Home' RTCA Documents for Review : Addressing Human Factors/Pilot Interface Issues Contents 45
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aural alert portion while still keeping the source system powered. On some
applications, the fastest and easiest way for the integrator to mute the aural alerts
is to suppress the power to the system. Instead of designing the system to mute
the necessary aural and visual alerts alone such that it cancels only the alerting
information to the crew, the whole system becomes inoperative. Neither the
switch labeling nor annunciations gave any expectation that the whole system
would be inoperative because it had been labeled “INHIBIT” vs. “OFF”, which
was its true state.
Frequent false alerts have the tendency to desensitize the flightcrew to their
presence and/or impede or slow the required immediate or subsequent action.
o Warnings for allowable exceedances associated with normal powerplant
operation can be perceived as a nuisance. For example, lack of hysteresis on
engine instruments getting close to the out of limit threshold has caused
nuisance display or aural alerting, distracting the crew when engine operation
is normal, or in the case of an allowable transient, no pilot action is required.
o Repetitive windshear warnings that post when windshear conditions do not
exist or result in false TAWS alerts, have caused pilot to either react
inappropriately, become desensitized to actual windshear warnings over time,
or turn off the warning system.
o An electrical power bus failure that supplied multiple systems annunciated
failures of systems that remained operational because they had power sources
from multiple busses.
o Aural alerts: Inappropriate volume setting(s) during both high and low ambient
sound conditions have made aural alerting volumes insufficient. Factors that
diminished the aural alert effectiveness have included: high dynamic air
pressure, engine or propeller noise, environmental control system (ECS) noise,
and background radio communications. Ineffective designs have included:
The RADALT (radio altitude) altitude alert and the intercom system
had separate volume controls. The volume settings were inappropriate
during both high and low ambient sound conditions.
On the ground, during preflight test, the sound was sufficient,
however, once in flight, the ambient noise was too high to hear the
o Warnings that could be muted by the flightcrew to avoid nuisance alerting
generated by some flight conditions, such as overspeed in turbulence, have
been deemed to be unacceptable in previous certifications as the warning may
be missed by the crew during different flight conditions if there is no indication
that the aural is cancelled.
Approved Design Examples
If the system had an alerting function that was meant to inhibit aural and visual
alerts only, labeling on the inhibit switch was consistent with the intended function.
If the system did not have an inhibit function to mute aural and visual alerting
alone, but rendered the entire system OFF, the switch was labeled appropriately to
ensure the derived inhibit function is clearly and unambiguously documented (e.g.,
Dynamic displays that provided system status information such as engine
parameters displays were appropriately tuned and applied a detection threshold and
tolerance that minimized the potential for nuisance alerting.
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