Aircraft Suspension Systems

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Ernest Heath
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1 AVIM 103D Aircraft Suspension Systems

2 Suspension Systems Provide controlled flexibility to the landing gear systems while maintaining their structural integrity Up to a point they will eliminate the unusual loads incurred during landing and takeoff operations They can also reduce or eliminate ground operation vibrations from uneven or rough taxi surfaces

3 Suspension Systems Suspension vs. Absorbtion Suspension systems are devices that allow flexibility or bounce to occur between the ground and a vehicle This can include low pressure tires, springs (torsion, flex, coiled)(rubber, metal, plastic), telescoping struts

4 Suspension Systems Suspension vs. Absorbtion Absorbtionis a suppression or restriction to flexibility or bounce The most common form are air / oil filled telescoping struts Less commonly are stiffeners such as plastic or wood straps attached to flexing type gear

5 Suspension Systems Very early aircraft had rigidly mounted gear As technology progressed two main forms of suspension came into being Rubber bungee mechanical lever systems Flexible metal tapered bars or shafts

6 Suspension Systems The main advantage of these two systems are: they are light easy to maintain relatively inexpensive fairly areodynamicallyclean The main disadvantage is they provide no permanent shock absorbtion

7 Suspension Systems Air-Oleo struts were then designed to: suspend, or provide bounce and to truly absorb the shock energy, or prevent spring-back. Note: FAA test questions handle this badly Springs and bungees only delay the shock energy, but eventually spring back.

8 Suspension Systems Bungee System

9 Elastic Shock Ring Endless elastic ring of multiple strand rubber 1/4-inch to 13/16-inch diameter; 400 lb. to 1160 lb test rating Cotton braid cover Rubber Cure Date: See AC65-9, Page 166 Two like color code threads year Single thread quarter Blue 2008 Red 1 st Yellow 2009 Blue 2 nd Black 2010 Green 3 rd Green 2011 Yellow 4 th Red 2012

10 Shock Ring Notes: Remove boot for thorough inspection Beware safety hazard Oil stained cotton cover damaged Necked diameter worn, broken elastic Replacement considered preventive maintenance FAR 43 Appendix A

14 Bungee Cord

15 Bungee Installation / Removal Tools

16 Spring Systems Flexible tapered bars and shafts

17 Spring Systems Flexible tapered bars and shafts

18 Spring Systems Less commonly there are numerous versions of coiled spring, rubber disc, torsion bar, plastic flexible bar, etc. assemblies that all provide some form of flexibility to the landing gear

19 N Flexible Gear Servicing Includes checking all fittings for security, tightness, and appropriate free play Inspect main gear for signs of corrosion, fatigue, hard landing damage or taxi damage Inspect auxiliary gear and steering connections for damage and corrosion Repair all worn or failed parts

20 N Flexible Gear Servicing Some tubular structures may be repairable by welding All spring type structures are not repairable by welding Spring type may have serial numbers and may be matched pairswhich means they are replaced in pairs

21 Struts

22 Suspension Systems Oleo Telescoping Strut Over all aircraft it is the most commonly used suspension system Range from 1" in shaft diameter to 10", 12", etc. Can be used as main gear, or as auxilary gear Can be steerable, fixed or free castoring

23 Suspension Systems Oleo Strut -Basic principle of operation A telescoping strut that contains compressed gases and fluid, usually a light oil The compressed gas causes the strut to extend thereby sustaining the changing weight of the aircraft (suspension)

24 Suspension Systems For the strut to change length the oil must pass through a restricted orifice Due to the nature of hydrostatic lock this restriction of oil flow "meters" the rate at which the strut can change length (shock absorbtion) The tapered metering pin determines the rate of compression

25 Suspension Systems Torque or scissors links maintain wheel alignment May have a flapper return valve that allows the strut to extend quicker then it compresses Very slight seepage of seals is normal to lubricate the piston

26 Oleo Strut

27 Oleo strut telescoping

28 Oleo strut telescoping

29 Oleo strut telescoping

30 Oleo strut telescoping

31 Oleo Strut Parts Main Strut, outer tube Piston, piston rod, inner cylinder Upper or inner strut seal rings Upper inner bearing Snubber or return valve (sometimes) Lower outer collar, bearing and gland nut

32 Oleo Strut Parts Oil and gas fill plug / valves Neoprene V-ring seals Orifice tube Orifice or snubber plate Tapered metering pin Oil Dry gas

33 Oleo Strut Notes Strut service is preventitive maintenance Earlier struts used O-ring seals Newer use stacked V-ring seals Fluid pressure is applied to the inside of the V Or D-rings with round side facing movement Piston is hardened polished and or chromed steel Gland nuts are bronze (may or may not be adjustable)

34 Oleo Strut Notes Deflating struts will protect piston from corrosion Piston may have a splineor cam that aligns the nose gear for retraction Strut extentiondistance at a given weight is the common method for determining gas charge Seal compatibility determines type of oil Strut should have a data plate attached

35 Oleo Strut Notes Dried Nitrogen is the gas of choice Inert Inexpensive No moisture Three type of filling Valves MS most common AN 6287 AN 812 older models

MS 28889 Fill Valve Has no valve core Base nut and swivel are 3/4" Has a roll pin to keep swivel

36 MS Fill Valve Has no valve core Base nut and swivel are 3/4" Has a roll pin to keep swivel valve in place Base nut torque is 110 in/lbs Swivel nut torque is 70 in/lbs Pressure rated to 5000psig

37 MS Fill Valve

38 AN 6287 Fill Valve Has high pressure valve core (stamped H) and a swivel nut valve Base nut is 3/4", Swivel nut is 5/8" Base nut torque is 110 in/lbs Swivel nut torque is 70 in/lbs Pressure rated to 3000 psig Do not interchange with MS 28889

39 AN 6287 Fill Valve

40 AN 812 Fill Valve Has only a valve core Base nut is 5/8" Med. press. valve core short type stamped H Base nut torque is in/lbs Pressure rated to 1500psig Do not use in place of MS28889 or AN6287

41 AN 812 Fill Valve

42 Fill Valve Warning All the fill vavesare interchangeable DO NOT DO INTERCHANGE THEM DO NOT ATTEMPT TO USE AUTOMOTIVE VALVE CORES WITH EITHER THE AN 6287 OR THE AN 812 DO NOT INTERCHANGE VALVE CORES OR CAPS BETWEEN ANY OF THEM

43 Oleo Strut Servicing Servicing data may come from current maintainencemanual, or data plates Depressurize, remove from plane Disassemble and clean, inspecting for any damage, corrosion or cracks Replace all rubber seal components, worn bushings, and failed parts

44 Oleo Strut Servicing Reassemble, add oil to level with filler opening, bleed air out, and seal Reinstall and repressurizewith nitrogen 100hr / annual must include checking strut fluid and gas levels Typical pressures range from psi You will not be able to service a strut with shop air sources

45 Oleo Strut Servicing Use a nitrogen charged bottle, or a strut pump (12:1) Cycle pressurized strut several times to ensure seal seating and air bubble removal Struts can have slow gas leaks, recheck fill after 24 hours Always rock the aircraft prior to measuring strut extension

46 Strut Servicing

55 Strut Service Bogert Strut Service Tool Clean way to service struts. Strut valve core is NOT removed. Hose fitting releases pressure & fluid into the bottle. Raise & lower strut a few times, then apply charge. Deluxe model has 2 large tanks Deluxe model keeps old and new fluid separate.

Strut Service Schrader 556 High Pressure Strut

USA 556 Tee-coupler attaches directly to the

56 Strut Service Schrader 556 High Pressure Strut Coupler -1/8" NPT Male -3,000 PSIG The Schrader USA 556 Tee-coupler attaches directly to the strut valve to inflate or deflate the strut via the built-in core depressor pin. It attaches securely by hand or wrench to the valve threads ( thread size) and forms a perfect seal at up to 3000 psi. The airline connector is a 1/8" NPT male thread.

58 Strut Inflation Bogert Strut and Tire Pump Boost air to 1000psi WITHOUT an Air Compressor. Very portable. No air compressor needed! Use air bottle, spare tire or tire pump as pumps air source. Can also use with air compressor for air source. Detailed instructions. Equipped with 7 foot hose

59 Strut Inflation Bogert Aviation 15 Air or Nitrogen Boost Pump Boost Air or Nitrogen to 2250psi. 36" hose. Comparable to pumps priced in the thousands. Nitrogen adapter makes using bottled nitrogen efficient.

60 Strut Inflation Our new Aircraft Strut Pump delivers up to 1,500 PSI of pressure to raises struts without the need to place the airplane on jacks. Safety one-shot air delivery system gives you maximum control over the entire process --each stroke of the delivery valve injects a small shot of high pressure air into the strut so that you can raise it in controlled increments. There is virtually no chance of you accidentally overpressurizing and possibly damaging your struts. Newly engineered design now features 6061-T6 Aluminum cylinder construction for lightweight durability Operates on 50 to 150 psi air pressure (100 psi recommended) Enhanced high pressure valve system rated at over 3,000 psi True 10X pressure magnification to produce over 1,500 psi from a 150 psi air source Also works with Nitrogen delivery systems Safety "one-shot" system delivers pressure to strut in controlled increments. 24" High Pressure Delivery Hose MADE IN U.S.A.

62 B737 Main Landing Gear

70 END SECTION TWO

Aircraft Maintenance Prof. A.K Ghosh Prof. Vipul Mathur Department of Aerospace Engineering Indian Institute of Technology, Kanpur

Aircraft Maintenance Prof. A.K Ghosh Prof. Vipul Mathur Department of Aerospace Engineering Indian Institute of Technology, Kanpur Lecture 05 Aircraft Landing Gear System Now, coming to the next aircraft