Engine instruments, fire protection and installation

Engine instruments

• Tachometer: crankshaft rpm, with a red line at the limit.
• Manifold pressure: the absolute pressure in the intake, in inches of mercury. With the engine stopped it reads the outside air pressure, about 29.92 at sea level on a standard day.
• Oil pressure and oil temperature: the first things to check after start.
• Cylinder head temperature (CHT): usually an iron-constantan thermocouple.
• Exhaust gas temperature (EGT): a chromel-alumel thermocouple in the exhaust, used to lean the mixture.
• Fuel flow and fuel pressure.

A thermocouple is two different metals joined at the hot end; it makes its own small voltage, so it needs no power. Its leads are part of the calibrated circuit, so don't shorten or splice them.

Fire protection

The firewall separates the engine from the rest of the aircraft and is made of fireproof material such as stainless steel. Anything passing through it uses fireproof fittings and grommets so a fire can't get through. Fuel and oil lines in the engine compartment are fire resistant and kept away from the exhaust. Larger aircraft add the detection and extinguishing systems from the Airframe course.

Mounts and engine changes

Engines hang on welded steel tube mounts through rubber shock mounts that soak up vibration. Inspect mounts for cracks at welds and the rubber for sagging or oil damage. Changing an engine means draining and disconnecting everything (fuel, oil, controls, wiring, exhaust), hoisting with the right sling, fitting the new engine with new hardware where required, reconnecting everything, checking control travel, then a run-up and leak check, all recorded in the engine's records.

Storing an engine

An engine that won't run for a while is preserved with corrosion-preventive oil in the cylinders and oil system. Desiccant plugs go in the spark plug holes: their silica gel is blue when dry and turns pink when it has absorbed moisture, so pink plugs mean the protection needs renewing.