Re: A 777 with four engines.

Date:         26 Jul 98 23:57:17 
From:         k_ish <kenish@ix.netcom.com>
Organization: ICGNetcom
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The Warden wrote:
> Me who doesn't have much faith in new technology thinks that 4 engines are
> agreater safety factor, despite the extra cost.

Most failures will result in a loss of one engine; in this case, the
added engines buy you nothing in terms of safety factor.  Actually, the
added engines do increase the probability of a single engine out
incident.  There are other scenarios that can cause the loss of all
engine power (misfueling, ingestion of volcanic ash, maintenance error
on all engines).  In these cases, it doesn't matter how many engines you
have.

IMHO, four engines create slightly more than twice the chance for an
engine failure of some sort, and they buy you no added safety factor
when it does happen.

> Also, if something happens
> and rudder control is lost, the two outboard engines could be pressed into
> providing sideways control (can't remember the technical term for it) and
> would do a better job than having two engines closer to the center line.

Compare a 777 to a 747.  The 747's outboard engines are mounted further
outboard, but the 777 has almost double the thrust per engine.  The
overall moment created by the differential thrust is roughly the same.
Also, an airplane is perfectly flyable with the rudder inactive.  I can
fly around all day without use of rudder.  A rudder assists in making
coordinated turns, but it is not essential.

> For
> instance, if UAL 232 had been a 707, 747, A340, or even a DC-8, (first off,
> the problem wouldn't have happened, but that's another story), they may have
> been able to get a bit more directional movement, and may have been able to
> correct for that gust of wind that pushed them off of the runway and made
> the wing dig into the ground (if that had not happened, they would have
> landed safely). Personally, I think the 777 should be a trijet, at least...

Coincidentally read a transcript of a lecture by Captain Haynes to
engineers at NASA Dryden.  The reason the wing dug into the ground had
nothing to do with crosswinds.  With no elevator control, the aircraft
was exhibiting a phugoid oscillation around its trim speed.  They
discovered that in the dive, you had to apply power, and at the top of
the oscillation you had to chop power (very counter-intuitive).  But
when power was applied, the plane would roll to the right.
Unfortunately, the dive portion of a phugoid began about 300 feet AGL.
(Or to paraphrase Capt. Haynes, they had been very lucky, but their luck
ran out right there.)

The reason he was at Dryden was to test fly a simulator where instead of
the control yoke controlling the usual surfaces, there was a reversion
mode that used thrust for control of yaw and pitch.  The simulator was
a F-15, which has both engines almost on the centerline.  The distance
from the centerline isn't a big factor if you have lots of thrust!

Ken Ishiguro