Q 1. Give an explanation why pneumatic power is used on aircraft in place of mechanical, hydraulic or electric powered systems (this will include advantages and disadvantages)
Solution 1:
Pneumatic powered system is almost similar to Hydraulic, both are based on Pascal’s Law (Experimentalaircraft.info, 2017) with compressed air instead of fluid to transfer power. The air inside may be compressed inert gas / normal air or compressed air in the form of liquid.
Major advantages:
The cost of pneumatic circuits is less and simple circuits (hydraulicspneumatics.com, 2006). Whereas in case of Hydraulic, the piping systems are more complex in design.
Pneumatic driven machines are much quieter, as its power source (air compressor) is installed in enclosed container remotely away from the machine resulting less noise.
Pneumatic operated systems are always cleaner.
Leaks in hydraulic system can cause fire issue (instruction.greenriver.edu, 2017), which is not there in case of Pneumatic.
Major disadvantages:
Air is compressible in nature (dtic.mil, 1971), thus pneumatic driven control valves or actuators cannot hold a pressure rigidly resulting less accuracy level (nptel.ac.in, 2017).
Maintenance cost is more.
Due to low conductivity of air, it cannot dissipate heat as much as hydraulic.
Q 2. Explain the function and operation of aircraft pneumatic systems and their associated status indicators, your answers should include sources of pneumatic power supplies and the systems they may be used to operate such Thermal anti icing, hydraulic reservoir pressurisation, oxygen systems and their anti g systems. Also give details of the components within the systems and their constructional features.
Solution 2:
Three different power source of supply is High-Pressure systems, the pressure ranges between 1,000 to 3,000 psi (faa.gov, 2017), stored in metal cylinders with the inlet valves used for recharging air by compressors at ground and the other one is flow Control Valve acts as shut-off and contains
Figure 1
(High-pressure pneumatic system)
Source: https://www.faa.gov/regulations_policies/handbooks_manuals/aircraft/amt_airframe_handbook/media/ama_Ch12.pdf
trapped air inside the bottle at high pressure. These cylinders are light in weight and used for emergency like actuating landing gear or brakes.
Pressure ranges between 50 to 150 psi in Medium-Pressure system, the source is bleed air (Tenning & Cass, 2014) which is used for deicing of wings and engines, start one engine from other, maintain cabin pressure and temperature, hydraulic reservoir pressure and lavatory water tank. Bleed air is readily available, reliable and important source of supply for pneumatic power system.
In case of Low-Pressure system, the pressure ranges between 1 to 20 psi are produced by mechanical process of air pump. In this system the vane 1 & 2 keeps on changing causing change in size
Figure 2
(Schematic of vane-type air pump)
Source: https://www.faa.gov/regulations_policies/handbooks_manuals/aircraft/amt_airframe_handbook/media/ama_Ch12.pdf
of chambers with trapped air increasing the pressure into the system from single chamber at a time which is used for boot system of deicing of wings, door seal etc.
Thermal anti-icing system:
Heated bleed air are passed through the inner surface of leading edges of aircraft wings to prevent ice formation or deicing. As per faa.gov, (2017) anti icing system acts with “Wing Anti Ice (WAI) system” in which large amount of bleed air is passed through the ducts, manifolds and valves into the area where deicing is required.
Figure 3
(Schematic sketch of WAI system)
Source: https://www.faa.gov/regulations_policies/handbooks_manuals/aircraft/amt_airframe_handbook/media/ama_Ch15.pdf
Different parts of WAI system are the “Ducts” through which the bleed air passes are mainly made of titanium, aluminum alloy, stainless steel or fiber glass and are attached to each other by bolted flange joints. “Valves” control the flow and are controlled electrically and actuated pneumatically. “Pressure Sensors” sense the pressure in ducts, which sends signal for opening/closing of valves. “Control System” is ACIPS (airfoil and cowl ice protection system) computer cards, which controls the signals and proper operation of WAI system. “Indication system” indicates various parameters against the permissible limits in monitor. “BITES” is a circuit which independently checks the WAI system and continuously sends data to AICPS for maintenance messages.
Hydraulic reservoir pressurization system:
Engine bleed air is used to pressurize fluid for its proper flow to pump suction. Reservoirs are cylindrical in shape. Main parts are “Pressure relief valve (PRV)” which pops up when the pressure inside reservoir increases more than design. “Sight glass” helps maintenance team to see the condition of reservoir. “Sample valve” helps to collect sample from reservoir for testing purpose. “Drain valve” helps to drain the reservoir for maintenance or cleaning. “Temperature transducers” indicates temperature of reservoir and “Quantity transmitter” indicates the fluid present in reservoir. Pressurizing system is located near the reservoir and consist of Filters, PRVs, check valve, pressure switch, bleed valve, gauge port and test port.
Oxygen system:
As the altitude of flight increases the oxygen content decreases progressively. Hypoxia can be avoided by increasing the pressure or quantity of oxygen in air. Various source like “Gaseous oxygen” stored in green colored cylinder, “LOX” liquefied oxygen (skybrary.aero, 2017) is stored by pressurizing gaseous and converted to liquid , “Solid Oxygen” is sodium chlorate which has a unique characteristic of producing oxygen only when it is ignited and are kept as emergency back-up in pressurized aircrafts and “OBOGS” that is Onboard Oxygen Generating System is the molecular sieve separation method used to separate oxygen from rest of the air and these are installed in aircrafts.
“Gaseous oxygen system” is the most common in aviation sector and are stored in high pressure cylinders of 2400 psi pressure withstanding capacity. To minimize loss, pressure demand flow system is used and consists of flow indicators, plumbing and valves to control flow and conserve oxygen.
Figure 4
(Demand type regulators and mask design to control flow and conservation of oxygen)
Source: https://www.faa.gov/regulations_policies/handbooks_manuals/aircraft/amt_airframe_handbook/media/ama_Ch16.pdf
“Solid oxygen” are stored in portable type gaseous oxygen cylinder as back-up and are safe for carrying and maintenance. The “LOX” system is similar to the gaseous.
Anti-G system:
This system provides high level protection against change in gravitational force over a very short period of time due to supersonic fighter aircrafts, helps in maintaining proper fluid movement through the body by providing proper pneumatic pressures through pre sated logics in aircrafts which regulates the pressure accordingly. This combined anti-g and pressure suit has helmet with respiratory mask (Row R, 1991) for proper flow of oxygen, a suit from neck to foot which regulates proper pressure at different parts of body in positive and negative G acceleration. It increase the G tolerance of pilot/aviator.
Q 3. Explain the operation of cabin conditioning systems for both pressurise and un pressurised aircraft, and the pressurisation system for a large passenger aircraft.
Solution 3:
Cabin conditioning system in large passenger aircraft:
Figure 5
(Cabin Conditioning System)
Source: https://www.faa.gov/regulations_policies/handbooks_manuals/aircraft/amt_airframe_handbook/media/ama_Ch16.pdf
The air pressure is increased in case of large aircraft carrier and quantity is increased in case of small or medium size aircrafts which are designed for cabin without pressurization. “Air cycle air conditioning system” uses bleed air and regulated by “Pack valve” which maintains proper flow of air, “Bleed air Bypass” maintains proper temperature, “Primary and Secondary heat exchangers” also maintains proper temperature, “Water separator” is used to separate water present in cold air and maintain dryness, “Refrigeration bypass valve” is used to maintain anti icing and “Cabin Temperature control system” is used to control and maintain comfortable cabin environment. It is generally located in lower half or tail section of aircraft. The “Vacuum cycle air conditioning system” is used in absence of bleed air, i.e. either when flight is on ground or it is reciprocating engine aircraft. It is used solely to transfer heat from cabin to outside.
Pressurization system in large passenger aircraft:
As the altitude of flight increases, the atmospheric pressure decreases and even the temperature too with an average of -2˚C per 1000 ft. of increase in altitude and pressure deceases progressively from 14.7 to 1.7 psi at 0 to 50,000 ft. altitude progressively (faa.gov, 2017). Cabin crew member can control pressure either by maintaining same cabin altitude, say if they maintain 8000ft of cabin altitude (ncbi.nlm.nih.gov, 2017) then irrespective of the outside pressure the pressure inside cabin will always remain 10.92 psi. But if the differential pressure increases the withstanding limits of the structure can cause problem. So, to avoid cabin crew member can also opt for the feature to maintain constant differential pressure inside cabin which is the difference of air pressure inside and outside cabin as constant from 3.5 to 9 psi (faa.gov, 2017) depending on the requirement. Now a days, automatic systems with built in logic developed which can maintain the inside pressure of cabin preferably in constant and as the difference increases it switches to differential type to maintain safety of aircraft structures.
Q 4. Explain the operation and constructional features of the components in pressurisation, cabin conditioning and refrigeration systems.
Solution 4:
Pressurization system:
Three main sources of air “Supercharger” located at upstream of fuel delivery system were used in older reciprocating engine aircrafts. “Turbocharger” takes engine exhaust air as intake and after adequate compression of cabin and these are used for modern reciprocating engines and “Engine driven compressor” minimizes the drawbacks but at the same time adds weight on aircraft. Turbine engine aircrafts has overcome all the issues related to reciprocating and can fly above 25,000 ft altitude (ncbi.nlm.nih.gov, 2017) and also its engine bleed air which is comparatively less prone to contamination is used as a perfect source of air for cabin pressurization.
Cabin pressure controller is the systems which controls all signals using FMS (flight management system), by regulating “Pressure regulator” & “Outflow valve” which allows the entry of air in cabin and maintain balance differential pressure and are operated through pneumatic pressure. “Pressure safety Valve” prevents over pressurization of for structural safety and opens up at differential pressure of 8 to 10 psi. “Pressure gauge” indicates the data wherever installed as per requirements. These control system normally located in cockpit for preliminary manual control. The safety valve is attached to WOW switch which keeps valve always open till load is there on wheel allows to always maintain the cabin depressurized when at ground. So with all these systems the pressure inside cabin is controlled and the air distribution system is located above seats with automatic oxygen mask dropper whenever pressure decreases the desired one.
Cabin conditioning system:
Its primary function is to provide comfort to passengers by maintaining comfortable cabin temperature at all altitudes. Engine bleed air is too hot to directly sent in cabin, so it is first passed through primary heat exchanger and then through secondary, after further pressurization passed through water separator and after checking the temperature it is again pressurized and then sent to cabin through air circulation system. For conducting this process pack valve, bleed air bypass valve, primary heat exchanger, water separator, refrigeration bypass valves are used for different stages of its cooling as discussed above.
Refrigeration system:
Refrigeration system works on vapor cycle, where refrigerant changes its state and cools down the cabin temperature. The components are verities of “Refrigerant” which can be used according to design requirements and most commonly used refrigerant is R134a (lockheedmartin.com, 2017). “Receiver dryer” is used to store extra refrigerant and acts as reservoir for the cycle. Now refrigerant after passing through dryer passes through “expansion valve” where it sprays into “Evaporator” which is having fan outside and refrigerant flows through inside tube resulting in absorbing heat and converts into low pressure vapor before entering in “Compressor” where the low pressure vapor is compressed to increase temperature and further passes through “Condenser” where it dispose its heat to outside air. Modern engine driven compressors are located at engine. Condensers is a heat exchanger with fins attached for effective heat transfer and all this system is regulated by two “Service valves” one at high end and other at low to regulate the flow of refrigerant.
References:
Experimentalaircraft.info. (2017). Pneumatic Systems. [online] Available at: http://www.experimentalaircraft.info/articles/pneumatic-systems.php [Accessed 14 February 2017]
hydraulicspneumatics.com. (2006). Chapter 5: Pneumatic and hydraulic systems. [ebook] Available at: http://hydraulicspneumatics.com/other-technologies/chapter-5-pneumatic-and-hydraulic-systems?page=1 [Accessed 14 February 2017]
instruction.greenriver.edu. (2017). Pneumatic systems. [online]. Available at: http://www.instruction.greenriver.edu/aviation/downloads/AVIA112_files/Pneumatics.pdf [Accessed 14 February 2017]
dtic.mil. (1971). Engineering design handbook, Hydraulic fluids. [ebook]. Available at: http://www.dtic.mil/dtic/tr/fulltext/u2/884519.pdf [Accessed 14 February 2017]
nptel.ac.in. (2017). Lecture 33 Introduction to pneumatics. [ebook page 23]. Available at: http://www.nptel.ac.in/courses/112106175/Module%204/Lecture%2033.pdf [Accessed 14 February 2017]
faa.gov. (2017). Chapter 12 Hydraulic and pneumatic power systems. [ebook]. Available at: https://www.faa.gov/regulations_policies/handbooks_manuals/aircraft/amt_airframe_handbook/media/ama_Ch12.pdf [Accessed 14 February 2017]
Tenning P, Cass G. (2014). Aircraft pneumatics training aid and methods. US8794969 B2, USA
faa.gov. (2017). Chapter 15 Ice and rain protection. [ebook]. Available at: https://www.faa.gov/regulations_policies/handbooks_manuals/aircraft/amt_airframe_handbook/media/ama_Ch15.pdf [Accessed 14 February 2017]
skybrary.aero. (2017). Oxygen systems. [online]. Available at: http://www.skybrary.aero/index.php/Oxygen_Systems [Accessed 14 February 2017]
Row R. (1991). Combination anti-g and pressure suit. US5007893 A, USA
ncbi.nlm.nih.gov. (2017). Environmental Control Systems on Commercial Passenger Aircraft. [ebook]. Available at: https://www.ncbi.nlm.nih.gov/books/NBK219009/ [Accessed 14 February 2017]
faa.gov. (2017). Chapter 16 Cabin environmental control systems. [ebook]. Available at: https://www.faa.gov/regulations_policies/handbooks_manuals/aircraft/amt_airframe_handbook/media/ama_Ch16.pdf [Accessed 14 February 2017]
faa.gov. (2017). Aviation maintenance technician handbook – powerplant, volume 1. [ebook]. Available at: https://www.faa.gov/regulations_policies/handbooks_manuals/aircraft/media/FAA-H-8083-32-AMT-Powerplant-Vol-1.pdf [Accessed 14 February 2017]
lockheedmartin.com. (2017). Supplemental rapid cooling system C-130 application FAA STC No. SA02365CH. [ebook]. Available at: http://www.lockheedmartin.com/content/dam/lockheed/data/aero/documents/global-sustainment/product-support/2010HOC-Presentations/Wed_1700_NASI-Supplemental_Rapid_Cooling-Wesley_Plattner.pdf [Accessed 14 February 2017]
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