Commercial

Safety in Avionics: Watching Your (Aircraft) Weight

By David Evans | April 1, 2001

In an industry where safety is based on the twin pillars of precise procedures and accurate measurements, it seems strange that "informed guesstimates" are used to determine an aircraft’s weight and balance. Pilots are provided an estimated weight before pushback, but they have no way to verify the information in real-time. Yet the technology is available for precisely such information, and exploiting it would benefit economics, as well as safety.

The weight and balance issue is particularly important for cargo operations. Two fatal cargo plane crashes occurred in the United States in recent years because of a failure to load the airplane within weight and/or center of gravity (CG) limits. The first unnecessary crash involved the Aug. 7, 1997 fiery impact moments after takeoff of a Fine Air DC-8 cargo jet at Miami. The second, still under investigation by the U.S. National Transportation Safety Board (NTSB), involved the Feb. 16, 2000 crash seconds after takeoff from Rancho Cordova, Calif., this time of an improperly loaded Evergreen Worldwide Airlines DC-8.

To be frank, the inability to get something as basic as weight and balance "right" is, one might say, "excuseless." In the Fine Air crash, 45 tons of denim on pallets destined for a clothing factory in the Dominican Republic was loaded in such a way that a stall was practically inevitable. In the dry wording of the NTSB findings, it would have taken "exceptional skills and reactions" to save the aircraft. The findings included this pointed observation: "If the flightcrew had had an independent method for verifying the accident airplane’s actual weight and balance and gross weight, in the cockpit, it might have alerted them to the loading anomalies, and might have prevented the accident."

NTSB investigators are so concerned (actually angry) about the second cargo plane crash in recent years that they plan to hold a full-blown public hearing in coming months on the Evergreen case.

The problem of uncertain weight and balance certainly is not restricted to cargo operations. Passenger pilots sometimes grouse about the lack of precision in weight and balance, as well. Current dispatching procedures can calculate the aircraft weight based on historical survey data that may encompass four or more variables: average passenger weight, average carry-on bag weight, average checked baggage weight, and fleet average aircraft weight.

One B777 first officer observed that underestimating the weight of carry-on baggage by, say, 3 pounds per passenger spread over 300 passengers pushes the error up to nearly 1,000 pounds for the calculated takeoff weight. That may be just the tip of the iceberg of uncertainty.

The differences between carrier practices are nothing short of startling. One airline applies a winter figure of 185 pounds per passenger (including carry-on bags), and a summertime figure of 180 pounds. One operator may apply planning figures like these across an entire row of seats, while another "refines" the data to each occupied seat.

Consider the economic aspect. Assume, for illustrative purposes, a 3% variance between fuel gauges and the amount of fuel measured by actual dipstick readings. Let’s say the individual cockpit gauges sum to 140,000 pounds (20,000 gallons at 7 pounds per gallon) and the dipstick readings sum to 19,400 gallons (3% less). The variation of 600 gallons equals about 4,200 pounds (or 600 gallons multiplied by 7). In other words, the airplane could be 4,200 pounds lighter or 4,200 pounds heavier. This range falls within the weight and balance tolerances established for the aircraft, prompting perhaps the premature conclusion that the variance is no big deal.

Ah, but assume the lighter side; the fuel load is 600 gallons less than the gauges indicate. That 4,200-pound under-reading equals about 19 passengers, including baggage. At, say, $300 a ticket, those 19 extra passengers spell another $5,700 of revenue for each flight.

Two approaches exist for weighing a loaded airplane in real-time: "on-ground" and "on-wing." The on-ground system involves taxiing the airplane onto steel and concrete weighbridges, in a manner similar to the way heavy trucks are weighed by the roadside. One such system, with four weighing pads for the main gear and one for the nose gear, was installed at Bogota’s El Dorado International Airport in 1995. Writing in the January/February 2001 issue of the International Civil Aviation Organization Journal, Walter Young, chairman of Connecticut-based Emery Winslow Scale Co., claims the system his company installed at Bogota can accurately weigh transport category airplanes in 30 seconds, up to the size of a Boeing 747.

The on-wing approach taken by Texas-based Trinity Airweighs incorporates the weighing equipment into the landing gear. Briefly, the system uses metering cylinders to exercise the landing gear strut seals. Pressure sensors and an on-board computer measure the actual weight supported by each landing gear strut. Basically, the Airweighs system converts the landing gear struts into scales. The system automatically corrects for strut-seal friction by injecting and then withdrawing a small volume of nitrogen gas into the landing gear struts while strut pressure is monitored. A readout in the cockpit displays weight and balance to the pilot.

The system’s accuracy was demonstrated in static testing of the Saab 340A, a 67-foot long airplane with a fore-and-aft CG limit of 10 inches. The on-wing system measured the airplane’s CG to an accuracy of half an inch.

The on-ground and the on-wing approaches offer advantages and disadvantages. The on-ground system can measure every airplane, but not at the gate, when last minute passengers and late baggage loading can alter the weight and balance. The on-wing system can be used at any time, at any gate, but provides the information only for those operators who equip their airplanes.

Real-time weight and balance measuring addresses a number of issues:

  • Some airplanes can exceed their aft CG envelope before they reach maximum takeoff weight. A more refined measure of actual weight and distribution can permit greater use of the CG envelope.

  • Loading with a more aft CG can reduce the amount of nose-down trim in flight, lowering drag and hence fuel burn.

  • A CG located further aft allows for decreased landing speeds, permitting shorter field length requirements.

One final point seems worth stressing: With real-time measuring of weight and balance, one can load to the safe maximum for the aircraft.

For more on the on-ground system, visit www.emerywinslow.com, and for the on-wing unit, www.trinityairweights.com .

David Evans is editor of the award-winning newsletter Air Safety Week. Direct comments to devans@phimedia.com.

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