Preflight Descriptions:  More Than Merely  ‘Check,’ Part 2

Nose wheel strut
A properly serviced nose strut should provide the proper damping to minimize any oscillations.
Credit: Patrick Veillette

In Part 1, we discussed properly checking windshield seals and nosewheel shimmy.

Do your struts need servicing?

When a business jet’s nose is lowered to the runway on landing, it is important that the nose remains stable regardless of whether the nosewheel abruptly rolls over imperfections in the pavement. The same applies for takeoff. A properly serviced nose strut should provide the proper damping to minimize any oscillations.

“Oleo struts” are commonly used on business jets for their ability to dampen these oscillations. Oleo struts use a combination of air (compressed nitrogen) and hydraulic fluid contained within a cylinder. As the piston (attached to the nose fork) is pushed up into the cylinder upon landing, the hydraulic fluid flows through a small orifice. A metering pin restricts the flow of hydraulic fluid as the piston nears the top limit of its travel, providing more resistance to the piston’s travel. Compressed air on the top side of the piston resists this motion and pushes the hydraulic fluid to flow in the opposite direction (toward extension). This combination of forces dampens oscillations and is designed to return the piston to the proper extension.

The proper amount of fluid and compressed nitrogen are important to the functioning of the oleo strut. Rubber seals in the cylinder will become hardened and brittle over time, especially when exposed to cold weather. Properly serviced struts help to soften landings and prevent damage to the airframe. Keeping the struts in good shape will pay off big in the long run.

What are some symptoms that your nosewheel oleo struts need servicing? During preflight there should be a specified amount of extension in the nosewheel. Sometimes this will be expressed in the number of inches of bright shiny metal visible on the nose strut. If the oleo strut is over-serviced, then an abnormally long amount of shiny metal will be visible. If the nose strut doesn’t comply with the length limitations, bring this to the attention of your maintenance technician.

Cessna Citation Service Letter SL560-32-08 provides instructions for measuring the unloaded landing-gear strut extension. This requires jacking the airplane until all three tires clear the ground and the struts are fully extended. Measure the nose gear strut extension, from the bottom of the upper barrel to the top of the fork. If the dimension is out of tolerance, the strut must be disassembled, inspected and repaired in accordance with the component maintenance manual.

If upon landing the nose strut stays extended and then suddenly collapses, you likely have a low hydraulic level. Any sort of knocking noise from a nose strut during taxi or landing indicates that the nose strut is “bottoming out” and needs to be properly serviced with fluid and/or air.

Be Careful When Cleaning
When you see advertisements for “high-pressure cleaning” devices in aviation catalogs, don’t be quick to buy one and crank up the power to wash the undercarriage of your aircraft. High-pressure water can penetrate bearing seals and begin corrosion of bearing surfaces. Bearings are very susceptible to fretting damage that begins when direct impingement of pressurized water or solvents forces contaminants into the joints, causing accelerated wear, breakdown of internal surfaces and corrosion of rolling elements. 

Service Difficulty Reports have documented deterioration of roller bearing elements, bearings, bushings lined with TFE (tetrafluoroethylene), landing gear joints, electrical components and structural elements.

High-pressure washing can also remove the semi-permanent film from the very-high-strength steels in landing gear components. That film is an essential preventive cover against corrosion. The protective surface should be reapplied at regular maintenance intervals with the proper compounds.

Bonding Straps
Bonding provides an electrical connection between two or more objects not otherwise adequately connected. These metal straps are often found between moving parts of an aircraft, providing a connection between the ailerons and wing.

According to Advisory Circular AC 21-99, “Aircraft Wiring and Bonding,” properly designed and maintained bonding straps and grounding connections are vital for the protection of the aircraft and its occupants. These critical components guard against hazards from lightning discharge and from shock hazard. In the event of a lightning strike, bonding straps allow current to pass through the airframe with minimum arcing. They also prevent the accumulation of static charge, which can interfere with radio and navigational equipment.

Bonding connections must be done so that vibration, expansion or contraction, or relative movement incident to normal service will not break or loosen the connection. They are purposely designed to be flexible and to move with the control surface. However, over time they are susceptible to fatigue and corrosion. Inspect the grounding and bonding straps to ensure that they are free of corrosion, which will adversely affect performance, and that they are not frayed or cut more than 25% of the original strap. 

If a bonding strap does not comply with these limits, then the aircraft isn’t properly protected, and thus it isn’t airworthy. Write it up and have a maintenance technician repair it.

In the final part of this article, we’ll discuss leaking fluids and TKS panel maintenance.

Preflight Descriptions, Part 1:…

Patrick Veillette, Ph.D.

Upon his retirement as a non-routine flight operations captain from a fractional operator in 2015, Dr. Veillette had accumulated more than 20,000 hours of flight experience in 240 types of aircraft—including balloons, rotorcraft, sea planes, gliders, war birds, supersonic jets and large commercial transports. He is an adjunct professor at Utah Valley University.