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Can You Re Engine A 757?

Posted: Mon Aug 02, 2004 10:42 am
by dc8jet
Suppose one of the airlines operating 737-300s wanted to sell them. Say NW was interested in them but all of the others have RR engines as opposed to NWs P&Ws. Would it be possible to re engine them with P&Ws? If so would it be worth it from a cost standpoint?

RE: Can You Re Engine A 757?

Posted: Mon Aug 02, 2004 11:18 am
by greasespot
Well lets say it this way. You could but the cost would be more than the whole airplane is worth.

The entire wing, Pylon, nacelle Bleed system, hydraulics, fuel,would have to be re designed and re-certified.

Since engines are not meant to be switched you would also have to change all the black boxes and engine controls.

Since this would be done after market you would also need a new STC to do it.

Plus you would need two engines.

Therefore not a economical proposition.
Greasespot


RE: Can You Re Engine A 757?

Posted: Mon Aug 02, 2004 12:02 pm
by citationjet
Like Grease said, it is possible, but not economical.
I worked at Boeing Wichita as a structures engineer when the CFM-56 were added to the KC-135 to make the KC-135R model.
It is a lot of work.
Some of the items required to certifiy a new engine on an airplane include: updated aircraft loads analysis (flight maneuver loads, dynamic gust loads, dynamic landing loads), updated flutter analysis, updated mass properties to support flutter analysis, ground vibration test (to validate dynamic model for flutter analysis), flight flutter testing (envelope expansion), fatigue analysis, flyover noise certification. Not to mention designing the hardware to accomodate the new engine.

RE: Can You Re Engine A 757?

Posted: Mon Aug 02, 2004 1:07 pm
by cdfmxtech
If I'm not mistaken, the Pratss for the B757s are FADEC. The RRs are mechanical.

RE: Can You Re Engine A 757?

Posted: Mon Aug 02, 2004 5:10 pm
by dl757md
A couple of years back when DL 757s were having bad problems with 10th/11th stage stator cracks there was talk of re-engining all 121 of DL's PW 757s with RR. The cost I believe was estimated at 12 million dollars per plane.

Dl757md

RE: Can You Re Engine A 757?

Posted: Mon Aug 02, 2004 9:30 pm
by AUAE
For the 757, the only structrual difference is the pylon. All the cost comes from rewiring, reconfiguring and recertification of the a/c. Oh, and the cost of the new engines.

Other aircraft may require more structual changes to the wing, which really make it expensive.

Shawn

RE: Can You Re Engine A 757?

Posted: Mon Aug 02, 2004 10:31 pm
by broke
Another kicker on re-engining the 757; P &WA engines rotate clockwise, RR engines rotate counterclockwise (anticlockwise in the UK).
Pylon problems (and there are some) that are related to the torque on the pylon and wing result in problems occurring on one side on a Pratt powered airplane and on the opposite side of the Rolls powered airplane.
Swapping engines would result in an airplane that has been torqued in both directions and would likely give you long term problems that would be unique to the modified airplanes.

RE: Can You Re Engine A 757?

Posted: Mon Aug 02, 2004 10:36 pm
by lapper
Am I right in saying that the engines for a 7E7 will be inter-changeable?

RE: Can You Re Engine A 757?

Posted: Mon Aug 02, 2004 11:44 pm
by LMP737
Lapper:

Yes you are correct. In an effort to keep costs down the 7E7 will have a common pylon that can mount both the RR or the GE engine.

RE: Can You Re Engine A 757?

Posted: Mon Aug 02, 2004 11:50 pm
by lapper
Do some of the problems that Greasepot and Citationjet mentioned occur, or are they all overcome with this enhancement?

Also, why was that never done before?

RE: Can You Re Engine A 757?

Posted: Tue Aug 03, 2004 3:05 am
by JeffDCA
Yes you are correct. In an effort to keep costs down the 7E7 will have a common pylon that can mount both the RR or the GE engine.

Not only that, Boeing are also looking at the ability to have 1 GENX and 1 Trent 1000 on the same aircraft, making it a lot easier for airlines when an engine goes tech, and they have one available from a different manufacturer.

Very good idea!

Cheers,

Jeff

RE: Can You Re Engine A 757?

Posted: Tue Aug 03, 2004 4:54 am
by LMP737
JeffDCA:

That will be interesting to see. An airline would have to be preety desperate to fit another type engine on it. Changing an engine twice costs money so my bet would be on them waiting for the same type engine to show up. But like I said before it would be fun to see it.  Smile

RE: Can You Re Engine A 757?

Posted: Tue Aug 03, 2004 5:03 am
by JeffDCA
LMP737,
I think the idea of it is that if a RR operator has an aircraft with a bust up engine somewhere and there's no replacement, but there's a GE operator at that airport with a spare GE engine, then rather than delay the flight and wait for a RR engine to be flown in, they can fit the GE and carry on with the flight.

It certainly would be fun to see!  Smile

Cheers,

Jeff

RE: Can You Re Engine A 757?

Posted: Tue Aug 03, 2004 5:07 am
by Thrust
Of course you can reengine, but that don't come cheap, cookie  Big grin  Laugh out loud

AA wanted to retrofit TWA's 757s with RR engines, but again, the excessive costs made it inefficient and uneconomical to do so. My one question to you is...why was it much cheaper to retrofit the 707s with turbofans over turbojets rather than do an engine change today? Why were many more airlines willing to do this with their earlier 707s? Obviously to maintain engine consistency in the fleet, but AA didn't seem to have a problem operating TW's 757s for 4 years. Please help me out. Thanks.

--Thrust

RE: Can You Re Engine A 757?

Posted: Tue Aug 03, 2004 6:45 am
by LMP737
AA never wanted to reengine the TWA 757's with Rolls Royce motors. The cost of doing so on leased aircraft that were eventually going to be returned anyway would not have made sense. As we speak AA is in the process of returning all the TWA 757's to the lessors.

RE: Can You Re Engine A 757?

Posted: Tue Aug 03, 2004 7:20 am
by QantasA332
In the case of four-holers, something else needs to be checked out before re-engining: resonant vibration. Depending on the engines' vibration frequency and the spacing between the engines on each wing, the vibration could be resonant and start shaking the wings with increasing amplitude, possibly to the extent of structural damage (like flutter). If it happens that the frequency of the new engines to be fitted are resonant at the spacing of the old engines on the wing, the whole engine hardpoints, fuel lines, systems, etc., would all have to be moved to rectify that - a big expense. Better just to stick with what you've got!

Cheers,
QantasA332

RE: Can You Re Engine A 757?

Posted: Wed Aug 04, 2004 1:45 am
by broke
The early 707-100's and 720's were converted to the "B" version by exchanging the JT3C turbojet engine for the JT3D turbofan engine. The reason was the dramatic improvement in fuel consumption, with an increase in take-off thrust, and a reduction in engine noise (not very noticeable).
Both engines used the same mounts, as they are variants of each other.
The down side of the conversions was an increase in engine weight, an increase in frontal area (more drag), modifying the pylon to accommodate the fan portion of the nacelle, and a loss of thrust at higher altitudes.
Turbofan engines suffer a greater percentage loss of engine thrust than turbojet engines as ambient pressure decreases (altitude increase).

RE: Can You Re Engine A 757?

Posted: Wed Aug 04, 2004 1:27 pm
by Korg747
It's a great idea with the 7E7 to have changeable engines. It would be a bad idea though to have on GE on a side and one RR on another side. I'm sure there will be weight differences and thrust differences too. How would one deal with that? Unless GE and RR agree on one specific numbers for every thing on both engines, I can see problems with that.

RE: Can You Re Engine A 757?

Posted: Thu Aug 05, 2004 3:56 am
by techspec
Do not get too carried away with the idea of "interchangeable" motors on the 7E7. This is driven by the leasing companies for "marketability issues" not a maintenance convenience. I highly doubt you will ever see a 7E7 flying with a mixed set of motors.

RE: Can You Re Engine A 757?

Posted: Thu Aug 05, 2004 5:32 am
by JeffDCA
I've seen the idea discussed on the interent somewhere (i believe it was the Boeing website), also, i found this from www.airmech.co.uk (an Aircraft Mechanic forum):

"Yes, the aim is to be able to have a mix of both engines on one aircraft. No need to have a double engine change because brand GE or brand RR is not available at the airport where an engine failure occurs. Hook up the available brand, EGR, Alles!!

It is all being worked out at the design stage, to ensure that it goes as easily as other interchangeable parts we are currently familiar with."


Whether or not we will ever see a 7E7 with both engines attached at the same time remains to be seen, but it is certainly being discussed as a possibility.

Cheers,

Jeff