Graybeard,
There is a "feature" of HF that renders your comment pretty much meaningless, at least for reception.
The aircraft would be operating well below 10MHz and probably around 5MHz or even 3MHz at that hour of the night in June of 2008. Above those frequencies the propagation would simply not be there. At those frequencies you can use a receive antenna that is only 10% efficient or even less and still receive as well as a "good" antenna would receive. Ambient noise that cannot be escaped makes up the difference. At 3MHz it's on the rough order of 20dB above "Boltzman" or theoretical minimum noise levels. (When on those bands I always intentionally put in 10dB to 20dB of attenuator following the antenna. It makes the receiver's so called "intermodulation distortion" (birdy level) lower.) This noise is an issue until you get up around 100MHz give or take a little.
The aircraft would HEAR DAKAR just fine. And unless DAKAR had storm static to worry about they'd be able to hear the plane as well as they ever hear the plane.