Engine and temperature instruments

Navigation · Chapter 35 · DGCA CPL and ATPL

What a jet engine is, why a temperature probe reads warm, and how the cockpit knows the thrust, health and fuel of the engine.

Remember

TAT = SAT + ram rise; rise ≈ (TAS ÷ 100)² °C; grows with TAS, not height.
With Mach: TAT = SAT × (1 + 0.2 Kr M²) in kelvin.
Thrust: N1, EPR (P7 ÷ P2), torque (turboprop). Health: EGT, N2, oil, vibration.
EPR falls in the take-off run. Set it before about 60 kt. Iced P2 probe: over-reads.
EGT thermocouples (chromel/alumel) in parallel: the average; no power needed.
Low to high pressure: capsule, bellows, Bourdon tube (oil).
Fuel flow impeller; fuel quantity capacitance: failure to zero, water past full.

How does an aircraft feel the temperature outside?

Two thermometers

Why a pilot needs it • Ice: will the wings pick up ice? • Engine power: hot air is thin, so the engine gives less. • Speed and height: the computers need it to work out true speed.
Small aircraft: two metals stuck together • Brass stretches a lot when warm. Invar hardly stretches. • Stuck together they make a bimetallic strip, which can do only one thing: curl. • Move the slider and watch the strip.
Wind it into a spring • Wound into a helix, the strip twists as it warms. • The twist turns a pointer. No wires, no power. • Fine for a small aircraft. A big one needs an electric signal, and a hole in a pressurised windscreen is weak.
Big aircraft: the TAT probe • A small probe on the nose, reaching out past the slow air on the skin. • Inside, a fine platinum wire. Warmer wire, more electrical resistance. • So temperature becomes an electric signal for the computers.
Keeping water out • The air has to turn a sharp corner to reach the wire. • Water drops are heavy. They cannot turn, so they fly straight out of the back.
Keeping ice, sun and slow air off • Heater: stops ice. As it warms it takes less current, so it looks after itself. • Bleed holes: suck away the slow, warm air along the wall. • Shield: the strut keeps the sun off the wire. • The heater’s own warming adds an error of less than 1 °C.
Standing on the ground • No wind on a hot apron: the probe just soaks up heat and reads too high. • So big probes have an aspirator: bleed air from the APU or an engine sucks outside air through it.

Why does the probe read warmer than the air outside?

SAT, TAT and ram rise

The real air • SAT: the true temperature of the air outside. • It is what you would feel floating along with the air. • A probe moving at 450 kt never feels that. It hits the air first.
Stopped air warms up • The probe stops the air. The air behind keeps coming and squeezes it. • Squeezed air gets warm. That warming is the ram rise.
Rubbing warms it too • Air rubbing along the metal also warms it: kinetic heating. • Simple thermometer on the windscreen: mostly rubbing. • TAT probe: mostly squeezing (adiabatic heating).
Three names, one idea • SAT: the real air. The coldest of the three. • TAT: the air stopped completely. The warmest. • RAT: what a real probe reads. Just under TAT. • Ram rise = TAT − SAT.
It is speed, not height • Twice the speed gives four times the rise. • The rise does not care how high you are. Only how fast.