Home' RTCA Documents for Review : Addressing Human Factors/Pilot Interface Issues Contents 65
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Approved Design Examples
Approved designs of vertical speed and flight path angle modes have shared a
common control interface and clearly differentiated the two modes. These
approved implementations clearly labeled the units, selected mode, and mode state
in a collocated display window on the flight guidance panel, and also annunciated
the selected mode near the altitude tape and in the FMA field on the PFD.
FMS functions that allow manual changes to speed or altitude constraints should
honor any pilot entry once accepted, as long as the constraint can be achieved by
the system logic. If the constraint cannot be met, then indication of that limitation
should be annunciated to the crew to avoid a system commanded flight path
deviation, or last minute manual control adjustment.
AC 25.1329-1(), Approval of Flight Guidance Systems, provides information on
AC 27-1(), Certification of Normal Category Rotorcraft, Section AC 27.1329
AC 29-2(), Certification of Transport Category Rotorcraft, Section AC 29.1329
Retrofit Considerations: Flight Directors and Flight Path Markers
Avionics systems that implement an existing Flight Path Marker (FPM) (predictive flight
guidance) or Flight Director (FD) (pitch or path based guidance) on larger display formats
may need to tune the gains of the FPM or FD for the new display size to avoid excessive
“hunting” or extraneous pilot control movements during manual flight path control.
Depending on the level of complexity, independent computations may come from multiple
Flight Guidance computers drive the FD display, causing guidance from the FDs to be
different between the left and right sides. Multiple flight directors displayed at the same
time may force the crew to spend additional time and effort understanding which FD is
providing the correct guidance during a failure.
Specific Certification Issues
Manual flight path control of flight path guidance (FPM or FD) was adversely
affected by a primary flight display that incorporated an expanded pitch scale,
where the FPM or FD gains were not appropriately tuned.
Implementation of some “Highway in the Sky” (HITS) guidance symbology
contributed to a lack of piloting precision during tracking tasks if used in place of
primary guidance, as compared to traditional flight director targets.
Left seat / right seat Flight Directors driven by independent Flight Guidance
computers were displayed on the same flight deck, but on different PFDs, without
clear indication of which FD was controlling the aircraft.
Approved Design Examples
On dual PFD or HUD equipped flight decks, there have been approved
implementations where one side displays a flight path vector referenced FD to an
ILS source while the other side displaying a pitch referenced FD to the same ILS
source (or HUD symbology versus pitch-based symbology heads down). The gains
and offsets of the guidance cues from their respective targets may appear slightly
different, but following either symbology type results in the same aircraft control
and flight path. Implementations that allow this type of design have significantly
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