Get The Balance Right

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

By Bill Cox

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.

I turned around to discover that my big Cessna turboprop twin had become a taildragger. The airplane had fallen back onto its tailcone, crushing the tail tiedown ring up into the aluminum and suspending the nosewheel high in the air. The fueler was still standing precariously on the airplane’s airstair bottom clamshell, its aft lip now resting against the ramp. He was holding the fuel hose in his hand, obviously confused by what had just happened.

It was all too obvious. He’d climbed up onto the airstair and begun refueling the first tank he saw, in this case, the aft ferry tank. With wing fuel well down and the three ferry tanks empty, the result was inevitable. Loading 730 pounds into the aft tank with so little in the front containers was more than the CG could handle.

It was my fault, of course. The young man actually doing the fueling spoke little English, and his supervisor hadn’t translated my instructions on how to fuel the airplane. On many delivery flights, we often fuel the ferry tanks ourselves to make certain there are no errors. I’d been complacent by counting on someone else to do it right.

Most weight-and-balance problems aren’t that dramatic, but many pilots are aware that improper balance can be deadly. Overloading is a no-no as well, but it’s usually more of a venial rather than a cardinal sin. Usually.

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.

Overstressing the airframe in flight isn’t the only risk. Several times, I’ve been forced to return and land shortly after takeoff with a major fuel overload. The need to touch down with a load that’s 1,000 to 2,000 pounds over the limit gives you a strong incentive to grease it on. If you don’t do it right, the stress on the shock struts, brakes and aircraft center section can be well beyond the limits, resulting in destructive forces. I once saw a Cessna 402 parked on the ramp at Honolulu with the gear punched half way up into the wings from a poorly executed overweight landing.

For that very reason, many airplanes are saddled with maximum landing weights that may be several hundred pounds below the aircraft’s approved takeoff gross. The Piper Mirage has a max takeoff weight of 4,340 pounds and a max landing weight of 4,123 pounds. The majority of aircraft, especially single-engine models, concentrate the bulk of their weight in the fuselage and store their fuel in the wings, so another occasional limitation is “maximum except fuel,” a restriction on the amount of weight that may be concentrated in the aircraft center section. These limits usually apply to heavier twins. For the average pilot, most of these paperwork limits will rarely prove to be operational problems.

None of the above is to suggest that weight isn’t important, and it most definitely shouldn’t be construed as suggesting you should violate your airplane’s approved gross weight, not by one pound or 100, much less thousands as delivery pilots do regularly (under an FAA ferry permit). In addition to the obvious possible certificate action, insurance companies might use a known overgross condition to deny coverage in the event of an accident. The simple fact is that additional weight isn’t as liable to get you into serious trouble as is an unbalanced CG.

For that very reason, aircraft with ferry tanks installed are configured to keep the extra weight inside the envelope. Some models become less stable when you load them heavy and near the forward or aft limit. (To offer the pilot the option of carrying more fuel on an especially long leg, some ferry companies used to employ the expedient of installing 55-gallon drums with a placard on the aft tank that stipulated, “Maximum capacity 55 gallons, maximum allowable capacity 20 gallons” or whatever the appropriate number was to maintain a CG within limits. This left it up to a pilot concerned about strong headwinds to exceed the limit if he dared. No one ever said it was smart or legal, but if the winds weren’t cooperating, it often was the only way to get the job done.)

Senecas, Bonanzas and Malibus sometimes need a tailstand in ferry configuration to keep them from falling over on their tails during fueling (like my Conquest in Tenerife). A Seneca often will rest so low in the rear during taxi with all ferry tanks full that it will drag belly-mounted antennas on the asphalt. Load too much weight too far aft in a Malibu, and you may induce a very slight bending moment in the fuselage, not enough to see but enough to make it difficult to close the tight-fitting door.

I once escorted a pilot with a Malibu JetPROP on a round-trip voyage across the Atlantic from Monterey, Calif., to Berlin, Germany. The ferry tank wasn’t large, but it had been beautifully installed in the worst possible place: far back below the rear seat. The result was we had to carry a half-dozen precut, plywood tailstands to allow us to climb aboard, get the door closed, start the engine and taxi away without problems. This, of course, left the tailstand lying on the ramp. When we came back thru Iceland and Greenland, the rampers had saved our tailstands for us.

Another common problem for some airplanes in normal configuration is a tank location that moves the CG aft as fuel is burned off. Bonanzas store most of their fuel in the wing’s leading edge, a station that’s well forward of the typical CG location. This means, by definition, the CG moves aft as fuel is burned off. If you depart at max gross weight with full fuel in most four-place Bonanzas, the CG will slowly move toward the rear as you fly, and it may be beyond the aft limit if the flight is more than an hour or two. This can lead to unusual sensitivity in pitch and, in the worst case, a tendency to phugoid (or porpoise) on landing.

For that reason, you need to make two CG calculations for some Bonanzas, one for takeoff and another for landing. A standard rule for many Bonanza pilots is to keep the heaviest passengers as far forward as possible to prevent the CG from moving aft out of limits.

But don’t overdo it. Load too much weight too far forward, and the airplane may run out of both down elevator and trim and be difficult or impossible to flare. Any flap application may only exacerbate the problem, most often tending to pitch the nose farther down.

Conversely, a CG that’s near the limit isn’t always a bad thing. All airplanes benefit from an aft CG in terms of cruise speed. According to Jim LoPresti of LoPresti Speed Merchants in Vero Beach, Fla., loading the airplane as far aft as possible (but within the allowable limit) cuts drag by reducing the download on the tail, an airfoil designed to fly down as the wing flies up. This doesn’t mean you should fly with any more weight than you have to, by the way. On some airplanes, Jim commented, the difference between a max aft load and a more forward balance point can be three knots.

Many pilots, this one included, feel the Piper Cherokee Six and Saratoga models fly better with a more aft CG. Like the Malibu, the Six/Saratoga offers a nose baggage compartment to help balance the load. Piper is the only manufacturer of piston singles I can think of that provides such a hedge.

If there’s a message here somewhere, it may be that violating either weight or balance limits is a bad idea. Flying overweight can compromise all parameters of performance, and operating the aircraft outside the balance point can result in control problems that may be impossible to counter.

We like to think of flying as a relatively safe occupation/pastime, and it is, but only if you live by the limitations that some very smart folks have learned by the best- or perhaps worst-possible method: trial and error.