Placards for Battery-Excited Alternators Installed in Light Aircraft

AC 20-40 · FAA

Public domain · FAAAdvisory Circulars

Overview

The Placards for Battery-Excited Alternators Installed in Light Aircraft (AC 20-40) is a public-domain FAA advisory circular, republished here as a free chaptered HTML edition with a linked table of contents and the official PDF.

Publisher: FAA
Document: AC 20-40
Pages: 1

Key points

This advisory circular provides guidance on placarding for battery-excited alternators in light aircraft.
It addresses compliance with Federal Aviation Regulations 23.1541(a)(2) and 27.1541(a)(2).
Battery-excited alternators can lose functionality if turned off in flight due to battery discharge.
A recommended placard should advise pilots not to turn off alternators in flight except in emergencies.
The document was published by the FAA and is effective as of August 11, 1965.

Frequently asked questions

What is the purpose of this advisory circular?

The purpose is to set forth an acceptable means of complying with placarding rules for battery-excited alternators in light aircraft.

What regulations are affected by this advisory circular?

The affected regulations are Federal Aviation Regulations 23.1541(a)(2) and 27.1541(a)(2).

What is the risk associated with battery-excited alternators?

If the alternator is turned off in flight, a subsequent loss or discharge of the battery can result in the loss of the alternator.

What should the placard indicate regarding the alternator?

The placard should advise the pilot not to turn off the alternators in flight, except in case of emergency.

When was this advisory circular published?

This advisory circular was published by the FAA and is effective as of August 11, 1965.

Document

AC NO: AC 20-40 .AIRCRAFT EFFECTIVE : 8/11/65 SU BJ E CT : PLACARDS FOR BA'l'TERY- EX.CITED ALTERNATORS INSTALLED IN LIGHT AIRCRAFT 1. PURPOSE: This advisory circular sets forth an acceptable means of complying with placarding rules in Federal Aviation Regulations 23 and 27 with respect to battery-excited alternator installations.

2. REGULATIONS AFFECTED: Federal Aviation Regulations 23.l54l(a)(2) and 27.l54l(a)(2).

3. BACKGROUND: Since an alternator is but one class of generator, approval of alternator installations in light aircraft have been made by the Agency under those rules in FAR 23 and 27 which deal with generator systems. It has been found, however, that certain types of alternators (which are apparently used in a number of automotive applications) obtain their field excitation from the battery. When field excitation is obtained in this wa:y, there is the danger that, if the alternator is turned off in flight, subsequent loss or discharge of the battery would result in loss of the alternator as well. This unusual design feature invokes the provisions in §s23.154l(a)(2) and 27.l54l(a)(2).

4. ACCEPTABLE MEANS OF COMPLIANCE: With respect to battery-excited alternator installations, an acceptable method of complying with §§23.154l(a)(2) and 27.1541(a)(2) is to provide a placard advising the pilot not to turn off the alternators in flight, except in case of emergency.