Process for the replacement of a hydraulic fluid contained in a control circuit such as an aircraft circuit

The hydraulic fluid contained in a control circuit such as an aircraft circuit is replaced by expelling the spent fluid with new fluid. For this purpose the tank (10) is disconnected from the circuit after depressurizing and emptying it. The suction branch (18) is then connected to a new fluid supply (42) to return branch (34) a casing of a high pressure filter (30) and a reservoir (48) for recovering the spent fluid. The fluid is transferred sectionwise by operating pumps (22, 4) and by manipulating the control devices (28a, 28b). A check on the transferred fluid volume makes it possible to ensure that the replacement has taken place. Following a disconnection of the supply device (42) from the reservoir (48), the tank (10) is connected to the control circuit, filled and repressurized.

BACKGROUND OF THE INVENTION 
1. Field of the Invention P The invention relates to a process for making 
it possible to replace the hydraulic fluid contained in a possibly complex 
control circuit such as a hydraulic control circuit of an aircraft. 
2. Description of the Prior Art 
Although the process according to the invention is particularly suitable 
for the replacement of the hydraulic fluid contained in the hydraulic 
control circuits of aircraft,, it can also be used in numerous other 
technical fields without passing outside the scope of the invention. 
A hydraulic control circuit generally includes a tank in which the 
hydraulic fluid is stored under pressure. The hydraulic fluid contained in 
the tank is sampled or tapped by one or more pumps through a suction 
branch and is then delivered under pressure to each of the control means 
of the circuit by a delivery branch, before being returned to the tank by 
a low pressure return branch. 
In complex hydraulic control circuits such as those equipping aircraft, one 
or more high pressure filters are placed in the delivery branch. Moreover, 
a general filter is placed in the return branch immediately upstream of 
the tank. 
The presence of one or more filters in the hydraulic control circuits makes 
it possible to filter any particles present in the hydraulic fluid 
Consequently the pollution of said fluid by solid particles is maintained 
within acceptable limits. Moreover, in the particular case of circuits 
equipping aircraft, a ground connection system makes it possible to 
connect the circuits to an external filtering installation designed for 
eliminating this type of pollution. 
However, these filtration means have no effect in the case of a chemical 
pollution of the hydraulic fluid contained in the circuit. Such a 
pollution can be caused by the introduction into the circuit of an 
incompatible fluid, acid formation or overheating leading to the 
deterioration of the characteristics of the fluid. 
When such a chemical pollution occurs, only a complete change of the 
hydraulic fluid contained in the circuit restores the quality of said 
fluid. It may also prove necessary to replace the fluid present in the 
circuit in order to change the fluid type used. 
In both cases, the replacement of the hydraulic fluid contained in the 
circuit can at present not take place without having recourse to at least 
one of two existing processes. 
A first of these processes comprises of emptying the tank, filling it with 
the new fluid, operating the control means and repeating this cycle 
several times. It is clear that this process only permits a partial 
replacement of the hydraulic fluid. Moreover, a satisfactory replacement 
of the fluid can only be obtained by multiplying the number of cycles. 
Another known process for emptying the hydraulic fluid contained in the 
control circuit comprises of emptying the tank and disconnecting the pipes 
forming the circuit at several locations, so as to bring about a 
sectionwise elimination of the spent hydraulic fluid. However, this method 
leads to the introduction of air and particle-type pollution into the 
circuit It can also lead to leaks or to the forgetting to reconnect all 
the pipes. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a simplified process 
making it possible to replace the hydraulic fluid contained in a control 
circuit without suffering from the disadvantages of existing processes, 
i.e. it permits the replacement of substantially all the fluid without 
dismantling the circuit and without risking the introduction into it of 
air or particle pollution and without risking the formation of leaks or 
giving rise to a possibility of forgetting reconnect the sections. 
This result is obtained by a process for the replacement of a hydraulic 
fluid contained in a control circuit including a tank, at least one 
suction branch connecting the tank to at least one pump, at least one 
delivery branch connecting the pump to at least one control means, and at 
least one return branch connecting the control means to the tank, said 
process comprising the following steps: 
depressurizing, emptying and disconnecting of the tank, 
connection of a new fluid supply means and a spent fluid recovery reservoir 
respectively on the suction branch and on the return branch, substantially 
in place of the tank, 
replacing of the hydraulic fluid by operating the pump, so as to force the 
spent fluid into the recovery reservoir with new fluid being drawn from 
the supply means, 
disconnecting of the supply means and the recovery reservoir, 
connecting the tank to the suction branch and to the return branch and then 
filling and pressurizing the tank. 
A feature of this process, is that only the connectors connecting the tank 
to the remainder of the circuit are disconnected. The fluid is replaced by 
operating the pump or pumps. This operation has the effect of forcing back 
the spent fluid by the new fluid successively into the different sections 
of the circuit. Thus, the spent fluid is replaced without the new fluid 
mixing with the spent fluid and without the air or particles entering the 
circuit. 
Another feature of the invention is that the control circuit includes at 
least one high pressure filter placed between the delivery branch and at 
least one supply branch of the control means, the disconnecting step of 
the tank is followed by connecting the recovery reservoir to the trough of 
the high pressure filter and to the outlet of the general return filter 
and the replacement step for the hydraulic fluid includes a first step of 
replacing the fluid contained in the suction and delivery branches and a 
second step of replacing the fluid contained in the supply and return 
branches. 
The first step of replacing the fluid is advantageously performed by 
operating the pump, whereas the second fluid replacement step is performed 
by an operation of the pump accompanied by at least one manipulation of 
the control means. The control means is generally manipulated several 
times after recovery of a given spent fluid quantity. 
In practice, the connection step of the recovery reservoir is preceded by a 
step of dismantling a filter feed trough containing a filtering element 
and a step of fitting a connection tool container to the casing of the 
high pressure filter. This connection tool container makes it possible to 
connect the recovery reservoir to the casing of the high pressure filter 
and maintain it open if there is an isolating valve for the high pressure 
filter, which is normally closed when the filter feed trough containing 
the filtering element is removed. 
According to a feature of a preferred embodiment of the invention, the step 
of connecting the supply means and the recovery reservoir are performed by 
using flexible tubes. 
During the step of replacing the hydraulic fluid, it is advantageous to 
check the displaced fluid volume, so as to stop the replacement of the 
fluid when said volume reaches a predetermined threshold. 
During the first step of replacing the fluid, it is possible to stop the 
fluid replacement by closing a tap valve placed in the flexible tube 
ensuring the connection of the recovery reservoir to the high pressure 
filter casing. 
Another feature of the present invention is that when the control circuit 
also has a general filter placed in the return branch, the recovery 
reservoir is connected downstream of said filter and a filtering element 
contained in said general filter is replaced after disconnecting the 
recovery reservoir.

DETAILED DESCRIPTION 
FIG. 1 shows in simplified manner one of the hydraulic control circuits 
equipping an aircraft. This circuit is used for hydraulically controlling 
certain of the members of the aircraft, generally in combination with one 
or more other hydraulic control circuits with an identical design. Among 
the hydraulic controls which can be performed by the circuit of FIG. 1 
servocontrols for engine thrust reverses, the servocontrols serving the 
right and left wings, the servocontrols serving the rear part of the 
aircraft, the controls for the flaps and wing slats, the braking circuits, 
the front wheel orientation circuits, the hatch and landing gear 
manipulating circuits, the cargo door opening and closing circuits, etc. 
The hydraulic control circuit illustrated in FIG. 1 has a tank 10 which can 
be pressurized and which normally contains a certain hydraulic fluid 
volume. The tank 10 has an entrance pipe 12 and an exit pipe 14. The tank 
exit pipe 14 is connected by a tight connector 16 to a suction branch 18 
of the circuit. In the vicinity of the tight connector 16, the suction 
branch 18 has a bypass 19 terminated by a ground connection 20, which is 
accessible from the outside of the aircraft. 
Downstream of the bypass 19, the suction branch 18 is subdivided into 
several sections, each of which is connected to a suction port of a pump. 
In the embodiment illustrated in FIG. 1, a first section 18a of the 
suction branch 18 is connected to a suction port of a mechanical pump 22 
and a second section 18b of the suction branch 18 is connected to the 
suction port of an electric pump 24. 
The hydraulic control circuit illustrated in FIG. 1 also has a deliver 
branch 26 forming the high pressure part of the circuit. This delivery 
branch 26 connects the delivery port of each of the pumps 22, 24 to one or 
more control means such as 28a, 28b, etc. serving to actuate or manipulate 
the aircraft member or members controlled by this circuit. 
In the embodiment illustrated in exemplified manner in FIG. 1, the delivery 
branch 26 has a common section connected upstream by two sections 26a, 26b 
to the delivery ports of the pumps 22, 24 respectively. The common section 
of the delivery branch 26 is connected downstream to a high pressure 
filter 30, which incorporates an interchangeable filtering element 32. 
It should be noted that when the circuit has several mechanical pumps 
identical to the pump 22, each of them is connected to a high pressure 
filter 30 by a separate delivery branch. An interchangeable filtering 
element 32 is then associated with each of the branches. 
Downstream of the high pressure filter 30, the circuit has the same number 
of supply branches 27a, 27b, etc. as the circuit has control means 28a, 
28b, etc. Downstream of these control means, the branches 27a, 27b, etc. 
are connected to a low pressure manifold 36. 
The hydraulic control circuit illustrated in FIG. 1 also has a return 
branch 34 by which the low pressure manifold 36 is connected to the 
entrance pipe 12 of the tank 10. The return branch 34 is connected to the 
entrance pipe 12 of the tank 10 by a tight connector 38. A general filter 
40 having a detachable filtering element is placed in the return branch 34 
in the vicinity of the tight connector 38. 
Finally, the circuit illustrated in FIG. 1 also has draining branches 41a, 
41b, which drain the lubricating fluid from the housing of the pumps. 
These branches 41a, 41b respectively connect a drainage port formed in the 
housing of each of the pumps 22, 24 to the low pressure manifold 36. In a 
hydraulic circuit like that described hereinbefore with reference to FIG. 
1, the manipulation of any random one of the members controlled by the 
control means 28a, 28b, etc. is ensured after at least one of the pumps 
22, 24, etc. has been put into operation. 
When it is necessary to replace the hydraulic fluid contained in such a 
circuit, either for changing the fluid type, or to take account of a 
chemical pollution of the fluid present in the circuit, the following 
procedure is proposed according to the invention. 
Firstly the operator depressurizes the tank 10 and ensures the emptying 
thereof by means of the valves and taps equipping said tank. The operator 
then separates the tank 10 from the remainder of the circuit by 
disconnecting the tight connectors 16 and 38, as illustrated in FIG. 2. 
A new fluid supply means 42 is then connected to the ground connection 20 
by means of a flexible tube 44. Said supply means 42 has a large capacity 
reservoir (e.g. 100 to 150 dm.sup.3). The discharge flow rate of the new 
fluid supply means 42 is e.g. approximately 4 m.sup.3 /h under a pressure 
of approximately 300 kPa. It is necessary to fill the reservoir of the 
supply means 42 when the new fluid quantity contained therein drops below 
20 dm.sup.3. Moreover, the discharge pressure must not descend below about 
200 kPa. 
A manual tap valve 46 is preferably placed at the exit of the supply means 
42 in order to make it possible to isolate the circuit should this prove 
necessary (e.g. in the case of depressurization of the circuit or for 
safety reasons). 
In order to be able to replace the hydraulic fluid contained in the 
circuit, to the latter is also connected a spent fluid recovery reservoir 
48. More specifically, the reservoir 48 is connected by a flexible tube 50 
to that part of the tight connector 38 fixed to the end of the circuit 
return branch 34. The recovery reservoir 48 is also connected to the 
casing of the high pressure filter 30 by means of a flexible tube 52. 
The two entrance lines for the recovery reservoir 48 and to which are 
connected the flexible tubes 50, 52 are provided with check valves 57a, 
57b with a low opening threshold. 
More specifically, before connecting the flexible tube 52 to the casing of 
the high pressure filter 30, dismantling takes place of the filter trough 
32 and its filtering element (FIG. 1) and they are replaced by a 
connection tool container 54 (FIG. 2). This connection tool container 54 
is designed for fitting in place of the filter trough 32, so as to keep 
open an isolating valve of the high pressure filter 30, which is normally 
closed when the filter trough is dismantled. Moreover, the connection tool 
container 54 is designed to permit the tight connection of the flexible 
tube 52 to the casing of the high pressure filter 30. 
In the case where the high pressure filter 30 has several filter troughs 
each containing a filtering element, the connection tool container 54 can 
be fitted in place of any random one of said troughs. 
A manual tap 56 is placed in the flexible tube 52 at the entrance of the 
recovery reservoir 48. In addition, not shown means are respectively 
associated with the supply means 42 and the reservoir 48, in order to 
measure the fluid volume leaving the supply means 42 and admitted into the 
recovery reservoir 48. 
When the different connections described hereinbefore with reference to 
FIG. 2 have been completed, the hydraulic fluid can be replaced in the 
circuit in sectionwise manner in a way to be described hereinafter. 
Firstly replacement takes place of the fluid in the suction branch 18, the 
delivery branch 26, in the drainage branches 41a, 41b and in the return 
branch 34. 
For this purpose, operation takes place in turn of each of the pumps 22, 
24, by progressively opening the tap valve 56 placed at the entrance of 
the recovery reservoir 48. During the operation of each of the pumps, the 
tap valve 56 is closed when a hydraulic fluid volume slightly above the 
overall capacity of the suction 18, delivery 26, drainage 41a or 41b and 
return 34 branches is reached. The same operation is repeated for each of 
the pumps 22, 24. It is then ensured that the spent hydraulic fluid has 
been replaced by a new hydraulic fluid in each of the branches in 
question. 
As is illustrated in mixed line form in FIG. 2, the high pressure part of 
the circuit is then restored to its normal configuration. In other words, 
the flexible tube 52 is disconnected from the connection tool container 
54, the latter is dismantled and a filter trough 32 containing a new 
filtering element is put into place. In the case where the high pressure 
filter 30 has several filtering elements, the latter are all replaced by 
new filtering elements. 
The second step of replacing the hydraulic fluid in the circuit is then 
performed sectionwise with only the return branch 34 of the circuit 
connected to the recovery reservoir 48. 
Thus, the electric pump 24 is operated and one of the control means 28a, 
28b, etc. is completely manipulated in general several times. The number 
of manipulations of each of the control means essentially depends on the 
nature of said means. It is necessary to ensure that all the hydraulic 
fluid contained in the supply branches 27a, 27b etc. of the control means 
28a, 28b etc. between the high pressure filter 30 and the low pressure 
manifold 36, as well as in the return branch 34 is replaced. If necessary, 
a check on the thus replaced fluid volume is carried out by volumetric 
measuring means associated with the supply means 42 and the recovery 
reservoir 48. 
If necessary, the operation of the electric pump 24 is preceded by the 
manipulation of ancillary members such as taps, pressure switches, 
computers, etc. 
Following the replacement of the fluid in the supply branch associated with 
the control means such as 28a, 28b etc., which have just been manipulated, 
the electric pump 24 is stopped. 
When the hydraulic fluid contained in all the supply branches 27a, 27b etc. 
associated with the control means such as 28a, 28b, etc. has been 
replaced, the flexible tubes 44, 50 are respectively disconnected from the 
ground connection 20 and the part of the tight connector 28 fixed to the 
end of the return branch 34. The tank 10 is then connected to the circuit 
by tight connectors 16 and 38, followed by filling and pressurization. The 
filtering element of the general filter 20 is also replaced 
The electric pump 24 is then started up and manipulations of the control 
means 28a, 28b, etc. take place whilst monitoring the level of the 
hydraulic fluid in the tank 10 until it has stabilized. Optionally filling 
top-ups take place. 
A substantially total replacement of the hydraulic fluid contained in the 
control circuit can thus be carried out in a particularly simple and 
reliable manner. The risks of introducing air or creating leaks in the 
circuit are in practice eliminated. Moreover, the handling of certain 
fluids which can be irritating for the skin, eyes and respiratory system 
is minimized. 
In the case of a control circuit without a ground connection, the supply 
means 42 can be directly connected to that part of the tight connector 16 
which is fixed to the suction branch 18.