# U.S. Department of Transportation![](media/image1.wmf){width="0.5506944444444445in" height="0.5708333333333333in"}

## Federal Aviation Administration

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  Subject:   **[INFORMATION]{.underline}**: Policy      Date:   May 30, 2003          
             Statement on Standardization of                                          
             Application of 14 CFR Part 23, § 23.1309                                 
             Regarding Hazardous Misleading Heading                                   
             Information for Attitude-Heading Reference                               
             Systems (AHRS); PS-ACE100-2002-003                                       

                                                                                      

  From:      Manager, Small Airplane Directorate,       Reply   Erv Dvorak            
             ACE-100                                    to      (816) 329-4123        
                                                        Attn.                         
                                                        of:                           

                                                                                      

  To:        See Distribution                                                         
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**Purpose**

The purpose of this memorandum is to clarify Federal Aviation
Administration (FAA) certification policy on the application of Advisory
Circular (AC) 23.1309-1C, Equipment, Systems, and Installations in Part
23 Airplanes, regarding hazardous misleading heading information.

The issue in question is specifically about the application of AC
23.1309-1C for an airplane with a certification basis of amendment 23-41
or later. This clarification is limited to installations approved for
operation in Instrument Meteorological Conditions (IMC) under Instrument
Flight Rules (IFR).

For operations in Visual Meteorological Conditions, a misleading heading
indication is not considered hazardous.

**Current Regulatory and Advisory Material**

-   14 CFR part 23, § 23.1309, Equipment, systems, and installations.

-   Technical Standard Order (TSO) TSO-C151b, Terrain Awareness and
    Warning System.

-   AC 23.1309-1C, Equipment, Systems, and Installations in Part 23
    Airplanes.

-   AC 23-17A, System and Equipment Guide For Certification of Part 23
    Airplanes.

-   AC 23-18, Installation of Terrain Awareness and Warning System
    (TAWS) Approved for Part 23 Airplanes.

**Summary**

A Functional Hazard Assessment (FHA) and the related safety assessments
should be made for the specific airplane type and configuration, as
there can be a large number of combinations of failures, various
mitigating factors, and other functions available to a pilot. Because of
the numerous factors that affect the criticality of the heading
indication, a safety assessment process using AC 23.1309-1C should be
used to classify the failure conditions for misleading heading
information.

A hazardously misleading heading is usually when the accuracy error is
greater than 10 degrees on the primary heading instrument and it is an
undetected error from the Attitude-Heading Reference Systems (AHRS). The
safety assessment process should consider appropriate mitigating factors
that might alleviate failure conditions effects, including:

-   availability, accuracy, and reliability of magnetic compass;

-   independent heading instrument;

-   independent navigation information;

-   moving map if approved for IFR operations;

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-   TAWS, if approved (for design and performance) in accordance with
    TSO-C151b, Class A or B, and following AC 23-18 (for airworthiness
    approval for the installation);

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-   air traffic control monitoring of the aircraft; and

-   exposure time.

**Safety Assessment Process**

A hazardously misleading heading is usually when the accuracy error is
greater than 10 degrees due to a malfunction on the primary heading
system using the AHRS. The normal accuracy should be within the
guidelines under section 23.1301 in AC 23-17A, dated June 27, 2002. For
part 23 airplanes, the failure condition for misleading hazardous
heading is usually considered major per AC 23.1309-1C.

For a major failure condition on Class I airplanes, the probability for
the system to provide hazardously misleading heading information to the
display should be less than or equal to 10^-4^ per flight hour due to
undetected or latent failures. For Class II, III, and IV airplanes, the
probability for the system to provide hazardously misleading heading
information to the display should be less than or equal to 10^-5^ per
flight hour due to undetected or latent failures. For major failure
conditions, a qualitative analysis is usually sufficient.

Note: There is a difference between hazardous as used in general policy
or regulations and hazardous failure condition as used in an FHA. When
the term \"hazard\" or \"hazardous\" is used in general policy or
regulations, it is generally defined as follows:

\"Any condition that compromises the overall safety of the airplane or
that significantly reduces the ability of the flight crew to cope with
adverse operating conditions.\"

When used in an FHA, \"hazardous failure condition\" is one of the five
different failure condition classifications in AC 23.1309-1C. Hazardous
failure conditions would reduce the capability of the airplane or the
ability of the crew to cope with adverse operating conditions to the
extent that there would be the following:

(1) A large reduction in safety margins or functional capabilities;

\(2\) Physical distress or higher workload such that the flight crew
cannot be relied upon to perform their tasks accurately or completely;
or

\(3\) Serious or fatal injury to an occupant other than the flight crew.

### Mitigating Factors

During the safety assessment process, mitigating factors should be
considered if they are independent of the AHRS. These factors should
provide additional qualitative and quantitative credit and alleviate
failure conditions. The mitigating factors may include the following:

-   availability, accuracy, and reliability of magnetic compass;

-   independent heading instrument;

-   independent navigation information;

-   moving map if approved for IFR operations;

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-   TAWS, if approved (for design and performance) in accordance with
    TSO-C151b, Class A or B, and following AC 23-18 (for airworthiness
    approval of the installation);

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-   air traffic control monitoring of the aircraft; and

-   exposure time.

Some of these factors may not be appropriate for higher performance and
complex airplanes.

Normally, the pilot should be scanning the instruments for cues to
determine misleading information. Therefore, the locations of primary
instruments and secondary instruments should be within the primary and
secondary fields-of-view. This is defined in the General Aviation
Manufacturers Association (GAMA) Publication No. 10, Recommended
Practices and Guidelines for Part 23 Cockpit/Flight Deck Design, dated
September 2000.

GAMA Publication No. 10 is available free of charge from the GAMA web
site: www.generalaviation.org.

### Magnetic Compass or Independent Heading Instrument

The usability of the magnetic compass, as a mitigating factor, is
dependent on the airplane installation on a case-by-case basis. The
following guidelines should be considered. A magnetic compass or
independent heading instrument is considered acceptable if the accuracy
is within 10 degrees and it is located in the secondary field-of-view.
However, the compass may not be usable if it is susceptible to large
errors induced by other onboard equipment. For example, an airplane
approved for known icing may have an unusable compass when a windshield
heater is affecting the compass; therefore, additional mitigating
factors may be necessary to meet the appropriate safety level.

One general concern is that Class II airplanes may have a number of
systems that can affect the magnetic compass that are not typically
found in Class I airplanes. For this reason, the magnetic compass should
be usable for many Class I airplanes, but it may not be usable for many
Class II airplanes or higher. An evaluation should be conducted to
determine if the magnetic compass is usable and accurate within 10
degrees.

**Navigation Information**

Credit should also be given for moving maps displaying position
information from independent navigation systems such as Global
Positioning Systems (GPS) that are approved for IFR operations. The
moving map should depict the airplane relative to the desired navigation
track and the navigational track's heading.

Navigation systems such as IFR-approved GPS systems can provide an
independent relative heading source that can be used to verify the
primary heading indication. Furthermore, the FAA expects that pilots
will naturally crosscheck their moving map displays because they are
easier to use for navigation than traditional deviation needles.

The moving map should be in the primary field-of-view (preferably) or
the secondary field-of-view to get credit for the safety assessment
process by AC 23.1309-1C.

**TAWS**

TAWS with a display provides the pilot with sufficient information and
alerts to detect potentially hazardous terrain situations so that the
pilot may take effective action to prevent Controlled Flight Into
Terrain (CFIT) events. Although a TAWS with a display is not approved
for navigation under normal circumstances, under emergency conditions
TAWS provides acceptable situational awareness to prevent CFIT.

TAWS may not be usable to determine misleading heading information, but
TAWS should reduce or prevent a CFIT accident, which will mitigate the
failure effect. TAWS that meet TSO‑C151b, Class A or B, and AC 23-18
should be acceptable when pilot proficiency is achieved.

**Air Traffic Control Monitoring**

For most IFR operations, the airplane will be in controlled airspace and
under radar coverage. Air Traffic Control (ATC) has been the primary
mitigating factor for heading indicator accuracy for many decades. ATC
maneuvers airplanes based on a ground track. Heading errors in
traditional mechanical directional gyros (DG) are still identified by
ATC when pilots do not maneuver in the direction ATC expects. Most of
the time ATC will identify heading errors when the pilot fails to
adequately update their DG or their Horizontal Situation Indicator has
failed.

However, there are locations at some airports where radar coverage is
not available. In these situations, there is usually some type of
navigation signal that pilots can use to get into and out of the
airport. These are the types of departures and approaches where heading
accuracy may be important. Although ATC may be a mitigating factor,
specific guidelines are not being provided at this time. Also, this may
not be a mitigating factor under Free Flight operation concept.

**Exposure Time and Risk Assessment**

Exposure time and risk factors that may be considered are as follows:

-   automatic pre-engagement self test;

-   monitoring capabilities;

-   maintenance intervals; and

-   phase of flight, and so forth.

In the Society of Automotive Engineers Standard ARP4761, Guidelines and
Methods for Conducting the Safety Assessment Process on Civil Airborne
Systems and Equipment, exposure time is defined as:

> \"The period of time between when an item was last known to be
> operating properly and when it will be known to be operating properly
> again.\"

In this policy statement, risk is also being considered.

Risk assessment is the composite of:

-   the outcome or effect of the failure condition;

-   the average probability of the failure condition per flight hour;
    and

-   

-   the exposure time.

For risk assessments, a credit should be factored into the analysis that
is proportional to the probability that a given failure condition will
occur on the aircraft. This credit is related to the system level of the
failure condition, the operational and environmental conditions, and the
phase of flight.

To determine exposure time, the actual time that the pilot would only be
using heading information to navigate should be considered. Only a
percentage of a typical IFR flight is flown navigating solely on the
heading instrument. The critical times for heading are when the pilot is
not navigating with a navigation instrument but is only using the
heading instrument.

The details on how to calculate the "Average Probability Per Flight
Hour" for a Failure Condition are given in Appendix C of AC 23.1309-1C.
A common assumption for 14 Code of Federal Regulations (CFR) part 23
airplanes is that the average flight duration is one hour.

The FAA's General Aviation and Air Taxi Activity Survey gives estimated
flight hours for IMC operations for specific airplanes. When researching
the operations of specific airplane models, the result indicates that
Class I airplanes are operated in IMC conditions about 20 percent of
their total flight hours and Class II airplanes about 30 percent. Also,
the FAA believes that only portions of most IFR flights are flown by
navigating solely on the heading instruments.

Using the percentages for "phases of flight" from a Boeing accident
study, the FAA estimates that on any given IFR flight, navigation solely
by the heading instrument would only occur about 20 percent of the time.
For this analysis, Class III and Class IV airplanes are assumed to be
operating IFR nearly 100 percent of the time.

### Acceptable Configurations for IFR Use

Some acceptable configurations for IFR approval include:

Configuration A:

(1) One dependable magnetic compass.

\(2\) A reliable magnetic gyroscopically stabilized heading system that
together meets the AC 23.1309-1C reliability levels, such as 10^-4^ per
flight hour for Class I airplane.

Configuration B:

(1) A magnetic compass that meets the airworthiness requirements but
    cannot be given credit for AC 23.1309-1C purposes for the reasons
    stated above.

\(2\) One magnetic gyroscopically stabilized heading system that meets
the AC 23.1309-1C reliability levels, such as 10^-4^ per flight hour for
Class I airplane, alone or in combination with one or more of the
mitigating factors addressed above such as moving map or TAWS with a
display.

Configuration C:

(1) A magnetic compass that meets the airworthiness requirements but
    cannot be given credit for AC 23.1309-1C purposes.

\(2\) Two magnetic gyroscopically stabilized heading systems that
together meet the AC 23.1309-1C reliability levels, such as 10^-4^ per
flight hour for a Class I airplane.

Configuration D:

(1) A magnetic compass that meets the airworthiness requirements but
    cannot be given credit for AC 23.1309-1C purposes.

\(2\) Two magnetic gyroscopically stabilized heading systems with each
being less reliable than required by AC 23.1309-1C (such as 10^-4^ or
10^-5^).

\(3\) A comparator monitor. In this case, the warning from the
comparator monitor would alleviate a hazardous misleading heading.
However, if the system still has a reliability problem, numerous
nuisance alarms would cause the pilot to ignore the warnings.

## 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.

Applicants should expect that the certificating officials will 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**

There are numerous factors that affect the severity of hazardously
misleading heading indication, so a safety assessment process using AC
23.1309-1C should be used to classify this failure condition.

The safety assessment process should consider appropriate mitigating
factors that might alleviate failure conditions, including:

-   availability and reliability of magnetic compass;

-   independent heading instrument;

-   navigation information;

-   moving map;

-   terrain awareness warning system;

-   air traffic control monitoring of the aircraft; and

-   exposure time.

For questions and assistance regarding this matter, please contact Mr.
Ervin Dvorak at (816) 329-4123, by fax (816) 329-4090, or by e-mail at
erv.dvorak@faa.gov.

s/

Michael Gallagher

Manager, Small Airplane Directorate

Aircraft Certification Service