Re: Please answer - 757/767 Hydraulic Question (LONG!)

From: (Robert Dorsett)
Organization: Capital Area Central Texas UNIX Society, Austin, Tx
Date:         23 Feb 93 00:10:17 PST
References:   1 2 3
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In article <airliners.1993.188@ohare.Chicago.COM> (Michael Woodhams) writes:
>So if a 747 (or other airplane with no RAT) has a loss of all engines
>in such a way that they no longer spin, the airplane is without power,
>and it is time to start dictating your will into the cockpit voice

It depends on the airplane.  Older 747's have four engine-driven pumps,
and four air-driven pumps.  If you lose engine power, the power output from
the engine-driven pumps is reduced, but you'd get some power from the air-
driven pumps, provided that there's adequate airflow through the engine.  
You need an airspeed above 160 knots for control with *windmilling* engines 
for this to work.  But if there's a lot of dust, it's entirely plausible that 
the entire system could get corrupted.

The 747-400, in comparison, has four independent systems, as with the older
airplanes, but the #3 and #4 systems have AC electric pumps: therefore, the
airplane isn't critically dependent on proper engine airflow in order to
keep the sunny side up.  I wouldn't be surprised if the change was due to
the volcanic dust near-disasters of the late 70's/early 80's, to maximise
controllability if even one engine or the APU remains online.

>The only single cause I can think of for this would be
>flying into a volcanic ash cloud. Does this clog the engines
>sufficiently to prevent generating power from windmilling? (I guess
>not, as a 747 has survived this scenario.) 

There have been many more airliner-volcanic-ash incidents than have been
reported into the media, including at least six 747's.  By all rights, at
least a few of them have been *real* lucky, with several flights exper-
iencing complete loss of power.  Fortunately, jet engines are fairly simple,
and can continue operating, despite massive damage.  Still, remarkable that
many of these cases (such as the KLM 747-400) were able to relight, though.

>If a 757, say, lost both
>engines in ash, would the ash prevent the RAT from operating?

It's my understanding that the ash acts as an abrasive in the normal engine
mechanism, which suggests it interferes with normal moving parts and the
clearances between the fan tips and the nacelle and enclosing structures.
RAT's often don't have a duct, so there wouldn't be a clearance problem.  But
I really don't know.

In the 757 case, on the other hand, there are no "air-driven pumps," as with
the 747: just two engine-driven pumps, plus two electric pumps to go along 
with the engine-driven pumps, plus two more electric pumps for the center 
system.  But here the problem is power: if you lose the engines, you've lost 
the generators--and how long would the APU last in such conditions?  So the 
RAT could still be your last hope...

But then again, we've got the 727, which has a "manual reversion" mode, by
aerodynamic trim tabs.  *Small* pulleys move the tabs on the surfaces, which
then move via an aerodynamic effect.  Sure, the controls get REAL heavy,
but it'll get you back on the ground.  :-)  I'm fond of this approach. :-)

Robert Dorsett!!rdd