FAA Aviation Mechanic Powerplant (AMP)Reciprocating EnginesEasy
A technician is preparing to start a horizontally opposed reciprocating engine that has been sitting outside overnight in freezing temperatures. Company procedures require engine preheating before starting. What is the primary reason for preheating the engine under these conditions?
- ATo prevent the magneto impulse coupling from slipping in cold weather
- BTo increase the compression ratio so the engine starts more easily
- CTo reduce spark plug fouling caused by cold combustion chambers
- DTo reduce the viscosity of congealed oil so adequate lubrication reaches internal parts during start
Show answer & explanationAnswer & explanation
Correct answer: D. To reduce the viscosity of congealed oil so adequate lubrication reaches internal parts during start
Cold oil becomes thick (high viscosity) and may not flow adequately to bearings and other moving parts, risking damage from inadequate lubrication during start. Preheating warms the oil and metal components so oil can flow and lubricate properly before and during engine start.
Why the other options are wrong
- A. Impulse couplings are mechanical devices unaffected significantly by cold soak in this manner.
- B. Compression ratio is a fixed mechanical dimension and is not changed by preheating.
- C. Preheating does not primarily target spark plug fouling, which is caused by combustion deposits, not cold temperature alone.
Engine Preheating
Warming a cold-soaked reciprocating engine before starting to reduce oil viscosity and prevent damage from inadequate lubrication and thermal shock.
- Cold oil is thick and slow to circulate
- Preheating reduces the risk of bearing/cylinder scoring on start
- Also helps prevent thermal-shock cracking of aluminum crankcases
Memory trick: Cold oil is like cold honey — warm it up before it has to flow!