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DO-253D Change 1
All power levels specified for the VDB are measured as the average power over the period
of the synchronization and ambiguity resolution portion of the message at the receiver input
port. All power levels specified for other emissions (i.e., VHF Omnirange (VOR),
Instrument Landing System (ILS) localizer and Frequency Modulation (FM) broadcast)
are measured as the power in the Radio Frequency (RF) carrier at the receiver input port.
1. See Figure 1-3 for a representative block diagram.
2. Determination of the Message Failure Rate (MFR) requires additional test
equipment to verify the Cyclic Redundancy Check (CRC), as the CRC is not
verified by the VDB receiver function. Instead, it is verified by the PAN function
to reduce the hazard classification of the VDB receiver.
The hardware and software shall [LAAS-034] be designed and developed such that the
probability of providing misleading information (MI) and the probability of loss of function
are acceptable based on the system integrity and continuity requirements, respectively.
This requirement must be met when the equipment is in its installed configuration for the
most stringent operation supported. To demonstrate compliance, it will be necessary to
conduct a safety assessment to evaluate the system’s implementation against known failure
conditions. This safety assessment should be based upon the guidance of AC 23.1309-1()
for Part 23 aircraft, AC 25.1309-1() for Part 25 aircraft, AC 27-1() for normal category
rotorcraft, and AC 29-2() for transport category rotorcraft.
An acceptable means to demonstrate integrity compliance for the VDB receiver is to show
that no failure of the equipment can result in misleading information (MI). This may be
accomplished by integration with a PAN that a) provides the capability for the pilot to
independently verify that the correct approach is selected, and b) verifies the LAAS
Demonstration of compliance with the continuity requirements depends on the required
probability of a detected failure. The required probability will depend on the level of
airborne redundancy. Hardware design assessment must consider the MTBF of the
equipment as well as the independence of failure modes for redundant equipment.
FAA Advisory Circular (AC) 20-115C, which references RTCA/DO-178C, or later
revision, provides an acceptable means for showing that software complies with applicable
airworthiness requirements. One acceptable means to demonstrate integrity compliance
for the VDB receiver is to show that no design failure of the equipment can result in
misleading information, and to develop all software that affects VDB functions to at least
Level D criteria. Another acceptable approach is to substantiate software levels required
based on a system safety assessment.
Note: Demonstration of compliance with the continuity requirements depends on the
required probability of a detected failure. The required probability will depend
on the level of airborne equipment redundancy. For applications beyond CAT I,
(e.g., autoland), software development of level D classification may not be
sufficient to ensure the probability of detected failure is low enough such that
system continuity requirements are met. Furthermore, most redundancy methods
rely on failures in redundant architectures to be independent. Software level D
classification may not be sufficient to ensure independence of detected failures.
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