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Old 17th Jun 2013, 01:25   #1 (permalink)
 
Join Date: Jul 2003
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Kingair C90 Prop Governor

The C90 has 3 governors.

1. Normal governor
2. Over-speed governor
3. Fuel topping governor.

Its does seem to have a lot of equipment for such a simple device.

Why is the over-speed governor required, shouldn't the normal governor be able to do the task.

In the latest turbo props that have an electronic engine management system, is the mechanical fuel topping governor replaced by software?
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Old 17th Jun 2013, 14:33   #2 (permalink)
 
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I'm giving a generic answer here. perhaps someone can make it more C90-specific.

There are two rotor systems in the PT6 (& many other engines), and each rotor is subject to its own failure modes. For example, if the prop fails to flat pitch, the free turbine is unloaded and can overspeed at a cruise power setting. In this case, the prop governor is out of control, and a very rapid fuel cut is necessary to keep the free turbine or prop from destroying themselves.

And there may be modes in the main fuel controller wherein the core rotor overspeeds, so a redundant o/s protection may be needed.

I'd think a good PT6 pilot fam course will fill in the blanks.
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Old 17th Jun 2013, 16:15   #3 (permalink)
 
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nomorecatering

There are actually 4 governors on the PT6A

1. Hydraulic Propeller Governor
2. Hydraulic Propeller Overspeed Governor
3. Pneumatic Propeller Fuel Topping Governor
4. Fuel Control Unit Governor.

They are all interconnected and perform various functions.

The reason for the Hydraulic Propeller Overspeed Governor is primarily for a back up for the failure of the Propeller Governor - It is not field adjustable - has no input from the flight crew and basically sits there like a lump until needed. If needed you will be very glad it was installed. In 40 years of working on the PT6 I have only seen a couple of Propeller Governor complete failures - so system is pretty robust. I would assume that part of the reason for having the overspeed governor has to do with back up protection as required by the pesky regulations set by the authorities to cover as many possible failure modes as they deemed a requirement.

Here is a link to some very good stuff on the PT6 small engine.

File Catalog - FreeBee


I don't know of any small turboprop engines that have electronic Fuel Topping functions but it is a possibility - all you would need would be electronic speed sense for propeller speed - and the computer to tell the fuel control to reduce fuel flow and it should work just fine - unless of course the computer control has been compromised and you are now running on "Manual" . I would think that those pesky authorities would take that into consideration and then require a manual backup for the computer failure. Probably just as easy to just make the one unit a manual one.

Also don't forget that many PT6 engines have/had a "Beta Back-up" system - This was used to prevent the propeller blades from going below the Flight Idle blade angle with the Power Lever at Flight Idle or above. Electric blade position sense plus microswitches adjusted on the Power Lever - activates a "Lock Pitch" solenoid on the propeller governor to move blades towards coarse. This system prevents un-commanded blade movement towards reverse while in flight - which would not be a good thing on the Beech!!!

Hope this helps.
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Old 17th Jun 2013, 17:46   #4 (permalink)
 
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The overspeed governor is a second prop governor that works like the primary governor but is set to one specific RPM (2080 on the C90 I fly). If the primary governor fails but the rest of the system is working properly, then the secondary governor will prevent the prop RPM from going above 2080.

If on the other hand the primary governor is working fine but is unable to control the prop RPM due to a failure of the prop pitch mechanism, then it will command the fuel topping governor to reduce power coming from the engine in order to prevent a prop overspeed.

Last edited by ahramin; 17th Jun 2013 at 17:48.
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Old 17th Jun 2013, 21:13   #5 (permalink)
 
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It all boils down to system redundancy. Certain failure modes are very bad things that can ruin your day, so it's best to have Plan B.
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Old 18th Jun 2013, 02:07   #6 (permalink)
 
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Quote:
Originally Posted by MX Trainer View Post

There are actually 4 governors on the PT6A

1. Hydraulic Propeller Governor
2. Hydraulic Propeller Overspeed Governor
3. Pneumatic Propeller Fuel Topping Governor
4. Fuel Control Unit Governor.

They are all interconnected and perform various functions.

The reason for the Hydraulic Propeller Overspeed Governor is primarily for a back up for the failure of the Propeller Governor - It is not field adjustable - has no input from the flight crew and basically sits there like a lump until needed. If needed you will be very glad it was installed. In 40 years of working on the PT6 I have only seen a couple of Propeller Governor complete failures - so system is pretty robust. I would assume that part of the reason for having the overspeed governor has to do with back up protection as required by the pesky regulations set by the authorities to cover as many possible failure modes as they deemed a requirement.
Thanks, info similar to above on the fuel topping governor would be nice as well.

Quote:
Originally Posted by MX Trainer View Post

Also don't forget that many PT6 engines have/had a "Beta Back-up" system - This was used to prevent the propeller blades from going below the Flight Idle blade angle with the Power Lever at Flight Idle or above. Electric blade position sense plus microswitches adjusted on the Power Lever - activates a "Lock Pitch" solenoid on the propeller governor to move blades towards coarse. This system prevents un-commanded blade movement towards reverse while in flight - which would not be a good thing on the Beech!!!
The King Air and Beech 99 I flew did not have the Beta back up system like the Twin Otter, I wonder why?

Also in your info on various PT-6 engines....Confirm that the early models did not have reverse capability. Never heard of that before.
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Old 18th Jun 2013, 15:16   #7 (permalink)
 
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Reverse capability is a prop-specific feature - controls & interlocks, tied in with the engine controls, but not really part of the engine. Given that the PT6 was on the DHC-6 from the start, I suspect there were very few -6's made without reverse. Can't speak to the King Air though.
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Old 18th Jun 2013, 17:20   #8 (permalink)
 
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Reverse capability as posted above is manufacturer specific. All of the PT6A series "could" be used as non reversing if the reversing governor and linkages specific to the reverse function were removed and a non-reversing propeller governor installed.

The only aircraft that I recall seeing that had the non-reversing propellers was a modified Beech 18 that was converted sometime in the late 1970's. The original non-reversing system was replaced with the reversing props at a later date.

Just a quick note on reversing propeller installations - there are 3 limitations on reverse power made available - the first is the internal limitations of the engine. Internal temps go up quickly in reverse and in some cases the torque limit on the gearbox can be the limiting factor - just like in forward flight.

The second limitation is structural - so engine mount - nacelle - wing structures may not be capable of absorbing the max output from the powerplant in reverse.

The 3rd limitation is for lack of a better term "Installation Geometry". The Twin Otter has this issue being a high wing with a tricycle landing gear. If not limited the powerplant could sit the aircraft on its' tail in full reverse. C of G location obviously has a role here as well.

The Beech 90 and the 99 have / had a "Beta Backup System" - they just didn't call it that - Beech term for the same system is " Secondary Low Pitch Stop System" - it works exactly the same as the Beta Backup System. I have been away from the aircraft a while but I believe there is a Service Bulletin that Beech has that allows the system to be removed - not sure if it is still on the newer aircraft so someone with some current time on the new aircraft can confirm this.


Fuel Topping Governor has 1 function - to bleed the FCU Control pressure - usually referred to as Py when it senses an overspeed condition.

The fuel topping governor is connected to the propeller reversing linkage and the speed setting is changed depending on the operational mode of the engine. In constant speed range "Alpha" the reset linkage sets the Fuel Topping to 5-6% above what the Propeller Lever has commanded from the Propeller Governor. So in this case the Fuel Topping governor will reduce fuel flow to the FCU at 105-106% Propeller speed when the propeller control is set to command 100% propeller speed. If in cruise for example the propeller control is set to 80% Np then the fuel topping governor would reduce fuel flow at 84-85% propeller speed. It is important to remember that the fuel topping governor "Trip" speed in the constant speed range is a function of the propeller lever input to the propeller governor.

The fuel topping governor is reset to 5-6% less than the selected propeller control lever command to the propeller when we have reached maximum reverse position of the Power Lever "Beta Range - Full Reverse". In this case it will limit the propeller speed to about 94-95% in full reverse - when the propeller lever is in the 100% condition. Many aircraft have an interlock system installed that will not allow you to physically move the Power Lever below the flight idle gate on landing unless certain conditions are met. One of those conditions on some aircraft is the Propeller Control must be fully forward at the 100% position.

In reverse the Fuel Topping Governor will prevent interference from the Propeller Governor. If in reverse the propeller speed should reach the 100% that the propeller governor is set for the overspeed sensing of the propeller governor speeder spring would cause the internal valve to move to block of and dump oil pressure delivered to the propeller. This would have the effect of moving the blades to a greater pitch - works perfectly when in constant speed mode - but when in reverse - movement towards a greater pitch has the opposite effect as the blade angle moves from say -14 deg to 0 deg. The result would be a severe overspeed and catastrophic failure of the engine.

Hope this helps.
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Old 19th Jun 2013, 08:29   #9 (permalink)
 
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Awesome information MX Trainer.

I knew that in reverse the prop would never get "on speed" but nobody could ever explain to me the reason why! Not something that comes up in the POH either, other than some simple statement saying that the FTG controls prop speed in reverse.


Hoping you might be able to clarify something for me. I've never experienced a governor failure, all I know is what I've read and been taught.

Obviously the FTG can take over prop control at a lower RPM than the OSG. Is it true that the prop will somewhat "cycle" at this FTG-limited RPM? I've been lead to believe that it will cycle because as the prop overspeeds, the FTG will catch it, bleed air and thus it will slow off the limit, then the bleeding stops and prop will speed up again where the FTG catches it again and the process repeats.
Also, is it best to stay at the FTG-limited RPM or push the prop lever forward and have the OSG control the prop rpm, even though it would then be a higher rpm?

As an aside - NEVER EVER SELECT BETA OR REVERSE WHILST THE ENGINE IS NOT RUNNING!!!!
Someone else did that to one of our aircraft. We were not happy!

A little while ago I did read an article about some early Beech aircraft that had PT6s without reverse. If I can find it again I'll post the models.

Last edited by MyNameIsIs; 19th Jun 2013 at 08:38. Reason: Something went silly and put in lots of codes making it hard to read.
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Old 19th Jun 2013, 10:32   #10 (permalink)
 
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Quote:
Originally Posted by MX Trainer View Post
Many aircraft have an interlock system installed that will not allow you to physically move the Power Lever below the flight idle gate on landing unless certain conditions are met. One of those conditions on some aircraft is the Propeller Control must be fully forward at the 100% position.
Thanks.

I remember that the Twin Otter had the interlock but not the King Air and beech 99. Never understood why.

The interlock system was a potential trap for the unwary and could lead to an accident. On the Twin Otter, full forward on the prop levers was actually considered to be 96% selection with no reverse capability available(due to the interlock) unless a prop lever position above 91% was selected.

Many captains didn't want the prop levers selected forward until touchdown to avoid excessive noise. Twice, it happened to me where, there was some sort of a hangup between the power levers and the prop levers due to reverse being attempted while the prop levers were still being moved forward. This meant no reverse available.

I was lucky....in both cases we were on long runways. But many of our flights were to extremely short offstrip locations where you needed max reverse.

We had one accident out on the ocean ice where they could not get reverse and damaged the aircraft. The pilots swore that they props had been selected forward for landing and maybe they were, but no mechanical or electrical malfunction could be found. I believe there is a friction selector but I don't know whether it was tight or loose for the prop levers in that accident.

So perhaps the props were selected forward but moved slightly aft by themselves prior to reverse selection on touchdown. There is not very much propeller lever travel between 91 and 96%.

Bottom line, after these experiences, when landing offstrip, I would have props selected full forward and have my hand giving positive forward pressure on both prop levers on touchdown, always, even if the captain said not to move them forward until touchdown. Never had another problem after that.

Last edited by JammedStab; 19th Jun 2013 at 10:37.
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Old 19th Jun 2013, 12:30   #11 (permalink)
 
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The KingAir has the "Reverse Not Ready" master caution annunciator light to tell you the props aren't full forward. No physical stop to reverse selection, but my understanding is that it probably won't be beneficial for all the linkages when reversing whilst the prop levers are not in the full forward position.

There are, of course, gates or other mechanisms (twisty-grips in the Twotter etc) to help prevent inadvertent reverse selection.

Last edited by MyNameIsIs; 19th Jun 2013 at 12:33.
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Old 22nd Jun 2013, 17:16   #12 (permalink)
 
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Everyone

The differences between the DHC-6 TwinOtter and the Beech could just be in the certification process - DHC had to be certified to the Canadian standards while the Beech was certified to the USA standards.

Also the TwinOtter is missing the Weight On Wheels system - many aircraft spend a good portion of their life on floats - so possibly this is the reason for the different interconnects that needed to be made before allowing the power lever into reverse. IIRC the TwinOtter had 3 things that had to happen - prop lever fully forward, flaps selected and in the approach position, and activation of the twist grip to get the power lever below the flight idle position. It is easy to get into reverse in the air with the TwinOtter - and no it is NOT APPROVED!!!

I have some right seat time in the -6 and one of the things that was my responsibility was to follow up the captains big paws on the power levers on takeoff and to maintain positive forward pressure on the propeller levers on the landings. My left hand stayed on the prop levers right from the "Full Fine" command from the left seat. By having clearly defined cockpit procedures it eliminated the uncertainty of who was doing what.

Many other aircraft require the weight on wheels system - ie "Squat Switches" to indicate the aircraft is on the ground before allowing the power lever below the flight idle. At the end of the day it really doesn't matter what system is being used as you are trained for that specific aircraft type.

Regarding the propeller action when on the Fuel Topping Governor - All mechanical governors "Hunt" during operation. I use the word hunt rater than cycle because to me something that cycles turns on and off while hunting is the variations in speed (and torque) within a narrow range.

One of the maintenance checks that can be done is a check of the fuel topping governor speed. This is accomplished by the removal of the air bleed reset link to the integrated governor and locking the fuel topping governor reset arm to the reverse position with lockwire. This has the effect of simulating the operation of the fuel topping governor in reverse but operating the engine in the forward range. It has been a while since I have had to do this but I don't remember the propeller cycling but I do believe it will hunt a bit more due to the little bit more lag in the system as we are controlling the propeller speed by changing fuel flow from the FCU.

What does cycle is the Beta Backup System - or equivalent. On that system you can see and feel the cycling when you do the ground test. The operation of the ground test simulates the system in flight mode while on the ground. It changes the logic of the power lever below flight idle switch to indicate flight mode. So with the switch in the test position bringing the power lever below the flight idle detent will drive the propeller blades to the low pitch sensor. The sensor will activate the system which sends an electrical command to the lock pitch solenoid. The lock pitch solenoid will close blocking off the oil flow to the propeller and the propeller blades will now move towards a greater blade angle. Once moved to the point where the low pitch sensor is de-activated the electrical command signal is removed from the lock pitch solenoid. This allows governor oil pressure once again to the propeller which causes the blades to move towards a lower pitch and the cycle starts again. You can see, hear, and watch the indicator lights come on and off as the system cycles.

Regarding a propeller system failure and ending up with the fuel topping governor in charge I would be very irresponsible to suggest anything other than what is written in the flight manual. There are too many variables to consider - follow the checklist is the only advice I can give here.

Hope that helps!!
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Old 24th Jun 2013, 03:46   #13 (permalink)
 
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Originally Posted by MX Trainer View Post
Everyone

The differences between the DHC-6 TwinOtter and the Beech could just be in the certification process - DHC had to be certified to the Canadian standards while the Beech was certified to the USA standards.

Also the TwinOtter is missing the Weight On Wheels system - many aircraft spend a good portion of their life on floats - so possibly this is the reason for the different interconnects that needed to be made before allowing the power lever into reverse. IIRC the TwinOtter had 3 things that had to happen - prop lever fully forward, flaps selected and in the approach position, and activation of the twist grip to get the power lever below the flight idle position. It is easy to get into reverse in the air with the TwinOtter - and no it is NOT APPROVED!!!
Thank s Mx. Could be that the Canadian certification authorities insisted on some sort of blocking mechanism concerning propeller lever position and reverse capability. Whatever the reason, I think it is a safety hazard based on the STOL type work done by the DHC-6.

As for flap position having anything to do with reverse on the DHC-6....I don't think there is any relation between the two as can be seen in this cool video of a Twin Otter going backwards with the flaps up.

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Old 24th Jun 2013, 14:56   #14 (permalink)
 
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Flaps and Reverse on DHC-6

JammedStab

The old 100 and 200 series aircraft that did not have Mod 6-1223 had the flap switches that were interconnected to the "Reset Props Lights". Without the flaps down the light would be on even if the other electrical conditions were met - but there was no preventative system installed to physically prevent the movement of the power lever into reverse. Just the "Reset Props Light. The switch was rigged to actuate at 7 1/2 deg. flap down. With the Mod the switches could be removed which left just the propeller control switches operating the lights.

Most aircraft have this system removed and actually the only place that I remember it being described is in TAB (Technical Advisory Bulletin) 610/2.

Quote - DHC TAB 610/2 - Section 31 - Page 1
""Mandatory Mod 6-1223 provides an interlock witch makes impossible the selection of reverse with propeller levers set below 95% . With Mod 6-1223 incorporated, functioning Reset Props light cautions the pilot that "Reversing" will be impossible, unless propeller levers are advanced to Max RPM setting.""

Old airplanes often have lots of history - and some of us helped make that history. About 1975 or 76 I was right seat in the -6 on a Medevac run in the dark of night at about -30 Deg F. On the takeoff run with the nurse and patient on board we had an uncommanded right engine auto-feather right at liftoff. Severe "Black Hole" effect and there was some pretty fancy hand and foot work in the cockpit. We clipped the trees with the landing gear on the way out - straightened everything out and flew back to base. We were just one of a few that this had happened to - DHC solution was an upgraded autofeather system relay - so no one else has this issue.

The constant progression of aircraft reliability is often a very direct function of feedback from incidents like this.

There are reasons for the operation of the aircraft with adherence to the Flight Manual - Reducing noise or a short field landing that is not within the normal operation limits published for the aircraft are not acceptable reasons to deviate from the approved procedures. If the checklist calls for "Props Max RPM" on the approach then that is where they should be. Operating the aircraft outside of the requirements will bite your a$$ at some point. You might think this system is a hazard - and you should voice your opinion about it - but do it to someone who can do something about it - the manufacturer!!!
Write them with all the details of what you have for concerns - and expect that they will stand behind their published procedures - those procedures cost them a lot of money and were done by men better able to access the dangers than I.

Don't know about you because for all I know "You are a certified "Test Pilot".

(No I am not being sarcastic - I really don't know what your qualifications are.)

Hope this helps a bit.
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