Aeroengine Safety

Fig. "Wide cut fuel" (Lit. 22.2.1-2, Lit. 22.2.1-3, Lit. 22.2.1-5 and Lit. 22.2.1-6): Fuels for aeroengines are produced today by fractional destillation in a tower like facility (sketch above). Thereby the different evaporation temperatures of the crude oil components are used. A „fraction“ of about 15 % (depending from the crude oil type) of the total amount of the crude oil are kerosenes. The relatively small percentage is problematic for the security of supply. After the destillation the cerosene is subject of a chemical process (dissolver, sulphuric acid). With this should be achieved:

• Reduction of the sulfur, residues from reactions with the air („gums”) and resins.
• „Cracking“ of long chained molecules to gain a higher kerosene percentage. This takes place thermal and catalytic.
• Shorter chains can be connected to longre chains by polymerisation.
• Introduction of additives which serve to get certain properties of the fuel:
  
  • Preventing of bacterial growth, Preventing of ice formation, thermal stability, lubtrication capability, as corrosion protection (inhibitor), reducing static charging, thermal stability, oxidation stability (metallic activators).
    

The historical development of many fuels for aeroengines should guarantee suitable operation properties with secured supply. So mixtures/blends arose (e.g., with gasoline), which only partly contain kerosene. In the military field „JP fuels” have been developed. They correlate the MIL-J-5624 specification. In civil use, the „Jet-A and Jet-B“ fuels after the ASTM spezification D-1655, are known. For aeroengine fuels there are national, miilitary and company-owned labelings/identifications (table).
Typical aeroengine fuels and its properties:
JP-1 fuel is a kerosene with low freezing point. Characteristic are low vapour pressure, a good lubrication effect and a high energy content. From a higher ignition temperature an improved safety was expected. Unfortunately this fuel has olso disadvantages. Starts during cold weather have been extremely difficult because of the bad ignition, caused by low vapour pressure. In great hights the extinction of the aeroengine could occur. Then a restart was almost no more possible. As kerosene, the fuel keeps water anf solid particles floating what complicates the filtration and promotes the ice formation.
JP-3 fuel is a blend/mixture from about 70% gasoline and 30% kerosene. Its properties are similar to gasoline. With this the cold start and restart properties in great hights have been improved. Disadvantages have been evaporation losses and the formation of vapour bubbles (danger of cavitation, influencing of the control system) and bad lubrication properties.

JP-4 fuel frequently is used in the civil field (Jet B) and military field. Concerned is cerosene with additions of „naphtha” and gasoline. The low vapour pressure reduces the evaporation losses and the formation of bubbles. However it worsens the lubrication effect and can lead to increased wear. Additionally the combustion during cold and/or at great hights will be deteriorated.

JP-5 is a „heavy“ kerosene. It is mixed with gasoline. Thereby we get a fuel, similar to JP-4. The lower evaporation enables a safe storage. This is also true for the transport. The cold start properties which can be expected are in the limit range and the unsuitable restart during flight is controlled with the high energy ignition of today.

JP-6 and JP-7 Fuels are developed for military use, especially for the supersonic region. They are distinguished by a very low freezing point. This predestines them for the use in a cold environment and great height.

JP-8 is the military counterpart of Jet A-1. It contains additional additives against corrosion and ice formation. Jet A and Jet A-1 are the mostly used civil fuels. They are kerosenes. Jet A-1 has merely a 10°C lower freezing point as Jet-A. With this it is suitable for colder operating conditions.

Jet B fuel correlates JP-4.

Fig. "Terms to fuel properties" (Lit. 22.2.1-1 and Lit. 22.2.1-2): This summary should merely serve as an information. It contains for the operation of an aeroengine relevant fuel properties (Fig. "Properties of fuels 1" up to Ill. 22.2.1-7). Here a detailed definition was disclaimed and pointed at expert literature (norms, spevifications).

Fig. "Influences of fuels at quality and failures" (Lit. 22.2.1-2): On the way from the basic product/crude oil, up to the operation in the aeroengine the quality of the fuel can be influenced in many ways (Fig. "Sedimentation of a fuel").

• In the refinery (distillation and chemical processing) physical and chemical properties are adjusted (Illl. 22.2.1-2). For this additives are used (Fig. "Wide cut fuel").

Also traces of undesirable additions like sulfur, metals (e.g., copper) and nitrogen compounds can be significant influenced by the refinery.

• Between transport of the fuel to the fuel depot and/or to the refuelling of the airplane switched cleanings (filter, water separator) can remove

already introduced contaminations. From unsufficient maintained transport tanks, contaminations or corrosion products can get into the fuel. To these belong contaminations wich extremely reduce the surface tension. Concerned are unsuitable cleaning agents or auxilary materials. They can deteriorate the effectiveness of the filters. A further effect are slimy deposits. Especially the storage in tanks over a long period of time can be quite problematic. Corrosion of the tank walls, microorganisms (sulfur enrichments), unsuitable sealing media (Lit. 22.2.1-7) can get into the fuel. Before the refueling, deicer can be added. During long storage periods with access of air, a reaction of the fuel with the air leads to sticky depositions (gum).

• Also in the tanks of the airplane, carelessness can enable contaminations or reactions with not enough durable materials.
• In the aeroengine itself, the fuel can change under operation influences up to the fuel injector/nozzle. For this primarily a high operation temperature is responsible (Fig. "Problems by overheated fuel 1" and Fig. "Problems by overheated fuel 2").

Fig. "Critical fuel properties" (Lit. 22.2.1-6, Lit. 22.2.1-7 and Lit. 22.2.1-8): This chart shows problematic respectively deteriorating effects of the fuel properties (Fig. "Properties of fuels 1" up to Fig. "Properties of fuels 3") at an aeroengine. Connections and effects are discussed more in detail at an other site. A summary of the cross references contains Ill. 22.2-1.

Fig. "Properties of fuels 1", Fig. "Properties of fuels 2", Ill. 22.2.1-7 (Lit. 22.2.1-1, Lit. 22.2.1-2 and Lit. 22.2.1-6): In these pictures properties of the fuels (Fig. "Terms to fuel properties") with the corresponding terms are explained more in detail.

Fig. "Safety relevant fuel ignition temperatures" (Lit. 22.2.1-4 and Lit. 22.2.1-7): The ignition temperature (flash point) is not only important for the start of the aeroengine and the combustion. It is also a safety relevant value. Danger of ignition respectively explosion exists, if the tank is not filled. The vapour above the fuel forms with air a good ignitable mixture with a flash point of 35-40 °C (Fig. "Terms to fuel properties" and Fig. "Properties of fuels 3"). These temperatures can already occur in the wing tanks due to air friction during supersonic flight (volume 2, Ill. 9.2-3 and Ill. 9.2-4). Are due to availability reasons so called „wide-cut” fuels used, the flash point can be markedly lower. From the view of safety a high flash point should be persued (volume 2, chapter 9.3). A dangerous ignition can occur in a row of events:

• Flight accidents.
• Fuel leaks, with ignition at hot surfaces.
• Explosios and fires in tanks by lightning (volume 1, Ill. 5.1.3-2).
• Electric sparks during short-circuits.This issue gets actuality with the introduction of light evaporating fuels.
  

To minimise the ignitabilty/flamability, fuels like JP-8 (AVTUR, F-34, Fig. "Wide cut fuel") have less fugitive components and with this a relatively high ignition temperature/flash point of about 40°C. An ignition of liquid fuel respectively of a very rich or meager fuel/air mixture, demands higher temperatures (>800°C). They can be expected at overheated casing walls and during electric sparks. In the diagram it can be seen, that during fast heating, the igniton point/ignition temperature rises. This may be connected with a delayed forming of vapour/evaporation.