From: firstname.lastname@example.org (Terrell D. Drinkard) Organization: Boeing Commercial Airplane Group Date: 28 Nov 93 16:39:03 PST References: 1
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In article <email@example.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 firstname.lastname@example.org "Anyone who thinks they can hold the company responsible for what I say has more lawyers than sense."