(b) No, that's not correct. The law does not simply distinguish between trivial and non-trivial harm. Rather, the courts will take account of the degree of seriousness of the potential harm in the circumstances of each case. The more serious the likely harm, the greater the extent of the obligation to prevent it from occurring.
In addition, the seriousness of the harm may be assessed in light of some characteristic or vulnerability of the plaintiff, of which the defendant was or ought to have been aware.
In the case-study, it is strongly arguable that the potential harm would be extremely grave, involving serious injury or death to a large number of people who live near the runways and flight paths. The gravity of the harm would therefore be a factor in favour of requiring the responsible authority to prevent it from happening.