LANDING GEAR, TIRES, WHEELS, BRAKES LANDING GEAR LANDING GEAR CONFIGURATIONS Two main types: Conventional, and Tricycle TRICYCLE Has nose wheel, which may be steerable Main gear, on either side Example: Cessna Keeps aircraft level during take-off and landing The most important advantage is its ease of ground handling. CONVENTIONAL Two main wheels One tail dragger wheel Reduced drag in the air Reduced landing gear weight Requires more skill in ground taxiing The most important advantage is the ability to operate the aircraft over rough terrain. CLASSIFICATION OF LANDING GEAR MAIN LANDING GEAR Cushions landing impact Heavily stressed area Main Landing Gear consists of the main weight-bearing structure Auxiliary landing gear includes tail wheels, skids, nose wheels, etc. NON-ABSORBING LANDING GEAR Includes Rigid landing gear, Shock-cord landing gear, Spring landing gear Rigid: helicopters, sailplanes. No flexing other than the structure. Shock cord system: uses “Bungee” cords Spring type uses spring steel (some Cessna’s) SHOCK-ABSORBING LANDING GEAR Dissipates landing energies by forcing fluid through a restriction This fluid generates heat, dissipated into the atmosphere Two types: Spring Oleo, and Air-Oil Oleo Spring Oleo is history by now Air Oleos are all very similar: a needle valve restricts fluid flow Air in the oleo holds the weight of the a/c on the ground Air Oleos present in both retractable and fixed gears 1
FIXED GEAR Non retractable, usually bolted on to the structure Often uses fairings or wheel pants Cessna 152 Advantages: Lighter weight. Less complex and Least costly RETRACTABLE GEAR Designed to eliminate drag (the greatest advantage) Can be either fully or partially retractable Direction of retraction depends on airframe model Methods of retraction: hydraulic, electric, mechanical, pneumatic Critical area of aircraft maintenance for safety reasons HULLS AND FLOATS Can be single float, or multiple Definition may include floating hulls (ex. “Lake” aircraft) Floating hulls may only require wing tip floats Skis used for snow and ice (wood, metal, composites) Skis may use shock cord to assist angle of ski attack Skis are mounted on the same strut as tires LANDING GEAR COMPONENTS Exact definitions of some components will vary The Oleo strut is the widely used form of shock absorption on aircraft landing gear. TRUNNIONS Portion of the landing which attaches to the airframe Supported at the ends by bearings Landing gear traditionally extends from the center STRUTS Vertical member, contains the shock absorbing mechanism Top of the strut mounts onto the trunnion Strut forms the cylinder for the oleo (“outer” cylinder) Piston is the moving portion (aka piston rod, tube or inner cylinder) Oil is forced from the lower portion of the strut to the upper Oil flow is restricted or varied according to a metering pin Final weight of a/c rests on air in the top of the strut Snubbers are used to prevent a sudden dropping of gear on takeoff Metering pin controls the flow of fluid between the chambers. The shock of landing is absorbed by the fluid being forced through a metered orifice. The metering pin gradually reduces the size of the orifice as the shock strut extends, which avoids a rapid extension after the initial shock of landing and related bounce.
Chevron seals are used in shock struts to prevent the oil from escaping On nose wheel struts, a cam is built into the strut for the purpose of straightening the nose wheel before retraction. Filling a shock strut: “exercise” the strut in order to seat the seals, and remove air bubbles from the fluid. Most shock strut oil levels are checked by releasing the air, bottoming the strut, and checking to see if the oil is at the level of the filler plug. Information about shock struts: see: Manufacturer’s maintenance manual Information decal located on the strut Mfr’s overhaul manual TORQUE LINKS Also called scissors assembly Two A-frame members Connects and aligns upper and lower cylinders Connects the strut cylinder to the piston Restricts extension of piston during retraction Correctly aligns axle to the strut TRUCKS Located at the bottom of the strut piston Axles are mounted on the truck Trucks can tilt fore or aft to allow for a/c attitude changes DRAG LINKS Stabilizes landing gear longitudiannly May be hinged to allow retraction Also called a drag strut SIDE BRACE LINKS Stabilize gear laterally May be hinged to allow retraction Can be called a side strut OVERCENTER LINKS (AKA DOWNLOCK MECHANISM) Use to apply pressure to the center pivot joint in a drag or side brace link Overcenter link is hydraulically retracted to allow gear retraction Also called a downlock, and/or a jury strut SWIVEL GLANDS Flexible joint with internal passages Route hydraulic fluid to the wheel brakes Used where space limitation eliminate flex hoses 3
SHIMMY DAMPERS Hydraulic snubbing unit Reduces tendency of nose wheels to oscillate PISTON TYPE DAMPERS Piston and rod filled with hydraulic fluid Piston has an orifice restricting speed of travel Slow movement has no restriction Large shimmy dampers incorporate temperature compensation VANE TYPE DAMPERS Employ stationary vanes and rotating vanes Small passages restrict fluid movement Central shaft rotation is restricted from moving quickly DAMPER INSPECTIONS Check for leakage & effectiveness of operation Check mounting bolts and hardware Most dampers are fairly reliable STEERING SYSTEMS Some a/c have free castering nose wheels; most have steerable. MECHANICAL STEERING SYSTEMS Uses foot power to steer the aircraft – no assistance Some types will disengage when the gear is retracted Some types have an automatic centering device when weight is off the a/c TAIL WHEEL Conventional gear use the tail wheel to steer May be a castering type with no steering capabilities (rudder steers) May be lockable, for parking purposes POWER STEERING SYSTEMS Used where large amounts of force are required to steer Controlled by pilots rudder pedals, OR By a steering wheel, OR By a combination of both Most will require a towing bypass valve which allows Ground crews to to the a/c without damaging the system RETRACTION SYSTEMS Purpose: reduce drag, or adapt a/c for landing on different surfaces
(consider retractable wheels on float systems) MECHANICAL SYSTEMS Crank mechanism, or uses a lever pulled by the pilot This method may use a mechanical latch system to lock wheels “up” No emergency backup available for this system ELECTRICAL RETRACTION SYSTEMS Uses a central motor and push-pull rods Uses microswitches to detect when gear is down/locked, or up/locked HYDRAULIC RETRACTION SYSTEMS Most common system of retraction for most sizes of a/c Used exclusively where landing gear is too large to be retracted by other methods May use ED pumps, electric pumps, hand or wind-driven pumps EMERGENCY LANDING GEAR SYSTEMS Four possible methods of dropping gear when hydraulics are lost: air bottle “blows” the gear down hand crank or ratched separate hydraulic system (may be hand pump) mechanical system which releases UP locks, and gear free-falls LANDING GEAR OPERATION 4 main components: shock strut the wheel the brake assembly the trunnion and side/drag brace scissors (torque links) actuating cylinder down & up locks the bungee system Using hydraulics, landing gear retraction requires greater energy than lowering Gear rotates on the trunnion pin Extending landing gear requires a release of the UP lock first, then The gear can begin free falling, slowed by the snubber in the orifice check valve Final few degrees of travel may require hydraulic pressure assistance Bungee system is used for emergency operation: Gear doors must be operated before extension & after retraction
LANDING GEAR POSITION INDICATOR SYSTEMS Positive indication must be provided to the pilot that gear is down & locked Safety system includes squat switches and other microswitches Squat switches tell pilot when weight of a/c is on the wheels Squate switches are electrically “open” when on the ground Some a/c use warning horns: they sound when: If gear is retracted, and throttle retarded to below cruise Landing gear position indicators: show position of Landing gear May use a system of different color indicator lights Transport Aircraft Landing Gear Systems Corporate Jets and Dual-Wheeled Transports Most have retractable tricycle-type gear, 2 wheels on each Nose gear will probably be a dual-wheel steerable type Gear will become completely enclosed when retracted
HELICOPTER LANDING GEAR Basic skid gear is common for small & mediums Wheel gear is used on sikorsky aircraft Retractable or cushioning gear may impart ground resonance Skid tubes are replaceable, and repairable Bending and deforming limits are established, and occasionally liberal Skid protectors are available, as are “bear paws” snow shoes Ground handling wheels are bolt-on towing additions INSPECTION AND MAINTENANCE OF LANDING GEAR The following must be carefully inspected: Attachments to fuselage or wings Struts, Wheels, Brakes, Gear doors Actuating mechanisms for gear Hydraulic systems FIXED GEAR INSPECTION Inspect for wear, deterioration, corrosion, alignment Jack up the aircraft to relieve the weight on the gear Shock cord should be inspected for age & fraying (5 years, retire) Shock Cord MIL-C-5651A. See diagram for year and quarter. Check oleos for bottoming: air charge has been lost.
RETRACTABLE LANDING GEAR INSPECTION Similar to fixed gear inspection, but add inspections for: Wear or looseness in joints or trunnions Leakage of fluids Smoothness of operation Operational check performed by jacking the airplane & operating gear Check for clearance of new tires in the wheel wells Check for operation of gear doors Check operation and adjustment of microswitches If a oleo bottoms upon initial landing, but operates normally during taxiing, it is likely an indication of low fluid. Check your fluid levels. ALIGNMENT OF MAIN GEAR WHEELS Check for Camber and Toe-in. Camber is the tilt of the top of the wheel inboard or outboard Toe is the angle of the forward edge of the tire Oleo type gear often use angled shims or washers A common Toe setting is 0º, with a tolerance of ½º Some a/c may require alignment to be set while weight is on the gear. Use grease plate for this INSPECTION OF FLOATS AND SKIS Consider pop out floats and fixed floats (helicopters) Standard float assemblies for small a/c are sensitive to salt water Check for corrosion thoroughly Use standard sheet metal repair techniques Check for leaks TIRES AND WHEELS AIRCRAFT TIRE OPERATION CHARACTERISTICS Tires for aircraft must endure higher loads and higher speeds than automobiles and trucks; the safety issue is much higher as well. Heat generation is higher in aircraft tires Rubber, the major material used, dissipates heat slowly Underinflating or overinflating increases shear forces in between the plies: tension will be higher in outer plies than in inner. Type I tires: smooth contour Type II tires: high pressure III: low pressure IV: extra low pressure
V: not applicable VI: low profile VII: tires are constructed for extra high pressure; jet aircraft. VIII: extra high pressure, low profile, low speed or high speed. Types I, II, IV and VI are being phsed out.
AIRCRAFT TIRE NOMENCLATURE Tire ply rating refers to the maximum static load and its inflation pressures Tire markings: manufacturer, country of mfr, design type, load rating, tube or tubeless, tire size, part number, ply rading Number of recaps used to be stamped on the sidewall, but not all models have provision for this. Chafers are used to protect the wheel rim-to-tire bead chafing. The most important part of an aircraft tire is the bead. A tire with smooth tread is used for very light aircraft, grass runways, and locations where braking is only used as an aid to taxiing. Ribbed tread tires are used for directional stability, good tread wear, and to allow water to escape from between the tire tread and runway. A Chine sided tire is used to deflect water or slush away from the intake of the jet engines. Double or single chine wheels exist. Inboard halves of an aircraft wheel are different from the outboard by the provisions made for mounting & securing the disk brake assembly, and the presence of fusible plugs. The fusible plug is used to prevent tire blow-out due to heat build-up. The plug has a low melting point core which melts under high temperatures which may build up during heavy braking. TIRE STRUCTURE Steel wire beads form the inner diameter Plies are diagonal layers of rubber coated nylon cord fabric Chafers protect the tire during mounting/demounting. They also provide a good seal between tire and wheel. Breakers are used to increase structural strength Inner liner acts as a built-in tube; prevents air from seeping through the casing plies. Beads anchor the plies and provide mounting surfaces for the wheel Tread is the surface of the tire for runway contact Most treads have groove patterns with 3 – 6 ribs, depending on size and type of service The light point on a tire is indicated by a red dot on the tire.
TIRE STORAGE Tires should be stored vertical, in racks, in cool dry places. Ensure there are no sources of light or electrical appliances (for the ozone) nearby. Same with chemical fumes. TUBES Made of rubber sections vulcanized together. Air valve is vulcanized to the tube for inflation/deflation Tubes can be checked for leaks by immersing in a water through with a light inflation pressure applied. AIRCRAFT WHEEL CONSTRUCTION Aircraft tires are too stiff to stretch over wheel rims. Damage will occur to the bead of the tire Split rims are used, and the two halves are sealed with an O-ring Most wheels are constructed of forged aluminum or magnesium alloys Wheels rotate on two tapered roller bearings; the cups are shrink fitted into the hub. Wheels are constructed with three fusible plugs, equally spaced around the wheel. The bead seat area is strenthened by rolling: it pre-stresses the surface Nose wheels are smaller in diameter and width than mains: rarely have brakes DEMOUNTING AND MOUNTING TIRES Safety procedures: Deflate the tire beforee loosening the axle nut. This must occur in case the rim is cracked or the wheel bolts fail. Before we loosen the wheel half retaining bolts, we must deflate the tire, and remove the valve stem. Break the tire beads away using a bead breaker – do not use sharp tools Then dismantle the wheel halves & inspect the bolts Always use a safety cage when inflating a tire. Bearings are generally a tapered roller bearing in hubs. Grease seals are used to prevent foreign material from entering and contaminating the wheel bearing grease. Stuck wheel bearing? Don’t beat it to death with a hammer & punch ~ use a bearing puller. The only place to obtain proper inflation pressures is in the A/C M/M. That manual will quote pressures for the aircraft when loaded. (all-up weight) Installing tubes requires a dusting of talc into the tire for lubrication purposes. Tube should be placed so that the yellow strip (the heavy spot) is adjacent to the red dot (the light spot) on the tire. If using a tubeless tire; inspect the new tire to make sure it is a tubeless type.
Install bolts and nuts ensuring they are installed in the correct direction. Use an alternating sequence to tighten. Only partially inflate the first time to allow the tire bead to seat to the wheel. A strap around the surface of the tread may be used to prevent the tire from expanding radially. Nitrogen is preferred, but not always necessary. Tires should be allowed to set for 12 to 24 hours before being installed. This allows for “growth” and natural stretch, and the air pressure to settle. Do not use soap/water installation liquids; may cause slippage of tire on the rim during landing. TIRE AND WHEEL INSPECTION Inspect outer tread while mounted on the a/c. Certain types of landing gear wear the tread unevenly (spring gear) Check for over or under-inflation, damage to sidewalls, cracks, weather checking, flat spots, chafing, thinning, valve stem movement (indicator slip marks) Dye Penetrant is not always the best answer: in some cases a crack may not show up because of the extreme pressures imposed by inflation and a/c weight. When the tire is deflated & inspected, some cracks will close up, and not show with Dye Penetrant. The most corrosion prone area of a wheel is any area which is exposed to the direct entry of moisture. This includes split lines, cavities, milled areas. Any fuse plugs showing deformation must be replaced. All of ‘em. The most critical areas on the wheel bolts is where the shank joins the head, or where the shank joins the threads. Pressures of a tire/wheel that is NOT installed on the a/c should read 4% below the recommended pressure. Any single burned patches on a tire tread would likely indicate hydroplaning. WHEEL INSTALLATION If the Mfr’s wheel balance weights have to be removed for any reason, you must mark their position, and replace the same weight in the same place.
AIRCRAFT BRAKE ASSEMBLIES Light a/c can use simple shoe brakes or single discs; low weight & speed INTERNAL EXPANDING SHOE BRAKES Older a/c and home-builts
One-way (single servo type), or Two way (dual servo) Can be similar to automotive types EXPANDER TUBE BRAKES Four parts: brake frame, expander tube, return springs, and brake blocks Older aircraft Hydraulically operated Each brake block is independent; no tendency to grap SINGLE DISK BRAKES One of the most popular types Disk held in the wheel by teeth or keys Linings on either side of the disk; compressions forms braking action One lining is attached to axle structure, the other moves according to hydraulic pressure May have multiple pistons (& therefore multiple linings) Cleveland is one type of popular manufacturer Removal of air requires a brake bleeder valve MULTIPLE DISK BRAKES Used where a substantial amounf of braking force is required Typically will have multiple rotors and pistons, stator plates, a pressure plate and a torque tube (see page 9-22 in Jeppesen) Wear indicators are often included in this design SEGMENTED ROTOR DISK BRAKES Heavy duty brakes design for use with high pressure systems using power brake control valves, or power boost master cylinders Uses stationary high-friction linings with rotating rotor segments CARBON COMPOSITE BRAKES Weigh 40% less than conventional steel segmented rotor brakes. Natural gas is used in forming the brake discs to add carbon Strength does not decrease at elevated temps Carbon against carbon performs excellent as a friction material Carbon brakes can exceed 3000 degrees F BRAKING HEAT ENERGY Huge amounts of heat energy on braking Pilots figure out gross weight of aircraft to establish a ground cooling time for brakes Somewhat reduced by using thrust reversers
TROUBLESHOOTING Pilot reports excessive brake pedal travel. Check the brake fluid level. Which way should the chevron seals face? The inside of the chevron should face the pressure. An aircraft is reported as having excessive brake travel, but the brakes are still hard and effective. The probable cause is worn brake linings. AIRCRAFT BRAKE SYSTEMS Modern aircraft brakes are classified as single or multiple disk Mechanically operated, or hydraulically operated, or pneumatically operated Mechanical is the older smaller systems, using pulleys, cables, bell cranks. Many a/c use hydraulic system pressures to actuate their brakes. Some have an entirely independant system. Pneumatic brake systems use air only; some hydraulics use air as a backup pressure supply. INDEPENDENT BRAKE SYSTEMS Basic systems require a reservoir, a master cylinder actuated by pedal or handle, a brake assembly at the wheel, and all related hosing/tubing. Master cylinder is the energizing unit, usually one for each main gear wheel. Parking brakes are often a simple ratchet affair for holding the pedal or handle in place, which continues to supply pressure to the brakes. Various models of master cylinders; some mount on top of the rudder pedals. Function remains the same. Heel operated brakes or hand-brakes. Parking brake mechanism may be interrelated with the main brakes, but setting of parking brakes when hot may be an issue. POWER BOOST SYSTEMS Power boost systems are used on a/c with high landing speeds. Power boost is halfway between manual brakes and power brakes. Power boost uses hydraulic pressure from the main system to the brakes via a check valve. May use a shuttle valve to route emergency air pressure. Larger aircraft require more braking power than can be applied through a master cylinder. Extra pressure can be exerted on the brake system by allowing hydraulic system pressures to act through a spool valve.
A Brake Debooster serves the purpose of decreasing system pressure to a useable level in the brake system. It also has the effect of increasing the volume of hydraulic fluid flowing. POWER BRAKE SYSTEMS Used where manual and boosted brakes are not adequate. Uses a power brake control valve to direct hydraulic system pressure to the brakes. Power brake control valve is also called a brake metering valve; generally one for each main landing gear brake. Typical system has four lines to each valve; pressure, return, brakes, and automatic braking. Automatic braking is used to stop wheel rotation during retraction on take-off. Sources pressure from the landing gear UP position in hydraulic system. DEBOOSTER VALVES Used to reduce the system hydraulic pressure to a lesser pressure in the braking system. Generally exchanges High Pressure/Low Volume into Low Pressure/High Volume. MULTIPLE POWER BRAKE-ACTUATING SYSTEMS Brake actuation systems get very complex at this level. Requires a lot of research and careful work on the AME’s part to learn these individual systems Brakes are operated by 2 independent systems. (#1, and #2) Each systems consists of daul-brake-control valves, pressure accumulators, brake-pressure transmitters and indicators, brake quantity-limiter valves, a skid-control manifold for each gear, and a parking-brake valve. All contribute the actuation of the independent cylinders in the eight main wheel brakes. Either system is capable of stopping the airplane on a maximum gross-weight landing. CONSTRUCTION OF BRAKES Sintered brake linings are another term for metallic linings. Segmented rotor discs produce three benefits: Eliminate heat buildup in the disc Produce more efficient braking Allow for longer braking action. Floating calipers are used to adjust for brake lining wear. Those types of aircraft which use a large amount of fluid to operate the brakes will incorporate a power brake control valve. Carbon disk brake lining material is usedfor light weight, better wear resistance, and better heat resistance. Automatic adjusters are installed in modern systems to maintain a set clearance between the disc and brake lining. 13
Heavy steel plates called Pressure Plates are used to act as a backing against which the linings are forced by the pistons. BRAKE TEMPERATURE Certain a/c have brake temperature readouts in the cockpit Temp ranges are relative to a scale of 0 to 9 Brake temperatures can increase even after brakes have been applied and released due to heat soaking Temp values above a “5” illuminates a BRAKE TEMP light BRAKE MAINTENANCE Some types of bonded linings are not fully cured at the time they are installed. The curing process requires they be installed and then used in a moderate-to-heavy application of the brakes. BCIT students will learn more about curing processes in the a/c composites section of level 3. For routine maintenance, check indicator pins for brake pad wear. Check lugs or keys holding rotor disks Check fusible plugs in the wheels for yeilding or cracks Examine fittings for leakage Ensure you are servicing the brakes with the appropriate fluids. Inspect hoses fro swelling, leakage, sponginess Check for reports of dragging brakes, fading brakes, excessive pedal travel, pedal creep or non responsive braking. Dragging brakes? Check for air in the system, sticking valves, and weak or worn return springs Grabbing brakes? Check for oil or FOD on linings. Fading brakes? Check for overheated linings and glazing Excessive travel? Check for lining wear limits, lack of system fluid, air in the system, or maladjusted brakes. Pedal creep? Inspect for leaks in a master or slave cylinder BRAKE BLEEDING Purpose is to remove any air from the braking fluids system and all related valves and cylinders Air will cause sponginess or dragging Gravity bleeding uses a clear plastic tube, attached at one end to the bleed fitting at the brakes, and the other end is immersed in a container of fluid. Apply pressure to brakes, and open the bleed fitting. Trapped air bubbles will be removed with the fluid, and can be seen in the container. Maintain fluid levels in the reservoir. Pressure bleeding uses special tooling for the specific aircraft. Most types use a pressurized reservoir attached to the brake bleed fitting. Fluid is forced through the system back to the reservoir. ANTI-SKID SYSTEMS 14
Several reasons apply why anti-skid systems aare in use on many modern aircraft: 1. They prevent wheel lockup 2. They prevent skidding 3. They reduce the chance of hydroplaning 4. They help reduce excessive heat build up A successful anti-skid system will have two main features: A form of wheel sensor that can detect a change in the rate of deceleration A valve system that can rapidly apply and release the brakes, which will prevent a skid The Three main Components of an anti-skid system: 1. Wheel speed sensor(s) 2. Control unit (computer) 3. Control valves Two types of wheel speed sensors are: 1. The AC sensor, which creates a variable frequency AC current 2. A DC unit, (basically a DC generator)
ANTISKID SYSTEM OPERATION • • • • Antiskid systems are generally armed by a switch in the cockpit. System will utilize the squat switch to prevent current from flowing to the system during flight. System allows full pilot control over braking at speeds below 20 mph. System will perform its function when the wheel deceleration indicates an impending skid.
FACTS ABOUT OLEO-PNEUMATIC STRUT Oleo is trade name for an air-oil cylinder, or shock absorber Also applies to an oil-only (no gas charge) cylinder Oleos work on the fact that liquids are practically incompressible Function of the oleo is to absorb and dissipate kinetic energy.
Oleos are anchored to hard points on airframes to pass the stresses of landing and taxiing The most common type of oleo features a close-tolerance fitted piston capable of sliding within the cylinder, much like a piston engine A seal is established within this fit to prevent oil and nitrogen from escaping under pressure Lower part of oleo, the piston, is kept in alignment using a collapsible, extendable pair of links called scissors The scissors also prevent over-extension of the strut once the aircraft is airborne, and weight comes off the landing gear Scissors can also be called torsion links. The upper link is anchored to the cylinder, and the lower link is bolted to the piston assembly. Wheels, axles, trucks, brakes, etc are mounted on the lower piston assembly On small aircraft, the upper cylinder bore has a smaller diameter cylinder mounted within, almost like a small drinking straw inserted into a big one. This smaller straw is called the piston tube while the outer is called the cylinder. The hollow piston slides up into the cavity between the smaller piston tube and the larger cylinder. Cavities at the bottom of the hollow piston are filled with hydraulic fluid. Cavities at the top of the cylinder are filled with nitrogen. Also from the bottom of the hollow piston, centered inside, is a long pin called a metering pin. On some models, this metering pin appears uniform in diameter from bottom to top. On others, the diameter of the pin will vary along the length of the pin. This metering pin protrudes through a hole in the bottom cap of the piston tube. The diameter of the metering pin serves to open or close the metering orifice, which controls the flow of hydraulic fluid from the lower piston chamber up into the upper air chamber.
Strut servicing (filling with hydraulic fluid and/or nitrogen) is performed at fully bottomed condition: the piston is fully retracted into the cylinder body. According to manufacturer’s servicing manuals, a determined level of fluid is added to the strut, usually by way of removing the charge valve on top of the cylinder body. Adding nitrogen; consult servicing manual, then apply aircraft weight to the strut. Charge nitrogen slowly through the charge valve. Pressures and/or quantities may be available on data plates on the cylinder body. Some manufacturers will specify pressures to be applied. Others will ask that the pressure is applied in increasing quantities until a length of piston is visible below the cylinder body. After takeoff, the weight is removed from the strut. The piston extends fully, as limited by the scissors. Rate of extension may be governed by the metering pin/orifice, or by a relief valve in some oleo models. In some cases, during this extension, cam surfaces or hydraulic steering actuators may be used to straighten nose wheels in alignment with aircraft centerline, in preparation for stowage or landing. After takeoff, fluid drains past the metering orifice, from the upper cylinder chamber to the lower piston oil chamber. At the point of impact during landing, the piston is forced up into the cylinder body, which reduced the available volume of the piston oil chamber. Oil is forced through the metering orifice into the upper chamber. Because of the varying diameters of the metering pin, the rate of oil flow will change relative to the orifice size. When load is applied, oil is forced through a small orifice; energy is dissipated, not stored. No rebound or bounce. A strut with insufficient oil will easily bottom on landing. A strut with low nitrogen pressure will provide a rough ride during taxiing. HYDRAULIC SYSTEM
• • • •
In hydraulics, we consider liquids to be incompressible The study of Pneumatics becomes a parallel subject, except for this fact. The study of both is called Pneudraulics
PASCAL’S LAW Pressure applied to any part of a confined liquid is transmitted with undiminished intensity to every other part
ADVANTAGES OF HYDRAULICS
• • • • • • • • •
Ease of installation Minimal maintenance requirements Light weight Hydraulic systems are quite efficient, and losses are negligible.
MINIMUM MATHEMATICS REQUIREMENTS Ensure you are comfortable with these formulas A Force per unit area is a measure of Pressure Pressure: A force applied over a given area. Quoted in pounds per square inch, or grams per square centimeter. Volume: when fluid is contained, the amount of space it consumes is quoted in cubic units. Power: product of force and distance. But consider the amount of time it took to perform the work. Power is the force times distance, divided by time.
• • • •
How do we determine the amount of pressure exerted by a column of liquid? Determine the height of the column Do not let the volume of the column distract you
“The static pressure exerted by a column of fluid is proportional the height of the fluid, but NOT affected by its volume.” All the above pressure gauges will read exactly the same given the similarities in the height of the fluid levels. . • In fluid systems, power may be quoted in a flow rate of gallons per minute. One gallon is 231 cubic inches. Multiply this by the pressure, in pounds per square inch to get the force/distance/time relationship. • One gallon per minute of flow, with a pressure of one psi will yield
0.000583 horsepower. Therefore: • Horsepower = GPM x PSI x 0.000583 Force/Pressure/Area Relationship
• • • •
Force = Pressure X Area Pressure = Force / Area Area = Force / Pressure Mechanical Advantage: Check this diagram: • L1 is 40 inches, L2 is 20 inches. W1 is 50 lbs and W2 is 100 lbs. Balance.
• If we want to move W2 a foot, W1 must move 2 feet. The work done on
side 1 is two feet times 50 lbs, or 100 foot pounds. Using hydraulics, the same mechanical advantage may use pistons to illustrate the principle. Side 1 piston has an area of one square inch, and side 2 has 10 square inches. Apply a force of ten pounds to side one, you get ten pounds per square inch. Remember Pascal; pressure is the same throughout the system. So therefore we have a pressure of 100 pounds on piston 2. When piston 1 is moved an inch, a cubic inch of fluid transfers from side one to side two. But this is spread over an area of ten square inches, so the piston only moves one tenth of an inch. Piston 1 would have to move 10 inches to produce a single inch of movement in piston 2. With how much force will it travel?
• • • • • • • • • • •
Hydraulic Fluids Fluids must have good lubricating capabilities to avoid component wear. Fluids must have good anti-foaming characteristics to avoid vapour lock. Fluids must be totally compatible with the components, seals, flex lines, etc. Must be able to transfer force, above all.
Desirable properties of a good hydraulic fluid High Fire Point High Flash Point Chemical Stability Low Viscosity
Types of Hydraulic Fluids
• • • • • • •
3 main types: Vegetable based Mineral based Synthetic fluids
Vegetable Base Fluids Seals are made from Natural Rubber Flush these systems with Alcohol Veggie-based fluids (MIL-H-7644) was essentially castor oil mixed with alcohol, dyed blue for identification. Almost extinct now.
Mineral Base Fluids
• • •
Seals are made from Neoprene or Buna-N Flush them with Mineral Spirits Dyed Red. Flush systems using 5606 with naptha, or varsol, or stoddard solvent. Serviceable with Neoprene seals and hoses. Flammabilty problems. “Old” 5606 has a sour smell, and is darker in color.
• • •
Seals are made with Butyl Flush them with Triclorethylene Colored light purple, although other grades of Skydrol are colored green or amber
• Skydrol is susceptible to contamination by water from the atmosphere. Cans must be kept fully sealed. Skydrol liquids attack the insulation on wires, and can easily damage or discolor the paint on an aircraft. Use Skydrol with Butyl, Silicone rubber, or Teflon seals, but check the compatibility list as Skydrol is incompatible with many types of rubbers
Hydraulic Fluid Colors
• • •
Mil-H-5606 Red Skydrol Light Purple Originally developed to provide a fire resistant liquid for use in high performance aircraft
• Fluids must be kept absolutely clean; any contamination causes seal failures on close fitting parts. Keep all reservoir lids tightly closed. Use the proper caps and plugs on hydraulic lines when removed. Any contaminated or old fluids must be changed out, and the system flushed. For synthetic fluid systems, special microscopes are used to examine the liquid for contamination. If any system liquids are suspect, change them in accordance with the manufacturer’s recommendations.
Basic Hydraulic Components • A basic system must have at least:
– A pump, or pumps – Lines and valves – actuators
Simple Hydraulic Systems
• The simplest was a sealed brake system which used a master cylinder to transmit force to the brakes
Unloader Valve • Maintains pressure on the system
Allows pump to freely flow fluid back to the reservoir
Power Pack • Any one single unit which contains the hydraulic pump, the reservoir, control
valve, and any of the auxiliary valves is known as a Power Pack system.
Low Altitude Aircraft • Only require unpressurized hydraulic reservoirs High Performance Aircraft • Require pressurized reservoirs • These prevent hydraulic pump cavitation from foaming and air bubbles in the
Constant Displacement Pump • Delivers Fixed quantities of fluid per revolution Hydraulic Fluid Draining? • Try checking the tank scupper if you just recently refilled your reservoir • Or it could be a worn hydraulic pump shaft seal, draining into an overboard line Deadheading
• Please, no jokes about my lazy brother • Deadheading is actually a condition when a pump is trying to force liquid into an already fully pressurized system. This can cause rupture of lines or components • Deadheading of a pump may cause violent failure • Many pumps are protected by a shear section (a necked area) on the drive shaft
Relief Valve • Required when a constant displacement pump is used • Prevents failure of components, or rupture of hydraulic lines from excessive
Selector Valves • Used for directing fluid to a particular end of an actuating cylinder • Simultaneously directs return fluid to the reservoir from the other end Open Center Valves • Allows fluid to flow through the valve when in the “Off” position Closed Center Valves • Do not permit flow when in the “Off” position • Systems require an unloader valve to relieve pressure • Most Closed Center systems work with accumulators in the system Bourdon Tube • Used as the mechanism behind a pressure gauge • Gauge acts upon the uncurling action of the tube due to pressure • Most gauges require a snubber (orofice) in line to dampen out pressure
Accumulator • Backs up the pump when it is under high load demands from the system • 3 types of accumulator:
– Piston type – Bladder type – Diaphragm type
Accumulator • One compartment of the accumulator is connected to the hydraulic system • The other maintains a pressure head using compressed air or nitrogen • More on accumulator a bit later Valve Cores • Types of valve cores include Short and Long • Low Pressure and High Pressure • High Pressure valve cores have an “H” stamped on the head of the stem Main Hydraulic Components • Reservoir • Stores fluid. Acts as an expansion chamber where it relieve any accumulated • •
air. Two types: Pressurized, and non-pressurized
Non-Pressurized Reservoirs • Strainers at the fill tube to catch FOD • Baffles or drip plates to help remove air bubbles • Stand pipes are used as emergency supply sources for hand pumps Stand Pipe • Creates a fluid reserve for emergency or auxiliary pumping. Pressurized Reservoirs • Pressure keeps foaming and bubbles to a minimum • Early models used an air-injection system to help scavenge the oil back to the •
reservoir, but then would have to remove the air again. Older models kept about 12 psi on the fluid
Pressurized Reservoirs • Later models maintain a head of 30 psi on the fluid • Diaphragms separate the fluid from the air pressure, or piston types provide a
Pressurized Reservoirs • Smaller pressurized systems merely let the fluid cool itself at the reservoir. Hydraulic Pumps • Consist of hand pumps, and power-driven pumps. Hand Pumps • Two types: • Single action, where fluid is moved only when the pump shaft is moved in one •
directions. Double action pumps move fluid when the shaft is moved in both directions.
Power Pumps • May be driven by engines (direct gear drive), electrical sources, pneumatics, • • •
or any other energy source Two main types Variable Displacement type Constant Displacement type
Constant Displacement sub-types:
1. Vane type 2. Gear type 3. Gerotor type
Vane type • used to move a large volume of fluid under a lower pressure • Vanes are free floating paddles held in place by a rotor, and kept in place by
the walls of the pump. The rotor is built off-center of the walls to allow certain quadrants of the pump area to have different volume capacities than others.
Vane type • Uses for this pump are the air pump (pneumatics for instruments and de-icing),
and occasionally low pressure hydraulics.
Vane Type Gear type pumps Spur type gear pump Gerotor type pump • Gerotors use a unique shape of impeller to move liquid from one crescent
shaped port to another. It uses an impeller with one less lobe than the housing; that is, in the above picture, there are six lobes, and seven “teeth’ on the driving gear. This type of pump is also used for medium pressures.
Gerotor type High pressure pumps • High pressure but low volume can be accomplished with a piston style pump.
On the radial piston engines, these pumps had either 7 or 9 axially-drilled holes in the rotating cylinder block. They were connected by small ball joints on the ends of the piston rods to a drive plate.
Piston type pump Variable Displacement • One type of pump found in a closed center hydraulic system • Does NOT require an unloading valve •
Hydraulic oil coolers were mounted in the fuel tanks to help cool the fluid, and heat the fuel
Variable Delivery pumps • Up until now, we have been talking Constant displacement type pumping;
this is to say; a set amount of fluid delivered per RPM of the engine. This type of pumping requires an unloading valve of some sort in the system to route unrequired pressure back to the reserevoir. Now we look at Variable delivery pumps.
Variable Delivery pumps • The most popular of the Variable Displacement pumps is the StratoPower •
demand pump. . This type of pump also used axially oriented pistons and cylinders. Pistons are driven by a wedge cam, and instead of ball joints for con rods, they use
ball joint slippers
Variable Delivery pumps • . The wedge cam rotates and provides the pumping energy • The stroke is always the same, regardless of the amount of fluid required, but
the “Effective Length” of the stroke is what controls volume of fluid pumped.
Variable Delivery pumps • Effective length is variable due to a compensator piston. • . This compensator piston is situated in the middle of all nine fluid pistons, so •
it can equally access all cylinders. . The compensator piston moves a spider and sleeve that surrounds each piston.
Variable Delivery pumps • . The sleeve is what determines how much fluid will be taken in or pumped out
from each piston stroke. Any excess pumping is vented back into the the pump body.
• Consist of Flow Controlling Valves and Pressure Controlling valves. • Flow Control type valves are available in several model types: • Selector Valves • Sequence Valves • Priority Valves • Check valves • Hydraulic Fuses
Selector Valves • determine the direction of flow (such as “Gear Down” or “Gear UP”). • There is two types of Sector valves: Open Center, and Closed Center. • Open Center allows fluid to flow through the center of the valve when in the
neutral position. Fluids flowing through the center return to the reservoir.
Closed Center valves • permit no flow when in the neutral position • Both Open and Closed center types direct fluid to and from the actuators
simultaneously when a selection is made. 26
One example of a closed center valve is the Poppet style valve, which uses small pistons moved by mechanical and hydraulic pressure.
Poppet style valves • This style of valve uses a crankshaft arrangement to open poppets to permit
flow to various ports. The poppets are opened and closed according to the position of the lobes on the crankshaft.
Sequence Valve • Permits a certain hydraulic operation to take place and complete, before a •
second or subsequent operation may start For example, wheel well doors can only close after the landing gear has been retracted
Sequence valves • Another type of sequence valve •
This type of valve requires a mechanical plunger to actuate its function.
Priority Valves • Priority Valves are rather similar to sequence valves, but they are opened by
hydraulic pressure rather than mechanical movement.
Priority valves operate landing doors (first in sequence, and lower pressure) before permitting the main landing gear pressures to flow.
Quick Disconnects • Line disconnect valves (similar to the Quick Disconnect fittings in the Fluid • • •
Lines & Fittings section) prevent the loss of fluid when disconnecting units from the main system. They also prevent FOD ingestion. They are precision built to close tolerances, and as such are not easily field repairable. prevent damage to system components should line blockage or serious leaks occur. The spring keeps the main passage open for normal pressures. If pressure at side B should drop, the piston will slide to close the valve and prevent loss of fluid.
Check Valve • This style of valve permits flow in one direction, then prevents flow in the • • • •
opposite direction. Three common types exist: Ball Check Valve Cone Check Valve Swing check valve
A fourth check valve • But there is a fourth that permits a small amount of fluid to flow in the opposite •
direction which may be desirable for thermal relief. Called an Orifice Check Valve
Types of check valves: Ball Check Valve Cone check valve Swing check valve Orifice Check Valve • A type of valve which permits fluid to flow freely in one direction, yet restricts
the rate of flow in the reverse direction
Hydraulic Fuse • Designed to remain open during normal flow rates
Will close if the flow increases beyond a specified rate.
Hydraulic Fuse • See page in your text Pressure Control Valves • Pressure Controlling Valves have several types: • Relief Valves • Pressure Regulators • Pressure Reducers
Relief Valves • usually used as a backup rather than a main pressure regulating device • (this is due to the heat generated and power dissipated when pressure •
relieves in this manner.) Relief Valves are set to a certain value, and above that value they release excess pressure.
Relief Valves • . Thermal pressure relief valves are used in places where fluid may become
trapped, and expand when heated. Thermal valves then relieve the pressure back into the return line.
Thermal Relief Valve • Prevents movement (such as flap actuation) past a selected position • Ensures park brakes don’t reset themselves after being released, due to
Pressure Regulators • Specific to closed center systems, these regulators keep pressures within • •
specific ranges, and allow the pump to unload when no actuators are selected. Regulators are often used with accumulators. . When pressure from the pump has built up, and thus charged the accumulator, the regulator is required to maintain system pressure.
• A condition of balance occurs when the system reaches 1500 lbs; there is a force of 1500 pounds pushing down on the piston, resisted by 1000 pounds spring force plus 500 pounds applied to the ball seat. Should more than 1500 pounds be exerted on on the system, the piston will move up and unseat the ball.
Pressure Reducer Valve • Used when some portion of the hydraulic system requires lower pressures to • •
operate than the system provides Examples: brake systems
Pressure reducing valves perform this function using a balance of hydraulic
and spring forces
Incoming pressure is 1500 lbs. It bleeds through a small hole in the piston. The 1500 lb force also acts on a piston shoulder area of 1/2" inch. It is resisted by a spring of 100 lbs force, plus the force of 750 lbs hydraulic fluid acting on the top of the piston which has an area of 1 inch.
Pressure Reducer Valve • The bleed hole is used as a hydraulic snubber to prevent ratcheting of the
Shuttle Valve • Used for directing fluid for either the normal source, or an emergency source Accumulators • Three types • Bladder type (hollow sphere) • Diaphragm type (hollow sphere) • Piston type (cylinder shape) Accumulators • Accumulators are charged with air or nitrogen to about 1/3 the system
pressure. When system hydraulic pressure increases, the gas in the accumulator is further compressed, and exerts an increased force against the hydraulic fluid.
Air Valves • Air valves in accumulators take on a special importance.
Most have similar appearances. But high pressure valve cores can be distinguished by the letter “H” stamped into the end of the valve stem.
• The basic valve, an AN812 style, is very similar to automotive valves. But to deflate the accumulator with the AN812, merely unwind the valve body to expose the small hole in the side.
AN812 • But to deflate the accumulator with the AN812, merely unwind the valve body
to expose the small hole in the side.
AN6287 • Uses a swivel nut to open and close, in addition to the stem valve AN6287 • To deflate air using the AN6287 type valve, turn the swivel nut one turn and
press the valve stem. For charging, you must also loosen the swivel nut one turn. The additional step allows for redundancy via a metal-to-metal seal.
MS28889 • MS28889, is very similar to the 6287 but there is no valve core, and the swivel
nut is the same size as the housing. A roll pin prevents the housing being threaded too far into the body.
Filters • Cleanliness is a prime requisite for hydraulic fluid. •
All solid particulate must be removed to prevent damage to high pressure components. Filter capacity is measured in microns (one millionth of a meter, or 39 millionths of an inch). 10 microns is the desirable filtering level.
• Three types of filters are available • Surface filters trap fod on the surface of the element • Sintered metal is quite popular • Micronic filters use cellulose elements are installed in return lines. Micronic describes the size of particles trapped. • Some of the micronic style have a stainless steel wire mesh around the outside which catches the majority of contamination.
Cuno Filters • Features a simple cleaning mechanism rather than removing the filter • Rotate the handle on the top, and flush with fresh fluid Paper Filters • Pleated paper micronic filters are installed in the return portion of the hydraulic •
system A safety feature, which prevents paper FOD from causing hydraulic component failure
Paper filters are thrown away & replaced with new elements
Actuators • An Actuator is the unit which transforms pressure into mechanical movement • Linear actuators are those which produce straight-line movement • Most common linear actuator is the Double acting unbalanced type. Rotational Actuators • Also called a Hydraulic Motor • Produce a continuous rotational force Comparison • Consider: 2 actuating cylinders, both connected to the same source of • • •
hydraulic pressure. Each has the same piston area. But they have different stroke lengths Will they travel at the same speed? Will they have equal amounts of force?
Comparison • Consider: 2 actuating cylinders, both connected to the same source of • • •
hydraulic pressure. Each has the same piston area. But they have different stroke lengths Will they travel at the same speed? Will they have equal amounts of force?
– Yes, and Yes. But one has farther to travel.
Troubleshooting • Your pilot reports that operating hydraulic pressures are normal, but when the
pump is stopped, no pressure is available. What is the problem?
Troubleshooting • Your pilot reports that operating hydraulic pressures are normal, but when the
pump is stopped, no pressure is available. What is the problem?
• If you have a pressurized accumulator, you may have a leaking air valve Air Leaks in Pump Inlets • During engine runup, your pilot notices a “chattering” of the hydraulic pump • This is an indication of probable air leaking into the inlet, and cavitating the
Air Removal • In many aircraft, air bubbles do not present a big problem as they are “worked”
out of the system over repeated component cycling.
Troubleshooting • You’ve just installed a rebuilt hydraulic hand pump.
The handle moved once only, and cannot be further moved in the pumping direction. What is the problem?
Troubleshooting • You’ve just installed a rebuilt hydraulic hand pump. •
The handle moved once only, and cannot be further moved in the pumping direction. What is the problem? Check the hand pump outlet port check valve – it’s probably installed backwards.
Troubleshooting • Your system has been serviced with the wrong fluid. Troubleshooting • Your system has been serviced with the wrong fluid. • Drain
the system. Flush all fluids. seals in all components.
What can be done?
What can be done?
Change all affected
Troubleshooting • Your aircraft is running up after a major inspection.
The pilot shows you that the flaps cannot be lowered using the main hydraulics, but can be slowly actuated by using the emergency hand pump. What is the problem?
Troubleshooting • Your aircraft is running up after a major inspection. •
The pilot shows you that the flaps cannot be lowered using the main hydraulics, but can be slowly actuated by using the emergency hand pump. What is the problem? Most likely answer? Your reservoir fluid level is low.
Pneumatics • Low pressure pneumatics are used to power de-icing boots and some
Air is supplied by engine-driven vane-type pumps, occasionally called vacuum pumps. Level 3 students will be looking at these systems in detail, in another level
Fairchild F-27 • Uses a closed-center pneumatic system • Special compressor on engine gear box • 3,000 psi • Excess dumped overboard Pneumatics, Advantages • No supply problems; air supply is all around • Most components are light weight • Systems do not require a return path • Few temperature problems • No fire hazards • Contamination is minimal Pneumatics, Disadvantages • Moisture in the air will freeze • Systemic noise is a factor • Storage bottles may be heavy • Leaks are nard to find • System complexity