Weighing C-5s a complex process
Aircraft workers position jacks in order to raise the aircraft for weighing. U. S. Air Force photo by Sue Sapp
by Wayne Crenshaw
7/22/2011 - ROBINS AIR FORCE BASE, Ga. -- A lot of complex work is done to each aircraft which undergoes programmed depot maintenance at Robins, but one of the last steps is something everyone can understand, even if it's not as simple as it sounds.
The final step before an aircraft goes out for functional test is to weigh it.
The three people in the Weight and Balance Shop oversee the intricate process of weighing each F-15, C-130, C-17 and C-5 following PDM.
Weighing the aircraft is important because, if the weight is off, there can be a problem, said Melvin Appling, weight and balance technician, as he prepared to weigh a C-5 last week.
The most important reason for weighing it is the process of adding, modifying and removing parts during PDM can change the aircraft's weight. Operational crews need to know the final weight and center of gravity because it impacts how the plane is fueled and loaded.
But, it's not just a matter of rolling the planes onto a giant scale. Jacks capable of bearing 60 tons each are used to carefully hoist the aircraft completely off the ground. The jacks give readings to a central unit which gives a readout of the weight on each jack and a total weight of the plane.
For a C-5, one of the largest aircraft in the world, its unloaded and unfueled weight is 370,000 pounds, or 185 tons.
For C-17s and C-5s, the process takes about two hours. The smaller F-15s and C-130s can be done faster. While the Weight and Balance Shop directs the weighing process, the hands-on work is done by a crew of mechanics.
A mechanic mans each of the six jacks for a C-5, one works the manifold that operates the jacks, and another is inside the plane watching a plumb bob which shows the plane's center of gravity.
The plumb bob is key to weighing the plane. As the six hydraulic jacks begin to lift the plane, the person watching the plumb bob relays its position to the manifold operator, who then adjusts pressure to the jacks to ensure the plane is kept level during lifting.
"You are basically the eyes and ears for the manifold operator on the ground," said Wesley Connell, a C-5 hydraulic mechanic who monitored the plumb bob during the weighing last week.
After getting the weight, the plane is lowered to the ground, the jacks are zeroed out, and it is weighed again to ensure an accurate measurement.
The two readings on the plane last week differed by only 100 pounds, which is considered close considering the overall weight of the plane.
At one time, some planes were weighed with a roll-up scale, which was taken to each plane and the planes towed onto it. Today, all are weighed with jacks.
"Jacks are easier and a lot safer," Appling said.