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

From: (Robert Dorsett)
Organization: Capital Area Central Texas UNIX Society, Austin, Tx
Date:         17 Feb 93 14:09:32 PST
References:   1 2 3
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In article <airliners.1993.178@ohare.Chicago.COM> William Wood writes:
>during flight on a 757.  The person to which I was speaking,
>informed me that "it would be no terrible problem for a 757 
>to suddenly loose an engine whilst in flight!"

Karl's written a series of articles demonstrating various thrust-to-weight
ratios, which should cover any "thrust" questions you might have.  Suffice
it to say, the 757 has enough power to handle an engine-out situation.

>   Amazing to me as this sounded, I had to be the one to pose
>the question of 'why this wouldn't be a problem?'  I was in-
>formed by the other party that the 757 has a "pseudo-engine"
>that is 'tucked-under' the wings,  [...]
>    Could this be a truth?  I would appreciate your con-
>sideration towards addressing this question.  Also, if
>it is a truth, would this also be the case in the design
>of the 767 as well??

What you're referring to is a ram air turbine.

The 757 operations manual states the following:

"The ram air trubine (RAT) hydralic pump is located in the body fairing
aft of the right main gear.  Inflight the RAT automatically deploys into
the airstream when airspeed is above 80 knots and both engines fail.  The 
RAT then supplies power to the center system.  Once extended, the RAT can
only be retracted on the ground.  At speeds above 130 knots, the RAT pro-
vides adequate power for normal center syste[ operation.  A Ram Air Turbine
Pressure Light indicates the RAT is providing hydraulic power.  The UNLKD 
light indicates teh RAT is not locked in the stowed position.  Manual control 
for extending the RAT is provided by the guarded Ram Air Turbine Switch."

The center hydralic system includes the center autopilot servos, spoilers, 
elevators, rudder, yaw dampers, stab trim, and elevator feel.  Note that it 
doesn't handle the landing gear. 

Appended is a series of posts which address this specific question in 
detail, the last time it surfaced.

Aeronautics Digest 4.31                                        August 14, 1992


   Solly Ezekiel          Re: Ram jet engine in Commmercial aircraft 
   Mary Shafer            Re: Ram jet engine in Commmercial aircraft 
   Ken Hoyme              Re: Ram jet engine in Commmercial aircraft
   Robert Dorsett         Re: Ram jet engine in Commmercial aircraft
   Bob Furtaw             767 flight control (Re: Ram jet engine...)



Date: 3 Aug 92 19:35:51 GMT
From: (Solly Ezekiel)
Subject: Re: Ram jet engine in Commmercial aircraft like Boeing..

In article <> writes:

>I remember once being told in a course that Boeing and other commercial
>aircrafts have Ram-jet engine for emmergency supply of power for vital
>sysems in flight. Where is this engine situated on a 747? What power
>does it produce and does it help the aircraft in controlling its decent
>during a engine failure. I wonder if any one can enlighten me on this topic?

I believe that what you are referring to is called a Ram Air Turbine.
It isn't a ramjet, but just a fan that is dropped into the airstream to
generate power for such things as hydraulics if the engines fail.  If
memory serves, the one on the 767 drops from the belly of the aircraft
(and I would guess that the one on the 747 does similarly).  An example of
an occasion during which this little gadget came in useful was the
infamous "Gimli Glider" incident, during which an Air Canada 767 ran
out of fuel in flight and landed on a race track.


Date: 3 Aug 92 19:49:04 GMT
From: (Mary Shafer)
Subject: Re: Ram jet engine in Commmercial aircraft like Boeing..

I think that you may be referring to the RAT or Ram Air Turbine.
Perhaps you've misremembered?

This is just a little turbine that folds out of the side of the plane
(more usually fighters than transports) and uses the ram air to spin
the turbine and generate enough power to keep the plane flying.  In
the F-8, for example, the RAT would keep your hydraulics going, as
well as the more important avionics.

It's not a ram-jet engine and 747s don't have it anyway.  They (747s
and all other transports that I know of) have APUs (Auxiliary Power
Units) to do the same thing.  The APU just burns Jet-A to make
electricity and power the hydraulics.

The F-16 (and the X-29 and X-31) has an EPU (Emergency Power Unit)
which burns hydrazine to make the power.  The EPU doesn't last 
very long; it's just designed to get you to a good place in the
ejection envelope.


Date: 5 Aug 92 15:34:46 GMT
From: (Ken Hoyme)
Subject: Re: Ram jet engine in Commmercial aircraft like Boeing..

The Boeing 777 *will* have a RAT, and that is exactly what it is called.
Also, the A320 has a RAT.

> Whether any or all of the 747 line have them I can't say.

Since there are electrical generators on each engine, a 4 engine plane
is less likely to have one since the probability of total loss of
electrical power is much lower.  As far as I know, the RATs on
commercial airplanes are for electrical power, not hydraulic.

Any twin engine plane with fly-by-wire will undoubtedly have a RAT.  I
can't imagine the certifying agencies approving the plane without it.  

I don't have knowledge about the 757/767 configuration.  They are twins,
but are not fly-by-wire, so continuous electrical power is not quite as


Date: Mon, 10 Aug 1992 04:09:33 GMT
From: (Robert Dorsett)
Subject: Ram jet engine in Commmercial aircraft like Boeing..

> I remember once being told in a course that Boeing and other commercial
> aircrafts have Ram-jet engine for emmergency supply of power for vital
> sysems in flight. Where is this engine situated on a 747? What power
> does it produce and does it help the aircraft in controlling its decent
> during a engine failure. I wonder if any one can enlighten me on this 
> topic?

[ I've been in a sort of net.purgatory for the last week, and composed 
  this early on; apologies if the discussion has lapsed. ]

As others have noted, what you're referring to is a ram air turbine.  This 
isn't the same as a ram jet engine.  The RAT is a miniature turbine, usually
with a propeller blade in the front, which is placed in the free airstream, 
to develop power.  A ramjet is a hypersonic propulsion device.

A conventional jet engine is basically a bunch of fan blades mounted
on two or more shafts.  Most airplane systems (electrical, hydraulic, 
pneumatic) are centered on the engine.  "Accessories" are the devices 
that produce these services, and are physically arranged around the engine,
within the engine nacelle.  They use gear reduction mechanisms, hooked up 
to a shaft, to obtain motive power.  Electrical generators and *engine-
driven* hydraulic pumps are two such types of "accessories."

The important thing to note about this arrangement is that all these
accessories require the engine blades to be spinning, NOT necessarily
for the engine to be ON.  I.e., they aren't "steam-driven" from the
engine turbine.  An engine that is OFF will still spin: back in the good
old days, when we got to walk to airplanes, one would get to see the front-
stage blades of an "inert" engine spin in even a light headwind (these days,
of course, we're lucky if they tell us what kind of airplane we're flying
on).  An engine hooked up to an airplane in a 200 kt glide will obviously 
spin much faster, and will still supply services, some of which may be 

In a normal airplane, each engine powers one electric generator.
In addition, on most airplanes, an auxilliary power unit, a gas-powered
generator, is provided for ground operations, engine startup, and 
emergencies.  This is usually mounted in the tail empennage.  On some 
airplanes, such as the 727-200, the APU isn't certified for in-flight use; 
on others, like most 747-200's, the APU is either not certified for in-flight 
use, or has strict envelope restrictions on its use.  

The APU does two things: it produces electricity, and pressurizes the 
pneumatic system (useful in ground starts/air conditioning/etc).  

Most airplanes have one electrical generator per engine, an APU, and
emergency (15-30 minutes) battery power.

On the "control" issue, note that on most aircraft, hydraulic power is 
obtained via a combination of two or three methods:
	1.  Engine-driven pumps.
	2.  Electrically driven.
	3.  Air-driven.  Engine bleed air is used to drive a pump.

Most airplanes tend to match one hydraulic system per engine: thus, a 
747 has four, a DC-10 three, etc.  There are exceptions (a 727 only has
two (and one standby), while a Tristar has four--but the 727 has a manual 
backup flight control system, like the 707, while the Tristar is all-
hydraulic, like the DC-10 and 747).  

For each hydraulic system, one can generally count on two power sources.  
Thus, on the 747-200, there are four engine-driven hydraulic pumps and four 
air pumps, or eight sources in all.  Even if all four engines fail due to 
something weird, like fuel starvation, there will likely be enough hydraulic 
pressure, via both the engine-driven and air-driven pumps, to continue to fly 
the airplane, due to windmilling effects.  

When we start taking away engines, though, the RAT becomes an attractive 
alternate power source.  If one loses all four engines on, say, a 747, 
one is still developing a lot of independent power.  If one loses one or
both engines on a twin, one has suddenly put a very large control burden 
on the hydraulic system, one which a windmilling or air-driven pump may not
be able to meet.  One may not have the time to turn on the APU and get it
on-line.  In this context, a RAT provides a margin of safety: it's designed 
*explicitly* for a specified purpose (hydraulic and/or electrical power 
generation), and may permit one to do things that available pressure 
wouldn't make possible, such as lowering the flaps or landing gear.

In general, the fewer the engines, and the longer the range, the more
likely it is that you'll find a RAT.  The RAT *supplements* existing 
systems; it doesn't really *replace* any "standard" level of redundancy.
An airplane without a RAT isn't necessarily less safe than one with them:
the 747, for instance, but also the 737/DC-9-class airplanes, which have
a lower hydraulic demand requirement.

Most Boeing airplanes do not have a RAT; the two exceptions are the 
757/767.  On these airplanes, the RAT is used as a source of standby
hydraulic power.  As airplanes become more complex, the need for a last-
ditch *electrical* source is also manifesting itself.  On the A320, the RAT 
powers the "blue" hydraulic system (one of three); Blue services a core 
set of control surfaces.  Blue can also run a separate "hydraulic generator," 
for 5KVA of electrical output (as opposed to 90 KVA for each of the engine-
based generators).  It thus serves a dual purpose.  The 777 will also have
a RAT, which will follow similar considerations.

On both the 757/767 and the A320, the RAT either pops out automatically in 
an emergency or may be commanded by the pilot.  In both cases, it must be 
stowed on the ground.

Off the top of my head, these airplanes have or will have RATs:
These don't:
	DC-9/MD-80 (?)
So there's no hard and fast "manufacturer culture" rule we could use
to figure out that an airplane will have a RAT; it's more driven by 
mission requirements.  And, again, they're such a *marginal* power source 
that it's best not to screw up bad enough that it's necessary to fall back 
on one.  I would tend to view them as "safety margins," not something I'd 
ever want to have an airplane I'm flying in depending on to get me home.

[ I subsequently learned that the A340 has a RAT... --rdd ] 


Date: 7 Aug 92 21:11:58 GMT
From: (Bob Furtaw)
Subject: 767 flight control (Re: Ram jet engine...)

In article <furtaw-030892152029@>, (Bob
Furtaw) wrote:
> The device I think you are referring to is called a RAT (Ram Air Turbine
> ?).  It consists of a propeller driving a hydraulic pump that falls by
> pilot command from just behind the right main landing gear of the 757 &
> 767.  It supplies partial hydraulic power when all engines fail. 
> Retraction of this device is done by ground crew only.


Here are some notes about the hydraulic system(s) from 767 training done in
Seattle in '86.

1. Type of System

Each of the three hydraulic systems of the 767 operates with a maximum
pressure of 3000 psi.

2. Location

The three hydraulic systems are identified by location.  They are Left,
Right and Center System.

The Left system is located in the left engine strut and contains a total
of approximately 17 gallons of hydraulic fluid.

The Right system is located in the Right engine strut and contains a total
of 20 gallons of hydraulic fluid.

The Center system is located in or near the right main wheel well.  It
contains approximately 40 gallons of fluid for a total 767 capacity of 
approximately 77 gallons.

3. Multiple Systems

The three separate systems - Left, Right and Center - have no

4. Multiple Pressure Sources

Two engine driven (EDP) hydraulic pumps are used, one on each engine. 
These are primary pumps in the Left and Right systems

Four electrically driven pumps ( ACMP) are used.  Two serve as primary
pumps in the Center system, while one each serve as demand pumps in the Left 
and Right systems.

An air driven pump (ADP) is the demand pump in the center system.  Air
power comes from the airplane pneumatic system, which in turn may be supplied 
by engine bleed, APU or a ground source such as an air cart.

A RAM Air turbine (RAT) in the center system serves as an emergency source
of hydraulic pressure and is powered by the slipstream.

The Right and Left primary system pumps are rated at 37 GPH at takeoff

The Center primary system AC motor pumps are rated at 7 GPH.

The secondary demand pumps are turned on automatically when the primary
pumps can not maintain sufficient pressure.

The Center system RAT is rated at 11.3 GPH.

There is also a generator that is driven by a hydraulic motor, operation is
automatic on loss of power from both electrical generators.

Devices driven by each system:

	Roll, Pitch, Yaw
	Lateral Central Control Actuators (LCCA)
	L & R Elevators

Left and Center only:
	Elev Feel
	Stab trim
	Yaw damper

R&L only:
	Thrust Reversers

R only:
	Normal Brakes

L only:
	Ratio changer *

* editors note:  this is a mechanical arrangement that changes the ratio
of command input versus rudder travel so that at high speed the rudder does 
no travel as far.  This  keeps the rudder from ripping of the airplane.  Nice 
feature :-).

Center only:
	Alternate Brakes
	Reserve brakes
	Inbd / outdb slats
	Inbd / outdb flaps
	Normal and reserve steering
	ldg gear doors
	hyd motor generator.

The RAT controls:  Roll, Pitch and Yaw, LCCA, L&R Elev, Rudder, Elev Feel,
Stab trim, and Yaw damper. ONLY

 * editors note: my experience in the simulator is that you don't get all
these at one time.  Sometimes  the pump can't keep up.  You get most 
operations, then you have to wait a second.  Tricky but one can get the hang 
of it.