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5 Revising the Predicate-Action Diagrams
The predicate-action diagramrepresentation of the FCOMdescription can easily
be completed, in the following steps:
• list all state predicates occurring in any diagram;
• determine the values of all the state predicates in every state denoted, and
list them with the state;
• add transitions each way between all pairs of states;
• notate each transition with its action description (where possible);
• disambiguate the action descriptions from a given state (if necessary);
• note those transitions between pairs of states that cannot occur, and re-
move these transition arrows.
6
Antiskid No antiskid
A/S on
A/S off OR
speed < 20kts OR
yellow lowpress
Figure 3: The Antiskid Condition from the FCOM Specification
AND spoilers retracted
spoilers armed
AND spoilers extended
spoilers armed
(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)
spoilers NOT armed
( (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) )
speed brake levers
moved into up position
Figure 4: Ground Spoiler Deployment from the FCOM Specification
Using these principles, we have revised the predicate-action diagrams as in
Figs 5, 6 and 7.
6 Further Analysis
Some expressions on the FCOM refer to state predicates holding over some
explicit period of time. For example, a full logical expression of the description
contained in the FCOM must include some logical means of handling assertions
of thrust lever position and temporal duration, as noted in [Lad95]. This can
be done straightforwardly in TLA [AL94]. In fact, by using TLA, we believe
the approach we are suggesting here is extendable to all forms the specification
may take, since TLA allows arbitrary mathematics to be included.
When asserting formulas concerning variables which change with time, it’s
a good idea to list all the potential variables. The list of variables referred to
in the FCOM description appears in Table 1. One well-known source of poten-
tial inaccuracies is discrepancy between system states which represent certain
environmental variables by values derived from sensors, and the actual val-
ues of those variables themselves. One example, expressed crudely, is that the
Lufthansa A320 suffered delayed deployment of braking systems because, for
various reasons expressed in the report [MCAAI94], the sensors didn’t detect
that the plane was on the runway. See also [FI.93a]. We describe now what
must be done with the variables.
7
NOT autobrk avail
(NOT power supply or BSCU failure) AND
Antiskid AND
(A/S - N/W STRG switches on) AND
(NOT Prkbrk) AND
yellow pressure AND
NOT green pressure AND
(NOT Normal-brk) AND
Alt with A/S Alt w’out A/S
NOT autobrk avail
(NOT Antiskid) AND
power supply or BSCU failure ] AND
[ NOT (A/S - N/W STRG switches on) OR
(NOT Prkbrk) AND
NOT yellow pressure AND
NOT green pressure AND
(NOT Normal-brk) AND
(NOT power supply or BSCU failure) AND Normal
green pressure
NOT green pressure =>
(A/S - N/W STRG switches on) AND
(NOT Prkbrk) AND
green pressure AND
Normal-brk AND
Antiskid AND
Autobrk avail
Normal w’out A/S
EXCEPT
(NOT Antiskid)
As for state ‘Normal’
Speed => (20kt OR >20kt)
Speed => (<20kt)
( (green pressure => NOT green pressure) AND
(yellow pressure => NOT yellow pressure) )
BSCU fails OR
power supply fails OR
A/S and N/W STRG turned off OR
(green pressure =>
NOT green pressure)
AND
(yellow pressure => yellow pressure)
NOT yellow pressure
yellow pressure =>
????
????
Figure 5: The Revised Braking Modes
Parkbrake Alternate brk
Normal brk or
Prkbrk-handle => OFF
Prkbrk-handle => ON
Prkbrk AND
NOT Normal-brk AND
NOT Antiskid
Figure 6: The Revised Parking Brake Mode
8
Variable name Values
antiskid-mode activated, deactivated
autobrake-mode disarmed, armed, active
braking-on? Boolean
braking-system-failure? Boolean
sensed-skid-onset? Boolean
anti-skid-switch-position ON, OFF
main-gear-tachometer-left real-number
main-gear-tachometer-right real-number
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