After initial touchdown on the main runway at Edwards Air Force Base on its final flight in the space shuttle Approach and Landing Test series, the prototype shuttle Enterprise bounced back into the air and rolled to the right due to an unintended pilot-induced oscillation. (NASA photo)
(Editor's Note: NASA's Dryden Flight Research Center at Edwards Air Force Base in Southern California's high desert played a significant role in the space shuttle's development. With the conclusion of the Space Shuttle Program, we are publishing a series of historical retrospectives that explore some of the center's contributions to the shuttle's design, development and flight validation. In this third article, Dryden historian Peter Merlin recalls how the pilot-induced oscillation problem that occurred on the last flight in the Approach and Landing Tests in 1977 was resolved by use of Dryden's F-8 Digital Fly-By-Wire research aircraft.)
F-8 DFBW Helped Solve Shuttle Oscillation Problem
NASA Dryden and the Space Shuttles
The fifth and final free flight in the shuttle Approach and Landing Test program took place at Dryden Oct. 26, 1977. In the last few minutes of the flight, as the prototype space shuttle orbiter Enterprise crossed the threshold of the main runway at Edwards Air Force Base, it seemed to float at an altitude of just four feet above the airstrip as if reluctant to touch down. Commander Fred Haise attempted to overcome this tendency by pushing forward on his control stick, but to no effect. The orbiter’s altitude then ballooned slightly before the main wheels gently kissed the concrete some 1,000 feet beyond the planned touchdown point at a speed of 180 knots. Enterprise skipped into the air and rolled to the right, surprising the crew.
While pilot Gordon Fullerton attempted to level the wings, a lateral pilot-induced oscillation – or PIO -- developed. As the oscillations became more pronounced, the crew realized that roll commands through the rotational hand-controller were abnormally large and control response was lagging. Fighting his instincts, Haise stopped making command inputs and allowed the roll rate to damp out naturally to nearly wings-level.
About six seconds after first skipping into the air, Enterprise touched down for a second time. The crew was forced to accept a higher-than-normal sink rate because of concern about airspeed bleed-off, which had reduced the orbiter’s speed to 155 knots. Consequently, the orbiter landed harder than expected. The left main wheels also lifted slightly on the rebound but quickly settled onto the runway.
The unintended PIO provided quite a show for spectators and invited guests. For this particular event selected VIPs had been taken to a viewing site relatively close to the edge of the runway. Among those watching was Great Britain’s Prince Charles, himself a pilot.
Analysis of the data from the flight indicated the oscillations resulted from control stick inputs made during the last eight seconds before touchdown. Pilot inputs to control sink rate resulted in a sequence of large elevon deflections prior to touchdown that caused the orbiter to skip back into the air. Since Haise was unaware of any problem beyond the fact that he was landing long, he applied forward stick-pressure to halt the ballooning. This inadvertently initiated a roll command. Because the orbiter’s center of pitch is near the cockpit, and due to cockpit visibility limitations, small changes in pitch attitude were not readily apparent to the crew and neither crewmember detected the oscillation.
Dryden engineers launched an all-out effort to comprehend and resolve the orbiter’s oscillation problem prior to the beginning of orbital flight tests. In early 1978, Milton O. Thompson, director of Dryden’s Research Projects branch, drafted a plan to “obtain a current database that will sharpen our awareness of all factors (subtle and obvious) that might influence a low L/D [lift-to-drag] orbiter runway landing in demanding situations.” Part of the program called for application of ALT data to computerized simulators for the purpose of familiarizing shuttle pilots with the gain settings needed for landing. At the same time, Dryden’s F-8 Digital Fly-By-Wire test bed – an ex-Navy F-8 Crusader fighter jet with highly modified flight controls connected to the same AP-101 computer as used in the shuttle – provided flight-test data to determine how delayed computer response to human input might be reduced or eliminated.
Five research pilots flew PIO data flights, making a total of 60 landings to simulate the orbiter’s control characteristics. They found that lags as short as 200 milliseconds between pilot input and discernible control surface response profoundly affected the aircraft’s handling qualities.
The solution was a software filter that dampened, without affecting handling qualities or causing control time delays, the types of pilot inputs most likely to cause oscillations. These software changes worked, reducing, if not entirely eliminating, PIO tendencies. Greater landing control, however, came at the expense of some degree of control-stick responsiveness.
Development of the space shuttle orbiter produced the first reusable spacecraft capable of returning from orbit and landing on a conventional runway. This bold, pioneering effort forced engineers to confront complex challenges in developing a vehicle with longitudinal flying qualities required for landing the orbiter manually in an operational environment. The ALT program was the final hurdle before the first orbital mission. Based on ALT flight data and orbiter crew evaluations, all objectives of the program were successfully accomplished.
For their exceptional efforts, the four Enterprise crewmen were later honored with the Society of Experimental Test Pilots Kincheloe Award. The PIO suppression filter was added to orbiter flight control system software prior to the first flight of Columbia, setting the stage for a successful landing at the end of STS-1, the shuttle’s first orbital flight.
Peter W. Merlin, Historian
NASA Dryden Flight Research Center