Re: passenger stairs in aircraft tail

From:         ehahn@bass.mitre.org (Ed Hahn)
Organization: The MITRE Corporation, McLean, Va.
Date:         27 Jun 95 01:43:18 
References:   1
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In article <airliners.1995.873@ohare.Chicago.COM> Martin Fiddler <entmlf@staffs.ac.uk> writes:

   Certain aircraft - the DC9 / MD80 family, and the british BAC-111, have
   stairs that lower from beneath the tail.   Obviously these are very useful
   at remote airfields, as portable stairs are not needed for the passengers
   to enter and exit.  Possibly there are other aircraft with the same
   facility too.

   Why don't all aircraft have them?   Is it something to do with the rear
   mounted engines on the above two models?    I guess it's much more complex
   from the rear pressure bulkhead aspect, but that doesn't explain why, say,
   the B737 doesn't have them but the DC9 does.

=====
The DC9 and MD80 are also commonly fitted with forward airstairs,
which retract under the forward entry door (L1).

The B727 also has aft airstairs, and the DC9/MD80 is not always
equipped with rear stairs.

Safety and security considerations have generally stopped use of the
rear stairs for passenger boarding of aircraft.

The reason most newer or larger aircraft no longer have them is
two-fold:

First, at least in the US, jetways have eliminated the need for
airstairs in the first place.  Internationally, they still have use,
as many airports do not have enought gates or jetways serve the
necessary traffic.  However, even those airports without jetways still
generally have movable stairs for boarding.  (Such as at FRT - I
boarded an LH A320 via movable stairs in 1993.)

Second, there is a significant weight penalty to carry these stairs
around, especially if the first reason means that they aren't used
very often.  Furthermore, the weight penalty would be much worse if
fitted to a larger (actually, taller) aircraft, such as the B757, due
to the extra structure needed.  Widebody airstairs would be right out -
these aircraft are generally operated on routes where the fuel
requirements mean that the aircraft can't carry maximum payload - the
extra dead weight of the stairs takes away from the number of
passengers or additional cargo the aircraft otherwise could carry
(i.e. revenue).

Aft airstairs are generally not removable, and thus are helpful
to maintenance crews, when the aircraft is off the gate.

Additional remarks about forward airstairs:
American Airlines a few years back actively removed the forward
airstairs from their MD80s to save weight, as they tend not to fly
into non-jetway cities.  TWA was also considering the same action with
their DC9/MD80s.

Interestingly enough, the plan for TWA's short DC9s (-10 and -15) had
them retain their forward airstairs because the removal of that weight
so far forward had severe impact on the balance.  Ballast weight would
have been needed in the nose to keep the aircraft's CG within limits.

My $0.02,
ed

--------   Ed Hahn | ehahn@mitre.org | (703) 883-5988   --------
The above comment reflects the opinions of the author, and does not
constitute endorsement or implied warranty by the MITRE Corporation.
Really, I wouldn't kid you about a thing like this.