You're choices are limited - Air France has 2 flights a week into Chad, and Air Afrique (if you want to chance it) has one flight a week from Paris, as well as a flight from Abidjan, (on the odd chance you might be in another part of Africa and are still interested in going to Chad). From East Africa Ethiopian Air flies in once or twice a week (depending upon demand). With all due respect for the other two, we recommend you take the Air France flight.
From Chicago, you'll have an
overnight 8 hour trip to Paris - arriving at about 8 am local time - and then an
all-day layover. If you're adventuresome like Diane, you catch a train and go
into the city to see the Eiffel Tower or eat crêpes. On the other hand, if
you're like Mark, you're just as happy to avoid the adventure and spend the day
in the airport - knowing that the adventure will find you sooner or later in any
case so there's no sense in provoking it. 
Around midnight, about 24 hours after you left Chicago, you board the flight to N'Djamena. It's never been real clear why they have this compulsion to fly at night over the Sahara, but they almost always do. On the not-so-odd chance your plane is delayed and you fly during the day, you're in for a treat - the desert is a place of magnificent beauty (at least from 35,000 feet).
Arrival at N'djamena airport can be a bit of shock to the system. It has never failed to make an impression on us - even after several years and numerous trips. The heat, the soldiers with their assault weapons, the tension of going through customs, and just the thought of being in what is very much a frontier outpost in the heart of Africa.
The
plane parks a good 200 yards from the the terminal, which is actually
pretty nice, and quite modern, even if it has only two gates - one gate for
arrivals and one for departures.
Unlike the USA, where they make you "declare" what you are bringing in, and then for the most part take your word for it with the promise of terrible consequences if they catch you lying, In Chad they just open each of your suitcases to see for themselves. And so it becomes a game of cat-n-mouse.
Need to find your way around N'djamena? http://www.lib.utexas.edu/Libs/PCL/Map_collection/world_cities/NDjamena.jpg
World Map (minus the route) courtesy of www.theodora.com/maps used with permission.