US oxygen connectors and couplings

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Left: CRU-8/P used by USAF for MBU-3/P and early MBU-5/P masks. Centre and right:
CRU-94/P (left) and CRU-60/P (right) connector blocks seen from both sides. CRU-60/P
has been the USAF standard connector block for more than 40 years, used with both
MBU-5/P and MBU-12/P oxygen masks. CRU-94/P is the connector block used in connection
with the Combat Edge system and MBU-20/P oxygen mask. It has an extra port for the
counter-pressure vest.

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Different types of oxygen connectors with integrated emergency oxygen bailout port. #1
and #2 are early 191A2 connectors, one without anti-flailing strap. #3 is a British version
of the 191A2 distinguishable by its extra black flange around the green plug-in coupling.
#4 and 5 look just like MC-3A connectors but for some unknown reason their plug-in
coupling are natural metal instead of red. #6, 7 and 8 are MC-3 or MC-3A connectors. #7 is
in standard configuration with anti-flailing strap. #6 has been modified with a snap hook for
attachment to the Saab LV-20 life preserver used in RDAF Draken aircraft. #8 has been
modified with a French EFA coupling for attachment to the Secumar HK10 life preserver used
in RDAF F-104 and F-100 aircraft. The picture to the right show the connectors in the same
order but with the other side up.

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The difference between an MC-3 and an MC-3A connector lies in the detail. The MC-3 had a
tendency to get stuck in the aircraft oxygen supply hose coupling during ejections and emer-
gency egress unless the pull was straight in line with the connector centreline. Worst case the
connector would be pulled off of the oxygen mask hose and thereby cut the pilot off from his
emergency oxygen supply. The cause of the problem seems to have been that the small pin
pushing open the disconnect warning valve had too much room for movement. The problem
was solved by fixing the pin to the disconnect warning valve centre rod with a nut, ensuring
that the pin stayed at right angles to the rod. In the pictures above you can see the MC-3 to
the left and the MC-3A to the right. The nut holding the pin to the rod is clearly visible on the
MC-3A. Note that the MC-3A pin is thicker than the MC-3 pin.

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USAF three-pin connector as used together with CRU-8/P, CRU-43/P, CRU-60/P and CRU-94/P
oxygen blocks. Introduced in 1968, the current connector is the MS27796 distinguishable by its
black colour and round inserted pins. The earliest connector, 57B3661, was natural metal and
its pins were square, milled out of the same piece of aluminium as the main body. The two
black connectors in the pictures are both MS27796, one is just more worn than the other.