One would think that the worst a poorly designed interface could do is cause frustration. But actually, the worst it can do is kill people.
On August 21st, 2017, 10 sailors died, and 48 were injured when a US Navy warship collided with a Liberian oil tanker (source / source / source). It was a result of many mistakes made by the crew members. However, a significant factor that led to those mistakes was the warship’s touchscreen interface.
It all started when the captain set the ship’s navigation system to “backup manual mode.” He found that the system’s automated functions were too hard to use. The backup manual mode gave him more direct control over the navigation. However, he didn’t know that the mode put the system in an emergency state that removed the steering control safeguards. This meant crew members at other stations could inadvertently take control of the steering.
That fatal night the wrong stations took control of the steering. The steering shifted from the bridge to the aft station to the bridge and back to the aft station — a total of three times in 16 seconds. As this happened, the ship veered off course toward the shipping lanes and crashed into the oil tanker.
The captain and crew members were at fault for their actions. However, this accident could’ve been avoided if the interface was easier to use.
Hidden Conditions in Modes
The first usability issue was the hidden condition in backup manual mode. The captain and crew members did not know that the mode allowed other stations to transfer steering unilaterally. In normal mode, both stations had to press a confirmation button before a steering transfer could occur. When a sailor transferred steering control to the wrong station, it resulted in a perceived loss of steering.
The system did what it was designed to do, but the design didn’t do what the sailors expect. No one expected other stations to take over steering without warning. Had they known of this hidden condition in backup manual mode, they could’ve regained control of the ship in time.
Destroy the Concept of Modes
The concept of modes can confuse anyone. Modes don’t describe what happens to the system when they’re activated. Instead, users have to interpret what happens through an ambiguous label. The sailors couldn’t interpret what happens in “backup manual mode” because the label doesn’t communicate it.
What does “backup manual mode” even mean? What specific conditions and limitations occur in this mode? It’s important to destroy the concept of modes for better usability. When you think of the interface in modes, you’ll tend to use non-descriptive labels with the word “mode” at the end. It may make sense to you as the designer, but it doesn’t inform users of the mode’s caveats.
Instead of thinking in modes, think in options. Designate conditional actions as options instead of hiding them inside a mode. If the ship’s interface had the steering transfer condition as an option, they would’ve been able to diagnose the error.
The option could be labeled “Allow any station to control steering.” When users see that the option is active, they know what’s happening. Replacing the concept of modes with the concept of options prevents ambiguous labels and hidden conditions.
A switch should be used for the option instead of a checkbox because it signifies an immediate activation. It also mimics industrial interface controls that users are familiar with. The green color for the active state indicates completion. Any other color wouldn’t have the same effect.
All these design decisions make the interface more intuitive. Had the ship’s interface designers made the same decisions, those ten sailors might still be alive today.
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