Aircraft Structural Health Monitoring (SHM)

It is extremely important that the level of fatigue, imposed on an aircraft structure and associated components can be monitored and recorded so that the respective fatigue lives are not exceeded. Several methods have been developed to assist in the vital tasks involved with SHM

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Fatigue Meters

Fatigue meters are used to check overall stress levels on aircraft and to monitor the fatigue history of the aircraft. Fatigue meters also allow a check to be made on the moment in time when the aircraft exceeds the design limits imposed on it.

Strain Gauges

Strain gauges may be used to monitor stress levels on specific aircraft structures. Strain gauges are thin-foil electrical resistor elements that have been bonded to the aircraft structure during manufacture. Their resistance varies proportional to the applied stress loading.

Fatigue Fuses 

Fatigue fuses are metallic fuses, which are bonded to the structure and which fail at different fatigue stresses. The electrical current flowing through the fuse, will vary and so provide an indication of the stress level.

Intelligent Skins Development

Modern developments in aircraft structures will allow the structures to be designed and built with a variety of sensors and systems embedded into the structure and skin. This would mainly be restricted to structures manufactured from composite materials. One major benefit of this is to allow the structure to monitor it's own loads and fatigue life.

Quick-release Couplings

Quick-release couplings are required at various points in aircraft systems. Typical uses are in fuel, oil, hydraulic and pneumatic systems. Their purpose is to save time in the removal and replacement of components; to prevent the loss of fluid and to protect the fluid from contamination. The use of these couplings also reduces the maintenance cost for the system involved.

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A coupling consists of a male and female assembly. Each assembly has a sealing piston/poppet valve that prevents the loss of fluid when the coupling is disconnected.

When the coupling is disconnected a spring pushes the conical shaped poppet onto a seat to ensure no fluid can escape. When the coupling is connected the piston faces compress the springs and lift the poppet off its seat allowing fluid to pass through.


It is extremely important to ensure the open parts of a disconnected coupling do not become contaminated. This can be achieved by placing blanking caps on the end of the couplings as soon as they have been separated.

Three checks may be used to verify a positive connection. These involve an audible, visual and tactile indication. A click may be heard at the time the coupling is locked and indicator pins will extend from the outer sleeve upon locking, which can be seen and felt.

Chobert Rivets

Chobert rivets are made with either snap or countersunk heads.  The action of closing a Chobert rivet is illustrated.  When the mandrel is pulled through the tapered hole the rivet head is formed.  As the mandrel continues pulling through the rivet, the shank expands and fills the hole.

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Sealing pins can be fitted in the rivet bore.  This increases the shear strength of the rivets and prevents moisture entering.

The manufacturer’s rivet part number is in the form of a code giving rivet material and diameter and grip length in ¹/₃₂ in.

Chobert rivets with oversize shanks are available for repair work on aircraft.

The chobert rivet is closed with a steel mandrel which forms part of the riveting tool.  A different sized mandrel is used with each diameter of rivet.  Before use it should be checked for diameter using an approved gauge.  The mandrel must also be inspected for scratches or other damage and must be clean and lightly lubricated.  Mandrels must be replaced when the head diameter has been reduced by 0.002 inch.

Impact Testing of Metals

The impact test is designed to determine the toughness of a material and the two most commonly used methods are those using the ‘Charpy’ and ‘Izod’ impact-testing machines.

The Izod test  uses a notched specimen supported on one side only. The pendulum strikes the specimen that is not held by the machine. The Charpy test also uses a notched sample, but this is supported on both sides with the pendulum hammer striking the middle of the specimen.

Both tests use specimens of standard dimensions, which record the energy absorbed by the test piece on impact to give a measure of toughness. A brittle material will break easily and will absorb little energy, so the swing of the pendulum (which is recorded against a calibrated scale) will not be reduced significantly. A tough material will absorb considerably more energy, and greatly reduce the recorded pendulum swing.



Most materials show a drop in toughness with a reduction in temperature, though some materials show a rapid drop as the temperature is progressively reduced. This temperature range is called the Transition Zone, and components, which are designed for use at low temperature, should be operated above the material’s Transition Temperature.

Identification Of Heat-Treated Aluminium Alloys

Aluminium alloys that have been subjected to heat-treatment are usually identified by markings that indicate the heat-treatments involved. Three typical identification systems are those of the British Standards Institute (BS), the Ministry of Supply (MoS), and the American systems as can be seen below,


Identification Markings of Heat Treated Aluminium Alloys

BS System Meaning
M As manufactured state
O Annealed state
OD Annealed and lightly drawn
T Solution-treated, no precipitation required
W Solution-treated, can be precipitated
WP Solution-treated and precipitation treated

MoS System Meaning
A Annealed state
N Solution-treated, no precipitation required
W Solution-treated, and requires precipitation
WP Solution-treated and precipitation treated

American System Meaning
T3 Solution-treated and cold worked
T4 Solution-treated only (naturally aged)
T6 Solution-treated and artificially aged
T8 Solution-treated, cold worked and artificially aged
T9 Solution-treated, artificially aged and cold worked

An example of one of these marking systems would be an alloy with the designation 2024-T4, which indicates an aluminium/copper alloy that has been solution-treated only, and then naturally aged

Apart from these systems, many other exist world-wide, but the British systems are broadly confined to the following for light alloys.

• British Standards for general engineering use BS 1470 -1475. In this series the prefix N is used to denote non-heat-treatable aluminium alloys and prefix H for the heat-treatable alloys.

• British Standards for aerospace use the L series such as BS 3 L72, which indicates the 3rd amendment to the basic L 72 specification whilst LM indicates a cast material. The wrought materials are commonly abbreviated to L71, L72 and L 73 et al.

Examples of some of these aircraft BS codes are:

• 159 Dural Solution-Treated and Artificially aged
• L163 Alclad Solution-Treated and Naturally aged

Dural is a Trade name for an 2017 Al/Cu/Mg/Si/Mn alloy, originally manufactured by the Duren Aluminium Company (Germany) for the Zeppelin Airships. It is often used as a generic name for similar alloys, regardless of source of manufacture.

DTD Specifications are material identification numbers issued by the Directorate of Technical Development (a Ministry Department) for specialised applications, when widespread use is not anticipated. If such a material finally becomes commonly used, a British Standards specification is compiled and issued.

Aircraft Control Cable Tensioning

The correct tension for a control cable is specified in the Aircraft Maintenance Manual.  It is checked using a tensiometer and adjusted using the turnbuckles.

The tensiometer must have the riser installed that is specified for the cable size being checked.  The cable is slipped between the riser and the two anvils.  The lever is then closed against the housing.  The reading on the scale is then applied to a chart to obtain the cable tension.  A pointer lock can be used to hold the instrument reading while it is being removed from the cable.  The tension is checked when there is no load on the controls and in the middle of the longest stretch of cable between pulley.

Before using the tensiometer make sure that the serial number of the chart is the same as that of the tensiometer.  Make sure that the tensiometer is ‘in date’ ie. not due for calibration.

Changes in temperature will affect the tension of the cables and this must be taken into account when adjusting the cable tension.  It is usual for a temperature/tension graph to be given by the aircraft manufacturer.  Using the graph, the correct tension can be applied to the cable.  In colder countries the required load will be lesser as compared to the hotter countries, this is mainly due to temperature effect on materials which causes it to expand or contract.

An example of a graph is shown below. 

Air Transport Association 100 Chapter System (ATA100)

ATA 100 CHAPTER AND SECTION HEADINGS

AIRCRAFT GENERAL

ATA Number	Chapter Name
ATA 01	INTRODUCTION
ATA 05	TIME LIMITS/MAINTENANCE CHECKS
ATA 06	DIMENSIONS AND AREAS
ATA 07	LIFTING AND SHORING
ATA 08	LEVELING AND WEIGHING
ATA 09	TOWING AND TAXIING
ATA 10	PARKING, MOORING, STORAGE AND RETURN TO SERVICE
ATA 11	PLACARDS AND MARKINGS
ATA 12	SERVICING - ROUTINE MAINTENANCE

AIRFRAME SYSTEMS

ATA Number	ATA Chapter name
ATA 20	STANDARD PRACTICES – AIRFRAME
ATA 21	AIR CONDITIONING AND PRESSURIZATION
ATA 22	AUTOFLIGHT
ATA 23	COMMUNICATIONS
ATA 24	ELECTRICAL POWER
ATA 25	EQUIPMENT/FURNISHINGS
ATA 26	FIRE PROTECTION
ATA 27	FLIGHT CONTROLS
ATA 28	FUEL
ATA 29	HYDRAULIC POWER
ATA 30	ICE AND RAIN PROTECTION
ATA 31	INDICATING / RECORDING SYSTEM
ATA 32	LANDING GEAR
ATA 33	LIGHTS
ATA 34	NAVIGATION
ATA 35	OXYGEN
ATA 36	PNEUMATIC
ATA 37	VACUUM
ATA 38	WATER/WASTE
ATA 46	INFORMATION SYSTEMS
ATA 49	AIRBORNE AUXILIARY POWER

STRUCTURE

ATA Number	ATA Chapter name
ATA 51	STANDARD PRACTICES AND STRUCTURES - GENERAL
ATA 52	DOORS
ATA 53	FUSELAGE
ATA 54	NACELLES/PYLONS
ATA 55	STABILIZERS
ATA 56	WINDOWS
ATA 57	WINGS

POWER PLANT

ATA Number	ATA Chapter name
ATA 71	POWER PLANT
ATA 72	ENGINE
ATA 73	ENGINE - FUEL AND CONTROL
ATA 74	IGNITION
ATA 75	BLEED AIR
ATA 76	ENGINE CONTROLS
ATA 77	ENGINE INDICATING
ATA 78	EXHAUST
ATA 79	OIL
ATA 80	STARTING
ATA 81	TURBINES (RECIPROCATING ENGINES)
ATA 82	WATER INJECTION
ATA 83	ACCESSORY GEAR BOXES (ENGINE DRIVEN)
ATA 84	PROPULSION AUGMENTATION
ATA 91	CHARTS

Screw Thread Gauges

The simplest method of checking threads in the production shop is to use screw thread limit gauges.  These gauges possess the same thread form as the mating thread.  The gauges are screwed on to the thread being checked.

Thread gauges make sure that screw threads are of the correct size as specified in the applicable standard.  A GO and NOT GO screw plug gauge will make sure that an internal thread is correct.

Plain GO and NOT GO gauges will check the diameter of an internal thread.

GO and NOT GO screw ring gauges  are used to check that an external thread is correct.  Plain ring or calliper gauges are used to check the diameter of an external thread.

RING SCREW GAUGES

For the gauging of bolts or external threads the equivalent mating gauges are known as ring screw gauges.  As in the case of plug screw gauges a limit system can be provided by a full-form GO and NOT GO effective ring gauge.  As the factors involved are exact counterparts of the gauging of internal threads, the GO ring gauge has a full-form thread.  The NOT GO gauge is truncated on the minor diameter, and cleared on the major diameter at the root of thread.

GO and NOT GO ring screw gauges are used to check that an external thread is correct.  Plain ring or caliper gauges are used to check the diameter of an external thread.

PLUG SCREW GAUGES

When gauging nuts or internal threads of full-form a GO plug gauge is used.  The GO gauge is accurately made to the minimum dimensions of the required thread.  It will assemble with the component and will make sure that the major, minor, and effective diameters are not below the minimum dimensions.  The GO gauge also makes sure that any errors in pitch, angle and thread form are within limits.

The GO end of a plug screw gauge is longer than the NO GO end.  This will indicate a defect deeper into the hole.