The primary hydraulic flight control system of an Embraer 15 comprises of elevators, rudder and ailerons. While the rudder and ailerons are hydraulically powered, its elevators are mechanically actuated. This airplane is equipped with two hydraulic systems (1 and 2) that operate independently, and each one of them is powered by an electric motor-driven pump and an engine driven-pump (Pratt 1999). The rudder and ailerons may also be mechanically actuated in the event of loss of both hydraulic systems. Both hydraulic systems are similar, except for the systemic functions of each and a priority valve placed in hydraulic system one. Like Embraer 145, Boeing also has two hydraulic systems (A and B). Both systems A and B hydraulically power the primary flight control operations of the airplane by altering the positions of the ailerons, rudders and elevators (Cook, 2009). Either hydraulic system A or B can operate all the functions of primary flight control.
The hydraulic systems of these aircrafts can be compared by reviewing their roll control operations. Looking at the roll control of an Embraer 145, it is noted that this operation is enabled by the hydraulically-actuate ailerons. The ailerons are controlled by any of the two control wheels (Pratt 1999). As for a Boeing 737, roll control comprises of two elements: the hydraulically-actuated ailerons and the flight spoilers, which, like in the case of an Embraer, are controlled trough rotation of either control wheel in the cockpit. The ailerons of an Embraer 145 are controlled by the control wheels of the pilot that are joined by cables and a torque tube to take mechanical input to two different hydraulic actuators. Each aileron actuator is supplied by the two hydraulic systems, each of which has the capability of full power control (Cook, 2009). In a Boeing 737, ailerons are also positioned using the control wheels of the pilot that are connected through cables that supply mechanical input to the two different control units of hydraulic actuation. Hydraulic systems A and B of a Boeing avail pressure to the power control units that operate the ailerons.
It also imperative to notice that the hydraulic system of an Embraer 145 can be shut off when necessary through a button positioned on the overhead control panel. However, loss of power in both hydraulic control systems, the control wheels can mechanically control the position of ailerons of both Embraer 145 and Boeing 737 (Cook, 2009). In a Boeing 737, there are two flight control switches in control of the shut off valves for every aileron, and the switches can also control hydraulic pressure of the rudder and elevators. In the event of aileron jamming, both control panels of an Embraer 145 may be disconnected by means of a handle installed on the control pedestal to free the other aileron and allow it to be commanded (Pratt 1999). The failure of either aileron in a Boeing 737 leaves the first flight control officer with the choice of bypassing the control system of ailerons because roll control is only possible through operation of the flight spoilers.
Concerning the spoiler system responsible for roll control, the spoiler system of an Embraer 145 include a speed brake and subsystems of ground spoilers. Speeds brakes facilitate an increase in the descent rate and decelerate the aircraft, while ground spoilers destroy the lift effect, thereby enabling better effectiveness in braking. The spoilers of this aircraft are commanded electrically and actuated hydraulically. The spoilers of a Boeing 737 include four 737NG, two positioned respectively on the upper surface of every wing. Hydraulic systems A and B power the spoilers through hydraulic shut off valves controlled by the pilot through two Flight Spoiler Switches (Cook, 2009). These spoilers get hydraulically actuated in response to positioning of aileron controls. A spoiler mixer is connected to the cable drive of the aileron to control the hydraulic control centers on each spoiler so that the movement of the spoilers is proportional to that of the ailerons (Pratt 1999).
The hydraulic systems can also be compared in terms of pitch control. An electrically-controlled horizontal stabilizer commanded by the Pitch Trim System and mechanically-positioned elevators controls the pitch of an Embraer 145. The pitch control surfaces of a Boeing 737 include hydraulically-actuated elevators and a stabilizer powered by electricity. Cables join the control columns of the pilot to the hydraulically-powered Power Control Units of the elevator. The power comes from four inputs of systems A and B – autopilot, neutral shift and mach trim (Pratt 1999). Normally, the stabilizer is controlled either by the autopilot or the switches of the stabilizer trim. In essence, this discussion shows that the aircraft hydraulic systems of Embraer 145 and a Boeing 737 are similar in most aspects. The chief observation is that Embraer 145 has two hydraulic systems (1 and 2) controlling ailerons and rudder, while Boeing 737 also has two hydraulic systems (A and B) controlling ailerons, rudder and elevators. The elevators of Embraer 145 are actuated by mechanical means.
Cook, M. V. (2009). Aerospace engineering desk reference. Amsterdam: Elsevier.
Pratt, R. (1999). Flight control systems: Practical issues in design and implementation. London: Institution of Electrical Engineers.