AC 21-99 Aircraft Wiring and Bonding
Sect 2 Chap 13
SECTION 2
CHAPTER 13
BONDING AND GROUNDING
INTRODUCTION
1.
Bonding and grounding connections are made for
the following purposes:
a.
Splice, Conductor, Disconnect,
Crimp Style, Copper, Insulated
Barrel, Type II, Class 1
SAE AMS-M-3171
Magnesium Alloy; Processes
for Pre-treatment and
Prevention of Corrosion
SAE AS 50881
Wiring, Aerospace Vehicle
SAE AS 7928
Terminals, Lug and Splices,
Conductor Crimp Style,
Copper, General Specification
To protect aircraft and personnel against hazards
from lightning discharge.
b. To provide current return paths.
c.
MS27429
To prevent development of RF potentials.
d. To protect personnel from shock hazard.
e.
To provide stability and homogeneity of radio
transmission and reception.
f.
To prevent accumulation of static charge.
g. To provide fault current return paths.
2.
This chapter describes and illustrates the
recommended procedures to be followed in the
preparation and installation of bonding and grounding
connections.
DEFINITIONS
Bonding
4.
The electrical connecting of two or more
conducting objects not otherwise adequately connected.
Grounding
REFERENCE SPECIFICATIONS
5.
The electrical connecting of conducting object to
primary structure or earth electrode, for return of current.
3.
The following specifications are applicable to
bonding and grounding connections:
Primary Structure
A-A-857
Thinner, Dope, & Lacquer
(Cellulose-nitrate)
6.
The main frame, fuselage, and wing structure of
the aircraft (commonly referred to as ground).
MIL-C-83413/8
Connectors and Assemblies,
Electrical, Aircraft Grounding:
Type IV Jumper Cable
Assembly, Lead Electrical
MIL-DTL-22520
Crimping Tools, Wire
Termination, General
Specification For
MIL-PRF-23377
Primer Coatings, Epoxy, High
Solids
MIL-PRF-85582
Primer Coatings, Epoxy,
Waterborne
MIL-STD-464
Electromagnetic
Environmental Effects
Requirements for Systems
MIL-T-81714
GENERAL PRECAUTIONS AND PROCEDURES
7.
When making bonding or grounding connections
in aircraft, observe the following general precautions and
procedures:
a.
Bond or ground parts to the primary aircraft
structure where practicable.
b. Make bonding or grounding connections in such a
way as not to weaken any part of the aircraft
structure.
c.
Bond parts individually wherever possible.
d. Make bonding or grounding connections against
smooth, clean surfaces.
Terminal Junction System,
Environment Resistant,
General Specification For
1
AC 21-99 Aircraft Wiring and Bonding
Sect 2 Chap 13
e.
f.
Install bonding or grounding connections so that
vibration, expansion or contraction, or relative
movement incident to normal service use will not
break or loosen the connection.
Locate bonding and grounding connections in
protected areas whenever possible. Locate
connections, whenever possible, near hand holes,
inspection doors, or other accessible areas to
permit easy inspection and replacement.
g. Do not compression-fasten bonding or grounding
connections through any non-metallic material.
h. Inspect the grounding and bonding straps to
ensure that they are free of corrosion which will
adversely affect performance, and are not frayed
or cut more than 25% of the original strap.
i.
No more than four ground wires should be
connected to a common ground stud. Ground
modules in accordance with MIL-T-81714 may be
used for multiple grounds. No more than 16
ground wires should be connected in a ground
module. Each ground for electric power sources
(primary, secondary, emergency) should be
connected to separate ground points. Grounds for
utilisation equipment may be connected to a
common ground point only when supplied from
the same power source, provided this equipment
does not perform duplicate or overlapping
functions.
SELECTION OF HARDWARE
8.
Hardware used to make bonding or grounding
connections is selected on the basis of mechanical
strength, current to be carried, and ease of installation.
Where connection is made by aluminium or copper
jumpers to structure of dissimilar material, a washer of
suitable material should be installed between the
dissimilar materials so that any corrosion which may
occur will occur in the washer, which is expendable,
rather than in the structure.
Hardware Material and Finish
9.
Select hardware material and finish from Table
13–1, Table 13–2 or Table 13–3, depending on material
of structure to which attachment is made, and material of
jumper and terminal specified for the bonding or
grounding connection.
Selection of Stud
10. Use either an AN screw or bolt of the proper size
for the specified jumper terminal. Length of screw or bolt
should be such that when bonding or grounding
connection is fully tightened, approximately 3.7mm of
screw protrudes beyond top of nut.
Selection of Nuts
11. Use AN nuts, either plain or self-locking, where
indicated in Figure 13–1 and Figure 13–3. Use an allmetal, self-locking nut if practicable. Always use an allmetal, self-locking nut where current will be present.
Where installation conditions require, use an AN nutplate, riveted to structure.
Selection of Washers
12. Use AN plain washers and split lock washers as
indicated in Figure 13–1, Figure 13–2 and Figure 13–3.
Unless otherwise directed by applicable equipment
technical order, use split lock washers with nuts, either
plain or self-locking. With aluminium terminals, use a
plain washer of at least the diameter of the terminal
tongue, next to the aluminium terminal. If an AN washer
does not meet this requirement, use a washer of the
SAE heavy series, or a special washer made for this
application.
Selection of Cable Clamp
13. For bonding or grounding to cylindrical surfaces,
use an AN735 clamp. Where an AN735 clamp is not
available, or where installation conditions do not allow its
use, a non-cushioned AN742 clamp may be substituted.
NOTE
When repairing or replacing existing bonding or
grounding connections, use the same type of
hardware as used in the original connection.
2
CAUTION
Do not use cushioned clamps in any bonding or
grounding connection.
AC 21-99 Aircraft Wiring and Bonding
Sect 2 Chap 13
Figure 13–1 Stud Bonding or Grounding to Flat Surface
Table 13–1 Hardware for Stud Bonding or Grounding to Flat Surface
(Refer to Figure 13–1)
Aluminium Terminal and Jumper
Structure
Screw or Bolt;
Locknut
Plain Nut
Washer A
Washer B
Washer
C&D
Lockwasher E
Lockwasher F
Aluminium
Alloys
Cadmium
Plated Steel
Tin Plated
Brass
Aluminium
Alloy
Aluminium
Alloy
Cadmium
Plated Steel
or Aluminium
Cadmium
Plated Steel
Cadmium
Plated Steel
Magnesium
Alloys
Cadmium
Plated Steel
Cadmium
Plated
Steel
Magnesium
Alloy
Magnesium
Alloy
Cadmium
Plated Steel
or Aluminium
Cadmium
Plated Steel
Cadmium
Plated Steel
Steel,
Cadmium
Plated
Cadmium
Plated Steel
Cadmium
Plated
Steel
None
None
Cadmium
Plated Steel
or Aluminium
Cadmium
Plated Steel
Cadmium
Plated Steel
Steel,
Corrosion
Resisting
Corrosion
Resisting Steel
Cadmium
Plated
Steel
None
None
Cadmium
Plated Steel
or Aluminium
Corrosion
Resisting Steel
Cadmium
Plated Steel
Lockwasher E
Lockwasher F
Cadmium
Plated Steel
Cadmium
Plated Steel or
Aluminium
Tinned Copper Terminal and Jumper
Structure
Aluminium
Alloys
Screw or Bolt;
Locknut
Cadmium
Plated Steel
Plain Nut
Cadmium
Plated
Steel
Magnesium
Alloys
Washer A
Aluminium
Alloy
Washer B
Aluminium
Alloy
Washer
C&D
Cadmium
Plated Steel
AVOID CONNECTING COPPER TO MAGNESIUM
Steel,
Cadmium
Plated
Cadmium
Plated Steel
Cadmium
Plated
Steel
None
None
Cadmium
Plated Steel
Cadmium
Plated Steel
Cadmium
Plated Steel
Steel,
Corrosion
Resisting
Corrosion
Resisting Steel
Corrosion
Resisting
Steel
None
None
Cadmium
Plated Steel
Corrosion
Resisting Steel
Corrosion
Resisting Steel
3
AC 21-99 Aircraft Wiring and Bonding
Sect 2 Chap 13
Figure 13–2 Plate Nut Bonding or Grounding to Flat Surface
Table 13–2 Hardware for Plate Nut Bonding or Grounding to Flat Surface
(Refer to Figure 13–2)
Aluminium Terminal and Jumper
Structure
Screw or Bolt;
Nut Plate
Rivet
Lockwasher
Washer A
Washer B
Aluminium Alloys
Cadmium Plated
Steel
Aluminium Alloy
Cadmium Plated
Steel
Cadmium Plated
Steel or Aluminium
None
Magnesium Alloys
Cadmium Plated
Steel
Aluminium Alloy
Cadmium Plated
Steel
Cadmium Plated
Steel or Aluminium
None or
Magnesium Alloy
Steel, Cadmium
Plated
Cadmium Plated
Steel
Corrosion
Resisting Steel
Cadmium Plated
Steel
Cadmium Plated
Steel or Aluminium
None
Steel, Corrosion
Resisting
Corrosion
Resisting Steel or
Cadmium Plated
Steel
Corrosion
Resisting Steel
Cadmium Plated
Steel
Cadmium Plated
Steel or Aluminium
Cadmium Plated
Steel
Tinned Copper Terminal and Jumper
Structure
Aluminium Alloys
Screw or Bolt;
Nut Plate
Cadmium Plated
Steel
Magnesium Alloys
Rivet
Aluminium Alloy
Lockwasher
Cadmium Plated
Steel
Washer A
Cadmium Plated
Steel
Washer B
Aluminium Alloy
AVOID CONNECTING COPPER TO MAGNESIUM
Steel, Cadmium
Plated
Cadmium Plated
Steel
Corrosion
Resisting Steel
Cadmium Plated
Steel
Cadmium Plated
Steel
None
Steel, Corrosion
Resisting
Corrosion
Resisting Steel
Corrosion
Resisting Steel
Cadmium Plated
Steel
Cadmium Plated
Steel
None
4
AC 21-99 Aircraft Wiring and Bonding
Sect 2 Chap 13
Figure 13–3 Bolt and Nut Bonding or Grounding to Flat Surface
Table 13–3 Hardware for Bolt and Nut Bonding or Grounding to Flat Surface
(Refer to Figure 13–3)
Aluminium Terminal and Jumper
Structure
Screw or Bolt;
Locknut
Lockwasher
Washer A
Washer B
Washer C
Aluminium Alloys
Cadmium Plated
Steel
Cadmium Plated
Steel
Cadmium Plated
Steel or Aluminium
None
Cadmium Plated
Steel or Aluminium
Magnesium Alloys
Cadmium Plated
Steel
Cadmium Plated
Steel
Magnesium Alloy
None or
Magnesium Alloy
Cadmium Plated
Steel or Aluminium
Steel, Cadmium
Plated
Cadmium Plated
Steel
Cadmium Plated
Steel
Cadmium Plated
Steel
Cadmium Plated
Steel
Cadmium Plated
Steel or Aluminium
Steel, Corrosion
Resisting
Corrosion
Resisting Steel or
Cadmium Plated
Steel
Cadmium Plated
Steel
Corrosion
Resisting Steel
Cadmium Plated
Steel
Cadmium Plated
Steel or Aluminium
Tinned Copper Terminal and Jumper
Structure
Aluminium Alloys
Screw or Bolt;
Locknut
Lockwasher
Cadmium Plated
Steel
Cadmium Plated
Steel
Magnesium Alloys
Washer A
Cadmium Plated
Steel
Washer B
Aluminium Alloy
Washer C
Cadmium Plated
Steel
AVOID CONNECTING COPPER TO MAGNESIUM
Steel, Cadmium
Plated
Cadmium Plated
Steel
Cadmium Plated
Steel
Cadmium Plated
Steel
None
Cadmium Plated
Steel
Steel, Corrosion
Resisting
Corrosion
Resisting Steel or
Cadmium Plated
Steel
Cadmium Plated
Steel
Corrosion
Resisting Steel
None
Cadmium Plated
Steel
5
AC 21-99 Aircraft Wiring and Bonding
Sect 2 Chap 13
PREPARATION OF BONDING OR GROUNDING
SURFACES
14. Clean bonding and grounding surfaces thoroughly
before making the connection. Remove paint, anodic or
conversion coating film and surface corrosion from
planned attachment area with abrasive mat, A-A-58054.
CAUTION
Do not use abrasives such as emery cloth, crocus
cloth, steel wool, etc. These may leave particles
imbedded in the surface or scattered in the area
which may cause corrosive action.
Cleaning Procedure for Steel Surfaces
17. When the surface is corrosion resisting or plated
steel, clean bonding or grounding surface as described
in paragraph 16, steps a and b.
CAUTION
Do not remove zinc or cadmium plate from steel
surfaces.
Cleaning Procedure for Aluminium Surfaces
15. Apply a coating of petrolatum compound to
bonding or grounding surface of aluminium structure and
clean surface thoroughly, using stainless steel wire
brush with pilot as shown in Figure 13–4. Wipe off the
petrolatum compound with a clean dry cloth.
Cleaning Procedure for Magnesium Alloy Surfaces
16. Prepare magnesium alloy surfaces for bonding or
grounding as follows:
WARNING
Cleaning solvents are toxic to skin, eyes and
respiratory tract. Skin and eye protection is required.
Avoid repeated or prolonged contact. ensure
adequate general ventilation and avoid breathing
fumes generated by solvents.
a.
Remove grease and oil from surface with P-D680, Type III.
Figure 13–4 Stainless Steel Wire Brush With Pilot for
Cleaning Aluminium Surfaces
WARNING
A-A-857 is flammable and toxic to eyes, skin, and
respiratory tract. Skin and eye protection is required.
Avoid prolonged contact. Use only with adequate
ventilation.
b. If present, remove paint or lacquer from surface
with lacquer thinner, A-A-857.
c.
Brush area liberally with chrome pickle solution,
SAE AMS-M-3171, Type I for one minute, then
rinse within five seconds by brushing with clean
water.
d. Dry thoroughly.
6
METHODS OF BONDING OR GROUNDING
18. Bonding or grounding connections are made
directly to a flat surface of basic structure, or to a
cylindrical surface of basic structure.
Connection to Flat Surfaces
19. Bonding and grounding of through bolts or
screws, where installation has easy access. There are
three types of bolted connection, as follows:
a.
Stud Connection. In this type of connection, a
bolt or screw is locked securely to structure, thus
becoming, in effect, a stud. (See Figure 13–1 and
Table 13–1.) Grounding or bonding jumpers can
be removed or added to the shank of stud without
removing stud from structure. Not more than four
lugs should be connected to any one stud.
AC 21-99 Aircraft Wiring and Bonding
Sect 2 Chap 13
b. Nut Plate and Bolt Connection. Nut plates are
used where access to the nut for repairs may be
difficult. Nut plates are riveted or welded to a
clean area of the structure. (See Figure 13–2 and
Table 13–2.) Cleaning of structure is done in
accordance with paragraphs 15 through 17 as
applicable.
c.
Nut and Bolt Connection. In this connection the
bolt or screw is not attached permanently to
structure. (See Figure 13–3 and Table 13–3.)
When jumpers are to be added or removed, the
entire connection is remade. The table lists
materials and platings that are compatible with the
structure to which they are mounted. These
materials are selected so that corrosion, if it
occurs, will occur in the washers, which are
expendable, rather than in the structure.
Connection to Tab Riveted to Structure
Bonding Conduit to Structure
22. Bond aluminium alloy or corrosion-resisting steel
conduit to structure as shown in Figure 13–8. If
installation conditions require, an AN742 clamp may be
used instead of an AN735 clamp, using same hardware.
Tightness of Connections
23. Make sure that all connections are tight, as
evidenced by the split lockwashers being completely
compressed.
CAUTION
When terminal is under head of screw or bolt (as
shown in Figure 13–2), it is preferable not to install
more than one terminal. Otherwise, the screw may
loosen and cause improper operation of equipment.
20. For bonding leads carrying high current, (size
AWG 4 or larger), do not make the connection directly to
the structure, but to a tab of suitable size adequately
bonded to the aircraft structure. (See Figure 13–5.)
When a bonding or grounding connection is made to a
tab riveted to the structure rather than directly to the
structure, clean the bonding or grounding surface and
make the connection exactly as though the connection
were being made to structure. If it is necessary to
remove the tab for any reason, replace rivets with one
size larger. Make sure mating surfaces of structure and
tab are clean and free of anodic film.
Connection to Cylindrical Surfaces
21. Make bonding or grounding connections to
aluminium alloy, magnesium alloy, or corrosion resisting
steel tubular structure as shown in Figure 13–6 and
Figure 13–7. Figure 13–6 shows the arrangement of
hardware for bonding with an aluminium jumper.
Because of the ease with which aluminium is deformed,
it is necessary to distribute screw and nut pressure by
means of plain washers as shown. Figure 13–7 shows
the arrangement of hardware for bonding with a copper
jumper. No extra washers are used. If installation
conditions require, use an AN742 clamp (noncushioned) instead of an AN735 clamp. Do not change
any other hardware if this substitution is made.
Figure 13–5 Bonding Tab Riveted to Structure
7
AC 21-99 Aircraft Wiring and Bonding
Sect 2 Chap 13
Figure 13–6 Aluminium Jumper Connection to Tubular Structure
Figure 13–7 Copper Jumper Connection to Tubular Structure
Figure 13–8 Bonding Conduit to Structure
BONDING AND GROUNDING JUMPERS
Fabricating Bonding and Grounding Jumpers
24. To accomplish the purpose of bonding or
grounding, it is necessary to provide a conductive path
where direct electrical contact does not exist. Jumpers
are used for this purpose in such applications as
between moving parts, between shock-mounted
equipment and structure, and between electrically
conducting objects and structure. Keep jumpers as short
as possible; if practical, under 76mm. Do not use two or
more jumpers in series.
25. Jumpers of tinned copper wire are fabricated in
accordance with MIL-C-83413/8. For smaller size wire,
terminate with MS25036 insulated copper terminal lugs
of appropriate size. Use M22520/5-01 tool with
M22520/5-100 for crimping terminals to wire. For larger
wire size, terminate with MS20659 uninsulated copper
terminal lugs, crimped to the wire with M22520/24-10 or
MS25441 tool. The appropriate size copper wire braid
should be selected from Table 13–4.
8
AC 21-99 Aircraft Wiring and Bonding
Sect 2 Chap 13
Table 13–4 Tinned Copper Woven Braid for Fabrication of Electrical Grounding and Bonding Leads
Construction
Strand Diameter
Width
Part Number
Lays
Strands
Mil
SWG
mm
Ins
Area
Cir Mils
Current
Rating
Terminal
Lug Size
G154-8-3-.122
8
3
0.122
40
1.19
.047
380
4
26-24
G154-16-3-.122
16
3
0.122
40
1.58
.063
1000
7
22-18
G154-24-3-.1226
24
3
0.122
40
2.78
.110
1900
10
22-18
G154-24-4-.122
24
4
0.122
40
3.18
.125
2550
13
16-14
G154-24-6-.1228
24
6
0.122
40
4.76
.187
3800
15
16-14
G154-24-8-.122
24
8
0.122
40
6.35
.250
5100
20
16-14
G154-24-14-.122
24
14
0.122
40
9.52
.375
7650
30
12-10
G154-32-16-.122
32
16
0.122
40
12.70
.50
10200
40
12-10
G154-32-20-.122
32
20
0.122
40
15.88
.625
12700
50
12-10
G154-48-16-.122
48
16
0.122
40
19.05
.75
14300
60
8
G154-48-32-.122
48
32
0.122
40
25.40
1.0
21700
80
8
Note
For RF bonding, a rule of thumb for achieving
minimum bond strap inductance is that the length-towidth ratio of the strap should be 5:1 or less.
bonding of RF components is required, the resistance
should be a maximum 0.0025 ohms (2.5 milliohms)
(Reference – MIL-STD-464). Test is made after the
mechanical connection is completed, and consists of a
milli-ohmmeter reading of the overall resistance between
the cleaned areas of the object and the structure.
Quick-Disconnect Jumpers
Resistance Test Procedure
26. Where a quick disconnect jumper is required,
crimp an MS27429 electrical disconnect splice into a
copper wire jumper, fabricated as described in
paragraph
25,
using
M22520/5-01
tool
with
M22520/5-100 die. Note that the disconnect splice is not
centred in the jumper, but is installed so that the coupler
remains on the short end when the jumper is
disconnected.
28. Measurements of the specified resistance value
are made with special calibrated low-range ohmmeters.
The Avtron T477W or functional equivalent meeting UL913 explosion-proof test criteria should be used in
enclosed areas where hazards exist, such as explosive
vapours from fuel systems. The AN/USM-21A can be
used in non-hazardous areas. Proceed as follows,
observing the precautions emphasised in the meter
instruction manual.
TESTING BONDS AND GROUNDS
Resistance Tests After Connection
NOTE
The resistance figures provided below are for general
electrical and RF bonding. Specific requirements
detailed in aircraft or component publications should
take precedence.
27. The resistance across a bonding or grounding
jumper is required to be 0.1 ohms or less for general
electrical bonding whether using bonding jumpers or
where metallic components are directly attached. Where
a.
With the Function Control OFF, set the Range
Control to the 0.1 ohm position.
b. Attach the instrument test clips for good electrical
contact with the cleaned areas immediately
adjacent to the jumper terminal lugs of object and
structure.
c.
Set the Function Control to CALIBRATE, then use
the Calibration Adjustment Control to obtain a 0.1
ohm full scale deflection.
d. Set the Function Control to OHMS position, and
note the bond (only) reading. It should be less
than 0.1 ohm.
9
AC 21-99 Aircraft Wiring and Bonding
Sect 2 Chap 13
REFINISHING
Refinishing Metal Surfaces
WARNING
Appropriate personal protective equipment should be
worn when handling and using cleaning and corrosion
control solutions.
29. Following the connection and successful testing of
bonding or grounding leads, the bare metal surface must
be protected from corrosion.
10