Aircraft Landing Gear

Conventional Gear Advantages:

1. Allows adequate ground clearance for a larger propeller
2. More desirable for operations on unimproved fields

Conventional Gear Disadvantages:

1. With the CG located behind the main gear, directional control of this type of aircraft becomes more difficult while on the ground
   • If the pilot allows the aircraft to swerve while rolling on the ground at a low speed, there may not be sufficient rudder control available, and the CG will attempt to get ahead of the main gear, which may cause the airplane to ground loop
2. Lack of good forward visibility when the tailwheel is on or near the ground

Tricycle Gear Advantages:

1. It allows the more forceful application of the brakes during landings at high speeds without causing the aircraft to nose over
2. It permits better forward visibility for the pilot during takeoff, landing, and taxiing
3. It tends to prevent ground looping (swerving) by providing more directional stability during ground operation since the aircraft's CG is forward of the main wheels
   • The forward CG keeps the airplane moving forward in a straight line rather than ground looping

Fixed Gear Advantages:

• Always deployed
• Low cost

Fixed Gear Disadvantages:

• Creates constant drag, mitigated by the use of a cover called a fairing

Retractable Landing Gear Disadvantages:

• Increased weight
• Increased cost
• Limited to high-performance aircraft

Retractable Landing Gear Functionality:

• The landing gear, if retractable, may function with either electrical or hydraulic power

• Electrical Functionality:

  • An electrical landing gear retraction system utilizes an electrically driven motor for gear operation
  • When moving a switch in the cockpit to the UP position, the electric motor operates
    • Through a system of shafts, gears, adapters, an actuator screw, and a torque tube, a force is transmitted to the drag strut linkages
    • The gear retracts and locks
    • The struts that open and close the gear doors are also activated
  • When moving the switch to the DOWN position, the motor reverses, and the gear moves down and locks
  • Once activated, the gear motor will continue to operate until an up or down limit switch on the motor's gearbox is tripped

• Hydraulic Functionality:

  • A hydraulic landing gear retraction system utilizes pressurized hydraulic fluid to actuate linkages to raise and lower the gear
  • When the switch is moved to the UP position, hydraulic fluid is directed into the gear-up line
  • The fluid flows through sequenced valves and down-locks to the gear-actuating cylinders
  • A similar process occurs during gear extension
  • The pump that pressurizes the fluid in the system can be either engine-driven or electrically-powered
  • If using an electrically powered pump to pressurize the fluid, the system is called an electro-hydraulic system
  • The system also incorporates a hydraulic reservoir to contain excess fluid and to provide a means of determining the system fluid level
  • Regardless of its power source, the hydraulic pump operates within a specific range
  • When a sensor detects excessive pressure, a relief valve within the pump opens, and hydraulic pressure routes back to the reservoir
  • Another type of relief valve prevents excessive pressure that may result from thermal expansion
  • Limit switches also regulate hydraulic pressure
  • Each gear has two limit switches-one dedicated to extension and one dedicated to retraction
  • These switches de-energize the hydraulic pump after the landing gear has completed its gear cycle
  • In the event of a limit switch failure, a backup pressure relief valve activates to relieve excess system pressure

Landing Gear Switches and Indicators:

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• A switch in the cockpit controls the landing gear position
• In most airplanes, the gear switch is shaped like a wheel to facilitate positive identification and to differentiate it from other cockpit controls, such as the flaps [Figure 7]
• Landing gear position indicators vary with different makes and models of airplanes, but the most common types of landing gear position indicators utilize a group of lights
  • One type consists of a group of three green lights, which illuminate when the landing gear is down and locked [Figure 8]
  • Another type consists of one green light to indicate when the landing gear is down and an amber light to indicate when the gear is up
  • Still, other systems incorporate a red or amber light to indicate when the gear is in transit or unsafe for landing
  • The lights are usually of the "press to test" type, and the bulbs are interchangeable
• Other types of landing gear position indicators consist of tab-type indicators with markings "UP" to indicate the gear is up and locked, a display of red and white diagonal stripes to show when the gear is unlocked, or a silhouette of each gear to indicate when it locks in the DOWN position

Operational Preflight:

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• Because of their complexity, retractable landing gears demand a close inspection before every flight [Figure 9]
• The inspection should begin inside the cockpit
• The pilot should first make certain that the landing gear selector switch is in the GEAR DOWN position
• The pilot should then turn on the battery master switch and ensure that the landing gear position indicators show that the gear is down and locked
• External inspection of the landing gear should consist of checking individual system components
  • The landing gear, wheel wells, and adjacent areas should be clean and free of mud and debris
  • Dirty switches and valves may cause false safe light indications or interrupt the extension cycle before the landing gear is completely down and locked (to prevent collapse while taxiing)
  • The wheel wells should be clear of any obstructions, as foreign objects may damage the gear or interfere with its operation
  • Bent gear doors may be an indication of possible problems with normal gear operation
  • Ensure proper shock struts inflation and that the pistons are clean
  • Check main and nose gear up-and down-lock mechanisms for general condition
  • Power sources and retracting mechanisms should be checked for general condition, obvious defects, and security of attachment
  • Hydraulic lines should be checked for signs of chafing and leakage at attachment points
  • Warning system micro switches (squat switches) should be checked for cleanliness and security of attachment
  • Actuating cylinders, sprockets, universals, drive gears, linkages, and any other accessible components should be checked for condition and obvious defects
  • The airplane structure to which the landing gear is attached should be checked for distortion, cracks, and general condition
  • All bolts and rivets should be intact and secure

Retractable Landing Gear Safety Systems:

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• Most airplanes with retractable landing gear have a gear warning horn that will sound when configuring the airplane for landing, and the landing gear is not down and locked
• Normally, the horn is linked to the throttle or flap position and/or the airspeed indicator so that when the airplane is below a certain airspeed, configuration, or power setting with the gear retracted; the warning horn will sound
• Such devices may prevent accidental retraction of landing gear as mechanical down-locks, safety switches, and ground locks
• Mechanical down-locks are built-in components of a gear retraction system and are operated automatically by the gear retraction system
• Electronically operated safety switches prevent accidental operation of the down-locks and inadvertent landing gear retraction while the airplane is on the ground
• A landing gear safety switch, sometimes referred to as a squat switch, is usually mounted in a bracket on one of the main gear shock struts [Figure 10]
• When the weight of the airplane compresses the strut, the switch opens the electrical circuit to the motor or mechanism that powers retraction
• In this way, if the landing gear switch in the cockpit is placed in the RETRACT position when weight is on the gear, the gear will remain extended, and the warning horn may sound as an alert to the unsafe condition
• Once the weight is off the gear, however, such as on takeoff, the safety switch will release, and the gear will retract
• Many airplanes are equipped with additional safety devices to prevent a collapse of the gear when the airplane is on the ground
• These devices are called ground locks
• One common type is a pin installed in aligned holes drilled in two or more units of the landing gear support structure
• Another type is a spring-loaded clip designed to fit around and hold two or more units of the support structure together
• All types of ground locks usually have red streamers permanently attached to them to indicate installation

Emergency Gear Extension Systems:

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• The emergency extension system lowers the landing gear if the main power system fails
• Some airplanes have an emergency release handle in the cockpit, which connects to the gear up locks through a mechanical linkage
• When operating the handle, it releases the up-locks and allows the gears to free fall or extend under their weight [Figure 11]
  • Due to the wind stream, airspeed limitations may apply to ensure the gear lock when extended

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• The release of the up-lock may be accomplished using compressed gas directed to up-lock release cylinders [Figure 12]

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• In some airplanes, design configurations make an emergency extension of the landing gear by gravity and air loads alone impossible or impractical, and so forceful gear extension in an emergency is required
• Some installations allow either hydraulic fluid or compressed gas provide the necessary pressure, while others use a manual system such as a hand crank for emergency gear extension [Figure 13]
• Hydraulic pressure for emergency operation of the landing gear comes from an auxiliary hand pump, an accumulator, or an electrically powered hydraulic pump, depending on the design of the airplane

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Landing Gear Operations:

• Takeoff and Climb:

  • The landing gear should be retracted after lift-off when the airplane has reached an altitude where, in the event of an engine failure or other emergency requiring an aborted takeoff, the aircraft can no longer be landed on the runway
    • Landing gear retraction should be pre-planned, taking into account the length of the runway, climb gradient, obstacle clearance requirements, the characteristics of the terrain beyond the departure end of the runway, and the climb characteristics of the particular airplane
    • The landing gear should not be retracted until achieving a positive rate of climb
  • If the airplane has not attained a positive rate of climb, there is always the chance it may settle back onto the runway with the gear retracted
    • This is especially so in cases of premature lift-off
  • The pilot should also remember that leaning forward to reach the landing gear selector may result in accidental forward pressure on the yoke, which will cause the airplane to descend
  • As the landing gear retracts, airspeed will increase, and the airplane's pitch attitude may change
    • The gear will take several seconds to retract, and becoming familiar with the sounds and feel of normal gear retraction so that any abnormal gear operation can be readily discernible
  • Sounds and feels associated with gear retraction and locking (and extension and locking) are unique to the specific make and model airplane
    • Abnormal landing gear retraction is most often a clear sign that the gear extension cycle will also be abnormal
  • If operating in winter conditions, keeping the gear down (or, alternatively, cycling the gear up and down several times) during the climb allows the airflow to fling off snow and slush

• Approach and Landing:

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  • The operating loads placed on the landing gear at higher airspeeds may cause structural damage due to the forces of the airstream
  • Limiting speeds (not found on the airspeed indicator), therefore, are established for gear operation to protect the gear components from becoming overstressed during flight
  • They are published in the Airplane Flying Manual/Pilot Operating Handbook for the particular airplane and are usually listed on placards in the cockpit [Figure 14]
    • The maximum landing extended speed (V_LE) is the maximum speed at which the airplane can be flown with the landing gear extended
    • The maximum landing gear operating speed (V_LO) is the maximum speed at which the landing gear may be operated through its cycle
  • Pilots extend the landing gear by placing the gear selector switch in the GEAR DOWN position
  • As the landing gear extends, the airspeed will decrease, and the pitch attitude may increase
  • During the several seconds it takes for the gear to extend, the pilot should be attentive to any abnormal sounds or feel
  • The pilot should confirm that the landing gear has extension and locking by the normal sound and feel of the system operation as well as by the gear position indicators in the cockpit
  • The landing gear should be extended by the time the airplane reaches a point on the downwind leg that is opposite the point of the intended landing
  • The pilot should establish a standard procedure consisting of a specific position on the downwind leg at which to lower the landing gear
    • Operation of an airplane equipped with a retractable landing gear requires the deliberate, careful, and continued use of an appropriate checklist
    • When on the downwind leg, the pilot should make it a habit to complete the landing gear checklist for that airplane
  • Standardization:
    • Supports muscle memory, avoiding forgetting to lower the gear
    • Increases the pilot's awareness of the landing gear's status, checking before landing
  • Unless acceptable operating practices dictate otherwise, complete the landing roll and clear the runway before operating any levers or switches - especially the flaps - as it allows the pilot to focus attention on the after-landing checklist and identify the proper controls
  • This will ensure safety switches are actuated, deactivating the landing gear retract system

• Transition:

  • Pilots transitioning to retractable gear airplanes should be aware of some common errors that lead to accidents:
    • Forgetting (neglecting) to extend the landing gear
    • Inadvertently retraction of the landing gear
    • Activating gear but failed to check gear position
    • Misuse of the emergency gear system
    • Retracting gear prematurely on takeoff
    • Extending gear too late
  • To minimize the chances of a landing gear-related mishap, pilots should:
    • Use an appropriate checklist
      • A condensed checklist placard visible to the pilot serves as a reminder and easy reference
    • Be familiar with and periodically review the landing gear emergency extension procedures for the particular airplane. Be familiar with the landing gear warning horn and warning light systems for the particular airplane. Use the horn system to cross-check the warning light system when an unsafe condition is noted
    • Review the procedure for replacing light bulbs in the landing gear warning light displays for the particular airplane so that you can properly replace a bulb to determine if the bulb(s) in the display is good. Check to see if spare bulbs are available in the airplane spare bulb supply as part of the preflight inspection
    • Be familiar with and aware of the sounds and feel of a properly operating landing gear system

Bungee Struts:

• Bungee struts slowly distribute forces to the airframe at acceptable rates to reduce any bouncing tendencies

Spring Struts:

• Bungee struts slowly distribute forces to the airframe at acceptable rates to reduce any bouncing tendencies

Oleo Struts:

• Oleo struts are composed of oil and air (typically nitrogen)
• A piston absorbs the shock during operations
• Oil absorbs landing shocks
• Air absorbs taxi shocks