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| ![](media    | Federal Aviation                                      |
| /image1.wmf) |                                                       |
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| **           |                                                       |
| Memorandum** |                                                       |
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Date: January 26, 2007

To: SEE DISTRIBUTION

From: Manager, Small Airplane Directorate, ACE-100 S/Kim Smith

Prepared by: Robin L. Sova

Subject: Final Policy Statement on Applying Advisory Circular 20-152,
"RTCA, Inc., Document RTCA/DO-254, Design Assurance Guidance for
Airborne Electronic Hardware," to Title 14 Code of Federal Regulations,
Part 23 Aircraft; PS-ACE100-2005-50001

##### Summary

This final policy statement sets up Federal Aviation Administration
(FAA) certification policy on applying Advisory Circular (AC) 20-152 to
complex airborne electronic hardware (CEH) installed in part 23 aircraft
or in airships. The specific issues addressed concern selecting and
applying hardware design assurance levels (HDAL) to CEH.

## **Current Regulatory and Advisory Material**

## 

Part 23 aircraft CEH is subject to Title 14 of the US Code of Federal
Regulations (14 CFR) sections (§§) 23.1301(a, d) and 23.1309(a) (1).
These sections state equipment must be "...appropriate to its intended
function..." to "...Function properly when installed..." and "...not
adversely affect...safe operation." Although 14 CFR § 23.1309 does not
exactly address development of software or CEH, RTCA, Incorporated has
developed two documents the FAA recognizes in ACs 20-115B and 20-152 as
acceptable means for showing compliance. These documents are:
"RTCA/DO-178B, Software Considerations in Airborne Systems and Equipment
Certification" and "RTCA/DO-254, Design Assurance Guidance for Airborne
Electronic Hardware."

Figure 2 of AC 23.1309-1C allows a downward adjustment in the
correlation between failure conditions and software development
assurance levels (SDAL) from that identified in DO-178B. The adjustments
allowed are dependent on both failure condition and airplane
classification. A joint FAA, National Aeronautics and Space
Administration (NASA), and industry team, in consideration of general
aviation (GA) accident statistics, developed this variable adjustment
approach. That team reasoned that wider availability of later
technologies to GA, through better affordability gained by reducing
SDAL-related certification costs, would yield significant safety

benefits. The FAA has since reasoned that allowing these same decreases
to HDALs should yield the same expected benefits.

**Relevant Past Practice**

### 

### Like DO-178B, DO-254 does not consider variable host aircraft platforms or environments in assigning suitable HDALs, only failure condition classifications. The release of AC 23.1309-1C added the ability to reduce SDALs in consideration of these variables. Before the release of 

### DO-254 and AC 20-152, the FAA used Issue Papers (IP) to address CEH compliance. These IPs required either comprehensive testing or analyses of CEH, or use of a development process acceptable to the FAA, such as DO-178B.

### 

### For part 23 projects using DO-178B (or later, DO-254), these IPs also allowed a downward HDAL adjustment based on the same logic used in AC 23.1309-1C for SDAL adjustments. The next update of AC 23.1309 will also include allowance of this HDAL adjustment. Meanwhile, this policy is required to continue to allow that downward adjustment of HDALs to preserve the consistency between part 23 equipment SDALs and HDALs in equivalent failure condition classifications.

### 

The second reason for needing this policy is that AC 20-152 relieves the
applicant from an FAA review of life cycle data for HDAL "D" CEH (having
Minor failure conditions) if developed according to DO-254. This
policy's allowance of reducing HDALs "B or C" to "D" combined with AC
20-152's exclusion from FAA review of HDAL "D" CEH could wrongly result
in an inappropriate level of development oversight of devices having
Major or Hazardous failure conditions. Therefore, this policy must
prevent possible future problems by disallowing

AC 20-152's HDAL "D" review exclusion for any CEH that originally (i.e.,
before this policy memorandum-based downward adjustment) would be
classified as HDAL "B or C."

## **Policy**

The Small Airplane Directorate previously determined and documented in
AC 23.1309-1C that differing classes of aircraft or operations do not
warrant the same SDALs. That same reasoning will also apply to selecting
HDALs for part 23 projects.

Similarly, AC 20-152 provides an exclusion from FAA review for HDAL "D"
CEH developed under DO-254. Therefore, because this policy will allow
reducing higher HDALs to "D", the AC 20-152 exclusion will only be
allowed for CEH originally classed as HDAL "D" without a downward
adjustment based on this policy memo. Applicants and FAA certification
authorities should follow this policy as summarized below for all Part
23 aircraft (or Airship) certification or related equipment approval
projects.

"Certification/Approval of aircraft or airborne equipment containing
complex electronic hardware (CEH) requires they perform their intended
functions safely. One means of showing CEH complies to this requirement
is to apply an FAA accepted development process such as one recognized
by AC 20-152 (RTCA/DO-254). When assigning hardware design assurance
levels (HDALs, "A-D") for CEH installed in Part 23 aircraft (or
Airships), one may select the same levels used for software development
assurance levels (SDALs) based on the guidance in AC 23.1309-1C, Figure
2. An HDAL assigned by this

approach may be less stringent than one arrived at by applying the
guidance contained in DO-254, Table 2-1. Additionally, AC 20-152's
exclusion from FAA review of life cycle data for HDAL "D" CEH, will not
apply to CEH that is classified as "D" because of this policy's downward
adjustment allowance (i.e., CEH that have Major or Hazardous failure
conditions). The AC's exclusion will only apply to CEH originally
classed as "D" per DO-254 (i.e., having Minor failure conditions)."

## **Effect of Policy**

The general policy stated in this document does not constitute a new
regulation or create what the courts refer to as a "binding norm." The
office that implements policy should follow this policy when applicable
to the specific project. Whenever an applicant\'s proposed method of
compliance is outside this established policy, it must be coordinated
with the policy issuing office, for example, through the issue paper
process or equivalent. Similarly, if the implementing office becomes
aware of reasons that an applicant\'s proposal that meets this policy
should not be approved, the office must coordinate its response with the
Small Airplane Directorate.

Applicants should expect that the certificating officials would consider
this information when making findings of compliance relevant to new
certificate actions. Also, as with all advisory material, this policy
statement identifies one means, but not the only means, of compliance.

**Conclusion**

Because of likenesses in software/hardware: guidance (i.e., DO-178B and
DO-254), their failure condition classification definitions, development
processes, and common safety goals; the rationale used by AC 23.1309-1C
for reducing SDALs should also be applicable to CEH HDALs. Therefore,
this policy statement allows decreases in the HDALs for CEH depending
upon aircraft class and operating category. Also, to clarify AC 20-152's
application to part 23 aircraft, only CEH initially classified as HDAL
"D" per DO-254 prior to any HDAL reduction allowed by this policy are
excluded from FAA review of life cycle data.

**Contact**

For questions and support on this policy statement, please contact Mr.
Robin Sova at

\(816\) 329-4133, by fax (816) 329-4090, or by e-mail at
robin.sova@faa.gov.

#### Distribution:

+-----------------------------------------------------------------------+
| Manager, Aircraft Engineering Division, AIR-100                       |
|                                                                       |
| Manager, Brussels Aircraft Certification Staff, AEU-100               |
|                                                                       |
| Manager, Boston Aircraft Certification Office, ANE-150                |
|                                                                       |
| Manager, New York Aircraft Certification Office, ANE-17O              |
|                                                                       |
| Manager, Airplane Certification Office, ASW-150                       |
|                                                                       |
| Manager, Special Certification Office, ASW-190                        |
|                                                                       |
| Manager, Atlanta Aircraft Certification Office, ACE-115A              |
|                                                                       |
| Manager, Chicago Aircraft Certification Office, ACE-115C              |
|                                                                       |
| Manager, Wichita Military Certification Office, ACE-115M              |
|                                                                       |
| Manager, Wichita Aircraft Certification Office, ACE-115W              |
|                                                                       |
| Manager, Anchorage Aircraft Certification Office, ACE-115N            |
|                                                                       |
| Manager, Seattle Aircraft Certification Office, ANM-100S              |
|                                                                       |
| Manager, Denver Aircraft Certification Office, ANM-100D               |
|                                                                       |
| Manager, Los Angeles Aircraft Certification Office, ANM-100L          |
|                                                                       |
| Manager, Regulations and Policy, ACE-111                              |
|                                                                       |
| Manager, Project Support, ACE-112                                     |
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