Weight and balance is no trivial matter
The answer made him laugh. The locals weren't just spinning yarns about one of their number behind his back. Turns out that one of them was preparing to take his private pilot checkride, and the big fellow was the designated examiner. The student had run several theoretical weight-and-balance calculations for their flight using various weight estimates -- and had gone into a mild panic. At the more conservative estimates, the training aircraft's gross weight could be made to come out within limits by manipulating the fuel load. But the center of gravity was unacceptably far forward, and it remained so no matter what the student did with the fuel.
Knowing this, on checkride day the examiner would lead the student through a discussion of aircraft loading that would cause the student to realize that by adding weight to a rear station of the loading envelope, the flight could depart within CG limits -- an amount of weight about equal to that of a case of engine oil.
The local flight instructors never knew exactly what method the examiner would use to "make it come out right," but they knew he'd come up with something. So they merely told their students to present the problem to the check airman and "work out a solution together." For years, guessing the examiner's weight and jockeying the load was a rite of passage for local pilots -- and about as good an education on load management as you could want for your students.
Like real engine failures, spins, and weather encounters, the potential risks of flying outside an aircraft's loading envelope is a subject approached only in theoretical terms. That is to say, no one intentionally overloads an aircraft -- or centers its gravity aft of its CG limit, then stalls the airplane to demonstrate how hard it is to recover from a stall when the elevator isn't getting the nose down.
Besides, clues to the repercussions of weight-and-balance abuse can be found in the normal realm. Students training in two-seaters are routinely cautioned at the point of their first solo to expect surprisingly enthusiastic climb performance from the aircraft, once it is lightened by 25 to 50 percent of its useful load without the CFI in the right seat. Students flying four-seat trainers with only their instructor aboard for the first dozen or so flight hours will perceive how performance erodes and trim inputs change when a back-seat observer rides along for the first time. Add a second observer, or some luggage, and a student may experience flight at max weight for the first time. Part of the weight-and-balance discussion on the checkride includes a requirement that the applicant exhibit knowledge of "the adverse effects of exceeding limitations." In theory, then, they know what they need to know to stay safe. Unfortunately, the oil-case adventure or equivalent training exercise turns out to be the last time many pilots give serious consideration to weight-and-balance needs of their aircraft.
That's unfortunate, because most trainers and four-place singles are straightforward from a loading point of view and don't hint at issues you might face later. Get the CG-and-weight combination right, make sure it will be right at the end of the flight (when weight will be lower), and the mission is accomplished. High-performance aircraft can be trickier. There may be additional restrictions on load distribution to manage CG constraints, or constraints on the sequence in which fuel must be consumed from auxiliary and main fuel tanks. Multiengine aircraft can be even more demanding, right down to how fuel must be added to the tanks. This is a big part of learning how to fly advanced aircraft.
Exceeding weight-and-balance tolerances can be the hidden time bomb not directly causing mishaps but rearing up to place an aircraft out of control after a disturbance like turbulence. Or, it can end a flight almost before it starts without an external catalyst. On April 10, 2002, a twin-engine Beech E18S took off from Juneau, Alaska, with a load of construction materials. It crashed during the initial climb, killing the 22,000-hour-plus pilot. It was a clear, cool day with 8-kt winds. The investigators focused on the load.
"During a brief on-scene conversation with the National Transportation Safety Board (NTSB) investigator-in-charge on April 11, a personal acquaintance of the accident pilot related that the purpose of the flight was to deliver a load of wooden roofing shakes to a friend's remote lodge. He said that the accident flight was the pilot's second load of roofing shakes that day to the lodge. Two individuals who had helped the accident pilot to load the airplane before the accident flight estimated that about 50 bundles of roofing shakes were loaded on board, but they were unsure exactly how many bundles were ultimately loaded. Both individuals said that the airplane was completely full, and the retaining straps on some of the shingle bundles were cut to allow individual shingles to fill as much of the cabin as possible," the NTSB accident summary said.
Witnesses "consistently reported that the airplane departed Runway 8 and climbed to about 200 to 300 feet above the ground. During the climb, as the airplane approached the departure end of the runway, the nose of the airplane abruptly pitched up about 70 degrees and drifted to the right. The witnesses added that the airplane continued to turn to the right as the nose of the airplane lowered momentarily. As the airplane flew very slowly in a southerly direction, the landing gear was extended. The nose of the airplane pitched up again, the right wing dropped, and the airplane descended...into an area of tidal mud flats and subsequently struck shallow ocean water."
A Beech 18 is certified to operate at a maximum gross weight of 9,700 pounds. This aircraft had been modified to conduct operations at 10,100 pounds. The useful load for this airplane was listed in its weight-and-balance documentation at 3,590.2 pounds, the NTSB said. When a weight-and-balance calculation was performed, including only fuel that had been placed on board before the flight, the 160-pound pilot, and 77 bundles of roofing shakes (a number determined by interviewing a lumber yard employee where the shakes were purchased), the flight was determined to be "1,400.8 pounds in excess of the maximum takeoff gross weight of 10,100 pounds." CG could not be pinpointed because the load was scattered around the accident site. Probable cause: "The pilot's excessive loading of the airplane that precipitated an inadvertent stall/mush during the initial climb."
Don't get the impression that it takes extreme weights to destroy the stability characteristics of an aircraft. On April 22, 1999, a Wells Baby Lakes biplane operating from a private airstrip at Monroe, Washington, lost control maneuvering at low altitude and crashed, killing the pilot. Said the NTSB, "A witness, located approximately one mile east of the accident site, reported seeing a red biplane flying 'quite low over our farm.' The witness stated it appeared the aircraft was practicing touch-and-go landings, 'but never quite touching the ground.' The aircraft continued to maneuver in the area for approximately 20 minutes. The witness reported that the aircraft was maneuvering approximately 40 feet above the ground when it abruptly nosed over and spiraled into the ground."
As for weight and balance: "The aircraft's electric starter and battery had been removed and the original wooden propeller had been replaced with a McCauley metal propeller. The aircraft's logbooks did not reflect the above changes, and no evidence of a revised weight and balance calculation for the aircraft was found. A calculated weight and balance, using the most current data, revealed that with a 210-pound pilot and 12 gallons of fuel (full fuel), the center of gravity is approximately 19.2 inches aft of datum. With the same pilot weight and six gallons of fuel, the aircraft's center of gravity is approximately 17.27 inches aft of datum." The recommended CG range was 12.5 inches to 15.5 inches aft of datum, leading the NTSB to conclude that the probable cause was "the pilot's failure to maintain aircraft control. A factor was attempting to fly the airplane outside of its weight and balance limits."
Destructive potential increases when the pilot aggravates the situation by tackling a short-field departure with an "informal" takeoff technique. A high-performance single-engine aircraft could not overcome its pilot's decision-making in Vi-burnum, Missouri, on April 25, 2002. It hit a hill during an aborted takeoff that put a number of people, including three passengers, at risk. This was the gist of the NTSB report: "The pilot stated that he used an informal short-field takeoff procedure using 10 degrees of trailing-edge flaps. He also stated that the airplane was 80 pounds over its maximum gross weight. After realizing the airplane had a less-than-expected climb rate, the pilot performed an aborted takeoff on the remainder of Runway 27 in an attempt to avoid hitting a church along the climb path. The airplane's landing gear was sheared off by contact with a hill. The right main landing gear struck the horizontal stabilizer." Credit the pilot for aborting the takeoff. There were no injuries.
The report noted: "According to the pilot's operating handbook [POH] for the airplane, 25 degrees of flaps is the proper flap setting for a short-field takeoff. The POH does not contain takeoff performance data for a takeoff with 10 degrees of flaps." Probable cause: "The pilot's improper short-field takeoff planning and procedure. The takeoff weight above the maximum gross weight and the hill were contributing factors."
Pilots are taught their aircraft are designed with stable flight characteristics -- meaning, in part, that the aircraft won't make a flight condition worse as a result of its design. This assumes that the pilot doesn't "redesign" it. Even if a pilot believes he's solved performance problems that the engineers and test pilots couldn't, he owes it to passengers and any people sitting in the church along the departure path to exclude them from his experiments.
Dan Namowitz is an aviation writer and flight instructor. A pilot since 1985 and an instructor since 1990, he resides in Maine.
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