A relatively expensive, relatively heavy, and relatively nonflammable hydraulic fluid (chlorotrifluorethylene) is used in an aircraft ground wheel brake system between the piston (72) of a deboost device (68) and a set of wheel brakes (196). A relatively cheaper and relatively lighter, conventional hydraulic fluid (MIL-H-5606), which is also relatively flammable, is used in the remainder of the system. A replenish system for the relatively nonflammable fluid includes a reservoir divided into two chambers by a piston. One chamber contains a quantity of the relatively nonflammable fluid and the other chamber is connected to the system pressure. The deboost device includes a replenish valve which is opened in response to a position of the deboost piston. Replenishment only occurs when the brakes are applied and replenishment is necessary. At other times, all portions of the brake system are at return pressure. The relatively nonflammable hydraulic fluid acts as a buffer between heat generated at the brakes and the relatively flammable fluid.

DESCRIPTION 
1. Technical Field 
The present invention relates to hydraulic systems for aircraft and the 
like, such as aircraft ground wheel braking systems. More particularly, it 
relates to a two-fluid system and method for preventing fire damage and 
danger caused by the ignition of hydraulic fluid on hot surfaces. 
2. Background Art 
The Air Force has become increasingly concerned about the danger and dollar 
loss caused by aircraft hydraulic fires. During the 1965-1979 time period, 
the Air Force experienced approximately 153 noncombat hydraulic fires with 
an associated dollar loss of over 179 million. A major cause of these 
fires is the ignition of hydraulic fluid on hot surfaces. During the 1970 
to 1975 time period, about sixty-three percent of the hydraulic fluid 
fires occurred in the wheel well and/or landing gear area. Most of these 
fires were related to the ignition of hydraulic fluid on hot brakes. 
The hydraulic fluid currently used on most military aircraft is a 
petroleum-based mineral fluid, per military specification MIL-H-5606, 
which has a low manifold ignition temperature and high heat of combustion, 
and burns quite readily. On some recent aircraft, a synthetic hydrocarbon 
fluid per MIL-H-83282 is being used because of its gunfire resistance and 
somewhat lower overall flammability characteristics. However, it also has 
a relatively low hot-surface ignition temperature. 
Although these fluids are used throughout such aircraft, the brake, 
steering, and landing gear hydraulic actuation systems are statistically 
the most vulnerable. For example, when a hydraulic failure occurs in which 
these hydrocarbon fluids contact a hot brake, rapid ignition of the fluid 
occurs creating intense heat which ignites other fuel sources (such as the 
tire) that sustain the fire after the hydraulic fluid source is depleted. 
In an effort to reduce the occurrence of aircraft hydraulic fires, the Air 
Force initiated a program to develop a nonflammable hydraulic fluid. This 
effort led to the development of chlorotrifluoroethylene (CTFE) base 
hydraulic fluids. Although virtually nonflammable, the principal 
disadvantage of the CTFE hydraulic fluid is its high specific gravity 
(density) which is 2.11 times that of the MIL-H-5606 fluid. In addition, 
the CTFE fluid is not compatible with most seal elastomer materials 
commonly used in MIL-H-5606 fluid hydraulic systems, and CTFE fluid cost 
for high production quantities is very high compared to aircraft hydraulic 
fluids presently in use. 
The use of CTFE fluid in aircraft hydraulic systems would greatly alleviate 
fire danger and result in a significant improvement in aircraft safety. 
However, replacing MIL-H-5606 fluid with CTFE fluid throughout the entire 
aircraft hydraulic system would result in a significant weight penalty due 
to the increase in fluid density (e..g., +1700 lbs for the YC-14 advanced 
medium STOL cargo aircraft). This weight penalty can be reduced to 
approximately 64 lbs for a cargo/transport aircraft and 30 lbs for a 
fighter aircraft by employing the two-fluid nonflammable brake hydraulic 
system of the present invention in which the heavy CTFE fluid is used only 
in the hydraulic lines to the wheel brakes. 
DISCLOSURE OF THE INVENTION 
In accordance with the present invention, two different hydraulic fluids 
are used in a control system. A first hydraulic fluid, which is relatively 
flammable but otherwise desirable, is used in the system upstream of a 
piston in a pressure transmitter which serves to mechanically separate the 
two fluids. A relatively nonflammable second hydraulic fluid, having 
characteristics making it undesirable as a single fluid in the system, is 
used in the system between the isolator piston and an actuator which is in 
use adjacent a load which produces a considerable amount of heat. An 
example of such a load is the friction surfaces of an aircraft ground 
wheel brake. 
In accordance with an aspect of the invention, makeup nonflammable 
hydraulic fluid is introduced into the system between the piston and the 
actuator whenever the quantity of such fluid drops below a predetermined 
value. 
A preferred replenish system comprises a reservoir and means for delivering 
the hydraulic fluid from the reservoir into the system when needed. 
The system may include a replenish value which is adapted to open in 
response to a need for additional second hydraulic fluid in the system. 
The replenish reservoir may comprise a housing and a follower in the 
housing dividing the housing into a reservoir chamber for the second 
hydraulic fluid on one side of the follower and a feed pressure chamber on 
the opposite side of the follower. In preferred form, system pressure, 
supplied by the first hydraulic fluid, is introduced into the feed 
pressure chamber at the same time that it is applied against the piston, 
and replenishment occurs during operation of the actuator. In a braking 
system, this would occur whenever (1) the brakes are applied and (2) there 
is a need to replenish the second hydraulic fluid portion of the actuator 
system. 
In accordance with another aspect of the invention, the mechanical divider 
between the two hydraulic fluids also performs a deboost function. 
Yet another aspect of the invention relates to a construction of the 
mechanical isolator which permits the use of two different types of seals. 
A first seal constructed from a material which is compatible with the 
first hydraulic fluid is used to seal between the pressure transmitter 
piston and its surrounding portion of the pressure transmitter housing. A 
second seal constructed from a second material that is compatible with the 
second hydraulic fluid is used to seal between the pressure transmitter 
piston and its surrounding portion of the housing. In preferred form, an 
air chamber is provided in the housing between the two seals so that, if 
any leakage occurs through either one or both of the seals, the leakage 
will be into the air chamber. 
Further aspects of the invention are set forth in the detailed description 
of the preferred embodiment, all of which details are considered to be a 
part of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION 
An example of an existing aircraft ground wheel braking system is shown by 
FIGS. 1-4. A system of this type is used in the KC-135 military 
tanker/transport aircraft. This particular aircraft employs a truck-type 
main landing gear with paired-wheel brake control. That is, the brake 
pressure associated with each forward and aft wheel pair on one side of 
the truck is controlled by a single antiskid valve and control system. 
FIG. 1 is a schematic diagram of the entire system. FIG. 2 is a schematic 
diagram of only that portion of the system which is associated with a 
single tandem-wheel pair. 
Referring to FIGS. 3 and 4, the prior art system includes a mechanism 10 
which is termed a "deboost valve". It comprises a differential housing 12 
containing a differential piston 14. Specifically, housing 12 comprises a 
relatively small first-end portion 16 and a larger second-end portion 18. 
Housing portion 16 defines a chamber 20 in which a small-diameter portion 
22 of the piston reciprocates. Housing portion 18 defines a larger chamber 
24 in which the larger second-end portion 26 of piston 14 reciprocates. 
The piston 14 includes a shoulder 28 defined where the small and 
large-diameter portions of the piston 14 meet. Housing 12 includes a 
shoulder 30 formed where the small and large-diameter portions 16, 18 of 
the housing 12 meet. A variable-volume chamber 32 is defined axially 
between the two shoulders 28, 30 and radially between piston portion 14 
and housing portion 18. This space 32 is vented to the atmosphere via a 
vent 34, so that the air in it will not be trapped and retard the movement 
of piston 14. The small-diameter end of piston 14 is formed to include an 
axial opening in which a replenish valve 36 is situated. This valve 
includes an axial passageway 38 leading to a valve seat 40 (FIG. 4). A 
ball closure member 42 is normally held into a seated position by fluid 
within chamber 20. 
The piston 14 is hollow. A central orifice 44 provides a way of 
communicating passageway 38 with the interior 46 of piston 14. 
As shown by FIGS. 1 and 2, a conduit 48 is connected to the first end 50 of 
the housing 12. Hydraulic fluid from conduit 48 flows into and outfrom 
chamber 20 via an end opening 52. 
The opposite end of the housing 12 is closed by means of an end cap 54. End 
cap 54 comprises a central opening 56 which is in communication with a 
conduit 58 which connects the second end of deboost valve 10 with the 
wheel brakes (FIGS. 1 and 2). A plurality of ports 60 communicate the 
interior 46 of piston 14 with passageway 56. A central replenish pin 62 is 
secured at its base end 64 to a central portion 66 of end cap 54. The 
opposite end of replenish pin 62 is aligned with orifice 44. 
The KC-135 aircraft employs MIL-H-5606 hydraulic fluid on both sides of the 
piston 14. The piston 14 is provided to perform a deboost function. The 
system pressure upstream of piston 14 is relatively high, e.g. 
approximately 3,000 psi. The differential piston 14 reduces this pressure 
in that portion of the system between the piston 14 and the wheel brake. 
The area ratio of piston 14 is approximately three-to-one. 
The ball member 42 remains seated and closes the orifice 44 during movement 
of the piston 14 between the position shown by FIG. 3 and any other 
position in which piston 14 is located closer to end wall 50. However, 
whenever piston 14 moves towards end cap 54 beyond the position shown by 
FIG. 3, the upper end of pin 62 will contact and hold valve ball 42 in 
position. The valve seat will be moved away from ball 42. This will open 
the orifice 44 and will allow hydraulic fluid in chamber 20 to flow 
through the orifice 44 into the space 46. Such fluid will continue to flow 
until a pressure build-up which occurs on the brake side of the piston 14 
causes movement of the piston 14 back into the position shown in FIG. 3. 
When this happens, ball 42 will again become seated and will close the 
orifice 44. In this manner, a proper amount of hydraulic fluid is 
maintained in the system between the piston 14 and the hydraulic brakes. 
The two-fluid concept of the present invention utilizes the deboost 
device's ability to function as an isolator for the two fluids. Referring 
to FIGS. 7 and 9, the pressure transmitter 68 may be composed of many 
components of the deboost device 10. The housings 70 and 12 are identical, 
and the pistons 72 and 14 are identical. The replenishment valve 36 has 
been replaced by a plug 74. The old end cap 54 and the replenish pin 62 
have been replaced by a new end cap 76 and a stand pipe 78. In the 
two-fluid system shown by FIGS. 5-9, the standard hydraulic fluid (e.g. 
MIL-H-5606) is still used upstream of piston 72. The nonflammable 
hydraulic fluid, e.g. CTFE, is used between the piston 72 and the brake or 
other actuator, depending upon the use to which the system is put. 
Referring to FIG. 9, the small-area end portion 80 of piston 72 carries an 
O-ring or other elastomer seal 82 within a circumferential groove. A seal 
material is used which is compatible with the hydraulic fluid within 
chamber 84. By way of example, a Buna N Nitrile seal can be used when 
MIL-H-5606 fluid is used within chamber 84. 
A similar O-ring or other elastomer seal 86 is carried within a 
circumferential groove formed in the large area end of the piston 72. The 
material used for seal 86 is compatible with the hydraulic fluid which 
exists within the inner space 88 of piston 72. The same material is used 
for seals 90 and 92. At this time, phosphonitrilic fluoroelastomer (PNF), 
marketed by the Firestone Company, is the best material found for O-rings 
and other elastomer seals intended for use in the region of the CTFE 
fluid. As in the system shown by FIG. 3, the region 94 which is defined 
axially between piston shoulder 96 and housing shoulder 98 and radially 
between piston 72 and housing 70, is an air space and is preferably vented 
to the atmosphere via vent passageways 100. Thus, any leakage occurring 
across seal 82 or seal 86 will be into the space 94 and not into the 
region of the other seal 82 or 86. If leakage is not excessive, the 
stepped construction of the housing will help prevent leakage from one 
seal region to reach the other seal region. 
Each tandem-wheel pair has its own nonflammable hydraulic fluid 
replenishment system (FIG. 5). The schematic diagram for the entire brake 
system would look very much like the system disclosed by FIG. 1, but 
modified in accordance with the changes shown in FIG. 5, for each 
tandem-wheel pair. 
The replenish system comprises a replenish valve 102 which is incorporated 
in the end cap 76. Valve 102 includes a housing 104 having an inner end 
which is received within an axial opening 106 provided in end cap 76. The 
outer end of housing 104 is connected to a conduit 108 which is 
innerconnected between valve 102 and a reservoir for additional 
nonflammable hydraulic fluid. Valve 102 includes a valve seat at 110. A 
ball type closure element 112 is normally seated against valve seat 110 by 
a spring 114. A poppet element 116 is positioned inwardly of ball element 
112. It includes a first-end portion 118 which extends towards piston 72, 
a second-end portion 120 which extends towards ball element 112, and a 
flange 122 between the two end portions 118 and 120. A spring 124, 
positioned between flange 122 and an internal shoulder 126 within valve 
102, normally biases the poppet 116 towards the piston 72 and away from 
the ball element 112. 
Referring now to FIG. 6, the reservoir 128 for additional nonflammable 
hydraulic fluid may comprise a cylindrical housing 130 having an end cap 
132 at one of its ends and an end cap 134 at its opposite end. A piston 
type follower 136 is positioned within the interior of housing 130. It 
includes a radial end wall 138 and a cylindrical sidewall 140. A 
high-pressure seal 142 is located within a first circumferential groove 
formed in the sidewall of housing 130. A second lower-pressure seal 144 
may be provided in a second circumferential groove formed in the sidewall 
130. A third seal 146 may be formed in a third circumferential groove 
formed in sidewall 130. 
The additional or makeup fluid is located in chamber 148, between end cap 
132 and follower end wall 138. Thus, seal 142 which is located closest to 
this chamber 148 is a high-pressure seal and is made from a material that 
is compatible with the particular hydraulic fluid which is stored in 
chamber 148. In the example system, in which CTFE fluid is provided in 
chamber 148, the seal material for seal 142 is the same material that is 
used in device 68 for seal 86, i.e. Firestone phosphonitrilic 
fluoroelastomer (PNF). The center seal 144 may be made out of the same 
material. Seal 146 which is positioned closest to end cap 134 is 
constructed from a material which is compatible with the particular 
pressure fluid that is introduced into chamber 150. In the illustrated 
example, MIL-H-5606 fluid is introduced into chamber 150 and the seal 
material used for seal 146 is the same material that is used in device 68 
for seal 82, i.e. Buna N Nitrile. 
Seal 142 should, by itself, provide against leakage from chamber 148 
towards seal 146. However, should leakage occur, an annular groove formed 
in the sidewall 130 collects the leakage and directs it to vent holes 152. 
In similar fashion, should leakage occur from chamber 150 across seal 146, 
the fluid would be collected by an annular groove formed in sidewall 130 
between seal 146 and seal 144, and from such groove would be directed to a 
vent opening 154. The presence of seal 144 between the two annular grooves 
provides additional safeguard against leakage. 
Preferably, the reservoir 128 is provided with a sight gauge 156 which is 
provided for indicating the quantity of fluid within chamber 148. The 
particular sight gauge that is illustrated comprises a tubular housing 158 
which is mounted at its inner end to end cap 134. Housing 158 is axially 
directed and receives an axially extending indicater rod 160 which is 
connected at its inner end to follower end wall 138. Housing 158 need be 
formed to include a viewing port or window 162. The outer end portion 164 
of rod 160 may be colored a bright color, such as red, and/or may be 
marked with indicia as the word "service". As the quantity of fluid within 
chamber 148 decreases, the rod 160 is moved to the left (as pictured) 
bringing the end portion 164 into registry with the window 162. The 
separate color of end portion 164 and/or the indicia on such end portion 
will signal that it is necessary to add additional fluid into chamber 148. 
A suitable seal 166 is provided to seal between end cap 134 and rod 160. 
Each reservoir end cap, 132 and 134, contains two ports. End cap 134 
includes a port 168 which is connected to a line 170 which supplied a 
pressure feed fluid. The second port receives a bleed valve 172. End cap 
132 includes a fill port 174, in which a fill valve 176 is received, and 
an outlet port 178. The second end of conduit 108 is connected to the port 
178. 
As shown by FIGS. 6 and 8, two radial slots are machined in the inner 
surface of end cap 132. Slot 180 communicates with the inner end of port 
178, and slot 182 communicates with the inner end of port 174. Referring 
to FIG. 6, a bleed port 184 is provided in end cap 134. 
The chamber 148 is sized to account for volumetric changes due to 
temperature, brake wear, and normal fluid loss by leakage. 
Referring to FIG. 5, in the preferred embodiment, the pressure fluid that 
is introduced into chamber 150 for pressure feeding additional relatively 
nonflammable fluid into the system is the same fluid that is used in the 
system on the control side of pressure transmitter 68, i.e. the relatively 
flammable fluid. 
Referring now to FIG. 5, the system that is shown includes a first conduit 
186 which is at supply pressure. This conduit and a return conduit 188 are 
connected to a three-way pilot-controlled metering valve 190. The third 
conduit that is connected to valve 190 extends to the inlet of pressure 
transmitter 68. This conduit 192 includes an antiskid valve 193 which 
forms no part of the present invention. The antiskid valve 193 is shown in 
schematic form in FIG. 1. 
The fourth conduit of the system is conduit 194 which connects the brake 
side of pressure transmitter 68 with the wheel brakes 196. This conduit 
194 may include a conventional pressure relief valve 198. Conduit 108 
which extends between reservoir chamber 148 and the brake side of piston 
72 is a fifth conduit in the system. Conduit 170 is a sixth conduit in the 
system. It is innerconnected between conduit 192 and the feed pressure 
inlet 168 for the replenish reservoir 128. Conduit 170 includes a 
restrictor 200. 
The pressure within conduit 192 will be herein referred to as pilot metered 
pressure. This pressure is supplied to conduit 170 through restrictor 200 
for the purpose of eliminating the dynamic effect which the addition of 
fluid from conduit 192 into chamber 150 would have upon the response and 
performance of the control system. 
The use of pilot metered pressure to feed additional relatively 
nonflammable second fluid into the system when needed eliminates the need 
for a high-pressure air-charged accumulator for performing the replenish 
function. The use of such an accumulator would cause the brakes to lock if 
leakage occurred through the replenish valve 102. In the system shown by 
FIG. 5, the reservoir chamber 148 is pressurized and replenishment occurs 
only when braking is commanded. Thus, when braking is not commanded (as in 
flight) the entire brake system (pilot metered pressure, the pressure in 
chambers 148 and 150, and the pressure between piston and the brakes) is 
at return pressure and no brake pressure build-up can occur due to 
replenish valve leakage. 
Replenishment, as in the prior art system shown by FIGS. 1-4, occurs only 
when the piston 72 is within a small distance from a completely bottomed 
condition at the low-pressure end of the pressure transmitter 68. By way 
of example, this distance may be about 0.125 inches. 
Referring to FIG. 9, replenishment occurs when the piston 72 contacts end 
portion 118 of poppet 116 and moves it against ball member 112, moving 
ball member 112 away from its seated position. Replenishment fluid from 
reservoir 148 enters the low-pressure end of pressure transmitter 68 at 
the same pressure (not considering antiskid activity) as the original 
system pressure which in the example system is approximately 3,000 psi. In 
addition, during braking, the piston 72 rides or functions at the same 
level (near the replenishment level) and with the same stroke as an 
unmodified deboost valve in a system of the type shown by FIGS. 1-4. 
For aircraft brake hydraulic systems that operate at the system pressure of 
3,000 psi (and therefore do not use deboost valves) a mechanical isolation 
and replenishment system identical to that shown in FIGS. 5-7 would be 
used. However, in that case, a near equal-area isolation piston would 
replace the differential area piston shown in FIGS. 5, 6 and 9. Piston 72 
in the example system has an approximately three-to-one piston area ratio. 
The replenish system is pressurized normally by pilot-metered pressure. 
However, in the event of a failure, the system can be converted over to be 
pressurized by co-pilot-metered pressure. The necessary switchover valves 
are designated 202 and 204 in FIG. 5. The complete switchover system is 
shown in FIG. 1 in conjunction with the prior art system. 
Since the fluid flow path from the high-pressure side to the low-pressure 
side of the pressure transmitter has been plugged in the two-fluid system, 
the need for quantity-measuring hydraulic fuses upstream of the deboost 
valve (shown in FIGS. 1 and 2) is eliminated. 
The system approach shown by FIGS. 5-9 has two distinct advantages: (1) the 
brake hydraulic system is virtually unchanged and (2) it functions exactly 
the same as the original system. No modifications have been made which 
effect or change the dynamic operation of the pressure transmitter (known 
as a deboost valve in the prior system) or brake system. During normal 
brake system operation, the original brake system and the modified 
two-fluid brake hydraulic system are identical. The differences which 
exist between the two configurations involve only the replenishment 
system. The replenishment valve that, in the original system, is located 
within the pressure transmitting piston has been moved from such piston to 
the end cap at the brake end of the pressure transmitter. Since the 
replenish valve is closed (blocking the replenish path in both systems) 
during normal braking activity, the configurations of the two systems are 
identical. Thus, the brake and deboost valve modifications do not affect 
the normal operation of the brake system for the stopping performance of 
the aircraft. Similarly, other brake system operating modes such as 
parking, refuse takeoff, manual braking and emergency braking are not 
affected by the hardware modifications. 
In summary, the modifications necessary to convert a KC-135 deboost valve 
to a fluid isolator, and the replenishment system additions which are 
necessary, to create the two-fluid system, are: 
(1) The original replenish valve in the deboost piston is removed and a 
solid plug is installed in its place to eliminate the original fluid 
interchange path (such path being shown in FIG. 3); 
(2) The original brake path at the end of the deboost valve, and the 
replenish pin, are discarded; 
(3) A new end cap assembly is provided. It includes a bleed/output 
standpipe and a replenish valve assembly (FIG. 9); 
(4) The end cap and the low-pressure piston seals are changed to a material 
which is compatible with the relatively nonflammable hydraulic fluid; 
(5) A replenish system is added which includes a fluid reservoir, a refill 
valve for servicing, and a fluid-level indicator; and 
(6) Provision is made for bleeding the isolated second fluid. 
Filling and bleeding the brake system is accomplished by ground servicing 
of the first-fluid portion of the system and then the second-fluid portion 
of the system. Servicing the first-fluid portion is formed by adding 
maximum pressure and cracking the reservoir bleed valve to circulate the 
first fluid through the brake system. The second-fluid portion is then 
serviced by opening the brake bleed valve and pumping the second fluid 
through the reservoir and the brakes. After bleeding, additional second 
fluid is added to fill the replenish reservoir chamber 148. 
When the second-fluid portion of the system is serviced, the 
pressure-transmitter piston 72 is bottomed against the end cap 54. This 
causes an opening of the replenish valve and positions the inner end of 
the standpipe 78 within a cavity formed in the plug 74. Second fluid is 
then pumped through the fill valve 176 into the reservoir 148, and through 
the replenish valve 102 into the chamber 88, into the plug cavity, down 
the standpipe 78 and into the brakes and out of the brake bleed port. As 
second fluid passes through its portion of the system, any air in such 
portion of the system will be forced out through the brake bleed port. For 
example, air in the pressure transmitter 68 rises and collects in the plug 
cavity. This air is forced down the standpipe 78 and out the brake bleed 
port as the volume 88 fills with the second fluid. 
Several features have been included in the two-fluid brake hydraulic system 
configuration to improve system safety. 
Namely, four replenish systems, one for each tandem-wheel pair have been 
included in the system designed to prevent the loss of braking capability 
in the event of a failure. For example, if the hydraulic line between a 
pressure transmitter 68 and a set of hydraulic brakes were to burst, only 
the braking capability and fluid associated with that wheel pair and its 
replenishment system are lost. Normal braking capability and replenishment 
capacity is maintained on the other three paired-wheel sets. The 
replenishment reservoir is pressurized normally by pilot-metered pressure, 
and in the event of a failure, by the co-pilot-metered pressure through a 
shuttle valve system. This configuration prevents the loss of 
replenishment capability when pilot-metered pressure is lost.