Preparation for Entry Into the Fuel Tank
It is most important to ensure that all persons who are engaged in the process of Fuel Tank Entry are fully trained on all aspects of the following information to ensure that all precautions are taken and all risks minimised.
Steps that must be taken include the following:
a) Ensure the aircraft is electrically grounded
b) Ensure that Fire Extinguishers are available (typically CO2 would be used for a Fuel Fire
c) Deactivate all Electrical Systems on the Aircraft and suitable placard
d) Defuel the aircraft using the Aircraft Maintenance Manual Procedures
e) Deliver a safe atmosphere for maintenance personnel by ensuring the following:
i/ Ensure adequate ventilation.
ii/ Follow recommended ventilation techniques.
iii/ Properly monitor the air in fuel tanks.
The Importance of Ventilation
Ventilation is the single most important method of controlling the fire, explosion, and toxic hazards associated with working in an open fuel tank.
The physical structure of airplane fuel tanks presents some unavoidable challenges in ensuring adequate ventilation.
Some of the challenges include dead air spaces and small openings between tank sections that inhibit airflow, ventilation equipment that may not be selected or set up properly, or suspension of ventilation before the entry work is completed. Planning and execution are critical to providing adequate ventilation.
Ensuring that sufficient clean fresh air is present in the fuel tank, the safer the environment will be for maintenance personnel.
Continuously pushing fresh air into a fuel tank to ensure positive airflow helps to prevent a fuel vapor concentration from reaching its LFL, thus preventing fires and explosions.
The terms used about the flammability are known as the lower flammability limit (LFL) or lower explosive limit (LEL). (Usually expressed as a percentage by volume.)
Fresh air also dilutes the vapor concentration of toxic chemicals, reducing the risk of hazardous exposures.
Issues with Oxygen Depletion
High volumes of fresh air will prevent a condition known as oxygen deficiency.
The normal concentration of atmospheric oxygen in the air is about 21 percent. At oxygen-deficient levels (19.5 percent and below) a person will begin to exhibit signs of oxygen starvation, including headache, nausea, drowsiness, and slurred speech.
At increasingly lower oxygen concentrations, more severe reactions occur. Ultimately, death by asphyxiation is possible.
Oxygen deficiency is often caused by physically displacing the oxygen in a space. For example, pumping nitrogen into a fuel tank to prevent fire ignition will cause the oxygen concentration to decrease.
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