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DO-253D Change 1
© 2019 RTCA, Inc.
OPERATIONAL PERFORMANCE REQUIREMENTS
Alerts can be presented in a variety of ways depending on the installed avionics equipment
of a particular aircraft. Some aircraft will be equipped with Electronic Flight Instruments
while others will be equipped with mechanical instrumentation and discrete indicators.
Alerts may result in the form of a mechanical flag dropping into view on a mechanical
instrumented aircraft to a more sophisticated presentation on an Electronic Flight Control
equipped aircraft. A common form of presentation for alerts may be used; for example,
the vertical flag may be used to indicate vertical integrity alerts.
The GPS/LAAS avionics equipment interfaces must be designed such that when an aircraft
on the final stage of the precision approach passes the GPIP (reference Section
22.214.171.124.1 .2), the precision approach vertical guidance indication to the pilot (e.g., the
vertical course deviation indicator needle) shall [LAAS-276] be removed or indicate full-
scale fly up.
1. The intent of this requirement is to avoid having the pilot display indicate a fault
in the SIS or airborne receiver.
2. The guidance indication in this requirement refers to that indication provided to
the pilot of the aircraft’s position relative to the desired path. A flag or other
indication of the validity or status of this guidance is not considered guidance.
Guidance Reference Point
Deviations are defined for a guidance reference point (GRP). The GRP for lateral and
vertical deviations may be different. The GRP may be the phase-center of the GPS antenna,
a fixed offset (in the along-track and vertical axis), or a separate point referenced to the
If the GRP is the phase-center of the antenna or a fixed offset, there is no additional position
error resulting from this offset. However, the offset must be determined to be operationally
acceptable at time of installation.
If the GRP is translated to another point on the aircraft, there is an additional error term
introduced in the guidance by the input of aircraft pitch, roll and yaw. This error must be
addressed at time of installation.
Missed Approach Guidance
It is required that all LAAS equipped aircraft provide missed approach guidance capability
in order to support the lowest landing minimum. LAAS equipped aircraft will most likely
provide this capability by integrating the LAAS equipment with a RNAV system (e.g.,
For non-RNAV equipped aircraft, the LAAS equipment may sequence based on an input
commanding missed approach or by automatic sequencing to missed approach guidance.
The automatic sequence will occur when all of the following conditions are satisfied: 1)
the aircraft position is past the LTP/FTP (i.e., when the bearing to the LTP/FTP is more
than ±90 deg from the FAS bearing), 2) the aircraft altitude is greater than the touchdown
zone elevation (TDZE) (or LTP/FTP altitude) plus 200 feet, and 3) the proportional vertical
deviation is full-scale fly down (≥ 150 μAmps).
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