"As soon as possible" is the same as immediately , a paddock , road, or whatever is available, essentially for when failure may be imminent . "As soon as practicable", means land as soon as you have somewhere suitable to land.
Perhaps if you look at it like this......
1. A fire in flight..Land as soon as possible, you don't care where, as long as you aren't in a burning missile.
2. Your oil pressure is not normal, but there are no other indicators of a problem, head for the nearest strip, to "Land as soon as practicable" but be ready to land ASAP if things worsen.
Or you could use your computer to find the following.
AS soon as possible
It often takes the form as soon as possible, meaning at the earliest possible moment, as in He'll finish the work as soon as possible. This expression employs possible in the sense of “if it can or could be,” a usage dating from the late 1600s.
Definition of As soon as practicable
As soon as practicable means as soon as possible and practical taking into account all of the facts and circumstances in the individ- ual case.