Ariane flight V88

Flight 501, which took place on June 4, 1996, was the first test flight of the European Ariane 5 expendable launch system. It was not successful. Due to a malfunction in the control software the rocket veered off its flight path 37 seconds after launch. It was destroyed by its automated destruct system when high aerodynamic forces caused the core of the vehicle to disintegrate.

The breakup caused the loss of the payload: four Cluster mission spacecraft, resulting in a loss of more than US$370 million.

It is one of the most famous computer bugs in history.

Summary

The Ariane 5 software reused the specifications from the Ariane 4, but the Ariane 5's flight path was considerably different and beyond the range for which the reused code had been designed. Specifically, the Ariane 5's greater acceleration caused the back-up and primary inertial guidance computers to crash, after which the launcher's nozzles were directed by spurious data. Pre-flight tests had never been performed on the re-alignment code under simulated Ariane 5 flight conditions, so the error was not discovered before launch.

Because of the different flight path, a data conversion from a 64-bit floating point to 16-bit signed integer value caused a hardware exception (more specifically, an arithmetic overflow, as the floating point number had a value too large to be represented by a 16-bit signed integer). Efficiency considerations had led to the disabling of the software handler (in Ada code) for this error trap, although other conversions of comparable variables in the code remained protected. This led to a cascade of problems, culminating in destruction of the entire flight.

Although the report identifies a software bug as the direct cause, other investigators see the causes as system design failures and management issues..

h) On the basis of those calculations the main computer commanded the booster nozzles, and somewhat later the main engine nozzle also, to make a large correction for an attitude deviation that had not occurred.

i) A rapid change of attitude occurred which caused the launcher to disintegrate at 39 seconds after H0 due to aerodynamic forces.

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m) The inertial reference system of Ariane 5 is essentially common to a system which is presently flying on Ariane 4. The part of the software which caused the interruption in the inertial system computers is used before launch to align the inertial reference system and, in Ariane 4, also to enable a rapid realignment of the system in case of a late hold in the countdown. This realignment function, which does not serve any purpose on Ariane 5, was nevertheless retained for commonality reasons and allowed, as in Ariane 4, to operate for approx. 40 seconds after lift-off.

n) During design of the software of the inertial reference system used for Ariane 4 and Ariane 5, a decision was taken that it was not necessary to protect the inertial system computer from being made inoperative by an excessive value of the variable related to the horizontal velocity, a protection which was provided for several other variables of the alignment software. When taking this design decision, it was not analysed or fully understood which values this particular variable might assume when the alignment software was allowed to operate after lift-off.

o) In Ariane 4 flights using the same type of inertial reference system there has been no such failure because the trajectory during the first 40 seconds of flight is such that the particular variable related to horizontal velocity cannot reach, with an adequate operational margin, a value beyond the limit present in the software.

p) Ariane 5 has a high initial acceleration and a trajectory which leads to a build-up of horizontal velocity which is five times more rapid than for Ariane 4. The higher horizontal velocity of Ariane 5 generated, within the 40-second timeframe, the excessive value which caused the inertial system computers to cease operation.

q) The purpose of the review process, which involves all major partners in the Ariane 5 programme, is to validate design decisions and to obtain flight qualification. In this process, the limitations of the alignment software were not fully analysed and the possible implications of allowing it to continue to function during flight were not realised.

r) The specification of the inertial reference system and the tests performed at equipment level did not specifically include the Ariane 5 trajectory data. Consequently the realignment function was not tested under simulated Ariane 5 flight conditions, and the design error was not discovered.

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t) Post-flight simulations have been carried out on a computer with software of the inertial reference system and with a simulated environment, including the actual trajectory data from the Ariane 501 flight. These simulations have faithfully reproduced the chain of events leading to the failure of the inertial reference systems.

3.2 CAUSE OF THE FAILURE

The failure of the Ariane 501 was caused by the complete loss of guidance and altitude information 37 seconds after start of the main engine ignition sequence (30 seconds after lift- off). This loss of information was due to specification and design errors in the software of the inertial reference system.

The extensive reviews and tests carried out during the Ariane 5 Development Programme did not include adequate analysis and testing of the inertial reference system or of the complete flight control system, which could have detected the potential failure.

Aftermath

Flight 501's high profile disaster brought the high risks associated with complex computing systems to the attention of the general public, politicians, and executives, resulting in increased support for research on ensuring the reliability of safety-critical systems. The subsequent automated analysis of the Ariane code was the first example of large-scale static code analysis by abstract interpretation.

The failure also harmed the excellent success record of the European Space Agency's rocket family, set by the high success rate of the Ariane 4 model. Only recently have Ariane 5 launches become as reliable as those of the predecessor model.

See also

• Computer bug

External links

• Ariane 501 Inquiry Board report
• Spaceflight Now - Cluster II - Ariane 501 explodes — Footage of the final seconds of the rocket flight (216 kB QuickTime file).
• Wired - History's Worst Software Bugs — An article about the top 10 software bugs. The Ariane 5 Flight 501 software glitch is mentioned as one of the these bugs.
• Template:De icon Ariane 5 - 501 (1-3) — A good article (in German) where the actual code in question is given