Lubricants, Fuels & Lubrication Systems
Mineral vs ashless-dispersant oils and their jobs, wet- and dry-sump systems, and AVGAS grades.
Lubricants, fuels & lubrication, summary notes
- Aircraft piston-engine oils are either straight MINERAL or ASHLESS-DISPERSANT (AD): mineral oil is used for the break-in of a new/overhauled engine, then AD oil (which holds combustion by-products in suspension for the filter to catch) for normal service.
- Oil does six jobs: it lubricates, COOLS, cleans (carries debris to the filter), seals the piston rings, protects against corrosion and cushions shock loads; it is graded by viscosity (SAE/commercial aviation grades).
- A WET-SUMP system keeps the oil in the crankcase, simple and light, standard on most light aircraft. A DRY-SUMP system holds oil in a separate tank with scavenge pumps returning it, giving reliable supply at any attitude (aerobatic and larger engines).
- The oil system has a pressure pump, filter, oil cooler, pressure relief valve and pressure/temperature indication; the oil pressure gauge is the engine's vital sign.
- Aviation gasoline (AVGAS) is graded by anti-knock (octane/performance number); 100LL ('low lead', dyed blue) is the common grade, using too low a grade invites detonation, and grades must not be mixed carelessly.
- ⚠ Exam trap: a WET-sump keeps oil in the crankcase (light aircraft); a DRY-sump uses a separate tank + scavenge pumps for supply at any attitude (aerobatics); a NEW/overhauled engine is run in on straight MINERAL oil before switching to ashless-dispersant.
Why is a freshly overhauled cylinder run on straight mineral oil rather than ashless-dispersant oil?
During break-in the piston rings must bed into the cylinder wall, which needs a little controlled wear. Straight mineral oil lets the rings seat properly, whereas ashless-dispersant oil's additives can glaze the bore and prevent the rings sealing. Once the engine is bedded in, it is changed to AD oil for normal service.
Lubrication & fuels concept map
Lubricants, Fuels & Oil Systems
Lubricants, fuels & lubrication quiz
Lubricants, Fuels & Lubrication Systems, quiz
1. An oil system holds 30 litres and consumes 0.6 litres per hour. Ignoring any top-up, the endurance before the tank is empty is:
50 hours18 hours0.02 hours2. A newly overhauled engine is run in on:
Straight mineral oilAshless-dispersant oilTurbine oil3. Ashless-dispersant (AD) oil is used because it:
Holds combustion by-products in suspension for the filterGlazes the bore quicklyContains lead4. A wet-sump lubrication system keeps the oil:
In the crankcaseIn a separate remote tankIn the fuel tank5. A dry-sump system is preferred for aerobatics because it:
Supplies oil reliably at any attitudeIs lighterNeeds no pump6. Which is NOT a function of engine oil?
Generating the ignition sparkCoolingSealing the rings7. The common aviation gasoline grade is:
100LL (low lead, dyed blue)Jet A-1Diesel8. Using too low a fuel grade risks:
DetonationBetter economyLower CHT