, turn and bank, rate of climb and a Sperry autopilot which held the plane on the course selected. The magnetic compass was a standby compass and had a deviation card which indicated the compass heading to steer a true heading.

?? I don't understand what is meant by the directional gyro allowing her to fly a true course.  Didn't the DG have to be set to match the magnetic compass?  The deviation card for the magnetic compass would only give corrections for magnetic errors unique to the plane, not for magnetic variation.

Mona
Yes,the DG must be set to the mag compass. This is one of the final checks a pilot would make before takeoff.  The gyro is either electrically or vacuum operated. The primary advantage of the heading indicator is that it is much easier to make standard rate turns and roll out on a specific true heading as opposed to using a mag compass. In my experience, the heading indicator does not need to be reset all that often. On an instrument check ride, one of the emergency procedures is to be able to fly on the mag compass if the DG fails. I don't recall reading anywhere that AE had a problem with her flight instruments. The only problem recorded, alludes to the Cambridge Exhaust Gas Analyzer.


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Walter Runck

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True Virgins Make Dull Company So, Add Whiskey!
« Reply #35 on: February 11, 2011, 05:49:15 PM »

A couple of comments on statements earlier in this thread:
Quote
The original Line of Position (LOP) seems to have been derived from a dawn sighting.  I presume, but do not know as a matter of fact, that the tables used for celestial navigation would have given results in true rather than magnetic terms.  Drawing the line that indicates one's location at dawn isn't at all dependent on compass readings but a trigonometric calculation based on visual information (seeing the sun break the horizon), altitude, and time.  The line comes as the answer to the question, "Where in the world would you have to be to see the sun come over the horizon at that time of day?"

Marty is correct.  Celestial navigation is a matter of mathematics, not earth science, so all directions derived from it are True (meaning referenced to the North Pole).  The art of celestial navigation is based on spherical trigonometry and the relative motion of the earth and other heavenly bodies, not on the wandering location of our Magnetic North Pole.  Latitude and longitude are measured with reference to the True North and South Poles, the Equator and an arbitrary line through the Royal Observatory in Greenwich.  
Quote
Good point.  Some definition clarifications:
Heading = The direction the plane's nose is pointing.  This can obviously be very different from the plane's actual direction of travel, especially with wind correction angles.  Pilots call this phenomenon 'crabbing'.
Bearing = The direction the plane is actually traveling across the ground (or in this case water).  Sometimes referred to as track or course.
* when speaking of these two, it is extremely important to differentiate between TRUE and MAGNETIC *
Magnetic Variation = This is the angle between TRUE north and MAGNETIC north.  Sometimes also called magnetic declination.  This number has an infinite number of possibilities depending upon the earth's magnetic field and your geographic location.  
Magnetic Deviation = Similar to variation, this is the correction angle to account for localized magnetic disturbances of the metal in the airplane itself.  This number is usually a fixed number and does not change as long as you are always in the same airplane.

Personally I like the term track to describe where someone or something has actually been.  Bearing can mean a couple of things in navigation; usually the direction of a sighting of something besides whatever you're in or on.  You can have bearings on different things at the same time, a lighthouse, a radio beacon and a point of land.  Three-arm protractors have three arms for a reason.  Plus they're fun to play with.  More fun than "Bearing constant, range decreasing!", which is navspeak for a collision course.  
For my money, 337-157 is pair of reciprocal directions.  Nothing more, until you give me a point that it passes through.  Now it is a LOP.  If you're on it and travelling SSE, now it is a heading of 157 and the 337 is your six o'clock .  Hopefully there is some land ahead.
Magnetic North is somewhere in Canada at the moment and travelling Northwest.  Magnetic compasses point to it wherever it happens to be, so you have to account for the difference in polar locations in order to get from a celestial observation or other True direction, to a magnetic one.  At any particular location, this difference is an angle and it is called Variation, and it changes depending on where you are in the world.  Variation is marked on a marine or aeronautical chart by a compass rose, with True North marked on the outer ring and Magnetic on an inner.  The variation is listed inside the rose (at least on a NOAA chart) along with the year it was determined and the rate and direction of annual change.  So, to get from True to Magnetic, you need to know roughly where you are (Charleston, SC  32 47' N  079 55' W) and the year (2011).  The harbor chart 11524 shows variation of 7 deg 15' W in 2008 with an annual increase of 3'.  3' per year for the three years since 2008 gives me a 2011 variation of 7 deg 15' W plus (3 x 3) equals 7 deg 24' W.
So if I take a magnetic compass and sight across the center and 007 degrees 24 minutes, I will be looking at True North.
Unless there's some iron nearby.  Cuz that can mess up a compass.  So now we need to account for local conditions.  Metallic aircraft or boat structure or wires carrying electric current can deflect a compass by interfering with the earth's magnetic field.  Once a compass is installed in a ship or aircraft, you can only account for the local (as-installed) weirdness by preparing a Deviation Card.
Deviation can and will change depending on your heading, so a card will show deviations at a number of directions all around the compass.  Most recreational marine compasses have adjustments that allow you to limit deviation with small magnets; big units for ships have large iron balls that are used to compensate for local effects.  Either way, unless deviation is zero all around, the only way to accurately account for it is to develop, maintain and use a deviation card.
This is done by "swinging the compass" - actually turning the boat, ship or plane in known directions and reading the compass indications.  Take the readings in each direction, compute the errors and appropriate corrections and retain for future use on a deviation card.
Not that I think it had anything to do with the loss, but it's a common mistake to think that if the compass is off by plus 5 degrees heading east, it will also be off by plus five heading west.  If you have a tool or wire to the right of the compass, it might make the compass magnet at its north pole deflect to the right (east) when you're heading north, resulting in a low compass reading , but pull it to the right (west) when you are heading south, leading to a high reading .
Check the DC illumination wires on a compass, they are probably twisted together to try and cancel out magnetic effects.  The same for other series circuit wiring in a well done binnacle.
Now for the mnemonic:
True
Virgins
Make
Dull
Company
So,
Add
Whiskey
True plus Variation = Magnetic, Magnetic plus Deviation = Compass.  Westerly corrections are added, so that's the last part.  Works in reverse, too, for when you/re trying to get from the real world to a nautical almanac.
We tend to ignore stuff like this in the days of satellite and other electronic navigation because our guidance systems either are not subject to them or do the compensation for us, but in 1937, this was find it or don't type stuff.  Maybe even life or death type stuff and FN took it seriously.  They swung the compasses in the Miami hanger before they left on the last test flight and rechecked them once in the air over ranges familiar to FN.  He may have been willing to bet his life that the RDF would work, but he wasn't missing any bets on being able to do his own DR.  Remember, he'd made numerous RDF departures and approaches, so he would know just how lost they would be without it.  
« Last Edit: February 12, 2011, 08:35:27 AM by Walter Runck »
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h.a.c. van asten

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Re: Navigating the LOP with the offset method.
« Reply #36 on: April 23, 2011, 04:21:49 AM »

Mark Petersen . If a navigator lets communicate his craft is on an Line of Position , after having messaged to be over a small island , two conditions are clear : 1. The LOP runs over the island´s coordinates , 2. The APL (Aircraft Progression Line) was along the LOP. Most probaly the One Line Approach (Noonan was experienced with it) was used from a point 150 st.mls. off Howland on the main 072 T course with a 102 mls offset lane to compass 055 T. The risk , namely , to miss a very small target by direct approach is so considerable that the "Find the Island" (name given by Francis Chichester) fashion is the first only with a good reliability , first demonstrated bij captain Sumner in 1837 by one observation on the sun , the LOP in the direction of a lightvessel which actually showed up after some time sailing the line along. At 1744 Z Earhart asked for a bearing "on the hour" , to verify the aircraft having Howland straight ahead when still on the main course (072) , at 1815 she asked for a bearing at 1845 to assure the offset being northward when nearing the TOP (turn Off point) for steering 157 T. Howland did not run in sight for reasons explained elsewhere , by which the target was 16 miles on the port bow in lieu of below the APL at 1912 GMT. It is true that a "Fixed Square Search" , set in if a target does not run up , is the most reliable under all circumstances. It is however , very waisteful on fuel  so that it can only be used if reserves are for several hours. One of the indications that fuel was low (45 US gall. all in) at 1912 Z is that a FSS was not tried : such operation , namely , has a 100 percent succes rate.
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Ric Gillespie

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Re: Navigating the LOP with the offset method.
« Reply #37 on: April 23, 2011, 08:41:20 AM »

If a navigator lets communicate his craft is on an Line of Position , after having messaged to be over a small island ,

Neither Earhart nor Noonan ever messaged that they were over a small island.

The risk , namely , to miss a very small target by direct approach is so considerable..

You have a basic misunderstanding of Earhart's plan for finding Howland Island. It was never intended that Noonan would find the island by celestial/DR.  His job was to get the flight close enough for the final run in to be accomplished by Radio Direction Finding. By the time Earhart realized that RDF was not going to work she was already on the advanced LOP ("We must be on you..." - too late to use any kind of offset.
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h.a.c. van asten

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Re: Navigating the LOP with the offset method.
« Reply #38 on: April 23, 2011, 02:27:12 PM »

T.Campbell / Moleski. A sunset fix was established at 175453 GMT over coordinates 178deg47´-W/00deg09´N from 1,000 ft altitude with the mariner´s sextant. The 200 miles out message originated from Nikunau Atoll having been reached at 1540 GMT and concerned 300 st.mls made good at 1740 GMT. The precomputed distance Nikunau-sunrise on board was 348 mls whereas 50 miles , 20 min. later ("100 mls out" , 181453 GMT ,  radio 1815) the offset course 055 T was set in for a 102 mls distance to the turn-off-point on the 157-337 line at 30 mls northwest of Howland´s assumed position. Arriving in the Howland region could never be done by RDF at random so that astro was necessary , as well as a one-line approach , especially since the aircraft was not flown over the great circle but by loxodromic (rhumb line) curves. Up to 1815 speed over ground was 150 mph , afterwards 140 mph , to "arrive" over Howland 1912 GMT. For island not running in sight see elsewhere. It has been recorded (letter capt. Irving Johnson who 1940 visited the Gilberts for search) that an aircraft flew , high altitude , over the island Tabiteuea in the same 1deg23´ latitude as Nikunau , the aircraft (Earhart´s) coming from an initial point 123 mls southwest of Nauru ("Lights in sight ahead" , radio) for a distance of 766 mls.
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Ric Gillespie

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Re: Navigating the LOP with the offset method.
« Reply #39 on: April 23, 2011, 06:05:23 PM »

A sunset fix was established at 175453 GMT over coordinates 178deg47´-W/00deg09´N from 1,000 ft altitude with the mariner´s sextant.

Baseless speculation.

The 200 miles out message originated from Nikunau Atoll having been reached at 1540 GMT and concerned 300 st.mls made good at 1740 GMT.

Nikunau is not an atoll and it's 506 statute miles from Howland.

The precomputed distance Nikunau-sunrise on board was 348 mls whereas 50 miles , 20 min. later ("100 mls out" , 181453 GMT ,  radio 1815) the offset course 055 T was set in for a 102 mls distance to the turn-off-point on the 157-337 line at 30 mls northwest of Howland´s assumed position.

Baseless speculation.

Arriving in the Howland region could never be done by RDF at random so that astro was necessary,

Yes, that was Noonan's job, to use astro and DR to get the flight to the Howland region  - i.e. within RDF range.  That is exactly how the Pan Am flights were made and how Noonan and Manning navigated the Oakland/Honolulu flight in March 1937.

as well as a one-line approach , especially since the aircraft was not flown over the great circle but by loxodromic (rhumb line) curves.

It is irrelevant whether Noonan used a great circle or rhumb line curves. The fact is that he clearly got the airplane to with radio reception range of Howland.


 Up to 1815 speed over ground was 150 mph , afterwards 140 mph , to "arrive" over Howland 1912 GMT.

You're backing into the numbers.


 For island not running in sight see elsewhere. It has been recorded (letter capt. Irving Johnson who 1940 visited the Gilberts for search) that an aircraft flew , high altitude , over the island Tabiteuea in the same 1deg23´ latitude as Nikunau,

In his letter to Bessie Young of June 4, 1940, Irving Johnson says only that a missionary on Beru told him that he had from a native that "...it was believed that the Earhart plane had flown eastward high up over the island of Taputeouea."  The missionary cautioned that "it was hard to tell whether some ignorant native had actually seen an airplane or wished he had.."  Maybe a plane was heard or seen, maybe not, but to say that it was reported "in the same 1deg23´ latitude as Nikunau" is incorrect.  Tabiteuea is a long skinny atoll stretching from 1° 06 ' in the north to 1° 32' in the south. Johnson's third-hand account says nothing about where on the atoll the plane was supposedly heard or seen.


 the aircraft (Earhart´s) coming from an initial point 123 mls southwest of Nauru ("Lights in sight ahead" , radio) for a distance of 766 mls.

The reported message heard by Nauru was "Ship in sight ahead." Probably SS Myrtlebank but no way to be sure.
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h.a.c. van asten

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Re: Navigating the LOP with the offset method.
« Reply #40 on: April 24, 2011, 05:28:44 AM »

R.G. "Mariner´s sextant for back up only" . Whereas a bubble sextant is needed to observe from higher altitudes (no visible horizon) , the mariner´s sextant is useful c.q. needed  for two reasons : 1. at lower (p.e. 1,000 ft) altitudes with visible horizon. 2. If sunrise of the upper limb is to be observed. Almanac tables register "sunrise" in LMT for U.L. on the horizon. Only the U.L. is useful since due to refraction the lower limb suffers severe distortion. Thence ,  for a sunrise position check flying low and using the mariner´s sextant are unconditional necessities. An artificial horizon sextant is unable to register on the upper limb. A sunrise fix at p.e. 1744-45 GMT at 200 mls off Howland was impossible since the sun was for the longitude still below the horizon. As a third point  mariner´s sextant observation features  considerably more precision w.r.t. operating the bubble sextant. The ship´s sextant was p.e. also used by Francis Chichester for one line approaches ("Find the Island") to small islands (a.w. Lord Howe) when crossing the Tasman Sea by airplane.
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h.a.c. van asten

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Re: Navigating the LOP with the offset method.
« Reply #41 on: April 24, 2011, 05:45:17 AM »

It has btw been said by researchers that Noonan´s bubble sextant observations may have been injured by Coriolis disfigurement of the bubble position w.r.t. the vertical crosshair. Coriolis displacement (to r.h.side for N-hemisphere) is a function of the Coriolis force , this being very small , and the speed of the aircraft which consequently must be very high to induce miscalculation. The maximum Lockheed Electra airspeed was by far too slow for occasioning any measurable bubble shift.
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h.a.c. van asten

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Re: Navigating the LOP with the offset method.
« Reply #42 on: April 25, 2011, 12:00:07 AM »

"lights in sight ahead" . In an article , National Geographic , 1935 , Earhart confirms the non-visibility of ships by night from an aircraft 8,000 ft up : she thought it unlikely that she would ever see any vessel generally on the surface of the large sea when flying thousands of feet above it. When flying the great circle (chord no.9) it would have been possible to see the ship Myrtlebank the more or less ahead. But the great circle was not flown and it seems best to follow those who heard "Lights in sight ahead" : the lights of Nauru at 1030 GMT from 123 mls distance.
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Ric Gillespie

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Re: Navigating the LOP with the offset method.
« Reply #43 on: April 25, 2011, 09:04:21 AM »

"When flying the great circle (chord no.9) it would have been possible to see the ship Myrtlebank the more or less ahead. But the great circle was not flown and it seems best to follow those who heard "Lights in sight ahead" : the lights of Nauru at 1030 GMT from 123 mls distance.

I say again, the correct quote is "ship in sight ahead."
The original source is a State Department telegram from Sydney, Australia dated July 3m 1937, which reads: “Amalgamated Wireless state information received that report from ‘Nauru’ was sent to Bolinas Radio ‘at 6.31, 6.43 and 6.54 PM Sydney time today on 48.31 meters (6210 kHz), fairly strong signals, speech not intelligible, no hum of plane in background but voice similar that emitted from plane in flight last night between 4.30 and 9.30 P.M.’ Message from plane when at least 60 miles south of Nauru received 8.30 P.M. Sydney time, July 2 saying ‘A ship in sight ahead.’ Since identified as steamer Myrtle Bank sic which arrived Nauru daybreak today."

the lights of Nauru at 1030 GMT from 123 mls distance.

"New Nauru fixed light latitude 0.32 S / longitude 166.56 E five thousand candlepower 560 ft above sea level visible from shios to naked eye at 34 miles"

Earhart was heard to say "Ship in sight ahead" at 1030Z.  Roughly three hours earlier she had said she was at 8,000 feet "over cumulus clouds."  We don't know her altitude or the cloud conditions at 1030Z but you're suggesting that she saw "lights" (not a light) from 123 miles away.
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Walter Runck

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Re: Navigating the LOP with the offset method.
« Reply #44 on: April 25, 2011, 08:10:30 PM »

The problem I see with the concept of making this flight by the offset method is based in the uncertainty associated with flight parameters and the accumulated errors produced by them.

Offsets only make sense if they don't add more risk or error than they compensate for.  For example, if you're flying from Havana to Key West (roughly 020T) and for some reason don't have enough fuel to continue on to mainland Florida if you miss to the west, it makes sense to shade your course to the right (say 030T) and turn west when you see the Keys.  The Keys are fixed, visible to the naked eye, they lead to Key West and you have a very high probabliity of finding them, so you're not introducing new error or uncertainty.  You are, however, guaranteeing a longer flight.

Doing the same thing based on a DR track buys you very little, and at quite a price in terms of risk and uncertainty.  For simplicitys sake, consider a flight from a point on the equator to a small island 1000 nm east on the equator.  If you think an offset is a good idea, go ahead and fly to a spot in the ocean (not an island, coastline, depth contour or lighted range, just a random-looking patch of ocean) exactly 100 nm south of the island, then hang a left.  The island will be exactly 100 nm ahead of you.  Piece of cake.

If:  You were able to hit the correct longitude.  At night.  No identifiable landmarks.  No decent winds aloft data.  Maybe a single LOP a couple hours before ETA.

If AE and FN intended to try this, it would require that they not home in on the RDF bearings they intended to be picking up.  As Itasca and/or Howland came into range, they would have to watch the bearing move from almost dead ahead to straight off the port wing and then and only then only then turn at that point.  Then fly into their destination from a direction 90 degrees from their origin.

Without telling the people that would be looking for them that they would be coming in from the south. 

Seems a bit of a stretch to me.
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