Basic Principal
The basic principal of a thrust reverser is to redirect the gas flow from a rearward direction (which provides normal forward thrust) to a forward direction, providing reverse thrust, i.e. using the engine power as a decelerating force.
Ideally, the gas stream should be directed in a completely forward direction; however, this is not possible (except with reverse pitch propellers) mainly due to aerodynamic reasons. A discharge angle of between 90 and approximately 45 degrees forward is common, resulting in a proportionally less effective reverse thrust than the thrust of the same engine in its normal direction; however, reverse thrust can be up to 50% of full forward thrust.
There are various types of thrust reverser, and each type can be operated by a variety of mechanisms. All designs invariably have locking mechanisms and safety features to prevent uncommanded deployment, an event which would normally have catastrophic consequences, however there are at least two exceptions to this which are discussed later.
This section concentrates on describing the types of thrust reversers rather than the detail of the systems required to operate them.; one page is dedicated to selection, sequencing  and safety features.
Effects
The effect of using thrust reversers is to reduce the landing roll out. In civilian aircraft this is primarily to save wear and tear on the brakes, and allow the aircraft to clear what can be busy runways and to achieve quick turn-rounds, i.e. keeping passenger delays to a minimum.
In military aircraft the above can apply, and also to give shorter landings for tactical reasons, i.e. the aircraft can land and unload supplies quickly minimizing risk in hostile environments.
Note 1: Civilian aircraft are certified to operate without thrust reversers.
Note 2: Some aircraft have the ability, and the authorisation to use the thrust reversers to reverse the aircraft out of a parking berth, but the following reservations need to be considered:-

• Some engine manufacturers are not in favour of this operating procedure, as the risk of re-ingesting hot exhaust gases can raise the internal temperatures of the engine affecting engine component lives.
• Some airports are not in favour of it as the forward jet blast can throw debris against the airport buildings.
• Aircraft manufacturers may also not be in favour of this procedure. One aircraft type, with rear mounted engines issued the following proviso -‘using the reversers to back out an aircraft can be used in extreme circumstances, but do not use the brakes’!

Thrust Reversers – Selection, Sequencing and Safety Features

General
The illustration below shows two types of throttle mechanism. They are titled Hydro Mechanical and FADEC (Full Authority Digital Electronic Control) controlled systems, but are not restricted to either control methods. There are as many designs as there are aircraft manufacturers, even when the same engine is installed in aircraft from different manufacturers.
Predominantly however, the type of mechanisms do more or less, fall under the control system types.
Selection - Hydro Mechanical Mechanism
In the illustration on the left, selecting reverse thrust is controlled by a mechanical cam, with a cam follower fixed to the throttle assembly. It can be seen that the throttle has to be in the forward idle position to select deployment via the piggy-back levers.
Lifting the piggy-back levers moves the main throttle slightly rearwards. This selects deployment of the reverser. Now the throttle cannot be pushed forward to accelerate the engine, this is achieved by pulling the throttle further rearwards.
However, this rearward action of the throttle is baulked (prevented) by a stop until the reverser has deployed. This stop is signalled to release either by a feedback cable mechanism or an electrical sensor indicating thrust reverser position and a solenoid to remove the stop.

Selection – FADEC System
With this type of system, selecting reverse thrust could be done at any time in the flight (not to be recommended as a failure in the system may allow deployment!). However, the engine EEC (Electronic Controller) should not normally allow deployment unless certain conditions are met, these can be:-

• Aircraft on ground and on some systems forward speed above a certain value signalled by the nose wheel mounted ‘weight-on-wheel’ (WOW) and ‘wheel spin up’ (WSU) sensors respectively.
• Engine RPM below a certain value, signalled by RPM sensors on the engine.
• On some systems a throttle interlock prevents the piggy-back lever being moved to increase engine power until the thrust reverser is at a certain position in the deployment cycle, sensed by position sensors on the reverser.

Safety
Mechanical locks (some systems feature Primary and Secondary Locks) are provided to hold the thrust reverser in the stowed position, and on some systems a lock to hold the deployed position. There are also isolator valves on the supply of the operating medium, i.e. either pneumatic or hydraulic pressure.

Thrust Reversers – Translating Cowl Type

Design
This type of thrust reverser was quite a common design in early passenger aircraft and in comparison to modern systems, and can be quite a complicated mechanism.
However, in simple terms the translating sleeve is mounted on multiple tracks, and is powered through a number of actuators either by hydraulic pressure or mechanical drive from an air driven motor, and mechanically synchronized ensure all actuators move at the same time and rate.
As the translating cowl moves backwards, it takes back blocker doors. which are pulled across the bypass duct by a door link, to close off the rearward flow of bypass air.
Simultaneously, as the translating cowl traverses rearwards, it exposes cascade assemblies. These are multi vane castings or fabrications which direct the bypass air to through the cowling in an almost forward direction, thereby providing the reverse thrust.
Early designs of this system also featured a thrust reverser on the hot core flow as well as the bypass air, this was commonly of the target door type thrust spoiler, see page 10 for a brief description.
As most of the thrust from a high bypass engine is from the fan, reversing the fan flow only is usually sufficient to provide the required reverse thrust. Additionally on engines with a common nozzle (where the hot and cold streams are mixed before passing through the propelling nozzle to atmosphere), removing the cold stream from the nozzle reduces the thrust from the hot stream.
C17 Deployment
This aircraft is extremely unusual in that it can deploy the thrust reversers in flight for a tactical descent from high altitude into a hostile zone, hereby minimising the threat of anti-aircraft fire etc in the area. The aircraft has to be immensely strong to withstand the forces of this type of operation.

Thrust Reversers – Pivoting Door Type

Design
This system is a relatively simple system with much less working parts than the afore mentioned translating cowl design.
The illustration below shows the system as fitted to the RAF Sentinel R17, a derivative of the Bombardier Global Express executive transport aircraft. It comprises of two doors on this system, but multi door systems are beginning to replace the translating cowl designs on high by-pass fan engines
Each door is actuated by its own hydraulic actuator, each of which incorporates an internal safety lock and the travel stops at the fully deployed position. Although the doors on this particular version are not synchronized, the EEC can recognise if the doors are not in time with each other, sensed  by door mounted position sensors.
Safety Features
Door Locks
As well as the primary lock, internal to the actuators, each door has a ‘Tertiary Lock’ (secondary lock), an electrical solenoid operated device which is energised to release upon EEC command.
Throttle Interlock
The thrust reverser is mechanically prevented from being selected unless the throttle levers are at the idle position, only then can the throttle levers be moved to the reverse select quadrant area.
Hydraulic Lock
The hydraulic pressure supply is inhibited by an Isolation Valve by the EEC.
Sequence
The full sequence is controlled by the engine electronic control unit (EEC) after pilot selection via the throttle levers. This unit ensure that the thrust reverser does not actuate unless it is safe to do so, and then prevents engine acceleration in reverse unless both thrust reverser doors have deployed past a certain position.
Deployed Locks
On this system there aren’t any, the thrust reverser is kept in the deployed position by the jet blast pushing against the doors and the door mounted ‘Kicker Plate’. In addition, at the deployed state, the hydraulic pressure is also switched off.
Should the doors become unlocked whilst the engine is running, the jet blast hitting the rear inner end of the doors, will keep the doors in the near stowed position. When the commanded deployment takes the doors past a certain point, (around 35-40% of travel) the jet blast can be greater on the front end of the door compared to the rear end, at this point the doors will be pushed open by the jet blast. Because of this, there is no need for deployed locks or hydraulic pressure to hold the deployed position, it can be achieved simply by using the jet blast.

Thrust Reversers – Clam Shell Door Type

Design
This is a relatively old design featured on the VC10 aircraft Rolls Royce Conway Engines. All four VC10 engines have the thrust reverser fitted, but on the RAF aircraft, the two inboard engine thrust reversers have been inhibited because the jet blast from all four engines whilst in reverse created too much buffeting on the fin and elevator surfaces.
The design name comes from the shape of the doors, although only the cross section is shown below, they are actually shaped like the proverbial clam shell.
The cascades are mounted top and bottom and can be seen from the outside of the engine casing, the doors being mounted internally in the jet pipe.
Safety
Stowed position locks will prevent the doors from deploying uncommanded, and locks would be required in the deployed position, to prevent the jet blast forcing the doors back towards the stowed position.

Thrust Reversers – Target Door Type

Design
The target doors are mounted on the outside surface of the engine nacelle, and when deployed lift outwards and rearwards to the rear of the engine.
In the deployed state, the target doors can be designed to redirect the jet blast either in a slightly forward or in an up or down direction. The latter, commonly fitted to large fan engines, are not strictly thrust reversers, they are more of a combined forward thrust spoiler and drag feature.
The system shown below is the type fitted to the Tornado combat aircraft. This system does redirect the jet thrust slightly forward, witness the exhaust staining on the aircraft fuselage and tail.
Safety
Safety locks need to be fitted to hold the target doors in the stowed and the deployed positions. Failure of the stowed locks would be catastrophic in flight.
NASA Shuttle Flight Crew Training
One version of the Rolls Royce Spey powered Gulfstream Business Jet is flown with the thrust reverser (Gulfstream manufactured) deployed and used as a shuttle simulator to train shuttle pilots in the in-atmosphere glide and landing of the shuttle.

Thrust Reversers – Other Methods

Reverse pitch Propellers
Aircraft braking distances can be reduced by reversing the pitch of the propeller blades, sending the thrust in a totally forward direction. On turbo-props, the jet stream from the engine continues to give a forward thrust element; but it is only a small amount compared to the thrust from the prop.
Drogue Parachutes
Parachutes can be deployed which increase the drag and increase the deceleration of the aircraft. They are only a one shot operation, the drogue is dropped and then recovered for repacking before the next use, the aircraft can still attain a fast turn around by fitting another pre packed drogue.