OXYGEN SYSTEM SERVICING, INSPECTION AND MAINTENANCE PRACTICES

Oxygen System Servicing, Inspection And Maintenance Practices

 INTRODUCTION

Oxygen system requires some careful maintenance activities. This week highlight different type of  servicing, inspection and maintenance procedures.

  Oxygen System Servicing: GENERAL

Care and attention to detail is the mark of professional aviation maintenance, and nowhere is this characteristic more important than when servicing aircraft oxygen systems. Compressed gaseous oxygen demands special attention be­cause of both its high storage pressure and its extremely active chemical nature.

When possible, all oxygen servicing should be done outdoors, or at least in a well ventilated area of the hangar. Oxygen systems having removable or portable supply cylinders should have these containers removed from the operation, and all electrical work within the aircraft should be suspended during the servicing. In all cases the manufacturer's service information must always be used while performing service, maintenance or inspection on aircraft oxygen systems.

Servicing Gaseous Oxygen Systems

  Leak Testing Gaseous Oxygen Systems:  Leaks should be searched out by using a special leak detector material which is a form of non-oily soap solution. Spread this solution over every fitting and at every place a leak could possibly occur, and the presence of bubbles will indicate a leak. If a leak is found, release the pressure from the system, and check the fittings for proper torque. It is especially true of flare less fittings where over tightening can intensify a leak. If the fitting is properly torqued and still leaks, remove the fitting and examine all of the sealing surfaces for indication of damage. It may be necessary to replace the fitting and reflare the tube or install a new flare less fitting.

Draining The Oxygen System: Draining of the oxygen system should normally be done after the high pressure bottle has been removed or isolated from the system. Either out­doors or in a well ventilated hangar, open the aircraft's doors and windows, then bleed the system's pressure off by opening the appropriate fitting to allow the oxygen pressure to bleed off. Normally a system will require purging after the system has been drained. All the safety precau­tions mentioned later in this chapter should be followed during any oxygen draining procedure.

 Filling An Oxygen System: Fixed base operators who do a considerable amount of oxygen servicing will usually have one of the larger oxygen servicing carts such as the one seen in Figure 7.1. This cart carries six large cylinders, each holding approximately 250 cubic feet of aviators breathing oxygen. A seventh cylinder, facing the opposite direction and filled with compressed nitrogen, is normally carried to charge hydraulic accumulators and de-icer cylinders. Fittings on the nitrogen cylinders are different from those on the oxygen cylinders, to minimize the possibility of using nitrogen to fill the oxygen system, or of servicing the other systems with oxygen.

If you ever fill a low-pressure oxygen system from a high-pressure supply, be sure the proper regulator is installed and the output pressure is adjusted to that required for the system.

Various manufacturers of oxygen equipment use different types of connections between the supply and the aircraft, and a well equipped ser­vice cart should have the proper adapters. These adapters must be kept clean and protected from damage. Never improvise when adapting a supply cart to the aircraft. Leakage during the filling operation is not only costly, but it is hazardous as well.

Before filling any aircraft oxygen system, be sure that all of the cylinders are of the approved type, and that they have all been hydrostatically tested within the required time interval.

No oxygen system should be allowed to become completely empty. When there is no pressure in­side the cylinder, air can enter, and most air contains water vapour. When the water vapour is mixed with the oxygen and expanded through the small orifices in the system, the water is likely to freeze and shut off the flow of oxygen to the masks. Water in a cylinder can also cause it to rust on the inside and weaken it so it will fail with catastrophic results. A system is considered to be empty when the pressure gets down to 50 to 100 psi. If the system is ever allowed to get completely empty, the valve should be removed and the cylinder cleaned and inspected by an FAA-approved repair station.

When filling an airplane from a large supply cart, start with the cylinder having the lowest pressure. The pressure should be written on the container with chalk or a record kept with the cart. Momen­tarily crack the valve on the cylinder and allow some oxygen to purge all of the moisture, dirt and air from the line; then connect the line to the aircraft filler valve and slowly open the valve on the cylinder. Most all filler valves have restrictors that prevent too high a flow rate into the cylinder. When the pressure in the aircraft system and that in the cylinder with the lowest pressure stabilizes and there is no more flow, mark this pressure on the cylinder with chalk and close the cylinder valve. Slowly open the valve on the cylinder having the next lowest pressure and allow oxygen to flow into the system until it again stabilizes. Continue this procedure until the pressure in the aircraft system is that which is required.

The ambient temperature determines the pres­sure that should be put into the oxygen system, and a chart similar to the one in should be used to determine the pressure needed. For example, if the ambient temperature is 90 °F and you want a stabilized pressure in the system of 1,800 psi, you should allow the oxygen to flow until a pressure of 2,000 psi is indicated on the system pressure gauge. When the oxygen in the system drops to its standard temperature of 70 °F, the pressure should stabilize at 1,800 psi. If the ambient temperature is low, you must stop filling the system at a lower pressure, because the oxygen will expand and the pressure will rise when it warms up to its standard temperature.\

Purging A Gaseous Oxygen System: If the oxygen system has been opened for servic­ing, you should purge it of any air that may be in the lines. To purge a continuous flow system, plug masks into each of the outlets, turn on the oxygen supply valve, and allow the oxygen to flow through the system for about ten minutes. Diluter demand and pressure demand systems may be purged by placing the regulators in the EMERGENCY posi­tion and allowing the oxygen to flow through them for about ten minutes. After the system has been thoroughly purged, fill the cylinders to the re­quired pressure.