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CH-47 Damper-Protecting a Precious and Limited Resource


CH-47 Damper – Protecting a Precious and Limited Resource

The CH-47 Lag Damper is one of the more frequently overlooked maintenance items - generally only subject to a pre-flight visual inspection for cracks, leaks and visible discolored fluid. However, problems may be lurking that will not be detected visually. Proactive maintenance will safeguard the damper’s proper function and help keep your aircraft operational.

Many current fleets are comprised of helicopters purchased in the surplus market from the military with potentially high airframe time. As a part that is serviced on condition, the dampers on these CH-47s have likely not been serviced in a considerable period of time and are prone to failure, grounding the aircraft. Also, many of these surplus CH-47s have been operated in extreme environments, hastening further deterioration.

A complex system, the blade lag shock absorber consists primarily of the housing assembly and piston assembly. The housing assembly consists of a housing, bushing, self-aligning bearing, 2 LEE check valves, jets, and pin plugs. A bearing sleeve, cylinder sleeve and an end cover retainer are installed in large bore of the housing. A control valve assembly and two check valves assemblies are installed in small bores in lower portion of the housing. A fluid tank and vent valve assembly are attached to the inboard end of the housing. The piston assembly consists of a piston, four LEE check valves, springs, spring seats, screw assemblies, and locking screws which constitute the four cushion control valves. The piston assembly extends through the open end of the housing assembly and through the end cover cylinder sleeve and bearing sleeve. A self-aligning rod end bearing is installed at the exposed end of the piston assembly. The head of the piston assembly contains piston rings and expanders.

The lag damper is in use from startup and shutdown. It is one of the most used components of the helicopter that works wells and lasts a long time – until it doesn’t. Fluid contamination is a significant problem. Repair and replacement is the usual strategy, but is expensive and dependent on locating existent parts.

The main areas of concern are the Damper Piston Cushions, Control Valve and Check Valves in the Damper Housing. With a diameter of 0.187 inches and an internal passage way of approximately 0.06 inches in the check valve; not much sediment is required to generate an issue.

The most significant problems are sediment and water intrusion which impedes the proper operation of the damper and will destroy the usability of the damper. As the dampers are not readily available, the replacement supply is limited to existing inventory, a dwindling resource. If the housing is damaged beyond usability, replacements are limited.

Little field maintenance can be accomplished on the damper and is limited to pre-flight inspections for overall condition - checking fluid levels, searching for cracks, leaks and any other obvious maladies. Rod end bearings may be replaced in the field while housing bearings will need to be done in a shop with further inspections being performed while the bearing is removed.

Due to the construction of the reservoir and internal components, withdrawing the visible fluid only allows for a small percentage of the total fluid to be replaced. Contamination in the damper can only be removed with a complete tear down and cleaning followed by an extensive testing after reassembly.

Further complicating the condition assessment is that only 5% of CH47 dampers indicate date of last service. Completing a preventative basic cleaning/repair on these dampers creates a known baseline for further continuous operations removing the guesswork regarding the condition in which they were inherited into the system for usage.


  • Leaking

  • Unusual vibrations

  • Visual indication the damper is extending/retracting more than usual

  • Visual condition of the fluid. If it looks dark in the sight glass, the bulk of the fluid will be exponentially darker and more deteriorated

  • Rod end or housing bearing showing sign of deterioration

Sunset Aviation Services can keep you up and running to protect your damper assets. They can perform basic repairs which include- pretest, disassembly, cleaning- clear all interior passageways of sludge, replacement all O-rings and replacement of seals as needed as well as relapping main check valves for proper operation. Sunset can complete further repairs including stripping and repaint, new data tags, replacement of rod end and housing bearings followed by a return to service post testing per manufacturer’s specifications.


Robert Peterson is recognized as a world-renowned authority on hydraulic systems with 55 years of experience in aviation. Service of five years in the Air Force and 31 in Oregon National Guard offered versatile and expansive hydraulic systems experience on many aircraft including F-4, F-100, F-101, F-102, F-104, F105, F-106, B-52, KC-135, C-130, C-47, C-54, T-26, H-19, H-43, and F-15, among others.

As Hydraulic Shop Chief for more than 30 years, Peterson innovated and improved upon systems such as the serviceability of the BV-107 and BV-234 hydraulic pumps, working in conjunction with Boeing engineering and Pall Corp to develop more effective filtration. His efforts significantly increased flight hours on Columbia Helicopters’ Vertol B107/234 fleet. Peterson served as a member of the Standards of American Engineering (SAE) for Aerospace Actuation, and Control and Fluid Power Systems. Peterson is currently applying his extensive experience and expertise as Director of Maintenance at Sunset Aviation Service.

Tony Lindstrom, President of Sunset Aviation Service, combines experience from 16 years as a helicopter pilot and 27 years as an A&P mechanic. He opened Sunset Helicopters as a 133/135/137 flight school in 2002 which evolved in 2013 to include a repair station. His capabilities include Bell 206, Huey UH-1, CH-46, CH-47, CH-54 and S-61 components.

Sunset Aviation Service, Inc. can accept all aircraft hydraulic components for evaluation and repair covered in its repair manual.  Federal Aviation Administration's 145 Repair Station Certificate 9SHR799L Hydraulic services include standard repairs and overhaul, seal change and exchange units. Various other components can be addressed upon request provided manuals and support documents are available.

Sunset Aviation Service.

23115 Airport RD NE #12

Aurora OR 97002


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