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BOEING 757-300 FLIGHT TESTS PROCEED AT FULL SPEED

In an exclusive interview at the beginning of December, Art Fanning, chief engineer responsible for the 757-300 flight test programme, gave his view on the progress so far.

FLUG REVUE: Describe the 757-300 flight test program from beginning to end. What are the stages af the certification program? What major tests do we put emphasis on? What do we have to do?

Art Fanning: The 757-300 test program started as the first 757-300 was being assembled in the factory and will conclude shortly after type certification, which is expected early next year. The test effort includes more than 1,200 hours of dedicated ground testing and about 850 hours of flight testing. Testing naturally falls into several overlapping stages: pre-flight testing, initial airworthiness demonstrations, flight envelope expansion and configuration refinement, type certification demonstrations, and operational and customer-unique demonstrations.

There are three 757-300s involved in the test program. Two of the airplanes have instrumentation and test equipment installed in place of the normal customer interior. That allows us to create and control certain test conditions as well as to record the information about the tests. The third airplane is fitted with a normal interior tailored to the needs of Condor, the first customer airline, and has a very limited set of test instrumentation on board. This third airplane is used for testing that requires a normal airline interior. Some tests in this category show proper functioning of cabin temperature and air conditioning, cabin noise suppression, smoke detection and fire protection systems, lavatories and water systems, emergency escape systems, cabin management systems and in-flight entertainment equipment.

The pre-flight stage, using the first test airplane, involves about one-third of the time of the total ground test program. The emphasis at the tests in this stage is on ensuring basic airworthiness and making sure that the basic functioning of the systems is what the designers had in mind. Tests are conducted in a building block fashion working from the more simple to the highly complex, so that any unexpected results can be tracked back to their cause. There is a preflight stage preceding the first flight of each test airplane. This phase emphasizes the same basic airworthiness of the airplane and checks out increased levels of airplane functionality.

The initial airworthiness demonstration stage begins with the first flight. During this time, we first establish that the aircraft functions as we expected in a basic set of maneuvers carefully selected to indicate that the airplane is ready for more extensive exercise of its capabilities without undue risk. Basic airworthiness testing shows that the aircraft controls function in an expectable manner. The portions of the flight envelope that impose the greatest challenges on the structure of the airplane are cautiously explored to make sure that we have not exceeded the expectations of the engineers.

The flight envelope expansion and configuration refinement stage includes extensive examination of the characteristics of the airplane. All the elements of the airplane are checked during this phase. Any deviations from the expected characteristics are noted and, if necessary, changes in the configuration of the airplane are made to refine the characteristics of the airplane. This refinement process is where the airplane is brought to a standard of performance that meets the requirements for products of The Boeing Company and which we are confident will satisfy the requirements of the regulatory agencies when we demonstrate the airplane to them.

During the type certification demonstration phase, we show that the airplane performs in a manner that satisfies the requirements of the regulatory agencies who issue type certificates for airplane models, Those include the Federal Aviation Administration (FAA) and the Joint Aviation Authorities (JAA). Representatives of those agencies participate in the tests, and in some cases conduct the tests to ensure that the requirements are satisfied. About 225 flight test hours are used in doing these certification demonstrations. That is just under one-quarter of the overall flight test time.

While most of the testing done during the airworthiness and type certification phases is done within the state of Washington, we also take the airplane to what we call remote locations when certain logistic or weather conditions must be met. Performance testing, for example, was done at a U.S. Air Force base in the California desert that is known for its long runway. Ten days of high intensity radiated fields (HIRF) testing was done in lightly populated, rural Montana because there the chance was less of interfering with air traffic signals, cellular phones and other such uses which share the airways. And just this month, the second flight test airplane had to go as far as Iceland to obtain the high velocity surface crosswinds needed to test the airplane's autoland system. Winds of up to 25 knots were required; the airplane landed in crosswinds of up to 38 knots.

The operational and customer-unique demonstrations generally occur toward the end of the program. Testing of customer unique equipment which is not required for basic type certification of the aircraft model can be delayed until late in the program. This equipment includes items such as the in-flight entertainment system and alternate interior arrangements to meet the needs of an individual customer. Operational demonstrations include showing that the procedures in the maintainance manuals accurately reflect how the airccraft needs to be maintained, showing that the planned air crew training is correct to bring the pilot familiarity with the aircraft to the proper level, and that the aircrew procedures published for the airplane's crew are complete, accurate and result in the expeded airplane response.

FLUG REVUE: What are the differences between a flight test certification program for a derivative aircraft like the 757-200 and one for an all-new airplane model?

Art Fanning: The certification program for a derivative differs from the certification program for an all-new model in that for the derivative, the unchanged portions of the airplane do not necessarily need to be tested again. The tests that were conducted in the original certification can be used to show continued compliance by the derivative airplane. As one might expect, this reduces the number and extent of the testing and demonstrations that are required for a derivative. The result is a shorter test program and so, frequently fewer test airplane are involved. Functions of the airplane that result from new portions of the airplane interacting with unchanged portions of the basic airplane typically must be demonstrated for the derivative in much the same manner that they would be demonstrated for an all-now airplane. The difference in duration and content of a test program for an all-new airplane model and for a derivative can vary greatly depending on how much of the derivative airplane is newly designed.

FLUG REVUE: How many missions and how many flight hours?

Art Fanning: We use a unit that we call ìtest items" to group our tests together. A test item is a set of test conditions that is required to meet a certain test objective - for example: all tests that are necessary to show that the smoke detection system in the cargo bay functions property.

The program currently includes 347 test items. One test item may extend over several flights or test sessions. Also, several test items may be tested during a single test flight or session. The whole program includes about 850 planned, dedicated flight test hours. In addition, the tests that are conducted concurrently with other dedicated tests would require about 800 hours of additional flying if we were not able to do more than we test at a time.

The overall ground test effort includes about 1250 planned, dedicated ground test hours. Similar to the flight test situation, we perform some ground tests concurrently but the percentage is much smaller, about 150 hours in this program. As of early November, we had completed 270 flight test hours on the first airplane, NU701; about 175 flight test hours on the second airplane, NU721; and about 5 hours on the third airplane, NU722. (To help you differentiate, NU701 is painted in Boeing livery; the others are painted in Condor's new colors.)

The total flight test time completed so far is about 450 hours, and about 1050 ground test hours have been completed. Test completion rates are very close to our planned rate and we are proceeding through the test program generally as expected.

Our plan included some moderate allowance for test program growth to investigate and retest areas that we investigate during the initial testing. In general, we are staying within that anticipated growth.

FLUG REVUE: What problems or challenges came up during the flight-test program? Were there any small or large events during the program?

Art Fanning: The greatest challenge in our test program is keeping the airplanes flying at the rates that are necessary to meet our objective of completing the development and certiftation of the 757-300 in much shorter time than for other recent derivative airplanes.

The test rates we have achieved in the first several months of our program are the result of several factors. Our test airplanes are truly workhorses, ready to fly again and again with less maintenance than is typical early in a test program.

The crews who maintain, and configure the airplane, who plan and coordinate the tests, who operate the test airplane and other test workhorses who analyze the test data to distill out the pertinent information and the engineers who use the test information to make decisions about how to change the airplane configuration to achieve the desired characteristic, all have had to meet a series of challenges to yield the results we have had this far.

Each of the first two airplanes in our test force flew about 120 hours in the first month after we started operating them. The greatest challenge in our program up to this time was to have our people understand that this was what was necessary if we were going to meet our objective and then got them what was needed to let them achieve it.

While achieving the high test rates, we have had to overcome a number of unanticipated events. Several key tests needed to keep on the planned schedule coincided with unavailability of remote test sites that we needed to use. The sites were unavailable for a variety of reasons: scheduled airshows, runway resurfacing, dedication to other test programs, and holidays which closed down airports.

Although the weather has been generally favorable for our program, when we needed stiff winds to test crosswind performance, the winds in our part of the United States were too calm. Our test team met some of these challenges by changing where we performed the test in order to meet the required schedule. We have been to Iceland and Ireland to conduct tests because the weather and the winds were what we needed.

While meeting these challenges, we have also dealt with the more routine challenges of developing an airplane. We've identified places where our airplane didn't act exactly the way we wanted it to and we've tried several candidate configurations to improve the characteristics the way we wanted. We have chosen the candidate that provided the most effective improvement for Boeing and our customer and then implemented that configuration refinement in the airplane design.

An example involved the roll characteristics of the airplane. Our original aerodynamic configuration had a characteristic that under some sideslip conditions with the flaps extended, the roll rate did not vary quite as we wanted it to with increasing control input. By adding some aerodynamic configuration refinements to the flaps, we were able to achieve the control characteristic that we believe is correct.

A second example is the modifications we have made to the control column to minimize the changes in control forces. We've balanced the control column so that the aircraft accelerations do not cause noticeable changes in control forces and adjusted the elevator feel computer settings so that different aircraft loadings do not result in too large a variation in control pressures needed to perform a given maneuver. As a result, the forces are similar to that of the 757-200.

From page 26 of FLUG REVUE 1/99