Plane & Pilot
Monday, October 1, 2007

Get The Balance Right

If you think weight and balance are boring and unimportant, you need to read the following

It was 1985, and I was refueling a Cessna 425 Conquest I at Tenerife in the Canary Islands on my way to Johannesburg, South Africa. I’d instructed the fueler to fill the wing tanks first, then begin topping the three 110-gallon internal ferry tanks starting with the front tank. I turned away to fill out the necessary paperwork, heard the pump running for a few minutes and as I finished the fuel request, heard a sickening crunch behind me.
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The airplane doesn’t know that it’s over the maximum allowable gross weight and may not manifest any noticeable differences in handling or performance until the overgross condition reaches about 5% to 10%, 150 to 300 pounds on a typical single. At that level, climb can become sluggish, service ceiling is reduced, stall speed rises and the airplane may lose five to 10 knots or more in cruise.

The heaviest I’ve flown above the limit was in a Beech Duke I ferried back and forth to Amman, Jordan and Abu Dhabi, UAE, a half-dozen times in the ’80s. Topped off with ferry fuel, the airplane was about 2,000 pounds over gross. Fortunately, the Duke handled the weight reasonably well because the ferry tanks were mounted at stations in the cabin that kept the extra fuel well forward. Still, the weight had a pronounced effect on all flight parameters. The Duke demanded at least half again its normal runway requirement (which was already substantial), lost at least half its normal climb and suffered an initial 30 knots to the heavy load, slowly accelerating to its normal speed at the end of the flight.

Flying overweight can present more problems than simply performance and handling, however, even if it’s only 100 pounds. Normal-category aircraft are certified for a maximum positive G-loading of 3.8. To use the simplest possible example, a 2,000-pound gross weight aircraft is approved for a G-load equivalent to roughly a 7,600-pound load (3.8x2,000). (Ultimate load is theoretically 1.5 times that, or 5.6 G’s, but that’s another story.) Increase the weight of the aircraft to 3,000 pounds, and allowable G-tolerance drops to approximately 2.5 (7,600/3,000). Double the weight to 4,000 pounds, and the G-limit is a mere 1.9.

Believe it or not, such a heavy loading isn’t unheard of. Back in the ’50s and ’60s, Max Conrad, a famous ocean flyer and former contributor to this magazine, flew his Comanche from Casablanca, Morocco, to Los Angeles, Calif., with an amazing 104% overload.

Under the best circumstances, a little extra weight may not present the problems you might imagine, though G-loads inside severe weather such as thunderstorms may easily reach destructive levels, no matter what the aircraft’s weight. Back in the last century, I installed a G-meter in a Globe Swift and was amazed at how little G was generated during what I regarded as moderate turbulence. In those days, I flew back and forth to the Reno Air Races up California’s Owens Valley. The bumps seemed spectacular, sometimes alternately slamming me into the seat, then bouncing charts, luggage and occasionally people off the ceiling. Before I installed the G-meter, I assumed the positive loads were three to four G’s. After I installed it, I was surprised to learn I was experiencing only 1.5 to 2 G’s maximum. In the four years I flew the Swift with the G-meter, I never registered more than 2 G’s. Although turbulence may not be the problem you imagined in terms of excess G-loading, it shouldn’t be ignored.


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