PT6A-67B/P (PC-12) Questions
06-20-2016, 06:24 AM
#11
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Joined APC: Feb 2007
Posts: 461
Originally Posted by PT6driver
It's my understanding that this engine was derated from 1,600SHP to 1,200SHP (5 minute limitation) and then to 1,000SHP continuous.
How was this derating achieved? ITT limit? TQ limit? Both?
I ask because some of my coworkers run the engine well below it's maximum continuous ITT limit for climbs and cruise and I just don't understand the justification behind it other than, "we only have one so we'll take it easy" and, "this is the way "everyone" else runs it."
In the Series 9, Max. Continuous ITT is 760*C. We run it at 720*C.
In the NG, Max. Continuous is 820*C. We run it at 780*C.
It's also my understanding that Pratt & Whitney says you don't need to observe any specific TQ values in cruise so long as you stay under the Max. Continuous ITT. Pilatus provides a maximum TQ chart in cruise and have heard this is only a CYA move by Pilatus for liability reasons. I run the engine exactly as I've been taught but I just don't understand the "why" aspect of it all.
Why are we being taught to be easy on an engine that is already derated? I don't do well with "just run it this way." I need to understand the reasoning behind it.
How was this derating achieved? ITT limit? TQ limit? Both?
I ask because some of my coworkers run the engine well below it's maximum continuous ITT limit for climbs and cruise and I just don't understand the justification behind it other than, "we only have one so we'll take it easy" and, "this is the way "everyone" else runs it."
In the Series 9, Max. Continuous ITT is 760*C. We run it at 720*C.
In the NG, Max. Continuous is 820*C. We run it at 780*C.
It's also my understanding that Pratt & Whitney says you don't need to observe any specific TQ values in cruise so long as you stay under the Max. Continuous ITT. Pilatus provides a maximum TQ chart in cruise and have heard this is only a CYA move by Pilatus for liability reasons. I run the engine exactly as I've been taught but I just don't understand the "why" aspect of it all.
Why are we being taught to be easy on an engine that is already derated? I don't do well with "just run it this way." I need to understand the reasoning behind it.
Reduces turbine erosion
The PT6-67P on the NG is rated to 1200hp all the way up due to the extra temperature margin.
(also has that nifty 2nd 300 amp alternator... and split electrical system on the later models)
It is possible that the lower temperatures are a precaution because... you know... one engine.
06-23-2016, 09:25 PM
#12
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Joined APC: Feb 2015
Position: Left seat bizjet
Posts: 293
We have a 45 and a 47E and run the same temps as you do. As I understand it running hotter means more blade erosion and a more expensive hot section inspection. We had BE99 engines at a past operator I flew for and those things sometimes would barely make required tq at max N1 and would temp out early. I was told it had to do with worn turbine blades and pilots abusing the hell out of them.
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06-24-2016, 08:55 AM
#13
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Joined APC: Apr 2009
Position: A-320 FO
Posts: 693
PT-6 fail:
Many operators run their PT-6's at less than maximum ITT (T5) for a number of different reasons. Some are newer operators, who are looking
to increase tbo times. Others, as mentioned, are attempting to take some of the sting out of a hot section replacement.
What everyone failed to remember here, is the simple fact that there is a reduction gear box connected to this thing, hence the maximum continuous horsepower ratings. If you have ever seen one come apart, it's uncontained, and parts will wander through the fuselage.
to increase tbo times. Others, as mentioned, are attempting to take some of the sting out of a hot section replacement.
What everyone failed to remember here, is the simple fact that there is a reduction gear box connected to this thing, hence the maximum continuous horsepower ratings. If you have ever seen one come apart, it's uncontained, and parts will wander through the fuselage.
06-24-2016, 10:16 AM
#14
Disinterested Third Party
Joined APC: Jun 2012
Posts: 6,023
Originally Posted by PT6driver
It's also my understanding that Pratt & Whitney says you don't need to observe any specific TQ values in cruise so long as you stay under the Max. Continuous ITT.
Torque is a limiting value, as is temperature.
In many cases, turbine installations have torque limits that aren't due to the engine, but for the airframe.
Torque is usually a limiting factors at lower altitudes and cooler days, and temp at higher altitudes and hotter days.
If you hit a torque limit or a temp limit first, you stop. Some days you're torque limited, others temp limited, depending on the conditions and location, for departure, and nearly always temp limited in cruise at higher altitudes.
Be careful about the rumors and wives tales that you'll her about Pratt said this or that...I recently talked to a pilot who was convinced that while he had a five minute allowance, he could simply back off to a lower temperature and then go back to the five minute overage again...and again...and again. His reasoning was that it was five minutes at a time, and that so long as he only did five minutes, he could keep doing it. Not so.
06-28-2016, 08:52 AM
#15
Line Holder
Thread Starter
Joined APC: Jun 2016
Posts: 28
Originally Posted by clipperskipper
What everyone failed to remember here, is the simple fact that there is a reduction gear box connected to this thing, hence the maximum continuous horsepower ratings. If you have ever seen one come apart, it's uncontained, and parts will wander through the fuselage.
Originally Posted by JohnBurke
Who told you that?
Torque is a limiting value, as is temperature.
In many cases, turbine installations have torque limits that aren't due to the engine, but for the airframe.
Torque is usually a limiting factors at lower altitudes and cooler days, and temp at higher altitudes and hotter days.
If you hit a torque limit or a temp limit first, you stop. Some days you're torque limited, others temp limited, depending on the conditions and location, for departure, and nearly always temp limited in cruise at higher altitudes.
Be careful about the rumors and wives tales that you'll her about Pratt said this or that...I recently talked to a pilot who was convinced that while he had a five minute allowance, he could simply back off to a lower temperature and then go back to the five minute overage again...and again...and again. His reasoning was that it was five minutes at a time, and that so long as he only did five minutes, he could keep doing it. Not so.
Torque is a limiting value, as is temperature.
In many cases, turbine installations have torque limits that aren't due to the engine, but for the airframe.
Torque is usually a limiting factors at lower altitudes and cooler days, and temp at higher altitudes and hotter days.
If you hit a torque limit or a temp limit first, you stop. Some days you're torque limited, others temp limited, depending on the conditions and location, for departure, and nearly always temp limited in cruise at higher altitudes.
Be careful about the rumors and wives tales that you'll her about Pratt said this or that...I recently talked to a pilot who was convinced that while he had a five minute allowance, he could simply back off to a lower temperature and then go back to the five minute overage again...and again...and again. His reasoning was that it was five minutes at a time, and that so long as he only did five minutes, he could keep doing it. Not so.
As I've said, I was just looking to find the reasoning behind the operation. Platus's POH says one thing, we operate another way, and then Finoff Aviation's calculator has their own numbers. (seen here: Free Pilatus PC-12 Power Calculator from Finnoff Aviation). Just making sense of it all.
The unused range from our operating ITT to the Max. Continuous ITT is just mainly for longevity and ease of operation on the engine. It won't kick itself out of the airframe for running there for however long necessary assuming we're also under the TQ limitation.
07-01-2016, 09:50 AM
#16
Disinterested Third Party
Joined APC: Jun 2012
Posts: 6,023
Engine manufacturers will establish operating parameters for their product, but once put into an aircraft, the airframe manufacturer may establish additional limitations which will be more conservative than those of the engine manufacturer. When mated with a particular propeller (in the case of a turboprop), additional limitations may apply, which are typically either torque or RPM limits (or caution bands), and wind limits during ground operations. Operators frequently place additional limitations on the way the aircraft will be used or operated.
When I'm running a PT6A-67, I'm usually carrying 800 ITT continuously during takeoff and climb, and sometimes for a good portion of the flight, and I'll return to it frequently when maneuvering. In cruise I back off if able, but there I run torque and fuel flow.
When I'm running a PT6A-67, I'm usually carrying 800 ITT continuously during takeoff and climb, and sometimes for a good portion of the flight, and I'll return to it frequently when maneuvering. In cruise I back off if able, but there I run torque and fuel flow.
07-16-2016, 02:32 PM
#17
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Joined APC: Jul 2008
Position: 747 Captain (Ret,)
Posts: 804
Other possible reasons that the airframe manufacturer may have established lower engine torque operating limits than the engine manufacturer is that the rudder, vertical stabilizer, and engine mounts may not be stressed to take the extra torque of full available engine power, or you may have controllability issues with not enough rudder authority at low airspeed.
Many WWII single engine fighters would torque roll uncontrollably if the pilot applied immediate max power during a slow speed go around.
Joe
Many WWII single engine fighters would torque roll uncontrollably if the pilot applied immediate max power during a slow speed go around.
Joe
Last edited by joepilot; 07-16-2016 at 02:33 PM. Reason: clarity
07-16-2016, 08:23 PM
#18
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Joined APC: Jun 2012
Posts: 6,023
Torque doesn't affect the rudder and vertical stab; one isn't going to overtorque them.
Excess torque at low speeds does pose controllability issues in some aircraft, but this discussion is about cruise torque limitations or recommendations. Manufacturers pose "nominal" values for cruise as means of limiting long term thermal exposure for the engine, which helps limit creep and blade erosion.
The modern Air Tractor 802 has approximately the same weight and power as the P-51, but the P-51 experienced very different effects at low speed with rapid power application; that was a matter of a small rudder and an airplane designed to go fast, not slow. It was subject to a torque roll in which rapid power application resulted in yaw and roll in excess of available control authority to counteract. The solution; feed the power in more slowly. This was not, however a torque limitation, but a pilot technique. The 802, by comparison, at the weight and power, doesn't exhibit that characteristic.
The PT6A-67AG has a nominal cruise number of 720 degrees, but for climb, and at cruise, and at any time needed, it can be run indefinitely at 800 degrees, and often is when heavy.
Airframe manufacturers set torque limits for the structural integrity of the airframe, which includes the engine mount assembly and primarily the attach points at the firewall.
As an example, an airplane which has used radial engines with a power limitation may be retrofitted with a turbine engine capable of a much greater power output, but may be limited to the power of the original radial engine installation due to structural limitations. The engine can do more, but the airframe not necessarily so.
In cruise, the issue of rudder size really doesn't have an impact from torque and power settings, especially when talking reduced nominal cruise settings.
Reduced nominal cruise are strictly to "save the engine."
Excess torque at low speeds does pose controllability issues in some aircraft, but this discussion is about cruise torque limitations or recommendations. Manufacturers pose "nominal" values for cruise as means of limiting long term thermal exposure for the engine, which helps limit creep and blade erosion.
The modern Air Tractor 802 has approximately the same weight and power as the P-51, but the P-51 experienced very different effects at low speed with rapid power application; that was a matter of a small rudder and an airplane designed to go fast, not slow. It was subject to a torque roll in which rapid power application resulted in yaw and roll in excess of available control authority to counteract. The solution; feed the power in more slowly. This was not, however a torque limitation, but a pilot technique. The 802, by comparison, at the weight and power, doesn't exhibit that characteristic.
The PT6A-67AG has a nominal cruise number of 720 degrees, but for climb, and at cruise, and at any time needed, it can be run indefinitely at 800 degrees, and often is when heavy.
Airframe manufacturers set torque limits for the structural integrity of the airframe, which includes the engine mount assembly and primarily the attach points at the firewall.
As an example, an airplane which has used radial engines with a power limitation may be retrofitted with a turbine engine capable of a much greater power output, but may be limited to the power of the original radial engine installation due to structural limitations. The engine can do more, but the airframe not necessarily so.
In cruise, the issue of rudder size really doesn't have an impact from torque and power settings, especially when talking reduced nominal cruise settings.
Reduced nominal cruise are strictly to "save the engine."
11-13-2016, 08:25 AM
#19
IflyApanes
Joined APC: Apr 2008
Position: Former Air Ambulance, Flt Inst, Check Airman, Current CRJ CA
Posts: 187
720 ITT is a recommended climb temp in the PC12 PIM, Normal Procedures section 4.11 "Climb", Also Flight Safety teaches this habit. They say the recommendation comes from the manufacturer but I can't find any evidence of that.
Like said before. Has to do with taking care of the engine. We had a catastrophic engine failure in a TBM and had to dead stick it 60 miles for a safe landing. You DON'T want that to happen to you. You have one engine, take care of it.
Like said before. Has to do with taking care of the engine. We had a catastrophic engine failure in a TBM and had to dead stick it 60 miles for a safe landing. You DON'T want that to happen to you. You have one engine, take care of it.
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