Eyes Inside

By By Frank Lombardi | August 8, 2014
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Looking at a panel full of instruments is like looking at the heartbeat of your aircraft. Thinking back to primary flight training brings up basic images of how they work, such as the altimeter with its expandable bellows that moves a dial; or the attitude indicator, with a gyro inside that looks like the one on the shelf in science class. Those simple pictures help bring understanding and may aid in troubleshooting a problem or failure, but as an aircraft gets more complicated, so do the systems that monitor and report its condition. Designing the indicating systems of today’s airplanes and helicopters is a challenge to a host of engineering disciplines.

Pilots are only as good as the decisions they make. Since many of those decisions are based on information presented, it must be accurately gathered, presented in a clear and timely manner, and readily accessible. Too much information will overload our own CPU, and too little deprives us of critical information. Either way, errors follow. Although the vast majority of cockpit indicators are digital electronics working at the speed of light, there still exists a measurable amount of time that passes before signals are processed and displays present their information. This is called latency, and is a very important aspect of system design. This delay in signal processing must be taken into account. Add this to the time it takes for a pilot to perceive, process, and react to the info presented, and the total delay grows. This is especially critical when designing an IFR-certified aircraft, since there will often be a lack of outside visual information to provide the aircraft’s state to the pilot. The delay in receiving and reacting to an indication has the potential to cause issues with handling qualities such as a pilot-induced oscillation.

As indicators get more complex, engineers must examine all possible failure modes and decide upon the likelihood of each failure condition during the design process. The electronic age has made it more difficult to troubleshoot failures in the cockpit. On one occasion, I squawked an engine that was becoming increasingly slow to start. After checking out rigging and other mechanical possibilities, our maintenance team found that a temperature transducer had “gone bad.” This transducer is a device that reads engine temperature, converts it to a voltage, processes the signal, and then displays it back as a temperature in the cockpit. It was providing erroneous temperature readings, causing the pilots to incorrectly modulate the throttle. Luckily, it failed in a mode that displayed a higher-than-normal temperature, avoiding a very costly repair.


Human factors exist in every aspect of aviation, and instrument design is no different. Digital electronics allow infinite ways to display information. With this in mind, it is important for displays to be intuitive and clear. An increasing value displayed on a gauge that moves in a decreasing fashion is counter-intuitive and can lead to confusion.

The ability of modern electronics to be able to display hundreds of parameters creates a dilemma when deciding what is important enough to show the pilot. Sometimes the system allows the pilot to choose what is displayed on the panel. This can help to reduce clutter, but has the potential to increase workload, since the pilot must now remember what is not displayed and how to obtain it if needed. Getting “cognitively lost” in a computerized menu system while trying to maintain situational awareness has caused accidents. Some indicators incorporate a “display by exception,” where a minimum amount of info is displayed when conditions are in a normal range, but when conditions are exceeded, the display will then present the information that is out of limits.

Having one indicator that monitors many parameters and concisely presents the info greatly reduces pilot workload. But when needles start splitting and lights start flashing, the info being displayed must be quickly interpreted. Size, color, symbology, location, and trending must be thought out so pilots can make well-informed decisions. Although the robustness of today’s digital equipment has greatly improved, the idea of redundancy cannot be overlooked. No critical display should ever provide a single point of failure in the system. The displays must serve the pilot, not the other way around.

Take the time to truly understand the information that is presented to you by your instruments, in both normal and failure modes. An emergency is no time to start digging through a manual.

Related: Avionics News

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