NASA Office of Logic Design

NASA Office of Logic Design

A scientific study of the problems of digital engineering for space flight systems,
with a view to their practical solution.

Shuttle ALT Flight 1A: GPC 3 Failure


General purpose computer 3 failed during preflight checks for captive-active flight 1A on June 17, 1977, at 14:33:04. The central processing unit and input-output processor both stopped executing. No built-in test equipment error indications were generated.

Each general purpose computer consists of two electronic packages; a central processing unit and an input-output processor (fig. 6-7). Computer memory is split between the two packages, as shown in the figure. The central processing unit contains the main memory control circuits.

The central processing unit and input-output processor operate essentially independently. Each has access to the shared memory during alternate 900-nanosecond cycles. During high input or output activity, the input-output can take over exclusive control of memory and the central processing unit clock logic will become static (central processing unit will stop and wait until input-output use of the memory has been completed).

Two possible causes of the failure have been identified:

First, the central processing unit clock oscillator or clock logic may stopped or hung at a time when the central processing unit was accessing memory.  If this occurred, the central processing unit would not release the memory for the next input-output processor memory cycle and the input-output processor would stop.

Second, the memory control circuits in the central processing unit may not have responded to the input-output memory advance signal (signal that releases memory to the central processing unit) after an input-output processor memory access cycle. In this case, the central processing unit clock logic would go static and wait for memory access and the input-output processor would also stop the next time it required memory access.

Troubleshooting, including thermal cycling, has not caused the problem to recur.  The problem cannot be further isolated by analysis, so the actual cause cannot be determined.

This anomaly is closed.

Figure 6-7. General-purpose computer block diagram.


  1. "Space Shuttle Orbiter Approach and Landing Test Evaluation Report: Captive-Active Flight Test Summary," NASA-TM-75011; JSC-13045, 19770901; Sep 1, 1977.  See section 6.6.

Space Shuttle Computers and Avionics Page

Other related reports from the NASA Technical Reports Server (no digital version available for some).

Approach and Landing Test Program Status; of Space Shuttle Orbiter

Haise, F. W., Jr.
JAN 1, 1977

This paper discusses captive-active tests of the Shuttle Orbiter's landing capability. Tests include: free-flight, in which a flare maneuver was undertaken to evaluate the control stick steering mode; separation, in which the pilot pitches up and rolls at a 20 deg bank angle; up and away flight, to test vehicle handling and stability; 2nd rollout, to test the anti-skid system and braking. Attention is given to bare airframe handling qualities, stressing the role of the pilot in controlling the craft.

The Approach and Landing Test program of the Space Shuttle Orbiter 101

M. J. Sugano1 and T.R. Brice2
1NASA Johnson Space Center
2Ford Aerospace and Communication

Aircraft Systems and Technology Conference
Los Angeles, Calif., Aug. 21-23, 1978

Abstract (excerpt)
   The Approach and Landing Test was conducted to verify the landing capability of the Orbiter. The preliminary mated test phases led up to the free flight tests and were used to check out software and hardware systems in a constrained environment.  The tests were conducted with the Orbiter mounted atop a modified 747 commercial airliner, and verified the systems required for separation. The first free flight of the Orbiter occurred on August 12, 1977 and was manned by astronauts Fred Haise and Gordon Fullerton. Free flight two occurred on September 13, 1977, with astronauts Joe Engle and Richard Truly flying the Orbiter. Three other flights were eventually flown with the two crews alternating flights.
   The preparations for the flight, the engineering problems and considerations which arose before and during the flights, and the unique approach to the flight testing are discussed in this paper. Some comparisons of predicted and flight results are presented, and excerpts from the pilot reports highlight the paper.

Space Shuttle Orbiter Approach and Landing Test:
Final Evaluation Report

NASA-TM-79404; JSC-13864
Feb 1, 1978

The Approach and Landing Test Program consisted of a series of steps leading to the demonstration of the capability of the Space Shuttle orbiter to safely approach and land under conditions similar to those planned for the final phases of an orbital flight. The tests were conducted with the orbiter mounted on top of a specially modified carrier aircraft. The first step provided airworthiness and performance verification of the carrier aircraft after modification. The second step consisted of three taxi tests and five flight tests with an inert unmanned orbiter. The third step consisted of three mated tests with an active manned orbiter. The fourth step consisted of five flights in which the orbiter was separated from the carrier aircraft. For the final two flights, the orbiter tail cone was replaced by dummy engines to simulate the actual orbital configuration. Landing gear braking and steering tests were accomplished during rollouts following the free flight landings. Ferry testing was integrated into the Approach and Landing Test Program to the extent possible. In addition, four ferry test flights were conducted with the orbiter mated to the carrier aircraft in the ferry configuration after the free-flight tests were completed.

Space shuttle program orbiter approach and landing test

Feb 11, 1977

The orbiter approach and landing test (ALT) reports are published to provide senior NASA management with timely information on ALT program plans and accomplishments. The ALT reports will be comprised of this pre-ALT report, ALT pre-flight memoranda, and an ALT post-flight report following each flight. The purpose of this pre-ALT report is to provide an overview of the ALT program, describing the flight vehicles involved and summarizing the planned flights.

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