This phrase is commonly used in any safety decision, which will not be someones bright idea but carefully researched and risk assessed.
Reasonably practicable
What is
reasonably practicable is a
question of fact. The
Court of Appeal held in
1949 that:
[13][14] ... in every case, it is the risk that has to be weighed against the measures necessary to eliminate the risk. The greater the risk, no doubt, the less will be the weight to be given to the factor of cost.
– Lord Justice Tucker
— and:
Reasonably practicable is a narrower term than 'physically possible' and seems to me to imply that a computation must be made by the owner in which the quantum of risk is placed on one scale and the sacrifice involved in the measures necessary for averting the risk (whether in money, time or trouble) is placed in the other, and that, if it be shown that there is a gross disproportion between them - the risk being insignificant in relation to the sacrifice - the defendants discharge the onus on them.
– Lord Justice Asquith
or in other words if the cost of improving safety is high in relation to the actual improvement or the risk is low it can be discounted.
I'm fairly sure the aviation authorities are aware of this!