The module closes with the gas turbine's report card (how its thrust and fuel consumption move with altitude, speed and temperature, and the thrust-management ideas built on those curves), and then with the smallest gas turbine on board: the APU (auxiliary power unit) in the tail.
Three questions from this lesson — one straightforward, one that applies it, one that pushes a little further. Nothing is recorded, and you can draw a new set as often as you like.
For an APU (auxiliary power unit), maximum starting altitude and maximum operating altitude are:
Climbing at a constant TAS (true airspeed), the thrust and SFC (specific fuel consumption) of a turbojet or turbofan vary as follows:
A flat-rated engine (flat-rating temperature ISA+15, fifteen degrees above the standard day) makes a full-rated take-off on a day with an OAT (outside air temperature) well above ISA+15. Compared with a take-off at exactly ISA+15, the engine delivers:
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Climbing at constant TAS (true airspeed), the engine breathes ever thinner air: mass flow falls with density, and thrust falls steadily with altitude. The SFC (specific fuel consumption), fuel flow per unit of thrust, improves slightly on the way up, because the colder air raises the cycle's efficiency, and stops improving at the tropopause, where the temperature stops falling. This pairing is the whole case for cruising high: less thrust is available, but each newton of it is bought with less fuel, and the aeroplane covers more distance on every kilogram of it.
Accelerating at constant altitude, two effects fight: the jet's advantage over the intake air (the momentum difference that is thrust) shrinks as the aeroplane catches its own exhaust up, pulling thrust down, while ram compression at the intake raises the mass flow, pushing it back up. The result is the shallow sag and recovery of the right-hand chart: thrust falls gently at first, then recovers at high TAS as the ram effect wins — the shallow sag that the first lesson's approximately-constant description smooths over. SFC rises with TAS throughout: the faster the aeroplane, the more fuel each newton of thrust costs.
A flat-rated engine is one whose take-off thrust is held constant over a range of OAT (outside air temperature) — flat up to a stated temperature, often around ISA+15, fifteen degrees above the standard day. The engine could give more on a cold morning, but the rating is capped below its thermal capability, and the control system simply feeds enough fuel to hold the certified figure. As the day warms, holding constant thrust in thinner, hotter air costs more: TIT (turbine inlet temperature) and rpm rise with OAT along the flat part. At the flat-rating temperature the TIT reaches its limit, and from there the priorities swap: TIT is held at the limit and take-off thrust falls as OAT rises further, the rpm easing down with it. One curve is bought with the other.
The thrust-setting parameter of the previous lesson gets its definition here: EPR (engine pressure ratio) is the ratio of the pressure at the turbine outlet (in the jet pipe) to the pressure at the compressor inlet — the engine's output pressure compared with what it was given, a direct measure of how hard the gas generator is working and therefore of thrust.
Most take-offs do not need the full rating, and two schemes spend the surplus on engine life:
Reduced (flexible) thrust — uses the flat-rated curve backwards: the crew tells the engine the day is hotter than it is (the assumed temperature), and the engine obligingly gives the lower thrust it would give on that hotter day; full thrust remains available at a push of the levers.
Derated thrust — selects a lower certified rating outright, with its own limits, as if a smaller engine were fitted.
The advantages over a full-rated take-off: lower TIT and rpm every departure, so hot-section life stretches, deterioration slows, and maintenance costs fall (across a fleet, enormously). The disadvantages: more runway is used and the initial climb is shallower, so the reduction is only allowed where the calculated performance still clears every requirement, and flexible thrust is forbidden where the calculation cannot be trusted — contaminated runways above all.
Bleed air is a performance tax: air extracted from the compressor for the cabin or anti-ice has been paid for (compressed) but never reaches the turbine to give its work back. For the same lever position, thrust falls; to restore it, rpm, EGT (exhaust gas temperature) and fuel flow all rise, and the SFC worsens — which is why take-off performance tables carry separate columns for bleeds on and off, and why packs are sometimes switched off for the take-off itself.
The APU is a small gas turbine engine, usually in the tailcone, that produces no thrust. Its tasks: driving a generator for electrical power and supplying bleed air for air conditioning and for main-engine starting, making the aircraft independent of ground carts — and serving as a backup source of electrics and air in flight.
The two inlet types — a flush inlet, a door opening into the fuselage surface, cheap in drag but poor at catching air in flight; and a scoop inlet standing into the airstream, better fed at speed at the cost of drag.
Maximum starting altitude — the highest altitude at which the APU may be started; maximum operating altitude, the highest at which it may run. The second is higher: a running APU keeps running in air too thin for a reliable start.
Control and monitoring — a start/stop switch and, typically, rpm as a percentage, EGT, oil indications and fault lights; on modern types a page on the ECAM (electronic centralised aircraft monitoring) or EICAS (engine indication and crew alerting system) displays.
Automatic shutdown protection is the APU's defining habit: nobody sits watching its gauges, so it watches them itself and shuts down without asking when they go wrong: fire, overspeed, low oil pressure, overtemperature. A main engine with a sick oil pressure presents the problem to the crew and lets them weigh it; the unattended APU simply protects itself.
The four curve questions (altitude, TAS, flat rating, bleed) are drawn straight from the two figures; the flex-versus-derate distinction and the APU's two altitudes are the classic word-trap pairs. The overview, following the learning objectives:
Describe thrust and SFC variation with altitude at constant TAS, and with TAS at constant altitude
Explain the flat-rated engine: take-off thrust, TIT and rpm against OAT, and define EPR
Explain reduced (flexible) and derated take-off thrust with their advantages and disadvantages, and the effects of bleed air on rpm, EGT, thrust and SFC
State the APU's nature and tasks, the two inlet types, the two maximum altitudes, the monitoring instruments and the automatic shutdown protection