The Duncan Download Blog: Business Aviation Advice & Observations

Contributed by Jerry Cable, Accessory Tech Rep.

Most definitely. Yes.

Landing gear overhauls, inspections and restorations are sometimes called a commodity service. Any shop that has received an OEM Maintenance Manual and follows FAA standards is free to advertise that it can inspect and overhaul aircraft landing gear. The FAA states "a thorough inspection of the landing gear involves the entire structure of the gear, including attachments, struts, wheels, brakes, actuating mechanisms for retractable gears, gear hydraulic system and valves, gear doors, and all associated parts. The manufacturer’s inspection procedures should be followed where applicable."

But not all landing gear service is equal. There are many additional service and capability requirements related to landing gear maintenance that can cut expenses and downtime that can’t be found at every aircraft landing gear shop.

The landing gear is a behind-the-scenes inspection, most often scheduled alongside, but taking a backseat to, a major airframe inspection. However, it can have a huge impact on the entire project’s schedule if the landing gear restoration runs into major squawks. You will want to use a shop that has invested heavily in the landing gear customer and understands full well the impact a landing gear overhaul or inspection has on an airframe inspection. After all, an aircraft can’t go anywhere without its legs.

Controlling Costs

At Duncan Aviation, we take several extra steps that help reduce costs. For example, on select inspections and restorations, we ask customers to leave the harness and brake pipes on the gear when removing it from the aircraft. By leaving the gear intact, this will save customers approximately 35 labor hours when removing and installing the gear. Duncan Aviation has also established some special pricing programs that help provide assurance to customers through pricing controls. These are just a sampling of the above-and-beyond landing gear available at Duncan Aviation.

Feel free to reach out and contact Duncan Aviation's landing gear experts. We will address the most common questions and concerns that customers ask us daily regarding landing gear work.

1. How much will it cost?
2. How long with the inspection take?
3. What about corrosion removal?

Jerry Cable is an Accessories Tech Rep located at Duncan Aviation’s Lincoln, Nebr., facility. He is a landing gear and accessory components and systems specialist. His aviation career began in 1991.

Contributed by Jerry Cable, Accessory Tech Rep.

When experiencing an intermittent nose wheel steering fault, the first thought is generally, “what is wrong with the steering servo, computer, valve, actuator or strut?” But don’t overlook a very important part of the system—Weight on Wheels (WOW). This system is very easy to test and replace. It has been my experience that WOW is the cause for more than half of these types of faults.

The WOW System

Most aircraft utilize some type of WOW Sensor or Switch that activates when the aircraft is on the ground. They come in many different sizes shapes and technologies and can be in various positions in the aircraft and landing gear. The one thing they all have in common is they complete the circuitry required to do many other things on the aircraft. From thrust reversers to nose wheel steering and many in between, the aircraft relies on these switches to operate correctly. A faulty or incorrectly adjusted switch/sensor may cause vital systems to not function or function intermittently.

The Wow System

There two basic types: mechanical switches and proximity sensors. Mechanical switches are easier to test; however, they fail more often because they rely on mechanical contacts to create the circuit. Proximity sensors do not utilize a direct mechanical contact but instead use circuitry to decipher when the magnetic field is interrupted. The proximity sensors are more reliable, but more difficult to troubleshoot. Most mechanical switches are either open or closed. Yet due to the circuitry of a proximity sensor it can be partially open and partially closed at the same time. So a proximity sensor that is known to be good but set to an incorrect clearance can cause very erratic behavior of other systems.

Systems that are run through the WOW system are exclusively used either in the air or on the ground, not both. Systems that utilize the WOW system include but not limited to, thrust reversers, nose wheel steering, trim and autopilot.

Jerry Cable is an Accessories Tech Rep located at Duncan Aviation’s Lincoln, Nebr., facility. He is a landing gear and accessory components and systems specialist. His aviation career began in 1991.

Contributed by Jerry Cable, Accessory Tech Rep

Challenger landing gear requires a detailed inspection at 60 months for the 600 and 601 and 96-months for the 604 and 605. It must then undergo a 120-month restoration for the 600 and 601 and 192 month for the 604/605.

The reason for these inspections is to check for cracks, wear and corrosion. Years of water, dirt, salt air, fluid and various other electrolytes have taken their toll. The gear is very susceptible to corrosion and even though steps may have been taken to lessen the effects, some corrosion still forms.

Corrosion is normally found throughout the gear but can often be repaired using standard repair schemes available in the CMM or through engineering concessions. Sometimes, however, the corrosion has exceeded the limits of those repair tolerances.

Some of the more severe corrosion found during these inspections is detailed in the following photographs.

	MLG Trailing Link CL600/601 Axle Bore corrosion found in 35% of the restorations we perform.*
	MLG Side Strut Pin CL600/601 Corrosion on OD radius found in 50% of restorations.*
	MLG Hinge Pin CL600/601 Corrosion on the ID mating with brake line swivels found in 70% of the restorations.*
	MLG Oleo Pin CL600/601 Corrosion on the ID found in 65% of the overhauls.*
	MLG Oleo Cylinder CL600/601 ID corrosion found in 50% of the restorations.*
	NLG Main Fitting CL600/601 Corrosion under steering sleeve found in 50% of restorations.*
	NLG Plunger Tube CL600/601 Corrosion, lower band mating with axle found on 75% of restorations.*
	NLG Axle Barrel CL600/601 Corrosion mating with plunger tube found on 75% of restorations.*
	NLG Steering Cuff CL600/601 Corrosion in the ID Bore found in 60% of restorations.*
	NLG Axle Transverse Bore CL600/601 Corrosion found on 50% of the restorations.*
	NLG Steering Rack CL600/601 Tooth damage going to caster mode found during 90% of restorations.*
	MLG Side Strut Rod End CL600/601 Bearing bore corrosion found in 85% of the restorations.*
	MLG Side Strut CL600/601  Bearing bore corrosion found in 85% of the restorations.*

Jerry Cable is an Accessories Tech Rep located at Duncan Aviation’s Lincoln, Nebr., facility. He is a landing gear and accessory components and systems specialist. His aviation career began in 1991.

Contributed by Dave Schiver, Airframe Tech Rep.

Corrosion Ignored never stops!

Due to its location and exposure to the elements, landing gear is highly susceptible to corrosion. And corrosion, even in small amounts, will not stop growing until it is effectively removed. If left unattended, landing gear corrosion can grow and jeopardize the functionality and airworthiness of the unit leading to expensive repairs or the premature removal of components.

As a Learjet (non 40/45) owner/operator, I want to advise you of a corrosive situation that can occur on all Learjet model aircraft, excluding the 40/45. These aircraft are susceptible to corrosion in the sockets for the main landing gear forward trunnion pins on the bottom side of the wing. Corrosion in this area is serious if left unattended, with the potential to cause cracks in the weakened casting and eventual failure resulting in a potential gear collapse.

Five consequences of ignoring Learjet (non 40/45) landing gear trunnion corrosion.

1. Increased repair costs requiring the sending of the wing to the OEM for repair or even complete wing replacement.
2. FAA violations for not following the maintenance manual which requires the repair of all corrosion.
3. If corrosion is ignored for too long, the castings will have to be replaced.
4. Corrosion can spread into the adjacent spar which translates into a longer downtime and more money to replace the spar, if not scrapping the entire wing.
5. Catastrophic failure of the Main Landing Gear (MLG) socket resulting in a takeoff, landing or taxi incident leading to injury or loss of life.

Two solutions to trunnion casting corrosion

Replace Trunnion Castings. To replace trunnion castings, the wing will need to be de-mated and shipped to the OEM in Wichita, KS. There it will be placed in the production wing fixture to remove the upper skin and replace the trunnion castings. Depending on the demand for the fixture, we have seen downtimes as long as six months and at a significantly high repair cast.   

Remove Corrosion from Trunnion Casting. Without having to remove the wing, Duncan Aviation is able to remove the corrosion with an on-wing fixture that allows qualified airframe technicians to bore the trunnion and insert a bushing, while staying within allowable limits. This repair process generally takes only 5-7days to complete, eliminating the need for wing removal, shipping, months of down time and at a dramatically less overall cost.

If there is sufficient Ground Support Equipment (GSE) available at your location, this repair can be performed in your hangar by Duncan Aviation’s Engine AOG Rapid Response Teams.

You can read more about this innovative on-wing fixture in the Duncan Debrief. If you have any questions about this or other Learjet technical issues, contact me at anytime. For scheduling and pricing information, contact Learjet Airframe Service Sales Representative Brad Lennemann.

Dave Schiver is an Airframe Technical Representative at Duncan Aviation’s Lincoln, Nebr. (LNK) facility. He specializes in Learjet aircraft. His aviation career began in 1981.

Jerry Cable, Accessory Tech Rep

Messier is aware that the current configuration and greasing procedure of the main landing gear (MLG) main fitting may not allow grease to be forced into the joint between the MLG main fitting and trailing link assembly. The lack of grease on these parts can lead to abnormal and excessive wear on the bushings. A supplemental greasing procedure and associated tooling has been developed by Messier to lubricate this joint. The procedure will allow grease to reach the affected areas, but is somewhat cumbersome to perform.

Bushing Design Improvements

Messier and Bombardier are in the process of releasing Service Bulletins that improve the bushing design to eliminate the need for a supplemental greasing procedure. It is recommended that this improvement be installed during the next 96-month or 192-month inspection interval.

The improvement not only rectifies the problem of grease passage to the proper area but also implements a new installation procedure. The installation procedure, which uses wet sealant on all areas of the bushings, will help to alleviate the corrosion problem present on many of the main fittings under the bushings.

While the bulletins have not yet been released, the improvements can be implemented and approved through a Messier concession and subsequent Bombardier SRPSA approval. Consider these improvements and the next scheduled inspection to prevent further or future problems with the MLG that could lead to costly repairs or even more costly replacement. Contact Duncan Aviation's landing gear experts for more information. 

Jerry Cable is an Accessories Tech Rep located at Duncan Aviation’s Lincoln, Nebr., facility. He is a landing gear and accessory components and systems specialist. His aviation career began in 1991.