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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 community.

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 goal.

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 marshmallows.

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 fuel 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 most aircraft, serve a second purpose, letting you choose between the tank located in the left wing and the tank located in the right wing. (I prefer flying aircraft that also have a "Both" position, so all this gas selection can be avoided.)

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 veering 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-optimistic reading.
  3. The third method is adding fuel to the tanks, which Denver failed to do. Because the Long-EZ has very large tanks, the common practice is to add an amount of fuel suffient for the flight, plus a healthy margin. It may be John Denver was told not to fill the tanks, but was not told of this partial-filling practice.

The valve: The builder not only placed the valve 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 full 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 its long history of setting aside findings, human factors or otherwise, that might conflict with a verdict of pilot error, ruled that the responsibility for this crash lay with the pilot. The interface was relegated to a mere "factor." Had John Denver fueled his aircraft in spite of evidence indicating he had sufficient fuel, had he somehow managed to thoroughly familiarize himself with the idiosyncrasies of this uniquely-assembled experimental aircraft sans manual, 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 when their fuel starved out and, therefore, a bit more time to react.

With all of 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, who aren't really invited. Until that changes, general aviation will continue to experience both a high fatality rate and a continuing drop in new pilots.

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 commercial and military 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. They'd be a refreshing change in the world of general aviation, too.

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