Sunday, November 13, 2016

Heat Treatment: Critical Points of Steel

When a piece of steel is heated at a uniform rate, the temperature of the steel will, at first, rise steadily.  When the temperature reaches approx.700°C (a dull red colour) it will remain there for certain time then continue to rise again at a uniform rate.  If the heating is continued further then occurs a second arrest in the rise in temperature.  After this, if the heating is continued, the temperature will continue to rise at approximately the initial rate.

During these periods of arrest, the metal absorbs heat, but instead of raising the temperature, the heat brings about a structural change in the steel.  The temperatures at which these periods occur are called ‘critical’ or ‘arrest’ points.

If the steel is heated to 900°C (a bright reddish yellow colour) then removed from the furnace and observed in a darkened room, it will be seen that, as it cools, it will lose its brilliance.  At the points where it received its checks in heating the metal is seen to glow more brightly and it will seem that the cooling has stopped.  In fact the steel will be seen to take on an extra glow as though it was reheated.  After this the rate of cooling will be normal until the metal is cold to room temperature or becomes equal to environmental temperature. 

The temperature at which the changes start (lower critical point) is the same for all steels and is about 700°C.  At this temperature, the internal structural change is as that the pearlite disappears and the laminae of ferrite and cementite of which it is composed, dissolves and forms the solid solution Austenite.  The finishing point of the transformation is known as the upper critical point this point varies according to the steel carbon content.  


Saturday, September 26, 2015

TORQUE LOADING

The purpose of torque loading is to make sure the correct and efficient clamping together of two surfaces.  This prevents over-stressing, distortion, and shearing of bolts, studs, nuts etc.  The majority of bolts, nuts and unions on aircraft are subject to a standard torque loading.  Special bolts are subjected to torque loading
, which are specified in the aircraft maintenance manual. There will be a range for the torque value in most cases an  Torque load values are normally determined by friction, type of thread, material, lubrication and finish of the surfaces of the fasteners.


Formula
Torque  =  Force X Length


Under tightening of screw fasteners may result in lack of firmness between the separate parts of the assembly causing fretting corrosion due to relative movement , and early failure of a component may occur through fatigue or mechanical breakdown.  Conversely, over-tightening is likely to cause immediate failure of the bolts, distortion of one or more parts of the assembly leading to eventual failure, or to stress corrosion, or cracking this will result by the high stressed areas cause by over torque. Due to the varying effects of friction under different conditions of assembly it is important that torque be applied in accordance with the manufacturer's instructions.  The pre-load applied to a fastener at a specified lubricated torque would be considerably higher than if the same torque were applied dry.


Before even start to use torque wrench make sure All torque wrenches that are used on aircraft are regularly inspected, tested and calibrated by a facility equipped to do so.if the serviceable tag indicates as expired. DO NOT USE THE TOOL. 

How to do it :
  •  Clean, and  lubricate the threads(if instructed on AMM)  and mating surfaces of nut, bolt and washer.
  • Tighten the nut to half the specified torque value.
  • Loosen the nut then finally re-tighten to the specified torque value.
  • When the torque-loaded fastener is to be secured by means of a split pin or lock wire, tighten first to the low side of the torque range.  If necessary, tighten the fastener so that the next slot aligns with the hole, ensuring that the maximum torque is not exceeded.  If the maximum torque is reached and the slot in the nut does not line up with the hole in the bolt, the nut and/or washer must be changed.


Tuesday, December 16, 2014

Types Of Landing Gears

The various types of landing gear are dependent on the design and manufacture of the undercarriage units, 

The main type of landing gear for aircraft over 5700 Kg is termed the TRICYCLE UNDERCARRIAGE.

The tricycle undercarriage landing gear comprises two main undercarriage units and one nose undercarriage unit.  However, there are THREE main variations of the tricycle undercarriage as follows:


Standard (Boeing 737)


·         Two main undercarriage units
·         One nose undercarriage unit







Centreline (Airbus A340)


·         Two main undercarriage units
·         One center-line undercarriage unit
·         One nose undercarriage unit


Wing/Body (Boeing 747)


·         Two main (outer) wing mounted, undercarriage units
·         Two main (inner) body mounted, undercarriage units
·        
One nose undercarriage unit




Friday, July 11, 2014

Solid Rivets

In the construction of a metal air frame, permanent joints are made either with rivets or bolts. To securely attach structures together, rivets are cheaper to use, lighter and more rapidly fitted than nuts and bolts, but in the case of power operated machine riveting, more extensive equipment is usually required to make the permanent joints.

Solid rivets have the greatest strength and are therefore preferable to any other type of rivet, but they can only be used where there is access to both sides of the structure.

Rivets are always supplied to the operator with one head already formed and the shank plain to permit insertion into the hole, the opposite end being formed into a head by manual or mechanical means. The size of a rivet is expressed as the diameter and length of its shank; the exception is the countersunk rivet where the length is inclusive of the head




  1. SNAP HEAD : for general purposes where strength is required but not a streamline finish.
  2. MUSHROOM HEAD: for skin covering to give maximum strength.
  3. FLAT HEAD: for internal work where heads are not easily accessible
  4. COUNTERSUNK:  for flush finish (90°, 100°, 120° head) in aviation mostly used 100°
  5. RAISED COUNTERSUNK : for more streamlined surfaces.
  6. UNIVERSAL HEAD
  7. 100° COUNTERSUNK TRUNCATED RADIUS HEAD:  


Tuesday, June 10, 2014

Ball Bearings

These bearings are used where it is not practical to use plain bearings, and where a high degree of reliability and precision is required.
The advantages are:
·         Low frictional losses
·         Wide range of loads may be accepted
·         Simple lubrication requirements
Ball Bearings

The most common type of bearing used on aircraft.  The ball bearing has many variations in design allowing it to be used in a wide variety of situations/

They carry radial loads and moderate axial loads in both directions and where a high axial load may be experienced, the balls run in a deep groove in the races.Two types of ball bearings are in general use.  The caged type and the crowded type.

Caged Ball Bearings

In general use, for engine applications and for equipment with rotational speeds in excess of 100 rpm.  When used within engine/gearbox casings they are lubricated with engine oil supplied by jets or by splash.  When used outside casings, they are lubricated by the application of grease which may be applied at specified intervals by grease gun, or may be of the pre-packed type where lubricating grease is packed and sealed into the bearing on assembly.

Crowded Ball Bearings

This bearing has filling slots in one or both races and has no cage or separators.  The balls therefore touch each other during operation, hence the term ‘crowded’.  They are suitable only where slow rotation or part rotation (oscillations) are found, and are usually of the sealed or pre-packed type.

Angular Contact Ball Bearings

Accept radial loads, and axial loads in one direction where a single bearing may be used.  For axial loads in both directions  an opposed pair of bearings is often used.


Friday, December 13, 2013

Thrust Reversers


There will be cases when the aircraft is required to land on runways that are shorter than those for which the aircraft can normally land.  It is also beneficial to use less runway than that available in order to have a safety margin in the event of a failure.  Although the brakes on the aircraft are designed to be adequate under all normal operating conditions, the engine can assist in shortening the landing distance by using the thrust reversal systems.In this system the engine thurst is vectored to the front to slow down the aircraft.



On a propeller engine this is very simply done by reversing the pitch of the propeller blades so that they now thrust air forward instead of backwards. Therefore the prop aircraft can change the direction of thrust by 180 degrees. But, This is impossible to achieve with a jet engine, The result of this is that the reversed thrust from a jet engine can never be as fully effective as that from a propeller.  The thrust level obtained from the engine can be as high in reverse as it is in forward thrust but, because of the angle involved, the effect can never be the same as the forward thrust.

types of reversers

As far as jet engines are concerned there are three basic types of thrust reverser: 


  • Clamshell Door
  • Retractable Ejector
  • Cold Stream Cascade Reverser






 

Friday, December 6, 2013

Gas Turbine Combustion Systems


.
The combustion system is designed to burn the fuel as efficiently as possible over the whole range of engine operating condition.  All the energy released by the fuel is converted into heat and velocity energy. Very high temperatures exist in the combustion system, the burning temperature of the fuel being in the region of 2,000°C.  To protect the material from which the system is manufactured, about 60% of the total air flow is used for cooling and the rest is used for fuel burning.

 The combustion chamber is quite short therefore it must be efficient in completing the combustion. To achieve this fuel air mixture is made with it best ratios.

The combustion process requires 15 unit of air to 1 unit of fuel for a successful combustion.  This is known as an air/fuel ratio of 15:1 by weight



The air flow leaving the compressor is first split into two, approximately 20% - 40% being used for combustion, the other 60% - 80% is further divided for combustion support and the greater proportion for gas cooling.

These three flows are known as:-


  • Primary air flow, for mixing with the fuel and to support combustion.(20% )
  • Secondary air flow to shape the flame and complete combustion. ( 20%)
  • Tertiary air flow, to cut off the flame and reduce gas temperature to a figure acceptable to the turbine. ( Cooling 40% & Dilution air 20%)