Rationale: This Aerospace Information Repor (AIR) provides guidelines for the design and installation of point fuel level sensing devices, as well as for its integration in the fuel system.
Fuel level point sensing has been used for many years on commercial and military airplanes to provide alerts and information to the pilot of a low or high level of fuel which may impact the mission or the correct operation of the aircraft or to send a signal controlling the aircraft fuel system such as:
- high level sensor preventing overshoot during ground refueling
- low level sensor advising of potential engine fuel starvation
- low level sensor interrupting fuel depletion in a tanker
- low level sensor interrupting the operation of a pump to prevent it from running dry
- point level sensors acting in parallel of the fuel measurement system for fuel transfer control to increase the integrity of the function.
To-day the sensing devices are often electrically powered. Several technologies can be used, each with its own benefits and drawbacks.
Historically, the point level sensing functions were achieved by using the Fuel Quantity Measurement System. But the aircraft increased availability and reliability required by the operators, as well as the increased levels of safety required by the regulators, have led to an implementation of point level sensing independent from the Fuel Quantity Measurement System.
Because these functions are electrically achieved, the more stringent safety requirements ( reduced energy into the tank, fault tolerance, etc) necessary to preclude ignition of the fuel vapors in the tank have led to an evolution in the technology and in the sensing concept.