tings are designed with straight threads to receive Tube flared tube connections. Tubing-to-component (cylinder, regulator, and indicator) fittings have straight threads on the tubing end and external pipe threads (tapered) on the other end for attachment Sleeve to the component, as shown in figure 14-50.
Nut hex
• Tube Recessed tube seat Fitting hex
- Sleeve Double tube flare External pipe thread • Fitting nut • Nut hex Nipple FIcURE 14-51. A typical flareless fitting.
Fitting hex External pipe thread FIGURE 14-50. Sectional view of a typical oxygen system hitting.
Oxygen system fittings may be made of aluminum alloy, steel, or brass. These fittings may be either of two types, flared or flareless. A typical flared fitting is shown in figure 14-50. A flareless fitting is shown in figure 14-51.
The sleeve in a flareless fitting must be preset before final installation in a flareless seat. Presetting causes the cutting edge of the sleeve to grip the tube sufficiently to form a seal between the sleeve and the tubing. The end of the tubing bottoms on the seat of the flareless fitting to provide tube end support after installation.
To seal oxygen system tapered pipe thread connections and to prevent thread seizure, use only an approved hread compound. Never use a mixture containing oil, grease, or any other hydrocarbon on any fittings used in oxygen systems.
Replacement Lines The same cutting and bending methods described in Chapter 5, "Fluid Lines and Fittings," of AC 65-9A, Airframe and Powerplant Mechanics General Handbook, also apply to oxygen lines. As a general rule oxygen lines are double flared. The double flare makes the connection stronger and able to withstand more torque.
When installing a line, make sure there is proper clearance. The minimum clearance between oxygen plumbing and all moving parts should be 2 in. It is desirable to maintain a 6-in. clearance between oxygen tubing and electrical wires. When this is not possible, fasten all electrical wires securely with clips so that they cannot come to within 2 in. of the oxygen tubing.
OXYGEN VALVES Five types of valves are commonly found in high-pressure gaseous oxygen systems. These are filler valves, check valves, shutoff valves, pressure reducer valves, and pressure relief valves. Low-pressure systems will normally contain only a filler valve and check valves.
Filler Valves The oxygen system filler valve is located on most aircraft close to the edge of an access hatch or directly behind a cover plate in the skin. In either location, the valve is readily accessible for servicing. It is usually marked by a placard or a sign stenciled on the exterior, reading: &OXYGEN FILLER VALVE.' There are two types of oxygen filler valves in use, a low-pressure filler valve and a high-pressure filler valve.
The low-pressure filler valve, figure 14-52, is used on systems equipped with low-pressure cylinders. When servicing a low-pressure oxygen system, push the recharging adapter into the filler valve casing. This unseats the filler valve and permits oxygen to flow from the servicing cart into the aircraft oxygen cylinders. The filler valve contains a spring-loaded locking device which holds the recharging adapter in place until it is released. When the adapter is removed from the filler valve, reverse flow of oxygen is automatically stopped by a check valve. A cap is provided to cover the filler opening and prevent contamination.
The high-pressure valve has a threaded fitting to receive the oxygen supply connector and a manual valve to control the flow of oxygen. To service an oxygen system that uses a high-pressure filler valve, screw the recharging adapter onto the aircraft filler valve. Open the manual valve on the filler valve and the servicing bottle. When recharging is completed, close the valves, remove the recharging adapter, and screw a valve cap on the valve to prevent contamination.
Check Valves Check valves are installed in the lines between cylinders in all aircraft that have more than one storage cylinder. They are provided to prevent a reverse flow of oxygen or to prevent the loss of all oxygen as the result of a leak in one of the storage cylinders. Check valves permit the rapid flow of oxygen in only one direction. The direction of unrestricted flow is indicated by an arrow on the valves.
Of the two basic types of check valves commonly used, one type consists of a housing containing a spring-loaded ball. When pressure is applied to the Valve cap Locking device Low-pressure oxygen.
Fill to 425 Ibs. pressure.
Do not exceed 450 Ibs.
pressure.
CAUTION Keep clean, dry and a free of all oils.
Oxygen system filler valve Access door FicuRE 14-52. Low-pressure gaseous oxygen filler valve.
inlet side, the ball is forced against the spring, thus breaking the seal and allowing oxygen to flow.
When pressure is equalized, the spring re-seats the ball, preventing any reverse flow of oxygen. The other type is a bell-mouthed hollow cylinder fitted with a captive ball in its bore. When pressure is applied at the bell-mouthed end (inlet), the ball will permit oxygen to flow. Any tendency of reverse flow causes the vall to move onto its seat, covering the inlet and preventing a reverse flow.
Shutoff Valves Manually controlled two-position (on, off) shutoff valves are installed to control the flow of oxygen being emitted from a cylinder or & bank of cylinders. For normal operation, the knobs which control the valves are safetied in the "on" position.
When necessary, such as changing a component, the appropriate valve can be closed. As a precaution, when opening a valve, the knob should be turned slowly to the "on" position. Otherwise, the sudden 593
rush of highly pressurized oxygen into a depleted system could rupture a line.
Pressure-Reducer Valves In high-pressure oxygen systems, pressure-reducing valves are installed between the supply cylinders and the cockpit and cabin equipment. These valves reduce the high pressure of the oxygen supply cylinders down to approximately 300 to 400 p.s.i. required in the low-pressure part of the system.
Pressure-Relief Valves A pressure-relief valve is incorporated in the main supply line of a high-pressure system. The relief valve prevents high-pressure oxygen from entering the system downstream of the pressure reducers if the reducer fails. The relief valve is vented to a blowout plug in the fuselage skin.
REGULATORS Diluter-Demand Regulators The diluter-demand regulator gets its name from the fact that it delivers oxygen to the user's lungs in response to the suction of his own breath. To prolong the duration of the oxygen supply, the oxygen is automatically diluted in the regulator with suitable amounts of atmospheric air. This dilution takes place at all altitudes below 34,000 ft.
The essential feature of a diluter-demand regulator is a diaphragm-operated valve called the demand valve (figure 14-53), which opens by slight suction on the diaphragm during inhalation and which closes during exhalation. A reducing valve upstream from the demand valve provides a controlled working pressure. Downstream from the demand valve is the diluter control closing mechanism. This consists of an aneroid assembly (a sealed, evacuated bellows) which controls the air inlet valve. When the diluter lever is set in the position marked "normal oxygen," atmospheric air at ground level is supplied with very little oxygen added. As altitude increases, the air inlet is grad ually closed by the bellows to give a higher concen- Emergency metering control Pressure reducing valve Diaphragm Demand valve Aneroid Oxygen - Diluter control metering closing mechanism port Air metering port lever Air iniet check valve FIGURE 14-53. Schematic of a diluter-demand regulator.
tration of oxygen until at about 34,000 ft. the air inlet is completely closed and 100% oxygen is supplied. As altitude decreases, this process is reversed.
The diluter control as shown in figure 14-54, can be set by turning the lever to give 100% oxygen at any altitude. At moderate altitudes, however, this causes the oxygen supply to be consumed much more rapidly than normal. The diluter control should be set at "normal oxygen" for all routine operations. It can be set at "100 percent oxygen" for the following purposes: (1) Protection against exhaust gases or other poisonous or harmful gases in the aircraft, (2) to avoid the bends and chokes, and (3) to correct a feeling of lack of oxygen.
Emergency knob (red)
OILUTER DE Diluter lever FIcURE 14-54. Diluter-demand regulator control.
The diluter-demand regulator is provided with an emergency valve, operated by a red knob (figure 14-54) on the front of the regulator. Opening this valve directs a steady stream of pure oxygen to the The following paragraphs illustrate a typical procedure for checking the operation of a diluter-demand regulator. First, check the oxygen system pressure gage, which should indicate between 425 and 450 p.s.i.; then check out the system using the following steps:
(1) Connect an oxygen mask to each diluterdemand regulator.
(2) Turn the auto-mix lever on the diluter-demand regulator to the 100 percent oxygen" position and listen carefully to make certain that no oxygen is escaping.
(3) Breathe oxygen normally from the mask.
The oxygen flowmeter should blink once for each breath. (Figure 14-55 shows a 250
BLINKER 500
OXYGEN FLOW
INDICATOR FICURE 14-55. Flow indicator and pressure gage.
typical oxygen fowmeter and pressure gage.)
(4) With auto-mix lever in "100 percent oxygen" position, place the open end of the mask-to-regulator hose against the mouth and blow gently into the hose. Do not blow hard, as the relief valve in the regulator will vent. There should be positive and continued resistance, if not, the diaphragm or some part of the air-metering system may be leaking.
(5) Turn the auto-mix lever to normal oxy.
gen" position.
(6) Turn the emergency valve on the diluterdemand regulator to the "on" position for a few seconds. A steady flow of oxygen should result, ceasing when the emergency valve is turned off.
(7) Safety wire the emergency valve in "off" position with Federal Specification QQ-W-341, or equal, annealed copper wire, 0.0179-in. diameter.
Another type of diluter-demand regulator is the narrow panel type. This type regulator face (figure 14-56) displays a float-type flow indicator which signals oxygen flow through the regulator to the mask.
The regulator face also displays three manual control levers. A supply lever opens or closes the oxygen supply valve. An emergency lever is used to obtain oxygen under pressure. An oxygen selector lever is used for selecting an air/oxygen mixture or oxygen only.
Figure 14-57 illustrates how the narrow panel oxygen regulator operates. With the supply lever in the "on" position, the oxygen selection lever in the "normal" position, and the emergency lever in the "off" position, oxygen enters the regulator inlet.
The addition of air may be cut by turning the oxygen selection lever to "100%." When this lever is in "normal," air enters through the air inlet port, and the required amount is added to the oxygen to form the correct air/oxygen mixture.
OXYGEN REGULATOR GILUTER DEMAMO obtained by turning the emergency lever to "on." Positive pressure at the regulator: outlet may be This mechanically loads the demand diaphragm to provide positive outlet pressure.
Continuous-Flow Regulator Continuous-flow regulators of the hand-adjustable and the automatic type are installed for the Emergency lever crew and passenger oxygen supply respectively.
Flowmeter The hand-adjustable, continuous-flow regulator Oxygen lever delivers to the user's mask a continuous stream of oxygen at a rate that can be controlled. The system Supply lever usually contains a pressure gage, a flow indicator, and a manual control knob for adjusting the oxy- FicURE 14-56. Typical narrow panel oxygen regulator. gen flow. The pressure gage indicates the p.s.i. of oxygen in the cylinder. The flow indicator is calibrated in terms of altitude. The manual control Demand diaphragm knob adjusts the oxygen flow. The user adjusts the manual control knob until the altitude of the flow Relief valve indicator corresponds to the cabin altimeter read- On ing- Emergency lever Off • Demand valve The automatic continuous-flow regulator is used Inlet air cover in transport aircraft to supply oxygen automatically 100% Supplementary to each passenger when cabin pressure is equivalent Oxygen selection lever oxygen valve to an altitude of approximately 15,000 ft. Operation Normal of the system is initiated automatically by means of On • Flow indicator an electrically actuated device. The system can also Supply lever be actuated electrically or manually should the au- Off tomatic regulator malfunction.
Oxygen inlet Air inlet valve Upon actuation, oxygen flows from the supply
- Air metering valve cylinders to the service units. A typical passenger FIcURE 14-57. Schematic of a narrow panel: service unit is shown in figure 14-58. During the oxygen regulator.
When there is sufficient differential pressure across the demand diaphragm, the demand valve Mask stowage box opens to supply oxygen to the mask. This pressure Mask stowage box door differential exists during the user's inhalation cycle.
After passing through the demand valve, the oxygen is mixed with air that enters through the air inlet port. The mixture ratio is determined by an aneroid-controlled air metering valve. A high oxygen ratio is provided at high altitudes and a high air ratio at lower altitudes. The air inlet valve is set to permit the airflow to begin at the same time as the oxygen flow. FIGURE 14-58. Typical passenger service unit.
first few seconds of oxygen flow, a pressure surge of 50 to 100 p.s.i. causes the oxygen mask box doors to open.
Each mask assembly then falls out and is suspended by the actuating attachment on the flexible tubing. The action of pulling the mask down to a usable position withdraws the outlet valve actuation pin, opening the rotary valve, allowing oxygen to flow to the mask.
OXYGEN SYSTEM FLOW INDICATORS Flow indicators are used in oxygen systems to give visual indications that oxygen is flowing through the regulator. They do not show how much oxygen is flowing. Furthermore, their operation does not indicate that the user is getting enough oxygen.
In the blinker type indicator, figure 14-59, the eye opens and closes each time the user inhales and exhales. To check the flow indicator, set the diluter lever to "100% oxygen" and take several normal breaths from the mask-to-regulator hose. If the blinker opens and closes easily with each breath, it is in operating condition.
BLINKER
OXYGEN FLOW
INDICATOR FICURE 14-59. Oxygen flow indicator.
PRESSURE GAGES Pressure gages are usually of the Bourdon tube type. Figure 14-60 shows the faces of two oxygen gages: (1) A low-pressure gage and (2) a highpressure gage.
200 300 1000 100 400 FULL LBS. PER $0.500 • 2000 ÖXYGEN O OXYGEN e CYLINDER
PRESSURE PRESSURE Low pressure gage High-pressure gage FIGURE 14-60. Oxygen pressure gages.
Because of their connection into a system, the gages do not necessarily show the pressure in each cylinder. If the system contains only one supply cylinder, the pressure gage will indicate cylinder pressure. In systems where several cylinders are interconnected through check valves, the gage will indicate the pressure of the cylinder having the highest pressure.
Immediately after the system has been filled, pressure gage accuracy can be checked by comparing the aircraft pressure gage with the gage on the servicing cart. On low-pressure systems, the aircraft gage should read within 35 p.s.i. of the 425 p.s.i.
servicing cart pressure. The same check can be used for high-pressure systems, but servicing pressure is 1,850 p.s.i. and a tolerance of 100 p.s.i. is allowed.
The tolerances shown for pressure gage accuracy are typical and should not be construed as applying to all oxygen systems. Always consult the applicable aircraft maintenance manual for the tolerances of a particular system.
OXYGEN MASKS There are numerous types of oxygen masks in use which vary widely in design detail. It would be impractical to discuss all of the types in this hand book. It is important that the masks used be compatible with the particular oxygen system involved.
In general, crew masks are fitted to the user's face with a minimum of leakage. Crew masks usually contain a microphone. Most masks are the oronasal type, which covers only the mouth and nose.
Large transport aircraft are usually fitted with smoke masks for each crew position. The smoke masks are installed in stowage containers within easy grasp of the individual. These masks provide crew protection in an emergency and are not used frequently like the demand and continuous-flow masks. Smoke mask equipment consists of a full- 597
tace mask, a flexible breathing tube, and a coupling. The coupling connects to a demand regulator.
A microphone is permanently installed in the mask.
Passenger masks, figure 14-61, may be simple, cup-shaped rubber moldings sufficiently flexible to obviate individual fitting. They may have a simple elastic head strap or they may be held to the face by the passenger.
Valve actuating attachment Base plate Head strap Reservoir bag Face piece FicURE 14-61. Passenger oxygen mask.
All oxygen masks must be kept clean. This reduces the danger of infection and prolongs the life of the mask. To clean the mask, wash it with a mild soap and water solution and rinse it with clear water. If a microphone is installed, use a clean swab, instead of running water, to wipe off the soapy solution.
The mask must also be disinfected. A gauze pao, which has been soaked in a water solution of merthiolate can be used to swab out the mask. This solution should contain one-fifth teaspoon of merthiolate per quart of water. Wipe the mask with a clean cloth and air dry.
SERVICING GASEOUS OXYGEN SYSTEMS The servicing procedures for a gaseous oxygen system depend upon the type of system. Before charging an aircraft system, consult the aircraft manufacturer's maintenance manual. Precautions such as purging the connecting hose before coupling to the aircraft filler valve, avoiding overheat.
ing caused by too rapid filling, opening cylinder valves slowly, and checking pressures frequently during charging should be considered.
The type of oxygen to be used, the safety precau tions, the equipment to be used, and the procedures for filling and testing the system must be observed.
Gaseous breathing oxygen used in aircraft is a special type of oxygen containing practically no water vapor and is at least 99.5% pure. While other types of oxygen (welder, hospital) may be pure enough, they usually contain water, which might freeze and block the oxygen system plumbing especially at high altitudes.
Gaseous breathing oxygen is generally supplied in 220- to 250-cu. ft. high-pressure cylinders. The cylinders are identified by their dark green color with a white band painted around the upper part of the cylinder. The words &OXYGEN AVIATORS' BREATHING" are also stenciled in white letters, lengthwise along the cylinders.
Oxygen Service Safety Gaseous oxygen is dangerous and must be handled properly. It causes flammable materials to burn violently or even to explode. Listed below are several precautionary measures to follow:
(1) Tag all reparable cylinders that have leaky valves or plugs.
(2) Don't use gaseous oxygen to dust off clothing, etc.
(3) Keep oil and grease away from oxygen equipment.
(4) Don't service oxygen systems in a hangar because of the increased chances for fire.
(5) Valves of an oxygen system or cylinder should not be opened when a flame, electrical arc, or any other source of ignition is in the immediate area.
(6) Properly secure all oxygen cylinders when they are in use.
Gaseous Oxygen Servicing Trailers Even though several types of servicing trailers are in use, each recharging system contains supply cylinders, various types of valves, and a manifold that connects the high-pressure cylinders to a purifier assembly. In the purifier assembly, moisture is removed from the oxygen. Coarse particles are trapped in the filter before reaching a reducing valve. The reducing valve has two gages which are used to monitor inlet and outlet pressures respectively. The reducing valve also has an adjusting screw for regulating the outlet pressure. This pres- 598
sure is discharged into a flexible hose which connects to the charging valve and the adapter. The charging valve controls oxygen flowing away from the servicing trailer, and the adapter connects the recharging equipment to the aircraft filler valve.
On many aircraft a chart is located adjacent to the filler valve which shows the safe maximum charging pressure for the ambient temperature. This must be observed when charging the system.
It is common practice to have a warning placard cautioning against using oil or grease on the filler connections. Oxygen ground equipment should be maintained to a standard of cleanliness comparable to that of the aircraft system.
Leak Testing Gaseous Oxygen Systems This test is performed at different times, depending on the inspection requirements for the particular type of aircraft. The system is allowed to cool, usually 1 hr., after filling before the pressures and temperatures are recorded. After several hours have elapsed, they are recorded again. Some manufacturers recommend a 6-hr. wait, others a 24-hr. wait.
The recorded pressures are then corrected for any change in temperature since filling. Figure 14-62 is typical of the graphs provided in the aircraft main-
• 50 - 25 45
- 20 CHANGE IN TEMPERATURE-°C +15.
- 10 5 10 15 20 25 30 35 40
CHANGE IN PRESSURE-PSI
CORRECTION OF PRESSURE DURING LEAKAGE
TEST FOR CHANGE IN TEMPERATURE
ADD PRESSURE CHANGE IF TEMP.
ERATURE RISES:
SUBTRACT PRESSURE CHANGE IF TEMP.
ERATURE FALLS FicURE 14-62. Pressure/temperature correction chart.
tenance manual to aid in making pressure/temperature corrections.
As an example for using the chart, assume that the oxygen system was recently charged. An hour later, the oxygen pressure gage read 425 p.s.i. at a temperature of 72° F. When the pressure gage was read 6 hrs. later, the pressure was 430 p.s.i. with a temperature of 79° F. By referring to figure 14-57, we see that the 7° temperature rise should have caused the pressure to increase 5 p.s.i., making the pressure gage read 430 p.s.i.
When oxygen is being lost from a system through leakage, the gage reading will be less than shown on the pressure/temperature correction chart. Leakage can often be detected by listening for the distinct hissing sound of escaping gas. If the leak cannot be located by listening, it will be necessary to soap-test all lines and connections with a castile soap and water solution or specially compounded leak-test material. To make this check, apply the test solution to areas suspected of leakage. Watch for bubbles. Make the solution thick enough to adhere to the contours of the fittings.
Any leak no matter how small must be found and repaired. A small leak may not cause trouble but if leak continues over a period of time, the surroundings and atmosphere become saturated Such conditions are especially dangerous because personnel may not be aware that oxygen-enrichment exists. Oxygen-enriched conditions are almost always present in poorly ventilated areas.
NO ATTEMPT SHOULD BE MADE TO
TIGHTEN A LEAKING FITTING WHILE THE SYSTEM IS CHARGED.
Draining the Oxygen System When it is necessary to drain the system, it can be done by inserting a filler adapter into the filler valve and opening the shutoff valves. Do not drain the system too rapidly as this will cause condensa tion within the system. An alternate method of draining the system is opening the emergency valve on the demand oxygen regulator. Perform this job in a well-ventilated area and observe all fire precautions.
Cleaning the Oxygen System Always keep the external surfaces of the components of the oxygen system, such as lines, connections, and mounting brackets, clean and free of corrosion and contamination with oil or grease.
As a cleaning agent, use anhydrous (waterless) ethyl alcohol, isopropyl alcohol (anti-icing fluid), 599
or any other approved cleaner. If mask-to-regulator hoses are contaminated with oil or grease, the hoses should be replaced.
Cleaning Compound, Oxygen System An approved cleaning formula for use on oxygen systems is available. This mixture of chlorinated, fluorinated hydrocarbons (freon) and isopropyl alcohol is safe for cleaning oxygen system components in aircraft, and for rinsing, flushing, and cleaning oxygen lines. Skin contact and prolonged inhalation of vapors should be avoided.
Purging the Oxygen System An oxygen system needs to be purged if: (1) It has been depleted and not re-charged within 2 hrs.,
(2) if any line or component is replaced, requiring the draining or opening of the system for more than 2 hrs., or (3) it is suspected that the system has been contaminated.
The main cause of contamination in the system is moisture. Moisture in the system may be due to damp charging equipment. In very cold weather the small amount of moisture contained in breathing oxygen can cause contamination, due to repeated Although the introduction of moisture into the aircraft oxygen system can be considerably reduced by using the correct charging procedure, cumulative condensation in the system cannot be entirely avoided. There have been instances where oxygen systems, unused for long periods, have developed an unpleasant odor which necessitated purging to clear the system of moisture.
The procedure for purging may vary somewhat with each aircraft model. Generally speaking, on aircraft having the filler lines and the distribution lines commonly connected to one end of the storage cylinder, the system can be purged by filling the system with oxygen and then draining it at least three times. On aircraft that have the filler lines connected on one end of the cylinder and distribution lines connected to the opposite end of the cylinder, purge the system as follows: With all the regulator emergency palves open, pass oxygen at a pressure of 50 p.s.i. at the filler valve through the system for at least 30 min. Perform this job in a well-ventilated area and observe all fire precautions.
Dry nitrogen and/or dry air may also be used to purge oxygen systems. All open lines must be capped after use, also the system lines must be purged of the nitrogen by use of oxygen.
PREVENTION OF OXYGEN FIRES OR EXPLO SIONS Many materials, particularly oils, grease, and non-metallic materials, are likely to burn when exposed to oxygen under pressure. To avoid fire or an explosion it is essential that all oxygen equipment be kept clean and free from oil or grease.
An oxygen fire or explosion depends on a combination of oxygen, a combustible material, and heat.
The danger of ignition is in direct ratio to the concentration of oxygen, the combustible nature of the material exposed to the oxygen, and the temperature of the oxygen and material. Oxygen itself does not burn but it supports and intensifies a fire with any combustible material.
When working on an oxygen system it is essential that the warnings and precautions given in the aircraft maintenance manual be carefully observed. In general, before any work is attempted on an oxygen system the following fire precautions should be taken:
(1) Provide adequate fire-fighting equipment.
(2) Display "NO SMOKING" placards.
(3) Avoid checking aircraft radio or electrical systems.
(4) Keep all tools and oxygen servicing equipment free from oil or grease.
Oxygen System Inspection and Maintenance Oxygen system inspection and maintenance should be accomplished according to these precautionary measures and any in addition to the manufacturer's instructions.
1. Never attempt maintenance until oxygen supply is turned off.
2. Fittings should be unscrewed slowly to allow residual pressure to dissipate.
3. Plug or cap all open lines immediately.
4. Do not use masking tape to seal openings:
use caps or plugs designed for that purpose.
5. Maintain at least 2 inches clearance between oxygen lines and all moving equipment/ parts within the aircraft to prevent the possibility of wearing oxygen lines.
6. Maintain at least 2 inches clearance between oxygen lines and all electrical wiring in the aircraft.
7. Provide adequate clearance between oxygen lines and all hot ducts, conduits and equip- 600
ment to prevent heating of the oxygen system.
8. Maintain at least 2 inches clearance between oxygen lines and all oil, fuel, hydraulic, or other fluid lines to prevent contamination.
* U.S. G.P.O.: 1996-873-S/20104 601
9. Do not use lubricants unless specifically approved for oxygen system use.
10. A pressure and leak check must be performed each time the system is opened for maintenance.