Member 'Efftup' is 100% correct.
Here are more photo examples:
http://www.airliners.net/photo/Airb...35448/L/&sid=2e272e428af44c1fa6a676dcb4dac2b1
External Quote:
F-WWEA (cn
009) This A380 was in ADD for high altitude test for four days. The elevation of the airport is 7,630 ft (2.326 m) and the 3.800 m RWY is capable for 450 t. Here you can see the ballast barrels.
AND:
http://www.airliners.net/photo/Boeing/Boeing-747-8JK/1941472/L/&sid=34d5881856e4d96541f141e486890461
External Quote:
N6067E (cn
38636/1434) The rear section of the main deck. The barrels are used to hold water for weight balance purposes.
I suppose two examples are enough....this is a well-known method used in airplane flight certification testing. New airplanes, or new designs (modified) from previous models.
The barrels hold water (to add weight, and simulate the weights of passengers, their luggage, and any cargo) and also the barrels are inter-connected, so the water weight can be 'shifted' during a flight....thus saving time because the "balance" is altered for any flight-test mission....saves time, no need to land, re-distribute weight, take-off again...etc, etc, etc......
Adding: It's important to remember the weight of water....it is about 8 pounds per gallon (about 3.7 kg/U.S. gallon). Very useful for this purpose...and of course, non-hazardous!!! (unless you are drowning in the ocean....but, this isn't the topic....)