Gnomon
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PW4090: Why The Lower Mgtow?

Tue Nov 15, 2005 1:21 pm

I never knew, until recently, that 772ERs powered by the PW4090 are certified only to a 648k max gross T/O weight, instead of the standard 656k.

I noticed this lower certified MGTOW carries with it a substantial decrease in range, from ~7,700 nm to a hair more than 5,900, if I understand things correctly. That really diminishes some PW 772ER operators' capabilities to use the aircraft on longer missions that some of their competitors are launching with GE- and RR-powered machines.

Given that the PW4090 produces roughly the same thrust (90k lbs) as the Trents and GEs that power higher-MGTOW 772ERs, why was the PW4090 subject to the reduced MGTOW in its certification? Is the reduced MGTOW simply a "paperwork" issue that could be modified if the operators so desired, or is there more to the lighter MGTOW than that?

(A search on both this forum and Civil Aviation turned up extensive discussions of some of UA's and KE's issues with the PW line, but nothing on why, exactly, the PW4090's certification won't support a higher gross weight.)

Thanks for the answers!
 
LAXintl
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RE: PW4090: Why The Lower Mgtow?

Tue Nov 15, 2005 3:15 pm

In recent years and particularly true with the 777, operators could purchase various "paper" weight options on Boeing aircraft. While the aircraft are basically structurally the same except some small variations like brakes the same aircraft in theory could be upgauged or downgauged in weights during its life.
The same somewhat goes with modern engines that get derated via restrictions in their software.

An operator generally chooses the lightest of the required weights as higher weights cost more money to acquire and often maintain.
I know of two operators where one purchased additional weight capability for their B757 fleet, while another operate derated and lowered weights on several B767s.

In addition depending on the country, overflight and landing charges are based on the aircraft max take off weights. Hence one reason you will see many low weight aircraft operated particularly with European airlines.

As to how this relates to your specific B777 question, its likely no operator specifically requested the weight and PW engine combo as the GE and RR operators have.
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Gnomon
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RE: PW4090: Why The Lower Mgtow?

Wed Nov 16, 2005 2:55 am

That's interesting, Laxintl. Thanks so much for the information.

I take it from your informative reply that the lighter MGTOW on the PW4090 birds is not so much a matter of a mechanical limitation on the engine itself and more a "paperwork" formality.

The example that comes most readily to mind is UAL, which ostensibly might need the additional range for its 772ER fleet at some point in the future (not to rehash multiple discussions about potential UAL DEL service over in other forum).

Could UAL -- or other carriers with PW4090s at lighter MGTOWs -- increase the range of those machines just by amending some paperwork and, if necessary, amending the on-board software?
 
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lightsaber
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RE: PW4090: Why The Lower Mgtow?

Wed Nov 16, 2005 5:09 am

Quoting Gnomon (Thread starter):
Given that the PW4090 produces roughly the same thrust (90k lbs) as the Trents and GEs that power higher-MGTOW 772ERs, why was the PW4090 subject to the reduced MGTOW in its certification?

Actually, not quite the same thrust. The Trent and GE-90 are available at slightly higher thrusts. While nominally the engine might produce the same thrust as the pw4090, during an engine out situation, the extra thrust (94k with the GE-90A IIRC, 92k for the Trent) makes a big difference in allowed MTOW.

The pw4098 was supposed to burn 2% less fuel than the pw4090. Instead, it burns 2% MORE fuel... (4% fuel burn miss). Thus, a 773 or a 772ER actually has more range with the pw4090 than the pw4098 despite the lower MTOW. Recall, the pw4098 core is also heavier than the pw4090 core. If the pw4098 had worked as planned, Pratt would have sold many more engines on the 773's. (To UA, JAL, ANA, KE, and possibly NW. Recall, northwest didn't like the economics of the 772 and the 773 doesn't do MSP-NRT with Pw4090's.)

Also, recall there is more in the MTOW than meets the eye. The GE-90A burns about 2% less fuel than the PW4090. So even though its a slightly heavier engine, for the same MTOW, the GE-90 will generally have a longer range even though its carrying less fuel.

And note that the Trent 895 is *much* lighter (2,000lbm per engine) than the pw4090! While the trent has the highest fuel burn on the 777, its really hard to overcome a 4,000 lbm (almost 2 metric tons) weight advantage. Weight savings=fuel savings.

The pw4098 fuel burn debacle was the first of three *major* strikes by pratt. (2nd was the pw4172 compressor vent overtemp, 3rd was the pw6022/6024 fuel burn miss). KE is the only operator of the pw4098 and is pissed with the higher fuel burn. While the MTOW allowed is impressive, KE bought those airframes to fly to Europe and the pw4098 fuel burn issue has really hurt the airframe economics.

Just to add a bit more: The pw4098 miss resulted in so many Pratt 777 customers already deciding to "jump ship" that there was no support for a follow on 777 Pratt engine.  cry  RR had a loyal following who was willing to launch a larger diameter Trent properly sized for 115k thrust. (Recall, the Trent on the 777 has the smallest diameter of all of the offered engines.) But GE demanded/paid for an exclusive on the high MTOW frames. If Pratt had made it onto the high MTOW 777 you would have seen a contra-Rotating engine. (A la F119)

Lightsaber
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LTU932
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RE: PW4090: Why The Lower Mgtow?

Sat Nov 04, 2006 8:43 am

Quoting Lightsaber (Reply 3):
KE is the only operator of the pw4098 and is pissed with the higher fuel burn. While the MTOW allowed is impressive, KE bought those airframes to fly to Europe and the pw4098 fuel burn issue has really hurt the airframe economics.

Isn't KE working with PW to get those engines down to PW4090 standard? I don't know in which topic it was mentioned though, but I know I read it here.
 
MarkC
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RE: PW4090: Why The Lower Mgtow?

Sat Nov 04, 2006 1:57 pm

Quoting LTU932 (Reply 4):
Isn't KE working with PW to get those engines down to PW4090 standard? I don't know in which topic it was mentioned though, but I know I read it here.

The 4098 is physically different than the 4090. It has an extra compressor stage. Those were not derated, they were exchanged outright over time. Its all done now. Some of the engines were dontated to museums. It was only 9 engines on 4 airplanes.

Hey Lightsaber, was PW really working on a bigger fan version? What was the fan dia to be? I'm sure the size of the core would have had to finally change.
 
brendows
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RE: PW4090: Why The Lower Mgtow?

Sun Nov 05, 2006 12:49 am

Hi folks!

As Lightsaber wrote, the PW4090-birds are limited because of less thrust in an engine out situation. But why are the thrust limit on the PW4090 lower? scratchchin 
The answers, thanks to Lightsaber, can be found in this good ol' post: More PW4090 Questions (by ConcordeBoy May 24 2005 in Tech Ops)

I quote:

Quoting Lightsaber (Reply 13):
The reason for the pw4090 thrust limit is simple: Emissions. The pw4090 at 90,000lbf of thrust puts out 99.2% of the allowed ICAO NOx emissions. This is the highest certified thrust that Pratt is allowed to sell on the market. You are not allowed to fly into/out of a FAA or JAA airport without meeting ICAO emissions. In other words, the pw4090 is certified for 91,700lbf of thrust in every criteria *except* emissions. But you're not allowed to miss even one certification criteria...

Thanks Lightsaber praise 

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