AVIA 222 Pneumatic and Pressurization Systems
Pneumatic Systems Pneumatic systems: systems that use air (or other gases, such as nitrogen) to do some form of work.
Pneumatic System Components Some common components of a pneumatic system are: • Compressors • Air distribution systems • After coolers • Receivers • Dryers • Check valves • Cylinders • Motors • Flow controls • Directional valves • Regulators • Silencers • Quick exhaust valves • Filter systems • Excess flow valves and boosters
Tires Oleo Vacuum gauges De-ice systems Pressurization Stored energy Back up systems for hydraulic failure Landing gear Flaps Brakes Cargo door actuators Aircraft Pneumatic SystemsSome common uses on an A/C:
Pneumatic Tires • The air filled rubber tires that are used on aircraft and cars are a pneumatic system. The pressure in the tire acts as a shock absorbing/dampening device to aid the landing gear, or suspension, in maintaining control of the a/c during ground movements. A heavy landing is the most common Cause of a pneumatic tire failure
Oleo • The pneumatic oleo strut system is the most common type of landing gear dampener used on light aircraft. • Pressurized nitrogen gas is the most common gas used to fill the oleo. On a Cessna 172, if the oleo is serviceable , the oleo collar is lifted about 2” off the wheel assembly. • A recharge valve stem allows for easy maintenance.
Vacuum Gauges • Vacuum lines connected to the low pressure intake manifold, or engine driven air pump or electric powered pump, provide a engine sourced power supply for some standard gauges. Specifications Power Requirements: Vacuum or Pressure, 4.5 to 5.2 in. Hg Minimum Air Flow: 2.2 CFM Air Filtration: 3 microns, 95% Autopilot pick-off: AC, linear transformer, 5Khz, *VAC (pp) Weight: 3.4 lbs. Dimensions: 3.38" x 3.38" x 8.35" Internal Lights: 14/28 VDC
Attitude Indicator On the Cessna 172 the Attitude Indicator and Heading Indicator are both air driven. About 2.5 psi is required to power the gyro to full rpm. A regulator reduces the suction when the engine is at higher rpms.
Pneumatic De-Ice boots. • Rubber “boots” fitted to the leading edges of aircraft are powered by air pressure to inflate and crack/brake off any layer of ice that may of formed.
This B/E Aerospace de-ice boot system uses 22 PSI to inflate. Inflation rates and cycle times vary with each system.
Typical layout of a pneumatic pump systems for a single engine piston driven aircraft.
Here is a vacuum pump system with a back up exhaust drive meant for only temporary, emergency use.
Pressurization Systems • Aircraft that carry passengers at an altitude of over 10,000 – 13,000 feet often utilize a cabin pressurization system that allows for comfortable travel (without mask systems) at high altitudes. • When traveling higher an aircraft can: • Avoid most weather • Increase fuel economy • Navigate more efficiently
Why do we need Pressurization • At lower atmospheric pressures, although the air may contain sufficient oxygen content, the pressure is not sufficient enough to allow the lungs to exchange gases at the membrane level.
How The pressurized portion of the aircraft must be made relatively air tight. This is accomplished by the use of seals around tubing, ducting, bolts, rivets, and other hardware that pass through or pierce the pressure tight area.
While Climbing or Descending • Important part during this phase is the pressure rate of change, which is controlled in reasonable limits for the passenger's comfort by the pressurization system. • Too large of a deviation from the required pressure rate of change will cause direct consequences to passengers Ex. ear-ache • The human physiology dictates that the pressure rate of change must not exceed 18 mbar per minute during a climb phase. This speed allows air to flow inside human head (nose, throat, internal ear ) so that we may have the same pressure on each side of our ear drum.
As an a/c climbs and the pressure reduces, the cabin pressurization system allows the cabin pressure to drop only to the pre-set, or computer controlled level (5000’-10,000’). In most cases the cabin pressure “ outflow valve ” will close and prevent any further loss of pressure, and allow the compressor bleed air (or other pressure supply system) to maintain or increase as required.
Pressure Relief Valves prevent the cabin from becoming excessively pressurized. • Negative Pressure Relief Valves prevent the outside pressure from exceeding the cabin pressure. • At 40,000 feet a pressurization of approximately 8.5 psi is required to maintain a cabin pressure of 8,000 ft. • Aircraft certified for pressurized operation must have warning systems installed that alert the pilot when the system fails.
Cabin Pressure Control Systems control automatically cabin pressure to ensure passenger and crew comfort. Key components are electric or pneumatic, or electro-pneumatic outflow valves Sample outflow valves made by Liebher (used on Dash 8)
Rate of Climb • Pressurization rate required is dependent on the rate of climb of the aircraft. If an a/c climbs very quickly, the pressure supply system must be able to pressurize the cabin at a specific rate. This is calculated to stay within the comfort zone of 18 mbar per minute.
Calculation!!! • To determine the cabin rate of climb required you need to know: • Destination altitude minus start altitude • Planed Cruise flight level subtract airport elevation • Expected Rate of Climb • Feet per minute • Desired Cabin altitude • Cabin pressure altitude to be maintained
Example: Airport elevation: 2000 feet Planned Cruise: FL330 Rate of Climb: 1500’/min. Desired Cabin Pressure: 8000’ 31,000’ = 20.66 minutes to altitude. 1500’/min 6000’ (cabin pressure change) = 290’ feet per min. change 20.66min.
Don’t think this stuff is important? WASHINGTON --Loss of cabin pressure and failure to obtain oxygen incapacitated the crew of golfer Payne Stewart's plane, leading to the crash last year that killed all six aboard the chartered Learjet. The yearlong investigation was hampered by the plane's extensive damage, its lack of a flight data recorder and the short half-hour duration of the cockpit voice recorder, Board Chairman Jim Hall said. The accident happened Oct. 25, 1999 after Stewart's chartered Learjet 35 left Orlando, Fla. Flying at 23,000 feet, the pilot acknowledged permission to climb to 39,000 feet in the last contact with the plane. It eventually climbed to more than 40,000 feet and flew on autopilot for four hours before running out of fuel and crashing near Aberdeen S.D. Military pilots sent to observe the unresponsive craft reported that the cockpit windows were iced up. The loss of cabin pressure could cause this, as well as the loss of enough oxygen to cause unconsciousness. Emergency oxygen was available, but in the older-style plane it had to be activated manually by the crew. Dr. Mitchell Garber, the board's medical officer, said that many pilots believe that when pressure fails they have a minute or two to take action before they need oxygen. But impairment begins within seconds, he said, and the longer the crew waits to activate the oxygen the less likely they are to make the right decision. In a depressurization, he said, the first thing a pilot should do is reach for the oxygen mask.
At altitudes above 25,000’, sudden cabin depressurization will give you on average less than 60 seconds to get an oxygen mask on and working….also expect it to be quite cold!! Review the different types of hypoxia and their effects!
Don’t forget about the assigned reading from the Turbine manual One more class and your mid term!
Homework: • TPFM:ch11 • ATPLprep:sec4