Fourth landing gear on MD aircraft ?

From:         Jean-Francois Mezei <MEZEI_JF@Eisner.DECUS.Org>
Organization: Digital Equipment Computer Users Society
Date:         05 May 95 03:27:11 
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On a publication I receive was a picture of an MD aircraft wearing KLM colours
just about to land.

I do not know what model of MD aircraft it is, but it has the following
characteristics:
	2 engines on wings
	1 engine in tail
	winglets (up only)

Most aircraft I have seen have 3 landing gears, one at the nose and 2 under the
wings near fuselage.

However, on that KLM MD plane, there was a 4th landing gear with 2 wheels only,
located under the fuselage and just between the 2 gears that are under the
wings.

QUESTIONS:
	Is the 4th gear a feature that only MD now uses (as is the tail
	mounted 3rd engine) or will that feature also be used by other
	major manufacturers ?

	Is the purpose of that gear to provide additional breaking power
	or as a structural feature to support the weight of aircraft during
	touchdown ?

	If the 4th gear was required to support the weight of aircraft, was
	this required because they used less structural metal in the fuselage,
	or because of a weird and new design or because they wanted the plane
	to be able to carry more weight (cargo/passengers/luggage) ???