Turbines and Exhaust
Domain 5 - Turbines and Exhaust
The exam tests turbine stage design, cooling, clearance control and exhaust hardware.
Core rules
- Impulse stages take the whole pressure drop across the nozzle guide vanes; reaction stages split it between NGV and rotor. Aircraft turbines are usually about 50% reaction to keep relative Mach numbers and losses low.
- Cooling: HP nozzle and blade internal passages, film cooling holes and thermal barrier coatings protect metal well below gas temperature. Cooling air is compressor bleed and costs cycle efficiency; blocked cooling holes are a life-limiting defect.
- Tip clearance is set cold and grows with centrifugal and thermal growth. Active clearance control cools the case to close the gap at cruise and improve specific fuel consumption.
- Creep elongation, hot corrosion (sulphidation) and thermal fatigue cracking at leading edges and shrouds are the classic distress modes.
- Exhaust: convergent nozzle, exhaust cone, struts and turbine rear frame; thrust reversers of the target, cascade or bucket type; noise-suppressing chevrons. EGT/TGT is measured downstream of the last turbine stage and is the crew's main limit.
Worked example
An engine has 25 °C EGT margin at takeoff; after 3,000 cycles the margin is 8 °C. Borescope shows blade tip rub and opened tip clearance. The clearance leak bypasses work extraction, so the turbine needs a higher temperature to produce the same power, which consumes margin. The fix is a hot-section clearance check, not a fuel trim.
Traps
- Calling T4 the EGT; EGT is measured after the LP turbine and is hundreds of degrees lower.
- Assuming any visible crack is rejectable; blades are judged against manual limits.
- Treating cracks in non-structural exhaust liners as automatically airworthy, or automatically unairworthy.