Magnetic Chip Detectors

The third component we will be discussing under AircraftLubrication systems is the Magnetic Chip Detector (MCD’s). As the name implies, it is a detector, which functions on the principles of magnetism.

Main function of the magnetic chip detector is to provide condition monitoring of the engine and its lubrication system components. To give an early warning of bearing failure, magnetic chip detectors are fitted in the system. 

They are normally of the bayonet type fitting with auto sealing adaptors thus,  can be removed, inspected and replaced very quickly, with no oil spillage. They are often found  in gearboxes and in the scavenge pump return lines where the metal debris are often found.

See below for details of a lubrication pump MCD.


 
 

In earlier versions of engines, the detection is done visually by removing and inspecting the chip detector at periodic intervals. However, On modern engines this is done electronically. If there are metallic debris that comes into contact, it completes an electrical pathway and provides a cockpit indications. Magnetic Chip Detectors (MCD’s) alerts / indication to the flight deck informs the pilot of contamination that requiring attention. 

Often the low resistance across the MCD, can be caused by small carbon deposits or tiny metallic deposits that are insignificant as far as engine wear is concerned. In such cases, there may be a ‘fuzz burner’ fitted that will pass an sufficient electrical charge through the MCD and clear off the fine debris and leave only major particles. If the warning light still remains ‘ON’ then the pilot knows that the debris is of a more serious nature.
 

Below illustration shows the location of MCDs in a Lubrication Pump scavange lines.

Aircraft Engine Lubrication Unit ( Lubrication Pump)

Lubrication unit Or simply said the Lubrication pump system is the main driving force behind the pressurized lubricants in a jet engine. Its main function is to provide oil under the required pressure for lubrication of the engine components and spool bearings, then perform the scavenging (returning) of the oil after lubrication process then finally to directs it to the oil tank via the oil/fuel heat exchanger.

In order to pressurize and to maintain positive displacement flow, it uses a series of pumps which are driven by a single shaft. Often the pump is a Gerortor pump (Click on to read on Gerotor pump construction) which is driven by a single shaft. Number of pumps in the shaft depends on the number of supply and scavenge systems available.  

Lubrication units has its main supply inlet from the Main Oil Tank , one or more Supply lines and one or more scavenge lines for returning oil. It also has various tapping for temperature and pressure measuring sensors. (See below)

The return lines are equipped with scavenge filter screens/mesh to remove any debris from entering the pump system. Each scavenge line is often provided with oil condition monitoring magnetic chip detectors. Magnetic chip detectors works by attracting metal substances to its tip and provide good indications on possible failures in the system ( based on metal analysis). It also aid in reducing  the amount of powdery metallic debris in oil system by attracting the metallic substances.(See below)

Aircraft Engine Lubrication Systems

As the gas turbine engines have limited number of moving components their lubrication requirements are much simpler to meet. This is because the oil does not lubricate any parts that are directly heated by combustion (i.e Combustion chambers, Turbine Blades/Vanes...etc) 

However, for a sustained and prolonged operation, the engine requires ample supply of lubricating oil at all the bearings, gears and driving splines. This supply should be a continuous flow of clean filtered oil and an appropriate temperature, pressure and viscosity, throughout the engine operation.
Main goals of  engine lubricating oils are to:

•         Lubricate
•         Cool/ carry away the heat
•         Prevent corrosion
•         Resist oxidation at high temperatures
•         Possess suitable viscosity at all operating temperatures

Gas turbine engines uses specially formulated low viscosity synthetic oil that does not originate from mineral oil. military specification MIL-PRF-7808 commonly referred as Type I jet oil and MIL-PRF-23699 commonly referred as Type II Jet oil are some examples of such synthetic aviation oils. It is essential that only those oils specified by the engine manufacturer are used. 
 
The ultimate decision of which oil to be used is decided by the operation characteristics and the loads of the engine including the bearing temperatures. Often the selection of the appropriate type of oil is a results of multiple engine tests and lab analysis of the oil. There are wide range of components being used in the lubrication system and basic overview is given in the below diagram. More details of each components will be discussed in later posts.

Following video is created to provide overview of the lubrication system : 

 
 

Aircraft Engine Oil Tanks

Engine Oil tanks are the main focal point in the aircraft lubrication system. As the name implies, its main function is to provide storage for the lubrication and thermal control oil being used in the engine and its components. Oil tanks are also used for providing the additional space required for expanding fluids due to temperature rise during its operation cycles. Engine Oil tanks are usually attached to the frontal portion of the engine often called Fan cases.

Oil tanks have few major sub-assemblies which include:  

1. Filling port and an overflow port – For Filling / Top-up purposes
2. Sight gage- To check fluid levels visually
3. Oil-out / Supply port equipped with a strainer – To supply Oil to the inlet of the lubrication pump
4. Drain plug – To drain the tank for emptying
5. Oil-in / Return port – To connect return line from the scavenge circuit
6. De-aerator /  air separator – To separate any trapped air from returning oil
7. Tank vent port – To provide pressure equalizing vents
8. Oil level transmitter – An electronic measuring system for remote indications
9. Internal walls – To strengthen the oil tank structure and also to reduce oil sloshing during operation cycles.

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.  

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.

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

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:  

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.

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

 

Gas Turbine Combustion Systems

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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%)