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sensed-reference-speed real-number
brake-release-order-in-effect Boolean
wheel-slip-value real-number
green-hydraulic-pressure available, insufficient
yellow-hydraulic-pressure available, ??
antiskid-power-electric? Boolean
autobrake-arming lo, med, max
speedbrake-control-lever-position armed, disarmed
sensed-MLG-touch-down? Boolean
takeoff-run? Boolean
speed->-72-kt? Boolean
ground-spoilers-state armed, extended, in-transit, retracted
thrust-lever-positions {1, 2} × { reverse, idle, < 4◦, [4◦, 20◦], > 20◦ }
number-rudder-pedals-depressed 0,1,2
in-flight? Boolean
speed-brakes-control-lever-position up-armed, down-disarmed
sensed-speed < 20-kts, [20kts, 72kts], > 72-kts
A/SKID ON, not-ON
N/W-STRG ON, not-ON
PARKING-BRAKE ON, not-ON
power-supply-failure? Boolean
BSCU-failure? Boolean
Table 1: The variables used in the actuation logic
Variable name Values
sensed-skid-onset? Boolean
sensed-reference-speed real-number
wheel-slip-value real-number
sensed-MLG-touch-down? Boolean
takeoff-run? Boolean
speed->-72-kt? Boolean
in-flight? Boolean
sensed-speed < 20-kts, [20kts, 72kts], > 72-kts
Table 2: Internal variables corresponding to environmental situations
9
(both thrust levers in reverse) )
(one thrust lever idle AND one thrust lever in reverse) OR
( (both thrust levers idle) OR
WHEN
MLG touchdown OR (T.O. AND speed > 72kt)
( (both thrust levers in reverse) AND
(speed brake control lever pushed down) )
OR
( (one thrust lever advanced above 20 deg) OR
(one thrust lever advanced for > 3 sec between 4 deg and 20 deg) )
Spoilers disarmed
Spoilers armed
and retracted
Spoilers extended
(spoilers armed) AND
(speed brake levers up) AND
NOT (spoilers extended)
(spoilers armed) AND
(speed brake levers up) AND
(spoilers extended)
speed brake levers => UP speed brake levers => DOWN
(NOT speed brake levers up) AND
NOT (spoilers extended)
(NOT spoilers armed) AND
Figure 7: Revised Ground Spoiler Deployment Diagram
Variable name Values
sensed-skid-onset? Boolean
sensed-reference-speed real-number
wheel-slip-value real-number
sensed-MLG-touch-down? Boolean
takeoff-run? Boolean
in-flight? Boolean
Table 3: Reduced Set of Internal variables corresponding to environmental sit-
uations
10
Listing the Variables
Firstly, all the variables mentioned in the FCOM description should be listed,
along with the types of values they can have, which may be determined from the
description. These variables determine the possible states, given by their com-
binations of values. This may be a larger set than needed, because of logical
dependencies between some variables. This list may then be surveyed to de-
termine variables which are internal representations of environment variables.
Variables which correspond to environmental properties are listed in Table 2
and a reduced set in which variables which are restatements of values of others
variables, and thus logically dependent on them, are listed in Table 3.
The three steps involved in analysing the variables are:
• List all variables appearing in the description being considered;
• identify variables which are representations of environmental conditions,
and for each such variable [variable-name], add a new variable named
sensed-[variable-name];
• identify logical dependencies amongst the variables listed, and reduce the
set of variables by elimating those whose values may be expressed as values
of others (the choice of which variables to eliminate and which one to retain
is arbitrary, subject only to the condition that the eliminated variables
may be defined in terms of the retained ones).
7 Conclusions
We have suggested that the description in the Flight Crew Operating Manual of
an aircraft such as the A320 may be considered as a high-level system specifica-
tion of the usual sort. We have shown how predicate-action diagrams, a simple
graphical technique based on rigorous logical methods, may be used to analyse
the specification, and to express it better.
References
[AL94] M. Abadi and L. Lamport. An old-fashioned recipe for real time. ACM
Transactions on Programming Languages and Systems, 16(5):1543–
1571, Sep 1994.
[Eco94] Air crashes: But surely ... The Economist, 331(7866):92–93, June 4th
- 10th 1994.
[FB92] J.H. Fielder and D. Birsch. The DC-10 Case: A Study in Applied
Ethics, Technology and Society. State University of New York Press,
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