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Author Topic: The flight plan, magnetic course, headwinds.  (Read 439590 times)

John Ousterhout

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Re: The flight plan, magnetic course, headwinds.
« Reply #420 on: February 26, 2012, 11:23:06 AM »

"Noonan's iPhone batteries" would have been just about the right A-H capacity to recharge with a hand-cranked generator.  Too bad they left it behind ;)
Cheers,
JohnO
 
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Gary LaPook

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Re: The flight plan, magnetic course, headwinds.
« Reply #421 on: February 26, 2012, 11:29:52 AM »

An air courier service makes a daily stop at the airport where I work (Grant County, MWH), on their way from Missoula to Seattle.  They fly an old Beechcraft B18, which has 2 radial engines (I'm not sure which ones this particular aircraft have - likely P&W R-985's which are smaller and likely less noisey than Amelia's engines).  On a still day I can hear them on final approach at a distance of about less than 10 miles.  If there is any wind noise at all, they're virtually silent on approach.

I'm curious how you estimated the distance of 10 miles. Do me a favor, the next time you watch the Twin Beech take off start you stop watch and time it until you can't hear it anymore. You climb a Twin Beech at about 120 so are you sure that you can hear it for a full five minutes? According to Google Earth I live 11.3 SM from Camarillo airport, which is quite busy and I don't hear any planes in the pattern over there. It is also quite common to have T-6s fly directly over my house, often in formations of several planes, and I don't hear them until they are near. I always go outside when I hear them, I love the sound of round engines. I also plan to do the same experiment, I will start my stop watch-----------------while I was typing this I heard one coming, I went outside, watched it pass over and timed it until I couldn't hear it any longer, 44 seconds. Figure about 1.8 SM at 150 mph.

gl
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John Ousterhout

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Re: The flight plan, magnetic course, headwinds.
« Reply #422 on: February 26, 2012, 12:23:15 PM »

Gary,
Timing them is a good idea.  I was estimating distances just based on visual approximation, and knowing accurate distances to some landmarks at 5 and 10 miles.  You're right though, I am just guessing that the Beech is over the landmarks.
http://aeronav.faa.gov/d-tpp/1202/00961AD.PDF
Our plant is just north of rwy 22, making an aircraft on final to 14L about 5 miles from me, in a straight line. 
Cheers,
JohnO
 
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Gary LaPook

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Re: The flight plan, magnetic course, headwinds.
« Reply #423 on: February 26, 2012, 08:44:04 PM »



Update - After thinking on it a bit, does it really matter if they flew a GC or rhumb line flight plan? The celestial navigation accuracy is limited to about 15SM, so does it really matter? Perhaps this is splitting hairs? The only real difference is the number of computations to get you back to the flight line if and when errors are detected. Formulas for both exist and were probably available to FN as well.
A minor but important quibble. The uncertainty in celestial fixes is 10 NM (11.5 SM) and very few fixes fall beyond 10 NM from the actual position. The uncertainty of any one LOP is 7 NM or else you could not achieve 10 NM fix accuracy. You can thank Pythagoras for this.

gl
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Gary LaPook

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Re: The flight plan, magnetic course, headwinds.
« Reply #424 on: February 26, 2012, 09:43:16 PM »


There is also the statement about independent verification of the magnetic variation data used by Williams. Can anyone point me to the source material that was used to verify this data? Would this have came from the U.S. Hydrographic Office or the U.K. Hydrographic Dept that was producing the Air Almanac?
You spend a lot of time worrying about variation, did you ever wonder how the variation was determined in the first place so that it could be displayed on charts? Actually it is pretty simple, you compute what the true azimuth of the sun is and then measure, with an accurate compass, the magnetic azimuth to the sun and the difference is the variation at that point. Of course, in order to be able to compute the true azimuth of the sun you have to know where you are and in order to get a valid compass reading you must not be near any large metal objects since they distort the earth's magnetic field and give erroneous magnetic compass readings. Then after you get your readings you just connect the points of equal variation and draw in the isogonic lines. To be away from large metal objects you must be standing on dirt, not on the deck of a steel ship. Now you might have noticed that there are not very  many spots with dry dirt in the pacific so it is not possible to get many of the variation readings so it is unlikely that you get many with exactly the same variation. So, when you draw in the isogonic lines you end up using a lot if imagination because variation does not necessarily change smoothly between the data points.

It is obviously important on ships to have accurate compasses so normal practice is to compare the ship's compass with the computed azimuth of the sun every morning at sunrise when the sun is low on the horizon making it easier to acccurate measure its azimuth. We can see that they did this on the Itasca. For example, looking at the deck log for July 1, 1937 we see "Gyro compass error 0° by amplitude of sun." (See attached.) Amplitude is a special case of azimuth which is the azimuth at sunrise which can be computed from simpler tables. You find the same entries for the other days in the deck log. Although not in the log they also check the ship's master magnetic compass at the same time.

Here is a link to a description of variation (declination to surveyors.) Remember that they did not have computer models of variation in 1937. Here is the warning that comes with the models.


"Magnetic reference field models give results that are typically accurate to about 30 minutes of arc, but the difference between the model value and the true value of the magnetic field at a given location is dependent on a number of factors:

    * The accuracy of the model will worsen at locations close to the magnetic poles.
    * As the time from the epoch of the model increases, uncertainties in the estimate of secular variation will result in an increasingly large difference.
    * Magnetic minerals in local geological formations cause magnetic anomalies that can sometimes be very large. These cannot be reproduced by reference field models.
    * Large magnetic storms can temporarily cause large changes in the magnetic field, especially at high latitudes."


But my point is that you put way to much effort into your computations as they relate to variation, which can not be perfectly known at sea.

gl
« Last Edit: February 26, 2012, 10:08:47 PM by Gary LaPook »
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Gary LaPook

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Re: The flight plan, magnetic course, headwinds.
« Reply #425 on: February 27, 2012, 12:45:00 AM »



Of course there are factors that affect what you can hear.  Gary, was your lunch inside a restaurant or outside?  What way was the wind blowing?  How close was the runway? 


I was on the patio, 2500 feet from the end of the runway where the takeoff roll started and the wind was blowing from the west, basically from the airplane to me.

gl
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Heath Smith

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Re: The flight plan, magnetic course, headwinds.
« Reply #426 on: February 27, 2012, 04:26:04 AM »

Quote
But my point is that you put way to much effort into your computations as they relate to variation, which can not be perfectly known at sea.

Gary,

Thank you for the background information on how the magnetic variations were obtained. It is interesting that back in the day, the actual latitude and longitude of Lae itself was not exactly determined from what I had read on the TIGHAR page The World Flight, Second Attempt: Intended Route to Howland. While that was probably not important for this discussion, it is interesting that not even the lat/lon was precisely determined in the day.

The magnetic variations that I used are from the Williams strip chart. Although I did play with the model data it was not used in the rhumb line example. The model data and the actual observations of the data seem to be fairly close. I found an old page that you had posted previously that gave some basic information about the Lae airport. The magnetic variation was listed as 5.5 in 1935. The model said 5.48 for that date at Lae. I think that is a pretty good agreement. Whatever source they were using, it was probably fairly accurate.

As to the importance of the magnetic variation data, yes, I believe it was just as important as the accuracy of the great circle course or the rhumb line course itself and one cannot be considered without taking the other in to account. Errors in one or the other could land you tens of miles off course which is what we have been discussing. So I believe you cannot do any sort of meaningful attempt at flight reconstruction without considering the magnetic variation. You can use model data or you can use the Williams data, the results are much the same. The model data puts you a bit closer to Howland and this might be a bit more accurate description of where the intended flight line was on that day (in my opinion).

I did read a snippet from a book where FN said that 10NM was an implied error. I thought that I had recently read another one of his writings that said that a 15NM error was implied. I will try to dig that up after work today. To run with the rhumb line example for a moment, if you have an assumed error of 11.5SM, and you were directly on a flight line that terminated 15SM North of Howland, does that not imply that a celestial fix cannot tell you if you were 3.5SM or 26.5SM away from your target? Maybe I am missing the meaning of "implied" error. I took this to mean a maximum accuracy.

My question is now whether this seemingly small error or bias to the North could suggest that this is where they ended up, North of Howland, narrowly missing it, maybe within visual range. From what I understand most of the theories out there have them falling short and to the South. I have never really read any backing information on this other than the suggestion that the winds were stronger than they had anticipated. If FN was able to get a fix that morning, this seems like a very unsatisfactory explanation. I believe TIGHAR now thinks they were far more Southerly than anyone had thought previously although I do not know the rational behind that either.
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Gary LaPook

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Re: The flight plan, magnetic course, headwinds.
« Reply #427 on: February 28, 2012, 01:04:41 AM »


I did read a snippet from a book where FN said that 10NM was an implied error. I thought that I had recently read another one of his writings that said that a 15NM error was implied. I will try to dig that up after work today. To run with the rhumb line example for a moment, if you have an assumed error of 11.5SM, and you were directly on a flight line that terminated 15SM North of Howland, does that not imply that a celestial fix cannot tell you if you were 3.5SM or 26.5SM away from your target? Maybe I am missing the meaning of "implied" error. I took this to mean a maximum accuracy.


Let's say, that when you drive to the airport to catch a flight it usually takes you about one hour to get there but a couple of times it has taken you as long as two hours when traffic was bad. You have to catch a 3 p.m. flight tomorrow to go for a job interview in Chicago for a job that will pay $200,000 a year if you get it. What time will you leave for the airport tomorrow? I'll bet you won't wait to leave your house until 2 p.m. allowing only the usual one hour to get to the airport since you might run into traffic tomorrow and you can't afford to miss that flight. You probably won't wait until 1 p.m. either since tomorrow might be the day to set a new record for a traffic delay so you might actually leave at noon to allow even extra time to get to the airport because of the importance of catching the flight.

Then tomorrow you leave at noon and it takes the usual one hour to get to the airport so you kill two hours waiting for your flight.

The 10 NM "uncertainty" in the fix is analogous to the three hours "uncertainty" in the time it might take you to get to the airport, it is a number you use for planning, to make sure you plan your course of action in a way that will make sure that you safely arrive at your destination. But most of the time the fixes will be much more accurate, just like most of the time you will get to the airport in just one hour. About two-thirds of the time the fix will be within 5 NM so only one-third of the time will the fix exceed five NM and almost all of those will be within 10 NM. So you have it exactly backwards, it is not the maximum accuracy but it defines the limit of the maximum error that is likely to occur in the fix. It may be possible that you might, in a very rare case, shoot a fix that is more in error than 10 NM but it is very unlikely, just like it might take you more than three hours to drive to the airport but that is very unlikely based on your prior experience. But you still allowed the full three hours just in case.

For more information about fix accuracy go here.

gl
« Last Edit: February 28, 2012, 01:07:37 AM by Gary LaPook »
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Heath Smith

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Re: The flight plan, magnetic course, headwinds.
« Reply #428 on: February 28, 2012, 04:04:26 AM »


I found that snippet from the Fred Noonan’s PanAm Memo.

I should have not used the term maximum but rather average as you suggest. With an average error window of +/- 10NM to 15NM, with an offset of 15SM from Howland, it is understandable why they missed Howland. After everything I have looked at I believe they were very close and not short at the end of the flight. This brings us back to why they did not find Howland on the subsequent pass or passes running N/S on the 157/337. As we had discussed previously, if they did not double back after 19:12GMT and before 19:28GMT, this would have put them out of visual range on their N/S pass. Given that they were received with a signal strength of 5 and hour later, they might have gone North for 30 minutes then turned South, or vice versa. This trip would have cost them a full hour of fuel without making any progress. In any case, I think they were long on the target and passing Howland on the East side as they searched. Where they went from there is anyone's guess. Perhaps we could work on a fuel consumption estimates using the Lockheed 487 report? I started a thread over in the power-plant section of the forum here. I am curious to know what you guys think about a simple procedure to estimate full consumption.
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Gary LaPook

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Re: The flight plan, magnetic course, headwinds.
« Reply #429 on: February 28, 2012, 01:13:55 PM »




I should have not used the term maximum but rather average as you suggest. With an average error window of +/- 10NM to 15NM, with an offset of 15SM from Howland, it is understandable why they missed Howland. After everything I have looked at I believe they were very close and not short at the end of the flight.
Here is a letter Noonan sent to Weems saying that 10 miles is the average accuracy.

gl
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John Ousterhout

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Re: The flight plan, magnetic course, headwinds.
« Reply #430 on: February 28, 2012, 02:15:40 PM »

Timing audibility of a twin Beech - The B18 began its takeoff roll at a distance of a little more than 2 miles from where I was standing to the North of runway 22.  From first visible motion until I could no longer hear the engines at all was 2 minutes, 51 seconds.  At 120 mph, that's a distance travelled of only 5.7 miles, made even shorter by virtue of taking off somewhat towards me, then turning and departing the vicinity nearly straight away from my position.  My previous eyeball estimate method would have assumed it was about 10 miles away when I could no longer hear it or see it, but now I believe it was less than 5 miles.  It was very hard to make out against the scattered cloud layer with the naked eye.

Ambient noise level was relatively low at the time - no other traffic, very light wind, some light industrial noises in the distance.  Weather cold (below freezing), scattered clouds at a couple thousand feet AGL, early morning light (<0800).  Under the conditions I experienced this morning, a Lockheed Electra flying 5+ miles from my location at 1000 feet would have been impossible for me to spot by unaided eye, or hear be ear.

Cheers,
JohnO
 
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Heath Smith

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Re: The flight plan, magnetic course, headwinds.
« Reply #431 on: February 28, 2012, 02:33:08 PM »


I should have not used the term maximum but rather average as you suggest. With an average error window of +/- 10NM to 15NM, with an offset of 15SM from Howland, it is understandable why they missed Howland. After everything I have looked at I believe they were very close and not short at the end of the flight.

Here is a letter Noonan sent to Weems saying that 10 miles is the average accuracy.

gl

I am not sure that it really matters but wasn't this discussion about the same round trip from Hawaii to California? It would appear that FN certainly said 10 to 15NM in his Pan Am memo. Perhaps he later changed it the phrase when writing Weems who later wrote a book and included Fred's writing or perhaps Weems just edited the book as he saw fit. In any case, I think this is splitting hairs, 10 or 15NM should not change anything.
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Gary LaPook

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Re: The flight plan, magnetic course, headwinds.
« Reply #432 on: February 28, 2012, 02:58:07 PM »

Timing audibility of a twin Beech - The B18 began its takeoff roll at a distance of a little more than 2 miles from where I was standing to the North of runway 22.  From first visible motion until I could no longer hear the engines at all was 2 minutes, 51 seconds.  At 120 mph, that's a distance travelled of only 5.7 miles, made even shorter by virtue of taking off somewhat towards me, then turning and departing the vicinity nearly straight away from my position.  My previous eyeball estimate method would have assumed it was about 10 miles away when I could no longer hear it or see it, but now I believe it was less than 5 miles.  It was very hard to make out against the scattered cloud layer with the naked eye.

Ambient noise level was relatively low at the time - no other traffic, very light wind, some light industrial noises in the distance.  Weather cold (below freezing), scattered clouds at a couple thousand feet AGL, early morning light (<0800).  Under the conditions I experienced this morning, a Lockheed Electra flying 5+ miles from my location at 1000 feet would have been impossible for me to spot by unaided eye, or hear be ear.
That seems more reasonable regarding hearing the round engines. It is hard to spot airplanes in flight. I took the deposition of an FAA visibility expert witness, in a mid-air case, who testified that airplanes are rarely spotted at more than one mile. And this will scare you pilots out there, I forced him to admit, all under oath, that a pilot, using the FAA's recommended technique for scanning for traffic, would be unlikely to see an incoming plane prior to the mid-air collision. An incoming plane, closing at 120 knots, would only be visible for 30 seconds from the time it came within the one mile visibility range until the collision. The recommended scanning procedure calls for stopping the gaze for several seconds at spots every 10°. So if you looked out past the right wingtip and then made about 18 stops for three seconds each as you shifted your gaze from the right wingtip, past the nose, and on to the left wingtip scanning a total of 180°, it would take about a minute before you could again look for that incoming collision threat out by the right wingtip that had not been close enough to be seen the last time you looked that way. So you all be careful out there now.

gl
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Gary LaPook

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Re: The flight plan, magnetic course, headwinds.
« Reply #433 on: February 28, 2012, 03:04:48 PM »


I should have not used the term maximum but rather average as you suggest. With an average error window of +/- 10NM to 15NM, with an offset of 15SM from Howland, it is understandable why they missed Howland. After everything I have looked at I believe they were very close and not short at the end of the flight.

Here is a letter Noonan sent to Weems saying that 10 miles is the average accuracy.

gl

I am not sure that it really matters but wasn't this discussion about the same round trip from Hawaii to California? It would appear that FN certainly said 10 to 15NM in his Pan Am memo. Perhaps he later changed it the phrase when writing Weems who later wrote a book and included Fred's writing or perhaps Weems just edited the book as he saw fit. In any case, I think this is splitting hairs, 10 or 15NM should not change anything.
I just try to be accurate and the 10 mile number is consistent with the standard flight navigation texts and the Federal Aviation Regulations that I referred you to and with my own experience. And it does make a difference in trying to figure out their actions near Howland. Adding 5 NM to the uncertainty, which is a radius around the fix, increases the diameter of the uncertainty circle by 10 NM making the distance needed to search along the LOP 10 NM longer if they used the landfall procedure and adding 15 NM if they didn't. 

Even if you want to use 15 NM for the uncertainty of a fix for your computations, remember that the important thing when using a single LOP to find an island is not the uncertainty in a fix but the uncertainty in the LOP which is about 10 NM if the fix uncertainty is 15 NM. So, for planning the approach to Howland, even if Noonan used the 15 NM fix, 10 NM LOP uncertainty, then he would have been confident that he would pass within 10 NM of the island and actually expect to be closer.



gl
« Last Edit: February 28, 2012, 06:16:12 PM by Gary LaPook »
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Heath Smith

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Re: The flight plan, magnetic course, headwinds.
« Reply #434 on: February 29, 2012, 03:52:29 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?

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?

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?
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