Jet Propulsion/Engine ratings

Engines are certified to deliver standard thrusts depending upon flight conditions. Thrust is typically measured in kN or lbs. A 'rating' is a predefined power setting that the pilot can select which may be appropriate for particular flight conditions. Rating terminology differs between civil and military aircraft, reflecting the different requirements of these types of aviation.

Civil Aircraft Ratings

The following ratings are typical of commercial airliners. The aircraft/engine manufacturer will have to declare two principal ratings to the certifying authorities, since these define the safe limits of operation of the engine/aircraft - these are the Maximum Take-Off (MTO) rating, and the Maximum Continuous Thrust (MCT or MCN) rating.

Maximum Takeoff thrust (MTO)

This is the maximum thrust that the engine can deliver for 5 minutes in the take-off envelope of the aircraft. Peak thrust is usually achieved when the engine is static, however the most demanding condition for a modern turbofan engine is end-of-runway or lift-off conditions, typically at about 0.25Mn. This condition usually generates the highest stresses and temperatures in the engine, hence use of this rating is only permitted for up to 5 minutes of operation.

It is used, as the name suggests, for take-off when the aircraft is at its heaviest and has to be accelerated to take-off speed in a finite runway distance. The higher the thrust available from the engine, the shorter the runway can be, or the greater the aircraft payload can be. This affects which airports an aircraft can be operated from, and the economics of operation. As an alternative to payload, a higher thrust rating allows more fuel load to be carried into the air, so extending range of operation. These trade-offs between available thrust, runway length, aircraft weight and range may need to be assessed for each flight, and is part of a commercial pilot's preparation prior to take-off. An aircraft may take-off with less than maximum take-off thrust to reduce wear on the engine and extend its life. This is usually termed a 'reduced thrust' take-off, and is used to reduce engine maintenance costs.

It is a condition of certification that an aircraft should be able to take-off if one engine fails at the most critical point in the take-off run, when it is going too fast to be able to come to a safe stop in the remaining runway. In the case of twin engine aircraft, they have to be capable of taking off on one engine, so that in normal operation 'de-rate' is usually applied as an excess of thrust is available.

If an engine exceeds its 'redline' speeds or temperatures when running at MTO thrust, it is no longer considered airworthy.

Sometimes referred to as 'TOGA' thrust, short for take-off/go-around.

Maximum Continuous thrust (MCT)

Outside the MTO flight envelope, the MCT rating defines the maximum thrust that can be demanded by the pilot from the engine. As such, it has particular significance with respect to engine failure in flight, as the aircraft will have to proceed to its destination or nearest diversion airport at max continuous thrust. If the engine cannot achieve this thrust level whilst staying within its operating limits for engine speed and temperature, (the 'amber line'), it is no longer considered airworthy.

Maximum Climb thrust (MCL)

This is the thrust rating the manufacturer recommends be used during the climb phase of a typical flight. It may be the same as max continuous thrust, and usually is for a three or four engined aircraft. The top of the climb phase is typically the most challenging condition for a turbofan engine outside the take-off regime, and is a critical design requirement. De-rate can be applied to MCL thrust to extend engine life, but at the cost of a slower time to climb and slightly increased trip fuel consumption.

Maximum Cruise thrust (MCR)

Sometimes defined, but not a particularly useful rating since in cruise the pilot/autopilot will use the thrust required to maintain constant altitude and air speed to meet with air traffic control requirements.

Flight Idle

The idle rating is the minimum thrust that can be used whilst the aircraft is in flight. It is largely defined by the requirement to keep the engine running, possibly supplying secondary services to the aircraft such as hydraulic and electrical power, and, especially at high altitude, to supply passenger air at a minimum pressure. The flight idle rating is important in that the lower it is, the quicker the aircraft can descend (without going into a dive). It is often determined by stability considerations such as flutter and surge margin.

High or Approach idle

In the final phases of approach to landing it is important to be able to provide rapid response to throttle movements. This may require the engine to be running at a higher speed than flight idle to be able to provide rapid acceleration if required. There may be a maximum response time requirement to achieve 'TOGA' thrust if a landing is aborted.

Ground Idle

Used for maneuvering on the ground. Typically defined by the need to keep the engine running and supplying power and services to the aircraft. Generally, the lower this value the better, since brake wear is a significant factor in aircraft running/maintenance costs.

Example ratings

The figure below shows the typical behavior of a modern turbofan. The orange curves show maximum cruise thrust at altitude. The take-off thrust is about 25% higher than the cruise thrust at sea level since it is permitted for short durations only.

Military Ratings

Combat aircraft have very different requirements to civil aircraft, and different rating terminology is used, especially for aircraft using reheat or afterburning for thrust augmentation.

Military Thrust

Typically used to define the maximum available thrust without use of reheat. Sometime referred to as max 'dry' thrust.