4 Engine Multiple power loss / Engine Roll Back checklist

[Chris Down Under]

Hi all

Got a Q on this 4 engine roll back checklist. Lockheed Safety Supp C-130 No 64

Step 1 is Go to NULL on the TD's..

Step 2 is GO to Mech Gov on the props

Step 3 is to Turn Sinc Master Switch to OFF

It’s this step that gets me..

Why if you are already in Mech Governing do you need to turn the Sync Sw to Off??

especially when in the preamble it says Synicrophaser induced RPM reductions do not result in significant power loss

Going to Mech Governing gets rid off all the Electrons out at the prop as why the sep 3 then???

Enquiring minds would like to know..

Chris

[Dan Wilson]

All I can tell you is NOBODY knows what causes the four engine roll backs

For decades lackheed denied they even happened

A couple I know of they had to pull the syncrophaser out of the rack to get the power back

they thought it was the tube type syncrophaser that caused it, this was the reason they switched to solid state syncrophasers; the result was fewer rollbacks but they were more sever when they happened and your syncrophaser didn't work for crap. Man we missed the tube type, they at least worked.

[Lkuest]

Going to Mech Governing gets rid off all the Electrons out at the prop as why the sep 3 then???

You really need to look at a wiring schematic and signal diagram to understand what's going on in the system, but put simply, the only thing selecting Mechanical does in the system is opening the circuit to the reference winding (or reference circuit) within the Speed Bias Servo Motor itself. The reference winding just gives the speed bias servo motor a reference for which direction to rotate, so the synchrophaser can send a signal to the motor and know which direction to rotate (it's A/C power, so it needs a reference frequency, and offset the frequencies to change direction). This is not to be confused with the Feedback Potentiometer, which is meant to dampen servo motor action and set synchrophaser output voltage. This means that if the synchrophaser is malfunctioning, it still has a direct line of sight to the motor, even in Mechanical Governing. The master sync switch is not wired even close to the Mech/Gov switch, so if it is the Master board that is malfunctioning inside the synchrophaser, de-selecting a master may resolve that problem, even in Mechanical Governing.

The synchrophaser receives a feedback voltage of up to 5 volts, but if there is a major sync malfunction in excess of 5 volts, the propeller will either drop 4% or rise 6% RPM. There are mechanical stops that physically prevents the speed bias servo motor from going any further. Decreasing RPM to 96% won't be too bad for engine operation, but going up to 106% could decrease power output due to hitting the Fuel Governor at about 105ish. High RPM is the danger, but easily remedied by either going to mechanical, turning the synchrophaser off, or as Dan said, if all else fails physically removing the synchrophaser to break that direct connection.

The entire USAF C-130 Fleet had been re-wired for internal prop balance wiring. As they were doing that, they changed out all the synchrophaser wiring and eliminated the solid state synchrophaser adapter. That seems to have enhanced Sync system reliability.

From what I understand, the rollback steps in the checklist were written in reponse to the King 56 trajedy which suffered a 4 engine power loss and flameout in flight. The Synchrophaser, Temp Datum, and fuel systems were investigated to see what could cause an engine to flameout. The synchrophaser was the only one that was eliminated outright. The Temp Datum system caused a flameout during interruption of A/C power (which took out a Pope aircraft a few years ago in Iraq, and almost brought down another in Arkansas), and the fuel system can cause a flameout with a fuel system back pressure of 3 psi above pump output pressure (cabin pressure being pumped into an on-board benson tank).

Simply put, a Synchrophaser-caused rollback is not severe enough to cause an engine to flameout, but it is possible to cause a power loss due to excessive RPM even in mechanical and master selected.