[Robert A Cook PE]

Problem was, Noonan was good, but not particularly great navigator (Amelia was a terrible!) and had drinking problems at the time.

That left the two as a solo pilot and a solo navigator. Either make a mistake, and the other could not correct it, and may not even be able to recognize that an error had been made!

[DesertRhino]

Actually, he was very good at open water navigation, one of the very best, and was a pioneer in the field. The slanders against him loomed large because people were really not wanting to blame Amelia.
But there was zero margin for error on either of their parts.
It was a leg that was essentially flying from Los Angeles to Washington DC. The target was hundreds of miles from nowhere isolated. The target at that distance was a flat featureless, uninhabited island much smaller than most typical airports in America today.
If Fred made the slightest error, or if Amelia looking out into the featureless haze blinked, they missed it. The whole mission was an exceedingly dangerous one and really before its time to be done safely except by giant flying boats.

Noonan was damned good.

[ExSES]

Problem was, Noonan was good, but not particularly great navigator (Amelia was a terrible!) and had drinking problems at the time.

My Father knew Noonan back then..., and always said that he probably screwed up the navigation due to too much booze!