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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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.





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