When Interfaces Kill
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From: Peter Langston <psl>
Date: Thu, 3 Jun 99 12:57:11 -0700
Subject: When Interfaces Kill
When Interfaces Kill: What Really Happened to John Denver
On October 12, 1997, John Denver, popular folk singer and amateur pilot, at
the controls of a newly-purchased experimental aircraft, died after crashing
into Monterey Bay, in California. He died in an aircraft that had already
done its best to kill two previous pilots, an aircraft with a human
interface flaw so fundamental, so profound, that it finally managed to kill.
The Long EZ is a kit aircraft designed by Burt Rutan, one of the world's
greatest aerospace designers. Rutan was responsible for the Voyager, the
first aircraft to circumnavigate the globe without refueling. He is
currently working on a reusable spacecraft for commercial and tourist
operations that can fly into space in the morning, be checked out and
refueled over lunch, and fly again that very afternoon. One of his Long EZ
planes, similar to John Denver's, holds the altitude record for conventional
aircraft. It is a brilliant design, and is well respected in the aviation
Experimental aircraft kits, however, need not be built as the designer
intended. Indeed, the flaws that led to Denver's death were the work of the
builder, and had nothing to do with Burt Rutan. These flaws led from the
builder's sincere desire to improve on Rutan's work, a goal that could
actually be said to have been accomplished from an engineering perspective,
even if it did kill the pilot.
Aircraft are designed to be as safe as possible. This sounds pretty obvious,
but if you look back to the history of the motorcar, you can see quite a
contrast with aviation. The car companies required government intervention
before adding, while still kicking and screaming, such esoteric safety
equipment as headlights, windshield wipers, and seat belts. The aviation
community, on the contrary, from the beginning made safety their primary
Car fires are a common enough occurrence along America's freeways. A gas
line breaks under the hood and soon the engine is engulfed in flames. The
cure? Pull over, get out, find a long stick, and start roasting
That same fire in an aircraft at 10,000 feet is a far more serious affair.
It can take several minutes to "pull over," during which time that fire can
be pouring inky black smoke into the cockpit, blinding the pilot, making a
crash inevitable. As a result, aircraft have full shutoff valves in the
cockpit. Flip the valve and find a nice, friendly field somewhere below
where you can safely land your plane.
These shutoff valves, on some aircraft, serve a second purpose, letting you
choose between a tank located in the left wing and a tank located in the
right wing. I've never received a lucid explanation for why this is a good
thing, but a lot of planes have it, so I guess it must be good. (I prefer
flying aircraft that have a "Both" position, so all this gas selection can
The Bad Interface
John Denver's aircraft had a fuel selection valve with only three positions:
Off, Left, and Right. Burt Rutan's design called for that valve to be placed
on the front panel of the aircraft, making it easy to switch among the
options. The builder of the aircraft, however, elected to place the valve
back behind the pilot's left shoulder. He did so with the best of
intentions. By placing the valve behind the pilot's compartment, on the
other side of the back firewall, with only a long rod leading to the handle
behind the pilot's left shoulder, he avoided running the gas lines through
the passenger compartment, eliminating any possibility of a gasline rupture
occuring inside the compartment.
He did so, however, at a terrible cost to the human interface, because the
only way to switch tanks was to let go of the controls, twist your head to
the left to look behind you, reach over your left shoulder with your right
hand, find the valve, and turn it. As the National Transportation Safety
Board (NTSB) discovered, it was difficult to do this without bracing
yourself with your right foot_by pressing the right rudder pedal all the
way to the floor. And that's what killed John Denver. His plane was seen
vearing to the right and plunging into the ocean from only a few hundred
feet up, consistent with the NTSB's reconstruction.
Making things worse
The fuel: Denver had three ways to ensure he had enough fuel. Evidence
suggests he made use of two of them:
1. He had fuel gauges in the rear of the aircraft, behind the pilot, and a
mirror (!) used to look at them. However, the fuel gauges were not linear
and had no markings to indicate that apparently half-full was really
close to empty.
2. He dipped a rod into the fuel tanks while pre-checking the plane before
flight to test the fuel level. He may not have been aware, however, that,
because of the way the Long EZ rests, the fuel tends to slosh toward the
fuel tank filler port, giving a highly-optomistic reading.
3. The third method is filling the tanks, which Denver failed to do. I never
fly with anything but full tanks, and most pilots I know act likewise.
The valve: The builder not only placed the value in a non-standard location,
he also rotated it in such a way that turning the valve to the right turned
on the left fuel tank. This ensured that a pilot unfamiliar with the
aircraft, upon hearing the engine begin missing and spotting in his mirror
that the left fuel tank was empty, would attempt to rotate the fuel valve
to the right, away from the empty tank, guaranteeing his destruction.
Lessons to be Learned
John Denver learned the biggest lesson of all, even if he only had a few
seconds to appreciate it: Let the User Beware! And, indeed, the NTSB, as
per it's long history of ignoring human factors in aviation accidents,
blamed the entire matter on him. Had he bothered to fuel his aircraft, had
he spent the time to thoroughly familiarize himself with the ideosycracies
of an experimental aircraft, he would be alive and well today.
However, to those of us versed in even rudimentary human factors, it is easy
to see that the design of this fuel system was a disaster waiting to happen,
as was borne out not only by what Denver experienced, but by incidents
reported by two previous pilots of this same plane who almost met death
under the same circumstances. Presumably, they had a bit more altitude and,
therefore, a bit more time to react.
With all of general aviation's emphasis on safety, the human factors of
small planes and the environment in which they fly would be laughable, if
it weren't so dangerous. Why? Because the whole thing is awash in "macho."
Just as with Unix, just as with DOS, the more confounding everything is,
the better it is, because it helps separate the men from the boys_and the
girls. Until that changes, general aviation will continue to experience both
a high fatality rate and a continuing drop in new pilot starts.
We in the PC and web worlds have a lot to learn from this, too. We have a
lot of bad design floating around that is just as perverse as fuel valves
that face the wrong way, hidden behind firewalls. And it is not all to be
found in freeware and shareware programs, where one might argue that, as
with experimental aircraft, "let the user beware." Indeed, some of the most
egregious examples of design are apparent in the most expensive, mainstream
operating systems and applications. Fortunately for the corporations behind
them, our screw-ups generally don't kill people outright. Instead, we
specialize in driving our users slowly insane.
If you approach software design the way experts in cockpit human factors
approach their craft, you will end up with designs that are fast, familiar,
and forgiving. Such designs would be a refreshing change in the ghastly
world of PC software.
Copyright 1998, 1999 Bruce Tognazzini
© 1999 Peter Langston