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 principal 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.