Originally Posted by
tdracer
Not familiar with the specifics of the F135 engine, but it's standard design practice on jet engines that the high-pressure fuel doesn't only go into the burner - it's used as muscle for various actuators such as variable vanes and bleeds, etc. - so lots of fuel lines snaking their way around the engine. Typical pressure downstream of the fuel metering unit is around 300 psi - so any crack in a fuel line is going to result in a large fuel leak - not only do you have a big risk of fire if a line fails, you lose muscle pressure to the various actuators. In other words, bad things are likely to happen.
Smoke (or flames) is a pretty common indicator...
F-35 Air Vehicle Technology Overview 2018 LM people
"...The F-35B 3BSM consists of a STOVL LOAN and a three-bearing swivel mechanism. The mechanism can deflect the exhaust flow through 95 degrees in the pitch axis and ±12.25 degrees in the yaw axis as a function of pitch angle. The 3BSM can vector up to 23,900 pounds of thrust at the maximum rearward thrust split. The 3BSM forward (No. 1) bearing is powered by twin
fueldraulic actuator motors through a gearbox and drive train. The middle (No. 2) bearing is likewise powered by a twin
fueldraulic actuator motor and gearbox/drive train system. A transfer gearbox links the middle and aft (No. 3) bearings with an efficient, compact, epicycle gear train. The twin actuator motors on the No. 1 and Nos. 2 and 3 bearings, respectively, are designed with a fail-degraded capability (full torque, half rate).This is one of the key differences between this design and that of the X-35B. In the X-35B, the Nos. 2 and 3 bearings were braked following a first failure, with no ability to continue vectoring the aft thrust post. This did not satisfy operational requirements requiring an ability to perform a shipboard vertical landing following a first failure. The dual redundancy on the fueldraulic motors enabled that fault tolerance on the F-35B...."
https://www.lockheedmartin.com/conte...y_Overview.pdf