EngineeringClicks

Boeing 747 Landing Gear Design

Boeing 747 isolated on white background

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?

Compared to hydraulics, pneumatic systems offer several advantages:

So why doesn’t the Boeing 747 Landing Gear use pneumatics?

The reasons are as follows:

  1. 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.
  2. 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?

Notable Complications

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
Load rating High Low
Life cycle Very high Moderate
Acceleration Very high Moderate
Operating speed Low Very high
Maintenance Requirement Low
(oil lubricates system components)
High
(no self-lubrication)
Stiffness High Low
Other effects Oil leaks Extreme noise
Efficiency 50-80% 50%<