RETARDATION METHODS INTRODUCTION 1. For many years fixed wing aircraft relied purely on wheel brakes to retard their forward movement after landing. Over the years, advancements in design and the introduction of new materials has led to increased capabilities and carrying capacities, which has inevitably led to faster and heavier aircraft. Increased flying speeds have led in turn to higher landing speeds, which has increased the burden placed on the wheel brakes. 2. Without some form of additional assistance in retarding the aircraft, the wheel brakes would need to be applied in a heavy and sustained manner leading to rapid wear of the brakes and tyres, or runways would need to be extended to unacceptable lengths. To help reduce brake and tyre wear and to maintain runways at an acceptable length, additional retardation methods based on the principle of aerodynamic braking have been devised to supplement the wheel brakes. 3. Methods of emergency retardation have also been devised to arrest the forward motion of the aircraft on the ground in the event of failure of the wheel brake system or during an aborted take-off where normal braking methods would be inadequate. WHEEL BRAKES 4. Wheel brakes form the primary method of retarding the forward movement of most aircraft when on the ground and, in common with most braking systems, they rely on the principle of energy conversion for their operation. In this method, friction is used to convert the forward motion of the aircraft (kinetic energy) into heat, which is then dissipated, to the surrounding air. 5. Wheel brakes and their associated operating systems are covered in Section 3 Chapters 1 and 3 of this Volume. AERODYNAMIC BRAKING 6. The term aerodynamic braking means using the airflow to assist in retardation by selectively increasing the amount of drag produced by the aircraft. This increase in drag is normally applied, in conjunction with operation of the wheel brakes, immediately after landing or during an aborted take-off and is produced by one, or sometimes a combination of more than one, of the following methods: • Flying control surfaces. • Thrust reversal. • Brake parachute. • Angle of Attack. Flying control surfaces 7. The flying control surfaces which can be used to help retard the aircraft through aerodynamic braking are: • Airbrakes. • Elevators • Flaps. • Tailerons • Spoilers. • Foreplanes 8. As the airbrakes, flaps or spoilers are moved out into the airflow they cause an increase in drag. The further into the airflow they protrude, the greater the drag produced and, just as in flight, any increase in drag slows down the forward progress of the aircraft. It should be noted that only the airbrake is specially designed to slow the aircraft; for the other controls this is only a secondary function. The flying control surfaces which provide aerodynamic braking are shown at Fig 1. Thrust reversal 9. Thrust reversal is used to assist in aircraft retardation by using engine power as a deceleration force. 10. Propeller driven aircraft. On propeller driven aircraft, thrust reversal is achieved by reversing the pitch of the propeller blades so that the thrust generated by the propeller is directed forwards instead of rearwards, the degree of braking assistance being controlled by use of the engine throttle lever(s). 11. Turbo-jet aircraft. Thrust reversal on turbo-jet aircraft is achieved by changing the direction of the exhaust gas stream through more than 90 degrees, but less than 180 degrees. One method of deflecting the exhaust gas stream is shown at Fig 2. During flight the bucket-type doors are held in the open position as shown in Fig 2A, allowing rearward passage of the exhaust gas stream. When braking assistance is required, the doors are moved into the gas stream by hydraulic or pneumatic jacks as shown in Fig 2B, uncovering apertures through which the gas is deflected in a slightly forward direction to give 'thrust reversal'. Brake parachute 12. Brake parachutes are normally to be found on fast jet aircraft and produce aerodynamic drag to assist the wheel brakes in retarding the aircraft during its landing run. A typical brake parachute installation consists of the parachute assembly in housing with associated release/ jettison controls and mechanisms (Fig 3). 13. The parachute assembly. The parachute assembly consists of the main parachute, the auxiliary (or drogue) parachute and the streamer cable. A ring on the free end of the streamer cable connects the parachute assembly to the airframe through a release unit. The drogue may be a small, spring-loaded version of the main parachute or connected to a rigid conical cap, which forms the tip of the a/c tail cone during flight. The main parachute and streamer cable are packed into a canvas bag or light alloy canister (hopper), whilst the drogue may be packed with the main parachute and streamer cable or may be located separately. The main parachute canopy is made of vented nylon panels or ribbons, connected to the streamer cable by nylon rigging lines and a connector block. 14. The parachute housing. A typical brake parachute housing is located as near as possible to the end of the rear fuselage. The housing comprises of a compartment designed to accommodate the parachute assembly or hopper with a spring loaded door or detachable cap, which closes off the opening to the compartment. 15. The release and jettison controls and mechanisms. Control of the brake parachute is normally effected mechanically by a Teleflex or Bowden control cable system, but it may be achieved electrically or, on some aircraft, by a combination of both methods. Cockpit selection for both the release and jettison of the brake parachute is usually made from a single control, except for combination systems where the parachute is released mechanically by a lever or handle and a separate switch is used to operate an electrical jettison unit. The cockpit control is normally guarded to prevent inadvertent operation. 16. Principles of operation When 'release' or 'stream' is selected at the cockpit control, the door or end cap of the compartment housing the brake parachute opens to release the drogue into the airstream as shown in Fig 4A. 17. The drag of the drogue parachute withdraws the main parachute from its pack along with a streamer cable, as the cable is pulled taut the main parachute develops (opens) to provide the braking drag as shown in Fig 4B. NOTES (1) On brake parachute installations fitted with a door, the door is usually designed to hinge open under the influence of a damped spring when the parachute release control is operated, allowing the spring loaded drogue parachute to deploy and withdraw the main parachute. (2) On those installations where the parachute housing is closed off by a cap, the cap is connected to the drogue chute by nylon cords. When the parachute release control is operated on this type of installation, the cap is completely released from the airframe structure and, as it is carried away by the airflow, it withdraws the drogue chute which withdraws the main parachute. 18. Once the aircraft has slowed to a speed where the brake parachute is no longer effective, it is jettisoned onto the runway or taxiway to be collected by a brake parachute recovery team. Angle of attack. 19. On some aircraft types, during landing, the pilot of the aircraft will hold the nose of the aircraft high off the runway, rolling along on his main wheels only. This high angle of attack produces a lot of aerodynamic drag, which assists the retardation of the aircraft. EMERGENCY RETARDATION 20. Large aircraft normally have relatively low landing speeds and can be brought to a controlled standstill in an emergency either by aerodynamic braking alone or, on some airfields, by an area at the end of the runway referred to as a 'run off area'. This area is either a stretch of rough ground designed to absorb the forward momentum of the aircraft, or a shallow pit filled with pebbles, both types being designed to slow the aircraft down on entry without causing significant damage. 21. Small, fast jet aircraft present more of a problem during emergencies on landing due to their often high landing speeds and the use of shorter runways. The undercarriage of small aircraft also tend to be less robust than those of large aircraft and their engine air intakes are normally quite close to the ground, two factors which make the use of 'run off areas' unsuitable due to the high risks of seriously damaging the undercarriage and of ingesting debris into the engine air intake(s). To overcome these problems, many small aircraft are fitted with Arrestor hooks, which work in conjunction with the Rotary Hydraulic Arrestor Gear (RHAG) installed on the runway. Many airfields from which fast jet aircraft operate are also equipped with crash barriers located at the ends of the runway. NOTE As a last resort some aircraft utilise emergency undercarriage (u/c) up selection to enable the underside of the a/c to be used as a retard medium. The arrestor hook 22. The arrestor hook was adopted from the Royal Navy, who used it as the primary method of retardation when landing aircraft on aircraft carriers. The arrestor hook is a strong metal arm with a hook on the end. The arm is attached to a strong point on the rear of the fuselage and is lowered to engage with the RHAG arrestor wire stretched across the runway. Control of an arrestor hook system may be mechanical, electrical or electro-hydraulic. 23. Mechanically operated arrestor hooks. In these installations the arrestor hook is controlled from the cockpit by a handle or lever. The cockpit control is connected by a Teleflex or an open control cable system, to a release unit which normally holds the hook in the retracted position. When the cockpit control is pulled, the release unit is tripped and the hook is forced rapidly downwards, either by a spring jack (Fig 5A) or by the inherent spring of the hook blade (Fig 5B). The spring jack or a torque tube acting at the arm pivot stops the hook from bouncing as it is trailed along the ground after release. 24. Electrically operated arrestor hooks. The hook is held in the up position by the jaws of the release unit. When a hook down selection is made the solenoid of the release unit is energised which opens the jaws allowing the hook to lower. The jaws are locked around the roller on the upper surface of the hook by the roller operating the jaws when the hook is manually moved to the up position. 25. Indication. All types of arrestor hook installation usually have a cockpit indicator light, which illuminates when the arrestor hook is released. The light is energised by a micro-switch on the release unit and also serves as a warning light to indicate that the hook is not securely locked up. On some aircraft there is also a mechanical indicator on the release unit which normally lies flush with the aircraft structure but protrudes when the release unit is unlocked. 26. Raising the arrestor hook. On aircraft fitted with mechanically or electrically operated hooks, the hook usually has to be raised manually by the ground crew using a lifting rig or mechanical hoist. This is done after the cockpit control lever has been returned to the 'hook up' position to reset the release unit so that it is ready to receive and lock the hook in the retracted position. NOTE On installations where the arrestor hook has to be raised manually, it is vital, for safety reasons, to ensure that the hook is securely 'locked up' before removing the lifting force. Bulb filaments can become defective therefore it would be wrong to rely on the cockpit indicator light alone. A visual check of the up-lock must always be made using if necessary, an extending inspection mirror and a torch. 27. Safety devices. To prevent the accidental release of the arrestor hook, a safety device is fitted either on the spring jack, to stop the jack from extending, or as a safety bar fitted to the aircraft structure to hold the hook in the up position. This safety device will be fitted at all times whilst the aircraft is on the ground, only to be removed before flight or for testing. A secondary safety pin may also be fitted to the cockpit control to prevent it from being moved. A warning pennant is attached to each of the safety devices. Rotary Hydraulic Arrestor Gear (RHAG) 28. The RHAG is designed to arrest aircraft fitted with arrestor hooks after an aborted take off or an emergency landing with a wheel brake or brake parachute failure. There are two RHAG assemblies, normally positioned approximately 1300 feet (400 metres) from each end of the runway, both of which may be engaged from either landing direction. 29. A typical RHAG installation (Fig 6) comprises of a cable suspended across the runway a little above the surface, and two energy-absorbing units, one located at each side of the runway. As shown, each end of the cable is connected, via a heavy nylon tape, to one of the energy absorbing units which is mounted on a concrete block. The tape is guided by a runway sheaf mounted on a second concrete block. Each energy- absorbing unit consists of a tape drum connected to a vaned rotor operating between two sets of stationary vanes in an oil filled chamber. 30. Normally, the cable is over-run by an aircraft, but when its arrestor hook has been lowered and engages the cable, the pull on the tape causes the tape drums and rotors to revolve. The resulting oil turbulence created provides the retarding force by converting the kinetic energy of the aircraft into heat. 31. An electrically powered retrieve system is fitted close to each energy absorption unit. This is used to wind the tape back onto the drum and re-tension the cable after it has been disengaged from the aircraft's arrestor hook. The crash barrier 35. The crash barrier (Fig 7) is designed to stop a small or medium sized turbo-jet aircraft which, because of some emergency, is about to overrun the end of the runway. The crash barrier is never used to stop a propeller driven aircraft. The barrier is sited at the end of the runway and takes the form of a fence made up of vertical nylon ropes attached to upper and lower steel cables. 36. The upper cable is spliced into the lower cable at each end of the barrier and the ends of the extended lower cable are taken, via shock absorber cables, to two rotary brake units. At each end of the barrier a suspension cable is attached to the upper steel cable by clamps and a shear pin. The suspension cables pass over hinged stanchions which are normally lowered so that the barrier lies on the ground. The stanchions are raised electrically by remote control to bring the barrier into a vertical position as shown when there is a possibility that it will be required. 37. When the barrier is engaged, its lower cable is overrun by the aircraft's wheels and its upper cable passes over the cockpit canopy. The nylon barrier ropes then fold over the aircraft's wings, pulling the upper barrier cable downwards causing the pins attaching the suspension cables to shear. The continuing pull on the barrier, transmitted via the shock absorber cables, pulls on brake cables wound on the brake drums of the rotary brake units, thus causing the drums to rotate. Rotation of the drums releases a compressed air supply to apply the brakes and effect a steady retardation of the aircraft. Section 1 Chapter 1 RAF PTC CN 3787 1-1-6 06-528a/01/B50 06-528a/01/B50 1-1-7