Boeing is haunted by a 50-year-old feature of 737 jets

A set of stairs may have never caused so much trouble in an aircraft.

First introduced in West Germany as a short-hop commuter jet in the early Cold War, the Boeing 737-100 had folding metal stairs attached to the fuselage that passengers climbed to board before airports had jetways. Ground crews hand-lifted heavy luggage into the cargo holds in those days, long before motorized belt loaders were widely available.

That low-to-the-ground design was a plus in 1968, but it has proved to be a constraint that engineers modernizing the 737 have had to work around ever since. The compromises required to push forward a more fuel-efficient version of the plane - with larger engines and altered aerodynamics - led to the complex flight control software system that is now under investigation in two fatal crashes over the last five months.

Boeing's problems deepened Thursday,