Subject: ATTITUDE INDICATOR Date: 6/25/03 

Initiated By: AFS-400

AC No.: 91-75

Change: 

1. PURPOSE. The purpose of this advisory circular (AC) is to provide a method to substitute 

an approved attitude indicator for the rate-of-turn indicator mandated by Title 14 of the Code 

of Federal Regulations (14 CFR) part 91, section 91.205(d)(3). This AC excludes airplanes 

covered by section 91.205(d)(3)(i) and (ii). This AC is applicable to part 23-certificated 

airplanes (or airplanes certificated under earlier equivalent regulations) that weigh less than 

12,500 pounds and are operated under part 91. Like all advisory material, this AC is not in itself 

mandatory and does not constitute a regulation. One may follow an alternate FAA-approved 

method. Mandatory words such as “shall” or “must” apply only to those who seek to 

demonstrate compliance to a specific rule by use of a method prescribed in this AC without 

deviation. 

2. RELATED READING MATERIALS. 

a. Sections 23.1303(f), 23.1331(b), and 91.205(a) and (d). 

b. AC 23-17A, Systems and Equipment Guide for Certification of Part 23 Airplanes, 

section 23.1331. 

c. TSO-C4c, Bank and Pitch Instruments. This document may be ordered from: The U.S. 

Department of Transportation, Subsequent Distribution Center, Ardmore East Business Center, 

3341 Q 75th Avenue, Landover, MD 20785. 

d. SAE AS396B, Bank and Pitch Instruments (Indicating Stabilized Type) (Gyroscopic 

Horizon, Attitude Gyro). This document may be ordered from: SAE, 400 Commonwealth 

Drive, Warrendale, PA 15096-0001. 

e. Safer Skies, Weather Joint Safety Implementation Team Final Report, Intervention 2-2b, 

March 2000.

3. BACKGROUND. 
AC 91-75 6/25/03

a. Section 91.205(a) requires “instruments and equipment ... or FAA-approved 

equivalents... .” A Safer Skies initiative recommendation authored by the Federal Aviation 

Administration (FAA) and industry highlighted vacuum system failures as a significant cause or 

contributor to fatal accidents in instrument meteorological conditions (IMC). Additionally, the 

FAA and the Aircraft Owners and Pilots Association (AOPA) have conducted research to further 

analyze the potential hazards of partial panel operations in IMC. The FAA’s Civil Aeromedical 

Institute conducted simulations studying pilot responses to vacuum system failures. These 

studies included pilots with varying experience levels in both high-performance and lowperformance aircraft. AOPA’s Air Safety Foundation conducted parallel flight studies in a Piper 

Archer and a Beech Bonanza. 

b. Those studies showed that a vacuum system or gyro failure is insidious because the gyro 

fails slowly, making the failure difficult to recognize. Data indicates that pilots may not have the 

proficiency to safely recover and land the airplane, even though instrument-rated pilots receive 

partial panel training. This is particularly true in high-performance airplanes. 

4. DISCUSSION. 

a. The attitude indicator is centrally located in the pilot’s primary field of view. It provides 

easily interpreted pitch and bank information in one instrument. For this reason, most pilots tend 

to rely heavily on the attitude indicator in IMC to maintain aircraft control. Most small general 

aviation airplanes typically have an attitude and heading indicator powered by a single vacuum

source. These airplanes normally do not have redundant vacuum systems or a second attitude 

indicator. Therefore, to recognize that a failure condition exists and isolate which instrument has 

failed, the pilot must cross-check other instruments that indicate pitch or bank information. 

NOTE: Standby vacuum pumps and attitude indicators with failure 

warning flags are available in newer model aircraft and are slowly making 

their way into the general aviation fleet as retrofits. Many of the newly 

manufactured aircraft intended for use in IMC are being delivered with 

these systems. Additionally, some new aircraft only have electric gyro 

instruments and are equipped with redundant, independent electrical power 

sources. The systems described above offer mitigation to the risk of pilots 

being misled by a failed attitude indicator. 

b. The rate-of-turn indicator (either a turn coordinator or turn and bank indicator) is used to 

cross-check the attitude indicator and directional gyro for bank information. Turn coordinators 

also include a failure warning flag that provides an indication if the gyro power source is lost. 

The rate-of-turn indicator typically has an independent power source to provide redundancy for 

the attitude indicator and directional gyro. However, the rate-of-turn indicator only provides 

information that the aircraft is in a turn and whether the turn is at the standard rate (a standard 

rate turn is 3 degrees per second, or 2 minutes for a full 360-degree circle). It does not provide 

information on the bank angle. 

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6/25/03 AC 91-75 

c. A disagreement between the vacuum-powered gyro instruments and the rate-of-turn 

indicator is typically the first indication of a failure condition. To confirm which gyro has failed, 

the pilot must cross-check other flight instruments. 

d. Loss of aircraft control can occur: 

(1) When the pilot does not recognize that a failure condition exists and continues 

following erroneous instrument indications. 

(2) During the time it takes the pilot to determine which system has failed. 

(3) During subsequent partial panel instrument flying, which can be confusing if the 

pilot cannot cover the failed instrument(s). 

e. Loss of control is most likely to happen during a high workload environment such as 

instrument departure or instrument approach. The risk increases in aircraft with higher 

complexity and speed. 

5. RATIONALE FOR CHANGE. 

a. Substituting a second attitude indicator (with a power source independent from the 

primary attitude indicator) for the rate-of-turn indicator will provide an increased level of safety. 

It will replace a gyro that only indicates direction and rate of turn with one instrument that 

presents turn direction, bank angle, and pitch attitude information. Also, a second attitude 

indicator will be less confusing during partial panel operations because it presents pitch and bank 

information in the same manner as the primary attitude indicator. The pilot’s scan and 

instrument interpretation during partial panel operations becomes easier because pilots will still 

be able to rely on an attitude indicator for pitch and bank reference just as they did during full 

panel operations. Recognition time that a failure condition exists will be equivalent to current 

system configurations. 

b. Replacing the rate-of-turn indicator will mean losing an easy reference for standard rate 

turns. However, in today’s air traffic control system, there is little need for precisely measured 

standard rate turns or timed turns based on standard rate. Maintaining a given bank angle on the 

attitude indicator for a given speed will result in a standard rate turn. Pilots using this AC to 

substitute an attitude indicator for their rate-of-turn indicator are encouraged to know the bank 

angle needed for a standard rate turn. 

NOTE: The FAA preamble language for the 1970 amendment to 

section 91.33, re-codified to section 91.205, states: “[T]he FAA believes, and 

all other commenters apparently agree ... the rate-of-turn indicator is no 

longer as useful as an instrument which gives both horizontal and vertical 

attitude information.” 

6. COMPLIANCE REQUIREMENTS. 

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AC 91-75 6/25/03

a. This AC is only applicable to part 23-certificated airplanes of less than 12,500 pounds 

operated under part 91, subparts A through D that do not require more than one crewmember. 

b. This AC provides an Administrator-approved method of compliance with the requirement 

to have a gyroscopic rate-of-turn indicator specified by section 91.205(d)(3), excluding aircraft 

covered by section 91.205(d)(3)(i) and (ii). 

c. Part 91 airplanes with an FAA-approved minimum equipment list (MEL) must have the 

MEL modified to reflect changes accomplished under this AC. 

d. An FAA-approved attitude indicator with a failure warning flag that monitors power 

supply is acceptable under this AC. This attitude indicator must have an independent power 

source from the primary attitude indicator. This provides an equivalent level of safety to existing 

rate-of-turn gyro arrangements. 

If your airplane is equipped with... Then the rate-of-turn indicator may be 

replaced with... 

vacuum-powered primary attitude indicators 

and electrically-powered rate-of-turn indicators

• an electrically-powered attitude indicator, 

or 

• a vacuum- or pressure-powered attitude 

indicator that is connected by independent 

lines to a second, independent vacuum or 

pressure pump 

pressure-powered primary attitude indicators 

and electrically-powered rate-of-turn indicators

• an electrically-powered attitude indicator, 

or 

• a vacuum- or pressure-powered attitude 

indicator that is connected by independent 

lines to a second, independent vacuum or 

pressure pump 

all-electric gyro systems that have two 

independent electrical power sources in 

accordance with AC 23-17A, section 23.1331 

an electrically-powered attitude indicator 

electrically-powered primary attitude 

indicators and vacuum- or pressure-powered 

rate-of-turn indicators 

a vacuum- or pressure-powered attitude 

indicator 

electrically-powered primary attitude 

indicators and vacuum- or pressure-powered 

rate-of-turn indicators and has two independent 

electrical power sources in accordance with 

AC 23-17A, section 23.1331 

an electrically-powered attitude indicator 

e. An attitude indicator installed in accordance with this AC must be located within the 

pilot’s primary field of view. 

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6/25/03 AC 91-75 

f. The slip/skid indicator required by section 91.205(d)(4) must be permanently mounted on 

the panel or on a flight instrument within the pilot’s primary field of view. 

g. Aircraft with autopilots that receive inputs from a turn coordinator require that the 

applicant do one of the following: 

(1) Obtain a new Supplemental Type Certificate for the autopilot system to receive 

inputs from an electrically powered attitude indicator; 

(2) Deactivate or remove the autopilot in a manner acceptable to the Administrator; or 

(3) Relocate the rate-of-turn instrument and maintain the autopilot connection in 

accordance with data acceptable to the Administrator (generally viewed as a MINOR alteration). 

7. RECOMMENDATIONS. To provide an increased level of safety, the FAA recommends, 

but does not require, the following: 

a. An approved second attitude indicator with a failure warning flag that monitors both gyro 

revolutions per minute (RPM) and power supply. 

b. A primary attitude indicator that has a failure warning flag. 

c. A blinking, low-vacuum indicator light located in the pilot’s primary field of view. 

d. A vacuum gauge located in the pilot’s primary field of view. 

e. A method to cover failed gyro instruments. 

f. The slip/skid indicator should be mounted: 

(1) Internally in the lower portion of the electric attitude indicator; 

(2) Externally on the lower bezel of the electric attitude indicator; or 

(3) On the panel directly underneath the electric attitude indicator. 

/s/ John M. Allen for 

James J. Ballough 

Director, Flight Standards Service 

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