Use of Society of Automotive Engineers (SAE) Class H11 Bolts

AC 20-127 · FAA

Public domain · FAAAdvisory Circulars

Overview

The Use of Society of Automotive Engineers (SAE) Class H11 Bolts (AC 20-127) 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 20-127
Pages: 2

Key points

This Advisory Circular (AC) provides guidance on the use of Society of Automotive Engineers (SAE) Class H11 bolts in primary structure on all aircraft.
The service history of H11 bolts indicates a higher than normal failure rate due to stress corrosion cracking.
The use of H11 bolts in primary structure is discouraged and should not be considered for new type design aircraft.
H11 bolts should be replaced with different types of bolts if exposed to high temperatures, corrosive environments, or high sustained tensile stress.
Encapsulation of H11 bolts against environmental effects is advised but should be used with discretion.

Frequently asked questions

What is the purpose of this Advisory Circular?

The purpose of this Advisory Circular is to provide guidance on the use of SAE Class H11 bolts in primary structures of aircraft.

Why are H11 bolts discouraged for use in aircraft?

H11 bolts are discouraged due to their higher than normal failure rate attributed to stress corrosion cracking, which may pose safety problems.

What should be done if H11 bolts are exposed to high temperatures?

H11 bolts should be replaced with a different type of bolt and nut of equivalent strength and fatigue rating if exposed to high temperatures above 250°F.

Can H11 bolts be used in corrosive environments?

No, H11 bolts should not be used in corrosive environments such as moisture or salt air, as they are more sensitive to environmental influences.

What is recommended if encapsulation of H11 bolts is not practical?

If encapsulation is not practical, a substitute material for H11 bolts should be used.

Document

Advisory

US. Deportment of Transportation

Circular

Federal Aviation AdmlnlstraNon Date: 7 /8/87 Subject: USE OF SOCIETY OF AUTOMOTIVE AC No: 20- 127 Initiated by: ANM-110 ENGINEERS (SAE) CLASS Hll BOLTS Chance: 1 .. PURPOSE. This Advisory Circular (AC) provides guidance on the use of Society of Automotive Engineers {SAE) Class Hll bolts in primary structure on all aircraft, including gliders and manned free balloons, and on aircraft engines and propellers.

2. RELATED FAR SECTIONS. This guidance is provided for showing compliance with the requirements of Sections 21.21, 23.603, 25.603, 27.603, 29.603, 33.15, and 35.17 of the Federal Aviation Regulations (FAR).

3. BACKGROUND. The service history of Hll bolts used in primary structure indicates a higher than nonnal failure rate. These failures are attributed to stress corrosion cracking and may become a safety problem. Hll bolts and companion nuts are more sensitive to environmental influence than bolts made from other materials. The use of Hll bolts in primary structure· is therefore discouraged and should not be considered for use on new type design aircraft.

There are other fasteners available which have demonstrated satisfactory service experience. Inconel 718 and stainless steel bolts are examples of acceptable substitutes.

4. GENERAL CONSIDERATIONS.

a. Hll bolts and nuts should be replaced with a different type of bolt and nut of equivalent strength and fatigue rating. if any of the following conditions exist: (1) The bolts are exposed to high temperature {above 250° F) and possible simultaneous exposure to hydraulic fluids.

(2) The bolts are exposed to a corrosive environment such as moisture, salt air, exhaust gases, etc., at ambient temperature.

(3) The bolts are used primarily in tension application with a high sustained tensile stress, in any environment continually exposed to weather.

(4) The bolts are used in any application. including both tension or shear, in which a single failure could be catastrophic.

(5) The bolts are used in any application which requires frequent inspections of the bolts. · (6) The bolt service history indicates a need for frequent replacement.

7/8/87 AC 20-127 b. Hll bo1ts, when used, should be completely encapsulated against environmental effects. Since encapsulation is not a complete or certain solution to avoid stress corrosion cracking in Hll bolts, encapsulation should be used only with discretion based on factors such as the specific environment, the maintenance requirements in the immediate area, and the inspection requirements for the Dolts and adjacent structural members. Where complete encapsulation is not practical, a substitute material for Hll bolts should be used.

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