AIRFRAME SYSTEMS

REQUIREMENTS OF EMERGENCY EQUIPMENT

  Orders on provisioning aircraft equipment came in the ANO under Equipment of Aircraft (articles 19, 20, 21) and emergency equipments are covered here in article 19. Corresponding schedules specifying such equipment are schedule 4.

ANO Article 19:

Article 19 (2):  In the case of any aircraft registered in the United Kingdom the equipment required to be provided (in addition to any other equipment required by or under this Order) shall:

(a) Be that specified in such parts of Schedule 4 as are applicable in the circumstances;

Article 19 (4): The equipment carried in compliance with this article shall be so installed or stowed and kept stowed, and so maintained and adjusted, as to be readily accessible and capable of being used by the person for whose use it is intended.

Article 19 (5): The position of equipment provided for emergency use shall be indicated by clear markings in or on the aircraft.

Article 19 (6): In every public transport aircraft registered in the United Kingdom there shall be provided individually for each passenger or, if the CAA so permits in writing, exhibited in a prominent position in every passenger compartment, a notice which complies with paragraph (7).

Article 19 (7): A notice complies with this paragraph if it:

(a) is relevant to the aircraft in question;

(b) contains pictorial instructions on the brace position to be adopted in the event of an emergency landing;

(c) Contains pictorial instructions on the method of use of the safety belts and safety harnesses as appropriate;

(d) contains pictorial information as to where emergency exits are to be found and instructions as to how they are to be used; and (e) contains pictorial information as to where the lifejackets, escape slides, life rafts and oxygen masks, if required to be provided by paragraph (2), are to be found and instructions as to how they are to be used.

1.2.3 Schedule 4:  Schedule 4 gave a table at paragraph 5 with 3 columns: Description of aircraft, circumstances of flight and scale of equipment required. Scales of equipment are specified in paragraph 6 with headlines Scale A, Scale B……etc.

Scale A came in the form of:  A (1), (2), (3) ……etc listing the equipment as required in the table paragraph 5. Emergency equipment is listed in A(3), B, H, I, J, K, Scale KK, L1, U, V, W, X, Y, Z

Chemical Oxygen Systems

Sodium chlorate mixed with appropriate binders and a fuel is formed into a block, called a candle.

When this candle is burned, it releases oxygen. The shape and com­position of the candle determines the oxygen flow rate. An igniter, actuated either electrically or by a spring, starts the candle burning, and as the sodium chlorate decomposes, it produces oxygen by a chemical action that looks something like this:

2 NaClO₂ + HEAT®  2 NaCl + 2O₂

The core of the candle is insulated to retain the heat needed for the chemical action and to prevent the housing from getting too hot, and filters are located at the outlet to prevent any contaminants entering the system.

The long shelf life of unused chemical oxygen generators makes them an ideal source of oxygen for occasional flights where oxygen is needed, and for the emergency oxygen supply for pressurized aircraft where oxygen is required only as a standby in case cabin pressurization is lost.

The emergency oxygen systems for pressurized aircraft have the oxygen generators mounted in either the overhead rack, in seat backs, or in bulkhead panels. The masks are located with these generators and are enclosed, hidden from view by a door that may be opened electrically by one of the flight crew members or automatically by an aneroid valve in the event of cabin depres­surization. When the door opens, the mask drops out where it is easily accessible to the user. At­tached to the mask is a lanyard that, when pulled, releases the lock pin from the flow initiation mechanism, so the striker , can hit the igniter and start the candle burning. Once a chemical oxygen candle is ignited, it must burn until it is exhausted.

Airframe system

Liquid Oxygen Systems (LOX)

Civilian aircraft do not generally use liquid oxygen, or LOX, systems because of the difficulty in handling this form of oxygen, and because it is not readily available to the fixed-base operators who service general aviation aircraft. The military, on the other hand, uses liquid oxygen almost exclusively because of the space and weight savings it makes possible. One litre of liquid oxygen will produce approximately 860 litres of gaseous oxygen at the pressure required for breathing.

The regulators and masks are the same as those used for gaseous oxygen systems, the difference in the systems being in the supply. Shown in Figure 9.8 is a sketch of a typical LOX converter and supply system. Liquid oxygen is held in the spheri­cal converter and in normal operation the build-up and vent valve is back-seated so some of the LOX can flow into the build-up coil where it absorbs enough heat to evaporate and pressurize the sys­tem to the amount allowed by the container pres­sure regulator, normally about 70 psi. This gaseous oxygen maintains a relatively constant pressure in the converter and supplies the oxygen to the regulator.

When the supply valve on the regulator is turned on, LOX flows from the converter into the supply evaporator coil where it absorbs heat and turns into gaseous oxygen.

If, for any reason, excessive pressure should build up in the system, it will vent overboard through one of the relief valves.

Airframe system

Masks

(a) Continuous Flow Masks

Almost all of the masks used with a continuous flow oxygen system are of the rebreather type and vary from the simple bag-type disposable mask used with some of the portable systems to the rubber bag-type mask used for some of the flight crew systems.

Oxygen enters a rebreather mask, shown in  , at the bottom of the bag, and the mask fits the face of the user very loosely so air can escape around it. If the rebreather bag is full of oxygen when the user inhales, the lungs fill with oxygen. Oxygen continues to flow into the bag and fill it from the bottom at the same time the user exhales used air into the bag at the top. When the bag fills, the air that was in the lungs longest will spill out of the bag into the outside air, and when the user inhales, the first air to enter the lungs is that which was first exhaled and still has some oxygen in it. This air is mixed with pure oxygen, and so oxygen rich air is always breathed with this type of mask. More elaborate rebreather-type masks have a close-fitting cup over the nose and mouth with a built-in check valve which allows the air to escape, but prevents the user breathing air from the cabin.

The oxygen masks that automatically drop from the overhead compartment of a jet transport aircraft in the event of cabin depressurization are of the rebreather type. The plastic cup that fits over the mouth and nose has a check valve in it, and the plastic bag attached to the cup is the rebreather bag. 

Demand-type Masks

All demand-type masks must fit tightly to the face so no outside air can enter to disturb the metering of the regulator, as illustrated in . Demand masks all connect to the regulator with a large diameter corrugated hose, whereas the continuous flow masks all use a small diameter tube to carry the oxygen to the mask.

A full-face mask is available for use in case the cockpit should ever be filled with smoke. These masks cover the eyes as well as the mouth and nose, and the positive pressure inside the mask prevents any smoke entering.

Plumbing And Valves

Most of the rigid plumbing lines that carry high­-pressure oxygen are made of stainless steel, with the end fittings silver soldered to the tubing. Lines that carry low-pressure oxygen are made of aluminium alloy and are terminated with the same type fittings used for any other fluid-carrying line in the aircraft. The fittings may be of either the flared or flare less type. It is essential in any form of aircraft maintenance that only approved com­ponents be used. This is especially true of oxygen system components. Only valves carrying the cor­rect part number should be used to replace any valve in an oxygen system.

Many of the valves used in oxygen systems are of the slow-opening type to prevent a rapid in-rush of oxygen that could cause excessive heat and become a fire hazard. Other valves have restrictors in them to limit the flow rate through a fully open valve.

Continuous Flow Regulators

1) Manual Continuous Flow Regulator

A typical manually adjusted continuous flow oxygen regulator is shown in  . The gauge on the right shows the pressure of the oxygen in the system and indicates indirectly the amount of oxygen available. The gauge on the left is a flow indicator and is adjusted by the knob in the lower center of the regulator. As the airplane ascends into the less dense air, the occupants need more oxygen, and with this type of regulator the user is able to adjust the flow to correspond with the altitude being flown, and the regulator will meter the correct amount of oxygen.

2) Automatic Continuous Flow Regulator

An automatic regulator, such as the one in   has a barometric control valve that automat­ically adjusts the oxygen flow to correspond with the altitude being flown. The flight crew need only open the valve on the front of the regulator, and the correct amount of oxygen will be metered into the system for the altitude being flown.

(b) Demand Regulators

The simple demand-type oxygen regulator, such as the one seen on the cylinder in meters oxygen to the user only during inhale. This type of regulator is far more economical of the oxygen than the continuous flow type, but there are regulators that are even more efficient.

Oxygen is almost always supplied to the crew of an aircraft by an efficient system that uses one of the demand-type regulators. Demand regulators allow a flow of oxygen only when the user is inhaling and shuts it off during exhale. There are several types of these regulators, as we will see

(c) Diluter Demand Regulators

The oxygen regulator used by the flight crews for most commercial jet aircraft are of the diluter demand type. In we have a very basic schematic of this type of regulator. When the supply lever is turned on, oxygen can flow from the supply into the regulator. There is a pressure reducer at the inlet of the regulator that decreases the pressure to a value that is usable by the regulator. The demand valve shuts off all flow of oxygen to the mask until the wearer inhales and decreases the pressure inside the regulator. This decreased pressure pulls the demand diaphragm over and opens the demand valve so oxygen can flow through the regulator to the mask.

A diluter demand regulator dilutes the oxygen supplied to the mask with air from the cabin. This air enters the regulator through the inlet air valve and passes around the air metering valve. At low altitude, the air inlet passage is open and the passage to the oxygen demand valve is restricted so the user gets mostly air from the cabin. As the aircraft goes up in altitude, the barometric control bellows expands and opens the oxygen passage while closing off the air passage. At an altitude of around 34,000 feet, the air passage is completely closed off, and every time the user inhales, pure oxygen is metered to the mask.

If there is ever smoke in the cabin, or if for any reason the user wants pure oxygen, the oxygen selector on the face of the regulator can be moved from the NORMAL position to the 100% position. This closes the outside air passage and opens a supplemental oxygen valve inside the regulator so pure oxygen can flow to the mask.

An additional safety feature is incorporated that bypasses the regulator. When the emergency lever is placed in the EMERGENCY position, the demand valve is held open and oxygen flows con­tinuously from the supply system to the mask as long as the supply lever is in the ON position.

AIRFRAME SYSTEMS

Gaseous oxygen systems and components

 Storage Cylinders

(a) Low-pressure Cylinders

Most military aircraft at one time used a low-pressure oxygen system in which the gaseous oxygen was stored under a pressure of ap­proximately 450 psi in large steel cylinders painted yellow. These cylinders were so large for the amount of oxygen they carried that they never became popular in civilian aircraft, and even the military has stopped using these systems.

(b) High-pressure Cylinders

Today, almost all gaseous oxygen is stored in green painted steel cylinders under a pressure of between 1,800 and 2,400 psi. All cylinders ap­proved for installation in an aircraft must be ap­proved by the Department of Transportation (DOT) and may be of either the DOT 3AA 1800 or the DOT 3HT 1850 type.

Both types of cylinders must be hydrostatically tested to 5/3 of their working pressure, which means that the DOT 3AA cylinders are tested with water pressure of 3,000 psi every five years and stamped with the date of the test. DOT 3HT cylinders must be tested with a water pressure of 3,083 psi every three years, and these cylinders must be taken out of service after 15 years, or after they have been filled 4,380 times, whichever comes first.

All oxygen cylinders must be stamped near the filler neck with the approval number, the date of manufacture, and the dates of all of the hydros­tatic tests. It is extremely important before servic­ing any oxygen system that you ensure that all cylinders are proper for the installation and that they have all been inspected within the ap­propriate time limit.

Oxygen cylinders may be mounted permanently in the aircraft and connected to an installed oxygen plumbing system, or for light aircraft where oxygen is needed only occasionally, they may be carried as a part of a portable oxygen system. The cylinders for either type of system must meet the same requirements, and should be painted green and identified with the words AVIATORS BREATH­ING OXYGEN written in white letters on the cylinder.

9.2.2 Regulators: It is the oxygen regulator that determines the type of system we have. There are two basic types of regulators in use, and in each type we have variations. For low-demand systems, such as are used in the smaller piston-engine powered general aviation aircraft, we normally use a con­tinuous flow regulator that allows oxygen to flow from the storage cylinder regardless of whether the user is inhaling or exhaling. Continuous flow systems are not economical of the oxygen, but their simplicity and low cost make them desirable when the demands are low. The emergency oxygen systems that drop the mask to the pas­sengers of large jet transport aircraft in the event of cabin depressurization are also of the con­tinuous flow type.

Simultaneous equations

Simultaneous equations

Simultaneous equations involve more than one variable or unknown. We can solve a simple linear equation with one unknown using the laws of algebra, which you have already learnt. It is often required to represent an engineering problem that involves more than one unknown.

Graphs of linear equations

Graphs of linear equations

In the above example all values of the coordinates are positive. This is not always the case and to accommodate negative numbers, we need to extend the axes to form a cross (Figure 4.8), where both positive and negative values can be plotted on both axes.

Graphical axes, scales and coordinates

Graphical axes, scales and coordinates

To plot a graph, you know that we take two lines at right angles to each other (Figure 4.6). These lines being the axes of reference and they are known as Cartesian coordinates or rectangular coordinates where their intersection at the point zero is called the origin. When plotting a graph a suitable scale must be chosen, this scale need not be the same for both axes. In order to plot points on a graph we need to identify them by their coordinates. The coordinate points (2, 4) and (5, 3) are shown in Figure 4.6b. Note that the x-ordinate or independent variable is always quoted first. Also remember that when we use the expression plot s against t. Then all the values of the dependent variable s are plotted up the vertical axis and the other independent variable (in this case t) are plotted along the horizontal axis.

Solution of linear equations

Solution of linear equations

Although you may not have realized, you have already solved linear equations analytically. However, before we start our study of the graphical solution of equations. Here is an example, which shows that in order to solve simple equations analytically, all we need to do is apply the techniques you have learnt when transposing and manipulating formula. The important point about equations is that the equality sign must always be present!