Part-66 Module 15: Gas Turbine Engine

A gas turbine does the same four things a piston engine does: it takes air in, squeezes it, burns fuel in it and throws it out, but it does all four at the same moment, in four different places along one engine, continuously rather than in strokes. That idea has to land properly at topic 15.1, because everything later depends on it; 15.2 then turns it into numbers: gross and net thrust, ram drag, propulsive efficiency, specific fuel consumption, bypass ratio, and what happens to the thrust on offer when the day is hot, the airfield is high or the aeroplane is already moving fast. Both rows sit at level 2, so a formula and a graph are fair game.

The syllabus then walks the gas path from the inlet, a duct doing real work: it recovers ram pressure at speed and hands the compressor an even flow, and its lip is heated because ice shed from it goes straight down the engine. Compressors are the longest stop: axial and centrifugal types, stages and pressure ratio, why a compressor stalls and surges, and the variable inlet guide vanes and bleed valves that exist to stop it doing so while the engine accelerates. Combustion turns out to be a lesson in sharing out air, because only a fraction of what arrives actually burns, while the rest cools the flame tube and dilutes the gas down to something the turbine can survive. The turbine takes the energy back out through nozzle guide vanes and cooled blades, under the creep and temperature limits that govern its life. The exhaust then does the last of the work: it straightens the gas and, on a turbojet, speeds it out through the propelling nozzle.

The second half of the module is the engine’s plumbing and wiring, and this is where the questions get specific: a dry sump oil system with pressure, scavenge and breather subsystems, magnetic chip detectors and a filter that can bypass; a fuel system running from the LP pump through the HP pump and control unit out to the burners; an air system bleeding compressor air away to cool turbine discs, pressurise bearing chambers, seal, supply the aeroplane and hold ice off; a starter driven by air, a high energy ignition unit, and a start sequence you follow when the engine misbehaves; and the indication set of N1, N2, exhaust gas temperature, fuel flow, oil pressure and vibration, the only thing telling you what is happening inside a casing nobody can see into.

Topics 15.16 to 15.18 keep the same core and change what it drives. You get the turboprop with its reduction gearbox, the turboshaft with its free power turbine, and the auxiliary power unit, which runs on the ground and supplies bleed air and electrical power of its own. What is left after those is the engine as an installed article: the mounts, cowlings, firewalls and drains that hold it on the pylon, the detection loops and extinguishing bottles that deal with a fire, and the inhibiting, blanking and desiccant it needs when it comes off the wing and goes into storage. Sitting among them is 15.21, engine monitoring and ground operation. That is the one row in the module that asks for a conclusion rather than an account, and it is why the deepest thing this syllabus wants from you sits out here among the installation work rather than back in the cycle.

92 questions, 115 minutes: the second largest paper in the category B1.1 set, and they come off the whole syllabus, from what a gas turbine is at 15.1 through to how one is inhibited and blanked for storage at 15.22, with the EASA text marking none of it as background reading. Of the table, one row sits at level 1, one at level 3, and everything between them at level 2, so a paper this size over a table with almost no shallow ground in it is the whole character of the module.

To pass at 75% you need 69 right, and 23 can go wrong, an allowance that is wide by the standards of the set, but wide because the paper is long, not because the module is forgiving. Spread it across 22 topics and it stops looking like room to move: a thin film of tolerance over a very large surface, enough to absorb the odd misread anywhere on the engine, nowhere near enough to absorb a whole section of it.

Read this table for its order rather than for a comparison: topics 15.3 to 15.7 follow the air through the engine, and from 15.8 onwards the syllabus doubles back to the shaft and works outwards through the systems that serve it. There is little else to set side by side, because the section under the table covers everything the two authorities do differently here; only two titles are worded differently, one trivially and one seriously, and the serious one is 15.15.

In one row, and in that row completely. The useful comparison here is depth rather than question count: the two texts carry 22 topics apiece, numbered alike and ordered alike, and in our reading of the assimilated text, row for row, each asks a UK candidate for exactly what it asks an EASA one. The paper matches too, at 92 questions in 115 minutes and no letter after the module number, so Module 15 joins the nine agreeing papers on the strength of its level column, not only its arithmetic.

Topic 15.15 is the exception, and it is a different subject, not different wording. The EASA table calls it “Alternate turbine constructions” where the UK table calls it “Power augmentation systems”, and power augmentation is the older business of injecting water, or water mixed with methanol, and of afterburning: ways of getting more out of an engine than its ordinary cycle will give. Those are two subjects sharing one number, so if you revise the EASA title and then sit a UK paper you’ve revised the wrong topic, a mistake easy to make precisely because every other row lines up.

The other difference is a space: EASA writes “Power plant installation” at 15.19 where the UK writes “Powerplant installation”, one subject spelled two ways, and it changes nothing you have to know about mounts, cowlings or firewalls.

EASA puts 15.15 at level 1, the shallowest the syllabus goes, where knowing the main elements and being able to describe them simply is the whole requirement, and the UK text reads the same way to us. That caps what a wrong guess can cost you without making it free, and it is why this difference is a footnote rather than a reason to buy two sets of revision material.

The wider split between the two authorities is on the EASA versus UK CAA page: mutual recognition, which licence to go for, and what happened to the conversion route.

This page covers one of the 13 papers. The rest of the guide covers the route they sit inside: what the exam is, what else you must have done besides passing it, and how the two authorities differ.

Both authorities require the same thirteen subjects. Only the numbering and some of the titles differ, and the UK still uses the A suffix on four of them where EASA has dropped it. Each has a page of its own.