To my understanding, you're trying to estimate the population with the sample, because normally you don't have the whole population. For example if you're trying to estimate the average height of all men in the world ever existed, you obviously can't measure every single one of them. But you can take a sample measure, say 1,000,000 men and then you average their height. As the number of samples increases you're approaching the real population.
In your case I guess yes you would have to measure every time interval between planes landing ever in order to get the whole population...So your 250 observations would be the sample I think, if planes still kept landing after your observations :)
Here's another definition of population and sample:
When we think of the term “population,” we usually think of people in our town, region, state or country and their respective characteristics such as gender, age, marital status, ethnic membership, religion and so forth. In statistics the term “population” takes on a slightly different meaning. The “population” in statistics includes all members of a defined group that we are studying or collecting information on for data driven decisions.
A part of the population is called a sample. It is a proportion of the population, a slice of it, a part of it and all its characteristics. A sample is a scientifically drawn group that actually possesses the same characteristics as the population – if it is drawn randomly.