The inquiry board investigating the loss of the first Ariane 5 launcher has
presented its conclusions. I have not seen the actual report of the board,
but I have access to an official summary. Even the summary is a rather
lengthy document, so I have extracted the parts which directly concern the
sequence of events, and the causes of the failure. The extracts are
verbatim, except for my possible typos.

- During the launch preparations and the count-down no events occurred which
were related to the failure. The meteorological conditions were acceptable,
and no other external factors have been found to be of relevance.

- At 36.7 seconds after H0 (approx. 30 seconds after lift-off) the computer
within the back-up inertial reference system, which was working on stand-by
for guidance and attitude control, became inoperative. This was caused by an
internal variable related to the horizontal velocity of the launcher
exceeding a limit which existed in the software of this computer.

- Approx. 0.05 seconds later the active inertial reference system, identical
to the back-up system in hardware and software, failed for the same reason.
Since the back-up inertial system was already inoperative, correct guidance
and attitude information could no longer be obtained and loss of the mission
was inevitable.

- As a result of its failure, the active inertial reference system
transmitted essentially diagnostic information to the launcher's main
computer, where it was interpreted as flight data and used for flight
control calculations.

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

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

- Destruction was automatically initiated upon disintegration, as designed,
at an altitude of 4 km and a distance of 1 km from the launch pad.

- The inertial 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. The
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.

- 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 analyzed or fully
understood which values this particular variable might assume when the
alignment software was allowed to operate after lift-off.

- 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.

- 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 to
cease operation.

- 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.

- 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.