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Locking the TD during fuel governing and pitch lock check


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Hi to all members and guests,

Looking for an answer to the following...

On "fuel governing and pitch lock check procedures" almost all manuals -on hand- are asking to lock the TD; some manuals required to have it "locked" at a governing speed (above cross-over), others asking to lock it once retarding the throttles to below cross-over.

The main concept of locking the TD to my understanding is to avoid any sudden change of power when passing through cross-over point..

 However which is not clear to me the idea behind locking the TD below cross-over;

1- Is the TD valve going to "Lock" below cross over? Hence the 66 coordinator switch is not actuated yet?

2- How the TD system is behaving once locked below cross-over?

3- How the TD system is behaving once throttles are advanced and passing through the 66 deg….. ?


Any highlight on this will be appreciated.

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Below crossover, the brake lock function is locked out through the brake lockout relay, so as pjvr states, going to locked below crossover would only have overtemp protection and nothing more. This same relay kicks in when you overtemp in locked, and that's why the fuel correction light stays on after overtemp. Potentially, you could have a 100 rich and 70 lean engines on opposite wings cause issues with pulling the throttles back unevenly, but you still have that problem because the props don't exactly all pitchlock at the same blade angle anyway, so the procedure may be just as effective regardless of what throttle position you flip the switch to locked. I prefer going to locked above crossover just to eliminate variables, but both procedures may give you the same information for the op check.

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I have been having the same idea on how TD system works as you have it, until I came across the following which mix me up….

Quoting TM 382C-2-3 section 3-page 3: "The other switch "66 degrees coordinator microswitch" is actuated at a throttle lever setting of about 66 degrees and opens a circuit to prevent the temperature datum valve brake from being locked at a throttle lever setting below 66 degrees".

Therefore according to the above; the TD brake should not go to lock below cross-over (open circuit)

In which my questions arise… what if passing across the 66, while TD switch was already on locked position?

Is it going to lock?

And if it is supposed to lock; normally it should lock in a position having no any correction/trimming (Null position).

As for the correction light, they supposed to remain illuminated hence the system is locked below cross-over.  

On the other hand, is there any holding circuit (relay) to maintain the lock position on the case of cycling the throttle below and above the 66 switch to prevent the cycling of the TD brake?


As for how the system will act if locked?

  I agree with you and with "pjvr" regarding the existence of over temperature protection.    

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Tenten, I understand your frustration, but there really is no hope in logically understanding the system without looking at the schematic. Doing so allows you a much greater understanding of why the system behaves the way it does under all conditions. In this specific case, it is important to understand that the TD Valve brake does not get energized in order to engage, it actually is energized into the "released" position. If the TD Valve brake solenoid loses power, it moves the the "applied' position, locking in the fuel correction. In this case, if you were to move the TD switch to the "locked" position, this energizes the brake lock relay, which is meant to cut the power going from the brake lock relay switch to the TD Valve brake solenoid. It just so happens that, below 66 degrees, the coordinator also sends power to the TD valve brake solenoid, thus, the brake is still "energized" into the released position. Why select locked below crossover then? Because when the Brake Lock relay becomes energized, it sends power to the limit selector relay, telling the TD Amp to ignore Bias and Slope and  to only start pulling fuel once the TD system reaches the max limit, which on a -15 engine is 1077. Once you advance the throttle above 66 degrees, the power no longer comes from the coordinator to release the TD Valve brake solenoid, and lock the brake. So, is the brake locked? Nope. The electronic fuel correction light is on below crossover due to being below the 66 degree switch, but remains on while advancing throttles above crossover because the brake lockout relay is already energized by the below 66 degree switch, which not only provides its own circuit once energized below crossover, but also powers the electronic fuel correction light, and also the TD Valve brake solenoid, keeping the TD Valve brake released.

In short, placing the TD Control Switch to "Locked" below crossover accomplishes nothing other than providing overtemp protection, and the brake is not locked at any time, no matter the throttle position. If the brake lockout relay is energized by the coordinator below 66 degrees before going to "locked", it always powers the TD valve brake to release. You must allow the coordinator to go above the 66 degree switch in order to prevent the brake lockout relay from energizing before the brake lock relay. You do this by first putting the TD switch to "Auto", advance above crossover, then go to "Locked". This prevents the brake lockout relay from keeping itself energized, and removes all possible power sources from the TD Valve brake solenoid, allowing it to apply and lock fuel correction.

Now, for your second question, if the system were to be locked above crossover, and the throttle were moved below crossover, would the brake hold below crossover? The answer is, yes. If the brake lock relay becomes energized without the brake lockout relay first becoming energized, the brake lock relay prevents the 66 degree switch from ever receiving power, thus it cannot power the TD valve brake solenoid to release it. It also does not have power to energize the electronic fuel correction light. This is why the light stays off below crossover once the TD system is locked above crossover. How can you "reset" the system? Downspeed the motor. This allows the 94% switch to energize the brake lockout relay, restoring power to the 66 degree switch and the TD Valve brake solenoid to the released position. Or, you could just put the TD switch back to Auto, which restores power to the brake lockout relay. Then, you'd have to advance the throttle above 66 degrees to deenergize the brake lockout relay in order to lock the TD valve again.

Edited by Lkuest
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I think we might be missing part of the procedure here.  In our checklist, the test is the Fuel Governor and Pitchlock Check. As far as the pitchlock goes, it doesn't matter if the TD system is in auto, null, or locked.  Of course, auto and locked will permit overtemp protection.

However, the fuel governor check requires that we can compare a specific fuel flow to a specific RPM.  Apparently, those who provide us the chart for adjusting the fuel control governor to get the proper fuel flow do not want the TD system altering the fuel flow.  It appears they want "uncorrected" fuel flow for whatever RPM the fuel control limits the speed to.

It is easy to do, painless, and that is what our procedures require.  Why not?

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Thanks "Lkuest" for your technical details which help me a lot to have a look at the TD electrical circuits; and by going over the "twisted circuits" it confirmed all the points that you stated in wards.

Therefore-in short- can we say…

1- TD valve cannot be locked if TD switch is placed to "lock" below cross-over.

2- Advancing throttle to above cross over while TD switch being locked below cross-over will have no any effect on the TD valve brake.(brake will remain unlocked)

3- Correction light will remain illuminated on the entire throttle range if TD locked below cross-over.

4- The only advantages of locking the TD below cross-over are:

     a- To arm/energize the "normal limit circuit" on the entire range of throttle. (for TIT not to exceed 1077*).

     b- To deactivate the BIAS & SLOP corrections.(No fuel trimming at cross-over).


I agree with you on the point that TD system is not an effective issue to have the pitch locked; it may only help "if in AUTO" on the case of rpm dropping to below 94% once throttle is retarded while the pitch is locked (over-temp. protection).

As for your second point, I understand that you prefer to have the TD locked below cross-over to avoid any value of fuel trim in order to check "uncorrected fuel flow" against fuel governing chart/table.          

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