Overview
This section presents mechanical design and installation considerations with an emphasis on interface and coordination between engineering functions. The mechanical systems covered in this section include flight controls, hydraulics, landing gear, and the water and waste systems. Some of the interface and/or requirements data come from, or are related to, aerodynamics, autoflight, proximity and position indication, structural interfaces, human factors, and aircraft real estate considerations.
Flight controls
The basic building blocks of today’s flight control systems consist of mechanical input devices in the cockpit, a closed loop cable system to transfer the pilot’s input motion, and an output mechanism that operates a hydraulic valve or control surface tab. These controls actuate the various flight surfaces of the aircraft.
As we add features to meet growing safety requirements, more demanding performance requirements, and decreased pilot workload, these mechanical systems become increasingly complex.
Fly-by-wire
It should be noted that flight control definitions and descriptions are changing as fly-by-wire control systems have come into widespread usage in commercial transport aircraft. Current fly-by-wire control systems require extremely precise input devices, which present a special challenge for the mechanical systems designer.
Contrary to popular belief, fly-by-wire does not generally save weight; it is usually heavier. Additionally, fly-by-wire is not automatically safer. Safety is generally related to system architecture. The value in fly-by-wire is that good system architecture is easier to implement using electronic components.
Some other considerations are addressed in Table 7.5.3.
Table 7.5.3
Design requirements
Many of the basic system performance requirements are set by the Aerodynamics and Avionics departments. Some of these requirements include:
- • stick position vs surface position
- • artificial load feel requirements
- • system stiffness and frequency response
The FAA, JAA, and CAAC also set many basic safety and operational requirements. FARs 25.671 and 25.1309 relate to single failure and probability of multiple failures related to hazard class. FARs 25.779 and 25.781 relate to shape, color, and motion requirements of control levers and knobs.
Figure B-1. FAR 25.671 General
- (a) Each control and control system must operate with the ease, smoothness, and positiveness appropriate to its function.
- (b) Each element of each flight control system must be designed, or distinctively and permanently marked, to minimize the probability of incorrect assembly that could result in the malfunctioning of the system.
- (c) The airplane must be shown by analysis, test, or both, to be capable of continued safe flight and landing after any of the following failure or jamming in the flight control system and surfaces (including trim, lift, drag, and feel systems) within the normal flight envelope, without requiring exceptional piloting skill or strength. Probable malfunctions must have only minor effects on control system operation and must be capable of being readily counteracted by the pilot.
- (1) Any single failure, excluding jamming (for example, disconnection or failure of mechanical elements, or structural failure of hydraulic components, such as actuators, control spool housing, and valves).
- (2) Any combination of failures not shown to be extremely improbable, excluding jamming (for example, dual electrical or hydraulic system failures, or any single failure in combination with any probable hydraulic or electrical failure).
- (3) Any jam in a control position normally encountered during takeoff, climb, cruise, normal turns, descent, and landing unless the jam is shown to be extremely improbable, or can be alleviated. A runaway of a flight control to an adverse position and jam must be accounted for if such runaway and subsequent jamming is not extremely improbable.
- (d) The airplane must be designed so that it is controllable if all engines fail. Compliance with this requirement may be shown by analysis where that method has been shown to be reliable.
A low rigload minimizes friction, but too low a rigload may lead to slack cables when the aircraft cold soaks. Cold soaking is a condition where an object absorbs (is soaked with) a cold ambient or static temperature. Hot soaking refers to the absorbing of a hot ambient temperature. Low rigload also leads to degraded system response. When cables are routed through areas of the aircraft that experience significant bending, such as the wing, it is very important to route the cables as close to the neutral axis of bending so that any inputs to the control system caused by bending are minimized.
Another consideration in cable system design is the stored energy in the rigged cable system. If one of the cables fails, the opposite cable will shorten by the amount it was deflected due to rig load. This shortening can cause an input to a control valve, forcing a hard over of flight surfaces and other equipment.
One common practice in cable routing that has come under scrutiny lately is the use of large multiple pulley brackets where pulleys for several different systems share a common pulley bracket. This practice minimizes weight and cost of the control systems but could lead to the loss of several systems if the bracket were to fail.
Mechanisms
Mechanisms design and installation is the second major area of flight control systems design. These mechanisms require a more creative approach to design than cable systems and have some important design guidelines of their own. These mechanisms are used for everything from pilot input devices, to overrides and load feel mechanisms that are required to meet safety requirements. In mechanisms design, ultimate strength of the parts is seldom a concern as stiffness/deflection requirements will usually determine the size of parts. Mechanisms must also be designed to minimize the number of parts utilized. This not only minimizes cost but also reduces the number of failure conditions that must be considered. One very important consideration in controls system design, both mechanisms and cables, is the prevention of jamming. This is typically one of the worst failure conditions for a control system because it often leads to a hardover surface. Hardover is a condition when, uncommanded, the flight control surface goes to full extension and stays there. Flight control systems must be designed to prevent jamming and also to have overrides and disconnects to minimize or eliminate the effect of a jammed system.
Interfacing considerations
When designing a system to interface with a mechanical control system, free play and deflection must be considered. This is critical in the interface of the autoflight system with the flight control system. The flight control system does not provide a perfect ground (anchor) point and will deflect under load. When trying to limit deflection, the structural interface must be considered. Taking great care in designing a stiff mechanism with minimum backlash is useless if you mount it to a flexible structural panel. The flight control system designer must coordinate with his structural counterpart to ensure the mounting surfaces are suitably stiffened. The FAA requires a slack cable test on all new aircraft. This test consists of driving the control system hard over and then manually pulling on the slack return cable and trying to get it to hang up on any surrounding structure or components. This test must be considered when routing and installing components near control cables. Functional requirements for the flight control system are generally set by other groups—Aerodynamics and Avionics are the primary requirements drivers. These requirements need to be carefully analyzed and coordinated with the flight control designers. The more stringent and complex the requirements, the more difficult and expensive the control system will be. An area where requirements are often not defined in enough detail is performance after failures. This is an area that needs much greater attention than in the past. As the performance of our aircraft and the complexity of our systems increase, performance after certain failures that result in degraded control system performance must be carefully considered. Flight control system design is always a balance between performance and complexity—proper coordination between the systems designers and the requirements creators is critical.
Hydraulics
As aircraft weight, size, and speed have increased, a powerful, efficient, and highly reliable power source was needed for flight control surface actuation. In today’s aircraft, hydraulic power systems have found widespread acceptance for use in surface actuation (along with other applications requiring high-output forces from a small lightweight actuator). Some of the advantages of hydraulic power are:
- • large power output
- • minimal weight and space
- • efficient power amplification.
- • smooth, vibrationless power output
- • little affected by load variations
- • hydraulic fluid carries away unwanted heat
- • hydraulic fluid acts as a lubricant, increasing component life
Design requirements
As the hydraulic actuator is usually the final link between the flight control system and the control surfaces (ailerons, flaps, etc.), the hydraulic system is subject to many of the same requirements that govern the flight control system. The aerodynamic requirement for surface rates and forces are used to size the hydraulic system. Rate is the speed at which a flight control surface changes position. It is only after knowing forces and flow rates for all the hydraulic components, along with the knowledge of which components must be capable of being operated simultaneously, that the hydraulic system pumps can be properly sized. FAR 25.1309, AC20-128, and the other aforementioned documents apply to the hydraulic system just as they do the flight controls. The hydraulic system is also subject to a regulation that requires that no single failure can cause the loss of more than one hydraulic system. In the event of a failure, flight control surfaces must be properly assigned to the redundant hydraulic systems.
Detail design considerations
The hydraulic system consists of several separate types of components that combine to create a complete actuation system.
Hydraulic pumps
The pump is the source for pressurizing the hydraulic fluid to do the work. Pumps are commonly driven by the aircraft engine, an electric motor, or by another hydraulic system. Pump selection determines the normal system operating pressure and flow characteristics; 3000 psi systems are currently the standard, but 4000 and even up to 6000 psi systems are under development. To maximize efficiency, most current hydraulic pumps are pressure compensated variable delivery types, i.e., they decrease the output flow as the pressure approaches normal system operation pressure.
Reservoirs
The reservoir stores an extra supply of hydraulic fluid to account for differential volumes of actuating cylinders, accumulator volume, thermal expansion and contraction of fluid, and system leakage between service intervals. Minimum reservoir size is an important consideration as Skydrol weighs approximately 9 lbs/gal. (Skydrol is a brand name for the hydraulic fluid used in aircraft.) Reservoirs are also designed to minimize fluid foaming and to ensure positive pressure to the pump inlet.
Accumulators
Accumulators are used to store pressurized fluid that can be used to supplement pump output during peak and demand, and to provide hydraulic power when the pump is inoperative.
Accumulators can also be used to damp out pressure spikes caused by rapid valve closure in high-flow-velocity systems.
Valves
The control valve is the control device that receives the input commands (pilot inputs) and ports fluid to the appropriate chambers of the output device (cylinder or motor). The input commands are typically mechanical or electrical (solenoid or motor operated).
Output devices
These devices are typically linear actuators/cylinders or rotary actuators/motors. The force output is approximately equal to the pressure differential, times the cross-sectional area.
Hydraulic piping
The piping system is used to transfer the fluid around the aircraft to all the different components. As maximum force at the actuator is the design goal, piping design is a compromise between the large pressure drops associated with small diameter pipes, and the increased weight of larger diameter pipes and the weight of the additional fluid. To prevent the loss of more than one hydraulic system due to a single failure, the latest engine rotor burst requirements present many challenges in hydraulic system layout. Careful system separation and routing are the first priorities in hydraulic system layout. Because loss of hydraulic fluid is a major safety concern, proper component location and piping layout is critical and must be considered carefully. Hydraulic fuses and shutoff valves are used only as secondary methods of preventing hydraulic fluid losses.
Interfacing system considerations
There are two primary concerns when designing or routing systems near hydraulic components:
- 1. Skydrol is a very corrosive fluid; it will attack and destroy many metallic and nonmetallic substances. It is very important to use Skydrol-resistant materials in the vicinity of hydraulic components. Even if hydraulic system leaks are rare, in-service component replacements typically result in Skydrol spills.
- 2. High potential energy is stored within the hydraulic system. With 3000 psi pressure acting on all components, a large amount of energy can be released if a component fails. Such failures must be considered by system designers as part of the Zonal Analysis and Events Reviews (ZA&ER) when placing components near hydraulic system components.
Like the flight control system, the hydraulic system requirements are strongly influenced by the aerodynamic requirements of the aircraft. Having accurate aerodynamic load information is essential to properly size the hydraulic actuators. If the loads are higher than expected, the system will not meet the rate and travel requirements. If the loads are lower than expected, the actuator will be heavier and larger than necessary. Specifying the minimum requirements is just as important to the hydraulic system designers as it is to the flight controls system designer. The more stringent the requirements, the higher the cost. Finally, careful coordination with the structural designers is imperative as the actuators exert large forces on the structure. Structural deflection is a major concern and must be coordinated with the structural designer along with the maximum loads.
Landing gear
The undercarriage of the aircraft is comprised of the landing gear, wheels, tires, and brakes. The purposes of these components are to:
- • allow the aircraft to taxi while on the ground,
- • absorb the vertical component of the aircraft kinetic energy upon touchdown, and
- • provide a means of retarding the forward motion of the aircraft.
The landing and braking energies associated with today’s large transports are enormous. They provide the landing gear designer with a significant challenge. It is clear that with the loads and energies involved, the landing gear and its mounting structure must be very substantial. For this reason, the main gear is usually mounted very close to the wing rear spar. The gear actuation mechanism must be designed to minimize the possibility of inadvertent gear extension and retraction at all aircraft weights and speeds.
Design requirements
Besides FAR 25.1309 mentioned earlier, the following FAA regulations apply to landing gear:
- • FAR 25.721 requires that the gear must break away during an aft-upward overload without causing enough fuel spillage to present a significant fire hazard.
- • FAR 25.729 requires that the gear must positively lock in the extended position. Also, an alternate means of extension must be provided that will extend the gear after any reasonably probable failure or any single failure of hydraulic, electrical, or equivalent energy supply failure.
- • FAR 25.111 requires that the landing gear retraction after takeoff must be quick enough to allow the aircraft to meet the minimum height requirements of the takeoff path.
Two major functional requirements that determine landing gear layout are pavement loading and tire scrubbing. As aircraft weights have increased, pavement loading (distribution of weight on the pavement) has become a major factor in determining landing gear layout. The major parameters used to determine pavement loading are aircraft weight, numbers of tires, size of tires, and tire layout (the position of each tire relative to the others). Tire scrubbing is another major concern in landing gear layout. As more tires have become necessary to meet pavement loading requirements, the relative spacing along the Y-axis (length of the aircraft) has increased, causing tires to scrub sideways as the aircraft turns.
Tire scrubbing not only causes excessive wear but also increases strut seal wear, strut bending, and the force required to tow or taxi the aircraft. In extreme cases, the main landing gear must be designed to steer with the nose gear to minimize scrubbing.
Detail design considerations
Due to the enormous loads exerted on the landing gear, sizing the gear to have an infinite fatigue life would result in too great a weight penalty. For this reason, many gear parts are “life limited” parts, meaning that they must be replaced at a specific number of cycles.
These same parts are generally made of very high-strength steels that are sensitive to fatigue cracks. Therefore, great care must be taken during the design and fabrication of the parts to minimize stress concentration. Due to the high cost and large size of landing gear parts, they are often designed with extra material and oversized bosses. This enables the airline to repair parts with minor wear, damage, or corrosion. This leads to slightly heavier parts but saves the airlines from having to replace expensive parts due to minor corrosion or damage.
Interfacing systems considerations
The wheel/tire/brake assembly represents a high-energy, high-temperature source that, when retracted into the wheel well, presents a major hazard to surrounding systems. When routing components within the wheel well, both tire and wheel failures must be considered. Other damage within the wheel well can be caused by foreign object debris (OBD) from the runway being thrown up and into the wheel well by the tires. As mentioned earlier, the structural attachment of the landing gear to structure is critical, and the structure must be designed to minimize deflection. The antiskid system must also be carefully designed to not create resonant frequencies within the landing gear. If the resonant frequencies are not avoided, landing gear extension and retraction is one of the biggest demands on the hydraulic pumps.
Landing gear indication is an area where the requirements are currently under review. In the past, the primary indication was by indicator lights in the cockpit, with secondary indication via viewing tubes in the main cabin that allowed the flight crew to visually determine landing gear position. With the advent of two-man crews in large transport aircraft, certification authorities have been reluctant to allow a crew member to leave the cockpit to verify gear position. The current trend is toward more reliable electronic indication systems with no mechanical backup, and/or remote video..
Water and waste
Potable water, as discussed here, refers to water that meets the requirements of the United States Public Health Service (USPHS) drinking water standards. Potable water is used for drinking, hand washing, food preparation, and the rinsing of vacuum waste toilets. Waste water is generally broken into two categories: gray water and black water. Gray water is water that drains from lavatory and galley sinks and drinking fountains. In some installations, it is vented overboard through heated drain masts. Black water is the water used in the flushing of toilets and is collected with other waste products in waste tanks for disposal during ground servicing of the aircraft.
Design requirements and considerations
Potable water
USPHS approval of the aircraft potable water system must be obtained prior to aircraft delivery. This approval signifies that the system meets the USPHS design requirements and that it has been properly sterilized and serviced.
Sizes for potable water and waste tanks are based on the number of passengers and maximum duration of the aircraft flight (approximately 0.045 gal/h, per passenger). Water pressure requirements are usually determined to provide maximum toilet cleaning during flushing. There are no requirements for number of lavatories in the aircraft, but a general rule of thumb is one lavatory for every 40 passengers. The potable water system must be designed so that it can be completely drained, and the tanks must have access for cleaning.
Waste water
There are two types of toilet/waste systems in common use in modem aircraft: the recirculating type of system, and the vacuum waste system. The vacuum waste system is generally lighter but more expensive than a recirculating type system. The potable water and waste systems must be designed to prevent freezing, both during flight and while the aircraft is parked on the ground. Corrosion, of course, is a major concern in the water and waste systems. For this reason material must be very carefully chosen. Some of the more commonly used materials are stainless steel, plastics, and composites.
Interfacing system design
With plumbing throughout the aircraft, corrosion protection of equipment from leaking water is critical. One critical area of concern is the electrical power center in the CAC. The Water/Waste group works with the Interiors group to minimize or eliminate plumbing in this area. Where plumbing can’t be eliminated, plumbing lines should be shrouded and dams installed to divert water away from the critical area. Floor panels are also sealed to help prevent spilled liquids from seeping through floors. Yet even with these precautions, the systems designer should be aware that liquid spills/leaks will occur, and so should be considered during design and installation. For the vacuum waste system, the differential pressure that is generated when the toilet is flushed must be considered in lay design. There must be adequate venting in the lay enclosure.
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