: Flight Controls and Environmental Control Systems
Table of Contents:
Relevance of Flight Controls Air-Conditioning Systems and Commercial Aviation
In this chapter, it will be explained how pilots control the aircraft in-flight using a series of movable surfaces fitted to the airframe, the structure of the aircraft. The purpose of understanding this technical content is to recognise how these moving surfaces change the flight aerodynamics locally, the general design logic behind the control, and the most obvious differences between Airbus and Boeing (being the two most significant commercial aircraft manufacturers).
The pressurisation and ventilation systems on a large aircraft are considered to be essential to support life, and these systems will be explained in this chapter.
Both the development and technological advances of flight controls and the use of pressurisation systems have historical influences in the First World War and Second World War, especially in the later stages of the Second World War. As the wars progressed, the technology of the aircraft developed during both conflicts, respectively. The general trend was that the military aircraft became larger, flew further and carried more mass (i.e. bombs/fuel/ structure). Also, to improve their survivability in combat, these military aircraft flew at much higher altitudes to evade being shot down. By the end of the Second World War the largest technological advances were seen, with pilots and crews being pushed to the limit of their physical performance. Afterwards that journey of development continued, albeit at a slower rate.
Comparing the Underlying Philosophy of Flight Controls for Airbus and Boeing
Aircraft have been increasing in size ever since the first flight of the Wright Brothers Kittyhawk plane in 1903 (Figure 4.1). That initial flight was possible due to a combination of a gasoline motor coupled with human-controlled and 'powered' flight control technology. The wings were deformed inflight by 'wing warping', which used cables to twist the wings to bank the aircraft. Yawing was achieved using two small vertical surfaces, again controlled by cables. The simplicity of the flight controls in the Wright Flyer allowed for those early achievements, and further enhancements followed thereafter.
As aircraft evolved throughout the two World Wars, a common theme that is apparent was the introduction of larger aircraft that could carry more mass, with an equally larger wing and control surface area.
Early aircraft were controlled with cable runs that were coupled between the controller (the yoke) and the moving surface (i.e. elevator, ailerons). Simple aircraft, such as single-engine piston planes such as the Cessna 172 or Piper PA28, still continue to use this technology, as illustrated in Figure 4.2. One such advantage for using this simple arrangement is the forces felt by
Wright Flyer, Kitty Hawk, first flight 1903.
Cessna C172 simplified drawing of Aileron controls using wire cables coupled to the yoke. (FAA.)
the pilot when flying. If the aircraft increases speed significantly, the forces needed to move the surface increase exponentially: the pilot can feel with their hands the forces necessary to move the controls while flying the aircraft.
With much larger and heavier aircraft, the need arose to use significantly larger control surfaces with bigger wings. It became apparent to the aircraft designers that human pilots were approaching their physical limits. Bigger military pilots were starting to struggle to move the aircraft controls (i.e. the yoke), so new technologies were introduced post-Second World War to rectify this limitation. The cable-run technologies that had proved so popular earlier evolved to incorporate the introduction of hydraulically-moved surfaces. Cables were retained because the technology was mature, and more importantly they were much lighter than having long pipes full of hydraulic fluid that would otherwise need to run from the flight deck to the extremities of the aircraft. Weight is always a significant factor for aviation, as demonstrated by the military in both World Wars.
The use of hydraulics was a natural choice because the cables that connect the yoke ran to a valve block that was close to the moving surface. The moving surface was directly coupled to the hydraulic actuator, which in turn is controlled by the respective valve blocks. The introduction of hydraulic technologies resulted in pilots losing the ability to feel how the aircraft was flying, so a workaround solution was the introduction of 'Q-feeT - or artificial feedback into the flight control circuit. This Q-feel operated by measuring the airspeed of the plane, and mechanically making the input to the yoke more difficult at higher speeds. An additional development was to reduce the number of moving surfaces on the wings at high-speed flight compared to take off and landing, which reduces the wing loadings (and the forces acting upon them).
The incorporation of hydraulic actuators into flight controls has resulted in another technological development, namely the autopilot system. If an electrical or electronic control system is incorporated into the hydraulic valves (for the autopilot), the hydraulic actuator can directly control the movement of the surface. What this means is that the pilot’s previous need to hold the yoke at all times is reduced, especially when the autopilot function is engaged.
Small corrections to an aircraft's moveable surfaces are made with trimming devices, and these trims are fitted to the elevator, ailerons and rudder, and operate independent of one another. Once again the trimming function was initially with cable-control, leading to hydraulic actuation, the same development process as with the main moving surfaces, including inputs from the autopilot.
All passenger flights on large commercial aircraft will require the pilots to make use of the Flight Management System, in addition to the navigation systems that are fitted on board the aircraft. When pilots prepare an aircraft for a journey, one of the tasks that will be undertaken is to programme the navigation computer with all the necessary details. During the flight, the aircraft calculates its current position from navigational inputs. These include Very High Frequency (VHF) radio signals, including VHF Omni-directional Range (VOR) signals; flight performance data, such as the altitude and airspeed from the Air Data Computer; magnetic compass readings; and inertia-based systems such as ring laser gyros.
The actual 'hands-on' control of the aircraft will be taxiing the aircraft from the stand (the terminal area) to the runway, the take-off roll and the initial climb. The usual commercial practice is to immediately engage the autopilot system in the post take-off climb, and to let the Flight Management System (the autopilot and the navigation System) fly the aircraft all the way to the destination. The controls for the autopilot are usually located on the glareshield, which is the central panel that is between the instrument displays and the front windshield. An example of this system is illustrated in Figure 4.3, representing the B747-400 aircraft.
The use of the autopilot system is a reliable, proven technology that, once activated, allows the pilots to reduce their own workload by using this system. The system can control all the axis of the flight, in addition to the speed of the aircraft and the rate of descent. The aircraft will fly the route towards the destination airfield, as per the pre-programmed plan. At around 10 nautical miles away from the airfield, the aircraft would usually be configured for a possible landing, meaning that a steady rate of descent (circa 3 degrees down) has been established. Usually, Air Traffic Control will be instructing the Pilots via radio commands as to what airspeed should be flown,
B747-400 Glareshield autopilot controls.
Aircraft approaching runway using the Instrument Landing System.
headings, etc., to ensure the safe operation of all the aircraft in this zone. It is at this time the aircraft's Instrument Landing System (ILS) has 'captured' the airport's precision landing radio signals, as illustrated in Figure 4.3 (vertical glide slope and the horizontal localiser radio signals). The use of the ILS as a precision landing tool is normal, as commercial aircraft fly in all weathers, and often the pilots cannot see the runway as they are approaching from a large horizontal distance. Before the landing, usually at a preagreed 'Decision Height' (which is based on the airports published Category class, such as Cat I, II, Ilia, Illb or IIIc), the Pilot flying will make it known to their colleague their intention to continue to land, or to climb away and try again. Assuming that the pilots are continuing with the approach beyond the Decision Height, some modern aircraft may continue to be flown fully on this autopilot and Autoland system to the actual landing (i.e. the aircraft fully transitioning the mass to 'weight on wheels', if the airport and corresponding runway permits).
Lastly, the Airbus and Boeing flight control systems have very different underlying control philosophies. Up to the introduction of the Boeing 777 aircraft in the late 1990s, the Boeing flight control philosophy has always been to combine cables and hydraulics that are connected to the yokes, so if a total computing failure occurred on a Boeing aircraft, the pilots could land the plane manually.
Airbus has a very different philosophy. Namely, the flight deck does not have yokes, but two side sticks, like the devices used in computer games. The pilots move the sidesticks which change a voltage, and this change is interpreted by a flight-control computer. The Airbus flight computer flies the aircraft at all times, and the computer decides whether to allow the hydraulically-controlled surfaces to move in the direction that the pilot requests. If the Airbus pilot's sidestick movement is outside of the safe flight envelop during a flight, the pilot's sidestick input is ignored.
These two philosophies are drawn from the technical literature provided by the manufacturers, e.g. Flight Crew Operating Manuals, Aircraft Maintenance Manuals.