The Crash Position Indicator
Aviation Safety

By Sadiq Hasnain, April 1979

Almost 25 years of effort at NRC's National Aeronautical Establishment (NAE), has resulted in the design of a unique system for locating a downed plane, its passengers and the flight recorder. Called the Crash Position Indicator (CPI), this brainchild of Harry Stevinson, an engineer with NAE's Flight Research Laboratory, is being manufactured and marketed by the Avionics Division of Leigh Instruments Ltd., at Carleton Place, Ontario.

When planes crash in remote areas of the world, such as Canada's north, searching for them without the aid of an emergency radio beacon's distress signal is like looking for a needle in a haystack. Rescue may take days when the chances of survival of the injured may hinge on only a few hours. The early recovery of passengers is, of course, one of the key arguments for using emergency radio beacons on aircraft. But another important consideration is the recovery of the aircraft or its flight recorder so that the problem causing the crash can be determined and thereby avoided in future flights.

Twenty-five years ago aviation people were aware of the need for a reliable emergency radio beacon. Beacons which were carried inside air craft were practically useless in many types of crashes because they were either destroyed, buried or sunk. It was obvious that a successful beacon would somehow have to escape from the plane in the fraction of a second that the aircraft took to crash. The best system at the time fired the beacon device from a mortar, but never gained popular acceptance. There were simply too many vulnerable devices involved a parachute, a shock absorber, two orienting arms, an external extendable antenna, and a flotation bag; added to this, the mortar system did not necessarily have the time to fire the device clear.

Harry Stevinson, electrical engineer-cum-inventor who joined the Flight Research Laboratory in 1945, was aware of the shortcoming of these beacon systems and set out to design a new Crash Position Indicator. Knowing what not to do, lie decided to try to build an escape device system without moving parts and yet contain the transmitter, antenna and the delivery system all in one package. He believed that, if the device were mounted externally on the plane's body and attached by a spring-loaded latching mechanism which released on impact, the air rushing against the CPI's leading edge would strip it away from the plane almost instantaneously. At the same time its shape would provide enough lift to carry it a safe distance away from the crash scene but produce enough speed-reducing drag to land safely nearby. The outer protective skin and shock absorbing foam would have to be tough, transparent to radio waves, and the antenna capable of transmitting a signal whatever its orientation on the earth's surface. Finally, the package would have to float and be fire resistant.

After a year or two of experiments in which Stevinson and his NAE co-workers considered the multitude of problems involved in designing a successful transmitter, they came up with the first facsimile of the modern CPI, a paper model which was first tested by dropping it from the balcony of the hangar laboratory. The disc-shaped device, which tumbled as it fell, gave encouraging results, and he built the next version out of aluminum. The model's performance, tested by releasing it from a speeding car, convinced Stevinson that the tumbling airfoil principle was almost ideally suited to this complex task.

While Stevitison and his co-workers were adapting the system to various aircraft, the Division of Electrical Engineering developed his ideas for a radio transmitter and an omni-directional antenna small enough to tuck into the CPI.

When the first reinforced plastic CPI (containing transmitter and antenna) was assembled, it looked remarkably like the stubby section of an airplane wing. In one set of trials it was attached to a rocket sled and fired at a cliff at speeds up to 370 km/h. To everyone's pleasure, the model detached from the sled as predicted. While the sled was crashing into the cliff, the CPI flew in an arc over the site, slowed, and gently landed above the cliff, with only minor scrapes to its tough outer skin. The transmitter worked without a hitch.

Tests that included a series of drops from aircraft over all manners of surfaces proved the performance of the CPI.

Yet the device did not enjoy an auspicious commercial beginning. Licenses for manufacturing rights were bounced from company to company, finally to Dominion Scientific Instruments of Ottawa, which contracted Leigh Instruments of Carleton Place to manufacture the system. Leigh, at the time a fledgling company looking for a product, committed itself almost entirely to CPI production. Their success eventually allowed them to buy Dominion Scientific Instruments, which set the Course for Leigh becoming a diversified, multimillion dollar corporation.

Early production models of the CPI were fitted mainly on aircraft operating in Canada's north. When some of these crashed, the CPI was instrumental in their early location and recovery. In one particular case, a plane crash in the Yukon mountains, location would have been impossible without the CPI. The internal antenna was able to get the signal between the mountain peaks to the search aircraft.

In another example, the United States Air Force credited the CPI with the early location of one of their aircraft which crashed at night in the ocean. The life of at least one critically injured person was saved by the quick rescue (Leigh Instruments received a letter of thanks, which was forwarded to NAE).

The next development by Leigh in the CPI story involves the inclusion of a flight recorder with the emergency signal transmitter. The flight recorder, or "black box", is an electronic recording device which monitors the aircraft's systems and operations, information vital to identifying the cause of aircraft accidents. Most heavy aircraft carry a "black box" as a permanent internal fixture, but they are prone to damage in a crash. In one case, it took many hours of painstaking effort at NAE's Flight Recorder Playback Center to extract the recorder's information from the charred i-nagnetic tape. Although the Center was able to reconstruct the doomed flight, a lot of time and effort could have been saved if the recorder had been contained in a CPI.

The success of the Crash Position Indicator is now well established. The Canadian armed forces are committed to using them, as are some of the American Armed Forces. European jet manufacturers are placing CPI's on planes such as PanAvia's Tornado, and many private planes flying Canada's remote northern routes carry them.

For Leigh Instruments of Carleton Place, CPI spells commercial success. Manufacturing and marketing the product has created a healthy financial picture for Leigh's Avionics Division, with CPI sales of six million dollars in 1978 alone.

Reprinted courtesy of National Research Council.