897. Learning to Live With the OBAWS


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A M Filipov: 897. Learning to Live With the OBAWS. 1971.



OBAWS, the on-board aircraft weighing system which is on the Boeing 747 and is presently being installed on the Douglas DC-10 aircraft, has accumulated a large number of flight hours on many aircraft in commercial service. During this time period, tests have been conducted on some aircraft, and a number of the airlines have compiled comparisons of 747 OBAWS and manifest weight and CG values. BLH Electronics, Inc. has been further analyzing the data with the help of a Time-Sharing Computer. A summary of the data is presented in this paper.
Sixty-two sets of data, collected from forty-eight airplanes, have been analyzed. Comparisons of OBAWS values with manifest values for fifty-one sets of data taken from thirty-six of the forty-eight airplanes indicate that the distribution of discrepancies between OBAWS and manifest values has a standard deviation of +/- 1.07% of full scale, or +/- 7,597 pounds.
Since both OBAWS and manifest values have errors associated with them, the discrepancies between the values can be expected to be greater than the individual error spreads. However, to date, neither system has an established accuracy, which can only be obtained from a series of actual weighings.
The formal tests made on the 747-100 aircraft indicated that OBAWS could be expected to meet the targeted accuracy requirement under normal operating conditions. The tests were carried out under normal and abnormal conditions, and the accuracy of the system under the latter conditions appeared to be somewhat degraded. However, the comparisons of the variations in discrepancies between OBAWS and manifest indicate that abnormal conditions which would degrade the accuracy are infrequent.
A number of interesting anomalies can be seen in the data, and some inferences can be made. Maintenance actions on the system can often be recognized, as can ‘paper’ errors. Some studies have been made in an attempt to determine what happens at different stations.
A brief outline of future programs that should resolve the differences between OBAWS and manifest values is presented.


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