A Lufthansa Airbus landing in Warsaw in 1993 encountered bad weather conditions. Upon landing, the aircraft's braking system did not activate for 9 seconds, leaving insufficient runway to stop. This caused the plane to run into an embankment, killing 2 people and injuring 54. The accident was caused by outdated wind information provided to the pilots, their failure to notice inconsistent wind data, and the braking software's inability to handle a landing where only one wheel initially touched down due to weather. The software functioned as specified but did not anticipate this condition.

1. Warsaw airbus accident 1993
Ian Sommerville
Warsaw aircraft accident, 1993
Slide 1

2. What happened
• A Lufthansa Airbus on a flight from Frankfurt
landed at Warsaw Airport in bad weather (rain
and strong winds)
• On landing, the aircraft’s software controlled
braking system did not deploy when activated
by the flight crew and it was about 9 seconds
before the braking system activated
Warsaw aircraft accident, 1993
Slide 2

3. • There was insufficient runway remaining
to stop the plane and the aircraft ran
into a glass embankment
• Two people were killed and 54 injured
Warsaw aircraft accident, 1993
Slide 3

4. Causes of the accident
•
As with most accidents, there were multiple factors
that contributed to this accident. The three main
contributory causes were:
– The aircraft pilots were given outdated information
on the wind speed and direction by the landing
controllers
– The aircrew failed to notice that the on-board
information about the wind direction was
inconsistent with that provided by the controllers
Warsaw aircraft accident, 1993
and that their approach speed was higher than Slide 4

5. • The aircraft braking control software
specification had failed to take into
account the landing conditions
encountered
Warsaw aircraft accident, 1993
Slide 5

6. Focus on software
• The braking control system on the
Airbus behaved exactly as specified
• There were no bugs or errors in the
software
• This is an example of a situation of
where a reliable software system was
unsafe
Warsaw aircraft accident, 1993
Slide 6

7. Aircraft braking
• Aircraft braking depends on deployment
of spoilers which are flaps on the wings
that are deployed to slow down the
plane
• It also makes use of ‘reverse thrust’
which means that the engines are run
‘backwards’ so that their effect is to
Warsaw aircraft accident, 1993
Slide 7

8. • It is critical to the safety of the flight that
neither the spoilers nor the reverse
thrust is deployed while the plane is in
the air
• Therefore, the braking system software
includes checks to ensure that the plane
has landed before the braking system is
deployed
Warsaw aircraft accident, 1993
Slide 8

9. Weight on wheels
• The landing gear includes sensors that
can detect if the wheel struts are
compressed i.e. that there is weight on
the wheels.
• The software specification was that
landing could be recognised if there was
weight on both wheels
Warsaw aircraft accident, 1993
Slide 9

10. Wheel rotation
• Each wheel included sensors that
checked whether the wheel was rotating
or not.
• The software specification was that the
aircraft had landed if the speed of wheel
rotation was greater than 72 knots
Warsaw aircraft accident, 1993
Slide 10

11. • The braking system could be deployed if
either of these conditions were true
• This was checked by the braking
system control software
Warsaw aircraft accident, 1993
Slide 11

12. • The software specification did not anticipate a
situation where neither of these conditions
would hold during landing
IF weight-on-both-wheels OR
(left-wheel-turning OR right-wheel-turning) THEN
braking-system-deployment := permitted
Warsaw aircraft accident, 1993
Slide 12

13. • In this case, because of the weather
conditions, the plane landed at an angle
so that one wheel touched the runway
first
• The runway was wet and that wheel
‘acquaplaned’ so skidded along the
runway without turning
Warsaw aircraft accident, 1993
Slide 13

14. What went wrong?
• The pilots were told that there was a
crosswind across the runway
• Standard procedure for a crosswind landing
to bank the aircraft so that initial touchdown is
on one wheel and the crosswind then acts on
the wing to push the other wheel onto the
runway
Warsaw aircraft accident, 1993
Slide 14

15. •
However, in this case, the wind had changed
direction so that it was a tailwind rather than a
crosswind
•
This meant that the landing speed was higher than
normal and there was no need for a single wheel
touchdown
This was not noticed by the pilots and the higher speed
was a contributory factor to the accident
Warsaw aircraft accident, 1993
Slide 15

16. Warsaw aircraft accident, 1993
Slide 16

17. Warsaw aircraft accident, 1993
Slide 17

18. • The Warsaw Airbus landed on one wheel but
there was no crosswind to push down the
other wheel so, for 9 seconds, the plane was
landing on a single wheel
• Because there was only weight on a single
wheel, the on-ground condition of weight on
both wheels in the braking system did not
hold
Warsaw aircraft accident, 1993
Slide 18

19. Acquaplaning
Warsaw aircraft accident, 1993
Slide 19

20. •
The single wheel on the ground was acquaplaning
rather than turning so the condition that one or both
wheels should be rotating at more than 72 knots did
not hold
•
After about 9 seconds, the 2nd wheel made contact
with the runway and the braking system deployed
•
But it was too late to stop the aircraft and the accident
occurred
Warsaw aircraft accident, 1993
Slide 20

21. Warsaw aircraft accident, 1993
Slide 21

22. Conclusions
•
In practice, it is impossible to make any system
completely safe
•
It is impossible for system designers to anticipate
every possible condition and they have to make
assumptions such as the pilots being given correct
wind information
•
No blame in this case was associated with the
software but it was modified to take this particular
situation into account should it happen again
Warsaw aircraft accident, 1993
Slide 22