Landing gear employing a rear-mounted wheel is called conventional or a tailwheel/dragger [Figure 1]
Tailwheel landing gear aircraft have two main wheels attached to the airframe ahead of its Center of Gravity (CG) that support most of the weight of the structure
Conventional Gear Advantages:
Allows adequate ground clearance for a larger propeller
More desirable for operations on unimproved fields
Conventional Gear Disadvantages:
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
Lack of good forward visibility when the tailwheel is on or near the ground
Landing gear employing a front-mounted wheel is called tricycle landing gear [Figure 2]
Tricycle landing gear aircraft have two main wheels attached to the airframe behind its CG that support most of the weight of the structure
Additionally, a nose wheel will typically provide snose wheel steering control through either castering or mechanicanl linkage designs
Nose wheels may be accompanied by a shimmy damper
Tricycle Gear Advantages:
It allows the more forceful application of the brakes during landings at high speeds without causing the aircraft to nose over
It permits better forward visibility for the pilot during takeoff, landing, and taxiing
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
Pontoons:
One or more pontoons, or floats, are mounted under the fuselage to provide buoyancy [Figure 3]
Fixed gear simplifies design and operation [Figure 5]
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:
Retractable gear streamlines the airplane by allowing the landing gear to stow inside the structure during cruising flight [Figure 6]
The primary benefits of being able to retract the landing gear are increased climb performance and higher cruise airspeeds due to the resulting decrease in drag
Retractable landing gear systems may be operated either hydraulically, electrically, or may employ a combination of the two systems
Warning indicators are provided in the cockpit to show the pilot when the wheels are down and locked and when they are up and locked or if they are in intermediate positions
Emergency operation systems provide additional security
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:
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:
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:
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:
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
The release of the up-lock may be accomplished using compressed gas directed to up-lock release cylinders [Figure 12]
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
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:
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
The maximum landing extended speed (VLE) is the maximum speed at which the airplane can be flown with the landing gear extended
The maximum landing gear operating speed (VLO) 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
Landing Gear Components & Equipment:
There are varying different parts, linkages, and hoses that are necessary for landing gear to provide their function, including:
Struts
Struts:
Struts transmit shock loads of landing, takeoff, and taxi to the airplane structure
There are three types of landing gear struts:
Bungee
Spring
Oleo
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
Steering Linkages:
Steering is typically controlled through the rudder pedals, but in larger aircraft, separate controls are used
A steerable nosewheel or tailwheel permits the airplane to be controlled throughout all operations while on the ground
Steerable wheels are linked to the rudders by cables or rods, while caster wheels are free to swivel
In both cases, the aircraft is steered using the rudder pedals
Aircraft with castering wheels may require the pilot to combine the use of the rudder pedals with the independent use of the brakes
Tow Bars:
Tow bars attach to the aircraft to better steer the aircraft during ground tows [Figure 15]
Tow bars range from bars you drag to drill-powered and even remote tug devices
The towbar should never be attached to the plane unless the pilot is attached to the towbar
Aircraft Brakes:
Brakes are necessary to slow the aircraft down after touchdown on the runway to a speed at which they can turn onto a taxiway
In the case of most modern airplanes, airplane brakes are generally disc brakes
They consist of multiple (one for each pedal) hydraulically actuated pads (called caliper pads) squeezed toward each other with a rotating disk (called a rotor) between them
The pads place pressure on the rotor, which is turning with the wheels
As a result of the increased friction on the rotor, the wheels inherently slow down and stop turning
The disks and brake pads are made either from steel, like those in a car, or from a carbon material that weighs less and can absorb more energy
Because airplane brakes are used principally during landings and must absorb enormous amounts of energy, their life is measured in landings rather than miles
Typically located on the main gear only
Applied by either a hand control or by foot pedals (toe or heel)
Foot pedals operate independently, allowing for differential braking, and can supplement nosewheel/tailwheel steering for ground operations
Disc brakes are most common on trainers
Most brake systems are hydraulically actuated
Air brakes may be used to slow the aircraft for landing and while in flight
Breaks controlled by the top of the rudder pedal to apply pressure
When pressure is applied, fluid from the reservoir is directed via the master cylinder to the calipers
A parking brake helps keep the brake applied during ramp operations, as do aircraft chocks [Figure 16]
Landing Gear and Tire Servicing & Maintenance:
Tire Pressure:
Service tires with an air compressor or nitrogen for almost every general aviation aircraft
Commercial aircraft require nitrogen to compensate for lower temperatures at altitude, increasing tire pressure stability and overall physical integrity
Tire pressure changes by about 1 psi per 10° F (pressure increases with increased temperature, and vice versa)
Improperly inflated tires will not only result in undesirable performance but also increased wear
Wheel Hardware:
The hub and bearing bore are susceptible to corrision if left unmaintained
The hub bearings specifically must remain greased (not oiled) to minimize friction and resist moisture
Untreated bearings will lead to corrision on the wheel halves
Breaks & Brake Assemblies:
Brake fluid is hydroscopic, meaning the fluid can absorb and retain moisture
Moisture, typically entered through the vented reservoir, can accumulate in the lower portions of the system, causing corrision
The lowest part of the system is typically a caliper
Bleeding the brakes avoids moisture buildup; however, note that bleeding is not considered preventative maintenance and requires an A&P
Tire Surface Condition:
Landing in crosswinds increases the risk of flat spots, reducing the life of tires
Cracks in the sidewall are unacceptable
Cracks on the tread are acceptable until they interfere with the tread
Tire Storage:
Reduce, where possible, sun exposure to tires, as ultraviolet light will break down rubber over time
Low-wing aircraft will have less of a problem with UV light than high-wing
Landing Gear Malfunctions:
Treat any indication of a malfunction as actual
The pilot should not recycle the landing gear if indications or sounds suggest an irregularity
Consider landing at an airfield with Crash Fire Rescue
Before landing, exhaust all avenues for correcting the problem (maintenance, ATC, any other instructors)
Consider exhausting all fuel possible to reduce the risk of fire
After landing, remain on the runway and contact the maintenance department to inspect the gear before it is taxied or towed back to the ramp
Landing Gear Fails to Retract:
When the landing gear will not retract after takeoff, the pilot should leave the landing gear extended
Trying to force the landing gear to retract may cause the landing gear to become stuck in the retracted position
Landing gear position may be confirmed by the tower or other aircraft
If the landing gear appears locked down, then flight may be continued at reduced performance
Consideration should be given to rescue services at the destination in the event of further emergency
Consideration should be given to inspecting the landing gear before taxiing, following landing
Landing Gear Fails to Extend:
When the landing gear will not extend, the pilot should try to manually extend the landing gear
Landing gear position may be confirmed by the tower or other aircraft
If a gear-up landing is required, consideration should be given to pavement vs. grass to ensure a smoother landing (no bumps, etc.
Consideration should also be given to fields with the appropriate services desired after an emergency landing
Parking Brake Fails to Disengage:
If the parking brake fails to disengage in a tricycle gear aircraft, more than likely, the aircraft won't move or it will try to rotate about the stuck wheel
The risk to the aircraft is relatively minimal, of course, unless there are objects very nearby
Tail dragger airplanes risk a prop strike if attempted to move, as the aircraft will immediately rotate forward
Shimmy Damper Failure:
Dampener shimmy will be felt in the rudder pedals when malfunctioning
Blown Tire:
A blown tire will produce a turning moment toward the blown tire when on the surface
Pilots should be suspicious of other damage that could have occurred when the wheel blew, especially when operating retractable landing gear
Common Training Aircraft Landing Gear Characteristics:
Cessna-172:
Tricycle type w/ steerable nose wheel
Shock absorption is provided by tubular spring steel main landing gear struts and the air/oil nosegear shock strut
Each main gear is equipped with a hydraulically actuated disc-type brake on the inboard side of each wheel
Each brake is connected by hydraulic lines to a master cylinder, which is attached to each of the pilot's rudder pedals
Effective steering is accomplished through nosewheel steering by using the rudder pedals (ground)
The nosewheel turns about 10° on each side of center
Applying left or right brake results in differential braking
The minimum turning radius using differential brakes is about 27 feet
Piper Arrow:
Retractable tricycle landing gear
Hydraulically actuated by an electrically powered reversible pump
The operation takes about 7 seconds
Has emergency gear extension lever to equalize system pressure
The nosewheel is spring-assisted during freefall
Nose gear steerable 30° arc on each side of center
WARNING GEAR UNSAFE displays when:
Gear up and power reduced below approximately 14" manifold pressure,
Gear selector UP on the ground with the throttle in retarded position, or
Flaps extended beyond 10° without gear down and locked
Warning horn sounds at 90 Hz
The main gear uses Cleveland single-disc hydraulic breaks
Toe breaks and the parking brake use separate cylinders, but a common reservoir
Private Pilot (Airplane) Operation of Aircraft Systems Airman Certification Standards:
.
Objective: To determine the applicant exhibits satisfactory knowledge, risk management, and skills associated with safe operation of systems on the airplane provided for the flight test.
Private Pilot (Airplane) Operation of Aircraft Systems Risk Management:
The applicant is able to identify, assess, and mitigate risk associated with:
PA.I.G.R1:
Detection of system malfunctions or failures.
PA.I.G.R2:
Management of a system failure.
PA.I.G.R3:
Monitoring and management of automated systems.
Private Pilot (Airplane) Operation of Aircraft Systems Skills:
The applicant exhibits the skill to:
PA.I.G.S1:
Operate at least three of the systems listed in K1a through K1l appropriately.
PA.I.G.S2:
Complete the appropriate checklist(s).
Private Pilot (Airplane) Systems and Equipment Malfunctions Airman Certification Standards:
Objective: To determine the applicant exhibits satisfactory knowledge, risk management, and skills associated with system and equipment malfunctions appropriate to the airplane provided for the practical test
Causes and remedies for smoke or fire onboard the aircraft.
PA.IX.C.K4:
Any other system specific to the airplane (e.g., supplemental oxygen, deicing).
PA.IX.C.K5:
Inadvertent door or window opening.
Private Pilot (Airplane) Systems and Equipment Malfunctions Risk Management:
The applicant is able to identify, assess, and mitigate risk associated with:
PA.IX.C.R1:
Checklist usage for a system or equipment malfunction.
PA.IX.C.R2:
Distractions, task prioritization, loss of situational awareness, or disorientation.
PA.IX.C.R3:
Undesired aircraft state.
PA.IX.C.R4:
Startle response.
Private Pilot (Airplane) Systems and Equipment Malfunctions Skills:
The applicant exhibits the skill to:
PA.IX.C.S1:
Describe appropriate action for simulated emergencies specified by the evaluator, from at least three of the elements or sub-elements listed in K1 through K5 above.
PA.IX.C.S2:
Complete the appropriate checklist(s).
Conclusion:
Follow established procedures and practice to avoid distractions, which can lead to landing with the gear up
Depending on the type of landing gear, an understanding of the aircraft's electrical and/or hydraulics & pneumatic systems is required
Landing gear may also be used to increase rates of descent by increasing drag
Aerodynamic breaking, that is, the raising of flaps, trying to hold the nose up, are effective ways to slow the aircraft before the pilot must apply brakes