Re: DC-8s in service; no 707s?

Date:         30 Jan 97 00:36:25 
From:         Steve Lacker <>
Organization: applied research laboratories
References:   1 2
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A number of people have written along these same lines:

Bizfixer wrote:
> CFM-56 engines on DC-8s allow them to meet current noise requirements and
> are also very efficient.  As far as I know, hush-kits for the 707 are
> marginal on noise and the old engines so equipped are very
> fuel-inefficient.  I believe USAF re-engined some 707s (AWACS?), but too
> expensive for commercial use.

I still don't think this is the whole or primary reason we see more
cargo DC-8's in the US. Why? Because the majority of cargo DC-8's I see
are powered not by CFM-56's, but by non-hushkitted P&W JT3D's, making
them either rather loud Stage-II or quiet Stage-I aircraft (I believe
they are Stage II, but I'm not sure).

Both the 707 and DC-8 started life with the turbojet powerplants, P&W
J-57 (commercially designated JT-3) or a larger P&W pure jet- the
J-75/JT4 I believe, but I'm not positive. Starting with the 707-100B,
the JT3D TurboFAN engines were used. I'm not sure which DC-8 first used
these engines, and I'm neglecting a small number of 707's that got RR
Conway engines.

My point is that the "most common" configuration for both the 707 and
DC-8 through the bulk of their domestic lives was to have JT3D
low-bypass turbofans under the wings. Although some civilian DC-8's and
now freight-carrying DC-8's received CFM-56 re-enginings, a lot are
still out there with JT3D's just like many of the surviving 707's.
However, most of the 707's now operate overseas carrying passengers.

Thats why I think (not a fact, just my opinion!) that the DC-8 is more
popular as a freighter here. Its less in demand elsewhere for passengers
(cheaper to buy), has great freighter economics, and yes, it does have a
certified upgrade "path" to CFM-56 engines- but thats less of a factor
since having JT3D's does NOT severely hamper operations... YET! Now,
Boeing did build and fly the 707-700 with CFM 56 engines, but decided
not to market it commercially, although the later AWACS and
foreign-sales military 707 derivatives were based on it (NOT on the
KC-135, by the way!) I don't know what rules/certification requirements
must be met in order to upgrade a JT3D powered 707 FREIGHTER to
CFM-56's. Clearly it can and has been done for military applications and
NOT for passenger applications.

KC-135's are another ball of wax. Many were re-engined to JT3D's
(Military TF33), and have since been re-engined again to CFM-56's.

I've left out a lot, and probably got something wrong- the history of
the 707 and KC-135 is truly a tangled web, and I'm going largely from
memory :-) I recommend "Boeing Aircraft since 1916" by Peter Bowers for
a good story of the evolution of the 367-80/707/717/KC-135/720 family as
well as other Boeing aircraft.

Stephen Lacker
Applied Research Laboratories, The University of Texas at Austin
PO Box 8029, Austin TX 78713-8029