Density

Density

Air is thus not a continuum. If it were, the density at a point would be defined as follows : considering the mass M of a small volume V of air surrounding the point, the density would be
the limiting ratio of M/V as V vanishes. But we must suppose that the volume V enclosing the point is contracted only until it is small compared with the scale of variation of density, while it still remains large compared with the mean distance separating the molecules. Clearly,however, V can become very small before the continuous passage of molecules in all directions across its bounding surface can make indefinite the number of molecules enclosed and M or M/V uncertain. Density is thus defined as the ratio of the mass of this very small, though finite, volume of air i.e. of the aggregate mass of the molecules enclosed to the volume itself. Density is denoted by p, and has the dimensions M/Z.

In Aerodynamics it is convenient to use the slug-ft.-sec. system of units.* At 15 C. and standard pressure 1 cu. ft. of dry air weighs 0-0765 Ib. This gives p = 0-0765/g = 0-00238 slug per cu. ft.
It will be necessary to consider in many connections lengths, areas, and volumes that ultimately become very small. We shall tacitly assume a restriction to be imposed on such contraction as discussed above. To take a further example, when physical properties are attached to a '
point' we shall have in mind a sphere of very small but sufficient radius centred at the geometrical point.

AIR AT REST, THE ATMOSPHERE AND STATIC LIFT

AIR AT REST, THE ATMOSPHERE AND STATIC LIFT


Air at sea-level consists by volume of 78 per cent, nitrogen, 21 per cent, oxygen, and nearly 1 per cent, argon, together with traces of neon, helium, possibly hydrogen, and other gases.

Although the constituent gases are of different densities, the mixture is maintained practically constant up to altitudes of about 7 miles in temperate latitudes by circulation due to winds. This lower part of the atmosphere, varying in thickness from 4 miles at the poles to 9 miles at the equator, is known as the troposphere. Above it is the stratosphere, a layer where the heavier gases tend to be left at lower levels until, at great altitudes, such as 50 miles, little but helium or
hydrogen remains. Atmospheric air contains water-vapour in varying proportion, sometimes exceeding 1 per cent, by weight. 

From the point of view of kinetic theory, air at a temperature of C. and at standard barometric pressure (760 mm. of mercury) may be regarded statistically as composed of discrete molecules, of mean diameter 1-5 X 10 ~ 5 mil (one-thousandth inch), to the number of 4-4 x 1011 per cu. mil. These molecules are moving rectilinearly in all directions with a mean velocity of 1470 ft. per sec., i.e. onethird faster than sound in air. They come continually into collision with one another, the length of the mean free path being 0-0023 mil.

Helicopter Fuel Systems

Helicopter Fuel Systems

Figure 1.7 illustrates a typical fuel system found in a light turbine powered helicopter. This system incorporates, a single bladder type fuel cell, located below and aft of the rear passenger seat. Installed in the fuel cell are two submersible centrifugal-type boost pumps, an upper and lower fuel quantity indicating probe, and a solenoid operated sump drain.

The boost pumps are connected so that their outlet ports join to form a single line to the engine. Either pump is capable of supplying sufficient fuel to operate the engine. Check valves are installed at the outlet of each pump, and a pressure switch located in the outlet port of each pump will il­luminate the FUEL BOOST CAUTION LIGHT in case of a pump failure.

An electrically operated shut-off valve is in­stalled in the fuel line running from the tank to the engine. A fuel selector valve is not necessary be­cause only one tank is used in this system.

Fuel is filtered twice before entering the engine, and each filter is equipped with a warⁿing light to indicate filter^- clogging. Additional provisions are made in the system for a fuel pressure gauge, vent system and a fuel quantity indication.

Figure1.7: Fuel system schematic for light turbine powered helicopter.

Jet Transport Aircraft Fuel Systems

Jet Transport Aircraft Fuel Systems

A large jet transport aircraft, such as the Boeing 727 has a relatively simple fuel system that sup­plies its three engines from three fuel tanks.

Tanks No. 1 and No. 3 are integral tanks, that is, part of the wing is sealed off and fuel is carried in the wing structure itself. Each of these tanks holds about 12,000 pounds of fuel. A fuselage tank consisting of either two or three bladder type fuel cells holds another 24,000 pounds of fuel.

Each of the wing tanks has two 115-volt AC electric boost pumps and the fuselage tank; tank No. 2 has four such pumps.

Each of the three engines may be fed directly from one of the three fuel tanks, or all of the tanks an^d engines may be opened, into a cross-feed manifold.

Fuelling is accomplished by connecting the fuel supply to a single-point fuelling receptacle located under the leading edge of the right wing. Fuel flows from this receptacle through the fuelling and dump manifold into all three tanks through the ap­propriate fuelling valves. When the tanks are com­pletely filled, pressure shutoff valves sense the amount of fuel and shut off the fuelling valve. This prevents the tank being overfilled or damaged. If only a partial fuel load is required, the person fuelling the aircraft can monitor a set of fuel quan­tity gauges at the fuelling station and can shut off the flow al fuel to any tank when the desired level is reached.

The airplane tanks may be refuelled by connect­ing the fuel receiving truck to the manual refuelling valve, closing the engine shutoff valves, and opening the cross-feed valve from the tank to be emptied. The fuel may be cither pumped out of the tank with the boost pumps, or it may be pulled from the tank by suction from the receiving truck. If it Is pulled out by section, it leaves the tank through the boost pump bypass valve.

Fuel may be dumped in-flight by opening the fuel dump valve for the tanks to be dumped and then opening the fuel dump nozzle valve in the wing tip through which the fuel is to leave the airplane. Fuel can be dumped from either wing tip or from both tips at the same time.

There is a fuel dump limit valve in each of the three systems that will shut off the flow if the pressure drops below that. -needed to supply the he engine with adequate fuel. It will also shut off the dump valve when the level in the tank gets down to the preset dump shutoff level. This dump sys­tem is capable of dumping about 1300 pounds of fuel per minute when all of the dump valves are open and the entire boost pumps are operating,

This fuel system has provisions for heating the fuel before it enters the fuel filter if its temperature is low enough for there to be danger of ice forming on the filter.

Figure 1.6: Fuel system for the Boeing 727 transport airplane.

Large Reciprocating Aircraft Fuel Systems

Large Reciprocating Aircraft Fuel Systems

Transport-category aircraft powered by reciprocating engines are rapidly disappearing from the active fleet. One exception seems to be the venerable. Douglas DC -3. This aircraft has seen a working life of more than 50 years and is still being used for passenger and cargo applications. The fuel system installed on the DC--3 is illustrated to figure 1.5 and is typical for aircraft using large radial-type engines.

Fuel is supplied from two resin tanks and two auxiliary tanks mounted in the centre wing section of the airplane. The capacity of each main tank is 202 gallons, and the auxiliary tank holds 200 gallons each. Provisions are made for the installation of from 2 to 8 long-range tanks, each holding 100 gallons. This makes it possible to carry a fuel load of 1604 galleons, in a total of 12 tanks.

The fuel quantity is measured by the liquid meter system which consists of a float as­sembly and a liquid meter tank unit in each tank. These are connected electrically to the fuel gauge on the right instrument panel in the pilots' com­partment. There are two tank selector valves, operated by dial and handle controls in the pilots' compartment. Ordinarily the left hand engine draws fuel from the left tanks, avid the right engine draws fuel from the right tanks, but, by using the selector valves fuel may be supplied from any tank to either engine.

Two hand-operated wobble pumps are used to raise the fuel pressure when starting the engines, or before the engine-driven pumps are in opera­tion. The fuel flows from the wobble bumps through lines to the strainers located in each nacelle, through the engine-driven pumps, and from there, under pressure, into the carburettors. A cross-feed line is connected on the pressure side of each engine-driven pump, and the two cross­-feed valves in this line are operated by a single control in the pilots' compartment. The cross-feed system enables both engines to receive fuel from one engine-driven pump in case either pump fails.

On later model airplanes the wobble pumps are replaced by two electric booster pumps. Each fuel strainer is located in the centre wing near each selector valve. The fuel, therefore, flows from the selector valves, through the strainers, through the booster pumps, through the engine driven fuel pumps into the carburetor. On airplanes equipped with electric booster pumps, there is no cross-feed system. The booster^- pumps will furnish ample pressure and supply for operation of the airplane in case either engine-driven pump fails.

A vapour overflow line connects from the top chamber of the carburetor to the main tanks, and a fuel line from the back of each carburetor operates the fuel pressure gauge in the pilots' compartment. This pressure gauge normally shows from 14 to 16 pounds pressure. On some airplanes a pressure warning switch is installed in they fuel pressure gauge line. When the fuel pres­sure drops below 12 pounds, the switch il­luminates a warning light on tyre instrument panel.

A restricted fitting on the fuel pressure gaugeIine connects to the oil-dilution solenoid. This unit releases fuel into the engine oil system and the propeller feathering oil, to aid in cold weather starting. Another solenoid valve in the fuel pres­sure gauge line releases fuel into the eight upper cylinders of the engines for priming.

Vent lines from each tank vent overboard, and a vapour line connects each main tank with its cor­responding auxiliary tank.

Figure 1.5: DC-3 Fuel system schematic diagram.

Small Multi-Engine Aircraft Fuel Systems

Small Multi-Engine Aircraft Fuel Systems

The diagram in figure 1.4 shows a typical fuel system for a twin-engine airplane using an RSA fuel injection system. This fuel injection system does not return fuel to the tank like the system we have just discussed.

Each wing has two fuel tanks that are connected together and sense as a single tank, and the selec­tor valves allow either engine to operate from the tanks in either wring. From the selector valve, the fuel flows to the fuel filter and then to the electric fuel pump, on to the engine-driven pump, into the fuel injection system and to the cylinders.

The instrumentation for this system consists of the fuel quantity, fuel pressure, and fuel flow gauges. The fuel quantity gauges show the total amount of fuel in the two tanks in each wing. The two fuel pressure gauges show the pressure produced by the fuel pumps. This pressure is measured at the inlet of the fuel metering unit. The fuel flow indicator is a pressure gauge that reads the pressure drop across the fuel injector nozzles and is calibrated in either gallons per hour or in pounds per hour of fuel burned

Figure 1.3: Typical fuel system for a high-performance single-engine airplane using a Teledyne-Continental fuel injection system.

Figure 1.4: Typical fuel system for a twin-engine airplane using an RSA fuel injection systems.

Small Single-Engine Aircraft Fuel Systems

Small Single-Engine Aircraft Fuel Systems

Single-engine aircraft may utilize any of several types of fuel systems, depending upon the fuel metering unit (carburetor or fuel injector) used and whether the aircraft is a high-wing or low-wing design.

a. Gravity-feed Systems

The most simple aircraft fuel system is that found on the small high-wing single-engine train­ing-type airplanes. This type of system is illustrated in figure 1.1. These systems normally use two fuel tanks, one in either wing. The two tank outlets are connected to the selector valve that can draw from either tank individually, or both tanks can feed the engine at the same time. A fourth position on the selector valve turns off all fuel to the engine. Since both tanks can feed the engine at the same time. the space above the fuel in both tanks must be interconnected, and this space vented outside of the airplane. The vent line nor­mally terminates on the underside of the wing where the possibility of fuel siphoning is mini­mized.

After the fuel leaves the selector valve, it passes through the main strainer and on to the carburetor inlet. Fuel for the primer is taken from the main strainer.

Figure 1.1: Typical gravity-feed fuel system for a small single-engine, high-wing airplane.

b. Pump-feed systems

Low wing airplanes cannot use gravity to feed the fuel to the carburetor, and these airplanes use a fuel system similar to that in figure 1.2. The selector valve used in these systems can normally select either tank individually, or shut off all flow to the engine. But they do NOT have a both position, because the pump would pull air from an empty tank rather than fuel from a full tank. After leaving the fuel selector valve, the fuel flows through the main strainer and into the electric fuel pump. You will notice that the engine-driven pump is in parallel with the electric pump. so the fuel can be moved by either pump, and there is no need for a bypass feature to allow one pump to force fuel through the other. In order to assure that both pumps are functioning. note the fuel pressure produced by the electric pump before starting the engine, and then, with the engine running. turn the electric pump off and note the pressure that is produced by the engine driven pump.

The electric pump is used to supply fuel pres­sure for starting the engine and as a backup in case the engine-driven pump should fail and to assure fuel flow when switching from one tank to the other.

Figure 1.2: Typical pump-feed fuel system for a small single-engine, low-wing airplane.

c. High-wing Airplane Using A Fuel Injection System

The fuel injection system requires an engine driven fuel pump, and the system in uses a Teledyne-Continental system that returns part of the fuel from the pump back to the 

fuel tank. This fuel contains any vapours that could block the system, and by purging all of these vapours from the pump and returning them to the tank they cannot cause any problems in the en­gine.

Fuel flows by gravity from the wing tanks through two feed lines, one at the front and me at the rear of the inboard end of each tank, into two small accumulator (reservoir) tanks, and from the bottom of these tanks to the selector valve.

The selector valve directs fuel from the desired reservoir tank to the engine, and at the same time directs the fuel vapour from the engine-driven pump back to the selected reservoir tank. This vapour then returns to the wing tank that supplies the reservoir tank.

Tine electric auxiliary fuel pump picks up the fuel at the discharge of the selector valve and forces it through the strainer and on to the inlet of the engine-driven fuel pump. From the engine driven fuel pump, the fuel flaws to the heel-air control unit where the fuel that is needed for engine operation goes to the cylinders. and all of the excess fuel returns to the inlet side of the pump. Some of the fuel that. is taken into the engine driven pump has vapour in it and this fuel is returned to the selector valve through the fuel return check valve.

AIRCRAFT FUEL SYSTEMS

 AIRCRAFT FUEL SYSTEMS

The aircraft fuel system stores fuel and delivers the proper amount of clean fuel at the right pressure to meet the demands of the engine. A well designed fuel ensures positive and reliable fuel flow through all phases of flight. This must include changes in altitude, violent manoeuvres and sudden acceleration and deceleration. Furthermore, the system must be reasonably free from tendency to vapour lock. Such indicators as fuel pressure gauges, warning signals and tank quantity gauges are provided to give continuous indications ions of how the system is functioning.

We will examine here the fuel system used in several different types of aircraft. They will range f^rom the simple to (lie complex. and represent the variety offered by today’s civilian fleet.

BASIC FUEL SYSTEM REQUIREMENTS

BASIC FUEL SYSTEM REQUIREMENTS

The requirements for the fuel system design are specified in detail in the parts of the Federal Avia­tion Regulations under which the aircraft was built. Since the vast majority of airplanes in the general aviation fleet are built under FAR Part 23, "Airworthiness Standards: Normal, Utility, and Acrobatic Category Airplanes," we will list a few of the more basic requirements for the fuel system of these airplanes.

1. No pump can draw fuel from more than one tank at a time, and provisions must be made to prevent air from being drawn into the fuel supply line. (23.951)

2. Turbine-powered aircraft must be capable of sustained operation when there is at least 0.75 cc. of free water per gallon of fuel, and the fuel is cooled to its most critical condition for icing. The system must incorporate provisions to prevent the water which precipitates out of the fuel freezing on the filters and stopping fuel flow to the engine.

3. Each fuel system of a multi-engine aircraft must be arranged in such a way that the failure of any one component (except the fuel tank) will not cause more than one engine to lose power. (23.953)

4. If multi-engine aircraft feed more than one engine from a single tank or assembly of intercon­nected tanks, each engine must have an inde­pendent tank outlet with a fuel shutoff valve at the tank. (23.953)

5. Tanks used in multi-engine fuel systems must have two vents arranged so that they are not likely to both become plugged at the same time. (23.953),

6. All filler caps must be designed so that they are not likely to be installed incorrectly or lost in-flight. (23.953)

7. The fuel systems must be designed to prevent the ignition of fuel vapours by lightning. (23.954)

8. A gravity feed system must be able to flow 150% of the takeoff fuel flow when the tank con­tains the minimum fuel allowable, and when the airplane is positioned in the attitude that is most critical for fuel flow. (23.955)

9. A pump feed fuel system must be able to flow 125% of the takeoff fuel -flow required for a reciprocating engine. (23.955)

10. If the aircraft is equipped with a selector valve that allows the engine to operate from more than one fuel tank, the system must not cause a loss of power for more than ten seconds for a single-engine or twenty seconds for a multi-engine airplane, between the times one tank is allowed to run dry and the time at which the required power is supplied by the other tank. (23.955)

11. Turbine-powered aircraft must have a fuel system that will supply 100% of the fuel required for its operation in all flight attitudes, and the flow must not be interrupted, as the fuel system auto­matically cycles through all of the tanks or fuel cells in the system. (23.955)

12. If gravity feed system has interconnected tank outlets, it should not be possible for fuel feeding from one tank to flow into another tank and cause it to overflow. (23.957)

13. The amount of unusable fuel in an aircraft must be determined and this must be made known to the pilot Unusable fuel is the amount of fuel in a tank when the first evidence of malfunction occurs. The aircraft must be in the attitude that is most adverse for fuel flow. (23.959)

14. The fuel system must be so designed that it is free from vapour lock when the fuel is at a temperature of 110 °F under the most critical operating conditions. (23.961)

15. Each fuel tank compartment must be ade­quately vented and drained so no explosive vapours or liquid can accumulate. (23.967)

16. No fuel tank can be on the engine side of the firewall, and it must be at least one-half inch away from the firewall. (23.967)

17. No fuel tank can be installed inside a per­sonnel compartment of a multi-engine aircraft. (23.967)

18. Each fuel tank must have a 2% expansion space that cannot be filled with fuel, and it must also have a drainable sump where water and ^­contaminants will normally accumulate when the aircraft is in its normal ground attitude. (23.969 and 23.9^_71)

19. Provisions must be made to prevent fuel spilled during filling the tank from entering the aircraft structure. (23.973)

20. The filler opening of an aircraft fuel tank must be marked with the word "FUEL" and, for aircraft with reciprocating engines, with the min­imum grade of fuel. For turbine-powered aircraft, the tank must be marked with the permissible fuel designation. If the filler opening is for pressure fuelling, the maximum permissible fuelling and defuelling pressure must be specified. (23.1557).

21. If more than one fuel tank has intercon­nected outlets, the airspace above the fuel must also be interconnected. (23.975)

22. If the carburetor or fuel injection system has a vapour elimination system that returns fuel to one of the tanks, the returned fuel must go to the tank that is required to be used first. (23.975)

23. All fuel tanks are required to have a strainer at the fuel tank outlet or at the booster pump. For a reciprocating engine, the strainer should have an 8 to 16 mesh element, and for turbine engines, the strainer should prevent the passage of any object that could restrict the flow or damage any of the fuel system components. (23.977)

24. For engines requiring fuel pumps, there must be one engine driven fuel pump for each engine. (23.991)

25. There must be at least one drain that will allow safe drainage of the entire fuel system when the airplane is in its normal ground attitude. (23.999)

26. If the design landing weight of the aircraft is less than that permitted for takeoff, there must be provisions in the fuel system for jettisoning fuel to bring the maximum weight down to the design landing weight. (23.1001)

27. The fuel jettisoning valve must be designed to allow personnel to close the valve during any part of the jettisoning operation.

Elements of a Human Factors Programme

  Elements of a Human Factors Programme

(a)      Figure 1.5 (adapted from ATA Specification 113: Maintenance Human Factors Program Guidelines) shows how the various elements of a human factors programme should interact:

(b)     The key elements of a human factors programme are:

·         Top level commitment to safety and human factors.

·         A company policy on human factors.

·         Human factors training (of all appropriate personnel, including managers - not just certifying staff).

·         Reporting, investigation and analysis scheme which will allow reporting of errors, actual & potential safety risks, inaccuracies and ambiguities with Maintenance Manuals, procedures or job cards (not just those which have to be reported as Mandatory Occurrence Reports or MORs).

·         A clear disciplinary policy stressing that genuine errors will not result in punishment.

·         Human factors and ergonomics audits / Line Operations Safety Audits (LOSA) (of workplaces, lighting, noise, tooling, adequacy of procedures, actual compliance with procedures, manpower, adequacy of planning, etc.).

·         The resources and willingness to act upon the findings arising from occurrence reports and audits, and to provide fixes where appropriate.

·         A mechanism for reporting problems to the Type Certificate Holder.

·         A mechanism for ensuring that internal procedures and work instructions are well designed and follow best practice.

·         A means of providing feedback to staff on problems and fixes.

·         Abolition of any ‘double standards’ concerning procedural violations.

·         A policy for management of fatigue.

·         Motivation of staff to support the initiatives.

(c)      Health and safety would normally be considered separate to human factors, although there are areas of overlap.

 

Figure 1.5