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convair880mfan
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Posts: 213
Joined: Fri Jun 25, 2021 12:33 am

Thrust limits for derated jet engine?

Tue Aug 03, 2021 7:04 pm

I don't know how to put this question since I lack the technical vocabulary but I hope I can make myself understandable.

McDonnell Douglas produced a DC-10 product for two Mexican airlines called the DC-10-15. It was designed for "hot and high" operations and tailored to Mexico City operations. It was basically a DC-10-10 with more powerful engines that were derated.

So my question is this. If a, say, 50,000 pound thrust engine is derated to say, 45,000 pounds of thrust for takeoff, can a pilot get the full 50,000 pounds of thrust in a pinch or is it mechanically impossible? Or if mechanically possible, is it inadvisable or dangerous?

My apologies if I have phrased any of this incorrectly.
 
LH707330
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Joined: Fri Jun 15, 2012 11:27 pm

Re: Thrust limits for derated jet engine?

Tue Aug 03, 2021 8:14 pm

Short answer is " it depends," on a couple of variables:
1. If it's mechanically constrained (maybe the DC-10-15 engine was, I don't know), then it wouldn't be adjustable and in a pinch you'd get pinched. I'm not sure why that D10 model was limited to 45k #, so I couldn't tell you the specifics. Maybe it was flat-rated or something.
2. If it's a software-controlled limit, many types have an override for TOGA or OEI conditions that gives you the full thrust that the engine is capable of, i.e. bypassing the normal thrust limit
3. Some types (think A318/736) have shorter tail lever arms, so the max allowable thrust for those is constrained for VMCa and VMCg reasons. So you may have (making up numbers) an engine that is capable of 28k # of thrust with a plug for 22k #, and a TOGA override that only allows 24k # so it doesn't go off the side of the runway.
4. Some types also have max thrust allowable for VMCg at low weights, so you're limited to x thrust/N1/EPR/some other parameter for either the whole takeoff roll or until you reach climb speed or some other threshold value at which point the software decides you're fast enough that there's enough rudder authority for an OEI case at more thrust.
 
CanukinUSA
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Joined: Sun Oct 25, 2020 5:06 pm

Re: Thrust limits for derated jet engine?

Wed Aug 04, 2021 2:15 am

It is not clear to me that you understand what derate means.
Every engine has been tested to determine the maximum thrust it can produce (typically on an engine test stand). This is usually more thrust than the engine is rated for due to Engine life and reliability considerations. Then taking in account any number of reasons the maximum rated thrust for certification of the engine is determined with the Aviation regulators. The reasons may be the maximum Mechanical/Pressure limits of the engine at certain lower temperatures (called flat rated) and is typically limited by internal temperatures usually in the turbine section of the engine at higher temperatures (called temperature limited).
Once the engine is installed on an aircraft there may be a lower rated thrust approved which is the rated thrust for that engine/aircraft combination. These limits are typically determined by aircraft structure, aircraft controllability, etc.). For most newer engines different thrust limits are available for a given aircraft and the airline customer may obtain thrust levels depending on what they negotiate with the engine manufacturer and pay for. This usually only involves a software change in the electronic engine control and is determined by a plug attached to the engine. Obviously, the engine manufacturer will charge you more for specific features (like thrust increases) if you want or need them. Other special features are things like thrust bumps that are available for special conditions like hot and high mentioned (i.e. for airports like Mexico City) or high temperatures at low altitudes (for airports like Dubai), etc. for airlines that need them and are willing to pay for them.
A derate is when you operate the engine at a lower percentage of its rated thrust during takeoff (high power) for better engine life. Typical derates are 90%, 80%, 70% and 60% of rated thrust. The airline again typically selects which derates it would like to purchase on its aircraft with the engine/aircraft manufacturer.
If an airline/flight crew decides to use derated thrust for takeoff they are limited to that thrust as the takeoff planning has been done for that derate thrust level. If you try to increase it you may cause issues with aircraft controllability etc. as your takeoff has been based on that derated thrust limit.
 
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Starlionblue
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Re: Thrust limits for derated jet engine?

Wed Aug 04, 2021 4:01 am

As discussed above, there are thrust limitations in the installation itself. These cannot be overridden by the pilots. We simply get up to the maximum "installed".

We typically use reduced thrust (that is, less than the "installed" max thrust) to reduce engine wear. Mostly this is in the form of flex/assumed-temp thrust. There is an important difference between the operational use of fixed derate thrust and flex/assumed-temp thrust.

Both fixed derate and flex/assumed-temp are reduced thrust settings. However, in general, derate is used on contaminated runways where VMC in an engine-out scenario is limiting. Therefore with a fixed derate, TOGA is not permitted below a certain speed (F speed on Airbus) because going to TOGA may exceed rudder authority. However, with flex/assumed-temp , TOGA may be selected at any time.

There is one exception to the "no TOGA" rule with derate, which is a windshear event. In that case, go TOGA because not doing so is more risky than the chance of an engine failure at that precise time.
 
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zeke
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Joined: Thu Dec 14, 2006 1:42 pm

Re: Thrust limits for derated jet engine?

Wed Aug 04, 2021 3:55 pm

convair880mfan wrote:
So my question is this. If a, say, 50,000 pound thrust engine is derated to say, 45,000 pounds of thrust for takeoff, can a pilot get the full 50,000 pounds of thrust in a pinch or is it mechanically impossible? Or if mechanically possible, is it inadvisable or dangerous?


Those older engines had mechanical thrust linkages, it would have been possible to exceed the normal thrust limits.

These days with more modern engines with FADEC if they thrust is rated at 45,000 lb that is all the engine will deliver, that thrust rating is normally specified at a temperature, e.g. 45,000 lb at 33 deg C, so it will deliver the 45,000 lb to an equivalent density height of 2100 ft. I know some engines these days will deliver their full rated thrust up to 8000 ft.
 
BoeingGuy
Posts: 6633
Joined: Fri Dec 10, 2010 6:01 pm

Re: Thrust limits for derated jet engine?

Sat Aug 07, 2021 5:26 am

Starlionblue wrote:
As discussed above, there are thrust limitations in the installation itself. These cannot be overridden by the pilots. We simply get up to the maximum "installed".

We typically use reduced thrust (that is, less than the "installed" max thrust) to reduce engine wear. Mostly this is in the form of flex/assumed-temp thrust. There is an important difference between the operational use of fixed derate thrust and flex/assumed-temp thrust.

Both fixed derate and flex/assumed-temp are reduced thrust settings. However, in general, derate is used on contaminated runways where VMC in an engine-out scenario is limiting. Therefore with a fixed derate, TOGA is not permitted below a certain speed (F speed on Airbus) because going to TOGA may exceed rudder authority. However, with flex/assumed-temp , TOGA may be selected at any time.

There is one exception to the "no TOGA" rule with derate, which is a windshear event. In that case, go TOGA because not doing so is more risky than the chance of an engine failure at that precise time.


Boeing also has selectable detates on most models, designated as TO 1 and TO 2. If a crew uses a derate, rather than an Assumed Temperature thrust reduction (what Airbus calls Flex Power) then it’s considered a limit for that takeoff for the same reasons you describe.

You can physically select TO/GA or manually push the thrust levers up to remove the derate and select maximum takeoff thrust. Similar to your windshear comment, Boeing manuals state that a derate thrust should only be exceeded if ground contact is imminent.

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