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

INTEGRATED APPROACH TO HUMAN FACTORS AND SAFETY

INTEGRATED APPROACH TO HUMAN FACTORS AND SAFETY

 Integrated Approach

(a)      Human factors initiatives will be more effective if they are integrated within existing

company processes, and not treated as something additional or separate or short-term. Human factors initiatives have sometimes failed in the past because they have been marginalized and regarded as a temporary ‘fashion’. Much of human factors, in the context of maintenance organizations and JAR145 requirements, are common sense, professionalism, quality management, safety management – i.e. what organizations should already have been doing all along.

(b)     The “human factors” initiatives in the context of JAR145 are really “safety and airworthiness” initiatives, the aim being to ensure that maintenance is conducted in a way that ensures that aircraft are released to service in a safe condition. The organization should have a safety management system in place, many of the elements of which will need to take into account human factors in order to be effective.

(c)      Ideally, human factors best practice should be seamlessly and invisibly integrated within existing company processes, such as training, quality management, occurrence reporting and investigation, etc. Sometimes it is a good idea to re-invent an initiative under a new name if it has failed in the past, but we should be cautious about unnecessarily duplicating functions which may already exist (e.g. occurrence reporting schemes / quality discrepancy reporting/ etc.). It may only be necessary to slightly modify existing processes to meet the JAR 145 human factors requirements.

(d)      Human factors training is probably an exception to the advice given above, in that it is usually so new and different to any existing training that it warrants being treated as a separate entity, at least for initial training. Recurrent training, however, is probably better integrated within existing recurrent training. 

NEED TO ACCOUNT FOR HUMAN FACTORS PROBLEM IN WORKPLACE

 NEED TO ACCOUNT FOR HUMAN FACTORS PROBLEM IN WORKPLACE

(a)      Humans have performance limitations - therefore they make errors. Principles of HF establishes that policy of zero error tolerance is unlikely to be an effective safeguard against errors. Therefore, it is needed that effective program must be there to account for human factors problem in workplace – to established a policy of error management to create a safety net to trap human errors, to establish such  practices and procedures that human mind naturally follows and thus improve quality and safety. It is the human factors program that caters for this need.

(b)     An Effective Human Factors Programs train staff and put systems in place to pick up those errors – therefore, those errors don't result in delays, incidents or accidents.

(c)      Fewer errors by engineers mean reduced delays, incidents, and accidents. Therefore, the company is safer and more cost efficient.

(d)      A safer, more cost efficient, company means:

-    Fewer delays

-    Fewer injuries

-    Better company performance

-    And, therefore, better job security for its workers.

(e)      The study of human factors model establishes that humans are in the center of the working environment interacting with other elements. To have proper matching with peripheral components, proper practices and procedures are essentially needed, otherwise humans will not be able to perform in best way, rather there will be higher rates of error resulting in accidents and incidents, endangering aviation safety.

(f)      History of aviation accidents and incidents established already that it is the human factors chapter that needs to be studied, improved and practiced in aviation industry because accidents/incidents still happen in almost the same trend of statistics as those happened previously in spite of huge technological improvement in aviation. It implies that accidents don’t happen merely for technical failure, but those do happen mostly due to the failure of the human mind and management in working environment.

(g)      In 1940, it was calculated that approximately 70% of all aircraft accidents were the results of human errors.

(h)      International Air Transport Association (IATA) reviewed the situation 35 years later,  they found that there had been no reduction in the human error component of accident statistics. (Figure 1.4). 

Figure 1.4: The dominant role played by human performance in civil aircraft accidents Source: IATA, 1975

(i)       A study was carried out in 1986, in the by Sears, looking at significant accident causes in 93 aircraft accidents. These were as follows:

Causes/major contributory factors                                          %

of accidents in which this was a contributing factor

•        Pilot deviated from basic operational procedures            33

•        Inadequate cross-check by second crew member           26

•        Design faults                                                            13

•        Maintenance and inspection deficiencies                        12

•        Absence of approach guidance                                    10

•        Captain ignored crew inputs                                        10

•        Air traffic control failures or errors                                9

•        Improper crew response during abnormal conditions       9

•        Insufficient or incorrect weather information                  8

•        Runways hazards                                                      7

•        Air traffic control/crew communication deficiencies                    6

•        Improper decision to land                                          6

(j)      As can be seen from the list, maintenance and inspection deficiencies are one of the major contributory factors to accidents.

(k)      The UK CAA carried out a similar exercise in 1998 looking at causes of 621 global fatal accidents between 1980 and 1996. Again, the HF error in “maintenance or repair” area was found among the top 10 primary causal factors to accidents/incidents.

(l)       It is clear from such studies that human factors problems in aircraft maintenance engineering are a significant issue to be taken into serious consideration.

The SCHELL Model

The SCHELL Model

(a)      In 1994, Professor Graham Hunt from Massey University in New Zealand proposed that all of these interactions take place within a cultural context. While many commentators see this aspect as part of the environment, Professor Hunt believes that the organisational, national and ethnic backgrounds of individuals play a profound part in the interactions of the SHELL model and are currently not addressed satisfactorily in most cases. He has proposed the   SCHELL (pronounced skell) model. This is still a debatable point. (Figure 1.3)

Figure 1.3: SCHELL Model

The SHELL Model

The SHELL Model

(a)   In 1984, Frank Hawkins proposed that the interactions between people are also a significantarena for error generation. He proposed the addition of another Liveware to the model to take into account the interactions between people forming the interface Liveware-Liveware. This made the SHELL Model. (Figure 1.2)

Figure 1.2: SHELL Model

(b)     Liveware–Liveware: This is the interface between people.

In aviation, maintenance and aircrew training and proficiency testing have traditionally been conducted on individual basis. If each individual engineer/air crewmember is proficient, then it is normally assumed that the team of maintenance/operation team comprising those individuals is also be effective. But this is not always the case.  Interactions among team members play an important role in determining team performance.

In this L-L interface, Human Factors concentrates on errors caused by miscommunication between individuals, poor teamwork in small group situations, ineffective leadership by supervisors and managers that generate errors on the workshop floor. Staff management relationships are also within the scope of this interface.

HUMAN FACTORS MODELS

(a)              Fundamentals of human factors are better understood by different models postulated by experts.

(b)              In this section, some models of human factors will be highlighted as a beginning of the basic elements of the subject.

 The SHEL Model

(a)      Perhaps the most common way of expressing complex systems is to use simple models to illustrate the ideas. In aviation, Elwyn Edwards (1972) proposed the SHEL model to identify the components and interactions within our complex industry (see Figure 1.1).

 

Figure 1.1: Shel Model

(b)     The acronym identifies the components with the following meanings:

Software: manuals, rules, procedures, spoken words, etc., which are part and parcel of standard operating procedures in an organization;

Hardware: aircraft, machinery, tools, control and display systems;

Environment: physical, social and economic climate in which the organization and individuals operate; and

Liveware: the human beings i.e. engineers, flight crew, cabin crew, ground crew, management and administration people - in the system.

(a)              Interactions between components are represented in the model by interfaces. There are 3 interfaces in SHEL model: Liveware-software, liveware-hardware, and liveware-environment (see Figure 1.1), Human factors concentrates on theses interfaces and - from a safety viewpoint, on the elements that can be deficient, e.g.:

Deficiency in S: likelihood of misinterpretation of procedures, badly written manuals, poorly designed checklists, untested or difficult-to-use computer software etc.

Deficiency in H: not enough tools, inappropriate equipment, poor aircraft design for maintainability etc.

Deficiency in E: Uncomfortable workplace, inadequate hangar space, extreme temperatures, excessive noise, poor lighting etc.

Deficiency in L (the central component): Shortage of manpower, lack of knowledge or skill, lack of supervision, lack of support from managers, general nature of human fallibility and so on.

Note: Practical deficiencies may be identified/listed by participants that they experience in their own working environment and their impact on performance may be highlighted for realization.

(a)              Notably, Liveware is at the hub of the SHEL model. Liveware has to interact with other elements in the model forming the interfaces: Liveware-Hardware, Liveware-Software, Liveware-Environment. Suitable design and matching of the interfacings is very important to have optimum level in the performance output in the working system. 

Although modern aircraft are now designed to embody the latest self-test and diagnostic routines that modern computing power can provide, one aspect of aviation maintenance has not changed: maintenance tasks are still being done by human beings. However, man has limitations. Re-designing of aircraft with modern manufacturing techniques are making the aircraft more and more reliable but it is not possible to re-design the human being: we have to accept the fact that the human being (liveware) is intrinsically unreliable due to its natural tendency of fallibility. However, we can guard against this unreliability of human by careful design and suitable matching in the interfaces to assist his performance and respect his limitations. If these two aspects are ignored, the human - in this case the maintenance engineer - will not perform to the best of his abilities, may make errors, and may jeopardize safety.

          Liveware-Hardware: This interface is most commonly known as the human-machine systems e.g. the design of seats to fit the sitting characteristics of the human body; design of displays to match the sensory and information-processing characteristics of the user; design of controls with proper movement, force and location.

Deficiency in this interface (L-H deficiency) is to be removed to reduce the error rate. Controls that require extreme physical strength, displays that are easy to misread and hours of boring monitoring contribute to high error rates in this interface.

User-friendly controls and displays and a better understanding of the relative strengths of humans and machines provide a platform for reducing error rates considerably.

Human Factors ergonomically deals with issues arising from this interface.

Liveware-Software: This encompasses the interface between humans and the non-physical aspects of the system such as procedures, manual and checklist layout, symbology, and computer programs. The problems may be less tangible than those involving the L-H interface and consequently more difficult to detect and resolve (e.g. misinterpretation/misunderstanding of technical literature or symbology). Many of the manuals engineers were expected to use were not user friendly, policies were interpreted differently by different supervisors and rules were often ignored because they conflicted with common sense.

Liveware-Environment In the maintenance of aircraft, environment ranges from physical environment (e.g. noise, light, temperature, humidity etc) of working place to broad managerial, political, and economic constraints of the organization. These aspects of the environment interact with the human via this interface. Although modifications to some of these factors may fall beyond the function of Human Factors practitioners, they should be considered and addressed by those in management with the ability to do so.

MEANING OF HUMAN FACTORS

What is Human Factors?

(a)      Human Factors as a term and as a subject has to be clearly defined. But no single definition seems to meet all of the needs.

(b)     As a term, it refers to any factor related to humans when a human being is in a working environment.

(c)      Human element is treated as a central component of any working system. It is the most flexible, adaptable and valuable part;but it is also prone to various influences giving adverse affect to its performance. To understand this nature and reveal predictable capabilities and limitations of humans and to apply this understanding in real-life working situations, there has been research and investigations on Human Factors and it has evolved as a complete subject of study. The subject has been progressively developed, refined and institutionalized since the end of the last century and today, the subject has been backed by a vast store of knowledge which can be applied to enhance the safety of the complex system as well as increase the efficiency of the workman.

HUMAN FACTORS

The subject “Human Factors and Error Management” is most commonly termed as “Human Factors”.

(b)     The principles behind this subject area reflect the fact that as humans we all make errors. If we can accept that we all make errors, then a policy of zero error tolerance is unlikely to be an effective safeguard against errors. Therefore, a policy of error management is much more likely to result in safe operations.

(c)      This subject tells us to recognize various factors, commonly called “Human Factors” that affect us as humans, both positively and negatively, affecting our work performance and imposing limitations on our capabilities.

(d)       Research and study revealed today much about human capabilities and limitations. An understanding of these predictable human capabilities and limitations and the application of this understanding in working environment are the primary objectives of Human Factors. The result will be twin: safety and efficiency, although the major focus is on SAFETY; safety has always been the primary rationale for HF programs.

(e)      The elements of Human Factors are not new, although the subject as a matter of study is new; in most cases, human factors are application of “common sense” in working situation. The spirit of applying these common senses aim at utilization of all available resources - equipment, procedures and people - to promote safety and enhance the efficiency of flight operations as well as promote/improve quality of performance. In cockpit environment, this is called Crew Resource Management (CRM), while in maintenance, this is Maintenance Resource Management (MRM).

(d)      The CRM or MRM is concerned not so much with the technical knowledge and skills required to fly/operate an aircraft but rather with the cognitive and interpersonal skills needed to manage the flight/maintenance operations within an organized aviation system.