Database
[{AirportID:"1001",delayMinutes:"15.0"},{AirportID:"1004",delayMinutes:"3.0"},{AirportID:"1001",delayMinutes:"20.0"},{AirportID:"1002",delayMinutes:"6.0"},{AirportID:"1002",delayMinutes:"25.0"},{AirportID:"1004",delayMinutes:"55.0"},]
Query
db.collection.aggregate([{$group:{_id:"$AirportID",delayMinutes:{$sum:{"$toDouble":"$delayMinutes"}}}},{$sort:{delayMinutes:-1}},{$limit:2}])