Up close and personal with DC-7 tankers: former passenger cabin, ballast

 The image above is interesting to me for a couple of reasons. First, remember that Butler's DC-7's once saw service with the airlines. Obviously, part of the conversion process from passenger plane to tanker has been stripping the old passenger cabin. Moving on. Now in this photo it is obvious that there is something going on. This is Butler's tanker 66 and the gentlemen in the photo is Brian Lash, the pilot of T-66. Larry Kraus tells me that in this photo, taken on May 26, 2008, Brian is doing a preflight check for a maintenance test flight of T-66. More importantly, Larry sent me this picture because it shows ballast secured in place on the aircraft.

Ballast represents additional items (for example the tires in back of the cabin in the above photo), is carefully stowed and secured at specific points called fuselage stations to add weight to bring the aircraft within manufacturer designated weight and balance parameters. It isn't just the weight of the plane that is important but how the weight is distributed within the plane that is crucial for safe operations. For example, weight that is distributed too far forward will put the plane into a nose down situation with possible fatal consequences for the plane and crew. Aviation mechanics, aka airframe and power train or A&P mechanics, periodically reweigh the plane and then check the planes center of gravity. Once the center of gravity is known, there are calculations that are performed that determine weight and balance parameters. Then additional ballast is often needed to bring the plane within these weight and balance parameters.

The photo above, a little fuzzy, shows one of these fuselage stations (F.S. 860) for tanker 62. What this means, according to Larry, is that this station is "860 inches aft of the weight and balance datum (F.S. 0, located at the tip of nose of the DC-7)." As I understand the weight and balance datum is also known as reference datum and is:

an imaginary vertical plane from which all horizontal distances are measured for balance purpose (FAA, Aircraft Weight and Balance Handbook (2007), Glossary p. 5).

From this ballast station, F.S. 860, additional ballast stations (fuselage stations) and their assigned weights are calculated to assure that tires, parts, ladders, etc equal the required ballast. According to Larry T-62 carries somewhere around 1200 lbs. of ballast. Most of it is behind (aft) of F.S. 860 and some of it is forward of F.S. 860.

Larry wanted to show you a graphic from one of his DC-7 manuals (electronic files) showing the location of the fuselage stations. Unfortunately, computer troubles with the computer where these manuals are stored prohibit him from doing so. Thanks for the effort, Larry.

I want to close this article on ballast by letting Larry share with us about how the weight of the retardant tank (1,500 pounds empty and an additional 27,000 pounds when carrying 3,000 gallons of retardant) affects tanker 62's center of gravity (CG).

With the retardant tank installed, the DC-7 is out of the forward center of gravity range in the empty condition unless several hundred pounds of ballast is located right aft, or a higher total weight spread over the aft part of the fuselage. We originally used sandbags at the rear pressure bulkhead,which is as far back as you can go inside the pressurized cabin, just behind the seats in the old passenger lounge at the rear of the plane.

The weight varies from airplane to airplane. I think that it's around 660 lbs on tanker 62.We now carry spare tires.tools,parts and other supplies that are carefully weighed in place with the fuselage stations noted (in inches from the bulkhead forward of the cockpit) to be sure that we get it right.

The mechanics changed things around in T-62 this year after doing a new weight and balance and I think that they got the CG slightly too far forward. At least it felt that way on take-offs and landings.There's a heavy box that we can move fore and aft over several feet to easily adjust the CG, but it wasn't bad enough to adjust, although I thought about moving it a couple of feet aft to see if I was right, but the season ended before I got around to moving it.

References

A wikipedia entry on an aircrafts center of gravity and the related concept of weight and balance may be found here. A somewhat more technical publication (cited above) called the Aircraft Weight and Balance Handbook (2007) is published by the FAA and freely available on the web, Much of it is written for A&P mechanics and other aviation experts. But the first chapter or two have an overview that I could follow as well as a glossary.