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Replacing Rate-of-Turn With Backup ADI

September 24, 2003

Replacing Rate-of-Turn Indicator With Second Attitude Indicator (source: FAA)

Want a second attitude indicator but don't have the panel space?  FAA Advisory Circular 91-75, dated 6/25/03, provides this new guidance:

If your airplane is equipped with:

1)  Vacuum-powered primary attitude indicator(s) and electrically powered rate-of-turn indicator(s) (turn coordinator or turn-and-bank), then you may replace the rate-of-turn indicator with

     a. an electrically powered attitude indicator OR

     b. a vacuum- or pressure-powered attitude indicator that is connected by independent lines to a second, independent vacuum or pressure pump.

2)  Pressure-powered primary attitude indicator(s) and electrically powered rate-of-turn indicators, then you may replace the rate-of-turn indicator with

     a.  an electrically powered attitude indicator OR

     b.  a vacuum- or pressure-powered attitude indicator that is connected by independent lines to a second, independent vacuum or pressure pump.

3)  All-electric gyro systems that have two, independent electrical power sources in accordance with AC 23-17A, section 23.1331, then you may replace the rate-of-turn indicator with an electrically powered attitude indicator.

In all cases a slip-skid indicator required by 91.205d(4) must be permanently mounted on the panel or in a flight instrument in the pilot's primary field of view.

From ABS:  Experts differ on the wisdom of replacing the turn coordinator or turn-and-bank with a second attitude indicator.  If you decide to make the switch, it may not be a simple remove-and-replace job...depending on the power source of your instruments, your panel may require a significant change in pneumatic air plumbing or electrical systems to meet the redundancy rules.  See www.faa.gov and AC 91-75 for more information.

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