fspro
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Full Power - Piston vs Turboprop or Jet

Sun Jun 03, 2018 12:52 am

Why is it that in piston aircraft, full throttle is used commonly for takeoff and climb, but when flying a jet powered aircraft (and I think turboprop as well,) you never use full throttle except in emergencies?
 
BoeingGuy
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Re: Full Power - Piston vs Turboprop or Jet

Sun Jun 03, 2018 1:23 am

fspro wrote:
Why is it that in piston aircraft, full throttle is used commonly for takeoff and climb, but when flying a jet powered aircraft (and I think turboprop as well,) you never use full throttle except in emergencies?


That isn’t correct. Modern day jets often use a derate takeoff thrust to save engine wear and maintenance cost by keeping EGT lower. However, it is not accurate to say they never use full rated thrust for takeoff. When performance requires it or the runway is contaminated, they will use full rated thrust.
 
fspro
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Re: Full Power - Piston vs Turboprop or Jet

Sun Jun 03, 2018 1:41 am

Does "full rated thrust" mean that the thrust levers are all the way forward?
 
GalaxyFlyer
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Re: Full Power - Piston vs Turboprop or Jet

Sun Jun 03, 2018 2:30 am

Depends, if the engines are FADEC controlled or not. Without FADECs, the throttles will be wherever required to produce the calculated thrust, which will normally not full forward.

GF
 
Max Q
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Re: Full Power - Piston vs Turboprop or Jet

Sun Jun 03, 2018 4:04 am

Question hasn’t been answered completely though


Piston engine aircraft generally don’t have significant power to spare, I can’t speak for the biggest engines on the ultimate piston airliners like the Dc7 / Constellation
etc but on smaller light aircraft there’s also no benefit in terms of wear and tear by using a reduced power setting on take off, plus the manufacturer doesn’t provide charts for such settings and finally once again there’s just little power to spare



For example on a light twin, say a Beech Baron if you have an engine failure after take off at maximum weight you will barely be able to climb at all, you can expect around a 300 fpm rate at best, and that’s if you do everything just right


All these calculations are based on a full power take off, remember this as well, high elevation/ temperature will also increase density altitude so you’re losing some power anyway



Jets have so much reserve thrust that if one fails after V1 when using a reduced setting you can continue the take off without needing to increase thrust on the remaining engine(s)
The best contribution to safety is a competent Pilot.
 
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kitplane01
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Re: Full Power - Piston vs Turboprop or Jet

Sun Jun 03, 2018 5:50 am

fspro wrote:
Why is it that in piston aircraft, full throttle is used commonly for takeoff and climb, but when flying a jet powered aircraft (and I think turboprop as well,) you never use full throttle except in emergencies?



Lets assume you want full thrust.

On a large turbocharged piston engine, throttles all the way forward might be *more than* full thrust. It might exceed some limit. So one would advance the throttles until some limit (manifold pressure, exhaust temp, rpm) was reached.

All of the above is also true with early jet and early turbine engines.

In a more modern engine, there are computers (FADEC) between the throttle and the actual engine. On those engines, one can advance the throttle to the physical limit, and expect the computer (FADEC) to run the engine up to some limiting factor (max temp, rpm, torque, etc), but no more.

The difference is not between piston and jet, it's between FADEC and manual.

Also, as some pointed out above, it's become customary to take off with reduced thrust where doing so will reduce future maintenance, and is allowed by conditions.
 
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TripleDelta
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Re: Full Power - Piston vs Turboprop or Jet

Sun Jun 03, 2018 7:12 am

fspro wrote:
Does "full rated thrust" mean that the thrust levers are all the way forward?


On FADEC controlled engines, the position of the power/thrust levers is not always an indication of the actual power/thrust produced. Aircraft equipped with these engines generally have several predefined positions on the levers' travel range where engine control is handed over fully to the FADEC - which then automatically varies engine parameters to give the maximum desired power without any crew intervention.

A practical example: the power levers on the Dash 8 Q400 have a "Rating Detent". It is a small gate at about 80% of the levers' full travel range. Whenever you put the power levers into that position, you tell the FADEC that you want rated power. On take-off, the default is Normal Take-Off Power (NTOP), which gives 90% of maximum rated torque. However, if you need more (and the particular airplane is so equipped), you can press a button forward of the throttle quadrant to get what's called Maximum Take-Off Power (MTOP) and get up to 106% torque. Likewise, if you need less power, you can push another button and get Reduced Take-Off Power (RTOP), which can be between 80 and 88% torque.

So, you have a take-off power range of between 80 and 106% torque (roughly between 4050 and 5050 HP) without ever moving the power levers out of their single detent position.

However, should you move the levers past the Rating Detent and into what's called the "overtravel range", you get Emergency Power (up to 125% torque). However, this also disables virtually all of the engine protection functions, and requires a thorough engine inspection after use.
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BravoOne
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Re: Full Power - Piston vs Turboprop or Jet

Sun Jun 03, 2018 11:52 am

I'll throw in some big piston radial information just for grins. The R2800, R4360 and Curtiss Wright 3350 all used BMEP gauges (Brake Mean Effective Pressure), to measure the power being generated by the engines, This in turn was backed up by Manifold Pressure and RPM gauges. The engines had thrust loaded plates that when power was applied pushed them outward thus measure against a pressure plate and generating a specific pressure.

Check out the top row of gauges on this C6Br

http://www.airliners.net/photo/Red-Bull ... /2049936/L
 
BravoOne
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Re: Full Power - Piston vs Turboprop or Jet

Sun Jun 03, 2018 2:02 pm

Since reduced power takeoffs are pretty much the norm for domestic legs you might actually find yourself in a spot where a full power takeoff had not been accomplished for a couple of weeks, or more. Where I worked we did a lot of SAN / HNL trips and it was not uncommon to find out that MAX EPR was not quite achievable on a MAX power setting. We did not reject the takeoff, but did write it up as such in the logbook.
 
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Qantas94Heavy
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Re: Full Power - Piston vs Turboprop or Jet

Mon Jun 04, 2018 1:25 am

BravoOne wrote:
Since reduced power takeoffs are pretty much the norm for domestic legs you might actually find yourself in a spot where a full power takeoff had not been accomplished for a couple of weeks, or more. Where I worked we did a lot of SAN / HNL trips and it was not uncommon to find out that MAX EPR was not quite achievable on a MAX power setting. We did not reject the takeoff, but did write it up as such in the logbook.


Are aircraft required to do a full power takeoff every so often to check this, or is it possible an aircraft could never use its full capabilities after initial testing?
 
Okcflyer
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Re: Full Power - Piston vs Turboprop or Jet

Mon Jun 04, 2018 2:29 am

Qantas94Heavy wrote:
BravoOne wrote:
Since reduced power takeoffs are pretty much the norm for domestic legs you might actually find yourself in a spot where a full power takeoff had not been accomplished for a couple of weeks, or more. Where I worked we did a lot of SAN / HNL trips and it was not uncommon to find out that MAX EPR was not quite achievable on a MAX power setting. We did not reject the takeoff, but did write it up as such in the logbook.


Are aircraft required to do a full power takeoff every so often to check this, or is it possible an aircraft could never use its full capabilities after initial testing?


Climb thrust usually isn’t derated. Although net thrust isn’t the same as take off, core power is essentially at max continuous once you’re a couple thousand feet off the ground. Degrading performance should be visible to cockpit during these phases. Not that it matters that much, engines monitor performance and remotely self report to Maintenance. They’ll know small changes long before it’ll be visible in the cockpit.
 
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Starlionblue
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Re: Full Power - Piston vs Turboprop or Jet

Mon Jun 04, 2018 3:41 am

Okcflyer wrote:
Qantas94Heavy wrote:
BravoOne wrote:
Since reduced power takeoffs are pretty much the norm for domestic legs you might actually find yourself in a spot where a full power takeoff had not been accomplished for a couple of weeks, or more. Where I worked we did a lot of SAN / HNL trips and it was not uncommon to find out that MAX EPR was not quite achievable on a MAX power setting. We did not reject the takeoff, but did write it up as such in the logbook.


Are aircraft required to do a full power takeoff every so often to check this, or is it possible an aircraft could never use its full capabilities after initial testing?


Climb thrust usually isn’t derated. Although net thrust isn’t the same as take off, core power is essentially at max continuous once you’re a couple thousand feet off the ground. Degrading performance should be visible to cockpit during these phases. Not that it matters that much, engines monitor performance and remotely self report to Maintenance. They’ll know small changes long before it’ll be visible in the cockpit.


I can only speak for our operation but we use derated climb thrust in most cases.

TripleDelta wrote:
fspro wrote:
Does "full rated thrust" mean that the thrust levers are all the way forward?


On FADEC controlled engines, the position of the power/thrust levers is not always an indication of the actual power/thrust produced. Aircraft equipped with these engines generally have several predefined positions on the levers' travel range where engine control is handed over fully to the FADEC - which then automatically varies engine parameters to give the maximum desired power without any crew intervention.

A practical example: the power levers on the Dash 8 Q400 have a "Rating Detent". It is a small gate at about 80% of the levers' full travel range. Whenever you put the power levers into that position, you tell the FADEC that you want rated power. On take-off, the default is Normal Take-Off Power (NTOP), which gives 90% of maximum rated torque. However, if you need more (and the particular airplane is so equipped), you can press a button forward of the throttle quadrant to get what's called Maximum Take-Off Power (MTOP) and get up to 106% torque. Likewise, if you need less power, you can push another button and get Reduced Take-Off Power (RTOP), which can be between 80 and 88% torque.

So, you have a take-off power range of between 80 and 106% torque (roughly between 4050 and 5050 HP) without ever moving the power levers out of their single detent position.

However, should you move the levers past the Rating Detent and into what's called the "overtravel range", you get Emergency Power (up to 125% torque). However, this also disables virtually all of the engine protection functions, and requires a thorough engine inspection after use.



The Airbus thrust levers also have detents which do not always indicate thrust level. The thrust levers will typically stay in the CLB detent from acceleration height to flare, with thrust varying based on autothrust commands.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
BravoOne
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Re: Full Power - Piston vs Turboprop or Jet

Mon Jun 04, 2018 9:15 am

Okcflyer wrote:
Qantas94Heavy wrote:
BravoOne wrote:
Since reduced power takeoffs are pretty much the norm for domestic legs you might actually find yourself in a spot where a full power takeoff had not been accomplished for a couple of weeks, or more. Where I worked we did a lot of SAN / HNL trips and it was not uncommon to find out that MAX EPR was not quite achievable on a MAX power setting. We did not reject the takeoff, but did write it up as such in the logbook.


Are aircraft required to do a full power takeoff every so often to check this, or is it possible an aircraft could never use its full capabilities after initial testing?


Climb thrust usually isn’t derated. Although net thrust isn’t the same as take off, core power is essentially at max continuous once you’re a couple thousand feet off the ground. Degrading performance should be visible to cockpit during these phases. Not that it matters that much, engines monitor performance and remotely self report to Maintenance. They’ll know small changes long before it’ll be visible in the cockpit.


That is simply wrong.
 
mmo
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Re: Full Power - Piston vs Turboprop or Jet

Mon Jun 04, 2018 9:16 am

Okcflyer wrote:

Climb thrust usually isn’t derated. Although net thrust isn’t the same as take off, core power is essentially at max continuous once you’re a couple thousand feet off the ground. Degrading performance should be visible to cockpit during these phases. Not that it matters that much, engines monitor performance and remotely self report to Maintenance. They’ll know small changes long before it’ll be visible in the cockpit.


Not sure where you are getting your information from, but, it is wrong. The logic on the FMS for both Boeing and Airbus is pretty similar. If you do a reduced thrust or assumed temp takeoff, your initial climb will be with the same reduction. As you climb, both Boeing and Airbus blend the thrust so passing a certain altitude you will be climbing in full climb thrust. The reason being is a full thrust climb will result in substantial fuel savings and relatively increased maintenance.

Depending on the governing authority, the airline is required to do a full thrust takeoff within a certain period of time. One airline I worked at, the first takeoff of the first day of the new month was a full thrust takeoff and there had to be a logbook notation confirming it was done satisfactorily.
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Starlionblue
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Re: Full Power - Piston vs Turboprop or Jet

Mon Jun 04, 2018 9:49 am

On the 330 and 350, we typically use assumed temperature thrust to acceleration altitude, and after that derated climb thrust. Full climb thrust normally only if required by atmospheric conditions, SID constraints and so on.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
mmo
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Re: Full Power - Piston vs Turboprop or Jet

Mon Jun 04, 2018 4:26 pm

Starlionblue wrote:
On the 330 and 350, we typically use assumed temperature thrust to acceleration altitude, and after that derated climb thrust. Full climb thrust normally only if required by atmospheric conditions, SID constraints and so on.


I think you will find, once passing FL200 or so, there will be little if any difference in full climb and derated climb thrust.
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BoeingGuy
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Re: Full Power - Piston vs Turboprop or Jet

Mon Jun 04, 2018 9:52 pm

mmo wrote:
Okcflyer wrote:

Climb thrust usually isn’t derated. Although net thrust isn’t the same as take off, core power is essentially at max continuous once you’re a couple thousand feet off the ground. Degrading performance should be visible to cockpit during these phases. Not that it matters that much, engines monitor performance and remotely self report to Maintenance. They’ll know small changes long before it’ll be visible in the cockpit.


Not sure where you are getting your information from, but, it is wrong. The logic on the FMS for both Boeing and Airbus is pretty similar. If you do a reduced thrust or assumed temp takeoff, your initial climb will be with the same reduction. As you climb, both Boeing and Airbus blend the thrust so passing a certain altitude you will be climbing in full climb thrust. The reason being is a full thrust climb will result in substantial fuel savings and relatively increased maintenance.

Depending on the governing authority, the airline is required to do a full thrust takeoff within a certain period of time. One airline I worked at, the first takeoff of the first day of the new month was a full thrust takeoff and there had to be a logbook notation confirming it was done satisfactorily.


This isn't totally correct either.

The initial climb very well may not be the same reduction as takeoff. In fact, on the 757 and 767 it probably most often is not assuming they usually do an Assumed Temperature Derate.

On Boeing airplanes, Assumed Temperate Derates are baseline for Takeoff. Fixed Derates for Climb are baseline. Fixed Derates for Takeoff are optional on the 757 and 767; and baseline on other models.

On the 757 and 767, you cannot mix and match fixed derates (if equipped). If you do full TO, you get full CLB. If you do TO 1 (usually 10% derate), you get CLB 1. If you do TO 2 (usually 20% derate), you get CLB 2.

On the 757 and 767, if you do an Assumed Temperate Derate, you get full CLB. You cannot select CLB 1 or CLB 2. So you do not get the same relative derate.

On the 777 and 787, you can mix and match derates. If you select TO, TO 1, or TO 2, the system will select the same corresponding climb thrust by default, CLB, CLB 1, or CLB 2 respectively. However, you can select a different Climb Thrust in any combination. For example, you can have TO and CLB 2, or TO 2 and CLB 1.

On the 777 and 787, if you select an Assumed Temperature Derate the system will default to the Climb Thrust - CLB, CLB 1, or CLB 2 - that most closely approximates the Takeoff Assumed Temp Derate. But again, you can select a different Climb derate if you wish.

To make matters more complicated, you can select an Assumed Temperature Takeoff Derate on top of a Fixed Derate for takeoff.

I don't know the 747 and 737 as well, but I understand they work the same as I described for the 777 and 787.

The comment about Climb Thrust blending is correct for Boeing airplanes. It's usually called the "Climb Derate Washout". Baseline is the Climb Derate (if one is selected) washes out linearly to full Climb Thrust between 10,000 - 12,000 feet. There is an option to wash it out slower from 10,000 to 30,000 feet. I've heard one engine manufacturer call this the "Fast Taper" or "Slow Taper".
 
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Starlionblue
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Re: Full Power - Piston vs Turboprop or Jet

Tue Jun 05, 2018 3:20 am

mmo wrote:
Starlionblue wrote:
On the 330 and 350, we typically use assumed temperature thrust to acceleration altitude, and after that derated climb thrust. Full climb thrust normally only if required by atmospheric conditions, SID constraints and so on.


I think you will find, once passing FL200 or so, there will be little if any difference in full climb and derated climb thrust.


True, but it's a long way from acceleration height to FL200.
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GalaxyFlyer
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Re: Full Power - Piston vs Turboprop or Jet

Tue Jun 05, 2018 3:21 am

To be pedantic, it’s ATM Reduced Thrust, not a derate. Reduced and derate are very diffferent animals.

GF
 
BoeingGuy
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Re: Full Power - Piston vs Turboprop or Jet

Tue Jun 05, 2018 5:11 am

GalaxyFlyer wrote:
To be pedantic, it’s ATM Reduced Thrust, not a derate. Reduced and derate are very diffferent animals.

GF


Correct. There are significant differences between them, as I’m quite sure you are familiar with. We tend to use the term “derate” colloquially for ATM also, so I did so in my last post.
 
mmo
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Re: Full Power - Piston vs Turboprop or Jet

Tue Jun 05, 2018 5:54 am

BoeingGuy wrote:
This isn't totally correct either.

The initial climb very well may not be the same reduction as takeoff. In fact, on the 757 and 767 it probably most often is not assuming they usually do an Assumed Temperature Derate.

On Boeing airplanes, Assumed Temperate Derates are baseline for Takeoff. Fixed Derates for Climb are baseline. Fixed Derates for Takeoff are optional on the 757 and 767; and baseline on other models.

On the 757 and 767, you cannot mix and match fixed derates (if equipped). If you do full TO, you get full CLB. If you do TO 1 (usually 10% derate), you get CLB 1. If you do TO 2 (usually 20% derate), you get CLB 2.

On the 757 and 767, if you do an Assumed Temperate Derate, you get full CLB. You cannot select CLB 1 or CLB 2. So you do not get the same relative derate.

On the 777 and 787, you can mix and match derates. If you select TO, TO 1, or TO 2, the system will select the same corresponding climb thrust by default, CLB, CLB 1, or CLB 2 respectively. However, you can select a different Climb Thrust in any combination. For example, you can have TO and CLB 2, or TO 2 and CLB 1.

On the 777 and 787, if you select an Assumed Temperature Derate the system will default to the Climb Thrust - CLB, CLB 1, or CLB 2 - that most closely approximates the Takeoff Assumed Temp Derate. But again, you can select a different Climb derate if you wish.

To make matters more complicated, you can select an Assumed Temperature Takeoff Derate on top of a Fixed Derate for takeoff.

I don't know the 747 and 737 as well, but I understand they work the same as I described for the 777 and 787.

The comment about Climb Thrust blending is correct for Boeing airplanes. It's usually called the "Climb Derate Washout". Baseline is the Climb Derate (if one is selected) washes out linearly to full Climb Thrust between 10,000 - 12,000 feet. There is an option to wash it out slower from 10,000 to 30,000 feet. I've heard one engine manufacturer call this the "Fast Taper" or "Slow Taper".


TBH, I hadn't even thought about the 757/767 as it's been over 20 years since I was in the pointy end of the 757 (never flew the 767). But your last paragraph was what I was referring to with respect to climb thrust. On all the PW4000 400s I've flown they seem to begin the taper just after 10,000 and have it complete by FL200. The 320 appears to be the same on both the IAE and the CFM.
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