Poor UI Design Can

KILL

A SHORT PRESENTATION BY SANDESH BHETWAL

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Air Inter Flight 148

CASE 1 :2

On January 20, 1992, Air Inter Flight 148 crashed into the

Vosges Mountains while approaching Strasbourg

Airport in Strasbourg, France. 87 of the 96 crew and

passengers on board were killed.

The Airbus A320 had the most advanced cockpit in the

world. The majority of the gauges and instruments in

front of the pilots in this machine were digital.

The Accident

Flight 148 crashed because the pilots inadvertently left the autopilot

set in Vertical Speed mode (instead of Flight Path Angle mode) then

entered "33" for "3.3° descent angle", which for the autopilot meant a descent rate of 3,300 feet per minute.

When the aircraft was approaching the runway through thick

clouds, the captain thought he had set a descent angle of a mild

3.3 degrees for a gentle descent above the Vosges Mountains.

However, unbeknownst to him the plane was descending at a rate

of 3,300 feet per minute, much faster than the descent would have

been at 3.3°.

The aircraft struck a mountain top and exploded.

3

The Cause

The problem was that the airbus A320 used a single display for two different descent modes – Vertical Speed (VS) Mode and Flight Path Angle (FPA) mode.

The display showed FPA as two digits separated by a decimal point, and VS as just two digits (and both had a minus sign where their values were negatives).

This simple design error led the pilot to believe he was in one mode while he was in another. The crash almost certainly would not have happened had the aircraft’s user interface been more clear.

This UI design error is one of a well-known class of errors called mode errors.

4

The Aftermath

Airbus modified the interface of the autopilot so that

a vertical speed setting would be displayed as a four-

digit number, preventing confusion with the Flight Path Angle mode.

The flight data recorder was upgraded so that it was

able to withstand higher temperatures and for longer.

Also recommended that pilot training for the A320

should be enhanced.

The story of the disaster was featured on the ninth

season of Cineflix television show Mayday in the

episode entitled "The Final Blow"

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The Kegworth Air DisasterCASE 2 :

6

In 1989, a Boeing 737 crashed into the side of the M1

motorway near Kegworth, England.

The Accident

The problems started when the jet began vibrating, indicating that

one of its two engines was malfunctioning.

Unfortunately, the pilots got the impression that the good engine

was at fault and turned it off.

Without any functioning engines, the jet became a large piece of

metal suspended in the air and did what large pieces of metal

suspended in the air tend to do.

It went down !

7

The Cause

Some months earlier, the cockpit instrumentation in this type of plane was updated

to give them a slicker-looking digital design.

At one point during the Kegworth flight, the dial indicating the vibration in the left

engine rose to the maximum and stayed there for three minutes -- but the pilots

never saw it, because it was too small.

The dial itself was already hard to see, and on top of that it had a digital pointer that

barely existed.

Also the fact that the whole place was vibrating at the time.

In the confusion, the pilots never saw the dials.

8

The Cause

The older model had a large, clearly visible

mechanical pointer that was pretty hard to

miss and can read those dials.

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The newer model had a slicker, small, digital dials.

The dials were hard to see, and on top of that it had a digital pointer that barely existed.

In a zoomed view we can see the pointers.

The Aftermath

Many people died because of this silly dial error.

Redesign of the dial was done to make it readable than look stylish.

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