Not long after the Electra entered service,
there were three crashes, two of which involved wing separation (the other
was attributed to pilot error). As a result, performance restrictions
were imposed and Lockheed instigated a modification program which came
to be known as LEAP. Several references decode the acronym as 'Lockheed
Electra Achievement Program' but Lockheed sources state that the correct
title is 'Lockheed Electra Action Program'. The following account of LEAP
is extracted from "Beyond the Horizons - The Lockheed Story"
by Walter J. Boyne (St. Martin's Press, New York 1998)
“Investigation revealed that under certain conditions of engine
nacelle or power-plant damage, a phenomenon known as 'whirl mode' could
occur. ‘Whirl mode’ refers to the results of the application
of a force to gyroscopic characteristics of a rotating propeller. When
such a force is applied, precession occurs; that is, like a gyroscope,
the propeller reacts ninety degrees out of phase to the applied force.
This causes the structural resistance of the engine mounting system to
apply a nose-down pitching moment. This forces the propeller disc (as
viewed from the rear) to turn to the left due to precession.This in turn
causes a nose-down propeller disc yawing to the right, which causes a
nose-up pitch, completing the cycle.
“This combination of effects is termed the "whirl
mode", and its direction of rotation is opposite to that of the propeller.
In a normal aircraft, the whirl mode could operate only within the limits
of the flexibility of the engine mounts. If, however, some structural
element of the power plant, the power-plant mounting system, or the nacelle
was in a damaged or weakened condition, the whirl mode would not damp
out, but could become more violent, increasing damage to the structure,
and could approach the natural frequency of the wing. This would perpetuate
the whirl mode in a form of induced flutter and lead to catastrophic failure.
“John Margwarth, another University of Michigan man, was director
of safety for Lockheed, and it was his insight that led to an investigation
revealing that the Electra's fatal flaw was in the three member structure
connecting the gearbox and the engine, a part supplied by the engine manufacturers.
“When one member of that structure failed, the engine mount became
flexible. On an outboard engine, at the Electra's original cruise speed,
failure of the strut induced immediate, violent flutter that tore the
wing off. Technically, Lockheed could have passed the problem off to the
engine manufacturer, disclaiming responsibility. Instead, it redesigned
the wing structure so that it would not flutter when such a failure occurred.
(Allison also redesigned the strut so that it would not fail.)
“Additional mounts were added to stabilize the propeller in the
event that any mount failed, or if breakage occurred between the gearbox
and the power section. The nacelle structure was also strengthened by
the addition of reinforcements and diagonal braces.
“Lockheed was rocked by the three crashes and their adverse publicity.
For weeks there was one meeting after another to handle the latest problem.
It was soon evident that engineering the wing modification was not going
to be as difficult as finding a way to pay for it. Carl Kotchian recalled
coming out of a meeting with Robert Gross, the latter sunk deep in thought.
(Gross had been under considerable strain for some time; he would die
less than two years later.)
“In the garage, Gross asked, 'How much do you think the modification
is going to cost?' Kotchian hesitated and said, 'Well, I think it's going
to cost maybe $25 million.' Gross turned white, then replied, 'Well, we've
got to do it.'
“And Lockheed did, instituting the Lockheed Electra Action Program
(LEAP) and modifying all Electras at its own expense, whether they were
within warranty or not. The LEAP program came in just under Kotchian's
$25 million estimate.”