DESC:1. Title of invention
Aircraft fuel tank gravity filler cap integrated with pressure status indication.
2. Field of invention
The present invention relates to aircraft mechanical system in general and more particularly to the aircraft fuel system.
3. Background of invention
To transfer fuel in an aircraft from transfer tanks to supply tank (engine feed tank), pressurization of fuel at transfer tank is one of the reliable and safe method. During pressurization of fuel tank, conditioned air from aircraft’s Environmental Control system (ECS) is provided at top of the free surface of fuel in tank, at estimated pressure (e.g. 5psig). The supply tank is generally kept at atmospheric pressure, or pressure less than transfer tank pressure. This pressure difference pushes the fuel from transfer tank to supply tank.
The pressure built-up inside transfer tank during pressurization, act on all the surfaces of the tank including gravity fuel filler cap. Transfer tanks hold this pressure, till the pressurization system is depressurized. Depressurization of the system is performed once the aircraft is landed and parked.
It has been observed, that many times, ground operators in hurriedness of preparing aircraft for next sortie and to refuel the aircraft, try to open gravity fuel filler cap before checking the depressurization status. This action, causes serious safety risk to the operator, and may cause severe injuries to the operator, as the filler cap once loosen can hit the operator due to pressure inside the tank. This possibility of incident calls for an easy and simple indication of fuel tank pressurization status, which does not require any electrical power and can be easily read by the ground crew while preparing aircraft.
Some of the patents published on similar invention: -
1. US5183173A, “Auto venting fuel cap”
A fuel cap for closing a threaded filler neck of a fuel tank. When the cap is installed on a filler neck, the driver is in an upper position and the crown freely rotates relative to the threaded housing in a cap-removal direction. When the driver is depressed, a pressure relief valve is opened and a driving relationship is created between the crown and the threaded housing in a cap-removal direction, permitting the cap to be removed from the filler neck. When the crown is rotated in a cap installation direction, the driver is moved to the upper position and the pressure relief valve is closed.
2. CN203111515U, “Lightning preventing gravity filler cap device”
The utility model provides a lightning preventing gravity filler cap device During oiling, a product can be opened by rotating the handle of the top part of the gravity filler cap, and then the filler cap seat is connected with ground oiling equipment to fill fuel oil to the fuel tank of an airplane.
3. GB2435032A, “Fuel filler cap with warning to prevent incorrect fuelling”
An Audio Command Diesel Fuel Filler Cap to prevent incorrect fuelling comprising of a printed circuit board, speaker and battery.
The cap once turned, gives an audio reminder or alert that the user should fill with diesel fuel. The fuel cap could be programmed to say which fuel should be used, and could be programmed in any language.
Above mentioned arts, discussed about various ways of improving applications of gravity filler cap; but none of them deliberated on identifying, weather fuel tank is pressurized or not, through an integrated pressure sensing mechanism, which is the scope of current work.
4. Summary of the present invention
The primary objective of the present invention is to provide an easy and reliable indication of fuel tank pressurization status, directly on gravity fuel filler cap to assist ground operators in knowing the status of tank quickly, and to help them in taking appropriate actions.
In the present invention of gravity fuel filler cap, there is a small transparent window covered with acrylic glass (sight glass). The colour inside is changing depending upon the status of pressurization inside the tank. “GREEN” colour or “SAFE” state, for indication of a depressurized tank, i.e. tank is safe to open. SAFE state is the default state of indication on the fuel filler cap.
“RED” colour or “UNSAFE” state, indicates a pressurized tank, i.e. tank is holding pressure and unsafe to open. Above a pre-decided positive pressure value (e.g. 3 psig typically), the mechanism inside the filler cap changes the colour from GREEN to RED. The operating pressure for the mechanism inside gravity filler cap is dependent on the rated pressure inside the tank.
The present invention finds applicability in aircraft fuel system and aircraft maintenance.
5. Brief description of drawings
FIG-1 illustrates a typical gravity fuel filler cap, main features are machined body (1) of filler cap, lever (2) for opening and locking of cap, and standard markings (3).
FIG-2 shows the position of proposed indication (4) for fuel tank pressurization status.
FIG-3(a) describes the mechanism integrated inside gravity filler cap, for sensing of fuel tank pressure in default mode (depressurized tank).
FIG-3(b) illustrates the pressure sensing mechanism, under the influence of pressure inside the fuel tank.
6. Detailed description of invention
Gravity refueling is filling the aircraft with required volume of fuel under the influence of gravity alone. To gravity refuel an aircraft, multiple openings are provided all over the aircraft structure wherever fuel tanks are located. These openings are required to be sealed before aircraft’s operation, this is achieved by means of Gravity fuel filler caps. Gravity fuel filler caps are provided on all the aircrafts, even if they use pressure refueling primarily, in these cases gravity filler caps are used for emergency operations, in event of unavailability of pressure refueling system.
A typical gravity filler cap is shown in FIG-1. It consists of a machined body (1), with one lever to open and close the locking mechanism (2). A plurality of markings (3) are provided on the surface of gravity filler cap as per standards for positions and orientation.
To transfer fuel from transfer tanks to supply tank (engine feed tank), pressurization of fuel at transfer tank is one of the reliable and safe method. During pressurization of fuel tank, conditioned air from aircraft’s Environmental Control system (ECS) is provided at top of the free surface of fuel in tank, at estimated pressure (e.g. 5psig). The supply tank is generally kept at atmospheric pressure, or pressure less than transfer tank pressure. This pressure difference pushes the fuel from transfer tank to supply tank.
The pressure built-up inside transfer tank during pressurization, act on all the surfaces of the tank including gravity fuel filler cap. Transfer tanks hold this pressure, till the pressurization system is depressurized. Depressurization of the system is performed once the aircraft is landed and parked.
It has been observed, that many times, ground operators in hurriedness of preparing aircraft for next sortie and to refuel the aircraft, try to open gravity fuel filler cap before checking the depressurization status. This action, causes serious safety risk to the operator, and may cause severe injuries to the operator, as the filler cap once loosen can hit the operator due to pressure inside the tank. This possibility of incident calls for an easy and simple indication of tank pressurization, directly on fuel filler cap to assist ground operators in knowing the status of tank quickly, and to help them in taking appropriate actions.
This present invention provides a simple integrated indication for fuel tank pressurization status.
Refer FIG-2, In the present invention, there is a small transparent indication window covered with acrylic glass (sight glass) (4). The colour inside changes depending upon the status of pressurization inside the tank. “GREEN” colour or “SAFE” state, for indication of a depressurized tank, i.e. tank is safe to open, refer FIG-2 (a). SAFE state is the default state of indication on proposed fuel filler cap.
“RED” colour or “UNSAFE” state, indicates a pressurized tank, i.e. tank is holding pressure and unsafe to open, refer FIG-2 (b). Above a decided positive pressure value (e.g. 3 psig typically), the mechanism inside the filler cap changes the colour from GREEN to RED. The operating pressure for the mechanism inside gravity filler cap is dependent on the rated pressure inside the tank.
The mechanism to actuate the colour from GREEN to RED in influence of pressure depends upon stiffness of a calibrated spring. The stiffness of spring needs to be selected, based on rated pressure inside the tank.
The required stiffness of spring is estimated by following equations, where force due to pressure and force to overcome stiffness of spring is equated.

Fp=P*A ..........(a)

Here, Fp = Force due to pressure, P = Pressure inside the tank, A = Area of the seat in mechanism.
Similarly, for spring, we can write,
Fs=-k*x ........(b)
Here, Fs = Spring force, k = spring constant, x = displacement of spring in mechanism, this is dependent on the space availability, and estimated geometrically based on size constraints.
Equating both the forces from equation (a) and (b),
P*A=k*x ......(c)
Or
k= -(P*A)/x......(d)
Required spring constant is estimated using equation (d) to select appropriate spring for the mechanism.
This is theoretical estimation, and additional factors (friction, temperature etc.) are considered while designing of mechanism and selecting the spring material.
FIG-3 shows the preferred embodiment (of present invention) of internal mechanism inside the gravity filler cap. A guided wedge (8) is located with one end attached to the selected spring (10) (using equation (d), other end of the spring (10) is attached to a fixed stopper (11). The other end of the wedge (8) is kept in contact with a pressure sensing plunger (9) exposed to tank pressure. Top of the guided wedge is coloured “GREEN” (6) and “RED” (7) for indication. “GREEN” colour or “SAFE” state, for indication of a depressurized tank, i.e. tank is safe to open. SAFE state is the default state of indication on the fuel filler cap. “RED” colour or “UNSAFE” state, indicates a pressurized tank, i.e. tank is holding pressure and unsafe to open. Above a pre-decided positive pressure value (e.g. 3 psig typically), the mechanism inside the filler cap changes the colour from GREEN to RED.
FIG-3 (a) is showing the default position, which is “SAFE” state, green colour visible from the sight glass (5)indicates that it is safe to open the gravity filler cap.
In FIG-3 (b), the position of mechanism is shown under the influence of tank pressure, which is “UNSAFE” state. Red colour visible from the sight glass (5) depicts that it is unsafe to open the gravity filler cap and tank is in pressurized state.
The present invention embodied to gravity filler cap provides a safe and simple indication of pressurization status for an aircraft’s fuel tank, improving safety of crew during aircraft’s ground operation and maintenance.
,CLAIMS:We claim:
1. An aircraft fuel tank gravity filler cap integrated with pressure status indication used on aircraft fuel tank, comprising:
- a filler cap body (1);
- a lever locking mechanism (2);
- a plurality of markings (3);
- a small transparent indication window covered with acrylic glass (sight glass) (5);
- a guided wedge (8) inside of filler cap body (1);
- a spring (10) inside fuel filler cap body (1) with its one end attached to the guided wedge (8) and another end attached to a fixed stopper (11);
- a pressure sensing plunger (9) kept in contact with the guided wedge (8) at one end and another end is exposed to tank pressure; and
- “GREEN” (6) and “RED” (7) coloured indications on top of the guided wedge (8) are provided.
2. The aircraft fuel tank gravity filler cap is integrated with pressure status indication, as claimed in claim 1, wherein, the colour indication on the sight glass (5) will change depending upon the status of pressurization inside the tank, with “GREEN” (6) colour for indication of a depressurized tank, and “RED” (7) for indication for pressurised tank.
3. The aircraft fuel tank gravity filler cap integrated with pressure status indication, as claimed in claim 1, wherein, it provides a safe and simple colour coded indication of pressurization status for an aircraft’s fuel tank by virtue of actuation of an integrated mechanism; and thereby improving safety of crew during aircraft’s ground operation and maintenance.