Some aircraft lavatory holding tanks have a blue chemical added to the water to deodorize the water and break down solid waste. Occasionally, the holding tank or drain tube develops a leak. If this happens at high altitudes, the water will freeze once it hits outside air. However, if blue ice falls from an aircraft, the ice will usually break up and melt before it hits the ground. If the ice doesn't fall off, it will melt as the airplane descends for landing. Then it usually dissipates into small droplets.
Many people assume that aircraft lavatories dump overboard when they are flushed; they do not. The aircrew cannot dump the wastewater in flight because the waste valve is located on the exterior of the aircraft and only ground crew can operate valve.
Blue Water, Blue Ice or ???
We at the Chicago O'Hare FSDO receive many telephone calls and FAA Hotline complaints regarding blue water or blue ice, especially during autumn. The calls are mostly from people who live under one of the main flight paths into Chicago O'Hare International Airport or Chicago's Midway Airport.
One possible explanation during autumn is that birds, particularly Canada geese, are flocking together and preparing for their migration south. As fruit trees are also ripening during this time of year, the birds eat the fruit. If the fruit is blue, it will come out blue when the bird passes it.
While this is one possible explanation, if you see any object fall off an aircraft in flight, please call your local FAA Flight Standards District Office immediately. Provide us with any information that we can use to identify the aircraft or airline.