The DC-9 aircraft was designed in the early 1960s and introduced into commercial service in 1965. The original version had a maximum takeoff weight of 76,000 lb and could carry 80 passengers. Growth versions of the DC-9 increased the aircraft capacity to 121,000 lb maximum takeoff weight in the Series 50. Today’s airplane, the MD-80, at 160,000 lb, has a takeoff weight more than double that of the first DC-9s.

The original aircraft hydraulic design principles were to keep the hydraulics:
• simple
• reliable, and
• easy to operate and maintain.

The concept of the DC-9 hydraulic system was very simple: a minimum number of components and maximum of efficiency. The DC-9 required only half as many hydraulic components as were used in the DC-8, and substantially fewer than used in contemporary small jets. Simplification of mechanisms paid off in a corresponding improvement in reliability and ease of operation and maintenance.

Component improvement
Historic hydraulic system problems were analyzed and either solved or greatly minimized. For example:
• packing failure problems were virtually eliminated by Douglas’ pioneering in the development of ethylene propylene compounds for use with phosphate ester fire-resistant fluid
• backup rings used with all static and dynamic seals greatly reduced O-ring nibbling
• elimination of positionable, universal-type fittings in boss connections increased reliability
• while flareless-type tube fittings were offered as an optional feature, flared fittings were recommended as standard and accepted by virtually all DC-9 operators
• corrosion-resistant tubing was used in all pressure applications and in return lines smaller than in OD
• coiled tubing replaced hoses where possible
• where hoses were used, they had Teflon-lined, stainless steel wire braid construction, and
• careful attention to design detail greatly reduced the ever-present problems of system surges and corrosion.

Family overview

The family of DC-9 aircraft has two independent and completely separate hydraulic systems, Figure 1. Fluid from one system is never allowed to mix with fluid from the other. The systems are known as Right or Left; each receives its primary power from an 8-gpm variable displacement pump mounted on the respective right or left engine. The Right system is also equipped with an 8-gpm electric-motor-driven auxiliary pump. The Left system received auxiliary power from two fixed displacement, mechanically connected pumps to provide a reversible drive between the two systems. Each system also contains a hand pump for ground servicing.

The engine pumps are dual-range-compensated and may be selected to either 3000 psi or 1500 psi. During normal takeoff and landing operations, all pumps are switched ON and the engine pumps are in the 3000-psi mode. During cruise, after cleanup, the electric pump and reversible pumps are switched OFF and the engine pumps are switched to the 1500-psi mode. This reduced pressure greatly increases component life.

Hydraulic systems

The systems are as air-free as modern technology permits. Closed system, boot-strap reservoirs are used with ample air trap volume, and simple pushbutton bleeding valves purge air quickly, Figure 2.

Figure 2

Excellent return filtration is achieved with identical 15-µm absolute disposable-element system filters. All return fluid is filtered as it enters the reservoir. Fluid from the electric and engine pumps is filtered on the pressure and case drain lines. All fluid entering the system is filtered, whether a ground service connection or hand pumps is used. Each filter incorporates a visual, differential pressure indicator pop-up button to signal when an element must be replaced.

The hydraulic and landing gear systems for the DC-9 family have changed as the aircraft grew. Series 10 established the baseline design. Series 30 added a hydraulically-driven wing leading edge slat system, increased reservoir capacity, and strengthened the landing gear. Series 40 retained the series 30 hydraulic system but changed the wing incidence angle. The size of the main landing gear was increased, repositioned in the wing, and used a new rolling assembly. Series 50 required only a change to the elevator system to increase hinge moment capability.

The MD-80 represents more substantial change. The wing span was increased, adding to both the leading edge slats and trailing edge flaps. The main landing gear is re-positioned in the wing, has a larger diameter and length, and a new rolling assembly. Also added is a new inboard ground spoiler system. Reservoir capacity was increased again and most hydraulic components were re-designed or re-located. The auxiliary pump was changed to the DC-10 air-cooled unit; a new reversible power transfer unit was added. Through all of the system evolution, the engine hydraulic pumps remained the same.