When it comes to aircraft, hydraulic systems are often the preferable choice when compared to electrical and mechanical systems. This is certainly the case with the Boeing 747 landing gear. However, this still leaves the question: why would we not use a pneumatic system instead of hydraulics?
- Hydraulic systems requires oil to perform the operation whereas pneumatic systems work on compressed air. Use of a pneumatic system over a hydraulic system eliminates the need to carry 100 litres of hydraulic oil on every flight. It is a well known fact that an increase in an aircraft’s weight increases the fuel consumption, and hence is undesirable.
- A return line is not required (air can be directly exhausted into the atmosphere).
- Minor leakage does not lead to catastrophic results.
- The velocity of a pneumatic system can be very high.
So why doesn’t the Boeing 747 Landing Gear use pneumatics?
The reasons are as follows:
- Pneumatic systems are not designed to operate at very high pressures. It takes around 3 minutes to generate pressure of 6 bar in a closed 450 litre system, so imagine the time it will take to reach 300 bar! This indicates the reaction time of the system. Comparatively, hydraulic systems are extremely fast, offering very low reaction time.
- Low pressure pneumatic systems can reach the desired pressure pretty quickly compared to high pressure pneumatic systems (it takes less time to reach 50 bar pressure than to reach 300 bar pressure). Therefore, one could argue that the aircraft industry could use low-pressure pneumatics to improve response time. So why don’t they do it then?
a) To generate the same actuating force, low pressure hydraulic systems need a considerably large cylinder and large hoses (F=P*A). As pressure (P) decreases, cross section area (A) should increase to maintain same force. This increases the space consumed by the system.
b) The large size of cylinder and hoses increases the volume of the system. Increasing the volume increases the amount of input air. With a fixed speed compressor the amount of air supply per minute is constant and hence the system takes more time to build the required low pressure with an increase in volume. So, even a low pressure pneumatic system has a very high reaction time.
c) The size of a pneumatic reservoir is large compared to a hydraulic reservoir (on average it is twelve times larger for the same application). A hydraulic reservoir stores liquid at atmospheric pressure whereas the pneumatic reservoir stores gas at high pressure and so it needs to be strong and bulky. This makes the pneumatic system bulky, consuming space that the airline industry cannot afford.
d) An aircraft operates under dynamic conditions. On the ground it might sit at an ambient temperature of 30ºC, and at a cruising altitude of 35,000 ft temperatures can reach -50°C. This changes the pressure of the system, and so the position of the piston can change with altitude. If it is unconstrained, the landing gear may open directly without any external force at high altitudes!
e) The control will change continuously from ground position to cruising position. Fluids also undergo changes in viscosity with temperature changes, but the pressure of oil in the system remains the same (closed system!). So the system is stable even with manual control at different altitudes. That said, for control it does need a computer to filter the input, based on the behavior of the fluid at different altitudes control.
Summary of a hydraulic system versus a pneumatic system:
|Hydraulic system||Pneumatic system|
|Life cycle||Very high||Moderate|
|Operating speed||Low||Very high|
(oil lubricates system components)
|Other effects||Oil leaks||Extreme noise|