Engine instruments

Engine instruments monitor engine parameters and are mandatory minimum instrumentation per CS-23 and NCO.IDE.A.120.

Overview

Temperature measurement in the cockpit

The following cockpit gauges involve temperature measurements:

• Engine lubricant (oil temperature).
• Engine cooling fluid (in liquid-cooled engines).
• Cylinder head temperature (CHT).
• Exhaust gas temperature (EGT).
• Outside air temperature (OAT).
• Cabin air temperature.

CHT sensor — thermocouple

Cylinder head temperature sensors work electrically via thermocouples:

• A thermocouple is a point sensor of two metals (typically copper-constantan or iron-constantan) that generate a small voltage on a temperature difference (Seebeck effect).
• The thermocouple is mounted on a cylinder head (in a spark-plug bore or directly on the head).
• The voltage is amplified and sent to the CHT gauge.

CHT responds faster than oil temperature

The CHT reacts noticeably faster to engine temperature changes than the oil temperature — reason and consequence:

• The cylinder head is directly heated by combustion → CHT follows load changes almost immediately.
• The engine oil is a thermal buffer — absorbs heat and releases it through the cooler; changes take minutes.
• → Pilot benefit: the highest and lowest permissible CHT can be read off immediately, allowing fast countermeasures (mixture richer, cowl flaps open, reduce climb angle).
• Oil temperature is a trend indicator over longer periods.

Manifold pressure gauge — detail

The manifold pressure gauge (MAP) measures the air pressure behind the throttle valve in the intake manifold:

• Operating principle: barometric — essentially an aneroid barometer connected to the intake manifold.
• With engine running: MAP reads below atmospheric (typically 15-29 inHg, depending on power and altitude).
• With engine shut down: the MAP reads the atmospheric pressure at the aircraft's position — typically 29.92 inHg at sea-level ISA, less at altitude.
• → On shutdown MAP is a surrogate for QFE (local ground pressure) at the current position.

Exhaust Gas Temperature (EGT) — use

The EGT gauge is used primarily for:

• Mixture-adjustment check — pilot leans the mixture, EGT rises to peak, then drops (too lean). Peak EGT = leanest without detonation risk.
• Maximum performance of a carburetted engine — EGT is significant for max performance of a carburetted engine. At peak EGT setting, optimal combustion is achieved.

Fuel gauge — one per tank

Per CS-23.1337: a fuel indicator must be available for each tank. The indicator must be calibrated to read zero during level flight when the remaining fuel equals the unusable amount:

• "Zero" on the gauge ≠ empty tank, but = only unusable sump fuel remains.
• Consequence: at "0" on the gauge the pilot must not count on a usable reserve — the tank is effectively empty.

Float-type fuel indicator

A float-type fuel indicator measures fuel volume — a float on the surface gives a height that is converted to volume:

• Drawback: volume changes with fuel temperature — warm fuel expands, cold fuel contracts. A constant mass of fuel reads different volumes depending on temperature.
• Consequence: in very cold weather (after overnight frost) the float-type shows less volume than in warm weather even with the same mass.

→ The most reliable method to determine fuel on board on the ground is a direct look into the tank (visual or dip stick) — no float effect, direct quantity reading (in litres on a calibrated scale).

Marks on scale instruments

Per CS-23.1545 (Airspeed indicator) and CS-23.1549 (Powerplant and accessory instruments):

• Green arcs — normal operating range
• Yellow arcs — caution / transition range
• Red lines / arcs — max or min limit, not to be exceeded