Re: A320, MD-80, 727

From:         drinkard@bcstec.ca.boeing.com (Terrell D. Drinkard)
Organization: Boeing Commercial Airplane Group
Date:         28 Nov 93 16:39:03 PST
References:   1
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In article <airliners.1993.764@ohare.chicago.com>,
Brandon Walts <bwalts@lamar.ColoState.EDU> wrote:
>I have 2 completely unrelated questions that I've always been
>curious about.  Perhaps someone here knows the answers...
>
>1)  Why do 727s and MD-80s always have their ventral stairs down
>when parked at the gate.  I never see any service or maintinence people
>using them.  Is it for faster emergency egress in case of a refuelling 
>accident?  Does it somehow hold the tail up when there's nobody in the front
>of the plane (ala DC8 and IL-62)?  It seems like a lot of wear on
>the hydraullics and mechanical systems, so there must be a good reason...

Typically it is to keep the airplane from tipping back on its tail.  Rear
engined airplanes have chronic problems with CG location empty.  They have
other chronic problems, but I won't go into that.  :-)

The CG problem stems from having the CG of the empty airplane well aft
of the CG of the payload (the passengers and baggage).  When summed
together, the CG of the airplane system must be within a relatively small
range defined by the stability and control requirements and tail power
available.  The landing gear like to be pretty close to the CG of the
loaded airplane in order to allow easy rotation at takeoff.  So, when the
airplane is NOT loaded, the CG moves aft - very close to the main gear - 
and someone walking around in the back, or loading cargo into the aft
cargo compartment, can easily cause the airplane to tip back.

In fact, if one pays close attention to cargo airplanes with the main deck
cargo door aft of the wing you will notice the ground crew using a tail
prop (called a 'pogo stick') to keep the airplane from tipping back.  This
is regardless of the engine configuration.  Shoving a heavy pallet onto the
main deck gives a dynamic loading of twice the actual pallet weight, and
can cause an empty unpropped airplane to tip.  In this particular case, tip
back is quite serious with an 8,000 lb (or heavier) pallet running the length
of the cabin to impact the aft structure.  This is considered poor form by
most crews.  :-)

No doubt this is more than anyone really wanted to know about tipping
airplanes back on their tails.  :-)


-- 
Terry
drinkard@bcstec.boeing.com
"Anyone who thinks they can hold the company responsible for what I say has
more lawyers than sense."