Strength of Riveted Joints

The factors that govern the strength of a joint are:

Plate specification             This will be of such a material and gauge as to  successfully withstand tensile and bearing loads.

Rivet Specification             This will be selected to withstand shear loads.  In cases where the

specification of the rivet is not given, use a rivet of the same material as the plate, with a diameter

of 2¹/₂ where T is the thickness of the plate.

Rivet Pitch    This is important as too great a pitch will result in insufficient rivets to take the shear loads and too small a pitch will result in lowering the resistance of the plate to tensile loads.

Types of Rivet Spacing

Single Chain            Used chiefly on attachment and lightly stressed joints.

Multiply Chain         Used on watertight joints and in places of high stress where thick gauge plate is used.

Staggered Riveting            Used as an alternative to multiple chain in watertight joints, circular patches etc.

Aluminium Alloy Identification

Aluminium and its alloys are internationally classified into eight groups identified by a four figure series number.  

 The first digit indicates the principal alloying element.  For example any alloy in the 2000 series such as 2117 or 2024 has copper as its main alloying element.  7075 has zinc as its main alloy.

Aluminium Alloys grouped by main alloying elements.	99.0% minimum	1000
	Copper	2000
	Manganese	3000
	Silicon	4000
	Magnesium	5000
	Magnesium and Silicon	6000
	Zinc	7000
	Lithium and others	8000

 

The second digit identifies the alloy modification state, 0 indicates that the alloy is original.  1 indicates that the alloy has been modified once.  The third and fourth digits identify the specific aluminium alloy.  In the case of 2024, the alloy consists of 4.5% copper, 1.5% magnesium, 0.1% chromium and the remainder aluminium.

There are two basic divisions of aluminium alloys: 

• NON-HEAT TREATABLE – those that can be softened but not hardened by heat treatment.  These alloys are strengthened by controlled working, eg cold rolling. 

• HEAT TREATABLE – those that can be softened and hardened by heat treatment.

If non-heat treatable alloys are heated to their annealing temperature and allowed to cool slowly they will be softened to their annealed, or –0 condition.

The 2000, 4000, 6000, 7000 and 8000 series alloys are strengthened by solution treatment followed by age hardening.

The 1000, 3000 and 5000 series alloys are cold worked to increase their strength.

IDENTIFYING STRAIN HARDENING PROCESSES

Non-heat treatable aluminium alloys can be hardened by strain hardening.  This is usually done by rolling the sheets of metal.

The cold worked temper of wrought alloys is indicated by the letter –H followed by a number.  Tempers in the –H series are not applicable to castings.

The first digit following –H indicates the form of the strain hardening.

A second digit 2, 4, 6 or 8 indicates the final degree of hardness.  The fully hard condition is 8.4 indicates material having a strength midway between fully annealed –0 and full hard i.e. half hard.  -0  Indicates the soft, annealed condition.  Applies only to the wrought alloys.

   5052 – H24----------->Strain hardened +  Half hard Then partially annealed

IDENTIFYING HEAT TREATMENT PROCESSES

The heat treatable alloys can be hardened and strengthened by solution heat treatment or by precipitation heat treatment (artificial ageing).

The heat treated temper of aluminium alloys is indicated by the letter –T followed by one or more numbers.  The number following the letter T shows the type of heat treatment.  Any variations are indicated by a second number.

-T3      Solution heat treated and strain hardened (cold rolled).

-T36    Solution heat treated and cold rolled to reduce the thickness of the sheet by 6 per cent.

-T4      Solution heat treated then naturally aged.

-T42    Solution heat treated by the user regardless of the previous temper.

2024   –  T4--------------> Heat treated alloy + Solution treatand +naturally aged

Heat Treatment Of carbon Steel

Heat treatment is a series of operations involving the heating and cooling of steel in the solid state. Its purpose is to change the mechanical properties of the steel so that it will be harder, stronger or more resistant to impact. It can also make a steel softer and more ductile.

No single heat treatment can produce all these characteristics. Some properties may be improved at the expense of others eg. when being hardened a steel may become ‘brittle’.

Heat treatments are normally carried out by heating and cooling the steel. The temperature to which the steel is heated and the rate of cooling is most important.
Plain high carbon steels can be:

• ANNEALED
• NORMALISED
• HARDENED
• TEMPERED

Annealing

This is a softening heat treatment. In general terms the steel is heated to its upper critical point and then ‘allowed to cool very slowly in the furnace’.

Normalising

This is a form of heat treatment in which the steel is heated to its upper critical point and ‘allowed to cool slowly in still air’. Normalising restores the crystalline structure and relieves stresses in the steel.

Hardening

This is a heat treatment whereby steel is made hard and brittle by heating to the upper critical point and immediately quenching in water or oil.

Tempering

This is a heat treatment process in which some of the hardness is removed from the steel to increase its toughness and decrease its brittleness. After hardening, the steel is reheated to a fairly low temperature (below the lower critical point) then quenched in water.

The temperature depends on the purpose of the tool. The higher the tempering temperature, the less the hardness but the greater the toughness. Thus the purpose of the tool or article must be considered.
The temperature required may be judged from the temper colours which appear on the bright surface of steel which is heated slowly.

Aircraft Fasteners

The word FASTENER is used to describe all of the various types of fastening device used in the construction of an aeroplane. This is because the meanings of the words BOLT and SCREW for example, may be interpreted differently by separate organisations.

A bolt is an externally threaded fastener designed for insertion through holes in the assembled parts. Bolts usually have a plain (unthreaded) portion on the shank. A bolt is normally intended to be tightened or released by turning or torquing the nut.

 A screw is an externally threaded fastener capable of being screwed into preformed threads in the assembled parts. It is normally intended to be tightened or released by turning or torquing the head. A screw has a fully threaded shank. Certain types of screw such as self tapping screws and woodscrews form a thread when they are screwed into place.

A standard fastener comprises of a threaded portion, a head and sometimes a plain shank or grip.  Between the shank and the thread is a small tapered transition zone.  Between the shank and the head is stress relieving fillet radius.

A fastener will be defined by a number of features which are given in detail in it’s specification.  These include it’s Thread Form, Shank Diameter, Head Style, Grip Length, Material, Surface Finish and Locking Facilities.

EASA Part-145 Approved Maintenance Organisations

In order to be able to carry out maintenance on aircraft, which are operated under the influence and control of EASA, organizations within that area of influence must apply to the relevant competent authority for approval to maintain aircraft, their components and to issue certificates of release to service in accordance with Part-145.

If the organization or organizations have their principal place or places of business within the borders of a Member State then that state will designate the authority.

If the organization or organizations have their principal place or places of business outside the borders of the Member States then the European Aviation Safety Agency (EASA) will be the relevant competent authority.

Organizations Approval Class and Rating System

•       An organization must be granted an approval ranging from a single class and rating with limitations to all classes and ratings with limitations.

•   the Part-145 approved maintenance organization is required to indicate scope of work in the maintenance organization exposition.

•  Within the approval class (as) and rating(s) granted by the Member State, the scope of work specified in the maintenance organization exposition defines the exact limits of approval. It is therefore essential that the approval class (as) and rating(s) and the organization’s scope of work are compatible.

77w air Extended-range Twin-engine Operational Performance Standards ETOPS

The ETOPS Extended-range Twin-engine Operational Performance Standards or EXTENDED RANGE OPERATIONS (EROPS).  A two-engine aeroplane obviously loses 50% of its available power whenever there is an engine shutdown in flight.  Further, with only one engine operating, safety margins are much reduced since if the other engine should fail than an accident is almost inevitable. One of most known ETOP aircraft is Boeing 777 ER or 77W

With the very good reliability of modern turbine engines it has become possible to approve transoceanic flights with certain two-engine aeroplanes without compromising safety requirements.  Three levels of approval can be given to a particular aeroplane type with each level being designated by:

the time in flight which a twin engine aeroplane might be from a suitable emergency airfield at normal single engine cruise speed.

The levels are 60 minute, 120 minutes and 180 minutes.

Historically such approvals have only been given after considerable time in service when sufficient evidence of reliability has been achieved.  Experiences of granting and monitoring such approvals, advances in reliability and proven MSG procedures have led to the situation where up to 120 minutes ETPOS approval can be given for a new type in-service evidence.

Initially the onus is on the design of the aircraft with engine reliability, adequate redundancy, alternative electrical supplies, a more restrictive MEL(Minimum equipment list), etc. being given due attention.

A maintenance programme for aeroplanes used for an ETOPS operation will be based on the same maintenance programme as for aeroplanes used for other operations.  The programme will, however, be enhanced and amended to ensure that the operations are safe.  Procedures will prevent identical action being taken on the same item on both engines, for example a fuel control unit change on each engine.  Maintenance personnel must be aware of the special nature of ETOPS and qualified persons used to sign for checks identified as being significant for ETOPS.

Airframe Construction-Helicopters

The airframe of the Helicopter covers a wide range of materials, mainly due to the advances in technology that have taken place in the 50 years since the first helicopters were manufactured. For the most part the materials used in helicopter construction are the same as those used in fixed wing aircraft.

There are 3 (three) basic types of construction normally used for the fuselage, tail cone and pylon, these are:

• Tubular or Truss type
• Monocoque or Semi-Monocoque type, Sheet Metal Construction
• Bonded Construction  

There are two types of tubular-truss construction, known as the PRATT TRUSS and the WARREN TRUSS.  In both cases the structure is built up around longerons which provide the main strength to resist Torsional and Bending loads

 

Aircraft Structure Zoning System

Zoning of large aircraft is specified by the Air Transport Association of America in the ATA-100 Specification.
A zone is identified by one of three indicators, depending upon whether it is a major zone, major sub-zone, or simply a zone.

 Major zones are identified by three digit numbers as follows:
Major Zone                                       Area
 No.                                 
100              Lower half of the fuselage to the rear pressure bulkhead (below the main cabin deck).
200              Upper half of the fuselage to the rear pressure bulkhead.
300              Empennage, including fuselage aft of the rear pressure bulkhead.
400              Power plants and struts or pylons.
500              Left wing.
600              Right wing.
700              Landing gear and landing gear doors.
800              Doors.
900              Reserved for uncommon differences between aircraft types not covered by standard series numbers.

The standard series is from 100 to 800 and the special series numbers are in the 900 bracket. \

Aircraft Maintenance Requirements

Aircraft maintenance requirements fall into two main categories:

•   Scheduled – where we know, in advance, when and what will cause the maintenance to be due.
•  Unscheduled – where we do not know when the  maintenance will be due although we may be able to anticipate a certain number of unscheduled maintenance events over a period of time from the analysis of reliability records.

In both of the above cases certain factors or events trigger maintenance as illustrated.

Consider, first, the factors which trigger scheduled maintenance.  The primary trigger is the MAINTENANCE SCHEDULE itself, this is usually part of the MAINTENANCE PROGRAMME.  The schedule must be approved by the Airworthiness Authority.

The maintenance schedule or programme specifies:

• which maintenance tasks are to be carried out and.
• when they are to be carried out.

The specification of WHEN is stated in periods between maintenance events measured in:

•   days, weeks, months or years (calendar time).
•  aircraft flying hours.
• engine or propeller hours.
•  number of landings.
•  number of cycles (eg engine start/stop cycle or pressurisation cycle).

The time related intervals may be combined so, for example, a maintenance task may have to be carried out every 50 flying hours or 62 days whichever is the sooner.