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Calculating Weight And Balance In Advance

On a hot day in July, a portly pilot and his rotund passenger landed their Cessna 152 at a small Maryland airport to top off the fuel tanks. After filling up, they perused the local pilot shop, purchased two cases of oil, and enjoyed soft drinks out of the vending machine. Once back in the airplane, the pilot, knowing the 152 was heavy but not how heavy, faced a dilemma. Which way should he take off? The airport had Runway 31/13, which was 1,845 feet long.

The wind was varying from a direct crosswind to barely favoring Runway 13. This runway has a 400-foot overrun at the departure end but runs slightly uphill with 50-foot trees at its terminus. Runway 31, which slopes slightly downhill, has a 200-foot overrun that drops off into a 60-foot-deep ravine with tall trees that rise on the other side. The pilot elected to depart on Runway 13 because of the longer overrun and the varying favorable headwind. He told his passenger that he would abort the takeoff if he had not achieved 55 knots airspeed by the time he was midway down the runway. He believed he could then roll harmlessly to a stop on the grassy overrun before running into the trees.

He started his takeoff roll. The acceleration was agonizingly slow, but the small airplane had reached 55 knots at the runway midpoint. The pilot pulled back on the yoke, and the aircraft left the ground. But that's about all it did. The pilot noticed that he was not climbing and that the airspeed was deteriorating. He could not climb out of ground effect. He pushed the yoke forward slightly to regain his airspeed. The main gear touched down on the runway again, and then the airplane bounced back into the air. By this time, the pilot saw the threshold pass under him as he headed for the trees. There was no time to stop now; he was committed. He resisted the urge to pull back on the yoke until the airspeed began to build. He was somehow able to milk the little airplane over the trees as it began an anemic climb. The pilot looked at his passenger, who had terror written all over his face. The pilot was not feeling too well himself. Neither spoke for the rest of the flight.

Let's take a look at what this pilot had asked the little airplane to do. The basic empty weight of the Cessna 152 with full fuel was 1,336 pounds. Add the pilot and passenger weight of 430 pounds and 45 pounds for two cases of oil in the back. That put the takeoff weight at 1,811 pounds. Since the maximum gross weight for the 152 is 1,670 pounds, the airplane was 141 pounds over maximum gross weight. At maximum gross weight under those conditions, the aircraft would have barely made it over a 50-foot obstacle by the time it ran out of runway.

The pilot and his passenger were very lucky. The pilot should have realized that he was at maximum gross weight when he landed, even before topping off the tanks and taking on baggage.

Instructors who attend the AOPA Air Safety Foundation's Flight Instructor Refresher Clinics see a videotape of the crash of a Cessna 152 that had made an emergency landing on a residential street in Florida. The aircraft had run out of gas. The pilot and his passenger landed safely after passing under electric wires and had refueled with gas purchased from a local airport. But instead of leaving the passenger behind to take a taxi to the airport, the pilot decided to take off on the short residential street with his passenger, in full view of a TV news crew that was doing a story on the earlier power-off landing. Not surprisingly, the aircraft crashed into trees at the end of the street. Now the TV crew had a real story. The two airplane occupants were not seriously hurt, and the pilot, while being interviewed by the TV station, said that he wished he hadn't tried to take off with his passenger. The pilot should have calculated the weight of the aircraft with the passenger, then walked off the length of the street, and from the pilot's operating handbook determined whether the aircraft would clear a 50-foot obstacle at the end of the street. An even better solution would have been to take the wings off (without the assistance of the trees) and truck the aircraft to an airport. This pilot didn't seem to comprehend the effect that weight has on takeoff performance, an essential item in the instruction of new pilots.

After a recent Beech Baron accident that occurred during high winds and turbulence in southwest Virginia, officials calculated that the aircraft was almost 400 pounds over maximum gross weight and that the center of gravity was almost five inches aft of the limit at the time of the crash. Witnesses said that they saw the aircraft spinning before it crashed. The pilot may have gotten away with flying out of weight-and-balance limits before, but not this time.

The last time many pilots do a weight-and-balance calculation is for their checkride. They have a vague notion of what their airplane will carry, and they assume that as long as they don't load an iron anvil into the baggage compartment, the CG is reasonably in range. They have gotten away without doing the formal calculation, but sooner or later, perhaps on a hot day at a high elevation airfield or in high winds and turbulence, this lack of attention to detail will catch up with them.

The point of this article could be to convince instructors to have their students calculate weight and balance each time that they fly. But that's like the dentist telling you to floss your teeth every day. It's a good idea, but many people don't do it. My students often ask as they prepare the navigation logs and weight-and-balance data before a cross-country if "real pilots" do all this paperwork prior to each flight. My answer is they should, but they often don't.

Is there another way? I recommend that my students try doing ballpark weight and balance calculations with various scenarios in advance for airplanes that they fly regularly. For example, can they safely fly a particular Cessna 172 with full fuel, three passengers and minimal baggage? Let's say that the 172 they rent regularly has a basic empty weight with full fuel of 1,740 pounds. The pilot's weight and the weight of three 170-pound passengers is 680 pounds. Let's put 40 pounds in the baggage compartment. This results in a total weight of 2,460 pounds or 160 pounds over the allowable weight. Now they know that they can't carry three passengers with full fuel and some baggage in this Cessna 172. In fact, they can only carry 13 gallons of fuel to be within the weight and balance envelope.

I suggest my students write down on a 3-by-5 card under what conditions they can fly without exceeding the maximum gross weight for each aircraft they use. I also have them write down under what conditions they can avoid exceeding the forward or aft CG limits of the aircraft. If they have a 350-pound friend, do they put him in the front or the back? They will have to make allowances for children and others who don't meet the 170-pound standard profile. What is the takeoff distance with maximum weight on a 90-degree day with no wind at their home airport? Have them write it down on the card.

Encourage your students to calculate all of that in advance. It will save them time when they go to the airport, and the card will cause them to think about their weight-and-balance situation each time they fly.

By Richard Hiner

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