February 26, 1997

Testing may give F-5E

longer lease on life

By Jennifer O'Connor


Testing under way on a Northrop F-5E "Tiger II" aircraft hopes to indicate that the airframe's projected service life can be increased.

Last year, VFC-13 sent the F-5E to the Naval Air Warfare Center Aircraft Division (NAWCAD) for flight tests to help increase the airframe's projected service life. VFC-13 is one of two squadrons which maintains F-5E/F aircraft for use in the adversary training program.

When an aircraft rolls off the production line, it's expected to be in service for a certain number of flight hours, which constitutes the "fatigue life" of the airframe. This fatigue life projection assumes a certain usage pattern for the aircraft, in terms of the type and quantity of maneuvers it performs while in service. The Naval Air Systems Command (NavAir) keeps track of the usage pattern of the planes, and that information is used to determine how quickly the airframe fatigue life is being expended.

Recently, the algorithms used by NavAir to monitor F-5 fatigue life expended (FLE) were updated. The new algorithms were used with recent analytical loads data from Northrop to calculate the fatigue life remaining to the fleet of F-5 aircraft, which unfortunately resulted in a significant life reduction from what had previously been expected.

It is hoped that the "real-life" data collected in this test program will demonstrate that the numbers used in the FLE calculation were overly conservative, and the test data can be used with the new FLE algorithm to provide some additional life for the F-5 fleet.

The test airplane arrived at Naval Air Station Patuxent River right after Labor Day, and was promptly moved to Hangar 101 to be instrumented by the Test Article Preparation Department. Dick Labare headed up the instrumentation effort, which was no simple task because the airplane was not a test asset in its full-time job, so it had to be instrumented from scratch. Several strain gauges were installed on internal structural members, and complete signal conditioning and telemetry encoding systems were added. Pilot's hot mike was included in the telemetry package to allow easy communication with engineers at the Real-Time Telemetry Processing System (RTPS) ground station. The instrumentation effort was completed in mid-December.

Two Navy test pilots were assigned to the project. Lt. Cmdr. Joe "Spanky" Capstaff from Naval Strike Aircraft Test Squadron (Strike) was the project officer, and Lt. Cmdr. Darryl "Spike" Long (Senior Fixed Wing instructor, U.S. Naval Test Pilot School) was a project pilot. Long also served as test pilot school liaison for the project team, an important job since the aircraft was being hosted by test pilot school during its stay at Pax.

Both officers went to NAS Fallon at the end of December to receive F-5 flight training from VFC-13 squadron pilots. Once the pilots returned to Pax, NATOPS quals in hand, the project was ready to get under way.

The first test flight took place on Jan. 27. It proved to be an eye-opener for the test team because the aircraft was much more fuel efficient than was originally anticipated. This allowed the project to be completed in just 10 flights, all of which took place during the week of Jan. 27.

Test points consisted mainly of high positive and negative-g maneuvers which were designed to stress the appropriate internal structural members. Test instrumentation worked superbly throughout, and all the necessary data were collected.

Engineers supporting the flight tests were Jennifer O'Connor, lead project engineer; Lisa Oswald, Bob Bunton, Amy Perusse and Mary Botting, all from T&E Engineering; and Phil Conjelko from Air Vehicle Engineering.

The question everyone is asking now is "Will the newly measured strain data provide some additional fatigue life for the fleet?" The answer: "We don't know yet." Structural engineers from Air Vehicle Engineering are working with the data, but there is a lot of number crunching still to be done.

If the answer turns out to be "yes," the fleet may be able to keep the F-5 in service for several years to come. That would be great news for proponents of dissimilar aircraft combat training, as well as for all the pilots who enjoy flying and fighting these feisty airplanes.

Last updated: 2.26.97