Module 15 Gas Turbine Engine common mistakes
Module 15 Gas Turbine Engine · Understand the traps and how to avoid them.
When a question gives compressor pressure ratio and asks for ideal thermal efficiency or work, candidates plug pressure ratio straight into the Brayton efficiency formula without converting to the temperature ratio, or they use the pressure ratio as if it were the temperature ratio.
The ideal Brayton relation is efficiency = 1 - 1/(PR)^((gamma-1)/gamma), and the temperature ratio across a stage or the cycle equals (PR)^((gamma-1)/gamma). Candidates memorise 'efficiency depends on pressure ratio' and then treat the two ratios as interchangeable, especially when the question supplies only one of them and expects the exponent step.
What to do instead
Write the exponent explicitly: T2/T1 = (P2/P1)^((gamma-1)/gamma), with gamma = 1.4 and (gamma-1)/gamma = 0.286 for air. Compute the temperature ratio first, then the efficiency. Practise with PR = 10, 20, 30 and check the answers land near 0.48, 0.58, 0.65. If a question gives temperatures, compute the ratio from them and never assume it equals the pressure ratio.
Candidates answer thrust or SFC questions using the wrong rating basis, mixing takeoff thrust with maximum continuous or maximum climb, or assuming SFC is constant across the whole operating envelope when comparing a high-bypass turbofan with a turbojet.
Engine performance questions often present a table-like scenario and ask which rating applies to a given flight phase or how SFC changes with bypass ratio. Candidates remember 'turbofans are more efficient' but cannot state that SFC falls as bypass ratio rises at a given thrust, and they confuse flat-rated takeoff thrust (constant below a break temperature) with the actual thermodynamic thrust the core produces.
What to do instead
Build a one-page rating map: takeoff/go-around (5 min), maximum continuous, maximum climb, cruise, idle, and note that takeoff thrust is flat-rated up to a break temperature and then decreases about 1 percent per degree above it. For SFC, remember it is thrust-specific fuel consumption (kg of fuel per N of thrust per hour or lb/lbf/hr), so higher bypass ratio at the same core gives lower SFC. Read the flight phase in the stem before choosing a rating.
When asked to identify a compressor stall or surge symptom or cause, candidates treat stall and surge as synonyms and pick whichever term appears first, or they blame surge on a single cause such as 'too much fuel' without linking it to the compressor map and the operating line.
Both are instabilities of the same compressor, and training material often mentions them together. Candidates miss the distinction: rotating stall is a local, often partial-span breakdown of flow that may be steady and less violent, while surge is a full-annulus, axial oscillation of flow that can reverse direction and produces loud bangs and EGT spikes. Causes include inlet distortion, too-high angle of attack, bleed valve malfunction, acceleration schedule errors, and damaged or dirty blades.
What to do instead
Memorise the distinction by the observable: stall gives rumble, reduced thrust, and possible mild vibration; surge gives a bang, flow reversal, and severe EGT rise. Then link causes to the compressor map: any event that moves the operating point above the surge line (rapid throttle increase, bleed extraction changes, inlet distortion) causes surge. For each answer option, ask whether it moves the operating point or damages the map, and reject options that describe turbine or combustor faults.
For combustion chamber questions, candidates choose the chamber type or fuel nozzle by name recognition rather than matching the described hardware. For example, they label an annular chamber as can-annular because it has multiple fuel nozzles, or they confuse a duplex nozzle with a simplex or air-blast nozzle.
Annular chambers use many fuel nozzles around a single annular liner, so 'many nozzles' is not a reliable cue for can-annular. Can-annular has separate cans joined by interconnectors, and can-type has individual cans with individual casings. Simplex nozzles have one fuel circuit and are prone to poor atomisation at low flow; duplex and air-blast nozzles handle wide flow ranges.
What to do instead
Learn three structural signatures: can (separate cans, each with its own casing and liner), can-annular (cans inside a common casing, linked by interconnectors), annular (one continuous liner and one common casing). For nozzles, match the flow range: simplex for narrow range, duplex (primary and secondary orifices) for wide range, air-blast for good atomisation with low fuel pressure. Ask what the stem describes structurally, not how many nozzles it mentions.
On turbine questions, candidates assume turbine blade cooling air comes from the compressor discharge and is then recovered, or they answer that turbine clearances should be as tight as possible in all conditions without accounting for thermal growth and rub risk.
Turbine cooling air is indeed compressor air, but it is usually extracted at intermediate compressor stages and sometimes at discharge, and it is not recovered into the cycle; it bypasses the combustor and is discharged into the gas path, which costs cycle efficiency. Clearance questions test the trade-off: tighter clearance improves efficiency but risks rub during thermal transients and casing distortion.
What to do instead
Trace the cooling air path: HP compressor bleed, external or internal passages, nozzle guide vanes and blades, then into the gas path. State the efficiency penalty of using compressor air for cooling. For clearances, remember active clearance control uses modulated cooling of the casing to shrink clearance at cruise and open it during transients; the correct answer usually balances efficiency against rub risk, not 'minimum possible'.
Candidates describe a thrust reverser as simply 'reversing the fan or exhaust flow' and cannot distinguish a cascade reverser from a target or bucket reverser, or they forget that reversers are prohibited below a certain speed and are not used for sustained braking.
Reverser questions often present a description of translating sleeves, blocker doors, and cascade vanes, and candidates pick the wrong type because they only remember 'reverses thrust'. They also miss operational limits: reversers are for deceleration on the ground, are ineffective or unsafe at low speed, and must not be deployed in flight on most aircraft.
What to do instead
Learn the mechanisms: cascade reverser uses translating sleeve, blocker doors, and cascade vanes to redirect fan air forward; target or bucket reverser uses clamshell buckets that swing into the exhaust. For operation, state that reverse thrust is used after touchdown, is most effective at higher ground speed, and is stowed before taxi. Reject options that describe in-flight use or sustained braking.
For bearing, seal, and lubrication questions, candidates mix up which bearing carries radial versus axial loads, or they answer that a labyrinth seal is a contact seal or that oil is used only for lubrication and not for cooling.
Rolling-element bearings are described by the load they take: ball bearings handle radial and some axial load, roller bearings handle primarily radial load, and thrust bearings handle axial load. Labyrinth seals are non-contact, clearance-based seals that rely on a tortuous path. Oil serves both lubrication and heat removal, and oil system questions often hinge on that dual role.
What to do instead
Make a three-column table: bearing type, load carried, typical location. Add a seal table: labyrinth (non-contact, clearance), carbon (contact, low leakage, limited temperature), and brush or finger seals. For oil, always state both functions and note that oil cooling removes heat from bearings and gears, so a low oil flow can cause overheating even when lubrication film seems adequate.
In fuel system and FADEC questions, candidates say the FADEC meters fuel directly or that the hydro-mechanical unit is redundant, instead of describing the FADEC as a computing and scheduling authority that commands a fuel metering unit, with the hydromechanical unit providing backup or limiting functions.
FADEC is an electronic control that computes required fuel flow from throttle position, engine parameters, and limits, then commands the fuel metering valve. Candidates conflate the computer with the actuator. They also forget that many systems retain a hydromechanical backup or overspeed and over-temperature protection independent of the electronic channel.
What to do instead
Describe the chain: throttle and sensors to FADEC (dual channel, with fault detection and reversion) to fuel metering unit or servo valve to fuel nozzles, with the hydromechanical unit providing backup scheduling and limits. For any FADEC question, identify which element computes and which meters. Remember the FADEC also controls variable stator vanes, bleed valves, and ignition on many engines.
On air systems and starting questions, candidates confuse bleed air used for anti-ice with bleed air used for cabin pressurisation or engine cooling, or they state that the starter motor cranks the engine to self-sustaining speed without mentioning ignition and fuel scheduling.
Bleed air serves several consumers: anti-ice (wing and engine inlet), cabin air, and sometimes cooling. Starting involves cranking to a minimum speed, igniting, introducing fuel on schedule, and accelerating to self-sustaining speed, then disengaging the starter. Candidates compress this into 'starter turns the engine and it starts', missing the sequence and the role of ignition and fuel scheduling.
What to do instead
List bleed air consumers separately: anti-ice, cabin, and engine cooling, and note that anti-ice bleed reduces available thrust. For starting, memorise the sequence: starter cranks, ignition on, fuel on at scheduled speed, light-off confirmed by EGT rise, acceleration to idle, starter disengaged. For each option, ask which phase it belongs to and whether it is a bleed, ignition, or fuel event.
For indication and monitoring questions, candidates treat EGT as a direct measure of turbine inlet temperature, or they assume a vibration spike always means a bearing failure rather than considering fan imbalance, blade damage, or sensor fault.
EGT is measured at the exhaust, downstream of the turbine, so it lags and is lower than turbine inlet temperature; it is a proxy used for limit and trend monitoring. Vibration can come from many sources, and a single spike may be a sensor or transient. Trend monitoring compares baseline data over time, so one reading is not a trend.
What to do instead
State that EGT is an exhaust-gas temperature used as an indirect limit for turbine temperature, with a lag and a different value from TIT. For vibration, list sources: fan or compressor imbalance, blade damage, bearing degradation, and sensor or wiring faults, and note that trend monitoring needs repeated data points against a baseline. Reject answers that equate EGT with TIT or that diagnose a single spike as a specific failure.