Fermi estimate: aircraft movements per day at Mumbai's main airport

In a product manager interview at Flipkart, you are asked for a quick estimate: on a typical day, how many aircraft depart from and arrive at Mumbai's airport? You have no data to look up. Lay out your approach, state every assumption with round numbers, do the arithmetic out loud, and check whether your final figure is plausible. ```hint Count it two ways One estimate can start from how much traffic the runways can physically handle; another can start from how many travelers pass through the airport. If the two land close together, your number is far more convincing. ``` ### Clarifying Questions - Should the count cover only the city's main airport, or every airport serving the Mumbai region? - Does the interviewer want total movements, with each takeoff and each landing counted separately, or only the number of departing flights? - Should cargo, private and military flights be included, or only scheduled passenger flights? - Is the target an ordinary day, or a peak day such as one during holiday travel season? ### What a Strong Answer Covers - A clear definition of what is being counted: which airport, movements versus departures, and which kinds of flights - A structured equation whose assumptions are labeled and use round, defensible numbers - Correct arithmetic carried through to a final figure, with the split between departures and arrivals - A second, independent method or a sanity check against a physical limit such as runway capacity - Awareness of which assumptions move the answer most, such as night operations, aircraft size and how full flights are ### Follow-up Questions - Which single assumption, if wrong, would change your answer the most, and how would you check it quickly? - How would your estimate change for the day before a major festival holiday? - How would you adapt the method to estimate the number of flights across all of India in a day?