Aircraft jet engines turbine outlet temperature indication

Aircraft jet engines turbine outlet temperature (TOT) or exhaust gas temperature (EGT) or interturbine temperature (ITT) indicating system  has its principle on the seebeck or thermoelectric effect. The seebeck effect states, when a conductor (metal) is heated an electromotive force (emf) or voltage is generated across the conductor.Due to the produced emf current runs through the conductor, because electrons tend to move away from the hot end towards the cold end of the heated metal (iron constitutes an exception- reverse electron flow). By heating two dissimilar conductors (metals) a potential or voltage difference, proportional to the temperature sensed, is generated causing  an electron flow/ current to circulate along the hot and cold sections of the formed close loop.The joined conductors at the hot section form a thermocouple which is in fact a heating sensing element and across the cold ends, an indicator measuring the sensed temperature/heat is connected.

• Applications

Thermocouples used as probes or heating sensing elements wherever conventional sensors such as thermometers  or thermistors ( resistance varies according to temperature variations) cannot be used due to high or extreme temperatures developed.

• Thermocouple probe types

There are several probe types used depending on chemical properties, melting point, stability and of course cost. These are iron - constantan, platinum- rhodium alloys and Chromel ( chromium/ nickel)- Alumel ( aluminium/nickel).The most commonly used in turbine engine aircrafts are the Chromel- Alumel probes. Chromel- Alumel probes have extended operating life at temperatures up to 1100 degrees Celsius on the one hand and present smooth and linear characteristics on the other (produced emf increases linearly to heating sensed).

• Indicating system layout

The system comrises:

1. Thermocouples
2. Matching resistors
3. Cabling
4. Indicator

• Thermocouples

They are minimum four, electrically connected in parallel operation and mounted radially around the gas generator outlet in the gas flow to deliver a homogeneous measurement.Should a thermocouple be defective, the parallel connection ascertains indicating system's proper functionality.However, loss of two or more thermocouples can cause destabilised and erroneous indications. 

• Matching resistors

Matching resistors or trim compensators allow a corrected exhaust gas temperature for a given turbine inlet temperature for all same type engines. A batch of same type engines does not guarantee all engines, for a given gas generator turbine (N1) rpm, fuel flow  and given outside temperature (OAT), will produce exactly similar turbine inlet temperatures. Pilots need standard  indications for operational parameters, such as N1&N2 rpm, TOT, OAT, EPR, TORQUE. etch., for a fleet  of same type aircrafts and engines.

• Cabling

The larger the temperature difference, consequently the greater the distance chromel- alumel cabling extends between  the hot and cold sections of the closed loop, the larger the emf generated, thus more stabilized temperature indications produced at the indicator.  

• Indicator

It is connected in series to the formed electrical loop at its cold ends and the indicator is graded in degrees Celsius increments pertaining to the emf produced.

• Failures 

TOT chromel-alumel  indicating systems are generally reliable with long service life and at maximum temperatures.However, there are reasons for causing TOT indicating systems to fail, such as:

1. Wiring insulation failures, electrical resistance/ leak  between one of the elements and the ground, in particular at the hot section.    
2. Continuity problems between the joined metals.
3. Broken matching resistors or disabled trim compensators.
4. Failed indicators.

• Maintenance

Following are preventative maintenance practices based upon aircraft& engine maintenance manuals (AMM& EMM)  that contribute to optimum system performance:

1. Insulation resistance checks, using proper tooling ( insulation testers).
2. Continuity checks at thermocouples& Cabling, using proper tooling (Ohmeters).
3. Visual checks for oxidized or broken components.
4. Calibrating or testing indicators and Cabling,using proper tooling (test sets)