What if L'Oiseau Blanc had a tailwind?
In my original analysis, I created a flight plan which plausibly accounted for the reports of the aircraft passing Kilrush in Ireland and Harbour Grace in Newfoundland, using the wind reports from the French report in the Tighar archive which suggested a headwind.
https://tighar.org/smf/index.php?topic=2251.0
To summarise, the elapsed time from Kilrush to Belle Isle was 24hr 46min giving an ETA at Belle Isle of 10:39UT (07:09AM, an hour or two after sunrise). However, if they failed to lay-off the drift they would end up somewhere between Belle Isle and Harbour Grace, position C1. The aircraft was seen over Harbour Grace at 13:00UT (9:30AM Local) therefore, the elapsed time from Position C1 to Harbour Grace was 2hr 21min and point C1 was about 235nm North of Harbour Grace or about 65nm South of Belle Isle.
In response, Arthur Rypinski posted a chart depicting the weather situation on the 9th May 1927 and indicating a tailwind. I have rewritten the flight plan using the winds from this chart to examine the effect of a tailwind. This gives an elapsed time of 16hrs 18min from Kilrush (observed at 09:53UT) to Belle Isle ETA 02:11UT on the 9th (22:41PM on the 8th about 6hrs before sunrise). (02:11+24:00-09:53=16hr 18min). This leaves 10hrs 49min to be accounted for, between landfall at 02:11UT and when the aircraft is seen at Harbour Grace at 13:00 UT.
If the crew correctly allowed for the drift and made landfall at Belle Isle at 02:11UT, then it should only take about 3 hrs to fly 300nm to Harbour Grace leaving 7hrs 49min to be accounted for.
If the crew did not allow for drift and because of the shape of the coast, they would make landfall at Position D1 about 240nm North of Belle Isle about 2hrs later (04:11UT) still in the dark. The flight to Harbour Grace would then be about 540nm taking 5hrs 24min and leaving about 3hrs 25mins unaccounted for. (13:00UT-04:11UT-5hr:24min=3hrs 25min)
The difficulty with this scenario is that if they made landfall North of Belle Isle and their only option was to go South, how did they miss Belle Isle and end up at Harbour Grace? During the night of the 8th/9th there was a half-moon in the West so there should have been enough light to see the coast. Possible reasons for the unaccounted time could be time spent searching for the Straight of Belle Isle or looking for another way around Newfoundland or avoiding weather or waiting for sunrise.
If they had found the Strait of Belle Isle soon after landfall and crossed the Gulf of St Lawrence to Nova Scotia they would have made it to New York at 14:15UT (09:15AM NY) with 677litres of fuel remaining after a total flight time of 33hrs 57mins. But that did not happen.
As it was, whether they had a headwind or a tailwind, they ended up near Harbour Grace at 13:00UT with insufficient fuel to make New York.
These two scenarios result from the guesswork as to where the low-pressure system East of Newfoundland was exactly located. If it was a bit South, it would allow them to gain a tailwind as they passed North of it. If it was a bit North and they passed South of it, they would have headwinds. I think the latter case is simpler, more likely and leaves fewer unanswered questions.