There was a second airspeed indicator mounted on the copilot's side of the instrument panel which was the reason for the second pitot tube. Noonan' could have been taped into either one. I don't know if there were two separate static ports for the two altimeters, the copilot had one too, they could have been using just one static port and Noonan's would have used the static pressure line for his altimeter.
This would be very useful to know especially if we believe that the belly antenna was indeed ripped off at take off at Lae. Just as a theory, maybe they did extend the co-pilot instrument back to Noonan and when the antenna was ripped off, this tweaked his readings as compared to what Earhart might have seen.
If this was the actual pitot tube, is it possible that a piece of the back of the tube was sheared off in the process? Could this cause Noonan to read higher indicated air speeds than was reality?
For whatever reason, I believe that they were achieving a ground speed of 124.5 all the way from Lae to where the Ontario was spotted. The timing is too coincidental to be otherwise (my opinion). If you assume they were achieving 150mph after departing from Nukumanu Island (since they reported a head wind of 23 knots), you are somewhat forced in to assuming that the Myrtl Bank was the ship spotted (or some yet unknown mystery ship) even though very little is known about her true where abouts. This would put the Myrtle Bank about 29 miles off the planned flight path, compared to the Ontario at 8 miles off. I suppose you could think that they flew far North of the Ontario and spotted the Myrtle Bank instead but this would infer a major navigational malfunction. This seems highly unlikely since they had a good fix 3.2 hours earlier and were right on the flight line.
Assuming that they did in fact spot the Ontario, which makes a lot of sense since she was only 8 miles north of the flight path, the distance from that point at 10:30GMT to the "200 miles out" message at 17:42 would be about 1077 miles. This would mean that the ground speed achieved was 149.58 mph on the trip beyond spotting the ship to being near Howland. The 150 mph make sense in that this was the original speed desired in the original flight plan. If you disregard the 10:30GMT ship spotted, and calculate the ground speed achieve from Nukumanu Island to "200 miles out" would be about 141.7 mph that does not seem to fit well from what I can see.
So the basic theory is that a speed discrepancy was discovered when the ship was spotted and the data seems to suggest that this could indeed be the case. How the ground speed achieved could have been mis-read or calculated, I have no idea. A damaged pitot tube seems to be a good culprit. Noonan might have ran some calculations, found a huge error and asked Amelia "Hey, what is your indicated air speed?".
The idea that the belly antenna was torn off is based on the rear support mast being close to the ground while in the three point attitude, tail on the ground. In this same attitude the pitot tubes are well above the ground (head high, see attached photo) and this continues after the tail comes up.
If the rear of the pitot had been ripped off, opening it to the air, then the airspeed indicator attached to that pitot tube would read
zero. There is no particular reason for Noonan's airspeed indicator to be plumbed into the pitot tube for the copilot's ASI, it could just as easily have been connected to Earhart's. if it was connected to the copilot's then Noonan's would read the same as the copilot's and Earhart could have noticed the discrepancy between the speed displayed on her ASI compared to that displayed on the copilot's ASI.
A general comment. Many on this forum attempt to do navigation computations to an impossibly high level of precision that was never achievable in flight, carrying out calculations to such a level accomplishes nothing. Just because your calculator has ten decimal places doesn't mean you should use all of them. Mr. van Asten was the poster child for this, calculating the time to fly from Lae to Howland to be exactly 18 hours, 50 minutes and
8 seconds! The same for Google Earth, just because it gives data to the 1/100th of a degree and to the 1/100th of a mile doesn't mean that those extra decimal places have any meaning in the real world. A hundredth of a nautical mile is only 60 feet, this level of precision only became available with GPS. A hundredth of a degree change in heading would result in being off course only four-tenths of a statute mile after flying the 2556 SM from Lae to Howland.
People also get hung up on the idea of staying exactly on the course line but in flying long distances over the ocean there is no reason to do this since there is only an extremely small penalty even for wandering far off course. For example, flying 2556 SM from Lae to Howland, if you wandered off course 100 SM at the midpoint of the flight it would add only 8 SM to the total route, only three minutes and twelve seconds to the flight. If you find yourself off course you don't make a large change in your heading to get back on the course line that you had drawn on your chart but change heading to head directly towards the destination, never regaining the course line until you arrive at the destination. You do it this way because that is the shortest distance from your off course position to the destination. Trying to regain the course line creates a dog leg, adding distance. For example, the mid-point of the Lae to Howland leg is 1278 SM from Howland. If at that point you discover that you are actually one hundred miles off course and you then make a sharp turn of ninety degrees to intercept the course line at the midpoint you would have to fly a total of 1378 SM to get to Howland, the 100 SM to get to the course line plus the remaining 1278 SM along the course line. Instead you adjust your heading to head directly to Howland from the off course fix and the distance you have to fly is only 1282 SM, 4 SM longer than if you had stayed on course and 96 SM shorter than regaining the course line.
Studying Noonann's chart work on the flight from Oakland to Hawaii shows that they did not try to stay on the course line. Noonan plotted six fixes on that flight, 0317 Z, 60 NM left of the great circle; 0446 Z, 60 NM left of the great circle; 0738 Z, 25 NM left of the great circle; 1007 Z, 65 NM right of the great circle; 1328 Z, 130 NM right of the great circle; and 1520 Z, 20 NM right of the great circle. They got off the pre-planned course line early and just made gradual changes in heading to keep the destination ahead of them and they never regained the pre-planned course line until they arrived in Hawaii.
Looking at
Noonan's chart for the Natal to Dakar leg the 1341 Z running fix showed them to be 130 NM left of the pre-planned course line. They made no attempt to regain the course line but changed heading to intercept the course line and the coast of Africa south of Dakar, both at the same time. See d
iscussion of the navigation on this flight here.Another thing that people get hung up on is about the need to fly the great circle course instead of the rhumb line course. A rhumb line maintains the same true direction for the entire flight while to follow the great circle you must calculate and then make periodic changes in your heading. The great circle is shorter than the rhumb line so that is why people think you must follow the great circle. However this really only makes a difference at higher latitudes but makes virtually no difference when flying near the equator. The great circle distance between the exact coordinates used by Williams for this leg, 06° 47.000' south, 147° 00.000' east for Lae and 00° 49.000' north, 176° 43.000' west for Howland is 2556.1 SM and the rhumb line is 2556.2 SM, exactly one-tenth of a statute mile longer. I can see poor Mr. Williams computing each segment (14 in all) of the great circle between Lae and Howland by hand using logarithmic trig tables only to save 1/10th of a statute mile. Leaving Lae, the initial great circle course is 079.4° true and it changes in steps so that the GC course approaching Howland is 077.6° true. The rhumb line for the entire flight is 078.1° true, only 1.3° difference. And the two course lines lie close to one another, never more than 9 SM apart which is so close that Noonan would not have been able to tell the difference, he would not know if he was on the great circle course line or on the rhumb line course line. Here is a
link to Mr. Williams chart. and his data form is attached.
The reason that I specified those coordinates so exactly was so that I could compute the distances to the nearest one-tenth of a statute mile. Williams and all flight navigators would only use coordinates to the nearest one minute of latitude and longitude, one nautical mile of precision. When the input data is only accurate to one nautical mile it is improper to calculate a distance to a greater precision than that of the original data but many people do this and it is not valid. Using the coordinates as Williams did, only good to one minute, would make the distance for the GC course 2556 SM and for the rhumb line also 2556 SM, there is
no difference based on the level of precision of the data used by flight navigators.
So when you do your calculations give some thought as to what the numbers actually signify.
gl