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AP4ATCO - Jet Engine

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Gain an understanding of:

  • Jet engine structure, operation principles and basic terms.
  • Advantages and disadvantages in terms of traffic flow and ATC.
  • Malfunctions and their handling.


Gas Turbine Engine

A Jet Engine is a reaction engine - that is, an engine which provides propulsion or thrust by expelling a reaction mass - and works in accordance with Newton's third law of motion: "For every action (force) there is an equal and opposite reaction (force)".

Most jet engines used in aviation are air breathing, axial flow, gas turbine engines. A gas turbine is a rotary engine that extracts energy from a flow of combustion gases. Ambient air is drawn into the engine intake where an axial or a centrifugal compressor (or both) increases both the pressure and temperature of the air before feeding it into the combustion chamber. In the combustion chamber, fuel is added to the hot, compressed air and ignited. Once ignition has occurred, it is self sustaining as the constant flow of air and fuel provide for continuous combustion. A high energy exhaust stream (reaction mass), produced by burning fuel/air mixture, leaves the combustion chamber passing through one or more turbines which serve to drive the compressor(s). The remaining exhaust gas is ejected through a nozzle providing thrust (force) to propel the aircraft forward.

A turbojet engine is most efficient when the speed of the aircraft that it propels approximates the speed of the exhaust gas. In many cases, aircraft are designed for speeds much slower than that of typical jet exhaust so the engine turbines are also used to drive other components such as a fan, propeller or other machinery. In this way, turboprop, turbofan and turboshaft engines are optimised for the speed and the type of the aircraft that they power.

Engines under development for very high speed applications eliminate the need for a powered compressor. In a "ram" engine such as a ramjet or a scramjet, air entering the engine is compressed due to the intake and compressor section geometry and the high forward speed of the aircraft. As a consequence, these types of engine do not require a compressor or the turbine to drive it but the engine cannot operate while the aircraft is stationary.

Types of Jet Engine

Turbo Jet: A turbojet engine is a jet engine which produces all of its thrust by ejecting a high energy gas stream from the engine exhaust nozzle. In contrast to a turbofan or bypass engine, 100% of the air entering the intake of a turbojet engine goes through the engine core.

Turbofan: A turbofan is a jet engine with a large fan in front of the compressor. It has been developed from the by-pass engine with a significant increase in the amount of air that passes by the engine (high ratio by-pass engine). The outer fan diameter is greater than the diameter of the rest of the engine and thus air that has been accelerated by the fan blades passes by the engine. The fan is driven by the free turbine at the rear of the engine (a part of the power from hot gases is tapped to drive the fan), but the additional thrust of the fan is significantly greater. That is why the turbofan engine is more powerful during take-off, in climb and in cruise. It has lower specific fuel consumption as well. Its additional thrust created by the fan is apparent especially during take-off. In addition, the envelope of colder air from the fan surrounds the hot expanding exhaust gases, allowing them to expand more slowly. This reduces noise levels significantly, without reducing thrust.

Turboshaft: A turboshaft engine is a variant of a jet engine that has been optimised to produce shaft power to drive machinery instead of producing thrust. Turboshaft engines are most commonly used in applications that require a small, but powerful, light weight engine, inclusive of helicopters and auxiliary power units.

Characteristics of a Jet Engine

Engine Control

A throttle lever, more often referred to as a thrust lever or power lever, is the means by which the pilot controls the amount of fuel provided to the engine with which it is associated. There is normally one throttle lever for each engine and, depending upon the flight deck or cockpit configuration, they may be installed on the centre console, side console, on the dash board or mounted on the aircraft ceiling. In some two pilot flight decks, each pilot station has its own set of throttle levers. In some older aircraft, the pilots shared one set of throttles and a second set was installed at the flight engineer station. In both of these cases, the levers are linked and moving one set of levers results in a similar movement of the other.

Dependant upon the installation, throttle levers may incorporate provisions for selecting reverse thrust, have a fuel cut-off position or have some means of preventing beta (ground) range selections whilst the aircraft is in flight.

Engine Pressure Ratio (EPR) is a means of measuring the amount of thrust being produced by a jet engine. As there is a finite limit on the amount of pressure that an engine is designed to produce, EPR can be used to provide feedback to the pilot as the thrust lever is moved or to the Full Authority Digital Engine Control (FADEC), when installed, to ensure that engine limitations are not exceeded. An alternate method of limiting engine thrust production is based on compressor/fan speed and is referred to as N1.

In an axial flow jet engine, N1 refers to the rotational speed of the low speed spool which consists of the fan, the low pressure compressor and the low pressure turbine which are connected by a concentric shaft.

On many jet engines, N1 is the primary indication of engine thrust as an alternative to Engine Pressure Ratio (EPR)

Reverse Thrust

Reverse thrust is thrust projected in the opposite direction to normal and is used to decelerate an aircraft after landing, in the event of a rejected take off or, in some limited cases, in flight.

On many aircraft types, reverse thrust capability is installed to augment wheel brakes in decelerating the aircraft. This feature can significantly increase deceleration rates and reduce landing distance or, in the event of a rejected take off, reduce stopping distance. On some aircraft, reverse thrust can be used to enable the aircraft to back up under its own power. On a limited number of aircraft types, such as the C17 Globemaster, reverse thrust can be utilised in flight to significantly increase descent rate without a corresponding increase in airspeed.

On a Jet Engine, reverse thrust can be generated by a target reverser or a cascade reverser installation. A target reverser is a reverse thrust system most typically installed on turbojet and low bypass ratio turbofan engines.

A target reverser is a hydraulically actuated system that uses bucket type doors to reverse the flow of the engine hot gas stream. Whilst in forward thrust, the bucket doors form the final tail pipe nozzle of the engine. When reverse thrust is selected by the pilot, actuators close the buckets over the hot gas stream deflecting it forward. A mechanical lock holds the buckets in their extended position to allow thrust to be increased without the associated risk of an uncommanded, asymmetric retraction of the buckets.

Fokker 70 with target reverser deployed

A cascade reverser is a reverse thrust system most typically installed on high bypass ratio turbofan engines.

A cascade reverser incorporates radially arranged openings near the aft edge of the fan cowl of a turbofan engine. Within each of the openings is mounted a cascade set of air flow turning vanes. A blocking door and its associated actuating system are positioned flush with the inner wall of the fan cowl adjacent to each opening. The outer surface of the cascade sets are covered by a "sleeve like" translating (or sliding) section of the cowl. When the reversers are activated, the actuating system causes the translating cowl to move aft uncovering the cascades. The linkage between the translating cowl and the blocking doors move the doors into the bypass air stream blocking its normal path and diverting it out through the cascades which redirect it forward to help slow the aircraft.

Boeing 777 with Cascade Reverser deployed

Engine Failure

Engine Fire: Fire in the air is one of the most hazardous situations that a flight crew can be faced with. A fire can lead to the catastrophic loss of that aircraft within a very short period of time. An engine fire is normally detected in a timely fashion and in most cases, contained satisfactorily by the aircraft fire detection and suppression systems. However, in certain circumstances (e.g. an explosive breakup of the turbine), the nature of the fire is such that onboard systems may not be able to contain the fire and it may spread to the wing and/or fuselage. Heat from such fire could cause deformation of wing surfaces, affect the aircraft systems, and ultimately compromise the structural integrity of the aircraft leading to loss of control. Where an engine fire has been successfully contained, there is still a risk that the fire may reignite and therefore it is still advisable for the crew to land the aircraft as soon as possible and allow fire crews and technical personnel to carry out an inspection of the engine.

Engine Flameout: The run down of a jet engine for reasons other than intentionally shutting off the fuel supply. Engines can flame out for a variety of reasons:

  • Fuel starvation or exhaustion
  • Compressor Stall
  • Ingestion of foreign objects such as volcanic ash, hail, ice,birds or an exceptionally large quantity of liquid water
  • Mechanical Failure

Impact of a loss of Power on Aircraft Performance

If an aircraft is at its cruise ceiling, where it is most efficient in terms of fuel consumption, then the loss of a power plant will mean an immediate descent because the aircraft will have insufficient power to maintain cruise altitude. However, the descent can be gradual...

The SKYbrary article Engine Failure: Guidance for Controllers provides details about the impact of an engine failure on crew workload and what the crew may require in terms of support from ATC.

Quiz Questions:

Q1: The turbines of a jet engine during normal operation are driven by

  1. the compressors
  2. electric power
  3. exhaust gas
  4. compressed air before the fuel is injected and burned





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