Re: ETOPS Question

Date:         09 Dec 97 03:54:25 
From:         Seth Dillon <bdillon1@mindspring.com>
Organization: MindSpring Enterprises
References:   1 2
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WIENI wrote:
> For all ETOPS flights the APU is a NO-GO- item.

True

> Therefore it's of course certified for operation and starting in
> cruising altitude. For this reason all APU's I know have an oil
> heater

Not on the 767/757/727/737/A310/L1011/MD11/MD88/MD90

> which is permanently on to reduce the drag when starting in high
> altitude.  A very important fact for APU starting capability in high
> altitude is the pressure ratio of intake vs. exhaust. This is a
> major driver when choosing the APU intake position.

Most APU's I have encountered will windmill at 10-20% in flight if the
inlet door actuator fails open.

> The APU bleed is not only necessary for engine restart

On high bypass engines inflight restart is performed using windmill
rotation only.  This is typiclly around 25-30% N2.  The 767/757 when
sensing engine speed below flight idle will pop up a restart
altitude/airspeed envelope matrix on EICAS.  It should be noted that
crash engaging the starter at these RPMs could have catastrophic results
to the gearbox.

> but also for cabin
> ventilation and pressurization ( perhaps more important).
> In case of no electrical power from the main engines there is also
> a ram air turbine on AIRBUS A/C delivering power for the basic systems.

A310/757/767/L1011 the RAT provides hydraulic power only.  DC10/MD11
have an electric generator driven by the RAT.  This in turn powers an
electrically driven hyd. pump.  The 767ER has a hyd powered generator,
driven by the center hyd system.  This system is pressurized by either
elec. hyd pumps or air driven pumps.  the air coming from the A/C
pneumatic system.

> [Moderator's note: Boeing's twins also have a ram air turbine.  -- Karl]

-Seth