4 June 1996, 12:34:38 UTC, T + 00:00:39

Liftoff of Ariane 5 L501, 4 June 1996. (ESA)

4 June 1996: The first Ariane 5 heavy launch vehicle, L501, was launched from the Ensemble de Lancement Ariane 3 (Ariane Launch Area 3) at the Centre Spatial Guyanais (CSG), northwest of Kourou, French Guiana, at 12:33:59 UTC, (9:33:59 a.m., local time).

Everything proceeded normally until T + 00:00:36.7. At that time, the backup Inertial Reference System computer failed. 0.05 seconds later, the primary IRS computer also failed.

Having lost its spatial reference, the guidance system began swiveling the engines to correct a perceived attitude change, which, in fact, had not occurred. This caused the rocket to veer off course.

Once the Ariane 5’s angle of attack reached 20°, at T plus 39 seconds, aerodynamic forces caused a structural failure. The two solid rocket boosters broke away. As the rocket began to break apart, the automatic destruct system was activated. L501 exploded at approximately 4,000 meters (13,123 feet), about 1 kilometer (0.6 miles) from the launch pad. Debris fell, covering an area of approximately 5 × 2.5 kilometers (12.5 square kilometers/4.8 square miles).

Explosion of Ariane 5 L501, 4 June 1996 (ESA)


. . . The internal SRI software exception was caused during execution of a data conversion from 64-bit floating point to 16-bit signed integer value. The floating point number which was converted had a value greater than what could be represented by a 16-bit signed integer. This resulted in an Operand Error. The data conversion instructions (in Ada code) were not protected from causing an Operand Error, although other conversions of comparable variables in the same place in the code were protected. . .


The failure of the Ariane 501 was caused by the complete loss of guidance and attitude 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.

ARIANE 5, Flight 501 Failure, Report of the Inquiry Board, Paris 19 July 1996

When designing the Ariane 5, the same software used in the the Ariane 4 guidance system was used. But the Ariane 5 accelerates in a way that causes horizontal velocity to increase at a rate 5 times that of the Ariane 4. This excessive value could not be processed and the computers shut down.

© 2019, Bryan R. Swopes

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About Bryan Swopes

Bryan R. Swopes grew up in Southern California in the 1950s–60s, near the center of America's aerospace industry. He has had a life-long interest in aviation and space flight. Bryan is a retired commercial helicopter pilot and flight instructor.

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