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Upgrading engines on 707-120

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Upgrading engines on 707-120

Old 29th Aug 2016, 16:02
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Upgrading engines on 707-120

I understand from some reading that I was doing that most Boeing 707-120s were upgraded to 120Bs with JT3D turbofans which were considerably more powerful than the original JT3C turbojets shortly into their life. The same was true of 720s but that it was not feasible for the 320 series of the aircraft even though the JT3D which powered the 320B/C was was not that much more powerful than the JT4A. I have a number of questions doubtless showing my ignorance:

Was a lot of work required to re-certify the 120 for turbofan engines? Did the original design envisage fitting new engines or was it just good fortune and conservative design that allowed this?

Was the main reason for fitting new power plants cheaper operating costs, enhanced performance or both? (I suspect that given the low cost of fuel in those days and the poor performance of early 707s it was the latter.)

Why couldn't 320s be re-engined to become 320Bs? Was the 320 at the limit of its design? (And indeed could you have fitted JT3Ds to Conway powered 420s? which would still have been an upgrade?)

Moving on (and this should possibly be in the Questions forum), could you fit upgraded engines to 320 ceos to make them neos once now that the new engines are certified presuming that the engine mass and thrust is similar (and ditto for 737s)?

Emirates would like to see a 380neo. Rolls Royce seems possibly keener than Airbus. Would it actually require a lot of work by the latter? It seems that during the 80s quite a lot of incrementally upgraded engines with ever longer designations by all three manufacturers were fitted to 747-200s.

Last edited by Peter47; 29th Aug 2016 at 16:37.
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Old 30th Aug 2016, 09:11
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Boeing 707 transition from turbojet to turbofan [Archive] - PPRuNe Forums
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Old 1st Sep 2016, 12:08
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The improvement in performance by replacing 13,500lb s.t. JT3Cs with 17,000lb s.t JT3Ds was marked. Conversion of a 707-120 also involved installation of the reprofiled inner wing leading edge, as already introduced on the 720, and greater tailplane span. Re-engining the 720 produced an even greater performance improvement as they were originally fitted with 12,000lb s.t JT3C-7s.
There was no similar imperative to re-engine the 707-320 as they were fitted with 17,500lb s.t JT4As and the fan engines would not have provided a significant thrust increase. The -320B was not just a re-engined -320, they had a wing with redesigned tips and leading edge high lift devices and were certified for higher weights. The 17,500lb s.t. Rolls Royce Conway in the -420 was, of course, a turbofan anyway.
Boeing did propose a re-engined 707 with CFM56s, but it was really too much engine for the airframe and stretching the 707 was not economically viable as it was already a stretch of a stretch. However, many 707 family aircraft were built or re-engined with CFM56s for military purposes, e.g. E-3 Sentrys and KC-135s.
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Old 2nd Sep 2016, 21:19
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The 707-120/720 upgrade was not done by all operators, but the large (in fact huge for the time) fleets of both which American Airlines had built up very early on in the jet age were fully done. Some of their older turbojets were only months old when done and American changed over at the end of 1960 in mid-delivery of their fleet as soon as able to do so. I believe the only operators to actually do the upgrade were Pan Am, American, Qantas, and a few USAF VIP aircraft.

I did understand that once the orders for re-engining suddenly started Boeing realised their mistake, and the 707-320B upgrade from the 707-320, in hand at the time, was deliberately re-engineered, with spar and structure mods etc, on command from the marketing team, so simple engine upgrades would not be possible on the same certificate, and the operators would have to buy complete new aircraft instead. As this was actually what happened, there seems some credence to this. Although the 707-320B upgrade was not as marked for power, fuel consumption reduction, so greater range, was significant, and also smoke and noise reduction, which the early 707s had caused to become a major political issue.

Were they wholly new engines or just reworked with a fan on the front ?

Regarding converting the RR Conway aircraft to P&W power, I believe no 707s were done but one or two DC8s were. It proved not cost-effective.
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Old 3rd Sep 2016, 20:57
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Originally Posted by WHBM View Post
Were they wholly new engines or just reworked with a fan on the front ?
The engines were upgraded as well, converted engines carried an MC6 or MC7 suffix on the data plate. Depending on the original JT3C-6 or -7 model number.
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Old 5th Sep 2016, 18:51
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As stated Qantas upgraded. There was no more requirement for water assisted take offs. Hurrah!
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Old 5th Sep 2016, 21:42
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Qantas were the only intercontinental operator to go for the original 707-120, whose performance on their long-haul route structure was so marginal for them that they had a special shortened (and lighter structural weight) version done, the 707-138. Boeing seemed to do things like this in those early jet days. As soon as the new engines were available they put them on, but then ordered the mainstream 707-320C and sold their first generation off. They were some of the first secondhand jets to come onto the market, and became a particular favourite of the transatlantic charter companies. John Travolta's personal 707 is one of this small batch, it has Qantas livery (unless done up for yet another period movie) but is a genuine ex-Qantas aircraft.
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