12.2 Temperature Instruments: CHT, EGT & Thermocouple Principles

12.2 Temperature Instruments: CHT, EGT & Thermocouple Principles

Quick Answer: Aircraft engine temperature instrumentation relies primarily on thermocouples operating on the Seebeck effect—a thermoelectric phenomenon where heating the junction of two dissimilar metal conductors produces an electromotive force (EMF) in millivolts directly proportional to the temperature differential between the hot (measuring) junction and the cold (reference) junction. Thermocouple circuits are completely self-powered, operating independently of aircraft battery or alternator bus power. Cylinder Head Temperature (CHT) systems employ Type J (Iron-Constantan) thermocouples, installed as spark plug gasket rings or spring-loaded bayonet well probes, operating normally between 300°F and 400°F with a redline limit of 460°F to 500°F. Exhaust Gas Temperature (EGT) systems utilize Type K (Chromel-Alumel) thermocouples installed 2 to 4 inches downstream of the exhaust valve, operating from 1200°F to 1650°F to guide mixture leaning. Thermocouple leads are calibrated to a precise total circuit resistance (2 to 8 ohms); technicians must NEVER cut, shorten, or splice thermocouple leads, as altering lead length destroys calibration accuracy.

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Thermoelectric Operating Principles: The Seebeck Effect

Unlike resistive temperature detectors (RTDs) or ratiometer oil temperature gauges that require external DC electrical power, cylinder head and exhaust gas temperature systems are based on thermoelectric voltage generation.

                      The Seebeck Thermocouple Circuit

     Hot Measuring Junction                         Cold Reference Junction
     (Cylinder Head or Exhaust)                     (Cockpit Instrument Meter)

     [ Dissimilar Metal A: Iron / Chromel (+) ]
     *=======================================================\
     |                                                       ( Galvanometer )
     *=======================================================/
     [ Dissimilar Metal B: Constantan / Alumel (-) ]

     Thermoelectric Voltage (EMF in Millivolts) Drives Moving-Coil Pointer
     ZERO AIRCRAFT BUS ELECTRICAL POWER REQUIRED (SELF-CONTAINED SYSTEM)

The Thermoelectric Circuit

Discovered by physicist Thomas Johann Seebeck in 1821, the Seebeck effect dictates that when two wires composed of chemically dissimilar metals are joined at both ends to form a continuous closed loop, and a temperature difference exists between the two junctions, an electromotive force (EMF / voltage) is generated.

• Hot Junction (Measuring Junction): Placed at the engine location whose temperature is to be monitored (the cylinder head casting or the exhaust gas stream).
• Cold Junction (Reference Junction): Located inside the cockpit indicator case where the thermocouple leads attach to the meter movement terminals.
• Millivolt Generation: The voltage produced is microscopic—typically between 15 and 55 microvolts per degree Fahrenheit (a few thousandths of a volt total). This millivoltage drives a sensitive D'Arsonval moving-coil galvanometer mechanism. Because the thermocouple generates its own current, the system functions perfectly during a total electrical bus failure.

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Cold Junction (Reference Junction) Compensation

A fundamental challenge in thermocouple instrumentation is that the Seebeck voltage is proportional to the temperature difference ($T_{hot} - T_{cold}$) between the engine probe and the instrument case.

• If the cockpit cabin warms from 30°F on a winter morning to 100°F on a hot summer afternoon, the temperature difference between the hot probe and the meter shrinks, causing the gauge to display an erroneously low temperature unless compensated.

+-------------------------------------------------------------------------+
|                    COLD JUNCTION COMPENSATION MECHANISMS                |
|                                                                         |
|   ANALOG INDICATORS  --> Calibrated bimetallic hairspring mechanically  |
|                          biases the pointer to room ambient temperature.|
|   DIGITAL INDICATORS --> Solid-state thermistor/RTD at terminal block   |
|                          digitally adds ambient temp to the ADC reading.|
+-------------------------------------------------------------------------+

Analog Bimetallic Hairspring Compensation

In analog cockpit gauges, the cold junction is compensated by a bimetallic spiral hairspring attached to the D'Arsonval pointer movement. As cockpit ambient temperature changes, the unequal thermal expansion of the bimetal strip winds or unwinds the hairspring, shifting the pointer's mechanical zero position.

• Pre-Flight Sanity Check: When an aircraft engine has been shut down overnight and is at ambient temperature, the CHT and EGT needles will not rest at zero; they will indicate the current ambient air temperature of the cockpit flight deck.

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Cylinder Head Temperature (CHT) Indicating Systems

Cylinder Head Temperature (CHT) monitors the thermal loading of the engine's air-cooled combustion chambers, serving as the primary metric to prevent engine overheating and structural degradation.

Sensor Probe Configurations

1. Spark Plug Gasket Probe: A specialized thermocouple shaped like an annular copper ring. During maintenance, it replaces the standard solid copper spark plug crush washer under the spark plug of the hottest-running cylinder (traditionally the rear cylinder on horizontally opposed engines, such as cylinder #5 or #6, due to restricted baffling airflow). Technicians must ensure that only one gasket is installed; stacking a standard washer with a thermocouple ring causes thread reach errors and poor heat transfer.
2. Bayonet Well Probe: Modern high-performance reciprocating installations utilize spring-loaded bayonet probes. The probe tip is inserted into a pre-drilled, threaded thermowell boss in the cylinder head casting and retained by a slotted bayonet adapter nut under spring compression. This provides superior accuracy by sensing internal aluminum metal temperature rather than external spark plug boss temperature.

Thermal Limits & Metallurgical Dangers

Aircraft cylinder heads are cast or forged from high-strength aluminum alloys that undergo precise artificial aging and heat treatment during manufacture.

• The 435°F Threshold: Sustained operation with CHT above 435°F (224°C) rapidly anneals the aluminum alloy. Overheated cylinder heads lose their structural yield strength.
• Structural Failure Modes: Chronic high CHT results in warped valve guides, dropped exhaust valve seats (causing immediate catastrophic engine seizure), loosened spark plug helicoil inserts, and cylinder head-to-barrel separation along the interference fit threads.
• Preignition & Detonation: Excessively hot combustion chambers ignite incoming fuel charges before the spark occurs (preignition), driving peak cylinder pressures to destructive levels.

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Exhaust Gas Temperature (EGT) Indicating Systems

Exhaust Gas Temperature (EGT) measures the temperature of combustion gases as they exit the cylinder exhaust valve, serving as the most sensitive and immediate indicator of fuel-air mixture combustion efficiency.

                     EGT Leaning Curve Dynamics

     Exhaust Gas
     Temperature
     (EGT)
        ^                      [ Peak EGT ]
        |                  (Stoichiometric ~15:1)
        |                         /\
        |     Best Power         /  \
        |    (50°-100° ROP)     /    \       Best Economy
        |         *            /      \     (Peak to 50° LOP)
        |        / \          /        \        *
        |       /   \        /          \      / \
        |      /     \______/            \____/   \
        +--------------------------------------------------->
               FULL RICH       STOCHIOMETRIC       LEAN
                             [ Fuel-Air Ratio ]

Installation Constraints: 2 to 4 Inches

EGT probes must be clamped into a hole drilled into the exhaust stack 2 to 4 inches from the cylinder head exhaust port flange:

• If mounted closer than 2 inches, the probe is exposed to unburned flame fronts and radiant heat directly from the exhaust valve face, producing artificially high and fluctuating readings.
• If mounted further than 4 inches, exhaust gases cool as they expand and mix with cooler boundary air, producing an erroneously low reading and masking cylinder-to-cylinder mixture imbalances.

Mixture Leaning Protocol

• Full Rich: At full rich (takeoff and climb), excess fuel cools combustion, yielding low EGT (1250°F–1350°F).
• Leaning toward Peak EGT: As the pilot pulls the mixture control lean, the air-fuel ratio approaches the chemically ideal stoichiometric ratio (15:1 by weight). Combustion becomes increasingly complete, releasing maximum thermal energy; EGT steadily rises to Peak EGT.
• Best Economy vs. Best Power:
  • Leaning past peak introduces excess air that absorbs combustion heat, causing EGT to drop on the Lean-of-Peak (LOP) side.
  • Best Economy is achieved at Peak EGT or 25°F–50°F Lean of Peak.
  • Best Power is operated 50°F to 100°F Rich of Peak (ROP), providing excess fuel that cushions against cylinder detonation during high-power cruise.

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The Ironclad Thermocouple Lead Wire Calibration Rule

A critical subject of FAA certification examinations is the maintenance and handling of thermocouple lead wiring.

+-------------------------------------------------------------------------+
|             THERMOCOUPLE LEAD WIRE INTEGRITY RULES (AC 43.13-1B)        |
|                                                                         |
|   1. NEVER CUT OR SHORTEN   --> Decreases resistance; gauge reads HIGH  |
|   2. NEVER LENGTHEN/SPLICE  --> Increases resistance; gauge reads LOW   |
|   3. NEVER SOLDER TERMINALS --> Adds third metal; produces parasitic EMF|
|   4. COIL EXCESS WIRE       --> Coil in smooth loops; clamp to airframe |
|   5. MATCH CIRCUIT OHMS     --> Must match gauge face rating (2-8 ohms) |
+-------------------------------------------------------------------------+

Ohm's Law and Circuit Resistance

Because a thermocouple circuit generates microscopic millivolt potentials, the current ($I$) flowing through the galvanometer movement is governed strictly by Ohm's law: $I = \frac{E_{thermo}}{R_{total}}$ Where $R_{total}$ is the sum of probe resistance, lead wire resistance, and internal meter resistance. The indicator dial calibration assumes a fixed, permanent resistance value—typically 2 ohms or 8 ohms, clearly stamped on the meter face.

Why Cutting Leads Causes High Readings

If a technician finds a replacement CHT lead wire is 3 feet too long and cuts the wire to fit:

1. Shortening the wire reduces total circuit resistance ($R \downarrow$).
2. By Ohm's law, a smaller resistance allows greater current to flow ($I \uparrow$) for the exact same engine temperature.
3. The increased current drives the galvanometer pointer further across the scale, causing the gauge to display an erroneously HIGH temperature reading.
4. Conversely, adding splices or lengthening leads increases resistance, causing the gauge to read erroneously LOW, potentially allowing the engine to overheat without the pilot's knowledge.
5. Solder Splices Create False Junctions: Solder contains tin and lead. Splicing a thermocouple wire with solder introduces a third dissimilar metal junction that creates its own parasitic thermoelectric voltage, distorting the entire reading.
6. Mandatory Practice: If replacement lead wires are longer than required, the technician must coil the excess wire into neat, smooth loops (minimum 3-inch bend radius) and clamp them securely to the engine mount or airframe structure.

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Independent Prep Note

Independent FAA AMT Powerplant prep by OpenExamPrep. Not sponsored by or affiliated with the Federal Aviation Administration (FAA). Technical data compiled from FAA-H-8083-32B, FAA AC 43.13-1B, and 14 CFR Parts 33, 43, and 65.